A variety of heap closure solutions at one mine site

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A4   |   Letter


SRK News | Issue 58
Mine Closure: Can closure create opportunities?

Breese Burnley, Principal Engineer     

 

SRK has assisted a client with closing four gold heap leach pads at the same mine through various phases of operation and ownership since 1998. The continued expansion of the pit and waste rock disposal areas (WRDAs) has necessitated closing or relocating the heap leach facilities. The different approaches to closure for each leach pad underscore the complexity of waste and water management requirements for successful mining: “there are no one-size-fits-all solutions in mine closure”.

SRK prepared comprehensive closure plans for the mine site between 1999 and 2004. The original closure studies for the heap leach pads focused on rinsing and offloading spent oxide ore for use as cover material over WRDAs. However, the low permeability of the ore, coupled with onerous rinse water management requirements, pointed to closure in place as the most feasible alternative. 

Updated closure plans were prepared and implemented as part of expanded mining in the open pit. Each heap was to be overdumped by WRDA expansions around the pit perimeter. Draindown was captured in gravel drains constructed around the perimeter of each leach pad, using existing geomembrane-lined perimeter channels. Heap sideslopes were graded over the drains to protect them during overdumping. 

Collection drains were routed to a centrally-located sump and then to transfer tanks. All pipelines and tanks were buried to protect from freezing. As draindown flows were initially too high to manage by evaporation, collected draindown flows were pumped from one transfer tank to another and ultimately to the process circuit. Nearby process ponds were re-purposed as emergency overflow ponds. 

After implementing these closure measures, the following modifications to the closure plans have been implemented:

Pad 1 – To accommodate the pit expansion, the mine relocated 2.8 million tons of spent ore from Pad 1 to a containment cell on top of a nearby WRDA. The ore was graded to drain infiltrating meteoric water away from the spent ore and a geomembrane liner was constructed over the spent ore. The containment cell is currently being overdumped with 50 to 150 feet of waste rock. 

Pad 2 – The need to stabilise a pit wall called for relocating Pad 2 to prevent it from failing into the pit and releasing process solution to the environment. Pad 2 spent ore was placed in a vertical expansion over the Pad 4 liner. 

Pad 3 – This is the only heap closed as originally planned by overdumping with waste rock. The constructed perimeter drain is operational, and an evaporation cell was constructed in 2018 to manage long-term draindown.

Pad 4 – To facilitate mining in the existing pit, about 13 million tons of tailings had to be removed. The tailings were mixed with waste rock and placed in a disposal cell formed by a waste rock containment berm constructed directly over the existing Pad 4 geomembrane liner. The Pad 4 liner captures the tailings draindown and routes it to the perimeter draindown collection system. Pad 4 and the disposal cell were incorporated into an expanded WRDA. Plans are currently being developed to manage draindown in a lined evaporation cell. 

Breese Burnley: bburnley@srk.com

SRK Latin America