Method for manufacturing lithium hydroxide

Abstract

[Problem] To provide an efficient method for manufacturing high-purity lithium hydroxide, the method making it possible to raise the yield rate of lithium hydroxide and to reduce the amount of byproducts generated simultaneously with removing alkali metal impurities. [Solution] A method for producing lithium hydroxide, the method being configured from steps indicated by (A) through (F) below. (A) A step for producing a lithium hydroxide aqueous solution and sulfuric acid from lithium sulfate; (B) a crystallization step for producing lithium hydroxide crystals through crystallization from the lithium hydroxide aqueous solution; (C) a solid-liquid separation step for distilling away a slurry in the crystallization step and separating out the lithium hydroxide crystals, and a cleansing step for cleansing a solid fraction; (D) a carbonation step for distilling away a crystallization mother liquid in the crystallization step and reacting the resultant substance with carbonic acid gas to obtain a slurry including a lithium-containing carbonic acid compound, and a solid-liquid separation step for separating a lithium-containing carbonic solid fraction from the slurry; (E) an acid dissolution step for producing a lithium sulfate aqueous solution from the lithium-containing solid fraction; and (F) a step for reusing the lithium sulfate obtained in the acid dissolution step as a raw material in step (A).

Description

How to manufacture lithium hydroxide

[0001] The present invention relates to a method for producing lithium hydroxide using lithium sulfate as a raw material, and more particularly to a production method for removing sodium and potassium contained as impurities and obtaining lithium hydroxide in high yield.

[0002] In recent years, in order to manufacture batteries with higher performance and higher energy density, the development and practical application of lithium-based battery materials has been progressing not only for secondary batteries but also for primary batteries. As the production volume of such battery materials increases, there is a growing demand for lithium hydroxide, which has better reactivity than the conventionally widely used lithium carbonate.

[0003] Lithium chloride and lithium sulfate are widely used as raw materials for lithium hydroxide production. However, when lithium chloride is used as the raw material, it is necessary to handle hydrochloric acid and chlorine gas as by-products and reaction materials, which inevitably increases the burden on the equipment. In addition, chlorine is easily contaminated into the lithium hydroxide product, and further purification steps are required to use it as high-purity lithium hydroxide, which reduces economic viability and is undesirable.

[0004] Electrochemical membrane separation processes, such as electrodialysis and compartmentalized electrolysis, have been developed and put to practical use as processes for producing lithium hydroxide, but when chloride is contained in the raw material, handling highly corrosive and reactive chloride ions places many restrictions on the types of metals that can be used in electrodes and the types of membranes that can be used in membrane separation, making efficient operation difficult.On the other hand, when lithium sulfate is used as the raw material, the equipment can be constructed using general-purpose materials, which significantly reduces the restrictions on the electrochemical membrane separation process and enables efficient operation.

[0005] In a typical purification process accompanying an electrochemical membrane separation process, separation of polyvalent metals present as impurities can be carried out relatively easily using known techniques, whereas efficient removal of sodium and potassium, which are alkali metals like lithium, is not easy.

[0006] For example, methods that use ion-exchange resins or media that selectively adsorb lithium are known as methods for removing alkali metal impurities such as sodium and potassium from aqueous solutions containing lithium as the main component. However, because high-purity lithium hydroxide is required in the process of regenerating the ion-exchange resin that has adsorbed impurities or in the process of increasing the alkali metal ion selectivity of the ion-exchange resin, there are problems such as a significant decrease in the yield of lithium hydroxide that can be used as a product, the introduction of new impurities during the process of desorbing lithium from the lithium-selective adsorbent, and limitations on the type of reaction solution that can be used for desorption. Therefore, it is difficult to say that an efficient impurity removal technology that can be achieved with a high yield and a minimum number of steps has been established.

[0007] It is a well-known technique that the purity of lithium hydroxide can be increased by a crystallization operation. However, in order to maintain a high purity of the lithium hydroxide crystals obtained as a product, it is necessary to discharge alkali metal impurities such as sodium and potassium that are concentrated in the mother liquor from the system, and an operation to discharge a portion of the crystallization mother liquor from the system is generally required (this operation is called blowing or bleeding, but in this invention it will be called blowing). Because this blowing operation also discharges high-concentration lithium from the system, even though it is possible to purify the lithium hydroxide crystals, it is not possible to increase the yield of lithium hydroxide.

[0008] Patent Document 1 discloses a technique for concentrating brine to precipitate sodium chloride and potassium chloride as a technique for removing alkali metals such as sodium and potassium from a raw material aqueous solution in order to recycle a portion of the crystallization mother liquor to the raw material system and thereby reduce the amount of sodium and potassium remaining in the lithium hydroxide production process. However, this technique is only effective for systems using lithium chloride as a raw material and cannot be used in systems in which lithium sulfate is the main component.

[0009] Furthermore, from the perspective of separating sodium and potassium from lithium, a method is known in which lithium hydroxide is reacted with carbon dioxide to obtain high-purity lithium carbonate from which sodium and potassium have been removed, utilizing a reaction that changes the compound form. Patent Document 2 is an example of this method. However, in order to obtain high-purity lithium carbonate in a high yield, it is necessary to use less carbon dioxide than the equivalent amount. Furthermore, because lithium carbonate dissolves and flows out during the process of washing and separating the reaction mother liquor from the lithium carbonate obtained as a solid, the proportion of lithium in the raw material recovered as purified lithium carbonate is around 70%. Furthermore, if strict washing procedures or repeated purification procedures are performed to obtain lithium carbonate of even higher purity, the lithium yield further decreases. While it is possible to produce lithium hydroxide using lithium carbonate purified in this way as a raw material using known methods, the overall yield of lithium hydroxide relative to the recycled lithium raw material does not exceed the above-mentioned yield of lithium carbonate, which significantly reduces the economic viability of lithium hydroxide production.

[0010] In order to improve the yield of lithium, Patent Document 3 discloses a method in which an aqueous lithium hydroxide solution obtained from an electrodialysis step is concentrated and crystallized, and a part of the mother liquor from this concentrated and crystallized step is extracted and reacted with carbon dioxide to obtain lithium carbonate. However, this method increases the amount of lithium compound that can be used as a product in the form of lithium carbonate, but does not increase the yield of lithium hydroxide obtained from the aqueous lithium solution that is the raw material.

[0011] Generally, the unit price of high-purity lithium hydroxide is higher than that of lithium carbonate, so even a 1% change in the yield of lithium hydroxide can result in a large difference in the resulting economic value. This trend has become more pronounced in recent years as demand for lithium hydroxide has increased. Therefore, it is more important to increase the yield of lithium as lithium hydroxide than to increase the yield of lithium as lithium carbonate.

[0012] Furthermore, a method is known in which lithium carbonate, which is produced together with lithium hydroxide as described above, is reacted with calcium hydroxide to convert it into an aqueous lithium hydroxide solution, which is then returned to the lithium hydroxide concentration and crystallization process. However, the conversion rate to lithium hydroxide is not necessarily high (hence resulting in a decrease in yield), and new impurities are introduced from the calcium hydroxide, reducing the purity of the lithium hydroxide product, making this method difficult to call an efficient lithium recycling method. Furthermore, since a large amount of calcium carbonate is produced as a by-product, the waste disposal costs result in a loss of economic viability, making this method difficult to call an efficient lithium recycling method.

[0013] Therefore, there is a need to establish a method for producing lithium hydroxide using lithium sulfate as a raw material that achieves high yield and efficient removal of impurities.

[0014] JP 2011-518257 A JP 2004-196607 A JP 2021-507864 A

[0015] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a method for efficiently producing high-purity lithium hydroxide, which method increases the yield of lithium hydroxide and enables removal of alkali metal impurities including sodium and potassium, while at the same time extremely reducing the amount of by-products generated, in the production of lithium hydroxide using lithium sulfate as a raw material.

[0016] As a result of extensive investigations conducted by the present inventors to solve the above problems, the following findings were obtained.

[0017] In order to continuously obtain high-purity lithium hydroxide crystals in high yield, it is necessary to configure a process that includes a mechanism for purifying lithium hydroxide, a mechanism for efficiently discharging alkali metal impurities from the system with a minimum number of steps and with a minimum amount of by-products generated, and a mechanism for recovering and recycling lithium with a reduced impurity content in high yield.

[0018] It is well known that crystallization is an effective means for obtaining high-purity lithium hydroxide crystals, but as explained above, it is not a technology that can obtain high-purity lithium hydroxide in high yields through continuous operation. Therefore, in systems that use lithium sulfate as a raw material and have no established technology for recycling lithium hydroxide crystallization mother liquor that is concentrated with impurities, the conventional technical level has been to use a part of the lithium hydroxide crystallization mother liquor as a raw material for lithium carbonate in order to increase the amount of lithium that can be used as a product, as disclosed in the prior art.

[0019] Furthermore, even if lithium hydroxide is produced via lithium carbonate by combining known techniques, as long as the conventional techniques are used, the yield in the step of obtaining high-purity lithium carbonate limits the overall yield of lithium obtained as lithium hydroxide, and therefore, the yield of lithium hydroxide significantly decreases as one tries to obtain a high-purity product. This limitation could not be overcome.

[0020] As a result of extensive investigations, the inventors have found that in a carbonation reaction step using an aqueous lithium hydroxide solution derived from a lithium hydroxide crystallization mother liquor as a raw material, rather than thoroughly removing alkali metal impurities, allowing a certain amount of impurities to be mixed into the recycled lithium, thereby increasing the lithium yield as much as possible in the lithium and alkali metal impurity separation step, and that reusing such recycled raw material in the lithium hydroxide crystallization step through an electrochemical membrane separation step is advantageous for the production of high-purity lithium hydroxide and can significantly increase the yield of lithium hydroxide. In other words, this is a lithium hydroxide production method that could not be achieved by applying conventional technologies alone or simply combining them. Instead, the inventors have developed a method for reusing (recycling) a substance similar to waste liquid, which would have been disposed of as waste in conventional technologies, as a raw material through a special process, and have found a lithium hydroxide production method that can continuously produce high-purity lithium hydroxide in a high yield while removing alkali metal impurities.

[0021] Because lithium hydrogen carbonate has a higher solubility than lithium carbonate, sodium hydrogen carbonate has a lower solubility than sodium carbonate, and potassium hydrogen carbonate has a lower solubility than potassium carbonate, in the prior art, in order to obtain high-purity lithium carbonate in as high a yield as possible, it was necessary to control the reaction pH in the carbonation step so that unreacted lithium hydroxide remained in the reaction mother liquor in the carbonation reaction, that is, to prevent the reaction pH from decreasing too much. However, in the carbonation step of the present invention, conditions can be selected that optimize the lithium yield rather than the degree of purification by carbonation, and therefore the yield of lithium-containing carbonate compounds including lithium carbonate can be increased.

