Purification method of low-impurity lithium carbonate

Abstract

The invention discloses a purification method of low-impurity lithium carbonate. The purification method comprises seven steps of leaching, impurity removal, purification, evaporation and concentration, lithium precipitation, centrifugal desalination and mother liquor circulation. During leaching, the lithium-containing material and water are mixed and then subjected to supergravity treatment in a rotating packed bed, the temperature is regulated and controlled in two stages, and a leaching solution is obtained. In the impurity removal process, LiS is added firstly to remove heavy metal, then deep impurity removal is conducted through a composite system of modified D751 chelate resin and free EDTA, and the modified D751 chelate resin is prepared by grafting EDTA groups and loading nano zirconium dioxide. The purification adopts two-stage treatment of gradient pore composite resin balls and a double-layer ceramic membrane. According to the method, through deep impurity removal of the modified D751 chelate resin, two-stage purification of the gradient pore composite resin balls and the double-layer ceramic membrane and combination of mother liquor circulation, the purity, the impurity removal rate and the lithium recovery rate of lithium carbonate can be remarkably improved, and the method is suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of industrial-grade lithium carbonate preparation technology, specifically a method for purifying low-impurity lithium carbonate. Background Technology

[0002] Lithium carbonate, as an important inorganic chemical raw material, is widely used in lithium-ion batteries, ceramics, glass, and pharmaceuticals. In the lithium-ion battery industry, high-purity lithium carbonate is a core raw material for preparing cathode materials (such as lithium iron phosphate and ternary materials), and its impurity content directly affects the cycle life and safety of the battery.

[0003] In existing technologies, lithium carbonate purification processes rely excessively on single resins or chemical precipitation methods for impurity removal. This limitation makes it difficult to simultaneously and efficiently remove heavy metals and alkaline earth metal ions such as calcium and magnesium. The result is often high levels of residual impurities, directly impacting product purity and failing to meet low-impurity requirements. In single-stage filtration or adsorption processes, if the adsorbent material has a simple pore structure, its flow rate is limited by high resistance, or its effectiveness is reduced due to insufficient adsorption depth, making it difficult to improve purification efficiency. Furthermore, poorly designed filter membranes suffer from high filtration resistance and low flux, failing to effectively trap small colloidal particles, making it difficult to control solution turbidity at low levels. In addition, the mother liquor is usually directly discharged, causing serious waste of lithium resources and exacerbating environmental pollution, increasing environmental pressure. These technical challenges result in lithium carbonate products produced by existing methods having high impurity content, far from meeting the high-end demand for low-impurity materials.

[0004] Therefore, it is necessary to provide a method for purifying lithium carbonate with low impurities to solve the above-mentioned technical problems. Summary of the Invention

[0005] The purpose of this invention is to provide a method for purifying lithium carbonate with low impurities, in order to solve the problems of low lithium leaching rate, incomplete impurity removal, insufficient product purity, and low resource utilization in the prior art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for purifying low-impurity lithium carbonate, comprising the following steps: Step 1: Leaching The lithium-containing material is mixed with water and then subjected to high-gravity leaching in a rotating packed bed. The temperature of the mixture during the leaching process is controlled in two stages to promote the dissolution of lithium, and finally the leachate is obtained. Step 2: Impurity Removal Li2S is first added to the leachate obtained in step one to remove heavy metal ions, and then a composite impurity removal system consisting of modified D751 chelating resin and free ethylenediaminetetraacetic acid is added. After the impurity removal is completed, the impurity removal solution is obtained. The modified D751 chelating resin is prepared by grafting ethylenediaminetetraacetic acid groups and loading nano-zirconia on the surface. Step 3: Purification The impurity removal solution obtained in step two is purified in two stages using composite resin balls and a precision ceramic membrane: the first stage uses an ion exchange column filled with composite resin balls to adsorb ionic impurities and remove most of the ionic impurities in the impurity removal solution to obtain a first-stage purified solution; the second stage uses ceramic membrane filtration, and the first-stage purified solution is filtered through the ceramic membrane after passing through the ion exchange column to obtain a second-stage purified solution. Step 4: Evaporation and Concentration The secondary purified liquid obtained in step three is evaporated into a concentrated liquid with a lithium concentration of 20-30 g / L; Step 5: Lithium precipitation Na2CO3 was added to the concentrated solution obtained in step four to carry out a lithium precipitation reaction, followed by solid-liquid separation to obtain crude lithium carbonate and mother liquor. Step Six: Centrifugal Desalination The crude lithium carbonate obtained in step 5 is first pre-washed with hot water, and then centrifuged to obtain low-impurity lithium carbonate. Step 7: Mother liquor circulation The mother liquor obtained in step five is returned to step one. The lithium concentration in the mother liquor is controlled at 1-2 g / L, which is lower than the leaching equilibrium concentration. The resulting concentration gradient drives the continuous dissolution of lithium.

[0007] Preferably, in the first leaching step, the temperature of the mixture is controlled as follows: the initial temperature is controlled at 20-40℃ for 1-2 hours to inhibit the dissolution of impurities such as calcium and magnesium, and the subsequent temperature is controlled at 60-80℃ for 3-5 hours.

[0008] Preferably, in step two, the amount of Li2S added is 1.1-1.3 times the total molar amount of heavy metal ions in the solution, and the reaction temperature is controlled at 35-45℃. After the reaction is completed, the mixture is allowed to stand for 1-2 hours, and then the mixture is filtered to separate the precipitate.

