POST-O₂ <em>EUCALYPTUS</em> PULP IN A/D(EP)DP BLEACHING SEQUENCE WASHED WITH RECIRCULATION OF THE ALKALINE FILTRATE

Abstract

Circuit closure in the pulp manufacturing process may mitigate water scarcity. This work aimed to analyse the feasibility of reusing the alkaline filtrate (carryover) from the (EP) stage of producing hardwood pulp in the A/D(EP)DP bleaching sequence. Pulp from Eucalyptus grandis and Eucalyptus grandis × Eucalyptus urophylla wood and the alkaline effluent from (EP) bleaching stage were collected in a pulp mill. Bleaching was conducted both without this effluent (as a control) and with 15 kg odt-1 of the alkaline filtrate from the (EP) stage. The consumption of reagents during bleaching and the chemical, physical, mechanical and optical pulp properties were evaluated. The alkaline filtrate increased the pulp kappa number from 1.4 to 2.2 with a fixed reagent dosage. The consumption of chlorine dioxide, sulfuric acid and the brightness reversion were 13.7, 17.3 and 20.1% higher in treatments using carryover, respectively to achieve a brightness level of 89 ± 0.2% ISO in the bleaching process. The viscosity, hemicelluloses content and the pulp properties did not vary between these two treatments. The adoption of effluent recirculation should consider the environmental and economic gain of reducing the water consumption and the negative effect of pulp chemical changes and the increase in reagent consumption.