abstract
In kraft pulp production, the bleaching sequence is an imperative step in obtaining high ISO pulp brightness. This sequence consists on several stages with chlorine dioxide or sodium hydroxide, with pulp washing between each stage. The optimization of Cacia bleaching sequence D0EPD1D2 consists on evaluate the current washing equipment (washers, wash press and atmospheric diffusers) identifying operation problems and suggesting alternatives for this equipment. Finally, is intendend to reduce the chlorine dioxide charge on D2 stage, without the final pulp brightness being compromised. The evaluation stage of the implemented system was based on the analysis of pulp consistency, in COD analysis of the samples collected and the analysis of the NaOH consumption between 2013 and 2017. At the same time as the evaluation of the implemented system, presses and DD washers were simulated at laboratory scale to determine the relationship between the pH of the pulp obtained and the NaOH consumption. To meet the last objective, by manipulating chlorine dioxide charge, the brightness and residuals of chlorine dioxide in the resulting pulps were analysed. From the analysis of this system, it was found that D0 stage washers need to be replaced because of the increased NaOH consumption at 3,12 kg·tAD-1 derived from increased bleaching pulp production. In the case of stage D2 washers, the efficiencies obtained indicate the need to repair or replace the equipment, since the obtained equivalent displacement ratio (EDR) values are outside the range of 0.58 to 0.77. To replace the D0 stage washers it was proposed to replace the current configuration (three rotary drum washers) with three new configurations (one configuration without the washer 2, replacing the 3 washers with a press or a DD washer). It was verified that the three cases imply a sodium hydroxide charge reduction on extraction stage, respectively 7,15 ± 1,00 kg·tAD-1, 9,25 ± 2,17 kg·tAD-1 and 7,85 ± 1,02 kg·tAD-1, with 95 % confidence. Related to the D2 washers, it was found that a DD washer would be preferable to a wash press due to high efficiencies obtained (EDR between 0,808 and 0,939) compared to the wash press (EDR between 0,393 and 0,564). Regarding the decrease of ClO2 charge in the last bleaching stage sequence, it was concluded that to achieve an 90,5 %ISO brightness, a charge of 2 kg·tAD-1 is sufficient, identifying that the load of this stage is in excess. It is suggested the chlorine dioxide charge redistribution by D0 and/or D1 stages, setting in 2 kg·tAD-1 the same reagent charge on D2 stage. In this way, the substitution of D0 washing equipment will allow a reduction of the NaOH charge and for D2 stage it was found the potential to reduce the chlorine dioxide consumptions to be confirmed in the future with industrial tests. Thus, Cacia's bleaching line can be optimized, leading to considerable reductions in production costs and produced effluents.
authors
Inês Da Silva Ferreira Mendes
our authors
Inês da Silva Ferreira Mendes
PhD Studentsupervisors
João Manuel Da Costa E Araújo Pereira Coutinho