Membrane capacitive deionization module for treating brine with high magnesium-lithium ratio as well as preparation method and application of membrane capacitive deionization module

Abstract

The invention provides a membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio as well as a preparation method and application of the membrane capacitive deionization module. The magnesium-lithium ratio of brine is 300-700, and the preparation method comprises the following steps: firstly, mixing and ball-milling an acid-washed lithium titanate molecular sieve, a binder, a conductive agent and a solvent, coating the surface of a whisker carbon nanotube membrane with the obtained slurry to form a coating, and then taking the whisker carbon nanotube membrane comprising the coating as an electrode, and assembling to obtain the membrane capacitive deionization module. Wherein the weight ratio of the acid-washed lithium titanate molecular sieve to the whisker carbon nanotube film is 1: (2-4). According to the membrane capacitive deionization module, efficient adsorption of lithium ions is achieved by utilizing high selectivity of the acid-washed lithium titanate molecular sieve, the whisker carbon nanotube membrane serves as a current collector, the module structure is simplified by utilizing high conductivity and large specific surface area of the whisker carbon nanotube membrane, rapid transmission of charges and uniform adsorption of the lithium ions are promoted, and the membrane capacitive deionization module is suitable for brine treatment; the method is especially suitable for brine with high magnesium-lithium ratio.

Description

Technical Field

[0001] The present invention relates to the technical field of brine lithium extraction, and in particular, to a membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio, a preparation method thereof, and an application thereof. Background Art

[0002] Since the advent of capacitive deionization (CDI) technology, it has received extensive attention due to its potential advantages in water treatment and ion recovery. Traditional CDI technology is mainly based on the principle of electric double layer adsorption, using porous electrode materials to adsorb and release ions in water under the action of an electric field, thereby realizing water desalination or selective extraction of specific ions.

[0003] However, when dealing with brine resources with a high magnesium-lithium ratio, traditional CDI technology faces many challenges, which limit its wide application in the field of lithium extraction. 1. Limited adsorption capacity: The specific surface area and pore structure of traditional CDI electrode materials determine their adsorption capacity, and these materials usually do not have enough lithium ion storage sites, resulting in limited lithium ion adsorption and restricting the efficiency of lithium extraction from brine. 2. Poor selectivity: Electrode materials usually do not have the ability to selectively adsorb specific ions with high selectivity. Especially in brine with multiple ions coexisting, it is difficult to selectively adsorb lithium ions, and non-selective adsorption of other ions often occurs, which reduces the purity of lithium recovery. 3. Poor electrode cycle stability: During repeated charging and discharging processes, the structural stability of traditional CDI electrode materials is easily damaged, resulting in a decline in electrode performance, a short cycle service life, and an increase in the cost and complexity of the lithium extraction process. 4. Low efficiency: The ion adsorption and desorption processes of traditional CDI technology are usually relatively slow, requiring a long treatment time, and consuming a large amount of energy during the desorption stage, with a low overall energy efficiency, which is not conducive to large-scale industrial applications.

[0004] In view of the above problems, it is urgent to develop new electrode materials and optimize process parameters to improve the lithium ion selective adsorption ability and efficiency of CDI technology when treating brine with a high magnesium-lithium ratio, while ensuring the cycle stability and long life of the electrode. Summary of the Invention

[0005] The main object of the present invention is to provide a membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio, a preparation method thereof, and an application thereof, so as to solve the problems of limited adsorption capacity, poor adsorption selectivity for lithium ions, poor electrode cycle stability, and low efficiency in the existing capacitive deionization technology when treating brine with a high magnesium-lithium ratio.

[0006] To achieve the above object, according to one aspect of the present invention, there is provided a method for preparing a membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio, where the magnesium-lithium ratio of the brine is 300 to 700. The preparation method includes the following steps: Step S1, sequentially mix pickled lithium titanate molecular sieve, binder, conductive agent, and solvent, and perform first ball milling to obtain a slurry; coat the slurry on the surface of the whisker carbon nanotube membrane and dry it to form a coating on the surface of the whisker carbon nanotube membrane; Step S2, use the whisker carbon nanotube membrane including the coating as an electrode, and assemble the electrode, ion exchange membrane, diaphragm, module frame, power supply, and fluid delivery system to obtain a membrane capacitive deionization module; wherein, the weight ratio of the pickled lithium titanate molecular sieve to the whisker carbon nanotube membrane is 1:(2 to 4).

[0007] Further, the preparation method of the pickled lithium titanate molecular sieve includes the following steps: treat the lithium titanate molecular sieve with an acid solution to obtain the pickled lithium titanate molecular sieve; wherein, the acid in the acid solution includes one or more of hydrochloric acid, sulfuric acid, and nitric acid, the concentration of the acid is 0.1 to 0.6 mol / L, and the solid-liquid ratio of the lithium titanate molecular sieve to the acid solution is 1 g:(200 to 300) mL; the treatment is carried out under stirring, the temperature is 10 to 60 °C, the rotation speed is 10 to 50 rpm, and the time is 6 to 48 h; and / or, the preparation method of the lithium titanate molecular sieve includes the following steps: sequentially perform second ball milling, roasting, and cooling on the lithium source and the titanium source to obtain the lithium titanate molecular sieve; wherein, the lithium source includes one or more of lithium hydroxide, lithium carbonate, and lithium chloride, the titanium source includes titanium oxide and / or titanium chloride, and the molar ratio of the lithium source to the titanium source is (1.5 to 2.5):1; and / or, the rotation speed of the second ball milling is 300 to 600 rpm, and the time is 0.5 to 12 h; and / or, the roasting temperature is 700 to 1000 °C, and the holding time is 2 to 24 h.

[0008] Further, in the slurry, the weight ratio of the pickled lithium titanate molecular sieve to the binder is (70 to 85):(5 to 15); and / or, the binder includes one or more of polyvinylidene fluoride, carboxymethyl cellulose, and polytetrafluoroethylene; and / or, the weight ratio of the pickled lithium titanate molecular sieve to the conductive agent is (70 to 85):(5 to 15); and / or, the conductive agent includes one or more of conductive carbon black, Ketjen black, and carbon nanotubes; and / or, the weight ratio of the pickled lithium titanate molecular sieve to the solvent is (70 to 85):(100 to 300); and / or, the solvent includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide, and propylene glycol.

[0009] Further, the thickness of the whisker carbon nanotube membrane is 60 to 70 μm, and the density is 70 to 90 g / m 3, the porosity is 20-60%; and / or, the conductivity of the whisker carbon nanotube film is 0.015-0.018 kS / cm, and the specific surface area is 12-18 m 2 / g.

[0010] Furthermore, in step S1, the slurry is coated on one or both sides of the whisker carbon nanotube film; and / or, the area percentage of the coating in the whisker carbon nanotube film is 50-100%; and / or, the thickness of the coating is 10-250 μm, and the coating density is 70-90 g / m 3 , and the specific surface area is 4-16 m 2 / g.

