A process for the preparation of a lyocell fibre dissolving pulp

Abstract

The application provides a preparation method of a dissolving pulp for lyocell fibers, and belongs to the technical field of pulp refining of artificial fibers. The bleached chemical pulp is used as raw material, and the bleached chemical pulp is treated with an acid solution and a chelating agent in sequence, so that metal ions can be effectively removed; the pulp is pretreated by mechanical beating and hemicellulase treatment to obtain modified pulp; and the modified pulp is treated with an NMMO solution, so that hemicellulose in the modified pulp can be removed. The preparation method provided by the application is a green method with solvent recycling and recycling, the prepared pulp has low metal ion content, high alpha cellulose content, low hemicellulose content and high pulp yield. The main solvent NMMO used in the application is the same as the solvent used in the existing lyocell fiber production process, and can be effectively combined with the existing process route, and has the application prospect of industrialization and large scale.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of refining of pulp for artificial fibers, and particularly relates to a dissolving pulp for lyocell fibers and a preparation method thereof. BACKGROUND

[0002] Lyocell fiber is developed in the process of searching for a solvent to replace viscose fiber. The production process of lyocell fiber is a pure physical process, which is non-toxic and non-polluting. The NMMO (N-methyl morpholine aqueous solution) solvent used can be recycled and is known as the "green fiber" of the 21st century. Lyocell fiber has high requirements for dissolving pulp, and has higher requirements for the content of alpha-cellulose, the content of metal ions, the content of pentosan, whiteness and intrinsic viscosity.

[0003] Traditional methods for preparing dissolving pulp often use wood and bamboo as raw materials, and adopt acid sulfite method and pre-hydrolysis kraft method to prepare dissolving pulp after cooking and bleaching of the raw materials. However, the traditional process for preparing dissolving pulp has long process flow, complex process and low yield.

[0004] Some researchers use chemical pulp as raw material, and upgrade it to dissolving pulp through further treatment and regulation. This has become a new method for preparing dissolving pulp. For example, the patent with publication number CN115094660A completes the purification and upgrading of pulp through alkali extraction, enzyme treatment and acid treatment.

[0005] The process of preparing lyocell fiber from chemical pulp can be regarded as a physical process of hydrogen bond breaking and reconfiguration. In the spinning process, all components in the pulp enter the spinning solution. However, in the process of preparing lyocell fiber spinning solution, cellulose and hemicellulose in the pulp can be dissolved in NMMO / H2O solution at the same time, and then coagulate and precipitate at the same time in the fiber forming process. However, the presence of hemicellulose will cause the increase of the viscosity of the spinning solution, insufficient impregnation of the pulp, the decrease of the strength performance of the product and the increase of the load of solvent recovery. In addition, the metal ions in the pulp will also cause oxidation-reduction reaction with NMMO to produce N-methyl morpholinyl, accelerate the breaking of hydroxyl groups in cellulose, and cause the degradation of cellulose, thereby reducing the degree of polymerization of cellulose. At the same time, metal ions are effective inducers of NMMO homolysis reaction, which will accelerate the decomposition of NMMO, resulting in the decrease of the viscosity of the spinning solution and the deterioration of the spinnability. If the content of metal ions is too high, the spinning solution will accelerate the decomposition and release a large amount of heat, which will cause explosion if out of control. Therefore, the separation of components in chemical pulp and the control of the content of metal ions are the key to preparing dissolving pulp for lyocell fibers. SUMMARY

[0006] Therefore, the present application aims to provide a new preparation method of lyocell fiber dissolving pulp.

[0007] To achieve the above-mentioned purpose, the present application provides the following technical solutions.

