Top 10 Papers

An environmentally friendlier approach to hydrometallurgy: highly selective separation of cobalt from nickel by solvent extraction with undiluted phosphonium ionic liquids	Removal of transition metals from rare earths by solvent extraction with an undiluted phosphonium ionic liquid: separations relevant to rare-earth magnet recycling
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Recycling of rare earths: a critical review	Continuous ionic liquid extraction process for the separation of cobalt from nickel
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Highly efficient separation of rare earths from nickel and cobalt by solvent extraction with the ionic liquid trihexyl (tetradecyl) phosphonium nitrate: process relevant to the recycling of rare earths from permanent magnets and nickel metal hydride batteries	Rare-earth recycling using a functionalized ionic liquid for the selective dissolution and revalorization of Y2O3:Eu3+ from lamp phosphor waste
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Separation of rare earths by split-anion extraction	Antimony recovery from the halophosphate fraction in lamp phosphor waste: a zero-waste approach
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Non-aqueous solvent extraction of rare-earth nitrates from ethylene glycol to n-dodecane by Cyanex 923	Solvometallurgy: an emerging branch of extractive metallurgy
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TOP 10 papers

(1) An environmentally friendlier approach to hydrometallurgy: highly selective separation of cobalt from nickel by solvent extraction with undiluted phosphonium ionic liquids

Wellens, B. Thijs, K. Binnemans
Green Chemistry 14, 1657–1665 (2012).
DOI: 10.1039/C2GC35246J

A green solvent extraction process for the separation of cobalt from nickel, magnesium and calcium in chloride medium was developed, using undiluted phosphonium-based ionic liquids as extractants. Cobalt was extracted to the ionic liquid phase as the tetrachlorocobaltate(II) complex, leaving behind nickel, magnesium and calcium in the aqueous phase. The main advantage of this ionic liquid extraction process is that no organic diluents have to be added to the organic phase, so that the use of volatile organic compounds can be avoided. Separation factors >50,000 were observed for the cobalt/nickel separation from 8 M HCl solution. After extraction, cobalt can easily be stripped using water and the ionic liquid can be reused as extractant, so that a continuous extraction process is possible. Up to 35 g L^-1 of cobalt can be extracted to the ionic liquid phase, while still having a distribution coefficient higher than 100. Instead of hydrochloric acid, sodium chloride can be used as a chloride source. The extraction process has been upscaled to batch processes using 250 mL of ionic liquid. Trihexyl(tetradecyl)-phosphonium chloride (Cyphos IL 101) turned out to be the best option as the ionic liquid phase, compromising between commercial availability, separation characteristics and easiness to handle. This paper shows for the first time that ionic liquids are suitable for separation of metal ions by solvent extraction on an industrial scale.

(2) Removal of transition metals from rare earths by solvent extraction with an undiluted phosphonium ionic liquid: separations relevant to rare-earth magnet recycling

Vander Hoogerstraete, S. Wellens, K. Verachtert, K. Binnemans
Green Chemistry 15, 919-927 (2013).
DOI: 10.1039/C3GC40198G

An environmentally friendly process for the separation of the transition metals copper, cobalt, iron, manganese and zinc from rare earths by solvent extraction with the ionic liquid trihexyl(tetradecyl) phosphonium chloride has been developed. The solvent extraction process is carried out without the use of organic diluents or extra extraction agents and it can be applied as a sustainable hydrometallurgical method for removing transition metals from neodymium-iron-boron or samarium-cobalt permanent magnets. Transition metals are efficiently extracted, while the rare earths remain in the raffinate. The method was tested for the removal of cobalt and iron from samarium and neodymium, respectively.

(3) Recycling of rare earths: a critical review

Binnemans, P.T. Jones, B. Blanpain, T. Van Gerven, Y. Yang, A. Walton, M. Buchert
Journal of Cleaner Production 51, 1-22 (2013).
DOI:        http://doi.org/10.1016/j.jclepro.2012.12.037

Setting the scene for rare-earth recycling, this highly-cited paper provides an overview of the literature on the recycling processes for rare earths, with emphasis on three main applications: permanent magnets, nickel metal hydride batteries and lamp phosphors. The state of the art in preprocessing of End-of-Life materials containing REEs and the final REE recovery is discussed in detail. Both pyrometallurgical and hydrometallurgical routes for REE separation from non-REE elements in the recycled fractions are reviewed. The relevance of Life Cycle Assessment (LCA) for REE recycling is emphasized. The review corroborates that, in addition to mitigating the supply risk, REE recycling can reduce the environmental challenges associated with REE mining and processing.

