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Supersaturation Phenomena and the Formation of Fluorapatite in Aqueous Suspensions of Phosphate Rock¹

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.

¹ Journal Paper No. J-7937 of the Iowa Agr. and Home Ec. Exp. Sta., Ames, 50010, Project 1868. Contribution from the Dep. of Agronomy.

² Former Graduate Assistants and Professor. Dr. Chien is now a Research Associate at Washington University, St. Louis, Missouri. Dr. Wier is now a Research Chemist with the Phillips Petroleum Company, Bartlesville, Oklahoma.

Received for publication July 12, 1974. Accepted for publication October 7, 1974.