[0022] Furthermore, in order to recover lithium carbonate and lithium hydrogen carbonate dissolved in the carbonation reaction mother liquor, the slurry obtained in the carbonation step can be subjected to solid-liquid separation, and the liquid fraction obtained can be concentrated to separate and recover the precipitated lithium-containing solid fraction, which can then be returned to the series of lithium hydroxide production steps.

[0023] One of the features of the present invention is that the liquid fraction obtained in the concentration and crystallization step of the mother liquor of the carbonation step and the subsequent solid-liquid separation can be converted into a sulfate solution by reacting it with sulfuric acid, and then further concentrated and crystallized, thereby transferring sodium and potassium into the solid fraction. Sodium sulfate and potassium sulfate each have the property of forming double salts with lithium sulfate, and separation is often difficult in a concentration and crystallization operation. However, by separating a sufficient amount of lithium from sodium and potassium in the carbonation step before the concentration and crystallization operation of the sulfate solution, it is possible to prevent the formation of a double salt with lithium sulfate during the concentration and crystallization operation of the sulfate.

[0024] The double salt or mixture of sodium sulfate and potassium sulfate thus obtained can be subjected to an appropriate washing process to significantly reduce the lithium content, and can be reused as a raw material containing sodium or potassium by known techniques. This makes it possible to effectively utilize sodium and potassium as resources, which would otherwise be discarded as impurities.

[0025] When the amounts of sodium and potassium are reduced in the aqueous sulfate solution converted from the mother liquor of the carbonation step, the remaining aqueous sulfate solution mother liquor can be reused in the electrochemical membrane separation step. This makes it possible to effectively utilize lithium that was not recovered in the carbonation step and the carbonation mother liquor concentration and crystallization step, thereby further increasing the yield of lithium obtained as the product lithium hydroxide.

[0026] The lithium-containing solid content obtained from the solid-liquid separation step in the carbonation step or the step of concentrating the carbonation reaction mother liquor in the present invention can be regenerated into an aqueous lithium sulfate solution by adding sulfuric acid prepared so as not to introduce new impurities. Although high-purity sulfuric acid may be newly prepared as the sulfuric acid used in this step, it is preferable to use sulfuric acid that is produced simultaneously with the aqueous lithium hydroxide solution in the electrochemical membrane separation step included in the present invention after adjusting the concentration as necessary.

[0027] Understanding the following points will make clear the differences between the technology disclosed in this invention and the prior art.

[0028] As far as the inventors know, there has been no known technique for producing a solid content of a lithium-containing carbonate compound from lithium hydroxide produced using lithium sulfate as a raw material by using carbon dioxide gas, and then converting this solid content of the lithium-containing carbonate compound into a sulfate solution with sulfuric acid and directly returning it to a process for producing lithium hydroxide again for reuse. However, the term "directly" here means that the aqueous solution obtained by sulfation is used to produce lithium hydroxide in the electrochemical membrane separation process together with a raw lithium sulfate aqueous solution that is newly supplied as a raw material for the electrochemical membrane separation process, without being subjected to a chemical impurity removal treatment in a separate process before the production of lithium hydroxide.

[0029] When producing high-purity lithium carbonate from lithium hydroxide using conventional techniques, it was necessary to obtain lithium hydroxide crystals in which the impurity concentration had been reduced by lithium hydroxide crystallization, and then redissolve these in water before carrying out a carbonation reaction. This is because the proportion of alkali metal impurities that can be removed by the carbonation reaction from a lithium hydroxide solution in which alkali metal impurities are dissolved at high concentrations is insufficient. Therefore, it can be seen that there is a crucial difference in the properties of the lithium hydroxide handled compared to the carbonation reaction used in the present invention. That is, in the conventional method for producing high-purity lithium carbonate, crystals obtained by lithium hydroxide crystallization are redissolved and used as the raw material for the carbonation reaction, whereas in the present invention, a crystallization mother liquor in which impurities have been concentrated by lithium hydroxide crystallization is used as the raw material for the carbonation reaction, which is a major difference.

[0030] In the prior art, when lithium carbonate is recovered as a product from the mother liquor of lithium hydroxide crystallization, it is preferable to reduce the concentration of alkali metal impurities contained in the lithium carbonate in order to use the lithium carbonate as a raw material in other processes. Therefore, in order to obtain high-quality lithium carbonate with reduced impurities, it is necessary to dilute the impurities contained in the lithium carbonate with lithium as much as possible, i.e., to increase the proportion of lithium blown from lithium hydroxide crystallization to the carbonation step, or to reduce the proportion of the aqueous lithium hydroxide solution supplied to lithium hydroxide crystallization and increase the proportion of the aqueous lithium hydroxide solution supplied to the carbonation step, or to redissolve the lithium hydroxide crystals obtained by crystallization and use them as a raw material in the carbonation step. This inevitably reduces the yield of lithium hydroxide as a product.

[0031] On the other hand, the process disclosed in the present invention is specialized in improving the yield of lithium hydroxide, and therefore does not require the production of high-purity lithium carbonate, which significantly differs from the method of simultaneously producing lithium hydroxide and lithium carbonate known in the prior art.

[0032] A first gist of the present invention resides in a method for producing lithium hydroxide, characterized by comprising at least the following steps (A) to (F): (A) a step of producing an aqueous lithium hydroxide solution and sulfuric acid by electrochemical membrane separation using lithium sulfate as a raw material, (B) a crystallization step of producing lithium hydroxide crystals by crystallization using the aqueous lithium hydroxide solution obtained by the electrochemical membrane separation step as a raw material, (C) a solid-liquid separation step of extracting a part of the slurry from the crystallization step and separating it into lithium hydroxide crystals and a crystallization mother liquor, and a washing step of washing the solids, (D) a carbonation step of extracting a part of the crystallization mother liquor from the crystallization step and reacting it with carbon dioxide gas to obtain a slurry containing a lithium-containing carbonate compound as a solid component, and a solid-liquid separation step of separating the slurry into a lithium-containing solid component and a liquid component, (E) an acid dissolution step of reacting the lithium-containing solid component with sulfuric acid to produce an aqueous lithium sulfate solution, and (F) a mixing step of reusing the lithium sulfate obtained in the acid dissolution step as a raw material for step (A).

[0033] The second aspect of the present invention resides in the method for producing lithium hydroxide according to the first aspect, which includes the following step (G) in addition to the steps (A) to (F), and includes an operation of introducing the solid fraction obtained in the solid-liquid separation step into the step (E): (G) a carbonate concentration and crystallization step of concentrating and crystallizing the liquid fraction obtained from the carbonation step and the solid-liquid separation step (D) to obtain a slurry, and a solid-liquid separation step of separating the slurry into a lithium-containing solid fraction and a liquid fraction.

[0034] The third aspect of the present invention resides in the method for producing lithium hydroxide according to the first or second aspect, which includes the following step (H) in addition to the steps (A) to (G), and includes an operation of introducing the liquid fraction obtained in the solid-liquid separation step into the step (E) or (F): (H) a step of adding sulfuric acid to the liquid fraction obtained in the carbonate concentration and crystallization step and the solid-liquid separation step of (G) to produce a sulfate solution, a sulfate concentration and crystallization step of concentrating and crystallizing the sulfate solution to obtain a slurry, and a solid-liquid separation step of separating the slurry into a solid fraction and a liquid fraction.

[0035] A fourth aspect of the present invention resides in a method for producing lithium hydroxide according to any one of the first to third aspects, comprising a washing step of washing any or all of the solid fractions obtained in the carbonation step and solid-liquid separation step of (D), the solid fraction obtained in the carbonate concentrating and crystallization step and solid-liquid separation step of (G), and the solid fractions obtained in the sulfate concentrating and crystallization step and solid-liquid separation step of (H), as a solid-liquid separation step or a step subsequent to the solid-liquid separation.

[0036] A fifth aspect of the present invention resides in the method for producing lithium hydroxide according to any one of the first to fourth aspects, which includes an operation of introducing a part or all of the washing filtrate obtained in the washing step according to the fourth aspect into the step (E).

[0037] A sixth aspect of the present invention resides in the method for producing lithium hydroxide according to any one of the first to fifth aspects, wherein the electrochemical membrane separation step (A) is an electrodialysis method.

[0038] A seventh aspect of the present invention resides in the method for producing lithium hydroxide according to any one of the first to sixth aspects, further comprising: (I) a redissolving step of redissolving the lithium hydroxide crystals obtained in the crystallization step (B) in an aqueous medium; (J) a recrystallization step (second lithium hydroxide crystallization step) of producing lithium hydroxide crystals by further crystallization; and (K) a solid-liquid re-separation step of separating the slurry obtained in the recrystallization step into lithium hydroxide crystals and a crystallization mother liquor, and a washing step of washing the solids, wherein the crystallization mother liquor obtained in the solid-liquid re-separation step is supplied to the crystallization step (B) and / or the carbonation step (D) and the recrystallization step (J).

[0039] The present invention has the following features: The present invention discloses a method for effectively separating, recovering, and recycling lithium that is discharged together with highly concentrated alkali metal impurities as a lithium hydroxide crystallization mother liquor, and makes it possible to significantly improve the yield of lithium hydroxide obtained as a product.

[0040] The present invention discloses a method for efficiently recovering lithium contained in the mother liquor of a carbonation process, which utilizes a method for concentrating and crystallizing the carbonation mother liquor, and a method for sulfating the concentrated carbonation mother liquor and concentrating and crystallizing the sulfate. This method not only increases the recycling rate of lithium and further increases the yield of lithium hydroxide, but also makes it possible to produce a mixture of sodium sulfate and potassium with a significantly reduced lithium content, thereby enabling the effective utilization of the resource value of not only lithium but also sodium and potassium as alkali metals.

[0041] The present invention makes it possible to significantly reduce the amount of waste material to be treated, since the only chemical species used to remove alkali metal impurities in the process for producing high-purity lithium hydroxide are carbonic acid and sulfuric acid, and carbonates and sulfates derived from the alkali metals contained in the raw materials are simply discharged as by-products containing alkali metals. Furthermore, unlike conventional techniques that use sodium carbonate, such as the use of sodium carbonate to obtain lithium carbonate or the use of sodium carbonate as a pH adjuster, there is no need to introduce any alkali metals into the system other than the alkali metals supplied from the raw material aqueous solution, and therefore the amount of alkali metals discharged as waste material outside the system is significantly reduced, thereby realizing a reduction in the amount of waste material to be treated.

