THE MANUFACTURE OF PHOSPHORIC ACID. 163 



react on the dissolved sulpho-phosphate of potash, the gypsum is 

 precipitated in the same state, and its ehmination from the Hquid 

 by filtration presents no ditficiilty. The free sulphm-ic acid of 

 the sulpho-phosphate combines with the lime to form gypsum ; by 

 addition of phosphoric acid, the K.,SO^ is again transformed into 

 KHSO^ + KH0PO4, and the acid of this new bisulphate is in its 

 turn removed by CaCOg. The sum of these reactions is represented 

 by the following equation : — 



K,SO, + 2H3PO, + CaCOg = 2KH,P0, + CaSO, + CO, + H,0 



However, these reactions are not effected in such a simple manner. 

 Thus the direct action of calcium carbonate on phosphoric acid is to 

 form insoluble phosphate of lime. Jf it was desired to treat a mere 

 solution of potassium sulphate in dilute phosphoric acid there would 

 be a considerable loss of phosphoric acid. Moreover, the washing of 

 the precipitated gypsum containing P..,0-, and the evaporation of the 

 wash water would entail great expense, capable perhaps of absorb- 

 ing all the profit of the manufacture. An elegant and simple solu- 

 tion to the proiDlem has been found i5y the use of grey phosphatic 

 chalks. These phosphates, for which formerly no use could be 

 found in spite of all the researches and tentatives made, contain 

 about 30 per cent of Ca3(P04)2, and a preponderant quantity of 

 CaCOg ; they are sold cheap. It is clear that, if in the process now 

 described, calcium carbonate be added as phosphatic chalk, there 

 will be obtained as a filtration residue a mixture of phosphate and 

 gypsum, the phosphoric acid content of which is low but which can 

 be perhaps utilized for the extraction of phosphoric acid. And if the 

 phosphoric acid extracted from it is afterwards used in the manu- 

 facture of sulpho-phosphate of potash, it will suffice to treat the above 

 phosphatic gypsum in the filter press to eliminate the adherent 

 solution of potassic phosphate, but it would be useless to wash it, 

 because the remainder of the potassic phosphate which it contains 

 returns to the process by the extraction of the phosphoric acid. In 

 this way the small amounts of phosphoric acid precipitated by 

 the chalk would also be utilized. This detail also brings out better 

 the advantages resulting from the use of phosphatic chalk in this 

 case; in fact the amount of phosphoric acid precipitated is exceed- 

 ingly small, because the conversion is effected in a less violent and 

 more regular manner. Another very disagreeable drawback due to 

 the carbonate of lime, is eliminated by phosphatic chalk ; it is the 

 entrainment of soluble potash by the gypsum sludge, owing, no 

 doubt, to the formation of a double phosphate of calcium and 

 potassium. The process just enunciated was the subject of a 

 patent delivered to H. & E. Albrecht ; it is applied on the large scale 

 at the Biebrich factorv, in the manner about to be described. 



