PULP PROCESSING
 
I. GENERAL OVERVIEW. Pulps are exposed to numerous processing steps, all of which depend on their method of preparation. The processing parameters involved in mechanical and semi-mechanical pulping will differ significantly from that of sulfite or kraft pulping. However, screening, thickening, and storage operations are necessary in all pulping processes, and cleaning is usually required where appearances are important (such as the sheet that this is written on). Defibering is required for all semimechanical and high yield chemical grade pulps. Deknotting is usually necessary in the production of clean bleachable chemical pulps. All of the aforementioned processes require pumps to move the pulp around. These processes fall under the general category of pulp processing and are mentioned in outline form below.
 
II. DEFIBERING.
  1. High-yield chemical/semichemical pulps require defibering.
  2. Similar to chip refining (same general refiners used).
  3. Energy required depends on the pulp yield.
  4. Works best when liquor is still present.
 
III. DEKNOTTING.
  1. All chemical pulps have residual unpulped knots (reaction wood).
  2. Pressure screen knotter. Requires pressure/vacuum pulses and a secondary screen.
 
IV. BROWN STOCK WASHING.
  1. Remove residual liquor from pulp (would contaminate further operations).
  2. Recover maximum amount of chemicals with minimum dilution.
  3. Standard Method: Counter-current rotary vacuum washers. This consists of a round cylinder under vacuum. The pulp adheres to cloth-covered drum and is washed with water. It is about 80% efficient, and therefore about 3-4 stages are needed for 99% removal. Foaming can be a problem here.
  4. Newer methods. Diffusion washing has a long contact period between cooked chips and moving wash liquor allowing diffusion to occur. Trapped air is avoided which limits foam formation and the unit can be place at the end of a continuous digester (high temperature means good diffusion). Pressure washers are alot like vacuum washers. However, they are more efficient than as dewatering accomplished by pressure. Finally belt washers look alot like a fourdriner paper machine with vacuum underneath and a water wash on top.
 
Relative Efficiencies of Various Washing Equipment
Equipment Efficiency
Vacuum Drum Dryer 2.5--4
Single-stage diffuser 3-5
Kamyr chip digester
---1.5 hr retention
---3 hr retention

 

4--6

7--11

 
V. SCREENING.
  1. Remove over-sized materials (smaller than knots).
  2. Based on a perforated barrier.
  3. Pressure screen (basic principle of knotter) is the most common screening method. The inlet feed is exposed to centrifugal (outward) or centripetal (inward) flow. In combination with a cylindrical perforated plate and a rotating hydrofoil, for the centrifugal-based screen, the radial force puts smaller stuff through screen. The hydrofoil breaks up the mat so the unit doesn't clog up. Good removal efficiencies requires a high reject rate so a cascade arrangement is typically used.
 
VI. CENTRIFUGAL CLEANING.
  1. Liquid cyclone, hydrocyclone, vortex cleaner, centricleaner.
  2. Removes contaminants by centrifugal force and fluid shear.
  3. Pump is the power source.
  4. Pulp suspension is introduced radially. Centrifugal force puts dirt to outside (higher density than fibers). As diameter narrows the flow is forced inward increasing the centrifugal force. The less dense fibers (accepts) rise and the dirt (rejects) go down.
  5. Plugging is a problem.
  6. Typically a cascade design.
 
VII. THICKENING.
  1. A. Previous processes were low consistency (<1%).
  2. B. Effective pulp movement is typically at 4-8%.
  3. C. Several processes available. The decker is most common.
 
Thickening Equipment Capabilities
Equipment Discharge Consistency (%)
Slusher 3.5--4
Decker (gravity thickener) 4--8
Vacuum filter 12-15
Multidisc filter 10--12
Screw extractor >20
Various press designs >20
 
VIII. STOCK PUMPING AND HANDLING
  1. A. Standard centrifugal pumps are good up 6-7% consistency.
  2. B. Modified centrifugal pumps for higher consistency pumping are of limited use.
  3. C. Medium Consistency Pumping Technology (8-15%). High shear forces cause pulps to fluidize and act as Newtonian fluids. Air separation and discharge is essential for operation (ie., cavitation). This technology can be applied to pressure screening (mechanical pulps).
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