[0009] Preferably, in the composite impurity removal system, the mass ratio of the modified D751 chelating resin to the free ethylenediaminetetraacetic acid is 4-6:1, the pH value of the impurity removal process is controlled at 8-9, and the preparation steps of the modified D751 chelating resin include: (1) Resin pretreatment Take D751 chelating resin, mix it with 4-6% hydrochloric acid solution at a solid-liquid ratio of 1:9-11, stir it in a constant temperature water bath at 55-65℃ for 1.5-2.5h to remove residual impurities and free ions on the resin surface, filter it and wash it with deionized water until neutral. The resin is then transferred to a 4-6% sodium hydroxide solution and stirred at 55-65℃ for 1.5-2.5 hours at a solid-liquid ratio of 1:9-11 to activate the functional groups of the resin. After filtration, the resin is washed with deionized water until neutral and then vacuum dried to constant weight. (2) Amine functionalization grafting The pretreated D751 resin and ethylenediaminetetraacetic acid dianhydride were added to N,N-dimethylformamide solvent at a mass ratio of 1:1.8-2.2, wherein the solid-liquid ratio of resin to N,N-dimethylformamide was 1:7-9. The mixture was heated to 75-85℃ under nitrogen protection and stirred for 5-7 hours. After the reaction is complete, the resin is collected by filtration and washed 2-4 times with anhydrous ethanol to remove unreacted ethylenediaminetetraacetic acid dianhydride. The resin is then dried under vacuum at 55-65℃ to graft ethylenediaminetetraacetic acid groups onto the resin surface, thus obtaining anamine-functionalized resin. (3) Nano-zirconia loading Prepare a 0.4-0.6 mol / L zirconium oxychloride solution, impregnate the amine-functionalized resin in it at a solid-liquid ratio of 1:14-16, and stir and adsorb at 25-35℃ for 3-5 hours. Subsequently, 0.8-1.2 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH of the system to 8.5-9.5, so that the zirconium ions adsorbed on the resin surface would hydrolyze to form nano-zirconium dioxide particles. After stirring for 0.5-1.5 h, the mixture was filtered, washed with deionized water until the filtrate was free of chloride ions, and then vacuum dried at 75-85 °C to obtain the composite resin loaded with nano-zirconium dioxide. (4) Crosslinking stabilization treatment The above composite resin was added to a 4-6% (w / w) glutaraldehyde solution and mixed at a solid-liquid ratio of 1:11-13. The mixture was stirred and crosslinked at 35-45°C for 1.5-2.5 hours to allow the functional groups on the resin surface to be connected by covalent bonds to form a stable structure. After filtration, the resin was washed with deionized water until no aldehyde residue was found. The resin was then vacuum dried at 55-65°C to constant weight to obtain the modified D751 chelating resin.

[0010] Preferably, in the purification process of step three, the composite resin ball has a gradient pore structure: the outer layer has large pores of 40-110μm, which allows the solution to flow through quickly and initially adsorb large-particle impurities; the inner layer has micropores of 4-11μm, which enhances the deep adsorption of trace ions such as calcium and magnesium through capillary action. The preparation of the composite resin balls includes: (1) Resin mixing Weigh LSC-500, D451 and S930 resins according to a mass ratio of 2-4:5-7:1, place them in a mixing tank and stir at 250-350 r / min for 25-35 min to ensure that the three resin particles are evenly mixed. (2) Preparation of outer substrate Add 4-6% polyvinyl alcohol solution by mass to the mixed resin obtained in step (1), control the solid-liquid ratio of resin to solution to be 1:2-4, stir in a water bath at 45-55℃ until a uniform paste is formed, and the paste is extruded and molded by a granulation mold. Due to cross-linking, polyvinyl alcohol forms macropores of 40-110μm, which constitute the outer skeleton of the composite resin ball. (3) Preparation of inner layer substrate Take another portion of the mixed resin from step (1), add 7-9% by mass of gelatin solution, control the solid-liquid ratio to 1:1-3, stir at 35-45℃ until the gelatin is completely swollen and coats the resin particles, so that the gelatin forms a 4-11μm microporous structure on the surface of the resin particles as an inner adsorption carrier. (4) Freezing and setting The outer skeleton of the composite resin ball obtained in step (2) and the inner adsorption carrier obtained in step (3) are assembled in a layered structure, that is, the outer skeleton is used as the base and the inner adsorption carrier is uniformly covered on the base. Then, it is placed in a freezer at -25 to -15℃ for 3-5 hours to solidify the colloidal structure of polyvinyl alcohol and gelatin. (5) Drying control The frozen semi-finished product from step (4) is transferred to a vacuum drying oven at 55-65℃ and dried at a vacuum of -0.09 to -0.07 MPa for 5-7 hours. The pore structure is stabilized by water sublimation, and the overall porosity of the composite resin ball reaches 40-50%. (6) Outer layer granulation The semi-finished product after drying in step (5) is crushed into 1-4 mm particles, and then mixed with a polyvinyl alcohol solution of 4-6% by mass at a solid-liquid ratio of 1:1-3. The mixture is then placed in a rotary granulator and granulated at a speed of 10-20 r / min. During the rolling process, the outer layer of the particles is wrapped with a polyvinyl alcohol film to form spheres with a diameter of 3-5 mm. The outer macroporous structure of 40-110 μm is naturally maintained by the surface tension during granulation. (7) Inner layer pore formation After granulation in step (6), the spheres are immersed in a gelatin solution with a mass fraction of 7-9% for 25-35 minutes to allow the gelatin to penetrate into the spheres. After granulation, the freeze-drying process in steps (4) and (5) is repeated. That is, the spheres are first frozen at -25 to -15℃ for 3-5 hours, and then vacuum dried at 55-65℃ for 5-7 hours. Through the shrinkage of the gelatin and the evaporation of water, a microporous structure of 4-11 μm is formed inside the spheres, and finally a composite resin sphere with a gradient pore structure is obtained.

[0011] Preferably, in the purification process of step three, the ceramic membrane has a double-layer structure, with a dense outer layer with a pore size of 0.8-1.2μm to trap residual small colloidal particles, and a loose inner layer with a pore size of 4-6μm to reduce filtration resistance, thereby reducing the turbidity of the solution to below 0.1 NTU. The preparation steps of the ceramic membrane include: (1) The surface layer is made of 0.4-0.6μm ceramic powder and sintered at 1150-1250℃ to form a dense surface layer; (2) The bottom layer uses 4-6μm ceramic powder. It is first pre-sintered at 950-1050℃ for 1h to form a porous substrate. Then the dense surface layer blank prepared in step (1) is covered on the surface of the bottom substrate. It is sintered at the same temperature and kept warm for 1.5-2.5h so that the two layers are firmly bonded through diffusion sintering, and finally a loose bottom layer is formed with a membrane flux of 10-15L / m²・h.

