Optimization, bleach reactions

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. 

After conversion to the proper DE, the reaction is stopped in the neutralizer tank by raising the pH with soda ash (sodium carbonate) to 4.5-5.0. This pH is critical not only to optimize the conditions under which the proteins and fats can be removed, but also to reduce the risk of unnecessary color development. At this point, the liquor may be pumped to an enzyme tank for further enzyme-catalyzed conversion, or clarified, bleached and evaporated. 

The last type of chemical degradation to be considered is that encountered by natural fibers in their purification by agents such as acids, alkali, and bleaches. Several excellent reviews and books are available on this subject (96,97,98,99), all of which stress the proper choice of temperature, pH, and other reaction conditions to optimize removal of impurities from fibers while minimizing damage. 

The most promising procedure so far was introduced by Jacobsen and coworkers in 1990 [20] and has been reviewed [19]. The method uses chiral, C2-symmetric (salen)Mn complexes, such as shown in Scheme 8.6. Such materials are very easily prepared by the condensation of a chiral diamine with a substituted salicylaldehyde, followed by coordination of the metal. The ready availability of both components and the swift synthesis of the target complexes permits easy access to a great many catalyst variations, which facilitates reaction optimization. The starting Mn(III) complex is subjected to in situ oxidation with the stoichiometric oxidant, usually NaOCl (bleach ). The use of this inexpensive and relatively safe oxidant is another virtue of this system. 

The phenomenon of silver tarnishing increased in automatic dishwashers with the introduction of low-alkaline formulations containing oxygen bleaches. As a consequence, the oxidation potentials were shifted to favor the formation of black silver oxide during the cleaning process. To suppress this reaction, the concentration of the bleach system has to be optimized, and antitarnishing agents such as benzotriazoles have to be added [198]. 

---