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Microfluidic Platform for High-Throughput Screening of Leach Chemistry

Yang, Die, Priest, Craig
Analytical chemistry 2018 v.90 no.14 pp. 8517-8522
activation energy, adhesion, ammonia, ammonium hydroxide, annealing, chromium, copper, copper sulfate, gold, leaching, monitoring, screening, temperature, thiosulfates
We demonstrate a microfluidic screening platform for studying thiosulfate leaching of Au in a transparent microchannel. The approach permits in situ (optical) monitoring of Au thickness, reduced reagent use, rapid optimization of reagent chemistry, screening of temperature, and determination of the activation energy. The results demonstrate the critical importance of the (1) preparation and storage of the leach solution, (2) deposition and annealing of the Au film, and (3) lixiviant chemistry. The density of sputter-deposited Au films decreased with depth resulting in accelerating leach rates during experiments. Atomic leach rates were determined and were constant throughout each experiment. Annealing above 270 °C was found to prevent leaching, which can be attributed to diffusion of the chromium adhesion layer into the Au film. The microfluidic analysis revealed leach rates that are sensitive to the stoichiometric ratio of thiosulfate, ammonia, and copper in the leach solution and optimized for 10 mM CuSO₄, 1 M Na₂S₂O₃, and 1 M NH₄OH. The temperature dependence of the leach rate gave an apparent activation energy of ∼40 kJ mol–¹, based on Arrhenius’s relationship.