Pulp and paper industry is highly dependent on water resources. It also causes serious water pollutions in many countries around the world. As a result, strict regulations has been issued by governments on discharging paper effluent into receiving waters, especially for bleached pulp mills. These regulations mainly focus on the biotoxicity and AOX concentrations present in the final discharges. Moreover, the bleaching process of pulp uses chemical treatments including the use of chlorine to remove residual lignin and chromophore groups. The use of chloride increases the BOD and COD of wastewater and leads to environmental hazards. Facing these problems, papermaking enterprises started to look for alternative solutions to traditional chlorine bleaching. And ozone pulp bleaching is the best.
Ozone Pulp Bleaching Process
Ozone is strongly oxidizing. It is applicable for sterilization and odor removal in food processing industry, wine industry, beverage production enterprises, agriculture, medical institutions, etc. In addition, bleaching is another core function of ozone. Ozone has been used for pulp bleaching for more than 20 years. As a green bleaching method, ozone is considered to be one of the most economical, safe and environment-friendly solution. Ozone pulp bleaching is used in pulp mills in many countries. The use of ozone to bleach the highly concentrated pulp allows the ozone gas to be fully absorbed by the fibers, resulting in an optimal whiteness of the pulp.
Using ozone as a bleaching agent has been a long-standing research project for scientists. Ozone bleaching provides higher brightness and strength through its ecologically clean process, as well as reducing pulp production costs. Economically, the effect of each kilogram of ozone can equal the effect of 2-4 kilograms of pure carbon dioxide. So the use of ozone makes paper mills more profitable than adopting other means. Environmentally, ozone treatment can also significantly reduce the consumption of fresh water and the production of waste water during the production process. By doing this, pulp manufacturers can make themselves more compliant with environmental regulations.
Ozone is an extremely unstable gas, which can easily be affected by temperature and quickly be decomposed. When the temperature is between 0-10℃, ozone can achieve a sufficient delignification efficiency and whiteness improvement. Therefore, to make the best use and reduce the decomposition of ozone, it is recommended to restrict the time length of ozone pulp bleaching within 10-30 minutes.
Benefits of Ozone Pulp Bleaching
The following are the benefits of using ozone for pulp bleaching:
- Ozone can achieve a TCF (total chlorine free) bleaching, ensuring that there is no AOX in the bleached wastewater
- Ozone can react fast and efficiently with the pulp. Among all the pulp bleaching agents, ozone is the most efficient. There will not be any secondary pollution either.
- Ozone can achieve a higher whiteness of the pulp. Due to the high reactivity of ozone, the whiteness of pulp can easily be as high as 90% after the treatment.
- Ozone pulp bleaching can reduce the costs. Studies have shown that 1 kilogram of ozone has the equal bleaching power in 2-3 kilograms of carbon dioxide. And to produce the same weight of ozone and carbon dioxide costs almost the same.
- Compared with traditional pulp bleaching methods, ozone bleaching can reduce the discharge of wastewater.
- Ozone bleaching can significantly prevent the bleached pulp from turning yellow again.
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