Method for preparing high-purity nickel sulfate solution

Abstract

This invention provides a method for preparing a high-purity nickel sulfate solution, the method comprising the following steps: i. providing an aqueous feed solution containing nickel, cobalt, calcium, and magnesium; ii. extracting cobalt, calcium, and a portion of magnesium from the aqueous feed solution using a first solvent containing a first alkylphosphine acidic extractant, thereby obtaining an aqueous raffinate containing nickel and magnesium; iii. extracting magnesium from the aqueous raffinate solution containing nickel and magnesium using a second solvent containing a second alkylphosphine acidic extractant, thereby obtaining a high-purity nickel sulfate aqueous solution containing nickel and magnesium; iv. stripping the first loading solvent containing cobalt, calcium, and magnesium with an aqueous solution containing an inorganic acid.

Description

Technical Field

[0001] This invention relates to a novel method for producing a high-purity nickel sulfate aqueous solution, which can be further processed in a crystallization unit to produce high-purity nickel sulfate crystals with sufficient purity for electroless deposition of nickel metal layers or for the production of battery materials. Background Technology

[0002] The development of lithium-ion batteries, specifically the use of nickel-manganese-cobalt and nickel-cobalt-aluminum cathode materials, has increased the demand for high-purity nickel sulfate in solid or solution form. In fact, impurities in cathode materials can significantly affect battery performance. Therefore, considerable effort has been devoted to producing high-purity nickel sulfate using industrially feasible methods.

[0003] In this regard, US 2014 / 322109 provides a method for obtaining high-purity nickel sulfate with low impurity levels (particularly low levels of magnesium and chloride ions) through the following steps: introducing a selective nickel sulfide precipitation step and redissolving the nickel sulfide into a nickel sulfate solution. The concentration of the acidic organic extractant and the pH or acid concentration during the treatment are adjusted, and the solution is further purified by solvent extraction to remove cobalt and magnesium impurities. This processing strategy is cumbersome for concentrated nickel sulfate solutions because it requires intermediate precipitation and redissolution of the nickel block, followed by solvent extraction to remove cobalt and magnesium impurities. The nickel sulfide step is hazardous due to the risk of hydrogen sulfide formation. Furthermore, the solvent extraction is only used to remove cobalt and magnesium, but crude nickel feedstock typically contains more impurities.

[0004] CN 107162067 relates to the field of solid waste recycling, and specifically discloses a method for recovering high-purity nickel sulfate from nickel-containing waste batteries. The method includes the following steps: disassembling the nickel-containing waste batteries into battery powder, dissolving the battery powder with acid to obtain a metal-containing solution, adding alkali metal sulfate, removing iron through an oxidation precipitation process, further removing impurities through a solvent extraction process to obtain a magnesium-containing nickel solution, passing the magnesium-containing nickel solution through a chelating resin exchange column to selectively adsorb nickel ions and allowing the magnesium-rich solution to flow out for treatment, desorbing the nickel ions to obtain a nickel sulfate solution, evaporating, cooling, crystallizing, filtering, and finally drying the nickel sulfate solution to obtain a purified nickel sulfate product. This lengthy and complex process ensures that the recovered nickel sulfate is a high-purity product with a nickel content of over 99.5% and an impurity (i.e., magnesium) content of less than 0.005%. However, three different solvent extraction units are proposed to remove copper, manganese, and cobalt in different steps. Besides the high investment cost, other impurities such as calcium and magnesium are not even assumed to be removed. Finally, nickel is recovered by adsorption onto the resin, which requires a fourth separation step and consumes a neutralizing agent equivalent to the amount of adsorbed metal ions. Overall, the method is considered neither simple nor efficient.

[0005] EP 1252345 describes a method for extracting cobalt from a cobalt-nickel solution using a nickel-loaded solvent to obtain a purified nickel sulfate stream. However, it does not mention how to remove impurities such as calcium and magnesium to very low levels in order to produce a purified nickel sulfate solution for electroless nickel plating or battery applications. It appears more likely to be a solvent extraction method that avoids the formation of insoluble ammonium sulfate / nickel double salts.

[0006] EP 2784166 describes a method for producing pure nickel sulfate solution in multiple process steps, including a sulfidation step, a redissolution step, a purification step by precipitation, and a solvent extraction step. Particularly noteworthy are the sulfidation and redissolution steps, which use a sulfiding agent to produce a nickel sulfide intermediate. These steps are costly operations, and both products are toxic and can potentially generate highly toxic, gaseous hydrogen sulfide through contact and reaction with inorganic acids. Finally, the purified nickel sulfate solution still contains 50 mg / L of magnesium impurities, which is excessive for battery-grade nickel sulfate and indicates a lack of selectivity in the proposed method.

[0007] EP 3733884 describes a solvent extraction method that allows for the selective separation of magnesium from an acidic aqueous solution of sulfuric acid. The solvent extraction method involves contacting an acidic aqueous solution of sulfuric acid containing nickel, cobalt, and magnesium with an organic solvent under very specific extraction conditions to selectively extract magnesium into the organic solvent: extracting magnesium with a concentrated solvent containing 40 to 60% alkylphosphonic acid as the extractant at a relatively low pH of 1.5 to 2, or with a solvent containing a lower extractant concentration of 20 to 50% alkylphosphonic acid at a higher pH of 2.0 to 2.5. This step is intended only to remove magnesium, without separating cobalt from the nickel solution. Notably, under the same extraction conditions, up to 46% of magnesium was removed from the nickel sulfate solution, with approximately 9% of nickel being co-extracted. Under such conditions, only a Mg / Ni separation factor of 8 to 23 was obtained. When the extractant concentration in the solvent used was reduced to below 40% by volume, a higher Mg / Ni separation factor of up to 35 was obtained, but the removal of magnesium was much lower, below 28%.

[0008] EP 3222735 discloses a method for separating cobalt and magnesium from a nickel-containing feed solution by liquid-liquid extraction, wherein the organic solvent used contains an alkylphosphine acid as an extractant. Both cobalt and magnesium are extracted along with some nickel. First, nickel is washed from the loading solvent with an acidic solution. Since the resulting nickel solution may contain some cobalt, it is returned to the feed solution. Subsequently, magnesium is washed from the solvent with an acidic solution. The resulting magnesium solution may contain some cobalt, which is then processed elsewhere. Cobalt is stripped from the solvent with a diluted aqueous solution of acid to form a cobalt stripping solution. Besides cobalt and magnesium, the patent does not address the removal of other metallic contaminants from the nickel sulfate solution, such as calcium, zinc, cadmium, copper, manganese, and iron. Given the release of acidic protons during extraction with the acidic extractant, the patent also does not detail how to achieve the desired pH for extracting cobalt and magnesium from the nickel sulfate solution.

