A method for selectively extracting cobalt from an aqueous nickel-cobalt solution.

Abstract

The present invention provides a multi-step solvent extraction process for selectively extracting cobalt from an aqueous mixed metal solution containing nickel and cobalt, characterized in that each solvent extraction step maintains a magnesium concentration in the aqueous phase above a predetermined magnesium concentration.

Description

Technical Field

[0001] The present invention relates to a novel method for selectively extracting cobalt from an aqueous feed solution containing nickel using a dialkylphosphinic acid extractant in an organic solvent.

Background Art

[0002] The development of lithium-ion batteries, particularly the use of nickel-manganese-cobalt and nickel-cobalt-aluminum cathode materials, has increased the demand for high-purity cobalt sulfate, either as a solid or in solution.

[0003] Impurities in cathode materials strongly affect the performance of the battery. Thus, much effort has been devoted to producing high-purity cobalt sulfate in an industrially feasible process. Cobalt is usually separated from nickel by solvent extraction, and particularly when the nickel content in the feed solution is high, nickel and cobalt compete with respect to the extraction capacity. Process solutions consisting of high-nickel matrices are certain to be derived from leached battery materials using NMC battery chemistries with Ni:Mn:Co ratios of 6:1:1, 8:1:1, or higher.

[0004] Furthermore, the recycling of battery-grade nickel sulfate requires an efficient extraction process, resulting in a nickel sulfate raffinate solution with low levels of residual cobalt present. Thorough removal of cobalt from the feed solution requires, in addition to providing a sufficient number of extraction stages, a sufficient excess of solvent volume, which further increases the co-extraction of nickel.

[0005] CA1,170,839 discloses an improved method for separating cobalt and nickel using an organic phosphoric acid within a pH range of 4 to 7. The extractants used are 20% by volume of di-(2-ethylhexyl) phosphoric acid and 5% by volume of tributyl phosphate, 20% by volume of 2-ethylhexyl-ylphosphonic acid mono-2-ethylhexyl ester and 5% by volume of tributyl phosphate.

[0006] CN 112 375 910 describes a method for recycling waste battery powder, which includes leaching nickel, cobalt, and manganese from the battery powder. The extract used is Cyanex 272, namely bis(2,4,4-trimethylpentyl)phosphinic acid.

[0007] Nevertheless, there is still a need to improve the separation of cobalt and nickel from aqueous solutions, including the thorough removal of cobalt from raffinate, under stable process conditions and in the presence of a high nickel content in the feed solution, and all of this must be done under stable process conditions. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Canadian Patent Application Publication No. 1170839 [Patent Document 2] Chinese Patent Application Publication No. 112375910 Specification [Overview of the project] [Means for solving the problem]

[0009] The inventors have found that maintaining a sufficiently high magnesium concentration in an aqueous phase containing cobalt and nickel increases the efficiency of a solvent extraction method using a dialkylphosphinic acid extract for cobalt extraction. The improved efficiency stems from the fact that magnesium is extracted preferentially by nickel. By reducing competition with nickel, higher solvent volumes can be utilized without falling into the dangerous zone where the organic phase becomes unstable. Indeed, it was observed that solvent solidification occurred with the phosphinic acid extract at higher nickel loads, and the treatment range depended primarily on the extractant concentration and temperature in the solvent. More efficient utilization of solvent volume leads to two important advantages: i. It can process supply solutions containing relatively high nickel content. ii. Thoroughly remove cobalt from the supply solution to obtain a nickel raffinate sulfate solution with minimal residual cobalt traces.

