Method for preparing dissolving pulp

Abstract

The invention discloses a method for preparing dissolving pulp, which comprises the following steps: 1) adding eucalyptus into an ultralow acid-phenoxyethanol two-phase solvent system for two-phase pretreatment, and filtering to remove filtrate to obtain a solid material; (2) adding the solid material into a NaOH-Na2S solution, and cooking by a sulfate process to obtain sulfate pulp; and 3) performing ECF bleaching on the sulfate pulp to obtain the dissolving pulp. The method for preparing the dissolving pulp has the advantages of being good in raw material compatibility, high in cellulose retention rate, high in lignin and hemicellulose removal rate, small in medicine dosage and the like, and the prepared dissolving pulp is high in quality and suitable for large-scale industrial application.

Description

Technical Field

[0001] The present invention relates to the technical field of pulp and paper making, and specifically relates to a method for preparing dissolving pulp. Background Art

[0002] Dissolving pulp (characterized by an α-cellulose content exceeding 90% and extremely low contents of hemicellulose and lignin) is the main raw material for preparing cellulose-derived products, and is mainly used for producing products such as viscose fiber, nitrocellulose, cellulose acetate, cellophane, carboxymethyl cellulose (CMCC), etc. At present, the production of dissolving pulp mainly uses the acid sulfite (AS) method and the prehydrolysis kraft (PHK) method. However, the AS method has problems such as limited raw material adaptability and low recovery efficiency of chemicals in the hydrolysis liquor after the reaction. Although the PHK method has better raw material compatibility compared with the AS method, the PHK method cannot fully extract hemicellulose from the raw materials in the prehydrolysis stage, which will affect the overall utilization rate of the raw materials. In summary, the existing methods for producing dissolving pulp all have obvious defects and are difficult to fully meet the actual application requirements.

[0003] Therefore, it is of great significance to develop a method for preparing dissolving pulp with good raw material compatibility, high retention rate of cellulose, high removal rate of lignin and hemicellulose, and less chemical consumption. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing dissolving pulp.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for preparing dissolving pulp includes the following steps:

[0007] 1) Adding eucalyptus wood into an ultra-low acid-phenoxyethanol biphasic solvent system for biphasic pretreatment, and then filtering to remove the filtrate to obtain a solid material;

[0008] 2) Adding the solid material into a NaOH-Na2S solution for kraft cooking to obtain kraft pulp;

[0009] 3) Performing ECF bleaching on the kraft pulp to obtain dissolving pulp.

[0010] Preferably, the mass-volume ratio of the eucalyptus wood to the ultra-low acid-phenoxyethanol biphasic solvent system in step 1) is 1 g: 15 mL to 25 mL.

[0011] Preferably, the ultra-low acid (ULA)-phenoxyethanol biphasic solvent system in step 1) is composed of a sulfuric acid solution with a mass fraction ≤ 0.1% and phenoxyethanol (EPH) in a volume ratio of 1: 0.8 to 1.2.

[0012] Preferably, the two-phase pretreatment in step 1) is carried out at a temperature of 150°C to 180°C, and the pretreatment time is 15 min to 60 min.

[0013] Preferably, the mass-volume ratio of the solid material to the NaOH-Na2S solution in step 2) is 1 g: 3 mL to 7 mL.

[0014] Preferably, the alkali content of the NaOH-Na2S solution in step 2) is 5% to 15% by mass percentage of Na2O, and the sulfidity is 20% to 30%.

[0015] Preferably, the kraft pulping in step 2) is carried out at a temperature of 160°C to 170°C, and the pulping time is 10 min to 60 min.

[0016] Preferably, the ECF bleaching in step 3) includes the following operations: chlorine dioxide bleaching, hydrogen peroxide bleaching, and chlorine dioxide bleaching.

