Process for the preparation of a battery grade sodium carboxymethyl cellulose with low gel and low insolubles

Abstract

This invention relates to the field of cellulose ether preparation control technology, particularly a method for preparing battery-grade sodium carboxymethyl cellulose with low gel content and low insoluble matter. The method includes the following steps: S1, gas-phase controlled oxidation to reduce viscosity of the cellulose raw material; S2, stepwise gradient alkalization by adding alkali solution in two stages; S3, transferring the material to a kneader, discharging and collecting incompletely reacted fiber clumps through a bottom valve, and then sending the main purified material to an etherification machine; S4, temperature gradient directional etherification; S5, carbonyl reduction end-capping; S6, replacing counterions and transition metal ions coordinated with carboxyl groups to easily elutable forms, then precisely neutralizing with dilute alkali and washing to neutrality; S7, drying and pulverizing to obtain the finished product. The sodium carboxymethyl cellulose of this invention exhibits a uniform molecular chain distribution, low gel rate, and low insoluble matter content: gas-phase micro-oxygen viscosity reduction avoids localized over-reaction of hydrogen peroxide, and combined with low-temperature long-term wetting, results in a narrower molecular weight distribution.

Description

Technical Field

[0001] This invention relates to the field of cellulose ether preparation control technology, and in particular to a method for preparing battery-grade sodium carboxymethyl cellulose with low gel content and low insoluble matter. Background Technology

[0002] Sodium carboxymethyl cellulose (CMC-Na) is commonly prepared industrially as an aqueous binder for the negative electrode of lithium-ion batteries using the solvent method. This method uses refined cotton pulp or wood pulp as raw materials, which are alkalized and etherified in an ethanol-water medium. For oxidation and viscosity reduction, high-concentration hydrogen peroxide is often added directly, while for alkalization, a high-concentration liquid alkali is added in one go. The material is discharged from a kneader and transferred to an etherifier to complete the etherification. After washing, drying, and pulverizing, the finished product is obtained. The equipment is usually a production line with two kneaders and two etherifiers connected in series.

[0003] However, in actual production, after the finished product is formulated into negative electrode slurry, the simultaneous exceedance of gel and insoluble matter often occurs. Furthermore, extending the process time to improve uniformity leads to a decrease in production capacity and supply interruption. Existing methods have obvious shortcomings in terms of reaction uniformity control and rational production organization.

[0004] Insufficient uniformity in the reaction process is the main cause of gelation and insoluble matter formation. Localized addition of liquid hydrogen peroxide leads to uneven oxidant concentration distribution and significant differences in the degree of molecular chain breakage. Incompletely degraded chain segments during dissolution easily form surface gels. Furthermore, the aldehyde and ketone groups introduced during oxidation can trigger inter-chain cross-linking through hemiacetal bonding during dry storage, further exacerbating the gelation tendency. One-time high-concentration alkali treatment causes a rapid formation of a high-viscosity alkali cellulose layer on the fiber surface, hindering the penetration of alkali solution. Insufficient alkali treatment of the core results in residual fibrous insoluble matter after etherification.

[0005] In addition, the lack of process rationality in material transfer and production organization makes it difficult to fundamentally solve the above problems in large-scale production. The dead corner of the bottom valve of the kneader cannot be reached due to stirring. The attached incompletely alkalized fiber clumps have insufficient reaction activity after entering the etherification machine with the main material, and finally remain as insoluble matter.

[0006] Based on this, it is technically necessary to develop a method for preparing battery-grade sodium carboxymethyl cellulose with low gel content and low insoluble matter that combines gas-phase oxygen control and viscosity reduction, stepwise gradient alkalization and bottom valve purification, and has flexible production capabilities. Summary of the Invention

[0007] To solve one of the aforementioned technical problems, the present invention employs the following technical solution: a method for preparing battery-grade sodium carboxymethyl cellulose with low gel content and low insoluble matter, comprising the following steps: S1, subjecting cellulose raw materials to gas-phase controlled oxidation to reduce viscosity; S2, after oxidative degradation is completed, adding ethanol medium and wetting and dispersing agent to the reaction system, and then adding alkali solution in two stages for stepwise gradient alkalization; S3, after alkalization, transferring the material to a kneader, before discharging the material to the etherification machine, first discharging and collecting incompletely reacted fiber clumps through a bottom valve, then closing the bottom valve and sending the main pure material into the etherification machine; S4, ... After purification, the material undergoes temperature gradient directional etherification treatment; S5, the etherified product is subjected to carbonyl reduction end-capping treatment: after the etherification reaction is completed, a carbonyl reducing agent is added to the reaction system to eliminate the active sites for hemiacetal crosslinking between molecular chains; S6, after reduction is completed and preliminary neutralization is performed, the product is first soaked and washed with an ethanol-water mixture containing hydrochloric acid to replace the counter ions electrostatically trapped by the CMC molecular chains and the transition metal ions coordinated with the carboxyl groups into easily elutable forms, and then precisely neutralized with dilute alkali and washed until neutral; S7, the washed product is subjected to segmented rate-controlled drying treatment, and after drying, it is pulverized to obtain the finished product.

[0008] Preferably, S1 is performed according to the following steps: first, the reactor is evacuated to a vacuum degree of -0.085 to -0.095 MPa, and carbon dioxide is introduced to atmospheric pressure to complete the pre-replacement; after evacuating again, nitrogen is introduced to atmospheric pressure, and then the replacement is repeated 1 to 2 times in the evacuation-nitrogen filling method, and finally the residual oxygen volume fraction in the system is adjusted to 0.5% to 2.0%, and nitrogen filling is maintained at a slight positive pressure of 0.005 to 0.02 MPa;

[0009] First, let it stand at 15-20℃ for 15-20 minutes, then raise the temperature to 30-40℃ at 0.5-1℃ / min and maintain it for 20-40 minutes for staged heating oxidation; after the oxidation is completed, immediately introduce cooling water into the jacket to reduce the system temperature to below 15℃ within 10-20 minutes to complete the rapid cooling termination.

[0010] Specifically, the design of carbon dioxide pre-replacement is based on the dual requirements of optimizing the economy of the nitrogen replacement process and precisely controlling the oxygen concentration. In the traditional vacuum-nitrogen replacement process, the first replacement requires a large amount of high-purity nitrogen to remove air from the reactor. However, carbon dioxide, as an industrial gas with a significantly lower cost than high-purity nitrogen, can replace most of the air by first filling the reactor. Subsequent fine replacement requires only a small amount of nitrogen, reducing nitrogen consumption and the operating load of the nitrogen generator without affecting the final residual oxygen control accuracy. The residual oxygen volume fraction is controlled within a narrow range of 0.5% to 2.0%, which is based on the high sensitivity of the cellulose oxidative degradation reaction to oxygen concentration: when the oxygen concentration is below 0.5%, the driving force of the oxidation reaction is insufficient, the degree of polymerization decreases slowly, and the batch-to-batch fluctuations increase; when the oxygen concentration is above 2.0%, the reaction rate is too fast, the difference in the degree of local molecular chain breakage increases, the molecular weight distribution broadens, and the number of aldehyde and ketone groups introduced by oxidation increases, increasing the load on subsequent reduction and end-capping. Cellulose gas-solid oxidation is limited by the mass transfer of oxygen into the fiber interior. This step avoids severe chain breakage caused by local overconcentration of liquid hydrogen peroxide by controlling the oxygen concentration. Combined with staged heating to ensure a smooth start of the oxidation reaction and rapid cooling to prevent excessive degradation by residual heat, and finally with the reaction uniformity control in the subsequent etherification stage, the product molecular weight distribution PDI ≤ 1.90 is achieved.

[0011] A slight positive pressure of 0.005 to 0.02 MPa prevents outside air from seeping in and causing oxygen concentration to run out of control, while also avoiding excessive pressure that would increase the burden on equipment seals.

[0012] Preferably, after oxidation is terminated, under the condition of maintaining a slight positive pressure of nitrogen, the inner wall of the reactor is sprayed with an ethanol solution with a volume fraction of 90% to 95% to rinse the reactor wall for 5 to 10 minutes, so that the oxidized material adhering to the reactor wall is flushed into the reaction system, and the rinsing solution is incorporated into the alkalization medium.

