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Bulletin No, 69 |
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JEMINA JiiXTRACTION
ILLINOIS STATE GEOLOGICAL SURVEY
3 3051 00000 1630
STATE OF ILLINOIS
DWIGHT H. GREEN, Governor
DEPARTMENT OF REGISTRATION AND EDUCATION
FRANK G. THOMPSON', Director
DIVISION OF THE
STATE GEOLOGICAL SURVEY
M. M. LEIGHTON, Chief URBANA
BULLETIN NO. 69
AMENABILITY OF VARIOUS TYPES OF CLAY MINERALS TO
ALUMINA EXTRACTION
BY THE LIME SINTER AND LIME-SODA 0^
SINTER PROCESSES G^°V°G *
R. E. Grim, J. S. Machin, and W. F. Bradley
Released for publication by the Office of Production Research and Development of the War Production Board, Research Project NRC-523 under contract WPB-38 with hie University of Illinois
PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS
URBANA, ILLINOIS
1945
ORGANIZATION
STATE OF ILLINOIS
HON. DWIGHT H. GREEN, Governor DEPARTMENT OF REGISTRATION AND EDUCATION
HON. FRANK G. THOMPSON, Director
BOARD OF NATURAL RESOURCES AND CONSERVATION
HON. FRANK G. THOMPSON, Chairman NORMAN L. BOWEN, Ph.D., D.Sc, LL.D., Geology ROGER ADAMS, Ph.D., D.Sc, Chemistry LOUIS R. HOWSON, C.E., Engineering *WILLIAM TRELEASE, D.Sc, LL.D., Biology EZRA JACOB KRAUS, Ph.D., D.Sc, Forestry ARTHUR CUTTS WILLARD, D.Engr., LL.D. President of the University of Illinois
GEOLOGICAL SURVEY DIVISION
M. M. LEIGHTON, Chief
^Deceased.
(82471— 4M— 5-45)
SCIENTIFIC AND TECHNICAL STAFF OF THE
STATE GEOLOGICAL SURVEY I) I V S I O N
100 Natural Resources Building, Urbana
M. M. LEIGHTON, Ph.D., Chief
Enid Townley, M.S., Assistant to the Chief
Velda A. Millard, Junior Asst. to the Chief
Helen E. McMorris, Secretary to the Chief
Effie Hetishee, B.S., Geological Assistant
GEOLOGICAL RESOURCES Coal
G. H. Cady, Ph.D., Senior Geologist and Head L. C. McCabe, Ph.D., Geologist (on leave) R. J. Helfinstine, M.S., Mech. Engineer Charles C. Boley, M.S., Assoc. Mining Eng. Heinz A. Lowenstam, Ph.D., Assoc. Geologist Bryan Parks, M.S., Asst. Geologist Earle F. Taylor, M.S., Asst. Geologist
(on leave) Rai ph F. Strf.te, A. M., Asst . Geologist M. W. Pullen, Jr., M.S., Asst. Geologist Robert M. Kosanke, M.A., Asst. Geologist Robert W. Ellingwood, B.S., Asst. Geologist George M. Wilson, M.S., Asst. Geologist Arnold Eddings, B.A., Research Assistant
(on leave) Henry L. Smith, A.B., Asst. Geologist Raymond Siever, B.S., Research Assistant (on leave) John A. Harrison, B.S., Research Assistant
(on leave) Mary E. Barnes, B.S., Research Assistant Margaret Parker, B.S., Research Assistant Elizabeth Lohmann, B.F.A., Technical Assistant
Industrial Minerals
J. E. Lamar, B.S., Geologist and Head H. B. Willman, Ph.D., Geologist
Robert M. Grogan, Ph.D., Assoc. Geologist
Robert T. Anderson, M.A., Asst. Physicist
Robert R. Reynolds, M.S., Asst. Geologist
Margaret C. Godwin, A.B., Asst. Geologist
Oil and Gas
A. H. Bell, Ph.D., Geologist and Head Carl A. Bays, Ph.D., Geologist and Engineer Frederick Squires, B.S., Petroleum Engineer Stewart Folk, M.S., Assoc. Geologist (on leave) Ernest P. Dubois, Ph.D., Assoc. Geologist David H. Swann, Ph.D., Assoc. Geologist Virginia Kline, Ph.D., Assoc. Geologist Paul G. Luckhardt, M.S., Asst. Geologist
(on leave) Wayne F. Meents, Asst. Geologist James S. Yolton, M.S., Asst. Geologist Robert N. M. LIrash, B.S., Research Assistant Margaret Sands, B.S., Research Assistant
Areal and Engineering Geology
George E. Ekblaw, Ph.D., Geologist and Head Richard F. Fisher, M.S., Asst. Geologist
Subsurface Geology
L. E. Workman, M.S., Geologist and Head Carl A. Bays, Ph.D., Geologist and Engineer Robert R. Storm, A.B., Assoc. Geologist Arnold C. Mason, B.S., Assoc. Geologist
(on leave) C. Leland Horberg, PhD., Assoc. Geologist Frank E. Tippie, B.S., Asst. Geologist Merlyn B. Buhle, M.S., Asst. Geologist Paul Herbert, Jr., B.S., Asst. Geologist Charles G. Johnson, A.B., Asst. Geologist
(on leave) Margaret Castle, Asst. Geologic Draftsman Marvin P. Meyer, B.S., Asst. Geologist Robert N. M. Urash, B.S., Research Assistant Elizabeth Pretzer, A.B., Research Assistant Ruth E. Roth, B.S., Research Assistant
Stratigraphy and Paleontology
J. Marvin Weller, Ph.D., Geologist and Head Chalmer L. Cooper, M.S., Assoc. Geologist
Petrography
Ralph E. Grim, Ph.D., Petrographer
Richards A. Rowland, Ph.D., Asst. Petrographer
(on leave) William A. White, B.S., Research Assistant
Physics
R. J. Piersol, Ph.D., Physicist
B. J. Greenwood, B.S., Mech. Engineer
GEOCHEMISTRY
Frank H. Reed, Ph.D., Chief Chemist Elizabeth Ross Mills, M.S., Research Assistant
Coal
G. R. Yohe, Ph.D., Chemist
Herman S. Levine, B.S., Research Assistant
Industrial Minerals
J. S. Machin, Ph.D., Chemist and Head Delbert L. Hanna, A.M., Asst. Chemist
Fluorspar
G. C. Finger, Ph.D., Chemist
Oren F. Williams, B. Engr., Research Assistant
X-ray and Spectrography
W. F. Bradley, Ph.D., Chemist
Chemical Engineering
Harold W. Jackman, M.S.E., Chemical Engineer P. W. Henline, M.S., Assoc. Chemical Engineer James C. McCullough, Research Associate James H. Hanes, B.S., Research Assistant (on leave) Leroy S. Miller, B.S., Research Assistant (on leave)
Analytical
O. W. Rees, Ph.D., Chemist and Head L. D. McVicker, B.S., Chemist Howard S. Clark, A.B., Assoc. Chemist William F. Wagner, M.S., Asst. Chemist Cameron D. Lewis, B.A., Asst. Chemist Herbert N. Hazelkorn, B.S., Research Assistant William T. Abel, B.A., Research Assistant Melvin A. Rluenstorf, B.S., Research Assistant Marian C. Stoffel, B.S., Research Assistant Jean Lois Rosselot, A.B., Research Assistant
MINERAL ECONOMICS
W. H. Voskuil, Ph.D., Mineral Economist Douglas F. Stevens, M.E., Research Associate Nina Hamrick, A.B., Research Assistant Ethel M. King, Research Assistant
PUBLICATIONS AND RECORDS
George E. Ekblaw, Ph.D., Geologic Editor Chalmer L. Cooper, M.S., Geologic Editor Dorothy E. Rose, B.S., Technical Editor Meredith M. Calkins, Geologic Draftsman ['.I i i \n Featherstone, B.F.A., Asst. Geologic
Draftsman Willis L. Busch, Principal Technical Assistant Portia Allyn Smith, Technical Files Clerk Leslie D. Vaughan, Asst. Photographer
Consultants: Ceramics, Cullen W. Parmalee, M.S., D.Sc, and Ralph K. Hursh, B.S., University of Illinois
Mechanical Engineering, Seiciii Konzo, M.S., University oj Jllinoti Topographic Mapping in Cooperation with the United States Geological Survey.
May, 1, 1945
Digitized by the Internet Archive
in 2012 with funding from
University of Illinois Urbana-Champaign
http://archive.org/details/amenabilityofvar69grim
CONTKNTS
Page
Introduction 9
Clay mineral concept 10
Objectives of the investigation 10
Acknowledgments 11
Materials studied 11
Clays 11
Selection of samples 11
Collection and preparation of samples 11
Analysis of samples 13
Limestone 13
Anorthosite 13
Lime-sinter process 14
Experimental methods 14
Preparation of sinter samples 14
Chemical analysis of sinters 15
Extraction of alumina from sintered material and analysis of extracts 15
Experimental results 15
Effect of type of clay minerals 15
Optimum yield of alumina 15
Lime-alumina ratio 16
Sintering temperature 19
Effect of time held at sintering temperature 27
Effect of furnace atmosphere 28
Dusting of sinters 29
Silica in the alumina extract 30
Effect of minor components present in the raw materials 30
General comments 30
Effect of magnesium 30
Effect of sulfur 32
Effect of phosphorus 32
Effect of iron 33
Effect of different types of limestone 34
Mineralogical analyses of sinters 35
Microscopic analysis 35
General comments 35
Effect of variations in lime content in sinters fired one hour at 1360° C 35
Effect of variations in sintering temperature on batches with a lime-to-alumina ratio
of 1.66 and held at sintering temperature for one hour 37
Effect of variations in sintering time of batches with a lime-to-alumina ratio of 1.66
fired at 1360° C 37
Effect of variations in kiln atmosphere 38
Effect of miscellaneous variations in composition 38
Effect of variation in the character of the limestone 38
X-ray analysis 39
General comments 39
Predominant phases 39
Low temperature syntheses 40
Sintering process 40
Differential thermal analyses 41
Summary and conclusions 44
CONTENTS — Continued
Page
Lime-soda sinter process 47
Experimental methods 47
Preparation of sinter samples 47
Chemical analysis of sinters 48
Extraction of alumina from sintered material and analysis of extracts 48
Experimental results 48
Effect of type of clay minerals 48
Optimum yield of alumina 48
Sintering temperature 50
Effect of time held at top sintering temperature 54
Effect of heating rate 57
Effect of furnace atmosphere 57
Effect of soda-to-alumina ratio 57
Effect of lime-to-alumina ratio 60
Silica in the alumina extract 60
Effect of minor components present in the raw materials 60
General comments 60
Effect of phosphorus 64
Effect of titania 64
Effect of iron 64
Effect of magnesia 64
Mineralogical analyses 64
Microscopic analysis 64
Effect of variations in top sintering temperature 64
Effect of variations in the ratio of soda to alumina 65
Effect of miscellaneous variations in composition 68
Effect of reducing atmosphere 68
Effect of variation in time batch held at top sintering temperature 69
Effect of variation in heating rate 69
Effect of variations in the ratio of lime to silica 69
X-ray analysis 70
General comments 70
The phases 70
The sintering process 70
Differential thermal analyses 71
Discussion and summary of study of phases present in the sinters 73
Summary and conclusions 74
References 77
ILLUSTRATIONS
Figure Page
1 A-E — Percent A1203 extracted versus mole ratio CaO/Al203 16—18
2 A-E — Percent A1203 extracted versus temperature at which sinter was held for
one hour 19—21
MgO
3 — Percent A1203 extracted versus 100 ., ., in sinter mix 31
4 — Percent A1203 extracted versus percent S03 in sinter mix 32
5 — Percent A1203 extracted versus percent P205 in sinter mix 33
6 — Percent A1203 extracted versus percent Fe203 in sinter mix 34
7 — Photomicrographs of lime-clay sinters 36
8 — Differential thermal analyses of lime-clay sinters 42
9 — Scale for determining the temperature differences recorded by peaks of the
differential thermal curves 43
10 A-B — Comparison of extraction values for lime-soda sinters prepared from clays
containing more than one clay mineral type with hypothetical mixtures 49
11 A-E — Percent A1203 extracted versus temperature at which sinter was held for
10 minutes 50-52
12 A-D — Percent A1203 extracted versus time sinter was held at 1100° C 55—56
13 A-D — Percent A1203 extracted versus mole ratio Na20/Al203 58—59
14 A-B — Percent A1203 extracted versus mole ratio CaO/Si02 61
15 — Typical record of a sinter heating schedule as traced from recorder chart 62
16 — Percent A1203 extracted versus percent P205 in mix 63
17 — Percent A1203 extracted versus Ti02 in mix 63
18 — Percent Al2Os extracted versus sinter temperature 63
19 — Percent A1203 extracted versus percent Fe203 in mix 63
% MgO
20 — Percent A1203 extracted versus 100 ~, . , ^ in sinter 64
21 — Photomicrographs of soda-lime-clay sinters 66
22 — Photomicrograps of lime-soda-clay sinters 67
23 — Differential thermal analyses of lime-soda-clay mixtures 72
21 — Equilibrium diagram of the system CaO— A1203— Si02 76
TABLES
1 — Common clay minerals 10
2 — Location and clay mineral composition of clay samples 11
3 — Chemical composition of clay samples 12
A — Location and texture of limestone samples 13
5 — Chemical composition of limestone samples 13
6 — Chemical composition of anorthosite sample 13
7 — Sinter composition and extraction data 22—26
8 — The effect of time held at sintering temperature 27
9 — The effect of furnace atmosphere on extractability 28
10— The effect of phosphorus on the extractability of lime-clay sinters 33
11 — Effect of different types of limestone on extractability of clay-lime sinters 34
12 — Effect of sintering temperature on percent A12C):; extracted with various types of clay 53
AMENABILITY OF VARIOUS TYPES OF
CLAY MINERALS TO
ALUMINA EXTRACTION
BY THE LIME SINTER AND LIME-SODA
SINTER PROCESSES
By
R. E. Grim, Petrographcr J. S. Machin, Physical Chemist W. F. Bradley, X-ray Analyst
INTRODUCTION
THE usual ore from which aluminum is recovered is bauxite, an essentially silica-free alumina hydrate. In order to meet the normal peacetime needs of the United States, it has been necessary to import bauxite. With the outbreak of the war, government agencies at once recognized the seriousness of the situation that would result if bauxite imports were halted and began studies of the possible use of other materials as sources of alumina. Particular attention was directed to clays because they are abundant, they contain considerable quantities of alumina, and a considerable amount of information concerning clays was already in hand.
There are two general processes for ex- tracting alumina from clays: An acid proc- ess that uses an acid to selectively dissolve the alumina, usually after the clay has been roasted, and an alkaline process that uses water or a dilute alkaline solution to selec- tively dissolve the alumina from a sintered mixture of lime and clay or of lime, soda, and clay. Most past experimental work with these processes had been limited to high-alumina clays of the kaolin or bauxitic- kaolin type. A study of the occurrence of clays indicated that deposits of kaolinite clays of sufficient purity and size for an alumina plant are not now known. Kaolin- ite clays are abundant, but in the main they contain varying, although frequently small,
amounts of other clay minerals. Further, other types of clay, notably the illite shales, are widespread in enormous relatively uni- form deposits, and some of them have alumina contents not very much smaller than the average in the large deposits of kaolin.
It became obvious, therefore, that re- searches were needed on the clay minerals other than kaolinite as possible sources of alumina. Accordingly in January 1943, the War Metallurgy Committee of the Nation- al Academy of Sciences suggested that the Illinois State Geological Survey conduct an investigation of the "amenability of clay mineral types to lime sinter and lime-soda sinter alumina processes" for the War Production Board.
The Illinois State Geological Survey had been and still is conducting extensive re- searches on the clay mineral composition and properties of the clays and shales of Illinois. Since the properties of clay materi- als in general are controlled largely by their clay mineral composition, the Survey has studied extensively the relation between the clay mineral composition of all types of clays and shales and their properties and uses. The investigation undertaken for the National Academy of Sciences was, there- fore, essentially an extension of these re- searches.
9 I
10
ALUMINA EXTRACTION
Clay Mineral Concept
Recent researches in several laboratories have shown that natural clay substances are composed of extremely small crystal- line particles of members of any one or more of a few groups of minerals known as the "clay minerals." The "clay minerals" (table 1) are hydrous aluminum silicates, frequently with some replacement of alumi- num by iron and magnesium and with small amounts of alkalies and alkali earths. In rare instances magnesium and iron com- pletely replace the aluminum. In addition to the "clay minerals," variable but usually small amounts of quartz, limonitic material, gibbsite, diaspore, organic material, pyrite, feldspar, etc., may also be present in clay materials. Grim1* has recently published a summary of the composition, properties, structure and occurrence of the various clay minerals.
Table 1. — Common Clay Minerals
|
Name |
Composition formula |
|
Kaolinite group*. . |
(OH)8Al4Si4O10 |
|
Illite groupb |
(OH),Ky(Al4-Fe4-Mg4-Mgs) |
|
(ol8-y - Aly)U20 |
|
|
Montmorillonite |
|
|
group0 |
(OH)4Al4Si8O20-nH2O |
|
Hydrated halloy- |
|
|
sited |
(OH)8Al4Si4Oio-2H20 |
|
Halloysite |
(OH)8Al4Si4O,0 |
|
Attapulgitee |
(OH2yOH)2Mg5Si8O20-4H2O |
a Dickite and nacrite with the same composition also belong to this group but their occurrence is extremely rare. Anauxite with a slightly higher silica content is usually classified in the kaolinite group — its occurrence is also rare.
b As indicated by the formula the composition of the illite group is variable. The y in the formula is frequently equal to about 1. The illite group has not yet been divided into distinct mineral species.
c The A1+++ of the montmorillonite is replaceable by Fe+++ or Mg++. When the replacement is relatively complete the resulting mineral species are nontronite and saponite, respectively.
d Hydrated halloysite inverts to halloysite irreversibly at about 60°C.
e In attapulgite A1+++ can replace Mg++ and Si+++ to a limited extent.
The type of clay known as kaolin is com- posed of minute ( 1 micron ± ) crystalline particles of the mineral kaolinite. Some such clays contain also small amounts of mont- morillonite and illite. The ceramic ball clays and fireclays are usually composed of kaolinite with small amounts of illite. The high-alumina clays are mixtures of kaolin-
*See references at end of report.
ite and diaspore or gibbsite. Large deposits of halloysite clays are not known, but the mineral may be present in small amounts in many clays that are made up primarily of other clay minerals.
Illite is the dominant mineral in all the shales that have been studied mineralogical- ly. Bentonites are composed primarily of extremely minute particles of montmorillo- nite, and attapulgite is the component of certain fuller's earths. The clay mineral composition of surficial soil materials is variable depending on the conditions under which the soil has formed and on the parent material.
Objectives of the Investigation
Objectives of the investigation were as follows :
1. To determine if any clays other than those composed of kaolinite are promising raw materials for alumina extraction by the alkali methods, and if so how the con- ditions governing sintering and extraction of other clays differ from those governing the sintering and extraction of kaolinite types.
2. To provide some of the necessary data for the solution of problems that will arise in the large scale use of clays as a source of alumina because of the inevitable variations in clays. It is unlikely that a de- posit of any clay, kaolin or some other type, large enough for several years supply of alumina, is to be found anywhere that is composed throughout of a single clay min- eral. Variations in kaolin deposits, for ex- ample, may be expected by the sporadic occurrence of small amounts of illite or montmorillonite.
3. To throw light on the mechanism of the extraction of alumina from clays by the alkali methods. It seemed probable that the study of a variety of materials might provide clues that would not appear in investiga- tions limited to one type of clay.
After the investigation was started it was enlarged to include the study of the effect on alumina extraction of certain non- clay-mineral impurities apt to be found in clays and of variations in the character of limestone.
