阅读说明：本技术 一种单分散碳酸锂晶体的结晶方法及采用多级梯度结晶提高产品收率的方法 (Crystallization method of monodisperse lithium carbonate crystal and method for improving product yield by adopting multistage gradient crystallization ) 是由 龚俊波 汤伟伟 赵绍磊 侯宝红 王艳 吴送姑 尹秋响 于 2020-05-28 设计创作，主要内容包括：本发明提供了一种单分散碳酸锂晶体的结晶方法及采用多级梯度结晶提高产品收率的方法。首先采用硫酸锂和碳酸钠反应结晶的方法,通过出晶瞬间取样的方法,得固体经干燥后得到单分散碳酸锂晶体。同时提出由多个结晶釜串联,通过多级结晶和母液循环的方法提高产品收率。混合料液进入第一级结晶釜,控制釜温,浆料从结晶釜底流出,进入固液分离系统,固体分离后母液进入下一个结晶釜,控制釜温最终浆料从最后结晶釜底流出,进入固液分离系统,固体分离后母液返回至混合釜循环利用,循环量为母液体积的40～80％。本发明制备的单分散碳酸锂晶体形貌完整且几乎无聚结、晶体粒度小且分布均匀,结晶方法操作简单、生产效率高、适合工业化大规模生产。(The invention provides a crystallization method of monodisperse lithium carbonate crystals and a method for improving product yield by adopting multi-stage gradient crystallization. Firstly, a method of reaction crystallization of lithium sulfate and sodium carbonate is adopted, and a method of crystal-appearing instant sampling is adopted to obtain solid, and monodisperse lithium carbonate crystals are obtained after the solid is dried. Meanwhile, a plurality of crystallization kettles are connected in series, and the product yield is improved by a multi-stage crystallization and mother liquor circulation method. And (2) allowing the mixed feed liquid to enter a first-stage crystallization kettle, controlling the kettle temperature, allowing the slurry to flow out of the bottom of the crystallization kettle, allowing the slurry to enter a solid-liquid separation system, allowing the mother liquid after solid separation to enter the next crystallization kettle, controlling the kettle temperature, allowing the slurry to flow out of the bottom of the last crystallization kettle, allowing the slurry to enter the solid-liquid separation system, allowing the mother liquid after solid separation to return to the mixing kettle for recycling, wherein the recycling amount is 40-80% of the volume of the mother liquid. The monodisperse lithium carbonate prepared by the method has the advantages of complete crystal shape, almost no coalescence, small crystal granularity, uniform distribution, simple operation of the crystallization method, high production efficiency and suitability for industrial large-scale production.)

1. A crystallization method of monodisperse lithium carbonate crystals is characterized in that a one-step reaction crystallization method with equal molar volume of lithium sulfate and sodium carbonate is adopted, the temperature of a crystallization kettle is 65-95 ℃, the concentration of a lithium sulfate solution is 5.3-9.0% by weight, and the concentration of a sodium carbonate solution is 5.1-8.7% by weight, and the monodisperse lithium carbonate crystals are obtained by a method of crystal extraction instant sampling, namely, slurry is taken out for solid-liquid separation at the instant when the solution becomes turbid from clarification, and the obtained solid is dried.

2. A multi-stage gradient crystallization method of monodisperse lithium carbonate crystals is characterized by comprising the following steps:

(1) pumping a raw material lithium sulfate solution and a sodium carbonate solution into a mixing kettle through a three-stage crystallization kettle mother liquor circulation method, mixing the raw material lithium sulfate solution and the sodium carbonate solution with a circulation mother liquor in the mixing kettle to react, wherein the temperature of the mixing kettle is 20-40 ℃, and then feeding the mixed material liquid into a first-stage crystallization kettle;

(2) the temperature of the first-stage crystallization kettle is 65-75 ℃, monodisperse lithium carbonate crystals are generated by crystallization, slurry flows out from the bottom of the crystallization kettle by a method of crystal-out instant sampling, the slurry enters a solid-liquid separation system, the lithium carbonate crystals are separated and dried to be used as a product discharge system, and mother liquor enters a second-stage crystallization kettle;

