Preparation method of battery-grade lithium hydroxide monohydrate

Abstract

The invention discloses a preparation method of battery-grade lithium hydroxide monohydrate, which comprises the following steps: causticizing a lithium sulfate concentrated solution, freezing and crystallizing to obtain a frozen solution, precisely filtering the frozen solution, filtering by adopting an alkali-resistant nanofiltration membrane, collecting a penetrating fluid, and when the permeability of the alkali-resistant nanofiltration membrane is reduced, filtering by adopting an alkali-resistant nanofiltration membrane to obtain the battery-grade lithium hydroxide monohydrate. The alkali-resistant nanofiltration membrane is cleaned by lithium bicarbonate thermal decomposition mother liquor of a lithium carbonate production line, and all collected penetrating fluid is subjected to evaporative crystallization and centrifugal drying to obtain the battery-grade lithium hydroxide monohydrate. By adopting a method of nanofiltration membrane impurity removal and primary evaporative crystallization, the nanofiltration membrane impurity removal is used for replacing the primary evaporative crystallization in the prior art, so that the energy consumption can be effectively reduced, the production cost is saved, the Si content of the nanofiltration solution is lower than 3ppm and lower than that of the primary evaporative crystallization heavy solution, and the method can be used for preparing high-quality battery-grade lithium carbonate. The Si content of the prepared lithium carbonate is lower than 20 ppm.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and in particular relates to a method for preparing battery-grade lithium hydroxide monohydrate. Background Art

[0002] Lithium is known as "industrial MSG" in the industry. It is the lightest metal in nature. Since lithium has the highest standard oxidation potential among all elements, it has broad application prospects in the fields of batteries and energy. In recent years, thanks to the rapid expansion of the lithium-ion battery market, the global demand for lithium salts has increased significantly.

[0003] Lithium hydroxide (LiOH), as an important cathode material for lithium-ion batteries, is widely used in ternary lithium batteries. The current mainstream methods for extracting lithium hydroxide from spodumene in the industry include lithium carbonate causticization and lithium sulfate causticization. The lithium sulfate causticization method is to roast and transform spodumene concentrate, and then produce battery-grade lithium hydroxide through leaching purification, evaporation concentration, causticization, freezing crystallization, evaporation crystallization and other processes. Battery-grade lithium hydroxide can be carbonized with carbon dioxide to prepare battery-grade lithium carbonate due to its low impurity content.

[0004] When preparing battery-grade lithium hydroxide by sulfuric acid method, the lithium sulfate concentrate is causticized to form a mixed solution of sodium sulfate, lithium sulfate, sodium hydroxide, and lithium hydroxide. After freezing and crystallization, most of the sodium sulfate is precipitated in the form of Glauber's salt. The frozen liquid is separated and purified by two evaporation crystallizations to obtain lithium hydroxide products. Since it is impossible to deeply remove Si when preparing lithium sulfate solution in the front-end process, the alkali solution will also introduce some silicon during causticization. If there is no silicon removal process in the back-end, the lithium hydroxide heavy solution obtained after evaporation, crystallization and re-dissolution has a high Si content, which will affect the product quality when used to prepare battery-grade lithium carbonate (Si≤20ppm). As the lithium hydroxide mother liquor is reused in the evaporation and crystallization process, Si will gradually enrich in the system, which will cause scaling of the lithium hydroxide MVR evaporator, making it difficult to clean the silicon scale, affecting the production efficiency and the quality of lithium hydroxide products. The frozen liquid is a strong alkaline solution, and the existence of silicon in the solution is in the form of soluble silicon and insoluble silicon. Conventional silicon removal methods such as anion exchange resin, flocculation, and chemical precipitation methods in a strong alkaline environment have limited effects and it is difficult to achieve deep silicon removal (Si≤5ppm).

[0005] When the frozen liquid evaporates and crystallizes, it is difficult to meet the battery-grade lithium hydroxide impurity requirements (such as SO4 2- , Na, K), so the first crystal needs to be redissolved and then purified by secondary evaporation and crystallization, which has high energy consumption cost.

