Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1042—Use of chelating agents
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21B—FIBROUS RAW MATERIALS OR THEIR MECHANICAL TREATMENT
  • D21B1/00—Fibrous raw materials or their mechanical treatment
  • D21B1/04—Fibrous raw materials or their mechanical treatment by dividing raw materials into small particles, e.g. fibres
  • D21B1/12—Fibrous raw materials or their mechanical treatment by dividing raw materials into small particles, e.g. fibres by wet methods, by the use of steam
  • D21B1/14—Disintegrating in mills
  • D21B1/16—Disintegrating in mills in the presence of chemical agents
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• Pulping processes can be put into either of two broad classifications; high yield pulping and chemical pulping.
• High yield pulping processes use mechanical destructuring (i.e. grinding, refining) of the raw material to produce individual fibers or pulp from lignocellulosic materials, usually in chip form, sometimes with mild chemical pretreatment of the chips.
• Chemical pulping or low yield processes that primarily use chemical reactions to produce individual fibers from chips.
• Within the high yield category there are many different combinations of mechanical, chemical, and thermal treatments. Each specific combination of mechanical, chemical and thermal treatments has a different effect on fiber separation, lignin removal, fiber brightness and papermaking strength of paper made from the resulting fibers.
• This invention concerns high yield pulping comprising chemical pretreatments using peroxide in combination with mechanical treatment to produce pulp from chips. More particularly, this invention reduces peroxide decomposition associated with the pretreatment of chips with peroxide prior to refining of the chips to produce high yield pulp with peroxide modification of the lignin in the fibers. With this invention pulp having desired papermaking properties, particularly a combination of high strength and brightness is obtained with reduced chemical and energy consumption.
• Pulp produced by mechanical refining alone without chemical pretreatment results in extremely high yields (about 95% or higher) but results in fibers containing almost all of the original lignin in essentially a chemically unmodified form and fiber damage/fines generation due to the somewhat indiscriminate mechanical action on the lignocellulosic raw material.
• Such unmodified lignin imparts relatively low brightness to the fibers, and, due to its hydrophobic nature, the lignin inhibits the development of paper strength through fiber collapse and fiber to fiber bonding (hydrogen bonding).
• Such fibers are stiffer than partially or completely delignified fibers from the same lignocellulosic raw material.
• the papermaking strength of high yield pulps can be increased by sulfonation of the lignin, particularly when the wood chips are treated with the sulfonation chemicals (usually sodium sulfite with or without sodium hydroxide) prior to mechanical defibration (refining).
• the resulting fibers can also be bleached economically as with alkaline peroxide and/or sodium hydrosulfite to give both improved brightness and papermaking strength.
• the higher levels of sulfonation required for high strength result in pulps which respond less well to bleaching than similar non-sulfonated or low-sulfonated pulps, and therefore such highly sulfonated pulps have high strength but have lower bleached brightnesses and higher bleach chemical demand.
• low-sulfonation pulps are more bleachable but have lower strength.
• sulfonation processes require the removal and disposal of environmentally objectionable sulfur compounds from process waste streams.
• the need for separate sulfonation, refining, and post-refiner bleaching equipment makes the capital equipment and operating costs for such a system very significant.
• An alternative to sulfonation of lignin for increasing papermaking strength of high yield fibers is carboxylation and brightening of lignin achieved by the combined swelling and brightening action of alkaline peroxide prior to and/or during defibration.
• carboxylation results in lignin containing sulfonate groups
• carboxylation results in lignin with carboxylate groups.
• Both the sulfonate and the carboxylate groups are capable of participating in hydrogen bonding which increases the strength of paper made from such high yield pulps (papermaking strength).
• alkaline peroxide pretreatment of chips softens the lignocellulosic raw material resulting in easier fiber separation (less energy consumption and less fines generation and fiber fragmentation) during refining.
• refiner brightening with alkaline peroxide can eliminate the need for separate post-refiner bleaching equipment, due to the facts that refiners are excellent mixers of pulp and brightening agents, and the temperature within the refiner (about 100°C or higher) causes brightening to occur extremely fast relative to typical post-refiner alkaline peroxide bleaching steps (approximately 50°C to 80°C).
• Peroxide decomposition The primary drawback to brightening chips within the refiner with alkaline peroxide is peroxide decomposition.
• Peroxide decomposition is hastened by the high temperatures reached in refiners and by metal contaminants, particularly manganese, iron, and copper, which are contained in significant quantities in lignocellulosic raw materials and in lesser quantities in process water. Partial removal or inactivation of such metal contaminants in lignocelluslosic raw material can be effected by introducing chelating agents into the wood chips and then removing the chelant-metal complexes.
• the physical entrapment and chemical attraction of such metals by fiber components within the chips make complete removal of the metals impractical and require additional peroxide protection.
• a common method for circumventing the problem of peroxide decomposition in alkaline peroxide bleaching of chips during refining is to add the bleaching agent directly to the refining zone to minimize the contact time between the chip and alkaline peroxide, and in some cases to allow mare intimate contact between metal contaminants and silicate and/or magnesium ion stabilizer flocs (See for example, U.S. Patent Non. 3,023,140-Textor, 3,069,309-Fennell, 4,022,965-Coheen, et al., 4,270,976-Sandstorm, et al., 4,311,553-Akerlund, et al., Japanese Patent Application No. 80-72091, and Federal Republic of Germany Patent No. 2818-320).
• wood chips have been pretreated by impregnation and/or refining with chelants (U.S. Patents Nos. 3,023,140-Textor, 4,311,553-Akerlund, et al., Japanese Patent Application No. 80-7209, and Federal Republic of Germany Patent No. 2818-320) or with sodium silicate (U.S. Patent Nos. 3,069,309-Fennell, 4,311,553-Akerlund, et al.), or with magnesium salts (U.S. Patent Nos.
• the present invention is based in part upon the hypothesis that with processes employing alkaline peroxide addition directly into the refiner, the majority of the defibration occurs before the alkali and peroxide can react with the wood fibers, thereby reducing the potential for papermaking strength development imparted with requiring more energy for refining and increasing the generation of fines, all of which could be avoided if alkaline peroxide could be inserted into and stabilized within the chip and allowed to react with the chip prior to defibration.
• Impregnation of chips with alkaline peroxide prior to refining has also been practiced (U.S. Patents 4,187,141-Ahrel, and 4,270,976-Sandstorm, et al., and Canadian Patents 1,078,558, and 1,173,604).
• the brightness obtained was comparable to that obtainable with post refiner alkaline peroxide bleaching.
• metal contaminants are not removed or deactivated; rather, peroxide decomposition is reduced by lower alkalinity (U.S. Patent Nos. 4,270,976-Sandstorm, et al., Canadian Patent 1,070,558, and 1,173,604) or by minimizing refining temperature (U.S. Patent No.
• Alkaline peroxide stabilizers like water soluble alkaline sodium silicate and magnesium sulfate are often utilized in the peroxide bleaching of high yield pulps to further reduce peroxide decomposition caused by metal contaminants.
• the silicate-magnesium combination forms a floc in alkaline peroxide solutions and this floc attracts and absorbs the metal ions thereby reducing the ability of such ions to decompose peroxide.
• Magnesium ions also reduce peroxide decomposition by electronically deactivating the metal ions, thereby reducing the potential of the metal ions to decompose peroxide.
• the alkaline peroxide impregnated chips with the "in-situ" formed stabilizing floc within the cellular structure are refined in one or more stage(s) under atmospheric pressure or superatmospheric pressure and the corresponding saturated steam temperature. (The refining pressure is usually associated with steam added to or generated within the refining device.)
• the resulting pulp is dewatered and washed to remove bleaching and stabilizing chemicals and dissolved wood substances to result in a non-sulfonated pulp having a unique combination of properties including high yield, superior brightness and papermaking strength, and low fines content.
• Recyclable alkaline peroxide is obtained from the dewatering of the pulp after refining and preferably before acidification.
• the alkaline peroxide obtained from the post refiner dewatering step can be reused as makeup in the impregnation step in which peroxide is added to the chips.
• the present invention is based upon the discovery that the key to the realization of successful peroxide stabilization and brightening utilization lies in (1) substantial reduction of metallic ions content in the chips, such as iron, copper, and (in particular) manganese content in the chips (these metals catalyze peroxide decomposition) prior to impregnation of chips with peroxide and (2) the maintenance of high level of magnesium in the chips with the alkaline peroxide prior to and preferably during refining.
• the present invention provides an alkaline peroxide process for the production of high yield pulps from wood chips having a reduced manganese concentration prior to peroxide impregnation and increased magnesium concentration prior to or simultaneously with alkaline peroxide impregnation, followed by refining to produce a pulp of high brightness and pulp strength with a minimum of peroxide consumption.
• This invention is directed toward the removal of metallic ions such as manganese from lignocellulosic material in chips form prior to treatment with alkaline peroxide and to the utilization of high levels of magnesium (Mg) within the chips while in contact with alkaline peroxide.
• the present invention is particularly useful for pulps from wood species naturally high in metallic ions such as eastern black spruce, eastern balsam fir, lodgepole pine/white spruce mixtures and aspen.
• the present process is characterized by at least a two stage and preferably three or more stage impregnation process of chips prior to refining.
• wood chips which preferably have been purged of entrapped air, are impregnated with a chelating agent such as diethylenetriaminepentaacetic acid (DTPA), ethylene diaminetetraacetic acid (EDTA), hydroxyethylethylenediamine triacetic acid (HEEDTA), and nitrilotriacetic acid (NTA), sodium tripolyphosphate (STPP), and phosphonic acid derivatives or other similar compounds known in the art for such functionality or a combination of such chelating agents.
• DTPA diethylenetriaminepentaacetic acid
• EDTA ethylene diaminetetraacetic acid
• HEEDTA hydroxyethylethylenediamine triacetic acid
• NTA nitrilotriacetic acid
• STPP sodium tripolyphosphate
• the concentration of the chelating agent in the impregnating solution should be preferably from 0.1 gram/liter to 20 grams/liter (expressed as 100% chelating agent in the solution) to give preferably 0.01% to 2% chelant based on the dry weight of the chips with from 0.05% to 0.5% being particularly preferred.
• the temperature of the first inpregnation step is preferably between 15°C and 80°C.
• the pH is preferrably between 4 and 12 with between 8 to 11 being particularly preferred. Adjustments to the solution pH can be made with any suitable acid or alkaline substance which does not react with peroxide, cause darkening of the chips, or cause any of the components of the impregnation solution to be precipitated or to lose chelating ability such as sulfuric acid or sodium hydroxide.
• the chips are preferably squeezed to expel liquid and any remaining air and then allowed to expand in contact with the impregnation solution.
• the quantity of solution absorbed in an impregnation step is influenced by the impregnation device (primarily the degree of squeezing of the chips) and the particular material being impregnated.
• the level of chemical addition into the chips is primarily controlled by the concentration of the particular chemical in the impregnating solution and the quantity of solution absorbed.
• the concentration of chelant and quantity of solution absorbed is preferably adjusted to give a chemical addition of 0.01% to 2% based on the dry weight of lignocellulosic material being treated.
• the first impregnation solution is expressed either prior to or as the initial part of the second impregnation step.
• Metallic ions are removed from the chips with the expressed first impregnation solution which removal is enhanced by the presence of the chelating agents.
• Preferably at least about 25% of the first impregnation solution is expressed from the chips with from about 33% to about 50% being particularly preferred.
• the effectiveness of the first impregnation step to remove metallic ions is enhanced by providing some retention time between impregnation and expressing the impregnation solution from the chips. This enhanced effectiveness is observed with a retention time of one half hour although several hours is more effective.
• the first impregnation solution should consist esstentially of a chelating agent because it is expressed from the chips.
• the presence of substantial quantities of peroxide ( e.g. above 1%) or stabilizers in the first impregnation solution would be wasteful of such chemicals since they would be removed from the chips when the first impregnation solutions is expressed from the chips.
• Metallic stabilizers, particularly magnesium should be avoided in the first stage because they interfere with the primary purpose of the chelating agent.
• the second impregnation step can be a conventional alkaline peroxide impregnation stage with stabilizers and chelating agents.
• the second impregnation stage is with an aqueous solution of soluble magnesium salts (e.g., epsom salt - MgSO4.7H2O).
• soluble magnesium salts e.g., epsom salt - MgSO4.7H2O
• the epsom salt addition level is 0.05% to 2% based on oven dried (o.d.) wood expressed as MgSO4.7H2O.
• This can be readily accomplished with an impregnation solution containing 0.5 grams per liter to 20 grams per liter.
• the purpose of this second step is to allow the soluble magnesium salts to penetrate the chip interstices.
• the temperature of the second impregnation step is preferably between 15°C and 80°C, and the pH is maintained below 10 to keep the magnesium in a soluble form. Adjustment to the solution pH is preferably made in the same manner as described for the first impregnation solution.
• the second impregnation solution can contain alkaline peroxide with silicate stabilizers.
• the third impregnation solution is an aqueous alkaline peroxide solution which contains peroxide stabilizers such as silicates.
• the third impregnation solution contains magnesium and silicate stabilizers.
• Silicates are preferably in a concentration range of 1 gram per liter to 50 grains per liter to yield between 0.1% to 5% by weight of silicates (calculated as SiO2) based upon the dry weight of the chips.
• Magnesium based on weight of Mg in salt is preferably in a concentration of from 0.01 grams per liter to 2 grams per liter to result after impregnation in from 0.001% to 0.2% by weight Mg based upon the dry weight of the chips.
• the third impregnation solution contains preferably hydrogen peroxide although any other peroxygen compound suitable for lignin brightening may be used.
• the peroxygen compound is preferably in the concentration range of between 5% and 100 grams per liter (calculated and expressed as hydrogen peroxide) to give additional levels of preferably between 0.5% and 10% of the dry weight of chips expressed as hydrogen peroxide.
• an alkaline substance preferably sodium hydroxide
• impregnation sequence is the three stage sequence previously described and presented in greater detail in the examples. However, it should be understood that the invention is not limited to the generally described three stage sequence.
• a two-stage impregnation sequence in which the first impregnation stage is practiced as previously described and the second impregnation (Mg compounds) solution is combined with the third impregnation solution (alkaline peroxide and peroxide stabilizers such as silicates) and used as the impregnation solution in the second stage, preferably at a pH from 9 to 13 can also be employed.
• the invention is not limited to the concentration and addition level ranges previously described for stabilizers and chelating agents, since differences in metal contamination levels of lignocellulosic raw materials or process water could justify stabilizer or chelating agent usages outside of the specified preferred ranges.
• the chemically treated chips can be allowed to react with or without the application of heat, prior to entering the refining stage by retaining the impregnated chips for some finite time in any suitable vessel.
• a retention time of 5 minutes to 60 minutes is particularly suitable.
• Each impregnation is preferrably followed by a short drainage period and additional time for diffusion and/or reaction to occur within the chips. The amount of diffusion/reaction time required depends on the type of wood processed.
• the chemically treated chips are mechanically refined in a suitable defibration apparatus in one or more stages in accordance with conventional processes and equipment.
• the steam pressure and corresponding temperature during refining are optional and can be at atmospheric and/or superatmospheric pressure, depending on the species being pulped and the desired pulp properties. Atmospheric pressure refining is preferred.
• the pulp After refining, it is preferred that the pulp remain alkaline so that the silicate and other pH-sensitive materials can be removed prior to washing and neutralization. This prevents silicate deposition which can impair final pulp properties.
• the pulp may be allowed to continue bleaching as long as is practical prior to expelling the impregnation solutions.
• the amount of peroxide used in the impregnation steps is preferably preselected to result in some residual peroxide remaining after refining in order to maintain high brightness.
• the refined pulp is concentrated, e.g., by compressing or thickening, to remove residual impregnation solution containing potentially recyclable alkaline peroxide, then diluted with water, washed and acidified preferably with sulfur dioxide, sodium bisulfite, sulfurous acid, or sulfuric acid, to a pH between 5.5 and 6.0. In particular, washing prior to acidification is preferred for removal of alkali soluble wood components and silicate.
• the residual peroxide extracted from the pulp after refining can be recycled as a source of peroxide in one of the impregnation solutions particularly if the process is practiced continuously or in sequential batches.
• the washed pulp is preferably screened and cleaned by conventional means to result in a pulp suitable for the production of paper products.
• wood chips from eastern black spruce, eastern balsam fir, lodgepole pine/white spruce 50/50 mixture (all softwoods) and/or aspen (hardwood), are treated by both the process of the present invention and that of Gentile, et al., U.S. Patent 4,849,058 and a comparison of the results tabulated.
• the process of U.S. Patent 4,849,058 was chosen for comparison because it represents the what is believed to be the best of the prior art processes for stabilizing alkaline peroxide in chips prior to high yield mechanical pulping. In the examples, all parts are by weight unless specified otherwise.
• Example A Screened eastern black spruce chips (2000 grams o.d. of wood at a solid content of 49%) were steamed for 15 minutes.
• the hot chips solid content 40%
• the impregnator was then closed, filled with 55 liters of impregnation liquor and the chips released (allowed to expand) into the liquor.
• the inpregnated chips were removed from the impregnator and drained for 5 minutes to remove all excess liquor. The same procedure was repeated for all three impregnation stages.
• Example A the preferred three-stage impregnation process of U.S. Patent 4,849,058 was employed.
• Example 1 the 3 impregnation steps were conducted according to the process of the present invention.
• Table 1 lists the impregnation conditions, the amount and type of chemicals added, and the test results comparing the process of the present invention (Example 1) with the control (Example A).
• the magnesium salt used in these examples was Epsom salt (MgSO4.7H20).
• the reported amounts of pentasodium salt of diethyienetriamine-pentaacetic-acid (DTPA) are based on a solution with 40.2% active ingredient.