MATERIALS STUDIED
11
Acknowledge ents
'{'he authors wish to express their appre- ciation to the War Metallurgy Committee of the National Academy of Sciences and the War Production Board for sponsoring
the project and their continued interest in the work. They also wish to make acknowl- edgment to the Project Committee which supervised the project, particularly to its chairman, Mr. John D. Sullivan, for many helpful suggestions and comments.
MATERIALS STUDIED
CLAYS
Selection of Samples
On the basis of a large amount of analyt- ical data in the files of the Illinois State Geological Survey, 19 samples of clay (table 2) were selected to represent the important types of clay minerals and mixtures of these minerals. Particular at- tention was given to clays containing some kaolinite because such clays tend to have the highest content of alumina. The samples selected to represent mixtures illustrate the
combinations of clay minerals that are apt to be encountered in large deposits of the common types of clays. Further consider- ation was given to the selection of samples that would represent important types of clay that occur widely in large deposits.
Collection and Preparation of Samples
All of the samples except 865, 868, 881, 882, and 883 were collected personally by Drs. Grim and Bradley. About 100 lbs. of each sample was obtained in lump form.
Table 2. — Location and Clay Mineral Composition of Clay Samples
Sample No.
Type of clay
Location
Mineral composition
865 866
Kaolin Ringgold, Tenn.
Underclay Grundy County,
Kaolinite
111 Illite, kaolinite (20%±), quartz (15%=b), trace
of pyrite, gypsum, and calcite
Halloysite clav Eureka, Utah Hallovsite and hydrated halloysite, gibbsite
(10%±)
Bentonite Clay Spur, Wyo Montmorillonite, quartz (10%±)
Kaolin Union County, 111 Kaolinite with unusual lattice structure proper- ties, quartz (10%±)
Shale Menard County, 111 Illite (high iron variety), quartz (15%±), trace
of pyrite, limonite, kaolinite
Plastic fireclay Mexico, Missouri Kaolinite with small amount of illite, montmoril- lonite and organic material
Diaspore Swiss, Missouri Diaspore, trace of kaolinite, anatase, rutile
Flint clay New Florence, Missouri. Kaolinite, trace of anatase and montmorillonite
(?) Bauxitic kaolin Dry Branch, Ga Kaolinite with small amount of gibbsite, trace of
anatase
Bauxite Irvington, Ga Gibbsite with small amount of kaoinlite, trace of
anatase Hard kaolin Gordon, Ga Kaolinite, very fine grain size, trace of montmoril- lonite
Soft kaolin Dry Branch, Ga Kaolinite, medium grain size
Plastic kaolin Dry Branch, Ga Kaolinite, montmorillonite (25%±)
Ball clay Whi dock, Tenn Kaolinite, illite, quartz (10%±), organic ma- terial
Ball clay Atwood, Tenn Kaolinite, illite (?), gibbsite (?)
Kaolin Aiken, S. Carolina Kaolinite, trace of alunite
Fuller's earth Quincy, Florida Attapulgite, montmorillonite (10%±)
Kaolin Hobart Butte, Oregon. . . Kaolinite with low degree of crystallinity
881 882 883
12
ALUMINA EXTRACTION
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MATERIALS STI'DIKP
13
Table 4. — Location and Texture of Limestone Samples
Sample No.
Aye and Location
Size of calcite particles
R7 St. Louis limestone, Alton, Maximum diameter 5 microns; most particles less than 3 mi-
lllinois crons
R2 St. Louis limestone, Alton, Maximum diameter 15 microns; particles range from 3 to 15
Illinois microns
MC Burlington limestone Maximum diameter 70 microns; most particles larger than 20
Marblehead, Illinois microns
A small quantity of the lump material of each sample was retained for reference, and and the remainder was ground to minus 10-mesh in a disk pulverizer in order to obtain thorough mixing. Ten-pound lots of each sample were ground further to minus 80-mesh in preparation for chemical analysis and sinter experiments.
Analysis of Samples
The clay mineral composition of each sample was determined by X-ray diffrac- tion, differential thermal, and optical analy- ses with the results given in table 2. Chemi- cal analyses of all samples are given in table 3.
LIMESTONE
Precipitated CaCOs was used in prepar- ing all of the sinters except those especially made to investigate the effect of variations in the textural character of the limestone on alumina extraction. For this purpose three limestones were selected. Table 4 gives the location of the limestones and the size of the particles of calcite that make up each of them. Chemical analyses of the lime- stones are given in table 5.
ANORTHOSITE
Anorthosite was included in the part of the investigation concerned with lime-soda sinters. A sample was supplied by Mr. D.
R. Williams of the Monolith Portland Midwest Company from a deposit near Laramie, Wyoming. A chemical analysis of the sample is given in table 6. Mineralogi- cally the anorthosite is composed of a plagioclase feldspar of about labradorite composition.
Table 5. — Chemical Composition of Limestone
Samples
(In percent)
R7
Si02 2.59
Ti02 02
AL03 55
Fe203 14
MgO 33
CaO 53.77
Na20 06
K2O 07
P205 00
SO3 14
Loss on Ign 42.53
Total 100.20
H2O-(<110°C). .24
C02 42.30
Table 6. — Chemical Composition of Anortho- site Sample (In percent)
Si02 52.50 MgO... 0.21 P2Os... 0.08
TiOs 23 CaO... 11. 71 Loss on
Ign 32
Fe203 2.28 Na20....3.94
AI2O3. . . .28.48 K2Q 43 Total. .100. 16
|
R2 |
MC |
|
0.46 |
0.35 |
|
.02 |
.00 |
|
.27 |
.14 |
|
.14 |
.07 |
|
.23 |
.27 |
|
55.36 |
55.65 |
|
.05 |
.08 |
|
.03 |
.00 |
|
.00 |
.00 |
|
.06 |
.04 |
|
43.67 |
43.72 |
|
100.29 |
100.32 |
|
.08 |
.11 |
|
43.51 |
43 . 69 |
14
ALUMINA EXTRACTION
LIME-SINTER PROCESS
This process in brief consists in mixing ground siliceous alumina bearing ore with ground calcium carbonate in the proportions to give a mixture corresponding approxi- mately in percentage composition (ignoring volatile constituents present) to a mixture of dicalcium silicate and pentacalcium tria- luminate. This mixture is then heated at a suitable temperature (of the order 1375°C.) and for such period of time as may be necessary to develop dicalcium sili- cate and some calcium aluminate compound or compounds which can be acted upon by dilute alkali carbonate solution in such manner as to dissolve alumina and leave undissolved all or nearly all of the silica, lime, and any other materials present in the sintered material. The dicalcium silicate plays an extremely important dual role. First, its development is complete enough to tie up nearly all of the silica present in a form not soluble in the leach liquor. Second, it undergoes a crystallographic transformation on cooling below about 675 °C. with an increase in volume which results in reduction of the sintered mass to a powder, making the soluble alumina compounds easily accessible to the action of the leach liquor without grinding. This phenomenon is commonly called "dusting."
The clay samples used in these experi- ments after mixing with CaCOs and heating gave sintered materials in which the com- position ranged about as follows in most cases :
AhO 10 to 30 percent CaO 52 to 62 percent SiO 14 to 28 percent
The fusion points of mixtures represent- ed by such ranges in composition are well above 1375°C. so that in general, it is to be expected that a state of chemical equilib- rium will be approached but not reached unless the mixtures are kept at the sintering temperature for very long periods of time. The longer the period of time and the high- er the temperature, the closer will be the
approach to a state of equilibrium. The presence of varying, even though minor, amounts of alkalies, iron, magnesia, phos- phorus, and titania also might be expected, through fluxing action or otherwise, to in- fluence the final state reached. Finally the various clay minerals, with their differences in reactivity and refractoriness might be ex- pected to exert a major effect, at least up to some critical temperature which would probably be different for each clay mineral type. This critical temperature might be the fusion point of the sinter mixture or might be a lower temperature. The data presented in this report were collected from experiments designed to study the influence of these factors on the lime-sinter process.
The equilibrium diagram of the ternary system CaO- AI2O3— SiC>2 of Rankin and Wright as revised by Schairer9 (page 76) is included for the convenience of the reader in visualizing the compositions of the sin- ters discussed in the lime-sinter section of this report. The shaded section includes the range of compositions included in our ex- periments.
EXPERIMENTAL METHODS
Preparation of Sinter Samples
All of the clay samples except 865 and 872 were calcined at 800°C. Sample 872 was not calcined because calcination of diaspore clay results in crystal changes that take place below 800° C. which it was desired to avoid. Sample 865 was the clay used most extensively in certain experiments at the T. V. A. laboratory,3 and it was de- sired to follow their experimental method. In this instance the calcination temperature was 900° C.
Calcined clays were ground to pass 80- mesh sieves and then mixed thoroughly with the desired quantity of precipitated calcium carbonate and enough water to make a good briquet. The mixtures were pressed into cylindrical briquets about \j4, inches in diameter under a pressure of
LIME-SINTER PROCESS
15
approximately 5000 pounds per square inch. These briquets were dried, placed in a cold Globar-heated furnace, heated up to the desired temperature, and held for a chosen period at that temperature. The cooling rate was such that approximately 30 min- utes elapsed while the furnace cooled down to 1100°C After this the power was turned off. The sintered briquets were removed from the furnace when the temperature had dropped to about 700°— 800°C and were stored in closed bottles as soon as they were cool enough to handle. Usually the briquets dusted in the cooling process.
Most sinters prepared as described dusted to a powder which easily passed a 200-mesh screen with one percent or less oversize. A few had considerable plus 200-mesh mate- rial. Some of those which did not so disinte- grate were ground to pass 200 mesh and subsequently extracted in the same manner as the others.
Chemical Analysis of Sinters
The sintered material was analyzed by gravimetric methods for SiOa, Al^O^, CaO, FesOs and TiCX About three quarters of all sinters were so analyzed.
Extraction of Alumina from Sintered Material and Analysis of Extracts
The extraction procedure used was basi- cally similar to that used by Walthall 2 and coworkers at the T. V. A. laboratory, Wil- son Dam, Alabama. Ten-gram samples of each sinter were extracted with three per- cent sodium carbonate solution using me- chanical stirring sufficiently vigorous to pre- vent settling. The extraction time was 15 minutes and the temperature was 65 °C. Sufficient sodium carbonate solution was used to provide 1.66 moles of Na;CO< for each mole of Al^Oa calculated to be present in the 10-gram sample. The extract was immediately filtered off and the residue washed. The extract was analyzed for alumina and silica using gravimetric meth- ods for alumina and colorimetric methods for silica.
EXPERIMENTAL RESULTS
Effect of Type of Clay Minerals optimum yield of alumina
The factors which may be varied at the will of the operator of a plant producing alumina by the lime-sinter process are the lime-to-clay ratio, the temperature, the du- ration of the heating period, and to a much lesser extent the furnace atmosphere. Proper adjustment of these variables with relation to one another might be expected to result in an optimum yield for a given raw material, assuming good extraction practice. Data are presented in graphic and tabular form in figures 1 and 2, and in tables 7, 8, and 9 which bear on the effect of these operating variables when the various types of clays are used in the sinter mix under laboratory con- ditions. Variables connected with the ex- traction process were not considered in these experiments except that care was taken to keep the extraction conditions constant.
The data indicate that gibbsite-kaolinite clays and kaolinite clays may yield above (some well above) 90 percent of their alumina with little to choose between them. Note data on clay samples, 865, 869, 873, 874, 875, 876, 877, 880, and 881.
Clay sample 883 did not quite measure up to the other kaolinite clays in yield of alumina. Two possible causes of the lower yields from this clay can be suggested; the first and more probable is that it differed considerably from the typical kaolins insofar as its content of true kaolinite is concerned ; second, the titania content was rather high. Clay sample 865 also had rather high titania content and gave slightly lower yields than some of the best kaolinite clays. It is not clear, however, why titania should cause lower yields. The presence of calcium titan- ate in the sinters (see page 38) suggests that better yields might be expected with the higher lime-to-clay ratios, but this sugges- tion is not confirmed experimentally. Impure kaolins, containing illite and/or montmo- rillonite, gave lower yields roughly in pro- portion to the amounts of these clay miner- als present.
The illite and montmorillonite clays gave lower percentage yields than the
16
ALUMINA EXTRACTION
kaolinite clays although it is not proved that this was due to the clay mineral type. Sin- ters made with such clays tended to dust slowly and poorly, possibly because of their high alkali content. That is, the dusted sin- ter contained larger amounts of coarse sandy material. The iron, as indicated in another part of the report, appeared in the sinter as tetracalcium alumino ferrite from which alumina is not readily extractable with dilute sodium carbonate solutions. The illites reacted adversely to low lime ratios (fig. 1, samples 866 and 870), probably be- cause of their high iron content.
Halloysite clay gave low yields except at very high temperatures. This clay contained remarkably small amounts of oxides other than alumina and silica. The sinters pre- pared from it dusted fairly quickly but in- completely and contained much coarse sandy material, indicating either incomplete in- version of dicalcium silicate, incomplete re- action between the clay and the lime, or the presence of stable glass.
Diaspore clay gave very unsatisfactory sinters. Attempts to sinter it with lime at temperatures above 1300°C. resulted in par- tially fused glassy masses which did not dust. Upon grinding through 200 mesh these sinters yielded about 70 percent of their alumina. Both the fusion and the failure to dust were expected. The sinters prepared from this clay contained 6 to 7 percent silica, about 40 percent alumina, 48 to 50 percent lime, and the balance iron, titania, and alkalies. The fusion point of such a composition should be below 1335° C, and it is not surprising that the relative- ly small amount of dicalcium silicate that could be formed should be insufficient to cause dusting.
Attempts to prepare sinters from the attapulgite clay resulted in cinder-like mass- es which disintegrated slowly into coarse sand-like particles. These sinters were so unpromising that no further work was done on this clay.
LIME-ALUMINA RATIO
Data presented graphically in figure 1 show the effect of varying the CaO/ALO
ratio on the percentage of alumina extracted from different types of clay. The CaO/Ab Os ratios consider only that part of the total CaO above that required to react with the silica to produce dicalcium silicate. These same data are listed in table 7 together with the analyses of the sintered materials and of the clays from which they were prepared.
The data show that in general pure kaolinite clays are not very sensitive to moderate variation in the CaO/ALOs ratio. Ratios between 1.5 and 1.8 appear to be favorable for most kaolins, whereas ratios outside these limits usually result in lower yields of alumina.
Illite clays, probably because of high iron, sometimes show considerable improvement in yield with increased lime (note sample 870 in figure 1). Illite sample 866, which gave a low yield with a CaO/AbOs ratio of 1.4, showed an increase in yield when the ratio was increased to 1.5, but no further increase with larger amounts of CaO.
Neither the halloysite clay, nor the mont< morillonite clay showed any decided sen- sitivity to the CaO/AbOs ratio. The data were somewhat erratic. Such erratic ex- traction values are common if the dusting is incomplete, as it was with most sinters prepared from these two clays.
Gibbsite-kaolinite (bauxitic) clays be- haved very much like the pure kaolinites. Like these clays, they reacted unfavorably to much excess lime.
|
1- A |
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Q U |
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KAOLIN |
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1.4 1.5 1.6 1.7 1.8 1.9 2.0 |
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CaO/AI203 |
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Fig. |
1.— p |
art A |
Percent A1203 extracted versus mole ratio CaO/ A1203 (considering only that part of the CaO beyond that required to react with the silica to form 2CaO • Si02). Sinter temper- ature 1360°
LIME-SINTER PROCESS
17
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40 |
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1.4 1.5 16 1.7 18 19 2 0 1.4 1.5 1.6 17 1.8 19 2.0
Ca O /Al2 03
Fig. 1.— Pan, B and ('
Percent Al2Oa extracted versus mole ratio CaO/Al^Os (considering only that part of the CaO
bevond that reauired to react with the silica to form 2CaO.SiOv). Sinter temperature 1360°C.
18
ALUMINA EXTRACTION
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HARD KAOLIN (KAOLINITE ) |
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Fig. 1.— Parts D and E
Percent A1203 extracted ver us mole ratio CaO/Al203 (considering only that part of the CaO
beyond that required to react with the silica to form 2CaO.Si02). Sinter temperature 1360°C.
LIME-SINTER PROCESS
19
SINTERING TEMPERATURE
The data presented in table 7 and figure 2 indicate that for most clays sintering tem- peratures between 1340° and 1380°C. are favorable. Higher temperatures are with few exceptions not necessary unless for the purpose of shortening sintering time.
Kaolinite clays usually give good yields when sintered at 1360° to 1380°C. There was a pronounced tendency to show a de- creased yield with lowered sintering tem- perature which sometimes was manifest at 1340°C. (869, 873, 876, 877, 883), and sometimes not until somewhat lower tem- peratures were reached (865, 871, 874, 878, 879, 880, 881). Sinters prepared from pure kaolinite clays showed little or no tendency to overburn.
Sinter mixes prepared from gibbsite- kaolinite and diaspore clays, due to their high alumina and low silica content, were apt to melt. The composition of sinters pre- pared from sample 875 — if only alumina, lime and silica are considered — should not have melted much below 1600°C. However, the small amounts of other oxides present lowered the fusion point of the mixture considerably. All sinters from sample 875 melted partially when the sintering tem- peratures were only as high as 1360°C. As stated above, sinters from the diaspore clay melted at even lower temperatures.
Sinters prepared from illite and mont-
morillonite clays were somewhat sensitive to sintering temperature. Thej appeared to give lowered yields with temperatures on either side of an optimum which was in the range 1360° to 1380°C. They showed a definite tendency to overburn at higher temperatures.
Sinters prepared from halloysite clay had to be heated to high temperatures in order to give even moderately good yields. Even when heated to 1450° (J. where fusion or semifusion took place the disintegrated sin- ter still felt somewhat gritty.
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NEUTRAL ATMOSPHERE |
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1340 1380 1380 1400
TEMPERATURE DEGREES C
Fig. 2.— Part A Percent A1203 extracted from sinter versus tem- perature at which sinter was held for one hour.
20
ALUMINA EXTRACTION
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TEMPERATURE DEGREES C
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TEMPERATURE DEGREES C.
Fig. 2.— Parts B and C Percent A1203 extracted from sinter versus temperature at which sinter was held for one hour.
LIME-SINTER PROCESS
21
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TEMPERATURE DEGREES C
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1400 1320 1340
TEMPERATURE DEGREES C
Fig. 2.— Parts I) and E Percent A1203 extracted from sinter versus temperature at which sinter was held for one hour.
22
ALUMINA EXTRACTION
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23
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24
ALUMINA EXTRACTION
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Mole Ratio CaO/Al203 |
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LIME-SINTER PROCESS 25
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26
ALUMINA EXTRACTION
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LIME-SINTER PROCESS
27
EFFECT OF TIME HELD AT SINTERING TEMPERATURE
A number of sinters were prepared using selected clay samples for which the time that the sinter was held at maximum tem- perature was varied. For all of these sinters, the lime-alumina ratio was 1.66 and the temperature was 1360°C. The data are re- corded in table 8.
Kaolinite clay sinters all showed reduced extractability when the time held at 1360° C. was only 10 minutes, but there appeared to be no advantage in heating them for periods greater than one hour.
Halloysite clay sinters showed better extractability with increasing time. This clay is not well suited to treatment by the lime-sinter process. The highest temperature used and the longest heating periods were insufficient to give good yields from sinters made with this clay.
The extractability of the gibbsite-kaolin- ite clay was not affected by variations in the sinter time within the range of times used.
