Anodic electrochemical process in NaCl-KCl-Al2S3 molten salts

Abstract

Al₂S₃ electrolysis process in NaCl-KCl molten salts is advantageous for aluminum production due to the low electrolysis temperature, reduced energy consumption, and no greenhouse gas emissions. It is S^2- discharging on the anode and gaseous sulfurs is generated. However, the origin of S^2- from either free S^2- or S^2- in complex ions has rarely been investigated, and the anodic electrochemical mechanism remains unclear. Herein, the discharge sequence of sulfur ions at a graphite anode in equimolar NaCl-KCl molten salts containing 1 wt.% Al₂S₃ electrolyte at 953 K was investigated by quantum chemical calculations, and the electrochemical mechanism was verified by linear sweep voltammetry, square wave voltammetry, and chronoamperometry. The results indicate oxidation of free S^2- and S^2- in AlSCl₃^2- to produce gaseous sulfurs at the anode, with free S^2- more prone to oxidation than S^2- in AlSCl₃^2- during the anodic electrochemical reaction. The oxidation reactions of both free S^2- and S^2- in AlSCl₃^2- follow one-step two-electron transfer processes. Free S^2- follows a quasi-reversible process, controlled by diffusion, with a charge transfer coefficient of 0.667 and a diffusion coefficient of 3.248 × 10^-5 cm²·s^-1. By comparison, S^2- in AlSCl₃^2- follows an irreversible process, controlled by diffusion, with a charge transfer coefficient of 0.561 and a diffusion coefficient of 1.351 × 10^-5 cm²·s^-1. Overall, these theoretical findings look very promising for future studies and the development of green electrolytic aluminum production processes using NaCl-KCl-Al₂S₃ molten salts.