Generation of Chlorine Dioxide

Chlorine dioxide is always manufactured on site because of the risk of rapid

decomposition. In most of the processes, chlorine dioxide is produced in strong

acid aqueous solutions from either sodium chlorite or sodium chlorate. Sodium

chlorite is used in small- and medium-scale production units mainly for water

treatment and disinfection applications that require high-purity waters. Sodium

chlorate is utilized in those applications not requiring high-purity waters, but very

large amounts of chlorine dioxide, such as in pulp bleaching.

Generally, chlorine dioxide manufacture is based on the reduction of sodium

chlorate at high acidity. In nonintegrated processes, sodium chlorate is purchased

as a crystalline solid or as a concentrated (40–45%) aqueous solution, whereas in

an integrated process sodium chlorate is prepared by the electrolysis of a sodium

chloride solution according to the following equation:

NaCl _ 3 H 2 O _ NaClO 3 _ 3 H 2_6 H _ _ 6 e __ _69_

The evolving hydrogen (H2) is recycled to produce hydrochloric acid, which is

used in the integrated processes, known as R6, Lurgi or Chemetics processes, to

reduce chlorate to chlorine dioxide. The redox reaction in the chlorine dioxide generation

can be written as:

NaClO 3 _ 2 HCl _ ClO 2 _ 0_5 Cl 2 _ NaCl _ H 2 O _70_

where the byproduct sodium chloride is recirculated to the electrolysis cell [Eq.

(70)].The excess chlorine gas evolved from Eq. (70) is fed to the HCl synthesis unit

together with make-up chlorine, where it is burned together with the hydrogen

produced from the chlorate electrolysis unit to produce hydrochloric acid (HCl).

The three unit operations – chlorate electrolysis, chlorine dioxide generator and

hydrochloric acid synthesis – comprise the key steps in an integrated chlorine

dioxide plant [21]. The advantage of an integrated system is that it prevents the

formation of any sulfur-based byproducts. However, an integrated system produces

a chlorine dioxide solution with a relatively high level of residual elemental

chlorine in the range 11–23% of the chlorine dioxide concentration (Tab. 7.31).

Since the first commercial use of chlorine dioxide in pulp bleaching in Canada

and Sweden in 1946, both the technology of chlorine dioxide manufacture and the

requirements on pulp bleaching have been changed considerably. Until the 1960s

and 1970s, chlorine dioxide was preferentially used in the final bleaching stages

according to CEHDED or later CEDED sequences for the production of fully

bleached kraft pulp. At that time, chlorine as a byproduct from chlorine dioxide

7.4 Chlorine Dioxide Bleaching 741

Tab. 7.31 An overview of the different chlorine dioxide generation technologies.

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