LIMITATIONS, CHALLENGES AND ENVIRONMENTAL IMPACT OF HALIDE LEACHING
Though the highly corrosive acidic environment can be tackled by special stainless steel and rubber-lined equipment and difficulties in transportation can be handled by in-situ chlorine generation, the poisonous nature of chlorine is still a problem to be controlled to avoid any health risk. The use of chlorine for commercial operations has been successfully established in Nevada, with safety measures, since 1971 (Marsden and House, 2006), and was recently employed by Sumitomo Metal Mining Co. in the leaching of precious metals (Brent and Atluri, 1998). But obtaining selectivity other than controlled ORP leaching seems to be difficult, and without it, in the highly oxidizing environment of acidic media, the maximum of base dissolved in leach liquor can present big challenges during downstream processing. Moreover, control of ORP at production level would be challenging, as Cu2+ itself provides a favourable oxidizing environment for gold leaching. Temperature plays an important role in chlorine adsorption in HC1 solution, hence controlled temperature reactions are required to lower operational costs. Adsorptive dissolution of chlorine has already been identified as playing a major role in chlorine leaching; therefore, it would be interesting to look at replacing HC1 with other cheap and comparatively less corrosive reagents like NaCl.
Iodine has been identified as a good oxidant (< 11 pH) and better than HOC1 with higher leaching kinetics of gold (Davis and Tran. 1991). Nevertheless, iodine continues to be underemployed as a lixiviant for gold leaching, maybe largely because of cost (Syed, 2012 ; Yannopoulos, 1991). Besides this, the non-toxic and non-corrosive nature of iodine and the stability of gold-iodide complex at a wide range of pH can be advantageous if the research community regards it as a 'green' alternative to the highly toxic cyanidation process. This possibility may be similar to the one presented by Kim et al. (2016) for the selective chlorine leaching of gold in two stages (by controlling the potential). Controlled chlorine leaching at the front followed by iodide leaching at the back end may be an alternative, subject to process costs and compatibility of the two media at the two different stages.
This work was supported by the Brain Pool Programme through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant No. 2019H1D3A2A02101993) and the Basic Science Research Programme through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Project no. 2020R 111 Al AO 1074249).
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