JP7559566B2 - How to make cobalt sulfate - Google Patents

Description

The present invention relates to a method for producing cobalt sulfate. More specifically, the present invention relates to a method for producing highly pure cobalt sulfate by removing impurity elements contained in a cobalt chloride solution.

Cobalt is a valuable metal that is widely used for industrial purposes, not only as an additive element in special alloys, but also as a raw material for magnetic materials and lithium-ion secondary batteries. In particular, lithium-ion secondary batteries have recently come to be widely used as batteries for mobile devices and electric vehicles, and as a result, the demand for cobalt has expanded rapidly. However, most cobalt is produced as a by-product of nickel and copper smelting, so separation of cobalt from impurities such as nickel and copper is an important elemental technology in cobalt production.

For example, when recovering cobalt as a by-product in the hydrometallurgy of nickel, the raw material is first leached or extracted into a solution or subjected to a dissolution treatment using a mineral acid, an oxidizing agent, etc., to obtain a solution containing nickel and cobalt. Furthermore, the nickel and cobalt contained in the obtained acidic solution are often separated and recovered by a solvent extraction method using various organic extractants according to a conventionally known method.
However, the resulting cobalt solution often contains various impurities derived from the raw materials used in the process.

Therefore, it becomes necessary to further remove impurity elements such as manganese, copper, zinc, calcium and magnesium from the cobalt solution after nickel has been separated and recovered by the above-mentioned solvent extraction method.
Furthermore, in order to produce a high-purity cobalt product with a low impurity content, it was necessary to remove impurity elements from the cobalt solution that had been separated and recovered from the nickel solution containing cobalt, and then to commercialize the cobalt through an electrolysis process or crystallization, etc.

As a method for removing impurity elements from a cobalt solution, there are conventional techniques described in Patent Documents 1 and 2.
Patent Document 1 discloses a method for purifying a cobalt solution, including: (1) a copper removal step of adding a sulfiding agent to a cobalt solution and adjusting the oxidation-reduction potential (ORP) (based on Ag/AgCl electrode) to 50 mV or less and the pH to 0.3 to 2.4 to obtain a copper sulfide precipitate and a copper-depleted purified liquid; (2) a demanganization step of adding an oxidizing agent and a neutralizing agent to the copper-depleted purified liquid and adjusting the oxidation-reduction potential (based on Ag/AgCl electrode) to 950 to 1050 mV and the pH to 2.4 to 3.0 to obtain a manganese precipitate and a demanganized purified liquid; and (3) a solvent extraction step of extracting and separating zinc, calcium and trace impurities in the demanganized purified liquid using alkyl phosphoric acid as an extractant.
Patent Document 2 describes a technique in which a cobalt chloride solution having a hydrochloric acid concentration of 2 to 6 mol/L is brought into contact with an anion exchange resin, and metal impurities such as iron, zinc, and tin that form complexes with a distribution coefficient for the anion exchange resin that is larger than that of a cobalt chloride complex are adsorbed and separated.

The solvent extraction method using alkyl phosphoric acid as an extractant described in the above Patent Document 1 has high separation performance for zinc and calcium. However, in the case of a cobalt chloride solution with a hydrochloric acid concentration of 2 to 6 mol/L, an ion exchange method using an anion exchange resin and a solvent extraction method using an amine-based extractant have higher separation performance for zinc and cobalt than the above solvent extraction method using alkyl phosphoric acid.
Furthermore, when removing very small amounts of zinc from a cobalt chloride solution, the ion exchange method is more efficient and economical because the process and operation are simpler.

From this viewpoint, a method has been proposed in which the purification method of Patent Document 1 and the separation technique of Patent Document 2 are combined to remove these impurity elements from a cobalt chloride solution containing manganese, copper, and zinc (for example, Patent Document 3).
The method for producing high-purity cobalt chloride described in paragraph 0022 of Patent Document 3 includes a solvent extraction step for separating nickel and cobalt, a demanganese step for removing manganese, a decopper step for removing copper, a dezincification step for removing zinc, and an electrolysis step.
In the dezincification process, the aqueous cobalt chloride solution obtained in the decopperization process is brought into contact with an anion exchange resin to adsorb and remove zinc. In the electrolysis process, the high-purity aqueous cobalt chloride solution obtained in the dezincification process is used as an electrolytic feed solution to produce metallic cobalt (also called electrolytic cobalt).