The montmorillonite clay and the illite clay (sample 870) appeared to be rather critically sensitive to overtime heating. The reason for the bad effect of long time and high temperature heating is probably related
Table 8. — The Effect of Time Held at Sintering Temperature
|
Sinter No. |
Time at 1360°C. in minutes |
AI2O3 extracted in percent |
Clay Sample No. |
Type of clay |
|
|
201 .. . 12... 225... |
10 60 240 |
32] 88 865 90j |
kaolinite |
||
|
290... 288... 291... |
10 60 240 |
771 78 866 79j |
illite, kaolinite |
||
|
22... 203... 233... |
60 120 240 |
451 61 867 70j |
halloysite |
||
|
216... 19... 295... 234. . . |
10 60 150 240 |
611 87 1 868 67 f 56j |
montmorillonite |
||
|
217... 56... 226... |
10 60 240 |
571 88 869 90J |
kaolinite |
||
|
218... 26... 227... |
10 60 240 |
721 71 59 |
870 |
high iron illite |
|
|
219... 32... 228... |
10 60 240 |
321 88 873 93j |
kaolinite (flint clay) |
||
|
296... 41... 299... |
10 60 240 |
921 95 874 92j |
gibbsite, kaolinite (bauxitic kaolin) |
||
|
221... 47... 230... |
10 60 240 |
491 91 876 92J |
kaolinite |
||
|
222... 59... 231... |
10 60 240 |
451 91 879 90J |
kaolinite (ball clay) |
||
|
202... 65... 232... |
10 60 240 |
46^ 93 93; |
881 |
kaolinite |
28
ALUMINA EXTRACTION
to the relatively high content of alkali in both clays. Although the alkali would tend to be lost by vaporization from the sinter at high temperatures and with prolonged heating periods, its presence no doubt in- directly inhibits inversion of dicalcium sili- cate by causing the formation of appreciable amounts of stable glass. The other illite clay (sample 866) did not show this sensitivity to sintering time. The reason for the differ- ent behavior of this illite clay is obscure although it may be because it contains an appreciable amount of kaolinite and con- siderably less iron than sample 870.
EFFECT OF FURNACE ATMOSPHERE
In a great many tests made by the writers and also by others, the briquetted sinter mixes were placed upon a graphite block because graphite is the only material so far
found to which the sintered briquets do not stick. It was noted on many occasions that the end of the briquet which was ad- jacent to the graphite block was different in color from the opposite end and that the color changed more or less gradually as the distance from the graphite increased. Furthermore, it was noticed that in some cases there was a pronounced odor of hy- drogen sulfide when the sodium carbonate extracts were acidified in preparing to de- termine their alumina content. When the iron content of the sinters was considerable, the sintered material usually showed a gain in weight when ignited in oxidizing atmos- pheres if the briquet was sintered on a graphite block. All of these phenomena in- dicated that more or less reduction was taking place during the sintering process. It was therefore decided to determine, if
Table 9.
-The Effect of Furnace Atmosphere on extractability
|
CaO/AhOs Mole Ratio |
Atmosphere |
||
|
Oxidizing |
Reducing |
||
|
Clay Sample No. |
Graphite Crucible |
Natural Gas |
|
|
Alumina extracted in percent |
865 kaolinite
35
|
1.50 |
||
|
1 . 66 |
||
|
1.83 |
||
|
866 |
1.40 |
|
|
illite |
1.50 1.66 1.83 |
|
|
868 |
1.40 |
|
|
montmor |
llonite |
1.50 1 . 66 1.83 |
|
870 |
1.40 |
|
|
illite |
1.50 1.66 1.83 |
|
|
871 |
1.40 |
|
|
kaolinite illite |
1.50 1.66 |
|
|
montmor |
llonite |
1.83 |
83
92 88 84
63 78 81
77
83
77 87
5-s 57 71 69
73 80 85 83
83
89 89 92
59 67 77 76
70 80 86
84
63 69 81 82
62 73 79 64
83
93 71 88
59 64 71 78
77 77
84
67 72 81 83
85 81 70
58
LIME-SINTER PROCESS
29
possible, whether furnace atmosphere exert- ed any considerable influence on the amount of extractable alumina. Tests were made with five different clays.
Oxidizing atmospheres were maintained by placing the briquets in an electrical 1\ heated muffle furnace which contained no combustible material whatever. Reducing atmospheres were maintained in two ways : (1) the briquets were sintered in covered graphite crucibles; (2) the briquets were placed in the furnace on a graphite block and a slow current of natural gas was passed into the furnace during the sintering period. The sintering temperature wTas 1360° in all cases.
The clays chosen for test were numbers 865, 866, 868, 870, and 871. Sinters with lime-to-alumina ratios varying from 1.4 to 1.83 were made with each of the five clays using oxidizing atmospheres and also re- ducing atmospheres. The extraction data are presented in table 9.
With clay 865 (kaolinite) there was no significant difference in the amount of alumina extracted which could be correlated with furnace atmosphere. With clay 871 (kaolinite, illite, and a little montmorillo- nite), wThich was chosen because it con- tained considerable iron, the data are erratic. The high-lime sinters gave less satisfactory yields in reducing atmospheres.
Sinters made from clay 866 (largely illite high in iron) gave slightly better extraction in oxidizing atmosphere when the lime-to- alumina ratio was low, but there was no significant difference when higher lime-to- alumina ratios wTere used. Sinters prepared from clay 870 (illite clay very high in iron) gave definitely better yields of alumina from sinters burned in reducing atmospheres.
There were no significant differences in alumina yields from sinters prepared from clay 868 (montmorillonite) which could be correlated with atmosphere.
DUSTING OF SINTERS
The speed of dusting is erratic even for sinters made with the same type of clay and fired to the same temperature. The com- pleteness of dusting is occasionaly erratic
especially for sinters made with clays con- taining alkalies and or considerable iron. High temperatures and long heating periods favor rapid and complete dusting. However, it was onl\ at the highest temperature (1400°C.) used that the dusting was notice- ably faster. The average dusting time for sinters heated to 1360°, 1340° and 1320 C. was nearly the same for the same type clays. There were some sinters prepared at all of these temperatures which required several days to dust.
Sinters prepared with kaolinite clays usually dust fairly completely in 5 to 15 minutes. None of the sinters prepared with kaolinite clays 869, 871, 873, 876, 878, 879, 880, and 881 required more than 30 min- utes to dust. A few sinters prepared with kaolinite clays 865 and 877 required from 2 hours to overnight to dust. Only one sin- ter prepared from a kaolinite clay and sin- tered at a temperature of 1320° or above did not dust at all.
The dusting characteristics of sinters prepared with gibbsite-kaolinite clays were much like those of sinters prepared with kaolinite clays. Sinters prepared from hal- loysite clay 867 dusted quickly but contained considerable gritty material. Sinters pre- pared with illite and montmorillonite clay frequently dusted slowly and incompletely.
Although it is usually true that quick dusting sinters are apt to extract well, they do not always do so. For example, sinter 16 from clay 866 was incompletely dusted after standing overnight yet yielded 81 percent of its alumina. Sinter 194 from the same clay dusted completely in 10 minutes and yielded 77 percent of its alumina. Another sinter from this same clay dusted in 4 min- utes but yielded only 50 percent of its alumina. Examples of less extreme varia- tion with all types of clay are common. In some instances the explanation is obvious. The dusting may be quick but incomplete. In other instances there seems to be no ob- vious reason why one sinter dusts in 10 minutes and another requires 24 hours al- though both give good extraction yields. In some cases dusting time is related to the presence of iron or magnesia, as described on pages 32, 33.
30
ALUMINA EXTRACTION
As might be expected, gritty particles in the sinter dust are not associated with the best yields of extractable alumina. Such particles are sometimes glassy and some- times resemble particles of burnt clay, suggesting that the clay and lime have not always been sufficiently mixed, even though all batches were tumbled for several hours in mixing jars, and in most cases were sieved afterward.
SILICA IN THE ALUMINA EXTRACT
Under the extraction conditions which prevailed in these studies, sinters from kaolinite clays gave alumina extracts in which the average values of the ratio (100xSiO»)/(Al.O.+SiO0 usually ranged from 1.5 to 1.7. The weight of silica ex- tracted from 10 grams of sinter averaged from 0.02 to 0.03 gram. High sintering temperatures resulted in lowered solubility of silica, and low temperatures favored in- creased solubility of silica. The amount of soluble silica decreased with increasing lime- to-alumina ratio, except that some sinters appeared to have more soluble silica when the CaO/AbOs ratio was increased beyond 1.83.
Sinters made with gibbsite-kaolinite clay behaved like those made with kaolinite clays, so far as silica extraction was con- cerned, except that the lime-to-alumina ratio did not clearly affect the amount of soluble silica.
Sinters prepared with illite and with montmorillonite clays gave extracts in which the value of the ratio (lOOXSiO)/ (AbOi+SiO.) was 2-0 to 2.5. This high value is the result of low soluble alumina rather than high soluble silica. The average weight of silica extracted from 10 grams of sinter was only 0.015 gram. The response to changes in lime-to-alumina ratio was similar to that noted for kaolinite clays. It could not be observed that the amount of soluble silica varied with sinter temperature.
Extracts from sinters prepared with the halloysite clay had about the same soluble silica content as those prepared with kaolins. There was no definite variation of soluble silica with either temperature or lime-alu- mina ratio.
Effect of Minor Components Present in the Raw Materials
general comments
All clays and limestones likely to be chosen as raw materials for the lime-sinter process are apt to contain magnesia, sulfur, phosphorous, and iron in varying quanti- ties. It was decided, therefore, to make some tests for the purpose of determining wheth- er these elements are likely to exert any important influnce on the efficiency of the extraction. The study was limited to one kaolinite clay. Only in a pure mono-mineral clay could the effects be easily detected. Clay 877 was chosen because it was the purest and cleanest kaolinite clay available.
Sinter mixes were prepared using precipi- tated calcium carbonate as a source of lime. The mole ratio of lime-to-alumina was 1.66 in all test batches. Quantities of iron, sulfur, phosphorous and magnesia were added to these batches in the form of Fe2C>3 (rouge), CaSCK (precipitated), Cas (PO*)2 (pre- cipitated) and MgCOs (precipitated), and in amounts calculated to cover the ranges most likely to be encountered in practice. The batches were briquetted and sintered for one hour at 1360°C. (except as other- wise noted below).
EFFECT OF MAGNESIUM
Other workers 3 have reported the ad- verse effect of MgO on the amount of extractable alumina in lime-clay sinters. Since MgO is a common impurity in both the limestone and clays likely to be used in the lime-sinter process, it was given more attention than the other minor components.
Precipitated magnesium carbonate was added to the sinter mixes in such quantities that the amount of MgO (calculated) varied from about 1.5 percent to about 37 percent of the weight of AUOs in the fin- ished sinter. One series was sintered at 1360° and another at 1400°C, both for one hour. The results are shown graphically in figure 3, which gives percent of alumina ex- tracted as a function of the ratio MgO/Ah Os expressed as weight percent. One sinter not shown on the figure contained MgO
LIME-SINTER PROCESS
31
|
1 1 |
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3 |
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• SINTER TEMPERATURE I360°C ® " " I400°C Ca 0/ai203 1.66 CLAY 877 |
|||||||||||||||||||
|
<8> |
|||||||||||||||||||
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
Al203
MgO Fig. 3. — Percent A1203 extracted versus 100 A, ~ in sinter mix.
AI2U3
equal to 242 percent of the weight of alu- mina. This sinter was made at 1400/C. Sixty-three percent of the alumina it con- tained was extracted.
The data show that the alumina yield de- creased as the MgO increased up to about 14 percent MgO. With further increases in MgO the yield increased until about 28 percent MgO was present. Larger amounts of MgO caused no further change in alumina yield. The scattering of points about the 1400°C. curve on figure 3 is due in part to the fact that it was necessary to make two heats in collecting the data for this curve. If the data from each heat are considered separately, there is much less scattering of the points. This suggests that some phenomena which is very sensitive to slight changes in conditions may be involved.
It is of interest to note that the effect of MgO on the extractability was greater at the higher temperature. This suggests that the phenomenon may be related to the solu-
bility of periclase in the liquid portion of the sinter. Presumably, there is more liquid in the 1400°C. sinter and hence more dissolved MgO. Possibly one reason why more AhOs is extractable in the presence of excess MgO than in the presence of smaller quantities is that this excess MgO inhibits supersaturation of the liquid part of the sinter with MgO. This would mean that the saturation point is reached near the minimum on the curve where the MgO is about 14 percent of the alumina present. Reports of the optical examinations, how- ever, note the presence of periclase in sinters on both sides of the minima. This would not rule out the supersaturation hypothesis, however, unless equilibrium conditions pre- vailed, and it is most unlikely that complete equilibrium was reached in these sinters. Reports of the X-ray examinations indicate the presence of an unidentified compound in some but not in all of these sinters. At 1360°C. approximately 65 percent of the
32
ALUMINA EXTRACTION
Q 70
< 60
< ^3C
|
<» — i |
> — i |
> |
— i |
|- — |
4 |
||||||||||||||
|
!=*•= |
|||||||||||||||||||
|
S03 ADDED AS Ca S04 Ca o/a^Oj 1.66 TEMPERATURE 1360° CLAY 87 7 |
|||||||||||||||||||
|
|
|||||||||||||||||||
.2 .3 .4 .5 .6 .7 .8
% S03 IN SINTER MIX (CALCULATED TO IGNITED BASIS)
Fig. 4. — Percent A1203 extracted versus percent S03 in sinter mix.
alumina is extractable in the neighborhood of the minimum. The unextractable alu- mina and the MgO present are, therefore, in the proper proportions to form the spinel MgO • AI2O3 on that part of the 1360°C. curve to the left of the minimum.
However, if the formation of MgO.Ab Os was the whole explanation, we should not expect the extractability to improve when furtherMgO is added nor should we expect the increased temperature to affect the extractability so markedly, although the increased temperature might promote solution of alumina in MgO.AUOs, and explain the portion of the curves to the left of the minima. Similar objections might be raised to any explanation based on the hypothesis of a phase composed of any compound involving aluminum and mag- nesium oxides.
Whatever the explanation of the peculiar behavior of sinters containing magnesium, it seems clear that the presence of mag- nesum in the quantities most likely to be encountered is bad and that its effect is aggravated at higher temperatures.
It was observed in the course of this series of tests that when a group of sinters was prepared under precisely identical con- ditions, increasing the quantity of magnesia shortened the time required for the sintered briquet to "dust."
EFFECT OF SULFUR
Precipitated calcium sulfate was added to the mixtures to produce sinters contain- ing up to one percent of sulfur as S(X The six sinters prepared gave yields of extract- able alumina which were the same within the precision of the work. Sulfur added as calcium sulfate apparently has little or no harmful effect. The test data are presented in figure 4.
EFFECT OF PHOSPHORUS
Phosphorus is present as an impurity in many limestones and in some clays, and because it has been reported 4'5 to inhibit the transformation of p— C2S to the y-form, it was desirable to obtain some information concerning its influence on the lime-sinter process.
Six sinters were made with added Cas (PCX) 2. The P2O5 content of the sinters (ignited basis) varied from zero to one percent by weight. The results presented in table 10 and figure 5 show that the ex- tractability falls off as the phosphorus in the sinter increases.
The effect of phosphorus was worse than the numerical data indicate. Sinter 312 was very poorly dusted and contained much coarse sandy material. Sinter 313 was a hard glasslike lump that showed no evidence
LIME-SINTER PROCESS
33
Table 10. The Effect oi Phosphorus on rHi
Ex ik \r r \uiu in OF 1 .IME-Cl 11 SlN rERS
|
Sinter No. |
%P20, in Sinter |
% A120.; E xtracted |
Sinter Temp. |
Dusting Time |
|||||||||||||||||
|
303 309 310 311 312 313 |
.01 94 1360 1.5 hours .05 93 1360 1.5 hours .10 88 1360 overnight .29 91 1360 overnight 4(; 62 1360 several davs 1.00 72 1360 did not dust |
||||||||||||||||||||
|
100 < 90 80 |
• \ j |
4 |
5 |
||||||||||||||||||
|
• 1 |
|||||||||||||||||||||
|
<> |
|||||||||||||||||||||
|
UJ 1- u < 60 a. t- |
4 |
> |
|||||||||||||||||||
|
uJ 5 0 |
|||||||||||||||||||||
|
O |
|||||||||||||||||||||
|
< |
P205 ADDED AS Ca3(P04)_, |
||||||||||||||||||||
|
v° |
CaO/AI203 1.66 |
||||||||||||||||||||
|
TEMPERATURE I360°C |
|||||||||||||||||||||
|
10 |
CLAY 877 |
||||||||||||||||||||
.2 .3 .4. .5 .6 .7 .8
% Pa05 IN SINTER MIX (CALCULATED TO IGNITED BASIS)
Fig. 5. — Percent ALOa extracted versus percent P^O-, in sinter mix.
of dusting after several days. It was necessary to crush and grind it through a 200-mesh sieve before it could be extracted at all. The tests indicate that no limestone containing phosphorus should be used as raw material in the lime-sinter process without very careful testing.
EFFECT OF IRON
The amount of iron present in good kaolinite clays usually does not exceed one or two percent as Fe^O, but may run much higher in other clays and shales. Examina- tion of figure 6 indicates that the effect of iron in the form added is not serious unless the quantity present in the sinter is greater than two or three percent. The optical examination usuallv shows the presence of
the compound 4Ca().AUO<.Fe;0.< in clay- lime sinters which contain appreciable quan- tities of iron. This compound does not easily yield its alumina to dilute sodium car- bonate solutions so that it is not surprising that iron reduces the yield of alumina from clay-lime sinters. The reduction in yield appears to be somewhat greater than is ac- counted for by the quantity of alumina in- volved in tetra-calcium-alumino-ferrite. The difference may be partly due to the fact that considerable lime is also tied up in the com- pound. This, of course, would have the effect of changing the lime-alumina ratio.
It was observed that increasing the iron accelerated the dusting rate when con- ditions under which the sinters were pro- duced were otherwise identical.
34
ALUMINA EXTRACTION
90
Q 70
y
< 60
CC h- X
LU SO
o
_N40 <
20
|
6 |
|||||||||||||
|
Fc203 ADDED AS SUCH Ca 0/AI203 1.66 TEMPERATURE I360°C CLAY 877 |
|||||||||||||
12 3 4 5 6
Vo F«203 IN MIX (CALCULATED TO IGNITED BASIS) Fig. 6. — Percent AljO:; extracted versus percent Fe-03 in sinter mix.
Effect of Different Types of Limestone
Varying results have been reported" when different limestones were used as sources of lime in the lime-sinter process. It was thought that the differences might be due to the size of the ultimate crystals of calcite of which the limestone is composed. Ac- cordingly, three samples of limestone were selected which are designatd R7, R2, and MC. R7 is fine-grained, R2 is intermediate- grained, and MC is coarse-grained. R7 and R2 were from the St. Louis limestone taken from a quarry near Alton, Illinois. MC was from the Burlington limestone taken from a quarry near Marblehead, Illinois. The analyses of the limestones are given in table 5 on page 13-
The limestones were ground so that 95 percent passed through a 200-mesh sieve. Sinters were prepared using illite clay 870 and kaolinite clays 877 and 880. The lime- alumina ratio chosen was 1.50. This low ratio was selected so that small or moderate effects would not be obscured by the presence of excess lime. The sinters were burned at 1360° C. for one hour following which they were extracted in the usual manner.
The results presented in table 1 1 indicate no basis of choice between the three lime- stones. For a given clay any one of the limestones or a precipitated calcium car- bonate which is extremely fine-grained gives equally good results within experimental error. The sinters all dusted well and with- in a reasonable time.
It must be concluded that differences in limestone insofar as they affect the efficiency of the lime-clay-sinter process are due to factors other than the grain size.
