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Mine Water Treatment

The new owner of a mine site had 4 critical questions answered using ITHACA simulation software

The new owner of a mine site had significant legacy environmental issues to remedy prior to restarting the mine and process plant. Chief among these was how to manage several gigalitres of contaminated water held in a disused pit while at the same time using the pit to store newly generated tailings slurry. Weather events were also a source of concern due to the total catchment area draining into the pit far exceeding the liquid surface area available for evaporation.

Case Study

Client: [confidential]

Industry: Mining & Minerals

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The client was looking to answer the following questions:

Size

Given the proposed mining schedule how large should the paste plant (used for diverting a portion of tailings slurry to underground fill) be in order to provide four years’ safe tailings deposition into the pit?

Time

How many operating evaporators would be required at any given time over the next five years in order to control pit water level to below maximum – given extreme rainfall in the first year of operation?

Recoverables

How much valuable metal contained in dissolved form in pit water could be recovered and how much would be lost to precipitation and entrapment on contact with high-pH tailings?

Quality

How concentrated would salt levels in pit water become over the course of five years?

Five-year simulations of a flowsheet comprising the process plant, pit, several additional rainfall catchments and the evaporators were carried out. Historical rainfall and evaporation data was adjusted to provide extreme weather events as required, and applied to all catchments and the evaporator performance model.

Five-year simulations

Results and recommendations resulting from the simulations were:

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Loss of metal values

Up to 24% loss of metal values could be expected due to chemical precipitation and physical entrapment in deposited tailings.

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Dissolved salt concentration

The dissolved salt concentration in pit water would increase by 260% in 4 years and 400% in 5 years. If the water treatment plant was not run at optimal pH the pit water would become unusable as process water within 2-3 years.

Evaporators

Five evaporators would be needed to control water level in the first year, however given the criticality of this duty six evaporators were recommended. Four evaporators thereafter were sufficient to reduce free pit water to zero within 3-4 years.

26 kilotonne per month

A paste plant capacity of 26 kilotonne per month (dry solids) would allow tailings deposition into the pit for four years, under the tailings density assumption employed. Flexing settled tailings slurry density by ±20 % yielded ±8 month variation in available deposition time. Further work would be required to determine likely compacted tailings slurry density in order to confirm the paste plant capacity recommendation.

Graph presenting a summary of five-year ITHACA simulation output.

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