[0012] Preferably, in step three of the purification process, the two-stage purification process involving the composite resin balls and the ceramic membrane includes: (1) Adsorption and purification by composite resin balls The impurity-removing solution obtained in step two is pumped into an ion exchange column filled with composite resin balls. The solution flow rate is controlled at 15-20 BV / h. The composite resin balls allow the solution to flow rapidly through the outer 40-110 μm macropores. At the same time, LSC-500, D451, and S930 type resins are used to adsorb and remove more than 85% of calcium, magnesium, and heavy metal ions in the solution, resulting in a first-stage purified solution. (2) Precision ceramic membrane depth filtration The effluent from the ion exchange column enters the ceramic membrane filtration system, which uses an alumina ceramic membrane module with a pore size of 0.08-0.12μm. The operating pressure is set at 0.15-0.25MPa and the cross-flow velocity is 1.2-1.8m / s. Under pressure, the primary purified liquid flows at high speed along the membrane surface. The residual trace colloidal particles, resin debris, and unadsorbed nanoscale impurities are trapped by the membrane, and the permeate is the secondary purified liquid.

[0013] Preferably, in step four, the evaporation and concentration process, the evaporation process is heated by steam, and the secondary steam generated during the evaporation process is recycled and reused in step one. The mixture of lithium-containing material and water is preheated to 55-65°C before entering the rotating packed bed.

[0014] Preferably, in step five, the lithium precipitation process, Na2CO3 is added in excess by 10-15%, the stirring rate is 300-500 rpm, the generated crystal particle size is 15-55 μm, and the sodium ion residue is ≤10 ppm; the temperature during the lithium precipitation process is controlled at 30-90℃ and the pH value is controlled at 10-11, and solid-liquid separation is performed after 1-5 hours of reaction to obtain crude lithium carbonate and mother liquor.

[0015] Preferably, in step six, the crude lithium carbonate obtained in step five is first pre-washed with hot water at 50-70°C, and then centrifuged to obtain low-impurity lithium carbonate. The solid-liquid ratio during washing is 1:4-6. Preferably, in step seven, during the mother liquor circulation, after every 4-6 cycles, 15-25% of the mother liquor is taken for electrodialysis desalination, with a lithium recovery rate ≥90%, to avoid impurity accumulation and a chloride ion concentration ≤20ppm.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. The ethylenediaminetetraacetic acid groups grafted onto the surface of the modified D751 chelating resin in this invention can form stable chelates with calcium, magnesium, and heavy metal ions through multidentate coordination effects. Meanwhile, the supported nano-zirconia, with its high specific surface area and strong adsorption of surface hydroxyl groups, physically adsorbs and chemically complexes residual trace metal ions, forming a dual barrier of chelation and adsorption. Compared with traditional single resin or chemical precipitation methods, the removal rate of calcium and magnesium ions is increased by more than 20%, and the residual amount of heavy metal ions can be reduced to below 0.5 ppm, thereby reducing the impurity content of the product from the source.

[0017] 2. This invention utilizes the gradient pore structure of composite resin spheres to achieve a staged treatment process from pre-adsorption to deep purification. The outer 40-110μm macropores reduce fluid resistance, ensuring a high solution flow rate (15-20 BV / h) while simultaneously adsorbing large-particle impurities. The inner 4-11μm micropores enhance the capture of trace ions through capillary action, achieving a calcium and magnesium ion removal rate of over 99%. Combined with a dual-layer ceramic membrane design: a dense 0.8-1.2μm surface layer precisely traps colloidal particles and resin debris, while a porous 4-6μm bottom layer reduces filtration resistance, maintaining a membrane flux of 10-15 L / m²·h and a stable solution turbidity below 0.1 NTU. This two-stage purification synergistically solves the problem of balancing efficiency and precision inherent in traditional single-stage purification.

[0018] 3. This invention avoids the formation of inclusions by calcium and magnesium ions co-precipitating with lithium carbonate through deep removal, resulting in a more uniform lithium precipitation reaction, a stable crystal particle size of 15-55 μm, sodium ion residue ≤10 ppm, and a product purity fluctuation range reduced to ±0.02%. Simultaneously, the low resistance of the double-layer ceramic membrane reduces filtration energy consumption by approximately 30%, and the high-efficiency adsorption performance of the composite resin balls reduces material consumption by more than 15%.

[0019] 4. This invention improves the lithium recovery rate to over 90% by combining mother liquor recycling with electrodialysis desalination, further reducing the raw material consumption per unit product. Attached Figure Description

[0020] Figure 1 A schematic diagram of the process flow for the purification method of low-impurity lithium carbonate provided by the present invention. Figure 2 Line graph comparing the lithium leaching rate in the purification methods provided in the embodiments and comparative examples of the present invention; Figure 3 Line graph comparing the Ca²⁺ removal rate in the purification methods provided in the embodiments and comparative examples of the present invention; Figure 4 Line graph comparing the Mg²⁺ removal rate in the purification methods provided in the embodiments and comparative examples of the present invention; Figure 5 Line graph comparing the purity of lithium carbonate in the purification methods provided in the embodiments and comparative examples of the present invention; Figure 6 This is a line graph comparing the total lithium recovery rate in the purification methods provided in the embodiments and comparative examples of the present invention. Detailed Implementation

[0021] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] The purification method for low-impurity lithium carbonate provided by this invention can be specifically implemented through the following Examples 1-3, and requires prior preparation of the following raw materials and equipment: the lithium-containing material is lepidolite powder, wherein the content of Li2O is 2.8%, the content of Ca²⁺ is 0.35%, the content of Mg²⁺ is 0.22%, and the content of Pb²⁺ is 0.005%; the auxiliary materials are analytical grade Li2O and ethylenediaminetetraacetic acid (EDTA), modified D751 chelating resin, and industrial grade Na2CO3 with a purity of 99.5%; the equipment includes a rotating packed bed, an ion exchange column with a diameter of 50 mm and a height of 800 mm, a vacuum drying oven, a rotary granulator, and a ceramic membrane filtration system made of alumina.

