1. Alkali recovery system is an integral part of modern alkali pulping production. The alkali recovery system is the recovery system of chemical drugs and thermal energy in 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 processes are: extraction of black liquor from cooked pulp → pretreatment before black liquor evaporation → black liquor evaporation → black liquor combustion (concentration 60%-75%) → green liquor causticization (green liquor is black liquor in alkali After the combustion in the recovery furnace, the molten material flows out of the melt and is dissolved with a thin white liquor or water) → The alkali is recovered (The white mud and NaOH, which are main components of CaCO3 in the caustic reaction, 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 process can be divided into high concentration oxygen bleaching (25%-28%), medium concentration 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 its non-pollution to the environment. For example, it can be bleached with O-Z-E-P 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 with a medium-concentration atmospheric H2O2 bleaching can make bleaching wheat straw weeds particularly difficult. The pulp bleached from 22.8% ISO to 70.5% ISO brightness, and its wastewater can meet the national emission standards. (to be continued)

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