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In the production of packaging paper (primarily boxboard, corrugated base paper, grey-based white board paper, etc.), it is often necessary to classify OCC raw materials. Common classification types include long fiber + short fiber or long fiber + medium fiber + short fiber. For some papermaking practitioners, questions may arise: Why is it necessary to classify OCC raw materials? What are the differences in processing methods for different grades of pulp slurry? What are the distinctions in the pulp mixing section of the paper machine?
The primary equipment used for fiber classification is the "classification screen." This device represents a new generation of pulp fiber classification screening equipment developed by integrating advanced foreign technology with domestic production practices. It is primarily applied in the pulping process to separate long and short fibers for individual treatment during production. Its advantages in waste paper pulping applications are particularly notable.
The working principle of the fiber grading screen involves continuous pressure screening in a closed state. Pressurized pulp enters the screen body tangentially through an inlet pipe at the lower part of the shell, moving from bottom to top. Under centrifugal force, heavy impurities are discharged downward along the inner wall of the screen into the heavy collection tank located in the lower shell (intermittent slag discharge). The slurry then enters the screen drum in the upper shell. Under the action of the rotor, short fibers pass through the screen gaps and are discharged via the slurry outlet pipe in the middle of the shell. Long fibers, unable to pass through the screen gaps, continue rising, enter the top of the screen, and are discharged via the long fiber slurry outlet pipe at the top. Light impurities rise continuously inside the screen drum under centrifugal force and are discharged from the top alongside the long fibers without entering the area between the rotor and the screen drum.
As a result, the pulp after fiber classification is divided into three parts:
1. Heavy impurities deposited in the heavy impurity collection tank at the bottom;
2. Short fibers passing through the tiny gaps of the sieve drum;
3. Long fibers and light impurities that do not pass through the fine gaps of the sieve drum.
Common screen slit sizes and their corresponding classification ratios for long and short fibers are as follows:
| Sieve Seam Size (mm) | Proportion of Long Fibers (%) | Proportion of Short Fibers (%) |
|
0.15 |
60 |
40 |
|
0.20 |
50 |
50 |
|
0.25 |
45 |
55 |
|
0.30 |
40 |
60 |
The purpose and function of the fiber grading screen include:
1. Extracting higher-strength long fibers from waste paper raw materials with limited strength;
2. Processing long and short fibers separately, allowing short fibers to bypass thermal dispersion or disc grinding processes, thereby reducing pulping energy consumption and equipment investment;
3. Obtaining cleaner short fibers through grading, optimizing purification and screening efficiency while ensuring stable paper machine operation (mainly addressing adhesive material issues);
4. Achieving finer fiber subdivisions (e.g., long lower fibers, medium fibers, and short fibers) through one or two grading stages, providing a rational basis for pulp mix optimization in multi-layer cardboard manufacturing;
5. Avoiding excessive cutting of some fibers or insufficient processing cleanliness by treating long and short fibers separately (following the long fiber processing procedure would fragment medium and short fibers excessively, while following the short fiber procedure would compromise adhesive removal effectiveness, impacting paper machine operation and increasing black spot degradation in finished paper);
6. Enhancing the addition and utilization efficiency of chemicals (e.g., precise addition of adhesive control agents to the pulp pool before long fiber grinding).
Common application problems and cases related to grading screens include:
1. Classifier screen selection is typically determined during paper machine design. However, optimizing the proportion of each layer's pulp in actual production is common. Therefore, reserving design capacity for subsequent classifier screen processing (e.g., thermal dispersion, white water multi-tray, and desizing) is crucial. Process design should consider the maximum pulping equipment capacity to avoid additional technical modifications caused by changes in classification ratios and increased total pulping capacity during future operations.
2. Excessive long-fiber classification can lead to increased processing energy consumption, deeper fiber cutting and fragmentation in the medium and short ranges, web section dewatering difficulties, increased steam consumption, and intensified paper layering and bubbling tendencies.
3. Excessive short-fiber classification (larger screen gap size) introduces more small adhesive substances into the short fibers. These substances significantly impact paper machine operation, causing surface defects when adhered externally and operational issues when adhered internally.
The discussion regarding the impact of long-fiber surface coating on paper folding endurance has a long history. Theoretically, long-fiber surface coating should enhance folding endurance, evident in high-strength multi-layer paperboard (e.g., liquid-coated base paper), but this effect is often difficult to demonstrate in low-strength multi-layer paperboard (e.g., boxboard). Consequently, more mills prefer short-fiber surface coating due to its cleanliness. From the perspective of optimizing low-strength paper folding endurance, the following strategies yield better results:
- Dot optimization (the formed plate must achieve sufficient pre-dewatering);
- For multi-layer cardboard, the surface layer/surface layer slurry web velocity ratio should be controlled at 1.01 or higher;
- Increasing sizing agent penetration enhances folding endurance (moisture content before sizing agent should not fall below 8.5%). The glue boiling temperature should not be lower than 95°C, and the viscosity of the rubber compound at the application point should be reduced.
