Lithium carbonate production method

Abstract

Provided is a production method by which high-purity lithium carbonate can be obtained at low cost without using a large amount of chemicals.　The present invention involves sequentially executing a lithium adsorption step (1), a lithium elution step (2), an impurity removal step (3), and a conversion step (4), wherein, in the impurity removal step (3), an oxidizing agent is added to a second lithium-containing solution to oxidize manganese in the second lithium-containing solution into a form of insoluble manganese dioxide, an alkali is added to the second lithium-containing solution after an oxidation step (3A) is executed to precipitate and remove, as hydroxides, mainly magnesium in the second lithium-containing solution and manganese remaining after the oxidation step, thereby obtaining a neutralized solution with reduced magnesium and manganese, and the neutralized solution is brought into contact with an ion exchange resin to remove mainly calcium and aluminum and to remove the remaining magnesium and manganese, thereby obtaining a high-purity lithium carbonate-containing solution.

Description

Lithium carbonate manufacturing method

[0001] The present invention relates to a method for producing lithium carbonate. More specifically, the present invention relates to a method for producing lithium carbonate that can produce high-purity lithium carbonate at low cost.

[0002] In recent years, the demand for lithium-based secondary batteries has been increasing for use in automotive batteries. Lithium is extracted from salt lake brine or lithium ore (Li ２ O.Al ２ O ３ 2SiO ４ Lithium has been produced by refining ores containing lithium, such as uranium dioxide. Because the lithium content of these raw materials is low compared to other components, they are first subjected to a concentration process to increase the lithium concentration, followed by a conversion process to convert it into lithium carbonate. For example, when using brine from salt lakes as the raw material, solar concentration is the main method used in major lithium producing countries such as Chile and Argentina.

[0003] In this solar concentration method, first in the concentration process, salt lake brine is concentrated in a concentration pond called a solar pond, where the main components, sodium and potassium, are precipitated and removed as salts such as sodium chloride and potassium chloride, concentrating the highly soluble lithium chloride in aqueous solution. After concentration, impurities are removed as needed in a liquid purification process, and in the next conversion process, sodium carbonate is added to convert the lithium chloride into lithium carbonate, which has low solubility. In this conversion process, lithium carbonate precipitates as a solid due to its low solubility. This method makes it possible to recover lithium carbonate.

[0004] On the other hand, methods such as solvent extraction, ion exchange, and chromatographic separation have been used to produce lithium carbonate and other compounds that are not affected by climatic conditions or the properties of salt lake brine. However, salt lake brine, which is rich in lithium, contains impurities such as sodium, potassium, magnesium, and calcium in overwhelmingly greater amounts than lithium. Therefore, when solvent extraction is used in the concentration process, the extractant has a problem of low lithium selectivity. That is, impurities must first be removed from the impurity-rich liquid using large amounts of chemicals, resulting in high overall costs. Ion exchange also has a problem of low lithium selectivity of ion exchange resins, and similar to solvent extraction, the concentration process is costly. Chromatographic separation is a method for fractionating and recovering lithium by utilizing the difference in elution rate between lithium and impurities other than lithium in the liquid (Patent Document 1). This method also has problems with the concentration process, such as the use of large amounts of chemicals and the need for multiple steps, resulting in high equipment costs.

[0005] In contrast to the above-mentioned methods, Non-Patent Document 1 discloses a lithium recovery method using a lithium-selective adsorbent. Specifically, lithium is adsorbed using a lithium-selective adsorbent in a concentration step, followed by the addition of an alkaline carbonate in a conversion step to remove impurities, followed by heating and concentration, and finally the addition of a carbonate to cause precipitation, thereby obtaining powdered lithium carbonate. However, the lithium carbonate obtained by the method of Non-Patent Document 1 contains high impurity concentrations (e.g., magnesium and sodium) ranging from several hundred to several thousand ppm. Furthermore, since this method does not include a step of removing calcium after the selective adsorption of lithium, when treating brine containing a large amount of calcium, the trace amounts of calcium remaining after the selective adsorption of lithium cannot be removed, and the concentration in the final lithium carbonate product is similar to that of magnesium and sodium. Such impurity concentrations are unacceptable when used as a raw material for lithium-ion batteries, the demand for which has recently increased rapidly.

[0006] In each of the above-mentioned conventional technologies, when lithium carbonate is produced, if a conventional solvent extraction method or the like is used for the lithium concentration step, there is a problem that the cost is high in order to obtain lithium from a liquid containing a large number of impurities. Furthermore, the conventional technology of Non-Patent Document 1 discloses a method for obtaining lithium carbonate by using a lithium-selective adsorbent in the conversion step, but as mentioned above, it is not possible to obtain high-purity lithium carbonate having a quality that can be used as a battery raw material.