[0011] Furthermore, the adsorption capacity q + of the electrode for Li e is 5-17 mg / g, the adsorption rate is 15-60 mg / (g·h), and the electrode capacity attenuation amount after 250 cycles of use is ≤5%; and / or, the ion exchange membrane includes a Fumasep FAA-3-50 anion exchange membrane and a FuMA-Tech cation exchange membrane; and / or, the separator includes polyvinyl fluoride and / or polypropylene.

[0012] According to another aspect of the present invention, there is provided a membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio, which is obtained by the above preparation method.

[0013] Furthermore, the lithium adsorption rate of the membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio is 20-70%, the magnesium removal rate is 95-99.9%, and the lithium enrichment multiple is 1-10 times.

[0014] According to another aspect of the present invention, there is provided a method for treating brine with a high magnesium-lithium ratio, including the following steps: forming a closed loop by connecting the above membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio to a DC voltage circuit, feeding the brine with a high magnesium-lithium ratio into the membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio, first adsorbing for 10-35 min under the conditions of a pressure of 0.8-1.6 V and a temperature of 5-30 °C, and then desorbing for 10-35 min under the conditions of a pressure of -0.8 to -1.6 V, a temperature of 5-30 °C, and an acidic condition, and circulating 1-300 times.

[0015] Furthermore, in the composition of the brine with a high magnesium-lithium ratio, the content of Li + is 200-700 mg / L, the content of Mg 2+ is 20000-200000 mg / L, the content of K + is 100-3000 mg / L, the content of Na + is 300-6000 mg / L, and the content of Ca 2+The content is 500 to 7000 mg / L, and the salinity is 20 to 40%; and / or, the pH of the high magnesium-lithium ratio brine is 3 to 6; and / or, the flow rate of the high magnesium-lithium ratio brine through the membrane capacitive deionization module for treating high magnesium-lithium ratio brine is 5 to 300 mL / min.

[0016] The membrane capacitive deionization module of the present invention includes electrodes with a specific composition. Utilizing the high selectivity of lithium titanium oxide molecular sieve in the electrodes, efficient adsorption of lithium ions is achieved. The electrodes use whisker carbon nanotube membranes as current collectors. Their high conductivity and large specific surface area not only simplify the module structure but also promote the rapid transmission of charges and the uniform adsorption of lithium ions. The combination of this electrode design and the MCDI module not only improves the electrochemical performance, such as the charging / discharging rate and adsorption capacity, but also has the characteristics of environmental friendliness and economy, and is applicable to the treatment of brines with different salinities, especially brines with a high magnesium-lithium ratio. Brief Description of the Drawings

[0017] The specification drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0018] Figure 1 The SEM diagram of the cross-section of the whisker carbon nanotube membrane of Example 1 of the present invention is shown;

[0019] Figure 2 The SEM diagram of the surface of the whisker carbon nanotube membrane of Example 1 of the present invention is shown;

[0020] Figure 3 The SEM diagram of the cross-section of the electrode of Example 1 of the present invention is shown;

[0021] Figure 4 The SEM diagram of the surface of the electrode of Example 1 of the present invention is shown;

[0022] Figure 5 The EDS diagram of the electrode of Example 1 of the present invention is shown;

[0023] Figure 6 The cyclic voltammogram of the electrode of Example 1 of the present invention at a scan rate of 1 mV / s is shown;

[0024] Figure 7 The photo of the uncut electrode of Example 1 of the present invention is shown. Detailed Embodiments

[0025] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments.

[0026] As described in the background art of the present invention, in the prior art, there are problems in the capacitive deionization technology when treating brine with a high magnesium-lithium ratio, such as limited adsorption capacity, poor adsorption selectivity for lithium ions, poor electrode cycle stability, and low efficiency. To solve the above problems, in a typical embodiment of the present invention, a preparation method of a membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio is provided. The magnesium-lithium ratio of the brine is 300-700, and the preparation method includes the following steps: Step S1, mixing pickled lithium titanate molecular sieve, binder, conductive agent and solvent in sequence and performing first ball milling to obtain a slurry; coating the slurry on the surface of the whisker carbon nanotube membrane and drying to form a coating on the surface of the whisker carbon nanotube membrane; Step S2, using the whisker carbon nanotube membrane including the coating as an electrode, and assembling the electrode with a cathode, an ion exchange membrane, a diaphragm, a module frame, a power supply and a fluid delivery system to obtain a membrane capacitive deionization module; wherein, the weight ratio of the pickled lithium titanate molecular sieve to the whisker carbon nanotube membrane is 1:(2-4). It should be noted that in this application, the magnesium-lithium ratio refers to the weight ratio of magnesium to lithium.

[0027] The raw materials of the coating in this application include pickled lithium titanate molecular sieve (HTO), binder, conductive agent and solvent. Among them, the pickled lithium titanate molecular sieve serves as the active center of the electrode, can participate in electrochemical reactions, store charges, and promote the insertion and extraction of ions, so as to achieve high-capacity lithium ion adsorption. It should be noted that this application particularly uses pickled lithium titanate molecular sieve as the active substance for the following reasons: First, the metal oxides and impurity ions on the surface of lithium titanate are effectively removed after pickling, the micropores are expanded and can provide new pores, so as to provide more adsorption sites; Second, during the pickling process, hydrogen ions displace lithium ions, increasing specific lithium ion adsorption sites, enabling lithium ions to be adsorbed directionally on the surface or in the pores, significantly improving the adsorption capacity and rate of lithium ions; moreover, the active sites introduced by pickling can optimize the conductivity and electrochemical stability of the material, ensuring high charge efficiency during charge and discharge cycles; Third, the pickled lithium titanate has enhanced hydrophilicity and a high surface charge density, which is beneficial to the adsorption of lithium ions.

[0028] In summary, in brine with a high magnesium-lithium ratio, the electrode material in the membrane capacitive deionization module of this application, due to using pickled lithium titanate molecular sieve as the active center, while adsorbing ions based on the double-layer principle, can also specifically adsorb lithium ions through ion intercalation reactions and ion exchange reactions. In contrast, the adsorption effect on magnesium ions is average. Therefore, when treating brine with a high magnesium-lithium ratio, the enrichment of lithium ions can be selectively achieved.

[0029] In addition, the slurry also includes a binder, which plays a bridging role between the active substance, the conductive agent and the whisker carbon nanotube membrane matrix, can ensure the integrity and mechanical strength of the composite electrode structure, prevent the shedding of the active substance during charge and discharge, and ensure the long-term stability and cycle performance of the electrode.

[0030] The addition of conductive agents can effectively improve the conductivity of the electrode, optimize the overall electron transfer path of the electrode, and ensure the uniform distribution of current in the electrode, thereby effectively improving the power density and charge and discharge rate of the electrode. In addition, the presence of conductive agents can promote ion exchange between the electrode and the electrolyte, enhance the adsorption and desorption efficiency of lithium titanate for lithium ions, thereby improving the processing capacity and selectivity of membrane capacitor deionization (MCDI) modules, especially when treating brine with a high magnesium-lithium ratio, it can effectively reduce the magnesium-lithium ratio and achieve lithium ion enrichment. The stable chemical properties of conductive agents can also provide additional structural support for the electrodes, helping to maintain the morphological integrity of the electrodes during the cycle, thereby effectively extending the service life of the module and improving the cycle stability (e.g., the capacity decay of the electrode after 250 cycles is ≤5%).

