MANUFACTURE OF SOLUBLE PHOSPHATES. 79' 



If sulphuric acid be employed in excess all the phosphoric acid is 

 liberated. 



2FeP0, + 3H,S0, -^ 2H,P0, + Fe,(S0j;3 



The decomposition of the phosphate of iron therefore gives phosphoric 

 acid and sulphate of iron. A portion of this sulphate ot n-on reacts 

 on the acid phosphate of lime and occasions a gelatmous precipitate, 

 whilst a small fraction remains inactive and can be detected m the 

 aqueous extract of the superphosphate. This amount is about 2 per 

 cent Consequently the presence of 2 per cent of oxide ot iron m 

 (raw) phosphate has practically no bad effect, that amount remammg^ 

 as soluble sulphate if the acid be slightly increased ; as much as 3 to 4 

 per cent of oxide of iron always causes a loss of soluble phosphoric 

 acid Bevond 4 per cent the phosphate becomes unfit for super- 

 phosphate manufacture. In his excellent treatise on superphosphates, 

 Schucht insists on this special point and explains the precipitation 

 of the phosphate of iron thus : — 



CaH,(POJ-^H.,0 + Fe2(S0,)3 + sh.O ==FeP0,2H,0 + 

 ' CaS0/2H.0 + 2H,S0, 

 H.SO, + CaH,(POJ,H,0 +''H,0 = CaS0,2H,0 +^|^H ^0, 

 ' 2(FePO,2H,0) + 4H3PO, = FeP0,2H,P0, + 4H,0 



He demonstrates the course of these reactions by the following 

 small experiment. If one adds to a solution of a superphosphate 

 say an acid solution of a salt of oxide of iron, phosphate ot iron is 

 precipitated. But according to their degree of concentration super- 

 phosphate solutions act differently on solutions of oxide ot iron ; thus- 

 a 2 per cent solution precipitates immediately all the oxide ot iron a 

 4 per cent solution gives a weaker reaction, and m a 6 per cent solu- 

 tion the reaction occurs slowly. Consequently the greater the amount 

 of free phosphoric acid in a superphosphate solution, the more it 

 retains the iron in solution. If a solution of a ferruginous supei- 

 phosphate be evaporated to dryness and the residue taken up with 

 water, a clear solution is not obtained, but a solution with a con- 

 siderable precipitate. This hydrated phosphate of iron may pass m 

 the superphosphate into the completely insoluble state when it& 

 constitutional water is removed from it by the crystallization ot 

 the amorphous suljDhate of lime. 



FeP0,2H,0 + CaSO, = CaS0,2H.p + FePO, 



This explains why an incompletely dissolved or retrograded 

 superphosphate cannot be improved by fresh treatment with 



