Corrosion problems in the hottest pulp and paper i

Corrosion problems in pulp and paper industry pulp and paper waste water treatment and recycling

corrosion problems in pulp and paper industry continuous digesters, thermomechanical pulp systems, intermittent digesters, chlorine dioxide generators, tar oil recovery systems, steam condensing systems, dryers, chippers, chip transportation systems, overflow equipment, boiler flue gas dust collectors, and pulp washers all have corrosion. Here are some of the most important corrosion problems in the pulp and paper industry

stress corrosion cracking of continuous digester

alkaline cooking is carried out at 1.1MPa and 175 ℃ in a liquid medium containing about 100g/l sodium hydroxide and 35g/l sodium sulfide and a large amount of sodium carbonate, sulfate and thiosulfate. The welded structure of carbon steel continuous digester usually breaks, which is caused by stress corrosion cracking of welds that have not been stress relieved in alkaline medium

corrosion of chemical recovery furnace tubes

the use of recovery boilers that use waste heat from flue gas to improve the temperature of primary and secondary air entering the furnace is increasing year by year, and water-cooled walls, superheaters and generator (boiler) pipes play different roles in different boilers

the first is the recycling of cooking chemicals, which includes the concentration, evaporation and combustion of the waste liquid extracted from the pulp to produce steam and molten salts at the bottom of the furnace

the industrial trend of controlling the outflow and discharge as accurately as possible is that the optimal combustion rate and higher pressure boilers have changed the composition of molten salt or "smelting" and combustion gas, and increased the temperature of some parts of the boiler

in this case, the most likely corrosion problem is the corrosion of the external pipeline caused by vulcanization and oxidation due to smelting, as well as overheating, fatigue, wear corrosion and corrosion on one side of the cooling pipe

equipment corrosion in bleaching and washing processes

in the multi-stage bleaching production line, the pulp must be cleaned in each stage to prevent the residual chemicals in each process. The most common bleaching agent is chlorine ion based on oxidizing drugs, which causes corrosive local corrosion of stainless steel cleaning drums and pipes used for cleaning. This kind of corrosion is particularly serious in cracks, under pulp deposits and welds

corrosion of vacuum press roller

paper dehydration is the basic process of papermaking. Before the paper passes through the steam dryer, dehydration is completed by drainage, vacuum water absorption, pressing and wool cloth water absorption. Vacuum roller is an important part of many paper machines. There are dense holes on the surface of this kind of roller to make the absorbed water pass through the roller wall. The damage of vacuum roller is a long-term problem of paper machine, and the cause of damage is generally corrosion fatigue

structural corrosion and damage

the atmosphere of local processing places in pulp and paper workshops contains water vapor, chloride, chlorine dioxide, sulfur dioxide, hydrogen sulfide and other corrosive gases, which are highly corrosive. In addition, spillage and leakage often occur, resulting in serious corrosion of steel and concrete structures such as ground, walkway, column, beam and roof foundation, while pump foundation, cable tray, pipe hanger Control equipment and many other components also often need to be repaired or replaced

the most effective way to solve these problems is to properly control the comprehensive measures such as corrosive flue gas, steam and overflow, including ventilation. Appropriate protective coatings can be used for structural steel, such as primers containing polychlorinated ester, vinyl, epoxy and zinc rich

according to the principle of conservation of matter, the sum of the total amount of matter in the product and the total amount of matter in the waste is certain, which is equal to the total amount of matter in the raw material. It can be said that pollutants are also a form of raw materials, but this form of existence reduces the amount of available resources, damages people's economic interests, and also affects people's health. Since substances can be transformed, as long as appropriate measures are taken, substances existing in pollutants may become forms that can be used. Therefore, people have been looking for effective and reasonable methods to treat pulping and papermaking wastewater, and make resource utilization of the treated wastewater and the useful substances contained in the wastewater as much as possible

I. sources and characteristics of pulping and papermaking wastewater

1.1 waste liquid of cooking section

that is, black liquor produced by alkali pulping and red liquor produced by acid pulping. Most paper mills in China use alkaline pulping to produce black liquor. Here, black liquor is the main research object. The pollutants contained in black liquor account for more than 90% of the total pollution emissions of the paper industry, and have the characteristics of high concentration and difficult degradation. Its treatment has always been a major problem. There are three main components in black liquor, namely lignin, pentose and total alkali. Lignin is a kind of non-toxic natural macromolecular material, which has a wide range of uses as chemical raw materials. Pentose can be used as livestock feed

1.2 intermediate water

pulping intermediate wastewater refers to the wastewater discharged from the screening, washing, bleaching and other processes of cooking pulp after black liquor extraction. Its color is dark yellow, accounting for 8%~9% of the total pollution discharge of the paper industry, and the COD load per ton of pulp is about 310kg. The concentration of middle water is higher than that of domestic sewage, and the ratio of BOD to cod is between 0.20 and 0.35. The biodegradability is poor, and the organic matter is difficult to biodegrade and difficult to treat. The organic matter in the middle water is mainly lignin, cellulose, organic acids, etc., mainly soluble COD. Among them, the most serious environmental pollution is the chlorine containing wastewater produced in the bleaching process, such as chlorination bleaching wastewater, hypochlorite bleaching wastewater, etc. Hypochlorite bleaching wastewater mainly contains trichloromethane, as well as more than 40 other organic chlorides, most of which are various chlorinated phenols, such as dichlorophenols, trichlorophenols, etc. In addition, bleaching waste liquid contains dioxin, a highly toxic carcinogen, which poses a serious threat to the ecological environment and human health

