Introduction

For many years, kraft pulping has maintained its status as the most crucial method for processing wood, primarily due to its low operating costs, which can be attributed in part to efficient chemical recovery systems. Other factors contributing to its success include its relatively high selectivity of delignification and its broad tolerance for various raw materials (Ragnar et al. 2013). Despite this wide range of acceptable raw materials, not all wood types are pulped at the same rate. The lignin structure is a key factor, with softwood, and particularly compression wood, delignifying at a slower rate than hardwood. This slower rate is a result of a higher content of stable condensed bonds in the lignin (Gonzalo Epelde et al. 1998).

In addition to lignin structure, covalent bonds between lignin and polysaccharides, known as “lignin carbohydrate complexes” (LCC), significantly influence delignification rates. These complexes, which exhibit variations between different tree species, serve as rate-limiting factors for delignification (Lawoko et al. 2004).

One less studied aspect in the field of kraft pulping is the impact of the inorganic content of wood on the kinetics of the process. An interesting example of this concerns the calcium oxalate content present in Eucalyptus dunnii wood; recent research has revealed that wood samples of this species with a higher calcium content exhibit suboptimal performance during kraft pulping. This is of particular interest given that kraft pulp from eucalyptus fibers has probably become the worlds most used short fiber in papermaking due to, among other advantages, its high cellulose content and low production cost (Colodete et al. 2005). Specifically, it was found that samples of Eucalyptus dunnii with high calcium contents displayed slower delignification rates and a more significant degree of cellulose degradation (Vegunta et al. 2022). In laboratory-scale experiments, pulping samples with the highest calcium content, which reached levels of 4.7 g of calcium per kilogram of wood, proved to be a challenge. This finding highlights the importance of considering the inorganic content of wood when optimizing kraft pulping processes. These results observed in the study by Vegunta et al. corroborates earlier observations made by Senthilkumar (2019), who found calcium to have progressively detrimental effects on the delignification rate as well as the pulp yield when applying the kraft process to Eucalyptus dunnii and by Fariña (2014), who correlated higher calcium content with lower yield at a constant kappa number and with a higher kappa number and lower ISO brightness at constant cooking conditions, particularly for Eucalyptus dunnii.

Interestingly, this issue was less pronounced, yet still significant, at the industrial scale. One possible explanation is that industrial white liquor may be “less pure” due to incomplete causticizing, meaning it contains carbonates. Alternatively, the addition of black liquor to fresh chips before pulping could partially alleviate the calcium-related challenges in some manner (Vegunta et al. 2022).

Kraft pulping of wood chips using black or green liquor is a potential solution to address challenges associated with high calcium content in wood (Johakimu et al. 2011, 2013, 2021). Since green liquor is an intermediate in the chemical recovery system, it is readily available in the kraft mill alongside black liquor (Ragnar et al. 2013). It has been suggested to apply green liquor for a variety of purposes, such as increase of sulfidity or for pH control (Ban et al. 2003). Improved delignification in kraft pulping has been reported by several scientists (Ban et al. 2003; Andrews and Chang 1985; Svedman and Tikka 1998). It can also be used as a pretreatment prior to enzymatic degradation (Wang et al. 2015). In this study we investigate green liquor addition as a way to solve calcium related problem in kraft pulping.

Previous experiments with green liquor kraft pulping have demonstrated encouraging results, such as increased delignification rate, higher pulp yield, enhanced strength, improved selectivity, and reduced chemical consumption (Andrews and Chang 1985; Ban et al. 2004; Klevinska and Treimanis 1997; Svedman and Tikka 1998). Nevertheless, the exact mechanisms behind these improvements remain elusive.

In this study, we examine the impact of adding black and green liquor prior to kraft pulping on the delignification kinetics of E. dunnii wood with varying calcium contents including as high as of 4.7 g/kg—a quality that is difficult to pulp under normal conditions in laboratory (Vegunta et al. 2022).

Materials and methods

Materials

Eucalyptus dunnii wood chips of standard kraft pulping size with a calcium content of 3366 mg/kg were acquired from the Soriano farm at Bequilo district, Uruguay. E. dunnii, with a calcium content of 3756 mg/kg, was acquired from the Grecco farm in the Los Cercos district, Uruguay. E. dunnii, with a calcium content of 4668 mg/kg, was also acquired from the Grecco farm in the Los Cercos district, Uruguay. Sodium carbonate (99.5%) was acquired from Sigma Aldrich, St. Louis, USA. Nitric acid (65%) was acquired from Sigma Aldrich, Burlington, USA. Hydrogen peroxide (30% H2O2) was acquired from Sigma Aldrich St. Louis, USA. Sulphuric acid (72%) was acquired from Alfa Aesar, Kendel, Germany.

