Professor Chen Kefu, Dean of School of Paper and Environmental Engineering at South China University of Technology, said:
I. Major sources of pollution in the paper industry The pollution of the paper industry in China is mainly due to the harmful waste discharged. In 1994, 5 billion tons of hazardous waste was discharged, accounting for 1/6 of the national wastewater discharge, of which organic pollutants occupied 1/1. 4, ranked third. There is also pollution from exhaust gas and solid waste, but it is not essential compared to hazardous waste fluids. Harmful waste rate liquid comes from the following three aspects: (1) The waste liquid from the pulping section is the black liquor from the alkaline pulping (pulping) and the red liquor from the acid cooking pulp (pulping). At present, only some paper mills in China use acid pulping, and most paper mills use alkaline pulping. The black liquor discharged is mainly the degradation products of lignin and carbohydrates. For example, lignin, formic alcohol, dimethyl sulfide, and dimethyl disulfide are degradation products of lignin. They are all odor- and even odor-causing compounds. They are major contaminants causing high COD values ​​in waste liquids. The degradation products of carbohydrates are in the state of heterogenous sugar acid, which is the main contaminant causing high BOD value in the waste liquid. Acidic cooking contaminants, also degradation products of lignin and carbohydrates, are the main reasons for high COD and BOD formation. (2) Contaminants containing chlorine bleaching waste liquid The most serious pollution from paper mills is the discharge of chlorine bleaching waste liquid. The waste liquid contains not only COD and BOD, but also other highly toxic substances. Thousands of original paper mills producing bleached pulp in China have adopted chlorine-based bleaching technologies, such as chlorinated bleaching and hypochlorite bleaching, in addition to individual plants. Chloroform generated during the bleaching of hypochlorite contains approximately 150-250 g of waste liquor discharged from each ton of bagasse slurry, and approximately 700 g of wood pulp per ton of bleach. Chlorination bleaching In addition to the production of chloroform, the waste liquid also contains more than 40 organic chlorides, of which the most is a variety of chlorinated phenols, such as dichlorophenols, trichlorophenols, also produces dioxins. ) And furan products, more than 10 species are highly toxic. (3) The pollution of the middle-stage wastewater in the pulping and papermaking process is not serious compared with the pollution of the retort pulping black liquor and the chlorine bleaching waste liquor. The waste water pollution in other sections of the pulp and papermaking process is not serious. This part of the waste water is generally called the middle-stage waste water. Due to technical and economic reasons, the current alkaline pulp production process cannot be fully closed and zero emissions. There are wastes discharged from other sections, as well as all kinds of pollutants, such as waste water generated from stock preparation, white water from paper machines, and waste water caused by drip, run, run, and run of pipelines in plant equipment. If these waste waters are not treated, they will also cause environmental pollution. However, these waste waters can generally meet the standard through proper physical treatment. In another place, a semi-closed circulation system can be used. The previous process uses the waste water of the next process as much as possible to reduce the discharge of waste water. Second, on the pulp and paper industry to solve the environmental pollution method 1. Alkali recovery system is an important part of the modern alkali pulping production. The alkali recovery system is the recovery system of chemical drugs and thermal energy in the alkaline pulping black liquor. The alkali in the black liquor is fully recovered and used for the alkali pulp production of the plant. The organic matter in the black liquor is converted into energy, and heat and electricity are produced, so that the discharged waste water reaches the standard. Its main process is: from the digested pulp after extraction black liquor black liquor before evaporation pretreatment black liquor evaporation black liquor combustion (concentration 60% - 75%) green liquor causticization (green liquor is black liquor in the alkali recovery furnace (After burning, the molten material is dissolved in diluted white liquor or water) The alkali is recovered (Caustic reacts to produce white clay and NaOH, the main component of which is CaCO3, and alkali can be recovered from the white mud). Alkaline pulp production plants in foreign countries are equipped with alkali recovery systems, which have attracted much attention in recent years in China. Some large and medium-sized paper mills have established or are establishing an alkali recovery system. According to statistics, nearly 90 pulp mills have established alkali recovery systems, of which 40 are wood pulp mills. The black liquor discharged from wood pulp production has a very mature alkali recovery technology abroad. However, for grass pulp production, due to the low strength of grass pulp, poor filtration performance, plus silicon interference in black liquor and characteristics of black liquor, such as small fine fibers, high polysaccharide content, complex lignin structure, etc. Harder to extract. The low concentration of black liquor requires too much energy to evaporate, while a slight increase in the concentration