Measurement of pH in the vicinity of a cathode during the chloride electrowinning of nickel
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I.
INTRODUCTION
ELECTRODEPOSITION of metallic elements from aqueous electrolytes is usually affected by the hydrogen ion concentration (pH) of the medium. The evolution of hydrogen gas, which lowers the cathodic current efficiency of metal deposition, is more or less inevitable during electrodeposition of base metals such as nickel, zinc, and chromium. Sometimes, the pH in the vicinity of the cathode or in the cathodic diffusion layer, increases with a hydrogen evolution, and a metal hydroxide layer forms on the cathode. To avoid this increase in pH, various kinds of buffering agents have been tried. We studied, for example, chromium electrodeposition from divalent chromium aqueous electrolytes containing buffering agents such as formic acid, glycine, and citric acid.[1,2] In contrast, Sumitomo Metal Mining Co., Ltd. has developed a new nickel electrowinning process, MCLE (matte chlorine leach electrowinning), in which electrolytic nickel is obtained from chloride baths with pH 1.1 to 1.5 in the absence of any buffering agents (Table I). The potential-pH diagram of the Ni-H2O system[3] (Figure 1) implies that hydrogen evolution precedes the electrodeposition of nickel at such a low pH. Many industrial nickel electrolytic processes (e.g., electrorefining and electroplating) operate using baths with pH 3 to 6. The MCLE process, however, achieves a relatively high current efficiency, 94 to 97 pct, for nickel electrowinning despite the low pH. The notion that such a low-pH electrolyte reduces the current efficiency is a commonly held view in the field. The MCLE process is, thus, a new concept that contradicts this viewpoint. In the MCLE process, the pH near the cathode surface may not increase sigKUNIAKI MURASE, Research Associate, TAKESHI HONDA, Graduate Student, TETSUJI HIRATO, Associate Professor, and YASUHIRO AWAKURA, Professor, are with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. Manuscript submitted December 22, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
nificantly in spite of the lack of a buffering agent, because hydrogen evolution may be suppressed. Measurement of the pH in the vicinity of a cathode is, therefore, a key to the theoretical understanding of many industrial electrodeposition processes where the optimum conditions for electrolysis are determined only empirically on the basis of deposit quality and current efficiency. Recently, we assembled an antimony microelectrode system and measured pH values in the vicinity of the cathode evolving hydrogen gas.[4] Here, we demonstrated directly that a rise in pH in the vicinity of the cathode due to hydrogen evolution is depressed in the presence of glycine— one of the typical buffering agents.[4] In the present work, the microelectrode was employed for the measurement of pH in the vicinity of the cathode during the galvanostatic chloride electrowinning of nickel as a conventional hydrometallurgical process. The resultant pH profiles across the cathodic diffusion layer were discussed in terms of the de
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