Black Pine Mining Heap Leach Closure

Black Pine Mining, Inc. (BPMI) contracted in 1995 with WESTEC, Inc. (whose operations were merged with SRK Consulting in 1998) to prepare a closure plan for their 34-million ton heap leach facility located in the south-eastern corner of Idaho, USA. The heap, a valley fill design, had 50 million gallons of solution in process containing cyanide and other constituents in concentrations greater than safe drinking water standards.

Background data was collected to characterize the physical, geochemical, and other environmental aspects of the heap. Solution treatment methodologies were researched and their respective costs evaluated. The results of this work were used to develop a risk assessment to address the potential impacts that the spent ore-solution system could have on the local environment. Black Pine Mining, Inc., stopped adding cyanide to the heap in January 1998 and began recirculating solution. Evaporation proved to be so effective that land application of spent solution was not utilized. Further, the weak-acid dissociable (WAD) cyanide level has been consistently declining and was projected to meet the 0.2 mg/l standard in March 1999. Other constituents previously occurring at concentrations greater than safe drinking water standards have also declined.

“All in all,” said BPMI’s General Manager Crellin Scott, “closure activities at the mine proceeded at or better than projections. Advanced planning allowed us to proceed with a minimum of capital and operating outlay. This project demonstrates, among other things, that intensive solution management can shorten the pad closure time requirements, enhance gold recovery, reduce solution treatment costs and control the solution balance.”

“We interacted closely with BPMI to identify the most effective means of providing the best quality service while keeping costs down”, said Val Sawyer, SRK Principal Consultant. “We were able to work directly with BPMI technical and operating staff to collect samples, conduct metallurgical testing, analyze results, and then form a long-term strategy.”