Method for preparing high-purity lithium sulfate from lithium-sodium mixed solution

Abstract

The invention provides a method for preparing high-purity lithium sulfate from a lithium-sodium mixed solution. The method comprises the following steps: providing a lithium-sodium mixed sulfate solution; the method comprises the following steps: based on a Li2SO4-Na2SO4-H2O ternary system phase diagram, evaporating and concentrating a lithium-sodium mixed sulfate solution to be saturated, and then freezing and denitrifying to obtain a denitrified solution; evaporating the denitrated solution to a co-saturation point of lithium-sodium mixed sulfate, carrying out lithium precipitation, and separating to obtain a lithium precipitation mother solution and a lithium sulfate monohydrate crude product; purifying the lithium sulfate monohydrate crude product to obtain a high-purity lithium sulfate product; freezing and crystallizing the lithium precipitation mother solution, and separating to obtain a lithium-sodium mixed sulfate hydrate and a frozen solution; the lithium-sodium mixed sulfate hydrate is returned to the lithium-sodium mixed sulfate solution, and the frozen liquid is returned to the denitrated liquid. According to the method, the process is simplified, the lithium recovery rate is increased, cost reduction and efficiency improvement are achieved, material recycling can be achieved in the whole process, and the development prospect is good.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion purification technology, specifically relating to a method for preparing high-purity lithium sulfate using a lithium-sodium mixed solution. Background Technology

[0002] With the rapid expansion of the electric vehicle market, lithium-ion batteries, as their core power source, directly impact the range and safety of new energy vehicles. Currently, traditional lithium-ion batteries are facing increasingly prominent bottlenecks in energy density improvement and safety assurance, making it difficult to meet the urgent demands of future electric vehicles for long range and high safety. Against this backdrop, solid-state lithium-ion batteries, due to their promise of achieving higher energy density and inherent safety, are widely considered an important development direction for next-generation power batteries.

[0003] Lithium sulfide, as a key precursor for sulfide solid electrolytes and high-capacity cathode materials, plays a crucial role in advancing the development of solid-state batteries. Lithium sulfate is the main raw material for lithium sulfide preparation; therefore, researching efficient lithium sulfate preparation processes and optimizing its performance is essential for promoting the application of solid-state battery technology and improving the performance and safety of new energy electric vehicles.

[0004] Currently, the industrial production of high-purity lithium sulfate mainly employs the reaction of lithium carbonate with sulfuric acid. However, this process faces two major problems in practical applications: first, the reaction consumes a large amount of sulfuric acid, leading to high production costs; second, lithium carbonate, as a raw material, is relatively expensive, further exacerbating the economic burden. Furthermore, the process faces technical challenges such as complex steps and low lithium recovery rates, resulting in high production costs and significant resource waste. In addition, the use of large amounts of acidic reagents in the process increases the environmental burden and causes pollution. Overall, the industrial production of high-purity lithium sulfate still faces certain bottlenecks in terms of both economic efficiency and environmental friendliness.

[0005] Therefore, how to reduce the use of chemicals such as sulfuric acid, reduce dependence on high-cost lithium carbonate, and improve the economic efficiency and environmental friendliness of lithium sulfide preparation processes are urgent technical challenges that need to be addressed. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing high-purity lithium sulfate using a lithium-sodium mixed solution. Based on the ternary phase diagram of Li₂SO₄-Na₂SO₄-H₂O, this invention proposes a process for directly preparing high-purity lithium sulfate from a lithium-sodium mixed sulfate solution. This process effectively controls the precipitation of double salts, eliminates the cumbersome steps of converting lithium to lithium carbonate followed by sulfuric acid acidification, simplifies the process, and reduces lithium loss. Furthermore, this method reduces dependence on chemicals such as lithium carbonate and sulfuric acid, reduces acid consumption, and improves the economics and environmental friendliness of the process. In addition, the entire process allows for material recycling, further improving lithium recovery rates. The widespread application of this method is expected to promote the green, low-cost, and sustainable industrialization of high-purity lithium sulfate, especially against the backdrop of increasing demand for battery-grade lithium sulfide, providing a new technological path for the large-scale preparation of key materials for next-generation solid-state batteries.

[0007] To achieve this objective, the present invention adopts the following technical solution: This invention provides a method for preparing high-purity lithium sulfate using a lithium-sodium mixed solution, the method comprising the following steps: A lithium-sodium mixed sulfate solution is provided.

