This review describes the historical development and current state of metals leaching and sulfide mineral biooxidation by the minerals industries. During the past 20 years commercial processes employing microorganisms for mineral recovery have progressed from rather uncontrolled copper dump leaching to mineral oxidation and leaching in designed bioheaps for oxidation of refractory gold ores and for copper recovery. Also during this period of time, stirred tank bioleaching has been commercialized for cobalt recovery and for biooxidation of refractory gold ores. Chalcopyrite bioleaching in stirred tanks is on the verge of commercialization. Commercial applications of biohydrometallurgy have advanced due to favorable process economics and, in some cases, reduced environmental problems compared to conventional metal recovery processes such as smelting. Process development has included recognition of the importance of aeration of bioheaps, and improvements in stirred tank reactor design and operation. Concurrently, knowledge of the key microorganisms involved in these processes has advanced, aided by advances in molecular biology to characterize microbial populations.

Column testing was conducted to evaluate the potential for copper extraction from composited samples of Newmont Mining Corporation's Yanacocha Verde deposit in Peru. The Mineral Liberation Analysis (MLA) revealed that the composites contained various amount of copper sulfide minerals. Comparative bioleaching was conducted with consortia of mesophilic iron-oxidizing bacteria containing species of Acidithiobacillus and Leptospirillum for bioleaching at ambient laboratory temperature (20–22°C) and with a mix of a thermophilic archaea culture containing Acidianus, Metallospheara and Sulfolobus at 65°C. Enargite- and covellite-rich composites exhibited low copper extraction ranging from 7.3% to 27.1% based on column head/residue assays from columns after more than 300days of bioleaching at room temperature with mesophilic bacteria. Chalcocite rich composites responded to mesophilic bacteria bioleaching with extractions ranging from 50 to 90% based on head/residue assays. High copper recovery from secondary chalcocite and low copper recovery from the primary copper minerals enargite and covellite are consistent with the performance of mesophilic bacteria. Thermophilic archaea proved advantageous for bioleaching primary enargite and covellite at 65°C with copper extractions ranging from 60 to 98% over 335–346days. Copper extractions from chalcocite-rich composites using thermophilic archaea were similar to those with room temperature mesophilic bacteria bioleaching. Semiquantitative XRD analysis and Mineral Liberation Analysis (MLA) were performed for mineralogical investigation with respective heads, and mesophilic and thermophilic residues. MLA confirmed only chalcocite leached under mesophilic conditions with partial leaching of covellite; and all copper minerals were leached under thermophilic conditions.

The importance of comprehensive laboratory evaluation for development of an ore body to commercial processing using biohydrometallurgy cannot be understated. Laboratory evaluation for a biohydrometallurgical process must include the microbiological component and definition of operating parameters for the engineers to design the commercial plant. Failure to meet commercial production at a mine site can be a consequence of incomplete understanding of biohydrometallurgical technologies for processing a specific ore. One example is the inability of a copper bioleach process to meet the design criteria in part because of lack of sufficient testing to demonstrate the ramifications of fluoride toxicity to the microbial component of the bioleach process. Laboratory research has demonstrated toxicity of low levels of fluoride to Acidithiobacillus species. However, laboratory determined toxicity values are not always relevant to field conditions at commercial bioleach operations. This is the case with fluoride toxicity where complexing reactions increase the amount of fluoride required for toxicity. Consequently, the toxic fluoride concentrations at field sites can be significantly higher than toxic levels reported in the laboratory, but still achieve concentration inhibitory for the microorganisms.

The importance of comprehensive laboratory evaluation for development of an ore body to commercial processing using biohydrometallurgy cannot be understated. Laboratory evaluation for a biohydrometallurgical process must include the microbiological component and definition of operating parameters for the engineers to design the commercial plant. Failure to meet commercial production at a mine site can be a consequence of incomplete understanding of biohydrometallurgical technologies for processing a specific ore. One example is the inability of a copper bioleach process to meet the design criteria in part because of lack of sufficient testing to demonstrate the ramifications of fluoride toxicity to the microbial component of the bioleach process. Laboratory research has demonstrated toxicity of low levels of fluoride to Acidithiobacillus species. However, laboratory determined toxicity values are not always relevant to field conditions at commercial bioleach operations. This is the case with fluoride toxicity where complexing reactions increase the amount of fluoride required for toxicity. Consequently, the toxic fluoride concentrations at field sites can be significantly higher than toxic levels reported in the laboratory, but still achieve concentration inhibitory for the microorganisms.

My perceptions of the biohydrometallurgical field span four decades and stem from being a professional microbiologist conducting academic research and research for process development and applications. My experiences have given me an appreciation for knowledge gained through fundamental research and the transfer of this knowledge to development of commercial scale applications of microbial processes.The symposia series for international activities in biohydrometallurgy has been a major factor in advancing knowledge and applications for microbial bioleach systems. The first international biohydrometallurgy meeting was held in Braunschweig, Germany in 1977. This was the predecessor for the International Biohydrometallurgy Symposia. As evident from the Symposia, advances in development and applications of biohydrometallurgy technologies follow an evolutionary, rather than revolutionary progression from demonstration of knowledge at the laboratory scale to engineering commercial plants.

This paper reports evaluation of a composite sample from Newmont Mining Corporation's Kupfertal deposit located at the Peruvian Minera Yanacocha mining operation to amenability for use of bioleaching to extract the copper occurring primarily as the mineral covellite (CuS). Column testing was performed to compare bioleaching at ambient mesophilic temperature (20–23 °C) using Acidithiobacillus/Leptospirillum/Sulfobacillus species and at elevated thermophilic temperature (60–65 °C) conditions with archaea Acidianus and Metallospheara. Over a period of 346 days, bioleaching copper extraction by mesophilic bacteria ranged from 12 to 20%, determined by residue assays. Increasing the temperature of bioleaching to 60–65 °C increased copper extraction to 62–65%. Copper extraction was continuing to increase at termination of the column tests. The advantage of use of thermophilic archaea for bioleaching covellite is consistent with their use for bioleaching the highly refractory chalcopyrite (CuFeS2). This paper discusses the comparative conditions and results for bioleaching of covellite.