[0042] Since the present invention uses carbon dioxide gas to obtain the solid content of the lithium carbonate compound, unlike the case of producing lithium carbonate using sodium carbonate, contamination by sodium attached to or adsorbed on the solid content of the lithium carbonate compound can be kept at a low level, making it possible to control the amount of alkali metal impurities entrained in the lithium recycled into the system with only minimal washing. The carbonation step, the carbonation mother liquor concentration and crystallization step, and the solid-liquid separation step and washing steps associated with the subsequent sulfate concentration and crystallization step according to the present invention not only function to recover lithium but also to discharge alkali metal impurities from the system, making it possible to reduce the overall number of steps and control the form and amount of waste generated simply by adjusting the number of crystallization steps and the degree of washing of the solid content.

[0043] The present invention having the above-mentioned characteristics can significantly improve the yield of lithium hydroxide, minimize the number of steps, and minimize the amount of waste generated, thereby achieving the effect of significantly improving the economic efficiency of lithium hydroxide production.

[0044] 1 is a process flow diagram illustrating an embodiment of the present invention.

[0023] FIG. 2 is a process flow diagram illustrating an embodiment of the present invention, including the function of a crude purification step.

[0045] Possible embodiments of the present invention will be described in more detail below. However, these are only examples of possible embodiments, and the configuration of the unit operations that make up the actual process is not limited to these examples. Those skilled in the art with experience in such technology can make modifications within the scope of the present invention.

[0046] Each step described below can be carried out by batch, continuous, or semi-batch operation as needed. A person skilled in the art who practices the present invention can select an appropriate operation method while considering the balance of facility capacity, such as operating one step by batch operation and another step by continuous operation.

[0047] The lithium sulfate aqueous solution is appropriately selected and used from among those obtained by dissolving lithium through acid leaching, those obtained by dissolving lithium sulfate crystals, etc. As will be described below, the lithium sulfate aqueous solution obtained in the acid dissolution step in which lithium-containing solid matter is reacted with sulfuric acid is mixed and used as a raw material for the electrochemical membrane separation step.

[0048] Impurities such as polyvalent metal ions contained in the lithium sulfate aqueous solution raw material are preferably removed by a known method. Furthermore, if insoluble components are present, they are preferably filtered and separated by a known method. In the present invention, Na and / or K are contained as impurities. The amounts of these impurities are such that the Na / Li weight ratio is less than 3.3, more preferably less than 0.70, and even more preferably less than 0.11, and the K / Li weight ratio is less than 5.6, more preferably less than 2.0, and even more preferably less than 0.21. This is because, if the impurity amount exceeds Na / Li = 3.3 or K / Li = 5.6, more than half of the alkali metals in the alkaline aqueous solution produced in the electrochemical membrane separation step will be derived from the impurities, resulting in a decrease in energy utilization efficiency. However, if the process is operated in a configuration including a crude purification step as described below, the amount of impurities contained in the lithium sulfate aqueous solution raw material is not limited to this.

[0049] The lithium sulfate aqueous solution thus prepared is used as a raw material in (A) an electrochemical membrane separation step, particularly an electrodialysis step, to produce a lithium hydroxide aqueous solution and sulfuric acid. The sulfuric acid obtained in this step can be used for a reaction with a lithium-containing carbonate compound after appropriate concentration adjustment, such as by a concentration operation. It may also be used as a raw material in another step.

[0050] When electrodialysis is used as an electrochemical membrane separation process, the temperature of the salt solution is preferably kept below 40°C, and more preferably between 10 and 30°C. In a typical electrodialysis device, if the temperature of the salt solution exceeds 40°C, not only may the service life of the membranes and components be shortened, but also there may be problems such as a decrease in the airtightness of the dialysis membrane stack due to distortion caused by differences in the thermal expansion coefficients of the components. Naturally, this does not apply to devices designed to be capable of high-temperature operation.

[0051] The lithium hydroxide solution obtained in the electrochemical membrane separation process is used as the raw material for the crystallization process. The separation membranes used in electrodialysis and compartmentalized electrolysis are selective for the valence of ions and cannot separate lithium, sodium, and potassium, which are all alkali metals. Therefore, if sodium or potassium is present in the raw lithium sulfate solution, the sodium and potassium will migrate directly into the lithium hydroxide solution produced in this process without being selectively separated.

[0052] The lithium hydroxide aqueous solution thus obtained is subjected to crystallization, particularly concentrated crystallization, to obtain crystals of (B) lithium hydroxide by a known method. For concentrated crystallization, it is preferable to apply evaporative crystallization, which involves evaporation of water as the solvent. When applying evaporative crystallization, it is preferable to maintain the temperature of the evaporator at 60°C or higher, and more preferably to maintain it constant between 70 and 100°C. Although evaporative crystallization can be performed even when the temperature of the evaporator is below 60°C, there is a problem in that production efficiency decreases.

[0053] Alternatively, instead of carrying out such concentration and crystallization, it is also possible to concentrate the aqueous lithium hydroxide solution obtained from the electrochemical membrane separation step to a degree that does not cause crystal precipitation, and then obtain high-purity lithium hydroxide crystals as a product from this concentrated solution by cooling and crystallization. Therefore, in the present invention, concentration and crystallization includes not only an operation in which lithium hydroxide crystals are produced while simultaneously removing water as a solvent, but also an operation in which concentration and cooling and crystallization are carried out in combination.

[0054] The lithium hydroxide slurry obtained in this crystallization step is subjected to solid-liquid separation and washing by a known method (C), where a portion of the slurry is extracted and separated into lithium hydroxide crystals and a crystallization mother liquor. A centrifuge is generally used for the solid-liquid separation operation, but other methods may also be employed. The washing operation may be performed in the same apparatus as the solid-liquid separation operation, or may be performed as a separate step after solid-liquid separation by passing wash water through the solids. Furthermore, even if performed in the same apparatus, the washing operation may be performed in an apparatus equipped with a mechanism for separately recovering the liquid fractions resulting from solid-liquid separation and washing. The manner in which solid-liquid separation of the crystallization slurry and washing of the resulting solid fraction are performed may be appropriately selected depending on the steps in which the solid-liquid separation filtrate and washing filtrate are to be reused.

[0055] For example, the filtrate obtained by solid-liquid separation of the crystallization slurry and the washing filtrate generated in the subsequent washing operation can both be returned to the lithium hydroxide crystallization step. However, it is more efficient in terms of operational operation to introduce a part or all of the filtrate obtained by solid-liquid separation of the crystallization slurry into the subsequent carbonation step.

[0056] The weight ratio of Na / Li and K / Li in the high purity lithium hydroxide thus obtained was 1.2 × 10 －３ More preferably, it is less than 3.0 × 10 －４ and more preferably less than 1.2 × 10 －４ The weight ratios of Na / Li and K / Li in the high-purity lithium hydroxide are less than 1.2 × 10 －３ In the above cases, the content of alkali metal impurities exceeds 200 ppm, which causes problems such as being unsuitable for producing battery materials using lithium hydroxide as a raw material, or limiting the amount that can be used.

[0057] The amount of slurry extracted from the lithium hydroxide crystallization step may be appropriately selected within an allowable range for continuous crystallization. When lithium hydroxide crystallization is carried out in a continuous system, the amount of lithium contained in the extracted slurry during steady-state operation and the lithium hydroxide crystals recovered as a product is adjusted by setting the extraction amount so that the fluctuation in the amount of lithium remaining in the system is within 20%, preferably within 10%, per hour, and / or so that the average amount of lithium remaining is constant, taking into account the amount of lithium contained in the aqueous lithium sulfate solution newly introduced into the system as a raw material and the lithium hydroxide yield. When lithium hydroxide crystallization is carried out in a semi-batch system, the amount of lithium contained in the crystals in the extracted slurry temporarily exceeds the amount of lithium newly introduced into the system, but the amount of crystals extracted on average during continuous operation is preferably adjusted by setting the extraction amount so that the amount of lithium remaining in the system remains within a substantially constant range (preferably within a constant range as the average value of a semi-batch operation cycle).

[0058] In the lithium hydroxide crystallization described above, a portion of the crystallization mother liquor is withdrawn and used as a raw material for the subsequent carbonation step. The Na / Li weight ratio of the mother liquor is less than 0.57, preferably less than 0.29, and more preferably less than 0.12, and the K / Li weight ratio is less than 1.6, preferably less than 0.80, and more preferably less than 0.32. If the Na / Li weight ratio is 0.57 or higher or the K / Li weight ratio is 1.6 or higher, the impurity contents of Na and K contained in the lithium hydroxide crystals obtained as a product are likely to exceed 100 to 200 ppm, making it difficult to produce high-purity lithium hydroxide. However, if the present invention is utilized including a crude purification step as described below, the impurity contents of the mother liquor are not limited to this. The mother liquor can be withdrawn by a known method. As described above, the liquid fraction obtained during solid-liquid separation of the product lithium hydroxide crystals may be utilized, or a portion that is not affected by stirring may be created within the crystallizer, and the mother liquor may be removed from the crystallizer as a supernatant liquid by sedimentation.

[0059] The amount of this mother liquor extracted depends on the Na / Li and K / Li weight ratios of the lithium hydroxide aqueous solution flowing into the lithium hydroxide crystallization process and the required purity of the lithium hydroxide crystals obtained as the product. It is extremely important in the present invention to recover lithium as a solid or liquid from the extracted mother liquor, convert it to sulfate, and recycle it. That is, the weight ratios of the Na / Li and K / Li impurities contained in the recycled lithium are smaller than those of the lithium hydroxide crystallization mother liquor, and preferably smaller than those of the raw material aqueous solution. Therefore, if a sufficient amount of recycled lithium is mixed with newly introduced raw materials and passes through an electrochemical membrane separation process to enter the lithium hydroxide crystallization process in the form of an aqueous sodium hydroxide solution, it will have a dilution effect that reduces the Na / Li and K / Li weight ratios of the lithium hydroxide crystallization mother liquor. Therefore, it is possible to allow a certain amount of alkali metal impurities to be mixed into the recycled lithium, as long as the amount does not impair this dilution effect, thereby achieving a continuous increase in the yield of lithium hydroxide as much as possible.

[0060] The lithium hydroxide solution used as a raw material in the carbonation step is preferably adjusted in concentration before use to facilitate the operation of the carbonation step. This is because, depending on the configuration of the reaction apparatus, solids produced by the reaction between carbon dioxide and lithium hydroxide may clog the raw material introduction nozzle. On the other hand, in order to reduce the total amount of dissolved lithium after the carbonation reaction, it is preferable to increase the concentration of the lithium hydroxide solution used in the carbonation step as much as possible. The concentration of the lithium hydroxide aqueous solution used in the carbonation step can be appropriately selected while taking these factors into consideration. Furthermore, a portion of the mother liquor from the carbonation step can be used to adjust the concentration of this lithium hydroxide aqueous solution. When using a method in which carbon dioxide gas is blown into the lithium hydroxide aqueous solution, the concentration of this lithium hydroxide aqueous solution is preferably 4 to 8 wt%. When the carbonation reaction is carried out at a concentration exceeding this range, it is advisable to appropriately check whether the tip of the nozzle through which carbon dioxide gas is blown is clogged, and replace or clean the clogged nozzle tip as necessary.

