MIRARCO CEO touts bioleaching
A more environmentally friendly way to extract metal from sulphide-based and iron-containing ores and mineral concentrates is receiving more recognition in the mining industry.
Bioleaching, sometimes called bio-oxidation, is growing in popularity as a way to process certain metals using bacteria. These living organisms have the ability to break down, or oxidize iron and sulphides in metal-containing ores to produce ferric iron and sulphuric acid. This in turn, converts the insoluble sulphides of metals like copper, nickel and zinc to soluble metal sulphates that can be recovered from solution, explained Stephen Hall, president and CEO of Sudbury’s Mining Innovation, Rehabilitation and Applied Research Corporation (MIRARCO).
“Their food source is the process of putting metal into solution from sulphide minerals, so that is how they feed,” Hall explained. “Like all living organisms, they breed, require nutrients and will in fact take carbon dioxide from the air as a source of carbon to grow. … The process will also generate heat, which from a researcher’s viewpoint, is quite an interesting by-product.”
Hall, who is a metallurgist and studied bioleaching in Australia and Africa prior to arriving in Canada last year, said the microbial action can also be used to break down sulphide minerals that lock up very fine gold particles. Then a process using a cyanide solution allows access to the gold.
Currently, this technology is being used extensively in South Africa for arsenic-rich gold ores that are more complicated to process.
Although perceived as a new technology, using microorganisms to decompose a variety of mineral deposits is an old process that dates back to Roman times. In the 1970s, work performed in South Africa brought the process from laboratory novelty to a commercially viable operation in the mid-80s at the Fairview Mine near the town of Barberton. Initially designed to treat 10 tonnes of concentrate a day, it has been increased to treat 55 tonnes per day.
There are two bioleaching methods: one is the heap or dump method and the other is a stirred-tank reactor. There are pros and cons to both methods.
The stirred-tank process involves vigorously aerating a finely-milled mineral suspension. A controlled environment produces robust microbial action within a few days. Capital and operational costs are higher, and the quantity of solids maintained in suspension is limited to 20 per cent. Consequently, only higher-value mineral concentrates are used in this method.
The heap-reactor method is a much slower process, taking months rather than days, and is generally used for lower-grade ores. Crushed ore is piled onto an impermeable base and irrigated with an acidic-leaching solution that percolates through the ore. A collection system retrieves the metal-containing leach solutions. Creating optimal pH and temperature levels is more complicated and may result in uneven leaching and breakdown of the sulphides. This method results in lower overall metal recoveries, for example 55 per cent compared to 85 per cent, Hall said.
The most commonly-used bacteria operate best around 30 degrees Celsius, but will take themselves (heat up) to a temperature above 60 degrees Celsius and go dormant, be less efficient, or die if they are not cooled down. At this time, the processing plants using the stirred-tank reactor technology have to use energy to cool the bacteria down.
A process to capture the heat generated from these microbes has not been engineered, but it is something that is being considered, Hall said. He added that if the heap leach process can be sped up (which is presently being researched), it can become more cost effective for historical tailings material, which is where he’d like to see the technology applied.
“It may be cost effective to deal with the material that is generating acid mine drainage and pay for the treatments through some of the metal recovery.”
Hall sees the potential to mitigate environmental liability, produce heat as a by-product and earn carbon credits resulting from the capture of carbon.
Researchers are always looking for alternative processes, particularly in light of growing environmental concerns, said Hall. In 2007, a feasibility study for a bio-oxidation plant began for Independent Nickel Corp’s Lynn Lake nickel property in Manitoba.
While the technology is advancing and bio-oxidation plants are getting larger, the biggest problem preventing heap leaching from getting greater acceptance in the mining industry is that it takes time.
“If you engineer the process in a simple heap of crushed ore, it takes several months to get a reasonable extraction of the metal of interest,” Hall said.
Historically, the heap-leaching method was used in Elliot Lake, Ontario for the recovery of uranium. Currently, it is being used in Chile to extract copper from low-grade ores.
As environmental concerns come to the fore, biomining may become the method of choice, if it is economically feasible.
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