Preparation method of high-purity lithium chloride

Abstract

The invention provides a preparation method of high-purity lithium chloride, and relates to the technical field of lithium chloride purification. Comprising the following steps: providing a hydrophilic porous fiber rope with a capillary effect, vertically arranging the porous fiber rope on the liquid level of a lithium chloride solution, immersing one end of the porous fiber rope in the lithium chloride solution, carrying out evaporative crystallization to obtain a lithium chloride salt shell, dissolving the lithium chloride salt shell in water, filtering, and drying to obtain high-purity lithium chloride, according to the method, no chemical agent is used in the lithium enrichment process, and the problem that sodium is difficult to remove in the lithium chloride purification process is fundamentally solved.

Description

Technical Field

[0001] The present invention relates to the technical field of preparing high-purity lithium chloride, and particularly relates to a method for preparing high-purity lithium chloride. Background Art

[0002] Lithium is the lightest metal in the world. Due to some special properties, lithium and its compounds have a wide range of special uses and are known as "energy metals" and "metals that drive the world forward". It plays an extremely important role in the energy industry, aerospace industry, metal smelting and manufacturing industries, refrigeration, ceramics, glass and other industries. At present, the global implementation of the dual-carbon and new energy policies has an urgent demand for lithium products, and the demand for lithium has been increasing rapidly year by year.

[0003] High-purity lithium and lithium compounds generally require a purity of > 99.9%. The starting materials for producing lithium are lithium chloride and lithium carbonate, so strict requirements are also imposed on their purity. The impurities that are difficult to separate in lithium and lithium compounds are potassium, sodium, calcium, and magnesium, especially sodium. Due to the extremely similar properties of alkali metals, sodium in lithium and lithium compounds is extremely difficult to remove. In the existing application environments of lithium compounds, the presence of impurity sodium ions in lithium compounds is required to be relatively strict, generally ≤ 15 ppm. Therefore, in a sense, removing impurity sodium ions has become one of the keys to the quality control of high-purity lithium and lithium compounds, and it is also difficult to completely eliminate impurity sodium ions. Summary of the Invention

[0004] (I) Technical Problems to be Solved

[0005] Aiming at the deficiencies of the prior art, the present invention provides a method for preparing high-purity lithium chloride, which solves the technical problems that in the existing methods for preparing high-purity lithium chloride, chemical agents are used and sodium is difficult to remove.

[0006] (II) Technical Solutions

[0007] To achieve the above object, the present invention is realized through the following technical solutions:

[0008] The present invention provides a method for preparing high-purity lithium chloride, including the following steps: providing a porous fiber rope with capillary effect and hydrophilic material, the porous fiber rope is suspended in groups above the lithium chloride solution surface, and one end is immersed in the lithium chloride solution. Each component in the solution migrates to different positions above the rope due to different capillary forces of the fiber rope. Controlling the evaporation crystallization conditions of the saturated salt of the fiber rope to obtain a lithium chloride salt shell, dissolving the lithium chloride salt shell in water, filtering and drying to obtain high-purity lithium chloride.

[0009] In the present invention, a porous fiber rope is suspended above the liquid level of a lithium chloride solution, with one end immersed in the lithium chloride solution for evaporation crystallization to obtain a lithium chloride salt shell. The lithium chloride salt shell is dissolved in water, filtered, and dried to obtain high-purity lithium chloride. No chemical agents are used in the process of preparing high-purity lithium chloride, fundamentally solving the problem of difficult removal of sodium during the purification of lithium chloride.

[0010] Preferably, the porous fiber rope includes a rope core and multiple groups of porous fiber bundles, and the multiple groups of porous fiber bundles are arranged on the outer peripheral part of the rope core.

[0011] The porous fiber rope includes a rope core and multiple groups of porous fiber bundles. The rope core provides a supporting function and improves the tensile strength of the porous fiber rope.

[0012] Preferably, the porous fiber bundle includes porous fibers, and the porous fibers are modified cellulose acetate / polyacrylonitrile blend fibers.

[0013] In this application, the hydrophilicity of the modified cellulose acetate / polyacrylonitrile blend fiber is appropriate. When the brine migrates upward along the porous fiber bundle of this application, the water evaporation rate is moderate, causing sodium chloride and potassium chloride to reach supersaturation while lithium chloride does not reach saturation. Therefore, sodium chloride and potassium chloride precipitate at the section 0.5 - 0.6 m above the liquid level of the porous fiber bundle. Then, the brine without sodium chloride continues to migrate upward along the porous fiber bundle, and water continues to be evaporated. During the upward migration process, the water evaporation rate is moderate, causing lithium chloride to reach supersaturation. Therefore, lithium chloride precipitates at the section 0.8 - 1.5 m, thus realizing the purification of lithium chloride.

[0014] Preferably, the modified cellulose acetate / polyacrylonitrile blend fiber includes cellulose acetate, acrylonitrile, vinyl acetate, and a cross-linking agent. The mass ratio of cellulose acetate to the other components is 5 - 40:95 - 60. The capillary pore size distribution of the fiber is 1 nm - 30 nm, the porosity is 20% - 50%, and the contact angle of pure water on the fiber surface is less than 90°;

[0015] In this application, the surface contact angle of the modified cellulose acetate / polyacrylonitrile blend fiber is less than 90°. When the brine migrates upward along the porous fiber bundle of this application for 50 cm, the water evaporation rate is moderate, causing sodium chloride and potassium chloride to reach supersaturation while lithium chloride does not reach saturation. Therefore, sodium chloride and potassium chloride precipitate at the section 0.5 - 0.6 m above the liquid level of the porous fiber bundle. Then, the brine without sodium chloride continues to migrate upward along the porous fiber bundle, and water continues to be evaporated. During the upward migration of 20 cm, the water evaporation rate is moderate, causing lithium chloride to reach supersaturation. Therefore, lithium chloride precipitates at the section 0.8 - 1.5 m, thus realizing the purification of lithium chloride.

[0016] Furthermore, for the modified cellulose acetate / polyacrylonitrile blend fiber, the mass ratio of cellulose acetate to other components is 10-30:90-70. The capillary pore size distribution of the modified cellulose acetate / polyacrylonitrile blend fiber is 1 nm - 20 nm, the porosity is 25% - 45%, and the contact angle of pure water on the surface of the modified cellulose acetate / polyacrylonitrile blend fiber is 30 - 60°.

[0017] The surface contact angle of the modified cellulose acetate / polyacrylonitrile blend fiber in this application is 30 - 60°. When the brine migrates upward along the porous fiber bundle of this application, the water evaporation rate is moderate, making sodium chloride and potassium chloride reach supersaturation and lithium chloride not reach saturation. Therefore, sodium chloride and potassium chloride precipitate at the section 0.5 - 0.6 m above the liquid level of the porous fiber bundle. Then, the brine without sodium chloride continues to migrate upward along the porous fiber bundle, and water continues to be evaporated. During the upward migration, the water evaporation rate is moderate, making lithium chloride reach supersaturation. Therefore, lithium chloride precipitates at the section 0.8 - 1.5 m, thus realizing the purification of lithium chloride.

