Pressures Force Changes in Disposal Practices
In most countries around the world, it has become extremely difficult if not impossible to dispose of tailings in a lake or wetland. Marine disposal is seldom even considered because of international opposition. In some jurisdictions, valley deposition is discouraged, particularly if groundwater or surface water can be adversely impacted or if the flow to downstream users is reduced. In a dry climate the conservation of water is a very important consideration.
This trend is being driven by regulatory, technical and sustainability issues and also the Equator Principals that lending agencies now adhere to. Project financing is linked to sustainable development. Because of stronger awareness, there is also political pressure and pressure from stakeholders including regulators, the public, NGOs and from within the mining industry itself.
As a result, miners are searching for new ways to respond to growing demands for safer tailings and waste rock disposal facilities that will have less risk and be more responsive to the ever-increasing environmental and social concerns. Failures and non-compliance can have serious adverse downstream environmental and social impacts, a loss of production, loss of reputation and a high cost of cleanup, remediation and loss of business.
The trends in risk-mitigation today are to reduce or eliminate ponded water on top of tailings, to thicken tailings to reduce water loss, and to consider alternative methods of disposal. Tailings are being thickened to high density, non-segregating slurries or a paste and even a filtered product. Most thickened, non-segregating slurries can be pumped with a centrifugal pump. Paste normally has to be pumped with a positive displacement pump and moves through a pipeline as plug flow. Filtered tailings are transported by conveyor belt or truck.
High density thickened slurries can be beached to reduce the size of containment dams that would be required for conventional slurry deposition. Cemented paste tailings can be used for backfill in underground mines. As well, the co-disposal and even co-mingling of tailings and waste rock in the same facility is frequently being considered today at open pit mines, and the concept of paste rock is being developed.
Thickened tailings can be conveniently located in acceptable areas with small collecting watersheds, thus minimizing expensive water management and treatment facilities. Paste and filtered tailings can even be stacked using thin-layer deposition. This allows the tailings to desiccate and increase in strength before placing the next layer, a well recognized procedure.
Thickening has many benefits including reduced risk, no particle segregation, minimal liquids/solids separation, denser tailings mass, smaller footprint, less seepage to groundwater, greater chemical stability, water conservation, easier progressive closure, better control against wind and water erosion, reduced need for large expensive dams, less water to manage and treat in the disposal area, and easier extension of the life of an existing facility. While the capital and operating costs are higher, in most cases they are less than the status quo (i.e., the cost of dam construction for conventional slurry disposal and water recovery systems).
Dr. Eli Robinsky at the University of Toronto in the early 1970s was a pioneer in the field of thickening tailings for surface disposal. An early application was a cone of thickened tailings at the Kidd Creek concentrator near Timmins, Ont., which is still in operation after more than 30 years.
Universities have been carrying out research into the properties and application of thickened tailings technologies. In Canada the Universities of British Columbia, Toronto and Montreal as well as Carleton University are active in this field, to name a few. Many mining companies are encouraging research, and the mining associations are also interested. There is a definite trend toward change in the disposal and management of mine tailings world wide, and the mining companies are quickly adapting and responding.
Don Welch, P.Eng. of Golder Associates Ltd. ([email protected]; 905-567-4444), is group leader, project manager, advisor and review consultant on tailings, mine waste, dams, hydraulic structures and water management. Frank Palkovits, P.Eng. ([email protected]; 705-524-5533) is senior project engineer with Golder Paste Technology.