Leicester Ionic Liquids Group
Green Solutions
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The main research interests of the group are firmly based around green chemistry with particular emphasis on electrochemical processes. It is active in developing novel solvent systems with industrial applications such as metal deposition and dissolution. It collaborates strongly with industry and much of the work to date has been in the development of novel processes using ionic liquids.

Metal Oxide Processing

The processing of metal oxides is essential for metal extraction, waste recycling, and catalyst preparation. One of the major difficulties with metal oxides is that they are insoluble in most molecular solvents and generally require strong aqueous mineral acids for their dissolution. Separation has usually been achieved using solvent extraction with specific chelating agents for given metals. An increasing number of groups are now using ionic liquids for solvatometallurgical processing.

We have shown that our choline chloride based deep eutectics can dissolve a range of metal oxides and they can be used to separate metals from a complex mixture using electrochemistry. These liquids are, however, all totally miscible with water and cannot be used for biphasic extraction. The solubility of 17 metal oxides in the elemental mass series Ti through Zn has been reported in three of these liquids based on choline chloride. This has also been applied to the practical application of recovery of Zn and Pb from a waste material produced by the electric arc furnace (EAF).

An EAF is one of the most commonly used methods for the production of steel and is primarily used in the remelting of scrap steel. The EAF used in steel making and recycling has become a very intensive and rapid process. It involves passing a high-energy electric current (450-500 KWh/tonne) through ferrous scrap and alloying elements to produce molten steel. Approximately 15-20 kg of dust is formed per tonne of steel but the high content of metals such as Pb and Cd mean that the waste is classified as toxic. The dust forms when volatile metals, such as zinc and lead, are vapourized at the high operating temperature and several million tonnes of dust are produced per annum. We have optimized the extraction process and the selective cementation of lead in deep eutectics

The process developed for treating EAF dust involves three steps:

  1. extraction of lead and zinc from matrix,
  2. isolation of lead,
  3. recovery of zinc.

Schematic diagram of the cell for extracting lead and zinc from electric arc furnace (EAF) dust together with a photograph of the pilot plant built for 5 kg dust extraction batches.

A common process used for extraction in aqueous solutions is cementation. In this process, a sacrificial metal with a more negative redox potential than the metal being recovered is immersed in the solution. The metal ions in the liquid are reduced and deposited onto the sacrificial metal, which in turn is oxidized and goes into solution. In aqueous solutions, this tends to be an inefficient process because the immersion deposit is dense and precludes dissolution of the underlying metal. For the current case, the process is Pb2+ + Zn(s) → Pb(s) + Zn2+, This is ideal because zinc metal is an inexpensive substrate that will be recovered in the final stage of the process. A sample of EAF dust was extracted using the 1 ChCl:1.5 EG:0.5 urea mixture at 50°C for 2 days. The resultant liquid was filtered and a zinc sheet was immersed for 2 days at 25°C. At the end of this time, a black deposit was produced and energy dispersive X-ray spectroscopy (EDAX) analysis of the sample showed the only metal present on the surface was lead. This shows that cementation can be used as a method for achieving effective separation of Pb from Zn in the EAF dust.

There are two approaches to recovering the zinc can from the liquid. The first is electrowinning, however, this has proven to be slow and current-inefficient and probably economically unfavourable on a large-scale owing to the low value of zinc metal. An alternative approach would be to precipitate the metal from solution with a complexing agent that forms an insoluble species. This has the potential advantage that a higher-value product could be obtained.

Scanning electron microscopy (left) and photograph (right) of cathode after 7 h electrowinning at 50°C.

It is well known that ammonia solution is an extremely effective precipitating agent for zinc in aqueous solutions. Various concentrations of ammonia solution were added to aliquots of the leachate and the results in show that dilute solutions are extremely effective at precipitating Zn from solution. This is most likely due to partial ammination of Zn that is not as soluble in the ionic liquid/water as the fully amminated Zn species. Small-scale tests showed that this liquid could be regenerated by filtration and then evaporation of the ammonia solution from the liquid. The resulting liquid was slightly depleted of urea owing to decomposition at higher temperatures but this could be easily added to bring the liquid back to its original composition.

In conclusion, effective separation can be achieved by selective dissolution through appropriate choice of the hydrogen bond donor (HBD), cementation of the metal in solution, electrowinning of the metal in solution or precipitation of the metal by addition of an aqueous-based complexing agent. This is a generic technology that could be applied to a wide variety of matrices. The simplicity in terms of synthesis and constituents means that this can be applied to large-scale extraction.