(3) the temperature of the second-stage crystallization kettle is 75-85 ℃, monodisperse lithium carbonate crystals are generated by crystallization, slurry flows out from the bottom of the crystallization kettle by a method of crystal-out instant sampling, the slurry enters a solid-liquid separation system, the lithium carbonate crystals are separated and dried to be used as a product discharge system, and mother liquor enters a third-stage crystallization kettle;

(4) and the temperature of the third-stage crystallization kettle is 85-95 ℃, monodisperse lithium carbonate crystals are generated by crystallization, slurry flows out from the bottom of the crystallization kettle by a method of crystal-out instant sampling, the slurry enters a solid-liquid separation system, the lithium carbonate crystals are dried after separation and serve as a product discharge system, part of mother liquor is returned to the mixing kettle by a pump for cyclic utilization, and the circulation amount is 40-80% of the volume of the mother liquor.

3. The multistage gradient crystallization method according to claim 2, wherein a three-stage crystallization kettle is changed into a four-stage gradient crystallization method, wherein the stage temperatures of a first-stage crystallization kettle, a second-stage crystallization kettle, a third-stage crystallization kettle and a fourth-stage crystallization kettle are 65-75 ℃, 75-80 ℃, 80-85 ℃ and 85-95 ℃ respectively; and (3) after the slurry of the fourth-stage crystallization kettle enters a solid-liquid separation system, drying the solid, discharging the solid out of the system, returning part of the mother liquor to the mixing kettle by a pump for continuous cyclic utilization, wherein the circulating amount is 40-80% of the volume of the crystallization mother liquor.

4. The multistage gradient crystallization method of claim 2, wherein a three-stage crystallization kettle is changed into a two-stage gradient crystallization method, the temperatures of a first-stage crystallization kettle and a second-stage crystallization kettle are 65-75 ℃ and 85-95 ℃ respectively, after slurry of the second-stage crystallization kettle enters a solid-liquid separation system, solid is discharged out of the system after being dried, part of mother liquor is returned to a mixing kettle by a pump for continuous recycling, and the recycling amount is 40-80% of the volume of the crystallization mother liquor.

5. The crystallization method as claimed in any one of claims 1 to 4, wherein the sampling at the moment of crystal growth is controlled by a laser method, the sampling is carried out at the moment of crystal growth, a laser beam is transmitted through the solution by a laser transmitter, when the laser intensity signal suddenly drops, the slurry is taken out for solid-liquid separation, and at the moment, the solid content of the slurry in each stage of the crystallization kettle is 0.3 to 1.0 percent by weight.

6. A crystallization method according to any one of claims 2 to 4, characterised in that the concentration of the starting lithium sulphate solution in step (1) is between 12% and 32% wt and the concentration of the starting sodium carbonate solution is between 11% and 30% wt, both being pumped in equimolar equal volumes.

7. The crystallization method according to any one of claims 2 to 4, wherein the pumping flow rates of the raw material lithium sulfate solution and the sodium carbonate solution in the step (1) are 10% to 30% per hour of the volume of the solution in the mixing tank.

8. The crystallization method according to any one of claims 2 to 4, wherein a mixed solution containing 1.8 wt% to 2.2 wt% of lithium sulfate and 1.6 wt% to 2.0 wt% of sodium carbonate is prepared in the start-up stage of step (1) and pumped into the mixing kettle as a circulating mother solution, and after the system is stabilized, a part of the mother solution in the last stage of the crystallization kettle is used as the circulating mother solution, and the pumping flow rate is 40% to 80% per hour of the volume of the solution in the mixing kettle.

9. The crystallization method according to any one of claims 1 to 4, wherein after separation of the slurry in the final crystallization vessel, the obtained solid is air-dried; the temperature of the forced air drying under normal pressure is 40-60 ℃, and the time of the forced air drying is 8-12 h.

Technical Field

The invention belongs to the field of industrial production processes of lithium carbonate, and relates to a crystallization method of monodisperse lithium carbonate crystals and a method for improving product yield by adopting multi-stage gradient crystallization.

