Electrorecovery of metals

Abstract

A method and ionic liquid for the electrorecovery of metal from a metal salt including at least one metal ion. The method includes the steps of dissolving the metal salt in an ionic liquid, the ionic liquid including an ionic liquid cation and an ionic liquid anion; whereby the metal ion of the metal salt forms a metal complex in solution with at least the ionic liquid cation; and subjecting the metal complex to an electrical potential between a cathode and anode to recover metal at the cathode. The ionic liquid includes an ionic liquid cation and an ionic liquid anion, wherein the ionic liquid cation has an affinity for the metal ion which is at least about equal to that of the ionic liquid anion for the metal ion.

Claims

1. A method for the electrorecovery of metal from a metal salt, the method including the steps of: dissolving the metal salt in an ionic liquid to form a solution of metal ions in the ionic liquid, the ionic liquid including an ionic liquid cation and an ionic liquid anion, whereby the metal ions in solution form metal complexes in solution with at least the ionic liquid cation, wherein the ionic liquid cation includes at least one heteroatom having a positive charge and at least one donor centre (D) having a lone pair of electrons with sufficient Lewis basicity for coordination to each metal ion to form a neutral or positive charge on the metal complexes or to reduce a size of a negative charge on the metal complexes as compared to their charge if coordination had occurred by the ionic liquid anion of the ionic liquid alone, wherein an affinity of the ionic liquid cation for the each metal ion is at least about equal to that of an affinity of the ionic liquid anion for each metal ion to be recovered; and wherein the ionic liquid cation is such that a complex formation constant between each metal ion to be recovered and the ionic liquid cation allows sufficient bond formation with each metal ion in the dissolution process to form the metal complexes and ease of bond breaking allows metal recovery from the metal complexes at a cathode; and subjecting the metal complexes to an electrical potential between the cathode and an anode to recover metal from the metal complexes at the cathode; and wherein the metal is aluminium.

2. The method of claim 1, wherein the heteroatom is selected from the group consisting of group VA and group VIA elements of the periodic table.

3. The method of claim 1, wherein the donor centre (D) includes an atom selected from the group consisting of the group VA to group VIIA elements of the periodic table.

4. The method of claim 3, wherein the donor centre (D) includes an atom selected from the group consisting of O, N, S and P.

5. The method of claim 3, wherein the donor centre (D) includes an oxygen atom.

6. The method of claim 1, wherein the ionic liquid cation is 1-alkyl-1,4-diazabicyclo[2.2.2]octane (C.sub.nDABCO).

7. The method of claim 6, wherein the alkyl group (C.sub.n) is C.sub.1 to C.sub.14 moiety that is saturated, unsaturated, branched or contains other functionalities.

8. The method of claim 1, wherein the ionic liquid anion is bis(trifluoromethylsulfonyl)amide.

9. The method of claim 1, wherein the metal complexes further include the ionic liquid anion.

10. The method of claim 1, wherein the metal complexes are not negatively-charged.

11. The method of claim 1, wherein co-ordination by the ionic liquid cation of the ionic liquid to the at least one metal ion forms a neutral or positively-charged metal complex among the metal complexes.

12. The method of claim 1, wherein the donor centres (D) are present in a sufficient number to allow interaction with the at least one metal ion in preference to the ionic liquid anion that would otherwise co-ordinate to the metal ion.

13. A method of accelerating the rate of reduction of a metal complex at a negatively charged cathode during electrorecovery of metal from a metal salt in an ionic liquid, the method comprising: wherein the metal salt includes at least one metal ion, and the ionic liquid includes an ionic liquid cation and an ionic liquid anion, wherein the ionic liquid anion forms a first negatively charged metal complex with the at least one metal ion, forming a metal complex less negatively charged than the first negatively charged complex by: adding to the first negatively charged metal complex at least one ionic liquid cation including at least one heteroatom having a positive charge and at least one donor centre (D) having a lone pair of electrons with sufficient Lewis basicity for co-ordination to the at least one metal ion to form the less negatively charged metal complex; or substituting at least one ionic liquid anion of the first negatively charged metal complex with an ionic liquid cation including at least one heteroatom having a positive charge and at least one donor centre (D) having a lone pair of electrons with sufficient Lewis basicity for co-ordination to the at least one metal ion to form the less negatively charged metal complex; and wherein formation of the less negatively charged metal complex assists in the approach of the less negatively charged metal complex to the negatively charged cathode such that current density of the less negatively charged metal complex reaching the cathode per unit area is from 3 to 5 times higher than current densities achievable when non-coordinating cations are used.

