“The use of ionic liquids & deep-eutectic solvents in extractive metallurgy is a dead-end street”. This inconvenient truth forms the conclusion of a new Open Access paper in Journal of Sustainable Metallurgy, entitled: “Ionic Liquids and Deep-Eutectic Solvents in Extractive Metallurgy: Mismatch Between Academic Research and Industrial Applicability”. The paper is authored by Koen Binnemans and Peter Tom Jones (SOLVOMET ISC & SIM2 KU Leuven). In this article we provide some background on this paper and we publish some testimonials from several experts, who provide their take on this controversial matter.

Why this paper?

The lead author, Prof. Koen Binnemans, has published about 200 papers on the use of ionic liquids and deep-eutectic solvents. For many years he has been promoting their adoption in sustainable metallurgy and he has written or participated in a multitude of European research projects on the matter. So why has Binnemans given up on these neoteric solvents? In this new paper the authors elaborate on 8 reasons: (1) issues with high viscosity; (2) limited chemical stability under the conditions of metallurgical processes; (3) difficulties with recycling and reuse; (4) a lack of demonstrated unit processes and flowsheets on the pilot scale; (5) insufficient material-property data available for engineering purposes; (6) the administrative burden of obtaining licenses and safety permits; (7) very high costs for large-scale operations; (8) minimal added value compared to state-of-the-art hydrometallurgical processes.

In his recent interview, in the context of being awarded a second ERC Advanced Grant for his project on Circular Hydrometallurgy (CIRMET) [Read interview here], Koen Binnemans provided more background on why he made this remarkable U-turn, summed up in this quote:

“Genuine breakthroughs in hydrometallurgy will not come from the use of neoteric solvents like ionic liquids or deep-eutectic solvents, but rather from a deep understanding of hydro-processes at a molecular level. Hydrometallurgy needs to evolve to low-energy-input circular hydrometallurgy.”

Photo: Prof. Koen Binnemans (SOLVOMET, KU Leuven). Credits: Elisabeth De Decker

The catalyst for this U-turn

Although the U-turn took some time to fully mature, the trigger for this change of heart can be brought back to a "killer comment" we received several years ago. It came from an anonymous reviewer of an ionic-liquid-based metal recovery project proposal we submitted to the KU Leuven authorities. At that time we were rather surprised as we were convinced that ILs were going to be the breakthrough solution in metallurgy. Now we fully understand and endorse what the critical reviewer wrote at the time. It is an eye-opener (cited in the paper):  

“Ionic liquids are not the panacea. They have been around for 20  years, thousands of papers have been published on their use in separations, yet no large industrial operator is using them. Drumming the use of ionic liquids year after year without any examples of large-scale success makes it more difficult to present this approach as a reasonable option over time. If ionic liquids are so good for everything, why is no one willing to use them in the industry?” 

Testimonials by (industry) experts

Following the launch of the paper, the authors have received several reactions from a diversity of industry and academic experts. Some were public, but most of these were sent as private LinkedIn or email messages (somehow researchers tend to stay away from controversy?). Given the huge importance of this matter, we hereby provide an overview of some of the public testomonials:

Kathy Sole (Consulting Hydrometallurgist):

"It is truly admirable when an academic who has spent many years of thought and effort on advancing a particular technology is able to step back and consider that an alternative approach would perhaps be better. An excellent example of how real scientific progress is made, based on hypothesis, validation or refutation of the hypothesis, and reformulation of a new hypothesis."

Aaron Wilson (Research Scientist at Idaho National Laboratory):

“There are "zombie technologies" in water treatments people keep pushing for advantage x, y, or z without addressing the known process limitation of the technology. Effectively diverting the push for dead end technology is an interesting challenge. I'll be watching this line of conversation.”

Ajay B. Patil (Head of the department of process metallurgy @HIF):

“Prof. Koen Binnemans, it is a courageous and admirable move and paper. I am happy to see our aligning opinions on the hydrometallurgical process implementation: key is the ease of operations and scale up and economic feasibility. Which requires the maximum efficiency and minimum waste generation and losses. I really hope that fundamental solution chemistry and equilibrium processes will evolve from the pressure of becoming green with already compromised competitive process developments chances in western world. Important to say that, fundamental understanding and traction you achieved with solvometallurgy activities in past years has brought the great reputation to hydrometallurgy in chemistry research in Europe. Therefore, as the member of hydrometallurgy and solution chemistry community, I very much appreciate that.  More power and support to you for developing new and basic approaches.”

Benjamin Sprecher (Assistant Professor at TU Delft, Industrial Design Engineering):

“I applaud this work! The message is unfortunate of course but really good that you are explicit about this.”

Steven De Meester (Professor at Ghent University):

“One of the nicest papers I read in a long time. Academics shouldn’t be hipsters following trends and beliefs but focus on facts. Congrats for this down to earth and important message!”

Anonymous industrial hydrometallurgist (cited in paper):

“Why do we keep taking new, self-proclaimed technologies such as ionometallurgy seriously? I cannot see that anyone in industry would realistically consider their adoption in a flowsheet.”

Full reference & acknowledgements

Binnemans, K., Jones, P.T. Ionic Liquids and Deep-Eutectic Solvents in Extractive Metallurgy: Mismatch Between Academic Research and Industrial Applicability. J. Sustain. Metall. (2023). https://link.springer.com/article/10.1007/s40831-023-00681-6

The authors thank the Industrial Research Fund (IOF KU Leuven) and the LRD Division RARE3 of KU Leuven for financial support. They also acknowledge the anonymous reviewers of project proposals who were very critical about the use of ionic liquids in extractive metallurgy; their comments were eye-openers. The authors also wish to thank the many colleagues from industry with whom they had discussions about the topic of this article. The authors acknowledge SOLVOMET Group colleagues (Sofia Riano, Nand Peeters, Xiaohua Li, Stijn Raiguel, Brecht Dewulf) for their internal review and valuable comments.