The quantity of substance which remains after burning the bones depends of course on the quality of the material used. Tubular bones of old animals contain the largest quantity of mineral substance, and give a much more abundant yield of bone-ash than the spongy bones of younger animals. On an average 100 parts by weight of fresh bones yield 55 parts by weight of bone-ash. The composition of the latter is as follows:
Basic calcium phosphate, 80 to 84 per cent.
Basic magnesium phosphate, 2 to 3 per cent.
Calcium carbonate, } 10 to 14 per cent.
Calcium fluoride, }
The bone-ash thus obtained is converted into a coarse powder by means of machinery, a bone-mill being best suited for the purpose. Experience has shown that the granules obtained by grinding should be the size of lentils. With the use of larger pieces the acid, with which the ash is treated later on, does not penetrate the entire thickness of the bone mass and a portion of the latter remains undecomposed. If the granules are too small, lumps are formed when the ash is brought together with the acid, and the mass would have to be constantly stirred in order to, make the action of the acid effective.
Decomposition of the bone-ash by sulphuric acid. When the basic calcium phosphate—the constituent of the bone-ash which comes here into consideration—is brought in contact with an acid of sufficient strength to effect its decomposition, calcium sulphate (gypsum) is formed, and a solution of acid calcium phosphate. If the latter be mixed with powdered charcoal, evaporated to dryness, and the mixture exposed, with the exclusion of air, to a strong red heat, the acid calcium phosphate is first converted into calcium metaphosphate, water being yielded. At this high temperature the calcium metaphosphate is by the action of the carbon decomposed to basic calcium phosphate and phosphorus; the latter escapes in the form of vapor and may be caught in suitable condensing vessels.
Hence three separate processes have to be distinguished: 1. The formation of acid calcium phosphate from the basic calcium phosphate contained in the bone-ash. 2. The conversion of the acid calcium phosphate into calcium metaphosphate. 3. Decomposition of the calcium metaphosphate, phosphorus being liberated, while basic calcium phosphate remains behind.
Expressed in chemical symbols these processes may be embodied in the following equations:
| I. | Ca3(PO4)2 Basic calcium phosphate. | + | 2H2SO4 Sulphuric acid. | = | 2CaSO4 Calcium sulphate. | + | CaH4(PO4)2 Acid Calcium phosphate. (gypsum). |
| II. | CaH4(PO4)2 Acid calcium phosphate. | = | 2H2O Water. | + | Ca(PO3)2 Calcium metaphosphate. |
| III. | 3Ca(PO3)2 Calcium metaphosphate. | + | 10C Carbon. | = | 10CO Carbon. monoxide. | + | Ca3(PO4)2 Basic calcium phosphate. | + | P4 |
If the processes mentioned under II. and III. would pass off in the practice exactly as there stated, two-thirds, or 13.3 per cent., of the total quantity of phosphorus contained in the basic calcium phosphate originally present would be obtained. However, besides these processes, others take place which cause a loss of phosphorus. By the action of the red heat upon the acid calcium phosphate, reciprocal action takes place between the latter, the water and carbon, so that a portion of the water is decomposed, and in addition to carbon monoxide, phosphoretted hydrogen is formed; the phosphorus contained in the latter must be considered as lost. Furthermore, a portion of the phosphorus is lost in the form of vapor, even with the use of the best condensing contrivances. In consequence of these losses the actual yield of phosphorus is between 8 and 11 per cent.