[0031] In addition, the addition of solvent to the slurry can not only make the active material, conductive agent and binder uniformly mixed to form a stable slurry, but also promote the uniform coating of the slurry on the current collector.

[0032] In summary, the components in the slurry work together to effectively improve the lithium adsorption capacity, cycle stability and electrochemical performance of the electrode.

[0033] The slurry is coated on the surface of the whisker carbon nanotube film and dried to form a coating on the surface of the whisker carbon nanotube film. It should be noted that the present application particularly uses the whisker carbon nanotube film as a current collector. Compared with carbon nanocloth and graphene film, the whisker carbon nanotube film is a multi-micro-nanopore film composite material with a large number of micropores on the surface. It is used as a lithium ion battery current collector to replace metal copper foil / aluminum foil. During the coating process, the coating can penetrate into the interior of the substrate, greatly increasing the contact interface, and it belongs to a gradient interface. Therefore, there is no obvious distinguishable interface between the active material in the coating and the whisker carbon nanotube film. On the one hand, the impedance of the electrode can be reduced by multiples. On the other hand, the multiple pores in the whisker carbon nanotube film can absorb a large amount of electrolyte, thereby greatly improving the battery charge and discharge capacity. While maintaining high conductivity and large specific surface area, it exhibits excellent mechanical strength, toughness, and electrochemical stability.

[0034] In addition, as a carrier of the coating, the whisker carbon nanotubes can not only expand the effective contact area between the electrode and the electrolyte, promote the rapid diffusion of lithium ions, but also ensure the uniform distribution of the slurry and avoid agglomeration. Moreover, as a multi-micro-nanopore film composite material, the whisker carbon nanotube film has a large number of micropores on the surface, and the whisker carbon nanotube film has good hydrophilicity, including the acid-washed lithium titanate molecular sieve slurry can penetrate into the pores of the base material, providing a channel for lithium ion transportation, which is conducive to improving the adsorption capacity of lithium.

[0035] In addition, the present application specifically defines the weight ratio of pickled lithium titanate molecular sieve to whisker carbon nanotube film as 1:(2 - 4). Typically but not limitedly, the weight ratio of pickled lithium titanate molecular sieve to whisker carbon nanotube film is 1:2, 1:2.5, 1:3, 1:3.5, 1:4 or a range value composed of any two of these numerical values. This ratio enables the pickled lithium titanate molecular sieve to be fully adsorbed on the whisker carbon nanotube film with high conductivity and large specific surface area, forming a tight and uniform coating. The high conductivity of the whisker carbon nanotubes can significantly enhance the overall electron transfer efficiency of the electrode, while its nano-scale pore structure helps to improve the ion diffusion rate, thereby enhancing the electrochemical reaction rate and efficiency of the electrode. In addition, the mechanical strength and flexibility of the whisker carbon nanotubes provide a stable support structure for the lithium titanate molecular sieve, preventing the shedding of active substances during charge and discharge cycles and ensuring the cycle stability and service life of the electrode. This composite material with such a weight ratio can also balance the adsorption capacity and mechanical properties of the electrode, enabling the electrode to exhibit excellent selective adsorption and efficient desorption of lithium ions during the membrane capacitive deionization process, thereby improving the efficiency and purity of separating and enriching lithium ions from brine.

[0036] The whisker carbon nanotubes and the pickled lithium titanate molecular sieve interact synergistically, not only improving the lithium ion adsorption capacity and efficiency of the MCDI module, but also enhancing the electrochemical stability and charge transfer ability. Even after a long period of charge and discharge cycles, the decline in electrode performance is extremely limited, thus ensuring the long-term high efficiency of the module during the electrochemical lithium extraction process.

[0037] In summary, the membrane capacitive deionization module of the present application utilizes the high selectivity of the pickled lithium titanate molecular sieve to achieve efficient adsorption of lithium ions. Using the whisker carbon nanotube film as a current collector, its high conductivity and large specific surface area can not only simplify the module structure, but also promote the rapid transfer of charges and the uniform adsorption of lithium ions. The combination of this electrode design and the MCDI module can not only improve the electrochemical performance, such as the charge / discharge rate and adsorption capacity, but also demonstrate the characteristics of environmental friendliness and economy, and is suitable for treating brines with different salinities.

[0038] In a preferred embodiment, the preparation method of the pickled lithium titanate molecular sieve includes the following steps: treating the lithium titanate molecular sieve with an acid solution to obtain the pickled lithium titanate molecular sieve; the acid in the acid solution includes one or more of hydrochloric acid, sulfuric acid and nitric acid, and this type of acid can more effectively dissolve the basic impurities on the surface of lithium titanate, create more vacancies for lithium ion adsorption, and enhance the adsorption performance.

[0039] In a preferred embodiment, the concentration of the acid is 0.1 - 0.6 mol / L. Typically but not restrictively, the concentration of the acid is 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, or a range value composed of any two of these values. Within this range, the surface modification effect of lithium titanate can be more effectively improved, while over-corrosion can be avoided, which is more conducive to maintaining the structural integrity and stability of the material. It should be noted that if the acid concentration is too low, it may affect the degree of surface modification of lithium titanate, thereby affecting the generation of lithium-ion adsorption sites and reducing the adsorption efficiency; if the concentration is too high, it may over-corrode the material, affect the crystal structure, and thus affect the adsorption performance and service life.

[0040] In a preferred embodiment, the solid-liquid ratio of lithium titanate molecular sieve to the acid solution is 1 g:(200 - 300) mL. Typically but not restrictively, the solid-liquid ratio of lithium titanate molecular sieve to the acid solution is 1 g:200 mL, 1 g:250 mL, 1 g:300 mL, or a range value composed of any two of these values; within this range, the pickling reaction can proceed more fully, ensuring the uniformity of surface modification, and thus being more conducive to improving the selectivity and efficiency of lithium-ion adsorption. If the solid-liquid ratio is too low, it may affect the uniformity of the pickling reaction, thereby affecting the surface modification effect of lithium titanate and the distribution of lithium-ion adsorption sites; if it is too high, it will increase the treatment cost and may reduce the effective contact area and pickling efficiency at the same time.

[0041] In a preferred embodiment, the treatment is carried out under stirring, the temperature is 10 - 60 °C, the rotation speed is 10 - 50 rpm, and the time is 6 - 48 h. Under these conditions, ion exchange can be more effectively promoted, the purity and activity of lithium titanate can be further improved, and it is more conducive to enhancing the ion adsorption performance of the electrode material.

[0042] All in all, controlling the pickling parameters within the above range can not only significantly improve the lithium-ion adsorption capacity and rate of the lithium titanate molecular sieve after pickling, but also further optimize its electrochemical performance and extend its service life. More importantly, this treatment method can significantly enhance the lithium-magnesium separation ability of lithium titanate in high magnesium-lithium ratio brines, providing strong support for the application of MCDI technology in lithium resource recovery.

