152 CHEMICAL MANUEES. 



Ca,(P0,)2 + 3H.,S0, = 3CaS0, + 2H,P0, 

 310 2M 408 196 



604 604 



By a strange coincidence, the old manuals state that by treat- 

 ing tricalcic phosphate with dilute sulphuric acid, monocalcic phos- 

 phate is obtained. Now this statement is erroneous because, as will 

 be seen further on, only a small amount of phosphoric acid enters 

 into solution. But even by using sulphuric in insufficient amount 

 hke that calculated for the preparation of monocalcic phosphate, free 

 phosphoric acid is obtained seeing that a part of the Ca^fPO^)- re- 

 mains undecom posed. As far back as 1870, the firm of H. & E. 

 Albrecht at Biebrich, installed an experimental factoiy for the manu- 

 facture of phosphoric acid, but the affair turned out badly because 

 at that time suitable appliances for separating the phosphoric acid 

 from its insoluble residue were not in existence. It was only by 

 the use of the filter press already known at that time in sugar 

 works, that the manufacture of phosphoric acid became lucrative, 

 for the conversion of dilute phosphoric acid into double super- 

 phosphates did not in the beginning present insurmountable diffi- 

 'Culties. Now this double superphosphate — with a minimum of 40 

 per cent P^Og — soon became in very great demand, as much by 

 agriculturists as by manure manufacturers, to whom it afforded a 

 ■simple and easy means of securing a high percentage of P.,0_v 

 But let us examine the manufacture of phosphoric acid a little more 

 closely. 



Manufacture of Pliosplwric Acid. — A 50 per cent phosphate 

 (CaaP^Oj,) is used. If damp, it is dried on metal plates. If lumpy 

 the lumps are crushed by a jaw-breaker and then reduced to the re- 

 quisite fineness by a flatstone mill or a ball mill. The ground phos- 

 phate is then fed into the vats by a chain elevator, 1 to 2 tons per 

 charge ; the sulphuric acid and the necessary water (wash water con- 

 taining P.O.) to dilute the acid to 16° B., is run in at the same 

 time. The charging of the vats lasts twenty minutes, and during this 

 time decomposition is completely effected, the phosphoric acid is 

 liberated, the sulphuric acid has taken its place and combined with 

 the lime to form gypsum, which has to be separated from the 

 liquid. For this purpose the muddy liquids from the decomposi- 

 tion vats are collected in a collecting tank lower down, fitted with 

 an agitator, its role is to maintain the solid particles in suspension 

 ointil the mixture is forced to the filter presses bv a membrane 

 pump, m which the acid liquid alone comes in contact with rubber 

 or lead. The filter presses used have 50 plates, and are capable 

 of treating 14 tons of phosphate in twenty-four hours (say three tons 

 •^2^5)- The filter plates are of pitch-pine, the frames are of the 

 same wood or of oak. The gutters are of plum-tree wood. When 

 the filter press is charged, its contents are washed with water under 



