Recovery of fluorine from waste gases

Abstract

Fluorine in phosphate ores is present as the mineral fluorapatite. When the ores are digested with sulfuric acid a slurry is formed which can be filtered to make wet-process phosphoric acid. But fluorine compounds are discharged into the surrounding air during digestion and filtration. The fluorine-containing air is scrubbed with water to prevent air pollution. Scrubber water is cooled in ponds and is recycled to the scrubber. However, fluorine compounds are emitted from the cooling ponds and cause air pollution. In the present invention, processes are disclosed for treating fluorine-containing gases without use of cooling ponds thereby eliminating the air pollution problem. A facility is provided for treating fluorine-containing gases wherein fluosilicic acid or fluosilicate salts are converted to a fluoride salt by the addition of an alkaline substance. A slightly acidic scrubber water is concentrated in an absorption tower. When the scrubber water contains nutrients, ammonium is the preferred alkaline substance and the scrubber water is recovered in solid ammonium phosphate fertilizers or in suspension fertilizers. Sodium hydroxide or sodium carbonate are the preferred alkaline substances when sodium cryolite or aluminum fluoride are produced.