[0008] Based on the ternary phase diagram of Li2SO4-Na2SO4-H2O, the lithium-sodium mixed sulfate solution was evaporated and concentrated to saturation, and then subjected to freeze denitration to obtain the denitrated solution.

[0009] The denitrified liquid was evaporated to the cosaturation point of the lithium-sodium mixed sulfate to precipitate lithium, and the lithium precipitation mother liquor and crude lithium sulfate monohydrate were separated.

[0010] The crude lithium sulfate monohydrate was purified to obtain a high-purity lithium sulfate product.

[0011] The lithium precipitation mother liquor is subjected to freeze crystallization to separate lithium-sodium mixed sulfate hydrate and frozen liquid; wherein, the lithium-sodium mixed sulfate hydrate is returned to the lithium-sodium mixed sulfate solution, and the frozen liquid is returned to the denitrification liquid.

[0012] This invention, based on the Li₂SO₄-Na₂SO₄-H₂O ternary phase diagram, proposes a process for directly preparing high-purity lithium sulfate from a lithium-sodium mixed sulfate solution. This process effectively controls the precipitation of double salts, eliminates the cumbersome steps of converting lithium to lithium carbonate followed by sulfuric acid acidification, simplifies the process, and reduces lithium loss. Furthermore, this method reduces dependence on chemicals such as lithium carbonate and sulfuric acid, decreases acid consumption, and improves the economics and environmental friendliness of the process. In addition, the entire process allows for material recycling, further improving lithium recovery rates. The widespread application of this method is expected to promote the green, low-cost, and sustainable industrialization of high-purity lithium sulfate, especially given the increasing demand for battery-grade lithium sulfide, providing a new technological pathway for the large-scale preparation of key materials for next-generation solid-state batteries.

[0013] The mechanism of this invention using the Li₂SO₄-Na₂SO₄-H₂O ternary system phase diagram lies in controlling the change path of the liquid phase composition of the lithium-sodium mixed sulfate solution, thereby achieving efficient separation and directional crystallization of lithium and sodium. Specifically, the Li₂SO₄-Na₂SO₄-H₂O ternary system exhibits specific solubility curves, co-saturation points, and complex salt crystallization regions at different temperatures. Based on this phase diagram characteristic, this invention first evaporates and concentrates a lithium-sodium mixed sulfate solution to saturation (near the double salt crystallization zone), precisely controlling the precipitation of the double salt. Then, it performs freeze-drying, utilizing the significantly reduced solubility of sodium sulfate at low temperatures to remove sodium ions, yielding a lithium-rich denitrated solution. Next, the denitrated solution is further evaporated to the co-saturation point of the lithium-sodium mixed sulfate, where the crystallization driving force of lithium salts is greatest, thus preferentially precipitating crude lithium sulfate monohydrate, achieving efficient lithium-sodium separation. Finally, the lithium precipitation mother liquor is freeze-crystallized to precipitate a lithium-sodium mixed sulfate hydrate, which is returned to the initial mixture as a raw material, while the frozen solution is returned to the denitrated solution for reuse. In summary, based on the Li₂SO₄-Na₂SO₄-H₂O ternary system phase diagram, precise control of the lithium extraction path can be achieved, forming a closed loop throughout the process. This reduces lithium loss and achieves resource-based separation of sodium salts, significantly improving the recovery rate and purity of lithium sulfate.

[0014] It should be noted that "double salt" refers to lithium sodium sulfate double salt; "co-saturation point" refers to the state in which the solution is saturated with respect to both lithium sulfate monohydrate and sodium salt. At this point, selective crystallization of lithium sulfate monohydrate can be achieved, thereby realizing efficient separation of lithium and sodium.

[0015] It should be noted that the source of the lithium-sodium mixed sulfate solution is not limited in this invention. For example, it can be the leachate of lithium ore after sulfuric acid processing, the lithium-containing solution obtained after impurity removal during the recycling of waste lithium batteries, or the leachate of glass powder, etc.

[0016] Preferably, in the lithium-sodium mixed sulfate solution, the mass concentration of lithium ions is 5~7 g / L, for example, 5 g / L, 5.2 g / L, 5.8 g / L, 6.1 g / L, 6.8 g / L or 7 g / L, and the mass concentration of sodium ions is 50~60 g / L, for example, 50 g / L, 52 g / L, 56 g / L, 59 g / L or 60 g / L.

[0017] Preferably, the pH of the lithium-sodium mixed sulfate solution is 6-7, for example, it can be 6, 6.2, 6.4, 6.6, 6.8 or 7.

