Method for preparing lithium hydroxide monohydrate

Abstract

The invention relates to an improved method for preparing lithium hydroxide monohydrate suitable for battery applications. Potassium hydroxide, KOH, is preferably added to an aqueous solution of lithium sulphate, Li2SO4, and the aqueous solution obtained is adjusted to a temperature in a range from 0 to 25°C. After quantitative separation of K2SO4, LiOH can be crystallised out at high yield and purity.

Description

[0001] Manufacturing process of lithium hydroxide monohydrate

[0002] An improved manufacturing process for lithium hydroxide monohydrate suitable for battery applications is provided.

[0003] Lithium hydroxide monohydrate (LiOH H2O) is an important starting material for the production of cathode material for lithium-ion batteries.

[0004] There are several ways to produce LiOH H2O, including electrochemical processes, pyrometallurgical processes, ion exchange processes and hydrometallurgical processes.

[0005] In the electrochemical method, LiOH is produced by electrolysis of LiCl (from salt solutions) or Li₂SC>4 (mineral ore leaching). This method can produce a high-purity product with a high yield. However, careful preparation of the pure starting material is required, as impurities can block the membrane, and ion exchange membranes are expensive and incur high maintenance costs.

[0006] In the pyrometallurgical process, lithium-rich mineral ores are mixed with limestone (CaCOs) and roasted at 1000 °C, followed by leaching to produce the crude LiOH solution. The process is simple and straightforward, but energy consumption is high and product purity is low.

[0007] Ion exchangers can also be used to produce LiOH H₂O. The lithium salt solution (e.g., LiCl or Li₂SC>4) is brought into contact with an anion exchange resin containing hydroxide ions (OH⁻), and the anion in the lithium salt is replaced by the OH⁻ in the resin, resulting in a LiOH solution. This method is cost-effective and can produce a highly pure product because no foreign ions are introduced. However, due to the low capacity of the resin and the low exchange rate, this process is very time-consuming.

[0008] In comparison to the methods mentioned above, hydrometallurgical processes continue to be preferred industrially due to their relatively low energy consumption, simple operating procedures and good product quality.

[0009] The conventional hydrometallurgical technique for producing LiOH H₂O is the causticization of Li₂CO₃. Li₂CO₃, obtained from salt solutions or mineral ores, is limed with lime (Ca(OH)₂) to create a slurry containing LiOH(aq) and calcium carbonate (CaCO₃(s)), from which CaCO₃ precipitates are filtered. While this process can produce a high-purity product, the yield of LiOH (3–3.5 wt%) is low due to the low solubility of both reactants. Furthermore, this process requires U₂CO₃, which cannot be directly obtained from primary lithium sources (mineral ore leaching or natural brines), thus complicating the process.

[0010] Another causticization method involves mixing lithium sulfate (U₂SO₄) with sodium hydroxide (NaOH) and then removing the sodium sulfate (Na₂SC>4) by cooling crystallization. Evaporation then yields solid LiOH H₂O. This method has a high product yield but low purity, as the highly soluble sodium salt is difficult to remove. The byproduct of this method is Na₂SC>4, which has little value in large quantities. Furthermore, there is a significant loss of lithium due to inclusion in the precipitate Na₂SC>4(s), primarily in the form of the double salt NaLiSC(s). This leads to inefficient use of lithium resources. The use of barium hydroxide (Ba(OH)₂) and Li₂SC>4 solution for the production of LiOH has also been proposed. However, the shelf life of alkaline earth hydroxide solutions, such as Ba(OH)₂, is very limited.The presence of carbon dioxide from the air quickly leads to a high proportion of carbonate, which makes the production of LiOH more difficult due to contamination with other anions.

[0011] Therefore, there is an urgent need to develop a hydrometallurgical LiOH H2O production method that enables high product yield and product purity with minimal lithium loss and minimal generation of chemical waste.

[0012] SUMMARY OF THE INVENTION

[0013] Surprisingly, it has now been found that the production of lithium hydroxide, preferably lithium hydroxide suitable for battery applications, can be improved with the aid of potassium hydroxide (KOH). According to the invention, a process for the production of lithium hydroxide by reacting lithium sulfate, lithium nitrate, lithium carbonate, or lithium chloride, preferably lithium sulfate, with potassium hydroxide is therefore provided. According to the invention, KOH is preferably added to an aqueous solution of lithium sulfate, Li₂SO₄, and / or KOH and Li₂SO₄ are dissolved in water.

