Polysulfide and anthraquinone methods are well-known technologies available today to increase pulp yields of kraft cooking. Since polysulfide is produced by oxidation of white liquor, Na2S concentration in the liquor decreases as the oxidation proceeds. This decrease of Na2S concentration sometimes causes increases of kappa numbers of pulp and screened rejects in polysulfide cooking. For developing highly concentrated polysulfide cooking, we investigate the effects of anthraquinone addition at

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... arious polysulfide and Na2S concentrations. Statistical analysis using the multivariate analysis of variance method was carried out to examine in detail the effects of polysulfide and anthraquinone, which have been reported to exert a probable synergistic effect. The result showed that the anthraquinone addition mainly reduced kappa numbers of pulp at a given active alkali charge without any decrease of pulp yields, and that the increase of polysulfide concentration raised screened pulp yields with a slight increase of kappa numbers. Thus, each of anthraquinone and polysulfide contributed to the increase of pulp yields at given kappa numbers individually. There was no interaction between the effects of polysulfide and anthraquinone. It is considered that a synergistic effect of polysulfide and anthraquinone should be explained by the fact that anthraquinone can compensate for increases of kappa number and screened rejects caused by the lack of Na2S concentrations in polysulfide cooking. Therefore, anthraquinone addition is particularly effective for highly concentrated polysulfide cooking to obtain high pulp yields.