(4) Continuous ionic liquid extraction process for the separation of cobalt from nickel

Wellens, R. Goovaerts, C. Möller, J. Luyten, B. Thijs, K. Binnemans
Green Chemistry 15, 3160–3164 (2013).
DOI: 10.1039/C3GC41519H

A continuous ionic liquid extraction process using the ionic liquid trihexyl(tetradecyl)phosphonium chloride (Cyphos® IL 101) has been developed for the selective extraction of cobalt from nickel. The performance of this continuous extraction process is competitive with that of currently applied industrial processes. Moreover, the elimination of volatile odorous compounds from the extraction phase leads to environmentally friendlier and healthier working conditions.

(5) Highly efficient separation of rare earths from nickel and cobalt by solvent extraction with the ionic liquid trihexyl(tetradecyl)phosphonium nitrate: process relevant to the recycling of rare earths from permanent magnets and nickel metal hydride batteries

Vander Hoogerstraete and K. Binnemans
Green Chemistry 16, 1594–1606 (2014).
DOI: 10.1039/C3GC41577E

A solvent extraction process with the ionic liquid trihexyl(tetradecyl)phosphonium nitrate has been developed to extract rare earths and separate them from nickel or cobalt. Rare earths are extracted to the ionic liquid, whereas transition metals remain in the raffinate. The process is environmentally friendlier than traditional solvent extraction processes, since no volatile and flammable diluents have to be used. Compared to conventional ionic liquid metal extraction systems, the advantage of using the new ionic liquid is that expensive and persistent fluorinated ionic liquids can be avoided. The ionic liquid can be prepared by a simple metathesis reaction from the commercially available ionic liquid trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101). The extraction is facilitated by an inner salting-out effect of a highly concentrated metal nitrate aqueous phase. The separation of lanthanum and samarium from nickel or cobalt, out of highly concentrated metal salt solutions by solvent extraction, is of importance for the recycling samarium–cobalt permanent magnets or nickel metal hydride (NiMH) batteries.

(6) Rare-earth recycling using a functionalized ionic liquid for the selective dissolution and revalorization of Y₂O₃:Eu³⁺ from lamp phosphor waste

Dupont and K. Binnemans
Green Chemistry 17, 856–868 (2015).
DOI: 10.1039/C4GC02107J

The supply risk for certain rare-earth elements (REEs) has sparked the development of recycling schemes for end-of-life products like fluorescent lamps. In this paper a new recycling process for lamp phosphor waste is proposed based on the use of the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf₂N]. This innovative method allows the selective dissolution of the valuable red phosphor Y₂O₃:Eu³⁺ (YOX) without leaching the other constituents of the waste powder (other phosphors, glass particles and alumina). A selective dissolution of YOX is useful because this phosphor contains 80 wt% of the REEs although it only represents 20 wt% of the lamp phosphor waste. The proposed recycling process is a major improvement compared to currently used hydrometallurgical processes where the non-valuable halophosphate (HALO) phosphor (Sr,Ca)₁₀(PO₄)₆(Cl,F)₂:Sb³⁺,Mn²⁺ is inevitably leached when attempting to dissolve YOX. Since the HALO phosphor can make up as much as 50 wt% of the lamp phosphor waste powder, this consumes significant amounts of acid and complicates the further processing steps (e.g. solvent extraction). The dissolved yttrium and europium can be recovered by a single stripping step using a stoichiometric amount of solid oxalic acid or by contacting the ionic liquid with a hydrochloric acid solution. Both approaches regenerate the ionic liquid, but precipitation stripping with oxalic acid has the additional advantage that there is no loss of ionic liquid to the water phase and that the yttrium/europium oxalate can be calcined as such to reform the red Y₂O₃:Eu³⁺ phosphor (purity >99.9 wt%), effectively closing the loop after only three process steps. The red phosphor prepared from the recycled yttrium and europium showed excellent luminescent properties. The resulting recycling process for lamp phosphor waste consumes only oxalic acid and features a selective leaching, a fast stripping and an immediate revalorization step. Combined with the mild conditions, the reusability of the ionic liquid and the fact that no additional waste water is generated, this process is a very green and efficient alternative to traditional mineral acid leaching.

(7) Separation of rare earths by split-anion extraction

Larsson and K. Binnemans
Hydrometallurgy 156, 206–214 (2015).
DOI: 10.1016/j.hydromet.2015.04.020

Split-anion extraction is a new approach to the separation of mixtures of rare earths by solvent extraction. The rare-earth ions are extracted from a concentrated chloride aqueous phase to an organic phase, consisting of a water-immiscible thiocyanate or nitrate ionic liquid. This allows for efficient extraction of trivalent rare-earth ions from a chloride aqueous phase, without the need of using acidic extractants. The process is called split-anion extraction because the aqueous and organic phases contain different anions. Thiocyanate and nitrate anions have a strong affinity for the organic phase, while chloride anions have a strong affinity for the aqueous phase. In split-anion extraction, the source of complexing anions is the organic phase which allows for the use of chloride aqueous feed solutions and easy stripping of the rare-earth ions from the loaded ionic liquid phase by water (instead of strong inorganic acids). The principle of the new extraction approach is described in detail for the extraction of rare earths from aqueous chloride solutions by the ionic liquids tricaprylmethylammonium thiocyanate and trihexyl(tetradecyl)phosphonium thiocyanate. Rare-earth and chloride concentrations can be varied to optimize the separation process. Separation factors between the end members of the lanthanide series (La–Lu) exceed the value of 200,000.

