Oxygen Delignification Stage Research Articles

Effect of residual alkali level in softwood kraft cooking

The hypothesis was that low residual alkali after cooking would cause lignin re-precipitation during washing and in turn affect the subsequent oxygen delignification stage negatively. To test the hypothesis, kraft cooks were performed in lab-scale to different residual alkali levels, ranging from 5 to 15 g/L and the pulps were subjected to washing with either water or 0.1 M NaOH and then oxygen delignified. The results show that even at low residual alkali and washing with water, the pH in the liquor after washing was above 11 which is sufficiently high to keep lignin in solution. No effect of residual alkali level was observed on the performance of the oxygen delignification stage.

Bleaching of bagasse-pulp using short TCF and ECF sequence

Abstract More than 70 % bleached chemical pulp is produced in India through elemental chlorine-free bleaching in which chlorine-based compounds like chlorine dioxide is a dominant chemical which generates chlorinated organic toxins harmful to the environment. Present studies demonstrate short sequence of bleaching combined with acid treatment, followed by pressurized oxygen delignification. It was found that efficiency of oxygen improved by adding hydrogen peroxide as an additive in oxygen delignification with subsequent treatment with ozone or chlorine dioxide as bleaching agents. It was observed that by using additive in ODL process, pulp can achieve 70±1 (%ISO) brightness. Reduction attains in kappa number 65–70 % as compared to 45–50 % in control oxygen delignification stage. Through AOpZ and AOpD bleaching sequences, full brightness achieved 84–85 (%ISO) without considerable loss in mechanical strength properties compared to DEpD sequence. A potential reduction in COD, color, and AOX was 28, 53.3, and 88 % respectively were observed in AOpZ short bleaching sequence compared to DEpD bleaching.

Effect of different elemental chlorine-free bleaching sequences on pulp, effluent properties and their impact on index of global pollution.

Six elemental chlorine-free bleaching sequences were studied to see their effect on quality of pulp produced and quantity of pollutants generated during bleaching of wheat straw soda pulp. Impact of parameters like adsorbable organic halogens (AOX), chemical oxygen demand (COD), biochemical oxygen demand (BOD) and sodium adsorption ratio, pH and total dissolved solids generated during bleaching on environment as index of global pollution (IGP) was also studied. The purpose of the study was to investigate six elemental chlorine-free bleaching sequences to compare the pulp quality, effluent properties and their impact on environment. An oxygen delignification stage before D0EOPD bleaching sequence reduced the AOX, COD, colour and BOD by 44%, 42%, 36% and 33%, respectively, whereas an ozonation stage before bleaching reduced the same by 70%, 66%, 73% and 60%, respectively, as compared to those of control. The generation of pollutants as AOX, COD, colour and BOD was reduced further by 88%, 80%, 90% and 77%, respectively, with the use of short bleaching sequences (DD, DP and PD) as compared to those of control. The environmental impact as IGP with D0EOPD bleaching sequence was 2.65 which got reduced to 1.44 with bleaching sequence OZPD. Bleaching sequence OZD0EOPD was found the most effective for improving brightness and whiteness of the pulp, whereas short bleaching sequence OZPD found the most effective for reducing generation of pollutants during bleaching among different bleaching sequences studied. Graphical abstract.

Oxygen delignification: laboratory evaluation of the impact of dissolved organic matter, sodium carbonate and sodium thiosulfate

Abstract Dissolved matter (DM) in the oxygen delignification (O2Del) stage affects the lignin and carbohydrate degradation. To understand this topic better, laboratory O2Del experiments were conducted with controlled addition of either unoxidized dissolved matter (UnoxDM) originating from the cooking stage or oxidized dissolved matter (OxDM) recycled from the post-O2Del washing. The presence of UnoxDM decreased the delignification, while the presence of OxDM showed the opposite effect, both compared to a well-washed pulp and at the same alkali charge level. However, both types of DM had a negative impact on the carbohydrate degradation. The distribution between these DMs will affect the resulting lignin degradation, because the filtrate accompanying the fibers into the O2Del stage is a mixture of UnoxDM and OxDM. It is proposed that the positive impact on the delignification by OxDM is due to the high carbonate ion concentration in the filtrate. Further, the high content of thiosulfate ions in the UnoxDM was one of the reasons for the lower delignification in its presence.