[0010] The inventors have found that the magnesium concentration in the aqueous phase should be maintained at a level high enough to avoid nickel content in the organic phase that is above the threshold for solvent solidification. This threshold depends on temperature, extractant concentration in the organic phase, and sodium concentration in the aqueous phase. In the absence or presence of low levels of magnesium alone, nickel is the only candidate for co-extraction. If magnesium is also sufficiently present in the aqueous feed solution, magnesium is co-extracted instead of nickel. In this case, higher nickel content in the aqueous feed solution can be handled, for example, by increasing the extractant concentration or increasing the flow rate ratio of the organic phase to the aqueous solution, or by using a higher solvent volume. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 shows a graphical representation of the stability zone as a function of nickel and cobalt concentrations in the organic phase consisting of 45 vol% Cyanex 272 in Escaid 110 (Example 1). The sodium content in the aqueous phase varies from 21 g / L to 26 g / L. [Figure 2] Figure 2 shows the mass balance of cobalt, nickel, and magnesium in an extraction section consisting of four stages in a countercurrent configuration of a low magnesium feed solution (Example 2). Example 2 shows the solidification of the organic phase through low levels of magnesium. [Figure 3] Figure 3 shows the mass balance of cobalt, nickel, and magnesium in a four-stage countercurrent extraction section for a feed solution having the same cobalt and nickel content as in Example 2, but with a higher magnesium concentration (Example 3). Example 3 does not show solidification of the organic phase. [Modes for carrying out the invention]

[0012] Unless otherwise defined, all terms used to disclose this invention, including technical and scientific terms, have the meanings that are commonly understood by those skilled in the art to which this invention pertains. Further guidance provides definitions of terms to better understand the teachings of this invention.

[0013] As used herein, the following terms have the following meanings:

[0014] As used herein, "A," "an," and "the" refer to both singular and plural unless the context explicitly indicates otherwise. For example, "a section" refers to one or more sections.

[0015] As used herein, “about” when referring to measurable values ​​such as parameters, quantities, and temporal durations means that variations from specific values ​​of ±20%, preferably ±10%, more preferably ±5%, even more preferably ±1%, and even more preferably ±0.1% or less are appropriate for the previously disclosed variations to be carried out at the disclosed values, provided that such variations are appropriate for the carrying out of the invention. However, it should be understood that the values ​​referred to by the modifier “about” are themselves specifically disclosed.

[0016] "Comprise," "comprising," "comprises," and "comprise of," as used herein, are synonymous with "including," "including," or "contain," "containing," and "contains," and are inclusive or open terms that identify the presence of at least a subsequent one, and do not exclude or preclude the presence of additional, undisclosed components, features, elements, members, or steps known in the art or disclosed herein.

[0017] The enumeration of numerical ranges by endpoint includes all numbers and fractions contained within that range, as well as the enumerated endpoints.

[0018] All percentages should be understood as being abbreviated as "weight %" for weight percentages, "volume %" for volume percentages, "atomic %" for atomic percentages, or "mol %" for mole percentages, unless otherwise defined, or unless a different meaning is obvious to a person skilled in the art from its use, and unless a different meaning is obvious to a person skilled in the art in the context in which it is used.

[0019] For organic phases, the following terms are used to identify their components or the whole: i. In the context of the present invention, the term “organic phase” is understood to be synonymous with the term “solvent” or “solvent mixture,” and refers to a liquid composition comprising one or more extractants, diluents, and optionally one or more modifiers. ii. An "extractant" (or "extracting agent") is an active component in the organic phase that chemically binds metal species to the organic phase, forming a metal extractant complex that dissolves better in the organic phase than in the aqueous phase, thereby extracting the metal species into the organic phase. iii. A “diluent” is an organic molecule, or usually a mixture of different organic molecules added to an organic phase, which dilutes the extract, allows for the dissolution of metal complexes, improves the physical properties of the organic phase (particularly phase separation phenomena), and reduces its cost, given that diluents are usually less expensive than extracts. Diluents are often kerosene fractions, and may be aliphatic or aromatic hydrocarbons, naphthenes, etc., or mixtures thereof. Diluents are preferably kerosene-based petroleum fractions such as Escaid, Elixore, Shellsol, Isopar. The term “organic solvent” is used interchangeably with “diluent.” iv. The organic phase may also contain a "modifier". The modifier may be added to improve the solubility of the metal complex in the organic phase, and to change the physical properties of the solvent to avoid clad formation or third-phase formation, since these phenomena are undesirable in solvent extraction. The addition of a modifier can prevent the chemical decomposition of the extract or diluent. However, since the modifier can participate in complex formation with the extract, it can compromise the selectivity of the organic phase.

