Kraft pulping of Portuguese Eucalyptus globulus:: Effect of process conditions on yield and pulp properties

The major goal of the present work was to study the response of Eucalyptus globulus Labill crowing in Portugal to changes in kraft cooking process variables and their relationship with yield, Kappa number and intrinsic viscosity of the resulting pulps. For this purpose, approximately one hundred cooks were performed in a laboratory digester using a homogeneous sample of E. globulus wood chips. Mathematical models successfully quantified the effect of cooking conditions on pulp properties. H factor and Tasman factor (EAxlog(10)S) were sufficient to describe the Kappa number response. However, for total yield modelling an extra term including effective alkali was required. Viscosity was found to be dependent on effective alkali. with the cross term EAxT being the other main factor. To predict the selectivity of the process, yield and viscosity were also related to Kappa number by empirical models. Eucalypt kraft pulps are becoming increasingly important as a source of paper-making fibres. Many studies have been published, mainly for the Australian (1-2) and Brazilian species (3-4). concerning the response of this hardwood to changes in cooking parameters. It is well known that this response is highly affected by the morphology,. density and chemical composition of the wood, which in turn significantly vary within this genus. Thus, distinct behaviours are to be expected during pulping and bleaching of different species. Moreover. even for the same eucalypt species there is a clear variation in the physico-chemical composition which can produce pulps with dissimilar properties depending upon age (5-6), genetic factors (5,7), growing environment conditions (5, 8-9) and pulping process conditions (3,10). The eucalypt species growing in Portugal, E. globulus Labill, has specific structural features that may affect the technological properties of the corresponding pulps. An unusual chemical structure of the xylan was found for this wood (11), which is composed of galactosyl residues in addition to glucuronosyl and xylosyl residues, being 1: 10 the molar ratio of the latter (11), instead of 1:6 as for the same species growing in Australia (12). However, no attempt is made in this 7 study to compare the pulping properties of Portuguese E. globulus with any other source of this tree species. Although Portugal is the second largest producer of bleached eucalypt pulp, which has a reputation of very good quality for office papers, there are not many studies concerning the behaviour of this species under different cooking conditions (13-14). Furthermore, there is still a lack of systematic research devoted to predicting pulp properties that could be used to improve pulping selectivity (which can be defined as pulp viscosity at a fixed Kappa number). An extensive study of the effects of some kraft cooking conditions on the yield and Z properties of Portuguese E. globulus kraft pulps, which covered a wide ranges of alkaline charges, sulfidities and temperatures, has been reported previously (15-16). Additionally, a significant number of pulps from this study with identical Kappa numbers, at a level close to industrial practice, were produced and used to investigate pulp bleachability (15,17). The present paper focuses on the experimental results from this study concerning the effect of cooking parameters on yield, Kappa number and intrinsic viscosity.