[0018] This invention limits the pH of the lithium-sodium mixed sulfate solution to 6-7, which can ensure the stability of phase changes during evaporation and crystallization, and effectively inhibit impurity precipitation and equipment corrosion, providing a favorable chemical environment for the efficient and high-purity separation of lithium sulfate.

[0019] Preferably, during the evaporation and concentration of the lithium-sodium mixed sulfate solution, the temperature is 80~120℃, for example, it can be 80℃, 90℃, 100℃, 110℃ or 120℃.

[0020] Preferably, during the evaporation and concentration of the lithium-sodium mixed sulfate solution, the pressure is 0.01~0.09 MPa, for example, it can be 0.01 MPa, 0.02 MPa, 0.03 MPa, 0.04 MPa, 0.05 MPa, 0.06 MPa, 0.07 MPa, 0.08 MPa or 0.09 MPa, etc.

[0021] This invention selects suitable temperature and pressure conditions for evaporation and concentration of lithium-sodium mixed sulfate solution, which can ensure that the evaporation process is stable and controllable, and allows the solution to reach the saturation point along the preset ternary system phase diagram path, effectively controlling the precipitation of double salts.

[0022] Preferably, the temperature for cryogenic denitrification is -1 to -10°C, for example, it can be -1°C, -2°C, -3°C, -4°C, -5°C, -6°C, -7°C, -8°C, -9°C, or -10°C.

[0023] The present invention selects a suitable freezing denitrification temperature. Within this low temperature range, the supercooling of sodium sulfate increases and the crystallization driving force is significantly enhanced, which can maximize the denitrification effect and reduce lithium entrainment loss.

[0024] Preferably, the freezing and denitrification time is 3 to 8 hours, for example, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours.

[0025] Preferably, the temperature during the evaporation of the denitrified liquid is 80~120℃, for example, it can be 80℃, 90℃, 100℃, 110℃ or 120℃.

[0026] Preferably, the purification process includes recrystallization.

[0027] Preferably, the purity of the high-purity lithium sulfate product is ≥99.99%, for example, it can be 99.99%, 99.991%, 99.993%, 99.995% or 99.996%, etc.

[0028] Preferably, the temperature during the freeze-crystallization process is -1 to -10°C, for example, it can be -1°C, -2°C, -3°C, -4°C, -5°C, -6°C, -7°C, -8°C, -9°C, or -10°C.

[0029] Preferably, the freezing and crystallization time is 3 to 8 hours, for example, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours.

[0030] Preferably, the evaporation process of the denitrified liquid is carried out using a gradient evaporation method, and the steps include: The first stage of evaporation is carried out at a first pressure and a first temperature, and then the second stage of evaporation is carried out at a second pressure and a second temperature.

[0031] Wherein, the first pressure is greater than the second pressure, and the first temperature is greater than the second temperature.

[0032] In this invention, the first stage of evaporation is carried out under relatively high pressure and temperature, which can improve the concentration efficiency and shorten the evaporation time; the second stage of evaporation is carried out under relatively low pressure and temperature, and the evaporation process is gentle. This allows for precise control of the solution composition to approach the cosaturation point of the lithium-sodium mixed sulfate, avoiding the large-scale co-precipitation of sodium salts caused by exceeding the cosaturation point. This ensures the selective crystallization quality of lithium sulfate monohydrate and improves the purity of the product.

[0033] Preferably, the first pressure is atmospheric pressure, and the second pressure is 0.01~0.09MPa, for example, it can be 0.01MPa, 0.03MPa, 0.05MPa, 0.07MPa or 0.09MPa, etc.

[0034] It should be noted that "normal pressure" is also known as one standard atmosphere, which is 101325 Pa.

[0035] Preferably, the first temperature is 90~120℃, for example, it can be 90℃, 100℃, 110℃ or 120℃, etc., and the second temperature is 80~120℃, for example, it can be 80℃, 90℃, 100℃, 110℃ or 120℃, etc.

[0036] Preferably, the method includes the following steps: (1) Provide a lithium-sodium mixed sulfate solution with a pH of 6-7; wherein the lithium-sodium mixed sulfate solution has a lithium ion mass concentration of 5-7 g / L and a sodium ion mass concentration of 50-60 g / L.

[0037] (2) Based on the ternary phase diagram of Li2SO4-Na2SO4-H2O, the lithium-sodium mixed sulfate solution is evaporated and concentrated to saturation at a temperature of 80~120℃ and a pressure of 0.01~0.09MPa, and then subjected to freeze denitrification at a temperature of -1~-10℃ for 3~8h. After precipitation and filtration, the denitrified liquid and sodium sulfate are obtained.

