Cyclic regeneration method of cellulose coagulating bath

Abstract

The invention provides a cyclic regeneration method of a cellulose coagulating bath, and belongs to the technical field of cyclic regeneration of coagulating baths. According to the invention, sodium bicarbonate / sodium carbonate / urea coagulating bath is used as a core circulation hub to construct a'urea-NaOH-NaHCO3 'ternary closed-loop circulation system, which is embodied in that: (1) urea is efficiently recovered, high-yield and high-purity recovery of urea is realized by adopting a separation strategy of'wrapping resistance-gradient dissolution-low-temperature crystallization', and the risk of urea decomposition in a causticizing environment at 80 DEG C is avoided; and (2) sodium bicarbonate coagulating bath in-situ regeneration: performing causticization reaction on the residual sodium carbonate / sodium bicarbonate after urea separation and CaO to regenerate NaOH, and recycling CaO regenerated by calcining CaCO3 generated by the reaction for causticization to realize calcium element internal circulation. And (3) carbon is directionally recycled, CO2 released by calcining CaCO3 is used for coagulating bath carbonization, sodium carbonate generated in a regeneration process is converted into a regeneration coagulating bath, and closed-loop utilization of the carbon element is realized.

Description

Technical Field

[0001] This invention relates to the field of coagulation bath recycling technology, and more specifically to a method for recycling a cellulose coagulation bath. Background Technology

[0002] Cellulose, as the most abundant biomass polymer, has seen its green processing technology become a core pathway to replace petroleum-based plastics. The alkali / urea solvent system, due to its disruptive environmental advantages, is widely recognized as the next-generation cellulose molding technology. However, research shows that the composition of the coagulation bath in the alkali / urea system (such as the concentrations of sodium bicarbonate, sodium carbonate, and urea) significantly affects the mechanical properties of cellulose hydrogels and films: as the NaHCO3 concentration increases to 100 g / L, the stress-elongation, fracture work, and Young's modulus of the material gradually increase, and the performance is superior to that of a sulfuric acid coagulation bath system with the same concentration; when the NaHCO3 concentration is fixed, increasing the sodium carbonate (Na2CO3) concentration can further enhance the material strength through the physical coagulation-salting-out effect; appropriate accumulation of Na2CO3 and urea can optimize performance, but excessive amounts will lead to weakened mechanical properties of the film.

[0003] However, achieving fiber / film formation using a sodium bicarbonate coagulation bath presents three major industrial challenges: ① Unsustainable reagent consumption: Urea accounts for 35%–40% of solvent costs, but traditional recovery technologies (distillation / stepwise crystallization) suffer from low urea recovery rates and purity due to the urea-carbonate eutectic effect. ② Uncontrolled material performance: The need for precise control of coagulation bath components, coupled with the generation of large amounts of residual urea and sodium carbonate, can loosen the hydrogen bond network of molecular chains, leading to reduced material performance. ③ High carbon emissions during existing coagulation bath regeneration: Key components (sodium hydroxide, CO2) fail to form a closed loop, requiring external supplementation of raw materials and easily generating waste.

[0004] Therefore, the recovery of urea in the sodium bicarbonate / sodium carbonate / urea coagulation bath and the recycling of the coagulation bath are of great significance for the green and low-cost production of cellulose. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a method for the recycling and regeneration of a cellulose coagulation bath. The recycling and regeneration method provided by this invention can achieve efficient recovery of urea and realize the closed-loop utilization of calcium and carbon elements.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for recycling and regenerating a cellulose coagulation bath, comprising the following steps: The cellulose coagulation bath is subjected to a first reduced pressure rotary evaporation and a first cooling crystallization. After solid-liquid separation, a first crystal and a residual liquid are obtained. The components of the cellulose coagulation bath include urea, sodium bicarbonate, sodium carbonate, and water. The components of the first crystal include sodium bicarbonate. The residual liquid is subjected to a second reduced pressure rotary evaporation to dryness to obtain a second crystal, the second crystal comprising urea, sodium carbonate and sodium bicarbonate; The second crystal is ground and mixed with a surfactant to obtain a ground mixture; The grinding mixture is mixed with a first portion of urea solvent to perform the first urea dissolution. After solid-liquid separation, a first filter cake and a first filtrate are obtained. The first filter cake was washed with a second portion of urea solvent to obtain a second filtrate and a second filter cake. The second filter cake is mixed with a third portion of urea solvent to perform a second urea dissolution. After solid-liquid separation, a third filter cake and a third filtrate are obtained. The third filter cake is washed to obtain a third crystal, which is a mixture of sodium bicarbonate and sodium carbonate. The first, second, and third filtrates were combined and subjected to a third vacuum rotary evaporation, a second cooling crystallization, and alcohol washing to obtain urea crystals; The first and third crystals were mixed with CaO and water to carry out a causticizing reaction. After solid-liquid separation, NaOH solution and CaCO3 solid were obtained.

[0007] Preferably, after obtaining solid CaCO3, the method further includes calcining the solid CaCO3 at high temperature to obtain CaO and CO2, wherein the CaO is recycled to the causticization reaction and the CO2 is used for the carbonization regeneration reaction of the waste Na2CO3 coagulation bath; The carbonization regeneration reaction includes the following steps: Waste coagulation bath containing a large amount of Na2CO3 is subjected to carbonization regeneration reaction with CO2 to obtain a regenerated coagulation bath containing NaHCO3.

[0008] Preferably, the concentration of urea in the cellulose coagulation bath is 50-300 g / L; the concentration of sodium bicarbonate is 80-100 g / L; and the concentration of sodium carbonate is 5-50 g / L.

[0009] Preferably, the temperature of the first vacuum rotary evaporation is 50~70℃, and the vacuum degree is -0.07~-0.1 MPa; the first vacuum rotary evaporation is carried out to 1 / 5~3 / 5 of the original volume; The temperature for the first cooling crystallization is 0~20℃; The temperature of the second vacuum rotary evaporation is 50~70℃, and the vacuum degree is -0.07~-0.1 MPa.

