Dickenson close to decision on using bioleaching

Is Dickenson Mines (TSE) poised to become the operator of the first commercial-size bioleach operation in North America?

President Robert McEwen is making no public commitment, but the company did go to the trouble recently of hosting a luncheon in Toronto for a group of financial analysts and media representatives where the speaker, Pieter van Aswegen, was introduced as the world’s foremost authority on bioleaching. The speculation is that Dickenson is in the final stages of making a positive decision and that an announcement is to be anticipated in the near future. Bioleaching, or bacterial oxidation (BIOX), is the technology whereby refractory gold ores are broken down by bacteria and subsequently made amenable to conventional gold recovery by cyanidation.

Researchers in Europe, Australia and North America have tried for years to commercialize the activities of a particular bacterium that thrives on the sulphur of pyritic and arsenical ores but to little avail. To them, the promise of a low-cost, environmentally benign process has all but seemed to be the chasing of a will-o’-the-wisp.

The micro-organisms were too slow, they could be killed off all too easily by traces of particular metals, they were sensitive to temperature changes and they lost their appetites if there was insufficient acid to go along with the sulphur.

However, a company in South Africa had also been researching the same field since the late 1950s. Initially, General Mining Corp. (Gencor), South Africa’s largest gold producer after Anglo-American, had uranium extraction in mind; it switched to gold 20 years later.

Gencor’s work was carried out with little fanfare and in 1986 it constructed its first commercial plant at the Fairview mine in the east Transvaal. The company discovered the key that had escaped the others — the cultivation of a bacterial species that was resistant to a range of toxins and a sulphuraholic to boot.

Van Aswegen is manager of minerals technology for General Mining, Metals and Minerals (Genmin), a wholly owned subsidiary of Gencor. Genmin is actively seeking participation wherever refractory gold ores are mined. At Sao Bento, Brazil, the capacity of a 900-ton-per-day autoclave plant has been doubled by bacterially pre-oxidizing the ore. Australia’s Harbour Light mine will use BIOX.

Dickenson’s Arthur W. White mine at Red Lake, Ont., produces about 75,000 oz. gold per year from the milling of 780 tons of ore per day. The ore grades 0.32 oz. per ton and mill recovery is in the mid-80s. Cyanidation is responsible for the present production but a flotation concentrate was produced until recently. The concentrate grades about 1 oz. per ton and it is refractory. A high arsenic content makes it unacceptable for smelting and BIOX appears to be the only process which meets Dickenson’s requirements. Dickenson’s neighbor, the Campbell mine of Placer Dome (TSE), recently brought an autoclave on-stream (pressure oxidation) but their throughput and ore grade permit the higher capital and operating expense.

BIOX at Dickenson would probably push the percentage of gold recovery into the mid-90s. It would increase the profit margin per ounce. and automatically reduce the economic cutoff grade of the ore reserves.

Whether the latter would lead to an increase in ore reserves is not known but it is a possibility. (The company quotes a proven and probable reserve sufficient for 11 years at current production rates.)

And the cost of a BIOX plant? Van Aswegen noted capital and operating costs for a 100-ton-per-day plant. At half of that size (the size that Dickenson would need), the costs pro-rate to R6.6 million ($2.8 million) and R144 ($61) per ton respectively. Inevitably there would be some upward modification because of scaled-down factors and to differences in South African and Canadian working conditions.