Recovering precious metals from industrial waste

AN inexpensive, non-toxic method has been developed to recover silver and palladium from industrial waste.

Precious metals such as silver and palladium are used in many industries. For example, silver is used in nanomedicine and electronics, and palladium is important as a catalyst. Demand for these metals is increasing as natural resources are depleted, so recovering them from waste is an ideal solution. Recycling the metals also reduces pollution, as silver and palladium ions are toxic to living organisms.

Extracting precious metals from waste is challenging because base metals are often contained with the waste, which are difficult to separate. Conventional methods include electrolysis, solvent extraction, and membrane filtration. These methods are complex, have low selectivity, are costly, and generate secondary waste.

Researchers in Japan from Kanazawa University, Fukushima University, and Daicel Corporation have developed a new method that uses adsorption to capture the silver and palladium. Adsorption has the advantage of being easy to implement, is low cost, and doesn’t leave any waste residue. Biomass-based adsorbents for the extraction of silver and palladium have previously been developed but these can have a low adsorption capacity. The team used a cellulose adsorbent – dithiocarbamate-modified cellulose (DMC) – to recover the metals.

They used the DMC to adsorb the metal ions. The DMC also partially adsorbed copper and lead base metals, but these could be removed by washing the DMC with nitric acid, leaving only the precious metal ions. The DNC was then incinerated at 500^oC and 850^oC for silver and palladium respectively to produce elemental metal powder.

"The adsorbent selectively chelated the soft acid silver and palladium cations," said study lead author Foni Biswas, a PhD candidate at Kanazawa University. "Of the 11 competing base metals we tested, only copper and lead cations were also adsorbed, but we removed them with ease."

The powder could then be converted into metal pellets by further heating to 950^oC and 1,600^oC for silver and palladium respectively. The yield for recovery of both metals as pellets is around 99%, showing that the DMC has a high selectivity for silver and palladium. As well as recovering pure metals, the method also has the advantage that cellulose is being cost-effective and highly combustible. It also doesn’t require toxic eluents or any reductants to retrieve the metals in elemental form.

The process was also tested on real industrial waste. "We removed nearly all of the silver and palladium from real industrial waste samples," said Biswas. "Obtaining pure and elemental metals proceeded as smoothly as in our trial runs."

The process can potentially be applied at larger scale.

Chemical Engineering Journal, https://doi.org/fq4j