4
C.V.G. Esteves: Oxygen deligni fi ed fi bers as a greener alternative for tissue paper
The higher the response from the sensor the lower is the paper softness. The top microphone algorithm was used in this work.
Table : Summary of the kraft cooking and oxygen deligni fi cation trials. Cooking temperature was ° C, sul fi dity %, e ff ective alkali %and liquor-to-wood ratio . l/kg. For the single oxygen trials the pressure was . MPa and the temperature was ° C, while for the double oxygen trials the pressure was MPa and the temperature was ° C for the fi rst stage and . MPa and ° C for the second stage.
2.3.2 Total fi ber charge increase calculation
To quantify how much the increase in the fi ber charge after oxygen deligni fi cation was compared to the kraft cooking, a regression fi t was made for the kraft cooked pulps as a reference, given by Equation (1). TTFC cook ( y ) = 0 . 88x + 58 . 9 (1) TTFC Cook is the T heoretical value for the T otal F iber C harge of a kraft cooked pulp for a given kappa number. The fi ber charge increase obtained by oxygen deligni fi cation is then calculated according to Equation (2): TFC oxygen − TTFC cook TTFC cook (2) where TFC Oxygen is the total fi ber charge of pulp after oxygen deligni fi cation to kappa number “ x ” , andTTFC Cook is the theo- retical total fi ber charge of pulp after kraft cooking until the kappa number (x) was the same as the oxygen deligni fi edpulps.
Samples
H factor
Kappa no.
Total yield (cooking stage) (%)
Residual alkali (g/l)
–
K K
. . .
. . .
– – –
K
Samples
Time, min
Alkali charge, %
Kappa no.
Total yield (oxygen stage) (%)
EndpH
. % . % . % . % . % . %
. . . . . .
K _O
. .
K _OO /
.
.
K _O
. . . . . . . .
K _OO /
K _O
K _OO /
3.2 Total fi ber charge
3 Results and discussion 3.1 Deligni fi cation
It is known that the carboxylic acid groups are the main charged groups found in the chemical pulp fi bers (Dang et al. 2006; Zhang et al. 2005). However, the charges su ff ermodi- fi cations during the di ff erent pulping processes that can eventually lead to their decline. Figure 2 illustrates the di ff erent behaviors that occur during Kraft cooking, oxygen deligni fi cation, and bleaching. The linear decrease in the charged groups as the lignin content is reduced during kraft cooking is clear (Figure 2) and it is well documented (Buchert et al. 1997; Chai et al. 2003; Dang et al. 2006; Esteves 2022). This is caused by the lignin removal during the deligni fi cation and by some xylan dissolution (Buchert et al. 1997; Chai et al. 2003; Dang et al. 2006; Esteves et al. 2021a) since they are the main compo- nents where the charged groups are present. For oxygen deligni fi cation, the total fi ber charge devel- opment is not straightforward, but the increase in total fi ber charge when compared to the kraft cooked pulps for the same lignin content is evident in the higher kappa numbers. The higher values for the charge content were obtained for the pulp with the highest kappa number from the cooking, which was expected. This is mainly due to the oxidation reactions in lignin that lead to a higher content of new charged groups on the pulp due to the higher lignin content. These groups are largely from the muconic acid structures created in lignin (Dang et al. 2006; Snowman et al. 1999; Yang et al. 2003), but
The cooking trials were aimed to obtain three di ff erent kappa numbers: a high kappa number (109), a medium- high kappa (61) and a low kappa number (29) – Table 3. The kappa numbers aimed for are based on results from previous studies (Esteves 2022; Esteves et al. 2021a; Mai 2021) where it was seen that the higher the starting kappa number is after cooking the higher will be the fi ber charge increase. Having kraft-cooked pulps between kappa 109 and 29 will also allow us to evaluate the in fl uence of the lignin content on the fi nal absorption and strength properties in connection with the fi ber charge. The pulp with the highest kappa number (K109) was de fi brated in a Sprout Waldron re fi ner before the oxygen deligni fi cation. The oxygen trials were aimed to have extended deligni fi cation (more than 60 min) with a high alkali charge (more than 3 % NaOH) and an end pH between 10 and 11.5. The goal was to achieve the highest possible total fi ber charge. However, for the lower cooked kappa number (K29) the end pH was higher than it was aimed at since the alkali charge was too high for the deligni fi cation e ffi ciency.
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