The Origin and Control of Pulp Stress During High-Consistency Refining

The trend toward higher-value added grades of paper demands the application of sufficient specific energy to produce pulp with well developed, flexible, and conformable fibres. Failure to do this results in fibre cutting and a pulp containing poorly developed material. Analysis of a possible mechanism by which the refiner and the pulp may interact has led to the prediction that, for a given pulp, stress in refining is dependent on both the density at a given point in the process and on the specific energy per impact, or intensity, at which the refiner is operating. The analysis suggests that as the pulp density increases with accumulating specific energy, the stress per impact rises at a given intensity, which may in practice limit additional energy application. INTRODUCTION AND THEORY The trend towards higher value added grades is leading to demands for higher mechanical pulp quality. In addition, for the Canadian industry, the problem with meeting such demands is compounded by the increasing need to use wood supplies which produce pulp of less than optimum quality such as jack pine. The issue is centered on the need to apply sufficient specific energy at the target freeness to obtain a pulp with long, well-developed, flexible, and conformable fibres that will contribute to the sheet’s strength without negatively affecting the surface roughness. Failure to achieve this, results in fibre cutting and pulp containing short, coarse material. The latter condition is likely to be associated with the generation of excessive levels of stress as the refiner attempts to apply the needed specific energy. The basis for the stress must lie in part with how the refiner actually works on the material, in conjunction with the properties of the material itself and of the fibres which comprise it. This report focuses specifically on the refining of mechanical pulp after the main fibre separation phase. Discussions of the mechanism for energy absorption in mechanical pulping based on experimental studies of the material properties, have to a large extent focused on the properties of solid, water-saturated wood, see e.g. Salmén and Fellers [1] and Höglund et al. [2]. This is not necessarily relevant for the large amount of energy for fibre development and fines production which is being applied to the pulp after the fibre separation. Pulp nodules or flocs at high consistency have much lower density than solid wood and a very different structure. Thus, the material properties would be expected to be different. It is well established that the refiner acts on the pulp with a combination of compressive and shear forces. It is also well established that the refiner works on nodules or flocs and not single fibres, shown by the fact that the refiner gap is larger than a single fibre. Figure 1 shows examples of the structure of pulp nodules. Compression of the pulp against the edges and the surfaces of the bars, implies a compaction and densification of the pulp, as well as rubbing of the fibres against each other, and compression, bending and shearing of the single fibres inside the nodule. The tangential shear component would provide a reorientation of the nodules and possibly, if the pulp sticks to the bar edges as suggested by Atack [3], a brushing action. As for compaction of porous, cellular materials, the stress rises sharply as the density approaches that of the wall material, in this case the density of the fibre wall and the moisture that was not pressed out during the impact [4]. Several approaches have been made to measuring the pressure or the force acting on the pulp in the refining zone of high consistency refiners [5–9]. This is a very difficult task, for several reasons, and the results vary widely. The investigations have not yet been carried to the level where the stresses can be accurately measured and studied as a function of refining conditions and wood properties. The analysis outlined in this paper offers a possible explanation for the mechanism of the energy application and the response of the pulp on an overall level. It is based on experimental observations and on the previously developed concept of refining intensity for high consistency refining [10,11]. The refining intensity provides an estimate of the average specific energy per impact from the overall balance of forces determining the pulp flow through the refiner, which is necessary in order to connect the properties of the pulp material to the refiner action. The analysis suggests how the properties of the ingoing pulp being refined may interact with the refiner, to affect the total energy absorption and the stress level at which it takes place. The objective is to develop a practical understanding in order to assist in optimization and troubleshooting of the refiner operation. Figure 1. Pulp nodules from black spruce TMP produced in Paprican’s pilot plant at 3011 kWh/odt. The samples were taken from the refiner without disintegration at about 20% consistency, freeze dried after solvent exchange to methanol and tributhyl alcohol and coated with gold. The microscope used was a scanning electron microscope (SEM), model JEOL JSM 840A. EXPERIMENTAL OBSERVATIONS AND IMPLICATIONS Experimental studies carried out on Paprican’s Bauer 400 atmospheric refiner revealed two interesting interactions between plate gap, pulp quality and plate dimensions: • At a given specific energy, the plate gap decreased with each successive stage of refining under otherwise similar conditions, as shown in Figure 2. Thus, for the same load on the refiner, the gap responded to changes in the properties of the material as it was being processed. • The presence of a filling in the grooves required a much lower plate gap to draw the same motor load and specific energy, as seen in Figure 3. The grooves in the plates were filled with a rubber compound so that the groove depth was greatly reduced, in a manner similarly observed in [12]. Figure 2. For a given application of specific energy, the plate gap decreased with each successive stage of refining. The pilot trials were done using an atmospheric Bauer 400 double disc refiner (36” diameter) at a rotational speed of 1200 rpm. The plate pattern used was Bauer 36104 (NiHard) and the target discharge consistency was 25%. This gives a refining intensity of 0.062 kWh/odt (0.223 kJ/kg) per impact. 0.0 0.2 0.4 0.6 0.8 1.0 0 50