(8) Antimony recovery from the halophosphate fraction in lamp phosphor waste: a zero-waste approach

Dupont, K. Binnemans
Green Chemistry 18, 176-185 (2016).
DOI: 10.1039/C5GC01746G

Antimony is becoming an increasingly critical element. Antimony production is primarily concentrated in China (90%) and as the industrial demand for this metal surges, attention has to turn towards the recovery of antimony from (industrial) waste residues and end-of-life products in order to guarantee a sustainable supply of antimony. Although lamp phosphor waste is usually considered as a source of rare earths, it also contains significant amounts of antimony in the form of the white halophosphate phosphor (Ca,Sr)₅(PO₄)₃(Cl,F):Sb³⁺,Mn²⁺ (HALO). HALO phosphor readily dissolves in dilute acidic conditions, making antimony far more accessible than in the main production route which is the energy intensive processing of stibnite ore (Sb₂S₃). HALO makes up 50 wt% of the lamp phosphor waste, but it has been systematically overlooked and treated as an undesired residue in the efforts to recover rare earths from lamp phosphor waste. In this paper, the feasibility of antimony recovery is discussed and an efficient process is proposed. The HALO phosphor is first dissolved in dilute HCl at room temperature, followed by a selective extraction of antimony with the ionic liquid Aliquat® 336. The remaining leachate is valorized as apatite which is a feed for the phosphate and fertilizer industry. A zero-waste valorization approach was followed, meaning that no residue or waste was accepted and that all the elements were converted into useful products. This paper thus emphasizes the potential of lamp phosphor waste as a secondary source of antimony and describes a sustainable process to recover it. The process can be integrated in lamp phosphor recycling schemes aimed at recovering rare earths.

(9) Non-aqueous solvent extraction of rare-earth nitrates from ethylene glycol to n-dodecane by Cyanex 923

N.K. Batchu, T. Vander Hoogerstraete, D. Banerjee, K. Binnemans
Separation and Purification Technology 174, 544–553 (2017).
DOI:        http://doi.org/10.1016/j.seppur.2016.10.039

Non-aqueous solvent extraction is based on the distribution of metal ions between two immiscible organic phases, in contrast to conventional solvent extraction where the metal ions are distributed between an aqueous phase and an immiscible organic phase. Very few non-aqueous solvent extractions systems have been described in the literature, but it offers opportunities to develop new separation processes for metal ions. Kumar Batchu and Koen Binnemans developed a solvent extraction system where the more polar organic phase was ethylene glycol with dissolved rare-earth nitrate salts and lithium nitrate, while the less polar phase was a solution of the neutral extractant Cyanex 923 dissolved in n-dodecane. When compared to aqueous feed solutions, the light rare-earth elements (LREEs) are less efficiently extracted and the heavy rare-earth elements (HREEs) more efficiently extracted from an ethylene glycol feed solution, resulting into the easy separation of HREEs from LREEs. The separation factors between neighboring elements are higher for this non-aqueous solvent extraction process than for extraction from an aqueous feed solution.

(10) Solvometallurgy: an emerging branch of extractive metallurgy

Binnemans and P.T. Jones
Journal of Sustainable Metallurgy (2017) in press. DOI: 10.1007/s40831-017-0128-2This position paper shows that solvometallurgy is complementary to pyrometallurgy and hydrometallurgy. However, this new approach offers several advantages. Firstly, the consumption of water is very limited offering a major advantage in regions where there is a shortage of water. Secondly, the leaching and solvent extraction can be combined in a single step, which leads to simplified process flow sheets. Thirdly, solvent leaching can be more selective than leaching with acidic aqueous solutions, leading to reduced acid consumption and less purification steps. Fourthly, solvometallurgy is useful for the treatment of ores that are rich in soluble silica (such as eudialyte) as no silica gel is formed. Hence, solvometallurgy is in a position to help develop near-zero-waste metallurgical processes, and with levels of energy consumption that are much less than with high-temperature processes. The Technology Readiness Level (TRL) of this emerging branch of extractive metallurgy is still low (TRL = 3-4), which is a disadvantage for short-term implementation, but a great opportunity for research, development and innovation, in order to tackle the resource challenges of the future.