Method of extracting zinc from ores

摘要

Zinc is extracted from zinc ores comprising or consisting of silicates soluble under the reaction conditions, by leaching the ore with aqueous sulphuric acid in the presence of aluminium ions, digesting the reaction mixture to coagulate the silica and separating the resultant zinc sulphate solution. According to the first aspect of the invention, 1-50% by weight of aluminium ions based on the soluble silica present is employed, preferably at or above 70 DEG C., said ions being supplied in the form of an aluminium salt or generated in situ by the addition of an ore containing alumina to the reaction mixture. Coagulation takes place at a pH of 3.5-4: this is reached by neutralization of the acid with the ore or if desired by neutralizing with limestone. The sulphuric acid employed may be a spent electrolyte which should not contain more than 50 g/l. H2SO4. According to the second and preferred aspect of the invention, said zinc ores and the aqueous sulphuric acid are alternately charged to the reactor, each addition of ore and of acid being followed by digestion at above 70 DEG C.: in this modification of the invention the aluminium ions may be supplied from the zinc ore without further addition, if the aluminium content is sufficient. Part of the digested suspension may be removed from the reactor as further charges are added and allowed to settle or thickened; part of the separated mud (preferably that part which has not become granular, and which takes longer to settle) may be returned to the leaching tank. The sulphuric acid used may be a spent electrolyte and contain 10-30% H2SO4, and within this range it is preferred to commence operations with lower concentrations. For either embodiment of the invention, a suitable aluminium salt solution may be obtained by reacting bauxite with a spent electrolyte at 70 DEG C. If it is desired to remove any silica remaining in the final solution, this may be done by adding ferric and/or aluminium ions and digesting the mixture.