Dec '11/Jan '12

Winning electrowinning

Barrick’s Zaldivar refinery puts Optibar technology through its paces

By G. Lanktree

An Optibar intercell bar installed on one of Compañia Minera Zaldivar's refining cells in northern Chile

More than 40,000 tonnes of copper have been refined in a year-long test run of Optibar Inc.’s new technology at Barrick Gold’s Zaldivar copper plant in northern Chile. The new segmented intercell bars being used in the Compañia Minera Zaldivar refinery have slashed energy costs, reduced short circuits during refining and allowed the production of heftier, purer sheets of copper – gains that all promise to boost the plant’s margins.

A copper refining tank house containing the cell circuits typically consumes 2,000 kWh of electricity to produce a single tonne of grade-A copper. Yet, monitoring the energy efficiency of the Optibar technology after increasing the weight of harvested copper sheets, the project’s engineers recorded a 92 kWh-per-tonne cut in energy consumption, a savings of 4.6 per cent. “Today, with the power prices we have, Optibar technology represents a potential savings for the whole plant of US$1.5 million per year,” says Robert Mayne-Nicholls, executive general manager of Barrick Gold’s Chilean operations.

The savings generated by a single circuit weighs in at US$375,000 per year. Installing one will cost US$650,000, Mayne-Nicholls says, but that investment is paid back in less than three years. There are other savings to be made as well, he adds. “We think that Optibar will also extend the life of cathodes, anodes and capping boards.” The technology promises to do this by reducing metallurgical short circuits during the refining process.

“High-intensity short circuits cause deterioration of anodes, copper contacts and capping boards,” says Eduardo Wiechmann, Optibar’s general manager and the principal researcher of the experimental technology. “They also impact production, energy consumption and the quality of cathode sheets.

Even current yields higher quality copper

During the electrowinning process used to refine high-grade copper, copper ions dissolved in an electrolyte and charged with a direct electrical current attach to cathode surfaces. Traditional Walker intercell bars, which run the length of the many cathode-anode pairs that make a cell, disperse the electrical current unevenly throughout the refining cell array. This results in significant weight differences between finished cathode sheets and favours the creation of short circuits, reducing the copper’s quality and leading to frequent replacement of parts. Optibar’s segmented bars force the current to be balanced between all cathode-anode pairs throughout the circuit and to use preferred electrical paths. Because the current is distributed evenly, current densities can be increased and harvest times extended, which produces larger, higher quality sheets of copper.

“The new bars were installed progressively during regular maintenance of the copper refining cells starting in October 2010,” says Wiechmann. The modifications made to one of four 92-cell circuits in the plant’s refining tank house did not disrupt production, he explains, because Optibar simply modified the original Walker intercell bars connecting cathodes and anodes. “Not even a single modification to cells, electrodes, cranes, lifting devices, short circuit frames or other equipment was required,” he wrote in a paper describing the technology.

Working at full capacity, the Zaldivar refinery produces 140,000 tonnes of copper cathode sheets each year. To power production, the plant uses 280 GWh of electricity generated by coal power plants at an annual cost of US$42 million.

Testing demonstrated that the optimal harvest cycle for Walker bars is 110 hours, with a final cathode sheet weight of 39 kilograms. With the same current density, Optibar produced 42-kilogram cathodes in 116 hours. “Short-circuit occurrences are down one-third and their intensity also to one-third,” says Wiechmann. “Therefore, their overall impact on the process is only one-ninth of what it was with Walker bars.”

New technology is no replacement for routine maintenance

Optibar’s technology is not the only way that refineries can achieve better performance. “We started measuring our operational practices during the tests,” says Mayne-Nicholls, who notes improved housekeeping, such as the regular maintenance and cleaning of intercell bars, anode-cathode hanger bars, cell structures and electrode contacts, can have a large impact. When these routines were applied to Walker intercell bars, engineers measured a reduction in energy consumption from 1,985 to 1,965 kWh per tonne and saw fewer short circuits occur throughout the cells.

Even with these results in hand, Mayne-Nicholls says the Zaldivar plant will postpone installation of Optibar in its remaining circuits. “We have learned that the main savings with Optibar are generated with higher current densities,” he explains. “The original goal was to reach a reduction in power consumption from two to five per cent. At the beginning, we almost reached five per cent but since then, we have consistently applied a two per cent savings.” Next year, the plant will continue to use the same current density. With that in mind, further installation of Optibar will be delayed until the plant’s current density is increased. “Despite this,” Mayne-Nicholls says, “we strongly recommend that all new electrowinning facilities be fitted with Optibar from the beginning.”

Refining the science

The positive results at Zaldivar contrast with some previous modelling that had been done. Two research papers, which were published before the device was installed in the Zaldivar plant, challenged the work done by Optibar’s Wiechmann and his team at the University of Concepción.

In a 2009 paper, Finnish researcher Ilkka Laitinen, of the Tampere University of Technology, found that the Walker system offers uniform distribution of an electrical current, while Optibar’s is much more uneven. And in a 2010 paper, researchers Anthony Blackett and Michael Nicol of Australia’s Murdoch University had success replicating Optibar’s current distribution, but found that during simulated short circuits, the technology passed the short circuit effect to adjacent copper cathodes.

Laitinen, Blackett and Nicol’s research had one flaw that prevented them from predicting realistic results, says Wiechmann. “Their mathematical models did not include a compensating bar used to interconnect both ends of the intercell bar,” an element that he says is crucial.

Until now, mathematical models were all that Optibar had, but with the positive results from Barrick Gold’s Zaldivar copper works, Wiechmann says that Optibar is close to a deal to test the technology at BHP Billiton’s Spence plant nearby. He believes this is just the beginning, because the technology can also be applied to zinc refineries and, he explains, his team is working to further increase energy efficiency by an additional eight per cent. However, to get the most out of this new technology, Wiechmann says plant designers need to consider its use from the very beginning, “Full advantage of Optibar technology requires more freedom during the design stage of a new project.”

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