3. Results and discussion
3.3. Fiber charge
The chlorine dioxide bleaching was significantly more selective (increased delignification with lower yield loss) than the hydrogen peroxide bleaching for both HAI and REF. This was expected, as previous publications have established that chlorine dioxide selectively removes lignin without significantly damaging the carbohydrates in the pulp (Lachenal et al. 1995, Lachenal 1996, Reeve 1996a, Loureiro et al. 2011, Sevastyanova et al. 2012).
Interestingly, the difference between the yields of the different bleaching sequences was greater for the REF pulps than HAI pulps. In REF impregnated pulps, the DED had a 2.4 %-units higher yield compared to PP, whereas an increase of less than 1 %- unit was observed for HAI pulps. HAI pulps bleached with hydrogen peroxide were also noted to suffer lower yield loss than the REF pulps.
carboxylic groups in xylan (Bhardwaj et al. 2004, Dang et al. 2006) are considered to be the main contributors to the charge in unbleached pulps. Even though kraft cooking can create new phenolic hydroxyl groups in lignin, as α and β alkyl aryl ether linkages are cleaved, additional delignification lowers the amount of phenolic groups as the lignin is degraded and dissolved into the black liquor (Gellerstedt and Lindfors 1984, Chai et al. 2003). The removal of uronic acids from the xylan backbone (and/or xylan molecules) as well as the degradation and dissolution of xylan into the cooking liquor, contribute to the observed charge decrease (Jafari et al. 2014).
Oxygen delignification increases the total fiber charge of the pulp when compared to pulps obtained only by kraft delignification (when pulps with similar kappa numbers are compared). This increase in fiber charge obtained by oxygen delignification is well reported in the literature (Laine 1997, Zhang et al. 2005, Zhang et al. 2006). It has been proposed that the increase in charge is due to oxidation reactions that occur during oxygen delignification, which increase the number of carboxylic acid groups in the pulp. These occur both in the lignin, such as in the form of muconic acids (Figure 8), and as new carboxylic groups in the carbohydrates (Snowman et al. 1999, Yang et al. 2003, Zhang et al. 2005, Dang et al. 2006, Zhang et al. 2006, Tao et al.
2011, Genco et al. 2012, Zhao et al. 2016).
As expected, the fiber charge tends to decrease with bleaching, as most of the lignin is removed and carbohydrates are also partially dissolved. Even though chlorine dioxide and hydrogen peroxide are theoretically capable of generating new carboxylic groups in the pulps, the majority of these groups are removed during the bleaching step (Lachenal et al. 1995, Laine 1997, Zhang et al. 2006, Sevastyanova et al. 2012). This decrease in charge during bleaching is due to the large removal of the oxidized lignin. In bleached pulps, the amount of residual lignin is almost negligible and therefore the remaining charge can be assumed to originate solely from carboxylic acid groups (Zhang et al. 2006).
3.3.2. Process conditions of oxygen delignification
The total fiber charge for oxygen-delignified pulps, derived from kraft-cooked pulps with different initial kappa numbers, is shown in Figure 20. The pulps presented were obtained from a single oxygen delignification trial where the alkali charge, reaction time and degree of delignification were varied. The increase in fiber charge, quoted as a percentage increase compared to kraft-cooked pulps, was found to depend on the oxygen delignification conditions and the initial kappa number, as illustrated by the dashed lines in Figure 20.
Figure 20: Total fiber charge of unrefined pulps with different kappa number after kraft cooking (black squares and solid line) and a single oxygen delignification step (circles and dashed lines) at different alkali charges, given as % NaOH in the figure; the “iKa-no” is the initial kappa number of the kraft cooking pulp that was oxygen- delignified.
From the data it can be concluded that the greater the kappa number of the initial kraft-cooked pulp, the more charged groups can be introduced during oxygen delignification, most likely due to oxidation of the lignin. Froass et al. (1996) reported that it is possible to greatly increase the number of carboxylic acid groups in kraft-cooked pulps with high kappa numbers, even though they have low phenolic contents. From the data in Figure 20, we can see that the fiber charge increases with the alkali charge up to a point, after which it decreases, indicating that there is a maximum total fiber charge. Similar behavior has been reported by Zhang et al.
(2006) and Lucia (2005). In the work published by Zhang et al. (2006) the highest fiber charge from oxygen delignification was achieved with an alkali charge of 2.5 %, however, the pulps used had a much lower initial kappa numbers of 32. In the present study, the influence of alkali charge was not investigated in detail, rather, the final kappa number after delignification was used as the main parameter by which the various pulp samples were compared.
To understand how the oxygen delignification conditions impact on the increase in fiber charge, the alkali charge (NaOH), time and kappa number reduction (Δkappa) were correlated. The fiber charge increase induced by oxygen delignification was calculated according to Equation 3:
TFCoxygen −TTFCcook
TTFCcook [3]
In which the theoretical values of total fiber charge (TTFCCook) for kraft-cooked pulps were given by Equation 4, obtained from the regression fit in Figure 19.
TTFCcook (𝑦) = 1.5342𝑥 + 37.781 [4]
In these equations, TFCOxygen is the total fiber charge of pulp after oxygen delignification to kappa number “𝑥”, and TTFCCook is the theoretical total fiber charge of pulp after kraft cooking until the kappa number (𝑥) was the same as the oxygen- delignified pulps.
Increases in the alkali charge (Figure 21a), reaction time and reduction of the kappa number (Figure 21b), were all found to increase the total fiber charge. This suggests that in order to significantly increase carboxylic acid groups content, the reaction time and alkali concentration should be increased and as great a reduction in kappa number as possible should be obtained during delignification.
Figure 21: Effect of a) alkali charge, b) time and delignification degree during oxygen delignification, on the increase in total fiber charge.