Losses of alkali would be to recirculate the pressed lye to the sodium hydroxide

Circuit for re-use in CCE treatment. A closed loop operation, however, inevitably

Leads to an accumulation of dissolved hemicelluloses in the lye circulation system.

Depending on the amount of hemicelluloses removed from the pulp and the leaks

From the circuit (e.g., the discharge with the press cake), a certain level of dissolved

Hemicelluloses is allowed to be reached under equilibrium conditions. It

Has been reported that the extent of purification is much deteriorated by the presence

of dissolved hemicelluloses and other impurities [4]. Surprisingly, if an (E/O)

Treated hardwood acid sulfite dissolving pulp is subjected to mild cold caustic

extraction at 5% NaOH concentration in the presence of 10 g L–1 hemicelluloses,

The xylan content even slightly increases, clearly due to xylan reprecipitation

(Fig. 8.9).

As expected, the purification efficiency increases when raising the NaOH concentration

to 90 g L–1 while keeping the ratio to the hemicellulose concentration

constant at 5:1. Nonetheless, the presence of hemicelluloses significantly impairs

Pulp purification (see Fig. 8.9). In the light of the previous discussion about the

Nature of hemicelluloses, it was interesting to examine which of the two hemicellulose

Fractions would have the greater impact on purification efficiency. The

Gamma-cellulose fraction was separated by nanofiltration, while the beta-cellulose

Was prepared by precipitation upon acidification. The data in Fig. 8.10 show that

The presence of the low molecular-weight gamma-cellulose fraction during CCE

Treatment does not affect purification, whereas the presence of the high molecular-

Weight beta-cellulose clearly impedes xylan removal.

This observation strengthens the presumption that xylan, when exceeding a certain

Molecular weight, precipitates onto the surface of the pulp fiber even at rather

high NaOH concentration (2.5 mol L–1). Xylan redeposition is clearly the main reason

For a reduced purification efficiency, if CCE is carried out with a lye containing

Dissolved beta-cellulose.

Cold Caustic Extraction

0 20 40 60 80 100

0.0

hemicellulose containing lye: 10 g/l at 50 g NaOH/l; 18 g/l at 90 g NaOH/l

Pure lye

Xylan content [%]

NaOH concentration [g/l]

Fig. 8.9 Effect of hemicelluloses in the lye on the xylan

Removal efficiency during CCE treatment of hardwood acid

sulfite dissolving pulp (HW-S) in the range of 0 to 100 g L–1

NaOH [28]. HW-S: (E/O) pretreated, kappa 1.6, 2.7%xylan.

pure Lye 20 g/l Gamma 20 g/l Beta

0.0

0.5

1.0

1.5

2.0

Xylan content [%]

Lye Purity

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