A method for purifying low-impurity lithium carbonate

Abstract

The application discloses a purification method of low-impurity lithium carbonate, which comprises seven steps of leaching, impurity removal, purification, evaporation concentration, lithium precipitation, centrifugal desalination and mother liquor circulation. The lithium-containing material is mixed with water and then subjected to hypergravity treatment in a rotating packed bed to obtain a leaching solution by controlling the temperature in two sections. In the impurity removal process, Li2S is added to remove heavy metals, and then a deep impurity removal is performed through a composite system of modified D751 chelating resin and free EDTA, wherein the modified D751 chelating resin is prepared by grafting EDTA groups and loading nano zirconium dioxide. The purification is performed through two-stage treatment of gradient-pore composite resin balls and double-layer ceramic membranes. The modified D751 chelating resin is used for deep impurity removal, the gradient-pore composite resin balls and the double-layer ceramic membranes are used for two-stage purification, and the mother liquor circulation is combined, so that the purity of the lithium carbonate, the impurity removal rate and the lithium recovery rate can be remarkably improved, and the method is suitable for industrial production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of industrial-grade lithium carbonate preparation, in particular to a purification method of low-impurity lithium carbonate. BACKGROUND

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

[0003] In the prior art, the existing lithium carbonate purification process excessively relies on a single resin or chemical precipitation method in the impurity removal process. This limitation makes it difficult to simultaneously and efficiently remove heavy metals and alkaline earth metal ions such as calcium and magnesium, resulting in high residual impurity content, which directly hinders product purity and makes it difficult to meet low-impurity requirements. In the purification process, under the mode of single-stage filtration or adsorption, if the adsorption material has a single pore structure, either the flow rate is limited due to high resistance, or the effect is compromised due to insufficient adsorption depth, and the purification efficiency is always difficult to improve. If the filter membrane is not reasonably designed, it will be trapped in a quagmire of high filtration resistance and low flux, and it is also unable to effectively intercept small colloidal particles, resulting in difficulty in controlling the solution turbidity at a low level. In addition, the mother liquor is usually directly discharged, which not only causes serious waste of lithium resources, but also exacerbates environmental pollution and increases environmental pressure. These technical problems make the impurity content of lithium carbonate products produced by the existing method too high, far from meeting the demand for low-impurity materials in high-end fields.

[0004] Therefore, it is necessary to provide a purification method of low-impurity lithium carbonate to solve the above technical problems. SUMMARY

[0005] The purpose of the present application is to provide a purification method of low-impurity lithium carbonate to solve the problems of low lithium leaching rate, incomplete impurity removal, insufficient product purity, and low resource utilization rate in the prior art.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solution: a purification method of low-impurity lithium carbonate, comprising the following steps:

[0007] Step one: leaching

[0008] After mixing the lithium-containing material with water, perform supergravity leaching treatment in a rotating packed bed; the temperature of the mixture in the leaching process is controlled in two stages to promote the dissolution of lithium elements, and finally obtain a leaching solution;

[0009] Step two: impurity removal

[0010] Li2S is first added to the leaching solution obtained in step one to remove heavy metal ions, and then a composite impurity removal system composed of modified D751 chelating resin and free ethylenediaminetetraacetic acid is added, and after the impurity removal is completed, an impurity removal solution is obtained; wherein the modified D751 chelating resin is prepared by grafting ethylenediaminetetraacetic acid groups on the surface and loading nano zirconium dioxide;

[0011] Step three: purification

[0012] The impurity removal solution obtained in step two is subjected to two-stage purification treatment by using a composite resin ball and a precision ceramic membrane: in the first stage, an ion exchange column filled with a composite resin ball is used to adsorb ion impurities, removing most of the ion impurities in the impurity removal solution to obtain a first-stage purified solution; in the second stage, ceramic membrane filtration is used, and the first-stage purified solution is filtered through the ceramic membrane after the water outlet of the ion exchange column to obtain a second-stage purified solution.

[0013] Step four: evaporation and concentration

[0014] The second-stage purified solution obtained in step three is evaporated to a concentrated solution with a lithium concentration of 20-30 g / L.

[0015] Step five: lithium precipitation

[0016] Na2CO3 is added to the concentrated solution obtained in step four to perform a lithium precipitation reaction, and then solid-liquid separation is performed to obtain a lithium carbonate crude product and a mother liquor.

[0017] Step six: centrifugal desalination

[0018] The lithium carbonate crude product obtained in step five is first prewashed with hot water, and then low-impurity lithium carbonate is obtained through centrifugal operation.

[0019] Step seven: mother liquor circulation

[0020] The mother liquor obtained in step five is returned to step one, and the lithium concentration in the mother liquor is controlled at 1-2 g / L, which is lower than the leaching equilibrium concentration, and the concentration gradient formed drives continuous dissolution of lithium.

[0021] As a preferred, in the step one leaching process, the temperature of the mixture is controlled in the following way: the temperature of the front section is controlled at 20-40℃ for 1-2h to inhibit the dissolution of impurities such as calcium and magnesium, and the temperature of the rear section is controlled at 60-80℃ for 3-5h.

[0022] As a preferred, in the step two impurity removal process, 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℃, and after the reaction is completed, the mixture is allowed to stand for 1-2h, and then the mixture is filtered to separate the precipitate.

