Oxygen Delignification Chemistry and Its Impact on Pulp Fibers

Abstract

Two southern pine kraft pulps with kappa numbers of 30.0 (SW1-0) and 48.0 (SW2-0) were oxygen delignified by 30–60% by varying the reaction temperature (78–110°C) and charge of sodium hydroxide (1.6–4.4%). O-bleachability was found to be correlated to the incoming kappa number and charge of sodium hydroxide employed. In general, a lower charge of caustic and a higher brownstock kappa number improved pulp bleachability. The residual lignin in the brownstocks and O-delignified kraft pulp samples was isolated and characterized by ¹³C and ³¹P NMR. ¹³C NMR analysis of the residual lignin samples indicated that the post-oxygen delignified pulps were enriched with α-carbonyl groups and carboxylic acid groups. The content of β-O-aryl structures was increased by 23–36% depending on the extent of oxygen delignification. The post-oxygen delignified pulps were also shown to have increased substituted aryl carbons. ³¹P NMR indicated that the relative content of condensed phenolic units increased by 9–20% after the oxygen delignification, depending on the severity of the O-stage. This observation was probably due to the accumulation or formation of 5,5-biphenyl structures in the process. The physical strength properties of brownstock and post-oxygen delignified pulps were assessed in terms of zero-span strength, tensile strength, tear strength, and burst strength. Oxygen delignification led to a slight increase in the curls and kinks of the pulp fibers. The O-stage was shown to cause a 4.8–15.6% decrease in zero-span strength. In contrast, oxygen delignification increased tensile strength. This result could be explained as the improvement of fiber bonding after the oxygen bleaching.     

The Influence of Sulfidity in Kraft Pulping on the Nature of Residual Lignin

The influence of sulfidity in kraft pulping of Norway spruce chips on the characteristics of residual lignin has been examined at two levels 25 and 40%. Comparing with the reference kraft cook at similar kappa number, the high sulfidity pulp lignin displayed a significantly lower phenolic hydroxyl group content, and a lower response to O₂ delignification, but it gave a slightly higher yield of nitrobenzene oxidation products, and was more responsive to a neutral sulfite treatment.     

Mechanistic Differences Between Kraft and Soda/AQ Pulping. Part 1: Results from Wood Chips and Pulps

Abstract

Soda pulping catalyzed by anthraquinone (AQ) or 2-methylanthraquinone (MAQ) can produce hardwood chemical pulps similar to kraft pulps in all respect but for bleachability. Results accumulated in our laboratory suggest that the residual lignin in pulps from anthraquinone catalyzed processes is less reactive toward bleaching chemicals than that in kraft pulps. Analyses of pulps by periodate and permanganate oxidations suggest that the residual lignin from the non-sulfur processes contained more condensed structures than kraft residual lignin. The low reactivity of these structures is believed to be responsible for the lower brightness of bleached soda-AQ (SAQ) pulps. Pulping and bleaching trials with hardwood chips demonstrated that shortening of the cooking time and/or increasing the alkalinity is one strategy for improving bleachability of SAQ pulps. When sugar maple (Acer saccharum) chips were SAQ cooked for 1.0 and 2.0 h at 165°C, the higher kappa number pulp produced after 1.0 h of cooking bleached to a significantly higher brightness with a small increase in the chlorine dioxide application.     

Effect of Soda-Additive Pulping on Molecular Characteristics of Residual Pulp Lignin

The soda-additive pulps of pine wood were compared to corresponding soda and kraft pulps in respect to the macromolecular properties of the residual pulp lignins. Results of measurements of number and weight—average molecular weights as well as branching parameters show that the presence of additives such as anthraquinone, methanol and O₂—pretreatment significantly causes the breaking of cross-links in the pulp lignin. On the other hand, ethylenediamine and hydroeulfide ion contribute mainly to extensive lignin degradation. The influence of used additives on the condensation reactions of the residual lignin has also been studied.     