[0010] Preferably, the surfactant is one or more of sodium dodecyl sulfate, sodium dodecyl sulfonate, and alkyl polyglucoside; The mass ratio of the second crystal to the surfactant is 50~200:1.

[0011] Preferably, the volume ratio of the first part of the urea solvent, the second part of the urea solvent, and the third part of the urea solvent is (4~5):(2~3):(2~3); The mass ratio of the grinding mixture to the volume ratio of the first urea solvent is 50-64 g: 150 mL; The temperature of the first urea solvent is 20~30℃; The temperature of the second urea solvent is 0~10℃; The temperature of the third urea solvent is 20~30℃; The urea solvent includes one or more of N,N-dimethylformamide, dimethyl sulfoxide, dimethylacetamide, and ethanol.

[0012] Preferably, the temperature of the third vacuum rotary evaporation is 50~70℃, the vacuum degree is -0.07~-0.1 MPa, and the temperature of the second cooling crystallization is 0~10℃.

[0013] Preferably, the total mass ratio of the first crystal and the third crystal to CaO is 15:9.3~11.2; the causticizing reaction is carried out at a temperature of 70~100℃ for 1~4h.

[0014] Preferably, the high-temperature calcination temperature is 800~1200℃ and the time is 1~12h; The carbonization regeneration reaction is carried out at a temperature of 30~70℃, a pressure of 2~10 bar, and a time of 0.5~6h.

[0015] Preferably, the NaOH solution obtained from the causticizing reaction is used to dissolve regenerated cellulose.

[0016] This invention provides a method for the recycling and regeneration of a cellulose coagulation bath. Using a sodium bicarbonate / sodium carbonate / urea coagulation bath as the core recycling hub, this invention creatively constructs a ternary closed-loop recycling system of "urea-NaOH-NaHCO3," achieving a breakthrough across the entire chain from precise molecular recovery to efficient process regeneration in the field of cellulose coagulation bath recycling and regeneration. Specifically, the innovative reconstruction of this invention is reflected in the following three aspects: ① Efficient urea recovery: Before the critical causticization reaction (where high temperatures easily lead to urea decomposition), a unique three-in-one separation strategy of "anti-encapsulation-gradient dissolution-low-temperature crystallization" is adopted to achieve high urea yield and high purity recovery, avoiding the risk of urea decomposition in the 80℃ causticization environment, and ensuring that the recovered urea can be directly reused in the preparation of cellulose solutions. ② In-situ regeneration of sodium bicarbonate coagulation bath: A unique "calcium carrier recycling chain" (CaO→Ca(OH)2→CaCO3→CaO) is used. The sodium carbonate / sodium bicarbonate residue remaining after urea separation undergoes a causticization reaction with CaO at 80℃. This efficiently regenerates the NaOH required for cellulose dissolution. The resulting CaCO3 precipitate is calcined, and the regenerated CaO is reused for causticization, achieving an internal calcium cycle. ③ Directed carbon recycling: The CO2 released from calcining CaCO3 is used for coagulation bath carbonization, converting the sodium carbonate (Na2CO3) produced in the cellulose regeneration process into the NaHCO3 coagulation bath needed for regeneration, achieving closed-loop utilization of carbon.

[0017] This invention seamlessly integrates five key processes: urea recovery, NaOH regeneration, NaHCO3 coagulation bath regeneration, CaO regeneration, and CO2 utilization. This forms a self-sufficient ternary closed-loop cycle of urea-NaOH-NaHCO3, truly achieving a coagulation bath regeneration scheme with "zero carbon emissions" (internal CO2 recycling), "zero solid waste" (CaCO3 calcination regeneration), and "high-efficiency urea recovery" (high-efficiency separation before causticization).

[0018] Furthermore, the sodium hydroxide and urea recovered by this invention can be directly used for cellulose dissolution. After regeneration in a CO2-carbonized sodium bicarbonate coagulation bath, the tensile strength of the resulting cellulose film remains at 193.06±10.27MPa (new bath standard 203.88MPa). Moreover, after five cycles of regeneration, the performance of the resulting cellulose film remains stable, proving that the coagulation bath has highly stable performance.

[0019] The cellulose coagulation bath recycling method provided by this invention has the advantages of high efficiency, greenness, and low cost, resulting in significant economic benefits. It has broad application value in the fields of cellulose membrane materials (separation membranes, packaging membranes) and medical sanitary materials (non-woven fabrics, dressings). Attached Figure Description

[0020] Figure 1 A schematic diagram of the recycling process of the cellulose coagulation bath; Figure 2 The images show the actual urea recovered in Example 1 and the sodium hydroxide regenerated by causticization. Figure 3 The XRD patterns are of the urea recovered in Example 1 and the sodium hydroxide regenerated by causticization. Figure 4 The purity and yield of the urea recovered in Example 1 and the sodium hydroxide regenerated by causticization; Figure 5 X-ray powder diffraction patterns of urea recovered in Example 2 and sodium hydroxide regenerated by causticization; Figure 6 The image shows the XRD pattern of the crystals obtained by coagulation bath crystallization and drying after carbonization in Example 2. Figure 7 The graph shows the purity and yield of the urea recovered in Example 2 and the sodium hydroxide regenerated by causticization. Figure 8 The XRD patterns are of the urea recovered in Example 3 and the sodium hydroxide regenerated by causticization. Figure 9 The graph shows the purity and yield of the urea recovered in Example 3 and the sodium hydroxide regenerated by causticization. Figure 10 The changes in sodium bicarbonate and sodium carbonate content in the regenerated coagulation bath obtained in Example 3 before and after 1, 2, 3, 4, and 5 cycles of reaction are shown. Figure 11 The stress-strain mechanical properties of the coagulated and regenerated film after 1, 2, 3, 4, and 5 cycles of reaction in the coagulation bath of Example 3 are shown. Detailed Implementation

