Main processes and methods of chemical pulping
Pulping includes four processes: material preparation, cooking, washing and screening, and bleaching. In order to prepare slurry suitable for different performance and strength requirements, the removal degree of impurities in slurry is also different. Chemical pulp is pulped and bleached by delignification.
Cooking; This is my business.
According to the different pH values of liquid medicine, it can be divided into alkaline method (sulfate method, caustic soda method and alkaline sulfite method), neutral salt method and acidic method (acidic sodium sulfite method and bisulfite method). Among them, sulfate method is the main method of pulping, followed by acid sulfite method.
Kraft pulping
Pulping method with sodium hydroxide and sodium sulfide as cooking agents. In the process of alkali recovery, cheap sodium sulfate can be used as a supplement. The advantages of sulfate method are: strong adaptability to raw materials, higher pulp strength than sulfite method, and it can be used to make all kinds of tough paper, paperboard and tissue paper; Short cooking cycle, high drug recovery rate and mature waste liquid recovery technology. The disadvantage is that the pulp is dark in color, difficult to bleach, poor in opacity and low in pulp yield. The pulping process is as follows:
The parameters affecting the cooking process include wood species, chip specifications and quality, active alkali dosage, liquid concentration, vulcanization degree, temperature and cooking time. These parameters influence and restrict each other.
Sulfate method is suitable for all kinds of needles, hardwoods and bamboo. Conifer wood has high lignin content and dense wood structure; The proportion of hard broad-leaved trees is large, and the cooking conditions and cooking results are very different. The amount of active alkali used for cooking varies with the kind of wood and the purpose of pulp. Generally, the amount of alkali used for chemical pulping of hardwood is 13 ~ 20% (calculated by sodium oxide), and that of softwood is 18 ~ 28%. Active alkali refers to sodium hydroxide and sodium sulfide in liquid medicine, usually converted into sodium oxide meter. The active alkali concentration (g/L) of cooking liquor is an important factor to determine the cooking speed. When the amount of alkali used is constant, the concentration of liquid medicine is controlled by the liquid ratio. The liquid ratio should be determined according to the cooking method, raw material type, pulp quality requirements and steam quality, and is generally 1: 3 ~ 4.
Sodium sulfide can accelerate the breaking of ether bonds between lignin structural units, accelerate its dissolution speed, shorten cooking time and improve pulp yield and quality. The content of sodium sulfide (percentage of active alkali) in cooking liquid is expressed by the degree of vulcanization.
(in Na2O)
Generally, the vulcanization degree of coniferous wood is about 25%, and that of broad-leaved wood is about 20%.
Increasing cooking temperature can speed up cooking and shorten cooking cycle. Under the same other cooking conditions, the higher the cooking temperature, the faster the cooking speed, but the higher the temperature, the lower the pulp yield and pulp strength. Wood cooking temperature is generally around 65438 070℃. Cooking time consists of heating time and heat preservation time. The heating time is related to the penetration of liquid medicine, which is generally 1.5 ~ 2.5 hours. The holding time at the highest temperature is related to the kind of raw materials, the amount of alkali used, the degree of vulcanization, the concentration of alkali, the highest cooking temperature and the kind of pulp. Cooking temperature and time can be expressed by parameter H factor. H factor is the integral area of cooking relative reaction rate constant to cooking time. Under the same cooking conditions, pulp with the same lignin content and yield can be obtained as long as the H factor is the same. H- factor is often used to control the cooking end point in cooking.
Black liquor extraction equipment (usually vacuum pulp washing system) extracts black liquor from cooked pulp. Then the black liquor is evaporated to a concentration of 50-60% in a multi-effect vacuum evaporator and a direct evaporator. Concentrated black liquor is mixed with mirabilite, which is sprayed into alkali recovery furnace for combustion, and mirabilite is reduced to sodium sulfide. The melt flows out from the bottom of the furnace, and its components are sodium carbonate and sodium sulfide. When dissolved in water, it is called green liquid. The heat generated by the combustion of the alkali recovery furnace is recovered by the steam boiler. The clarified green liquor is causticized with lime milk to convert sodium carbonate into sodium hydroxide, and the generated calcium carbonate (white mud) is precipitated to obtain white liquor (sodium hydroxide and sodium sulfide) for cooking.