[0020] In the context of the present invention, the term "aqueous feed solution" should be understood to be synonymous with the terms "aqueous mixed metal feed" or "aqueous mixed metal solution", and may include any one or combination of the processed raw materials and recycled materials. Examples of recycled materials include used cathode materials, and materials derived from recycled lithium-ion batteries or lithium-ion battery manufacturing scrap, which are collectively referred to herein as "black mass", but are not limited thereto.

[0021] The method of the present invention is also suitable for extracting cobalt from an aqueous feed solution containing nickel and cobalt obtained from the treatment of the raw material "MHP" or "mixed hydroxide precipitate". · Mixed hydroxide precipitate (MHP) is an intermediate product of nickel metal formed from the treatment of laterite ores, mainly containing nickel and a small amount of cobalt. MHP is usually a solid product produced by extracting nickel and cobalt from laterite ores. Alternatively, or additionally, MHP can be obtained from nickel and / or cobalt-containing materials generated as production waste during the manufacture of cathode materials, or from battery recycling processes. · "CHIP" or "cobalt hydroxide intermediate precipitate". The cobalt hydroxide intermediate mainly consists of cobalt and usually has a cobalt content of 25% to 40% by weight based on the total weight of the intermediate product. Typically, the said CHIP contains a significant amount of nickel. CHIP is known to have a very low amount of impurities and is attractive for the method according to the present invention.

[0022] An aqueous mixed metal solution containing nickel and cobalt can be derived from various sources such as mixed hydroxide precipitate, crude nickel sulfate, or suitable or optionally suitable resources, which can be used as they are or optionally processed into a suitable feed solution. The treatment of raw materials or recycled materials may include leaching, selective leaching, dissolution, precipitation processes, and / or any other type of pretreatment process. This combination is possible. For example, when a pretreated battery recycling material containing nickel, cobalt, manganese, and lithium is processed with this flowsheet and at least leaching and ultimately pretreatment lithium removal are included in the pretreatment, a pure nickel sulfate solution and a pure cobalt salt solution can be produced.

[0023] In the context of the present invention, the term "continuous process" should be interpreted as a process in which the generated solution has a substantially constant outflow and composition. Specifically, a continuous process is a process in which the generated solution has a constant composition within the range considered as normal process variations. More specifically, the generated solution has a composition in which the concentration of each component is within the range of ± 20% or less, preferably ± 10% or less, more preferably ± 5% or less, and even more preferably ± 3% or less of its average concentration.

[0024] In a first aspect, the present invention is a multi-stage solvent extraction process for selectively extracting cobalt from an aqueous mixed metal solution containing nickel and cobalt, and optionally sodium, wherein each solvent extraction stage i. maintaining the magnesium concentration in the aqueous phase above a predetermined magnesium concentration ii. A step of contacting the aqueous phase of step i with an organic phase containing a dialkylphosphinic acid extract and an organic diluent, and iii. A method is provided which includes the step of separating the organic phase obtained in step ii from the aqueous phase, thereby obtaining a cobalt-rich organic phase and an aqueous nickel sulfate raffinate solution.

[0025] Specifically, the present invention is characterized by ensuring that the predetermined magnesium concentration is sufficiently high and that nickel remains mainly in the aqueous phase.

[0026] The inventors observed that at higher nickel concentrations loaded onto the organic phase, complex formation of nickel by phosphinic acid extracts contributes to the solidification of the organic phase. This complex formation and solidification occur particularly when the aqueous feed solution containing cobalt and nickel also contains sodium. As a result of this solidification phenomenon, solvent extraction methods for separating cobalt and nickel are limited in efficiency with respect to solvent volume, removal yield, and acceptable nickel content in the feed solution. Solidification of Ni-containing solvents is particularly pronounced at temperatures below 20°C, and temperatures below 20°C may occur when the process temperature is high and the equipment is temporarily shut down. The threshold nickel load at which solvent solidification occurs with decreasing temperature depends on the extractant concentration in the organic phase and the sodium concentration in the aqueous phase.

