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You are here : Home > Training > EFPG Days > 11- Effect and future of the bleaching chemicals (slides 16-23)

Last update : September 12, 2003

			Third Session : Bleaching
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					XI - Effect and future of the bleaching chemicals (slides 16-23) Dominique Lachenal (EFPG)

B - Perspectives (slides 16-23)

	16 - Introduction There exists a great potential of cost saving in bleaching, if one knows how to limit the extent of secondary reactions. One way is to avoid excess of chemical reagent and run x y x y x y x... type of sequence, in which x and y stand for stages with low chemical charge.				17 - (de)(de)(de)... sequence (1)   DE (DE) (DE) (DE) ClO2 charge (%) 2.4 0.8 0.8 0.8 ClO2 consumed (%) 2.4 0.8 0.8 0.8 D final pH 2.7 3.0 3.6 3.4 NaOH charge (%) 3 1.5 1.5 1.0 E final pH 12.1 11.1 11.3 11.2 E kappa number 4.5 13.9 5.6 1.6 E brightness (%) 49 28 45 68 AOX (kg/t) 1.1 - - 0.46 D: 60 min, 10% consistency, 70 °C E: 60 min, 70 °C

	18 - (de)(de)(de)... sequences (2) The figure represents the variation of the kappa number as a function of chlorine dioxide concentration in a classic DE process and in a (de)(de)(de)(de) process.				19 - OOOO... sequences Variation of DPv as a function of kappa number for a OOO sequence (2% NaOH) and a OOOOOO sequence (1% NaOH).

	20 - Role of chlorine dioxide Chlorine dioxide has become the universal bleaching agent. Example: DEDED. Its high delignification power comes from the release of chlorine.				21 - The coloured groups Oxygen is the most economical way to extend delignification. However, its selectivity is poor. Example: ODEDED, OODED. Neither chlorine dioxide nor oxygen have a direct action on the coloured groups in lignin. Consequently, the chemistry of these two reagents must be completed by other chemistries.

	22 - Elimination of the coloured groups The colored groups are highly conjugated systems containing carbon-carbon double bonds and carbonyl groups. One example is the quinone chromophore: This type of chromophore can be destroyed by ozone or hydrogen peroxide.				23 - Conclusion More generally, ozone is the best chemical to destroy C=C bonds, and hydrogen peroxide the best one to destroy C=O groups. As a conclusion, the ECF bleaching sequences should include the use of ozone or hydrogen peroxide, depending on the nature of the chromophores.

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