Washing and Screening

Washing

Pulp washing is the most common operation in pulp mill fibre lines.

This washing process is designed to separate black liquor from the pulp fibres as efficiently as possible with a minimum of dilution of the liquor. The equipment used in this operation can be wash filters, wash presses or continuous diffusers.

Washing also takes place in the high heat washing zone of a continuous digester, after oxygen delignification, as well as in the different bleaching stages.

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Screening

Screening systems remove uncooked chips, large knots, shives, fibre bundles, stones and sand.

Several different types of equipment can be used. These all work on different principles such as filtering, centrifugal forces or fluidised beds.

 

In most screening systems, the “rejects” are then screened in a secondary screen while the “accepts” are re-circulated to the primary screen. If two screens connected in this way, they are considered to work in cascade. The “rejects” from the secondary screen go to a reject screen. The knots are then taken back to the digester for re-cooking.

Some examples of NAF valves successfully used in this process

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