Process for selectively extracting cobalt from an aqueous nickel-cobalt solution

Abstract

The present invention provides a multi-stage solvent extraction process for selectively extracting cobalt from an aqueous mixed metal solution comprising nickel and cobalt, whereby each solvent extraction stage is characterized by maintaining a magnesium concentration in an aqueous phase above a predetermined magnesium concentration.

Description

[0001] PROCESS FOR SELECTIVELY EXTRACTING COBALT FROM AN AQUEOUS NICKEL-COBALT SOLUTION

[0002] TECHNICAL FIELD

[0003] The present invention relates to a novel method for selectively extracting cobalt from an aqueous feed solution containing also nickel using a dialkyl phosphinic acid extractant in an organic solvent.

[0004] INTRODUCTION

[0005] The development of lithium-ion batteries, and specifically the use of nickel-manga- nese-cobalt and nickel-cobalt-aluminium cathode materials, has increased the demand for high-purity cobalt sulphate, either as a solid or in solution.

[0006] Impurities in the cathode materials strongly affect the performance of the batteries. As such, much effort has been devoted to producing high-purity cobalt sulphate in an industrially viable process. Cobalt is typically separated from nickel by solvent extraction, whereby nickel and cobalt compete for the capacity of the extractant, especially at higher nickel content in the feed solution. Process solutions composed of a high nickel matrix certainly originate from leaching battery materials with NMC battery chemistries with Ni :Mn:Co ratios of 6: 1 : 1, 8: 1 : 1 or even higher.

[0007] Moreover, recycling towards battery-grade nickel sulphate requires an efficient extraction process yielding a nickel sulphate raffinate solution with low presence of residual cobalt. In addition to providing a sufficient number of extraction stages, deep removal of cobalt from the feed solution requires a sufficient excess of solvent capacity, thus increasing co-extraction of nickel even more.

[0008] CA 1,170,839 discloses an improved method of separating cobalt and nickel using an organic phosphoric acid within a pH range of 4 to 7. The extractants used are di-(2- ethylhexyl)phosphoric acid 20 vol% and tri-n-butyl phosphate 5 vol%, 2-ethylhex- ylphosphonic acid mono-2-ethylhexyl ester 20 vol% and tri-n-butyl phosphate 5 vol% CN 112 375 910 describes a method for recycling waste battery powder and involves leaching nickel, cobalt, and manganese from the battery powder. The extractant used is Cyanex 272, i.e. bis(2,4,4-trimethylpentyl)phosphinic acid.

[0009] Yet, there is a need for improved separation of cobalt and nickel from an aqueous solution, until deep removal of cobalt from the raffinate, in presence of a high nickel content in the feed solution, all this under stable process conditions.

[0010] SUMMARY

[0011] The inventors found that maintaining a sufficiently high magnesium concentration in an aqueous phase containing cobalt and nickel increases the efficiency of a solvent extraction process with a dialkylphosphinic acid extractant for the extraction of cobalt. The improved efficiency originates from the fact that magnesium is extracted in favour of nickel. By reducing competition from nickel, a higher solvent capacity can be exploited without entering the risk zone where the organic phase becomes instable. Indeed, it was observed that solvent solidification occurs with phosphinic acid based extractants at higher nickel loadings, with the processing window depending mainly on extractant concentration in the solvent and temperature. Two significant benefits result from a more efficient exploitation of the solvent capacity: i. feed solutions comprising a comparatively higher nickel content can be processed; and ii. deep removal of cobalt from the feed solution, resulting in nickel sulphate raffinate solutions with low residual traces of cobalt.

[0012] The inventors found that the magnesium concentration in the aqueous phase should be maintained at a sufficiently high level to avoid a content of nickel in the organic phase higher than the threshold value for solvent solidification. This threshold value depends on the temperature, the extractant concentration in the organic phase, and the sodium concentration in the aqueous phase. In the absence or in the presence of only low levels of magnesium, nickel is the only candidate for co-extraction. If magnesium is also sufficiently present in the aqueous feed solution, magnesium will be co-extracted instead of nickel. In that case, a higher nickel content in the aqueous feed solution can be processed or a higher solvent capacity can be used, e.g. by increasing the extractant concentration or the organic-to-aqueous flow ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 shows a graphical representation of the stability zone as a function of nickel and cobalt concentrations in the organic phase composed of 45 vol% Cyanex 272 in Escaid 110 (Example 1). The content of sodium in the aqueous phase varies from 21 g / L to 26 g / L.

