DELIGNIFICATION AND DEGRADATION OF KRAFT PULP DURING OXYGEN BLEACHING

Oxygen bleaching of the spruce kraft pulp prepared by modified process es to a kappa number of 21 and 25 has been studied. To stress the infl uence of the oxygen reaction on the pulp properties under extreme expe rimental conditions there have been used: time of reaction 0.5-3.0 hr. , alkali concentration of 0.05-0.5 mol NaOH, initial oxygen pressure o f 0.6 and 1.2 MPa, reaction temperature of 120-degrees. It has been pr oved that the reaction of delignification goes on in two different pha ses. In the first delignification phase, realized for 30-45 minutes an d/or using an alkali concentration of 0.05-0.1 mol NaOH in solution, a maximum of 60 per cent - lignin of the unbleached pulp is removed. In the second degradation phase, realized by prolonging the reaction tim e over 30-45 minutes and/or using more alkali than 0.1 mol NaOH in sol ution, no more delignification is performed, e.g. this phase does not remove any more lignin, but degradation of cellulose occurs, expressed by the drop of yield and DP. The extent of cellulose degradation may be lowered by retarding effect of Mg++ ions which, in the presence of ferric ions cause also a decrease of alkali consumption during oxygen bleaching. On the reaction of delignification and mainly on degradatio n, the alkali concentration in solution is of decisive influence, to w hich reaction time and initial oxygen pressure are further added. Part ial chlorination with Cl2 and a mixture of Cl2 + ClO2 of kraft pulp, b efore the oxygen stage, on applying 5-25% active Cl2 on the kappa numb er of unbleached pulp on the properties of oxygen bleached pulp has al so been studied. It has been proved that partial chlorination removes the trace elements, mainly Fe+++ present in unbleached pulp and increa ses the efficiency of the oxygen stage. The application of 5-10% activ e Cl2 on the kappa number of unbleached pulp increase the efficiency o f the oxygen stage to such an extent that it removes 77-85 per cent of the lignin of unbleached pulp, increases the brightness of pulp up to 62-64% MgO and lowers the losses and the degradation of the carbohydr ate part of pulp. By application of 25% active Cl2 on the kappa number , up to 95% of the lignin present in unbleached pulp has been removed in the following oxygen stage (kappa number dropped up to values of 1. 0-1.3) and the brightness of pulp increased up to 72% MgO. It follows that partial chlorination can be used instead of an acid washing stage before an oxygen stage.