Reference

19.0 none SW-K 34.0 1280 LC ZE(O) 9.5 850 1.72 14.2 Lindholm [52]

38.0 none SW-K 34.0 1280 LC ZE(O) 3.1 700 2.93 10.6 Lindholm [52]

11.0 none Radiata pine-KO 16.0 968 HC Z 6.7 785 1.11 8.4 Ruiz et al [76]

11.0 98% HCOOH Radiata pine-KO 16.0 968 HC Z 4.1 816 0.88 13.5 Ruiz et al [76]

11.0 80% AcOH Radiata pine-KO 16,0 968 HC Z 4.9 820 0.85 13.1 Ruiz et al [76]

11.0 98% HCOOH Radiata pine-KO 16.0 968 HC ZP 1.7 747 1.42 10.1 Ruiz et al [76]

20.0 1.4-dioxane SW-K 30.0 1060 HC ZE(O) 5.0 950 0.48 52.1 Ni and Ooi [81]

6.0 none SW-KO 11.2 940 HC AZE 2.5 715 1.57 5.5 Johansson et al. [79]

6.0 25 wt% MeOH SW-KO 11.2 940 HC AZE 3.4 815 0.75 10.4 Johansson et al. [79]

6.0 25 wt% EG SW-KO 11.2 940 HC AZE 3.2 870 0.39 20.6 Johansson et al. [79]

11.5 none SW-KO 17.7 935 HC ZE 3.9 665 2.06 6.7 Bouchard et al. [83]

11.5 MeOH G-P SW-KO 17.7 935 HC ZE 3.4 710 1.59 9.0 Bouchard et al. [83]

11.5 MeOH impr SW-KO 17.7 935 HC ZE 2.8 865 0.39 37.9 Bouchard et al. [83]

11.5 none SW-KO 17.7 950 HC ZRE 3.6 770 1.14 12.4 Bouchard et al. [83]

11.5 MeOH G-P SW-KO 17.7 950 HC ZRE 3.3 828 0.71 20.3 Bouchard et al. [83]

11.5 MeOH impr SW-KO 17.7 950 HC ZRE 2.3 920 0.15 100.0 Bouchard et al. [83]

20.0 none Hemlock-KFB n.d. 910 HC Z n.d. 800 0.70 Kang et al. [84]

20.0 70% MeOH Hemlock-KFB n.d. 910 HC Z n.d. 830 0.49 Kang et al. [84]

20.0 70% t-BuOH Hemlock-KFB n.d. 910 HC Z n.d. 820 0.56 Kang et al. [84]

20.0 none Hemlock-K 27.7 1020 HC Z 9.9 695 2.14 8.3 Kang et al. [84]

20.0 70% MeOH Hemlock-K 27.7 1020 HC Z 7.0 885 0.67 31.0 Kang et al. [84]

20.0 70% t-BuOH Hemlock-K 27.7 1020 HC Z 7.3 785 1.34 15.2 Kang et al. [84]

SW-K softwood unbleached kraft pulp

SW-KO softwood oxygen-delignified kraft pulp

KFB kraft pulp fully bleached

EG ethylene glycol

MeOH G-P methanol addition to the ozone gas stream corresponding to an amount of approximately 3% on pulp

MeOH impr pulp suspension diluted to 2% with 50% aqueous methanol at pH 2, mixed for 5 min and pressed to a consistency of

about 40%

R-stage 1% charge of sodium borohydride mixed with the pulp at 10% consistency, 15 °C for 24 h.

