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Magnetic extraction

Sustainable and with lower pollution levels, new process uses nanoparticles to capture and produce metals

Superparamagnetic nanoparticles bound to copper salts in an aqueous solution are attracted to a magnet outside the container

Superparamagnetic nanoparticles bound to copper salts in an aqueous solution are attracted to a magnet outside the container

A new technology for processing ore, developed at the University of São Paulo (USP), could revolutionize mining, especially copper extraction. Called magnetic nanohydrometallurgy (MNHM), it is 100% Brazilian and considered “green” because the environmental impact is minimal compared to other industry practices. The technique is performed in an aqueous medium, using supermagnetic nanoparticles instead of the organic solvents such as kerosene, for example, that are typically used in conventional processes. Coordinated by chemist Henrique Toma, professor at the USP Institute of Chemistry (IQ-USP), the article summarizing the group’s work that led to the development of the technology was published in the journal Green Chemistry and resulted in a patent application.

Developed by Ulysses Condomitti as part of his PhD research, carried out under Toma’s supervision and defended at IQ-USP, the new process uses nanotechnology to renew and perfect a procedure already used by the mining industry called hydrometallurgy. “Among the advantages of nanohydrometallurgy is the ability to perform all of the steps in a single sequential procedure, in the same reactor operating under ambient conditions, eliminating traditional extraction using organic solvents, acid treatments and concentration stages, in addition to reducing the production of tailings,” explains Toma. “The metal-capturing nanoparticles fully regenerate after the procedure, without further treatment.”

The new process developed at USP is an alternative to conventional hydrometallurgy, which starts with leaching, a procedure for separating minerals using acid or, alternatively, bacteria, followed by chemical treatment with metal-complexing agents, which form compounds with metals. The difference is that the chemical complexing agent and the solvent used for the extraction of metal are replaced with superparamagnetic nanoparticles controlled by a magnetic field produced using magnetite (iron oxide). They are mixed in an aqueous solution rich in copper salts. After a few minutes, the dissolved copper ions (Cu2+) attach to the nanoparticles through the complexing agent found on their surface, and they are attracted to an electrode, also immersed in the solution, because of the force of a magnet located outside the solution. Thus, the nanoparticles with the copper salts become concentrated on the surface of the electrode. Then electricity is applied to the electrode, which causes the migration and transfer of the electrons from the copper ions to the electrode, generating metallic copper (Cu).

After about five minutes of electrical current, the nanoparticles release almost all of the metal that had attached to them. “Electrodeposition provides copper with a 99.9% purity level,” says Toma. “The nanoparticles, in turn, can be used again.” According to Toma, the experiment conducted in the laboratory proved to be advantageous when compared to traditional hydrometallurgy processes in terms of process time. Using nanotechnology, metallic copper can be obtained in just a few minutes, while the current industrial process takes seven days.

Another advantage is that all processing can take place in the same container (see infographic). The particles are attracted to the metal plates of the copper electrodes, and electrodeposition takes place in a localized manner. There is no need to transport the material to another reactor, as in conventional hydrometallurgy. This means cost reduction, process simplification and greater reasonableness. “Magnetic nanohydrometallurgy also generates less waste,” says Toma. “Automation may also be possible, using smaller reactors.” According to the researcher, the new technique could also be used to process various strategic metals, including rare earth metals, and this is being studied at IQ-USP. The group’s next step is to transfer the technology to a company. He says that two have already shown interest: 3M and Caraíbas, the only mining company in Brazil that produces metallic copper, although the quantities are insufficient for the domestic market.

Toma lists other arguments to demonstrate the significance of the new nanotechnology, beginning with the importance of copper—the main metal to which the new process applies—which is fundamental in sectors such as construction, the electrical industry, machines, transportation and electronics. In the specific case of Brazil, although the country is rich in minerals, it does not have a lot of copper. “Chile has the largest reserves of this element, with 38% of the total,” Toma says. “Brazil has about 1.5% and does not have ore rich in this element. What exists has a low percentage of copper, about 4% to 5%, but this is enough for the new technology. Worldwide, according to the chemist, mining of copper—and even other metals—is facing some problems now. Among them are the gradual depletion of high-grade ore and the increase in environmental impact caused by both extraction and processing.

Supramolecular chemistry and nanotechnology (No. 2013/24725-4); Grant Mechanism: Thematic Project; Principal investigator: Henrique Toma (USP); Investment: R$940,870.28 (FAPESP).

Scientific article
Toma, H. E. Magnetic nanohydrometallurgy: a nanotechnological approach to elemental sustainability. Green Chemistry. Published on-line on February 12, 2015.