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        Last update : June 19, 2003
                  Second Session : The Yield Issue                  
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            10 - Using recycled papers as raw material for incremental increase in chemical pulp production            
C. Chirat, M.T. Viardin, D. Lachenal (EFPG) et T.N.B. Nguyen (Ho Chi Minh Ville University)

Logo EFPGThe concept presented in this study is the use of recycled fibers as a raw material to produce chemical pulps. This study has been driven by two incentives: first, it is well known that as the recycling rate increases, the quality of recycled fibers usually decreases, unless a sufficient input of virgin fibers of high quality (like kraft fibers) is ensured, particularly on brown grades. The second incentive is the economy: wood costs represents about one third of a chemical pulp production cost. Lignin containing recycled paper prices are varying significantly over the time but they are lower than that of wood.

Recycled papers contain fibers of various origins (from chemical bleached and unbleached, or mechanical pulps), in addition to diverse contaminants (ink, fillers, and so on). The application of a mild cooking (kraft or soda/AQ) should delignify these pulps and thus lead to the production of unbleached and bleached "chemical" pulps. The idea would be to have a parallel line in a kraft pulp mill, so that the cooking liquor and the bleaching effluent are taken care of.

Literature results on cooking of recycled papers deal mainly with the use of OCC (Old Corrugated Container) from North America. This type of recycled paper is composed of more than 50 % of unbleached kraft fibers, and is easily delignified either by alkaline cooking (kraft or soda) or by oxygen delignification stages, with pulp yield higher than 75 % for a final kappa of 25 or higher. In some cases bleached pulp up to 87 % ISO was produced. No data of yield after full bleaching is available.

The present study presents the effect of delignification (kraft cooking or oxygen delignification) and bleaching on different pulp mixtures prepared from kraft liner, mechanical pulp and bleached chemical pulp (for the make up to 100 %). The different pulp mixtures were easily delignified by a mild kraft cooking down to 20-30 kappa number, at a pulp yield between 65 and 79, to be compared to a yield close to 50 when wood chips are cooked. The reason for this are the more severe conditions used to delignify wood chips. The pulp mixtures responded however differently to oxygen delignification: mixtures containing only chemical pulp were delignified more extensively than those containing mechanical pulp. The reasons for that could be differences in lignin structure and accessibility.

The different mixtures were bleached after kraft cooking, with a sequence usually applied to chemical pulps – ODEDED - up to brightness levels between 86 and 88 % ISO. The final yields were between 55 and 60 % (to be compared to 45-50 % when wood chips are the raw material).

The strength properties of these two bleached pulps were intermediate between those of a bleached softwood kraft and a bleached hardwood kraft pulps.

The economy of this process is dependant upon the price of recycled papers but should be favorable most of the time.


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