Grand Challenges for Bioleaching
Yuandong Liu, Guanzhou Qiu*, Weimin Zeng
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China
Key Laboratory of Biometallurgy of Ministry of Education, Changsha 410083, China
*Corresponding author at: Key Laboratory of Biohydrometallurgy of Ministry of Education, Changsha, Hunan 410083, China. Tel.: +86 731 88877472.
E-mail address: email@example.com (Guanzhou Qiu)。
Abstract: With the development of human activities, the needs for non-renewable metal resources are larger and the demands for environment protection are higher, bioleaching is an ideal alternative to traditional technologies. However, bioleaching is a complex procedure with diversities of mineral raw, microbes, chemical reactions, operating conditions, which give many challenges in fundamental researches, industrial applications, exploitation range, advanced biotechnologies transfer and so on. Its final goal is to efficiently improve the metal exploit ratio to unlock mineral resources value.
Key Words: bioleaching; biotechnology; mineral resource; utilization
The first United Nations Conference on the Human Environment (UNCHE) was held in Stockholm, Sweden in 1972. The UNCHE emphasized that rational utilization of resources and proper protection of environments must become a goal to be pursued by people in whole world . In the aspect of utilization resources, the problem of non-renewable metal mineral resources utilization is particularly prominent, because with the economic development, the demand for various metals are increasing, however, with the reduction of high-grade ore resources, the metal grade for mining are gradually cut down, the conventional methods and equipment, which involve many expensive steps such as roasting and smelting and require sufficient concentrations of elements in ores, are difficult to meet the requirements of the development of mineral resources. For instance, with the transfer of processing industry in the world, the consumption of copper in China in 2008 was 6,937,000 tons, while only 951,000 tons of copper was produced by themselves with current technologies . So, from now on we should be prepared to develop new theories and new methods for the mineral processing area of the next century . Bioleaching uses bacteria to extract precious metals, such as copper, gold, from ores in which it is embedded . It is low temperature, environmentally friendly, simple, capable to processing low-grade ores, and so on. Therefore, bioleaching is an ideal alternative to smelting or roasting to unlock mineral resources value.
The system of bioleaching mainly includes minerals, microbes, solutions and air aspects. Generally, it has the four diversities of mineral kinds, microbial species, biological and chemical reactions, and operating conditions.
- The diversity of mineral raw materials.
The minerals in a bioleaching system usually mainly include: the primary ore and secondary ore for all kinds of sulfide minerals, such as pyrite, chalcopyrite, chalcocite, bornite, covellite and so on; the various types of minerals in oxidation zone; Gangue mineral.
- The diversity of microbial species.
The microorganisms used in bioleaching mainly include bacteria, archaea and fungi. According to the difference of their growth temperature, they can be grouped into mesophiles, moderate thermophiles and extreme thermophiles. However, we now know that mining biotopes show a great microbial diversity with at least 11 putative prokaryotic divisions living at acid mine drainage sites . During the course of bioleaching, they change with the leaching time and space changes, and each play a role in mineral decomposition. In one same area, different sampling points are also differences in microbial populations.
- The diversity of biological and chemical reactions.
The reactions mainly include: oxidation of dissolved sulfide ore (including the S2- and Fe2+ oxidation); alternate mineralization reactions for solutes and some reduction reactions (for instances, Ｃu２+ →Ｃu+ ，Ｆe３+ →Ｆe２+ ); acid-base neutralization reaction for dissolved minerals in oxidation zone (consumption of acid reaction); produce acid reaction of oxidized and dissolved sulfide minerals (lower pH of slurry).
(4) The diversity of operating conditions for leaching procedure.
For heap leaching, the operating conditions mainly include methods of building stack, heap geometry, heap size, permeability, moisture, sprinkler system, ore hardness, mass of reserves, etc; for tank leaching, the operating conditions mainly include pulp density, Cu species solubility, acid balance, Fe balance, heat generation, temperature, pH, potential, hydrodynamic parameters of mixing, amenability of ore to a bio-process; toxicity issues; microbe populations, etc.