[0021] This invention provides a method for recycling and regenerating a cellulose coagulation bath, comprising the following steps: The cellulose coagulation bath is subjected to a first reduced pressure rotary evaporation and a first cooling crystallization. After solid-liquid separation, a first crystal and a residual liquid are obtained. The components of the cellulose coagulation bath include urea, sodium bicarbonate, sodium carbonate, and water. The components of the first crystal include sodium bicarbonate. The residual liquid is subjected to a second reduced pressure rotary evaporation to dryness to obtain a second crystal, the second crystal comprising urea, sodium carbonate and sodium bicarbonate; The second crystal is ground and mixed with a surfactant to obtain a ground mixture; The grinding mixture is mixed with a first portion of urea solvent to perform the first urea dissolution. After solid-liquid separation, a first filter cake and a first filtrate are obtained. The first filter cake was washed with a second portion of urea solvent to obtain a second filtrate and a second filter cake. The second filter cake is mixed with a third portion of urea solvent to perform a second urea dissolution. After solid-liquid separation, a third filter cake and a third filtrate are obtained. The third filter cake is washed to obtain a third crystal, which is a mixture of sodium bicarbonate and sodium carbonate. The first, second, and third filtrates were combined and subjected to a third vacuum rotary evaporation, a second cooling crystallization, and alcohol washing to obtain urea crystals. The first and third crystals were mixed with CaO and water to carry out a causticizing reaction. After solid-liquid separation, NaOH solution and CaCO3 solid were obtained.

[0022] This invention involves subjecting a cellulose coagulation bath to a first vacuum rotary evaporation and a first cooling crystallization process, resulting in solid-liquid separation to obtain first crystals and a residual liquid. In this invention, the cellulose coagulation bath is preferably a waste cellulose coagulation bath, and the components of the cellulose coagulation bath include urea, sodium bicarbonate, sodium carbonate, and water. The concentration of urea in the cellulose coagulation bath is preferably 50-300 g / L, more preferably 100-200 g / L, and even more preferably 150 g / L; the concentration of sodium bicarbonate is preferably 80-100 g / L, more preferably 100 g / L, and the concentration of sodium carbonate is preferably 5-50 g / L, more preferably 30-50 g / L.

[0023] In this invention, the distillation equipment used for the first vacuum rotary evaporation and subsequent second and third vacuum rotary evaporations can be a scraped-film evaporator or a vacuum distillation kettle. In this invention, the temperature of the first vacuum rotary evaporation is preferably 50~70℃, more preferably 60℃, the vacuum degree is preferably -0.07~-0.1 MPa, more preferably -0.1 MPa, and the rotation speed of the first vacuum rotary evaporation is preferably 100 rpm. In this invention, the first vacuum rotary evaporation preferably evaporates to 1 / 5~3 / 5 of the original volume, more preferably 1 / 3.

[0024] In this invention, the preferred temperature for the first cooling crystallization is 0-20°C, more preferably 10°C. In this invention, the solid-liquid separation method is preferably vacuum filtration, and the temperature of the Buchner funnel used during vacuum filtration is preferably 10°C. This invention utilizes the difference in solubility between carbonates and urea in water through a first reduced-pressure rotary evaporation and a first cooling crystallization process to precipitate some sodium bicarbonate crystals from the mother liquor. In this invention, the main component of the first crystal is sodium bicarbonate, and the main components of the residual liquid are urea, remaining sodium bicarbonate, sodium carbonate, and water.

[0025] The present invention involves subjecting the residual liquid to a second vacuum rotary evaporation to obtain a second crystal, the second crystal comprising urea, sodium carbonate, and sodium bicarbonate. In this invention, the temperature of the second vacuum rotary evaporation is preferably 50~70℃, more preferably 60℃; the vacuum degree is preferably -0.07~-0.1 MPa, more preferably -0.1 MPa; and the rotation speed of the second vacuum rotary evaporation is preferably 100 rpm.

[0026] In this invention, the second crystal is ground and mixed with a surfactant to obtain a ground mixture. In this invention, the surfactant is preferably sodium dodecyl sulfate; the mass ratio of the mixed crystal to the surfactant is preferably 50-200:1, more preferably 100-150:1. In this invention, the grinding rate is preferably 50 rpm, and the grinding time is preferably 5 min. In this invention, the sodium dodecyl sulfate surfactant can effectively break the encapsulation effect of urea on sodium bicarbonate, which is beneficial for the subsequent separation of urea and sodium bicarbonate.

[0027] In this invention, the grinding mixture is mixed with a first portion of a good urea solvent to perform a first urea dissolution. After solid-liquid separation, a first filter cake and a first filtrate are obtained. In this invention, the good urea solvent preferably includes one or more of N,N-dimethylformamide, dimethyl sulfoxide, dimethylacetamide, and ethanol, and more preferably N,N-dimethylformamide.

[0028] In this invention, the mass ratio of the grinding mixture to the volume of the first urea solvent is 64g:150mL; the temperature of the first urea solvent is preferably 20-30°C, more preferably 25°C. Preferably, the first urea dissolution is carried out under stirring conditions, and the stirring time is preferably 10 minutes. During the first urea dissolution process, some of the urea in the grinding mixture dissolves in the urea solvent. In this invention, the solid-liquid separation is preferably vacuum filtration, and the main components of the obtained first filter cake are crude sodium bicarbonate and crude sodium carbonate, while the first filtrate contains urea.

