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Vaihtoehtoalkalit mekaanisen massan peroksidivalkaisussa

Hietanen, Tomi (2007)

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Diplomityö

Hietanen, Tomi
2007

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Tiivistelmä

The most efficient way to bleach mechanical wood pulps is to use oxidizing peroxide bleaching under strongly alkaline conditions. Traditionally this alkalinity has been produced with Sodium Hydroxide and Sodium Silicate. However using these chemicals dissolves great amounts of ligning off the pulp, which decreases yield and increases organic carbon content and chemical oxygen demand of bleaching wastewaters. Ever higher pulp brightness demands and more stringent water use regulations have produced a need to come up with better alkalis to pulp bleaching.

Based on literature knowledge the following alkalis were selected for experimental: Magnesium Hydroxide, Magnesium Oxide, CalciumHydroxide and Calcium Oxide. Their applicability as oxidizing peroxide bleaching alkali was determined by conducting bleaching experiments with them. Experiments were carried out both using Sodium Silicate stabilation and without it. It was found out that the most suitable alternative alkali is Magnesium Hydroxide. Mg(OH)2 was then used on high-consistency laboratory bleaching experiments. According to both medium- and high consistency bleaching results using Mg(OH)2 as replacement for Sodium-based chemicals results in about one percent (ISO) lower brightness. To counter-balance brightness loss Mg(OH)2-alkali dissolves only some 45% of organic carbon compared to Sodium-based process. These results were further tried by carrying out bleaching experiments using groundwood pulp and water samples obtained directly from a Finnish paper mill. Bleaching conditions were also set to match those of a real pulp producing plant. The results are in accordance with laboratory experiments: there is a brightness loss of about one percent (ISO) but the total organic carbon content of bleaching filtrate is under 50 % of that when Sodium-based chemicals were used.
 
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