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Title:
PROCESS FOR THE EXTRACTION OF SCANDIUM FROM SCANDIUM CONTAINING MATERIALS
Document Type and Number:
WIPO Patent Application WO/2014/032095
Kind Code:
A1
Abstract:
A method for recovering scandium comprises the steps of providing an aqueous solution containing one or more dissolved EDTA salts, contacting the solution with a scandium-containing material so that scandium and at least one other impurity metal are transferred into the solution to produce a scandium loaded solution, separating a depleted scandium-containing material from the scandium loaded solution, treating the scandium loaded solution to remove one or more of the impurity metals whilst avoiding substantial precipitation of scandium and further treating the scandium loaded solution from step (iv) to precipitate scandium.

Inventors:
NAKON DAVID GREGORY (AU)
Application Number:
PCT/AU2013/000959
Publication Date:
March 06, 2014
Filing Date:
August 28, 2013
Export Citation:
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Assignee:
SCANDIUM PTY LTD (AU)
International Classes:
C22B3/16; C22B59/00
Foreign References:
US4968504A1990-11-06
US4718995A1988-01-12
US4898719A1990-02-06
Attorney, Agent or Firm:
CULLENS PATENT AND TRADE MARK ATTORNEYS (Brisbane, Queensland 4001, AU)
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Claims:
CLAIMS.

1. A method for recovering scandium comprising the steps of: a) providing an aqueous solution containing one or more dissolved EDTA salts, b) contacting the solution with a scandium-containing material so that scandium and at least one other impurity metal are transferred into the solution to produce a scandium loaded solution; c) separating a depleted scandium-containing material from the scandium loaded solution; d) treating the scandium loaded solution by increasing pH to selectively precipitate one or more of the impurity metals whilst avoiding substantial precipitation of scandium; and e) further increasing pH of the aqueous solution to precipitate scandium.

2. A method as claimed in claim 1 wherein the at least one other impurity metal comprises aluminium, calcium, cobalt, chromium, copper, iron, magnesium, manganese, nickel, lead, arsenic, antimony, silicon, sodium, potassium, titanium, zinc and zirconium, or mixtures of two or more thereof.

3. A method as claimed in claim 1 or claim 2 wherein the salts of EDTA comprise sodium, potassium, ammonium, barium, calcium or magnesium salts of EDTA, or mixtures thereof.

4. A method as claimed in any one of claims 1 to 3 wherein the , salts of EDTA are in mono-protonated form, di-protonated form or tri-protonated form.

5. A method as claimed in any one of claims 1 to 4 wherein the concentration of EDTA salts are adjusted such that the concentration is as high as possible without causing precipitation of any of the salts.

6. A method as claimed in any one of the preceding claims wherein the solution containing one or more dissolved EDTA salts is recycled.

7. A method as claimed in any one of the preceding claims wherein one or more reducing agents are also be present in the solution.

8. A method as claimed in claim 7 wherein the reducing agents present in the solution comprise metabisulphite salts, sulphite salts, sulphur dioxide or sulfurous acid, or metallic iron or zinc powder. The one or more reducing agents may be added at any stage of the process.

9. A method as claimed in any one of claims 1 to 6 wherein one or more oxidising agents are present in the solution.

10. A method as claimed in claim 9 wherein the one or more oxidising agents comprise hydrogen peroxides, potassium permanganate, sodium hypochlorite, calcium hypochlorite, air or oxygen, chlorine or sodium peroxide.

11. A method as claimed in any one of the preceding claims wherein one or more modifiers are added to the solution, the one or more modifiers being selected from the fluoride salts of sodium, potassium, ammonium, calcium, barium or magnesium; the oxalate and or hydrogen oxalate salts of sodium, potassium, ammonium, calcium, barium or magnesium; hydrofluoric acid; oxalic acid or mixtures thereof.

12. A method as claimed in any one of the preceding claims wherein the scandium containing material comprises a scandium loaded ion-exchange resin, a scandium- loaded liquid organic extractant, a solid scandium containing material such as a residue, waste or intermediate arising from the treatment of an ore or concentrate or arising from the treatment of another solid material.

13. A method as claimed in any one of the preceding claims wherein contacting of the scandium containing material with the aqueous solution occurs under a blanket gas, such as nitrogen or argon.

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14. A method as claimed in any one of the preceding claims wherein the solution containing one or more dissolved EDTA salts and the scandium containing material is mixed in a ratio to ensure that most of the scandium present in the scandium containing material is transferred into the aqueous phase and is significantly bound in an EDTA complex.

15. A method as claimed in claim 14 wherein the ratio is adjusted such that there are insufficient EDTA salt(s) present to bind impurity metal ions with formation constants less than that of scandium.

16. A method as claimed in any one of the preceding claims wherein one or more EDTA salts are present in step (b) in sufficient quantity to bind not only scandium but also other extractable metal ions that have formation constants in the range of or greater than scandium, to ensure that a deficit of EDTA salt(s) for binding scandium does not occur.

17. A method as claimed in any one of the preceding claims wherein step (b) is operated at ambient temperature or at elevated temperature.

18. A method as claimed in any one of the preceding claims wherein transfer of the scandium from the scandium containing material into the aqueous solution also

. results in the transfer of some impurjty metals including iron and aluminium into the solution.

