I. Sources and characteristics of papermaking wastewater
There is a lot of water in the process of paper making. It takes about 300~500 m3, to produce 1 paper. In paper wastewater, it not only contains a large number of raw materials (about 20% of raw materials are lost with wastewater), but also contains a lot of chemicals and other impurities.
For the limited treatment of papermaking wastewater. First of all, we must understand the water quality of papermaking wastewater. There are three kinds of wastewater from alkaline papermaking:
1. cooking wood pulp (or straw pulp) generated waste liquid, also known as black liquid.
2. beating machine and pulping machine discharged wastewater, called beating wastewater.
3. paper machine wastewater, which can be directly used called white water.
The main pollutants contained in these wastewater are as follows:
1. suspended
Includes collapsible and non-collapsible suspensions, mainly fibers and fine fibers (i.e. broken fiber fragments and miscellaneous cells)
2. biodegradable organic compounds
Including low molecular weight hemicellulose, methanol, acetic acid, formic acid, sugar and so on.
3. refractory organic matter
It mainly comes from lignin and macromolecular carbohydrates in fiber raw materials.
4. toxic substances
Black liquor contains rosin acid and unsaturated fatty acids and so on.
5. acid-base poisons
The ph value of alkali pulping wastewater is 9~10, and that of acid pulping wastewater is 1.2~2.0.
6. chroma
The residual lignin in pulping wastewater is highly colored.
II. Recycling of papermaking wastewater
Because papermaking wastewater consists of three kinds of wastewater: black liquid, pulping machine wastewater and papermaking machine wastewater, its recovery and utilization is mainly aimed at these three kinds of wastewater.
- black liquor recycling
(1) Traditional alkali recovery (combustion)
The paper industry uses a large amount of alkali, each production of 1 t pulp needs 200~400 kg caustic soda. About 35% of the black liquor discharged after cooking is inorganic, the main components of which are free NaOH、Na2S、Na2SO4 and other sodium salts combined with organic matter. The purpose of alkali recovery is to convert these sodium salts into NaOH and Na2S recycling to reduce costs and reduce pollution to water bodies.
(2) Wet oxidation
Wet air oxidation refers to the process in which organic matter in wastewater is oxidized and decomposed at high temperature and high pressure, the degree of oxidation depends on the temperature and pressure used. This method is suitable for caustic soda black liquor.