WO2004099079A1

WO2004099079A1 - A method for producing an electrolytic solution containing vanadium

Info

Publication number: WO2004099079A1
Application number: PCT/AU2004/000621
Authority: WO
Inventors: Nikolai Zontov
Original assignee: Clean Teq Pty Ltd
Priority date: 2003-05-12
Filing date: 2004-05-12
Publication date: 2004-11-18
Also published as: AU2003902298A0

Abstract

A method of producing an electrolytic solution containing vanadium as positive and negative electrode active material, with the electrolytic solutions being suitable for a redox battery. The method includes: (1) dissolving vanadium compound(s) into solution under alkaline or neutral conditions; (2) correcting the pH to 2.0 - 5.5 using mineral acid; (3) adsorbing vanadium from this solution using a macroporous resin such as the resin described in Australian patent no. 758690; (4) washing the vanadium loaded resin with distilled or demineralised water; (5) desorbing the vanadium from the loaded resin using 1 - 6 M sulphuric acid solution containing a reduction agent such as sulphur dioxide, thiourea, sulphurous acid, oxalic acid, hydrogen sulphide, organic acids, alcohols, saccharide, and the like; and (6) separating the desorption solution from the resin to form a vanadium electrolytic solution. According to the present invention, a high purity vanadium electrolytic solution can be economically produced from raw materials including vanadium magnetite concentrate, fly ash, vanadium pentoxide, sodium or ammonium metavanadate etc.

Description

A METHOD FOR PRODUCING AN ELECTROLYTIC SOLUTION CONTAINING

VANADIUM

FIELD OF THE ISWEHTIO The present invention relates to a method of extracting vanadium compounds from a mixture containing impurities and forming an electrolytic solution using the vanadium compounds . ks is explained in greater detail below, concentrated solutions of vanadium and compounds thereof can be used as an electrolytic solution in the positive and negative half-cells of a vanadium redox battery.

BACKGROUND TO THE PRESENT INVENTION In recent years, global environmental pollution, such as acid rain and the greenhouse effect, is increasingly being seen as a problem confronting both present and future generations . In response to community concerns over environmental pollution, energy providers who have traditionally generated electrical power by burning hydrocarbons, which is a major contributor to acid rain and greenhouse gases, are now looking toward inexhaustible and clean energy sources including solar energy, heat reclamation, wind energy, and the energy of ocean currents or temperature differences of seawater to generate electrical power.

One of the difficulties in commercialising electrical power generated from alternative energy sources is that the rate at which electrical power can be generated is dependent on climatic conditions. The coordinated use of clean energy sources such as solar energy and effective storage batteries is required to overcome fluctuations in the rate at which electrical power can be generated and in addition, fluctuations in the rate of consumption of electrical power by the end users.

Numerous types of secondary batteries have been proposed as a method of storing electrical power. One battery type, namely redox batteries, is preferred because they are effective and do not pollute the environment.

Redox batteries are a high-volume battery, capable of operating at room temperature and atmospheric pressure, and have a longer life than other secondary batteries. In addition, redosε batteries are not prone to automatic self- discharge and can thus be left dormant for a period of time with only minimal effect on the performance of the battery. Vanadium redox flow batteries have especially high outputs and can recover quickly when being recharged. These batteries use vanadium electrolytic solutions in both the positive and negative half-cells. However, in order to produce suitable vanadium electrolytic solutions, high purity vanadium material, which is expensive and difficult to obtain, is required. Therefore, the commercialisation of vanadium redox flow batteries requires a cost-effective method of producing a purified source of vanadium. It is known that the purification of raw materials containing vanadium can be carried out by converting the vanadium compounds in the raw material to ammonium metavanadate, dissolving the ammonium metavanadate in hot water, and subsequently filtering and crystallising a more pure form of ammonium metavanadate. However, in order to obtain a high purity ammonium metavanadate, the dissolving and crystallising steps must be repeated several times and therefore is not cost effective.