[0009] When using EHEHPA (also known as PC88A) as an extractant, the extraction behavior for magnesium or calcium is similar to that for nickel. JP 10-310437 discloses an example of separating nickel and cobalt by solvent extraction using PC88A as an extractant to extract cobalt along with other impurities such as calcium, copper, zinc, iron, and magnesium. When solvent extraction is performed on solutions containing high concentrations of nickel, a decrease in the extraction efficiency of magnesium or calcium occurs. Difficulty in removing magnesium from nickel sulfate solutions is mentioned. When containing 90 g / L to 117 g / L of nickel, the final impurity output concentration in the purified nickel sulfate solution is still 3 mg / L to 26 mg / L of cobalt, 2 mg / L to 7 mg / L of calcium, and 10 mg / L to 27 mg / L of magnesium. The present invention solves the problem of insufficient calcium extraction by selecting operating conditions that favor calcium extraction while simultaneously reducing magnesium extraction. This reduction in magnesium extraction is offset by additional, separate solvent extraction of magnesium with a more favorable extractant and more favorable operating conditions.

[0010] JP 2021 / 031729 illustrates the treatment of a crude nickel sulfate solution in a solvent extraction process, in which an attempt is made to remove all cobalt, magnesium, and calcium from the nickel sulfate solution together. The ratio of the amount of nickel loaded onto the solvent to the concentration of cobalt in the nickel sulfate solution must vary depending on the desired impurity removal. However, as can be seen from the examples, it is impossible to remove all contaminants, as the purified nickel sulfate solution still contains 1 mg / L to 60 mg / L of cobalt, 1 mg / L to 20 mg / L of magnesium, and 1 mg / L to 15 mg / L of calcium. Furthermore, magnesium removal appears negligible, as the input concentration of magnesium in the crude nickel sulfate solution is very low, only 19 mg / L to 31 mg / L, compared to the very high concentration of cobalt (8 to 12 g / L). This principle of co-extracting trace amounts of magnesium with large amounts of cobalt (even incompletely) demonstrates that a large amount of nickel must be used on the solvent. This is only an assumption that magnesium removal can be better achieved by increasing the amount of nickel loaded onto the solvent compared to the cobalt concentration in the crude nickel sulfate solution. In a similar patent JP 2021 / 031730, it is assumed that the amount of nickel selected in the solvent, compared to the concentration of cobalt present in the crude nickel sulfate solution, can affect the amount of magnesium reported in the cobalt eluent by co-extraction into the solvent. A similar embodiment is found in JP 2021 / 031729. The purified nickel sulfate solution may still contain impurities up to 1 mg / L to 60 mg / L of cobalt, 1 mg / L to 20 mg / L of magnesium, and 1 mg / L to 15 mg / L of calcium. Furthermore, the amount of co-extracted magnesium reported in the cobalt eluent can vary significantly.

[0011] US 6,149,885 describes a process for removing impurities such as cobalt, calcium, copper, and zinc from a crude nickel sulfate solution via solvent extraction. A method is disclosed for loading nickel onto a solvent, which can then be used to remove impurities from the crude nickel sulfate solution. However, only a small amount of magnesium is removed. In one embodiment, the purified nickel sulfate solution still retains 34 mg / L of magnesium, which is generally considered too impure for battery-grade nickel sulfate quality. In another embodiment, the purified nickel sulfate solution retains even 354 ppm of magnesium relative to 100% nickel. Removal of other metals, such as cadmium and manganese, from the crude nickel sulfate solution is not even considered.

[0012] JP 2021 / 105206 discloses a solvent extraction method that improves the separability between nickel and cobalt in a nickel recovery stage. The proposed solvent extraction method includes a nickel recovery stage in which an acidic extractant carrying nickel and cobalt is contacted with acid for back-extraction of nickel to obtain a nickel recovery solution. The extraction temperature in the nickel recovery stage is set to 47°C-60°C. Because the extraction temperature in the nickel recovery stage is set to 47°C or higher, the partition rate of cobalt to the organic solvent can be increased while maintaining a low partition rate of nickel to the organic solvent, thereby improving the separability between nickel and cobalt. While JP 2021 / 105206 shows that the amount of magnesium and calcium in the nickel solution is affected by the extraction process that separates nickel from cobalt, it does not teach how to optimally reduce the amount of magnesium and calcium impurities in the nickel solution.

[0013] US 2008 / 0003154 describes a two-step solvent extraction loop for the selective removal of impurity metals zinc and cobalt from the precious metal nickel. For selective extraction of zinc, the separation between zinc and cobalt in the Cyanex 272 system must be adequate. Similarly, for cobalt and nickel, the separation factor must be sufficiently large to obtain a pure nickel product. The solvent extraction process for the impurity metals is carried out at a temperature between 80°C and 100°C. It is thus recognized that cobalt can be selectively extracted from nickel, and any iron, copper, zinc, manganese, and magnesium are adequately co-extracted with cobalt. Further removal of impurities from nickel is not recommended, as such impurities are considered to be present at very low levels.

[0014] In summary, there is a need for a simple and practical method to achieve high-purity nickel sulfate with low levels of cobalt, calcium, magnesium, and other impurities, and for producing nickel sulfate suitable for applications requiring high purity, such as electroless deposition of nickel metal layers or as a precursor for battery cathode materials. One object of the present invention is to provide a novel method for producing a high-purity nickel sulfate solution from an aqueous nickel solution containing cobalt, magnesium, and calcium, and optional impurities such as iron, zinc, copper, cadmium, and manganese. Furthermore, an object of the present invention is to readily produce nickel sulfate of stable quality. Finally, an object of the present invention is to provide a method for producing a cobalt-rich aqueous solution suitable for further processing from crude nickel raw materials. Summary of the Invention

[0015] The present invention provides a solution to at least one of the above-mentioned problems by providing a method for preparing a high-purity nickel sulfate solution as described in claim 1.

[0016] The advantage of this invention is that if elements cobalt, zinc, manganese, cadmium, aluminum, copper, calcium, and magnesium are all present, they are all completely separated from nickel. This invention consists of a two-step solvent extraction process, wherein in the first step all the mentioned impurities except magnesium are completely removed, and in the second step residual magnesium is removed.

