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Towards efficient oxygen delignification

The overall aim of the project is to improve the oxygen delignification process for the manufacture of chemical pulps. Delignification with oxygen in alkaline solution is a very useful method for the removal of lignin from pulp and, accordingly, most of the pulp and paper mills in Europe use oxygen delignification to produce pulps for bleaching. However, both the efficiency and selectivity of oxygen delignification need to be improved in order to make full use of its potential, since only half of the lignin can be dissolved. This is mainly because non-phenolic b-aryl ether structures, the network building structure in lignin, are virtually non-reactive under oxygen-alkali conditions.

This project is aiming to find conditions under which degradation of all the different structural features present in lignin can be achieved, in order to reach a greater extent of lignin degradation.
The first specific goal is to find and test active chemical or biochemical agents to enhance the cleavage of the networking non-phenolic ether linkage in lignin. The second is to activate kraft pulps before the oxygen delignification with these selected agents in order to improve the selectivity of the oxygen delignification process. Significant improvements might be reached by 'advanced oxidation technologies' (AOT).
An evaluation of the pulp properties will be made for this improved modified oxygen delignification process, and an estimate of the costs will be calculated.

Progress to Date
The Oxydelign project investigated basic reaction mechanisms for the formation of certain radical cations as a base for a better understanding of the action of enzymes (especially laccase) during pulp delignification. A large number of lignin model compounds were prepared and examined. A number of structures were identified and evaluated as possible mediators in the enzymatic oxidation of lignin model compounds, particularly in the laccase-catalysed reactions. The information collected on the efficiency of these structures was implemented into new oxidative strategies for activating lignin structures in pulps. These new oxidative strategies were applied to wood pulps by using appropriate pre-treatments to activate lignin structures towards subsequent oxygen delignification. The project clarified the efficiency of various artificial mediators in oxidation reactions. Experimental results obtained supported the fact that suitable phenols can act as 'natural' mediators of laccase. This explains the apparent paradox that some ligninolytic fungi do not excrete the stronger oxidising enzymes such as lignin peroxidase or manganese peroxidase, and only rely on laccase to carry out the oxidative degradation of lignin. The best mediator identified for laccase, and for the oxidation of non-phenolic substrates, was violuric acid. Subsequently, the most satisfactory laccase/mediator combination, i.e. the one with violuric acid, was applied as an efficient pre-treatment of pulps to enable the second stage of the oxygen delignification treatment to proceed more extensively.
Moreover, the Oxydelign project provided fundamental information on the reactivity of aromatic radical cations and alkoxyl radicals, reactive intermediates, which have been shown to play an important role in the chemical and enzymatic oxidative degradation of lignin model compounds.

The Oxydelign project achieved in-depth knowledge on the behaviour of certain lignin and cellulose structures in advanced oxidation technology systems, for example, carbonate ion radical and laccase enzyme mediator systems, as well as with these systems combined with oxygen delignification treatment. The reaction mechanisms prevailing in the advanced oxidation technology systems were clarified using model compounds under controlled reaction conditions. The delignification selectivity of the advanced oxidation technology and enzyme (laccase) mediator system, when applied to softwood kraft pulps in combination with oxygen delignification and final ECF bleaching to full brightness, has now been demonstrated. Evidently, the delignification selectivity is often higher for the enzyme/mediator systems than that for the carbonate ion radical system, but the brightness of the pulps obtained with this system are lower. Many interesting delignifying/activating substances and systems have been evaluated within this project, but more research is needed before these new methods can be developed further into industrial scale processes.


Scientist responsible for the project

Associate Professor OLOF Dahlman
drottning Kristinas väg 61 Box 5604
114 86 Stockholm
Sweden - SE

Phone: +46 8 6767120
Fax: +46 8 108340


Project ID QLRT-1999-01277
Organisation Stiftelsen STFI Skogsindustrins Tekniska Forskning
Area 5.3.2
Start date 21 January 2000
Duration (months) 42
Total cost 2 280 700 €
Total EC contribution   1 625 966 €
Status Completed
Web address of the project

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