Another known purification process, in which vanadium oxychloride is produced from vanadium raw material, is also not cost effective due to the complicated production process.

A difficulty that may inhibit the purification of some vanadium raw materials, such as ash produced by combustion of heavy oil fuels, is the difficulty in removing silicon from the raw material. A method for producing a high purity vanadium electrolytic solution for use in the positive and negative half-cells of a redox battery has been described in United States patent 5,587,132. The method described comprises. dissolving a vanadium raw material in a solvent under alkaline or neutral conditionsj thermal precipitation of a polyvanadate compound containing vanadium ions under acidic conditions filtering the precipitate from the filtrate? oxidising or reducing the precipitate to remove ammonium under elevated temperature conditions; and dissolving the oxidised/reduced precipitate; and optionally converting trivalent vanadium to tetravalent vanadium using suitable reductants.

A difficulty with the method described in the US patent is the complexity of the method.

It is an object of the present invention to provide an alternative method for producing a high purity vanadium solution.

SUMMARY OF THE PRESENT INVENTION

The present invention is a method of extracting vanadium-containing compounds from a mixture containing impurities, the method including the steps of: a) adsorbing anionic vanadium compounds from the mixture onto an anion-exchange substrate; b) desorbing vanadium compounds from the substrate by treating the substrate with a reducing agent to convert the anionic vanadium compounds adsorbed on the substrate to cationic compounds and thereby facilitating release of vanadium compounds from the substrate; and c) using a solvent to wash the vanadium compounds released from the substrate in step b) and thereby form a solution of vanadium compounds .

It will be appreciated that the steps b) and c) may be carried out concurrently or that any one of steps a) to c) may be carried contiguously or disjunctively in which the steps may be performed at different locations and at different times.

Throughout this specification the term "vanadium compound" is to be understood to mean any compound having two or more atoms in which at least one of the atoms is a vanadium atom.

Similarly, the term "silicon compound^ means any compound having two or more atoms in which at least one of the atoms is a silicon atom.

The following is a list of commercially available anion-exchange substrates that can be used in the present invention: i) Purolite A-830 ii) Purolite A-100 iii)Dow Dowex M43 iv) Dow Dowex M56 v) Rohm & Haas Amberlite IRA67 vi) Rohm & Hass Amberlite IRA68 vii)Bayor Lewatit AP43 viii)Mitsubishi Dianon A-11 ix) Sybron Ionic A 375

It is preferred that the substrate be a particulate material that can be used in a moving bed.

It is preferred that the substrate be a macroporous resin. It is preferred that the substrate be an amphoteric macroporous resin.

It is preferred that the substrate has a loading capacity of approximately 120 grams of vanadium compound per litre of the substrate. We have found that the anion exchange resin described in our earlier Australian patent application, namely application no. 77469/98 (serial no. 758690) is a particularly suitable substrate. The complete specification of application 77469/98 is hereby incorporated into the present specification.

It is preferred that the reducing agent used for treating the substrate in step b) includes one or a combination of any of the following: sulphurous acid, sulphur dioxide, oxalic acid, thiourea, hydrogen sulphide, organic acid, an alcohol, or a saccharide.

Although it is possible that any suitable solvent may be used in step c) , it is preferred that the solvent be sulphuric acid. In the situation when sulphur dioxide is used as the reducing agent in step b) , one of the advantages of using sulphuric acid is that it can increase the solubility of vanadium compounds by changing the valence of the vanadium atoms in the compounds released from the substrate from a pentavalent atom to a quatrivalent atom as illustrated in the following reaction scheme :

R4H2V10O28 + H₂S0₄ + S0₂ -* R2-SO4 + V0²⁺S0₄ ^2~

It is preferred that the method also includes the step of treating the substrate with substitute anions to replace the vanadium compounds released from the substrate in step b) . One of the benefits arising from using a dedicated substitute anion is that the substrate is prevented from becoming contaminated with anions that may form string bonds with the substrate and thereby reduce the load capacity of the substrate for vanadium anionic compounds .