Sulfite pulping
Pulping method using bisulfite and sulfurous acid as cooking agent. According to the different pH value of cooking liquid, it can be divided into acid sulfite method, bisulfite method, neutral sulfite method and alkaline sulfite method. Sulfite method in a narrow sense refers to acid sulfite method.
Acidic sulfite pulping is light in color, easy to bleach and high in pulp yield. Softwood sulfite waste liquid can be used to make adhesives such as alcohol, yeast and vanillin, cement water reducer and viscosity reducer for petroleum drilling mud. However, the adaptability to raw materials is poor, cooking time is long, and acid-resistant equipment is needed; It is difficult to recycle waste liquid and solve the pollution problem. This method is gradually replaced by sulfate method. Soluble base (such as magnesium, ammonium, sodium, etc. ) instead of calcium base to solve the problem of waste liquid recovery; Multi-stage cooking is adopted, which expands the application range of raw material varieties. Acidic sulfite pulping process flow is as follows:
Sulfuric acid and bisulfite are used to form cooking liquor. Its composition is represented by total sulfur dioxide, combined sulfur dioxide and free sulfur dioxide. The penetration of liquid medicine into wood chips is an important process of sulfite cooking, and the penetration temperature is about 1 10℃. Insufficient penetration of liquid medicine will cause wood chips to "black cook" at cooking temperature. The factors affecting the cooking process include cooking temperature, concentration and composition of cooking liquid, pH value, alkali type of cooking agent, cooking time and so on. The acid sulfite cooking temperature of wood is 140 ~ 150℃.
Alkaline sulfite pulping
A new pulping method. NaOH and Na2SO3 were used as cooking agents, and the pH value of the liquid medicine was 10 ~ 13. This method has strong adaptability to raw materials, high pulp yield, good bleaching performance, light color of unbleached pulp and good paper strength.
When pine is cooked by alkaline sulfite method, the total amount of chemicals is 17 ~ 2 1% (calculated by Na2O), the degree of sulfitation is 80% (the calculation method is similar to that of sulfate method), the cooking temperature is 175℃, and the cooking time is 180 minutes. Under the same delignification rate, the yield of pulp is 6 ~ 12% higher than that of sulfate method, and the beating performance is good, which can save energy by 50 ~ 70%. The strength of pulp is similar to that of kraft pulp except that the tearing degree is slightly lower than that of kraft pulp. The main disadvantage of this method is that it is difficult to recover waste liquid.
Pulp bleaching
The color of pulp is mainly due to the existence of lignin in pulp. The bleaching process is mainly to remove colored substances from pulp and eliminate its chromophoric groups. Bleaching methods can be divided into two categories: ① using oxidative bleaching agent to destroy the residual lignin in pulp to improve the whiteness of pulp, which is called delignification bleaching and is mostly used in chemical pulp; (2) Using chemicals to change the structure of chromophoric groups of colored substances and decolorize them is called lignin retention bleaching, which is mostly used for high-yield pulp.
