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Design and Operation for Closure at Kennecott Ridgeway
SRK, through its long-standing ties with the Kennecott Ridgeway Mine in South Carolina, has helped make this mine a prime example of successful design and operation for closure.
“From the prefeasibilty stage of design, strong emphasis was given to the need to minimise the actual and contingent liability associated with operations and closure,” explains Rob Dorey, SRK Corporate Consultant. “Closure planning has been periodically updated throughout the mine life, with the detail of the plan increasing as closure approached.”
“The potentially acid generating (PAG) nature of the host rock of this epithermal deposit required the adoption of proactive waste rock management. Use of the waste rock in construction of the tailings impoundment, with compaction and encapsulation of PAG waste rock, was incorporated in the design. The phased development of the two pits also allowed the partial backfilling of one pit during mining of the other.”
As part of the closure planning, SRK has performed a number of studies. These include a surface water mixing and water quality model, project-wide groundwater and contaminant transport model and tailings cover model. A detailed evaluation of the methods and viability of achieving long-term closure was also completed.
The presence of PAG material in the pit walls required the management of both backfill material and water used to flood the pit. “Lime softening has been used, with a target alkalinity of the pit water to buffer acid generated by the pit walls and backfill and contact runoff water from the site,” Rob expands. “To close the tailings impoundment, a modification of the deposition system to regrade the impoundment surface to create positive drainage was adopted.”
“With a net positive water balance at closure, the need to rapidly cover the tailings was accommodated by placing the cover as a slurry, using the existing tailings deposition system. To date all indications are that performance criteria for the closure are being met.”