Method for producing high-purity nickel sulfate

Abstract

Provided are: a method for removing magnesium, contained as an impurity, from nickel sulfate; and a method for manufacturing high-purity nickel sulfate. The present invention is a manufacturing method characterized by comprising, as steps for manufacturing an aqueous solution of nickel sulfate in which magnesium has been removed from the nickel sulfate, steps indicated by the following (1) through (3): (1) a carboxylation step in which an aqueous solution of nickel sulfate and lithium carbonate are blended to obtain a slurry that contains nickel carbonate as a solid component; (2) a solid / liquid separation step in which the slurry obtained in the carboxylation step is separated into a solid component and a liquid component; and (3) a dissolution step in which the solid component obtained in the foregoing step is dissolved in a solution that contains sulfuric acid.

Description

Technical Field

[0001] The present invention relates to a method for producing high-purity nickel sulfate. Specifically, the present invention relates to a method for removing magnesium contained as an impurity from nickel sulfate. The present invention can be applied to an aqueous nickel sulfate solution generated in nickel extraction from ore, a recycling process of a lithium-ion secondary battery, an acid treatment process of a lithium nickel composite, or the like.

Background Art

[0002] Nickel sulfate is obtained as a product or a by-product through nickel extraction from ore, a recycling process of a lithium-ion secondary battery, an acid treatment process of a lithium nickel composite, or the like. The nickel sulfate thus obtained is used as a synthetic raw material for a lithium-ion secondary battery positive electrode material, a primary battery positive electrode material, various catalysts, and the like.

[0003] When nickel sulfate is used as a raw material for the above applications, high purity is one of the important physical properties of nickel sulfate. In order to meet this requirement, crystallization methods, solvent extraction methods, precipitation methods using alkali hydroxides, and the like have been developed as purification methods for nickel sulfate.

[0004] With these developed purification methods, it is possible to efficiently remove impurity elements of major polyvalent metals excluding magnesium, but it is difficult to say that an industrial process capable of efficiently removing only magnesium has been established.

[0005] Regarding magnesium removal by the crystallization method, in Non-Patent Document 1, it has been reported that sodium, chlorine, and magnesium tend to be mixed into nickel sulfate crystals by cooling crystallization, and it has been shown that magnesium is more likely to remain in nickel sulfate than other elements. As shown in this example, removing magnesium using the crystallization method is not an efficient technique.

[0006] Regarding magnesium removal by solvent extraction, Patent Document 1 describes a method in which an organic phase holding nickel is brought into countercurrent contact with a crude nickel sulfate solution containing impurities, and the impurities are separated from the nickel by a substitution reaction. Although the amount of magnesium in the nickel solution is reduced by this method, the removal rate is low because the reaction behavior of magnesium is similar to that of nickel. As seen in this example, solvent extraction is not a technique that can efficiently remove magnesium from nickel sulfate.

[0007] Regarding magnesium removal by precipitation, Patent Document 2 describes a method in which an alkaline hydroxide such as calcium hydroxide is added to an aqueous solution of nickel sulfate containing magnesium as an impurity, nickel is recovered as a nickel hydroxide precipitate by solid-liquid separation, magnesium is separated into a filtrate, and this dissolved magnesium is separated and recovered as a precipitate by neutralization.

[0008] This method utilizes the property that nickel and magnesium precipitate at different pH levels in aqueous solutions. However, the pH difference is not significant, and operating this process while avoiding coprecipitation of magnesium and maximizing nickel yield requires extremely high technical skill, making it far from an advantageous method. Furthermore, the nickel hydroxide precipitate obtained by adding alkali hydroxide tends to precipitate as fine particles. When processing large quantities of nickel sulfate, this presents practical challenges, such as reduced efficiency due to slow filtration rates, the need for relatively large solid-liquid separation equipment to achieve appropriate filtration rates, or decreased economic viability due to the introduction of specialized filtration equipment. Therefore, it is not an efficient technology.

[0009] Furthermore, Patent Document 3 discloses a method combining a carbonation step, a solid-liquid separation step, and a neutralization step to selectively separate and remove magnesium contained in an aqueous nickel sulfate solution. While the use of carbonation is expected to make pH adjustment easier and improve the filterability of the precipitate compared to using alkali hydroxide, the problem is that it does not yield the high magnesium removal rate that can be obtained with the pH adjustment method described in Patent Document 2. Another problem is that operating under conditions that increase the magnesium removal rate significantly reduces the yield of nickel, which is the main component. Therefore, although magnesium reduction is possible, it is not an efficient technology for magnesium removal that balances nickel yield and magnesium removal rate.