of the black liquor results in a large viscosity, which makes it difficult to deliver and store it. Therefore, the concentration of the black liquor entering the alkali recovery furnace can only be reduced, so that the black liquor dry matter has a low calorific value and cannot be fully burned. In this way, there is a series of difficulties in black liquor extraction, evaporation, causticization, and white mud washing, and it is particularly difficult to extract and evaporate black liquor. Some experience, but so far there is no demonstration project worthy of promotion. In view of the above reasons, and taking into account that China's current pulp and paper mill has a large proportion of grass materials, China has decided to shut down small plants with an annual output of 5,000 tons of pulp, and will shut down and transfer pulp with an annual output of 10,000 tons or less of pulp. The paper mill no longer builds a new pulp mill with an annual output of tens of thousands of tons, so that the amount of pulp black liquor reaches the requirements of the alkali recovery system, and the problem of solving the black liquor alkali recovery of straw pulp is included in the National Science and Technology Project of the Ninth Five-Year Plan. . Summarizes the experience of black liquor alkali recovery in domestic straw pulp, and draws lessons from the black liquor alkali recovery technology from abroad, and tackles key technologies such as black liquor extraction, thickening, viscosity reduction, descaling, and causticization. Grass black liquor contamination can also be solved. 2. The traditional method of replacing whole-chlorine bleached pulp with a new medium-high-concentration chlorine-free bleaching or less-chlorine bleached pulp pulp. Chlorine-free bleaching (TCF), also known as non-polluting bleaching, uses bleach-free substances such as O2, H2O2, and O3 as bleaching agents. The agent bleaches the pulp under medium and high concentration conditions; low chlorine bleaching (ECF) uses CIO2 as a bleaching agent to bleach the pulp under medium consistency conditions. Chlorine-free and low-chlorine bleaching are used to replace the chlorination and hypochlorite bleaching of low-concentration pulps in severely polluted environments, which are still used in paper mills in China. (1) Chlorine-free bleaching 1 Oxygen bleaching uses oxygen bleaching for environmental protection. Since oxygen is non-toxic and does not pollute the environment itself, after the oxygen delignification, the amount of bleaching agent and bleaching wastewater in the latter stage can be reduced by 50%. In recent years, interest in oxygen bleaching has been increasing at home and abroad in response to demands for environmental protection. The Swedish government uses oxygen delignification as a measure to control pollution in newly-built bleached pulp mills. Oxygen bleaching has been shown to significantly reduce the BOD, COD, chroma, and total organochlorine content in bleaching wastewater. It plays an important role in reducing the pollution of modern pulp bleaching wastewater. Oxygen bleaching can also save on the consumption of other chemicals and can increase pulp yield compared to chlorine bleach bleached pulp. Oxygen bleaching processes can be classified as high-oxygen bleaching (25% - 28%), medium-oxygen bleaching (7% - 15%). The initial oxygen bleaching system uses high concentration, because of the high cost of the high-concentration system, and the investment cost is large. Later, it gradually developed toward the medium and strong. The reason for the strong development is that the medium-concentration bleaching process is simpler, safer, more reliable, easier to implement, and has a lower investment cost. Medium-high oxygen bleaching is carried out at a pressure of 0.6-0.8 MPa. 2 Hydrogen peroxide bleaching Hydrogen peroxide is often used in the post-stage of multi-stage bleaching of chemical pulp to improve the whiteness of the pulp and the stability of bleached pulp whiteness as well as the bleaching of mechanical pulp. Alkaline hydrogen peroxide can remove most of the chromophoric groups on the side chains of lignin. The lignin structure can also be broken to produce a colorless ester compound without degrading the lignin to a soluble level, so that hydrogen peroxide can be used in groundwood pulp, preheated wood pulp, chemical ground wood pulp, Neutral sulfite semi-chemical pulp and bleached pulp with high lignin content play an increasingly important role. In addition to reducing pollution, the use of hydrogen peroxide bleaching reduces costs and improves pulp quality. Hydrogen peroxide bleaching process also has the advantages of medium concentration and high concentration. According to domestic and foreign data reports, both types of bleaching have their own advantages. 