Preparation method of 4n / 5n grade ultra-high purity sodium chloride suitable for multi-source raw material brine

Abstract

The application provides a 4N / 5N grade ultra-high purity sodium chloride preparation method suitable for multi-source raw material brine, proposes a raw material differentiation pre-purification method to prepare 3N grade sodium chloride intermediate, combines a two-stage gradient purification overall route of membrane technology interception, multi-stage sodium type ion exchange deep impurity removal and controlled speed crystallization deep purification, and through the gradient purification route, the product purity can be stably reached to 4N grade, and under the optimal condition, can be reached to 5N grade, the total impurity can be as low as below 0.8 ppm, and the overall process total yield is not less than 85%, which is much higher than the yield level of less than 60% of the melting method.

Description

Technical Field

[0001] This invention relates to the field of high-purity sodium chloride technology, specifically to a method for preparing 4N / 5N grade ultra-high purity sodium chloride suitable for multi-source raw material brine. Background Technology

[0002] High-purity sodium chloride can be classified into three grades according to purity: 3N (NaCl≥99.9%), 4N (NaCl≥99.99%), and 5N (NaCl≥99.999%). It is a key basic material supporting high-end manufacturing and strategic industries in many fields, and its application scenarios have extended from traditional chemical and pharmaceutical fields to core fields such as nuclear energy, semiconductors, optics, and new energy.

[0003] In the nuclear energy industry, 4N grade sodium chloride is the core electrolytic raw material for preparing nuclear-grade metallic sodium. Metallic sodium is the main coolant and heat transfer medium for sodium-cooled fast neutron reactors. Trace amounts of impurities such as calcium, magnesium, and fluorine can exacerbate the corrosion of loop equipment and affect the safety and heat transfer efficiency of reactor operation. It is a key strategic material supporting the independent development of my country's fourth-generation nuclear power technology. It is also widely used in the preparation of high-purity reaction media and reagents in the nuclear chemical system.

[0004] In the field of electronic semiconductors, sodium chloride of grade 4N and above is the core raw material for preparing wet electronic chemicals such as electronic grade hydrochloric acid, electronic grade sodium hydroxide, and electronic grade sodium hypochlorite. It is widely used in key processes such as integrated circuit wafer cleaning, etching, and photoresist stripping. It is also indispensable in the manufacturing of third-generation semiconductors, advanced packaging, and photovoltaic cells. Its impurity level directly affects chip yield and device performance.

[0005] In the pharmaceutical and biomanufacturing fields, high-purity sodium chloride is a core raw material for high-end injectables and excipients for injectable drugs. It is also used in the preparation of cell culture media, antibody and protein purification buffers, in vitro diagnostic reagents, and genetic engineering research systems in biopharmaceuticals. Trace amounts of heavy metals and impurity ions can cause adverse drug reactions and interfere with biological reaction systems. The purity requirements for high-end biopharmaceuticals and clinical drug use scenarios are constantly increasing.

[0006] In the field of optics and special materials, ultra-high purity sodium chloride can be grown into large-size infrared optical single crystals, which are used as infrared windows, prisms and spectrometers in aerospace remote sensing, infrared detection and security monitoring. It is also a key raw material and flux for the preparation of other high-purity chlorides, special glasses and precision ceramics.

[0007] In the field of high-end metrology and analysis, 5N grade sodium chloride can be used as a reference reagent, a spectroscopically pure reagent and an ion chromatography standard substance for the calibration of trace analysis instruments, environmental monitoring and food testing traceability, with extremely high requirements for purity uniformity and stability.

[0008] In the field of new energy storage, the metallic sodium and molten salt system prepared from high-purity sodium chloride is the core medium for sodium-based energy storage batteries and large-scale high-temperature molten salt energy storage technology, playing a crucial role in improving energy storage cycle life and system efficiency. Currently, my country's supply capacity of high-end, high-purity sodium chloride is insufficient, and nuclear-grade and electronic-grade high-purity salts have long relied on imports, indicating a significant shortcoming in domestically developed preparation technology.

[0009] Currently, the main technologies for preparing high-purity sodium chloride at home and abroad are divided into four major routes: melt purification method, chemical precipitation-recrystallization method, ion exchange-crystallization combined method, and membrane separation purification method. Each of these has its own applicable scenarios and technical limitations.

[0010] The melt purification method is a dry high-temperature process that utilizes the difference in melting and boiling points between sodium chloride and impurity halides to remove impurities step by step through graded gradient sweating and segmented high-temperature vaporization. No solvents or chemical reagents are added throughout the process, and 4N grade products can be stably prepared. However, it has limitations such as high energy consumption, low product yield, stringent equipment requirements, and difficulty in large-scale mass production.

[0011] Chemical precipitation-recrystallization is the traditional mainstream process for preparing industrial refined salt and laboratory pure salt. It removes impurities such as sulfate, magnesium ions, and calcium ions stepwise by using precipitants such as barium chloride, sodium hydroxide, and sodium carbonate. The product is then obtained by evaporation and crystallization. The process is mature and has low cost, but the highest purity can only reach about 99.7% analytical grade, which cannot break through the 3N grade bottleneck. It is also easy to introduce secondary impurities and generate a large amount of saline wastewater and sludge.

[0012] The ion exchange-crystallization combined method utilizes ion exchange resins or selective adsorbents to deeply remove trace ionic impurities, and further improves purity by combining multi-stage recrystallization. Under laboratory conditions, alkaline earth metal impurities can be reduced to the level of 0.1 ppm. It is a classic laboratory route for preparing ultra-high purity salts. However, when directly processing brine, the resin load is high, the operating cost is high, and there is a risk of adsorbent impurities leaching out. At present, it has not yet achieved large-scale industrial application.

[0013] Membrane separation purification is an emerging technology in the salt chemical purification field in recent years. It utilizes the sieving effect and charge selectivity of membranes to remove impurities. Representative processes include monovalent selective electrodialysis and two-stage membrane filtration purification. It can be used to prepare pharmaceutical-grade and industrial refined salts. It has the advantages of continuous process and large processing capacity. However, the existing technology can only improve the purity of the product to 98%~99.5%, and cannot stably reach the 3N level. It is difficult to support the preparation of ultra-high purity products above 4N when used alone.

