Bioleaching of chalcopyrite
This research is concerned with the bioleaching of chalcopyrite (CuFeS\(_2\)) by Thiobacillus ferrooxidans (ATCC 19859), which has been carried out in shake flasks (250 ml) and a 4-litre stirred tank bioreactor. The effects of experimental factors such as initial pH, particle size, pulp density and shake flask speed have been studied in shake flasks by employing cell suspensions in the chalcopyrite concentrate with the ATCC 64 medium in the absence of added ferrous ions. The characterisation of T. ferrooxidans on chalcopyrite concentrate was examined by investigating the adsorption isotherm and electrophoretic mobility. Subsequently, a mechanism for copper dissolution was proposed by employing relevant experiments, including the chemical leaching of chalcopyrite by sulphuric acid and ferric sulphate solutions, bioleaching of chalcopyrite in the presence of added ferric ions, and cell attachment analysis by scanning electron microscopy. Following the above, the work then focused on the bioleaching of the chalcopyrite concentrate in a stirred tank bioreactor for the purpose of scaling up, and investigated the effects of agitation speeds. Finally, the bioleaching of low-grade copper ores has been briefly studied. The results show that the rate of copper dissolution has a positive relationship with bacterial growth, particularly with respect to bacterial attachment, which has an important role based on adsorption isotherm and scanning electron microscopy studies. However, it is not only bacteria that play an important role in copper dissolution; also the strength of sulphuric acid can influence copper solubility. For example, copper dissolution can be achieved using a sulphuric acid solution of pH 1.5, giving a concentration of about 1 g/l copper after 25 days. The results obtained from the adsorption isotherm of T. ferrooxidans and the electrophoretic mobility of chalcopyrite particles before and after interaction with each other has proved the fact that the changes in surface chemistry occurred when bacterial interactions on the mineral surface took place. Furthermore, agitation speed have a significant influence on cell growth, metal dissolution and cell adsorption ratio when carried out in shake flasks and a stirred tank bioreactor. The bioleaching results for different shake flask speeds (i.e. 100, 200 and 300 rpm) in shake flasks displayed that shake flask speed above 100 rpm was detrimental to bacterial growth and thus copper dissolution. For the bioreactor experiments, agitation was performed within a rotor speed range of 50, 100, 150 and 200 rpm. A rotor speed of 150 rpm represents the most suitable conditions for bacterial growth and the percent extraction of copper dissolution amongst those considered. In conclusion, the concentration of copper dissolution for all pulp densities reached its maximum at a concentration of 4.8 ± 0.2 g/l after 30 days leaching time. This indicted that copper dissolution has a limited solubility; this may be because the chalcopyrite particle surface was covered by mineral and bacterial deposits over the period of bioleaching time as described in the SEM analysis of the bioleaching surface. Finally, this work attempted to extract copper from a low-grade ore using bioleaching techniques. However, initial bioleaching tests proved that T. ferrooxidans could not leach copper and iron from the low-grade copper ores due to the chemical composition of the gangue minerals (mainly carbonates). This is due to the neutralising action of carbonates, which create an environment in which the pH is too high for the acidophilic bacteria to grow.