[0061] In this way, (D) the mother liquor extracted from the lithium hydroxide crystallization step is reacted with carbon dioxide to produce a lithium-containing carbonate compound, and the resulting slurry is subjected to solid-liquid separation by a known method.

[0062] The carbonation step may be carried out by a known gas-solid contact reaction method. For example, a certain amount of the extracted mother liquor may be filled into a reaction vessel, and carbon dioxide gas may be blown into the vessel to cause the reaction. Alternatively, a packed tower containing a packing material such as Raschig rings or an absorption tower using a spray or shower may be used, or a combination of these may be employed. The concentration of carbon dioxide gas used in the reaction may be varied depending on the gas-solid contact step employed. As will be described in the examples below, carbon dioxide gas may be diluted with nitrogen gas. If the gas-solid contact step can tolerate a larger gas flow rate, ordinary air may be introduced into the carbonation step after appropriate pretreatment (such as removal of suspended solids). The carbon dioxide-depleted air obtained in this manner may be used in a separate or subsequent step.

[0063] Regardless of the reaction type used to carry out the carbonation step, it is preferable to control the reaction of lithium with carbon dioxide in an amount equivalent to or 90% or more of the amount of lithium used in the reaction during the process of extracting the solid content. This is because if the reaction between lithium hydroxide and carbon dioxide is insufficient, the amount of lithium that can be recovered as a solid content in the carbonation step decreases.

[0064] This carbonation reaction is preferably carried out in a temperature range of 60°C or higher, and since the higher the reaction temperature, the higher the solubility of lithium hydroxide and the lower the solubility of lithium carbonate, it is more preferably carried out in a temperature range of 70 to 90°C. The reaction between the aqueous lithium hydroxide solution and carbon dioxide gas is exothermic, and this heat of reaction may be used to maintain the liquid temperature. If the temperature of the carbonation reaction is lower than 60°C, the solubility of lithium carbonate increases, resulting in problems such as a decrease in the yield of lithium recovered and reused as a solid content or an increased load on the carbonate concentration and crystallization step and the sulfate concentration and crystallization step described below.

[0065] The solid fraction obtained in the carbonation step is primarily composed of lithium carbonate, but may also contain lithium bicarbonate and water of crystallization. Furthermore, the reaction mother liquor and washing water may be attached. In the prior art, since the carbonation step is a purification step, the inclusion of lithium solid fractions in forms other than lithium carbonate is not a preferred embodiment. However, the present invention differs significantly from the prior art in that it is sufficient to separate lithium from other alkali metal impurity elements through the carbonation reaction. This is because the amount of lithium recycled into the system after the solid fraction is treated in the acid dissolution step described below is particularly important. The Na / Li weight ratio of the solid fraction obtained here is 60% or less, preferably 40% or less, and more preferably 20% or less of the Na / Li weight ratio of the lithium hydroxide crystallization mother liquor. Similarly, the K / Li weight ratio of the lithium hydroxide crystallization mother liquor is 60% or less, preferably 40% or less, and more preferably 20% or less of the K / Li weight ratio of the lithium hydroxide crystallization mother liquor. If the weight ratio of Na / Li or K / Li in the solids exceeds 60% of the weight ratio of Na / Li or K / Li in the lithium hydroxide crystallization mother liquor, the amount of Na and K returned to the system increases, which is disadvantageous for the production of high-purity lithium hydroxide. Therefore, for example, in the case of a reaction at 75°C, the end-point pH of the carbonation reaction may be lowered to 8.3 to 9.0, at which point the amount of lithium hydrogencarbonate produced begins to increase. If the carbonate concentration and crystallization step described below, or the sulfate concentration and crystallization step described below is additionally performed, the end-point pH of the carbonation step may be set in the range of 7.5 to 9.5. The slurry produced in this carbonation step is subjected to solid-liquid separation by a known method, and the solids can be washed with water or warm water as needed.

[0066] Another feature of the present invention is that the impurity separation process disclosed in the present invention (i.e., the carbonation process, and the carbonate concentration and crystallization process and sulfate concentration and crystallization process described below) is positioned after the lithium hydroxide crystallization process and before the recycled lithium is mixed with the raw material aqueous solution. In other words, the lithium hydroxide crystallization process not only obtains purified lithium hydroxide crystals, but also has the function of concentrating impurities, and the required level of impurity removal can be achieved by introducing only a portion of the lithium flowing into the crystallization process into the impurity separation process and the lithium recovery process. Therefore, since it is not necessary to pass lithium equivalent to the entire amount of lithium hydroxide obtained as a product through the impurity removal process, the scale of the process required for impurity removal can be significantly reduced.

[0067] The liquid fraction obtained by solid-liquid separation after the carbonation step contains a large amount of dissolved lithium, and if the carbonation reaction has progressed sufficiently, the amount of dissolved lithium is usually equivalent to 10 to 20% of the lithium introduced into the carbonation step. It is believed that the dissolved lithium is not only contributed by the dissolution of lithium carbonate produced by the carbonation reaction, but also by the dissolved lithium as lithium bicarbonate. The Na / Li weight ratio of this liquid fraction is less than 9.0, preferably less than 5.0, and the K / Li weight ratio is less than 20, preferably less than 12. If the Na / Li weight ratio of the liquid fraction is 9.0 or more, or if the K / Li weight ratio of the liquid fraction is 20 or more, there is a problem in that sodium and potassium are likely to precipitate as carbonate compounds along with lithium in the carbonate concentration and crystallization step described below.

[0068] This liquid fraction can be subjected to a concentration and crystallization operation to further recover lithium as a solid fraction of a lithium-containing carbonate compound. In the present invention, this step is referred to as (G) carbonate concentration and crystallization step. The slurry obtained in the carbonate concentration and crystallization step is subjected to a solid-liquid separation operation.

[0069] This carbonate concentration crystallization is preferably carried out by evaporation crystallization using either heating or reduced pressure, or a combination of both. This is because, particularly in the presence of dissolved lithium derived from lithium bicarbonate, heating promotes the decomposition of lithium bicarbonate, increasing the amount of lithium recovered as lithium carbonate and enhancing the lithium yield. This evaporation crystallization is preferably carried out at a temperature of 60°C or higher, more preferably at a temperature of 70 to 90°C. Evaporative crystallization temperatures below 60°C can result in problems such as reduced productivity due to a slower evaporation rate and slower decomposition of lithium bicarbonate. Furthermore, to reduce the energy required for concentration, a concentration operation using membrane separation can be combined. The resulting slurry is subjected to solid-liquid separation using a known method to obtain a lithium-containing solid content. This solid content can be washed with water or warm water as needed. The Na / Li weight ratio of this solid content is 60% or less, preferably 40% or less, and more preferably 20% or less, of the Na / Li weight ratio of the lithium hydroxide crystallization mother liquor. Similarly, the weight ratio of K / Li in this solid content is 60% or less, preferably 40% or less, and more preferably 20% or less of the weight ratio of K / Li in the lithium hydroxide crystallization mother liquor. If the weight ratio of Na / Li or K / Li in the solid content exceeds 60% of the weight ratio of Na / Li or K / Li in the lithium hydroxide crystallization mother liquor, the amounts of Na and K returned to the system increase, which is disadvantageous for the production of high-purity lithium hydroxide.

[0070] The lithium-containing solids produced and separated in the carbonation step and carbonate concentration and crystallization step are used as raw materials for the acid dissolution step (E). In this step, the lithium-containing solids obtained in the previous steps are decomposed with sulfuric acid to produce an aqueous solution containing lithium sulfate as a main component. The sulfuric acid used in this step may be purchased fresh, but it is more preferable to use sulfuric acid produced together with lithium hydroxide in the electrochemical membrane separation step. This is because using sulfuric acid produced in the electrochemical membrane separation step can prevent impurities from being mixed in from outside the system. The amount of sulfuric acid used in this operation can be adjusted as appropriate, but particularly when electrodialysis is used as the electrochemical membrane separation step, it is preferable to adjust the amount used so that the pH of the lithium sulfate solution is 4 or less. In addition, to adjust the concentration of this lithium sulfate aqueous solution, a low-concentration salt solution (a low-concentration lithium sulfate aqueous solution) obtained in the electrochemical membrane separation step can be added.

[0071] When the lithium-containing solid content is decomposed in the acid dissolution step, carbon dioxide gas is generated. This gas can be recovered and reused in the carbonation step. Because carbon dioxide gas is generated rapidly by the reaction of the lithium-containing solid content with sulfuric acid, it is preferable to design the reaction vessel to be able to accommodate pressure changes. To mitigate pressure changes, it is preferable to consider a buffer mechanism that provides a large space above the reaction vessel, a mechanism that protects the reaction vessel from sudden pressure increases by installing a pressure relief valve, or a mechanism that operates under reduced pressure while constantly venting with a blower. Furthermore, to control the amount of carbon dioxide gas generated, it is preferable to use a mechanism that gradually adds the lithium-containing solid content to a solution containing a predetermined amount of sulfuric acid, or a mechanism that gradually adds sulfuric acid to the lithium-containing solid content, or a combination of these mechanisms.

[0072] The lithium sulfate aqueous solution obtained in the acid dissolution step is reused via a step (F) of mixing with a lithium sulfate aqueous solution supplied as a raw material for the electrochemical membrane separation step, thereby completing one embodiment of the treatment cycle disclosed in the present invention. The Na / Li weight ratio of this reused lithium sulfate aqueous solution is 60% or less, preferably 40% or less, and more preferably 20% or less of the Na / Li weight ratio of the lithium hydroxide crystallization mother liquor. The K / Li weight ratio of the reused lithium sulfate aqueous solution is 60% or less, preferably 40% or less, and more preferably 20% or less of the K / Li weight ratio of the lithium hydroxide crystallization mother liquor. If the Na / Li weight ratio or the K / Li weight ratio of the reused lithium sulfate aqueous solution exceeds 60%, the amounts of Na and K returned to the system increase, making it difficult to produce high-purity lithium hydroxide.

[0073] The liquid fraction remaining after the carbonate concentration and crystallization step and the subsequent solid-liquid separation is converted into a sulfate aqueous solution by adding sulfuric acid, and this sulfate aqueous solution is then subjected to a concentration and crystallization operation, thereby separating sodium and potassium from lithium and recovering lithium as a liquid fraction after solid-liquid separation. In the present invention, this step is referred to as (H) sulfate concentration and crystallization step. The slurry obtained in this step is subjected to a solid-liquid separation operation.

