Bioleaching technology 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 and concentrates.

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 metal encapsulated in sulphides in as little as 5-6 days, as opposed to many years, in their natural habitat.

The historical approach to treating refractory ores has been accomplished by using 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. A side effect of burning sulphides is the creation of arsenic trioxide (As203) which is partially released into the atmosphere. Since bioleaching works in the absence of heat, there are no arsenic trioxide 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.

Additional work will be required to optimize the recovery of all metals. The bio leach process has many real advantages over the conventional refractory processes such as roasting, pressure oxidation and nitric acid leaching. These include:

• Improved rates of recovery.
• Significantly lower capital costs.
• Low running costs.
• Robust technology that is suited to remote areas.
• Low level of skills required for operation.
• Environmentally friendly.
• Ongoing process development and improvement.

Solvent Extraction / The Electrowinning (SX/EW) process will follow the Bioleach process to produce 99.9% pure marketable nickel. Click here to read more about the solvent extraction / electrowinning process.

In addition to Lynn Lake there are several other base metal mines in Central Manitoba (or Northern Minnesota) that would benefit from a low cost metal extraction process.

There are three nickel smelters in Canada, eight in Africa, which include PGM plants, three in Russia and one each in Australia, Brazil, Finland, and China (which is the only country possibly considering construction of these new plants).

Applying Bioleach Tank Reactor at Lynn Lake

Stirred-Tank Reactor Technology

The Stirred-Tank process uses highly aerated, continuous-flow reactors. Finely ground mineral concentrate or ore is added to the first tank together with inorganic nutrients in the form of ammonia- and phosphate-containing fertilizers.

The stirred suspension flows through a series of pH- and temperature-controlled aeration tanks in which the mineral decomposition takes place. Mineral decomposition takes only days in stirred-tank reactors compared with weeks or months in heap reactors.

Stirred tank reactors that operate at 40°C and 50°C have proven to be highly robust and very little adaptation is required for the treatment of different mineral types.

One of the major constraints concerning the operation of stirred tank reactors is that the quantity of solids (pulp density) that can be maintained in suspension is limited to 20%. At pulp densities in excess of 20%, physical and microbial problems occur due to constraints on the oxygen mass transfer process. Not only does the liquid become too thick for efficient gas transfer but the shear force induced by the impellers also causes physical damage to the microbial cells.

This limitation in solids concentration, plus considerably higher capital and running costs compared to heap reactors, has meant that the use of stirred reactors has been restricted to high value minerals or mineral concentrates such as the rich Lynn Lake concentrate.

Bio Leach vs Shipping Concentrate to Smelter

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A bulk concentrate is normally more difficult to market since smelter contracts typically pay for only the highest-cost ingredients, since secondary metals are often easily lost to slag, making them more expensive to recover. This provokes two options:

a. Conventional filtering, drying and shipment to a toll smelter.
b. Onside leaching of concentrate.

In addition, hydrometallurgical processing off-site also requires high transportation costs, or an equally high cost of transporting acid to the mine site.

An alternative to transporting acid in is to produce acid on site. Heap leaching has been evaluated but it had been estimated that the capital cost of creating impermeable leach pads would be more expensive than the cost of building a flotation plant and grinding mill. However, a tank reactor using the bio-leach process allows for multiple streams to be processed simultaneously and also has greater sustainability for Lynn Lake flotation concentrates.

Lynn Lake is a project with proven reserves, existing power and rail access. The project can be operated year-round with a low Cap-Ex cost of CDN$148 million. The overall cost margin is well defined at US$7/lb nickel, inclusive of cap-ex. Ore can go through the onsite bio-leach SX/EW option to produce 99.9% pure marketable nickel onsite, or it can ship to a smelter in concentrate form (9% Ni) after milling. The location is in a safe jurisdiction and the extensive prefeasibility study validates the economics of the project. Lynn Lake is a relatively low-risk project on path to production within 2 to 3 years.

Bioleaching Process

Bioleaching is a process, whereby metals are leached from ore as a result of bacterial action. In nature, bioleaching is triggered spontaneously by micro-organisms in the presence of air and water. Commercially applied bioleaching technologies utilize the same phenomenon, but accelerate this natural process. Several physicochemical and microbiological process parameters are modified in order to enhance and speed up the metal recovery process.

Typically, primary and secondary sulphides are associated with pyrite which, when oxidised, has the potential to release sufficient quantities of heat. The biological oxidation of the sulphide components of minerals is an exothermic reaction which releases substantial amounts of energy. This process needs to be carefully managed to maximise effective metal recovery. To reach and maintain the temperatures required for enhanced sulphide mineral leaching, different microbial populations are required to be present over time and the microbial growth rates need to be optimum.