GB2642033A

GB2642033A - Gallium separation process

Info

Publication number: GB2642033A
Application number: GB2408801.5A
Authority: GB
Inventors: Murray Beth; Byrne Emily; Holbrey John; Swadzba-Kwasny Malgorzata
Original assignee: Queens University of Belfast
Current assignee: Queens University of Belfast
Priority date: 2024-06-19
Filing date: 2024-06-19
Publication date: 2025-12-31
Also published as: GB202408801D0; WO2025262425A1

Abstract

A process for the separation of gallium from a mixture of metals, the process comprising: providing an aqueous acidic solution comprising a mixture of metals including gallium, contacting said solution with an extractant composition comprising a deep eutectic solvent system, and obtaining a gallium enriched product from the first phase, wherein the deep eutectic solvent system comprises: a trialkylphosphine oxide of formula R3PO where each R = C6+ alkyl; a hydrogen bond donor comprising a carboxylic acid of formula Z-L-COOH wherein Z is a -COOH or -C(O)CH3 and L is a C1-4 alkyl linker; and a reducing agent to provide a first phase into which gallium is extracted. The hydrogen bond donor may comprise malonic acid. The reducing agent may be a polyol. An extractant composition for the separation of gallium from an aqueous acidic zinc leachate solution, comprising a deep eutectic solvent system which comprises a trialkylphosphine oxide of formula R3PO where each R = C6+ alkyl, and ascorbic or erythorbic acid. A further extractant composition for the separation of gallium from an aqueous acidic zinc leachate solution and a process for the separation of gallium from a mixture of metals are further defined.

Description

[0001] GALLIUM SEPARATION PROCESS

[0002] The present invention relates to the separation of gallium from other metals. In particular, the present invention relates to the separation of gallium from a mixture of metals using a deep eutectic solvent system.

[0003] Background

[0004] Gallium is a metallic element that serves as a key component in several compounds that are used in advanced electronic devices. The rapid growth of this technology sector and its substantial impact on the modern economy have generated an increased need for gallium. This surge in demand has made gallium a critical raw material.

[0005] Gallium is typically obtained as a product of the aluminium mining industry, however in areas with relatively low abundance of aluminium this can lead to a lack of supply chain resilience and security. Gallium can also be found in zinc mining and processing residues, for example in acidic leachate solutions from the hydrometallurgical processing of zinc minerals. However, such solutions can contain only low concentrations of gallium of up to 0.3%, together with zinc and other metals.

[0006] Processes for extracting gallium from zinc residues have typically involved complex multistep processes involving high energy demands for elevated temperatures, and/or the use of toxic or polluting materials presenting environmental and safety concerns.

[0007] There is therefore a need for improved processes for selectively separating gallium from other metals

[0008] Summary

[0009] An aspect of the present invention provides a process for the separation of gallium from a mixture of metals, wherein the process comprises: providing an aqueous acidic solution comprising a mixture of metals including gallium; contacting said aqueous acidic solution with an extractant composition comprising a deep eutectic solvent system, the deep eutectic solvent system comprising: i) a trialkylphosphine oxide of formula R3P0, where -2 -each R group is independently selected from C6+ alkyl; ii) a hydrogen bond donor comprising a carboxylic acid of formula Z-L-COOH, wherein Z is -COOH or -C(0)CH3, and L is a Ci4 alkyl linker; and iii) a reducing agent, to provide a first phase into which gallium is extracted; and obtaining a gallium enriched product from the first phase.

[0010] It has been surprisingly found that by the use of a deep eutectic solvent system comprising a trialkylphosphine oxide and a hydrogen bond donor together with a reducing agent, it is possible to obtain efficient and selective extraction of gallium from an aqueous acidic solution, such as a zinc leachate solution.

[0011] A deep eutectic solvent system is used in the present process. Deep eutectic solvents are known in the art and it will understood that the term deep eutectic solvent as referred to herein is used to mean a eutectic mixture of two or more substances, in which the melting point of the mixture is lower than that of the individual substances that form the mixture. A deep eutectic solvent may for example have a melting point of less than 100°C, for example less than 60°C such as less than 30°C. A deep eutectic solvent may for example be liquid at room temperature (around 20°C to 30°C). A deep eutectic solvent system as referred to herein will be understood to refer to a solvent system comprising a deep eutectic solvent, optionally together with other substances that may form, but will not necessarily form, a part of the eutectic mixture. A deep eutectic solvent system may, for example, comprise a mixture of two substances that form a deep eutectic solvent, with a third substance that may itself also form a part of the deep eutectic solvent in a tri-component mixture, for example as a further hydrogen bond donor.