[0014] In this process, potassium sulfate, K2SO4, is preferably precipitated or crystallized at low temperatures, preferably between 0 and 25 °C, preferably between 0 and 10 °C, most preferably between 0 and 5 °C, and separated from the liquid phase, wherein the precipitation or separation of K2SO4 is preferably carried out quantitatively.

[0015] The terms "crystallization" and "precipitation" are both used in the context of this description and can be used interchangeably throughout, as both alternatives are included in the invention. In each case, a solid phase of K₂SO₄, "K₂SO₄(s)", is formed, which is separated from the aqueous, liquid phase. "Quantitative" here means that after the K₂SO₄ separation, the sulfate ions are present in the liquid phase in negligible concentrations or below a concentration at which UKSO₄ or other sulfate salts precipitate.

[0016] At the temperatures mentioned above, it has been shown that pure K2SO4 can be separated in the form of the resulting solid phase and, surprisingly, the double salt, KUSO4, is neither formed nor present in the solid phase of K2SO4.

[0017] According to the invention, the formation of sulfate salt precipitates, particularly LiKSO4(s), and the formation of sulfate salt crystals, particularly LiKSO4(s), are especially favorably avoided when KOH is present in a small stoichiometric excess relative to the sulfate ions in the reactant solution. Specifically, 2.00 mol to 5.00 mol of KOH, preferably 2.00 mol to 2.80 mol, more preferably 2.00 mol to 2.30 mol, and most preferably 2.00 mol to 2.10 mol of KOH are added per 1.00 mol of sulfate (or U2SO4). In this way, only K2SO4(s) forms as a sulfate precipitate.

[0018] “Reactant solution” refers to the aqueous solution in which U2SO4 and KOH are placed before K2SO4 forms as a solid phase.

[0019] In the case of a stoichiometric deficit of potassium ions (KOH) relative to sulfate in the reactant solution, i.e., less than 2.00 mol KOH per 1.00 mol sulfate, there is a risk of undesired double salt formation, KLiSO4(s). With a further decrease in potassium concentration in the reactant solution, experiments quickly show an increasing amount of KLiSO4(s) formed during the formation of the K2SO4 solid phase.

[0020] With a further increased excess of potassium ions or KOH relative to sulfate (more than 5.00 mol KOH per 1.00 mol sulfate, but possibly even more than 2.80 mol KOH per 1.00 mol sulfate), there is a risk of LiOH crystallization during K₂SO₄ crystallization or precipitate formation, which would lead to lithium losses during separation of the solid K₂SO₄. Experiments show that with increasingly higher KOH excesses, more and more lithium is lost as LiOH during K₂SO₄ precipitation or crystal formation into the solid K₂SO₄ phase.

[0021] In a particularly preferred embodiment, the KOH excess relative to sulfate in the aqueous starting material, as preferred according to the invention, is achieved in a circulating process by adding potassium hydroxide and U₂SO₄, in an equimolar ratio to each other, to the starting material. This occurs after an initial excess of potassium hydroxide relative to sulfate has been established at the beginning of the process. This excess is not consumed during the separation of K₂SO₄(s) and subsequently LiOH(s), wherein the mother liquor recycled after the LHM crystallization always ensures a superstoichiometric KOH to sulfate ratio preferred according to the invention. For example, the starting material then contains an excess of KOH(aq) maintained in a concentration range of 0.1–400 g / 1000 g H₂O, preferably 1–100 g / 1000 g H₂Ü, and particularly preferably 10–40 g / 1000 g H₂O.

[0022] Impurities may necessitate an additional consumption of hydroxide ions. This consumption must then be added to the stoichiometric ratio to determine the total potassium hydroxide requirement.

[0023] To limit the purification effort, in a further preferred embodiment, when dealing with complex lithium starting solutions comprising U₂SO₄ or U₂SO₄ H₂O, together with one or more salts such as UKSO₄, Li₂KNa(SO₄)₂, or other double or triple salts, with the cations lithium and other alkali metals, as well as sulfate as an anion, the complex starting solutions can be fractionally crystallized, optionally separated from the mother liquor, and resoluble with water or pure circulating solutions before the conversion process of the present invention begins with the K₂SO₄ separation. In this way, the lithium cation can be effectively separated from impurities that are readily water-soluble and others that are difficult to remove by alkaline purification processes. Separate treatment of the mother liquor, e.g., by precipitation of U₂CO₃, can optionally ensure a high overall lithium yield.

