Sivola Processes

The Sivola sulfite cooking and recovery process was developed in the early 1950s

to produce either dissolving or high-grade sulfite pulp at Rauma from spruce and

balsam woods [15–17]. The sodium-based, multi-stage cooking concepts of the

“Sivola” type comprise the bisulfite-neutral sulfite (soda), the acid bisulfite-neutral

sulfite (soda), and the bisulfite-acid sulfite-soda processes. The bisulfite-neutral

sulfite process is characterized by a change of the pH level from around 3.5–4.0 in

the first stage up to 7.0–8.5 (and higher) in the second stage. Differences in pulp

quality are observed if the first-stage liquor is drained before alkali is added to the

second stage. Pulps of superior burst and tear levels are obtained when the alkali

is introduced directly into the first-stage liquor. However, when the first-stage

liquor is drained before alkali is added, tearing resistances close to that of kraft pulps

are obtained, but the burst was not further improved. The effect of a two-stage bisulfite-

soda cooking process as compared to conventional acid sulfite and bisulfite pulping

methods using aspen as a raw material is detailed in Tab. 4.64 [15].

Tab. 4.64 Comparative evaluation of acid sulfite, bisulfite and

bisulfite-soda (Sivola) cooks of aspen (according to [15]).

Acid stage Alkaline stage Unbleached pulps

Liquor

PH

Max.

Temp.

[°C]

Time at

Max.

Temp.

[h]

Liquor

PH

Max.

Temp.

[°C]

Time at

Max.

Temp.

[h]

Total

Time

[h]

Kappa

Calc.

Brightness

[% G.E.]

Scr.

Yield

[%]

Tearing

Resistancea)

[%]

Tensile

Strengtha)

[kN m–1]

1.8 140 3.0 8.0 26.2 44.4 52.9 58 4.8

3.7 160 4.3 7.3 20.5 45.2 54.8 62 5.8

3.7 160 2.5 8.5 169 1.5 7.0 27.2 60.5 53.2 100 5.8

a) after 20 min of beating time.

468 4 Chemical Pulping Processes

The two-stage cook showed the highest unbleached brightness and superior

tearing resistance compared to the one-stage cooked pulps. The yield, however,

was lower than that of the bisulfite pulp when comparing at the same kappa number

level.

As expected, the presence of thiosulfate exerts no undesirable effects up to very

high concentration levels (up to 6–8% on o.d. wood).

A very comprehensive comparison of multi-stage sulfite pulping of both softwoods

and hardwoods was presented by Sanyer et al. [18,19]. Selected results

using spruce wood Jack pine as a raw material and sodium as a base are shown in

Tabs. 4.65 and 4.66.

Tab. 4.65 Unbleached pulp yield and physical properties of

one-stage and multi-stage spruce pulps in comparison to a

spruce kraft pulp with a permanganate number of about 20

(according to [18]).

Processes Yield Tear factor Breaking

Length

Дата добавления: 2015-10-21