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Mining

Microbes that mine

The University of São Paulo (USP) and Vale conduct research to discover microorganisms that can extract copper from ore

MARCELO COELHO / VALETailings pond at the Sossego mine, Canaã dos Carajás municipality in the state Pará, that receives the residues generated through copper beneficiationMARCELO COELHO / VALE

The strategy of employing microorganisms to recover copper from rock waste brought the University of São Paulo, the Vale mining company, and the National Bank for Economic and Social Development (BNDES) to the Sossego mine in the Canaã dos Carajás municipality of the state of Pará, where they embarked on a joint research project.  By exploiting the ability of certain bacteria and fungi to feed on substances found in rock containing copper, USP and Vale researchers hope to facilitate the extraction of the metal. To this end, they are looking in the mine itself for microorganisms that can be used to extract copper from waste.   Besides yielding greater profits from mining, the technology can reduce the environmental impact of the mining itself. The Sossego mine, which Vale opened in 2004, produced 109,000 tons of copper in 2011. Its tailings pond, where samples of fungi and bacteria are collected, contains approximately 90 million tons of 0.07% copper-grade detritus. If this entire mineral content were recovered, Vale could earn gross receipts of approximately US$1.4 billion, more than the entire US$1.2 billion the company invested between 1997 and 2004 to make the mine operational.

With the project still in its initial stages, the researchers’ greatest challenge is finding microorganisms that can solubilize copper and to understand how this process works.  Cláudio Oller, chemical engineer and professor at the USP Polytechnical School and one of the research coordinators of the team that works in collaboration with the Institute of Biomedical Sciences of his university, says “During initial tests, we found approximately 35 microorganisms with bio-mining potential at the Sossego mine tailings dam.”  The dam – a large pond containing 20 million cubic meters of water – receives the waste from the mineral-extraction process through a mixture of water and pulverized rock containing low-grade copper.  “The residual copper in the pond is dissolved and the solid material decanted at the bottom of the dam,” explains Oller.  The team, composed of 20 researchers, including biologists, chemists, and engineers, gathers and selects bacteria and fungi at the dam for classification. Team members also have the task of developing a mineral-extraction technology. Up to now, the microorganisms selected by the researcher have not been revealed for confidentiality purposes.

The project, which is expected to go on for five years, will receive funding in the amount of approximately R$15 million, R$ 3 million of which will be contributed by Vale and R$12 million –  dispersed directly to the University of São Paulo’s Support Foundation (Fusp) – from BNDES.  “This project is part of BNDES’s larger strategy of backing technological innovation in products and processes,” says Márcio Macedo, the bank’s Environmental Division chief, adding that “it is an approach that stimulates innovation in Brazilian universities, benefits businesses and, moreover, favors the environment and society at large.”  According to Luiz Eugênio Mello, Director of the Vale Institute of Technology (ITV), who spoke on behalf of the company, “This is a new project, but not only for Vale –also for the world. Although the so-called ‘bioleaching’ technology has been successfully applied on a commercial scale at a gold mine in Minas Gerais, up to now, as far I know, there isn’t a single commercial operation in progress – not for gold, or for copper, or for any other mineral.” Mello believes that in the future, Vale will use this very technology to recover other minerals the company works with. “That’s the idea,” he says. “However, it’s impossible for us to know at this point whether we’ll succeed with copper, let alone with the other minerals. One step at a time, we will broaden the scope of our efforts,” he adds.

A unique feature of the project is the fact that the research is conducted at the actual mining site. “There are many studies in the field of bioleaching,” notes Oller, “but few advance to the commercial-application stage. In our case, we will be carrying out research in a pilot-tank – one that we developed – at the mine itself. We hope to begin this phase next year, as this will allow us to better assess the activity of the bacteria and fungi already selected by our team.”  Bioleaching technology is in use today on an industrial scale in South Africa and in Chile.

092-095_Extracao de minerios_200_novoAlexandre Affonso

Nuclear pioneering
Research in fields now referred to as  bioremediation and  biomining can lay claim to something of a tradition in Brazil. One of the first scholars to dedicate himself their study was the biologist and former São Paulo State University (Unesp) professor, Oswaldo Garcia Junior, who died in 2010. In the 1980’s, while working at the Brazilian Nuclear Companies – now Brazilian Nuclear Industries (Nuclebrás), the state-run  concession that oversees the country’s nuclear sector – Garcia conducted pioneering studies in Latin America in bioleaching. Garcia’s achievements ranged from bench-runs to a pilot project for uranium extraction utilizing a bacterial process. Denise Bevilaqua, the researcher’s widow and Unesp professor who continues her husband’s work, says “Oswaldo gained international renown for his research.  He developed and patented a uranium-extraction method using bacteria. In 1986, he came to the university and established a research program at the Unesp Institute of Chemistry at Araraquara.”

