Monterrey, Mexico — Having spent the past 15 years developing bioleach applications to treat refractory sulphide gold ores, only to see gold prices plummet to historic lows,
The Toronto-based junior, in partnership with Mintek, a leading South African metallurgical research organization, is at the threshold of proving that its bioleaching technology is a viable commercial alternative to the traditional concentrate treatment processes of smelting, roasting and pressure-oxidation. The pair aim to demonstrate and advance the capabilities of their copper technology through separate joint ventures with
Peoles, a dominant player in Mexico’s mining, refining and industrial chemical sector, is putting the finishing touches on a newly constructed, US$4.9-million test plant at its research facilities in Monterrey. “The pilot plant will provide credibility for the technology to the mining industry,” says BacTech President Geoffrey Donohue.
Over the next four months, the pilot plant will test bioleaching of polymetallic concentrates as a viable, environmentally safe treatment option. The plant will process about half a tonne of copper-zinc concentrate per day.
Peoles intends to run a mix of four concentrates through the plant, with three of the concentrates coming from its own mines.
During a recent visit of the Mexican facilities, The Northern Miner learned that Peoles will undertake a feasibility study of a commercial, 25,000-tonne-per-year copper and zinc bioleach plant. The study, which is due in late February 2002, is expected to confirm an internal rate of return of 20% or greater on a projected investment of US$50 million.
Under a 1999 agreement with BacTech and Mintek, Peoles holds the exclusive right to participate, with a 55% interest, in commercial bioleaching operations for copper in Mexico plus five other Latin American countries, including Venezuela, Bolivia, Guatemala, Argentina and Colombia.
Peoles will have the option to increase its interest to 60% in any Mexican project by funding half of BacTech and Mintek’s share of capital requirements. BacTech and Mintek would then each hold 20%.
Peoles
Founded in 1887, Peoles is a mining and metallurgical company, with additional experience in industrial chemicals. It is the world’s top producer of refined silver, metallic bismuth and sodium sulphate, the leading Latin American producer of refined lead and zinc, and Mexico’s largest producer of refined gold.
Peoles operates 15 underground mines and one open-pit mine, as well as the world’s fourth-largest metallurgical complex, Met-Mex, in Torreon. Last year, company sales totalled US$9.37 billion, with silver accounting for 37.8% of that total, followed by gold at 19.2%, zinc at 14.5%, and lead at 9.1%.
Its mining division posted record volumes for each of its metals, producing 44.7 million oz. silver in 2000 (up 6.2% from 1999), 298,300 oz. gold (up 16.6%), 173,200 tonnes zinc (up 16%) and 74,400 tonnes lead (up 4.5%).
In the metallurgical refinery division, gold production increased 37.6% over 1999 to 645,700 oz., while silver production, at a record 74 million oz., rose 15.3%.
Peoles posted a loss of US$137 million (or 34 per share) in 2000 after taking US$483 million in writedowns, compared with a 1999 net profit of US$796 million ($2 per share). The company’s shares are listed on the Mexican Stock Exchange. At year-end, Peoles had 397 million shares outstanding.
Although copper has not been a strategic metal for Peoles historically, the company is investing in two such projects. At the Milpillas project, in Sonora state, the major has drilled off an indicated resource of 30 million tonnes of leachable oxide grading 2.5% copper, equivalent to 750,000 contained tonnes. A prefeasibility study is to be completed by year-end, with startup projected for 2003. Yearly production is pegged at 60,000 tonnes of cathode copper, with a total investment of US$180 million.
Elsewhere in Sonora, the company is engaged in an exploration joint venture with Chile’s state-owned entity Codelco (Corporacion Nacional del Cobre de Chile). The pair have identified 454 prospects, 162 of which will be examined further. Last year, they drilled 23 anomalies and spent US$3.2 million on exploration. A further US$5 million is budgeted for 2001.
In terms of regional exploration, Peoles is reviewing projects in Chile, Bolivia, Colombia, Brazil, Guatemala and the Dominican Republic.
