JP2025149953A - Method for producing metal sulfate crystals and metal sulfate - Google Patents

Abstract

The present invention provides a method for producing metal sulfate crystals that can obtain high-quality metal sulfate crystals that are free from free _H2SO4 adhesion and have low amounts of impurities such as Na from a metal sulfate solution _that contains free _H2SO4 and has a pH of less than _4.0 .
[Solution] A method for producing metal sulfate crystals, in which metal sulfate crystals are crystallized and recovered from a metal sulfate solution containing metal sulfate and having a pH of less than 4.0, is characterized by comprising: an alkaline metal compound addition step of adding an alkaline metal compound of a metal constituting the metal sulfate to the metal sulfate solution having a pH of less than 4.0 to adjust the pH to 4.5 or higher; a crystallization step of crystallizing the metal sulfate crystals from the metal sulfate solution with a pH of 4.5 or higher to obtain a metal sulfate crystal-containing slurry containing the metal sulfate crystals; and a solid-liquid separation step of separating the metal sulfate crystal-containing slurry into the metal sulfate crystals and a residual liquid.
[Selection diagram] None

Description

The present invention relates to a method for producing metal sulfate crystals by crystallizing and recovering metal sulfate crystals from a metal sulfate solution having a pH of less than 4.0, and to metal sulfates.

In recent years, lithium has been recovered from crushed lithium-ion batteries and reused. When recovering lithium from lithium-ion batteries, it is common to use a recycled raw material called black mass, which is obtained by sintering and crushing used lithium-ion batteries.
The above-mentioned fired lithium-ion battery product contains valuable metals other than lithium, such as cobalt, nickel, manganese, etc. Therefore, techniques have been proposed for recovering valuable metals such as cobalt, nickel, and manganese from the fired lithium-ion battery product.

For example, Patent Document 1 discloses a method for obtaining sulfates of cobalt, nickel, and manganese by adjusting the pH of an acidic solution containing at least cobalt, nickel, manganese, and aluminum obtained by wet processing of lithium-ion battery waste and performing solid-liquid separation to remove aluminum, dissolving the resulting neutralized residue containing cobalt, nickel, and manganese in a sulfuric acidic solution, heating and concentrating or cooling this sulfuric acidic solution, and performing solid-liquid separation.

Furthermore, as a method for producing cobalt sulfate crystals used as a raw material for lithium-ion batteries, for example, Patent Document 2 discloses a method in which a cobalt chloride solution containing impurities is subjected to sulfide treatment and solvent extraction to remove the impurities, the cobalt is extracted into an organic solvent, and then back-extracted with sulfuric acid to obtain a cobalt sulfate solution, which is then subjected to a crystallization process to obtain cobalt sulfate crystals.

Patent No. 7232119 Japanese Patent Application Laid-Open No. 2023-161557

In Patent Documents 1 and 2, a sulfuric acidic cobalt sulfate solution (metal sulfate solution) is heated and concentrated or cooled to crystallize and recover cobalt sulfate crystals (metal sulfate crystals).
Here, in the above-mentioned cobalt sulfate solution (metal sulfate solution) acidified with sulfuric acid, _{free H2SO4} coexists. Cobalt sulfate crystals (metal sulfate crystals) recovered from such _a cobalt sulfate solution (metal sulfate solution) acidified with sulfuric acid become highly acidic crystals with free _H2SO4 attached to their surfaces.
When such highly acidic crystals are supplied as a raw material for another product, if they are supplied to a wet manufacturing process, a large amount of neutralizing agent such as alkali will need to be added, and if they are supplied to a dry manufacturing process, there is a risk of problems such as corrosion of equipment due to sulfur oxide gas derived from the free _{H2SO4 that} is generated.

A specific example of subjecting the recovered cobalt sulfate crystals (metal sulfate crystals) to a wet production process is to dissolve the cobalt sulfate crystals (metal sulfate crystals) produced by crystallization in water to form an aqueous cobalt sulfate solution (aqueous metal sulfate solution), and then add various alkalis or other chemicals to recover solid cobalt particles (solid metal particles), such as cobalt hydroxide Co(OH) ₂ , cobalt oxyhydroxide CoOOH, or cobalt oxide Co ₃ O ₄ , which can then be further processed into a positive electrode active material or the like.
If excessive free _H2SO4 adheres to the cobalt sulfate crystals (metal sulfate crystals) _and the dissolved cobalt sulfate aqueous solution (metal sulfate aqueous solution) becomes acidic, the amount of alkaline agent, such as sodium hydroxide (NaOH), added when producing the cobalt solid particles (metal solid) will increase, resulting in high costs. Furthermore, if the amount of sodium hydroxide (NaOH) added increases, there is a concern that Na may migrate to the recovered cobalt solids (metal solids) and the Na impurity content may increase.

Furthermore, in a process for crystallizing and recovering cobalt sulfate crystals (metal sulfate crystals) from a crystallization raw material solution containing a large amount _of free _H2SO4 , i.e., a low-pH, sulfuric acid-acidic cobalt sulfate aqueous solution (metal sulfate aqueous solution), by subjecting the solution to a heating and concentration treatment, if the reaction vessel is made of metal, the metal parts may be corroded by heating. For example, SUS304 and SUS316, which are commonly used in industrial-scale chemical manufacturing process equipment, are known to be slightly corroded under sulfuric acid acidity. Metal corrosion of the reaction vessel is synonymous with a reaction in which elements such as Cr and Fe dissolve into the solution due to corrosion, and there is also a concern that the recovered cobalt sulfate crystals may be contaminated by elements derived from the metal vessel. Naturally, corrosion can also lead to concerns about reduced equipment life, operational troubles, reduced productivity, and increased processing costs. While there is a method for fabricating the reaction vessel from a more corrosion-resistant metal material, this method has the drawback of high equipment costs.

The simplest method for increasing the pH of a cobalt sulfate solution (metal sulfate solution) that contains a large amount of free H ₂ SO ₄ and is acidic due to sulfuric acid and has a low pH is to add an alkaline solution such as sodium hydroxide. However, this method generates Na ₂ SO ₄ (aq) in the solution during the neutralization treatment, resulting in a large amount of Na being dissolved in the cobalt sulfate solution (metal sulfate solution).
H ₂ SO ₄ (aq) + 2NaOH (aq) → Na ₂ SO ₄ (aq) + H ₂ O (l)
Cobalt sulfate crystals (metal sulfate crystals) produced from a cobalt sulfate solution (metal sulfate solution) having a large amount of dissolved Na and having an increased pH by this method will have a very low purity and contain a large amount of Na.

The present invention has been made in view of the above-mentioned circumstances, and aims to provide _a method for producing metal sulfate crystals, which can obtain high-quality metal sulfate crystals that are free from adhesion _of free _H2SO4 and have low amounts of impurities such as sodium, from a metal sulfate solution containing free _H2SO4 and having a pH of less than 4.0, and a metal sulfate.

In order to solve the above problems, the method for producing metal sulfate crystals of aspect 1 of the present invention is a method for producing metal sulfate crystals by crystallizing and recovering metal sulfate crystals from a metal sulfate solution containing metal sulfate and having a pH of less than 4.0, and is characterized by comprising: an alkaline metal compound addition step of adding an alkaline metal compound of the metal that constitutes the metal sulfate to the metal sulfate solution having a pH of less than 4.0 to adjust the pH to 4.5 or higher; a crystallization step of crystallizing the metal sulfate crystals from the metal sulfate solution with a pH of 4.5 or higher to obtain a metal sulfate crystal-containing slurry containing the metal sulfate crystals; and a solid-liquid separation step of separating the metal sulfate crystal-containing slurry into the metal sulfate crystals and a residual liquid.

According to the method for producing metal sulfate crystals of Aspect 1 of the present invention, an alkaline metal compound of a metal constituting a metal sulfate is added to a metal sulfate solution containing a metal sulfate and having a pH of less than 4.0 to adjust the pH _to 4.5 or higher, so that the free _H2SO4 can be sufficiently reduced by reacting the free _H2SO4 with the alkaline metal compound. Furthermore, because the reaction produces a metal sulfate, it is possible to prevent impurities such as sodium from being mixed into the _metal sulfate solution.
The method includes a crystallization step in which metal sulfate crystals are crystallized from the metal sulfate solution having a pH of 4.5 or higher to obtain a metal sulfate-containing slurry containing the metal sulfate, and a solid-liquid separation step in which the metal sulfate-containing slurry is separated into the metal sulfate and a residual liquid. _This makes it possible to obtain a high-quality metal sulfate that is free from adhesion of free _H2SO4 and has a small amount of impurities such as sodium.

