Washing and Screening


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

The washing process is designed to separate the black liquor from the pulp fibers as efficiently as possible with a minimum of dilution of the liquor. The equipment used can be wash filters, wash presses, continuous diffusers etc.

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

Click to enlarge image.


Screening systems removes uncooked chips, large knots, shives, fiber bundles, sand and stones.

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

In most screening systems, the “rejects” are screened in a secondary screen while the “accepts” are re-circulated to the primary screen. Two screens connected in this way are said 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.

Examples of NAF valves used in this process

  • NAF Duball DL full bore ball valve – Pulp line applications, reject lines, drum wash filter valves, cooling water to spray tower, sealing water, knot washer junk trap
  • NAF Trunnball DL full bore ball valve – Pulp line applications, buffer filtrate tank return
  • NAF Setball V-port ball sector valve, MC-pulp version – Washer feed
  • NAF Setball V-port ball sector valve- Black liquor washing, cleaner feed, knotter feed, reject valves, filtrate circulation
  • NAF Torex triple offset butterfly valve – Effluent with fibres to tanks and bypass, vacuum tank shutoff, screening feed valves, knots to cooking dilution, dilution pump valves, filtrate pump valves
  • NAF Turnex pneumatic actuator- For all control and on/off service on NAF rotary valve
  • NAF Duball DL Pocket ball valve – Sand separator dump valve
  • NAF Duball DL Ceramic lined control ball valve- Concentrated magnesium sulphate
  • NAF Check- general service check valve service for pulp, steam, black liquor lines
  • NAF Triball- drain valves, sampling valves