Peoles’ assistant vice-president of research and development, Ricardo Benavides Perez, says the company’s decision to invest in the bioleaching copper technology was influenced by the environmentally responsible manner in which the sulphide concentrate can be treated.
This procedure is also an economic alternative to concentrate copper smelting. “I don’t think there is any copper sulphate that won’t leach out,” explains Tony Pinches, manager of Mintek’s biotechnology department.
The bacteria employed are naturally occurring micro-organisms that can only be seen under a microscope. They live on inorganic material and are harmless to living creatures. Their diet consists of pyrite, arsenopyrite and other metal sulphides, including chalcocite and chalcopyrite.
Thermophiles and other types of bacteria may be found in acidic environments produced by the oxidation of sulphur, in and around hot springs, volcanic regions and sulphide-rich areas.
The process that thermopiles and other rock-eating bacteria use to oxidize sulphide minerals is not fully understood. Chemical and biological forces work together to oxidize the metal sulphide to form acid-soluble sulphates. Iron, arsenic and base metals (copper, cobalt and zinc) pass into solution, which can be separated from the residue and treated by conventional processing methods, such as solvent extraction.
Bioleaching in tanks involves feeding a continuous stream of slurried concentrate into primary reactors, or tanks, containing a suspension of bacteria in a mildly acidic environment. The use of controlled conditions, such as agitated, aerated tanks, results in rapid oxidation of metal sulphides.
Most of the leaching occurs in the primary reactors. As concentrate is added to the primary reactor, partially oxidized material flows into secondary-stage reactors, where the final oxidation occurs. The leached material then flows from the secondary reactors into thickening tanks for separation. Typically, the separation of the solution and residue is done through a counter-current decantation circuit using thickeners. The solution is treated for the recovery of copper using solvent extraction-electrowinning. Zinc goes to zinc sulphur and can be precipitated out. The residue can also be treated, either for recovery of precious metals and lead, or for disposal as tailings.
“Fundamentally, what we’re really talking about is an efficient way of burning off the sulphides, compared with smelting or any other technology,” says Peter Scott, director of marketing for Mintek.
Optimum temperature, nutrients, oxygen supply, pulp density and residence time need to be established for the best results. The leaching process is carried out at near-ambient temperature and atmospheric pressure, thereby lowering capital and operating costs, diminishing safety and health hazards, and lessening environmental impacts.
Capital and operating costs are influenced by:
– the rate of reaction and the level of sulphide oxidation required for acceptable metal recovery, which determines the size and number of tanks, plus the aeration and agitation requirements;
– the reagent usage for pH (hydrogen ion concentration) control and solution neutralization; and
– the salinity of site water and the requirement for acid-resistant materials.
In Mexico, three reactors will be tested to determine differences in capital and cost savings. One of the reactors, to which BacTech holds the exclusive rights, was developed by Pacques Bios
ystems of the Netherlands. This reactor requires no agitation and, instead, uses high-pressure air.
BacTech-Mintek
BacTech’s relationship with South Africa’s Mintek goes back some seven years. The pair was first brought together by Anglo American, and in April 1997, they formed a joint venture to pool their respective bacterial oxidation technology. The relationship has evolved over time, so that BacTech is now primarily responsible for developing the commercial end and targeting joint-venture deals for base metal and gold bioleaching technology. Mintek’s responsibility is to keep the technology at the forefront and implement it for industry.
BacTech is reviewing project opportunities around the world, especially those that occupy mineral belts in Mexico, Spain, Portugal and Peru. These contain multiple deposits exhibiting complex or refractory ore, with high smelter and transportation costs. “We’re looking at doing deals with mid-size companies that don’t have smelter facilities, but have the resources,” says Donohue.
BacTech has operated since 1988, with its roots in Western Australia. It went public in Canada in September 1997 and currently has 11 million shares outstanding, or 16.1 million fully diluted. It sits at 50 in a thinly traded 52-week range of 90-30. Rothschild Australia, the largest shareholder, owns 18% of the shares outstanding, or 38% on a fully diluted basis. The company has recently drawn down $300,000 from a $1 million convertible note.
