10 BULLETIN 312, IT. S. DEPARTMENT OF AGRICULTURE. 



distances. In order to produce a more concentrated phosphatic 

 fertilizer, the following method is employed: 



The ground phosphate rock is mixed with dilute sulphuric acid, and 

 the phosphoric acid thus produced is separated from the gypsum and 

 impurities both by decantation and filtration. The acid is then con- 

 centrated by evaporation and sold as such or used to treat another 

 batch of phosphate rock in the production of double acid phosphate, 

 which contains as high as 40 per cent of phosphoric acid (P 2 5 ) . The 

 equations showing these reactions may be represented thus: 



Ca3(P0 4 ) 2 +3H 2 S0 4 +6H 2 0=2H,P0 4 +3(CaS0 4 .2H 2 0); 

 4H 3 PO 4 +Ca3(PO 4 ) 2 =30aH 4 (PO 4 ) 2 . 



A list of the patents on this subject, arranged in chronological order, 

 is given in Table III, Appendix. 



Several of the processes cited in Table III, if they accomplish what 

 is claimed for them, should make it possible to produce soluble 

 phosphate more cheaply than by the methods now generally used, or 

 to produce a more concentrated f ertilizer, which will admit of shipping 

 it long distances. The following processes have features of interest 

 from either a scientific or an economic standpoint. 



The process of Designolle 1 consists in treating phosphate rock sus- 

 pended in water with sulphur dioxide under pressure, producing 

 thereby a solution of monocalcium phosphate and sulphite of lime — 



thus : 



Ca 3 (P0 4 ) 2 +2S0 2 +2H 2 0=CaH 4 (P0 4 ) 2 +2CaS0 3 . 



The suspended matter is allowed to settle in some suitable vessel 

 and the solution is boiled with steam to drive off the excess of sulphur 

 dioxide and to precipitate the calcium sulphite. The solution of 

 monocalcium phosphate is then poured off, evaporated to a sirupy 

 consistency, and treated with plaster of Paris to take up the excess 

 of water. 



Bergmann 2 claims that dicalcium phosphate free from calcium sul- 

 phite is obtained by first mixing phosphate rock with sulphurous acid 

 in the cold, then adding monocalcium phosphate to the solution, and 

 finally boning the solution to precipitate the dicalcium phosphate and 

 drive off sulphur dioxide. 



In the process of Machalske, 3 phosphate rock is subjected to the 

 action of sulphur dioxide in a small quantity of water. The resulting 

 mass is leached with a dilute solution of sulphur dioxide to extract 

 the soluble phosphates, and the residue, which is said to contain a 

 large percentage of calcium sulphite, is calcined to recover the sulphur 

 dioxide, which can be used again. 



i United States Patent No. 196881 (1877). 

 2 United States Patent No. 852371 (1907). 

 s United States Patent No. 902425 (1908). 



