Method of recovering a metal

Abstract

The disclosure relates to a method of recovering a target metal, a compound comprising the target metal or a salt thereof, from a composition comprising the target metal or a first oxidised form thereof. The method comprises contacting the composition and an extraction liquid to extract the target metal or the first oxidised form thereof from the composition and to thereby produce a first solution. The extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the first solution comprises a second oxidised form of the target metal dissolved therein. The method further comprises conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal.

Description

[0001]Method of Recovering a Metal The present invention relates to a method of recovering a metal. In particular, the method may be used to recover a metal from an ore; a waste by-product of an industrial process, such as a slag; or from an end-of-life product, such as a battery or magnet. There is a global trend to reduce the impact on the environment of industrial processes. In view of this, the industrial sector has focused on reducing waste, reducing energy consumption and / or otherwise mitigating the environmental impact of industrial processes. Around 9% of all global CO2 emissions can be attributed to the iron and steel production industry. Efforts to decarbonise the iron-making process include transitioning from using solid fossil carbon in iron reduction reactions to using direct gas reduction by hydrogen. There have also been steps to decarbonise the high temperature heating required to prepare vessels or heat treat steel. In both cases there is a need to scale-up and invest in the production of low-carbon hydrogen to support the transition to low-carbon steelmaking. Slag is a by-product material that is produced during the process of smelting or refining ore in metallurgical industries, such as iron and steel production. It consists of impurities and non-metallic components that are separated from the desired metal during the refining process. During steelmaking, metal-containing slag is produced. The presence of different metals in the slag prevents it from being recycled in the iron industry’s internal processes, and requires it to be treated or landfilled. There are approximately 6.7 million tonnes of zinc / iron-rich slag stockpiled in the UK and 90,000 tonnes produced annually. Existing slag treatment processes are energy intensive and offer little in the way of added value. The present invention arises from the inventors’ work in attempting to overcome the problems associated with the prior art. In a first aspect of the invention, there is provided a method of recovering a target metal, a compound comprising the target metal or a salt thereof, from a composition comprising the target metal or a first oxidised form thereof, the method comprising: - contacting the composition and an extraction liquid to extract the target metal or the first oxidised form thereof from the composition and to thereby produce a first solution, wherein the extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the first solution comprises a second oxidised form of the target metal dissolved therein; and - conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal. Advantageously, the method of the first aspect allows the target metal, a compound comprising the target metal or a salt thereof, to be separated from the composition. In some embodiments, the composition comprises or consists of a water insoluble material or a material which is sparingly soluble in water. In some embodiments, the target metal, or the first oxidised form thereof, is water insoluble or sparingly soluble in water. The composition may comprise a polymetallic composition. Accordingly, the composition may comprise the target metal or a salt thereof and a second target metal or a salt thereof. The composition may further comprise a third target metal or a salt thereof. The composition may be an impure composition. The impure composition may be understood to comprise the target metal or the first oxidised form thereof and an impurity. The impure composition may comprise or consist of a waste material from an industrial process. The composition may comprise or consist of an ore. The composition may comprise or consist of material originating from a mining process. For instance, the composition may comprise or consist of a mining tailing. The composition may comprise or consist of material originating from a battery or battery production. Accordingly, the composition may comprise or consist of black mass. It may be appreciated that the term “black mass” as used herein, may refer to a composition comprising or consisting of electrode materials. The electrode materials may originate from batteries or battery production. The batteries or battery production may be alkaline batteries or alkaline battery production. The batteries or battery production may be lithium-ion batteries or production thereof. The black mass material may originate from new batteries, waste batteries, spent batteries, battery production waste or scrap materials and battery electrode materials. The electrode materials may be shredded. The black mass material may comprise zinc or a salt thereof, manganese or a salt thereof and / or iron or a salt thereof. The polymetallic composition and / or the black mass may comprise between 1 and 90 wt%, between 5 and 80 wt%, between 10 and 70 wt%, between 12 and 60 wt%, between 14 and 50 wt%, between 16 and 40 wt%, between 18 and 35 wt%, between 20 and 30 wt%, between 22 and 28 wt% or between 24 and 26 wt% zinc or an oxidised form thereof. The polymetallic composition and / or the black mass may comprise between 1 and 95 wt%, between 5 and 90 wt%, between 10 and 80 wt%, between 15 and 70 wt%, between 20 and 60 wt%, between 22 and 50 wt%, between 24 and 45 wt%, between 26 and 40 wt%, between 28 and 38 wt%, between 30 and 36 wt% or between 32 and 34 wt% manganese or an oxidised form thereof. The polymetallic composition and / or the black mass may comprise between 0.1 and 20 wt%, between 0.5 and 10 wt%, between 1 and 7.5 wt%, between 1.5 and 5 wt%, between 2 and 4 wt%, between 2.5 and 3 wt% iron or an oxidised form thereof. The method may comprise pre-treating the composition. Pre-treating the composition may comprise heating the composition. The composition may be heated to at least 50 °C, at least 70 °C, at least 90 °C, at least 100 °C, at least 110 °C or at least 120 °C. The composition may be heated to between 50 and 500°C, between 70 and 300°C, between 90 and 200°C, between 100 and 150°C or between 110 and 130°C. The composition may be heated for at least 1 hour, at least 4 hours, at least 8 hours, at least 12 hours, at least 16 hours, at least 20 hours, at least 22 hours, or at least 24 hours. The composition may be heated for between 1 hour and 4 weeks, between 4 hours and 1 week, between 8 and 72 hours, between 12 and 48 hours, between 16 and 36 hours, between 20 and 30 hours or between 22 and 26 hours. In some embodiments, pre-treating the composition comprises heating the composition until it is substantially dry. The composition may be considered substantially dry if it has a moisture content of less than 10 wt%, less than 8 wt%, less than 6 wt%, less than 5 wt%,less than 4 wt% or less than 3 wt%. It will be appreciated that the moisture content of the sample may be determined by any suitable method. The moisture content may be determined using the following equation: In some embodiments, pre-treating the composition comprises homogenising the composition particle size. Homogenising the composition particle size may comprise crushing the composition and / or sieving the composition. Homogenising the composition particle size may comprise crushing the composition and subsequently sieving the composition. Sieving the composition may comprise removing particles above a predetermined maximum particle size. Sieving the composition may comprise removing particles below a predetermined minimum particle size. The composition may comprise or consist of particles with a particle diameter between 50 and 5000 nm, between 100 and 3000 nm, between 150 and 2500 nm, between 200 and 2000 nm, between 250 and 1900 nm, or between 300 and 1800 nm. The composition may be considered to be homogenous if at least 80%, at least 90% or at least 99% of particles in the composition have a particle diameter between 50 and 5000 nm, between 100 and 3000 nm, between 150 and 2500 nm, between 200 and 2000 nm, between 250 and 1900 nm, or between 300 and 1800 nm. In some embodiments, the composition is considered to be homogenous if at least 80%, at least 90% or at least 99% of particles in the composition have a particle diameter between 300 and 1800 nm. The composition may comprise or consist of material originating from a magnet or magnet production. The material originating from the magnet or magnet production may comprise neodymium. The material originating from the magnet or magnet production may further comprise iron, boron and / or praseodymium. In an embodiment, the composition comprises or consist of a waste material from a metallurgical process. The metallurgical process may be iron or steel production. In an embodiment, the composition may comprise or consist of slag. The composition may comprise or consist of slag produced as a byproduct in the industrial production of iron or steel. The composition may be a solid or a liquid. In some embodiments, the composition is a solid. The composition may comprise one or more further metals and / or a first oxidised form thereof. Contacting the extraction liquid with the composition may cause one or more further metals and / or the first oxidised form thereof to be extracted from the composition. Accordingly, the first solution may further comprise the second oxidised form of one or more further metals. In some embodiments, the method may comprise preferentially extracting the target metal, the compound comprising the target metal or the first oxidised form thereof. The target metal may be understood to be preferentially extracted if the proportion of the target metal, the compound comprising the target metal or the first oxidised form thereof extracted from the composition relative to the total present in the composition is greater than the proportion of the further metal or the first oxidised form thereof extracted from the composition relative to the total present in the composition. The method may comprise selecting the extraction liquid and / or reaction conditions used during the extraction to preferentially extract the target metal, the compound comprising the target metal or the first oxidised form thereof. The target metal, the compound comprising the target metal or the first oxidised form thereof in the composition may have an oxidation state of zero or a low oxidation state. The first oxidised form of the target metal may be understood to have a low oxidative state if it can be further oxidised through an electrochemical or chemical reaction. The first oxidised form of the target metal may have an oxidation state of +1, +2, +3, +4, +5, +6, +7 or +8. In some embodiments, the first oxidised form of the target metal has an oxidation state of +1, +2 or +3. The target metal or the first oxidised form thereof present in the composition may comprise or consist of actinium, aluminium, barium, beryllium, bismuth, cadmium, caesium, calcium, cerium, chromium, cobalt, copper, dysprosium, erbium, europium, francium, gadolinium, gallium, gold, hafnium, holmium, indium, iridium, iron, lanthanum, lead, lithium, lutetium, magnesium, manganese, mercury, molybdenum, neodymium, nickel, niobium, osmium, palladium, platinum, potassium, praseodymium, promethium, protactinium, radium, rhenium, rhodium, rubidium, ruthenium, samarium, scandium, silver, sodium, strontium, tantalum, technetium, terbium, thallium, thorium, thulium, tin, titanium, tungsten, uranium, vanadium, ytterbium, yttrium, zinc, zirconium, a first oxidised form of any of the previously mentioned metals and / or a combination thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition may comprise or consist of a rare earth metal or a first oxidised form thereof. It may be appreciated that a rare earth metal may be cerium, dysprosium, erbium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, niobium, praseodymium, promethium, samarium, scandium, tantalum, terbium, thulium, ytterbium or yttrium. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or consists of copper, zinc, lead, calcium, iron, neodymium, praseodymium, magnesium, aluminium, dysprosium, manganese, cerium, titanium, a first oxidised form of any of the previously mentioned metals or a combination thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or consists of magnesium, copper, zinc, lead, calcium, iron, a first oxidised form of any of the previously mentioned metals or a combination thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or consists of iron, neodymium, praseodymium, a first oxidised form of any thereof or a combination thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or is iron or a first oxidised form thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or is magnesium or a first oxidised form thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or is copper or a first oxidised form thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or is zinc or a first oxidised form thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or is neodymium or a first oxidised form thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or is praseodymium or a first oxidised form thereof. In some embodiments, the target metal or the first oxidised form thereof present in the composition comprises or is calcium or a first oxidised form thereof. The first oxidised form of the target metal may comprise oxygen, nitrogen, phosphorous, a halogen, sulphur, selenium, carbon and / or hydrogen. Accordingly, the first oxidised form of the target metal may be an oxide, a hydroxide, a nitrate, a phosphate, a fluoride, a chloride, an iodide, a bromide, a sulphide, a sulphate, a selenide or a carbonate. The further metal or the first oxidised form thereof present in the composition may comprise or consist of aluminium, barium, beryllium, bismuth, cadmium, caesium, calcium, cerium, chromium, cobalt, copper, dysprosium, erbium, europium, gadolinium, gallium, gold, hafnium, holmium, indium, iridium, iron, lanthanum, lead, lithium, lutetium, magnesium, manganese, mercury, molybdenum, neodymium, nickel, niobium, osmium, palladium, platinum, potassium, praseodymium, rhenium, rhodium, rubidium, ruthenium, samarium, scandium, silver, sodium, tantalum, terbium, thorium, thulium, tin, titanium, tungsten, uranium, vanadium, ytterbium, yttrium, zinc, zirconium, a first oxidised form of any of the previously mentioned metals and / or a combination thereof. In some embodiments, the further metal or the first oxidised form thereof present in the composition comprises or consists of lead, calcium, iron, magnesium, manganese or zinc a first oxidised form of any of the previously mentioned metals or a combination thereof. In some embodiments, the further metal or the first oxidised form thereof present in the composition comprises or consists of lead, calcium, iron or manganese a first oxidised form of any of the previously mentioned metals or a combination thereof. In some embodiments, the further metal or the first oxidised form thereof present in the composition comprises or consists of lead, calcium, iron, a first oxidised form of any of the previously mentioned metals or a combination thereof. In some embodiments, the further metal or the first oxidised form thereof present in the composition comprises or is iron or a first oxidised form thereof. In some embodiments, the further metal or the first oxidised form thereof present in the composition comprises or is lead or a first oxidised form thereof. In some embodiments, the further metal or the first oxidised form thereof present in the composition comprises or is calcium or a first oxidised form thereof. The first oxidised form of the further metal may comprise oxygen, nitrogen, phosphorous, a halogen, sulphur, selenium, carbon and / or hydrogen. Accordingly, the first oxidised form of the further metal may be an oxide, a hydroxide, a nitrate, a phosphate, a fluoride, a chloride, an iodide, a bromide, a sulphide, a sulphate, a selenide or a carbonate. The impurity may comprise or consist of one or more non-metal or metalloid impurities. The one or more non-metal or metalloid impurities may comprise or consist of boron, germanium, arsenic, antimony, tellurium, polonium, sulfur, silicon, phosphorous or a first oxidised form thereof. The first oxidised form of the non-metal or metalloid impurity may comprise oxygen, nitrogen, phosphorous, a halogen, sulphur, selenium, carbon and / or hydrogen. Accordingly, the first oxidised form of the non-metal or metalloid impurity may be an oxide, a hydroxide, a nitrate, a phosphate, a fluoride, a chloride, an iodide, a bromide, a sulphide, a sulphate, a selenide or a carbonate. The second oxidised form of the target metal may be the same or different to the first oxidised form of the target metal. The second oxidised form of the target metal may have a low oxidation state. The second oxidised form of the target metal may have an oxidation state of +1, +2, +3, +4, +5, +6, +7 or +8. In some embodiments, the second oxidised form of the target metal may have an oxidation state of +1, +2 or +3. The second oxidised form of the target metal may be understood to be soluble in the first solution. The compound comprising the target metal may be an oxide. In some embodiments, compound comprising the target metal is an iron oxide, magnesium oxide, neodymium oxide, praseodymium oxide or zinc oxide. The solid salt of the target metal may comprise oxygen, nitrogen, phosphorous, a halogen, sulphur, selenium, carbon and / or hydrogen. Accordingly, the solid salt of the target metal may be a hydroxide, a nitrate, a phosphate, a fluoride, a chloride, an iodide, a bromide, a sulphide, a sulphate, a selenide or a carbonate. In some embodiments, the solid salt of the target metal is a hydroxide. In some embodiments, the solid salt of the target metal is a chloride. In some embodiments, the solid salt of the target metal is a sulphate. In some embodiments, the solid salt of the target metal is copper hydroxide, copper (I) chloride, copper (II) sulphate, iron chloride, iron sulphate, an iron hydroxide, magnesium hydroxide, lead (II) sulphate, zinc carbonate, or zinc hydroxide. In some embodiments, the solid salt of the target metal is neodymium hydroxide or praseodymium hydroxide. In some embodiments, the solid salt of the target metal is calcium hydroxide. The target metal, the compound comprising the target metal or the solid salt of the target metal may comprise a particle or a plurality of particles. The hydrodynamic diameter of the or each particle may be determined by dynamic light scattering (DLS). The or each particle of the target metal, the compound comprising the target metal or the solid salt of the target metal may have a hydrodynamic diameter of between 10 and 10,000 nm, between 100 and 7,500 nm, between 200 and 5,000 nm, between, 300 and 2,000 nm, between 400 and 1,000 nm, or between 500 and 900 nm. The or each particle of the target metal, the compound comprising the target metal or the solid salt of the target metal may have a hydrodynamic diameter of more than 50 nm, more than 100 nm, more than 200 nm, more than 300 nm, more than 500 nm, more than 750 nm, more than 1000 nm, more than 1,500 nm, more than 2,000 nm, more than 5,000 nm or more than 7,500 nm. The or each particle of the target metal, the compound comprising the target metal or the solid salt of the target metal may have a hydrodynamic diameter of less than 10,000 nm, less than 7,500 nm, less than 5,000 nm, less than 3,000 nm, less than 2,000 nm, less than 1,000 nm, less than 750 nm, less than 500 nm, less than 300 nm, less than 200 nm or less than 100 nm. In some embodiments, the extraction liquid comprises or consists of an ionic liquid. Ionic liquids are sometimes called liquid salts. Ionic liquids are ionic compositions comprising or consisting of an anion and a cation. The ionic liquid may have a melting point of less than 350°C, less than 300°C, less than 250°C, less 200°C or less than 150°C, more preferably less than 100°C, less than 50°C, or less than 25°C. The ionic liquid may have a melting point between -300°C and 350°C, between -250°C and 300°C, between -200°C and 250°C, between -150°C and 200°C or between -100°C and 150°C, or between -50°C and 100°C. In some embodiments, the ionic liquid is a protic ionic liquid. It may be appreciated that a protic ionic liquid may be produced by contacting a Brønsted acid (i.e. a proton donor) with a Brønsted base (i.e. a proton acceptor). Accordingly, a protic ionic liquid consists of an anion and a cation. The anion may be formed from the Brønsted acid when it donates or loses a proton (i.e. a deprotonated Brønsted acid). The cation may be formed from the Brønsted base when it accepts or gains a proton (i.e. a protonated Brønsted base). In some embodiments, the Brønsted base is water soluble. Alternatively, in some embodiments the Brønsted base is insoluble or sparingly soluble in water. For example, the Brønsted base may be trihexylamine, cyclohexylamine, tributylamine, aniline, N-methylaniline, diphenylamine and benzylamine. The cation may be a molecule comprising a positively charged atom. The positively charged atom may be a nitrogen (N), phosphorous (P) or sulphur (S). The cation may be an organic or inorganic molecule or atom. The cation may comprise a metal. , wherein R1to R14are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-24 cycloalkyl, an optionally substituted C6-12 aryl, -OR15, -SR15, -CN, -NR15R16, -SO3R15, -OSO3R15, -COR15, -COOR15, -NO2, -Cl, -Br, -F, or –I, or two of R1to R14, together with the atoms to which they are attached, form an optionally substituted 3 to 15 membered ring, wherein R15and R16are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-6 cycloalkyl or an optionally substituted