[0007] International Publication No. 2012 / 163791

[0008] Yu, Eihei, "Lithium Recovery System from Brine," [online], June 11, 2010, Kagawa Industrial Support Foundation [November 22, 2018], Internet (https: / / www.kagawa-isf.jp / wp-content / uploads / 2022 / 02 / 21tang.pdf)

[0009] In view of the above circumstances, an object of the present invention is to provide a method for producing lithium carbonate that can obtain high-purity lithium carbonate at low cost without using large amounts of chemicals.

[0010] The method for producing lithium carbonate of the first invention includes the steps of: (1) lithium adsorption step: a step of bringing a first lithium-containing solution having a low lithium concentration as a raw material into contact with a lithium-selective adsorbent to adsorb lithium onto the lithium-selective adsorbent; (2) lithium elution step: a step of eluting lithium from the lithium-selective adsorbent to obtain a second lithium-containing solution; (3) impurity removal step: a step of removing a part of metal ions from the second lithium-containing solution to obtain a third lithium-containing solution; and (4) conversion step: a step of converting lithium salt contained in the third lithium-containing solution to lithium carbonate to obtain crystals of the lithium carbonate, the impurity removal step being carried out in this order; (3B) a neutralization step: adding an alkali to the second lithium-containing solution after the oxidation step to precipitate and remove mainly magnesium in the second lithium-containing solution and manganese remaining after the oxidation step as hydroxides, thereby obtaining a neutralized solution with reduced magnesium and manganese; and (3C) an ion exchange step: bringing the neutralized solution obtained in the neutralization step into contact with an ion exchange resin to remove mainly calcium and aluminum, and also to remove magnesium and manganese remaining after the neutralization step, thereby obtaining the third lithium-containing solution. A second aspect of the present invention is a method for producing lithium carbonate in accordance with the first aspect of the present invention, characterized in that the conversion step comprises sequentially carrying out the following steps: (4A) a carbonation step: a step of adding a carbonate source to the third lithium-containing solution obtained in the impurity removal step to obtain crude lithium carbonate; (4B) a hydrogencarbonation step: a step of blowing carbon dioxide into a slurry containing the crude lithium carbonate to obtain a lithium hydrogencarbonate solution; and (4C) a decarbonation step: a step of heating the lithium hydrogencarbonate solution to obtain purified lithium carbonate. A third aspect of the present invention is a method for producing lithium carbonate in accordance with the second aspect of the present invention, characterized in that the temperature in the decarbonation step of the conversion step is 50°C or higher and 100°C or lower.A fourth aspect of the invention is a method for producing lithium carbonate, wherein the lithium-selective adsorbent in the lithium adsorption step is a manganese oxide obtained by contacting lithium manganate with an acid and desorbing lithium from the lithium manganate. A fifth aspect of the invention is a method for producing lithium carbonate, wherein the hydrogen ion concentration of the acid used in the lithium elution step is 0.3 mol / L or more and 2.0 mol / L or less. A sixth aspect of the invention is a method for producing lithium carbonate, wherein the pH of the neutralized solution in the neutralization step of the impurity removal step is 8.5 or more and 12 or less. A seventh aspect of the invention is a method for producing lithium carbonate, wherein the ion exchange resin used in the ion exchange step of the impurity removal step is an iminodiacetic acid type chelating resin.

[0011] According to the first invention, by performing four basic steps and three of the steps, from the oxidation step to the ion exchange step, in the impurity removal step, impurities such as manganese, magnesium, calcium, and sodium can be removed with high accuracy. Therefore, high-purity lithium carbonate can be obtained at low cost without using large amounts of chemicals. According to the second invention, by performing three steps in the conversion step, calcium carbonate and sodium contained in the third lithium-containing solution can be removed at high levels, thereby obtaining high-purity lithium carbonate. According to the third invention, by setting the temperature in the decarbonation step to 50°C or higher and 100°C or lower, the decarbonization reaction proceeds sufficiently, resulting in high-purity purified lithium carbonate. According to the fourth invention, by using manganese oxide from which lithium has been desorbed, ion exchange with lithium can be performed, and lithium adsorption can be efficiently performed. According to the fifth invention, by setting the hydrogen ion concentration of the acid in the range of 0.3 mol / L or higher and 2.0 mol / L or lower, it can be used in place of the mineral acid used in the lithium elution step. According to the sixth aspect of the present invention, magnesium and manganese can be efficiently precipitated and removed by adjusting the pH of the neutralized solution to 8.5 or more and 12 or less. According to the seventh aspect of the present invention, the calcium concentration after treatment can be reduced by using an iminodiacetic acid type chelating resin as the ion exchange resin.