[0014] Figure 2 shows a mass balance for cobalt, nickel and magnesium in an extraction section consisting of four stages in a counter-current configuration for a feed solution low in magnesium (Example 2). Example 2 shows solidification of the organic phase through the low levels of magnesium.

[0015] Figure 3 shows a mass balance for cobalt, nickel and magnesium in an extraction section consisting of four stages in a counter-current configuration for a feed solution similar to Example 2 in respect of cobalt and nickel but high in magnesium (Example 3). Example 3 does not show solidification of the organic phase.

[0016] DETAILED DESCRIPTION OF THE INVENTION

[0017] Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

[0018] As used herein, the following terms have the following meanings:

[0019] "A", "an", and "the" as used herein refers to both singular and plural referents unless the context clearly dictates otherwise. By way of example, "a compartment" refers to one or more than one compartment.

[0020] "About" as used herein referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of + / - 20% or less, preferably + / -10% or less, more preferably + / -5% or less, even more preferably + / -1% or less, and still more preferably + / -0.1% or less of and from the specified value, in so far such variations are appropriate to perform in the disclosed invention. However, it is to be understood that the value to which the modifier "about" refers is itself also specifically disclosed.

[0021] "Comprise," "comprising," and "comprises" and "comprised of" as used herein are synonymous with "include", "including", "includes" or "contain", "containing", "contains" and are inclusive or open-ended terms that specifies the presence of what follows at least and do not exclude or preclude the presence of additional, non-recited components, features, elements, members, or steps known in the art or disclosed therein.

[0022] The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within that range, as well as the recited endpoints.

[0023] All percentages are to be understood as percentage by weight, abbreviated as "wt%" or as volume percentage, abbreviated as "vol%" or as atomic percentage, abbreviated as "at%" or as molar percentage, abbreviated as "mol%", unless otherwise defined or unless a different meaning is obvious to the person skilled in the art from its use and in the context wherein it is used.

[0024] Regarding the organic phase, the following terms are used to identify its components or the whole: i. In the context of the present invention, the term "organic phase" is to be understood as synonymous for the term "solvent" or "solvent mixture" and designates a liquid composition comprising one or more extractants, diluents, and optionally one or more modifiers. ii. The "extractant" or extracting agent is the active component in the organic phase that extracts the metal species to the organic phase by chemically binding with it and forming a metal-extractant complex that is better soluble in the organic phase than it is in the aqueous phase. iii. The "diluent" is an organic molecule or usually a mixture of different organic molecules added to the organic phase to dilute the extractant and allow for dissolution of the metal complexes, improve the physical properties of the organic phase (especially phase-separation phenomena), and decrease its cost, given that diluents are usually cheaper than extractants. Diluents are frequently kerosene fractions and can be aliphatic or aromatic hydrocarbons, naphthenes, etc., or mixtures thereof. The diluent is preferably a kerosene- based petroleum fraction such as Escaid, Elixore, Shellsol, Isopar, etc. The term "organic solvent" is used interchangeably with "diluent". iv. The organic phase may also contain a "modifier". A modifier is sometimes added to improve solubility of metal complexes into the organic phase, to alter the physical properties of the solvent such to avoid crud formations or third- phase formation as these phenomena are unwanted in solvent extraction. A modifier can also be added to prevent chemical degradation of extractant or diluent. However, modifiers may impair the selectivity of the organic phase as these may participate in the complex formation of the metals with the extractant.

[0025] In the context of the present invention, the term "aqueous feed solution" is to be understood as synonymous to the term "aqueous mixed metal feed" or "aqueous mixed metal solution" and may comprise any one or a combination of processed raw materials and recycled materials. Examples of recycled materials include, but are not limited to, spent cathode material, and materials derived from recycled lithium-ion batteries or lithium-ion battery manufacturing scrap, collectively, referred to herein as 'black mass'.

