AMERICAS — International challenges for Canadian

The problem with bioleaching of refractory gold ores is that it only works in the Southern Hemisphere — or such is the opinion of a good many Canadian metallurgists who were key to the bioleaching research effort over the past 10-15 years.

The bioleaching process was tested at five pilot plants in Canada: at the Congress and Equity mines in British Columbia, the Arthur White and Campbell in Ontario, and the Salmita in the Northwest Territories. A further seven pilot projects, employing Canadian expertise to a greater or lesser extent, operated in South Africa, the U.S., Sweden and Zimbabwe. Yet, today, there is not a single gold mine in North America using bioleaching as its primary recovery system. Nor is the recently inaugurated bio-heap-leach of Newmont (NYSE) in Nevada an exception. It is an adjunct to an existing mill, and leaches mineralization too low in grade to be treated otherwise. For operating biooxidation plants one must follow the trail of South Africa’s Gencor. There are five operating world-wide, and all use Gencor-licensed technology. Two are in Australia, one in South Africa, and one in Brazil. By far the largest is the 760-tonne-per-day plant at Ashanti, Ghana, which came on line a few weeks ago. Purists would look askance at placing Ghana in the Southern Hemisphere, but it scarcely alters the process’s apparent preference for southern climes.

Obviously, geographic location has nothing to do with viability. Character of the refractory ore and economics are what count. But, it is suggested, North American mining executives consider bioleaching to be unproven technology. It is also believed there may be serious misunderstandings with regard to its environmental acceptability.

These concerns, and others, were addressed in a paper delivered by Richard Lawrence at the annual meeting of BIOMINET, in October, 1993. Lawrence’s paper gave an overview of bioleaching development from its earliest days in Canada. (BIOMINET is the federal ministry of Energy, Mines and Resources’ agency created to administer and oversee biotechnology development.) Lawrence is an associate professor of mining and mineral processing at the University of British Columbia, and has been involved in biotechnology for many years, notably at B.C. Research and later at Coastech Research, as well as with provincial and private research organizations.

Until a decade ago, roasting (calcination) was the traditional method for unlocking gold from refractory ores. In the mid-1970s, gold prices rocketed. Companies scrambled for the many known, but unmined, refractory gold deposits. But by then, environmental concerns had come to dominate the agenda. Roasting, as then practised, was no longer acceptable. Alternatives had to be found quickly, and autoclaving (pressurized leaching) promised the answer. The greater part of autoclaving theory and practice was elucidated by Sherritt (TSE) in the late 1940s and early 1950s. That company’s work centred on the extraction of copper and nickel from sulphide ores. While autoclaving sulphidic-arsenical gold mineralization presented its own unique problems, the basic ground work had already been covered. Homestake Mining (NYSE) was the first company to apply autoclaving to gold ores. Its plant at the MacLaughlin mine turned over in 1985. Rated capacity is 2,700 tonnes per day. (MacLaughlin also conventionally mills an additional 2,900 metric tonnes per day of oxide ore.)

A further eight autoclave plants have been introduced worldwide since 1985. The largest is operated by American Barrick (TSE) at its Goldstrike mine in Nevada. Installed capacity exceeds 13,600 tonnes per day.

On the other hand, bioleaching development was at a disadvantage. Apart from early studies concerning the critical role played by the bacterium Thiobacillus ferro-oxidans in the generation of acidic, coal-mine-waters, researchers had to start from scratch. The first Canadian pilot plant was tested at Equity Silver (TSE), and in 1986 the first commercial bioleach plant came on-stream at the Fairview mine in East Transvaal. Fairview applied processing that had been quietly and painstakingly developed over 10 years by Gencor, South Africa’s second-largest gold producer. Today, Gencor has a virtual monopoly on technology in the commercial bioleach field. North America’s first industrial-size plant was built by US Gold at its Tonkin Springs gold mine in Nevada. A US$28-million mill, centering on a 1,400-tonne-per-day bioleach circuit, was scheduled to be brought into production in June, 1990. But financial problems caused commissioning to be aborted, and operations were halted eight months later.

The result of the untimely plant closure was that the technical merit of whole-ore bio-oxidation remains to be tested. (Whole-ore denotes crushed and ground, but otherwise untreated, mine ore. In contrast, most pilot projects and all Gencor-licensed plants use flotation concentrates as feedstock.) Prior laboratory-scale testwork on Tonkin Springs’ ore assaying 4.3 grams gold per tonne and 1.5% sulphur indicated 60 hours bioleaching would oxidize 80-90% of the sulphur, after which the liberated gold would be recovered by carbon-in-leach cyanidation.

Because of the limited reserves at Tonkin Springs, the likelihood of the plant being completed and brought into production is not encouraging. Lawrence says the technology will take root in the Northern Hemisphere sooner rather than later. “We have the research, the technology and the personnel,” he said. “All we need are suitable ores, and we have those, too, in Canada.” — David Scott is a freelance writer based in Victoria, B.C.