Approaches for converting sugarcane trash, a promising agro residue, into pulp and paper using soda pulping and elemental chlorine-free bleaching

Industrialization and urbanization are the key drivers to threaten the environmental quality and natural resources. Traditionally, pulp and paper industry has been highly reliant on forest based resources for its escalation. Shortage of forest based raw materials, strict environmental regulations and policies have compelled the industry to seek for alternate raw materials for its production. Mills are now focusing on exploring the potential of different agro wastes such as cereal straws and bagasse etc. In present research, sugarcane trash, one of the most abundant agro residues in India, was investigated for its capability in pulping and bleaching. Chemical characterization of sugarcane trash showed that it contains 40.4% cellulose, 33.2% hemicelluloses, 17.4% lignin and 6.45% ash content with 2.76% silica. To cook sugarcane trash with soda pulping method, 14% alkali charge, 15 min time and 162 °C temperature was found to be optimum and pulp of kappa no. 19.8 was prepared that exhibited good strength properties. Bleaching of pulp was carried out using elemental chlorine (Cl2), hypochlorite and chlorine dioxide (ClO2) based sequences. Replacement of Cl2 with ClO2 has significant impact on strength properties and effluent characteristics. Oxygen delignification stage was incorporated prior to ClO2 that resulted in improved optical properties of pulp with noticeable reduction in wastewater load. This paper elaborates the need of using sugarcane trash in paper mills with environment friendly pulping and bleaching processes to combat with the problem of waste management, wastewater load reduction during bleaching and substitute for wood based raw materials in papermaking.

Effects of lignin chemistry on oxygen delignification performance

The present work focused on characterizing the chemical and structural properties of isolated lignin from six hardwoods and their kraft pulps in an attempt to better understand the relationship between lignin’s chemical properties and resultant oxygen delignification performance. Several hardwood samples were cooked under the same conditions with varying alkali charges to obtain unbleached pulps with kappa numbers between 19 and 20. These pulps were then subjected to an oxygen delignification stage. Both processes were evaluated for pulp quality, residual lignin, and O-stage delignification efficiency. The oxygen delignification stage was carried out under fixed conditions and evaluated with regards to kappa number, which was corrected for hexenuronic acid (HexA) contributions.Results revealed that different hardwood species exhibited differing oxygen delignification efficiencies. A high correlation was found between the O-stage delignification efficiency and the content of phenolic groups in the unbleached lignin, which confirmed that free phenolic groups are the reactive site for molecular oxygen attack. When different hardwood species were compared, the HexA contents were not found to affect O-stage delignification efficiencies.

Using online bubble size and total dissolved solids measurements to investigate the performance of oxygen delignification

The main target of oxygen delignification is to continue delignification that started in cooking in a more selective manner than occurs in the digester (i.e., remove a substantial fraction of the residual lignin using oxygen and alkali at a moderate temperature). Delignification with oxygen is a gentler way of reducing the kappa number than extended cooking. Lowering inlet kappa to bleaching also decreases bleaching chemicals consumption and, because of this, reduces organic wastewater load from bleaching. We studied the performance of oxygen delignification by installing bubble size imaging systems and refractometers, which measure dissolved dry solids in the oxygen stage feed. Based on these measurements, we gathered information about gas dispersion (bubble size distribution) and the behavior of dissolved matter in the hardwood mill’s oxygen delignification stage. Our goal was to investigate the effects of different variables on the oxygen stage’s gas dispersion, kappa reduction, yield, and pulp quality. Gas dispersion improved (i.e., average bubble size decreased) when the chemical mixer speed increased. Increasing the mixer speed and the amount of oxygen yielded higher kappa number reductions and increased the amount of dissolved organic matter.

Improvement in rice straw pulp bleaching effluent quality by incorporating oxygen delignification stage prior to elemental chlorine-free bleaching.

