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A study on iron oxidation in leaching of copper ore with pyrite using by mesophilic bacteria
Abstract
Bioleaching/biooxidation processes have been commercially applied for the recovery of copper, gold and uranium for two decades. Concerning these processes by mesophiles and thermophiles, academic and commercial applications have been extensively increasing in laboratory, pilot, full scale operations. Several bacterial species are used in many commercial operations in South America, Australia, South Africa, India, China. In near future Turkish copper and gold mines will probably use these processes as commercial applications due to the economical and environmental reasons. Therefore, the close relationship between biooxidation and cyani-dation with mineralogical composition is important for the commercial selection of these processes. In addition to lab tests, full-scale feasibility studies being performed to determine the impacts of climate and environmental factors for potential mining areas will also be completed in the near future. This paper presents an investigation of the potential bioleaching developments in Turkey.
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The extraction of copper from chalcopyrite has for centuries been limited to pyrometallurgical methods. Smelting of chalcopyrite is an efficient process but costly both in terms of capital investment, operating costs and environmental compliance. Biological extraction appeared as an appealing alternative. Unfortunately, traditional mesophilic biological extraction methods have met with little success. The chalcopyrite quickly becomes passivated and unacceptable copper extractions are achieved. It was not until the adoption of thermophilic systems that the biological leaching of chalcopyrite became a reality. Several questions remain as to the applicability of the thermophilic system for chalcopyrite; can the system operate auto-thermally; can high extraction rates be achieved; is the process sensitive to mineralogy or grade; and can the precious metals be recovered? GeoBiotics, LLC has embarked on an extensive program to develop the GEOCOAT ® bioleaching system to chalcopyrite ores. This program encompasses mathematical heap modeling, laboratory amenability and column tests, and large scale field trials. The GEOCOAT ® process involves the coating of concentrates onto a suitable substrate, usually barren rock, then stacking the coated material in a conventional heap fashion. The heap is irrigated with acidic solutions containing iron and nutrients while low pressure ambient air is applied at the heap base. To-date, copper extractions in excess of 97% have been achieved in approximately 140 days. Excellent gold extractions have been achieved from the biooxidation residue by cyanidation. Modeling indicates that obtaining thermophilic temperatures within the GEOCOAT ® heap is not a problem. Development is continuing, focusing on the heap design parameters and additional copper concentrates including enargite. Plans are now underway for the first large scale field test in the fall of 2002.
Bioleaching/biooxidation is essentially a dissolution process with the involvement of acidophilic bacteria acting as the "catalyst" to accelerate the dissolution of metals from sulfide minerals. The contribution of bacteria to the metal dissolution is closely controlled by the growth of bacteria, which is itself affected by the physico-chemical conditions within the bioleaching environment. There are a number of operating parameters controlling bioleaching processes, which are required to be maintained within a certain range in the leaching environment whereby the activity of bacteria with the resultant oxidation of sulfide minerals can be optimized. In this regard temperature, acidity, oxidizing conditions, availability of nutrients, oxygen and carbon dioxide, surface area and presence of toxic ions are of prime importance for control and optimization of bioleaching of sulfide ores/concentrates. Bioleaching processes are temperature and pH dependent with optimum metal dissolution occurring in a particular range where the bacterial strain is most active e.g. mesophiles at 35-40°C and pH 1.6-2.0. Provision of nutrient salts is required to maintain the optimum growth and hence metal dissolution with the quantity of nutrients apparently being dependent on the availability of substrate i.e. head grade/pulp density of an ore/concentrate. Oxygen transfer is one of the most critical factors since the oxygen levels below 1-2 mg/l may adversely affect the oxidizing activity of bacteria. Bioleaching rate tends to improve with increasing the surface area at low pulp densities but, in practice, the pulp density is limited to ~20% w/v. Increasing concentrations of ions such as Cl -may also adversely affect the oxidative activity of bacteria.
Modern commercial application of biohydrometallurgy for processing ores became reality in the 1950s with the advent of copper bioleaching at the Kennecott Copper Bingham Mine. Early application entailed dump leaching of low-grade, low-value, run-of-mine material. Dump bioleaching has evolved into a commercially accepted option for bioheap copper leaching of higher-grade, higher value ores. This commercial practice is exemplified by at least 11 mining operations. Paradoxically, application of biohydrometallurgy in the pretreatment of refractory gold ores began with processing high value concentrates, using biooxidation-tank processes and was followed by extension to processing low-grade, lower value ores in heaps. Now, bioleaching has been extended to the commercial extraction and recovery of cobalt. Even with the current success of biohydrometallurgical applications in the mining industry, the real potential of biotechnology in mining remains to be realized. As confidence in commercial bioprocessing grows and experience extends the application's knowledge base, innovations and new commercial practices will emerge. Near-term future commercial applications will likely remain focused on recoveries of copper, gold and possibly nickel. Recent technical advances show that very refractory chalcopyrite can be successfully bioleached. Processes for copper recovery from this mineral will include both heap and stirred-tank reactors. Next generation technologies for pretreatment of refractory gold ores will be based on use of thermophilic bacteria for sulfide oxidation. For biohydrometallurgy to commercially advance, the microbiologist must work cooperatively with the practitioners of the technology for mutual understanding of operational limitations and practical constraints affecting the microbiological component. q 2001 Elsevier Science B.V. All rights reserved.
