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UBC Theses and Dissertations

Multistage fibre length fractionation of softwood chemical pulp using a pressure screen equipped with smooth-holed screen cylinder Mohamad, Masita


Often, fibre fractionation produce a higher value long-fibred reject stream and a lower value short-fibred accept stream simultaneously. Fractionation is only practical when a mill can make use of all obtained fractions. This study sought to demonstrate the potential of upgrading the reject fraction through multiple stages of fractionation while creating a new market for the remaining low value pulp for an efficient use of the raw materials. In this study, an NBSK pulp was fractionated on the basis of fibre length using a small industrial pressure screen Beloit MR-8 in multiple consecutive stages to isolate the lowvalue fines fraction from the feed pulp using the best combination of operating parameters. The best conditions to carry out fractionation were determined by conducting experiments to investigate the effect of varying volumetric reject ratio, Rv aperture velocity, Vs aperture diameter and rotor tip speed, Vt on reject thickening and passage ratio using several smooth-holed screen cylinders. This work shows that in general, increasing fines percentage in the accept and increasing fibre length in the reject were obtained by using the screen cylinder with 0.5 mm apertures, the highest Rv at 0.6 and the smallest Vs at 0.3 ms-¹. The strength properties of the unfractionated pulp were compared to the reject pulp produced from the multistage fractionation. The tensile strength of the final reject pulp (which is 95 wt-% of the feed pulp) was increased up to 40% through the removal of only a small amount of fines. The TEA, burst and tear indexes also improved. The Gurley air resistance was decreased up to 50%. The final accept fraction contains a significantly higher proportion of fines and it was analyzed by FPInnovations for its potential suitability as a raw material for a novel fibre based product, Nanocrystalline Cellulose (NCC).

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