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Mining with microbes

Bacteria are used to recover valuable metals from electronic scrap and mine tailings

Photos Eduardo Cesar Illustrations Pedro HamdanBrazil is one of the world champions in e-waste generation.  A recent study by the Brazilian Agency for Industrial Development (ABDI) of the federal government found that about 1 million tons of electronic scrap, from computer monitors, cell phones, printers and cameras, among other equipment, is discarded each year in the country. Only a small portion is recycled because current techniques that make recycling feasible are costly and polluting. This situation may change if a method to recover metals such as copper and gold in printed circuit boards—the greenish glass fiber boards present in most electronic devices—can be made sustainable in both the economic and environmental sense. The technique, known as bio-hydrometallurgy, was developed by a group of Brazilian researchers and uses bacteria harmless to humans to extract the metal in these boards during one of its stages.

“Bacteria are already being used to bioprocess metals in mines or to recover metallic tailings in dams. Our idea was to use the method to recover copper from scrap,” says Jorge Tenório, a professor of metallurgical engineering at the Polytechnic School of the University of São Paulo (USP). By comparison, Tenório says the copper ore mined by Vale at its mines has a concentration of less than 1% copper, while a printed circuit board contains about 30% copper (see Pesquisa FAPESP Issue No. 200, October 2012).

Currently the copper and other metals in printed circuit boards can be reused using chemical processes that employ acids to extract them, or using a pyrometallurgical process in which the metals are recovered at high temperatures, resulting in the emission of greenhouse gases. “The advantage of our technique is that it is cheaper than conventional methods and does not harm the environment,” says Luciana Yamane, who did her doctorate in Tenório’s group with a grant from FAPESP. Her dissertation, Recuperação de metais de placas de circuito impresso de computadores obsoletos através de processo bio-hidrometalúrgico (Recovery of metals from printed circuit boards from obsolete computers through a bio-hydrometallurgical process) won an honorable mention for the Dow-USP Sustainability Innovation Award in 2012. In this process, Yamane explains, the first step is the mechanical processing of the circuit boards. They are shredded and ground in a mill into grains up to 2 mm in diameter. Then, a magnetic separator is used to remove the parts containing iron and nickel. “We only work with non-magnetic waste, which is where the copper is,” says Yamane. The next step is to add the grains of circuit board to an aqueous solution containing a soluble form of iron (ferrous ion or Fe+2). When the LR strain of the bacterium Acidithiobacillus ferrooxidans is inoculated into this medium, it oxidizes the ferrous ion, turning it into the ferric ion (Fe+3). This eventually oxidizes the copper, which is released from the granules of the board and dissolved in the solution—a process known as bioleaching. The final step, separating the solubilized copper, is performed by already-established processes.

Ana Paula Campos

Yamane’s greatest challenge was how to condition the microorganisms, whose natural habitat are rocks containing iron, to survive and reproduce in the liquid medium containing the grains of circuit boards. “When we added the shredded boards to the culture medium containing the bacteria, they died. Certain board components, such as fiberglass, resins and ceramics, are toxic to them,” says Yamane. The solution was to let the microorganism adapt to the boards slowly. “We started by mixing 1.25 grams of circuit board to each liter of solution containing the bacteria. We selected the resistant microorganisms, increased their population and raised the concentration. We repeated this process several times until, at the end of the adaptive stage, we were able to add 28 grams of circuit board per liter. The higher the concentration, the more productive the copper recovery process is. This means that more boards can be processed at once,” says Yamane. The novelty of the process led the researchers to file a patent application with the National Industrial Property Institute (INPI).

According to Prof. Jorge Tenório, the bio-hydrometallurgical process permits the extraction of 99% of the copper present in the printed circuit board powder. Interestingly, the initial objective of the research was not to simply recover copper from printed circuit boards. The intention was to create a sequence of steps that would ultimately leave only gold embedded in the crushed boards. This metal is also present in printed circuit boards, but in a low concentration of 0.01%. This may seem to be an insignificant level, but 1 metric ton of boards contains 100 grams of gold. “However, cyanidation, a method for extracting gold, cannot occur in the presence of other metals, especially copper. Hence the importance of first recovering the copper, then extracting the gold from the boards,” says Yamane.