[0024] If sodium is also present in a process solution, the process according to the invention partially or completely produces glaserite 3Na(SO4)2 instead of K2SO4. This is not undesirable according to the invention. The glaserite is a valuable raw material and can subsequently be used, for example, to produce K2SO4 using known methods.

[0025] In a further preferred embodiment, a molar lithium yield of at least 97% of LiOH or LHM per unit of Li used, e.g., in the form of 1 U2SO4, is achieved. More preferred is a molar lithium yield of at least 98%, and most preferred is at least 99%.

[0026] According to the invention, such molar lithium yields are achieved particularly when the temperatures for K₂SO₄(s) formation and the formation of LiOH(s) or LHM(s) are set in the ranges preferred according to the invention, preferably by slow temperature adjustment, and when working with only slightly supersaturated solutions. It has been shown that this avoids the formation of adhesive alkalis and other product losses. To reduce the negative effects on the molar lithium yield caused by the alkalis adhering to the crystallized K₂SO₄ and LHM, known washing processes for the crystallizates can be used, applying technical and economic criteria.

[0027] The adjustment of the molar ratios is preferably carried out in a cyclic process according to the invention, e.g. by mixing a U2SO4 solution with the mother liquor from the LHM crystallization returned to the beginning of the process and adding an equimolar amount of KOH to the sulfate in the form of U2SO4, so that an excess of KOH to sulfate according to the invention is maintained in the reactant solution - as explained above - after which the system is brought to an equilibrium state at low temperature in which K2SO4 crystallizes as the only stable mineral.

[0028] In a preferred embodiment, the mother liquor ideally already contains a high concentration of LiOH, so that after the addition of KOH, in addition to sulfate, quantitative crystallization occurs in the form of K₂SO₄(S), leaving low concentrations of KOH in solution. This facilitates the establishment of a suitable equilibrium state as a stable operating range. This is advantageous for minimizing lithium losses without complex post-treatment steps. The resulting losses, which are solely due to the lye, can be reduced to negligible lithium losses with minimal washing effort.

[0029] Unlike the production of lithium hydroxide using sodium hydroxide, the invention avoids lithium loss through the inclusion of precipitate (Na₂SO₄ or Na₂SO₄ or other double salts). This empirical finding is all the more surprising because the inventors initially had to assume that they would first have to precipitate the double salt KUSO₄ in a stepwise process and then subsequently redissolve it by adding further potassium hydroxide.

[0030] Surprisingly, it has been shown that the formation of the double salt KUSO4 can be avoided according to the invention.

[0031] Furthermore, compared to the use of NaOH, the advantage is that only KUSO4 exists as a double sulfate, whereas when using NaOH several double salts occur and must be separated.

[0032] Preferably, the process according to the invention is carried out with a lithium sulfate solution (as the reactant solution) containing up to 30 wt.% lithium sulfate. KOH is added in a ratio specified according to the invention. The process is preferably carried out with an amount of water such that, after the crystallization of K₂SO₄, the equilibrium concentration of LiOH H₂O has not yet been reached, i.e., lithium hydroxide saturation is not present. In a preferred case, at least 80%, in a particularly preferred case at least 85%, and in a very particularly preferred case at least 90% of the molar equilibrium or saturation concentration of LiOH H₂O is achieved under the given process conditions.

[0033] After the separation of K₂SO₄(s), lithium hydroxide is crystallized from the aqueous solution. In certain embodiments, lithium hydroxide is crystallized as lithium hydroxide monohydrate (“LHM”).

[0034] According to the invention, the crystallization of LiOH, preferably LHM, is preferably carried out by evaporation, e.g., at room temperature under vacuum, or at the boiling point below atmospheric pressure. Operating at temperatures as close as possible to the boiling point of the solution is even more preferred. High temperatures are particularly preferred, since the solubility of potassium sulfate increases more sharply than that of lithium hydroxide with increasing temperature, thus allowing more lithium hydroxide or LHM to be crystallized from the solution. Suitable crystallization conditions lie within a temperature range of 25 to 110 °C. Sufficient water removal from the mother liquor after potassium sulfate (K₂SO₄(s)) deposition leads to the equilibrium concentration of lithium hydroxide monohydrate. Further water removal causes LHM to begin crystallizing.At a constant temperature, the water removal can be continued, but not to the point where the remaining potassium sulfate begins to crystallize alongside LHM. Ideally, the process according to the invention is carried out at the highest possible temperature between 75 °C and 110 °C. Maintaining a sufficient safety margin above 110 °C allows the anhydrous form, LiOH, to crystallize instead of LHM. The following table shows the pairs of values ​​for the concentrations of LiOH and K₂SO₄ at which both K₂SO₄ and LHM crystallize for various temperatures.