Storage of the samples

The black liquor obtained after pulping was stored in a refrigerator. Prior to analysis, the samples were left on the table to reach room temperature and thoroughly mixed.

Methods

Kraft cooking

Three sets of kraft pulping experiments were conducted, using E. dunnii wood chips with varying calcium contents: the first with 3366 mg/kg, the second with 4668 mg/kg, and the third with 3756 mg/kg. In the first set of experiments, different kraft pulping process conditions were assessed, and the effects of black or green liquor additions on pulping efficiency were evaluated. The second set of experiments involved E. dunnii wood chips with a very high calcium content of 4668 mg/kg, which were challenging to pulp in the laboratory using a conventional protocol. In this case green liquor additions were applied during impregnation as well as during cooking. The objective was to further examine and confirm the role of green liquor on the pulping performance of wood with very high calcium content. The third set of experiments utilized E. dunnii wood chips with a calcium content of 3756 mg/kg, similar to the wood chips in the first set. This study aimed to assess the effects of green liquor on the bleachability of the produced kraft pulp. Both batches had calcium contents too high to achieve reasonable defibration in our laboratory using standard pulping techniques (Vegunta et al. 2022).

The kraft pulping experiments were conducted using 1000 ml stainless steel autoclaves installed into a steam heated rotative polyethylene glycol bath, with the process heating controlled by a computer system and a PID-regulator system. Specifically, 100 g of wood chips were degassed under vacuum for 30 min after which the liquor was drawn in and the autoclaves were positioned in the glycol bath. After 10 min of temperature acclimation, impregnation was performed at a temperature of 110 °C for a duration of 90 min. The, temperature was ramped up to the requisite temperature for the sample in question (as per the following subsections and summarized in Tables 1, 2 and 3) and kraft cooking was performed using a liquid-to-wood ratio of 4:1 and effective alkali and sulfidity charges set to 18% and 35%, respectively. After cooking, the delignified wood was extracted, washed overnight and defibrated using a NAF defibrator (Nordiska Armaturfabrikerna, Linköping, Sweden) utilizing a water jet at 2.3 bar to separate the fibers.

A stock solution of Na2S was prepared by dissolving technical-grade flakes of Na2S, and similarly, a stock solution of NaOH was prepared by dissolving technical-grade flakes of NaOH.

Preparation of black liquor for impregnation experiment

Black liquor production was conducted in the laboratory for impregnation experiments, utilizing E. dunnii wood chips containing 3366 mg/kg of calcium. The kraft cooking conditions for producing black liquor included an impregnation temperature of 110 °C for 90 min, a 4:1 liquid-to-wood ratio using a liquor with 18% effective alkali (EA) and 35% sulfidity, and kraft cooking at 145 °C for 1 h. Consequently, the laboratory-produced black liquor, containing hydroxyl and sulfide ions, closely resembled industrial black liquor.

Preparation of green liquor

Synthetic green liquor was prepared by adding sodium carbonate during the preparation of the liquor such that the resulting solution had a final concentration 0.52 M sodium carbonate, in addition to an effective alkali of 18% and a sulfidity of 35%. The liquid-to-wood ratio was maintained at 4:1. White liquor, a precursor to green liquor, is created from sodium hydroxide and sodium sulfide stock solutions.

Kraft cook series 1

E. dunnii wood chips with a calcium content of 3366 mg/kg were used in this study. Kraft cooks were conducted using either reference white liquor, a combination of black and white liquor, or synthetic green liquor with similar concentrations of alkali and hydrogen sulfide. The impregnation and cooking chemicals were the same as previously described. Kraft cooking was performed at various H-factors, with a constant time of 210 min and temperatures ranging from 138 to 170 °C.

Table 1 Kraft cooking conditions for pulp samples using wood chips with a calcium content of 3366 mg/kg:
Full size table

Kraft cook series 2

E. dunnii wood chips containing 4668 mg/kg of calcium were used. Kraft cooking was performed using synthetic green liquor. The impregnation and cooking chemical conditions remained the same as previously described, followed by the kraft cook. The kraft cooking conditions were set at 145 °C for 210 min.

