Manufacturing method of nickel sulfate aqueous solution

Abstract

To provide a manufacturing method of a nickel sulfate aqueous solution that can reduce heating energy while sufficiently dissolving crude nickel sulfate raw material.SOLUTION: A manufacturing method of a nickel sulfate aqueous solution comprises an evaluation step of evaluating the solubility of a sample collected from crude nickel sulfate raw material, and a dissolution step of dissolving the crude nickel sulfate raw material in water at a dissolution treatment temperature set based on the result obtained in the evaluation step to obtain a crude nickel sulfate aqueous solution. The dissolution treatment temperature can be set low within a range where the crude nickel sulfate raw material is sufficiently dissolved according to the solubility of the crude nickel sulfate raw material, and thus heating energy can be reduced.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing an aqueous nickel sulfate solution. More specifically, the present invention relates to a method for producing an aqueous nickel sulfate solution from a crude nickel sulfate raw material. [Background technology]

[0002] The following method is known as a method for producing a high-purity nickel sulfate aqueous solution (for example, Patent Document 1). First, a crude nickel sulfate raw material is dissolved to obtain a crude nickel sulfate aqueous solution (dissolution step). Next, the crude nickel sulfate aqueous solution is reacted with a sulfiding agent in a pressurized reaction tank to remove zinc as a sulfide precipitate (sulfidation step). Next, iron contained in the crude nickel sulfate aqueous solution is removed as a neutralized precipitate by a neutralization reaction to obtain a neutralization end solution (neutralization step). Finally, nickel in the neutralization end solution is extracted and stripped by solvent extraction to obtain a high-purity nickel sulfate aqueous solution (solvent extraction step). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2024-56178 Summary of the Invention [Problem to be solved by the invention]

[0004] In the dissolution process, the crude nickel sulfate raw material is dissolved in water to obtain a crude nickel sulfate aqueous solution. At this time, the aqueous solution is heated to promote dissolution of the crude nickel sulfate raw material. If the crude nickel sulfate raw material remains undissolved in the dissolution process, it is discharged as residue outside the system, resulting in nickel loss. Therefore, the dissolution temperature is set so that the crude nickel sulfate raw material does not remain undissolved. The higher the dissolution temperature, the easier it is to dissolve the crude nickel sulfate raw material, but the more energy required for heating, resulting in higher production costs.

[0005] In view of the above circumstances, an object of the present invention is to provide a method for producing an aqueous nickel sulfate solution that can reduce the energy required for heating while sufficiently dissolving a crude nickel sulfate raw material. [Means for solving the problem]

[0006] The method for producing a nickel sulfate aqueous solution of the first aspect is characterized by comprising: an evaluation step of evaluating the solubility of a sample collected from a crude nickel sulfate raw material; and a dissolution step of dissolving the crude nickel sulfate raw material in water at a dissolution treatment temperature set based on the results of the evaluation step to obtain a crude nickel sulfate aqueous solution. The method for producing a nickel sulfate aqueous solution of the second aspect is characterized in that, in the first aspect, the evaluation step includes one or more of a nickel content determining step of determining the nickel content of the sample, an ammonia content determining step of determining the ammonia content of the sample, and a dissolution test step of dissolving a reference weight of the sample in water at a reference temperature for a reference time to test whether or not any residue remains. A third aspect of the method for producing a nickel sulfate aqueous solution is the same as that of the second aspect, wherein the dissolution temperature in the dissolution step is a first temperature when the nickel content of the sample is equal to or greater than a reference nickel content; a second temperature that is equal to or lower than the first temperature when the nickel content of the sample is less than the reference nickel content, the ammonia content of the sample is equal to or greater than the reference ammonia content, and the sample remains undissolved in the dissolution test step; a third temperature that is lower than the second temperature when the nickel content of the sample is less than the reference nickel content, the ammonia content of the sample is equal to or greater than the reference ammonia content, and the sample completely dissolves in the dissolution test step; and a fourth temperature that is equal to or lower than the third temperature when the nickel content of the sample is less than the reference nickel content and the ammonia content of the sample is less than the reference ammonia content. A fourth aspect of the method for producing an aqueous nickel sulfate solution is the method for producing an aqueous nickel sulfate solution of the third aspect, characterized in that the first temperature is 79°C or more and 90°C or less, the second temperature is 45°C or more and 90°C or less, the third temperature is 35°C or more and 40°C or less, and the fourth temperature is 20°C or more and 40°C or less. A fifth aspect of the method for producing an aqueous nickel sulfate solution is characterized in that, in the third or fourth aspect, the standard nickel content is a value set within a range of 22 to 26% by weight (dry weight basis), the standard ammonia content is a value set within a range of 0.1 to 0.5% by weight (dry weight basis), the standard temperature is a value set within a range of 40 to 45°C, the standard time is a value set within a range of 0.5 to 1.5 hours, and the standard weight is a value set within a weight range such that the specific gravity of an aqueous nickel sulfate solution obtained by dissolving nickel sulfate hexahydrate in water is 1.35 to 1.42. [Effects of the Invention]