[0010] Furthermore, Patent Document 4 describes a method combining a hydroxide step, a carbonation step, a solid-liquid separation step, and a neutralization step. Using this technology, the precipitation of hydroxide with alkali hydroxide can be performed under conditions where pH can be easily adjusted, and the small amount of dissolved nickel after this operation is subsequently recovered as a precipitate in the carbonation step. This technology avoids the problem of pH adjustment which requires high control and improves the nickel yield and magnesium removal rate. However, if the amount of carbonation additive used in the carbonation step is increased, the amount of alkali metal originating from the carbonation additive mixed into the process of repeated nickel reuse increases, and the amount of alkali metal incorporated into the nickel increases, so ultimately the nickel sulfate product becomes contaminated with alkali metal. For this reason, the amount of carbonation additive that can be used in this technology is limited, and most of the nickel sulfate must be recovered as nickel hydroxide by adding alkali hydroxide. Consequently, for the reasons mentioned above, the load on the solid-liquid separation step increases, and from a practical standpoint, it is difficult to say that this is an efficient technology.

[0011] When obtaining nickel as a precipitate, it is preferable from the viewpoint of solid-liquid separation to carry out the precipitation operation as a nickel carbonate-containing solid, such as basic nickel carbonate. For example, Patent Document 5 describes a method for producing basic nickel carbonate with excellent filterability by using an alkali carbonate and adding an alkali hydroxide as needed. In the examples in this publication, sodium carbonate is used as the alkali metal carbonate.

[0012] However, as mentioned above, using sodium carbonate as a carbonation additive inevitably leads to the contamination of nickel with alkali metals, namely sodium, making it unsuitable for reuse as a nickel raw material. Thus, it is difficult to efficiently remove magnesium using conventional solvent extraction or crystallization methods, and even with precipitation methods, it is difficult to say that an economical method has been established that simultaneously satisfies the requirements of precipitate filterability, high magnesium removal efficiency, and high nickel yield.

[0013] Not only are there challenges in efficiently removing magnesium, but conventional technologies also present challenges in obtaining high-purity nickel sulfate from nickel sulfate that is contaminated with alkali metals such as sodium after magnesium removal. Solvent extraction is one method for removing sodium from nickel to obtain high-purity nickel sulfate when sodium is present in the nickel. For example, Patent Document 1 discloses a method for reducing impurities in a nickel solution using an exchange reaction. This technique can separate sodium contained in an aqueous nickel sulfate solution. Although the amount of by-products generated is reduced by applying solvent extraction with pH adjustment to a portion of the nickel sulfate that needs to be processed, it is still necessary to repeat the extraction and back-extraction operations at a certain ratio relative to the raw nickel, so it is unavoidable that a large amount of neutralization salt will be generated as a by-product from the acid and alkali used. When sulfuric acid and sodium hydroxide are used as pH adjusters, a large amount of sodium sulfate will be generated. Therefore, the method of removing sodium from an aqueous nickel sulfate solution by solvent extraction is not economically viable because it requires additional processing of a huge amount of by-products.

[0014] Furthermore, as seen in Non-Patent Document 1 mentioned above, a method of removing sodium by crystallization to obtain high-purity nickel sulfate crystals is also conceivable. However, even if high-purity nickel sulfate crystals can be temporarily obtained by crystallization, continuing this operation will concentrate sodium in the crystallization mother liquor, and the amount of sodium mixed into the nickel sulfate crystals will gradually increase. When a certain nickel-sodium concentration ratio is reached, double salts of nickel sulfate and sodium sulfate begin to precipitate, and it becomes impossible to obtain high-purity nickel sulfate crystals by the crystallization operation anymore.

[0015] Therefore, in conventional crystallization purification operations, it is necessary to discharge a certain percentage of the mother liquor, which is concentrated with impurities, from the system. This operation results in the disposal of a large amount of nickel sulfate that has reached saturation concentration, or it being transferred to another impurity removal process, which significantly reduces the economic viability of obtaining high-purity nickel sulfate. For this reason, crystallization is not a suitable technique for economically removing sodium. [Prior art documents] [Patent Documents]

[0016] [Patent Document 1] Japanese Patent Application Publication No. 10-310437 [Patent Document 2] Japanese Patent Publication No. 2013-151717 [Patent Document 3] Japanese Patent Publication No. 2013-203646 [Patent Document 4] Japanese Patent Publication No. 2014-144877 [Patent Document 5] Japanese Unexamined Patent Publication No. 49-91996 [Non-patent literature]

[0017] [Non-Patent Document 1] Ina Beate Jenssen, Seniz Ucar, Oluf Bockman, Ole Morten Dotterud, Jens-Petter Andreassen, "Impurity Uptake During Cooling Crystallization of Nickel Sulfate", Rare Metal Technology 2020, p.191-199 [Overview of the project] [Problems that the invention aims to solve]

[0018] As is clear from the above explanation, various methods have been attempted to remove magnesium impurities from nickel sulfate, but a practical, efficient, and economically viable technology has not yet been established.

[0019] Regarding the technology of recovering high-purity nickel by adding alkali hydroxide or alkali carbonate, which is considered to be superior to the crystallization method or solvent extraction method, it is necessary to realize an economically efficient impurity removal process that can recover nickel in a high yield while efficiently removing magnesium, and to realize a process that facilitates the handling of the precipitate due to the excellent filtration properties of the nickel precipitate obtained in the magnesium separation process. A technology that can achieve these simultaneously has not been established. Furthermore, even when the carbonated nickel is repeatedly used, it is necessary to realize a process that keeps the amount of impurities mixed in the regenerated nickel sulfate at a low level and maintains the quality as high-purity nickel sulfate. A technology that satisfies all of these issues has not been established.

