146 CHEMICAL FERTILIZERS 



According to more modern ideas, the reaction is supposed 

 to take place in two stages, first according to the equation 

 used for excessive sulphuric acid. i.e. (i), (2) or (3), and then 

 the free phosphoric acid reacts according to the following 

 equations : 



(7) 4H 3 P0 4 + Ca 3 P 2 8 + sH 2 O = 3CaH 4 P 2 O 8 . H 2 O 



(8) 7H 3 P0 4 + Ca 6 (P0 4 ) 3 F + 5H 2 O = 5CaH 4 P 2 O 8 . H 2 O+HF 



(9) 7H 3 P0 4 + Ca 5 (P0 4 ) 3 OH + 4 H 2 O = 5 CaH 4 P 2 O 8 . H 2 O 



Observations under working conditions show that the 

 following equation represents better the main reaction in 

 practice : 



(10) 5Ca 8 P 2 O 8 + nH 2 SO 4 + 26H 2 O 



= 4CaH 4 P 2 O 8 .H 2 O + 2H 3 PO 4 + nCaSO 4 .2H 2 O 

 The free phosphoric acid reacts on more phosphate to form 

 a little di-calcium phosphate, and mono-calcium phosphate 

 decomposes to a little di-calcium phosphate and free phos- 

 phoric acid. Excess of acid reduces the amount of di-calcium 

 phosphate. 



Calcium Carbonate. The equations given to represent 

 the action of sulphuric acid on calcium phosphate require 

 modification to include the numerous impurities, such as 

 calcium carbonate, magnesium carbonate, etc., which absorb 

 sulphuric acid. It is hardly correct to describe calcium 

 carbonate altogether as an impurity in this case, since without 

 some calcium carbonate the resulting superphosphate 

 would be very pasty and unsuitable for use. In treating 

 phosphates with sulphuric acid, it is the carbonates of lime 

 and magnesia that are attacked first. Carbonate of lime 

 is invariably present, though carbonate of magnesia is also 

 met with in a few cases. The decomposition of calcium 

 carbonate by sulphuric acid forms, in the first instance, 

 anhydrous calcium sulphate, and, later on, the hydrated 

 form, gypsum, and is the main cause of the increased tempera- 

 ture of the mass. At the same time, much carbon dioxide is 

 evolved, to the great advantage of the mechanical condition 

 of the finished superphosphate. Phosphates, which contain 

 little carbonate of lime, heat only slowly and slightly in contact 



