Solvent extraction of metals: Role of ionic liquids and microfluidics
Introduction
Metal ion extraction has always drawn extensive attention from researchers due to a wide range of applications in the industry including metallurgy, electronics, pigments and ceramics, catalysis and medical treatment, nuclear technologies, synthetic chemicals, transport, and communication sectors [2], [3], [4]. It has stimulated rapid growth of industries, and delivered a large economical profit, yet also raised environmental problems. Metal extraction is applied for three main business cases: a) extraction of metal ions from natural resources, b) recycling the materials from secondary resources, and c) treatment of wastewater [5], [6], [7], [8], [9].
Presently, one of the major industrial-scale techniques for the separation and recovery of metals is Solvent Extraction (SX), which is part of hydrometallurgical processing. The principle of the SX technique is based on the preferential distribution of the dissolved target substance between two immiscible phases. Generally, aqueous and organic phases are used for this purpose [10]. The transport of dissolved substances in the aqueous phase from the aqueous phase to the organic phase is provided by using an extracting agent called extractant [11]. This method has become popular because of its several advantages including high selectivity, simple operation, high metal purity, and low power consumption, and has been increasingly applied in the separation of metal ion process [12]. There are many different kinds of extractant employed in separation processes that were continuously improved during the decades and have been utilised for extractors as diverse as batch units, counter-current stages, and microfluidic devices [13], [14].
Many reviews have been presented on the constitutional technologies of this review, which are metal extraction, ionic liquids, and microfluidic devices [10], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Most of them constrict, however, to the extraction process for special groups of metals or solvents. To the best of our knowledge, no manuscript compiles recent improvements in metal extraction technology, advanced microfluidic device design, and the optimised ionic liquid selection and how this affects the separation performance. To fill such a knowledge gap, we hereby present a review providing oversight about the improvement in metal extraction from the past until the present, with particular focus on both utilising microscale technology and ionic liquids to increase the efficiency of metal extraction. The purpose of this review study is to answer the following questions:
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What has been the progress of the metal extraction process so far?
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Which solvents and technologies were used in the past and now?
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What are the effective parameters in the process and how effective is each method?
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What advantage may new emerging reactors provide over old ones?
Section snippets
Extraction of metal ions in the past
The extraction of metal ions using the solvent extraction method has been started in the 1950s with the extraction of uranium from its ores and later for tantalum and niobium [25], [26].
Organic solvents such as xylene [27], kerosene [28], n-heptane [29], methyl-isobutyl ketone (MIBK) [30] and chloroform [31] are commonly used for the extraction process. From the 1980s onwards, the development of powerful extracting agents added much value to increase the performance of the extraction process
Ionic liquid in the metal extraction process
From an environmental point of view, the finding of green solvents, e.g. with lower toxicity and vapor pressure than conventional solvents, has attracted much interest [21]. Ionic liquids (ILs) could establish a prime role to be considered as future solvent or extractant in the metal extraction process. Ionic liquids consist of organic cations in combination with various inorganic anions. Depending on the cationic or anionic structure of ILs, they can be miscible with water and/or nonpolar
Ionic liquids as a solvent for extraction of metal ions in microfluidic system
The limitations of using ILs for extractions have already been pointed out as high production cost and high viscosity. The first issue might also pose an environmental problem due to the need for a larger solvent amount and the corresponding accruing waste. The latter might be known to cause lower mass transfer efficiency in conventional extractors [141]. To overcome these obstacles, a novel technology with high performance and a low solvent holdup is needed. Microflow technology with its
Process intensification of microfluidic - ionic liquids extraction
Reducing plant size while keeping or increasing its production efficiency was the original aim of process intensification and indeed the microfluidic chemical plants commonly have smaller footprints [144]. The motivation of microfluidics for metal extraction is to improve the mass transfer between the two immiscible liquid phases. Because of their large specific surface to volume ratio, interfacial mass transfer is largely facilitated [144]. Microfluidics benefits from unique, highly regular
Conclusions
Nowadays, the resourcing and recovery of metals are big challenges for manifold major global industries, both economically and environmentally, facing an increasing demand for declining resources (element criticality). This work investigated different methods and technologies used to extract metal ions from the aqueous solution from the past until now, in the context of minerals processing for mining;
The following conclusions can be drawn:
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Past – successful implementation: Metal extraction
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Volker Hessel acknowledges the support of the start-up grant provided by The University of Adelaide, Australia.
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