Abstract:
Flocculation of fibres plays havoc in the paper industry. The entanglement of wood pulp fibres greatly reduces the efficiency of most processing operations including washing, screening, hydrocycloning, and pumping. Various attempts have been made to minimise the effects of flocculation by reducing fibre concentration, using different fibre types and distributions, and by selective chemical addition. Ecreasing liquid viscosity offers a further alternative as it has been shown that fibre network strength can be reduced markedly or even be eliminated completely in a viscous medium. E the initial part of this investigation various liquids were tested in a screen-like device to ascertain their ability to reduce flocculation. It was decided that carboxymethyl cellulose (CMC) solution had the greatest overall advantage. It was shown that at Iow fibre concentrations flocculation effects are reduced because of the higher dissipation of fibre elastic bending energy at higher viscosities. At elevated fibre concentrations flocculation is reduced because of the increased lubrication of the fibre surfaces. The shear behaviour of fibre suspensions at high viscosities was evaluated by several means. It was found that the higher the viscosity the more the suspension behaves Iike a homogenous liquid which should Iead to much improved processing capabilities. The flow regimes observed in pulp-water suspension flow are not present in a pulp-CMC solution suspension at high viscosities and consistencies. Average floc sizes in channel flow obtained from photographic analysis are greatly reduced in a viscous medium, similar to a reduction of disruptive shear stress gathered from a quasi-static shear tester. Significant improvements in pressure screening could be achieved when using a viscous suspending liquid. Reject thickening could almost be eliminated and hence screen capacity could be enhanced. The higher fibre acceptance in a viscous medium was shown to be caused by a combination of a higher number of separated fibres in the suspension and a more favourable flow field for individual fibres to pass through the screen. It could be shown that the floc sizes or porosity of the suspension in front of the screen apertures rather than the amount of fibres inside the apertures are responsible for the accept pressure drop. The increase in screen capacity is accompanied by reduced fibre length fractionation efficiency because the passage of Iong fibres increased with increasing viscosity. Hydrocyclone operation cannot be enhanced by the use of viscous media.