[0020] The present invention has been made in view of the above circumstances, and aims to simultaneously solve these problems related to magnesium removal and high purity, and to significantly improve the efficiency and economy of the production of nickel sulfate from which magnesium impurities have been removed and the production of high-purity nickel sulfate.

Means for Solving the Problems

[0021] As a result of intensive studies to solve the above problems, the present inventors have obtained the following findings.

[0022] The means disclosed by the present invention is a production method characterized by selecting an appropriate chemical species as a carbonation additive for separating nickel and magnesium, and applying a two-stage crystallization method as a production process for nickel sulfate.

[0023] Specifically, lithium carbonate, which has not been practically utilized until now, is used as a carbonation additive. The precipitate obtained in this process has a high settling rate and excellent filterability, making solid-liquid separation easy. The solid recovered by solid-liquid separation contains a certain amount of lithium as an impurity. This precipitate is regenerated into an aqueous nickel sulfate solution using sulfuric acid or an aqueous nickel sulfate solution containing an excess of sulfuric acid. This aqueous solution is then supplied to a process that alternately repeats a concentration crystallization operation and a cooling crystallization operation, where high-purity lithium sulfate crystals are obtained from the concentration crystallization operation and high-purity nickel sulfate crystals are obtained from the cooling crystallization operation.

[0024] Since lithium sulfate and nickel sulfate are dissolved in the cooled crystallization mother liquor, this mother liquor is recycled for concentrated crystallization. Because lithium sulfate and nickel sulfate do not form double salts, concentrated crystallization and cooled crystallization can be operated continuously with high efficiency without substantial loss of raw materials. The absence of by-product generation is also a factor that enables highly efficient continuous operation.

[0025] After the reaction between the lithium carbonate aqueous solution and the raw material solution, solid-liquid separation yields a liquid solution containing dissolved magnesium, trace amounts of nickel, and lithium. Lithium hydroxide is added to this solution, and the magnesium and trace amounts of nickel are recovered as solid components. The dissolved lithium sulfate can then be introduced into the concentration and crystallization process after appropriate pH adjustment.

[0026] In other words, the first gist of the present invention lies in a manufacturing method characterized by comprising the steps (1) to (3) below as a step to produce an aqueous nickel sulfate solution from which magnesium has been removed from nickel sulfate. (1) Carbonation process to obtain a slurry containing nickel carbonate by mixing an aqueous nickel sulfate solution with lithium carbonate. (2) A solid-liquid separation step for separating the slurry obtained in the carbonation step into solid and liquid. (3) Dissolution step in which the solid obtained in the above step is dissolved in a solution containing sulfuric acid.

[0027] The second gist of the present invention is a method for producing an aqueous nickel sulfate solution as described in the first gist, further comprising a concentration and crystallization step to obtain a slurry in which lithium sulfate is the solid component by concentration and crystallization of the aqueous lithium-containing nickel sulfate solution obtained in the dissolution step of (3) dissolving in a solution containing sulfuric acid, and a solid-liquid separation step to separate the slurry obtained in the concentration and crystallization step into solid and liquid to obtain the solid component of lithium sulfate crystals and the crystallization mother liquor.

[0028] The third gist of the present invention is a method for producing an aqueous solution of nickel sulfate as described in the second gist of the present invention, further comprising a cooling crystallization step of obtaining a slurry in which nickel sulfate is the solid component by cooling crystallization of the crystallization mother liquor separated in the concentration crystallization step, and a solid-liquid separation step of separating the slurry obtained from the cooling crystallization into solid and liquid to obtain the solid component of nickel sulfate crystals and the crystallization mother liquor, thereby extracting nickel sulfate as crystals.

[0029] The fourth gist of the present invention lies in the method for producing an aqueous nickel sulfate solution as described in the second or third gist, which includes the operation of returning the crystallized mother liquor separated in the cooling crystallization step back to the concentration crystallization step.

[0030] The fifth gist of the present invention lies in a method for producing an aqueous nickel sulfate solution as described in any of the second to fourth philosophies, which involves performing pH adjustment and solid-liquid separation on the liquid obtained in the solid-liquid separation step after the carbonation step to obtain a solution from which dissolved carbon dioxide and polyvalent metals have been removed, and introducing the obtained solution into the concentration and crystallization step.

[0031] The sixth gist of the present invention lies in the method for producing an aqueous nickel sulfate solution described in any of the second to fifth philosophies, wherein the operating temperature in the concentration crystallization step is 40°C or higher.

[0032] The seventh gist of the present invention lies in the method for producing an aqueous nickel sulfate solution described in any of the third to sixth philosophies, wherein the operating temperature in the cooling crystallization step is 20°C or more lower than the operating temperature in the concentration crystallization step. [Effects of the Invention]

[0033] In the carbonation process according to the present invention, a solid containing nickel carbonate is obtained as a precipitate. By appropriately controlling the reaction equivalent and reaction temperature in this process, it is possible to recover nickel as a solid in high yield and prevent magnesium from co-precipitating with the solid simultaneously and easily.

