ARTIFICIAL MANURE MANUFACTURE 61 



as nearly as is practicable, CaH^PO^ being 

 soluble in water. Calcium carbonate, a com- 

 mon impurity in phosphates, consumes sulphuric 

 acid in this treatment. Other objectionable 

 impurities in phosphates are the ferric oxide 

 and alumina, because both oxides form in- 

 soluble phosphates, so that the proportion of sol- 

 uble phosphates in the prepared superphosphates 

 is diminished by their presence. Various plans 

 have been proposed for removing calcium carbon- 

 ate e.g., treatment with an aqueous solution of 

 62 or S(>2 in which calcium carbonate is soluble; 

 or by causticising the lime by heat, and acting upon 

 it with ammonium salt e.g., the chloride the am- 

 monia being recovered. Redonda phosphate (es- 

 sentially A1P04) has been converted into an avail- 

 able form by treatment with sulphuric acid, 

 yielding aluminium sulphate and phosphoric acid, 

 or by heating with sodium chloride and superheated 

 steam, whereby a portion of the phosphoric acid 

 is converted into sodium phosphate. 



"Superphosphate" is a mixture of calcium sul- 

 phate as gypsum with the acid phosphate of lime 

 CaH 4 (PO4)2 which is the essential manurial con- 

 stituent. The crude calcium phosphate to be 

 converted into superphosphate should be, as 

 stated above, as free as possible from iron and 

 aluminium compounds (not more than 3 per cent 

 of Fe2Os A^Os) and from calcium carbonate. 

 The material is ground very finely e.g., to pass 

 a sieve having 80 meshes per linear inch and is 



