Having completed the first large-scale batch test using bio-oxidation, Newmont Gold (NGC-N) is beginning a second test designed to improve operating techniques.
The first test involved 731,000 tons of low-grade sulphide material. The two types of material tested in the first batch were a “non-preg-robbing” ore, which can be leached using cyanide, and “preg-robbing” ore, which requires Newmont’s patented ammonium thiosulfate (ATS) recovery method.
The company hoped to achieve a recovery rate of 60% but managed only 50% from cyanide ores and 44% from the ATS method. Overall gold production from both ores is about 13,000 oz.
Thomas Logan, Newmont’s bio-oxidation project manager, says that while optimum oxidation was not reached, the ore contained two and a half times more initial sulphides than the facilities would normally treat. “In all, we were pleased with the results.”
For the second batch, the company intends to stack another 500,000 tons of ore containing 2% sulphide, which is more typical of the low-grade material.
About 300,000 tons have been laid out since construction was started in the first quarter. Bio-oxidation of both types of ore should last 10-12 months.
“We are tinkering around with the designs to optimize the results,” Logan adds. The company is looking at different designs for pad construction and water treatment.
The bio-cyanidation process costs about US$4 per ton; the ATS process, about US$5 per ton.
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