[0012] The deep eutectic solvent system comprises a trialkylphosphine oxide of formula R3P0, where each R group is independently selected from C6+ alkyl, preferably C8+ alkyl. Preferably, the trialkylphosphine oxide comprises 3 linear alkyl groups, preferably 3 linear Cs+ alkyl groups. More preferably, the trialkylphosphine oxide is trioctylphosphine oxide.

[0013] As referred to herein, the term "alkyl" refers to a linear or branched-chain alkyl groups and encompasses unsaturated moieties such as alkenyl and alkynyl groups. Preferably where an alkyl group is referred to, the alkyl group is a saturated hydrocarbon chain. Alkyl groups may be substituted or unsubstituted, preferably unsubstituted. The term "substituted" as -3 -used herein, in the context of a chemical structure describes a group being bonded to any other atom or functional group other than hydrogen. The term "unsubstituted" as used herein, in the context of a chemical structure describes a group being bonded to no other atom or functional group except hydrogen.

[0014] The deep eutectic solvent system also comprises a hydrogen bond donor comprising a carboxylic acid of formula Z-L-COOH, wherein Z is -COOH or -C(0)CH3, and L is a C14 alkyl linker. The alkyl linker may be substituted or unsubstituted and may be branched or linear, preferably the alkyl linker is unsubstituted and linear. The hydrogen bond donor may for example comprise malonic acid, levulinic acid or succinic acid. Most preferably, the hydrogen bond donor comprises or consists essentially of malonic acid.

[0015] It will be appreciated that the trialkylphosphine oxide and the hydrogen bond donor will be mixed in a ratio so as to provide a liquid composition under the reaction conditions. The trialkylphosphine oxide and the hydrogen bond donor are therefore suitably mixed in a ratio so as to provide a deep eutectic solvent. It will be appreciated that the optimal ratio may vary depending on the specific compounds being used. The ratio of the trialkylphosphine oxide to the hydrogen bond donor is preferably from 0.5:0.5 to 0.75:0.25, preferably from 0.55:0.45 to 0.7:0.3, for example from 0.6:0.4 to 0.7:0.3. Such ratios have been found to provide a suitable liquid mixture, in particular for a hydrogen bond donor such as malonic acid and succinic acid. Where the hydrogen bond donor is levulinic acid, for example, the ratio of the trialkylphosphine oxide to the levulinic acid is preferably from 0.3:0.7 to 0.5:0.5, preferably around 0.5:0.5. It will be appreciated that the ratio referred to is a molar ratio of the trialkylphosphine oxide to the hydrogen bond donor.

[0016] The deep eutectic solvent system also comprises a reducing agent. Without wishing to be bound by any particular theory, it is believed that the addition of a reducing agent leads to reduction of metal species in the system, this can alter the metal ions' properties such as ionic radius, which affects coordination properties and can aid in the separation of gallium from other metals in the system. In particular, this aids in separation of gallium from copper, usually present in the feedstock as Cu2+, which is chemically similar and difficult to separate from Ga3+. The inclusion of a reducing agent in the deep eutectic solvent system -4 -has been found to provide a certain synergy for the separation of gallium in combination with the trialkylphosphine oxide and hydrogen bond donor used in the present process The reducing agent may be any suitable reducing agent. Preferably, the reducing agent is an organic reducing agent, more preferably a polyol. A polyol as referred to herein will be understood to refer to an organic compound comprising two or more hydroxyl groups. Where the reducing agent is a polyol this has the additional benefit that it may act as a further hydrogen bond donor in the deep eutectic solvent system, forming a tri-component eutectic mixture with the trialkylphosphine oxide and the hydrogen bond donor. Preferably the polyol is a C2 to Cio polyol, for example a C4 to Cs polyol. The polyol preferably comprises 2 to 4 hydroxyl groups, preferably 4. The polyol may be formed from only carbon, hydrogen and oxygen. Preferably, the polyol is a heterocyclic polyol. Most preferably, the reducing agent is ascorbic acid or erythorbic acid. It will be appreciated that ascorbic acid or erythorbic acid are stereoisomers and are believed to be chemically interchangeable for the purposes of the present process. Preferably, the reducing agent is ascorbic acid.

