The Effect of Magnesium Ions and Chelants on Peroxide Bleaching

Abstract

Summary We recently reported that during peroxide bleaching, magnesium is substantially more effective when in a complex form with either pulp or a chelant. We also speculated that different magnesium forms affected the catalytic activity of transition metals towards peroxide decomposition to different degrees. As oxygen-delignified pulps still containing lignin were used, it was impossible to separate the catalytic peroxide decomposition by transition metals from the peroxide reaction with lignin, and thus to determine where in the pulp-liquor system magnesium or chelants, or both, were deactivating transition metals. In this paper, we studied the peroxide decomposition kinetics with different modes of addition of the P-stage chemicals in the presence of fully-bleached kraft pulps which are virtually lignin-free, in alkaline filtrates, in P-stage filtrate and in water. We found that most of the peroxide decomposition occurring during a P-stage applied to chemical pulps takes place through interaction with transition metals in the pulp rather than with transition metals in the soluble bulk phase. We also concluded that in any component of a peroxide bleaching system, magnesium is extremely efficient at reducing the rate of peroxide decomposition, while a chelant becomes more valuable when complexing magnesium.