Energy-saving and emission-reducing production process and process method for alkaline chemical pulping

Abstract

The invention relates to an energy-saving and emission-reducing production process and process method for alkaline chemical pulping, which belongs to the technical field of pulping, cooking and washing, and adopts the technical scheme that a dipping pot, a cooking pot, a pulp washing pot, a washing tank and the like are connected with a pulp pump, a valve and a pipeline through a vacuum and compressed air system; the method has the beneficial effects that the high-concentration and high-temperature black liquor is directly extracted from the steamer and sent to the alkali recovery evaporation section, the black liquor is firstly reduced to the temperature required by evaporation through flash evaporation, and secondary steam is reused for production, so that the evaporation steam consumption is greatly reduced; the steam produced by alkali recovery can meet the requirements of evaporation, causticization and cooking, and the residual steam can be used by other workshops; real zero emission of bamboo and wood pulping wastewater is realized, and ultralow emission of straw pulping wastewater is realized; the amount of water for washing the bamboo wood pulp is 0 (clear water), and the amount of water for washing the straw pulp is ultra-low; the two problems of water pollution and high energy consumption of alkaline pulping are solved satisfactorily.

Description

Technical field

[0001] The invention relates to a fiber raw material steaming and washing production process technology, an alkaline chemical pulping energy-saving and emission-reducing production process and a process method. Background Art

[0002] For a long time, the production process of alkaline chemical pulping, cooking and washing at home and abroad usually involves the following: after the cooking is completed in the cooking pot, the slurry in the pot is under high temperature and high pressure, and the entire pot of pulp (including black liquor) is quickly sprayed into the spray pot. Then, dilute black liquor is injected into the spray pot to dilute the pulp to a certain concentration, and then the slurry in the spray pot is pumped to the vacuum pulp washer through a pulp pump for black liquor extraction and pulp washing. However, over the years, some pulping companies in my country (mainly bamboo pulp) have changed their cooking equipment from ordinary vertical pots to DDS displacement cooking vertical pots, which has reduced energy consumption. However, the pulp washing equipment still uses multiple vacuum pulp washers in series to extract and wash black liquor, and operates in a countercurrent washing method. The black liquor extracted by the first vacuum pulp washer has a high concentration and temperature, and is directly sent to the evaporation section of the alkali recovery workshop for alkali recovery treatment. The concentration and temperature of the black liquor extracted by the subsequent vacuum pulp washers decrease with each subsequent one, and the extracted dilute black liquor is used as countercurrent washing liquid. This pulping, cooking and washing production process and process method is commonly adopted by various chemical pulping companies for intermittent pulping. This conventional traditional chemical pulping process technology method (including DDS cooking vertical kettle replacing ordinary vertical kettle, but the pulping production process remains unchanged and is also an intermittent pulping production process) has the following main disadvantages:

[0003] 1. The traditional black liquor extraction method has low concentration, low temperature, large amount of black liquor, and higher steam consumption for evaporation than for cooking.

[0004] 2. The discharge volume of pulping wastewater (black water) is large, the treatment cost is high, and there is a certain degree of environmental pollution problem.

[0005] 3. High-temperature full-pressure spraying after cooking significantly weakens the pulp fiber strength, especially for wood pulp. Although DDS cooking is cooled and pumped after cooking, the spraying temperature is still relatively high, which still has a certain impact on weakening the fiber strength.

[0006] The above three points are the main reasons for high pulping costs, low added value of pulp and poor economic benefits.

[0007] Therefore, in order to improve the social and economic benefits of chemical pulping, it is necessary to reform the traditional production process, increase the concentration and temperature of black liquor extraction, achieve zero discharge of pulping wastewater and zero water use for pulp washing (meaning no clean water is used), reduce the pulp spraying temperature as much as possible, and increase the strength and added value of pulp. These process methods are important technical measures to achieve energy conservation and emission reduction in chemical pulping and improve social and economic benefits. Summary of the Invention

[0008] The purpose of the present invention is to provide an energy-saving and emission-reducing production process and process for the technical renovation, expansion, and new construction of wood and non-wood alkaline chemical pulping production. Based on the problems of traditional steaming and washing production processes and processes, such as low black liquor extraction concentration, low temperature, high evaporation and concentration steam consumption, large pulping wastewater (black water) discharge, high treatment costs, and water pollution, it is necessary to research and design an energy-saving and emission-reducing production process and process for alkaline chemical pulping that can improve and overcome the shortcomings and defects of traditional chemical pulping production and improve its social and economic benefits.