[0023] As preferred, 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 step of the modified D751 chelating resin comprises:

[0024] (1) Resin pretreatment

[0025] Take D751 chelating resin, first mix with a mass fraction of 4-6% hydrochloric acid solution at a solid-liquid ratio of 1:9-11, stir in a constant temperature water bath at 55-65°C for 1.5-2.5h to remove surface residual impurities and free ions, then wash with deionized water to neutral after filtration;

[0026] Then transfer the resin into a mass fraction of 4-6% sodium hydroxide solution, also mix at a solid-liquid ratio of 1:9-11 at 55-65°C for 1.5-2.5h to activate the functional groups of the resin, then wash with deionized water to neutral after filtration, and vacuum dry to constant weight;

[0027] (2) Amine functionalization grafting

[0028] Put the pretreated D751 resin and ethylenediaminetetraacetic dianhydride into N,N-dimethylformamide solvent at a mass ratio of 1:1.8-2.2, wherein the solid-liquid ratio of the resin to N,N-dimethylformamide is 1:7-9, and heat to 75-85°C under nitrogen protection, and stir for 5-7h;

[0029] After the reaction is completed, collect the resin by filtration, wash with anhydrous ethanol for 2-4 times to remove unreacted ethylenediaminetetraacetic dianhydride, and vacuum dry at 55-65°C to graft ethylenediaminetetraacetic acid groups on the surface of the resin to obtain amine functionalized resin;

[0030] (3) Nano zirconium dioxide loading

[0031] Prepare a 0.4-0.6mol / L zirconyl chloride solution, immerse the amine functionalized resin in it at a solid-liquid ratio of 1:14-16, and stir for 3-5h at 25-35°C;

[0032] Then slowly add 0.8-1.2mol / L sodium hydroxide solution to adjust the pH of the system to 8.5-9.5, make the zirconium ions adsorbed on the surface of the resin hydrolyze to form nano zirconium dioxide particles, continue to stir for 0.5-1.5h, then filter, wash with deionized water until there is no chloride ion in the filtrate, and vacuum dry at 75-85°C to obtain a composite resin loaded with nano zirconium dioxide;

[0033] (4) Crosslinking stabilization treatment

[0034] The above composite resin is added into a glutaraldehyde solution with a mass fraction of 4-6%, mixed at a solid-liquid ratio of 1:11-13, stirred and cross-linked at 35-45°C for 1.5-2.5h, so that the functional groups on the surface of the resin are connected by covalent bonds to form a stable structure, the resin is filtered and washed with deionized water until no aldehyde group is left, and then dried at 55-65°C under vacuum until the weight is constant, to obtain the modified D751 chelating resin.

[0035] Preferably, in the purification process of step three, the composite resin ball has a gradient pore structure: the outer layer is a macropore with a size of 40-110μm, allowing the solution to flow quickly and preliminarily adsorb large-particle-size impurities; and the inner layer is a micropore with a size of 4-11μm, enhancing the deep adsorption of trace ions such as calcium and magnesium through capillary action.

[0036] The preparation of the composite resin ball comprises:

[0037] (1) Resin mixing

[0038] LSC-500, D451 and S930 resins are weighed at a mass ratio of 2-4:5-7:1, placed in a stirred tank and stirred at a speed of 250-350r / min for 25-35min, so that the three kinds of resin particles are uniformly mixed;

[0039] (2) Outer substrate preparation

[0040] A polyvinyl alcohol solution with a mass fraction of 4-6% is added to the mixed resin prepared in step (1), and the solid-liquid ratio of the resin and the solution is controlled at 1:2-4, and the mixture is stirred in a water bath at 45-55°C until a uniform paste is formed. The paste is extruded into a pellet through a pelletizing die, and the polyvinyl alcohol forms macropores with a size of 40-110μm, constituting the outer framework of the composite resin ball;

[0041] (3) Inner substrate preparation

[0042] Another portion of the mixed resin of step (1) is taken, a gelatin solution with a mass fraction of 7-9% is added, and the solid-liquid ratio is controlled at 1:1-3, and the mixture is stirred at 35-45°C until the gelatin is completely swollen and wrapped around the resin particles, so that the gelatin forms a microporous structure with a size of 4-11μm on the surface of the resin particles, serving as an inner adsorption carrier;

[0043] (4) Freeze setting

[0044] The outer framework of the composite resin ball prepared in step (2) and the inner adsorption carrier prepared in step (3) are assembled in a layered structure, i.e. the outer framework serves as a substrate, and the inner adsorption carrier is evenly covered on the substrate, and then they are placed in a freezer at -25~-15°C for 3-5h, so that the colloid structure of the polyvinyl alcohol and the gelatin is solidified;

[0045] (5) Drying and pore control

[0046] The semi-finished product after freezing in step (4) is transferred into a vacuum drying oven at 55-65°C, dried under a vacuum degree of -0.09 to -0.07 MPa for 5-7 h, the pore structure is stabilized by sublimation of water, and finally the overall porosity of the composite resin ball is 40-50%;

[0047] (6) Outer layer granulation

[0048] The semi-finished product after drying in step (5) is crushed into 1-4 mm particles, then mixed with a polyvinyl alcohol solution with a mass fraction of 4-6% at a solid-liquid ratio of 1:1-3, and placed into a rotary granulator for granulation at a speed of 10-20 r / min. The particles are wrapped with a polyvinyl alcohol film during rolling to form balls with a diameter of 3-5 mm, and the outer macroporous structure of 40-110 μm is naturally maintained by surface tension during granulation.

[0049] (7) Inner layer pore formation

[0050] The balls after granulation in step (6) are immersed in a gelatin solution with a mass fraction of 7-9% for 25-35 min to allow the gelatin to penetrate into the balls. After taking out, the freezing and drying processes in steps (4) and (5) are repeated, i.e., first frozen at -25 to -15°C for 3-5 h, and then vacuum dried at 55-65°C for 5-7 h. Through the shrinkage of gelatin and the evaporation of water, a microporous structure of 4-11 μm is formed in the balls. Finally, a composite resin ball with a gradient pore structure is obtained.

