Reduction of AOX in the Bleach Plant of a Pulp Mill

Abstract

The reconfiguration of an existing five-stage bleaching D0E1D1E2D2 sequence is proposed to ensure the minimal formation of organochlorine compounds, expressed as AOX, in the bleach plant of a kraft pulp mill processing Eucalyptus globulus wood. This reduction of the load of AOX in the effluents can be achieved without introducing new and expensive technologies in the bleaching process. In practice, this goal can be achieved by eliminating the washing step between the D0 and the E1 stages. With this strategy, the total AOX in the liquid effluent of the bleach plant can be reduced by almost 65% while maintaining a level of 90% ISO brightness of the pulp and even diminishing the degradation of its polysaccharides. With this process redesign, the total AOX discharge from the bleach plant can be decreased from 1.2 to 0.42 kg/t prior to any biological treatment. Furthermore, the proposed modified (D0E1)D1E2D2 sequence enables the reduction of the total flow rate of effluent to be treated and also the total consumption of water, which is quite critical in these types of plants. Moreover, with this strategy, one is able to move the first drum washer of a conventional bleaching sequence to the entrance of the bleach plant. This gives rise to a cleaner pulp and therefore to lower requirements of chlorine dioxide, which in turn will enable further reductions in the global AOX generation.