[0014] All four existing technical routes have significant shortcomings and cannot simultaneously meet the industrial demands for wide raw material compatibility, high purity, low cost, high yield, and easy scaling. The core defects are reflected in multiple aspects.

[0015] Firstly, the raw material compatibility is limited, failing to cover all types of brine resources. Most existing processes are developed for single raw materials. The melting method uses industrial refined salt as raw material, while the chemical precipitation and membrane methods are for specific well and mine salt brines. The ion exchange method is only suitable for laboratory pure salt raw materials. However, my country's salt resources cover three major categories: well and mine salt, sea salt, and lake salt. The impurity composition of different brines varies greatly. The magnesium and potassium content in lake salt is several to tens of times that of well and mine salt. A single process cannot be adapted to all raw materials, resulting in low resource utilization.

[0016] Secondly, there is an irreconcilable contradiction between purity and cost / yield. The melting method can achieve a purity of 4N, but the ultra-high temperature melting consumes a lot of energy, and the sweating and vaporization processes result in a product yield of generally less than 60%. The equipment has strict requirements for high temperature corrosion resistance, and the cost alone is 3 to 5 times that of the wet method. The chemical precipitation method and membrane method have low cost and high yield, but the highest purity is only 99.7%, which cannot break through the 3N bottleneck and cannot meet the needs of high-end scenarios. The ion exchange method can achieve trace impurity removal, but the resin load is extremely high when directly treating brine, resulting in a large amount of resin, frequent regeneration, high operating costs, and the inability to efficiently remove all anionic impurities.

[0017] Third, the risk of introducing secondary impurities is high, affecting the improvement of the ultimate purity. Chemical precipitation methods require the addition of reagents such as barium chloride, sodium carbonate, and sodium hydroxide. Excessive ions can easily form new impurities, and the purity of the precipitant itself will limit the final purity of the product. Adsorbents such as ammonium zeolite have the risk of ammonium ion dissolution, which will introduce nitrogen-containing impurities and is not suitable for pharmaceutical and electronic applications. Ordinary stainless steel and carbon steel equipment are prone to dissolving metal impurities such as iron, chromium, and nickel in salt water environments, which is a key bottleneck restricting the upgrading of products from 3N grade to 4N grade and 5N grade.

[0018] Fourth, large-scale production is difficult and the industry is not well implemented. The melting method requires special reactors that are resistant to high temperatures and corrosion. The capacity of a single piece of equipment is limited, making it difficult to achieve mass production of tens of thousands of tons. The laboratory ion exchange plus multi-polar recrystallization process is an intermittent operation with low production efficiency, high labor costs, and poor purity stability after scale-up. The existing deep purification process has a low degree of continuity and lacks mature industrial complete technology.

[0019] Finally, energy consumption and environmental pressure are prominent. The melting method operates at ultra-high temperature, and the energy consumption per unit product is more than 5 times that of the wet evaporation process, resulting in high carbon emissions. The chemical precipitation method generates a large amount of saline wastewater and chemical sludge, which has high environmental treatment costs and does not meet the requirements of green manufacturing.

[0020] In summary, there is a need to develop a method for preparing 4N / 5N grade ultra-high purity sodium chloride that is compatible with brine from multiple sources. Summary of the Invention

[0021] In view of the problems existing in the prior art, the present invention provides a method for preparing 4N / 5N grade ultra-high purity sodium chloride adapted to multi-source raw material brine. It proposes a two-stage purification route for preparing 3N grade sodium chloride intermediate by differentiating raw materials, and combining membrane technology retention, multi-stage sodium ion exchange for deep impurity removal and rate-controlled crystallization for deep purification. Through the step-by-step purification route, the product purity can be stably reached at 4N grade, and can reach 5N grade under optimized conditions. The total impurities can be as low as below 0.8 ppm, which fully covers the needs of nuclear power, electronics, pharmaceuticals and reference reagents. The overall process yield is not less than 85%, which is much higher than the yield level of less than 60% of the melt method.

[0022] To achieve this objective, the present invention adopts the following technical solution: The purpose of this invention is to provide a method for preparing 4N / 5N grade ultra-high purity sodium chloride suitable for multi-source raw material brine. The preparation method includes the following steps: preparing raw material brine, and obtaining 3N grade sodium chloride intermediate through a pre-purification process; dissolving the 3N grade sodium chloride intermediate in water, and sequentially performing membrane separation, ion exchange, and rate-controlled crystallization on the obtained crude sodium chloride brine to obtain 4N grade and above ultra-high purity sodium chloride.

[0023] This invention provides a method for preparing 4N / 5N grade ultra-high purity sodium chloride adapted to multi-source brine. It proposes a two-stage purification process: differentiated pre-purification of raw materials to prepare 3N grade sodium chloride intermediates, combined with membrane technology for impurity retention, multi-stage sodium-type ion exchange for deep impurity removal, and rate-controlled crystallization for deep purification. In the first stage, a differentiated pre-purification process is matched to the impurity characteristics of well salt, sea salt, and lake salt to prepare 3N grade sodium chloride intermediates at low cost and high yield, reducing the burden on subsequent deep purification. In the second stage, using 3N salt as raw material, a graded membrane technology is used to retain impurities and divalent salt ions, significantly reducing the ion exchange load. Combined with multi-stage sodium-type ion exchange for deep removal of trace impurities, and finally, controlled-rate crystallization suppresses lattice entrainment, achieving the preparation of 4N to 5N grade ultra-high purity sodium chloride.

[0024] As a preferred technical solution of the present invention, in the preparation method, the material of all material contact parts is at least one of polytetrafluoroethylene, high-purity titanium TA2, high-purity quartz or Teflon, and the material of all material contact parts is a low-leaching material to prevent the leaching of equipment impurities from the source.

[0025] As a preferred technical solution of the present invention, the raw material brine is well and mine salt brine, and the pre-purification process includes precision filtration, multi-effect evaporation, evaporation crystallization, and solid-liquid separation performed sequentially.

