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Conventional Disposal vs. Thickened Tailings Disposal
Conventional Disposal
The "conventional" approach
is to build confining or perimeter dams in a valley or flat area to create a
basin that is supposed to last in perpetuity. The tailings, which often contain
a
great deal of process water, are discharged continuously from the ore processing
plant
into
the basin, where they settle out in a very loose structure like lake-bottom mud.
In spite of the many tailings spills and dam failures that are still occurring,
there has been little evolution of the conventional system, except that dam construction
is receiving more attention. However, this has not stopped the failures.
Negative Features of Conventional Disposal
The main critical features of the older or "conventional" disposal system is the presence of highly vulnerable dams built of unconsolidated tailings, the presence of an extremely loose deposit of tailings, and a superimposed settling pond that promotes seepage downwards and into the surrounding environment and does not allow the tailings to consolidate by drying. The reason why the failure of a "conventional" tailings dam is so disastrous environmentally is not so much the dam itself but the fact that the dam retains a mass of extremely loose unconsolidated tailings and a great deal of water, or process fluid. If the dam fails, say because of earth tremors, poor construction, or inadequate inspection and maintenance, the contents liquefy completely as the tailings and process fluid flow through the breach. In this liquid state they can flow many miles downstream. If the dam itself was built of tailings material, as is often the case, it too will liquefy and join the flow.
Strengthening the Tailings for a TMS
To adopt the TMS, the tailings must be strengthened to enable them to stack at a slope. This can be done by the removal of most of the process water which has been used in separating the ore from the tailings. This is attained by passing the tailings through high density thickeners. Most of the process water is taken off the thickeners and recycled back into the plant. The thickening process must be sufficient to change the tailings and process water from a mixture to a non-segregating, but pumpable, slurry.
Behaviour of Thickened Tailings
When
the tailings are released, in spite of their heavy consistency and thus high
viscosity, they will still flow, without segregation. Eventually the flow stops
at a gentle slope. The slope is controlled by the degree of thickening. The
aim is to attain a slope of 2 to 6 percent (1.1° to 3.4°) in moderate climates.
Such slopes are sufficiently gentle to avoid excessive erosion, yet provide
good drainage for future revegetation. One of the aims of the system is to
provide
sufficient
surface area during deposition to allow drying of the discharged tailings,
thus strengthening
them considerably. In very dry climatic conditions even steeper slopes may
be contemplated. The non-segregating property of thickened tailings is also
responsible for bonding the tailings particles, both in the wet state and after
desiccation, thus reducing both erosion and dusting potential.
TMS is Acceptable in any Topography
Tailings disposal sites may consist of valleys or flat terrain somewhere in the vicinity of the process plant. To form a sloping tailings deposit in a valley the thickened tailings would be discharged at the head of the valley or along one of the side hills. The heavy slurry will flow down the valley until it encounters a slope flatter than it's own, or alternatively, until it is stopped by a small dam. On flat terrain thickened tailings would be discharged from an artificial ramp or tower, resulting in a ridge or cone of tailings, respectively. Only a low perimeter dyke is required to direct precipitation and a small amount of extruded process water to a pond, ideally located beyond the limits of the tailings deposit, for recycling. Typically, because of thickening, such a pond will receive only 1/3 of the amount of process water that flows to conventional ponds.
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