C6-12 aryl. The optionally substituted 3 to 15 membered ring formed by two of R1to R14, together with the atoms to which they are attached, may be an optionally substituted C3-15 cycloalkyl, an optionally substituted 3 to 15 membered heterocycle, an optionally substituted 5 to 15 membered heteroaromatic or an optionally substituted C6-12 aryl. , wherein R1to R6are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-6 cycloalkyl, an optionally substituted C6-12 aryl, -OR15, -SR15, -CN, -NR15R16, -SO3R15, -OSO3R15, -COR15, -COOR15, -NO2, -Cl, -Br, -F or –I, or two of R1to R6, together with the atoms to which they are attached, form an optionally substituted 3 to 15 membered ring; and R15and R16are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-6 cycloalkyl or an optionally substituted C6-12 aryl. The cation may be: R1to R6may independently be H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C6-12 aryl or an optionally substituted C3-6 cycloalkyl. In some embodiments, R1to R6are independently H, an optionally substituted C1-12 alkyl, an optionally substituted C2-12 alkenyl, an optionally substituted C2-12 alkynyl, optionally substituted C6-12 aryl or an optionally substituted C3-6 cycloalkyl. In some embodiments, R1to R6are independently H, an optionally substituted C1-6 alkyl, an optionally substituted C2-6 alkenyl, an optionally substituted C2-6 alkynyl, an optionally substituted phenyl or an optionally substituted C3-6 cycloalkyl. In some embodiments, R1to R6are independently H, an optionally substituted C1-6 alkyl, an optionally substituted phenyl or an optionally substituted cyclohexyl. In some embodiments, R1to R6are independently H, an optionally substituted C1-6 alkyl or an optionally substituted cyclohexyl. In some embodiments, at least one of R1to R6is H. In some embodiments, the alkyl, alkenyl, alkynyl or cycloalkyl is substituted with a C6-12 aryl, an -OR23group or a -COOR23group. In some embodiments, the alkyl, alkenyl, alkynyl or cycloalkyl is substituted with a phenyl, an -OR23group or a -COOR23group. In some embodiments R23is H. In some embodiments, the optionally substituted C6-12 aryl is a phenyl. In some embodiments, the cation is: wherein, R1to R4are defined as above. R1may be methyl. R2may be methyl. R3may be methyl. R4may be -CH2COOH. Accordingly, the cation may be (carboxymethyl)trimethylammonium. R1may be ethyl. R2may be ethyl. R3may be ethyl. R4may be H. Accordingly, the cation may be triethylammonium. R1may be methyl. R2may be methyl. R3may be H. R4may be cyclohexyl. Accordingly, the cation may be dimethylcyclohexylammonium. R1may be -CH2CH2OH. R2may be -CH2CH2OH. R3may be -CH2CH2OH. R4may be H. Accordingly, the cation may be triethanolammonium. R1may be hexyl. R2may be hexyl. R3may be hexyl. R4may be H. Accordingly, the cation may be trihexylammonium. R1may be hydrogen. R2may be hydrogen. R3may be hydrogen. R4may be . Accordingly, the cation may be α-methylbenzylammonium (MBA). In some embodiments, the cation is trihexylammonium, cyclohexylammonium, tributylammonium (TBA), anilinium, N-methylanilinium, diphenylammonium, benzylammonium or α-methylbenzylammonium (MBA). In some embodiments, the cation comprises an acidic labile proton. The acidic proton may be bound to an atom with a formal positive charge. For example, dimethylcyclohexylammonium may comprise an acidic labile proton bound to the positively charged nitrogen atom. The cation may comprise a functional group comprising an acidic proton. For example, (carboxymethyl)trimethylammonium may comprise a carboxylic acid (carboxy) group comprising an acidic labile proton. The anion may be a halide or a molecule comprising a negatively charged atom or a delocalised negative charge. The molecule may be an organic or inorganic molecule. Accordingly, the anion may be F-, Cl-, Br-, I-, ClO4-, BrO4-, NO3-,NC-, NCS-, NCSe-, R22are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-6 cycloalkyl, an optionally substituted 5 to 10 membered heteroaromatic ring, an optionally substituted C6-12 aryl, -OR15, -SR15, -CN, -NR15R16, -SO3R15, -OSO3R15, - COR15, -COOR15, -NO2, -Cl, -Br, -F or –I, or two of R17to R22, together with the atoms to which they are attached, form an optionally substituted 3 to 15 membered ring; and R15and R16are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-6 cycloalkyl or an optionally substituted C6-12 aryl. The optionally substituted 3 to 15 membered ring formed by two of R17to R22, together with the atoms to which they are attached, may be an optionally substituted C3-15 cycloalkyl, an optionally substituted 3 to 15 membered heterocycle, an optionally substituted 5 to 15 membered heteroaromatic or an optionally substituted C6-12 aryl. In some embodiments, the anion is In some embodiments, the anion is F-, Cl-, Br- or I-. The anion may be Cl-. In some embodiments, the anion is: R17may be H, an optionally substituted C1-12 alkyl, an optionally substituted C2-12 alkenyl, an optionally substituted C2-12 alkynyl, an optionally substituted C3-6 cycloalkyl, an optionally substituted C6-12 aryl, -OR15, -SR15, -CN, -NR15R16, -SO3R15, - OSO3R15, -COR15, -COOR15or –NO2. R17may be -OR15, -SR15or an optionally substituted phenyl. In one embodiment, R17is –OR15. R15may be H, an optionally substituted C1-12 alkyl, an optionally substituted C2-12 alkenyl, an optionally substituted C2-12 alkynyl. In one embodiment, R15is H. In one embodiment, R17is an optionally substituted phenyl. The phenyl may be unsubstituted or substituted with a C1-6 alkyl, a C2-6 alkenyl or a C2-6 alkynyl. The phenyl may be unsubstituted or substituted with a C1-3 alkyl, a C2-3 alkenyl or a C2-3 alkynyl. In an embodiment, the anion is p- toluenesulfonate (PTSA). In some embodiments, the anion is Cl- or hydrogen sulphate (HSO4-). In some embodiments, the anion comprises an acidic labile proton. For example, hydrogen sulphate (HSO4-) may be understood to comprise an acidic labile proton. In some embodiments, the extraction liquid comprises or consists of a DES. A deep eutectic solvent (DES) may be understood to be a system formed from a eutectic mixture of Lewis or Brønsted acids and bases which can contain a variety of anionic and / or cationic species. A DES may comprise a hydrogen bond donor and a hydrogen bond acceptor. In some embodiments, the DES comprises more than one hydrogen bond donor. In some embodiments, the DES comprises two hydrogen bond donors (e.g. a first hydrogen bond donor and a second hydrogen bond donor). In some embodiments, the DES comprises two hydrogen bond acceptors (e.g. a first hydrogen bond acceptor and a second hydrogen bond acceptor). The hydrogen bond acceptor may comprise or consist of a zwitterionic species. The hydrogen bond acceptor may comprise or consist of a salt. Accordingly, the hydrogen bond acceptor may comprise or consist of an anion and a cation. The anion and cation may be defined as above. The hydrogen bond acceptor may be betaine or a complex thereof, choline chloride, glycerol, glucose, ethylene glycol, an organic acid, imidazole, betaine, acetamide, an ammonium salt, an alkyl ammonium salt, a dialkylammonium salt, a trialkylammonium salt, a 1-H-alkylimidazolium salt, a 1-methyl-3-alkylimidazolium salt, an imidazolium salt, a metal salt (e.g. zinc chloride or calcium chloride), a phosphonium salt (e.g. tetrabutylphosphonium chloride) or a compound of formula (I): wherein X1is O, S or NR28; R25is a C1-6 alkyl, a C2-6 alkenyl, a C2-6 alkynyl or NR29R30; R26to R30are each independently H, a C1-6alkyl, a C2-6alkenyl or a C2-6alkynyl; or a salt of any of the aforementioned compounds or a complex of any of the aforementioned. The hydrogen bond acceptor may be betaine or a complex thereof, choline chloride, glycerol, urea, glucose, ethylene glycol or an organic acid or a salt thereof. The organic acid may be citric acid, lactic acid, malic acid, acetic acid, tartaric, tricarboxylic acid, benzoic acid, ascorbic acid, itaconic acid, cinnamic acid, phenylacetic acid, hexanoic acid, coumaric acid, stearic acid, oleic acid, linoleic acid, decanoic acid, suberic acid or oxalic acid. A complex of betaine may be betaine hydrochloride. In some embodiments, the hydrogen bond acceptor is choline chloride. The hydrogen bond acceptor may be a salt comprising a cation and an anion. The cation may be: some embodiments, R1to R14are independently H, a C1-12 alkyl, a C2-12 alkenyl or a C2-12 alkynyl. In some embodiments, R1to R14are independently H, a C1-6 alkyl, a C2-6 alkenyl or a C2-6 alkynyl. In some embodiments, R1to R14are independently H, a C1-3 alkyl, a C2-3 alkenyl or a C2-3 alkynyl. The anion may be a halide. In some embodiments, the anion may be a fluoride, chloride, bromide or iodide. In some embodiments, the anion may be a chloride. In some embodiments, the or each hydrogen bond acceptor is ammonium chloride, an alkylammonium chloride, a dialkylammonium chloride, a trialkylammonium chloride, a 1-H-alkylimidazolium chloride, or a 1-methyl-3-alkylimidazolium chloride. In some embodiments, the hydrogen bond acceptor is ammonium chloride. The or each hydrogen bond donor may comprise a carboxylic acid, a hydroxyl and / or an amine group. The amine may be a primary amine, a secondary amine, a tertiary amine or a quaternary amine. The hydrogen bond donor may be betaine or a complex thereof, choline chloride, glucose, an organic acid, an alcohol, a diol, a polyol, an ether substituted with one or more OH groups, a polyether substituted with one or more OH groups or a compound of formula (I): wherein X1is O, S or NR28; R25is a C1-6 alkyl, a C2-6 alkenyl, a C2-6 alkynyl or NR29R30; R26to R30are each independently H, a C1-6 alkyl, a C2-6 alkenyl or a C2-6 alkynyl; or a salt or complex of any of the aforementioned compounds. The alcohol may be a C1-12 alcohol, a C1-6 alcohol or a C1-3 alcohol. The diol may be a C1-12 diol, a C1-6 diol or a C1-3 diol. The polyol may be a C1-12 polyol, a C1-6 polyol or a C1-3 polyol. The ether substituted with one or more OH groups may be a C2-12 ether, a C2-6 ether or a C3-5 ether. The polyether substituted with one or more OH groups may be a C3-12 polyether, a C4-8 polyether or a C5-7 polyether. The alcohol, diol, polyol, ether or polyether may be ethylene glycol, propylene glycol, glycerol, diethylene glycol, triethylene glycol, diethyleneglycol monomethyl ether or ethyleneglycol monomethyl ether. In some embodiments, hydrogen bond acceptor may be ethylene glycol. In some embodiments, the hydrogen bond donor or the first hydrogen bond donor is an alcohol, a diol, a polyol, an ether substituted with one or more OH groups or a polyether substituted with one or more OH groups. In some embodiments, the hydrogen bond donor or the first hydrogen bond donor is ethylene glycol, propylene glycol, glycerol, diethylene glycol, triethylene glycol, diethyleneglycol monomethyl ether or ethyleneglycol monomethyl ether. In some embodiments, the hydrogen bond donor or the first hydrogen bond donor is ethylene glycol. The compound of formula (I) or a salt or complex thereof may be urea, thiourea, acetamide, thioacetamide, guanidine, guanidine hydrochloride or tetramethylguanidine. In some embodiments, the hydrogen bond donor or the second hydrogen bond donor is a compound of formula (I) or a salt or complex thereof. In some embodiments, the hydrogen bond donor or the second hydrogen bond donor is urea, thiourea, acetamide, thioacetamide, guanidine, guanidine hydrochloride or tetramethylguanidine. In some embodiments, the hydrogen bond donor or the second hydrogen bond donor is urea. For example, the or each hydrogen bond donor may be acetamide, oxalic acid, malonic acid, malic acid, xylitol, urea, 1,1-dimethylurea, D-isosorbide, tartaric acid, tricarballylic acid, thiourea, trifluoroacetamide, benzoic acid, itaconic acid, citric acid, imidazole, 2-imidazolione, benzamide, 4-hydroxybenzoic acid, cinnamic acid, ethylene glycol, propylene glycol, propylene urea, resorcinol, phenylacetic acid, D-sorbitol, lactic acid, glycerol, succinic acid, hexanoic acid, coumaric acid, stearic acid, oleic acid, linoleic acid, decanoic acid, or suberic aid. In some embodiments the hydrogen bond donor comprises at least one hydroxyl group. The hydrogen bond donor may comprise more than one hydroxyl group. Accordingly, the hydrogen bond donor may be ethylene glycol. In some embodiments, the DES is a non-ionic DES. It may be understood that a non- ionic DES does not comprise, or consist of, an ionic species. In some embodiments, the DES is an ionic DES. It may be understood that an ionic DES comprises an ionic species. An ionic DES may comprise more than one ionic species. The molar ratio of hydrogen bond acceptor to the or each hydrogen bond donor in the DES may between 1:1 and 1:10, between 1:2 and 1:8, between 1:3 and 1:5 or between 2:7 and 2:9 hydrogen bond acceptor : the or each hydrogen bond donor. The molar ratio of hydrogen bond acceptor to the hydrogen bond donor or a first hydrogen bond donor may be about 1:4 hydrogen bond acceptor : the hydrogen bond donor or the first hydrogen bond donor. In some embodiments, the DES may comprise (i) an alcohol, a diol, a polyol, an ether substituted with one or more OH groups or a polyether substituted with one or more OH groups; (ii) a compound of formula (I) or a salt or complex thereof; and (iii) a salt comprising a cation and an anion. Each component may be as defined above. The molar ratio of the alcohol, diol, polyol, ether substituted with one or more OH groups or polyether substituted with one or more OH groups to the compound of formula (I) or a salt or complex thereof may be between 10:1 and 1:5, between 5:1 and 1:2, between 5:2 and 1:1.5, between 5:3 and 1:1 or about 5:4 the alcohol, diol, polyol, ether substituted with one or more OH groups or polyether substituted with one or more OH groups : compound of formula (I) or the salt or complex thereof. The molar ratio of the alcohol, diol, polyol, ether substituted with one or more OH groups or polyether substituted with one or more OH groups to the salt comprising a cation and an anion may be between 1:5 and 20:1, between 1:2 and 15:1, between 1:1 and 10:1, between 2:1 and 8:1, between 3:1 and 7:1, between 4:1 and 6:1 or about 5:1 the alcohol, diol, polyol, ether substituted with one or more OH groups or polyether substituted with one or more OH groups : the salt. The ionic liquid or DES may be selected on the basis that it selectively extracts the target metal or the first oxidised form thereof. For example, the inventors have found that the ionic liquids [TEA][HSO4], [TEA]Cl and [BET][HCl] show good selectivity for zinc over iron. Additionally, the inventors have found that non-metallic impurities do not dissolve in [TEA][HSO4], [TEA]Cl or [BET][HCl]. The inventors have also found that [BET][HCl] and EtGly:Ur:NH4Cl show good selectivity for zinc over manganese. In particular, the inventors have found that preferential extraction of zinc over iron and manganese can be achieved by using an extraction liquid which does not comprise an acid. Accordingly, in some embodiments the ionic liquid or DES is basic and / or does not comprise an acid. The inventors have found that preferential extraction of zinc can be achieved using a DES comprising ethylene glycol. In some embodiments, the DES comprises ethylene glycol, urea and ammonium chloride. In some embodiments, the DES comprises ethylene glycol, urea and ammonium chloride in a molar ration of about 5:4:1 ethylene glycol: urea: ammonium chloride. The extraction liquid may comprise a further solvent in addition to the ionic liquid and / or the DES. In some embodiments, prior to contacting the extraction liquid and the composition, the extraction liquid may consist substantially of the ionic liquid and / or DES and the further solvent. In some embodiments, the further solvent is water. Advantageously, the further solvent decreases the viscosity of the extraction liquid. The concentration of the ionic liquid and / or DES in the extraction liquid may be at least 0.1 wt%, at least 1 wt%, at least 2.5 wt%, at least 5 wt%, at least 7.5 wt%, at least 10 wt% or at least 12.5 wt%. In an embodiment, the concentration of the ionic liquid and / or DES in the extraction liquid may be at least 15 wt%, at least 20 wt%, at least 25 wt% or at least 30 wt%. In an embodiment, the concentration of the ionic liquid and / or DES in the extraction liquid may be at 35 wt%, at least 45 wt%, at least 50 wt% or at least 55 wt%. In an embodiment, the concentration of the ionic liquid and / or DES in the extraction liquid may be at 60 wt%, at least 65 wt% or at least 70 wt%. The concentration of the ionic liquid and / or DES in the extraction liquid may be less than 99 wt%, less than 95 wt%, less than 90 wt%, less than 85 wt%, less than 80 wt%, less than 75 wt%, less than 70 wt% or less than 65 wt%. In an embodiment, the concentration of the ionic liquid and / or DES in the extraction liquid may be less than 60 wt%, less than 55 wt%, less than 50 wt%, less than 45 wt% or less than 40 wt%. In an embodiment, the concentration of the ionic liquid and / or DES in the extraction liquid may be less than 35 wt%, less than 30 wt%, less than 25 wt%, less than 20 wt% or less than 15 wt%. The concentration of the ionic liquid and / or DES in the extraction liquid may be between 0.1 and 99 wt%, between 1 and 95 wt%, between 2.5 and 90 wt%, between 5 and 85 wt%, between 7.5 and 80 wt% or between 10 and 75 wt%. In an embodiment, the concentration of the ionic liquid and / or DES in the extraction liquid is between 12.5 and 70 wt%, between 15 and 65 wt%, between 17.5 and 60 wt%, between 20 and 55 wt%, between 22.5 and 50 wt%, between 25 and 45 wt%, between 27.5 and 40 wt%, or between 30 and 35 wt%. In an alternative embodiment, the concentration of the ionic liquid and / or DES in the extraction liquid is between 20 and 85 wt%, between 30 and 80 wt%, between 40 and 75 wt%, between 45 and 70 wt%, between 50 and 65 wt%, between 55 and 62.5 wt% or between 59 and 61 wt%. In an alternative embodiment, the concentration of the ionic liquid and / or DES in the extraction liquid is between 10 and 95 wt%, between 20 and 90 wt%, between 50 and 85 wt%, between 55 and 80 wt%, between 60 and 75 wt%, between 65 and 72.5 wt% or between 69 and 71 wt%. The concentration of the further solvent in the extraction liquid may be at least 0.1 wt%, at least 1 wt%, at least 5 wt%, at least 10 wt%, at least 15 wt%, at least 20 wt%, at least 25 wt% or at least 30 wt%. In an embodiment, the concentration of the further solvent in the extraction liquid may be at 35 wt%, at least 45 wt%, at least 50 wt% or at least 55 wt%. In an embodiment, the concentration of the further solvent in the extraction liquid may be at 60 wt%, at least 65 wt%, at least 70 wt%, at least 80 wt% or at least 85 wt%. The concentration of the further solvent in the extraction liquid may be less than 99 wt%, less than 95 wt%, less than 90 wt%, less than 85 wt%, less than 80 wt%, less than 75 wt%, less than 70 wt% or less than 65 wt%. In an embodiment, the concentration of the further solvent in the extraction liquid may be less than 60 wt%, less than 55 wt%, less than 50 wt%, less than 45 wt% or less than 40 wt%. In an embodiment, the concentration of the further solvent in the extraction liquid may be less than 35 wt%, less than 30 wt%, less than 25 wt%, less than 20 wt% or less than 15 wt%. The concentration of the further solvent in the extraction liquid may be between 0.1 and 99 wt%, between 1 and 95 wt%, between 2.5 and 90 wt%, between 5 and 85 wt%, between 7.5 and 80 wt% or between 10 and 75 wt%. In an embodiment, the concentration of the further solvent in the extraction liquid is between 30 and 87.5 wt%, between 35 and 85 wt%, between 40 and 82.5 wt%, between 45 and 80 wt%, between 50 and 77.5 wt%, between 55 and 75 wt%, between 60 and 72.5 wt%, or between 65 and 70 wt%. In an alternative embodiment, the concentration of the further solvent in the extraction liquid is between 15 and 80 wt%, between 20 and 70 wt%, between 25 and 60 wt%, between 30 and 55 wt%, between 35 and 50 wt%, between 37.5 and 45 wt% or between 39 and 41 wt%. In an alternative embodiment, the concentration of the further solvent in the extraction liquid is between 5 and 70 wt%, between 10 and 60 wt%, between 15 and 50 wt%, between 20 and 45 wt%, between 25 and 40 wt%, between 27.5 and 35 wt% or between 29 and 31 wt%. The inventors have found that the amount of the target metal or the first oxidised form thereof extracted from the composition varies with the concentration of the ionic liquid and / or DES in the extraction liquid. In particular, increasing the concentration of the ionic liquid and / or DES in the extraction liquid increases the amount of the target metal or the first oxidised form thereof extracted. The inventors also found that varying the concentration of the ionic liquid and / or DES in the extraction liquid can increase the proportion of the target metal or the first oxidised form thereof extracted relative to the proportion of the further metal or the first oxidised form thereof which is extracted. Accordingly, the method may comprise selecting a composition configured to optimise extraction of the target metal or the first oxidised form thereof and / or minimise extraction of the further metal or the first oxidised form thereof. Contacting the extraction liquid and the composition may cause at least 5 wt%, at least 10 wt%, at least 20 wt%, at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, at least 80 wt% or at least 90 wt% of the target metal or the first oxidised form thereof present in the composition to be extracted therefrom. It may be appreciated that all of the target metal or the first oxidised form thereof extracted from the composition may be present as the second oxidised form of the target metal dissolved in the extraction liquid. Contacting the extraction liquid and the composition may cause between 5 and 95 wt%, between 10 and 90 wt%, between 20 and 88 wt%, between 30 and 85 wt%, between 40 and 80 wt% or between 50 and 70 wt% of the target metal or the first oxidised form thereof present in the composition to be extracted therefrom. Contacting the extraction liquid and the composition may cause between 10 and 99 wt%, between 50 and 97 wt%, between 