[0035] Table 1: Limit values ​​of LiOH, KOH and K2SO4 concentrations in the aqueous phase after K2SO4(s) separation in the presence of crystalline LHM and K2SO4 at different temperatures

[0036] Table 1 shows that ideally, work is carried out at temperatures above 60°C, preferably above 75°C, since in this range LiOH crystallization begins at more than 130 g LiOH per 1000 g water or at more than 139 g LiOH per 1000 g water, and K2SO4 remains in solution in amounts up to 144 g per 1000 g water or up to 156 g per 1000 g water.

[0037] A forced circulation evaporator is preferably used for evaporative crystallization. A "forced circulation evaporator" is a unit in which the medium contained in the evaporator is fed to a heat exchanger by means of a conveying device, e.g., a pump, and then flows back into the evaporator, so that the thermal power required to maintain the evaporation process is supplied by this external heat exchanger. Standard forced circulation evaporators can be used according to the invention.

[0038] Preferably, the lithium hydroxide monohydrate obtained is of battery quality according to the invention. More preferably, the lithium hydroxide monohydrate obtained contains less than 50 ppm potassium, and even more preferably less than 10 ppm potassium. Typical desired specifications of LHM for battery applications are listed in Table 2 below.

[0039] Table 2: Desired specifications of LiOH for battery applications

[0040] *The weight fraction of LiOH in the obtained product LHM is conventionally determined without the

[0041] The amount of water of crystallization is calculated and specified.

[0042] An advantage of the manufacturing process of the present invention is the provision of high-purity lithium hydroxide monohydrate while simultaneously utilizing the by-product potassium sulfate.

[0043] Potassium sulfate is a high-priced starting material that, unlike sodium sulfate, is in demand for commercial use. A further advantage is that the known production process for potassium sulfate starting from sodium sulfate and KCl can be avoided. Therefore, the production of LHM and K₂SO₄ according to the invention can significantly reduce necessary investments and production costs.

[0044] Potassium sulfate is used as a high-quality fertilizer. With sufficient purity, it can also be used in the construction materials and chemical industries. Major consumers of Na₂SO₄ are the detergent and glass industries. Due to the large and ever-increasing supply of Na₂SO₄, the requirements for its marketing are considerably more demanding than for K₂SO₄, for which there is a growing global demand.

[0045] DESCRIPTION OF THE FIGURE

[0046] Figure 1 shows the process concept for the production of LHM, LiOH H2O, by adding KOH to U2SO4 solution.

[0047] The figure illustrates the process of coupled production of LHM and K₂SO₄ in a cycle according to a preferred embodiment of the invention. It shows the mixing of the starting materials Li₂SO₄ solution and potassium hydroxide (KOH, here 50 wt%) with the mother liquor, with simultaneous K₂SO₄ crystallization as the first process step. Cooling is required for this. Ideally, a slight stoichiometric excess of KOH to sulfate is used in the reactant solution.

[0048] The resulting suspension is subjected to a solid-liquid separation process in a second step, during which the resulting filter cake is washed with water. The washing procedure can be adjusted to the desired purity of K₂SO₄. It also serves to reduce lithium losses. Ideally, K₂SO₄(s) is separated quantitatively. In a third step, the liquid phase obtained after K₂SO₄ crystallization is evaporated by evaporative crystallization, for example, in a forced-circulation evaporator, resulting in the crystallization of lithium hydroxide monohydrate (LHM). In principle, all other water removal methods can also be used. However, a forced-circulation evaporator is best suited to prevent scale formation.

[0049] The resulting suspension is subjected to a solid-liquid separation process to remove LHM(s). The remaining LiOH-containing mother liquor is fed back to process step 1.

[0050] DETAILED DESCRIPTION OF THE INVENTION

[0051] The process according to the invention describes a new process variant for the production of lithium hydroxide, preferably lithium hydroxide monohydrate (LiOH H2O, LHM), ideally of battery quality, which is realized starting from a lithium salt source, preferably LiX, X = Cl, NO3; lithium carbonate, U2CO3; or lithium sulfate, U2SO4.

[0052] By mixing in an aqueous solution, potassium hydroxide (KOH) and the lithium salt, preferably U2SO4, are directly converted into LiOH H2O (LHM) and potassium salt, preferably K2SO4, and subsequently obtained, ideally in a few process steps.