[0029] This invention uses a second portion of urea-based solvent to wash the first filter cake, obtaining a second filtrate and a second filter cake. In this invention, the temperature of the second portion of urea-based solvent is preferably 0-10°C, more preferably 0-5°C. This invention uses a cold urea-based solvent for washing, which effectively dissolves and removes impurities (urea and surfactants) while, due to its low-temperature characteristics, maximally suppressing recontamination of the product by urea and the decomposition of the product itself during the washing process. In this invention, the main components of the second filtrate are urea and solvent, and the main components of the second filter cake are sodium bicarbonate and sodium carbonate.

[0030] In this invention, the second filter cake is mixed with a third portion of a urea solvent to perform a second urea dissolution. After solid-liquid separation, a third filter cake and a third filtrate are obtained. The third filter cake is washed to obtain a third crystal, which is a mixture of sodium bicarbonate and sodium carbonate. In this invention, the temperature of the third portion of the urea solvent is preferably 20-30°C, more preferably 25°C. The second urea dissolution is preferably performed under stirring conditions. In this invention, the solid-liquid separation is preferably performed by vacuum filtration.

[0031] In this invention, the preferred volume ratio of the first, second, and third portions of the urea solvent is (4-5):(2-3):(2-3), more preferably 5:2:3. The key design of this invention is the addition of the urea solvent in three stages, aimed at achieving efficient separation and purification of urea and carbonates. First, the first portion of the urea solvent is added and stirred at room temperature. Its main function is to dissolve most of the urea, achieving the main separation of urea and carbonates, resulting in a urea-rich filtrate and crude sodium bicarbonate and sodium carbonate filter cakes. Subsequently, the filter cake is washed with the second portion of the low-temperature urea solvent. This low-temperature operation effectively removes residual urea and surfactants while significantly reducing the solubility of urea, preventing the re-dissolution of impurities during washing and thus deeply purifying sodium bicarbonate and sodium bicarbonate. Finally, the third portion of the urea solvent ensures that all residual urea is completely dissolved, further improving the purity of sodium bicarbonate and sodium carbonate, and maximizing the urea recovery rate. This meticulous, phased, and temperature-controlled operation collaboratively achieves the dual goals of high product purity and high yield.

[0032] This invention combines the first, second, and third filtrates and subjectes them to a third vacuum rotary evaporation, a second cooling crystallization, and alcohol washing to obtain urea crystals. In this invention, the temperature of the third vacuum rotary evaporation is preferably 50-70°C, more preferably 60-70°C, and the vacuum degree is -0.07 to -0.1 MPa, more preferably -0.1 MPa. The third vacuum rotary evaporation is preferably concentrated to 1 / 10 to 1 / 6 of the original volume. The temperature of the second cooling crystallization is 0-10°C, more preferably 4°C, and the time is preferably 2 hours. In this invention, after the third vacuum rotary evaporation and second cooling crystallization of the third filtrate, the crystals are separated, and the resulting residual liquid is subjected to the same process of vacuum rotary evaporation, cooling crystallization, and crystal separation until the residual liquid is evaporated to dryness.

[0033] In this invention, the alcohol washing is a cold ethanol washing, and the temperature of the cold ethanol is preferably 0°C. After the alcohol washing, the resulting crystals are preferably vacuum dried, and the vacuum drying temperature is preferably 50°C, and the vacuum degree is preferably -0.1 MPa.

[0034] In this invention, the first and third crystals are mixed with CaO and water to undergo a causticizing reaction. After solid-liquid separation, a NaOH solution and a CaCO3 solid are obtained. In this invention, the mass ratio of the total mass of the first and third crystals to the mass of CaO is preferably 15:9.3~11.2, more preferably 15:9.5~10.85, and even more preferably 15:10. In this invention, the mixing process is preferably as follows: first, the first and third crystals are mixed with a first portion of water to obtain a mixed solution of Na2CO3 and NaHCO3; then, CaO is mixed with the remaining water to obtain a calcium hydroxide solution; finally, the mixed solution of Na2CO3 and NaHCO3 and the calcium hydroxide solution are mixed.

[0035] In this invention, the temperature of the causticizing reaction is preferably 70-100°C, more preferably 80°C, and the time is preferably 1-4 hours, more preferably 2 hours. In this invention, the solid-liquid separation is preferably performed by vacuum filtration. In this invention, the NaOH solution obtained from the causticizing reaction is preferably used for dissolving regenerated cellulose.

[0036] After obtaining solid CaCO3, the present invention further includes calcining the solid CaCO3 at high temperature to obtain CaO and CO2. The CaO is recycled to the causticization reaction, and the CO2 is used for the carbonization regeneration reaction of the waste Na2CO3 coagulation bath.

[0037] In this invention, the temperature of the high-temperature calcination is preferably 800~1200℃, more preferably 1000℃, and the time is preferably 1~12h, more preferably 3~8h.

[0038] In this invention, the carbonization regeneration reaction includes the following steps: Waste Na2CO3 coagulation bath is subjected to carbonization regeneration reaction with CO2 to obtain a regenerated coagulation bath containing NaHCO3.

[0039] In this invention, the waste Na2CO3 coagulation bath is preferably a waste Na2CO3 coagulation bath that has been coated once using a casting method, and the concentration of Na2CO3 in the waste Na2CO3 coagulation bath is preferably 10~50 g / L. In this invention, the carbonization regeneration reaction is preferably carried out in a high-pressure reactor, the temperature of the carbonization regeneration reaction is preferably 30~70℃, more preferably 60℃, the pressure is preferably 2~10 bar, more preferably 5~10 bar, and the time is preferably 0.5~6 h, more preferably 1~3 h. In this invention, the carbonization regeneration reaction process is Na2CO3 + CO2 + H2O → 2NaHCO3.

[0040] In this invention, the NaHCO3-containing regeneration coagulation bath comprises sodium bicarbonate and sodium carbonate, wherein the concentration of sodium bicarbonate is preferably 95-100 g / L and the concentration of sodium carbonate is preferably 5-10 g / L.