[0026] As a preferred technical solution of the present invention, the filter pore size of the precision filter is 4~6μm, such as 4μm, 4.5μm, 5μm, 5.5μm or 6μm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0027] And / or, the multi-effect evaporation includes a 2- to 4-effect evaporation system, using a co-current flow process, with the heating steam pressure of the first effect controlled at 0.2~0.4MPa (e.g., 0.2MPa, 0.25MPa, 0.3MPa, 0.35MPa, or 0.4MPa, etc.), and the evaporation temperatures of each effect distributed in a gradient of 110℃, 95℃, 80℃, and 65℃. That is, if a 2-effect evaporation system is used, the evaporation temperatures of each effect are set to 110℃ and 95℃ respectively; if a 3-effect evaporation system is used, the evaporation temperatures of each effect are set to 110℃, 95℃, and 80℃ respectively; if a 4-effect evaporation system is used, the evaporation temperatures of each effect are set to 110℃, 95℃, 80℃, and 65℃ respectively.

[0028] And / or, the NaCl concentration of the concentrate obtained by the multi-effect evaporation is 310~330 g / L, such as 310 g / L, 315 g / L, 320 g / L, 325 g / L or 330 g / L, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0029] And / or, the evaporation crystallization includes: using a mechanical vapor recompression crystallizer, an evaporation temperature of 70~80℃ (e.g., 70℃, 71℃, 73℃, 75℃, 76℃, 78℃ or 80℃, etc.), controlling the evaporation rate by adjusting the frequency of the vapor compressor, and maintaining the supersaturation of the solution in the metastable range of 0.2%~0.5% (e.g., 0.2%, 0.3%, 0.4% or 0.5%, etc.).

[0030] And / or, the solid-liquid separation includes: first centrifuging the filter cake with a centrifuge, then washing the filter cake countercurrently with pure water, and returning the resulting centrifuged mother liquor to the multi-effect evaporator for recycling.

[0031] As a preferred embodiment of the present invention, the raw brine is sea salt brine and / or lake salt brine, and the pre-purification process includes sedimentation, filtration, isothermal evaporation crystallization, and solid-liquid separation performed sequentially; the isothermal evaporation crystallization is based on Na… + -K + -Mg 2+ / / Cl - -SO4 2--H2O pentahydrate-salt system phase diagram, determine the boundary between the saturated crystallization region of sodium chloride and the unsaturated regions of all other associated salts, lock the target salt precipitation point where all other associated salts are in an unsaturated state when sodium chloride reaches saturation, and once the degree of isothermal evaporation crystallization reaches the target salt precipitation point, immediately perform the solid-liquid separation to obtain 3N grade sodium chloride intermediate.

[0032] As a preferred technical solution of the present invention, the target temperature for isothermal evaporation crystallization is 25~50℃, such as 25℃, 30℃, 35℃, 40℃, 45℃ or 50℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0033] And / or, the solid-liquid separation includes: first centrifuging the solids with a centrifuge, and then washing the filter cake with a saturated sodium chloride pure solution.

[0034] The first stage of the preparation method described in this invention is the differentiated pre-purification of raw materials, which is used to prepare 3N grade sodium chloride intermediate. Based on the source of brine and the composition of impurities, two targeted process routes are adopted, both of which can stably produce 3N grade sodium chloride intermediate with NaCl ≥ 99.9%.

[0035] For well and mine brine, a coupled rate-controlled crystallization process using multi-effect evaporation and mechanical vapor recompression crystallizer (MVR) is employed. The raw brine first undergoes a precision filtration process with a resolution of 4-6 μm to remove suspended solids and insoluble impurities. The clarified brine then enters a 2-4 effect evaporation system, employing a co-current flow. The heating steam pressure in the first effect is controlled at 0.2-0.4 MPa, and the evaporation temperatures in each effect are distributed in a gradient of 110℃, 95℃, 80℃, and 65℃, concentrating the brine to a near-saturated state with a NaCl concentration of 310-330 g / L. The concentrated solution then enters the mechanical vapor recompression crystallizer. The evaporation temperature is controlled at 70~80℃. The evaporation rate is controlled by adjusting the frequency of the steam compressor to maintain the supersaturation of the solution in the metastable range of 0.2%~0.5%, which promotes regular crystal growth and reduces lattice entrainment. The crystallizer is equipped with a flow guide tube and a crystal growth zone to ensure that the average residence time of the crystals is not less than 4 hours. After the crystal slurry is separated by centrifugation, the filter cake is washed countercurrently with a small amount of pure water to remove the mother liquor impurities attached to the surface. The centrifuged mother liquor is returned to the multi-effect evaporation system for recycling. 3%~5% of the mother liquor containing enriched impurities is periodically discharged to maintain the impurity balance of the system.

[0036] For marine and lake brine with high levels of associated impurities such as potassium, magnesium, and sulfate, a precise point-control salt precipitation process using a water-salt phase diagram is employed. The raw brine first undergoes sedimentation and filtration to remove suspended solids. Then, based on the ionic composition of the brine and the corresponding Na content at the appropriate temperature, a specific salt precipitation process is used. + -K + -Mg 2+ / / Cl - -SO42- The phase diagram of the H2O pentahydrate-salt system was used to determine the boundary between the saturated crystallization region of sodium chloride and the unsaturated regions of potassium, magnesium, and sulfate salts. The target salt precipitation point was identified where all associated salts are in an unsaturated state when sodium chloride reaches saturation. Isothermal evaporation was carried out at a target temperature of 25-50℃, with strict control of the concentration ratio. When the brine was concentrated to the target salt precipitation point, solid-liquid separation was immediately performed. At this point, only sodium chloride crystals precipitate, while impurities such as potassium, magnesium, and sulfate remain dissolved in the liquid phase, with no eutectic inclusions. After centrifugation of the crystal slurry, the filter cake was washed with a saturated pure sodium chloride solution to further remove surface adsorbed impurities.

[0037] As a preferred embodiment of the present invention, the water used to prepare the crude sodium chloride brine has a resistivity ≥ 18.2 MΩ. cm.

[0038] And / or, the concentration of the crude sodium chloride brine at 25°C is 200~250 g / L, such as 200 g / L, 210 g / L, 220 g / L, 230 g / L, 240 g / L or 250 g / L, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0039] And / or, the membrane separation includes sequential ultrafiltration and nanofiltration, wherein the ultrafiltration employs a ceramic membrane and / or an organic ultrafiltration membrane, and the nanofiltration employs a polyamide composite nanofiltration membrane.

[0040] And / or, the ion exchange adopts a series process of cation exchange plus anion exchange, and all the ion exchange resins used are pre-converted to high-purity sodium form to prevent the leaching of impurities from the resin itself.