The presence of some anions and cations at certain levels in the bioleaching environment may exert an inhibitory effect on the growth and hence leaching activity of a bacterial culture. In this respect, the quality of process water available with particular reference to salinity can be of prime importance for the application or development of a bioleaching process for a particular feed at an operation site.
The current study investigates the extent to which salinity up to 8% Cl- (~80 g/l) affects the bioleaching activity of mesophilic, moderately and extremely thermophilic strains of bacteria during the bioleaching of a complex Zn/Pb sulphide ore. The results indicated that salinity can adversely influence the “optimum” bioleaching activity of mesophiles and moderate thermophiles; the extent being dependent upon the strain (and type) of bacteria and the concentration of chloride. The mesophilic WJM strain was found to oxidise the complex ore at concentrations of up to 0.8% Cl- (~8 g/l) without any significant effect on the extraction of zinc while the limited extraction of zinc by DSM 583 strain occurred at 0.2% Cl-. It was noted that mesophiles can be adapted to tolerate 0.8-1% Cl- (~8-10 g/l) in solution. The bioleaching ability of the strains of moderate themophiles was adversely influenced even at 0.2% Cl- (~2 g/l). On the other hand, the extreme thermophiles were shown to perform well under saline conditions up to 5% Cl- (~50 g/l). This probably indicates the halophilic peculiarity of the extreme thermophiles compared with the mesophiles and the moderate thermophiles.
Strategies for efficient start-up of a continuous process for biooxidation of refractory gold ore and concentrate obtained from Hutti Gold Mines Limited (HGML), India are discussed in this work. The biooxidation of the concentrate at high pulp density (10%) with wild strain of Thiobacillus ferrooxidans isolated from HGML mines is characterized by significant lag phase (20 days) and incomplete oxidation (35%) even after prolonged operation (60 days). Two strategies, biooxidation with concentrate adapted cells and a step leaching strategy, in which the pulp density is progressively increased from 2% to 10% were considered and the latter resulted in efficient biooxidation of concentrate. Conversion of such a process from batch to continuous operation is shown to result in complete biooxidation of the concentrate and gold extraction efficiency in excess of 90%.
The present article illustrates the increased interest which is manifested in the microorganisms, Thiobacillus ferrooxidans, involved in the biohydrometallurgical extraction processes. The wide varieties of problems currently studied are very important in order to gain a better understanding about the factors which are governing the growth of microorganisms, and as a consequence, the metal dissolution phenomena. In several mining sites, the microbiological leaching techniques are currently practiced at industrial-scale, especially for recovery of copper and uranium from low-grade materials. However, an accurate assessment of further potential possibilities for the application of microorganisms in leaching metal sulfides requires a more fundamental knowledge about the interactions of the physical and chemical factors with the growth of T. ferrooxidans in pure and mixed cultures including heterotrophic and thermophilic cohabitants. Altogether, the future industrial exploitation of these microbiological leaching techniques are very attractive in many countries of the world.
In the history of Turkey the first use of cyanide for gold recovery has been at the Ovacik Gold Mine. During one-year test period, this mine has successfully been mining and processing after a complicated and extensive environmental impact procedure. In Turkey about 2500 ton of sodium cyanide are used with about 240 ton of sodium cyanide being used at this mine annually. During the test period, it has been shown that an effluent quality (CNWAD) between 0.06 ppm (min) and 1 ppm (max) was achievable after cyanide destruction with the Inco Process. It was also found that treated effluent values (CNWAD) of process water (decant) were between 0.04 ppm (min) and 0.59 ppm (max). This paper presents a review of the cyanidation and cyanide destruction processes at the Ovacik Gold Mine.
Biomining is the use of microorganisms to extract metals from sulfide and/or iron-containing ores and mineral concentrates. The iron and sulfide is microbially oxidized to produce ferric iron and sulfuric acid, and these chemicals convert the insoluble sulfides of metals such as copper, nickel and zinc to soluble metal sulfates that can be readily recovered from solution. Although gold is inert to microbial action, microbes can be used to recover gold from certain types of minerals because as they oxidize the ore, they open its structure, thereby allowing gold-solubilizing chemicals such as cyanide to penetrate the mineral. Here, we review a strongly growing microbially-based metal extraction industry, which uses either rapid stirred-tank or slower irrigation technology to recover metals from an increasing range of minerals using a diversity of microbes that grow at a variety of temperatures.
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