Common pulp washing fundamentals – How and why do we wash pulp?

Feb 8, 2022

Although washer designs may differ, each type of washer functions to wash pulp using common fundamental principles. Washing of pulp is a critical unit operation in the fiberline design that has great influence on the economic
profitability of a mill. For this reason, it is important to understand the fundamentals of pulp washing to identify and resolve issues in existing installations or select the best alternatives for future improvements or new installations.

Why do we wash pulp?

The purpose of pulp washing in the brown stock area is to recover the maximum amount of cooking chemicals and dissolved wood components with the least amount of fresh water, limiting steam for evaporation of weak liquor to recovery.

Brownstock washing and the liquor cycle

The diagram is a simplified interpretation of the brown stock washing system and liquor cycle with the balance of the mill.

The purpose of pulp washing in the bleach plant is to make bleaching easier and more effective by removing the organic and soluble material left in the pulp between bleaching stages.

Good washing can reduce the cost of chemicals and steam and reduce the volume of bleach effluent that must be treated if washers can achieve high discharge efficiency and operate with lower shower flows.

How do we wash pulp?

Cooked brown stock pulp from the digester is composed of chemically separated, suspended wood fibers and black liquor containing the spent cooking chemicals to dissolve wood products in water.

Brown stock washing separates and concentrates the dissolved solids in the black liquor from suspended fibers and delivers washed pulp either to the bleach plant in the bleach mill or to the machine in the brown mill. Black liquor from washing is processed in the recovery cycle, first in evaporators to further concentrate the liquor, then in the recovery boiler to produce energy.

Green liquor, after the recovery boiler, is regenerated as white liquor for reuse in cooking. The recovery cycle includes makeup chemicals that are lost or carried forward from the washing system. Pulp and liquor character from the digester directly affect the operation and efficiency of the brown stock washing system. Similarly, the performance of brown stock washing system directly impacts the operation of the bleach plant. As liquor washed from the pulp is recovered, the brown stock washing directly affects the recovery area.

Rely on a legacy of trustRely on a legacy of trust

Valmet has long history of process knowledge, system design and equipment manufacturing for pulp washing. There are many washing devices with varying designs installed around the world that were manufactured directly by Valmet, or the legacy OEM companies purchased by Valmet, since the early 1950s.

Valmet has the OEM resources and expertise for legacy pulp washing brands such as Beloit, IMPCO, GL&V, Rauma and others.

For a deeper dive into the principles of pulp washing, watch our Principles of Pulp Washing webinar. Learn why Valmet is the world’s leading supplier of pulp washers.