Table 2 Kraft cooking conditions for pulp samples using wood chips with a calcium content of 4668 mg/kg:
Full size table

Kraft cook series 3

E. dunnii wood chips with a calcium content of 3768 mg/kg were used. Kraft cooking was conducted using either reference white liquor, a combination of black and white liquor, or synthetic green liquor with similar concentrations of alkali and hydrogen sulfide, aiming to achieve similar kappa numbers in each case (13–14). The impregnation and cooking chemical conditions were consistent with prior descriptions, followed by the kraft cook. The kraft cooking conditions for the three cooking liquors were as follows: white liquor (170 °C, 210 min), black and white liquor (170 °C, 210 min), and green liquor (145 °C, 210 min).

Table 3 Kraft cooking conditions for pulp samples made for the purpose of bleaching experiments:
Full size table

Bleaching

A DEDED bleaching sequence was conducted on kraft pulp (with similar kappa numbers) obtained from E. dunnii wood using reference white liquor, a combination of black and white liquor, and synthetic green liquor with similar concentrations of alkali and hydrogen sulfide. The details of the respective bleaching steps of this sequence are summarised in Table 4. The drainability (ISO 5267-1), water retention value (WRV) for 100 mesh (ISO 23,714), tensile index (ISO 1924-2), tensile stiffness (ISO 1924-2), and tear index (ISO 1974) were evaluated for both unbleached and bleached pulps.

Table 4 Chemical composition details for DEDED bleaching sequence on kraft pulps. Bleaching consistency was 2% for the acid step, and 10% for all the preceding steps. The acid wash step was done using sulfuric acid
Full size table

Lignin isolation

Concentrated sulfuric acid (97 to 98% H2SO4) was added to 20 ml of impregnation liquor until the pH of the solution reached 2–3. The mixture was then centrifuged at 4800 rpm for ten-minute intervals, with 5-minute breaks in between. After centrifugation, the supernatant was separated from the precipitate, which was subsequently washed twice with a sulfuric acid solution at pH 2 and centrifuged again. The resulting pellet was dried in an oven overnight at 80 °C, and the weight was recorded. The precipitation process was performed in duplicate.

Analysis

Determination of oxalic acid using HPLC

Approximately 100 mg of dried black liquor samples were diluted with a dilution factor of 1000 using Milli-Q water. The diluted samples were filtered through a 0.25 μm nylon filter and transferred to HPLC vials. Oxalic acid content was determined using high-performance liquid chromatography (HPLC) with a Thermo Fischer Scientific system (USA) equipped with a ROA-organic acid column (Thermo Fischer Scientific, USA) and a refractive index detector. The mobile phase consisted of a sulfuric acid solution with a pH between 2 and 8 (0.45 M) at a flow rate of 0.5 ml/min. The oven temperature was set at 50 °C.

Determination of dry solid content and ash content

Approximately 5 ml of black liquor was placed in a ceramic crucible and kept in an oven at a temperature of 105 °C until a constant mass was achieved (typically within 12–15 h). The mass was then recorded. Following this, the residue from the 105 °C drying process was heated to 600 °C for 6 h in the oven, and the resulting weight, representing the ash content, was documented.

Viscosity

The intrinsic viscosity of pulp samples was determined according to the ISO 5351:2010 standard.

Klason lignin and sugar composition analysis

All pulp samples were Wiley milled using a 40 mesh screen and then subjected to acid hydrolysis to determine lignin and sugar content. Initially, 3 ml of 72% H2SO4 was added to samples of 100 mg of pulp, followed by placement in a vacuum desiccator for 1 h and 20 min with occasional stirring. The mixtures were then diluted with 84 ml of Milli-Q water and autoclaved at 125 °C for 1 h. Subsequently, the samples were filtered through a glass fiber filter using a 3-piece filtration setup. The filtrates were diluted at a 1:10 ratio for sugar analysis and acid-soluble lignin determination. The insoluble (Klason lignin) fraction was dried in an oven at 105 °C and weighed.

Acid-soluble lignin was measured using a Shimadzu UV-2550 UV-VIS spectrophotometer at an absorbance of 205 nm. Carbohydrate content was determined using a Dionex ICS-3000 high-performance anion-exchange chromatography system with pulsed amperometric detection (HPAEC-PAD), featuring a CarboPac PA1 column (Thermo Scientific, USA), an injection volume of 25 µl, and a flow rate of 1 ml/min. External sugar standards based on the sample were used for calibration. The results were reported as anhydrous sugars and performed in duplicate.