[0034] One of the effects of this invention is that nickel can be recovered in high yield and with high purity even when the amount of lithium carbonate added is less than the theoretical equivalent. In conventional techniques, it was necessary to add an equivalent or more of a carbonating agent to obtain basic nickel carbonate that recovers nickel in high yield, but this is unnecessary to recover the solid content obtained in this invention, making it possible to obtain nickel carbonate-containing solid content more economically.

[0035] The nickel carbonate-containing solid obtained in this way has a large aggregate particle size, resulting in a high sedimentation velocity and excellent filterability, allowing for easy solid-liquid separation. Therefore, the solid can be efficiently recovered even with a general-purpose filtration system.

[0036] This nickel carbonate-containing solid is further dissolved in an aqueous solution containing sulfuric acid. This solution contains nickel sulfate and a trace amount of lithium sulfate. By performing a concentration crystallization operation on this aqueous solution, lithium sulfate crystals are obtained as solid matter, and nickel sulfate is concentrated in the mother liquor. By performing appropriate washing on the solid matter, high-purity lithium sulfate crystals can be obtained. The lithium sulfate obtained in this process is of a quality suitable for reuse as a raw material for producing lithium carbonate and lithium hydroxide.

[0037] The concentrated crystallization mother liquor is then transferred to a cooling crystallization process, where nickel sulfate is obtained as crystals. By performing appropriate washing on these crystals, high-purity nickel sulfate crystals can be obtained.

[0038] Because no by-products are generated during the crystallization process, and lithium sulfate and nickel sulfate do not form double salts, it is possible to repeatedly perform concentrated crystallization and cooling crystallization, resulting in a highly economical crystallization process in which virtually no nickel or lithium is lost from the product.

[0039] The liquid produced during the carbonation process, after the magnesium component is removed, becomes a lithium sulfate solution, which can then be introduced into the concentration and crystallization process. Therefore, the wastewater treatment from the carbonation process and the recovery of lithium sulfate, a valuable material, can be handled in a single process, simplifying the process and increasing its economic efficiency.

[0040] The process of dissolving nickel carbonate-containing solids in sulfuric acid generates carbon dioxide. To reduce carbon dioxide emissions, the carbon dioxide can be absorbed by a reaction with lithium hydroxide to synthesize lithium carbonate. The lithium carbonate obtained in this way can be reused as a carbonation additive for nickel sulfate. Therefore, carbon dioxide is repeatedly used within the process, and the amount continuously emitted outside the process can be significantly reduced. [Brief explanation of the drawing]

[0041] [Figure 1] This is a production flow chart for high-purity nickel sulfate and high-purity lithium sulfate according to the present invention. [Figure 2] This figure shows the relationship between the magnesium removal rate and the carbonation temperature in the solid content obtained in the carbonation step in an example using the present invention. [Figure 3] This figure shows the relationship between the yield of solids obtained in the carbonation step and the carbonation temperature in an example using the present invention. [Figure 4] This figure shows the relationship between the yield of solids obtained in the carbonation process and the magnesium removal rate for examples using the present invention and comparative examples using known techniques. [Modes for carrying out the invention]

[0042] To illustrate possible embodiments of the present invention, a manufacturing flow consisting of a carbonation step, a decarboxylation step, a neutralization step, a dissolution step, a concentration crystallization step, a cooling crystallization step, and a solid-liquid separation step is given as an example. However, the combination of unit operations that constitute the actual process is not limited to this example, and those skilled in the art who have experience in such art can make modifications without departing from the spirit of the present invention.

[0043] The following explanation will follow the flowchart shown in Figure 1. The carbonation step involves mixing nickel sulfate containing magnesium as an impurity with an aqueous lithium carbonate solution to precipitate nickel carbonate-containing solids. At this time, the equivalent ratio of lithium carbonate to nickel is preferably 1 or less, and more preferably 0.9 or less. With this control, the pH of the mixed slurry becomes 8 or less, and if the equivalent ratio is 0.86, the pH becomes 7.3 or less.

[0044] The deposition temperature is preferably 50°C or higher. While the magnesium removal rate is not significantly affected by temperature differences, the nickel yield does change. Therefore, from the viewpoint of nickel yield, a temperature of 70°C or higher is more preferable. Although it is possible to operate at temperatures higher than 70°C, there are many limitations on the materials and equipment design that can be used, so it is more advantageous to carry out the operation in the temperature range of around 70°C. From the above viewpoint, the upper limit of the deposition temperature is preferably 110°C.

[0045] The concentration of the raw material solution can be determined arbitrarily, but higher nickel sulfate concentrations are preferable for greater efficiency. At room temperature, a solution with a concentration of 26% by weight of nickel sulfate can be easily prepared. A raw material solution with a higher concentration of dissolved nickel sulfate may also be prepared by heating it to a higher temperature. If the reaction is performed at 70°C, the raw material solution may also be heated to 70°C and dissolved, for example, 35% by weight of nickel sulfate. The upper limit of the concentration of the raw material solution is preferably below the saturation concentration at the temperature at which the solution is prepared, allowing for stable handling.