[0041] And / or, the controlled-rate crystallization includes: maintaining the supersaturation of the solution at 0.2%~0.5% (e.g., 0.2%, 0.3%, 0.4% or 0.5%) by adjusting the evaporation rate and the feed rate, controlling the crystal growth time at 1~3 hours (e.g., 1 hour, 1.5 hours, 2 hours, 2.5 hours or 3 hours), and discharging the mother liquor enriched with impurities every 1~2 hours (e.g., 1 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours or 2 hours), with the discharge amount being 3%~8% (e.g., 3%, 4%, 5%, 6%, 7% or 8%) of the effective volume of the crystallizer.

[0042] The second stage of the preparation method described in this invention uses 3N salt as raw material. A graded membrane technology is used to remove impurities and divalent salt ions, significantly reducing the ion exchange load. Combined with multi-stage sodium-type ion exchange, trace impurities are deeply removed. Finally, lattice entrainment is suppressed through limited-rate crystallization, achieving the preparation of 4N to 5N grade ultra-high purity sodium chloride. The prepared crude sodium chloride brine is fed into a graded membrane separation unit. First, a ceramic membrane or compact organic ultrafiltration membrane unit is used to remove small molecule impurities such as colloids through sieving. This step serves as a pre-filter for nanofiltration, greatly reducing the organic fouling load on subsequent nanofiltration membranes and extending the membrane module's lifespan. Then, the resulting permeate enters the nanofiltration membrane unit, where residual divalent impurity ions are effectively removed through sieving and the Donnan effect, thereby significantly reducing the subsequent ion exchange load. The compact organic ultrafiltration membrane element uses alumina or zirconium oxide ceramic nanofiltration membranes or organic ultrafiltration membranes such as PES, PVDF, and PS, with a pore size controlled at 0.5~1nm and a molecular weight cutoff of 200~500Da. The operating pressure is set at 0.1~0.4MPa, the cross-flow velocity at 1~2m / s, and the operating temperature at 25~40℃. This unit completely removes suspended solids and colloidal particles, and the turbidity of the produced water does not exceed 0.1NTU. The nanofiltration membrane element uses an antifouling composite nanofiltration membrane element, with an operating pressure set at 0.6~1.2MPa, a cross-flow velocity at 1~2m / s, and an operating temperature at 25~40℃. It achieves a total removal rate of ≥98% for divalent cations and sulfate ions. After the operating cycle, it is acid-washed with a 0.5% citric acid and 0.1% EDTA disodium solution, and then alkaline-washed with a 0.1% sodium hydroxide solution to restore membrane flux. Deep impurity removal is achieved through multi-stage resin ion exchange, employing a series of cation and anion exchange processes to thoroughly remove trace amounts of monovalent and divalent cations and anions. All resins undergo pre-conversion to high-purity sodium form to prevent the leaching of impurities from the resin itself. Sodium-form conversion of the resin is a core step in ensuring product purity. Taking strongly acidic cation exchange resin as an example, the resin is first loaded into a high-purity titanium exchange column. The resin layer is backwashed with ultrapure water to remove fine particles. Then, a 5wt% high-purity sodium chloride solution is countercurrently passed through the resin layer at a flow rate of 1-2 BV / h, with three times the resin volume of the conversion solution introduced each time. After soaking for 2-3 hours, the solution is drained. This conversion process is repeated at least three times to thoroughly replace calcium, magnesium, iron, and other impurity cations on the resin functional groups. Finally, the resin layer is forward-washed with ultrapure water until the chloride ion concentration in the effluent matches that in the influent. After sodium-form conversion, the resin is sealed for later use.During continuous impurity removal, the cation exchange unit uses three stages of sodium-type strong acid cation exchange resin in series, controlling the space velocity to 1-2 BV / h and the operating temperature to 25-35℃. This effectively removes trace amounts of potassium, calcium, magnesium, strontium, and heavy metal ions, with total cation impurities at the outlet not exceeding 50 ppb. The anion exchange unit uses a single stage of strong base anion exchange resin pre-converted to the chloride form using high-purity sodium chloride, connected in series after the cation exchange unit to remove trace amounts of sulfate, nitrate, and other anionic impurities. When effluent impurities exceed the threshold, both the cation and anion exchange resins are regenerated with high-purity sodium chloride solution. The regeneration solution is prepared entirely with ultrapure water to avoid introducing impurities during regeneration. Rate-controlled crystallization is a core process ensuring the final product reaches 4N and 5N standards. The purified brine enters the crystallization unit, where crystallization kinetics are strictly controlled to minimize lattice encapsulation and surface adsorption impurities. The crystallization equipment uses an MVR crystallizer or a multi-effect evaporator crystallizer, with high-purity titanium or polytetrafluoroethylene lining for the material contact parts. By adjusting the evaporation rate and feed rate, the supersaturation of the solution is maintained at 0.2%~0.5% to avoid explosive nucleation and promote slow and complete crystal growth. A crystal growth zone is set up in the crystallizer, and the crystal growth time is controlled at 1~3 hours. The mother liquor enriched with impurities is discharged every 1~2 hours, with a discharge volume of 3%~8% of the effective volume of the crystallizer, to maintain the concentration of impurities in the liquid phase at an extremely low level. After the crystal slurry is centrifuged by a titanium centrifuge, the crystals are sprayed and washed with a small amount of hot ultrapure water to remove the trace amounts of mother liquor adhering to the surface. The washing liquid is returned to the upstream salt dissolving process for reuse.

[0043] It should be noted that there are several alternative implementation methods for the core process route of this invention, all of which do not depart from the core inventive concept. The ion exchange resin unit can use an aminophosphonic acid-type sodium chelating resin instead of a strongly acidic cation exchange resin, which offers stronger selectivity for alkaline earth metals and heavy metals, making it suitable for scenarios with large fluctuations in raw material impurities and requiring deep removal of trace heavy metals. The rate-controlled crystallization process can use a gradient cooling crystallization process instead of evaporation crystallization, controlling the solution supersaturation through slow cooling at 0.5~1℃ / h, suitable for preparing small batches of ultra-high purity products sensitive to evaporation energy consumption. Regarding raw materials, this process can directly use industrial refined salt as raw material, omitting the first-stage pre-purification step and directly proceeding to the second-stage deep purification, thus producing 4N and 5N grade ultra-high purity sodium chloride products.