[0006] Chinese patent document CN117963951A discloses a monohydrated lithium hydroxide and its preparation method and application. The preparation method includes the following steps: precipitating lithium from a lithium-containing solution, and then performing solid-liquid separation to obtain a lithium salt precipitate; causticizing the lithium salt precipitate, and then performing solid-liquid separation to obtain a lithium hydroxide solution and a filter residue; evaporating and crystallizing the lithium hydroxide solution once and re-dissolving it to obtain a heavy solution; performing heat exchange and solid-liquid separation on the heavy solution, and then removing impurities through a nanofiltration membrane and performing secondary evaporation and crystallization to obtain monohydrated lithium hydroxide. In the comparative document, the lithium carbonate precipitate obtained by precipitating lithium from a lithium sulfate solution is washed (mainly to remove impurities of sodium, potassium and sulfur), then causticized, and evaporated and crystallized once to obtain a lithium hydroxide solution. After two evaporation and crystallization, a nanofiltration membrane is also required to remove silicon. The method is complicated and energy-intensive, and the nanofiltration membrane is only used to remove impurity silicon and does not play a wider role.

[0007] Chinese patent document CN106006683A discloses a method and system for purification, separation and membrane concentration of lithium hydroxide, which reacts the lithium sulfate solution obtained by roasting, acidifying, pulping, leaching, etc. of spodumene with the added sodium hydroxide to obtain a mixed solution of sodium sulfate and lithium hydroxide with extremely high salt content (high-valent ions). When the nanofiltration membrane is directly filtered, the osmotic pressure during the nanofiltration membrane treatment is proportional to the salt content in the water. The higher the salt content, the higher the osmotic pressure. When the water inlet pressure remains unchanged (the pressure of the alkali-resistant nanofiltration membrane is generally not higher than 60 bar), the net pressure will decrease and the water production will decrease. And the salt permeability is proportional to the salt concentration difference on both sides of the membrane. The greater the concentration difference, the lower the desalination rate. On the other hand, lithium, sodium and potassium are all monovalent ions of the same family, with similar particle sizes, and the nanofiltration salt separation effect is poor, and the direct salt separation effect of lithium hydroxide and sodium sulfate cannot be achieved. Summary of the invention

[0008] In order to overcome the problems in the prior art, the present invention provides a method for preparing battery-grade lithium hydroxide monohydrate. The battery-grade lithium hydroxide is prepared by causticizing and freezing a lithium sulfate concentrate to obtain a frozen liquid, and then removing impurities with a nanofiltration membrane and performing a single evaporation crystallization method. The energy consumption is effectively reduced and the production cost is saved. At the same time, the permeate removed by the nanofiltration membrane has a lower silicon content than the conventional lithium hydroxide single evaporation crystallization heavy solution, and can meet the Si≤20ppm requirement when used for preparing battery-grade lithium carbonate.

[0009] In order to solve the above technical problems, the technical solution proposed by the present invention is: The present invention provides a method for preparing battery-grade lithium hydroxide monohydrate, comprising the following steps: The lithium sulfate concentrate is causticized and frozen to obtain a frozen liquid. The frozen liquid is precisely filtered and then filtered using an alkali-resistant nanofiltration membrane to collect the permeate. When the permeability of the alkali-resistant nanofiltration membrane decreases, the alkali-resistant nanofiltration membrane is cleaned using the lithium bicarbonate thermal decomposition mother liquor of the lithium carbonate production line. All the collected permeates are evaporated, crystallized and centrifuged to obtain battery-grade lithium hydroxide monohydrate.

[0010] The lithium sulfate concentrate is causticized to form a mixed solution mainly containing sodium sulfate, lithium sulfate, sodium hydroxide, and lithium hydroxide. After freezing and crystallization, most of the sodium sulfate will precipitate as mirabilite (sodium sulfate decahydrate), while lithium sulfate, lithium hydroxide, and sodium hydroxide will not precipitate. Insoluble particles are removed after precise filtration. The frozen liquid in the present invention contains lithium hydroxide as the main component, and has high impurity sulfate, sodium, potassium, and silicon contents. The frozen liquid is filtered by an alkaline-resistant nanofiltration membrane mainly to remove sulfate, so that it meets the concentration requirements of sulfate in the heavy solution of lithium hydroxide once evaporated and crystallized, and at the same time, the silicon content is reduced to less than 3ppm. Compared with the method of using two evaporation crystallizations in the prior art, the present invention uses nanofiltration membrane filtration instead of one evaporation crystallization to remove high-concentration sulfate and silicon, effectively reducing energy consumption and saving production costs. The solution from which sulfate ions have been removed (mainly a mixed solution of lithium hydroxide, sodium hydroxide and potassium hydroxide) is then evaporated and crystallized. Since the solubility of sodium hydroxide and potassium hydroxide is much higher than that of lithium hydroxide, the evaporation crystals will not precipitate (lithium hydroxide will crystallize when concentrated to a certain multiple, but the concentrations of lithium hydroxide and potassium hydroxide are much lower than their saturation concentrations). The precipitated lithium hydroxide crystals are washed away with a small amount of mother liquor (containing sodium hydroxide and potassium hydroxide) during the back-end washing to ensure the requirements for sodium and potassium impurities in the product and obtain battery-grade lithium hydroxide monohydrate.

