Aspects of extended impregnation kraft cooking for high-yield pulping of hardwood

Abstract: The long-term trend regarding wood is an increase in price. Because wood contributes to a large part of production costs, the efficient utilisation of wood is greatly desired to reduce production costs for kraft pulp producers. During the 1990s, the development of improved modified kraft cooking began, which led to higher yields. There was also a trend of terminating kraft cooking at a higher kappa number to maximise the overall yield. For hardwood, the defibration point became a critical setback in allowing this termination at a high kappa number. This thesis discusses how this issue has been tackled in the laboratory by using improved modified kraft cooking combined with extended impregnation to enable a decrease in reject content and shift the defibration point towards a higher kappa number for hardwood. This lab concept is referred to as extended impregnation kraft cooking (EIC), and this thesis reveals that EIC cooking efficiently reduces the reject content for both birch and eucalypt. By using EIC cooking, the defibration point was shifted to a kappa number of ca. 30 from ca. 20 using conventional kraft cooking. This study demonstrates the great potential for achieving a higher overall yield for eucalypt by terminating the EIC cooking at a high kappa number, but with the conditions used in this thesis, no improvement in yield was observed for birch. An important issue is that the termination of kraft cooking at high kappa number increases the demand for extended oxygen delignification to reach a similar kappa number into bleaching, i.e., due to cost and environmental reasons. Extended oxygen delignification was shown to be possible for both birch and eucalypt EIC pulps (i.e., from kappa number 27 to 10) with an acceptable pulp viscosity number. The other part of this thesis addresses aspects regarding the limitations in oxygen delignification. It has previously been shown in the literature that a high xylan yield of kraft cooking could negatively affect the efficiency of subsequent oxygen delignification. In this work, the increased xylan content in eucalypt kraft pulp within the range of 8–18% had only a marginally negative impact on the oxygen delignification efficiency after correcting for the HexA contribution to the kappa number. It is also desired to extend the oxygen delignification towards lower kappa number, i.e., below kappa number 10 to decrease the bleaching chemical requirement. In this study, the hypothesis that the reduced efficiency of oxygen delignification at low kappa numbers could partly be due to the formation of oxidisable carbohydrate-related structures (i.e., HexA and/or other non-lignin structures) was also tested. No formation was established. On the other hand, a final oxygen delignification stage in the bleaching could be an attractive alternative for reducing yellowing and enhancing brightness; in fact, this has led to the development of a patent (SE 528066).