1 alkali recovery method
Alkali recovery treatment is an effective method to solve the black liquor problem. Through the four main sections of black liquor extraction, evaporation, combustion and caustic treatment, SS, COD and BOD in black liquor can be completely removed, and alkali can be recovered to generate secondary steam (energy). However, the alkali recovery system has high technical requirements and high equipment investment. Because the small and medium-sized paper mills are generally unable to bear the high cost of building alkali recovery system, the alkali recovery system is only mainly used in large paper mills. In addition, the silica content in the white mud produced by the straw pulp plant is high, and it is not easy to burn back to lime, and the white mud may cause secondary pollution.
Through the experimental study of simulated solution, Du Zhaonian and helianjuan selected an effective desilicant and applied to the real black liquor. After adding desilicant, the silicon content in black liquor can be reduced by more than 95%, while the alkali loss is only about 5%. The process basically solves the problem of high silicon content in the recovery of white mud. Aitianzhao et al. Has improved the technology of the traditional alkali recovery and causticization process of black liquor in paper making, which avoids the production of waste residue (white mud) fundamentally, and makes the alkali recovery method eliminate the pollution treatment process of white mud, and directly produces the functional calcium carbonate with high added value and has achieved better economic and environmental benefits.
2 acid analysis
The traditional acid analysis 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. Compared with alkali recovery, the biggest advantage of this technology is that the equipment investment is less, and can be used in small and medium sized paper mills. However, the lignin ash separated by this method is high, and there are many impurities, so it is difficult to use. The process has the advantages of high acid content, high cost, serious equipment corrosion, easy to cause acid leakage accident, and harm the subsequent biochemical treatment unit.
It is another method to treat black liquor by acid black liquor from flue gas in recent years. The acid analysis of flue gas from black liquor of cooking has the characteristics of strong acid and weak acid analysis. The purification effect can reach the level of sulfuric acid method, while 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. Based on the study of the conditions for the separation of lignin and silicic acid in black liquor and the colloidal characteristics, 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 analysis and purification of black liquor flue gas - electrodialysis of single cation membrane". The process adopts the method of waste treatment, which not only eliminates the flue gas pollution, but also avoids the phenomenon of lignin precipitation and plugging, thus improving the recovery rate of alkali and reducing the power consumption per ton of alkali. The removal rate of lignin was 85% - 97%, the removal rate of chroma, COD and silicon was 75.94%, 63.18% and 87.32% respectively. Chen Junzhi and others concentrated the black liquor extracted by extrusion by flue gas, and then used the black liquor as the bonding reinforcement agent for building materials after chemical modification.
The experiment shows that the addition of the modified black liquor can improve the forming and drying properties of the green obviously, improve the compressive strength of the finished products after baking, reduce the water absorption performance, and save a lot of groundwater for the building materials industry. 3 ultrasonic method
The degradation of organic pollutants in water by ultrasonic is a physical chemical degradation process, which is mainly caused by the physical and chemical changes caused by ultrasonic cavitation. The ultrasonic cavitation process of liquid is a process of concentrating the energy of sound field and releasing rapidly. That is, the liquid produces cavitation bubbles under ultrasonic radiation. These cavitation bubbles absorb the energy of sound field and collapse and release energy in a very short time, and generate high temperature and high pressure, strong shock wave and micro jet in the very small space around it. The water vapor entering the cavitation bubble reacts at high temperature and high pressure to produce hydrogen and oxygen radicals, while the organic pollutant vapor entering the bubble can also undergo similar combustion thermal decomposition reaction. The water molecules on the surface layer of the cavitation bubble can form supercritical water, which increases the chemical reaction rate. Organic pollutants are degraded by hydrogen oxygen free radical oxidation, combustion decomposition in bubble and supercritical water oxidation. Zhou Shan et al. Studied the degradation rate of black liquor by supergeneration method. The results showed that the degradation rate of black liquor was proportional to ultrasonic time, and the initial concentration had a certain influence on the degradation effect; when 30 ℃ ± 2 ℃ and pH = 12, the ultrasonic degradation effect was the best in 4h; the degradation efficiency could be improved by adding hydrogen peroxide and Fenton test agent. The removal rate of COD and TOC in black liquor is 47.9% and 45.8%, respectively. Ultrasonic method is expected to be a pretreatment technology for biochemical treatment of papermaking wastewater.
4 combustion method
The process of combustion is to evaporate and concentrate black liquor by using residual heat of flue gas and coal heat, and then burn organic matter such as lignin and recover alkali. The industrialization technology of this process has been relatively mature. The investment of black liquor per ton of combustion method is lower than that of alkali recovery method, but the operating cost is higher. Yin Guoxun and others improved the combustion method. They used the red mud with high CaO content and black liquor of paper making with high organic content to develop the sulfur fixing combustion aids for the powder coal, which can achieve the effect of sulfur fixation and combustion support, especially for low sulfur coal at 1050 ℃. This method has achieved the effect of turning waste into treasure, and has good environmental protection significance and economic benefits.
5 coagulation method
Coagulation is a method to put a certain amount of coagulant into wastewater, which makes it difficult to naturally precipitate colloidal pollutants and a part of fine suspended substances through the reaction process of desstable, coagulation and bridge erection, forming a flocculation with certain size, and then settling and separating in subsequent sedimentation tank, so as to separate colloidal pollutants from wastewater. The common coagulants are inorganic coagulants (such as aluminum salt, iron salt, etc.) and organic coagulant (such as polyacrylamide). The slag composite coagulant was prepared by the industrial waste residue extracted by sulfuric acid and hydrochloric acid. The effects of the types, acid concentration and temperature of the waste slag on the coagulation effect of black liquor were investigated. The results show that the coagulant made from fly ash has the best coagulation effect, and the acid concentration used in the extraction should not be too high. The temperature rise during the extraction is conducive to improve the coagulation effect. They put forward a practical black liquor treatment process.
The cost of COD removal was 0.62 yuan / kg, which was only 3.6% of the treatment cost of common aluminum salt coagulant. The results show that COD removal rate is 50%, turbidity removal rate is 65.41%. The treatment of black liquor by this method can not only reduce the organic load of black liquor, but also reduce the dosage of coagulant, which provides a feasible way to reduce the cost of pollution treatment for paper enterprises with tight capital.
The treatment of black liquor by coagulation and other treatment methods can achieve better treatment effect. Sunjiashou and others used the methods of acidification flocculation and cross-linking bentonite adsorption to decolorize the black liquor of paper making. The results showed that the organic crosslinked bentonite adsorbent had better adsorption and decolorization performance on acidified black liquor. When the dosage was 24g/l, the decolorization rate was 99.86%, COD removal rate was 71.4% and the effluent quality was better.