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Detection and evaluation of decay in pulp and paper fibre supplies Stirling, Roderick Anthony

Abstract

Decay of wood chips is a significant economic threat to the pulp and paper industry. It results in reduced wood chip quality, which in turn causes decreased pulp yields and poorer pulp properties. The currently accepted methods to measure decay content in wood, such as buffering capacity and 1% caustic solubility, are too laborious to be of practical use in mill environments. The present research developed and investigated methods for rapidly quantifying decay in wood chips. Partial Least Squares (PLS) models were developed, based on FTIR and NIR spectra, to predict the extent of decay in wood chips, using caustic solubility, buffering capacity and basic wood density as indicators of decay. Data from these models were found to be highly correlated with existing methods (r2 > 0.80). In order to provide further validation for the PLS models, changes in wood chemistry underlying the decay were also examined. Extent of decay, as shown by the PLS models, was found to correlate with changes in the concentration of acetyl groups and lignin in the wood samples. Following the development and validation of the PLS models, the impacts of inoculum size and storage time on chip and thermo-mechanical pulp properties were determined. Results showed that both sound chips and chips incubated with varying quantities of a brown rot decay fungus for up to four months showed little decay and few changes in thermo-mechanical pulp properties. The most significant change was a drop in pulp brightness that was accounted for by the storage time, rather than fungal inoculation, of the wood chips. In order to understand the effects of pulping samples with high levels of brown-rot decay, softwood chips were heavily decayed by a brown rot fungus under ideal growth conditions in the laboratory. Both kraft and refiner mechanical pulps produced from these heavily decayed chips showed significant differences from pulps produced from equivalent sound chips. Mechanical pulps were prepared to a given freeness with less energy, but had significantly poorer strength and optical properties. Kraft pulps were produced in lower yields and consumed more alkali. The length of kraft fibres decreased with increasing decay and resulted in poorer strength properties. Existing research has shown that kraft pulping of chips heavily decayed by brown rot fungi is not economical (Hunt, 1978b). These results confirm this existing research and extend it to mechanical pulping, showing that heavily decayed chips exhibit similar detrimental effects on both kraft and mechanical pulping. The PLS models developed to estimate extent of decay were able to identify wood chips with poor pulping properties.

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