Effect of Alkaline Extraction

Ozone-bleached pulp may contain considerable amounts of degraded lignin that

can be removed by different after-treatments. It has been shown that degraded lignin

which does not contribute to the light absorption of the pulp can be removed

by neutralization, followed by washing and hot water extraction [94]. The removal

of colorless lignin thus shifts the brightness values to lower values at given kappa

numbers. Alkaline extraction following an ozone stage removes additional lignin

fragments which, contrary to neutralization, does not change the relationship between

light absorption coefficient and kappa number [82,95–97]. The effect of

kappa number reduction caused by various after-treatments is plotted against the

kappa number measured after the ozone stage (Fig. 7.98).

2 4 6 8 10 12 14 16

Ozone post-treatments:

neutralization&washing

neutralization&hot water extraction

neutralization&alkaline extraction

Δ Kappa number

Kappa number after Z

Fig. 7.98 Kappa number reduction induced by

different after-treatment procedures in relation

to the kappa number directly after the ozone

stage (according to [94]). Substrate: oxygendelignified

pine kraft pulp, kappa number 20,

intrinsic viscosity 1010 mL g–1. Ozone treatment:

LC; after treatment, neutralization: pH

5.5 at 1% consistency, dewatered, rediluted;

alkaline extraction: 2% NaOH, 70 °C, 60 min;

hot-water extraction: similar to alkaline extraction,

but without addition of NaOH.

824 7Pulp Bleaching

The amount of leachable kappa number is relatively constant for each post-treatment,

unless the kappa number after Z-stage approaches a very low value.

Ozone is known to introduce carbonyl groups along the polysaccharide chain,

and this promotes peeling reactions under alkaline conditions. The latent chain

sensitivities emerge during highly alkaline viscosity measurements (cuene).

Reduction of the pulps with borohydride prior to viscosity measurement provides

viscosity values that are 20 to 130 mL g–1 higher than without reduction, depending

on the ozone dose [98,99]. Borohydride reduction after alkaline extraction does

not affect the viscosity values, which correspond more or less to the unstabilized

viscosity values after the ozone stage. Thus, alkaline extraction after ozone treatment

decreases the molecular weight of the carbohydrates by splitting the alkalisensitive

linkages. As expected, alkaline treatment of ozone-bleached pulps in

general reduces fiber strength. Again, strength reduction is more severe for HC

as compared to LC ozone-bleached pulps [82]. The data in Tab. 7.41show that

alkaline extraction significantly reduces the kappa number of the ozone-treated

pulp, but the viscosity level is only moderately reduced. Lignin removal can be

further enhanced by using alkaline extraction reinforced with hydrogen peroxide,

EP, or with oxygen, EO.

Table 7.41 Effect of alkaline extraction (E), alkaline extraction

reinforced with hydrogen peroxide (EP) or with oxygen (EO)

after an ozone stage on kappa number and viscosity of a pine

kraft pulp according to C.-A. Lindholm[82]. Z-stage: 1%

consistency, prewash at pH 3, room temperature: E-Stage: 2%

NaOH on od. pulp, 60 °C, 60 min; EP-stage: 2% NaOH on od.

pulp, 0.5% H2O2 on od. pulp, 60 °C, 60 min; EO-stage: 2 %

NaOH on od. pulp, oxygen pressure 0.2 MPa, 90 °C, 60 min.

Ozone

Дата добавления: 2015-10-21