Catalytic recovery of the oxidative power of the chlorate residual in the chlorine dioxide delignification
Creators: name: Heynen, Ian F.
Publisher: name: University of British Columbia
Year: 2009

description: Chlorine dioxide has traditionally been used extensively in the brightening stages of kraft pulp bleaching. More recently, however, environmental pressures have resulted in increased substitution of chlorine dioxide for chlorine in the first bleaching stage, otherwise known as the chlorination or delignification stage. It is now common, in Canadian bleached kraft mills, for the bleach plant to be operated with 100 percent chlorine dioxide substitution in the first stage. One of the few disadvantages of using chlorine dioxide in the bleaching of chemical pulp is the formation of chlorate. Chlorate formation accounts for up to 30 percent of the chlorine dioxide charge. Since chlorate is inactive as a bleaching chemical, chlorate formation represents a loss in chlorine dioxide bleaching power, and increased bleaching costs. The formation of chlorate has an environmental cost as well. Chlorate is a known fungicide and herbicide, and the chlorate contained in bleached kraft mill effluent has resulted in the elimination of the Bladder Wrack population in the areas surrounding bleached kraft pulp mill outfalls in the Baltic Sea. The purpose of this study was to examine the potential for vanadium pentoxide to recover the bleaching power of the chlorate residual in the chlorination stage, when the chlorination stage was operated at high substitution. Vanadium pentoxide has been shown to catalyze the transfer of electrons from chlorate to pulp by cycling between the +5 and +4 oxidation states. This catalytic transfer of electrons has loosely been termed the "activation" of the chlorate residual. By "activating" the chlorate residual, we hoped to decrease the final chlorate concentration and recapture the bleaching power lost as chlorate. The investigation was conducted using factorial experimental design at two levels. The experimental conditions were chosen to reflect practical bleaching conditions. The experiments were performed using the bag bleaching method with an oxygen delignified pulp. The factors included in the factorial design were temperature, retention time, pH, chlorine dioxide substitution, and vanadium pentoxide charge. In order to measure the "activation" of the chlorate residual, the response of the following variables were measured: final chlorate concentration, brightness, kappa number, pulp viscosity, and pulp physical properties. The addition of vanadium pentoxide to the chlorination stage had a large impact on the final chlorate concentration. For bleaching runs performed at 80°C, for four hours and at a catalyst charge of 0.005 to 0.01 weight percent, reductions in chlorate concentration of up to 95 % were obtained. Despite the large drops in chlorate concentration, the addition of vanadium did not substantially increase the oxidizing power of the first bleaching stage. When compared to uncatalyzed bleaching runs at the same temperature, the addition of the catalyst resulted in brightness gains of up to 1 percent. The catalyst had no significant impact on the (D+C)E kappa number of the bleached pulp. A serious disadvantage of using vanadium pentoxide to "activate" the chlorate residual in chlorine dioxide delignification is the loss in pulp viscosity. The viscosity dropped from the uncatalyzed values of 22 to 23 cps to as low as 16 cps. These viscosity losses, however, did not result in significant losses in pulp strength properties. From this study it is recommended that further investigations of recovery of the bleaching power of chlorate in the delignification stage be performed. Of particular interest are organic catalysts for application in closed cycle mills where the solids content of the bleach plant effluent will be fired in the recovery boilers. Another recommendation focused on the use of vanadium pentoxide for chlorate elimination and effluent improvement. The chlorination stage recycle would be passed through a vanadium pentoxide catalyst stage prior to re-entering the chlorination stage. The vanadium pentoxide could be in the form of plates or immobilized catalyst. This would result in reductions in chlorate concentration in the chlorination stage without entraining vanadium pentoxide in the effluent stream.