SIMS studies of oxidation mechanisms and polysulfide formation in reacted sulfide surfaces

doi:10.1016/S0892-6875(00)00074-1

Minerals Engineering

Volume 13, Issues 8–9, August 2000, Pages 857-870

https://doi.org/10.1016/S0892-6875(00)00074-1 Get rights and content

Abstract

The surface oxidation of metal sulfides in air and aqueous solution is of central importance in mineral separation and environmental control of acid mine drainage. Mechanisms of oxidation, dissolution and surface restructuring have been extensively studied using XPS. High binding energy components in S 2p XPS spectra have been attributed to metal-deficiency, formation of polysulfide S_n²⁻, elemental sulfur and electronic defect structures (ie Cu(I)/ZnS). The assignment of these components in S 2p XPS spectra has, however, left significant uncertainties particularly in the formation of SS bonding in polysulfide species requiring confirmation from other surface analytical techniques.

The use of static ToF-SIMS has provided a new avenue for identification of these species and their development in oxidation of the sulfide surfaces. For the iron sulfides, there is a systematic change in the FeS₂/FeS fragment ratio from troilite (FeS) through pyrrhotite (Fe_1−xS) to pyrite (FeS₂) with ratios varying from 0.59, 1.2 to 32 respectively. Similarly, high ratios for FeS_n/FeS are found for pyrite compared with pyrrhotite and troilite mirrored in the S_n/S fragment ratios. Changes in surface oxidation, represented in atomic concentrations and S 2p XPS spectra, are seen in the ToF-SIMS signals for S_n/SO_n ratios in the same iron sulfide sequence. These mass markers, reflecting increased SS bonding, increase in surfaces after oxidation giving further confidence in XPS assignment to polysulfide species.

Freshly cleaved galena PbS surfaces reacted in pH8 aqueous solution for increasing periods of time have also shown a systematic increase in S_n/S ratios with increasing at.% of oxidised S_n²⁻ species from XPS spectra. Statistical analysis of oxidised galena has shown that the ratios ²⁰⁶PbO⁺/²⁰⁶Pb⁺ and ²⁰⁸PbOH⁺/²⁰⁸Pb⁺ directly reflect the degree of oxidation of the surface lead species whilst the O⁻/S⁻, S⁻/total — ion yield and SO₃⁻/S⁻ are the best measures for following the oxidation of sulfur species. Results from these ratios suggest that initial air oxidation takes place predominantly on the S sites rather than Pb sites but, in solution at pH9, both sites are oxidised.

The ToF-SIMS results appear to directly reflect the surface chemistry of the metal and sulfur species and are not consistent with recombination or fragmentation of secondary neutral or ionic species. The results strongly suggest increasing polymerisation of SS species with increasing oxidation in accord with the XPS assignment to polysulfide of increasing chain length.

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^☆: Presented at Flotation 2000, Adelaide, Australia, March 2000

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SIMS studies of oxidation mechanisms and polysulfide formation in reacted sulfide surfaces☆

Abstract

Applied Surface Science

Hydrometallurgy

Applied Surface Science

J. Electroanalytical Chemistry

Geochimica Cosmochimica Acta

Inorganica Chimica Acta

Geochimica Cosmochimica Acta