Investigating the pathway and fate of inorganic impurities in a bisulfite dissolving pulp production process.
Loading...
Date
2011
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
This study sought to investigate the pathway and fate of metals and Si in a bisulfite pulp
production process, at the same time providing a means to mitigate on the residual metals
bound in the final pulp. The metals pathway and fate was traced from the Eucalyptus
plantation, through the pulp production process. Principal component analysis was employed
to determine the correlation between the observations and the variable of interest. It was
established that Fe is a limiting factor in the growth of the Eucalyptus trees in the forest under
study. The main pathway for the metals and silicon was found to be the exchangeable soil
fraction. The residual metals in the pulp material during pulping and bleaching process are
influenced by the media pH, the accessibility of the active sites, the affinity of the metal
towards the active sites and the degree of delignification and hemicelluloses extraction. Metal
desorption under the influence of acetylacetone at both pH 6 and 8 were best described by
Langmuir desorption model. Pulp metal desorption under the influence of EDTA was also
best described by the Langmuir model except for Al (R2) 0.572 and 0.004, at pH 6 and 8
respectively. This implies that most of the metals in the dissolving pulp sample were
chemically adsorbed on the surface of the pulp. However, aluminum was poorly described by
the Langmuir isotherm model. This is because polyvalent metals like Al are hard Lewis acids,
capable of strong and specific bonding to hard Lewis base functional groups on organic
molecules. The Freundlich model described Mg, Al and Cu desorption suggesting some
fraction of these metals to have been physically adsorbed onto the pulp material. It is possible
that the metal fraction being physically desorbed was introduced onto the pulp during the
pulp bleaching process. Voltammetry was employed to investigate the effect of organic
ligands on metal mobility in bleaching filtrates. It was observed that filtrate samples obtained
from the alkaline-oxygen bleaching stage produced higher current peak suppression at pH 2
as compared to the filtrate samples obtained from alkaline bleaching stage; this indicates the
presence of stronger none-labile metal ligands. At a higher pH of 3.6 the voltammograms of
the filtrates obtained from the alkaline-oxygen delignification produced higher cathodic peak
shifts. The extent to which the peak potential shifts cathodically is indicative of the
magnitude of the stability constant. This may imply that the samples obtained from the
alkaline-oxygen delignification comprise of ligands that form strong metal-ligand complexes
Description
Ph. D. University of KwaZulu-Natal, Durban 2011.
Keywords
Sulfite pulping process., Wood-pulp--Bleaching., Voltammetry., Theses--Chemistry.