Options for Bleaching Mechanical Pulp with Lower COD Load

Abstract

Peroxide bleaching requires alkaline conditions. In mechanical pulp bleaching this results in an increased solubilisation of wood substances, visible in an increase of the effluent COD. Because bleaching to very high brightness requires a high input of caustic soda for the activation, the resulting COD load becomes very high. As an alternative for the application of caustic soda other compounds like sodium carbonate (soda ash) and magnesium oxide have been recommended. Within a narrow range, sodium carbonate is capable to replace caustic soda by weight in equivalent amounts. Similar brightness is achieved at a COD level about 20 % lower. The buffering effect of soda ash results in a lower turnover of the H{2}O{2} added. However, it is impossible to make use of the higher residual. Thermal activation, retention time increases or an initially higher soda ash input are not resulting in a higher brightness. The reason for this is the decomposition of the intermediately generated percarbonic acid at the normal bleaching temperature. This is verified by a comparison of the stability of buffered peroxide solutions in the presence or absence of carbonate or bicarbonate. This side reaction is a limitation for the application of carbonates at the 'normal' (60 C to 70 C) temperature level in mechanical pulp bleaching. The application of sodium bicarbonate at low temperature (40 C) results in very moderate COD values, however, it requires days to reach the top brightness. Magnesium oxide and magnesium hydroxide are alternatives to caustic soda. Their disadvantage in the first place is their poor solubility. This requires very effective mixing to guarantee an even distribution and in addition a low particle size. Mg(OH){2} not only replaces NaOH but in addition most of the sodium silicate. The COD is between 30% and 40% lower. For very high brightness, a slightly higher peroxide charge is needed.