Ionometallurgy

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Mineral processing and extraction of metals are very energy-intensive processes, which are not exempted of producing large volumes of solid residues and wastewater, which also require energy to be further treated and disposed. Moreover, as the demand for metals increases, the metallurgical industry must rely on sources of materials with lower metal contents both from a primary (e.g., mineral ores) and/or secondary (e.g., slags, tailings, municipal waste) raw materials. Consequently, mining activities and waste recycling must evolve towards the development of more selective, efficient and environmentally friendly mineral and metal processing routes.

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Mineral processing operations are needed firstly to concentrate the mineral phases of interest and reject the unwanted material physical or chemically associated to a defined raw material. The process, however, demand about 30 GJ/tonne of metal, which accounts about 29% of the total energy spent on mining in the USA. [1] Meanwhile, pyrometallurgy is a significant producer of greenhouse gas emissions and harmful flue dust. Hydrometallurgy entails the consumption of large volumes of lixiviants such as H2SO4, HCl, KCN, NaCN which have poor selectivity. [2] Moreover, despite the environmental concern and the use restriction imposed by some countries, cyanidation is still considered the prime process technology to recover gold from ores. Mercury is also used by artisanal miners in less economically developed countries to concentrate gold and silver from minerals, despite its obvious toxicity. Bio-hydro-metallurgy make use of living organisms, such as bacteria and fungi, and although this method demands only the input of O2 and CO2 from the atmosphere, it requires low solid-to-liquid ratios and long contact times, which significantly reduces space-time yields.

Ionometallurgy makes use of non-aqueous ionic solvents such ionic liquids (ILs) and deep eutectic solvents (DESs), which allows the development of closed-loop flow sheet to effectively recover metals by, for instance, integrating the metallurgical unit operations of leaching and electrowinning. It allows to process metals at moderate temperatures in a non-aqueous environment which allows controlling metal speciation, tolerates impurities and at the same time exhibits suitable solubilities and current efficiencies. This simplify conventional processing routes and allows a substantial reduction in the size of a metal processing plant.

Metal extraction with 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 °C, and they exhibit similar physico-chemical properties to traditional ILs, while being much cheaper and environmentally friendlier. Most of them are mixtures of choline chloride 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, [3] and the DES formulated from these anions are typically much more viscous and can have higher conductivities than for choline chloride. [4] 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. [5] Similarly, Ethaline (a 1: 2 mixture of choline chloride and ethylene glycol) facilitates metal dissolution in electropolishing of steels. [6] DESs have also demonstrated promising results to recover metals from complex mixtures such Cu/Zn and Ga/As, [7] and precious metals from minerals. [8] 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, [9] while metal ions can be simultaneously separated from the solution by electrowinning. [10]

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. [11]

Sperrylite (PtAs2) and moncheite (PtTe2), 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 FeS2, arsenopyrite FeAsS, chalcopyrite CuFeS2) 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, [12] [13] ). The presence of Cl− ions has been suggested to alter the morphology of any sulfide surface formed, allowing the sulfide mineral to leach more easily by preventing passivation. [14] DESs provide a high Cl− 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 (Cu2S) and covellite (CuS) produce a yellow solution, indicating that [CuCl4]2− complex are formed. Meanwhile, in the solution formed from chalcopyrite, Cu2+ and Cu+ species co-exist in solution due to the generation of reducing Fe2+ 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. [15]

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. [16]

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 TiO2, 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). [17] Hence, different solubilities can be obtained by using, for instance, different carboxylic acids as HBD. [18]

Outlook

Currently, the stability of most ionic liquids under practical electrochemical conditions is unknown, and the fundamental choice of ionic fluid is still empirical as there is almost no data on metal ion thermodynamics to feed into solubility and speciation models. Also, there are no Pourbaix diagrams available, no standard redox potentials, and bare knowledge of speciation or pH-values. It must be noticed that most processes reported in the literature involving ionic fluids have a Technology Readiness Level (TRL) 3 (experimental proof-of-concept) or 4 (technology validated in the lab), which is a disadvantage for short-term implementation. However, ionometallurgy has the potential to effectively recover metals in a more selective and sustainable way, as it considers environmentally benign solvents, reduction of greenhouse gas emissions and avoidance of corrosive and harmful reagents.

