Studies on Bleaching of Pulps Part 2

Comparison of Calcium Hypochlorite and Sodium Hypochlorite in Hypochlorite Bleaching Stages

Abstract

In the preliminary study, it was found that the use of calcium hypochlorite in H-stage of a special bleaching sequence [HCEHD] produced less pollutant than that of sodium hypochlorite. So that, the detailed studies on the differences between both hypochlorite bleaching were carried out.
Hardwood kraft pulps were bleached with calcium hypochlorite or sodium hypochlorite at various levels of available chlorine. For the same (free alkali/available chlorine) ratio in each hypochlorite stage, it was found that the final pH of calcium hypochlorite bleaching was lower than that of sodium hypochlorite bleaching, and the former consumed more available chlorine.
The considerable differences between both hypchlorite bleaching were recognized not in pulp properties but in bleaching effluent. Calcium hypochlorite bleaching yields less pollution load than sodium hypochlorite bleaching. In addition, it was found that changes in the molecular weight distribution of pollutant components with the addition of avalable chlorine was greatly influenced by the kinds of hypochlorite. These suggest that the kind of cation influences seriouly on the mechanism of dissolution of pollutant components from pulps into effluents.
Bleaching by calcium hypochlorite gave slightly higher pulp yield than that by sodium hypochlorite for hardwood kraft pulps. The pulp yield estimated on the basis of total carbon dissolved in the effluent agreed approximately with the measured value.
When the mixture of both hypochlorites was used, the characteristics of the effluent showed resemblances to those produced with calcium hypochlorite only.
The addition of inorganic compounds of alkaline earth metals in sodium hypochlorite bleaching was significantly effective to improve the nature of the effluents. The effluents produced in this case were similar to those produced with calcium hypochlorite.