BacTech first acquired its proprietary bacterial oxidation technology from King’s College in London. The first commercial application of the company’s technology was implemented in 1994 at the Youanmi Deeps gold mine in Western Australia, boosting refractory recoveries to more than 90%, compared with 50% prior to the application. The plant produced 6,000 oz. gold per year before the mine closed in 1998.
Mintek was founded more than 60 years ago, having starting in the phosphate industry and then the uranium industry during the Second World War. The company later moved into gold, platinum group metals and, more recently, ferro-alloys. Up until 1989, it was solely funded and owned by the South African government. Today, 60% of its funding comes from private hands, with half of that coming from outside South Africa. Mintek has a total staff of 660, including scientists and engineers.
The company specializes in setting up laboratory-scale pilot plants in which to test the flow process sheets. Lately, the thrust has been pyrometallurgical research on ferro-chromes and ferro-nickel. Another important area for the Mintek has been the recovery of metal from ferro-alloy slags. It builds, owns and operates slag recovery plants worldwide.
Mintek has been researching bioleaching since 1981 and now employs 20 people in this department. The company uses small, 1-to-2-litre laboratory-scale test plants to provide clients with estimates for continuous-flow steady-state systems, explains Pinches, manager of Mintek’s biotechnology department. The next step up are 75-litre continuous flow mini-plants, which incorporate solvent extraction-electrowinning recovery systems. Some 300 different rock types have been tested. Recent tests have focused on zinc electrowinning.
In the late 1980s, Mintek set up an intermediate-stage, 20-tonne-per-day demonstration bioleaching gold recovery plant at the Vaal Reefs gold mine in South Africa. It was an expensive endeavor to build and run, says Scott, Mintek’s director of marketing, but based on that work, the company is now able to design a full-scale gold plant and bypass the intermediate stage.
Commenting on the difficulties that BacTech experienced going from a 300-litre test plant to a 3,000-cubic-litre plant at Youanmi, consultant Michael Rhodes says: “Essentially, the bacteria worked as expected. The things we learned were more in the areas of construction and engineering aspects.”
Since those tests, two additional bioleach gold plants have been built under licence at Beaconsfield in Tasmania, Australia, and more recently at the Laizhou metallurgical plant in China’s Shandong province, where a 65,000-oz.-per-year custom plant is being commissioned. The licensing deal with Laizhou could be worth about US$1 million a year over 10 years for BacTech and Mintek.
The Beaconsfield plant, which was built in 1999, ran into problems, with dramatic cost overruns. BacTech’s Donohue says the problems are mechanical and engineering-related in nature, and stresses that there have been no problems with the technology.
Noranda has teamed with Mintek and BacTech to study bioheap-leaching of chalcopyrite material, and Mintek and BacTech have developed chalcopyrite bioleaching in stirred tanks in a laboratory setting. “We have applied our copper concentrate knowledge to heap simulations, with excellent results,” says Pinches. Over the course of the next two years, Noranda will invest $400,000 in work aimed at developing the technology and establishing its commercial viability.
“To be able to heap-leach chalcopyrite would be a major technological breakthrough for the industry,” said John Peacey, Noranda’s director of innovation, during the project’s launch last February. “At the end of our two-year commitment, we will have a better understanding of the process and its potential applications and economics. If we are successful, the next step would be a pilot heap test at a mine site.”
Under the working agreement between BacTech and Mintek, all rights to biotechnical innovations and developments are shared equally.
Bioheap-leaching involves crushing the ore-bearing material, stacking it on plastic-lined pads and spraying it with a dilute sulphuric acid solution containing high-temperature thermophilic bacteria and nutrients. The solution drains through the heap, is recovered and re-sprayed over the heap.
Base metal sulphides are released into solution and recovered by conventional processes prior to the return of the solution to the heap. If the ore is gold-bearing, the solution is recycled until sufficient sulphides have been oxidized to expose the gold. The residue is washed with water to remove acid and metals, treated with lime to neutralize any remaining acid and then sprayed with cyanide to recover the gold. Better gold recoveries may be achieved by processing the oxidized residue through a conventional milling-and-cyanidation circuit.
Noranda is collecting chalcopyrite samples from many sources for lab testing.
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