Boron Compounds-Added Kraft Pulping from Scots Pine Kraft celuloza od borovine s dodatkom spojeva bora

• In this study, effects of KBH 4 (PB), NaBH 4 (SB), Etibor-48 (E48), Etidot-67 (E67), and colemanite (Col) on kraft pulp and paper properties of Scots pine (Pinus sylvestris L.) wood were evaluated. The control and boron compound-added kraft pulps were obtained under constant cooking conditions. The boron compounds were used as digester additives in different ratios (2 % and 4 %). The addition of boron compounds to kraft cooking liquor resulted in increases screened and total yield of pulps. The highest screened yield (52.05 %) and total yield (55.09 %) were obtained from PB-4 pulp. The lowest reject ratio (0.61 %) and kappa number (34.60) were deter - mined from PB-2 pulp. Furthermore, the highest tensile properties of handsheets were obtained from E67-4 pulp. Also, E48-4 pulp had the highest burst index and tear index values. E48, E67, and Col are cheaper than PB and SB. From these boron compounds, pulps with relatively low pulp yield but stronger can be obtained.


INTRODUCTION 1. UVOD
Scots pine is a significant trading tree species that is extensively planted for conservation and industrial activities such as live snow hedges and soil erosion protection (Şevik and Topaçoğlu, 2015).Scots pine forests cover 28 million hectares in Europe (Bußkamp et al., 2020).Scots pine covers 1.41 million hectares of Türkiye forests and has a 6.15 % share of the Turkish forested area (OGM, 2021).
Türkiye has 73 % of the world's boron reserves based on B 2 O 3 content.It is also the world's largest producer of boron compounds (57 % of the boron market).Türkiye's annual boron compound production is 1.73 million tons.The main boron minerals in Türkiye are ulexite ( The pulp industry has commercially used chemical or mechanical pulping methods, or a combination of the two, to obtain pulps with desirable properties.Chemical pulp accounts for approximately 75 % of the world's wood pulp production.Among the chemical pulp production methods, kraft pulping is the most widely used method and the strength properties of kraft pulp are higher compared to other methods.This process produces very high-strength papers.However, the process also has some negative aspects such as giving relatively low yields, being capital and energy-intensive, producing relatively low-value by-products, and generating problematic wastes (Das and Houtman, 2004).
In 2020, the world's paper and paperboard production was 401 million metric tons.The global chemical pulp production volume in 2020 was 148 million metric tons (FAO, 2021).Due to the rapid increase in paper consumption and demand, chemical pulp mills are aiming to find new lignocellulosic raw materials for pulp production (Kaur et al., 2018) or to increase pulp yield with various cooking liquor additives.In the chemical pulping process, cooking chemicals (NaOH, Na 2 S, etc.) combined with heat break down the lignin.During pulping, carbohydrate losses and thus the pulp yield losses also occur.The oxidation or reduction of carbohydrate reducing end can prevent these losses.For this purpose, SB was extensively studied as cooking liquor additive in the several lignocellulosic biomass such as Pinus radiata (Meller, 1963;Meller and Ritman, 1964), Pinus brutia (Akgul et al., 2007;Copur and Tozluoglu, 2008;Tutuş et al., 2012;Saraçbaşı et al., 2016), Pinus pinaster (İstek and Gonteki, 2009), Pinus pinea (Gümüşkaya et al., 2011), Abies bornmulleriana (Akgul and Temiz, 2006), Picea orientalis (Tutus et al., 2010a; Erişir et al., 2015), Populus tremula (İstek and Ozkan, 2008), Diospyros kaki (Tutuş et al., 2014), Prunus armeniaca (Tutuş et al., 2016), Cas-ing temperature, and 170 °C cooking temperature.The 2 % and 4 % (oven-dried wood) of each boron compound were added to kraft cooking liquor.Abbreviations of 2 % and 4 % KBH 4 -added cookings were PB-2 and 4% PB-4.The cookings of other boron compounds (SB, E48, E67, and Col) were carried out with similar abbreviations.The boron compound-free (C) was also done as a control.In each cooking, 700 g