Analysis and comparison of cyanide detoxification methods from spent heaps

Abstract

This work entailed the testing of cyanide degradation of microorganisms isolated by the U.S. Bureau of Mine's Salt Lake Research Center (SLRC) and native strains isolated from an Alaskan mine, development of conceptual designs for in situ biological detoxification of leached heaps, and comparison of common in situ and ex situ heap detoxification processes using a cost comparison, sensitivity analysis, and Monte Carlo simulation. Microorganisms from an Alaskan mine site and SLRC were tested for cyanide degradation. Testing of samples from the Ryan Lode site indicated that several bacterial colonies could tolerate cyanide, but no degradation was found. Once degradation was confirmed in the SLRC sample, a winter survival rate test and an in situ heap detoxification test were performed. Winter survival experiments indicated approximately 5-20% of the bacterial population in heap agglomerate samples were viable after a winter and subsequent coldroom wait. In addition, bacteria colonized the agglomerate where nutrients were available. During August 1993, a 250 ton test heap was constructed on a heap that was undergoing detoxification using the INCO air-SO$\sb2$ process. The test heap was inoculated with bacteria in October 1993 and September 1994. Unfortunately, the test was terminated at an early stage, limiting the conclusions. The detoxification processes chosen for comparison were in situ biological, peroxide, and chlorination, and ex situ biological, INCO air-SO$\sb2,$ peroxide, and chlorination. To compare the costs, a hypothetical heap of 1 million tons was used. In situ biological detoxification had a cost of $0.41 per ton of ore treated, followed by ex situ biological detoxification, with a cost of \$0.92 per ton. The remaining methods ranged from $1.05 to \$1.35 per ton. The biological detoxification methods were most sensitive to the rinsing rate, labor costs, and capital costs. The chemical detoxification methods were most sensitive to oxidant cost, oxidant to cyanide ratio, and starting cyanide concentration. For the Monte Carlo simulation, in situ biological detoxification had a cost of $0.63 per ton treated, followed by in situ peroxide with a cost of \$1.09 per ton. The remaining methods ranged from $1.11 to \$1.45 per ton.