[0051] Preferably, in the purification process of step three, the ceramic membrane is a double-layer structure, the surface dense layer has a pore size of 0.8-1.2 μm to trap residual small colloidal particles, and the bottom loose layer has a pore size of 4-6 μm to reduce the filtration resistance, so that the turbidity of the solution is reduced to below 0.1 NTU;

[0052] The preparation steps of the ceramic membrane include:

[0053] (1) The surface layer is formed by sintering 0.4-0.6 μm ceramic powder at 1150-1250°C to form a surface dense layer;

[0054] (2) The bottom layer is formed by pre-sintering 4-6 μm ceramic powder at 950-1050°C for 1 h to form a porous substrate. Then, the surface dense layer blank prepared in step (1) is covered on the surface of the bottom layer substrate, and sintering and heat preservation are continued at the same temperature for 1.5-2.5 h to make the two layers firmly combined by diffusion sintering, and finally a bottom loose layer is formed, and the membrane flux is 10-15 L / m 2 h of the precision ceramic membrane.

[0055] Preferably, in the purification process of step three, the two-stage purification treatment of the composite resin ball and the ceramic membrane includes:

[0056] (1) Composite resin ball adsorption purification

[0057] The impurity removal 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 quickly through the outer layer of 40-110 μm macropores, and meanwhile, more than 85% of calcium, magnesium ions and heavy metal ions in the solution are removed by adsorption using LSC-500, D451 and S930 resins, so as to obtain a first-stage purified solution;

[0058] (2) Precision ceramic membrane depth filtration

[0059] The water outlet of the ion exchange column enters a ceramic membrane filtration system, the system adopts an alumina ceramic membrane assembly with a pore size of 0.08-0.12 μm, and is set to have an operating pressure of 0.15-0.25 MPa and a cross-flow speed of 1.2-1.8 m / s; under pressure driving, the first-stage purified solution flows along the membrane surface at a high speed, and the residual trace colloidal particles, resin debris and nanoscale impurities not adsorbed are intercepted by the membrane, and the permeate is the second-stage purified solution.

[0060] Preferably, in the evaporation and concentration process of step four, steam heating is used in the evaporation process, and the secondary steam generated in the evaporation process is recycled for use in step one; the mixture obtained by mixing the lithium-containing material with water is preheated to 55-65 ℃ before entering the rotating packed bed.

[0061] Preferably, in the lithium precipitation process of step five, the addition amount of Na2CO3 is 10-15% in excess, the stirring speed is 300-500 rpm, the crystal particle size is 15-55 μm, and the residual sodium ion content is ≤10 ppm; the temperature in the lithium precipitation process is controlled at 30-90 ℃, the pH value is controlled at 10-11, and the solid-liquid separation is performed after 1-5 h of reaction, so as to obtain lithium carbonate crude product and mother liquor.

[0062] Preferably, in step six, the lithium carbonate crude product obtained in step five is first prewashed with hot water at 50-70 ℃, and then low-impurity lithium carbonate is obtained by centrifugation, and the solid-liquid ratio during washing is 1:4-6;

[0063] Preferably, in the mother liquor circulation of step seven, 15-25% of the mother liquor is taken out for desalination by electrodialysis every 4-6 times of circulation, the lithium recovery rate is ≥90%, the impurity accumulation is avoided, and the chloride ion concentration is ≤20 ppm.

[0064] Compared with the prior art, the present application has the following beneficial effects:

[0065] 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.

[0066] 2. This invention utilizes the gradient pore structure of composite resin spheres to achieve a tiered 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 through its porous structure, maintaining a membrane flux of 10-15 L / m³. 2 The turbidity of the solution remained stable below 0.1 NTU for h. The synergistic effect of two-stage purification solved the problem of balancing efficiency and precision inherent in traditional single-stage purification.

[0067] 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%.

[0068] 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

[0069] Figure 1 A schematic diagram of the process flow for the purification method of low-impurity lithium carbonate provided by the present invention.

[0070] Figure 2 Line graph comparing lithium leaching rates in the purification methods provided in the embodiments and comparative examples of the present invention;

[0071] Figure 3 The purification methods for Ca provided in the embodiments and comparative examples of the present invention 2+ Line chart comparing removal rates;

[0072] Figure 4Mg 2+ The line contrast chart of removal rate;

[0073] Figure 5 The line contrast chart of lithium carbonate purity in the purification method provided by the embodiment and the comparative example of the present application;

[0074] Figure 6 The line contrast chart of total lithium recovery rate in the purification method provided by the embodiment and the comparative example of the present application. DETAILED DESCRIPTION

[0075] The technical solutions of the present application will be described in detail below in combination with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0076] The purification method of low-impurity lithium carbonate provided by the present application can be implemented through the following embodiments 1-3, and the following raw materials and equipment preparation are required in advance: lithium-containing material is lepidolite powder, wherein the content of Li2O is 2.8%, the content of Ca 2+ The content of Mg 2+ The content of Pb 2+ The content of 0.005%; the auxiliary materials are analytical pure Li2O and ethylenediaminetetraacetic acid (EDTA), modified D751 chelating resin, industrial grade Na2CO3 with a purity of 99.5%; and 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 aluminum oxide.

[0077] Embodiment 1: First, the lithium-containing material and deionized water are mixed at a solid-liquid ratio of 1:5, stirred uniformly, and then pumped into the rotating packed bed for leaching treatment under supergravity conditions. The rotating speed of the rotating packed bed is 1200 r / min. The temperature is controlled in two stages, the first stage is controlled at 20℃ for 1h to inhibit the dissolution of calcium and magnesium, and the second stage is controlled at 60℃ for 3h to promote the dissolution of lithium. After leaching, the leaching solution is obtained, wherein the concentration of Li + The concentration of Ca 2+ The concentration of Mg 2+ The concentration of 0.08 g / L.