[0012] Fig. 1 is a basic process diagram in a method for producing lithium carbonate according to one embodiment of the present invention; Fig. 2 is a flow diagram of an impurity removal process in a method for producing lithium carbonate according to one embodiment of the present invention; Fig. 3 is a flow diagram of a conversion process in a method for producing lithium carbonate according to one embodiment of the present invention.

[0013] Next, embodiments of the present invention will be described with reference to the drawings. However, the embodiments described below are merely examples of production methods for embodying the technical concept of the present invention, and the production method of lithium carbonate in the present invention is not limited to the following.

[0014] The method for producing lithium carbonate according to the present invention comprises the following basic steps (1) to (4), which are carried out in this order: (1) Lithium adsorption step: A step of bringing a first lithium-containing solution, which is a raw material having a low lithium concentration, into contact with a lithium-selective adsorbent to adsorb lithium onto the lithium-selective adsorbent; (2) Lithium elution step: A step of eluting lithium from the lithium-selective adsorbent to obtain a second lithium-containing solution; (3) Impurity removal step: A step of removing a portion of the metal ions from the second lithium-containing solution to obtain a third lithium-containing solution; (4) Conversion step: A step of converting lithium salt contained in the third lithium-containing solution to lithium carbonate to obtain lithium carbonate crystals. The impurity removal step (3) comprises the following three steps (3A) to (3C), which are carried out in this order: (3A) Oxidation step: A step of adding an oxidizing agent to the second lithium-containing solution to oxidize manganese in the second lithium-containing solution to an insoluble form of manganese dioxide. (3B) Neutralization step: A step of adding an alkali to the second lithium-containing solution after the oxidation step (3A) to precipitate and remove mainly magnesium in the second lithium-containing solution and manganese remaining after the oxidation step as hydroxides, thereby obtaining a neutralized solution with reduced magnesium and manganese. (3C) Ion exchange step: A step of contacting the neutralized solution obtained in the neutralization step (3B) with an ion exchange resin to remove mainly calcium and aluminum, as well as magnesium and manganese remaining after the neutralization step, thereby obtaining the third lithium-containing solution.

[0015] In the present invention, the impurity removal step (3) includes three steps, namely, an oxidation step (3A), a neutralization step (3B), and an ion exchange step (3C), thereby making it possible to reliably remove metals other than lithium. Furthermore, an oxidizing agent is used in the oxidation step (3A), and an alkali is used in the neutralization step (3B) to remove impurities, and then an ion exchange resin is used to remove the remaining impurities. However, most of the impurities are removed by the lithium adsorption step (1), which selectively adsorbs lithium. Therefore, high-purity lithium carbonate can be obtained at low cost without using a large amount of chemicals in the impurity removal step (3).

[0016] In the present invention, the conversion step (4) preferably comprises the following three steps (4A) to (4C): (4A) carbonation step: a step of adding a carbonate source to the third lithium-containing solution obtained in the impurity removal step (3) to obtain crude lithium carbonate; (4B) hydrogen carbonate step: a step of blowing carbon dioxide into a slurry containing the crude lithium carbonate to obtain a lithium hydrogen carbonate solution; and (4C) decarbonation step: a step of heating the lithium hydrogen carbonate solution to obtain purified lithium carbonate.

[0017] By the conversion step (4), sodium and potassium contained in the third lithium-containing liquid can be removed at a high level, and the purity of the obtained lithium carbonate can be further increased.

[0018] (First embodiment) Fig. 1 shows a method for producing lithium hydroxide according to one embodiment of the present invention. Each step will be described in detail below with reference to Figs. 1 to 3. <Lithium adsorption step (1)> The lithium adsorption step (1) shown in Fig. 1 is a step of bringing a lithium-selective adsorbent into contact with a first lithium-containing solution having a relatively low lithium concentration, such as salt lake brine, which is the raw material, and selectively adsorbing lithium from this first lithium-containing solution onto the lithium-selective adsorbent.

[0019] The reaction formula for the lithium adsorption step (1) is shown in Equation 1. Here, H is used as the lithium selective adsorbent. １．６ Mn １．６ O ４ Although the reaction formula using the formula (I) is shown, it is not particularly limited to this. For example, it is also possible to use a lithium-selective adsorbent obtained from lithium manganese oxide having a spinel structure. Furthermore, the lithium-selective adsorbent obtained from lithium manganese oxide is preferably a manganese oxide obtained by contacting lithium manganese oxide with an acid and removing lithium from the lithium manganese oxide. By using manganese oxide from which lithium has been removed, ion exchange with lithium can be performed, and lithium adsorption can be carried out efficiently.

