(Originally printed in CIM Magazine)


Title: Thickened tailings pave the way

Sub-title: Incorporating this relatively unused technology into Osisko’s Canadian Malartic development helped get the project approved by regulatory authorities.


When managers from Montreal-based mining developer Osisko first began to study the Canadian Malartic gold deposit, one of their first challenges 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’Environment,” says Jean-Sébastien David, Osisko’s vice-president (sustainable development). “The process took place in a politically charged atmosphere so we knew that the solutions that we proposed had to be perfect.”


The Canadian Malartic property is an orphan site, with an existing tailings pond, left over from operations carried out a by series of previous companies that did work there. 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, has mostly been used to back-fill old underground mines.


Building a mountain

The Osisko thickened tailings would then 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 meters, high 2.5 kilometres long, cover 450 hectares in area that will eventually be covered in vegetation.”


According to David, converting effluent to thickened tailings may cost the company more than conventional disposal methods, such as building new a traditional tailings pond and storing them there. However the new method’s positive environmental impact was a key element in getting the operation approved. (The Quebec Government gave the project final authorization on August 19, 2009).


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,” says David. “Otherwise we would have had to build huge dams or dig a new reservoir to hold a traditional tailings pond.” 


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


Another key attribute of thickened tailings are their hydric properties, which allow them to remain saturated with water. This in turn enables them to slow the diffusion of oxygen and to curb acid drainage.


A partnership to clean up the site

Key to the initiative was a partnership deal that Osisko struck with the Quebec’s Ministère des Ressources Naturelle et de la Faune (MRNF) to split the cost of cleaning up a legacy tailings site already on the property, just adjacent to the outskirts of the town of Malartic.


The work will consist broadly of covering and neutralizing the existing acid-generating tailings with the non-acid generating thickened tailings produced from the Canadian Malartic mine. To do this, water in the entire existing tailings pond will be fully drained, into a reclamation pond, where it will re-cycled and used in Osisko’s mining operations. Company officials are particularly proud of the fact that this reclaimed water will supply all of the mining facility’s production needs.


When full production rates are reached the new mine is expected to generate 55,000 tons of thickened tailings per day. At this pace all of the legacy East Malartic site will be covered with a three-meter thick layer within three years. A stone wall will be built around the deposit to make sure that the mixture does not spread. Then, additional layers will be added one at a time, as the thickened tailings accumulate, and each layer will have a stone wall built around it.


According the David, because of its costs, the thickened tailings disposal method is only very rarely used. The Kidd Creek mine in Timmins Ontario is one of the few in the country that resorted to the process, due to the geographical impracticality of building a conventional tailings pond there.

This will be the first time that thickened tailings will be used in this manner in Quebec.


According to Quebec government documents, the Malartic cleanup deal was central to enabling Osisko to “begin operations at the site immediately, to minimize the amount of natural resources (such as sand, gravel, clay and so on) that would be needed to rehabilitate the site and to sidestep the need to build a new large scale mine tailing site, which would disturb other regions.”


A win-win situation

The deal was also good for the Quebec government, which had been responsible for the East Malartic site since 2004, following the bankruptcy of McWatters Mining Company, its previous owner. The government, which would have been on the hook for the $23 million in projected costs needed to rehabilitate the site, will now have considerable resources to invest elsewhere.


According to David, in all 860 hectares will be reforested it the Malartic site. This will include 700 hectares for the tailings pond, 25 hectares of roads and the mine plant site, 35 hectares around the open pit, and 100 hectares of the polishing pond area. This in turn will effectively provide a new carbon sink.


As noted above, the thickened tailings project comes at a cost. According to a recent investor relations presentation published by Osisko, the company has budgeted tailings and water management expenses at $26.2 million.





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