14. The method of claim 13, wherein the approach of the less negatively charged metal complex to the negatively charged cathode is assisted by reducing the Coulombic resistance to movement of the less negatively charged metal complex toward the negatively charged cathode.

15. The method of claim 13 wherein the metal is selected from the group consisting of transition metals, group IIIA to VIA metals, alkali earth metals, rare earth metals, and metalloids.

16. A method for the electrorecovery of metal from a metal salt including at least one metal ion, the method including the steps of: dissolving the metal salt in an ionic liquid, the ionic liquid including an ionic liquid cation and an ionic liquid anion, whereby the anion is selected from: ##STR00005## and whereby the metal ion of the metal salt forms a metal complex in solution with at least the ionic liquid cation, wherein the ionic liquid cation includes at least one heteroatom that has a positive charge and at least one donor centre (D) having a lone pair of electrons with sufficient Lewis basicity for co-ordination to the metal ion to form a metal complex less negatively charged than if co-ordination had occurred by the anion of the ionic liquid alone; and subjecting the metal complex to an electrical potential between a cathode and anode to recover metal at the cathode; wherein the metal is selected from the group consisting of transition metals, group IIIA to VIA metals, alkali earth metal; rare earth metals, and metalloids.

17. The method of claim 16 wherein the metal complex includes only the ionic liquid cation and not the ionic liquid anion.

18. The method of claim 16 wherein the ionic liquid is fluid enough at operating temperatures to allow transport of the complexed metal ions during the electrorecovery process.

19. A method for reducing the bulk concentration of metal ions required in the electrorecovery of metal from a metal salt including at least one metal ion, the method including the steps of: dissolving the metal salt in an ionic liquid, the ionic liquid including an ionic liquid cation and an ionic liquid anion, whereby the metal ion of the metal salt forms a metal complex in solution with at least the ionic liquid cation, wherein the ionic liquid cation includes at least one heteroatom that has a positive charge and at least one donor centre (D) having a lone pair of electrons with sufficient Lewis basicity for co-ordination to the metal ion to form a metal complex less negatively charged than if co-ordination had occurred by the anion of the ionic liquid alone; and subjecting the metal complex to an electrical potential between a cathode and anode to recover metal at the cathode; wherein the metal is selected from the group consisting of transition metals, group IIIA to VIA metals, alkali metals, alkali earth metal, rare earth metals, and metalloids; wherein the minimum concentration of metal ions in the ionic liquid is reduced to half or less than half of the concentration of metal ions in the ionic liquid for electrodeposition to occur compared to a method using an ionic liquid having non-coordinating cations.

20. The method of claim 19 wherein the rate of the electrorecovery method is accelerated compared to an electrorecovery method using an ionic liquid having non-coordinating cations.

21. The method of claim 19 wherein the metal is selected from the group consisting of transition metals, group IIIA to VIA metals, alkali earth metals, rare earth metals, and metalloids.

22. The method of claim 19 wherein the metal is selected from the group consisting of transition metals, group IIIA to VIA metals, alkali earth metals, rare earth metals, and metalloids.

23. A method for the electrorecovery of metal from a metal salt, the method including the steps of: dissolving the metal salt in an ionic liquid to form a metal ion in the ionic liquid, the ionic liquid including an ionic liquid cation and an ionic liquid anion, whereby the metal ion in solution forms a metal complex in solution with at least the ionic liquid cation, wherein the ionic liquid cation includes at least one heteroatom having a positive charge and at least one donor centre (D) having a lone pair of electrons with sufficient Lewis basicity for coordination to the metal ion, and wherein the donor centres (D) are present in a sufficient number and are of sufficient Lewis basicity such that the ionic liquid cation interacts with the metal ion to be recovered in preference to the at least one ionic liquid anion that would otherwise co-ordinate to the metal ion such that interaction of the ionic liquid cation with the metal ion takes place to form a neutral or positive charge on the metal complex or to reduce a size of the negative charge on the metal complex as compared to their charge if coordination had occurred by the ionic liquid anion of the ionic liquid alone; and wherein the ionic liquid cation is such that a complex formation constant between the metal ion and the ionic liquid cation allows sufficient bond formation with the metal ion in the dissolution process to form the metal complex and ease of bond breaking allows metal recovery from the metal complex at the cathode; and subjecting the metal complex to an electrical potential between a cathode and anode to recover metal from the metal complex at the cathode; and wherein the metal is selected from the group consisting of transition metals, group IIIA to VIA metals, alkali metals, alkali earth metals, rare earth metals, and metalloids.