[0078] Furthermore, Li2S is added to the leaching solution to remove heavy metals, and the amount of Li2S added is Pb 2+1.1 times of total molar quantity, the reaction was stirred at 35℃ for 1h, and filtered after standing for 1h to remove the precipitates such as PbS; then the composite impurity removal system was added to the filtrate, the mass ratio of modified D751 chelating resin to free EDTA in the system was 4:1, the resin dosage was 50g / L, the EDTA dosage was 10g / L, the pH of the system was adjusted to 8 with 0.1mol / L NaOH, and the system was stirred at 40℃ for 2h; then the impurity removal liquid was obtained by filtration, in which Ca 2+ concentration≤0.005g / L, Mg 2+ concentration≤0.003g / L.

[0079] Before purifying the impurity removal liquid, the composite resin ball needs to be prepared in advance, and the preparation method is as follows: first, LSC-500, D451 and S930 resins are weighed and mixed according to a mass ratio of 2:5:1, and stirred at a speed of 250r / min for 25min; then 4% polyvinyl alcohol solution is added to the mixed resins, and the solid-liquid ratio of the resins to the solution is 1:2; the mixture is stirred in a 45℃ water bath until it forms a paste, and then it is extruded into a granulating mold to form an outer layer substrate with an outer layer macropore of 40μm; another portion of the mixed resins is taken, 7% gelatin solution is added, and the solid-liquid ratio of the resins to the solution is 1:1; the mixture is stirred at 35℃ until the gelatin swells and wraps the resins, forming an inner layer substrate with an inner layer micropore of 4μm; the outer layer skeleton and the inner layer carrier are assembled in layers, and then solidified by freezing at-25℃ for 3h; then the mixture is transferred into a vacuum drying oven, and dried at a vacuum degree of-0.09MPa and a temperature of 55℃ for 5h, so that the porosity reaches 40%; the dried semi-finished product is broken into 1mm particles, mixed with 4% polyvinyl alcohol solution according to a solid-liquid ratio of 1:1, and then put into a rotary granulator to form balls with a diameter of 3mm at a rotating speed of 10r / min; the balls are immersed in 7% gelatin solution for 30min, frozen at-25℃ for 3h, and then dried in a vacuum drying oven at 55℃ for 5h, to finally obtain composite resin balls with gradient porosity.

[0080] The two-stage purification treatment is carried out by the composite resin ball and the ceramic membrane system; in the first-stage purification, the impurity removal liquid is passed through the ion exchange column filled with the composite resin ball at a flow rate of 15BV / h, to obtain the first-stage purified liquid, in which Ca 2+ concentration≤0.0005g / L; in the second-stage purification, the first-stage purified liquid enters the ceramic membrane system, the surface layer of the ceramic membrane has a pore size of 0.8μm, the bottom layer has a pore size of 4μm, the operating pressure is 0.15MPa, and the cross-flow velocity is 1.2m / s; after filtration, the second-stage purified liquid is obtained, and its turbidity is 0.08NTU.

[0081] Furthermore, the second-stage purified liquid is pumped into an evaporator, heated to boiling by steam, and concentrated by evaporation to a Li + concentration of 20g / L; the secondary steam generated in the evaporation process is recovered and used for preheating the leaching raw materials, and the mixture is preheated to 55℃.

[0082] Furthermore, Na₂CO₃ was added to the concentrated solution 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, centrifugation was performed at 3000 rpm for 10 minutes to obtain crude lithium carbonate and mother liquor. The mother liquor contained Li₂CO₃. + The concentration is 1.2 g / L.

[0083] 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.

[0084] 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.

[0085] 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 packed bed was 1200 r / min. The leaching temperature was controlled in two stages: the first stage temperature was 30℃ for 1.5 h, and the second stage temperature was 70℃ for 4 h, resulting in a leachate containing Li. + The concentration is 4.5 g / L, Ca 2+ The concentration is 0.11 g / L, Mg 2+ The concentration is 0.07 g / L.

[0086] Furthermore, Li₂S was added to the leachate in an amount equal to Pb. 2+ The total molar amount was 1.2 times, reacted at 40℃ 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 the resin amount being 50 g / L and the EDTA amount being 10 g / L. The pH of the system was adjusted to 8.5, and the mixture was stirred at 40℃ for 2 hours to obtain the impurity-removed solution, in which Ca... 2+ Concentration ≤0.003g / L, Mg 2+ Concentration ≤ 0.002 g / L.

[0087] Before the purification and impurity removal liquid, need to prepare composite resin ball in advance, its preparation method is: according to the mass ratio 3:6:1, LSC-500, D451, S930 type resin is mixed, stirring at 300 r / min for 30 min;5% polyvinyl alcohol solution is added to the mixed resin, and the solid-liquid ratio of resin and solution is 1:3, stirring at 50 DEG C, forming the outer layer matrix with 75 μm outer layer macropore; another mixed resin is taken, 8% gelatin solution is added, and the solid-liquid ratio of resin and solution is 1:2, stirring at 40 DEG C, forming the inner layer matrix with 7 μm inner layer micropore; the outer layer skeleton and the inner layer carrier are assembled in layers, and solidification is carried out at-20 DEG C for 4 h; it is transferred into a vacuum drying oven, dried at-0.08 MPa vacuum degree and 60 DEG C for 6 h, so that the porosity reaches 45%; the dried semi-finished product is broken into 2 mm particles, mixed with 5% polyvinyl alcohol solution at a solid-liquid ratio of 1:2, put into a rotary granulator at a rotating speed of 15 r / min, and granulated to form a spherical body with a diameter of 4 mm; the spherical body is immersed in 8% gelatin solution for 30 min, frozen at-20 DEG C for 4 h, and then dried at 60 DEG C for 6 h in vacuum, to obtain the composite resin ball.