of wood chips (oven-dried) were cooked in a laboratory-type rotary digester.After digestion, pulps were washed with tap water and were disintegrated in a pulp mixer.Somerville-type pulp screen (TAPPI T 275) was used in the screening of pulp samples.Pulp samples were then beaten to 25 °SR in a Valley Beater according to TAPPI T 200.Freeness, viscosity, screened yield, and kappa number of pulps were determined according to ISO 5267-1, SCAN CM 15-62, TAPPI T 210, and TAPPI T 236, respectively.75 g/m 2 handsheets made by a Rapid-Kothen Sheet Former (ISO 5269-2) were conditioned according to TAPPI T 402.Tensile properties (tensile index, stretch, and TEA) (ISO 1924-3), tear index (TAPPI T 414), burst index (TAPPI T 403), and brightness (TAPPI T 525) of the handsheets were determined according to standard methods.The data of the handsheet properties for all pulp samples were statistically analyzed using analyses of variance (ANO-VAs) and the Duncan test at a 95 % confidence level.In Figure 1-6, the different letter lowercase indicates that the difference in the mean values of properties among the compared groups was statistically significant (P<0.05).

REZULTATI I RASPRAVA
The chemical composition and fiber morphology of Scots pine are given in Table 1 and Table 2, respectively.These results showed that both the chemical composition and fiber morphology of Scots pine used in this study were similar to previous studies.
The pulp yield of kraft pulping is relatively low.Therefore, some researchers have aimed to increase pulp yield using digester additives such as anthraquinone, polysulfur, NaBH 4 .The screened yields of pulps (except for E67-4 pulp) were increased with the addition of boron compounds.The highest screened pulp yield, increasing by 13.97 % (6.38 points from 45.67 % to 52.02 %), was obtained from PB-4 pulp.E67-4 pulp (45.51 %) had a lower screened yield than the control pulp (45.67 %) (Table 3).The screened pulp yields of all boron compounds-added pulps were increased with increasing boron addition ratios (except for E67-4 pulp).However, the holocellulose contents of E67-4, Col-2, and Col-4 pulps were lower than the control pulp (Table 3).These results showed that hemicellulose retention during pulping was significantly increased with PB, SB, and E48 additions (both 2 % and 4 %) to cooking liquor.However, hemicellulose retention was slightly increased in the E67-2 pulping.The total pulp yields of all pulps were also increased with boron additions (Table 3).The highest total pulp yield, increasing by 15.43 % (8.50 points from 46.59 % to 55.09 %), was obtained from PB-4 pulp.These results can be ascribed to the preven- Increases in pulp yield provide a significant economic contribution to the pulp mill.The addition of boron compounds also enables more effective use of forests (Gulsoy and Eroglu, 2011).The reject ratios of pulps provide information about cooking liquor penetration into chips and pulping efficiency.The boron compounds added to pulps had a higher reject ratio (except for PB2 pulp).The reject ratios of pulps increased with the increasing addition ratio of boron compounds.The lowest and highest reject ratios were observed for SB-2 pulp (0.73 %) and PB-4 pulp (3.04 %), respectively (Table 3).E48, E67, and Col had a negative effect on the kraft pulps of Populus tremula and Pinus nigra (Kilic Pekgözlü et al., 2017).The reject ratio increases in the SB-added pulps were observed in the Pinus nigra (Gulsoy and Eroglu, 2011) and Pteridium aquilinum (Gülsoy and Şimşir, 2018) kraft pulping.On the contrary, some authors noted that SB addition caused reject ratio decreases (Ayata, 2008 The kappa number of pulp is related to the effectiveness of pulping and the delignification degree of pulp.The boron compounds-added pulps had a higher kappa number (except for PB-2 and E48-2 pulps) than that of the control pulp.There was a linear relationship between the kappa number of pulps and the addition ratio of boron compounds.The lowest and highest kappa numbers were found in PB-2 (34.60) and Col-4 (46.60), respectively (Table 3).Kilic Pekgözlü et al.