24. The method of claim 23 wherein the rate of the electrorecovery method is accelerated compared to an electrorecovery method using an ionic liquid having non-coordinating cations.

25. The method of claim 23 wherein the metal is selected from the group consisting of transition metals, group IIIA to VIA metals, alkali earth metals, rare earth metals, and metalloids.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1.sup.15N NMR spectra of (a) pure [C.sub.2DABCO][NTf.sub.2] and (b) [C.sub.2DABCO][NTf.sub.2] containing 0.5 molal AlCl.sub.3 acquired at 110 C. Assignments for the nitrogen signals are shown on the structure.

(2) FIG. 2Relative rates of electrorecovery of aluminium from known ionic liquid solution [C.sub.4mpyr][NTf.sub.2] (dotted line) and from [C.sub.2DABCO][NTf.sub.2] in accordance with the present invention (solid line).

DETAILED DESCRIPTION OF THE EMBODIMENTS

(3) In the vast majority of cases, metal ions dissolved in ionic liquids co-ordinate with the anions of the ionic liquid and produce negatively-charged complexes in solution. In the present invention, the chemistry of the cation of the ionic liquid is modified so that it co-ordinates, as a ligand, with the metal ion in solution. The co-ordination of any neutral or positively charged ligands with the metal ion produces metal complexes which are less negatively-charged, or even positively charged, in solution. This change is particularly desirable if the metal complex is electrochemically reduced at an electrode. Lowering the net negative charge, or forming a positive charge, on the complex assists the approach of the complex to the negatively-charged cathode during reduction and thereby accelerates the rate of reduction of the metal.

(4) The affinity of the ionic liquid cation for the metal ion to be recovered is adjusted so that the ionic liquid cation binds with the metal ion to be recovered in the electrolyte. More specifically, the ionic liquid cation must be able to successfully compete with other potential ligands in the ionic liquid electrolyte, including the ionic liquid anion (and any anions introduced with the metal salt that is added to the ionic liquid, eg Cl.sup. in AlCl.sub.3). Successful competition means that actual interaction of the ionic liquid cation with the metal ion takes place to some degree.

(5) However, the ionic liquid cation must not bind so strongly with the metal ion to be recovered that the complex formed is too stable to be electrochemically reduced within the electrochemical window of the ionic liquid. That is, in order to avoid reduction of the ionic liquid itself, the metal complex formed by the ionic liquid cation and the metal ion to be recovered must be reducible within the electrochemical window of the ionic liquid. The complex formation ability of the ionic liquid cation is tailored in a way to allow for sufficient bond formation with the metal ions in the dissolution process and for ease of the bond breaking in the deposition step. That is, the complex formation constant between the metal ion and the ligand (in this case, the ionic liquid cation) is neither too high nor too low to enable this process. Interaction of a ligand with a metal ion to be recovered can be measured using techniques such as mass spectroscopy and NMR.

(6) The donor centre of the cation must be carefully chosen from functional groups or atoms that have an affinity for the metal ion to be recovered at the cathode by an electrorecovery process. A donor centre with a high affinity for the metal ion in solution will form a metal complex that is too stable and will not be reducible within the electrochemical window of the ionic liquid.

(7) Further, the ionic liquid must be fluid enough at the operating temperature to allow the transport of the complexed metal ions during the electrorecovery process.