Another advantage in using sulphuric acid as the solvent is that step c) is that the sulphate anions of sulphuric acid can also act as the substitute anions.

Another advantage in using sulphuric acid to carry out step c) is that the sulphuric acid can aid the action of a sulphur dioxide reducing agent when used to carry out step b) .

It is preferred that the concentration of sulphuric acid used in step c) have a concentration ranging from 1 to 6 M.

Yet another advantage in using sulphuric acid as the solvent in step c) is that the solution obtained from washing the substrate can be used as an electrolytic solution in the half cells of a vanadium redox flow battery. Accordingly, it is preferred that the method also includes separating the solvent that has been used to wash the substrate in step c) and thereby provide a sulphuric acid and vanadium containing solution.

In order for the solution to be suitable for use in both half-cells of a redox battery, it may be necessary to treat the solution with a reducing agent to adjust the proportions of trivalent and tetravalent vanadium in the solution.

It is preferred that the concentration of vanadium contained in the solution ranges from 50 to 200 grams per litre (1 to 4 M) and that the concentration of sulphuric acid in the solution ranges from 100 to 600 grams per litre (1 to 6 M) .

Moreover, steps b) and c) and the steps of treating the substrate with substitute anions and separating the solvent that has been used to wash the substrate from the substrate to form a solution can be carried out concurrently using sulphur dioxide as a reducing agent and sulphuric acid as a solvent, whereby the sulphuric acid: i) assists in the reduction of anionic compounds sorbed on the substrate, ie. aides the action of the sulphur dioxide when used in step b) ; ii) provides substitute anions in the form of sulphate ions; iii) increases the solubility of vanadium compound desorbed from the substrate in step b) ; and iv) acts as a solvent and forms the basis of a solution that is capable of being used in the half-cells of a redox battery.

In the situation in which the steps are not carried out concurrently, it is preferred that the substrate be rinsed with a liquid between at least steps a) , b) and c) . It is preferred that the liquid a distilled or demineralised water.

It is preferred that the method also includes the step of forming the mixture by dissolving a raw material containing vanadium compounds in a solvent under alkaline or neutral conditions.

It is preferred that an alkali such as ammonia or caustic soda be used to create alkaline or neutral conditions while forming the mixture. The raw material used in forming the mixture may include any one of the following materials containing vanadium or compounds containing vanadium: vanadium magnetite, fly ash, vanadium pentoxide, and sodium or ammonium metavanadate. It is preferred that the raw material be a vanadium magnetite concentrate that has been roasted with a sodium salt.

It is preferred that the raw material be a vanadium magnetite concentrate that has been roasted with a calcium or magnesium salt or a mixture thereof.

It is preferred that the solvent in which the mixture is formed is water and that the impurities include water-soluble metal salts, silicates, colloidal silicon or mixtures thereof. In the situation in which the mixture contains silica, it is preferred the method include reducing the concentration of soluble silicon compounds in the mixture to lOOOppm or less. A difficulty that may be encountered in using a crude mixture having a silica concentration greater than lOOOppm is that the silica may adsorb onto the substrate and thereby reduce the available cites for vanadium compounds .

It is preferred that the concentration of silica be reduced to 500 ppm or less. It is preferred that the step of reducing the concentration of soluble silica be carried out by reaction the silica with aluminium sulphate to form a precipitate. It is preferred that the method include adjusting the pH of the crude mixture to range from 2.0 to 5.5 prior to carrying out step a) . The purpose of this step is to transform metavanadate compounds to polyvanadate compounds in which vanadium atoms occur at a higher proportion. The following figures illustrate the change in composition of vanadium compounds with changes in pH for solutions having a concentration of vanadium in the range of 10 to 50 grams per litre.

