Electrolytic Removal of Chloride from Acidic Nitrate Solutions

Abstract

Chloride containing ores are among other things often used as the raw material in metal electrowinning from aqueous solutions. The dissolution of this ore may cause problems as chlorine gas will be evolved either as a side reaction on the working electrode, or as a main reaction on the counter electrode. Either way, the presence of chlorine is damaging both for the production inventory and for the environment. The amount of chloride present in the process water in the production needs therefore to be reduced.The aim of this thesis was to study the feasibility of electrolytic removal of chloride, from an acidic nitrate solution, using a customized dimensionally stable anode (DSA) with an active component of iridium oxide. Voltammetry was carried out using different electrode set ups and electrode materials. Both a rotating disc electrode (RDE) and a rotating ring disc electrode (RRDE) were utilized. The RDE was used for kinetic studies of the chlorine and oxygen evolution, using a commercial titanium supported IrO2-Ta2O5 electrode. The RRDE for current efficiency measurements using both a Pt-Pt electrode (both disc and ring in platinum), and an IrO2-Pt electrode (iridium oxide coated glassy carbon disc, platinum ring). Prior to the electrolysis experiments, a titanium supported IrO2-Ta2O5 electrode was prepared by thermal decomposition. Different application techniques of the coating was investigated, including application by brush and dip coating. The synthesized electrodes were analysed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), and compared to a similar, commercial electrode from Permascand.The electrolysis experiments were carried out in both a stagnant cell and in a designed flow cell, testing both a synthetic electrolyte and process water. Different working electrodes were utilized, including the synthesized IrO2-DSA and a commercial RuO2/IrO2-mesh. The voltammetry experiments using both a RDE and a RRDE indicated that chlorine evolution on iridium oxide based electrodes was only to small degree affected by mass transport. The electrolysis experiments showed proof of concept, yielding a reduction in the chloride content in the process water of 23 % and a current efficiency of 3 %, using the flow cell and the commercial RuO2/IrO2-mesh electrode. The synthesized electrode showed low selectivity towards chlorine evolution. This was found to be most likely due to low crystallinity and/or high porosity of the IrO2-Ta2O5 coating. During long time, stagnant, electrolysis in the process water a white deposit was formed on the counter electrode. This deposit was analysed by EDS indicating a high content of calcium phosphate.