[0008] The present application provides a preparation method of lyocell fiber dissolving pulp, comprising the following steps:

[0009] The bleached chemical pulp is mixed with an acidic solution for acid treatment to obtain an acid-treated pulp;

[0010] The acid-treated pulp is mixed with a chelating agent for chelation treatment to remove the formed metal chelate to obtain a pulp with low metal content;

[0011] The pulp with low metal content is sequentially subjected to mechanical refining and hemicellulase treatment to obtain a modified pulp;

[0012] The modified pulp, NMMO solution and antioxidant are mixed for purification, and in the purification process, the hemicellulose in the modified pulp is dissolved into the NMMO solution, and solid-liquid separation is performed to obtain the lyocell fiber dissolving pulp; the mass concentration of the NMMO solution is 60-80%.

[0013] Preferably, the temperature of the acid treatment is 10-50℃, and the time is 10-90min.

[0014] Preferably, the pH value of the acid treatment is 1-5.

[0015] Preferably, the temperature of the chelation treatment is 25-50℃, and the time is 10-60min.

[0016] Preferably, the refining revolution of the mechanical refining is 4000-14000r, and the beating degree of the pulp after the mechanical refining is 13-81°SR.

[0017] Preferably, the hemicellulase includes xylanase and / or mannanase.

[0018] Preferably, the temperature of the hemicellulase treatment is 30-60℃, the time is 10-120min, and the pH value of the system is 5-8.

[0019] Preferably, the mass of the modified pulp is 2-15% of the total mass of the modified pulp, NMMO solution and antioxidant.

[0020] Preferably, the antioxidant is 0.04-0.1% of the total mass of the modified pulp, NMMO solution and antioxidant.

[0021] Preferably, the purification is performed at 80-110°C for 20-120 minutes.

[0022] Preferably, after the solid-liquid separation, the obtained solid is washed with NMMO solution at 80-110°C.

[0023] The present application provides a preparation method of lyocell fiber dissolving pulp, comprising the following steps: mixing bleached chemical pulp with an acidic solution to perform acid treatment, obtaining acid-treated pulp; mixing the acid-treated pulp with a chelating agent to perform chelation treatment, removing the formed metal chelate, obtaining pulp with low metal content; sequentially performing mechanical refining and hemicellulase treatment on the pulp with low metal content, obtaining modified pulp; mixing the modified pulp, NMMO solution and antioxidant to perform purification, in which the hemicellulose in the modified pulp is dissolved into the NMMO solution, and then the solid-liquid separation is performed, obtaining the lyocell fiber dissolving pulp; the mass concentration of the NMMO solution is 60-80%.

[0024] The present application uses ordinary bleached chemical pulp as raw material, and sequentially uses an acidic solution and a chelating agent to treat the bleached chemical pulp, which can effectively remove metal ions (Fe ions, Cu ions, Ca ions and Mg ions); the mechanical refining and hemicellulase treatment are used to pretreat the pulp to obtain modified pulp; the NMMO solution is used to treat the modified pulp, which can remove the hemicellulose in the modified pulp, so that the cellulose and hemicellulose are separated.

[0025] The preparation method provided by the present application is a green and solvent recyclable method, and the prepared pulp has low metal ion content, high alpha-cellulose content, low hemicellulose content and high pulp yield. The results of the examples show that the lyocell fiber dissolving pulp prepared by the present application has an alpha-cellulose mass percentage of 96%, a hemicellulose mass percentage of <3.4%, an Fe ion content of 2.58 ppm, a Cu ion content of 0.033 ppm, a Ca ion content of 16.36 ppm, a Mg ion content of 12.05 ppm and a whiteness of 94%, which meets the requirements for alpha-cellulose content, hemicellulose content, metal ion content and whiteness (generally, the requirements are alpha-cellulose content ≥95%, hemicellulose content ≤4%, metal ion content Fe ≤4 ppm, Cu ≤0.05 ppm, Mg ≤15 ppm, Ca ≤20 ppm and whiteness ≥92%).

[0026] The molecular weight of cellulose in chemical pulp is large and the crystallinity is high, the molecular weight of hemicellulose is small and the crystallinity is zero, due to the difference in molecular weight and crystallinity, the dissolution sequence and rate of them in NMMO solution are quite different, the present application uses the above difference, and innovatively proposes to use NMMO dilute solution with a mass concentration of 60-80% to treat chemical pulp, by controlling the condition parameters (NMMO concentration and system temperature), hemicellulose is dissolved in NMMO solution prior to cellulose, and after separation, the purpose of removing hemicellulose is achieved.