[0026] The process of the present invention is also suitable for extracting cobalt from aqueous feed solutions comprising nickel and cobalt obtained from processing of the following raw materials:

[0027] ■ "MHP" or "mixed hydroxide precipitate." Mixed hydroxide precipitate (MHP) is an intermediate product of nickel metallurgy derived from processing laterite ores which contains primarily nickel and a minor amount of cobalt. MHP is a solid product which is typically prepared by extracting nickel and cobalt from laterite ores. Alternatively, or additionally, MHP may be obtained from nickel and / or cobalt containing materials produced as production waste during preparation of cathode materials or obtained from battery recycling processes.

[0028] ■ "CHIP" or "cobalt hydroxide intermediate precipitate." Cobalt hydroxide intermediate is comprised primarily of cobalt, and typically has a cobalt content of 25 wt% to 40 wt%, relative to the total weight of said intermediate product. Typically, said CHIP comprises a significant amount of nickel. CHIP'S are known to have a very low amount of impurities, which render them attractive for processes according to the present invention. The aqueous mixed metal solution containing nickel and cobalt can originate from various kinds of resources like mixed hydroxide precipitates, crude nickel sulphate or any other type of suitable resource which is suitable, or which has optionally been processed into a suitable feed solution. The processing of the raw material or recycled material can include leaching, selective leaching, dissolving, precipitation steps and / or any other type of pre-treatment step. Combinations hereof are possible. For example, a pre-processed battery recycling material containing nickel, cobalt, manganese, and lithium can be treated in this flowsheet to produce a pure nickel sulphate solution and a pure cobalt salt solution if at least leaching and eventually upfront lithium removal is included in the pre-processing.

[0029] In the context of the present invention, the term "continuous process" is to be considered as a process in which the produced solution has a substantially constant outflow and composition. Specifically, a continuous process is a process in which the produced solution has a constant composition within the range of what are considered normal process variations. More specifically, the produced solution has a composition wherein the concentration of each ingredient is within the range of + / -20% or less, preferably + / -10% or less, more preferably + / -5% or less, even more preferably + / - 3% or less of its average concentration.

[0030] In a first aspect, the present invention provides a multi-stage solvent extraction process for selectively extracting cobalt from an aqueous mixed metal solution comprising nickel and cobalt and optionally sodium, whereby each solvent extraction stage comprises the steps of: i. maintaining a magnesium concentration in an aqueous phase above a predetermined magnesium concentration; ii. contacting the aqueous phase of step i. with an organic phase comprising a dialkylphosphinic acid extractant and an organic diluent; and iii. separating the organic phase obtained in step ii. from the aqueous phase, thereby obtaining a cobalt rich organic phase and an aqueous nickel sulphate raffinate solution.

[0031] Specifically, the invention is characterized in that said predetermined magnesium concentration is sufficiently high to ensure that nickel remains predominantly in the aqueous phase. The present inventors have observed that at higher nickel concentrations loaded onto the organic phase, complexation of nickel by the phosphinic acid extractant is responsible for solidification of the organic phase. This complexation and solidification particularly occurs when the aqueous feed solution containing cobalt and nickel also contains sodium. As a consequence of this solidification phenomenon, the solvent extraction process to separate cobalt and nickel is limited in efficiency with regard to solvent capacity, removal yield, and the allowable nickel content in the feed solution. Ni-containing solvent solidification is especially noticed at a temperature below 20 °C. If the process temperature is higher, temperature values below 20 °C may still occur upon temporary shutdowns of the installation. The threshold nickel loading value at which solvent solidification occurs upon decreased temperature depends on the extractant concentration in the organic phase and the sodium concentration in the aqueous phase.

[0032] In a preferred embodiment, the present invention provides a process according to the first aspect of the invention, whereby said predetermined magnesium concentration corresponds to at least 25 mol% of the amount of nickel in said aqueous phase, preferably at least 40 mol%, more preferably at least 50 mol%, and even more preferably at least 80 mol%. As outlined above, a sufficiently high magnesium concentration suppresses co-extraction of nickel, this way ensuring an optimal processing window for cobalt.

