Increase equally affects purification, as would a decrease in NaOH concentration

by 17 g L–1, from 70 to 53 g L–1, at 30 °C, respectively. The processability of the

CCE treatment is worsened at low temperature because washing is deteriorated

Due to an increased lye viscosity. At a given washer capacity, this may result in

Additional alkali losses. In industrial praxis, a compromise must be found between

Economic considerations and pulp quality demand. In most cases, the temperature

level is adjusted to about 35 °C, which might fulfill both targets.

8.3.3

Presence of Hemicelluloses in the Lye

CCE treatment requires a comparatively high dosage of NaOH. Maintaining a

NaOH concentration of 80 g L–1 (74 kg NaOH t–1) at 10% pulp consistency

requires a total NaOH charge of 666 kg odt–1. The total alkali loss to the sewer is

Economically by no means acceptable, however, and consequently methods to reuse

The entire quantity of the lye must be evaluated. In the case of cold caustic

Purification of a PHK pulp, the excess lye may be completely recycled to the kraft

Cook, provided that the demand of alkali for cooking is not lower than the amount

Of alkali originating from the CCE treatment. In this particular case, white liquor

Must be used as the alkali source. The efficacy of the white liquor with respect to

Purification efficiency is equal to a pure NaOH if the strength of the white liquor

Pulp Purification

Is calculated as effective alkali (EA). Assuming total EA losses (including EA consumption

through CCE treatment and washing losses of about 50 kg odt–1), an

amount of 616 (equals 666–50) kg odt–1 of EA is recycled to the cooking plant

(note that the CCE filtrate must be evaporated in order to reach the white liquor

EA-concentration). Supposing a bleached yield of 35% (o.d.), this amount of alkali

corresponds to an EA charge of 216 kg odt–1 wood which, for cooking, seems to be

A rather too-low than a too-high amount (depending on the wood species, cooking

Technology and intensity of prehydrolysis, the required EA amount for cooking

ranges from 22% to 26% on o.d. wood). This brief example shows that the excess

Lye of cold alkali purification balances quite well with the demand in PHK cooking.

However, the situation is different when combining acid sulfite cooking with

A CCE treatment. There, the opportunities to re-use the excess lye quantitatively

Are limited to special cases. For example, one possibility of disposing of the excess

Lye from the CCE treatment would be to use it for hot caustic extraction, provided

That the production of hot alkali-purified pulp considerably exceeds that of cold

Alkali-purified pulp. If this is not the case, the only chance of preventing too-high

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