[0027] In addition, the present application uses mechanical refining and hemicellulase treatment to pretreat the chemical pulp, and controls the specific surface area and accessibility of the chemical pulp fibers, promotes the penetration of NMMO solution, and effectively improves the removal effect of NMMO solution on hemicellulose, and in the whole process, the molecular structure and crystal form of cellulose do not change at all, the paper pulp yield is high, and the selectivity of hemicellulose removal is high.

[0028] NMMO is an important solvent in the production process of lyocell fibers, and its recovery rate is more than 99%. The recovery process of NMMO is to change the low-concentration NMMO solution in the coagulation bath into high-concentration NMMO through vacuum concentration for further use, and a large amount of NMMO solution is generated in this process. In the present application, NMMO solution is the key solvent for the separation of chemical pulp components, and this solvent is already present in large quantities in the production process of lyocell fibers, and can be used on site. Therefore, the main solvent NMMO used in the present application is the same as the solvent used in the existing lyocell fiber production process, and can be effectively combined with the existing lyocell fiber production process, and has the application prospect of industrialization and large scale. DETAILED DESCRIPTION

[0029] The present application provides a preparation method of lyocell fiber dissolving pulp, comprising the following steps:

[0030] The bleached chemical pulp is mixed with an acid solution for acid treatment to obtain an acid-treated pulp;

[0031] The acid-treated pulp is mixed with a chelating agent for chelation treatment to remove the formed metal chelate, and a pulp with low metal content is obtained;

[0032] The pulp with low metal content is sequentially subjected to mechanical refining and hemicellulase treatment to obtain a modified pulp;

[0033] The modified pulp, NMMO solution and antioxidant are mixed for purification, and in the purification process, the hemicellulose in the modified pulp is dissolved into the NMMO solution, and solid-liquid separation is performed to obtain the lyocell fiber dissolving pulp; the mass concentration of the NMMO solution is 60-80%.

[0034] The acid treatment pulp is obtained by mixing the bleached chemical pulp with an acid solution.

[0035] In the present application, the bleached chemical pulp is preferably prepared by the following steps: tearing the chemical pulp board into 1cm*1cm paper pieces, soaking in deionized water, and defibrating with a paper pulp defibrator until the fibers are evenly dispersed; wringing off the water, tearing into pieces and drying in an oven to obtain the bleached chemical pulp. The bleached chemical pulp is preferably placed in a self-sealing bag to balance the moisture content, and the moisture content is measured for later use.

[0036] In the present application, the bleached chemical pulp preferably includes one or more of wood pulp, bamboo pulp, straw pulp and hemp pulp. In the present application, the acid solution preferably includes a sulfuric acid solution, a hydrochloric acid solution or an acetic acid solution; the acid treatment time is preferably 10-90min, further preferably 20-70min, and more preferably 30-50min; the acid treatment temperature is preferably 10-50℃, further preferably 10-30℃.

[0037] In the present application, the pH value of the acid treatment is preferably 1-5, further preferably 2-4. The present application does not have special requirements for the concentration and amount of the acid solution, and the pH value of the acid treatment can be adjusted to 1-5.

[0038] The mass concentration of the pulp during the acid treatment in the present application is preferably 1-10%, further preferably 2-7%, and more preferably 3-5%. The present application controls the water content in the acid solution system to make the mass concentration of the pulp 1-10%.

[0039] After the acid treatment is completed, the obtained pulp is preferably washed to a pH value of 5-7.

[0040] By adding an acid solution, the present application can dissolve the metal ions in the bleached chemical pulp, and then remove a part of the metal ions through washing.

[0041] After obtaining the acid treatment pulp, the present application mixes the acid treatment pulp with a chelating agent to perform chelation treatment, removes the formed metal chelates, and obtains pulp with low metal content.

