Copper bioleaching, a sustainable way to develop copper industries

Increasing in copper price in international markets causes a great interest in the production ofthis metal as an important material with a vast application. In mining industry, chalcopyrite(CuFeS۲) is the most abundant and refractory copper sulphide mineral that is the source ofmore than ۷۰% of produced copper in the world.Presently, pyro metallurgical methods are preferred to produce copper from sulphideconcentrates containing chalcopyrite as the main copper sulphide. Hydrometallurgicalalternatives are under consideration by copper producer. Hydrometallurgical methods are saferthan pyro metallurgical methods regarding the environmental concerns. In recent decades,bacterial leaching of sulphide minerals has been extensively taken into consideration, as it is aprocess with low capital and operating costs and environmental benefits. This technology isnot applied for chalcopyrite, because of its natural refractory against the chemical andbiological leaching that decrease the rate of dissolution.Bioleaching consists of the application of sulfur and iron oxidizing microorganisms foraccelerating the dissolution of valuable metal species, like copper, zinc, cobalt or nickel fromsulfide ores and flotation concentrates. Oxidation of metal bearing ores is accomplished bychemical reaction with ferric ions and/or protons (equations ۱ and ۲).Microorganisms have catalytic effect and reproduce oxidation agent. It means they oxidizeferrous into ferric ions (equations ۳); they also have the ability of oxidizing elemental sulfur tosulfate and producing acid (equations ۴).(...)Bacterial leaching of metal sulfides is used in commercial scale and has a lot of economic benefits in comparison with other technologies such as high pressure oxidation. Bioleaching of metal bearing sulfides includes agitated tank, heap and dump leaching. Selection of the leaching process is affected by grain size and ore grade. Historically, chalcopyrite has proved a stubborn candidate for obtaining high recoveries using normal bioleaching practices, either in ore heap leaching or using agitated reactors for concentrate treatment.Metal extraction from high grade ores causes the decrease in these resources. So, the metal extraction from low grade ores has been necessary. Large amount of low grad ores is produced every year while the usage of common processing methods is not economically viable because of the low concentration; therefore, other techniques must be employed. Bioleaching is an economical and a simple technique which attracts many attentions in mining industries. Leaching in heaps offers some advantages like simple equipment, minor investment and operational costs and reasonable process recovery; hence they are suitable for low grade ore treatment. Bioleaching of low grade chalcopyrite ore (۰.۲۴% Cu as chalcopyrite, and ۲.۱۹% Fe as pyrite) have been investigated using moderate thermophilic bacteria in the column reactors. The results showed that ۶۹.۶۸% of copper were extracted during ۶ months by the use of particles finer than ۱۲.۰۷mm. To develop this technology to industrial scale, a ۱۰۰۰ tone pilot heap was constructed. The ore contained ۰.۲۹۸% Cu as chalcopyrite, and ۴.۶% Fe as pyrite. Over than ۵۰% copper recovery was achieved in ۱۲۰ days of optimum operation.Over the past two decades, the optimization of bioleaching processes for the treatment of chalcopyrite ores and concentrates has been the subject of many research programs. Using a flotation concentrate containing ۴۶% chalcopyrite ۲۳% pyrite, the high pulp density (۱۵%) bioleaching tests were carried out at ۴۷⁰C using a set up consisted of ۳ continuous stirred tank bioreactors in series. To increase the copper recovery in contrast to the conventional bioleaching (~۳۹.۶۲%), the effect of redox potential on the chalcopyrite bioleaching was investigated by electrochemically controlled bioleaching. The results showed that by controlling the redox potential, faster copper leach kinetics could be achieved. At last, reducing the redox potential from high levels to optimum window (۴۲۰-۴۴۰mV SCE) caused an increase in copper recovery from around ۳۹% to higher than ۶۹% (over ۲۵ g/L Cu۲+)

کلیدواژه ها:

bioleaching ، Heap and Tank leaching ، Chalcopyrite ، Moderate thermophiles