|Bioleaching: A Game Changing Technology in the Tailings Reclamation Industry
Bacterial oxidation technology, also known as bioleaching, originally developed by BacTech Mining Corporation (re-named REBgold Corporation effective December 2, 2010), employs the use of naturally occurring bacteria, harmless to both humans and the environment, to liberate precious and base metals from difficult to treat ores, concentrates and tailings. However, this is only one of the benefits of using bioleaching for the remediation of tailings and other mining stockpiles. Through its ability to neutralize the source of Acid Mine Drainage and stabilize toxic heavy metals, BACOX bioleaching addresses several tailings-related environmental issues at once, while generating revenue that can effectively fund the remediation process.
Put simply, bioleaching uses naturally-occurring bacteria in reactors (tanks) to oxidize sulphides. The key is that by providing the bacteria with optimal operating and living conditions in reactors, they are capable of oxidizing sulphides in as little as 5-6 days, as opposed to many years in their natural habitat. This is a common residence time in a commercial bioleach plant and the control of this and other parameters, such as the particle size of feed, are relatively simple and readily managed by operators with the correct training. The registered trademark for REBgold's bioleaching technology is "BACOX".
BacTech Environmental Corporation (BEC) was created in December 2010 following completion of a Plan of Arrangement originated by BacTech Mining Corporation (now called REBgold). Under the Plan of Arrangement, BacTech Mining shareholders were given 1 share of REBgold for each BacTech share they held, and 1/5 of one share of BacTech Environmental; that is how BEC came into existence.
BacTech Environmental was granted a perpetual, exclusive licence by REBgold to use the BACOX technology for the remediation and reclamation of tailings and waste rock left by previous mining operations.
Cleantech Alternative to Conventional Methods
The historical approach to treating refractory arsenic ores has been to use smelting and/or roasting. This entails subjecting the sulphide ore to intense heat, whereby the sulphides are burned off, leaving the desired metals for recovery. There are strict limits on the amount of arsenic that can be burned through a mineral treatment process, and the general industry rule is that smelters cannot treat base or precious metal concentrates that contain more than 1.5% arsenic. Furthermore, they are also subject to a total tonnage limit that can be produced in a given year.
Since bioleaching works in the absence of direct heat, there are no arsenic trioxide (As203) gases produced through the process. In fact, bioleaching can treat concentrates with much higher levels of arsenic than what would be allowed using a pyrometallurgical process. Precious metal concentrates produced from tailings in a region of Northern Ontario regularly return readings for arsenic of over 10%, thereby eliminating them from consideration for smelting.
Bioleaching, given access to a ready supply of iron, either internally generated or through the addition of pyrite from external tailings, can handle much higher levels of arsenic for neutralization. Bioleaching converts the arsenic present in the tailings into ferric arsenate which is a stable and environmentally benign end product.
Previous testing of bioleaching precipitate residues using the Toxicity Characteristic Leaching Procedure (TCLP) has been undertaken in conformance with EPA Standards, which are recognized internationally as the test method for environmental stability of such materials. The results serve to confirm the environmental suitability of the precipitates for disposal. The EPA TCLP is the procedure used by Ontario Regulation 558 to determine the suitability of a byproduct for land disposal.
Bioleach plants built by REBgold in the gold industry treat multiple types of arsenical concentrates and readily achieve the EPA TCLP criteria. Staff also routinely monitors any ferric arsenate produced according to the TCLP protocol to ensure the material remains stable.
In addition to the avoidance of S02 airborne emissions, bioleaching can be an effective method for treating Acid Mine Drainage through the oxidation of sulphides, which are responsible for leaching metals into surrounding areas. As opposed to 'band-aid' solutions that treat contaminated water and/or soils indefinitely, bioleaching acts as a 'cure' by targeting the source, the sulphides in tailings, and thus preventing future acid generation from occurring.
There are numerous technologies that are being developed using a hydromet process, but only REBgold and Gold Fields have successfully commercialized their gold bioleach processing technologies to date. BacTech Environmental intends to apply this proven technology on tailings for base metal extraction, Acid Mine Drainage remediation and arsenic stabilization.
An example of an alternative hydromet process to bioleaching is pressure leaching, in which high temperatures and pressures are used to "crack-open" sulphides. However, these processes are expensive and complicated, using sophisticated equipment requiring a high skill level, making it difficult to train local people as operators. The high cost of a pressure leaching operation also makes it more vulnerable to low metal prices, making the long term viability of pressure oxidation more questionable in these times of economic uncertainty.
A common theme for many environmental applications of pressure leaching is the inability to make a high enough grade of concentrate containing sufficient metal value to sustain an expensive treatment process such as pressure leaching. Secondly, if a high grade concentrate can be made, then only a small proportion of the metals can be collected into the concentrate, making a potential remediation operation ineffective, as much of the metal value and harmful elements such as arsenic are still left in the original feed material. As bioleaching is able to process lower grades of concentrate, this makes the operation far more amenable to remediation applications. The concentrate can recover more of the metal value and harmful elements if there is less emphasis on the need to meet specific criteria for the grade of concentrate to be produced.
