10 BULLETIN 144, U. S. DEPAETMENT OF AGEICULTTJEE. 



This last reaction partly explains why acid phosphate in excellent 

 mechanical condition, but with a relatively high percentage of phos- 

 phoric acid insoluble in water is often made from rock containing 

 large quantities of iron and aluminum. 



Compounds of aluminum react in a manner similar to those of iron, 

 but to a less marked degree. 



Fertilizer manufacturers and authorities differ widely on the 

 question of what constitutes the maximum quantity of iron and 

 alumina that a phosphate rock can contain and still be useful in the 

 manufacture of acid phosphate. ■ Wyatt * says that phosphates 

 containing from 6 to 8 per cent of iron and alumina may be used, 

 provided there is sufficient carbonate of lime present to produce a 

 dry, pulverulent mass. Schucht 2 and Fritsch 3 are inclined to con- 

 sider any quantity of iron and alumina in excess of 3 per cent as 

 undesirable. Stillwell 4 states that phosphates containing from 4 

 to 6 per cent of these oxides can be handled, but that the presence of 

 more than 2 per cent is objectionable. 



Thousands of tons of high-grade acid phosphate, however, are now 

 annually made from Tennessee brown rock phosphate containing as 

 high as 5 per cent of the combined oxides of iron and aluminum, and 

 though the handling of such phosphates necessitates an increased 

 consumption of sulphuric acid, there seems little reason why they 

 should not be used in making acid phosphate, provided they are so 

 manipulated that a dry, readily workable product is obtained. 



CARBONATES OF LIME AND MAGNESIA. 



Carbonates are frequently very desirable impurities in phosphate 

 rock, provided they do not occur in quantities so great that the per- 

 centage of phosphoric acid present is materially reduced. The 

 carbonic acid is usually combined with lime, and it is in this form 

 that it is considered here. Sulphuric acid acts upon calcium carbon- 

 ate to form calcium sulphate, water or steam, and carbon dioxide, 

 which escapes as a gas. The reaction may be represented thus. 



Sulphuric Calcium Calcium Water or Carbon 

 acid. carbonate, sulphate. steam. dioxide. 

 H 2 S0 4 + CaC0 3 = CaS0 4 + H 2 + C0 2 



If sufficiently diluted sulphuric acid is used the excess of water 

 combines with the calcium sulphate to form gypsum. Modifying 

 the above equation therefore, we obtain: 



Sulphuric Calcium Carbon 



Acid. Water, carbonate Gypsum. dioxide. 

 H 2 S0 4 + H 2 + CaC0 3 = CaS0 4 .2H 2 0+ C0 2 



1 Phosphates of America, pp. 111-116 (1S91). 

 - Die Fabrikation des Superphosphates, pp. 79-83 (1909). 

 3 Manufacture of Chemical Manures, pp. 78-80 (1911). 

 < Industrial Chemistry, Rogers & Aubert, p. 403 (1912). 