[0018] Preferably, the dry preparation method of the modified cellulose acetate / polyacrylonitrile blend fiber includes the following steps:

[0019] S1. After melting cellulose acetate, add polyacrylonitrile, vinyl acetate, maleic anhydride graft copolymer, and methylbenzenesulfonic acid, and continue melting and kneading to obtain a kneaded material. After cooling and pressing the kneaded material into slices, pre-crystallize the slices, and then dry them until the moisture content of the slices < 30 ppm, and perform melt spinning to obtain cellulose acetate / polyacrylonitrile blend fiber;

[0020] S2. Immerse the cellulose acetate / polyacrylonitrile blend fiber completely below the liquid level of a 25% - 27% by mass ethylenediamine solution, and modify the cellulose acetate / polyacrylonitrile blend fiber to obtain the modified cellulose acetate / polyacrylonitrile blend fiber.

[0021] The method of the present invention uses ethylenediamine modification to increase the proportion of hydrophilic groups of the modified cellulose acetate / polyacrylonitrile blend fiber and control its hydrophilicity within a suitable range. The surface contact angle of the modified cellulose acetate / polyacrylonitrile blend fiber is 30 - 60°. When the brine migrates upward along the porous fiber bundle of this application, under the evaporation conditions of this application, the water evaporation rate is moderate, making sodium chloride and potassium chloride reach supersaturation and lithium chloride not reach saturation. Therefore, sodium chloride and potassium chloride precipitate at the section 0.5 - 0.6 m above the liquid level of the porous fiber bundle. Then, the brine without sodium chloride continues to migrate upward along the porous fiber bundle, and water continues to be evaporated. During the upward migration, the water evaporation rate is moderate, making lithium chloride reach supersaturation. Therefore, lithium chloride precipitates at the section 0.8 - 1.5 m, thus realizing the purification of lithium chloride.

[0022] Preferably, the wet preparation method of the modified cellulose acetate / polyacrylonitrile blend fiber comprises the following steps:

[0023] S1. Blend cellulose acetate, polyacrylonitrile, vinyl acetate, and maleic anhydride graft copolymer in different ratios, and prepare a spinning dope using dimethylacetamide (DMAC) or dimethylformamide (DMF) as a solvent. Adopt the wet spinning process, use a DMAC solution or ethanol solution with a certain concentration as a coagulation bath, and perform wet spinning through stretching, relaxation, and winding to obtain the as-spun cellulose acetate / polyacrylonitrile blend fiber;

[0024] S2. Immerse the cellulose acetate / polyacrylonitrile blend fiber completely below the liquid level of a 3%-5% crosslinking agent HQ solution by mass fraction, and carry out thermal crosslinking treatment on the cellulose acetate / polyacrylonitrile blend fiber under certain temperature and time conditions; then place it in a 2%-10% alkali solution for hydrolysis, controlling the alkali solution concentration, temperature, and time. After hydrolysis, take it out and neutralize its surface with a weak acid, and then wash it repeatedly with distilled water to obtain the modified cellulose acetate / polyacrylonitrile blend fiber.

[0025] The method of the present invention uses 5% crosslinking agent HQ and 5% alkali solution for modification to increase the proportion of hydrophilic groups in the modified cellulose acetate / polyacrylonitrile blend fiber and control its hydrophilicity within a suitable range. The surface contact angle of the modified cellulose acetate / polyacrylonitrile blend fiber is 30-60°. When the brine migrates upward along the porous fiber bundle of the present application, under the evaporation conditions of the present application, the water evaporation amount is moderate, so that sodium chloride and potassium chloride reach supersaturation and lithium chloride does not reach saturation. Therefore, sodium chloride and potassium chloride precipitate at the 0.5-0.6 m section above the liquid level of the porous fiber bundle. Then, the brine without sodium chloride continues to migrate upward along the porous fiber bundle, and the water continues to be evaporated. During the upward migration process, the water evaporation amount is moderate, so that lithium chloride reaches supersaturation. Therefore, lithium chloride precipitates at the 0.8-1.5 m section, thereby realizing the purification of lithium chloride.

[0026] Preferably, the conditions for evaporation crystallization include: the temperature is 20-50°C, the humidity is above 75%, and the evaporation amount in 24 hours is 100-300 mm.

[0027] The evaporation conditions of this application are as follows: the temperature is 20 - 50°C, the humidity is above 75%, the evaporation amount in 24 hours is 100 - 300 mm, and the surface contact angle of the modified cellulose acetate / polyacrylonitrile blend fiber is 30 - 60°. When the brine migrates upward along the porous fiber bundle of this application, under this evaporation condition, the water evaporation amount is moderate, enabling sodium chloride and potassium chloride to reach supersaturation while lithium chloride does not reach saturation. Therefore, sodium chloride and potassium chloride precipitate at the section 0.5 - 0.6 m above the liquid level of the porous fiber bundle. Then, the brine without sodium chloride continues to migrate upward along the porous fiber bundle, and water continues to be evaporated. During the upward migration process, the water evaporation amount is moderate, causing lithium chloride to reach supersaturation. Therefore, lithium chloride precipitates at the section 0.8 - 1.5 m, thus realizing the purification of lithium chloride.

[0028] Preferably, industrial-grade lithium chloride with a purity of not less than 97 wt% is dissolved in water to obtain the lithium chloride solution; the mass ratio of the industrial-grade lithium chloride to water is 1.5 - 2.5:1.0, and the dissolution temperature is 25 - 35°C.

[0029] Preferably, the density of the lithium chloride solution is 1.1 - 1.4 g / cm 3 。

[0030] Preferably, the purity of the high-purity lithium chloride is greater than 99.95%.

[0031] Preferably, the core is a steel wire rope core. The steel wire rope core is convenient for improving the tensile strength of the porous fiber.

[0032] (III) Beneficial effects

[0033] The present invention provides a method for preparing high-purity lithium chloride. Compared with the prior art, it has the following beneficial effects:

[0034] (1) It simplifies and shortens the process flow, greatly reduces energy consumption, the equipment is easy to configure, install and transfer, and is extremely easy to promote and apply, and can carry out industrial demonstration and large-scale production;

[0035] (2) No chemical agents are used in the process of preparing high-purity lithium chloride, and clean production is realized as much as possible; fundamentally solves the problem that it is difficult to remove sodium during the purification process of lithium chloride.

[0036] (3) After removing the salt crust, the porous fiber rope can be reused during the purification process; the lithium chloride brine can be reused, and the comprehensive recovery rate is high.

