Chlorine dioxide bleaching conditions

The commercial importance of this metal was first recognized in 1950s when its high strength/density ratios were found attractive in aerospace applications. The corrosion resistance in a variety of conditions led to its use in wet chlorine gas coolers for chlor-alkali cells, chlorine and chlorine dioxide bleaching equipment in pulp/paper mills, and reactor interiors for pressure acid leaching of metallic ores. The metal and its alloys were used in seawater power plant condensers, with over 400 million feet installed in application.65,66 The most commonly used alloys and their composition are given in Table 4.48. 

The stoichiometric relationship between chlorine dioxide added and color removed during bleaching is nonlinear, but it is independent of temperature, pH, and pulp concentration under conditions normally used. Models used to explain the kinetics and stoichiometry show a strong dependence on chromophore concentration that probably results from differences in the reaction rates of the various chromophores present in the pulps (80). 

Pulp bleaching with chlorine dioxide is most often performed at an acidic pH, so that the final pH of the bleach Hquor is in the range of 2—5. Under these conditions, the residual concentration of chlorite and chlorate ions in the bleach Hquor are minimized and chloride ion is the predominant chlorine species in the spent bleach (77). In addition to direct addition to pulp in bleaching, chlorine dioxide also finds use in wastewater treatment from pulp mill operations as a means to remove effluent color (85). 

Chlorine dioxide, prepared from sodium chlorate by reduction, reacts quite selectively with lignin and is therefore widely used for pulp bleaching. At slightly acidic conditions (pH 4-5) chlorine dioxide can be reduced to chloride ions ... 

The oxidation pathways of chlorine dioxide under actual conditions are complex because a number of species including chlorine, hypochlorous, chlorous, and chloric acids are formed as intermediates. A rapid conversion of chlorine dioxide to chloride and chlorite (chlorous acid, pK 2.0) may first take place, followed then by a slow phase during which mainly the chlorite reacts with the pulp components. However, continuous generation of chlorine dioxide during bleaching takes place, for example, by the reaction between chlorite and chlorine (or hypochlorous acid) ... 

Chlorine dioxide, CIO2, contains a central chlorine atom with an oxidation number of +4. It has an odd number of electrons and is a paramagnetic yellow gas that is used to bleach paper pulp. It is highly reactive and may explode violently under the right conditions. 

Electrochemical protection of bleach washers has permitted the continued use of lower alloyed stainless steels, while washing conditions have become more corrosive. Electrochemical protection was applied successfully in an early chlorine dioxide washer [101], Electrochemical protection at slighdy cathodic potentials was commercially appUed for C- and D-stage washers [51,102-106], The effectiveness of electrochemical protection has been assessed by means of coupons [104],... 

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