Na2SO4 Cl2

[t/t ClO2] [t/t ClO2] [in 10 gpl ClO2]

Mathieson Non integrated Atmospheric SO2 3.5

Solvay Non integrated Atmospheric CH3OH 3.5

R2 Non integrated Atmospheric NaCl 6.8 0.6 1.0

R3. SVP Non integrated Vacuum NaCl 2.3 0.6 2.0

R3H Non integrated Vacuum HCl 1.1 0.6 1.5–2.5

R6 Lurgi Chemetics Integrated Vacuum HCl 0.0 (0.7) 1.1–2.3

R8. SVP-MeOH Non integrated Vacuum CH3OH 1.1 0.01 0.1

R9 Non integrated Vacuum CH3OH 0–1.1 0.01 0.1

R10 Non integrated Vacuum CH3OH 0.9 0.010.1

manufacture could be completely recycled to both C- and D-stages. With growing

concern about the environmental impact of chlorinated organic compounds, elemental

chlorine has been replaced by chlorine dioxide which produces approximately

five times less AOX than the former. With the banishment of elemental

chlorine from pulp bleaching sequences, the production of chlorine as a byproduct

became an important criterion for the selection of chlorine dioxide generators.

Hence, chlorine dioxide generation technology has advanced substantially

during the past 50 years. The processes of the first generation, the Mathieson, the

Solvay, and the R2, use sulfur dioxide, methanol, and sodium chloride, respectively

as reducing agents. The corresponding overall reactions for the Mathieson,

Solvay and R2 processes are displayed in Eqs. (71) to (73), respectively:

2 NaClO 3 _ SO 2 _ H 2 SO 4 _ 2 ClO 2 _ 2 NaHSO 4 _71_

4 NaClO 3 _ CH 3 OH _ 2 H 2 SO 4 _ 4 ClO 2 _ 3 H 2 O _ HCOOH _ 2 Na 2 SO 4 _72_

NaClO 3 _ NaCl _ H 2 SO 4 _ ClO 2 _ 0_5 Cl 2 _ Na 2 SO 4 _ H 2 O _73_

All three processes operate at atmospheric conditions, and thus require high

charges of sulfuric acid to assure a high chlorine dioxide yield (defined as molar

percentage of the sodium chlorate converted to chlorine dioxide). The drawback of

these processes is the generation of large amounts of spent acid solution,

expressed as tons of Na2SO4 losses per ton chlorine dioxide produced (see

Tab. 7.31).

742 7Pulp Bleaching

The only advantage of the R2 process over the Mathieson and Solvay processes

is the much faster reaction, which results in lower investment costs. However, the

specific spent acid solution production is almost twice that of the Mathieson and

Solvay technologies. Moreover, the R2 process generates 0.6 t of Cl2 t–1 of ClO2,

with about 1g L–1 chlorine in the chlorine dioxide solution. The residual 0.5 t

Cl2 t–1 of ClO2 is primarily used to produce sodium hypochlorite.

The R3 and SVP (single-vessel process) processes were developed in a response

to reduce the amount of waste acid produced during the course of chlorine dioxide

manufacture. As these processes operate under vacuum, at higher temperatures,

and employ a catalyst, both high production and high yield are maintained at a far

lower solution acidity as compared to the R2 process, which enables neutral anhydrous

sodium sulfate to be crystallized. The neutral saltcake can be filtered-off

and removed from the system. Thus, the saltcake production is approximately

34% less as compared to the Mathieson and Solvay processes, and about 66% less

than that of an R2 process (see Tab. 7.31) [22]. Nonetheless, the amount of elemental

chlorine formation is high due to the addition of sodium chloride as a reducing

agent. The urgent need to eliminate chlorine and to further reduce the

amount of sodium sulfate during chlorine dioxide production promoted the development

of a new methanol-based process. This process, known as R8, SVPMeOH

and SVP-LITE processes, account for the majority of the installed ClO2 capacity

[23]. The plants are similar with respect to the overall operation and the process

technology employed. The generation system consists of a generator and a

reboiler. In the generator, the sodium chlorate is reduced to chlorine dioxide while

sodium sesquisulfate is formed as a byproduct. The chlorine dioxide is separated

as a gas which, after a concentration process, is dissolved in chilled water, where a

concentration of approximately 10 g L–1 is adjusted. The chlorine dioxide solution

is pumped to storage tanks for subsequent use in the bleach plant. The sodium

sesquisulfate is filtered off from the generator slurry, dissolved to a saturated,

acidic aqueous solution which is stored in a tank for further use (e.g., in tall oil

plant). The overall reaction comprising the R8 process may be expressed according

to:

3 NaClO 3 _ 2 H 2 SO 4 _ 0_85 _ CH 3 OH _ 3 ClO 2 _ Na 3 H SO 4 _ _2_2_3 H 2 O

_ 0_8 HCOOH _74_

Model experiments revealed that the generation of chlorine dioxide in the

methanol-chlorate process involves three distinct phases, namely initiation, startup,

and steady-state [24]. In the initial phase, no chlorine dioxide is formed, while

chlorous acid and elemental chlorine are generated. Chlorous acid is generated

continuously from the reduction of chlorate by methanol and then further

reduced to hypochlorous acid, which then is converted to elemental chlorine.

