(214a) Silver Recovery from Catalysts Using a Leaching and Emulsion-Liquid-Membrane Hybrid Process

Silver is a precious metal that is widely used in many industries including the petrochemical, electrical, electronics, jewelry, and photography¹. Recovery of silver from waste materials such as photographic and X-ray films, jewelry components, and deactivated catalysts is of economic and environmental interests². To extract silver from solid wastes, many methods have been used^3-6. Among them, leaching processes are more popular^7-8. However, a leaching process transfers all types of metal ions present in the solid phase into a liquid phase. So, this process must be combined with a complementary process to separate silver ions from other types of metal ions present in the leach solution. In recent years, the membrane technology, in particular the emulsion liquid membrane (ELM) technology, has received more attention as a promising technology that is more environmentally friendly, is more efficient, and has lower operating and capital costs^9-11.

This paper introduces a hybrid process for an efficient silver recovery from the deactivated industrial catalysts. The hybrid process combines leaching and ELM processes. Leaching first transfers silver from a catalyst to an aqueous solution. In a single separation step, ELM then extracts silver ions from the leach solution. 2-ethylhexyl phosphoric acid ester (MEHPA, a mixture of 55% mono- and 45% di-esters) is used as the carrier in membrane phase (facilitated transport) to extract silver ions from the aqueous solution. A paraffinic and naphthenic solvent is used as the diluent. As a major problem with ELM is emulsion instability resulting from poor surfactants like Span 80, a new surfactant, polypropylene glycol (PPG)-PEG-PPG triblock copolymer, is synthesized. Two stability methods are used to evaluate the ELM stability and the compatibility of the ELM process components (diluent, surfactant, and carrier). Finally, effects of process operating conditions such as carrier, surfactant and internal phase (receiving phase) concentrations, pH of the external phase (feed phase), striping speed, volume ratio of the internal phase to the membrane phase, and volume ratio of the emulsion phase to the external phase (emulsion phase holdup) on the silver recovery are investigated to arrive at the optimal operating conditions that maximize the silver recovery.

References

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