(8) The cationic entity of the ionic liquid may be, for instance, as in Scheme 2. may be of an alicyclic or cyclic nature where multiple carbon chains are linked to heteroatoms in a way creating a positive charge for the entity. The carbon chains are of a length that creates a stable system which is liquid in the anticipated temperature range of operation. The heteroatoms may be elements of group VA and VIA of the periodic table of the elementsthe arrangement is obvious for those skilled in the art. The carbon chains may be of a saturated or unsaturated (alkenes, alkynes) nature. The carbon chain may be a linear or a branched array of carbon atoms. In addition, the cyclic compounds may also be of an unsaturated or aromatic nature, including condensed ring systems. Furthermore, the cyclic compounds may be of multiple ring cycles and spirocyclic structures. Therefore, the cationic entity of the ionic liquid may be chosen from the compound classes: ammoniums, antimoniums, arsoniums, imidazoliums, morpholiniums, oxazoliums, oxoniums, phosphoniums, pyridiniums, pyrrolidiniums, piperidiniums, piperaziniums, pyraziniums, seleniums, sulfoniums, teluriums, thiazoliums, triazolium and the like.

(9) The ionic liquid anion may be any known to those skilled in the art (Scheme 3). For instance, the anion could be bis(trifluoromethylsulfonyl)amide(NTf.sub.2.sup.).

(10) The current density is a measure of the number of electroreducible metal complexes reaching the cathode per unit area. In the case of an electrorecovery process, the current density is a reflection of the rate of the electrorecovery. As mentioned, the movement of the metal ions to be recovered towards the cathode will be affected by the charge on those complexed metal ions (ie the metal ion as part of a complex with components of the ionic liquid and other ligands present in the system). In a prior art ionic liquid electrorecovery process, the charge on the metal complex is negative, thus hindering movement of the metal complexes towards the negative cathode and thus reducing the current density. In the present invention, the charge on the metal complex is less negative, neutral, or positive, than in the prior art, thus increasing the ease with which the cations may move toward the negative cathode and thus increasing the current density. For the present invention, current densities on the order of 3 to 5 times higher than those achievable for the prior art ionic liquids are achievable.

(11) In the prior art ionic liquid systems, the aluminium ion concentration in the ionic liquid electrolyte must be relatively high in order to allow electrorecovery. Increasing the concentration of negatively-charged metal complexes in solution can overcome the coulombic repulsion issues by placing the metal complexes in sufficient close proximity to the negatively-charged cathode to enable electrorecovery to take place. In the present invention, the concentration of the aluminium ions can be lower than that of the prior art. Typically, for ionic liquids including the ionic liquid cation of the present invention, the aluminium ion concentration may be about half that needed to conduct electrorecovery using a prior art ionic liquid.

(12) The efficacy of the electrorecovery process may be affected in that the minimum concentration of metal ions required in the ionic liquid for electrorecovery to occur is lowered, preferably by a factor of 2. Alternatively, for a fixed concentration of metal ions in solution the rate of electrorecovery is 2 to 3 times faster compared to that in known ionic liquid solutions.

(13) The cathode may be of any electrically conducting material suitable for use in electrorecovery. Thus it may be smooth, reticulated, or porous. It may also have a geometry that facilitates mass transport and minimises electrical impediments in the cell in which it is used. Thus it may be planar or cylindrical or another geometry which meets these criteria.

EXAMPLES

Example 1

NMR Evidence of Ionic Liquid Cation Interaction with a Metal Ion

(14) In ionic liquids where the ionic liquid cation possesses a nitrogen donor centre such as in [C.sub.nDABCO][NTf.sub.2], .sup.15N NMR spectroscopy may be used to obtain evidence for ionic liquid cation (C.sub.nDABCO.sup.+) coordination to metal ions in solution e.g. Al.sup.3+.

(15) In FIG. 1, the .sup.15N NMR spectra of the pure ionic liquid [C.sub.2DABCO][NTf.sub.2] and that of a sample of [C.sub.2DABCO][NTf.sub.2] containing 0.5 molal AlCl.sub.3 acquired at 110 C. are shown. All three nitrogen atoms of [C.sub.2DABCO][NTf.sub.2], two in the ionic liquid C.sub.2DABCO.sup.+ cation and one in the ionic liquid NTf.sub.2.sup. anion, were detected. The resonance at 238 ppm was assigned to the amide nitrogen in the ionic liquid NTf.sub.2.sup. anion, the resonance at 332 ppm was assigned to the quaternary amine nitrogen in the ionic liquid C.sub.2DABCO.sup.+ cation and the resonance at 372 ppm was assigned to the tertiary amine nitrogen (ie. the donor centre or Lewis basic site) in the ionic liquid C.sub.2DABCO.sup.+ cation. After the addition of 0.5 molal AlCl.sub.3 to [C.sub.2DABCO][NTf.sub.2] the resonances at 238 and 332 remained unchanged but the resonance assigned to the tertiary nitrogen shifted by 5 ppm to 367 ppm. This shift in this resonance is likely due to the interaction of the tertiary nitrogen atom with Al.sup.3+ resulting in the formation of an aluminium complex with a more positive charge.