New Insights into Lignin Modification during Chlorine Dioxide Bleaching Sequences (III): The Impact of Modifications in the (EO) versus E Stage on the D1 Stage

Abstract

Chlorine dioxide delignification (D₀) modifies kraft residual lignin by oxidizing phenolic groups to both quinone and muconic acid structures. Alkaline extraction (E), in addition to removing solubilized lignin, converts quinone moieties to polyphenols. These polyphenols are easily oxidized by oxygen in an (EO) stage or by ClO₂ in a D₁ stage to hydroxyquinones (～1.8 mmol/g lignin). Pulps treated by D₀E consume considerably more ClO₂ in the D₁ than D₀(EO), and have lower bleachability, as was quantified by a simple bleaching model. Both D₀E and D₀(EO) pulps approach a common brightness ceiling (～83 ISO) when excess ClO₂ is applied. Examination of the post‐D₁ b* values indicates that D₀E and D₀(EO) also have similar asymptotic b* values (～6), indicating that both pulps have similar residual chromophores. Hydroxyquinone structures appear to be eliminated in the D₁ stage for D₀(EO) pulps, and at high ClO₂ levels for D₀E pulps.     

SO2-Ethanol-Water (SEW) Pulping: I. Lignin Determination in Pulps and Liquors

Abstract

Quantitative determination of lignin in SO₂-ethanol-water (SEW) pulps and spent liquors is described. The methods developed for conventional sulfite pulping are successfully applied to the SEW process. Linear correlations between Klason/total lignin content and kappa number are found over a wide pulp yield range for spruce, beech, and wheat straw. Lignin content of the spruce spent SEW liquors is determined using either hydrogen peroxide to remove SO₂ and dilution by 3% sulfuric acid or simply by dilution with 0.1M sodium hydroxide. The recommended wavelength is 280 nm. The experimentally found values for the extinction coefficient of dissolved lignin in 3% sulfuric acid and in 0.1M NaOH are 19 and 23 L/(g·cm), respectively. The interference of furanic compounds is eliminated by reduction with sodium borohydride.     

Preliminary Results on an Approach for the Quantification of Lignin-Carbohydrate Complexes (LCC) in Hardwood Pulps

Abstract

The literature on biomass research contains many references to lignin-carbohydrate complexes (LCC) decreasing the rate of delignification in chemical pulp production, decreasing the yield of cellulosic ethanol via fermentation, and decreasing forage digestibility. However, it is difficult to find correlations between rates of the processes above and initial LCC concentration. One of the main reasons for the lack of such correlations is the absence of methods for accurate quantification of LCC. In this investigation, repeatable and reproducible determinations of bound sugars at monomeric concentrations as low as 0.3 wt% on enzymatic lignin (EL) have been achieved. The bound sugars are hydrolyzed by H₂SO₄, most likely as low molecular weight oligomers. In the same H₂SO₄ treatment, the oligomers are hydrolyzed to monomers which are subsequently quantified by ¹H NMR analyses. A significant enrichment of bound arabinan was previously reported when a crude milled wood lignin (MWL) was compared to the starting wood meal. A similar arabinan enrichment was observed for ELs from kraft and soda-AQ (SAQ) pulps in the present study. Also, well-resolved cross-peaks have been obtained in 2D HSQC NMR analyses of ELs. It has so far been confirmed that the EL from a 30.6 kappa number SAQ pulp from sugar maple contained ～30% more benzyl ethers linked to primary-OH groups in sugar units than the corresponding EL from a 33.7 kappa number kraft pulp.     

Optimized Catalytic Bleaching Sequences for Eucalyptus Kraft Pulp

Contemporary multi-stage bleaching processes partially remove residual lignin and hexenuronic acid from cellulosic pulps. The reactions in the steps could be faster and consume smaller amounts of chemicals. Catalytic bleaching (H_cat), utilizing hypochlorite (H), triethylenediamine (DABCO) and its derivative N-carboxymethyl triethylenediamine (CM-DABCO), is a new discovery that has the potential to improve the chemical and energetic efficiency of bleaching processes in chemical pulp mills, e.g. through reducing the reaction time of the bleaching processes. The objective of this study was to clarify if new kraft pulp bleaching sequences with initial stage of chlorine dioxide (ClO₂; D) and an intermediate stage of H_cat could provide fully bleached pulps. The bleaching sequences of the studied eucalyptus pulps include D₀E_(OP)H_cat(Q)P and H_catZ/DP, which attained a final brightness of 88 and 89% ISO, respectively. H_catZ/DP showed to be the best sequence for the catalytic bleaching of eucalyptus kraft pulps. This study may open new doors to future bleaching of cellulose pulps with fewer towers and decreased use of chemicals.     