[0037] (4) In the lithium chloride product produced by the present invention, the purity of lithium chloride > 99.9%, and the impurity content can be reduced to: Na ≤ 10 ppm, Ca ≤ 20 ppm, Mg ≤ 10 ppm, Fe ≤ 7 ppm. Description of the drawings

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0039] Figure 1 Process flow chart of the preparation method of high-purity lithium chloride provided for Example 1;

[0040] Figure 2 Schematic structural diagram of the capillary crystallization lithium extraction pool;

[0041] Figure 3 Schematic structural diagram of the porous fiber rope group;

[0042] Figure 4 Schematic cross-sectional structure diagram of the porous fiber rope;

[0043] Figure 5 Schematic structural diagram of the lithium extraction insulation chamber;

[0044] Figure 6 Schematic structural diagram of the control system. Specific embodiments

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0046] By providing a preparation method of high-purity lithium chloride in the embodiments of the present application, the technical problems in the existing methods for preparing high-purity lithium chloride, including the use of chemical agents and the difficulty in removing sodium, are solved.

[0047] The general idea of the technical solutions in the embodiments of the present application to solve the above technical problems is as follows:

[0048] Removing sodium chloride from lithium chloride is the bottleneck in the preparation process of high-purity lithium chloride. To develop a more cost-effective separation process route, scientific and technological personnel at home and abroad have conducted extensive exploration and research on this issue. In salt lake lithium chloride and lithium chloride recovered from waste, there are often relatively high contents of sodium chloride impurities. General purification methods include solvent extraction method, ion exchange method, salting-out method, thermal concentration crystallization method, cooling crystallization method, etc. These methods all have their defects. Currently, in the existing technologies and patents, sulfate removal and calcium removal operations need to be carried out on the lithium chloride solution at the front end. Usually, barium chloride is added to remove sulfate, then calcium or excess barium is removed with carbonate, then excess carbonate is removed with hydrochloric acid, and then the pH is back-adjusted to neutral with alkali. Then, impurity removal operations are carried out on the lithium chloride solution to obtain qualified anhydrous lithium chloride products.

[0049] (1) Solvent extraction method: The solvent extraction method for removing impurity sodium chloride is mainly based on the solubility difference between lithium chloride and sodium chloride in different solvents, that is, lithium chloride dissolves in organic solvents while inorganic impurities such as sodium, potassium, magnesium, and sulfates do not dissolve in them. By using the extraction effect of organic solvents on lithium chloride, the impurities are left in the mother liquor to achieve the purpose of removing impurity sodium chloride. Generally, alcohol extractants are used to dissolve and extract lithium chloride, and the lithium chloride product is obtained by evaporation to dryness and separation. This method has good effects, can obtain relatively pure lithium chloride products, and can also be used to produce battery-grade lithium chloride products. However, alcohol extractants consume a large amount of energy during evaporation to dryness, and at the same time, a large amount of flammable and explosive organic gases are generated, with extremely high safety risks, complex production routes, high costs, and also the risk of environmental pollution.

[0050] (2) Ion exchange method: The ion exchange method mainly uses Li + in the solid ion exchanger containing lithium to + exchange with Na + in the solution to achieve the purpose of removing impurity sodium in the solution. It is found that compounds with a LiTi2(PO4)3 framework structure have the best lattice size for Li + migration and have high selectivity for Na + in aqueous solution. When Li + with a relatively high diffusion coefficient and conductivity is replaced by Na

[0051] in the solution, LiTi2(PO4)3 will be transformed into NaTi2(PO4)3, thus achieving the purpose of removing impurity sodium chloride. The ion exchange adsorption method for removing impurity sodium chloride has the characteristics of mild conditions, remarkable effects, and environmental friendliness; however, there are problems such as complex preparation of ion exchange adsorbents requiring multiple high-temperature calcinations, small particles being difficult to recover, not being reusable, and high treatment costs.(3) The salting-out method is a method that reduces the precipitation concentration of a certain substance in the system and the remaining concentration of the precipitate in the system in the presence of a salting-out agent. Although the salting-out method has a simple process flow and low cost, currently, only by adding an alkaline substance (lithium hydroxide) can the mass fraction of sodium chloride in the finally obtained solid-phase lithium chloride be reduced to 0.25%. However, only by adding lithium hydroxide can lithium chloride be generated through the subsequent reaction with hydrochloric acid. If other alkaline substances are added, new impurities will be introduced, increasing the post-treatment process. The acid precipitation method will cause equipment corrosion problems, and it is difficult to find suitable materials in the industrialization process. There are problems such as insufficient removal rate of sodium chloride by the salting-out method, the product purity not reaching the high-purity level, and the high cost of adding lithium hydroxide.

[0052] (4) Thermal concentration crystallization method: The lithium chloride solution containing sodium chloride is evaporated to supersaturation, and using the common ion effect, at this time, sodium chloride crystallizes out first and is separated to remove sodium chloride. This method is extremely difficult to control the precipitation concentration of sodium chloride, and when the concentration is relatively high, sodium chloride and lithium chloride will crystallize out simultaneously, and the final removal effect of sodium chloride is very poor, and the obtained product cannot meet the standard requirements of industrial-grade lithium chloride;

[0053] (5) Cooling crystallization method: Lithium hydroxide is added to the solution concentrated to supersaturation, and sodium chloride precipitates out under freezing conditions and is separated to remove. This method has a relatively thorough precipitation of sodium chloride and can obtain industrial-grade lithium chloride. However, there are also problems such as high energy consumption, low removal rate of sodium chloride, and high cost of adding lithium hydroxide.

[0054] For the preparation of high-purity lithium chloride, under the synergistic effect of the porous fiber rope prepared from modified cellulose acetate / polyacrylonitrile blend fiber and the evaporation crystallization conditions of this application, the salt crust formed at the 0.5 - 0.6 m section above the liquid level of the porous fiber rope is sodium chloride, potassium chloride, etc. (containing part of lithium chloride), and the salt crust formed at the 0.8 - 1.5 m section above the liquid level of the fiber rope is lithium chloride. Without using chemical agents, sodium and potassium are removed, realizing the purification of lithium chloride.

[0055] The inventors of the present application innovatively combine the capillary action of hydrophilic porous fibers on salt solutions, evaporation concentration, and the control of the evaporation process by industrial hot and cold air blowers to control the crystallization rate and salt content. They make full use of the capillary force differences of hydrophilic porous fibers for different chloride salts (such as lithium salts, sodium salts, potassium salts, etc.) and the characteristics of large solubility differences to purify lithium chloride from brine pools. The lithium salt with high concentration and high solubility crystallizes at the top of the fiber rope end after the fiber ropes in different brine pools are saturated by adsorption. This realizes the capillary crystallization separation of different salts in the horizontal and height directions of the fiber rope. It simplifies and shortens the technological process, greatly reduces energy consumption and the technical difficulty of purification, is easy to configure, install, and transfer equipment, is extremely easy to promote and apply, and can carry out industrial demonstration and large-scale production. No chemical agents are used in the process of separating and enriching lithium, and clean production is achieved as much as possible. It fundamentally solves the problem that it is difficult to remove sodium during the purification process of lithium chloride.

