Secondary Reduction of Refractory Metals near the Smooth Cathode during Molten Salt Electrolysis. 1. Derivation of Funda
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ndary Reduction of Refractory Metals near the Smooth Cathode during Molten Salt Electrolysis. 1. Derivation of Fundamental Equations for the Process Model A. P. Khramova, *, A. A. Chernysheva, b, **, A. V. Isakova, and Yu. P. Zaykova, b aInstitute
of High Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620137 Russia b Ural Yeltsin Federal University, Institute of Chemical Technology, Yekaterinburg, 620002 Russia *e-mail: [email protected] **e-mail: [email protected] Received July 17, 2019; revised November 21, 2019; accepted January 31, 2020
Abstract—A model of the steady-state process of the refractory metal ion secondary reduction with an alkali or alkaline earth metal formed on a smooth cathode during the electrolysis of molten salt is presented. The secondary reduction is localized near the cathode. The model takes into account the molecular diffusion of the initial components and the thermodynamics of the secondary chemical reaction, the Fick’s first law, the Faraday law, and the Nernst equation. Expressions are obtained for the calculating of concentration profiles, the velocity profile of the secondary reduction reaction inside the diffusion layer, and the secondary reduction current distribution profile over the thickness of the diffusion layer. Keywords: model of the cathode process in an alkali halide melt, model of secondary reduction at the cathode by alkali or alkaline earth metal DOI: 10.1134/S1023193520090050
INTRODUCTION The development of electrochemical technologies in molten salts is pertinent to processes of preparation of metals with different structure. These cathodic products vary from sound deposits [1–3] to dendrite deposits and powders [4, 5]. Among evolving practices are metallothermic reduction reactions in alkali and alkali earth metal halides’ melts [6–9]. The alkali metals are able being dissolved in molten salts [10, 11]. These solutions are strongly reductive, the chemical reduction processes often proceed at a high rate [9]. Thus, during high-temperature electrolysis in molten salts, metals are dissolved to a greater or lesser degree in molten electrolytes; also, sputtering of metals occurs during their cathodic deposition. These phenomena are tightly interrelated: the formation of finely dispersed metal deposits, weakly adherent to cathode, during the high-current-density electrolysis is due to the secondary reduction of the plateable metal ions by the alkali metal dissolved in the electrolyte [12]. The secondary reduction lowers the cathodic current efficiency. On the other hand, similar processes during electrolysis were used for the manufacturing of fine-dispersed powder materials of different purposes [13–16]. The refractory metal cathodic deposition from its salt solution in alkali-halide melts used to be studied, but not be limited to, by the registering of steady-state
polarization curves. These studies always give important information; and the electrolysis under the conditions of secondary reduction is no exception. To be plott
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