[0013] Specifically, after the gas-phase oxidation viscosity reduction is completed, the cellulose material is in a loose, flocculent state. Some fine fibers adhere to the inner wall, lid, and agitator shaft of the reactor under the action of stirring and airflow. Because this part of the material is separated from the main liquid phase environment, it cannot fully react with the alkali solution and etherifying agent in the subsequent alkalization and etherification processes. Ultimately, it enters the finished product as unreacted fibrous insoluble matter. In traditional processes, the material is directly discharged to the next equipment, and the material adhering to the wall remains in the next batch or is not cleaned periodically, which causes material loss and affects product quality. In long-term continuous production, the material adhering to the wall may also cause local overheating and coking. After the coking particles fall off, they mix into the finished product and form black spot impurities. Using 90% to 95% ethanol to spray and wash the inner wall of the reactor, the wetting and penetration effect of ethanol on cellulose loosens the fibers adhering to the wall and causes them to fall off and merge into the main material. The washing time of 5 to 10 minutes is sufficient to cover the entire area of ​​the reactor wall and does not significantly extend the process cycle. The ethanol concentration is chosen to be 90%–95% instead of anhydrous ethanol because a small amount of water helps enhance the wetting effect on the fibers adhering to the wall surface, while avoiding the evaporation loss and safety risks associated with high-concentration ethanol. The washing solution is directly incorporated into the alkalization medium rather than discharged separately, ensuring that the adhering material enters the subsequent alkalization process together with the main material, obtaining sufficient reaction opportunities in the stepwise gradient alkalization and eliminating the potential for insoluble substances caused by residual material on the wall surface. This step is carried out under a slightly positive pressure of nitrogen after oxidation has ended, preventing air from entering and affecting the subsequent alkalization atmosphere, achieving quality improvement and material recovery without the need for additional equipment.

[0014] Preferably, the first stage in S2 is carried out according to the following steps: A refrigerant is introduced into the jacket of the reactor to pre-cool the system to 5-10°C; an ethanol aqueous solution with a volume fraction of 85%-95% is added, wherein the ethanol aqueous solution contains 0.5%-2.0% polyethylene glycol 200 wetting and dispersing agent relative to the mass of ethanol, and the mixture is stirred for 5-10 minutes until homogeneous; then, the first part of the alkali solution is added to make the NaOH concentration of the system 10%-20%; after the alkali is added, the mixture is intermittently stirred and ultrasonically assisted in wetting at 10-20°C for 60-90 minutes, wherein the intermittent stirring is stopped for 5 minutes every 10 minutes of stirring, the ultrasonic frequency is 20-40 kHz, and the power density is 0.1-0.3 W / cm², and the ultrasonic treatment is turned on during the period when the stirring is stopped.

[0015] Specifically, the combination of pre-cooling, wetting and dispersing agents, intermittent stirring, and ultrasonic assistance is aimed at ensuring uniform alkalization. The jacket is pre-cooled to 5-10°C by circulating a cooling medium, so that the system is in a low-temperature state before adding alkali. When alkali is added, the exothermic reaction of alkali cellulose formation is absorbed by the low-temperature environment, avoiding local temperature rise that would cause a high-viscosity alkali cellulose layer to form rapidly on the fiber surface, thus hindering the penetration of alkali solution. This temperature control is a prerequisite for the first stage of low-temperature, low-alkali wetting to be achieved.

[0016] When PEG-200 is added to the ethanol medium as a wetting and dispersing agent, its function is to reduce the surface tension of the fiber and promote the spreading and penetration of the alkali solution on the fiber surface. At the same time, the steric hindrance effect of PEG molecules can inhibit fiber aggregation, making the contact between the alkali solution and the fiber more uniform.

[0017] The dosage of PEG-200 is 0.5%–2.0% of the ethanol mass. Below 0.5%, the wetting and dispersion effect is insufficient; above 2.0%, the liquid phase viscosity is too high, affecting the alkali diffusion rate. The combination of intermittent stirring and ultrasonic assistance is crucial in this step: while continuous stirring is beneficial for macroscopic dispersion, the shearing effect may damage the fiber morphology and produce excessive fine powder. When stirring is stopped, ultrasound is activated, utilizing the micro-jets generated by the ultrasonic cavitation effect to promote the alkali solution into the crystalline and amorphous regions of the fiber, achieving enhanced penetration at the microscale. The 10-minute stirring and 5-minute pause cycle balances macroscopic dispersion and microscopic penetration. An ultrasonic frequency of 20–40 kHz and a power density of 0.1–0.3 W / cm² ensures cavitation effectiveness while avoiding excessive energy that could lead to cellulose molecular chain breakage.

[0018] In the first stage, at a low alkali concentration of 10%–20% and a low temperature of 10–20°C, the alkali solution penetration rate is greater than the alkali cellulose formation rate, allowing NaOH to fully diffuse into the fiber before the reaction occurs, thus avoiding the defect of external curing and internal rawness in traditional one-time high alkali treatment.

[0019] Preferably, the second stage in S2 is carried out according to the following steps: a second part of the alkaline solution is sprayed into the system to raise the NaOH concentration of the system to 20% to 30%. The second part of the alkaline solution is formed by mixing and dissolving liquid alkali and flake alkali in a mass ratio of 3 to 5:1. The liquid alkali part contains 30% to 50% of the alkaline solution that was recovered from the solid-liquid separation in the previous batch of production and then concentrated by membrane filtration. During the alkali addition process, the system temperature is controlled below 15°C. After the alkali addition is completed, the temperature is raised to 25 to 40°C and maintained for 30 to 60 minutes.

[0020] Specifically, after the first stage of low-temperature, low-alkali impregnation, the fiber interior has been fully penetrated by the alkali solution. At this point, it is necessary to replenish the alkali and increase the temperature to complete the thorough activation of the cellulose crystal regions. The second part of the alkali solution is added via spraying. Compared to traditional one-time pouring or pipeline injection, spraying disperses the alkali solution into fine droplets, increasing the contact area with the material and allowing the alkali solution to be evenly distributed throughout the material system in a short time. This avoids the formation of a surface alkali crust and uneven reaction caused by a sudden increase in local alkali concentration. Liquid alkali and caustic soda flakes are used in a mass ratio of 3–5:1. The liquid alkali provides a rapidly dispersed alkali source, while the caustic soda flakes dissolve slowly to maintain a stable alkali concentration in the system. The combination of the two ensures that the NaOH concentration in the second stage rises steadily to 20%–30%, avoiding concentration fluctuations caused by excessively high concentration peaks after adding only liquid alkali or delayed dissolution of caustic soda flakes. 30%–50% of the alkali-containing ethanol-water solution from the previous batch, which was recovered through solid-liquid separation and concentrated by membrane filtration, is mixed into the liquid alkali. This recovered solution originates from the filtrate produced after solid-liquid separation of the material from the previous batch. After being concentrated to a suitable NaOH concentration by nanofiltration membrane, it is reused, reducing both the consumption of fresh alkali solution and the amount of alkali-containing wastewater to be treated. Membrane filtration can retain fine cellulose powder and impurities in the filtrate, ensuring that the quality of the recovered solution does not affect the alkali treatment effect. During the alkali addition process, the system temperature is controlled below 15℃. The heat of alkali dilution and the heat of reaction for alkali cellulose generation are removed by jacket cooling to prevent local temperature rise during the alkali addition stage from disrupting the uniform permeation state established in the first stage. The second stage of alkali treatment is carried out at a temperature of 25–40℃ for 30–60 minutes. Under the higher alkali concentration and moderate temperature after alkali replenishment, the cellulose crystal regions gradually swell and activate, laying the foundation for the uniform entry and reaction of the subsequent etherifying agent. If the temperature is too low, the crystal regions will not be sufficiently activated; if the temperature is too high, the alkali cellulose will undergo side reactions such as peeling and oxidative degradation, affecting the molecular weight and purity of the product.

[0021] Preferably, S3 is performed according to the following steps: After alkalization, the material is transferred to a kneader and stirred at 15-20 rpm for 2-3 minutes; then the stirring speed is reduced to 5-10 rpm and the kneader is rotated alternately in both directions for 1-2 minutes, while the vibrator on the outer wall of the kneader is turned on to assist in material discharge with an amplitude of 2-5 mm and a frequency of 20-30 Hz; then the bottom valve is opened for 2-3 seconds to discharge the dead corner material to a special collection device, and the discharged dead corner material is crushed and mixed into the next batch of alkalized material at a ratio of 5%-10% for reuse; after closing the bottom valve, normal stirring is resumed and the main material is sent to the etherification machine at a normal discharge rate.