[0017] The overall process according to the invention is effective in providing a nickel solution with high purity, i.e., at least 99.8 atomic% nickel relative to the metal content of the solution, while minimizing material loss due to minimal co-extraction of the matrix element nickel; this avoids the formation of complex nickel-containing mixtures. Therefore, the process according to the invention is environmentally friendly. The processing strategy avoids the presence of undesirable ions in the final solution, such as calcium from calcium alkali, sodium from sodium alkali, and chloride ions from hydrochloric acid, derived from reagents used in the nickel refining process in the nickel sulfate final solution. Thus, the nickel sulfate solution obtained from the proposed method is readily further processed by crystallization or spray drying to form easily transportable nickel sulfate crystals or particles, respectively. Advantageously, the invention also allows for the production of cobalt-rich washes, which can be further processed separately, for example, for the production of high-purity cobalt salts such as cobalt chloride, cobalt sulfate, or others. The method of the invention is simple, environmentally friendly, and provides high-purity nickel sulfate. Attached Figure Description

[0018] The accompanying drawings are included in a further illustrative manner to better understand the teachings of the invention. These drawings are intended to aid in the description of the invention and are in no way intended to be limiting of the invention currently disclosed. The numbers and symbols contained herein have the meanings commonly understood by one of ordinary skill in the art to which this invention pertains.

[0019] Figure 1 The method according to the first aspect of the invention is illustrated schematically. Detailed Implementation

[0020] Unless otherwise defined, all terms used in disclosing this invention, including technical and scientific terms, have the meanings commonly understood by one of ordinary skill in the art to which this invention pertains. Definitions of terms are included in a further guiding manner to better understand the teachings of this invention.

[0021] As used herein, the following terms have the following meanings:

[0022] Unless the context clearly indicates otherwise, as used herein, “a,” “an,” “the,” and “the” refer to both the singular and plural indicators. For example, “compartment” means one or more compartments.

[0023] As used herein, “about” refers to measurable values, such as parameters, quantities, durations, etc., and is intended to cover 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, and such variations within this range from the specified value are suitable for implementation in the disclosed invention. However, it should be understood that the values ​​referred to by the modifier “about” are themselves specifically disclosed.

[0024] As used herein, “comprising” is synonymous with “including” or “containing” and is an inclusive or open-ended term that indicates the presence of what follows and does not exclude or prevent the presence of any other undescribed components, features, elements, members or steps known in the art or disclosed herein.

[0025] The range of values ​​listed by endpoints includes all numbers and fractions contained within that range, as well as the listed endpoints. Unless otherwise defined, or unless those skilled in the art would clearly understand a different meaning from its use and in the context of its use, all percentages should be understood as weight percentage (abbreviated as “weight%”) or volume percentage (abbreviated as “volume%”) or atomic percentage (abbreviated as “atomic%”).

[0026] Regarding organic phases, the following terms are used to define their components or as a whole:

[0027] i. An "extractant" or extraction reagent is an active component in an organic phase that extracts the metal substance into the organic phase by chemically binding with it and forming a metal-extractant complex, the metal-extractant complex being more soluble in the organic phase than in the aqueous phase.

[0028] ii. A “diluent” is an organic molecule, or typically a mixture of different organic molecules, added to an organic phase to dilute the extractant and allow the metal complex to dissolve, improve the physical properties of the organic phase (especially phase separation), and reduce its cost (given that diluents are generally cheaper than extractants). Diluents are typically kerosene fractions and can be aliphatic or aromatic hydrocarbons, cycloalkanes, etc., or mixtures thereof.

[0029] iii. The organic phase may also contain "modifiers". Modifiers are sometimes added to improve the solubility of metal complexes in the organic phase, to alter the physical properties of the solvent to prevent scaling or third-phase formation, as these phenomena are undesirable in solvent extraction. Modifiers may also be added to prevent chemical degradation of the extractant or diluent. However, because these modifiers may participate in the formation of metal-extractant complexes, they may impair the selectivity of the organic phase.

[0030] iv. "Organic phase" is another term used to define "solvent" or "solvent mixture" and includes a mixture of (one or more) extractants, (one or more) diluents and optionally (one or more) modifiers.

[0031] The "selectivity" S of an extractant for one metal relative to another can be expressed as the ratio of the distribution coefficients D of the two metals:

[0032] S Mg / Ni = D Mg / D Ni

[0033] The "distribution factor" of a metal should be understood as the ratio of the equilibrium concentration of that metal in the organic phase to that of the same metal in the aqueous phase.

[0034] D M = [M] O / [M] A

[0035] Where M is a metal such as nickel or magnesium, O refers to the organic phase, and A refers to the aqueous phase.

[0036] In the context of this invention, "solvent extraction loop" should be understood as synonymous with the terms "solvent extraction," "solvent extraction process," "solvent loop," "solvent circuit," or "solvent extraction circuit," referring to a series of one or more solvent extraction sections, each consisting of one or more solvent extraction stages. While each extraction section may use a different set of process parameters such as temperature, pH characteristics, and solvent-to-water ratio, the solvent extraction loop uses only one and the same organic phase. The organic phase composition of the solvent extraction loop is fixed, as characterized by a single set of parameters, such as the type of extractant, the type of diluent, and the extractant-to-diluent ratio.

[0037] In a first aspect, the present invention provides a method for preparing a high-purity nickel sulfate solution, the method comprising the following steps:

[0038] i. Provide an aqueous feed containing nickel, cobalt, calcium and magnesium, and if present, zinc, manganese, cadmium and / or copper;

[0039] ii. Extracting cobalt, calcium, and at least a portion of magnesium from the feed aqueous solution using a first organic phase comprising a first alkylphosphine extractant (I) and a first diluent, thereby obtaining an aqueous raffinate solution (A1) containing nickel and residual magnesium, and a cobalt-rich organic phase (O1), said organic phase typically comprising calcium, magnesium, and nickel, and, if present, zinc, copper, cadmium, and manganese; and

[0040] iii. Magnesium is extracted from the aqueous raffinate solution (A1) using a second organic phase (O2) containing a second alkylphosphine extractant (II) and a second diluent, thereby obtaining a magnesium-poor high-purity nickel sulfate aqueous solution (A2) and a magnesium-rich organic phase.

[0041] The advantage of this invention is that elements cobalt, magnesium, calcium, and, if present, zinc, manganese, cadmium, iron, aluminum, and copper are completely separated from nickel in a single process. The process provides a high-purity aqueous nickel sulfate solution (A2) containing nickel at a concentration between 40 g / L and 200 g / L and magnesium at a concentration of up to 10 mg / L, two loaded organic phases: a cobalt-rich organic phase (O1) containing calcium, magnesium, and nickel, and, if present, zinc, copper, cadmium, and manganese, and a magnesium-rich organic phase (O2) containing nickel and magnesium. Preferably, the high-purity nickel sulfate aqueous solution contains up to 5 mg / L, or even more preferably up to 1 mg / L, magnesium. The first and second organic phases may contain modifiers.

[0042] The aqueous raffinate solution A1 contains nickel sulfate and magnesium at a concentration between 20 mg / L and 20 g / L, preferably between 20 mg / L and 2 g / L, and more preferably between 20 mg / L and 500 mg / L.

[0043] Typically, the residual magnesium content in the obtained aqueous raffinate solution (A1) is too high for high-purity applications, so a second solvent extraction step is performed.