Table 11. — Effect of Different Types of Lime- stone ON EXTRACTABILITY OF CLAY-LlME
Sinters
|
Clay Sample |
Limestone |
Sinter |
Percent A1203 extracted |
|
No. |
Sample No. |
No. |
|
|
870 |
R2 |
314 |
58 |
|
illite |
R7 |
315 |
59 |
|
MC |
316 |
54 |
|
|
Fpt CaC03 |
25 |
57 |
|
|
877 |
R2 |
317 |
92 |
|
kaolinite |
R7 |
318 |
88 |
|
MC |
319 |
90 |
|
|
Ppt CaC03 |
49 |
93 |
|
|
880 |
R2 |
320 |
91 |
|
kaolinite |
R7 |
339 |
86 |
|
MC |
340 |
89 |
|
|
Ppt CaCO, |
61 |
93 |
LIME-SINTER PROCESS
35
MINERALOGICAL ANALYSES OF SINTERS
Microscopic Analysis
Microscopic studies were made with the petrographic miscroscope on mounts pre- pared by immersing the sinter material in liquids of known refractive index. Magni- fications up to 900 X were used.
GENERAL COMMENTS
The principal phases developed in the sinters were CS(2CaO.SiO->) and C5A3 (5Ca0.3Al20;:). The former was present in euhedral grains and the latter as inter- stitial material between grains. In some sinters prepared with high alumina (gibbs- ite-kaolinite) clays, a small amount of euhedral CAs could be identified.
In sinters prepared with kaolinite clays the C2S was usually in grains 5 to 10 mi- crons in diameter (fig. 7B). The C2S tended to be smaller in sinters prepared with clays composed of well crystallized kaolinite than in those prepared with clays composed of poorly crystallized kaolinite. In sinters pre- pared from clays composed of other clay minerals the same component was frequent- ly 20 to 60 microns in diameter (fig. 7A).
The OS was usually in the y-form al- though sinters prepared from all clay samples contain some uninverted ft— CS.
Sinters prepared from all types of clay contained a yellow-brown pigmentary ma- terial that was present usually in particles less than 2 microns in diameter. It was im- possible to be certain of the identification of all of the pigmentary material, but a considerable part at least was 4CaO.Al*03. Fe^Os. The amount of this material was small in sinters prepared with halloysite or kaolinite clays because such clay minerals, unlike illite and montmorillonite, do not have appreciable amounts of iron in their lattice structure.
Sinters prepared with some clays at low temperatures contained some material that showed little or no development of the new phases. This material was isotropic in appearance which suggests that the reaction
of the components had not been carried to completion.
EFFl.ci OF VARIATIONS l\ LIME CONTEN1 IN SINTERS FIRED ONE HOUR AT 1360°C.
Kaolinite clays. — The inversion of C»S
to the y-phase was most complete (90'/ ±) in sinters with a lime-to-alumina ratio of 1.83. Sinters prepared with a lime-to-alu- mina ratio less than 1.66 frequently con- tained small amounts of CA - the amount increasing as the ratio decreased below 1 .66. A component identified as CA was present in small amounts in sinters with a lime-to- alumina ratio of 1.83.
In general the amount of C A> was about the same in sinters prepared with lime-to- alumina ratios of 1.66 and 1.83. The amount of this component decreased as the ratio decreased below 1.66.
The amount of pigmentary material in- creased as the lime content decreased.
Illite and montmorillonite clays. — Sinters prepared with such clays showed about the same general effects of changes in lime con- tent as those prepared with kaolinite cla\ except that they had less tendency to develop CA or CA. Also the variation in the amount of inversion of CS to the y-phase with changes in the lime content was great- er for illite and montmorillonite clays than for kaolinite clays. Sinters of the former type clay with high or low lime content fre- quentlv contained as much as 70 percent
p-OS.
Diaspore clays. — Sinters prepared with a lime-to-alumina ratio of 1.66 were composed largely of CsAs. A small amount {25% ±) of indistinct material, which may be incom- pletely reacted, was present also. Sinters with a lime-to-alumina ratio of 1.83 seemed to be about the same as the foregoing, but the indistinct material became more prom- inent as the lime-to-alumina ratio decreased below 1.66.
Gibbsite-kaolinite clays. — Sinters pre- pared with varying lime-to-alumina ratios up to 1.83 all appeared about the same, being composed almost wholly of CS and C«A». Sinters with a higher ratio were somewhat different because of the presence of indis-
36
ALUMINA EXTRACTION
V
•"^i
--i*;a:
|
few' |
cj !# |
|
.'# t |
|
|
^ |
«# |
*
,o
Sg^ \* -
Fig. 7. — Photomicrographs of
A. Illite-clay sinter fired to 1360° C, show- ing distinct development of large parti- cles of 7C2S, and pigmentary ferrite (dark material).
B. Kaolinite-clay sinter fired to 1360° C. showing distinct development of small particles of 7C2S. The C5A3 is interstitial between the silicate grains and is not evident on the photomicrograph.
lime-clay sinters, 300 X.
C. Halloysite-clay sinter fired to 1360° C. showing the development of some C^S, and large aggregate masses of material that appear to be incompletely reacted.
D. Kaolinite-clay sinter fired to 1320° C. showing the distinct development of small particles of C2S, and large aggre- gate masses in which the new phases are less clearly developed than in B.
LIME-SIN 7 ' E R PROC i SS
37
tinct material which seemed to be incom- pletely reacted.
Hal lay site clay. — All sinters fired at 1360°C. contained a large amount (40' , ±) of material which appeared to he in- completely reacted.
EFFECT OF VARIATIONS IN SINTERING TEMPERATURE OX BATCHES WITH \ LIME-TO-ALUMINA RATIO OF 1.66 AND HELD AT SINTERING TEMPERATURE FOR ONE HOUR
Kaolinite clay. — Sinters fired at 1320°C. contained a considerable amount of material (40% — ) that suggested poor development of new phases (fig. 7D). The amount of this material decreased as the temperature increased until in sinters fired at 1360° C. almost the entire mass was composed of rather distinct new components, i.e., C->S and CsAa.
An increase in sintering temperature was accompanied by an increase in size of the y— C2S grains from 5 to about 15 microns and a decrease in the amount of /?— C2S. In version of C2S to the y-phase was about complete in sinters prepared at 1400°C.
Illite and montmorillonite clays. — Sinters fired at 1320°C. were composed entirely of new reaction products. As the sintering tem- perature increased the size of the individual units of y— C2S increased from about 20 to 60 microns, and the amount of (3— C2S de- creased slightly. The presence of illite or montmorillonite in kaolinite clays lowered the temperature required for complete re- action and increased the size of the C-S formed at a given temperature.
Diaspore clay. — Sinters fired at 1320°C. like those fired at 1360°C. were composed of OAs with a relatively small amount (25% ±) of indistinct material that could not be positively identified.
Gibbsit e-kaolinit e clay. — Sinters m ide from clays of this type reacted to variation of sintering temperature in much the same manner as did those made from kaolinite clays except that the new phases showed slightly better development at 1320°C and 1340°C.
Halloysite clay. — Sinters fired at 1360° C. were composed of about 50 percent 01 isotropic material that suggested incomplete reaction between components (fig. 7C). The remaining 50 percent was C-S and CnAs. About 20 percent of the sinter had the appearance of incomplete reaction after firing to 1400°C. and only after firing to 1450°C. were the phases about completely developed. The presence of halloysite in a clay would increase the necessary sintering temperature.
EFFECT OF VARIATIONS IN SINTERING TIME OF BATCHES WITH A LIME-TO- ALUMINA RATIO OF 1.66 FIRED AT 1360°C.
Kaolinite clays. — Sinters held at 1360° (J. for only ten minutes contained a consider- able amount (40% ±) of material showing poor development of new phases. The re- maining 60 percent was made up of distinct particles of C2S and interstitial OAa.
Sinters held at 1360°C. for one hour were composed almost completely of new phases, and those fired four hours at 1360° C. differed from the one-hour sinters only in having more complete inversion of the C2S to the y-form and slightly larger indi- vidual units of C2S.
Illite and montmorillonite clays. — Sinters fired for ten minutes and 60 minutes at 1360°C, both showed nearly complete de- velopment of new phases.
Gibbsit e -kaolinite clays. — Aggregates of isotropic material without the distinct de- velopment of new phases entirely composed sinters fired for only 10 minutes. Curiously the sinter of this clay (unlike those of the kaolinite clays) fired for 10 minutes, gave high extraction yields. Sinters fired one hour and four hours were both completely composed of new phases — the four-hour sinter contains a much larger portion of the C2S inverted to the y-form.
Halloysite clay. — Sinters fired at 1360° C. for four hours were composed almost entirely of new phases whereas those fired at this temperature for one hour contained about 50 percent of material with incom- plete new phase development.
38
ALUMINA EXTRACTION
EFFECT OF VARIATIONS IN KILN ATMOSPHERE
Kaolinite clays. — Sinters with a lime-to- alumina ratio of 1.66 and fired in reducing atmospheres differed from similar batches fired under definitely oxidizing conditions by showing more complete reaction at a given firing temperature, more complete in- version of C2S to the y-form, and less pig- mentary material. The differences between the sinters fired under the two sets of con- ditions were, however, very slight, and no new phases were found in the sinters fired in reducing atmospheres.
The foregoing statement does not hold for sinters with high lime contents. For ex- ample, sinters with a lime-to-alumina ratio of 2.00 fired under oxidizing conditions showed more complete reaction, more in- version of C2S to the y-form, and less pig- ment than those fired in a reducing atmos- phere.
Illite and montmorillonite clays. — Sinters with a lime-to-alumina ratio of 1.66 fired in reducing atmosphere at a given temper- ature showed better development of new phases, more inversion of C2S to the y-form, and less pigment than those fired under oxidizing conditions. The differences be- tween sinters fired under the two sets of conditions were pronounced although no new phase could be found in the sinters fired in reducing atmosphere.
Like sinters composed of kaolinite clays, the foregoing statements do not hold for sinters of either illite or montmorillonite clays when the lime-to-alumina ratio is high, e.g., 2.00. In such sinters, there was better phase development, more inversion of OS to the y-form, and less pigment when the firing was done under oxidizing conditions.
Gibbsite-kaolinite clays. — A comparison of sinters fired under oxidizing and reducing conditions gives the same results as those noted above for kaolinite clays.
EFFECT OF MISCELLANEOUS VARIATIONS IN COMPOSITION
The following data were obtained on sinters prepared with kaolinite clay 877 to which various components were added. The
batches had lime-to-alumina ratios of 1.66 and were held for one hour at 1360°C.
Titanium. — No distinct TiO* phase could be found in any sinters even when a large percentage of titania had been added to the sinter batch. X-ray diffraction data, however, indicate perovskite (CaO.TiOz) in sinters prepared with added T1O2.
Phosphate. — Sinters to which 0.05 per- cent P2O5 had been added showed consider- able reduction in the amount of C2S invert- ed to the y-form. As the amount of P2O5 in the sinters increased, the amount of GS inverted to the y-form decreased until in sinters with one percent P2O5, all of the C2S remains in the /?-form, and therefore no dusting took place.
Iron. — In general the only difference that can be detected in the sinters as the content of iron increased is an increase in the amount of the compound 4CaO.Al2O3.Fe2 Os. In clays with a low iron content this compound was present in minute (2/ndz) pigmentary units whereas with larger amounts of iron (6%±Fe203) it occurred in larger masses interstitially between the C2S grains.
Magnesia. — Sinters prepared from batch- es with added MgCOs frequently contained periclase (MgO) in distinct euhedral grains with a diameter of about 60 microns. The calcium silicate in such sinters often had optical characteristics somewhat un- usual, such as higher birefringence, suggest- ing some solid solution phenomenon. It has not been possible, however, to establish any definite correlation between extraction results and the character of such sinters as seen under the microscope.
EFFECT OF VARIATION IN THE CHARACTER OF THE LIMESTONE
No differences in character of compo- nents could be detected in sinters prepared with either coarse, medium, or fine crystal- line limestone. Further, no difference could be detected between such sinters and those prepared with precipitated calcium carbon- ate. The foregoing statements appear to hold regardless of the clay mineral composi- tion of the clay used in preparing the sinters.
LIME-SINTER PROCESS
39
The only conclusion justifiable on the above data is that the size of the calcite units composing a limestone does not deter- mine its suitability for the lime-sinter proc- ess.
X-ray Analysis general comments
The method of examination consisted of the registration at room temperature of powder diffraction diagrams of representa- tive whole sinter preparations. Such patterns consist of superpositions of the diffraction patterns of the several individual compo- nents, each developed in proportion to the proportion of that component in the sinter.
The ease and accuracy with which a given component can be recognized depends not only on the component itself, but on the accessories which happen to be present in a particular instance. For phases like CaO and MgO, 2 or 3 percent may be clearly evident; for phases like the cubic alumi- nates, 5 or 10 perecent may be necessary for identification, and for phases like the lime silicates, 20 or 30 percent may be somewhat ambiguous- Under such circumstances, little value can be attached to the examination of any one sinter, yet generalizations drawn from a sufficiently varied group of sinters, viewed collectively, may be taken with some confidence.
All of the important phases encountered in the lime sinters are relatively well known from the extensive literature on the con- stitution of Portland cement.
PREDOMINANT PHASES
Calcium orthosilicates. — Of the three di- calcium silicates, only /3-GS and y— OS are of significance in the lime-sinter process. Of the many studies of the /3 to y inversion (and its failure) the most recent one'' ad- vances the idea that the inversion may be inhibited chemically, on one hand, by foreign ions in the crystal lattice, or, on the other hand, physically, by the isolation from each other of enormous numbers of fine-grained /?^OS particles.
i lie behavior of the lime-sinter prepara- tions seems to be consistent with such a concept. For sinters prepared at 1360°C, those made with kaolinite clays usually in- verted fairly rapidly, and to about the same degree. A small amount of /3~CS invariabh remained uninverted, but it did not keep the sinters from appearing to be completely "dusted." For sinters prepared with clays composed mainly of montmorillonite or illite, however, dusting usually was slow, and occasionally failed altogether. In these sinters the fi~ C2S grains were comparatively large, and in those in which y— C2S was developed it was also clearly larger than in sinters prepared with kaolinite clays. One is led to the conclusion that the more abundant impurities in the latter two types of clays included ions with inhibiting effect.
When the sintering temperature was re- duced below about 1300°C, dusting first slowed up, then stopped, even for the kaolinite sinters. In these sinters f$-C& was abundantly developed, but the primary particle size was much smaller (by a factor of 100 or more). One can hardly say whether the composition was typical, but the physical condition was clearly conducive to physical inhibition. The material reported by microscopic analysis as showing incom- pleted development of new phases probably was composed to a considerable extent of such fine particles.
For both the y— C2S, whose crystallization is of the olivene type, and for the /?— C2S, whose crystallization is not known, minor variations in the cell dimensions were ob- served which could be attributed to slight variation in chemical composition, but no attempt was made to correlate or interpret such variations. It seemed quite possible that the failure of low temperature (i~~ OS preparations to invert might be due to small departures from the ideal lime-to-silica ratio as well as to physical inhibition.
Calcium a! u mi nates. — Three stable alu- minates have compositions such that they would be anticipated in materials of the compositions of the various sinter prepara- tions. They are the tri-, penta-, and mono-, referred to respectively as OA, GA* and CA. It is commonly claimed that the CeAa
40
ALUMINA EXTRACTION
phase actually runs slightly higher in lime than the formula indicates.
In the region of the compositions em- ployed, there are two eutectic compositions ; one involving C-'S, C>A», and CA, another C*S, C5A3, and OA. Both of these melt below 1360°C, the normal sintering tem- peratures used in our studies. In any sinter then, along with the C2S and CoAs either some CsA or some CA should also be present. Probable amounts, though, are only from about 10 down to 1 or 2 percent, and detection by the X-ray method is erratic. The CsA phase was clearly observed only in one high-lime and high-alumina sinter, and the CA phase was apparent in only about half of the sinters examined (those sinters which were low in lime).
Comparison of this trend with the trend of extraction data suggests that batch com- positions had best be made up with residual lime-to-alumina ratio in the range 1.55 to 1.66 (which includes the C2S— C5A3— CA eutectic) rather than with a higher ratio (which would include the CaS- CsAs— CaA eutectic).
LOW TEMPERATURE SYNTHESES
The course of sinter reactions in the temperature range between the decarbona- tion of limestone and the normal finishing temperature is best followed by diffraction methods. In order to study these reactions, powder diffraction diagrams were registered for a variety of synthetic mixtures fired at different temperatures. The following con- clusions are the result of this work.
Lime-silica. — Lime-silica mixtures, in the ratio of C2S show extensive reaction at 1000°C. The chief phase developed is ft- C2S which is fine-grained and not subject to inversion. An additional phase is appar- ently wollastonite, the /3~CS, but the amount present does not permit positive identification. Unreacted lime does not ex- ceed 2 or 3 percent. Even as low as 800°C. some reaction is evident, the new phase also apparently being wollastonite.
Lime-alumina. — Lime-alumina mixtures, in the ratio of CsAs react slowly over a range from 1000° or 1100° on to 1300°C.
or possibly higher. At 1100° it has been possible to observe diffraction effects from C3A, CsAa, and CA in a single sinter. Con- tinued temperature rise increases the amount of GA.t at the expense of the other alumi- nates.
The source of alumina is a highly sig- nificant factor. Natural aluminous clays resist attack by lime more than does chem- ically prepared hydrated alumina. Diaspor- ite, for example, dehydrates to corundum at about 500° C, and some unattacked corundum is still evident in lime sinters fired to 1200°C. Other aluminous clays dehydrate first to an "active" y-oxide which either may or may not invert to corundum before it can be attacked by free lime. Chemically prepared alumina and hydrated alumina remain active to 1200°C. or higher.
Lime-kaoliniie. — In the critical tempera- ture range from about 900° to 950°C, de- hydrated kaolinite is not a simple mixture of silica and alumina, but possesses a certain degree of association. This material has been called "metakaolin." Prepared lime- kaolinite mixtures in sinter proportions were fired to a series of intermediate tempera- tures. At 1000°C. gehlenite is an abundant- ly developed phase and unreacted free lime is prominent. Little ft— C2S has yet been de- veloped, and the presence of any aluminate is doubtful. From 1000° to 1300° or 1350° C, free lime and gehlenite gradually dis- appear and the ft— C2S, CA. and CsAs develop in the order named. The ft— C2S does not be- come subject to inversion until the firing temperature exceeds 1300°C, and apparent- ly not until the aluminate phases have ad- justed their proportions to the sinter com- position. It is suggested that the composition of ft— C2S may also be undergoing some re- adjustment in this temperature interval.
SINTERING PROCESS
The conditions for the preparation of the standard lime sinters, and of the deliber- ately under-burned sinters described above, differ from the conditions under which thermal curves are registered in that more time is available for progress of the slower reactions. It is apparent in a qualitative way on the differential thermal curves (see page
LIME-SIM ER PROCESS
41
42) themselves where the thermal effects on reactions involving only one mineral are generally much more prominent than are those involving two solids.
High alumina and kao Unite clays. — The endothermic effects up to about 95()°C. are all known effects ascribable to individual
components.
The intensity of the exothermic effect near 950° C. seems clearly to be correlated with the abundance of kaolinite minerals in the clay, and in turn with the abundance of gehlenite present in sinters finished off at only slightly higher temperatures. It seems not unlikely that the formation of gehlenite from lime and "metakaolin" could be a sufficiently fast process to become apparent in the thermal curves. The even more prom- inent exothermic effect in the thermal curve of pure kaolinite ' at this temperature is considered by the authors to represent the collapse of "metakaolin" to mullite. Com- parisons of the respective compositions also suggest that comparable heat effects would result from the transition from metakaolin to mullite, or to gehlenite, respectively. It is not possible to say whether this same ex- othermic effect may also include the forma- tion of some wollastonite.
The further attack of free lime on the gehlenite, or any extraneous material, is a slow, gradual process for which no effects are observable in the curves.
Endothermic effects near the finishing temperature probably represent the melting of some of the material.
lllite and montmorillonite. — Curves for the clays composed of these minerals differ from those composed of kaolinite in the lesser significance of the exothermic effect around 950°, and in the presence of prom- inent effects in the neighborhood of 1200°.