V0₃ ^" -» HV0₄ ^2" -* V₂0₉ ^{2 "} -» HVι₀θ28^5" -*H₂Vιoθ28^{4 "} -* V0₂ ⁺ pH > 13 8 . 0 - 13 . 0 6 . 6 - 6 . 8 3 . 5 - 6 . 0 2 . 0 - 3 . 5

It is preferred that the pH of the mixture be adjusted using a mineral acid.

It is preferred that the mineral acid be sulphuric acid. Examples of other suitable mineral acids include hydrochloric and nitric acids .

It is preferred that the substrate be recyclable so that is can be used in carrying out the method on more that one occasion within the method.

According to the present invention there is also provided a plant for carrying out the method including any one or a combination of the aspects described above, the plant including: i) an absorption column in which step a) can be carried out; and ii) a desorption column in which steps b) and c) can be carried out. According to the present invention there is also provided a redox flow battery in which one or both of the half-cells of the battery contains a vanadium solution for generating electrical power, wherein the vanadium solution is made according to the method described above and may include any one or a combination of the preferred features thereof . DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described in detail together with a series of examples . According to the preferred embodiment, the method is capable of producing an electrolytic solution containing vanadium compounds that can be used in the positive and negative half-cells of a vanadium redox battery. The preferred embodiment involves the following method.

(1) Dissolving a technical grade vanadium material containing impurities (including vanadium magnetite concentrate, fly ash, vanadium pentoxide, sodium or ammonium metavanadate etc.) in solvent under alkaline or neutral conditions to form a crude mixture from which vanadium compounds can be extracted.

(2) In the situation in which the crude mixture contains more than 1000 ppm of silicon compounds, (which may be present through the hot alkali leaching of silicate minerals in the magnetite concentrate), it is recommended that the concentration of soluble silica compounds be reduced to less than 500 ppm before sorption. To achieve this, soluble silica can be precipitated using aluminium sulphate at a controlled pH followed by filtering the precipitate from the crude mixture.

(3) The pH of the crude mixture is then corrected using mineral acid (sulphuric acid etc.) to a pH range from 2.0 to 5.5. This step converts metavanadate compounds to polyvanadate compounds . (4) The crude mixture is then feed into a sorption column in which vanadium compounds are removed from the mixture by a macroporous anion-exchange resin having the structure described in our earlier Australian patent application, namely application no. 77469/98 (serial no. 758690). The macroporous resin has maximum loading capacity for vanadium compounds and allows vanadium compounds to be removed selectively from the mixture. (5) The resin once loaded with vanadium compounds is then washed with distilled or demineralised water in a wash column before being stripped in the desorption column. (6) Upon completion of washing, the resin is stripped by 1-6 M sulphuric acid solution containing a reduction agent (sulphur dioxide, oxalic acid, hydrogen sulphide, thiourea, organic acids, alcohols, saccharide etc.) using either the continuous or batch process desorption column. (7) The stripped resin is separated from the electrolytic vanadium solution using filtration, when using the batch process in the desorption stage.

(8) After separation, the resin is again washed with distilled or demineralised water before returning to the sorption column.

The method produces an electrolytic solution containing 50 -200g/l (1-4M) of vanadium compounds and 100-600g/l (1-6M) of sulphuric acid. The valency of the vanadium atoms of the vanadium compounds is a mixture of vanadium (+III) and vanadium (+IV) .

EXAMPLE 1

The starting material used in this example is vanadium magnetite which contains a large amount of impurities such as are silicon, sodium, potassium, calcium, magnesium, iron, chromium, aluminium and compounds thereof. The vanadium magnetite was first roasted with sodium oxalate slurry to convert the vanadium compounds to a sodium metavanadate, which is then dissolved in water to form a crude mixture.