[0044] The solvent extraction steps ii. and iii. can be carried out in any suitable equipment and are not specifically limited thereto. Solvent extraction equipment typically includes at least one or more devices consisting of a mixing settler, column contactor, centrifugal contactor, or any other type of contactor. Preferably, the extraction is carried out in a countercurrent configuration.

[0045] Preferably, the present invention also provides a method according to a first aspect of the invention, the method further comprising step iv., wherein the cobalt-rich organic phase (O1) containing calcium, magnesium, and nickel, and if present, zinc, copper, cadmium, and manganese, is stripped with an aqueous solution containing an inorganic acid. This effectively causes cobalt, calcium, magnesium, and if present, zinc, copper, cadmium, and manganese to dissolve from the first solvent. Preferably, the inorganic acid is selected from one or more of the following: hydrofluoric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, phosphoric acid, sulfuric acid, boric acid, and perchloric acid. More preferably, the inorganic acid is selected from one or more of the following: hydrochloric acid, hydrobromic acid, nitric acid, phosphoric acid, and perchloric acid. In another embodiment, the inorganic acid is sulfuric acid. Most preferably, the inorganic acid is hydrochloric acid. This allows a concentrated eluent solution containing cobalt, calcium, magnesium, and if present, zinc, copper, cadmium, and manganese to be obtained from the first solvent.

[0046] The stripping step can be performed in any suitable equipment, and there are no specific limitations. Stripping equipment typically includes at least one or more devices consisting of a mixing settler, column contactor, centrifugal contactor, or any other type of contactor. Preferably, the stripping is performed in a counter-current configuration.

[0047] In a preferred embodiment, the present invention provides a method according to a first aspect thereof, wherein iron and / or aluminum are removed from a leaching solution comprising nickel, cobalt, magnesium, and iron and / or aluminum, and optionally calcium, zinc, copper, cadmium, and manganese, to obtain the feed aqueous solution comprising nickel, cobalt, calcium, magnesium, and optionally zinc, copper, cadmium, and manganese. The iron and / or aluminum can be advantageously removed by adding an alkaline reagent, such as a hydroxide, to the aqueous solution, thereby forming an iron and / or aluminum hydroxide precipitate. Potentially, this iron and / or aluminum removal step may include the addition of an oxidizing agent, such as oxygen or hydrogen peroxide.

[0048] In a preferred embodiment, the iron and / or aluminum are removed by precipitation using a calcium base, such as calcium hydroxide, calcium oxide, calcium carbonate, calcium bicarbonate, or any other calcium-containing alkaline reagent. The use of a calcium base is advantageous because calcium forms calcium sulfate, also known as gypsum, in this step of the method, which has low water solubility. Therefore, using an excess of calcium base will not adversely affect the purity of the resulting nickel sulfate solution. Only a limited amount of calcium will remain in the nickel solution sent to the solvent extraction step ii. The latter step is designed to allow for complete removal of calcium from the nickel solution. The formation of calcium sulfate during the iron and / or aluminum precipitation process enhances the filterability of the iron and / or aluminum precipitate. Therefore, it is preferable to use a stoichiometric excess of calcium base relative to the amount of iron and / or aluminum impurities present in the feed aqueous solution containing nickel, cobalt, magnesium, and iron and / or aluminum.

[0049] In another preferred embodiment, the base used may be a nickel hydroxide or carbonate or any other nickel-containing basic reagent, thereby introducing beneficial nickel ions into the nickel sulfate solution. Other preferred nickel bases are nickel bicarbonate and nickel hydroxysulfate.

[0050] In another preferred embodiment, the base used may be a magnesium hydroxide or carbonate or any other magnesium-containing basic agent, because magnesium is efficiently and effectively removed in subsequent steps of the method of the present invention. Other preferred magnesium bases are magnesium bicarbonate and magnesium hydroxide.

[0051] In another preferred embodiment, impurities such as iron and / or aluminum can be separated by precipitation using a combination of two or more precipitants selected from calcium alkali, magnesium alkali and nickel alkali.

[0052] Furthermore, impurities such as iron and / or aluminum can be removed by precipitation in two or more precipitation steps, where different precipitants can be used in each precipitation step. In a preferred embodiment, a nickel alkali is used in the first precipitation step, and a calcium alkali is used in subsequent precipitation steps.

[0053] Alternatively, impurities such as iron and / or aluminum can be separated by other methods, such as neutralization. However, using alkali metal bases such as sodium hydroxide or potassium hydroxide introduces metallic impurities into the feed aqueous solution, which cannot be extracted by subsequent solvent extraction processes and may therefore complicate potential crystallization or granulation processes at the end of the process.

[0054] In another embodiment, calcium is already present in the nickel feed solution entering the solvent extraction step ii, because it is introduced in advance by the raw material or before entering the solvent extraction step ii by using a calcium-containing reagent such as calcium hydroxide, calcium oxide, calcium carbonate, calcium bicarbonate or other calcium-containing alkaline reagents.

[0055] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the feed aqueous solution provided in step i. comprises nickel in an amount of at least 60 atomic% relative to the total metal content of the feed aqueous solution, and cobalt in an amount of at most 40 atomic% relative to the total metal content of the feed aqueous solution. Preferably, the feed aqueous solution comprises at least 70 atomic% of nickel and at most 30 atomic% of cobalt; more preferably, the feed aqueous solution comprises at least 80 atomic% of nickel and at most 20 atomic% of cobalt; and most preferably, the feed aqueous solution comprises at least 90 atomic% of nickel and at most 10 atomic% of cobalt.

[0056] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the feed aqueous solution provided in step i. further comprises calcium, magnesium, zinc, copper, cadmium, and manganese in a total amount of up to 25 atomic% relative to the total metal content of the feed aqueous solution. Preferably, the feed aqueous solution further comprises calcium, magnesium, zinc, copper, cadmium, and manganese in a total amount of up to 10 atomic% and even more preferably up to 5 atomic% of the total amount of calcium, magnesium, zinc, copper, cadmium, and manganese.

[0057] Therefore, the feed aqueous solution can be derived from a variety of sources, such as mixed hydroxide precipitates, crude nickel sulfate, or any other suitable source that is itself suitable or has optionally been processed into a suitable feed solution. This processing may include leaching, selective leaching, dissolution, precipitation steps, and / or any other type of pretreatment step. These may be combined. For example, pre-processed battery recycling material containing nickel, cobalt, manganese, and lithium can be treated in such a process to produce a pure nickel sulfate solution if the pre-processing includes at least leaching and final pre-removal of lithium. Alternatively, lithium can be removed at the end of step iii. by, for example, a lithium-ion exchange column.