60 and 95 wt%, between 65 and 92 wt% or between 70 and 90 wt% of the target metal or the first oxidised form thereof present in the composition to be extracted therefrom. Contacting the composition and the extraction liquid may cause one or more chemical reactions to occur. A gas may be generated as a product of the one or more chemical reactions. The method may comprise collecting the gas. It will be appreciated that collecting is another word for capturing. The gas may be purified. The gas may be stored and / or used in further applications. The gas may be hydrogen gas (H2), nitrogen gas (N2), carbon dioxide (CO2), carbon monoxide (CO) or a combination thereof. In some embodiments, the gas is or comprises hydrogen gas. It will be appreciated that multiple applications use hydrogen gas, such as energy production. Accordingly, the production of hydrogen gas as a product could be beneficial. It will be appreciated that hydrogen gas may be formed from acidic labile protons which are released from the ionic liquid or DES. Accordingly, contacting the composition and the extraction liquid may cause a component of the ionic liquid or DES to lose a proton, which is reduced to produce hydrogen gas. The proton which is lost from the ionic liquid or DES may have been a proton from the cation or a proton from the anion. In an embodiment, the ionic liquid or DES and the target metal or the first oxidised form thereof react. In an embodiment, the ionic liquid or DES and the further metal or the first oxidised form thereof react. In an embodiment, the ionic liquid or DES preferentially react with the target metal or the first oxidised form thereof as opposed to the further metal or the first oxidised form thereof. In embodiments where the composition comprises the target metal, contacting the composition and the extraction liquid may cause the target metal to be oxidised to provide the second oxidised form of the target metal. The second oxidised form of the target metal once formed may dissolve in the extraction liquid, to provide the first solution. In embodiments where the composition comprises a first oxidised form of the target metal, contacting the composition and the extraction liquid may cause the first oxidised form of the target metal to be oxidised or reduced to provide the second oxidised form of the target metal. The second oxidised form of the target metal once formed may dissolve in the extraction liquid, to provide the first solution. Alternatively, in embodiments where the composition comprises a first oxidised form of the target metal, contacting the composition and the extraction liquid may not affect the oxidation state of the first oxidised form of the target metal. The first oxidised form of the target metal may dissolve in the extraction liquid, to provide the first solution. Accordingly, in this embodiment, the first oxidised form of the target metal may be the same as the second oxidised form of the target metal. In embodiments where the composition comprises the further metal, contacting the composition and the extraction liquid may cause the further metal to be oxidised to provide the second oxidised form of the further metal. The second oxidised form of the further metal once formed may dissolve in the extraction liquid, to provide the first solution. In embodiments where the composition comprises a first oxidised form of the further metal, contacting the composition and the extraction liquid may cause the first oxidised form of the further metal to be oxidised or reduced to provide the second oxidised form of the further metal. The second oxidised form of the further metal once formed may dissolve in the extraction liquid, to provide the first solution. Alternatively, in embodiments where the composition comprises a first oxidised form of the further metal, contacting the composition and the extraction liquid may not affect the oxidation state of the first oxidised form of the further metal. The first oxidised form of the further metal may dissolve in the extraction liquid, to provide the first solution. Accordingly, in this embodiment, the first oxidised form of the further metal may be the same as the second oxidised form of the target metal. The inventors have found that the amount of the target metal or the first oxidised form thereof extracted from the composition varies with the length of time for which the composition and the extraction liquid are contacted. The composition and the extraction liquid may be contacted for at least 1 minute, at least 15 minutes, at least 30 minutes, at least 1 hour or at least 2 hours. The composition and the extraction liquid may be contacted for less than 7 day, less than 48 hours, less than 24 hours, less than 12 hours, less than 6 hours, less than 4 hours or less than 2 hours. The composition comprising the metal source and the ionic liquid may be contacted for between 5 minutes and 7 days, between 10 minutes and 48 hours, between 15 minutes and 24 hours, between 30 minutes and 12 hours, between 45 minutes and 6 hours, between 1 and 3 hours or between 90 minutes and 150 minutes. The inventors have found that amount of the target metal or the first oxidised form thereof extracted from the composition varies with the temperature at which the composition and the extraction liquid are contacted. Additionally, the inventors have found that the temperature also impacts the overall gas yield and the initial rate of gas evolution. The composition and the extraction liquid may be contacted at a temperature of at least -20°C, at least -10°C, at least 0°C, at least 10°C, at least 20°C, at least 30°C, at least 40°C, at least 50°C, at least 60°C, at least 70°C or at least 80°C. The composition and the extraction liquid may be contacted at a temperature of between -20°C and 200°C, between -10°C and 155°C, between 0°C and 100°C, between 15°C and 90°C or between 20°C and 85°C. In an embodiment, the composition and the extraction liquid may be contacted at a temperature of between 20°C and 200°C, between 25°C and 150°C, between 30°C and 100°C, between 45°C and 55°C or between 48°C and 52°C. In an alternative embodiment, the composition and the extraction liquid may be contacted at a temperature of between 30°C and 200°C, between 50°C and 100°C, between 70°C and 90°C, between 75°C and 85°C or between 78°C and 82°C. The method may comprise contacting the composition and the extraction liquid in a ratio of between 0.1 grams and 1 kg of the composition per 1 litre of the extraction liquid, between 1 and 750 grams of the composition per 1 litre of the extraction liquid, between 10 and 500 grams of the composition per 1 litre of the extraction liquid, between 30 and 200 grams of the composition per 1 litre of the extraction liquid or between 50 and 150 grams of the composition per 1 litre of the extraction liquid. Prior to conducting the reaction to cause the second oxidised form of the target metal dissolved in the extraction liquid to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal, the method may comprise separating the first solution from residual solid material. It may be appreciated that the residual solid material may be any part of the composition which has not been extracted into the extraction liquid. Accordingly, the method may comprise: - contacting the composition and an extraction liquid to extract the target metal or the first oxidised form thereof from the composition and to thereby provide a first solution, wherein the extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the first solution comprises a second oxidised form of the target metal dissolved therein; - separating the first solution from solid material; - conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal. The first solution may be separated from the residual solid material by filtration and / or centrifugation. Depending upon the composition particle size of the residual solid material, the method could comprise using conventional filtration, ultra filtration or nano filtration. In embodiments where the first solution is separated from the residual solid material by centrifugation, the first solution may be cooled prior to centrifugation. The first solution may be cooled to below 50°C, below 40°C, below 30°C, below 20°C, below 15°C, below 10°C, below 8°C or below 6°C. The first solution may be cooled to between 0°C and 15°C, between 1°C and 10°C, between 2°C and 8°C, between 3°C and 7°C or between 4°C and 6°C. The first solution may comprise the ionic liquid and / or the DES. Alternatively, or additionally, in embodiments where the ionic liquid is or comprises a protic ionic liquid, the first solution may comprise a deprotonated form of the ionic liquid. It may be understood that the deprotonated form of the ionic liquid may not be an ionic liquid. The first solution may comprise the anion of the ionic liquid, or a deprotonated form thereof. The first solution may comprise the cation of the ionic liquid or a deprotonated form thereof. In embodiments where the extraction liquid comprises a further solvent, the first solution may also comprise the further solvent. As noted above, the composition may comprise one or more further metals and / or a first oxidised form thereof. As further noted above, prior to conducting the reaction to cause the second oxidised form of the target metal dissolved in the extraction liquid to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal, the method may comprise separating the first solution from residual solid material. The residual solid material may be understood to be residual solid material from the composition which has not been extracted. In some embodiments, the one or more further metals and / or the first oxidised form may not be extracted from the composition when the composition and extracting liquid are contacted. In such embodiments, the residual solid material may comprise a further target metal or a first oxidised form thereof. Accordingly, the method may comprise further processing the residual solid material to recover the further target metal, a compound comprising the further target metal or a solid salt of the further target metal. Accordingly, in some embodiments, the method may comprise: - contacting the composition and a first extraction liquid to selectively extract a first target metal or the first oxidised form thereof from the composition and to thereby produce a first solution and a residual solid material, wherein the first extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES), the first solution comprises a second oxidised form of the first target metal dissolved therein and the residual solid material comprises the second target metal or a first oxidised form thereof; - separating the first solution from the residual solid material; - contacting the residual solid material and a second extraction liquid to extract the further target metal or the first oxidised form thereof from the residual solid material and to thereby produce a third solution, wherein the second extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the third solution comprises a second oxidised form of the target metal dissolved therein; - conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal; and - conducting a reaction to cause the second oxidised form of the further target metal to be converted to the further target metal, the compound comprising the further target metal or a solid salt of the further target metal. Embodiments of conducting the above method of further processing the residual solid material to recover the further target metal, the compound comprising the further target metal or the solid salt of the further target metal may be analogous to embodiments described above of recovering the target metal, a compound comprising the target metal or a salt thereof, from the composition comprising the target metal or a first oxidised form thereof. Accordingly, definitions of the target metal, the compound comprising the target metal or the solid salt of the target metal provided above may be applied to both the first target metal, the compound comprising the first target metal or the solid salt of the first target metal and the further target metal, the compound comprising the further target metal or the solid salt of the further target metal. The definition of the extraction liquid provided above may be applied to both the first extraction liquid and the second extraction liquid. The definition of the first solution provided above may be applied to the third solution. It will be appreciated that in embodiments where the residual solid material comprises a further target metal or a first oxidised form thereof, the composition also comprises the further target metal or a first oxidised form thereof. Advantageously, the inventors have found that by tuning the first and second extraction liquids, they can preferentially extract the first, and then the further, target metal from the composition. Advantageously, this provides a first and further target metal with high purity. In some embodiments, the first target metal is different to the further target metal. In some embodiments, the first target metal is zinc. In some embodiments, the further target metal is manganese or iron. In some embodiments, the further target metal is manganese. In embodiments where the composition comprises a first oxidised form of the further metal, contacting the composition and the second extraction liquid may cause the first oxidised form of the further metal to be oxidised or reduced to provide the second oxidised form of the further metal. The second oxidised form of the further metal once formed may dissolve in the extraction liquid, to provide the third solution. In embodiments where the composition comprises the further metal, contacting the composition and the second extraction liquid may cause the further metal to be oxidised to provide the second oxidised form of the further metal. The second oxidised form of the further metal once formed may dissolve in the extraction liquid, to provide the third solution. As explained above, the inventors have found that preferential extraction of a target metal can be achieved by using an extraction liquid which is basic and / or does not comprise an acid. Accordingly, in some embodiments, the first extraction liquid is basic and / or does not comprise an acid. In some embodiments, the second extraction liquid does comprise an acid. In some embodiments, the first extraction liquid comprises or consists of a deep eutectic solvent. In some embodiments, the first extraction liquid comprises ethylene glycol, urea and ammonium chloride. In some embodiments, the second extraction liquid comprises a chelating amine. In some embodiments, the second extraction liquid comprises triethanolammonium. In some embodiments, the second extraction liquid comprises triethanolammonium hydrogen sulfate [TEA][HSO4]. Prior to conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the method may comprise diluting the first solution. Diluting the first solution may comprise combining the first solution and a further solvent. The further solvent may be as defined above. The method may comprise combining the first solution and the further solvent in a volumetric ratio between 2:1 and 1:50, between 1:1 and 1:20, between 1:2 and 1:15, between 1:3 and 1:12 or between 1:4 and 1:10. It may be appreciated that conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal may provide the target metal, the compound comprising the target metal or the solid salt of the target metal and a second solution. The second solution may comprise the ionic liquid and / or the DES. Alternatively, or additionally, in embodiments where the ionic liquid is or comprises a protic ionic liquid, the second solution may comprise a deprotonated form of the ionic liquid. It may be understood that the deprotonated form of the ionic liquid may not be an ionic liquid. The second solution may comprise the anion of the ionic liquid, or a deprotonated form thereof. The second solution may comprise the cation of the ionic liquid or a deprotonated form thereof. In embodiments where the extraction liquid comprises a further solvent, the second solution may also comprise the further solvent. Conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal may comprise conducting a precipitation reaction. Conducting a precipitation reaction may comprise contacting the first solution and a precipitating agent. It may be appreciated that the precipitating agent may react with the second oxidised form of the target metal to provide the target metal, the compound comprising the target metal or a solid salt of the target metal. In some embodiments, the precipitating agent reacts with the second oxidised form of the target metal to provide the solid salt of the target metal. Accordingly, the method may comprise: - contacting the composition and an extraction liquid to extract the target metal or the first oxidised form thereof from the composition and to thereby provide a first solution, wherein the extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the first solution comprises a second oxidised form of the target metal dissolved in the ionic liquid and / or a deep eutectic solvent (DES); and - contacting the first solution and a precipitating agent to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal. In some embodiments, the method comprises: - contacting the composition and an extraction liquid to extract the target metal or the first oxidised form thereof from the composition and to thereby provide a first solution, wherein the extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the first solution comprises a second oxidised form of the target metal dissolved in the ionic liquid and / or a deep eutectic solvent (DES); and - contacting the first solution and a precipitating agent to cause the second oxidised form of the target metal to be converted to the solid salt of the target metal. The precipitating agent may comprise or be a metal, a base, a salt and / or a gas. In embodiments where the precipitating agent is a metal, the metal may be a reducing agent. It may be appreciated that a metal precipitating agent may reduce the second oxidised form of the target metal to the target metal. The base may be a hydroxide, an oxide, a carbonate or a nitrate. In some embodiments, the base comprises a metal cation. The salt may be a metal salt. Accordingly, the metal salt may comprise a metal cation. The salt may be an organic salt or an inorganic salt. The salt may be a halide, a carbonate, a nitrate, a phosphate, a sulphide, a sulphate or a sulphite. The halide may be chloride, fluoride, iodide, or bromide. The gas may be carbon dioxide. The metal cation may be a group 1 metal cation or a group 2 metal cation. In some embodiments, the metal is calcium or sodium. The method may comprise providing the precipitating agent at a concentration between 0.1 and 10 M, between 0.5 and 5 M, between 1 and 3 M, between 1.5 and 2.5 M, or between 1.9 and 2.1 M. The method may comprise mixing the first solution and the precipitating agent. The method may comprise sonicating the first solution and the precipitating agent to thereby mix them. The method may comprise sonicating the first solution and the precipitating agent in an ultrasonic bath. The method may comprise mixing the first solution and the precipitating agent (e.g. sonicating them) for at least 30 seconds, at least 1 minute, at least 2 minutes, at least 5 minutes, at least 10 minutes or at least 15 minutes. It may be appreciated that contacting the first solution and a precipitating agent, may alter the pH of the first solution. In some embodiments, the precipitating agent is alkaline and increases the pH of the first solution. Contacting the first solution and the precipitating agent may comprise contacting the first solution and the precipitating agent at a ratio to cause the first solution to reach a target pH. The target pH may be between 6.0 and 14.0, between 7.0 and 13.5, between 8.0 and 13, between 9.0 and 12.5, between 10.0 and 12.0, or between 10.5 and 11.5. It may be appreciated that the target pH may be measured at a temperature of 20°C. Advantageously, the claimed method may be used to provide the target metal, the compound comprising the target metal or a solid salt of the target metal with different morphological properties, and therefore suitable for different applications. In particular, the precipitation conditions, such as the concentration of the precipitating agent, the concentration of the target metal in the first solution and the target pH, may be tuned in order to provide the target metal, the compound comprising the target metal or the solid salt of the target metal with particular morphological properties. For example, the precipitation conditions may be tuned to provide particles of the target metal, the compound comprising the target metal or the solid salt of the target metal with a particular hydrodynamic diameter. Conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal may comprise conducting an electrochemical reaction on the first solution. Conducting an electrochemical reaction on the first solution may comprise: - providing a first electrode and a spaced apart second electrode; - at least partially disposing the first and second electrodes in an electrolyte, wherein the electrolyte comprises or consists of the first solution; and - applying a voltage across the first and second electrodes, to thereby cause the target metal, the compound comprising the target metal or a salt of the target metal to be deposited on the first electrode. Accordingly, the method may comprise: - contacting the composition and an extraction liquid to extract the target metal or the first oxidised form thereof from the composition and to thereby provide a first solution, wherein the extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the first solution comprises a second oxidised form of the target metal dissolved in the ionic liquid and / or a deep eutectic solvent (DES); - providing a first electrode and a spaced apart second electrode; - at least partially disposing the first and second electrodes in an electrolyte, wherein the electrolyte comprises or consists of the first solution; and - applying