Con_ _1_

where: con = consistency (%; validity range 5–15%); T = temperature (°C; validity

range 80–97 °C);NaOHch= NaOHcharge (kg odt–1; validity range 34–228 kg odt–1);

and t = time (h; validity range 0–4 h).

Hot Caustic Extraction 953

HCE is usually carried out at medium consistency of 10–18%, though in some

cases a consistency of 25–30% is practiced. The presence of oxygen at elevated

Pressure during HCE, aiming to reduce the kappa number parallel to pulp purification,

Clearly impairs the degree of purification (Fig. 8.16).

0 30 60 90 120

E-stage EO-stage

R18 content [%]

NaOH charge [kg/odt]

Fig. 8.16 R18 content as a function of NaOH charge comparing

E- and (EO)-treatments of hardwood sulfite dissolving

pulp (HW-S) [33]. HW-S: kappa number 5.1, 91.8%R18 content.

Process conditions: E: 90 °C, 0–120 kg NaOH odt–1,

90 min; (EO): equal to E plus oxygen: 8.4 bar (abs) at t = 0.

The data in Fig. 8.16 indicate clearly that purification levels off at about 94%

R18 if (EO) is applied. At a given alkali charge, temperature and time are adjusted

To achieve a minimal caustic residual. The amount of NaOH consumed relates to

Both the gain in R18 and pulp yield. The curve characterizing the increase in R18

As a function of the caustic consumed is comparable for spruce and beech sulfite

pulps; these data are in agreement with the report of Leugering [30].

When oxygen delignification follows HCE treatment without interstage washing

[characterized as (E/O)], the relationship between R18 and the amount of

Caustic consumption proceeds parallel to pure HCE treatment (E), with a shift to

Higher NaOH consumption due to an additional consumption during oxygen

Delignification (Fig. 8.17). When oxygen delignification and HCE occur simultaneously,

the degree of purification is leveled off at ca. 95% R18. By further intensifying

The reaction conditions during (EO) treatment through increased temperature

And caustic charge, no additional gain in R18 content can be attained while

Caustic consumption continues to increase. This unselective behavior of (EO) is

Also reflected in the relationship between purification yield and R18 content (see

Fig. 8.18). As anticipated, E and (E/O) treatments with hardwood sulfite pulps

Pulp Purification

0 30 60 90 120

HW-Sulfite SW-Sulfite

E-stage EO-stage (E/O)-stage E-stage

R18 content [%]

NaOH consumption [kg/odt]

Fig. 8.17 R18 content as a function of the

Amount of NaOH consumed comparing E-,

(EO)- and (E/O)-treatments of hardwood sulfite

Dissolving pulp (HW-S) and E-treatment of

spruce sulfite dissolving pulp (SW-S) [33]. HWS:

kappa number 4.6–7.1, 91.4–92.0%R18 content;

SW-S: kappa number 4.6–12, R18 content:

90–91.6%. Process conditions: E: 82–110 °C,

40–120 kg NaOH odt–1, 90–240 min; (EO): 85–

110 °C, 150–300 min, 35–145 kg NaOH odt–1,

8.4 bar (abs) at t = 0; (E/O): 90–110 °C, 30–

120 kg NaOH odt–1, 90–240 min, 8.4 bar (abs)

at t = 0.

Follow the same pattern in terms of yield versus R18 content. The gain in R18 content

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