OCC – Fine Screening
OCC plants convert recycled old corrugated containers into pulp that can be used on paper and board machines to make valuable pulp and paper products. After repulping, the OCC pulp contains some contaminants, such as staples, glass debris, and sand, as well as fiber flakes and bundles that were not separated into individual fibers during repulping. To remove these undesirable materials, the pulp slurry goes through one or more screening processes to remove the contaminants that are larger than fibers. This course describes the second, “fine screening” process used to remove the smaller, more flexible contaminants from the OCC pulp.
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Learning Objectives
• Explain the purpose of fine screening
• Explain why centrifugal cleaners are often used ahead of fine screens
• Describe why fine screening systems often use multiple stages
• Identify and describe major components of a cylindrical fine pressure screen
• Describe the process flows for fine screening
• List two important geometries for bar-type screen baskets
• List safety hazards and guidelines associated with fine screening systems
Specs
| Course Level | Intermediate |
| Languages | English, Portuguese |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
What is the goal of fine screening in an OCC facility?
Fine screening follows coarse screening and is intended to remove contaminants that were allowed by the coarse screens but are still larger than the fibers.
What is the purpose of the rotor in a cylindrical screen?
Shaped elements on a spinning rotor in a cylindrical screen create pressure pulses which assist with fiber flow through the screen and help prevent screen plugging.
What process variables can be used to determine if a screen is plugged?
If the pressure differential between the inlet to the screen and the screen accepts stream increases, there is a good chance that the screen is plugging.
What are some differences between coarse screens and fine screens?
Fine screens have smaller openings their screen baskets and operate at lower feed consistencies than coarse screens.
Why is the design of the final fine screen in a multi-stage screening system often a different design?
The feed stream to the final stage has a high concentration of contaminants which needs to be handled by a screen with a different physical configuration.
Sample Video Transcript
The screen basket and rotor designs are not easily changed. But the clearance between the basket, and rotor should be checked regularly, and adjustments made if necessary. Suppliers can help with equipment design modifications and maintenance, and also provide recommendations regarding feed consistencies, pressures, and flows. During operation, the feed consistency, feed pressure, and accepts pressure are typically monitored. If the pressure drop between the feed and accepts is too high this can indicate a problem with plugging. The screen can undergo a flush cycle to clear the openings. If that doesn’t work the screen will need to be shut down and opened up.
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