[0023] Example 1: First, lithium-containing materials were mixed with deionized water at a solid-liquid ratio of 1:5. After thorough stirring, the mixture was pumped into a rotating packed bed for leaching under hypergravity conditions. The rotation speed of the packed bed was 1200 r / min. The temperature was controlled in two stages: the first stage temperature was maintained at 20℃ for 1 hour to inhibit the dissolution of calcium and magnesium, and the second stage temperature was maintained at 60℃ for 3 hours to promote lithium dissolution. After leaching, the solution was filtered to obtain a leachate with a Li⁺ concentration of 4.2 g / L, a Ca²⁺ concentration of 0.12 g / L, and a Mg²⁺ concentration of 0.08 g / L.

[0024] Furthermore, Li₂S was added to the leachate to remove heavy metals. The amount of Li₂S added was 1.1 times the total molar amount of Pb²⁺. The mixture was stirred at 35°C for 1 hour, allowed to stand for 1 hour, and then filtered to remove precipitates such as PbS. A composite impurity removal system was then added to the filtrate. In this system, the mass ratio of modified D751 chelating resin to free EDTA was 4:1, with the resin dosage being 50 g / L and the EDTA dosage being 10 g / L. The pH of the system was adjusted to 8 with 0.1 mol / L NaOH. The mixture was stirred at 40°C for 2 hours and then filtered to obtain the impurity-removed solution, in which the Ca²⁺ concentration was ≤0.005 g / L and the Mg²⁺ concentration was ≤0.003 g / L.

[0025] Before purifying the impurity-removing solution, composite resin balls need to be prepared in advance. The preparation method is as follows: First, weigh LSC-500, D451, and S930 type resins and mix them in a mass ratio of 2:5:1. Stir at 250 r / min for 25 min. Add 4% polyvinyl alcohol solution to the mixed resin, with a resin-to-solid ratio of 1:2. Stir in a 45℃ water bath until a paste is formed. Extrude the paste through a granulation mold to form an outer layer substrate with macropores of 40 μm. Take another portion of the mixed resin and add 7% gelatin solution, with a resin-to-solid ratio of 1:1. Stir at 35℃ until the gelatin swells and coats the resin, forming an inner layer with micropores of 1:1. The inner layer substrate is 4μm thick. The outer skeleton and inner carrier are assembled in layers and cured by freezing at -25℃ for 3 hours. Then, it is transferred to a vacuum drying oven and dried at -0.09MPa vacuum and 55℃ for 5 hours to achieve a porosity of 40%. The dried semi-finished product is crushed into 1mm particles and mixed with 4% polyvinyl alcohol solution at a solid-liquid ratio of 1:1. The mixture is then placed in a rotary granulator and granulated at a speed of 10r / min to form spheres with a diameter of 3mm. The spheres are immersed in 7% gelatin solution for 30 minutes, frozen at -25℃ for 3 hours, and then vacuum dried at 55℃ for 5 hours to finally obtain composite resin spheres with a gradient pore structure.

[0026] A two-stage purification process using a composite resin ball and ceramic membrane system is employed. In the first stage, the impurity-removing solution is passed through an ion exchange column packed with composite resin balls at a flow rate of 15 BV / h to obtain a first-stage purified solution with a Ca²⁺ concentration ≤ 0.0005 g / L. In the second stage, the first-stage purified solution enters a ceramic membrane system with a surface pore size of 0.8 μm and a bottom pore size of 4 μm. The operating pressure is 0.15 MPa, the cross-flow velocity is 1.2 m / s, and the filtered solution yields a second-stage purified solution with a turbidity of 0.08 NTU.

[0027] Furthermore, the secondary purified liquid is pumped into the evaporator, heated to boiling by steam, and evaporated and concentrated to a Li⁺ concentration of 20 g / L. The secondary steam generated during the evaporation process is recovered and used to preheat the leaching raw materials, preheating the mixture to 55°C.

[0028] Furthermore, Na2CO3 was added to the concentrate to carry out a lithium precipitation reaction, with Na2CO3 in excess by 10%. The mixture was stirred at 300 rpm at 30°C, and the pH of the system was adjusted to 10 with 1 mol / L NaOH. After reacting for 1 hour, the mixture was centrifuged at 3000 rpm for 10 minutes to obtain crude lithium carbonate and mother liquor. The Li⁺ concentration in the mother liquor was 1.2 g / L.

[0029] Furthermore, the crude lithium carbonate was washed twice with hot water at 50°C at a solid-liquid ratio of 1:4 to remove surface sodium salts. Then, it was centrifuged at 7000 r / min for 15 min to obtain a wet product, which was then vacuum dried at 60°C for 2 h to obtain low-impurity lithium carbonate.

[0030] Finally, the mother liquor is returned to the leaching process in step one. After every 6 cycles, 15% of the mother liquor is taken for electrodialysis desalination, and the chloride ion concentration is controlled to be ≤20ppm.

[0031] Example 2: First, lithium-containing materials and water were mixed at a solid-liquid ratio of 1:5 and pumped into a rotating packed bed. The rotation speed of the rotating packed bed was 1200 r / min. The leaching temperature was controlled in two stages: the first stage temperature was 30℃ and maintained for 1.5 h, and the second stage temperature was 70℃ and maintained for 4 h, resulting in a leachate with a Li⁺ concentration of 4.5 g / L, a Ca²⁺ concentration of 0.11 g / L, and a Mg²⁺ concentration of 0.07 g / L.

[0032] Furthermore, Li₂S was added to the leachate at an amount 1.2 times the total molar amount of Pb²⁺. The mixture was reacted at 40°C for 1 hour, allowed to stand for 1.5 hours, and then filtered. A composite impurity removal system was added to the filtrate, in which the mass ratio of modified D751 resin to EDTA was 5:1, with 50 g / L of resin and 10 g / L of EDTA. The pH of the system was adjusted to 8.5, and the mixture was stirred at 40°C for 2 hours to obtain the impurity-removed solution, in which the concentration of Ca²⁺ was ≤0.003 g / L and the concentration of Mg²⁺ was ≤0.002 g / L.

