High Temperature Resistant Chelants in Peroxide Bleaching of Kraft Pulps, 1996 International Pulp Bleaching Conference Proceedings

Traces of transition metal ions, particularly iron, manganese and copper ions, occurring in pulp reduce the effect of hydrogen peroxide bleaching. Much effort is usually expended on removing these metals prior to peroxide bleaching by using chelating agents such as EDTA or DTPA in a separate Q stage. Rendering the metal ions inactive by adding organic stabilizers directly to the bleach suspension is another promising approach to reduce the negative influence of metal ion catalysed peroxide decomposition. In this paper the stabilization of hydrogen peroxide in bleaching of softwood kraft pulp with alkaline hydrogen peroxide is discussed for different stages of various bleaching sequences. It is shown that the addition of small amounts of stabilizer formulations based on hydroxycarboxylic acids and phosphonic acids has a major influence on successful bleaching. As these substances act as metal chelating agents even under strongly alkaline conditions, the peroxide stability can be extremely improved, higher brightness levels can be reached and viscosity losses can be minimized. Furthermore an activator was tested in the above mentioned stabilized (OP) stage. A softwood kraft pulp with a Kappa no. of 23.5 could be bleached to a final brightness of 88.2% ISO, by only charging 4% hydrogen peroxide in a high temperature (OP) stage of an OA(OPHT ) sequence.