[0043] In a preferred embodiment, the lithium titanate molecular sieve includes Li 2 TiO 3 and / or Li 4 Ti 5 O 12 . The above types of lithium titanate molecular sieves are all suitable for making the lithium titanate coated electrode of the present application.

[0044] In a preferred embodiment, the preparation method of lithium titanate molecular sieve comprises the following steps: successively performing secondary ball milling, calcination and cooling on a lithium source and a titanium source to obtain the lithium titanate molecular sieve; wherein, the lithium source includes one or more of lithium hydroxide, lithium carbonate and lithium chloride, the titanium source includes titanium oxide and / or titanium chloride, and the molar ratio of the lithium source to the titanium source is (1.5 - 2.5):1. Typically but not limitedly, the molar ratio of the lithium source to the titanium source is 1.5:1, 2:1, 2.5:1 or a range value composed of any two of these values. Under the above conditions, it is more conducive to ensuring the integrity of the molecular sieve structure and the formation of lithium ion vacancies, thereby further optimizing the electrochemical performance of the electrode.

[0045] In a preferred embodiment, the rotation speed of the secondary ball milling is 300 - 600 rpm and the time is 0.5 - 12 h, which can promote the deep mixing and refinement of the lithium source and the titanium source, thus being more conducive to the formation of uniform precursor particles, and / or, the calcination temperature is 700 - 1000 °C. Typically but not limitedly, the calcination temperature is 700 °C, 800 °C, 900 °C, 1000 °C or a range value composed of any two of these values, and the heat preservation time is 2 - 24 h, preferably 12 - 24 h; such conditions can more precisely control the crystal structure to form lithium titanate with a higher specific surface area and more lithium ion exchange sites, thereby further enhancing the lithium adsorption capacity and rate. It should be noted that if the calcination time is too long or the temperature is too high, although it is beneficial to increase the site density, it may lead to excessive crystal growth and changes in the pore structure, thereby affecting the effectiveness of ion exchange sites. If the temperature is insufficient, it may affect the chemical conversion rate of the reaction, ultimately easily resulting in uneven composition distribution of the material, possible residues of unreacted precursors, and low purity and activity of the material. If the time is too short, it may not be sufficient to make the material reach the required structure and crystal maturity, resulting in too small grain size or incomplete crystal form, affecting the conductivity, ion exchange ability and stability of the material.

[0046] The inventors further optimized the composition of the slurry. In a preferred embodiment, in the slurry, the weight ratio of pickled lithium titanate molecular sieve to binder is (70-85):(5-15); and / or, the binder includes one or more of polyvinylidene fluoride, carboxymethyl cellulose, and polytetrafluoroethylene; and / or, the weight ratio of pickled lithium titanate molecular sieve to conductive agent is (70-85):(5-15); and / or, the conductive agent includes one or more of conductive carbon black, Ketjen black, and carbon nanotubes; and / or, the weight ratio of pickled lithium titanate molecular sieve to solvent is (70-85):(100-300); and / or, the solvent includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide, and propylene glycol. Under the above formula, while ensuring that the electrode has sufficient ion adsorption sites, the structural integrity and conductivity of the electrode can be further optimized, ion transport can be promoted, the electro-chemical reaction efficiency can be improved, and the slurry is easy to coat, so as to ensure the selective adsorption of lithium ions by the electrode in the membrane capacitive deionization (MCDI) module, enhance the cycle stability and service life of the electrode, reduce the internal impedance of the electrode, and achieve fast charge and discharge and high energy density. In a preferred embodiment, in the slurry, the weight ratio of pickled lithium titanate molecular sieve, polyvinylidene fluoride, conductive carbon black, and N-methylpyrrolidone is 80:10:10:200.

[0047] In a preferred embodiment, the thickness of the whisker carbon nanotube membrane is 60-70 μm, and the density is 70-90 g / m 3 , this condition is more conducive to ensuring the structural stability and light weight of the membrane, the porosity is 20-60%, typically but not limited to, the porosity is 20%, 30%, 40%, 50%, 60% or the range value composed of any two of these values, this condition is more conducive to the rapid migration of ions in the electrolyte. The conductivity of the whisker carbon nanotube membrane is 0.015-0.018 kS / cm, this condition can further reduce the internal impedance of the electrode, improve the electro-chemical reaction rate, and the specific surface area is 12-18 m 2 / g, under this condition, there are more adsorption sites for lithium ions and the adsorption efficiency is faster.

[0048] In a preferred embodiment, in step S1, the slurry is coated on one or both sides of the whisker carbon nanotube membrane; and / or, the area percentage of the coating on the whisker carbon nanotube membrane is 50-100%; and / or, the thickness of the coating is 10-250 μm, and the coating density is 70-90 g / m 3 , and the specific surface area is 4-16 m 2 / g, under this condition, the adsorption efficiency and transport speed of lithium ions can be further improved.

[0049] In a preferred embodiment, the drying temperature is 40 - 60 °C and the time is 4 - 8 h. These conditions can ensure the shaping of the coating and the complete removal of the solvent, which is more conducive to improving the electrochemical stability and mechanical strength of the electrode.

[0050] In a preferred embodiment, the adsorption capacity q of the electrode for Li + is 5 - 17 mg / g, the adsorption rate is 15 - 60 mg / (g·h), and the attenuation of the electrode capacity is ≤ 5% after 250 cycles of use. e

[0051] In order to better adapt the components in the membrane capacitive deionization module, in a preferred embodiment, the ion exchange membrane includes Fumasep FAA-3-50 anion exchange membrane and FuMA-Tech cation exchange membrane; and / or, the separator includes polyvinyl fluoride and / or polypropylene.

[0052] In another typical embodiment of the present invention, a membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio is also provided, which is obtained by the above preparation method. The membrane capacitive deionization module of the present application can efficiently adsorb lithium ions in the brine with a high magnesium-lithium ratio, and at the same time can effectively block magnesium ions, significantly improving the recovery rate and purity of lithium resources. Moreover, the high conductivity and large specific surface area of the module can enhance the ion transport efficiency, and its low energy consumption and long cycle life can ensure the stability of operation.

[0053] In a preferred embodiment, the lithium adsorption rate of the membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio is 20 - 70%, the magnesium removal rate is 95 - 99.9%, and the lithium enrichment multiple is 1 - 10 times.

[0054] In another typical embodiment of the present invention, a method for treating brine with a high magnesium-lithium ratio is also provided, including the following steps: forming a closed loop by connecting the above-mentioned membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio to a DC voltage circuit, feeding the brine with a high magnesium-lithium ratio into the membrane capacitive deionization module for treating brine with a high magnesium-lithium ratio, first adsorbing for 10 - 35 min under the conditions of a pressure of 0.8 - 1.6 V and a temperature of 5 - 30 °C, and then desorbing for 10 - 35 min under the conditions of a pressure of -0.8 - 1.6 V, a temperature of 5 - 30 °C and an acidic condition, and circulating 1 - 300 times. As mentioned above, the electrode material of the present application has high selectivity and can effectively adsorb lithium ions while repelling magnesium ions, thereby greatly improving the recovery efficiency and purity of lithium. The high conductivity and large specific surface area of the module can optimize the ion transport path, reduce energy consumption and extend the service life, especially suitable for treating complex brine with a high magnesium-lithium ratio.

