Electrochemical Processes at the Bi0–Bi3+ Interface in Chloride Melts

Abstract

For rational design of technological schemes for production of high-grade bismuth and to improve electrochemical bismuth refining, knowledge of physicochemical properties of low-valent bismuth compounds and the mechanisms underlying processes occurring at the Bi⁰–Bi³⁺ interface in molten salts is necessary. For this purpose, the kinetics and mechanism of formation of low-valent bismuth compounds in the Bi⁰–BiCl₃–ZnCl₂–NH₄Cl system by UV–Vis spectroscopy was studied. Chemical interactions between metallic bismuth and Bi³⁺ species in the melt give rise to Bi+ intermediates and [Bi₅]³⁺ clusters, which are registered by their characteristic absorption bands at approximately 18000 and 14000 cm^–1. The kinetic parameters for the Bi⁺ and [Bi₅]³⁺ formations are estimated from the dependence of intensities of these absorption bands on the time of contact between metallic bismuth and Bi³⁺ species in the melt. The rate constants characterizing the Bi⁺ and [Bi₅]³⁺ formations are estimated to be 3.33 × 10^–3 and 2.31 × 10^–3 s^–1, respectively. The mechanism of anodic bismuth dissolution is proposed, and the cathodic and anodic current efficiencies for bismuth are determined.