Evaluating the Pulpability and Paper Performance of Soda and Soda-Anthraquinone Pulping of Acacia Mearnsii (2025)

Sample collection and preparation

Sampling material was collected from Debre Markos town, East Gojjam Zone (10°20′N 37°43′E), Amhara Regional State, Ethiopia (“Debre Markos,” 2022). The material for analysis was gathered from healthy trees between the ages of 8 to13 years. A total of five trees were selected. From each tree, a 2.5 m long log was taken and transported to the Forest Products Innovation Center of Excellence (FPICE) located in Addis Ababa. From each log a slices of 5 cm were cut using a circular band saw. All slice samples were dried outdoors under shade and then ground in a Wiley-type mill (LABQUIP International Ltd, United Kingdom) to obtain a particle size of 60 mesh sieve (250 µm) for chemical composition characterization. For pulping and pulp yield determination, the particle size of a wood sample above 9 mm was used. The prepared A. mearnsii sample was leveled and placed in a plastic bag for subsequent chemical analysis and pulping.

Characterization of the chemical composition of sieved samples of A. mearnsii

The chemical composition of A. mearnsii sieved sample was characterized using the American Society for Testing and Materials (ASTM) standards, except for cellulose content, which was determined according to Kurchner-Hoffer method as cited in (Tolessa, Woldeyes, and Feleke Citation2017). All the chemical composition analyses were conducted in triplicates. presents the standards and methods used to analyze each chemical composition.

Pulping condition

presents pulping conditions for experimental setup. To pulp sieved samples of A. mearnsii, a typical concentrated solution of sodium hydroxide (soda) and soda-anthraquinone (soda-AQ) was serially diluted with distilled water to make soda and soda-AQ cooking liquor. Using soda and soda-AQ pulping methods; 100 g of oven-dry wood chips were weighed and charged into a rotary reactor (PL1–00, Xianyang Taist Test Equipment Co., Ltd, China) with the required amounts of chemical solution at a material-to-liquor ratio of 1:5. Treated sieved samples of A. mearnsii were digested by soda and soda-AQ processes at 15%, 20%, and 25% active alkali (NaOH) charge, with 0.1% AQ. In all cases, cooking temperatures were maintained at 160 °C and the reactor heating time was 3 hr. All cooking analyses as well as yield and Kappa number determinations were conducted in triplicates.

The resulting pulp was thoroughly washed with water. The pulp was manually screened through a 250 µm mesh sieve size and separated for screening yield and rejection. The pulp yield was determined gravimetrically by drying it at 105 ± 3 °C until a constant weight was achieved in the oven.

Pulp yield determination

The pulp yield was determined by weighing the pulps (W) and collecting two representative samples of 50 g each to determine the dryness. The samples were weighed and dried in an oven at 105 °C overnight. The dried samples were weighed, and the yield was calculated following EquationEquations (1)(1) $\mathit{Dryness}\left(\mathrm{\%}\right)=\left(\frac{\mathit{A}}{\mathit{B}}\right)\times 100$(1) and (Equation2(2) $\mathit{Total}\mathit{pulp}\mathit{yield}\mathrm{\%}=\left(\frac{\mathit{WA}}{\mathit{OD}\mathit{sample}}\right)\times 100\mathit{\hspace{0.17em}}$(2) ).

(1) $\mathit{Dryness}\left(\mathrm{\%}\right)=\left(\frac{\mathit{A}}{\mathit{B}}\right)\times 100$(1)

(2) $\mathit{Total}\mathit{pulp}\mathit{yield}\mathrm{\%}=\left(\frac{\mathit{WA}}{\mathit{OD}\mathit{sample}}\right)\times 100\mathit{\hspace{0.17em}}$(2)

Where A is oven dry weight of pulp taken for dryness; B is air-dry weight of the pulp taken for dryness (50 g); WA is the total air-dry weight of the pulp; and OD is oven-dry weight of the pulp.