[0007] According to the present invention, the dissolution treatment temperature can be set low in accordance with the solubility of the crude nickel sulfate raw material within a range in which the crude nickel sulfate raw material is sufficiently dissolved, thereby reducing the energy required for heating. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is an overall process diagram showing a method for producing a nickel sulfate aqueous solution according to one embodiment. [Figure 2] 1 is a flowchart of an evaluation process in one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Next, an embodiment of the present invention will be described with reference to the drawings. 1, a method for producing a nickel sulfate aqueous solution according to one embodiment of the present invention includes a dissolving step S1, a sulfurizing step S2, a neutralizing step S3, and a solvent extraction step S4. However, it is sufficient that at least the dissolving step S1 is performed, and the other steps S2, S3, and S4 may be performed as needed. Furthermore, other steps may be added.

[0010] As the raw material for crude nickel sulfate, for example, crude nickel sulfate crystals obtained as a by-product of copper smelting are used. The raw material for crude nickel sulfate contains impurities such as zinc and iron.

[0011] In the dissolution step S1, a crude nickel sulfate raw material is dissolved in water to obtain a crude nickel sulfate aqueous solution. The dissolution step S1 is performed as a batch process. Specifically, a predetermined amount of crude nickel sulfate raw material is charged into a dissolution tank containing water. The crude nickel sulfate raw material is then dissolved while heating the aqueous solution in the dissolution tank to a set dissolution temperature. The water used to dissolve the crude nickel sulfate raw material can be industrial water or a low-concentration nickel sulfate aqueous solution generated in the system. The heating method is not particularly limited, and steam injection or electrical heating may be used. In addition, an alkali such as calcium carbonate or sodium hydroxide is added to the dissolution tank to neutralize the sulfuric acid contained in the crude nickel sulfate raw material and adjust the pH of the crude nickel sulfate aqueous solution. The dissolution time per batch is not particularly limited, but is set within a range of 2 to 20 hours. The crude nickel sulfate aqueous solution obtained in the dissolution step S1 contains impurities such as zinc, iron, and cadmium.

[0012] In the sulfurization step S2, a sulfurizing agent is added to the aqueous solution of crude nickel sulfate in a pressurized reaction tank to generate a sulfide precipitate containing zinc, cadmium, etc., and obtain a sulfide slurry. Hydrogen sulfide gas, for example, is used as the sulfurizing agent. The sulfide precipitate is removed from the sulfide slurry after the reaction by solid-liquid separation to obtain a final sulfurization solution (aqueous solution of nickel sulfate containing impurities). Note that if any crude nickel sulfate raw material remains undissolved in the dissolution step S1, the undissolved crude nickel sulfate raw material is removed together with the sulfide precipitate and discharged outside the system. Therefore, if any crude nickel sulfate raw material remains undissolved, it results in nickel loss.