[0044] As a preferred technical solution of the present invention, at least one of the following (1) to (5) is satisfied: (1) The ceramic membrane includes an alumina ceramic membrane or a zirconia ceramic membrane.

[0045] (2) The organic ultrafiltration membrane is made of at least one of PES, PVDF or PS.

[0046] (3) The pore size of the ultrafiltration is controlled at 0.5~1nm (e.g., 0.5nm, 0.6nm, 0.7nm, 0.8nm, 0.9nm or 1nm, etc.), the molecular weight cutoff is controlled at 200~500Da (e.g., 200Da, 250Da, 300Da, 350Da, 400Da, 450Da or 500Da, etc.), the operating pressure is set at 0.1~0.4MPa (e.g., 0.1MPa, 0.2MPa, 0.3MPa or 0.4MPa, etc.), the cross-flow velocity is 1~2m / s (e.g., 1m / s, 1.2m / s, 1.4m / s, 1.6m / s, 1.8m / s or 2m / s, etc.), and the operating temperature is 25~40℃ (e.g., 25℃, 28℃, 30℃, 32℃, 35℃, 38℃ or 40℃, etc.).

[0047] (4) The turbidity of the water produced by the ultrafiltration does not exceed 0.1 NTU.

[0048] (5) The operating pressure of the nanofiltration is set to 0.6~1.2MPa (e.g. 0.6MPa, 0.7MPa, 0.8MPa, 0.9MPa, 1.0MPa, 1.1MPa or 1.2MPa, etc.), the cross-flow velocity is 1~2m / s (e.g. 1m / s, 1.2m / s, 1.4m / s, 1.6m / s, 1.8m / s or 2m / s, etc.), and the operating temperature is 25~40℃ (e.g. 25℃, 28℃, 30℃, 32℃, 35℃, 38℃ or 40℃, etc.).

[0049] As a preferred technical solution of the present invention, at least one of the following (6) to (8) is satisfied: (6) The total cation impurities at the outlet of the cation exchange shall not exceed 50 ppb.

[0050] (7) The anion exchange uses a strong base anion exchange resin that has been converted from high-purity sodium form to chloride form.

[0051] (8) The high-purity sodium conversion includes: loading ion exchange resin into the exchange column, first backwashing the resin layer with ultrapure water to remove fine particles, then using 5wt% high-purity sodium chloride solution for conversion, and finally forward washing the resin layer with ultrapure water until the chloride ion concentration of the effluent is consistent with that of the influent, thus completing the high-purity sodium conversion, and sealing it for later use.

[0052] As a preferred technical solution of the present invention, the conversion using a 5wt% high-purity sodium chloride solution includes: using a 5wt% high-purity sodium chloride solution to flow countercurrently through the resin layer at a flow rate of 1~2 BV / h (e.g., 1 BV / h, 1.2 BV / h, 1.4 BV / h, 1.6 BV / h, 1.8 BV / h, or 2 BV / h, etc.), introducing a conversion solution three times the volume of the resin each time, soaking for 2~3 hours (e.g., 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, or 3 hours, etc.) and then draining, repeating this conversion operation at least 3 times.

[0053] Compared with existing technical solutions, the present invention has at least the following beneficial effects: (1) This invention has made a comprehensive breakthrough in addressing the core problems in the prior art, and its technical effects cover multiple dimensions such as raw material compatibility, purity cost, impurity control, large-scale production and environmental benefits.

[0054] (2) In terms of raw material compatibility, this invention targets the impurity characteristics of three types of brine: well salt, sea salt, and lake salt. It matches two pre-purification processes: multi-effect evaporation plus MVR rate-controlled crystallization and phase diagram-controlled point precipitation. Both processes can stably produce 3N grade intermediate products, breaking the limitation that a single process can only be adapted to a single type of raw material. The raw material adaptability covers the vast majority of salt mine resources in my country.

[0055] (3) In terms of synergistic optimization of purity, yield and cost, through the step-by-step purification route, the product purity can be stably reached 4N level, and under optimized conditions it can reach 5N level, and the total impurities can be as low as 0.8ppm, fully covering the needs of nuclear power, electronics, medicine and reference reagents. The overall process yield is not less than 85%, which is much higher than the yield level of less than 60% of the melt method. The yield of the pre-purification stage is not less than 90%, and the yield of the deep purification stage is not less than 92%. The membrane technology pretreatment reduces the amount of ion exchange resin used by more than 70% compared with the direct column process, and extends the resin regeneration cycle by 3 to 5 times. The coupling process of multi-effect evaporation and MVR reduces the energy consumption per unit product by 70% to 80% compared with the melt method. The comprehensive manufacturing cost is only one-third to one-half of the melt method.

[0056] (4) In terms of impurity control, no chemical precipitants or flocculants are added throughout the process, thus avoiding contamination from reagents; the resin is pre-converted to high-purity sodium form to completely eliminate the risk of impurity leaching from the resin itself; low-leaching materials are used in all material contact parts to prevent the leaching of metal ions from the equipment, thus ensuring the stability and reliability of trace purity from the source.

[0057] (5) In terms of large-scale production, the core units are all industrialized mature equipment, the process can achieve continuous operation throughout the entire process, and a single production line can be easily scaled up to an annual capacity of 10,000 tons, solving the industry pain points of limited capacity and difficulty in scaling up the melting method and intermittent recrystallization.

[0058] (6) In terms of environmental benefits, the process is a closed loop, the mother liquor is reused in stages, there is no large amount of saline wastewater discharge, and no chemical precipitation sludge is generated. The environmental treatment cost is reduced by more than 90% compared with the traditional chemical precipitation method, which is in line with the development direction of green manufacturing in the salt chemical industry. Detailed Implementation

[0059] To better illustrate the present invention and facilitate understanding of its technical solutions, typical but non-limiting embodiments of the present invention are as follows: Example 1 This embodiment provides a method for preparing 5N grade high-purity sodium chloride suitable for well and mine brine, the preparation method including the following: 5N grade ultra-high purity sodium chloride was prepared using well brine as raw material. The raw material was a well brine with a NaCl content of 280 g / L and impurities including calcium ions 0.8 g / L, magnesium ions 0.3 g / L, sulfate ions 1.2 g / L, and potassium ions 0.15 g / L.

