Effect of Sodium Borohydride after Treatment

The use of organic solvents with radical-scavenging properties (e.g., methanol or

multivalent alcohols) to replace part of the aqueous phase of a pulp suspension is

not (yet) a realistic option for an industrial application to protect pulp viscosity

during an ozone treatment. The advantage of higher strength properties does not

justify the high investment and operational costs connected with the additional

equipment needed to recover the solvents. The problem of finding a cheap, effective

commercial inhibitor of carbohydrate depolymerization during ozone delignification

remains to be solved [62]. The combined use of ozone and hydrogen peroxide

for the production of a fully bleached pulp inevitably causes severe cellulose

degradation. In some special cases, when ozone bleaching must be applied to the

production of pulps comprising both low kappa number and high viscosity (e.g.,

high-purity dissolving pulps), additional measures must be undertaken to preserve

viscosity. To date, the only economically feasible way to compensate in part

for the viscosity loss during ozonation is a post-treatment based on sodium borohydride.

It has been discussed previously that the ozonation of pulp introduces

carbonyl groups within the anhydroglucose unit (AHG), giving rise to b-elimination

reaction in a subsequent alkaline treatment that results in cleavage of the carbohydrate

chain and, thus, a loss in viscosity. The carbonyl groups can be partly

reduced by borohydride in a strongly alkaline environment. The chain scissions

after borohydride treatment roughly correspond to the so-called direct scissions

caused by the oxidation of molecular ozone on carbons C(1) and C(4) [79]. Analytically,

the treatment of an ozonated pulp with sodium borohydride prior to viscosity

measurement is a well-known procedure to obtain more reliable information

on the respective chain length. Lindholm has established the following relationship

between the viscosity before and after borohydride treatment of an oxygendelignified

softwood kraft pulp [90]:

_g ZR _0_88 _ g_ Z _180 _105_

822 7Pulp Bleaching

where [gZ] is the limiting viscosity after ozone treatment (Z), and [gZR] is the limiting

viscosity after borohydride treatment (R).

It is interesting to note that, according to Eq. (105), the viscosity preservation is

more pronounced for a low-viscosity pulp (e.g., 600 –1, or 18% increase) than for a

higher-viscosity pulp (e.g., 800 → 884 mL g–1, or 11% increase). The conditions

for the reductive treatment in the laboratory with a charge of sodium borohydride

of far more than 2 kg odt–1 and long retention times are not applicable in industrial

practice, due to high costs. For commercial applications, sodium borohydride

is provided in the form of a strongly basic aqueous solution, containing 12% sodium

borohydride and 40% sodium hydroxide, sold under the name Borol™[91,92]. Borol

can be applied immediately after ozonation, without any intermediate washing, provided

that the initial pH exceeds a level of 10, preferably about 10.5. The reductive

action can be optimized when the temperature is adjusted to about 70 °C and the

retention time extended to at least 30 min [93]. Pulp viscosity increases almost linearly

with an increasing dosage of Borol, although improvements in viscosity

gradually level off when charges >10 kg odt–1 are applied (Fig. 7.97).

Figure 7.97 illustrates that the addition of 1% Borol solution (on o.d. pulp)

improves pulp viscosity by more than 50 to 60 units. This may be decisive to provide

either better strength or solution properties as a macromolecule, as in the

case of a cellulose ether. At the same time, Borol serves as a bleaching agent, as

demonstrated by an increase of brightness by 2–3 ISO points. Similar results with

0 3 6 9 12

Viscosity

Chain scissions

Viscosity [ml/g]

Borol charge [kg/odt]

0.6

0.9

1.2

1.5

Chain scissions

Fig. 7.97 Effect of Borol charge on the viscosity

of an OP-prebleached softwood kraft pulp after

ZE- and ZER-treatments [93]. OP-delignified

softwood kraft pulp: kappa number 8.2, viscosity

890 mL g–1; Z-stage: 45% consistency, pH 2,

20 °C, 8.5 kg O3 odt–1; OP-Z-treated softwood

kraft pulp: kappa number 2.8, viscosity

660 mL g–1; E-stage: 10% consistency, 70 °C,

30 min, adjusted to pH 11 by appropriate

NaOH addition; R-stage: 10% consistency,

70 °C, 30 min, pH adjustment either by NaOH

or by H2SO4.

7.5 Ozone Delignification 823

respect to viscosity preservation and bleaching efficiency have been observed for

ozone-treated, high-purity eucalyptus PHK pulps [28]. An increase of 50–70 units

in viscosity may be sufficient to meet the specifications given for high-purity, acetate-

grade pulps.

Although Borol is expensive, its application has proved to be advantageous for

some special applications, not least because of the very simple and inexpensive

additional equipment required for its use.

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