Screening: Experimental Approach and Modelling, 1998 Pulping Conference Proceedings

An experimental pressure screen has been built for the study of the mechanisms of screening. The screen has been designed in such a way to facilitate the measurements and investigations about the effect of the most relevant parameters, and to simulate current screening technology on industrial scale. Extensive trials were performed on two types of fibres and contaminants. The experimental screen was equipped with a conventional foil rotor and slotted baskets with machined profiles, in centrifugal flow configuration.

The effects of pulp consistency (1 to 3 %), slot width (0.10 to 0.25 mm) and passing velocity (0.5 to 4 m/s) were investigated on a chemical pulp mixture with added flat shaped impurities. The latter parameters showed a large influence on the screening efficiency and fibre passage ratio while the effect of consistency was limited, except for the hydraulic screen load limits. A too large increase of the passing velocity reduced the efficiency and improved fibre passage, even more than increasing the slot width. However, the finest slots were clearly the most efficient for a given screening capacity, but the long fibres were rejected under the conditions tested. The relations between fibre passage ratio and fibre length were also investigated.

Additional screening trials were performed on a softwood TMP. The effects of consistency and passing velocity were investigated on the experimental screen equipped with 0.15 and 0.25 mm slots. As for the chemical pulp, the increase of the passing velocity improved fibre passage but reduced the efficiency in terms of shives removal. The mechanical fibres showed however to be more difficult to screen than the chemical fibres.

The analysis of the screening process shows that the rotor and screen plate designs should affect the screening results as well as the effects of the operating parameters. The possibilities to develop a new screening model based on particle passing probabilities are discussed.