Dec '09/Jan '10

Engineering Exchange

Cover systems for reactive mine waste

By H. Ednie

O’Kane Consultants help Vale Inco manage acid drainage at Whistle Mine

 

A view of Whistle Mine after implementing the closure plan 


Upon closure, many mines need to prevent acid rock drainage (ARD) with a dedicated system to control sulphide oxidation. Mine waste that contains sulphidic material, such as pyrite, becomes oxidized upon exposure to water and oxygen, resulting in accumulation of acidic pore-water and increases in metal concentrations. The mine waste can also contain minerals that neutralize the acidic pore-water; however, if the acid-neutralizing balance is not favourable, ARD will result as precipitation and snow melt flushes the pore-water.

Managing ARD through collection and treatment, as well as through strategies that prevent sulphidic oxidation and migration of acidic pore-water, are key aspects of mine closure. Mine waste cover systems provide the opportunity to control sulphide oxidation and migration of acidic pore-water, and are the particular expertise of O’Kane Consultants.

Since Vale Inco’s Whistle Mine, located about 30 kilometres north of Sudbury, Ontario, was shutdown in 1998, a multi-stage reclamation project has been successfully implemented. Over its approximate seven-year operating life, about seven million tonnes of acid-generating waste rock were stockpiled on the Whistle site.

A common method to control sulphide oxidation, and thus ARD, is to put the waste rock under water. The challenge lies in maintaining the water cover. “In Ontario, as well as many parts of Canada, we generally have a moisture surplus on an annual basis; however, it varies throughout the year,” explains Mike O’Kane, president of O’Kane Consultants. “We need to create an environment to keep the waste permanently under water, through topography, constructing dams, or backfilling the material into open pits. There are many cases in Canada where that is done successfully. In many situations, however, climate and site-specific constraints make this option challenging, which is when a cover system is required.”

Several complicating issues were associated with decommissioning the Whistle Mine waste rock dumps (WRDs) in-place. First, due to the proximity of the WRDs to the open pit and a nearby creek, re-grading some of the WRD slopes to facilitate construction of an earthen cover system was not feasible. Second, a properly designed and constructed cover system would substantially reduce the infiltration of meteoric water at the cover surface, but would still allow for preferential ingress of oxygen through the underlying glacial till at the toe of the slope. Finally, seepage collection and treatment from the toe of the covered WRDs would be required for an indefinite period of time. Coupled with this fact, a spill from the collection system could result in significant environmental damage to the surrounding wetlands.

In light of the environmental and economic liabilities associated with release of ARD to the surrounding ecosystem, Vale Inco elected to relocate the two WRDs to the open pit as a means of mitigating environmental damage post-closure. Geochemical modelling demonstrated the effectiveness of backfilling the open pit and covering it in terms of isolating the acid-generating material from water and oxygen. Although the climate for this region has a moisture surplus on an annual basis, a water cover is not feasible for this site because of the absence of a pit lake and the relatively steep slope of the backfilled pit surface.

Vale Inco implemented the project in phases. First, waste rock was backfilled into the mined-out pit between July 2000 and December 2001, with lime being added during backfilling to neutralize existing acidic pore-water. The water level within the backfilled pit will rise over time such that a portion of the waste rock would remain saturated.  However, given that the upper portion of the backfilled pit will remain above the long-term water table elevation, an engineered cover system is required to control the influx of atmospheric oxygen to the acid-generating waste rock. The third phase is a collection and treatment system for any contaminated waters that discharge from the pit overflow.

O’Kane engineers designed and tested a multi-layer cover system, which includes a layer that remains tension-saturated, and that inhibits the ingress of oxygen into the waste rock. O’Kane states that “the idea is to create a ‘blanket of water’, such that water is held in place by a layer of the cover system, as opposed to topography or dam walls, thus limiting oxygen ingress.” Data from the monitoring system that was built into the cover system indicate that it has substantially reduced oxygen and water ingress.  Furthermore, the quality of water requiring treatment has improved substantially in the few short years since the cover was constructed.

“The Whistle Mine project is the latest example of the full-scale application mine closure technology, the development of which is being led by Canadian researchers and practitioners,” says O’Kane, adding that the  project won the Canadian Land Reclamation Association’s Tom Peters Memorial Reclamation Award.

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