Abstract
Index levels of the process of pulping wheat straw (WS), industrial hemp stalks (HS), and Miscanthus x giganteus (MS) using the nitric acid-alkali (NA-A) method (HNO3/KOH and HNO3/NH4OH two-stage sequential delignification systems) were determined. The research showed, as a rule, a lower total yield of NA-A pulps than kraft pulps from WS, HS, and MS, and a lower level of the tensile index and tear resistance compared to the properties of soda-AQ and kraft pulps presented in other studies, as well as the author’s own studies. Additionally, the levels of nitrogen (in different forms) and potassium in the combined post-pulping filtrate were determined. Their levels were found to be higher than in the fertilizer solution used to fertilize tomato crops grown on soilless substrates in greenhouses. However, application of filtrates from pulping of nonwoody plant straw using NA-A method requires determining the effectiveness of this type of fertilizer solution, its impact on the horticulture and agriculture plants, and the properties of their output including grains, tubers, and fruits.
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Nitric Acid-Alkali Two-stage Pulping of Wheat Straw, Industrial Hemp, and Miscanthus × giganteus
Dariusz Danielewicz *
Index levels of the process of pulping wheat straw (WS), industrial hemp stalks (HS), and Miscanthus × giganteus (MS) using the nitric acid-alkali (NA−A) method (HNO3/KOH and HNO3/NH4OH two-stage sequential delignification systems) were determined. The research showed, as a rule, a lower total yield of NA−A pulps than kraft pulps from WS, HS, and MS, and a lower level of the tensile index and tear resistance compared to the properties of soda-AQ and kraft pulps presented in other studies, as well as the author’s own studies. Additionally, the levels of nitrogen (in different forms) and potassium in the combined post-pulping filtrate were determined. Their levels were found to be higher than in the fertilizer solution used to fertilize tomato crops grown on soilless substrates in greenhouses. However, application of filtrates from pulping of nonwoody plant straw using NA-A method requires determining the effectiveness of this type of fertilizer solution, its impact on the horticulture and agriculture plants, and the properties of their output including grains, tubers, and fruits.
DOI: 10.15376/biores.18.4.7629-7644
Keywords: Wheat straw; Industrial hemp stalks; Miscanthus × giganteus stalks; Nitric acid, KOH, and NH4OH; Pulping conditions; Pulp properties; Post-pulping filtrates; Nitrogen and potassium
Contact information: Fibrous Papermaking Pulps Technology Team, Centre of Papermaking and Printing, Lodz University of Technology, Wolczanka 223 Str., 90-924 Lodz, Poland;
* Corresponding author: dariusz.danielewicz@p.lodz.pl
INTRODUCTION
The two-stage method of pulping plant fibrous raw materials (PFRM) consisting of the nitration (N) of lignin in the first stage with nitric acid and its extraction (E) with alkali in the second stage, i.e., the nitric acid-alkali (NA−A) method, is a known method of obtaining fibrous papermaking pulps. This method of producing this type of pulp was first patented in Great Britain in 1861 (Kalish 1967). In 1937, using a similar method, the I. G. Farben concern in Wolfen (Germany) started the production of nitrogen cellulose from beech wood, called N-Zellstoff, which was to be a substitute for nitrocellulose from cotton linters. However, production of this product was discontinued after World War II (Kalish 1967). An example of an attempt to introduce the NA-A pulping process into industrial practice is also the Delbay process. The pulping of wood in this process was performed in a special lying-type reactor, which is divided into two parts. In the first one, called the nitrator, wood chips are treated with 42% nitric acid and then extracted/neutralized with NaOH (Brink 1961).
In 1961, Brink (1961) proposed the use of black liquors from the pulping of white spruce using the NA−A method (HNO3/NH4OH system) as fertilizer for agricultural purposes. In the second part of the paper under Brink’s direction (Brink et al. 1961), the authors completed several attempts to pulp white spruce with the use of a 10% nitric acid solution, i.e., dosing of this acid at approximately 100% to oven-dry (o.d.) wood with the use of alkaline extraction with ammonia as the second stage of the process. The third part of work (Brink et al. 1962) concerned the balance of nitrogen in the process of pulping of white spruce with the NA−A method.
In the mid-1960s, pulping of softwood and hardwood using the NA−A method was discussed by J. H. Kalisch. He developed a new industrial-scale process for pulping this PFRM using this method in the so-called rapid cycle (NARC – Nitric Acid Rapid Cycle). In this process, the wood wafers were nitrated at two stages using 12.0 to 14.5% and 0.9 to 2.1% of HNO3 solutions, respectively, washed, and extracted in the E stage (Kalish 1967). In contrast, Kurrle (1963) processed red spruce wood from the trunk, tips, roots, and branches of this species into NA−A pulps using HNO3 water solutions with a concentration of 16 to 17% at a liquid−to−wood ratio of 6:1 (i.e., using approximately 114 to 122% HNO3 for wood), followed by alkaline extraction.
Relatively large amounts of HNO3 on o.d. on PFRM were also used by Biyani et al. (1967), i.e., 52.2% and 66.8% on o.d. of bamboo (the nitric acid concentration in the cooking liquor was 8 to 10%, cooking liquor-to-PFRM of 6:1). An important result of this study was that the pulping of bamboo using the NA−A method gives pulps that are stronger if the pulping temperature is lower, e.g., 75 °C rather than 85 °C, and that NA−A pulps have low pentosane content, which predisposes the NA−A process to produce dissolving kinds of pulps.
In the 1970s, a method of pulping PFRM (wood, straw) using the NA−A method was developed in Poland (Nitramcel process), using the black liquor from such pulping as fertilizer. In this process, PFRMs were first impregnated with a solution containing 0.1% dimethyl oleic acid amide and 0.05% imidazolium inhibitor or ammonium nitrate for cellulose protection, then nitrated using of 15 to 18% nitric acid solution, and finally extracted with alkali (Kin 1990).
