Technical and environmental improvement of the bleaching sequence of dissolving pulp for fibre production

Abstract

Reactivity of dissolving pulp is one of the main parameters to determine its availability to be transformed into viscose. It is related to the use of carbon disulphide (CS2). An industrial sequential totally chlorine free bleaching process is used as case study. It is carried out in two stages: (1) Alkaline extraction (EOP) and (2) peroxide bleaching (PO). In order to assess how to decrease the use of carbon disulphide, several experiments were performed at laboratory scale for the two stages mentioned before by modifying the operating conditions: NaOH and H2O2 dosages, time and temperature. Reactivity using a modified Fock’s method and pentosan content was analysed along with quality pulp parameters: α-cellulose, viscosity and lignin content (kappa number). Results showed that reactivity increases through the bleaching process and varies with the chemical dosage in both stages. Pulp obtained at the best conditions had the following characteristics: reactivity, 95.3%; α-cellulose 91.17%; intrinsic viscosity, 448 mL/g; kappa number, 1.81 and pentosan content 2.86%, and as a result, CS2 usage was reduced by 11.88%. At the best conditions obtained in this work, NaOH dosage in PO stage was reduced to zero and temperature was slightly lower, when compared with industrial operating conditions.