Background

Lithium carbonate is a key raw material of electrode materials (lithium cobaltate, lithium manganate, lithium iron phosphate, etc.) of lithium ion batteries, electrolytes (lithium hexafluorophosphate, etc.) and additives (lithium dioxalate borate, etc.). In recent years, with the increase of global new energy development, lithium ion batteries for power and energy storage are in a trend of high-speed development, so that the rapid increase of lithium carbonate market consumption is promoted, and lithium carbonate and preparation thereof are widely concerned by the industry.

Currently commercially used lithium ion battery positive electrode materials are mainly classified into the following three types according to structures: lithium cobaltate with a hexagonal layered crystal structure; ② lithium manganate with cubic spinel crystal structure; ③ lithium iron phosphate with an orthorhombic olivine crystal structure. The anode material is mainly synthesized by using battery-grade lithium carbonate as a raw material and corresponding cobalt, manganese and iron compounds by adopting a high-temperature solid-phase synthesis method. The high-temperature solid-phase synthesis method does not need to additionally use a solvent, so that the introduction of impurities in the preparation process of the electrode material is avoided, and the method is generally used in industrial production. The method mainly comprises the steps of mixing, spray drying, sintering, crushing, mixing and baking. Because the synthesized product is required to reach a certain granularity, more importantly, the cobalt, manganese and iron compounds are very fine products, and thus the lithium carbonate used in the high-temperature solid-phase synthesis method is required to have small granularity, narrow distribution and good dispersibility, so that the reactants can be uniformly mixed in the mixing process, the reaction is thorough in the sintering process, and the obtained product has good charge-discharge effect. In addition, lithium carbonate is added into the battery as an additive of the electrolyte, so that the film forming property of the battery can be improved, and the cycle performance and the low-temperature discharge performance of the battery are improved. In summary, lithium carbonate is used as a basic raw material for manufacturing lithium ion batteries, and has strict quality requirements, and battery grade lithium carbonate needs to have good dispersibility, complete crystal morphology, almost no coalescence, small crystal particle size, uniform distribution, and high purity.

The carbonization thermoeduction method is used as a preparation method of battery-grade lithium carbonate which is widely applied in industry, and the process flow mainly comprises three stages of lithium precipitation, carbonization and thermoeduction and a post-treatment process. In the lithium precipitation stage, the solution rich in lithium ions reacts with sodium carbonate to crystallize a primary lithium carbonate product; in the carbonization stage, primary lithium carbonate is mixed with deionized water to form aqueous solution slurry, then high-purity carbon dioxide gas is introduced into the aqueous solution slurry to convert the insoluble lithium carbonate into lithium bicarbonate with higher solubility, then the slurry is filtered to remove insoluble impurities, and the impurities dissolved in the filtrate can be removed by methods such as ion exchange and extraction; in the thermal precipitation stage, heating the purified lithium bicarbonate solution to cause decomposition reaction, and precipitating to generate lithium carbonate crystals; and performing post-treatment operations such as filtering, washing, drying, jet milling and the like to obtain the battery-grade lithium carbonate micro powder. Although the product obtained by the carbonization thermal precipitation method can basically meet the quality requirements of lithium carbonate for lithium batteries on purity and granularity, the process route is complicated and long, and the lithium carbonate product crushed by air flow is seriously crushed and has incomplete crystal morphology. If the lithium-rich solution and the sodium carbonate can be reacted and crystallized in one step to directly generate a high-quality lithium carbonate product in the lithium precipitation stage, the process route can be greatly shortened, and the defects of the existing high-purity lithium carbonate preparation method are overcome. However, in the traditional lithium precipitation reaction crystallization process, materials stay in a single kettle for a long time at low temperature and high degree of over saturation, and due to the nature of a lithium carbonate system, secondary crystallization mainly based on agglomeration is serious, so that a series of problems of large product particle size, uneven particle size distribution, poor crystal morphology, serious agglomeration behavior, low purity and the like obviously exist, and the application of the lithium precipitation reaction crystallization process is greatly limited.

