Method for removing reducing sugar in industrial lignin

Abstract

The invention provides a method for removing reducing sugar in industrial lignin, which comprises the following steps: (1) dissolving industrial lignin, and carrying out first solid-liquid separation to obtain clear liquid; (2) mixing the clear liquid with hemicellulase, and carrying out an enzymatic hydrolysis reaction to obtain a reacted solution; and (3) adjusting the pH value of the reacted solution to be acidic, separating out the industrial lignin, and carrying out second solid-liquid separation to obtain the purified industrial lignin. The method is easy to operate, low in treatment cost and mild in reaction condition, reducing sugar in industrial sodium lignin sulfonate is efficiently removed, and subsequent high-value utilization of industrial lignin is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of resource utilization of industrial waste, and particularly to a method for removing reducing sugars from industrial lignin. Background Art

[0002] Lignin is a natural polymer second only to cellulose, mainly existing in plant cell walls, with a wide source and rich yield. Industrial lignin is a by-product generated in the papermaking process. Currently, the pulping method mostly used in papermaking is the sulfite pulping method. Therefore, industrial lignin mainly refers to sodium lignosulfonate. Industrial sodium lignosulfonate contains a large number of functional groups such as sulfonic acid groups, carboxyl groups, and hydroxyl groups. Its cost is low, but it is mostly used as a dispersant for concrete or directly incinerated. Incinerating industrial sodium lignosulfonate may produce harmful gases that pollute the air, and the high-value utilization of waste has not been achieved. Therefore, in order to avoid secondary pollution of industrial lignin and at the same time realize the high-value utilization of industrial lignin, the application of industrial lignin hydrogel as a product in the treatment of heavy metal pollution has gradually attracted people's attention.

[0003] However, due to the low purity of industrial lignin and the presence of impurities such as hemicellulose, insoluble substances, and inorganic salts. Hemicellulose belongs to reducing sugar substances, which will affect the synthesis of lignin materials, such as the crosslinking degree of hydrogels, and thus affect the performance of the materials. Therefore, if industrial lignin is used as a raw material for material synthesis, industrial lignin needs to be purified.

[0004] CN101845064A discloses a method for purifying industrial lignin. First, weigh silica gel and fill it in a chromatography column. Then, dissolve the industrial lignin to be purified in water, pour the industrial lignin solution on the top layer of silica gel in the chromatography column. After it penetrates into the silica gel, place a layer of silica gel on the top of the silica gel, add deionized water for elution, and receive the elution solution according to the color distribution. Receive the brown elution solution in a second container, and receive the light-colored part before the brown elution solution in a first container. Then, add acid solution to the silica gel column and wash until the silica gel in the column turns white, and receive the acid elution solution in a third container. Evaporate the brown part of the elution solution in the second container to obtain the purified lignin product. Evaporate the light-colored part before the brown elution solution in the first container to obtain the inorganic salt part. Evaporate the acid elution solution in the third container to obtain the hemicellulose and oligosaccharide part.

[0005] CN117659434A discloses a method for purifying lignin with organic solvents and a method for preparing lignin-based degradable plastics. The method includes: adding industrial lignin to an acetone solution, filtering to remove insoluble substances, and adding an acidic aqueous solution after rotary evaporation of the filtrate; the lignin precipitated after standing is the purified lignin; and using polyvinyl alcohol and purified lignin to prepare degradable plastics.

[0006] CN109535442A discloses a method for separating lignin, cellulose, and hemicellulose from straw using an organic solvent. The specific steps are as follows: (1) Dry straw powder and an acidic ethanol solution are added to a high-pressure reactor. After purging with nitrogen to remove oxygen, a reaction is carried out, and a mixed solution A is obtained after the reaction; (2) The mixed solution A obtained in step (1) is cooled to room temperature, and then solid-liquid separation is carried out to obtain a liquid phase and a solid phase. The solid phase is cellulose; (3) An aqueous solution with a pH value of 2 to 7 is added to the liquid phase obtained in step (2). After the reaction, a mixed solution B is obtained, and then solid-liquid separation is carried out to obtain a solid phase and a liquid phase. The solid phase is lignin, and the liquid phase is a liquid containing hemicellulose. This separation method has the advantages of simple process, rapid and efficient separation, and low separation cost.