Environmental degradation by industrial and other developmental activities is alarming for imperative environmental management by process advancements of production. Pulp and paper mills are now focusing on using nonwood-based raw materials to protect forest resources. In present study, rice straw was utilized for pulp production as it is easily and abundantly available as well as rich in carbohydrates (cellulose and hemicelluloses). Soda-anthraquinone method was used for pulp production as it is widely accepted for agro residues. Bleaching process during paper production is the chief source of wastewater generation. The chlorophenolic compounds generated during bleaching are highly toxic, mutagenic, and bioaccumulative in nature. The objectives of study were to use oxygen delignification (ODL) stage prior to elemental chlorine-free (ECF) bleaching to reduce wastewater load and to study its impact on bleached pulp characteristics. ODL stage prior to ECF bleaching improved the optical properties of pulp in comparison to only ECF bleaching. When ODL stage was incorporated prior to bleaching, the tensile index and folding endurance of the pulp were found to be 56.6±1.5Nm/g and 140, respectively, very high in comparison to ECF alone. A potential reduction of 51, 57, 43, and 53% in BOD3, COD, color, and AOX, respectively was observed on adding the ODL stage compared to ECF only. Generation of chlorophenolic compounds was reduced significantly. Incorporation of ODL stage prior to bleaching was found to be highly promising for reducing the toxicity of bleaching effluents and may lead to better management of nearby water resources. Graphical abstract ᅟ.

Impact of dissolved matter in the oxygen delignification stage

The presence of dissolved matter in the pulp suspension in an oxygen (O2) delignification stage, consisting mainly of dissolved lignin, is normally considered to negatively affect the delignification rate due to the competing reactions between the fiber bound lignin and the lignin dissolved in the filtrate. Recirculated oxidized filtrate from the post-O2 washing is usually considered to be less harmful to the delignification efficiency than unoxidized dissolved matter originating from the cooking stage. To develop a better understanding of the mechanisms of the dissolved matter’s reactions and impact on the O2 stage performance, laboratory oxygen delignification experiments with varying levels of unoxidized and oxidized dissolved matter were conducted. The results showed that unoxidized dissolved matter had a negative impact on the delignification in the O2 stage, whereas oxidized dissolved matter actually had a positive effect. The delignification efficiency of the laboratory experiments thus depends on both the amount of dissolved matter and its origin. The pulp viscosity decreased with increasing dissolved matter content irrespective of its origin but at higher COD levels; however, the viscosity drop was larger for the unoxidized dissolved matter. In terms of selectivity, the oxidized filtrate had a similar impact as additional NaOH charge. Both types of filtrates consumed hydroxide and the final pH decreased with increasing dissolved matter content. The final pH was significantly lower in the unoxidized filtrate experiments at higher COD levels, indicating a high reactivity between the unoxidized dissolved matter and the oxygen in the reactor. Based on the results, further understanding is achieved about the relation between pre-O2 washing performance and process configuration in an actual mill case, as well as the impact of dissolved matter on delignification. The importance of efficient removal of harmful unoxidized dissolved matter is verified, but the results also suggest that the oxidized dissolved matter that is recirculated from post-O2 washing actually has a significant positive impact on the delignification and is not just a potential problem in case of carryover to the bleach plant. Subsequently, pulp washing efficiency is critical both pre- and post-O2.

Using white liquor as the alkali source in oxygen delignification

This paper considers the potential impact on oxygen delignification stage performance when unoxidized white liquor and partially oxidized white liquor containing thiosulfate, or (OWL[T]), are used as the alkali source. A literature review was conducted to estimate the oxygen consumption of the pulp, the raw black liquor carryover, and the sulfur compounds in the charged white liquor and in the postoxygen washer filtrate recirculated to the oxygen stage. These values were used to calculate the total oxygen consumption (kg O2/bone dry ton [BDT]) as a function of oxygen stage Δ kappa. The maximum oxygen charge was estimated using the generally accepted void volume (Xg) limit of 20%. A delignification model was developed using an empirical equation for the rate of change of lignin in the pulp as a function of operating parameters. The initial (OH-) concentration was adjusted to achieve 40% delignification at 0.70 MPa (100 psig) and 50% delignification at a pressure of 1.03 MPa (150 psig) for a 30 kappa pulp. The model predicted oxygen exhaustion and hence a reduction in achievable Δ kappa at operating pressures of less than 150 psig when OWL(T) was used as the alkali sources and the raw black liquor carryover was 30 kg/BDT