[0022] Specifically, after alkalization, the material is transferred to a kneader for pretreatment before etherification. The discharge valve area at the bottom of the kneader is inaccessible to the stirring blades, creating a dead zone for material flow. Fiber clumps adhering to the valve plate and seat cannot be carried away during the main material discharge, remaining in the next batch or being intermittently discharged with subsequent batches. This dead zone material, due to its uncertain residence time in the kneader and lack of sufficient stirring, exhibits uneven alkalization levels, resulting in insufficient reactivity upon entering the etherification machine, ultimately remaining as fibrous insoluble matter. This solution involves stirring at 15–20 rpm for 2–3 minutes before discharge to ensure uniform material distribution, followed by alternating forward and reverse rotation at 5–10 rpm for 1–2 minutes. This low-speed forward and reverse rotation causes the material to tumble and shear within the kneader, loosening the material near the bottom valve. Simultaneously, the external vibrator vibrates at an amplitude of 2–5 mm and a frequency of 20–30 Hz. This mechanical vibration is transmitted through the kneader wall to the bottom valve area, causing the fiber clumps adhering to the dead zone of the valve plate and seat to detach from the wall under the force of vibration. Open the bottom valve for 2-3 seconds to initiate micro-discharge, using the material's own weight to expel the loosened dead material to a dedicated external collection device. The discharge time must be strictly controlled within 2-3 seconds; too short a time results in incomplete removal of dead material, while too long a time increases the loss of the main material. The discharged dead material is then crushed and added at 5%-10% to the next batch of alkalization material, allowing it to have a sufficient reaction opportunity in the next alkalization process, thus achieving waste resource utilization. After closing the bottom valve, normal stirring is resumed, and the main material is fed into the etherification machine at the normal discharge rate, removing dead material sources that cause insoluble matter in the finished product. This step does not change the main process route; significant quality improvement can be achieved simply through optimized operation sequence and simple vibration assistance.

[0023] Preferably, the first stage in S4 is carried out according to the following steps: 20% to 30% of the total etherification dose of chloroacetic acid is first added to the etherification machine, and the reaction is carried out at 25 to 30°C for 15 to 20 minutes to complete the pre-etherification; then the remaining first-stage etherification agent, which accounts for 20% to 25% of the total etherification dose, is slowly added dropwise at a relative rate of 1% to 3% / min. During the dropwise addition, the temperature is maintained at 25 to 35°C and microwave-assisted etherification with a power density of 0.2 to 0.5 W / cm² is turned on. After the dropwise addition is completed, the reaction continues for 15 to 30 minutes.

[0024] Specifically, etherification is a nucleophilic substitution process involving chloroacetic acid and hydroxyl groups in alkali cellulose. The dehydrated glucose units of cellulose have three substituted hydroxyl groups at C2, C3, and C6. The C6 position is the primary hydroxyl group, exhibiting the least steric hindrance and the highest reactivity. The C2 and C3 positions are secondary hydroxyl groups, with reactivity decreasing sequentially. Traditional processes involve adding all chloroacetic acid at once and reacting at high temperatures. This leads to the rapid consumption of a large amount of etherifying agent at the C6 position, while the C2 and C3 positions suffer from insufficient substitution due to lower reactivity and decreased etherifying agent concentration. The final product exhibits uneven substitution distribution, poor solubility, and poor gelation performance.

[0025] This method first involves adding 20%–30% of the total etherification dose of chloroacetic acid and reacting it at 25–30°C for 15–20 min. At this temperature, the primary hydroxyl group at the C6 position has relatively high reactivity, which is beneficial for increasing the initial substitution ratio at the C6 position, while avoiding the slow reaction rate at low temperatures that would affect efficiency. Subsequently, 20%–25% of the total etherification dose of chloroacetic acid is added dropwise at a relative rate of 1%–3% / min. The slow addition maintains the concentration of chloroacetic acid in the system at a low and stable level, avoiding excessively high local concentrations and side reactions such as chloroacetic acid hydrolysis caused by a one-time addition. If the dropping rate is lower than 1% / min, the reaction time will be too long, affecting the production capacity; if it is higher than 3% / min, the concentration control effect will be weakened. During the dropwise addition phase, maintain the temperature at 25–35°C and turn on the microwave at 0.2–0.5 W / cm². The selective heating effect of the microwave preferentially provides energy to polar reaction sites such as the hydroxyl groups of alkali cellulose and the carboxyl groups of chloroacetic acid, promoting a uniform substitution reaction. Simultaneously, the non-thermal effect of the microwave lowers the activation energy, achieving a satisfactory reaction rate at a lower temperature. Microwave power below 0.2 W / cm² has little auxiliary effect, while power above 0.5 W / cm² may lead to localized overheating and uncontrolled temperature rise of the material. After the dropwise addition is complete, continue the reaction for 15–30 minutes to ensure the added etherifying agent is fully consumed, laying a uniform substitution foundation for the subsequent heating phase.

[0026] Preferably, the second stage in S4 is carried out according to the following steps: the remaining chloroacetic acid is added to the system, and the system is kept at 35-45°C for 20-30 min, then the temperature is increased to 55-65°C in steps of 1-2°C / min, and the reaction is carried out at 55-65°C for 60-120 min.

[0027] Specifically, after the pre-etherification and dropwise addition stages, the C6 position has achieved sufficient substitution, but residual chloroacetic acid still needs to react with the secondary hydroxyl groups at the C2 and C3 positions. Due to steric hindrance, the C2 and C3 positions require higher temperatures to achieve sufficient reactivity. However, directly jumping from the 25–35°C of the dropwise addition stage to the final reaction temperature of 55–65°C would cause a rapid increase in the reaction rate, with localized exothermic reactions and uneven concentrations further broadening the substitution distribution. In this scheme, after adding the residual chloroacetic acid, the temperature is first maintained at 35–45°C for 20–30 minutes. This intermediate temperature range allows the reaction at the C2 and C3 positions to gradually initiate, ensuring uniform diffusion and contact with the active sites under mild conditions, avoiding runaway reactions caused by sudden temperature increases. The 20–30 minute holding time ensures a relatively complete reaction at this intermediate temperature, providing a uniform reaction substrate for subsequent temperature increases. The temperature is then increased to 55–65°C at a rate of 1–2°C / min and reacted for 60–120 min. This slow heating allows the reaction rate to gradually increase with temperature, resulting in a uniform and controllable exothermic reaction. 55–65°C is the suitable temperature range for cellulose etherification. Below 55°C, the reaction rate at the C2 and C3 positions is too slow, and the reaction time is too long. Above 65°C, chloroacetic acid hydrolysis intensifies, the effective utilization rate of the etherifying agent decreases, and byproducts increase. The reaction time of 60–120 min is adjusted according to the amount of material and the target degree of substitution to ensure sufficient substitution at the C2 and C3 positions while avoiding over-reaction that leads to a decrease in molecular weight. This stepped heating regime results in a more reasonable reaction rate distribution throughout the etherification process, with a more balanced ratio of substitution degrees at the C6, C2, and C3 positions. After product dissolution, the molecular chains are evenly extended in the solution, reducing entanglement and gel formation caused by uneven substitution distribution. This temperature control can be achieved simply by programmed heating in the jacket of the etherification machine.

[0028] Preferably, S5 is carried out according to the following steps: After the etherification reaction is completed, the system temperature is first lowered to 15-25°C, and the pH is adjusted to 9.5-10.5 with dilute acetic acid; after purging with nitrogen for 10-15 minutes, sodium borohydride is prepared into a 0.5%-1.0% alkaline aqueous solution and added to the system in 3-5 batches, with an interval of 5-8 minutes between each batch, and the total amount added is 0.1%-0.3% relative to the dry cellulose mass; after the addition is completed, the mixture is stirred for 15-30 minutes, and finally, oxygen diluted with nitrogen is introduced with an oxygen volume fraction of 5%-10%, and the reaction is terminated after 5-10 minutes; after the reduction reaction is completed, the material is centrifuged to separate the materials and the reduction waste liquid is collected; calcium chloride is added to the reduction waste liquid to precipitate borate, and after filtration, the filtrate is treated with a cation exchange resin to remove calcium ions and then reused in the washing process or alkali preparation process that does not directly contact the product.