At present, the research on the lithium carbonate reaction crystallization process at home and abroad mainly focuses on controlling the purity, granularity and appearance of the product. For example, patent CN110028088A discloses a method for preparing battery-grade lithium carbonate, which comprises preparing lithium aluminum hydrotalcite from lithium carbonate-containing brine lithium extraction mother liquor, transferring lithium ions to an aqueous solution by an acidification method to realize separation of lithium ions from impurity ions, and removing aluminum and concentrating to obtain battery-grade lithium carbonate; patent CN102408119A discloses a method for preparing lithium carbonate ultrafine powder by elution-reaction crystallization, wherein under the action of introduced elution agent, water-soluble lithium salt reacts with reactant, and lithium carbonate ultrafine powder is prepared by reaction crystallization; patent CN106517259A discloses a spherical lithium carbonate and a preparation method thereof, wherein a spherical lithium carbonate crystal is obtained by controlling process conditions and adding a surface treatment agent, and the obtained spherical lithium carbonate has high purity and good fluidity, and is beneficial to mixing and processing in subsequent processes. In the prior art, the water phase reaction crystallization method is adopted to prepare the monodisperse lithium carbonate crystal with complete crystal morphology, which is still blank in the field, in combination with literature and patent reports.

On the basis of controllable preparation of the monodisperse lithium carbonate crystals, the invention provides a multistage gradient crystallization process for realizing large-scale production of the monodisperse lithium carbonate crystals by a method of series operation of a plurality of crystallization kettles and mother liquor circulation, realizes controllable preparation of the lithium carbonate crystals with complete monodisperse morphology by controlling the reaction crystallization process, and has important application value.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a lithium carbonate crystal crystallization method which can prepare monodisperse crystals, has complete morphology and almost no coalescence, small crystal particle size, uniform distribution, larger specific surface area and stable product quality, and a multistage gradient crystallization process for improving the product yield.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for crystallizing monodisperse lithium carbonate crystals adopts a one-step reaction crystallization method with equal molar volume of lithium sulfate and sodium carbonate, the temperature of a crystallization kettle is 65-95 ℃, the concentration of a lithium sulfate solution is 5.3-9.0 wt%, and the concentration of a sodium carbonate solution is 5.1-8.7 wt%.

A multi-stage gradient crystallization method of monodisperse lithium carbonate crystals comprises the following steps:

(1) pumping a raw material lithium sulfate solution and a sodium carbonate solution into a mixing kettle through a three-stage crystallization kettle mother liquor circulation method, mixing the raw material lithium sulfate solution and the sodium carbonate solution with a circulation mother liquor in the mixing kettle to react, wherein the temperature of the mixing kettle is 20-40 ℃, and then feeding the mixed material liquid into a first-stage crystallization kettle;

(2) the temperature of the first-stage crystallization kettle is 65-75 ℃, monodisperse lithium carbonate crystals are generated by crystallization, slurry flows out from the bottom of the crystallization kettle by a method of crystal-out instant sampling, the slurry enters a solid-liquid separation system, the lithium carbonate crystals are separated and dried to be used as a product discharge system, and mother liquor enters a second-stage crystallization kettle;

(3) the temperature of the second-stage crystallization kettle is 75-85 ℃, monodisperse lithium carbonate crystals are generated by crystallization, slurry flows out from the bottom of the crystallization kettle by a method of crystal-out instant sampling, the slurry enters a solid-liquid separation system, the lithium carbonate crystals are separated and dried to be used as a product discharge system, and mother liquor enters a third-stage crystallization kettle;

(4) and the temperature of the third-stage crystallization kettle is 85-95 ℃, monodisperse lithium carbonate crystals are generated by crystallization, slurry flows out from the bottom of the crystallization kettle by a method of crystal-out instant sampling, the slurry enters a solid-liquid separation system, the lithium carbonate crystals are dried after separation and serve as a product discharge system, part of mother liquor is returned to the mixing kettle by a pump for cyclic utilization, and the circulation amount is 40-80% of the volume of the mother liquor.

