 | Why Use Bioleaching?Over the last two decades, the application of bioleaching to the treatment of refractory gold deposits has been of major interest to mining companies throughout the world. The majority of the gold in these deposits is locked up in a sulphide matrix within the ore, and provides for very poor gold recoveries unless the sulphide minerals are removed first. The minerals of pyrite and arsenopyrite are the two most common sulphides associated with these refractory gold ores. In the past, the treatment method used has first involved making a sulphide concentrate that contains the gold, and then roasting this concentrate at a high temperature to drive off the sulphides as gases. This is then followed by recovery of the gold from the residue that is left behind. Roasting is, however, becoming increasingly unacceptable due to the production of sulphurous gases, i.e., emissions that often contain arsenic and cause damage to the environment. Bioleaching is an environmental alternative technique to roasting that does not create gaseous emissions, and is often a cheaper alternative to both roasting and other methods (such as pressure leaching) that might also be considered. In the commercial gold applications for which bioleaching has been used to date, the technique has shown that improvements in gold recovery from 20% without bioleaching pretreatment, to greater than 90% are readily achievable.
As well as the demonstrated use of bioleaching for the pretreatment of refractory gold concentrates, bioleaching also has potential application for the treatment of refractory base metal sulphides. Bioleaching can be used to oxidize metal-containing sulphides into soluble sulphate forms, and metals such as copper, nickel, cobalt and zinc can be extracted and purified into a saleable form directly at the minesite. Of particular current interest is the use of this technique for extracting copper from extremely refractory sulphides such as chalcopyrite. Traditionally, extraction of base metals from sulphides has been undertaken by the transport of concentrates from a mine site to an off-site smelter, which, similar to a roasting operation, can create severe environmental problems due to toxic gaseous emissions. For many mine sites, it also means the loss of control of the saleable metal production.
Therefore, bioleaching could be considered as an all-encompassing process for the treatment of very complex deposits containing multiple metals, and it can also be adapted to a variety of scales for different mines. The treatment of complex copper/gold deposits by bioleaching is a good example of how bioleaching may be applied in the future for treating deposits containing more than one metal of value.
What is Bioleaching and BacTech's Commercial Process Technology?
The terms bioleaching, biooxidation and bacterial oxidation are all used to describe the process for metal extraction that harness specific types of bacteria that degrade sulphide minerals naturally. BacTech's technology takes bacteria cultured from the environment and applies them in a way that speeds up the natural process of sulphide degradation by up to 500,000 times. There is no "re-engineering" or manipulation of these natural bacteria, but BacTech uses its' knowledge and experience of how to isolate and apply the bacteria, and create the perfect environment for them to operate effectively. This converts a slow natural process into a commercial economic process for metal extraction, while protecting the environment by not using the alternative techniques such as roasting. The only byproduct of bioleaching is an inert iron or iron arsenate precipitate, which is environmentally benign.
What are Bioleach Bacteria Like and How Do They Work?
The types of bacteria used in bioleaching are quite unique in that, unlike most living things that derive energy for growth from organic carbon, they gain energy for growth by oxidising sulphide minerals. They are not harmful to animals or plant life and are only visible using a microscope, being about 1um to 2 um in size. Some types of bioleach bacteria are rod shaped, while others are spherical, and yet others are spiral in form. In a typical bioleaching process, the culture grows and divides rapidly, resulting in the presence of millions of cells for each gram of sulphide to be oxidised. As they live in an aqueous environment, the technique of bioleaching is known as a hydrometallurgical method of extraction.
How these "rock-eating" bacteria actually work has been the subject of investigation by scientists since the 1940's when they were first isolated in the laboratory. A number of theories have been advanced, and the knowledge base is growing in evaluating the mechanisms involved in which both chemical and biological forces work together to give the individual reactions for sulphide oxidation. The bacteria work both directly on the sulphide minerals, as well as being active in solution. They live as mixed populations, with different types having slightly different requirements and abilities, thereby creating symbiotic and competitive relationships to degrade the sulphide substrates.
How are the Bacteria Employed in a BacTech Commercial Process?
The bacteria must be provided with air, small amounts of simple fertililser type nutrients and be in good contact with the minerals to be oxidised. For this reason a series or chain of stirred aerated tanks are used. A ground pulp of the minerals (or a concentrate) is introduced into the first tank with the oxidized product withdrawn from the last tank. Oxidation of the sulphides occurs as the pulp progresses through the series of tanks over a period typically lasting between 4 to 6 days.
Bacteria are lost continuously from the system during oxidation. By choosing the correct residence time, the bacteria can grow and divide quickly enough to maintain a high population within the tanks in order to oxidize the fresh incoming sulphide feed.