[0029] Specifically, the design of carbonyl reduction end-capping and waste liquid borate recovery addresses the carbonyl side effects introduced by gas-phase oxidation viscosity reduction and the resource utilization of boron-containing waste liquid. During the oxygen oxidation degradation of cellulose, glycosidic bonds break, introducing aldehyde and ketone groups at the ends and in the chain. These carbonyl groups can form hemiacetal bonds with hydroxyl groups on adjacent chains during subsequent drying and storage, leading to inter-chain crosslinking. This results in increased gel particles and insoluble matter during product reconstitution. Existing technologies often indirectly mitigate this by controlling the degree of oxidation. This scheme includes a dedicated carbonyl reduction step after etherification to reduce the aldehyde and ketone groups introduced by oxidation to hydroxyl groups, eliminating the active sites of hemiacetal crosslinking from a chemical structural perspective. Before reduction, the temperature is lowered to 15–25°C, and the pH is adjusted to 9.5–10.5 with dilute acetic acid. Sodium borohydride exhibits good stability and suitable reducing activity under weakly alkaline conditions. Reduction efficiency decreases above pH 10.5, and sodium borohydride becomes ineffective due to acid-catalyzed decomposition below pH 9.5, producing hydrogen gas. Nitrogen is used to purge oxygen from the system, preventing oxygen from consuming sodium borohydride. Sodium borohydride is prepared as a 0.5%–1.0% alkaline aqueous solution and added in 3–5 batches. Adding in batches avoids excessively high local reducing agent concentrations, violent hydrogen release, and ineffective decomposition of the active ingredient caused by a single addition. Each batch is added after a 5–8 minute interval to allow sufficient reaction time. The total amount is 0.1%–0.3%, adjusted according to the degree of oxidation. After the reaction, excess sodium borohydride is slowly oxidized by passing nitrogen-diluted oxygen (5%–10% oxygen by volume) for 5–10 minutes, terminating the reaction. Under these conditions, the ethanol vapor concentration is below the lower explosive limit, avoiding the risk of combustion and explosion. After reduction, the waste liquid is collected by centrifugation. This waste liquid contains boron-containing species such as sodium metaborate and borax. Direct discharge wastes boron resources and increases the difficulty of wastewater treatment. Calcium chloride is added to convert borate into slightly soluble calcium borate precipitate. After filtration, the filtrate is treated with a cation exchange resin to remove calcium ions before being reused in the alkaline solution preparation or initial washing process, preventing calcium impurities from being introduced into the product.

[0030] Preferably, S6 is carried out according to the following steps: after reduction, the pH is adjusted to 7-8 with acetic acid, and most of the liquid phase is removed by centrifugation;

[0031] First acid exchange: Add an ethanol-water solution with a volume fraction of 70%–85% ethanol and a mass fraction of 0.5%–1.0% hydrochloric acid, stir at 20–40°C for 15–30 min, and then centrifuge. Second acid exchange: Repeat the above acid exchange operation once. Subsequently, disperse the material in ethanol-water to prepare a sodium carboxymethyl cellulose dispersion with a solid-liquid mass ratio of 1%–3%, and desalinate with the assistance of electrodialysis at a voltage of 10–20 V and a flow rate of 0.1–5 m / s. 3 / h; then neutralize to pH 6.5-7.5 with an ethanol-water solution containing 0.5%-1.0% sodium hydroxide, centrifuge, soak in anhydrous ethanol for 10-15 min to complete ethanol replacement, and centrifuge to remove ethanol.

[0032] Specifically, after reduction, the system is weakly alkaline. The pH is first adjusted to 7-8 with acetic acid, followed by centrifugation to separate most of the liquid phase from the sodium carboxymethyl cellulose solid phase, creating suitable starting conditions for subsequent acid exchange. Acid exchange washing is the core innovation of this step: when the carboxyl groups on the sodium carboxymethyl cellulose molecular chain exist in the form of sodium salts, Na... + As counterions, they electrostatically bind to carboxyl groups. Simultaneously, transition metal ions such as iron and chromium introduced from fiber raw materials and processing equipment can coordinate with carboxyl groups. These ions are difficult to completely remove during conventional water washing due to the Donnan equilibrium effect. Residual metal ions not only affect product purity but may also catalyze electrolyte decomposition in battery applications. Acid exchange is performed using an ethanol-water solution containing 0.5%–1.0% hydrochloric acid and 70%–85% ethanol. + The sodium form of CMC is converted to the hydrogen form, while electrostatically retained Na is also converted. + The coordinated metal ions are replaced into a free state and enter the liquid phase. Ethanol medium inhibits the hydrogen-form swelling of CMC, keeping the particles dispersed and preventing blockage of ion diffusion channels after swelling. Hydrochloric acid concentrations below 0.5% result in insufficient ion exchange, while concentrations above 1.0% increase acid consumption and subsequent neutralization load. An ethanol volume fraction of 70%–85% balances the inhibition of swelling and ion migration rate. Two acid exchanges ensure thorough replacement. After centrifugation, CMC is dispersed in ethanol-water to prepare a low-concentration dispersion with a solid-liquid ratio of 1%–3%, which is further removed by electrodialysis to remove free H+. + Cl - Na + Plasma electrodialysis, driven by an electric field, enables selective ion separation through a membrane. Under low concentration conditions, it exhibits low concentration polarization and high desalination efficiency, with water consumption significantly lower than conventional multiple water washes. After desalination, the material is neutralized to pH 6.5–7.5 using ethanol-water containing 0.5%–1.0% sodium hydroxide, precisely converting the hydrogen-form CMC back to the sodium-form. Controlling the final pH prevents the product from becoming too acidic or alkaline, which could affect battery applications. Finally, residual moisture is replaced by soaking in anhydrous ethanol, reducing subsequent drying energy consumption and preventing CMC from swelling and clumping under high water content. After centrifugation, the material enters the drying process.

[0033] Preferably, the ethanol-water waste liquid generated from acid exchange and neutralization washing is separated and recovered into ethanol by a distillation column, with a recovered ethanol concentration of ≥95%, and reused in the alkalization and washing process; the residue at the bottom of the distillation column is evaporated and concentrated to recover sodium chloride and sodium acetate by-products.

[0034] Specifically, the production of sodium carboxymethyl cellulose generates a large amount of washing wastewater, mainly composed of water, ethanol, sodium chloride, sodium acetate, and a small amount of organic matter. Traditional processes directly send this wastewater to wastewater treatment plants, wasting valuable ethanol as a solvent. Treating saline wastewater is costly and difficult due to the challenges of biochemical treatment. This solution uses distillation to separate the ethanol-water wastewater generated during acid exchange and neutralization washing. Utilizing the boiling point difference between ethanol (78.4℃) and water (100℃), the ethanol is recovered to a concentration ≥95% via a distillation column. The recovered ethanol can be directly reused in the alkalization and washing processes, achieving an ethanol recovery rate of over 90%, significantly reducing the purchase of fresh ethanol and VOC emissions. The ethanol concentration recovered by distillation is controlled at ≥95%, which meets the requirements for preparing 85%–95% ethanol aqueous solutions for alkalization media and 70%–85% ethanol aqueous solutions for washing liquid, eliminating the need for further dehydration to anhydrous ethanol, thus reducing distillation energy consumption and operational complexity. The bottom liquid of the distillation column is an aqueous solution containing sodium chloride and sodium acetate. Both salts have recycling value: sodium chloride, after evaporation, concentration, crystallization, and recrystallization, can reach industrial-grade standards and be used in the chlor-alkali industry or as a de-icing agent; sodium acetate, after concentration and crystallization, can be used as a chemical raw material or a carbon source for wastewater treatment. Byproduct recovery significantly reduces solid waste emissions throughout the production process, achieving a resource recycling model that turns waste into treasure. Before recovery, the bottom liquid must be filtered to remove suspended cellulose powder and mechanical impurities to ensure the quality of the crystalline product. This recovery system operates in parallel with the main production line, without affecting continuous production of the main process. It has a short investment payback period and significant economic benefits in large-scale production. It also aligns with national policies promoting green and low-carbon development in the chemical industry, reducing the company's environmental compliance risks.

[0035] Preferably, S7 is carried out according to the following steps: adding 0.05% to 0.1% of hydrogen bond modifier urea relative to the oven-dry CMC mass to the washed material, mixing evenly, and then extruding and granulating to a particle size of 2 to 5 mm. After drying, the residual urea content is ≤50 ppm.

[0036] The first stage involves drying at 80-100℃ until the solid content is about 30%, followed by turning the material once using an online turning device.

[0037] The second stage involves drying at 60–80°C until the solid content is approximately 50%.

[0038] The third stage involves drying at 70–90°C until the moisture content is below 8%.

[0039] The waste heat from the drying exhaust gas is recovered by a heat exchanger and used to preheat the first stage drying air intake. After drying, the material is pre-cooled to 0-10℃, and cold air at 0-10℃ is introduced for low-temperature pulverization. The temperature after pulverization is controlled not to exceed 40℃. After being screened through a 200-mesh sieve, the coarse material is returned to the pulverization chamber.