Examination of the sinters reveals that the (i~CS developed in them is remarkably coarse-grained. It is suggested that the small alkali (and other impurity) contents of these materials exercise enough mineralizing influence to render the formation of /3~OS (and possibly the aluminates) evident in these curves.
DIFFERENTIAL THERMAL ANALYSES
Differential thermal analysis consists oi heating a sample at a constant rate of in- crease of temperature and recording the temperature at which thermal reactions take place and their intensity. The minerals in a clay may be identified usually from a differential thermal curve,7 and the develop- ment of phases that form when a clay-lime mixture is heated may be studied by the differential thermal curve of such a mix- ture.
In figure 8 differential thermal curves are given for batches of various types of clay with lime equal to a lime-to-silica ratio of 2.0 and lime-to-alumina ratio of 1.66. The temperature in the analyses was carried to 1400°C. with a rate of increase of about 10°C. per minute. The downward deflec- tions of the curves indicate endothermic reactions, and upward deflections indicate exothermic reactions. A vertical scale for determining the temperature difference in- dicated by the deflections of the curve is given in figure 9.
Diaspore clay 872, sinter mixture CS35. — The endothermic reaction between about 500°C. and 600°C. corresponds to the loss of lattice water (OH) from the diaspore. The endothermic reaction between about 800° C. and 970° C. is due to the loss of CO2 from the lime carbonate.
Except for a very slight endothermic re- action at about 1325°C, which might be partial fusion, the curve between 970° C. and 1400° C. shows no thermal effect. The reaction at 1325°C. is of such low intensity that it is unlikely that it corresponds to a major reaction of components. Further, if the formation of new phases was accompan- ied by a distinct thermal reaction, it would be expected to be exothermic rather than endothermic. X-ray and other data show the presence of new phases in material heat- ed to no more than 1360°C, and therefore since the thermal curves do not show dis- tinct reactions, it must be concluded that the new phases form from the lime and de- hydrated diaspore without a thermal effect showing on the thermal curves. It would
42
ALUMINA EXTRACTION
Fig. 8. — Differential thermal analyses of lime-clay sinters. See also fig. 9.
LIME-SINTER PROCESS
43
|
LU |
— |
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|
o ° |
■ |
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U 20 DC |
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|
u_ 24 L. Q 28 UJ 5 32 $ cr LjJ Q. UJ 1- |
200
500 600 700 800 900 TEMPERATURE - DEGREES C
1200 1300
Fig. 9.
-Scale for determining the temperature differences recorded by peaks of the differential thermal curves (figs. 8 and 23).
seem that the velocity of such a reaction must be low.
Kaolinite clay 877, sinter mixture CS50. — The endothermic reaction with a peak at 600° C. is due to the loss of lattice water (OH) from kaolinite. The endothermic reaction between about 800° C. and 970° C. corresponds to the loss of CO2 from the lime carbonate.
The reaction due to loss of CO2 is followed immediately by an exothermic effect in- dicating that in this clay there is a reaction between components as soon as CaO is formed. X-ray data indicate that this re- action is the formation of gehlenite. Again in this material there is no thermal effect shown by the curves corresponding to the formation of CsAs and C2S and it follows that these new phases are probably formed gradually and slowly.
Gibb site -kaolinite clay 875, sinter mix- ture CS44. — The initial endothermic peak is due to loss of adsorbed water and sug- gests that halloysite as well as kaolinite is present in this clay. The endothermic peaks at about 340° and 590°C. are the result of loss of lattice (OH) water from the gibbsite and kaolinite (and halloysite), respectively.
The endothermic reaction between 800° C. and 970°C, due to loss of CO* from the lime carbonate, is followed immediate- ly by an exothermic effect which is inter-
preted as resulting from a reaction between the dehydrated kaolinite component and the CaO leading to the formation of gehlenite. The intensity of this exothermic reaction fits with the intensity of the kaolinite de- hydration reaction. If this exothermic effect represented a reaction between the CaO and both the dehydrated kaolinite and gibbs- ite, it would be expected to have greater in- tensity. Sinters made with this type of clay show the development of C*S and C5A3 at least by 1320°C, and since the curves show no thermal effects corresponding to these reactions it would seem that the new phases form gradually and slowly. The endother- mic effect at the end of the curve may represent partial melting.
Illite clay 870, sinter mixture CS26. — The initial endothermic reaction is due to the loss of adsorbed water, and the endo- thermic peak at about 575 °C. corresponds to the loss of lattice (OH) water from the illite.
The endothermic reaction corresponding to loss of CO* is not followed by a distinct exothermic reaction suggesting that dehy- drated illite, unlike dehydrated kaolinite, does not react with lime as soon as the CO* is driven off. The thermal reactions between 1150°C. and 1225°C. are probably the result of the formation of (3— C2S and C5A3.
It would seem that the kind of reaction and temperature of the reaction is different
44
ALUMINA EXTRACTION
for the CaO and dehydrated kaolinite, de- hydrated illite, dehydrated gibbsite, and diaspore.
Kaolinite -illite {ball) clay, sinter mix- tare CS59. — The initial endothermic peak corresponds to the loss of adsorbed water by the illite, the exothermic reaction be- tween 200°C. and 500°C. is the result of burning off of organic material, and the endothermic peak at about 600° C. is due to the loss of lattice water (OH) from the kaolinite and illite.
The endothermic reaction due to the loss of CO* is followed immediately by a small exothermic effect which is interpreted as the result of a reaction between the de- hydrated kaolinite portion of the clay and lime to form gehlenite. The exothermic re- action just above 1200°C. is interpreted as the result of a reaction between the de- hydrated illite and the lime. This clay com- posed of two clay minerals, kaolinite and illite, affords an excellent illustration of the different temperatures required for the re- action between the various clay minerals and lime.
Montmorillonite clay 868, sinter mixture. — The endothermic peaks at about 150°C. and 700° C. are due to loss of adsorbed water and lattice water (OH), respectively, from the montmorillonite.
Like the illite clay, the reaction due to loss of CO* is not followed immediately by an exothermic reaction. Rather there are exothermic reactions at about 1200°C, suggesting that this temperature is required for the reaction of components and the de- velopment of new phases.
Montmorillonite is like illite and unlik^ either kaolinite, gibbsite, or diaspore in the temperatures at which dehydrated compo- nents react with the lime to form new pha- ses. The higher temperature to bring about the reaction is probably due to the higher temperature required for the complete destruction of the montmorillonite and illite lattice as compared to the kaolinite lattice. This will be discussed in more detail for the lime-soda-sinters where a similar corre- lation of reaction temperature and clay min- eral type prevails.
SUMMARY AND CONCLUSIONS
Kaolinite clays and gibbsite-kaolinite (bauxitic) clays gave higher percentage yields of alumina than clays composed of other clay minerals. When such clays were composed of poorly crystallized kaolinite, the yield of alumina was reduced slightly. Halloysite clays gave yields comparable to those for kaolinite clays only if they were heated to unusually high temperatures. The diaspore-clay sinters fused and did not dust so that they are less satisfactory for this process as commonly practiced.
It is probably significant that the more desirable sinters all resulted from those two clay minerals which yield loose active products on roasting, i.e., metakaolin from kaolinite and y— AhOs from gibbsite.
Within the range of variations in the CaO/ALOs ratios from 1.5 to 1.8, only the illite clays, perhaps because of their high iron content, showed considerable variation in the yield of alumina. Illite clays frequent- ly gave considerably higher yields with in- creased amounts of CaO. In the case of pure kaolinite clays, however, the lime in the mixture can be reduced considerably below the quantity required to give the CsAa com- position without seriously affecting extract- ability.
Kaolinite, illite, and montmorillonite clays all showed optimum yields when sin- tering temperatures were around 1360° to 1380°C. Sinters made with illite and montmorillonite clays were more sensitive to overburning than others. At 1360°C. sinters made with diaspore and gibbsite- kaolinite clays fused whereas those made with halloysite clays required sintering tem- peratures of 1400°C. and higher for mod- erately good yields.
The yield of alumina from kaolinite and montmorillonite clays was not affected by furnace atmosphere. The effect of variations in the furnace atmosphere on illite clays was erratic except that high iron illite clays gave improved yields when sintered under reducing conditions.
Sinters prepared with illite and mont- morillonite clavs tended to dust less com-
LIME-SINTER PROCESS
45
pletelv and less rapidly than sinters made with other types of clay.
The values for (100XSiO«)/(Al»Oa+ SiOa) were greater for ill i te and mont- morillonite clays than for clays of other types, primarily because of low soluble alumina rather than high soluble silica. Only in the case of kaolinite clays was the silica content decreased by increasing the sintering temperature. Varying the CaO/ Al-O* ratio caused small variations in the silica in the extracts from kaolinite, illite, and montmorillonite clay sinters.
In kaolinite-clay sinters fired at a given temperature the yield of alumina decreased as the MgO increased up to about 14 per- cent of the alumina. With further increases in MgO up to about 28 percent, the yield of alumina increased. Larger amounts of MgO caused no change in alumina yield. The adverse effect of MgO on extract- abilitv was greater in sinters fired at 1400° C. than at 1360°C.
No change in the amount of extractable alumina was encountered when sulphate was added in amounts up to one percent sulphur as SO^.
Phosphate even in small quantities in- hibited dusting and sharply reduced the yield of alumina. Iron in the form of Fe^O- did not reduce yields of alumina much un- less it exceeded one or two percent. In larger amounts iron caused considerable reduction in alumina yield.
Differences in limestone, insofar as they affect the efficiency of the lime-clay-sinter process, are due to factors other than the grain size of the calcite units of which they are composed.
The principal phases developed in the sinters were GS (2CaO.SiO0 in euhedral grains and OA^CaOJAhOO as inter- stitial material. The GS was usually in the y-form and attained a maximum size of about 10 microns in kaolinite-clay sinters; 60 microns in illite- or montmorillonite- clay sinters.
The compound 4CaO.Al-XXFe^ was present in sinters made with clays contain- ing appreciable iron. The amount of ferrite increased as the lime content increased and was greater in sinters fired under oxidizing
conditions than under reducing conditions. The compounds CaO.Al^Oa and 3CaO.Al2 Os were identified in some sinters in which the lime-to-alumina ratio varied below and above 1.66.
Kaolinite-clay sinters fired at 1320°C contained a considerable amount (40% ±) of material that suggested poor development of new phases. At 1360°C. the new phases were completely developed, but on increas- ing the sintering temperature to 1400°C the units of GS became larger and more completely inverted to the y-form. New phases were completely developed in illite- and montmorillonite-clay sinters fired to 1320°C Higher sintering temperatures served only to increase the size of the GS units. New phases were completely developed also in diaspore clay sinters fired at 1320° C. Gibbsite-kaolinite-clay sinters showed about the same relation to sintering tem- perature as kaolinite-clays except that there was slightly better phase development at the lowrer temperature. In the case of halloy- site-clay sinters, extensive new phase de- velopment was not attained until about 1450°C. The amount of material with poor phase development in sinters made with kaolinite and halloysite clays was reduced by increasing the time the sinter was held at the top temperature.
X-ray diffraction analysis indicates that the material suggesting poor development of new phases was largely /}— GS in ex- tremely fine units in which minor variations in cell dimensions have been observed that could retard inversion to the y-form.
X-ray and differential thermal analyses suggest that in diaspore clay sinters the re- action of components to form new phases began as soon as the CO^ was driven off and continued slowly without pronounced thermal effects. In the case of kaolinite clays the loss of CO^ was followed immediately by the formation of gehlenite which was accompanied by a sharp endothermic re- action at about 950° C. In such mixtures (kaolinite clay) fired 1000°C, gehlenite and free lime were prominent phases. As the temperature was carried from 1000° to 1300°C free lime and gehlenite disappeared gradually and the /8-CaS, GA and GAs
46
ALUMINA EXTRACTION
developed in the order named, the latter at the expense of CsA.
X-ray and differential thermal analyses for mixtures of illite and montmorillonite clays did not indicate the formation of gehlenite. Apparently there was little re- action of components until about 1200°C. when the f3~CS and possibly the aluminates
developed sharply with a pronounced ther- mal effect. The higher temperature required for the reaction of illite and montmorillo- nite with lime than for kaolinite and lime was probably the result of the higher tem- peratures required for the destruction of the lattice structure of the former clay minerals.
LIME-SODA-SINTER PROCESS
47
LIMK-SODA-SINTER PROCESS
From the chemical viewpoint the lime- soda-sinter process is not fundamentally different from the lime-sinter process. The ground siliceous alumina-bearing ore is mixed with calcium carbonate and sodium carbonate in the proportions to give a mix- ture corresponding approximately in per- centage composition (ignoring volatile con- stituents) to a mixture of dicalcium silicate and sodium aluminate, NaAlO--. The mix- ture is then heated to such temperature and for such a period of time as may be neces- sary to render a maximum proportion of alumina and soda and a minimum propor- tion of other materials present soluble in dilute alkali-carbonate solution. The tem- perature necessary is not so high and not so critical as for the lime-sinter process. As in the lime-sinter process the primary function of the lime is to tie up the silica in a form not soluble in the leach liquor. The transformation of dicalcium silicate when or if it occurs does not result in dust- ing of the sinter. The sintered mass is usually considerably less dense than that produced when the lime-sinter method is used. It is rather friable, porous, and easily ground when not overburned.
The clay samples used in these experiments, after mixing with CaCOs and NaaCOs and heating, gave sintered materials in which the compositions in percent for the im- portant oxides were about as follows in most cases.
II lite and Kaolinite clays Montmorillonite clays (percent) (percent)
SiO^ 23 26
Al2Oa 20 9
CaO 44 53
Na20 + K,0 12 6
Sinters made from the very high alumina clays and bauxites, of course varied con- siderably from the above values. Halloysite clay sinters were similar to kaolinite clay sinters in composition.
Equilibrium data on the four-component system AUO^-SiO^-Na^O-CaO are not available on which estimates of fusion points
can be based, but experiment has shown that most of these sinters are only partially fused at 1300°C. Hence, it is to be predict- ed that an equilibrium state will probably not be reached at lower temperatures un- less the mixtures are held at those temper- atures for considerable periods of time. The presence of iron, magnesia, phosphorus, and titania, even in minor amounts, cannot safely be presumed to be without influence on the final state reached under a given set of operating conditions. The nature of the various clay mineral types, which react differently when heated, might be expected to have an important effect on the final state reached when such mixtures are heat- ed unless the heating is carried far enough to produce complete or nearly complete fusion of the sinter mix. The data presented were collected from experiments designed to study the influence of some of these factors in the lime-soda-sinter process.
EXPERIMENTAL METHODS
Preparation of Sinter Samples
Clay samples were ground to pass 80 mesh, calcined at 800°C. (Sample No. 872 diaspore was not calcined, see page 14), mixed carefully with the appropriate quan- tities of precipitated CaGO and C. P. grade anhydrous NaaCCX The batches were then pressed into cylindrical briquets 1^ inches in diameter under a pressure of 5000 pounds per square inch.
Except in work planned to study the effect of varying amounts of lime or soda, batches were made up with sufficient CaCO- to furnish two moles of CaO for each mole of SiOa and enough Na*CO;= to supply one mole of Na-^O for each mole of AUO:. In calculating the amount of NaaCOa to be added, account was taken of NaaO and JGO occurring naturally in the clay. IGO was considered equivalent to Na*0 mole for mole.
The briquets were placed in a cold electrically-heated muffle furnace, heated in a manner indicated bv the recorder trace
48
ALUMINA EXTRACTION
(fig. 15) except as noted otherwise in special instances. The briquets were placed in tightly closed bottles as soon as they became cool enough to handle.
A few sinters were made with the briquets resting on a graphite block which in turn rested on the floor of the muffle. The color of such briquets indicated that the furnace atmosphere was reducing under these conditions. Since oxidizing conditions are normal in commercial kilns, measures were taken to insure oxidizing conditions in the test furnace except as otherwise noted for studies on the effect of furnace atmos- phere. The briquets thereafter were sup- ported in small clay dishes with a thin briquet of composition identical with that of the test briquet interposed between the dish and the test briquet. After sintering, the thin briquet was discarded. No trouble was encountered until the temperature 1300°C. was reached when the corrosive action of the briquet on the clay dishes began to be serious with briquets made from most of the clays. Briquets from clays con- taining considerable iron showed signs of incipient fusion at 1300°C. Briquets from fairly clean kaolins could probably be heat- ed to temperatures somewhat higher than 1 300° C. without much distortion if support- ed on graphite or otherwise protected from the slagging action of the clay dish. As a rule the sinters shrank during the heating process, although some prepared at the lower temperatures swelled noticeably.
Chemical Analysis of Sinters
The sintered material was analyzed for silica and R-Os by standard methods. The values for alumina were obtained by cor- recting the R*Oa values for Fe2C>3 and TiO* as calculated from the batch.
Extraction of Alumina from Sintered Material and Analysis of Extracts
After some preliminary experimenting the method of extraction described below was adopted as the standard procedure for this series of tests. The sintered briquet was ground to pass 65 mesh. A 10-gram sample
was weighed out, and the quantity of AhO present in the sample was calculated from the batch composition. A quantity of Na2 COs (in the form of a standard solution) was taken, sufficient so that the total Na20 available including that in the sinter amounted to 1.8 moles Na20 per mole of AhOs in the 10-gram sinter sample. This NasCOs solution was diluted to 100 cc, heated to 65 °C, and maintained at this temperature while the ground sinter was introduced in such manner that no lumps were formed. The suspension was stirred with a motor stirrer for 15 minutes, at a rate which would prevent sedimentation, and filtered immediately with suction. The solution was made up to 250cc. and ana- lyzed gravimetrically for ALOs and colori- metrically for SiCX
EXPERIMENTAL RESULTS
Effect of Type of Clay Minerals optimum yield of alumina
The factors which may be varied at the will of the operator of a plant producing alumina by the lime-soda sinter process are the lime-clay and soda-clay ratios, the tem- perature, the duration of the heating period, and to a lesser extent the furnace atmos- phere. Proper adjustment of these variables with relation to one another might be ex- pected to result in an optimum yield for a given raw material, assuming good ex- traction practice. Data are presented in graphic and tabular form in table 12 and figures 11, 12, 13, and 14, which bear on the effect of these operating variables when various types of clays are used in the sinter mix under laboratory conditions. Variables connected with the extraction process were not considered except that care was taken to keep the extraction conditions standard- ized.
The data indicate that the kaolinite clays and gibbsite-kaolinite (bauxitic) clays may yield more than 90 percent of their alumina when clean, that is, when such clays are relatively free from iron and clay minerals other than kaolinite and gibbsite. Halloy- site and montmorillonite clays and anortho-
LIME-SODA-SINTER PROCESS
49
|
lOA |
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• — EXTRACTION VALUES |
ACTUAL SINTERS |
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— $ — EXTRACTION VALUES |
HYPOTHETICAL SINTERS |
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REST - KAOLINITE |
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TEMPERATURE- DEGREES C
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ESTIMATED 80% ILLITE 20% KAOL 1 M 11 |
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300 1000 1100
TEMPERATURE - DEGREES C
Fig. 10.— Parts A and B Comparison of actual extraction values for lime-soda sint:rs prepared from clays containing more than one clay mineral type with hypothetical mixtures of clay: 877 kaolinite, 870 illite, and 868 montmorillonite.
50
ALUMINA EXTRACTION
site yielded nearly as great a percentage of their alumina as the kaolins, but it was somewhat more difficult to get top yields. The top yields from illite clays ran 10 or 12 percent below those from the other clays. The single diaspore clay studied did not yield quite as high a percentage of its alumina as most of the kaolins.