[0038] (3) The denitrified liquid is evaporated to the cosaturation point of lithium-sodium mixed sulfate at a temperature of 80~120℃ and a pressure of 0.01~0.09MPa to precipitate lithium. After solid-liquid separation, lithium precipitation mother liquor and crude lithium sulfate monohydrate are obtained.

[0039] (4) The crude lithium sulfate monohydrate is recrystallized and then dried at 140~180℃ (e.g., 140℃, 150℃, 160℃, 170℃ or 180℃, etc.) for 4~8h (e.g., 4h, 5h, 6h, 7h or 8h, etc.) to obtain a high-purity lithium sulfate product with a purity ≥99.99%.

[0040] The lithium precipitation mother liquor was subjected to freeze crystallization at a temperature of -1 to -10℃, and after precipitation and filtration, Li2SO4·3Na2SO4·12H2O and the frozen liquid were obtained; wherein, the Li2SO4·3Na2SO4·12H2O was returned to the lithium-sodium mixed sulfate solution, and the frozen liquid was returned to the denitrification liquid.

[0041] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0042] Compared with the prior art, the present invention has the following beneficial effects: This invention, based on the Li₂SO₄-Na₂SO₄-H₂O ternary phase diagram, proposes a process for directly preparing high-purity lithium sulfate from a lithium-sodium mixed sulfate solution. This process eliminates the cumbersome steps of converting lithium to lithium carbonate followed by sulfuric acid acidification, simplifying the process and reducing lithium loss. Furthermore, this method reduces dependence on chemicals such as lithium carbonate and sulfuric acid, decreasing acid consumption and improving the process's economics and environmental friendliness. In addition, the entire process allows for material recycling, further improving lithium recovery rates. The widespread application of this method is expected to promote the green, low-cost, and sustainable industrialization of high-purity lithium sulfate, especially given the increasing demand for battery-grade lithium sulfide, providing a new technological pathway for the large-scale preparation of key materials for next-generation solid-state batteries. Attached Figure Description

[0043] Figure 1 The phase diagram of the Li2SO4-Na2SO4-H2O ternary system provided by the present invention.

[0044] Figure 2 The XRD diffraction pattern of high-purity lithium sulfate prepared in Example 1 of this invention.

[0045] Figure 3 This is a process flow diagram provided in Embodiment 1 of the present invention. Detailed Implementation

[0046] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0047] The scope of this invention can be defined by lower and upper limits. The selected lower and upper limits define the boundaries of a specific range. The range defined in this way can be defined by the inclusion or exclusion of endpoints. Any endpoint can be independently selected for inclusion or exclusion, and all lower and upper limits can be arbitrarily combined to form new ranges. That is, any lower limit can be combined with any upper limit to form an effective range. For example, if the ranges of 60~120 and 80~110 are listed for specific parameters, it should be understood that the ranges of 60~110 and 80~120 also fall within the scope of this invention. In addition, if the minimum range values ​​1 and 2 are listed, and the maximum range values ​​3, 4 and 5 are also listed, then all ranges of 1~3, 1~4, 1~5, 2~3, 2~4 and 2~5 fall within the scope of this invention. In this invention, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0~5" means that all real numbers between 0 and 5 have been fully listed in this document, and "0~5" is only a shortened representation of this set of numerical combinations. When a parameter is expressed as an integer ≥2, it is equivalent to listing positive integers that meet the requirements, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. When a parameter is expressed as an integer selected from "2~10", it is equivalent to listing any integer among 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0048] In this invention, "a combination of at least two" refers to a quantity greater than or equal to 2 unless otherwise specified. For example, "any one or a combination of at least two" means that any one of the listed items can be selected, or a combination of at least two of the listed items formed in a manner that does not conflict and enables the implementation of this invention. In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" cover any one of two or more related listed items, as well as any and all combinations of the related listed items. The arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" means a set consisting of A, B, and combinations of A and B, where "containing A and / or B" can be understood, depending on the context of the statement, as containing A, containing B, or simultaneously containing both A and B. In this invention, "optional" means that the corresponding feature, component, step or solution is not necessary, that is, it is selected from either "with" or "without". If there are multiple "optional" limitations in a technical solution, unless otherwise specified and there is no technical conflict or mutual constraint, each "optional" limitation is independent and does not affect the others.