19. A method as claimed in claim 18 wherein step (d) comprises increasing the pH to precipitate iron from solution and increasing pH to precipitate aluminium from solution and separating precipitates of iron from solution and separating precipitates of aluminium from solution.

20. A method as claimed in claim 19 wherein one or more other impurities selected from titanium, chromium, silicon and zirconium precipitate with the iron and/or aluminium.

21. A method as claimed in claim 19 or claim 20 aluminium is precipitated by increasing pH to about 10±0.5.

22. A method as claimed in any one of the preceding claims wherein precipitated impurities are removed prior to step (e) to leave behind a solution loaded with scandium that was transferred into the loaded solution.

23. A method as claimed in any one of the preceding claims wherein step (e) comprises increasing the pH of the solution to greater than 10.5 to recover scandium in the form of scandium hydroxide.

24. A method as claimed in claim 23 wherein the pH is increased to between 10.5 and 12.6 to recover scandium in the form of scandium hydroxide.

25. A method as claimed in claim 23 or claim 24 wherein the scandium hydroxide is separated from the liquid phase using a solid/liquid separation process.

26. A method as claimed in any one of the preceding claims wherein the scandium precipitated in step (e) is separated from the solution.

27. A method as claimed in claim 26 wherein the solution remaining after scandium removal contains other dissolved metals that form complexes with EDTA and remain in solution during precipitation of scandium hydroxide and the other dissolved metals are removed from solution by increasing the pH to cause precipitation of the other metals.

28. A method as claimed in any one of the preceding claims wherein the pH is increased to cause precipitation of impurity metals or scandium by adding an alkaline material to the scandium loaded solution.

29. A method as claimed in claim 28 wherein the alkaline material is selected from include caustic soda (sodium hydroxide), sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, potassium carbonate, potassium hydrogen carbonate, ammonia, ammonium hydroxide, ammonium carbonate, ammonium hydrogen carbonate, calcium oxide, calcium hydroxide, calcium carbonate, magnesium hydroxide, magnesia, magnesium carbonate, barium hydroxide, barium oxide and barium carbonate.

30. A method as claimed in any one of the preceding claims wherein impurity metals selected from copper, zinc, nickel or manganese are removed using sulphide precipitating agents.

31. A method as claimed in claim 30 wherein sulphide addition takes place at a pH of around 5, and this will result in partial to complete removal of copper, zinc, nickel and manganese from solution.

32. A method for recovering scandium comprising the steps of: i) providing an aqueous solution containing one or more dissolved EDTA salts, ii) contacting the solution with a scandium-containing material so that scandium and at least one other impurity metal are transferred into the solution to produce a scandium loaded solution; iii) separating a depleted scandium-containing material from the scandium loaded solution; iv) treating the scandium loaded solution to remove one or more of the impurity metals whilst avoiding substantial precipitation of scandium; and v) further treating the scandium loaded solution from step (iv) to precipitate scandium.

33. A method as claimed in claim 32 wherein step (iv) occurs at a pH of 10.5 or less, which assists in maintaining scandium in solution whilst the one or more impurity metals are precipitated from solution.

34. A method as claimed in claim 32 or claim 33 wherein step (v) comprise increasing the pH of the solution to greater than 10.5, preferably from 10.5 to 12.6, to cause precipitation of scandium hydroxide in high purity.

35. A method as claimed in claim 34 wherein the scandium hydroxide that is precipitated is at least 90% pure, more preferably at least 95% pure, even more preferably at least 99% pure.

36. A method as claimed in any one of claims 32 to 35 wherein step (iv) comprises adding a precipitating agent to the loaded solution.

37. A method as claimed in claim 36 wherein the precipitating agent is selected from an alkaline material, a sulphide material, a phosphate, a mono-hydrogen phosphate, a di-hydrogen phosphate, an oxalate, a hydrogen oxalate, a carbonate, a hydrogen carbonate and a fluoride.

38. A method as claimed in any one of the preceding claims wherein the scandium depleted EDTA salt solution remaining after scandium removal is recycled for reuse in the process.

39. A method as claimed in claim 39 wherein the pH of the recycled solution is lowered prior to contacting the recycled solution with the scandium-containing material.

40. A method as claimed in claim 38 or claim 39 wherein part or all of the depleted scandium solution is treated to lower its pH to level at which sparingly soluble tetra-protonated EDTA is formed and an EDTA-containing precipitate is separated from the solution and the EDTA depleted liquor is discarded from the process to regulate build-up of impurities that would otherwise arise from recycling the solution.

41. A method as claimed in claim 40 wherein the EDTA precipitate is re-dissolved and returned to the step of contacting the EDTA salt solution with the scandium containing material.

Description:
PROCESS FOR THE EXTRACTION OF SCANDIUM FROM SCANDIUM CONTAINING MATERIALS

FIELD OF THE INVENTION

The present invention relates to a process for recovering scandium from solution. In some embodiments, scandium may be recovered from solution in the form of scandium hydroxide or it may be further treated to yield scandium oxide or other scandium compounds.

BACKGROUND TO THE INVENTION

Scandium is a high value metal, typically supplied in the form of scandium oxide. Annual world production of scandium is quite small, totalling approximately 2 tonnes per year. Demand for scandium beyond that provided by annual production is currently being met by a drawdown from a relatively large stockpile in Russia (this stockpile was generated by the former USSR during the Cold War).