The amount of pulp manufactured per cubic meter of wood (known as pulpwood productivity) was calculated using the wood’s basic density (BD) and pulp yield, EquationEquation (3)(3) $\mathit{Pulpwood}\mathit{productivity}\left(\frac{\mathit{kg}}{\mathit{m}}\right)=\left(\mathit{BD}\right)\times \mathit{Pulp}\mathit{Yield}$(3) . Before pulping, moisture content and the basic density of wood from each sample tree were determined. The difference between the green and oven-dried weights, divided by the green weight, was used to determine the moisture content. Basic density was measured using oven-dried weight and green-saturated volume, determined by water immersion.

Pulpwood productivity is the amount of oven-dry pulp generated per cubic meter of green wood. It is the result of the basic density of wood and pulp yield. Pulp yield indicates the percentage of pulp obtained from a given weight of wood used as raw material, essentially meaning how much pulp is produced relative to the initial wood weight used in the process; higher yield means more pulp is extracted from the wood, but it may not necessarily imply higher productivity because it may take longer to achieve that yield.

(3) $\mathit{Pulpwood}\mathit{productivity}\left(\frac{\mathit{kg}}{\mathit{m}}\right)=\left(\mathit{BD}\right)\times \mathit{Pulp}\mathit{Yield}$(3)

Where BD stands for bulk density of A. mearnsii wood.

Determination of moisture content in pulp

The moisture content was determined using TAPPI; Determination of equilibrium moisture in pulp, paper, and paperboard for chemical analysis (TAPPI Citation2013a) in triplicate and average values were used for reporting in percentage. The pulp specimens were conditioned in the air and 2 g was weighed in a tared dried weighing bottle and then dried in an oven for 120 min. Then the samples were cooled in a desiccator and weighed until two successive weighs did not differ by more than 0.1% of the weight of the specimen. The present moisture content of the pulp is calculated using EquationEquation (4)(4) $\mathit{Percent}\mathit{moisture}\mathit{content}=\frac{({\mathit{W}}_2-{\mathit{W}}_1)}{({\mathit{W}}_2-{\mathit{W}}_{\mathit{T}})}\times 100\mathit{\hspace{0.17em}}$(4) .

(4) $\mathit{Percent}\mathit{moisture}\mathit{content}=\frac{({\mathit{W}}_2-{\mathit{W}}_1)}{({\mathit{W}}_2-{\mathit{W}}_{\mathit{T}})}\times 100\mathit{\hspace{0.17em}}$(4)

Where W₂ = weight of bottle and specimen prior to drying; W₁ = weight of bottle and specimen after drying; and W_T = weight of bottle

Kappa number determination

To determine the kappa number, 3 g of pulp (dry weight) was mixed and filtered into a pad using a Buchner funnel to avoid any loss of fibers. The test specimen was then disintegrated into 500 ml distilled water until free of fiber bundles. The disintegrated test specimen was transferred to a 2000 ml reaction beaker and washed thoroughly with distilled water until the total volume reached 795 ml. Then, 100 ml of potassium permanganate solution (0.1 N) and 100 ml of the sulfuric acid solution (4 N) were pipetted in a 250 ml beaker. The mixture was brought to 25 °C quickly and was added immediately to the disintegrated test specimen, while a stopwatch was started. Then, beakers were washed with 5 ml of distilled water and bringing the total volume to 1000 ± 5 mL. After exactly 10 min, the reaction was stopped by adding 20 ml of potassium iodide solution (1.0 N) from a graduated cylinder, and immediately after mixing, the free iodine was titrated with the sodium thiosulfate solution (0.2 N), with a few drops of the starch indicator added toward the end of the reaction. A blank determination was carried out using the same method as above without taking any pulp sample (TAPPI Citation2013b).

The Kappa number was calculated as follows using EquationEquations (5)(5) $\mathit{K}=\frac{\mathit{P}\times \mathit{f}}{\mathit{w}}$(5) and (Equation6(6) $\mathit{P}=\frac{\left(\mathit{b}-\mathit{a}\right)\times \mathit{N}}{0.1}$(6) ):

(5) $\mathit{K}=\frac{\mathit{P}\times \mathit{f}}{\mathit{w}}$(5)

(6) $\mathit{P}=\frac{\left(\mathit{b}-\mathit{a}\right)\times \mathit{N}}{0.1}$(6)

Where K is kappa number; f is the factor for correction to a 50% permanganate consumption, dependent on the value of p; w is the weight of moisture-free pulp in the specimen, g; p is the amount of 0.1 N permanganate consumed by the test specimen, mL; b is the amount of the thiosulfate consumed in the blank determination (without adding pulp), mL; a is the amount of the thiosulfate consumed by the test specimen, mL; and N is the normality of the thiosulfate solution.

