Добавил:
Upload Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз: Предмет: Файл:
Макулатура стр 1147.doc
Скачиваний:
8
Добавлен:
20.11.2018
Размер:
5.28 Mб
Скачать

8.4Hot 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

91

93

94

95

96

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

954 8 Pulp Purification

0 30 60 90 120

91

92

94

96

98

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

during HCE of spruce sulfite pulps appears to develop slightly more selectively

as compared to beech sulfite pulps (see Fig. 8.18). The reaction of purification

can be divided into two phases: first, a more-selective course; and second, a

less-selective course. Transition between the two phases appears for E and (E/O)

stages at R18 values of 95.5–96.0%, and in the case of (EO) treatment at R18 values

of 94.0–94.5%.

A yield loss of about 3% per 1% increase in alpha-cellulose content has been

reported elsewhere [4,27,30]. Recent studies on beech and spruce dissolving pulps

have confirmed this “rule-of-thumb” in general. However, small deviations are

experienced as the yield loss is related to R18 content which, in contrast to alphacellulose

or R10 values, is rather independent of viscosity in the range investigated.

A summary of the specific yield losses and NaOH consumption values is

provided in Tab. 8.3.

The data in Tab. 8.3 show that HCE is very unselective at R18 values greater

than 96%. The one-stage hot purification and oxygen delignification behaves

slightly less selectively when exceeding R18 values of 94%. NaOH consumption is

a good indication for the degree of purification. Similar to kraft cooking,