MINING NEWS: Agencies approve Fort Knox heap leachStringent monitoring and prevention measures, including the use of technology developed for cold climates, should make facility safe Sarah Hurst For Mining News
Alaska’s Department of Natural Resources issued permits July 3 for Fort Knox gold mine’s heap leach project, which will have a total capacity for leaching 161 million tons of ore. The heap leach facility will be in the Walter Creek drainage on about 310 acres of land, and the haul road to the pad will cover another 40 acres. DNR and the Department of Environmental Conservation responded to numerous public comments about the project that offered suggestions and expressed concerns.
“The project is based on sound engineering and environmental control technologies and proven concepts that have been successfully applied at numerous sites around the world,” state agencies wrote. “The heap leach facility does not have exposed areas ... where cyanide solution will be allowed to pool and pose a risk to wildlife. Cyanide solution is pumped to and from the heap leach facility in a double-walled piping system. When being dispersed within the heap, wildlife will not have access to the solution because it will be held in the interstitial pore spaces throughout the ore heap.”
The facility’s embankment dam includes an emergency spillway for a worst-case scenario, the agencies noted. If the dam were to overtop, the spillway could safely pass the runoff from a 100-year, 24-hour storm without risk of a catastrophic dam failure. If the heap leach embankment were nevertheless to fail, the maximum volume of fluid that could be released is 100.5 million gallons. This would go to the tailings storage facility, which has a capacity for 206 million gallons.
Frozen ore in heap a risk One of the risks in a cold climate is that frozen ore can develop in the heap, causing a failure of the leaching process, the agencies wrote. Utilizing mill water is recommended to reduce the potential for frozen ore to develop. The heat inventory of the heap will ensure that its Leak Collection and Recovery System isn’t subject to freezing. Heat traces and thermisters are included on monitoring sumps to prevent and monitor freezing.
Historically some heap leach facilities have failed for identifiable reasons, according to the agencies. All the facilities that failed were designed and constructed in the 1980s, when design procedures, construction materials and construction techniques were still being developed for cold climate conditions, the agencies wrote. The construction of these facilities occurred in less than optimal climatic conditions, resulting in damage to construction materials; and the design didn’t adequately estimate the water balance and containment requirements, thus under-sizing the facilities.
Newer facilities have fail-safe mechanisms “Since these facilities were built, the designs, construction materials and techniques have been refined,” the agencies wrote. “Among others, facilities such as Minera Yanacocha, Cripple Creek and Victor, Kemess, Montana Tunnels and Kidd Creek have been successfully operating without experiencing chemical releases. Like the (Fort Knox) heap leach facility, these facilities have been designed and constructed with fail-safe mechanisms, such as leak detection devices, pump back systems, and excess containment structures to ensure compliance with environmental regulations.”
The designers of the Fort Knox facility used Nevada’s stringent heap leach pad regulations and standards as guidelines, in addition to complying with Alaska dam safety and solid waste disposal regulations, the agencies noted. No significant areas of permafrost were observed in the proposed footprint of the heap leach pad. A small area of suspected discontinuous permafrost on the south slope of Walter Creek in the project area will be investigated during construction and removed if high moisture contents are discovered.
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