Autoclaving, bioleaching replacements for roasting

It’s difficult to specify how much of the world’s gold reserves are tied up in refractory ores, which do not yield their gold to cyanidation without prior treatment. But South Africa’s General Mining Corp. (Gencor) has probably as good a figure as any. One of that company’s senior metallurgists, Pieter van Aswegen, recently quoted 15-30% as a likely figure.

We hear a great deal about these ores but they just haven’t suddenly appeared upon the scene. They’ve been around for close to a century. Earlier operators would drop properties when they found the ore was not free-milling. Why go to the extra expense when other properties were readily available? Some owners were more farsighted. A large orebody assured a long and profitable life despite the higher capital and operating costs. They erected roasters to eliminate the ore’s sulphur and arsenic and so overcame the problem.

It’s not a route that can be taken easily today. Yet there are three ultra-modern roasters in operation in North America now. One is in British Columbia and there are two (or soon will be) in Nevada. They are fitted with the latest control devices and are environmentally benign. The earlier generation of roasters have all but gone.

It was the dramatic increase in gold prices that brought refractory ores sharply into focus. From 1972 until 1980, the price of gold rose to US$612 per oz. from the US$35 level where it had languished since 1934. There was a hectic round of activity to discover if there were ways to avoid the roaster. Two avenues were explored and both have produced commercial solutions. One uses bacteria and the other chemical treatment in the autoclave (finely tuned pressure cooking on a grand scale).

Autoclaving is the dominant process at present but bacterial leaching is starting to make headway. Both have their distinctive applications. There is, in fact, one mine using both.

Autoclaving technology came from work undertaken by Sherritt Gordon (TSE) during the Second World War. They were seeking means of recovering copper and nickel from their Lynn Lake, Man., ores without resorting to smelting. Many of Sherritt’s early autoclaving patents have expired and this has opened the door to others. Subsequently, a number of companies have developed their own proprietary processes. These include Fluor-Daniel-Wright and Kilborn as well as Sherritt’s newer developments in Canada, Davy-McKee in the U.S. and South Africa’s Gencor.

Process details vary, but the basics are easily described. At the Campbell mine of Placer Dome (TSE) at Red Lake, Ont., Sherritt technology is applied to autoclave 78 tons per day of high arsenic/sulphur flotation concentrates which assay about 13 oz. gold per ton. The concentrates are reground to -325 mesh and cooked at a pressure of 300 lb. per square inch at a temperature of 190C. Pure oxygen is jetted into the pulp and after two hours the sulphur and arsenic are extracted leaving a residue that is readily cyanided. The sulphur and arsenic compounds are further treated to produce innocuous, stable materials that can be safely consigned to the tailings dam. Campbell’s plant went into operation Oct. 3, 1991. Gold recoveries are 98%. According to Ken Norris, acting mill superintendent, an additional bonus has been the drop in cyanide consumption from 700 lb. per day with the roaster to 75 lb. today. The decrease is attributed to the autoclave’s superior capabilities.

Campbell was Canada’s first gold autoclaving operation and Nerco’s (NYSE) Con mine at Yellowknife, N.W.T., will be the second. It uses Fluor-Daniel-Wright technology and will come on stream in July or August. Capacity will be 95 tons per day.

Project manager John Wright quotes a mixed bag of feed for the Con plant: 30% arsenical sludge from the discontinued roaster, 30% calcines from the same source (the roaster was discontinued during the mid-1960s) and 40% flotation concentrates. The Con orebody changed from refractory to free milling at depth but the company is finding fresh reserves in the upper levels of the refractory section of the mine.

There is a strong probability that International Corona’s (TSE) Eskay Creek will host Canada’s third.

There are four established autoclave plants in the U.S., with one more in the offing. Existing plants are at the McLaughlin mine in California of Homestake (NYSE) (this was the first in North America, starting in 1985); the Mercur mine in Utah and the Goldstrike mine in Nevada of American Barrick Resources (TSE); and the Getchell mine in Nevada of FirstMiss Gold (NASDAQ). The next plant will probably be at the Lone Tree mine in Nevada of Santa Fe (NYSE). Offshore, Porgera in Papua New Guinea and Sao Bento in Brazil are operating autoclaves; the latter is using bioleaching as an assist. The first is owned in part by Placer Dome (TSE), and the second, also in part, by Gencor.