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A bleach plant is made up of many stages. Each stage is made up of four components: a pump to move the pulp, a mixer to blend the bleaching chemicals and sometimes steam into the pulp, a reaction or retention tower, which the pulp moves through slowly allowing time for the bleaching reactions to occur, and a washer to remove the bleaching chemicals and dissolved lignan from the pulp. Some stages operate at low consistency, 0 to 6% pulp, and some operate at medium consistency, 6% to 18% pulp. Some bleach towers are up flow towers, some are down flow, and some are both. The equipment and process conditions of each stage can vary considerably depending on the bleaching chemical and the goals of the stage.