Editing Extractive metallurgy (section)

=== Metal extraction with [[Ionic liquid|ionic fluids]] ===
DESs are fluids generally composed of two or three cheap and safe components that are capable of self-association, often through hydrogen bond interactions, to form eutectic mixtures with a melting point lower than that of each individual component. DESs are generally liquid at temperatures lower than 100&nbsp;°C, and they exhibit similar physico-chemical properties to traditional [[Ionic liquid|ILs]], while being much cheaper and environmentally friendlier. Most of them are mixtures of [[choline chloride]] (ChCl) and a hydrogen-bond donor (e.g., urea, ethylene glycol, [[malonic acid]]) or mixtures of choline chloride with a hydrated metal salt. Other [[choline]] salts (e.g. acetate, citrate, nitrate) have a much higher costs or need to be synthesised,<ref>{{cite book |last1=Endres |first1=F |last2=MacFarlane |first2=D |last3=Abbott |first3=A |title=Electrodeposition from Ionic Liquids |date=2017 |publisher=Wiley-VCH |url=https://www.wiley.com/en-gb/Electrodeposition+from+Ionic+Liquids-p-9783527315659}}</ref> and the DES formulated from these anions are typically much more viscous and can have higher conductivities than for choline chloride.<ref>{{cite journal |last1=Bernasconi |first1=R. |last2=Zebarjadi |first2=Z. |last3=Magagnin |first3=L. |title=Copper electrodeposition from a chloride free deep eutectic solvent |journal=Journal of Electroanalytical Chemistry |date=2015 |volume=758 |issue=1 |pages=163–169 |doi=10.1016/j.jelechem.2015.10.024 |hdl=11311/987216 |url=https://doi.org/10.1016/j.jelechem.2015.10.024|hdl-access=free }}</ref> This results in lower plating rates and poorer throwing power and for this reason chloride-based DES systems are still favoured. For instance, Reline (a 1:2 mixture of choline chloride and [[urea]]) has been used to selectively recover Zn and Pb from a mixed metal oxide matrix.<ref>{{cite journal |last1=Abbott |first1=A. |last2=Collins |first2=J. |last3=Dalrymple |first3=I. |last4=Harris |first4=R.C. |last5=Mistry |first5=R. |last6=Qiu |first6=F. |last7=Scheirer |first7=J. |last8=Wise |first8=W.R. |title=Processing of Electric Arc Furnace Dust using Deep Eutectic Solvents |journal=Australian Journal of Chemistry |date=2009 |volume=62 |issue=4 |pages=341–347 |doi=10.1071/CH08476 |url=https://doi.org/10.1071/CH08476}}</ref> Similarly, Ethaline (a 1: 2 mixture of choline chloride and [[ethylene glycol]]) facilitates metal dissolution in electropolishing of steels.<ref>{{cite journal |last1=Abbott |first1=A. |last2=Capper |first2=G. |last3=McKenzie |first3=K.J. |last4=Glidle |first4=A. |last5=Ryder |first5=K.S. |title=Electropolishing of stainless steels in a choline chloride based ionic liquid: an electrochemical study with surface characterisation using SEM and atomic force microscopy |journal=Phys. Chem. Chem. Phys. |date=2006 |volume=8 |issue=36 |pages=4214–4221 |doi=10.1039/B607763N |pmid=16971989 |bibcode=2006PCCP....8.4214A |url=https://pubs.rsc.org/en/content/articlehtml/2006/cp/b607763n|hdl=2381/628 |hdl-access=free }}</ref> DESs have also demonstrated promising results to recover metals from complex mixtures such Cu/Zn and Ga/As,<ref>{{cite journal |last1=Abbott |first1=A. |last2=Harris |first2=R.C. |last3=Holyoak |first3=F. |last4=Frisch |first4=G. |last5=Hartley |first5=J. |last6=Jenkin |first6=G.R.T. |title=Electrocatalytic recovery of elements from complex mixtures using deep eutectic solvents |journal=Green Chem. |date=2015 |volume=17 |issue=4 |pages=2172–2179 |doi=10.1039/C4GC02246G |url=https://pubs.rsc.org/en/content/articlelanding/2015/gc/c4gc02246g|hdl=2381/31850 |hdl-access=free }}</ref> and precious metals from minerals.<ref>{{cite journal |last1=Jenkin |first1=G.R.T. |last2=Al-Bassam |first2=A.Z.M. |last3=Harris |first3=R.C. |last4=Abbott |first4=A. |last5=Smith |first5=D.J. |last6=Holwell |first6=D.A. |last7=Chapman |first7=R.J. |last8=Stanley |first8=C.J. |title=The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals |journal=Minerals Engineering |date=2016 |volume=87 |pages=18–24 |doi=10.1016/j.mineng.2015.09.026 |bibcode=2016MiEng..87...18J |doi-access=free |hdl=10141/603645 |hdl-access=free }}</ref> It has also been demonstrated that metals can be recovered from complex mixtures by electrocatalysis using a combination of DESs as lixiviants and an oxidising agent,<ref>{{cite journal |last1=Abbott |first1=A. |last2=Harris |first2=R.C. |last3=Holyoak |first3=F. |last4=Frisch |first4=G. |last5=Hartley |first5=J. |last6=Jenkin |first6=G.R.T. |title=Electrocatalytic recovery of elements from complex mixtures using deep eutectic solvents |journal=Green Chemistry |date=2015 |volume=17 |issue=4 |pages=2172–2179 |doi=10.1039/C4GC02246G |url=https://pubs.rsc.org/en/content/articlelanding/2015/gc/c4gc02246g|hdl=2381/31850 |hdl-access=free }}</ref> while metal ions can be simultaneously separated from the solution by electrowinning.<ref>{{cite journal |last1=Anggara |first1=S. |last2=Bevan |first2=F. |last3=Harris |first3=R.C. |last4=Hartley |first4=J. |last5=Frisch |first5=G. |last6=Jenkin |first6=G.R.T. |last7=Abbot |first7=A. |title=Direct extraction of copper from copper sulfide minerals using deep eutectic solvents |journal=Green Chemistry |date=2019 |volume=21 |issue=23 |pages=6502–6512 |doi=10.1039/C9GC03213D |s2cid=209704861 |url=https://pubs.rsc.org/en/content/articlelanding/2019/gc/c9gc03213d}}</ref>

