CHAPTER VII.

BLEACHING.

The methods of bleaching being essentially the same for all pulps, it will only be necessary to describe one in detail.

The process depends upon the use of a solution of calcium hypochlorite, prepared by dissolving bleaching powder in water. This solution is best prepared in a vessel provided with stirrers or agitators, whereby a thorough mixture of the bleaching powder and water is obtained. Bleaching powder always contains a certain amount of free lime and calcium carbonate, which remain undissolved in the water. This should be allowed to settle to the bottom of the vessel, and the clear liquor run off. The residue may then be again treated with water, or with weak liquor obtained by washing the residue. If the washings are too weak to be conveniently used for bleaching, they may be stored in a separate tank and used for dissolving fresh powder. In this way a strong stock solution can be readily prepared, the powder at the same time being thoroughly exhausted. The washing should be repeated as frequently as possible, after which the residue may be allowed to drain. This operation is best conducted on a filter bed provided with an air-pump similar to that described in Chapter XII. The residue is obtained in this way as a hard cake, containing about 60 per cent. of water. If the above operations have been properly conducted, it should not contain more than about 0·25 per cent. of chlorine.

The original powder should contain from 33·0 to 35·0 per cent. of “available chlorine,” that is, chlorine which is effective in the bleaching process. {111}

Owing to the ease with which it decomposes it should be kept cool and not exposed to the air.

If possible, one or more large store tanks should be provided for the strong bleach liquor, so that time may be given for the complete separation of the insoluble portions. The liquor can be drawn off with a “siphon” pipe without disturbing the residue.

It is of the utmost importance that only clear liquor be used, as a milky solution of calcium hypochlorite is much more sluggish in its action than one which is perfectly clear and bright.

A convenient strength for the stock solution is 6° Twaddle: this corresponds to about half a pound of bleaching powder to the gallon.

The method of testing the powder and its solution will be described in