[0053] If other lithium salts, such as LiCI, UNO3, or U2CO3, are used as reactants instead of U2SO4, it has been shown in analogous process execution that the formation of the double salts KLiCh(s), KLi(NOs)2(s) or KLiCO3(s) can be prevented analogously to KUSO4 when using U2SO4.

[0054] Furthermore, the use of U₂CO₃ as a reactant offers a particular advantage. Because K₂CO₃ and KOH are highly soluble under the set reaction conditions, the separation of UOH and H₂O is surprisingly straightforward.

[0055] The process according to the invention is characterized by its simplicity, since no mixed salts are produced that have to be separated or processed in complex process steps.

[0056] Furthermore, potassium sulfate is preferentially produced as a high-quality second end product alongside LHM, with significant market potential. This is not the case with currently used industrial processes for the production of LHM, as sodium sulfate or calcium carbonate are produced as byproducts, both of which are of lower quality and more difficult to market due to market saturation.

[0057] Surprisingly, it was found that the formation of the mineral KUSO4, which is stable over wide concentration and temperature ranges, can be avoided when adjusting the KOH concentration according to the invention. At the same time, sulfate can be quantitatively precipitated with respect to the previously added amount of U2SO4.

[0058] Depending on the nature and extent of the deviation from the concentrations specified in this invention, K2SO4 and LHM may no longer crystallize in a stoichiometric ratio, unlike in the process steps according to the invention, and therefore further process steps may be necessary to regain this stoichiometric ratio of the products.

[0059] According to the invention, preferably an "excess" of KOH relative to sulfate (U₂SO₄) is provided in the reactant solution. The "excess" refers to the fact that the potassium concentration in the solution is greater than the stoichiometrically corresponding sulfate concentration. The stoichiometric molar ratio of the amount of substance of K + the amount of SO4 2 The reaction ratio is 2:1. Preferably, the reaction is carried out at a high lithium concentration. This allows the lithium in the aqueous phase to be stoichiometrically completely allocated to the hydroxide ions, and the excess of hydroxide ions to the remaining potassium. Depending on the temperature of the evaporation crystallization of LiOH, this calculated excess of KOH is present in the reactant solution at a concentration of between 0 and 60 g / 1000 g H₂O. Preferably, a calculated KOH concentration of 5 to 20 g / 1000 g H₂O is maintained in the reactant solution.

[0060] Low temperatures are required to separate K₂SO₄ as quantitatively as possible and thus ensure low potassium sulfate concentrations in the aqueous solution for the subsequent crystallization of LHM. A sufficiently low potassium sulfate concentration in this remaining solution is achieved by the reaction of U₂SO₄ and KOH (U₂SO₄ + KOH).

[0061] The reaction of K₂SO₄(S) + LiOH(aq) is carried out at low temperatures of less than 25 °C, preferably at temperatures below 10 °C and particularly preferably at 0 °C. Only K₂SO₄(S) precipitates.

[0062] At sufficiently low potassium sulfate concentrations in the lithium hydroxide solution remaining after K₂SO₄ separation, only lithium hydroxide monohydrate crystallizes out. Lithium hydroxide monohydrate is preferably obtained at temperatures between 25 °C and the boiling point of the solution under normal conditions, generally up to 10 °C above the boiling point of water. Preferably, evaporative crystallization is carried out in a temperature range of 60 °C to 110 °C, more preferably between 75 °C and 110 °C.

[0063] According to the invention, at least 25 wt.% of the lithium hydroxide solution present after K₂SO₄ crystallization and before LHM evaporation crystallization is crystallized as LiOH(s) or as lithium hydroxide monohydrate (LHM(s)). Preferably, at least 30 wt.% and particularly preferably at least 32.5 wt.% are crystallized as lithium hydroxide monohydrate (LHM). Depending on the amount of water introduced with U₂SO₄ and KOH and removed with any solids from cleaning processes and any reject solutions, as well as the LHM, water must be removed. This is preferably done by evaporation crystallization under vacuum or atmospheric pressure. Concentration continues until the entire amount of water introduced into the process, less the water losses with the discharge of the products, any solids from cleaning processes, or reject solutions, has evaporated.

[0064] Because the high concentration of hydroxide ions displaces a large amount of lithium hydroxide from the aqueous solution, no crystallization of the double salt KUSO4 can occur in a closed-loop process when the mother liquor is returned to the beginning of the process and subsequently mixed with fresh lithium sulfate solution and potassium hydroxide solution at a suitably chosen operating temperature. This means that only K2SO4 crystallizes, and the entire process can be closed. Therefore, in addition to adjusting the stoichiometric ratio of potassium (in the form of KOH) to sulfate (in the form of U2SO4) in the reactant solution, operating the process with a high hydroxide content in a closed-loop system is advantageous.

