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Effects of metal catalysed peroxide decomposition on the bleaching of mechanical pulp

posted on 2023-05-27, 07:03 authored by Brown, DG
Hydrogen peroxide is widely used as an industrial bleaching agent, particularly for the bleaching of mechanical pulps. Under the alkaline conditions required for effective bleaching a number of transition metal ions, including iron, copper and manganese, catalytically decompose peroxide resulting in a loss in the concentration of active bleaching reagent, with a subsequent reduction in the bleaching efficiency. In this study the effects of transition metal ions on the decomposition of hydrogen peroxide have been studied, both in the presence and absence of pulp. The influence of various additives, including sodium silicate and magnesium, on the rates of metal catalysed peroxide decomposition have also been studied. The results of studies of the stability of alkaline hydrogen peroxide solutions in the absence of pulp have demonstrated that the behaviour of transition metal ions towards decomposition depends on the hydrolysis species of the metal present. Changes in the rate of decomposition over the course of the reaction can be attributed to changes in the catalytically active species present. The addition of a number of additives to peroxide solutions containing added transition metal ions can also lead to changes in the observed catalytic activity, either accelerating the rate of decomposition or stabilizing the systems. In the presence of these additives a correlation was observed between the catalytic activity of the transition metals and the formation of transition metal-additive complexes, which could be monitored by UV-visible spectroscopy. This implies that a direct interaction between the metal ions and additives is responsible for the observed changes in the catalytic activity. It is proposed that the mechanism by which the additives operate is by the formation mixed hydrolysis species with the transition metal ions, with the resulting species having altered catalytic properties. Studies of the effects of metal catalysed decomposition in the presence of pulp were conducted using a P. radiata TMP. The addition of the desired metal ions to prechelated pulp enabled their catalytic activities to be determined individually. The effects of sodium silicate and magnesium on the rates of metal catalysed decomposition were also determined in the presence of pulp. Comparison of the behaviour of these systems in the presence and absence of pulp reveals that the inclusion of pulp can have a significant influence on the catalytic activities. These results demonstrate that it is not possible to predict the peroxide decomposition behaviour during bleaching from studies conducted in the absence of pulp.


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Copyright 1993 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Thesis (Ph.D.)--University of Tasmania, 1994. Includes bibliographical references (p. 219-230)

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