[0020] [Math 1] H １．６ Mn １．６ O ４ +1.6LiCl → Li １．６ Mn １．６ O ４+1.6HCl

[0021] The method for contacting the lithium-selective adsorbent with the first lithium-containing solution is not particularly limited, as long as contact can be achieved. For example, a column method, a batch mixing method, or the like can be arbitrarily adopted. In the column method, when the lithium-selective adsorbent is used as a fine powder, the liquid flow resistance is high, and continuous liquid flow is often difficult. For this reason, it is preferable to use pellets prepared by kneading the lithium-selective adsorbent (including a precursor before becoming the lithium-selective adsorbent) with a binder such as an alumina binder and sintering the mixture.

[0022] In the lithium adsorption step (1), the pH of the first lithium-containing solution is preferably 3 or more and 10 or less. Because the reaction in the lithium adsorption step (1) is a reaction in which lithium is adsorbed to produce an acid, a low pH of the aqueous solution may slow the reaction rate or prevent the reaction from occurring. Therefore, it is desirable to increase the pH of the first lithium-containing solution before contacting the first lithium-containing solution with the lithium-selective adsorbent. However, if the first lithium-containing solution contains magnesium, magnesium hydroxide precipitates if the pH becomes too high. This magnesium hydroxide covers the surface of the lithium-selective adsorbent, physically inhibiting the lithium adsorption reaction.

[0023] When a column method is employed as a method for contacting the first lithium-containing solution with a lithium-selective adsorbent, blockages due to magnesium hydroxide often occur within the column. From this viewpoint, the pH of the first lithium-containing solution is preferably 10 or less. Furthermore, if the pH is less than 3 after passing through the column, the lower part of the column may not be able to efficiently adsorb lithium. For these reasons, in order to effectively promote the lithium adsorption reaction, the pH of the first lithium-containing solution is preferably 3 or more and 10 or less.

[0024] When a circulation method is used as a method for contacting the first lithium-containing solution with the lithium-selective adsorbent, it is preferable to add a neutralizing agent to the effluent after passing through the column to adjust the pH to the above range and then return it to the column. When using a column method, the passing rate can be changed depending on the required throughput. If a precipitate occurs after neutralization, a filtration device such as a filter press or check filter can be used as appropriate to smoothly pass the solution through the column. After the adsorption operation, the lithium-adsorbed lithium-selective adsorbent may be washed with water as needed to perform the next lithium elution step (2). When passing through a column, water washing can be performed by passing relatively high-purity distilled water through the column after passing the first lithium-containing solution through the column and extruding and washing the first lithium-containing solution remaining inside. When using batch mixing, water washing can be performed by pouring water onto the lithium-adsorbed lithium-selective adsorbent after solid-liquid separation, to remove the first lithium-containing solution that adheres to the adsorbed lithium.

[0025] <Lithium Elution Step (2)> In the lithium elution step (2) shown in Figure 1, the lithium-selective adsorbent that has adsorbed lithium is contacted with a mineral acid such as hydrochloric acid to obtain a second lithium-containing solution. The lithium-selective adsorbent after lithium adsorption is in the form of, for example, lithium manganate, and lithium is eluted by contacting this lithium-selective adsorbent after lithium adsorption with a mineral acid such as hydrochloric acid. The contact method is generally a column system, but a batch mixing system may also be used, and any contact method is possible. The reaction formula during lithium elution is shown in Equation 2.

[0026] [Math 2] Li １．６ Mn １．６ O ４ +1.6HCl → H １．６ Mn １．６ O ４ +1.6LiCl

[0027] In the above reaction formula, Li is used as lithium manganate. １．６ Mn １．６ O ４However, the present invention is not limited to this. For example, it is possible to use a lithium-selective adsorbent obtained from lithium manganese oxide having a spinel structure. The mineral acid concentration should be sufficient to elute lithium; however, if the concentration is too high, the lithium manganese oxide used in the lithium-selective adsorbent will dissolve and be worn out. If the concentration is too low, lithium will not be eluted. The mineral acid can be hydrochloric acid, sulfuric acid, or nitric acid. When using any of these acids, the acid concentration is preferably 0.3 mol / L or more and 2.0 mol / L or less. In this case, lithium elution is efficient and the lithium-selective adsorbent is less likely to be worn out. When using a column system, the liquid flow rate can be adjusted depending on the required throughput.

[0028] The eluate obtained in the lithium elution step (2), i.e., the second lithium-containing solution, contains sodium, potassium, magnesium, and calcium, which are small amounts of the elements that accompany the lithium adsorption step (1). It also contains manganese eluted from the lithium-selective adsorbent. In addition, when granules using an alumina binder are used as the lithium-selective adsorbent, the second lithium-containing solution contains aluminum. Among these impurities, polyvalent metals other than sodium and potassium cause problems, such as shortening the membrane life in the subsequent conversion step (4), and are therefore removed in the impurity removal step (3). Note that, after lithium adsorption, the lithium-selective adsorbent returns to the state it was before lithium adsorption through the elution operation, making it capable of adsorbing lithium again. Therefore, it can be reused in the lithium adsorption step (1), as indicated by arrow a in Figure 1 . When reused, the lithium-selective adsorbent is preferably washed with water.