[0017] It is a particular advantage that the present process may be performed using naturally occurring, non-toxic, organic compounds including carboxylic acids such as malonic acid and organic polyols such as ascorbic acid. In addition to the use of a deep eutectic solvent system that can be used without organic solvents, this can minimise safety hazards and environmental risks associated with conventional metal extractions.

[0018] The reducing agent may be present in the extractant composition at any suitable level.

[0019] Preferably, the reducing agent is present in the extractant composition at a concentration of at least 0.05M, preferably at least 0.1M, for example from 0.05M to 0.5M, such as from 0.1M to 0.3M.

[0020] The extractant composition may further comprise water, preferably in an amount of up to 5 wt.%, preferably up to 3 wt.%, for example water may be added to the extractant composition in an amount of from 0.5 wt.% to 5 wt.%, for example from 1 wt.% to 3 wt.%. It will be appreciated that this refers to the total water content of the extractant composition, and while some components added to the mixture may comprise very small amounts of -5 -water, the total water content described previously may be equivalent to an amount of water added to the extractant composition, together with the trialkylphosphine oxide, the hydrogen bond donor and the reducing agent.

[0021] The extractant composition may therefore comprise a deep eutectic solvent system comprising the trialkylphosphine oxide and the hydrogen bond donor, the reducing agent (which may or may not form a part of the deep eutectic solvent system) and water. The extractant composition may therefore comprise or consist essentially of the trialkylphosphine oxide, the hydrogen bond donor, the reducing agent and water.

[0022] The aqueous acidic solution suitably comprises an acidic solution comprising a mixture of metals including gallium. Acids are typically used for dissolving metal residues in ores or recycled materials such as LEDs and the aqueous acidic solution may for example comprise a zinc leachate solution, which is a by-product from hydrometallurgical zinc processing. The acidic solution may be any suitable acidic solution, and it will be appreciated that the nature of the aqueous acidic solution will vary depending on its source. Preferably, the aqueous acidic solution comprises a solution of the mixture of metals in HCI, preferably HCI at a concentration of at least 3M, preferably at least 4M, more preferably at least 5M, for example approximately 6M. In some examples, it has been found that gallium extraction is improved as the concentration of the acid increase up to around 4M HCI, above which extraction efficiency remains high, 6M HCI being found to provide optimal concentration. The extractant composition may be contacted with an aqueous acid to pre-condition the extractant composition before contacting with the aqueous acidic solution comprising the mixture of metals, for example an acid corresponding in nature and/or strength to the acidic solution comprising the mixture of metals. Such pre-conditioning of the extractant composition may be performed for 10 minutes or more, for example 20 minutes or more, such as about 30 minutes.

[0023] The mixture of metals may be any suitable mixture and it will be appreciated that the present process is surprisingly effective for the extraction of gallium, regardless of the other metals that are present. The present process has been found to be particularly effective where the aqueous acidic solution comprises zinc, copper and gallium, and optionally further comprises iron, aluminium and indium. For example, where the aqueous acidic -6 -solution is a zinc leachate solution, it may comprise gallium in a mixture of metals comprising some or all of zinc, copper, iron, aluminium and indium. The concentration of gallium is not particularly limited, however the present process may be advantageously used where gallium is only present at low concentrations, such as in zinc leachate solutions. For example, in some preferred embodiments, gallium is present in the aqueous acidic solution at an amount of 1 wt.% or less, preferably 0.5 wt.% or less, for example 0.3 wt.% or less or 0.1 wt.% or less, or as low as 0.01 wt.% or less. The other metals may, for example, be present in an amount of 1 wt.% or higher in the aqueous acidic solution, for example 2 wt.% or higher, for example up to 3 wt.% or up to 5 wt.%.

[0024] The step of contacting the aqueous acidic solution with the extractant composition may be performed in any suitable way and using any suitable apparatus. Such methods and apparatus for contacting a biphasic mixture to extract substances between phases are known to the skilled person. For example, the aqueous acidic solution and the extractant composition may be mixed using an agitator or stirrer. The mixing apparatus may comprise equipment specifically designed for multi-phase mixing such as high shear mixing apparatus.