Professor Bevilaqua’s research team concentrates primarily on the bio-mining of copper, whereby the mineral is recovered from chalcopyrite, the world’s chief source of copper.  To extract the copper mineral, Bevilaqua employs oxidizing microorganisms from iron and sulfur, especially the Acidithiobacillus ferrooxidans bacterium.  “We’re the only research team in Brazil that keeps a cell-bank of these bacteria strains, all of which were examined at the molecular level by the team of Professor Laura Ottoboni at the State University of Campinas (Unicamp),” says Bevilaqua. “We were able to increase our rate of carbon extraction from 30% to 60% in bench-tests and hope to improve these figures,” she adds.

According to Bevilaqua, bioleaching presents advantages over conventional mining techniques. Because nothing is burned in the process, no gases are released that would pollute the atmosphere. In addition, bioleaching technology is relatively easy to apply and its operating costs are much lower than that of traditional thermal metallurgical methods, by which the mineral-rich material is burned in high-temperature ovens until the metal is released. Once the metal liquifies, it is extracted before it returns to its solid form. A number of research teams throughout the world, teams usually associated with large mining interests like Chile’s Codelco or BHP Billiton in Australia, look to chalcopyrite deposits for an efficient and economically viable technology, but they have yet to find a solution that can be applied on a commercial scale. “Chile, the world’s leading copper producer, is at the forefront in metals-extraction through bioleaching, but is using only other copper ores like calcocite, covelite and bornite,” says Bevilaqua. Though the most abundant copper-ore on earth, the problem with chalcopyrite is that it is also the most resistant to chemicals and microbes.

A fundamental difference between the research conducted at Unesp into recovering the mineral from solid waste, and the Vale/USP project that investigates liquid-waste alternatives, lies in the type of waste each uses in extracting copper.  While the USP researchers look to the copper that is diluted in a liquid environment in the tailings dams, the Unesp team attempts to recover the residual copper embedded in heaps of solid waste. These gigantic mounds, each containing thousands of tons of pulverized rock, are collected at the mines themselves from crude ore deposits that are less than 0.3% copper grade. For processing to be viable, the ore should have a copper grade between 0.3% and 1%. Conventional thermal methods do not make sense, then, given the low-grade of the copper. This is where the miner-microbes come into play.

Besides the operations in the mines themselves, mining companies and the Unesp Institute of Chemistry at Araraquara have put microorganisms to use in recovery operations involving industrial effluents that contain precious rare-earth metals, lanthanide chemical elements used as raw material in the manufacture of screens for tablets and smartphones. Professor Sandra Sponchiado conducts research on fungi that recover metals through the biomass generated by microorganisms with a unique capacity to attach themselves to these same metals.  “The focus of my research is the biosorption of metals by utilizing a mutant strain of the Aspergillus nidulans fungus, as well as to create optimum conditions for this process to occur,” says Sponchiado. “We want to assess the use of this biomass for future recovery efforts of rare-earths from the effluents of industries that extract these metals,” the Unesp researcher adds.  The process technology can be considered an “environmentally-correct” in view of the reduced chemical and biological waste generated when compared to alternative recovery processes.

Although the research is still in its academic stage, it has sparked the interest of the marketplace. “Several mining companies, including the National Steel Corporation, Nuclebrás and a mining interest in Manaus, have contacted us concerning practical applications for the new technology,” says Sponchiado.

Project
Chalcopyrite (CuFeS2) bioleaching: bacteria/mineral surface mechanism and interactions (nº 2011/19868-5); Coordinator Denise Bevilaqua – Unesp; Grant mechanism Regular line of research project award; Investment R$ 51,470.55 and US$ 73,676.31 (FAPESP)

Scientific article
BEVILAQUA, D. et al. Utilization of electrochemical impedance spectroscopy for monitoring bornite (Cu5FeS4) oxidation by Acidithiobacillus ferrooxidans. Minerals Engineering. v. 22, p. 254-62. 2009.

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