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Fibre fractionation in hydrocyclones Rehmat, Tazim

Abstract

Literature on fibre fractionation in hydrocyclones is reviewed. A force balance on an idealized particle moving in an idealized centrifugal field is used to show that the radial velocity of a fibre or other type of particle moving inside a hydrocyclone is slower for particles having higher values of specific surface. Thus, in theory, the rejects stream is more likely to contain material having lower specific surface than the feed and the accepts stream is more likely to contain material having higher specific surface material. It is also shown that fibre coarseness is inversely related to specific surface. Fractionation of various pulps are described showing evidence of fractionation by length and coarseness. Sheet property measurements, showing that sheets made from hydrocyclone accepts are always stronger than those made from hydrocyclone rejects, are also presented. Multistage fractionation of mechanical and chemical pulps has been investigated to show the degree of separation achievable. This was quantified by the measurement of fibre (length, coarseness, microscopy, width, shape factor) and paper (tensile, tear, burst, roughness) properties. For tests performed with mechanical pulp, it was shown that the hydrocyclone tested in these experiments resulted in rejects fibres which were coarser and shorter than fibres reporting to the accepts. In these tests fibre fines reported to the rejects. A different hydrocyclone was tested to fractionate chemical pulp. In these tests it was found that fibre fines and earlywood fibres reported to the accepts and latewood fibres reported to the rejects. Refining of fractionated chemical pulp was performed. These tests illustrated that earlywood fibres develop at lower refining intensity than latewood fibres. It was also demonstrated that latewood fibres could be upgraded to usable fibre through refining.

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