Multi-stage bleaching of chemical wood pulp has the advantages of less bleaching agent, high whiteness and little damage to fiber strength. The simplest multistage bleaching includes three stages: chlorination (C), alkali treatment (E) and hypochlorite bleaching (H). With the development of bleaching technology, new bleaching agents, such as chlorine dioxide (D) and hydrogen peroxide (P), have appeared, and the bleaching level has reached 7 ~ 8, including repeated chlorination and alkali treatment, as well as the use of various bleaching agents, such as C-E-H-D-E-D and so on. So that the pulp has a high and stable whiteness (above 90). The purpose of pulp chlorination is to remove residual lignin from pulp and lay the foundation for later bleaching. The slurry is directly mixed with chlorine in the slurry chlorine mixer for reaction. The chlorine consumption of sulfate pulp accounts for about 60 ~ 70% of the total chlorine consumption, the pulp concentration is 3%, the reaction time is 65438 0 hours at room temperature, and the final pH value is 2. After chlorination, a part of lignin is dissolved in water in chlorination stage, and a part needs to be dissolved in alkaline medium. Therefore, it is necessary to set up an alkali treatment stage after chlorination. When treated with alkali, the pulp concentration is 10%, the pH value is above 9 ~ 10, the temperature is 60 ~ 70℃, and the time is about 1 ~ 2 hours. Bleaching agents used in pulp bleaching include hypochlorite and chlorine dioxide. (1) Hypochlorite: It is the earliest bleaching agent used. Lignin and colored substances in pulp are removed by hypochlorite oxidation. In the bleaching process, cellulose and hemicellulose are oxidized at the same time, which reduces the physical strength of pulp and discolors pulp. By controlling pH value, temperature, pulp concentration and chlorine dosage, the purpose of improving pulp whiteness and minimizing cellulose damage can be achieved. Increasing the bleaching temperature can speed up the bleaching process, but it will increase the oxidative damage of cellulose, so the bleaching temperature should not exceed 40℃. PH value is the most important factor affecting the bleaching process. The composition and properties of bleaching solution are different with different pH values. When the pH value is higher than 9, hypochlorite is the main bleaching agent. When the pH value is lower than 4, chlorine is dominant; When the pH value is 4 ~ 6.5, hypochlorous acid is the main component. Different bleaching solutions have different oxidation potentials, among which hypochlorous acid has the highest oxidation potential. Therefore, when bleaching in the pH range of 5 ~ 7, cellulose degradation is the most serious, pulp yield and strength decrease, and bleached pulp is easy to turn yellow. When bleaching under alkaline conditions, the oxidation potential is the smallest and the damage to cellulose is the smallest. When bleaching, adjust the pH value with alkali and control the bleaching end point at 8.0 ~ 8.5. When the pH value is too high, the bleaching speed slows down, but the whiteness is stable. ② Chlorine dioxide: It is an excellent pulp bleaching agent. Its aqueous solution is a strong oxidant, which has strong selective reaction ability with lignin and less damage to fiber and hemicellulose. Especially in acidic medium, chlorine dioxide hardly reacts with carbohydrates. Chlorine dioxide bleaching can make pulp obtain high whiteness without reducing fiber strength and yield. The bleaching conditions of chlorine dioxide are: pH 3.5 ~ 6.0, temperature 60 ~ 80℃. Adding a small amount of chlorine dioxide in the chlorination stage of pulp can improve the chlorination effect and reduce the content of toxic organic chlorine in chlorinated wastewater.
Improvement of chemical pulping technology
Due to the main technical problems of chemical pulping, one is to improve the yield of pulp to save wood, and the other is to study the pulping method with no pollution or less pollution. Therefore, an improved kraft process technology was developed, which mainly includes: ① Adding 0.03 ~ 0. 1% anthraquinone (calculated by dry wood) to kraft cooking liquor can increase the pulp yield by about 2%, and reduce the alkali consumption or shorten the cooking time. This method has been widely popularized in kraft pulp mills. At present, anthraquinone is being studied to replace all sodium sulfide (that is, caustic soda-anthraquinone method) in order to reduce and eliminate the atmospheric pollution caused by malodorous gas discharged from sulfate pulp factory. (2) Cooking with sodium polysulfide can reduce the stripping reaction of carbohydrates in alkaline cooking and improve the yield of alkaline cooking pulp. Compared with sulfate method, this method can generally increase the pulp yield of softwood by 8% and hardwood by 3%. Sodium polysulfide is not widely used because of its corrosiveness and smell. ③ Gas phase cooking. The method is to cook wood chips at the pressure of 0.0 196 ~ 0. 196 MPa for 3 ~ 6 minutes, then fully penetrate into the cooking liquid, and then cook them at 160 ~ 170℃ for 20 ~ 40 minutes under the gas phase condition. It is characterized by low alkali consumption, easy bleaching and beating of pulp, slightly higher pulp yield and better pulp quality than ordinary sulfate method. ④ In order to reduce the content of toxic organochlorine in bleaching wastewater, an improved sulfate method (advanced delignification method) was put forward at the end of 1970s. Two-stage cooking is adopted. The concentration of alkali liquor in the first stage is lower than that in the conventional method, and the concentration of cooking liquor in the second stage is higher than that in the conventional method. Sodium sulfide is mainly added to the cooking liquor in the first stage. This method can reduce the kappa number of pulp from 30 ~ 32 to 25 while maintaining the fiber viscosity, thus reducing the amount of chlorine used in bleaching and the content of organic chlorine in bleaching wastewater by 20%. This method has been widely used in industry. In addition, energy-saving cooking methods have been popularized in industry since 1980s. In this method, the washing water is used to replace the hot black liquor in the cooking pot and recycled, which can save cooking steam by 60-65%, shorten cooking time and improve pulp strength.
