Effect of cell wall thickness and fines on bleaching of softwood kraft pulp

The effectiveness of various bleaching chemicals on softwood kraft pulp fibres of different cell-wall thickness was studied and the effect of primary fines on bleaching was investigated. Softwood kraft pulps of different average cell-wall thickness were obtained by fractionating with hydrocyclones. After the fractionation unbleached softwood kraft pulp fractions were treated with oxygen, chlorine dioxide, hydrogen peroxide and ozone. In the oxygen stage, kappa reduction increased and the consumption of sodium hydroxide per unit decrease in kappa number decreased with cell wall thickness at a given primary fines content. The consumption of hydrogen peroxide per unit decrease in kappa number increased with cell wall thickness at the same primary fines content. The consumption of chlorine dioxide and ozone per brightness unit gained decreased with cell wall thickness. In addition, the results showed that primary fines adversely affected the hydrogen peroxide bleaching of the pulp.