Biooxidation of refractory arsenic-bearing gold ores
Author(s)
Nyashanu, Ridge Mukorera
Abstract
During recent years bacterial oxidation has gained popularity as a pre-treatment process for refractory gold-bearing arsenical ores. Research and development has been focused on various aspects of bacterial oxidation of refractory arsenical gold ores because the majority of these ores are ones in which gold is locked in mineral sulphides pyrite and arsenopyrite. However bioleaching of these ores produce a mixture of arsenic(III) and arsenic(V). Salts of the latter are much more stable for subsequent disposal of the waste residues and therefore it is important that all the arsenic reports as arsenic(V) at the end of the oxidation stage. The bacterial leaching and mineralogy of refractory arsenic-bearing gold ores from different deposits were investigated to determine the fate of arsenic during processing and to assess the effect of biological pre-treatment on gold extraction. Experiments were conducted using a mixed culture of iron and sulphur oxidising bacteria dominated by Thiobacillusferrooxidans. The speciation of arsenic during bioleaching of arsenopyriterich concentrates was studied using polarography. Leaching of a concentrate which was predominantly arsenopyrite resulted in 78% of the leached arsenic reporting in solution as arsenic(III) at the end of the bacterial oxidation stage. It has been found that adding pyrite and iron(III) to arsenopyrite concentrates promotes the formation of arsenic(V) leaving less than 1% of the extracted arsenic as arsenic(III). Study of the oxidation of concentrates from different ore deposits with various ratios of pyrite to arsenopyrite showed that the amount of pyrite was an important factor in determining the dominating arsenic oxidation state in the leach solutions at the end of the oxidation stage. These results demonstrate the importance of ore composition in terms of pyrite to arsenopyrite ratio in the production of environmentally acceptable biooxidation residues containing arsenic(V). Iron(III) alone was found not to be able to oxidise arsenic(III), instead pyrite was needed as a catalyst. Furthermore a strong dependence of pyrite leaching and arsenic(III) oxidation was demonstrated. Actual mineral degradation during the oxidation process has been studied using optical microscopy and a scanning electron microscope (SEM). SEM and optical microscopy examination of the biooxidation residues revealed the development of corrosion patterns and pores oriented along the crystallographic planes in some of the pyrite grain. A study of the stability of the arsenical solid residues emanating from the bacterial oxidation of these arsenical concentrates has also been incorporated. The composition of the residues was examined with acid treatment of the bioresidues, chemical, and x-ray diffraction (XRD) analysis. Arsenic solubility of the solid under study were evaluated using the standard United States Environmental Protection Agency (U.S. EPA) Toxicity Characteristic Leaching Procedure (TCLP) test. The stability of the solid bacterial oxidation residues was found to be directly related to the mineralogical composition of the feed concentrate, in particular the ratio of pyrite to arsenopyrite and also the extent of arsenic extraction and oxidation during the bacterial oxidation stage. The potential of biooxidation for the production of environmental acceptable residues from arsenical gold ores has been shown.
Version
Open Access
Date Awarded
1998
Copyright Statement
Attribution-Non Commercial-No Derivatives 4.0 International Licence (CC BY-NC-ND)
Advisor
Monhemius, A.J.
Buchanan, D.L.
Sponsor
Institution of Mining and Metallurgy; Stanley Elmore Fellowship, the ORS award committee; the Authur Bensusan Memorial Trust.
Publisher Department
Department of Earth Science & Engineering