On the other hand, as mentioned above, the demand for cobalt as a raw material for lithium ion secondary batteries has recently increased, and the form of a cobalt sulfate solution or cobalt sulfate crystals is desired.
To obtain cobalt sulfate crystals from metallic cobalt obtained by the conventional technique of Patent Document 3, metallic cobalt is dissolved in sulfuric acid to obtain a cobalt sulfate solution, and this solution is then crystallized to obtain cobalt sulfate crystals. However, this manufacturing method increases the manufacturing cost due to an increase in the number of steps and chemical costs. Furthermore, plate-shaped metallic cobalt, which is used in corrosion-resistant alloys, dissolves slowly in sulfuric acid, and in order to dissolve it in a short time, it is necessary to turn the plate-shaped metallic cobalt into a powder by atomization or the like.
For this reason, a method for obtaining a cobalt sulfate solution directly from a cobalt chloride solution without going through metallic cobalt has been desired.

JP 2004-285368 A JP 2001-020021 A JP 2020-19664 A

The present invention has been proposed in view of the above-mentioned circumstances, and aims to provide a method for producing highly pure cobalt sulfate by separating impurities and cobalt from a cobalt chloride solution containing impurities without using an electrolysis process.

The method for producing cobalt sulfate of the first invention includes a copper removal step in which a sulfiding agent is added to a cobalt chloride solution containing one or more impurities selected from the group consisting of copper, zinc, manganese, calcium, and magnesium , and the oxidation-reduction potential is adjusted to −100 to 200 mV (based on an Ag/AgCl electrode) and the pH to 1.3 to 3.0 to precipitate copper sulfide, which is then separated and removed; a first solvent extraction step in which the pH of the cobalt chloride solution that has been subjected to the copper removal step is adjusted to 1.5 to 3.0, and the solution is contacted with an organic solvent containing an alkyl phosphoric acid extractant, and zinc, manganese, and calcium are extracted into the organic solvent for separation and removal. a step of adding an alkali solution to the cobalt chloride solution that has been subjected to the first solvent extraction step to adjust the pH to 5.0 to 7.0 and extracting cobalt into an organic solvent using a carboxylic acid extractant; and a second solvent extraction step of contacting the organic solvent from which cobalt has been extracted with a sulfuric acid solution to adjust the pH to 2.0 to 4.5 and stripping the cobalt into the sulfuric acid solution to obtain a cobalt sulfate solution, the extraction step of the second solvent extraction step being characterized in that a non-oxidizing atmosphere is maintained by purging the gas phase with an oxidation-suppressing gas .
The method for producing cobalt sulfate according to the second aspect of the present invention is characterized in that, in the first aspect of the present invention , the cobalt sulfate solution obtained through the second solvent extraction step is subjected to a crystallization step to obtain cobalt sulfate crystals.
A third aspect of the present invention is a method for producing cobalt sulfate according to the first or second aspect of the present invention, characterized in that in the first solvent extraction step, the alkyl phosphate-based extractant is bis(2-ethylhexyl) hydrogen phosphate .