[0033] Before purifying the impurity-removing solution, composite resin balls need to be prepared in advance. The preparation method is as follows: weigh LSC-500, D451 and S930 type resins in a mass ratio of 3:6:1 and mix them. Stir at 300 r / min for 30 min. Add 5% polyvinyl alcohol solution to the mixed resin, with a resin-to-solid ratio of 1:3. Stir at 50℃ to form an outer layer substrate with macropores of 75 μm. Take another mixed resin and add 8% gelatin solution, with a resin-to-solid ratio of 1:2. Stir at 40℃ to form an inner layer substrate with micropores of 7 μm. The outer skeleton and inner carrier were assembled in layers and cured by freezing at -20℃ for 4 hours. The mixture was then transferred to a vacuum drying oven and dried at -0.08MPa vacuum and 60℃ for 6 hours to achieve a porosity of 45%. The dried semi-finished product was crushed into 2mm particles and mixed with a 5% polyvinyl alcohol solution at a solid-liquid ratio of 1:2. The mixture was then placed in a rotary granulator and granulated at a speed of 15r / min to form spheres with a diameter of 4mm. The spheres were immersed in an 8% gelatin solution for 30 minutes, frozen at -20℃ for 4 hours, and then vacuum dried at 60℃ for 6 hours to obtain composite resin spheres.

[0034] In the first stage of purification, the impurity-removing solution is passed through an ion exchange column at a flow rate of 17 BV / h to obtain a first-stage purified solution with a Ca²⁺ concentration ≤ 0.0003 g / L. In the second stage of purification, the first-stage purified solution enters a ceramic membrane system with a surface pore size of 1.0 μm and a bottom pore size of 5 μm. The operating pressure is 0.2 MPa, the cross-flow velocity is 1.5 m / s, and the filtered solution yields a second-stage purified solution with a turbidity of 0.05 NTU.

[0035] Furthermore, the secondary purified liquid is evaporated and concentrated to a Li⁺ concentration of 25 g / L, and the secondary steam generated from the evaporation is used to preheat the raw materials to 60°C.

[0036] Furthermore, Na2CO3 was added to the concentrate, with an excess of 12.5%. The mixture was stirred at 400 rpm at 60°C, and the pH was adjusted to 10.5. After reacting for 3 hours, solid-liquid separation was performed to obtain crude lithium carbonate and mother liquor. The Li⁺ concentration in the mother liquor was 1.5 g / L.

[0037] Furthermore, the crude lithium carbonate was washed with hot water at 60°C at a solid-liquid ratio of 1:5 and centrifuged at 7000 r / min for 15 min.

[0038] Finally, after every 5 cycles, 20% of the mother liquor was taken for electrodialysis desalination.

[0039] Example 3: First, lithium-containing materials and water were mixed at a solid-liquid ratio of 1:5 and pumped into a rotating packed bed. The rotation speed of the rotating packed bed was 1200 r / min. The leaching temperature was controlled in two stages: the first stage temperature was 40℃ and maintained for 2 hours, and the second stage temperature was 80℃ and maintained for 5 hours, resulting in a leachate with a Li⁺ concentration of 4.6 g / L, a Ca²⁺ concentration of 0.13 g / L, and a Mg²⁺ concentration of 0.09 g / L.

[0040] Furthermore, Li₂S was added to the leachate at an amount 1.3 times the total molar amount of Pb²⁺. The mixture was reacted at 45°C for 1 hour, allowed to stand for 2 hours, and then filtered. A composite impurity removal system was added to the filtrate, in which the mass ratio of resin to EDTA was 6:1, with 60 g / L of resin and 10 g / L of EDTA. The pH of the system was adjusted to 9, and the mixture was stirred at 40°C for 2 hours to obtain a purified solution with Ca²⁺ concentration ≤0.004 g / L and Mg²⁺ concentration ≤0.003 g / L.

[0041] Before purifying the impurity-removing solution, composite resin balls need to be prepared in advance. The preparation method is as follows: weigh LSC-500, D451, and S930 type resins in a mass ratio of 4:7:1 and mix them. Stir at 350 r / min for 35 min. Add 6% polyvinyl alcohol solution to the mixed resin, with a resin-to-solid ratio of 1:4. Stir at 55℃ to form an outer layer substrate with macropores of 110 μm. Separately, take the mixed resin and add 9% gelatin solution, with a resin-to-solid ratio of 1:3. Stir at 45℃ to form an inner layer substrate with micropores of 11 μm. The outer skeleton and inner carrier were assembled in layers and cured by freezing at -15℃ for 5 hours. The mixture was then transferred to a vacuum drying oven and dried at -0.07MPa vacuum and 65℃ for 7 hours to achieve a porosity of 50%. The dried semi-finished product was crushed into 4mm particles and mixed with a 6% polyvinyl alcohol solution at a solid-liquid ratio of 1:3. The mixture was then placed in a rotary granulator and granulated at a speed of 20r / min to form spheres with a diameter of 5mm. The spheres were immersed in a 9% gelatin solution for 35 minutes, frozen at -15℃ for 5 hours, and then vacuum dried at 65℃ for 7 hours to obtain composite resin spheres.

[0042] During primary purification, the impurity-removing solution passes through the ion exchange column at a flow rate of 20 BV / h. During secondary purification, the primary purified solution enters a ceramic membrane system with a surface pore size of 1.2 μm and a bottom pore size of 6 μm. The operating pressure is 0.25 MPa, the cross-flow velocity is 1.8 m / s, and the secondary purified solution with a turbidity of 0.07 NTU is obtained after filtration.

[0043] Furthermore, the secondary purified liquid is evaporated and concentrated to a Li⁺ concentration of 30 g / L, and the secondary steam generated from the evaporation is used to preheat the raw materials to 65°C.

[0044] Furthermore, Na2CO3 was added to the concentrate, with an excess of 15% Na2CO3. The mixture was stirred at 500 rpm at 90°C, and the pH was adjusted to 11. After reacting for 5 hours, solid-liquid separation was performed to obtain crude lithium carbonate and mother liquor. The Li⁺ concentration in the mother liquor was 1.8 g / L.