[0033] To avoid excessive competition of magnesium extraction over cobalt extraction, it is preferred to avoid too high magnesium concentrations in the aqueous phase, such as a magnesium concentration higher than 100 mol% of the amount of cobalt in the aqueous phase, preferably the magnesium concentration in the aqueous phase is lower than 75 mol% of the amount of cobalt in the aqueous phase, most preferably even lower than 50 mol% of the amount of cobalt in the aqueous phase.

[0034] In a preferred embodiment, the present invention provides a process according to the first aspect of the invention, whereby said predetermined magnesium concentration is sufficiently high to ensure that at least 75 mol% of nickel, relative to the total amount of nickel, remains in the aqueous phase, preferably at least 85 mol% and most preferably at least 95 mol%. Avoiding that nickel concentrations in the organic phase are too high avoids the occurrence of solidification phenomena of organic phase, this way limiting the processing window for cobalt.

[0035] In one embodiment, the present invention provides a process according to the first aspect of the invention, whereby said aqueous mixed metal feed comprising 0.1 to 30 g / L sodium. The inventors noticed that the problem of solidification of the organic phase during cooldown of the system to temperatures well below the regular operating temperature, e.g. well below 40°C, became more significant when sodium was present in the aqueous mixed metal feed. Yet, it was found that the maintenance of a sufficiently high magnesium concentration allowed to avoid the solidification. In specific embodiments, the specified sodium concentration was 0.5 to 30 g / L, or 1 to 30 g / L, or 2 to 30 g / L, or 5 to 30 g / L, or 5 to 25 g / L.

[0036] In a preferred embodiment, the present invention provides a process according to the first aspect of the invention, the organic phase employed in step ii. of the inventive process comprising a dialkylphosphinic acid extractant and an organic diluent. Specifically, said dialkylphosphinic acid is used as a single extractant, meaning that no further extractants are employed; the extractant consists essentially of dialkylphosphinic acid.

[0037] Each solvent extraction stage consists of contacting an organic phase with an aqueous phase. Preferably, the O / A flow ratio, i.e. the relative ratio of the incoming organic and aqueous feed to the solvent extraction unit, is between 0.1 and 10, preferably between 0.2 and 5, and most preferably between 0.3 and 3.

[0038] In a preferred embodiment, the present invention provides a process according to the first aspect of the invention, whereby said multi-stage solvent extraction process is operated in counter-current mode.

[0039] In a preferred embodiment, the present invention provides a process according to the first aspect of the invention, whereby magnesium is added to an aqueous phase in one or more stages of the multi-stage solvent extraction process to achieve a magnesium concentration above a predetermined magnesium concentration. Accordingly, the process can be actively controlled to ensure a good selectivity for cobalt extraction and a sufficiently broad processing window. The amount of magnesium added may depend on the desired selectivity for extraction of cobalt over nickel, and the number of stages in the multi-stage solvent extraction process, as well as on the amount of magnesium present in the aqueous phase prior to addition of magnesium. Preferably, the present invention provides a process according to the first aspect of the invention, whereby magnesium is added in step i. to said aqueous phase an amount of at least 2 mol%, relative to the amount of nickel in said aqueous phase, preferably in an amount of at least 5 mol%, relative to the amount of nickel in said aqueous phase.

[0040] In a preferred embodiment, the content of magnesium in the aqueous feed solution is maintained or magnesium is added to achieve a content of nickel in the organic phase lower than the threshold value of 5 g / L for a typical system, such as from 1 g / L to 5 g / L, however, the limit value depending on the extractant concentration in the organic phase and the sodium concentration in the aqueous phase.

[0041] The inventors found that the efficacy of the extractant can be increased through the maintenance of a certain level of magnesium in the aqueous mixed metal solution containing nickel and cobalt. In particular, no solidification of the organic phase occurs through the maintenance of magnesium at a critical level in the aqueous feed solution containing nickel and cobalt.

[0042] In a preferred embodiment, the diluent is a hydrocarbon. More generally, any organic, water-immiscible solvent capable of dissolving the extractant can be used. Hence, the diluent is not specifically limited.

[0043] In one embodiment, the diluent is a kerosene-based compound, which can be aliphatic, naphthenic, aromatic, or even mixtures thereof.