Papermaking and paper testing

Chemical composition of A. mearnsii

The chemical compositions of A. mearnsii and other literature reported values are summarized in and . The average moisture content of the A. mearnsii samples used for the chemical composition analysis ranged from 3.35% to 5.83%. Lignin is one of the main factors in wood pulpability; it affects the kappa number of pulp and the bleachability of pulp (Costa and Colodette Citation2007). Lignin, an undesirable component of pulpwood, is removed during the pulping process. The Klason-lignin content of A. mearnsii obtained in this study ranged from 14.84 to 22.17%, with an overall average value of 17.31% (). A. mearnsii has quite different quantities of lignin (Giesbrecht et al. Citation2022). reported 16.20% (Rizaluddin et al. Citation2015), reported 20.30% (Yadav, Singh, and Prasad Citation2007), reported 19.63% (Furtado et al. Citation2015), reported 38.73 and (Salaghi Citation2019) reported 21.4%. The Klason-lignin content of A. mearnsii found in this study is lower comparable to that reported in other countries for A. mearnsii and other acacia species. Low-lignin raw materials are beneficial to the pulping process because they require less chemicals, resulting in more efficient pulping in the delignification process and bleaching (to attain a high kappa number under softer pulping conditions) (Haque, Aziz, et al. Citation2019; Haque, Moinul, et al. Citation2019).

The cellulose content of A. mearnsii obtained in this study ranged from 44.14 to 56.93%, with an overall average value of 48.22% (). The higher cellulose content of A. mearnsii was found at the middle (51.26%) position followed by the bottom (48.27%). The lower cellulose content was observed at the top position (45.17%). The overall cellulose content of A. mearnsii found in this study is higher than the same species reported in the literature and very comparable to values published in the literature for other acacia and hardwood species ( and ). Hemicellulose is also an important component of plant cell walls. However, the hemicellulose content of A. mearnsii was not measured in this investigation, but according to (Furtado et al. Citation2015), the A. mearnsii sawdust fraction has 13.78% of hemicelluloses in total attributable matter. The hemicelluloses in the secondary wall of hardwood cells are primarily glucuronoxylan or 4-O-methyl-glucuronoxylan with some acetyl groups, meanwhile the hemicelluloses in softwood cells are primarily galactose glucose mannan or O-ethyl-galactose glucose mannan as reported in (Furtado et al. Citation2015). Thus, exploring the structure-properties relationship and mechanism of hemicellulose and its derivatives using modern instruments is crucial in further study.

The average ash content of A. mearnsii in this study was 1.24, 0.76, and 0.74% for top, middle, and bottom, respectively, with an overall average value of 0.91% (). The ash content of A. mearnsii increases from bottom to top. The ash content of A. mearnsii in this study is slightly higher when compared to values reported in the literature for the same species in different countries, and comparable to values published in the literature for other acacia and hardwood species. The value of the ash content varies due to the edaphoclimatic factor, which indeed affects the presence of ash-forming inorganic elements that are essential (macronutrients and micronutrients) and non-essential (Si, Al, and Na) for plant growth (Longui et al. Citation2024; Werkelin, Johan Skrifvars, and Hupa Citation2005). Therefore, the slightly higher ash content of A. mearnsii in this study shows the presence of non-essential inorganic elements.

The extractive content of A. mearnsii increases from bottom to top position. This trend is also reported for A. mangium and Leucaena leucocephala grow in Malaysia (). A. mearnsii extractive content in this study is 4.83%, 5.60%, and 6.62% for the bottom, middle, and top, respectively, with overall average value of 5.68% (). This value is higher than that reported in different countries, but comparable with some acacia species like A. auriculiformis (2.60 to 5.96%), A. mangium (1.40 to 5.38%), and A. hybrid (2.90 to 4.70%) reported in Indonesia, Malaysia, and Bangladesh. The high extractive content may negatively impact the soda pulping processes. Wood compounds can induce corrosion of equipment due to acidic extractives and complexes with metallic materials (Giesbrecht et al. Citation2022).