The issue of pulping wood by the NA−A method was also extensively studied in the Soviet Union. For example, Sorokin and Bakina (1977) studied the extraction of nitric acid−nitrated birch and poplar wood with different alkalis, while Gorbunova et al. (1993a,b,c) discussed the chemical composition of black liquors from NA−A pulping.
In 1990, Ngamveng and Ndikontar presented the results of pulping cassava stems (Manihot esculenta Crantz) using the NA−A method with concentration of nitric acid in the nitration stage of 7.5% (i.e., about 32% o.d. PFRM) and extraction of nitrated PFRM with alkali.
More recently, Yang et al. (2019) found that the optimal conditions for pulping birch wood (Betula platyphylla) in nitration stage are a nitric acid concentration of 9.2%, a liquor-to-wood ratio of 10:1 (i.e., approximately 101% HNO3 on o.d. PFRM), 85 °C, and 4.3 h. Even greater amounts of HNO3 in the stage of nitration on o.d. PFRM [(black mustard (Brassica nigra L.) and camelina (Camelina sativa L.)], i.e., approximately 127% to the o.d. PFRM (concentration of HNO3 solution of 6%, with a cooking liquor-to-PFRM ratio of 20:1), were used by Hájkova et al. (2021).
Attempts to pulp PFRM by the NA−A method were also performed with lower concentrations of nitric acid in the N stage. For example, Ivanow et al. (1978) showed that wood can be processed into pulp by the NA-A method using a concentration of nitric acid of 2 to 6% in the nitration stage, while Shishonok et al. (1997) that such effect can be obtained using concentration HNO3 of 1 to 2% in this stage.
It was also proposed to use the NA−A method to obtain chemical-mechanical pulps from bagasse (El-Taraboulsi et al. 1983a) and rice straw (El-Taraboulsi et al. 1983b).
In the works mentioned above, the temperature and time conditions of the nitration and extraction processes, as well as the type of substance used for the extraction of lignin in the E stage, were also presented. The review of these conditions shows that in order to obtain the desired pulping degree of PFRMs, it was necessary to conduct these processes at temperatures elevated to the ranges 75 to 100 °C and 75 to 130 °C, respectively, for one to four hours. For the removal of lignin from the nitrated raw material, hot 1 to 5% solutions of alkali were used. As the source of alkali, sodium-containing alkalis (NaOH or in one work also Na2CO3, and NaHCO3) were used. Only in works of Brink et al. (1961), Kin (1990), and Sorokin and Bakina (1977), NH4OH was used instead of NaOH.
The current study aims to determine the level of basic processing indices of wheat straw (WS), industrial hemp (Cannabis sativa L.) stalks (HS), and Miscanthus × giganteus (MS) (i.e., non-woody plant materials growing well in the northern temperate climate zone, such as in Europe) pulping using the NA-A method in temperature-time conditions described in works of other authors. In addition to the type of PFRM, this work is distinguished from the work of other authors by dosing of HNO3 as percentage on the o.d. weight of PFRMs (i.e., as in the case of industrial production of kraft pulps) at a liquor-to-PFRMs ratio at a constant level of 5, and by using KOH or NH4OH as the alkalizing agents in the E stage. The difference is also the determination of the content levels of total nitrogen, nitrate nitrogen, ammonia nitrogen, and potassium in the combined post-cooking filtrate (CPF) obtained by combining the filtrates from the N, E and washing stages using contemporary analytical methods.
EXPERIMENTAL
Fibrous Raw Materials
Unsorted WS (with leaves and ears) cut into sections of approximately 20 mm, similarly cut MS stalks (without leaves), and HS stalks (also without leaves) cut into 3 to 6 mm sections were used in the study. The purpose of cutting fibrous raw materials was to facilitate the penetration of nitric acid into these raw materials and to remove nitrated lignin from them at the alkaline extraction stage. In the case of hemp stalks, their greater fragmentation was caused by the need to shorten the long hemp bast fibers to reduce their tendency to spin during the formation of paper handsheets and deteriorate their formation. These PFRM were obtained locally from small plantations owned by Polish farmers. Birch (Betula pendula) (B) and pine (Pinus sylvestris) (P) chips were obtained from one of the Polish pulp mills (Fig. 1).
Fig. 1. WS (A), HS (B), and MS (C)
Pulping of PFRM Using the NA−A Method
The pulping of WS, MS, and HS using the NA−A method was performed in a Hatoo rotary digester (LD8; Hatoo Oy, Helsinki, Finland) in autoclaves with a capacity of 3000 cm3, usually 100 g of o.d. of these raw materials. Nitric acid was dosed in terms of the pure compound in the amount of 5 to 30% on the o.d. of PFRMs (in most cases 10 to 30% on o.d. PFRM). The liquid−to−PFRMs ratio in these experiments was always 5. The range of concentrations of nitric acid in the digestion process was therefore in the range of 1 to 6% (in most cases of 2 to 6%). Before starting the heating, the fibrous raw materials were left in the autoclaves for 1 h to supersaturate them with a nitric acid solution on the principle of penetration. After this period, the digester heating was started. The time of heating to the maximum temperature (60 to 90 °C) was the same for all experiments and amounted to 1 h, while the time of pulping ranged between 2 or 3 h. After the nitration stage, the cooking acid was drained, and then the PFRM was poured twice with tap water in a pile and left for 3 h to wash the raw material from acid residues by diffusion (two stages of such washing) and filtered on a Buchner funnel lined with cheesecloth. After nitration and washing, the pulped material was subjected to alkaline extraction (E) with aqueous potassium hydroxide (KOH), or in selected experiments ammonia hydroxide (NH4OH) to extract nitrated lignin from it. Extraction was also performed in a Hatoo rotary digester. The KOH and NH4OH were dosed into these autoclaves in the amounts of 2 to 10% on the o.d. PFRM (in most cases 10% on o.d. PFRM). The liquid-to-PFRM ratio in the E stage was 5:1, while the concentration range of KOH or NH4OH in the E stage was in the range of 0.4 to 2.0% (in most cases 2%). Extraction experiments were performed at 80 or 90 °C. Heating time and extraction time were respectively, 1 h and 2 to 3 h. After extraction, a strong alkaline filtrate was drained, and then the raw material was washed with tap water in a bucket, left for 3 h for diffusion washing (two stages of such washing), centrifuged, and stored in plastic bags for further research. After washing, pulped raw materials were disintegrated in a laboratory disintegrator and screened on a Weverk screen. Uncooked parts of raw materials, after separated from pulp, were refined in a laboratory Bauer disc refiner and then added to screened pulp. The pulp was then centrifuged, and after determining its dryness, it was stored in a wet condition in a refrigerator in closed polyethylene bags.