[0058] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the extractant used in steps ii. and iii. comprises an alkylphosphine. Suitable alkylphosphines include bis(2-ethylhexyl)phosphate (D2EHPA), (2-ethylhexyl)phosphonate mono(2-ethylhexyl) ester (EHEHPA, PC88A), bis(2,4,4-trimethylpentyl)phosphonic acid (CYANEX 272 or IONQUEST 290), and diisooctylphosphonic acid (DOPA). Alkylphosphines act as chelating extractants due to the presence of coordinating phosphorus and oxygen atoms in these molecules. Among elements in aqueous solution, elements that form corresponding chelating compounds with higher stability are more conducive to extraction efficiency than elements that are less likely to form chelating compounds.

[0059] When using EHEHPA (PC88A) as the extraction solvent, the extraction behavior of magnesium and calcium is similar to that of nickel. Therefore, solvent extraction of solutions containing high concentrations of nickel results in reduced extraction efficiency for both magnesium and calcium. This invention addresses the problem of insufficient calcium extraction by selecting operating conditions that favor calcium extraction while simultaneously reducing magnesium extraction. The reduced magnesium extraction is offset by additional, separate solvent extraction of magnesium using a more favorable second extraction solvent and operating conditions that further enhance magnesium extraction.

[0060] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the second extractant (II) has a higher selectivity for magnesium than the first extractant (I). In other words, the second extractant (II) has a higher affinity for magnesium than the first extractant (I). Furthermore, the second extractant (II) has a higher selectivity for magnesium than for nickel. Most preferably, the second extractant (II) comprises an alkylphosphinolic acid, such as IONQUEST 290.

[0061] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the first extractant (I) has a higher selectivity for calcium relative to nickel than the second extractant (II). In other words, the first extractant (I) has a higher affinity for calcium relative to nickel than the second extractant (II). Furthermore, the first extractant (I) has a higher selectivity for calcium than for nickel. Most preferably, the first extractant (I) comprises an alkylphosphonic acid, such as PC88A. Preferably, the first alkylphosphonic extractant (I) comprises an alkylphosphonic acid and / or its nickel salt, and the second alkylphosphonic extractant (II) comprises an alkylphosphonic acid and / or its nickel salt.

[0062] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the first and second diluents are hydrocarbons. More generally, any organic, water-immiscible solvent capable of dissolving the extractant can be used. Therefore, there is no particular limitation on the diluent. Examples of diluents include kerosene compounds, which can be aliphatic, cycloalkane, aromatic compounds, or even mixtures thereof.

[0063] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the first organic phase used in step ii. comprises a first extractant (I) in an amount of 5 vol% to 50 vol% relative to the total volume of the first organic phase, and a first diluent in an amount of 50 vol% to 95 vol% relative to the total volume of the first organic phase. More preferably, the first organic phase comprises a first extractant (I) in an amount of 30 vol% to 40 vol% and a first diluent in an amount of 60 vol% to 70 vol%.

[0064] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the second organic phase used in step iii. comprises a second extractant (II) in an amount of 5 vol% to 50 vol% relative to the total volume of the second organic phase, and a second diluent in an amount of 50 vol% to 95 vol% relative to the total volume of the second organic phase. More preferably, the second organic phase comprises a second extractant (II) in an amount of 10 vol% to 25 vol% and the diluent in an amount of 75 vol% to 90 vol%. It has been found that the extractant concentration in the organic phase allows for optimal extraction of magnesium without compromising the processability of the solvent.

[0065] In a preferred embodiment, the extractant used in steps ii. and iii. is neutralized with an alkali metal hydroxide and preloaded with nickel at a high pH before being used for extraction in steps ii. and iii., wherein the preloaded nickel-containing organic phase is contacted with an aqueous feed solution containing impurities. In this case, an exchange reaction occurs, through which elements more likely to be extracted than nickel are transferred to the solvent, while nickel in the organic phase is transferred to the aqueous phase. Thus, the nickel concentration in the resulting raffinate solution is increased while removing impurities from the feed aqueous solution, thereby largely avoiding the introduction of alkali metals from the neutralizing agent into the main process (raffinate) stream. Sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc., can be used as the alkali metal hydroxide. However, sodium hydroxide is preferably used as the alkali metal hydroxide.

[0066] It has been found that the nickel-preloaded extractant used in steps ii. and iii. allows for optimal and improved extraction without compromising the processability of the extractant. During this preloading step, the partially neutralized extractant (i.e., in an alkali metal conversion form) exchanges the alkali metal (typically sodium) on the extractant for nickel from the aqueous nickel sulfate solution. Preferably, the residual alkali metal on the preloaded solvent is as low as possible to limit the transfer of residual alkali metal from the preloaded solvent to the feed aqueous solution when extracting impurities from the feed aqueous solution.

[0067] A portion of the nickel can be replaced by another harmless metal, which will exchange with impurities extracted from the aqueous nickel sulfate solution to be purified. This can be an alkali metal such as sodium or potassium, or a similar substance such as ammonium. However, these other metals can impart extraction of such metals present in the aqueous nickel sulfate solution to be purified, or even contaminate the nickel sulfate solution by exchanging it with the impurities to be extracted.

[0068] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein, prior to extraction, the extractant is converted into nickel salts corresponding to the appropriate conversion of the extractant, thereby comprising nickel in an amount between 20% and 70% of the available extractant capacity and residual sodium at a concentration of up to 2 g / L, preferably up to 0.5 g / L, more preferably up to 0.1 g / L. Preferably, nickel is preloaded to an amount between 25% and 60% of the available extractant capacity, preferably greater than 30% of the available extractant capacity, and the concentration of residual sodium is up to 0.5 g / L. More preferably, nickel is preloaded to an amount between 30% and 50% of the available extractant capacity, and the concentration of residual sodium is up to 0.1 g / L.

[0069] Therefore, the preferred nickel concentration on the preloaded solvent depends on the extractant concentration and the degree of conversion. Both are determined by the target pH in the feed aqueous solution and are thus a function of the total amount of impurities to be removed. A higher degree of conversion of the extractant results in a higher pH during the extraction process, allowing for the extraction of more impurities (and nickel) from the nickel-containing feed solution, while a lower degree of conversion of the extractant results in a lower pH during the extraction process, allowing for better selectivity for impurities relative to nickel.

[0070] In a preferred embodiment, the preloaded solvent containing nickel and possibly some other metals (e.g., sodium, potassium, or other metals) or other cations (e.g., ammonium) may be contacted again with a pure nickel-containing solution (e.g., nickel sulfate or nickel chloride solution) to further exchange the metallic sodium, potassium, ammonium, or other metals on the solvent with the nickel from the pure nickel-containing solution. The preloaded nickel operation may be carried out in two or more stages, preferably in a countercurrent operation, using at least a pure nickel sulfate solution to remove any alkali metals from the used base that may have been co-extracted from the solvent. As a result, a nickel preloaded solvent containing significantly fewer other metals is obtained for use in steps ii. and iii., thus minimizing undesirable metal contamination of the nickel aqueous solution.

