AbstractThe effect of adsorbed and soluble carboxymethyl cellulose (CMC) on dispersing, dewaterability, and fines retention of pulp fibre suspensions was investigated. CMC was added to a suspension in the presence of electrolytes, causing its adsorption to the fibre surfaces, or to a suspension without electrolytes, so that it stayed in the liquid phase. Both the CMC adsorbed on fibre surfaces and that in the liquid phase were able to disperse the fibre suspension due to the ability of CMC to reduce fibre-to-fibre friction in both phases. Adsorbed CMC promoted the formation of a water-rich microfibrillar gel on the fibre surfaces through the spreading out of microfibrils, leading to a decrease in friction at the fibre-fibre contact points and to the increased dispersion of fibres. CMC in the liquid phase of the suspension was in turn thought to prevent fibre-to-fibre contacts due to the large physical size of the CMC molecules. CMC in both phases had detrimental effects on dewatering of the pulp suspension, but adsorbed CMC caused more plugging of the filter cake, and this was attributed to its ability to disperse fibre fines, in particular. Thus, adsorbed CMC also reduced fines retention considerably more than did CMC in the liquid phase of a suspension.