Biohydrometallurgical techniques are quite established for the treatment of certain sulphide minerals and others have been demonstrated at laboratory or pilot plant scale level, where iron and sulphuroxidising bacteria are utilised for the leaching of low grade sulphide ores and the pretreatment of gold ores and concentrates. The majority of the microorganisms which are used for bacterial leaching are autotrophic aerobes, therefore carbon dioxide and oxygen are essential nutrients for their growth and survival, adequate supplies must be ensured in order for the oxidation process to be successful through an effective gas mass transfer process from air or other sources to the gas/liquid/solids interface in the pulp which can then be readily available to the microorganisms. Gas mass transfer limitation is one of the major problems faced by the industry therefore, most commercial biooxidation plants currently operate at low pulp densities at around 20 - 25% to minimise gas mass transfer limitations. Three cultures, a mesophile (DSM 583), a moderate thermophile (Y1FI) and a extreme thermophile iSulfolobus BC65) were used to bioleach the Ok Tedi copper concentrate under optimum Ch and CO2 enriched air (30% Ch and 10% CO2) based on volume ratios. The positive effect of using oxygen and carbon dioxide enriched air to bioleach the copper mineral was clearly demonstrated where an increase in Cu leaching rate by a factor of2.8, 2.1 and 1.9 for DSM 583, YTFI and Sulfolobus respectively were observed compared with the shake flask tests. The lag phase observed with the other test conditions such as in shake flask or with air, was eliminated when leached under optimum gas enrichment and 1t;80%of the Cu was extracted in the first 120 to 150 hours of leaching for all three cultures. The average dissolved oxygen concentrations observed in the pulp for these tests were 11.8, 8.0 and 7.2 mgll for DSM 583, YTFI and Sulfolobus respectively, indicating that O2 was not limiting in these tests. The Fe released during the bioleaching of copper was non stoichiometric for a predominantly chalcopyrite concentrate and the low redox potential values (max 638mV) suggested galvanic interaction during the leaching of copper where chalcopyrite was leached preferentially due to the presence of pyrite. Bioleaching of copper under optimum gas enrichment at different pulp densities showed near total copper extraction occurred for pulp densities of 3, 10 and 20% (w/v). At high pulp densities, more so at 40% than 30% (w/v), the leaching rate decreased rapidly for tests with YTFI and Sulfolobus. Monitoring of dissolved Ch shows that O2 demand increased with increasing pulp density and the average dissolved O2 concentration observed for DSM 583, YTFl and Sulfolobus at 40% solids concentration were 3.0, 2.6 and 2.4 mgll respectively, suggesting that oxygen and carbon dioxide are not limiting. The decrease in leaching rate observed at high pulp densities (30 & 40%) under optimum gas enrichment was accompanied by low metal dissolution and low redox potential values which was attributed to shear stress due to attrition of cells by high solids concentration, toxicity of metal ions especially Ag, Fe and Cu, and the formation of precipitates and jarosites as confirmed from XRD analysis of leach residues hence impeding the leaching process rather than as a result of O2 and C~ limiting conditions. The addition of flowing air increased the overall iron release rate for the Porgera gold-bearing pyrite concentrate by a factor of AJ 2.8, 1.8 and 1.5 for DSM 583, YTFl and Sulfolobus respectively compared with the shake flask tests, and the lag time was observed to decrease by about 80% for all cultures, suggesting O2 and CO2 limiting conditions in the shake flask. The average dissolved ~ concentration observed for the pulp with the different cultures were 8.7, 5.8 and 5.0 mgll for DSM 583, YTFI and Sulfolobus respectively indicating oxygen was not limiting in these tests. Biooxidation of pyrite concentrate under optimum gas enrichment at various pulp density shows DSM 583 performing well exhibiting the highest oxidation rate for all pulp densities, followed by YTFt and Sulfolobus, except at 3% pulp density where the rates are similar for all cultures. At high pulp density, above 30%, the oxidation rate decreased rapidly for YTFt and Sulfolobus, with DSM 583 showing a moderate decrease. Monitoring of dissolved oxygen showed that oxygen demand increased with increasing pulp density and at one point the dissolved oxygen concentration for the test with YTFI at 40% and Sulfolobus at 30 and 40% solids concentration decreased below t.O mgll, suggesting oxygen and carbon dioxide limiting conditions. The decrease in leaching rate at high pulp densities above 30% under optimum gas enrichment was accompanied by low Fe dissolution and low redox potential which was attributed to shear stress due to attrition of cells by high solids concentration, toxicity of metal ions especially Ag, and Fe. and the formation of precipitate andjarosites hence impeding the leaching process. Oxygen and CO2 limiting conditions due to high pulp density and viscosity were only observed for YTFt and Sulfolobus cultures. The cyanide leaching of the pretreated pyrite concentrate shows that the extent of gold and silver recovery increased with the degree of pyrite oxidation and Au and Ag recoveries in excess of 80% were achieved compared with that of only 18.1% and 35.94'10 for gold and silver respectively, from the original. untreated concentrate by direct cyanidation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:302672 |
| Date | January 2000 |
| Creators | Witne, John Yagbinan |
| Publisher | University of Exeter |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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