Bleaches for wood pulp

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 ... 

Much of the CI2O manufactured industrially is used to make hypochlorites, particularly Ca(OCl)2, and it is an effective bleach for wood-pulp and textiles. CI2O is also used to prepare chloroisocyanurates (p. 324) and chlorinated solvents (via mixed chain reactions in which Cl and OCl are the chain-propagating species).Its reactions with inorganic reagents are summarized in the scheme opposite. 

Chlorine is used to produce many commercially important products. Tremendous amounts of it are used in extractive metallurgy and in chlorinating hydrocarbons to produce a variety of compounds (such as polyvinyl chloride, a plastic). Chlorine is present as CI2, NaClO, Ca(C10)2, or Ca(C10)Cl in household bleaches as well as in bleaches for wood pulp and textiles. Under carefully controlled conditions, CI2 is used to kill bacteria in public water supplies. 

Hydrosulphit . [BASF AG] Sodium dithionile-based reducing bleaches for wood pulps and wood-based old paper. 

Sulfur dioxide is produced on a large scale by burning sulfur, Sg. It is also obtained as a by-product of the roasting of sulfide ores (such as FeS2, CuS, ZnS, and PbS). Most of this sulfur dioxide is used to prepare sulfuric acid. Some is used as a bleach for wood pulp and textiles (Figure 22.46) and as a disinfectant and food preservative (for example, in wine and dried fruit). Its use as a food preservative depends on the fact that sulfur dioxide is especially toxic to yeasts, molds, and certain bacteria. Because some people are allergic to sulfur dioxide, foods containing it must be properly labeled. 

A particularly interesting reaction (and one of considerable commercial value in the BOROL process for the in situ bleaching of wood pulp) is the production of dithionite, S204 , from SO2 ... 

US military requirements and tests for wood pulp(sulfite) intended for manuf of NC are as follows(Ref 8) a)The material shall be bleached sulfite pulp from coniferous wood which has been washed to remove the purifying chemicals and formed into sheets or laps suitable for fluffing or shredding and subsequent nitration. 

Calcium and sodium hypochlorite are both used in the bleaching of wood pulp, primarily as a step in multistage bleaching. The conditions used are a pH of 10-11, temperatures around 100°C for 2-3 hr. Average chlorine use is about 1.5% based on the pulp. 

Used to disinfect and deodorize, a bleach for wood, paper, pulp, cotton, and many other products... 

Sulfur dioxide, SO2, is of considerable technological importance as a fungicide, a bleach, a refrigerant, and a digesting agent for wood pulp. It is generally prepared by burning sulfur or sulfides smaller amounts may be prepared in the laboratory by acidification of sulfites. The molecule is nonlinear ... 

An electrolytic cell permitting gas-liquid reaction has promise for direct conversion of 02 into 2% H202 in 1 M NaOH, a solution used for bleaching of wood pulp. The cell reactions are... 

Chlorine dioxide is to an increasing extent replacing elemental chlorine as a bleaching agent for wood pulp. 

The major market for sodium chlorate is for the preparation of the chlorine dioxide used for bleaching of wood pulp (Chap. 15). This market has recently risen by an order of magnitude every 10 years from the demand to replace of part, or all of the chlorine previously used for bleaching [13]. Other smaller uses of sodium chlorate are as a weed killer, for the making of matches and fireworks, and for the tanning of hides. A small market exists for sodium (and potassium) perchlorate, made by electrolysis of sodium chlorate using platinum electrodes (Eq. 8.30). 

Uses Bleaching agent for wood pulp, leather, textiles, beeswax, fats, and oils biocide color removal, taste/odor control, disinfection,... 

The synthesis of chlorine dioxide has eui extensive literature, especially in the patent area. This is doubtless because it is widely used as a bleach, particularly for wood pulp and flour. The primary source of chlorine dioxide is sodium chlorate. Sodium chlorite also serves as a source, but since this compound itself is always made from chlorine dioxide, its advantages derive from its ease of application or from the purity of the chlorine dioxide produced. 

A rather extensive review of chlorine dioxide has been published in French by Masschelein (155). This review includes the physical and chemical properties of chlorine dioxide, methods of synthesis, and different analytical methods for determining chlorine dioxide, as well as the industrial applications of chlorine dioxide. A review by Rapson (180) of the history of the application of chlorine dioxide to the bleaching of wood pulp covers the most important industrial use of chlorine dioxide and gives an indication of the economics of its use. 

Chlorine forms a number of well-known oxides (Table VII) which are generally unstable and prone to explosion. Only CIO2 and CI2O are employed to any extent commercially, where they are used to make chlorinated solvents and chemicals for water treatment, or function as important bleaching agents for wood pulp and textiles. 

Finally, we look at an example where environment-friendly catalytic oxidation is desired. In the paper industry, bleaching of wood pulp by hypochlorite is an important step. The chloride-containing effluent of the bleaching process increases the E factor and is of environmental concern. The heteropolyoxo anion [AlVWjjO J has been proposed as a potential homogeneous catalyst for a chloride-free, oxygen-based bleaching process. 

Hydrogen peroxide in aqueous solution has many uses, because the products from its reaction are either water or oxygen, which are generally innocuous. The chief use is bleaching of textiles, both natural and synthetic, and of wood pulp for paper. Other uses are the oxidation of dyestuffs, in photography and in the production of... 

Bleaching. The largest single use for hydrogen peroxide in the United States and North America is wood pulp bleaching, but consumption for the manufacture of chemicals, environmental appHcations, and for bleaching cotton (qv), wool (qv), and other textiles (qv) is significant. 

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