a voltage across the first and second electrodes, to thereby cause the target metal, the compound comprising the target metal or a solid salt of the target metal to be deposited on the first electrode. In some embodiments, the electrolyte consists of the first solution. It may be appreciated that when the voltage is applied across the first and second electrodes an electrolysis reaction occurs. A reduction reaction may occur at the first electrode and an oxidation reaction may occur at the second electrode. In particular, the target metal, the compound comprising the target metal or a solid salt of the target metal may be generated at the first electrode. Accordingly, the second oxidised form of the target metal may be reduced in the electrolysis reaction to provide the target metal, the compound comprising the target metal or a solid salt of the target metal. The target metal, the compound comprising the target metal or the solid salt of the target metal which is produced at the first electrode may be disposed on the first electrode. In particular, the target metal, the compound comprising the target metal or the solid salt of the target metal may form a film disposed on the first electrode. The voltage may be applied for at least 1 minute, at least 5 minutes, at least 10 minutes, at least 30 minutes, at least 60 minutes or at least 90 minutes. The voltage may be applied for between 1 minute and 12 hours, between 10 minutes and 6 hours, between 30 minutes and 5 hours or between 45 minutes and 4 hours. In some embodiments, the voltage may be applied for between 60 minutes and 3 hours, between 90 minutes and 150 minutes. In some embodiments, the voltage may be applied for between 110 minutes and 130 minutes. The inventors have found that controlling the voltage at the first electrode provides improved control over target metal deposition. The voltage applied at the first electrode may be between 0.1 and 2.5 V, between 0.3 and 2 V, between 0.5 and 1.5 V, between 0.8 V and 1.2 V or between 0.9 and 1.1 V. It will be appreciated that voltage applied at the first electrode may be measured relative to a reference electrode. In addition to providing the first electrode and the spaced apart second electrode, the method may comprise providing a spaced apart reference electrode. The reference electrode may be understood to be a different electrode to both the first and second electrodes. The method may comprise at least partially disposing the reference electrode in the electrolyte. The reference electrode may comprise any material capable of providing a stable reference. For instance, the reference electrode may comprise a silver / silver chloride (Ag / AgCl) electrode. The first electrode may have a substantially smooth surface or have a substantially textured or porous surface. Accordingly, the first electrode comprise a sheet, a rod, a mesh or a felt. The first electrode may be a polymeric electrode, a ceramic-based electrode, a carbon-based electrode or a metal-based electrode. The metal-based electrode may comprise or consist of a pure metal, a metal-containing compound, or an alloy. The carbon-based first electrode may comprise graphite. Advantageously, graphite is in the electrolyte solution, and so does not cause contamination thereof. The second electrode may have a substantially smooth surface or have a substantially textured or porous surface. Accordingly, the second electrode comprise a sheet, a rod, a mesh or a felt. The second electrode may comprise a carbon-based electrode or a metal-based electrode. The metal-based second electrode may comprise platinum. After conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal, the method may further comprise separating the target metal, the compound comprising the target metal or the solid salt of the target metal and the second solution. The target metal, the compound comprising the target metal or the solid salt of the target metal may be separated from the second solution by filtration and / or centrifugation. Depending upon the target metal particle size, the method could comprise using membrane filtration, conventional filtration, ultra filtration or nano filtration. The second solution may further comprise an oxidised form of a second target metal dissolved therein. It may be appreciated that the oxidised form of the second target metal may have been present in the first solution. The method may further comprise conducting a reaction on the second solution to cause the oxidised form of the second target metal to be converted to the second target metal, a compound comprising the second target metal or a solid salt of the second target metal. Conducting a reaction on the second solution may comprise conducting a precipitation reaction or an electrochemical reaction on the second solution. Accordingly, the method may comprise: - conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal and to provide the target metal, the compound comprising the target metal or the solid salt of the target metal, and a second solution; and - conducting a reaction on the second solution to cause the oxidised form of the second target metal to be converted to the second target metal, the compound comprising the second target metal or the solid salt of the second target metal. In some embodiments, conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal comprises conducting a precipitation reaction and conducting a reaction on the second solution comprises conducting an electrochemical reaction. Definitions of the first target metal, the compound comprising the first target metal or the solid salt of the first target metal provided above may be applied to the second target metal, the compound comprising the second target metal or the solid salt of the second target metal. In one embodiment, the second target metal is different to the first target metal. The precipitation reaction or electrochemical reaction may be as defined above. Similarly, it may be appreciated that conducting a reaction to cause the oxidised form of the second target metal to be converted to the second target metal, the compound comprising the second target metal or a solid salt of the second target metal may provide a further solution. The further solution may comprise the ionic liquid and / or the DES. Alternatively, or additionally, in embodiments where the ionic liquid is or comprises a protic ionic liquid, the further solution may comprise a deprotonated form of the ionic liquid. It may be understood that the deprotonated form of the ionic liquid may not be an ionic liquid. The further solution may comprise the anion of the ionic liquid, or a deprotonated form thereof. The further solution may comprise the cation of the ionic liquid or a deprotonated form thereof. The further solution may also comprise the further solvent. The further solution may further comprise an oxidised form of a further target metal dissolved therein. It may be appreciated that the oxidised form of the further target metal may have been present in the first solution and / or the second solution. Accordingly, the method may further comprise conducting a reaction on the further solution to cause the oxidised form of the further target metal to be converted to the further target metal, a compound comprising the further target metal or a solid salt of the further target metal. Conducting a reaction on the further solution may comprise conducting a precipitation reaction or an electrochemical reaction on the further solution. This step may be repeated. Definitions of the first target metal, the compound comprising the first target metal or the solid salt of the first target metal provided above may be applied to the further target metal, the compound comprising the further target metal or the solid salt of the further target metal. In one embodiment, the further target metal is different to the first target metal. In one embodiment, the further target metal is different to the second target metal. Each time the step is repeated, the further target metal which was in an oxidised form and was converted to the further target metal, a compound comprising the further target metal or a solid salt of the further target metal may be different to the target metal(s) which were previously converted. The precipitation reaction or electrochemical reaction may be as defined above. Conducting a reaction multiple times may enable multiple different target metals (and / or compounds comprising the target metals and / or salts of the target metals) to be obtained with high purity. Similarly, the above processes may be used to provide a second solution and / or further solution wherein the second solution and / or further solution substantially consists of the IL and / or DES. Accordingly, the above processes may be used to regenerate the IL and / or the DES. The method may comprise further purification steps to regenerate the IL and / or DES. It may be understood that in embodiments where the ionic liquid is a protic ionic liquid, i.e. comprises a cation derived from a Brønsted base and an anion derived from a Brønsted acid, the above method steps, including contacting the composition and the extraction liquid, conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal, conducting a reaction on the second solution and / or conducting a reaction on the further solution may result in the de- protonation of the IL cation. It may be understood that the IL cation derived from the Brønsted base is a protonated Brønsted base, and de-protonation thereof will provide the Brønsted base. Accordingly, the second solution and / or further solution may comprise the Brønsted base. Re-generating the IL may comprise separating the Brønsted base from the second solution and / or further solution. The Brønsted base may be separated from the second solution and / or further solution using liquid-liquid extraction, distillation or adsorption. Alternatively, in embodiments where the Brønsted base is insoluble or sparingly soluble in water and the resulting second solution and / or further solution comprises an aqueous solution, the Brønsted base may be immiscible with the second solution and / or further solution. The Brønsted base may comprise or consist of a solid or liquid. Advantageously, this allows for simplified separation of the Brønsted base from the second and / or further solution. Re-generating the IL may comprise separating the Brønsted base from the second and / or further solution. For example, the Brønsted base may be separated by decantation or filtration. Regenerating the IL may comprise contacting the Brønsted base with an acid (e.g. a Brønsted acid). It may be appreciated that when the Brønsted base is contacted with an acid, the Brønsted base may be protonated to provide a cation and the acid may be deprotonated to form an anion. The Brønsted base and the acid may be contacted after the Brønsted base has been separated from the second solution and / or further solution. In alternative embodiments, the method may comprise contacting the second solution and / or further solution comprising the Brønsted base with the acid. Advantageously, the re-generated IL and / or DES may be re-used. The term “alkyl” as used herein, unless otherwise specified, refers to an optionally substituted, saturated straight or branched hydrocarbon. The or each optionally substituted alkyl may be an optionally substituted C1-12alkyl or an optionally substituted C1-6 alkyl. The term “alkenyl”, refers to an optionally substituted, olefinically unsaturated hydrocarbon group which can be unbranched or branched. The or each optionally substituted alkenyl may be an optionally substituted C2-12 alkenyl or an optionally substituted C2-6 alkenyl. The term “alkynyl” refers to an optionally substituted, acetylenically unsaturated hydrocarbon group which can be unbranched or branched. The or each optionally substituted alkynyl may be an optionally substituted C2-12 alkynyl or an optionally substituted C2-6 alkynyl. The or each alkyl, alkenyl and / or alkynyl can be unsubstituted or substituted with one or more of an optionally substituted C3-6 cycloalkyl, an optionally substituted phenyl, oxo, -OR23, -SR23, -CN, -NR23R24, -SO3R23, -OSO3R23, -COR23, -COOR23, -NO2, -Cl, -Br, -F or –I, wherein R23and R24are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-6 cycloalkyl or an optionally substituted phenyl. “Aryl” refers to an optionally substituted, aromatic 6 to 12 membered hydrocarbon group. An optionally substituted aryl may be an optionally substituted phenyl. The aryl can be unsubstituted or substituted with one or more of an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-6 cycloalkyl, an optionally substituted C6-12 aryl, - OR23, -SR23, -CN, -NR23R24, -SO3R23, -OSO3R23, -COR23, -COOR23, -NO2, -Cl, -Br, -F or –I, wherein R23and R24are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-24 cycloalkyl or an optionally substituted C6-12 aryl. “Cycloalkyl” refers to an optionally substituted, non-aromatic, saturated, partially saturated, monocyclic, bicyclic or polycyclic hydrocarbon membered ring system. “Heteroaryl” or “heteroaromatic ring” refers to an optionally substituted, monocyclic or bicyclic aromatic ring system in which at least one ring atom is a heteroatom. The or each heteroatom may be independently selected from the group consisting of oxygen, sulfur and nitrogen. “Heterocycle” or “heterocyclic ring” refers to an optionally substituted, monocyclic, bicyclic or bridged molecules in which at least one ring atom is a heteroatom. The or each heteroatom may be independently selected from the group consisting of oxygen, sulfur and nitrogen. The or each cycloalkyl, heterocycle / heterocylic ring and / or heteroaryl / heteroaromatic ring can be unsubstituted or substituted with one or more of an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-6 cycloalkyl, an optionally substituted C6-12 aryl, oxo, -OR23, -SR23, -CN, -NR23R24, -SO3R23, -OSO3R23, -COR23, -COOR23, -NO2, -Cl, -Br, -F or –I, wherein R23and R24are independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C3-24 cycloalkyl or an optionally substituted C6-12 aryl. The inventors believe that their method of selective extraction is novel and inventive per se. Accordingly, in a second aspect, there is provided a method of selectively extracting first and second target metals from a polymetallic composition comprising the first target metal or a first oxidised form thereof and the second target metal or a first oxidised form thereof, the method comprising: - contacting the composition and a first extraction liquid to selectively extract the first target metal or the first oxidised form thereof from the composition and to thereby produce a first solution and a residual solid material, wherein the first extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the first solution comprises a second oxidised form of the first target metal dissolved therein and the residual solid material comprises the second target metal or a first oxidised form thereof; - separating the first solution from the residual solid material; and - contacting the residual solid material and a second extraction liquid to extract the second target metal or the first oxidised form thereof from the residual solid material and to thereby produce a third solution, wherein the second extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the third solution comprises a second oxidised form of the second target metal dissolved therein. Contacting the composition and the first extraction liquid and contacting the residual solid material and the second extraction liquid may be as defined in relation to the first aspect. Similarly, the polymetallic composition, the first extraction liquid and the second extraction liquid may be as defined in relation to the first aspect. In some embodiments, the polymetallic composition is a black mass material. In some embodiments, the first extraction liquid is less acidic than the second extraction liquid. In some embodiments, the first extraction liquid has a higher pH than the second extraction liquid. In some embodiments, the first extraction liquid is basic or does not comprise an acid. All features described herein (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which: Figure 1 shows a scanning electron microscope (SEM) micrograph of electrodeposited material recovered from the liquid fraction of Solid A after treatment with [Et3NH][HSO4] solution; Figure 2 shows an energy-dispersive X-ray (EDX) spectrum of the electrodeposited material recovered from the liquid fraction of Solid A after treatment with [Et3NH][HSO4] solution; Figure 3 shows SEM micrographs of electrodeposited materials recovered from the liquid fractions of a) Solid A and b) Solid B after treatment with [BET][HCl] solution; Figure 4 shows EDX spectra of the electrodeposited materials recovered from the liquid fractions of a) Solid A and b) Solid B after treatment with [BET][HCl] solution; Figure 5 shows an SEM micrograph of electrodeposited material recovered from the liquid fraction of Solid A after treatment with [DCA][HSO4] solution; Figure 6 shows the EDX spectrum of the electrodeposited material recovered from the liquid fraction of Solid A after treatment with [DCA][HSO4] solution; Figure 7 is a graph showing volume of hydrogen produced versus time when was zinc treated with [Et3NH][HSO4] at 80°C; Figure 8 provides graphing showing volume of displaced water recorded versus time at 80°C for a) BOF-US powders and b) LC powders, treated with [TEA]Cl solution, [TEA][HSO4] solution, and [BET][HCl] solution, wherein the volume of water recorded correlates to the volume of gas produced; Figure 9 provides graphing showing volume of displaced water recorded versus time at 80°C for a) BOF-US powders and b) LC powders, treated with 14% [TEA][HSO4] solution, 30% [TEA][HSO4] solution, and 60% [TEA][HSO4] solution, wherein the volume of water recorded correlates to the volume of gas produced; Figure 10 provides graphing showing volume of displaced water recorded versus time at 80°C for a) BOF-US powders and b) LC powders treated with 14% [BET][HCl] solution, and 35% [BET][HCl] solution, wherein the volume of displaced water recorded correlates to the volume of gas produced; Figure 11 provides graphing showing the volume of displaced water recorded versus time at 80°C, 50°C and room temperature for BOF-US powders treated with a) [TEA]Cl solution and b) [TEA][HSO4] solution, wherein the volume of displaced water recorded correlates to the volume of gas produced; Figure 12 is a graph showing volume of water recorded versus time at 80°C for LC original and ground powders treated with 14% [TEA]Cl solution, wherein the volume of water recorded correlates to the volume of gas produced; Figure 13 provides graphing showing a) iron extraction and b) zinc extraction versus time for LC treated with H2SO4, [BET][HCl], [TEA][HSO4] and [TEA]Cl as leaching mediums; Figure 14 provides graphing showing a) iron extraction and b) zinc extraction versus time for BOF-US treated with H2SO4, [BET][HCl], [TEA][HSO4] and [TEA]Cl as leaching mediums; Figure 15 provides graphing showing a) iron extraction and b) zinc extraction versus temperature for BOF-US treated with [TEA][HSO4] and [TEA]Cl as leaching mediums; Figure 16 provides graphing showing a) iron extraction and b) zinc extraction versus [TEA][HSO4] concentration, for BOF-US treated with [TEA][HSO4] as the leaching medium; Figure 17 provides graphing showing a) iron extraction and b) zinc extraction versus [TEA][HSO4] concentration, for LC treated with [TEA][HSO4] as the leaching medium; Figure 18 provides graphing showing mass spectra (scan mode) for four hydrogen producing waste samples treated with 14% [BET][HCl] at 80°C; a) BM, b) BOS, c) SD-3 and d) FD-2. Baseline levels were obtained by nitrogen sweep gas flow without the solid sample; Figure 19 is a mass spectrum (multiple ion detection mode) for waste sample SD-3 treated with 14% [BET][HCl] at 80°C; Figure 20 shows XRF results for zinc samples treated with [BET][HCl] and precipitated (using conditions summarised in Table 18), both before and after calcination; Figure 21 shows XRD data for zinc samples treated with [BET][HCl] and precipitated (using conditions summarised in Table 18) before calcination (samples 1-5 precipitated with NaOH, and sample 6 precipitated with Na2CO3), signals lower that 30° correspond to impurities in the system; Figure 22 shows XRD data for zinc samples treated with [BET][HCl] and precipitated (using conditions summarised in Table 18) after calcination (samples 1-5 precipitated with NaOH, and samples 6-10 precipitated with Na2CO3); Figure 23 is SEM images of zinc samples treated with [BET][HCl] and precipitated (using conditions summarised in Table 18) before and after calcination (samples 1-5 precipitated with NaOH, and samples 6-10 precipitated with Na2CO3); Figure 24 shows the Fe, Pr and Nd compositions (determined by XRF) of leachates after NdPrFeB magnet material was subject to leaching using the IL [BET][HCl] and H2SO4 and HCl controls; Figure 25 shows the Fe, Pr and Nd compositions (determined by ICP-MS) of supernatant samples after leachates obtained by treating NdPrFeB with [BET][HCl] were subject to pH-dependant precipitation using Na2CO3; Figure 26 shows XRF measurements of Mg(OH)2 (left) and MgO (right) materials synthesized in [TEA]HSO4 and [BET][HCl]; Figure 27 shows SEM images of selected MgO samples synthesised in [TEA][HSO4] and [BET][HCl]; Figure 28 is a schematic diagram showing the experimental set up for gas evolution experiments; Figure 29 is an XRD spectra of Ca(OH)2 synthesized in ionic liquids. Blue squares indicate peaks corresponding to Ca(OH)2, while green circles represent peaks from CaCO3; Figure 30 shows SEM images of Ca(OH)2 synthesized in ionic liquids (left) and commercially available Ca(OH)2 (right); Figure 31 shows extraction yields for BM with water, 1 M H2SO4, 30% [BET][HCl] and 30% [TEA][HSO4] at 2 h, 4 h and 24 h; Figure 32 shows results of extractions performed with different solvent systems at 80 °C for 2 h (red: total extraction yield, i.e. mass loss percentage from the original BM after leaching; yellow and purple: mass percentage of Mn and Zn extracted from BM, respectively); Figure 