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Electrochemically mediated oxygen bleaching of pulp Perng, Yuan-Shing


Two electrochemical reactors, one operated at atmospheric pressure, the other at superatmospheric pressure, were used in a study of electrochemically mediated oxygen bleaching of unbleached Kraft softwood pulp. The atmospheric reactor was used to help elucidate the effects of reactants in electrochemically mediated oxygen bleaching. In addition, the electrochemical behaviour of the reactants, which was measured by cyclic voltammetry, was correlated with the bleaching effects. Ions of the transition metals Fe, Mn, Co, Cr, Cu, V, complexed by various ligands, as well as permanganate, dichromate, and anthraquinone were examined at pH levels of 9.4, 11.2, and 14. The simple salts of the abovetransition metals, together with ferricyanide, permanganate, anthraquinone, benzoquinone, bromate, and PdC12/CuC12 were also investigated in 0.18 M H2SO4. Of all the hydrolytically stable metal complex reactants investigated in alkaline conditions, only those with a reduction potential at or above that of the ferri/ferrocyanide couple at the pH of reaction, and with reversible redox behaviour, showed a synergistic effect with oxygen in pulp bleaching. Specifically, these were ferricyanide (Fe(CN)63-, hexacyanoferrate(III)), iron(III)/2,21-dipyridyl(Fe(dipy)33+, tris(2,2'-dipyridyl)iron(III)), and ferrate (Fe042-, tetraoxoferrate(VI)). In the super atmospheric reactor study of oxygen bleaching mediated by electrochemically generated ferricyanide, four sets of 24 factorial experiments were performed to illuminate the process mechanism. In addition, kinetic information was obtained via a set of parametric runs. Finally, stoichiometric experiments, designed to determine the ratios of the decrease in lignin content and cellulose degradation to ferricyanide consumption, were executed. In these experiments, five process variables; current, oxygen pressure, pH, temperature, and time, were investigated. Extensive experiments with the ferri/ferrocyanide reactant system showed that oxygen bleaching mediated by electrochemically generated ferricyanide increased the rate of delignification and cellulose degradation and gave a small increase in selectivity. As well, electrochemical oxygen bleaching provided milder operating conditions than employed in conventional oxygen bleaching. The highest selectivity in the electrochemical bleaching process was obtained by in-situ generation of ferricyanide with zero oxygen pressure (nitrogen sparge). However, the delignification rate measured under nitrogen was about one third to half the rate measured under oxygen with the same current. In the presence of oxygen, the selectivity was increased by application of a current together with increased pH and temperature, but increasing the current decreased the selectivity. The presence of ferricyanide in the oxygen bleaching process shifted the rate determining steps for both the lignin and cellulose oxidation from reaction with oxygen to a diffusion controlled reaction with ferricyanide and made the reaction rates nearly independent of oxygen pressure above 0.1 MPa. The stoichiometry of the reactions of ferricyanide with lignin and cellulose varied with operating conditions but results indicated that both reactions are non-catalytic. A mechanism has been proposed for electrochemically mediated oxygen delignification in which the reaction of lignin with oxygen is initiated by oxidation of the lignin with the higher valent form of the metal complex. This initiation reaction has been identified as the mechanism's rate determining step. It is recognized that the metal complexes might also catalyze peroxide decomposition via the "Fenton" reaction, thus promoting cellulose degradation. This study of electrochemically mediated oxygen bleaching of pulp has showed how a reversible redox couple promoted oxygen bleaching reactions in alkaline conditions. The delignifying action of this redox couple was sustained by current. These results have opened a door to further exploration of both the effects of metal complexes on bleaching reactions and the kinetics and mechanism of electrochemically mediated oxygen bleaching.

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