[0049] In this invention, technical features or solutions described using open-ended terms such as "comprising" or "including" do not exclude additional non-conflicting elements beyond the listed elements unless otherwise specified. They are considered to disclose both closed-ended features or solutions consisting solely of the listed elements and open-ended features or solutions that may include additional non-conflicting elements beyond the listed elements. For example, if A includes a1, a2, and a3, unless otherwise specified, this means that A can consist only of a1, a2, and a3, or it can include other non-conflicting elements based on a1, a2, and a3. This corresponds to the disclosure of technical solutions such as "A consists of a1, a2, and a3," "A is selected from a1, a2, and a3," and "A not only includes a1, a2, and a3, but may also include other non-conflicting elements." All embodiments and optional embodiments of this invention, unless otherwise specified and without technical conflict, can be combined to form new technical solutions, and such combinations fall within the scope of this invention. The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various locations throughout the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this invention can be combined with other embodiments that do not conflict with the technology. The ordinal numbers "first," "second," "third," and "fourth," etc., used in the expressions "first aspect," "second aspect," "third aspect," and "fourth aspect" in this invention are for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly specifying the importance or quantity of the indicated technical features. They serve only as a non-exhaustive enumeration and do not constitute a closed limitation on quantity.

[0050] In this invention, the order in which the steps are written in the methods described in each embodiment does not imply a strict execution order. The actual execution order of each step should be determined based on its function and possible internal logic. Unless otherwise specified, all steps of this invention can be executed in the order they are written, or in any order without technical conflict. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) executed sequentially, or it may include steps (b) and (a) executed sequentially. If the method also includes step (c), then step (c) can be added to the method in any order without conflict, including but not limited to the execution order of steps (a), (b), and (c), steps (a), (c), and (b), steps (c), (a), and (b), etc.

[0051] It should be noted that in the following embodiments, the lithium-sodium mixed sulfate solution comes from the leaching solution of lithium ore after the sulfuric acid process, and its composition and content are shown in Table 1 below.

[0052] Table 1 Example 1 This embodiment provides a method for preparing high-purity lithium sulfate using a lithium-sodium mixed solution, the method comprising the following steps: (1) Provide a lithium-sodium mixed sulfate solution with pH adjusted to 6.5 after impurity removal. Its composition and component content are shown in Table 1.

[0053] (2) Based on such Figure 1 The phase diagram of the ternary system Li2SO4-Na2SO4-H2O shown is used to add 5L of the lithium-sodium mixed sulfate solution to the MVP evaporator and evaporate and concentrate it to 1600mL at a temperature of 100℃ and a pressure of 0.08MPa to reach saturation. Then, it is transferred to a freeze crystallizer and subjected to freeze denitration at a temperature of -5℃ for 5h. After precipitation and filtration, the denitrated liquid and sodium sulfate are obtained.

[0054] (3) Transfer 1010 mL of the denitrified liquid to the MVP evaporator and evaporate it to 600 mL at a temperature of 100 °C and a pressure of 0.08 MPa to reach the cosaturation point of the lithium-sodium mixed sulfate for lithium precipitation. After hot filtration, lithium precipitation mother liquor and crude lithium sulfate monohydrate are obtained.

[0055] (4) The crude lithium sulfate monohydrate is recrystallized and then dried at 160°C for 6 hours to obtain a high-purity lithium sulfate product (anhydrous lithium sulfate product) with a purity of 99.99%.

[0056] The lithium precipitation mother liquor is transferred to a cooling crystallizer and subjected to freeze crystallization at a temperature of -5°C. After filtration, Li2SO4·3Na2SO4·12H2O and the frozen liquid are obtained. The Li2SO4·3Na2SO4·12H2O is returned to the lithium-sodium mixed sulfate solution for recycling, and the frozen liquid is returned to the denitrification liquid for recycling.

[0057] Figure 2 The XRD diffraction pattern of the high-purity lithium sulfate prepared in this embodiment is shown. As can be seen from the figure, the diffraction peak positions of the sample prepared in this embodiment are all consistent with the standard card, and the sample has excellent crystallinity, no obvious impurities, and high purity.

[0058] Figure 3 A process flow diagram provided in this embodiment is shown.

[0059] Example 2 This embodiment provides a method for preparing high-purity lithium sulfate using a lithium-sodium mixed solution, the method comprising the following steps: (1) Provide a lithium-sodium mixed sulfate solution with pH adjusted to 6 after impurity removal. Its composition and component content are shown in Table 1.