[0055] More preferably, hydrochloric acid is used as the desorbing solution during desorption. More preferably, the pH is controlled to be 3 - 5 during desorption.

[0056] There are many types of brines applicable to the membrane capacitive deionization module of the present application. In a preferred embodiment, in the composition of the high magnesium-lithium ratio brine, the content of Li + is 200 - 700 mg / L. Typically but not limitedly, the content of Li + is 200 mg / L, 300 mg / L, 400 mg / L, 500 mg / L, 600 mg / L, 700 mg / L or the range value composed of any two of these numerical values. The content of Mg 2+ is 20000 - 200000 mg / L. Typically but not limitedly, the content of Mg 2+ is 20000 mg / L, 50000 mg / L, 100000 mg / L, 150000 mg / L, 200000 mg / L or the range value composed of any two of these numerical values. The content of K + is 100 - 3000 mg / L. Typically but not limitedly, the content of K + is 100 mg / L, 500 mg / L, 1000 mg / L, 1500 mg / L, 2000 mg / L, 2500 mg / L, 3000 mg / L or the range value composed of any two of these numerical values. The content of Na + is 300 - 6000 mg / L. Typically but not limitedly, the content of Na + is 300 mg / L, 1000 mg / L, 2000 mg / L, 3000 mg / L, 4000 mg / L, 5000 mg / L, 6000 mg / L or the range value composed of any two of these numerical values. The content of Ca 2+ is 500 - 7000 mg / L. Typically but not limitedly, the content of Ca 2+ is 500 mg / L, 1000 mg / L, 2000 mg / L, 3000 mg / L, 4000 mg / L, 5000 mg / L, 6000 mg / L, 7000 mg / L or the range value composed of any two of these numerical values. The salinity is 20 - 40%, typically but not limitedly, the salinity is 20%, 30%, 40% or the range value composed of any two of these numerical values; and / or, the pH of the high magnesium-lithium ratio brine is 3 - 6, typically but not limitedly, the pH of the high magnesium-lithium ratio brine is 3, 4, 5, 6 or the range value composed of any two of these numerical values; and / or, the flow rate of the high magnesium-lithium ratio brine through the membrane capacitive deionization module for treating the high magnesium-lithium ratio brine is 5 - 300 mL / min, typically but not limitedly, the flow rate is 5 mL / min, 50 mL / min, 100 mL / min, 200 mL / min, 300 mL / min or the range value composed of any two of these numerical values.

[0057] The present application will be further described in detail below in conjunction with specific embodiments, and these embodiments should not be construed as limiting the scope claimed in the present application.

[0058] Example 1

[0059] Preparation of pickled lithium titanate molecular sieve: According to the stoichiometric molar ratio (Li:Ti = 2:1), weigh the lithium source (Li 2 CO 3 ) and the titanium source (TiO 2 ), and then ball-mill them for 1 h under the condition of 600 rpm. Subsequently, take them out and place them in a ceramic crucible, and prepare the precursor in a muffle furnace, controlling the heating rate to be 3 °C·min -1 , calcine at a temperature of 800 °C for 4 h, and then control the cooling rate to be 3 °C·min -1 . Then cool it to 25 °C in air to obtain Li 2 TiO 3 , denoted as LTO. Subsequently, use a hydrochloric acid solution with a concentration of 0.2 mol·L -1 to pickle for 24 h at 25 °C and a rotation speed of 25 rpm. The solid-liquid ratio of the lithium titanate molecular sieve to the acid solution is 1 g:250 mL, and replace Li + with H + , then wash, filter, and dry in a vacuum drying oven for 12 h to obtain the pickled lithium titanate molecular sieve (i.e., H 2 TiO 3 , denoted as HTO).

[0060] Preparation of the electrode: According to the weight ratio of 80:10:10:200, mix the pickled lithium titanate molecular sieve, binder (PVDF), conductive agent (conductive carbon black), and solvent (N-methylpyrrolidone) under the condition of 25 rpm for 6 h, and then ball-mill for 2 h under the condition of 600 rpm, controlling the solid particle size in the slurry to be ≤100 μm, then screen and defoam to obtain the slurry. Use an MSK-AFA-DE400-M5 multi-functional coater to coat it on one side of the whisker carbon nanotube film (with a thickness of 60 μm, a density of 80 g / m 3 , a porosity of 50%, a conductivity of 0.017 kS / cm, a specific surface area of 16.98 m 2 / g, and the weight ratio of the pickled lithium titanate molecular sieve to the whisker carbon nanotube film is 1:3). The area percentage of the coating on the whisker carbon nanotube film is 50%, the coating thickness is 25 μm (the substrate thickness is 70 μm), the coating density is 80 g / m 3 , the specific surface area is 12 m 2 / g, the coating width is 220 mm, and the environmental dew point is -30 °C. After coating, dry at 25 °C to obtain the electrode.

[0061] Preparation of the CDI module: Assemble the above-mentioned electrodes, anion exchange membrane (Fumasep FAA-3-50 anion exchange membrane), cation exchange membrane (FuMA-Tech cation exchange membrane), diaphragm (polyvinyl fluoride), module frame, power supply and fluid delivery system to obtain the CDI module, and form a closed loop with a DC voltage circuit. Control the voltage range applied to the anode and cathode by the DC voltage circuit to be 1V.

[0062] Electrochemical desalination application of the CDI module: Use a peristaltic pump to send the saline solution to be treated from the reservoir into the CDI module and finally back to the reservoir. The concentration of the saline solution is 20 g / L, and the flow rate of the saline solution is 15 mL / min. Apply a voltage of 1V to the module using the DC voltage circuit, control the adsorption time to be 20 min, and the temperature to be 25°C for ion adsorption. Then apply the opposite voltage of -1V, control the desorption time to be 20 min, and the temperature to be 25°C for desorption in an acidic environment (pH about 3 - 5) (hydrochloric acid is the desorption solution). Use a conductivity meter to detect the conductivity of the saline solution to be adsorbed in real time at the outlet of the CDI module, and perform ICP testing on the desorbed solution to determine the adsorption capacity.

[0063] The pH of the brine before treatment is 5.4. Specifically, the composition of the brine before and after treatment is shown in Table 1. From the change in the magnesium-lithium ratio, it can be seen that the electrode in this application has a significant effect on reducing the magnesium-lithium ratio and enriching lithium ions. The adsorption capacity of lithium ions can reach more than 6.39 mg / g, and the electrode capacity decreases by less than 5% after 250 cycles.

[0064] Among them, the SEM image of the cross-section of the whisker carbon nanotube membrane is shown in Figure 1 and the SEM image of the surface is shown in Figure 2 From the figure, the fibrous characteristics of the matrix (whisker carbon nanotube membrane) can be seen.

