Table 1. Brightness achieved with ECF sequences using ozone for different pulps. Table 2. Brightness achieved with TCF sequences using ozone for different pulps. Figure 1. Year wise development trend of global ozone bleached pulp production capacity. Table 3. Adoption of different bleaching sequences for producing hardwood/softwood pulp on commercial scale. Reactions of ozone with lignin and carbohydrates Ozone has the highest oxidation potential (2.07 V) among all the bleaching chemicals used in paper industry like hydrogen peroxide (1.77 V), chlorine dioxide (1.57 V), chlorine (1.36 V), oxygen (1.23 V) and hypo chlorite (0.94 V). Numerous types of chemical reactions take place during ozone treatment of the pulps due to the heterogeneity in the pulps and very high degree of reactiv- ity of ozone. Figure 2. Generation of ozone from oxygen in dielectric ozone generators. Figure 3. Reactions of ozone with olefins. The hydrocarbons that contain one or more pairs of carbon-carbon double bonds are called olefins. Oxidative cleavage of these carbon-carbon double bonds with ozone (ozonolysis) is an important reaction during the course of ozone-lignin reactions. Reaction mechanism of ozone with olefins was proposed by Criegee (1975). Later, the mechanism of ozonolysis was revisited with the use of '7O-NMR spectroscopy confirms the Criegee mechanism (Geletneky and Berger 1998). Three reaction pathways were proposed for ozonolysis of olefins. Initially, ozone reacts with olifins to form initial ozonide followed by formation of zwitterions as a reactive intermediate. These zwitter- ions can follow three separate pathways to produce reaction products. The most important pathway of ozonolysis of olefins is recombination of the zwitterions with the carbonyl fragment to form final ozonide. In second reaction pathway the zwitterion formed Figure 6. Ozonation of side-chain in lignin model compound. Side chain oxidation of lignin molecule depends on the nature of substituent. Researchers reported that elec- ron releasing groups accelerates the oxidation, while electron withdrawing groups hinders the reactivity Kojima, Miura, and Kayama 1978; Kratzl, Claus, and Reichel 1976). Side chain oxidation of lignin model compounds with the reaction of ozone normally resulted in formation of aldehyde or carboxylic func- ional groups (Kojima, Miura, and Kayama 1978) as shown in Figure 6. Ozone is superior compared to chlorine and chlorine dioxide in terms of dissolution of lignin and is much more reactive towards lignin than carbohydrates. But intermediate inorganic by-products formed during Figure 4. Selective ring cleavage reaction for ozone oxidation of lignin model. Figure 5. Reaction of ozone with aromatic bonds in lignin via electrophilic substitution. Figure 8. Ozone-catalyzed hydrolysis of cellulose. Researchers have reported that chemical reactions of ozone are more selective towards lignin as compared to carbohydrates. The reaction rate of ozone with carbohy- drates in acidic medium is reported to be very slow (k - = 0.1-0.3 M™ s') as compared to the reaction rate of ozone with ignin (k = 10* M's‘) at 20 °C (Hoigné and Bader 1983; Jablonsky et al. 2016; Nompex et al. 1991; Ragnar 2000). Study involving ozonation of a-methyl glu- coside, me nonspecific and alcoho hyl cellulose concluded that ozone is highly oxidizing agent, and attacks keto, aldehyde groups with similar ease. Ozone-catalyzed hydrolysis reaction mechanism producing glucose as the primary product was proposed (Katai and Schuerch 1966) as shown in Figure 8. as shown in Figure 8. Ozone bleaching of pulp has been studied on three distinct consistency regions based on the pulp mixing equipment used like low consistency (0.5-5.0%), medium consistency (5.0-12%) and high consistency (25-40%). Most of the Figure 9. Ozone-catalyzed electrophilic hydrolysis of glycosidic linkages.