[0027] In a preferred embodiment, the present invention provides a method according to a first aspect of the present invention, wherein a predetermined magnesium concentration corresponds to at least 25 mol%, preferably at least 40 mol%, more preferably at least 50 mol%, and even more preferably at least 80 mol%, of the amount of nickel in the aqueous phase described above. As outlined above, a sufficiently high magnesium concentration suppresses the co-extraction of nickel, thus ensuring an optimal processing range for cobalt.

[0028] To avoid excessive competition between magnesium extraction and cobalt extraction, it is preferable to avoid excessively high magnesium concentrations in the aqueous phase, such as magnesium concentrations higher than 100 mol% of the amount of cobalt in the aqueous phase. Preferably, the magnesium concentration in the aqueous phase is less than 75 mol% of the amount of cobalt in the aqueous phase, and most preferably less than 50 mol% of the amount of cobalt in the aqueous phase.

[0029] In a preferred embodiment, the present invention provides a method according to a first aspect of the present invention, wherein the predetermined magnesium concentration is high enough to ensure that at least 75 mol%, preferably at least 85 mol%, and most preferably at least 95 mol%, of the total amount of nickel remains in the aqueous phase.

[0030] By avoiding an increase in temperature, the solidification phenomenon of the organic phase is prevented, thereby limiting the range of cobalt that can be processed.

[0031] In one embodiment, the present invention provides a method according to a first aspect of the invention, wherein the aqueous mixed metal feed contains 0.1 to 30 g / L of sodium. The inventors found that the problem of solidification of the organic phase during cooling of the system to temperatures far below the normal operating temperature, for example, far below 40°C, becomes more significant when sodium is present in the aqueous mixed metal feed. However, it was found that solidification could be avoided by maintaining a sufficiently high magnesium concentration. In certain embodiments, the specified sodium concentrations were 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.

[0032] In a preferred embodiment, the present invention provides a method according to a first aspect of the present invention, wherein the organic phase used in step ii of the method comprises a dialkylphosphinic acid extract and an organic diluent. Specifically, the dialkylphosphinic acid is used as a single extract, meaning that no further extracts are used, and the extract consists essentially of dialkylphosphinic acid.

[0033] Each solvent extraction step consists of bringing the organic phase into contact with the aqueous phase. Preferably, the O / A flow rate ratio, i.e., the relative ratio of the organic and aqueous feeds flowing into the solvent extraction unit, is 0.1 to 10, preferably 0.2 to 5, and most preferably 0.3 to 3.

[0034] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the multi-step solvent extraction process is operated in a countercurrent mode.

[0035] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein magnesium is added to the aqueous phase at one or more stages of a multi-stage solvent extraction process to achieve a magnesium concentration exceeding a predetermined magnesium concentration. Thus, this method can be actively controlled to ensure good selectivity and a sufficiently wide processing window for cobalt extraction. The amount of magnesium added is:

[0036] The desired selectivity for the extraction of cobalt from nickel may depend on the number of steps in the multi-step solvent extraction process and the amount of magnesium present in the aqueous phase before the addition of magnesium. Preferably, the present invention provides a method according to a first aspect of the present invention, wherein in step i, magnesium is added in an amount preferably at least 2 mol% and at least 5 mol% relative to the amount of nickel in the aqueous phase.

[0037] In preferred embodiments, the magnesium content in the aqueous feed solution is maintained, or magnesium is added to adjust the nickel content in the organic phase to a threshold range of 5 g / L (e.g., from 1 g / L to 5 g / L) in a typical system, where this limit depends on the extractant concentration in the organic phase and the sodium concentration in the aqueous phase.