Clays which are composed of mixtures of clay minerals behaved about as would be predicted from a consideration of the be- havior of the nearly pure types. The graphs in figure 10 are comparisons of actual yields from clays which contain considerable amounts of more than one type of clay mineral with hypothetical mixtures of pure types. For example, clay 879 was estimated to contain 25 percent illite and 75 percent kaolinite. The yield from clay 877 (pure kaolinite) at 1200°C. was 94 percent; that from clay 870 (nearly pure) illite was 76 percent. Seventy-five and twenty-five per- cent of these values were 70.5 and 19
respectively. The sum of these is 89.5 which is taken as the yield of a hypothetical mix- ture containing three parts by weight of clay 877 and one part by weight of clay 870.
Hypothetical yields from mixtures of 877 (kaolinite) with 870 (nearly pure illite) and with 868 (nearly pure montmorillo- nite) are plotted on the same diagrams (fig. 10) with actual yields from clays 866, 871, 878, and 879. The agreement is considered good, taking into account the difficulty of making accurate estimates of the relative amounts of the clay minerals in a natural mixture and that there are factors other than the clay mineral types which are diffi- cult or impossible to evaluate.
SINTERING TEMPERATURE
Fourteen representative types of clay materials and one anorthosite comprising samples 865, 866, 867, 868, 869, 870, 871, 872, 875. 877, 878, 879, 881, 883, and 884
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MOLE RATIO Na20/A|203=l ii Ca 0 / Si 02 =2 |
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TEMPERATURE -DEGREES C
Fig. 11. — Part A Percent AI2O3 extracted versus temperature at which sinter was held for ten minutes. See pp. 52.
LIME-SODA-SINTER PROCESS
51
|
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DIASPORE ( MISSOUR 1 ) 872 |
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TEMPERATURE - DEGREES C
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a. X UI |
< f- X u |
|||||||||||||||
|
U z o |
IU z o |
||||||||||||||||
|
KAOLIN, RINGGOLD, TENN. |
1 BENTONITE |
||||||||||||||||
|
f k aoi initf ) |
( MDNTMORII 1 ONITF ) |
||||||||||||||||
|
865 |
868 |
||||||||||||||||
|
/ / |
1000 I I 00 1200 1300 1000 1100 1200
TEMPERATURE - DEGREES C
Fig. 11.— Parts B and C Percent AI2O3 extracted versus temperature at which sinter was held for ten minutes
52
ALUMINA EXTRACTION
LU
h-
U 50
<
CE h- X Id <*> ° 90
(M <
K80
|
lll-D |
|||||||||||||||||
|
J |
i |
r^ |
|||||||||||||||
|
1 |
|||||||||||||||||
|
KAOLIN AIKIN,S. Ckini i mitp 1 |
c |
SOFT KAOLIN ( KAOLINITE ) 877 |
|||||||||||||||
|
88 1 |
|||||||||||||||||
|
^ - |
- - 1 |
>- ^ > |
|||||||||||||||
|
x / |
► |
/ |
"> |
> |
|||||||||||||
|
ir |
t^ |
||||||||||||||||
|
1 |
r |
||||||||||||||||
|
BALL CLAY ( KAOLINITE . ILLITE1 |
|||||||||||||||||
|
879 |
KAOLIN, HOBART BUTTE, ORE. ( K AOI IN ITF ") |
||||||||||||||||
|
883 i |
1200 1300
TEMPER AT U
were mixed with CaCOs and Na2C(X The mole ratio of soda to alumina was one and that of lime to silica was two. Briquets made from each were heated up to top temperatures ranging from 1000°C. to 1300°C. as indicated in table 12, held there for ten minutes, allowed to cool, ground, and extracted. The data are presented in table 12 and in figure 11.
It will be noted that the curves showing the behavior of clays of a given type are quite similar in shape although the yields of soluble alumina vary considerably. For some clays the yields from sintering at 1000°C. and 1100°C. are about the same but there is a definite increase in yield when the temperature is raised to 1200°C. and higher. The reason for this break is partly that, since it required longer to heat to the higher temperature, the effect involves both a time and a temperature factor, which circumstance tends to magnify any increase in extractability due to increased tempera- ture. In some cases where the variation was most extreme a new series of sinters was made for which the time that the sinters were kept above 1000°C. was held constant
RE
1000 1100
DEGREES C
<
cr so
|
ll-E |
||||||||
|
ANC |
)RTH0SIT |
E |
/ |
> |
||||
|
884 |
/ |
|||||||
|
> |
/ |
|||||||
1000 1100 1200 1300
TEMPERATURE - DEGREES C
Fig. 11.— Parts D and E Percent AI2O3 extracted versus temperature at which sinter was held for ten minutes.
and equal to three hours, as nearly as this could be done. This was done for clays 865, 866, 870, 871, and 879, and the resulting data appear on figure 11 as dotted line curves.
Kaolinite clays, if quite pure, gave nearly the same percentage yield of alumina for all temperatures in the range 1100°C. to 1300° C. when the time was the same. The opti-
LIME-SODA-SINTER PROCESS
53
mum temperature is probably about 1200° C, Yields ranging upward from 75 percent of total alumina are obtainable from good kaolinite clays even at 1000°C. Imperfectly crystallized kaolinite clays such as 869 and 883 did not yield so well, especially at the lower temperatures.
Sinters prepared from gibbsite-kaolinite clays are less sensitive to temperature varia- tion than those from kaolinite clays. The yields were about the same at 1000°C. as at 1300°C despite the fact that the heating period was longer at the higher temperature.
Sinters prepared from diaspore clay be- have toward variation of temperature much like those from gibbsite-kaolinite clays. The small rise in yield with increasing temper- ature noted for 872 was probably mostly due to the longer period required to reach the higher temperatures.
No alumina could be extracted from sinters prepared from montmorillonite clay at temperatures up to 1100°C. Even at 1200°C. only half of the alumina was made soluble. The extraction yield went up to 90 percent at 1300°C.
Table 12 — Effect of Sintering Temperature on Percent AI2O1 Extracted with Various Types of
Clay.
Time at top temperature — 10 minutes
Na20/Al203 = 1; CaO/Si02 = 2; see Figure 14
|
Clav Sample No. |
Sinter |
Sinter Temp. Degrees C. |
Percent AI2O3 |
Percent AI2O3 |
100 Si02 |
|
No. |
in Sinter |
Extracted |
AhOi+SiOj |
||
|
in extract |
|||||
|
865 |
51 |
1000 |
19.7 |
74 |
1.2 |
|
37 |
1100 |
« |
76 |
2.4 |
|
|
70 |
1200 |
« |
87 |
2.1 |
|
|
82 |
1300 |
« |
90 |
1.8 |
|
|
866 |
52 |
1000 |
9.7 |
52 |
0.2 |
|
38 |
1100 |
ft |
44 |
1.4 |
|
|
71 |
1200 |
« |
75 |
2.7 |
|
|
83 |
1300 |
ft |
85 |
1.7 |
|
|
867 |
53 |
1000 |
21.2 |
71 |
1.8 |
|
39 |
1100 |
« |
72 |
2.0 |
|
|
72 |
1200 |
« |
83 |
1.9 |
|
|
84 |
1300 |
« |
89 |
1.7 |
|
|
868 |
54 |
1000 |
9.2 |
|
|
|
40 |
1100 |
ft |
— |
— |
|
|
73 |
1200 |
« |
44 |
1.5 |
|
|
85 |
1300 |
ft |
89 |
1.7 |
|
|
869 |
55 |
1000 |
15.0 |
69 |
1.3 |
|
41 |
1100 |
« |
69 |
.2.5 |
|
|
69 |
1200 |
« |
82 |
2.4 |
|
|
86 |
1300 |
» |
90 |
2.2 |
|
|
870 |
56 |
1000 |
9.5 |
13 |
2.4 |
|
42 |
1100 |
« |
22 |
0.0 |
|
|
75 |
1200 |
« |
75 |
3.8 |
|
|
87 |
1300 |
« |
79 |
1.8 |
|
|
871 |
57 |
1000 |
13.3 |
65 |
2.3 |
|
43 |
1100 |
« |
64 |
2.5 |
|
|
76 |
1200 |
« |
79 |
2.2 |
|
|
88 |
1300 |
« |
91 |
1.8 |
|
|
872 |
58 |
1000 |
46.1 |
80 |
2.6 |
|
44 |
1100 |
« |
81 |
2.1 |
|
|
66 |
1200 |
« |
82 |
2.1 |
|
|
89 |
1300 |
« |
86 |
1.6 |
54
ALUMINA EXTRACTION
Table 12 — Continued
|
Clay Sample No. |
Sinter |
Sinter Temp. Degrees C. |
Percent AI2O3 |
Percent AI2O3 |
100 Si02 |
|
No. |
in Sinter |
Extr acted |
AhOa+SiO, |
||
|
in extract |
|||||
|
875 |
59 |
1000 |
32.1 |
85 |
2.5 |
|
45 |
1100 |
32.2 |
87 |
1.8 |
|
|
77 |
1200 |
32.2 |
92 |
2.2 |
|
|
90 |
1300 |
34.3 |
85 |
2.0 |
|
|
877 |
64 |
1000 |
19.4 |
80 |
2.9 |
|
50 |
1100 |
19.6 |
82 |
2.3 |
|
|
65 |
1200 |
19.9 |
94 |
1.9 |
|
|
95 |
1300 |
20.2 |
96 |
1.8 |
|
|
878 |
60 |
1000 |
17.7 |
73 |
2.7 |
|
46 |
1100 |
17.7 |
76 |
2.3 |
|
|
78 |
1200 |
17.9 |
86 |
1.9 |
|
|
91 |
1300 |
18.0 |
91 |
1.9 |
|
|
879 |
61 |
1000 |
16.5 |
76 |
2.0 |
|
47 |
1100 |
16.5 |
74 |
2.1 |
|
|
79 |
1200 |
16.7 |
90 |
2.0 |
|
|
169 |
1300 |
16.7 |
89 |
1.9 |
|
|
881 |
62 |
1000 |
18.7 |
77 |
2.3 |
|
48 |
1100 |
18.6 |
76 |
2.2 |
|
|
80 |
1200 |
19.0 |
86 |
2.3 |
|
|
93 |
1300 |
19.1 |
93 |
2.0 |
|
|
883 |
63 |
1000 |
17.0 |
62 |
2.2 |
|
49 |
1100 |
17.4 |
64 |
2.1 |
|
|
81 |
1200 |
17.5 |
82 |
1.8 |
|
|
94 |
1300 |
17.7 |
87 |
1.8 |
Sinters prepared from illite clays yielded poorly at temperatures up to 1100°. The yield was much higher at 1200°C. but little was gained by increasing the temperature above 1200°C.
EFFECT OF TIME HELD AT TOP SINTERING TEMPERATURE
A series of briquets was prepared in which the mole ratio of soda to alumina was one and that of lime to silica was two. These briquets were then heated to 900 °C. at a rate of about 50° per hour. The rate was then increased to approximately 250° per hour and continued until the temper- ature reached 1100°C. The temperature of 1100°C. was held for 10 minutes with one set of briquets, 60 minutes for another set, and 120 minutes for a third set. The temperature of 1100°C. was selected be- cause the extraction values for short sinter- ing periods at this temperature were low
enough to allow improvement with more extended periods.
The results obtained from this series of tests are presented graphically in figure 12. All samples except the montmorillonite clay showed some improvement in yield of alumina with increased time of heating. Anorthosite showed more improvement than any of the clays. This is probably because of the relatively large size of the anortho- site particles in comparison with the clay mineral particles that make up clays. Al- though the anorthosite was ground so that 90 percent passed the 200-mesh sieve, in- dividual particles are relatively large. Each 200-mesh particle of anorthosite can be visualized as a single fragment of this min- eral. On the other hand, a 200-mesh par- ticle of clay is composed of a large number of smaller single particles aggregated to- gether. Furthermore, the breakup of the clay structure on heating might be expected to result in a porous material which pre-
LIME-SODA-SINTER PROCESS
55
|
100 90 80 |
12-A |
|||||||||||||||||
|
Ci |
UNDERCLAY LLITE, KAOLINITE 866 |
0 |
MOLE RATIO Na2o/AI203=l " ii CQ0A1O2 -Z |
|||||||||||||||
|
O *n |
||||||||||||||||||
|
h- o < 50 a \- X |
||||||||||||||||||
|
UJ (0 ° on |
< |
> |
||||||||||||||||
|
N90 < |
||||||||||||||||||
|
• |
p |
^AOLI |
N |
|||||||||||||||
|
(KA |
OLIN 865 |
TE) |
Challoysite) |
|||||||||||||||
|
OD / |
||||||||||||||||||
|
40 |
20
20 20 60
TIME (MINUTES) HELD AT I I00°C
|
PO |
12-B |
||||||||||||||||||
|
(1 |
HAL LLIT |
:) |
|||||||||||||||||
|
W* |
1 KAOLIN |
870 |
|||||||||||||||||
|
(K/ |
UDLIN 869 |
TE) - |
|||||||||||||||||
|
60 70 |
c \- < a > - L u \ |
1 |
c b |
1 j |
c h |
> J |
|||||||||||||
|
: c |
K X |
< er 1- X |
I— |
||||||||||||||||
|
1 |
2 0 2 |
UJ 2 O 2 |
|||||||||||||||||
|
50 |
r |
MON- |
BEN l"MOR |
TONITE ILLONITE |
) |
(KAOLI |
MITE, |
FIRECLAY ILLITE, MONTK |
rfORIl |
.LONI |
TE) |
||||||||
|
868 |
e |
>7I |
|||||||||||||||||
60 120 20 60
TIME (MINUTES) HELD AT IIOO°C
Fig. 12.— Parts A and B Percent A1203 extracted versus time sinter was held at 1100°C.
56
ALUMINA EXTRACTION
|
100 |
! » |
_____ |
12-C |
|||||||||||||||
|
• |
4 |
> |
||||||||||||||||
|
BAUXITE |
SOFT KAOLIN . |
|||||||||||||||||
|
Q _~ |
(Gl |
3BSITE, KAOLINI1 |
rE) |
(KAOLINITE) ail |
||||||||||||||
|
UJ 1- o |
O / _> |
|||||||||||||||||
|
< 50 or \- X |
||||||||||||||||||
|
UJ |
||||||||||||||||||
|
< |
f |
i |
4 |
* |
||||||||||||||
|
J> 80 |
||||||||||||||||||
|
70 |
D |
IASPC 872 |
)RE |
PLASTIC KAOLIN (KAOLINITE, MONTMORILLONITE") |
||||||||||||||
|
878 |
||||||||||||||||||
|
40 |
120 20 60
TIME (MINUTES') HELD AT I 100-C
|
100 |
12-D |
|||||||||||||||||
|
^ — < |
, ( |
i |
||||||||||||||||
|
•^ |
■" |
K |
AOLIt |
g |
||||||||||||||
|
(KA( |
DLINI 881 |
rE) |
||||||||||||||||
|
Id o |
AOLIf 883 |
*JITE |
||||||||||||||||
|
o. \- X |
||||||||||||||||||
|
_ i ° SO |
||||||||||||||||||
|
<NJ90 < ^ 80 |
||||||||||||||||||
|
r |
BALL CLAY ^AOLI MITF. II 1 IT |
fO |
||||||||||||||||
|
879 |
||||||||||||||||||
|
CA |
SJORTHOSIT 884. |
E) |
||||||||||||||||
|
40 |
60 12.0 20 60
TIME (MINUTES) HELD AT I 100-C
Fig. 12.— Parts C and D Percent A1203 extracted versus time sinter was held at 1100°C.
LIME-SODA-SINTER PROCESS
57
sumabh could absorb molten sodium car- bonate so as to allow relatively quick re- action all the way to the center of the particles. The less porous anorthosite unit would be exposed to the action of soda and lime only at the surface of the particle.
Sinters prepared from pure kaolinite clays showed continuous though moderate improvement with increasing time. The yield from those prepared from imperfectly crystalized kaolinite clays (869 and 883) improved more with increasing time than did the others.
Sinters prepared from diaspore clay were less affected by the time of sintering than those prepared from kaolinite clay.
Sinters prepared from gibbsite-kaolinite clay behaved much like those prepared from kaolinite clays toward time variation ex- cept that the improvement in alumina yield was somewhat less.
Sinters prepared from illite clay 870 showed great and nearly linear improve- ment with increased sintering time but those from clay 866 (estimated to contain 80 percent illite) improved only moderate- ly with increased time. Possibly the reason for the difference is related to the fact that the iron content is very high in clay 870.
No alumina could be extracted from sinters prepared from the montmorillonite clay heated to 1100°C. even after sintering for 120 minutes at this temperature.
HFFECT OF HEATING RATE
A series of briquets, covering the same samples and made up to the same specifica- tions as those used in the series discussed in the preceding section were heated to 1 100°C on a heating schedule in which the rate of increase averaged about 50° C per hour over the range from 900° to 1100°C. The part of the heating schedule below 900° was similar to that illustrated in iigure 15. The yields of soluble alumina were almost exactly the same in nearly all cases as those shown on figure 12 for the period in which the sinters were held at 1100°C. for 60 minutes. These sinters (figure 12, 60 minutes) were above 1000°C. for about 95 minutes, whereas
the sinters heated at the slower rate were above 1000° (J. for about 120 minutes. There was, therefore, no considerable effect on the yield which could be ascribed to the slow rate of heating. The character of the briquets was, however, noticeably different in that the) were more friable than similar briquets heated to the same top temperature at a more rapid rate. Some of them had increased in diameter as much as \/% inch. Others, although showing some shrinkage, did not shrink as much as similar briquets which had been heated more rapidly.
EFFECT OF FURNACE ATMOSPHERE
Sinters were made at 1200°C. using the same composition for the sinter mixes as in the tests described in the two preceding sections with the furnace atmosphere ren- dered strongly reducing by passing a current of natural gas into the furnace. Comparison of the extraction yields from these sinters with those from similar sinters burned un- der the normal oxidizing conditions at the same temperature disclosed no differences which could certainly be correlated with atmosphere. There wrere twTo cases in which considerably better yields were obtained with reducing atmospheres than with oxidiz- ing atmospheres. These were from sinters prepared with the montmorillonite clay and with the anorthosite. However, it is not considered that the small amount of data justify more than a tentative conclusion in these two instances.
EFFECT OF SODA-TO-ALUMINA RATIO
A series of sinters was made in which the mole ratio of soda to alumina was 0.8, 1 .0, and 1.2. The results obtained from these tests are presented in figure 13. In all cases the effect of increasing the Na*0/Al-0 ratio was an improvement in the yield of soluble alumina. The degree of improve- ment, however, varied greatly. The data are erratic in some cases. This frequently happened when some of the sinters were rather completely vitrified. When such a sinter is ground to pass 65-mesh in prepa- ration for extraction it will contain more particles close to 65-mesh size and fewer
58
ALUMINA EXTRACTION
|
90 eo 70 60 to 40 |
3-A |
|||||||||||||||||
|
t |
>^ |
MOLE RATIO Ca O/Si 02 = 2 ® REDUCING ATMOSPHERE |
||||||||||||||||
|
/ |
^ |
|||||||||||||||||
|
UNDERCLAY (1 LL ITE. KAOLINI |
rE) |
|||||||||||||||||
|
866 |
||||||||||||||||||
|
90 |
||||||||||||||||||
|
70 60 50 40 |
1 |
r |
||||||||||||||||
|
\ (KA |
<AOU OLIN |
M TE) |
(HAL LOYSITE) Rfi7 |
|||||||||||||||
|
865 |
||||||||||||||||||
MOLE RATIO Na20/Al203
U 50
<
a:
.eo
|
13-B |
|||||||||||||||||
|
< |
$ ^^ |
||||||||||||||||
|
Cka |
(AOLI OLIN |
N TE) |
< |
/' |
< |
dHAL LLI T 87C |
E |
||||||||||
|
869 |
|||||||||||||||||
|
<s |
> |
||||||||||||||||
|
/ |
i |
Dl |
ASPO 872 |
RE |
|||||||||||||
|
/ |
(M |
BEN ONTN |
TONI /lORIL |
TE LONI" |
m |
||||||||||||
|
N |
1 |
368 |
MOLE RATIO Na20/Al203
Fig. 13.— Parts A and B
Percent A1203 extracted versus mole ratio Na20/Al203
Sinter temperature 1200° C. ; sinter time 10 minutes at top temperature.
