Study on Cu2S mineral surface modification by low temperature Ar/O2 plasmas

Abstract

The oxidation of mineral and synthetic chalcocite (Cu2S), using low temperature plasmas, has been investigated and compared to thermodynamic calculations. The main aim of this work is to understand the fundamental interaction between mineral surfaces and low temperature plasmas, with a view to improving froth flotation by mineral pretreatments. Capacitive radio frequency (RF) discharge Ar/O2 plasmas, operating at different external parameters, have been used to treat powder samples, which resulted in surface modifications. These were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). In situ mass spectrometry (MS) was used to determine oxidation reaction rates. The energy flux density from the plasma on the sample surface was determined by active thermal probe measurements, and the density of atomic oxygen produced within the plasma zone was obtained by optical emission spectroscopy (OES). Sulfur dioxide (SO2), one reaction product of mineral Cu2S plasma oxidation, was emitted with a certain delay, which depends on energy flux density and atomic oxygen density. In contrast, no delay was found as synthetic Cu2S was treated. This indicates that the contamination by pyrite (FeS2), found in mineral samples, plays an important role, significantly influencing the mechanisms of plasma surface interaction. Comparisons of mass spectrometry (MS)-, XPS- and XRD-measurement results with thermodynamic calculations give evidence for a stepwise plasma processing, whereas the transition of sulfur atoms from FeS2 to Cu2S could be identified as a first step in forming cupric sulfate ( CuSO4). This effect might be used to develop selective plasma surface pretreatments for mineral mixtures in order to improve their separation efficiency of froth flotation.