The multistage gradient crystallization method can change a three-stage crystallization kettle into a four-stage gradient crystallization method, wherein the stage temperatures of a first-stage crystallization kettle, a second-stage crystallization kettle, a third-stage crystallization kettle and a fourth-stage crystallization kettle are 65-75 ℃, 75-80 ℃, 80-85 ℃ and 85-95 ℃ respectively; and (3) after the slurry of the fourth-stage crystallization kettle enters a solid-liquid separation system, drying the solid, discharging the solid out of the system, returning part of the mother liquor to the mixing kettle by a pump for continuous cyclic utilization, wherein the circulating amount is 40-80% of the volume of the crystallization mother liquor.

The multistage gradient crystallization method can change a three-stage crystallization kettle into a two-stage gradient crystallization method, wherein the temperatures of a first-stage crystallization kettle and a second-stage crystallization kettle are 65-75 ℃ and 85-95 ℃ respectively, after slurry of the second-stage crystallization kettle enters a solid-liquid separation system, solid is discharged out of the system after being dried, part of mother liquor is returned to a mixing kettle by a pump for continuous recycling, and the recycling amount is 40-80% of the volume of the crystallization mother liquor.

The method for crystal-out instant sampling is characterized in that a laser method is adopted to control crystal-out instant rapid sampling, a laser transmitter is used for transmitting a beam of laser through a solution, when a laser intensity signal suddenly drops, slurry is taken out for solid-liquid separation, and at the moment, the solid content of the crystal slurry in each stage of crystallization kettle is 0.3-1.0 wt%.

The concentration of the raw material lithium sulfate solution in the step (1) of the multistage gradient crystallization method is 12-32 wt%, the concentration of the raw material sodium carbonate solution is 11-30 wt%, and the raw material lithium sulfate solution and the raw material sodium carbonate solution are pumped in equal molar equal volumes.

The pumping flow rate of the raw materials of the lithium sulfate solution and the sodium carbonate solution in the step (1) of the multistage gradient crystallization method is 10-30%/h of the volume of the solution in the mixing kettle.

In the step (1) of the multistage gradient crystallization method, a mixed solution containing 1.8-2.2 wt% of lithium sulfate and 1.6-2.0 wt% of sodium carbonate is prepared in the start-up stage and pumped into a mixing kettle as a circulating mother solution, part of the mother solution in the last stage of crystallization kettle is used as the circulating mother solution after a system is stabilized, and the pumping flow rate is 40-80% per hour of the volume of the solution in the mixing kettle.

After the slurry of the last stage of crystallization kettle is separated, the obtained solid is dried by air blast; the temperature of the forced air drying under normal pressure is 40-60 ℃, and the time of the forced air drying is 8-12 h.

The concrete description is as follows:

a crystallization method of monodisperse lithium carbonate crystals is characterized in that a one-step reaction crystallization method with equal molar volume of lithium sulfate and sodium carbonate solution is adopted, the temperature of a crystallization kettle is controlled to be 65-95 ℃, the concentration of the lithium sulfate solution is 5.3-9.0% by weight, and the concentration of the sodium carbonate solution is 5.1-8.7% by weight, so that the instantaneous rapid sampling of crystal production is controlled, namely slurry is taken out at the moment when the solution becomes turbid from clarification, solid-liquid separation is carried out, and the obtained solid is dried to obtain the monodisperse lithium carbonate crystals.

In order to further improve the yield of the monodisperse lithium carbonate product, a method for improving the product yield by adopting multi-stage gradient crystallization is provided, which specifically comprises the following steps:

(1) pumping a raw material lithium sulfate solution and a sodium carbonate solution into a mixing kettle, mixing the raw material lithium sulfate solution and the sodium carbonate solution with a circulating mother solution (a mixed solution of lithium sulfate and sodium carbonate with a certain concentration is required to be prepared as the circulating mother solution in a start-up stage, and the mother solution of the last-stage crystallization kettle is adopted as the circulating mother solution after the solution is stabilized) in the mixing kettle to perform a chemical reaction, wherein the temperature of the mixing kettle is 20-40 ℃, and the mixed material liquid enters the first-stage crystallization kettle;