[0042] In the present application, the chelating agent preferably includes ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) or diethylenetriaminepenta(methylene phosphonic acid) (DTPMPA). In the present application, the chelation treatment time is preferably 10-60min, further preferably 20-40min; the chelation treatment temperature is preferably 25-50℃, further preferably 30-45℃. In the present application, the amount of the chelating agent added is preferably 0.2-0.7% of the absolute dry mass of the acid treatment pulp, further preferably 0.4-0.6%.

[0043] The mass concentration of the pulp during the chelating treatment is preferably 3-10%, and more preferably 4-7%. During the chelating treatment, the mass concentration of the pulp is controlled to be 3-10% by controlling the water content in the acid-treated pulp and / or adding a certain amount of water.

[0044] After the chelating treatment, the metal chelate formed is removed to obtain pulp with low metal content. The obtained pulp is preferably washed to neutral and dried to remove the metal chelate.

[0045] The present application can effectively remove metal ions (Fe ions, Cu ions, Ca ions and Mg ions) by treating the bleached chemical pulp with an acidic solution and a chelating agent to obtain pulp with low metal ion content.

[0046] After obtaining the pulp with low metal content, the present application performs mechanical refining on the pulp with low metal content to obtain refined pulp.

[0047] Before the mechanical refining, the present application also preferably soaks the pulp with low metal content in deionized water for 24 h, then adds deionized water for defibration, and adjusts the mass concentration of the pulp to 10% after defibration.

[0048] In the present application, the refining revolution of the mechanical refining is preferably 4000-14000r, and more preferably 6000-12000r, and even more preferably 8000-10000r; and the beating degree of the refined pulp is preferably 13-81°SR, and more preferably 20-60°SR, and even more preferably 30-50°SR.

[0049] The present application can increase the specific surface area and accessibility of chemical pulp fibers by mechanical refining.

[0050] After the mechanical refining, the present application preferably washes and dries the obtained refined pulp with deionized water, and stores it in a self-sealing bag after balancing the moisture content.

[0051] After obtaining the refined pulp, the present application performs hemicellulase treatment on the refined pulp to obtain modified pulp.

[0052] In the present application, the hemicellulase treatment preferably includes: mixing citric acid buffer solution with the refined pulp and hemicellulase, and placing the obtained mixture in a water bath to perform hemicellulase treatment.

[0053] In the present application, the pH value of the citric acid buffer solution is preferably 4-7. The present application does not have a particular requirement for the amount of the citric acid buffer solution, and the slurry mass concentration of the hemicellulase treatment system can be adjusted to 10%. In the present application, the buffer pair of the citric acid buffer solution is citric acid and sodium hydrogen phosphate.

[0054] In the present application, the hemicellulase preferably includes xylanase and / or mannanase; the amount of the hemicellulase is preferably 200-1000 U / g, further preferably 350-800 U / g, and more preferably 500-650 U / g. In the present application, the amount of the hemicellulase is preferably based on the absolute dry mass of the pulp after refining.

[0055] In the present application, the temperature of the hemicellulase treatment is preferably 30-60°C, and further preferably 40-50°C; the time of the hemicellulase treatment is preferably 10-120 min, further preferably 20-100 min, and more preferably 50-80 min.

[0056] During the hemicellulase treatment, the present application preferably kneads the pulp every 10 min to mix it evenly, and after the enzyme treatment, the pulp is soaked in hot water at 90-95°C for 30 min for inactivation, then washed with deionized water and suction filtered with a sand core funnel, and the filter cake is dried and placed in a self-sealing bag for water balance before use.

[0057] In the present application, during the hemicellulase treatment, the hemicellulase specifically treats hemicellulose, gradually degrading hemicellulose polysaccharide into oligosaccharide, which facilitates better dissolution into NMMO solution.

[0058] The present application uses mechanical refining and hemicellulase treatment to pretreat chemical pulp, controls the specific surface area and accessibility of chemical pulp fibers, promotes the penetration of NMMO solution, effectively improves the removal effect of hemicellulose by NMMO solution, and the molecular structure and crystal form of cellulose do not change during the whole process, the pulp yield is high, and the selectivity of hemicellulose removal is high.