According to the first aspect of the present invention, the following effects are achieved.
In the copper removal step, by adjusting the oxidation-reduction potential to −100 to 200 mV (based on an Ag/AgCl electrode) and the pH to 1.3 to 3.0, copper sulfide can be precipitated and sufficiently removed from the cobalt chloride solution, and coprecipitation of cobalt can be suppressed.
In the first solvent extraction step, by adjusting the pH to 1.5 to 3.0, it is possible to extract zinc, manganese and calcium and separate them from cobalt, while leaving cobalt in the aqueous phase.
In the second solvent extraction step, the pH is adjusted to 5.0 to 7.0, and cobalt can be extracted into the organic phase by contacting with an organic solvent containing a carboxylic acid extractant. The cobalt extracted into the organic phase is then contacted with a sulfuric acid solution and the pH is adjusted to 2.0 to 4.5, allowing the cobalt to be back-extracted into the sulfuric acid solution.
Furthermore, by maintaining a non-oxidizing atmosphere during cobalt extraction in the second solvent extraction step, a decrease in the amount of cobalt stripped during stripping can be suppressed.
By carrying out the above steps in order, a high-purity cobalt sulfate solution from which impurities have been separated can be obtained. In addition, high-purity cobalt sulfate can be produced without using an electrolysis process .
According to the second aspect of the present invention, by further carrying out a crystallization step, high-purity cobalt sulfate crystals can be obtained from the cobalt sulfate solution.
According to the third invention, bis(2-ethylhexyl) hydrogen phosphate is used as the alkyl phosphate extractant, which makes it easier to separate the impurities zinc, manganese and calcium from the cobalt solution .

FIG. 1 is a process diagram showing a method for producing cobalt sulfate according to the present invention. FIG. 1 is a process diagram showing a method for producing cobalt sulfate according to one embodiment of the present invention. FIG. 3 is a detailed process diagram of the first solvent extraction step S2 shown in FIG. 2. FIG. 3 is a detailed process diagram of the second solvent extraction step S3 shown in FIG. 2. 1 is a graph showing the amount of cobalt stripped in a non-oxidizing atmosphere.

Specific embodiments of the present invention are described in detail below. Note that the present invention is not limited to the following embodiments, and various modifications are possible without departing from the gist of the present invention.

(Basic principle of the present invention)
The method for producing cobalt sulfate according to the present invention will be described with reference to FIG.
This manufacturing method is characterized by carrying out the following steps in order.
(1) A copper removal step S1 in which a sulfiding agent is added to a cobalt chloride solution containing one or more impurities of copper, zinc, manganese, calcium, and magnesium to generate a precipitate of copper sulfide and separate and remove it;
(2) a first solvent extraction step S2 in which an organic solvent containing an alkyl phosphoric acid-based extractant is brought into contact with the cobalt chloride solution from which copper has been removed by the copper removal step S1, and zinc, manganese and calcium are extracted into the organic solvent to be separated and removed;
(3) A second solvent extraction step S3 is carried out in which the cobalt chloride solution from which copper, zinc, manganese and calcium have been removed by the first solvent extraction step S2 is contacted with an organic solvent containing a carboxylic acid extractant to extract cobalt into the organic solvent under a non-oxidizing atmosphere, and the cobalt is then stripped with sulfuric acid to obtain a cobalt sulfate solution.

In the present invention, after each of the steps S1 to S3, a crystallization step S4 of precipitating crystals from the cobalt sulfate solution is carried out as necessary.
Although not shown in FIG. 1, after the first solvent extraction step S2 and/or after the second solvent extraction step S3, a step of subjecting the liquid to an oil-water separation device such as an activated carbon column to separate and remove organic components that have become mixed in the liquid may be added.

The cobalt chloride solution used as the starting material in the present invention contains one or more of the impurity elements copper, zinc, manganese, calcium, and magnesium. The application of the present invention is not limited to any cobalt chloride solution containing such impurities, but it is particularly suitable for use in cobalt chloride solutions obtained after nickel is separated and recovered from a cobalt-containing nickel solution using an alkyl phosphate extractant or an amine extractant in the solvent extraction process of nickel smelting.

According to the present invention, a high-purity cobalt sulfate solution can be obtained by removing copper from a cobalt chloride solution by forming a copper sulfide precipitate in a copper removal process S1, separating and removing zinc, manganese and calcium in a first solvent extraction process S2, and separating and removing magnesium in a second solvent extraction process S3. Therefore, a high-purity cobalt sulfate solution can be produced directly by separating impurities and cobalt without using an electrolysis process.