[0065] The SEM image of the cross-section of the electrode is shown in Figure 3 It can be seen that an acid-washed lithium titanium oxide ion sieve coating is formed on the surface of the matrix; the SEM image of the surface is shown in Figure 4 It can be seen that the acid-washed lithium titanium oxide ion sieve is evenly covered, indicating that the lithium titanium oxide ion sieve has been evenly coated on the matrix and no serious caking has occurred; the EDS image is shown in Figure 5 Figure 5 ​Figure A in it is an electron microscope image magnified 10,000 times, Figure B is the distribution map of Ti element, Figure C is the distribution map of F element, Figure D is the distribution map of O element, and Figure E is the distribution map of C element. It can be seen that various elements are evenly distributed, and the active slurry is successfully coated on the substrate. Thanks to the coating with high pseudocapacitance properties (especially the pickled lithium titanate molecular sieve and conductive carbon black among them), this morphology can greatly promote the rapid transmission of charges in the electrolyte solution, so that ions can be better adsorbed during the subsequent brine adsorption process; the cyclic voltammogram of the electrode at a scanning rate of 1 mV / s is shown in Figure 6 , thanks to the property of pickled lithium titanate to store high-density charges and generate Faraday pseudocapacitance, the electrode exhibits great capacitance. The photo of the uncut electrode after coating is shown in Figure 7 .

[0066] Example 2

[0067] The difference from Example 1 is only that:

[0068] After coating, the thickness of the electrode is 100 μm (excluding the substrate).

[0069] Example 3

[0070] The difference from Example 1 is only that:

[0071] After coating, the thickness of the electrode is 150 μm (excluding the substrate).

[0072] Example 4

[0073] The difference from Example 1 is only that:

[0074] After coating, the thickness of the electrode is 200 μm (excluding the substrate).

[0075] Example 5

[0076] The difference from Example 1 is only that:

[0077] The process of pickled lithium titanate molecular sieve is different. Specifically, according to the stoichiometric molar ratio (Li:Ti = 1.5:1), weigh the lithium source (lithium hydroxide) and the titanium source (TiO 2 ), and then mix them at 300 rpm for 12 h. Then take it out and place it in a ceramic crucible to prepare the precursor in a muffle furnace, control the heating rate at 3 °C·min -1 , calcine at 700 °C for 24 h, and then control the cooling rate at 3 °C·min -1 . Then cool it to 25 °C in air to obtain lithium titanate. Subsequently, use a concentration of 0.1 mol·L -1The hydrochloric acid solution was pickled for 48 h at 60 °C with a rotation speed of 10 rpm. The solid-liquid ratio of lithium titanate molecular sieve to acid solution was 1 g: 200 mL. Replace Li + with H + , then wash, filter, and dry in a vacuum drying oven for 12 h to obtain pickled lithium titanate molecular sieve.

[0078] Example 6

[0079] The difference from Example 1 is only that:

[0080] The process of pickling lithium titanate molecular sieve is different. Specifically, according to the stoichiometric molar ratio (Li:Ti = 2.5:1), weigh the lithium source (lithium chloride) and the titanium source (TiCl 4 ), and then mix them at 600 rpm for 0.5 h. Then take it out and place it in a ceramic crucible to prepare the precursor in a muffle furnace, control the heating rate at 3 °C·min -1 , calcine at 1000 °C for 2 h, and then control the cooling rate at 3 °C·min -1 . Then cool it to 25 °C in air to obtain lithium titanate. Subsequently, use a nitric acid solution with a concentration of 0.6 mol·L -1 , pickling for 6 h at 10 °C with a rotation speed of 50 rpm. The solid-liquid ratio of lithium titanate molecular sieve to acid solution is 1 g: 300 mL. Replace Li + with H + , then wash, filter, and dry in a vacuum drying oven for 12 h to obtain pickled lithium titanate molecular sieve.

[0081] Example 7

[0082] The difference from Example 1 is only that:

[0083] The composition and preparation method of the slurry are different. Specifically, according to the weight ratio of 70:5:15:100, mix HTO, binder (carboxymethyl cellulose), conductive agent (Ketjen black), and solvent (N,N-dimethylformamide) at 25 rpm for 6 h, then ball mill at 600 rpm for 2 h, control the solid particle size in the slurry ≤100 μm, then screen and defoam to obtain the slurry.

[0084] Example 8

[0085] The difference from Example 1 is only that:

[0086] The composition and preparation method of the slurry are different. Specifically, HTO, binder (polytetrafluoroethylene), conductive agent (carbon nanotubes), and solvent (dimethyl sulfoxide) are mixed at a weight ratio of 85:15:5:300 under the condition of 25 rpm for 6 h, and then ball-milled at 600 rpm for 2 h. The solid particle size in the slurry is controlled to be ≤ 100 μm, and then it is sieved and degassed to obtain the slurry.

[0087] Example 9

[0088] The difference from Example 1 is only that:

[0089] The parameters of the whisker carbon nanotube film and the coating process parameters are different. Specifically, the slurry is coated on one side of the whisker carbon nanotube film (with a thickness of 20 μm, a density of 90 g / m 3 , a porosity of 20%, a conductivity of 0.015 kS / cm, a specific surface area of 12 m 2 / g, and the weight ratio of acid-washed lithium titanate molecular sieve to the whisker carbon nanotube film is 1:2). The area percentage of the coating in the whisker carbon nanotube film is 100%, the coating thickness is 10 μm, the coating density is 70 g / m 3 , the specific surface area is 4 m 2 / g, the coating width is 220 mm, the environmental dew point is -30°C, and after coating, it is dried at 25°C to obtain the electrode.

[0090] Example 10

[0091] The difference from Example 1 is only that:

[0092] The parameters of the whisker carbon nanotube film and the coating process parameters are different. Specifically, the slurry is coated on one side of the whisker carbon nanotube film (with a thickness of 60 μm, a density of 70 g / m 3 , a porosity of 60%, a conductivity of 0.018 kS / cm, a specific surface area of 18 m 2 / g, and the weight ratio of acid-washed lithium titanate molecular sieve to the whisker carbon nanotube film is 1:4). The area percentage of the coating in the whisker carbon nanotube film is 50%, the coating thickness is 250 μm, the coating density is 90 g / m 3 , the specific surface area is 16 m 2 / g, the coating width is 220 mm, the environmental dew point is -30°C, and after coating, it is dried at 25°C to obtain the electrode.

[0093] Example 11

[0094] The difference from Example 1 is only that:

[0095] The flow rate of the brine, as well as the parameters of adsorption and desorption, are different. Specifically, a peristaltic pump is used to feed the salt-containing solution to be treated from the reservoir into the CDI module and finally back to the reservoir. The concentration of the salt-containing solution is 20 g / L, and the flow rate of the salt-containing solution is 5 mL / min. A voltage of 0.8 V is applied to the module using a DC voltage circuit, and the adsorption time is controlled to be 35 min, the temperature is 5 °C for ion adsorption. Then, an opposite voltage is applied, and the desorption time is controlled to be 35 min, the temperature is 5 °C for desorption. A conductivity meter is used to detect the conductivity of the salt solution to be adsorbed in real time at the outlet of the CDI module to determine the adsorption capacity. This step is repeated 1 time.