[0071] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the feed aqueous solution entering step ii. has a pH between 1.0 and 6.0, more preferably between 2.0 and 5.5, and most preferably between 3.0 and 5.0, before being contacted with the solvent containing extractant I to achieve a chemical equilibrium between the nickel aqueous solution and the solvent.

[0072] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the extraction in step ii. is carried out at a temperature between 25°C and 70°C, preferably between 30°C and 60°C, more preferably between 30°C and 55°C, or even between 30°C and 50°C. More preferably, the extraction in step ii. is carried out at a temperature between 35°C and 45°C. The inventors have found that lower temperatures improve the extraction of calcium from nickel solutions. Therefore, the extraction temperature in step ii. is preferably below 50°C, preferably below 45°C. However, at lower temperatures, the extraction efficiency of cobalt decreases. Therefore, it is preferable to use an extraction temperature above 25°C, preferably above 30°C, more preferably above 35°C. Therefore, most preferably, the extraction temperature in step ii. is above 25°C and below 45°C, preferably above 30°C and below 45°C, more preferably above 35°C and below 45°C. Specifically, the extraction temperature is any temperature between 36°C, 38°C, 40°C, 42°C, or 44°C.

[0073] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the aqueous raffinate solution obtained from step ii. and entering step iii. has a pH between 2.0 and 7.0, more preferably between 3.0 and 6.5, and most preferably between 4.0 and 6.0, before being contacted with the second solvent containing extractant II to achieve a chemical equilibrium between the nickel aqueous solution and the second solvent.

[0074] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the extraction in step iii. is carried out at a temperature of at least 25°C, at least 30°C, preferably at least 35°C, and at most 80°C. Preferably, the extraction in step iii. is carried out at a temperature between 40°C and 70°C, or even between 45°C and 65°C. More preferably, the extraction in step iii. is carried out at a temperature between 50°C and 60°C. The inventors have found that higher extraction temperatures improve the extraction of magnesium from the nickel solution. However, due to the processability and safety aspects of the organic solvent, the extraction temperature is preferably limited to below 80°C, below 70°C, or below 65°C.

[0075] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the extraction in step iii. is performed at a temperature higher than the extraction temperature in step ii. Preferably, the temperature in step iii. is at least 5°C higher than the temperature in step ii., more preferably at least 10°C higher, more preferably 10°C to 20°C higher, and most preferably about 15°C higher.

[0076] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the high-purity nickel sulfate aqueous solution obtained after step iii. is crystallized or granulated. Preferably, the nickel sulfate in the nickel sulfate solution is crystallized, thereby allowing for additional purification steps. In the case of granulation, any granulation technique known to those skilled in the art is suitable, such as, for example, spray drying.

[0077] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein nickel is removed from the cobalt-rich organic phase and / or from the magnesium-rich organic phase. To recover this co-extracted nickel from the loaded organic phase prior to impurity stripping, it is first selectively removed from these solvents by washing with an acidic solution (e.g., an aqueous sulfuric acid solution). Nickel is selectively removed by applying optimal pH conditions, specifically the acidity of the final washing solution, and by adjusting the amount of acid added to achieve the desired pH.

[0078] In a preferred embodiment, the present invention provides a method according to a first aspect of the invention, wherein the stripping step in step iv. is carried out with hydrochloric acid. The stripping step allows the dissolution of nickel and cobalt, calcium, magnesium, and, if present, zinc, copper, cadmium, and manganese from the first loading solvent (O1). Thus, the extractant is regenerated to obtain a metal-free solvent that can be reused for extraction or preloading. Given the presence of calcium in the obtained stripping solution, hydrochloric acid is preferably used. Stripping with hydrochloric acid forms water-soluble calcium chloride. In this way, the metal content from the solvent to the aqueous solution can be concentrated. Given the low solubility of calcium sulfate, the use of sulfuric acid may cause the formation of solid precipitates, thereby disrupting the solvent extraction process.

[0079] In a preferred embodiment, the concentration of the hydrochloric acid solution is at least 50 g / L, more preferably between 100 g / L and 300 g / L.

[0080] In a preferred embodiment, the peeling in step iv. is performed at a temperature between 40°C and 55°C, preferably between 40°C and 50°C, and more preferably at a temperature of about 45°C.

[0081] In a preferred embodiment, the present invention provides a post-processing step according to a first aspect of the invention, wherein the magnesium-poor high-purity nickel sulfate aqueous solution comprises nickel at a concentration of 40 g / L to 180 g / L and magnesium at a concentration of up to 5 mg / L, preferably up to 1 mg / L. Preferably, the high-purity nickel sulfate aqueous solution has the content of calcium, cobalt, iron, aluminum, zinc, manganese and / or cadmium, each individually in an amount of up to 15 mg / L, preferably up to 10 mg / L or even up to 5 mg / L.

[0082] In a preferred embodiment, the invention provides a post-processing step according to a first aspect of the invention, wherein after stripping with hydrochloric acid in step iv., the cobalt-rich organic phase is additionally washed with sulfuric acid. Preferably, the solvent is washed with sulfuric acid at a concentration of 10 g / L to 200 g / L contained in an aqueous solution. Washing with sulfuric acid allows the removal of chloride ions and possible residual metals such as iron or aluminum from the solvent. Thus, the solvent is regenerated and can be reused for extraction after preloading.

[0083] In a preferred embodiment, the present invention provides a method for stripping the second solvent containing magnesium and nickel with sulfuric acid. Preferably, the solvent is washed with sulfuric acid at a concentration of 10 g / L to 200 g / L contained in an aqueous solution. Washing with sulfuric acid also allows for the removal of any possible residual metals, such as iron or aluminum, from the solvent. Thus, the solvent is regenerated and can be reused for extraction after preloading.

[0084] Example

[0085] Examples are provided for each process step to further illustrate the invention. These examples are based on experimental data and are in no way intended to limit the scope of the invention.

[0086] Preparation of feed aqueous solution

[0087] The leaching solution is obtained from the leaching of crude NiSO4 raw material. In a subsequent step, the leaching solution is subjected to iron removal by neutralization with Ni(OH)2 and then with Ca(OH)2. A solid iron cake is formed, which is filtered and separated from the aqueous solution. The table below shows the composition of the nickel sulfate aqueous solution before and after these two iron removal steps. The resulting feed aqueous solution containing nickel, cobalt, calcium, magnesium, copper, zinc, and manganese is free of iron and aluminum and continues into the solvent extraction unit.

