Cleaner approach for medium consistency eucalyptus slab pulp production using ozone bleaching under turbulent mixing

Abstract Eucalyptus slab (ES), a cellulosic waste from wood processing is abundant and can be utilized for the pulping in many wood scarce countries. However, during chlorine dioxide (ClO2) bleaching, which is the current applied bleaching technique for medium consistency eucalyptus slab pulp (MC-ESP), toxic absorbable organic halides (AOX) is still generated, causing pollution to the water environment. Applying ozone gas (O3) to replace ClO2 for MC-ESP bleaching is a promising alternative way since O3 is a clean and powerful oxidizing agent but not producing AOX pollutant. In this work, the influences of the three most important O3-bleaching factors, i.e., O3-concentration, O3-dosage and mixing intensity to the bleaching properties of MC-ESP was systematically evaluated. It was found that increasing of O3-concentration and O3-dosage greatly improved the properties of MC-ESP. Turbulent mixing intensity had a effective effect on improving O3-bleaching of MC-ESP. The high O3-concentration (170 mg/L), 1.0% O3-dosage and 1200 rpm rotational speed were found to be optimum conditions for O3-bleaching of MC-ESP. Under this condition, the brightness, kappa number and viscosity of the pulp obtained were similar or even better than those of the ESP after bleaching with similar or higher ClO2-dosage, however, without generating toxic AOX and with lower pollution load of effluent. This research provided important supports for the development of MC-ESP cleaner production by reducing the pollutant from ClO2 bleaching. Such approach of using wood processing waste as raw material, O3 gas as AOX-free bleaching agent could provide the excellent options for eco-friendly pulp production.