Wood pulp bleaching

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 In some mills, much of the chlorine and hypochlorite has been replaced by chlorine dioxide to reduce the amount of chlorinated by-products.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. 

In a typical process for manufacture on a commercial scale bleached wood pulp or cotton linters are pretreated for 12 hours with 40-50% sulphuric acid and then, after drying, with acetic acid. Esterification of the treated cellulose is then carried out using a mixture of butyric acid and acetic anhydride, with a trace of sulphuric acid as catalyst. Commercial products vary extensively in the acetate/ butyrate ratios employed. 

As described in Section 4-1. one important class of chemical reactions involves transfers of protons between chemical species. An equally important class of chemical reactions involves transfers of electrons between chemical species. These are oxidation-reduction reactions. Commonplace examples of oxidation-reduction reactions include the msting of iron, the digestion of food, and the burning of gasoline. Paper manufacture, the subject of our Box, employs oxidation-reduction chemishy to bleach wood pulp. All metals used in the chemical industry and manufacturing are extracted and purified through oxidation-reduction chemistry, and many biochemical pathways involve the transfer of electrons from one substance to another. 

Organocell A combined process for delignifying and bleaching wood pulp. Developed by Technocell, Dusseldorf. 

When bleaching wood pulp in a paper mill, the amount of leftover or residual hydrogen peroxide present in the spent bleach liquor can be important. Sometimes the bleach liquor is used in a second bleach or recycled back to the first bleach. Also, the residual peroxide level can indicate whether the bleach liquor makeup needs adjusting. Laboratory scale bleach tests are performed on the wood pulp, and the residual peroxide levels are checked by titration of the spent bleach liquors using potassium iodide and sodium thiosulfate. By monitoring the residual peroxide levels of these bleaches, the bleach liquor makeup can be adjusted, giving the best level, as needed by each paper mill. 

Chlorine. Except to bleach wood pulp and flour, chlorine itself is rarely used as a bleaching agent. 

Bleaching processes. Historically, the use of chlorine to bleach wood pulp was an important source of PCDDs and PCDFs, both to the environment, and to paper, some of which was used in sensitive applications such as food packaging and disposable nappies. However, this type of bleaching is now usually done using non-chlorine based processes, for example by using hydrogen peroxide. 

EPA. 1990c. Dibenzo-para-dioxins/dibenzofurans in bleached wood pulp and paper product referral for action. U. S. Environmental Protection Agency. 55 Federal Register 248 53047-53049. 

The important parameters in bleaching wood pulp are the concentration (consistency) of pulp and bleaching chemicals, the reaction temperature and duration (residence time), the mixing of pulp and chemical, and the pH at which the reactions are carried out. Initial temperatures and concentrations are usually selected based upon experience with a particular pulps needs. Control is achieved by carefully balancing all these various factors to optimize bleaching with a minimum expenditure of chemical. 

There are several important industrial uses of C102. Most of these involve its bleaching action, but it is also a useful bactericide in water treatment. Bleaching wood pulp, textiles,... 

Dichlorine oxide, CI2O, is best prepared on a laboratory scale by reacting chlorine with mercury oxide (equation 94). On an industrial scale, it can be prepared by reacting chlorine with moist carbonate (equation 95). This oxide is readily soluble in water to form hypochlorous acid (equation 96). Its salts, the hypochlorites, are quite stable and are used in the textile industries and to bleach wood pulp. The low-temperature crystal structme of CI2O reveals an essentially molecular structure [d(O-Cl) = 1.7092(4) A] with weak secondary interactions [d(0- Cl) = 2.7986(4) A] affording a distorted tetrahedral coordination around the O atom. ... 

In preparing compositions use is made of organic and mineral fillers. The most important organic filler is bleached wood pulp. The pulp fiber length depends on the type of wood thus, pulp produced from beech has 1 mm-long fibers, whilst that obtained from conifers -3 mm-long fibers. 

Remove water from an aqueous slurry of bleached wood pulp. 

Use Bleaching wood pulp, fats, and oils controversial maturing agent for flour water treatment (purification and taste removal) swimming pools odor control biocide. 

Use Source of hydrogen and other borohydrides. Reduces aldehydes, ketones and acid chlorides. Bleaching wood pulp, blowing agent for plastics, precipitation of mercury from waste effluent (by... 

The rose on the left is in an atmosphere of sulfur dioxide, SO2. Gaseous SO2 and its aqueous solutions are used as bleaching agents. A similar process is used to bleach wood pulp before it is converted to paper. 

The dielectric strength of polyethylene has been measured as a function of additive concentration while in stabilised polymers the depth of degradation was found to be uniform . HALS have been found to be effective stabilisers for UV cured coatings and do not influence the cure rate . Bleached wood pulps can be effectively stabilised by ascorbic acid and wool by hydroxy-benzotriazoles and HALS . Hydroxybenzotriazole stabilisers also protect wood pulp and polyurethanes by co-reaction . The spectroscopic properties of monomeric and polymeric benzotriazoles have also been compared . UV absorbers are effective in clear coats and when grafted to wood . Dihydroxybenzophenone stabilisers inhibit the chain scission in the photodegradation of poly(methoxyacrylophenone) while tin stabilisers have also been found to photostabilise PVC . 

Purified bleached wood pulp or sheets of cellulose are steeped in an alkali solu-... 

With each of these materials the first step is the manufacture of alkali cellulose (soda cellulose). This is made by treating cellulose (either bleached wood pulp or cotton linters) with concentrated aqueous sodium hydroxide in a nickel vessel at elevated temperamre. After reaction excess alkali is pressed out, and the resultant cake is then broken up and vacuum dried until the moismre content is in the range 10-25%. The moisture and combined alkali contents must be carefully controlled as variations in them will lead to variations in the properties of the resultant ethers. 

There have been some notable advances in the elucidation of the reaction mechanisms of hydrosulfite itself. The importance of the radical anion species has been demonstrated in detail, and the implication of these discoveries has opened the way to new vistas to bleach wood pulps using reductive chemistry. 

Most of the sodium chlorate, NaC103, produced in the United States is converted into chlorine dioxide, which is then used for bleaching wood pulp. 

Table 1 summarizes some representative solvents or solvent systems for cellulose. The xanthate system has been used for a longtime to make viscose rayon and sponge. However, rayon production, where H2S is released to varying amounts during the recovery system of spent solvent, is shrinking because of environmental issues. On the other hand, cellulose is soluble in aqueous NaOH alone under limited conditions. Microcrystalline cellulose with DP 200—300 is soluble in 6—9% NaOH by freezing and defrosting procedures. Pretreatments of cellulose such as steam explosion are necessary for complete dissolution in the aqueous NaOH for normal bleached wood pulp and cotton linters with DP of more than 500. ... 

Bortek. [Covan Ltd.] Sodium borohy-dride source of and other borohy-drides bleaching wood pulp blowing agent for plastics decoloiizer for plasticizes. 

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