August 2012

Mill mods save money

Barrick’s systematic approach to comminution efficiency

By Alexandra Lopez-Pacheco

Primary ore grinding accounts for about 55 per cent of Barrick Gold’s electricity consumption and is the source of over 1.7 million tonnes of greenhouse gasses per year. Because of numbers like that, a team headed by Danny Hillier, senior manager of metallurgy at Barrick Gold Australia Pacific, decided to tackle comminution as part of the company’s energy efficiency efforts in 2007. That work has now paid off, saving the company over $15 million a year in energy.

But before any savings could be achieved, Barrick needed to overcome some major hurdles, says Lloyd Buckingham, Barrick Gold’s manager of metallurgy. “We didn’t have any effective standards to measure against,” he explains, “and that’s critical. If you don’t measure against the standard, you don’t know if you’re doing better, but the standard has to be specific to the ore you’re grinding in order to benchmark your capabilities.”

Fred Bond, an American engineer renowned for his contributions to the science of ore crushing and grinding, developed a set of standards in the 1950s, but those are not suited to the complexities of modern milling.

“In the last 20 years or so, SAG mills have absolutely dominated the industry and they weren’t around when Bond was around,” says Stephen Morrell, managing director at SMCC Pty Ltd., a comminution consulting company based in Australia. “Some people have attempted to modify Bond’s equations to apply to the machines we have today, but to my mind they weren’t adequate.”

Over the last 15 years, Morrell has gathered data from more than 130 plants across the globe to develop new rock tests and equations for standards based on modern equipment. His intention was primarily to use these standards for his work as a consultant in the design of new plants. Barrick’s team, however, saw the potential to use Morrell’s equations to benchmark en­ergy efficiency in Barrick’s existing plants. “I thought it was a novel way to take one tool for new plants and use it to assess existing plants,” says Morrell. But Morrell’s equations only measure a mill’s overall energy efficiency, based on rock tests in the laboratory. For Barrick, that was just one piece of the puzzle. The company wanted other tools to evaluate energy efficiency at its mills with far more detail and ­precision.

Appropriate measures

For this, Barrick turned to Robert McIvor from Metcom Technologies Inc. McIvor is a metallurgist and grinding systems specialist who has developed a method to measure energy efficiency in grinding circuits.

His method is based on the discovery that there are two independent efficiencies in a grinding circuit: the percentage of the mill energy used on coarse particles versus fine particles, and how well the energy being applied to the coarse particles is being used, based on comparisons to standards.

McIvor recognized that circuit production rates play a role in these two efficiencies. When the circulating load – or the ratio of the amount of solids going through the ball mill to the amount of solids going through the circuit – is low, the ball mill is full of fines, resulting in wasted energy. A high-circulating load, on the other hand, is full of coarse material so the mill wastes little energy on the fines and thus is far more efficient.

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