[0074] This sulfate concentration crystallization is preferably carried out by evaporation crystallization using either heating or reduced pressure, or a combination of both. In this case, crystallization is preferably carried out at a temperature of 60°C or higher, more preferably 70 to 90°C. To reduce the energy required for concentration, a concentration operation using membrane separation can also be combined. The resulting slurry is subjected to solid-liquid separation by a known method, and most of the lithium is recovered as a liquid fraction. The amount of Na contained in this liquid fraction is 50% or less, preferably 30% or less, and more preferably 20% or less, of the amount of Na contained in the mother liquor blown from the lithium hydroxide crystallization step. The same applies to the amount of K. If the amount of Na or K contained in this liquid fraction exceeds 50% of the amount of Na contained in the mother liquor blown from the lithium hydroxide crystallization step, the amount of Na or K returned to the system increases, which is disadvantageous for the production of high-purity lithium hydroxide. The liquid fraction separated and recovered by the sulfate concentration crystallization can be mixed when adjusting the concentration or pH of the sulfate aqueous solution in the acid dissolution step.

[0075] The solid fraction obtained by sulfate concentration crystallization and subsequent solid-liquid separation can be washed with water or cold water as needed. When sodium and potassium are the main impurities, the washing operation leaves behind sodium sulfate, potassium sulfate, a double salt of sodium sulfate and potassium sulfate, or a mixture thereof, with a significantly reduced lithium content. The Li content of this solid fraction is less than 500 ppm, preferably less than 100 ppm. Because of its low lithium content, this solid fraction can be used as an alkali raw material in other processes using known techniques.

[0076] In the carbonation step and carbonate concentration and crystallization step, it is important to recover, as solids, preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more of the lithium derived from the mother liquor blown from the lithium hydroxide crystallization step, and introduce it into the acid dissolution step to regenerate the lithium sulfate aqueous solution, which is then reused as a raw material for the electrochemical membrane separation step. This is because it not only improves the yield of lithium hydroxide obtained as a product, but also affects its purity. As seen in conventional techniques, discharging lithium-containing carbonate compounds out of the system as lithium carbonate not only reduces the lithium yield as lithium hydroxide crystals, but also reduces the recovery rate of crystals per lithium treatment amount in the lithium hydroxide crystallization step. Therefore, if the amount of lithium recovered as solids is less than 70%, there are problems such as a reduced lithium yield and a reduced production efficiency of high-purity lithium hydroxide.

[0077] That is, lithium and alkali metal impurities such as sodium and potassium are separated from each other through the carbonation step and the optional carbonate concentration and crystallization step and sulfate concentration and crystallization step, and most of the alkali metal impurities are discharged outside the system. Therefore, the sodium / lithium ratio and potassium / lithium ratio of the lithium sulfate aqueous solution regenerated in the acid dissolution step after these steps are significantly reduced through these steps. When the lithium sulfate aqueous solution with reduced impurity content is regenerated and reused as a raw material in the electrochemical membrane separation step and further reused in the lithium hydroxide crystallization step, the reused lithium has the effect of diluting the impurities concentrated in the lithium hydroxide crystallization mother liquor.

[0078] It is known that the amount of impurities contained in lithium hydroxide crystals obtained by crystallization is approximately proportional to the impurity concentration relative to lithium in the crystallization mother liquor, and therefore, reducing the impurity concentration in the crystallization mother liquor is advantageous for purifying the lithium hydroxide obtained as crystals. Therefore, the blow rate in lithium hydroxide crystallization can be controlled within a realistic and appropriate range, and the recovery rate of lithium hydroxide crystals per treatment amount can be improved.

[0079] That is, the present invention, which utilizes a carbonation reaction as a separation operation for lithium and alkali metal impurities, not only simply increases the yield of lithium hydroxide crystals as a product by reusing the lithium-containing solid content as a lithium raw material, but also improves the utilization efficiency of the equipment for lithium hydroxide crystallization.

[0080] By understanding the characteristics of the carbonation step disclosed in the present invention, it will be understood that washing is not essential in the carbonation step and the subsequent carbonate concentration and crystallization steps and sulfate concentration and crystallization steps, and that the degree of washing may be adjusted appropriately, or washing may not be necessary at all. For example, the solid fraction obtained in the carbonation step may be washed away with a small amount of water to the extent that the yield of the lithium-containing carbonate compound is not significantly reduced, and after the solid fraction obtained in the subsequent carbonate concentration and crystallization step is separated and recovered, washing may not be performed. Furthermore, when washing the solid fraction obtained in the sulfate concentration and crystallization step, the washing filtrate may be recycled within the system or discharged outside the system.

[0081] In this way, by determining whether or not a washing operation is performed in the carbonation step, the carbonate concentrating and crystallizing step, and the sulfate concentrating and crystallizing step, and by adjusting the amount of the washing liquid, it is possible to select whether the composition of the alkali metal impurities discharged to the outside of the system is carbonate or sulfate, and to control the ratio of the alkali metal impurities contained therein to the content of the raw material aqueous solution.

[0082] In the present invention, high-purity lithium hydroxide with reduced Na and / or K can be obtained. In the present invention, high purity means that the Na or K content is less than 200 ppm, preferably less than 50 ppm, and more preferably less than 20 ppm. High-purity lithium hydroxide that satisfies this Na and / or K content can be obtained in high yield. Here, high yield means that 70% or more, preferably 90% or more, of the lithium contained in the raw material lithium sulfate is converted into lithium hydroxide crystals.

[0083] The present invention will be described in more detail below by showing examples relating to the lithium separation and recovery process after the carbonation process. In order to explain the effects of the present invention, an explanation form is adopted in which experimental results are verified by simulation under conditions in which a certain value is obtained as the impurity content in the lithium hydroxide crystals. However, these values ​​are set for convenience and do not limit the conditions under which the present invention can be practiced. In practicing the present invention, the target impurity concentration and operation parameters can be optimized as appropriate.

[0084] In the present invention, the notations Na / Li and K / Li are used to indicate the relative contents of sodium and potassium as impurities relative to lithium, and unless otherwise specified, these represent the weight ratios of these elements.

[0085] The analytical methods used in the examples are as follows. The amounts of alkali metals in the raw material aqueous solution, the crystallization mother liquor, and the crystals were measured using an ICP optical emission spectrometer iCAP PRO XP Duo (manufactured by Thermo Fisher Scientific K.K.). The pH during the reaction was measured using a pH meter HM-30P (manufactured by DKK-TOA Corporation).

[0086] It is well known that the amount of impurities contained in the lithium hydroxide crystals obtained by crystallization of lithium hydroxide is reduced. The amount of impurities is known to be proportional to the concentration in the mother liquor, and a relational expression is obtained as the distribution ratio, which is expressed by the following equation (the distribution ratio D is a dimensionless value):

[0087]

[0088] In a preliminary study, lithium hydroxide crystallization was investigated under the conditions of Na / Li = 0.20 and K / Li = 0.087, and the distribution coefficient for sodium and potassium was 1.05 × 10 ―３ and 3.80 x 10 －４ were obtained respectively.

[0089] Assuming the flow diagram shown in Figure 1 as an example of an embodiment of the present invention, the following experiments were carried out to obtain characteristic values ​​that determine the capacity of a process configured as shown in this flow diagram. Simulations based on material balance were carried out based on these characteristic values.

[0090] In the present invention, the "return rate (represented by the symbol Z)" refers to the proportion of an element separated, recovered, and recycled from the lithium hydroxide crystallization mother liquor by a process including at least a carbonation step, and is a proportion relative to the amount of the element flowing into the lithium hydroxide crystallization step as a reference value. The proportion recovered as a solid in the carbonation step is designated Z1, the proportion recovered as a solid by carbonate concentration and crystallization is designated Z2, and the proportion recovered in the liquid obtained by sulfate concentration and crystallization is designated Z3. Depending on the recovery process used, the return rate Z is calculated as the sum of Z1 to Z3.

[0091] Furthermore, the "relative throughput" in the present invention refers to the amount of lithium per hour during steady-state operation that passes through the electrochemical membrane separation step and flows into lithium hydroxide crystallization, with the amount of lithium supplied per hour contained in the raw aqueous solution being taken as 1.

[0092] In the following explanation, to make the differences in numbers easier to understand, lithium is written with three significant digits and sodium and potassium with two significant digits (however, important settings and results are written with three to four digits as appropriate), but all were treated as double-precision numbers when conducting the simulation.

[0093] Example 1: (Return Rate of Lithium and Alkali Metal Impurities by Carbonation Reaction) Lithium hydroxide, sodium hydroxide, and potassium hydroxide reagents were dissolved in water to prepare a simulated lithium hydroxide crystallization mother liquor with a Na / Li ratio of 0.083, a K / Li ratio of 0.11, and a total alkali concentration of 4.8 mol / L (according to a known acid-base titration method). This simulated mother liquor was placed in a 1-L stainless steel reaction vessel and heated to 75°C using a mantle heater. A carbonation reaction was carried out by passing a mixture of nitrogen gas and carbon dioxide gas (at 0.5 NL / min and 1.2 NL / min, respectively) through the solution while stirring it using a turbine-type stirring blade. Due to the high initial lithium hydroxide concentration, the tip of the gas inlet nozzle was observed to be clogged with precipitates. The tip of the nozzle was cleaned whenever clogging occurred. As the reaction progressed, such clogging no longer occurred. Heat was generated during the carbonation reaction, and the solution temperature temporarily rose to 81°C. The reaction was terminated when the pH of the solution decreased to 9.27 (79°C), and solid-liquid separation was carried out by vacuum filtration using a Buchner funnel and filter paper No. 5C manufactured by Advantec.

[0094] From the weights of the unwashed solids and the filtrate (reaction crystallization mother liquor) and the results of ICP analysis, 92.1%, 7.8%, and 8.8% of the lithium, sodium, and potassium that were reacted as hydroxides were recovered as solids. If these solids were introduced into the acid dissolution step, the return rates Z of lithium, sodium, and potassium would be Z = Z1 (see Figure 1), which would be 92.1%, 7.8%, and 8.8%, respectively (Experiment No. 1A).

[0095] Furthermore, when this solid content was washed with approximately 100 g of warm water (80°C), the return rate Z=Z1 was calculated from the weights of the solid content and the washing filtrate and the analysis results, and the return rates Z of lithium, sodium, and potassium were 91.3%, 1.1%, and 0.39%, respectively (Experiment No. 1B).

[0096] Based on these figures, a simulation was performed using a material balance. When the supply rate of the lithium raw material aqueous solution was set to 1 per unit time, the amounts of lithium retained in the electrochemical membrane separation process, the lithium hydroxide crystallization process, and the carbonation process were all set to 100. The balance in each process was calculated assuming that these processes were operated continuously, and successive calculation cycles were repeated until the values ​​of Na / Li and K / Li in each process converged to values ​​between 99.9% and 100.1% of the results obtained 1000 cycles before the calculation (i.e., until steady-state operation was achieved).