A method for producing metal sulfate crystals according to a second aspect of the present invention is characterized in that, in the method for producing metal sulfate crystals according to the first aspect of the present invention, the pH after the addition of the alkaline metal compound is set in the range of 6 or more and 7 or less.
According to the method for producing metal sulfate crystals of Aspect 2 of the present invention, the pH after the alkaline metal compound step is set within a range of 6 to 7, thereby removing impurity elements and producing high-purity metal sulfate crystals. Furthermore, since the metal ions and sulfate ions in the metal sulfate solution after the alkaline metal compound step are in a ratio of approximately 1:1, the metal ion concentration can be estimated from the electrical conductivity in the subsequent crystallization step, making it possible to easily confirm the degree of metal concentration.

A method for producing metal sulfate crystals according to Aspect 3 of the present invention is characterized in that, in the method for producing metal sulfate crystals according to Aspect 1 or Aspect 2 of the present invention, the crystallization step comprises heat-treating the metal sulfate solution having a pH of 4.5 or higher, thereby crystallizing the metal sulfate crystals.
According to the method for producing metal sulfate crystals of Aspect 3 of the present invention, the crystallization step involves heat-treating the metal sulfate solution adjusted to a pH of _4.5 or higher, so that high-quality metal sulfate crystals can be obtained that are free of free _H2SO4 and have low amounts of impurities such as sodium. Furthermore, the metal sulfate crystals can be efficiently crystallized.

A method for producing metal sulfate crystals according to Aspect 4 of the present invention is characterized in that, in the method for producing metal sulfate crystals according to any one of Aspects 1 to 3 of the present invention, at least a portion of the alkaline metal compound added in the alkaline metal compound addition step is a metal hydroxide precipitate or a metal carbonate precipitate produced by adding sodium hydroxide, sodium carbonate, or sodium bicarbonate to a separated solution separated from a metal sulfate solution having a pH of less than 4.0.
According to the method for producing metal sulfate crystals of Aspect 4 of the present invention, at least a part of the alkaline metal compound added in the alkaline metal compound addition step is a metal hydroxide precipitate or a metal carbonate precipitate produced by adding sodium hydroxide, sodium carbonate, or sodium bicarbonate to a solution separated from a metal sulfate solution having a pH of less than 4.0. This allows for efficient recovery of metal sulfate crystals from the metal sulfate solution.

A method for producing metal sulfate crystals according to Aspect 5 of the present invention is characterized in that, in the method for producing metal sulfate crystals according to any one of Aspects 1 to 3 of the present invention, at least a portion of the alkaline metal compound added in the alkaline metal compound addition step is a metal hydroxide precipitate or a metal carbonate precipitate produced by adding sodium hydroxide, sodium carbonate, or sodium bicarbonate to the residual liquid separated in the solid-liquid separation step.
According to the method for producing metal sulfate crystals of Aspect 5 of the present invention, at least a part of the alkaline metal compound added in the alkaline metal compound adding step is a metal hydroxide precipitate or a metal carbonate precipitate produced by adding sodium hydroxide, sodium carbonate, or sodium bicarbonate to the residual liquid separated in the solid-liquid separation step. This makes it possible to produce metal sulfate crystals with high recovery efficiency from the metal sulfate solution.

A sixth aspect of the present invention is a method for producing metal sulfate crystals, which is characterized in that, in the fourth or fifth aspect of the present invention, the metal hydroxide precipitate or the metal carbonate is washed with water.
According to the method for producing metal sulfate crystals of Aspect 6 of the present invention, the metal hydroxide precipitate or the metal carbonate is washed with water, so that sodium adhering to the metal hydroxide precipitate or the metal carbonate can be removed, further suppressing the incorporation of sodium as an impurity, and enabling the production of metal sulfate crystals of even higher purity.

A seventh aspect of the present invention is a method for producing metal sulfate crystals according to any one of the first to sixth aspects of the present invention, characterized in that the metal sulfate crystals are cobalt sulfate crystals.
According to the method for producing metal sulfate crystals of Aspect 7 of the present invention, the metal sulfate crystals are cobalt sulfate crystals, and the method includes a metal compound addition step of adding at least one of cobalt hydroxide and cobalt _carbonate to a cobalt sulfate solution having a pH of less than 4.0 to adjust the pH to _4.5 or higher, so that the free _H2SO4 can be sufficiently reduced by reacting the free _H2SO4 with at least one of cobalt hydroxide and cobalt carbonate. Furthermore, because cobalt sulfate is produced by the reaction, impurities such as sodium can be prevented from being mixed into the cobalt sulfate solution.
The method includes a crystallization step of crystallizing the cobalt sulfate crystals from the cobalt sulfate solution having a pH of 4.5 or higher to obtain a cobalt sulfate crystal-containing slurry containing the cobalt sulfate crystals, and a solid-liquid separation step of separating the cobalt sulfate crystal-containing slurry into the cobalt sulfate crystals and a residual liquid, so that high-quality cobalt sulfate _crystals can be obtained that are free of free _H2SO4 and have a low amount of impurities such as sodium.

The metal sulfate of aspect 8 of the present invention is characterized in that when dissolved in water to give a metal concentration of 1% by mass, the pH is 5.4 or higher.

The metal sulfate of aspect 9 of the present invention is characterized in that when dissolved in water to give a metal concentration of 1% by mass, the pH is 6.0 or higher.

According to the present invention, it is possible to provide _a method for producing metal sulfate crystals, which can obtain high-quality metal sulfate crystals that are free from adhesion _of free _H2SO4 and have low amounts of impurities such as sodium, from a metal sulfate solution that contains a large amount of free _H2SO4 and has a pH of less than 4.0, and a metal sulfate.

1 is a flow diagram showing a method for producing a metal sulfate crystal (cobalt sulfate crystal) according to a first embodiment of the present invention. FIG. FIG. 2 is a flow chart showing a method for producing a metal sulfate crystal (cobalt sulfate crystal) according to a second embodiment of the present invention. 1 is a graph showing the relationship between pH and dissolved cobalt concentration in an example. 1 is a graph showing the relationship between electrical conductivity and cobalt concentration in Examples. 1 is a graph showing the relationship between electrical conductivity and nickel concentration in Examples.

An example of an embodiment of the present invention is described below.

The method for producing metal sulfate crystals according to this embodiment involves crystallizing and recovering metal sulfate from a metal sulfate solution having a pH of less than 4.0.
In this embodiment, the target metal sulfate solution is a metal sulfate solution obtained by sulfuric acid treatment of lithium ion battery slag (so-called black mass) obtained by crushing used lithium ion batteries or defective products (so-called black powder) generated in the manufacturing process of positive electrode materials for secondary batteries, etc.
Here, examples of metals constituting the metal sulfate of this embodiment include cobalt and nickel.

(First embodiment)
A method for producing metal sulfate crystals according to a first embodiment of the present invention will be described with reference to FIG.
In the method for producing metal sulfate crystals according to the first embodiment of the present invention, cobalt sulfate crystals (metal sulfate crystals) are recovered from a cobalt sulfate solution (metal sulfate solution).
As shown in FIG. 1, the method for producing metal sulfate crystals according to this embodiment includes at least an alkaline metal compound adding step S01, a crystallization step S02, and a solid-liquid separation step S03.

(Alkaline metal compound addition step S01)
In this alkaline metal compound addition step S01, an alkaline metal compound is added to a cobalt sulfate solution serving as a crystallization raw material liquid having a pH of less than 4.0 (for example, 0.5 or more and less than 4.0), thereby adjusting the pH of the cobalt sulfate solution to 4.5 or more.
As the alkaline metal compound, cobalt hydroxide (metal hydroxide) or cobalt carbonate (metal carbonate) can be used.
In this embodiment, the pH of the cobalt sulfate solution after the alkaline metal compound addition step S01 is preferably 5.0 or higher, more preferably 5.5 or higher, and even more preferably within the range of 6.0 to 7.0.

Here, a large amount of free H ₂ SO ₄ is present in a cobalt sulfate solution having a pH of less than 4.0. When cobalt hydroxide is added to this cobalt sulfate solution having a pH of less than 4.0, the free H ₂ SO ₄ reacts with the cobalt hydroxide and is consumed, as shown below, resulting in an increase in pH.
_H2SO4 (aq)+Co(OH) ₂ (s) _{→ CoSO4} (aq)+ _2H2O (l)

Furthermore, when cobalt carbonate is added to a cobalt sulfate solution having a pH of less than 4.0, the free H ₂ SO ₄ reacts with the cobalt carbonate and is consumed, resulting in an increase in pH, as shown below.
H ₂ SO ₄ (aq) + CoCO ₃ (s) → CoSO ₄ (aq) + H ₂ O (l) + CO ₂ (g)
The cobalt sulfate produced by the above reaction can also be used as a raw material for cobalt sulfate crystals.