(2017) noted that the kappa number of Populus tremula and Pinus nigra kraft pulp was decreased with the addition of E48 and increased with the addition of E67.The authors also noted that the effect of Col addition on the kappa number of Populus tremula and Pinus nigra kraft pulps changed depending on tree species.SB addition result in increasing delignification ( The pulp viscosity indicates the degree of polymerization of polysaccharides.The addition of boron compounds resulted in pulp viscosity decreases.The increase in the addition ratios of boron compounds caused pulp viscosity increase.The lowest pulp viscosity was found in SB-2 pulp with 818.36 cm 3 /g (Table 3).The cause of pulp viscosity decreases can be as- The addition of all boron compounds caused tensile index increases (Figure 1).The highest tensile index increase of 26.27 % (from 68.02 Nm/g to 85.88 Nm/g) was determined in the E67-2 pulp.Otherwise, tensile index losses were observed in the PB-2 pulp.However, this strength loss was statistically insignificant (P>0.05).Furthermore, increasing the addition ratios of boron compounds (except for PB-4 pulp) caused tensile index increases.This result can be ascribed to an increase in the hemicellulose content of the boron compounds-added pulps.The hemicellulose with high hydrophilic properties causes the increase of strength properties of pulp (Kilic Pekgözlü et al., 2017).The fibers with higher hemicellulose content swell more easily and have high flexibility (Shin and Stromberg, 2007).Flexible fibers provide more contact areas with adjacent fibers, which leads to strong interfiber bonds (Forsström et al., 2005) and a high tensile index (Santos et al., 2008).Kilic Pekgözlü et al. (2017) reported that E48, E67, and Col caused a decrease in the tensile index of Pinus nigra kraft pulp.The authors also noted that the tensile index of E48, E67, and Col added pulps of Populus tremula was found to be higher than that of the control pulp.The losses in the tensile properties with SB addition (Akgül and Temiz, 2006;Copur and Tozluoglu, 2008;Gulsoy and Eroglu, 2011) and PB addition (Gülsoy et al., 2016) were reported in the literature.Conversely, some authors noted that SB (Ayata, 2008;Gülsoy and Şimşir, 2018) and PB (Gülsoy and Şimşir, 2018; Çiçekler and Tutuş, 2019) caused increases in the tensile properties of pulps.
The boron compounds-added pulps had a higher stretch ratio than that of the control pulp (Figure 2) (P<0.05).The highest stretch increase was obtained from E67-4 pulp with 21.78 % (from 2.02 % to 2.46 %).Kilic Pekgözlü et al. (2017) noted that E48 and E67 had a positive effect on the stretch ratio of Pinus nigra and Populus tremula kraft pulps.They also reported that Col addition caused the increase of the stretch ratio of Populus tremula, while it led to the decrease of the stretch ratio of Pinus nigra.Gülsoy and Şimşir (2018) noted that PB and SB had a positive effect on the stretch ratio of Pteridium aquilinum kraft pulps.İstek and Özkan (2008) stated that the stretch ratio of Populus tremula kraft pulp decreased with SB addition.