Metal ion content analysis

The total metal ion content in the pulp samples was measured using ICP-OES (Thermo Scientific iCAP 7000 series). Before the ICP-OES measurements, approximately 100 mg of each sample was taken for analysis. Initially, 7 ml of aqua regia solution (2 ml H2O2 + 5 ml HNO3) was added to the samples. The tubes were sealed and then placed in an ultrasonic bath for several minutes. After this, the samples were left overnight for acid digestion. The samples were then filtered using filter paper and diluted to 50 ml with Milli-Q water. This solution was further diluted to a 1:50 ratio using 5% HNO3. Finally, 10 ml of the diluted sample was subjected to metal analysis using ICP-OES.

Kappa number determination

The kappa number of pulp samples was determined according to the ISO 302:2004 standard, on the screened pulp fractions.

Residual alkali determination

The residual alkali of black liquor samples was determined according to the SCAN-N 33:94 standard.

Brightness determination

ISO brightness of samples were determined by making standardised test sheets according to the ISO 3688:2022 standard, and then measuring these as per the ISO 2470-1:2016 standard.

PCC brightness was determined according to an internal, modified method for sheet making and drying, followed by measuring the brightness according to the ISO 2470-1:2016 standard. Normally PCC-brightness shows ~ 1–1.5-unit lower value compared to ISO brightness.

PFI-refining process

The obtained dry matter content of pulp after bleaching and dewatering was measured in accordance with the ISO 638-1:2021 standard and found to be between 31 and 33% for the samples.

Beating of pulp samples was done with a PFI-refiner, following the ISO 5264-2:2011 method, over 3000 revolutions.

Test sheets with grammage 60.0 g m−2, calculated on an oven-dry basis, were made from unrefined pulp as well as refined pulp of each sample as per the ISO 5269-1 for the purpose of physical testing.

Physical testing

The tensile index of paper sheets was determined according to the ISO 1924-2:2008 method, while the tear index of such sheets was determined according to the ISO 1974:2012 method.

Drainage resistance and freeness

The drainage resistance was determined according to the ISO 5267-1 method, and the pulp freeness according to the ISO 5267-2:2001 method.

Results and discussion

Kraft cooking with different cooking liquors

Previous research has demonstrated that higher calcium content results in slower delignification and more severe cellulose degradation in E. dunnii with calcium contents of 705, 870, and 1500 mg/kg, and has proposed possible mechanisms for these effects (Vegunta et al. 2022). Even under harsh conditions, kraft pulping with lab-made white liquor proved to be practically challenging for samples containing more than 3000 mg/kg calcium. However, high calcium content had a lesser impact on industrial-scale pulping, although severe problems did arise. The difference between laboratory and industrial-scale pulping lies in the fact that black liquor is typically added to wood chips in a pulp mill (Ragnar et al. 2013), whereas white liquor may become contaminated with green liquor due to insufficient causticization. As a result, the composition of the liquors used for impregnation and kraft cooking in the pulp mill and a lab varies significantly. In this study, we conducted kraft pulping on a laboratory scale using white liquor, a combination of black and white liquor, and synthetic green liquor with similar concentrations of alkali and hydrogen sulfide (prepared in our lab from pure chemicals). Further details on the composition can be found in the materials and methods section.

Table 5 Details of kraft pulping experiments using different cooking liquors. E. dunnii wood chips with a calcium content of 3366 mg/kg were used
Full size table

Table 5 shows the characteristics of kraft pulps produced using various cooking liquors at different H-factors. Kraft pulping was carried out with the aim of achieving a similar kappa number in the pulp. To achieve the desired kappa number, the cooking temperature was selected as a variable factor (H-factor). In this study, kraft pulping of E. dunnii wood chips with a calcium content of 3366 mg/kg was performed at temperatures ranging from 138 to 140 °C, 145 °C, 155 °C, 165 °C, and 170 °C.

The rate of delignification of high calcium-content wood chips is significantly influenced by green liquor. Despite a lower H-factor for green liquor pulping, high calcium-containing wood chips could be pulped with fewer rejects to achieve a lower kappa (as shown in Fig. 1). Additionally, the kappa numbers were considerably lower for the pulps pulped with green liquor. In contrast, the addition of black liquor did not produce any significant effects (as illustrated in Figs. 1 and 2).