The overall process is exothermic, which means that heat is generated, and the tanks must be cooled with water to maintain an optimum temperature for the bacteria to operate at. This means that the technology can be used in a variety of both hot and very cold climates, as the actual temperature of the process is well controlled. Sometimes reagents additions are added to the tanks to control the level of acidity of the pulp. Overall, it is a very simple process using non-sophisticated equipment. This makes it ideal for use at a variety of scales, and also in a variety of remote regions of the world where skill levels of operation may be lower.
What's BacTech's Commercial Experience?
BacTech was a pioneer in the early application of bioleaching processes using specially isolated bacterial cultures for treatment of refractory gold concentrates. It is one of only two companies in the world that have commercially demonstrated bioleach technology of this type.
BacTech's technology was first proven commercially at the Youanmi mine in Western Australia in 1994, and in this first application a "thermophilic" culture was used. As the name suggests, it was a culture that operated well by degrading minerals at a slightly higher temperature (around 50°c). Thermophiles and other bacteria are often found in acidic environments produced by the oxidation of sulphur, for example, in and around hot springs, volcanic regions and sulphide- rich areas.
Since the first application of BacTech technology in Western Australia, two other gold operations, one in Tasmania and one in China, have used BacTech technology for treatment of concentrates. These latter applications have both used "mesophiles" which operate at a lower temperature and demonstrate robustness for treating a wide range of sulphide minerals.
It is clear that one of the skills BacTech has developed over time is in matching the best natural culture to the type of minerals to be degraded for different applications. However, it is also the knowledge of how to apply these bacteria at a commercial scale that has provided the main focus for BacTech's technical expertise.
BacTech has also made considerable advancements in base metal leaching which, despite excess current smelter capacity, continues to be recognised by many as the future environmentally friendly alternative to smelting. The technology for the treatment of complex and "dirty" copper concentrates has been progressed by BacTech in partnership (Penoles-Mexico) on a demonstration scale plant in which the technical viability of a process integrating bioleaching with down stream metal recovery has been proven at 200 tpa annum cathode copper scale. This type of technology has been tested for treating extremely refractory copper Chalcopyrite that contain smelter penalty elements such as arsenic, zinc and bismuth, and it has now been shown that such concentrates are readily treatable by bioleaching.
What are the Attributes and Characteristics of a Bioleach Process?
Bioleaching is a natural process that has a remarkable ability to adapt to treating a variety of sulphide minerals from a single deposit. This gives it a unique quality in comparison to competing processes in being able to treat a variety of minerals through the life of a project. Importantly, bioleaching can often treat a lower grade of sulphide feed more economically than competing techniques, and this can allow an increase in overall metal recovery for a project.
Generally, the process tends to be self-managing and, once established, it is relatively simple to operate, which is in contrast to other techniques.
Bioleaching can also be uniquely selective in terms of the sulphides that are oxidized. This can be of particular value to some gold projects in which only a partial oxidation is required to maximize the release of gold values (arsenopyrite). As the cost of oxidation is directly related to the quantity of sulphide to be oxidised, bioleaching has further economic advantages to offer in cases where only a partial oxidation is necessary.
How Does Bioleaching Compare With Other Processes?
Roasting
Roasting has traditionally been used as the method for treating refractory ore and concentrates and, as already mentioned, is often environmentally damaging due to the production of sulphurous gases. Roasting employs temperatures of up to 600°c - 800°c to convert the sulphide values to sulphur gas. If arsenopyrite is present, a two-stage roaster is often required to drive off the arsenic as arsenic trioxide, and then oxidise the remaining sulphides. 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 is now much more difficult due to environmental concerns, and is a lengthy, burdensome and costly process. The only disposal alternative for recovered arsenic may be hazardous waste landfills due to the purity standard of the arsenic and the existing global market surplus of the metal. Landfill 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 Leaching
If there is a high enough grade of gold present, then an autoclave process involving the use of oxygen under high pressure and temperature can be used to oxidize the sulphide minerals. If there are large quantities of sulphides to be oxidized, then the use of pressure oxidation can be prohibitively expensive, as a larger oxygen plant is needed. This increases both operating and/or capital costs. Unlike bioleaching, pressure oxidation is not selective in the sulphides that are oxidized. As already mentioned, for some projects, it is only necessary to oxidize a portion of the sulphide values to maximise precious metal recovery. As the cost of oxidation is directly related to the quantity of sulphide to be oxidized, bioleaching has further economic advantages to offer in such cases. Even if a total sulphide oxidation is required, then bioleaching is usually the cheapest option to employ for oxidation, both in terms of capital and operating costs.
Autoclaves are capital and maintenance intensive because of the advanced materials needed for their construction and complexity of operation. They can require long lead times for fabrication and installation. The pressure leaching costs are further increased by the need for high operating skill labour requirements and increased safety requirement. In more recent times, these types of considerations have led to a greater preference for bioleach plants in comparison to considering the use of pressure oxidation.
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