[0038] The inventors have found that the effectiveness of the extract can be enhanced by maintaining a specific level of magnesium in an aqueous mixed metal solution containing nickel and cobalt. In particular, by maintaining magnesium at a critical level in the aqueous feed solution containing nickel and cobalt, solidification of the organic phase does not occur.

[0039] In preferred embodiments, the diluent is a hydrocarbon. More generally, any organic immiscible solvent capable of dissolving the extract can be used. Therefore, the diluent is not particularly limited.

[0040] In one embodiment, the diluent is a kerosene-based compound, which may be aliphatic, naphthenic, aromatic, or even a mixture thereof.

[0041] In preferred embodiments, the organic solvent is a kerosene-based petroleum fraction such as Escaid, Elixore, Shellsol, or Isopar.

[0042] In one embodiment, the diluent is present in an amount of 55% to 99% by volume relative to the total volume of the organic phase.

[0043] In preferred embodiments, the present invention provides a method according to a first aspect of the invention, wherein the dialkylphosphinic acid extract comprises bis-(2,4,4-trimethylpentyl)phosphinic acid. Commercial trademark names include Cyanex 272, P272, and lonquest 290. Alternatively or complementaryly, the dialkylphosphinic acid extract comprises diisooctylphosphinic acid (DOPA).

[0044] Alkylphosphorus acids act as chelate extracts because they contain coordinating phosphorus and oxygen atoms in their molecules. Among the elements in the aqueous supply solution, those that form the corresponding chelate compound with higher stability will have easier extraction efficiency compared to elements that are less likely to form chelate compounds.

[0045] In preferred embodiments, the dialkylphosphinic acid extractant is present in an amount of 5% to 50% by volume relative to the total volume of the organic phase, preferably at least 15% by volume, preferably at least 30% by volume, and more preferably at least 40% by volume. However, higher extractant concentrations allow for an increase in nickel on the loaded solvent without the risk of solidification of the organic phase.

[0046] In one embodiment, the aqueous mixed metal solution and / or the aqueous phase contains nickel in an amount of less than 120 g / L, preferably less than 60 g / L, and most preferably less than 10 g / L.

[0047] In one embodiment, the aqueous mixed metal solution and / or the aqueous phase contains cobalt in an amount of 1 g / L to 60 g / L, preferably 5 g / L to 50 g / L, and most preferably 10 g / L to 40 g / L.

[0048] In one embodiment, the aqueous mixed metal solution and / or the aqueous phase containing nickel and cobalt further contains sodium. Preferably, the aqueous mixed metal solution and / or the aqueous phase contains less than 50 g / L, more preferably less than 30 g / L, and most preferably less than 10 g / L of sodium. The inventors have found that the reason the entire organic phase solidifies at room temperature or below when in contact with an aqueous supply solution containing nickel is due to the formation of complexes between the functional groups in the dialkylphosphinic acid extractant and nickel.

[0049] This is especially true when the aqueous supply solution also contains sodium.

[0050] In one embodiment, the pH of the aqueous mixed metal solution containing nickel and cobalt is 1.0 to 5.0, more preferably 2.0 to 4.5, and most preferably 3.0 to 4.0 before contact with the organic phase.

[0051] In one embodiment, the aqueous feed solution containing nickel and cobalt further contains metals from the group including zinc, calcium, cadmium, iron, aluminum, and copper. Other impurities may also be present. In one embodiment, the aqueous feed solution containing nickel and cobalt further contains magnesium, manganese, zinc, copper, and cadmium in amounts up to 40 atomic%, preferably up to 25 atomic%, and most preferably up to 15 atomic% of the total metal content of the aqueous feed solution.

[0052] In one embodiment, the aqueous mixed metal solution containing nickel and cobalt further contains calcium, zinc, copper, and cadmium in a total amount of up to 25 atomic percent relative to the total metal content of the aqueous supply solution. Preferably, the aqueous mixed metal solution further contains calcium, zinc, copper, and cadmium in a total amount of up to 10 atomic percent, and more preferably up to 5 atomic percent.

