On the entire pulp substrate for purification.

The treatment is carried out at low caustic concentration, typically 3–18 g L–1

NaOH, with pulp consistencies of 10–15% at temperatures ranging from 70 °C to

120 °C (occasionally 140 °C). As mentioned previously, HCE is carried out solely

For sulfite pulps, because the same carbohydrate degradation reactions are

Involved in alkaline cooks (kraft, soda), at less severe conditions and thus avoiding

Alkaline hydrolysis reactions. Therefore, HCE does not contribute much to the

Purity of pulps derived from alkaline cooking processes. From the chemistry point

Of view, HCE should be placed before any oxidative bleaching stage, as the efficiency

Of purification is impaired as soon as aldehyde groups are oxidized to carboxyl

Groups. It has been found that the gain in alpha-cellulose is related to the

copper number (or carbonyl content) of the unpurified pulp [30]. Consequently, if

Measures are undertaken to stabilize the carbohydrates against alkaline degradation

Either by oxidation (HClO2) or reduction (sodium borohydride), virtually no

purification is achieved [31,32]. However, for the production of low-grade dissolving

Pulps with a focus on viscose applications, hot caustic extraction (E) and oxygen

Delignification (O) are often combined into one single stage (EO) to reduce

Costs. The reduction in purification efficiency is negligible, provided that the

degree of purification is limited to R18 values well below 94–95%.

Pulp Purification

8.4.1

Influence of Reaction Conditions on Pulp Quality and Pulp Yield

8.4.1.1 NaOHCharge and Temperature in E, (EO), and (E/O) Treatments

The NaOH charge is the most important parameter controlling the degree of purification

During HCE. At a given alkali dosage, the consistency determines the

alkali concentration in the purifying lye. According to Leugering [30], the gain in

Alpha-cellulose is accelerated with increasing consistency at a given alkali charge,

As shown in Fig. 8.15.

0 1 2 3 4

5% consistency 10% consistency

Alpha-Cellulose [%]

Time [h]

Fig. 8.15 Alpha-cellulose versus time, calculated according to

the empiric formula developed by Leugering [30], with the following

assumptions: Initial alpha-cellulose content 90%, 4%

NaOH charge, 90 °C.

The relationship between the gain in alpha-cellulose content (Da) and NaOH

Concentration multiplied by retention time has been derived on the basis of

spruce acid sulfite pulps:

Da _ __3_3 _ 0_1 _ con_ _ 0_13 _ _T _ 80__ _ _NaOHch _ con

Con _ t__ 1

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