Furthermore, according to the present invention, the cobalt sulfate solution after the steps S1 to S3 is subjected to the crystallization step S4, whereby crystals are precipitated from the solution to obtain cobalt sulfate crystals.

(Embodiment)
Hereinafter, an embodiment of the method for producing cobalt sulfate will be described with reference to Fig. 2 to Fig. 4. Fig. 2 shows the details of steps S1 to S3 shown in Fig. 1.

(Copper removal process S1)
The copper removal step S1 will be described with reference to FIG.
The copper removal step S1 is carried out by adding a sulfurizing agent to a cobalt chloride solution containing one or more impurities of copper, zinc, manganese, calcium, and magnesium as a starting material. The amount of sulfurizing agent added is adjusted, and an oxidizing agent and a neutralizing agent are added as necessary to adjust the redox potential of the cobalt chloride solution to −100 to 200 mV (Ag/AgCl electrode standard) and the pH to 1.3 to 3.0.
By this step, a precipitate of copper sulfide is generated and separated from the cobalt chloride solution, and a cobalt chloride solution from which copper has been removed can be obtained.

Copper in the cobalt chloride solution forms a precipitate of copper sulfide according to the following formula 1, 2 or 3, and is removed from the solution.
CuCl ₂ +H ₂ S→CuS↓+2HCl... (Formula 1)
CuCl ₂ +Na ₂ S→CuS↓+2NaCl... (Formula 2)
CuCl ₂ +NaHS→CuS↓+NaCl+HCl...(Formula 3)

In the copper removal step S1, if the redox potential of the cobalt chloride solution is adjusted to −100 to 200 mV (based on an Ag/AgCl electrode) and the pH is adjusted to 1.3 to 3.0, copper can be sufficiently removed as sulfide and coprecipitation of cobalt can be suppressed, thereby increasing the recovery rate of cobalt sulfate.
If the redox potential exceeds 200 mV, the removal of copper from the solution becomes insufficient, and if the redox potential is less than -100 mV, the amount of cobalt co-precipitated increases, which is not preferable. If the pH is less than 1.3, the removal of copper from the solution becomes insufficient and the filterability of the resulting sulfide precipitate deteriorates. If the pH exceeds 3.0, the amount of cobalt co-precipitated increases with the removal of copper, which is not preferable.

The oxidation-reduction potential can be adjusted by adjusting the amount of sulfurizing agent added. When the oxidation-reduction potential becomes lower than the desired value due to the addition of the sulfurizing agent, it can be adjusted by adding an oxidizing agent. For example, the oxidation-reduction potential can be adjusted by introducing air into the solution and stirring it, or by adding a hydrogen peroxide solution. The sulfurizing agent is not particularly limited, but hydrogen sulfide gas, sodium sulfide or sodium hydrosulfide crystals or aqueous solutions, etc., can be used.
When hydrogen sulfide or sodium hydrosulfide is used as the sulfiding agent, the pH is adjusted by adjusting the amount of the sulfiding agent and adding a neutralizing agent. The neutralizing agent is not particularly limited, and alkali salts such as sodium hydroxide, calcium hydroxide, sodium carbonate, and cobalt carbonate can be used.

(First solvent extraction step S2)
The first solvent extraction step S2 will be described with reference to FIGS.
The first solvent extraction step S2 is a step in which the cobalt chloride solution that has been subjected to the copper removal step S1 is contacted with an organic solvent containing an alkylphosphate-based extractant, and zinc, manganese and calcium are extracted into the organic solvent and separated and removed.

As the organic solvent, an alkyl phosphate extractant diluted with a diluent is used. Examples of alkyl phosphate extractants include bis(2-ethylhexyl) hydrogen phosphate (trade name D2EHPA), 2-ethylhexyl (2-ethylhexyl) phosphonate (trade name PC-88A), and di(2,4,4-trimethylpentyl)phosphinic acid (trade name CYANEX272). Among these, when separating zinc, manganese, and calcium from a cobalt chloride solution from which copper has been removed, it is preferable to use bis(2-ethylhexyl) hydrogen phosphate, which has high separability from cobalt, as an alkyl phosphate extractant. This is because bis(2-ethylhexyl) hydrogen phosphate has good properties for separating the impurities zinc, manganese, and calcium from a cobalt solution.

