Influence of the history of loading during beating on the evolution of the drainage resistance (SR)

Abstract This paper concerns the different refining loading conditions of a bleached softwood Kraft pulp on a Valley beater. The refining trials were performed in two stages; each stage was characterized by a constant normal force applied on the pulp in the gap clearance. In the first stage, the pulp was refined from an initial to a given pulp slowness (SR-45), with a force F_n1. Then, the refining was continued from the intermediate (SR-45) to the final pulp slowness with the force F_n2. By analogy with the pressing operation, we propose to introduce the concept of refining impulse defined as the product of the applied normal force by the duration time. This quantity is equal to the ratio of the refining energy to the peripheral speed. The refining intensity is proportional to the refining impulse divided by the number of crossing points. It was demonstrated that the pulp slowness evolution (SR) may be plotted as a master curve of the refining impulse. Consequently, the successive SR evolutions are only dependent on the refining impulse. It has to be noted that this quantity might be completed by the normal force itself to account for the prediction of the shortening of fibres.