Date of Award


Degree Name

Bachelor of Science


Pulp and Paper Technology


The objective of this research project was to investigate the effect of various bleaching agents and procedures on brightness, degree of yellowness and retention of the original high yield of the unbleached neutral sulphite semichemical pulp.

In order to avoid excessive losses in bleaching of the high yield pulp, it was decided to limit the selection of bleaching chemicals to hypochlorites, peroxides and hydrosulphites. Furthermore, caustic extraction was to be employed only under relatively mild conditions as to temperature and concentration.

Based on the experimental conditions and results described in this report and on the use of an unbleached pulp with relatively low brightness of 32 percent, the following conclusions were drawn: Bleaching of neutral sulphite semichemical pulp by means of one stage calcium hypochlorite treatment brought about only limited brightness development with concurrent relatively high yellowness.

Two stage bleaching with calcium hypochlorite was found to be more effective than one stage bleaching for a given amount of chemical. Best results were obtained when about 25 percent of the total hypochlorite were used in the first stage and the remaining 75 percent in the second stage.

Caustic extraction between two stages of hypochlorite bleaching improved brightness and reduced the yellow cast of the bleached pulp. With this procedure, best results were obtained by using 50 to 75 percent of the total hypochlorite in the first stage.

Pulp bleached with hypochlorite followed by acidification and hydrogen peroxide treatment produced encouraging results as to brightness and decrease yellow cast.

Zinc hydrosulphite, although it did not improve brightness was effective in reducing the yellow cast of the bleached pulp.

All experiments caused only modest shrinkage of the pulp used.