Effluent closure of hardwood pulp hydrogen peroxide bleaching - Focus on influence of metal ions

Several mill trials were made at Enocell kraft mill. Hardwood (birch, Betula) pulp was bleached with hydrogen peroxide. Some mill trials included the use of magnesium salt. One mill trial also included the recirculation of filtrates in order to save fresh water and reduce chemical consumption. Pulp and filtrate samples were collected before and after the chelation stage and the peroxide stages. The metal contents were determined by atomic absorption spectroscopy. According to the results the addition of magnesium salt had a positive effect on both pulp viscosity and hydrogen peroxide consumption. However, excessive magnesium seemed to slow down the bleaching reactions which resulted in lower final brightness. There was little or no decrease in pulp brightness when alkaline filtrates were recirculated countercurrently and part of the filtrates from the first hydrogen peroxide stage were led to the chelation stage. Accumulation of COD did not appear to be a major problem. The performance of chelation stage proved to be very important; even minor changes in pH had a significant effect on the transition metal contents. The major problems in bleaching performance were caused by high manganese content. Also Mg/Mn molar ratio seemed to have an important role in hydrogen peroxide bleaching.