IRON AND ALUMINA COMPOUNDS 279 



239. Reaction with Carbonates. Most mineral phosphates con- 

 tain calcium carbonate ' in varying quantities. This com- 

 pound is decomposed on treatment with sulfuric acid according 

 to the reaction: CaCO 3 +H 2 SO 4 CaSO 4 +H 2 O+ CO 2 . When 

 present in moderate amounts, calcium carbonate is not an objec- 

 tionable impurity in natural phosphates intended for acid phos- 

 phate manufacture. The reaction with sulfuric acid which takes 

 place produces a proper rise in temperature throughout the mass, 

 while the escaping carbon dioxid permeates and lightens the whole 

 mass, assisting thus in completing the chemical reaction by leaving 

 the residual mass porous, and capable of being easily dried and pul- 

 verized. Where large quantities of carbonate in proportion to 

 the phosphate are present the sulfuric acid used should be dilute 

 enough to furnish the necessary water of crystallization to the 

 gypsum formed. For each 100 parts, by weight, of calcium car- 

 bonate, 80 parts of sulfuric anhydrid are necessary, or 125 

 parts of acid of 1.710 specific gravity=6o Beaume. 



In some guanos a part of the calcium is found as pyrophos- 

 phate, and this is acted upon by the sulfuric acid in the follow- 

 ing way : Ca 2 P 2 O 7 +H 2 SO 4 =CaH 2 P,O 7 +CaSO 4 . 



240. Solution of the Iron and Alumina Compounds. Iron may 

 occur in natural phosphates in many forms. It probably is 

 most frequently met with as ferric or ferrous phosphate, seldom as 

 ferric oxid, and often as pyrite, FeS 2 . The iron also may sometimes 

 exist as a silicate. The alumina is found chiefly in combination 

 with phosphoric acid, and as silicate. 



Where a little less sulfuric acid is employed, as is generally 

 the case, than is necessary for complete solution, the iron phos- 

 phate is attacked as represented below : 



3FeP0 4 + 3 H 2 S0 4 =FeP0 4 .2H 3 P0 4 +Fe 2 (S0 4 ) 3 . 



When an excess of sulfuric acid is employed, the formula is 

 reduced to the simple one : 



2FeP0 4 + 3 H 2 S0 4 =2H 3 P0 4 +Fe 2 (S0 4 ) 3 . 



A part of the iron sulfate formed reacts with the acid calcium 

 phosphate present to produce a permanent jelly-like compound, 

 difficult to dry and handle. As much as two per cent, of iron 