[0088] In the primary purification, the impurity removal liquid passes through the ion exchange column at a flow rate of 17 BV / h, and the primary purification liquid is obtained, wherein the Ca 2+ In the secondary purification, the primary purification liquid enters the ceramic membrane system, the surface layer of the ceramic membrane has a pore size of 1.0 μm, the bottom layer has a pore size of 5 μm, the operating pressure is 0.2 MPa, the cross-flow velocity is 1.5 m / s, and after filtration, the secondary purification liquid with a turbidity of 0.05 NTU is obtained.

[0089] Furthermore, the secondary purification liquid is evaporated and concentrated to a Li + The concentration is 25 g / L, and the secondary steam generated by evaporation is used to preheat the raw material to 60 DEG C.

[0090] Furthermore, Na2CO3 is added to the concentrated liquid, the excess amount of Na2CO3 is 12.5%, stirring at 400 rpm at 60 DEG C, adjusting the pH to 10.5, and after 3 h of reaction, solid-liquid separation is carried out, to obtain the lithium carbonate crude product and the mother liquor, and the Li + The concentration is 1.5 g / L.

[0091] Furthermore, the lithium carbonate crude product is washed with hot water at 60 DEG C at a solid-liquid ratio of 1:5, and centrifuged at a rotating speed of 7000 r / min for 15 min.

[0092] Finally, 20% of the mother liquor is taken after every 5 cycles for electrodialysis desalination.

[0093] Example 3: First, the lithium-containing material was mixed with water at a solid-liquid ratio of 1:5, pumped into a rotating packed bed, the rotating speed of the rotating packed bed was 1200 r / min, the leaching temperature was controlled in two stages, the temperature of the first stage was 40℃ and lasted for 2h, the temperature of the second stage was 80℃ and lasted for 5h, and the leaching liquid was obtained, wherein the concentration of Li + The concentration of Ca 2+ The concentration of Mg 2+ The concentration of Mg

[0094] Furthermore, Li2S was added to the leaching liquid, the amount of Li2S added was 1.3 times the total molar amount of Pb 2+ The reaction was carried out at 45℃ for 1h, and after standing for 2h, the filtrate was obtained; a composite impurity removal system was added to the filtrate, the mass ratio of resin to EDTA in the system was 6:1, the amount of resin was 60g / L, the amount of EDTA was 10g / L, the pH of the system was adjusted to 9, and the system was stirred at 40℃ for 2h to obtain an impurity removal liquid, wherein the concentration of Ca 2+ The concentration of Mg 2+ The concentration of Mg

[0095] Before purifying the impurity removal liquid, a composite resin ball was prepared in advance, and the preparation method was as follows: LSC-500, D451 and S930 resins were weighed according to a mass ratio of 4:7:1, mixed and stirred at a speed of 350 r / min for 35 min; 6% polyvinyl alcohol solution was added to the mixed resin, the solid-liquid ratio of the resin to the solution was 1:4, and the mixed resin was stirred at 55℃ to form an outer layer substrate with an outer layer macropore of 110μm; another mixed resin was taken, 9% gelatin solution was added, the solid-liquid ratio of the resin to the solution was 1:3, and the mixed resin was stirred at 45℃ to form an inner layer substrate with an inner layer micropore of 11μm; the outer layer skeleton and the inner layer carrier were assembled in layers, and were solidified by freezing at-15℃ for 5h; then the vacuum drying box was entered, and the vacuum degree was-0.07MPa, the drying was carried out at 65℃ for 7h to make the porosity reach 50%; the dried semi-finished product was broken into particles with a size of 4mm, mixed with 6% polyvinyl alcohol solution according to a solid-liquid ratio of 1:3, and put into a rotary granulator to granulate at a speed of 20r / min to form balls with a diameter of 5mm; the balls were immersed in 9% gelatin solution for 35min, frozen at-15℃ for 5h, and then dried in a vacuum drying oven at 65℃ for 7h to obtain the composite resin ball.

[0096] In the primary purification, the impurity removal liquid passed through the ion exchange column at a flow rate of 20BV / h; in the secondary purification, the primary purified liquid entered the ceramic membrane system, the surface layer of the ceramic membrane had a pore size of 1.2μm, the bottom layer had a pore size of 6μm, the operating pressure was 0.25MPa, the cross-flow velocity was 1.8m / s, and after filtration, the secondary purified liquid with a turbidity of 0.07NTU was obtained.

[0097] Furthermore, the secondary purification liquid is evaporated and concentrated to Li + The concentration is 30 g / L, and the secondary steam generated by evaporation is used to preheat the raw material to 65°C.

[0098] Furthermore, Na2CO3 is added to the concentrated liquid, the excess of Na2CO3 is 15%, and the pH is adjusted to 11 at 90°C with stirring at 500 rpm. After 5h of reaction, solid-liquid separation is performed to obtain a crude lithium carbonate and a mother liquor. The Li + The concentration is 1.8 g / L.

[0099] Furthermore, the crude lithium carbonate is washed with hot water at 70°C at a solid-liquid ratio of 1:6 and centrifuged at 7000 r / min for 15 min.

[0100] Finally, 25% of the mother liquor is taken after every 4 cycles for electrodialysis desalination.

[0101] Comparative Example 1: Unmodified D751 resin, no grafting of EDTA and nano ZrO2

[0102] In detail, the impurity removal in Step 2 uses an unmodified D751 resin, which is only the original resin without grafting of EDTA groups and loading of nano ZrO2, and the remaining steps are consistent with Example 2.