[0056] In order to better understand the above technical solution, the following will specifically describe the above technical solution in conjunction with specific embodiments.

[0057] Example 1

[0058] This example provides a method for preparing high-purity lithium chloride, as Figure 1 , specifically including the following steps:

[0059] S1. Deionized water is added to industrial-grade lithium chloride with a purity of 97 wt% according to the mass ratio of 1.5:1 to water. After stirring at 25 °C for 3 hours, it is filtered by a plate and frame to obtain lithium chloride solution A. The density of lithium chloride solution A is 1.1 g / cm 3 .

[0060] S2. As Figure 2 , lithium chloride solution A is added to the capillary crystallization lithium extraction pool 1 in the lithium extraction insulation chamber until the liquid level of lithium chloride solution A reaches 1.1 m and then stops. The temperature of lithium chloride solution A in the capillary crystallization lithium extraction pool 1 is maintained at 25 °C, and the solution mixing equipment in the pool continues to operate. In the lithium extraction insulation chamber, the evaporation temperature is 25 °C, the humidity is above 75%, and the evaporation amount in 24 hours is controlled at 300 mm.

[0061] A group of porous fiber ropes 2 is suspended above the capillary crystallization lithium extraction pool 1 in the lithium extraction insulation chamber. As Figure 3 , the length of the porous fiber rope 21 in the group of porous fiber ropes 2 is 2.0 m, and its lower end is immersed 0.3 m below the liquid surface of lithium chloride solution A in the capillary crystallization lithium extraction pool. And the group of porous fiber ropes 2 drops as the liquid level of lithium chloride solution A drops, so as to keep the lower end of the porous fiber rope always immersed 0.3 m below the liquid surface of lithium chloride solution A. After evaporating for about 48 hours, the liquid level of lithium chloride solution A drops to 0.5 m, and the density of the brine increases to 1.25 - 1.35 g / cm 3, at this time, the salt crust formed at the section 0.5 - 0.6 m above the liquid level of the fiber rope is sodium chloride, potassium chloride, etc. (containing some lithium chloride), and the salt crust formed at the section 0.8 - 1.5 m above the liquid level of the fiber rope is lithium chloride.

[0062] S3. Collect the salt crust of lithium chloride at the section 0.8 - 1.5 m above the liquid level of each fiber rope 21. After phase identification and chemical analysis, the purity of lithium chloride is ≥99.9 wt%, and the impurity sodium is ≤10 ppm, which is high-purity lithium chloride.

[0063] Collect the salt crusts such as sodium chloride at the section 0.5 - 0.6 m above the liquid level of each fiber rope 21 and sell them as industrial salt.

[0064] S4. Add deionized water to the collected high-purity lithium chloride salt crust according to the mass ratio of 1.5:1.0 to water. After stirring at 25°C for 3 hours, heat and stir to fully dissolve, and use a 50-nm ceramic membrane cross-flow filtration operation to achieve solid-liquid separation to obtain a high-purity lithium chloride solution C.

[0065] S5. Dry the high-purity lithium chloride solution C at 200°C to obtain high-purity lithium chloride. After detection, the purity of high-purity lithium chloride is greater than 99.95%, and the impurity contents are Na 8 ppm, Ca 10 ppm, Mg 5 ppm, and Fe 3 ppm.

[0066] Such as Figure 3 , in this embodiment, the porous fiber rope group 2 includes a solid fiber mesh 24 and multiple porous fiber ropes 21. One end of the porous fiber rope 21 is fixedly connected to the solid fiber mesh 24, and the other end is immersed in the liquid level of the lithium chloride solution A.

[0067] Such as Figure 4 , the porous fiber rope 21 includes a steel wire rope core 211 and multiple groups of porous fiber bundles 212, and multiple groups of porous fiber bundles 212 are wound around the outer peripheral part of the steel wire rope core 211.

[0068] In this embodiment, multiple groups of porous fiber bundles 212 include multiple groups of porous fibers. The porous fibers are modified cellulose acetate / polyacrylonitrile blend fibers, including cellulose acetate, acrylonitrile, vinyl acetate, and a cross-linking agent. The mass ratio of cellulose acetate to the other components is 20:80. The capillary pore diameter of the fiber is distributed from 5 nm to 30 nm, the porosity is 20% - 30%, and the contact angle of pure water on the fiber surface is 30 - 50°.

[0069] Among them, the dry preparation method of the modified cellulose acetate / polyacrylonitrile blend fiber includes the following steps:

[0070] The cellulose acetate, polyacrylonitrile, and vinyl acetate are dried under vacuum until the moisture content is < 30 ppm. The temperature of the internal mixer temperature control system is set at 245 °C, the rotor speed is 45 r / min, the upper ram pressure is 0.6 MPa, and the filling coefficient is 0.5. First, 4 kg of cellulose acetate is added to the internal mixer. After mixing for 30 s, the temperature of the temperature control system is adjusted to 325 °C, and then 14 kg of polyacrylonitrile, 1 kg of vinyl acetate, 0.48 kg of maleic anhydride graft copolymer, and 0.52 kg of methylbenzenesulfonic acid are added. Mixing continues for 200 s to obtain a mixed material, which is cooled and pressed into sheets to obtain slices. The slices are pre-crystallized in a drying oven and then dried under vacuum until the moisture content of the slices is < 30 ppm. The dried slices are extruded and melted by a screw and then enter the spinning box. The temperature of the spinning box body is 320 °C, and the spinning speed is 800 m / min. After extrusion through a spinneret, it is cooled and formed by a blowing system with a blowing speed of 0.5 m / min, wound and oiled. Then, it is drawn at a drawing temperature of 72 °C, a drawing speed of 800 m / min, and a draw ratio of 3.3 times to obtain cellulose acetate / polyacrylonitrile blend fibers. Then, the cellulose acetate / polyacrylonitrile blend fibers are completely immersed below the liquid level of a 25% by mass ethylenediamine solution and kept at 70 °C for 12 h to modify the cellulose acetate / polyacrylonitrile blend fibers. After cooling to room temperature, the fibers are taken out, washed with deionized water and ethanol until neutral, and dried in a drying oven for 12 h to obtain modified cellulose acetate / polyacrylonitrile blend fibers.

