The purpose of washing is to separate the pulp from black liquor to wash out the residual substances such as alkali-lignin produced in the cooking process, purify the pulp for the purpose of screening and bleaching, and separate the pulp from black liquor. At the same time, the pulping waste liquor with high concentration and temperature was put forward to the wastewater treatment, and the organic or inorganic substance in the waste liquor was recovered by acid and alkali recovery process.
The principle of washing: unwashed pulp, most of the waste liquid exists in the flow space between fibers (about 70-80% of the total amount of liquid), partly in the inner channel of the fiber cell (about 15-20% of the total fluid), and only a few in the interstitial space of the fiber wall (5%). Pulp washing is actually a process of filtration and diffusion. Filtration and diffusion are inseparable.