Supersaturation Phenomena and the Formation of Fluorapatite in Aqueous Suspensions of Phosphate Rock

Abstract

Dilute HCl solutions equilibrated with Florida phosphate rock and Tennessee brown phosphate rock in the presence of solid fluorite (CaF2) developed a condition of supersaturation with respect to fluorapatite at 25C. In suspensions boiled before equilibration at 25C, however, the solutions were in equilibrium with fluorapatite, as evidenced by ion‐activity products for fluorapatite in the solutions. The pH and concentrations of calcium and phosphorus in solution decreased due to boiling a suspension of Florida phosphate rock, as expected if fluorapatite had been precipitated. A condition of supersaturation with respect to fluorapatite redeveloped upon long‐continued equilibration of boiled suspensions of Florida and Tennessee phosphate rocks at 25C. Dilute HCl solutions equilibrated with phosphate rock or with phosphate rock plus solid fluorite became supersaturated with respect to fluorite at low pH values (about pH 3) where considerable calcium and fluoride were released from the phosphate rock, and the supersaturation persisted until the end of the equilibration period of 47 days at 25C. At pH values above about 4, the solutions became saturated with fluorite if solid fluorite was present but remained unsaturated where only phosphate rock was present as the solid phase.

Activity of phosphorus of synthetic Fe-K-P compounds in superphosphate fertilizers as affected by pH and ionic strength

“…The amounts of released P when Catalão PR was used tended to decrease with increasing pH. This is consistent with solubility studies on other PR sources and agronomic evaluation of apatitic PR (Engelstad et al, 1974;Chien et al, 1975). For the Fe-P compounds the trend was opposite (Figures 3 and 4), with a slight increase in the amounts of P released with increasing pH.…”

Aqueous cadmium removal by hydroxylapatite and fluoroapatite

“…For both apatites the amount of released phosphate was the highest for the initial pH 3. This confirms that the apatite solubility increases with decreasing pH (Chien et al 1975, Arends et al 1987, Narasaraju et al 1971, Narasaraju & Phebe 1996. The higher solubility of the HAP as compared to the FAP is caused by the difference in solubility products (K SP ) of the minerals which are equal to 10 -58.23 and 10 -59.56 , respectively (Lindsay 1979).…”