The amount of lime used is generally somewhat in excess of the theoretical quantity; 106 parts of sodium carbonate (Na2CO3) require 56 parts of lime (CaO): it is necessary, however, to add about 60 parts. A very good plan is to conduct two or even three causticisings in the same vessel without cleaning out or removing the calcium carbonate, using in the first operation a large excess of lime. The causticising being completed, the calcium carbonate and excess of lime are allowed to settle down, and the clear liquor run off by an arrangement such as that already described in the dissolving process. Fresh solution is then run in and the whole mass heated for some time, until the excess of lime is converted into carbonate. Fresh lime is then added if necessary until the conversion of the carbonate of soda is complete. The liquor is then allowed to settle, and is run off as before: this operation may again be repeated.
The residual calcium carbonate, or “lime-mud” as it is called in alkali works, is then washed once or twice by running in water, boiling up, allowing to settle, and running off the clear liquor. If these liquors are too weak for use in boiling fibres, they may be used for diluting fresh recovered soda liquor before causticising, or for dissolving the soda.
Some arrangement should be provided for removing as much as possible of the liquor from the lime-mud before throwing it away or otherwise disposing of it. This is best done by throwing it on a filter made of layers of stones, {191} ashes and sand, and covered at the top with perforated iron plates. The filter is connected with a vacuum pump. In this way very perfect draining is accomplished, and the mud forms a hard mass on the surface of the filter, from which it can be easily removed with spades. In this form it contains only 50–60 per cent. of water. If properly washed it should contain in this state only about 2 per cent. of alkali (Na2O). By careful manipulation, even this amount can be reduced.
The importance of thoroughly washing the mud can hardly be too much insisted upon. Where proper means are not employed for draining, the washing should be made more perfect. The lime-mud consists chiefly of carbonate of lime, together with silicate, free lime, &c. The following analysis is of a mud obtained by causticising recovered soda derived from the liquors in which esparto and straw had been boiled:—
| Calcium carbonate | 40·02 |
|---|---|
| Calcium hydrate | 5·13 |
| Silica | 4·01 |
| Sodium hydrate | 2·13 |
| Oxide of iron and alumina | 0·30 |
| Water | 48·10 |
| Other constituents | 0·31 |
| 100·00 |
As already pointed out the liquors contain a certain amount of soda, as sodium sulphide and other sulphur compounds. The presence of the former, if in large quantities, is objectionable, as it is liable to discolour fibres boiled in liquors containing it. It is therefore best to remove it. This can be conveniently done by blowing air into the liquors during the process of causticising: this has the effect of oxidising it to sulphate of soda, in which form it is harmless.
The air can of course be blown into the liquor by means of a pump; the most economical way is to connect with the steam pipe an injector constructed on the principle of the injectors used for feeding boilers and for other purposes. By this means a strong current of air is drawn in, and {192} being forced with the stream to the bottom of the liquor, passes through it in a number of fine streams.
If the amount of sulphide present be very high it may be necessary to prolong the oxidising operation beyond the time necessary for complete causticising.
In many paper-mills the causticising is conducted in circular vessels furnished with mechanical agitators. These are more expensive than the simple form described above, and they possess no special advantages. The use of causticisers in which neither mechanical agitation nor agitation by means of air is provided for is exceedingly wasteful of labour, time, steam, and soda. The lime-mud settles at the bottom as a hard mass, very difficult to manipulate.
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