[0046] Since nickel carbonate-containing solids tend to precipitate, the reaction vessel must be properly stirred. Any known stirring method can be used and selected as appropriate.

[0047] The carbonation process may be carried out in batch, continuous, or semi-batch manner. However, it is preferable to ensure a residence time or reaction time of at least one hour for the slurry in the reaction vessel. If this time is too short, the reaction between lithium carbonate and nickel sulfate may not be completed. If the residence time or reaction time of the slurry is too long, the reaction will be completed, but it will be less efficient in terms of time, so it is usually sufficient to ensure a residence time of five hours or less.

[0048] During the reaction process, it is undesirable to exceed the above-mentioned equivalence ratio, even locally. This is because if nickel sulfate is mixed with an excess amount of lithium carbonate, magnesium will coprecipitate. However, if the raw material concentrations are known in advance, maintaining a predetermined flow rate and addition amount can be easily achieved with an appropriate flow meter and flow control device, such as a control valve. For the same reason, adding an aqueous solution of nickel sulfate to an aqueous solution of lithium carbonate is undesirable. However, it is possible to add lithium carbonate solution to an aqueous solution of nickel sulfate up to a predetermined equivalence ratio. Alternatively, the lithium carbonate solution and nickel sulfate solution may be mixed up to an amount below the predetermined equivalence ratio, and then a small amount of lithium carbonate solution may be added while monitoring the pH.

[0049] The nickel carbonate-containing solid obtained in the carbonation process is separated into solid and liquid components in a solid-liquid separation process. Suitable solid-liquid separation equipment can be selected, such as a vacuum filtration system or a pressure filtration system. Conventional methods for recovering nickel carbonate by carbonation differ significantly from the present invention in that a large amount of nickel remains dissolved under conditions that maximize magnesium removal, resulting in a very large amount of nickel not being recovered as liquid along with magnesium in the solid-liquid separation process.

[0050] The solid obtained in this process is regenerated into an aqueous nickel sulfate solution by adding sulfuric acid. The nickel sulfate concentration can be set arbitrarily, but it is preferable to set it to the highest possible concentration in order to favorably proceed with the subsequent concentration and crystallization process. In conventional carbonation methods, sodium derived from the carbonating agent is mixed into the nickel-containing precipitate, and even when attempting to obtain high-purity nickel sulfate by crystallization, the sodium sulfate concentrated in the mother liquor forms a double salt with nickel sulfate, making separation and purification difficult. However, in the present invention, by using lithium carbonate as a carbonation additive, the problem of double salt formation in the crystallization process is solved, and high-purity nickel sulfate can be obtained.

[0051] The lithium-containing nickel sulfate aqueous solution obtained in the dissolution process is subjected to a concentration and crystallization operation using a known method employing either heating, reduced pressure, or a combination of both. Since lithium sulfate's solubility decreases with increasing temperature, it is advantageous to perform the concentration and crystallization operation in a high temperature range. However, excessively high temperatures increase equipment costs, so practically, the temperature is maintained in the range of 40°C to 110°C, preferably 60°C to 90°C.

[0052] The lithium sulfate crystals obtained by the concentration and crystallization process are separated from the solid component using a solid-liquid separation device. A centrifuge is commonly used for this purpose, but other types of devices may also be used. Furthermore, the solid-liquid separation process involves washing the crystals using an aqueous medium such as water, hot water, or a highly purified aqueous solution of lithium sulfate. Since a washing solution that does not readily dissolve lithium sulfate, such as ethanol, can also be used, the selection of the washing solution should be made considering the trade-off with the increased cost of wastewater treatment. If washing is done with water, hot water, or an aqueous solution of lithium sulfate, the washing wastewater can be returned directly to the concentration and crystallization process.

[0053] A portion of the concentrated crystallization mother liquor is withdrawn and transferred to a cooling crystallization apparatus. When the solution, whose nickel concentration has been increased by the concentration crystallization process, is cooled, nickel sulfate precipitates as crystals due to changes in solubility.

[0054] These crystals are also washed using appropriate solid-liquid separation and washing equipment. Generally, a centrifuge is used, and a small amount of water, cold water, or a highly purified aqueous solution of nickel sulfate is used as the washing medium. This washing wastewater can be returned to the cooling crystallization process, but since the efficiency of cooling crystallization decreases, it is operationally more advantageous to return it to the concentration crystallization process.

[0055] A portion of the cooled crystallization mother liquor is withdrawn and returned to the concentration crystallization apparatus. The lithium sulfate remaining in the mother liquor will crystallize through the concentration crystallization process, and the nickel sulfate will be concentrated again.

[0056] Since the solubility of nickel sulfate decreases with decreasing temperature, it is preferable to perform the cooling crystallization operation at lower temperatures. However, setting the temperature too low tends to increase cooling costs, so it is generally maintained in the temperature range of 10°C to 60°C. If the difference between the operating temperatures of the concentration crystallization process and the cooling crystallization process is small, the efficiency of crystal precipitation in each process decreases, so it is preferable to set a temperature difference of 30°C or more. For example, if the concentration crystallization is operated at 70°C and the cooling crystallization is operated at 35°C, the heating and cooling load can be reduced.

