Low-silicon pulping black liquor oxygen alkali pulping method and pulp and application

Abstract

The application belongs to the technical field of pulping and papermaking, and particularly relates to an oxygen-alkali pulping method of low-silicon pulping black liquor, pulp and application, which comprises the following steps: cutting and washing non-wood fiber raw materials; preparing a cooking liquor, wherein sodium hydroxide, sodium carbonate and magnesium sulfate are added into the cooking liquor, the molar ratio of sodium hydroxide and sodium carbonate is 3-8:2-7, the amount of alkali in the cooking liquor is 18%-28% of the mass of the absolute dry raw material in terms of Na2O, and the mass of magnesium sulfate is 0.1%-1.0% of the absolute dry raw material; mixing and rubbing the non-wood fiber raw materials with the cooking liquor for soaking, and then cooking under 0.3 MPa-0.8 MPa oxygen; after the cooking is completed, the pulp and the pulping black liquor are separated. The method can inhibit the dissolution and migration of silicon elements to the pulping black liquor from the source, significantly reduce the silicon content of the pulping black liquor, improve the silicon retention rate in the pulp, and maintain good delignification effect and pulp fiber strength.

Description

Technical Field

[0001] This invention belongs to the field of pulp and paper technology, specifically relating to an oxygen-alkali pulping method for low-silica black liquor, and the pulp and its application. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Wheat straw is a relatively high-quality fiber raw material for pulping and papermaking; however, its chemical composition contains a high proportion of silicon dioxide, far exceeding that of wood fiber raw materials. In traditional alkaline pulping processes (such as the caustic soda process and the sulfate process), a large amount of silicon in wheat straw dissolves under high-temperature alkaline cooking conditions and enters the pulping black liquor along with degradation products. This leads to a series of serious silicon interference problems in wheat straw pulping black liquor, such as increased viscosity of the cooking liquor hindering uniform penetration of the alkali into the fibers, reducing delignification efficiency and pulp quality; causing difficulties in washing pulp and increased water consumption; causing severe scaling in the pulping black liquor evaporator; and reducing paper quality. These problems seriously restrict the sustainable development of the wheat straw pulping industry.

[0004] Oxy-alkali pulping is a clean pulping technology that uses oxygen for alkaline cooking, as oxygen promotes the delignification reaction. Currently, most publicly available oxy-alkali pulping processes use sodium hydroxide as the sole alkali source, and the final pH value after cooking is typically maintained at a high level. Under these pH conditions, a considerable proportion of silicon in wheat straw dissolves into the pulping black liquor as soluble sodium silicate, resulting in a limited reduction in the silicon content of the pulping black liquor, which is insufficient to meet the requirements of industrial alkali recovery systems for low-silica black liquor. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide an oxygen-alkali pulping method for low-silica pulping black liquor, as well as the pulp and its applications.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a method for preparing oxygen-alkali pulp from low-silica pulping black liquor, comprising the following steps: Cut and clean the non-wood fiber raw materials; Prepare the cooking liquor by adding sodium hydroxide, sodium carbonate, and magnesium sulfate, with a molar ratio of sodium hydroxide to sodium carbonate of 3~8:2~7; the amount of alkali used in the cooking liquor, calculated as Na2O, is 18%~28% of the oven-dry raw material mass; and the amount of magnesium sulfate is 0.1%~1.0% of the oven-dry raw material mass. The non-wood fiber raw material is mixed with the cooking liquor and kneaded and soaked. Then it is cooked under oxygen at 0.3 MPa to 0.8 MPa. After cooking, the pulp and pulping black liquor are separated.

[0007] Secondly, the present invention provides a pulp prepared by the oxygen-alkali pulping method of the low-silica pulping black liquor.

[0008] Thirdly, the present invention provides the use of the pulp in the preparation of paper products.

[0009] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows: This invention uses sodium carbonate and sodium hydroxide as a common alkali source and utilizes the pH buffering effect of sodium carbonate to regulate the final pH value of the cooking system, making the final pH value ≤ 9.8. This inhibits the dissolution and migration of silicon into the pulping black liquor, allowing more silicon from the wheat straw raw material to remain in the pulp, achieving a significant reduction in the silicon content of the black liquor and effectively alleviating the silicon interference problem in wheat straw pulping black liquor.