Impact of dissolved lignin in oxygen delignification and chlorine dioxide stages

While carryover of dissolved lignin between stages in the pulp mill fiber line is a well-known problem, it is still typically seen only as a minor disturbance factor or bias in the control of oxygen (O2) delignification and bleaching stages. The present study, however, reveals that it plays a larger role than anticipated, and that it should be properly analyzed in order to correctly control the process stages. This is especially important for the O2 and D0 stages as the lignin content is still high in these positions. The results of the study show that dissolved lignin carried over between stages may have a significant impact on the bleaching chemical consumption and, indirectly, on the pulp quality. Mill investigations have shown very large variations in the dissolved lignin content in the pulp before the oxygen delignification stage and before the D0 stage that have significantly influenced the bleaching chemical demand and, subsequently, the degree of delignification. In order to develop a better understanding of the mechanisms of the dissolved lignin’s reactions, laboratory O2 and D0 experiments with controlled levels of dissolved lignin were conducted. It was anticipated that a better feedforward control could be achieved using an online dissolved lignin measurement, and results from mill trials are presented. Chlorine dioxide laboratory experiments using different levels of carryover (i.e., different dissolved lignin contents) were conducted. It was concluded that the filtrate kappa number provides a relevant measure of the bleach demand due to the dissolved lignin and that, subsequently, the combined fiber and filtrate kappa number provides an appropriate measure for optimum feedforward control of the stages. Mill results support these findings, which show that the chemical consumption is reduced significantly using the total kappa number. The post-D or post-DE kappa number feedback control can most probably be eliminated by using this technology.

Oxygen Delignification of Acid Sulfite and Bisulfite Softwood Pulps

This study discusses alternatives for the production of sulfite pulps cooked to high kappa numbers, and a subsequent oxygen treatment by using the same cation in the cooking and in the oxygen stages. Magnesium was used as a cation during both the cooking and oxygen delignification stages. By using the same cation in the cook and in oxygen delignification, it is theoretically possible to recover filtrates and send them to the chemical recovery system, meaning that the discharge of pollutants will be very low. Acid sulfite and bisulfite pulps at yields between 56 and 57% (kappa number between 50 and 55) were produced and then delignified with oxygen and magnesium hydroxide to a kappa number between 25 to 30. It was found that the delignification process time was shorter and the yield and viscosity increased, compared with delignification to the same kappa number (25 to 30) for a single cooking stage. The tear and tensile strengths were however unchanged, regardless of delignification process used.

The influence of magnesium oxide charge on kappa number after oxygen delignification of softwood acid sulfite pulp

A laboratory study has been carried out on the utilization of magnesium oxide (MgO) as alkali in an oxygen delignification stage after acid sulfite pulping of softwood. The first objective was to reconfirm results from previous researchers to better understand the process mechanism concerning optimum MgO charge with respect to kappa number reduction. The second objective was to determine if for example the ionic strength could be an important variable in such a stage. It was found that the MgO charge should not be too high as this resulted in reduced degree of delignification which was unexpected. The results also suggested that the ionic strength in the liquor could be an important parameter in the oxygen delignification stage and that it should not be too high. Thus, ionic strength effects seemed to be present already at 0.02 mol/l which is below the normal ion concentration in a pulp mill. The results indicated that there are at least two chemical activities that are competing in an MgO based oxygen delignification stage. The alkalinity is one and a higher alkalinity improves the degree of delignification. The other seemed to be the ionic strength and if this is too high the degree of delignification will go down.