[0013] In the neutralization step S3, air is blown into the sulfurization end solution and an alkali is added to generate a neutralized precipitate of iron and the like through an oxidation-neutralization reaction, yielding a neutralized slurry. Calcium hydroxide, for example, is used as the alkali. The neutralized slurry after the reaction is subjected to solid-liquid separation to remove the neutralized precipitate, yielding a neutralization end solution (aqueous nickel sulfate solution containing impurities). The neutralized precipitate may be repulped with sulfuric acid to dissolve the nickel hydroxide contained in the neutralized precipitate, and the filtrate obtained after filtration may be repeated in the dissolution step S1. This reduces nickel loss even if nickel co-precipitates in the neutralization step S3.

[0014] In the solvent extraction step S4, the nickel contained in the final neutralization solution is extracted into an organic solvent, and then the nickel is stripped from the organic solvent by substitution with cobalt ions or the like to obtain a high-purity aqueous nickel sulfate solution.

[0015] As described above, if any crude nickel sulfate raw material remains undissolved in the dissolving step S1, it will be discharged outside the system together with sulfide precipitate, resulting in nickel loss. Therefore, the dissolving temperature is set so that no undissolved crude nickel sulfate raw material remains. The higher the dissolving temperature, the easier it is to dissolve the crude nickel sulfate raw material, but the more energy required for heating, resulting in higher production costs.

[0016] There are various types of crude nickel sulfate raw materials, each with different compositions, such as impurity content and nickel content. The present inventors have discovered that the solubility of a crude nickel sulfate raw material varies depending on its composition. For example, a raw material containing nickel sulfate monohydrate (NiSO4·H2O) has lower solubility (less soluble) than a raw material that does not substantially contain it. Furthermore, a raw material containing nickel ammonium sulfate hexahydrate ((NH4)2(Ni(H2O)6)(SO4)2) has lower solubility than a raw material that does not substantially contain it. Furthermore, a raw material containing both nickel ammonium sulfate hexahydrate and nickel sulfate heptahydrate (NiSO4·7H2O) has lower solubility than a raw material that contains nickel ammonium sulfate hexahydrate but does not substantially contain nickel sulfate heptahydrate. Thus, nickel sulfate monohydrate, nickel ammonium sulfate hexahydrate, and nickel sulfate heptahydrate are factors that determine the solubility of a crude nickel sulfate raw material.

[0017] When dissolving a crude nickel sulfate raw material with low solubility, it is necessary to set the dissolution temperature high so as not to leave any undissolved material. On the other hand, when dissolving a crude nickel sulfate raw material with high solubility, no undissolved material will remain even if the dissolution temperature is set low. Setting the dissolution temperature low allows for a corresponding reduction in heating energy.

[0018] Therefore, a sample is taken from the crude nickel sulfate raw material, and the solubility of the crude nickel sulfate raw material is evaluated using the sample (evaluation step). This allows the crude nickel sulfate raw material to be classified. Then, the dissolution treatment temperature in the dissolving step S1 is set based on the evaluation results. If the dissolution treatment temperature is set low within a range in which the crude nickel sulfate raw material is sufficiently dissolved according to the solubility of the crude nickel sulfate raw material, the heating energy can be reduced.

[0019] The evaluation step is carried out, for example, according to the procedure shown in FIG. First, the nickel content of the sample is determined (nickel content determination step). The method is not particularly limited, but for example, electrolytic gravimetry can be used. Determining the nickel content by electrolytic gravimetry is performed in the following procedure. First, the sample is dissolved in acid to obtain a solution. Next, a platinum electrode is immersed in the solution, and a current is passed through it to electrodeposit nickel onto the platinum electrode. The increase in the weight of platinum is taken as the weight of nickel, and the nickel content is calculated.

