Investigating the Pathway and Fate of Inorganic Impurities in A Bisulfite Dissolving Pulp Production Process

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