[0060] Preparation stage of 3N grade sodium chloride intermediate: After the brine is filtered through a 5μm precision filter, it enters a triple-effect evaporation system. The steam pressure of the first effect is controlled at 0.3MPa, and the evaporation temperatures of each effect are set to 110℃, 95℃, and 80℃ respectively, until the NaCl concentration is 320g / L. The concentrated solution enters an MVR crystallizer, where the evaporation temperature is 75℃, the supersaturation is controlled at 0.3%, and the crystal residence time is 4 hours. After centrifugation and washing with pure water, 3N grade sodium chloride is obtained with a purity of 99.92% and a crystallization yield of 92%.

[0061] Deep refining and purification stage: In a dissolving tank lined with polytetrafluoroethylene, using a resistivity ≥18.2MΩ... Ultrapure water was prepared at 25℃ with a concentration of 200 g / L sodium chloride crude brine, and then filtered through a 1 nm pore size alumina ceramic compact ultrafiltration membrane. The operating pressure was 0.15 MPa, the cross-flow velocity was 2 m / s, the operating temperature was 25℃, and the product water turbidity was 0.07. NTU; the permeate enters the polyamide composite nanofiltration membrane element, operating at a pressure of 0.6 MPa, a cross-flow velocity of 1.5 m / s, and a temperature of 25°C. The total calcium and magnesium content in the permeate is 0.08 mg / L, and the sulfate content is 0.15 mg / L. The permeate is then passed sequentially through a series system of three-stage sodium-form D001 cation exchange resin and one-stage chloride-form 201×7 anion exchange resin, with a space velocity controlled at 2 BV / h and a total metal impurity at the outlet not exceeding 18 ppb. The purified solution enters the MVR crystallizer, where the evaporation temperature is 70°C, the supersaturation is 0.3%, and crystals are grown for 2 hours. Every 2 hours, 5% of the effective volume of the crystallizer is discharged as mother liquor. After centrifugation, the crystals are rinsed with ultrapure water. Finally, the crystals are vacuum dried at 105°C for 12 hours and then packaged in a Class 100 clean environment.

[0062] The product's NaCl purity was found to be 99.9992%, reaching 5N grade, with a total impurity content of 0.8 ppm, including 0.3 ppm potassium ions, 0.1 ppm calcium ions, 0.08 ppm magnesium ions, and 0.2 ppm sulfate ions. The overall process yield was 86%.

[0063] Example 2 This embodiment provides a method for preparing 4N grade high-purity sodium chloride suitable for lake brine, the preparation method including the following: 4N grade ultra-high purity sodium chloride was prepared using lake brine as raw material. The raw material was brine from a certain salt lake with a NaCl content of 260 g / L and impurities including potassium ions 2.5 g / L, magnesium ions 5.8 g / L, calcium ions 0.6 g / L, and sulfate ions 3.2 g / L.

[0064] 3N grade sodium chloride intermediate preparation stage: After the brine is settled and filtered, based on Na... + -K + -Mg 2+ / / Cl - -SO4 2- The target salt precipitation point for the H2O pentahydrate-salt system phase diagram calculation is to control the evaporation endpoint and crystallize and separate the salts in the sodium chloride saturated and magnesium and potassium salt unsaturated region. Specifically, the NaCl concentration is concentrated at 35°C to 315 g / L. At this point, both potassium and magnesium salts are in the unsaturated region. After isothermal crystallization and separation, the salts are first separated by centrifugation, and then the filter cake is washed with a saturated sodium chloride pure solution to obtain 3N grade sodium chloride with a purity of 99.91% and a crystallization yield of 86%. The salt precipitation mother liquor is sent to the potassium and magnesium extraction process.

[0065] Deep refining and purification stage: In a dissolving tank lined with polytetrafluoroethylene, using a resistivity ≥18.2MΩ... Ultrapure water was prepared at 25°C with a concentration of 210 g / L in sodium chloride crude brine. The solution was filtered through a compact PES ultrafiltration membrane with a pore size of 0.8 nm, operating at a pressure of 0.1 MPa, a cross-flow velocity of 2 m / s, and a temperature of 25°C, resulting in a product water turbidity of 0.05 NTU. The permeate then entered a polyamide composite nanofiltration membrane element, operating at 0.8 MPa, a cross-flow velocity of 2 m / s, and a temperature of 25°C, achieving a total divalent ion removal rate of 98.5%. Subsequently, the permeate passed through a series system of two stages of sodium chelating resin and one stage of cation exchange resin, with a space velocity of 1.5 BV / h, resulting in an outlet total cationic impurity level not exceeding 45 ppb. The purified solution was crystallized using triple-effect evaporation at a final effect temperature of 65°C, a supersaturation of 0.4%, and a crystal growth period of 1.5 hours. 6% of the effective volume of the crystallizer was discharged every 1.5 hours as mother liquor. Finally, the solution was dried with clean hot air at 110°C.

[0066] The product's NaCl purity was found to be 99.993%, reaching the 4N grade, with a total impurity content of 6.8 ppm, including 2.4 ppm potassium ions, 0.9 ppm calcium ions, 0.7 ppm magnesium ions, and 1.6 ppm sulfate ions. The overall process yield was 82%.

[0067] Example 3 This embodiment provides a method for preparing 4N grade high-purity sodium chloride suitable for sea salt brine, the preparation method including the following: 4N grade ultra-high purity sodium chloride was prepared using sea salt brine as raw material. The raw material was brine from a certain sea salt field with a NaCl content of 250 g / L. Impurities included magnesium ions 4.2 g / L, sulfate ions 2.8 g / L, calcium ions 0.4 g / L, potassium ions 1.1 g / L, and trace amounts of organic matter.

[0068] 3N grade sodium chloride intermediate preparation stage: The brine undergoes ozone oxidation to remove organic matter (ozone dosage 8 mg / L), followed by precision filtration, based on Na... + -K + -Mg 2+ / / Cl - -SO4 2- The target salt precipitation point for the H2O pentahydrate salt system phase diagram calculation was determined by controlling the evaporation endpoint and crystallizing and separating the salts in the sodium chloride saturated and magnesium and potassium salt unsaturated region. Specifically, the salts were isothermally evaporated and concentrated at 35°C until the NaCl concentration was 320 g / L. At this point, both potassium and magnesium salts were in the unsaturated region. After isothermal crystallization and separation, the salts were first separated by centrifugation and then the filter cake was washed with a saturated sodium chloride pure solution to obtain 3N grade sodium chloride with a purity of 99.90% and a crystallization yield of 88%.

