High-Yield Production Strategy for Hardwood Kraft Pulps, 1997 Pulping Conference Proceedings

The Pulp and Paper Industry has seen an increasing trend in the production of low kappa pulps through the early to mid 1990’s. This trend has largely been driven by an attempt to reduce or eliminate chlorine chemicals. In the latter case, TCF bleaching sequences require the lowest possible kappa number of the pulp prior to the bleach plant if high brightnesses are to be achieved (1). A major concern with the production of low kappa pulps for TCF and ECF bleaching is an associated yield loss. Loss in yield creates obvious economic consequence for a mill (2,3) and is of particular concern to fibre-short mills in Northern United States and Western Canada. Oxygen delignification of kraft pulps is the most widespread method of producing low kappa pulps. In the case of hardwood pulps, oxygen-stage yields and fully-bleached yields can fall unacceptably, if delignification during cooking or oxygen treatment is too extensive (4,5).

Without question, the major advantage of bleaching low kappa pulps is the reduction in chlorine chemical requirement and an associated reduction in formation of chlorinated organic compounds in effluents (6). In many instances however, mills cannot take advantage of extended delignification due to limitations in recovery furnace capacity. Similarly, with the constraints of fibre costs and availability, an economic balance has to be reached between yield loss from extended delignification and savings from reductions in bleaching chemicals. A mill may reduce chemical costs and cut chlorinated compound release but the cost of yield loss can be too high.

Earlier work on oxygen delignification of hardwoods has shown that it is possible to increase pulp yields and still take advantage of the potential cuts to bleach plant chemicals. Jamieson and Fossum (4) recognized that, although oxygen-stage yields fall sharply beyond a certain delignification limit, the treatment of high kappa pulps with oxygen can increase yields. Similarly, Lindblad and Lindstrom identified that there is an optimal balance between cooking, oxygen delignification, and bleaching (5).