[0007] However, all of the above methods have certain defects. The silica gel material cannot be regenerated, and discarding it may cause environmental pollution. Moreover, the chromatography method is complex and time-consuming in industrial production, while the waste liquid of the acetone purification method is an organic pollutant and will cause greater harm to the environment. Therefore, developing a new method to obtain the final industrial lignin hydrogel product is of great practical significance for the high-value utilization of industrial lignin. Summary of the Invention

[0008] In view of the problems existing in the prior art, the present invention provides a method for removing reducing sugars from industrial lignin. Through the method of hemicellulose enzymolysis and acid solution precipitation, the effective removal of reducing sugars in industrial lignin is achieved, and the purified industrial lignin obtained can be used for subsequent high-value utilization; the method of the present invention has the characteristics of simple and easy operation, low treatment cost, etc., and is suitable for large-scale popularization and application.

[0009] To achieve this purpose, the present invention adopts the following technical solutions:

[0010] The present invention provides a method for removing reducing sugars from industrial lignin, and the method comprises the following steps:

[0011] (1) After dissolving industrial lignin, perform the first solid-liquid separation to obtain a clear liquid;

[0012] (2) Mix the clear liquid obtained in step (1) with hemicellulase and carry out an enzymolysis reaction to obtain a post-reaction solution;

[0013] (3) Adjust the pH of the post-reaction solution obtained in step (2) to acidic to precipitate industrial lignin, and after the second solid-liquid separation, obtain purified industrial lignin.

[0014] The method for removing reducing sugars from industrial lignin according to the present invention first dissolves the industrial lignin, and removes the insoluble substances therein through solid-liquid separation; then, for the hemicellulose, which is a reducing sugar impurity with a relatively high content in the industrial lignin, a hemicellulase is used for enzymatic hydrolysis reaction to remove the reducing sugars therein; finally, the solution is treated under acidic conditions to precipitate sodium lignosulfonate, and finally purified industrial lignin is obtained. The purified industrial lignin has a low reducing sugar content, the insoluble substances in the sodium lignosulfonate of industrial lignin completely disappear and the inorganic salts are relatively reduced. The purified industrial lignin reaches the purity for laboratory research. The reduction of reducing sugars is significantly helpful for the synthesis of hydrogel-like materials, and the reduction of inorganic salts is beneficial to the improvement of the purity of the synthesized materials, which is conducive to the high-value utilization of industrial lignin.

[0015] The method for removing reducing sugars from industrial lignin according to the present invention uses hemicellulase for enzymatic hydrolysis reaction, does not use products such as organic solvents or silica gel, has simple treatment steps, mild reaction conditions, and a small amount of hemicellulase is used, with low treatment costs and no pollutants generated.

[0016] Preferably, the industrial lignin in step (1) is sodium lignosulfonate of industrial lignin.

[0017] Preferably, the solvent used for dissolution in step (1) includes water, and ultrapure water or deionized water commonly used in the art can be used.

[0018] Preferably, the first solid-liquid separation includes centrifugation, filtration or sedimentation, and centrifugation is preferred.

[0019] The present invention preferably uses centrifugation to separate and remove the insoluble substances in the dissolved industrial lignin. The conditions for centrifugation can be 5000 - 10000 rpm, and centrifuge for 10 - 30 min at 25°C.

[0020] Preferably, the mass ratio of the supernatant to the hemicellulase in step (2) is (50 - 200):1. For example, it can be 50:1, 60:1, 80:1, 90:1, 100:1, 120:1, 150:1, 170:1 or 200:1, etc., but it is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0021] The present invention preferably sets the mass ratio of the supernatant to the hemicellulase as (50 - 200):1, which can achieve the efficient enzymatic hydrolysis and removal of reducing sugars in sodium lignosulfonate of industrial lignin. When the mass ratio of the supernatant to the hemicellulase is relatively high, that is, the amount of hemicellulase used is less, the enzymatic hydrolysis effect on the reducing sugars in sodium lignosulfonate of industrial lignin becomes worse, resulting in a lower removal rate of reducing sugars; when the mass ratio of the supernatant to the hemicellulase is relatively low, that is, the amount of hemicellulase used is more, the removal rate of reducing sugars in sodium lignosulfonate of industrial lignin will not continue to increase, but instead will cause an increase in treatment costs.