[0069] Deep refining and purification stage: In a dissolving tank lined with polytetrafluoroethylene, using a resistivity ≥18.2MΩ... Ultrapure water was prepared at 25℃ with a concentration of 250 g / L sodium chloride crude brine. Filtration was performed using a PVDF compact ultrafiltration membrane with a pore size of approximately 0.79 nm to remove residual colloids and organic matter. The operating pressure was 0.15 MPa, the cross-flow velocity was 2 m / s, the operating temperature was 25℃, and the product water turbidity was 0.06%. NTU; the permeate enters the polyamide composite nanofiltration membrane element, operating at a pressure of 0.9 MPa, a cross-flow velocity of 2 m / s, and an operating temperature of 25℃, achieving a total divalent ion removal rate of 98.5%; the permeate then passes sequentially through a series system of three stages of sodium-type cation exchange resin and one stage of anion exchange resin, with a space velocity of 1.8 BV / h, and the total cationic impurities at the outlet do not exceed 40 ppb; the purified solution enters the MVR crystallizer, evaporating at a temperature of 70℃, with a supersaturation of 0.3%, and crystallization for 2 hours, with 5% of the effective volume of the crystallizer discharged every 2 hours; the crystals are centrifuged and then rinsed with ultrapure water; finally, they are vacuum dried at 105℃ for 12 hours and packaged in a Class 100 clean environment.

[0070] The product's NaCl purity was found to be 99.991%, reaching the 4N grade, with a total impurity content of 8.7 ppm, including 3.4 ppm potassium ions, 1.2 ppm calcium ions, 1.1 ppm magnesium ions, and 1.6 ppm sulfate ions. The overall process yield was 83%.

[0071] Comparative Example 1 This comparative example provides a method for preparing ultra-high purity sodium chloride using the melting method described in the specific embodiment of CN106629780A. The same batch of 3N grade sodium chloride intermediate obtained in Example 1 is used as raw material. A four-stage melting sweating-gasification process is adopted, with target temperatures of 780℃, 1000℃, 1450℃ and 1700℃ respectively, to obtain ultra-high purity sodium chloride product.

[0072] The product NaCl was tested and found to have a purity of 99.992%, reaching the 4N grade, with a total impurity content of 8.7 ppm, including 3.4 ppm potassium ions, 1.2 ppm calcium ions, 1.1 ppm magnesium ions, and 1.6 ppm sulfate ions. However, the yield was only 58%, and the unit energy consumption was 4.2 times that of Example 1.

[0073] Comparative Example 2 This comparative example provides a method for preparing sodium chloride. Compared with Example 1, the difference is that the membrane separation (passing through an alumina ceramic compact ultrafiltration membrane and a polyamide composite nanofiltration membrane element in sequence) is completely omitted. That is, the prepared sodium chloride crude brine is directly passed through a series system of a 3-stage sodium-type D001 cation exchange resin and a 1-stage chloride-type 201×7 anion exchange resin.

[0074] In this comparative example, the ion exchange resin achieved breakthrough after 12 hours of operation, the regeneration frequency was 3.5 times that of Example 1, and the annual resin consumption increased by 230%.

[0075] Comparative Example 3 This comparative example provides a method for preparing sodium chloride using a chemical precipitation-recrystallization method. A three-step precipitation method is used to treat the same batch of well brine as in Example 1. Specifically, the method includes: taking clarified brine that has undergone a 5μm precision filter; adding excess barium chloride to remove sulfate ions under stirring at 45°C; adding sodium hydroxide to adjust the pH to 11.5 to remove magnesium ions; and adding excess sodium carbonate to remove calcium ions and excess barium ions. After each reaction, the brine is allowed to settle and the precipitate is separated. The clarified liquid is neutralized to pH 7.0 with high-purity hydrochloric acid, transferred to an atmospheric pressure evaporator and evaporated at 100°C for crystallization. After centrifugation and washing, the product is dried at 105°C by forced air drying to obtain the final product.

[0076] Testing revealed that the NaCl purity of the product was only 99.68%, failing to meet the 3N grade standard. Furthermore, the production of 1 ton of finished salt generates approximately 0.12 tons of barium-containing chemical sludge and approximately 8 cubic meters of saline wastewater. The precipitant is prone to introducing secondary impurities, which in principle limits the improvement of purity.

[0077] In summary, this invention provides a method for preparing 4N / 5N grade ultra-high purity sodium chloride suitable for multi-source brine. First, a differentiated pre-purification process is used to prepare a 3N grade sodium chloride intermediate. Then, a two-stage step-by-step purification process is combined with multi-stage membrane retention, multi-stage sodium-type ion exchange for deep impurity removal, and controlled-rate crystallization for deep purification. This is essentially a two-stage step-by-step purification process. Using any one of well-mineral brine, sea salt brine, or lake salt brine as raw material, 4N to 5N grade ultra-high purity sodium chloride can ultimately be prepared. Furthermore, the method can be adapted to different types of raw brine. The technical solution of the heterogeneous matching pre-purification process is as follows: well and mine brine is prepared into 3N grade intermediate by multi-effect evaporation and MVR coupled rate-controlled crystallization; marine and lake brine is prepared into 3N grade intermediate by selectively crystallizing sodium chloride in the unsaturated range of associated salt based on the water-salt phase diagram. The deep refining process adopts a combination of multi-stage membrane pretreatment to reduce the load, multi-stage sodium ion exchange for deep impurity removal, and limit-rate-controlled crystallization to suppress lattice entrainment, so as to realize the overall process concept of a purity leap from 3N grade to 4N / 5N grade.

[0078] Furthermore, the preparation method of this invention uses low-leaching materials in all material contact areas throughout the process, specifically including one or more of polytetrafluoroethylene lining, high-purity titanium, high-purity quartz, and Teflon pipes, to avoid equipment impurities leaching out and affecting the product's ultimate purity; the ion exchange resin is pre-converted to sodium form through repeated countercurrent transformation with high-purity sodium chloride solution, completely replacing impurity cations in the resin body, eliminating the treatment method of impurity leaching during operation; the pre-purification of seawater brine is based on the five-membered aqueous salt system phase diagram, controlling the concentration endpoint so that when sodium chloride crystallizes to saturation, associated components such as potassium, magnesium, and sulfate are in an unsaturated state, avoiding eutectic entrainment; during the limited-rate crystallization process, the solution supersaturation is controlled at 0.2%~0.5%, a crystal growth process is set up and the mother liquor enriched with impurities is periodically discharged to suppress the process parameters of lattice encapsulation and surface adsorption of impurities; the multi-stage membrane uses inorganic or organic ultrafiltration / nanofiltration membranes with a pore size of 0.5~1nm, an operating pressure of 0.6~1.2MPa, and a divalent ion removal rate of not less than 98%, which is a technical solution to reduce the load on the ion exchange unit.