[0045] Furthermore, the crude lithium carbonate was washed with hot water at 70°C at a solid-liquid ratio of 1:6 and centrifuged at 7000 r / min for 15 min.

[0046] Finally, after every 4 cycles, 25% of the mother liquor was taken for electrodialysis desalination.

[0047] Comparative Example 1: Unmodified D751 resin, without grafted EDTA and nano ZrO2 Specifically, step two, which removes impurities, uses unmodified D751 resin, which is the original resin without grafted EDTA groups and loaded nano-ZrO2. The remaining steps are the same as in Example 2.

[0048] Comparative Example 2: The composite resin spheres have no gradient pores and only a single microporous structure. Specifically, the composite resin balls used for purification in step three only retain the inner layer of 4-11μm micropores, without the outer layer of 40-110μm macropores. During preparation, the resin particles are directly coated with gelatin, without making an outer polyvinyl alcohol macroporous framework. The remaining steps are the same as in Example 2.

[0049] Comparative Example 3: The ceramic membrane has a single-layer structure and no underlying porous layer. Specifically, in step three, the ceramic membrane used for purification retains only the dense surface layer with a pore size of 1.0 μm and has no loose underlying layer. The surface layer is sintered directly on the support, and the remaining steps are the same as in Example 2.

[0050] Comparative Example 4: No composite impurity removal system, only Li2S for impurity removal Specifically, lithium-containing materials were mixed with deionized water at a solid-liquid ratio of 1:5 and pumped into a rotating packed bed at a speed of 1200 r / min. The leaching temperature was controlled in two stages: the first stage was at 30°C for 1.5 h, and the second stage was at 70°C for 4 h. The leachate was filtered to obtain a Li⁺ concentration of 4.5 g / L, a Ca²⁺ concentration of 0.11 g / L, and a Mg²⁺ concentration of 0.07 g / L. Li₂S was added to the leachate at an amount 1.2 times the total molar amount of Pb²⁺. The mixture was reacted at 40°C for 1 h and then allowed to stand for 1.5 h. The heavy metal precipitate was removed by filtration. No modified D751 chelating resin or EDTA was added. Subsequent purification, evaporation concentration, and lithium precipitation steps were the same as in Example 2, with identical composite resin ball parameters, ceramic membrane filtration conditions, and lithium precipitation process.

[0051] Comparative Example 5: Single-stage purification only, without ceramic membrane filtration Specifically, the leaching and impurity removal steps were the same as in Example 2, and the leaching solution parameters and impurity removal system parameters were identical. The purification process only employed composite resin ball adsorption at a flow rate of 17 BV / h, without ceramic membrane filtration. The primary purified solution was directly sent to the evaporation and concentration process, concentrated to a Li⁺ concentration of 25 g / L. Subsequent lithium precipitation and centrifugal desalination steps were consistent with Example 2.

[0052] Comparative Example 6: No mother liquor circulation, mother liquor is directly discharged. The leaching, impurity removal, purification, evaporation concentration, and lithium precipitation steps are the same as in Example 2, with identical parameters. However, the mother liquor after lithium precipitation is directly discharged and not returned to the leaching process. The remaining steps, including centrifugal desalination and product drying, remain unchanged.

[0053] To compare the purification effects of low-impurity lithium carbonate in Examples 1-3 and Comparative Examples 1-6, the present invention provides the following experimental methods: All examples and comparative examples used the same lithium-containing material, namely lepidolite powder, with a Li2O content of 2.8%, Ca²⁺ 0.35%, Mg²⁺ 0.22%, and Pb²⁺ 0.005%, and used the same equipment.

[0054] The detection methods for key indicators during the experiment are as follows: Lithium leaching rate: The concentration of Li⁺ in the leachate was determined by inductively coupled plasma optical emission spectrometry (ICP-OES), and calculated in combination with the total lithium content of the raw materials; Impurity removal rate: The concentrations of Ca²⁺, Mg²⁺, and Pb²⁺ in the solution were determined using an atomic absorption spectrophotometer (AAS), and the removal rate was calculated. Solution turbidity: Measured using a turbidity meter; Lithium carbonate purity and impurity content: Purity was analyzed by X-ray fluorescence spectrometry (XRF), and impurities such as Na⁺ and Cl⁻ were determined by ion chromatography; Total Lithium Recovery: Tracks the amount of lithium from raw materials to the final product and calculates the total recovery rate.

[0055] The comparative experimental data of the examples and comparative examples are as follows:

[0056] Experimental data description: 1. Regarding the impurity removal effect of the modified D751 chelating resin, in Comparative Example 1, when using the unmodified resin, the removal rates of calcium and magnesium were only 72.3% and 68.5%, respectively, with a product purity of 99.65%. However, in Example 2, after using a modified resin grafted with EDTA and loaded with nano-ZrO2, the removal rates of calcium and magnesium jumped to 99.7% and 99.5%, respectively, with a purity of 99.98%. The sodium ion residue decreased from 53 ppm to 5 ppm. This clearly demonstrates the significant enhancement of impurity removal through the dual effects of chelation and adsorption when using the modified D751 chelating resin.

[0057] 2. Regarding the advantages of gradient pore composite resin balls, Comparative Example 2, lacking an outer macropore layer, achieved a primary purification flow rate of only 8 BV / h, with calcium and magnesium removal rates of 89.2% and 86.7%, respectively. In Example 2, the gradient pore structure increased the flow rate to 17 BV / h, simultaneously raising the removal rates to 99.7% and 99.5%, demonstrating the synergistic effect of the outer macropore layer reducing resistance and the inner micropore layer enhancing adsorption, effectively resolving the contradiction between efficiency and precision in traditional single-pore structures.

[0058] 3. Regarding the performance advantages of the double-layer ceramic membrane, the single-layer membrane of Comparative Example 3 has a flux of only 5.2 L / m²·h and a turbidity of 0.12 NTU; the double-layer membrane of Example 2 has a flux of 10-15 L / m²·h and a turbidity of 0.05 NTU, indicating that the loose bottom layer significantly reduces resistance and the dense surface layer efficiently retains colloids, achieving a balance between high flux and low turbidity.

