GAS BLEACHING 45
At this stage the batch was given five more minutes, and the control valve was
then closed. A sample was taken and twaddled. It was then 85° F. and io.8c
Tw. and the batch was finished.
When a batch is completed, it is allowed to settle, and is run off and the
sludge washed in the normal way. The quantity of sludge is small, and con-
sists chiefly of impurities present in the original lime, with a little reserve lime
to maintain alkalinity in the solution.
In some mills where it is not possible to use liquid chlorine from tank
wagons, 17-cwt. cylinders may be used in an exactly similar manner. The
process control in these cases can be achieved, if desired, by weighing the
cylinder.
Gas Bleaching.—-In this older method of bleaching by chlorine gas, the
chemical reaction is again one of oxidation. The chlorine gas passes through
damp half stuff, and the chlorine takes up hydrogen from the water or moisture
present, forming hydrochloric acid and setting free oxygen. This oxygen is
very active indeed, and will both remove colouring matter and reduce shive
or hgnin, on which the ordinary bleach has had little or no effect.
Great care has to be taken that the action is not too drastic and that the
stuff does not heat, or serious damage will be done to the cellulose.
For bleaching low-coloured rags and for destroying the shive in linen, a
gas bleaching chamber is very useful. A chamber holding i ton of rags is a
very good size, about 6 feet square by 8 feet in height, and having cross-bars
of wood on which the pulp is laid in order to allow the gas to penetrate all
through the mass. This chamber is closed and the door made air-tight by a
packing of wet paper. The retort should be on the outside and open to the
air, to obviate the risk of the attendant being gassed while putting in the charge.
The retort is of heavy cast iron and heavily lead lined. The top is also of lead
and is designed to fit on the base with a water channel, similar to a gas tank.
A lead pipe from the top communicates with the inside of the gas chamber.
For a ton of low-grade rags the following quantities of chemicals are re-
quired: 30 quarts of sulphuric acid, J cwt of salt, J cwt. of manganese dioxide.
The salt is placed in the retort with 6 gallons of water. The manganese dioxide
is added and the mass stirred into a paste. The top is then put on and the pipe
connected.
The sulphuric acid is filled in through a funnel on the top in three separate
lots. Some steam is turned on and the chlorine gas and steam pass into the
bleaching chamber and mix with the rags.
' The process occupies about 12 hours and is very drastic. The quantities
•axe varied according to the stock being bleached.
Bleaching Methods.—There are two methods of bleaching, a rapid and "a