[0040] Specifically, after washing and replacement with anhydrous ethanol, the material still contains a certain liquid phase. Urea is added at 0.05%–0.1% of the oven-dried CMC mass as a hydrogen bond modifier. During drying, urea molecules form reversible hydrogen bonds with the hydroxyl groups of CMC, competitively inhibiting irreversible hydrogen bond association between CMC molecular chains. This reduces the formation of nano-aggregates due to molecular chain proximity and cross-linking caused by water evaporation during drying. These aggregates are difficult to disperse completely upon reconstitution and exhibit gel-like particles. If the urea dosage is below 0.05%, the hydrogen bond modulation effect is insufficient; if it is above 0.1%, residual urea affects product purity and battery applications. By controlling the urea addition and drying conditions, the residual urea content after drying is ensured to be ≤50ppm, and no urea-like byproducts are detected. After uniform mixing, the mixture is extruded and granulated to a particle size of 2–5mm. The granular material has good air permeability and uniform drying in the dryer, avoiding channeling and clumping during the drying of powdered materials. The staged, speed-controlled drying process adjusts the temperature according to changes in the material's moisture content: In the first stage, with high moisture content, a higher temperature of 80-100℃ is used for rapid drying to improve efficiency. When the solid content reaches approximately 30%, the material is turned over once by an online turning device to eliminate uneven drying across the material layer. In the second stage, with medium moisture content, the temperature is lowered to 60-80℃ to prevent surface crusting that hinders internal moisture diffusion, drying to approximately 50% solid content. In the third stage, with low moisture content, the temperature is raised to 70-90℃ to ensure the moisture content reaches below 8%, while avoiding prolonged high temperatures that could lead to thermal degradation and oxidative discoloration. The drying exhaust gas contains a large amount of sensible heat, which is recovered through a heat exchanger to preheat the first-stage drying air intake, reducing steam consumption. After drying, the material is pre-cooled to 0-10℃ before low-temperature pulverization. Cold air at 0-10℃ is introduced to control the post-pulverization temperature to not exceed 40℃, preventing molecular chain breakage and localized thermal cross-linking that could cause gel formation due to mechanical and chemical effects. A 200-mesh sieve ensures uniform product fineness, and coarse material is returned to the pulverizing chamber for re-pulverization. This drying-pulverizing system organically combines product quality control with energy consumption optimization and green production.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] 1. The sodium carboxymethyl cellulose of this invention has a uniform molecular chain distribution and extremely low gelation rate: gas-phase micro-oxygen viscosity reduction avoids local over-reaction of hydrogen peroxide, and combined with low-temperature long-term wetting, it makes the product molecular weight distribution (PDI) narrower, and almost no fish eyes are formed when dissolved in water. High purity and significantly reduced insoluble content: stepwise alkalization ensures the uniformity of degree of substitution (DS); in particular, the bottom valve micro-discharge process effectively removes unreacted fibers remaining in the dead corners of the equipment, significantly reducing the insoluble content of the finished product.

[0043] 2. Gas-phase controllable oxidation replaces liquid hydrogen peroxide to achieve uniform viscosity reduction. Carbonyl reduction end-capping eliminates aldehyde and ketone groups introduced by oxidation from a chemical structure perspective, blocking the hemiacetal crosslinking pathway between molecular chains. Bottom valve micro-discharge removes incompletely alkalized fiber clusters from the dead corners of the kneader, cutting off the source of insoluble matter at the source. Segmented speed-controlled drying combined with urea hydrogen bond regulation inhibits the formation of nano-aggregates by irreversible hydrogen bond association of molecular chains during the drying process, and the residual urea content is controlled below 50 ppm.

[0044] 3. Uniformly dispersed oxidation with trace amounts of oxygen, combined with segmented heating and rapid cooling termination, ensures uniform molecular chain breakage and a molecular weight distribution PDI ≤ 1.90. Stepwise gradient alkalization, combined with osmotic pressure control and ultrasonic cavitation assistance, allows the alkali solution to fully penetrate from the surface to the core, avoiding insufficient activation of the surface alkali shell and core caused by one-time high alkalization. Temperature gradient directional etherification enables a high initial substitution ratio at the C6 position at low temperatures, while the C2 / C3 positions complete substitution after heating, resulting in a balanced distribution of substitution degrees at all three positions. The viscosity variation coefficient (CV) for 10 consecutive batches is only 1.0%–1.5%, lower than traditional processes, providing a reliable guarantee for batch consistency and coating stability of the battery negative electrode slurry.

[0045] 4. Acid exchange deep washing breaks through the Donnan equilibrium limit of conventional water washing, replacing and removing counterions electrostatically trapped by the sodium carboxymethyl cellulose molecular chain and transition metal ions coordinated with carboxyl groups. Combined with electrodialysis, it further reduces the content of free ions, resulting in high product purity and low ash content. Less gel and less insoluble matter ensure uniform dispersion of binder in negative electrode slurry, complete coating of graphite particles, and stable electrode interface impedance.

[0046] 5. Carbon dioxide pre-replacement reduces the consumption of high-purity nitrogen. The recovered alkaline solution is concentrated by membrane filtration and reused in the second stage of alkalization. The washing waste liquid is distilled to recover ethanol with a concentration of ≥95% for recycling. The reduction waste liquid is treated with calcium chloride to precipitate borate, and the filtrate is decalcified by cation exchange resin and reused. The waste heat from the drying tail gas is used to preheat the air intake. The multi-stage resource recycling significantly reduces raw material consumption, energy consumption and emissions of waste gas, wastewater, and solid waste. Attached Figure Description

[0047] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or components are generally identified by similar reference numerals. In the drawings, the elements or components are not necessarily drawn to scale.

[0048] Figure 1 This is a temperature-time control curve for the key process of this invention.

[0049] Figure 2 This is a comparison diagram of the molecular weight distribution of the embodiments and comparative examples of the present invention.

[0050] Figure 3 This is a comparison diagram of the particle size distribution of gel particles in the embodiments and comparative examples of the present invention. Detailed Implementation

[0051] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore merely examples and should not be used to limit the scope of protection of the present invention. Specifically, as shown below... Figures 1-3 As shown.

[0052] To make the technical solution of this invention clearer, the invention will now be further described. This invention provides a method for preparing battery-grade sodium carboxymethyl cellulose with low gel content and low insoluble matter. The specific raw materials and equipment are as follows: refined cotton pulp (degree of polymerization approximately 1200, methyl cellulose content ≥99.5%); sodium hydroxide (liquid alkali mass fraction 32%, flake alkali purity ≥99%); chloroacetic acid (purity ≥99.5%); sodium borohydride (purity ≥98%); polyethylene glycol 200 (PEG-200, chemically pure); urea (analytical grade); hydrochloric acid (mass fraction 36%–38%); anhydrous ethanol (≥99.7%); carbon dioxide (≥99.9%); nitrogen (≥99.99%). The equipment includes a stainless steel reactor with a jacket and vacuum / gas filling interface, a kneader (with a bottom valve and external wall vibrator), an etherifier (with a microwave feed inlet), a centrifuge, an electrodialysis unit, a distillation column, a belt dryer (with an online material turning device and a tail gas heat exchanger), and a cryogenic pulverizer. The production system adopts an existing flexible configuration of three parallel kneaders and three parallel etherifiers.

[0053] Preparation method: Follow the steps described above, with each step performed as follows.

[0054] S1, Gas-phase controlled oxidation viscosity reduction: The reactor is evacuated to -0.085 to -0.095 MPa, and carbon dioxide is introduced to atmospheric pressure for pre-replacement. Low-cost carbon dioxide replaces most of the air in the reactor, reducing subsequent consumption of high-purity nitrogen and avoiding local oxygen concentration fluctuations during direct nitrogen purging. After evacuation again, nitrogen is introduced to atmospheric pressure. This evacuation-nitrogen purging process is repeated 1-2 times until the final residual oxygen volume fraction is controlled at 0.5%–2.0%. Cellulose gas-solid oxidation is limited by mass transfer of oxygen into the fiber interior. This step controls the oxygen concentration at 0.5%–2.0% to avoid severe chain breakage caused by excessive local concentration of liquid hydrogen peroxide. Combined with staged heating to ensure a smooth start-up of the oxidation reaction and rapid cooling to prevent excessive degradation by residual heat, this step is ultimately used in conjunction with subsequent etherification. To ensure uniformity of the reaction during the oxidation stage, the molecular weight distribution (PDI) of the product is controlled to be ≤1.90. Nitrogen is used to maintain a slight positive pressure of 0.005–0.02 MPa to prevent external air infiltration and maintain a stable oxygen concentration in the system. The system is first allowed to stand at 15–20°C for 15–20 minutes. This low-temperature standing allows oxygen to diffuse and balance fully in the cellulose pores, preventing uneven chain breakage caused by excessively high local oxygen concentrations after heating. Then, the temperature is increased to 30–40°C at a rate of 0.5–1°C / min and maintained for 20–40 minutes. This slow heating allows the oxidation reaction to start smoothly and prevents excessive differences in the degree of molecular chain breakage caused by sudden temperature rises. After oxidation, cooling water is circulated to reduce the temperature to below 15°C within 10–20 minutes, rapidly lowering the system temperature to terminate the oxidation reaction and preventing excessive degradation and broadening of the molecular weight distribution due to residual heat in the reactor.