[0011] At the same time, the present invention utilizes the lithium bicarbonate thermal decomposition mother liquor of the lithium carbonate production line to clean the alkali-resistant nanofiltration membrane with reduced permeability, and the regular cleaning method mainly plays a dilution and flushing role to prevent the local lithium concentration on the membrane surface from precipitating lithium hydroxide crystals to contaminate and damage the nanofiltration membrane, resulting in a decrease in impurity retention rate and membrane flux.

[0012] As an optional embodiment, in the preparation method provided by the present invention, the pore size of the alkali-resistant nanofiltration membrane is 1-2 nm, the molecular weight cutoff is 100-200 Da, the working pressure of the alkali-resistant nanofiltration membrane is 35-45 bar, and the working temperature is not higher than 30°C.

[0013] In the present invention, the higher the liquid temperature of the nanofiltration membrane, the greater the membrane flux, but the impurity retention rate decreases, so the operating temperature of the nanofiltration membrane is set to be no higher than 30°C so that the interception rate of Si impurities and other impurities meets the requirements. The higher the operating pressure, the higher the membrane flux, the higher the water production, and the higher the impurity retention rate. The optimal pressure range in the present invention is 35-45bar. The alkali-resistant nanofiltration membrane in the present invention is selected to be 1-2nm, with a molecular weight cutoff of 100-200 Da, alkali resistance of 20% NaOH / 10% KOH, the maximum membrane tolerance pressure of 60bar, and the maximum temperature of 60°C. The reference membrane selects AMS alkali-resistant membrane or KOCH alkali-resistant membrane.

[0014] As an optional embodiment, in the preparation method provided by the present invention, the amount of the lithium bicarbonate thermal decomposition mother liquor added is 30-50% of the volume of the frozen liquid added to the alkali-resistant nanofiltration membrane.

[0015] As an optional embodiment, in the preparation method provided by the present invention, the pore size of the filter used in the precision filtration process is 0.1-0.5 μm.

[0016] As an optional embodiment, in the preparation method provided by the present invention, the concentrated solution obtained after filtration through an alkali-resistant nanofiltration membrane and the washing solution obtained after cleaning the alkali-resistant nanofiltration membrane are mixed, lithium is extracted after precise filtration, and then a high-purity lithium bicarbonate solution is obtained after back extraction with carbon dioxide.

[0017] In the present invention, the concentrated solution obtained after filtration with an alkali-resistant nanofiltration membrane has a high lithium content, and also contains high concentrations of sulfate and impurities such as sodium, potassium, iron, aluminum, and silicon. The quality of the crude lithium carbonate prepared directly is low, and it is difficult to remove impurities when preparing battery-grade lithium carbonate in the later stage (impurities are generally controlled at the raw material end before lithium carbonate synthesis). Therefore, a high-purity lithium bicarbonate solution is prepared by extracting lithium, and the high-impurity raffinate is returned to the existing process of the previous stage to be pulped bottom water, and no wastewater is generated. The harmful impurity silicon is finally enriched in the waste residue and leaves the system. At the same time, the concentrated solution obtained after filtration with an alkali-resistant nanofiltration membrane has a high lithium content, and the extraction agent has a limited loading capacity (generally 1-3g / L) when extracting lithium. If the lithium concentration of the pre-extraction solution is too high, the extraction stage is too long, and the investment cost is high. Here, the low-lithium concentration wash water and the high-lithium nanofiltration concentrated solution are mixed to play a dilution role. At the same time, the low-concentration lithium in the lithium bicarbonate pyrolysis mother liquor is also recovered in the subsequent extraction process, and the overall process has a high lithium recovery rate.