A common query for any arsenic treatment process relates to the stability of the final arsenic product and its fate over time in the environment. The stability of the final ferric arsenate product has never been an issue at the REBgold arsenical gold projects nor, to the best of our knowledge, has it been a concern at any of our competitor's sites, resulting in a total of 20 successful projects worldwide since 1986.
Over this time, a variety of sources of published literature have confirmed the stability of the final products from commercial bioleach plants. One such reference entitled "Neutralization of Bioleach Liquors" by Nyombolo et al. of Mintek South Africa (a high-quality, accredited, world class research facility) tested the stability of ferric arsenate precipitates over a 7 year period and concluded the following:
"For all of the tests, the very long-term trend is for an improvement in arsenic stability. Precipitates that are stable initially tend to remain stable, and those that exhibit some instability in the first few years appear to gain in stability after about two years. This is a very encouraging finding, and indicates that fears of a decrease in stability could occur over the long term are unfounded."
We believe that a majority of the tailings could be processed in a commercial facility that will adopt a broad design envelope to cater to variations in tails characteristics. The REBgold plant in Laizhou, China was designed using a similar strategy for being able to treat "unknown" arsenical gold ores from a variety of origins. Bioleaching lends itself to this type of strategy and copes with greater fluctuations in feed types than smelting or pressure leaching processes.
In the commercial treatment scenario, each of the tailings piles would be sampled and tested in the laboratory prior to reclamation and processing in the commercial facility to ensure that the arsenic can be extracted/stabilized and the metal values recovered. This is similar to the strategy currently employed in the REBgold-built plant in China where samples are sent for testing prior to establishment of contracts for treatment. In the nine years of operation of the plant in China, it has been very rare for a concentrate to be rejected as not amenable to processing.
Evolution of Bioleaching
• Bioleach technology first investigated in the mid 1980s at King's College, London, England for the elimination of sulphur from coal
• Technology, still in its infancy, migrates to Perth, Australia, in the late 1980s, where it is funded privately
• 1994: A public company, Gold Mines of Australia, builds the first BacTech bioleach plant at the Youanmi Mine in Western Australia. Bioleaching provides additional mine life of three years processing refractory arsenic gold concentrates from the mine. The mine was closed in 1997 due to low gold prices, but BacTech had its first successful commercial application.
• 1998: A public company, Allstate Mining, licenses and installs the 2nd BacTech bioleach plant to process refractory arsenic ore from the Beaconsfield Mine in Tasmania (Australia). The mine is now in its 12th year of continuous operation.
• 2000: A Chinese company, Shandong Tarzan Biogold Co. Ltd. ("Biogold"), licenses and installs a bioleach plant capable of treating 100 tonnes of concentrate per day from mines both in China and abroad, demonstrating the diversity of the technology for treating non-homogenous feeds from various metallurgical backgrounds. Recently, the current owner, Sino Gold Mining Limited, doubled the capacity of the current plant to 200 tonnes per day.
• 2001: Industrias Peñoles S.A. de C.V. ("Peñoles"), the world's largest silver producer, contributed USD$5 million to build a demonstration plant in suburban Monterrey, Mexico, to test the technology's ability to treat dirty or complex base metal concentrates. The findings of the study proved the benefits of bioleaching with respect to (a) neutralizing deleterious elements in the concentrate, and (b) eliminating costly transportation of concentrates to smelters, thereby reducing the environmental footprint left by smelting and truck haulage usage.
It should be noted that since 2001, research and development carried out by REBgold has led to many improvements to the technology, and what exists today is considerably different and advanced compared to what was used in the past.
Intellectual Property: Patents & Expertise
BacTech has always protected its intellectual property through the application of process patents in countries where deemed necessary. Since bacteria are naturally occurring, we cannot apply for any protection on them as entities, but can apply for patents relating to their application. We will continue to advance our base metal bioleach technology and we will apply for patents as appropriate. However, we have found that the art of bioleaching is made up of 50% process patent and 50% know-how, which can only be acquired through a lengthy commercial track record.
Summary of Advantages
• The bacteria are naturally occurring and harmless to the environment and human health
• What is being treated is the actual source of acid generation and not the symptoms
• Prevention of future Acid Mine Drainage through sulphide neutralization
• Toxic heavy metals, like arsenic, are converted into stable, environmentally benign products
• No gaseous emissions (e.g. S02,As203)
• Efficient reuse of water from the process
• Remediation of tailings at no cost to government
• Revenues through the recovery of contained metals -- Au, Ag, Cu, Ni, Zn, Co, Pb
• Local opportunities for work, training, development and the start of a new 'green' industry
• Lower cost than the alternatives (e.g. pressure oxidation) and requires less capital
• Easy to expand to accommodate tailings from many types of mines within a region
• The technology is already commercially proven
• BacTech has "in-house" capacity for the design, construction and operation of the plant
• It is a continuous process which is safe, easy to operate and flexible
• Scalable in a short period of time and capable of processing a wide variety of low to high grade mine tailings