Process for the preparation of alkali metal phosphate solutions

US5124137A · kind A · utility

Assignee

• Hoechst GmbH · DE

Inventors

• Gunther Schimmel · Lechenich, DE
• Thomas Staffel · Grünstadt, DE
• Reinhard Gradl · Hürth, DE

Key dates

• Filing date: Mar 28, 1991
• Grant date: Jun 23, 1992
• Expiry date: Mar 28, 2011

Classification

• Technology area (CPC C): Chemistry; Metallurgy
• CPC primary: C01B25/308
• WIPO field: Materials, metallurgy
• WIPO sector: Chemistry

Abstract

Alkali metal phosphate solutions having a fluorine content of less than 20 ppm, based on P.sub.2 O.sub.5, are prepared by first digesting crude phosphate using sulfuric acid, and separating the calcium sulfate from the crude phosphoric acid. An alkali metal compound is then added to the crude phosphoric acid, and the precipitated solid is filtered off. The prepurified crude phosphoric acid is neutralized to a pH of at least 7, and the solid produced during this operation is separated off. The resultant alkali metal phosphate solutions having a P.sub.2 O.sub.5 content of from 10 to 30% by weight are adjusted to a pH of between 4.8 and 6.0 using phosphoric acid. The solutions are heated to from 60.degree. to 120.degree. C., and from 1 to 6% by weight, based on the weight of the solution, of an alkaline earth metal oxygen compound are added to the alkali metal phosphate solutions with stirring. Finally, the precipitated solid is filtered off from the alkali metal phosphate solutions after a residence time of from 0.5 to 4 hours.