The mechanism of copper activation of sphalerite

On the basis of recent SIMS and XAFS measurements in conjunction with alrea dy published XPS results, a mechanism for the adsorption/absorption of Cu o nto sphalerite is proposed. Under conditions of high pH and high nominal su rface coverage of the sphalerite by the Cu, Cu(OH), colloidal particles are observed on the sphalerite surfaces using SIMS. Under other conditions, SI MS measurements have indicated that adsorption of the Cu is essentially uni form over the sphalerite surface and is not related to low coordination sit es on the surface of the sphalerite. Depth profiling of sphalerite surfaces with Cu adsorbed under conditions that do not result in Cu(OH), colloidal particles show that the Cu adsorbed/absorbed on the sphalerite surface is l argely in the first few atomic layers. XAFS analysis of Cu activated sphale rite has indicated that the Cu occupies a distorted trigonal planar geometr y, coordinated to three S atoms, in both surface and bulk sites. In additio n Cu(ls), absorption edges in XAFS show that both bulk and surface adsorbed copper have an oxidation state less than fl with the surface Cu being slig htly more oxidised than the bulk absorbed Cu. On the basis of the combined XPS, SIMS, XAFS and solution studies, a model is proposed that, on surface adsorption of Cu, the surface Zn(II) atoms are replaced by Cu(II) atoms whi ch are then reduced in situ to Cu(ls). This reduction is accompanied by the oxidation of the three neighbouring S atoms to an oxidation state of appro ximately - 1.5. On bulk absorption of Cu atoms into the sphalerite lattice a distorted trigonal planar configuration is achieved through the breakage of a formerly tetrahedral Zn-S bond. The breakage of this bond results in a 3-fold coordinated Cu plus one S 3-fold coordinated to Zn atoms. The break age of this bond leads to a greater reduction of the Cu than on surface abs orption and also oxidation of the 3-fold coordinated S atom to an approxima tely -0.5 oxidation state. This model does not invoke any polysulfite or S- S bonded species to explain the higher binding energy components of the S(2 p) XPS spectra. (C) 1999 Elsevier Science B.V. All rights reserved.