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In July 2008, we visited the town of Cobalt in northern Ontario to learn about the mining industry's legacy in this once-productive mining camp. What we saw and learned have opened the door to a new application of bioleaching, that being the environmental remediation of polluted tailings and the subsequent recovery of metal for sale to market.
In 1903, silver was discovered in Cobalt and that set off a mining rush similar to the Klondike gold fields. Between 1903 and 1922, over 300 million ounces of silver were produced from 100 mines. Keep in mind, that a mine can be anything from a project with a 1,000 foot shaft, to a hole in the ground made by a lone miner following a silver-rich vein. The grades of some of the veins in this camp reached 1,000 ounces per ton, which allowed for a significant amount of silver to be left behind if it was deemed to be too difficult to recover. In addition, there were very few applications or uses for cobalt metal in those times, and so it was thrown out with the silver tails. Prior to the global recession, cobalt metal traded upwards of $50 per pound compared to historic prices closer to $3 per pound. All of this means that there is considerable value entombed in the tailings of this mining camp and the hard work of blasting and lifting rock to the surface has already been done.
But there is also a big problem associated with the Cobalt tailings... arsenic.
Conventional mineral extraction practices in most mining regions, including the silver and gold projects in Cobalt, involved virtually unregulated processing of the ores, with no disposal control systems for the unwanted rock (i.e., mine tailings). Most tailings in the world contain sulphides, other toxic elements, and where previous technology limitations existed, economically recoverable quantities of precious or other valuable metals. Sulphides readily react with the atmosphere to create an acidic solution referred to as acid mine drainage ("AMD"). This acidic solution is very efficient at liberating certain heavy metals such as arsenic from mine tailings, resulting in pollution of the surrounding watersheds.
An estimated eighteen million tonnes of tailings were left in lakes, on shorelines and in open areas over a large region. These tailings contain high levels of arsenic that have been leaching into local lakes, streams and the drinking water. With the exception of one of the lakes in the area, most of the lakes contain between 0.040 to 6.510 mg/L arsenic, and water wells have reported concentrations as high as 6.97 mg/L. The maximum acceptable concentration for freshwater aquatic life is 0.050 mg/L, and for human consumption, it is enforced by regulation at 0.010 mg/L.
Arsenic toxicity has been linked to cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate, while non-cancerous effects include thickening and discoloration of the skin, stomach pain, nausea, vomiting, diarrhea, numbness in hands and feet, partial paralysis and blindness. Clearly, this is a serious environmental problem that must be cleaned up.
Bioleaching plant proposed for Cobalt area
BacTech has over 20 years of experience in the field of bacterial oxidation or "bioleaching". The Company has successfully designed, engineered, licensed and built bioleach plants for clients in the gold industry. However, no such plants exist in the base metal industry. BacTech is now poised to transfer its technology to environmental remediation applications that offer favourable economics through metal recovery.
In the Cobalt area, BacTech intends to build and operate a demonstration plant capable of treating 200,000 tonnes per annum ("tpa") of mine tailings. The plant will effectively remove a source of arsenic pollution (93%) in the region, along with recovering cobalt, silver, and nickel for sale to market. Successful operation of the demonstration plant will lead to the construction of a plant capable of treating 1,000,000 tpa over 15 years or more.
Clean air, soil and water benefits
There will be numerous clean air, soil, and water benefits to the environment with the operation of a demonstration and full scale tailings processing plant. Each tonne of tails contains 0.15% sulphur and the process will capture 85% of the sulphur produced. Therefore, the quantity of sulphur captured equals 1.28 kg of sulphur per tonne of tails. The equivalent amount of sulphur dioxide is then 2.55 kg per tonne of tails or 0.0025 tonnes sulphur dioxide per tonne of tails. At 200,000 tpa, the demo plant will save 509 kg sulphur dioxide emissions per year. A 1 million tpa commercial plant saves 2,547 kg of sulphur dioxide per year.
Other clean air benefits are as follows. The Cobalt tails contain quantities of arsenic at 0.42% and trace quantities of mercury at 4 grams per tonne. Such pollutants are common for many types of tailings materials. These elements would normally report to flue gases in smelting, which for a 1 million tpa operation equates to 4,200 tpa of arsenic (equivalent to 11,089 tpa of arsenic triooxide in flue gas) and 4 tpa of mercury, if normal smelting processes were used.
If BacTech's technology were not used for treatment of the concentrate, traditional methods (i.e., roasting and smelting) would release noxious SO2 and As2O3 emissions into the atmosphere. For soil and water benefits (assuming a 1,000,000 tpa plant), each tonne of tails contains 0.42% arsenic, 0.13% cobalt, 0.06% nickel. An acceptable recovery of 85% arsenic and 80% for the other metals would equate to 3.6 million kg per year arsenic, 1.1 million kg per year cobalt, and 0.5 million kg per year nickel that will no longer be in the soils and be leached into the local watershed.
Additional information
Fact sheet on the proposed bioleach plant
Presentation by Dr. Paul Miller, BacTech's Vice President of Technology & Engineering, to the Alta 2009 Nickel-Cobalt, Copper and Uranium Conference held May 25-30, 2009 in Perth, Australia
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