[0020] The nickel content of a sample is used as an index to determine whether or not the sample contains nickel sulfate monohydrate. Samples containing nickel sulfate monohydrate tend to have a relatively high nickel content. Therefore, if the nickel content of a sample is equal to or greater than the reference nickel content, the sample is determined to contain a predetermined proportion of nickel sulfate monohydrate or more. Here, the reference nickel content is set to a value appropriate for determining whether or not the sample contains nickel sulfate monohydrate at a proportion that significantly reduces solubility. The reference nickel content is preferably set to a value within the range of 22 to 26 wt% (dry weight basis), for example, 26 wt%.

[0021] When the nickel content of the sample is equal to or greater than the reference nickel content, the dissolution temperature in the dissolution step S1 is set to the first temperature. The first temperature is set to a relatively high temperature so that even nickel sulfate monohydrate, which has low solubility, can be sufficiently dissolved. The first temperature is preferably 79°C or higher and 90°C or lower. However, since the lower the dissolution temperature, the more energy required for heating can be reduced, the more preferably the first temperature is 85°C or lower.

[0022] If the nickel content of the sample is less than the reference nickel content, it is determined that the sample does not substantially contain nickel sulfate monohydrate. In this case, the ammonia content of the sample is then determined (ammonia content determination step). The method is not particularly limited, but for example, Kjeldahl spectrophotometry can be used. The ammonia content determination by Kjeldahl spectrophotometry is performed in the following procedure. First, a solution is obtained by dissolving the sample in acid. An alkali is added while the solution is heated, and water vapor is blown in to distill ammonia gas. The ammonia is collected in acid, and the ammonia concentration is measured by spectrophotometry to determine the ammonia content.

[0023] The ammonia content of a sample is used as an indicator to determine whether the sample contains nickel ammonium sulfate hexahydrate. If the ammonia content of a sample is equal to or greater than the reference ammonia content, the sample is determined to contain a predetermined proportion or more of nickel ammonium sulfate hexahydrate. Here, the reference ammonia content is set to a value that significantly reduces solubility and is suitable for determining whether the sample contains nickel ammonium sulfate hexahydrate. The reference ammonia content is preferably set within the range of 0.1 to 0.5 wt% (dry weight basis), for example, 0.3 wt%.

[0024] If the ammonia content of the sample is equal to or greater than the reference ammonia content, a reference weight of the sample is dissolved in water at a reference temperature for a reference time to determine whether any residual material remains (dissolution test process). The dissolution test is performed, for example, using the following procedure. First, a water bath filled with water is placed on a magnetic stirrer, and a thermometer is attached. Next, water and the sample are placed in a beaker in the water bath. The amount of sample placed in the beaker is taken as the reference weight. For example, 400 mL of pure water is added to a 500 mL beaker, and 148 g of sample is added. A stir bar is placed in the beaker, and stirring begins. The water bath is turned on and the temperature is raised to the reference temperature. Once the reference temperature is reached, the sample is dissolved for the reference time while maintaining the reference temperature. Subsequently, visual observation is performed to determine whether any residual material remains in the beaker or whether it has completely dissolved. For example, if the liquid in the beaker is cloudy, it is determined that the sample remains, and if it is not cloudy, it is determined that the sample has completely dissolved.

[0025] Here, the reference temperature is preferably a value set within the range of 40 to 45° C., for example, 45° C. The reference time is preferably a value set within the range of 0.5 to 1.5 hours, for example, 1 hour.

[0026] The reference weight is set within the range of 1.35–1.42 for the specific gravity of the nickel sulfate solution obtained by dissolving nickel sulfate hexahydrate in water. If the concentration of the aqueous solution after dissolving the sample approaches the solubility of the nickel sulfate hexahydrate, some of the nickel sulfate will remain. Therefore, it is recommended to set the amount of sample so that the concentration of the resulting nickel sulfate solution is approximately 0.7–0.9 times the solubility of nickel sulfate hexahydrate (NiSO4·6H2O). For example, the solubility of nickel sulfate hexahydrate at 40°C is 49.2 g per 100 g of water. If the reference temperature is set to 40°C, 34.4–44.2 g of sample should be added per 100 g of water.