[0071] The porosity and pore size of the modified cellulose acetate / polyacrylonitrile blend fibers are tested according to "GB / T 33052-2016 Determination method for porosity of microporous functional films - Hexadecane absorption method" and "GB / T 42269-2022 Test method for pore size of separation membranes - Gas permeation method" respectively. The porosity is 20 - 30%, and the pore size is 5 nm - 30 nm. Tested according to "GB / T 24368-2009 Contact angle measurement method for detecting hydrophobic pollutants on glass surfaces", the contact angle of pure water on the fiber surface is 30 - 50°. The fiber materials are not completely formed uniformly, and the properties are determined within the range of the characterization data.

[0072] Cellulose acetate fiber has good liquid permeability and excellent adsorption properties. However, when cellulose acetate fiber absorbs water, its strength decreases. Adding polyacrylonitrile improves the fiber strength but reduces the proportion of hydrophilic groups. After modification with ethylenediamine, -CN in the fiber hydrolyzes in the alkaline ethylenediamine solution and reacts with the amino group in ethylenediamine to form amide bonds, thus grafting ethylenediamine on the fiber surface and making the fiber surface contain hydroxyl and amino molecular chains, thereby controlling the surface contact angle of the modified cellulose acetate / polyacrylonitrile blend fiber to be 30 - 50° (tested with pure water). At 25 °C, the solubility of sodium chloride and potassium chloride in water is about 30 g / 100 g, and the solubility of lithium chloride is about 83 g / 100 g. Under the evaporation conditions of this application, when the brine migrates upward along the porous fiber bundle of this application for 50 cm, the water evaporation rate is moderate, making sodium chloride and potassium chloride reach supersaturation state and lithium chloride not reach saturation state. Therefore, sodium chloride and potassium chloride precipitate at the section 0.5 - 0.6 m above the liquid level of the porous fiber bundle. Then, the brine without sodium chloride continues to migrate upward along the porous fiber bundle, and water continues to be evaporated. During the upward migration of 20 cm, the water evaporation rate is moderate, making lithium chloride reach supersaturation state. Therefore, lithium chloride precipitates at the section 0.8 - 1.5 m, thus realizing the purification of lithium chloride. Therefore, under this evaporation condition, the crystallization at the section 0.5 - 0.6 m above the liquid level of the porous fiber rope prepared from the modified cellulose acetate / polyacrylonitrile blend fiber is a salt shell such as sodium chloride and potassium chloride, and the crystallization at the section 0.8 - 1.5 m above the liquid level of the porous fiber rope is a lithium chloride salt shell.

[0073] Example 2

[0074] This example provides a method for preparing high-purity lithium chloride, which specifically includes the following steps:

[0075] S1. Deionized water is added to industrial-grade lithium chloride with a purity of 97 wt% according to the mass ratio of 2:1 to it. After stirring at 25 °C for 4 hours, plate and frame filtration is carried out to obtain lithium chloride solution A, and the density of lithium chloride solution A is 1.2 g / cm 3 .

[0076] S2. As Figure 1 , lithium chloride solution A is added to the capillary crystallization lithium extraction tank 1 located in the lithium extraction insulation chamber until the liquid level of lithium chloride solution A reaches 1.1 m and then stops. Keep the temperature of lithium chloride solution A in the capillary crystallization lithium extraction tank 1 at 30 °C, and the solution mixing equipment in the tank runs continuously. In the lithium extraction insulation chamber, the evaporation temperature is 30 °C, the humidity is above 75%, and the evaporation amount in 24 hours is controlled at 200 mm.

[0077] Above the capillary crystallization lithium extraction tank 1 in the lithium extraction insulation chamber, a group of porous fiber ropes 2 is suspended vertically. As Figure 2, in the porous fiber rope group 2, the length of the porous fiber rope 21 is 2.0 m, and its lower end is immersed 0.3 m below the liquid level of the lithium chloride solution A in the capillary crystallization lithium extraction pool. Moreover, the porous fiber rope group 2 descends as the liquid level of the lithium chloride solution A drops, so as to keep the lower end of the porous fiber rope always immersed 0.3 m below the liquid level of the lithium chloride solution A. After evaporating for about 48 hours, the liquid level of the lithium chloride solution A drops to 0.6 m, and the density of the brine increases to 1.3 - 1.35 g / cm 3 , at this time, the salt crust formed at the 0.5 - 0.6 m section above the liquid level of the porous fiber rope is sodium chloride, potassium chloride, etc. (containing some lithium chloride), and the salt crust formed at the 0.8 - 1.5 m section above the liquid level of the fiber rope is lithium chloride.

[0078] S3. Collect the lithium chloride salt crust at the 0.8 - 1.5 m section above the liquid level of each fiber rope 21. After phase identification and chemical analysis, the purity of lithium chloride is ≥99.9 wt%, and the impurity sodium is ≤10 ppm, which is high-purity lithium chloride;

[0079] Collect the salt crust such as sodium chloride at the 0.5 - 0.6 m section above the liquid level of each fiber rope 21 and sell it as industrial salt.

[0080] S4. Add deionized water to the collected high-purity lithium chloride salt crust according to the mass ratio of 1.5:1 to water. After stirring at 25°C for 3 hours, heat and stir to fully dissolve it, and perform cross-flow filtration operation with a 50 nm ceramic membrane to achieve solid-liquid separation, obtaining high-purity lithium chloride solution C.

[0081] S5. Dry the high-purity lithium chloride solution C at 200°C to obtain high-purity lithium chloride; after detection, the purity of the high-purity lithium chloride is greater than 99.95%, and the impurity contents are Na 6 ppm, Ca 10 ppm, Mg 3 ppm, and Fe 5 ppm.

[0082] For the porous fiber rope group in this embodiment, the preparation method of the modified cellulose acetate / polyacrylonitrile blend fiber is a wet preparation method, which is specifically as follows. Others are the same as in Example 1.

[0083] The wet preparation method of the modified cellulose acetate / polyacrylonitrile blend fiber includes the following steps:

[0084] 5 kg of cellulose acetate, 14 kg of polyacrylonitrile, 0.5 kg of vinyl acetate, and 0.5 kg of maleic anhydride graft copolymer were blended, and a spinning dope was prepared using dimethylacetamide (DMAC) as a solvent. The wet spinning process was adopted, and a 5% DMAC solution was used as the coagulation bath (the temperature of the coagulation bath was controlled at 30 - 35 °C, the bath entry time was 30 s - 60 s, and wet spinning was carried out through stretching, relaxation, and winding to obtain a primary cellulose acetate / polyacrylonitrile blend fiber roll; the cellulose acetate / polyacrylonitrile blend fiber was completely immersed below the liquid level of a 5% crosslinking agent HQ solution, and the cellulose acetate / polyacrylonitrile blend fiber was subjected to thermal crosslinking treatment at 40 - 50 °C for 48 h; then it was placed in a 5% alkali solution for hydrolysis, controlling the alkali solution concentration at 4 - 5%, the temperature at (20 - 30 °C), and for 48 h. After hydrolysis, it was taken out and neutralized with 2% oxalic acid on its surface, then washed 4 times with distilled water, and after each washing, the water on the fiber surface was fully drained and dried in the air, and dried at 30 - 40 °C to obtain the modified cellulose acetate / polyacrylonitrile blend fiber.