33 outlines a two-step process for the extraction of Zn and Mn from alkaline battery BM, achieving 100% Zn leaching, 79% Mn leaching and 98% Fe leaching; and Figure 34 outlines the precipitation process for the leachates S1 and S2 obtained from the two-step extraction process. Example 1 - Leaching Case A: Acidic proton in the anion Two solid streams from industrial waste, denoted as Solid A and Solid B, were used as feedstocks for a leaching process. In this process an ionic liquid (IL) where the anion comprised an acidic proton was used. The following leaching protocol was used: 1.5 g of solid feedstock was combined with 30 mL of an IL solution comprising triethylammonium hydrogen sulfate ([Et3NH]+[HSO4]-) and water (30% w / w determined by Karl Fisher titration). The combined mixture was incubated at 80 ºC for 3 hours. The dispersion was then cooled to 5 ºC, centrifuged, and the supernatant was subsequently collected. The solid material was then subjected to two washes with deionized water and one with ethanol, and left to dry at 80 ºC overnight. The reaction during the leaching process may be represented by the following general equation: eq. (1a)In which A is a conjugate base of an acid; B is a base; Mt is a target metal(s), Mu is an undissolved metal(s) / compound(s); and H2 is hydrogen. The collected liquid was subject to X-ray fluorescence (XRF) analysis to ascertain the extracted metal content. The results for Solid A and Solid B are shown in Tables 1 and 2, respectively. Table 1: Elemental contents determined by XRF analysis of Solid A before processing and of the extracted liquid fraction after processing with [Et3NH][HSO4] solution. Extracted liquid Solid A fraction [Et3NH][HSO4] Element (Before Processing) solution Composition St. deviation Composition St. deviation Fe 95.61 0.82 94.14 0.15 Cu 0.81 0.04 1.20 0.02 Ca 0.81 0.06 0.83 0.02 Mn 0.66 0.04 0.57 0.03 Si 0.46 0.03 0.25 0.01 Eu 0.34 0.06 0.31 0.04 Ni 0.24 0.01 0.27 0.01 Al 0.23 0.01 1.12 0.01 Zn 0.075 0.003 0.01 0.01 Te 0.014 0.005 0.29 0.05 La 0.0076 0.002 0.27 0.01 Cr 0.21 0.03 - - S 0.12 0.01 * * Mg 0.049 0.009 - - Br 0.005 0.001 - - Sr 0.024 0.005 - - Mo 0.025 0.003 - - Yb 0.091 0.007- -Pb 0.099 0.003- -(*) Signal removed and other elements normalized. Table 2: Elemental contents (determined by XRF analysis) of Solid B before processing and of the extracted liquid fraction after processing with [Et3NH][HSO4] solution. Extracted liquid fraction Solid B Element [Et3NH][HSO4] solution Composition St. deviation Composition St. deviation Fe 83.51 0.47 91.50 0.36 Ca 10.65 0.12 1.95 0.09 Zn 2.06 0.03 3.46 0.08 Si 1.55 0.12 0.53 0.03 Al 0.21 0.06 1.31 0.01 Mn 0.38 0.02 0.68 0.02 Te 0.016 0.003 0.34 0.01 Eu 0.20 0.02 0.22 0.01 Mg 0.45 0.02 - - S 0.33 0.04 * * La 0.023 0.005 - - Pb 0.16 0.01 - - K 0.14 0.02 - - Cl 0.057 0.007 - - Cr 0.065 0.003 - - Ni 0.045 0.004 - - Rb 0.052 0.008 - - Sr 0.011 0.002 - - Cd 0.006 0.002 - - Yb 0.065 0.007 (*) Signal removed and other elements normalized. S is present in the IL. The results showed varying leaching rates for different metals. During the reaction, the formation of bubbles was observed, attributed to the generation of hydrogen (substantiated by mass spectrometry) and other gases such as CO2. Tables 1 and 2 demonstrate that the IL system may can be used to separate Fe from other metals / metalloids in a waste stream. In particular, Table 1 shows that for solid A, Cu, Al, Te, La and Zn are extracted faster than Fe. This demonstrates that the IL system could be used to recover these metals. Additionally, Table 2 shows that for solid B, Zn, Al, Mn and Te are extracted faster than Fe. Table 2 also demonstrates that Ca has a low solubility in [Et3NH][HSO4]. This property may be used to dissolve other metals and leave behind a calcium-rich material. Example 2 - Electrodeposition Case A: Acidic proton in the anion The inventors wanted to investigate if it was possible to recover the metal from the extracted liquid fraction. First, the inventors investigated the use of an electrodeposition process. To investigate this, the inventors conducted an electrodeposition process on the extracted liquid fraction from Example 1 for Solid A. The electrodeposition reaction may be represented by the following general equation: eq. (2.1a) Note that eq. (2.1a) does not show the possible effects of the co-solvent (water) in this reaction. The following electrodeposition protocol was used: 10 mL of solution (the liquid from example 1 as recovered after filtration) was introduced into a 3-electrode setup. All electrodepositions occurred at -1 volt (vs. aqueous Ag / AgCl reference electrode) for a duration of 2 hours. The counter electrode was platinum, while the working electrode was a conductive carbon sheet with a diameter of 2 centimeters. The electrodeposited material was analyzed with scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX), and the results are shown in Figures 1 and 2. Electrodeposition at -1 V resulted in the co-deposition of copper (primary) and traces of iron, along with sulfate and chloride anions. Significantly, the liquid contained mainly Fe with a small proportion of Cu, see Table 1. However, the electrodeposition step recovered Cu with high selectivity. This demonstrates that the methods of the invention could be used for copper refining. The electrodeposition step could be repeated at a different (e.g. higher) potential to recover a second metal. Example 3 - Chemical Precipitation Case Next the inventors investigated if it was possible to recover the metal and regenerate the IL using a precipitating agent. In this example, sodium hydroxide (NaOH) at a concentration of 2M was used as the precipitating agent to adjust the pH to 11 of the extracted liquid fractions obtained in Example 1. The reaction during precipitation may be represented by the following general equation: !2[" # $]%[&]' %()*+ ,(-.) / 01 / 3* 4 ,(-* + 2[" # $][&]'()* eq. (2.2a) Where P is a precipitating agent. The precipitating agent may be an organic or inorganic salts, a hydroxide, a carbonate and / or a gas, such as CO2. Following precipitation, the metal content in the liquids was analyzed via X-ray fluorescence (XRF) to assess the method's efficiency in recovering extracted metals, and the results are summarized in Table 3. It was observed that the majority of the metals, including Cu, Mn, Si, Eu, Ni, Al and La, were precipitated. Ca, Fe and Te were not precipitated. from Solids A and BT Post-precipitation liquid from Post-precipitation liquid from Element Solid A Solid B Composition St. deviation Composition St. deviation S* 80.22 1.13 85.7 1.3 Na** 5.24 0.29 4.30 0.18 Ca 0.11 0.02 0.14 0.03 Fe 14.18 1.78 9.5 1.0 Te 0.18 0.02 0.16 0.02 Zn - - 0.29 0.02 * The observed sulfur is present in the anion of the IL. ** The observed sodium is present as the cation in the precipitating agent. Example 4 - Leaching Case B: Acidic proton in the cation – soluble base The inventors wanted to investigate if it would be possible to process the solid stream from industrial waste using alternative ILs. In particular, they wanted to look if it would be possible to extract and recover metal if an IL was used where the cation comprised an acidic proton and a soluble base (betaine). The base is described as being soluble because it is soluble in water. Solids A and B, described in Example 1, underwent processing using the same methodology described in Example 1, but the IL solution described in Example 1 was replaced with a solution of betaine hydrochloride [BET][HCl] and water (30% w / w). The reaction during the leaching process may be represented by the following general equation: eq. (1b)XRF analysis of the solids before processing and of the resultant liquids after processing is presented in Tables 4 and 5. Table 4: Elemental contents (determined by XRF analysis) of Solid A before processing and of the extracted liquid fraction after processing with [BET][HCl] solution. Extracted liquid fraction Solid A Element [BET][HCl] Solution Composition St. deviation Composition St. deviation Fe 95.61 0.82 83.09 1.51 Cu 0.81 0.04 2.40 0.09 Ca 0.81 0.06 5.26 1.00 Mn 0.66 0.04 0.78 0.05 Si 0.46 0.03 0.76 0.16 Eu 0.34 0.06 0.30 0.07 Ni 0.24 0.01 0.43 0.01 Al 0.23 0.01 2.10 0.04 Cr 0.21 0.03 0.17 0.01 S 0.12 0.01 1.05 0.02 Te 0.014 0.005 0.48 0.01 Br 0.0049 0.001 0.37 0.01 and of the extracted liquid fraction after processing with [BET][HCl] solution. Extracted liquid fraction Solid B [BET][HCl] Solution Element St. St. Composition Composition deviation deviation Fe 83.51 0.47 73.36 0.36 Ca 10.65 0.12 20.10 0.15 Zn 2.06 0.03 2.64 0.11 Si 1.55 0.12 0.61 0.01 Mg 0.45 0.02 0.21 0.01S0.33 0.04- -Al 0.21 0.06 0.90 0.01 Mn 0.38 0.02 0.72 0.01 Te 0.016 0.003 0.21 0.01 La 0.023 0.005 0.08 0.01 Eu 0.20 0.02 0.26 0.02 Pb 0.16 0.01 0.17 0.01 K 0.14 0.02 - - Cl 0.057 0.007 * * Cr 0.065 0.003 - - Ni 0.045 0.004 - - Rb 0.052 0.008 - - Sr 0.011 0.002 - - Cd 0.006 0.002 - - Yb 0.065 0.007 (*) Signal removed and other elements normalized. Cl is present in the IL. Advantageously, Tables 4 and 5 show that this IL system can be used to extract increased proportions of Cu and Ca compared to other metals. Example 5 - Electrodeposition Case B: Acidic proton in the cation – soluble base. The electrodeposition methodology described in Example 2 was applied to the liquid fractions obtained in Example 4. The electrodeposition reaction may be represented by the following general equation: &%&'!*+'+!, !2["][#]$ (%)-...... / (0) 12["][#]$ (%) eq. (2.1b) The electrodeposited materials were analyzed with SEM and EDX spectroscopy, and the results are shown in Figures 3 and 4. Morphological differences are observed depending on the metal type and content in the solvent. Figure 4A shows that the recovered material from Solid A exhibited primarily a Cu and Cl compound, likely CuCl. Figure 4B shows that the recovered material from Solid B showed the precipitation of a Pb salt, likely PbSO4, without the presence of the other metals. This demonstrates that it is possible to recover different metals (with a high degree of purity) from the solution. Example 6: Chemical Precipitation Case B: Acidic proton in the cation – soluble base The chemical precipitation methodology described in Example 3 was applied to the liquid fractions recovered in Example 4. The precipitation process may be represented by the following general equation: The metal content of the solutions post-precipitation, determined via XRF, is summarized in Table 6.It can be seen that, after adding the precipitating agent, most of the metals are removed from solution. This demonstrates that precipitation can be used as a strategy to purify the IL for recycling or to target specific metals for recovery. Table 6: Elemental contents from solids A and B Post-precipitation liquid from Post-precipitation liquid from Element Solid A Solid B Composition St. deviation Composition St. deviation Cl* 91.34 0.05 92.98 0.04 Na** 0.17 0.01 1.73 0.04 S 0.09 0.01 0.08 0.01 Ca 0.46 0.08 4.85 0.18 Fe 7.2 0.13 0.10 0.01 Br 0.03 0.005 0.07 0.001 Te - - 0.07 0.001 * The observed sulfur is present in the anion of the IL. ** The observed sodium is present as the cation in the precipitating agent. Example 7 - Leaching Case C: Acidic proton in the cation – insoluble base The inventors wanted to investigate if it would be possible to process the solid stream from industrial waste using further alternative ILs. In particular, they wanted to look if it would be possible to extract and recover metal if a water-soluble IL was used in which the cation comprised an acidic proton and a water-insoluble base. When reacted with an acid to form the IL, it becomes soluble. Solids A and B, described in Example 1, underwent processing using the same methodology described in Example 1, but the IL solution described in Example 1 was replaced with a solution of N,N-dimethylcyclohexylammonium hydrogen sulfate [DCA][HSO4] and water (30% w / w) . The reaction during the leaching process may be represented by the following general equation: eq. (1c)The particular, eq. (1c) shows that the system forms two immiscible liquid phases, identified as l-1 and l-2. In particular, in the present case, when the IL is reacted with the metallic material, the amine may deprotonate and become insoluble splitting phases, making it easier to regenerate the ionic liquid. This phase splitting can either be observed after extraction or after deposition or precipitation. The collected liquids were subjected to XRF analysis to ascertain the extracted metal contents. The results for Solid A are shown in Table 7 and the results for Solid B are shown in Table 8. Table 7: Elemental contents (determined by XRF analysis) of Solid A before processing and of the extracted liquid fraction after processing with [DCA][HSO4] solution Extracted liquid fraction Solid A Element [DCA][HSO4] Composition St. deviation Composition St. deviation Fe 95.61 0.82 91.50 0.16 Cu 0.81 0.04 1.63 0.06 Ca 0.81 0.06 3.76 0.42 Mn 0.66 0.04 0.67 0.06 Si 0.46 0.03 0.21 0.002 Eu 0.34 0.06 0.22 0.01 Ni 0.24 0.01 0.35 0.01 Al 0.23 0.01 1.00 0.09 Te 0.014 0.005 0.34 0.02 Zn 0.075 0.003 0.10 0.01 Table 8: Elemental contents (determined by XRF analysis) of Solid B before processing and of the extracted liquid fraction after processing with [DCA][HSO4] solution Extracted liquid fraction Solid B Element [DCA][HSO4] Composition St. deviation Composition St. deviation Fe 83.51 0.47 91.31 0.02 Ca 10.65 0.12 1.61 0.32 Zn 2.06 0.03 4.19 0.46 Si 1.55 0.12 0.69 0.20 Al 0.21 0.06 0.88 0.01 Mn 0.38 0.02 0.67 0.03 Te 0.016 0.003 0.31 0.01 Eu 0.20 0.02 0.20 0.01 This example shows that different ILs extract different elements from the solids. Accordingly, they can be tuned to enhanced performance. Example 8 - Electrodeposition Case C: Acidic proton in the cation – insoluble base The electrodeposition methodology described in Example 2 was applied to the liquid fraction recovered after processing of Solid A, as described in Example 7. The electrodeposition reaction may be represented by the following general equation: eq. (2.1c) The electrodeposited material was analyzed with SEM (scanning electron microscopy) and EDX (energy-dispersive x-ray spectroscopy), and the results are shown in Figures 5 and 6. Figure 6 shows that from Solid A, a Cu compound was deposited selectively without indication of co-precipitation from the other metals. This demonstrates that it is possible to deposit metallic compounds with a lower oxidation state. In this case, a compound containing Cu and S is obtained. The significance of these result is that it opens the door for the recovery of metals / metallic compounds from polymetallic mixtures via electrodeposition. It could also be used to purify the IL for reuse. Example 9 - Chemical Precipitation Case C: Acidic proton in the cation – insoluble base The chemical precipitation methodology described in Example 3 was applied to the liquid fractions recovered in Example 7. The precipitation process may be represented by the following general equation: eq. (2.2c)The metal content of the solutions post-precipitation, determined via XRF, is summarized in Table 9. Table 9: Elemental contents (determined by XRF analysis) of post precipitation liquids from solids A and B Element Post-precipitation liquid after Post-precipitation liquid after extraction of Solid A extraction of Solid B Composition St. deviation Composition St. deviation S* 95.26 0.70 94.89 0.23 Na** 4.08 0.47 4.48 0.04 P - - 0.05 0.003 Fe 0.49 0.16 0.29 0.05 Te 0.15 0.01 0.14 0.01 * The observed sulfur is present in the anion of the IL. ** The observed sodium is present as the cation in the precipitating agent. All the metals in solution were recovered except some traces of P, Fe, Te. In this example, a phase split was observed after the extraction. In particular, when the pH was increased to precipitate the metals the phase separation was observed. This shows that all metals present in the IL solution (Table 9) can be removed from the IL except for traces of elements. This opens the door to purification strategies for the IL. The IL may then be reused in the process and the generation of polymetallic solids what could be further processes to obtain target metals with the desire purity. Example 10 - Leaching Case B: Acidic proton in aqueous systems) The inventors wanted to investigate if it would be possible to process the solid stream from industrial waste using deep eutectic solvents (DESs). Solids A and B, described in Example 1, underwent processing using the same methodology described in Example 1, but the IL solution described in Example 1 was replaced with a mixture of betaine and ethylene glycol 1:4 molar ratio (1:4 [BET]:[EtGly]) DES. The reaction during the leaching process may be represented by general equation (1b), provided in Example 4. XRF analysis of the solids before processing and of the resultant liquids after processing is presented in Tables 10 and 11. processing and of the extracted liquid fraction after processing with 1:4 [BET]:[EtGly] DES Extracted liquid fraction Element Solid A DES 1:4 [BET][EtGly] Composition St. deviation Composition St. deviation Fe 95.61 0.82 15.4 2.1 Cu 0.81 0.04 12.0 1.2 Te 0.014 0.005 23.1 4.6 Sn traces - 48.5 2.3 Table by XRF analysis) of Solid B before processing fraction after processing with 1:4 [BET]:[EtGly] DES Extracted liquid fraction Element Solid B DES 1:4 [BET][EtGly] Composition St. deviation Composition St. deviation Fe 83.51 0.47 13.9 1.5 Ca 10.65 0.12 14.0 0.2 Te 0.016 0.003 72.0 1.8 In particular, Table 10 demonstrates that the DES processing system may be used to concentrate Te, Sn and Cu from Solid A. Similarly, Table 11 demonstrates that the DES system may be used to concentrate Te from Solid B. Example 11 - Hydrogen production rates from reaction with IL with acidic protons in the anion The treatment process described in the previous examples uses ILs or DESs to dissolve and extract different metals from metal-containing materials. During the dissolution and extraction step, gases are produced. The metals can subsequently be recovered from the ionic liquid using either a precipitation or an electroplating step, producing metal based compounds or elemental metals. The inventors wanted to determine the hydrogen production rate when a metal was treated with an ionic liquid with an acidic proton in the anion. To do this, the inventors exposed 0.1288 g metallic zinc (98% purity) to a 50 mL solution of [Et3NH][HSO4] and water (40% w / w) at 80°C under agitation. The volume of gas produced was measured using a water displacement method, whereby the volume of gas produced correlates with the volume of water displaced, see the methodology section below for more details. The gas produced was collected and measured over time, with confirmation by Mass Spectroscopy that it was hydrogen. The volume of water displaced was converted into the volume of hydrogen formed and the volume of hydrogen produced over time is shown in Figure 7. Example 12 – Comparison of different ionic liquids Three different IL solutions, were tested as the leaching medium for two waste substrates: BoF-US and LC (slags and muds obtained as steel industry by-products). The ILs used in the three different solutions were [TEA][HSO4], [TEA]Cl and [BET][HCl], and each solution contained 86% (w / w) water. In the experiments 1g of material and 30 ml of the solution was used. The reactions were carried out at 80°C. Figure 8 shows the results for these experiments. It can be seen that for both substrates, when the IL was [BET][HCl] the lowest amount of gas produced. [TEA][HSO4] and [TEA]Cl produced comparable results. However, the hydrogensulfate ionic liquid outperformed the chloride for both substrates, enabling either higher gas yields or faster initial flow rates. Table 12: Reaction conditions used to treat BOF-US and LC and results obtained, wherein the volume of water measured correlates to the volume of gas produced Waste IL solution Initial water Volume of Substrate flowrate water [mL] [mL / s] BOF-US 14% [TEA][HSO4] 0.387 91.051 BOF-US 14% [TEA]Cl 0.463 82.601 BOF-US 14% [BET][HCl] 0.355 73.054 LC 14% [TEA][HSO4] 0.582 91.918 LC 14% [TEA]Cl 0.565 91.919 LC 14% [BET][HCl] 0.553 73.095 Example 13 – Effect of ionic liquid concentration The effect of varying the ratio of the water to the IL in the IL solutions was investigated. 