Verfahren zur Gewinnung von Kryolith

Fluorine values are recovered in the form of cryolite from the smokes emitted in the electrolytic production of aluminium by separating the dusts from the smokes and washing all or part of the dust-free gases in a hydrofluoric acid medium, washing all the gases in an alkaline medium, passing the washed gases to waste, calcining the extracted dust with sodium carbonate, washing the calcined mixture with water and cycling the resultant sodium fluoride solution to the alkaline gas washing stage, neutralizing the effluent from the latter to a pH or 6-8 with at least a part of the liquor from the acid washing step, and adding aluminium sulphate to the mixture to precipitate cryolite. In a modification of the process a part of the liquor obtained in the acid washing process and containing hydrofluoric acid is reacted with sodium aluminate to precipitate cryolite the remainder being used to neutralize liquors from the alkaline washing step after which cryolite is obtained by precipitation with aluminium sulphate. In a further modification of the process the portion of the acid wash liquors not used for neutralization of the alkaline liquors is recycled to the acid washing process. The alkaline liquors used for washing the gases may contain sodium hydroxide and/or sodium carbonate.