Liquid Unit Black liquor Water

Temperature °C 20 80 20 80

Dynamic viscosity mPas 2.60 0.81 0.99 0.36

Surface tension mN m–1 32.6 27.7 67.3 63.0

Industrial pine (Pinus silvestris) was used for the impregnation tests and subsequent

Superbatch® cooking. The results showed that pre-steaming of chips and

increased pressure (from 2 to 9 bar) had a favorable effect on the efficiency of

black liquor impregnation into heartwood and sapwood. The effect of penetration

pressure was more pronounced for heartwood chips due to the high amount of air

initially present within the chips. By applying 9 bar overpressure during penetration

into sapwood chips, it was possible to reach a degree of penetration of 99.6%.

When pre-steaming is finished and the chip temperature is reduced, condensation

of water within the chip voids occurs. The vacuum thus formed facilitates the

uptake of condensate from chip surface into voids. The effect of pre-steaming can

4.2 Kraft Pulping Processes 137

be explained primarily by the removal of air from the chip voids. In addition, deaspiration

of heartwood pits during steaming could also be considered as a cause

of improved penetration. The results of the Superbatch® cooking experiments

confirmed that efficient liquor penetration has a favorable influence on cooking.

The application of chip pre-steaming and high pressure during the initial cooking

stages (warm black liquor impregnation) resulted in a lower amount of rejects

and kappa number, whereas the screened yield remained unaffected. In the case

of heartwood, the presence of entrapped air can be considered as the primary

cause of possible heterogeneity in delignification, whereas in the case of sapwood

there are diffusion limitations. With heartwood chips, the effect of liquor temperature

is significant. In addition to a lower viscosity, the softening by the warm

liquor of resin compounds present in the pine heartwood capillaries is likely to

cause faster penetration under higher temperatures. The unbleached pulps produced

with different modes of impregnation were subjected to ECF bleaching

using an O-D0ED1ED2 sequence [33]. To reach the full brightness of 88% ISO,

20 kg more of active chlorine (ClO2) must be used for pulp produced using no

pre-steaming and low-pressure impregnation during the HBL stage, as compared

to pulp which was pre-steamed and treated at higher pressure during the HBL

stage. The differences in chemical consumption are caused solely by differences

in the incoming kappa number of the pulps, and not by their bleachability. The

equivalent chlorine multiple remains 0.35 to achieve 88% ISO in all cases.