Recovery of fluorine from pond water of wet process phosphoric acid

Abstract

A process for obtaining calcium fluoride from the pond water of phosphoric acid plants and recycling the high phosphate defluorinated water to the pond is disclosed. Sufficient calcium carbonate is added to a solution having a pH of about 0.8, containing about 1% fluoride, 1% phosphate and less than 0.4% sulfate, to raise the pH thereof to 2-3, whereby calcium fluoride is precipitated as a major component, calcium phosphates and calcium sulfates are precipitated as minor components. The calcium fluoride precipitate obtained is washed with raw pond water to reduce the phosphate content and with ammonium carbonate solution to reduce the sulfate content. In this manner an industrially useful fluorspar is obtained. The low fluoride supernatant liquid is then mixed with sufficient raw pond water to bring the pH into the range 1.5-2.0 and the resulting suspension is then held in a lagoon for 5-21 days to permit separation of silica. After separation by sedimentation, the water containing unprecipitated phosphate is returned to the pond for further use. The silica-rich sediment may be further dewatered by centrifugation and used in the phosphoric acid plant to promote better recovery of fluoride. To minimize sulfate contamination of the calcium fluoride product, a separate pretreatment step may be provided to reduce the sulfate content of the pond water to approximately 0.4%. In this step, sufficient calcium carbonate is added to raise the pH of the pond water to about 2.0, and after adequate aging to cause precipitation of calcium sulfate with minimal precipitation of calcium fluoride which precipitates at a higher pH. After pretreatment, the suspension is treated by the first step of the process to remove calcium fluoride.