[0060] (2) Based on such Figure 1 The phase diagram of the ternary system Li2SO4-Na2SO4-H2O shown is used to add 5L of the lithium-sodium mixed sulfate solution to the MVP evaporator and evaporate and concentrate it to 1600mL at a temperature of 80℃ and a pressure of 0.05MPa to reach saturation. Then, it is transferred to a freeze crystallizer and subjected to freeze denitration at a temperature of -1℃ for 8h. After precipitation and filtration, the denitrated liquid and sodium sulfate are obtained.

[0061] (3) Transfer 1010 mL of the denitrified liquid to the MVP evaporator and evaporate it to 600 mL at a temperature of 80 °C and a pressure of 0.05 MPa to reach the cosaturation point of the lithium-sodium mixed sulfate for lithium precipitation. After hot filtration, lithium precipitation mother liquor and crude lithium sulfate monohydrate are obtained.

[0062] (4) The crude lithium sulfate monohydrate is recrystallized and then dried at 140°C for 8 hours to obtain a high-purity lithium sulfate product (anhydrous lithium sulfate product) with a purity of 99.994%.

[0063] The lithium precipitation mother liquor is transferred to a cooling crystallizer and subjected to freeze crystallization at a temperature of -1°C. After filtration, Li2SO4·3Na2SO4·12H2O and the frozen liquid are obtained. The Li2SO4·3Na2SO4·12H2O is returned to the lithium-sodium mixed sulfate solution for recycling, and the frozen liquid is returned to the denitrification liquid for recycling.

[0064] Example 3 This embodiment provides a method for preparing high-purity lithium sulfate using a lithium-sodium mixed solution, the method comprising the following steps: (1) Provide a lithium-sodium mixed sulfate solution with pH adjusted to 7 after impurity removal. Its composition and component content are shown in Table 1.

[0065] (2) Based on such Figure 1 The phase diagram of the ternary system Li2SO4-Na2SO4-H2O shown is used to add 5L of the lithium-sodium mixed sulfate solution to the MVP evaporator and evaporate and concentrate it to 1600mL at a temperature of 120℃ and a pressure of 0.09MPa to reach saturation. Then, it is transferred to a freeze crystallizer and subjected to freeze denitration at a temperature of -10℃ for 3h. After precipitation and filtration, the denitrated liquid and sodium sulfate are obtained.

[0066] (3) Transfer 1010 mL of the denitrified liquid to the MVP evaporator and evaporate it to 600 mL at a temperature of 120 °C and a pressure of 0.09 MPa to reach the cosaturation point of lithium sodium sulfate for lithium precipitation. After hot filtration, lithium precipitation mother liquor and crude lithium sulfate monohydrate are obtained.

[0067] (4) The crude lithium sulfate monohydrate is recrystallized and then dried at 180°C for 4 hours to obtain a high-purity lithium sulfate product (anhydrous lithium sulfate product) with a purity of 99.991%.

[0068] The lithium precipitation mother liquor is transferred to a cooling crystallizer and subjected to freeze crystallization at a temperature of -10°C. After filtration, Li2SO4·3Na2SO4·12H2O and the frozen liquid are obtained. The Li2SO4·3Na2SO4·12H2O is returned to the lithium-sodium mixed sulfate solution for recycling, and the frozen liquid is returned to the denitrification liquid for recycling.

[0069] Example 4 The difference between this embodiment and Embodiment 1 is that a gradient evaporation method is used during the evaporation of the denitrified liquid, and the steps include: The first stage of evaporation is carried out at a first pressure and a first temperature, and then the second stage of evaporation is carried out at a second pressure and a second temperature.

[0070] Wherein, the first pressure is atmospheric pressure, the second pressure is 0.05 MPa, the first temperature is 110℃, and the second temperature is 100℃.

[0071] The remaining methods and parameters are consistent with those in Example 1.

[0072] Example 5 The difference between this embodiment and Embodiment 1 is that the pH of the lithium-sodium mixed sulfate solution is adjusted to 5.

[0073] The remaining methods and parameters are consistent with those in Example 1.

[0074] Example 6 The difference between this embodiment and Embodiment 1 is that the pH of the lithium-sodium mixed sulfate solution is adjusted to 8.

[0075] The remaining methods and parameters are consistent with those in Example 1.

[0076] Example 7 The difference between this embodiment and Embodiment 1 is that the temperature for cryogenic denitrification is 5°C.

[0077] The remaining methods and parameters are consistent with those in Example 1.