[0097] The amount of impurities contained in the lithium hydroxide crystals obtained as a product is affected not only by the reconstitution rate after the carbonation step, but also by the impurity content of the raw aqueous solution and the blow rate in the lithium hydroxide crystallization step. Various combinations of impurity amounts in the raw aqueous solution to which the present invention can be applied are possible, but the following settings were used to determine the simulation conditions.

[0098] First, the distribution ratios of sodium and potassium in the lithium hydroxide crystallization step determined in advance were applied, and the return rate in the carbonation step was set to 0. In other words, considering the case where lithium hydroxide crystals are recovered by partial crystallization and the crystallization mother liquor with concentrated impurities is discarded outside the system, the blow rate in the lithium hydroxide crystallization step was set to 40%. Note that the blow rate in this invention refers to the proportion of lithium discharged to the next step as mother liquor out of the lithium flowing into the lithium hydroxide crystallization step. In this way, under the conditions where the blow rate is set to 40% by partial crystallization and no lithium is recycled to the electrochemical membrane separation step, the yield of lithium hydroxide obtained as a product is 60% of the lithium input as the raw aqueous solution (in this invention, this is expressed as a lithium hydroxide yield of 60%).

[0099] In this invention, the prior art for obtaining high-purity lithium hydroxide without recycling lithium derived from the blown lithium hydroxide crystallization mother liquor in the system is treated as Comparative Example 1.

[0100] Furthermore, when the values ​​of the raw material aqueous solution were determined by simulation so that the amounts of sodium and potassium contained in the product lithium hydroxide crystals (lithium hydroxide monohydrate) would be 20 ppm and 10 ppm, respectively, during steady-state operation, the results were Na / Li = 0.0464 and K / Li = 0.0640. Also, the relative amounts of sodium and potassium contained in the lithium hydroxide crystallization mother liquor during steady-state operation were Na / Li = 0.116 and K / Li = 0.160.

[0101] The amounts of sodium and potassium impurities contained in the raw aqueous solution were set to these values, and the simulation described above was re-run using the reconstitution rate obtained from the above experimental results. The impurity amounts in the product lithium hydroxide crystals were set to 20 ppm or less of sodium and 10 ppm or less of potassium, and the minimum blow rate that satisfied both of these conditions was adopted (Simulation Nos. 1A and 1B). The lithium hydroxide yield (the proportion of lithium obtained as lithium hydroxide crystals out of the lithium added as the raw aqueous solution) was 96.5% during steady-state operation for both Experiment Nos. 1A and 1B. However, it was assumed that all of the alkali metal was converted to a sulfate solution in the acid dissolution step (the same applies to the following simulations).

[0102] Example 2: The composition of the simulated lithium hydroxide crystallization mother liquor to which a carbonation reaction is applied was changed to Na / Li = 0.28, K / Li = 0.31, and a total alkali concentration of 5.29 mol / L, and the carbonation reaction was terminated when the solution pH reached 8.83 (85.6°C), and the same operations as in Example 1 were carried out, except that the carbonation reaction, solid-liquid separation, and washing operations were carried out in the same manner as in Example 1.

[0103] The reconstitution ratios Z=Z1 for the unwashed solids were calculated from the weights of the sample and filtrate and the analytical results, and were 94.2%, 11.2%, and 16.5% for lithium, sodium, and potassium, respectively (Experiment No. 2A).For the sample that had been washed once, the reconstitution ratios were 93.3%, 0.46%, and 1.3%, respectively (Experiment No. 2B).

[0104] When the calculation cycle was re-executed by applying these return rates to the simulation assumptions employed in Example 1, the lithium hydroxide yields during steady operation were 97.2% and 97.3% for Experiment Nos. 2A and 2B, respectively.

[0105] Table 1 shows the operating conditions applied in Examples 1 and 2, and Table 2 shows the results of a simulation performed based on these results.

[0106]

[0107]

[0108] From Tables 1 and 2, the effects of the present invention can be understood as follows. That is, by recovering lithium as a solid content in the carbonation step, and then recycling it to the electrochemical membrane separation step via the acid dissolution step, the lithium hydroxide yield can be increased from 60% to 95% or more compared to a case in which these steps are not performed. Furthermore, although the solid content that was washed once in the carbonation step contained impurities in an amount that could not be considered high-purity lithium carbonate (Experiment No. 1B), and the unwashed product even contained impurities at around 10% (Experiment No. 1A), simulation results showed that the lithium hydroxide yield exceeded 95% during steady-state operation in both cases.

[0109] The reason why crystallization can be carried out to maintain high purity of lithium hydroxide crystals despite the fact that the lithium-containing carbonate compound contains a large amount of impurities can be easily understood by considering the following points.

[0110] Purification in lithium hydroxide crystallization is governed by the distribution ratio, but as the crystallization operation continues, impurities become concentrated in the crystallization mother liquor, resulting in an increase in the impurity concentration in the crystals. As shown in the flow diagram exemplified in Figure 1, in addition to lithium supplied from the raw aqueous solution, lithium from which most of the impurities have been removed through a carbonation process is recycled to the lithium hydroxide crystallization process through an electrochemical membrane separation process, resulting in the effect of diluting the impurities in the lithium hydroxide crystallization mother liquor by the lithium with a reduced impurity concentration. Therefore, high-purity lithium hydroxide crystals can be obtained as a product without increasing the blow rate in the crystallization operation.

[0111] For example, a detailed analysis of the results of Simulation No. 1A reveals that while the impurities in the raw aqueous solution were Na / Li = 0.0464 and K / Li = 0.0640, the lithium-containing solid matter recovered in the carbonation step during steady-state operation had Na / Li = 0.010 and K / Li = 0.015. Similarly, the lithium-containing solution introduced from the electrochemical membrane separation step to the lithium hydroxide crystallization step has the effect of being diluted with the recycled lithium to Na / Li = 0.036 and K / Li = 0.050.

[0112] If the impurity concentration of the solution introduced into the lithium hydroxide crystallization step is reduced in this manner, it becomes easier to obtain high-purity lithium hydroxide as crystals. That is, the feature of the present invention is that it becomes easier to obtain high-purity lithium hydroxide by increasing the amount of lithium recycled into the system. In the prior art, there are known techniques in which the lithium hydroxide crystallization mother liquor, which is concentrated with impurities, is discharged outside the system, or the liquid remaining after recovering the solids by carbonation is recycled into the system. However, in the former technique, no lithium is recycled into the system, and in the latter technique, not only is the amount of recycled lithium small, but the recycled lithium has an increased content of alkali metal impurities relative to the lithium, so that the above-mentioned effect of the present invention cannot be obtained.

[0113] The operation of decomposing a lithium-containing carbonate compound with sulfuric acid and converting it into an aqueous sulfate solution with reduced impurities such as sodium and potassium can make the conversion rate of the lithium-containing solid content into an aqueous sulfate solution substantially 100% and can reduce the possibility of introducing new impurities from outside the system, and is therefore a very suitable means for efficiently realizing such a system.

[0114] Furthermore, the amount of lithium transferred to the carbonation step is the relative treatment amount multiplied by the blow rate, and in the case of simulation number 1A, if the amount of lithium supplied as the raw aqueous solution is set to 1 (per unit time), then the carbonation reaction will be carried out on a lithium hydroxide solution containing 0.44 amount of lithium (also per unit time), and it is not necessary to carbonate all of the lithium. Therefore, the scale of the impurity separation step can be reduced accordingly.

[0115] In Example 2, the lithium hydroxide crystallization mother liquor was concentrated with impurities at a higher concentration (approximately three times) than in Example 1, but the simulation results in Table 2 show that there is no significant difference from the simulation results based on Example 1, which also represents a feature of the present invention. In other words, even if the amount of impurities mixed in increases due to a change in the composition of the raw material aqueous solution, it is possible to continue producing high-purity lithium hydroxide without making any major changes to the operating conditions, thereby enabling stable operation.

[0116] Example 3: (Carbonate Concentration and Crystallization Procedure for Carbonation Reaction Mother Liquor) 850 g of the mother liquor after the carbonation reaction obtained in Example 1 was placed in a 1 L stainless steel container and heated to 70°C using a mantle heater. A concentration and crystallization procedure was carried out by reducing the pressure (operating at an absolute pressure of 0.025 to 0.04 MPa) while maintaining the temperature and stirring. As a result, solids were precipitated, and 97.6 g of a slurry was finally obtained. A solid-liquid separation procedure was carried out by vacuum filtration using a Buchner funnel and Advantec filter paper No. 5C.

[0117] Based on the sample weight and analysis results, 6.65%, 2.5%, and 3.2% of the lithium, sodium, and potassium added as raw materials in Example 1 were recovered as solids. The return ratio Z = Z1 + Z2 was calculated based on the results of Example 1 and was found to be 98.8%, 10%, and 12%, respectively (Experiment No. 3). Based on this return ratio, a simulation was performed in the same manner as in Examples 1 and 2 (Simulation No. 3). The results are shown in Tables 3 and 4, along with the results of 1A.

[0118]

[0119]

[0120] Table 4 shows that, compared to Simulation No. 1A, there was no significant change in the operating conditions for lithium hydroxide crystallization, and the simulation results showed that by increasing the relative throughput by 4%, the lithium hydroxide yield could be increased to 99.4%. As described above, it was again demonstrated that by increasing the lithium return rate into the system, high-purity lithium hydroxide can be easily obtained by the crystallization operation, and that there is no need to significantly change the operating conditions even if the amount of impurities returned to the system increases, resulting in excellent stability of the entire system.

[0121] Example 4: (Sulfate Concentration and Crystallization Procedure for Carbonate Concentration Crystallization Mother Liquor) The composition and concentration of the concentrated mother liquor obtained in Example 3 were Na / Li = 13.6, K / Li = 15.4, and the total alkali content determined by acid-base titration using a hydrochloric acid standard solution was 2.48 mol / L. Approximately 1 L of an aqueous solution was prepared by dissolving lithium carbonate, sodium carbonate, and potassium carbonate reagents in water to achieve the same composition and concentration. When 70% sulfuric acid was added to this solution in small amounts, a large amount of bubbles (carbon dioxide gas) was generated. Sulfuric acid was continued to be added until the pH of this solution reached 4.02 (33.8°C), thereby preparing an aqueous sulfate solution.

[0122] A 1058 g portion of this sulfate aqueous solution was taken and placed in a 1 L stainless steel container. Concentration was performed in the same manner as in Example 3, resulting in the formation of a precipitate. The operation was stopped when the slurry was concentrated to a weight of 427 g. The slurry was cooled to 5°C, and then subjected to solid-liquid separation by vacuum filtration using a Buchner funnel and Advantec filter paper No. 5C. A portion of the solids (98.9 g) was taken, dispersed in 30 g of water, and then subjected to solid-liquid separation to form a new solid cake. The crystals were washed by passing an additional 120 g of water through the mixture.