Due to the small annual production of scandium, it is a high value material, with prices for scandium oxides ranging from $900 per kilogram to $3250 per kilogram, depending upon purity.

Scandium is used as an alloying agent in aluminium alloys. Addition of scandium in amounts of up to 0.5% by weight to aluminium alloys can significantly improve the properties of the alloys. These alloys are used in aircraft manufacture, and sporting goods requiring high-strength, such as baseball bats, bicycle frames and bicycle components. Scandium is also finding use as a component used in mixed metal oxides in fuel cells. Scandium is also used in the manufacture of high intensity discharge lamps.

Scandium has typically been produced as a by-product of other metal recovery processes. For example, scandium has been produced from tungsten digestion sludge, uranium tailings, Bayer process red mud, titanium white hydrolytic solution, zircon ore, tantalum residues and niobium residues. Production of scandium products via hydrometallurgical pathways has typically been achieved using three main techniques or combinations of those techniques, these being ion exchange, solvent extraction or multistage precipitation and re-leaching to form an enriched scandium product from a feed solution containing scandium and a host of impurities.

US 4816233, assigned to GTE Laboratories Inc., describes a method for recovering scandium values from a tungsten ore residue. In this method, tungsten residues are contacted with an acidic solution to dissolve scandium, iron and manganese into the acidic solution. The acidic solution contains a reducing agent such that Mn + ions are converted to Mn 2+ ions. Trivalent iron ions (ferric ions, Fe 3+ ) are converted to divalent iron ions (ferrous ions, Fe 2+ ). The solution is then contacted with an ion exchange resin at a pH of from 1.9 to 2.1. Scandium is adsorbed onto the ion exchange resin. The loaded resin is then washed with dilute acid to remove any base metals and rare earth metals on the resin without removing scandium from the resin. This results in the loaded resin essentially only having scandium adsorbed thereon. The scandium is eluted from the resin using a chelating agent. Diglycolic acid is the preferred chelating agent, but other chelating agents such as carboxylic and hydroxy acids and EDTA may also be used. This forms a high purity scandium solution. Scandium is then precipitated from the solution, for example by adding ammonium hydroxide to increase pH to above 7.

US 4898719, assigned to GTE Laboratories Inc., describes a liquid extraction process for the recovery of scandium. In this process, a digestion solution containing dissolved scandium and other base metals is formed. Dissolved iron is brought to the divalent state by reduction and the pH is reduced to about 2. Scandium is selectively extracted from the solution using an organic extractant consisting of thenoyltrifluoroacetone (TTA) dissolved in an aromatic solvent. The scandium is described as forming a very stable neutral chelate complex with the TTA. The TTA shows a high degree of selectivity for scandium over the divalent transition metals, alkaline earth metals, alkali metals and rare earth metals in the system. Scandium is recovered from the organic phase by stripping with an acid, followed by precipitation of scandium as a hydroxide or oxalate from the acid solution. US 4765909, assigned to GTE Laboratories Inc., describes a process of separating scandium from thorium. This process requires the adsorption of both metals onto a cation exchange resin, followed by selective elution of scandium from the resin using a solution containing 0.1M diglycolic acid and 1.2M hydrochloric acid, followed by elution of thorium with 6N hydrochloric acid solution.

A particular difficulty that exists with many scandium recovery processes that utilise hydrometallurgy is that the dissolved scandium is normally present with a number of other dissolved metal ions. Separation of scandium from the other dissolved metals can be difficult, as is shown by the prior art references discussed above, which all involve a number of steps required to eventually obtain a substantially pure scandium- containing solution. For example, in processes that utilise ion exchange resins to remove scandium from solution, it is normally necessary to subject the solution to a reduction step to reduce any dissolved manganese or iron to the divalent state. In the divalent state, manganese and iron do not typically adsorb onto the ion exchange resins being utilised.

A number of prior art processes for recovery of scandium are also directed towards using solvent extraction. Such processes are described in, for example, US patent 4624703, US patent 4808384, US patent 5492680, US patent 5787332, US patent 5015447, US patent 5030424, US patent 5039336 and US patent 4898719. Many of these processes require reduction of the solution prior to the solvent extraction step. Others result in the production of a scandium precipitate upon stripping of the loaded organic. This is known to be problematic and is thus avoided in industrial scale extraction processes as the solids tend to foul the organic phase to form a troublesome crud layer. Additionally, the direct precipitation stripping methods afford no opportunity for impurities removal prior to the formation of the scandium precipitate. Another of these processes proposes the use of hydrofluoric acid solution at industrial scale. However, it is known that processes using hydrofluoric acid are technically challenging, expensive and dangerous due to the difficulties experienced in handling and maintaining a safe operating environment. Many of the solvent extraction processes also propose the use of organic extractants that are not practical for large industrial scale process, in which issues including extractant degradation, excessive aqueous solubility and environmental unsuitability are encountered. A further difficulty arises if the solution contains dissolved aluminium and/or dissolved iron along with dissolved scandium. In such cases, treating the solution to precipitate aluminium by increasing pH results in the simultaneous precipitation of significant amounts of scandium, thereby incurring heavy scandium losses. Similarly, it is known that precipitation of large quantities of iron typically induces kinetic co- precipitation of other metals further along the hydrolysis curve, thus increasing the potential for additional scandium losses. Thus, selective and separate precipitation of iron, aluminium and scandium has proven to be difficult.