The diluent is not particularly limited as long as it can dissolve the extractant. For example, naphthenic solvents and aromatic solvents can be used as the diluent. The concentration of the extractant is preferably adjusted to 10 to 60% by volume, and more preferably 20 to 50% by volume. When the concentration of the extractant is within this range, impurity elements with high concentrations and impurity elements with low distribution ratios (element concentration in organic solvent/element concentration in solution) can be sufficiently extracted. On the other hand, when the concentration of the extractant is less than 10%, impurity elements with high concentrations and impurity elements with low distribution ratios cannot be sufficiently extracted, and tend to remain in the cobalt chloride solution. In addition, when the concentration of the extractant exceeds 60%, the viscosity of the organic solvent increases, and the phase separation property after the extraction operation of the organic solvent (organic phase) and the cobalt chloride solution (aqueous phase) deteriorates.

An acidic extractant such as an alkyl phosphate extractant is an extractant that extracts metal ions by forming a metal salt through the substitution of the -H of the extractant with a cation in the aqueous phase, as shown in formula 4. In general, as the pH increases, metal ions are more easily extracted into the organic phase, and as the pH decreases, the reaction of formula 4 proceeds in the opposite direction, and the metal ions extracted into the organic phase are more easily back-extracted into the aqueous phase.
Since the pH at which metal ions are extracted varies depending on the type of metal ion, the target element is separated from impurity elements by controlling the pH in the solvent extraction process using an acidic extractant.
nRH _org + M ⁿ⁺ _aq → MR _norg + nH ⁺ _aq ... (Formula 4)
In the formula, RH represents an acidic extractant, Mn ⁺ represents an n-valent metal ion, org represents an organic phase, and aq represents an aqueous phase.

Therefore, in the first solvent extraction step S2, it is desirable to adjust the pH of the cobalt chloride solution from which copper has been removed to 1.5 to 3.0. In this pH range, the extraction rates of zinc, manganese and calcium tend to be higher than that of cobalt, and it is possible to separate these impurity elements from cobalt by extracting them into an organic phase while leaving cobalt in the aqueous phase.
If the pH is less than 1.5, the extraction rate of these impurities is low and separation from cobalt becomes difficult, and if the pH exceeds 3.0, the extraction rate of cobalt also increases and the separability from the impurities decreases. When the pH is adjusted to 1.5 to 3.0, some of the cobalt may be extracted, but it is also possible to reduce the loss of cobalt by contacting the organic phase after extraction with a hydrochloric acid solution having a lower pH than that used during extraction and recovering the cobalt by back-extraction.
Furthermore, when this organic phase is brought into contact with an acidic solution having a pH of 1 or less, most of the extracted metal ions can be back-extracted into the aqueous phase, and the organic phase after back-extraction can be reused.

In the first solvent extraction step S2, the solvent extraction process may be performed in a non-oxidizing atmosphere, as in the second solvent extraction step S3 described below.

(Second solvent extraction step S3)
The second solvent extraction step S3 will be described with reference to FIGS.
The second solvent extraction step S3 is a step of contacting the cobalt chloride solution that has been subjected to the first solvent extraction step S2 with an organic solvent containing a carboxylic acid extractant, extracting the cobalt into the organic solvent, and then stripping the cobalt with sulfuric acid to obtain a cobalt sulfate solution.
The organic solvent used is a carboxylic acid extractant diluted with a diluent. Examples of the carboxylic acid extractant include versatic acid and naphthenic acid. These are acidic extractants having COOH groups.