[0065] The cycle process according to the invention comprises the following steps: a) mixing the mother liquor from the LHM crystallization (“LiOH mother liquor”) with a Li₂SO₄ solution and potassium hydroxide solution, preferably with a slight stoichiometric excess of potassium ions (added as KOH) compared to the sulfate ions (added or present as U₂SO₄) in the reactant solution; b) thermostating the process at low temperatures below 25 °C; c) separating K₂SO₄ as a solid (“K₂SO₄(s)”); d) evaporating the remaining liquid phase after separation of K₂SO₄ at temperatures above 25 °C, ideally close to the boiling point; e) separating the lithium hydroxide monohydrate (LHM) crystallized during evaporation; and f) returning the LiOH mother liquor to the beginning of the process and closing the cycle.

[0066] Depending on the purity of the Li2SO4 solution used, it may be necessary to wash or recrystallize the resulting LHM(s) using known methods. Another embodiment of the invention takes this into account.

[0067] Furthermore, to avoid the accumulation effects of impurities such as rubidium or cesium sulfate, it can be advantageous to partially remove the LiOH mother liquor from the process and process and purify it separately (e.g., to remove Rb). + or Cs + to separate) or to dispose of them instead of returning them to the beginning of the process.

[0068] Additionally, it can be advantageous to integrate a filtration step into the evaporation crystallization process. In this step, a solution that is not yet saturated with LiOH, or the LiOH solution before LiOH crystallization, is hot-filtered and then cooled, after which LHM crystallizes. After the LHM is separated, the LiOH mother liquor, or a large portion of it, can be recycled back into the evaporation process, as shown in Figure 1. Impurities such as CaOH₂ or MgOH₂ are thus filtered out. The remaining LiOH mother liquor is then recycled back to the beginning of the process after the impurities have been removed and the LHM separated. Since this measure increases energy consumption, it is only advantageous if it allows the removal of impurities such as calcium or magnesium hydroxide, which precipitate before the LiOH during the evaporation process.

[0069] The process parameters for the production of potassium sulfate and LHM are ideally adjusted based on energy consumption. Surprisingly, it has therefore been shown that the amount of lithium in circulation should be kept low.

[0070] The invention ensures that the amount of lithium crystallized, in the form of LiOH, is at least 25 wt.% of the amount of lithium contained in the liquid phase after the K₂SO₄ separation and before the evaporation crystallization of LiOH. In a preferred embodiment, this amount of lithium is greater than 30 wt.%, and in a particularly preferred embodiment, greater than 32.5%. If external conditions, such as impurities, necessitate a lower percentage of lithium, the process can still be operated stably, but the mother liquor recirculation rates will increase. This will increase the specific energy consumption of the process and worsen its economic performance.

[0071] Depending on the type, number, and concentration of impurities introduced into the process with the input streams of hydroxide monohydrate (H₂SO₄), water, and potassium hydroxide (KOH), further process steps are provided for their removal without altering the core process. A repulsive solution stream, as described above, is necessary if there is a risk of accumulation of components that are highly soluble under the given process conditions (e.g., sodium). Impurities in the solution that form sparingly soluble compounds in the presence of hydroxide or sulfate ions may necessitate the introduction of one or more purification steps, which are known per se. These primarily include measures such as precipitation or ion exchange, preferably carried out immediately before the crystallization of lithium hydroxide monohydrate. Integrating these measures elsewhere is possible, but with lower efficiency in terms of the achievable purity.

[0072] The process according to the invention is not limited to lithium sulfate, but in principle also applies to other anions such as chloride (CI-), nitrate (NOa') and carbonate (COa). 2- ) applicable. This opens up broad application possibilities, especially in adapting the process to different chemical raw materials and site conditions.

