Pulp is washed at two junctures in the chemical pulping process. Brown Stock (pulp with residual lignin) is washed following the digester; and Bleached Stock is washed in the multi-stage bleaching unit. In each case, pulp moves through a series of washers and screens to remove residual chemicals and chips. Level controls usually monitor mass tanks and filtrate tanks in the washing units.
Precise level control of the two pulp washing lines ensures a consistent supply of wash water; maintains level stability; ensures correct dilution levels for the wash line; keeps filtrate flows in balance; and will lessen the chemical load and possible problems in downstream processing units caused by unwashed pulp.