73 WOOD RESEARCH 54 (2): 2009 73-82 OPTIMIZATION OF OZONE BLEACHING CONDITIONS OF OXYGEN DELIGNIFIED HARDWOOD KRAFT PULP WITH D-MANNITOL Michal Jablonský Slovak University of Technology in Bratislava, Faculty of Chemical and Food Technology, Institute of Polymer Materials, Department of Chemical Technology of Wood, Pulp and Paper, Bratislava, Slovak Republic ABSTRACT Te infuence of ozone charge (0 – 1.2 % O 3 on oven dry pulp), temperature (20 – 60 °C) and charge of D-mannitol (0 – 1 % D-mannitol on oven dry pulp) on the kappa number, viscosity and brightness in oxygen delignifed hardwood kraft pulp, was studied. Te central composite factorial design in order to identify the optimum operating conditions, equations relating the dependent variables of the operational variables of the ozone bleaching process were derived that reproduced the former with errors lower than 8 %, was used. On the basis of optimum values of factorial design was determined that the D-mannitol at the ozone consumption 0.05 % O 3 on oven dry pulp at 20 °C and 1 % charge of D-mannitol on oven dry pulp prevent the most marked decrease degree of polymerization (ΔDP about 39 unit). Te maximum the of lignin removal (Δkappa number ≈ 2.6) was achieved at ozone consumption 0.54 % of O 3 on oven dry pulp at temperature 21 °C and 0.87 % charge of D-mannitol on oven dry pulp. Te maximum increase of brightness (ΔB about 10.27 % ISO) was achieved at temperature 60 °C, ozone consumption 0.167 % of O 3  on oven dry pulp and charge of D-mannitol 0 % on oven dry pulp. KEY WORDS: ozone bleaching, cellulose protectors, statistical modelling INTRODUCTION Ozone represents a very efective bleaching agent. However, ozone is not selective in its attack on lignin as carbohydrates are also degraded, leading to a lower viscosity (Mbachu and Manley 1981). Within the frst minutes, ozone attacks the aromatic rings of lignin. In addition to primary oxidation reactions, secondary ones proceed on a remarkable scale, giving rise to carbonyl products and causing a gradual decrease in the average molecular weight (Katuscak et al. 1971). Te same has been proved in the efect of ozone in kraft pulp bleaching. Due to a high ozonization reaction rate the efects can be seen during the frst minute (Jablonsky and Vrska 2004, Jablonsky et al. 2004). A remarkable decrease in viscosity (up to 200 ml.g -1 ) was found with the ozone consumption higher than 3 kg.t -1 oven dry pulp. It is important to mention that the problem of ozone bleaching