==== Recovery of precious metals by ionometallurgy ====
Precious metals are rare, naturally occurring metallic chemical elements of high economic value. Chemically, the precious metals tend to be less reactive than most elements. They include gold and silver, but also the so-called platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum (see precious metals). Extraction of these metals from their corresponding hosting minerals would typically require pyrometallurgy (e.g., roasting), hydrometallurgy (cyanidation), or both as processing routes.
Early studies have demonstrated that gold dissolution rate in Ethaline compares very favourably to the cyanidation method, which is further enhanced by the addition of iodine as an oxidising agent. In an industrial process the iodine has the potential to be employed as an electrocatalyst, whereby it is continuously recovered in situ from the reduced iodide by electrochemical oxidation at the anode of an electrochemical cell. Dissolved metals can be selectively deposited at the cathode by adjusting the electrode potential. The method also allows better selectivity as part of the gangue (e.g., pyrite) tend to be dissolved more slowly.<ref>{{cite journal |last1=Jenkin |first1=G.R.T. |last2=Al-Bassam |first2=A.Z.M. |last3=Harris |first3=R.C. |last4=Abbott |first4=A. |last5=Smith |first5=D.J. |last6=Holwell |first6=D.A. |last7=Chapman |first7=R.J. |last8=Stanley |first8=C.J. |title=The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals |journal=Minerals Engineering |date=2016 |volume=87 |pages=18–24 |doi=10.1016/j.mineng.2015.09.026 |bibcode=2016MiEng..87...18J |doi-access=free |hdl=10141/603645 |hdl-access=free }}</ref>

Sperrylite (PtAs<sub>2</sub>) and moncheite (PtTe<sub>2</sub>), which are typically the more abundant platinum minerals in many orthomagmatic deposits, do not react under the same conditions in Ethaline because they are disulphide (pyrite), diarsenide (sperrylite) or ditellurides (calaverite and moncheite) minerals, which are particularly resistant to iodine oxidation. The reaction mechanism by which dissolution of platinum minerals is taking place is still under investigation.

==== Metal recovery from sulfide minerals with ionometallurgy ====
Metal sulfides (e.g., pyrite FeS<sub>2</sub>, arsenopyrite FeAsS, chalcopyrite CuFeS<sub>2</sub>) are normally processed by chemical oxidation either in aqueous media or at high temperatures. In fact, most base metals, e.g., aluminium, chromium, must be (electro)chemically reduced at high temperatures by which the process entails a high energy demand, and sometimes large volumes of aqueous waste is generated. In aqueous media chalcopyrite, for instance, is more difficult to dissolve chemically than covellite and chalcocite due to surface effects (formation of polysulfide species,<ref>{{cite journal |last1=Ghahremaninezhad |first1=A. |last2=Dixon |first2=D.G. |last3=Asselin |first3=E. |title=Electrochemical and XPS analysis of chalcopyrite (CuFeS2) dissolution in sulfuric acid solution |journal=Electrochimica Acta |date=2013 |volume=87 |pages=97–112 |doi=10.1016/j.electacta.2012.07.119 |url=https://www.sciencedirect.com/science/article/pii/S0013468612012996}}</ref><ref>{{cite journal |last1=Dreisinger |first1=D. |last2=Abed |first2=N. |title=A fundamental study of the reductive leaching of chalcopyrite using metallic iron part I: kinetic analysis |journal=Hydrometallurgy |date=2002 |volume=60 |issue=1–3 |pages=293–296 |doi=10.1016/S0304-386X(02)00079-8 |bibcode=2002HydMe..66...37D |url=https://www.sciencedirect.com/science/article/pii/S0304386X02000798}}</ref>). The presence of Cl<sup>−</sup> ions has been suggested to alter the morphology of any sulfide surface formed, allowing the sulfide mineral to leach more easily by preventing passivation.<ref>{{cite journal |last1=Pikna |first1=L. |last2=Lux |first2=L. |last3=Grygar |first3=T. |title=Electrochemical dissolution of chalcopyrite studied by voltammetry of immobilized microparticles |journal=Chemical Papers |date=2006 |volume=60 |issue=4 |pages=293–296|doi=10.2478/s11696-006-0051-7 |s2cid=95349687 }}</ref> DESs provide a high Cl<sup>−</sup> ion concentration and low water content, whilst reducing the need for either high additional salt or acid concentrations, circumventing most oxide chemistry. Thus, the electrodissolution of sulfide minerals has demonstrated promising results in DES media in absence of passivation layers, with the release into the solution of metal ions which could be recovered from solution.