Quantification of Lignin and Hexenuronic Acid in Bleached Hardwood Kraft Pulps: A New Calibration Method for UVRR Spectroscopy and Evaluation of the Conventional Methods

Abstract

Lignin content determination is an important task when pulp bleaching is studied. However, none of the conventional methods were developed for bleached pulps and therefore they are accurate mainly for unbleached pulps. This article describes a new, rapid method to determine lignin and hexenuronic acid contents of bleached hardwood kraft pulps based on UV resonance Raman (UVRR) spectroscopy. The lignin contents of pulp samples were determined from the aromatic band heights of the UVRR spectra. Therefore the measurements gave the content of aromatic lignin in pulp, and did not include extensively oxidized lignin structures. The method was applicable for hardwood kraft pulps with lignin content less than 1%. The measured lignin content correlated linearly with the kappa number. The lignin content (% on pulp) equaled 0.15κ+0.16. The constant 0.16 was presumably caused by the incomplete oxidation of the lignin in the kappa number determination. Klason lignin or total lignin determinations were not accurate for these kinds of pulps. Hexenuronic acid content was simultaneously determined from the UVRR band height of unsaturated C?O and C?C structures. The linear correlation of this band with hexenuronic acid indicated that the content of other unsaturated structures was constant in all the pulps, was proportional to the hexenuronic acid content of the pulps, or was insignificantly low. When compared to conventional methods, the UVRR spectroscopic method is fast, requires little sample and pretreatment, and the procedure has good repeatability. In addition, the accuracy of this technique increases with decreasing lignin content (<1%) making it a very attractive method for bleaching studies.     

Structure and Reactivity of Spruce Mechanical Pulp Lignins Part II. Organosolv Fractionation of Lignins in a Flow-Through Reactor

Various spruce mechanical pulps were subjected to delignification with ethanol-water (1 : 1, v / v ) containing 0.1 M acetic acid at 175°C in a flow- through reactor. A thermomechanical pulp (TMP) and the corresponding samples derived from its bleaching (BTMP) and yellowing ( YBTMP ) treatments were delignified to a similar extent, about 70% of delignification degree, as compared with spruce wood. A series of five successive lignin fractions was recovered from each pulp sample and then characterized by various analytical methods. Large structural variations were observed within these fractions. The number of phenolic structures appeared to be an important factor influencing the dissolution of lignins in the ethanol-water medium. These lignin fractions were found to be different in the amount of β-aryl ether structures and in the relative importance of main carbon-carbon bonding patterns. The results are discussed in relation to the lignin fractionation in the flow-through reactor.     

Investigation of the Chemistry of Oxygen Delignification of Low Kappa Softwood Kraft Pulp using an Organic/Inorganic Chemical Selectivity System

Abstract

Oxygen delignification (OD) of low kappa softwood kraft pulp was examined in two steps without inter‐stage washing as part of an overall program to evaluate the efficiency of a selectivity enhancement system consisting of phenol and magnesium sulfate. Black liquor carryover in the reaction system did not substantially affect delignification and the selectivity of these OD reactions. The residual lignins from both the original pulp and oxygen‐delignified pulp with and without the phenol/MgSO₄ selectivity enhancement system were prepared and characterized using NMR spectroscopy. The effluent lignins after oxygen delignification were also prepared and characterized. The lignin characterizations provided the basis for the rationalization of the selectivity observed. A significant finding of this study was that the phenol/MgSO₄ system in the oxygen delignification reaction appeared to hinder phenolic guaiacyl unit condensation. It also appeared to enrich the levels of p‐hydroxyphenyls in the residual lignin.     

Modified Kraft Pulping of Bagasse: Infrared Spectroscopy of Lignin

Abstract

The infrared spectroscopy of precipitated lignin from waste black liquors of bagasse pulping with kraft sulfite pulping process was investigated. Also the effect of anthraquinon and methanol addition in the soda, kraft and kraft-sulfite pulping liquor on the infrared specra of the precipitated lignin was studied. The presence of methanol in the pulping liquor causes an increase in the degradation as well as increase in the carboxylic group in the precipitated lignin. Also, the phenolic hydroxyl group in case of kraft lignin is higher than soda lignin. Presence of sulfite in the kraft-sulfite pulping liquor produces lignin hydroxyl groups.     