[0053] In a preferred embodiment, the extraction process is carried out at a temperature of 20°C to 80°C, preferably 20°C to 60°C, more preferably 30°C to 55°C, and most preferably 40°C to 50°C. When the extraction unit is stopped, the temperature may drop below room temperature, or even down to 0°C, under which the organic phase solidifies.

[0054] In a preferred embodiment, the extraction process is completed by separating the organic phase from the aqueous phase, and prior to separation, the aqueous phase is acidified to obtain a pH of less than 5, preferably less than 4. This is advantageous for obtaining better recovery of residual organic phase from the aqueous phase.

[0055] Solvent extraction can be carried out in any suitable apparatus, and is not specifically limited. Common examples of solvent extraction apparatus include at least one apparatus consisting of a mixer-settler, column contactor, centrifugal contactor, or any other type of contactor.

[0056] Extraction is preferably carried out in a countercurrent configuration.

[0057] In one embodiment, this method Back-extracting cobalt from the organic phase, The method further includes recovering cobalt in an aqueous medium.

[0058] The back-extraction step can be carried out in any suitable apparatus, and is not limited to any particular type. Back-stretching apparatuses generally include at least one apparatus consisting of a mixer-settler, column contactor, centrifugal contactor, or any other type of contactor. Stripping is preferably carried out in a counter-flow manner.

[0059] In a preferred embodiment, the back extraction of cobalt is carried out at a temperature of 40°C to 70°C, preferably 45°C to 65°C, and most preferably 50°C to 60°C.

[0060] In a preferred embodiment, the present invention provides a continuous process that operates under steady-state conditions.

[0061] A further aspect of the present invention is the use of magnesium to reduce or prevent solidification of the organic phase in a process for extracting cobalt from an aqueous feed solution containing nickel and cobalt, and more particularly comprising a dialkylphosphinic acid extract. [Examples]

[0062] Examples are given for each process step to further clarify the present invention. These examples are based on experimentally derived data and are not intended to limit the scope of the present invention. Example 1 - Maximum nickel load in the organic phase

[0063] Figure 1 shows a graphical representation of the solvent stability zone at 20°C as a function of nickel and cobalt concentrations on an organic phase consisting of 45% deposited Cyanex 272 in Escaid 110, in contact with an aqueous phase containing 21–26 g / L of Na.

[0064] From Figure 1, it can be derived that under these conditions, the upper limit for nickel in the organic phase to maintain the stability of the organic solvent composed of 45 vol% Cyanex 272 is approximately 5 g / L. This limit concentration will vary depending on the extractant concentration in the organic phase, the temperature, and the sodium content in the aqueous phase. Example 2 - Low magnesium in aqueous supply solution

[0065] Nickel is co-extracted along with the extraction of cobalt. In particular, when enhanced cobalt removal is required, a higher solvent volume is provided, and lower cobalt / nickel selectivity is achieved in the first extraction stage.

[0066] Figure 2 shows the mass balance of cobalt, nickel, and magnesium in a four-stage extraction section in a countercurrent configuration for an aqueous feed solution with a low magnesium concentration. Solidification of the extract occurred. Example 3 - High Magnesium in Aqueous Supply Solution

[0067] Figure 3 shows the mass balance of cobalt, nickel, and magnesium in the extraction section, which consists of a four-stage countercurrent configuration of the aqueous feed solution, similar to Example 2 with respect to cobalt and nickel, but with higher levels of magnesium. No solidification of the extract occurred. Example 4 - Sodium in aqueous supply solution

[0068] The formation of nickel complexes with phosphinic acid functional groups of organic extracts, when in contact with an aqueous phase (containing sodium) at room temperature, is involved in the formation of precipitates at room temperature.

[0069] In batch testing, several combinations were prepared to evaluate the stability of the organic phase (see Table 1).

[0070] The organic and aqueous phase samples were stored at 20°C for several days. [Table 1]

[0071] The first three examples, 4.1 to 4.3, were stable at 20°C for 13 days.

