Paper industry is a raw material industry with circular economy characteristics. Paper industry can not only drive the development of upstream industries such as forestry, chemical industry, machinery manufacturing and logistics, but also promote the development of downstream industries such as packaging, publishing, cultural communication and medical & health care. However, the bleaching process of pulp will produce a large amount of medium wastewater containing cellulose, resinate and other ingredients difficult to biodegrade. At the same time, there will be organic halides, dioxins and other carcinogens in the waste water after bleaching, which will cause serious environmental pollution. With the increasingly strict requirements of environmental protection in the world, chlorine-free bleaching has been the trend in the world. In chlorine-free bleaching methods, ozone bleaching has the advantages of short process, fast reaction rate, low bleaching cost and low waste water discharge.
Principle of Ozone Bleaching
Ozone, produced from oxygen molecules in the atmosphere by electric shocks, is a trace gas present in the earth’s atmosphere. Ozone at room temperature has the color of light blue and a special odor. Inhaling an excessive amount of ozone will do harm to human health. Short periods of exposure to high concentrations of ozone can cause symptoms such as coughing and breathing difficulties. But ozone is a powerful oxidant. It can react with aromatic compounds such as lignin and phenol as well as heterocyclic compounds and proteins. Therefore, based on ozone oxidation, sterilization, deodorization and decolorization, ozone has been widely used in chemical industry, paper making, pharmaceutical, food preservation, wastewater treatment and other fields. For more details of ozone, please refer to our previous blog: An Introduction of Ozone.
In the bleaching process of pulp, no matter for phenolic or non-phenolic lignin structure, ozone can cause the fracture of the carbon-carbon bond in the benzene ring, so as to achieve continuous degradation. Ozone can also break the olefin bonds and ether bonds on the side chain, so as to realize the oxidative degradation of lignin and thus achieve the purpose of bleaching. In addition, the use of ozone bleaching can ensure the strength of the pulp. The effect of ozone on the fiber is uniform. It will not affect the integrity of the fiber or lead to the crosslinking or crimping of the fiber.
Influencing Factors of Ozone Bleaching
In the application of ozone bleaching, ozone concentration, pulp concentration, pH value, bleaching temperature, bleaching time, additives and others will affect the bleaching effect.
The pulp concentration for ozone bleaching can be divided into low, medium and high, corresponding to 3%, 10-15% and 30-50% respectively. When the ozone concentration is low, the surface of the pulp fibers will be covered with a water film, which affects the diffusion of ozone molecules into the fibers. Inadequate contact between ozone and pulp material will result in a decrease in bleaching effectiveness. When the ozone concentration is high, ozone can fully combine with the pulp, which is beneficial to improve the bleaching efficiency of ozone on the pulp and the whiteness of the final pulp products.
Since ozone bleaching needs to be carried out in acidic medium, it is necessary to add sulfuric acid, sulfite and other inorganic acids or organic acids to the pulp for pretreatment before ozone bleaching. The optimal pH value of the pulp is 2-3. The acidic environment can protect the viscosity of the pulp and inhibit the production of harmful free radicals during ozone bleaching, thus improving the quality of pulp.
Due to the active chemical properties and strong oxidization of ozone, high temperature conditions will accelerate the decomposition of ozone. This will reduce the efficiency of oxidative degradation of lignin by ozone. Ozone bleaching effect is better at low temperature (0-10℃). Therefore, ozone bleaching is more suitable for use at low or normal temperature.
The reaction time of ozone bleaching is very short. When in full contact with the pulp, the effect of ozone bleaching can be fully shown in just 5-10 minutes. This can increase the utilization of ozone. In addition, too long bleaching time will also lead to the decline of the viscosity of the pulp.
In order to improve the selectivity of bleaching and avoid the decrease of the viscosity of the pulp, organic compounds such as acetic acid, oxalic acid, methanol and dimethylformamide are usually added as additives for the chemical reactions. The addition of these additives is beneficial to reduce the degradation of carbohydrate, which will significantly improve the whiteness of the pulp after the bleaching process.
Characteristics of Ozone Bleaching
The use of ozone shows that this technique is suitable for bleaching pulps containing different types of sulfates. Moreover, compared with traditional bleaching methods, ozone bleaching will be better for its low temperature and short reaction time required. In summary, the characteristics of ozone bleaching can be summarized as follows:
- The use of ozone bleaching method can save about 1/3 of the water for bleaching than traditional pulp bleaching methods. With less water being used for bleaching, there will also be less waste water.
- Ozone bleaching is a chlorine-free bleaching method. This method is safe and reliable, and can eliminate AOX and COD in wastewater.
- Ozone bleaching is also more efficient and stable. The reaction time between ozone and pulp will only be about a minute. The degree of whiteness of the pulp can easily reach 90% or higher. It will also avoid the yellowing of the pulp.
- Ozone is about 1000 times more reactive with lignin than with carbohydrates. Therefore, ozone is highly selective for the degradation of lignin.
- Since ozone bleaching can be carried out at room temperature, high temperature conditions are not required for the ozone bleaching process. This will effectively save steam and its economic costs.
As an isotope of oxygen, ozone has been widely applied in our lives because of its strong oxidizing property. In our previous blogs, we have talked about the great role ozone plays in removing of odors from kitchen, bathroom, furniture, etc. Ozone has also got its position in the water treatment field. For aquariums, dairy farms and bottled water treatment, ozone is helping a lot to keep us healthy by ensuring excellent water quality.