[0017] More preferably, the ECF bleaching in step 3) includes the following operations:

[0018] a) Chlorine dioxide bleaching: adding the kraft pulp into a reaction kettle, then adding chlorine dioxide, and then carrying out bleaching at a temperature of 60°C to 80°C;

[0019] b) Hydrogen peroxide bleaching: adding sodium hydroxide, magnesium sulfate, and hydrogen peroxide into the reaction kettle, and then carrying out bleaching at a temperature of 70°C to 90°C;

[0020] c) Chlorine dioxide bleaching: adding chlorine dioxide into the reaction kettle, and then carrying out bleaching at a temperature of 60°C to 80°C.

[0021] Preferably, the bleaching time in step a) is 40 min to 80 min.

[0022] Preferably, the bleaching time in step b) is 40 min to 80 min.

[0023] Preferably, the bleaching time in step c) is 40 min to 80 min.

[0024] The principle of the present invention:

[0025] The present invention utilizes an ultra-low acid-phenoxyethanol biphasic solvent system for the biphasic pretreatment of eucalyptus. The ultra-low acid (extremely low acidity) can effectively avoid the degradation of cellulose during the pretreatment process, and the ultra-low acid can effectively penetrate the cell wall and depolymerize hemicellulose, thus helping to improve the removal rate of hemicellulose. The removal of hemicellulose in plant cells is positively correlated with the cracks and voids on the cell wall. The successful removal of hemicellulose greatly increases the space between cellulose and lignin in the raw material, thereby promoting the depolymerization of lignin and providing a large enough space for the sufficient contact between lignin in the cell wall and phenoxyethanol, and then promoting the effective dissolution and extraction of lignin. In addition, the solubility of phenoxyethanol in water is also an important factor affecting the pretreatment effect. As the reaction temperature increases, the compatibility of the organic solvent and water increases, enhancing the mass transfer effect of the solvent. Finally, the ultra-low acid-phenoxyethanol biphasic solvent system successfully separates the three major lignocellulose components (cellulose, hemicellulose, and lignin) in eucalyptus. The solid material is rich in cellulose, and pentosan is enriched in the filtrate. It is precisely because a large amount of lignin is removed in the pretreatment stage that the chemical dosage for subsequent kraft cooking is reduced by about 68%, and only 8% of the alkali dosage is required to achieve a good kraft cooking effect.

[0026] The beneficial effects of the present invention are as follows: The method for preparing dissolving pulp of the present invention has the advantages of good raw material compatibility, high cellulose retention rate, high removal rates of lignin and hemicellulose, and less chemical dosage. The quality of the prepared dissolving pulp is high and is suitable for large-scale industrial applications.

[0027] Specifically:

[0028] 1) High cellulose retention rate: The present invention utilizes ultra-low acid (extremely low acidity) to selectively remove hemicellulose while effectively avoiding the degradation of cellulose, maximizing the retention of cellulose in eucalyptus, and the cellulose retention rate can reach over 93%;

[0029] 2) High hemicellulose removal rate: The present invention utilizes the effective degradation ability of ultra-low acid on hemicellulose, enabling approximately 90% of the pentosan in eucalyptus to enter the filtrate, and the pentosan content in the filtrate is high, which can be further subjected to high-value conversion;

[0030] 3) High lignin removal rate: The present invention utilizes the high-efficiency dissolution ability of phenoxyethanol on lignin, resulting in a relatively high removal rate of lignin, which helps to reduce the alkali dosage for subsequent kraft cooking;

[0031] 4) Less chemical dosage for cooking: The chemical dosage for kraft cooking in the present invention is reduced by about 68% compared with traditional kraft cooking of hardwood, which is more friendly to equipment and the treatment of black liquor after cooking;

[0032] 5) High quality of dissolving pulp: In the present invention, eucalyptus is treated with ultra-low acid. The ultra-low acid has an extremely low concentration, which causes less damage to cellulose and improves the quality of the obtained dissolving pulp. Detailed implementation manners

[0033] The present invention will be further explained and described below in conjunction with specific embodiments.

