Guillaume ZANTE1, Evangelia DASKALOPOULOU1, Chris ELGAR1, Rodolfo MARIN RIVERA1, Jennifer HARTLEY1, Andrew ABBOTT1
1School of Chemistry, University of Leicester, Leicester, LE1 7RH, UK, Leicester, United Kingdom
The transition to a decarbonized world is heavily dependent on critical metals. For instance, silver and aluminum are essential in the development of solar photovoltaic cells, whereas wind turbines significantly rely on rare earths, copper, aluminum and steel, among others. Likewise, the implementation of technologies such as thermoelectric materials will not be possible without a large increase in the production of antimony, tellurium and bismuth. Recycling metals from e-waste could help to alleviate potential shortages in the future while avoiding the release of harmful substances and metals into the environment.
Deep eutectic solvents (DESs) consist of a hydrogen bond donor and acceptor. The network of hydrogen bonds between the two components lowers the melting point and allows liquids to be synthesized by slightly raising the temperature up to 50°C, while mixing two cheap and readily available chemicals such as urea, choline chloride, ethylene glycol, or water. DES can solubilize metals and, different methdologies can be applied to recover metals from DES, making them useful tools for metal processing and a sustainable alternative to mineral acids for dissolving metals.
In order to oxidize and dissolve the target metals from the waste, an oxidizing agent such as iron(III) chloride, iodine or copper(II) chloride can be used. After the targeted dissolution of the metals in the solid waste, a difference of potential is applied between two electrodes, allowing to regenerate the oxidizing agent at the anode, while the target metal is recovered in its elemental oxidation state at the cathode.
This strategy was applied to the recycling of metals from solar cells. The solar cell was a polycrystalline cell consisting of an aluminum and a silver electrode on a silicon wafer. Firstly, the aluminum is removed with aluminum chloride in water, which completely removes the aluminum electrode in 5 minutes. Secondly, iron (III) chloride was used as an oxidizing agent, dissolved in a choline chloride brine composed of choline chloride and water (molar ratio 1 to 4). Using water as a solvent, the iron was unable to dissolve the silver due to the low chloride content. By increasing the chloride content, oxidation of silver and its dissolution becomes possible due to the formation of iron-chloride and silver-chloride complexes which have a different redox behavior and a higher solubility. Overall, silver is completely dissolved in about 10 minutes. The use of water as a hydrogen bond donor in DES results in low viscosity solvents (about 10 cP, 4 times lower than conventional DES), which decreases the resistance to mass transfer and leads to fast leaching processes. By combining different DESs and oxidizing agents, we have obtained different etching liquids capable of recovering antimony, bismuth, tellurium and copper from thermoelectric materials as well as nickel and rare earths from nickel-coated neodymium-iron-boron magnets.