PFRMs Pulping by the Sulfate Method (Kraft Pulping)
To compare the total yield of NA−A and kraft pulps from WS, HS, MS, B, and P, kraft pulps were obtained as a result of the pulping of these raw materials by the sulfate method (kraft pulping). Kraft pulping experiments of PFRM were made in a laboratory Santasalo-Solhlberg digester using 11.5 to 18.5%, 14 to 18%, 12 to 18%, 18 to 26%, and 18 to 26% of active alkali to o.d. raw material, respectively. The sulfidity of white liquor in these experiments and the liquor-to-wood ratio were 25% and 5:1 for WS, HS, and MS, and 4:1 for wood samples, respectively. The time of heating to the pulping temperature (165 °C) and the time of pulping at this temperature were 90 min in each case. After pulping, pulped PFRMs were washed with water preliminarily on a wire mesh, then by diffusion overnight in a bucket, and finally disintegrated in a laboratory disintegrator. Uncooked parts of raw materials, after separated from the pulp, were refined in a laboratory disc refiner and then added to screened pulp. The pulp was then centrifuged, and after determining its moisture content, it was stored in a wet condition in a refrigerator in closed polyethylene bags.
Content of Lignin and Cellulose in PFRMs and Pulps Properties of in Loose Form
The content of lignin and Seifert cellulose in WS, HS, and MS were determined according to PN-74/P-50092 (1992) standard. As for pulps, the degree of pulping, i.e., kappa number (KN) and intrinsic viscosity (ηCED) of pulps were determined. The KN was determined according to the PN 85/P 50095.02 (1985) and ISO 5351−11 (1981) standards.
Pulp Properties in the Form of Handsheets
The pulps KN 8.9 WS NA−A pulp (obtaining conditions Table 1, raw 5), KN 8.9 HS NA−A pulp (obtaining conditions Table 1, row 14), and KN 11.5 MS NA−A pulp (obtaining conditions Table 1, row 20) were used for these tests. The handsheets properties of these pulps were compared with the properties of handsheets of KN of 9.4 B kraft pulp and KN of 22.5 P kraft pulp. The beating of these pulps was completed in the Jokro mill according to the PN-EN 25264-3 (1999) standard. Before beating, the pulp samples were soaked in water for 18 h. The freeness of beaten pulps was expressed as Schopper−Riegler freeness (S-RF), measured according to the PN−EN ISO 5267−1 (2002) standard. After beating, the pulp sample was transferred to the laboratory distributor and diluted to a consistency of 2%. This slurry was used for making handsheets of paper with a basis weight of 75 ± 3 g/m2 in standard Rapid-Köthen apparatus according to PN-EN ISO 5269 (2001). The mechanical properties of pulps in the form of paper handsheets in static (e.g., tensile strength, stretch at break) and dynamic conditions (e.g., tear strength) were determined after their conditioning according to the PN-EN 20187 (2000) standard. Testing of these properties was conducted according to the PN-EN ISO 1924-2 (2008) and the PN-EN ISO 1974 (2012) standards using tensile strength tester BZ2.5/TN1S (Zwick/Roell, Ulm, Germany) and ProTear tester (Thwing-Albert, West Berlin, NJ, USA) apparatuses. For the measurement of the thickness of the handsheet, the papermaking micrometer was used with an accuracy of 0.001 mm.
Total Nitrogen, Nitrate Nitrogen, Ammonium Nitrogen, and Potassium Contents in the Combined Post-Cooking Filtrate
The fertilizing substances content was determined in filtrates from pulping of two samples of WS using the NA−A method, performed using 15% HNO3 to o.d. PFRM in the nitration stage and alkaline extraction with 10% KOH (experiment 5) and 10% NH4OH on o.d. WS. These tests were made with 50 g o.d. WS. The time and temperature conditions in these experiments corresponded to the conditions of experiment no. 5 and no. 13 from Table 1. For these tests, the filtrates from the nitration and alkaline extraction stages and from the process of washing the pulp WS twice after these processes with water in the amount of 250 mL in each of its stages were preserved, giving a total of six aliquots of filtrates from one pulping experiment. The filtrates from each of the two NA−A pulping experiments were then combined and neutralized to pH 7 using KOH or NH4OH. This gave approximately 1500 mL of combined post-cooking filtrate (CPF). This filtrate was analyzed for the content of total nitrogen (NT), nitrate nitrogen (N-NO3), ammonium nitrogen (N-NH4), and potassium (K). NT was determined using the conductometric method according to Dumas (method symbol PB-08) using the TruSpec CNS analyzer (LECO Europe BV, Geleen, Netherlands). N-NO3 and N-NH4 were determined potentiometrically. Potassium was determined by inductively coupled plasma atomic emission spectrometry (ICP-OES) (Optima 2000 DV ICP; Perkin-Elmer Inc., Waltham, MA, USA). The tests were carried out in Institute of Horticulture in Skierniewice (Poland).
