Chlorine dioxide, bleaching agent

Figure 16.2a summarizes the major uses of chlorine. Chlorinated organic compounds, including 1,2-dichloro-ethene and vinyl chloride for the polymer industry, are hugely important. Dichlorine was widely used as a bleach in the paper and pulp industry, but environmental legislations have resulted in changes (Figure 16.2b). Chlorine dioxide, CIO2 (an elemental chlorine-free bleaching agent), is prepared from NaC103 and is favoured over CI2 because it does not produce toxic effluents. ...

Modern chlorine-based bleaching agents include elemental chlorine (CI2 gas), a variety of hypochlorites (OCl ), certain N-chlorinated organic compounds, and chlorine dioxide (CIO2). The first three bleaching agents all hydrolyze to produce hypo-chlorous acid in aqueous solution, according to the equilibria in reactions [I]-[III]... 

The oxides of the halogens are generally highly reactive compounds. Chlorine oxide has been extensively studied due to its role in the destruction of stratospheric ozone. Chlorine dioxide is a powerful chlorinating and bleaching agent. Iodine pentoxide is used to quantitatively determine carbon monoxide concentrations. 

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. 

After World War I, other chlohne-based bleaches were developed. In 1921 the use of chlorine dioxide for bleaching fibers was reported followed by the development of the commercial process for large-scale production of sodium chlorite. In 1928 the first dry calcium hypochlorite containing 70% available chlorine was produced in the United States. This material largely replaced bleaching powder as a commercial bleaching agent. 

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. 

Manufacture. Chloric acid is the precursor for generation of chlorine dioxide for pulp bleaching and other appHcations (see Bleaching agents), and is formed m situ by reaction of sodium chlorate [7775-09-9], NaClO, and a strong acid, eg. 

Nearly 95% of the sodium chlorate produced ia North America is used to produce chlorine dioxide [10049-04-4] CIO2, for pulp (qv) bleaching (see Bleaching AGENTS, pulp and paper) (37). Minor amounts are used to produce other chemicals such as KCIO, NaC102, NaClO, etc, to recover uranium [7440-61-17, U (see Uraniumand uranium compounds) and for agricultural uses as a defoHant or herbicide (see Herbicides). 

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