The hydrometallurgical ferro-reductive decomposition of stibnite, galena and arsenopyrite/pyrite gold bearing flotation concentrates.

Abstract

The thesis describes experimental investigations on the reductive decomposition of three sulphide minerals, namely (a) stibnite (Sb2S3), a gold and silver bearing antimony sulphide; (b) galena (PbS), a mainly silver bearing lead sulphide; (c) arsenopyrite/pyrite (FeAsS/FeSi), n gold <wid silver bearing iron/arsenic sulphide; using elemental or metallic iron as the reducing agent. The first two sulphide minerals, stibnite and galena, where studied in both hydrochloric and sulphuric acid media whilst the arsenopyrite/pyrite flotation concentrate was only studied in the hydrochloric acid medium. The main objective was to establish and confirm experimentally, the thermodynamic and kinetic feasibility of reductively decomposing the sulphide matrix for precious metals recovery. For this purpose, cyanidation leach tests were conducted on the reductive leach residues. The thermodynamic and kinetic feasibility of stibnite decomposition has been demonstrated in both media. The reaction system was found to proceed via both the reductive and non - oxidative chemical decomposition reactions, producing elemental antimony, dissolved iron and the evolution of hydrogen sulphide gas. In the case of the non - oxidative chemical dissolution, a complexing - hydrolysis mechanism was suggested to describe the dissolution of stibnite that is otherwise not thermodynamically feasible under standard conditions of temperature and pressure. Desulphurisation levels in excess of 95% were achieved in the hydrochloric acid medium. Faster decomposition rates were confined to the first 60 minutes of the reaction, after which a rapid decrease in rates occurred. The reaction kinetics were also analysed using the unreacted shrinking core model and the reaction system was found to be mainly of mixed control. While both desulphurisation and the decomposition kinetics increased with decreasing pH and increasing iron to concentrate ratio, an apparent optimum combination of the variable parameters was established as pH values around 0.44 and iron to concentrate iatio ot 0.7. There was a strong interaction between pH and iron to concentrate ratio, which made the optimisation of the process complex. The complexity was due to the competing hydrogen evolution side reaction that consumed both iron and hydrogen ions. In the sulphuric acid medium, desulphurisation levels (< 40%) and leaching rates were significantly low relative to those achieved in the hydrochloric acid medium. The reaction mechanism was similar to that observed in the hydrochloric acid system. In both media, stibnite and iron formed a galvanic cell, where they formed a decomposing cathode and anode, respectively. Cyanidation leach tests of the reductive leach residues extracted only 27% gold even after 98% desulphurisation. This represents a 16% improvement in gold recovery relative to the 11% achieved without prior desulphurisation. The results demonstrated that the proposed reductive decomposition of stibnite as a pretreatment process for precious metals recovery is not as efficient as envisaged. Galena decomposition in both hydrochloric and sulphuric acid media followed both the reductive and non oxidative chemical decomposition reactions. In the hydrochloric acid medium, desulphurisation levels in excess of 98% were achievable. Chemical dissolution was aided by the chloride ion complexing effect, which also increased the decomposition kinetics of the system. Generally faster leaching rates during the first 60 minutes characterised the galena decomposition in the hydrochloric acid medium as compared to the sulphuric acid medium. Also, in the sulphuric acid medium, relatively low ( < 90%) desulphurisation levels were achieved and the chemical decomposition reaction proceeded with the formation of a sparingly soluble lead sulphate precipitate. The reaction system showed an inverse and direct relationship with pH and iron to concentrate ratio, respectively. As in the stibnite reaction system, pH and iron to concentrate ratio had a strong interactive behavior and their influences on the reaction system were always contradicting. The application of the unreacted shrinking core model showed that galena decomposition is of mixed control. Metallic or elemental lead from the reductive leach reaction subsequently dissolved with the formation of lead chloro - complexes thus the reaction depended on the rate of chemical reaction at the reaction surface. Gold and silver extraction from the reductive leach residue, had a direct relationship with desulphurisation levels and this increased (extraction) significantly to values in excess of 70% for both precious metals. These results evidently demonstrated that the sulphide matrix was solely responsible for the refractory nature of galena.The arsenopyrite/pyrite flotation concentrate, mainly studied in the hydrochloric acid medium, decomposed through the non - oxidative chemical dissolution reaction and the reductive decomposition reaction for the arsenopyrite and pynte components, respectively. Desulphurisation levels below 65% were achieved at pH values below 0.1 5 and iron to concentrate ratios above 1. Overall, the system was characterised by very slow kinetics although faster leaching rates were confined to the first 30 minutes of the reaction. The reaction system had a direct linear relationship with iron to concentrate ratio and an inverse relationship with pH. An analysis of the pyrite/iion galvanic system showed that pyrite forms a partially inert cathodic surface on which the anodic dissolution of iron occurs supported by the hydrogen evolution reaction. This phenomenon explained the low desulphurisation levels and the mineral decomposition seemed to be restricted to the non - oxidative chemical dissolution reaction for the arsenopyrite component. The cyanide leach of the reductive leach residues showed very little improvement in gold recovery. A 10% increase from 5% gold extraction prior to reductive decomposition to 15% after around 64% desulphurisation characterised the process. The reductive leach process for arsenopyrite/pyrite, has considerable limitations in terms of both desulphurisation and precious metals liberation.On the overall, the reductive decomposition of the three minerals clearly shows that the sulphide matrix is solely responsible for the refractoriness in galena but this is not true for stibnite and arsenopyrite/pyrite minerals. In the latter, gold occurs as invisible gold. This gold exits either in solid solution or chemical compounds such as aurostibnite (Au3Sb). In such circumstances a subsequent pretreatment process for the complete dissolution of the reductive leach residue becomes necessary.