Overall, the chemical composition of A. mearnsii wood growing in Ethiopia is not considerably different from that of the same species in other countries and other acacia species. Variations of such magnitudes are likely within single species because chemical composition varies with tree age and wood position in the trunk. The raw material characteristics demonstrated that A. mearnsii growing in the country is acceptable for pulp manufacturing since the Klason-lignin content of the species discovered in this study is slightly lower than that reported in other countries for A. mearnsii and other acacia species. The cellulose content discovered in this study is higher than that of the same species reported in the literature and is quite similar to values published in the literature for other acacia and hardwood species. The slightly higher ash and extractive content of A. mearnsii in this study suggests the existence of non-essential inorganic and organic components, which could have a negative impact on soda pulping operations.

Pulp properties of A. mearnsii

The moisture content and basic density of A. mearnsii employed for pulping were around 25% and 816 kg/m³, respectively. Different process parameters prior to pulping such as active alkali concentration and pulping methods (soda and soda-AQ) influence kappa number and pulp yield. shows that soda-AQ and soda pulping of A. mearnsii with a chemical charge of 15 to 25%, cooking time of 180 min, and temperature of 160 °C resulted in higher kappa numbers of 10.90 to 16.74 and 15.90 to 22.78, respectively. The pulp yield ranged from 398.37 to 440.64 and 391.68 to 434.11, respectively.

In the present study, the pulp yields of A. mearnsii are comparable to most values reported for the same species and other hardwoods with the mean screened pulp yield value ranges from 48 to 54% for 15, 20, and 25% active alkali concentrations (). shows a decrease in pulp yield as the active alkali concentration of soda-AQ increased from 15 to 25%. A higher pulp yield of 54% was obtained using a 15% active alkali concentration while a lower pulp yield of 48.00% was obtained using a 25% active alkali concentration. Kappa numbers indicate residual lignin in the pulp or bleachability of pulp. Low Kappa pulps are easier to bleach while high Kappa pulps normally require more energy in refining but often produce stronger paper or board (specifically regarding tear strength). The kappa number of A. mearnsii in this study is in the range between 10.90 and 16.74 () for 15 to 25% of active alkali concentration using soda-AQ. The kappa number starts to decrease as the concentration of active alkali starts to increase from 15 to 25%. Kappa number of A. mearnsii using soda pulping decreases from 22.78 to 15.90 as the active alkali concentration increases from 15 to 25%. Our results are in line with (Liu et al. Citation2022), who report that unbleached pulp has a high lignin content of up to 30% of its weight, whereas bleached pulp has less than 2% lignin. An “acceptable” lignin content in pulp typically falls within the range of 1–20%, depending on the desired application (paper quality) and the pulping process used, with lower levels generally preferred for high-quality paper production, where a lignin content below 5% is often considered ideal.