[0103] Comparative Example 2: Composite resin spheres without gradient pores, only single microporous structure

[0104] In detail, the composite resin spheres used for purification in Step 3 only retain the inner layer of 4-11 μm micropores without the outer layer of 40-110 μm macropores. During preparation, the resin particles are directly coated with gelatin without forming an outer layer of polyvinyl alcohol macroporous framework. The remaining steps are consistent with Example 2.

[0105] Comparative Example 3: Ceramic membrane with single-layer structure without bottom loose layer

[0106] In detail, the ceramic membrane used for purification in Step 3 only retains the surface dense layer with a pore size of 1.0 μm without the bottom loose layer. The surface layer is directly sintered on the support body, and the remaining steps are consistent with Example 2.

[0107] Comparative Example 4: No composite impurity removal system, only Li2S impurity removal

[0108] In detail, the lithium-containing material is mixed with deionized water at a solid-liquid ratio of 1:5 and pumped into a rotating packed bed. The rotating speed of the rotating packed bed is 1200 r / min. The temperature is controlled in two stages, with the first stage being 30°C for 1.5 h and the second stage being 70°C for 4 h. Filtration is performed to obtain a leaching solution, in which the Li + The concentration is 4.5 g / L, Ca 2+ The concentration is 0.11 g / L, Mg 2+The concentration of Li2S was 0.07 g / L. Li2S was added to the leaching solution, and the amount of Li2S added was 0.07 g / L 2+ 1.2 times the total molar amount, and after reaction at 40°C for 1 h, the heavy metal precipitate was removed by filtration without adding modified D751 chelating resin and EDTA. The subsequent purification, evaporation concentration, and lithium precipitation steps were consistent with Example 2, and the composite resin ball parameters, ceramic membrane filtration conditions, and lithium precipitation process were the same.

[0109] Comparative Example 5: Only single-stage purification without ceramic membrane filtration

[0110] In other words, the leaching and impurity removal steps were consistent with Example 2, and the leaching solution indicators and impurity removal system parameters were the same. The purification treatment only used composite resin ball adsorption with a flow rate of 17 BV / h, and no ceramic membrane filtration was performed. The first-stage purification liquid was directly sent to the evaporation concentration process, and the concentration was increased to Li + The concentration was 25 g / L, and the subsequent lithium precipitation and centrifugal desalting steps were consistent with Example 2.

[0111] Comparative Example 6: No mother liquor circulation, mother liquor directly discharged

[0112] The leaching, impurity removal, purification, evaporation concentration, and lithium precipitation steps were consistent with Example 2, and the parameters were completely the same. However, the mother liquor was directly discharged after lithium precipitation and was not returned to the leaching process. The remaining steps, including centrifugal desalting and product drying, were unchanged.

[0113] In order to compare the purification effects of low-impurity lithium carbonate in Examples 1-3 and Comparative Examples 1-6, the following test method is provided:

[0114] The same lithium-containing material, i.e., lithium mica powder, was used in all examples and comparative examples, with a Li2O content of 2.8%, Ca 2+ 0.35%, Mg 2+ 0.22%, Pb 2+ 0.005%, and the same equipment was also used.

[0115] During the experiment, the detection methods of key indicators were as follows:

[0116] Lithium leaching rate: The Li + concentration in the leaching solution was determined by inductively coupled plasma optical emission spectrometer (ICP-OES), and the total lithium content of the raw material was calculated;

[0117] Impurity removal rate: The Ca 2+ , Mg 2+ , and Pb 2+ concentrations in the solution were determined by atomic absorption spectrophotometer (AAS), and the removal rate was calculated;

[0118] Solution turbidity: Determined by turbidimeter;

[0119] Purity and impurity content of lithium carbonate: purity was analyzed by X-ray fluorescence spectrometer (XRF), and Na, Cl, etc. impurities were determined by ion chromatograph + -

[0120] Total recovery rate of lithium: the amount of lithium from raw material to final product was tracked to calculate the total recovery rate.

[0121] Comparative experimental data of examples and comparative examples are as follows:

[0122]

[0123] Experimental data description:

[0124] 1. In terms of impurity removal effect of modified D751 chelating resin, in comparative example 1, when using unmodified resin, the removal rates of calcium and magnesium are only 72.3% and 68.5%, and the purity of the product is 99.65%; while in example 2, after using the modified resin grafted with EDTA and loaded with nano ZrO2, the removal rates of calcium and magnesium increase to 99.7% and 99.5%, and the purity reaches 99.98%, and the residual sodium ion decreases from 53 ppm to 5 ppm, which directly reflects that the use of modified D751 chelating resin significantly enhances the removal of impurities through the dual action of chelation and adsorption.

[0125] 2. In terms of the advantages of gradient pore composite resin balls, in comparative example 2, due to the absence of an outer large pore, the first-stage purification flow rate is only 8 BV / h, and the removal rates of calcium and magnesium are 89.2% and 86.7%; the gradient pore structure of example 2 increases the flow rate to 17 BV / h, and the removal rates increase to 99.7% and 99.5% simultaneously, which proves the synergistic effect of the outer large pore reducing resistance and the inner micropore strengthening adsorption, effectively solving the contradiction between efficiency and precision of traditional single-pore structure.

[0126] 3. In terms of the performance advantages of double-layer ceramic membrane, the flux of single-layer membrane of comparative example 3 is only 5.2 L / m 2 ·h, and the turbidity is 0.12 NTU; the flux of double-layer membrane of example 2 reaches 10-15 L / m 2 ·h, and the turbidity decreases to 0.05 NTU, which shows that the bottom loose layer significantly reduces the resistance, and the surface dense layer efficiently retains colloids, achieving the balance of high flux and low turbidity.

