In the chamber are placed large, well-glazed, earthenware vessels which contain the liquor to be evaporated, the vapors evolved being carried off by the fire-gases through the chimney. Evaporation proceeds quite rapidly, and fresh liquor is from time to time introduced through an earthenware pipe, the operation being repeated until a sample taken from the full vessels shows on cooling the presence of an abundance of crystals of acid calcium phosphate.
The admission of the fire-gases into the chamber is then interrupted, and the contents of the vessels are brought into a wooden tank furnished with a stirrer, which is kept in constant motion so that when the liquor cools, only small crystals will be formed. When crystallization is complete, the mother-liquor is drawn off and again evaporated. By this operation more crystals of acid calcium phosphate are obtained, which are, however, less pure than those from the first liquor. The mother-liquor drawn off from this second yield of crystals might, on being again evaporated, give more crystals which, however, would be too impure to be used to advantage.
To obtain the calcium phosphate contained in the last mother-liquor, the latter is exactly neutralized with burnt lime, a white precipitate of basic calcium phosphate being thereby obtained. The precipitate is repeatedly washed in water and allowed to settle, and added in small portions to the acid liquors obtained by extracting the bones. As these liquors always contain a considerable quantity of hydrochloric acid in excess, the basic calcium phosphate, being in a finely divided state, is readily and completely dissolved.
The crystals of acid calcium phosphate are removed from the crystallizing tanks by means of wooden shovels and brought into baskets covered inside with stout sack-cloth. They are left in the baskets until no more mother-liquor drains off, when the cloths are folded together and the crystals further freed from liquor by pressure. They are then heated, with constant stirring, in shallow stoneware pans until they are so dry as to crumble of their own accord. In this manner small crystals of a mother-of-pearl lustre are obtained, which feel like sharp quartz sand and consist of pure acid calcium phosphate.
This mass is mixed with 25 per cent. of its weight of granulated charcoal. The mixture is heated until it is pulverulent, and then treated in the same manner as the distilling mass from bone-ash.
In place of stoneware vessels, shallow lead pans may be used for evaporating the liquor containing calcium phosphate. To prevent the melting of the lead the pans are bricked in under a very flat arch, so that the fire gases are forced to pass close over the liquor, the pans being kept constantly full. When crystallization is complete the liquor is drawn off and the pans are refilled.
In the manufacture of phosphorus there is left after every distillation a residue of basic calcium phosphate, and it is advisable to decompose it with hydrochloric acid, this being effected in a vat lined with lead or coated with paraffine. The mass is completely dissolved, and the black sludge remaining on the bottom of the vat consists of charcoal, which had been added to the distilling mass.
Distillation of the Phosphorus.—The distilling mass consists of acid calcium phosphate, charcoal and about 4 to 6 per cent. water. By heating in the retorts, the acid calcium phosphate is first converted into calcium metaphosphate, water being eliminated, according to the following equation:
CaH4(PO4)2 = Ca(PO3)2 + 2H2O.
By further heating to a white heat the calcium metaphosphate is so far reduced as to yield two-thirds of its content of phosphorus, while one-third remains behind as calcium phosphate, corresponding to the following equation: