Patent Application
20100269994
The present invention relates to a method for treating pectin-containing ligno-cellulosic raw materials in a high-yield pulping process utilizing one or more treatment stages at alkaline conditions by controlling the conditions. More particularly the present invention relates to a method for increasing the bleachability of the final pulp and also lowering the cationic demand of the process waters. The present invention also relates to pulp, paper, board or tissue obtained with the method
Mechanical pulping aims at transforming the raw material into fibers of sufficient quality without severe yield losses. Enhancement of the properties of mechanical pulps by chemical treatment can be achieved prior to refining, during refining, on the coarse fibers between refining stages, or in a post-treatment after refining. In order to produce high-yield pulp of high-quality it is generally important to increase the brightness by removing colored or chromophoric structures without sacrificing too much of the yield in the process. An extensive overview of the prior art can be found in Sundholm, J.: Mechanical pulping, Book 5, Fapet Oy 1999.
Generally alkaline treatments are utilized in the pre-treatment of wood chips, such as in chemithermomechanical pulping, or in peroxide bleaching of mechanical or chemimechanical pulps, such as groundwood, thermomechanical and chemithermomechanical pulps.
Chromophores in mechanical pulps originate to a major part from the lignin. These structures are partly removed or converted to non-chromophoric structures during the bleaching processes. However, during the course of mechanical pulping process stages at alkaline conditions, i.e. mainly chip pre-treatment or pulp bleaching, new chromophoric structures are also formed due to the prevailing process conditions, i.e. high temperatures and high alkalinities. Alkaline darkening is a known phenomenon involving formation of ortho-quinones and coniferaldehydes.
Pectin in the wood is one of the major sources of so called “anionic trash” and alkali-induced “unbleachable” chromophores in the final product. Dissolution of pectic acid, i.e. “anionic trash”, and new chromophoric structures are formed as a result of alkaline-induced cleavage of natively occurring pectins.
When pectin is cleaved, large pectin molecules split into smaller fragments which are able to diffuse out of wood (“anionic trash”) and new chromophoric (terminal) end groups are formed.
It was verified in relation to the present invention that the temperature is a key parameter controlling the degree of pectin cleavage at alkaline conditions. Higher temperature results in more intensive cleavage and smaller molar-mass fragments, producing more chromophoric end groups.
Analogously, lower temperature results in less intensive cleavage and larger molar-mass fragments producing less chromophoric end groups.
Alkaline-induced cleavage of pectin is a very fast reaction: once it has reacted (been cleaved) extended alkaline treatment of pectin at higher (or lower) temperature than the initial temperature does not results in additional cleavage.
The present invention relates to a method for improving bleachability in industrial scale by preventing the formation of stable chromophores and reducing the release of anionic trash in alkaline treatment of pectin-containing materials, such as in chip pre-treatment or in peroxide bleaching. The invention is based on the discovery that when adding alkali to the process the temperature should be low, e.g. 70° C. or below, because the demethylation of pectin occurs surprisingly fast and after that the alkaline-induced cleavage of pectin is minimized even if the temperature is raised or more alkali is applied. Compared to high temperature (˜100° C.) generally used in the processes it results in two times less “anionic trash” in process water and preserves about 2% ISO brightness.
The release of pectins from spruce TMP was doubled at high temperature treatments (80-100° C.) compared to low temperature treatment (20-70° C.) according to the present invention. This was also seen as lower cationic demand (˜25%) of the process waters. At the same time the brightness of the peroxide bleached spruce pulps were up to 1-2% ISO higher for the pulps impregnated under low temperature conditions compared to high temperature conditions before raising the temperature to desired reaction temperature.
The present invention provides a method for treating pectin-containing ligno-cellulosic raw materials in a high-yield pulping process utilizing one or more treatment stages at alkaline conditions wherein the alkaline chemicals are applied at a low temperature treatment stage (T1), meaning the point when alkali for the first time is in contact with the pectin-containing material, before one or more consecutive treatment stages at the same or higher temperature (T2). This provides e.g. improved brightness and lower amount of ionic substances release into process waters.
The present invention further provides pulp or bleached pulp obtained with the method of the invention and paper, board or tissue obtained from said pulp.
One aspect of the present invention relates to increasing the bleachability of pectin-containing material and therefore e.g. to increasing the brightness of bleached pulps.
Another aspect of the present invention relates to decreasing the release of anionic pectic substances.
Still another aspect of the present invention relates to lowering the cationic demand of the process water.
Still another aspect of the present invention relates to enhancing certain papermaking properties, such as the quality of the pulp.
Chemical reactions can generally be influenced by controlling the reaction pH and temperature. By controlling the chemical reactions in alkaline treatment of chips or pulps, some important papermaking properties may be enhanced.
Pectins are polydisperse polymers and have a backbone of partly methyl-esterified galacturonic acid (GalA) units interspersed with rhamnose (Rha) units. Spruce wood contains 1.5-2% pectins and aspen wood 2-2.5% (Pranovich, A. V. Sundberg, K. and Holmbom, B. (2003) Chemical changes in thermomechanical pulp at alkaline conditions. J. Wood Chem. Technol. 23(1):89-112; Sundberg, A., Sundberg, K., Lillandt, C. and Holmbom, B. (1996) Determination of hemicelluloses and pectin in wood and pulp fibres by acid methanolysis and gas chromatography. Nord. Pulp Pap. Res. J. 11(4):216-219, 226).
Release of pectic substances into the process waters will increase the amount of the so called “anionic trash” in the water circulation. This will increase the overall consumption of chemicals, such as paper chemicals in the paper machine (PM) wet-end and decrease the PM runnability.
Lignin is usually assumed to be the predominant source of chromophores responsible for the brightness ceiling observed for mechanical pulps, for softwoods ˜80% ISO and for hardwoods ˜85% ISO. Acidic hemicelluloses like pectins, xylans and other uronans may, however, also play a key role in limiting the maximum brightness obtained in peroxide bleaching of mechanical pulps.
Polymer chain splitting according to the β-elimination mechanism requires the presence of methyl-esterified carboxyl groups and is therefore inhibited by demethylation (Kiss J. (1974) β-eliminative degradation of carbohydrates containing uronic acid residues. Adv. Carbohyd. Chem. Biochem. 29:229-03; Renard, C. M. G. C. and Thibault, J.-F. (1996) Degradation of pectin in alkaline conditions: kinetics of demethylation. Carbohydr. Res. 286:139-150). Both demethylation and β-elimination reactions rates increase with pH. However, an increase in pH will increase demethylation more than β-elimination, while an increase in temperature will increase β-elimination more than demethylation. By controlling the temperature in the alkaline treatment process stages it is, therefore, possible to decrease the release of anionic pectic substances and also increase the brightness of bleached mechanical pulps.
Two parallel reactions happen with pectin in alkaline conditions: demethylation and β-elimination. The susceptibility of pectin to depolymerisation by β-elimination depends on the presence of an ester group on the galacturonic acid. When the methanol removal from pectin is complete, the β-elimination reaction stops. A double bond appears between C-4 and C-5 at the non-reducing end (Kiss 1974).
The release of pectins from spruce TMP is doubled at high temperature treatments (80-100° C.) compared to low temperature treatment (20-70° C.) of the present invention. This was also seen as lower cationic demand (˜25%) of the process waters. At the same time the brightness of the peroxide bleached spruce pulps are up to 1-2% ISO higher for the pulps impregnated under low temperature conditions compared to high temperature conditions before raising the temperature to desired reaction temperature.
The effect of low temperature pre-treatment for aspen mechanical pulps (BCTMP) has not been evaluated. It could, however, be assumed that the effect would be even more pronounced since aspen contains approximately 25% more pectins than spruce.
The present invention provides a method for treating pectin-containing ligno-cellulosic raw materials in a high-yield pulping process (before the paper machine head box) utilizing one or more treatment stages at alkaline conditions. The pectin-containing raw material may be any suitable material, such as wood, softwood or hardwood or combinations thereof, chopped raw ligno-cellulosic material such as chopped wood, straw, defiberized wood or high-yield pulp.
The alkaline treatment stage may be performed before mechanical defibration or consecutive alkaline bleaching stage(s). The alkaline chemicals may be any suitable alkaline chemicals which provide conditions sufficient to achieve significant demethylation of the methyl esters in native pectins, typically above pH 9 at room temperature. The alkali source may for example originate from hydroxides, carbonates or sulfites, preferably sodium, calcium, ammonium or magnesium hydroxides, carbonates or sulfites, or combinations thereof.
In the method of the present invention the alkaline chemicals are applied at a low temperature treatment stage (T1) meaning the point when alkali for the first time is in contact with the pectin-containing material before one or more consecutive treatment stages at the same or higher temperature (T2). It is essential that no previous alkali treatments have been applied to the pectin-containing material because the alkali-induced cleavage of pectin happens very fast. On the contrary, when the alkali is applied for the first time at low temperature the demethylation reaction occurs fast thus preventing further alkali-induced cleavage of pectin by β-elimination reactions. The dosage of alkali may be in the range of 0.1% to 10% (w/w) of the dry-based pectin-containing material.
It is preferred that the pectins in the treated material are methyl-esterified. Generally the methyl-esterification degree is 20-100%, determined as the percentage of galacturonic units containing one methyl-ester group, preferably 50-70%.
The method of the present invention improves the bleachability of the pectin-containing material. This will lead to improved brightness of the pulp and lower bleach chemical consumptions and costs needed to achieve a given brightness. Further, lower amount of ionic substances are released into process waters. Lower amount of dissolved substances in the water means less chemical costs in papermaking or water treatment and less environmental impact. It will also improve the paper machine performance. Also the pulp quality is better, e.g. there is less darkening.
The high-yield pulping process may refer to thermomechanical pulping or chemithermomechanical pulping with yield generally over 70%, preferably over 80% and more preferably over 85%.
The low temperature in stage T1 referred herein means temperature of or below 70° C., preferably below 60° C., more preferably below 50° C. Generally temperature range from room temperature (20° C.) to said 70° C. may be used. In one embodiment the temperature is 50-60° C. The length of said low temperature step or the interval between T1 and T2 may be in the range of 1 second to 24 hours, generally less than 4 hours. In one embodiment the interval is in the range of 1 second to 2 hours. In another embodiment the interval is in the range of 2 to 10 minutes, for example about 5 minutes. It is essential that said time allows the demethylation reactions to occur in the lower temperature.
In one embodiment in stage T1 also other chemicals are applied such as chelating, stabilizing and/or bleaching agents, preferably peroxygens, such as peroxide. The chemicals used in stage T1 may also be recycled to the subsequent stage(s).
In one embodiment in stage T2 chemicals are applied such as alkali, chelating, stabilizing and/or bleaching agents, such as EDTA, DTPA, silicate, magnesium sulfate, peroxygens, preferably hydrogen peroxide. In another embodiment the temperature in the T2 stage is in the range of 70-210° C.
The anionic substances released may contain for example galacturonic acid, glucuronic acid and 4-O-methylene glucuronic acid. The method of the invention mainly reduces the amount of galacturonic acid.
In one embodiment the alkaline treatment stage is an alkaline pre-treatment stage before mechanical defibration at elevated temperature (CTMP refining). In another embodiment the alkaline treatment stage is an alkaline pre-treatment stage before alkaline peroxide bleaching; before MC stage in two stage MC-HC bleaching sequence or before a single HC stage or as a MC stage with controlled temperature before a HC stage running at elevated temperatures.
“Chemithermomechanical pulps” (CTMP) are produced by treating a lignocellulosic material, commonly wood chips, with one or more chemical agents, in combination with the operations of heating and mechanical separation of fibers. In the combined operation indicated above of heating, chemical treatment and fiber separation, the chemical treatment with controlled temperature T1 may be carried out either before, during or after the fiber separation.
CTMP pulps are generally produced to a yield, i.e. dry weight pulp relative to the dry weight of starting material, of 70-90%, typically 85-90%, and with TMP pulps 85-95%, typically 90-95%.
Generally by “chemical pre-treatment” it is intended that operation, over the course of which the lignocellulosic material, most commonly wood chips, is treated with liquor containing either sulfite or mixture of sulfite and sodium hydroxide at a temperature equal to or greater than 100° C. under saturation water vapor pressure. Treatment with liquors containing mixtures of sodium hydroxide and hydrogen peroxide is typically performed at 60-80° C. The chemical treatment potentially includes conventional impregnation with steaming of the lignocellulosic material to facilitate a good penetration of the solution of the selected reagents into the material followed by consecutive screw pressing stage(s) to remove entrained air and to facilitate complete chemical uptake of the wood material.
The temperature at which the treatment is carried out generally does not exceed 200° C. and usually ranges from about 120 to 160° C. The treatment medium is at an initial pH usually ranging from 6 to 12.5.
The duration of the chemical pre-treatment depends on the selection of other process parameters, but generally does not exceed 1 hour.
Expressed in terms of SO2, the amount of the sulfite in the pre-treatment ranges, for example, from approximately 0.1% to 10%, most typically from 0.5% to 3% on oven dry lignocellulosic material and the sodium hydroxide amount from 0% to 7% depending on wood species and product and process requirements The amount of H2O2 of the alkaline peroxide pre-treatment may range from 0.5% to 12%, most typically from 3% to 5% and the sodium hydroxide amount from 0.5% to 10%, most typically from 2% to 7% depending on wood species and product and process requirements.
Certain chemical agents may be used in the pre-treatment together with the alkaline sulfite, or alkaline peroxide for example complexing or sequestering agents, such as diethylenetriaminepentaacetic (DTPA) acid, ethylenediaminetetraacetic (EDTA) acid, sodium silicate and magnesium sulfate (Epsom Salt)
State of the art bleaching of the TMP and/or CTMP pulps by means of hydrogen peroxide in an alkaline medium is typically carried out by introducing an amount of hydrogen peroxide of approximately 0.5% to 10%, in the presence of about 1% to 6% sodium silicate solution at a pH of from approximately 9 to 11 and at a temperature of from about 40 to 100° C. for about 0.5 to 2 hours, at a consistency of approximately 10% to 30%. The bleaching bath may also contain certain additives, principally one or more sequestering or complexing agents, such as, for example, DTPA.
Tower bleaching refers to bleaching which normally takes place at high consistency. 2-stage bleaching with recycling of HC (high consistency) bleach filtrate to the initial MC (medium consistency) stage, is normally employed when high brightness targets (>80% ISO) are required. The temperature at the initial contact between pulp and bleaching liquor is normally the on the same level as the temperature in the bleaching stage, i.e. well above 60° C., typically between 70° C. and 90° C.
Steep bleaching refers to bleaching at high stock consistency in a pulp pile at lower temperatures, normally 20° C. to 45° C., and for longer bleach times than for conventional tower bleaching.
Refiner bleaching refers to bleaching conducted during the refining stage by adding alkaline peroxide to the feed to either the primary or secondary refiner. This means that the temperature at the initial contact initial contact between pulp and bleaching liquor is normally very high, i.e. 100° C. to 160° C.
Commercial methylesterified pectin was used (1 mg/ml pectin, 94% methyl-esterified, 0.3 ml 1M NaOH, 20-100° C., 30 min). In
In
Thermomechanical pulp from Norway spruce (Picea abies L.) was sampled in a Finnish mill, after the second-stage refiner at approximately 35% consistency and the pulp was stored at −24° C. until use.
“Clean” TMP fibers were obtained by extraction in a Soxhlet apparatus with hexane-acetone (9:1 v/v) for 24 h to remove lipophilic material. Water-soluble substances (hemicelluloses and low-molar-mass aromatics) were removed by thorough washing. The pre-extracted TMP was suspended in distilled water (pH 5.5) at 60° C. at 2% consistency and agitated for 3 h with a blade propeller (˜200 rpm). The TMP suspension was filtered under vacuum on a Büchner funnel with a paper machine wire. To prevent the loss of fines, the filtrate was passed twice through the formed fiber mat. The TMP was re-suspended in distilled water and the washing procedure was repeated 5 times. The TOC value of the final filtrate was 4 mg/l. The washed TMP was air-dried and stored in the dark at +4° C.