[0044] In a preferred embodiment, the organic solvent is a kerosene-based petroleum fraction such as Escaid, Elixore, Shellsol, Isopar.

[0045] In one embodiment, the diluent comprises 55 vol% to 99 vol%, relative to the total volume of the organic phase. In a preferred embodiment, the present invention provides a process according to the first aspect of the invention, whereby the dialkylphosphinic acid extractant comprises bis-(2,4,4-trimethylpentyl) phosphinic acid. Commercial tradenames include Cyanex 272, P272 and lonquest 290. Alternatively or complementary, the dialkylphosphinic acid extractant comprises diisooctylphosphinic acid (DOPA).

[0046] Alkylphosphorus-based acids act as chelating extractants due to the presence of co- ordinative phosphorus and oxygen atoms in these molecules. Among the elements in the aqueous feed solution, an element that forms the corresponding chelate compound with a higher stability facilitates the extraction efficiency more compared to an element that is less likely to form the chelate compound.

[0047] In a preferred embodiment, the dialkyl phosphinic acid extractant is present in an amount of 5 vol% to 50 vol%, relative to the total volume of the organic phase, preferably in an amount of at least 15 vol%, preferably at least 30 vol%, even more preferably at least 40 vol%, given that a higher extractant concentration allows for higher amounts of nickel on the loaded solvent without the risk for solidification of the organic phase.

[0048] In one embodiment, the aqueous mixed metal solution and / or said aqueous phase comprises nickel in an amount of less than 120 g / L, preferably less than 60 g / L, most preferably less than 10 g / L.

[0049] In one embodiment, the aqueous mixed metal solution and / or said aqueous phase comprises cobalt in an amount between 1 g / L to 60 g / L, preferably from 5 g / L to 50 g / L, most preferably from 10 g / L to 40 g / L.

[0050] In one embodiment, the aqueous mixed metal solution and / or said aqueous phase containing nickel and cobalt further contains sodium. Preferably, the aqueous mixed metal solution and / or said aqueous phase contains sodium in an amount of less than 50 g / L, more preferably less than 30 g / L, most preferably less than 10 g / L. The inventors found that the complexation of nickel with the functional group in dialkylphosphinic acid extractants is responsible for solidification of the entire organic phase at room temperature or lower when in contact with an aqueous feed solution comprising nickel. This is particularly the case when the aqueous feed solution further contains sodium.

[0051] In one embodiment, the pH of the aqueous mixed metal solution containing nickel and cobalt is between 1.0 and 5.0 before being contacted with the organic phase, more preferably between 2.0 and 4.5, and most preferably between 3.0 and 4.0.

[0052] In one embodiment, the aqueous feed solution containing nickel and cobalt further contains metals from the group comprising zinc, calcium, cadmium, iron, aluminium, and copper. Other impurities may be present as well. In one embodiment, the aqueous feed solution containing nickel and cobalt further comprises magnesium, manganese, zinc, copper, and cadmium in an amount of at most 40 at%, preferably at most 25 at%, most preferably at most 15 at% relative to the total metal content of the aqueous feed solution.

[0053] In one embodiment, the aqueous mixed metal solution containing nickel and cobalt further comprises calcium, zinc, copper, and cadmium in a totalized amount of at most 25 at%, relative to the total metal content of the aqueous feed solution. Preferably, the aqueous mixed metal solution further comprises calcium, zinc, copper, and cadmium in a totalized amount of at most 10 at% and even more preferably in an amount of at most 5 at%.

[0054] In a preferred embodiment, the extraction process is conducted at a temperature is between 20 °C and 80 °C, preferably between 20 °C and 60 °C, more preferably between 30 °C and 55 °C, most preferably at a temperature between 40 °C and 50 °C. Upon shutdown of the extraction unit, the temperature may drop below room temperature and even down to 0 °C, under which conditions there is a solidification of the organic phase.

[0055] In a preferred embodiment, the extraction process is concluded by separating the organic phase from the aqueous phase, whereby prior to separation, the aqueous phase is acidified to obtain a pH lower than 5, preferably lower than 4. This is advantageous to obtain a better recovery of residual amounts of organic phase from the aqueous phase. The solvent extraction can be performed in any device suitable and is not specifically limited. Solvent extraction equipment generally includes at least one or more devices consisting of a mixer-settler, a column contactor, a centrifugal contactor, or any other type of contactor.

