The purpose of pulp washing is to separate the waste liquid from the pulp, so as to facilitate the post-processing of the slurry and the treatment of the waste liquid. The washing of pulp is also called the extraction of waste liquid, but the focus is different, the main purpose of washing pulp is to clean the pulp, and the extraction of waste liquid is to extract high temperature and high concentration of waste liquid under the premise of using as little water as possible, in order to facilitate the recovery or comprehensive utilization of chemicals.