A sample of 10 kg of the crude mixture was then leached by water using a percolation method. The produced leach solution contained ~50g/l of vanadium compounds and large concentrations of impurities (including 1.5g/l of silicon and compounds thereof) . The crude mixture had a pH of approximately 9.5. In this instance, the crude mixture contained more than lg/litre of silicon and compounds thereof may co- precipitated within the sorption process and be absorbed physically by the resin, thereby reducing capacity of the resin. In order to preserve the usefulness of the resin, the concentration of silica in the crude mixture was reduced to less than 0.5g/l before sorption using aluminium sulphate to precipitate silicon compounds. After precipitation and filtration the concentration of silicon and compounds thereof in the mixture was 0.2g/l. The crude mixture had a pH ranging from 8.0 to 8.5.

A detailed analysis of the composition of the crude mixture was then conducted and the results of the analysis are set out below

TABLE 1

The pH of the crude mixture was then corrected to range from 2.0 to 5.5 by adding mineral acid. Technical grade sulphuric acid was selected for pH correction for economic reasons. During the pH correction, it was observed that the colour of the crude mixture changed from a light yellow to a reddish brown which reflects the transformation of metavanadate to a polyvanadate compound (ordinary and complex decavanadate) in the mixture. Set out below is the change in composition of vanadium compounds with changes in pH for solutions having a concentration of vanadium in the range of 10 to 50 grams per litre. V0₃ ^{" ■}+ HV0₄ ^{2" ■}+ V₂0₉ ^2" -* HVio0₂8^5" -*H₂V₁₀O₂8^{4" ■}* V0₂ ⁺ pH > 13 8.0-13.0 6.6-6.8 3.5-6.0 2.0-3.5

The vanadium liquor entered the sorption column with a bed of macroporous anion-exchange resin having the structure described in application no. 77469/98 (serial no. 758690) . The resin selectively absorbs anionic compounds, primarily polyvanadate, in favour of impurities such as sodium, potassium, calcium, magnesium, aluminium, iron, nickel etc., because they present as cations in the solution.

The vanadium sorption-process may be illustrated by the following reaction:

2R₂ - S0₄ + H₂V₁₀O₂8^4" -» R4H2V10O28 + 2S0₄ ^{2 "}

wherein R is the polymolecular matrix of the resin.

It is preferable to use a continuous sorption process with the sorption column having a moving bed of the resin, as this type of process and equipment permits the maximum loading of vanadium onto the resin. The maximisation of sorption loading makes for a greater vanadium extraction step. Approximately 500 ml of vanadium-loaded resin containing 150g/l of vanadium was produced by the sorption stage. The vanadium- loaded resin was washed with distilled water in a wash column to exclude the possibility of carryover of impurities from raw liquor to the vanadium electrolytic solution. The washing water may later be used and recycled to the dissolution stage. The washed vanadium-loaded resin is then moved to the next stage, for the production of vanadium electrolytic solution. The vanadium electrolytic solution was produced by desorption of vanadium from the vanadium- loaded resin using 4M sulphuric acid with sulphur dioxide as a reduction agent in a continuous desorption process.

In this instance, the vanadium changes its valency from (+V) to (+IV) during the desorption process under reduction conditions, according to the approximate reaction below:

RH2 10O28 + H2 O +S0₂ -* R₂-SO₄ + V0²⁺S0₄ ^2"

Approximately 500ml of vanadium electrolytic solution was produced and an analysis of the solution shows the solution has the following composition:

TABLE 2

If it is necessary to produce a mixture of trivalent and tetravalent vanadium, additional sulphur dioxide may be injected through the vanadium electrolytic solution until the potential of the solution reached that of a 50/50 mixture of vanadium (IV) and vanadium (III) (approx. 0.3 V versus the Standard Calomel Electrode).

The stripped resin was then washed with distilled water using a continuous process, before being returned to the sorption column. The washing solution may be reused in the primary vanadium dissolution stage.