1 gram of the substrate, BOF-US or LC, were treated with 30 ml of an IL solution at 80°C. In one experiment, the IL solution was [TEA][HSO4] and water, where the [TEA][HSO4] was present at a concentration of 14, 30 or 60% (w / w) IL, and the results are shown in Figure 9. Concentrations of 14 and 30% (w / w) [TEA][HSO4] resulted in very similar gas yields and initial flow rates, while 60% (w / w) [TEA][HSO4] resulted in both higher initial flow rates and higher overall yields. The BOF-US and LC substrates were also treated with an IL solution comprising [BET][HCl] and water. The [BET][HCl] was provided at a concentration of 14 or 35% (w / w), and the results are shown in Figure 10. A marked increase in the overall gas yield (ca. 32%) was observed when the concentration of IL was increased from 14 to 35% (w / w). However, it is noted that water affects the viscosity and polarity of the system and the chemical interaction between the IL and metal. In highly viscous systems it will be more difficult to separate the unreacted solids and recover materials efficiently (via chemical precipitation or electrodeposition). Table 13: Reaction conditions used to treat BOF-US and LC and results obtained. Waste IL Solution Initial gas Volume of Substrate flowrate gas [mL] [mL / s] BOF-US 14% [TEA][HSO4] 0.387 91.051 BOF-US 30% [TEA][HSO4] 0.412 90.471 BOF-US 60% [TEA][HSO4] 0.480 109.281 BOF-US 14% [BET][HCl] 0.355 73.054 BOF-US 35% [BET][HCl] 0.386 97.797 LC 14% [TEA][HSO4] 0.582 91.918 LC 30% [TEA][HSO4] 0.546 93.996 LC 60% [TEA][HSO4] 0.648 104.056 LC 14% [BET][HCl] 0.553 73.095 LC 35% [BET][HCl] 0.670 98.431 Example 14 - Effect of temperature The effect of varying the temperature of leaching experiments using BOF-US as the substrate was investigated and two different solutions (14% [TEA][HSO4] and 14% [TEA]Cl were used. Experiments were conducted at 80°C, 50°C and room temperature using 0.5 g of the substrate and 30ml of the IL solution. Figure 11 shows the results of these experiments, and it can be seen that higher temperatures resulted in a higher the reaction rate and the more gas being produced. Using [TEA]Cl at room temperature did not result in any gas evolution from BOF-US and very sluggish gas release at 50°C. Meanwhile, [TEA][HSO4] resulted in some gas release at room temperature and faster release at 50°C. Based on the flattening out of the gas production in many of the experiments, it is suspected that different reaction mechanisms are at play, some of which require a higher activation energy. Table 14: Reaction conditions used to treat BOF-US and results obtained at different temperatures, wherein the volume of water measured correlates to the volume of gas produced IL Solution Temperature Initial Volume of [°C] water water [mL] flowrate [mL / s] 14% [TEA]Cl 80 0.388 39.101±0.091 14% [TEA]Cl 50 0.051 16.067±0.002 14% [TEA]Cl 20 - - 14% [TEA][HSO4] 80 0.371 42.771±0.321 14% [TEA][HSO4] 50 0.348 17.970±0.002 14% [TEA][HSO4] 20 0.222 3.312±0.012 Example 15 – Effect of particle size The effect of substrate morphology on leaching was studied. Waste material LC samples were ground into finer powders using a pestle and mortar and leaching experiments conducted. In particular, 1g of the waster substrate (original or ground) was treated with 30ml IL solution (14% [TEA]Cl) at 80°C. Figure 12 shows that, compared with the original powders, the ground powders produced more gas and had higher initial gas flow rates, although the effect was small. This met with expectations, since it was expected that grinding would reduce particle size, increase surface area and thus improve reactivity. However, surprisingly, surface area measurements taken using BET did not confirm an increase in surface area [Original material 57 m2 / g, grounded material 53 m2 / g]. Table 15: Reaction conditions and initial gas flow rate from ground and original LC samples, wherein the volume of water measured correlates to the volume of gas produced Waste Substrate Initial water Volume of flowrate water [mL] [mL / s] LC (original) 0.423 92.426 LC (ground) 0.505 98.401 Example 16 –Change in metal composition of substrate materials during treatment It was investigated how the substrate metal composition changed during the treatment process. It is desirable to produce a material that can be given back to the foundation industry, and so the inventors sought to leach out zinc selectively, while leaving iron in the material. For this study, 1.4 gram of a waste material substrate (BOF-US or LC) was treated using 10 ml of an IL solution or 10 ml 1.5 M sulfuric acid solution as the leaching medium at 80°C. Measurements were taken at 0, 30 and 120 minutes, and the experiments stopped at 120 minutes. Acid digestion followed by inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the content of iron, zinc and copper in the waste material substrates before and after treatment, as shown in Tables 17 and 18 for the BOF-US and LC substrates, respectively. Figures 13 and 14 show that treatment of the substrates LC and BOF-US with 1.5 M H2SO4, 14% (w / w) [BET][HCl], 14% (w / w) [TEA][HSO4] or 14% (w / w) [TEA]Cl. It is noted that when the 1.5 M solution of H2SO4 was used this resulted in significant extraction of iron (>70% for all substrates). In contrast, the ILs extracted only 20- 25% of iron from LC and ca.10% of iron from BOF-US. Zinc extraction from both substrates was very high using sulfuric acid (>90% after two hours), and the ionic liquids extracted ca.50-70% of zinc from LC and ca.60% of zinc from BOF-US. The leaching reactions are expected to be largely complete after two hours. Figures 13 and 14 shows that the sulfuric acid control leaches significant amounts of Fe. The overall extraction of Zn is less for ILs that the H2SO4, but the extracted Zn to extracted Fe ratio is improved. These results indicate that the ionic liquids [TEA][HSO4], [TEA]Cl and [BET][HCl] have a much higher selectivity for zinc extraction vs. iron extraction compared to sulfuric acid. Table 16: Contents of different metals in BOF-US substrate before and after treatment ll lafel] ] afvo]o] ] af* v o * v p o % o *o] % o % % m m[o % [i [mi [ [o % a et tm e eit[e F a e n u r Z a n S F r Z C a u R R R r C Original powder 58.45± - 2.85± - 7.25± - 3.95 0.04 1.18 After reaction with 14% 28.55± 51.16 1.85± 35.22 <0.00 >99.999 [TEA]Cl at 20°C 3.60 0.00 1 After reaction with 14% 24.84± 57.51 1.25± 56.05 <0.00 >99.999 [TEA]Cl at 50°C 0.28 0.48 1 After reaction with 14% 22.32± 61.82 1.07± 62.43 <0.00 >99.999 [TEA]Cl at 80°C 0.23 0.14 1 After reaction with 14% 29.28± 49.91 1.40± 50.93 <0.00 >99.999 [TEA]Cl at 20°C 0.53 0.12 1 After reaction with 14% 27.28± 53.33 1.02± 64.33 <0.00 >99.999 [TEA]Cl at 50°C 0.23 0.03 1 After reaction with 14% 25.51± 56.37 0.77± 72.43 <0.00 >99.999 [TEA][HSO4] at 80°C 0.61 0.07 1 After reaction with 30% 23.80± 59.28 0.72± 74.89 <0.00 >99.999 [TEA][HSO4] at 80°C 0.06 0.06 1 After reaction with 60% 19.74± 66.23 0.48± 83.34 <0.00 >99.999 [TEA][HSO4] at 80°C 0.71 0.017 1 After reaction with 14% 41.86± 28.38 0.98± 65.52 <0.00 >99.999 [BET][HCl] at 80°C 3.42 0.04 1 After reaction with 35% 27.56± 52.84 0.72± 74.61 <0.00 >99.999 [BET][HCl] at 80°C 1.22 0.02 1 Table 17: Contents of different metals in LC substrate before and after treatment ll lafel] ] afvo]o] ] af* v * v *o] p % o [ o % % o m mi [ [o % % o i[ [o % etmtmi [a e a e n e a n u etS F R r a u F Z R r Z C R r C Original powder 46.30± - 1.82± - 0.311± - 2.65 0.05 0.008 After reaction with 14% 34.50± 25.50 1.01± 44.84 <0.001 >99.999 [TEA]Cl at 80°C 2.01 0.02 After reaction with 14% 36.04± 22.16 0.51± 72.06 <0.001 >99.999 [TEA][HSO4] at 80°C 2.07 0.01 After reaction with 30% 31.39± 32.20 0.32± 82.55 <0.001 >99.999 [TEA][HSO4] at 80°C 3.07 0.02 After reaction with 60% 27.60± 40.39 0.20± 88.97 <0.001 >99.999 [TEA][HSO4] at 80°C 2.38 0.01 After reaction with 14% 32.03± 30.82 0.56± 69.21 <0.001 >99.999 [BET][HCl] at 80°C 2.37 0.00 After reaction with 35% 27.98± 39.58 0.28± 84.69 <0.001 >99.999 [BET][HCl] at 80°C 1.01 0.00 * The removal ratio was calculated on a mass balance basis, taking the overall mass loss of the material into account. From the results shown in Tables 16 and 17 it can be seen that, after reaction with all three ionic liquids and sulfuric acid, there was a significant reduction in iron and zinc, as well as complete removal of copper from both the LC and BOF-US substrates. Reducing the amount of Fe leached, reduces costs of downstream units which are required to selectively precipitate Fe. These results shows that extraction time, water content, type of IL, etc can be adjusted to maximize Zn extraction while reducing the Fe leached. Similar principles may be used to deal with other waste streams and ores. Example 17 – Effect of temperature on iron and zinc extraction from waste materials The effect of temperature on iron and zinc extraction from the BOF-US substrate was studied using 14% (w / w) [TEA][HSO4] and 14% (w / w) [TEA]Cl as leaching mediums. The results are shown in Figure 15, and it can be seen that for both ILs an increase in temperature caused increased extraction of both iron and zinc from BOF-US. Example 18 - Effect of ionic liquid concentration on iron and zinc extraction from waste materials The effect of ionic liquid concentration on iron and zinc extraction was studied using [TEA][HSO4] for BOF-US (Figure 16) and LC (Figure 17) substrates. Iron extraction was increased with increasing IL concentration for both substrates by about 10 to 15 percent points. For the BOF-US substrate, zinc extraction was also increased by approximately 15 percentage points, from ca. 70% to ca. 85%, when the concentration of the IL was increased from 14% (w / w) to 60% (w / w). The most dramatic increase however was registered for zinc extraction from the LC substrate, where extraction increased from around 20% at 14% (w / w) IL concentration to around 85% at 60% (w / w) IL concentration. It is noted that as the concentration of IL increases so does zinc extraction. While iron extraction also increased, it did so less than zinc extraction. Accordingly, extraction solutions with a high concentration of ILs favour zinc extraction. Example 19 – Analysis of composition of the treatment The compositions of gases produced during the treatment of four different waste material substrates; BM, BOS, SD-3 and FD-2; were analysed by mass spectrometry. Initially scanning was done the scan mode, scanning AMUs from 1 to 160, to investigate the nature of product gases and to determine if any ionic liquid escaped the reflux condenser. The mass spectrometry results are shown in Figure 18 and show that treatment of all four substrates generated a hydrogen product. Waste material substrates SD-3 and FD-2 produced comparatively large amounts of hydrogen, and no escaped ionic liquid was detected. Surprisingly, as well as hydrogen, signals for moisture, nitrogen and carbon dioxide were also detected. The spectra in Figure 18 show that the H2 and CO2 levels are higher than the baseline levels, indicating that they were evolved during the treatment reactions. Both the SD-3 and FD-2 samples produced pronounced levels of CO2 and H2. This indicates that CO32-or HCO3- ions may be present in the samples. From the scan experiments, there was no evidence of any complex hydrocarbon or other inorganic gases being released during the treatment. The waste material substrate SD-3 was further investigated under the continuous multiple ion detection (MID) mode, and the results are shown in Figure 19. Strong CO2 and H2 signals be seen during the reaction. Hydrogen evolution continues throughout the experiment, whilst CO2 evolution drops to zero. This suggests that a carbonate species in the sample is reduced to CO2 at the same time as hydrogen is generated. Since the treatment is a batch process, the evolution of hydrogen from the ionic liquid / substrate reaction may slowly decrease as the waste material is reacted completely. conditions on metal-oxide formed The inventors aimed to investigate how process conditions affect the particle morphology of metal oxides obtained by chemical precipitation after leaching. In this example, Betaine Hydrochloride ([BET][HCl]) was used as the ionic liquid. A solution containing 65 wt% water content was prepared by mixing with deionized (DI) water, as determined using a Karl Fischer Titrator. Metallic zinc granules were placed in 1 L of the IL solution at 85°C with magnetic agitation for 24 hours. This resulted in oxidation of the zinc, and the resulting zinc ions were dissolved in the IL solution and hydrogen gas formation. Residual solids were removed by filtration. The residual solids were not analysed, but it is thought that they resulted from impurities in the feedstock material. In some experiments, additional water was added to dilute the IL solution comprising the zinc ions. In these experiments, the water was added prior to the precipitating agents. Two precipitating agents were tested on the resultant IL solutions: sodium hydroxide (NaOH) and sodium carbonate (Na2CO3), each at a concentration of 2 mol / L prepared with DI water. The IL solution comprising the zinc ions was placed in a beaker with a magnetic stirrer while the precipitating agent solution was added dropwise using a fixed-speed syringe. The precipitation experiments were conducted at room temperature. A pH meter was used to monitor the pH at the endpoint, targeting around pH 10 for NaOH and around 9.3 for Na2CO3. The mixture was kept under agitation for 30 minutes. The resultant solid was separated from the suspension using a medium-speed centrifuge at 2000 rpm for 10 minutes. The solids were washed multiple times with DI water (5 times) and absolute ethanol (2 times) to remove the remaining solvent. After centrifugation, the samples were dried in an oven at 105°C overnight. The dried solids were ground into powder using a crusher. Samples precipitated with Na2CO3 underwent calcination to transform the metal carbonate into metal oxide and carbon dioxide. NaOH samples were also calcined for comparison. The particles were analysed using XRD, XRF, DLS and SEM. The results, shown in Tables 18 and 19 and Figures 20 to 23 demonstrate the different degrees of purity and particle morphology and size achievable using different conditions. After calcination, Figure 22a shows that all of samples 1 to 5 show high purity. Samples precipitated with NaOH show some impurities. This is significant as it demonstrates that precipitation conditions may be used to optimize material properties to meet different industry specifications. Additionally, this method can also be used for hydrogen generation. Figures 21 and 22 shows XRD data for the samples before and after calcination. It can be seen that signals lower that 30° (corresponding to impurities) disappeared after calcination. Additionally, signals showing the formation of zinc oxide can be observed in the samples after calcination. In particular, the XRD graphs show three main characteristic peaks that correspond to the following crystallographic planes in the hexagonal wurtzite structure: (100) Plane (this peak is usually observed around 2θ ≈ 31.7°); (002) Plane (this peak appears at 2θ ≈ 34.4°); and (101) Plane (this peak is found at 2θ ≈ 36.2°).These peaks are indicative of the primary crystal orientations in the wurtzite structure of ZnO. The signal at 47.0° corresponds to impurities in the 2 system, likely ZnCl. Table 18. Experimental conditions of each precipitation experiment. L L I er o ) n oIr ) n ni fL . e / i nie L o tt itn g e / m ( u. fla uo cbiclin g gtin n(o ataulon nt t dn ni ivso oo o Hro ot tp p c cLi ii ititi pt teI n n+ + c ct)) )l lu u2 2 / u u de el le eal li ir rn n n g g no o% m mEZ s d W ( Z s d Z ( P a P a ( 1 28.8 0% 28.8 590 NaOH 696 10.2 2 25.7 20% 20.5 765 NaOH 866 10 3 25.9 33% 17.3 755 NaOH 865 9.99 4 25.5 43% 14.5 750 NaOH 862 10.1 5 24.3 50% 12.1 770 NaOH 895 10.1 6 25.2 0% 25.2 750 Na2CO3 598 9.5 7 25.6 20% 20.5 750 Na2CO3 615 9.43 8 25.2 33% 16.8 780 Na2CO3 648 9.37 9 25.6 43% 14.5 800 Na2CO3 708 9.39 10 25.3 50% 12.7 800 Na2CO3 698 9.37 Table 19. Particle size (nm) before and after calcination determined by Dynamic Light Scattering (Litesizer). Sample Hydrodynamic diameter (nm) No. Before Calcination After Calcination 1 308 546 2 266 390 3 329 487 4 286 454 5 273 347 6 6823 403 7 3077 396 8 975 1527 9 1764 601 10 1723 1394 Example 21 – Selective recovery of metals from a permanent magnet material using IL extraction system The inventors aimed to investigate how ionic liquids can be used for the selective leaching and recovery of metals from a NdPrFeB permanent magnet material. In this example, Betaine Hydrochloride ([BET][HCl]) was used as the ionic liquid. An IL solution containing 65 wt% water was prepared by mixing [BET][HCl] with deionized (DI) water, as determined using a Karl Fischer Titrator. 1 g of NdPrFeB metal alloy powder (Nd: 25.17%, Pr: 6.54%, Fe:66.96% and B:1.02%) was leached in 30 ml of IL solution at 50°C with shaking for 2 hours. Two control samples of NdPrFeB were contacted with 2 mol / L HCl and 1 mol / L H2SO4, respectively. For each sample, unreacted solid was separated from the leachate by filtration. The leachate was placed in a beaker with a magnetic stirrer and a precipitating agent (sodium carbonate (Na2CO3), 2 mol / L) was added until the target pH was reached (pH = 5.3). The mixture was kept under agitation for 20 minutes. The solid was separated from the suspension using a medium-speed centrifuge at 2000 rpm for 10 minutes. Additional precipitating agent was added until the next target pH (pH 6.9) was reached, after which the resulting solid was isolated using the same procedure. This process was repeated for each subsequent target pH until the final target pH of 10 was achieved. The precipitated solids were washed multiple times with DI water (5 times) and absolute ethanol (2 times) to remove the remaining solvent. After centrifugation, the samples were dried in an oven at 105°C overnight. The leachates, supernatants and powers were analysed using ICP, XRF and XRD. The results, shown in Figures 24 and 25, show that BET-HCl differentially leaches the components of the NdPrFeB material compared to acid controls. Additionally, in the presence of [BET][HCl], pH dependant precipitation using 2M Na2CO3 at room temperature enables further selective precipitation (Table 20). This is significant as it demonstrates that ionic liquids can be optimized for improved leaching and recovery of rare earth elements. Additionally, this method can also be used for hydrogen generation. Table 20 – Composition of the Nd-Pr-Fe-B solids. Total recovered solid after Total in Total in precipitation Elemental 1 g of unreacted [mg] composition raw solid of the solid material [mg] [mg] pH 5.3 pH 6.9 pH 10 Pr 65.4 4.92 15.6 60.4 60.5 Nd 251.7 22.2 62.3 229.0 229.5 Fe 669.6 382.4 32.1 198.0 287.2 B 10.2 0.1 1.3 5.5 10.1 Solubilised - 587.3 471.6 94.5 0.0 Solutions of [BET][HCl] with a 70 wt% water content and triethanolammonium hydrogen sulfate [TEA][HSO4] with a 40 wt% water content were prepared. The water content of the solutions was confirmed using Karl Fischer titration. Experiments were conducted using the conditions outlined in Table 21. The following general procedure was used: 50 ml aliquots of IL solution was combined with an excess amount of magnesium carbonate (MgCO3) powder. The reaction between the ionic liquid solution and MgCO3 was vigorous, accompanied by the evolution of gas, presumed to be CO2. The unreacted solids were removed by filtration. In certain experiments, additional water was introduced into the Mg-saturated ionic liquid solution and mixed using an ultrasonic bath for 15 minutes to ensure homogeneity before the addition of the precipitating agent. 50 ml of a 2M sodium hydroxide (NaOH) solution (precipitating agent) was added to the Mg-saturated ionic liquid solution. After the precipitating agent was added, the solution was further mixed in an ultrasonic bath at room temperature for an additional 15 minutes and then allowed to remain undisturbed overnight. A white precipitate formed, which was separated by filtration, redispersed in 40 ml of water, and centrifuged at 3000 rpm for 30 minutes. The supernatant was discarded, and this washing procedure was repeated four times, with a final wash using 40 ml of ethanol. The resulting powder was dried overnight at 105°C. X-ray diffraction (XRD) analysis confirmed that the synthesized samples were magnesium hydroxide (Mg(OH)2). Table 21. Experimental conditions for magnesium oxide and magnesium hydroxide synthesis Initial Sonicated after Water Additional addition of IL Content Water [ml] precipitating [% w / w] agent? A [TEA][HSO4] 40% 0 No B [TEA][HSO4] 40% 0 Yes C [TEA][HSO4] 40% 200 Yes D [TEA][HSO4] 40% 300 Yes E [TEA][HSO4] 40% 500 Yes F [BET][HCl] 70% 0 Yes G [BET][HCl] 70% 200 Yes H [BET][HCl] 70% 300 Yes I [BET][HCl] 70% 500 Yes The Mg(OH)2 samples were subsequently calcined at 500°C for 4 hours in an ambient atmosphere, converting the material to magnesium oxide (MgO). X-ray fluorescence (XRF) measurements were performed to assess the initial purity of the samples, and the results are shown in Figure 26 and Table 22. The XRF analysis indicated that, under the described experimental conditions, traces of the anions from the ionic liquids were detected in the solid samples both before and after calcination. This suggests the formation of Mg-S and Mg-Cl compounds, stable at temperatures up to 500°C. Variations in relative intensities and peak widths suggest differences in the crystalline structure. The diffraction patterns of Mg(OH)2 samples A and H exhibited additional signals, indicating the presence of unidentified crystalline impurities. In contrast, the MgO diffraction patterns did not show evidence of other crystalline structures. Scanning electron microscopy (SEM) images of selected samples, revealing differences in morphology, are shown in Figure 27. It was observed that Sample A, which did not undergo sonication after the addition of the precipitating agent, exhibited a higher sulfur content and formed distinct flake-like structures, indicating that the method of mixing significantly impacts the particle morphology and chemical composition of the final product. Table 22. XRF results for the solids before and after calcination. Sample Mg (%) S (%) Si (%) Other (%) Mg(OH)2 78 20 1 1 A MgO 83 15 1 1 Mg(OH)2 90 7 2 1 B MgO 92 5 2 1 Mg(OH)2 94 3 2 1 C MgO 94 3 2 1 Mg(OH)2 95 3 2 (traces) D MgO 95 3 1 1 Mg(OH)2 95 3 1 1 E MgO 95 3 1 1 F Mg(OH)2 93 - 1 Cl – 3 Ca – 3 Cl – 3 MgO 94 - 1 Ca – 3 Example 23: Synthesis of Ca(OH)₂ with Specific Morphologies in Ionic Liquids from CaCO₃ In this example, 2 g of CaCO3 were stirred for 1 hour at room temperature with 10 mL of the ionic liquid [BET][HCl] with 70% w / w water content. The CaCO3 dissolved almost instantly. Upon the addition of 2M NaOH, the pH was adjusted to 10, leading to the precipitation of a white powder. The resulting powder was then filtered, washed with deionized water, and dried overnight at 100°C. The X-ray diffraction (XRD) analysis (Figure 29) confirmed that the precipitate was predominantly Ca(OH)2, with minor traces of unreacted CaCO3. Semi-quantitative analysis indicated that the concentration of Ca(OH)2 was greater than 98%. X-ray fluorescence (XRF) analysis further revealed that the Ca content was 99% pure, with trace amounts of Cl (0.4%) derived from the ionic liquid. The synthesized Ca(OH)2 particles were characterized by scanning electron microscopy (SEM) and compared to a commercial sample of Ca(OH)2. For SEM analysis, both samples were coated with a 15 nm layer of gold using sputtering. The Ca(OH)2particles produced using the inventor’s process exhibited a distinct flat tabular morphology, in contrast to the commercial product, which lacked a defined particle morphology (Figure 30). Example 24 – Metal recovery from ‘black mass’ of spent alkaline batteries using aqueous H2SO4 Black mass (BM), the crushed anode and cathode residue of the battery recycling process, can be a valuable secondary source of critical metals, such as Zn, Fe and Mn. In this example, preliminary extraction experiments were performed in H2O and 1 M H2SO4, in order to understand the leaching behaviour of BM in neutral and acidic conditions. The BM was pre-treated and subsequently treated with H2SO4 as the extraction solvent, as described in the methods section. Three time points (2, 4 and 24 h) and two temperatures (30 °C and 80 °C) were tested, while the solid-to-liquid ratio was kept constant at 1 g / 30 mL. The extraction yields are shown in Figure 31. The results demonstrated that the BM material has low solubility in water, with 8 – 10% maximum BM dissolution depending on the process parameters; ICP-MS results demonstrated no significant loss for the metals of interest (i.e. Mn, Zn, Fe), indicating that the soluble fraction is attributed to soluble salts (e.g. KOH residuals of the battery electrolyte) or residual organic matter. The extraction yields were significantly affected by temperature when 1 M H2SO4 was used as the extraction solvent. At 30 °C the extraction yield was 60% in 2 h, rising to 67% in 24 h, while at 80 °C the yield was 69% in 2 h, rising to 91% in 24 h. For all experiments with 1 M H2SO4, ICP-MS revealed no metal selectivity in the leachates. At 30 °C, the solution was saturated at 2 hours, while at 80 °C there was an increase in solubility up until 24 hours, leading to almost complete dissolution of the BM. It is therefore clear that an acidic solvent (here 1 M H2SO4) will achieve high solubilisation yields, without any selectivity. Example 25 – Metal recovery from ‘black mass’ of spent alkaline batteries using protic ionic liquids Next, the inventors investigated whether replacing the solvent with a protic ionic liquid would help achieve the targeted metal selectivity. The inventors tested 30% w / w aqueous solution of betaine hydrochloride ([BET][HCl]) and 30% w / w triethanolammonium hydrogen sulfate ([TEA][HSO4]) aqueous ionic liquid solutions as the extraction solvents, following the method described in the methods section. Triethanolammonium was selected as the cation because a) it is a non-hazardous material and b) it is known to be a chelating amine, forming coordination complexes with several transition metals, so it would be expected to boost the solubility capacity of the solution. [BET][HCl] was selected because it is a non-hazardous, zwitterionic compound with high water solubility. Additionally, together with choline it is a common component of deep eutectic solvents. Total extraction yield (mass loss) was determined using the following equation: The total extraction yields are shown in Figure 31. 