Related Research Articles

In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively charged sodium ions and negatively charged chloride ions.

An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dissolving, the substance separates into cations and anions, which disperse uniformly throughout the solvent. Solid-state electrolytes also exist. In medicine and sometimes in chemistry, the term electrolyte refers to the substance that is dissolved.

<span class="mw-page-title-main">Sodium hydroxide</span> Chemical compound with formula NaOH

Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations Na+ and hydroxide anions OH−.

<span class="mw-page-title-main">Solubility</span> Capacity of a substance to dissolve in a solvent in a homogeneous way

In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.

Extractive metallurgy is a branch of metallurgical engineering wherein process and methods of extraction of metals from their natural mineral deposits are studied. The field is a materials science, covering all aspects of the types of ore, washing, concentration, separation, chemical processes and extraction of pure metal and their alloying to suit various applications, sometimes for direct use as a finished product, but more often in a form that requires further working to achieve the given properties to suit the applications.

<span class="mw-page-title-main">Noble metal</span> Metallic elements that are nearly chemically inert

A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals are most often so classified. Silver, copper, and mercury are sometimes included as noble metals, but each of these usually occurs in nature combined with sulfur.

<span class="mw-page-title-main">Ionic compound</span> Chemical compound involving ionic bonding

In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions such as the sodium (Na+) and chloride (Cl−) in sodium chloride, or polyatomic species such as the ammonium (NH+
4) and carbonate (CO2−
3) ions in ammonium carbonate. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network. Ionic compounds usually form crystalline structures when solid.

<span class="mw-page-title-main">Flux (metallurgy)</span> Chemical used in metallurgy for cleaning or purifying molten metal

In metallurgy, a flux is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.

<span class="mw-page-title-main">Ionic liquid</span> Salt in the liquid state

An ionic liquid (IL) is a salt in the liquid state at ambient conditions. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as 100 °C (212 °F). While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses.

Neodymium(III) chloride or neodymium trichloride is a chemical compound of neodymium and chlorine with the formula NdCl3. This anhydrous compound is a mauve-colored solid that rapidly absorbs water on exposure to air to form a purple-colored hexahydrate, NdCl3·6H2O. Neodymium(III) chloride is produced from minerals monazite and bastnäsite using a complex multistage extraction process. The chloride has several important applications as an intermediate chemical for production of neodymium metal and neodymium-based lasers and optical fibers. Other applications include a catalyst in organic synthesis and in decomposition of waste water contamination, corrosion protection of aluminium and its alloys, and fluorescent labeling of organic molecules (DNA).

<span class="mw-page-title-main">Pitting corrosion</span> Form of insidious localized corrosion in which a pit develops at the anode site

Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions.

<span class="mw-page-title-main">Sulfamic acid</span> Chemical compound

Sulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, sulphamic acid and sulfamidic acid, is a molecular compound with the formula H3NSO3. This colourless, water-soluble compound finds many applications. Sulfamic acid melts at 205 °C before decomposing at higher temperatures to water, sulfur trioxide, sulfur dioxide and nitrogen.

Liquid–liquid extraction (LLE), also known as solvent extraction and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an organic solvent (non-polar). There is a net transfer of one or more species from one liquid into another liquid phase, generally from aqueous to organic. The transfer is driven by chemical potential, i.e. once the transfer is complete, the overall system of chemical components that make up the solutes and the solvents are in a more stable configuration. The solvent that is enriched in solute(s) is called extract. The feed solution that is depleted in solute(s) is called the raffinate. LLE is a basic technique in chemical laboratories, where it is performed using a variety of apparatus, from separatory funnels to countercurrent distribution equipment called as mixer settlers. This type of process is commonly performed after a chemical reaction as part of the work-up, often including an acidic work-up.