In this method the vanadium electrolytic solution, which contains high purity vanadium with the required valence, was produced directly from the vanadium magnetite concentrate, a raw material that may contain large amounts of different impurities. Moreover, the solution is a suitable electrolyte for use in substantially all vanadium redox flow batteries. It will be appreciated that the method for producing the vanadium electrolytic solution can be completely automated, can be a continuous process and has virtually no detrimental impact on the environment.

EXAMPLE 2

In this example, technical grade vanadium pentoxide was selected as a starting material for the production of the vanadium electrolytic solution. A crude mixture suitable for anion-substrate exchange was prepared by mixing 10 litres of pure water and 273 grams of a technical grade vanadium pentoxide in a 15-litre beaker. The pH of the solution was corrected to range from approximately 9.0 to 9.5 by the addition of caustic soda. The crude mixture was mixed until all visible vanadium pentoxide was dissolved. The composition of the crude mixture was then analysed and results of the analysis are set out below.

TABLE 3

The pH of this crude mixture was reduced to approximately 3.5 by adding sulphuric acid with continuous mixing.

The crude mixture was then fed into a batch sorption column with a volume of approximately 1.3 litres. The resin loading capacity reached approximately 120g/l of vanadium compound.

After sorption, the vanadium-loaded resin was washed with one litre of distilled water and moved to the next stage of producing the vanadium electrolytic solution.

Stripping the vanadium-loaded resin with a mixture of 1M sulphuric acid produced the vanadium electrolytic solution and 1M oxalic acid in a batch desorption column. The desorption column produced 1.4 litres of electrolytic solution which had the following composition. TABLE 4

EXAMPLE 3

In this example technical grade ammonium metavanadate was used as a raw material for producing a crude mixture containing vanadium compounds. The crude mixture was produced by mixing 10 litres of pure water and 250 grams of a technical grade ammonium metavanadate in a 15-litre beaker. The crude mixture was mixed until all visible vanadium metavanadate was dissolved. The composition of the crude mixture was then analysed and results of the analysis are set out below.

TABLE 5

The pH of the solution was corrected to approximately 4.0 by adding concentrated sulphuric acid.

After the pH correction, the vanadium metavanadate liquor entered a batch sorption column with the volume of approximately 0.8 litres. The resin loading capacity reached approximately 1 0g/l of vanadium compounds.

The vanadium-loaded resin was washed with 800ml of distilled water and moved to the next stage of the vanadium electrolyte production.

The vanadium electrolytic solution was produced by desorption of the vanadium-loaded resin with a solution of 1M sulphuric acid and 1M oxalic acid in the batch desorption column. 0.9 litres of electrolytic solution was drained from the desorption column and analysed. The composition of the solution is set out below.

TABLE 6

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of extracting vanadium-containing compounds from a mixture containing impurities, the method including the steps of: a) adsorbing anionic vanadium compounds from the mixture onto an anion-exchange substrate; b) desorbing vanadium compounds from the substrate by treating the substrate with a reducing agent to convert the anionic vanadium compounds adsorbed on the substrate to cationic compounds and thereby facilitating release of vanadium compounds from the substrate; and c) using a solvent to wash the vanadium compounds released from the substrate in step b) and thereby form a solution of vanadium compounds.

2. The method according to claim 1, wherein the substrate is a particulate material that can be used in a moving bed.

3. The method according to claim 1 or 2, wherein the substrate is a macroporous resin.

4. The method according to claim 3, wherein the substrate is an amphoteric macroporous resin.

5. The method according to any one of claims 1 to 4, wherein the substrate has a loading capacity of approximately 120 grams of vanadium compound per litre of the substrate.

6. The method according to any one of claims 1 to 5, wherein the reducing agent used for treating the substrate in step b) includes one or a combination of any of the following: sulphurous acid, sulphur dioxide, oxalic acid, thiourea, hydrogen sulphide, organic acid, an alcohol, or a saccharide.