[0029] Furthermore, the second lithium-containing solution can be used in place of the mineral acid used in the lithium elution step (2) by adding an acid to the second lithium-containing solution and adjusting the hydrogen ion concentration of the acid to a range of 0.3 mol / L or more and 2.0 mol / L or less. By repeatedly using the second lithium-containing solution obtained in the lithium elution step (2) again in the lithium elution step (2) as shown by arrow b in Figure 1 , the lithium concentration in the second lithium-containing solution can be increased, and the amount of the second lithium-containing solution used in the subsequent impurity removal step (3) can be reduced. This allows the equipment capacity for the impurity removal step (3) and subsequent steps to be reduced, thereby suppressing equipment costs and running costs for chemicals and the like.

[0030] <Impurity Removal Step (3)> In the impurity removal step (3) shown in Fig. 1, a portion of the metal ions is removed from the second lithium-containing solution to obtain a third lithium-containing solution. Fig. 2 shows the details of the impurity removal step (3). This impurity removal step (3) comprises an oxidation step (3A), a neutralization step (3B), and an ion exchange step (3C), which are carried out in this order. Note that the impurity removal step (3) is not limited to the steps shown here, as long as it can remove a portion of the metal ions.

[0031] <Oxidation Step (3A) in the Impurity Removal Step> The oxidation step (3A) shown in FIG. 2 is a step in which an oxidizing agent is added to the second lithium-containing solution obtained in the lithium elution step (2) (see FIG. 1) to oxidize the manganese in the second lithium-containing solution and convert it into insoluble manganese dioxide, thereby precipitating and removing the manganese dissolved in the solution. Manganese can also be removed in the neutralization step (3B) described below, but the oxidation step (3A) removes manganese before the neutralization step (3B), thereby reducing the burden of removing manganese in the neutralization step (3B). Furthermore, the manganese precipitated and removed in the oxidation step (3A) can be reused. The oxidizing agent used in the oxidation step (3A) can be air, oxygen, sodium hypochlorite, or the like. The redox potential of the second lithium-containing solution can be set to a pH and potential that fall within the manganese dioxide region on a potential-pH diagram.

[0032] Neutralization Step (3B) in the Impurity Removal Step The neutralization step (3B) shown in FIG. 2 is a step in which an alkali is added to the post-oxidation solution obtained in the preceding oxidation step (3A) to obtain a post-neutralization solution and a neutralized precipitate containing impurities. In the neutralization step (3B), the pH is increased by the added alkali, and mainly magnesium becomes hydroxide and precipitates as a neutralized precipitate. At the same time, manganese can also be precipitated and removed as hydroxide. For this reason, the pH of the post-neutralization solution after the neutralization step (3B) is preferably 8.5 or more and 12 or less. In this case, magnesium and manganese can be precipitated and removed efficiently. A pH of the post-neutralization solution exceeding 12 is undesirable because it increases the cost of the neutralizing agent.

[0033] 2 is a step of contacting the neutralized solution obtained in the preceding neutralization step (3B) with an ion exchange resin to obtain a third lithium-containing solution from which some of the impurities have been removed. The ion exchange step (3C) can remove metals that could not be removed in the steps upstream of the ion exchange step (3C), such as calcium that could not be removed in the neutralization step (3B), aluminum that remains depending on the pH of the neutralization step (3B), and trace amounts of manganese and magnesium that could not be completely removed in the neutralization step (3B).

[0034] The ion exchange resin used is preferably a chelating resin. For example, an iminodiacetic acid type resin can be used. By using an iminodiacetic acid type chelating resin, the calcium concentration after treatment can be reduced. A specific example of an iminodiacetic acid type chelating resin is Amberlite IRC748 (a trade name of Organo Corporation). A column method is preferably used as a method for contacting the ion exchange resin with the third lithium-containing solution. However, a batch mixing method may also be used.

[0035] <Conversion step (4)> The conversion step (4) shown in Fig. 1 is a step of converting the lithium salt contained in the third lithium-containing solution from which some of the impurities have been removed into lithium carbonate to obtain lithium carbonate crystals. As shown in Fig. 3, the conversion step (4) in this embodiment includes a carbonation step (4A), a hydrogencarbonation step (4B), and a decarbonation step (4C), which are performed in this order.

