The yield of glue obtained from cartilage after extraction of the mineral constituents varies according to the compactness of the bones used. Solid and compact bones yield, on an average, 15 per cent. of dry glue, but a comparatively large quantity of calcium phosphate. On the other hand, porous bones rich in cartilage yield from 20 to 25 per cent. of dry glue. The liquor obtained by treating the bones contains, as previously mentioned, calcium phosphate, magnesium phosphate and calcium chloride in solution, and may be utilized in the manufacture of fertilizers or of phosphorus.
For the first purpose which is not very remunerative, but is not very troublesome, the liquor is treated with milk of lime until it is slightly alkaline, whereby a finely divided precipitate of basic calcium phosphate is obtained, whilst calcium chloride remains in solution. The precipitate is allowed to settle, separated from the supernatant fluid and dried. The resulting product contains, on an average, 65 per cent. calcium phosphate, up to 20 per cent. water and 10 to 15 per cent. calcium carbonate, quick lime and accidental impurities. It forms an excellent fertilizer.
If the liquor is to be utilized in the manufacture of phosphorus, it is evaporated in shallow pans of glazed stoneware. In cooling crystals of acid calcium phosphate are formed, which are separated from the mother-liquor. This subject will be more fully referred to in the next chapter.
[CHAPTER VI.]
MANUFACTURE OF PHOSPHORUS.
In some instances the preparation of phosphorus is carried on in conjunction with other industries, for instance, glue-boiling, the preparation of sal ammoniac, yellow prussiate of potash, etc. Bone-ash is the chief material used by phosphorus makers. Many manufacturers do not burn the bones to ashes, but purchase bone-ash, large quantities of which are brought from South America, especially from the Argentine Republic.
The ordinary method of preparing phosphorus includes the following operations:
1. Burning the bones and grinding the bone-ash to powder.
2. Decomposition of the bone-ash by sulphuric acid, and evaporation of the acid phosphate previously mixed with charcoal.
3. The distillation of the phosphorus.