THE MANUFACTURE OF ACID PHOSPHATE. 27 
presence of fluorides is objectionable because of the obnoxious 
fumes evolved in treating with acid; otherwise this impurity is not 
objectionable. 
Compounds of iron and aluminum are the most dreaded of the 
impurities occurring in phosphate rock. These elements when 
present in small quantities are very apt to cause a certain amount of 
reversion to take place, and when present in large quantities may 
render the product sticky and unfit for use. By careful handling, 
however, phosphates high in iron and aluminum compounds may be 
made to produce high-grade acid phosphate. 
Carbonate of lime, which is present in nearly all phosphate rock, is 
a rather desirable impurity when the quantity is not excessive. The 
decomposition of this compound by sulphuric acid is attended with 
considerable heat which promotes chemical reaction between the 
more slowly acting substances in the mass; moreover, the calcium 
sulphate produced therefrom acts as a drier for the acid phosphate. 
In the manufacture of acid phosphate the rock is first ground to 
pass a 60-mesh sieve, and then mixed with an equal weight (approxi- 
mately) of "chamber acid." The quantity, strength, and tempera- 
ture of acid used have an important influence on the quality of the 
product. 
After thorough mixing in a cast-iron pan the material is discharged 
into a "den" just below the mixer or into a car which takes it to a 
shed and dumps it on a pile. When the "den" system is used the 
reactions take place rapidly and the product can be dug out in 24 to 
36 hours, practically ready for shipment. The method of emptying 
the "dens" by hand, however, is attended with some risk owing to 
the poisonous nature of the fumes evolved from the freshly made acid 
phosphate and to the danger of large masses of the material falling on 
the laborers. 
In the "open dump" system the acid phosphate requires a long 
time to reach its maximum availability, and unless it is properly 
made may never be fit for use. 
The storing of acid phosphate in large piles for protracted periods 
sometimes causes reversion owing to the pressure on the material in 
the lower part of the pile; this pressure also tends to compact the 
material. The storing of well-made acid phosphate in medium-sized 
piles, however, should cause no ill effects. 
Properly made acid phosphate should require no artificial drying, 
since the calcium sulphate formed in the process takes up the water 
to form gypsum. It is nearly always necessary, however, to disin- 
tegrate and screen the material before shipping. This is often done 
by simply throwing the product upon inclined screens, but some- 
times disintegrating machines must be employed. 