Fig. 1
figure 1

Reject (%) as a function of H-factor

Full size image
Fig. 2
figure 2

Kappa number as a function of H-factor

Full size image

E. dunnii wood chips with a calcium content of 3366 mg/kg were employed in this study. Additionally, the viscosity of the pulps generated using green liquor was higher at a given H-factor, as well as at a given kappa number (as depicted in Figs. 3, and 4, respectively). This indicates that green liquor, in some manner, protects cellulose from degradation during kraft pulping.

Fig. 3
figure 3

Intrinsic viscosity (ml/g) as a function of H-factor

Full size image
Fig. 4
figure 4

Intrinsic viscosity (ml/g) as a function of kappa number

Full size image

These findings are consistent with previous studies demonstrating that green liquor has a significant impact on the delignification rate and pulp quality in kraft pulping (Andrews and Chang 1985; Ban et al. 2004; Klevinska and Treimanis 1997; Maholanyiova et al. 2013; Svedman and Tikka 1998). While earlier research suggested that green liquor could be used to increase the sulfur content in pulping, our experiments did not support this explanation since the sulfur content was not increased by the addition of green liquor. Instead, we propose that it is the carbonate ions in green liquor that are responsible for the positive effects by forming inert calcium carbonate crystals, thereby preventing the negative effects of calcium ions during pulping. Figure 5 provides a schematic representation of this hypothesis.

However, if this hypothesis is correct, calcium must remain in the fiber even during kraft cooking and not be significantly transferred to the pulping liquor. The data in Fig. 6 support the hypothesis in Fig. 5, indicating that green liquor-treated pulp contains considerably more calcium than white and black liquor-treated pulp.

Fig. 5
figure 5

Hypothetical explanation for the strong positive effect of green liquor impregnation

Full size image
Fig. 6
figure 6

Calcium content in pulp (mg/kg) as a function of kappa number

Full size image

Table 6 presents the data on the residual alkali, oxalic acid, inorganic, organic, and lignin content of the black liquor collected after each kraft cook. The data indicates that black liquor always contains total dry solids (%) and lignin content (%), as well as inorganics. Additionally, a substantial amount of residual alkali was detected in the black liquor obtained using green liquor at a low kappa number, suggesting that kraft pulping selectivity is more favorable for this kraft cook at a low kappa number, particularly when compared to white liquor and black combined with white liquor.

Table 6 Characterization of black liquor collected from kraft cooks performed using different cooking liquors
Full size table

Figure 5 illustrates a hypothetical interpretation of the chemistry of cooking liquors with regard to calcium oxalate ions in wood chips. Previous research has identified calcium oxalate in the lumen of wood, and during kraft pulping with strongly alkaline white liquor, oxalate and calcium ions can migrate into the cell wall. According to our previous study (Vegunta et al. 2022), these calcium ions can accelerate polysaccharide degradation while delaying delignification. If this hypothesis is correct, calcium must remain in the fiber during kraft cooking and should not be significantly transferred to the pulping liquor. The data presented in Fig. 6 support this hypothesis, as green liquor-treated pulp contains considerably more calcium than white and black liquor-treated pulp.

Given the success of kraft pulping with high calcium content E. dunni wood, we tested pulping with wood containing even higher calcium content—4668 mg/kg wood—a quality that had been virtually impossible to pulp in the laboratory with conventional white liquor pulping. As expected, pure white liquor pulping did not result in defibrillation under the conditions used (see materials and methods), but adding green liquor allowed for pulping to generate pulp with low rejects and acceptable kappa numbers (Table 7).

Table 7 Details regarding pulp produced using high calcium-containing wood chips (4668 mg/kg) using green liquor
Full size table

Impact of green liquor on bleaching of pulp

In order to assess the impact of green liquor on the bleaching of high calcium content wood chips, additional batches of pulp were prepared as controls and subjected to bleaching experiments. The pulp was produced at a specific kappa number (13–14) to investigate the effects on pulp quality and strength. Wood chips with a calcium content of 3758 mg/kg were used for these experiments, and three different impregnation liquors were employed in the kraft cooking process. These experiments were conducted to determine whether the enhanced mineral content from green liquor would have any adverse effects on pulp bleaching, due to coprecipitated transition metal ions, among other factors.

Table 8 Characterization of kraft cooks subjected to bleaching experiments
Full size table

Table 8 summarizes the properties of the pulp obtained through kraft pulping with different impregnation liquors. The green liquor impregnation resulted in higher pulp yield, faster delignification, and higher viscosity at the same kappa number, consistent with previous experiments using wood chips with a calcium content of 3366 mg/kg. All pulps were kraft pulped under identical chemical conditions with different H-factors to achieve a similar kappa number. The total yield increased by 5% when green liquor impregnation was used, compared to white and black + white liquor. The hexeneuronic acid content was slightly lower in the pulp obtained using green liquor impregnation.