[0078] Example 8 The difference between this embodiment and Embodiment 1 is that the temperature is 60°C during the evaporation process of the denitrified liquid.

[0079] The remaining methods and parameters are consistent with those in Example 1.

[0080] Example 9 The difference between this embodiment and Embodiment 1 is that the temperature is 140°C during the evaporation process of the denitrified liquid.

[0081] The remaining methods and parameters are consistent with those in Example 1.

[0082] Comparative Example 1 The difference between this comparative example and Example 1 is that step (2) is not based on the Li2SO4-Na2SO4-H2O ternary system phase diagram for evaporation and concentration.

[0083] The remaining methods and parameters are consistent with those in Example 1.

[0084] Comparative Example 2 The difference between this comparative example and Example 1 is that step (3) is replaced by: The denitrification solution was mixed with sodium carbonate and subjected to a lithium precipitation reaction at 95°C. After filtration, lithium precipitation mother liquor and crude lithium carbonate were obtained. The remaining methods and parameters are consistent with those in Example 1.

[0085] Performance testing The products obtained based on the methods provided in the above embodiments and comparative examples were subjected to lithium recovery rate testing and purity testing.

[0086] 1) Lithium recovery rate test, the calculation formula is: Lithium recovery rate (%) = mass of lithium in the product / mass of lithium in the raw material × 100%. The lithium content can be determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0087] 2) Product purity test, the test method is ICP-OES.

[0088] The test results are shown in Table 2.

[0089] Table 2 analyze: As shown in Table 2, this invention proposes a process for directly preparing high-purity lithium sulfate from a lithium-sodium mixed sulfate solution based on the Li₂SO₄-Na₂SO₄-H₂O ternary system phase diagram. This process eliminates the cumbersome steps of converting lithium to lithium carbonate followed by sulfuric acid acidification, simplifying the process and reducing lithium loss. Furthermore, this method reduces dependence on chemicals such as lithium carbonate and sulfuric acid, decreasing acid consumption and improving the process's economic efficiency and environmental friendliness. In addition, the entire process allows for material recycling, further improving lithium recovery. Moreover, the purity of the product is also improved.

[0090] As can be seen from the comparison between Example 1 and Examples 5-6, if the pH of the lithium-sodium mixed sulfate solution is too low, the residual acidic impurity ions in the solution will be difficult to hydrolyze and precipitate, causing the impurities to enter the subsequent process and ultimately affecting the purity of the product; if the pH of the lithium-sodium mixed sulfate solution is too high, some lithium ions may co-precipitate in the form of lithium hydroxide or undergo basic salt precipitation, resulting in lithium loss and reducing the recovery rate.

[0091] A comparison of Examples 1 and 7 shows that if the temperature of cryogenic denitration is too high, the solubility of sodium sulfate in the solution will be relatively high, resulting in a low supersaturation, weakened crystallization driving force, reduced sodium removal efficiency, and impact on the product purity and yield of subsequent lithium precipitation processes.

[0092] As can be seen from the comparison between Example 1 and Examples 8-9, if the temperature is too low during the evaporation of the denitrification liquid, the evaporation efficiency will decrease significantly, the solution viscosity will increase, and local supersaturation will easily lead to the precipitation of impurity crystals, affecting the selective crystallization of lithium sulfate monohydrate. If the temperature is too high during the evaporation of the denitrification liquid, the evaporation rate will be too fast, and the solution will easily exceed the cosaturation point, causing premature co-precipitation of sodium salt, increasing the sodium impurity content in the product and decreasing its purity.

[0093] As can be seen from the comparison between Example 1 and Comparative Example 1, if the evaporation and concentration in step (2) are not based on the phase diagram of the Li2SO4-Na2SO4-H2O ternary system, the solution composition cannot be accurately controlled to enter the critical region for the formation of double salts, resulting in uncontrolled precipitation of double salts, reduced denitrification efficiency, poor lithium-sodium separation effect, and a significant decrease in the purity of the final product and the lithium recovery rate.

[0094] As can be seen from the comparison between Example 1 and Comparative Example 2, if step (3) is replaced by: mixing the denitrified liquid with sodium carbonate, carrying out a lithium precipitation reaction at 95°C, and obtaining lithium precipitation mother liquor and crude lithium carbonate after filtration, not only will sodium carbonate be introduced, increasing the raw material cost, but the crude lithium carbonate will also need to undergo subsequent acidification conversion to obtain lithium sulfate, which will prolong the process and increase lithium loss; at the same time, the addition of sodium carbonate introduces a large number of sodium ions, increasing the separation burden, and the overall economic efficiency and environmental friendliness are both poor.