LIME-SODA-SINTER PROCESS
59
|
100 |
1 |
13- C |
||||||||||||||||
|
i |
||||||||||||||||||
|
< |
A |
^ i |
t |
|||||||||||||||
|
80 |
SOFT KAOLIN |
1/ |
7A- |
|||||||||||||||
|
70 |
(KAOLIN A77 |
ITE) |
< |
|||||||||||||||
|
60 O u h |
(KAC |
)LINITE , |
MONT 878 |
MORILLONITE) |
||||||||||||||
|
U 50 < cr |
||||||||||||||||||
|
Ul |
« |
i |
||||||||||||||||
|
< |
/ |
^ |
! |
|||||||||||||||
|
K 8U |
" (Gl |
B/ |
\UXITE |
IITE") |
« |
r |
||||||||||||
|
70 |
BBSIT t, kaulit 875 |
1 BALL CLAY (KAOLINITE, ILLI |
Tt) |
|||||||||||||||
|
60 |
879 " |
|||||||||||||||||
|
50 40 |
. |
MOLE RATIO Nd20/Al203
|
too 90 80 |
13-D |
|||||||||||||||||
|
6 |
||||||||||||||||||
|
a |
||||||||||||||||||
|
KAOLIN KAOLINITE' |
||||||||||||||||||
|
60 |
(r |
; |
||||||||||||||||
|
86 |
*3 |
ANORTHOSITE 884 |
||||||||||||||||
|
90 80 70 60 |
^ |
► |
||||||||||||||||
|
B |
i ^^. |
« |
1 |
|||||||||||||||
|
< |
r^" |
KAOLI |
M |
|||||||||||||||
|
(KA |
DLINI 8RI |
TE) |
PLA Ckao MON |
5TIC F LINIT TMOR Q- |
IRECL :,ILLI ILLON |
AY |
||||||||||||
|
ITE) |
||||||||||||||||||
|
40 |
/ |
O / 1 |
MOLE RATIO N<i20/Al203
Fig. 13.— Parts C and D
Percent A1203 extracted versus mole ratio Na20/ALO:t
Sinter temperature 1200°C. ; sinter time 10 minutes at top temperature.
60
ALUMINA EXTRACTION
smaller particles than would a more friable material ground to pass the same sieve.
The typical kaolinite clays show some- what less variation in alumina yield with increasing Na^O/ALO^ ratio than do im- pure kaolinite clays 871, 878, 879 or clays with imperfectly crystallized kaolinite, such as 869 and 883.
The illite clays reacted to variation in soda-to-alumina ratio in much the same way as did kaolinite and gibbsite-kaolinite clays.
The data for montmorillonite clay were erratic but it is probable that more exten- sive tests would give results similar to those obtained with illite clay.
Sinters prepared from the diaspore clay were rather insensitive to variation of Na*0 /AbOa ratio insofar as extraction yields were concerned.
Anorthosite showed considerable im- provement in extractability with the high NasO/AhOs ratio.
EFFECT OF LIME-TO-ALUMINA RATIO
Data are presented graphically in figure 14 which indicate the variation of the per- cent alumina extracted with CaO/SiO* ratios 1.8, 2.0, and 2.2. The Na«0/A1.0. ratio used was 1.0 in all cases except for kaolin 877 where it was 1.2. These sinters were burned at 1200°C. according to the schedule shown in figure 15.
Sinters prepared from kaolonite or illite clays give sharply reduced yields of alumina if the CaO/SiO* ratio is less than two, but the yields are not improved by increasing the value of this ratio above two.
Sinters prepared from the montmorillo- nite clay showed improvement with excess lime. It is not clear why montmorillonite should react so differently as compared to other clays toward excess lime.
SILICA IN THE ALUMINA EXTRACT
Sinters from kaolinite clays gave alumina extracts in which the average values of the ratio 100XSiO/(Al2O3+SiO<) lie in the
range 2.0 to 2.3. The weight of the silica extracted from 10 grams of sinter averaged about 0.037 gram for pure kaolinite clays and somewhat less for imperfectly crystal- line kaolins such as 869 and 883. Sinters from kaolinite clays which contained appre- ciable amounts of illite or montmorillonite showed less silica in their extract than did pure kaolins. High lime-clay ratios decreased the silica in the extracts from sinters pre- pared from kaolinite clays.
Sinters prepared from illite clays and montmorillonite clays gave extracts in which the average values of the ratio 100X SiO2/(Al.O.+SiO0 were 2.3 to 3.2. The weights of silica extracted from ten grams of sinter averaged about 0.014 gram. High lime-clay ratios decreased the weight of silica in the extract in most cases.
Sinters prepared from halloyiste, dia- spore, and gibbsite-kaolinite clays gave ex- tracts in which the silica-to-alumina ratio was about the same as for kaolins.
Anorthosite was also much like kaolinite clays insofar as the amount of soluble silica in sinters prepared with it was concerned.
Effect of Minor Components Present
in the Raw Materials
general comments
Practically all clays and limestones con- tain compounds of some or all of the el- ements iron, magnesium, titanium and phos- phorous. Sinter mixes were prepared, burned, and extracted as described above with the exception that quantities of Fe^Os, Cas (PO.) 2, TiO, and MgCO were added. The quantities were chosen so that the ranges likely to be encountered in naturally occurring clays or limestones would be covered. Clay 877 was chosen for the tests for the reason that it contained unusually small amounts of all of these substances except TiOa and because it contained no clay mineral other than kaolinite. The tests were carried out at 1150°C.top temperature with a firing schedule as illustrated in figure 15. The data are presented graphically in figures 16, 17, 19, and 20.
LIME-SODA-SINTER PROCESS
61
|
14-A |
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BENTONITE |
MOLE RATIO N a2 0 /Al 2 03 = 1 . |
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FIG. 14.— Parts A and B
Percent Al-Oa extracted versus mole ratio CaO/SiU^
Sinter temperature 1200°C. ; sinter time 10 minutes at top temperature.
62
ALUMINA EXTRACTION
10
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0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
TEMPERATURE-DEGREES C. Fig. 15. — Typical record of a sinter heating schedule as traced from recorder chart.
LIME-SODA-SINTER PROCESS
63
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64
ALUMINA EXTRACTION
1 00
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MgO ADDED AS M9 COj SINTER TEMPERATURE 1150 °C |
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9
100 %M*° %AI203
12 TEMPERATURE
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24
Fig. 20. — Percent A1203 extracted versus 100 c, ., ^
7r MgO . 2Oa
EFFECT OF PHOSPHORUS (Figure 16)
The data indicate that the presence of phosphate in quantities ranging up to one percent of the weight of sintered material exerts no important effect on the amount of AbOs made extractable by lime-soda sintering. This is an expected result but is in contrast with results obtained when the lime only is used as the alkali in the sinter mix.
EFFECT OF T1TANJA (Figure 17)
The results of these tests indicate that titania in the sinter is not harmful and may possibly have a small beneficial effect. There is a suggestion that it may be desirable to allow an extra mole of Na^CO for each mole of TiOa present. The variations in yield are, however, of the same order as the probable precision of the data.
EFFECT OF IRON (Figure 19)
Iron apparently has a moderate adverse effect on the yield of extractable AkCX This effect is not serious, however, unless three percent or more FeaOs is present in the sinter.
EFFECT OF MAGNESIA (Figure 20)
The data indicate no effect of magnesia on the percent of extractable alumina even
when the amount of magnesia present is as much as 26 percent of the total alumina present in the sinter. This is markedly different from the results when the lime- sinter method is used.
MINERALOGICAL ANALYSES Microscopic Analysis
effect of variations in top sintering temperature
Mixtures with a lime-to-silica ratio of 2.0 and a soda-to-alumina ratio of 1.0 were fired for 10 minutes under oxidizing condi- tions according to the schedule in figure 15.
Kaolinite clay. — New reaction products were very indistinctly developed in sinters fired to 1000° C, and it appeared that the components had only started to react and that new phases had only begun to separate out (fig. 21C). Most of the material was an aggregate mass of faintly anistropic material with an index of refraction about equal to 1.650. Some material, also slightly anistropic, with an index of refraction of about 1.590 was present also.
Sinters fired at 1 200 °C. were composed of j8— C2S in grains with a maximum diameter of 3 microns, and a lesser amount of a slightly anistropic material with a mean index of 1.590 that was interpreted as CsAa with considerable soda in solid solution.
LIME-SODA-SINTER PROCESS
65
The sinters fired to 1300 C. (fig. 21 1) ) were similar to those fired to 1200°C. ex- cept that the ft— C*S had a "cleaner" appear- ance. A possible interpretation is that £— C»S contains solid solution components at 1200° C. which separate out when the temper- ature is increased to 1300°C.
Halloysite clay. — Sinters prepared with this type of clay had the same characteristics as those made with kaolinite clay.
Diaspore clay. — Sinters fired at 1000°C. were composed largely of NA in irregular aggregate masses of individual units less than about 1 micron in diameter (fig. 21 A). A small amount of /3— OS in grains with a maximum diameter of 3 microns was present also (the sample contained a little kaolinite and therefore some silica as an impurity).
Sinters fired to 1100°, 1200°, and 1300° C. had the same characteristics as the 1000°C. sinter (fig. 21B).
Gibbsite-kaolinite {bauxite) clay — NA (Na^O.AbOs) wTas present in rather in- distinct extremely minute grains, and some poorly developed /3— C2S was found also in sinters fired to 1000°C.
Sinters fired to 1100°, 1200°, and 1300° C. all showed well developed NA in aggre- gates of units less than about 1 micron, and ft— C2S in distinct grains with a maximum diameter of 5 microns.
Illite clay. — A mixture of particles of lime and dehydrated illite composed sinters fired at 1000° and 1100°C. (fig. 22A).
In sinters fired to 1200°C. the compo- nents were ft— C2S in units with a maximum diameter of 15 microns, a pigmentary mate- rial with the characteristics of -J-CaO.AbCX FezOs, and a colorless faintly anistropic in- terstitial material with a mean index of re- fraction of 1.60. The latter component was interpreted as CbAs with soda in solid solu- tion (fig. 22B).
Sinters fired at 1300°C. were like those fired at 1200°C. except that the /J-OS had a "cleaner" appearance as if some solid solution components had separated out (fig. 22C ).
Montmorillonite clay. — A mixture of particles of lime and dehydrated montmo- rillonite composed sinters fired at 1000° and 1100°C.
Sinters fired at 120" C. were composed chiefly of j3— OS in units with a maximum diameter of 15 microns. A small amount of pigmentarj material with the characteristics of 4CaO.AK'03Fe20< was present also. A component approaching CbA« ma\ have been present, but it could not be positively identi- fied.
C^S, chiefly fi-ionw (there was some in- version to y-form), in units with a maxi- mum diameter of 15 microns made up about all of the 1300°C. sinter. The ferrite ap- peared to be less abundant, and the alumi- nate was less certain than in the 1200°C sinter.
Attention should probably be directed to the fact that these and ensuing mineralogical comments relate only to crystallized ob- servable phases. In many cases it is obvious that the lime content is insufficient to de- velop both a silicate and an aluminate for the entire silica and alumina contents with even the most generous allowances for soda solution.
EFFECT OF VARIATIONS IN THE RATIO OF SODA TO ALUMINA
The soda-to-alumina ratio was varied from 0.8 to 1.2, in sinters writh a lime-to- silica ratio of 2.0 and fired to 1200°C. for 10 minutes under oxidizing conditions ac- cording to the schedule in figure 15.
Kaolinite clay. — Variations in the soda- to-alumina ratio from 0.8 to 1.2 caused no essential variations in the character of the sinters.
Diaspore clay. — Sinters prepared with a soda-to-alumina ratio of 1.2 were composed essentially of NA. They also contained a considerable amount of green-brown pig- mentary material that has many of the characteristics of the ferrite, and it is sug- gested that the excess soda is held in some such compound. A small amount of (3— C2S was present also.
An isotropic glassy material with an index of refraction roughly equal to the mean index of NA was the essential component in sinters prepared with a soda-to-alumina ratio of 0.8. A deficiency in soda appeared to hinder the crystallization of NA.
66
ALUMINA EXTRACTION
...
w * I
n '%
5» %
# #•
:3- 4" *'*
^$ I y'' ^^ "#
#"
.,, *''fc'
<»
«?:■ A^"<i*fc
Fig. 21. — Photomicrographs of soda-lime-clay sinters, 300 X.
A. Diaspore-clay sinter fired to 1000°C. es in which the formation of new phases showing aggregate masses of aluminate. is indistinct.
B. Diaspore-clay sinter fired to 1200°C. ^ ^ ,. . , . x , . ioaao/-. showing aggregate masses of aluminate D" Kaolimte-clay sinter fired to 1300 C somewhat better developed than in A. showing the distinct development of
C. Kaolinite-clay sinter fired to 1000° C. sma11 particles of C2S (mostly fl-form). showing the development of some parti- The aluminate is interstitial and indis- cles of new phases, and aggregate mass- tinct.
LIME-SODA-SINTER PROCESS
67
'j § * ,
Fig. 22. — Photomicrographs of lime-sod
A. Illite-clay sinter fired to 1000°C. show- ing the absence of new phase develop- ment. Aggregate masses of dehydrated p clay and lime are the components.
B. Illite-clay sinter fired to 1200° C. show- ing the development of small particles of #CvS and pigmentary ferrite. Large aggregate masses in which there is in-
a-clay sinters, 300 X.
distinct development of new phases are also present.
Illite-clay sinter fired to 1300°C. show- ing the excellent development of large particles of /3C2S and pigmentary fer- rite(dark material). An aluminate phase is not evident in the photomicrograph.
68
ALUMINA EXTRACTION
Gibb site -kaolinite (bauxite) clay. — Sin- ters prepared with a soda-to-alumina ratio of 1.2 were composed of NA and ft— C2S.
In sinters prepared with a soda-to-alu- mina ratio of 0.8, the component in addition to f$— OS was an isotropic glassy material with a mean index of refraction about equal to that of NA. Again the crystallization of NA appeared to be retarded by a deficiency of soda.
lllite clay. — An increase in the soda-to- alumina ratio from 1 to 1.2 caused the aluminate to be more prominent and to have properties more nearly approaching NA. Further, the OS became "dirty" as if it had taken something into solid solution.
In sinters prepared with soda equivalent to 0.8, ft— C2S, dirty in appearance, seemed to be about the only component. Aluminate could not be identified positively.
Montmorillonite clay. — Increasing the soda-to-alumina ratio from 1 to 1.2 caused a distinct increase in the prominence of the aluminate and gave it properties more nearly approaching NA. Further, the C2S in such sinters was "dirty" in appearance.
A decrease in the soda-to-alumina ratio from 1 to 0.8 was attended by a decrease in the abundance of aluminate and a trend toward material with the characteristics of CsAa. Further, the C2S was distinct and "clean" in units to 10 microns in diameter.
EFFECT OF MISCELLANEOUS VARIATIONS IN COMPOSITION
Varying amounts of Fe^Oo, MgCCX TiC>2, and Ca3(PO02 were added to batches prepared with kaolinite clay 877 with a lime-to-silica ratio of 2, and soda-to-alumina ratio equal to 1. The sinters were fired at 1150°C. for 10 minutes under oxidizing conditions.
The sinter with the added FesOs con- tained a considerable amount of minute dis- crete particles of iron oxide. There was no suggestion that the iron had reacted with any of the other components.
No distinct phase containing TiO* could be detected in the sinters to which this com- ponent was added. However, the C*S and aluminate phases were much less distinct,
suggesting that the TiO* had hindered their development.
The sinter with added magnesia up to 26 percent of total alumina, and phosphate up to 1 percent of total sintered material were like those without these added com- ponents.
EFFECT OF REDUCING ATMOSPHERE
Sinters with a soda-to-alumina ratio of 1 and a lime-to-silica ratio of 2 were fired to 1200°C. under reducing conditions ac- cording to the heating schedule given in figure 15.
Kaolinite clays — Sinters fired under re- ducing conditions were essentially the same as those fired under oxidizing conditions except that under the former conditions the new phases were better developed. Further, when any iron compound was present in the clay as an impurity, the aluminate in the sinter fired under reducing conditions had a dirty yellowish appearance and a higher index of refraction indicating that iron was held in solid solution.
Gibbsite and diaspore clay. — If the clay contained any iron component as an im- purity, the aluminate in sinters prepared under reducing conditions was dirty in appearance and had a higher index of re- fraction, indicating the presence of iron in solid solution.
lllite clay. — The sinters fired under re- ducing conditions exhibited an absence of any material that appeared unreacted and an enhanced development of new phases. The (3— C2S developed in units with a maxi- mum diameter of 20 microns as compared to 10 microns in sinters fired under oxidiz- ing conditions. Further the aluminate was more distinct and had characteristics sug- gesting that iron had been taken into solid solution.
Montmorillonite clay. — Sinters prepared under reducing conditions showed a larger percentage of definitely reacted material and better development of new phases. The (3— C2S was present in cleaner and larger units (20 microns ±), and the aluminate appeared to have taken iron into solid solu- tion.
l./MI -SOD.I-SINTER PROCESS
69
EFFECT OF VARIATION IN TIME BATCH HELD AT TOP SINTERING TEMPERATURE
Sinters prepared with a soda-to-alumina ratio of 1 and a lime-to-silica ratio of 2 were heated according to the regular firing schedule shown in figure 15 to 1100°C. One batch of sinters was held at 1100°C. for 10 minutes, another batch for 60 min- utes, and a final batch for two hours.
Kaolinite clay. — Only a slight difference could be detected between the sinters fired for 10 minutes and those fired for two hours. The 10 minute sinter was composed of a small amount of material that seemed unreacted and material that appeared to be in the initial stage of reaction. The two hour sinters showed a more advanced de- gree of reaction writh better development of new phases.
Gibbsite and diaspore clays. — Sinters fired at 10 minutes and at two hours both showed complete reaction of components with the development of new phases.
Illite clay.— The sinter held at 1100°C. for two hours was composed of unreacted components with a small amount of material that might have been in the initial stages of reaction. The 10 minute sinter was com- posed almost entirely of unreacted material.
Montmorillonite clay. — Sinters fired at 10 minutes and at two hours were both composed of essential}7 unreacted material.
EFFECT OF VARIATION IN HEATING RATE
The following data were determined on sinters with a lime-to-silca ratio of 2 and a soda-to-lime ratio of 1 fired under oxidiz- ing conditions, with a heating schedule of 50° C. per hour over the range from 900° to 1100°C. instead of the more rapid and standard rate shown in figure 15.
Kaolinite clay. — The C2S and aluminate appeared to be more distinct and in better developed units in the sinters prepared at the slower heating rate. The aluminate showed more distinct anisotropism, and the C2S was more definite and "cleaner", sug- gesting less solid solution effects. Also there seemed to be more inversion of C2S to the y-form.
Gibbsite and diaspore clays. — The sinters prepared at the slower heating rate were usually about the same as those prepared at the regular heating rate. In a few in- stances there was a suggestion of better phase development with the slower heating rate. Also, any C*S present, because of the presence of a silicate mineral in the clay, showed more inversion to the y-form.
Illite and montmorillonite clays. — No difference could be detected between sinters prepared at the regular and at the slow rate.
The more friable character of many of the sinters (see page 57) prepared with the slower heating rate was in accord with the slightly better phase development and the greater degree of inversion of OS to the y-form.
EFFECT OF VARIATIONS IN THE RATIO OF LIME TO SILICA
The lime-to-silica ratio was varied from 1.8 to 2.2 in sinters with a soda-to-alumina ratio of 1, and fired to 1200°C. for 10 minutes under oxidizing conditions accord- ing to the heating schedule given in fig. 23.
