rilOSPIIORUS FERTILIZERS 145 



conversion of the phosphoric acid into soluble forms depends 

 on various points, such as the nature of the rock phosphate, 

 its hardness, its structure, the method of grinding and the 

 proportion of fine powder to granular matter. Under 

 modern systems of grinding, superphosphates are prepared 

 which leave 0-5-07 % of insoluble calcium phosphate, whereas 

 at one time this proportion exceeded 2 %. It is not uncommon 

 to find some poorly made phosphates of modern manufacture 

 which contain much insoluble phosphate as a result of 

 coarse grinding. On the other hand, excessive fineness of 

 the meal tends to produce undesirable mechanical conditions 

 in the final product ; a certain amount of granular structure 

 is necessary. Hard rock phosphates yield products contain- 

 ing more free phosphoric acid than do the more friable phos- 

 phates, a result which is partly due to the finer material 

 produced from the hard rock phosphates, and partly due 

 to the fact that the hard rock phosphates are not so easily 

 penetrated by the sulphuric acid. 



Many hypotheses have been adopted from time to time to 

 explain the actions proceeding in the mixer and in the den. 

 Where sulphuric acid is used in excess, the reactions probably 

 occur in accordance with the equations : 



(1) Ca 3 P 2 O 8 + 3H 2 SO 4 + 6H 2 O = 3CaSO 4 . 2H 2 O + 2H 3 PO 4 



(2) Ca 5 (P0 4 ) 3 F + 5H 2 S0 4 + ioH 2 O 



= 5CaSO 4 .2H 2 O + 3H 3 PO 4 + HF 



(3) Ca fi (P0 4 ) 3 OH + 5H 2 S0 4 + 9H 2 O 



- 5CaS0 4 . 2H 2 + 3H 3 P0 4 



In equation (2) F may be replaced by Cl or organic group. 

 If the amount of sulphuric acid used is insufficient to com- 

 pletely decompose rock phosphate, the reaction is generally 

 supposed to take place in accordance with the following 

 equations : 



(4) Ca 3 P 2 8 + 2H 2 S0 4 + 5H 2 



= 2CaS0 4 . 2H 2 + CaH 4 P 2 8 . H 2 O 



(5) 2Ca 6 (P0 4 ) 3 F + 7H 2 S0 4 + i 7 H 2 O 



= 7CaSO 4 . 2H 2 O + 3CaH 4 P 2 O 8 .H 2 O + 2HF 



(6) 2Ca 5 (P0 4 ) 3 OH + 7H 2 S0 4 + i.5H 2 O 



-= ;CaSO 4 . 2H 2 O -f 3CaH 4 P 2 O 8 . H 2 O 

 v. 10 