The present applicant does not concede that the prior art discussed in this specification forms part of the common general knowledge in Australia or elsewhere.

Throughout this specification, the word "comprising" and its grammatical equivalence shall be taken to have an inclusive meaning unless the context of use indicates otherwise.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a method for recovering scandium.

In a first aspect, the present invention provides a method for recovering scandium comprising the steps of: a) providing an aqueous solution containing one or more dissolved EDTA salts, b) contacting the solution with a scandium-containing material so that scandium and at least one other impurity metal are transferred into the solution to produce a scandium loaded solution; c) separating a depleted scandium-containing material from the scandium loaded solution; d) treating the scandium loaded solution by increasing pH to selectively precipitate one or more of the impurity metals whilst avoiding substantial precipitation of scandium; and e) further increasing pH of the aqueous solution to precipitate scandium.

The at least one other impurity metal may be selected from, but is not limited to, aluminium, calcium, cobalt, chromium, copper, iron, magnesium, manganese, nickel, lead, arsenic, antimony, silicon, sodium, potassium, titanium, zinc and zirconium, or mixtures of two or more thereof.

The salts of EDTA that may be used include sodium, potassium, ammonium, barium, calcium or magnesium salts of EDTA, or mixtures thereof. The salts may take the mono-protonated form, di-protonated form or tri-protonated form. It is noted that the tetra-protonated form (i.e. tetraacetic acid form) will likely be only a minor component due to its limited solubility in water and relatively weak complexing ability. Preferably, the concentration of EDTA salts will be adjusted such that the concentration is as high as possible without causing precipitation of any of the salts. It will be understood that lower concentrations may also be used.

In some embodiments of the present invention, the solution containing one or more dissolved EDTA salts is recycled. This will tend to introduce other salts into the solution. Other salts that may be present in the solution include the sulphate, hydrogen-sulphate, chloride or nitrate of sodium, potassium, ammonium, calcium, barium or magnesium, or combinations thereof.

In some embodiments, one or more reducing agents may also be present in the solution. The reducing agents may be beneficial to the scandium purification process that is used. Reducing agents that may be present in the solution include metabisulphite salts, sulphite salts, sulphur dioxide and sulfurous acid. Other common reducing agents may also be used, for example the addition of metallic iron or zinc powder. The one or more reducing agents may be added at any stage of the process. If the reducing agents are added after step (b), recycle of solution to step (b) may result in the reducing agents being present in step (b). In other embodiments of the present invention, one or more oxidising agents may be present in the solution. The one or more oxidising agents may be selected so that oxidising agents that are beneficial to the scandium purification process are used. The one or more oxidising agents may include hydrogen peroxides, potassium permanganate, sodium hypochlorite, calcium hypochlorite, air or oxygen, chlorine and sodium peroxide. The one or more oxidising agents may be added at any stage of the process. If the oxidising agents are added after step (b), recycle of solution to step (b) may result in the oxidising agents being present in step (b).

One or more modifiers may also be added to the solution. This may be beneficial for decreasing the time required for transferring scandium into the solution and /or increasing the amount of scandium transferred into the solution. The modifiers may be selected from the fluoride salts of sodium, potassium, ammonium, calcium, barium or magnesium; the oxalate and or hydrogen oxalate salts of sodium, potassium, ammonium, calcium, barium or magnesium; hydrofluoric acid; oxalic acid or mixtures thereof. The modifiers may be added before or after step (b). If the modifiers are added after step (b), recycle of solution to step (b) may result in the modifiers being present in step (b).

Step (b) of the present invention involves contacting the aqueous solution containing one or more dissolved EDTA salts (and possibly other dissolved components) with a scandium containing material. This results in scandium going into solution.

The scandium containing material may comprise a scandium loaded ion-exchange resin, or it may comprise a scandium-loaded liquid organic extractant, or it may comprise a solid scandium containing material such as a residue, waste or intermediate arising from the treatment of an ore or concentrate or arising from the treatment of another solid material.

Transfer of scandium from the scandium containing material into the aqueous solution is achieved by mixing or contacting the aqueous solution with the scandium containing material for a period of time that is sufficient to allow the solution to become enriched with scandium. Typically, scandium ions will be transferred into the solution. Mixing or contacting of the scandium containing material with the solution may occur in a batch manner or in a continuous process. One or more mixing/contacting stages may be used. Co-current, counter current and/or crosscurrents contacting processes may be used.

Contacting of the scandium containing material with the aqueous solution may occur under a blanket gas, such as nitrogen or argon. This may be useful, for example, where it is desired to avoid oxidation during the contacting step, or where reducing agents are present in the solution. It will be appreciated that use of a blanket gas may not be required in other embodiments of the present invention.

The ratio of aqueous solution containing dissolved EDTA salts and the scandium containing material can encompass any particular ratio that is sufficient for transferring scandium into the solution.