[0055] S2, Stepwise Gradient Alkalization: Maintain a slight positive pressure of nitrogen, spray 90%–95% ethanol onto the inner wall of the reactor for 5–10 minutes to rinse the reactor wall, and incorporate the rinsing solution into the system. This washes away the oxidized cellulose adhering to the reactor wall and incorporates it into the main material, reducing material loss and preventing residual material on the wall from entering the finished product due to incomplete reaction and forming insoluble matter. Pre-cool the jacket to 5–10°C with a cooling medium to reduce the exothermic reaction in the initial stage of alkali addition, preventing local temperature rise from causing the rapid formation of a high-viscosity alkali cellulose layer on the fiber surface, which would hinder the penetration of the alkali solution. Add 85% ethanol... A 95%–95% ethanol aqueous solution (with PEG-200 pre-dissolved at 0.5%–2.0% of the ethanol mass) is prepared. PEG-200 acts as a wetting and dispersing agent, reducing fiber surface tension and promoting the spreading and penetration of the alkali solution on the fiber surface. Simultaneously, the steric hindrance effect of PEG molecules inhibits fiber aggregation, resulting in more uniform contact between the alkali solution and the fiber. Stir for 5–10 minutes. Add the first portion of the alkali solution to achieve a NaOH concentration of 10%–20%, and stir intermittently at 10–20°C. The process involves immersion for 60-90 minutes with a combination of 10-minute pauses followed by 5-minute breaks, and ultrasonic infiltration (20-40 kHz, 0.1-0.3 W / cm², activated when stirring is stopped). Under low-alkali and low-temperature conditions, the alkali penetration rate exceeds the reaction rate, ensuring that NaOH fully penetrates the fiber interior. Intermittent stirring prevents continuous shearing from damaging the fiber morphology. Ultrasonic infiltration during the pause utilizes cavitation to promote the alkali solution's entry into the fiber crystal region. This alternation of infiltration and dispersion achieves synergistic effects. Subsequently, a second portion of alkali solution (liquid alkali to flake alkali mass ratio 3-5:1, the liquid alkali containing 30%-50% alkali-containing ethanol-water solution recovered through membrane filtration, with the temperature ≤15℃ during alkali addition) is sprayed in. The spray method increases the alkali solution dispersion area and avoids sudden increases in local alkali concentration. The alkali solution is recycled for reuse to reduce alkali consumption and wastewater discharge. Temperature control during alkali addition prevents exothermic formation of an alkali shell on the surface. The NaOH concentration is raised to 20%-30%, and the temperature is increased to 25-40℃ and maintained for 30-60 minutes. After alkali replenishment, the temperature is raised again to complete the thorough activation of the cellulose crystal region, laying the foundation for subsequent uniform etherification.

[0056] S3, Bottom Valve Micro-Discharge Purification: The material is transferred to the kneader and stirred at 15-20 rpm for 2-3 minutes, then reduced to 5-10 rpm with alternating forward and reverse rotation for 1-2 minutes. Simultaneously, the external vibrator vibrates at an amplitude of 2-5 mm and a frequency of 20-30 Hz. The low-speed forward and reverse rotation ensures that the material is fully agitated, and the vibration loosens and removes the fiber clumps attached to the dead corners of the valve plate and valve seat. The bottom valve is opened for 2-3 seconds to discharge the dead corner material (after crushing, it is mixed into the next batch of alkalization material at 5%-10%). The dead corner of the bottom valve is an area that the stirring paddle cannot reach. If the attached incompletely alkalized fiber clumps enter the etherification machine with the main material, they will remain as insoluble substances due to insufficient reactivity. Micro-discharge removes this dead material source from the source, and the discharged material is crushed and reused in the next batch of alkalization process to avoid material waste. After closing the bottom valve, the main material is sent to the etherification machine.

[0057] S4, Temperature Gradient Directed Etherification: First, add chloroacetic acid at 20%–30% of the total etherification dose and react at 25–30℃ for 15–20 min. At this temperature, the primary hydroxyl group at C6 has relatively high reactivity, which is beneficial for increasing the initial substitution ratio at C6 and controlling the distribution of carboxymethyl groups on the dehydrated glucose unit. Then, add 20%–25% chloroacetic acid dropwise at a rate of 1%–3% of the total etherification dose per min, maintaining the temperature at 25–35℃ and turning on the microwave at 0.2–0.5 W / cm² during the dropwise addition. Slowly add the etherifying agent to control the concentration and avoid localized reactions. The reaction was initially violent. Microwave selective heating of the polar reaction sites promoted uniform substitution, shortened the reaction time, and reduced side reactions. After the addition was complete, the reaction continued for 15–30 min. The remaining chloroacetic acid was added, and the mixture was kept at 35–45 °C for 20–30 min. The intermediate temperature holding allowed the secondary hydroxyl groups at the C2 and C3 positions to gradually initiate the reaction at a moderate temperature. The temperature was then increased to 55–65 °C at a rate of 1–2 °C / min, and the reaction was carried out for 60–120 min. The stepwise temperature increase completed the substitution at the C2 and C3 positions, avoiding excessively wide substitution distribution and loss of the etherifying agent due to hydrolysis caused by a single high temperature.

[0058] S5, Carbonyl Reduction End-Capping: Cool to 15–25°C, adjust pH to 9.5–10.5 with dilute acetic acid. Sodium borohydride is stable and has suitable reducing activity under weakly alkaline conditions. If the pH is too high, the reduction efficiency will decrease; if the pH is too low, sodium borohydride will decompose and become ineffective. Purge with nitrogen for 10–15 minutes. Prepare a 0.5%–1.0% alkaline aqueous solution of sodium borohydride and add it in 3–5 batches (5–8 minutes apart), with a total amount of 0.1%–0.3% of the oven-dry cellulose. Adding in batches avoids localized violent hydrogen release and ineffective decomposition of the reducing agent caused by a single addition. This allows the aldehyde and ketone groups to be gradually reduced to hydroxyl groups, eliminating the inter-chain hemiacetal crosslinking active sites introduced by oxidative degradation. Stir for 15–30 minutes. Pour oxygen diluted with nitrogen (5%–10% by volume) and terminate the reaction in 5–10 minutes to slowly oxidize excess sodium borohydride. Under these conditions, the ethanol vapor concentration is below the lower explosive limit to avoid the risk of combustion and explosion.

[0059] S6, Deep washing with acid exchange: After reduction, adjust the pH to 7-8 with acetic acid, centrifuge to remove most of the liquid phase, collect the reduction waste liquid, add calcium chloride to precipitate borate, so that the borate in the waste liquid is converted into slightly soluble calcium borate precipitate for easy separation, the filtrate is reused in the washing process to reduce boron emissions and water consumption; wash with an ethanol-water solution (70%-85% ethanol) containing 0.5%-1.0% hydrochloric acid at 20-40℃ for 15-30 min, centrifuge, and repeat once; H + Replacement of electrostatically trapped Na on CMC molecular chains +Transition metal ions coordinated with carboxyl groups are incorporated, overcoming the Donnan equilibrium limit of conventional water washing. Ethanol medium inhibits CMC swelling, maintaining particle dispersion. Two acid exchanges ensure sufficient ion replacement. The material is dispersed in ethanol-water to prepare a sodium carboxymethyl cellulose dispersion with a solid-liquid mass ratio of 1%–3%, followed by electrodialysis (10–20V, 0.1–5m). 3 / h) desalination, further removing free H under the drive of an electric field. + Cl - Na + Plasma washing reduces product ash content and avoids the high water consumption problem of conventional water washing. Neutralization to pH 6.5-7.5 using ethanol-water containing 0.5%-1.0% sodium hydroxide precisely neutralizes the hydrogen-form CMC back to the sodium-form, controlling the final pH to prevent the product from becoming too acidic or alkaline. After centrifugation, soaking in anhydrous ethanol for 10-15 minutes removes residual moisture, reducing subsequent drying energy consumption and preventing CMC from swelling and clumping at high water content. Washing wastewater is distilled to recover ethanol (≥95%) for reuse. Ethanol recycling reduces solvent consumption and VOC emissions. Sodium chloride and sodium acetate are recovered from the bottom residue, achieving comprehensive utilization of by-product resources and reducing solid waste emissions.