[0027] If the sample remains undissolved in the dissolution test, it is determined to have relatively low solubility among samples containing nickel ammonium sulfate hexahydrate. For example, a sample containing both nickel ammonium sulfate hexahydrate and nickel sulfate heptahydrate. In this case, the dissolution temperature in the dissolution step S1 is set to the second temperature. The second temperature is equal to or lower than the first temperature. The second temperature is preferably 45°C or higher and 90°C or lower. However, since a lower dissolution temperature can reduce the heating energy required, the second temperature is more preferably 70°C or lower, and even more preferably 50°C or lower.

[0028] On the other hand, if the sample is completely dissolved in the dissolution test, it is determined to have a relatively high solubility among samples containing nickel ammonium sulfate hexahydrate. For example, the sample contains nickel ammonium sulfate hexahydrate and is substantially free of nickel sulfate heptahydrate. In this case, the dissolution treatment temperature in the dissolution step S1 is referred to as the third temperature. The third temperature is a temperature lower than the second temperature. The third temperature is preferably 35°C or higher and 40°C or lower.

[0029] If the ammonia content of the sample identified in the ammonia content identification step is less than the reference ammonia content, the sample is determined to be substantially free of nickel ammonium sulfate hexahydrate. In this case, the dissolution temperature in the dissolution step is set to the fourth temperature. The fourth temperature is the same as or lower than the third temperature. The fourth temperature is preferably 20°C or higher and 40°C or lower. However, since the lower the dissolution temperature, the more energy required for heating can be reduced, the fourth temperature is more preferably 30°C or lower, and even more preferably 25°C or lower.

[0030] As shown in Figure 2, only the nickel content determining step, ammonia content determining step, and dissolution test step that are necessary to evaluate the solubility of the sample need to be performed. Depending on the type of sample, one or more of these steps may be performed. For example, if the nickel content of the sample determined in the nickel content determining step is equal to or greater than the reference nickel content, it is not necessary to perform the ammonia content determining step and the dissolution test step.

[0031] To summarize the above, if the nickel content of the sample is equal to or greater than the reference nickel content, the dissolution temperature is the first temperature. If the nickel content of the sample is less than the reference nickel content, the ammonia content is equal to or greater than the reference ammonia content, and the sample remains undissolved in the dissolution test, the dissolution temperature is the second temperature. If the nickel content of the sample is less than the reference nickel content, the ammonia content is equal to or greater than the reference ammonia content, and the sample completely dissolves in the dissolution test, the dissolution temperature is the third temperature. If the nickel content of the sample is less than the reference nickel content and the ammonia content is less than the reference ammonia content, the dissolution temperature is the fourth temperature.

[0032] The evaluation process has been described based on the flowchart shown in Figure 2, but this flowchart merely shows one example of a systematic and rational evaluation method. The evaluation process of the present invention is not limited to the flowchart shown in Figure 2. The evaluation process may include one or more of a nickel content determination step, an ammonia content determination step, and a dissolution test step.

[0033] In the dissolution step S1, the crude nickel sulfate raw material is dissolved in water at the dissolution treatment temperature set as above to obtain a crude nickel sulfate aqueous solution. Since the dissolution treatment temperature can be set low in accordance with the solubility of the crude nickel sulfate raw material within a range in which the crude nickel sulfate raw material is sufficiently dissolved, it is possible to reduce the energy required for heating. [Example]

[0034] Next, an example will be described. Four types of crude nickel sulfate raw materials A, B, C, and D were prepared. Samples taken from each of the four raw materials were designated as samples A, B, C, and D.

