4.1. Outline of the experimental work

The experimental work was carried out to test the hypotheses introduced in Chapter 1. The primary work is divided into four parts, which explore respectively the effects of the operation parameters and screen plate design on fractionation, the role of fibre coarseness in fractionation and the role of sampling in experimental research. Before the primary work, the applicability of the laboratory analyses for characterising fractionation is assessed in the preliminary part. The sections of the experimental work and their links with the hypotheses and other objectives of the study are summed up in Table 1.

The preliminary part of the work addresses the analytical characterisation of fractionation. There are a number of analytical methods available for characterising the fractionation result, but many of them are laborious and cannot be used as for routine purposes in extensive research work. In addition, their accuracy is greatly dependent on the analytical skill of the laboratory technician, and therefore the variation in the results mostly remains unknown. It was therefore found advisable to study the reliability of the analyses, the possibilities for simplifying the analytical procedures and the possibilities for both modelling and modifying the results. The characterisation methods which were explored and chosen for use in this work are presented in section 4.2.

The first part of the primary work (section 4.3) is concerned with the operating parameters. Pressure screening is highly dependent on the combination of operating parameters chosen. It is unclear, however, whether any combinations of operating parameters exist in which the screening mechanisms could be altered under varying screening conditions due to interactions between these parameters. Such interactions could arise from the strength of the fibre network, for example, internal macroflows, pulsation and turbulence or mixing conditions on the screen plate. In other words, it is a question of whether it is possible to obtain different fractionation effects at constant mass and volumetric reject rates, or whether the situation is as assumed in hypotheses 1 and 2 of this work. The programme of experiments was designed so that fractionation effects could be studied with a variety of combinations of operating parameters. The aim was to cover an extensive part of the operating range of pressure screening with the given screen configuration and screen plate design.

The second part of the primary work (section 4.4) considers screen plate design, as this has been reported to have a considerable influence on screening efficiency and fractionation. The effects of slot width and contouring are usually related both to capacity and efficiency. Crucial factors are the dimensions of the slot and particles and turbulence conditions on the screen plate. This part of the work examines the role of screen plate design — contouring and slot width — in fractionation, the aim being to ascertain possible interactions between screening parameters, especially between pulp properties, foil tip speed and contouring, which might affect the fractionation mechanism so that the mass and volumetric reject rates would no longer be able to explain the fractionation effect, in contrast to hypotheses 3 and 4. Another purpose was to evaluate freeness as a measurement of fractionation and to find an interrelationship between this and Bauer-McNett fractions. The experiments were designed so that the fractionation effects could be studied over a wide range of combinations of operating parameters with different screen plate designs.

The role of coarseness in fractionation is considered in the third part of the primary work (section 4.5). The aim is basically to test the assumption in hypothesis 2 that a coarseness change is interrelated with a change in fibre length. These experiments were performed mainly with mill screening units, accompanied by laboratory screening experiments. The programme was designed to be carried out with different screen plate designs at different pulp passage ratios.

The fourth part of the primary work (section 4.6) considers the reliability of experimental work in general, and particularly of the experiments performed here. Attempts have seldom been made to assess the reliability of sampling in connection with an experimental arrangement, but it was found important to do this in the present case so that the reliability of the screening results could be confirmed. Manual ball valves were employed for sampling in the first half of the experiments, after which they were replaced with semi-automatic piston valves. This made it possible to compare the two sampling methods by reference to the mass balance errors over the screen.