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|You are here: Home > Technique > Processes > Scientific report of the LGP2 > Chemical processes > Behaviour of pulp mixtures containing chemical and mechanical fibres during hydrogen peroxide treatments with oxygen. Application to bleaching of recycled fibres||Update: January 8th 2007|
|Scientific report of the LGP2
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|Researchers of the LGP2 (EFPG, INPG, CNRS, CTP)
Documents taken from the
"Scientific Report of the Laboratory of Pulp and Paper Science and Graphic Arts - UMR 5518
Grenoble - France
January 2002-November 2005"
The increased production of high quality recycled fiber-based papers requires an optimal sequencing
of deinking and bleaching stages. Mixed office recovered papers are generally contaminated
with coloured papers and brown envelopes, containing substantial amounts of mechanical pulp
fibres. Consequently, the production of high brightness deinked pulps increases in difficulty,
and additionally to the elimination of dyes and inks, it requires adequate bleaching stages suited
to mixed pulps.
Pulps made from recovered papers are usually treated with hydrogen peroxide (P stage). However, this process is not efficient enough to bleach some furnishes, and hydrogen peroxide under oxygen pressure bleaching (OP) has been recently proposed as a better technique to destroy lignin chromophores. To examine the effect of oxygen bleaching on the different components of cellulosic pulps, pulp models containing various types of fibres – from mechanical and chemical pulps – have been bleached under variable conditions. Oxygen was found to have a beneficial effect on mixtures containing kraft pulp (brightness gain up to 9 % ISO), but the effect was much more limited with mechanical pulp fibres (maximum brightness gain of only 1 % ISO). The poor reactivity of oxygen on mechanical pulp mixtures could be explained by the differences in the chemical structure of kraft lignin and mechanical pulp lignin. Free phenolic groups and carboxyl groups in lignins were determined by 13C NMR spectroscopy, and quinone groups were analysed by 19F NMR spectroscopy.
Table 1 - Major Fonctional groups in
TMP = mechanical pulp,
TMP-O = O treated mechanical pulp,
K = unbleached chemical pulp,
K-O =O treated chemical pulp
Expressed in mole/ 200g of lignin (10% accruracy)
Mechanical pulp lignin and kraft lignin did not contain the same amounts of functional groups
[Table 1]. The mechanical pulp lignin contained a significantly lower amount of free phenolic groups;
additionally, the mechanical pulp lignin exhibited a much higher molar mass (measured by
size exclusion chromatography). Both effects provide a good explanation for the lower reactivity of
oxygen on the mechanical pulp lignin. As expected oxygen bleaching did not affect the molar mass of
the mechanical pulp lignin, whereas kraft lignin was depolymerised. The analysis also showed that
the oxygen treatment created some carbonyl groups on the machanical pulp lignin, which was
not the case for the kraft lignin. The poor discolouring effect of oxygen on mechanical pulp could
be explained by the creation of new chromophores. However, additional experiments indicated
that the coloration of mechanical pulp during the O stage was due to the effect of alkali, rather than
by the action of oxygen istself. Consequently, contrary to what was observed on chemical pulps,
oxygen addition in a P stage had no beneficial effect in the case of mechanical pulp-containing
mixtures of fibres.
Pulps made from recovered fibres are also usually contaminated by dyes. Hydrogen peroxide in alkaline medium applied on a fully bleached chemical pulp impregnated with dyes was generally not efficient even in the presence of oxygen. However, when H2O2 was applied in acidic medium in the presence of iron, discoulouration was strongly improved.
Industrial deinked pulps mixtures were also reacted in alkaline medium with hydrogen peroxide with or without oxygen added; the results obtained confirmed the beneficial effect of oxygen only in the case of a low content of mechanical pulp.
|Brightness gain (%)|
|DIP wood-free +10% PK||DIP wood-containing +10% PK|
Table 2 - Brightness gain obtained
after P and (OP) stages
applied on industrial deinked pulps (DIP) contaminated
with unbleched chemical pulp
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