[0022] Preferably, a water bath is used for heating during the enzymatic hydrolysis reaction in step (2).

[0023] Preferably, the temperature of the enzymatic hydrolysis reaction is 45 - 65°C. For example, it can be 45°C, 48°C, 50°C, 53°C, 60°C, 65°C, etc., but is not limited to the listed values. Other unlisted values within this range are also applicable.

[0024] The present invention preferably sets the temperature of the enzymatic hydrolysis reaction at 45 - 65°C. At this time, the activity of the hemicellulase is the highest, which is conducive to the efficient progress of the enzymatic hydrolysis reaction. When the temperature of the enzymatic hydrolysis reaction is relatively low, the activity of the hemicellulase is poor, the efficiency of the enzymatic hydrolysis reaction is poor, and the removal rate of reducing sugars in industrial lignosulfonate is low; when the temperature of the enzymatic hydrolysis reaction is relatively high, it will cause the inactivation of the hemicellulase and the reduction of the removal rate of reducing sugars.

[0025] Preferably, the time of the enzymatic hydrolysis reaction is 4 - 6h. For example, it can be 4h, 4.5h, 5h, 5.5h, 5.8h, 6h, etc., but is not limited to the listed values. Other unlisted values within this range are also applicable.

[0026] The present invention preferably sets the time of the enzymatic hydrolysis reaction at 4 - 6h to ensure the optimal enzymatic hydrolysis effect of the hemicellulase on the reducing sugars in industrial lignosulfonate. When the time of the enzymatic hydrolysis reaction is relatively short, the enzymatic hydrolysis reaction is not fully carried out, and the removal rate of reducing sugars is low; as the time of the enzymatic hydrolysis reaction increases, the enzymatic hydrolysis efficiency increases and reaches the optimal reaction efficiency. However, when the time of the enzymatic hydrolysis reaction continues to increase, the activity of the hemicellulase will decrease, and the enzymatic hydrolysis efficiency will instead decrease.

[0027] Preferably, the pH in step (3) is below 0.3. When the pH is relatively high, it will cause the incomplete precipitation of industrial lignin.

[0028] Preferably, an acidic solution is used to adjust the pH of the solution after the reaction to be acidic.

[0029] Preferably, the acidic solution includes any one or a combination of at least two of sulfuric acid, nitric acid, or hydrochloric acid. Typical but non-limiting combinations include a combination of sulfuric acid and nitric acid, a combination of hydrochloric acid and sulfuric acid, or a combination of nitric acid and hydrochloric acid. Sulfuric acid is preferred.

[0030] The present invention preferably uses sulfuric acid to adjust the pH of the solution after the reaction, which will not introduce additional impurity elements.

[0031] Preferably, the second solid-liquid separation in step (3) includes centrifugation, filtration, or sedimentation, and centrifugation is preferred.

[0032] The present invention preferably uses centrifugation to separate the precipitated industrial lignin. The conditions for centrifugation can be 5000 - 10000 rpm, and centrifuge for 10 - 30 min at 25°C.

[0033] Preferably, the method further includes dissolving the purified industrial lignin obtained in step (3) under alkaline conditions and then removing the moisture to obtain a purified industrial lignin product.

[0034] The purpose of dissolving the purified industrial lignin under alkaline conditions in the present invention is to prevent the purified industrial lignin from deteriorating and affecting subsequent utilization; removing the moisture is to obtain a solid purified industrial lignin product for easy storage and utilization.

[0035] Preferably, the pH of the alkaline conditions is above 5.5, such as 5.5, 5.7, 5.9, 6, 6.5 or 7, etc., but is not limited to the listed values, and other unlisted values within this range are equally applicable.

[0036] Preferably, an alkaline solution is added to create the alkaline conditions.

[0037] Preferably, the alkaline solution includes any one or a combination of at least two of sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution or sodium bicarbonate. Typical but non-limiting combinations include the combination of sodium hydroxide solution and potassium hydroxide solution, the combination of sodium carbonate solution and sodium bicarbonate, or the combination of sodium hydroxide solution and sodium carbonate solution. Sodium hydroxide solution is preferred.