When extracting metal ions, the carboxylic acid extractant releases protons corresponding to the valence of the metal into the aqueous phase, and the metal ion coordinates to the carbonyl oxygen. Many carboxylic acids form dimers and trimers by hydrogen bonding with each other in non-polar solvents. In addition, when extracting metal ions, carboxylic acids that are not acid-dissociated may coordinate to remove the hydration water coordinated to the metal ion. For example, the reaction in which a hexacoordinated divalent metal ion M2 ⁺ is extracted by _a dimeric carboxylic acid extractant _R2H2 is represented by formula 5.
[M(H ₂ O) ₆ ] ²⁺ _aq +3(R ₂ H ₂ ) _org
→ (MR ₂・4RH) _org +2H ⁺ _aq +6H ₂ O _aq ... (Formula 5)
In the formula, org represents an organic phase, and aq represents an aqueous phase.

(Extraction process)
In the second solvent extraction step S3, an alkaline solution is added as a neutralizing agent to the cobalt chloride solution from which copper, zinc, manganese, and calcium have been removed, to adjust the pH to 5.0 to 7.0. When the pH is in this range, the cobalt can be extracted into the organic phase by contacting the solution with an organic solvent containing a carboxylic acid-based extractant. In this case, magnesium is not extracted and remains in the aqueous phase. If the pH is less than 5, it is difficult to extract cobalt, while if the pH is more than 7, the solubility of cobalt decreases and precipitation may occur.

In the extraction process, when the organic phase and the aqueous phase are mixed, the air that is entrained promotes oxidation. When the cobalt extracted into the organic phase is oxidized by oxygen to become trivalent cobalt ions, the bonds with the carboxylic acid, which is a complex ion, become stronger, making back-extraction difficult.
In industrial operations, the organic phase after stripping is usually reused repeatedly as the organic phase for extraction, and if cobalt remains in the organic phase, the amount of cobalt extracted from the beginning decreases when the organic phase is used repeatedly. Furthermore, when the organic phase is used many times, trivalent cobalt ions, which are difficult to strip, accumulate in the organic phase, which hinders efficient production.

Therefore, the extraction step of the second solvent extraction step S3 is carried out in a non-oxidizing atmosphere to prevent oxidation.
In this specification, the term "non-oxidizing atmosphere" refers to an atmosphere in which the gas phase is purged with a gas capable of suppressing oxidation, such as argon, nitrogen, or carbon dioxide. Blowing an oxidation-suppressing gas into the extractant or aqueous phase is also effective in removing dissolved oxygen. For industrial purposes, it is suitable to use nitrogen gas as the oxidation-suppressing gas.

Figure 5 is a graph showing the advantage of maintaining the extraction process in a non-oxidizing atmosphere. The graph shows the relationship between the time elapsed after extraction and the ratio of the amount of cobalt stripped when stripping is performed after a certain time to the amount of cobalt stripped at zero time (when stripping is performed immediately after extraction). In Figure 5, the line representing a nitrogen atmosphere is an example of a non-oxidizing atmosphere, and the line representing an air atmosphere is a typical air atmosphere.
When stripping was performed under a nitrogen atmosphere, the amount of cobalt stripped decreased slightly, but when stripping was performed under a general air atmosphere, the amount of cobalt stripped decreased as time passed. Thus, it is clear that when the organic phase is used over a long period of time while repeating extraction and stripping, as in industrial operations, it is preferable to perform the stripping in a nitrogen atmosphere.

(Back extraction process)
After the extraction step, the cobalt extracted into the organic phase is contacted with a sulfuric acid solution to adjust the pH to 2.0 to 4.5. This allows the cobalt to be back-extracted into the sulfuric acid solution. Thus, a high-purity cobalt sulfate solution is obtained.
Although stripping is possible even at a pH of less than 2, this is not preferred because the amount of trace impurities extracted at a lower pH than cobalt increases, whereas a pH of more than 4.5 is not preferred because the rate of stripping cobalt decreases and the amount of cobalt recovered decreases.

Since the organic phase after extraction still contains fine aqueous phase particles even after phase separation, a washing step may be added before stripping to remove the aqueous phase. For the aqueous phase in the washing step, a cobalt sulfate solution may be used. The stripping step is preferably carried out in a non-oxidizing atmosphere, as in the extraction step. The organic phase after stripping can be reused as the organic phase in the extraction step.
By the above method, a high-purity cobalt sulfate solution with few impurities can be produced.