Elaboration of Results
The determination of YT, KN, ηCED, and S-RF of pulps was performed twice for each pulp. Therefore, the results of the determination of these properties presented are the average of the two determinations. The standard deviations of the measurements of these properties ranged in the following intervals: 0.4 to 0.8%, 0.3 to 0.7 units, 10 to 23 cm3/g, and 0.5 to 1.0 °SR, respectively. From WS, HS, and MS NA−A pulps and from B, and P kraft pulps, two sets of eight paper handsheets were prepared. The paper formation was then visually examined against a light source to choose two sets of six handsheets each, representing the best-formed paper samples. These two sets of paper handsheets for each pulp sample were then used for the determination of the mechanical properties of pulp and bulk. Ten determinations of these properties were performed for each set of handsheets, so after the testing, there were two mean values for each property. In Table 2, the arithmetic average of these two values is presented. The standard deviations of the measurements of these properties ranged in the following intervals: 3.1 to 6.7 N·m/g, 0.2 to 0.5 mN·m2/g, 0.1 to 0.3 %, and 0.021 to 0.056 cm3/g, respectively.
RESULTS AND DISCUSSION
Table 1 shows the conditions for processing wheat straw into NA−A pulps and their KN. This Table shows that under the conditions of the WS pulping using the NA−A method described in rows 1 to 4 of Table 1, the splinter-fibrous (S-F) form of WS was obtained after mechanical fiberization, indicating that the sufficient high point of fiber liberation of the WS for pulps and a low residual lignin content in this case was not obtained due to too mild conditions of the NA−A pulping process. For this reason, the KN of the resulting fiber intermediates was not determined. This situation was marked in Table 1 as an F-S shortcut. For greater certainty of obtaining well-pulped pulps from WS in subsequent experiments, the amount of HNO3 was increased to 15 to 30% of HNO3 on o.d. WS and the amount of KOH to 10% on o.d. of this PFRM. This change brought the desired effect because, in this case, after mechanical fiberization, fibrous semi-finished products were obtained, in which most of the WS could be defibrated to single fibers, which made it possible to determine the kappa number by the standard and to prepare paper handsheets of pulps.
The data in Table 1 show that the pulping of WS described in row 5 allowed for the preparation of KN 8.9 NA−A pulp. Experiments conducted in similar time and temperature conditions (experiment no. 6) but with the use of 20% HNO3 on o.d. WS made it possible to obtain a pulp with an even lower KN, i.e., 6.6 units. For trials conducted with 20% HNO3 on o.d. WS, the effects of pulping with the amount of KOH reduced to 5% and 2% in the E stage were also examined due to the separation of black liquor from WS after pulping in the N stage and double washing of nitrated material. However, it seems that such amounts of KOH used for the E stage were too small, because the KN of the obtained NA−A pulps increased to 26.9 and 34.3 units, respectively. Considering the observed increased level of splinters in WS digester pulp when the KN of the pulp was 26.9 to 34.3, it can be concluded that such a level of lignin content may be too high. Therefore, the amount of KOH in the E stage should be more than 5% on o.d. PFRM, i.e., for example, 10% on o.d. PFRM.
Table 1. Conditions of Processing WS, HS and MS into NA−A Pulps and Their KN
O.n. – ordinal number; F−S: fibrous-splinter from of pulp preventing marking of KN
Tests were also conducted with the dosing of 30% HNO3 on o.d. WS in the N stage. These tests were completed with the temperature of the process reduced to 80 °C. For experiment No. 9, an increase in the amount of HNO3 dosed made it possible to obtain a KN 5.3 pulp. Subsequent tests with this amount of dosed HNO3 were made to check how reducing the amount of KOH in the E stage at the increased dose of HNO3 affects the effects of delignification. These studies have again shown that by reducing this amount from 10% to 6%, 5%, and 3%, the KN of final pulp again increases from 5.3 units to 15.4, 14.9, and 21.0 units, respectively. These experiments confirm the necessity of dosing the appropriate amount of KOH in stage E (approximately 10% on o.d. PFRM) to fully use the oxidation potential of HNO3.
Table 1 shows also the conditions for processing HS and MS into NA−A pulps and their KN. These data show that by performing pulping of HS using the NA−A method under the conditions shown in the row 14, a well-pulped KN 8.9 pulp can be obtained. Increasing the amount of HNO3 dosed from 15% to 20% on o.d. HS, maintaining the remaining process parameters as in experiment No. 1, resulted in the deepening of the delignification of the NA−A Hs pulp to KN 4.6 units. When the KN of pulps after pulping by the NA−A method in the range of 5 to 10 units is too low (what may happen, e.g., because of the negative effect of too deep delignification at elevated temperature on the strength properties of the fibers), it can be increased to the range of 10 to 15 units by lowering the process temperature from 90 °C to 80 °C and extending the duration of the N and E stages (experiment 16, Table 1). Experiment no. 17 from Table 2 shows that increasing the amount of nitric acid from 20% to 30% in the N stage while reducing the time of the N and E stages to 2 h deepens the delignification of these stalks to KN 5.3, i.e., as much as can be obtained by increasing the temperature of the Hs NA−A pulping by 10 °C at 20% HNO3 on the o.d. Hs (experiment No. 15).
It was also checked how the delignification of WS and HS is affected by the replacement of KOH with NH4OH in the E stage (experiment no. 13 and no. 18, Table 1), which can be considered a lignin extractor for the need to obtain fertilizer solutions from black liquor with a higher nitrogen content. The test results indicate, however, that in such a case, less delignified pulps should be expected due to the lower capacity of NH4OH than KOH to extract nitrated lignin (the kappa numbers of WS and HS NA−A pulps were 19.7 and 14.2, respectively, compared to 6.6 and 4.6 using the HNO3/KOH delignification system).
Table 1 shows also the conditions for processing MS into NA−A pulps and their KN (rows 19-21). These data indicate the possibility of obtaining MS NA−A pulps with residual lignin content corresponding to pulps intended for full bleaching from hardwood, i.e., KN 11.8 to 17.5, when pulping is completed under conditions presented in rows 20 and 21. These data indicate a lower efficiency of MS delignification in the N stage performed at a lower process temperature, i.e., 80 °C.
