Effect of Pretreatments and Additives

A considerable amount of research has been devoted to finding an additive or a

pretreatment that would protect cellulose and make ozone react more selective

with the lignin. From among a huge list of different inorganic and organic chemicals,

only a few were identified slightly to improve delignification selectivity during

ozonation [24,62]. Many attempts were made to add a hydroxyl radical scavenger,

but without success. The probable explanation for this is that hydroxyl radicals

react very rapidly with carbohydrate structures in the pulp. The reaction rate

constants of hydroxyl radicals with cellulose model compounds are about

3. 1 09 M–1 s–1 (e.g., for Me-b-d-glucopyranoside, 3.2. 109 M–1 s–1 [72]), but are in

the range of 109 M–1 s–1 to 1010 M–1 s–1 with the most common hydroxyl radical

scavenging agents [73]. This implies that huge amounts of radical scavengers

must be added to achieve a viscosity-stabilizing effect. Moreover, many of these

scavenging agents react with ozone, which renders them ineffective [68].

The most promising way to improve selectivity is to exchange the associated

water of the pulp suspension with media that improve the solubility of molecular

ozone, act as radical scavengers, or prevent the formation of radicals, at least to

some extent. Furthermore, the presence of organic solvents improves the accessibility

of lignin while reducing it for cellulose; this again protects carbohydrates

against degradation [74]. As an example, replacing the aqueous phase with an

acetic acid medium yields an improvement in delignification performance while

cellulose degradation is diminished. The selectivity advantage is particularly observed

for HC ozone bleaching in 90% acetic acid in the presence of 0.95% pyrophosphate

[75]. The favorable effect on selectivity is ascribed to the better stability

818 7Pulp Bleaching

of ozone in acetic acid medium. Chemiluminescence measurements revealed a

decrease in the amount of hydroxyl radicals with increasing acetic acid concentration

[75]. The protective effect of formic acid against cellulose degradation during

ozonation has been found to be even slightly better than that with acetic acid (see

Tab. 7.40) [76]. The replacement of water in the pulp suspension with methanol or

tert -butyl alcohol during an ozone stage also suppresses radical reactions with cellulose

to some extent, as shown by ozone-bleaching studies in both heterogeneous

and homogeneous systems [50,77]. In model compound studies, it was shown

that the degradation of cellobiose in 50% methanol solution was retarded even in

the presence of phenolic lignin model compounds [78]. Methanol and tert -butanol

are more efficient in trapping hydroxyl radicals (higher rate constants) than acetic

acid [73]. Therefore, the concentration of acetic acid must be significantly higher

than that of methanol or tert -butanol in order to achieve the same scavenging ability

(90 wt.% acetic acid versus 1wt.% methanol versus 2.2 wt.% tert -butanol) [77].

Johansson et al. reported that the presence of ethylene glycol increased the selectivity

during HC ozone delignification of an oxygen-delignified softwood kraft

pulp considerably more than did methanol (Tab. 7.40) [79].

The stabilizing effect was optimal at pH 3 and 25 wt.% ethylene glycol, based

on the total reaction medium. The improved selectivity in the presence of ethylene

glycol was primarily attributed to a partial suppression of the free-radical reactions.

Van Heiningen and Ni found that the selectivity of HC ozone bleaching

could be significantly improved when the pulp was impregnated with a solution

of dioxane-water at a pH of about 2 [80]. Based on this technology, a conventional

softwood kraft pulp, kappa number 30 and viscosity 1050 mL g–1, was delignified

with 2% ozone in a ZE(O) sequence to kappa number 5 and at a viscosity at

950 mL g–1 [81]. The delignification selectivity, denoted as Dkappa number per

chain scissions, calculates to about 52 [(30 – 5)/(104/2800 – 104/3235)], which is

significantly better than the value of 14.2 obtained for the same sequence but

applying a conventional LC ozone stage (see Tabs. 7.40 and 7.41) [82].

The addition of a small amount of methanol (ca. 3% on o.d. pulp) in the ozone

gas stream (methanol in gas phase, MeOH G-P) appears to protect pulp viscosity

more efficiently than impregnating the pulp with the same amount of methanol

[83]. The effect on the preservation of viscosity of a reductive treatment with

sodium borohydride (R) prior to the alkaline extraction stage (E) is considerably

more pronounced for the MeOH G-P-ozonated pulp than for the control pulp (see

Tab. 7.40). However, impregnating with a 50% methanol solution renders the

pulp very selective towards an ozone treatment. Clearly, a high concentration of

methanol prevents cellulose depolymerization induced by reactions with radicals.

It has been speculated that the protective effect of large amounts of methanol

towards the reaction with ozone is possibly due to a decrease in swelling of cellulose,

thus reducing the accessibility to ozone [83]. The viscosity-preservation effect

of shrinkage has however not been detected so far.

Ozonation of fully bleached kraft pulps proceeds with significantly more selectivity

as compared to unbleached kraft pulps [84]. The data in Tab. 7.40 show that

the protective effect of both 70% methanol and 70% t -butanol on cellulose

7.5 Ozone Delignification 819

820 7Pulp Bleaching

Table 7.40 Effect of pretreatments on the selectivity of ozone treatment of unbleached, oxygen

delignified and fully bleached pulps. The results are summarized from different literature sources.

For more details with regard to experimental conditions it is referred to the cited literature.

Ozone

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