SE9604469D0 - Alkaline treatment for metal ion removal during pulp bleaching - Google Patents

Abstract

Method for treatment of cellulose pulp (for example Kraft pulp) before bleaching by utilization of bleaching chemicals that use metal ions to minimize or eliminate the need for chelating compound and alkali additives. The method consists of the following consecutive steps: (a) The pulp is treated in an acid step in order to remove the main part (preferably at least 80%) of the negatively loaded acid end-groups (typical hexuronic acid) from the pulp. (b) The pulp is treated after step (a) with an alkali source (preferably an alkali fluid containing liberated organic material in an amount of at least 5 kg/ton pulp, such as filtrate from an alkaline bleaching or delignifying step or black lye) containing components that combine with the positively loaded , bleaching chemical-utilizing metal ions for production of metal-content combinations. (c) The metal-content combinations are removed (washing and/or thickening step) from the pulp especially without or with a minimum amount of chelating compounds or added alkali. In step (a) the pulp has a typical concentration of approximately 8-20%, and this concentration is reduced by at least 3% to between approximately 5 - 8% in step (b) and the added alkaline fluid and pulp in the step (b) are held in contact with each other for at least 5 minutes (preferably 10 - 60 minutes) before step (c) is taken. The concentration of Fe<++> and/or Fe<+++> ions in the resulting pulp is below 15 ppm, preferably under 10 ppm.<IMAGE>