During extraction of copper from copper sulfide minerals with Ethaline, chalcocite (Cu<sub>2</sub>S) and covellite (CuS) produce a yellow solution, indicating that [CuCl<sub>4</sub>]<sup>2−</sup> complex are formed. Meanwhile, in the solution formed from chalcopyrite, Cu<sup>2+</sup> and Cu<sup>+</sup> species co-exist in solution due to the generation of reducing Fe<sup>2+</sup> species at the cathode. The best selective recovery of copper (>97%) from chalcopyrite can be obtained with a mixed DES of 20 wt.% ChCl-oxalic acid and 80 wt.% Ethaline.<ref>{{cite journal |last1=Abbott |first1=A. |last2=Al-Bassam |first2=A.Z.M. |last3=Goddard |first3=A. |last4=Harris |first4=R.C. |last5=Jenkin |first5=G.R.T. |last6=Nisbet |first6=J. |last7=Wieland |first7=M. |title=Dissolution of pyrite and other Fe – S – As minerals using deep eutectic solvents |journal=Green Chemistry |date=2017 |volume=19 |issue=9 |pages=2225–2233 |doi=10.1039/C7GC00334J |url=https://pubs.rsc.org/en/content/articlelanding/2017/gc/c7gc00334j|hdl=2381/40192 |hdl-access=free }}</ref>

==== Metal recovery from oxide compounds with Ionometallurgy ====
Recovery of metals from oxide matrixes is generally carried out using mineral acids. However, electrochemical dissolution of metal oxides in DES can allow to enhance the dissolution up to more than 10 000 times in pH neutral solutions.<ref>{{cite journal |last1=Pateli |first1=I.M. |last2=Abbott |first2=A. |last3=Hartley |first3=J. |last4=Jenkin |first4=G.R.T |title=Electrochemical oxidation as alternative for dissolution of metal oxides in deep eutectic solvents |journal=Green Chemistry |date=2020 |volume=22 |issue=23 |pages=8360–8368 |doi=10.1039/D0GC03491F |s2cid=229243585 |url=https://pubs.rsc.org/en/content/articlehtml/2020/gc/d0gc03491f}}</ref>

Studies have shown that ionic oxides such as ZnO tend to have high solubility in ChCl:malonic acid, ChCl:urea and Ethaline, which can resemble the solubilities in aqueous acidic solutions, e.g., HCl. Covalent oxides such as TiO<sub>2</sub>, however, exhibits almost no solubility. The electrochemical dissolution of metal oxides is strongly dependent on the proton activity from the HBD, i.e. capability of the protons to act as oxygen acceptors, and on the temperature. It has been reported that [[eutectic]] ionic fluids of lower pH-values, such as ChCl:oxalic acid and ChCl:lactic acid, allow a better solubility than that of higher pH (e.g., ChCl:acetic acid).<ref>{{cite journal |last1=Pateli |first1=I.M. |last2=Thompson |first2=D. |last3=Alabdullah |first3=S.S.M |last4=Abbott |first4=A. |last5=Jenkin |first5=G.R.T. |last6=Hartley |first6=J. |title=The effect of pH and hydrogen bond donor on the dissolution of metal oxides in deep eutectic solvents |journal=Green Chemistry |date=2020 |volume=22 |issue=16 |pages=5476–5486 |doi=10.1039/D0GC02023K |s2cid=225401121 |url=https://pubs.rsc.org/en/content/articlelanding/2020/gc/d0gc02023k}}</ref> Hence, different solubilities can be obtained by using, for instance, different carboxylic acids as HBD.<ref>{{cite journal |last1=Abbott |first1=A. |last2=Boothby |first2=D. |last3=Capper |first3=G. |last4=Davies |first4=D.L. |last5=Rasheed |first5=R.K. |title=Deep Eutectic Solvents formed between choline chloride and carboxylic acids: Versatile alternatives to ionic liquids |journal=J. Am. Chem. Soc. |date=2004 |volume=126 |issue=29 |pages=9142–9147 |doi=10.1021/ja048266j |pmid=15264850 |url=https://pubs.acs.org/doi/10.1021/ja048266j}}</ref>