Sustainable Recovery of Silver from Deactivated Catalysts Using a Novel Process Combining Leaching and Emulsion Liquid Membrane Techniques

Industrial deactivated Ag/alumina catalysts contain an appreciable amount of silver. As silver is a precious metal, it is of great economic and environmental interest to recover silver from the catalysts before disposing them. This paper introduces a novel hybrid process for efficient silver recovery from deactivated catalysts. The process combines leaching and emulsion liquid membrane (ELM) techniques. Leaching first transfers silver from catalyst particles to an aqueous solution. An ELM then extracts silver ions from the leach solution in a single separation step. To prevent emulsion instability in the ELM, a new surfactant, a relatively low number-average-molecular-weight (<6000 g/mol) polypropylene glycol-polyethylene glycol-polypropylene glycol triblock copolymer, was synthesized and characterized by (HNMR)-H-1, (CNMR)-C-13, FTIR, and GPC. While preventing the instability, this surfactant did not have any adverse effects on mass transfer. For the first time, 2-ethylhexyl phosphoric acid was used as the carrier in an ELM to facilitate silver ions transfer from the aqueous solution. A solvent consisting of paraffinic and naphthenic hydrocarbons with no aromatic components was used as the diluent. Compatibilities of the ELM components (diluent, surfactant and carrier) were evaluated using two stability methods. Ag, Ca, Si, Al, Ti, and K leaching efficiencies of 97.7, 40.0, 30.0, 15.0, 6.0 and 0.5%, respectively, were achieved at the following optimal conditions: a nitric acid concentration of 1 mol L-1, a solid/liquid ratio of 50 g L-1, a mixing speed of 500 rpm, and a temperature of 70 degrees C. The ELM showed a maximum Ag separation efficiency of 97% with an Ag-ion extraction time of 7.5 min, a surfactant concentration of 6% (w/v), an HNO3 strip-phase concentration of 0.6 M, a feed-phase pH of 6.5, a mixing speed of 500 rpm, a treatment ratio of 75/500, and a phase ratio of 6/5. This study points to the high potential of the hybrid process for sustainable recovery of silver from deactivated catalysts.