Research on the comprehensive treatment technology of papermaking black liquor III

1.6 Chemical Oxidation

Chemical Oxidation (AOP) is mainly based on reactive oxygen radical chain reaction, especially by using hydroxyl radical (·OH) with strong attacking force to destroy organic molecular structure, mineralize organics into CO2 and H2O, or form organic precipitates out of water. Phase, the method of complete degradation of organic matter, can greatly improve the efficiency of wastewater treatment. For the first time, Mohamed et al.[27] used UV/TiO2 photocatalytic oxidation technology to decompose soluble lignin in black liquor at room temperature and obtain valuable industrial raw materials such as vanillin.

1.7 Recycling of papermaking black liquor

Lignin recovered from papermaking black liquor has excellent physicochemical properties and is an important basic industrial raw material. It has broad application prospects in various fields. The study of lignin as a basic raw material for synthesizing various types of organics has a history of nearly one hundred years, and in particular, many recent researches and applied technological achievements have increased [28].

At present, the varieties and uses of lignin products developed in China are mainly in the following two categories: The natural properties of lignin and lignosulfonates are used as industrial binders, reinforcing agents, fillers, dispersants and admixtures. Such as [29], widely used in agriculture, construction and oil exploration, geological drilling, highway paving, metallurgical group, pulverized coal and other industries.

Further research on lignin utilization technology with large demand, advanced technology, good economic benefits, and market prospects in the future is an important prerequisite for promoting the development of the lignin industry. Strengthen the research and development of lignin utilization process technology, speed up its industrialization process, not only the variable waste is a useful resource, but also will promote the fundamental treatment of pulp and papermaking wastewater, thereby bringing significant environmental and economic benefits to human society. .

2 Recent progress in black liquor making technology

The treatment of papermaking black liquor is the key to solve the pollution of the entire paper industry. Because the black liquor contains difficult biodegradable lignin and other toxic substances, the black liquor treatment is very difficult and becomes a worldwide problem. The treatment of black liquor mainly lies in how to dispose of lignin rationally. At present, the treatment of lignin in small and medium sized grass pulp mills at home and abroad is still the main method [30], but the deficiencies are very obvious. For lignin treatment, biological methods should be used as the final outlet. Combining the acid precipitation method with the biological method, it is possible to obtain a good governance effect. Studies have shown [31] that white rot fungi are the most promising strains for the degradation of lignin and its derivatives at the present stage. They can exert the dual functions of acid production and degradation in alkaline black liquor, and can be used in black liquor for papermaking. Biological treatment. A small factory with an annual output of 10,000 tons or less of paper accounted for about 95% of the total number of factories. Pulp made from non-wood fiber materials such as rice straw and wheat straw accounted for more than 70% of the total output. In view of the fact that there are too many small factories in our country and the complexity of raw materials for pulp, it is of great significance to vigorously popularize biological methods and solve some key problems in the treatment of black liquor in biological methods. It can be predicted that the biological method will fundamentally control the pollution of the paper industry.

In the past 30 years, supercritical fluid technology has developed rapidly. In particular, supercritical water technology is a high technology that began in Japan in 1998 and has gradually been widely used in the field of environmental engineering [32]. Supercritical water has specific properties not found in water under normal conditions and can be mixed with oxygen in an arbitrary ratio to provide a solvent medium for the supercritical water oxidation reaction. The wastewater treatment, especially industrial wastewater, high-concentration organic wastewater, difficult-to-decompose hazardous organics in urban groundwater, difficult-to-decompose organic solid wastes, sludge treatment, etc., show outstanding advantages. In addition, the supercritical water technology has the characteristics of energy saving, high efficiency, and adjustable selectivity. At present, there is no report on the application of this technology to papermaking black liquor.

The steam used by paper mills comes from coal-fired boilers. The smoke from coal combustion contains a lot of dust and harmful gases, which causes environmental pollution. Supercritical boilers can significantly increase cycle efficiency, reduce fuel consumption, and greatly reduce the emission of harmful substances. In addition, heavy metal elements will not be decomposed after high-temperature combustion in ordinary boilers, and they will volatilize to form metal vapors at high temperatures. When the flue temperature decreases, they will condense to form submicron particles, which are difficult to capture by the dust collector. Supercritical boilers can overcome common boiler heavy metals. The lack of pollution of the environment. At present, foreign supercritical boiler technology has entered the stage of commercial application. With China's increasingly stringent environmental protection requirements, accelerating the research and development of supercritical boilers is the most realistic and effective way to solve the problem of low energy utilization and severe environmental pollution. Papermaking black liquor exists as a long-term public hazard, and the study of supercritical technology for the treatment of papermaking black liquor will be of epoch-making significance.

Supercritical water oxidation technology is a cost-effective method to treat toxic organic waste. As early as in 1982, Modell[33] conducted supercritical water experiments on DDT, dichloroethylene, biphenyl and other pollutants, which proved that the removal rate of these organic substances were both More than 99.9%, the decomposition of pollutants is also very rapid, and therefore has the characteristics of high efficiency, short homogeneous reaction time, toxic substances, wastewater treatment products, etc., indicating that supercritical water oxidation technology treatment of wastewater is a very effective method. The harmful components in papermaking black liquor are mainly organics (>65%). Supercritical water oxidation technology is the most advantageous new method for the treatment of papermaking wastewater. The dissolved oxygen in supercritical water reacts with organic wastes to produce C02, The non-toxic substances such as H20 and N2 need no external heating and can utilize the energy generated by the oxidation process, and have the advantages of thorough treatment, relatively low investment, low annual operation and maintenance costs, and low unit cost. As an emerging environmental protection technology, it has received extensive attention from governments of all countries and has carried out in-depth studies in developed countries such as Europe, the United States, and Japan. Industrial plants have been put into operation, and wastewater from papermaking industry has been successfully treated. China's understanding of this technology has been successful. Far from enough, research is in its infancy.

With the development of high-temperature and high-pressure technologies and the in-depth study of related thermodynamics and supercritical water chemistry kinetics and mechanisms, supercritical water technology will be continuously improved, and the potential of supercritical water oxidation to treat papermaking wastewater will be fully realized. This is obviously a promising research field. It will bring new opportunities for the development of China's paper industry.

3 Conclusion

In China, there is a shortage of wood. Pulp made from non-wood fiber raw materials such as rice straw and wheat straw accounts for more than 70% of the total output in the papermaking industry, and there are mostly small factories. Alkaline recycling wood pulp black liquor which is quite mature in foreign industrialized countries is not suitable for China's paper industry. Although many researches have been conducted on the treatment of black liquor from small straw pulp mills, many new methods have been proposed, but so far there has been no mature process of generalized value. The domestic paper industry must adapt to the fiercely competitive international market environment, and solving the serious environmental pollution problem is a top priority. It can be predicted that with the deepening of the understanding of complex black liquor and the maturation of black liquor processing conditions, the problem of black liquor contamination will be fundamentally solved. (Wen/Su Weifeng Chai Liyuan Wang Yunyan)