[0041] As a specific embodiment of the present invention, the schematic diagram of the cyclic regeneration process of the cellulose coagulation bath is shown below. Figure 1 As shown.

[0042] The following detailed description of the cellulose coagulation bath recycling method provided by the present invention, with reference to specific embodiments, should not be construed as limiting the scope of protection of the present invention.

[0043] Example 1 The cyclic regeneration of the cellulose coagulation bath involves the following steps: (1) Prepare a 300 mL waste coagulation bath containing 150 g / L urea, 100 g / L sodium bicarbonate, and 50 g / L sodium carbonate. Transfer the solution to a 500 mL rotary evaporator flask. Set the heating bath temperature of the rotary evaporator to 60 °C, evaporate to -0.1 MPa, and adjust the rotation speed to 100 rpm to start rotary evaporation. Stop rotary evaporation when 200 mL of water has been distilled off. Cool 70 mL of the concentrate to 10 °C to crystallize. Pre-cool the solution in a Buchner funnel to 10 °C and filter to obtain crystal A (main component NaHCO3) and residual liquid.

[0044] (2) The residual liquid was evaporated to dryness at 60℃, -0.1 MPa and 100 rpm to obtain mixed crystals (containing urea, Na2CO3 and residual NaHCO3). Crystal A and mixed crystals were vacuum dried at 50℃ to obtain 25 g of crystal A and 63.6 g of mixed crystals. 0.4 g of sodium dodecyl sulfate (SDS) was added to the dried mixed crystals and the agglomeration was broken by grinding for 5 minutes to obtain the ground mixture.

[0045] (3) Add 150 mL of DMF to 64 g of the ground mixture and stir for 10 minutes. Filter the mixture to obtain filter cake ① and filtrate ①. Filter cake ① consists of NaHCO3 and crude sodium carbonate, while filtrate ① contains dissolved urea. Wash filter cake ① with 60 mL of cold DMF (0 °C) to obtain filter cake ② and filtrate ②. Add 90 mL of DMF to filter cake ② and stir at 25 °C to dissolve residual urea. Filter the mixture again. Wash filter cake ③ with 15 mL of acetone and dry under vacuum at 50 °C to obtain 19.5 g of crystal B (a mixture of Na2CO3 / NaHCO3). Filtrate ③ mainly contains urea / DMF solution. Filtrates ①, ②, and ③ were combined and rotary evaporated at 70°C to 35 mL. The solution was allowed to stand at 4°C for 2 h to crystallize. Urea crystals were obtained by vacuum filtration. The filtrate was rotary evaporated and then cooled to precipitate crystals until the DMF was completely evaporated. Finally, the obtained crystals were washed with 5 mL of cold ethanol and dried under vacuum at 50°C to obtain 43.88 g of white urea crystals.

[0046] (4) Pour crystals A and B into a 500mL beaker and add 200mL of deionized water to dissolve them. Weigh 30.72g of calcium oxide (CaO:total molar amount of carbonate = 1.1:1) and pour it into another 200mL beaker. Add 100mL of water to convert it into calcium hydroxide. Then pour it into a beaker containing Na2CO3 and NaHCO3 solutions and heat it to 80℃ for causticization reaction for 2h. Filter to obtain NaOH solution and CaCO3 precipitate. Evaporate the NaOH solution to obtain NaOH solid and dry the CaCO3 precipitate in a vacuum drying oven at 80℃.

[0047] (5) CaCO3 precipitate is calcined in a tubular furnace at 1000℃ for 3 hours to generate CaO and CO2. CaO is directly recycled for causticization reaction, and the collected CO2 is used for carbonization regeneration of waste Na2CO3 coagulation bath. 2L coagulation bath (composition includes 13g / L sodium carbonate, 88g / L sodium bicarbonate and trace amount of urea) after being coated once by casting method is poured into high pressure reactor, and 10 bar CO2 is introduced to carbonize at 60℃ for 1 hour to obtain regenerated coagulation bath.

[0048] Repeat the above steps to conduct 5 parallel experiments, and calculate the urea recovery rate and purity, and the NaOH regeneration yield (causticization rate) respectively.

[0049] The actual images of the recovered urea and causticized sodium hydroxide are shown below. Figure 2 As shown, Figure 2 In the diagram, (a) is urea and (b) is sodium hydroxide.

[0050] X-ray powder diffraction (XRD) analysis diagrams of the recovered urea and causticized regenerated sodium hydroxide are shown below. Figure 3 As shown, the recovered urea exhibits characteristic diffraction peaks at 2θ = 22.319° and 29.218° that are basically consistent with those of PDF standard card No. 48-1070, with a half-peak width of 0.248°, proving that the recovered urea crystal structure is intact; the diffraction peaks of the regenerated sodium hydroxide crystal at 2θ = 38.628°, 31.820°, and 15.769° are comparable to the characteristic diffraction peaks of standard sodium hydroxide.

[0051] The purity and yield (causticization rate) of the recovered urea and causticized sodium hydroxide are as follows: Figure 4 As shown, by Figure 4 It can be seen that the purity of the recovered urea reached 97.82±1.29%, and the recovery rate was 96.95±1.23%; the purity of the regenerated sodium hydroxide was 99.31±1.41%, and the yield (causticization reaction conversion rate) was 97.74±1.95%.

[0052] Example 2 The cyclic regeneration of the cellulose coagulation bath involves the following steps: (1) Prepare a 300 mL waste coagulation bath containing 150 g / L urea, 100 g / L sodium bicarbonate, and 50 g / L sodium carbonate. Transfer the solution to a 500 mL rotary evaporator flask. Set the heating bath temperature of the rotary evaporator to 60 °C, evaporate to -0.1 MPa, and adjust the rotation speed to 100 rpm to start rotary evaporation. Stop rotary evaporation when 200 mL of water has been distilled off. Cool 70 mL of the concentrate to 10 °C to crystallize. Pre-cool the solution in a Buchner funnel to 10 °C and filter to obtain crystal A (main component NaHCO3) and residual liquid.

