Process for treating residues from the electrolytic processing of zinc, by recovery of the metals therein

A process for treating sulphated residues from the electrolytic processing of zinc, by selective extraction of the metals contained in these residues, comprising

• a. digesting the residues, at an elevated temperature, with a measured amount of hydrochloric acid in the presence of calcium chloride in slight stoichiometric excess over sulphate ions, removing the insoluble matter by filtration and recovering the elevated temperature solution of chlorides;
• b. cooling the solution of chlorides until lead chloride precipitates, recovering precipitated lead chloride;
• c. removing iron chloride from the solution of chloride from which the lead has been removed, by the following stages:
• c.sub.1. extracting the iron chloride by counter-current circulation of an organic solvent which is selective for iron chloride and is immiscible with water, and separating the aqueous solution, from which the iron chloride has been removed, from the selective solvent charged with iron chloride accompanied with alkali metal chlorides, alkaline earth metal chlorides and zinc chloride;
• c.sub.2. counter-currently washing the selective organic solvent charged with iron chloride, with an aqueous hydrochloric acid solution of at least 3N strength, and separating the aqueous solution containing the alkali metal chlorides, alkaline earth metal chlorides and zinc chloride, from the washed selective organic solvent;
• c.sub.3. counter-current re-extracting the iron chloride from the washed selective organic solvent with a dilute solution of hydrochloric acid and separating an aqueous solution of re-extracted iron chloride containing a substantially equimolar amount of hydrochloric acid from the selective organic solvent, from which the iron chloride has been removed, and which is re-cycled to stage (c.sub.1), while recovering iron in the form of ferric oxide;
• d. rendering the solution of chlorides, from which iron has been removed, emanating from stage (c.sub.1), alkaline with ammonia until silver has been re-dissolved as an ammine complex, recovering the precipitated metal hydroxides by filtration;
• e. precipitating silver sulphide by measured addition of sulphide solution, recovering the silver sulphide;
• f. displacing the ammonia from the solution emanating from operation (e) with a measured amount of lime, recovering the ammonia necessary for operation (d), separating the precipitated zinc hydroxide, copper hydroxide and cadmium hydroxide and re-cycling the calcium chloride of the remaining solution in operation (a).