Application of an experimental design to modeling of ozone bleaching stage in TCF processes

An oxygen delignified eucalyptus Kraft pulp was bleached in a totally chlorine-free (TCF) sequence using an ozone stage (Z) followed by a hydrogen peroxide stage (P). Ozone treatments were carried out at a laboratory scale with new automated equipment. The Z stage was modeled using a four-variable sequential statistical plan over the following ranges: 10-20 g o.d.p. mass pulp, 20-50 mg/L ozone inlet concentration, 80-220 LN h ozone flow, and 0.25-0.65% o.d.p. ozone dose. The influence of these variables on several pulp properties after the Z and P stages of the XZP sequence was examined. The models defined from the results obtained predicted, in the Z stage, variations in ISO brightness, viscosity, and kappa number of 74.6-85.6%, 496-876 mL/g, and 0.44-2.27, respectively. For the P stage, the models predicted variations in ISO brightness and viscosity of 84.2-92.7% and 568-857 mL/g respectively. It is possible to obtain bleached eucalyptus pulp of brightness over 89% ISO and viscosities over 800 mL/g. The variable most strongly influencing the pulp properties was found to be the ozone dose.