[0059] After obtaining the modified pulp, the modified pulp, NMMO solution and antioxidant are mixed for purification, and during the purification process, the hemicellulose in the modified pulp is dissolved into the NMMO solution, and solid-liquid separation is performed to obtain the dissolving pulp for lyocell fibers.

[0060] In the present application, the mass of the modified pulp is preferably 2-15% of the total mass of the modified pulp, NMMO solution and antioxidant, and further preferably 5-10%.

[0061] In the present application, the mass of the antioxidant is preferably 0.04-0.1% of the total mass of the modified pulp, the NMMO solution and the antioxidant, and more preferably 0.06-0.08. In the present application, the antioxidant is preferably propyl gallate. In the present application, the antioxidant can prevent the degradation of NMMO at high temperature and other side reactions, and is a stabilizer for the NMMO solution.

[0062] In the present application, the mass concentration of NMMO in the NMMO solution is 60-80%, and preferably 65-75%.

[0063] In the present application, the purification time is preferably 20-120 min, more preferably 30-100 min, and even more preferably 50-80 min, and the purification temperature is preferably 80-110°C, and more preferably 85-100°C.

[0064] The present application does not have a special requirement for the solid-liquid separation method, and a method well known in the art can be used. In the present application, the obtained pulp is preferably subjected to solid-liquid separation by suction filtration using a sand core funnel.

[0065] After the solid-liquid separation is completed, the obtained solid is preferably washed with an NMMO solution having a temperature of 80-110°C and a mass fraction of 60-80% for 1-2 times, and then washed with deionized water having a temperature of 90-95°C until the pH value of the filtrate is neutral, and dried in an oven at 60°C for 24 h to obtain the dissolving pulp for lyocell fibers.

[0066] In the present application, due to the large molecular weight and high crystallinity of cellulose in the chemical pulp, and the small molecular weight and no crystallinity of hemicellulose, there is a large difference in the dissolution order and rate of the two in the NMMO solution. The present application utilizes the above difference, and innovatively proposes to treat the chemical pulp with a NMMO dilute solution having a mass concentration of 60-80%, and controls the conditions (NMMO concentration and system temperature) to make the hemicellulose dissolve in the NMMO solution prior to the cellulose, so that the hemicellulose is removed after separation.

[0067] NMMO is an important solvent in the production process of lyocell fiber, and its recovery rate is more than 99%. The recovery process of NMMO is to change the low-concentration NMMO solution in the coagulation bath into high-concentration NMMO through vacuum concentration for further use, and a large amount of NMMO solution is generated in this process. In the present application, the NMMO solution is the key solvent for the separation of chemical pulp components, and this solvent is already present in large quantities in the production process of lyocell fiber and can be used on site. Therefore, the main solvent NMMO used in the present application is the same as the solvent used in the existing lyocell fiber production process, and can be effectively combined with the existing lyocell fiber production process, and has the application prospect of industrialization and large scale.

[0068] In order to further illustrate the present application, a preparation method of a dissolving pulp for lyocell fiber provided by the present application is described in detail below in combination with examples, but they should not be understood as limiting the scope of protection of the present application.

[0069] Example 1

[0070] The ordinary bleached chemical pulp board was torn and disintegrated by a disintegrator until the fibers were uniformly dispersed, and then the water was wrung out and torn into pieces and placed in an oven at 60℃ for 24h for drying, and then placed in a self-sealing bag for moisture balance. The moisture content of the pulp was measured for standby use.

[0071] The disintegrated bleached chemical pulp and hydrochloric acid solution were mixed for acid treatment, and the mass concentration of the obtained pulp was controlled to be 3wt%, to obtain an acid-treated pulp. In the acid treatment process, the temperature of the acid treatment was 10℃, the time was 60min, the concentration of the hydrochloric acid solution was 6mol / L, the amount was 1.5% relative to the mass of the absolutely dry pulp, and the pH value of the acid treatment was 4.6.

