Polyoxometalate-catalyzed oxygen delignification of kraft pulp: A pilot-plant experience

The results of the first pilot-scale kraft pulp oxygen bleaching catalyzed by polyoxometalate are presented. A modified heptamolybdopentavanadophosphate heteropolyanion (HPA) was used as the catalyst. Scale-up experiments clearly revealed the dependency of the pulp delignification rate on the mass- and heat-transfer phenomena showing a rate constant on the pilot equipment assay twice that on the laboratory reactor. The selectivity of the oxygen-catalyzed bleaching stage (O-HPA) was higher than that of the conventional oxygen alkaline bleaching (O). Pilot trials demonstrated similar consumption of chlorine dioxide and the strength properties of pulps bleached by OHPADED and ODED sequences. The chemical oxygen demand, absorbable organic halogens, and toxicity of the effluent streams were assessed for each bleaching stage. No catalyst retention on fully bleached pulp was detected. The practical aspects of the eventual industrial application of polyoxometalate-catalyzed oxygen bleaching are discussed.