Polysulphide Cooking and TCF Bleaching of Pine Employing On-Site Alkali Products, 1997 Pulping Conference Proceedings

A laboratory-scale study was conducted to produce TCF bleached polysulphide pulp simulating a process in which all the alkali needed for cooking, oxygen delignification and TCF bleaching is produced internally. Using theoretically calculated alkali charges for the different process stages fully bleached TCF polysulphide pulps of good quality were produced. An advantage of this kind of process is the higher bleached pulp yield compared with kraft pulping.

The impregnation polysulphide cook can be stopped at kappa 40 with the same amount of rejects as in conventional kraft cooks taken to kappa 25. The increase in bleached pulp yield is about 1.5% on wood in this case with the polysulphide charges employed. Delignification must be continued from kappa 40 with oxygen, preferably in two stages, to save bleaching chemicals. Under optimized process conditions TCF pulp strength can be maintained. Process simulations showed that the new on-site alkali production doesn’t significantly affect the sulphur to sodium ratio and might even decrease the inorganic load to the chemical recovery cycle.