Silicate-Free Peroxide Bleaching of Mechanical Pulps: Efficiency of Polymeric Stabilizers

摘要

Sodium silicate (waterglass) is commonly used to stabilize hydrogen peroxide in bleaching mechanical pulps to high brightness. Though efficient stabilizer, silicate causes problems in papermaking by contributing to silicate deposit formation, decreased retention and sheet strength and as well as increased anionicity leading to higher consumption of wet-end chemicals. In this study the efficiency of peroxide stabilizers consisting of various polymeric compounds with different molecular weights were compared to conventional silicate. Unbleached mechanical pulp samples were taken from pulp mills and used in laboratory bleaching experiments. In addition, different complexing and stabilizing agents were screened in an alkaline peroxide solution (without pulp) containing transition metals. Several polymeric compositions were found to match the efficiency of waterglass. It was found that results obtained in the alkaline peroxide solution test did not always reflect what is observed in real pulp bleaching environment, indicating that the behaviour of transition metals in free solution is somewhat different to a pulp suspension where the activity of metals depends on location, activity state, counter ions etc. The results of our project and the chemistry of different polymeric stabilizers are discussed in this paper.