Effects of Al3+ on thermochemical sulphate reduction (TSR) at 250°C to 350°C under vapour-saturated pressures: A Raman spectroscopic investigation

Abstract

In geological processes, thermochemical sulphate reduction (TSR) is a significant way to transform oxidising sulphur into reducing sulphur, such as H₂S, that can promote the formation of metal sulphide deposits. The occurrence of TSR is a complex process, where all kinds of sulphate, organic matter, and catalysis materials are involved, in which Al exists commonly in geological background. In order to figure out the function of Al in the TSR process, a series of experiments were conducted to investigate the TSR by using fused silica capillary capsules combined with Raman spectroscopy at temperatures ranging from 250°C to 350°C in this study. Ethanol (cracked into ethylene by heating) or acetic acid was used as reducing agents, and sodium sulphate or magnesium sulphate as oxidising agents, and the AlCl₃ was introduced as a variable to investigate its effect on the initiation of TSR. Raman spectra were collected from the quenched and in-situ experiments. The results indicate that the addition of AlCl₃ favours the initiation of TSR. In-situ Raman investigation reveals that HSO₄^- is the dominant sulphate species involved in TSR under our experimental conditions. This facilitating effect of AlCl₃ on TSR has been attributed to the increased acidity in solution caused by the release of H⁺ through the formation of Al³⁺-bearing minerals such as natroalunite, where the released H⁺ combines with SO₄^2- to form HSO₄^-. And SO₂ was detected as an intermediate product during the reduction of HSO₄^- by in situ Raman spectroscopy. The experimental results imply that it is possible that the TSR can occur and accumulate enough reduced sulphur in a short period of time in an aluminium-rich geological environment at temperatures as low as 250°C.