[0072] The obtained pulp was washed to a pH value of 5-7 while controlling the mass concentration of the pulp to be 3%, and then a metal ion chelating agent EDTA was added for chelation treatment to remove the formed metal chelate. Then the obtained pulp was washed to neutral and dried to obtain a pulp with low metal content. In the chelation treatment process, the temperature of the chelation treatment was 30℃, the time of the chelation treatment was 60min, and the amount of the chelating agent was 0.5% relative to the mass of the absolutely dry pulp.

[0073] 30g of absolutely dry pulp was soaked in deionized water for 24h, and then disintegrated by adding deionized water. After disintegration, the mass concentration of the pulp was adjusted to 10%, and the pulp was evenly placed in a PFI mill for refining treatment, and the refining revolution was adjusted to 8000r. Then the pulp was washed with deionized water, dried, placed in a self-sealing bag for moisture balance, and used as standby.

[0074] Take 10 g of the chemical pulp pulp treated by grinding, adjust the pulp mass concentration to 10% with citric acid buffer solution with pH value of 4.8, then add hemicellulase, and place it in a 50-55°C water bath for hemicellulase treatment. The amount of hemicellulase (xylanase) is 350 U / g, and the treatment time is 120 min. Every 10 min during the enzyme treatment, knead it once to make it evenly mixed. After enzyme treatment, soak it in 90-95°C hot water for 30 min for inactivation, then wash it with deionized water and filter it with a sand core funnel. Dry the filter cake and place it in a self-sealing bag to balance the moisture for later use.

[0075] Mix the modified pulp with propyl gallate and a 70% NMMO dilute solution. The amount of propyl gallate is 0.04 wt%. Finally, form a pulp / NMMO / H2O blend with a pulp mass fraction of 5 wt%. Treat the blend in a 90°C water bath for 30 min. Immediately after treatment, filter it with a sand core funnel. Wash the filter cake with a 90°C 70% NMMO solution. Wash 1-2 times, then wash the filter cake with 90-95°C deionized water until the filtrate pH is neutral. Dry it in a 60°C oven for 24 h to obtain a lyocell fiber dissolving pulp.

[0076] The Fe ion content in the lyocell fiber dissolving pulp obtained in this example is 2.58 ppm, the Cu ion content is 0.033 ppm, the Ca ion content is 16.36 ppm, the Mg ion content is 12.05 ppm, the hemicellulose content is 3.37%, and the yield is 87.15%.

[0077] Example 2

[0078] The preparation steps are the same as in Example 1, except that the 70% NMMO dilute solution is replaced by a 75% NMMO dilute solution. The step of purifying with NMMO solution is:

[0079] Mix the modified pulp with propyl gallate and a 75% NMMO dilute solution. The amount of propyl gallate is 0.04 wt%. Finally, form a pulp / NMMO / H2O blend with a pulp mass fraction of 5 wt%. Treat the blend in a 90°C oil bath for 30 min. Immediately after treatment, filter it with a sand core funnel. Wash the filter cake with a 90°C 75% NMMO solution. Wash 1-2 times, then wash the filter cake with 90-95°C deionized water until the filtrate pH is neutral. Dry it in a 60°C oven for 24 h to obtain a lyocell fiber dissolving pulp.

[0080] The lyscel fibre dissolving pulp obtained in the example has Fe ion content of 2.58 ppm, Cu ion content of 0.033 ppm, Ca ion content of 16.36 ppm, Mg ion content of 12.05 ppm, hemicellulose content of 3.12%, and yield of 72.16%.