[0034] Example 1:

[0035] A method for preparing dissolving pulp, the steps are as follows:

[0036] 1) Add 2.5 g of absolute dry eucalyptus, 25 mL of sulfuric acid solution with a mass fraction of 0.098%, and 25 mL of phenoxyethanol into a high-pressure reactor, then treat at a temperature of 160 °C for 30 min, and then filter to remove the filtrate to obtain a solid material;

[0037] 2) Add the solid material into the NaOH-Na2S solution. The mass-volume ratio of the solid material to the NaOH-Na2S solution is 1 g:5 mL. The alkali content of the NaOH-Na2S solution is 8% by mass percentage of Na2O, and the sulfidity is 25%. Then cook at a temperature of 165 °C for 30 min to obtain kraft pulp;

[0038] 3) Perform ECF bleaching on the kraft pulp, and the bleaching sequence is D1-E P -D2, and then the dissolving pulp is obtained.

[0039] Note:

[0040] The specific parameters of ECF bleaching are shown in the following table:

[0041] Table 1 Parameters of ECF bleaching

[0042]

[0043]

[0044] Example 2:

[0045] A method for preparing dissolving pulp, except that the pretreatment temperature in step 1) is adjusted from "160 °C" to "150 °C", the rest is exactly the same as in Example 1.

[0046] Example 3:

[0047] A method for preparing dissolving pulp, except that the pretreatment time in step 1) is adjusted from "30 min" to "15 min", the rest is exactly the same as in Example 1.

[0048] Example 4:

[0049] A method for preparing dissolving pulp, except that the pretreatment time in step 1) is adjusted from "30 min" to "60 min", the rest is exactly the same as in Example 1.

[0050] Example 5:

[0051] A method for preparing dissolving pulp, except that the pretreatment temperature in step 1) is adjusted from "160 °C" to "170 °C", the rest is exactly the same as in Example 1.

[0052] Example 6:

[0053] A method for preparing dissolving pulp, except that the pretreatment temperature in step 1) is adjusted from "160 °C" to "180 °C", the rest is exactly the same as in Example 1.

[0054] Comparative example:

[0055] A method for preparing dissolving pulp, except that "phenoxyethanol" is not added in step 1), the rest is exactly the same as in Example 1.

[0056] Performance test:

[0057] 1) Add 0.3 g of solid material to a beaker with a volume of 10 mL, then add 3 mL of sulfuric acid solution with a mass fraction of 72%, stir at 25 °C for 1 h, then dilute with 84 mL of deionized water and transfer to a pressure-resistant bottle. Then place the pressure-resistant bottle in an autoclave and hydrolyze at 121 °C for 1 h. Then filter with a G3 sintered glass crucible to obtain solid and hydrolyzate respectively. Then take 5 mL of the hydrolyzate, dilute it by an appropriate multiple, and measure the absorbance with a UV-visible spectrophotometer. The selected reference solution is sulfuric acid solution with a mass fraction of 4%. Select 205 nm as the measurement wavelength. Then take 5 mL of the hydrolyzate, filter it with a nylon 66 filter head, dilute it by an appropriate multiple, and detect the content of glucan in the hydrolyzate with a high performance liquid chromatograph (HPLC). Then wash the solid in the G3 sintered glass crucible with deionized water until neutral, then put the solid together with the G3 sintered glass crucible into an oven and bake at 105 °C for 4 h. Then take it out and cool it in a desiccator. Then calculate the content of acid-insoluble lignin in the solid material by measuring the mass change (the total lignin content is the sum of the acid-insoluble lignin content and the acid-soluble lignin content). The cellulose retention rate, hemicellulose removal rate and lignin removal rate obtained by the test are shown in the following table:

[0058] Table 2 Test results of cellulose retention rate, hemicellulose removal rate and lignin removal rate

[0059] Test Items Cellulose Retention Rate (%) Hemicellulose Removal Rate (%) Lignin Removal Rate (%) Example 1 90.13 90.42 91.18 Example 2 93.17 74.51 48.91 Example 3 91.63 88.00 73.40 Example 4 87.14 91.90 94.11 Example 5 83.13 93.64 95.27 Example 6 64.12 97.67 99.64 Comparative Example 96.65 85.13 21.77

[0060] Note:

[0061] The calculation formula for cellulose retention rate is as follows: Cellulose retention rate (%) = weight of glucan in solid material / weight of glucan in raw material × 100%.