[0038] The present invention preferably uses sodium hydroxide solution to create the alkaline conditions without introducing additional impurity elements.

[0039] Preferably, the method for removing moisture includes drying at 65 - 85°C or performing vacuum freeze-drying, such as 65°C, 66°C, 68°C, 70°C, 75°C, 80°C or 85°C, etc., but is not limited to the listed values, and other unlisted values within this range are equally applicable.

[0040] As a preferred technical solution of the present invention, the method includes the following steps:

[0041] (1) After dissolving sodium lignosulfonate in water, perform the first solid-liquid separation to obtain a clear liquid;

[0042] The first solid-liquid separation includes centrifugation, filtration or sedimentation;

[0043] (2) Mix the clear liquid obtained in step (1) and hemicellulase in a mass ratio of (50 - 200):1, and perform an enzymatic hydrolysis reaction at a temperature of 45 - 65°C for 4 - 6 h under water bath heating to obtain a reaction solution;

[0044] (3) Adjust the pH of the solution after the reaction to below 0.3 with an acidic solution to precipitate sodium industrial lignosulfonate, and after the second solid-liquid separation, purified sodium industrial lignosulfonate is obtained; dissolve the purified sodium industrial lignosulfonate under alkaline conditions with a pH above 5.5 and remove the moisture to obtain a purified sodium industrial lignosulfonate product;

[0045] The acidic solution includes any one or a combination of at least two of sulfuric acid, nitric acid or hydrochloric acid; the second solid-liquid separation includes centrifugation, filtration or sedimentation; adding an alkaline solution to create an alkaline condition; the alkaline solution includes any one or a combination of at least two of sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution or sodium bicarbonate; the method of removing moisture includes drying at 65-85 °C or performing vacuum freeze-drying.

[0046] Compared with the prior art, the present invention has at least the following beneficial effects:

[0047] The method for removing reducing sugars in industrial lignin provided by the present invention is simple in operation, low in treatment cost, mild in reaction conditions, reduces the reducing sugar content in sodium industrial lignosulfonate by more than 10%, the insoluble substances in sodium industrial lignosulfonate completely disappear and the inorganic salts relatively decrease, which is beneficial for subsequent high-value utilization. Description of the Drawings

[0048] Figure 1 It is a FTIR comparison chart of the purified sodium industrial lignosulfonate product obtained in Example 1 of the present invention and analytical pure grade lignosulfonate. Detailed Embodiments

[0049] The technical solution of the present invention will be further described below in conjunction with the drawings and through specific embodiments.

[0050] The present invention will be further described in detail below. However, the following examples are only simple examples of the present invention and do not represent or limit the scope of the protection of the present invention. The scope of protection of the present invention is subject to the claims.

[0051] The content of reducing sugars in sodium industrial lignosulfonate in the following examples and comparative examples is 13%.

[0052] Example 1

[0053] This example provides a method for removing reducing sugars in industrial lignin, and the method includes the following steps:

[0054] (1) After dissolving 1 g of industrial sodium lignosulfonate in 50 mL of ultrapure water, stir it thoroughly for 15 min to completely dissolve the soluble substances in the industrial sodium lignosulfonate. Then centrifuge the suspension at 10,000 rpm and 25 °C for 10 min in a centrifuge to separate the insoluble impurities, and obtain a clear solution of industrial sodium lignosulfonate without insoluble substances;

[0055] (2) Mix the clear solution of industrial sodium lignosulfonate without insoluble substances described in step (1) and 0.01 g of hemicellulase so that their mass ratio is 100:1. Then heat the mixture in a water bath at 60 °C for 6 h to obtain a reaction solution;

[0056] (3) After the reaction solution described in step (2) is cooled to room temperature, add 6.4 mL of sulfuric acid with a concentration of 98% to adjust the pH of the solution to 0.2, and precipitate sodium lignosulfonate without hemicellulose; centrifuge the suspension at 10,000 rpm and 25 °C for 10 min, discard the liquid, and obtain purified industrial sodium lignosulfonate; soak the purified industrial sodium lignosulfonate in 50 mL of ultrapure water, add sodium hydroxide solution to adjust the pH to 5.5, and remove the water through vacuum freeze-drying to obtain a purified industrial sodium lignosulfonate product.