[0085] Example 3

[0086] This example provides a method for preparing high-purity lithium chloride, which specifically includes the following steps:

[0087] S1. Deionized water was added to industrial-grade lithium chloride with a purity of 97 wt% according to the mass ratio of 2.5:1 of it to water. After stirring at 35 °C for 5 hours, plate-frame filtration was carried out to obtain lithium chloride solution A, and the density of lithium chloride solution A was 1.4 g / cm 3 。

[0088] S2. As Figure 1 , lithium chloride solution A was added to the capillary crystallization lithium extraction pool 1 located in the lithium extraction insulation chamber until the liquid level of lithium chloride solution A reached 1.1 m and then stopped. The temperature of lithium chloride solution A in the capillary crystallization lithium extraction pool 1 was maintained at 35 °C, and the solution mixing equipment in the pool continued to operate. In the lithium extraction insulation chamber, the evaporation temperature was 35 °C, the humidity was above 75%, and the evaporation amount in 24 hours was controlled at 100 mm.

[0089] Above the capillary crystallization lithium extraction pool 1 in the lithium extraction insulation chamber, a porous fiber rope group 2 was vertically suspended. As Figure 2 , the length of the porous fiber rope 21 in the porous fiber rope group 2 was 2.0 m, and its lower end was immersed 0.3 m below the liquid level of lithium chloride solution A in the capillary crystallization lithium extraction pool. Moreover, the porous fiber rope group 2 dropped as the liquid level of lithium chloride solution A dropped to keep the lower end of the porous fiber rope always immersed 0.3 m below the liquid level of lithium chloride solution A. After evaporation for about 48 hours, the liquid level of lithium chloride solution A dropped to 0.9 m, and the density of the brine increased to 1.3 - 1.4 g / cm 3, at this time, the salt crust formed at the section 0.5 - 0.6 m above the liquid level of the fiber rope is sodium chloride, potassium chloride, etc. (containing some lithium chloride), and the salt crust formed at the section 0.8 - 1.5 m above the liquid level of the fiber rope is lithium chloride.

[0090] S3. Collect the salt crust of lithium chloride at the section 0.8 - 1.5 m above the liquid level of each fiber rope 21. After phase identification and chemical analysis, the purity of lithium chloride is ≥99.9 wt%, and the impurity sodium is ≤10 ppm, which is high-purity lithium chloride;

[0091] Collect the salt crusts such as sodium chloride at the section 0.5 - 0.6 m above the liquid level of each fiber rope 21 and sell them as industrial salt.

[0092] S4. Add deionized water to the collected high-purity lithium chloride salt crust according to the mass ratio of 1.5:1.0 to water. Stir for 3 hours at 25°C, then heat and stir to fully dissolve. Perform cross-flow filtration with a 50 nm ceramic membrane to achieve solid-liquid separation, and obtain high-purity lithium chloride solution C.

[0093] S5. Dry the high-purity lithium chloride solution C at 200°C to obtain high-purity lithium chloride; after detection, the purity of high-purity lithium chloride is greater than 99.95%, and the impurity contents are Na 5 ppm, Ca 10 ppm, Mg 4 ppm, and Fe 6 ppm.

[0094] The porous fiber rope group in this embodiment is the same as that in Embodiment 1.

[0095] In addition, the remaining brine in step S2 of this embodiment is returned to S1, mixed with the chemically synthesized lithium chloride solution, and the feed concentration of the brine is controlled, and then enters the porous fiber capillary crystallization lithium extraction insulation chamber again; for the brine that enters the porous fiber capillary crystallization lithium extraction insulation chamber multiple times, the sulfate content continuously rises. When the sulfate exceeds 1 - 2 g / L, it enters the porous fiber capillary crystallization lithium extraction insulation chamber again. Control the water temperature of the brine pool at -5 - 5°C, and control the evaporation temperature of the lithium extraction insulation chamber at 10 - 20°C (low-temperature evaporation), so that mirabilite forms a salt crust at a lower position of the fiber rope above the liquid level, and the salt crust at a higher position (0.8 - 1.5 m) is still high-purity lithium chloride.

[0096] Comparative Example 1

[0097] The difference between this comparative example and Example 1 is that ethylenediamine is not used to modify the cellulose acetate / polyacrylonitrile blend fiber, and the cellulose acetate / polyacrylonitrile blend fiber is used to replace the modified cellulose acetate / polyacrylonitrile blend fiber to prepare the porous fiber rope, and the others are the same as in Example 1. Because it is not modified with ethylenediamine, the proportion of hydrophilic groups in the cellulose acetate / polyacrylonitrile blend fiber is relatively low, the capillary pore size distribution of the fiber is 30 nm - 50 nm, the porosity is 10% - 20%, and the contact angle of pure water on the fiber surface is 80 - 90°. Under this evaporation condition, the water evaporation rate is fast. When the brine migrates 50 cm upward along the porous fiber bundle, sodium chloride, potassium chloride, and lithium chloride all reach supersaturated states. Therefore, sodium chloride, potassium chloride, and lithium chloride all precipitate at the 0.5 - 0.6 m section above the liquid level of the porous fiber bundle, and the purification of lithium chloride cannot be achieved.

[0098] Comparative Example 2

[0099] The difference between this comparative example and Example 1 is that in the lithium extraction insulation chamber in S2, the evaporation temperature is 60 °C, and the evaporation amount in 24 hours is controlled at 400 mm, and the others are the same as in Example 1. Under this evaporation condition, when the brine migrates 50 cm upward along the porous fiber bundle, the water evaporation rate is fast, and sodium chloride, potassium chloride, and lithium chloride all reach supersaturated states. Therefore, sodium chloride, potassium chloride, and lithium chloride all precipitate at the 0.5 - 0.6 m section above the liquid level of the porous fiber bundle, and the purification of lithium chloride cannot be achieved. Therefore, under the synergistic effect of the porous fiber rope prepared from the modified cellulose acetate / polyacrylonitrile blend fiber and the evaporation crystallization conditions of the present application, the salt crust formed at the 0.5 - 0.6 m section above the liquid level of the porous fiber rope is sodium chloride, potassium chloride, etc. (containing part of lithium chloride), and the salt crust formed at the 0.8 - 1.5 m section above the liquid level of the fiber rope is lithium chloride. Without using chemical agents, sodium is removed and the purification of lithium chloride is achieved.