[0056] Preferably, the extraction is performed in a counter-current configuration.

[0057] In one embodiment, the process further comprises the steps of:

[0058] ■ back-extracting cobalt from the organic phase; and

[0059] ■ recovering cobalt in an aqueous medium.

[0060] The back-extraction step can be performed in any device suitable and is not specifically limited. Back-extraction equipment generally includes at least one or more devices consisting of a mixer-settler, a column contactor, a centrifugal contactor, or any other type of contactor. Preferably, the stripping is performed in a counter-current configuration.

[0061] In a preferred embodiment, back-extraction of cobalt is performed at a temperature between 40 °C and 70 °C, preferably between 45 °C and 65 °C, most preferably at a temperature between 50 °C and 60 °C.

[0062] In a preferred embodiment, the present invention provides a continuous process which operates under steady-state conditions.

[0063] A further aspect of the present invention is the use of magnesium for reducing or preventing the solidification of an organic phase, in particular composed of a dialkyl phosph i n ic acid extractant, in a process for extracting cobalt from an aqueous feed solution comprising nickel and cobalt.

[0064] EXAMPLES

[0065] For each process step an example is given to further clarify the present invention. These examples are based on experimentally derived data and nowhere intended to limit the scope of the present invention. Example 1 - Maximum nickel load in the organic phase

[0066] Figure 1 shows a graphical representation of the solvent stability zone at 20 °C as a function of nickel and cobalt concentrations on the organic phase composed of 45 vol% Cyanex 272 in Escaid 110, the organic phase being in contact with an aqueous phase comprising 21 to 26 g / L Na.

[0067] From Figure 1, an upper limit of about 5 g / L nickel on the organic phase can be derived under these conditions in order to keep the organic solvent composed of 45 vol% Cyanex 272 stable. Depending on the extractant concentration in the organic phase, the temperature, and the sodium content in the aqueous phase, this limit concentration will be different.

[0068] Example 2 - Low magnesium in the aqueous feed solution

[0069] Together with the extraction of cobalt, nickel is co-extracted. Especially when enhanced cobalt removal is required, a higher solvent capacity is provided, and a lower cobalt / nickel selectivity is achieved in the first extraction stage.

[0070] Figure 2 shows a mass balance for cobalt, nickel and magnesium in an extraction section consisting of four stages in a counter-current configuration for an aqueous feed solution that is low in magnesium. Solidification of the extractant occurred.

[0071] Example 3 - High magnesium in the aqueous feed solution

[0072] Figure 3 shows a mass balance for cobalt, nickel and magnesium in an extraction section consisting of four stages in a counter-current configuration for an aqueous feed solution similar to the Example 2 in respect of cobalt and nickel but high in magnesium. No solidification of the extractant occurred.

[0073] Example 4 - Sodium in the aqueous feed solution

[0074] Complexation of nickel with the phosphinic acid functional group of organic extractants is responsible for the formation of precipitates at room temperature when in contact with (sodium containing) aqueous phases. Several combinations were prepared in batch tests to evaluate the stability of the organic phases (see Table 1).

[0075] The samples of the organic and aqueous phase have been stored for several days at 20 °C.

[0076] Table 1: Impact of magnesium in the presence of sodium in the aqueous feed solution.

[0077] The first three examples 4.1 to 4.3 were stable for 13 days at 20°C.

[0078] The organic phases from examples 4.4 to 4.6 precipitated after 7 days. The effect of the sodium concentration has been further studied. Example 4.6 was repeated with an aqueous phase diluted 2 and 4 times and stored with the same organic phase.

[0079] The behavior as a function of time is given in Table 2. The effect of the sodium concentration seemed of kinetic nature. The sodium concentration influenced the time to form a precipitate, but not whether a precipitate formed or not. Table 2: Impact of sodium on solidification / precipitation.

[0080] For examples 4.7 and 4.8, only in contact with the aqueous phase containing 35 g / L sodium, a precipitate was formed after 7 days.