[0010] The present invention, by adding magnesium sulfate to the cooking liquid before cooking, can simultaneously maintain good delignification effect and cellulose fiber strength. Detailed Implementation

[0011] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0012] In a first aspect, the present invention provides an oxygen-alkali pulping method for low-silica pulping black liquor, comprising the following steps: cutting and washing non-wood fiber raw materials; Prepare the cooking liquor by adding sodium hydroxide, sodium carbonate, and magnesium sulfate, with a molar ratio of sodium hydroxide to sodium carbonate of 3~8:2~7; the amount of alkali used in the cooking liquor, calculated as Na2O, is 18%~28% of the oven-dry raw material mass; and the amount of magnesium sulfate is 0.1%~1.0% of the oven-dry raw material mass. The non-wood fiber raw material is mixed with the cooking liquor and kneaded and soaked. Then it is cooked under oxygen at 0.3 MPa to 0.8 MPa. After cooking, the pulp and pulping black liquor are separated.

[0013] The molar ratio of sodium hydroxide to sodium carbonate is 3:2, 3:3, 3:4, 3:5, 3:6, 3:7, 4:2, 4:3, 4:4, 4:5, 4:6, 4:7, 5:2, 5:3, 5:4, 5:5, 5:6, 5:7, 6:2, 6:3, 6:4, 6:5, 6:6, 6:7, 7:2, 7:3, 7:4, 7:5, 7:6, 7:7, 8:2, 8:3, 8:4, 8:5, 8:6, or 8:7.

[0014] The amount of alkali used, calculated as Na2O, is 18% to 28% of the dry weight of the raw material. For example, it can be 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, or 28%.

[0015] The oxygen pressure can be 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa or 0.8 MPa.

[0016] Magnesium sulfate is 0.1% to 1.0% of the oven-dry raw material mass, such as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 0.1%.

[0017] High-pressure oxygen in alkaline cooking liquor forms highly oxidizing hydrogen peroxide ions, which rapidly break the chemical bonds between lignin molecules, accelerating lignin dissolution and significantly reducing the kappa number of pulp. Under alkaline conditions, oxygen preferentially reacts with lignin, reducing the degradation of cellulose and hemicellulose. Combined with magnesium sulfate, it effectively inhibits the peeling reaction, improving pulp yield and viscosity.

[0018] Sodium hydroxide, a strong alkali, is the core delignifying component of the cooking liquor. It breaks and dissolves the bonds between lignin structural units in plant fiber raw materials, thereby achieving fiber dissociation. In the cooking of non-wood fiber raw materials, sodium hydroxide can effectively remove impurities such as pectin, nitrogenous substances, and waxy substances, improving pulp purity.

[0019] Sodium carbonate is weakly alkaline and can be used as an auxiliary alkaline source to supplement the alkalinity of the system. During the cooking process, sodium carbonate gradually hydrolyzes to produce sodium hydroxide, continuously providing an alkaline environment for the delignification reaction. Compared to sodium hydroxide, its effect is milder and can reduce excessive degradation of cellulose. In addition, sodium carbonate helps to regulate the pH stability of the cooking liquor, thereby inhibiting the dissolution and migration of silicon into the black liquor, allowing more silicon from the wheat straw raw material to remain in the pulp, and to some extent mitigating the corrosion of equipment by strong alkali.

[0020] Magnesium sulfate, used as a cooking aid, forms magnesium hydroxide precipitate in alkaline systems. This precipitate has adsorption properties, adsorbing lignin degradation products generated during cooking, reducing their negative impact on cellulose, and improving pulp yield. Simultaneously, the presence of magnesium ions enhances the selectivity of the delignification reaction, making lignin easier to remove while relatively reducing the loss of cellulose and hemicellulose.

[0021] Properly controlling the amount of sodium hydroxide and sodium carbonate used can prevent excessive alkalinity from causing excessive silicon dissolution. At the same time, appropriately increasing the degree of sulfurization can enhance the delignification effect, reduce the degradation of cellulose and hemicellulose, and thus protect fiber strength.