According to the Introduction section, only Shishonok et al. (1997) have so far discussed the processing of real non-wood fiber raw materials for European papermaking located in northern temperate climate zone (i.e., rye straw) into pulps with a low lignin content using the NA−A method. However, it is possible that no one has yet processed WS, HS, and MS into NA−A pulps. Regarding the work of Shishonok et al. (1997), the authors state that when using HNO3 solution of a concentration of 1 to 2%, liquor-to-PFRM ratio of 10:1 (and therefore approximately 11 to 22% HNO3 on o.d. of rye straw), a temperature in N stage of 80 to 90 °C, a time of this process of 3 to 4 h, and using alkaline extraction of nitrated rye straw with a 1.5% NaOH solution, KN 11−35 NA−A pulps can be obtained. Therefore, these results seem to be similar to those obtained in this work for WS. These results also demonstrate that KOH can be used as an extractor for nitrated lignin instead of NaOH.
The data in Table 1 also enable the performance of a preliminary comparison of the susceptibility to delignification of WS, HS, and MS using the NA-A method. They indicate a slightly better susceptibility to delignification of WS and HS than MS using this method, despite a slightly higher lignin content in WS than HS and MS (22.2% versus 18.2% and 20.6%, respectively). In the case of WS, this may be due to a much better availability of native and nitrated lignin for HNO3 and KOH, respectively, than in the case of HS and MS (small thickness of the walls of wheat straw stalks compared to the thickness of HS and MS stalks), while in the case of HS, a significant share (25 to 35%) of hemp bast in hemp stalks containing a small amount of lignin (3.0 to 9.7%) (Kovacs et al. 1992, Zomers et al. 1995).
Figure 2 shows a comparison of the total yield (YT) of WS, HS, and MS NA−A pulps compared to the YT of kraft pulps from these PFRM and B and P woods.
Fig. 2. Comparison of YT of WS, HS, and MS NA−A pulps with YT of WS, HS, MS, B, and P kraft pulps
These data indicate that the YT of NA−A pulps was slightly lower than the index of WS, HS, and MS kraft pulps at the KN 10 to 20 unit range. For the values of KN < 10, the YT index of NA−A pulps decreased more per unit of the KN reduction than for KN 10 to 20 NA−A pulps, indicating a decrease in the delignification selectivity in the KN < 10 range. Again, it can be stated that the obtained levels of YT of NA−A pulps from WS are comparable to the values of this index of NA−A pulp from rye straw presented by Shishonok et al. (1997), which were in the range of 42 to 49%. The higher yield of NA-A pulp from HS can be explained by its higher cellulose content (46.9%) than in the case of WS and MS (41.5% and 40.5%, respectively), resulting from the presence of hemp bast fibers in former PFRM. In turn, the lower yield of NA-A pulps from WS than MS is explained by the well-known higher content of mineral substances and hemicelluloses in WS among PFRMs and the greater susceptibility of considerable part of these chemical components to extraction or hydrolysis/extraction processes compared to cellulose and lignin.
As part of the work, the goal was to determine the level of properties of handsheets prepared from selected WS, HS, and MS NA−A pulps. Moreover, the results determining these properties were compared with the values of the properties of soda-AQ and kraft pulps from these PFRM, presented by other authors, and B kraft and P kraft pulps. The results of these studies are presented in Table 2.
The data in Table 2 indicate that WS, HS, and MS NA−A pulps showed different S-RF after beating in a Jokro mill for 4 and 16 min. This was due to the different contents of primary fine fraction particles in the initial raw materials, which is much higher in WS and HS than in MS, B, and P pulps (Guo et al. 2009; Fišerova et al. 2013; Danielewicz and Surma-Ślusarska 2019). The S−RF of these pulps, however, seems high enough for the possibility of obtaining the high tensile strength of these pulps. For example, Guo et al. (2009) report that the breaking length of the WS soda-AQ pulps in the range of 30 to 50 °SR is 6.0 to 7.2 km, and Maddern and French (1989) report that with the S-RF of 45 to 55 °SR, the tensile index of the WS kraft pulp is 63 to 65 N⋅m/g. Similar values of the breaking strength of HS, soda−AQ or kraft pulps, i.e., at the level of 60 to 80 N⋅m/g, were reported by Fišerova et al. (2013) and Danielewicz and Surma-Ślusarska (2017). In turn, for MS kraft pulps with an S-RF of 20 to 35 °SR, this property was in the range of 55 to 65 N⋅m/g (Danielewicz and Surma-Ślusarska 2019). Therefore, it can be concluded that the tensile strengths of the WS, HS, and MS NA−A pulps obtained in this work are lower than the values of the indices of this strength of the soda-AQ and kraft pulps obtained by other authors (they reach the level of 27.3 to 47.1 N⋅m/g), constituting only 47 to 76 % and 41 to 54% of the breaking strength of B, kraft and S, kraft pulps.
Table 2. Determining Selected Properties of Handsheets of NA−A and Kraft Pulps after Their Beating in a Jokro Mill for 4 min and 16 min
ηCED – determined in unbeaten state; A.a. – as above
For the tear resistance, it was defined for WS, HS, and MS soda-AQ or kraft pulps at the level of 3.3 to 4.2 mN⋅m2/g at an S−RF of 30 to 50 °SR (Maddern and French 1989; Guo et al. 2009), 6 to 15 mN⋅m2/g at an S-RF of 50 to 70 °SR (Fišerova et al. 2013), and 7.0 to 8.2 mN⋅m2/g at an S-RF of 20 to 35 °SR (Danielewicz and Surma-Ślusarska 2019). Therefore, the tear resistance of WS, HS, and MS NA−A pulps was comparable with these values only for WS NA−A pulp, while for HS NA−A, and MS NA−A pulps it was lower. The WS, HS and MS NA−A pulps were also worse compared to the more deeply delignified B kraft pulp (Tables 21). For WS and HS pulps beaten more intensively (e.g., up to S-R−F 70 °SR), this is may not be a disadvantage, because Guo et al. (2009) also found almost twice lower tear index values of WS soda-AQ pulp than eucalyptus kraft pulp at S−RF of 40 to 65 °SR. Fišerova et al. (2013) showed quite low tear resistance of the unbleached Hs soda−AQ pulps (only approximately 6 mN⋅m2/g) with its S−RF of 75 °SR.
