PHOSPHATE FERTILIZERS 1 79 



chanical mixer, driven by machinery. From the 

 mixer the material is passed into large tanks, where 

 two or three days are allowed for the completion of 

 the reaction. When the mass solidifies, it is ground 

 and sold as superphosphate. In the manufacture of 

 superphosphate, gypsum (CaSO .2H 2 0) is always pro- 

 duced. A ton of superphosphate prepared from high- 

 grade rock in the way outlined will contain about 40 

 per cent, of lime phosphates, equivalent to 18 per cent, 

 phosphoric acid. If a poorer quality of rock is used a 

 proportionally smaller amount of phosphoric acid is 

 obtained. A more concentrated superphosphate is ob- 

 tained by producing phosphoric acid from the phos- 

 phate rock, and then allowing the phosphoric acid to 

 act upon fresh portions of the rock, the reactions be- 

 ing as follows : 63 



Ca 3 (P0 4 ) 2 + 3H S0 4 = 3 CaS0 4 + 2H (POJ, 

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

 Ca 3 (P0 4 ) 2 +2H P0 4 +i2H, O = 3 [Ca 2 H (P0 4 ) 2 . 4 H O]. 



The phosphoric acid is separated from the gypsum be- 

 fore 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 prod- 

 uct is preferable. The terms 'acid 1 and 'superphosphate' 

 are generally used to designate both the first product 

 formed by the action of sulphuric acid and that pro- 



