CHAPTER V. 



THEOEY OF MANUFACTURE OF SOLUBLE PHOSPHATES. 



It has already been seen that to render the phosphoric acid in 

 raw phosphates soluble, when it exists as tribasic phosphate of lime 

 Ca3(P0J.,j i^ suffices to replace [each of] the two atoms of dyad 

 calcium by two atoms of hydrogen by means of sulphuric acid, the 

 monobasic phosphate of lime, or acid phosphate of lime CaH^(P0j2> 

 soluble in water, the calcium displaced by sulphuric acid forming 

 gypsum CaS042H20, according to the following equation : — 



Ca.iPOJ- + 2H2SO4 + 5H2O = CaHj(P04)2H20 + 2CaS042H20 



T_ . \ Sulphuric -r-rr x Monocalic Sulphate 



phosphate '- . ^ Water. u u 4. * f- 



'- £ ,. acid. phosphate. of hme. 



of hme. ^ ^ 



310 196 90 252 344 



596 596 



As the equation shows, the sulphuric acid used is sulphuric 

 acid which has not been concentrated, that is, it contains a certain 

 amount of water incidental to its manufacture. This water assists- 

 in the crystallization of the two new compounds formed, monocalcic 

 phosphate and gypsum ; it, moreover, assists in thinning the mix- 

 ing and thus facilitates decomposition. In fact, if the mass formed 

 by the mixing of phosphate and acid be too thick, the new salts, 

 especially gypsum, would set and harden rapidly and thus bury a 

 certain amount of the raw phosphate which would escape the action 

 of the acid. Gypsum forms, as it were, the skeleton of the super- 

 phosphate. But the conversion of raw insoluble phosphate into 

 soluble phosphate is not accomplished in such a complete and 

 adequate manner as the equation appears to indicate. Experiments 

 have proved that the insoluble phosphate is rendered soluble in two- 

 somewhat parallel stages. The first yielding free phosphoric acid 

 and gypsum only extends to two- thirds of the tricalcic phosphate. 

 In the second the free phosphoric acid acts on the remainder of 

 the tricalcic phosphate, and combines with it to form acid phosphate- 

 of lime or monocalcic phosphate. 



(73) 