[0060] S7, Segmented speed-controlled drying: The washed material is first pre-dried at 50-60℃ until the liquid content is 1.0-1.5 times the dry CMC mass, reducing the liquid content of the wet material to the appropriate range for extrusion granulation, avoiding particle adhesion and deformation caused by high liquid content. The pre-drying temperature is lower than the boiling point of ethanol so that the solvent evaporates smoothly and does not produce surface skin. Add 0.05%–0.1% urea by weight of oven-dried CMC, mix thoroughly, and then extrude to granulate 2–5 mm. Urea acts as a hydrogen bond modifier, forming reversible hydrogen bonds with the hydroxyl groups of CMC during drying, inhibiting irreversible hydrogen bond association between molecular chains, reducing drying-induced nano-aggregates, and thus reducing the tendency to gel during reconstitution. By controlling the amount of urea added and drying conditions, ensure that the residual urea content after drying is ≤50 ppm, and that no urea-like byproducts are detected. Dry at 80–100℃ to a solid content of approximately 30%, turning the material once online. Higher temperatures are used for rapid drying during the high-humidity stage to improve efficiency, and online turning eliminates uneven drying of the material layer. Dry at 60–80℃ to approximately 50%, cooling during the medium-humidity stage to prevent surface crusting and hinder internal moisture diffusion. Dry at 70–90℃ to a moisture content of ≤8%, appropriately raising the temperature during the low-humidity stage to ensure the moisture content meets the standard, while avoiding prolonged high temperatures that could lead to product thermal degradation. Waste heat recovery from exhaust gas preheats the intake air, recovering the sensible heat from the drying exhaust gas to reduce steam consumption. After drying, pre-cool to 0-10℃, then pulverize at low temperature (material temperature ≤40℃) by blowing cold air at 0-10℃. Control the material temperature during the pulverization process to prevent molecular chain breakage and local thermal cross-linking caused by mechanical and chemical effects, resulting in gel formation. Sieve through a 200-mesh sieve, and return coarse material to the powder.

[0061] The table below shows the parameter design for the five embodiments in the specific implementation details:

[0062] ;

[0063] Example 1: Prepared according to the above method, with residual oxygen 0.5%, NaOH concentration of 10% in the first stage, sodium borohydride dosage of 0.1%, acid exchange hydrochloric acid concentration of 0.5%, and urea dosage of 0.05%; other parameters were taken as the median values ​​of the range (micro-positive pressure 0.01 MPa, oxidation at 17℃ for 17 min, followed by raising to 35℃ at 0.7℃ / min and maintaining for 30 min, rapid cooling for 15 min to lower to 12℃, PEG-200 dosage of 1.25%, first stage wetting at 15℃ for 75 min, second stage NaOH 25%, maintaining at 32℃ for 45 min, etherification first stage pre-etherification at 27℃ for 17 min followed by dropwise addition of 22.5% chloroacetic acid (rate 2% / min), microwave 0.35 W / cm², second stage reaction at 60℃ for 90 min, reduction pH 10, divided into 4 batches at 6.5 min intervals, electrodialysis at 15V / 2.5m). 3 / h, pre-dry at 55℃ to a liquid content of 1.2 times, then dry in stages at 90 / 70 / 80℃.

[0064] Example 2: Residual oxygen 0.9%, first-stage NaOH concentration 12.5%, sodium borohydride dosage 0.15%, acid-exchange hydrochloric acid concentration 0.6%, urea dosage 0.06%; other parameters are the same as in Example 1.

[0065] Example 3: Residual oxygen 1.25%, first-stage NaOH concentration 15%, sodium borohydride dosage 0.2%, acid-exchange hydrochloric acid concentration 0.75%, urea dosage 0.075%; other parameters are the same as in Example 1.

[0066] Example 4: Residual oxygen 1.6%, first-stage NaOH concentration 17.5%, sodium borohydride dosage 0.25%, acid-exchange hydrochloric acid concentration 0.9%, urea dosage 0.09%; other parameters are the same as in Example 1.

[0067] Example 5: Residual oxygen 2.0%, first-stage NaOH concentration 20%, sodium borohydride dosage 0.3%, acid-exchange hydrochloric acid concentration 1.0%, urea dosage 0.1%; other parameters are the same as in Example 1.

[0068] Comparative Example 1: Traditional solvent extraction process was used. 100 parts of refined cotton pulp were oxidized and viscosity reduced by adding 27.5% hydrogen peroxide (3% of the oven-dry cellulose mass). Then, 32% industrial liquid alkali was added in one step to bring the final NaOH concentration to approximately 30%. The mixture was alkalized at 40°C for 45 minutes. After alkalization, the material was directly fed from a kneader into an etherifier (without a bottom valve for micro-discharge). Chloroacetic acid was added in one step, and etherification was carried out at 65°C for 90 minutes. No carbonyl reduction end-capping step was performed. The material was washed three times with conventional ethanol-water until neutral. After drying at 105°C, it was pulverized at room temperature. The equipment used a conventional configuration of two kneaders and two etherifiers (without redundancy).

[0069] Comparative Example 2: The existing mainstream solvent method for battery-grade CMC was used. Refined cotton pulp was oxidized with 1.5% hydrogen peroxide to reduce viscosity, followed by two-stage alkalization (first stage: low-temperature pre-wetting with 10% NaOH; second stage: high-temperature main alkalization with 25% NaOH). Chloroacetic acid was added in two batches (pre-etherification at 30℃ + main etherification at 60℃). After etherification, the pulp was washed with hydrochloric acid, desalted by electrodialysis, and dried at 80℃. This method avoids gas-phase controlled oxygen oxidation, bottom valve micro-discharge, carbonyl reduction end-capping, and urea hydrogen bond regulation. The equipment used a conventional configuration of two kneaders and two etherifiers.

[0070] Performance testing methods:

[0071] Viscosity of 2% solution: Prepare a 2% (mass fraction) CMC aqueous solution and measure it at 25℃ using an NDJ-79 rotational viscometer.

[0072] Viscosity batch CV value: After 10 consecutive batches of production, the viscosity of 2% of each batch was measured and the coefficient of variation was calculated.

[0073] Molecular weight distribution (PDI): determined by gel permeation chromatography (GPC), using dextran as a standard.

[0074] Insoluble matter content: 1% aqueous solution filtered through a 200-mesh standard sieve, dried at 105℃, and the residue on the sieve was weighed.

[0075] Transmittance of 2% solution: Measured by spectrophotometer at a wavelength of 420 nm for 2% aqueous solution.

[0076] Gel particles The number of particles larger than 100 μm in a 1% aqueous solution was counted using a Coulter particle counter, in units of particles / 100 mL.

[0077] First Coulomb efficiency A negative electrode slurry with a solid content of approximately 45% was prepared by mixing CMC, SBR, conductive carbon black, and artificial graphite in a mass ratio of 1.5:1.5:1.0:96 with deionized water as the solvent. The slurry was coated onto copper foil, dried, rolled, and then assembled with lithium metal sheets to form a CR2032 half-cell. The electrolyte was 1M LiPF6 (EC / DMC=1:1). The first coulombic efficiency was determined by charge and discharge at 0.1C.

[0078] Single-line capacity guarantee rate: The percentage of rated capacity maintained by the remaining equipment when a single kneader or etherifier is shut down for maintenance.

[0079] The test results are shown in the table below:

[0080] ;

[0081] As shown in the table above, Examples 1-5 are significantly superior to Comparative Example 1 in terms of viscosity uniformity (CV value 1.0%-1.5%), molecular weight distribution (PDI ≤ 1.90), insoluble content (≤ 0.10%), transmittance (≥ 95.8%), gel particles (0-1), and initial coulombic efficiency (≥ 92.9%). Among them, Example 3 (with core parameters taken at the midpoint of the range) has the best overall performance. Compared with Comparative Example 2, which uses the mainstream process of staged alkalization, batch etherification, acid washing, and electrodialysis, Example 3 of the present invention still has significant advantages in terms of viscosity batch CV value (1.0% vs. 2.1%), PDI (1.80 vs. 2.12), insoluble content (0.06% vs. 0.18%), gel particles (0 vs. 4), and initial coulombic efficiency (93.8% vs. 92.3%). This indicates that the technical effect of the present invention does not depend on comparison with outdated processes, but rather on a substantial improvement over the mainstream processes in the industry. All five embodiments employ a flexible configuration of three parallel kneaders and three parallel etherifiers, achieving a single-line capacity guarantee rate of 61%–65%, which is higher than the 40% of Comparative Example 1.

