THE MANUFACTURE OF ACID PHOSPHATE. 25 



DOUBLE ACID PHOSPHATE. 



The double acid phosphate now marketed contains from two to 

 three times as much soluble phosphoric acid as ordinary superphos- 

 phate, and is therefore very valuable in the manufacture of concen- 

 trated fertilizers. Before the discovery of extensive deposits of 

 high-grade phosphate rock, both in this country and abroad, the 

 making of double superphosphate was widely practiced, since it 

 afforded a ready means of utilizing low-grade phosphates. Now, 

 however, most of the commercial rock is so high in phosphoric acid 

 that it is unnecessary to resort to schemes for enriching the soluble 

 product obtained therefrom, but in Germany, France, and several 

 other foreign countries, as well as in the State of South Carolina, 

 where a comparatively low grade of phosphate is mined, this process 

 is still used with considerable success. 



The two main chemical reactions involved in the manufacture of 

 double acid phosphate are: First, sufficient dilute sulphuric acid is 

 added to phosphate rock to convert the hypothetical tricalcium 

 phosphate into phosphoric acid and gypsum; and second, the phos- 

 phoric acid thus obtained is used to convert the tricalcium phosphate 

 of a fresh supply of rock into monocalcium phosphate. The reactions 

 in their simplest form may be represented thus: 



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



(2) 4H 3 P0 4 + Ca 3 (P0 4 ) 2 = 3CaH 4 (P0 4 ) 2 



The process, however, is by no means as simple as it at first appears, 

 for there are several distinct operations which not only require the 

 watchfulness of a competent superintendent but the control of a 

 skillful chemist. 



The phosphate rock and dilute sulphuric acid (16° B.) are run into a 

 vat simultaneously and stirred thoroughly for 15 or 20 minutes. It 

 is inadvisable to use warm acid to decompose the rock or to stir for a 

 protracted period, since under such conditions the compounds of 

 iron and aluminum are dissolved only to be precipitated again later 

 on, causing the reversion of a part of the phosphoric acid. The 

 quantity of sulphuric acid required to bring about the desired reac- 

 tions should be carefully ascertained from analyses of the raw material, 

 since either an excess or an insufficient quantity will cause trouble 

 in the subsequent operations. 



The muddy solution is run into a tank from which it is pumped to a 

 filter press where the sediment and gypsum is separated, the clear 

 phosphoric-acid solution being run into evaporating pans. The 

 residue in the filter press is then washed with water till the washings 

 have a concentration of 0.25° B. or less. These washings are used to 

 dilute the sulphuric acid employed in the process. 



