Impactos do excesso de O₂ e licor branco não oxidado na deslignificação com oxigênio: estudo de caso aplicado a uma planta industrial de polpa Kraft de eucalipto

Abstract

The Kraft process is the main route for producing cellulosic pulp, characterized by its operational robustness and chemical recovery cycle. Oxygen delignification is an essential stage for reducing the Kappa number and the consumption of bleaching reagents. This study evaluated, at both laboratory and industrial scales, the feasibility of partially replacing sodium hydroxide with non-oxidized white liquor (WL), combined with the use of excess oxygen to intensify delignification and increase brightness, consequently minimizing costs and environmental impacts. In the tests with a Parr reactor, the WL fraction ranged from 22.2% to 45%, and the O₂ excess from -1.1 kg/ADt (deficit) to 7.9 kg/ADt. The O₂ excess increased brightness by up to 2.68 ISO points and delignification efficiency by +2 percentage points, but decreased viscosity by up to 34 dm³/kg and selectivity by 0.028 Kappa units per dm³/kg. White liquor by itself did not affect pulp quality; however, in the presence of excess oxygen, resulted in a tendency of viscosity and selectivity loss. Increasing excess oxygen enhanced brightness and efficiency gains, with potential to reduce chlorine dioxide demand in the bleaching stage, although excesses above the baseline resulted in slight losses of viscosity and selectivity. Although obtained under laboratory conditions, the results were relevant and provided concrete guidance for the industrial trial phase. In this phase, it was found that increasing the white liquor fraction from 22% to 30% did not alter pulp quality indicators and allowed a reduction of about 1.7 kg NaOH/ADt, corresponding to approximately 8% in savings of sodium hydroxide costs. Furthermore, reducing the oxygen excess by 2 kg/ADt did not compromise these parameters and resulted in an estimated 10% reduction in oxygen costs. The adjustments in white liquor fraction and oxygen charge are currently being implemented in the analyzed mill.