[0096] Example 12

[0097] The difference from Example 1 is only that:

[0098] The flow rate of the brine, as well as the parameters of adsorption and desorption, are different. Specifically, a peristaltic pump is used to feed the salt-containing solution to be treated from the reservoir into the CDI module and finally back to the reservoir. The concentration of the salt-containing solution is 20 g / L, and the flow rate of the salt-containing solution is 300 mL / min. A voltage of 1.6 V is applied to the module using a DC voltage circuit, and the adsorption time is controlled to be 10 min, the temperature is 30 °C for ion adsorption. Then, an opposite voltage is applied, and the desorption time is controlled to be 10 min, the temperature is 30 °C for desorption. A conductivity meter is used to detect the conductivity of the salt solution to be adsorbed in real time at the outlet of the CDI module to determine the adsorption capacity. This step is repeated 300 times.

[0099] Comparative Example 1

[0100] The difference from Example 1 is only that: the lithium titanate molecular sieve is not pickled.

[0101] Analysis shows that in the lithium ion sieve without pickling, specific lithium active adsorption sites are reduced, and only the double-capacitance adsorption mechanism of the electrode material is used to adsorb ions without selectivity, and the ion exchange reaction between the lithium titanate ion sieve and the brine solution does not occur, resulting in poor adsorption capacity for lithium ions.

[0102] Comparative Example 2

[0103] The difference from Example 1 is only that: the pickling concentration is different. Specifically, a hydrochloric acid solution with a concentration of 0.01 mol·L -1 is used to pickle for 24 h at 25 °C and a rotation speed of 25 rpm. The solid-liquid ratio of the lithium titanate molecular sieve to the acid solution is 1 g:250 mL. Replace Li + with H + , then wash, filter, and dry in a vacuum drying oven for 12 h to obtain the pickled lithium titanate molecular sieve.

[0104] Analysis shows that when the pickling concentration decreases, the relative number of lithium-ion adsorption vacancies in the lithium-ion sieve decreases compared to Example 1, and the adsorption effect deteriorates.

[0105] Comparative Example 3

[0106] The difference from Example 1 is only that a carbon cloth is used as the current collector, and the carbon cloth is an activated carbon fiber cloth (model HCP331P, with hydrophilicity).

[0107] Analysis shows that the adsorption effect decreases because the conductivity and hydrophilicity of the carbon cloth are weaker than those of the whisker carbon nanotube film, the slurry is not easily penetrated into the matrix material, and the ion channel conduction path decreases.

[0108] Comparative Example 4

[0109] The difference from Example 1 is only that a titanium sheet (with a thickness of 0.1 mm) is used as the current collector.

[0110] Analysis shows that compared with the whisker carbon nanotube film, the titanium sheet has poor conductivity, flexibility and bendability, and moreover, it has a small surface area, provides fewer adsorption active sites, and has poor adsorption of lithium ions.

[0111] The performance test results of the membrane capacitive deionization modules prepared in the above examples and comparative examples are shown in Table 2.

[0112] Test method:

[0113] Embedding capacity of lithium ions: Electrochemical tests were carried out using an electrochemical workstation with a three-electrode system. Using the lithium titanate@carbon nanotube film as the working electrode, a platinum sheet as the counter electrode, and Ag / AgCl as the reference electrode, within an electrochemical window of 0.01 V to 1 V, CV tests were carried out at a scan rate of 1 mV / s. The calculation formula for the specific capacitance C (F / g) is:

[0114] In the formula, V 1 、V 2 are the starting potential and the ending potential, with the unit of V,

[0115] i is the response current, with the unit of A,

[0116] m is the effective electrode mass, with the unit of g,

[0117] v is the scan rate, with the unit of V / s,

[0118] ΔV is the voltage change, with the unit of V.

[0119] The calculation formula for the embedding capacity of lithium ions (Γ, mg / g) is:

[0120] In the formula, C 0is the initial concentration of lithium ions, in mg / L, C t is the lithium ion concentration at time t (in s), in mg / L, V is the volume of the brine before treatment, in L, m is the total mass of the electrode (including the substrate), in g, (in this application, m is 17 mg).

[0121] Adsorption rate: The test of the saturated adsorption capacity of the salt solution was carried out using a customized MCDI module, in which the CHI-760e electrochemical workstation was used to operate the charging and discharging processes in the potentiostatic mode (1 V to -1 V). After assembling the electrodes into the MCDI module, the solution to be adsorbed (brine solution) was pumped into the above MCDI module by a BT100-3J peristaltic pump, and the conductivity of the solution was recorded online by a DDBJ-350F handheld conductivity meter. The external size of the membrane electrode is about 2 cm × 2 cm. The thickness of the electrode is about 100 μm. The adsorption was carried out for 20 minutes, and the changes in the ion content of the brine solution before and after adsorption were tested.

[0122] Amount of electrode capacity decay after 250 cycles of use: All GCD tests were carried out on a CHI-760E electrochemical workstation using a three-electrode system, which included the sample to be tested as the working electrode, a Pt sheet electrode as the counter electrode, and an Ag / AgCl electrode as the reference electrode, and 1 M LiCl was selected as the electrolyte solution in the GCD test. All GCD tests were carried out at different current densities (0.25, 0.5, 1, and 2 A / g) in the electrochemical window of -1 to +1 V based on Ag / AgCl as the reference electrode, and the cyclic stability of the electrode sample was tested at a current density of 1 A / g.

[0123] Lithium adsorption rate: The test of the saturated adsorption capacity of the salt solution was carried out using a customized MCDI module, in which the CHI-760e electrochemical workstation was used to operate the charging and discharging processes in the potentiostatic mode (1 V to -1 V). After assembling the electrodes into the MCDI module, the solution to be adsorbed (brine solution) was pumped into the above MCDI module at an injection rate of 20 mL / min by a BT100-3J peristaltic pump, and the conductivity of the solution was recorded online by a DDBJ-350F handheld conductivity meter. The external size of the membrane electrode is about 2 cm × 2 cm. Saturated adsorption was carried out, and the changes in the ion content of the brine solution before and after adsorption were tested.

[0124] Magnesium removal rate: During the test of the saturated adsorption capacity of salt solution using a customized MCDI module, a CHI-760e electrochemical workstation was used to operate the charging and discharging processes in a constant potential mode (from 1V to -1V). After assembling the electrodes into the MCDI module, the solution to be adsorbed (brine solution) was pumped into the above MCDI module at an injection rate of 20 mL / min by a BT100-3J peristaltic pump, and the conductivity of the solution was recorded online by a DDBJ-350F handheld conductivity meter. The external dimensions of the membrane electrode were approximately 2 cm × 2 cm. Saturated adsorption was carried out to test the changes in the ion content of the brine solution before and after adsorption.