The lower values of the breaking strength of WS, HS, and MS NA−A pulps in comparison to WS, HS, and MS soda−AQ or kraft pulps may be because the lower fiber strength of the first set of pulps than the second one, resulting from their lower intrinsic viscosity, which was low, especially for HS NA−A pulp (Table 2). Further reasons for this may be the reduced content of hemicelluloses, the increased content of cellulose in NA−A pulps, and lower degree of polymerization of hemicelluloses. Some authors claim that the NA−A method is a good method for obtaining dissolving pulps, i.e., with a reduced content of pentosans and an increased content of cellulose (Biyani et al. 1967; Kalish 1967). This indicates the need to optimize the NA−A pulping process of WS, HS, and MS. Some indication for this here may be the results of Ivanov et al.’s (1978) work concerning the production of NA−A pulps from wood, indicating that these pulps obtained at a concentration of 2.0 to 2.5 % HNO3 in the N stage but at temperature maximum of 81 °C in this stage are characterized by significantly higher viscosity than pulps obtained at an HNO3 concentration of 6 %. It should also be checked whether it is possible to obtain pulps of low KN at even lower temperatures in the N stage, including room temperature. This would probably involve the need to increase the concentration of HNO3 in the N stage. Such a modification of the process conditions could be beneficial from the point of view of the possibility of obtaining a higher concentration of nitrogen in CPF and the simplification of the pulping process in small, locally operating pulp mills. Another possibility is the use of special substances that limit cellulose depolymerization in the nitration process, as described by Kin (1990) or assumption of the use of NA-A pulps for other purposes (such as dissolving pulps or pulps intended for the production of nanocellulose) that do not require such high strength properties as in the case of kraft pulps.
As already mentioned in the experimental section, in this work the levels of the fertilizing elements were also determined, i.e., nitrogen and potassium in combined post-pulping liquor (CPF), including nitrogen in the form of total nitrogen (NT), nitrate nitrogen (N-NO3), and ammonium nitrogen (N−NH4). These determinations were performed for two pulping experiments of WS using the NA−A method, especially for this purpose. The results of these determinations are presented in Table 3.
Table 3. Nitrogen and Potassium Content in CPF from WS Pulping Experiments in HNO3/KOH and HNO3/NH4OH Systems
The data in Table 3 indicate that for filtrates from the pulping and washing of WS under the conditions of experiments No. 5 and 13 from Table 1, the content of N−NO3 nitrogen, N−NH4 nitrogen, and K was obtained at the levels of 645, 238, and 1644 mg/dm3, respectively, while for pulping in the HNO3/NH4OH system, the contents of these fertilizing elements were at levels of 1470, 1420, and 212 mg/dm3, respectively. The data in Table 3 also indicate that the use of alkaline extraction of the nitrated PFRM using NH4OH instead of KOH increases the level of total nitrogen content in the PFP sample twice, the N-NO3 nitrogen content 2.28 times, and the N−NH4 nitrogen content 5.97 times. In contrast, the content of potassium in CPF from the digestion of WS in the HNO3/KOH pulping system was 7.75 times higher than the content of this element in CPF from the pulping in the HNO3/NH4OH system. The relatively high content of potassium in CPF from pulping using the HNO3/NH4OH system can be explained partially by the leaching of this element from WS in the process of NA−A pulping, whose content in rye straw is as high as 379 mg/dm3, as reported by Kaniszewki et al. (2010), and by using tap water to wash the pulps after N and A stages.
It seems that such filtrates could be used as fertilizer in horticulture, e.g., for soilless cultivation of vegetables in greenhouses on various types of growing media (e.g., Rockwood or Gordan-Master mineral wool). The content of fertilizing elements in filtrates from HNO3/KOH pulping of WS is higher or similar to the content of N−NO3 nitrogen and of K in the nutrient solution used in this type of crop for fertilizing tomatoes, amounting to 200 to 250 mg/dm3 and 280 to 380 mg/dm3, respectively (Kaniszewski et al. 2010) and the content of N-NO3, N-NH4, and K in the root zone of plants cultivated with this method, amounting to 400 to 600 mg/dm3, 0 to 60 mg/dm3, and 300 to 800 mg/dm3 (Dyśko et al. 2012).
If it is intended to use the filtrates from PFRMs pulping using the NA-A method in agriculture and other forms of horticulture than mentioned above, then it would probably be necessary to dewater and granulate the dry substance of the post-pulping filtrates to avoid their storage and enable their sale and more convenient use, what it was practically done by Kin (1990). The author of this article is not a specialist in the field of agricultural fertilizers, because the papermaking industry has not yet taken seriously the possibility of using black liquor from pulping to fertilize crops in horticulture or agriculture. Apart from that, it seems that this type of fertilizer could have an advantage over conventional fertilizers in that they can release nitrogen and potassium into the soil more slowly due to the ability to bind of nitrogen and potassium by lignin; enrich it with organic carbon of prepared organic matter of PFRMs with the action of HNO3, what was suggested by Wrocławski (2014); and delay the drying out of the soil by the presence of hemicelluloses, which are known to have the ability to absorb water.