The addition of all boron compounds caused TEA increases.E67-4 pulp had the highest TEA increase with 36.69 % (from 77.24 J/m 2 to 105.58 J/m 2 ).Otherwise, TEA loss observed in the PB-2 pulp was statistically significant (P<0.05).Besides, increasing the addition ratios of E48, E67, and Col led to TEA increases (Figure 3).Kilic Pekgözlü et al. (2017) noted that TEA of Populus tremula handsheets increased with the addition of E48, E67, and Col (except for 8 % Col), while TEA of Pinus nigra handsheets decreased.Gülsoy and Şimşir (2018) noted that SB had a positive effect on the TEA of Pteridium aquilinum kraft pulp.The effect of SB on the TEA of Populus tremula kraft pulp was statistically insignificant (İstek and Özkan, 2008).The addition of all boron compounds resulted in the burst index increases (P<0.05).The highest burst index increase was obtained from E48-4 pulp with 33.02 % (from 3.18 kPa•m 2 /g to 4.23 kPa•m 2 /g).Besides, the increasing addition ratios of SB and PB caused the burst index losses, while the increasing addition ratios of E48 led to the burst index increase.The effect of increasing addition ratios of E67 and Col on the burst index was statistically insignificant (P>0.05).The burst index increases can be ascribed to more hemicellulose retention in the course of pulping (Rydholm, 1965).Kilic Pekgözlü et al. (2017) stated that E48, E67, and Col had an unfavorable effect on the burst index of Pinus nigra, while they had a positive effect on the burst index of Populus tremula.The burst index losses with SB addition were determined in kraft pulp of Abies bornmulleriana (Akgül and Temiz, 2006), Pinus brutia (Copur and Tozluoglu, 2008), Pinus nigra (Gulsoy and Eroglu, 2011), poppy stems (Tutuş et al., 2011), and Brassica napus (Akgül et al., 2018).Conversely, some authors noted that SB addition caused the burst index increases in Eucalyptus grandis and Eucalyptus camaldulensis (Ayata, 2008), Picea orientalis (Tutus et al., 2010a), Castanea sativa (Aytekin, 2011), Diospyros kaki (Tutuş et al., 2014), Prunus armeniaca (Tutuş et al., 2016), and Rhododendron luteum and Rhododendron ponticum (Birinci et al., 2020) kraft pulps.PB had a positive effect on the burst index of Pinus brutia (Çiçekler and Tutuş, 2019) and Pinus pinea (Erkan et al., 2020).Conversely, PB addition led to burst index loss in the kraft pulp of Pinus pinaster (Gülsoy et al., 2016).All boron compounds caused the tear index increase (P<0.05).The highest tear index increase was obtained from E48-4 pulp with 37.55 % (from 5.30 mN•m 2 /g to 7.29 mN•m 2 /g).The increasing addition ratios of PB and E67 caused the tear index losses, while the effect of increasing addition ratios of SB, E48, and Col on the tear index was statistically insignificant (P>0.05).E48, E67, and Col had a positive effect on the tear index of Pinus nigra and Populus tremula pulps (Kilic Pekgözlü et al., 2017).The negative effect of PB on the tear index was reported by several authors The addition of all boron compounds (except for E67 pulp) caused the pulp brightness increases (P<0.05).The lowest and highest brightness values were determined in E67-4 pulp with 18.00 % and SB-2 pulp with 21.35 %, respectively.The brightness of E67 pulp decreased in direct proportion to the increase of the E67 addition ratio. 2 % boron compounds addition caused the increase in handsheet brightness, while 4 % boron compounds addition led to the decrease in handsheet brightness.Kilic Pekgözlü et al. (2017) noted that E48 and Col had a positive effect on the handsheet brightness of Pinus nigra.On the contrary, E67 had a negative effect on the handsheet brightness of Pinus nigra.The authors also reported that the handsheet

ZAKLJUČAK
The results showed that boron compounds-added kraft pulping of Scots pine confers various advantages.The addition of boron compounds to kraft cooking liquor caused increases in screened and total yield of pulps.PB-4 pulp had the highest screened yield and total yield.The lowest reject ratio and kappa number were determined from PB-2 pulp.Besides, E67-4 pulp had the highest tensile properties of handsheets.In addition, the highest burst index and tear index values were obtained from E48-4 pulp.E48, E67, and Col are cheaper than PB and SB.The addition of these boron compounds resulted in relatively lower pulp yield than those of PB and SB pulps.However, they caused stronger pulps.E48, E67, and Col are cheaper than SB and PB.The effects of these boron compounds on the black liquor recovery process should be determined.The effects of E48, E67, and Col on other tree species, that are widely spread and important for the paper industry, should also be investigated.

Table 1
Chemical composition of Scots pine Tablica