This study is aimed at determining the optimum mixing ratio and solvent group to extract REEs from nitric leach solution, and the efficiency was tested using three different kinds of extractants, which were prepared to mix three different solvents of Cyanex272, D2EHPA, and HQ. In order to select the best mixture system, different volumes (ml) of solvent extractions were used under optimum conditions (.05 M concentration and pH of 2). The result indicated that the mixture system of Cyanex272 (8 ml) and D2EHPA (2 ml) had the highest separation factor and enrichment ratio in the separation of rare earth element, especially dysprosium. The degree of performance of mixture systems in separation of the rare earth elements indicated that the best extraction systems were in the order of Cyanex272 (8 ml) and D2EHPA (2 ml) > HQ (2 ml) and D2EHPA (8 ml) > Cyanex272 (8 ml) and HQ (2 ml).

The cyanidation process is the most common method applied for the extraction of gold and silver in the hydrometallurgy industry, in which, sodium cyanide is used as a leaching agent. Therefore, the wastewater of gold mines contains a wide variety of cyanide ions needing to be removed before these wastewaters can be discharged to the receiving environments. In this study, a fuzzy multi-attribute decision-making approach (Fuzzy Delphi AHP and Fuzzy TOPSIS) was used for selecting the best cyanide removal method from the wastewater of Muteh gold mine. According to the experts' opinion, three methods including calcium hypochlorite, hydrogen peroxide and sodium hypochlorite were selected as alternatives. Then, by introducing the criteria influencing decision making, including cyanide removal ability, cost of process, amount of material consumed, time, pH, ease of performance and safety, and performing separated experiments, the criteria for each of three methods were determined. Finally, sodium hypochlorite was proposed as the best method for eliminating cyanide from wastewater. It was found that the rank of methods was as sodium hypochlorite (0.517) > calcium hypochlorite (0.474) > hydrogen peroxide (0.463).

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