METHOD AND ARRANGEMENT FOR TREATING FILTRATE AFTER OXYGEN DELIGNIFICATION OF CHEMICAL PULP COOKED TO A HIGH KAPPA NUMBER

摘要

The present invention relates to a method and an arrangement for treating chemical pulp that has been cooked to a high kappa number. Pulp 'is produced by cooking in an alkaline cooking process to a kappa number of 50-120, preferably 60-100, the cooked pulp is washed, the washed pulp is led into an oxygen stage where the pulp is treated in the presence of oxygen and alkali for removing lignin, and the oxygen-treated pulp is washed, whereby lignin-containing washing filtrate is formed. In the oxygen stage the kappa number of the pulp decreases by at least 30 units. The filtrate downstream of the oxygen stage is treated for separating lignin, and the treated filtrate is led as washing liquid into the washing apparatus for the cooked pulp.