1.3 white water

white water is the waste water of the papermaking section. Vesta (now subordinate to Lubrizol Life Sciences Department) provides customized contract manufacturing services for the medical device industry, which comes from the paper-making process in the paper workshop. White water mainly contains fine fiber, filler, coating and dissolved wood components, as well as added rubber, wet strength agent, preservative, etc. it is mainly insoluble cod, with low biodegradability, and the preservative added has certain toxicity. The amount of white water is large, but its organic pollution load is far lower than that of cooking black liquor and middle stage wastewater. At present, almost all paper mills and paper workshops have adopted partial or full closed systems to reduce water consumption, save power consumption, improve the reuse rate of white water and reduce the discharge of excess white water

II. Treatment and recycling of pulping and papermaking wastewater

2.1 treatment and recycling of black liquor

2.1.1 alkali recovery treatment method

alkali recovery treatment method is a more effective method to solve the problem of black liquor at present. Through the four main sections of black liquor extraction, evaporation, combustion and causticization, SS, COD and BOD in black liquor can be completely removed together, and alkali can be recovered to produce secondary steam (energy). However, the alkali recovery system has high technical requirements and high equipment investment. Because small and medium-sized paper mills are generally unable to bear the high cost of building the alkali recovery system, the alkali recovery system is currently mainly used in large paper mills. In addition, the white mud produced by the straw pulp plant has high silicon content, which is not easy to be burned back into lime, and the white mud may cause secondary pollution

Du Zhaonian and he lianjuan selected an effective desilication agent and applied it to real black liquor through experimental research on simulated solution. After adding silicon remover and improving the process, the silicon content of black liquor can be reduced by more than 95%, while the alkali loss is only about 5%. This process basically solves the problem of high silicon content in white mud recovery. AI Tianzhao and others have made technical improvements to the traditional caustic recovery process of papermaking black liquor, fundamentally avoiding the production of waste residue (white mud), so that the alkali recovery method saves the white mud pollution treatment process, and directly produces caustic soda and high value-added series of functional calcium carbonate, which has achieved good economic and environmental benefits

2.1.2 acid precipitation method

the traditional acid precipitation method is to precipitate the alkaline black liquor with acid, separate lignin, and then mix the wastewater with the middle water for aerobic and anaerobic biochemical treatment. This process is relatively mature. Compared with the alkali recovery treatment method, the biggest advantage is that it has less equipment investment and can be applied in small and medium-sized paper mills. However, the lignin separated by this method has high ash content and many impurities, which makes it difficult to use. Moreover, this process uses a large amount of acid, costs high, and the equipment is seriously corroded, which is easy to cause acid leakage accidents and endanger the subsequent biochemical treatment sheet

acid separation of black liquor from flue gas is another method to treat black liquor in recent years. The flue gas acidizing process of cooking black liquor has the characteristics of strong acid and weak acid acidizing. The purification effect can reach the level of sulfuric acid acidizing method, but the end-point pH value is 2-2.5 higher than that of sulfuric acid method, which greatly reduces the pollution of secondary acid wastewater. On the basis of studying the precipitation conditions and colloidal properties of lignin and silicic acid in black liquor, the catalytic oxidation principle of sulfur dioxide gas in flue gas and the flocculation theory of colloidal particles, Zhang Yuyun proposed and designed the alkali recovery process of "acid precipitation purification of black liquor flue gas single cation membrane electrodialysis". The process adopts the method of treating waste with waste, which not only eliminates the pollution of flue gas, but also avoids the phenomenon of lignin precipitation blocking the tank, so as to improve the recovery rate of alkali and reduce the power consumption per ton of alkali. Using this method to treat papermaking black liquor, the removal rate of lignin is as high as 85%~97%, and the removal rates of chromaticity, COD and silicon are 75.94%, 63.18% and 87.32% respectively. Chen Junzhi and others used flue gas to concentrate the black liquor extracted by extrusion, and then chemically modified the black liquor to be used as a bonding enhancer for building materials

experiments show that the addition of the modified black liquor can significantly improve the forming and drying properties of green slabs, improve the compressive strength of finished products after baking, reduce the water absorption performance, and save a lot of groundwater for the building materials industry

2.1.3 ultrasonic method

ultrasonic degradation of organic pollutants in water is a physical-chemical degradation process, which mainly depends on the physical and chemical changes caused by ultrasonic cavitation effect to degrade pollutants. The ultrasonic cavitation process of liquid is a process of concentrating the sound field energy and releasing it rapidly, that is, the liquid produces cavitation bubbles under ultrasonic radiation. These cavitation bubbles absorb the sound field energy and collapse and release energy in a very short time, producing high temperature and high pressure, strong shock waves and micro jets in a very small space around them. The water vapor entering the cavitation bubble reacts under high temperature and pressure to produce hydroxyl radicals, while the organic pollutant vapor entering the bubble can also undergo a thermal decomposition reaction similar to combustion. The water molecules on the surface layer of the cavitation bubble can form supercritical water, which increases the rate of chemical reaction. Organic pollutants are degraded by three ways: hydrogen and oxygen free radical oxidation, combustion decomposition in bubbles, and supercritical water oxidation. Zhou Shan et al. Studied the degradation of black liquor by supergrowth method. The results showed that the degradation rate of papermaking black liquor was proportional to the ultrasonic time, and the initial concentration had a certain influence on the ultrasonic degradation effect; At 30 ℃± 2 ℃ and ph=12, the effect of ultrasonic degradation for 4H is the best; Adding hydrogen peroxide and Fenton reagent can improve the degradation efficiency. This technology can degrade macromolecular organics in papermaking black liquor to a certain extent. The removal rates of COD and TOC in black liquor are 47.9% and 45.8% respectively. Ultrasonic method is expected to become a pretreatment technology for biochemical treatment of papermaking wastewater

2.1.4 combustion method

the technological process of combustion method is beneficial