Reductive-oxidative pretreatment of a stibnite flotation concentrate: Thermodynamic and kinetic considerations

The thermodynamics and kinetics of reductively decomposing stibnite (Sb2S3), a refractory sulphide, using an acidified solution of chromium(II) ions in a chloride medium (for Au recovery) have been considered. Results show that well over 80% reduction of stibnite to antimony (Sb) and hydrogen sulphide (H2S) is achieved, the extent of reduction being pH and reductant concentration dependent. The contrasting temperature sensitivity above and below 90°C (363K) suggests that two rate controlling modes are operative: diffusion in the range 60–90°C (333–363 K) and chemical reaction above 90°C. Although up to 80% reduction of the sulphide host mineral (Sb2S3) occurred, very little gold (<16%) was liberated. A sequential reductive-oxidative pretreatment of the stibnite ore achieved 90% gold liberation. This strongly suggests that in the stibnitic ores, the sulphide species are not wholly responsible for the reported refractories. Gold may be in submicroscopic form as finely disseminated fine particles or in solid solution as aurostibnite (Au3S) which requires further oxidative treatment after the sulphide decomposition.