[0025] The process may comprise contacting the aqueous acidic solution with the extractant composition for any suitable length of time and it will be appreciated that the time may be varied depending on the speed of extraction. In preferred embodiments, the process comprises contacting the aqueous acidic solution with the extractant composition for 30 minutes or less, preferably 20 minutes or less, more preferably 10 minutes or less. It has been found that the extraction is typically at equilibrium within 10 minutes of contacting the aqueous acidic solution with the extractant composition.

[0026] Suitably, following the step of contacting the aqueous acidic solution with the extractant composition, the first phase into which gallium is extracted is a first non-aqueous phase, and the process comprises the step of separating the first non-aqueous phase from a first 30 aqueous phase.

[0027] The separation of the aqueous and non-aqueous phases may be performed by any suitable method, for example at a small scale by use of apparatus such as a separating -7 -funnel or Craig apparatus. It will be appreciated that the phases will suitably be allowed to settle prior to separation, for example under gravity or preferably accelerated by the use of additional equipment such as centrifuge. Alternatively, aqueous and non-aqueous phases may be separated by the use of apparatus which both contacts and separates the phases, for example a centrifugal extractor, a pulsed column, or a combined mixer-settler.

[0028] As will be appreciated, an amount of gallium may remain in the first aqueous phase following contacting and separation. Therefore, in some embodiments the first aqueous phase is recycled to provide at least a portion of the aqueous acidic solution. This may form a recycle that provides a part of the aqueous acidic solution in the same process or, alternatively, the first aqueous phase may be separated and contacted separately with a further extractant composition as defined herein to recover additional gallium from the first aqueous phase.

[0029] Obtaining a gallium enriched product from the first phase into which gallium is extracted may be performed in any suitable way. Preferably, the process further comprises contacting the first phase into which gallium is extracted, for example the first non-aqueous phase, with an aqueous stripping solution. It has been found that an extractant composition as described herein is particularly suited to permit the selective stripping of gallium. For example, the present process may not only provide effective extraction from the aqueous acidic solution, but it has also been found that use of an extractant composition as defined herein can enable improved selectivity for gallium extraction by permitting selective stripping of gallium from the first phase with an aqueous stripping solution. Preferably, the aqueous stripping solution is an alkaline stripping solution. The process suitably therefore may comprise contacting the first phase into which gallium is extracted with an aqueous stripping solution, to form a second aqueous phase into which gallium is extracted and a second non-aqueous phase.

[0030] The aqueous stripping solution is preferably an aqueous ammonia solution, preferably an aqueous ammonia solution comprising ammonia in a concentration of greater than 0.1M and less than 2M, more preferably from 0.5M to 1.8M, for example from 0.7M to 1.5M, such as from 0.8M to 1.2M. It has been found that an aqueous ammonia solution is particularly suited to efficiently and selectively stripping gallium from the first phase. -8 -

[0031] The step of contacting the first phase into which gallium is extracted with an aqueous stripping solution may be performed for any suitable length of time, for example for 10 minutes or more, for 20 minutes or more, such as about 30 minutes.

[0032] The process may further comprise separating the second non-aqueous phase from the second aqueous phase. As will be appreciated, the contacting with an aqueous stripping solution and separating the second non-aqueous phase from the second aqueous phase may be performed as described in relation to the first aqueous and non-aqueous phases.

[0033] Preferably, the process further comprises recovering gallium from the second aqueous phase. Recovery of gallium from the second aqueous phase may be performed in any suitable way, for example by distillation of the second aqueous phase to provide a gallium-containing residue, by precipitation of gallium from solution, or by any other suitable method.

[0034] The process may further comprise a step of removing one or more metals other than gallium prior to or after obtaining the gallium enriched product. For example, the aqueous acidic solution comprising the mixture of metals may be processed prior to the process to remove one or more metals. Alternatively, or in addition, the gallium-containing phase at any stage of the process may be further processed to remove, or to facilitate the removal of, metals other than gallium. For example, the one or more metals other than gallium may be removed by a post-processing step after obtaining the gallium enriched product, or the one or more metals other than gallium may be removed prior to a step of stripping gallium from the first phase. In this way, metals that are not separated from gallium as effectively as others may be removed to provide a gallium product of increased purity. The one or more metals other than gallium referred to in this context may comprise iron. The process may comprise adding a reagent to enhance separation of one or more metals other than gallium from the gallium enriched product, for example to enhance separation of said one or more metals from gallium during a step of stripping gallium from the first phase, and/or to inhibit extraction of said one or more metals from the aqueous acidic solution into the first phase. For example, the reagent may comprise oxalic acid, preferably wherein the one or more metals other than gallium comprises iron. -9 -

