Introduction

Among the oxygen-based bleaching chemicals, ozone is the most powerful oxidizing

agent, reacting readily with almost any organic material. The good delignifying

and brightening properties make ozone an attractive candidate to replace chlorine-

based bleaching agents. The use of ozone as a bleaching agent results in an

effluent which is free from organochlorine compounds and can be completely

recirculated to the chemical recovery system. Thus, ozone bleaching may be a prerequisite

for a closed-loop bleaching process. However, there are some difficulties

concerning the application of ozone bleaching in industrial practice. First, ozone

is an unstable gas which must be produced on site, most commonly by passing

oxygen gas through an electrical discharge where some of the oxygen molecules

are dissociated into oxygen atoms. In turn, oxygen atoms unite with oxygen molecules

to form ozone. Ozone generation technology in the early stages could produce

only 2–4% ozone by weight in an oxygen carrier gas. Later developments in

ozone generation technology could produce 5% by weight. In the early 1990s, concentration

of ozone could be raised to 8–12% by weight with power efficiency.

Recent advances in ozone generation which enable ozone concentrations up to

16% by weight, as well as the lowering of oxygen cost by means of on-site production,

have established ozone as a highly competitive bleaching chemical. The

ozone concentration can be further increased by compressing the gas mixture;

this improves the mass transfer from the gas into the liquid phase, which is a prerequisite

for an efficient bleaching process. Second, the high oxidation potential

of ozone makes it also prone to depolymerize and to degrade pulp polysaccharides.

In fact, its delignification selectivity is significantly lower than that of chlorine

dioxide. The prevalent view attributes this lack of selectivity to the generation

of highly reactive and nonselective hydroxyl radicals during the bleaching process.

The formation of hydroxyl radicals is usually ascribed to ozone self-decomposition

7.5 Ozone Delignification 777

in an aqueous system, to ozone decomposition catalyzed by transition metal ions,

and mostly to reactions between ozone and lignin structures, preferably containing

phenolic hydroxyls. Based on a huge research effort within the past decade,

the performance of ozone bleaching has been significantly improved with respect

to both selectivity and production costs, making ozone a competitive bleaching

agent. However, it has not yet been possible to increase the selectivity of ozone to

the same level exhibited by chlorine dioxide. This is a severe drawback for the production

of pulps where the high molecular weight of cellulose is a prerequisite to

attain the desired properties (paper-grade pulp: high-strength properties; dissolving-

grade pulp: high solution viscosity). Special emphasis will be given in future

research work to further improve the efficiency and selectivity of ozone bleaching.

Although the first implementation of ozone on industrial scale was until 1990,

when the first installation of an ozone bleach plant came on stream in Lenzing,

ozone has long been known as an efficient bleaching agent.

The reaction of ozone with textile fibers such as cotton and linen was studied as

early as 1868 [1]. In 1889, a method for bleaching “fibrous substances”, including

those used in the making of paper, with a mixture of chlorine and ozone gases

was patented by Brin and Brin [2]. Cunningham and Doree reported in 1912 that

ozone would preferably attack the lignin part in jute, but cellulose was also

affected [3]. In 1934, Campbell and Rolleston patented a process for bleaching

pulp by sequential treatment with chlorine and ozone [4]. Since the studies of Brabender

in 1949, in which he investigated some of the variables involved in ozonation

and patented a high-consistency ozone bleaching process, many reports and

patents on ozone bleaching have been published [5]. The breakthrough of ozone

bleaching was the invention and development of a technology to compress ozone

gas, and this is the prerequisite to apply ozone in medium-consistency technology.

Since the first industrial installation of an ozone plant in 1990, more than 25 pulp

mills with an annual production of about 8 million tons of pulp have implemented

ozone bleaching on industrial scale (see Tab. 7.39).

7.5.2

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