[0035] The nickel content and ammonia content of each of samples A, B, C, and D were determined. The nickel content was determined by electrolytic gravimetry. The ammonia content was determined by Kjeldahl spectrophotometry. The results are shown in Table 1.

[0036] XRD analysis was performed on each of samples A, B, C, and D. As a result, a peak for nickel sulfate hexahydrate was confirmed in all of samples A, B, C, and D. Nickel sulfate hexahydrate is the main component of crude nickel sulfate raw material. A peak for nickel sulfate monohydrate was confirmed in sample A, while no peak for nickel sulfate monohydrate was confirmed in the other samples B, C, and D. The nickel content of sample A was 27 wt%, which is higher than that of the other samples B, C, and D. This confirmed that the nickel content can be used as an indicator to determine whether a sample contains nickel sulfate monohydrate.

[0037] Furthermore, peaks for nickel ammonium sulfate hexahydrate were confirmed in samples B and C, but no peaks for nickel ammonium sulfate hexahydrate were confirmed in sample D. The ammonia contents of samples B and C were 0.56 wt % and 1.23 wt %, respectively, which are higher than the ammonia content of 0.04 wt % of sample D. This confirmed that the ammonia content can be used as an indicator to determine whether or not a sample may contain nickel ammonium sulfate hexahydrate.

[0038] Strictly speaking, the ammonia content determined by Kjeldahl spectrophotometry is the ammonia equivalent of the total nitrogen content. In other words, non-ammonia nitrogen is also counted in the ammonia content. Furthermore, even if ammonia is identified, it does not necessarily mean that the ammonia exists in the form of nickel ammonium sulfate hexahydrate. Therefore, an ammonia content determined by Kjeldahl spectrophotometry equal to or greater than the reference ammonia content is a necessary but not sufficient condition for confirming the presence of nickel ammonium sulfate hexahydrate. In other words, an ammonia content determined by Kjeldahl spectrophotometry equal to or greater than the reference ammonia content serves as a qualitative solubility indicator, but not a quantitative solubility indicator. Therefore, after screening based on ammonia content, the solubility of the sample is finally confirmed by a dissolution test.

[0039] Dissolution tests were conducted on samples B and C. Here, the reference temperature was 45°C and the reference time was 1 hour. As a result, some of the sample B remained undissolved. On the other hand, sample C was completely dissolved. As a result of XRD analysis, a peak for nickel sulfate heptahydrate was confirmed for sample B, but no peak for nickel sulfate heptahydrate was confirmed for sample C. From this, it can be inferred that a raw material containing both nickel ammonium sulfate hexahydrate and nickel sulfate heptahydrate has lower solubility than a raw material containing nickel ammonium sulfate hexahydrate but substantially no nickel sulfate heptahydrate.

[0040] [Table 1]

[0041] Samples A, B, C, and D were dissolved in water at dissolution temperatures of 79°C, 45°C, 35°C, and 20°C, respectively. As a result, it was confirmed that all of Samples A to D were completely dissolved without any remaining residue after 4 hours. This confirmed that solubility differs depending on the type of crude nickel sulfate raw material, and that highly soluble crude nickel sulfate raw materials are completely dissolved even when the dissolution temperature is set low.

[0042] Next, the crude nickel sulfate raw material was dissolved using the dissolving equipment of the nickel sulfate aqueous solution manufacturing plant. 3 Hereinafter, one dissolution tank will be referred to as the first tank and the other as the second tank.

[0043] Example 1 7.2m in the first tank 3 A predetermined amount of water was stored in the first tank, and raw materials A and B were added. Calcium carbonate slurry and sodium hydroxide were added to the first tank, and the pH was adjusted to 1 to 5. The temperature was raised to 79°C by blowing in steam, and dissolution treatment was carried out while maintaining that temperature.

