The effect of cooking kappa number on oxygen delignification of Eucalypt kraft pulp

One efficient and well-known way of increasing the overall pulp yield is to stop the cook at a higher kappa number and instead increase the delignification work carried out in the oxygen delignification and bleaching stages. Since it is usually desirable to avoid the use of expensive bleaching chemicals, it is preferable to extend the oxygen delignification. To reach a delignification degree of 60-65 percent in the oxygen delignification of softwood kraft pulp is fairly straightforward. In this study, the effects of a higher cooking kappa number with a Eucalypt raw material are explored. It is shown that a maximum in lignin-free and reject-free yield is reached at around the defibration point at kappa number 22. However, the efficiency of oxygen delignification is markedly reduced when the ingoing cooking kappa number is increased, probably partly because the higher yield of hemicellulose. In addition the limiting viscosity number of pulps having different cooking kappa numbers but evaluated at the same kappa number after oxygen delignification was found to differ widely. A high cooking kappa number is clearly a disadvantage if a high viscosity after oxygen delignification is required. The selectivity can be improved emphasizing on the temperature rather than the alkali charge.