Adopting soluble base and multistage cooking technology can expand the adaptability of sulfite cooking to raw materials and reduce pollution. Sulfites of ammonium, sodium and magnesium have higher solubility than traditional calcium salts. According to the requirements of paper species, the pH value of liquid medicine is selected, and the content of hemicellulose in pulp is controlled to improve the physical and chemical properties of pulp. Salt base recovery technology can be used to reduce environmental pollution. Many countries in the world use acid ammonium sulfite method to cook wood pulp. This method has strong adaptability to raw materials and high pulp yield, and the waste liquid can be directly used as agricultural fertilizer. Sulfite multi-stage cooking can improve the pulp yield, and the industrialized methods are Stora method and Sivola method.
Pulp bleaching technology is developing towards less pollution, high whiteness, high strength and high yield. If there is no chlorine bleach (oxygen, hydrogen peroxide, ozone, etc. ) to reduce pollution, chlorine-free bleaching technology based on oxygen bleaching has been widely used. As a bleaching stage instead of chlorination stage, oxygen bleaching can eliminate toxic chlorinated wastewater, or introduce oxygen into conventional alkali treatment stage to improve lignin removal ability in alkali treatment stage. In the modern two-stage pulping method, oxygen delignification is regarded as a stage to improve the pulp yield during pulping. That is, pulp with high lignin content (kappa number is 30 ~ 50) is prepared by alkaline cooking, and lignin is delignified to kappa number 10 ~ 12 by oxygen. Because the selective delignification ability of oxygen bleaching is stronger than that of alkali boiling, the yield of pulp can be increased by about 5%. When bleached with chlorine dioxide, the whiteness of pulp after oxygen bleaching can reach above 85 degrees. This method is especially suitable for hardwood pulping.
Huangdaba (192 1 ~)
China modern forest products chemical expert and forestry educator. A native of Beiliu County, Guangxi Zhuang Autonomous Region, was born in 192 1 year 1 1 day. 1944 graduated from the Department of Chemistry of Guangxi University. 65438-0945 Worked as a technician in Guangxi Industrial Laboratory. 1946, went to study in the Department of Chemical Engineering and the Department of Forest Products of the University of Washington, USA. 1949 obtained the master of science degree, and then stayed in school to be a chemist in phuket puree wood company. 195 1 year. He has served as an associate professor and professor at Wuhan University, Huazhong Agricultural College and Nanjing Forestry College (now known as Nanjing Forestry University). 1959 ~ 1984 deputy director and director of the department of forest products and chemical engineering of Nanjing forestry college. He has served as the fourth director of chinese society of forestry, the first and second vice-chairmen of China Forest Products Chemistry and Chemical Engineering Society, the first vice-chairman of Jiangsu Light Industry Federation and the chairman of Daily Chemistry Society, the chairman of the National Forest Products Chemistry Textbook Committee, and the member of the first and second academic evaluation groups of the State Council Academic Degree Committee.
During his teaching in Nanjing Forestry College, Huang Ba has carried out many scientific researches, among which "Study on Hydrolysis of Camellia oleifera Shell to Furfural" won the 1978 Jiangsu Science Conference Award, "Preparation of Maleic Acid Rosin and Its Application as Paper Sizing Agent" won the third prize of National Science and Technology Progress Award, and "Pilot Experiment of Herbicide Zhencaoning" won the first prize of 1984 Science and Technology Achievements of Forestry Department. He has published more than 20 papers on forest chemistry industry, translated tree extract technology, and edited plant hydrolysis technology and English-Chinese chemistry and chemical vocabulary supplement (forest chemistry part).
Activation furnace
See activated carbon.