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. 



