The use thermophilic organisms for the recovery of gold and copper from low grade ores

Abstract

Biooxidation studies were carried out on gold ores isolated from Jugan and Pejiru, Sarawak. The gold and arsenic contents of the Pejiru and Jugan ores are 29.4 ppm, 48.7% and 13.5 ppm and 58.29% respectively. Due to its high concentration of gold, the Pejiru ore was then subjected to biooxidation studies using L.ferrooxidans, A.ferrooxidans, mesophilic local isolate, S.thermosulfidiooxidans, A.brierleyi and thermophilic local isolates (5B and C cultures). Significant gold recoveries (39.5, 76.09 and 68.16%) were obtained using L.ferrooxidans, S.thermosulfidiooxidans and A.brierleyi respectively. Low gold recoveries from other strains used could be due to arsenic toxicity. Fe(III) pretreatment of the ores was carried out to study its effectiveness in dissolution of arsenic from the ores. The optimized parameters for Fe(III) leaching is as follow; 0.2 M Fe2(SO4)3, 45°C, pulp density 10% and a stirring speed of 200 rpm. Indeed higher amounts of gold (73.59, 76.49 and 80.4%) were recovered after biooxidation of Fe(III) leached ore using L.ferrooxidans , S.thermosulfidiooxidans and A.brierleyi. Abandoned or disused mines posses an environmental problem because of the possibility of heavy metals presents to be leached out into groundwater or natural river system. This issue needs to be addressed because of the toxicity effects that might affect humans. A possible solution is the use of bioleaching technology to treat the low-grade ores, which are normally left idle. For this study, ores from the disused Sungai Lembing mine in Pahang was subjected to ferric leaching and bioleaching using mesophilic and thermophilic bacteria namely A. ferrooxidans, A. thiooxidans, L. ferrooxidans and S. thermosulfidooxidans respectively. Optimization of ferric leaching was carried out using Fe2(SO4)3 and FeCl3 at concentrations ranging from 0.2 to 1.0 M. The results of the experiment showed that FeCl3 1M was the best solution for the ferric leached of copper with 86.70% Cu leached. The otherwise, result from bioleaching experiment showed that A.ferrooxidans was the highest among the others bacteria with 77.05% Cu was extracted. Parameters optimized during the bioleaching process include Eh, temperature and Cu and Fe solubilization. The copper from the bioleached ores will be recovered using solvent extraction and stripping process. With the increasing need of using semiconductor in this millennium, the disposal of printed circuit board has received much attention from the viewpoints of environmental protection and resource utilization. In this research both bioleaching and chemical leaching process for the recovery of copper in printed circuit board sample and sludge sample of the semiconductor solid wastes were attempted. The copper contents of the printed circuit board and sludge sample are 23.17% and 4.89%. In the bioleaching process, mesophilic bacteria; T.ferrooxidans, L.ferrooxidans, T.thiooxidans and moderate thermophilic/thermophilic; S.thermosulfidiooxidans and A.brierlyi were used. Chemical leaching involves the use of oxidizing agents such as ferric chloride, ferric sulphate, acetic acid, sodium thiosulphate and sodium hypochlorite. In bioleaching of the sludge sample, high copper recoveries were obtained using T.thiooxidans (5.58%) while for printed circuit board sample high recoveries were obtained using L.ferrooxidans (60.84%). Copper in printed circuit board sample and sludge sample were efficiently recovered by ferric chloride leaching (93.30%) and sodium thiosulphate leaching (15.30%) respectively. The gold mining industry is an exhaustive process whereby large amounts of ore have to be processed to extract the metal. Also, limitations of the techniques used in gold processing leads to the incomplete recoveries of gold, which normally ends up in the tailings. In this study, biooxidation using mixed cultures consisting of Thiobacillus thiooxidans (TT), Thiobacillus ferrooxidans (TF), Leptospirillum ferrooxidans (LF) and Caldibacillus ferrivorus (CF) was carried out in a Continuous Stirred Tank Reactor (CSTR) to recover gold from mines tailings. Biooxidation studies were first conducted using shake flasks, where a mixed culture consisting of TT, TF, LF and CF at a ratio of 3:1:1:3 was found to decrease the percentage of preg-robbing by a factor of 3 compared to the control. Biooxidation was also carried out in a bioreactor using the batch and continuous modes. Results from the batch experiments show that the solubilisation of iron for the 3:1:1:3 (TT: TF: LF: CF) cultures is 0.52 times higher than the 1:1:1:1 (TT: TF: LF: CF) cultures and 1.08 times higher than the control. For the continuous mode, the best iron solubilisation was obtained under the following operating conditions i.e. temperature, 38oC; stirring speed, 350 rpm; pulp density, 15%; bacterial ratio, 3:1:1:3 (TT: TF: LF: CF); pH, 2.00 and 3 days residence time. There was also a great reduction in the heavy metal content of the tailings after biooxidation i.e. 83.02% Fe, 53.09% Cd, 65.00% Cu, 30.16%Pb and 54.72% Zn were solubilised from the tailings. The recovery of gold from the biooxidation process was then carried out using cyanide. The highest gold recovery (>95%) was achieved under the following set of conditions; 30% pulp density, 1000-ppm cyanide and with aeration. However, the amounts of gold recovered after electrorefining process was low i.e. 46.71%.