[0035] A further aspect provides an extractant composition for the separation of gallium from an aqueous acidic zinc leachate solution, wherein the extractant composition comprises a deep eutectic solvent system comprising: i) a trialkylphosphine oxide of forrnula R3P0, where each R group is independently selected from C6+ alkyl; ii) a carboxylic acid of formula Z-L-COOH, wherein Z is -COOH or -C(0)CH3, and L is a C14 alkyl linker; and iii) a reducing agent.

[0036] As will be appreciated, the extractant composition may suitably be as defined previously herein.

[0037] A further aspect provides a process for the separation of gallium from a mixture of metals, wherein the process comprises: providing an aqueous acidic solution comprising a mixture of metals including gallium; contacting said aqueous acidic solution with an extractant composition comprising a deep eutectic solvent system, the deep eutectic solvent system comprising: i) a trialkylphosphine oxide of formula R3P0, where each R group is independently selected from C7+ alkyl, and ii) ascorbic or erythorbic acid, to provide a first phase into which gallium is extracted; and obtaining a gallium enriched product from the first phase.

[0038] It has been surprisingly found that, in addition to acting as an effective reducing agent in combination with a trialkylphosphine oxide and a carboxylic acid hydrogen bond donor as described previously, ascorbic acid (or its stereoisomer erythorbic acid) can also itself act as the hydrogen bond donor in a deep eutectic solvent system that is effective for the separation of gallium.

[0039] While ascorbic or erythorbic acid with the trialkylphosphine oxide can work to provide surprisingly effective extraction alone, as has been described previously a reducing agent such as ascorbic or erythorbic acid has been found to provide a synergy for the separation of gallium when combined with a further hydrogen bond donor. Thus, the deep eutectic solvent system may comprise a further hydrogen bond donor, which may comprise a -10 -hydrogen bond donor as defined previously herein. Preferably the further hydrogen bond donor comprises an organic polyol or a carboxylic acid, more preferably having the formula Y-L-Z, wherein L is a linker selected from Ci_6 linear, branched or cyclic alkyl or aryl optionally substituted with one or more -OH groups, and wherein Y and Z are independently selected from -COOH, -OH or C(0)CH3, preferably wherein at least one of Y or Z is -COOH, and L is unsubstituted Ci_3 alkyl, for example a carboxylic acid of formula Z-L-COOH, wherein Z is -COOH or -C(0)CH3, and L is a C14 alkyl linker, more preferably wherein the further hydrogen bond donor comprises malonic acid.

[0040] The ratio of the trialkylphosphine oxide to the ascorbic or erythorbic acid or the further hydrogen bond donor may be from 0.5:0.5 to 0.75:0.25, preferably from 0.55:0.45 to 0.7:0.3, for example from 0.6:0.4 to 0.7:0.3. As will be appreciated, the ascorbic or erythorbic acid may form the bulk of the hydrogen bond donor that forms a deep eutectic solvent with the trialkylphosphine oxide, or the ascorbic or erythorbic acid may be present in the extractant composition at a concentration of at least 0.05M, preferably at least 0.1M, for example from 0.05M to 0.5M, such as from 0.1M to 0.3M.

[0041] As will be appreciated, the process may in all other respects be as defined previously herein.

[0042] A further aspect provides an extractant composition for the separation of gallium from an aqueous acidic zinc leachate solution, wherein the extractant composition is a deep eutectic solvent system comprising: i) a trialkylphosphine oxide of formula R3P0, where each R group is independently selected from C6, alkyl; and ii) ascorbic or erythorbic acid.

[0043] As will be appreciated the extractant composition may comprise a further hydrogen bond donor and/or be as further defined previously herein.

[0044] A further aspect provides use of an extractant composition as defined herein for extracting gallium. Preferably, the use comprises extracting gallium from an aqueous acid solution, preferably an aqueous acid solution as defined previously herein.

[0045] The invention will now be described by reference to the following non-limiting Examples and Figures.