(Crystallization step S02)
In this crystallization step S02, cobalt sulfate crystals are crystallized from the cobalt sulfate solution adjusted to a pH of 4.5 or higher by heating and concentrating, cooling, or the like, to obtain a cobalt sulfate crystal-containing slurry.
In this embodiment, a cobalt sulfate solution having a pH of 4.5 or higher is heated and concentrated under reduced pressure to crystallize cobalt sulfate crystals, thereby producing a cobalt sulfate crystal-containing slurry.

Here, the heating temperature is preferably in the range of 30° C. to 40° C. The holding time at the heating temperature is preferably in the range of 2 hours to 24 hours. Furthermore, the pressure during decompression is preferably 100 hPa or less.
By setting the heating temperature to 40° C. or less, it is possible to suppress the reduction of the hydration water of the sulfate and obtain homogeneous cobalt sulfate crystals, while by setting the heating temperature to 30° C. or more, evaporation is promoted and efficient concentration is achieved.
Furthermore, in a cobalt sulfate solution whose pH is adjusted to 4.5 or higher during crystallization, there is almost _no free _H2SO4 and the ratio of cobalt ions to sulfate ions is approximately 1:1. Therefore, the cobalt ion concentration can be estimated from the electrical conductivity, and the concentration of the metal can be confirmed from the results of measuring the electrical conductivity.

(Solid-liquid separation step S03)
In this solid-liquid separation step S03, the cobalt sulfate crystals (solid components) produced in the above-mentioned crystallization step S02 are separated from the residual liquid (liquid component).
As a method for separating the cobalt sulfate crystals (solid component) from the residual liquid (liquid component), an existing solid-liquid separation method such as gravity settling, centrifugal separation, or filter cloth filtration using a filter press or the like can be used.

In this way, cobalt sulfate crystals are produced from a cobalt sulfate solution with a pH of less than 4.0.

In this embodiment, as shown in FIG. 1, at least a portion of the alkaline metal compound added in the alkaline metal compound addition step S01 is a cobalt hydroxide precipitate or cobalt carbonate precipitate obtained through the separation step S05, the carbonate source or hydroxide source addition step S06, the second solid-liquid separation step S07, and the washing step S08.

(Preparative separation step S05)
In this fractionation step S05, a fraction of a cobalt sulfate solution having a pH of less than 4.0 to be used as a crystallization raw material liquid, or a fraction of a cobalt sulfate solution having a pH of 4.5 or more obtained by subjecting the cobalt sulfate solution having a pH of less than 4.0 to a step of adding an alkaline metal compound to the cobalt sulfate solution to be used as a crystallization raw material liquid, is fractionated to obtain a fractionated solution.
Here, the ratio of the fractionated solution to the cobalt sulfate solution having a pH of less than 4.0 to be used as the crystallization raw material liquid or the cobalt sulfate solution having a pH of less than 4.0 to be used as the crystallization raw material liquid and having a pH of 4.5 or higher obtained by the alkaline metal compound addition step is not particularly limited, but is preferably, for example, within a range of 1.0% to 20.0% by mass.

The cobalt concentration in the separated solution is preferably in the range of 40 g/L to 90 g/L.
When adding a carbonate source or hydroxide source in the carbonate source or hydroxide source addition step S06 described below, if the cobalt concentration in the separated solution is 90 g/L or less, an increase in the viscosity of the resulting suspension slurry can be suppressed and uniform stirring can be achieved. This promotes the reaction between the cobalt in the solution and the carbonate source or hydroxide source, reduces the amount of carbonate source or hydroxide source added, and suppresses the incorporation of impurities. On the other hand, by setting the cobalt concentration in the separated solution to 40 g/L or more, the efficiency of the reaction with the carbonate source or hydroxide source is improved.

Furthermore, the electrical conductivity of a fractionated solution obtained by fractionating a cobalt sulfate solution having a pH of 4.5 or more, which is obtained by passing the alkaline metal compound addition step S01 through the cobalt sulfate solution having a pH of less than 4.0, which serves as a crystallization raw material liquid, is preferably within a range of 3.7 S/m or more and 5.4 S/m or less.
This separated solution contains almost _no free _H2SO4 , and the ratio of cobalt ions to sulfate ions is approximately 1:1. In the carbonate or hydroxide source addition step S06 described below, the cobalt ion concentration can be estimated from the electrical conductivity. If the electrical conductivity of the separated solution is 3.7 S/m or higher, it can be confirmed that the cobalt ion concentration in the separated solution is 40 g/L or higher, and if the electrical conductivity of the separated solution is 5.4 S/m or lower, it can be confirmed that the cobalt ion concentration in the separated solution is 90 g/L or lower.

(Carbonate source or hydroxide source addition step S06)
In this carbonate source or hydroxide source adding step S06, any one of sodium hydroxide, sodium carbonate, sodium bicarbonate, and carbon dioxide gas is added to the separated solution as a carbonate source or hydroxide source, thereby obtaining a suspension slurry containing a cobalt hydroxide precipitate or a cobalt carbonate precipitate and dissolved sodium sulfate, or any one of added and unreacted sodium hydroxide, sodium carbonate, and sodium bicarbonate.

By adding sodium hydroxide to the separated solution (cobalt sulfate solution), a cobalt hydroxide precipitate is produced through the following neutralization reaction:
H ₂ SO ₄ (aq) + 2NaOH (aq) → Na ₂ SO ₄ (aq) + H ₂ O (l)
CoSO ₄ (aq) + 2NaOH (aq) → Na ₂ SO ₄ (aq) + Co (OH) ₂ (s)

By adding sodium carbonate (Na ₂ CO ₃ ) to the separated solution (cobalt sulfate solution), a cobalt carbonate precipitate is produced by the following neutralization reaction:
H ₂ SO ₄ (aq) + Na ₂ CO ₃ (aq) → Na ₂ SO ₄ (aq) + H ₂ O (l) + CO ₂ (g)
CoSO ₄ (aq) + Na ₂ CO ₃ (aq) → Na ₂ SO ₄ (aq) + CoCO ₃ (s)

By adding sodium hydrogen carbonate (NaHCO ₃ ) to the separated solution (cobalt sulfate solution), a cobalt carbonate precipitate is produced by the following neutralization reaction.
H ₂ SO ₄ (aq) + 2NaHCO ₃ (aq) → Na ₂ SO ₄ (aq) + 2H ₂ O (l) + 2 CO ₂ (g)
CoSO ₄ (aq) + 2NaHCO ₃ (aq) → Na ₂ SO ₄ (aq) + CoCO ₃ (s) + H ₂ O (l) + CO ₂ (g)

Here, it is preferable that the pH of the separated solution after addition of the carbonate source or hydroxide source (sodium hydroxide, sodium carbonate, sodium bicarbonate, carbon dioxide gas) is within the range of 6.5 to 11.0.
By adjusting the pH of the separated solution after addition of the carbonate or hydroxide source to 6.5 or higher, it becomes possible to sufficiently generate a cobalt hydroxide precipitate or a cobalt carbonate precipitate, while by adjusting the pH of the separated solution after addition of the carbonate or hydroxide source to 11.0 or lower, it becomes possible to prevent the addition of an excessive amount of the carbonate or hydroxide source.
The pH of the separated solution after addition of the carbonate or hydroxide source is more preferably 6.5 or higher, and more preferably 7.3 or lower, and even more preferably 7.0 or lower.

The viscosity of the resulting suspension slurry is preferably in the range of 20 cP to 128 cP.
By setting the viscosity of the suspension slurry within the above range, a large amount of energy is not required during stirring, and the occurrence of problems such as blockage or breakage of pipes during transportation can be suppressed.

Here, when sodium carbonate or sodium bicarbonate is added to the acidic solution in the carbonate or hydroxide source addition step S06, carbonate ions are liberated as carbon dioxide gas into the atmosphere while generating bubbles in the early stage, which may result in problems such as an increased amount of sodium carbonate or sodium bicarbonate used and splashing of the liquid due to bubbles.
Therefore, when sodium carbonate or sodium bicarbonate is added, it is preferable to add the preparative solution to an aqueous sodium carbonate solution or an aqueous sodium bicarbonate solution and mix them.

(Second solid-liquid separation step S07)
In the second solid-liquid separation step S07, the cobalt hydroxide precipitate or cobalt carbonate precipitate (solid component) generated in the carbonate source or hydroxide source addition step S06 is separated from the liquid component, and the majority of the sodium component added in the carbonate source or hydroxide source addition step S06 can be separated and removed by the second solid-liquid separation step S07.
As a method for separating the cobalt hydroxide precipitate or cobalt carbonate precipitate (solid component) from the liquid component, an existing solid-liquid separation method such as gravity settling, centrifugal separation, or filter cloth filtration using a filter press or the like can be used.

