142 CHEMICAL FERTILIZERS 



these fumes, the dens are connected with a powerful fan, 

 which sucks the air from the dens and drives it through one 

 or more wash towers. Water is either sprayed into the 

 exhaust gases or allowed to trickle down a column of coke or 

 over boards set on end. Up to the present no use has been 

 found for the condensed gases. 



Jt is necessary now to consider some of the changes that 

 are taking place in the mixer and in the den as the result of 

 the action of sulphuric acid upon the rock phosphates. First 

 of all, as regards the main constituent, calcium phosphate. 

 In the system, phosphoric acid, calcium oxide and water, 

 there is normally a solid phase (see p. 162) consisting of 

 mono and di-calcium phosphates, and a liquid phase consisting 

 of phosphoric acid, water and mono-calcium phosphate. 

 The proportion of free phosphoric acid and di-calcium phos- 

 phate in the mixture depends, in practice, upon the quantity 

 of water which is introduced by the sulphuric acid. When 

 increasing quantities of mono-calcium phosphate are dis- 

 solved in a given weight of water at a constant temperature, 

 the proportion of free phosphoric acid continually increases, 

 and tends towards a limit in accordance with the equation : 



2 CaH 4 (PO 4 ) 2 ^ CaH 4 (PO 4 ) 2 + CaHPO 4 -f H 3 PO 4 



Up to the saturation point at 15 C. (59 F.), there 

 is a liquid phase, consisting of water, mono-calcium 

 phosphate and free phosphoric acid, with a solid phase con- 

 sisting only of di-calcium phosphate, formed by hydrolysis 

 of mono-calcium phosphate. Beyond the saturation point, 

 with increasing quantities of calcium oxide and phosphoric 

 acid, the liquid phase remains unaltered, whilst the solid 

 phase is continually increasing in amount by additions of 

 mono-calcium phosphate. On raising the temperature, the 

 decomposition of the mono-calcium phosphate is increased, 

 whilst the solubility of the di-calcium phosphate is increased 

 at the expense of the solid phase. 



Commercial Superphosphates. Experimental data 

 show that superphosphates may be considered as existing 

 in two given types those that are normal, and those that 