[0046] Brief Description of Figures

[0047] Figure 1 shows a schematic flow diagram of an example gallium separation process; Figure 2 shows a graph of the percentage gallium extraction for the compositions according to Examples 1 and 2 and Comparative Example 1.

[0048] Examples

[0049] Figure 1 shows a flow diagram of a general extraction and separation procedure using an extractant composition as referred to herein. At step 100, the liquid extractant composition is pre-equilibrated with an aqueous acidic solution 102. At step 200, the liquid extractant composition is then contacted with an aqueous acidic solution comprising gallium and a mixture of metals 202. A first aqueous phase 204 is then separated from the mixture to leave a first non-aqueous phase into which gallium has been extracted. At 300, the first non-aqueous phase is contacted with an aqueous stripping solution 302 and a second non-aqueous phase 304 is separated to leave a second aqueous phase into which gallium has been stripped. At 400, the second aqueous phase (e.g. the remaining stripping solution) is removed at 402 to leave a gallium enriched product.

[0050] Example 1: TOPO:malonic acid with ascorbic acid A liquid extractant composition was formed from a mixture of trioctylphosphine oxide (TOPO: (0)P(C81-117)3) and malonic acid in a TOPO:malonic acid molar ratio of 0.6:0.4, together with ascorbic acid as a reducing agent at a concentration of 0.15 mol dm-3, and 2.5 wt.% water. The mixture was heated to 60 °C whilst stirring until a homogeneous liquid was formed.

[0051] Example 2: TOPO:ervthorbic acid A liquid extractant composition was formed from a mixture of trioctylphosphine oxide and erythorbic acid in a TOPO:erythorbic acid molar ratio of 0.68:0.32. The mixture was heated to 60 °C whilst stirring until a homogeneous liquid was formed.

[0052] -12 -Comparative Example 1: TOPO:malonic acid A liquid extractant composition was formed from a mixture of trioctylphosphine oxide and malonic acid in a TOPO:malonic acid molar ratio of 0.6:0.4, without the addition of a reducing agent. The mixture was heated to 60°C whilst stirring until a homogeneous liquid was formed.

[0053] Example 3 -Extraction process The relevant liquid extractant composition was pre-equilibrated with a 6M HCI solution.

[0054] The liquid extractant composition was then mixed with a model zinc leachate solution formed from a 6M HCI solution containing a mixture of metals as follows: Metal Concentration (ppm) Zn 103500 Fe 4980 Al 5000 In 914 Cu 258 Ga 42 After 10 minutes of mixing, a first aqueous phase and first a non-aqueous phase were centrifuged and separated.

[0055] The first non-aqueous phase was then contacted with an alkaline stripping solution (aqueous 1M NH3) for 30 minutes, after which the mixture was centrifuged and a second non-aqueous phase separated to leave a second aqueous phase. The second aqueous phase was distilled to remove the aqueous ammonia and the residue dissolved in 6M HCI and analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES) Table 1 below and Figure 2 show a comparison of the proportion of gallium that is recovered when the process is performed with the extractant composition of Examples 1 and 2 as compared to Comparative Example 1. As can be seen, the extractant composition comprising erythorbic acid performed significantly better for gallium extraction than that where only malonic acid was used. In addition, when malonic acid was used with the -13 -addition of a low concentration of ascorbic acid (Example 1), the recovery of gallium was increased synergistically beyond either malonic acid or erythorbic acid used alone

[0056] Table 1

[0057] % Gallium 5 recovered

[0058] Example 2 43

[0059] Comparative Example 1 17

[0060] Example 1 60

[0061] The different specific metals present in the feedstock and following extraction were also analysed for the reactions using the extractant composition of Example 1 and Comparative Example 1. The results were as shown in Tables 2 and 3 below.