(Cleaning step S08)
In this washing step S08, the cobalt hydroxide precipitate or the cobalt carbonate precipitate is washed with water to remove sodium components adhering to the surface of the cobalt hydroxide precipitate or the cobalt carbonate precipitate.
Since the cobalt hydroxide precipitate or the cobalt carbonate precipitate is hardly soluble in water, the sodium sulfate adhering to the surface of the cobalt hydroxide precipitate or the cobalt carbonate precipitate can be selectively washed away.
In the washing step S08, washing with pure water is preferably performed until the electrical conductivity of the washing solution after washing is 200 mS/m or less. By washing until the electrical conductivity of the washing solution after washing is 200 mS/m or less, the sodium component adhering to the surface of the cobalt hydroxide precipitate or the cobalt carbonate precipitate can be sufficiently removed.

In this embodiment, the cobalt hydroxide precipitate or cobalt carbonate precipitate obtained as described above is added in the alkaline metal compound addition step S01.

The method for producing metal sulfate crystals of this embodiment configured as described above includes the alkaline metal compound addition step S01 of adding an alkaline metal compound (cobalt hydroxide or cobalt carbonate) to a cobalt sulfate solution having a pH of less than 4.0 to adjust the pH _to 4.5 or higher, so that the free _H2SO4 can be sufficiently reduced by reacting the free _H2SO4 with the alkaline metal compound. Furthermore, because cobalt sulfate is produced by the reaction, _it is possible to prevent impurities such as sodium from being mixed into the cobalt sulfate solution.

The method includes a crystallization step S02 in which cobalt sulfate crystals are crystallized from a cobalt sulfate solution having a pH of 4.5 or higher to obtain a cobalt sulfate crystal-containing slurry, and a solid-liquid separation step S03 in which the cobalt sulfate crystal-containing slurry is separated into cobalt sulfate crystals (solid component) and a residual liquid (liquid component). This makes it possible to obtain high-quality cobalt sulfate crystals that are free _of free _H2SO4 and have a low amount of impurities such as sodium.
Furthermore, when the cobalt sulfate crystals thus obtained are dissolved in pure water (pH 6.5 to 7.0) so that the cobalt concentration is 1% by mass or more, the pH of the solution is 5.4 to 7.0, or 6.0 to 6.5.

In the method for producing metal sulfate crystals according to this embodiment, if the pH of the cobalt sulfate solution after the alkaline metal compound addition step S01 is set within the range of 6.0 or more and 7.0 or less, impurity elements such as Al, Fe, Cu, and Zn can be sufficiently removed. Furthermore, by setting the pH of the cobalt sulfate solution after the alkaline metal compound addition step S01 to 4.5 or more, particularly within the range of 6.0 or more and 7.0 or less, the ratio of cobalt ions to sulfate ions is approximately 1:1, making it possible to estimate the cobalt concentration from electrical conductivity in the crystallization step S02.

In the method for producing metal sulfate crystals of this embodiment, when cobalt sulfate crystals are crystallized in the crystallization step S02 by heating a cobalt sulfate solution adjusted to a pH of 4.5 or higher and concentrating it under reduced pressure, it becomes possible to efficiently crystallize cobalt sulfate crystals from the cobalt sulfate solution.

In the method for producing metal sulfate crystals of this embodiment, if at least a portion of the alkaline metal compound added in the alkaline metal compound addition step S01 is converted into a cobalt hydroxide precipitate or cobalt carbonate precipitate obtained through the fractionation step S05, the carbonate source or hydroxide source addition step S06, the second solid-liquid separation step S07, and the washing step S08, cobalt sulfate crystals can be produced with high recovery efficiency from a cobalt sulfate solution having a pH of less than 4.0.

Furthermore, the second solid-liquid separation step S07 makes it possible to sufficiently remove sodium present in the liquid component.
Furthermore, when the cobalt hydroxide precipitate or the cobalt carbonate precipitate is washed with water in the washing step S08, sodium adhering to the cobalt hydroxide precipitate can be further removed, the incorporation of sodium as an impurity can be further suppressed, and cobalt sulfate crystals of even higher purity can be produced.

Second Embodiment
A method for producing metal sulfate crystals according to a second embodiment of the present invention will be described with reference to Fig. 2. The method for producing metal sulfate crystals according to the first embodiment of the present invention involves recovering cobalt sulfate crystals (metal sulfate crystals) from a cobalt sulfate solution (metal sulfate solution). Note that components similar to those in the first embodiment are designated by the same reference numerals, and detailed descriptions thereof will be omitted.
As shown in FIG. 2, the method for producing metal sulfate crystals according to this embodiment includes at least an alkaline metal compound adding step S01, a crystallization step S02, and a solid-liquid separation step S03.

In the second embodiment, as shown in FIG. 2, at least a portion of the alkaline metal compound added in the alkaline metal compound addition step S01 is a cobalt hydroxide precipitate or cobalt carbonate precipitate obtained through the residual liquid recovery step S15, the carbonate source or hydroxide source addition step S16, the second solid-liquid separation step S17, and the washing step S18.

(Residual liquid recovery step S15)
In the residual liquid recovery step S15, the residual liquid (liquid component) separated in the solid-liquid separation step S03 is recovered. This residual liquid contains a portion of unrecovered cobalt sulfate dissolved as cobalt sulfate crystals.
The cobalt concentration in the residual liquid is preferably in the range of 40 g/L or more and 90 g/L or less.
When adding a carbonate source or hydroxide source in the carbonate source or hydroxide source addition step S16 described below, if the cobalt concentration in the residual liquid is 90 g/L or less, an increase in the viscosity of the resulting suspension slurry can be suppressed and uniform stirring can be achieved. This promotes the reaction between the cobalt in the liquid and the carbonate source or hydroxide source, reduces the amount of carbonate source or hydroxide source to be added, and suppresses the incorporation of impurities. On the other hand, by setting the cobalt concentration in the residual liquid to 40 g/L or more, the efficiency of the reaction with the carbonate source or hydroxide source is improved.
The residual liquid may have a cobalt concentration of 150 g/L or more. In this case, the residual liquid is mixed with pure water to adjust the cobalt concentration to fall within a desired range.

Furthermore, the electrical conductivity of the residual liquid is preferably in the range of 3.7 S/m to 5.4 S/m.
This residual liquid has a pH of 4.5 or higher, is almost free _of free _H2SO4 , and has a cobalt ion to sulfate ion ratio of approximately 1:1. In the carbonate source or hydroxide source addition step S16 described below, the cobalt ion concentration can be estimated from the electrical conductivity. If the electrical conductivity of the separated solution is 3.7 S/m or higher, the cobalt ion concentration in the separated solution will be 40 g/L or higher, and if the electrical conductivity of the residual liquid is 5.4 S/m or lower, it can be confirmed that the cobalt concentration of the residual liquid is 90 g/L or lower.

(Carbonate source or hydroxide source addition step S16)
In this carbonate source or hydroxide source adding step S16, any one of sodium hydroxide, sodium carbonate, sodium bicarbonate, and carbon dioxide gas is added as a carbonate source or hydroxide source to the residual liquid recovered in the residual liquid recovering step S15, thereby obtaining a suspension slurry containing a cobalt hydroxide precipitate or a cobalt carbonate precipitate and dissolved sodium sulfate, or any one of added and unreacted sodium hydroxide, sodium carbonate, and sodium bicarbonate.
Addition of sodium hydroxide, sodium carbonate, or sodium hydrogen carbonate to the residual liquid results in the formation of a cobalt hydroxide precipitate or a cobalt carbonate precipitate.

Here, it is preferable that the pH of the residual liquid after adding the carbonate source or hydroxide source (sodium hydroxide or sodium carbonate, sodium bicarbonate, carbon dioxide gas) is in the range of 6.5 to 11.0.
By adjusting the pH of the residual liquid after the addition of the carbonate source or hydroxide source to 6.5 or higher, it becomes possible to sufficiently generate a cobalt hydroxide precipitate or a cobalt carbonate precipitate, while by adjusting the pH of the residual liquid after the addition of the carbonate source or hydroxide source to 11.0 or lower, it becomes possible to prevent the addition of an excessive amount of the carbonate source or hydroxide source.
The pH of the residual liquid after addition of the carbonate source or hydroxide source is more preferably 6.5 or higher, and more preferably 7.3 or lower, and even more preferably 7.0 or lower.

The viscosity of the resulting suspension slurry is preferably in the range of 20 cP to 128 cP.
By setting the viscosity of the suspension slurry within the above range, a large amount of energy is not required during stirring, and the occurrence of problems such as blockage or breakage of pipes during transportation can be suppressed.