Thiocyanate was evaluated for gold recovery from low-grade refractory gold ores following biooxidation for an acidic heap leaching approach. One of the important features in using thiocyanate is that the leaching can be performed in acidic media, thus avoiding problems related to neutralization and material handling as leaching with alkaline cyanide. Also, the acidic leaching enables the use of ferric ion or bioleach solution as oxidants. In this study, Newmont-Nevada low-grade refractory sulfidic ore was biooxidized in columns at a particle size of 100% passing 102 mm. Thiocyanate leach on the biooxidized ore sample was performed on both ground samples in pulps and coarse material in columns using ferric ion as an oxidant. The variables of thiocyanate and ferric ion concentrations were compared for effect on leaching. Thiocyanate leach results are encouraging and indicate potential for development of a practical process. Gold extractions by thiocyanate are comparable to cyanide leach. Thiocyanate consumption was higher than cyanide, but was in an acceptable level.

Operation of a commercial-scale biooxidation heap for pretreatment of refractory gold ores demonstrates the propensity of heaps to heat concurrent with pyrite oxidation. Heap temperatures can reach 75 °C. The high temperature is lethal for mesophilic iron-oxidizing bacteria. Column testing was conducted to compare effects on biooxidation pretreatment of a sulfidic refractory gold ore and the microbes' response to temperature when bacteria and archaea were grown at different temperatures. Five columns were operated at a different temperature regime: 20–23 °C; 35 °C; 50 °C; 60 °C; and one column varied from 20–23 to 60 °C. For the variable temperature column, the temperature was increased stepwise from 20–23 °C, to 35 °C, to 50 °C, to 60 °C in 2-week increments; after 2 weeks at 60 °C, the temperature was decreased every 2 weeks in the same stepwise manner. The following inocula were used: (1) columns at 20–23 and 35 °C, mixed culture of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans; (2) column at 50 °C, moderate-thermophilic iron-oxidizing Sulfobacillus-type bacteria; (3) column at 60 °C, hyper-thermophilic archaea, Acidianus and Metallospheara species; and (4) an equal mix of all of the above for the variable temperature column. The hyper-thermophiles did not increase in numbers until the temperature was increased to 50 °C and above. Lowering the temperature resulted in a decrease in population density of this group of microbes. The moderately thermophilic bacteria increased in number until the temperature was raised to 60 °C; at that point, numbers decreased and remained stable as further decrease in temperature occurred. Decimation of the mesophilic iron-oxidizing bacteria occurred when the temperature was increased to 50 and 60 °C. A low-level population concentration was detectable at increased temperatures attributable to their presence in the ambient temperature solution reservoir. However, this population increased as the temperature was lowered. There was little difference in the amount of sulfide oxidation at either ambient room temperature or 35 °C. Increasing the biooxidation temperature from 35 to 50 °C increased apparent sulfide oxidation from about 38% to 48%. At 60 °C, sulfide oxidation was highest at about 51%. The variable temperature column also had higher sulfide oxidation, 41%, attributable to periods of high temperature biooxidation. The data suggest that there is little difference among the groups of microbes used in this study in terms of sulfide oxidation and improved gold recovery, with some apparent slight advantage in using the hyper-thermophilic microbes.

This review describes the historical development and current state of metals leaching and sulfide mineral biooxidation by the minerals industries. During the past 20 years commercial processes employing microorganisms for mineral recovery have progressed from rather uncontrolled copper dump leaching to mineral oxidation and leaching in designed bioheaps for oxidation of refractory gold ores and for copper recovery. Also during this period of time, stirred tank bioleaching has been commercialized for cobalt recovery and for biooxidation of refractory gold ores. Chalcopyrite bioleaching in stirred tanks is on the verge of commercialization. Commercial applications of biohydrometallurgy have advanced due to favorable process economics and, in some cases, reduced environmental problems compared to conventional metal recovery processes such as smelting. Process development has included recognition of the importance of aeration of bioheaps, and improvements in stirred tank reactor design and operation. Concurrently, knowledge of the key microorganisms involved in these processes has advanced, aided by advances in molecular biology to characterize microbial populations.

The acidophilic thermophilic archaebacteria Sulfolobus and Acidianus have the potential for applid use in the recovery of metal values from ores through the process of baterial leaching. These microbes readily adapt to the conditions of low pH and high concentrations of metals required for bacterial leaching. In addition, these archaebacteria can exist at high temperatures which can occur during the oxidation of metal sulfides in bioleaching reactors. The acidophilic of copper and molybdenum from chalcopyrite and molybdenite minerals, respectively. The microbes can also enhance the recovery of gold by oxidation of pyrite which occludes gold preventing recovery by standard metallurgical procedures. The ability of this group of microbes to facilitate metals recovery is yet to be developed on a commercial scale.

Biotechnology is an alternative process for the extraction of metals, the beneficiation of ores, and the recovery of metals from aqueous systems. Currently, microbial-based processes are used for leaching copper and uranium, enhancing the recovery of gold from refractory ores, and treating industrial wastewater to recover metal values. Future developments, emanating from fundamental and applied research and advances through genetic engineering, are expected to increase the use and efficiency of these biotechnological processes.

Revue concernant les donnees actuelles sur la lixiviation bacterienne des metaux a partir des minerais et la description des technologies microbiennes en developpement pour leur recuperation