Influence of mineralogy on biohydrometallurgical processing of complex sulphide ore

Abstract

The mineralogical basis for understanding biohydrometallurgical processing of low-grade sulphide ores and means by which microwave processing improves their microbial recovery is investigated using a Nigerian low-grade complex sulphide ore as a case study. The study is approached through an applied mineralogical study of the ore and its influence on developing an optimal route for the microbial leaching of the low-grade complex ore under varying process parameters, and an investigation on the interaction between mineralogy, microwave processing and bioleaching. Bioleaching behaviour, mechanisms of bioleaching, and the interplay of mineralogy and microwave irradiation and their influence on bioleaching process were carried out using mixed cultures of Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillium ferrooxidans in a mechanically stirred glass reactor at varying bioleaching process parameters and through electrochemical studies. Mineralogical analysis of the ore revealed the presence of siderite, sphalerite, galena, quartz, and traces of pyrite and chalcopyrite, with the ore exhibiting fine to coarse grain intergrowths of the constituent crystalline phases both at the interstitials and the boundaries. Complexity in the mineralogy of the ore affected mineralogical and elemental distribution amongst varying size fractions that led to variation in the galvanic behaviour within these size fractions and influenced microbe-mineral’s reactivity and the different dissolutions behaviours. Bioleaching at optimal bioleaching parameters revealed the highest dissolution at a particle size fraction of 75 μm, while electrochemical studies revealed the highest dissolution at particle size fraction of 106 μm. This discrepancy was consistent with and attributed both to the physical and mineralogical influences. The combined effects of mineralogical variation, precipitation phenomenon as well as the physico-chemical effect of particle size, controlled bioleaching behaviour, while galvanic interaction resulting from variations in mineralogical distribution controlled the electrochemical behaviour. Ore mineralogy and microwave heating both showed dual influences on heating characteristics, size reduction, and the effectiveness of microwave treatment in improving dissolution. The increase in the dissolution rate and the overall dissolution of the microwave treated samples is attributed to phase changes in the ore which promoted galvanic interaction within the system, decrease in the amounts of sulphur contents, and an increase in electrochemical and microbial growth sites resulting from an increase in the number of cracks induced by microwave heating.