 | Bioleaching of Chalcopyrite for Copper - The Future
Bioleaching can be thought of as the acceleration of a natural environmental process using naturally occurring organisms that leach sulphide minerals. Although the phenomenon of bioleaching has been recognised for many years, it is a relatively new technology for mining companies to consider as a controlled process for copper extraction. Currently, companies are using bioleaching for ore treatment in heap leach operations, and programmes of process development are well advanced by BacTech for the future treatment of copper concentrates by bioleaching in agitated reactors.
The most prominent area of interest to the copper industry, is the treatment of the more refractory copper sulphide, chalcopyrite. Historically, chalcopyrite has proved a stubborn candidate for obtaining high recoveries using normal bioleach practices, either in ore heap leaching or using agitated reactors for concentrate treatment. The importance of providing a hydrometallurgical alternative to smelting for chlacopyrite has been the subject of earlier articles, and the quest for a successful method to bioleach chalcopyrite has proved a fertile area for research and development programmes in which much emphasis was placed on "passivation" phenomenon limiting copper recovery. Over the last few years, advances in understanding the requirements for successful bioleaching of chalcopyrite concentrates has led to experimental campaigns using continuous laboratory scale bioleach plant, to show that high recoveries in an economic time frame are achievable. These findings for concentrate treatment were confirmed on a larger demonstration plant, suggesting that bioleach facilities for chalcopyrite concentrate may be a commercial reality within the not too distant future.
Looking globally at the progress of bioleach technology, the economic application of bacterial oxidation in the gold industry at commercial scale has already been proven over a number of years. As a result of this experience, many of the process issues associated with the introduction of this new technology have already been resolved, and the knowledge gained in treating refractory gold concentrates is readily transferable to future bioleaching processes for the copper industry. Participants in the extractive industry are under increasing pressure to achieve and maintain bottom quartile costs, while meeting ever-higher environmental standards. From an environmental perspective, many people also consider that bioleaching will become a preferred method of beneficiation of sulphide deposits as the natural process produces stable benign products for disposal and there are no noxious gases.
Pilot Plants
So how close are we to seeing commercial scale operations for chalcopyrite bioleaching and what are the advantages and disadvantages of the technology foreseen at the current time?
Armed with the knowledge of how to overcome previous inhibitions noted to obtaining high copper recovery, a continuous small scale bioleach pilot plant was established to investigate the bioleaching of chalcopyrite concentrates in 1998 by BacTech at Mt. Lyell in Tasmania. The plant was integrated with downstream solvent extraction and electrowinning for copper recovery. Treating a few kilograms a day of concentrate over a period of a year, the plant achieved a 97% extraction of copper from chalcopyrite concentrate. Losses of copper in the recovery circuit were limited to about 1% to give an overall recovery of 96%. Although this plant operated on a clean chalcopyrite concentrate, it became apparent that one of the prime benefits of bioleaching would be in the treatment of "dirty" concentrates which otherwise attract high penalties using conventional smelting. Not only are such concentrates amenable to bioleaching, but penalty elements may be recovered as saleable commodities or disposed of as benign precipitates.
In 2001, the technology was progressed to demonstration scale by the joint technology partnership of BacTech and Mintek in conjunction with Peñoles S.A. de C.V of Mexico. The plant operated for a year with a capacity of 200tpa copper cathode production using commercial equipment, and demonstrated well the technical feasibility of a totally integrated process with high levels of copper recovery from a dirty concentrate blend. On completion of the demonstration trial, a feasibility study was conducted for a commercial bioleach plant currently envisaged as having an initial capacity of 25,000 tonnes of copper with a contingency to increase the capacity in the future. At that time, the low price of copper prevented BacTech from moving forward with the technology to commercial scale, but with dramatic increases in copper prices over more recent years, this situation has changed, and the economics of applying this environmentally friendly technology are much more favourable.
In this type of process for treatment of chalcopyrite concentrates, the material is finely ground and fed continuously into a series of agitated aerated temperature controlled reactors containing the bacteria. The residence time used of between 4 to 6 days is common to similar bacterial processes used in the gold industry. After separation of the oxidised residue, copper is then extracted from the liquor by conventional solvent extraction and electrowinning technology. Iron (and if necessary arsenic) is rejected from the liquor by neutralisation with limestone to give a stable solid of ferric or ferric arsenate for disposal which meets the strictest environmental requirements.
Potential benefits
The potential benefits of such a process for treating chalcopyrite concentrates can be summarised as follows:
- Bioleaching provides independence for on-site metal production and is particularly attractive when the cost of concentrate transport to the smelter is high, allows miner to salvage orphaned Sx-Ew plant
- Bioleaching can treat "dirty" copper concentrates which are either unacceptable to smelting or attract high penalties. Some of the elements penalised in smelting (for example, zinc) are solubilised in the bioleach process and can be recovered for sale.
- Bioleaching can readily treat concentrates containing arsenic, as this is solubilised and then stabilised as a ferric arsenate precipitate, which is readily disposable and environmentally benign.
- Bioleaching produces no noxious gases.
- Bioleaching can treat a lower grade of concentrate if required. This may often allow an increased recovery of a few percent in the production of the copper concentrate.
- Bioleaching can be used on both small or large projects for concentrate treatment
- Bioleaching uses conventional upstream and downstream process technology and the unit operation itself has been proven in the gold industry.
- Bioleaching is a relatively simple technology in terms of its equipment requirements and conditions of operation at ambient pressure and non-excessive temperatures close to ambient.
- Bioleaching can accept a variety of feed types over a period of time and is a more forgiving process due to the longer residence time compared to other treatment methods.
In conclusion, the next few years hold exciting prospects for the use of bioleaching in the copper industry for the treatment of chalcopyrite ores and concentrates. For many end-users, it may provide an economic and environmentally sound alternative to smelting and give their operations a greater flexibility and independence with on-site metal production.
|
| Technology
 | | Latest News
September 07, 2010
August 13, 2010
August 09, 2010
| Stay Informed |
|
