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You are here : Home > Training > EFPG Days > 11- Effect and future of the bleaching chemicals (slides 9-15)

Last update : September 12, 2003

			Third Session : Bleaching
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					XI - Effect and future of the bleaching chemicals (slides 9-15) Dominique Lachenal (EFPG)

A - Bleaching chemistry (slides 9-15)

	9 - Bleaching with hydrogen peroxide Hydrogen peroxide would react as OH°+OOH°: 2 H2O2   OH° + OOH° + H2O				10 - Summary of bleaching chemistry     2 molecules of Cl2 attack 1 aromatic ring 1 molecule of ClO2 attacks 1.25 aromatic ring 1 molecule of O3 attacks 1 aromatic ring (H2O2 neglected) 1 molecule of O2 attacks 1 aromatic ring (only 50% of the aromatic rings can be attacked) 1 molecule of H2O2 attacks 1 aromatic ring (only 50% of the aromatic rings can be attacked)

	11 - Bleaching chemicals requirements For the oxidation of one free phenolic aromatic ring : 142g Cl2   54g ClO2 <48g O3 32g O2 68g H2O2 For the oxidation of one phenolic aromatic ring: 142g Cl2 No reaction with O2 nor H2O2 54g ClO2 <48g O3				12 - Practical consequences     Peroxide delignification has no economical justification.   Free phenol must advantageously be attacked by oxygen. However, the potential of oxygen is limited (no action on the other phenols).   Chlorine dioxide attacks the free phenols as oxygen. However the other phenols are slowly degraded because in situ chlorine hydrolyses them into free phenols.   Ozone attacks any phenolic group. Cost of ozone charge shold be equal to - or lower than - that of chlorine dioxide charge, depending on the effect of in situ produced hydrogen peroxide.

	13 - Chlorine dioxide consumption     In a pulp of kappa number 30 the lignin content is around 4.5%.   100g of pulp contain 4.5 g lignin i.e. 0.02 aromatic unit (weight of 1 aromatic unit is about 200g).   Chlorine dioxide consumption should then be 54 x 0.02 = 1.08g. In practice, consumption is 2 to 3g.				14 - Chlorine dioxide efficiency In ECF bleaching based on the use of chlorine dioxide, at least half of the ClO2 charge is lost in useless reactions.

	15 - Explanation of chlorine dioxide loss The hypothesis is that ClO2 reacts with the primary products of ClO2 reaction with lignin: primary products + ClO2    secondary products

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