Mining Mirror February 2019 | Page 20

Mine excursion designed to run at 2.5 million tonnes and upgraded to 3.1 million tonnes per annum. Solar reduces costs Sion Kamati, plant metallurgist at Otjikoto. Last year, Otjikoto installed a 7MW solar plant to augment the supply of power from its HFO power plant and that, according to Barnard, has resulted in significant cost savings. “Thirteen per cent of our total costs were spent on HFO for power generation.” Before the construction of the solar plant, the mine relied on 24MW of installed generation capacity provided by four heavy fuel oil (HFO) diesel generators and three backup generators. With a consistent demand of 12.5MW per hour, the plant consumed about 21.7 million litres of HFO in 2017, at a cost of close to USD10.5-million. According to Dawe, the solar plant has made a big difference. “The ‘green’ energy generated by the solar plant has enabled us to shut down one HFO generator during the day, and that has resulted in substantial cost savings,” says Dawe. Blending to optimise Peter Mawoyo, production manager at Otjikoto. Eric Barnard, acting general manager at Otjikoto. [18] MINING MIRROR FEBRUARY 2019 The grades in different areas of Otjikoto’s two open pits — Otjikoto and Wolfshag — vary substantially and therefore, an efficient blending strategy is necessary to optimise the processing plant’s performance. Mawoyo explains that the high-grade blocks contain gold at about 2 grams per tonne, but he emphasises that these high-grade areas are few and far between, and that medium-grade and low-grade mining blocks dominate the surface areas of both pits. The high-grade ore needs to be blended down to about 1.5 grams per tonne, as these blocks are usually associated with a high sulphur content, the grades of which adversely affect the recoveries. “Usually, high grades are associated with ore containing high percentages of pyrrhotite, which, in turn, has a high sulphur content and impacts negatively on the recoveries. So, if we process the high-grade ore, the sulphur content would increase, with the result that the cyanide consumption and oxygen demand would be higher; we wouldn’t have enough oxygen and cyanide in the leach circuit; and the leach efficiencies would drop. To maintain the plus 98% recoveries, we have to blend the high-grade ore on the ROM pad and reduce the sulphur content in the process from approximately 4% to 2%, www.miningmirror.co.za