[0125] Lithium enrichment multiple: During the test of the saturated adsorption capacity of salt solution using a customized MCDI module, a CHI-760e electrochemical workstation was used to operate the charging and discharging processes in a constant potential mode (from 1V to -1V). After assembling the electrodes into the MCDI module, the solution to be adsorbed (brine solution) was pumped into the above MCDI module at an injection rate of 20 mL / min by a BT100-3J peristaltic pump, and the conductivity of the solution was recorded online by a DDBJ-350F handheld conductivity meter. The external dimensions of the membrane electrode were approximately 2 cm × 2 cm. Both adsorption and desorption were carried out for 20 minutes, and the adsorption and desorption cycles were repeated 10 times to test the changes in the ion content of the brine solution and the desorbed solution before and after adsorption and desorption.

[0126] Table 1

[0127]

[0128]

[0129] Table 2

[0130]

[0131] As can be seen from the above, the membrane capacitive deionization module of the present invention utilizes the high selectivity of pickled lithium titanate molecular sieve to achieve efficient adsorption of lithium ions. Using whisker carbon nanotube film as the current collector, its high conductivity and large specific surface area not only simplify the module structure but also promote the rapid transmission of charges and the uniform adsorption of lithium ions. The combination of this electrode design and the MCDI module not only improves the electrochemical performance, such as the charging / discharging rate and adsorption capacity, but also has the characteristics of environmental friendliness and economy, and is applicable to the treatment of brines with different salinities, especially brines with a high magnesium-lithium ratio.

[0132] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a membrane capacitor deionization module for treating brine with a high magnesium-to-lithium ratio, characterized in that: The magnesium-lithium ratio of the brine is 300-700, and the preparation method comprises the following steps: Step S1, sequentially mixing and first ball-milling the acid-washed lithium titanate molecular sieve, a binder, a conductive agent and a solvent to obtain a slurry; coating the slurry on the surface of the whisker carbon nanotube film, and drying to form a coating on the surface of the whisker carbon nanotube film; Step S2, using the whisker carbon nanotube film including the coating as an electrode, assembling the electrode, ion exchange membrane, diaphragm, module frame, power supply and fluid delivery system to obtain a membrane capacitive deionization module; Wherein, the weight ratio of the acid-washed lithium titanate molecular sieve to the whisker carbon nanotube film is 1:(2-4).

2. The preparation method according to claim 1, characterized in that: The method for preparing the acid-washed lithium titanate molecular sieve comprises the following steps: treating the lithium titanate molecular sieve with an acid solution to obtain the acid-washed lithium titanate molecular sieve; The acid in the acid solution includes one or more of hydrochloric acid, sulfuric acid and nitric acid, the concentration of the acid is 0.1-0.6 mol / L, the solid-liquid ratio of the lithium titanate molecular sieve to the acid solution is 1 g: (200-300) mL; the treatment is carried out under stirring at a temperature of 10-60° C., a rotation speed of 10-50 rpm, and a time of 6-48 h; and / or, The preparation method of the lithium titanate molecular sieve comprises the following steps: subjecting a lithium source and a titanium source to a second ball milling, roasting and cooling in sequence to obtain the lithium titanate molecular sieve; Wherein, the lithium source comprises one or more of lithium hydroxide, lithium carbonate and lithium chloride, the titanium source comprises titanium oxide and / or titanium chloride, and the molar ratio of the lithium source to the titanium source is (1.5-2.5):1; and / or, the rotation speed of the second ball mill is 300-600 rpm, and the time is 0.5-12 h; and / or, The calcination temperature is 700-1000° C., and the heat preservation time is 2-24 hours.

3. The preparation method according to claim 1 or 2, characterized in that: In the slurry, The weight ratio of the acid-washed lithium titanate molecular sieve to the binder is (70-85):(5-15); and / or, the binder comprises one or more of polyvinylidene fluoride, carboxymethyl cellulose and polytetrafluoroethylene; and / or, The weight ratio of the acid-washed lithium titanate molecular sieve to the conductive agent is (70-85):(5-15); and / or, the conductive agent includes one or more of conductive carbon black, Ketjen black and carbon nanotubes; and / or, The weight ratio of the acid-washed lithium titanate molecular sieve to the solvent is (70-85):(100-300); and / or the solvent includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide and propylene glycol.

4. The preparation method according to any one of claims 1 to 3, characterized in that The whisker carbon nanotube film has a thickness of 60 to 70 μm and a density of 70 to 90 g / m 3 , a porosity of 20 to 60%; and / or, The electrical conductivity of the whisker carbon nanotube film is 0.015-0.018 kS / cm, and the specific surface area is 12-18 m 2 / g.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In the step S1, the slurry is coated on one side or both sides of the whisker carbon nanotube film; and / or, The coating accounts for 50-100% of the area of ​​the whisker carbon nanotube film; and / or, The coating has a thickness of 10 to 250 μm and a coating density of 70 to 90 g / m 3 , with a specific surface area of ​​4 to 16 m 2 / g.

6. The preparation method according to any one of claims 1 to 5, characterized in that The electrode pair Li + The adsorption capacity q e The adsorption rate is 5-17 mg / g, the adsorption rate is 15-60 mg / (g·h), and the electrode capacity decay is ≤5% after 250 cycles; and / or, The ion exchange membrane comprises a Fumasep FAA-3-50 anion exchange membrane and a FuMA-Tech cation exchange membrane; and / or, The diaphragm includes polyvinyl fluoride and / or polypropylene.

7. A membrane capacitor deionization module for treating high magnesium-lithium ratio brine, characterized in that: The method is obtained by the preparation method according to any one of claims 1 to 6.

8. The membrane capacitor deionization module for treating high magnesium-lithium ratio brine according to claim 7, characterized in that: The membrane capacitor deionization module for treating brine with a high magnesium-to-lithium ratio has a lithium adsorption rate of 20-70%, a magnesium removal rate of 95-99.9%, and a lithium enrichment multiple of 1-10 times.

9. A method for treating brine with a high magnesium-lithium ratio, characterized in that: The following steps are involved: The membrane capacitor deionization module for treating brine with a high magnesium-to-lithium ratio as described in claim 7 or 8 is connected to a DC voltage circuit to form a closed loop, and the brine with a high magnesium-to-lithium ratio is fed into the membrane capacitor deionization module for treating brine with a high magnesium-to-lithium ratio. It is first adsorbed for 10 to 35 minutes at a pressure of 0.8 to 1.6 V and a temperature of 5 to 30° C., and then desorbed for 10 to 35 minutes at a pressure of -0.8 to -1.6 V and a temperature of 5 to 30° C. under acidic conditions, and the cycle is repeated 1 to 300 times.

10. The processing method according to claim 9, characterized in that: The high magnesium-lithium ratio brine contains Li + The content is 200~700mg / L, Mg 2+ The content is 20000~200000mg / L, K + The content is 100~3000mg / L, Na + The content is 300~6000mg / L, Ca 2+ The content is 500-7000 mg / L and the salinity is 20-40%; and / or, The pH of the high magnesium-lithium ratio brine is 3-6; and / or, The flow rate of the high magnesium to lithium ratio brine passing through the membrane capacitor deionization module for treating the high magnesium to lithium ratio brine is 5 to 300 mL / min.

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