Deep eutectic solvents – A warm relationship

If you have read my previous articles, you might have noticed that my PhD is about deep eutectic solvents and their application in electrochemical extraction of critical metals from industrial wastes (quite a mouthful, but allow me to explain 🙂 ).

Metallurgists and Chemists might be wondering “Why deep eutectic solvents for extraction of metals? Why? I could just use hydrochloric acid or aqua regia and dissolve everything.” Someone else might say: “Throw everything in a furnace, heat them above 1000 ^οC, and here you are, you have all your metals”. Me, on the other side, I would respond: “I know all of these techniques and yes they are effective, well established but maybe also outdated”.

Pyro and hydro-metallurgical extraction are known from thousands and hundreds of years respectively, they have proven themselves to be effective techniques, able to produce very pure metals. But, these processes have a big environmental footprint, because they have an extremely high energy consumption, they emit greenhouse gases and they produce a huge amount of aqueous wastewater, which has to be disposed of accordingly. Therefore, pyro and hydro-metallurgy are facing a lot of criticism from researchers all over the world.

Metallurgists have turned their interest to greener processes, like dissolving with ionic liquids. Ionic liquids are solvents consisting of solely ions and liquid at temperatures below 100 ^οC . [1] They have many beneficial properties like virtually non-flammability, low vapor pressure, high thermal, electrical and chemical stability among many other chemical properties with names that sound extra-terrestrial to your ears. Ionic liquids were firstly reported in 1914 with ethyl ammonium nitrate [EtNH₃] [NO₃]. [1] Their downside: ionic liquids are generally very expensive and they often require a very complex production and purification process.

Therefore, another kind of green solvent was developed in 2001 by Professor Andy P. Abbott (my supervisor) at the University of Leicester. He invented deep eutectic solvents, eutectic mixtures of quaternary ammonium salts with a hydrogen bond donor. Deep eutectic solvents are mixtures of two components that stick with each other through hydrogen bonding. The magic feature of these solvents is that when the two components (solid salts) are mixed together, they melt. For example, one of these solvents called reline has melting point at 12 ^οC, while its components urea and choline chloride have melting points at 133 ^οC and 302 ^οC respectively. [3]

Most importantly, they are very cheap and their production is extremely simple as you literally mix the two components at 60 ^οC for few hours et voilà: you have your solvent ready! They might be cheap but they show the same good and important properties like ionic liquids, and they are biodegradable and non-toxic at all. They have been used successfully in many processes with one of the most well-known and a large-scale example being the selective separation of zinc and lead from a complex matrix of electric arc furnace dust. [4]

My role in the SOCRATES project is to extract critical metals like Indium (In), Germanium (Ge) and Gallium (Ga) and industrial metals like Nickel (Ni), Zinc (Zn) and Cobalt (Co). Furthermore, I have a secondary but of most significance role which is to dissolve most of the amount of Lead (Pb) from these wastes, so that my colleague Stelios (ESR 05) will be able to recover and separate it from the waste streams. Through my project I hope to develop an efficient, scale-up method using green solvents and specifically I will try different deep eutectic solvents (Ethaline 200, Reline 200, Oxaline, Maline, Glyceline, Lactiline) in order to obtain the most efficient system for extraction of these metals.

Marianna

P.S. It is good to make a change for a better world, don’t stay on your comfort zone.

 

References

[1]	Frank Endres, D. M. and a. A. Abbott (2008). Electrodeposition from Ionic Liquids, WILEY-VCH Verlag GmbH & Co. KGaA.
[2]	http://www.eurare.eu/technologies/ionicLiquidExtraction.html
[3]	Andrew P. Abbott, G. C., David L. Davies, Raymond K. Rasheed and Vasuki Tambyrajah (2002). “Novel solvent properties of choline chloride/urea mixtures.” Chem Comm.
[4]	Andrew P. Abbott, John Collins, Ian Dalrymple, Robert C. Harris, Reena Mistry, Fulian Qiu, James Scheirer and William R. Wise, (2009). “Processing of Electric Arc Furnace Dust using Deep Eutectic Solvents”, Australian Journal of Chemistry