The wet-end additive system plays a crucial role in the operation of paper machines, ensuring high efficiency and high-quality paper production. Without a good wet-end additive system, even the most advanced paper machines cannot reach their full operational potential, especially for machines using recycled fibers and straw pulp. In such cases, the lack of a well-designed wet-end additive system can lead to significant production problems.

The Role of Wet-End Additives

A well-functioning wet-end additive system ensures that additives are added to the pulp system in a stable and continuous manner, resulting in high-quality paper and optimal paper machine performance. The selection of the additive addition point is critical to the effectiveness of wet-end additives. Once an addition point is chosen, it is rarely changed due to the significant economic risks involved. Incorrect placement of the addition point can lead to excessive additive usage, failure to perform as expected, and potential risks that are difficult to predict.

Factors Influencing Additive Addition Point Selection

Several factors influence the optimal selection of the additive addition point:

  1. Pulp Characteristics

    • Composition: The composition of pulp is complex, and the properties of the pulp change with the types and proportions of wet-end additives used. Fine fibers and fillers have a strong adsorption capacity for wet-end additives (such as sizing agents like AKD, ASA, dyes, etc.). During the papermaking process, a significant portion of the additives are adsorbed by fibers, fine fibers, and fillers. These components retain the additives in the paper sheet during the dewatering process, allowing them to perform their function. However, the retention of fine components in the wet-end is low, so the issue of retention must be addressed when applying wet-end additives.
    • Charge Properties: The charge properties of the pulp are important for good retention of additives in the wet-end. The pulp must maintain a certain level of negative charge during production. However, excessive negative charge can have a negative impact on retention and dewatering, as anionic contaminants (such as high anionic trash content) can hinder water drainage.

  2. Pulp Concentration

    • Before dilution with white water, the pulp concentration in the paper machine’s flow system is relatively high (about 3%), while after dilution, the pulp concentration is around 1%. Adding wet-end additives to concentrated pulp allows for faster adsorption onto fibers, extending the contact time between additives and pulp, thereby increasing retention.

  3. Pulp Shear Force

    • Shear force in the pulp comes from the pump-induced shear, the shear caused by the flow of pulp, and the shear during dewatering. Additionally, some paper machines have been upgraded to improve speed, and screens have been replaced with slot screens to achieve better purification, significantly increasing the shear force on the pulp. The shear force impacts the retention and filtration efficiency in the wet-end, and it is crucial to consider its effect when selecting retention and filtration systems. Methods like increasing the inlet concentration and using high molecular weight polymers as retention agents are commonly employed to reduce shear force.

Additive Addition Methods and Equipment

The proper addition method is key to ensuring the effectiveness of wet-end additives. Once added to the system, the additive should quickly mix with the fibers and maintain stable concentrations and flow rates. Some key points include:

  1. Addition Point: Wet-end additives are generally added at the elbow of the pipe, where strong turbulence ensures efficient mixing with the pulp. Traditional methods involve using a pump to directly add the dissolved additive into the pulp pipe, with an online dilution system using water.
  2. Modern Addition Systems: Advanced systems use a pump to divide the extracted pulp into two parts—one part mixes with the chemical additives, and the other part provides strong jet action to inject the well-mixed chemical additives into the pulp pipe. This ensures that additives reach the center of the pipe quickly, and uniform mixing is achieved by injecting at multiple points along the same position.
  3. Additive Equipment: Common equipment for wet-end additive addition includes screw pumps and metering pumps. Screw pumps are ideal for additives that need to be continuously and steadily added to the paper machine system. They provide stable flow, strong self-priming capabilities, and low maintenance requirements. Metering pumps are generally used for additives that can tolerate some pulse action, such as defoamers, biocides, and dyes.

Common Additive Addition Points

  • Dyes: To improve dye retention, the addition point should be set before the stock chest. The closer the addition point is to the stock chest, the higher the pulp cleanliness and the lower the interference from other additives, thus improving dye retention. Additionally, setting the addition point after the trash screen can reduce the impact of the screen’s circulation, reducing dye usage.
  • Fillers: When fillers are added to the stock chest or mixing chest, they are often lost through the trash screen, which decreases the screen’s efficiency. Adding fillers after the trash screen has less impact on sizing, as fillers do not pass through the screen, reducing losses and wear on the screen. However, when fillers are added after the trash screen, the pulp’s cleanliness may not be well-protected.
  • Sizing Agents: For AKD sizing agents (which hydrolyze slowly), the addition point should be before the stock chest and added to the concentrated pulp system. For ASA sizing agents, which are more reactive, the addition point is generally selected between the paper machine screen and the trash screen, to reduce the residence time of ASA in the system and minimize its hydrolysis impact.

Conclusion

Selecting the appropriate addition point for wet-end additives is a complex and crucial process. Factors such as pulp type, beating degree, pH, and additive type must all be considered. Proper addition points ensure the best performance of the additives, improve paper quality, and optimize paper machine efficiency. As paper machines become faster and product requirements become more diversified, optimizing wet-end additive systems will play an increasingly important role. The future of additive research will focus on improving economic efficiency, performance, and environmental sustainability.