The formation of acid calcium phosphate may be effected either cold or with the assistance of heat, less time being required in the latter case. The process without the assistance of heat is as follows:
The bone-ash is brought into a lead-lined wooden tank, and enough boiling water to cover it poured in. It is then thoroughly mixed with the water by vigorous stirring with wooden rakes, and the necessary quantity of sulphuric acid is then run in with constant stirring. When an intimate mixture has been effected, the tank is covered with a well-fitting lid and allowed to stand for a few hours. As heat has been liberated by the introduction of the sulphuric acid into the mixture prepared with hot water, the entire mass acquires a high temperature.
Decomposition is accelerated by stirring the contents of the tank every six hours, and the process may be supposed to be finished in 48 hours. With the use of fresh burnt ash no special phenomena are observed, but if the ash has been prepared for some time the caustic lime formed in burning the bones has been completely converted into carbonate of lime, and the carbonic acid escapes, causing a slight foaming of the mass. In addition to carbonic acid, there also escapes a certain quantity of hydrogen fluoride gas, which is liberated by the decomposition of the calcium fluoride present in the ash. This gas being, even in very small quantities, very injurious to health, the tanks should be placed in a thoroughly ventilated room.
When decomposition is complete, enough water is admitted for the mass to acquire by stirring a thick milky appearance, when it is allowed to rest until it clarifies and a perfectly clear solution of acid calcium phosphate stands over the precipitate of gypsum. The clear solution is drawn off, and the sediment washed with water to obtain the solution retained by it. For this purpose the gypsum is stirred up with water, and the thick fluid discharged into a filtering tank. Upon the bottom of the latter is a four-inch-deep layer of coarse quartz sand; upon this is placed a false bottom, and upon the latter is spread a linen cloth. The liquor first running off being milky is poured back into the tank. However, it runs off clear so soon as the pores of the filtering cloth have become somewhat contracted by the gypsum.
As a rule, the contents of several filtering vats are run into a common filter, and the mass is repeatedly allowed to drain off. The dilute solutions thus obtained are evaporated with the first liquor. A third lixiviation of the sediment yields a fluid which is used instead of water in a subsequent operation.
The residue of gypsum is taken from the filtering tanks, and may be used as a fertilizer.
In the warm way the decomposition of the bone-ash is effected by providing the decomposing tanks with lead pipes through which steam is introduced, decomposition being complete in 24 hours, and the first run of solution of acid calcium phosphate reaches the evaporating pan in a very hot state. The lixiviation of the gypsum residue is also effected with water heated by steam, the object of separating the acid calcium phosphate as much as possible from the gypsum being thus obtained more completely with a comparatively small quantity of water than is possible by washing with cold water.
A suitable apparatus for hot lixiviation is shown in Figs. 51 and 52 in cross section and profile. A lead-lined tank, 13 to 16 feet in diameter and 3½ feet deep, is fitted with a stirrer furnished with two or four paddles, and closed by a well-fitting lid. The stirrer is kept in motion during the entire operation.
Fig. 51.