(Crystallization step S4)
The crystallization step S4 will be described with reference to FIG.
In the crystallization step S4, crystals of cobalt sulfate are precipitated from the cobalt sulfate solution obtained in the second solvent extraction step S3. The crystallization method is not particularly limited, and can be performed using a general crystallization method.

For example, one method is to obtain crystals by placing a cobalt sulfate solution in a crystallizer and crystallizing it within the crystallizer. A crystallizer is a device that precipitates crystals by evaporating the water in the cobalt sulfate solution under a certain pressure, and for example, a rotary evaporator or a double propeller type crystallizer is used. The internal pressure is reduced using a vacuum pump or the like, and in a rotary evaporator, crystallization proceeds while the flask is rotated and stirred with a double propeller. Note that in the crystallizer, a slurry is formed in which the cobalt sulfate solution is mixed with cobalt sulfate crystals.

The slurry discharged from the crystallizer is separated into cobalt sulfate crystals and mother liquor by a filter, a centrifuge, etc. The cobalt sulfate crystals are then dried in a dryer to remove moisture.
By the above method, it is possible to produce cobalt sulfate crystals from a cobalt sulfate solution. Of course, since the cobalt sulfate solution is a high-purity solution with few impurities, the obtained cobalt sulfate crystals are also high-purity with few impurities.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

Example 1
(Copper removal process S1)
A sodium hydrosulfide solution was added as a sulfiding agent to 2 L of a cobalt chloride solution having the composition shown in original solution A in Table 1 and adjusted to pH 2.5, and the oxidation-reduction potential was adjusted to -50 mV (Ag/AgCl electrode standard) to generate a precipitate of copper sulfide. The precipitate was separated and removed using a filter, and a filtrate having the composition shown in post-sulfidation B in Table 1 was obtained. The copper concentration was less than 0.001 g/L, and copper was successfully separated and removed.

(First solvent extraction step S2)
An organic phase was prepared by diluting an alkyl phosphoric acid extractant (trade name D2EHPA, manufactured by Daihachi Chemical Industry Co., Ltd.) with a diluent (trade name Teclain N20, manufactured by JX Nippon Oil & Energy Corporation) so that the concentration was 40% by volume. 0.9 L of the aqueous phase consisting of the cobalt chloride solution obtained in the copper removal step S1 and 1.8 L of the organic phase were mixed, and a sodium hydroxide solution was added to adjust the pH to 1.7, and impurities were extracted. The same extraction operation was repeated with 0.9 L of the aqueous phase after extraction and 1.8 L of a new organic phase, and a total of three extraction operations were performed. As a result, a cobalt chloride solution having the composition shown in 1SX after C in Table 1 was obtained. The concentrations of zinc, manganese, and calcium were all less than 0.001 g/L, and these impurities were able to be separated and removed.

(Second solvent extraction step S3)
An organic phase was prepared by diluting a carboxylic acid extractant (product name Versatic Acid 10, manufactured by Oxalis Chemicals) with a diluent (Teclean N20) so that the concentration was 30% by volume. While blowing nitrogen gas from a cylinder into the extraction reaction tank at a flow rate of 1 L/min to maintain a non-oxidizing atmosphere, 0.44 L of the aqueous phase consisting of the cobalt chloride solution obtained in the first solvent extraction step S2 and 1 L of the organic phase were mixed, and a sodium hydroxide solution was added to adjust the pH to 6.5, and cobalt was extracted into the organic phase.
Next, while flowing nitrogen gas at 1 L/min, 0.6 L of the extracted organic phase was mixed with 0.6 L of a cobalt chloride solution with a cobalt concentration of 10 g/L, and the aqueous phase mixed in the organic phase after extraction was washed. Then, this organic phase was mixed with 0.09 L of pure water, and sulfuric acid was added to adjust the pH to 4, and cobalt was back-extracted. At this time, 99% or more of cobalt could be back-extracted. As a result, a cobalt sulfate solution with the composition shown in 2SX after D in Table 1 was obtained. The magnesium concentration was less than 0.001 g/L, and magnesium could be separated and removed.
By the above method, a high purity cobalt sulfate solution was obtained.

