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MEI Online: Plant Operation News: Australasia: October 21st 2009


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:: Avoca's Trident Gold Mine Paste Plant Commissioned on Time and Under Budget

Avoca Resources has announced that it has commenced placing cemented tailings (paste) into underground workings at its Trident gold mine yesterday, following the successful construction and commissioning of its paste plant. The plant was completed on time and approximately A$1.5 million under budget and is, according to Avoca's Managing Director, Rohan Williams, "critical to maximising the ore extraction from Trident."

The paste plant is designed to mix cement with thickened tailings from the Higginsville treatment plant and pump the cemented mixture underground, where it is then placed into mined stopes via an underground reticulation system. The paste cures over a four-week period before mining adjacent to the filled stope can commence. The paste plant should ultimately result in increased extraction of gold from the high-grade Trident orebody.

In addition to providing a high-strength fill medium that maximises the extraction of the Trident orebody, other benefits of the paste plant include:

  • Removes the requirement to place tailings into surface storage facilities, thereby reducing the environmental impact of ore treatment
  • Reduces future capital costs that would otherwise be associated with increasing the size of the surface tailings storage facilities with ongoing ore treatment
  • Allows the recycling of process water from the paste plant back to the treatment plant for ongoing ore treatment. The recycled water contains treatment plant reagents, thereby significantly reducing the requirement and costs of adding new reagents into the process water necessary for ore treatment.

Avoca originally planned to use cement aggregate fill (CAF) to support the mined out workings. It was estimated this would cost around A$60/t. However, investigation showed the potential for paste fill to cost just ~A$20/t. So an investment decision was made based on spending A$15m of capital to save $40/t of fill required. CAF requires crushing and screening of waste material hauled from the mine, which makes up a considerable amount of the A$60/t; but the placement of the product would also have been more expensive than paste. CAF would have needed separate holes drilled from surface to fill each mined out stope. Another difficulty with CAF, according to UBS Investment Research, is that Avoca is mining the deposit from the top down. "Once the top stopes are filled they wouldn't be able to 'see' the next stope below, thus requiring feeder holes from surface on the angle. Paste fill on the other hand can be conducted with a single hole (plus a contingency hole for emergencies) and then with piped reticulation around the mine."

There was also a saving to be made in cement costs. UBS estimated that the CAF would require between 5 and 6% cement whereas the paste will be 3-4% cement (depending on the material). "All the cement has to be trucked up from Perth so the cost differential is significant."

Avoca has grown rapidly since its listing in 2002, and its acquisition of the Higginsville exploration project in 2004, located 130 km south of Kalgoorlie, Western Australia. In the period of just four years following its acquisition of Higginsville, Avoca discovered the Trident deposit, completed a feasibility study, commenced mine development, constructed and commissioned a new 1 Mt/y CIL treatment facility (built on time and under budget) and poured its first gold bar.

Currently, Trident is Western Australia's third largest underground gold mine and is forecasting gold production of more than 190,000 oz in FY2010 at an operating cash cost of A$452/oz (not including royalties).




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