PHOSPHATE FERTILIZERS 



always produced. A ton of superphosphate prepared 

 from high-grade rock in the way outlined will contain 

 about 40 per cent of lime phosphate, equivalent to 18 

 per cent phosphoric acid. If a poorer quality of rock 

 is used, there is a proportionally smaller amount of phos- 

 phoric acid. A more concentrated superphosphate is 

 known as double superphosphate and is obtained by pro- 

 ducing phosphoric acid from the phosphate rock, and 

 then allowing the phosphoric acid to act upon fresh por- 

 tions of the rock, the reactions being as follows : 64 



Ca 3 (P0 4 ) 2 + 3 H 2 S0 4 = 3 CaSO 4 + 2 H 3 (PO 4 ). 

 Ca 3 (P0 4 ) 2 + 4 H 3 P0 4 + 3 H 2 O = 3 [CaH 4 (PO 4 ) 2 , H 2 O]. 



The phosphoric acid is separated from the gypsum 

 before acting upon the phosphate flour. In this way, 

 superphosphate containing from 35 to 45 per cent of 

 phosphoric acid is produced. When fertilizers are to be 

 transported long distances, this concentrated product is 

 preferable. The terms ' acid ' and * superphosphate ' 

 have been generally used to designate the first product 

 resulting from the action of sulphuric acid upon phos- 

 phate rock or bones, and the term ' double superphos- 

 phate ' to mean the concentrated product formed by the 

 action of phosphoric acid. 



233. Commercial Value of Phosphoric Acid. The com- 

 mercial value of phosphoric acid in fertilizers is deter- 

 mined by the value of the crude phosphate rock, cost 

 of grinding and treating with sulphuric acid, and cost of 