[0127] 4. In terms of the synergistic effect of two-stage purification, the turbidity of comparative example 5 is 0.82 NTU when only single-stage purification is performed, and the lithium recovery rate is 88.6%; after two-stage purification, the turbidity of example 2 is 0.05 NTU, and the recovery rate increases to 92.5%, which shows that the synergistic effect of composite resin balls and ceramic membrane not only deeply removes ionic and colloidal impurities, but also reduces lithium loss.

[0128] ​​It will be apparent to those skilled in the art that the application is not limited to the details of the above-exemplified embodiments and that the present application can be implemented in other particular forms without departing from the spirit or essential characteristics of the present application. The embodiments should therefore be considered in all respects as illustrative and not restrictive, the scope of the application being indicated by the appended claims rather than by the above description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

1. A purification method of low-impurity lithium carbonate, characterized by, Comprising the following steps: Step one: leaching After mixing the lithium-containing material with water, perform hypergravity leaching treatment in a rotating packed bed; the temperature of the mixture in the leaching process is regulated in two stages to promote the dissolution of lithium, and finally obtain a leaching solution; Step two: impurity removal Add Li2S to the leaching solution obtained in step one to remove heavy metal ions, and then add a composite impurity removal system composed of modified D751 chelating resin and free ethylenediaminetetraacetic acid to obtain an impurity removal solution after impurity removal is completed; wherein the modified D751 chelating resin is prepared by grafting ethylenediaminetetraacetic acid groups on the surface and loading nano-zirconium dioxide; 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, first mix with a mass fraction of 4-6% hydrochloric acid solution at a solid-liquid ratio of 1:9-11, stir in a constant temperature water bath at 55-65°C for 1.5-2.5h to remove residual impurities and free ions on the surface of the resin, filter and wash with deionized water until neutral; Then transfer the resin into a mass fraction of 4-6% sodium hydroxide solution, also mix at a solid-liquid ratio of 1:9-11 at 55-65°C for 1.5-2.5h to activate the functional groups of the resin, filter and wash with deionized water until neutral, and vacuum dry to constant weight; (2) Amine functionalization grafting Put the pretreated D751 resin and ethylenediaminetetraacetic dianhydride into N,N-dimethylformamide solvent at a mass ratio of 1:1.8-2.2, wherein the solid-liquid ratio of the resin to N,N-dimethylformamide is 1:7-9, and stir under nitrogen protection at 75-85°C for 5-7h; After the reaction is completed, filter and collect the resin, wash it with anhydrous ethanol for 2-4 times to remove unreacted ethylenediaminetetraacetic dianhydride, and vacuum dry at 55-65°C to graft ethylenediaminetetraacetic acid groups on the surface of the resin to obtain amine functionalized resin; (3) Nano-zirconium dioxide loading Prepare a 0.4-0.6mol / L zirconium oxychloride solution, immerse the amine functionalized resin in it at a solid-liquid ratio of 1:14-16, and stir for 3-5h at 25-35°C; Then add 0.8-1.2mol / L sodium hydroxide solution dropwise to adjust the pH of the system to 8.5-9.5, so that the zirconium ions adsorbed on the surface of the resin are hydrolyzed to form nano-zirconium dioxide particles, continue to stir for 0.5-1.5h, then filter, wash with deionized water until there is no chloride ion in the filtrate, and vacuum dry at 75-85°C to obtain a composite resin loaded with nano-zirconium dioxide; (4) Crosslinking stabilization treatment Put the above composite resin into a mass fraction of 4-6% glutaraldehyde solution, mix at a solid-liquid ratio of 1:11-13, and crosslink at 35-45°C for 1.5-2.5h to make the functional groups on the surface of the resin form stable structures through covalent bonds, filter, wash with deionized water until there is no residual aldehyde group, vacuum dry at 55-65°C to constant weight, and obtain modified D751 chelating resin; Step three: purification The impurity-removed solution obtained in step two is subjected to two-stage purification treatment: in the first stage, an ion exchange column filled with composite resin balls is used to adsorb ion impurities, thereby removing most of the ion impurities in the impurity-removed solution and obtaining a first-stage purified solution; in the second stage, a precision ceramic membrane is used for filtration, and the first-stage purified solution is filtered through the precision ceramic membrane after passing through the water outlet of the ion exchange column to obtain a second-stage purified solution; The composite resin balls have a gradient pore structure: the outer layer is composed of macropores with a size of 40-110 μm, which allows the solution to flow quickly and preliminarily adsorb large-particle-size impurities; and the inner layer is composed of micropores with a size of 4-11 μm, which enhances the deep adsorption of trace ions such as calcium and magnesium through capillary action; The precision ceramic membrane has a double-layer structure, in which the surface dense layer has a pore size of 0.8-1.2 μm to trap residual small colloidal particles, and the bottom loose layer has a pore size of 4-6 μm to reduce the filtration resistance, so that the turbidity of the solution is reduced to below 0.1 NTU; Step four: evaporation and concentration The second-stage purified solution obtained in step three is evaporated to a concentrated solution with a lithium concentration of 20-30 g / L; Step five: lithium precipitation Na2CO3 is added to the concentrated solution obtained in step four to perform a lithium precipitation reaction, and then solid-liquid separation is performed to obtain lithium carbonate crude and a mother liquor; Step six: centrifugal desalting The lithium carbonate crude obtained in step five is first prewashed with hot water, and then low-impurity lithium carbonate is obtained through centrifugal operation; Step seven: mother liquor recycling The mother liquor obtained in step five is returned to step one, and the lithium concentration in the mother liquor is controlled at 1-2 g / L, which is lower than the leaching equilibrium concentration, and the concentration gradient formed drives the continuous dissolution of lithium.

2. The method of claim 1, wherein, In the leaching process of step one, the temperature of the mixture is controlled in the following manner: the temperature in the front section is controlled at 20-40℃ for 1-2h to inhibit the dissolution of impurities such as calcium and magnesium, and the temperature in the rear section is controlled at 60-80℃ for 3-5h.

