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MEI Online: Plant Operation News: Africa: April 8th 2014


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:: Tweaking Stirred Mill Spins Gold from Historical Tailings at New Dawn Mining


In 2010, management at New Dawn Mining, like many miners, had been looking for ways to economically increase the output of their mines. The Turk gold mine, after nearly 80 years of intermittent production, was a good candidate for improvements. There were roughly 400,000 tonnes of historical tailings that could be ground finer and retreated, and the milling circuit looked like it would perform better in general with the addition of a fine grinding mill.

“Turk mine is a sulphide-hosted ore body," says Ian Saunders, president and CEO of New Dawn. “You can liberate the gold and get a reasonable recovery as long as you have the technology to get it to an ultra-fine state at a reasonable cost." But doing that had not always been possible.

Since purchasing the mine in 1996, New Dawn subsidiary Casmyn Mining Zimbabwe has operated conventional ball mills, using 80- to 100-millimetre (mm) steel balls to grind the particularly hard ore (a 22.5 Bond work index) to 45 microns (P90). But using ball mills to achieve particle sizes that small is neither energy-efficient nor operationally efficient. Saunders explains that “with any change in parameters from the ‘ideal,’ you get a rapid deterioration of the performance of the mill."

Fortunately, another milling technology has started to come into its own. Although stirred mills have been around in some form for years, mines have been slow to adopt them due to high capital costs and high energy consumption. A 2010 pilot project at Turk, however, showed that a new vertical-axis stirred mill, from South African company Deswik, could help recover gold from the company’s tailings at an incremental cost of roughly half the price of gold. New Dawn ordered a unit in 2011 from Knelson Milling Solutions, a joint venture between Deswik and Canadian firm Knelson.

Shortly thereafter, that partnership was acquired by FLSmidth. The new owners, wanting to properly model and optimize its new mills, proposed an on-site optimization study to get field data. Happy to get help wringing out every bit of performance from their purchase, New Dawn agreed.




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