(Crystallization step S4)
The cobalt sulfate solution was placed in a rotary evaporator, and the inside of the rotary evaporator was decompressed with a vacuum pump while maintaining the temperature at 40° C., and the water was evaporated while rotating the flask, so that crystals of cobalt sulfate were precipitated. After solid-liquid separation, the obtained cobalt sulfate crystals were dried in a dryer. As a result, high-purity cobalt sulfate crystals as shown in Table 2 were obtained.

Example 2
A cobalt sulfate solution was obtained in the same manner as in Example 1, except that nitrogen gas was not introduced to maintain a non-oxidizing atmosphere in the extraction step and the subsequent washing step in the second solvent extraction step 3.
The composition in this case is shown in Table 3. As shown in D after the second SX in Table 3, a high purity cobalt sulfate solution was obtained. The stripping rate of cobalt in the second solvent extraction step S3 was 99%.

Subsequently, crystallization was carried out in the same manner as in Example 1, and high purity cobalt sulfate crystals as shown in Table 4 were obtained.

The cobalt stripping rate in Example 1 was just over 99%, while in Example 2 it was 99%, a difference of about 1%. The experiments in Examples 1 and 2 were batch processes, so the difference in stripping rate was small, but the difference becomes larger when continuous processing is used. In continuous processing, the organic phase after stripping is used again to extract cobalt, and extraction and stripping are repeated, so that cobalt oxidized in the organic phase gradually accumulates in the organic phase. Furthermore, as the amount of accumulation increases, not only the stripping rate but also the extraction rate decreases. Therefore, the effect of using a non-oxidizing atmosphere from extraction to stripping is great.

The results of Examples 1 and 2 above show that the present invention makes it possible to obtain highly pure cobalt sulfate from which impurities have been sufficiently removed.

The highly pure cobalt sulfate crystals obtained by this invention can be used as a raw material for lithium-ion secondary batteries and for a variety of other purposes.

S1 Copper removal step S2 First solvent extraction step S3 Second solvent extraction step S4 Crystallization step

Claims (3)

a copper removal process in which a sulfiding agent is added to a cobalt chloride solution containing one or more impurities selected from the group consisting of copper, zinc, manganese, calcium, and magnesium , and the oxidation-reduction potential is adjusted to −100 to 200 mV (based on an Ag/AgCl electrode) and the pH to 1.3 to 3.0 to generate a precipitate of copper sulfide, which is then separated and removed;
a first solvent extraction step in which the pH of the cobalt chloride solution that has been subjected to the copper removal step is adjusted to 1.5 to 3.0, and the solution is contacted with an organic solvent containing an alkyl phosphoric acid-based extractant, and zinc, manganese and calcium are extracted into the organic solvent to be separated and removed;
a second solvent extraction step in which an alkaline solution is added to the cobalt chloride solution that has been subjected to the first solvent extraction step to adjust the pH to 5.0 to 7.0, and cobalt is extracted into an organic solvent using a carboxylic acid extractant; and following the extraction step, a back extraction step is carried out in which a sulfuric acid solution is brought into contact with the organic solvent from which cobalt has been extracted, the pH is adjusted to 2.0 to 4.5, and cobalt is back extracted into the sulfuric acid solution to obtain a cobalt sulfate solution;
Execute the following in order.
A method for producing cobalt sulfate, characterized in that in the extraction step of the second solvent extraction process , a non-oxidizing atmosphere is maintained by purging the gas phase with an oxidation-suppressing gas . 2. The method for producing cobalt sulfate according to claim 1 , further comprising subjecting the cobalt sulfate solution obtained through the second solvent extraction step to a crystallization step to obtain cobalt sulfate crystals. In the first solvent extraction step, the alkyl phosphate extractant is bis(2-ethylhexyl) hydrogen phosphate.
The method for producing cobalt sulfate according to claim 1 or 2 .