Temperatures. This explains why the alkali consumption does not correspond

To 1 mol per degraded monosaccharide, but rather to 1.6 mol, indicating

that fragmentation to smaller acids takes place [4].

Unlike PHK pulps, acid sulfite pulps require the application of both technologies

To achieve purification levels appropriate to produce high-tenacity regenerated

Fibers (e.g., continuous-filament industrial rayons), cellulose acetate or cellulose

Ethers of pure quality. Cold alkali purification is certainly the most selective way

Of increasing the alpha-cellulose content of the pulp. The yield losses are in the

range of 1.2–1.5% per increase of 1% in alpha-cellulose content [4]. In the case of

hot caustic extraction, a yield loss of about 3% per 1% increase in alpha-cellulose

Content is experienced. However, cold caustic extraction is rarely used on a technical

scale because of the huge amounts of alkali needed. When working at 10%

consistency and 10% NaOH concentration, 1 t NaOH odt–1 pulp is necessary to

Charge. In combination with a PHK process, part of the press-lye can be re-used

In the cooking process or, alternatively, white liquor can be used for the cold

Extraction process. Another means of employing the excess lye is to use it for hot

Alkaline purification, with the prerequisite that the production of hot alkali-purified

Pulp considerably exceeds that of cold alkali-purified pulp. Recirculation of

The lye (after pressing) significantly deteriorates the result of the purification, due

To an accumulation of impurities derived from short-chain carbohydrate degradation

Products, being characterized as beta- and gamma-celluloses. Beta-cellulose

Is defined as the precipitate formed upon acidification of an aqueous alkaline solution

Containing the dissolved pulp constituents, while gamma-cellulose comprises

The carbohydrate residue in solution. The former consists of higher molecular-

Weight, the latter of lower molecular-weight material.

These compounds can be (partly) removed by means of dialysis of (part of) the

press-lye [4,5]. In addition, inter- and even intramicellar swelling of pulps under

The conditions of cold caustic extraction (low temperature combined with high

Alkali concentration in the vicinity of the swelling maximum) impedes the

Removal of excess lye during the course of subsequent washing. An optimum between

Purification performance and limitation of fiber swelling can be found by

Adjusting the temperature and caustic charge.

The treatment of pulp with aqueous sodium hydroxide solution still represents

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