[0053] (2) The residual liquid was evaporated to dryness at 60℃, -0.1 MPa and 100 rpm to obtain mixed crystals (containing urea, Na2CO3 and residual NaHCO3). Crystal A and mixed crystals were vacuum dried at 50℃ to obtain 25.2 g of crystal A and 63.5 g of mixed crystals.

[0054] (3) Add 150 mL of DMF to the mixed crystals and stir for 10 minutes. Filter the mixture to obtain filter cake ① and filtrate ①. Filter cake ① consists of NaHCO3 and crude sodium carbonate, while filtrate ① contains dissolved urea. Wash filter cake ① with 60 mL of cold DMF (0℃) to obtain filter cake ② and filtrate ②. Add 90 mL of DMF to filter cake ② and stir at 25℃ to dissolve the residual urea. Filter the mixture again. Wash filter cake ③ with 15 mL of acetone and dry it under vacuum at 50℃ to obtain 20.2 g of crystal B (a mixture of Na2CO3 / NaHCO3). Filtrate ③ mainly contains urea / DMF solution. Combine filtrates ①, ①, and ③ and rotary evaporate to 35 mL at 70℃. Let the mixture stand at 4℃ for 2 h to crystallize. Filter the mixture to obtain urea crystals. Repeat the rotary evaporation of the filtrate and cooling to precipitate crystals until all DMF is evaporated. Finally, wash the obtained crystals with 5 mL of cold ethanol and dry them under vacuum at 50℃ to obtain 43.2 g of white urea crystals.

[0055] (4) Pour crystals A and B into a 500mL beaker and add 200mL of deionized water to dissolve them. Weigh 30.98g of calcium oxide (CaO: total molar amount of carbonate = 1.1:1) and pour it into another 200mL beaker. Add 100mL of water to convert it into calcium hydroxide. Then pour it into a beaker containing Na2CO3 and NaHCO3 solutions and heat it to 80℃ for causticization reaction for 2h. Filter to obtain NaOH solution and CaCO3 precipitate. Evaporate the NaOH solution to obtain NaOH solid and dry the CaCO3 precipitate in a vacuum drying oven at 80℃.

[0056] (5) CaCO3 precipitate is calcined in a tubular furnace at 1000℃ for 3 hours to generate CaO and CO2. CaO is directly recycled for causticization reaction, and the collected CO2 is used for carbonization regeneration of waste Na2CO3 coagulation bath. The 2L coagulation bath after being coated once by casting method is poured into a high-pressure reactor, and 10 bar CO2 is introduced to carbonize the reaction at 60℃ for 1 hour to obtain the regenerated coagulation bath.

[0057] X-ray powder diffraction (XRD) analysis diagrams of the recovered urea and causticized regenerated sodium hydroxide are shown below. Figure 5 As shown, the recovered urea exhibits characteristic diffraction peaks at 2θ = 22.319° and 29.218° that are basically consistent with those of PDF standard card No. 48-1070, with a half-peak width of 0.21°, proving that the recovered urea crystal structure is intact; the diffraction peaks of the regenerated sodium hydroxide crystal at 2θ = 38.628°, 31.820°, and 15.769° are comparable to the characteristic diffraction peaks of standard sodium hydroxide.

[0058] The XRD pattern of the crystals obtained by coagulation bath crystallization and drying after carbonization is shown below. Figure 6 As shown.

[0059] The purity and yield (causticization rate) of the recovered urea and causticized sodium hydroxide are as follows: Figure 7 As shown, the purity of the recovered urea reached 98.02±1.19%, and the recovery rate was 96.15±1.21%; the purity of the regenerated sodium hydroxide was 99.19±1.32%, and the yield (causticization reaction conversion rate) was 98.74±1.87%.

[0060] Example 3 The difference from Example 1 is that the mass of calcium oxide in step (4) is 32.12g (CaO: total molar amount of carbonate = 1.15:1), and the rest of the operations are the same.

[0061] X-ray powder diffraction (XRD) analysis diagrams of the recovered urea and causticized regenerated sodium hydroxide are shown below. Figure 8 As shown, the recovered urea exhibits characteristic diffraction peaks at 2θ = 22.319° and 29.218° that are basically consistent with those of PDF standard card No. 48-1070, with a half-peak width of 0.246°, proving that the recovered urea crystal structure is intact; the diffraction peaks of the regenerated sodium hydroxide crystal at 2θ = 38.628°, 31.820°, and 15.769° are comparable to the characteristic diffraction peaks of standard sodium hydroxide.

[0062] The purity and yield (causticization rate) of the recovered urea and causticized sodium hydroxide are as follows: Figure 9As shown, the purity of the recovered urea reached 97.92±1.17%, and the recovery rate was 98.06±1.29%; the purity of the regenerated sodium hydroxide was 99.19±1.24%, and the yield (causticization reaction conversion rate) was 99.06±1.78%.

[0063] The changes in sodium bicarbonate and sodium carbonate content in the regenerated coagulation bath before and after 1, 2, 3, 4, and 5 cycles are shown in the figure. Figure 10 As shown, after multiple carbonization cycles in the coagulation bath, the concentration of sodium bicarbonate (NaHCO3) stabilized at approximately 95 g / L, and the concentration of sodium carbonate (Na2CO3) stabilized at approximately 5-10 g / L. After neutralization with the cellulose gum, the concentration of sodium bicarbonate (NaHCO3) decreased to approximately 85 g / L, while the concentration of Na2CO3 increased to 15-20 g / L. In summary, after multiple carbonization-neutralization cycles in the coagulation bath, the concentrations of sodium bicarbonate and sodium carbonate fluctuated within a relatively small stable range, exhibiting an inverse relationship between their concentrations.

