Effect of bar edge conditions on fibre trapping in low consistency refining

In a low consistency refiner pulp fibres are trapped between the bar edges of the rotor and stator and worked into the narrow gap between rotor and stator. No refining occurs if the fibres are not trapped. A reduction in fibre trapping is believed to be in the common observation that refining becomes less efficient as the bars wear and lose their sharpness. This paper examines the effect of bar edge sharpness on relative fibre trapping in low consistency refining. The measurements were made on a bleached softwood kraft pulp at consistencies ranging from 1–6% using a conical laboratory refiner. Three sets of trials were done. One set of trials was done with a set fillings that had been worn in number of previous refining trials, while a second set of trials were done with a set of fillings where the bar edge had been sharpened by running the refiner with quartz sand. A third set of trials was done with fillings in which the bar edges had been manually rounded with abrasive paper. The results showed that the fraction of the refiner bar length that trapped fibres was approximately proportional to pulp consistency for all three trials, independent of bar edge sharpness. The reduction in fibre trapping, with lower consistency, increases the harshness of the refining on the fibres, at a given level of refining power. The results also showed that the artificially treated bars trapped generally had a lower fraction of the bar surface trapping fibres, which also increased the harshness of the refining process. . Introduction Low consistency refining is the principal treatment for improving the mechanical properties of paper made from chemical pulps. A pulp refiner changes fibre cell wall structure by trapping fibre mats between rotor and stator bars and mechanically treating the fibres by cyclic strain of high frequency. It is known that three separate forces are imposed by refiner bars on the fibres (1-3): a corner force exerted at the edges of the bars; a normal force compressing the fibre mat and a shear force generated by the bar surface moving over the compressed mat of fibres. The presence of fibres on refiner bars is a prerequisite for application of this old technology as only those fibres trapped between the bars are treated. Refiner action is usually characterised by the Specific Energy Consumption (SEC), which is the energy per unit mass of pulp and a measure of refining extent. The most commonly used measure of refining intensity is the Specific Edge Load (SEL)-the energy consumed when a unit length of rotor bar crosses over a unit length of stator bar (4). The refiner cannot apply forces on the fibres and change their structure if the fibres are not trapped between bar surfaces. No energy will be consumed where sections of rotor and stator bar cross unless a fibre mat is being trapped underneath. However, SEL does not consider the trapping of the fibres between the bars. SEL is an average value for a length of bar, independent of how much of the bar traps fibres. As an example of the importance of trapping, consider two refiners where the first refiner traps fibres on all of the length of the bar and the second traps fibres only along half the length of the bar. If the SEL is the same in both refiners, then same energy will be applied to only half the amount of fibres in the second refiner in comparison to the first and the treatment of the fibres in the second refiner will be twice as severe. The key bar parameters affecting fibre trapping are bar velocity, distance between opposite bars and bar edge wear or rounding (5-7) as well as the bar surface structure and possibly the fillings’ design. The shape of the leading bar edge is an important factor as this initially traps the fibres, forcing them to be draped across the moving edge. Both laboratory studies and mill practice have also indicated that new and sharp refiner bars shorten fibres to a greater extent (8, 9). It is