Characterization of a natural and an electro-oxidized arsenopyrite: a study on electrochemical and X-ray photoelectron spectroscopy

Abstract

This paper discusses the results of a detailed study on the electrochemistry of an arsenopyrite mineral and a concentrate as well as other mineral species contained in it in a chloride medium (NaCl 1.9 M+HCl 0.1 M) using cyclic voltammetry. The surface modification promoted by the anodic oxidation of arsenopyrite mineral samples (+0.8 V for 1 h) is also analysed. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy have been used to provide information on the chemical state of natural and electro-oxidized surfaces. The results showed that layers of elemental sulphur are produced on the oxidized surface of arsenopyrite by the formation of an intermediate metal-deficient sulphide Fe 1− x As 1− y S. Other surface oxidation products such as iron oxides, arsenic oxides and oxy-sulphur species have also been detected, confirming the interpretation of the voltammetric studies. The identified surface products show that arsenopyrite oxidation occurs by the diffusion of the metal atoms from the bulk to the interface region and by their interaction with air forming metal oxide layers (mainly iron and arsenic oxides) and leaving a predominant elemental sulphur layer. XPS intensity ratios yielded mineral surface stoichiometry before and after electrochemical treatment. Based on the fitted XPS spectra new data for the binding energies of core electrons in arsenopyrite are proposed: 707.3 eV for the Fe 2p 3/2 level, 162.3 eV for the S 2p 3/2 level and 41.5 eV for the As 3d 5/2 level. The effect that electrochemical pre-treatment of refractory arsenopyrite concentrate has towards the breakdown of the sulphide matrix required for the release of the occluded gold is also discussed, taking into account the results obtained in this study.