In some embodiments, the solution containing one or more dissolved EDTA salts and the scandium containing material is mixed in a ratio to ensure that most of the scandium present in the scandium containing material is transferred into the aqueous phase and is significantly bound by the EDTA complex. In some embodiments, the ratio may be adjusted such that there are insufficient EDTA salt(s) present to bind i

impurity metal ions with formation constants less than that of scandium. This may assist in minimising introduction of impurity ions into the solution. In other embodiments, the ratio may be adjusted such that there is an excess o ' f EDTA salt(s), should transfer of one or more impurity ions be desirable or beneficial to the overall process.

In some embodiments of the present invention, the one or more EDTA salts are desirably added in sufficient quantity to bind not only scandium but also other extractable metal ions that have formation constants in the range of or greater than scandium, to ensure that a deficit of EDTA salt(s) for binding scandium does not occur. In some embodiments, where a modifier is used to increase the amount of scandium transferred into solution, the solution containing one or more dissolved EDTA salts and the modifier are mixed with the scandium containing material in a ratio to ensure that most of the scandium present in the scandium containing material is transferred into the aqueous phase and is significantly bound by the EDTA and/or modifier. In some embodiments, the ratio may be adjusted such that there is insufficient EDTA salt(s) and/or modifier to bind impurity metal ions with formation constants less than that of scandium. This may assist in minimising introduction of impurity ions into the solution. In other embodiments, the ratio may be adjusted such that there is an excess of EDTA salt(s) and or modifier, should transfer of one or more impurity ions be desirable or beneficial to the overall process.

In some embodiments of the present invention, the one or more EDTA salts and/or modifier is desirably added in sufficient quantity to bind not only scandium but also other extractable metal ions that have formation constants in the range of or greater than scandium, to ensure that a deficit of EDTA salt(s) and/or modifier for binding scandium does not occur.

Step (b) may be operated at ambient temperature or at elevated temperature. Simple experiments will be able to be conducted to determine the optimal temperature at which step (b) can be operated.

Step (b) may be operated at any pH that is effective for transferring scandium from the scandium containing material into the aqueous solution containing one or more dissolved EDTA salts. The ideal pH will be dependent on the nature of the scandium containing material, with simple experiments able to determine the optimal pH at which step (b) can be operated.

In the present invention, following the step of contacting/mixing of the scandium containing material and the aqueous solution, the scandium loaded solution is separated from the depleted scandium containing material. Any separation technique known to a person skilled in the art may be used. The particular separation technique that is selected will largely depend upon the scandium containing material that has been treated. For example, if the scandium containing material is a solid material (such as an ion exchange resin or a solid residue, waste or intermediate), a solid-liquid separation technique may be used. Such techniques may be selected from filtration, centrifugation, settling, clarification, use of hydrocyclones, thickening and the like. In some instances, the solid scandium containing material may be in the form of a packed bed and the solution may simply flow through the packed bed to become loaded with scandium and subsequently flow out of the packed bed to thereby separate the loaded solution from the depleted scandium containing material. This embodiment may be used, for example, where the scandium containing material comprises a loaded ion exchange resin.

In embodiments where the scandium containing material comprises a liquid organic extractant, the aqueous solution containing one or more EDTA salts and the liquid organic extractant will typically be mixed in a stirred tank. Separation may take place by settling and decantation. Alternatively, contact may take place by counter current flow of the organic extractant and the aqueous solution through any device designed for the purpose, for example a packed bed or pulsed column, in which separation of the organic phase and the aqueous phase will occur due to the design of the packed bed or pulsed column apparatus.

Transfer of the scandium from the scandium containing material into the aqueous solution is likely to result in the transfer of some impurity metals into the solution as well. Scandium containing materials frequently contain iron and/or aluminium and it is typical for one or both of iron and aluminium to also be transferred into the solution. Prior to the present invention, this has caused difficulties in recovering scandium at high purity levels and/or at a good yield from the loaded solution. For any given leach solution, the extent of coprecipitation will, for the most part, be dictated by the metals' hydrolytic stability with respect to hydroxide formation, with those metals that are hydrolysed easily being more susceptible to coprecipitation. The hydrolysis curves for aluminium and scandium are similar and therefore it is unlikely that selective aluminium precipitation can be achieved without incurring heavy scandium losses. Additionally, it is well-known that the precipitation of large quantities of iron typically induces co-precipitation of other metals further along the hydrolysis curve, thus increasing the potential for additional scandium losses. With respect to scandium, it has previously been demonstrated that scandium directly substitutes into jarosites, with increased substitution being realised as the precipitation increases. Again, this leads to increased losses of scandium.

The present invention allows for the selective removal of iron and/or aluminium from the scandium loaded solution, followed by the selective precipitation of scandium to thereby form a scandium containing product of high purity. Additionally the present invention allows for the selective removal of titanium, chromium, silicon, and zirconium from the scandium loaded solution, followed by the selective precipitation of scandium to thereby form a scandium containing product of high purity.