30% w / w is close to the limit of aqueous solubility of [BET][HCl] and, although it yielded a significantly less acidic solution compared to 1 M H2SO4 (pH 2 compared to pH near 0), 30% w / w [BET][HCl] demonstrated a very similar leaching profile to 1 M H2SO4. Figure 31 shows that at 30 °C, their extraction yields are almost identical (~65%). At 80 °C [BET][HCl] achieves maximum dissolution of 81% after 24 h, compared to the 91% achieved by 1 M H2SO4. Figure 31 demonstrates that, at 30 °C, the leaching profile of [TEA][HSO4] is also almost identical to that of H2SO4. At 80 C however, [TEA][HSO4] achieves 94% BM dissolution within 2 h, higher than that of H2SO4 at 24 h. The case of [TEA][HSO4] demonstrates that introducing a chelating amine to the extraction system can significantly accelerate the dissolution rates and reduce the reaction time. This is an important finding particularly where complete dissolution of the solid matrix is desirable / required. Example 26 – Metal recovery from ‘black mass’ of spent alkaline batteries using protic ionic liquids: investigating metal selectivity A selection of ionic liquids were tested to compare selectivity towards the extraction of either Mn or Zn. In particular, the inventors investigated extraction yields of Mn and Zn at 30oC for [BET][HCl], [TEA][HSO4] and [MBA][HSO4] extraction solutions with different water contents and extraction times. The results are summarised in table 23. Metal (Mn / Zn) extraction yields were determined using the following equation: !"#$%&'"(#)"*+,%-*!$. =!"#$% / #00%*,%+1"21"%$*31*. !"#$% / #00%*,%*,21"%0+$*. Table 23 Extraction yields of Mn and Zn at 30oC for [BET][HCl], [TEA][HSO4] and [MBA][HSO4] with different water contents and extraction times. Water Content and Time Mn Extraction Zn Extraction Total Ionic Liquid [h] yield yield Extraction yield [%] 30% w / w [BET][HCl] 1 25.3% 70.0% 57.6 30% w / w [BET][HCl] 2 31.1% 97.6% 59.9 30% w / w [BET][HCl] 4 34.2% 79.6% 63.1 10% w / w [BET][HCl] 1 27.6% 71.4% 57.7 10% w / w [BET][HCl] 2 32.9% 69.7% 59.4 10% w / w [BET][HCl] 4 35.1% 82.0% 62.4 60% w / w [TEA][HSO4] 1 38.0% 70.9% 72.2 60% w / w [TEA][HSO4] 2 42.0% 75.5% 76.8 60% w / w [TEA][HSO4] 4 53.8% 77.9% 85.1 30% w / w [TEA][HSO4] 1 40.9% 71.6% 69.1 30% w / w [TEA][HSO4] 2 53.3% 100.0% 78.4 30% w / w [TEA][HSO4] 4 54.7% 100.0% 78.0 10% w / w [TEA][HSO4] 1 40.2% 77.4% 68.4 10% w / w [TEA][HSO4] 2 56.2% 75.2% 75.2 10% w / w [TEA][HSO4] 4 52.9% 78.37% 77.04 60% w / w 1 24.55% 75.72% 55.70 [MBA][HSO4] 60% w / w 2 27.27% 66.11% 54.89 [MBA][HSO4] 60% w / w 4 28.18% 57.93% 54.24 [MBA][HSO4] 30% w / w 1 24.73% 61.78% 52.44 [MBA][HSO4] 30% w / w 2 30.73% 89.18% 59.07 [MBA][HSO4] 30% w / w 4 33.82% 74.28% 62.17 [MBA][HSO4] 10% w / w 1 18.55% 55.29% 45.92 [MBA][HSO4] 10% w / w 2 24.91% 76.92% 54.22 [MBA][HSO4] 10% w / w 4 29.82% 72.12% 58.00 [MBA][HSO4] It is noted that for a number of the solvent systems, extended extraction times appear to reduce the Zn extraction yield. Without wishing to be bound by theory, the inventors hypothesis that with extended reaction times, side-reactions are promoted which result in the re-precipitation of some of the leached material. Example 27 – Metal recovery from ‘black mass’ of spent alkaline batteries: investigating metal selectivity A further selection of ionic liquids and deep eutectic solvents with different polarities and acidities were tested to compare selectivity towards the extraction of either Mn or Zn. In particular, the ionic liquids [BET][HCl], [TEA][HSO4] and [TBA][PTSA]; and the deep eutectic solvent 2:1 (molar ratio) choline chloride: lactic acid (2:1 ChCl:LA), were tested as extraction solvents following the method described in the methods section and using the best extraction parameters determined for [TEA][HSO4] (80 °C for 2 h). The results are shown in Figure 32. The best selectivity identified was seen with 30% water w / w [BET][HCl], which extracted almost 100% of Zn and 41% of Mn (leachate contains Mn: 2.2 ± 0.3 mg / mL and Zn: 4.0 ± 0.1 mg / mL). Using 30% water w / w [TBA][PTSA] (tributylammonium p-toluenesulfonate), the total extraction yield for BM was high (~90%), and no leaching selectivity was observed. These results demonstrated that when an acid is present in the solvent (HCl, H2SO4, PTSA, LA) the selectivity is low; likely because for all the studied solvents, the acids exist either in their neutral form (in [BET][HCl] and 2:1 ChCl:LA) or in an equilibrium between the neutral and ionic form (in protic ionic liquids [TEA][HSO4] and [TBA][PTSA]), which makes the acidic hydrogen readily available to react with the metals present in BM and dissolve them without preference. The presence of the bases, clearly affected the leaching (compared to neat H2SO4 and HCl which almost immediately dissolve more than 90% of the BM), but their effect was not sufficient to compensate for the acid effect. Example 28 – Metal recovery from ‘black mass’ of spent alkaline batteries using deep eutectic solvents Next, the inventors investigated use of deep eutectic solvents which do not involve a mineral or organic acid, as the extraction solvent. In particular, the inventors tested 5:4:1 (molar ratio) ethylene glycol: urea: ammonium chloride (5:4:1 EtGly:Ur:NH4Cl), following the method described in the methods section. The extraction results are shown in Figure 32. 5:4:1 EtGly:Ur:NH4Cl shows ~32% extraction yield, leaching 71% of the original Zn, while only 5% of the Mn from BM (leachate contains 3.0 ± 0.2 mg / mL Zn, 0.27 ± 0.05 mg / mL Mn and 0.12 ± 0.02 mg / mL Fe). The selectivity of 5:4:1 EtGly:Ur:NH4Cl towards Zn is a significant finding, as it can provide a starting point for the design of task-specific ionic liquids and deep eutectic solvents for the targeted leaching and separation of Zn from polymetallic waste. The inventors further modified the solvent system, replacing ethylene glycol with glycerol to create 5:4:1 glycerol : urea : ammonium chloride (5:4:1 Gly:Ur:NH4Cl). However, this lead to a dramatically reduced extraction yield (~10%) and a loss of Zn selectivity. This demonstrated that ethylene glycol was crucial component for the Zn extraction selectivity. Example 29 – Selective metal recovery from ‘black mass’ of spent alkaline batteries using multiple extraction steps The inventors created a two-step process for the selective extraction and separation of Mn and Zn from BM. The two-step process involved initial treatment of BM with 5:4:1 EtGly:Ur:NH4Cl, followed by treatment of the residual solid matter with 30% water w / w [TEA][HSO4] or Choline Chloride: Lactic Acid (2:1). In the first step of selective extraction, 5.62% of Mn, 71.42% of Zn, and 20% of Fe are extracted. The first leachate, S1 is, rich in Zn (3.0 ± 0.2 mg / mL Zn, 0.27 ± 0.05 mg / mL Mn and 0.12 ± 0.02 mg / mL Fe). S2 - Choline Chloride: Lactic Acid (2:1) If Choline Chloride: Lactic Acid (2:1) is used in the second step, an additional 21.59% of Zn, 75.96% of Mn, and 45.20% of Fe can be extracted. Through the two-step extraction process, the total extraction rates are 93.01% for Zn, 81.58% for Mn, and 61.20% for Fe. S2 - 30% water w / w [TEA][HSO4] Alternatively, if 30% water w / w [TEA][HSO4] is used in the second step, an additional 29.37% of Zn, 73.33% of Mn, and 78.50% of Fe can be extracted. With the two-step extraction, Zn is fully extracted, Mn reaches an extraction rate of 78.95%, and Fe is extracted at 98.50%. Using 30% w / w [TEA][HSO4], the second leachate, S2, is rich in Mn (1.2 ± 0.1 mg / mL Zn, 4.0 ± 0.3 mg / mL Mn and 0.40 ± 0.04 mg / mL Fe). The two step extraction process using 30% w / w [TEA][HSO4] is outlined in Figure 33. Further treatment of S1 and S2 (e.g. via liquid-liquid extraction, selective precipitation, or electrochemical deposition) enables further purification of these leachates. Example 30 – Precipitation of elective metal recovery from ‘black mass’ of spent alkaline batteries using multiple extraction steps Following the two step extraction processes, described in example 29, the leachates S1 and S2 were further treated using selective precipitation, as outlined in Figure 34. Fe and Mn were present as impurities in Zn leachate S1 containing Mn(0.3g / L), Zn(2.88g / L), and Fe(0.1g / L). Precipitation was performed following the method described in the methods section. In the final step, when the pH increased from 9 to 13, the final precipitate achieved a zinc purity of 92.44%, with an overall recovery rate of 43.96%. Conversely, when the pH increased from 10 to 13, the final precipitate's zinc purity reached 98.75%, but the overall recovery rate dropped to 20.66%. During the precipitation process, if 3M NaOH is directly added to Leachate S1 until the pH reaches 13, a large amount of ammonia gas is released. Additionally, both the yield and purity of Zn are significantly reduced. This phenomenon is attributed to the influence of the DES. S2 - Choline Chloride: Lactic Acid (2:1) Mn leachate S2 containing Mn(4.18g / L), Zn(0.90g / L), and Fe(0.23g / L). At pH 13.00, 34.78% Fe, 93.66% Mn, and 21.11% Zn were precipitated. The final precipitate achieved a Mn purity of 85.67%, with an overall recovery rate of 71.18%. S2 - 30% water w / w [TEA][HSO4] Mn leachate S2 contained Mn(4.03g / L), Zn(1.22g / L), and Fe(0.39g / L). At pH 13.00, 46.92% Fe, 83.80% Mn, and 14.18% Zn were precipitated. The final precipitate achieved a Mn purity of 83.29%, with an overall recovery rate of 61.40%. Compared to 30% water w / w [TEA][HSO4], Choline Chloride: Lactic Acid (2:1) demonstrated better results in terms of both purity and recovery rate of Mn during the precipitation process. Materials Name Abbreviation Obtained from Sulfuric acid H2SO4 Sigma-Aldrich Hydrochloric acid HCl Sigma-Aldrich Triethylamine Et3N Sigma-Aldrich Triethylammonium hydrogen [Et3NH][HSO4] Direct synthesis by sulfate mixing triethylamine and sulfuric acid (Sigma- Aldrich) Ethylene glycol EtGly Sigma-Aldrich Triethanolammonium [TEA][HSO4] or TEAHSO4 Direct synthesis by hydrogensulfate mixing Triethanolamine and sulfuric acid (Sigma- Aldrich) Triethanolammonium chloride[TEA]Cl or TEAHClDirect synthesis by mixing Triethanolamine and hydrochloric acid (Sigma-Aldrich) Triethanolamine TEA Sigma-Aldrich N,N-Dimethylcyclohexylamine DCA Sigma-Aldrich N,N- [DCA][HSO4] Direct synthesis by Dimethylcyclohexylammonium mixing N,N- hydrogen sulfate Dimethylcyclohexylamine and sulfuric acid (Sigma- Aldrich) Betaine BET Sigma-Aldrich Betaine hydrochloride [BET][HCl] or BETHCl Sigma-Aldrich Sodium Hydroxide NaOH Sigma-Aldrich Glycerine Gly Sigma-Aldrich Calcium carbonate CaCO3 Sigma-Aldrich Calcium hydroxide Ca(OH)2 Sigma-Aldrich Magnesium carbonate MgCO3 Sigma-Aldrich DL- α-methylbenzylamine MBA Sigma-Aldrich DL- α-methylbenzylammonium [MBA][HSO4] Direct synthesis by hydrogen sulfate mixing DL- α- methylbenzylamine and sulfuric acid (Sigma- Aldrich) tributylamine TBA Sigma-Aldrich tributylammonium p- [TBA][PTSA] Direct synthesis by toluenesulfonate. mixing tributylamine and p-toluenesulfonic. acid (Sigma-Aldrich) ILs were prepared by mixing equimolar quantities for the acid and the amine under agitation and cooling. Then water content was adjusted and confirmed by Karl Fisher Titration. Black mass The black mass was obtained from a UK based alkaline battery recycler. According to ICP-MS, the metal elements content (mass content) of the black mass were as follows: Mn: 33.05%±1.26% Fe: 2.81%±0.35% Zn: 24.96%±0.40% Methodology Experimental Setup for gas production experiments The reactions were conducted in a round bottom flask with heating jacket and magnetic stirrer where the waste material substrates or metal are immersed in the ionic liquid or acid medium. Released gases travel through the tubing and into a scrubber bottle filled with water. The pressure of the gases pushes water up the scrubber tube and into a measuring cylinder. The measuring cylinder in turn is placed on a pressure transmitter, recording an electric signal as a function of the hydrostatic pressure in the cylinder. The water volume is measured and subsequently the gas volume is determined. From pressure increase readings in the system, it was established that 17 mL of hydrogen were required to be produced before water displacement occurred. Calibration of gas production measurement The volume of gas produced during substrate / ionic liquid reactions was calculated by the water displacement method, where the gas displaces water in a scrubber bottle, pushing it into a measuring cylinder. The measuring cylinder in turn was placed on a pressure transmitter, which continuously recorded the hydrostatic pressure, which was proportional to the height of the water inside the cylinder. The volume of water was thus obtained. For this calibration, known amounts of water were added to the cylinder and the reading of the pressure transmitter recorded (in mA). The current increased linearly with the increase of the water volume, and the formula for calculating the volume of water was obtained via linear regression. In a second step, the system was calibrated for gas production. Some initial gas volume was required to increase the pressure in the tubing to push the water column. Known amounts of pure zinc were added into concentrated sulfuric acid in the round bottom flask, resulting in the production of predictable quantities of hydrogen. The amount of zinc was subsequently plotted against the amount of gas (mL) formed (as measured by the above method) and the following calibration equation obtained via linear regression: !" = # $ !"% + &!": The total volume of gas produced; !"%: The volume of H2O collected with the cylinder; β: 1.098±0.003 &: The volume of hydrogen consumed by rising pressure in the equipment used, 17.008±0.549 mL. Analysis of gas composition To analyse the composition of the produced gases in the process using slag materials as substrates, a set-up was used where the gas produced is analysed by mass spectrometry (MS). The experimental set up for gas evolution experiments coupled to mass spectrometry is shown in Figure 28. The following method was used: 12 g of the substrate was reacted with 200 mL of 14% BET[Cl] at 80°C in a three-neck flask (see Figure 28). Nitrogen (180 mL per minute) was used as a sweep for the product to be sampled by a Mass Spectrometer, with the exhaust being vented. XRF The waste slag samples were analysed by X-ray fluorescence spectroscopy (XRF, PANalytical Epsilon 003) for metal composition. XRD XRD patterns were obtained using a PaNalytical Pro X'Pert Pro X-ray diffractometer with Cu Kα irradiation (40 kV and 20 mA). Waste slag samples were directly transferred onto a metal holder and irradiated at 2θ angles ranging from 20° to 90° with a step size of 0.03 and a scan speed of 0.01 - 0.08 steps / s. Microwave Digestion The waste slag samples were digested in the MARS 6 microwave with standard IR and iWave. Samples of 10–20 mg were digested with 3 mL of HCl and 10 mL of HNO3. The temperature was ramped to 175°C in 5 min and held at 175°C for 5 min for the digestion. ICP-MS The concentrations of the following elements in liquid samples were determined using an Agilent 7900 ICP-MS (Agilent Technologies): Lithium (Li), Boron (B), Sodium (Na), Magnesium (Mg), Aluminium (Al), Potassium (K), Calcium (Ca), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Gallium (Ga), Barium (Ba), Lead (Pb) and Cadmium (Cd) . All samples were prepared by dilution with 2% HNO3 solution. Particle Size Analysis The particle size of the samples was determined using an Anton Paar PSA 1190 device. BET Surface Area The Brunauer–Emmett–Teller (BET) technique was used to estimate the total surface area of the solid samples. Black Mass Pre-treatment The black mass (BM) used was the crashed anode and cathode residue from alkaline batteries. The BM looked highly heterogeneous upon receiving. The powder was dried for 24 h at 120 °C to remove excess moisture and the powder was crushed using a mortar and pestle. To ensure particle homogeneity, two sieves were used, discarding the fractions outside the range of 300 nm to 1800 nm. The BM samples were stored in a sealed glass vessel. Before each extraction experiment, the moisture of the BM powder was checked, as described below, and if the moisture was beyond 3% w / w, the sample was dried again. Extraction of metals from Black Mass For the extraction experiments, 1 g of the dried and sieved BM was mixed in a 50 mL centrifuge tube with 30 mL of the extraction solvent. The system was mixed using a vortex mixer, in order to ensure powder dispersion in the solvent. Then the falcons are left stirring in an incubator at the desired temperature, while mixing at 400 rpm. In order to exclude the effects of heating time, the solvent was pre-heated to the desired temperature before mixing with the BM powder. All experiments were performed in duplicate. Upon completion of the extraction experiment, the samples were centrifuged in a GT2R Expert centrifuge by Fisher Scientific, at 4000 rpm for 15 min, in order to separate the solids. The solids were washed twice with de-ionised water and once with ethanol and then dried at 120 °C overnight. Sample Digestion for ICP-MS Solid samples were digested in a MARS 6 microwave digestion oven with standard IR and iWave. Samples of 10–20 mg were digested with 3 mL of HCL and 10 mL of HNO3. Ramping to 175 °C in 5 minutes and holding at 175 °C for 15 minutes. Liquid samples (leachates) were diluted in 2% HNO3 (100 μL in 10 mL) and were measured without further treatment. Moisture Content Determination For moisture content determination, 150 to 300 mg of target material was placed in a weighing bowl and placed in an oven heated at 120 °C for 8 to 10 h. Then the dried material was weighed again and the moisture content is calculated using the following equation: S1 Leachate Precipitation At lab scale, precipitation was performed in a 250ml beaker under magnetic agitation. Precipitation agent (3M NaOH) was added to 20ml of leachate S1 at pH from 8.0 to 10.0 at 50 °C, with a pH interval of 1.0. The solution was settled for 12h at 50 °C. After decantation, 5ml of supernatant were taken and analysed. The most suitable conditions were obtained at pH 9 and pH 10 with the addition of a 3M NaOH. At pH 9.00, 60.00% Fe, 44.00% Mn, and 26.18% Zn were precipitated; at PH 10, 74.00% Fe, 81.33% Mn, and 14.58% Zn were precipitated. The precipitate was then separated, and 3M NaOH was further added to the supernatant until the pH reached 13 at 50 °C. This process resulted in the formation of the final precipitate. S2 Leachate Precipitation - 30% water w / w [TEA][HSO4] used as second extraction liquid At lab scale, precipitation was performed in a 250ml beaker under magnetic agitation. Precipitation agent (3M NaOH) was added to 20ml of leachate S2 at pH 13. The solution was settled for 12h. After decantation, 5 ml of supernatant were taken and analysed. S2 Leachate Precipitation - Choline Chloride: Lactic Acid (2:1)used as second extraction liquid At lab scale, precipitation was performed in a 250ml beaker under magnetic agitation. Precipitation agents(3M NaOH) was added to 20ml of leachate S1 at pH 13. The solution was settled for 12h. After decantation, 5ml of supernatant were taken and analysed. Instruments Item Supplier Date Details TC-08 Single-Channel Pico N / A Measure voltages from 0 to 5 V or Terminal Board Technology 4-20mA loop currents USB TC-08 Pico N / A Measure voltages from 0 to 5 V or Thermocouple Data Technology 4-20mA loop currents Logger Pressure Transmitter RS PRO N / A Measure voltages from 0 to 5 V or (0-50 mBar) 4-20mA loop currents and pressure from 0-50 mBar Pressure Transmitter RS PRO N / A Measure voltages from 0 to 5 V or (0-10 Bar) 4-20mA loop currents and pressure from 0-10 Bar