[0081] Example 3

[0082] The preparation steps are the same as those in Example 1, except that the temperature of the purification treatment is replaced by 100°C, i.e. the purification step using NMMO solution is as follows:

[0083] The modified pulp is mixed with propyl gallate and a 70% NMMO dilute solution, the amount of propyl gallate is 0.04wt%, and finally a pulp / NMMO / H2O blend with a pulp mass fraction of 5wt% is formed. The blend is treated in a 100°C oil bath for 30 min, immediately after the treatment, suction filtration is performed using a sand core funnel, and the filter cake is washed with a 100°C 70% NMMO solution for 1-2 times, then the filter cake is washed with 90-95°C deionized water until the filtrate pH is neutral, and is dried in a 60°C oven for 24 h to obtain a lyscel fibre dissolving pulp.

[0084] The lyscel fibre dissolving pulp obtained in the example has Fe ion content of 2.58 ppm, Cu ion content of 0.033 ppm, Ca ion content of 16.36 ppm, Mg ion content of 12.05 ppm, hemicellulose content of 3.22%, and yield of 80.39%.

[0085] Example 4

[0086] The preparation steps are the same as those in Example 1, except that the time of the purification treatment is replaced by 60 min, i.e. the purification step using NMMO solution is as follows:

[0087] The modified pulp is mixed with propyl gallate and a 70% NMMO dilute solution, the amount of propyl gallate is 0.04wt%, and finally a pulp / NMMO / H2O blend with a pulp mass fraction of 5wt% is formed. The blend is treated in a 90°C water bath for 60 min, immediately after the treatment, suction filtration is performed using a sand core funnel, and the filter cake is washed with a 90°C 70% NMMO solution for 1-2 times, then the filter cake is washed with 90-95°C deionized water until the filtrate pH is neutral, and is dried in a 60°C oven for 24 h to obtain a lyscel fibre dissolving pulp.

[0088] The Fe ion content of the dissolving pulp for lyocell fiber obtained in this example is 2.58 ppm, the Cu ion content is 0.033 ppm, the Ca ion content is 16.36 ppm, the Mg ion content is 12.05 ppm, the hemicellulose content is 3.28%, and the yield is 83.48%.

[0089] Comparative Example 1

[0090] The preparation steps are the same as those of Example 1, except that the mechanical refining treatment is omitted.

[0091] Comparative Example 2

[0092] The preparation steps are the same as those of Example 1, except that the hemicellulase treatment is omitted.

[0093] Comparative Example 3

[0094] The preparation steps are the same as those of Example 1, except that the mechanical refining and hemicellulase treatments are omitted.

[0095] Comparative Example 4

[0096] The preparation steps are the same as those of Example 1, except that the concentration of the NMMO dilute solution is 50% during the NMMO dilute solution treatment.

[0097] Comparative Example 5

[0098] The preparation steps are the same as those of Example 1, except that the treatment temperature is 70°C during the NMMO dilute solution treatment.

[0099] Comparative Example 6

[0100] The preparation steps are the same as those of Example 1, except that after the NMMO dilute solution treatment, suction filtration is performed using a sand core funnel, and then washing is directly performed using deionized water at 90-95°C.

[0101] The results obtained in Examples 1-4 and Comparative Examples 1-6 are compared as follows, and the results are shown in Table 1.

[0102] Table 1 Test results of Examples 1-4 and Comparative Examples 1-6

[0103] No. Fe (ppm) Cu (ppm) Ca (ppm) Mg (ppm) Hemicellulose content (%) Yield (%) Original pulp 26.62 0.92 291.21 179.34 20.29 -- Example 1 2.58 0.033 16.36 12.05 3.37 87.15 Example 2 2.58 0.033 16.36 12.05 3.12 72.16 Example 3 2.58 0.033 16.36 12.05 3.22 80.39 Example 4 2.58 0.033 16.36 12.05 3.28 83.48 Comparative Example 1 2.58 0.033 16.36 12.05 6.36 88.32 Comparative Example 2 2.58 0.033 16.36 12.05 7.06 97.96 Comparative Example 3 2.58 0.033 16.36 12.05 10.19 89.33 Comparative Example 4 2.58 0.033 16.36 12.05 15.62 94.06 Comparative Example 5 2.58 0.033 16.36 12.05 13.06 90.37 Comparative Example 6 2.58 0.033 16.36 12.05 6.19 89.48

[0104] As can be seen from Table 1, although the preparation methods of Comparative Examples 1-3 are simple, the hemicellulose content of the cellulose pulp prepared is high, and the hemicellulose will dissolve in the solvent during the production of lyocell fiber, increasing the viscosity of the spinning solution, and further affecting the flowability and permeability, resulting in insufficient impregnation of the fiber pulp to form agglomerated particles, affecting the performance of the spinning solution and the solvent recovery load.