[0073] The block flow diagram of the process according to the invention in a preferred embodiment is shown in Figure 1. An exemplary embodiment of the invention has the following mass balance: Based on one ton of lithium in an approximately 20% U₂SO₄ starting solution (15–25 wt.% U₂SO₄), 40 tonnes of starting solution are required. This is mixed with 16 tonnes of a 50% KOH solution and 65 tonnes of the mother liquor recycled from the LHM crystallization process at a temperature of 0°C, under cooling. This produces 13 tonnes of potassium sulfate, which is separated, washed, and dried. Approximately 110 tonnes of the K₂SO₄-depleted solution remain, which is evaporated at a temperature of 60°C under forced circulation with a mass flow of approximately 500 tonnes, removing 39 tonnes of water. This yields a suspension from which 6 tonnes of LHM and 65 tonnes of mother solution are withdrawn from the circulating stream.The mother liquor is returned to the beginning of the process. In this way, 99% of the lithium originally introduced into the process is converted into LHM. The remaining loss of lithium is due to the process solution adhering to the K₂SO₄ and can be further reduced with additional or more intensive washing steps.

[0074] Process conditions that differ from this exemplary description will lead to altered mass flows.

[0075] The process according to the invention preferably leads to highly pure LiOH, wherein the lithium hydroxide obtained, preferably lithium hydroxide monohydrate, contains less than 50 ppm, preferably less than 30 ppm and most preferably less than 10 ppm potassium.

[0076] The process according to the invention preferably further leads to highly pure LiOH, wherein the lithium hydroxide obtained, preferably lithium hydroxide monohydrate, contains less than 3 ppm sodium, particularly preferably less than 1 ppm sodium.

[0077] According to the invention, a device for producing lithium hydroxide, LiOH, suitable for battery applications is further provided, comprising: a) a feed unit for lithium sulfate solution and potassium hydroxide solution, b) a cooled crystallization apparatus in which the reactants and the mother liquor are mixed and potassium sulfate is crystallized, c) an apparatus unit for separating the crystalline potassium sulfate from the remaining lithium-containing liquid, d) an evaporation apparatus for concentrating the lithium-containing liquid and crystallizing lithium hydroxide monohydrate by dehydration, e) a further apparatus unit for separating the crystalline lithium hydroxide monohydrate from the remaining mother liquor, and f) a storage container for collecting and conveying the mother liquor.wherein the apparatus at the extraction point of the lithium hydroxide monohydrate suspension includes a device for splitting the stream of the lithium hydroxide monohydrate suspension to obtain a low-solids stream, which is wholly or partially returned directly to the evaporation apparatus, and a high-solids stream, so that the liquid load of the subsequent solid-liquid separation apparatus is reduced.

[0078] The device according to the invention ideally comprises a hydrocyclone or a clarification apparatus.

[0079] In a preferred embodiment, the device according to the invention comprises a forced circulation evaporator, the pipe of which to the heating unit with the suspension circulating to maintain the evaporation process contains below the evaporation body a thickening with a bell projecting into it, from the underside of which a largely clarified mother liquor is taken, which is suitable for being mixed with the U2SO4 solution and the potassium hydroxide solution in order to close the process cycle preferred according to the invention and to increase the residence time of the crystallized lithium hydroxide monohydrate, so that the mean grain diameter of the lithium hydroxide monohydrate can increase to at least 0.6 mm.

[0080] A typical example of the implementation of the invention is shown below:

[0081] EXAMPLE OF EXECUTION

[0082] To a 20 wt% Li₂SO₄ solution, a slight excess of KOH is added, and the resulting suspension is cooled to 0 °C, causing the K₂SO₄ to precipitate. After cooling, the suspension is filtered to separate and wash the precipitated K₂SO₄. The remaining liquid phase, or reject solution, is then evaporated. During this process, the saturation concentration of LiOH is reached, and LiOH H₂O crystallizes during further evaporation within the specified temperature range. After the removal of the moist LHM, a portion of the LiOH mother liquor is recycled back to the beginning of the process for the recovery of K₂SO₄.

[0083] The concentrations of all solutions in the process are summarized in the table. Based on this concept, approximately 330 g of fertilizer-grade K₂SO₄ and approximately 160 g of battery-grade LiOH H₂O are produced from 1 kg of Li₂SO₄ solution (20 wt%). Table 3: Input and output concentrations of the process concept for the production of LiOH H₂O by adding KOH to U₂SO₄ solution

Claims

REQUIREMENTS 1. Use of potassium hydroxide, KOH, for the production of lithium hydroxide, LiOH, suitable for battery applications.

2. Process for the production of lithium hydroxide, LiOH, suitable for battery applications, by reacting lithium sulfate, lithium nitrate, lithium chloride or lithium carbonate, preferably lithium sulfate, with potassium hydroxide.

