Processes Mannose Pentosea)

[% on total sugars]

BS-NS 6.7 6.1

BS-AS 8.0 9.6

NS-AS 13.6 6.4

BS 7.2 7.1

K 7.6 10.7

a) includes arabinose

Further investigations showed that the combinations of bisulfite-acid sulfite

(BS-AS), bisulfite-acid sulfite-neutral sulfite (BS-AS-NS; not shown in Tabs. 4.65

and 4.66), and neutral sulfite-acid sulfite-neutral sulfite (NS-AS-NS, not shown)

are intermediate to the two extremes in yield and strength mentioned above.

Brightness levels are generally lowered by the second stage. The highest brightness

level was obtained with the Mg system, and the lowest with the ammonium

hydrogen sulfite/calcium hydroxide system [20].

The adding to a bisulfite-acid sulfite (BS-AS) process of an additional third stage

at moderate alkaline conditions by adding sodium carbonate converts the Sivola

process to a very flexible process for the production of dissolving pulps. The

Rauma process (BS-AS-Soda), a development which parallels the Sivola process,

consists of a three-stage cook for use on spruce and pine wood [21–24]. A typical

cooking schedule for the Rauma three-stage sulfite cook is illustrated in Fig. 4.185.

The first stage is a bisulfite stage, pH 3–4, primarily to ensure good chip

impregnation. After adding SO2, the second stage is carried out as a conventional

acid sulfite stage. At the end of this stage SO2 is relieved when the desired pulp

viscosity is reached. In the third stage, sodium carbonate is added to extract the

low molecular-weight carbohydrates. The degree of purification, denoted as alphacellulose

content, is controlled by temperature, time and pH of cooking liquor

according to Fig. 4.186. At 170 °C and a pH level of about 8, alpha-cellulose contents

above 95% have been obtained [24].

470 4 Chemical Pulping Processes

0 2 4 6 8

Temperature [. C]

pH pressure

pH / pressure (bar, abs)

Cooking time [h]

temperature

Fig. 4.185 Typical course of pH, temperature and pressure

during a Rauma three-stage sulfite cook (according to [24]).

140 150 160 170

pH 6 pH 7 pH 8

Alpha-cellulose content [%]

Cooking temperature in third stage [° C]

Fig. 4.186 Alpha-cellulose content as a function of third stage

pH and temperature (according to [24]).

4.3 Sulfite Chemical Pulping 471