[0062] Table 2: Amount of each metal from the feedstock recovered in the final aqueous phase using TOPO:malonic acid (Comparative Example 1) Al Cu Fe Ga In Zn Feedstock (ppm) 5000 258 4980 42 914 10350 Recovered in the final aqueous phase (ppm) 7.65 61.2 323.5 6.9 2.5 2.9 % of metal recovered 0.2 23.7 6.5 16.4 0.3 -0 Table 3: Amount of each metal from the feedstock recovered in the final aqueous phase using TOPO:malonic acid with 0.15 mol dm-3 ascorbic acid (Example 1) Al Cu Fe Ga In Zn Feedstock (ppm) 5000 258 4980 42 914 10350 Recovered in the final aqueous phase 22 10 2713 26 3 3 (PPrn) % of metal recovered 0.4 3.9 54.5 61.9 0.3 -0 -14 -As can be seen from Tables 2 and 3, the use of a reducing agent in the extractant composition not only leads to a large increase in the proportion of gallium recovered in the process, but also selectively extracts a higher proportion of gallium in comparison to other metals present in solution, particularly copper which is typically difficult to separate from gallium.

Claims

1. -15 -CLAIMS: 1. A process for the separation of gallium from a mixture of metals, wherein the process comprises: providing an aqueous acidic solution comprising a mixture of metals including gallium; contacting said aqueous acidic solution with an extractant composition comprising a deep eutectic solvent system, the deep eutectic solvent system comprising: i) a trialkylphosphine oxide of formula R3P0, where each R group is independently selected from Cs+ alkyl; fi) a hydrogen bond donor comprising a carboxylic acid of formula Z-L-COOH, wherein Z is -COOH or -C(0)CH3, and L is a C1-4 alkyl linker; and iii) a reducing agent, to provide a first phase into which gallium is extracted; and obtaining a gallium enriched product from the first phase.

2. A process according to Claim 1, wherein the trialkylphosphine oxide comprises 3 linear alkyl groups, preferably 3 linear Cs+ alkyl groups, more preferably wherein the trialkylphosphine oxide is trioctylphosphine oxide.

3. A process according to Claim 1 or Claim 2, wherein the reducing agent is an organic reducing agent.

4. A process according to Claim 3, wherein the reducing agent is a polyol, preferably a heterocyclic polyol such as ascorbic acid or erythorbic acid.

5. A process according to any one of the preceding claims, wherein the ratio of the trialkylphosphine oxide to the hydrogen bond donor is from 0.5:0.5 to 0.75:0.25, preferably from 0.55:0.45 to 0.7:0.3, for example from 0.6:0.4 to 0.7:0.3.

6. A process according to any one of the preceding claims, wherein the hydrogen bond donor comprises malonic acid.

7. A process according to any one of the preceding claims, wherein the aqueous acidic solution comprises a solution of the mixture of metals in HCI, preferably HCI at a -16 -concentration of at least 3M, preferably at least 4M, more preferably at least 5M, for example approximately 6M.

8. A process according to any one of the preceding claims, wherein the reducing agent is present in the extractant composition at a concentration of at least 0.05M, preferably at least 0.1M, for example from 0.05M to 0.5M, such as from 0.1M to 0.3M.

9. A process according to any one of the preceding claims, wherein the extractant composition comprises water in an amount of up to 5 wt.%, preferably up to 3 wt.%.

10. A process according to any one of the preceding claims, wherein said first phase into which gallium is extracted is a first non-aqueous phase, and the process comprises the step of separating the first non-aqueous phase from a first aqueous phase.

11. 11 A process according to Claim 10, wherein the first aqueous phase is recycled to provide at least a portion of the aqueous acidic solution.

12. A process according to any one of the preceding claims, further comprising contacting the first phase into which gallium is extracted with an aqueous stripping solution, preferably an alkaline stripping solution, to form a second aqueous phase into which gallium is extracted and a second non-aqueous phase.

13. A process according to Claim 12, wherein the aqueous stripping solution is an aqueous ammonia solution, preferably an aqueous ammonia solution comprising ammonia in a concentration of greater than 0.1M and less than 2M, more preferably from 0.5M to 1.8M, for example from 0.7M to 1.5M, such as from 0.8M to 1.2M

14. A process according to Claim 12 or Claim 13, comprising separating the second non-aqueous phase from the second aqueous phase and preferably recovering gallium 30 from the second aqueous phase.

15. A process according to any one of the preceding claims, wherein the process comprises contacting the aqueous acidic solution with the extractant composition for 30 -17 -minutes or less, preferably 20 minutes or less, more preferably 10 minutes or less.

16. A process according to any one of the preceding claims, wherein the aqueous acidic solution comprises zinc, copper and gallium, and optionally further comprises iron, aluminium and indium, for example wherein the aqueous acidic solution is a zinc leachate solution.

