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Unlocking Potential Water Quality Improvements in the Process Mill

SRK News | Issue 53: Metallurgy and Mineral Processing

A4   |   Letter

Take a look at the flowsheet above. Can you tell if this is a step in the process plant or a water treatment plant? Mining operations are building dedicated water treatment plants costing between 10s and 100s of millions of dollars, when it is possible that these are wasted expenses if the complete process is considered.

Mineral processing plants are generally designed to achieve the highest recovery of valuable material, with limited regard for the cost of managing waste products—both solid and water components. The highest recovery processing route can, in fact, lead to costly issues downstream, which can include water treatment. In addition, handling storage of mine waste to eliminate environmental impacts from acid rock drainage and neutral pH contaminant leaching. This is particularly true considering the significant mine closure costs that can remain in perpetuity.

Often the design and selection of metallurgical and waste management engineering solutions take place in technical ‘silos’ and do not consider synergistic opportunities. However, by having geochemists, water management engineers and metallurgists work together, there exists potential for improved project economics and reduced environmental risk without the need to design and build dedicated water treatment plants and, instead, use the processes in the mill to manage water quality.

Typical mineral processing steps, such as grinding and lime addition, provide the potential to either remove contaminant metals through surface sorption (i.e. higher surface area from ground ore) or contaminant co-precipitation from lime addition. These mill processes are the same strategies used in water treatment plants and, given the high throughput of most processing plants, reclaim water volumes are also high and the ability to ‘treat’ significant volumes of water is possible.

Process flow diagram of a Mill to also act as a Water Treatment Plant

To ensure that discharge water meets receiving water quality guidelines, two recent examples of using in-plant water treatment, as opposed to dedicated water treatment plants, include the Kitsault project in northern British Columbia, Canada, and the Sisson project in central New Brunswick, Canada.

At Kitsault, cadmium sorption in tailings slurry was demonstrated. The Sisson project is using lime to clarify water for the process plant, but it also demonstrated that arsenic coprecipitation after lime addition in the mill could off-set the need to construct a dedicated arsenic water treatment plant.

While not every mining project may have the potential to realise these benefits, the number of potential opportunities is likely extensive, given how rarely environmental and metallurgical teams collaborate. For a small amount of investment,significant gains could be realised.

Chris Kennedy:

SRK Latin America