[0018] As an optional embodiment, in the preparation method provided by the present invention, the high-purity lithium bicarbonate solution is pyrolyzed to prepare battery-grade lithium carbonate, and the obtained pyrolysis mother liquor is returned to the cleaning section for the alkaline-resistant nanofiltration membrane.

[0019] As an optional embodiment, in the preparation method provided by the present invention, a raffinate is obtained by carbon dioxide stripping, and the raffinate is returned to the leaching process in the preparation of the previous lithium sulfate concentrate and used as slurry bottom water.

[0020] As an optional embodiment, in the preparation method provided by the present invention, after the raffinate is returned to the previous leaching process during the preparation of lithium sulfate concentrate and used as slurry bottom water, the raffinate is transferred to the leachate to remove silicon impurities during leaching.

[0021] In the present application, when the raffinate is returned to the leaching process as pulping bottom water during the preparation of the lithium sulfate concentrate in the previous stage, the impurities in the raffinate are eventually transferred to the leachate. Calcium salts (such as calcium carbonate, calcium oxide, and calcium hydroxide) are added during leaching. The purpose is to neutralize the acid and increase the pH value of the leachate to form calcium silicate to remove silicon and aid filtration. At the same time, magnesium is an impurity in the acid clinker. During leaching, the pH is adjusted to an alkaline environment during neutralization to form magnesium hydroxide colloid, which is filtered and removed. Silicon is also adsorbed and removed in this process. Magnesium and silicon impurities are removed at the same time. Therefore, although the use of the raffinate as leaching pulping bottom water will cause the silicon content in the leachate to increase, it can be removed in the subsequent neutralization and magnesium removal purification process.

[0022] As an optional embodiment, in the preparation method provided by the present invention, the silicon content in the nanofiltration permeate is less than 3 ppm, and the nanofiltration permeate is carbonized with CO2 to precipitate lithium, and then centrifuged, washed, dried and packaged to prepare battery-grade lithium carbonate.

[0023] Downstream cathode material manufacturers have strict requirements for battery-grade lithium carbonate. Some manufacturers require Si to be lower than 20ppm (generally ≤30-50ppm). This puts forward a stricter requirement of ≤5ppm for the Si index of the front-end raw material lithium hydroxide solution. Conventional evaporation and crystallization are difficult to stably meet the requirements. The silicon content in the nanofiltration permeate prepared by the present invention is lower than 3ppm, which can be used for the preparation of battery-grade lithium carbonate.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) For the lithium sulfate concentrate obtained by causticizing and freezing crystallizing, the prior art usually adopts two evaporation crystallization treatments to achieve the purpose of purification. In the present application, nanofiltration membrane filtration is used instead of one evaporation crystallization. Compared with evaporation crystallization, the nanofiltration membrane filtration in the present invention can effectively reduce energy consumption and save production costs.

[0025] (2) The Si content of the liquid after nanofiltration in the present invention is less than 3 ppm, which is lower than the Si content of the heavy solution after single evaporation crystallization. The present invention can be used to prepare high-quality battery-grade lithium carbonate. The Si content of the prepared lithium carbonate is less than 20 ppm.

[0026] (3) The lithium content in the lithium bicarbonate thermal decomposition mother liquor of the lithium carbonate production line is low, and lithium cannot be directly precipitated. In the existing process, the carbonized pyrolysis mother liquor is generally returned to the front-end lithium carbonate slurrying process or the spodumene acid clinker leaching process as slurry bottom water. During the process, the slag carries lithium, which will cause lithium loss. The present invention uses the lithium bicarbonate thermal decomposition mother liquor of the lithium carbonate production line to clean the alkaline-resistant nanofiltration membrane with reduced permeability. On the one hand, the pyrolysis mother liquor has low impurity content and low lithium content, and can be used as a cleaning liquid to dilute and clean high-concentration feed liquid, effectively preventing membrane pollution caused by crystallization of lithium hydroxide, without the need to introduce additional flushing pure water. On the other hand, the low lithium concentration wash water obtained here is combined with the concentrated liquid obtained after filtration through the alkaline-resistant nanofiltration membrane. While diluting the concentrated liquid, the lithium in the two is recovered together in the subsequent extraction process, and the overall process has a high lithium recovery rate.