3 Ozone bleaching Ozone is an excellent strong oxidant. It has been widely used in the industry, and bleaching agents in the paper industry have only been reported in these years. Oxygen delignification and bleaching are very strong. In the pulp bleaching system, it can be used alone, or it can be combined with hydrogen peroxide, oxygen and other bleaching agents to form a multi-stage bleaching. The greatest attraction of ozone bleaching is that it has no pollution to the environment. For example, it can be bleached with O—Z—E—P, a chlorine-free bleaching process, which is ideal for any pulp and paper mill. The pulp concentration in the ozone bleaching stage also has a medium-high concentration, namely medium-density ozone bleaching and high-density bedford oxygen bleaching. Technically, the above-mentioned several stages of chlorine-free bleaching can be connected in series, and a multi-stage bleaching process can be used to make the whiteness of the pulp reach a predetermined target. For example, after mid-concentration atmospheric pressure O2, H2O2 bleaching, sulfite wood pulp can reach 85% ISO brightness, mechanical wood pulp can reach more than 90%, and bleaching can be particularly difficult with a mid-concentration atmospheric H2O2 bleaching process. The machine pulp is bleached from 22.8% ISO to 70.5% ISO brightness, and its wastewater can meet the national emission standards. After years of research in our country, the major problems of non-contaminated pulp bleaching technology have been basically solved. The key to the present is to develop a bleaching system that maintains continuous production. Until now, we have successfully developed more than half of stand-alone machines. The non-polluting bleaching technology and equipment developed by our country and adapted to the characteristics of the domestic paper industry can be applied to production soon. Effluent generated by chlorine bleach bleached pulp can be combined into the black liquor extraction section of the alkali recovery system, into the alkali recovery system, or incorporated into the general wastewater treatment of the subsequent stage. (2) The use of CIO2 as a bleaching agent in the low-chlorine bleaching technology has been a common practice in foreign countries in the 1980s: In North America, most pulp mills have used CIO2 as a bleaching agent to bleach pulp; in European countries, some manufacturers Chlorine-free bleaching is advocated, but it also gradually transitions to chlorine-free bleaching, but many pulp mills still use CLO2 as a bleaching agent. In China, several pulp mills currently use CIO2 bleach, but unfortunately some plants are currently in poor operating condition. After bleaching pulp with CIO2, the bleaching wastewater not only reduces AOX (adsorbable organic halides) and extremely toxic substances, but also reduces the resin barrier, but the pulp strength is basically not. change. Practice has proved that the higher the replacement rate of CI2, the smaller the amount of wastewater, AOX, and toxic substances. When the complete replacement of CI2 is reached, the bleaching wastewater has reached the current national minimum emission standards. Therefore, using CIO2 as a bleaching agent to bleach pulp is also one of the effective measures to reduce the pollution of bleaching wastewater. However, the implementation of the low-chlorine bleaching technology has the problem of large construction investment. First, the CIO2 generator is expensive; second, it is the bleaching system and The subsequent washing equipment has a high material requirement and is generally made of titanium steel. The above two issues also affect the promotion of low chlorine bleaching technology in China's pulp and paper industry. 3. Develop other (no less) contaminated pulping technologies to open up new avenues for eliminating paper industry pollution (1) The development of mechanical pulping uses mechanical methods instead of chemically pulping fiber raw materials to obtain pulp, which has low production costs The high yield (about 90% - 98%), no or less use of chemicals, the environmental pollution is much smaller than the chemical pulping method. However, in the previously developed ground wood pulp (MP), due to short fibers, most of the lignin and non-cellulose components were not removed, and the paper was susceptible to yellowing and becoming brittle and could not be stored for a long time, thereby being limited. In the late 1970s, the international papermaking industry began to develop the preheated wood chips milled wood pulp (TMP), and China began to develop in the era. TMP technology is to increase the temperature of the raw material before entering the mill (preheat 115-135 0C), so that the fiber can be easily separated, which not only reduces the power consumption, but also improves the quality of the pulp. Since the 1990s, chemical thermomechanical pulp (CTMP) has emerged. On the basis of TMP, about 5% of the chemicals used in chemical pulping chemicals are added. This can achieve the advantages of TMP, namely high yield and less pollution. , And to make pulp has the advantages of chemical pulp, that is, lignin content is small, the fiber is long and soft. Several major newsprint mills in China have carried out CTMP projects in recent years, but equipment mainly depends on the introduction. In the research of CTMP, we have carried out arduous research work, mainly on the basis of foreign CTMP technology combined with China's development and promotion of fast-growing wood and non-wood CTMP technology and devices, and has made progress. (2) Development of waste paper pulping In order to reduce pulping pollution, it is also an effective method to expand papermaking raw material resources and recycle waste paper pulp. In the world, the amount of waste paper used in papermaking is increasing. For example, the waste paper recycling rate in the United States is above 30%, Japan is 43%, and the United Kingdom is 45%. The United States expressly stipulates that the amount of raw materials put into paper mills must contain more than 25% of waste paper. In recent years, the utilization rate of waste paper in China has also been increasing, and the waste paper recycling rate in Guangdong Province has now reached more than 30%. The major problems of waste paper pulping technology