Method Reference

CE method for quantitative determination of sulfite, thiosulfate, sulfate ions [9]

Refractive index [10]

Color [11]

pH-value [12]

Conductivity [6], [10]

0 20 40 60 80

Fresh Wood:

0.76 mol/l ÓSO

, 0.32 mol/l free SO

0.91 mol/l ÓSO

, 0.53 mol/l free SO

1.12 mol/l ÓSO

, 0.73 mol/l free SO

Dry stored wood

0.76 mol/l ÓSO

, 0.32 mol/l free SO

Viscosity [ml/g]

Extinction at 430 nm [a.u.]

Fig. 4.158 Pulp viscosity of beech wood sulfite dissolving

pulp as a function of the extinction at 430 nm at a given

liquor-to-wood ratio (2.5:1), different cooking acid compositions,

and differently stored beech wood (according to [13]).

4.3 Sulfite Chemical Pulping 433

Surprisingly, the absorbency–pulp viscosity relationship is not influenced by different

cooking acid compositions. Clearly, the color formation is only the result of

a temperature- and pH-dependent reaction. However, there are several sources of

deviations in the relationship which require a current calibration of absorbance

with final pulp viscosity achieved. The main factors comprise the liquor-to-wood

ratio, the type of base used, the wood quality and, of course, different wood species

or blends of wood furnish. As an example, the influence of different storage

conditions of beech wood on the color–pulp viscosity relationship is demonstrated

in Fig. 4.158. Dry storage of beech logs for 12–15 months causes some change in

the relationship of liquor color and pulp viscosity as compared to the use of fresh

beech wood (Fig. 4.158).

The digester pressure is first relieved to a pressure of about 3–5 bars, depending

on the pressure on the hot-acid accumulator. The pressure relief is continued by

simultaneously introducing cold washing liquor into the digester. To avoid SO2

gas and heat losses during blowing, the temperature inside the digester must be

cooled to less than 100 °C. The pulp is finally blown or pumped into the blowpit

for storage.