CS chain scissions, calculated as 104

DPt _ 104

DPO _ in mmol AGU–1.

n.d. not determined

degradation is minimal for fully bleached pulp, indicating that the extent of swelling

is not a decisive factor for accessibility to ozone. In the case of unbleached

kraft pulp, the presence of high concentrations of methanol or t -butanol significantly

improves the selectivity of ozone bleaching. This may be attributed to the

radical-scavenging effect of the solvents. At the same ozone charge, the number

of chain scissions is three-fold higher for the unbleached than for the fully

bleached kraft pulp in the pure aqueous system. The ratio of chain scissions between

the unbleached and bleached kraft pulps decreases to 2.4:1, and to 1.4:1

when replacing 70% of the water by t -butanol and by methanol, respectively. The

lower advantage of selectivity for the bleached kraft pulp can be explained by a

shift to direct ozone attack since the dissolved ozone concentration increases. The

greater extent of cellulose degradation during ozonation of the unbleached compared

to the bleached pulp suggests that even the high concentration of methanol

is incapable of scavenging all of the radicals generated by the reaction between

ozone and residual lignin structures. The better selectivity of an ozone treatment

in the presence of methanol than in that of t -butanol can be explained by the

more efficient radical-scavenging effect of the former. According to Hoigne and

Bader, the rate constant of the reaction between hydroxyl radicals and t -butanol is

0.47. 109 M–1 s–1, while that of the reaction between hydroxyl radicals and methanol

amounts to 0.85. 109 M–1 s–1 [85]. By taking a 2.3-fold higher molar concentration of

methanol compared to t -butanol into account, the hydroxyl radical scavenging rate of

70% methanol is more than four-fold that of t -butanol at the same concentration.

The application of chelants renders the ozone treatment more selective than

simple acidification to pH levels below 3. Allison reported that the addition of

DTPA at pH 3 before the Z stage provided a slightly better viscosity preservation

than a sulfuric acid treatment alone [59]. In some cases, an additional EDTA treatment

after the ozone stage makes the subsequent alkaline peroxide stage more

selective. A reasonable explanation for this behavior may be the better physical

and chemical accessibility of transition metal ions after the reaction of ozone with

the pulp components [68].

Oxalic acid [52], acidified DMSO [52,59] and an acidic peroxide treatment at

pH 2–3 [62] improved the selectivity and efficiency of an ozone treatment only

slightly. A recent study showed oxalic acid to be the most efficient acid for this

pretreatment [86], with a charge as low as 0.05 kg odt–1 providing an effective

reduction in cellulose degradation during ozone treatment. The protective effect

of oxalic acid against viscosity loss is attributed to a combination of different factors.

Oxalic acid may act as a radical scavenger and an efficient hydrogen donor,

thus inhibiting the formation of hydroxyl radicals. Moreover, oxalic acid behaves

as a chelating agent. Oxalic acid, however, is formed in quite large quantities during

ozone bleaching as a final oxidation product (~0.5 kg odt–1 at an ozone charge

of 3 kg odt–1) [87], this being well above the amount needed to attain viscosity stabilization.

Oxalic acid is known to form crystals of calcium oxalate that can precipitate

and cause severe problems with scaling. In industrial praxis, oxalic acid formation

is clearly undesirable because it prevents closure of the water cycle of the

ozone stage. Therefore, partial recirculation of the effluent within the Z-stage to

7.5 Ozone Delignification 821

adjust for oxalic acid concentration will only be carried out if it provides a significant

increase in delignification selectivity and efficiency.

On occasion, an enzymatic treatment may represent an alternative choice to

reduce the charge of nonselective oxidants (e.g., ozone), thereby improving the

strength properties while maintaining target brightness. Xylanase treatment (Irgazyme

40 s, derived from Trichoderma longibrachiatum) of an oxygen-delignified

softwood kraft pulp prior to medium-consistency ozone bleaching was reported to

increase brightness at a given ozone charge, or to allow a reduction in ozone

charge by 3 kg odt–1 while maintaining the same brightness [88]. Ryynanen et al.

showed that xylanase treatment of an oxygen-delignified eucalypt kraft pulp prior

to a HC ozone stage in an OXZQ(PO) sequence produced only slightly higher

brightness levels. However, the bleaching yield was about 1–3% units lower,

depending on the wood sample and the pulping process used [89].

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