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Typical metallurgical extraction routes require high energy investments to process metals from constituent ores. Others, such as cyanidation of gold ore, are perceived negatively by the public due to the potential risk posed by cyanide toxicity. All extraction methods have an associated carbon-footprint, which increases with subsequent waste-treatment and remediation requirements.
Methods with lower overheads and environmental impacts, such as heap-leaching and biohydrometallurgical approaches, are a step towards low-energy metal extraction but do not apply to all ore types.
Ionic Liquids and “Ionometallurgy”.
Ionic liquids represent a potential, novel approach to processing numerous ore-types, including base and precious metal ores. These are typically processed through energy-intensive methods including pyrometallurgical treatment and cyanide-based hydrometallurgical leaching.
Ionic liquids are anhydrous salt compounds in a liquid state at temperatures below 100oC. Their ionic nature imparts them with strong electrolytic and solvation capacity and, being anhydrous, the formation of oxides and hydroxides through reaction with water is mitigated.
With proven applications in metal recovery from scrap and metal oxides, ionic solvents are superior to aqueous leach solutions due to higher solute capacity and a wider electrochemical window. This means that less solvent is required to dissolve the same amount of metal from an ore than with an aqueous solution, and that the solvents are stable over a greater voltage range. This is important as electrowinning many metal-bearing solvents generates gas through chemical reduction. With ionic solvents, their stability under high voltage ensures the system remains closed, and the solvent can be reused.
As with any industrial operation, reagent cost and availability are critically important, with large-scale extraction of metals from ores requiring readily available reagents. The current drive for environmentally sustainable industrial technologies and the power of public opinion and influence regarding the practices of the extractive industries also means that methods with minimal risk and environmental impact are desirable.
Several ionic liquids have been tested for their capacity to dissolve precious and base metals from their ores; however, none have been found that are economically and environmentally viable.
Deep Eutectic Solvents.
Researchers at the University of Leicester’s Department of Chemistry, headed by Professor Andrew Abbott, have been developing a new type of ionic liquid known as a Deep Eutectic Solvent or DES. While they are slightly different to conventional ionic liquids, being mixtures rather than single salt compounds, their anhydrous nature and similar properties to standard ionic liquids imparts them with a similar capacity for application in metal recovery.
DESs are composed of salts, such as choline chloride, and dipolar organic molecules, such as citric acid or urea, that donate a hydrogen bond to significantly lower the melting point of the two compounds when heated together. Ratios of the two components can be adapted to alter the properties of the liquid and optimize its properties to suit a specific process.
Collaborating with Researchers from the University of Leicester’s Department of Geology, headed by Professor Gawen Jenkins, the metal extraction capacity of a DES composed of choline chloride, ethylene glycol and iodine has been tested on samples of ore containing electrum, chalcopyrite, galena and pyrite from the Cononish gold deposit in Scotland.
Results have shown that not only can precious- and base-metals be extracted using DESs, but also that dissolution rates of gold are approximately forty times faster than via cyanidation. The process is also strongly selective with gold and silver, present as electrum, dissolving first, followed by base-metal sulfides, i.e., galena and chalcopyrite, while pyrite remained undissolved.
The availability of all the reagents used in the DES coupled with their low toxicities are advantages when compared to conventional precious- and base-metal extraction routes. Furthermore, the early positive results of Professors Abbott and Jenkins and their teams, including the rate of extraction and the selectivity of the process with respect to the order of metal dissolution indicate that “ionometallurgy” has potential to be developed into a large-scale industrial process augmenting or supplanting traditional extraction methods.
- Abbott AP, Frisch G, Hartley J, Ryder KS (2010) ‘Processing of metals and metal oxides using ionic liquids’ Green Chemistry 13, 471-481.
- Jenkin GRT, Al-Bassam AZM, Harris RC, Abbott AP, Smith DJS, Holwell DA, Chapman RJ, Stanley CJ (2010) ‘The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals’ Mineral Engineering 87, 18-24.
- Oxley A, Barcza N (2016) ‘Why heap leach nickel laterites?’ Minerals Engineering 88, 53-60.
- Smith SL, Grail BM, Johnson B (2017) ‘Reductive bioprocessing of cobalt-bearing limonitic laterites’ Minerals Engineering 106, 86-90.
- Smith EL, Abbott AP, Ryder KS (2014) ‘Deep Eutectic Solvents (DESs) and their applications’ Chemical Reviews