November 2011

Thickened tailings pave the way

Site rehabilitation at Osisko's Canadian Malarctic Mine key to fast-track production

By P. Diekmeyer

When managers from Montreal-based mining developer Osisko Mining Corporation first began to study the Canadian Malartic gold deposit, one of their first hurdles was how they would deal with effluent disposal. “We faced a considerable challenge in getting the project through hearings held by Quebec’s Bureau d’audiences publiques sur l’environnement,” says Jean-Sébastien David, Osisko’s vice-president of sustainable development. “The process took place in a politically charged atmosphere, so we knew that the solutions that we proposed had to be perfect.”

In the past, a number of companies had worked on what is now the Canadian Malartic property, and Osisko inherited a tailings pond left over from their operations. To manage the overall footprint of the operation, the company officials decided to convert the effluent produced by Osisko’s planned mining operations into thickened tailings, a paste-like substance that, in the past, had mostly been used to backfill old underground mines.

As the name implies, thickened tailings are similar to the standard tailings effluent produced by mining operations, except that much of the water content has been drained out. In Osisko’s case, this will be done at the mill site prior to disposal.

“Standard tailings are 40 to 50 per cent solid,” says David. However, in the case of thickened tailings, that ratio can be increased to the level desired, which is usually between 65 per cent and 70 per cent solid. The finished tailings produced will be non-acid generating and pumped over to the disposal site through a pipeline.”

The thickened tailing’s hydric properties allow them to remain saturated with water. This, in turn, enables them to slow the diffusion of oxygen and to curb acid drainage generated from previous operations.

Building a mountain

The thickened tailings will be spread out to cover the existing tailings pond and then piled high on top of it. “We are building a small mountain,” says David. “It will be 54 metres high, 2.5 kilometres long and cover 450 hectares that will eventually be covered in vegetation.”

Generating thickened tailings takes more time and requires more equipment than producing conventional tailings. The major change that Osisko made to its mill circuit consisted of the addition of a second thickener, which is basically a settling pond, to improve water recovery efficiency. The resulting effluent is solid slurry that is sent to the detox plant. There, the cyanide within is destroyed and the remaining matter is then pumped to the tailings pond. This “solid” slurry is thus “stackable,” a major improvement over conventional slurry, because it requires a far smaller disposal area.

Yet according to David, despite the increased costs of the thickened tailings disposal process, there is a strong business case to be made for it. “It will enable us to remain within the existing brownfield area,” he says. “Otherwise, we would have had to build huge dams or dig a new reservoir to hold a traditional tailings pond.”

The new method’s positive environmental impact has been a key element in getting the operation approved. The Quebec government gave the project final authorization on August 19, 2009.

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