Recovery of fluorine from pond water of wet process phosphoric acid plants and recycling of defluorinated water

A process for recovering a metallurgical grade fluorspar containing from about 60 to about 80% calcium fluoride from the waste water pond of phosphoric acid plants and returning the defluorinated water to the pond, said waste water having a pH of about 0.8 and containing from about 0.5 to about 1.5% fluoride, from about 0.5 to about 1.5% phosphates, and more than about 0.4% sulfates, comprising the steps of:

• (1) pretreating the waste water to reduce the sulfate content thereof by adding sufficient granular calcium carbonate to said waste water to raise the pH to a value below 2.0 and aging said pretreated solution for 24 hours to cause precipitation of the sulfates and to reduce the level of sulfates to about 0.4% with minimum precipitation of calcium fluoride;
• (2) adding sufficient calcium carbonate to the pretreated solution obtained in step (1) to obtain an adjusted waste water solution having a pH in the range 2-3, whereby calcium fluoride is precipitated without substantial precipitation of phosphates and sulfates;
• (3) washing the calcium fluoride precipitate obtained in step (2) with the pretreated waste water solution obtained in step (1) and with an ammonium carbonate solution to reduce the phosphate and sulfate content of said precipitate;
• (4) filtering the calcium fluoride precipitate washed in step (3) to recover a metallurgical grade fluorspar and to obtain a phosphate and sulfate containing filtrate;
• (5) mixing the phosphate and sulfate containing filtrate obtained in step (4) with sufficient pretreated waste water to reduce the pH thereof to a value in the range 1.5-2.0;
• (6) sedimentating the mixture obtained in step (5) for 5-21 days to separate silica therefrom to obtain a clear solution;
• and
• (7) returning the clear solution obtained in step (6) to the pond.

A process for recovering a metallurgical grade fluorspar containing from about 60 to about 80% calcium fluoride from the waste water pond of phosphoric acid plants, said waste water having a pH of about 0.8 and containing from about 0.5-1.5% fluoride, 0.5-1.5% phosphates and less than about 0.4% sulfates, comprising the steps of:

• (1) adding sufficient calcium carbonate to the waste water to obtain an adjusted waste water solution having a pH in the range 2-3, whereby calcium fluoride is precipitated without substantial precipitation of phosphates and sulfates;
• (2) washing the calcium fluoride precipitate obtained in step (1) with said waste water solution and with an ammonium carbonate solution to remove excess phosphates and sulfates from said precipitate;
• (3) filtering said washed precipitate obtained in step (2) to recover a metallurgical grade fluorspar and to obtain a phosphate and sulfate containing filtrate;
• (4) adding additional calcium carbonate followed by slaked lime or quicklime to the phosphate and sulfate containing filtrate in step (3) to raise the pH thereof to a value in the range 7-10, whereby substantially all of the fluoride, phosphate and silica in said filtrate is precipitated in gelatinous precipitate;
• (5) centrifuging the mixture obtained in step (4) to remove the said silica precipitate to obtain a clear solution;
• and
• (6) returning the clear solution obtained in step (5) to the pond or discharging it.

A process for recovering a metallurgical grade fluorspar containing from about 60 to about 80% calcium fluoride from the waste water pond of phosphoric acid plants, said waste water having a pH of about 0.8 and containing from about 0.5-1.5% fluoride, 0.5-1.5% phosphates and more than about 0.4% sulfates, comprising the steps of:

• (1) pretreating the waste water to reduce the sulfate content thereof by adding sufficient granular calcium carbonate to said waste water solution to obtain a pretreated solution having a pH below 2.0 aging said pretreated solution for up to 24 hours to cause precipitation of sulfates and to thereby reduce the concentration of sulfate to a level of about 0.4% with minimum precipitation of calcium fluoride;
• (2) adding additional granular calcium carbonate to said pretreated solution obtained in step (1) to obtain an adjusted solution having a pH in the range 2-3, whereby calcium fluoride is precipitated without substantial precipitation of phosphates and sulfates;
• (3) countercurrently washing and agitating the calcium fluoride precipitate formed in step (2) with said pretreated waste water solution obtained in step (1) and with ammonium carbonate to remove excess phosphates and sulfates from said calcium fluoride precipitate;
• (4) filtering the washed precipitate obtained in step (3) to recover a metallurgical grade fluorspar and to obtain a phosphate and sulfate containing filtrate;
• (5) adding additional calcium carbonate followed by slaked lime or quicklime to the phosphate and sulfate containing filtrate of step (4) to raise the pH thereof to a value within the range 7-10, whereby substantially all of the fluoride, phosphate and silica is precipitated in gelatinous form;
• (6) centrifuging the mixture obtained in step (5) to remove said silica precipitate therefrom to obtain a clear solution;
• and
• (7) returning the clear solution obtained in step (6) to the pond or discharging it.

A process for recovering a metallurgical grade fluorspar containing from about 60 to about 80% calcium fluoride from the waste water pond of a phosphoric acid plant and for returning the defluorinated water to the pond, said waste water having a pH of about 0.8 and containing from about 0.5-1.5% fluoride, having a concentration of phosphates such that the fluoride to phosphate weight ratio (F/P.sub.2 O.sub.5) is in the range 1.2 to 1.5 and having less than about 0.4% sulfates, comprising the steps of:

• (1) adding sufficient calcium carbonate to said waste water solution to obtain an adjusted solution having a pH in the range 2-3, whereby calcium fluoride is precipitated without substantial precipitation of phosphates and sulfates;
• (2) filtering the calcium fluoride precipitate obtained in step (1) to recover a metallurgical grade fluorspar and to obtain a phosphate and sulfate containing filtrate;
• (3) mixing the phosphate and sulfate containing filtrate obtained in step (2) with sufficient waste water to reduce the pH thereof to a value in the range 1.5-2.0;
• (4) sedimentating the mixture obtained in step (3) after 5-21 days to separate silica therefrom to obtain a clear solution;
• and
• (5) returning the clear solution obtained in step (4) to the pond.