Deep eutectic solvents or DESs are solutions of Lewis or Brønsted acids and bases which form a eutectic mixture. Deep eutectic solvents are highly tunable through varying the structure or relative ratio of parent components and thus have a wide variety of potential applications including catalytic, separation, and electrochemical processes. The parent components of deep eutectic solvents engage in a complex hydrogen bonding network which results in significant freezing point depression as compared to the parent compounds. The extent of freezing point depression observed in DESs is well illustrated by a mixture of choline chloride and urea in a 1:2 mole ratio. Choline chloride and urea are both solids at room temperature with melting points of 302 °C and 133 °C respectively, yet the combination of the two in a 1:2 molar ratio forms a liquid with a freezing point of 12 °C. DESs share similar properties to ionic liquids such as tunability and lack of flammability yet are distinct in that ionic liquids are neat salts composed exclusively of discrete ions. In contrast to ordinary solvents, such as Volatile Organic Compounds (VOC), DESs are non-flammable, and possess low vapour pressures and toxicity.

A multiphasic liquid is a mixture consisting of more than two immiscible liquid phases. Biphasic mixtures consisting of two immiscible phases are very common and usually consist of an organic solvent and an aqueous phase.

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<span class="mw-page-title-main">Molten salt</span> Salt that has melted, often by heating to high temperatures

Molten salt is salt which is solid at standard temperature and pressure but liquified due to elevated temperature. A salt that is liquid even at standard temperature and pressure is usually called a room-temperature ionic liquid, and molten salts are technically a class of ionic liquids.

<span class="mw-page-title-main">Chloroauric acid</span> Chemical compound

Chloroauric acid is an inorganic compound with the chemical formula H[AuCl4]. It forms hydrates H[AuCl4]·nH2O. Both the trihydrate and tetrahydrate are known. Both are orange-yellow solids consisting of the planar [AuCl4]− anion. Often chloroauric acid is handled as a solution, such as those obtained by dissolution of gold in aqua regia. These solutions can be converted to other gold complexes or reduced to metallic gold or gold nanoparticles.

<span class="mw-page-title-main">Metal halides</span>

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In 2015, the UN defined a new sustainability-focused development plan based on 17 sustainable development goals. It recognized the need of "green chemistry" and "green solvent" for a more sustainable chemistry in the future. At the same time, more and more companies want to go green, to ensure that their activities and products are part of a sustainable development process. It is in this context that so-called green, ecological, biodegradable, and sustainable solvents have emerged., which were developed as a more environmentally friendly solvents, or biosolvents, derived from the processing of agricultural crops, an alternative to petrochemical solvents.