Claims

Claims 1. A method of recovering a target metal, a compound comprising the target metal or a salt thereof, from a composition comprising the target metal or a first oxidised form thereof, wherein the method comprises: - contacting the composition and an extraction liquid to extract the target metal or the first oxidised form thereof from the composition and to thereby produce a first solution, wherein the extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the first solution comprises a second oxidised form of the target metal dissolved therein; and - conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal.

2. The method according to claim 1, wherein the target metal, or the first oxidised form thereof, is water insoluble or sparingly soluble in water.

3. The method according to any preceding claim, wherein the target metal or the first oxidised form thereof present in the composition comprises or consists of copper, magnesium, zinc, lead, calcium, iron, neodymium, praseodymium, a first oxidised form of any of the previously mentioned metals or a combination thereof.

4. The method according to any preceding claim, wherein the composition comprises one or more further metals and / or a first oxidised form thereof, and the method comprises preferentially extracting the target metal, the compound comprising the target metal or the first oxidised form thereof.

5. The method according to claim 4, wherein the further metal or the first oxidised form thereof comprises or is iron, lead, calcium, zinc, manganese or magnesium, or a first oxidised form thereof.

6. The method according to any preceding claim, wherein the ionic liquid and / or DES comprises the cation:wherein each of R1to R6is independently H, an optionally substituted C1-24 alkyl, an optionally substituted C2-24 alkenyl, an optionally substituted C2-24 alkynyl, an optionally substituted C6-12 aryl or an optionally substituted C3-6 cycloalkyl.

7. The method according to any preceding claim, wherein the ionic liquid and / orDES comprises the anionF-, Cl-, Br-, I- orwherein R17is H, an optionally substituted C1-12 alkyl, an optionally substituted C2-12 alkenyl, an optionally substituted C2-12 alkynyl, an optionally substituted C3-6 cycloalkyl, an optionally substituted C6-12 aryl, -OR15, -SR15, -CN, -NR15R16, -SO3R15, - OSO3R15, -COR15, -COOR15or –NO2.

8. The method according to any preceding claim, wherein the extraction liquid comprises or consists of an ionic liquid.

9. The method according to claim 8, wherein the ionic liquid is a protic ionic liquid.

10. The method according to claim 9, wherein the ionic liquid is [TEA][HSO4], [Et3N][HSO4], [TEA]Cl, [DCA][HSO4], [BET][HCl], [TBA][PTSA] or [MBA][HSO4].

11. The method according to any preceding claim, wherein the extraction liquid comprises or consists of a DES.

12. The method according to claim 11, wherein the DES comprises: (i) an alcohol, a diol, a polyol, an ether substituted with one or more OH groups or a polyether substituted with one or more OH groups; and (ii) a compound of formula (I):wherein X1is O, S or NR28; R25is a C1-6 alkyl, a C2-6 alkenyl, a C2-6 alkynyl or NR29R30; R26to R30are each independently H, a C1-6 alkyl, a C2-6 alkenyl or a C2-6 alkynyl; or a salt or complex thereof; (iii) a salt comprising a cation and an anion.

13. The method according to any preceding claim, wherein the extraction liquid comprises a further solvent in addition to the ionic liquid and / or the DES.

14. The method according to claim 13, wherein the further solvent is water.

15. The method according to claim 14, wherein the ionic liquid is a protic ionic liquid and (i) contacting the composition and the extraction liquid, and / or (ii) conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal, results in the de-protonation of the IL cation to provide a Brønsted base.

16. The method according to claim 15, wherein the Brønsted base is insoluble or sparingly soluble in water.

17. The method according to claim 15 or claim 16, wherein the method comprises regenerating the IL, and regenerating the IL comprises contacting the Brønsted base with an acid.

18. The method according to any preceding claim, wherein contacting the extraction liquid and the composition causes at least 5 wt%, at least 10 wt%, at least 20 wt%, at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, at least 80 wt% or at least 90 wt% of the target metal or the first oxidised form thereof present in the composition to be extracted therefrom.

19. The method according to any preceding claim, wherein contacting the composition and the extraction liquid causes one or more gas-generating chemical reactions to occur; and the method comprises collecting the gas which is generated.

20. The method according to claim 19, wherein, the gas is hydrogen gas (H2), nitrogen gas (N2), carbon dioxide (CO2), carbon monoxide (CO) or a combination thereof.

21. The method according to any preceding claim, wherein the composition comprising the metal source and the ionic liquid are contacted for between 5 minutes and 7 days, between 10 minutes and 48 hours, between 15 minutes and 24 hours, between 30 minutes and 12 hours, between 45 minutes and 6 hours or between 1 and 3 hours.

22. The method according to any preceding claim, wherein the composition and the extraction liquid are contacted at a temperature of between 20°C and 200°C, between 25°C and 150°C, between 30°C and 100°C, between 45°C and 55°C or between 48°C and 52°C.

23. The method according to any preceding claim, wherein the composition comprises one or more further metals and / or a first oxidised form thereof and contacting the composition and the extraction liquid selectively extracts the target metal from the composition.

24. The method according to claim 23, wherein prior to conducting the reaction to cause the second oxidised form of the target metal dissolved in the extraction liquid to be converted to the target metal, the compound comprising the target metal or a solid salt of the target metal, the method comprises separating the first solution from residual solid material, wherein the residual solid material comprises the one or more further metals and / or the first oxidised form thereof.

25. The method according to claim 24, wherein subsequent to separating the first solution from residual solid material, the method comprises: - contacting the residual solid material and a second extraction liquid to extract the further target metal or the first oxidised form thereof from the residual solid material and to thereby produce a third solution, wherein the second extraction liquid comprises or consists of an ionic liquid and / or a deep eutecticsolvent (DES) and the third solution comprises a second oxidised form of the target metal dissolved therein; and - conducting a reaction to cause the second oxidised form of the further target metal to be converted to the further target metal, the compound comprising the further target metal or a solid salt of the further target metal.

26. The method according to any preceding claim, wherein conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal comprises conducting a precipitation reaction on the first solution, and wherein conducting a precipitation reaction on the first solution comprises contacting the first solution and a precipitating agent.

27. The method according to any of claims 1 to 25, wherein conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal comprises conducting an electrochemical reaction on the first solution, and wherein conducting an electrochemical reaction on the first solution comprises: - providing a first electrode and a spaced apart second electrode; - at least partially disposing the first and second electrodes in an electrolyte, wherein the electrolyte comprises or consists of the first solution; and - applying a voltage across the first and second electrodes, to thereby cause the target metal, the compound comprising the target metal or a salt of the target metal to be deposited on the first electrode.

28. The method according to any preceding claim, wherein conducting a reaction to cause the second oxidised form of the target metal to be converted to the target metal, the compound comprising the target metal or the solid salt of the target metal, provides the target metal, the compound comprising the target metal or the solid salt of the target metal and a second solution, wherein the second solution comprises an oxidised form of a second target metal dissolved therein.

29. The method according to claim 28, wherein the method further comprises conducting a reaction on the second solution to cause the oxidised form of the second target metal to be converted to the second target metal, a compound comprising the second target metal or a solid salt of the second target metal, and to provide a further solution.

30. The method according to claim 29, wherein conducting a reaction on the second solution to cause the oxidised form of the second target metal to be converted to the second target metal, the compound comprising the second target metal or the solid salt of the second target metal comprises conducting a precipitation reaction or an electrochemical reaction.

31. A method of selectively extracting first and second target metals from a polymetallic composition comprising the first target metal or a first oxidised form thereof and the second target metal or a first oxidised form thereof, the method comprising: - contacting the composition and a first extraction liquid to selectively extract the first target metal or the first oxidised form thereof from the composition and to thereby produce a first solution and a residual solid material, wherein the first extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and first solution comprises a second oxidised form of the first target metal dissolved therein and the residual solid material comprises the second target metal or a first oxidised form thereof; - separating the first solution from the residual solid material; and - contacting the residual solid material and a second extraction liquid to extract the second target metal or the first oxidised form thereof from the residual solid material and to thereby produce a third solution, wherein the second extraction liquid comprises or consists of an ionic liquid and / or a deep eutectic solvent (DES) and the third solution comprises a second oxidised form of the second target metal dissolved therein.

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