Vancouver —
Several tonnes of cathode were produced at a demonstration plant in Monterrey, Mexico. The partners used an integrated bioleach and solvent extraction-electrowinning circuit with a copper cathode capacity of 500 kg per day.
“The production of commercial-sized cathodes is a clear demonstration that the BacTech-Mintek proprietary bioleach process technology is able to recover copper from sulphide concentrates,” says BachTech President Geoffrey Donohue. “Given the environmental, capital and operating cost advantages that the technology offers over traditional treatment processes, it is only a matter of time before bioleaching, combined with solvent extraction-electrowinning, becomes the process of choice in the US$7-billion smelting industry and for other base metal projects.”
The proprietary bioleach process uses naturally occurring bacteria to oxidize sulphides and liberate metals from refractory sulphide ores and concentrates. The bulk of the world’s base metal supply is derived from sulphidic ores that are sent through a milling and flotation circuit to produce a concentrate of higher metal grades. This concentrate is then treated by roasting or smelting and refining to produce a metal.
“The problem with this traditional method is that the roasting and smelting process generates large quantities of sulphur dioxide gases,” explains Donohue. “In many cases, so-called ‘dirty concentrates’ can also release toxic elements [such as arsenic] that are increasingly becoming environmentally unacceptable.”
BacTech says the production of copper cathodes represents a significant step in the completion of a feasibility study to evaluate the viability of building, owning and operating a commercial plant for toll treatment of concentrates.
The feasibility study, due in late February 2002, will test the viability of a commercial, 25,000 tonne-per-year copper and zinc bioleach plant. The findings are expected to confirm an internal rate of return of 20% or greater on a projected investment of US$50 million. Once the study is completed, the pilot plant will be used to test sulphide concentrates from the Americas.
Penoles will earn the rights to Mintek-BacTech copper bioleach technology in Mexico and five other Latin American countries by contributing $6.6 million toward building the plant and completing the feasibility study. According to a 1999 agreement with BacTech and Mintek, Penoles stands to earn a 55% interest in commercial bioleaching operations for copper in Mexico, Venezuela, Bolivia, Guatemala, Argentina and Colombia. The company will have the option to increase its interest in any Mexican project to 60% by funding half of BacTech and Mintek’s share of capital requirements. BacTech and Mintek would then each hold 20%.
Founded in 1887, Penoles 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.
Sixteen mines
Penoles operates 15 underground mines and one open-pit mine, as well as the world’s fourth-largest metallurgical complex, Met-Mex, in the northeastern city of 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%.
BacTech’s relationship with South Africa’s Mintek goes back some seven years. They were first brought together by Anglo American, and in April 1997 they formed a joint venture to pool their respective bacterial oxidation technology. BacTech is primarily responsible for developing the commercial end and targeting joint-venture deals for base metal and gold bioleaching technology, whereas Mintek is responsible for implementing the technology.
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 is private, 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. Lately, the thrust has been pyrometallurgical research on ferro-chromes and ferro-nickel. Another important area for the Mintek is the recovery of metal from ferro-alloy slags. It builds, owns and operates slag recovery plants worldwide.
Bioleach process
Bioleaching, also referred to as bacterial oxidation or bio-oxidation, is carried out by naturally occurring micro-organisms known as thermophiles. These microscopic miners measure only 0.5-2 micrometres long and 0.5 micrometre wide. The thermophiles’ diet consists of pyrite, arsenopyrite and other metal sulphides, such as chalcocite and chalcopyrite. They are naturally found in acidic environments produced by the oxidation of sulphur — for example, around hot-springs, volcanic regions and sulphide-rich areas.
How exactly the thermophiles oxidize sulphide minerals is not yet known. Both chemical and biological forces work together to oxidize the metal sulphide, forming acid-soluble sulphates.
Precious metals, which are not soluble, remain with the residue. Iron, arsenic and base metals, such as copper, cobalt and zinc pass into solution. The solution can then be separated from the residue and treated by conventional processing methods, such as solvent extraction, to recover base metals. The residue generated by the bacteria may contain precious metals that can be recovered by cyanidation.
There are two methods of bioleaching: tank bioleaching and heap bioleaching. The former occurs rapidly, over 500,000 times faster than oxidation by natural exposure to air and water in the absence of bacteria.
Bioleaching in tanks involves feeding a continuous stream of slurried concentrate into primary reactors containing a suspension of bacteria in a mildly acidic environment. Most of the leaching occurs in these 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 solid-liquid separation.
The solution is then treated either for the recovery of base metals or for disposal in an environmentally acceptable form. The residue is also treated, either for the recovery of precious metals or for disposal as tailings.
Heap bioleaching involves crushing ore, stacking it on plastic-lined pads and spraying it with a dilute sulphuric acid solution containing bacteria and nutrients. The solution drains through the heap and is recovered. It is then processed by conventional methods and re-sprayed over the heap. The ore 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.
As the ore in a heap-leach configuration is quite coarse (usually larger than 6.5 mm), the recovery is less than would be achieved in agitated and aerated tanks. Bacterial heap leaching is, therefore, generally undertaken when the economics cannot sustain the cost of making a concentrate or when the mineralogy is such that the ore cannot be concentrated.
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