Abstract
The amount of lignin reacting according to the slow residual phase,
i.e. the residual phase lignin, is in many perspectives an interesting
issue. The purpose of the present investigation was to develop a
mathematical model to show how the amount of residual phase lignin in the
kraft cooking of spruce chips (Picm ahies) depends on the conditions in the
earlier phases of the cook. The variables studied were hydroxide ion
concentration, hydrogen sulfide ion concentration and ionic strength. The
liquor-to-wood ratio during pulping was very high to maintain approximately
constant chemical concentrations throughout each experiment (so called
"constant composition" cooks). An increase in hydroxide ion concentration
andtor hydrogen sulfide ion concentration leads to a decrease in the amount
of residual phase lignin, while an increase in ionic strength, i.e. sodium
ion concentration, leads to an increase. A signiticant result is that the
hydrogen sulfide ion concentration has a pronounced influence on the amount
of residual phase lignin during a cook at a low hydroxide ion concentration.
The amount of residual phase lignin expressed as % lignin on wood, L,, can
be described by the following equation developed for "constant composition"
cooks (when cooking with a constant sodium ion concentration of 2 mol/L):
LT=0,55-0.32*[HO-](-1,3)*ln[HS-]
This equation is valid for a concentration of HO- in the range from 0.17 to
1.4, and a hydrogen sulfide ion concentration from 0.07 to 0.6
mol/L.