[0057] The FTIR comparison chart of the purified industrial sodium lignosulfonate product obtained in this example and analytical pure grade sodium lignosulfonate is as Figure 1 shown. From Figure 1 it can be seen that the surface functional groups of the sodium lignosulfonate product obtained by the method provided in this example are similar to those of analytical pure sodium lignosulfonate, indicating that the structure of the sodium lignosulfonate product obtained by this method is similar to that of analytical pure sodium lignosulfonate, and the purity reaches the level that can be used for laboratory research.

[0058] Example 2

[0059] This example provides a method for removing reducing sugars from industrial lignin. The difference between this method and Example 1 is only that the water bath heating time in step (2) is 4 h.

[0060] Example 3

[0061] This example provides a method for removing reducing sugars from industrial lignin. The difference between this method and Example 1 is only that the water bath heating time in step (2) is 5 h.

[0062] Example 4

[0063] This example provides a method for removing reducing sugars from industrial lignin. The method includes the following steps:

[0064] (1) After dissolving 3 g of industrial lignosulfonate in 100 mL of ultrapure water, stir it thoroughly for 10 min to completely dissolve the soluble substances in the industrial lignosulfonate. Then, perform vacuum filtration on the suspension to separate the insoluble impurities, and obtain a clear solution of industrial lignosulfonate without insoluble substances;

[0065] (2) Mix the clear solution of industrial lignosulfonate without insoluble substances described in step (1) and 0.015 g of hemicellulase so that their mass ratio is 200:1. Then, heat the mixture in a water bath at 45 °C for 4.5 h to obtain a post-reaction solution;

[0066] (3) After the post-reaction solution described in step (2) cools to room temperature, add nitric acid to adjust the pH of the solution to 0.1 to precipitate lignosulfonate without hemicellulose; perform vacuum filtration on this suspension and discard the liquid to obtain purified industrial lignosulfonate; soak the purified industrial lignosulfonate in 50 mL of ultrapure water, add potassium hydroxide solution to adjust the pH to 6, and dry it at 65 °C to obtain a purified industrial lignosulfonate product.

[0067] Example 5

[0068] This example provides a method for removing reducing sugars from industrial lignin, and the method includes the following steps:

[0069] (1) After dissolving 1 g of industrial lignosulfonate in 50 mL of ultrapure water, stir it thoroughly for 13 min to completely dissolve the soluble substances in the industrial lignosulfonate. Then, centrifuge the suspension at 9000 rpm and 25 °C in a centrifuge for 15 min to separate the insoluble impurities, and obtain a clear solution of industrial lignosulfonate without insoluble substances;

[0070] (2) Mix the clear solution of industrial lignosulfonate without insoluble substances described in step (1) and 0.005 g of hemicellulase so that their mass ratio is 200:1. Then, heat the mixture in a water bath at 65 °C for 5.5 h to obtain a post-reaction solution;

[0071] (3) After the post-reaction solution described in step (2) cools to room temperature, add 5 mL of sulfuric acid with a concentration of 98% to adjust the pH of the solution to 0.25 to precipitate lignosulfonate without hemicellulose; centrifuge this suspension at a centrifuge speed of 9000 rpm and 25 °C for 18 min and discard the liquid to obtain purified industrial lignosulfonate; soak the purified industrial lignosulfonate in 50 mL of ultrapure water, add sodium carbonate solution to adjust the pH to 6.5, and remove the moisture through vacuum freeze-drying to obtain a purified industrial lignosulfonate product.

[0072] Example 6

[0073] This example provides a method for removing reducing sugars from industrial lignin. The difference between this method and that of Example 1 is only that the mass ratio of the clear solution of sodium lignosulfonate without insoluble substances to hemicellulase in step (2) is 50:1.

[0074] Example 7

[0075] This example provides a method for removing reducing sugars from industrial lignin. The difference between this method and that of Example 1 is only that the mass ratio of the clear solution of sodium lignosulfonate without insoluble substances to hemicellulase in step (2) is 200:1.

