Role of Sulfur in Displacement Kraft Batch Cooking, 1995 Pulping Conference Proceedings

M. Svedman

Oy Lännen Laboratoriot -Western

Laboratories Inc.

P. Tikka

SciTech-Service Inc.

K. Kovasin

Sunds Defibrator Pori Oy

The role of sulfur in alkaline cooking is of great importance as it renders the non-selective soda cooking into a highly efficient kraft cooking. Furthermore it has been suggested that by emphasizing the role of sulfur in the front-end of kraft cooking, considerably better results can be achieved in terms of faster delignification, extended cooking and pulp quality. In modem displacement kraft batch cooking hydrosulfide residual of black liquor is employed at the front-end of the cooking by recirculating hot and warm black liquors. In the present study, the importance of hydrosulfide in different cooking stages on delignification was examined by using a laboratory scale displacement cooking system. The hydrosulfide concentration of black and white liquors was varied over a wide range corresponding to sulfidity values of 0- 100% in black liquors and 0-54% in white liquor. The results show that hydrosulfide in the recirculated black liquors plays a controlling role in terms of delignification rate and selectivity. Moreover, it was found that the hydrosulfide content in the initial part of the bulk delignification, shows the highest impact on delignification efficiency. On the other hand, the effect of hydrosulfide in white liquor and, thus, during the cooking-at-the temperature phase, is rather insignificant. The results clearly confirm the vital role of sulfur at the front-end of displacement kraft batch cooking as one of the driving forces behind the high cooking efficiency of the process. In addition, the paper discusses the importance of sulfidity and hydrosulfide recirculation for maintained pulp strength when cooking low kappa number softwood pulps.