[0064] The dissolved and centrifuged cellulose solution (containing 7% cellulose urethane and 1% carbon black) was slowly poured onto one end of a glass plate. A glass rod with copper wires of a certain fineness attached to both ends was used to horizontally scrape the solution from one end of the glass plate to the other, resulting in a flat layer of cellulose solution. The glass plate with the cellulose solution was then immersed in a coagulation bath. After 5 minutes of coagulation and regeneration, the solution was completely solidified, yielding a cellulose hydrogel. The obtained cellulose hydrogel was rinsed with distilled water to remove ions from the cellulose network until the conductivity was 0 μS / cm. Finally, the washed hydrogel was laminated onto an acrylic plate to obtain a cellulose film with a thickness of 35 μm. The stress-strain mechanical properties of the coagulated and regenerated film after 1, 2, 3, 4, and 5 cycles of coagulation bath reaction are as follows: Figure 11 As shown, the mechanical properties of the cellulose membranes regenerated through multiple cycles are not significantly different, with the strain values ​​generally within the range of 7-9% and the stress generally within 193.06±10.27 MPa. This indicates that the coagulation bath used in this invention for multiple cycles has little impact on the mechanical properties of the coagulated and regenerated cellulose membranes, and that the coagulation bath can be reused through multiple cycles.

[0065] Comparative Example 1 (1) Prepare a 300 mL waste coagulation bath containing 150 g / L urea, 100 g / L sodium bicarbonate and 50 g / L sodium carbonate, and transfer it to a 500 mL beaker; weigh 32.12 g of calcium oxide (CaO: total molar amount of carbonate equals 1.15:1) and pour it into another 200 mL beaker, add 100 mL of water to convert it into calcium hydroxide, then pour it into the beaker containing the waste coagulation bath, and heat it to 80 °C to react for 2 h to obtain a solution containing urea and NaOH and CaCO3 precipitate. Dry the CaCO3 precipitate in a vacuum drying oven at 80 °C.

[0066] (2) The CaCO3 obtained in the previous step is calcined in a tube furnace at 1000℃ for 3 hours to produce CaO and CO2. CaO can be directly recycled for causticization reaction, and the collected CO2 is used for coagulation bath carbonization. The 2L coagulation bath after being coated once by casting is poured into a high-pressure reactor, and 10 bar of CO2 is introduced to carbonize at 60℃ for 1 hour to obtain a regenerated coagulation bath.

[0067] Repeat the above steps to conduct 5 parallel experiments, and analyze the urea decomposition, calculate the yield of sodium hydroxide regeneration (causticization rate), and the concentrations of sodium bicarbonate and sodium carbonate in the coagulation bath before and after the carbonization reaction.

[0068] The results showed that, although NaOH could be regenerated by directly causticizing urea without prior separation, the urea recovery process had fundamental defects (high-temperature decomposition + impurity contamination), resulting in a urea decomposition rate of 62-75%. The actual recovery rate and purity were significantly lower than in Example 3, and byproducts interfered with system stability. Example 3, through pre-treatment with fine separation, simultaneously achieved efficient urea recovery and efficient causticization / carbonization operation, verifying the necessity of the "separation before regeneration" strategy.

[0069] Comparative Example 2 The difference from Example 3 is that the mass of calcium oxide in step (4) is 27.93 g (CaO:total molar mass of carbonate = 1:1), and the rest of the operations are the same. The decomposition of urea was analyzed, the yield of sodium hydroxide regeneration (causticization rate) was calculated, and the concentrations of sodium bicarbonate and sodium carbonate in the coagulation bath before and after the carbonization reaction were determined.

[0070] The results showed that the purity of the regenerated sodium hydroxide was 98.93±1.01%, and the yield (causticization reaction conversion rate) was 96.27±1.39%. Because the added calcium oxide (27.93g, CaO:total carbonate molar ratio = 1:1) was not in excess, the conversion rate in the causticization reaction stage was lower than that in Example 3 when 32.12g of calcium oxide (CaO:total carbonate molar ratio = 1.15:1) was added. In this invention, all process parameters were scientifically designed and precisely controlled. From the precise preparation of the waste coagulation bath and the accurate determination of its components, to the constant temperature and pressure operation of rotary evaporation concentration (60℃, -0.1 MPa), to low-temperature crystallization (10℃), selective dissolution and separation of urea in DMF (supplemented by SDS dispersion), and the optimized conditions of the causticization reaction (80℃) and CO2 carbonization regeneration (60℃, 10 bar), the entire recycling process was ensured to have minimal material loss, high product purity, and a reliable and scalable coagulation bath circulation method.

[0071] In summary, the "urea-NaOH-NaHCO3" ternary closed-loop recycling regeneration method of this invention, compared with the traditional coagulation bath regeneration process, achieves a disruptive breakthrough in core resource recovery and regeneration through an innovative synergistic mechanism of "calcium carrier recycling chain" and "carbon-oriented recycling". Compared with existing technologies: Urea recovery: The unique "gradient dissolution-anti-encapsulation-low temperature crystallization" pre-separation strategy enables urea purity to reach 97.92±1.17% (significantly improved by 4~5% compared with traditional methods), and the recovery rate reaches 98.06±1.29% (improved by 15~22% compared with traditional methods), effectively avoiding high-temperature causticization decomposition and ensuring that the recovered urea can be directly reused.