[0022] To ensure effective delignification, appropriately lowering the cooking temperature or shortening the holding time can reduce silicon loss and excessive fiber degradation. Magnesium hydroxide precipitate, formed from magnesium sulfate in an alkaline system, can adsorb silicon compounds, reducing silicon loss. Simultaneously, magnesium ions can protect carbohydrates, improving pulp yield and fiber strength.

[0023] In some embodiments, the non-wood fiber raw material is wheat straw, sugarcane bagasse, rice straw, or reeds.

[0024] Preferably, the non-wood fiber raw material is wheat straw.

[0025] Further preferably, after the non-wood fiber raw material is cut into segments, each segment is 3 cm to 5 cm in length.

[0026] Preferably, the wheat ears are removed from the wheat straw.

[0027] In some embodiments, the molar ratio of sodium hydroxide to sodium carbonate is 5~8:3~5.

[0028] In some embodiments, the cooking temperature is 105°C to 150°C, and the cooking time is 1 h to 5 h. The cooking temperature can be 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, or 150°C.

[0029] The cooking time can be 1 hour, 2 hours, 3 hours, 4 hours or 5 hours.

[0030] Preferably, the cooking temperature is 110°C to 130°C and the cooking time is 3 to 5 hours.

[0031] In some embodiments, the liquid-to-cook ratio is 1:4 to 6.

[0032] In some embodiments, the pH value of the black liquor is ≤9.8.

[0033] Secondly, the present invention provides a pulp prepared by the oxygen-alkali pulping method of the low-silica pulping black liquor.

[0034] Thirdly, the present invention provides the use of the pulp in the preparation of paper products.

[0035] The present invention will be further described below with reference to the embodiments.

[0036] Example 1 An oxygen-alkali pulping method for low-silica black liquor includes the following steps: (1) Weigh 50 g of completely dry wheat straw and place it in a polyethylene bag; (2) Based on an alkali dosage equivalent to 22% of the dry raw material mass (calculated as Na2O) and a sodium hydroxide to sodium carbonate molar ratio of 6:4, add 8.5 g sodium hydroxide, 7.5 g sodium carbonate and 0.25 g magnesium sulfate to 300 mL of deionized water to prepare the cooking liquor. (3) Add the cooking liquid prepared in step (2) to the polyethylene bag and knead it thoroughly to ensure that the liquid is evenly soaked. Transfer it to the cooking tank, remove the air in the tank and introduce 0.8 MPa oxygen. Keep it at 115°C for 4 hours. (4) After cooking, separate the pulp and pulping black liquor.

[0037] (5) The obtained pulp is washed and screened to obtain fine pulp; the fine pulp is then processed through pulping, mixing, papermaking and drying to obtain pulp paper.

[0038] Example 2 Under the same cooking conditions as in Example 1, samples were taken from the digester at 15 min, 30 min, 45 min, 60 min, 80 min, 100 min, 120 min, 180 min, 240 min and 300 min after the start of cooking (the first 60 min was the heating stage). The pH value and soluble silicon content of the pulping black liquor were measured, and the silicon content in the pulp was analyzed. The results are shown in Table 3.

[0039] Example 3 An oxygen-alkali pulping method for low-silica pulping black liquor includes the following steps: (1) Weigh 50 g of completely dry wheat straw and place it in a polyethylene bag; (2) Add sodium hydroxide, sodium carbonate and 0.45 g magnesium sulfate to 300 mL of deionized water to prepare the cooking liquor, with an alkali amount equivalent to 20% of the dry raw material mass (calculated as Na2O) and a molar ratio of sodium hydroxide to sodium carbonate of 3:7. (3) Add the cooking liquid prepared in step (2) to the polyethylene bag and knead it thoroughly to ensure that the medicine is evenly soaked. Transfer it to the cooking tank, remove the air in the tank and introduce 0.8 MPa oxygen. Keep it at 135°C for 2 hours. (4) After cooking, separate the pulp and pulping black liquor.

[0040] (5) The obtained pulp is washed and screened to obtain fine pulp; the fine pulp is then processed through pulping, mixing, papermaking and drying to obtain pulp paper.