[0082] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. For those skilled in the art, any alternative improvements or transformations made to the implementation of the present invention fall within the protection scope of the present invention.

[0083] Any aspects of this invention not described in detail are well-known to those skilled in the art.

Claims

1. A method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter, characterized in that, Includes the following steps: S1, gas-phase controlled oxidation to reduce viscosity of cellulose raw materials; S2, after the oxidation degradation is completed, add ethanol medium and wetting and dispersing agent to the reaction system, and then add alkali solution in two stages to carry out stepwise gradient alkalization. S3. After alkalization, the material is transferred to the kneader. Before the kneader discharges the material to the etherifier, the unreacted fiber clumps are discharged and collected through the bottom valve. Then the bottom valve is closed and the main pure material is sent to the etherifier. S4, temperature gradient directional etherification treatment of the purified material; The temperature gradient directional etherification process is divided into a first stage of low-temperature pre-etherification and a second stage of high-temperature deep etherification. S5, carbonyl reduction end-capping treatment of etherified products: After the etherification reaction is completed, a carbonyl reducing agent is added to the reaction system to eliminate the active sites of hemiacetal crosslinking between molecular chains; S6, after reduction and initial neutralization, is first soaked and washed with an ethanol-water mixture containing hydrochloric acid to replace the counterions electrostatically trapped by the sodium carboxymethyl cellulose molecular chain and the transition metal ions coordinated with the carboxyl group into easily elutable forms. Then it is precisely neutralized with dilute alkali and washed until neutral. S7. The washed product is subjected to segmented, speed-controlled drying. After drying, the product is pulverized to obtain the finished product.

2. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 1, characterized in that, S1 is performed according to the following steps: First, evacuate the reactor to a vacuum level of -0.085 to -0.095 MPa, then fill it with carbon dioxide to atmospheric pressure to complete the pre-replacement. After evacuating again, nitrogen is introduced to atmospheric pressure. Then, the purging-nitrogen replacement process is repeated 1 to 2 times. Finally, the residual oxygen volume fraction in the system is adjusted to 0.5% to 2.0%, and nitrogen is used to maintain a slight positive pressure of 0.005 to 0.02 MPa. First, let it stand at 15-20℃ for 15-20 minutes, then increase the temperature to 30-40℃ at 0.5-1℃ / min and maintain it for 20-40 minutes for staged heating oxidation; After oxidation is complete, cooling water is immediately introduced into the jacket to reduce the system temperature to below 15°C within 10–20 minutes to complete the rapid cooling termination.

3. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 2, characterized in that, After oxidation is terminated, under the condition of maintaining a slight positive pressure of nitrogen, spray the inner wall of the reactor with an ethanol solution of 90% to 95% by volume for 5 to 10 minutes to rinse the reactor wall, so that the oxidized material adhering to the reactor wall is flushed into the reaction system, and the rinsing solution is incorporated into the alkaline medium.

4. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 1, characterized in that, The first phase of S2 is carried out according to the following steps: Pre-cool the system to 5-10°C by introducing a refrigerant into the jacket of the reactor; add an ethanol aqueous solution with a volume fraction of 85%-95%, wherein the ethanol aqueous solution contains 0.5%-2.0% polyethylene glycol 200 wetting and dispersing agent relative to the mass of ethanol, and stir for 5-10 minutes until homogeneous; Then add the first portion of alkali solution to bring the NaOH concentration in the system to 10%–20%. After the alkali addition is complete, infiltrate the system at 10–20°C using intermittent stirring combined with ultrasonic assistance for 60–90 minutes. The intermittent stirring is performed by stopping for 5 minutes after every 10 minutes of stirring. The ultrasonic frequency is 20–40 kHz, and the power density is 0.1–0.3 W / cm². 2 The ultrasonic wave is activated during the period when the stirring is stopped.

5. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 4, characterized in that, The second phase of S2 is carried out according to the following steps: The second part of the alkaline solution is sprayed into the system to raise the NaOH concentration of the system to 20% to 30%. The second part of the alkaline solution is formed by mixing and dissolving liquid alkali and flake alkali in a mass ratio of 3 to 5:

1. The liquid alkali part contains 30% to 50% of the alkaline solution that was recovered from the solid-liquid separation in the previous batch of production and then concentrated by membrane filtration. During the alkali addition process, the system temperature is controlled below 15℃. After the alkali addition is completed, the temperature is raised to 25-40℃ and maintained for 30-60 minutes.

6. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 1, characterized in that, S3 is performed according to the following steps: After alkalization, the material is transferred to a kneader and stirred at 15-20 rpm for 2-3 minutes. Then, the stirring speed is reduced to 5-10 rpm and the kneader is rotated in both directions for 1-2 minutes. At the same time, the vibrator on the outer wall of the kneader is turned on to vibrate and assist in material discharge with an amplitude of 2-5 mm and a frequency of 20-30 Hz. Next, open the bottom valve for 2-3 seconds to discharge the dead corner material to the special collection device. After being crushed, the discharged dead corner material is mixed into the next batch of alkalized material at a ratio of 5%-10% for reuse. After closing the bottom valve, resume normal stirring and send the main material into the etherification machine at the normal discharge rate.

7. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 1, characterized in that, The first phase of S4 is carried out according to the following steps: First, add 20%–30% of the total etherification dose of chloroacetic acid to the etherification machine and react at 25–30°C for 15–20 minutes to complete the pre-etherification. Then, slowly add the remaining first-stage etherifying agent at a relative rate of 1% to 3% of the total etherification dose per minute. This part accounts for 20% to 25% of the total etherification dose. During the addition process, maintain the temperature at 25 to 35°C and turn on microwave-assisted etherification with a power density of 0.2 to 0.5 W / cm². After the addition is completed, continue the reaction for 15 to 30 minutes.

8. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 7, characterized in that, The second phase of S4 is carried out in the following steps: Add the remaining chloroacetic acid to the system, first keep it at 35-45℃ for 20-30 min, then increase the temperature to 55-65℃ in steps of 1-2℃ / min, and react at 55-65℃ for 60-120 min.

9. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 1, characterized in that, S5 is performed according to the following steps: After the etherification reaction is complete, first lower the system temperature to 15-25℃, and adjust the pH to 9.5-10.5 with dilute acetic acid; After purging with nitrogen for 10–15 minutes, prepare a 0.1%–0.3% alkaline aqueous solution of sodium borohydride and add it to the system in 3–5 batches, with an interval of 5–8 minutes between each batch. The total amount added is 0.1%–0.3% of the oven-dry cellulose mass. After adding the oxygen, stir for 15-30 minutes, and finally introduce nitrogen-diluted oxygen with an oxygen volume fraction of 5%-10% for 5-10 minutes to terminate the reaction. After the reduction reaction is complete, the materials are separated by centrifugation, and the reduction waste liquid is collected. Calcium chloride is added to the reduction waste liquid to precipitate borate. After filtration, the filtrate is passed through a cation exchange resin to remove calcium ions and then reused in washing processes or alkali preparation processes that do not directly contact the product.

10. The method for preparing battery-grade sodium carboxymethyl cellulose with low gelation and low insoluble matter according to claim 1, characterized in that, S6 is performed according to the following steps: After reduction, adjust the pH to 7-8 with acetic acid, and centrifuge to remove most of the liquid phase. First acid exchange: Add an ethanol-water solution with a volume fraction of 70%–85% ethanol and a mass fraction of 0.5%–1.0% hydrochloric acid, stir at 20–40°C for 15–30 min, and then centrifuge. Second acid exchange: Repeat the above acid exchange operation once; The material was then dispersed in ethanol-water to prepare a sodium carboxymethyl cellulose dispersion with a solid-liquid mass ratio of 1%–3%. Desalination was then assisted by electrodialysis at a voltage of 10–20 V and a flow rate of 0.1–5 m / s. 3 / h; then neutralize to pH 6.5-7.5 with an ethanol-water solution containing 0.5%-1.0% sodium hydroxide, centrifuge, soak in anhydrous ethanol for 10-15 min to complete ethanol replacement, and centrifuge to remove ethanol.

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