The pH at which the precipitation of impurities (e.g. Fe, Al, Ti, Cr, Si, Zr etc.) occurs can vary considerably depending on the experimental conditions employed (e.g. temperature, precipitant, solution Eh, ratio of aqueous solution containing dissolved EDTA salts to the scandium containing material etc.). Generally however the following pH ranges are typically observed for the present invention:

Iron: precipitation onset > pH 6, approaches completion by pH ~ 11

Titanium: precipitation onset > pH 5, approaches completion by pH ~ 10

Chromium: precipitation onset > pH 8, majority precipitated by pH 10.3+/- 0.5 units Silicon: precipitation onset > pH 6, approaches completion by pH ~ 10

Zirconium: precipitation onset > pH 7, approaches completion by pH ~ 10

Aluminium: precipitation onset > pH 7, approaches a minimum solubility ~ pH 10+/- 0.5 units, increases in solubility beyond this due to hydroxide ion complexation (this dissolution forms the basis of the digestion of bauxite in the Bayer process): A1(0H) 3(S ) + OH- (aq) → [Al(OH) 4 ]-(aq)

Therefore, when aluminium is present as an impurity in the loaded solution, it may be desirable to separate precipitated aluminium containing solids in the region of the minimum solubility point of aluminium. The precipitated aluminium containing solids may be separated using a solid/liquid separation technique, such as filtration. After separation of the precipitated aluminium containing solids, further increases in the pH of the loaded solution retains any remaining aluminium in the solution as [Al(OH)4] ' (aq ) and it thus does not report to the final scandium product. The precipitated impurities can be removed at any point, such as by filtration, to leave behind a solution still loaded with most, if not all, of the scandium that was transferred into the loaded solution.

Scandium may then be recovered from the solution. Scandium may be recovered by increasing the pH of the solution to greater than 10.5, preferably to between 10.5 and 12.6, to recover scandium in the form of scandium hydroxide. The scandium hydroxide can be separated from the liquid phase, for example, by filtration. The recovered scandium hydroxide may be sold as a product or it may be subjected to further treatments known to a person skilled in the art. Further treatments that the scandium hydroxide may be subjected to may include washing, drying, calcining to form a scandium oxide (if required) and packaging of a marketable high purity scandium product. The scandium hydroxide may also be subjected to further purification steps, depending upon the impurity level in the scandium hydroxide.

The solution remaining after scandium removal may contain other dissolved metals that form complexes with EDTA and remain in solution during precipitation of scandium hydroxide. These metals may include, but are not limited to calcium, cobalt, chromium, copper, magnesium, nickel, lead, sodium, potassium and zinc. As these metals may be mostly retained in solution during scandium precipitation, they cause little or no impurity problems in the solid scandium product. These impurity metals may be removed after scandium separation by further pH elevation, if desired.

Adjusting the pH of the solution to increase the pH into the required ranges to selectively precipitate impurity metals and scandium can be conveniently achieved by adding an alkaline material to the scandium loaded solution. Any alkaline material, or combination of alkaline materials may be used to increase the pH of the loaded solution. Examples include caustic soda (sodium hydroxide), sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, potassium carbonate, potassium hydrogen carbonate, ammonia, ammonium hydroxide, ammonium carbonate, ammonium hydrogen carbonate, calcium oxide, calcium hydroxide, calcium carbonate, magnesium hydroxide, magnesia, magnesium carbonate, barium hydroxide, barium oxide and barium carbonate. The alkaline material may be added in the form of a solution or a slurry or suspension. Step (d) of the present invention may comprise the selective precipitation of one or more of the impurity metals from the solution. It will be appreciated that, in step (d), some impurity metals may not be precipitated and therefore remain in solution. In such embodiments, step (e) preferably comprises the selective precipitation of scandium in high purity whilst avoiding or minimising precipitation of any impurity metals remaining in the solution.

In some embodiments, impurity metals may be removed using other precipitating agents. For example, sulphide type reagents such as sodium sulphide can efficiently remove copper (either before or after scandium removal has taken place). Other base metals, such as but not limited to, zinc and nickel may also be removed by sulphide addition. Sulphide addition may take place prior to precipitation of scandium in order to remove impurity metals from the solution before scandium precipitation takes place. In one embodiment, sulphide addition takes place at a pH of around 5, and this will result in partial to complete removal of copper, zinc, nickel and manganese from solution.

In a second aspect, the present invention provides a method for recovering scandium comprising the steps of: i) providing an aqueous solution containing one or more dissolved EDTA salts, ii) contacting the solution with a scandium-containing material so that scandium and at least one other impurity metal are transferred into the solution to produce a scandium loaded solution; iii) separating a depleted scandium-containing material from the scandium loaded solution; iv) treating the scandium loaded solution to remove one or more of the impurity metals whilst avoiding substantial precipitation of scandium; and 1 v) further treating the scandium loaded solution from step (iv) to precipitate scandium.

In the second aspect of the present invention, step (iv) may comprise increasing the pH to selectively precipitate impurities, as described with reference to the first aspect of the present invention. Step (iv) may also comprise adding a precipitating agent to precipitate one or more impurities (such as copper, zinc, nickel and manganese), whilst avoiding substantial precipitation of scandium. Suitably, step (iv) occurs at a pH of 10.5 or less, which assists in maintaining scandium in solution whilst the one or more impurity metals are precipitated from solution.

Step (v) may comprise increasing the pH of the solution to greater than 10.5, preferably from 10.5 to 12.6, to cause precipitation of scandium hydroxide in high purity. Suitably, the scandium hydroxide that is precipitated is at least 90% pure, more preferably at least 95% pure, even more preferably at least 99% pure.