[0062] The calculation formula for hemicellulose removal rate is as follows: Hemicellulose removal rate (%) = (weight of hemicellulose in raw material - weight of hemicellulose in solid material) / weight of hemicellulose in raw material × 100%.

[0063] The calculation formula for lignin removal rate is as follows: Lignin removal rate (%) = (weight of total lignin in raw material - weight of total lignin in solid material) / weight of total lignin in raw material × 100%.

[0064] As can be seen from Table 2:

[0065] a) The cellulose retention rates in Examples 1 - 6 are relatively high, and the removal rates of hemicellulose and lignin are also relatively high;

[0066] b) Compared with the comparative example, in Example 1, under the same process conditions, after adding phenoxyethanol, the removal rate of lignin has increased significantly, from 21.77% to 91.18%, indicating that the pretreatment of eucalyptus with the ULA - EPH biphasic solvent system has achieved unexpected effects;

[0067] c) Compared Example 1 with Example 2, when the pretreatment time is 30 min, as the pretreatment temperature increases from 150 °C to 160 °C, the removal rates of hemicellulose and lignin increase significantly. The hemicellulose removal rate increases from 74.51% to 90.42%, and the lignin removal rate increases from 48.91% to 91.18%. The reason is that: high temperature during the pretreatment process can generate high pressure in the reactor, enabling ULA to effectively penetrate the cell wall and depolymerize hemicellulose, thus contributing to a relatively high hemicellulose removal rate. The increase in the hemicellulose removal rate is conducive to the cleavage of the linkage bonds in the lignin - carbohydrate complex (LCC), making the cell wall more porous, thus promoting the contact between lignin and EPH. Also, because EPH has a relatively high solubility for lignin, the lignin removal rate is also relatively high. In addition, benefiting from the mild acidic environment of ULA, the hydrolysis degree of cellulose is relatively low, and the cellulose retention rate only decreases slightly, from 93.17% to 90.13%;

[0068] d) Compared Examples 3 - 4 with Example 1, when the pretreatment temperature is 160 °C, changing the pretreatment time has a greater impact on the lignin removal rate and a smaller impact on the cellulose retention rate;

[0069] e) Compared with Example 1, under the condition that the pretreatment time is 30 min for both, as the pretreatment temperature rises to 170 °C, the cellulose retention rate decreases to 83.13%, indicating that for the ULA-EPH biphasic solvent system, the pretreatment temperature has a greater impact on the cellulose retention rate;

[0070] f) Compared with Example 1, under the condition that the pretreatment time is 30 min for both, as the pretreatment temperature rises to 180 °C, the removal rates of hemicellulose and lignin reach 97.67% and 99.64% respectively, but the cellulose retention rate is only 64.12%, indicating that too high a pretreatment temperature will cause cellulose hydrolysis;

[0071] In summary, it can be seen that the present invention can maximize the removal of hemicellulose and lignin by reasonably controlling the temperature and time of biphasic pretreatment, and at the same time can maintain a high cellulose retention rate, and finally obtain high-quality dissolving pulp.

[0072] 2) The test results of the α-cellulose content, viscosity and whiteness of the dissolving pulp in Examples 1 to 6 and the comparative examples are shown in the following table:

[0073] Table 3 Test results of α-cellulose content, viscosity and whiteness of dissolving pulp

[0074] Test Items α-Cellulose Content (%) Viscosity (mL / g) Whiteness / ISO Example 1 94.38 383.97 86 Example 2 92.76 411.25 83 Example 3 92.95 397.62 84 Example 4 94.12 335.67 88 Example 5 94.78 266.72 88 Example 6 95.31 160.25 91 Comparative Example 92.88 432.00 82

[0075] As can be seen from Table 3:

