Biooxidation and Thiocyanate Leaching of Refractory Gold Ore in One-Step

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Authors
Azizitorghabeh, Atefeh
Keyword
Gold , Thiocyanate , Biooxidation , Refractory ore , Leaching
Abstract
Gold is among the precious metals and for more than a century, it has been extracted by the cyanidation process. Cyanide is highly toxic for humans and living organisms. Moreover, with increasing gold extraction over time, the average gold grade in minerals has decreased and simultaneously cyanide usage increased. Consequently, using non-cyanide reagents for gold extraction from low-grade ores has attracted attention. Among different reagents, thiocyanate has been selected as the promising alternative to cyanide. Compared to cyanide, thiocyanate is less toxic to humans and aquatic organisms, and safer for the environment. First, a feasibility study on gold extraction from an oxide ore by thiocyanate leaching was conducted. After a series of experiments designed by the response surface methodology, the optimum condition for maximum gold recovery, 96%, from an oxide ore has been identified: 500 mM thiocyanate, 100 mM ferric iron, and 50% pulp density at 25 °C and pH 2 for 24 h. In addition to environmental issues and high cyanide consumption, sometimes cyanide leaching is not technically or economically feasible; it is generally done in alkaline media, so the acidic oxidation slurries should be neutralized before cyanide leaching which often adds to the complexity of the process. Consequently, a sustainable method for gold extraction from low-grade refractory ore was developed by combining acidic biooxidation (a mixture of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans) and acidic thiocyanate leaching in one system to avoid neutralization and minimize the complexity of the process. Furthermore, thiocyanate leaching requires ferric iron as an oxidant for gold extraction and this can be provided by microorganisms during ore biooxidation. It was found that in-situ leaching by 0.2 M thiocyanate of the biooxidized ore in the presence of bacterial culture and no ferric iron addition resulted in the highest gold recovery, 86.9%. Finally, for selective gold extraction from thiocyanate PLS, anion exchanger resins were tested. For selective gold adsorption from synthetic and real PLS, 100 g/L MTA5011 resin at pH 2 for 24 h adsorbed 99% gold. Then a mixture of 0.01M thiourea with 1M sulfuric acid selectively eluted 70.1 % gold from loaded resins.
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