Chlorine dioxide pulp bleaching

Atlanta, Georgia measurement of mixing quality in chlorine dioxide pulp bleaching ... 

Uses. The primary (95%) use of sodium chlorate is in the production of chlorine dioxide for bleaching in the pulp (qv) and paper industry (qv). Uses are summari2ed in Table 10. 

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) ... 

STERNCHEM Polyester Mortar is formulated to provide superior service for acid brick constructions in the chemical process, pulp and paper, plating, textile, fertilizer, and metal refining industries. They are especially suited for chlorine dioxide towers, bleaching vats, trenches, sumps, pits, bleaching solutions of sodium hypochlorite, oxidizing solutions of nitric and chromic acids to 20% concentrations, hydrobromic acid, potassium and sodium dichromates, bromine water, salt solutions to pH 11. 

Figure 20-15 Typical pulp bleaching results, a) Kappa number versus reaction time for chlorine dioxide (Dq) delignification of a softwood kraft pulp (Cm = 0.031, initial pH 3). (From Tessier and Savoie, 1997.) (b) Kappa number following chlorine dioxide (Di) bleaching of a kraft softwood pulp. Pulp was bleached in the mill using a DEop sequence and laboratory bleached (Di) with hand mixing (at Cm = 0.10 and T = 74°C). (From Bennington et al., 2001.)...

Liquid chlorine dioxide, ClOj, boils at 284 K to give an orange-yellow gas. A very reactive compound, it decomposes readily and violently into its constituents. It is a powerful oxidising agent which has recently found favour as a commercial oxidising agent and as a bleach for wood pulp and flour. In addition, it is used in water sterilisation where, unlike chlorine, it does not produce an unpleasant taste. It is produced when potassium chlorate(V) is treated with concentrated sulphuric acid, the reaction being essentially a disproportionation of chloric(V) acid ... 

Uses. The dominant use of sulfur dioxide is as a captive intermediate for production of sulfuric acid. There is also substantial captive production in the pulp and paper industry for sulfite pulping, and it is used as an intermediate for on-site production of bleaches, eg, chlorine dioxide or sodium hydrosulfite (see Bleaching agents). There is a substantial merchant market for sulfur dioxide in the paper and pulp industry. Sulfur dioxide is used for the production of chlorine dioxide at the paper (qv) mill site by reduction of sodium chlorate in sulfuric acid solution and also for production of sodium dithionite by the reaction of sodium borohydride with sulfur dioxide (315). This last appHcation was growing rapidly in North America as of the late 1990s. 

The biggest use of chlorine dioxide is in bleaching wood pulp. In some mills, much of the chlorine and hypochlorite has been replaced by chlorine dioxide to reduce the amount of chlorinated by-products. Chlorine dioxide is also used to bleach textiles, flour, and edible fats and oils. 

Large-Scale Industrial Production. Large amounts of chlorine dioxide ate used in pulp bleaching and smaller quantities ate used for the manufacture of sodium chlorite. In these appHcations, sodium chlorate is the only commercially available taw material. Chlorine dioxide production from sodium chlorate is achieved by the reduction of the chlorate ion in the presence of strong acid. The reaction consumes acid, so that acid and reducing agents must be constantly added to maintain the reaction. 

In all cases, chlorine dioxide is produced at the point of use either from sodium chlorite or sodium chlorate. Production volume can be accurately estimated from total sodium chlorate consumption for chemical pulp bleaching because this use accounts for greater than 95% of all chlorine dioxide production. 

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