Example 2

Electrochemical Properties of an Ionic Liquid Cation of the Present Invention

(16) A 1.0 molal solution (1 mole of solute per kg of solvent) of anhydrous AlCl.sub.3 in [C.sub.2DABCO][NTf.sub.2] is prepared by slowly adding small quantities of the AlCl.sub.3 to molten [C.sub.2DABCO][NTf.sub.2] (whose melting point is approximately 70 C.) under an inert atmosphere. The solution is constantly stirred throughout. Aluminium can be electrorecovered from this solution at modest temperatures (80 C.-130 C.) using any of several standard electrochemical methods such as cyclic voltammetry (see FIG. 2), chronoamperometry, chronopotentiometry, etc. For example, aluminium may be electrorecovered from this solution at 110 C. during cyclic voltammetry on an abraded gold electrode. A peak current density of 150 A m.sup.2 occurs at about 1.5 volts (versus the ferrocene/ferrocenium redox couple).

(17) A 0.75 molal solution (0.75 moles of solute per kg of solvent) of anhydrous CuCl.sub.2 in [C.sub.2DABCO][NTf.sub.2] is prepared by slowly adding small quantities of the CuCl.sub.2 to molten [C.sub.2DABCO][NTf.sub.2] under an inert atmosphere. The solution is constantly stirred throughout. Copper can be electrorecovered from this solution at modest temperature (80 C.-100 C.) using any of several standard electrochemical methods such as cyclic voltammetry, chronoamperometry, chronopotentiometry, etc. For example, copper may be electrorecovered from this solution at 80 C. during cyclic voltammetry on an abraded gold electrode. A peak current density of 50 A m.sup.2 occurs at about 1.4 volts (versus the silver/silver.sup.+ redox couple).

(18) A 1.5 molal solution (1.5 moles of solute per kg of solvent) of anhydrous AlCl.sub.3 in N-ethyl-N,N-dimethyl-2-methoxyethylammonium bis(trifluoromethylsulfonyl)amide ([N.sub.2,1,1OMe][NTf.sub.2]) is prepared by slowly adding small quantities of the AlCl.sub.3 to [N.sub.2,1,1OMe][NTf.sub.2] (which is a liquid at room temperature) under an inert atmosphere. The solution is constantly stirred throughout. Aluminium can be electrorecovered from this solution at modest temperature (80 C.-130 C.) using any of several standard electrochemical methods such as cyclic voltammetry, chronoamperometry, chronopotentiometry, etc. For example, aluminium may be electrorecovered from this solution at 80 C. during cyclic voltammetry on an abraded gold electrode. A peak current density of 100 A m.sup.2 occurs at about 2.0 volts (versus the silver/silver.sup.+ redox couple).

(19) A 1.5 molal solution (1.5 moles of solute per kg of solvent) of anhydrous AlCl.sub.3 in 1-ethyl-1-methyl-4-methyl-piperazinium bis(trifluoromethylsulfonyl)imide [C.sub.2,C.sub.1mpipz][NTf.sub.2] is prepared by slowly adding small quantities of the AlCl.sub.3 to molten [C.sub.2,C.sub.1mpipz][NTf.sub.2] under an inert atmosphere. The solution is constantly stirred throughout. Aluminium can be electrorecovered from this solution at modest temperature (80 C.-110 C.) using any of several standard electrochemical methods such as cyclic voltammetry, chronoamperometry, chronopotentiometry, etc. For example, aluminium may be electrorecovered from this solution at 80 C. during cyclic voltammetry on an abraded gold electrode. A peak current density of 110 A m.sup.2 occurs at about 1.6 volts (versus the ferrocene/ferrocenium redox couple).

(20) It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.