The Critical Importance of Refiner Plate
Refining refers to the process in which coarse fibers are broken down through the shearing action of the toothed discs within a refiner plate. Fiber bundles aligned parallel to the disc teeth are separated into individual fibers, whereas those oriented perpendicular to the teeth are ground into fragments during the refining operation.
In the pulp and paper industry, the refiner plate is one of the most critical components within the refiner. It directly determines the fibrillation, brooming, and cutting of fibers, as well as the ultimate properties of the paper.
The objective of refining is to balance fiber fibrillation and fiber cutting.
During the refining process, different types of pulp exhibit significant variations in their responses to beating. Typically, long-fiber pulps undergo fibrillation more readily and demonstrate greater tolerance to refining intensity, whereas short-fiber pulps require a gentler refining approach to prevent excessive fiber cutting.
Pulling In & Pump Out
In modern refiner plate design, central conveying and peripheral refining have become key characteristics of the plates. Consequently, it is essential to understand their intake and pumping capabilities.
Pull-In Capacity in Refiner Plates
When the refiner plates rotate at high speed, a low-pressure zone forms in the central region, while a high-pressure zone develops at the periphery. Under the influence of this centrifugal force, the pulp will continuously be drawn into the refining zone.
Typically, the angle of the refiner plate teeth is one of the key factors influencing this process. If the inlet angle of its teeth is acute, the relative velocity difference increases, thereby enhancing the pulp drawn rate into the refining zone. For instance, a 12° tooth angle cutting fine refiner plate exhibits a pulp intake volume that is 25% higher than that of conventional tooth profiles.
The tooth design is also one of the key factors affecting the magnitude of the pulling force. During the free-flow pulping of newsprint, long fibers must be drawn in rapidly to prevent flocculation. Therefore, a serrated tooth design on the refiner plate is required to reinforce the inlet guide channels and enhance the pulling force.
If the pulling force is too weak, the pulp will not enter the grinding zone stably, resulting in empty grinding. This not only affects pulp concentration and quality but also causes abnormal wear on the refiner plate.
Pump-Out Capacity in Refiner Plates
Following treatment within the refining zone, the pulp slurry is discharged from the grooves of the refiner plates via the pump-out effect. The efficiency of this process directly influences the fibers’ retention time and degree of submersion.
The selection of an appropriate pump-out efficiency must be carefully tailored to the specific type of paper being produced and the raw pulp materials being processed. For instance, the viscous refining of kraft paper requires a suppression of pump-out efficiency to extend the time available for fibrillation; this necessitates the use of refiner plate designs featuring narrow and shallow grooves.
Conversely, an excessively high pump-out efficiency results in insufficient fiber retention time, leading to inadequate fibrillation and a consequent reduction in overall refining effectiveness.
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The Influential Refiner Plate Parameters
Achieving a proper balance between the refining plates’ pull-in pump-out is a crucial prerequisite for realizing efficient fine refining and ensuring stable paper quality.
Varying structural parameters of the refiner plates directly influence the flow dynamics of the pulp within the refining zone, the residence time of the fibers, and the shear forces to which the fibers are subjected.
Bar Height
Tooth height directly influences the effective volume of the refiner plate grooves as well as the pulp conveying capacity.
Generally, a greater tooth height enhances pulp conveying capacity, improves the suction efficiency of the refiner plates, and enhances the flow stability of long-fiber pulp within the refining zone. Consequently, a high tooth height design is typically better suited for long-fiber pulps or high-consistency refining processes.
However, an excessively high tooth height can also cause the pulp to flow through too rapidly, thereby reducing the residence time of the fibers within the refining zone and diminishing the effectiveness of fiber fibrillation. For short-fiber pulps or low-intensity fine-refining processes, a relatively lower tooth height is typically employed to regulate pumping capacity and improve the uniformity of fiber treatment.
Bar Surface Geometry
The geometry of refiner plate teeth is typically categorized into two structural types: curved teeth and straight teeth.
Curved teeth facilitate a smoother flow transition as the pulp enters the refining zone, thereby mitigating the instantaneous impact forces exerted upon the fibers. This design is generally better suited for fine-refining processes. For instance, the refiner plate for tissue paper requires gentle fiber treatment.
In contrast, straight teeth prioritize the rapid conveyance and direct discharge of the pulp. They are therefore suitable for fine-refining applications involving low freeness levels or those requiring high pumping efficiency.
Surface Roughness
The surface roughness of the refiner plate material also influences the flow behavior of the pulp within the refining zone.
Rougher tooth surfaces enhance the friction between the fibers and the refiner plates. It improves fiber capture and traction capabilities and facilitates more effective fiber fibrillation.
Conversely, smoother tooth surfaces promote the smooth discharge of pulp, minimizing fiber hang-up and accumulation within the grooves. This reduces the risk of clogging and improves the stability of the pulp flow.