However, the application of filtrates from PFRM pulping using the NA-A method would require checking the impact of fertilizing cultivated plants with these filtrates on the properties of the plants themselves and their fruits, such as the color of leaves, the color of e.g. tomatoes or potatoes etc., the content of harmful substances in them, and in the absence of them also the taste of vegetable or fruit. It is known that harmful substances (e.g. dioxins) may be formed as a result of chemical changes in lignin under the influence of some bleaching chemicals (e.g., chlorine). In the case of pulping of PFRMs in the HNO3 in the first stage, the possibility of producing a certain amount of HCN during the process should be taken into account. The possibility of formation of this type of chemical compound was mentioned by Gorbunova et al. (1993a). This fact and the possible inability to avoid the formation of HCN could significantly limit the applicability of this method in practice.
The transformation of cellulose into explosive cellulose nitrate during NA-A pulping seems to be the second disadvantage of the technology in question. However, it may be unlikely, because according to Heuser (1946), explosive cellulose would have to have a nitrogen content above 10%, and such celluloses are obtained using nitration solution which contains e.g., 24.29% HNO3, 65.8% H2SO4, and 9.91% water, i.e., conditions that are very different from those used in this work for the processing of PFRMs into fibrous pulps using the NA-A method.
CONCLUSIONS
- In this work, well-pulped WS, HS, and MS NA−A pulps (i.e., of KN 5 to 20 units) were obtained with 10 to 30% of HNO3 dosed on o.d. of these PFRM in the N stage and 10% of KOH or NH4OH dosed on o.d. of these PFRM in E stage, conducting N and the E processed within 2 to 3 h and at 80 to 90 °C.
- The yield of WS, HS, and MS NA−A pulps in the range of KN 10 to 20 showed slightly lower values compared to the yield of WS, HS, and MS kraft pulps. Below KN 10, however, the selectivity of the delignification of these raw materials by the NA−A method seemed to deteriorate.
- The tensile strength and tear resistance of the handsheets made of selected WS, HS, and MS NA−A pulps were usually lower than the values of these indices of WS, HS, and MS soda-AQ or kraft pulps presented by other authors. One of the reasons for this is the lower intrinsic viscosity of the former pulps, especially HS NA−A pulp, but e.g. a lower content of hemicelluloses in NA-A pulps or their lower degree of polymerization may also contribute to this, as is the case with sulfite pulps.
- The worse strength properties of WS, HS, and MS NA−A in comparison with soda−AQ and kraft pulps suggest the need for further research on the reduction of the hydrolytic effect of conditions of HNO3 pulping on carbohydrates.
- The contents of N−NO3, N−NH4, and K in CPF from the processes of WS pulping using the NA−A method were higher or comparable to the content of these fertilizing elements in the nutrient solution used to fertilize the growing media in soilless tomato cultivation in greenhouses. This makes it possible, in principle, to use these solutions for the nutrition of plants cultivated in horticulture and agriculture in the form of a solution or granules. However, this still requires determining the effectiveness of this type of fertilizer, its impact on the horticulture and agriculture plants, and the properties of their output, i.e., grains, tubers, and fruits.
ACKNOWLEDGMENTS
The work was not supported by any institution.
REFERENCES CITED
Biyani, B. P., Gorbatsevich, S. N., and Lorey, F. W. (1967). “Nitric acid pulping of bamboo,” TAPPI J. 50(1), 87A-91A.
Brink, D. L. (1961). “Pulping process studies. Aspects of an integrated nitric acid pulping process,” TAPPI J. 44(4), 256−262.
Brink, D. L., Valmis, J., and Merriman, M. M. (1961). “Pulping process studies. II. The two-stage nitric acid: Ammonium hydroxide process and evaluation of effluents as fertilizers,” TAPPI J. 44(4), 263−270.
Brink, D. L., Merriman, M. M., and Schwedtfeger, E. J. (1962). “Pulping process studies. III. Material balance of nitric acid in two-stage nitric acid: Ammonium hydroxide process,” TAPPI J. 45(4), 315−326.
Danielewicz, D., and Surma-Ślusarska, B. (2017). “Properties and fibre characterization of bleached hemp, birch, and pine pulps: A comparison,” Cellulose 24(2), 5173-5186. DOI: 10.1007/s10570-017-1476-6
Danielewicz, D., and Surma-Ślusarska, B. (2019). “Miscanthus × giganteus stalks as a potential non-wood raw material for the pulp and paper industry. Influence of pulping and beating conditions on the fibre and paper properties,” Ind. Crop. Prod. 141, article ID 111744. DOI: 10.1016/j.indcrop.2019.111744
Dyśko, J., Kaniszewski, S., Kowalczyk, W., Diedziczak, K., Kowalski, B., Moraczewski, A., Podsiedlik, W., and Wojtysiak, J. (2012). “Effect of nitration of organic materials on nitrogen availability and yield of tomato in soilless culture,” Problemy Eksploatacji 2, 37-56. DOI: 10.2478/v10032-010-0007-x
El-Taraboulsi, M. A., Nassar, M. M., and Abd El-Rehim, E. A. (1983b). “Rapid nitric acid pulping of rice straw,” J. Chem. Technol. Biotechnol. Chem. Technol. 33(8), 97-405. DOI: 10.1002/jctb.504330803
El-Taraboulsi, M. A., Nassar, M. M., and Abd El-Rehim, E. A. (1983a). “A modified method of nitric acid pulping of bagasse,” J. Chem. Technol. Biotechnol. Chem. Technol. 33(8), 387-396. DOI: 10.1002/jctb.504330802
Fišerova, M., Gigac, J., and Illa, A. (2013). “Soda-AQ pulping of hemp stalks and pulp properties,” Pap. Cell. 68(3), 10-13.
Gorbunova, O. F., Gerasimova, L. V., and Verbitskaya, S. A. (1993a). “Formation of cyanide ions in model compounds during nitric-acid delignification of wood,” Izv. VUZ. Lesn. Zh. 2(-3), 179-183.