Step (iv) may comprise adding a precipitating agent to the loaded solution. The precipitating agent may comprise an alkaline material, a sulphide material, a phosphate, a mono-hydrogen phosphate, a di-hydrogen phosphate, an oxalate, a hydrogen oxalate, a carbonate, a hydrogen carbonate and a fluoride. Salt forms of the precipitating agents (such as the sodium, potassium, ammonium, calcium or magnesium salts) may be used, or acid forms of the precipitating agents may be used.

In some embodiments of both aspects of the present invention, the scandium depleted EDTA salt solution remaining^ after scandium removal may be recycled for re-use in the process. Since step (e) or step (v) results in the solution having an elevated pH, it may be necessary to lower the pH to render the solution suitable for recycling. This may be conveniently achieved by the addition of sulphuric acid, although other acids may be used.

Additionally in one embodiment, part or all of the depleted scandium solution (which now has an elevated pH) is treated to lower its pH to level at which sparingly soluble tetra-protonated EDTA is formed. This may be achieved by adding an acid (such as a mineral acid, e.g. sulphuric acid, although other acids may also be used). This sparingly soluble tetra-protonated EDTA precipitates and may be separated from the aqueous liquor using known solid/liquid separation techniques. The EDTA depleted liquor may be discarded from the process, and thereby act as a means to regulate the build of various impurities should some of the liquor from step (e) or step (v) be recycled for re-use in the process.

The solid EDTA may be re-dissolved and returned to the step of contacting the EDTA salt solution with the scandium containing material. In one embodiment this may be conveniently achieved by directly re-dissolving the solid EDTA into liquor from step e) or step (v) that is recycled for re-use in the process, thereby regenerating the EDTA salt.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a flowsheet of an embodiment of the method of the present invention. DETAILED DESCRIPTION OF THE DRAWINGS

It will be understood that the attached drawing has been provided to illustrate preferred embodiments of the present invention. Therefore, the skilled person will appreciate that the present invention should not be considered to be limited solely to the features as shown in the attached drawings.

In the process flowsheet shown in figure 1, a scandium containing material 10 is mixed with an aqueous solution 12 containing one or more salts of EDTA, such as the sodium salts of EDTA. The scandium containing material 10 in this embodiment comprises an organic solvent extractant that has been used to extract scandium, iron and other impurities from pregnant leach liquor obtained from high-pressure acid leaching of a nickel laterite ore. Persons skilled in the art will understand that pregnant leach solutions obtained from high-pressure acid leaching of nickel laterite materials contain dissolved nickel, cobalt, iron and number of other impurities, including scandium (in this instance, "impurities" is used to describe metals in the nickel/cobalt leach circuit that are not nickel or cobalt). In order to remove impurities from the solution, solvent extraction is used in which the pregnant leach solution is ■ contacted with an organic solvent extractant. Following solvent extraction, the aqueous raffinate contains dissolved nickel and cobalt. The bulk of the scandium and various amounts of impurity metals are transferred into the organic phase. It is this loaded organic phase that forms the scandium containing material 10.

The organic phase 10 and the aqueous EDTA salt solution 12 are contacted in a mixer or contactor 14. Mixer or contactor 14 may be any suitable type of mixing or contacting apparatus. For example, the mixer or contactor may comprise one or more packed beds or one or more stirred tanks. The mixer or contactor 14 may, for example, comprise a multistage countercurrent liquid-liquid extraction process in which intimate mixing between the organic phase 10 and the aqueous phase 12 occurs. As a result of the intimate mixing of the organic phase 10 and the aqueous phase 12, scandium that is present in the organic phase 10 is transferred into the aqueous phase 12. Other impurity metals, such as iron, aluminium, titanium, zirconium, silicon and possibly nickel, zinc and manganese, are also transferred into the aqueous phase.

Following contacting, the mixture of the organic phase and the aqueous phase are passed to a settling tank 16. In the settling tank, the organic phase disengages from the aqueous phase. The organic phase, now depleted in scandium (and having a lower content of other impurity metals as well) is transferred via line 18 for re-use in treatment of the pregnant leach liquor from the high-pressure acid leach of the laterite ore.

The aqueous phase, which now comprises a solution of EDTA salts, also contains scandium, aluminium, iron, titanium, zirconium, silicon and other impurity metals and has a pH of typically around 2 to 3, is transferred via line 20 to precipitation stage 22. In precipitation stage 22, a precipitating agent is supplied via line 24 to cause the precipitation of zinc, manganese and nickel. In particular, the precipitating agent that is supplied via line 24 comprises an aqueous solution of sodium hydroxide and sodium sulphide. The precipitating agent is provided in an amount that is sufficient to increase the pH to around 5. This results in the formation of solid zinc sulphide, manganese sulphide and nickel sulphide. The slurry arising from this precipitation stage is sent to a solid/liquid separation stage 24. Solid/liquid separation stage 24 is suitably a filtration stage, although centrifugation, thickening, settling or clarification may also be used. The solid sulphide material 26 is removed for disposal or for further treatment to recover valuable metals therefrom.