[0072] The organic phases of Examples 4.4 to 4.6 precipitated after 7 days. The effect of sodium concentration has been further tested. Example 4.6 was repeated with aqueous phases diluted two and four times, and stored with the same organic phase.

[0073] Table 2 shows the behavior as a function of time. The effect of sodium concentration appeared to be kinetic. Sodium concentration affected the time it took for precipitate to form, but did not affect whether or not precipitate formed. [Table 2]

[0074] In Examples 4.7 and 4.8, a precipitate formed after 7 days of contact with an aqueous phase containing 35 g / L of sodium.

[0075] However, the inventors believe that waiting for a longer or much longer time will ultimately lead to precipitation, for example, at lower sodium concentrations such as 4.7.

[0076] The inventors found that when the multi-stage extraction process is interrupted, i.e., for maintenance purposes, and therefore the system is cooled to a temperature far below the process temperature, particularly at ambient temperature, solidification or precipitation occurs in the nickel-containing organic phase. This phenomenon was found to occur more rapidly when the aqueous mixed metal feed solution contains a higher amount of sodium. This is clearly illustrated by the observations shown in Table 2.

[0077] Therefore, the present inventors aimed to provide a method for extracting cobalt from an aqueous mixed metal solution containing at least cobalt and nickel using a dialkylphosphinic acid extractant. This makes it possible to effectively avoid or suppress the phenomenon of the organic phase solidifying when the temperature drops below the process temperature during the cooling process.

[0078] While not bound by any mechanical considerations, further investigation by the inventors led to the conclusion that this precipitation or solidification of the extract was a result of a large 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. Thus, solidification of the organic phase during system cooling was successfully avoided.

Claims

1. A multi-step solvent extraction process for selectively extracting cobalt from an aqueous mixed metal feed containing nickel, cobalt, and sodium, wherein each solvent extraction step is: i. A step of maintaining the magnesium content in the aqueous phase at an amount of at least 25 mol% of the nickel content in the aqueous phase during the solvent extraction. ii. A step of contacting the aqueous phase of step i with an organic phase containing a dialkylphosphinic acid extractant and an organic diluent, and iii. A process comprising the step of separating the organic phase obtained in step ii from the aqueous phase to obtain a cobalt-rich organic phase and an aqueous nickel sulfate raffinate solution.

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

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

4. The process according to any one of claims 1 to 3, wherein the aqueous mixed metal feed contains 0.1 g / L to 30 g / L of sodium.

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

6. The process according to any one of claims 1 to 5, wherein the multi-stage solvent extraction process is operated in countercurrent mode.

7. The process according to any one of claims 1 to 6, wherein magnesium is added to the aqueous phase in one or more steps of the multi-step solvent extraction process.

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

9. The process according to any one of claims 1 to 8, wherein the aqueous mixed metal feed and / or the aqueous phase contains nickel in an amount of 5 g / L to 120 g / L.

10. The process according to any one of claims 1 to 9, wherein the aqueous mixed metal feed and / or the aqueous phase further comprises less than 60 g / L of sodium.

11. The process according to any one of claims 1 to 10, wherein the aqueous mixed metal feed and / or the aqueous phase contains cobalt in an amount of 10 g / L to 60 g / L.

12. The process according to any one of claims 1 to 11, wherein the concentration of the dialkylphosphinic acid extractant in the organic phase may be in the range of 1% by volume to 45% by volume.

13. The process according to any one of claims 1 to 12, wherein the solvent extraction is carried out at a temperature of 20°C to 80°C.

14. The process according to any one of claims 1 to 13, wherein cobalt is back-extracted from the organic phase obtained in step iii, thereby regenerating the organic phase and recovering the cobalt in an aqueous medium.

15. The process according to any one of claims 1 to 14, wherein the pH of the aqueous phase containing nickel and cobalt is 1.0 to 4.0 before step ii.

16. The multi-stage solvent extraction process, operated in a continuous countercurrent mode, according to any one of claims 1 to 15.

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