[0057] Eutectic freeze crystallization can also be applied to the cooling crystallization process. Using this technique, water crystals (ice) are generated as suspended matter during the process of obtaining nickel sulfate crystals as a precipitate, and by separating these solids and liquids, the concentration of the crystallization mother liquor can be achieved simultaneously. As long as the cooling crystallization operation is performed under conditions in which lithium sulfate crystals do not precipitate, the present invention can be realized without departing from its original concept, and the evaporation energy required for the concentration of the solution as a whole can be reduced.

[0058] The liquid generated in the carbonation process and the subsequent solid-liquid separation process contains lithium sulfate and trace amounts of nickel and magnesium. Additionally, a very small amount of unreacted carbonate ions remains, so sulfuric acid is added first to lower the pH and liberate and remove the carbon dioxide. At this time, it is preferable to control the pH to 4 or below. Furthermore, a reduced pressure operation may be performed to accelerate the degassing of the generated carbon dioxide.

[0059] Next, the neutralization step removes trace amounts of dissolved nickel and magnesium as solids. Any alkaline hydroxide can be selected as the neutralizing agent, but lithium hydroxide is preferable if this liquid is to be treated in the crystallization step. If any other alkaline hydroxide is used to supply the liquid to the crystallization step, the impurity concentration in the lithium sulfate obtained in the crystallization step will increase.

[0060] The neutralization step involves adjusting the pH to a level where nickel and magnesium precipitate sufficiently. Preferably, the pH is 8 or higher, and more preferably, 10 or higher.

[0061] The liquid obtained through the neutralization and solid-liquid separation steps is an aqueous lithium sulfate solution. When introducing this solution into the crystallization step, sulfuric acid is added beforehand so that the lithium ions and sulfate ions are stoichiometrically equivalent. The pH of the lithium sulfate solution should be adjusted to approximately 3.5 to 6.0.

[0062] The magnesium content in the high-purity nickel sulfate obtained in this invention is typically 300 mg(Mg) / kg(Ni) or less, preferably 100 mg(Mg) / kg(Ni) or less, as the magnesium content normalized by the nickel content. [Examples]

[0063] The present invention will be described in more detail below with reference to examples relating to the carbonation and crystallization processes. The analytical methods used in the following examples are shown.

[0064] The nickel concentration in the raw material solution and the high-concentration nickel content in the solid recovered after the carbonation process were measured using a known chelate titration method with a copper ion selective electrode.

[0065] The nickel, lithium, and magnesium content at low concentrations was measured using an ICP emission spectrometer iCAP6500 Duo (manufactured by Thermo Fisher Scientific Co., Ltd.).

[0066] The pH of the slurry obtained by carbonation was measured using a pH meter HM-30P (manufactured by Toa DKK Co., Ltd.).

[0067] Examples 1-4: <Separation of nickel and magnesium in the carbonation process and nickel yield> Simulated raw material aqueous solutions were prepared to have a nickel sulfate concentration of 316 g / L and a magnesium sulfate concentration of 371 mg / L. Approximately 40 mL of this solution was measured out and transferred to a 1 L stainless steel container. An aqueous solution of lithium carbonate (concentration shown in Table 1) was prepared as a carbonation additive and added to the above simulated solution over approximately 90 minutes while maintaining the temperatures of 50°C (Example 1), 60°C (Example 2), 70°C (Example 3), and 80°C (Example 4) in the equivalent ratios shown in Table 1. During the preparation and execution of these operations, the contents of the container were kept sufficiently agitated. After the addition was completed, only the liquid was sampled at the predetermined holding time and the amount of magnesium contained in the liquid was analyzed. The pH of the slurry after a holding time of 5 hours was measured. Solid-liquid separation was performed by vacuum filtration using a Buchner funnel, and the resulting solid cake was washed with water. These processing conditions are shown in Table 1. The magnesium content shown in Table 1 is expressed as magnesium element content (mg(Mg) / kg(Ni)) normalized by the nickel element content.

[0068] [Table 1]

[0069] Table 1 shows that the magnesium concentration in the slurry held for 3 hours and 5 hours remained almost unchanged compared to the slurry held for 1 hour after the addition of the lithium carbonate aqueous solution. Therefore, it can be said that the reaction time required to complete the carbonation reaction is less than 1 hour.

[0070] Figure 2 shows the percentage of magnesium that was present in the simulated mother liquor but did not migrate into the nickel precipitate and remained dissolved in the liquid, i.e., the magnesium removal rate relative to the solid content. It can be seen that a high removal rate of approximately 90% was achieved under all treatment conditions.

[0071] Figure 3 shows the percentage of nickel recovered as solid matter, i.e., the nickel yield. It can be seen that the nickel yield is higher at processing temperatures of 70 and 80°C than at 50 and 60°C.

