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December 15, 2016
Jennifer Marcon, Ph.D. student of LGP2, defended her doctoral thesis: "Lignocellulosic pulp: study of a new ECF bleaching stage with low environmental impact".
This doctoral thesis was prepared at University Grenoble Alpes under the supervision of Professor Gérard Mortha, and of Nathalie Marlin, Associate Professor (Grenoble INP-Pagora / LGP2).
Chlorine dioxide is the most widely used bleaching agent for the production of bleached chemical pulps. However, its main drawback is the formation of chlorate, which decreases the delignification efficiency, and the reject of COD and toxic chloro-organic molecules (AOX) in mill effluents.
This study focused on the development of a new bleaching stage using chlorine dioxide (D stage) in non-conventional conditions, to reduce the environmental impact and production costs. The work was carried out on several softwood kraft pulps after cooking, taken at different stages of the bleaching sequence. The best results of the novel D stage were obtained for pulps at low kappa number, i.e. at the end of the bleaching sequence. The D stage was optimized and coupled with hydrogen peroxide addition. The same brightness and average degree of polymerization as for conventional D bleaching were obtained. Interestingly, a very important decrease of pollution load (70% of AOX and 20% of COD) was obtained, accompanied by a significant gain of productivity and energy saving (lower temperature and reaction time).
Chemical investigations on the reaction mechanism, carried out by different techniques (ESR, NMR and FTIR spectroscopies, HPAEC-PAD chromatography), allowed to reveal the main features of the reaction mechanism of chlorine dioxide and highlight the structural modifications brought to the pulp residual lignin during the novel unconventional D bleaching stage.