In the kraft chemical pulping process, wood chips are "cooked" in a mixture of alkaline chemicals in large pressure vessels called digesters. Due to irregularities in the wood, and contaminants in the incoming wood, not all of the material coming from the digester is suitable for papermaking. Brown stock screening is that part of the pulping process where oversized and contaminant particles are separated from good papermaking fibers. Pulp exiting a kraft digester is known as brown stock. Unwanted material, or debris, is generally removed in two steps, deknotting and screening.
Learning Objectives
At the end of this module, you will be able to:
- Identify the contaminants in unscreened brown stock
- Describe the purpose and theory of knotting and screening
- Identify the major components of a screen/ pressure screen
- Identify the major components of a knotter
- Describe the screening process
- Describe the benefits of a cascaded screening system
- Identify factors which affect screening performance
- Identify and describe the flows in a screen and knotter