[0072] Comparative Example 1: <Separation of nickel and magnesium and nickel yield in a conventional carbonation process> An experiment was conducted on magnesium removal using sodium carbonate as a carbonation additive, based on conventional techniques. The procedure was the same as in Example 1, except that the reaction vessel temperature was set to 40°C, an aqueous solution of sodium carbonate was used instead of an aqueous solution of lithium carbonate to prepare an additive solution concentration of 3.10% by weight, and the equivalent ratio of sodium carbonate to nickel sulfate was set to 0.68.

[0073] Comparative Example 2: In an experiment similar to Comparative Example 1, the equivalent ratio of sodium carbonate to nickel sulfate was set to 1.18. Figure 4 shows the relationship between the percentage of nickel recovered as solid content and the magnesium removal rate of solid content for the samples obtained in Comparative Examples 1 and 2, along with the results from Examples 1 to 4. In the examples to which the present invention is applied, both a high nickel recovery rate (approximately 80% or more) and a high magnesium removal rate (approximately 80% or more) as solid content are achieved, whereas in the comparative examples using the conventional technology, both cannot be achieved.

[0074] Example 5: <Filtration rate of the solids cake obtained in the carbonation process> The carbonation reaction was carried out in the same manner as in Example 4, except that approximately 75 mL of the raw material solution was used, a 2 L stainless steel container was used as the reaction vessel, and the holding time after the addition of lithium carbonate was set to 3 hours.

[0075] The obtained slurry was subjected to solid-liquid separation by vacuum filtration using a Buchner funnel and Advantec filter paper No. 5C (90 mm in diameter). After recovering all the solids as a cake on the filter paper, approximately 1.8 L of water in total was added to the funnel in three separate additions, and the filtration rate of the wash water was measured, yielding filtration rates of 191 to 257 g / min. These results are shown in Table 2.

[0076] Comparative Example 3: <Filtration rate of solid cake obtained by the alkali hydroxide method based on conventional technology> The precipitation reaction and filtration rate were measured in the same manner as in Example 5, except that 65 mL of the raw material solution was used, a 2 L stainless steel container was used as the reaction vessel, an aqueous lithium hydroxide solution was used as the precipitation additive with an equivalent ratio of 0.9 of the additive, and the amount of washing water was 200 mL. At this time, the lithium concentration in the aqueous lithium hydroxide solution was adjusted to be the same as the lithium concentration in the aqueous lithium carbonate solution in Example 5. The results are shown in Table 2.

[0077] Comparative Example 4: <Filtration rate of solid cake obtained by a conventional carbonation process> The carbonation reaction was carried out in the same manner as in Example 5, except that sodium carbonate was used as a precipitating additive.

[0078] The obtained slurry was subjected to solid-liquid separation by vacuum filtration using a Buchner funnel and Advantec filter paper No. 5C (90 mm in diameter). After recovering all the solids as a cake on the filter paper, approximately 1.8 L of water in total was added to the funnel in three separate additions, and the filtration rate of the wash water was measured, yielding a filtration rate of 116–167 g / min. These results are shown in Table 2.

[0079] [Table 2]

[0080] Table 2 shows that the nickel yield is almost the same in Example 5 and in Comparative Examples 3 and 4. In this case, it is clear that the solids cake obtained in Example 5 has significantly better filterability than that obtained in Comparative Example 3. Furthermore, it is clear that the filterability of Example 5 is significantly superior to that of Comparative Example 4, which used sodium carbonate as an additive.

[0081] Example 6: <Separation of nickel sulfate and lithium sulfate by crystallization> Even when lithium is concentrated in the crystallization mother liquor, lithium sulfate can be separated by the concentration crystallization method according to the present invention, and in order to confirm that high-purity nickel sulfate crystals can be obtained by subsequent cooling crystallization, a simulated mother liquor was prepared from nickel sulfate and lithium sulfate reagents. This simulated mother liquor was prepared to contain 5.08% by weight of nickel sulfate and 1.23% by weight of lithium sulfate, respectively, in terms of metallic nickel.

[0082] A 3.2 L simulated mother liquor was placed in a crystallization vessel equipped with a heat-insulating jacket. To heat the vessel, warm water adjusted to 90-93°C was circulated through the heat-insulating jacket at a flow rate of 5.5 L / min. Furthermore, to maintain the temperature inside the crystallization vessel at 80°C, the absolute pressure inside the vessel was controlled between 35-38 kPa by depressurization, and this operation was continuously performed throughout the concentration crystallization process. In addition, the solution inside the vessel was kept sufficiently agitated throughout the crystallization process.

[0083] When a raw material solution with the same composition as the simulated mother liquor was continuously supplied to the controlled crystallization vessel in this manner, lithium sulfate crystals formed after approximately 5.8 hours. A total of about 18 kg of raw material was supplied over 32 hours. After crystal formation began, the slurry was intermittently withdrawn from the vessel to maintain a solid content concentration of 12% by weight, and solid-liquid separation was performed using a centrifuge. The solid obtained in this operation was washed with a high-purity aqueous lithium sulfate solution.

[0084] After the supply of raw materials for the concentration crystallization was completed, the entire slurry in the crystallization vessel was removed and separated into solid and liquid components using a centrifuge. The resulting liquid was combined with the liquid obtained during the intermittent withdrawal operation during the concentration crystallization process and transferred to a container kept at 80°C, which was used as the raw material solution for the cooling crystallization.

