Re-evaluating the sulphidisation reaction on malachite surface through electrochemical and cryo XPS studies

Abstract

Sulphidisation is an electrochemical reaction involving sulphide ions to form metal sulphides on the surface of metal oxides. It has been widely applied to promote the flotation of malachite where a conductive copper sulphide layer was believed to form on the malachite surface. The formation of a copper sulphide layer can promote the interaction of sulphidised malachite with sulfhydryl and other collectors. However, due to the limitation of traditional potentiometry and surface analysis, the reaction route of sulphidisation on the malachite surface and its related change in electrical property are always controversial. This leads to the uncertainty of its application in the industry. In this study, Electrochemical Probe Approach curves (PACs) were employed to re-evaluate the sulphidisation reaction on malachite through electron-transfer rate constant simulation, and the corresponding surface speciation was characterised using Cryo X-ray photoelectron spectroscopy (Cryo-XPS). It is interesting to find that the sulphidisation on malachite can be classified into two routes. The first route is the mild sulphidisation at a low concentration of the sulphidising agent. In this route, although the modified malachite surface presents a partially positive current feedback, it is not as conductive as a typical copper sulphide and covered by a large amount of elemental sulfur and polysulphide as well as Cu(OH) Cu(I)S complex. The second route is the strong sulphidisation at a high concentration of the sulphidising agent. In this route, the sulphidised surface is covered by a Cu(I)S layer and presents a similar electrical conductivity to a copper sulphide, but the pulp potential can be too low for a sulfhydryl collector to oxidise and adsorb on sulphidised malachite. The new finding explains the limitation of current sulphidisation in malachite flotation and provides guidelines to improve its application.