Table 8 displays the total inorganic metal content in unbleached pulp samples produced using different impregnation liquors. The unbleached pulp obtained from green liquor impregnation has a slightly higher calcium content compared to those obtained from black liquor + white liquor and white liquor (Table 8). This suggests that the calcium content follows the fiber line instead of being washed out with black liquor. However, the presence of calcium carbonate crystals in the pulp may trap transition metal ions and hinder the bleaching process. Hence, bleaching experiments with the sequence DEDED were conducted on pulps cooked with white and green liquors to comparable kappa numbers to assess the impact of green liquor on bleaching efficiency. The green liquor impregnated pulp samples had significantly higher intrinsic viscosity compared to those impregnated with white liquor and black + white liquor. This indicates an improvement in the mechanical properties of the pulp. The findings are consistent with the tensile strength and tear index results obtained from unbleached and bleached pulp samples (see Figs. 8 and 9). The viscosity (Fig. 7) of unbleached and bleached pulp generated with white liquor and black liquor + white liquor is lower at a given kappa number than that of green liquor impregnated pulp. The use of higher H-factor in pulp obtained from white liquor and black liquor + white liquor could be a reason for the lower viscosity.

Fig. 7
figure 7

Intrinsic viscosity (ml/g) after kraft cooking and after bleaching

Full size image

The benefits of the green liquor treatment in achieving high viscosity were mostly retained even after bleaching, as depicted in Fig. 7. This is evident in the improved tensile index (Fig. 8) and tear index (Fig. 9) for the bleached pulps previously treated with green liquor, although the increased hemicellulose content of the green liquor impregnated pulp may also have contributed to the strength development by refining.

Fig. 8
figure 8

Tensile index (Nm/g) of pulps produced at targeted kappa number using different cooking liquor

Full size image
Fig. 9
figure 9

Tear index (mNm2/g) of pulp pulps produced at targeted kappa number using different cooking liquor

Full size image

Refining of pulp fibers through PFI refining enhances the paper-making properties of pulp fibers, by increasing the swelling in water of the fibers and thereby improving their binding ability. As shown in Figs. 8 and 9, both unbleached and bleached pulps processed with green liquor impregnation have improved mechanical properties. As known, a higher hemicellulose content of pulp makes refining easier. Thus, pulp with a higher hemicellulose content at the same refining revolutions give a higher oSR value as seen in Fig. 11. The bleaching of green liquor-treated pulps was found to be equally effective in developing brightness as it was for the control samples. This, together with the data in Figs. 7, 8, 9, 10 and 11, suggests that calcium carbonate crystals in pulps should not significantly affect the bleaching process.

Fig. 10
figure 10

PCC brightness of unbleached and bleached pulp

Full size image
Fig. 11
figure 11

Drainage resistance of pulps produced at targeted kappa number using different cooking liquor

Full size image

Conclusions

In this study, we carried out a series of experiments to investigate the pulping of wood chips with high calcium content, which were virtually impossible to process using conventional kraft cooking methods with white liquor at lab scale. Our findings revealed that using green liquor for kraft cooking enabled the pulping of these calcium-rich wood chips at a reduced H-factor. Additionally, the bleaching performance of pulps cooked with green liquor remained unaffected. The key conclusions from this research are as follows:

  • Compared to using reference white liquor or a combination of black and white liquor, green liquor impregnation yields pulp with remarkably low reject content and a reduced kappa number at a similar H-factor.

  • Pulp with a lower kappa number was obtained at a given H-factor when green liquor was added to the white liquor for pulping calcium-rich wood.

  • The tensile and tear index values of bleached pulps were seen to increase when green liquor was used during impregnation.

  • Adding green liquor to the kraft pulping process causes calcium to remain with the fiber, instead of being transferred to the black liquor.

  • Kraft cooking with green liquor results in improved selectivity of the kraft pulping.

  • Adding green liquor to the kraft pulping process could alleviate the challenges encountered when pulping Eucalyptus dunnii wood chips with high calcium content.

  • Pulps treated with green liquor could be effectively bleached, largely retaining the benefits observed in the unbleached pulp.

  • A hypothesis for the effects of green liquor based on the formation of calcium carbonate (Fig. 5) has been presented.