Chemical Structure of Residual Lignin from Kraft Pulp

Structural characteristics of wood lignin and residual lignin in conventional and modified kraft pulps were examined employing elemental and methoxyl analysis and ¹³C, ¹H, and ³¹P-NMR. The structural analysis revealed that kraft residual lignin differs significantly from wood lignin while differences between residual lignins in conventional and EMCC® pulps have similar structural characteristics at kappa numbers corresponding to the point where the selectivity of the cook becomes poor. NMR analysis suggested that residual lignin, compared to the wood lignin, is much less reactive towards pulping chemicals due to the low content of aryl ether linkages and the prevalence of condensed type structures.     

Mechanistic Differences Between Kraft and Soda-AQ Pulping of Hardwoods with Regard to Lignin–Carbohydrate Complexes (LCC)

The literature on biomass research contains many references to lignin–carbohydrate complexes (LCC) decreasing the rate of delignification in chemical pulping, decreasing the yield of cellulosic ethanol via fermentation, and decreasing forage digestibility. Regarding wood delignification, there are a few reports on the formation and/or cleavage of lignin–carbohydrate (L–C) bonds during alkaline pulping. The behavior of LCC was investigated to find a potential explanation for the differences between the soda-anthraquinone (soda-AQ or SAQ) and kraft processes with regard to delignification rate in the residual phase of pulping and in the bleaching process. Enzymatically isolated lignin (EL) was prepared from two soda, nine SAQ, and twelve kraft pulps from sugar maple, a hardwood. The range of kappa numbers, after correction for hexenuronic acid (HexA), was 10–60. The bound sugars on each EL were hydrolyzed and converted to monomers by H₂SO₄ at 121°C. There was evidence in the data suggesting that the bound glucan and xylan on the ELs from soda, SAQ, and kraft pulps were native to the wood. The bound galactan data were somewhat ambiguous, and there was no detection of bound mannan on any EL. The reproducibility and repeatability of bound arabinan attached to ELs (BA) were excellent. Although not conclusive, the totality of the data is suggestive of both L–C bond formation and cleavage involving arabinose units during both kraft and SAQ pulping. There was no decrease in BA when SAQ was used to lower the c-kappa number (HexA-corrected) from ～60 to ～25. The case was similar when kraft was used in the range of ～60 to ～40. However, there were significant decreases in BA content when c-kappa number was lowered below ～25 by both SAQ and kraft. A common mechanism was proposed to explain essentially no decrease in BA content at higher kappa numbers, but distinctly different mechanisms were proposed to explain BA cleavage at c-kappa number <25. A mechanism favorable to subsequent bleaching was proposed for kraft, but an unfavorable mechanism was proposed for SAQ.     

Characterization of Residual Lignins Isolated from Unbleached and Semibleached Softwood Kraft Pulps

Abstract

Residual lignins in an unbleached and a semibleached softwood (Pinus taeda L.) kraft pulp were isolated by enzymatic hydrolysis of polysaccharides in the pulps. After purification, the residual lignins were characterized. A dissolved lignin was also isolated from the alkaline extraction spent liquor by acidification and characterized.

Results of the characterization indicate that extensive degradation of residual lignin in kraft pulp occurred during the first two stages of bleaching. The results also strongly support the previous finding that stable covalent linkages between residual lignins and carbohydrates in pulp any be the most probable cause for the residual lignins to resist delignification during kraft pulping and prebleaching.     

The Characteristics and Reactivity of Acid-Labile Aryl Ether Units in Wood Lignin

Wood lignin contains significant amounts of acid-labile aryl ether units, which play a significant role in lignin modification or delignification processes. We have evaluated the rate and reaction kinetics on the acid-catalyzed cleavages of aryl ether structures for wood lignin in situ based on the formation of phenolic hydroxyl groups. The content of acid-labile aryl ether units was quite uniform for a variety of softwood wood lignins (～4% per C₉ unit) and it varied appreciably among hardwood species, ranging from 4% for aspen to 9% for beech wood lignin. These variations, however, appear to be related to the content of syringyl units in wood lignin. The reactivity of these reactive aryl ether structures was noticeably higher for the spruce than for the aspen wood lignin. This difference in reactivity, based on the behavior of lignin model compound reactions, can be attributed to the influence of syringyl moieties in aspen wood lignin. It appears that most of the acid-labile aryl ether units in hardwood were associated with the syringy moiety being present as a benzyl unit, which is much less reactive than the corresponding guaiacyl moiety.     