[0100] Comparative Example 3

[0101] The difference between this comparative example and Example 2 is that 5% cross-linking agent HQ and 5% alkali solution are not used to modify the cellulose acetate / polyacrylonitrile blend fiber, and the cellulose acetate / polyacrylonitrile blend fiber is used to replace the modified cellulose acetate / polyacrylonitrile blend fiber to prepare the porous fiber rope, and the others are the same as in Example 2. Because there is no modification, the proportion of hydrophilic groups in the cellulose acetate / polyacrylonitrile blend fiber is relatively low, the capillary pore size distribution of the fiber is 30 nm - 50 nm, the porosity is 10% - 20%, the contact angle of pure water on the fiber surface is 80 - 90°, and under this evaporation condition, the water evaporation rate is fast. When the brine migrates 50 cm upward along the porous fiber bundle, sodium chloride, potassium chloride, and lithium chloride all reach supersaturated states. Therefore, sodium chloride, potassium chloride, and lithium chloride all precipitate at the 0.5 - 0.6 m section above the liquid level of the porous fiber bundle, and the purification of lithium chloride cannot be achieved.

[0102] Preferably, as Figure 2-6 , the lithium extraction heat preservation chamber may further include multiple stages of brine pools. Preferably, it includes four stages of brine pools, namely the first-stage brine pool 101, the second-stage brine pool 102, the third-stage brine pool 103, and the fourth-stage brine pool 104. Each stage of brine pool is matched with a porous fiber rope group 2, a heat preservation chamber 3, and a control system 4. The porous fiber rope group 2, the heat preservation chamber 3, and the control system 4 adjust the corresponding data according to the different levels of the brine pool.

[0103] Each stage of brine pool includes a capillary crystallization lithium extraction pool 1. A brine mixing device 11 is installed at the bottom end or the slope around the capillary crystallization lithium extraction pool 1. A heat exchange module 12 is installed on the inner wall of the capillary crystallization lithium extraction pool 1. A square grid net 13 is detachably installed to completely cover the liquid surface of the capillary crystallization lithium extraction pool 1. The capillary crystallization lithium extraction pool 1 is installed in the heat preservation chamber 3.

[0104] The capillary crystallization lithium extraction pool 1 is used to hold brine. During the extraction process, the setting of the brine mixing device 11 makes the brine temperature field and concentration field more uniform, and the water quality change in the pool is almost the same during the three-dimensional crystallization process; the heat exchange module 12 has the function of heating or cooling, and is used to control the temperature of the main body of the brine in the pool; the square grid net 13 is used to separate the fiber ropes of the porous fiber rope group 2 to prevent entanglement.

[0105] Preferably, the depth of the capillary crystallization lithium extraction pool 1 is 1 - 3m. It can not only meet the requirements of large-volume brine treatment and mixing, but also facilitate the crystallization and storage of salts at the bottom of the capillary crystallization lithium extraction pool 1.

[0106] Furthermore, the depth of the capillary crystallization lithium extraction pool 1 is about 1.5 - 2m, which can not only meet the requirements of processing capacity and process, but also ensure operation safety.

[0107] It should be noted that the brine mixing device 11 is one or a combination of a stirring device and a pumping and injection device, and is used for mechanical disturbance or forced convection circulation of the brine in the regulating pool. Preferably, the brine mixing device 11 includes multiple groups of submersible pumps, and the multiple groups of submersible pumps are dispersedly installed at the bottom around the capillary crystallization lithium extraction pool 1.

[0108] It should be noted that the heat source of the heat exchange module 12 can be selected from photovoltaic thermal utilization, cold and hot air circulation, or heating devices, etc.

[0109] It should be noted that the capillary crystallization lithium extraction pool 1 is designed according to the process processing capacity, and the pool body can be built on the ground or dug in the ground, and the process control is anti-corrosion and anti-leakage.

[0110] Preferably, the square grid net 13 is made of stainless steel or hard plastic. The square grid net 13 made of stainless steel or hard plastic can prevent personnel from accidentally falling into the pool during operation, and the detachable design is convenient for later maintenance and pool cleaning.

[0111] The porous fiber rope group 2 includes multiple groups of porous fiber ropes 21 and a fixed fiber mesh 24. The rope heads of the porous fiber ropes 21 are detachably installed on the fixed fiber mesh 24 through rope head locking devices 22, and the bottom ends of the porous fiber ropes 21 are inserted below the brine liquid level in the capillary crystallization lithium extraction pool 1.

[0112] Preferably, the length of the porous fiber rope 21 is 1.5 - 2.5 m, and the bottom end of the porous fiber rope 21 extends 0.1 - 0.5 m below the brine liquid level.

[0113] Adjust the depth of the bottom end of the ground rope entering below the liquid level of the capillary crystallization lithium extraction pool 1 according to the brine density and composition. When the brine density is high and the process impurity components in the brine pool are more, reduce the depth of entering below the liquid level of the capillary crystallization lithium extraction pool 1 to prevent excessive crystallization and breakage of the salt absorption rope for a long time and reduce the process control difficulty.

[0114] Preferably, the rope head locking device 22 is selected as a steel wire rope clip. The rope head is fixed on the fixed fiber mesh 24 through the rope head locking device 22. After the rope head locking device 22 is unlocked, the rope head is separated from the fixed fiber mesh 24.

[0115] Preferably, it further includes a lifting and moving device 23, such as an electric hoist. The fixed fiber mesh 24 is installed on the output end of the lifting and moving device 23. The fixed fiber mesh 24 can be fixed at a certain position or can be driven by the lifting and moving device 23 to lift or translate.

[0116] The insulation chamber 3 further includes an insulation chamber main body 31. The insulation chamber main body 31 is internally provided with heat insulation boards. The capillary crystallization lithium extraction pool 1 is installed in the insulation chamber main body 31. A cold and warm air circulation device 32 is installed on the insulation chamber main body 31. The cold and warm air circulation device 32 is installed on both sides of the capillary crystallization lithium extraction pool 1. A temperature adjustment window 33 for auxiliary temperature adjustment is installed above the roof or side wall of the insulation chamber main body 31.

[0117] The insulation chamber main body 31 is used to provide a controllable heat preservation environment for the capillary crystallization lithium extraction pool 1. The cold and warm air circulation device 32 is used to adjust the temperature in the insulation chamber main body 31, and the temperature adjustment window 33 is used for auxiliary temperature adjustment.

[0118] Preferably, the insulation chamber main body 31 is connected with a vacuum pumping device. The output end of the vacuum pumping device is connected with a water vapor condenser, and the water vapor condenser is connected with a distilled water temporary storage tank; the vacuum pumping device is connected to the exhaust port of the insulation chamber main body 31 to pump out the water vapor from the insulation chamber 3. The output end of the vacuum pumping device is connected to the tube side inlet of the water vapor condenser. The condensed liquid is connected with the distilled water temporary storage tank. The raw brine entering the primary brine pool 101 goes through the water vapor process, is heated up by preheating and then connected to the primary brine pool 101; the vacuum pumping device pumps out the water vapor to reduce the pressure on the liquid surface, which helps to accelerate the concentration of the liquid. The pumped out water vapor is condensed to by - produce distilled water, which is suitable for remote areas lacking pure water.