[0009] The technical solution adopted by the present invention to solve its technical problems is: connecting the equipment such as the immersion pot (1), the cooking pot (2), the pulp washing pot (3), the washing tank (4) through a vacuum, compressed air system and a pulp pump, valves, and pipelines to form a fully enclosed cooking and washing production process and process method, and the equipment used are all existing technical equipment.

[0010] The beneficial effects of the present invention are:

[0011] ① The high-concentration and high-temperature black liquor is automatically extracted directly from the cooking pot and quickly transported to the alkali recovery evaporation section. The high-temperature black liquor is first reduced to the evaporation temperature through flash evaporation, and the secondary steam is reused for production. According to process calculations, the initial concentration of wood pulp black liquor entering the evaporator reaches about 21%, and the initial concentration of bamboo pulp black liquor entering the evaporator reaches about 22.5%, which is significantly higher than the black liquor concentration and temperature of traditional black liquor extraction, with less heat loss and significantly reduced steam consumption for black liquor evaporation.

[0012] ② In the process of extracting high-concentration and high-temperature black liquor directly from the steamer, no waste gas or wastewater leakage is generated, and no pollution is caused to the environment.

[0013] ③ This black liquor extraction method maximizes the benefits of alkali recovery. According to process calculations, if existing pulping plants switch from traditional black liquor extraction methods to the method of this invention, the steam generated by the alkali recovery furnace can meet the steam needs of evaporation, causticizing, and cooking, achieving self-sufficiency in steam for pulping (including alkali recovery), with even excess steam available for use in other workshops. This allows for truly low-energy pulping (total energy consumption far lower than that of DDS technology), significantly reducing overall pulping costs. Furthermore, this reduces boiler room infrastructure investment and coal-fired flue gas emissions, contributing to environmental protection.

[0014] ④ The pulp washing adopts the in-pot washing process. According to process calculations, after multiple washings, the pulp is sprayed at ultra-low temperature (50-60°C) to avoid weakening the pulp fiber strength due to high-temperature spraying. After spraying, the pulp is decontaminated and dehydrated once. The cleanliness of wood pulp reaches about 99.72%, and the cleanliness of bamboo pulp reaches about 99.14%, which meets the cleanliness requirements of the industry. The pulp no longer needs to be washed. The wastewater (black water) containing black liquor squeezed out after the first dehydration is reused in production without any treatment, thus achieving zero discharge of pulping wastewater (black water) at the source of pulping, saving pulping wastewater treatment infrastructure investment and wastewater treatment costs, and significantly reducing pulping costs. For the pulping of grass fibers such as wheat straw, which have high black liquor viscosity, ultra-low discharge of pulping wastewater can also be achieved, and the treatment cost is low.

[0015] ⑤ According to process calculations, the pulp washing process of the present invention adopts a compressed air and vacuum suction countercurrent circulation washing process. In fact, the washing water is wastewater containing a small amount of black liquor solids. After multiple rounds of washing, the black liquor solid concentration of the washing wastewater is in a stable equilibrium state. From the engineering design and process calculations, it is shown that when using this wastewater for circulation washing, there is no need to add additional clean water, thereby achieving zero water use for pulp washing (meaning no clean water is used). This process is particularly suitable for wood pulp and bamboo pulp, which can save clean water costs, and the pulp cleanliness can meet industry requirements.

[0016] ⑥ After the present invention realizes zero discharge of wood and bamboo pulping wastewater (black water) and ultra-low discharge of straw pulping wastewater, the phenomenon of enterprises secretly discharging pulping wastewater (black water) can be put to an end, which is beneficial to environmental protection.

