Process Stages in Pulping and Papermaking and Their Functions

Pulp and Paper Manufacturing Process: Main Stages and Functions

The pulp and paper manufacturing process is relatively complex, but it can generally be divided into two main sections: Pulping and Papermaking.

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I. Pulping Process and Stage Functions

The objective of pulping is to separate cellulose fibers from raw materials and obtain pulp suitable for papermaking. Common raw materials include wood, grasses, and waste paper.

1.Raw Material Preparation

• Operations: debarking, chipping, washing.
• Function: Ensure uniform raw material size and remove sand, dirt, and other impurities.

2.Cooking / Chemical Treatment

• Methods: chemical processes such as Kraft (sulfate) or sulfite pulping, or mechanical pulping.
• Function: Dissolve lignin and release cellulose fibers.

3.Washing and Screening

• Process: wash pulp with water and remove oversized particles through screening.
• Function: Improve pulp cleanliness and ensure paper quality.

4.Bleaching (Optional)

• Chemicals: oxygen, hydrogen peroxide, chlorine dioxide, etc.
• Function: Increase pulp brightness, suitable for producing cultural paper and printing paper.

5.Refining (Beating)

• Process: mechanical treatment of fibers in refiners.
• Function: Improve fiber flexibility and bonding ability, enhancing paper strength.

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II. Papermaking Process and Stage Functions

After pulping, the fiber slurry is further processed into finished paper.

1.Stock Preparation and Mixing

• Process: pulp is mixed with fillers (e.g., calcium carbonate, talc) and additives (sizing agents, strength agents, dyes, etc.).
• Function: Improve the physical properties of paper, such as strength, stiffness, smoothness, water resistance, and color.

2.Forming

• Process: slurry is fed into the forming section (Fourdrinier or cylinder machine), fibers deposit on the wire, and water drains out to form a wet web.
• Function: Establish the initial paper sheet structure.

3.Pressing

• Process: the wet web passes through press rolls to squeeze out water.
• Function: Reduce water content and increase sheet density.

4.Drying

• Process: the paper web passes over a series of steam-heated drying cylinders.
• Function: Reduce moisture content to the required level (typically 5–8%).

5.Calendering

• Process: the paper passes through calender rolls.
• Function: Improve surface smoothness, gloss, and thickness uniformity.

6.Reeling / Cutting

• Process: the dried paper is wound into large rolls or cut into sheets according to specifications.
• Function: Produce the final product, ready for packaging and use.

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III. Summary

• Pulping process: Raw material preparation → Cooking → Washing & Screening → Bleaching → Refining.
• Papermaking process: Stock preparation → Forming → Pressing → Drying → Calendering → Reeling / Cutting.

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