Pulp leaving the digester wash unit retains a dark brown color due to residual lignin content that must often be bleached out. Bleach plants whiten pulp through three to five stages of bleaching and water washing. Typically, two pairs of chlorine dioxide and caustic extraction towers are followed by pulp washing stages.
Bleaching operations require level controls to maintain consistent levels in the bleach towers and manage pulp flow to successive stages by controlling tower outlet valves. Application challenges include variable pulp density, temperatures over +200 °F (+95 °C), harsh chemicals, and the need for easy cleaning of wetted parts. A point level switch ensures overfill protection.
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