[0076] Example 8

[0077] This example provides a method for removing reducing sugars from industrial lignin. The difference between this method and that of Example 1 is only that the temperature of water bath heating in step (2) is 40°C.

[0078] Example 9

[0079] This example provides a method for removing reducing sugars from industrial lignin. The difference between this method and that of Example 1 is only that the temperature of water bath heating in step (2) is 70°C.

[0080] Comparative Example 1

[0081] This comparative example provides a method for removing reducing sugars from industrial lignin. The difference between this method and that of Example 1 is only that after the soluble substances in sodium lignosulfonate in step (1) are completely dissolved, centrifugal separation is not carried out, and step (2) of mixing with hemicellulase is directly carried out.

[0082] Comparative Example 2

[0083] This comparative example provides a method for removing reducing sugars from industrial lignin. The difference between this method and that of Example 1 is only that step (2) is not carried out, that is, the clear solution of sodium lignosulfonate without insoluble substances in step (1) is not mixed with hemicellulase, and step (3) is directly carried out.

[0084] Comparative Example 3

[0085] This comparative example provides a method for removing reducing sugars from industrial lignin. The difference between this method and that of Example 1 is only that sulfuric acid is not added to adjust the pH in step (3).

[0086] Dissolve the purified sodium lignosulfonate products obtained from the above examples and comparative examples in water, and use ultraviolet spectrophotometry to test the reducing sugar content. The results are shown in Table 1.

[0087] Table 1

[0088] Reducing sugar content (%) Example 1 0.45％ Example 2 0.61％ Example 3 0.65％ Example 4 0.95％ Example 5 0.65％ Example 6 0.44％ Example 7 0.66％ Example 8 0.61％ Example 9 0.67％ Comparative Example 1 1.45％ Comparative Example 2 1.34％ Comparative Example 3 13.05％

[0089] It can be seen from Table 1 that:

[0090] (1) From a comprehensive view of Examples 1 to 5, it can be seen that the method for removing reducing sugars from industrial lignin provided by the present invention is simple in operation, achieving a substantial reduction in the reducing sugar content in sodium lignosulfonate, and the purified sodium lignosulfonate product obtained can be directly used for high-value utilization;

[0091] (2) From a comprehensive view of Example 1 and Examples 6 to 7, it can be seen that in Example 6, the mass ratio of the clear liquid of sodium lignosulfonate without insoluble substances to hemicellulase is small, that is, the dosage of hemicellulase is large, and the enzymatic hydrolysis effect on reducing sugars in sodium lignosulfonate should be better. However, the reducing sugar content is 0.44%, proving that the enzymatic hydrolysis effect has not improved significantly, the removal rate of reducing sugars in sodium lignosulfonate has not increased significantly, and it will also cause an increase in treatment costs; in Example 7, the mass ratio of the clear liquid of sodium lignosulfonate without insoluble substances to hemicellulase is large, that is, the dosage of hemicellulase is small, and too little hemicellulase leads to a poor enzymatic hydrolysis effect, and the reducing sugar content becomes 0.66%;

[0092] (3) From a comprehensive view of Example 1 and Examples 8 to 9, it can be seen that in Example 8, the water bath heating temperature is low, that is, the enzymatic hydrolysis reaction temperature is low, the activity of hemicellulase is poor, the enzymatic hydrolysis reaction efficiency is poor, the removal rate of reducing sugars in sodium lignosulfonate is low, and the reducing sugar content becomes 0.61%; in Example 9, the water bath heating temperature is high, that is, the enzymatic hydrolysis reaction temperature is high, which will cause the inactivation of hemicellulase, the removal rate of reducing sugars decreases, and the reducing sugar content becomes 0.67%.

[0093] (4) From a comprehensive view of Example 1 and Comparative Example 1, it can be seen that after the soluble substances in sodium lignosulfonate in Comparative Example 1 are completely dissolved, centrifugal separation is not carried out, and the insoluble substances are not removed, which will affect the subsequent enzymatic hydrolysis reaction effect, resulting in a decrease in the removal rate of reducing sugars, and the reducing sugar content becomes 1.45%;

[0094] (5) From a comprehensive view of Example 1 and Comparative Example 2, it can be seen that in Comparative Example 2, the clear liquid of sodium lignosulfonate without insoluble substances is not mixed with hemicellulase, and no enzymatic hydrolysis reaction is carried out, so reducing sugars cannot be removed well, and the reducing sugar content is 1.34%;

[0095] (6) From a comprehensive view of Example 1 and Comparative Example 3, it can be seen that the product in Comparative Example 3 precipitates prematurely without adjusting the pH, resulting in a large amount of reducing sugars still adhering to the surface of the industrial lignin after removing water, so that the reducing sugar content remains at a relatively high level.