3. The method of claim 1, wherein, In the impurity removal process of 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 1-2h, and then the mixture is filtered to separate the precipitate.

4. The method of claim 1, wherein, In the purification process of step three, the preparation of the composite resin balls includes: (1) Resin mixing LSC-500, D451, and S930 resins are weighed according to a mass ratio of 2-4:5-7:1, and are placed in a stirred tank for stirring at a speed of 250-350 r / min for 25-35 min to uniformly mix the three types of resin particles; (2) Outer layer substrate preparation A polyvinyl alcohol solution with a mass fraction of 4-6% is added to the mixed resin prepared in step (1), and the solid-liquid ratio of the resin to the solution is controlled at 1:2-4. The mixture is stirred in a water bath at 45-55℃ until a uniform paste is formed. The paste is extruded through a granulating die to form the outer layer framework of the composite resin balls. The polyvinyl alcohol forms macropores with a size of 40-110 μm due to cross-linking; (3) Inner layer substrate preparation Take another part of the mixed resin of step (1), add 7-9% by mass of gelatin solution, control the solid-liquid ratio to be 1:1-3, stir at 35-45℃ until the gelatin is completely swollen and wraps the resin particles, so that the gelatin forms a microporous structure of 4-11μm on the surface of the resin particles as an inner adsorption carrier; (4) Freeze-shaping The outer skeleton of the composite resin ball prepared in step (2) and the inner adsorption carrier prepared in step (3) are assembled in a layered structure, that is, the outer skeleton is used as a substrate, and the inner adsorption carrier is evenly covered on the substrate, and then it is placed in a-25~-15℃ freezing box for 3-5h to solidify the colloidal structure of polyvinyl alcohol and gelatin; (5) Drying and pore control The semi-finished product after freezing in step (4) is transferred into a 55-65℃ vacuum drying oven, dried at a vacuum degree of-0.09~-0.07MPa for 5-7h, and the pore structure is stabilized by sublimation of water, so that 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 broken into 1-4mm particles, and then mixed with 4-6% by mass of polyvinyl alcohol solution at a solid-liquid ratio of 1:1-3, and put into a rotary granulator at a rotating speed of 10-20r / min. The particles are wrapped with a polyvinyl alcohol film during rolling to form a spherical body with a diameter of 3-5mm, and the outer macroporous structure of 40-110μm is naturally maintained by the surface tension during granulation; (7) Inner layer pore forming The spherical body after granulation in step (6) is immersed in a 7-9% by mass of gelatin solution for 25-35min to allow the gelatin to penetrate into the interior of the spherical body. After taking out, repeat the freezing and drying processes in steps (4) and (5). Through the shrinkage of gelatin and the evaporation of water, a microporous structure of 4-11μm is formed in the interior of the spherical body, and finally a composite resin ball with a gradient pore structure is obtained.

5. The method of claim 4, wherein, In the third purification process, the preparation steps of the precision ceramic membrane include: (1) The surface layer is sintered at 1150-1250℃ using 0.4-0.6μm ceramic powder to form a surface dense layer; (2) The bottom layer uses 4-6 μm ceramic powder, and is pre-sintered at 950-1050 °C for 1 h to form a porous substrate, then the surface dense layer blank prepared in step (1) is covered on the surface of the bottom layer substrate, and is continuously sintered at the same temperature and kept for 1.5-2.5 h, so that the two layers are firmly combined through diffusion sintering, and finally a loose layer is formed at the bottom, and the membrane flux is 10-15 L / m 2 • Precision ceramic membranes of h.

6. The method of claim 5, wherein, In the third purification process, the steps of two-stage purification treatment of the composite resin ball and the precision ceramic membrane include: (1) Composite resin ball adsorption purification The impurity removal liquid obtained in step two is pumped into an ion exchange column filled with composite resin balls, and the solution flow rate is controlled at 15-20BV / h. The composite resin balls allow the solution to flow quickly through the outer 40-110μm macropores, while the LSC-500, D451, and S930 resins remove more than 85% of calcium, magnesium ions, and heavy metal ions in the solution, obtaining a first-stage purified liquid; (2) Precision ceramic membrane deep filtration The effluent of the ion exchange column enters the ceramic membrane filtration system. The system uses an alumina ceramic membrane assembly with a pore size of 0.08-0.12μm, and is set at an operating pressure of 0.15-0.25MPa and a cross-flow velocity of 1.2-1.8m / s. Under pressure driving, the first-stage purified liquid flows along the membrane surface at high speed, and the residual trace colloidal particles, resin debris, and unabsorbed nanoscale impurities are intercepted by the membrane. The permeate is the second-stage purified liquid.

7. The method of claim 1, wherein, In step four, the evaporation process is heated by steam, and the secondary steam generated in the evaporation process is recycled in step one. The mixture of lithium-containing material and water is preheated to 55-65℃ before entering the rotating packed bed.

8. The method of claim 1, wherein, In step five, the addition of Na2CO3 is 10-15% excess, the stirring rate is 300-500 rpm, the crystal particle size is 15-55 μm, and the residual sodium ion is ≤10 ppm. The temperature in the lithium precipitation process is controlled at 30-90℃, and the pH value is controlled at 10-11. After 1-5 hours of reaction, solid-liquid separation is performed to obtain lithium carbonate crude and mother liquor.

9. The method of claim 1, wherein, In step six, the lithium carbonate crude obtained in step five is first prewashed 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. And / or, in step seven, after every 4-6 cycles, 15-25% of the mother liquor is taken for electrodialysis desalination, with a lithium recovery rate of ≥90%, to avoid impurity accumulation and a chloride ion concentration of ≤20 ppm.

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