Mining industry is facing the challenge of finding simpler, higher grade ores. They seem less and less available, so, in order to get the materials we consume, we need to develop new processes to extract materials from low grade ores. Not only primary ores should be investigated, but also mine tailings, which is the leftover solids from past ore processing. These tailings can contain valuable metals, such as gold, silver, copper, nickel, cobalt, zinc, etc. but are currently seldom processed. What a valuable waste!
With the aim of to fully exploit the remaining valuable metals, we ran an experimental research on the tailings from flotation processes. We were looking for cyanide-free processes, due to its toxicity. The samples studied contained of low amount of iron, copper, and gold, cobalt, nickel, and zinc. At the beginning, we decided to use bioleaching as pretreatment process for the recovery of nickel, cobalt, and zinc, as well as for iron removal. Nickel, cobalt, zinc and iron were indeed extracted into the solution in 90%, 60%, 86% and 67%, respectively. However, gold and copper stayed in the residue.
But, what happens in bioleaching? In bioleaching, living microorganisms help to set free metals embedded within sulfide minerals, facilitating the following process (e.g. chloride leaching).
After bioleaching, we still had gold and copper in the residue! To extract these metals, we applied to cyanide-free chloride leaching. Gold and copper were extracted into the solution 98% and 63%, respectively. Also, residual nickel, cobalt, and zinc were dissolved into the solution with the extraction of 99%, 80%, and 90%, respectively. At the end, we observed in all chloride leaching experiments, the highest extractions copper, gold, nickel, cobalt, and zinc were gained with biologically pretreated feed (see the figure in below).
We also ran conventional cyanide leaching experiments on the bioleaching residue, and dissolutions of base metals were higher; however, 7%-unit more of gold could be extracted by cyanidation. It can be concluded that a combination of biological pretreatment and chloride leaching could provide a viable cyanide-free process route for improving the metal extraction and further recovery from low grade ores.
The study was presented in Biohydrometallurgy´18 Conference in Windhoek, Namibia on Jun 12-13, 2018. After the conference, it was published with title “Effect of biological pretreatment on metal extraction from flotation tailings for chloride leaching” as peer reviewed paper in Minerals Engineering, Volume 129, December 2018, Pages 47-53. You can find the full version of the paper in https://doi.org/10.1016/j.mineng.2018.09.012.
Figure 1. Effect of bioleaching pretreatment on gold, copper, cobalt, nickel and zinc extractions in chloride.