[0123] The washed crystals were dried at 60°C, and a portion was dissolved. From the alkali metal contents measured by ICP atomic emission spectrometry, the amounts of lithium, sodium, and potassium contained in the washed crystals were determined to be 39 ppm, 16 wt%, and 17 wt%, respectively, indicating that the washed solid content contained only a very small amount of lithium.

[0124] From the analysis results of the sample weight and liquid content, it was calculated that of the lithium, sodium, and potassium dissolved in the solution before concentration, the proportions dissolved in the liquid obtained by the solid-liquid separation operation after concentration were 57.4%, 13%, and 17%, respectively.

[0125] Similarly, the amounts of alkali metals eluted into the washing filtrate obtained when obtaining the above washed crystals were calculated from the analytical values ​​and weights of the crystals before and after washing, and were found to be 40.5%, 36%, and 4.4% for lithium, sodium, and potassium, respectively.

[0126] If the liquid fraction obtained by solid-liquid separation of the concentrated slurry in this example and the solid washing filtrate obtained at the same time were combined and returned to the electrochemical membrane separation process, 97.8%, 49%, and 22% of the lithium, sodium, and potassium introduced into the sulfate concentration and crystallization process would be returned to the electrochemical membrane separation process, respectively. If the solid fraction after washing contained only a very small amount of lithium (39 ppm as dry powder), more than 99% of the lithium should be contained in these solutions. However, the calculated result of 97.8% based on the analytical value is presumably due to experimental and analytical errors. However, if the lithium return rate was set low in the simulation, it would be unfavorable for the lithium hydroxide yield, so there is no risk of overestimating the simulation results.

[0127] Furthermore, if only the liquid obtained by solid-liquid separation of the slurry is returned to the electrochemical membrane separation step (the washing filtrate is discharged outside the system), these values ​​become 57.4%, 13%, and 17%, respectively.

[0128] From the return rate Z calculated in Example 3, the return rate Z3 was calculated by multiplying the amount (1-Z) remaining in the liquid fraction, i.e., 1.2%, 90%, and 88% for lithium, sodium, and potassium, by these values. When both the liquid fraction from the slurry solid-liquid separation and the washing filtrate were returned (Experiment No. 4A, Simulation No. 4B), the return rates were 1.22%, 44%, and 19%, respectively. When only the liquid fraction from the slurry solid-liquid separation was returned (Experiment No. 4B, Simulation No. 4B), the return rates were 0.72%, 11%, and 15%, respectively. Calculating the return rate Z = Z1 + Z2 + Z3 from these return rates Z3, the return rates Z for lithium, sodium, and potassium were 99.97%, 54%, and 31%, respectively, for Experiment No. 4A, and 99.5%, 22%, and 27%, respectively, for Experiment No. 4B. The results of simulations performed based on this result are shown in Tables 5 and 6 together with the results of Nos. 1A and 3 (simulation Nos. 4A and 4B).

[0129]

[0130]

[0131] From the results in Table 6, in Simulation No. 4A, which is the case where lithium is recovered to the maximum, the lithium hydroxide yield can be increased to 99.97%. Although the relative processing volume also increases, and the operational efficiency of the equipment decreases, in operations in areas where waste disposal costs are high, such operating conditions, which can reduce the total amount of waste even if only slightly, can be valuable.

[0132] In the case of 4B, in which the washing filtrate of the solids obtained in the sulfate concentration crystallization is discharged outside the system, it is shown that the increase in the relative throughput and the blow rate of lithium hydroxide crystallization is not very large.

[0133] The operation of Experiment 4A was performed with a large amount of water passed through the washing operation to demonstrate that the amount of lithium contained in the solids was reduced. When the present invention is applied to actual operations, the degree of washing of the solids should naturally be optimized, and therefore the amount of washing liquid can be reduced. Since such operating conditions are considered to exhibit a return rate between those of Experiments 4A and 4B, optimization can be performed by recycling the washing filtrate into the system, discharging it outside the system, or recycling a portion of it into the system, taking into account waste disposal costs and facility operating efficiency.

[0134] Reference Example 1: (Separation and recovery of lithium from an aqueous sulfate solution by a concentration and crystallization operation) A mother liquor discharged from a lithium hydroxide crystallization operation was neutralized with sulfuric acid to form a sulfate solution, and then it was verified whether it was possible to separate lithium from sodium and potassium by a concentration and crystallization operation and recycle the solution.

[0135] The experiment was carried out by concentrating a simulated solution of sulfate reagents in water. The solubility of lithium sulfate decreases as the solution temperature increases, while the solubility of sodium sulfate and potassium sulfate increases as the solution density increases. Therefore, it is expected that lithium sulfate can be obtained as a solid by heating and concentrating a mixed sulfate solution of these.

[0136] A simulated solution was prepared by dissolving lithium sulfate, sodium sulfate, and potassium sulfate so that the total concentration of the alkali metals obtained by dissolving each reagent was 0.226 mol / L, and the Na / Li ratio was 0.11 and K / Li ratio was 0.084. 1100 g of this simulated solution was placed in a 1 L stainless steel vessel and heated to 70°C using a mantle heater. While stirring, the absolute pressure was reduced to 0.02 PMa, and a concentration operation was performed.

[0137] The concentration operation was carried out for a total of 4.5 hours, with the liquid sample being sampled five times during the operation. As the concentration proceeded, solids precipitated. The slurry was subjected to solid-liquid separation by vacuum filtration using a Buchner funnel and Advantec filter paper No. 5C. The solids were recovered without washing, and the weights of the solids and liquid were recorded, followed by an analysis of their compositions.

[0138] When the material balance was calculated taking into account the amounts of elements taken out of the system by sampling during the concentration operation, it was found that the liquid recovered by solid-liquid separation contained only 13%, 53%, and 22% of the lithium, sodium, and potassium initially charged. From this result, if the unwashed solids are redissolved and returned to the electrochemical membrane separation process, the return rates Z of lithium, sodium, and potassium can be assigned to be 87%, 47%, and 78%, respectively.

[0139] When a simulation was performed using this return rate under the same conditions as in the example, the lithium hydroxide yield was 76.4%, the lithium hydroxide crystallization blow rate was 70.4%, and the relative processing amount was 2.58. In other words, even though the amount of lithium processed in the electrochemical membrane separation step increased to 2.58 times the amount supplied from the raw aqueous solution per unit time, only 76.4% of the lithium supplied from the raw aqueous solution was recoverable as lithium hydroxide, demonstrating that this is a production method in which the yield of lithium hydroxide is low despite the significant increase in the processing amounts in the electrochemical membrane separation step and the crystallization step.

[0140] It is conceivable that the amounts of sodium and potassium mixed into the solid content (mainly composed of lithium sulfate) can be reduced by stopping the concentrating operation of the sulfate solution shown in this example early. However, it is clear from the discussion of the example that the proportion of lithium that can be recovered as a solid content is significantly reduced, and therefore a decrease in the final yield of lithium hydroxide is unavoidable.

[0141] It is also known that lithium sulfate and potassium sulfate, and lithium sulfate and sodium sulfate form double salts. The composition of the mother liquor sample obtained during this concentration and crystallization operation is shown in Table 7. It can be seen that the K / Li value no longer fluctuates at the final stage of concentration.

[0142] If only lithium sulfate is precipitated by the concentration operation, the Na / Li and K / Li values ​​will increase, and this is shown in the data up to 4 hours in Table 7. However, as the concentration progresses, the K / Li value reaches the double salt formation composition, and so a double salt of lithium sulfate and potassium sulfate is produced and precipitated, which can be understood to have significantly increased the amount of potassium contained in the solid content.

[0143]

[0144] In this Reference Example, the raw material simulant solution was prepared to have similar Na / Li and K / Li values ​​(both approximately 0.1) as in Example 1. However, if the Na / Li and / or K / Li values ​​were larger as in Example 2 (both approximately 0.3 in Example 2), it is expected that the timing of double salt formation during the concentration process would be earlier, and the amount of sodium and / or potassium mixed into the solids would be greater than in this Reference Example. Therefore, it has been shown that separating and recovering lithium from a sulfate solution by concentration crystallization is not realistic. Thus, the difficulty in separating lithium and alkali metal impurities as the concentration of alkali metal impurities in the lithium hydroxide crystallization mother liquor increases, which is in stark contrast to the slight difference observed between Examples 1 and 2.

[0145] Reference Example 2: (Separation and recovery of lithium from an aqueous sulfate solution by cooling and crystallization) Assuming an aqueous solution with a lithium hydroxide concentration of 14.2% by weight (saturated solubility of lithium hydroxide alone at 80°C), and calculating the case where this is neutralized with a stoichiometric amount of sulfuric acid to convert it into an aqueous lithium sulfate solution, the required amount (weight) of sulfuric acid is 2.05 times the amount of lithium hydroxide in the solution, and the concentration of lithium sulfate contained in the solution after neutralization can be calculated to be 25.3%. At this concentration, the entire amount dissolves even in a solution at a low temperature (for example, below 25°C). This means that 3.19% by weight of lithium is dissolved as ions in this sulfate solution.

[0146] When producing such a solution, if the initial lithium hydroxide solution contains sodium such that Na / Li = 0.3, then the sulfate solution will contain 3.19 x 0.3 = 0.957 wt % sodium dissolved therein.

[0147] The solubility of sodium sulfate is about 4.8% by weight (about 1.5% by weight as sodium ions) at 0°C, so even if 0.957% by weight of sodium ions is dissolved, it is not possible to obtain sodium sulfate crystals by cooling to 0°C. It is known that the solubility of sodium sulfate increases when lithium sulfate coexists in a solution, so reducing the Na / Li value by precipitating sodium sulfate through cooling crystallization is not an efficient means.

[0148] If a similar consideration is made for potassium sulfate (the solubility of potassium sulfate at 0° C. is about 7.1% by weight, or about 3.2% by weight as potassium ions), a similar result can be predicted.

[0149] One possible method would be to prepare a sulfate solution with a high Na / Li and K / Li ratio (i.e., concentrated sodium and potassium) and then cool it, but the following experiment makes it clear that this is also not an effective method for separating and removing sodium and potassium.

[0150] Lithium sulfate monohydrate, anhydrous sodium sulfate, and anhydrous potassium sulfate were weighed out so that the impurity concentrations were Na / Li = 1.1 and K / Li = 0.83, and dissolved in water heated to 70°C while keeping the temperature constant. This resulted in the preparation of an aqueous solution containing 2.2 wt % of lithium, 2.4 wt % of sodium, and 1.8 wt % of potassium. When the aqueous solution was concentrated beyond this concentration, a precipitate was immediately observed. This concentration is significantly lower than the solubility of the sulfate alone, but it can be understood that the solubility of each alkali metal ion was significantly reduced because the double salt became an equilibrium phase at 70°C.

