Toronto-based
Since 1997, BacTech and its partner, Mintek, a South African government-owned minerals research company, have been using bacterial oxidation (or bioleaching) at a laboratory scale to produce copper metal from metallurgically problematic copper ores. Recovery rates are consistently above 95% over a residence time of about six days.
BacTech and Mintek built their first commercial bioleach plant at the now-closed Youanmi gold mine, near Mount Magnet in Western Australia. The US$6.5-million bioleaching portion of the processing plant only operated for three years, but it did boost gold recoveries from refractory ore to 92% from 40%.
Another such plant was erected at the newly opened Beaconsfield mine, a high-grade gold mine in Tasmania. Owned by two Australian companies, Allstate Explorations (52%) and Beaconsfield Gold (48%), the underground operation is expected to produce roughly 120,000 oz. gold annually.
“The technology, in our opinion, is proven,” says BacTech Chairman Alan Spence. “We’ve demonstrated the ability to scale up from a laboratory to a commercial plant in gold, and now we feel we can do it quite comfortably with base metals. We’re using effectively the same processes, the same equipment and the same facilities.”
Bioleaching speeds up ordinary biological weathering by creating an optimum environment for naturally occurring iron- and sulphur-eating bacteria. The process also yields sulphuric acid, which aids in the dissolution of ores. In contrast to roasting or pressure-oxidation techniques, bioleaching is carried out at atmospheric pressure and at temperatures below 100 C.
Researchers have found that individual species of bacteria thrive on different mineral assemblages. With this knowledge, BacTech tested more than 200 sulphide and refractory gold concentrates from around the world and built up a large collection of bacterial species classified according to their preferred ore types. The collection is maintained in cold storage in BacTech’s labs in Perth, Australia.
“What is unique about our bio-leaching approach is that we can handle multiple metals,” says Spence. “Therefore, we can treat a bulk concentrate as opposed to a pure concentrate.” He adds that the technological and cost advantages are especially evident in cases where it is difficult to separate copper from zinc — in super-fine-grained ores, for example.
In fact, Spence believes there are several advantages to applying bio-leaching to copper ores:
The process can simultaneously handle a complete range of copper-ore minerals (for example, chalcocite, bornite, covellite and chalcopyrite) by maintaining the acidity level of the leach material.
There is potential to recover accessory precious metals.
The process can treat lower-grade concentrate at little extra cost, allowing a mine operator to boost the total amount of metal recovered from a mine.
Because there is an electrowinning finish, bio-leaching of base metals is relatively harmless to the natural environment, compared with traditional smelting and its sulphurous air emissions.
Bio-leaching can convert toxic elements, such as arsenic or bismuth, into an environmentally acceptable form.
“On the cost side, this technology is young, and we think there is room for huge improvements at relatively minor research and development costs,” says Spence.
Pilot plant
Last summer, BacTech and Mintek advanced their copper bioleaching technology to a new level by signing an agreement with Mexican-listed
Penoles has agreed to spend a minimum US$4.4 million on the project, including feasibility study. The study will have two aims: to confirm lab results derived from copper byproducts mined at Penoles’ current operations, and demonstrate the viability of a jointly owned commercial treatment plant.
Eventually, the pilot plant will be used to test sulphide concentrates from elsewhere in the Americas and to evaluate several bioreactor (or tank) designs. In particular, BacTech and Mintek are keen to test a bioreactor developed by Paques Bio Systems, a Dutch-based treater of industrial waste. Paques’ newest bioreactor is characterized by gentle mixing, which is expected to produce better living conditions for the delicate bacterial species that are best suited to higher temperatures. (Higher operating temperatures in the bioleaching tanks will make the whole process speedier, thereby reducing capital and operating costs.) In return for its co-operation, Paques will be licensed to market the BacTech-Mintek bioleach technology in Europe and parts of Asia.
“The key in designing technology for metals is advancing it to the pilot-plant stage where the industry will accept it,” says Spence. “Having this facility in Mexico is a huge jump for us.”
Penoles, BacTech and Mintek are already contemplating building a commercial bioleaching plant, which would have an initial annual capacity of 15,000 tonnes of copper metal yet be expandable for treatment of concentrates from neighbouring mines. Ownership would be split between Penoles, with 55%, and the BacTech-Mintek partnership, with 45%. There is also talk of forming a three-way alliance for the purpose of building and operating commercial treatment plants in Mexico and five other Latin American countries.
For Penoles, the arrangement with BacTech and Mintek is consistent with its goal of expanding the copper side of its business while lessening its costly reliance on smelters operated by its Mexican competitors. Currently, Penoles is a major producer of silver, gold, zinc, lead and sodium sulphate. The new technology may also induce the company to consider developing some of Latin America’s more metallurgically troublesome base metal deposits, particularly those in central Mexico.
US20-30 per lb.
The operating cost of a moderately sized bioleaching plant will range from US20-30 per lb. copper, depending largely on power and reagent costs, says Spence. Looking at an individual case: tests performed on copper ores from the Mount Lyell mine in Tasmania indicate an operating cost of US17-20 per lb., and a capital cost of US$47 million, for a plant capable of producing 30,000 tonnes of copper metal annually. This translates into a total production cost of US68 per lb. copper.
BacTech is also researching the bioleaching of nickel-copper-cobalt ore. As with copper-zinc, such material can be bioleached as a bulk concentrate if a clean concentrate cannot be produced. Also, bioleaching tends to result in higher cobalt recoveries than those achieved by traditional smelting.
BacTech has already operated a small pilot plant using copper-nickel concentrates from Western Australia, and recoveries were in the range of 95%.
Beyond the bioleaching of base metals in tanks, Spence sees a future for the heap bioleaching of coarsely crushed, low-grade primary copper ore. He predicts there will be a trend similar to that seen in the gold industry over the past 10-15 years, when companies heap-leached progressively lower-grade material.
“We’ve had successful results in our laboratories and we’re definitely planning to move into heap leaching of chalcopyrite with the first opportunity,” he says.
(At the board-room level, BacTech’s Australian-based president, Richard Winby, died unexpectedly in December 1999, and the company is seeking a replacement.)
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