[0036] <Carbonation Step (4A) in the Conversion Step> In the carbonation step (4A) shown in Fig. 3, a carbonate source such as sodium carbonate is added to the third lithium-containing solution obtained in the preceding impurity removal step (3) (see Fig. 1), thereby precipitating lithium in the third lithium-containing solution as crude lithium carbonate. This is subjected to solid-liquid separation to separate the precipitated crude lithium carbonate from a supernatant, whereby alkali metals such as sodium or potassium and anion components such as boron contained in the supernatant can be removed.

[0037] Although sodium carbonate is used as the carbonate source in this embodiment, it is not limited to this, and it is also possible to use, for example, an alkali metal carbonate such as potassium carbonate.

[0038] In this embodiment, the carbonation step (4A) was carried out by maintaining the neutralized solution (see FIG. 2) obtained in the neutralization step (3B) at 80°C, but the temperature is not limited to this. It can also be carried out at room temperature. If the temperature of the neutralized solution increases, the solubility of lithium carbonate to be precipitated decreases, so it is preferable to increase the solution temperature within a range that does not cause boiling. For example, it is preferable to carry out the carbonation step within a range of 70°C or higher and 90°C or lower.

[0039] The amount of the carbonate source is preferably determined appropriately depending on the amount of lithium contained in the neutralized solution, and the upper limit is preferably determined depending on the solubility of the carbonate source.

[0040] <<Hydrogen Carbonation Step (4B) of the Conversion Step>> The hydrogen carbonation step (4B) shown in FIG. 3 is a step in which water is added to the crude lithium carbonate recovered in the carbonation step (4A) to form a slurry, and carbon dioxide is then blown into the slurry. As shown in Equation 3, the crude lithium carbonate is converted to highly soluble lithium bicarbonate by reacting with carbon dioxide and water, thereby obtaining a lithium bicarbonate solution. That is, the lithium bicarbonate dissolves in the liquid to form a lithium bicarbonate solution, and other poorly soluble impurities become solid. For example, such impurities include calcium carbonate, which is present in trace amounts in the crude lithium carbonate. By performing solid-liquid separation in this manner, impurities such as calcium carbonate can be removed.

[0041] [Math 3] Li ２ CO ３ +CO ２ +H ２ O → 2LiHCO ３

[0042] In the hydrogencarbonation step (4B) according to this embodiment, the temperature is preferably set to 20° C. or higher and 40° C. or lower. The amount of carbon dioxide to be blown in is preferably just enough to allow unreacted and insoluble carbon dioxide to start coming out as bubbles.

[0043] <<Decarbonation Step (4C) of the Conversion Step>> The decarbonation step (4C) shown in FIG. 3 is a step in which a lithium bicarbonate solution is heated to convert lithium bicarbonate into purified lithium carbonate with low solubility, followed by reprecipitation to obtain purified lithium carbonate. The lithium bicarbonate solution obtained in the preceding hydrogen carbonate step (4B) contains a large amount of sodium. Therefore, in order to reduce the sodium concentration, the lithium carbonate in the crude lithium carbonate is dissolved as lithium bicarbonate with high solubility in the hydrogen carbonate step (4B), and then converted back into lithium carbonate in the form of purified lithium carbonate in the decarbonation step (4C), thereby precipitating the purified lithium carbonate. In the precipitated purified lithium carbonate, sodium is almost completely removed, and the purity of the purified lithium carbonate can be increased. The precipitated purified lithium carbonate and the supernatant liquid are subjected to solid-liquid separation to obtain purified lithium carbonate as a solid.

[0044] The reaction formula for the decarboxylation step (4C) is shown in the following formula 4: [Formula 4] 2LiHCO ３ → Li ２ CO ３ +CO ２ +H ２ O

[0045] The decarboxylation step (4C) according to this embodiment may be carried out at any temperature, but is preferably carried out at 50°C or higher and 100°C or lower, and more preferably at 60°C or higher and 90°C or lower. By maintaining the temperature in the decarboxylation step (4C) at 50°C or higher and 100°C or lower, the decarbonation reaction proceeds sufficiently, and highly purified lithium carbonate can be obtained. The heating method is not particularly limited, and it is preferable to adopt a method appropriate for the scale of the reaction vessel. For example, heating using a fluororesin-coated heater (product name: Teflon (registered trademark) heater) or steam heating can be adopted. Furthermore, it is preferable to use a filter press for solid-liquid separation in the decarboxylation step according to this embodiment.

