Pulp washing processing is the essential process in paper pulp production. Pulp washing is the separation of the slurry and black liquor, so that washed out alkali lignin and other impurities from the reaction in the cooking processing, and purify the pulp, which is helpful for filtration and bleaching. Extract waste liquor with higher concentration and temperature while washing pulp thoroughly, and sent to the wastewater disposals for recovering the organic or inorganic matter in the waste liquid in the acid and alkali recovery process. So the pulp is suitable for the next processing. As for the way of pulp washing processing, including single-stage washing, multi-stage washing, and multi-stage countercurrent washing. Multi-stage countercurrent washing is the ideal way.
About 50% of the raw material of the plant fiber is dissolved in the cooking liquor to form pulping waste liquor. Unwashed pulp, most of the waste liquid exists in the free state of the flow space between the fibers (about 70-80% of the total amount of liquid). Some waste liquor exists in the channel of fiber cell (About 15-20% of the total amount of liquid). Only small part of the waste liquor exists in the hole of fiber wall (about 5% of the total amount of liquid). Therefore, pulp washing is actually a process of filtration, diffusion, and extrusion. Filtration and diffusion are two inseparable parts.
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