Raising social awareness and environmental specifications on cyanide application force gold industry to search for alternative leaching reagents. Therefore, researchers worldwide investigate cyanide alternatives for gold recovery since several decades. Often the research activities cannot be compared directly, since different input materials and experimental conditions are used. Over the course of this study, different promising cyanide alternative reagents were investigated in terms of their capability of pure gold dissolution at different temperatures. All experiments took place under identical conditions by using uniform samples of 99.99% gold disks, to enable a comparability. Thiosulfate as one of the most promising reagent thiosulfate according to literature revealed an insufficient leaching behavior. The gold dissolution was hindered due to the formation of a sulfide passive layer. Also in the thiourea trials, a surface precipitation took place, though gold dissolution did not stop. The halogens iodine, bromine and the well-known gold solvent aqua regia dissolved gold very fast (up to ~1,000 mg·h −1 ·cm −2 ). Methanosulfonic acid (MSA) was not capable to extract any gold. The experiments were compared with cyanide trials at identical conditions. The average dissolution rate of investigated reagents at 25 °C shows following order: aqua regia > iodine > bromine > cyanide > thiourea > thiosulfate > MSA.

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The importance of gold recovery from low grade ores and flotation tailings is continuously increasing due to raising gold demand and thereby high gold prices. However, due to raising social awareness of the ecological impacts of cyanidic gold extraction and environ-mental specifications, the development and the implementation of alternative hydrometallurgical extraction processes have been a focus for many research institutions in last decades. Present work gives a comparison between compatible extraction reagents, with focus on less harmful processes. The target of this review is to point out the best cyanide-free processes of following methods and reagents: Bioleaching, Chlorination, Aqua Regia, Bromine, Thiocyanate, Thiosulfate and Thiourea leaching. For this propose, the gold leaching reagents are described and discussed in terms of their environmental and economical points of view. As result of this comparison, thiourea stands out as the most promising alternative gold leaching reagent to cyanide.

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Very often, the production of silver causes devastating environmental issues, because of the use of toxic reagents like cyanide and mercury. Due to severe environmental damage caused by humans in the last decades, the social awareness regarding the sustainable production processes is on the rise. Terms like "sustainable" and "green" in product descriptions are becoming more and more popular and producers are forced to satisfy the rising environmental awareness of their customers. Within this work, an alternative environmental sound silver recovery process was developed for a vein type silver ore from Mina Porka, Bolivia. A foregoing characterization of the input material reveals its mineral composition. In the following mineral processing, around 92.9% silver was concentrated by separating 59.5 wt. % of non-silver minerals. Nitric acid leaching of the generated concentrate enabled a silver recovery of up to 98%. The dissolved silver was then separated via copper cementation to generate a metallic silver product of >99% purity. Summarizing all process steps, a silver yield of 87% was achieved in lab scale. A final upscaling trial was conducted to prove the process' robustness. Within this trial, almost 4 kg of metallic silver with a purity of higher than 99.5 wt. % was produced.