[0027] (4) In the present invention, the concentrated liquid obtained after filtration through the alkaline-resistant nanofiltration membrane and the washing water of the alkaline-resistant nanofiltration membrane of the lithium carbonate production line lithium bicarbonate thermal decomposition mother liquor are jointly processed and recovered. After extraction of lithium, the stripping liquid (lithium bicarbonate solution) can be thermally decomposed to prepare battery-grade lithium carbonate, and the raffinate is returned to the front-end leaching and lithium extraction process as slurry bottom water. Silicon is discharged from the system along with the slag in the leaching purification process. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 It is a technical roadmap for the processing technology of the embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings and preferred embodiments of the present invention, but the protection scope of the present invention is not limited to the following specific embodiments.

[0031] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0032] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0033] Example 1 The components of the frozen liquid and lithium bicarbonate thermal decomposition mother liquid used in this example are shown in Tables 1 and 2 below.

[0034] Table 1: Liquid composition after freezing (mg / L)

[0035] Table 2: Composition of lithium bicarbonate thermal decomposition mother liquor (mg / L)

[0036] (1) The frozen liquid obtained by causticizing a lithium sulfate solution and freezing and crystallizing a large portion of sodium sulfate was filtered through a filter with a pore size of 0.1 μm, and then removed impurities through an alkali-resistant nanofiltration membrane. The alkali-resistant membrane used was an Israeli AMS alkali-resistant membrane; the components of the concentrate and the permeate are shown in Table 3 below.

[0037] Table 3: Nanofiltration impurity removal concentrate and permeate composition table (mg / L)

[0038] The permeate is subjected to evaporation crystallization, centrifugal washing and drying, demagnetization and other processes to produce battery-grade lithium hydroxide monohydrate, the composition of which is shown in Table 4 below.

[0039] Table 4: Lithium Hydroxide Monohydrate Product Composition (%)

[0040] (2) The nanofiltration membrane adopts a two-stage system. The primary water enters the secondary nanofiltration membrane, and the secondary water is discharged (i.e., the permeate). The secondary concentrated water returns to the primary nanofiltration membrane, and the primary concentrated water is discharged (i.e., the concentrated liquid). The working pressure is 45 bar and the temperature is 25°C. When the membrane flux drops significantly, the system is shut down and drained, and cleaning begins. The cleaning liquid uses the lithium bicarbonate thermal decomposition mother liquor of the lithium carbonate production line, and the added amount is 30% of the volume of the liquid inlet after freezing; after the concentrated liquid is mixed with the washing water, it passes through a precision filter (precision filter pore size 0.1μm), and lithium is extracted and CO2 stripping is performed to obtain a high-purity lithium bicarbonate solution, and after thermal decomposition, a battery-grade lithium carbonate product (Si content 13.74ppm) is obtained.

[0041] (3) The extract is returned to the front-end spodumene acid clinker leaching process as slurry bottom water, and the impurities in the raffinate are finally transferred to the leachate. During leaching, calcium carbonate is used for neutralization to adjust the pH value. After filtration, most of the silicon in the leachate leaves the system in the form of silicon-calcium slag. The remaining silicon forms magnesium hydroxide colloid adsorption during subsequent alkalization and magnesium removal to avoid silicon enrichment in the system.

[0042] Example 2 The components of the frozen liquid and lithium bicarbonate thermal decomposition mother liquid used in this example are the same as those in Example 1.

[0043] (1) The frozen liquid obtained by causticizing a lithium sulfate solution and then freezing and crystallizing to precipitate a large portion of sodium sulfate is filtered through a filter with a pore size of 0.5 μm, and then removed by an alkali-resistant nanofiltration membrane. The alkali-resistant membrane used is an Israeli AMS alkali-resistant membrane; the components of the concentrate and the permeate are shown in Table 5 below.

[0044] Table 5: Nanofiltration impurity removal concentrate and permeate composition table (mg / L)

[0045] The permeate is subjected to evaporation crystallization, centrifugal washing and drying, demagnetization and other processes to produce battery-grade lithium hydroxide monohydrate, the composition of which is shown in Table 6 below.