[0088]

[0089] Preparation of preloaded nickel extractant I

[0090] A first solvent was prepared by combining (2-ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester (PC88A) and the hydrocarbon diluent Escaid 110 (ExxonMobil). The first solvent contained 36 vol% PC88A and 64 vol% hydrocarbon diluent. In two consecutive steps, the solvent was contacted with a high-purity aqueous solution of nickel sulfate containing 125 g / L nickel and 1.0 g / L sodium. A 125 g / L NaOH solution was added to a mixing settler, the volume of which was targeted at a preloaded solvent containing 11 g / L nickel. This corresponds to a conversion of 36% of the total extractant capacity. After one preload step, the solvent contained 10.4 g / L nickel and 0.82 g / L sodium. After a second preload step, a solvent containing 10.8 g / L nickel and a residual sodium concentration of 0.12 g / L was obtained.

[0091] Extraction of cobalt, calcium and other impurities

[0092] The feed aqueous solution was mixed in several stages with a first solvent containing PC88A and preloaded with 9.1 g / L nickel to extract cobalt, calcium, zinc, copper, manganese, and a portion of magnesium at 40°C. The extraction section consisted of four consecutive mixing settlers. In addition to sodium at a concentration below 1 g / L, a first loaded solvent (O1) containing nickel, cobalt, calcium, magnesium, copper, zinc, manganese, and cadmium, and an aqueous raffinate solution (A1) containing nickel sulfate and a significant amount of residual magnesium were obtained. The composition of the feed solution and raffinate is shown in the table below. The nickel concentration in the aqueous raffinate is increased compared to the nickel concentration in the feed aqueous solution because, during extraction, the nickel on the preloaded solvent is stoichiometrically exchanged for impurities in the feed solution.

[0093]

[0094] Extraction agent I was removed with hydrochloric acid.

[0095] Following extraction, a first loading solvent containing sodium, nickel, cobalt, calcium, magnesium, zinc, cadmium, and manganese was treated with an aqueous solution containing 200 g / L hydrochloric acid at 43°C. The dissolution section consisted of a multi-stage mixing settling tank. The solvent was regenerated, producing an aqueous eluent with 35 g / L residual hydrochloric acid. The composition of the loading and stripping solvents is shown in the table below.

[0096]

[0097] Wash the first solvent with sulfuric acid.

[0098] Following the dissolution phase, the first solvent can be washed with an aqueous solution containing sulfuric acid. The most important goal of this stripping solvent post-treatment is to remove the hydrochloric acid carried over from the preceding dissolution phase. Therefore, the stripping solvent is contacted with a sulfuric acid (33 g / L) solution. As a result, all residual contaminants are removed from the stripping solvent; all Ni, Co, Ca, Mg, Zn, Mn, Cd, Na, and Cl < 1 mg / L. Therefore, it can be regenerated for the next cycle.

[0099] Remove extractant I with sulfuric acid.

[0100] After extraction and before stripping, an additional scavenging step can be performed to prevent nickel loss into the eluent. The first loading solvent containing sodium, nickel, cobalt, calcium, magnesium, zinc, and manganese was combined with an aqueous solution containing 500 g / L sulfuric acid. The experiment was conducted at 40°C. The scavenging section consisted of three consecutive mixing settlers. The table below shows the composition of the solvent before and after scavenging. High selectivity was achieved: nickel was removed in 91% yield, magnesium in 13% yield, while all other impurities remained in the solvent.

[0101]

[0102] Preparation of preloaded nickel extractant II

[0103] A second solvent was prepared by combining bis(2,4,4-trimethylpentyl)phosphonic acid (IONQUEST 290) with the hydrocarbon diluent Escaid 110 (ExxonMobil). The second solvent comprised 15 vol% IONQUEST 290 and 85 vol% Escaid 110. In two consecutive steps, in a countercurrent configuration, the solvent was contacted with a high-purity aqueous solution of nickel sulfate containing 130 g / L nickel and 0.90 g / L sodium. A 125 g / L NaOH solution was used to obtain a preloaded solvent containing approximately 5.0 g / L nickel. This corresponds to a conversion of 38% of the total extractant capacity. After one preload step, the solvent contained 2.2 g / L nickel and 79 mg / L sodium. After a second preload step, a solvent containing 4.6 g / L nickel and a residual sodium concentration of 67 mg / L was obtained. Depending on the specific conditions, the preloaded solvent may contain more or less nickel or residual sodium.

[0104] In another embodiment, a higher extractant concentration was used. In one step, the second solvent, comprising IONQUEST 290 and Escaid 110, was contacted with a high-purity aqueous solution of nickel sulfate containing 97 g / L nickel, 40 mg / L magnesium, and 27 mg / L sodium. The pH was maintained constant at 6.0 using a 125 g / L NaOH solution. The table below shows the amount of nickel loaded onto the solvent as a function of the extractant concentration.

[0105]

[0106] Magnesium is extracted to form a high-purity nickel sulfate solution.

[0107] An aqueous raffinate (A1) containing 127 g / L nickel and 690 mg / L magnesium, obtained after the first solvent extraction, is mixed with a second solvent containing IONQUEST 290 preloaded with 6.0 g / L nickel to extract the remaining magnesium at 55°C. The extraction section consists of five mixing settlers configured in a countercurrent manner. Leaving the solvent extraction unit are (i) a second loaded solvent (O2) containing nickel and magnesium, and (ii) a high-purity aqueous nickel sulfate solution (A2) containing only 1.0 mg / L magnesium, which can then be further processed for crystallization or granulation to obtain a high-purity nickel sulfate product. The nickel concentration in the aqueous raffinate increases to 138 g / L because, during the extraction process, the nickel on the preloaded solvent is stoichiometrically exchanged for impurities in the feed solution.

[0108] Remove extractant II with sulfuric acid.

[0109] After extraction and before dissolution, an additional scavenging step can be performed to prevent nickel loss into the eluent. A second loading solvent containing 1.10 g / L nickel and 1.30 g / L magnesium was combined with an aqueous solution containing 70 g / L sulfuric acid. The experiment was conducted at 40°C. The scavenging section consisted of three consecutive mixing settlers. The nickel concentration in the scavenging solvent was reduced to 0.003 g / L, resulting in a scavenging yield of 99.7%. No magnesium was co-removed during this scavenging operation, resulting in a scavenging solvent containing 1.30 g / L magnesium.

[0110] Effect of temperature on the extraction of different impurities

[0111] In this example, the effect of temperature on the extraction of calcium, cobalt, and magnesium is illustrated. In two consecutive steps, a first solvent preloaded with 17 g / L nickel, containing 35 vol% PC88A and 65 vol% Escaid 110, was contacted with an aqueous feed containing 118 g / L nickel, 9 mg / L calcium, 68 mg / L cobalt, and 124 mg / L magnesium. The table below shows the extraction percentages of different impurities at 25°C and 65°C. Lower temperatures favored calcium extraction, while higher temperatures favored cobalt and magnesium extraction.

