1 70 SOILS AND FERTILIZERS 



known as phosphate flour, is treated with commercial 

 sulphuric acid to obtain soluble monocalcium phos- 

 phate. The amount of sulphuric acid used is deter- 

 mined by the composition of the rock. Impurities as 

 calcium carbonate and calcium fluoride react with sul- 

 phuric acid and cause a loss of acid. Ordinarily, a 

 ton of high-grade phosphate rock requires a ton of 

 sulphuric acid. The mixing is done in lead-lined 

 tanks. A weighed amount of phosphate flour is 

 placed in the tank, and the sulphuric acid added, 

 through lead pipes, from the acid tower. The mixing 

 of the acid and phosphate is done with a mechanical 

 mixer, driven by machinery. From the mixing tank 

 the material is passed into other 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 4 .2H 2 O) 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 phosphate, 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 : 64 



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

 Ca(PO ), + 4 H PO 3 H,0 = 3 [CaH (PO 4 ).,H.O]. 

 Ca (PO ;>.+ 2H 3 P0 4 + i2H,0=3[Ca,H,(P0 4 )., 4 H.O]. 



