Preliminary results on hydrogen peroxide addition to chlorine dioxide bleaching stages

There are convincing results in the literature indicating that the fiber yield for fully bleached hardwood kraft pulps can be increased by approximately 2% (measured on chip weight as basis) by pulping at milder Conditions and bleaching under more aggressive conditions. The kappa number of the unbleached pulp in this scheme would be higher by about 3-5 units. The viability of this process is predicated upon increased throughput in an existing bleach plant without a major retrofit. The feasibility of the principle of this process was investigated by adding hydrogen peroxide to the first two chlorine dioxide stages (D-0 and D-1) along with sodium molybdate as a catalyst. The incremental peroxide required for bleaching the higher kappa number pulps could in principle be added to either of the two stages without any change in their operating conditions. Our preliminary data suggest that peroxide action in the D, stage results in a lower rate of cellulose depolymerization and hence is preferable. The recovery of the molybdate catalyst is critical to the feasibility of this process. One possible process of molybdate recovery utilizing selective ion flotation with a cationic surfactant (dodecylamine) was investigated. A recovery of 58% was achieved after two preliminary trials using the column flotation technique. Other literature results suggest that a higher recovery, of the order of > 90% is achievable using ion flotation indicating that a highly efficient recovery process for both the molybdate catalyst and the flotation agent can be developed. Thus, this technique of obtaining enhanced yields could provide an economically feasible alternative for production enhancement in pulp mills.