Here, when sodium carbonate or sodium bicarbonate is added to the acidic residual liquid in the carbonate source or hydroxide source addition step S16, carbonate ions are liberated as carbon dioxide gas into the atmosphere while generating bubbles in the early stage, which may result in problems such as an increased amount of sodium carbonate or sodium bicarbonate used and splashing of the liquid due to bubbles.
Therefore, when sodium carbonate or sodium bicarbonate is added, it is preferable to add the residual liquid to the aqueous sodium carbonate or sodium bicarbonate solution and mix them.

(Second solid-liquid separation step S17)
In this second solid-liquid separation step S17, the cobalt hydroxide precipitate or cobalt carbonate precipitate (solid component) generated in the carbonate source or hydroxide source addition step S16 is separated from the liquid component. By this second solid-liquid separation step S17, most of the sodium component added in the carbonate source or hydroxide source addition step S16 can be separated and removed.
As a method for separating the cobalt hydroxide precipitate or cobalt carbonate precipitate (solid component) from the liquid component, an existing solid-liquid separation method such as gravity settling, centrifugal separation, or filter cloth filtration using a filter press or the like can be used.

(Cleaning step S18)
In this washing step S18, the cobalt hydroxide precipitate or the cobalt carbonate precipitate is washed with water to remove sodium components adhering to the surface of the cobalt hydroxide precipitate or the cobalt carbonate precipitate.
Since the cobalt hydroxide precipitate or the cobalt carbonate precipitate is hardly soluble in water, the sodium sulfate adhering to the surface of the cobalt hydroxide precipitate or the cobalt carbonate precipitate can be selectively washed away.
In the washing step S18, washing with pure water is preferably performed until the electrical conductivity of the washing solution after washing is 200 mS/m or less. By washing until the electrical conductivity of the washing solution after washing is 200 mS/m or less, the sodium component adhering to the surface of the cobalt hydroxide precipitate or the cobalt carbonate precipitate can be sufficiently removed.

In this embodiment, the cobalt hydroxide precipitate or cobalt carbonate precipitate obtained as described above is added in the alkaline metal compound addition step S01.

According to the method for producing metal sulfate crystals of this embodiment configured as described above, similar to the first embodiment, high-quality cobalt sulfate crystals can be obtained that are free _of free _H2SO4 and have a low amount of impurities such as sodium.
Furthermore, when the cobalt sulfate crystals thus obtained are dissolved in pure water (pH 6.5 to 7.0) so that the cobalt concentration is 1% by mass or more, the pH of the solution is 5.4 to 7.0, or 6.0 to 6.5.

In the method for producing metal sulfate crystals of this embodiment, if at least a portion of the alkaline metal compound added in the alkaline metal compound addition step S01 is converted into a cobalt hydroxide precipitate or cobalt carbonate precipitate obtained through the residual liquid recovery step S15, the carbonate source or hydroxide source addition step S16, the second solid-liquid separation step S17, and the washing step S18, cobalt sulfate crystals can be produced with high recovery efficiency from a cobalt sulfate solution having a pH of less than 4.0.

Furthermore, the second solid-liquid separation step S17 makes it possible to sufficiently remove sodium present in the liquid component.
Furthermore, when the cobalt hydroxide precipitate or the cobalt carbonate precipitate is washed with water in the washing step S18, sodium adhering to the cobalt hydroxide precipitate or the cobalt carbonate precipitate can be further removed, the incorporation of sodium as an impurity can be further suppressed, and cobalt sulfate crystals of even higher purity can be produced.

The above describes an embodiment of the present invention, but the present invention is not limited to this and can be modified as appropriate without departing from the technical concept of the invention.

In this embodiment, the metal sulfate is described using cobalt sulfate as an example, but the present invention is not limited thereto and may be applied to other metal sulfates. For example, when the metal sulfate is nickel sulfate, the above-described steps make it possible to crystallize and recover nickel sulfate crystals from a nickel sulfate solution having a pH of less than 4.0.
When the nickel sulfate crystals thus obtained are dissolved in pure water (pH 6.5 to 7.0) so that the nickel concentration is 1% by mass or more, the pH of the solution is 5.4 to 7.0, or 6.0 to 6.5.

Here, when nickel hydroxide is added as an alkaline metal compound to a nickel sulfate solution having a pH of less than 4.0, the free H ₂ SO ₄ reacts with the nickel hydroxide and is consumed, as shown below, resulting in an increase in pH.
_H2SO4 (aq)+Ni(OH) ₂ (s) _{→ NiSO4} (aq)+ _2H2O (l)

Furthermore, when nickel carbonate is added as an alkaline metal compound to a nickel sulfate solution having a pH of less than 4.0, the free H ₂ SO ₄ reacts with the nickel carbonate and is consumed, resulting in an increase in pH, as shown below.
H ₂ SO ₄ (aq) + NiCO ₃ (s) → NiSO ₄ (aq) + H ₂ O (l) + CO ₂ (g)
The nickel sulfate produced by the above reaction can also be used as a raw material for nickel sulfate crystals.

Furthermore, by adding sodium hydroxide to the separated solution (nickel sulfate solution) in the first embodiment or the residual solution (nickel sulfate solution) in the second embodiment, a nickel hydroxide precipitate is produced by the following neutralization reaction.
H ₂ SO ₄ (aq) + 2NaOH (aq) → Na ₂ SO ₄ (aq) + H ₂ O (l)
_NiSO4 (aq)+2NaOH(aq)→ _Na2SO4 ( _aq )+Ni(OH) ₂ (s)

By adding sodium carbonate (Na ₂ CO ₃ ) to the separated solution (nickel sulfate solution) or the remaining solution (nickel sulfate solution), a nickel carbonate precipitate is produced by the following neutralization reaction.
H ₂ SO ₄ (aq) + Na ₂ CO ₃ (aq) → Na ₂ SO ₄ (aq) + H ₂ O (l) + CO ₂ (g)
NiSO ₄ (aq) + Na ₂ CO ₃ (aq) → Na ₂ SO ₄ (aq) + NiCO ₃ (s)

By adding sodium hydrogen carbonate (NaHCO ₃ ) to the separated solution (nickel sulfate solution) or the remaining solution (nickel sulfate solution), a nickel carbonate precipitate is produced by the following neutralization reaction.
H ₂ SO ₄ (aq) + 2NaHCO ₃ (aq) → Na ₂ SO ₄ (aq) + 2H ₂ O (l) + 2 CO ₂ (g)
NiSO ₄ (aq) + 2NaHCO ₃ (aq) → Na ₂ SO ₄ (aq) + NiCO ₃ (s) + H ₂ O (l) + CO ₂ (g)

The viscosity of the suspension slurry obtained by adding sodium hydroxide, sodium carbonate, sodium bicarbonate, or carbon dioxide gas as a carbonate or hydroxide source to the fractionated solution and residual liquid is preferably in the range of 20 cP to 128 cP.
The electrical conductivity of the separated solution and the residual solution is preferably in the range of 3.7 S/m to 5.4 S/m.
Furthermore, it is preferable that the pH of the residual liquid after addition of the carbonate source or hydroxide source (sodium hydroxide or sodium carbonate, sodium bicarbonate, carbon dioxide gas) is within the range of 6.5 to 7.3.

In addition, in this embodiment, as at least a portion of the alkaline metal compound added in the alkaline metal compound addition step S01, a cobalt hydroxide precipitate or a cobalt carbonate precipitate obtained from a cobalt sulfate solution having a pH of less than 4.0 or a fractionated solution separated from a cobalt sulfate solution having a pH of 4.5 or higher obtained through the alkaline metal compound addition step S01 or a residual liquid separated in the solid-liquid separation step S03 is used. However, this is not limited to this, and there are no particular restrictions on the method for producing the alkaline metal compound to be added.

In addition, in this embodiment, the crystallization step S02 is described as being configured to crystallize cobalt sulfate crystals by heating a cobalt sulfate solution adjusted to a pH of 4.5 or higher and concentrating it under reduced pressure. However, the method for crystallizing cobalt sulfate crystals is not limited; for example, cobalt sulfate crystals may be crystallized by cooling a cobalt sulfate solution adjusted to a pH of 4.5 or higher.

Below, we will explain the results of confirmation experiments conducted to confirm the effectiveness of this invention.

Example 1
A predetermined amount of cobalt sulfate heptahydrate (special grade reagent) was weighed and dissolved in pure water to a constant volume so that the cobalt concentration was 1 mol/L. Before the constant volume adjustment, 47% by mass of sulfuric acid was added to adjust the pH of the entire solution to 1.0 (initial pH). This solution was used as the crystallization raw material solution (cobalt sulfate solution).
Here, the concentration analysis values of this crystallization raw material solution (cobalt sulfate solution) by ICP emission spectrometry were Co: 59 g/L, Na: 12 mg/L.