[0112]

[0113] Therefore, the experimental results show that lower extraction temperatures improve the extraction of calcium from nickel solutions. Therefore, the extraction temperature in this step is preferably below 50°C, more preferably below 45°C. However, at lower extraction temperatures, the extraction efficiency of cobalt also decreases. Therefore, it is preferable to use an extraction temperature above 25°C, more preferably above 30°C, and more preferably above 35°C. Most preferably, the extraction temperature is above 25°C and below 45°C, more preferably above 30°C and below 45°C, and more preferably above 35°C and below 45°C.

[0114] In another embodiment, the effect of temperature on magnesium extraction is shown. In one step, a second solvent containing 15 vol% IONQUEST 290 and 85 vol% Escaid 110 was contacted with a feed aqueous solution containing 127 g / L nickel and 180 mg / L magnesium. The pH was controlled at pH = 5.0. The table below shows that the percentage of magnesium extraction increased from 25°C through 44°C to 65°C, while nickel extraction remained low. As a result, the selectivity of magnesium relative to nickel increased with increasing temperature.

[0115]

[0116] The experimental results in the table above clearly demonstrate that using a higher extraction temperature, i.e., at least 25°C, preferably at least 30°C, or even at least 35°C, improves the extraction of magnesium from the nickel solution. More preferably, the extraction temperature is above 40°C, above 50°C, above 55°C, or even above 60°C. However, due to the processability of the organic solvent, its safety, and the need to avoid high energy input, the extraction temperature is preferably limited to below 80°C, below 70°C, or below 65°C.

Claims

1. A method for preparing a high-purity nickel sulfate solution, the method comprising the following steps: i. Provide an aqueous feed solution containing nickel, cobalt, calcium, and magnesium; ii. In the first solvent extraction loop, cobalt, calcium and at least a portion of magnesium, as well as zinc, copper, cadmium and manganese, if present, are extracted from the feed aqueous solution using a first organic phase comprising a first alkylphosphine extractant (I) and a first diluent, thereby obtaining an aqueous raffinate solution comprising nickel and residual magnesium content and a cobalt-rich and calcium-containing organic phase; iii. In the second solvent extraction loop, magnesium is extracted from the aqueous raffinate solution using a second organic phase comprising a second alkylphosphine extractant (II) and a second diluent, thereby obtaining a magnesium-poor, high-purity nickel sulfate aqueous solution and a magnesium-rich organic phase; and iv. Strip the cobalt-rich organic phase obtained in step ii) with an aqueous solution containing an inorganic acid. The first solvent extraction circuit and the second solvent extraction circuit are operated at different temperatures and / or with different extractants.

2. The method according to claim 1, wherein the inorganic acid is hydrochloric acid.

3. The method according to claim 1 or 2, wherein the feed aqueous solution further comprises zinc, copper, cadmium and / or manganese.

4. The method according to claim 1 or 2, wherein nickel is removed from the cobalt-rich organic phase and / or from the magnesium-rich organic phase.

5. The method according to claim 1 or 2, wherein the feed aqueous solution is obtained by removing iron and / or aluminum from a feed aqueous solution containing nickel, cobalt, calcium, magnesium, and if present, zinc, copper, cadmium and / or manganese, and also containing iron and / or aluminum.

6. The method of claim 5, wherein the iron and / or aluminum are removed by precipitation using a precipitant, the precipitant comprising a calcium base.

7. The method of claim 5, wherein the iron and / or aluminum are removed by precipitation using a precipitant, the precipitant comprising a magnesium base.

8. The method of claim 5, wherein the iron and / or aluminum are removed by precipitation using a precipitant, the precipitant comprising a nickel base.

9. The method of claim 5, wherein the iron and / or aluminum are removed by precipitation in two or more precipitation steps, wherein different precipitants or combinations of precipitants may be used in each precipitation step.

10. The method according to claim 1 or 2, wherein the first alkylphosphine extractant and the second alkylphosphine extractant comprise alkylphosphine acids and / or their nickel salts.

11. The method according to claim 1 or 2, wherein the second alkylphosphine extractant (II) is more selective for magnesium than the first alkylphosphine extractant (I).

12. The method according to claim 1 or 2, wherein the first alkylphosphine extractant (I) comprises alkylphosphonic acid and / or its nickel salt, and wherein the second alkylphosphine extractant (II) comprises alkylphosphonic acid and / or its nickel salt.

13. The method according to claim 1 or 2, wherein the first alkylphosphine extractant and the second alkylphosphine extractant are converted into their nickel salts, said nickel salts containing at least 20% nickel at a concentration of the available extractant capacity.

14. The method of claim 13, wherein the first alkylphosphide extractant and the second alkylphosphide extractant are converted into nickel salts, and wherein the first organic phase and the second organic phase contain less than 2 g / L of sodium.

15. The method according to claim 1 or 2, wherein the first organic phase and the second organic phase provided in steps ii. and iii. respectively comprise an amount of the first alkylphosphide extractant and the second alkylphosphide extractant in an amount of 5 vol% to 50 vol% relative to the total volume of the solvent, and an amount of the diluent in an amount of 50 vol% to 95 vol% relative to the total volume of the solvent.

16. The method according to claim 1 or 2, wherein the extraction in step ii. is carried out at a temperature between 25°C and 55°C.

17. The method according to claim 1 or 2, wherein the extraction in step iii. is carried out at a temperature between 40°C and 70°C.

18. The method according to claim 1 or 2, wherein the stripping step iv. is performed at a temperature between 40°C and 50°C.

19. The method according to claim 1 or 2, wherein the magnesium-poor high-purity nickel sulfate aqueous solution comprises nickel at a concentration of 40 g / L to 180 g / L and magnesium at a concentration of up to 5 mg / L.

20. The method according to claim 1 or 2, wherein the feed aqueous solution provided in step i. contains nickel in an amount of at least 60 atomic% relative to the total metal content of the feed aqueous solution, and wherein the feed aqueous solution provided in step i. contains cobalt, calcium, magnesium, and optionally zinc, copper, cadmium, and manganese in an amount of at most 40 atomic% relative to the total metal content of the feed aqueous solution.

21. The method according to claim 1 or 2, wherein the high-purity nickel sulfate aqueous solution obtained after removing residual magnesium in step iii. is crystallized or granulated.

22. The method according to claim 1 or 2, wherein after stripping with the inorganic acid, the first organic phase is washed with sulfuric acid.

23. The method of claim 22, wherein after washing or stripping with sulfuric acid, nickel is loaded into the first organic phase and / or the second organic phase using a solution of alkali metal hydroxide and nickel salt, and subsequently recovered in step ii and / or step iii, thereby closing the loop of the method.

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