[0046] Table 6: Lithium Hydroxide Monohydrate Product Composition (%)

[0047] (2) The nanofiltration membrane adopts a two-stage system. The primary water enters the secondary nanofiltration membrane, and the secondary water is discharged (i.e., the permeate). The secondary concentrated water returns to the primary nanofiltration membrane, and the primary concentrated water is discharged (i.e., the concentrated liquid). The working pressure is 35 bar and the temperature is 25°C. When the membrane flux drops significantly, the system is shut down and drained, and cleaning begins. The cleaning liquid uses the lithium bicarbonate thermal decomposition mother liquor of the lithium carbonate production line, and the added amount is 50% of the volume of the liquid inlet after freezing; after the concentrated liquid is mixed with the washing water, it passes through a precision filter (precision filter pore size 0.1μm), and lithium is extracted and CO2 stripping is performed to obtain a high-purity lithium bicarbonate solution, and after thermal decomposition, a battery-grade lithium carbonate product (Si content 17.49 ppm) is obtained.

[0048] (3) The extract is returned to the front-end spodumene acid clinker leaching process as slurry bottom water, and the impurities in the raffinate are finally transferred to the leachate. During leaching, calcium carbonate is used for neutralization to adjust the pH value. After filtration, most of the silicon in the leachate leaves the system in the form of silicon-calcium slag. The remaining silicon forms magnesium hydroxide colloid adsorption during subsequent alkalization and magnesium removal to avoid silicon enrichment in the system.

[0049] Example 3 The components of the frozen liquid and lithium bicarbonate thermal decomposition mother liquid used in this example are the same as those in Example 1.

[0050] (1) The frozen liquid obtained by causticizing a lithium sulfate solution and freezing and crystallizing a large portion of sodium sulfate was filtered through a filter with a pore size of 0.5 μm, and then removed impurities through an alkali-resistant nanofiltration membrane. The alkali-resistant membrane used was an American KOCH alkali-resistant membrane; the components of the concentrate and the permeate are shown in Table 7 below.

[0051] Table 7: Nanofiltration impurity removal concentrate and permeate composition table (mg / L)

[0052] The permeate is subjected to evaporation crystallization, centrifugal washing and drying, demagnetization and other processes to produce battery-grade lithium hydroxide monohydrate, the composition of which is shown in Table 8 below.

[0053] Table 8: Lithium Hydroxide Monohydrate Product Composition (%)

[0054] (2) The nanofiltration membrane adopts a two-stage system. The primary water enters the secondary nanofiltration membrane, and the secondary water is discharged (i.e., the permeate). The secondary concentrated water returns to the primary nanofiltration membrane, and the primary concentrated water is discharged (i.e., the concentrated liquid). When the membrane flux drops significantly, the system is shut down and drained, and cleaning begins. The cleaning liquid uses the lithium bicarbonate thermal decomposition mother liquor of the lithium carbonate production line, with an operating pressure of 35 bar and a temperature of 25°C. The amount added is 50% of the volume of the liquid inlet after freezing; after the concentrated liquid is mixed with the washing water, it passes through a precision filter (precision filter pore size 0.1μm), and lithium is extracted and CO2 stripped to obtain a high-purity lithium bicarbonate solution. After thermal decomposition, a battery-grade lithium carbonate product (Si content 15.15 ppm) is obtained.

[0055] (3) The extract is returned to the front-end spodumene acid clinker leaching process as slurry bottom water, and the impurities in the raffinate are finally transferred to the leachate. During leaching, calcium carbonate is used for neutralization to adjust the pH value. After filtration, most of the silicon in the leachate leaves the system in the form of silicon-calcium slag. The remaining silicon forms magnesium hydroxide colloid adsorption during subsequent alkalization and magnesium removal to avoid silicon enrichment in the system.

[0056] Comparative Example 1 The components of the frozen liquid used in the comparative example were the same as those in Example 1.

[0057] (1) A lithium sulfate solution is causticized and then frozen and crystallized to precipitate a large portion of sodium sulfate. The resulting frozen solution is evaporated and crystallized once to obtain lithium hydroxide crystals, which are then redissolved to obtain a lithium hydroxide heavy solution, the composition of which is shown in Table 9 below.

[0058] Table 9: Composition of lithium hydroxide heavy solution (mg / L)

[0059] (2) The lithium hydroxide heavy solution in (1) was used to precipitate lithium with CO2 to prepare battery-grade lithium carbonate. The Si content of the product was 28.52 ppm, which failed to meet the Si ≤ 20 ppm standard.

[0060] (3) The heavy lithium hydroxide solution in (1) is subjected to secondary evaporation crystallization and centrifugal drying to obtain battery-grade lithium hydroxide monohydrate, the composition of which is shown in Table 10 below.