[0151] The container containing this mixed aqueous sulfate solution was cooled in an ice bath until the temperature of the solution reached 2° C., but no precipitate was obtained. Considering the solubilities of sulfates alone (approximately 3.3 wt %, approximately 1.5 wt %, and approximately 3.2 wt % for lithium ions, sodium ions, and potassium ions, respectively, at 0° C., it is reasonable to expect that sodium would precipitate as sodium sulfate. However, it can be understood that the solubility was increased by the coexisting sulfates, and therefore sodium sulfate did not precipitate.

[0152] As shown in this example, even if sodium and potassium are concentrated as sulfates in lithium sulfate, in a system containing high concentrations of lithium sulfate, it is difficult to efficiently separate sodium and potassium from lithium and recover them from the aqueous sulfate solution by cooling crystallization. This is in contrast to a system in which the concentration of lithium sulfate is made low, as in Example 4.

[0153] The above shows the results of simulations based on the characteristic values ​​obtained from experiments in the case of a conventional technique in which the lithium hydroxide crystallization mother liquor, in which impurities have been concentrated, is not treated and recycled into the system (Comparative Example 1, hereinafter designated by simulation number C1), and in the examples and reference examples (hereinafter designated by simulation number R1). In addition to these results, Table 8 also shows the results of simulations conducted under operating conditions in which the impurity levels in the crystals obtained by lithium hydroxide crystallization are halved while the impurity content of the raw material aqueous solution is kept the same, i.e., sodium is 10 ppm or less and potassium is 5 ppm or less.

[0154]

[0155] In Table 8, the value shown as "lithium hydroxide yield / relative throughput" is the value obtained by dividing the lithium hydroxide yield, normalized to 0 to 1, by the relative throughput, and represents the relative value of the lithium hydroxide yield when the scales of the electrochemical membrane separation step and the lithium hydroxide crystallization step are set to the same standard. In other words, it is an index that represents the productivity of lithium hydroxide crystals when these steps are set to the same scale.

[0156] The results shown in Table 8 reveal the following. Specifically, in the system containing high concentrations of lithium as shown in Reference Example 1, converting lithium to sulfate and then concentrating and crystallizing the resulting solution can increase the yield of lithium hydroxide crystallization compared to the conventional technology (Comparative Example 1), but the value of "lithium hydroxide yield / relative throughput" decreases, resulting in a decrease in equipment utilization efficiency. On the other hand, when the present invention is used, the lithium hydroxide yield exceeds 90% in all cases. Furthermore, when attempting to halve the level of impurities contained in the product while keeping the composition of the raw material aqueous solution the conventional technology (Comparative Example 1) and Reference Example 1 significantly reduce not only the lithium hydroxide yield but also the equipment utilization efficiency. However, the results of using the present invention show that this reduction is much smaller than those of the conventional technology (Comparative Example 1) and Reference Example 1.

[0157] Therefore, it can be understood that by utilizing the technology disclosed in the present invention, it is possible to not only increase the yield of lithium hydroxide but also improve the utilization efficiency of facilities.

[0158] In the above description of the examples, a configuration in which high-purity lithium hydroxide crystals are obtained by applying only one lithium hydroxide crystallization step has been exemplified. However, if one understands the concept disclosed in the present invention, it will be easily understood that an application example such as that shown in FIG. 2 is also possible.

[0159] In Figure 2, the area surrounded by a dashed line indicates that the process is the same as that shown in Figure 1, except that the crystals obtained by lithium hydroxide crystallization are an intermediate product (this lithium hydroxide crystallization process will be referred to as the first lithium hydroxide crystallization process). Figure 2 also illustrates a process in which this intermediate product is (I) re-dissolved in an aqueous medium, and (J) lithium hydroxide crystallization is carried out again to obtain high-purity lithium hydroxide crystals (product) (this lithium hydroxide crystallization process will be referred to as the second lithium hydroxide crystallization process). The water used for re-dissolution may be recycled from the water discharged from the concentration process or the concentration and crystallization process.

[0160] The configuration illustrated in FIG. 2 is particularly effective when the raw lithium sulfate contains a high concentration of alkali metal impurities. Specifically, the lithium hydroxide crystals obtained in the first lithium hydroxide crystallization step within the dashed-dotted line are an intermediate product (crude product) with a reduced impurity content. These are re-purified in the second lithium hydroxide crystallization step, followed by (K) a solid-liquid separation step to separate the solids from the crystallized slurry and a washing step to obtain high-purity lithium hydroxide crystals as a product. The crystallization mother liquor blown from the second lithium hydroxide crystallization step is introduced, for example, into the first lithium hydroxide crystallization step. The crystallization mother liquor can also be introduced into the carbonation step. Since the mechanism by which alkali metal impurities are discharged outside the system is the same as in the configuration illustrated in FIG. 1, these steps can be carried out by the operations described above. Furthermore, the lithium contained in the mother liquor blown from the second lithium hydroxide crystallization step is recycled in the same manner as in the case of FIG. 1, thereby maintaining a high yield of lithium hydroxide as a product.

[0161] When the present invention is implemented using such a configuration having two crystallization steps, the first lithium hydroxide crystallization step functions to adjust the amount of alkali metal impurities remaining in the system, and the second lithium hydroxide crystallization step functions to adjust the impurity level in the lithium hydroxide crystals obtained as the product. The blowdown rate of each lithium hydroxide crystallization step is an important parameter for controlling these functions. That is, the higher the blowdown rate of the first lithium hydroxide crystallization step, the greater the amount of alkali metal impurities discharged from the system after the carbonation step. This reduces the amount of alkali metal impurities remaining in the system, making it possible to maintain an impurity concentration in the mother liquor that prevents the precipitation of sodium hydroxide and / or potassium hydroxide in the first lithium hydroxide crystallization step. Furthermore, the higher the blowdown rate of the second lithium hydroxide crystallization step, the more effectively the impurity concentration in the lithium hydroxide crystals obtained as the product can be reduced.

[0162] If the lithium sulfate raw material contains a large amount of alkali metal impurities, the electrical energy input to the electrochemical membrane separation step and the operating costs required to operate the carbonation step, and / or the carbonate concentration and crystallization step, and / or the sulfate concentration and crystallization step will be relatively high relative to the amount of lithium hydroxide obtained as a product. However, this method makes it possible to produce lithium hydroxide from low-grade lithium resources (resources with a low lithium content), and therefore, this method may be an important production method in the current situation where demand for lithium hydroxide is increasing significantly. Furthermore, if the sulfate concentration and crystallization step is carried out and the solid content obtained in this step is appropriately washed to recover alkali metal impurities as an alkali metal resource with a reduced amount of lithium contamination, the present invention will be an effective method for producing lithium hydroxide and a method for separating and recovering alkali metal resources, particularly when the lithium sulfate raw material contains a large amount of highly useful potassium.

[0163] The method for producing lithium hydroxide of the present invention makes it possible to recover lithium contained in raw materials in high yield as lithium hydroxide, to efficiently remove alkali metal impurities using existing equipment, and to reduce the amount of waste generated, thereby realizing a lithium hydroxide production process with extremely excellent economic efficiency.

Claims

1. A method for producing high-purity lithium hydroxide from lithium sulfate containing either sodium or potassium, or both, as impurities, comprising the following steps (A) to (F): (A) a step of producing an aqueous lithium hydroxide solution and sulfuric acid by electrochemical membrane separation using lithium sulfate as a raw material; (B) a crystallization step of producing lithium hydroxide crystals by crystallization using the aqueous lithium hydroxide solution obtained by the electrochemical membrane separation step as a raw material; (C) a solid-liquid separation step of extracting a part of the slurry in the crystallization step and separating it into lithium hydroxide crystals and a crystallization mother liquor, and a washing step of washing the solids; (D) a carbonation step of extracting a part of the crystallization mother liquor in the crystallization step and reacting it with carbon dioxide gas to obtain a slurry containing a lithium-containing carbonate compound as a solid component, and a solid-liquid separation step of separating the slurry into a lithium-containing solid component and a liquid component; (E) an acid dissolution step of reacting the lithium-containing solid component with sulfuric acid to produce an aqueous lithium sulfate solution; and (F) a mixing step of reusing the lithium sulfate obtained in the acid dissolution step as a raw material for the step (A).

2. The method for producing lithium hydroxide according to claim 1, further comprising: (G) a carbonate concentration and crystallization step in which the liquid fraction obtained from the carbonation step and solid-liquid separation step of (D) is concentrated and crystallized to obtain a slurry; and a solid-liquid separation step in which the slurry is separated into a lithium-containing solid fraction and a liquid fraction, and the solid fraction obtained in the solid-liquid separation step is introduced into the step of (E).

3. The method for producing lithium hydroxide according to claim 1 or 2, further comprising: (H) a step of adding sulfuric acid to the liquid fraction obtained in the carbonate concentrating and crystallizing step (G) and the solid-liquid separation step (G) to produce a sulfate solution; a sulfate concentrating and crystallizing step of concentrating and crystallizing the sulfate solution to obtain a slurry; and a solid-liquid separation step of separating the slurry into a solid fraction and a liquid fraction, wherein the liquid fraction obtained in the solid-liquid separation step is introduced into step (E) or (F).

4. The method for producing lithium hydroxide according to any one of claims 1 to 3, further comprising a washing step, as a solid-liquid separation step or a subsequent step of solid-liquid separation, for washing any or all of the solid fractions obtained in the (D) carbonation step and solid-liquid separation step, the solid fraction obtained in the (G) carbonate concentration and crystallization step and solid-liquid separation step, and the solid fractions obtained in the (H) sulfate concentration and crystallization step.

5. A method for producing lithium hydroxide according to any one of claims 1 to 4, which comprises introducing a part or all of the washing filtrate obtained in the washing step according to claim 4 into step (E).

6. The method for producing lithium hydroxide according to any one of claims 1 to 5, wherein the electrochemical membrane separation step (A) is an electrodialysis method.

7. A method for producing lithium hydroxide according to any one of claims 1 to 6, further comprising: (I) a redissolving step of redissolving the lithium hydroxide crystals obtained in the crystallization step (B) in an aqueous medium; (J) a recrystallization step (second lithium hydroxide crystallization step) of producing lithium hydroxide crystals by further crystallization; and (K) a solid-liquid re-separation step of separating the slurry obtained in the recrystallization step into lithium hydroxide crystals and a crystallization mother liquor, and a washing step of washing the solids, wherein the crystallization mother liquor obtained in the solid-liquid re-separation step is supplied to the crystallization step (B) and / or the carbonation step (D) and the recrystallization step (J).