158 CHEMICAL FERTILIZERS 



coarsely ground, a mechanical agitator is not absolutely 

 necessary. After soaking until the bones have become 

 semi-transparent, the solution is run off from a tap at the 

 bottom of the vat, and the residual gelatinous matter is 

 washed with weak hydrochloiic acid of about 4 % strength. 

 The weak acid after use for washing is retained for diluting 

 the hydrochloric acid used in preliminary extractions. The 

 strength of the acid used for the first extraction of the bones 

 varies according to the practice in different works from about 

 7-12 %. The amount of hydrochloric acid used should 

 be as little in excess as possible. When the bones are very 

 finely ground, and an agitator is used, a solution can be 

 effected in 10-15 minutes, but with larger bones the time 

 is correspondingly lengthened. The solution, which should 

 contain very little free hydrochloric acid, is transferred to a 

 lead-lined vat, in which it is neutralized by milk of lime. 

 Endeavours should be made to approximate to a precipitate 

 having the composition of di-calcium phosphate, but some 

 slight excess of lime must be used, so that the resulting material 

 is a mixture of di- and tri-calcium phosphates, produced 

 according to the equations : 



CaH 4 (P0 4 ) 2 .H 2 + Ca(OH) 2 + H 2 O = Ca 2 H 2 (PO 4 ) 2 .4H 2 O 

 CaH 4 (P0 4 ) 2 .H 2 + 2Ca(OH) 2 = Ca 3 P 2 O 8 + 5 H 2 O 



It is desirable that there should be sufficient chemical 

 control at this stage to prevent an excess of lime being used ; 

 samples can be taken from the vat, filtered and tested for 

 soluble phosphates. Traces of soluble phosphates are certain 

 to remain in solution unless excess of lime is used, hence a 

 compromise must be effected. The muddy liquor is then 

 run through a filter press, and the calcium chloride washed 

 out. It is important that the washing should be fairly 

 efficient. The cakes are then dried at a low temperature, 

 when a fine friable powder results, containing from 30 to 40 % 

 of phosphoric acid. The meal approximates to a mixture 

 of equal amounts of di- and tri-calcium phosphates. Precipi- 

 tated bone phosphate is very easily soluble in the weakest 

 citric acid solution, or in ammonium citrate. The meal is 