[0081] However, the inventors believe that waiting for a longer or much longer time would eventually lead as well to precipitation for example 4.7 with lower sodium concentration.

[0082] The inventors discovered that when interrupting a multi-stage extraction process, i.e. for maintenance purposes, and accordingly allowing the system to cool down to temperatures well below process temperatures, especially at ambient temperature, a solidification or precipitation occurred in the nickel loaded organic phase. This phenomenon proved to manifest faster when higher amounts of sodium were present in the aqueous mixed metal feed solution. This is well illustrated by the observations depicted in Table 2.

[0083] It was therefore an objective of the inventors to provide a process for the extraction of Co using a dialkylphosphinic acid extractant from an aqueous mixed metal solution comprising at least cobalt and nickel, whereby solidification of the organic phase during cooldown to temperatures below process temperatures is effectively avoided or suppressed.

[0084] Without being bound to any mechanistic considerations, further investigations by the inventors led to determine that this precipitation or solidification of the extractant was a result of the high amount of nickel present in the organic phase. The inventors proposed a solution to maintain a high concentration of magnesium in the aqueous phase during each solvent extraction step. Accordingly, solidification of the organic phase during cooldown of the system was successfully avoided.

Claims

CLAIMS1. A multi-stage solvent extraction process for selectively extracting cobalt from an aqueous mixed metal feed comprising nickel, cobalt and sodium, whereby each solvent extraction stage comprises the steps of: i. maintaining a magnesium content in an aqueous phase in an amount of at least 25 mol% of the amount of nickel in the aqueous phase during the solvent extraction; ii. contacting the aqueous phase of step i. with an organic phase comprising a dialkylphosphinic acid extractant and an organic diluent; and iii. separating the organic phase obtained in step ii. from the aqueous phase, thereby obtaining a cobalt rich organic phase and an aqueous nickel sulphate raffinate solution.

2. Process according to claim 1, whereby said magnesium content corresponds to at least 50 mol% of the amount of nickel in said aqueous phase, preferably to more than 80 mol%.

3. Process according to claim 1 or 2, whereby said predetermined magnesium concentration is sufficiently high to ensure that at least 75 mol% of nickel, relative to the total amount of nickel, remains in the aqueous phase.

4. Process according to any of claims 1 to 3, whereby said aqueous mixed metal feed comprising 0.1 to 30 g / L sodium.

5. Process according to any of claims 1 to 4, wherein the dialkylphosphinic acid extractant is bis-(2,4,4-trimethylpentyl)phosphinic acid.

6. Process according to any of claims 1 to 5, whereby said multi-stage solvent extraction process is operated in counter-current mode.

7. Process according to any of claims 1 to 6, whereby magnesium is added to an aqueous phase in one or more stages of the multi-stage solvent extraction process.

8. Process according to claim 7, whereby magnesium is added to said aqueous phase an amount of at least 2 mol%, relative to the amount of nickel in said aqueous phase.

9. Process according to any of claims 1 to 8, whereby said aqueous mixed metal feed and / or said aqueous phase comprises nickel in an amount of 5 to 120 g / L.

10. Process according to any of claims 1 to 9, whereby said aqueous mixed metal feed and / or said aqueous phase further comprises sodium in an amount of less than 60 g / L.

11. Process according to any of claims 1 to 10, whereby said aqueous mixed metal feed and / or said aqueous phase comprises cobalt in an amount of 10 g / L to 60 g / L.

12. Process according to any of claims 1 to 11, whereby the concentration of said dialkylphosphinic acid extractant in the organic phase can range from 1 to 45 vol%.

13. Process according to any of claims 1 to 12, whereby said solvent extraction is performed at a temperature of 20 °C to 80 °C.

14. Process according to any of claims 1 to 13, whereby cobalt is back-extracted from the organic phase obtained in step iii ., whereby the organic phase is regenerated and cobalt is recovered in an aqueous medium.

15. Process according to any of claims 1 to 14, whereby the pH of the aqueous phase containing nickel and cobalt is between 1.0 and 4.0 prior to step ii.

16. Process according to any of claims 1 to 15, whereby said multi-stage solvent extraction process is operated in continuous, counter-current mode.

Images