When compared to values given in the literature for the same species, A. mearnsii showed good pulping yields (). Marinho et al. (Citation2017) used the kraft process to manufacture pulp from A. mearnsii with a yield of 47.62% and a kappa number of 12.4, using 20% active alkali and 25% sulfidity. demonstrates that when a 20% active alkali load was applied to A. mearnsii, the pulp output was 50.67% and 50.00, and the kappa number was 11.30 and 19.59 for soda-AQ and soda pulping methods, respectively. Muneri (Citation1997) study indicated that A. mearnsii provided higher yields (323 kg/m³) than Eucalyptus grandis (224 kg/m³). Chan et al. (Citation2015) also reported pulpwood productivity of 294 to 337 kg/m³, 210 to 245 kg/m³, 247 to 302 kg/m³, and 220 to 288 kg/m³ for A. mearnsii, A. mangium, E. globulus, and E. nitens, respectively. These results revealed that the current study found that A. mearnsii has comparable but slightly higher pulpwood productivity, yielding 398 to 440 kg/m³ for soda-AQ and 391 to 434 kg/m³ for soda, based on the basic density of A. mearnsii growing in Ethiopia and the attained pulp yield. The considerable increase in pulpwood productivity in this study when compared to others is attributed to the highest basic density reported for Ethiopia (816 kg/m³) (Desalegn, Kelemwork, and Gebeyehu Citation2012), which is considered suitable density for pulp and paper) reported for A. mearnsii grown in Ethiopia. The density of A. mearnsii varies significantly throughout literature. Salaghi (Citation2019) and Salaghi et al. (Citation2019) found values of 611 and 525 kg/m³ for A. mearnsii and E. globulus, respectively. According to (Yadav, Singh, and Prasad Citation2007), the bulk density of A. Mearnsii De Wild and Eucalyptus Hybrid was 660 and 490 kg/m³, respectively. Giesbrecht et al. (Citation2022) found a basic wood density of 544 kg/m³ for A. mearnsii samples. Marinho et al. (Citation2017) reported a density of between 608 and 621 kg/m³ for A. mearnsii. Yahya et al. (Citation2010) also found a wood density of 490, 460 and 520 kg/m³ for A. hybrid, A. mangium, and A. auriculiformis, respectively.

Chan et al. (Citation2015) reported that the excellent yields obtained by A. mearnsii, together with its fiber diameters, demonstrate that this cellulosic material is very suitable for pulp manufacturing. According to the authors, acacia fibers are similar in length and width to Eucalyptus spp fibers. The high screened pulp yields achieved in this investigation can be attributed to the wood’s high carbohydrate content, which was maintained when low active alkali was used (15 to 25%). Colodette et al. (Citation2000) and Trugilho et al. (Citation2005) found that A. mearnsii outperformed Eucalyptus species in terms of pulp yield. The formers achieved a screened yield of 47.5% and a kappa number of 18, while the others had a total pulp yield of 54.3% and a kappa value of 16.3. A. mearnsii outperformed kraft pulping with high lignin removal and low kappa values. Thus, the lower kappa value the lower the lignin content in the pulp, making bleaching easier and more cost-effective (Tolessa, Woldeyes, and Feleke Citation2017). illustrates how the kappa number decreased as the active alkali increased for both pulping methods. The kappa number of bleaching grade pulps ranges between 16 and 18 in country like Brazil (Giesbrecht et al. Citation2022), and comparable with the results obtained in this investigation (16.74 for soda-AQ) with a low alkali charge (15%) and pulp yield of 54.00%. The lowest kappa number recorded was 10.90 at active alkali concentration of 25% for soda-AQ pulping method, with a screened yield of 48.33%. Furthermore, pulp with a kappa number of 15.90 was produced when a high alkali charge of 25% was used during soda pulping, and the yield was satisfactory with screened pulp yield of 48.00%. However, when the active alkali was increased from 15% to 25%, a slight kappa number reduction (from 22.78 to 15.90) and a pulp yield of 53.20 to 48.00 for soda pulping method. The pulp yield of A. mearnsii in this study is also comparable to the screened kraft pulp yield of A. manguim (40–50%) and A. crassicarpa A. Cunn. ex Benth (53.8%) reported in (Wan Rosli, Mazlán, and Law Citation2011) and (Martins et al. Citation2020), respectively. Overall, the ideal pulping condition was 15% active alkali with a kappa value of 16.74, resulting in a 54% screened unbleached pulp yield for soda-AQ pulp production. However, alkali concentration of 25% is the ideal alkali charge for A. mearnsii when the goal is to remove the most lignin during soda-AQ pulping while maintaining a high yield (48.33%).

