Fluoride toxicity in a chalcocite bioleach heap process

The importance of comprehensive laboratory evaluation for development of an ore body to commercial processing using biohydrometallurgy cannot be understated. Laboratory evaluation for a biohydrometallurgical process must include the microbiological component and definition of operating parameters for the engineers to design the commercial plant. Failure to meet commercial production at a mine site can be a consequence of incomplete understanding of biohydrometallurgical technologies for processing a specific ore. One example is the inability of a copper bioleach process to meet the design criteria in part because of lack of sufficient testing to demonstrate the ramifications of fluoride toxicity to the microbial component of the bioleach process. Laboratory research has demonstrated toxicity of low levels of fluoride to Acidithiobacillus species. However, laboratory determined toxicity values are not always relevant to field conditions at commercial bioleach operations. This is the case with fluoride toxicity where complexing reactions increase the amount of fluoride required for toxicity. Consequently, the toxic fluoride concentrations at field sites can be significantly higher than toxic levels reported in the laboratory, but still achieve concentration inhibitory for the microorganisms.

Research highlights
► Fluoride toxicity for bioleach microorganisms contributed to the failure of a commercial scale bioleach heap.
► Open cycle column testing for evaluating bioleaching is not adequate for detecting potential toxicity problems for the bioleach microbes.
► Viable Acidithiobacillus ferrooxidans can be found in bioleach solutions at apparent toxic fluoride concentrations because of complexation of fluoride with aluminum present in the solution.