[0041] Example 4 A method for oxygen-alkali pulping of low-silica black liquor includes the following steps: (1) Weigh 50 g of completely dry wheat straw and place it in a polyethylene bag; (2) Add sodium hydroxide, sodium carbonate and 0.1 g magnesium sulfate to 300 mL of deionized water to prepare the cooking liquor, with an alkali amount equivalent to 28% of the dry raw material mass (calculated as Na2O) and a molar ratio of sodium hydroxide to sodium carbonate of 4:1. (3) Add the cooking liquid prepared in step (2) to the polyethylene bag and knead it thoroughly to ensure that the liquid is evenly soaked. Transfer it to the cooking tank, remove the air in the tank and introduce 0.8 MPa oxygen. Keep it at 105°C for 5 h. (4) After cooking, separate the pulp and pulping black liquor.

[0042] (5) The obtained pulp is washed and screened to obtain fine pulp; the fine pulp is then processed through pulping, mixing, papermaking and drying to obtain pulp paper.

[0043] Comparative Example 1 The difference from Example 1 is that sodium carbonate is replaced with sodium hydroxide, otherwise it is the same as Example 1.

[0044] Specifically: (1) Weigh 50 g of completely dry wheat straw and place it in a polyethylene bag; (2) Based on the amount of alkali used (calculated as Na2O) being 22% of the dry raw material mass, add 15.6 g of sodium hydroxide and 0.25 g of magnesium sulfate to 300 mL of deionized water to prepare the cooking liquor; (3) Add the mixed alkaline solution prepared in step (2) to the polyethylene bag and knead it thoroughly to ensure that the solution is evenly soaked. Transfer it to a cooking tank, remove the air from the tank and introduce 0.8 MPa of oxygen. Keep it at 115°C for 4 hours. (4) After cooking, separate the pulp and pulping black liquor; (5) The obtained pulp is washed and screened to obtain fine pulp; the fine pulp is then processed through pulping, mixing, papermaking and drying to obtain pulp paper.

[0045] Comparative Example 2 (1) Weigh 50 g of completely dry wheat straw and place it in a polyethylene bag; (2) Based on the amount of alkali used (calculated as Na2O) being 15.5% of the dry raw material mass, add 10 g of sodium hydroxide and 0.05 g of anthraquinone to 300 mL of deionized water to prepare the cooking liquor; (3) Add the mixed alkaline solution prepared in step (2) to the polyethylene bag and knead it thoroughly to ensure that the solution is evenly soaked. Transfer it to a cooking tank and keep it at 170°C for 1.5 h. (4) After cooking, separate the pulp and pulping black liquor.

[0046] Comparative Example 3 The difference from Example 1 is that the amount of alkali remains the same, and sodium hydroxide is replaced with sodium carbonate, while everything else is the same as in Example 1.

[0047] Comparative Example 4 The difference from Example 1 is that magnesium sulfate is omitted, while everything else is the same as in Example 1.

[0048] Comparative Example 5 The difference from Example 1 is that the cooking temperature is 100°C, while everything else is the same as in Example 1.

[0049] Comparative Example 6 The difference from Example 1 is that the cooking temperature is 155°C, while everything else is the same as in Example 1.

[0050] Effect verification The silicon content was determined by gravimetric method. Approximately 1 g of pulp sample prepared in the examples and comparative examples, and approximately 5 mL of filtered pulping black liquor sample were placed in a pre-weighed crucible. After evaporation and concentration on a hot plate, sulfuric acid and nitric acid were added to thoroughly wet the sample. After cooling, the sample was filtered, and the residue was washed and returned to the crucible, then ignited in a muffle furnace at 800°C until constant weight. After cooling, the residue was weighed; the mass of the residue was the silicon dioxide content.

[0051] The pH value, soluble silicon content in the pulping black liquor, and silicon content in the pulp obtained from Comparative Example 1, Comparative Example 1, and Comparative Example 2 are shown in Table 1. The results show that the pH value of the pulping black liquor obtained in Example 1 is significantly lower than that of Comparative Example 1 and Comparative Example 2; the soluble silicon content in the obtained pulping black liquor is reduced by 53.4% ​​compared to Comparative Example 1 and by 67.1% compared to Comparative Example 2, indicating that more silicon is retained in the pulp.