• Trisodium salt of hydroxy-ethylethylene-diamine-triacetate (HEEDTA) was a 41.3% solution.
• the concentrations for silicate are calculated using a 37.6% solution with a SiO2/Na2O ratio of 3.22.
• Hydrogen peroxide usage or consumption (expressed as a weight percent of the dry weight of the chips) is based on the quantity of peroxide applied to the wood chips minus the quantity of residual peroxide in the pulp after refining.
• the percentage peroxide consumed is calculated as being equal to the quantity of peroxide consumed times 100 divided by the dry weight of the chips.
• the quantity of peroxide in a solution was determined by iodometric titration using starch as an end point indicator (Vogel, Quantative Inorganic Analysis, Wiley & Sons, 1961, p. 363).
• the magnesium and manganese concentration in the original wood chips and after impregnation was determined by atomic absorption spectroscopy.
• the process of the present invention led to an increase in manganese removal and magnesium retention, which in turn gave a 2% brightness increase and a 47% reduction in peroxide consumption, thus demonstrating the increased peroxide stability under the process of the present invention.
• Example B Two kilograms of screened eastern Canadian balsam fir chips with a solids content of 56.2% were impregnated and refined as in control Example A. The pH of the first stage liquor was adjusted to 7.0 and the 2nd stage liquor pH was 10.7. This experiment is designated as comparative Example B.
• Example 2 A three impregnation step sequence of the present invention was made on an identical batch eastern Canadian balsam fir chips as used in Example B.
• the general impregnation and refining conditions were the same as for Example 1 above.
• Table 2 lists the process conditions, chemical types and amounts added, and the comparative test results for both examples. This experiment is designated as Example 2.
• Example A Screened aspen (populus tremuloides) chips at a solid content of 58.2% were impregnated and refined using the general procedures described in Example A.
• the pH of the first stage liquor was adjusted to 7.0 and the 2nd stage liquor had a pH of 10.7.
• Example C An identical aspen chip charge was impregnated and refined by the general procedures of Example 1 above using the sequence of this invention. The specific additions used and results obtained are shown in Table III.
• Example 3 Although the original aspen chips had a low manganese content resulting in a low 3rd stage manganese level in the chips in both Examples C and 3, the third stage chips of Example 3 retained much more magnesium. As a result, brightness was improved by 1.2% while peroxide consumption was reduced by about 18% vs. the control.
• a lodgepole pine/white spruce (50/50%) chips mixture was subjected to both the 3-stage process of the present invention and that of the control process used in Example A.
• the same general impregnation and refining conditions were employed as in the previous examples along with comparable chemical applications while the specific conditions are given in Table IV.
• Table IV compares the brightness values, chemical compositions and Mn and Mg content for identical chip charges and both examples D, E, F, G (control) examples with 4, 5, 6 and 7, respectively (process of invention). In all cases, brightness is higher and peroxide consumption lower for examples 4, 5, 6, and 7. Also, chips impregnated in example 4, 5, 6 and 7 always have a lower Mn content and a higher Mg content after the 3rd impregnation stage, thus leading to greater peroxide efficiency in pulp brightening.
• Brightness gains attributable to the process of invention range from 1.8 to 6.2% ISO, while peroxide consumption is reduced by from 13 to 28% compared to the control.
• TABLE IV Comparative test results of 3-stage alkaline peroxide process of the present invention vs control 3-stage process. EXAMPLE No.
• the 3-stage preferred process of the present invitation increased brightness and decreased H2O2 consumption compared to the best prior art process for high yield peroxide pretreated pulp (U.S. Patent 4,849,058).
• Example 1 The best mode presently contemplated for practicing the invention is with a three stage impregnation process as examplified in Example 1 with chemical selection, concentration of impregnation solutions and chemical quantities impregnated into the chips in the ranges demonstrated in Examples 1 through 7.
• the present invention described in its broadest concept is on improvement to peroxide treatments of wood chips prior to refining in which the concentration of metal ions within the cellular structure of the chips is reduced in a first impregnation step as shown in examples 1 through 7 and followed by alkaline peroxide impregnation with stabilizers for peroxide including magnesium ions and then mechanical pulping (refining).

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• 1991
  • 1991-12-06 CA CA 2057231 patent/CA2057231A1/fr not_active Abandoned
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  • 1991-12-11 AU AU89609/91A patent/AU660435B2/en not_active Expired - Fee Related
  • 1991-12-13 EP EP91311614A patent/EP0494519A1/fr not_active Withdrawn
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• 1992
  • 1992-01-03 FI FI920048A patent/FI920048A/fi not_active Application Discontinuation
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