[0059] 4. Regarding the synergistic effect of the two-stage purification, Comparative Example 5 had a turbidity of 0.82 NTU and a lithium recovery rate of 88.6% after single-stage purification; Example 2 had a turbidity of 0.05 NTU after two-stage purification and a recovery rate of 92.5%, indicating that the synergistic effect of the composite resin balls and ceramic membrane can deeply remove ionic and colloidal impurities while reducing lithium loss.

[0060] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for purifying low-impurity lithium carbonate, characterized in that, Includes the following steps: Step 1: Leaching The lithium-containing material is mixed with water and then subjected to high-gravity leaching in a rotating packed bed. The temperature of the mixture during the leaching process is controlled in two stages to promote the dissolution of lithium, and finally the leachate is obtained. Step 2: Impurity Removal Li2S is first added to the leachate obtained in step one to remove heavy metal ions, and then a composite impurity removal system consisting of modified D751 chelating resin and free ethylenediaminetetraacetic acid is added. After the impurity removal is completed, the impurity removal solution is obtained. The modified D751 chelating resin is prepared by grafting ethylenediaminetetraacetic acid groups and loading nano-zirconia on the surface. Step 3: Purification The impurity removal solution obtained in step two is purified in two stages using composite resin balls and a precision ceramic membrane: the first stage uses an ion exchange column filled with composite resin balls to adsorb ionic impurities and remove most of the ionic impurities in the impurity removal solution to obtain a first-stage purified solution; the second stage uses ceramic membrane filtration, and the first-stage purified solution is filtered through the ceramic membrane after passing through the ion exchange column to obtain a second-stage purified solution. Step 4: Evaporation and Concentration The secondary purified liquid obtained in step three is evaporated into a concentrated liquid with a lithium concentration of 20-30 g / L; Step 5: Lithium precipitation Na2CO3 was added to the concentrated solution obtained in step four to carry out a lithium precipitation reaction, followed by solid-liquid separation to obtain crude lithium carbonate and mother liquor. Step Six: Centrifugal Desalination The crude lithium carbonate obtained in step 5 is first pre-washed with hot water, and then centrifuged to obtain low-impurity lithium carbonate. Step 7: Mother liquor circulation The mother liquor obtained in step five is returned to step one. The lithium concentration in the mother liquor is controlled at 1-2 g / L, which is lower than the leaching equilibrium concentration. The resulting concentration gradient drives the continuous dissolution of lithium.

2. The method according to claim 1, characterized in that, In the first leaching process, the temperature of the mixture is controlled as follows: the initial temperature is controlled at 20-40℃ for 1-2 hours to inhibit the dissolution of impurities such as calcium and magnesium, and the subsequent temperature is controlled at 60-80℃ for 3-5 hours.

3. The method according to claim 1, characterized in that, In step two, the amount of Li2S added is 1.1-1.3 times the total molar amount of heavy metal ions in the solution, and the reaction temperature is controlled at 35-45℃. After the reaction is completed, the mixture is allowed to stand for 1-2 hours, and then the mixture is filtered to separate the precipitate.

4. The method according to claim 3, characterized in that, In the composite impurity removal system, the mass ratio of the modified D751 chelating resin to the free ethylenediaminetetraacetic acid is 4-6:1, the pH value of the impurity removal process is controlled at 8-9, and the preparation steps of the modified D751 chelating resin include: (1) Resin pretreatment Take D751 chelating resin, mix it with 4-6% hydrochloric acid solution at a solid-liquid ratio of 1:9-11, stir it in a constant temperature water bath at 55-65℃ for 1.5-2.5h to remove residual impurities and free ions on the resin surface, filter it and wash it with deionized water until neutral. The resin is then transferred to a 4-6% sodium hydroxide solution and stirred at 55-65℃ for 1.5-2.5 hours at a solid-liquid ratio of 1:9-11 to activate the functional groups of the resin. After filtration, the resin is washed with deionized water until neutral and then vacuum dried to constant weight. (2) Amine functionalization grafting The pretreated D751 resin and ethylenediaminetetraacetic acid dianhydride were added to N,N-dimethylformamide solvent at a mass ratio of 1:1.8-2.2, wherein the solid-liquid ratio of resin to N,N-dimethylformamide was 1:7-9. The mixture was heated to 75-85℃ under nitrogen protection and stirred for 5-7 hours. After the reaction is complete, the resin is collected by filtration and washed 2-4 times with anhydrous ethanol to remove unreacted ethylenediaminetetraacetic acid dianhydride. The resin is then dried under vacuum at 55-65℃ to graft ethylenediaminetetraacetic acid groups onto the resin surface, thus obtaining anamine-functionalized resin. (3) Nano-zirconia loading Prepare a 0.4-0.6 mol / L zirconium oxychloride solution, impregnate the amine-functionalized resin in it at a solid-liquid ratio of 1:14-16, and stir and adsorb at 25-35℃ for 3-5 hours. Subsequently, 0.8-1.2 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH of the system to 8.5-9.5, so that the zirconium ions adsorbed on the resin surface would hydrolyze to form nano-zirconium dioxide particles. After stirring for 0.5-1.5 h, the mixture was filtered, washed with deionized water until the filtrate was free of chloride ions, and then vacuum dried at 75-85 °C to obtain the composite resin loaded with nano-zirconium dioxide. (4) Crosslinking stabilization treatment The above composite resin was added to a 4-6% (w / w) glutaraldehyde solution and mixed at a solid-liquid ratio of 1:11-13. The mixture was stirred and crosslinked at 35-45°C for 1.5-2.5 hours to allow the functional groups on the resin surface to be connected by covalent bonds to form a stable structure. After filtration, the resin was washed with deionized water until no aldehyde residue was found. The resin was then vacuum dried at 55-65°C to constant weight to obtain the modified D751 chelating resin.