[0044] 7.2m in the second tank 3A predetermined amount of water was stored in the second tank, and raw materials C and D were added. A calcium carbonate slurry and sulfuric acid were added to the second tank, and the pH was adjusted to 1 to 5. The temperature was raised to 35°C by blowing in steam, and dissolution treatment was carried out while maintaining that temperature.

[0045] (Comparative Example 1) 7.2m in the first tank 3 A predetermined amount of water was stored in the first tank, and a predetermined amount of raw material A was added. Calcium carbonate slurry and sodium hydroxide were added to the first tank, and the pH was adjusted to 1 to 5. The temperature was raised to 79°C by blowing in steam, and dissolution treatment was carried out while maintaining that temperature.

[0046] 7.2m in the second tank 3 A predetermined amount of water was stored in the second tank, and raw materials B, C, and D were added. Calcium carbonate slurry and sulfuric acid were added to the second tank, and the pH was adjusted to 1 to 5. The temperature was raised to 45°C by blowing in steam, and dissolution treatment was carried out while maintaining that temperature.

[0047] The raw materials were sufficiently dissolved in both Example 1 and Comparative Example 1. However, the amount of steam used for heating per melting was reduced by 54 kg in Example 1 compared to Comparative Example 1. This confirmed that the energy required for heating could be reduced by lowering the melting temperature.

Claims

1. an evaluation step of evaluating the solubility of a sample collected from the crude nickel sulfate raw material; a dissolving step of dissolving the crude nickel sulfate raw material in water at a dissolving treatment temperature set based on the results of the evaluation step to obtain a crude nickel sulfate aqueous solution.

1. A method for producing a nickel sulfate aqueous solution comprising the steps of:

2. The evaluation step includes: a nickel content determination step of determining the nickel content of the sample; an ammonia content determining step of determining the ammonia content of the sample; and a dissolution test step of dissolving a standard weight of the sample in water at a standard temperature for a standard time to determine whether or not any residue remains; Having one or more of the following:

2. The method for producing an aqueous nickel sulfate solution according to claim 1.

3. When the nickel content of the sample is equal to or greater than a reference nickel content, the dissolution treatment temperature in the dissolution step is set to a first temperature; When the nickel content of the sample is less than the reference nickel content, the ammonia content of the sample is equal to or greater than the reference ammonia content, and the sample remains undissolved in the dissolution test step, the dissolution treatment temperature in the dissolution step is set to a second temperature that is equal to or lower than the first temperature, When the nickel content of the sample is less than the reference nickel content, the ammonia content of the sample is equal to or greater than the reference ammonia content, and the sample is completely dissolved in the dissolution test step, the dissolution treatment temperature in the dissolution step is set to a third temperature that is lower than the second temperature; When the nickel content of the sample is less than the reference nickel content and the ammonia content of the sample is less than the reference ammonia content, the dissolution treatment temperature in the dissolution step is set to a fourth temperature that is equal to or lower than the third temperature.

3. The method for producing an aqueous nickel sulfate solution according to claim 2.

4. the first temperature is equal to or greater than 79°C and equal to or less than 90°C; the second temperature is equal to or greater than 45°C and equal to or less than 90°C; the third temperature is equal to or higher than 35°C and equal to or lower than 40°C; The fourth temperature is 20° C. or higher and 40° C. or lower.

4. The method for producing an aqueous nickel sulfate solution according to claim 3.

5. The reference nickel content is a value set within a range of 22 to 26 wt% (dry weight basis), The standard ammonia content is a value set within a range of 0.1 to 0.5 wt% (dry weight basis), The reference temperature is a value set within the range of 40 to 45°C, The reference time is a value set within a range of 0.5 to 1.5 hours, The reference weight is a value set within a weight range in which the specific gravity of an aqueous solution of nickel sulfate obtained by dissolving nickel sulfate hexahydrate in water is 1.35 to 1.

42.

5. The method for producing an aqueous nickel sulfate solution according to claim 3 or 4.

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