Method for hydrofluoric acid digestion of silica/alumina matrix material for the production of silicon tetrafluoride, aluminum fluoride and other residual metal fluorides and oxides

Abstract

A process for hydrofluoric acid digestion of inorganic silica/alumina matrix material for the production of silicon tetrafluoride and aluminum fluoride comprises the initial step of reacting the matrix material with hydrofluoric acid, to form silicon tetrafluoride gas and a solution/slurry containing aluminum fluoride, undigested oxides, and additional soluble and insoluble fluoride materials. The silicon tetrafluoride gas is purified of contaminants by condensing out impurities in cold traps and reacted with aqueous sodium fluoride to form a solution/slurry of fluorosilicate salt which is delivered to a filter press to separate sodium fluorosilicate powder from the solution/slurry. The sodium fluorosilicate powder is dried and delivered to a kiln at a temperature of about 600.degree. C. to 650.degree. C. to form substantially pure silicon tetrafluoride gas and sodium fluoride powder for collection. Recovery of aluminum fluoride is obtained by washing the solution/slurry containing aluminum fluoride, undigested oxides and additional soluble and insoluble fluoride materials with water to completely dissolve all water soluble materials; separating all undissolved materials by a filter press and drying the remaining solution containing aluminum fluoride and other water soluble fluorides in an evaporator while condensing water and excess hydrofluoric acid vapors in a condenser; recovering the aluminum fluoride or heavy metal fluorides from the dried solution and/or collecting high purity hydrofluoric acid from the liquid condensate by fractional distillation.

Dependencies are inferred from the claim text.