Chemical behavior of indigenous impurities (Al, Fe, Si, Mg, F, K, Na, NH(sub 3), Ca, and SO(sub 4)) during the production of filter-grade wet-process phosphoric acid.

摘要

The conventional (dihydrate process) extraction of phosphate rocks with sulfuric acid and subsequent filtration results in a dilute ''filter-grade'' wet-process phosphoric acid (WPA) containing many cationic and anionic impurities which can cause problems during fertilizer production. Most of these problems result because many of the impurities are present in amounts above saturation. This factorial study was designed to define the nature and amounts of components precipitating from simulated filter-grade (28% P(sub 2)O(sub 5)) phosphoric acid at 85, 68, and 25 C, with 2-day retention times. Results show that the precipitation of the iron ammonium and iron potassium phosphates generally provide the largest source of solids in filter-grade phosphoric acid; Mg and Ca fluoroaluminates and alkali fluorosilicates also provide a significant proportion of solids. The distribution of solids between these 3 groups is governed by the F/Si ratio in the acid. The study also delineates the deleterious effect of ammonia contamination (such as from ammonia leakage to gypsum ponds).