(2) controlling the temperature of the first-stage crystallization kettle to be 65-75 ℃, crystallizing to generate monodisperse lithium carbonate crystals, controlling the solid content of crystal slurry by a crystal outlet instant sampling method, enabling the slurry to flow out of the bottom of the crystallization kettle and enter a solid-liquid separation system, drying the lithium carbonate crystals after separation to serve as a product discharge system, and enabling mother liquor to enter a second-stage crystallization kettle;

(3) controlling the temperature of the second-stage crystallization kettle to be 75-85 ℃, crystallizing to generate monodisperse lithium carbonate crystals, controlling the solid content of crystal slurry by a crystal outlet instant sampling method, enabling the slurry to flow out of the bottom of the crystallization kettle and enter a solid-liquid separation system, drying the lithium carbonate crystals after separation to serve as a product discharge system, and enabling a mother liquor to enter a third-stage crystallization kettle;

(4) and controlling the temperature of the third-stage crystallization kettle to be 85-95 ℃, crystallizing to generate monodisperse lithium carbonate crystals, controlling the solid content of crystal slurry by a crystal-out instant sampling method, enabling the slurry to flow out of the bottom of the crystallization kettle and enter a solid-liquid separation system, separating the lithium carbonate crystals, drying the lithium carbonate crystals to serve as a product discharge system, returning part of mother liquor to the mixing kettle by a pump for cyclic utilization, wherein the cycle volume is 40-80% of the volume of the mother liquor.

In the invention, the method can be suitable for a multistage gradient crystallization process with the crystallization kettle stage number of 2-4, and the product indexes obtained by the processes with different stage numbers are not greatly different. Along with the increase of the number of the crystallization kettle stages, the yield can be improved by 5 percent per stage, but higher production cost is required at the same time, so the number of the crystallization kettle stages can be optimized according to the production scale and considering the factors such as equipment investment, operation cost and the like in actual production.

If a four-stage gradient crystallization method is adopted, the temperatures of the first, second, third and fourth-stage crystallization kettles are 65-75 ℃, 75-80 ℃, 80-85 ℃ and 85-95 ℃ respectively, after slurry of the fourth-stage crystallization kettle enters a solid-liquid separation system, the solid is dried and discharged out of the system, part of mother liquor is returned to the mixing kettle by a pump for continuous recycling, the circulation amount is 40-80% of the volume of the crystallization mother liquor, and monodisperse lithium carbonate crystals can be obtained.

If a two-stage gradient crystallization method is adopted, the temperatures of the first-stage crystallization kettle and the second-stage crystallization kettle are 65-75 ℃ and 85-95 ℃ respectively, slurry of the second-stage crystallization kettle enters a solid-liquid separation system, the solid is dried and discharged out of the system, part of mother liquor returns to the mixing kettle by a pump for continuous cyclic utilization, the circulating amount is 40-80% of the volume of the crystallization mother liquor, and monodisperse lithium carbonate crystals can be obtained.

In the invention, a laser method is adopted to control the crystal production instant and fast sampling, namely, a laser transmitter transmits a beam of laser through the solution, when the laser intensity signal suddenly drops, the slurry is taken out for solid-liquid separation, and at the moment, the solid content of the crystal slurry in each stage of crystallization kettle is 0.3-1.0% by weight.

In the invention, the concentration of the raw material lithium sulfate solution in the step (1) is 12-32 wt%, the concentration of the raw material sodium carbonate solution is 11-30 wt%, and the raw material lithium sulfate solution and the raw material sodium carbonate solution are pumped in equal molar equal volumes.

In the invention, the pumping flow rate of the raw materials of the lithium sulfate solution and the sodium carbonate solution in the step (1) is 10-30% per hour of the volume of the solution in the mixing kettle.

In the invention, the mixed solution containing 1.8-2.2 wt% of lithium sulfate and 1.6-2.0 wt% of sodium carbonate is prepared and pumped into the mixing kettle as the circulating mother solution in the start-up stage of the step (1), after the system is stable, the mother solution of the last stage of crystallization kettle is used as the circulating mother solution, and the pumping flow rate is 40-80%/h of the volume of the mixed kettle solution.