[0017] ⑦The present invention is applicable to the technical transformation, expansion and new construction projects of traditional intermittent alkali chemical pulping production. It can effectively improve and solve various problems existing in traditional alkali chemical pulping, overcome its shortcomings, retain and develop its advantages, and continuously and steadily enhance its social and economic benefits and profitability, thereby giving new vitality to traditional alkali chemical pulping. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a plan view of the production process of the fully enclosed steaming and washing process of the present invention.

[0019] In the figure, 1. the dipping pot, 2. the cooking pot, 3. the pulp washing pot, and 4. the washing tank are connected together through a vacuum, compressed air system, valves, and pipes to form a fully enclosed cooking and washing production process and process method. DETAILED DESCRIPTION

[0020] The present invention is realized by the following technical scheme: the immersion pot (1), the cooking pot (2), the pulp washing pot (3), the washing tank (4), these devices are all existing technical equipment, according to Figure 1Arrange and install, and connect through vacuum, compressed air system, slurry pump, valves, pipelines, etc.

[0021] ① Implementation method of extracting black liquor directly from the digester: The digester is an existing equipment that can directly extract high-concentration and high-temperature black liquor from the digester and quickly and automatically transport it to the alkali recovery evaporation section for treatment. The high-temperature black liquor is first reduced to the evaporation temperature through flash evaporation, and the secondary steam generated is reused in production. This black liquor extraction process, which is different from the traditional one, greatly increases the concentration and temperature of the extracted black liquor, greatly reduces the steam used for evaporation, and has a very significant energy-saving effect.

[0022] ② Implementation method of zero discharge of pulping wastewater (black water): After extracting black liquor directly from the steamer, immediately inject an equal amount of low-temperature black liquor into the steamer until it is full, and then transfer all the pulp in the steamer to the pulp washing pot. The pulp washing pot is an existing equipment. It is washed through multiple vacuum countercurrent cycles, and finally the entire pot of pulp is cold-sprayed (50-60°C) into a coarse pulp tank with a separation head by compressed air. After removing impurities, it is dehydrated once. According to engineering design and process calculations, the cleanliness of wood and bamboo pulp has reached industry requirements and does not need to be washed again. The pulp washing wastewater (black water) discharged from the first dehydration does not need to be treated in any way and is fully reused in production. This truly achieves zero discharge of pulping wastewater (black water) from the source of chemical pulping (applicable to wood and bamboo pulp). If the present invention is used for pulping grass fiber raw materials such as wheat straw, ultra-low wastewater discharge can also be achieved, and the treatment cost is low.

[0023] ③ Implementation method of using zero water (referring to clean water) for pulp washing: This pulp washing process utilizes the wastewater containing trace amounts of black liquor solids in each washing tank for multiple-pot circulation washing. According to engineering design and process calculations, the concentration of black liquor solids contained in the wastewater in each washing tank has been in a stable equilibrium state after multiple-pot circulation washing. The solid concentration contained therein has no longer increased and is very low. By using this wastewater circulation washing, the pulp cleanliness can meet industry requirements. This washing process method is particularly suitable for washing wood and bamboo pulp. During the entire washing process, no clean water needs to be added, achieving zero water (referring to clean water) for pulp washing and saving clean water costs.

Claims

1. An alkaline chemical pulping energy-saving and emission-reduction production process and process method, characterized in that Existing equipment such as an immersion pot (1), a cooking pot (2), a pulp washing pot (3), a washing tank (4) are connected together through a vacuum and compressed air system and a pulp pump, valves and pipelines to form a fully enclosed cooking and washing production process and process method.

2. Extract high-concentration and high-temperature black liquor directly from the cooking pot and send it to the alkali recovery and evaporation section.

3. The pulp washing adopts the in-pot circulation washing process. For wood and bamboo chemical pulp, it can achieve zero discharge of pulping wastewater (black water), and for straw pulp, it can achieve ultra-low discharge of pulping wastewater, with low treatment cost.

4. The pulp washing process of the present invention adopts a countercurrent circulation pot washing process combining vacuum and compressed air. The entire pulp washing process uses the original wastewater for circulation washing without adding additional clean water for washing, thereby achieving zero water (referring to clean water) for pulp washing, which is particularly suitable for chemical pulping of wood and bamboo.

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