Improving Extended Delignification Technology for Kraft Pulping. Part III: Yield Improvement from Two-Stage Kraft Cooking with Initial Polysulphide Treatment, 1998 Pulping Conference Proceedings

A two-stage kraft cooking technique developed recently at Paprican can extend delignification and improve pulp viscosity without increasing chemical consumption and cooking time. The unbleached yield, however, is about 1% lower than that of conventional pulp at kappa numbers above 20. To improve the pulp yield of the two-stage kraft cooking, we have combined it with modified multistage polysulfide cooking. In this procedure, wood chips are first treated at high polysulfide and sulfide concentrations in vapor-phase conditions, followed by two liquid-phase cooking stages. The effects of the two procedures on pulping results are complementary. The overall delignification rate is as fast as two-stage kraft pulping, and much faster than conventional kraft cooking. Pulp viscosity is 30% higher than that of conventional kraft pulp. Pulp yield at kappa number 18 is 2% higher than that of two-stage kraft cooking, and 1% lower than that of the modified multistage polysulfide cooking. Despite the 2% yield gain (mostly from hemicellulose), the drainage, beatability, and strength of the pulp are similar to those of conventional kraft pulp at the same kappa number. The tear-tensile strength of the pulp is much improved over that of the polysulfide pulp, but still needs a 5-10% improvement to reach the highest levels observed with conventionally cooked kraft pulp.