There are several bioleach pilot plants in existence. Noranda is involved in one in Mexico. Most of the majors are involved in some sort of pilot project,...
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http://www.mintek.ac.za/bulletin/feb2002/lme_cathodes.htm
'A' Grade copper cathodes produced at bioleach demo plant
MINTEK, BacTech Enviromet Corporation, and Industrias Peñoles SA de CV of Mexico have produced the first commercial-sized copper cathodes from copper concentrate using their proprietary bioleach process technology.
Vishal Deeplaul (Mintek), Mike Rhodes (BacTech), Teodoro Elicier Martinez (Peñoles), and Bakhulise Nyombolo (Mintek) with one of the 'A' Grade cathodes
Procesos Biometalúrgicos SA de CV (PBM), a joint venture company representing the three partners, has produced more than 40 t of cathodes at its demonstration plant in Monterrey, Mexico. The plant, which uses an integrated tank bioleach, solvent extraction and electrowinning (SX/EW) circuit, has a design capacity for 500 kg/day of copper (Mintek Bulletin no. 131).
A composite sample of one month's copper cathode production from the plant met the London Metals Exchange (LME) 'A' Grade specifications (ASTM Standard B 115-00 for Electrolytic Copper Cathode). The sample was analysed by three laboratories, all experienced in the assaying of cathode copper - Cobre de México (Mexico), Mexicana de Cobre (Mexico) and Ledoux & Company (USA). The copper content averaged 99.9965 per cent and all the minor elements were within specification.
Dr Tony Pinches, Manager of Mintek's Biotechnology division, said that the production of copper cathodes represents a significant step towards the completion of a feasibility study being conducted by PBM to evaluate the viability of a commercial plant to produce 25 kt/a of copper from concentrates produced in central Mexico. A pre-feasibility study, based on the performance of the demonstration plant, is expected to be completed in the first half of 2002.
BacTech-Mintek and PBM are also reviewing a number of other base and precious metals project opportunities that may be amenable to the proprietary process technology. 'This breakthrough on copper presents a tremendous opportunity for all partners of PBM to create substantial shareholder value through the acquisition of interests in such projects' said Mr Geoff Donohue, BacTech's CEO.
Historically, the bulk of the world's base metal supply has come from sulphidic ores. Typically, these ores are milled and floated to produce a concentrate, which is then treated by roasting or smelting and refining to produce metal. The problem with this traditional method is that the roasting and smelting process generates large quantities of sulphur dioxide gases. So-called 'dirty concentrates' can also release toxic elements such as arsenic which are increasingly becoming environmentally unacceptable.
'The BacTech - Mintek proprietary process provides mining companies with the ideal solution - an environmentally friendly technology that keeps them on the right side of the environmental debate and allows the production of metal from previously uneconomic ore or concentrate,' Mr. Donohue said. 'Peñoles, through their belief in the technology and financial commitment to PBM have played a pivotal role in the development of the process to recover copper from sulphide concentrates.'
http://www.bactech.com/details.html
Bioleaching of minerals (also known as bacterial oxidation or bio-oxidation) is carried out by naturally occurring micro-organisms. The micro-organisms which are employed in BacTech's technology are thermophiles. They are approximately 0.5-2.0 micrometers long and 0.5 micrometers wide and can only be seen under a microscope. Whilst most living things derive energy for growth and reproduction from organic carbon, the thermophiles grow on inorganic matter and are harmless to living creatures. Their diet consists of pyrite, arsenopyrite and other metal sulphides such as chalcocite and chalcopyrite. Thermophiles and other types of bacteria may be found in acidic environments produced by the oxidation of sulphur, for example in and around hot-springs, volcanic regions and sulphide-rich areas. The thermophile cultures developed by BacTech were originally isolated from a mine in Western Australia. This culture is robust, thriving in sulphuric acid environments with high dissolved metal concentrations under the high ambient temperatures found in many remote mining locations.
Thermophiles
How thermophiles and other "rock-eating" bacteria oxidize sulphide minerals is not precisely known. Both chemical and biological forces work together to oxidize the metal sulphide to form 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, such as copper, prior to neutralisation. The residue generated through the process may contain precious metals, exposed by the bacterial oxidation, that can be recovered by cyanidation.
There are currently two methods of application of bioleaching: tank bioleaching and heap bioleaching.
Tank Bioleaching
Bioleaching occurs rapidly, over 500,000 times faster than oxidation by natural exposure to air and water in the absence of bacteria. The use of controlled conditions, such as agitated, aerated tanks results in rapid and highly effective oxidation of metal sulphides. For example, an improvement in gold recovery from 30% to 90% may be achieved with 3 to 5 days of bioleaching oxidation prior to cyanidation.
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.
Typically, separation of the solution and residue is carried out through a counter current decantation circuit using thickeners. The solution is 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.