The xanthate method of grafting. III. Effect of lignin content on the graftability of wood pulp

Several commercial wood pulps of different chemical origin and with various lignin content were copolymerized with acrylonitrile using the xanthate grafting process. A number of experiments were carried out to evaluate the effects exerted by the residual lignin and by other wood components on the grafting reaction. The results obtained show that graft copolymers can be prepared in good yields with pulps containing as much as 23% lignin. With the aim to investigate the effect of lignin in more detail, two series of pulps were prepared by delignification of a crude sulfite pulp and a crude Kraft pulp to different levels of lignin content. Sodium chlorite was used as a bleaching agent. Copolymerization results obtained with these pulps indicate some fundamental differences in behavior between sulfite and Kraft pulps. In both cases, the copolymerization is afflicted by a short inhibition period whose duration, however, does not depend on the lignin content in the pulp.     

The Potential of 13C Cp/Mas Nmr in the Study of Kraft Pulping Kinetics

Abstract

Solid state ¹³C CP/MAS nuclear magnetic resonance spectra were obtained for unbleached kraft pulps prepared from white spruce with yields ranging from 47.6 to 96.1%. The observed trends of the relative peak intensities were found to correspond to established facts on kraft kinetics. These results suggest that ¹³C CP/MAS NMR of wood and pulp could be used to study kinetics in situ.     

Soda Pulping of Hardwoods Catalyzed by Anthraquinone and Methyl Substituted Anthraquinones

Abstract

Black liquor gasification (BLG) as well as the recovery of lignin and other organic compounds from pulping black liquor would be aided if an efficient sulfur‐free pulping process could be developed. This has provided new impetus for research on soda pulping with redox catalysts instead of sodium sulfide that is presently used in the kraft process. Soda/anthraquinone (AQ) pulping afforded white birch (Betula papyrifera) and sugar maple (Acer saccharum) pulps with equal if not superior strength to kraft pulps. However, the delignification rate was significantly lower for soda/AQ pulping. When AQ was replaced by 2‐methylanthraquinone (2‐MAQ) a delignification rate only slightly lower than that of kraft pulping was obtained at the same effective alkali (EA). At a kappa number of ～20, a soda/2‐MAQ pulp was produced from sugar maple at a higher yield (1.2% on chips) than for a kraft pulp. 2‐MAQ was synthesized, as a powder, at 75% yield using an AlCl₃–mediated Friedel‐Crafts reaction that is one of the methods used for commercial production of AQ.     

Sulfonation of Phenolated Kraft Lignin to Produce Water Soluble Products

Kraft lignin is water insoluble and has limited end-use applications. To produce water soluble lignin-based products, the modification of softwood kraft lignin through phenolation followed by sulfonation of sulfuric acid or sodium sulfite treatment were investigated in this work. Fourier transform infrared (FTIR) spectrophotometer, nuclear magnetic resonance (NMR), and thermo-gravimetric analysis were also considered for characterizing the lignin-based products. The results showed that phenolation pretreatment was effective to generate sulfonated lignin (SAP-lignin) by sulfuric acid treatment with a high charge density (3.12 meq/g) and solubility, which is due to the addition of sulfonation sites on the phenolic ring. However, sodium sulfite treatment of phenolated lignin generated sulfonated lignin (SSP-lignin) with the charge density of 1.20 meq/g due to hindered sulfonation by occupation of reactive α-position. SAP-lignin was soluble across the tested pH range of 1–13, but SSP became insoluble at a pH lower than 3. Thermogravimetric analysis revealed that phenolation pretreatment reduced the thermal resistance of modified lignin when compared to kraft lignin, while SAP-lignin exhibited the highest thermal resistance due to condensation under sulfuric acid treatment. SAP- and SSP-lignin were successfully used as a coagulant for dye removal from simulated solutions as they could remove 72.1 and 90.4% of ethyl violet from a simulated dye solution, respectively.