Leaching of oxidic zinc materials with chlorine and chlorine hydrate
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grade oxide zinc ores can either be processed by the electrolytic sulphate route or in the Imperial Smelting Furnace. However, there are a variety of low grade ores and oxidic residues which are not readily processed by either of the above major routes. These oxidic residues arise from two sources. During the refining of secondary copper, the zinc evaporates from the molten copper and is collected as zinc oxide together with substantial quantities of copper, tin and lead. ~Secondly, zinc oxide in the form of zinc ferrite fume is collected from electric arc steelmaking furnaces and basic oxygen furnaces. Recent estimates show that there are several hundred thousands tons of this material produced annually in the U.S.A. z The secondary copper fume is not readily treated by conventional extraction methods using a sulphuric acid leach because of the presence of large concentrations of leached tin in the electrolyte which is codeposited and leads to hydrogen evolution at the cathode in the zinc sulphate cell rather than zinc deposition. In the Imperial Smelting Furnace there is a limit to the amount of tin containing material that can be fed to the furnace, as most finds its way into the zinc product to the detriment of its mechanical properties. Two recovery methods have been developed for zinc ferrite residue. Leaching with sulphuric acid but this only achieves 80 pct recovery of the zinc with considerable extraction of the iron into solution. 2,3 The alternative process is to roast the zinc ferrite at 1200 ~ to produce leaded Waelz oxide which can only be treated in a zinc blast furnace? Other residues which cannot be readily treated are galvanisers' ashes, containing 70 pct zinc, flux skimmings, which are high in chlorides and fluorides. The latter are very troublesome in the electrolytic process and chlorides, not only cause pollution problems in pyrometallurgical treatment, but also attack the anodes in the electrowinning cell. Thorsen 5 leached galvanisers' ashes with a carboxylic acid (Versatic 911). The zinc was then stripped, with H2SO4, as zinc sulphate from this phase, leaving the chloride in solution. The resulting solution was found to be suitable for electrolysis.
B. K. THOMAS and D. J. FRAY are Graduate Student and University Lecturer, respectively, Department of Metallurgy and Materials Science, University of Cambridge, England. B. K. Thomas is now with IMI Rod and Wire, Birmingham, England. Manuscript submitted May 7, 1980. METALLURGICAL TRANSACTIONS B
None of the above methods for the treatment of these materials are entirely satisfactory and, as many sources of secondary zinc contain chlorides, it would appear logical to look for a zinc recovery route based on chloride leaching followed by the electrolysis of zinc chloride solution. This route is particularly pertinent as it is unlikely that iron oxide will be taken into solution in the presence of zinc oxide, the zinc chloride solution can be easily purified 6 and, lastly, zinc chloride can be electrolysed at high current densities to produc
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