Alternative solution purification in the

More than 80% of the world’s production of zinc from its concentrate is obtained by the roastleach-electrowin hydrometallurgical process. In the roast-leach process, very pure zinc sulphate solution must be prepared for zinc electrowinning, therefore, the purification process is one of the most important unit processes in zinc hydrometallurgy. The conventional methods for the zinc solution purification are the „hot-cold“ and the „reversed“ purification processes. In both methods the liquor is purified by cementation of harmful elements with zinc dust and activators in a continuous, multistage process. The disadvantages of these procedures are the high zinc dust consumption and even more the complex composition of the residues. Experimental work was carried out in laboratory scale to separate the metallic impurities, principally copper, cobalt and nickel from the leach solution by solvent extraction with hydroxyoximes. The separation of copper was excellent, with a final concentration in the raffinate of < 1 ppm. The extraction of cobalt and nickel wasn’t favorable because of the possible coextraction of zinc. Purification processes including copper solvent extraction and cementation of the other impurities for the hot-cold and the reversed solution purification methods are proposed. Proceedings of EMC 2001 1 Friedrich, Krüger, Méndez Bernal 1 Conventional zinc solution processing In the conventional electrolytic process for recovery of zinc from sulphide concentrates (see flow diagram in Fig. 1) the concentrate is first calcined by roasting to convert zinc sulphide to zinc oxide, which is readily solubilized in dilute sulfuric acid solutions. The calcinated material also contains iron and other elements, such as copper, cobalt, nickel, cadmium and antimony, which if allowed to remain with the zinc would interfere with the succeeding electrolytic process. Leaching the calcinated material in spent electrolyte produces an impure zinc sulphate solution, which, after the removal of iron, still includes as major impurities copper, cadmium, cobalt and nickel. These metal impurities must be substantially removed through succeeding solution purification. Apart from the necessity of such purification, the recovery of these metals, in particular copper and cadmium, is economically beneficial [1]. The zinc electrowinning process is unusual from a thermodynamic point of view because zinc metal has a more negative reduction potential than hydrogen: 2H + 2e → H2 e° = 0 V (1) Zn + 2e → Zn° e° = 0.76 V (2) One would therefore expect hydrogen gas to evolve at the expense of zinc deposition. However, zinc metal is electrowon economically from acidic zinc sulfate solution because hydrogen evolution has a high overpotential on zinc metal. In order to maintain this large overpotential, almost all impurities in the leach solution must be completely removed [2]. Any remaining impurities act as catalysts for hydrogen evolution causing large drops in current efficiency. Primary impurity removal in the roast-leach-electrowinning process takes place towards the end of the neutral leach stage by the neutralization-precipitation of a ferric hydroxide phase that acts as a scavenger for impurities like arsenic, antimony, germanium and tin [3]. Remaining traces of impurities in the zinc electrolyte are subsequently removed in the main purification section of the process by cementation with zinc dust [4]. Proceedings of EMC 2001 2 Alternative solution purification in the hydrometallurgical zinc production H2SO4 production H2SO4 Sulfidic zinc concentrate SO2 containing waste gas Oxidative roasting Unpurified neutral liquor Purified neutral liquor Cathodic zinc Zinc dust Cu,Cd residues to treatment Residue to treatment Cell acid Liquor purification