Tomra reported very positive results. Not only was the volume of material that would potentially be processed reduced by approximately 50%, but the grade also roughly doubled, and gold recovery was approximately 97%. The grade of material in the +38-65.5-mm size fraction went up to 22.9 g/t gold from an initial grade of 11.2 g/t. The grade of material in the +12.7-38-mm size was increased to 19.5 g/t gold from 11.8 g/t. When the coarser fraction was combined with the fine, -12.7-mm portion, the grade improved to 16.6 g/t gold.
Rejects from the sorting process represented 34% of the feed volume and graded 0.7 g/t gold, suitable for mine backfill.
Ore sorting has the potential benefits of reducing the amount of ore that must be milled, a savings in the size and cost of the mineral processing plant. With less material going through the plant, the volume of tailings is also reduced. There are also significant savings in trucking costs and green house gas emissions from diesel engines.
The Hammer down deposit was mined between 2000 and 2004, removing a total of 291,400 tonnes of ore grading 15.83 g/t gold and recovering 143,000 oz. of gold.
Maritime estimates the Hammerdown, and nearby Orion pit constrained measured and indicated resource at a 1.0 g/t cut-off is 1.7 million tonnes grading 6.65 g/t gold, containing 368,000 oz. The inferred resource (including Intervein) is 1.3 million tonnes at 4.77 g/t gold, containing 206,200 oz.
(This article first appeared in the Canadian Mining Journal)
