The effects of the order of chemical addition on the peroxide bleaching of an oxygen-delignified softwood kraft pulp

In pulp bleaching, while the peroxide-stage chemical charges and the physical operating conditions have been optimized, little attention has been given to the order in which these chemicals are added. We assessed the effects of chemicals, individually and combined, and the effects of the order of addition of these chemicals on peroxide bleaching performance in an acid-treated pulp and in a chelated pulp. We found that adding magnesium to an acid-treated pulp is essential for good peroxide bleaching, while adding magnesium to a chelated pulp provides only a marginal improvement in most additions. But adding magnesium and sodium hydroxide, or sodium hydroxide and magnesium sequentially into a bleaching solution before adding the solution into pulp, causes inefficient peroxide bleaching. This is particularly the case with a chelated pulp. This inefficiency can be avoided if a chelant is added between the additions of magnesium and sodium hydroxide. Magnesium is substantially more effective when in a complex form with either the pulp or a chelant, and the optimum concentration of magnesium for use in peroxide bleaching can be determined by following the peroxide residual.