40 MODERN PAPER-MAKING
chemical reaction which causes bleaching to take place, occurs. This method
is very slow and uncertain.
Chloride of lime or 'bleaching powder5 is still employed in many mills
as a bleaching agent. It is prepared by passing chlorine gas over slaked lime.
The lime absorbs a large quantity of the gas and holds it so long as it is kept
dry and in air-tight packages, which consist usually of large wooden barrels.
The compound thus formed is called "bleaching powder' and its formula is
taken to be CaOCl.Cl.
Ca—Calcium.
O—Oxygen.
Cl- Chlorine.
The powder contains about 35 per cent of available chlorine.
When this powder is mixed with water, two compounds are formed—
namely, calcium chloride, CaCL, and calcium hypochlorite, Ca(OCL). This
salt, Ca(OCl)a, is the actual bleaching agent, and forms oxygen by splitting
up thus:
Ca(OCl)2=CaCL+2O.
When the oxygen splits off from the calcium chloride it is in the nascent state,
and it is then that it oxidises the colouring matter in the rags, esparto or wood
fibres.
PREPARATION OF THE SOLUTION
The best way to prepare the solution for use in the mill is to have large
deep cast-iron tanks fitted with effective agitators. They should be tall and
narrow, and enclosed, except for an opening at the top to allow for the furnish-
ing oif the bleaching powder. The building in which the tops of the mixers
are situated should be well ventilated, as the operation of emptying the powder
from the barrel into the mixer is very unpleasant.
As the powder usually arrives at the mill in casks containing about 7 cwt.,
the capacity of the mixer should be not less than 1600 gallons. This will allow
plenty of room for sufficient water to be added to make the solution at standard
strength 6° Tw. (5 Ib. to 10 gallons of water) with one cask of bleaching powder,
and allow for the lime sludge, which will occupy some space at the bottom.
An efficient and economical mixing plant is shown in the illustration
0^8- 9}' Though this' seems a very elaborate arrangement and its working
at first sight rather complicated, it will be found that, by careful manipulation,
the utmost available chlorine will be extracted from the powder and the sludge
left quite innocuous, and there will be the minimum of waste.