SUPERPHOSPHATE 337 



from 20 to 40 per cent phosphoric acid. Tennessee phos- 

 phate contains from 30 to 35 per cent of phosphoric 

 acid. 



Basic slag, or, as it is also called, phosphate slag or 

 Thomas phosphate, is a by-product in the manufacture 

 of steel from pig-iron rich in phosphorus. The phos- 

 phorus present is in the form of tetracalcium phosphate, 

 (CaO) 4 P 2 5 . It also contains calcium, magnesium, 

 aluminum, iron, manganese silica and sulfur. On ac- 

 count of the presence of iron and aluminum, and because 

 its phosphorus is more readily soluble than the trical- 

 cium phosphate, the ground slag is applied directly 

 to the soil without treatment with acid. 



201. Superphosphate fertilizers. In order to render 

 more readily available to plants the phosphorus con- 

 tained in bone and mineral phosphates, the raw material, 

 purified by being washed and finely ground, is treated 

 with sulfuric acid. This results in a replacement of phos- 

 phoric acid by sulfuric acid, with the formation of 

 monocalcium phosphate and calcium sulfate, and a 

 smaller amount of dicalcium phosphate, according to the 

 reactions: 



Ca 3 (P0 4 ) 2 + 2H 2 S0 4 = CaH 4 (P0 4 ) 2 + 2 CaS0 4 and 

 Ca 3 (P0 4 ) 2 + H 2 S0 4 = Ca 2 H 2 (P0 4 ) 2 + CaS0 4 . 



The tricalcium phosphate being in excess of the sul- 

 furic acid used, a part of it remains unchanged. 



In the treatment of phosphate rock, part of the 

 sulfuric acid is consumed in acting upon the impurities 

 present, which usually consist of calcium and magnesium 

 carbonates, iron and aluminum phosphates, and cal- 