[0046] As described above, according to this embodiment, the basic steps of the lithium adsorption step (1), the lithium elution step (2), the impurity removal step (3), and the conversion step (4) are performed, and the impurity removal step (3) includes three steps, namely, the oxidation step (3A), the neutralization step (3B), and the ion exchange step (3C), thereby making it possible to remove impurities such as manganese, magnesium, calcium, and sodium with high accuracy. Furthermore, an oxidizing agent is used in the oxidation step (3A), and an alkali is used in the neutralization step (3B), after which the remaining impurities are removed using an ion exchange resin. However, most of the impurities are removed by the lithium adsorption step (1), which selectively adsorbs lithium. Therefore, high-purity lithium carbonate can be obtained at low cost without using a large amount of chemicals in the impurity removal step (3).

[0047] Hereinafter, examples of the method for producing lithium carbonate according to the present invention will be described.

[0048] Example 1 <Lithium adsorption step (1)> In the lithium adsorption step (1), H １．６ Mn １．６ O４ The lithium-selective adsorbent (500 mL, bulk density 1.0 g / mL) was packed into a cylindrical column with a capacity of approximately 650 mL, and the first lithium-containing solution shown in Table 1, which had a relatively low concentration, was passed through as a raw material to bring the first lithium-containing solution into contact with the lithium-selective adsorbent. The temperature was room temperature, the flow rate was SV15, and the flow volume was BV30. The pH of the first lithium-containing solution was approximately 8. SV stands for Space Velocity, and represents the flow volume BV per unit time (1 hour). BV stands for Bed Volume, and is a unit representing the volume of the lithium-selective adsorbent in the column. In this example, SV15 is 7.5 L / h, and the volume of the lithium-selective adsorbent is 500 mL, so BV30 is 15 L.

[0049]

[0050] <Lithium Elution Step (2)> In the lithium elution step (2), the lithium-adsorbed lithium-selective adsorbent used in the lithium adsorption step (1) was first washed with water. Then, a 0.5 mol / L aqueous hydrochloric acid solution was passed through the adsorbent to elute lithium, obtaining a second lithium-containing solution. After washing with water, the lithium-selective adsorbent was further subjected to repeated elution. For this repetition, the eluent obtained before the repetition, i.e., the second lithium-containing solution obtained before the repetition, was supplemented with hydrochloric acid to adjust the free acid concentration to 0.5 mol / L. The composition of the second lithium-containing solution obtained after this repetition 10 times is shown in Table 2. The lithium concentration increased from 0.710 g / L shown in Table 1 to 6.500 g / L shown in Table 2, demonstrating that the lithium-selective adsorbent used in this example was capable of selectively recovering lithium.

[0051]

[0052] <Oxidation Step (3A) in Impurity Removal Step (3)> In the oxidation step (3A), a post-oxidation solution was obtained by adding an aqueous sodium hydroxide solution with a molar concentration of 8 mol / L as a neutralizing agent and sodium hypochlorite with an effective concentration of 12% as an oxidizing agent so that the pH of the second lithium-containing solution was 5 and the oxidation-reduction potential was 730 mV at a silver-silver chloride electrode. This step (3A) was carried out entirely at room temperature (20°C). The composition of the metals contained in this post-oxidation solution is shown in Table 3. The manganese concentration (unit: g / L) in the post-oxidation solution decreased from 0.093 to <0.001, indicating that the manganese content was reduced.

[0053]

[0054] <Neutralization step (3B) in the impurity removal step (3)> In the neutralization step (3B), sodium hydroxide at a molar concentration of 8 mol / L was added as a neutralizing agent to the post-oxidation solution, and the pH was adjusted to 11 to obtain a post-neutralization solution. This step (3B) was carried out entirely at room temperature. The composition of metals contained in this post-neutralization solution is shown in Table 4. In the post-neutralization solution, the magnesium concentration (unit: g / L) decreased from 0.570 to <0.001, indicating that the magnesium and manganese contents were reduced.

[0055]

[0056] <Ion Exchange Step (3C) in Impurity Removal Step (3)> In the ion exchange step (3C), the neutralized solution obtained in the neutralization step (3B) was passed through a column packed with an iminodiacetic acid type chelating resin (Amberlite IRC748) (trade name of Organo Corporation) at SV5, and the resin was brought into contact with the solution to obtain a third lithium-containing solution. This step (3C) was carried out at room temperature (20°C). The composition of the metals contained in this third lithium-containing solution is shown in Table 5. It can be seen that the concentrations (g / L) of magnesium and calcium were dramatically reduced.

[0057]

[0058] <Carbonation Step (4A) in Conversion Step (4)> In the carbonation step (4A), sodium carbonate, which is a carbonate source, was added to the third lithium-containing solution obtained in the ion exchange step (3C) so as to give a weight concentration of 200 g / L, and crude lithium carbonate was obtained after solid-liquid separation. This step (4A) was carried out at a temperature of 80°C. The impurity concentrations (unit: ppm) in this crude lithium carbonate are shown in Table 6.