[0052] Table 1 Comparison of pH value and silica distribution of black liquor from different pulping processes

[0053] The fine pulp yield, properties, and physical properties of the pulp obtained in the examples and comparative examples are shown in Table 2. Compared with Comparative Example 1, the fine pulp yield of the pulp obtained in Example 1 increased by 5.6%, and the viscosity increased by 152.5 mL·g. -1The tensile index, tear index, bursting index, and ring crush index of the pulp obtained in Example 1 were increased by 26%, 12%, 8%, and 23%, respectively, compared to Comparative Example 1. This significant improvement in physical properties stems from the gentle action of the method of this invention on the fibers, reducing the degradation of cellulose fibers and improving the quality of the pulp fibers. The pulp of Comparative Example 1 had higher whiteness, directly related to its more thorough lignin removal. However, excessive delignification also led to severe fiber degradation and yield loss. The process of this invention, by protecting the efficient delignification under cellulose, better preserves the strength and yield of the pulp cellulose fibers while sacrificing some whiteness. For paper types such as corrugated base paper and kraft paper, which do not require high whiteness, the advantages of this process are particularly prominent.

[0054] Table 2 Comparison of Pulp and Pulp-to-Paper Properties

[0055] Table 3 shows the changes in pH and silicon distribution in the pulping black liquor during the cooking process in Example 2. Silicon leaching mainly occurred in the early stages of cooking (at higher pH levels). As the pH dropped below 9.8 during the cooking process, the silicon content in the pulping black liquor stabilized, indicating that silicon dissolution and migration were effectively suppressed. This dynamic change validates the technical rationale of controlling the final pH to ≤9.8 as an effective means of inhibiting silicon leaching.

[0056] Table 3. Changes in pH value and silicon distribution of black liquor during cooking under the conditions of Example 2.

[0057] In addition to wheat straw, this invention is also applicable to other non-wood fiber raw materials with similar characteristics, such as sugarcane bagasse, rice straw, reeds and other agricultural waste.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for producing oxygen-alkali pulp from low-silica black liquor, characterized in that: Includes the following steps: Cut and clean the non-wood fiber raw materials; Prepare the cooking liquor by adding sodium hydroxide, sodium carbonate, and magnesium sulfate, with a molar ratio of sodium hydroxide to sodium carbonate of 3~8:2~7; the amount of alkali used in the cooking liquor, calculated as Na2O, is 18%~28% of the oven-dry raw material mass; and the amount of magnesium sulfate is 0.1%~1.0% of the oven-dry raw material mass. The non-wood fiber raw material is mixed with the cooking liquor and kneaded and soaked. Then it is cooked under oxygen at 0.3 MPa to 0.8 MPa. After cooking, the pulp and pulping black liquor are separated.

2. The oxygen-alkali pulping method for low-silica pulping black liquor according to claim 1, characterized in that: The non-wood fiber raw materials are wheat straw, sugarcane bagasse, rice straw, or reeds.

3. The oxygen-alkali pulping method for low-silica pulping black liquor according to claim 2, characterized in that: The non-wood fiber raw material is wheat straw.

4. The oxygen-alkali papermaking method for low-silica pulping black liquor according to claim 3, characterized in that: After cutting the non-wood fiber raw materials into segments, each segment is 3 cm to 5 cm in length.

5. The oxygen-alkali pulping method for low-silica black liquor according to claim 4, characterized in that: The molar ratio of sodium hydroxide to sodium carbonate is 5~8:3~5.

6. The oxygen-alkali pulping method for low-silica pulping black liquor according to claim 1, characterized in that: The cooking temperature is 105°C ~ 150°C, and the cooking time is 1 h ~ 5 h; Preferably, the cooking temperature is 110°C to 130°C and the cooking time is 3 to 5 hours.

7. The oxygen-alkali pulping method for low-silica black liquor according to claim 1, characterized in that: The liquid-to-cook ratio is 1:4~6.

8. The oxygen-alkali pulping method for low-silica pulping black liquor according to claim 1, characterized in that: The pH value of the pulping black liquor is ≤9.

8.

9. A type of pulp, characterized in that: It is prepared by the oxygen-alkali pulping method of the low-silica pulping black liquor according to any one of claims 1-8.

10. The use of the pulp of claim 9 in the preparation of paper products.

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