[0085] A 1.52-fold concentrated version of the simulated mother liquor used in concentrated crystallization was used as the starting mother liquor for cooled crystallization, and 3.1 L of this concentrated solution was placed in the crystallization vessel. During cooled crystallization, the temperature of the cooling water flowing through the insulation jacket was controlled to maintain a temperature of 25°C inside the vessel.

[0086] When the raw material solution for cold crystallization was continuously supplied, nickel sulfate crystals precipitated. The raw material for cold crystallization was supplied continuously for approximately 17 hours. During the cold crystallization operation, the slurry was intermittently withdrawn to maintain a nearly constant volume of slurry liquid in the crystallization container. Solid-liquid separation of the withdrawn slurry was performed using a centrifuge, and the resulting solid component was washed with a high-purity nickel sulfate aqueous solution.

[0087] Table 3 shows the analysis results of the crystals obtained from the series of operations, the simulated mother liquor, and the crystallized mother liquor after cooling and crystallization.

[0088] [Table 3]

[0089] Table 3 shows that high-purity lithium sulfate and nickel sulfate crystals were obtained despite the use of a nickel sulfate simulating mother liquor containing a high concentration of lithium sulfate.

[0090] The concentration and crystallization process leads to the concentration of nickel sulfate and lithium sulfate, but since lithium sulfate precipitates as crystals, the lithium ratio in the concentrated crystallization mother liquor decreases. Furthermore, since the cooled crystallization mother liquor has the same nickel-lithium ratio as the raw material solution, it can be returned directly to the concentration and crystallization process and reused repeatedly for the crystallization of lithium sulfate crystals.

[0091] From the above results, it can be seen that by using lithium carbonate as an additive in the carbonation process according to the present invention, nickel and magnesium are effectively separated, and a precipitate with excellent filterability can be obtained. Furthermore, after this precipitate is regenerated into an aqueous nickel sulfate solution, the nickel sulfate and lithium sulfate contained in the aqueous solution are separated by the concentration crystallization and cooling crystallization according to the present invention, and high-purity nickel sulfate can be obtained. Magnesium contained in the raw materials can be removed from the system through a neutralization process, and lithium derived from the carbonation additive is recovered as lithium sulfate in the crystallization process, so impurities do not accumulate in the nickel sulfate purification process and affect the nickel sulfate crystal product. Therefore, nickel sulfate from which magnesium has been removed can be continuously obtained in high yield as an aqueous solution or crystals throughout the entire purification process. [Industrial applicability]

[0092] The present invention's method for producing high-purity nickel sulfate is easily adaptable to existing equipment, can produce nickel sulfate efficiently with high yield, and is extremely economical because it allows for the reuse of chemicals other than the target product generated in each step.

Claims

1. A manufacturing method characterized by comprising the steps (1) to (3) below as a process for producing an aqueous nickel sulfate solution from which magnesium has been removed from nickel sulfate. (1) Carbonation process to obtain a slurry containing nickel carbonate by mixing an aqueous nickel sulfate solution with lithium carbonate. (2) A solid-liquid separation step for separating the slurry obtained in the carbonation step into solid and liquid. (3) Dissolution step in which the solid obtained in the above step is dissolved in a solution containing sulfuric acid.

2. Furthermore, the method for producing an aqueous nickel sulfate solution according to claim 1, further comprising: a concentration and crystallization step to obtain a slurry in which lithium sulfate is the solid component by concentration and crystallization of the aqueous nickel sulfate solution containing lithium obtained in the dissolution step of (3) dissolving in a solution containing sulfuric acid; and a solid-liquid separation step to separate the slurry obtained in the concentration and crystallization step into solid and liquid to obtain the solid component of lithium sulfate crystals and the crystallization mother liquor.

3. Furthermore, the method for producing an aqueous nickel sulfate solution according to claim 2, further comprising a cooling crystallization step of obtaining a slurry in which nickel sulfate is the solid component by cooling crystallization of the crystallization mother liquor separated in the concentration crystallization step, and a solid-liquid separation step of separating the slurry obtained by cooling crystallization into solid and liquid components to obtain the solid component of nickel sulfate crystals and the crystallization mother liquor.

4. A method for producing an aqueous nickel sulfate solution according to claim 3, comprising the step of returning the crystallized mother liquor separated in the cooling crystallization step to the concentration crystallization step.

5. A method for producing an aqueous nickel sulfate solution according to claim 2 or 3, comprising the steps of (2) performing pH adjustment and solid-liquid separation on the liquid obtained in the solid-liquid separation step after the carbonation step to obtain a solution from which dissolved carbon dioxide and polyvalent metals have been removed, and introducing the obtained solution into the concentration and crystallization step.

6. A method for producing an aqueous nickel sulfate solution according to claim 2 or 3, wherein the operating temperature in the concentration crystallization step is 40°C or higher.

7. A method for producing an aqueous nickel sulfate solution according to claim 3 or 4, wherein the operating temperature in the cooling crystallization step is 20°C or more lower than the operating temperature in the concentration crystallization step.

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