[0095] It should be noted that the present invention is illustrated through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing high-purity lithium sulfate using a lithium-sodium mixed solution, characterized in that, The method includes the following steps: Provide a lithium-sodium mixed sulfate solution; Based on the ternary phase diagram of Li2SO4-Na2SO4-H2O, the lithium-sodium mixed sulfate solution was evaporated and concentrated to saturation, and then subjected to freeze denitration to obtain the denitrated solution. The denitrified liquid was evaporated to the cosaturation point of lithium and sodium sulfate mixture for lithium precipitation, and lithium precipitation mother liquor and crude lithium sulfate monohydrate were obtained by separation. The crude lithium sulfate monohydrate was purified to obtain a high-purity lithium sulfate product. The lithium precipitation mother liquor is subjected to freeze crystallization to separate lithium-sodium mixed sulfate hydrate and frozen liquid; wherein, the lithium-sodium mixed sulfate hydrate is returned to the lithium-sodium mixed sulfate solution, and the frozen liquid is returned to the denitrification liquid.

2. The method according to claim 1, characterized in that, In the lithium-sodium mixed sulfate solution, the mass concentration of lithium ions is 5~7 g / L and the mass concentration of sodium ions is 50~60 g / L; And / or, the pH of the lithium-sodium mixed sulfate solution is 6-7.

3. The method according to claim 1 or 2, characterized in that, During the evaporation and concentration of the lithium-sodium mixed sulfate solution, the temperature is 80~120℃; And / or, during the evaporation and concentration of the lithium-sodium mixed sulfate solution, the pressure is 0.01~0.09MPa.

4. The method according to any one of claims 1-3, characterized in that, The temperature for the cryogenic denitrification is -1 to -10°C; And / or, the freezing denitrification time is 3~8h.

5. The method according to any one of claims 1-4, characterized in that, During the evaporation process of the denitrified liquid, the temperature is 80~120℃.

6. The method according to any one of claims 1-5, characterized in that, The purification process includes recrystallization. And / or, the purity of the high-purity lithium sulfate product is ≥99.99%.

7. The method according to any one of claims 1-6, characterized in that, During the freeze-crystallization process, the temperature is -1 to -10°C; And / or, the freeze-crystallization time is 3~8 hours.

8. The method according to any one of claims 1-7, characterized in that, The evaporation process of the denitrified liquid employs a gradient evaporation method, and the steps include: The first stage of evaporation is carried out under a first pressure and a first temperature, and then the second stage of evaporation is carried out under a second pressure and a second temperature. Wherein, the first pressure is greater than the second pressure, and the first temperature is greater than the second temperature.

9. The method according to claim 8, characterized in that, The first pressure is atmospheric pressure, and the second pressure is 0.01~0.09 MPa; And / or, the first temperature is 90~120℃, and the second temperature is 80~120℃.

10. The method according to any one of claims 1-9, characterized in that, The method includes the following steps: (1) Provide a lithium-sodium mixed sulfate solution with a pH of 6-7; wherein the lithium-sodium mixed sulfate solution has a lithium ion mass concentration of 5-7 g / L and a sodium ion mass concentration of 50-60 g / L; (2) Based on the ternary phase diagram of Li2SO4-Na2SO4-H2O, the lithium-sodium mixed sulfate solution was evaporated and concentrated to saturation at a temperature of 80~120℃ and a pressure of 0.01~0.09MPa, and then subjected to freeze denitration at a temperature of -1~-10℃ for 3~8h. After precipitation and filtration, the denitrated liquid and sodium sulfate were obtained. (3) The denitrified liquid is evaporated to the cosaturation point of lithium-sodium mixed sulfate at a temperature of 80~120℃ and a pressure of 0.01~0.09MPa to precipitate lithium. After solid-liquid separation, lithium precipitation mother liquor and crude lithium sulfate monohydrate are obtained. (4) The crude lithium sulfate monohydrate is recrystallized and then dried at 140~180℃ for 4~8h to obtain a high-purity lithium sulfate product with a purity ≥99.99%; The lithium precipitation mother liquor was subjected to freeze crystallization at a temperature of -1 to -10℃, and after precipitation and filtration, Li2SO4·3Na2SO4·12H2O and the frozen liquid were obtained; wherein, the Li2SO4·3Na2SO4·12H2O was returned to the lithium-sodium mixed sulfate solution, and the frozen liquid was returned to the denitrification liquid.

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