The effect of an anionic polymer additive on fractionation of paper pulp

The effects of a fibre surface modification by a CMC chemical on fractionation of LWC furnish in pressure screen and hydrocyclone were studied. CMC adsorbs on the fibres and modifies their fibrillar surface, which affects their mechanical and hydrodynamic behavior. Due to the adsorption on fibres and the small amounts of CMC used, the effect of CMC on the pulp water viscosity was negligible. The fractionation results showed that long chained, low charge density CMC changed the operation point of the pressure screen (i.e. changed the reject thickening factor at constant volumetric reject rate) toward less fractionating conditions and increased capacity. The reason for the capacity increase was suggested to be the decrease of the friction in fibre-fibre contacts, which caused a decrease in fibre network strength and consequent easier break up of flocs, facilitating the passage of fibres. The operation point of hydrocyclone fractionation was changed toward more thickening and thus more fractionating conditions. The efficiency of fractionation was also increased, because CMC enhanced the specific surface differences of fibres by selective sorption to cellulose surfaces. Due to fibre surface modification, the addition of an anionic polymer enhances the pressure screen and hydrocyclone fractionation by increasing capacity or fractionation efficiency. If CMC is used, it may be beneficial to dose the chemical prior to fractionation at a paper or board mill.