[0105] Comparative Example 4 uses 50% NMMO dilute solution in the pretreatment process, and the removal of hemicellulose is slow. The reason is that the higher the mass fraction of NMMO, the easier it is to enter the interior of the cellulose molecule, causing the hemicellulose molecular chain to move and break, thereby more effectively removing the hemicellulose.

[0106] Comparative Example 5 uses a treatment temperature of 70°C in the NMMO pretreatment process, and the removal of hemicellulose is poor. The reason is that temperature affects the movement rate of NMMO molecules. The higher the temperature, the faster the movement rate of NMMO molecules, thereby enhancing the ability of NMMO solvent to diffuse into the interior of the cellulose molecule, causing the hemicellulose to degrade before the cellulose.

[0107] Comparative Example 6 lacks the step of washing the filter cake with the same concentration and temperature of NMMO dilute solution compared to Example 1, and only uses deionized water at 90-95°C for washing. At this time, the mass fraction of NMMO on the surface of the filter cake will rapidly decrease, causing the hemicellulose that has been dissolved in the NMMO solution to precipitate again, thereby affecting the removal of hemicellulose.

[0108] Examples 1-4 of the present application use a biological mechanical coupling method in conjunction with NMMO dilute solution for treatment, so that most of the hemicellulose in the pulp is removed, improving the purity of the pulp and increasing the accessibility of the NMMO solution. At the same time, a secondary washing method is used to remove the hemicellulose that has not been filtered and is attached to the surface of the filter cake. The present application solves the necking problem of lyocell fiber dissolving pulp and lays the foundation for the development of lyocell fiber dissolving pulp.

[0109] Although the above examples have made a detailed description of the present application, it is only a part of the embodiments of the present application, not all the embodiments, and other embodiments can be obtained according to the present embodiments without creativity, which are within the scope of protection of the present application.

Claims

1. A process for the preparation of a dissolving pulp for lyocell fibers, characterized in that, The method comprises the following steps: mixing the bleached chemical pulp with an acid solution to perform acid treatment to obtain an acid-treated pulp; the acid treatment is performed at a temperature of 10-50℃ for 10-90min, and the pH value of the acid treatment is 1-5; mixing the acid-treated pulp with a chelating agent to perform chelation treatment to remove the formed metal chelate to obtain a pulp with low metal content; the chelation treatment is performed at a temperature of 25-50℃ for 10-60min; the chelating agent comprises ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid or diethylenetriaminepenta(methylene phosphonic acid); performing mechanical refining and hemicellulase treatment on the pulp with low metal content in sequence to obtain modified pulp; the mechanical refining is performed at a refining revolution of 4000-14000r, and the beating degree of the pulp after the mechanical refining is 13-81°SR; mixing the modified pulp, an NMMO solution and an antioxidant to perform purification, in which the hemicellulose in the modified pulp is dissolved into the NMMO solution, the NMMO solution is removed by solid-liquid separation, the obtained solid is washed with an NMMO solution at a temperature of 80-110℃ to obtain the dissolving pulp for lyocell fiber; the mass concentration of the NMMO solution is 60-80%; the mass of the modified pulp accounts for 2-15% of the total mass of the modified pulp, the NMMO solution and the antioxidant; the purification is performed at a temperature of 80-110℃ for 20-120min.

2. The production method according to claim 1, characterized by, The hemicellulase comprises xylanase and / or mannanase.

3. The production method according to claim 1 or 2, characterized by, The hemicellulase treatment is performed at a temperature of 30-60℃ for 10-120min, and the pH value of the system is 5-8.