17. A process according to any one of the preceding claims, wherein gallium is present in the aqueous acidic solution at an amount of 1 wt.% or less, preferably 0.5 wt.% or less, for example 0.3 wt.% or less.

18. A process according to any one of the preceding claims, further comprising a step of removing one or more metals other than gallium prior to or after obtaining the gallium enriched product.

19. A process according to Claim 18, wherein the one or more metals other than gallium are removed by a post-processing step after obtaining the gallium enriched product, or wherein the one or more metals other than gallium are removed prior to a step of stripping gallium from the first phase

20. A process according to Claim 18 or Claim 19, wherein the one or more metals other than gallium comprises iron.

21. A process according to any one of Claims 18 to Claim 20, wherein the process comprises adding a reagent to enhance separation of one or more metals other than gallium from the gallium enriched product, for example to enhance separation of said one or more metals from gallium during a step of stripping gallium from the first phase, and/or to inhibit extraction of said one or more metals into the first phase.

22 A process according to Claim 21, wherein the reagent comprises oxalic acid, preferably wherein the one or more metals other than gallium comprises iron.

23. An extractant composition for the separation of gallium from an aqueous acidic zinc -18 -leachate solution, wherein the extractant composition comprises a deep eutectic solvent system comprising: i) a trialkylphosphine oxide of formula R3P0, where each R group is independently selected from C6+ alkyl; ii) a carboxylic acid of formula Z-L-COOH, wherein Z is -COOH or -C(0)CH3, and L is a C1-4 alkyl linker; and iii) a reducing agent.

24. A composition according to Claim 23, wherein the extractant composition is as defined in any one of Claims 2 to 6, 8 or 9.

25. A process for the separation of gallium from a mixture of metals, wherein the process comprises: providing an aqueous acidic solution comprising a mixture of metals including gallium; contacting said aqueous acidic solution with an extractant composition comprising a deep eutectic solvent system, the deep eutectic solvent system comprising: i) a trialkylphosphine oxide of formula R3P0, where each R group is independently selected from C7+ alkyl, and ii) ascorbic or erythorbic acid, to provide a first phase into which gallium is extracted; and obtaining a gallium enriched product from the first phase.

26. A process according to Claim 25, wherein the deep eutectic solvent system comprises a further hydrogen bond donor, preferably an organic polyol or a carboxylic acid, more preferably having the formula Y-L-Z, wherein L is a linker selected from C1_6 linear, branched or cyclic alkyl or aryl optionally substituted with one or more -OH groups, and wherein Y and Z are independently selected from -COOH, -OH or C(0)CH3, preferably wherein at least one of Y or Z is -COOH, and L is unsubstituted Ci_3 alkyl, for example a carboxylic acid of formula Z-L-COOH, wherein Z is -COOH or -C(0)CH3, and L is a 01-4 alkyl linker, more preferably wherein the further hydrogen bond donor comprises malonic acid.

27. A process according to Claim 25 or Claim 26, wherein the ratio of the -19 -trialkylphosphine oxide to the ascorbic acid or the further hydrogen bond donor is from 0.5:0.5 to 0.75:0.25, preferably from 0.55:0.45 to 0.7:0.3, for example from 0.6:0.4 to 0.7:0.3.

28. A process according to any one of Claims 25 to 27, wherein the ascorbic or erythorbic acid is present in the extractant composition at a concentration of at least 0.05M, preferably at least 0.1M, for example from 0.05M to 0.5M, such as from 0.1M to 0.3M.

29. A process according to any one of Claims 25 to 28, wherein the process is as further defined in any one of Claims 2 or 7 to 22.

30. An extractant composition for the separation of gallium from an aqueous acidic zinc leachate solution, wherein the extractant composition is a deep eutectic solvent system comprising: i) a trialkylphosphine oxide of formula R3P0, where each R group is independently selected from C6, alkyl; and ii) ascorbic or erythorbic acid.

31. A composition according to Claim 30, wherein the extractant composition is as defined in any one of Claims 2, 9 or 26 to 28.

32. Use of a composition as defined in any one of Claims 23, 24, 30 or 31 for extracting gallium.

33. Use according to Claim 32, wherein the use comprises extracting gallium from an aqueous acidic solution, preferably an aqueous acidic solution as defined in any one of Claims 7, 16 or 17.