Alternative Alkalis in Peroxide Bleaching of Mechanical Pulp, 2007 International Mechanical Pulping Conference

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Suitability of Magnesium Hydroxide, Magnesium Oxide, Calcium Hydroxide and Calcium Oxide as Sodium Hydroxide-replacing alkali in oxidizing hydrogen peroxide bleaching of mechanical pulp was studied. In the first stage, bleaching experiments of laboratory scale were carried out both using Sodium Silicate stabilizer and without it in medium consistency (10%). It was found that the most suitable alternative alkali is Magnesium Hydroxide, which needed no Sodium Silicate-stabilisation to achieve maximum brightness result of 77.0%-ISO whereas NaOH with Sodium Silicate gave 78.4%-ISO.

Mg(OH)₂ was then subjected to high-consistency (20%) laboratory bleaching experiments. According to the results replacing Sodium-based chemicals with Mg(OH)₂ produces about one percent (ISO) lower final brightness. To counter-balance this brightness loss Mg(OH)₂-alkali dissolves only some 45% of the organic carbon compounds compared to Sodium-based bleaching process. Similar results were obtained using technical grade Mg(OH)₂ in paper mill experiments. It was also found that it is possible, although not completely feasible to replace NaOH only partially with Mg(OH)₂. Where TOC in wastewaters of mechanical pulp peroxide bleaching stage pose a problem, one distinct and true possibility is to use Mg(OH)₂ as alkali.