References

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  2. ↑ Binnemans, Koen (2017). "Solvometallurgy: An Emerging Branch of Extractive Metallurgy". Journal of Sustainable Metallurgy. 3 (3): 571–600. doi: 10.1007/s40831-017-0128-2 . S2CID   52203805.
  3. ↑ Endres, F; MacFarlane, D; Abbott, A (2017). Electrodeposition from Ionic Liquids. Wiley-VCH.
  4. ↑ Bernasconi, R.; Zebarjadi, Z.; Magagnin, L. (2015). "Copper electrodeposition from a chloride free deep eutectic solvent". Journal of Electroanalytical Chemistry. 758 (1): 163–169. doi:10.1016/j.jelechem.2015.10.024. hdl: 11311/987216 .
  5. ↑ Abbott, A.; Collins, J.; Dalrymple, I.; Harris, R.C.; Mistry, R.; Qiu, F.; Scheirer, J.; Wise, W.R. (2009). "Processing of Electric Arc Furnace Dust using Deep Eutectic Solvents". Australian Journal of Chemistry. 62 (4): 341–347. doi:10.1071/CH08476.
  6. ↑ Abbott, A.; Capper, G.; McKenzie, K.J.; Glidle, A.; Ryder, K.S. (2006). "Electropolishing of stainless steels in a choline chloride based ionic liquid: an electrochemical study with surface characterisation using SEM and atomic force microscopy". Phys. Chem. Chem. Phys. 8 (36): 4214–4221. Bibcode:2006PCCP....8.4214A. doi:10.1039/B607763N. hdl: 2381/628 . PMID   16971989.
  7. ↑ Abbott, A.; Harris, R.C.; Holyoak, F.; Frisch, G.; Hartley, J.; Jenkin, G.R.T. (2015). "Electrocatalytic recovery of elements from complex mixtures using deep eutectic solvents". Green Chem. 17 (4): 2172–2179. doi:10.1039/C4GC02246G. hdl: 2381/31850 .
  8. ↑ Jenkin, G.R.T.; Al-Bassam, A.Z.M.; Harris, R.C.; Abbott, A.; Smith, D.J.; Holwell, D.A.; Chapman, R.J.; Stanley, C.J. (2016). "The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals". Minerals Engineering. 87: 18–24. doi: 10.1016/j.mineng.2015.09.026 . hdl: 10141/603645 .
  9. ↑ Abbott, A.; Harris, R.C.; Holyoak, F.; Frisch, G.; Hartley, J.; Jenkin, G.R.T. (2015). "Electrocatalytic recovery of elements from complex mixtures using deep eutectic solvents". Green Chemistry. 17 (4): 2172–2179. doi:10.1039/C4GC02246G. hdl: 2381/31850 .
  10. ↑ Anggara, S.; Bevan, F.; Harris, R.C.; Hartley, J.; Frisch, G.; Jenkin, G.R.T.; Abbot, A. (2019). "Direct extraction of copper from copper sulfide minerals using deep eutectic solvents". Green Chemistry. 21 (23): 6502–6512. doi:10.1039/C9GC03213D. S2CID   209704861.
  11. ↑ Jenkin, G.R.T.; Al-Bassam, A.Z.M.; Harris, R.C.; Abbott, A.; Smith, D.J.; Holwell, D.A.; Chapman, R.J.; Stanley, C.J. (2016). "The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals". Minerals Engineering. 87: 18–24. doi: 10.1016/j.mineng.2015.09.026 . hdl: 10141/603645 .
  12. ↑ Ghahremaninezhad, A.; Dixon, D.G.; Asselin, E. (2013). "Electrochemical and XPS analysis of chalcopyrite (CuFeS2) dissolution in sulfuric acid solution". Electrochimica Acta. 87: 97–112. doi:10.1016/j.electacta.2012.07.119.
  13. ↑ Dreisinger, D.; Abed, N. (2002). "A fundamental study of the reductive leaching of chalcopyrite using metallic iron part I: kinetic analysis". Hydrometallurgy. 60 (1–3): 293–296. doi:10.1016/S0304-386X(02)00079-8.
  14. ↑ Pikna, L.; Lux, L.; Grygar, T. (2006). "Electrochemical dissolution of chalcopyrite studied by voltammetry of immobilized microparticles". Chemical Papers. 60 (4): 293–296. doi:10.2478/s11696-006-0051-7. S2CID   95349687.
  15. ↑ Abbott, A.; Al-Bassam, A.Z.M.; Goddard, A.; Harris, R.C.; Jenkin, G.R.T.; Nisbet, J.; Wieland, M. (2017). "Dissolution of pyrite and other Fe – S – As minerals using deep eutectic solvents". Green Chemistry. 19 (9): 2225–2233. doi:10.1039/C7GC00334J. hdl: 2381/40192 .
  16. ↑ Pateli, I.M.; Abbott, A.; Hartley, J.; Jenkin, G.R.T (2020). "Electrochemical oxidation as alternative for dissolution of metal oxides in deep eutectic solvents". Green Chemistry. 22 (23): 8360–8368. doi:10.1039/D0GC03491F. S2CID   229243585.
  17. ↑ Pateli, I.M.; Thompson, D.; Alabdullah, S.S.M; Abbott, A.; Jenkin, G.R.T.; Hartley, J. (2020). "The effect of pH and hydrogen bond donor on the dissolution of metal oxides in deep eutectic solvents". Green Chemistry. 22 (16): 5476–5486. doi:10.1039/D0GC02023K. S2CID   225401121.
  18. ↑ Abbott, A.; Boothby, D.; Capper, G.; Davies, D.L.; Rasheed, R.K. (2004). "Deep Eutectic Solvents formed between choline chloride and carboxylic acids: Versatile alternatives to ionic liquids". J. Am. Chem. Soc. 126 (29): 9142–9147. doi:10.1021/ja048266j. PMID   15264850.
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