Gorbunova, O. F., Pivovarova, V. A., Chertovskaya, V. P., and Verbitskaya, S. A. (1993b). “Lignin-carbohydrate composition of spent liquors from nitrate acid-ammonia pulping of wood,” Izv. VUZ. Lesn. Zh. 2(3), 137−141.
Gorbunova, O. F., Pivovarova, V. A., Chertovskaya, V.P., and Verbitskaya, S. A. (1993c). “Formation of byproducts during concentration of nitric acid spent liquors,” Izv. VUZ. Lesn. Zh. 2(3), 142-147.
Gorbunova, O. F., Astyutchenko, L. V., and Bogomolov, B. D. (1992). “Polarographic determination of cyanide ions,” Izv. VUZ. Lesn. Zh. No. 6, 80-84.
Guo, S., Zhan, H., Zhang, C., Fu, S., Heijnesson-Hukten, A., Basta, J., and Greschik, T. (2009). “Pulp and fiber characterization of heat straw and eucalyptus pulps – A comparison,” BioResources 4(3), 1006-1016. DOI: 10.15376/biores.4.3.1006-1016
Hájkova, K., Bouček, J., Procházka, P., Kalous, P., and, Budsy, D. (2021). “Nitrate-alkaline pulp from non-woods plants,” Materials 14(13), article 3673. DOI: 10.3390/ma14133673
Heuser, E. (1946). The Chemistry of Cellulose, J. Wiley & Sons, Inc. Press, London.
ISO 5351−11. (1981). “Cellulose in dilute solutions − Determination of limiting viscosity number − Part 1: Method in cupri−ethylenediamine (CED) solution,” International Organization for Standardization, Geneva, Switzerland.
Ivanow, M. A., Sorokin, W. I., Stoljarowa, Ł. I., Bjejriez, E. J., Jewsjejew, P. A., and Zotow, I. M. (1978). “Improving conditions of cellulose pulping using nitric acid pulping method,” Bum. Promyszl. 11, 16-17. (In Russian)
Kalish, J. H. (1967). “Nitric acid pulping – A new rapid-cycle process,” TAPPI J. 50(12), 44-51.
Kaniszewski, S., Dyśkotu, J., Kowalczyk, W., Wojtysiak, J., Wrocławski, Z., and Diedziczak, K. (2010). “Effect of nitration of organic materials on nitrogen availability and yield of tomato in soilless culture,” Vegetable Crops Res. Bull. 72, 71-81. DOI: 10.2478/v10032-010-0007-x
Kin, Z. (1990). Manuscript of book entitled “Chemistry and Technology of Papermaking Pulps,” (Work unpublished) (in Polish).
Kovacs, I., Rab, A., Rusznak, I., and Annus, S. (1992). “Hemp (Cannabis sativa) as a possible raw material for the paper industry,” Cellulose Chemistry and Technology 26, 627-635.
Kurrle, F. L. (1963). “Nitric acid digestion of logging residues of red spruce,” TAPPI J. 46(4), 267-272.
Maddern, K. N., and French, J. (1989). “Papermaking properties of some Australian non-wood fibres,” APPITA J. 42(6), 433-437.
Ngamveng, J., and Ndikontar, M. (1990). “Pulping casava stalks by nitric acid process,” Cell. Chem. Technol. 24, 523−530.
PN−85/P50095.02 (1985). “Fibrous papermaking semi-finished products − Determination of degree of pulping of fibrous cellulose pulp − Determination of Kappa number,” Polish Committee for Standardization, Warsaw, Poland.
PN−EN −20187 (2000). “Paper, board and fibrous pulps – normalized conditions of conditioning and testing of samples,” Polish Committee for Standardization, Warsaw, Poland.
PN−EN 25264−3 (1999). “Fibrous pulps – Laboratory beating – The Jokro mill method,” Polish Committee for Standardization, Warsaw, Poland.
PN−EN ISO 1924−2 (2010). “Paper and paperboard. Determination of properties of paper and paperboard exposed to tensile forces – Part 2: The test at a constant stretching speed (20 mm/min),” Polish Committee for Standardization, Warsaw, Poland.
PN−EN ISO 1974 (2012). “Paper – Determination of tearing resistance – Elmendorf method,” Polish Committee for Standardization, Warsaw, Poland.
PN−EN ISO 5267−1 (2002). “Pulps – Determination of the degree of beating – Part 1: Schopper−Riegler method,” Polish Committee for Standardization, Warsaw, Poland.
PN−EN−ISO 5269−1 (2001). “Pulps – Preparation of laboratory paper handsheets for testing physical properties – Part 1: The conventional method of forming of handsheets,” Polish Committee for Standardization, Warsaw, Poland.
PN-74/P-50092 (1992). “Raw materials for the pulp and paper industry, wood, chemical analysis,” Polish Committee for Standardization, Warsaw, Poland
Shishonok, M. V., Torgashov, V. I., Gert, E. V., Zubets, O. V., and Kaputskii, F. N. (1997). “Investigation of conditions for nitric acid delignification of rye straw,” Cell. Chem. Technol. 31(5), 425-438.
Sorokin, W. I., and Bakina, G. G. (1977). “Selectivity of processes of delignification and pulping of aspen using nitric acid,” Khim. Driew. 4, 37-41. (In Russian)
Wrocławski, Z. Private correspondence.
Yang, D., Gao, T., and Mao, Z. (2019). “Study on the nitric acid pulping, delignification course, and waste-liquid recovery,” BioResources 14(4), 8892-8903. DOI: 10.15376/biores.14.4.8892-8903
Zomers, F. H. A., Gosselink, R. J. A., Van Dam, J. E. G., and Tjeerdsma, B. F. (1995). “Organosolv pulping and test paper characterization of fiber hemp,” TAPPI J. 78, 149-155.
Article submitted: August 24, 2023; Peer review completed: Sept. 16, 2023; Revised version received and accepted: Sept. 21, 2023: Published: September 27, 2023.
DOI: 10.15376/biores.18.4.7629-7644