[0059]

[0060] <Hydrogencarbonation Step (4B) in Conversion Step (4)> In the hydrogencarbonation step (4B), water was added to the crude lithium carbonate obtained in the carbonation step (4A) to obtain a slurry containing crude lithium carbonate. Carbon dioxide was blown into this slurry to obtain a lithium hydrogencarbonate solution. The flow rate was adjusted so that a small amount of unreacted and insoluble carbon dioxide bubbles were generated. This step (4B) was carried out at room temperature (20°C). The slurry containing crude lithium carbonate was subjected to solid-liquid separation, allowing the impurity calcium carbonate to be removed.

[0061] <Decarbonation Step (4C) in Conversion Step (4)> In the decarbonation step (4C), the lithium bicarbonate solution obtained in the hydrogen carbonate step (4B) was heated to 80°C to obtain purified lithium carbonate. The impurity concentrations (unit: ppm) in this purified lithium carbonate are shown in Table 7. Compared with Table 6, it can be seen that all impurities were reduced, and sodium in particular was reduced. The lithium carbonate purified in this manner was high-purity purified lithium carbonate that could be used as a battery raw material.

[0062]

[0063] From the above experimental results, it was found that by sequentially performing the steps of the present invention, high-purity lithium carbonate can be obtained more efficiently than conventional methods.

[0064] The lithium carbonate obtained by the present invention can be used to manufacture lithium-based secondary batteries. Typical applications of such lithium-based secondary batteries include those for vehicles, but they can also be used for PCs and other applications.

[0065] (1) Lithium adsorption step (2) Lithium elution step (3) Impurity removal step (4) Conversion step (3A) Oxidation step (3B) Neutralization step (3C) Ion exchange step (4A) Carbonation step (4B) Hydrocarbonation step (4C) Decarboxylation step

Claims

1. (1) Lithium adsorption process: A process in which a first lithium-containing solution with a low lithium concentration, which is the raw material, is brought into contact with a lithium selective adsorbent, and lithium is adsorbed onto the lithium selective adsorbent. (2) Lithium elution step: A step of eluting lithium from the lithium selective adsorbent on which lithium has been adsorbed to obtain a second lithium-containing solution. (3) Impurity removal step: A step of removing some of the metal ions from the second lithium-containing solution to obtain a third lithium-containing solution. (4) Conversion step: A step of converting the lithium salt contained in the third lithium-containing solution to lithium carbonate and obtaining the lithium carbonate crystals. Execute the following in order: The aforementioned impurity removal step is (3A) Oxidation step: A step of adding an oxidizing agent to the second lithium-containing solution to oxidize the manganese in the second lithium-containing solution to the form of insoluble manganese dioxide. (3B) Neutralization step: A step in which alkali is added to the second lithium-containing solution after the oxidation step to precipitate and remove magnesium and manganese remaining after the oxidation step as hydroxides, thereby obtaining a neutralized solution with reduced magnesium and manganese. (3C) Ion exchange step: A step in which the neutralized solution obtained in the neutralization step is brought into contact with an ion exchange resin to remove mainly calcium and aluminum, and magnesium and manganese remaining after the neutralization step, thereby obtaining the third lithium-containing solution. Execute the following in order: The aforementioned conversion process, (4A) Carbonation step: A step in which a carbon dioxide source is added to the third lithium-containing solution obtained in the impurity removal step to obtain crude lithium carbonate. (4B) Bicarbonation process: A process of blowing carbon dioxide into the slurry containing crude lithium carbonate to obtain a lithium bicarbonate solution. (4C) Decarboxylation step: A step of heating the lithium bicarbonate solution to obtain purified lithium carbonate. Execute the following in order: A method for producing lithium carbonate, characterized by the above.

2. (delete)

3. The temperature in the decarbonation step of the conversion step is 50°C or higher and 100°C or lower. A method for producing lithium carbonate according to claim 1.

4. The lithium selective adsorbent in the lithium adsorption step is a manganese oxide obtained by contacting lithium manganate with an acid and removing the lithium from the lithium manganate. A method for producing lithium carbonate according to any one of claims 1 or 3.

5. The hydrogen ion concentration of the acid used in the lithium elution process is 0.3 mol / L or more and 2.0 mol / L or less. A method for producing lithium carbonate according to any one of claims 1, 3, or 4.

6. The pH of the neutralized solution in the neutralization step of the impurity removal step is 8.5 or higher and 12 or lower. A method for producing lithium carbonate according to any one of claims 1, 3, 4, or 5.

7. The ion exchange resin used in the ion exchange step of the impurity removal step is an iminodiacetic acid type chelate resin. A method for producing lithium carbonate according to any one of claims 1, 3, 4, 5, or 6.