The aqueous phase 28 obtained from solid liquid separation stage 24 comprises an aqueous EDTA salt solution having scandium, iron, aluminium and other metal ion impurities dissolved therein (such that at least scandium and iron are present in solution as complexes of EDTA). This aqueous phase 28 is fed to precipitation stage 30. In precipitation stage 30, an alkaline material, such as a caustic soda solution 32, is fed to the precipitation stage in order to increase the pH to between 8 to 10.5. This results in the precipitation of aluminium, titanium, silicon, zirconium, chromium and iron. Following solid/liquid separation stage 33 the liquid phase 34 is sent to a scandium precipitation stage 36. The solid phase 35 may be sent to disposal or for further treatment. It will be appreciated that precipitation stage 30 and solid/liquid separation stage 33 may comprise two or more separate precipitation and solid/liquid separation stages, thereby yielding two or more separate precipitates.

The liquid phase 34 comprises an aqueous solution of EDTA containing dissolved scandium. A number of impurity metals, including iron, titanium, silicon, zirconium, chromium and aluminium, have been largely or quantitatively removed from the aqueous solution. In precipitation stage 36, sodium hydroxide solution 38 is added to increase the pH to between 10.5 and 12.6. This results in the selective precipitation of scandium (in the form of scandium hydroxide), such that scandium hydroxide of high purity, typically in excess of 95%, is obtained.

The slurry from precipitation stage 36 is sent to a filtration unit 40 (or any other suitable solid liquid separator). The solid scandium containing product 42 is recovered for sale or for further treatment (for example, to convert the scandium hydroxide into scandium oxide, or by subjecting the scandium hydroxide to further purification steps). The liquid phase 44, which now contains an EDTA solution at high pH, is sent to a liquor treatment step 46. Liquor treatment step 46 converts the liquor 44 into a form that is suitable for recycle to contacting step 14. This may include the addition of acid to decrease the solution pH thereby making it suitable for recycling and re-use in step 14. Additionally this may include treatment of some or all of the liquor with acid to the extent that solid EDTA precipitates, which is then separated from the EDTA depleted liquor, and recycled for re-use in the process. The EDTA depleted liquor may be discarded from the process, and thereby act as a means to regulate the build-up of various impurities.

The recycled liquor 48 will normally contain other salts introduced as a consequence of the processing steps to which the recycled liquor has been subjected. These salts may include the sulphate, hydrogen sulphate, chloride or nitrate of sodium, potassium, ammonium, calcium or magnesium, or combinations thereof.

EXAMPLE 1

The following example demonstrates the processing of a scandium loaded solution generated from contacting an EDTA salt solution with a scandium loaded liquid organic extractant. The EDTA salt solution used was in the partial sodium form and additionally contained sodium hydrogen oxalate (modifier), and sodium sulphate as a consequence of the liquor recycling. It is to be appreciated that the analytical data displayed in the following tables come from 1CP analysis, and while very accurate does suffer from variances typical to most analytical techniques.

Table 1 below displays the scandium loaded solution composition post separation from the organic extractant and post partial neutralisation with sodium carbonate to raise the pH from approximately pH 2.5 to pH 4.30, noting that this does not generate any precipitates. Beyond pH 4.30 caustic soda addition was employed to generate a bulk mixed impurity precipitate as demonstrated by Table 1. Table 1 : ICP analysis of scandium loaded solution at various pH levels during the removal of impurities with caustic soda, temperature 60°C +/- 5°C (concentrations given in mg/L).

As shown by Table 1 aluminium precipitates from solution whereby a minimum concentration- is obtained at pH -10.16, with further pH elevation resulting in the partial re-dissolution of aluminium due to complexation by hydroxide ions. Iron precipitation is close to quantitative by pH -10.47, as is the precipitation of titanium, zirconium and silicon by pH 9.93, where partial chromium precipitation has also occurred. Throughout the removal of these impurities it can be seen that little or no scandium losses have been incurred by the process.

Following this the system was filtered to remove the impurities prior to further caustic soda addition to precipitate scandium hydroxide from solution, the result of which is displayed in Table 2.

Table 2: ICP analysis of scandium loaded solution at various pH levels during scandium hydroxide precipitation with caustic soda, temperature 60°C +/- 5°C (concentrations given in mg/L).

pH Al Ca Co Cr Cu Fe Mg Mn Ni Sc Zn

10.44 7.7 1.8 0.9 0.9 3.0 3.4 38.4 5.2 6.70 534.5 4.5

1 1.10 7.2 1.7 0.9 0.9 2.9 0.0 33.4 3.7 6.60 211.5 4.3

11.17 8.2 2.0 1.0 1.0 3.1 0.0 33.2 4.1 7.40 53.4 4.7

11.31 8.3 2.0 1.0 0.9 3.3 0.0 31.2 0.3 7.50 3.9 4.7

11.30 7.9 1.7 1.0 0.7 3.0 0.0 28.2 0.4 7.10 3.4 4.4

11.28 8.5 2.0 1.0 0.5 3.3 0.0 29.9 0.0 7.70 3.1 4.7

11.28 9.7 2.9 1.4 0.0 4.2 0.0 24.4 0.0 9.00 1.2 6.0 As shown by Table 2 aluminium, calcium, cobalt, copper, nickel and zinc do not appreciably precipitate during scandium precipitation. The addition of caustic soda was stopped at pH 1 1.31 and allowed to equilibrate for a further 2.5 hours prior to filtration. Despite residual amounts of chromium, iron, magnesium and manganese co-precipitating during scandium precipitation, a scandium hydroxide precipitate of greater than 95% purity was formed, and that greater than 99.75% of the scandium was recovered during scandium precipitation step.

Persons skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.