| Fractionation of thermomechanical pulp in pressure screening: An experimental study on the classification of fibres with slotted screen plates | ||
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The aim was to solve the above research problems by testing the following hypotheses. Hypotheses 1 and 2 refer to problem 1, hypothesis 3 to problem 2, and hypothesis 4 to problem 3.
Fines will be fractionated in accordance with the volumetric reject rate. The particle dimensions of the fines are so small that these will freely follow the flow of water and their numbers in a certain unit volume of low consistency suspension can be considered constant everywhere within the screen and in the feed, accept and reject streams. Thus the amounts of fines drifting into the accept and reject streams can be predicted from the volumetric reject rate.
Fibres will be fractionated in accordance with the mass reject rate. The passing probability of a certain length class of fibres is an unambiguous function of the flow conditions on the screen plate determined mainly by the feed consistency, slot velocity and foil tip speed. From this — given constant feed pulp properties — it follows that with a given screen plate geometry and volumetric reject rate any combination of operating parameters will produce a constant length distribution of fibres on the accept side provided that the mass reject rate remains unchanged. The hypothesis entails the assumption of either a constant coarseness of fibres or a coarseness that is directly related to fibre length, as the mass flow of fibres is determined by the fibre length distribution and the average coarseness in each length class, together with the number of fibres per unit volume and the volumetric flow rate.
The geometry of the screen plate affects fibre fractionation, but it does not change the fractionation mechanism as such. The slot width and contouring of the screen plate will affect the passing probability of the fibres. A decrease in either the slot width or the height of contouring will increase pulp fractionation as compared with the situation at a constant volumetric and mass reject rate. There are no interactions between the screen plate geometry and the operating parameters, however, that would change the fractionation mechanism so that hypotheses 1 and 2 would no longer be valid.
The mass and volumetric reject rates together explicitly determine the fractionation of pulp. As a consequence of the above hypotheses, the two commonly used basic parameters of pressure screening, the mass and volumetric reject rates, can be used to predict the fractionation effect for a given screen plate design. By combining the reject rates mathematically, fractionation can be expressed with a single parameter, the passage ratio.