All the industrially applied gold leaching methods (historical chlorine gas based leaching, dominating state-of-the-art cyanide gold leaching, processes at precious metals plants) suffer from the characteristics related to aggressive and even toxic leaching media and high chemical consumption. This study targets environmentally sound cyanide-free gold leaching in mild chloride media in terms of minimizing chemical consumption. In the current study, it was investigated whether providing instant gold recovery (carbon-in-chloride-leach, CICl) could allow high gold recovery in a mild and non-toxic leaching environment. The investigated leaching parameters were S/L ratio, T, type of oxidant i.e. [Cu2þ]/[Fe3þ] and [Cl]. The results showed that gold could be dissolved in exceptionally mild conditions, when an appropriate adsorption/reduction (activated carbon) site was provided immediately after leaching. It was found that impurity metals iron and copper originating from the gold ore (Fe 1.6% and Cu 0.05%), were advantageous self-initiating oxidants and 87% of gold could be dissolved in pure calcium chloride (2.8 M) solution. In addition, no bromide, which is a commonly added aggressive additive in modern cyanidefree processes, was required. The lowest chloride concentrations applied were comparable (0.6 M) or even milder (0.3 M) than those typical of seawater chloride concentrations, and could still result in gold recovery, 72% and 64%, respectively, with copper as oxidant. Conventionally gold extraction is assumed to require highly aggressive leaching media, high redox potentials, and high gold complex stability in the solution. The findings presented can provide a competitive environmental and economic edge and therefore new horizons for future cyanide-free gold chloride technologies, suggesting that in future, even seawater can act as the basis for cyanide free gold leaching.

Introduction

All the known industrial gold leaching methods suffer from the use of aggressive leaching media and high chemical consumption, the challenge becoming more evident with increasingly impure low-grade ores. Chlorine gas was applied as early as the 19th century for gold leaching, with Cl2 originating from hydrochloride acid on manganese oxide (Habashi, 2005; Marsden and House, 2006). In addition, bromide, bromine, cyanide, thiosulfate, and thiourea were known as early as the late 1800s and early 1900s (Marsden and House, 2006). The triumphal march of the currently dominating gold cyanide leaching process started from the deep Witwatersrand ore. With cyanide leaching, gold extraction could be increased from 55 ‒ ۶۵%e90% (Marsden and House, 2006; Habashi, 2005). Later cyanide leaching has been operated in various modes – heap leaching, CIP (carbon in pulp), CIL (carbon in leach), and RIP/RIL (resin in pulp/leach) (Marsden and House, 2006). The environmental hazards and toxicity of cyanide have increased the interest in a new cyanide-free gold process (Aylmore, 2005). The typical challenges related to the new developmentstage cyanide-free gold processes are related to the high costs of chemicals, selectivity for gold leaching, and final gold recovery from the solutions (Hilson and Monhemius, 2006). The most promising alternatives to cyanide have been considered to be thiosulfate, thiourea, and halides (Hilson and Monhemius, 2006; Aylmore, 2005). Gold halide (iodine, bromide, and chloride) leaching has been observed to suffer from unstable gold complexes; therefore high halide concentrations and consequently high reagent consumption is typical for development-stage processes (Aylmore, 2005; Hilson and Monhemius, 2006; Marsden and House, 2006).