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The role of radical species in peroxide bleaching processes

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Hobbs, Graham Charles (1994) The role of radical species in peroxide bleaching processes. PhD thesis, University of Tasmania.

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Abstract

Analysis of expressions describing peroxide bleaching show that there are two distinct
mechanisms involved in peroxide bleaching. These routes can be associated firstly
with traditional bleaching by the perhydroxyl anion and the second mechanism appears
to involve the action of radical species. This analysis has been used as the basis for
experimental work to determine the existence and relative contribution of each of these
mechanisms.
Two-stage bleaching processes under acidic followed by alkaline conditions, without
an intermediate washingstage, have been investigated in order to separate the
contributions of anion and radical bleaching. The addition of chromium nitrate, which
is known to catalyse peroxide decomposition under acidic conditions, to such a
process enhances the brightness of the pulp compared with a single stage alkaline
peroxide bleaching process. The addition of small quantities of aluminium nitrate to a
two-stage bleaching process also enhances the brightness of the pulp, this process
depending on the presence of both aluminium and manganese. The results are
explained in terms of radical concentrations in the acidic stage and the proposed
mechanisms and active bleaching species in peroxide bleaching.
Many transition metal ions decompose peroxide, with some producing significant
quantities of hydroxyl radicals. The effects, of addition and removal of these metals on
peroxide bleaching have been investigated. Unusual negative effects are observed
upon chelation and the addition of manganese, copper and iron does not appear to
promote darkening reactions caused by the decomposition products of hydrogen
peroxide.
The effect of pulp consistency on brightness response has also been investigated. Work at constant conditions of peroxide and alkali, while varying either the total
consistency or the ratio of bleachable to inert fibre shows that consistency is not a
major factor in determining final brightness under alkaline conditions and indicates that
the effect of radical species is small at low consistencies.
A method of determining relative hydroxyl radical concentrations in solutions of
hydrogen peroxide has been developed, using N,N-dimethy1-4-nitrosoaniline. Under
alkaline conditions the effect of the hydroxyl radical appears to be negligible, whilst
under acidic conditions the concentration of radicals can be corelated to the observed
final brightness.

Item Type: Thesis (PhD)
Keywords: Wood-pulp, Papermaking, Peroxidation
Copyright Holders: The Author
Copyright Information:

Copyright 1994 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).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1994. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:28
Last Modified: 09 May 2017 06:25
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