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Investigating the effects of wood chemistry and cooking conditions on pulp properties at a dissolving wood pulp mill.

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Pulp viscosity is the main criteria used to determine the endpoint of the pulping process in acid bisulphite pulping for production of dissolving pulp (DP). However, pulping performance is highly dependent on the relative abundance, structure, and reactivity of wood chemical components, mainly cellulose, hemicellulose and lignin. Thus, the chemistry of wood components has become an important factor when selecting wood species for use in the production of DP. It is believed that the chemical composition of wood components in tree species can be used to predict the pulp properties after pulping the wood. Although this correlation has been reported in Kraft pulping studies, there is very limited knowledge of such correlations in the acid sulphite pulping processes. This study entailed an investigation to ascertain if there are correlations between wood chemistry components and the resulting pulp properties from acid sulphite pulping process. The study was conducted on wood samples from Eucalyptus species, coded in this study as (E.001, E.002, E.003, E.004), Acacia mearnsii (Wattle) and additional clones and hybrids of eucalyptus species (W962-71, G438-105, A189-97, 100 GN). The samples were pulped using the magnesium acid sulphite cooking process, at wood specific base charge of 2.5 and 3.0. The wood chemistry of the trees and properties of their resultant brown pulp samples were determined. The results showed that there were variations in wood chemical composition among the Eucalyptus species that were evaluated. A lower base ratio of 2.5 produced pulps with lower viscosity and lower kappa than those produced at base ratio 3.0. These pulping conditions resulted in about 1 to 2% lower pulp yields than those reported for standard mill pulping conditions. Comparison of data on wood chemistry composition and pulp properties seemed to indicate good correlations on samples cooked at base ratio of 3.0 with a correlation coefficient (r) maximum of 0.71: the correlation was between alpha cellulose and total pulp yield. Correlation coefficients above 0.5 are considered significant. The results suggested that for some eucalyptus species pulped at base ratio of 3.0, a higher syringyl to guaiacyl S/G ratio in wood resulted in a higher delignification rate, resulting in lower residual lignin (K number) and higher pulp brightness. Eucalyptus wood samples with higher total lignin produced lower total pulp yields and brightness values whereas those with higher alpha cellulose content produced higher pulp yields with higher brightness values. Analysis of a combination of various wood components, namely, S/G ratio, hemicellulose content, and alpha cellulose content led to derivation of a formula that showed these parameters could be used to predict the viscosity of resultant pulps: the correlation coefficient was approximately 0.7 was obtained at base ratio 3.0, whereas 0.67 was obtained at base ratio 2.5.


Masters Degree. University of KwaZulu-Natal, Durban.