Kaolinite and illite clays. — The sinters showed that the best phase development took place when the lime-to-silica ratio was 1.8. In such sinters the /?— C2S was in distinct units, and the aluminate, which approaches OAs in optical properties, was plainly vis- ible.
As the lime content increased, there was a decrease in the degree of phase develop- ment. For example, sinters with a lime-to- silica ratio of 2.0 frequently contained a considerable amount of material in aggre- gate masses of indistinct units with about the optical properties of fi— C2S but with few distinct grains of /?— C2S. Also the aluminate was less distinct and closer to C»Aa in op- tical properties.
With an increase in the lime-to-silica equal to 2.2 there was a further slight decrease in the degree of development of phases.
Montniorillonite clay. — In sinters pre- pared with clays containing montmorillo- nite, there appeared to be little or no dif-
70
ALUMINA EXTRACTION
ference in characteristics when the lime-to- silica ratio was varied from 1.8 to 2.2.
X-ray Analysis
general comments
In the application of the X-ray diffraction method to the lime-soda sinters, two un- satisfactory situations must be admitted: first, it is uncertain whether all of the phases in the four— component system are known, and second, the extent to which various solid solution possibilities may be realized cannot be established. Diffraction patterns are frequently not precisely typi- cal for a given phase, and should therefore not be relied upon for estimates of abun- dance of the phase.
THE PHASES
Calcium or tho silicate. — The lime-soda sinters do not dust. In none of the sinters examined did enough orthosilicate invert to the y-form so that the y— C2S could be found by diffraction methods. The develop- ment of /3-C2S was essentially parallel to the development outlined in the lime-sinter discussion. In the lower temperature sinters (1000°C.) a considerable amount of re- action products was evident, the material being of extremely small particle size, and the diffraction pattern of that resulting phase which we consider to be the silicate departed from the typical /3— C2S to such a degree that in ascribing the pattern to fi— C2S we could be in error. For the kaolin- ite sinters, development of this phase was gradual, and by 1200°C. the pattern was typical for (3— C2S whose particle size is about of the order of a micron. In the illite and montmorillonite clay sinters the same material developed abruptly above about 1200°C. and by 1300°C. attained a par- ticle size of 25 to 50 microns.
The large fi— C2S grains, which did not invert, no doubt held some extraneous ions in solution. The low temperature {3— C2S could vary from ideal composition to a much greater degree.
Sodium aluminate. — The sodium alumi- nate, NaAlO, is a well-established phase
for which the powder diffraction diagram is readily recognized but the crystallization has not been worked out. The phase was clearly developed and typical only in the sinters prepared with the highest alumina clays. In other cases, where the phase was observed at all, some lines had anomalous weak intensities or were missing, and the apparent abundance was not commensurate with the alumina content. It is our un- supported opinion that the typical NA is a variation of the cristobalite structure, and that the non-typical material is a combina- tion of this with y-AbOs. In some cases the development of NA was promoted by increased addition of Na2COs.
The complex aluminate. — A prominent feature in the diffraction diagrams, especial- ly of the kaolinite sinters at 1000° and 1100°C, was a pattern rather closely re- sembling that of CbAs. At higher temper- atures, after the full development of 0-CaS, this material was no longer apparent. In the CaO-Al203-Na20 system, according to Brownmiller and Bogue,8 OA3 does not dissolve soda. However, the published CsAa structure is of an open type, described as based on garnet, and it seems quite possible that soda and silica together could enter this crystallization. This material is assumed to be the nearly isotropic phase observed optically with the index 1.590.
The prominence of the above described pattern declined in sinters fired at 1200°C. and was not apparent at 1300°C. although it was still observed microscopically. One is led to the conclusion that the phase softened to a glass.
THE SINTERING PROCESS
In the lime-soda sinters, reaction is initi- ated at somewhat lower temperatures than in the lime sinters. The softening of the sodium-calcium double carbonate (appar- ently represented in the thermal curves by a flexion near the shoulder of the decarbon- ization peak) provides a liquid agent for attack on the clay. High alumina materials, even the highly resistant a—AhOs, react readily with the lime and soda, kaolinite
LIME-SODA-SINTER PROCESS
71
reacts less readily, and illite and mont- morillonite react relatively little.
It was not possible to identify the first reaction product in sinters made with high aluminous clays, but at 1100°C., NA had developed. On further heating, the NA grew in grain size and had a typical diffrac- tion pattern. Only a little /?— C2S devel- oped, as would be expected from the low silica content of these clays.
Kaolinite clay sinters have reacted at 1000°C. to form an extremely fine-grained mixture of the complex aluminate, "CsAa," and non-typical /?— C2S. Continued heating gradually developed more ft— C2S, which by the time the temperature reaches 1200° C. gave a typical diffraction pattern, while the crystalline structure of the complex alumi- nate was destroyed. The transition from the complex aluminate to the noncrystalline material was considered to be reflected in the greater extractability of aluminum in the higher temperature (or longer heated) sinters. The development of NA was either doubtful or unimportant in the sinters of kaolinite clays. When small amounts were indicated they appeared as well or better developed in the low temperature sinters as in the high.
In illite and montmorillonite clay sinters the complex aluminate, "GAs," was less prominent. The (3-C& development, as in the case of the lime sinters, showed a sudden marked growth to large particle size when temperatures of 1200° or 1300° C. were reached. This growth was probably not significant in itself from the stand- point of extraction, but apparently reflected the greater difficulty of attack on these more stable clays.
DIFFERENTIAL THERMAL
ANALYSES
Differential thermal analyses of batches of various types of clay with a lime-to-silica ratio equal to 2 and a soda-to-alumina ratio equal to 1 are given in figure 23.
The differential thermal curves show the relative intensity and the temperature of the reactions that take place when sinter mixtures are heated up to 1300°C. at a
uniform rate of approximate^ 10 C. per minute. The downward deflections of the curve indicate endothermic reactions and the upward deflections indicate exothermic reactions. A vertical scale for determining the temperature difference indicated by the deflections of the curve is given in figure 9.
Diaspore clay 872, sinter mixture SS89. — The endothermic reaction between 500° C. and 600° C. corresponds to the loss of lattice water (OH) from the diaspore. The part of the curve between 700° C. and 875 °C shows an endothermic reaction due to the loss of CO2 from the carbonates plus an exothermic reaction which indicates the formation of a new phase. The curve char- acteristics that result when loss of CO* is the only reaction taking place in this tem- perature interval can be seen in curves for SS85 and SS87.
The curve for sinter mixture SS89 shows that a reaction is taking place between the components of the diaspore clay and the carbonates, and that a new phase or phases develop at about 800° C. The formation of the new phase begins before the reaction corresponding to the loss of CO* is com- plete.
The portion of the curve above 900° C. cannot be interpreted in detail, but it prob- ably signifies changes in the form of the in- itial phases and/or the separation out of material held in solid solution.
Gibbsite-kaolinite {bauxite) clay 875, sinter mixture SS90. — The initial endother- mic peak is due to loss of adsorbed water and suggests that halloysite as well as kao- linite is present in this clay. The endother- mic peaks at about 340° C. and 590 °C. are the result of loss of lattice water (OH) from gibbsite and kaolinite (and halloysite), respectively.
The endothermic peak between about 790°C. and 840° C. is the result of loss of CO* from the carbonates. However, if loss of CO* was the only reaction in this temperature interval, the curve should be like those of SS85 and SS87 in the same temperature interval. It can be concluded, therefore, that an exothermic reaction due to the formation of a new phase or phases
72
ALUMINA EXTRACTION
C 1300" C
Fig. 23. — Differential thermal analyses of lime-soda-clay mixtures. The curves posits from data for the clay alone and for sinter mixtures. (See fig. 9.)
LIME-SODA-SINTER PROCESS
73
begins soon after the start of loss of CO*, i.e.. about 800°C.
The curve above 950° C. is probably the result of changes in the initial phases and/ or the separation out of material held in solid solution. It would seem that such changes take place without appreciable thermal effect. There appears to be no distinct thermal effect corresponding to the formation of the disilicate or the aluminate.
Kaolinite clay 877, sinter mixture SS67. — The endothermic reaction at about 600° C. corresponds to the loss of lattice water (OH) from the kaolinite. Again the endo- thermic reaction between about 750° C. and 875 °C. is not as large as would be expected if loss of CO2 were the only reaction during this temperature interval. It is probable that C2S and perhaps other phases begin to form before the destruction of the carbon- ate is complete. The new phases seem to develop at slightly higher temperatures in sinter mixes containing kaolinite clays than in those prepared with diaspore or gibbsite since the reaction due to loss of CO2 is less affected in such sinter mixes.
The curve above 950 °C. is like that for batch SS90, and the same explanation is offered.
Kaolinite-illite {ball) clay 879, sinter mixture SS92. — In this curve the initial endothermic peak corresponds to the loss of adsorbed water by illite, the exothermic reaction between about 200° C. and 500° C. is the result of the burning off of organic material, and the endothermic peak at about 600 °C. is caused by the loss of lattice water (OH) from the kaolinite and illite.
The portion of the curve between 750° and 1000°C. is about like that for the preceding sinter batch SS67 and the same ueactions are indicated. The curve above 1000°C. has some slight differences which are like those in the curve for the batch containing illite (SS87), and show the in- fluence of the small amount of illite in this clay.
Illite clay 870, sinter mixture SS87. — The broad initial endothermic reaction is due to loss of adsorbed water, and the endo- thermic peak at about 575° C. corresponds
to loss of lattice water (OH) from the illite.
The endothermic reaction between about 700°C. and 925° C. appears to be the result of a single reaction. Unlike the curves for the previous mixture, the removal of CO* from the carbonate seems to be the only reaction taking place in this temperature interval.
The sharp exothermic reaction at about 1200°C. represents the formation of C»S and perhaps other new phases. The temper- ature at which new phases form is several hundred degrees higher in sinter mixtures containing illite clays than in those made up of kaolinite, gibbsite, or diaspore clays. This is in accord with the extraction data which show very little alumina is extractable from illite clay sinters until firing temperatures reach 1200°C, whereas a high percentage of alumina is extractable from sinters con- taining kaolinite, gibbsite, or diaspore when the firing has been carried to only 1000°C.
Montmorillonite clay 868, sinter mixture SS85. — The endothermic peaks at about 150°C. and 700°C. are due to loss of ad- sorbed water and lattice water (OH), respectively, from the montmorillonite. The portion of the curve above 750° C. is simi- lar to that for the preceding mixture con- taining illite clay and the same reactions are indicated.
Like illite mixtures, batches containing montmorillonite do not yield new phases until a temperature of about 1200° C. is reached, and appreciable alumina is not ex- tractable until sinters have been fired to this temperature.
DrscussiON and Summary of Study of Phases Present in the Sinters
The X-ray and optical data concurred in the conclusion that in sinters with a lime- to-silica ratio of 2 and a soda-to-alumina ratio of 1, the compound Na*O.Al-Os as described by Brownmiller and Bogue8 is well developed only in sinters prepared with high alumina clays (containing diaspore or gibbsite). In kaolinite clays the aluminate appeared to be 5Ca0.3AU0: with soda and perhaps silica in solid solution. In illite and
74
ALUMINA EXTRACTION
montmorillonite clays the development of aluminate was very poor, but the compound again seemed to be more nearly like C5A3.
Soda in excess of that required for a ratio of soda to alumina equal to one caused an enhanced development of aluminate with characteristics more like NA. A decrease in the soda content caused a decrease in the development of aluminate and a trend toward characteristics like those of C5A3.
The C2S occurred in distinct units only a few microns in diameter in sinters made with kaolinite clay. In sinters made with illite or montmorillonite clays, the C2S attained a maximum diameter of over 20 microns. The C2S was in the /?-form with little inversion to the y-form.
The aluminate was found in irregular aggregates and interstitial masses composed of indistinct individual units less than one micron in diameter.
In sinters prepared with gibbsite or dia- spore clays new phases began to develop as soon as loss of CO2 released CaO and Na20, or perhaps sooner, under the attack of the fluid double carbonate, hence a large per- centage of alumina was extractable in sinters made at low temperatures, e.g., 1000° C. Again in sinters made with kaolinite clays, new phases began to form before the car- bonates were completely broken down al- though it would seem that the temperature was slightly higher than for the gibbsite or diaspore clays. As a consequence sinters containing kaolinite clay and fired at low temperatures had high percentages of ex- tractable alumina. In the kaolinite and gibbsite clays the phases appeared to con- tinue to develop throughout a temperature interval of several hundred degrees, and without sharp thermal effects.
When sinters were prepared containing illite or montmorillonite clays, new phases did not develop extensively until about 1200°C. and consequently little alumina was extractable unless the firing was carried to this temperature. Further there was a sharp thermal reaction which began at about 1175° corresponding to the formation of the new phases in illite and montmorillonite clay mixtures.
A plausible explanation for the difference in temperatures required for new phase development is as follows: In the case of gibbsite and diaspore the loss of (OH) water at about 325° C, and 525° C, respec- tively, results in the formation of free alumina which appears to be particularly susceptible to attack by fluid double carbon- ates. As a consequence new phases form at a very low temperature. The loss of (OH) lattice water from kaolinite at about 575° C. produces the active, but still crudely com- bined alumina and silica of metakaolin, and new phases are formed slightly later, after considerable development of free lime.
In the case of montmorillonite and illite, the loss of (OH) water at 575° and 675° C, respectively, is not accompanied by a destruction of the lattice of these minerals. A definite structural configuration is re- tained until a temperature is reached several hundred degrees in excess of that required for loss of (OH) water. It would be ex- pected that alumina and silica locked up in a definite structural configuration would be relatively unreactable, and that new phases would not develop until such a configuration is destroyed. New phases, then, would not be expected in sinters prepared with illite or montmorillonite clays until a higher temperature is reached (or until after a more protracted heating interval) than is required for similar development in batch- es containing the other types of clay mate- rials. It follows, of course, that in plant practice the presence of any illite or mont- morillonite in a clay would raise the sinter- ing temperature necessary for the best ex- traction.
All the data suggest that the new phases formed at low temperatures in the kaolin- ite, diaspore and gibbsite batches undergo changes as the temperature is raised. The )8— C2S can be seen under the microscope to become "cleaner" in appearance in sinters fired to higher temperatures, as if the ft— C2S were being freed of material in solid solu- tion. The diffraction pattern of ft— C2S is typical only in higher temperature sinters. In lower temperature sinters the pattern departs considerably from that typical for
LIME-SOD A-S1NTER PROCESS
75
this compound. The prominence of the pat- tern of the complex aluminate is reduced as the temperature of sintering is increased, suggesting; that the material is reduced to a kind of glassy suhstance.
SUMMARY AND CONCLUSIONS
The introduction of soda into the alkali extraction processes as practiced in the lime- soda method resulted in attack of the clay at temperatures much lower than when lime alone wras used. When the clays were of the high alumina variety (diaspores and bauxites) fairly good extractions were ob- tained with sintering temperatures of 1000° C. Kaolinite-clay sinters required slightly higher temperatures, whereas sinters pre- pared from illite or montmorillonite clays had to be heated to temperatures in the range 1200° to 1300° C. to give satisfactory yields. Unless raw materials containing sufficient soda to carry the process are avail- able there is little advantage in using the lime-soda as compared to the lime process with the two latter types insofar as sinter- ing temperature is concerned.
Kaolinite clays and gibbsite-kaolinite (bauxitic) clays may yield above 90 percent of their alumina by the lime-soda process when they are relatively free from iron and other clay minerals. Halloysite clay, mont- morillonite clay, and anorthosite may also yield over 90 percent of their alumina, but it is more difficult to get top yields. The highest yields for illite clays were 10 to 12 percent below those for the other clays. Yields for clays composed of mixtures of clay minerals were in agreement with results predicted from pure types.
Diaspore clays and gibbsite-kaolinite clays yielded nearly as much alumina on sintering to 1000°C. as to 1300°C. Pure kaolinite clays showed no increase in yield when sintered above 1100°C. other factors being the same, and yielded about 75 percent of their alumina when sintered to 1000°C. Montmorillonite clavs vielded none of their
alumina on sintering to 1100°C, 50 per- cent on sintering to 1200°C, and 90 per- cent on sintering to 13()0°C. Like mont- morillonite clay, illite clay required sinter- ing above 1200°C. for good alumina yields.
In sinters fired to 1100°C, increasing the time held at top temperature caused onl> slight increase in yield of alumina from diaspore clays, only moderate increases foi clays composed of kaolinite, gibbsite, and halloysite, and very great improvement in the yield from illite clays. Regardless of sintering time, montmorillonite clays yield- ed no alumina on firing to 1100°C.
Sinters of the various clays fired under strong reducing conditions gave the same yield as similar sinters fired under oxidizing conditions.
Although some of the results are erratic, the data show definite improvement in the yield of alumina from all types of clay except the diaspore as the Na20/Al20s ratio increased from 0.8 to 1.2.
Sinters of kaolinite clay and illite clay gave sharply reduced yield when the CaO/ SiOa ratio was less than two, but not im- proved yields with a ratio above two. Mont- morillonite clay showed greatly increased alumina yields with CaO/SiO^ ratio great- er than two.
Extracts from illite clays and mont- morillonite clay contained a slightly larger percent of silica than the extracts from other clays. Kaolinite clays which contained ap- preciable amounts of illite and montmo- rillonite gave extracts with lower silica con- tents than pure kaolinite clays, but the difference was small.
Addition of phosphate (Ca:>(PO02) or magnesia to a kaolinite clay batch does not affect the amount of extractable alumina. Added TiOa also was not harmful, but the data suggest that an extra mole of Na^O ought to be allowed for each mole of TiOz present. Added Fe*Os had a moderate ad- verse effect that was serious when the amount was more than three percent.
76
ALUMINA EXTRACTION
CoO.S.02 1544
3Co02S.02
2CoO.Si02 f0 2'30
3CoO.S.02
3AltO,2SiOt
Fig. 24. — Equilibrium diagram of the system CaO — A1203 — SiCK. Diagram of Rankin and Wright revised by Schairer (see reference 9) and reproduced with his permission. Shaded area includes compositions of all lime sinter batches discussed in this report. Numbers represent temperatures in degrees C.
REFERENCES 11
REFERENCES
1. Grim, R. E., Modern concepts of clay mate-
rials. Jour. Geol. 50, 225-275 (1942) ; Rept. of Inv. 80, 111. Geol. Survey (1942).
2. Walthall, J. H., Personal communication.
3. Copson, R. L., Walthall, J. H., and Hignett,
T. P., Final report on the extraction of alumina from clay by the lime-sinter modification of the Pedersen process. Serial W-103 War Metallurgy Committee of National Academy of Sciences (1943).
4. Bates, P. H., and Klein, A. A., Properties of
calcium silicates and calcium aluminates occurring in normal Portland cement. Tech. Paper 78, U. S. Bur. of Standards (1917).
5. Barrett, R. L., and McCaughey, W. J., The
system CaO-Si02-P205. Am. Min. 27, 680-695 (1942).
6. Zerfoss, S., and Davis, H. M., Observations
on solid-phase inversions of calcium or- thosilicate, constituent of dolomite-silica brick. Jour. Am. Cer. Soc. 26, 302-307 (1943).
7. Grim, R. E., and Rowland, R. A., Differential
thermal analyses of clay minerals and other hydrous materials. Amer. Mineralo- gist 27, 746-761, 801-818 (1942).
8. Brownmiller, L. T., and Bogue, R. H., The
system CaO— Na20— Al2Os. Am. Jour. Sci. XXIII, 501-524 (1932).
9. Schairer, J. F., The system CaO- FeC—Al20:!
— Si02: 1. Results of quenching experi- ments on five joins. Jour. Am. Cer. Soc. 25, 241-274 (1942).
Illinois State Geological Survey
Bulletin No. 69
1945