SUPERPHOSPIiATF.S AND EFFECT OF PHOSPHORIC ACID 8l 



are thoroughly mixed together by special machinery and con- 

 veyed to a pit where the mixture is allowed to remain until 

 ready for shipment. . 



Chemistry of the Process. — The phosphoric acid is in the form 

 of tricalcium phosphate in phosphates, or three parts of lime are 

 united with one part of phosphoric acid. When the sulphuric 

 acid is added it attacks the phosphate and dissolves it, setting 

 free two parts of lime (that were originally combined with the 

 phosphoric acid) which unite or combine with the sulphuric acid 

 forming superphosphate (one lime phosphate or mono-calcic 

 phosphate) and gypsum (sulphate of lime). In other words the 

 phosphoric acid in superphosphate is only combined with one 

 part of lime as the remaining two parts of lime, with which the 

 phosphoric acid was formerly combined, have been set free. From 

 the above it is evident that superphosphate is made up of one 

 lime (mono-calcic) phosphate and gypsum (sulphate of lime). 

 Or the reaction is : 



(sCaO P2O5) + 2(H,OS03) =: {Ca02H20P.,05) + alCaOSOj) 

 Tricalcic phosphate Sulphuric acid Monocalcic phosphate Gypsum 



Phosphates of Lime. — In the phosphoric acid fertilizers used 

 there are four different forms of phosphates of lime, all of differ- 

 ent availability. These phosphates of lime are known as the in- 

 soluble, soluble, reverted, and basic slag forms. 



1. Insoluble Phosphoric Acid. — The most common form of phos- 

 phate of lime is that which is found in bones, mineral phosphates, 

 guanos, etc., and is called insoluble. The lime and phosphoric 

 acid are combined a? three parts of lime and one of phosphoric 

 acid. This is called tricalcic, tribasic, bone phosphate and three 

 lime phosphate. We may represent this form as follows: 



Lime C 



Lime < Phosphoric acid. 



Lime ( 



This is the most insoluble form of phosphate of lime and is 

 called insoluble phosphoric acid. 



2. Soluble Phosphoric Acid. — When insoluble phosphate of lime 

 is acted upon by sulphuric acid, two parts of lime are replaced 

 by two parts of water and soluble phosphate of lime is formed. 



