Magnesium hydroxide; an alternative alkali source in hydrogen peroxide bleaching of wood pulp

摘要

After many lab trials and large scale trials, a reduction of chemical oxygen demand (COD) and higher amounts of residual hydrogen peroxide could be proved, if magnesium hydroxide had been used as alkali source in hydrogen peroxide bleaching of wood pulp. Additionally it could be proved, that the necessary amount of magnesium hydroxide and the bleaching results at a given amount of hydrogen peroxide were dependent from the purity and the fineness of the magnesium hydroxide. The magnesium hydroxide dissolves continuously during the bleaching process by consumption of alkali. Thus, the initial pH-value at the beginning of the bleaching process is lower compared with a caustic soda / silicate system. It amounts to 8. This fact probably causes the reduction of the chemical oxygen demand (COD) and influences the kinetics of the bleaching reaction in two ways: The rate of the undesired hydrogen peroxide decomposition into water and oxygen at a high concentration of perhydroxyl anions and thus at a high pH-value [1], is probably reduced and the amount of residual hydrogen peroxide increases. The higher amount of residual hydroperoxide effects, that nearly no brightness loss can be observed during unplanned shut downs. The speed of the bleaching reaction is lower, so that the best bleaching results are obtained after a bleaching time of 3-3,5 hours and the minimum bleaching time amounts to 2 hours, if magnesium hydroxide is used as the only alkali source.