THE FAIRVIEW FLOW: A gold mine in Eastern Transvaal represents a

The names Pilgrim’s Rest, Barberton and Murchison Range ring down through South African mining history in the same way as do Kirkland Lake and Cadillac in Canada. These were the first significant gold discoveries in South Africa, occurring several years before the greatest of them all, the Witwatersrand. A number of these relatively small producers still operate and are responsible for an annual production of about 200,000 oz.

In the company of several other countries (Canada included), South Africa has been vigorously pursuing ways to eliminate sulphide roasting; and one of the options followed by genmin was that of bacterial oxidation. This, in its simplest form, involves the introduction of specific bacterial cultures into the finely ground refractory ore. The operating conditions are such that the bacteria multiply rapidly; they feed upon the sulphur of the ore, thereby oxidizing it and presenting it in a form suitable for cyanidation.

General Mining, Metals & Minerals (the minerals arm of GENCOR) has been involved in biotechnological research for almost 30 years, and although interest was not directed toward gold extraction until the late 1970s, the company has the distinction of having brought the world’s first commercial plant into operation, in October, 1986, at the Fairview mine.

According to GENMIN, a pilot plant treating 1650 lbs. per day was commissioned in 1984 and operated for two years, after which time sufficient information had been accumulated to design a commercial-size unit with a capacity of 10 tonnes per day. This was consequently installed at the Fairview mine and today it handles 40% of the mill’s flotation concentrates, the remainder of the concentrates being roasted as before.

Due to its unique geology, Fairview produces a free milling ore from one section of the mine and refractory ore from the other. Both ores are passed to its own dedicated circuit rated at 72,000 tonnes per year and 168,000 tonnes per year, respectively (about 200 and 470 tonnes per day). The refractory run-of-mine ore averages 0.20 oz. gold, 1.35% sulphur and 0.5% arsenic trioxide and is processed by a flotation circuit to produce concentrate assaying 3.50 oz. gold, 29.0% sulphur and 8.0% arsenic trioxide.

Sixty per cent of the concentrate is roasted in two Edwards roasters and the balance of 10 tonnes per day delivered to the bioxidation plant. Here, the concentrate is first reground in a Sala ball mill working in closed circuit with a spiral classifier and the overflow pumped to surge tanks. The slurry is then adjusted to a density of 10% to 15% solids and delivered to a series of mechanically agitated bacterial oxidation biox tanks (each of which has a capacity of 100 cubic metres), where the inoculum (Thiobacillus ferrooxidans) is introduced. After being retained for about four days, the slurry flows to a series of thickeners where the acid ferric sulphate solution is washed out, the ferric sulphate diverted to a neutralization circuit, and the washed pulp batch cyanided.

The gold is recovered by the carbon-in-pulp process. Pulp temperature in the biox tank is kept at 40deg C and the ph maintained at 1.6 by the use of lime. Gold recovery is 95% and although operating costs are slightly in favor of roasting (which has a historic recovery of 90%), the inherent simplicity of the biox process, its high availability and environmental acceptability make it the preferred choice.

A number of interesting phenomena have come to light regarding Thiobacillus ferrooxidans, not the least of which has been the capability of the bacteria to adapt and mutate to the extent that the original retention time of 10 days was reduced to four days over the 2-year life of the pilot program and this with no loss in recovery, registering 97%. In a similar vein, the bacteria, over time, were able to adapt themselves from an arsenic strength of four grams per litre to one of 13 grams per litre — a major change in their living environment. Recoveries at Fairview require the oxidation of 85% of the sulphur to achieve gold recoveries of more than 95%.

It should be noted that the pilot plant’s recovery of 97% was not reached in the first two years of commercial operation, during which time 95% was the maximum achieved. It is believed that this shortfall is attributable to a batch rather than a continuous cyanidation plant. With capacity of the plant increased from an initial 10 tonnes per day to 12 and later 15 tonnes, it is expected that the several shortcomings of the plant revealed during two years of continuous operation will be remedied. Improvements include an increased degree of instrumentation, improvements in the ferric sulphate neutralization circuit and conversion of the cyanide plant.

With two years of pilot testing followed by a further two years of experience operating a commercial plant, genmin expects to advance its expertise in two main areas. First, because of the success that has attended the commercial operation at Fairview, management is now convinced they can abandon their 30-year-old Edwards roasters and treat all flotation concentrates by the biox process. This new plant came on stream in January, 1991.

Second, GENMIN is interested in joint ventures or “farming into” projects where there is a need for their specialized knowledge. In this regard, biox technology is to be introduced at the Santa Barbara mine in the state of Minas Gerais, Brazil. Here, an autoclave plant is treating an arsenical gold ore, but by subjecting the concentrate to a partial bacterial oxidation prior to autoclaving, it is expected to double capacity to 50,000 from 25,000 tonnes per month.