Selective removal of iron contaminations from zinc-chloride melts by cementation with zinc
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I. INTRODUCTION
THE development of modifications and improvements of the existing zinc process has been the object of a considerable amount of research and development effort. One such improvement is the Sheritt–Gordon zinc pressure-leach process,[1] in which sulfides are oxidized to elemental sulfur. However, the sulfur can be contaminated with mercury and selenium, rendering it unsalable and requiring it to be dumped. Neither have other suggestions resulted in completely satisfactory solutions. This study is part of a research program which will explore the possibilities of using mixtures of zinc chloride with alkali chlorides for the electrolytic production of zinc. Chlorination of zinc-sulfide concentrates results in the formation of elemental sulfur and metal chlorides. The metal chlorides should be dissolved in an alkali chloride solution which, after purification, will yield, on electrolysis, molten zinc metal and chlorine, with the chlorine being recycled. In a recent article, zinc chloride was electrolyzed with an energy consumption of 1.77 kWh/kg,[2] which is about 40 pct lower than the energy consumption for electrowinning from zinc-sulfate solutions. The impurity with the highest concentration is iron. When it is assumed that iron precipitates at an activity of 1, it can be calculated that cementation with zinc at 400 8C should result in an equilibrium iron concentration of 11 ppm.[3] When the activity coefficients of zinc chloride and iron chloride are altered by dissolving the metal chlorides in an alkali chloride solvent, the iron concentration could be reduced to 1 ppm at 450 8C.[4] However, Monk and Fray[5] concluded that it is not possible to purify an impure zincchloride melt with zinc at 450 8C, unless a substantial quantity of high-purity zinc is degraded. The iron concentration was reduced from 6428 to 1122 ppm in that work. Other techniques that were used to remove iron from a zinc-chloride melt, among others, are controlled-potential electrolysis, oxidation with oxygen, and volatilization. Electrolysis did not result in sufficient purification because of rechlorination of the iron deposit by anodically evolved R. A. DEVILEE, Student, A. VAN SANDWIJK, Associate Professor of Mining Chemisty, and M. A. REUTER, Professor of Raw Materials Technology, are with the Department of Raw Materials Technology, Delft University of Technology, 2628 RX Delft, The Netherlands. Manuscript submitted February 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
chlorine and chemical short-circuiting caused by repeated redox of ferrous and ferric iron.[6] The oxidation technique was found to be “satisfactory,”[6] but the hematite obtained was probably contaminated with large amounts of zinc oxide,[7] which makes it unsuitable for the subsequent processing in the iron blast furnace. Campbell and Ritchey[8] removed iron as ferric chloride from a zinc-chloride melt at 550 8C by bubbling chlorine through the melt, but the vapor also contained large amounts of zinc chloride. Subsequently, the condensed vapor had to
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