Xylan deposition onto eucalypt pulp fibers during oxygen delignification

Abstract A novel alternative was investigated for elevating the xylan content of eucalyptus pulp through xylan deposition in the course of the oxygen delignification stage. The pH in the range of 10–13 was the only variable evaluated, while the other variables were kept constant in a range similar to industrial practice. Xylans were obtained from unbleached and bleached eucalyptus pulps by cold caustic extraction (CCE), giving rise to brown xylan liquor (BXL) and white xylan liquor (WXL), respectively. The liquors were added to a commercial brown pulp during oxygen delignification. The xylan-enriched pulps were subsequently bleached to 90% ISO with the D(EP)D sequence, beaten in a PFI mill and evaluated for their physical and mechanical properties. Xylan deposition occurred at variable degrees depending upon the pH. Pulp bleachability was not impaired by WXL xylan deposition but was slightly negatively affected by BXL xylan. Pulp beatability was improved by both WXL and BXL xylan deposition. The deposited xylan was more stable across bleaching and beating with the WXL xylan than the BXL xylan deposits. At low energy consumption, the deposited xylan improved pulp physical and mechanical properties. Xylan extraction by CCE with subsequent deposition in the oxygen delignification is an interesting technique to manufacture eucalypt pulps of high xylan content. The potential for industrial application is high as no significant modification of current pulp mill installations is needed.

MEA/water/AQ-pulping of wheat straw

Abstract Annual plant pulping is faced with serious challenges. Traditional pulping processes with bleaching in chlorine based sequences and without appropriate chemical recovery system cannot be tolerated anymore. Alternative pulping processes with non-conventional chemicals, such as organic solvents, e.g., monoethanolamine (MEA), might be suited for an environmentally sound process with a closed mill system. Thus, pulping of wheat straw with MEA as the main delignifying agent was investigated in comparison to soda and soda/AQ pulps as references. The main focus was placed on reduction of the cooking temperature in order to avoid heat-induced MEA degradation. The temperature was reduced stepwise from 165°C to 130°C. At 150°C, a sufficient delignification was achieved. MEA was partly substituted by water and the resulting decrease of the delignification rate could be compensated by addition of anthraquinone (AQ). After optimization of the process, a pulp with a kappa number of 17 was obtained at a total yield of 56% based on o.d. straw. This pulp was selected for bleaching. Both, O/Q/OP/DQ/P (“ECF light”) and O/Q/OP/ZQ/P (TCFZ) bleaching were applied. NaOH was substituted by MEA as alkali source in the oxygen delignification stage. Up to 50% delignification rate was achieved under these moderate conditions. In the ECF light bleaching sequence, a brightness of 80% ISO was reached at a kappa number level below 3. In TCFZ bleaching, an even somewhat higher brightness was achieved at a kappa number level below 1. A yield of fully bleached screened pulp of around 50% (based on o.d. straw) was obtained. The strength properties of the MEA pulp were improved after bleaching. ECF bleaching resulted in slightly better pulp strengths than the TCFZ alternative. MEA forms in combination with water and AQ a very selective pulping system providing effective delignification and maintaining extremely high yields. MEA/water/AQ pulping should be regarded as a serious, environmentally friendly alternative to soda or soda/AQ pulping to produce high-quality pulp from wheat straw.

Reactions and Distribution of Birch Extractives in Kraft Pulp Oxygen Delignification

Extractives composition and distribution was examined around the oxygen delignification stage in a birch kraft pulp mill. Pulp suspensions were divided into different fractions with a dynamic drainage jar device to obtain fibers (+100 mesh), large fines (100–200 mesh), and small fines (200–400 mesh) fractions as well as the colloidal and dissolved fractions. The fractions were extracted and the resin components analyzed by gas chromatography. No significant oxidation reactions of wood resin were found to occur in oxygen delignification. Major removal of resin was achieved in the subsequent washer, where resin is removed with dissolved, colloidal, and fines fractions. Overall deresination in the subsequent washer was 58%.

Structural modification of eucalypt pulp lignin in a totally chlorine-free bleaching sequence including a laccase-mediator stage