Using this crystallization raw material solution (cobalt sulfate solution) with a pH of 1.0, a plurality of solutions were taken, and while thoroughly stirring, an 8 mol/L sodium hydroxide solution was added dropwise to adjust the pH of each solution to 4.0 to 11.0, followed by stirring for 1 hour.
After adjusting the pH and stirring, the slurry and solution, which may have formed a precipitate, were recovered and filtered through a 1.0 μm membrane. The cobalt concentration in the filtrate was measured. The dissolved cobalt concentration in the solution at each pH is shown in Figure 3.

As shown in Figure 3, at pH levels below 7.0, the production of cobalt hydroxide is insufficient and dissolved cobalt is detected in the solution. On the other hand, at pH levels above 7.0, dissolved cobalt is barely detected, confirming that the precipitation of cobalt hydroxide is progressing satisfactorily.

Example 2
A crystallization raw material liquid (cobalt sulfate solution) with a pH of 1.0 was prepared in the same manner as in Example 1. A portion of the crystallization raw material liquid (cobalt sulfate solution) with a pH of 1.0 was taken, and an 8 mol/L sodium hydroxide solution was added dropwise to the solution while thoroughly stirring, adjusting the pH of the solution to 11.0 to produce a cobalt hydroxide precipitate, thereby producing a cobalt hydroxide slurry.
The slurry was stirred for 1 hour and then filtered through a 1.0 μm membrane. Pure water was dropped into the cobalt hydroxide precipitate remaining on the membrane and filtered to wash away any solution adhering to the surface. After thorough washing with pure water and filtration, the cobalt hydroxide precipitate was allowed to stand and dry at 25° C., and the cobalt hydroxide precipitate was recovered. The Na concentration in the recovered cobalt hydroxide precipitate was less than 0.1% by mass.

The recovered cobalt hydroxide precipitate was added to a crystallization raw material solution (cobalt sulfate solution) having an initial pH of 1.0, and dissolved while stirring and mixing, until the pH was adjusted to 4.5.
_H2SO4 (aq)+Co(OH) ₂ (s) _{→ CoSO4} (aq)+ _2H2O (l)
(When cobalt hydroxide precipitate was added to the crystallization stock solution at 15 g/L, the Co concentration in the crystallization stock solution increased by 8 g/L and the Na concentration increased by 15 mg/L.)
The crystallization raw material liquid (cobalt sulfate solution) after being adjusted to pH 4.5 was subjected to a heat treatment and concentrated under reduced pressure until the cobalt concentration in the solution reached 200 g/L, thereby crystallizing cobalt sulfate heptahydrate.

The concentrated cobalt sulfate crystal-containing slurry containing the precipitated crystals was filtered, centrifuged, and then recovered and allowed to stand and dry at 25° C. to recover cobalt sulfate heptahydrate crystals.
The Co and Na contents of the recovered cobalt sulfate heptahydrate crystals were analyzed. The analysis results are shown in Table 1. As shown in Table 1, the Co content was 20.9 mass% and the Na content was less than 5 mass ppm, indicating that the crystals were of sufficiently high purity.
Approximately 10 g of the cobalt sulfate heptahydrate crystals recovered after standing and drying were further weighed and dissolved in pure water (pH 6.5 to 7) to a cobalt concentration of 1.00 mass%, and the pH was measured. The results are shown in Table 1. The pH of the solution was 6.0, confirming that crystals with sufficiently low _{free H2SO4} adhesion could be recovered.

Example 3
A crystallization raw material liquid (cobalt sulfate solution) was prepared in the same manner as in Example 1, except that the pH (initial pH) of the crystallization raw material liquid (cobalt sulfate solution) was set to 2.0.
A portion of the crystallization raw material solution (cobalt sulfate solution) with a pH of 2.0 was taken, and an 8 mol/L sodium hydroxide solution was added dropwise with thorough stirring to adjust the pH of the solution to 11.0, thereby forming a cobalt hydroxide precipitate and producing a cobalt hydroxide slurry.
The slurry was stirred for 1 hour and then filtered through a 1.0 μm membrane. Pure water was dropped into the cobalt hydroxide precipitate remaining on the membrane and filtered to wash away any solution adhering to the surface. After thorough washing with pure water and filtration, the cobalt hydroxide precipitate was allowed to stand and dry at 25° C., and the cobalt hydroxide precipitate was recovered. The Na concentration in the recovered cobalt hydroxide precipitate was less than 0.1% by mass.

The recovered cobalt hydroxide precipitate was added to a crystallization raw material solution (cobalt sulfate solution) having a pH of 2.0, and dissolved while stirring and mixing, and the pH was adjusted to 5.0.
_H2SO4 (aq)+Co(OH) ₂ (s) _{→ CoSO4} (aq)+ _2H2O (l)
(When cobalt hydroxide precipitate was added to the crystallization stock solution at 6 g/L, the Co concentration in the crystallization stock solution increased by 3 g/L and the Na concentration increased by 6 mg/L.)
The crystallization raw material liquid (cobalt sulfate solution) after being adjusted to pH 5.0 was subjected to a heat treatment and concentrated under reduced pressure until the cobalt concentration in the solution reached 200 g/L, thereby crystallizing cobalt sulfate heptahydrate.

The concentrated cobalt sulfate crystal-containing slurry containing the precipitated crystals was filtered, centrifuged, and then recovered and allowed to stand and dry at 25° C. to recover cobalt sulfate heptahydrate crystals.
The Co and Na contents of the recovered cobalt sulfate heptahydrate crystals were analyzed. The analysis results are shown in Table 1. As shown in Table 1, the Co content was 21.0 mass% and the Na content was less than 5 mass ppm, indicating that the crystals were of sufficiently high purity.
Approximately 10 g of the cobalt sulfate heptahydrate crystals recovered after standing and drying were further weighed and dissolved in pure water (pH 6.5 to 7) to a cobalt concentration of 1.00 mass%, and the pH was measured. The results are shown in Table 1. The pH of the solution was 6.1, confirming that crystals with sufficiently low _{free H2SO4} adhesion could be recovered.

Example 4
A crystallization raw material liquid (cobalt sulfate solution) was prepared in the same manner as in Example 1, except that the pH (initial pH) of the crystallization raw material liquid (cobalt sulfate solution) was set to 3.0.
A portion of the crystallization raw material solution (cobalt sulfate solution) with pH 3.0 was taken, and an 8 mol/L sodium hydroxide solution was added dropwise with thorough stirring to adjust the pH of the solution to 7.0, thereby forming a cobalt hydroxide precipitate and producing a cobalt hydroxide slurry.
The slurry was stirred for 1 hour and then filtered through a 1.0 μm membrane. Pure water was dropped into the cobalt hydroxide precipitate remaining on the membrane and filtered to wash away any solution adhering to the surface. After thorough washing with pure water and filtration, the cobalt hydroxide precipitate was allowed to stand and dry at 25° C., and the cobalt hydroxide precipitate was recovered. The Na concentration in the recovered cobalt hydroxide precipitate was less than 0.1% by mass.

The recovered cobalt hydroxide precipitate was added to a crystallization raw material solution (cobalt sulfate solution) having a pH of 3.0, and dissolved while stirring and mixing, and the pH was adjusted to 5.5.
_H2SO4 (aq)+Co(OH) ₂ (s) _{→ CoSO4} (aq)+ _2H2O (l)
(When cobalt hydroxide precipitate was added to the crystallization stock solution at 1 g/L, the Co concentration in the crystallization stock solution increased by 0.5 g/L and the Na concentration increased by 1 mg/L.)
The crystallization raw material liquid (cobalt sulfate solution) after being adjusted to pH 5.5 was subjected to a heat treatment and concentrated under reduced pressure until the cobalt concentration in the solution reached 200 g/L, thereby crystallizing cobalt sulfate heptahydrate.

The concentrated cobalt sulfate crystal-containing slurry containing the precipitated crystals was filtered, centrifuged, and then recovered and allowed to stand and dry at 25° C. to recover cobalt sulfate heptahydrate crystals.
The Co and Na contents of the recovered cobalt sulfate heptahydrate crystals were analyzed. The analysis results are shown in Table 1. As shown in Table 1, the Co content was 21.0 mass% and the Na content was less than 5 mass ppm, indicating that the crystals were of sufficiently high purity.
Approximately 10 g of the cobalt sulfate heptahydrate crystals recovered after standing and drying were further weighed and dissolved in pure water (pH 6.5 to 7) to a cobalt concentration of 1.00 mass%, and the pH was measured. The results are shown in Table 1. The pH of the solution was 6.2, confirming that crystals with sufficiently low _{free H2SO4} adhesion could be recovered.

