10 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 
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 1 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 
i Phosphates of America, pp. 111-116 (1891). 
2 Die Fabrikation des Superphosphates, pp. 79-83 (1909). 
s Manufacture of Chemical Manures, pp. 7-8-80 (1911). 
* Industrial Chemistry, Rogers & Aubert, p. 403 (1912). 