Abstract Structural modification of eucalypt pulp lignin was investigated in a totally chlorine-free (TCF) bleaching sequence including a laccase-mediator stage. This stage was applied after two oxygen delignification stages, and was followed by an alkaline peroxide stage. After two oxygen delignification stages, two more stages with a laccase mediator and alkaline peroxide were applied. The residual lignins were enzymatically isolated from the different pulps and analyzed by spectroscopic techniques and analytical pyrolysis. The latter revealed high amounts of syringyl units (>70%) in the lignins. 13C-1H heteronuclear single quantum correlation (HSQC) NMR indicated high amounts of β-O-4′ inter-unit linkages (>75% side-chains). Changes in lignin composition and inter-unit linkages were demonstrated in the course of the bleaching sequence. Moreover, oxidative modification of the major syringyl units was shown by C2,6-H2,6 HSQC correlations and by the presence of oxidized pyrolysis markers in pyrograms. The existence of both Cα keto and carboxyl groups in the residual lignin, together with normal (Cα-hydroxylated) units, was revealed by heteronuclear multiple bond correlation (HMBC) between aromatic H2,6 and side-chain carbons. These Cα-oxidized structures represent nearly 60% of total units in the lignin isolated from the enzymatically treated pulp. Analysis of residual lignin after the final peroxide stage compared with a simple alkaline treatment revealed that most of the oxidatively altered lignin was removed by the alkali used in the peroxide stage. Thus, the kappa number decreased and the final residual lignin was more structurally related to that found before the oxidative stages, although it contained less resinols and more carboxyl group-bearing units. However, the action of peroxide is necessary to attain the high brightness required (>90% ISO).

Effect of pulp delignification degree on fiber line performance and bleaching effluent load

Industrially made kraft pulps obtained by a modified cooking process may contain 60-75 mmol/kg of hexenuronic acids (HexAs), which represents 6-7.5 kappa units. HexAs do not react with oxygen and very little of it is actually removed across the oxygen delignification stage, causing low efficiencies in the range of 25-35%. In this study, an economical evaluation of the ECF bleaching processes was carried out, having none and double-stage oxygen delignification, when applied to eucalyptus pulps of kappa varying in the range of 14-21. The bleaching processes included sequences containing specific stages for HexAs removal (Z, A/D and DHT). Results indicated that the use of oxygen delignification was not economically attractive, particularly for HexA-rich low-kappa pulps, but processes without oxygen delignification present significant environmental challenges.

Phenol degradation and colour removal in submerged culture of Pleurotus sajor-caju with paper mill effluents

The fungus Pleurotus sajor-caju secretes phenol-oxidases that enable the use of recalcitrant compounds as substrates. The residues of paper manufacture contain high lignin levels, which gives the effluents a characteristic brownish colour. To test the potential of P. sajor-caju cultures on reducing these parameters, we used 90% of raw effluents from medium consistency oxygen delignification and bleaching stages plus 10% of mineral solution and different levels of glucose (5–15 g L−1) as substrate. We observed a greater fungal biomass in cultures using effluent than in controls. Cultures containing 10 to 15 g L−1 of glucose resulted in about 42% colour reduction. The polyphenol content was also reduced by 58.9% by the 13th day of culture. In addition, we observed the secretion of laccases (211.44 U mL−1 and 45.98 U mL−1 using ABTS and syringaldazine, respectively) and peroxidases (6.11 U mL−1-ABTS) both peaking at the 7th day of culture and with similar kinetics of production in different glucose concentrations.

Elucidating carboxylic acid profiles for extended oxygen delignification of high-kappa softwood kraft pulps

Abstract Two laboratory high-lignin-content softwood (SW) kraft pulps with kappa values of 48.0 and 49.5, prepared by cooking at high and low active alkali (AA), were used for the study of fiber charge development during two-stage oxygen delignification with inter-stage washing (OwO). It was established that the first oxygen delignification (O) stage increased total fiber charge by 2–4%, and further O-delignification via a second O-stage led to a 3–18% decrease in total fiber charge. Carboxylic acid content in pulp holocelluloses decreased by 12–26% with respect to a 35–70% kappa number reduction due to an O and OwO stage of delignification for high and low AA cooked SW kraft pulps. After an OwO-stage delignification, the residual lignin was found to exhibit a 50–100% increase in carboxylic acid content. 13C NMR spectral data for the residual lignin samples indicated that the unconjugated/conjugated acid ratio was approximately (3–4):1. Generally, the carboxylic acid content in low AA cooked softwood kraft pulp and the corresponding oxygen-delignified pulps was systematically higher (13–23%) than that in high AA cooked SW kraft pulp and the corresponding oxygen-delignified pulps. The experimental results also demonstrated that maximum acid-group content in total fiber occurred after 45–50% oxygen delignification of the SW kraft pulps studied.