In the invention, after the slurry of the last stage of crystallization kettle is separated, the obtained solid is dried by blowing air; the temperature of the forced air drying under normal pressure is 40-60 ℃, and the time of the forced air drying is 8-12 h.

In the invention, part of the mother liquor obtained by the last stage of solid-liquid separation enters the mixing kettle to be continuously circulated so as to further improve the yield.

In the invention, the monodisperse lithium carbonate crystal has complete appearance, almost no coalescence, small crystal size, uniform distribution, larger specific surface area and stable product quality.

In the traditional lithium precipitation reaction crystallization process, materials stay in a single kettle for a long time at low temperature and high degree of over-saturation, and because lithium carbonate has a multi-stage structure, secondary crystallization mainly caused by agglomeration is serious, so that the product performance is greatly influenced; particularly, the battery-grade lithium carbonate has high requirements on the particle size and the morphology, and a series of problems of large product particle size, uneven particle size distribution, poor crystal morphology, serious coalescence behavior and the like obviously exist in the traditional production and preparation process. And a multistage gradient crystallization method is adopted to separate the reaction process from the crystallization process, the crystallization process is maintained at a high temperature and a low supersaturation degree, the product is timely moved out of a crystallization system by controlling the retention time, and the problem that the lithium carbonate product is extremely easy to agglomerate can be fundamentally solved under the condition of ensuring the yield.

The product yield can be slightly improved by increasing the number of stages of the crystallization kettle, but the equipment investment cost can be improved, and the operation difficulty is increased. Reducing the number of stages of the crystallization kettle can reduce the equipment investment cost and the operation difficulty, but can reduce the product yield. Therefore, the actual number of crystallization kettle stages should be optimized according to the actual situation, and is generally ideal to be 2-4 stages.

Compared with the prior art, the invention has the following beneficial effects that the supporting data are shown in the specific implementation mode:

(1) the produced monodisperse lithium carbonate crystals have complete morphology and almost no coalescence, small crystal particle size, uniform distribution, larger specific surface area and stable product quality, and are beneficial to ensuring the consistency of the production process of the electrode material.

(2) The improvement of the quality such as the form, the granularity and the like of the lithium carbonate crystal reduces the resistance pressure drop of the filter cake and improves the efficiency of the post-treatment processes such as filtration (centrifugation), washing, drying and the like.

(3) The multistage gradient crystallization method adopts high-temperature low-supersaturation operation, and the product is timely moved out of the crystallization system by controlling the retention time, so that the problem of product coalescence caused by abnormal growth is effectively avoided, and the product yield can be further improved by multi-kettle operation and mother liquor circulation.

(4) The method is simple to operate, high in production efficiency and suitable for industrial large-scale production.

Drawings

FIG. 1 is a scanning electron micrograph of lithium carbonate crystals prepared in example 3 of the present invention, with a scale of 500. mu.m;

FIG. 2 is a scanning electron micrograph of a commercially available battery grade lithium carbonate product at 50 μm scale;

fig. 3 is a schematic flow chart of a three-stage gradient crystallization process of lithium carbonate according to the present invention;

FIG. 4 is a scanning electron micrograph of lithium carbonate crystals prepared in example 4 of the present invention, with a scale of 500. mu.m;

fig. 5 is a particle size distribution diagram of lithium carbonate crystals prepared in example 4 of the present invention;

FIG. 6 is a powder X-ray diffraction pattern of lithium carbonate crystals obtained by preparation of example 4 of the present invention;

fig. 7 is a schematic flow chart of a secondary gradient crystallization process of lithium carbonate according to the present invention;

fig. 8 is a schematic flow chart of the four-stage gradient crystallization process of lithium carbonate.

Detailed Description

The technical scheme of the invention is further explained by the specific implementation mode in combination with the attached drawings. The following examples are merely illustrative of the present invention and do not represent or limit the scope of the claims, which are defined by the claims.