Raw materials from which copper is recovered: computer boards and rocks such as chalcopyrite and malachite

Eduardo CesarRaw materials from which copper is recovered: computer boards and rocks such as chalcopyrite and malachiteEduardo Cesar

Mining company
The recovery of metals from waste rock with the same bacterium, A. ferrooxidans, including gold, copper, nickel and cobalt, was the motivation that led a group of researchers who had graduated from the Nuclear and Energy Research Institute (IPEN), the Institute of Chemistry of Universidade Estadual Paulista (Unesp), in Araraquara, the Mineral Technology Center (Cetem) and the USP Chemistry Institute to establish a start-up named Itatijuca Biotech in the incubator at the Center for Innovation, Entrepreneurship and Technology (Cietec) on the USP Butantã campus in March 2013. “We use bioleaching to recover metals in ores, reducing waste and the environmental impact of mining,” says chemist Érico Perrella, a partner at Itatijuca. “We offer a unique service in Brazil.” According to Perrella, there is no company in Brazil offering mineral processing and recovery of metals in mining tailings using bioleaching, a technique already used commercially in other countries, such as Chile and South Africa.

Due to her extensive knowledge of the bacterium A. ferrooxidans, Prof. Denise Bevilaqua, of Unesp- Araraquara, provides consulting services to the company. The bacteria feed on substances in the rocks in which the metal is embedded, facilitating its recovery. According to Perrella, bioleaching is a biotechnological alternative to conventional mineral processing methods, which release large amounts of carbon dioxide (CO2), sulfur dioxide and various other toxic materials into the environment, including arsenic. In addition to the environmental benefit, the new technology enables the processing of low-concentration ores, which are not economically viable when extraction is performed using traditional methods. During bioleaching, the pile of ore is continuously “irrigated” with a solution containing the bacterium that solubilizes the metals in it. This process is continuous, and when the metals amenable to bioleaching are removed, the embedded gold can be recovered.

Accessible gold
Another service offered by Itatijuca is a cyanide treatment that reduces the impact of gold mining. Cyanidation is a technique used on ore and waste rock to make the residual gold accessible. “Imagine a pile of ore containing gold and copper. With bioleaching, we remove the copper. Then, gold is recovered using cyanidation. But that pile of waste ends up with a high concentration of cyanide, which is a highly toxic substance—1.25 grams of it can kill a person. So we are developing a biotechnological treatment with a different bacterium, which we prefer not to name, to neutralize the cyanide and, at the same time, generate an environmentally innocuous and commercially valuable by-product from the tailings. This is what we call cyanide treatment,” explains Fábio Elias, a partner in Itatijuca. The cyanide neutralization process will lead to a patent filing.

The company is signing its first contract, to recover gold from an old mine in Minas Gerais that has a pyramid-shaped pile of tailings 200 m long by about 75 m wide and 6 m tall. “We will use bioleaching and other chemical processes such as cyanidation to recover the metals from the pile, which is out in the open,” says Elias. He says that the company expects to obtain profits of R$29 million per year starting in 2016 if certain conditions are met. “The business will achieve these profit levels if we recover metals in a mine with 350,000 metric tons of tailings, containing 4% copper and 2 parts per million (ppm) of gold.

If all goes well, the researchers at Itatijuca are interested in entering into another area, bioleaching of phosphate, a technique very similar to that used with metal ores. The main difference is that, in this case, it is used for commercial production of fertilizers. “Brazil currently imports a large amount of phosphate ore for use in agriculture, and this technique would help improve domestic production, which is low,” says Perrella. “We’re doing an initial survey to create a commercial process using bioleaching with fungi, rather than bacteria.” Itatijuca’s main challenge will be cost. Since phosphate has a low market value, to be economically viable the technique must process large volumes in short periods of time.

Recovery of gold from printed circuit boards from obsolete computers using a bio-hydrometallurgical process (nº 2010/51009-0); Grant Mechanism Regular Line of Research Project Award; Coord. Jorge Soares Tenório, USP; Investment R$19,550.00 (FAPESP).