10 g o.d. (oven-dry) TMP in 95 g suspension, in polyethylene plastic bags, was mixed well. All pulps were pre-heated at 70° C. for 3 hours before adjusting the target temperatures for the trials.
The pulps were preconditioned at different temperatures (4° C., 20° C. and 70° C.) for 1 hour before adding NaOH (see table 1). The pulps pre-treated at 4° C. and 20° C. had retention times of 2 and 1 hours respectively, before raising the temperature to 70° C. in a water bath. The pulp pre-treated at 70° C. was directly placed into water bath at 70° C. for 90 minutes (
Finally the pulps were cooled down in ice bath to 20° C. After measuring the end pH, the pulps were acidified with 6% SO2-water to pH 5. Samples of the pulp filtrates were taken for chemical analysis.
In
The pulp used in example 3 was the same as in example 2, except that in these trials it had not been hexane extracted or extensively washed. Part of the water-soluble substances (hemicelluloses and low-molar-mass aromatics) were, however, removed by diluting the pulp to 2%, agitating the pulp at 60° C. for 1 hour before thickening the pulp to 20% dryness on a Büchner funnel. To preserve fines the first portion of filtrate was recirculated through the fiber mat
Washed TMP, DTPA (0.25% calculated on dry pulp) and MgSO4, (0.05%) were mixed well in a polyethylene plastic bag and kept overnight at room temperature. The pulp was mixed well and divided into plastic bags containing 10 g o.d. pulp each. Three pulps were treated in parallel for each pre-treatment temperature test (2° C., 20° C. and 70° C.).
The treatment scheme for example 2 is shown in
All three series of pulps were placed into water bath at 70° C. for 90 minutes. The pulps were cooled down to 20° C. in an ice bath. After measuring end pH, samples were acidified with 6% SO2-water to pH 5 before sheet forming according to a modified ISO 5269-1979 standard.
In
The pulp used in example 4 was the same as in example 3. Part of the water-soluble substances (hemicelluloses and low-molar-mass aromatics) were removed by was agitating the pulp at 2% consistency at 60° C. for 2 hours. The pulp was thickened to 20% on a Büchner funnel. To preserve fines the first portion of filtrate was recirculated through the fiber mat.
Washed and filtered TMP was mixed with DTPA (0.25%) and MgSO4 (0.05%) and kept overnight before dividing the pulp into plastic bags containing 10 g o.d. pulp each.
The treatment scheme for some of the trials in example 4 is shown in
Each pulp was pre-heated for 1 h at desired temperature, 10 ml 3% H2O2 was added, then 3 ml Na-silicate (0.1 g/ml) and 2.5 ml 1M NaOH were added fast, mixed well (adding chemicals and mixing during 5 min) and the suspension was kept for 5 min more at pre-heating temperature (all together—10 min with alkali at pre-heating temperature). All series of bags were placed into water bath at 70° C. for 90 minutes.
All pulps were cooled down to 20° C. in ice bath. After measuring of the end pH, samples were acidified with 6% SO2-water to pH 5 before sampling the filtrate and sheet forming.
In
The analyses were performed using the following equipment and methodology.
The paper sheets from bleached pulp were examined by ISO brightness test according to SCAN.
The sugar composition of hemicelluloses and pectins was determined using methanolysis (2 M HCl in dry methanol), followed by gas chromatographic (GC) analysis of TMS-derivatives of corresponding sugar monomers formed (Sundberg et al. 1996). The samples were freeze-dried prior to methanolysis.
Cationic demand (CD) of TMP waters was determined by polyelectrolyte titration using 0.0005 N potassium polyvinyl sulfate (KPVS) as anionic polymer with a Mütek particle charge detector 03. TMP-water samples, containing dissolved and colloidal substances were mixed with 0.0005 N polybrene directly in the measuring cell and were then titrated with KPVS.
A pH-meter Handylab pH 12 (Schott-Geräte GmbH, Mainz, Germany) with Schott pH-electrode BlueLine 28 pH (pH 0-14/−5° C.-80° C./Ge) was used to monitoring the pH-value in water solutions/suspensions during alkaline treatment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20075729 | Oct 2007 | FI | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI08/50574 | 10/15/2008 | WO | 00 | 6/28/2010 |