Example 5
A crystallization raw material liquid (cobalt sulfate solution) was prepared in the same manner as in Example 1, except that the pH (initial pH) of the crystallization raw material liquid (cobalt sulfate solution) was set to 3.5.
A portion of the crystallization raw material solution (cobalt sulfate solution) with a pH of 3.5 was taken, and an 8 mol/L sodium hydroxide solution was added dropwise with thorough stirring to adjust the pH of the solution to 7.0, thereby forming a cobalt hydroxide precipitate and producing a cobalt hydroxide slurry.
The slurry was stirred for 1 hour and then filtered through a 1.0 μm membrane. Pure water was dropped into the cobalt hydroxide precipitate remaining on the membrane and filtered to wash away any solution adhering to the surface. After thorough washing with pure water and filtration, the cobalt hydroxide precipitate was allowed to stand and dry at 25° C., and the cobalt hydroxide precipitate was recovered. The Na concentration in the recovered cobalt hydroxide precipitate was less than 0.1% by mass.

The recovered cobalt hydroxide precipitate was added to a crystallization raw material solution (cobalt sulfate solution) having a pH of 3.5, and dissolved while stirring and mixing, and the pH was adjusted to 6.0.
_H2SO4 (aq)+Co(OH) ₂ (s) _{→ CoSO4} (aq)+ _2H2O (l)
(When cobalt hydroxide precipitate was added to the crystallization stock solution at 1 g/L, the Co concentration in the crystallization stock solution increased by 0.5 g/L and the Na concentration increased by 1 mg/L.)
The crystallization raw material liquid (cobalt sulfate solution) after being adjusted to pH 6.0 was subjected to a heat treatment and concentrated under reduced pressure until the cobalt concentration in the solution reached 200 g/L, thereby crystallizing cobalt sulfate heptahydrate.

The concentrated cobalt sulfate crystal-containing slurry containing the precipitated crystals was filtered, centrifuged, and then recovered and allowed to stand and dry at 25° C. to recover cobalt sulfate heptahydrate crystals.
The Co and Na contents of the recovered cobalt sulfate heptahydrate crystals were analyzed. The analysis results are shown in Table 1. As shown in Table 1, the Co content was 20.9 mass% and the Na content was less than 5 mass ppm, indicating that the crystals were of sufficiently high purity.
Approximately 10 g of the cobalt sulfate heptahydrate crystals recovered after standing and drying were further weighed and dissolved in pure water (pH 6.5 to 7) to a cobalt concentration of 1.00 mass%, and the pH was measured. The results are shown in Table 1. The pH of the solution was 6.2, confirming that crystals with sufficiently low _{free H2SO4} adhesion could be recovered.

(Comparative Examples 1 to 4)
The crystallization raw material solution (cobalt sulfate solution) having a pH of 1.0 to 3.5 used in Examples 2 to 5 was crystallized and recovered as cobalt sulfate heptahydrate under the same conditions as in Examples 2 to 5, without adjusting the pH by adding cobalt hydroxide, at a pH of 1.0 to 3.5.
The Co content and Na content of the recovered cobalt sulfate heptahydrate crystals were measured in the same manner as in Examples 2 to 5. Approximately 10 g of the recovered cobalt sulfate heptahydrate crystals were further weighed and dissolved in pure water (pH 6.5 to 7) to give a cobalt concentration of 1.00 mass%, and the pH was measured. The evaluation results are shown in Table 1.

As a result, the Co content and Na content of the cobalt sulfate heptahydrate crystals were of the same quality as those in Examples 2 to 5. However, the pH of the aqueous solution with a cobalt concentration of 1.00 mass % was significantly lower than those in Examples 1 to ₅ , and cobalt sulfate heptahydrate with a large amount of free _H2SO4 attached was recovered.
It was confirmed that if the crystallization raw material solution (cobalt sulfate solution) is subjected to crystallization treatment while remaining acidic with sulfuric acid without adjusting the pH, it is difficult to recover cobalt sulfate heptahydrate free of free H ₂ SO ₄ .

(Comparative Examples 5 to 8)
The crystallization raw material solution (cobalt sulfate solution) having a pH of 1.0 to 3.5 used in Examples 2 to 5 was adjusted to a pH of 4.5 to 6.0 by adding an aqueous sodium hydroxide solution instead of adjusting the pH by adding cobalt hydroxide, as in Examples 2 to 5, and cobalt sulfate heptahydrate was crystallized and recovered under the same conditions as in Examples 2 to 5.

The Co content and Na content of the recovered cobalt sulfate heptahydrate crystals were measured in the same manner as in Examples 2 to 5. Approximately 10 g of the recovered cobalt sulfate heptahydrate crystals were further weighed and dissolved in pure water (pH 6.5 to 7) to achieve a cobalt concentration of 1.00% by mass, and the pH was measured. The evaluation results are shown in Table 1. Note that in the comparative example, cobalt hydroxide was not produced, so the "pH adjustment" item in Table 1 is marked with "-."

As a result, the Co content and the pH of the aqueous solution with a cobalt concentration of 1.00 mass% were of the same quality as in the Examples, but the Na content was significantly higher than in the Examples, and cobalt sulfate heptahydrate with a high sodium impurity content was recovered.
Although adjusting the pH by adding an aqueous sodium hydroxide solution is a simple method, it excessively increases the sodium concentration in the crystallization raw material solution (cobalt sulfate solution), and also increases the amount of sodium mixed into cobalt sulfate heptahydrate. Therefore, it was confirmed that this method is unsuitable for recovering high-purity cobalt sulfate heptahydrate.

(Comparative Example 11, Examples 11 to 16)
Next, sodium carbonate was added to a cobalt sulfate solution having the composition shown in Table 3 to adjust the pH to 8.69, thereby obtaining a cobalt carbonate precipitate.
The cobalt carbonate precipitate obtained as described above was added to a crystallization raw material liquid (cobalt sulfate solution) having a pH of 2.0 and the composition shown in Table 2, and the pH was adjusted to the pH during crystallization shown in Table 4, thereby crystallizing cobalt sulfate crystals. The composition of the obtained cobalt sulfate crystals and the pH measured after dissolving the crystals in pure water (pH 6.5 to 7) to give a cobalt concentration of 1.00 mass% are shown in Table 4.

As shown in Table 4, in Examples 11 to 16, in which the pH during crystallization was 4.5 or higher, the content of Al, Fe, Mn, and Zn in the crystals was reduced compared to Comparative Example 11, in which the pH during crystallization was less than 4.5. Furthermore, in Examples 11 to 16, crystals with lower acidity were obtained than in Comparative Example 11.

(Examples 21 to 25)
Sodium carbonate was added to a cobalt sulfate solution having the composition shown in Table 3, and the pH was adjusted to the pH shown in Table 5 to obtain a suspension slurry containing cobalt carbonate precipitate and dissolved sodium sulfate or unreacted sodium carbonate. The cobalt concentration, electrical conductivity, and viscosity of this suspension slurry were measured. The measurement results are shown in Table 5.
The resulting suspension slurry was subjected to solid-liquid separation (gravity settling separation method) to obtain a cobalt carbonate precipitate, the composition of which is shown in Table 5.

Compared to Examples 24 and 25, where the pH was 7.5 or higher, Examples 21 to 23, where the pH was 7.0 or lower, showed a reduced content of Mn, Mg, and Ca in the cobalt carbonate precipitate.

(Example 31)
Next, pure water was added to a cobalt sulfate solution (simulating a fractionated solution or residual solution) with a pH of 4.72 after neutralization of free sulfuric acid with the composition shown in Table 6, to adjust the Co concentration as shown in Table 7. The electrical conductivity was measured (using a portable electrical conductivity measuring device, D-210C, manufactured by HORIBA, Ltd.) to confirm the relationship between the Co concentration and electrical conductivity. The evaluation results are shown in Table 7 and Figure 4.
When the Co concentration was in the range of 40 g/L or more and 90 g/L or less, the electrical conductivity was in the range of 3.7 S/m or more and 5.4 S/m or less, and it was confirmed that the Co concentration and the electrical conductivity were in a proportional relationship and that the Co concentration could be estimated by measuring the electrical conductivity.

(Example 41)
Pure water was added to a cobalt sulfate solution (a solution simulating a preparative solution or a residual solution) having a pH of 3.0 and a composition shown in Table 8 to adjust the Co concentration as shown in Table 8. Sodium carbonate was then added to the cobalt sulfate solution to obtain a suspension slurry containing cobalt carbonate precipitate and dissolved sodium sulfate or unreacted sodium carbonate after addition.
The suspension slurry was placed in a glass beaker and the viscosity was measured using a digital viscometer (manufactured by Eiko Seiki Co., Ltd., Brookfield, LVDV-E). The measurement results are shown in Table 8.

Table 8 confirms that slurry viscosities corresponding to Co concentrations of 40 g/L or more and 90 g/L or less are easy to stir.

