GB822494A - Separation of hydrofluoric and hydrochloric acid gases - Google Patents

Abstract

Hydrofluoric acid is absorbed from gaseous mixtures containing it, together with hydrochloric acid, by washing such mixtures with a saturated aqueous solution of hydrochloric acid. The aqueous acid solution may also contain metallic salts soluble therein, e.g. salts of calcium, aluminium, barium, lead, magnesium, or sodium, particularly chloride of calcium or aluminium, when metal fluorides may be formed. Mixtures of metal salts may be used, and if a mixture of sodium and aluminium is used cryolite is precipitated. The salts may be added to the acid as aqueous solutions or suspensions, or aqueous solutions or suspensions of the corresponding oxides may be used. The washing may be at ambient temperature, but preferably 50-100 DEG C., in countercurrent in an acid-resistant, preferably filled, column, with recycling of the washing acid. Washing acid may be continuously, or non-continuously, withdrawn from the cycle, and replaced by fresh hydrochloric acid solution containing metal salts, or by water, or aqueous solutions or suspensions of metallic salts or oxides, which immediately become saturated with hydrochloric acid. A decanter may be provided in the cycle in which metallic fluorides are precipitated. The hydrochloric acid remaining in the gas may be treated in known manner with water, or dehydrated. The gas mixture treated may be the anhydrous HCl containing HF evolved in the fluorination by exchange of halogen between HF and a halogenated derivative, e.g. chlorofluoromethanes from CCl4 or CHCl3, chlorofluoroethanes from chlorinated ethane or ethylene, or aluminium fluoride from aluminium chloride.