7. The method according to any one of claims 1 to 6, wherein the solvent used in step c) is sulphuric acid.

8. The method according claims 1 to 5, wherein sulphur dioxide is used as the reducing agent in step b) and sulphuric acid is the solvent used in step c) is sulphuric acid and thereby increase the solubility of vanadium compounds by changing the valence of the vanadium atoms in the compounds released from the substrate from a pentavalent atom to a quatrivalent atom in accordance with the following reaction scheme:

R4H2V10O28 + H2SO4 + S0₂ -* R2-SO4 + V0²⁺S0₄ ^2"

9. The method according to any one of claims 1 to 8, wherein the method includes a step of treating the substrate with substitute anions to replace the vanadium compounds released from the substrate in step b) and thereby prevent contamination of the substrate and thus the load capacity of the substrate.

10. The method according to claim 9, whereby when the solvent used in step c) is sulphuric acid, sulphate anions of sulphuric acid can act as the substitute anions.

11. The method according to claim 7, wherein the concentration of sulphuric acid used in step c) has a concentration ranging from 1 to 6 M.

12. The method according to claim 11, wherein the method also includes separating the solvent that has been used in step c) from the substrate and thereby provide a sulphuric acid and vanadium containing electrolytic solution that can be used in the half cells of a vanadium redox flow battery.

13. The method according to claim 12, wherein the method includes adding a reducing agent to the solution to adjust the proportions of trivalent and tetravalent vanadium in the solution.

14. The method according to claims 12 or 13, wherein the concentration of vanadium contained in the solution ranges from 50 to 200 grams per litre (1 to 4 M) and the concentration of sulphuric acid in the solution ranges from 100 to 600 grams per litre (1 to 6 M) .

15. The method according to any one of claims 1 to 14, wherein the method also includes a step of forming the mixture from which vanadium containing compounds are extracted by dissolving a raw material containing vanadium compounds in a further solvent under alkaline or neutral conditions.

16. The method according to claim 15, wherein an alkali such as ammonia or caustic soda is used to create alkaline or neutral conditions while forming the mixture.

17. The method according to claim 15 or 16, wherein the raw material used in forming the mixture includes any one or a combination of the following materials containing vanadium or compounds containing vanadium: vanadium magnetite, fly ash, vanadium pentoxide, and sodium or ammonium metavanadate.

18. The method according to claim 17, wherein the raw material is a vanadium magnetite concentrate that has been roasted with a sodium salt.

19. The method according to claim 17, wherein the raw material is a vanadium magnetite concentrate that has been roasted with a calcium or magnesium salt or a mixture thereof.

20. The method according to claim 15, wherein the further solvent is water and the impurities include water- soluble metal salts, silicates, colloidal silicon or mixtures thereof.

21. The method according to any one of claims 1 to 20, whereby when the mixture contains silica, the method includes reducing the concentration of soluble silicon compounds in the mixture to lOOOppm or less prior to treatment according to steps a) to σ) .

22. The method according to claim 21, wherein the step of reducing the concentration of soluble silica is carried out by reaction the silica with aluminium sulphate to form a precipitate.

23. The method according to any one of claims 1 to 22, wherein the method includes adjusting the pH of the mixture to range from 2.0 to 5.5 prior to carrying out step a) and thereby transform metavanadate compounds to polyvanadate compounds in which vanadium atoms occur at a higher proportion.

24. The method according to any one of claims 1 to 23, wherein the substrate can be recycled from said desorption step b) back to said adsorption step a) .

25. A plant for carrying out the method including any one of claims 1 to 24 the plant including: i) an adsorption column in which step a) can be carried out; and ii) a desorption column in which steps b) and c) can be carried out.

26. A vanadium electrolytic solution suitable for use in a redox flow battery made by the method according to any one of claims 1 to 25.

27. A redox flow battery in which one or both of the half-cells of the battery contains a vanadium solution for generating electrical power, wherein the vanadium solution is made by the method according to any one of claims 1 to

25.