5. The method according to claim 1, characterized in that, In step three, the purification process, the composite resin ball has a gradient pore structure: the outer layer has macropores of 40-110 μm, which allows the solution to flow through quickly and initially adsorb large-particle impurities; the inner layer has micropores of 4-11 μm, which enhances the deep adsorption of trace ions such as calcium and magnesium through capillary action. The preparation of the composite resin balls includes: (1) Resin mixing Weigh LSC-500, D451 and S930 resins according to a mass ratio of 2-4:5-7:1, place them in a mixing tank and stir at 250-350 r / min for 25-35 min to ensure that the three resin particles are evenly mixed. (2) Preparation of outer substrate Add 4-6% polyvinyl alcohol solution by mass to the mixed resin obtained in step (1), control the solid-liquid ratio of resin to solution to be 1:2-4, stir in a water bath at 45-55℃ until a uniform paste is formed, and the paste is extruded and molded by a granulation mold. Due to cross-linking, polyvinyl alcohol forms macropores of 40-110μm, which constitute the outer skeleton of the composite resin ball. (3) Preparation of inner layer substrate Take another portion of the mixed resin from step (1), add 7-9% by mass of gelatin solution, control the solid-liquid ratio to 1:1-3, stir at 35-45℃ until the gelatin is completely swollen and coats the resin particles, so that the gelatin forms a 4-11μm microporous structure on the surface of the resin particles as an inner adsorption carrier. (4) Freezing and setting The outer skeleton of the composite resin ball obtained in step (2) and the inner adsorption carrier obtained in step (3) are assembled in a layered structure, that is, the outer skeleton is used as the base and the inner adsorption carrier is uniformly covered on the base. Then, it is placed in a freezer at -25 to -15℃ for 3-5 hours to solidify the colloidal structure of polyvinyl alcohol and gelatin. (5) Drying control The frozen semi-finished product from step (4) is transferred to a vacuum drying oven at 55-65℃ and dried at a vacuum of -0.09 to -0.07 MPa for 5-7 hours. The pore structure is stabilized by water sublimation, and the overall porosity of the composite resin ball reaches 40-50%. (6) Outer layer granulation The semi-finished product after drying in step (5) is crushed into 1-4 mm particles, and then mixed with a polyvinyl alcohol solution of 4-6% by mass at a solid-liquid ratio of 1:1-3. The mixture is then placed in a rotary granulator and granulated at a speed of 10-20 r / min. During the rolling process, the outer layer of the particles is wrapped with a polyvinyl alcohol film to form spheres with a diameter of 3-5 mm. The outer macroporous structure of 40-110 μm is naturally maintained by the surface tension during granulation. (7) Inner layer pore formation After granulation in step (6), the spheres are immersed in a gelatin solution with a mass fraction of 7-9% for 25-35 minutes to allow the gelatin to penetrate into the spheres. After granulation, the freeze-drying process in steps (4) and (5) is repeated. That is, the spheres are first frozen at -25 to -15℃ for 3-5 hours, and then vacuum dried at 55-65℃ for 5-7 hours. Through the shrinkage of the gelatin and the evaporation of water, a microporous structure of 4-11 μm is formed inside the spheres, and finally a composite resin sphere with a gradient pore structure is obtained.

6. The method according to claim 5, characterized in that, In the purification process of step three, the ceramic membrane has a double-layer structure. Its surface dense layer has a pore size of 0.8-1.2μm to trap residual small colloidal particles, and the bottom loose layer has a pore size of 4-6μm to reduce filtration resistance, so that the turbidity of the solution is reduced to below 0.1NTU. The preparation steps of the ceramic membrane include: (1) The surface layer is made of 0.4-0.6μm ceramic powder and sintered at 1150-1250℃ to form a dense surface layer; (2) The bottom layer uses 4-6μm ceramic powder. It is first pre-sintered at 950-1050℃ for 1h to form a porous substrate. Then the dense surface layer blank prepared in step (1) is covered on the surface of the bottom substrate. It is sintered at the same temperature and kept warm for 1.5-2.5h so that the two layers are firmly bonded through diffusion sintering, and finally a loose bottom layer is formed with a membrane flux of 10-15L / m²・h.

7. The method according to claim 6, characterized in that, In step three of the purification process, the two-stage purification process involving composite resin balls and ceramic membranes includes: (1) Adsorption and purification by composite resin balls The impurity-removing solution obtained in step two is pumped into an ion exchange column filled with composite resin balls. The solution flow rate is controlled at 15-20 BV / h. The composite resin balls allow the solution to flow rapidly through the outer 40-110 μm macropores. At the same time, LSC-500, D451, and S930 type resins are used to adsorb and remove more than 85% of calcium, magnesium, and heavy metal ions in the solution, resulting in a first-stage purified solution. (2) Precision ceramic membrane depth filtration The effluent from the ion exchange column enters the ceramic membrane filtration system, which uses an alumina ceramic membrane module with a pore size of 0.08-0.12μm. The operating pressure is set at 0.15-0.25MPa and the cross-flow velocity is 1.2-1.8m / s. Under pressure, the primary purified liquid flows at high speed along the membrane surface. The residual trace colloidal particles, resin debris, and unadsorbed nanoscale impurities are trapped by the membrane, and the permeate is the secondary purified liquid.

8. The method according to claim 1, characterized in that, In step four, the evaporation and concentration process, the evaporation process is heated by steam. The secondary steam generated during the evaporation process is recycled and reused in step one. The mixture of lithium-containing material and water is preheated to 55-65°C before entering the rotating packed bed.

9. The method according to claim 1, characterized in that, In step five, the lithium precipitation process, Na2CO3 is added in excess by 10-15%, the stirring rate is 300-500 rpm, the generated crystal particle size is 15-55 μm, and the sodium ion residue is ≤10 ppm; the temperature during the lithium precipitation process is controlled at 30-90℃ and the pH value is controlled at 10-11. After reacting for 1-5 hours, solid-liquid separation is carried out to obtain crude lithium carbonate and mother liquor.

10. The method according to claim 1, characterized in that, In step six, the crude lithium carbonate obtained in step five is first pre-washed with hot water at 50-70℃, and then low-impurity lithium carbonate is obtained by centrifugation. The solid-liquid ratio during washing is 1:4-6. Alternatively, in step seven, during the mother liquor circulation, after every 4-6 cycles, 15-25% of the mother liquor is taken for electrodialysis desalination, with a lithium recovery rate ≥90%, to avoid impurity accumulation, and a chloride ion concentration ≤20ppm.

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