Bioleaching in tanks has to date been used to treat high value concentrates. The cost of tank oxidation is influenced by the following factors:
The rate of reaction and the level of sulphide oxidation required for acceptable metal recovery, determines the size and number of tanks and the aeration and agitation requirements and affects capital and operating costs.
The reagent usage for pH control and solution neutralisation affects the operating costs.
The salinity of site water and the requirement for acid resistant materials affects the capital costs.
Heap Bioleaching
Bactech and Mintek are conducting a program with Noranda Inc. of Canada to develop the technology for bioleaching of chalcopyrite copper bearing ores using high temperature thermophilic bacteria. This will create a very significant commercial opportunity if successful.
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 and resprayed over the heap.
Where the ore contains base metal sulphides the base metals are released into the solution and recovered by conventional processes prior to the return of the solution to the heap. Where the ore is gold bearing, the solution is recycled until sufficient sulphide has been oxidized to expose the gold. 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. If the ore contains sufficient gold, better recoveries can be achieved by processing the oxidized ore through a conventional milling and cyanidation circuit.
As the ore in a heap leach configuration is quite coarse, usually larger than 6.5 millimetres, the recovery is less than would be achieved in agitated and aerated tanks. Bacterial heap leaching is, therefore, generally considered when the economics cannot sustain the cost of making a concentrate or the mineralogy is such that the ore cannot be concentrated.
Comparison of Bioleaching to Other Processing Technologies
Processing of precious and base metal oxide ores is generally carried out by crushing and/or grinding the ore and subjecting it to a hydrometallurgical treatment to recover the metal of interest. Gold ores are treated with cyanide to dissolve the gold which is recovered on activated carbon, while base metal ores are leached with acid and the soluble metal is then recovered by methods such as solvent extraction and electrowinning. Due to the relative ease of processing these materials, much of the world’s metals have been produced from these ores.
As oxide resources are reduced, the remaining ores tend to be refractory in nature. Ores are considered to be refractory when a significant portion of the contained metal cannot be recovered by simple grinding and extraction and if the metal of interest is locked within other minerals or elements such as sulphide sulphur, or when elemental carbon is present which may interfere with the extraction process.
There are three principal pre-treatment processes for refractory ores:
Roasting (Smelting): Roasting of ores and concentrates has conventionally been used to break-down sulphide minerals. Roasting involves heating to 600°C - 800°C and can be capital and operating cost intensive. If arsenopyrite is present, a two-stage roaster is often required to drive off the arsenic (as arsenic trioxide) and then oxidize the remaining sulphide. Gas scrubbers are essential to contain sulphur dioxide and arsenic trioxide emissions, which are both of environmental concern. Two-stage roasters and emission control devices greatly increase capital costs. Securing permits for roasters can be a lengthy, burdensome and costly process. The only disposal alternative for recovered arsenic may be hazardous waste land fills, because the purity standard of the arsenic and the existing global market surplus might preclude sale. Land fill disposal increases operating costs and may result in perpetual liability for the then current landowner. Roasting arsenopyrite ores and concentrates also creates health and safety issues that must be addressed with increased vigilance.
Pressure Oxidation: If the grade is high enough, an autoclave process involving steam and oxygen injection under pressure can be used to oxidize the sulphide minerals. Autoclaves are capital and maintenance intensive because of the advanced materials needed for their construction and the need for an oxygen plant. Autoclaves require long lead times for fabrication and installation. The high level of operator training and skill which are necessary because of the complexity of operation and maintenance required, and increased safety requirements required to handle the high pressures and temperatures, increase the operating costs of this process.
Bioleaching: Bioleaching (bacterial oxidation) was first used in commercial practice for treating sulphidic ores and concentrates in 1986. In the case of gold bearing concentrates the process is a pre-treatment which oxidizes the sulphide minerals, and exposes the precious metals for subsequent cyanide leaching. Good recoveries (above 90% for concentrates) are achievable.
For base metal concentrates the valuable metal such as copper or nickel remains in the solution and when the solution is separated from the solid the metal can be recovered by conventional technology such as solvent extraction or precipitation.
Polymetallic deposits such as those containing copper and zinc provide ideal opportunities for bioleaching as both the copper and any zinc in the concentrates will be solublised and can be recovered from the solution. The ability to recover zinc in the solution may allow a mine owner to produce a lower grade concentrate with high zinc levels with higher recoveries which would be unsuitable for a smelter but well suited to bioleaching.
A similar situation exists with concentrates containing a base metal such as copper and gold where the copper is recovered from the solution and the gold from the residue.
Optimum temperature, nutrients, oxygen supply, pulp density and residence time need to be established for the best results. The 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.
New Developments
BacTech has recognized the need to reduce the cost of bioleaching by developing and testing two new reactors. The first reactor has been developed by Paques Biosystems of the Netherlands and this reactor will be tested during in conjunction with the copper bioleach demonstration plant in Mexico. A second reactor the BAR (BacTech Aerated Reactor) designed by BacTech will also be tested in Mexico. |