Chlorine dioxide is formed from the reaction between chlorine and chlorous acid

in the presence of chloride, which acts as a catalyst. Apparently, chlorine dioxide

is only generated when the chloride concentration exceeds a certain level. Chlorine

is usually present in a chlorine dioxide solution because of the necessary

7.4 Chlorine Dioxide Bleaching 743

presence of chloride in the generator. The presence of low concentrations of chloride

which actually leads to elemental chlorine formation is essential to avoid the

termination of chlorine dioxide production. The situation where chlorine dioxide

generation stops is known as a so-called “white-out” because a white gas consisting

of chlorine and water vapor appears. In industrial practice, the chlorine dioxide

solution contains elemental chlorine in a concentration ratio of 100:1, as

depicted in Tab. 7.31. At the same time, methanol is oxidized stepwise by chlorate

to form formaldehyde, then formic acid, and finally carbon monoxide in the case

of complete oxidation [25]. Some of the methanol and its oxidation products may

leave the generator along with chlorine dioxide.

Besides the very low concentration of chlorine, the R8 process has the advantage

of producing less saltcake (–52%) and higher chlorine dioxide yield (97% versus

93%) as compared to the R3/SVP processes. However, the development of the

methanol-based chlorine dioxide process continued by focusing on the further

reduction of the amount of saltcake. The sesquisulfate is electrolyzed to form

sodium hydroxide and sulfuric acid in the R9 process. There, the acid is recirculated

to the generator and the alkali fed to the bleach plant. Depending on the size

of the electrolytic cell, the amount of salt cake can be minimized in a range indicated

in Tab. 7.31. The metathesis of the sesquisulfate into neutral sodium sulfate

crystals is the key element of the R10 process. The remaining solution containing

residual salt and sulfuric acid is recirculated to the generator. The metathesis is

effected in such a manner as to minimize the additional evaporative load imposed on

the chlorine dioxide-generating process by the metathesis medium. The extent of

sulfate precipitation can be further enhanced by the addition of some methanol.

Hydrogen peroxide is another reducing agent which generates chlorine dioxide

from chlorate [26]. The stoichiometry of this process is represented in Eq. (75):

2 NaClO 3 _ H 2 O 2 _ H 2 SO 4 _ 2 ClO 2 _ Na 2 SO 4 _ 2 H 2 O _ O 2 _75_

The peroxide-based process exhibits faster reaction kinetics as compared to the

methanol-based process. Consequently, the former can be operated at a sulfuric

acid concentration of only 2 mol L–1 sulfuric acid as compared to that of 4.5–

5.0 mol L–1 for the methanol-based process. However, the high costs of hydrogen

peroxide limit the attractiveness of the peroxide-based process for chlorine dioxide

generation. Alternatively, combining hydrogen peroxide and methanol causes a

considerable increase in the rate of chlorine dioxide generation [27]. Furthermore,

it largely eliminates molecular chlorine as a byproduct. Even when substituting

only 10% of the methanol with hydrogen peroxide, the reaction rate of chlorine

dioxide formation is doubled. The high reaction rate can be explained by the rapid

reaction between hydrogen peroxide and chlorine, which results in chloride and

oxygen formation. Additionally, the reaction of hydrogen peroxide with dichlorodioxide

(Cl2O2) leads to a faster accumulation of the necessary chloride, which is a

catalyst in the methanol-based chlorine dioxide generation process.

Besides costs, the most important criterion for the selection of a chlorine dioxide

plant is the amount of byproducts generated. The level of elemental chlorine

744 7Pulp Bleaching

must be kept to the lowest possible amount to ensure low AOX formation, while

the quantities of sodium sulfate-sulfuric acid solution must be adjusted to fit the

Na/S ratio of the respective kraft mill.

During the past decade, chlorine dioxide has been increasingly delivered “overthe

fence” by companies specializing in chlorine dioxide generation. This concept

is achieving growing acceptance as the kraft mills are no longer involved in the

technology of chlorine dioxide generation.

7.4.4

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