EFFECT OF CONTROL VARIABLES AND LENGTH OF REFINING ZONE ON THE REFINING RESULT WITH A NEW PILOT REFINER

This article describes how the control variables used in a new pilot refiner affect the refining result. The new refiner treats a thin, evenly spread fiber network or individual fibers instead of fiber flocs. Due to the small dimensions of the refining zone the cutting length of the bars is great, which means that the refiner operates at low specific edge loads (< 2 J/m). The same refiner can be operated as a reference refiner when equipped with the tackle normally used in conventional low consistency (LC) refiners. In the new pilot refiner the only obvious effect that the specific edge load (SEL) had on the fundamental fiber properties was to increase the shrinkage potential of the fibers as SEL increased. A feature of the pilot refiner when operated with the new refining principle was that it developed a very good tear strength compared with the Escher Wyss (EW) refiner and the pilot refiner equipped with conventional low-consistency refiner tackle. An increase in pulp consistency in the new pilot refiner had no effect on paper properties, but it reduced the energy efficiency of refining. The effect of pulp volume flow on paper properties was dependent on disk geometry. With the shallow, straigh-bar tackle "A", an increase in the volume flow had no effect on the development of paper properties, although it did improve the energy efficiency of refining. With the shallow, curved-bar tackle "B", increasing the pulp volume flow caused the tear strength to fall but had no effect on the energy efficiency of refining. With the new refining principle, only when the rotor rotated in the pumping direction was flow through the refiner guaranteed. The speed of rotation seemed to be of minor importance with the new refining principle, if the peripheral speed was high enough. When the same refiner was operated with a typical low-consistency refiner tackle, increasing the rotor speed made the refining treatment more gentle. With this conventional disk type "C", the anti-pumping direction of rotation was better with respect to paper properties, but at the same time energy consumption increased. The length of the refining zone was significant, especially in terms of energy efficiency, when the pilot refiner was operated according to the new refining principle. For the new refiner to operate efficienctly, the refiner gap size had to be around the same thickness as a swollen fiber (20-50-mu-m) while the refining pressure had to be in the range 700-1,900 kPa (sofwood kraft pulp). In those cases where reducing the refining zone length improved the energy efficiency, the refining pressure and gap size were adjusted to with in these limits. In general, when the shallow curved-bar tackle "B" was used, it was difficult to maintain an even fiber film between the disks. This led to high local refining pressures. This, in turn, resulted in fiber crushing, as reflected by the lower tear strength values in comparison with refining with shallow straight-bar tackle "A".