When pulp yields for soda and soda-AQ pulping were compared, soda-AQ at 15% active alkali concentration produced the highest pulp yield. When comparing kappa numbers for soda and soda-AQ pulping, soda-AQ pulping displayed lower kappa numbers than soda pulping. It should be also observed that the amount of active alkali required for soda-AQ to achieve low kappa numbers was less than that necessary for soda, implying that soda-AQ improved the delignification process and significantly decreased kappa number. Anthraquinone acts as a catalyst in the pulp manufacturing process, significantly increasing the rate of delignification. This means that it effectively breaks down lignin in wood fibers, allowing for higher pulp yields with less chemical use while also stabilizing the cellulose component, resulting in improved pulp quality. In this study, both active alkali concentration and pulping methods (soda and soda-AQ) had a significant effect on pulp yield and kappa number. The kappa number of pulp is an important parameter in pulp manufacturing since it determines the pulp’s bleachability, or degree of delignification (Chai and Zhu Citation1999). It is a measure of chemical consumption in bleaching operations by total residue lignin in the pulp (Chai, Luo, and Zhu Citation2000). Therefore A. mearnsii reduced kappa number in this study makes it suitable for further pulp bleaching and paper production. Laboratory wood chips are usually more homogeneous than commercial wood chips since they are screened manually. Thus, because the A. mearnsii chips used in this study were obtained from a wood yard and no additional screening was performed, the reject rate is typical of the commercial process. The rejection rate value for modern pulp screening is typically between 1% and 3% by weight (Germgård, Sjöstrand, and Fiskari Citation2023). However, the rejection rate can be impacted by several factors, including pulp quality, pressure difference, screen room design, pulp consistency, and screen aperture size. Future research should consider evaluating pulp rejection rate value, viscosity, and optimal beating revolution, as these aspects were not considered in the current study.

Strengthen index properties of A. mearnsii unbleached pulp sheet

The drainability index found in this study varied between 20 and 21 °SR, which is comparable to the 26 °SR reported by (Marinho et al. Citation2017) for A. mearnsii at T30’ refining, which is near to ideal. and show A. mearnsii pulp sheet strength properties such as tensile, burst, and tear index for both soda and soda-AQ pulping.

shows that the tear index for A. mearnsii using soda-AQ is highest at 15% active alkali concentration, followed by 20% and 25%, respectively. The same trend was seen for soda pulping at 15, 20 and 25% active alkali concentrations. When tear strength was compared between soda and soda-AQ pulping, tear strengths were shown to be higher for 15 and 20% active alkali concentrations utilizing soda-AQ pulping. The tensile index ranged from 15.77 to 36.82 N.m/g for soda-AQ using active alkali concentrations of 15 to 25%, and from 20.95 to 26.50 N.m/g for soda pulping (). Tensile strength increases as active alkali concentration decreases. When comparing the tensile strength values for both chemical pulping methods, highest values were observed for 15% active alkali concentrations. The bursting index of A. mearnsii in this study follows a trend like tear and tensile strength. When pulping with soda-AQ, the bursting index decreases from 2.12 to 0.89 KPa.m²/g as the active alkali concentration rises from 15 to 25% (). The bursting strength of soda pulp decreases from 1.19 to 0.90 KPa.m²/g as the active alkali level increases from 20 to 25%.

Marinho et al. (Citation2017) evaluated bleached and unbleached Kraft pulp from the A. mearnsii De Wild species at various refining times. The study reported a pulp yield of 47.62% and Kappa values of 12.3 and 4.5 for unbleached and bleached pulps, respectively. The mechanical properties index for tensile were 25.24, 55.44, and 66.17 N.m/g in refining T0“, T15,” and T30” unbleached pulp, tear with 4.66, 12.94, and 11.66 mN.m²/g in refining T0‘, T15,’ and T30” unbleached pulp, and burst with 1.82, 3.93, and 4.58 kPa.m²/g in refining T0“, T15,” and T30’ unbleached burst. Given these characteristics, the study concluded that the species has industry potential in the production of sanitary “tissue,” decorative, and other papers. Therefore, the findings in this study revealed that A. mearnsii grown in Debre Markos, Ethiopia, has comparable mechanical properties at active alkali concentrations of 15 and 20%, but slightly lower when active alkali concentration is increased for both pulping methods. Soda-AQ pulping method outperformed the soda pulping method in terms of mechanical properties, particularly at low concentrations. Muneri (Citation1997) also found that A. mearnsii pulp had lower strength properties but higher opacity than E. grandis pulp at around the same Kappa value.