3. The method according to claim 1 or the use according to claim 2, wherein potassium hydroxide, KOH, is added to an aqueous solution of lithium sulfate, U2SO4, and the aqueous solution obtained is adjusted to a temperature in the range of 0 to 25°C, preferably 0 to 10°C and particularly preferably less than 5°C, whereby potassium sulfate, K2SO4, is crystallized or precipitated.

4. The method or use according to claim 3, wherein a stoichiometric excess of KOH to U2SO4 is established in the aqueous solution of KOH and U2SO4, preferably from 2.00 to 2.80 mol KOH per 1.00 mol U2SO4.

5. The method or use according to one of claims 3-4, wherein LHM is quantitatively crystallized, preferably at least 97 mol% LHM based on the molar amount of 1 U2SO4 used.

6. The method or use according to any one of claims 3-5, wherein no KUSO4 is precipitated.

7. The method or use according to any one of claims 3-6, wherein an aqueous lithium hydroxide solution containing between 15 wt.% and 25 wt.% lithium hydroxide is presented after separation of K2SO4 and prior to crystallizing LiOH.

8. The method or use according to any one of claims 3-7, wherein lithium hydroxide is crystallized as lithium hydroxide monohydrate from the aqueous solution after separation of K2SO4(s).

9. The method or use according to claim 8, wherein evaporation crystallization is carried out to produce lithium hydroxide monohydrate.

10. The method or use according to claim 9, wherein a forced circulation evaporator is used to produce lithium hydroxide monohydrate.

11. The method or use according to any one of claims 8 to 10, wherein the solution obtained after potassium sulfate separation is first evaporated to almost saturation of LiOH, then filtered against precipitated impurities and subsequently cooled.

12. The method or use according to any one of claims 9 to 11, wherein lithium hydroxide monohydrate is crystallized and separated from the aqueous solution and the solution thus obtained is partly returned to the evaporation process and partly returned to the beginning of the process.

13. The method or use according to any one of claims 9 to 12, wherein the evaporation crystallization is carried out in a temperature range of 60°C to 110°C, preferably between 75°C and 110°C.

14. The method or use according to any one of claims 3-13, which is carried out in a continuous circulating process, preferably characterized by a proportion of crystallizing lithium hydroxide in relation to the circulated quantity, expressed by a mass ratio of lithium in the crystallized lithium hydroxide monohydrate to lithium in the aqueous phase leaving the potassium sulfate crystallization, of at least 1.0 : 4.0, preferably at least 1.0 : 3.3 and particularly preferably at least 1.0 : 3.

0.

15. The method or use according to any one of claims 3-14, wherein the lithium hydroxide obtained, preferably lithium hydroxide monohydrate, contains less than 50 ppm, preferably less than 30 ppm and most preferably less than 10 ppm potassium.

16. Lithium hydroxide monohydrate produced by the process according to claim 15, wherein the sodium concentration is below 1 ppm.

17. Apparatus for the production of lithium hydroxide, LiOH, suitable for battery applications, comprising: a) a feed unit for lithium sulfate solution and potassium hydroxide solution, b) a cooled crystallization apparatus in which the reactants and the mother liquor are mixed and potassium sulfate is crystallized, c) an apparatus unit for separating the crystalline potassium sulfate from the remaining lithium-containing liquid, d) an evaporation apparatus for concentrating the lithium-containing liquid and crystallizing lithium hydroxide monohydrate by dehydration, e) a further apparatus unit for separating the crystalline lithium hydroxide monohydrate from the remaining mother liquor, and f) a storage tank for collecting and conveying the mother liquor, wherein the apparatus includes, at the point of withdrawal of the lithium hydroxide monohydrate suspension, a device for splitting the stream of the lithium hydroxide monohydrate suspension.to obtain a low-solids stream that is wholly or partially returned directly to the evaporation apparatus, and, to obtain a solids-rich stream so that the liquid load of the subsequent solid-liquid separation apparatus is reduced.

18. The device according to claim 17, comprising a hydrocyclone or a clarifier.

19. The device according to claim 17 or 18 for producing for suitable lithium hydroxide, LiOH, for battery applications, comprising a forced circulation evaporator, the pipeline of which to the heating unit with the suspension circulating to maintain the evaporation process below the evaporation body contains a thickening with a bell projecting into it, from the underside of which a largely clarified mother liquor can be extracted, which is suitable for being mixed with the U2SO4 solution and the potassium hydroxide solution in order to close the process cycle according to claim 14 and to increase the residence time of the crystallized lithium hydroxide monohydrate, so that the mean grain diameter of the lithium hydroxide monohydrate can increase to at least 0.6 mm.

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