[0096] The applicant declares that the above description is only a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for removing reducing sugars from industrial lignin, characterized in that: The method comprises the following steps: (1) After dissolving the industrial lignin, a first solid-liquid separation is performed to obtain a clear liquid; (2) mixing the clear solution of step (1) with hemicellulase to perform enzymatic hydrolysis to obtain a post-reaction solution; (3) Adjusting the pH of the solution after the reaction in step (2) to be acidic to precipitate the industrial lignin, and performing a second solid-liquid separation to obtain purified industrial lignin.

2. The method according to claim 1, characterized in that The industrial lignin in step (1) is industrial sodium lignin sulfonate.

3. The method according to claim 1 or 2, characterized in that: The solvent used for dissolving in step (1) includes water; Preferably, the first solid-liquid separation comprises centrifugation, filtration or sedimentation, preferably centrifugation.

4. The method according to any one of claims 1 to 3, characterized in that: The mass ratio of the clear liquid to hemicellulase in step (2) is (50-200):

1.

5. The method according to any one of claims 1 to 4, characterized in that: During the enzymatic hydrolysis reaction in step (2), water bath heating is used; Preferably, the temperature of the enzymatic hydrolysis reaction is 45-65°C; Preferably, the enzymatic hydrolysis reaction time is 4 to 6 hours.

6. The method according to any one of claims 1 to 5, characterized in that: The pH in step (3) is below 0.3; Preferably, an acidic solution is used to adjust the pH of the solution after the reaction to be acidic; Preferably, the acidic solution comprises any one of sulfuric acid, nitric acid or hydrochloric acid or a combination of at least two thereof, preferably sulfuric acid.

7. The method according to any one of claims 1 to 6, characterized in that: Step (3) The second solid-liquid separation comprises centrifugation, filtration or sedimentation, preferably centrifugation.

8. The method according to any one of claims 1 to 7, characterized in that: The method further comprises dissolving the purified industrial lignin in step (3) under alkaline conditions and removing water to obtain a purified industrial lignin product.

9. The method according to claim 8, characterized in that The pH of the alkaline condition is above 5.5; Preferably, adding an alkaline solution produces alkaline conditions; Preferably, the alkaline solution comprises any one of sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution or sodium bicarbonate solution or a combination of at least two thereof, preferably sodium hydroxide solution; Preferably, the method for removing moisture comprises drying at 65-85° C. or vacuum freeze drying.

10. The method according to any one of claims 1 to 9, characterized in that: The method comprises the following steps: (1) after dissolving industrial sodium lignin sulfonate in water, performing a first solid-liquid separation to obtain a clear liquid; The first solid-liquid separation includes centrifugation, filtration or sedimentation; (2) mixing the clear solution of step (1) with hemicellulase in a mass ratio of (50-200):1, and performing an enzymatic hydrolysis reaction at a temperature of 45-65° C. for 4-6 hours under heating in a water bath to obtain a post-reaction solution; (3) using an acidic solution to adjust the pH of the reaction solution to below 0.3, so that industrial sodium lignin sulfonate is precipitated, and undergoing a second solid-liquid separation to obtain purified industrial sodium lignin sulfonate; dissolving the purified industrial sodium lignin sulfonate in an alkaline condition with a pH of above 5.5 and removing water to obtain a purified industrial sodium lignin sulfonate product; The acidic solution includes any one of sulfuric acid, nitric acid or hydrochloric acid or a combination of at least two of them; the second solid-liquid separation includes centrifugation, filtration or sedimentation; adding an alkaline solution to produce alkaline conditions; the alkaline solution includes any one of sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution or sodium bicarbonate solution or a combination of at least two of them; the method for removing moisture includes drying at 65-85°C or vacuum freeze drying.

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