[0119] The cold and warm air circulation device 32 includes at least one set of fan groups oppositely installed on both sides of the capillary crystallization lithium extraction pool 1. The heat source of the cold and warm air circulation device 32 can be selected from heating devices, photovoltaic thermal utilization, hot air circulation, geothermal energy, etc., and the cold source is selected as compression refrigeration. The cold source and the heat source can be connected in series with the heat exchange module 12 to save energy.

[0120] The control system 4 includes a control panel 41. A temperature sensor 42, a humidity sensor 43, a liquid level sensor 44, a density sensor 45, and an evaporation amount sensor 46 are installed on the capillary crystallization lithium extraction pool 1, and all the sensors are electrically connected to the control panel 41. A height recognition device 47 and a weight recognition device 48 with reminder and alarm functions are also installed on the capillary crystallization lithium extraction pool 1.

[0121] Preferably, the control panel 41 is installed on the side wall of the heat preservation chamber body 31.

[0122] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of additional identical elements in the process, method, article or device including the said element.

[0123] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

[0124] The present invention uses the above embodiments to illustrate the detailed process flow of the present invention, but the present invention is not limited to the above detailed process flow, that is, it does not mean that the present invention must rely on the above detailed process flow to be implemented. Those skilled in the art should understand that any improvement to the present invention, equivalent replacement of the raw materials of the products of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for preparing high-purity lithium chloride, characterized in that: The method comprises the following steps: providing porous fiber ropes with capillary effect and hydrophilic material, wherein the porous fiber ropes are suspended in groups on the surface of lithium chloride solution, and one end of the porous fiber ropes is immersed in the lithium chloride solution, and each component in the solution migrates to different positions above the rope due to different capillary forces of the fiber ropes, and the saturated salt evaporation and crystallization conditions of the fiber ropes are controlled to obtain a lithium chloride salt shell, and the lithium chloride salt shell is dissolved in water, filtered, and dried to obtain high-purity lithium chloride.

2. The method for preparing high-purity lithium chloride according to claim 1, wherein The porous fiber rope comprises a rope core and a plurality of groups of porous fiber bundles, wherein the plurality of groups of porous fiber bundles are arranged at the outer periphery of the rope core.

3. The method for preparing high-purity lithium chloride according to claim 2, wherein The porous fiber bundle comprises porous fibers, and the porous fibers are modified cellulose acetate / polyacrylonitrile blended fibers.

4. The method for preparing high-purity lithium chloride according to claim 2, wherein The rope core is a steel wire rope core.

5. The method for preparing high-purity lithium chloride according to claim 3, wherein The modified cellulose acetate / polyacrylonitrile blended fiber comprises cellulose acetate, acrylonitrile, vinyl acetate and a crosslinking agent, wherein the mass ratio of the cellulose acetate to the other components is 5-40:95-60, the capillary pore size distribution of the modified cellulose acetate / polyacrylonitrile blended fiber is 1nm-30nm, the porosity is 20%-50%, and the contact angle of pure water on the surface of the modified cellulose acetate / polyacrylonitrile blended fiber is less than 90°; Preferably, in the modified cellulose acetate / polyacrylonitrile blended fiber, the mass ratio of cellulose acetate to other components is 10-30:90-70, the capillary pore size distribution of the modified cellulose acetate / polyacrylonitrile blended fiber is 1nm-20nm, the porosity is 25%-45%, and the contact angle of pure water on the surface of the modified cellulose acetate / polyacrylonitrile blended fiber is 30-60°.

6. The method for preparing high-purity lithium chloride according to claim 3, characterized in that: The dry method for preparing the modified cellulose acetate / polyacrylonitrile blended fiber comprises the following steps: S1, after melting cellulose acetate, adding polyacrylonitrile, vinyl acetate, maleic anhydride graft copolymer, and toluenesulfonic acid, continuing melt-kneading to obtain a mixed material, cooling the mixed material and pressing it into sheets to obtain slices, pre-crystallizing the slices, and then drying them until the moisture content of the slices is less than 30 ppm, and melt-spinning them to obtain cellulose acetate / polyacrylonitrile blended fibers; S2. The cellulose acetate / polyacrylonitrile blended fibers are completely immersed below the liquid level of a 25%-27% by mass ethylenediamine solution to modify the cellulose acetate / polyacrylonitrile blended fibers to obtain modified cellulose acetate / polyacrylonitrile blended fibers.

7. The method for preparing high-purity lithium chloride according to claim 3, characterized in that: The wet method for preparing the modified cellulose acetate / polyacrylonitrile blended fiber comprises the following steps: S1, blending cellulose acetate, polyacrylonitrile, vinyl acetate, and maleic anhydride graft copolymer in different proportions, preparing a spinning solution with DMAC or DMF as a solvent, adopting a wet spinning process, using a DMAC solution or an ethanol solution of a certain concentration as a coagulation bath, and performing wet spinning through stretching, relaxation, and winding to obtain primary cellulose acetate / polyacrylonitrile blended fibers; S2. The cellulose acetate / polyacrylonitrile blended fibers are completely immersed below the liquid level of a crosslinking agent HQ solution having a mass fraction of 3%-5%, and the cellulose acetate / polyacrylonitrile blended fibers are subjected to thermal crosslinking treatment under certain temperature and time conditions; then, the fibers are placed in a 2%-10% alkaline solution for hydrolysis, and the concentration, temperature and time of the alkaline solution are controlled. After the hydrolysis, the fibers are taken out and their surfaces are neutralized with a weak acid, and then they are washed with distilled water for multiple times to obtain modified cellulose acetate / polyacrylonitrile blended fibers.

8. The method for preparing high-purity lithium chloride according to claim 1, characterized in that: The conditions for the evaporation crystallization include: a temperature of 20-50° C., a humidity of more than 75%, and an evaporation volume of 100-300 mm in 24 hours.

9. The method for preparing high-purity lithium chloride according to claim 1, wherein Industrial-grade lithium chloride with a purity of not less than 97wt% is selected and dissolved in water to obtain the lithium chloride solution; the mass ratio of the industrial-grade lithium chloride to water is 1.5-2.5:1.0, and the dissolution temperature is 25-35°C.

10. The method for preparing high-purity lithium chloride according to claim 1, characterized in that: The density of the lithium chloride solution is 1.1-1.4 g / cm 3 ; The purity of the high-purity lithium chloride is greater than 99.95%.

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