The diversities of bioleaching make its procedures to become very complex and difficult to understand and control. So, it is full of challenges in the fundamental researches, industrial applications, exploitation range, advanced biotechnologies transfer of bioleaching.
In the fundamental research filed, the bioleaching mechanism, the actions among minerals, solution and microorganisms, the genetic composition and function relevant with bioleaching of the microorganisms are still not well understood. Some key points must be researched: the cooperative interactions of metallurgical microbes; the regulation of iron and sulfur metabolic system of microbes; chemical – biological mechanism of microbe-mineral multiphase interfacial actions; characteristics and succession rules of microbes in leaching process; the relevance of microbial populations and leaching environmental parameters.
In the industrial applications, the low bioleaching rate and the long industrial production cycle are difficult to attract the investment from the conventional fields. The bacterial leaching process is very slow compared to smelting. This brings in less profit as well as introducing a significant delay in cash flow for new plants. Currently it is more economical to smelt copper ore rather than to use bioleaching, since the concentration of copper in its ore is generally quite high. For some types of metal, such as copper, bioleaching is not always economically feasible or fast enough, even with its low cost. Moreover, how to maintain and control different microbial populations within these massive bioreactors to ensure effective leaching minerals to generate heat isn’t an easy thing. Heap bioleaching for low-grade, complex, polymetallic ores with differing mineralogy is also challenge.
In the exploitation range, successful industry application for bioleaching are only limited several metals, such as copper, uranium, gold, nickel at present. bioleaching for a lot of metals are still in its infancy with the preliminary results of some pilot tests having only recently been presented. Bio-treating for Ore, sheet ore, waste rock, slag, combustion ash, recovery of secondary metals, industrial electronics waste, sludge and so on also have not become commercial realities. So, a lot of works for bioleaching application must be done. However, bioleaching is not a simple process for application, it need cooperation of diverse disciplines: geologists – define the ore; mining engineers – recover the ore and design the heap; metallurgical engineers – process the ore heap leaching; microbiologists – evaluate potential for bioleaching; environmental engineers – evaluate “downstream” processes.
Moreover, biotechnology in the world has gotten tremendous progress, some advanced technologies, such as genomics, proteomics, metabolomics, bioinformatics, have been successfully applied in many fields. They are also very helpful to the research and application of bioleaching, such as functional gene detection, microbial community monitoring and so on. Dianzuo Wang thinked that some new technologies from microbial genomics will have significant impacts on biohydrometallurgy research and industrial applications, and will greatly advance our knowledge and capability in the future biohydrometallurgy .
These four aspects grand challenges for the future are certainly not the only ones, but they demonstrate that the unifying perspectives and big-picture questions of bioleaching reach beyond disciplinary boundaries.
The basic theory for conventional exploit methods of metal resources, such as smelting, hydrometallurgy or roasting, mainly includes physics, chemistry, which limits the ratio of metal resources extraction, so lots of resources are wasted and cannot be utilized. Correspondingly, the basic theory for the novel exploit methods of metal resources, biohydrometallurgy or bioleaching, is combination of physics, chemistry, biology and information, which can greatly increase the ratio of metal resources extraction, so lots of metal mineral resources can be transferred into metal reserves. Then, through improving comprehensive utilization ratio from reserves to materials and recycling from regrown materials, the utilization for the whole metal mineral resources will be further more improved. Figure 1 is the flow of comparison between traditional and novel methods in exploit ratio estimation, which show that the novel way can greatly improve resource utilization ratio and increase nearly 3 times relative to the traditional method.
Figure1. The final target for bioleaching
(Comparison between traditional and novel methods in exploit ratio estimation)
So, for the future, to support the economic sustainable development for the demand of metals, we must further develop bioleaching and the final goal for challenges of bioleaching is to improve mineral resource utilization ratio to realize the target of figure 1.
This work was supported by National Basic Research Program of China (Grant No. 2010CB630900) and National Natural Science Foundation of China (Grant No. 50904080).
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