(Example 101)
A predetermined amount of nickel sulfate hexahydrate (special grade reagent) was weighed and dissolved in pure water to obtain a nickel concentration of 1 mol/L. Furthermore, before the volumetric adjustment, 47% by mass of sulfuric acid was added to the solution to adjust the pH of the entire solution to 2.0. This solution was used as the crystallization raw material solution (nickel sulfate solution).
Here, the concentration analysis values of this crystallization raw material solution (nickel sulfate solution) by ICP emission spectrometry were Ni: 63 g/L, Na: 10 mg/L.

A portion of the crystallization raw material liquid (nickel sulfate solution) having a pH of 2.0 (initial pH) thus obtained was taken, and a 1.0 mol/L sodium carbonate solution was added dropwise to the solution while thoroughly stirring, thereby adjusting the pH of the solution to 11.0 to produce a nickel carbonate precipitate, thereby producing a nickel carbonate slurry.
The slurry was stirred for 1 hour and then filtered through a 1.0 μm membrane. Pure water was dropped thoroughly onto the nickel carbonate precipitate remaining on the membrane, and the solution adhering to the surface was washed away. After thorough washing with pure water and filtration, the mixture was left to dry at 25° C. to recover the nickel carbonate precipitate. The Na concentration in the recovered nickel carbonate precipitate was less than 0.1% by mass.

The recovered nickel carbonate precipitate was added to a crystallization raw material solution (nickel sulfate solution) of pH 2.0 and dissolved under stirring to cause the following reaction, and the pH was adjusted to 4.5.
H ₂ SO ₄ (aq) + NiCO ₃ (s) → NiSO ₄ (aq) + H ₂ O (l) + CO ₂ (g)
The crystallization raw material liquid (nickel sulfate solution) after being adjusted to pH 4.5 was subjected to a heat treatment and concentrated under reduced pressure until the nickel concentration in the solution reached 250 g/L, thereby crystallizing nickel sulfate hexahydrate.

The concentrated nickel sulfate crystal-containing slurry containing the precipitated crystals was filtered, centrifuged, and then recovered and allowed to stand and dry at 25° C. to recover nickel sulfate hexahydrate crystals.
The Ni and Na contents of the recovered nickel sulfate hexahydrate crystals were analyzed. The analysis results are shown in Table 9. As shown in Table 9, the Ni content was 22.3 mass% and the Na content was less than 5 mass ppm, indicating that the crystals were of sufficiently high purity.

Approximately 10 g of the recovered nickel sulfate hexahydrate crystals were weighed and dissolved in pure water (pH 6.5 to 7.0) to a nickel concentration of 1.00% by mass, and the pH was measured. The results are shown in Table 9. The pH of the solution was 5.4, confirming that crystals with sufficiently low _{free H2SO4} adhesion could be recovered.

(Examples 102 to 104)
The pH of the crystallization raw material solution (initial pH), the pH during preparation of the nickel carbonate precipitate (preparation pH), and the pH during crystallization of nickel sulfate (crystallization pH) were changed as shown in Table 9, and the other conditions were the same as those in Example 101. The evaluation results are shown in Table 9.

(Comparative Example 111, Examples 111 to 115)
Next, sodium carbonate was added to a nickel sulfate solution having the composition shown in Table 11 to adjust the pH to 8.77, thereby obtaining a nickel carbonate precipitate.
The nickel carbonate precipitate obtained as described above was added to a crystallization raw material solution (nickel sulfate solution) having a pH of 2.5 and a composition shown in Table 10, and the pH was adjusted to the pH shown in Table 12, thereby crystallizing nickel sulfate crystals. The composition of the obtained nickel sulfate crystals is shown in Table 12.

As shown in Table 12, in Examples 111 to 115, in which the pH during crystallization was 4.5 or higher, the content of Al, Fe, Mn, and Zn in the crystals was reduced compared to Comparative Example 111, in which the pH during crystallization was less than 4.5. Furthermore, in Examples 111 to 115, crystals with lower acidity than Comparative Example 111 were obtained.

(Examples 121 to 125)
Sodium carbonate was added to a nickel sulfate solution having the composition shown in Table 11, and the pH was adjusted to the pH shown in Table 13 to obtain a suspension slurry containing nickel carbonate precipitate and dissolved sodium sulfate or added, unreacted sodium carbonate. The nickel concentration, electrical conductivity, and viscosity of this suspension slurry were measured. The measurement results are shown in Table 13.
The resulting suspension slurry was subjected to solid-liquid separation (gravity settling separation method) to obtain a nickel carbonate precipitate. The composition of this nickel carbonate precipitate is shown in Table 13.

Compared to Examples 124 and 125, where the pH was 7.5 or higher, Examples 121 to 123, where the pH was 7.0 or lower, showed a reduced content of Mn, Mg, and Ca in the nickel carbonate precipitate.

(Example 131)
Next, pure water was added to a nickel sulfate solution (simulating a preparative solution or residual solution) with a pH of 4.9 after neutralization of free sulfuric acid having the composition shown in Table 14 to adjust the Ni concentration as shown in Table 15. The electrical conductivity was measured (using a portable electrical conductivity measuring device, D-210C, manufactured by HORIBA, Ltd.) to confirm the relationship between the Ni concentration and electrical conductivity. The evaluation results are shown in Table 15 and FIG. 5.
When the Ni concentration was in the range of 40 g/L or more and 90 g/L or less, the electrical conductivity was in the range of 3.7 S/m or more and 5.4 S/m or less, and it was confirmed that the Ni concentration and the electrical conductivity were in a proportional relationship and that the Ni concentration could be estimated by measuring the electrical conductivity.

(Example 141)
Pure water was added to a nickel sulfate solution (a solution simulating a preparative solution or a residual liquid) having a pH of 3.5 and a composition shown in Table 16, to adjust the Ni concentration as shown in Table 16. Sodium carbonate was then added to the nickel sulfate solution to obtain a suspension slurry containing nickel carbonate precipitate and dissolved sodium sulfate or added, unreacted sodium carbonate.
The suspension slurry was placed in a glass beaker, and the viscosity was measured using a digital viscometer (manufactured by Eiko Seiki Co., Ltd., Brookfield, LVDV-E). The measurement results are shown in Table 16.

Table 16 confirms that slurry viscosities corresponding to Ni concentrations of 40 g/L or more and 90 g/L or less are easy to stir.

As a result of the above confirmatory experiments, it was confirmed that the present invention can _{provide a} method for producing _metal sulfate crystals and a metal sulfate that can obtain high-quality metal sulfate crystals that are free from free _H2SO4 adhesion and have low amounts of impurities such as sodium from a metal sulfate solution that contains a large amount of free _H2SO4 and has a pH of less than 4.0.

Claims (9)

A method for producing metal sulfate crystals, comprising crystallizing and recovering metal sulfate crystals from a metal sulfate solution containing a metal sulfate and having a pH of less than 4.0, comprising:
an alkaline metal compound addition step of adding an alkaline metal compound of a metal constituting the metal sulfate to the metal sulfate solution having a pH of less than 4.0 to adjust the pH to 4.5 or higher;
a crystallization step of crystallizing the metal sulfate crystals from the metal sulfate solution having a pH of 4.5 or higher to obtain a metal sulfate crystal-containing slurry containing the metal sulfate crystals;
a solid-liquid separation step of separating the slurry containing the metal sulfate crystals into the metal sulfate crystals and a residual liquid;
A method for producing metal sulfate crystals, comprising: A method for producing metal sulfate crystals, characterized in that the pH after the alkaline metal compound addition step is within the range of 6 to 7. The method for producing metal sulfate crystals described in claim 1, characterized in that the crystallization step involves heat-treating the metal sulfate solution with a pH of 4.5 or higher to crystallize the metal sulfate crystals. The method for producing metal sulfate crystals described in claim 1, characterized in that at least a portion of the alkaline metal compound added in the alkaline metal compound addition step is a metal hydroxide precipitate or metal carbonate precipitate produced by adding sodium hydroxide, sodium carbonate, or sodium bicarbonate to a solution separated from a metal sulfate solution having a pH of less than 4.0. The method for producing metal sulfate crystals described in claim 1, characterized in that at least a portion of the alkaline metal compound added in the alkaline metal compound addition step is a metal hydroxide precipitate or metal carbonate precipitate produced by adding sodium hydroxide, sodium carbonate, or sodium bicarbonate to the residual liquid separated in the solid-liquid separation step. The method for producing metal sulfate crystals described in claim 4 or 5, characterized in that the metal hydroxide precipitate or the metal carbonate precipitate is washed with water. The method for producing metal sulfate crystals according to claim 1, characterized in that the metal sulfate crystals are cobalt sulfate crystals. A metal sulfate characterized by a pH of 5.4 or higher when dissolved in water to give a metal concentration of 1% by mass. A metal sulfate characterized in that when dissolved in water to give a metal concentration of 1 mass %, the pH is 6.0 or higher.