[0079] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0080] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0081] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0082] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing 4N / 5N grade ultra-high purity sodium chloride suitable for multi-source brine, characterized in that, The preparation method includes the following: Prepare raw brine and, through a pre-purification process, obtain 3N grade sodium chloride intermediate. Dissolve the 3N grade sodium chloride intermediate in water and then sequentially perform membrane separation, ion exchange, and rate-controlled crystallization on the resulting crude sodium chloride brine to obtain 4N grade or higher ultra-high purity sodium chloride.

2. The preparation method according to claim 1, characterized in that, In the preparation method, the material used for all material contact parts during the entire process is at least one of polytetrafluoroethylene, high-purity titanium TA2, high-purity quartz, or Teflon.

3. The preparation method according to claim 1, characterized in that, The raw material brine is well and mine brine, and the pre-purification process includes precision filtration, multi-effect evaporation, evaporation crystallization, and solid-liquid separation performed sequentially.

4. The preparation method according to claim 3, characterized in that, The pore size of the precision filter is 4~6μm; And / or, the multi-effect evaporation includes a 2 to 4-effect evaporation system, adopting a co-current flow process, with the heating steam pressure of the first effect controlled at 0.2~0.4MPa, and the evaporation temperatures of each effect distributed in a gradient of 110℃, 95℃, 80℃, and 65℃; And / or, the concentration of NaCl in the concentrate obtained by the multi-effect evaporation is 310~330 g / L; And / or, the evaporation crystallization includes: using a mechanical vapor recompression crystallizer, an evaporation temperature of 70~80℃, controlling the evaporation rate by adjusting the frequency of the vapor compressor, and maintaining the supersaturation of the solution in the metastable range of 0.2%~0.5%; And / or, the solid-liquid separation includes: first centrifuging the filter cake with a centrifuge, then washing the filter cake countercurrently with pure water, and returning the resulting centrifuged mother liquor to the multi-effect evaporator for recycling.

5. The preparation method according to claim 1, characterized in that, The raw material brine is sea salt brine and / or lake salt brine. The pre-purification process includes sedimentation, filtration, isothermal evaporation crystallization, and solid-liquid separation performed sequentially. The isothermal evaporation crystallization is based on Na... + -K + -Mg 2+ / / Cl - -SO4 2- -H2O pentahydrate-salt system phase diagram, determine the boundary between the saturated crystallization region of sodium chloride and the unsaturated regions of all other associated salts, lock the target salt precipitation point where all other associated salts are in an unsaturated state when sodium chloride reaches saturation, and once the degree of isothermal evaporation crystallization reaches the target salt precipitation point, immediately perform the solid-liquid separation to obtain 3N grade sodium chloride intermediate.

6. The preparation method according to claim 5, characterized in that, The target temperature for the isothermal evaporation crystallization is 25~50℃; And / or, the solid-liquid separation includes: first centrifuging the solids with a centrifuge, and then washing the filter cake with a saturated sodium chloride pure solution.

7. The preparation method according to claim 1, characterized in that, The water used to prepare the crude sodium chloride brine has a resistivity ≥18.2 MΩ. cm; And / or, the concentration of the crude sodium chloride brine at 25°C is 200~250 g / L; And / or, the membrane separation includes sequential ultrafiltration and nanofiltration, wherein the ultrafiltration uses a ceramic membrane and / or an organic ultrafiltration membrane, and the nanofiltration uses a polyamide composite nanofiltration membrane; And / or, the ion exchange adopts a series process of cation exchange plus anion exchange, and all the ion exchange resins used are pre-converted to high-purity sodium form to prevent the leaching of impurities from the resin body. And / or, the controlled crystallization includes: maintaining the supersaturation of the solution at 0.2%~0.5% by adjusting the evaporation rate and the feed rate, controlling the crystal growth time at 1~3 hours, and discharging the mother liquor enriched with impurities every 1~2 hours, with the discharge amount being 3%~8% of the effective volume of the crystallizer.

8. The preparation method according to claim 7, characterized in that, Satisfy at least one of the following (1) to (5): (1) The ceramic membrane includes an alumina ceramic membrane or a zirconia ceramic membrane; (2) The material of the organic ultrafiltration membrane includes at least one of PES, PVDF or PS; (3) The pore size of the ultrafiltration is controlled at 0.5~1nm, the molecular weight cutoff is controlled at 200~500Da, the operating pressure is set at 0.1~0.4MPa, the cross-flow velocity is 1~2m / s, and the operating temperature is 25~40℃; (4) The turbidity of the water produced by the ultrafiltration process does not exceed 0.1 NTU; (5) The operating pressure of the nanofiltration is set to 0.6~1.2MPa, the cross-flow velocity is 1~2m / s, and the operating temperature is 25~40℃.

9. The preparation method according to claim 7, characterized in that, Satisfy at least one of the following (6) to (8): (6) The total cationic impurities at the outlet of the cation exchange shall not exceed 50 ppb; (7) The anion exchange uses a strong base anion exchange resin that has been converted from high-purity sodium form to chloride form; (8) The high-purity sodium conversion includes: loading ion exchange resin into the exchange column, first backwashing the resin layer with ultrapure water to remove fine particles, then using 5wt% high-purity sodium chloride solution for conversion, and finally forward washing the resin layer with ultrapure water until the chloride ion concentration of the effluent is consistent with that of the influent, thus completing the high-purity sodium conversion, and sealing it for later use.

10. The preparation method according to claim 9, characterized in that, The conversion using a 5wt% high-purity sodium chloride solution includes: passing a 5wt% high-purity sodium chloride solution countercurrently through the resin layer at a flow rate of 1~2 BV / h, introducing a conversion solution with a volume of 3 times that of the resin each time, soaking for 2~3 hours, and then draining it. This conversion operation is repeated at least 3 times.