GB774319A - Improvements in or relating to the absorption of silicon tetrafluoride - Google Patents

Abstract

Silicon tetrafluoride is absorbed from gases containing it and phosphate impurities, e.g. den gases resulting from the treatment of phosphate material with acid, in an aqueous ammonia solution maintained at a temperature between its freezing point and 40 DEG C. and containing sufficient ammonia to maintain the pH of the resulting solution between 3.5 and 5.5, and the insoluble tricalcium phosphate and silica are separated by, e.g. centrifuging or filtering. Den gases may be continuously contacted with aqueous ammonia solution in a one or two-stage jet absorber or absorption tower, and aqueous or anhydrous ammonia is continuously injected to maintain a pH 4.5, while slurry is continuously withdrawn. The withdrawn material may be treated to produce ammonium fluosilicate, ammoninum fluoride or bifluoride, or alkali metal or other metal fluorides. For example, as described in Specification 765,901 the slurry is added to an aqueous solution containing sufficient ammonia to maintain a pH above 7.5 in the resulting solution, while the temperature is maintained below 40 DEG C. Precipitated silica is separated, and ammonium bifluoride may be prepared from the resulting ammonium fluoride solution by heating.