[0061] Table 10: Lithium hydroxide monohydrate composition

[0062] The nanofiltration impurity removal effect in the embodiment of the present invention is equivalent to the primary crystallization in the comparative example. As a substitute for primary crystallization, the silicon removal effect is significantly better than evaporation crystallization. Evaporation crystallization is difficult to reduce the Si content to meet the requirements of the back-end production of high-quality battery-grade lithium carbonate. The removal rates of other impurities such as sulfate are at the same level, and impurities such as sodium and potassium are even lower than the removal rates of the primary crystallization in the comparative example, which do not meet the battery-grade requirements and require evaporation and crystallization again. The problem of high sodium and potassium in the liquid after nanofiltration can be solved when the crystals are centrifuged and washed after evaporation and crystallization.

[0063] The nanofiltration membrane treatment in the present invention has significantly lower energy consumption and production costs than evaporative crystallization. When comparing nanofiltration and primary evaporative crystallization alone, the power consumption (kwh) of nanofiltration is about 1 / 4 of that of primary crystallization under the same frozen liquid treatment volume, and no steam consumption is required. For single frozen liquid treatment, the cost of nanofiltration is 20% of the cost of primary evaporative crystallization.

[0064] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the scope of protection of the present invention.

Claims

1. A method for preparing battery-grade lithium hydroxide monohydrate, characterized in that: The following steps are involved: The lithium sulfate concentrate is causticized and frozen to obtain a frozen liquid. The frozen liquid is precisely filtered and then filtered using an alkali-resistant nanofiltration membrane to collect the permeate. When the permeability of the alkali-resistant nanofiltration membrane decreases, the alkali-resistant nanofiltration membrane is cleaned using the lithium bicarbonate thermal decomposition mother liquor of the lithium carbonate production line. All the collected permeates are evaporated, crystallized and centrifuged to obtain battery-grade lithium hydroxide monohydrate.

2. The method for preparing battery-grade lithium hydroxide monohydrate according to claim 1, characterized in that: The pore size of the alkali-resistant nanofiltration membrane is 1-2 nm, the molecular weight cut-off is 100-200 Da, the working pressure of the alkali-resistant nanofiltration membrane is 35-45 bar, and the working temperature is not higher than 30°C.

3. The method for preparing battery-grade lithium hydroxide monohydrate according to claim 1, characterized in that: The amount of the lithium bicarbonate thermal decomposition mother liquor added is 30-50% of the volume of the frozen liquid added to the alkaline-resistant nanofiltration membrane.

4. The method for preparing battery-grade lithium hydroxide monohydrate according to claim 1, characterized in that: The pore size of the filter used in the precision filtration process is 0.1-0.5 μm.

5. The method for preparing battery-grade lithium hydroxide monohydrate according to claim 1, characterized in that: The concentrated solution obtained after filtering through the alkali-resistant nanofiltration membrane and the washing solution obtained after cleaning the alkali-resistant nanofiltration membrane are mixed, and lithium is extracted after precise filtration, and then a high-purity lithium bicarbonate solution is obtained after back extraction with carbon dioxide.

6. The method for preparing battery-grade lithium hydroxide monohydrate according to claim 5, characterized in that: The high-purity lithium bicarbonate solution is pyrolyzed to prepare battery-grade lithium carbonate, and the obtained pyrolysis mother liquor is returned to the cleaning section for the alkaline-resistant nanofiltration membrane.

7. The method for preparing battery-grade lithium hydroxide monohydrate according to claim 5, characterized in that: A raffinate is also obtained by carbon dioxide stripping, and the raffinate is returned to the leaching process in the preparation of the lithium sulfate concentrate in the previous stage and used as slurry bottom water.

8. The method for preparing battery-grade lithium hydroxide monohydrate according to claim 7, characterized in that: The raffinate is returned to the previous stage of the leaching process during the preparation of lithium sulfate concentrate and used as slurry bottom water. The raffinate is then transferred to the leaching solution to remove silicon impurities during leaching.

9. The method for preparing battery-grade lithium hydroxide monohydrate according to claim 1, characterized in that: The silicon content in the nanofiltration permeate is less than 3 ppm. The nanofiltration permeate is carbonized with CO2 to precipitate lithium, and then centrifuged, washed, dried and packaged to prepare battery-grade lithium carbonate.

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