[0076] a) The α-cellulose content of the dissolving pulp in Examples 1 to 6 is relatively high. Among them, the α-cellulose content of the dissolving pulp in Examples 1 and 4 to 6 reaches the standard of superior dissolving pulp of broad-leaved wood in the light industry (>93%);

[0077] b) Compared with the comparative example, Example 1 benefits from the high lignin removal rate brought by phenoxyethanol, and finally the α-cellulose content and whiteness of the finished dissolving pulp are higher;

[0078] c) Comparing Example 1 with Examples 2 to 3, since more lignin is removed during the pretreatment process, the α-cellulose content and whiteness of the final finished dissolving pulp are higher;

[0079] d) Comparing Examples 4 to 5 with Example 1, due to the degradation of cellulose caused by too strong pretreatment conditions, the viscosity of the finished dissolving pulp drops significantly and fails to meet the viscosity requirement of dissolving pulp (>380 mL / g);

[0080] In summary, it can be seen that the present invention realizes the preparation of high-quality broad-leaved wood dissolving pulp (with higher α-cellulose purity) by reasonably controlling the conditions of biphasic pretreatment and cooking conditions.

[0081] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for preparing dissolving pulp, characterized in that, It includes the following steps: 1) Add eucalyptus wood into an ultra-low acid-phenoxyethanol biphasic solvent system for biphasic pretreatment, and then filter to remove the filtrate to obtain a solid material; 2) Add the solid material into a NaOH-Na2S solution for kraft pulping to obtain kraft pulp; 3) Perform ECF bleaching on the kraft pulp to obtain dissolving pulp.

2. The method for preparing dissolving pulp according to claim 1, wherein: In step 1), the mass-volume ratio of the eucalyptus wood to the ultra-low acid-phenoxyethanol biphasic solvent system is 1 g: 15 mL to 25 mL.

3. The method for preparing dissolving pulp according to claim 1 or 2, characterized in that: The ultra-low acid-phenoxyethanol biphasic solvent system in step 1) is composed of a sulfuric acid solution with a mass fraction ≤ 0.1% and phenoxyethanol in a volume ratio of 1: 0.8 to 1.

2.

4. The method for preparing dissolving pulp according to claim 1 or 2, characterized in that: The biphasic pretreatment in step 1) is carried out at a temperature of 150 °C to 180 °C, and the pretreatment time is 15 min to 60 min.

5. The method for preparing dissolving pulp according to claim 1, characterized in that: In step 2), the mass-volume ratio of the solid material to the NaOH-Na2S solution is 1 g: 3 mL to 7 mL.

6. The method for preparing dissolving pulp according to claim 1 or 5, characterized in that: The alkali content of the NaOH-Na2S solution in step 2) is 5% to 15% in terms of the mass percentage of Na2O, and the sulfidity is 20% to 30%.

7. The method for preparing dissolving pulp according to claim 1 or 5, characterized in that: The kraft pulping in step 2) is carried out at a temperature of 160 °C to 170 °C, and the cooking time is 10 min to 60 min.

8. The method for preparing dissolving pulp according to claim 1, wherein: The ECF bleaching in step 3) includes the following operations: chlorine dioxide bleaching, hydrogen peroxide bleaching, and chlorine dioxide bleaching.

9. The method for preparing dissolving pulp according to claim 8, wherein: The ECF bleaching in step 3) includes the following operations: a) Chlorine dioxide bleaching: Add the kraft pulp into a reaction kettle, then add chlorine dioxide, and then carry out bleaching at a temperature of 60 °C to 80 °C; b) Hydrogen peroxide bleaching: Add sodium hydroxide, magnesium sulfate, and hydrogen peroxide into the reaction kettle, and then carry out bleaching at a temperature of 70 °C to 90 °C; c) Chlorine dioxide bleaching: Add chlorine dioxide into the reaction kettle, and then carry out bleaching at a temperature of 60 °C to 80 °C.

10. The method for preparing dissolving pulp according to claim 9, wherein: The bleaching time in step a) is 40 min to 80 min; the bleaching time in step b) is 40 min to 80 min; the bleaching time in step c) is 40 min to 80 min.

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