[0072] NaOH Regeneration: Based on the causticization reaction of the calcium cycle (CaO→Ca(OH)2→CaCO3→CaO), the NaOH regeneration rate reaches 98.06±1.29% (far higher than traditional inefficient regeneration methods), and the lime consumption is significantly reduced (close to zero replenishment). NaHCO3 Coagulation Bath Regeneration: By precisely capturing and utilizing the system's internally generated CO2 for carbonization, a high-quality NaHCO3 coagulation bath with the target concentration (100g / L) is stably regenerated. The regeneration efficiency and stability far exceed traditional methods that require external replenishment or are inefficient. Resource Recycling and Environmental Protection: Completely solves the pain point of broken resource recycling in traditional processes, realizing the internal circulation of calcium elements (zero solid waste, CaCO3 calcination to regenerate CaO) and the closed-loop utilization of carbon elements (zero carbon emissions, CO2 used for carbonization), making the system self-sufficient. Energy Consumption and Cost: The closed-loop design greatly reduces external dependence on raw materials (NaOH, CaO, CO2) and waste treatment costs. The overall energy consumption and material consumption are significantly reduced compared to traditional methods (estimated reduction >50%). Performance of Regenerated Materials: After five cycles of regeneration, the tensile strength of the cellulose film prepared using the regenerated coagulation bath remained stable at 193.06±10.27 MPa (new bath standard 203.88 MPa), demonstrating the highly stable and reliable performance of the regenerated coagulation bath. This technology not only completely solves the three core bottlenecks of traditional methods—low urea recovery rate / poor purity, low / unstable NaHCO3 regeneration efficiency, and broken resource recycling (dependence on external NaOH / CO2, generating solid waste)—but also achieves the revolutionary goals of "zero carbon emissions," "zero solid waste," and "high-efficiency urea recovery" by constructing a closed-loop system. This provides solid and reliable technical support for the green, low-cost, and sustainable large-scale production of cellulose-based materials (high-end membrane materials, medical and sanitary materials, etc.) and has broad potential for expanded applications.

[0073] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for recycling and regenerating a cellulose coagulation bath, characterized in that, Includes the following steps: The cellulose coagulation bath is subjected to a first reduced pressure rotary evaporation and a first cooling crystallization. After solid-liquid separation, a first crystal and a residual liquid are obtained. The components of the cellulose coagulation bath include urea, sodium bicarbonate, sodium carbonate, and water. The components of the first crystal include sodium bicarbonate. The residual liquid is subjected to a second reduced pressure rotary evaporation to dryness to obtain a second crystal, the second crystal comprising urea, sodium carbonate and sodium bicarbonate; The second crystal is ground and mixed with a surfactant to obtain a ground mixture; The grinding mixture is mixed with a first portion of urea solvent to perform the first urea dissolution. After solid-liquid separation, a first filter cake and a first filtrate are obtained. The first filter cake was washed with a second portion of urea solvent to obtain a second filtrate and a second filter cake. The second filter cake is mixed with a third portion of urea solvent to perform a second urea dissolution. After solid-liquid separation, a third filter cake and a third filtrate are obtained. The third filter cake is washed to obtain a third crystal, which is a mixture of sodium bicarbonate and sodium carbonate. The first, second, and third filtrates were combined and subjected to a third vacuum rotary evaporation, a second cooling crystallization, and alcohol washing to obtain urea crystals. The first and third crystals were mixed with CaO and water to carry out a causticizing reaction. After solid-liquid separation, NaOH solution and CaCO3 solid were obtained.

2. The recycling method according to claim 1, characterized in that, After obtaining solid CaCO3, the process further includes calcining the solid CaCO3 at high temperature to obtain CaO and CO2. The CaO is recycled to the causticization reaction, and the CO2 is used for the carbonization regeneration reaction of the waste Na2CO3 coagulation bath. The carbonization regeneration reaction includes the following steps: Waste coagulation bath containing Na2CO3 is subjected to carbonization regeneration reaction with CO2 to obtain regenerated coagulation bath containing NaHCO3.

3. The recycling method according to claim 1 or 2, characterized in that, The concentration of urea in the cellulose coagulation bath is 50-300 g / L; the concentration of sodium bicarbonate is 80-100 g / L; and the concentration of sodium carbonate is 5-50 g / L.

4. The recycling method according to claim 1, characterized in that, The temperature of the first vacuum rotary evaporation is 50~70℃, and the vacuum degree is -0.07~-0.1 MPa; the first vacuum rotary evaporation is carried out to 1 / 5~3 / 5 of the original volume; The temperature for the first cooling crystallization is 0~20℃; The temperature of the second vacuum rotary evaporation is 50~70℃, and the vacuum degree is -0.07~-0.1 MPa.

5. The recycling method according to claim 1, characterized in that, The surfactant is one or more of sodium dodecyl sulfate, sodium dodecyl sulfonate, and alkyl polyglucoside; The mass ratio of the second crystal to the surfactant is 50~200:

1.

6. The recycling method according to claim 1, characterized in that, The volume ratio of the first part of urea solvent, the second part of urea solvent, and the third part of urea solvent is (4~5):(2~3):(2~3); The mass ratio of the grinding mixture to the volume ratio of the first urea solvent is 50-64 g: 150 mL; The temperature of the first urea solvent is 20~30℃; The temperature of the second urea solvent is 0~10℃; The temperature of the third urea solvent is 20~30℃; The urea solvent includes one or more of N,N-dimethylformamide, dimethyl sulfoxide, dimethylacetamide, and ethanol.

7. The recycling method according to claim 1, characterized in that, The temperature of the third vacuum rotary evaporation is 50~70℃, the vacuum degree is -0.07~-0.1 MPa, and the temperature of the second cooling crystallization is 0~10℃.

8. The recycling method according to claim 1, characterized in that, The total mass ratio of the first and third crystals to CaO is 15:9.3~11.2; the causticizing reaction is carried out at a temperature of 70~100℃ for 1~4 hours.

9. The recycling method according to claim 2, characterized in that, The high-temperature calcination temperature is 800~1200℃, and the time is 1~12h; The carbonization regeneration reaction is carried out at a temperature of 30~70℃, a pressure of 2~10 bar, and a time of 0.5~6h.

10. The recycling method according to claim 2, characterized in that, The NaOH solution obtained from the causticization reaction is used to dissolve regenerated cellulose.

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