Cryolite prodn. from fluorosilicic acid - by forming alpha-aluminium fluoride soln. and digesting with slurry of crystalline sodium fluoride, followed by calcination of solid prod.

摘要

Cryolite is prepd. from fluorosilicic acid and/or sodium fluorosilicate by first forming alpha-aluminium fluoride soln. by a continuous or discontinuous process. Silica is eliminated by filtering or centrifuging, then the soln. is mixed in a digestion vessel with a stoichiometric amt. of a crystalline NaF slurry and digested for at least 60 mins. at 40-80 deg.C, pref. 50 deg.C. The reaction is carried out at an initial solids concn. of 10-50%, esp. 20%. The prod. is filtered off and calcined at above 500 deg.C to below 700 deg.C, esp. at 580 deg.C. Process is esp. useful for prodn. of cryolite from fluorosilicic acid which is obtd. as a by-prod. in industries producing e.g. phosphates and HF.