EP2176461B1 - Procédé de fabrication de papier et utilisation d'un traitement acide/base dans sa fabrication - Google Patents

Procédé de fabrication de papier et utilisation d'un traitement acide/base dans sa fabrication Download PDF

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Publication number
EP2176461B1
EP2176461B1 EP08760061A EP08760061A EP2176461B1 EP 2176461 B1 EP2176461 B1 EP 2176461B1 EP 08760061 A EP08760061 A EP 08760061A EP 08760061 A EP08760061 A EP 08760061A EP 2176461 B1 EP2176461 B1 EP 2176461B1
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Prior art keywords
pulp
acid
suspension
water
process according
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EP08760061A
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German (de)
English (en)
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EP2176461A1 (fr
Inventor
Aila Weaver
Risto Vahteri
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Linde GmbH
Stora Enso Oyj
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Linde GmbH
Stora Enso Oyj
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    • 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
    • D21C9/001—Modification of pulp properties
    • D21C9/002—Modification of pulp properties by chemical means; preparation of dewatered pulp, e.g. in sheet or bulk form, containing special additives
    • D21C9/004—Modification of pulp properties by chemical means; preparation of dewatered pulp, e.g. in sheet or bulk form, containing special additives inorganic compounds
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
    • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
    • D21H17/63—Inorganic compounds
    • D21H17/64—Alkaline compounds
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
    • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
    • D21H17/63—Inorganic compounds
    • D21H17/65—Acid compounds
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
    • D21H23/00—Processes or apparatus for adding material to the pulp or to the paper
    • D21H23/76—Processes or apparatus for adding material to the pulp or to the paper characterised by choice of auxiliary compounds which are added separately from at least one other compound, e.g. to improve the incorporation of the latter or to obtain an enhanced combined effect
    • 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
    • D21C9/1036—Use of compounds accelerating or improving the efficiency of the processes
    • 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
    • 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
    • D21C9/18—De-watering; Elimination of cooking or pulp-treating liquors from the pulp

Definitions

  • the present invention relates to a process for making paper, wherein a pulp suspension, which comprises mechanical pulp, is thickened in a dewatering device and is thereafter diluted with water and pumped to a storage tank, then diluted again and made into paper.
  • the pulp suspension is subjected to an acid-base treatment after said dewatering device.
  • the invention also relates to the use of an acid-base treatment in a process for making paper, wherein a pulp suspension, which has been thickened in a dewatering device, is subjected to an acid-base treatment for improving the process and/or the product.
  • the invention provides improvements in the brightness of the pulp suspension and/or reduces the need for bleaching agent.
  • the invention also decreases the dissolution of calcium carbonate filler in the process.
  • Mechanical pulps may be used in unbleached form, but they are typically subjected to bleaching with peroxide and/or dithionite to increase the brightness of the pulp. After peroxide bleaching the pulp suspension is subjected to a thickening step and thereafter it is diluted with process water. Alternatively, in the case of unbleached pulp the suspension is subjected, after the screening and washing stage, directly to a thickening step and then to dilution with process water.
  • the pH of the mechanical pulp is commonly reduced in order to minimize the darkening reactions of the pulp that decrease the brightness of the pulp and of the paper made from that pulp. These reactions include precipitation of dissolved substances together with metals, lignin etc. How severe these reactions are depend on pH, time, temperature, amount of dissolved metals and amount of dissolved and colloidal organic material.
  • the pulp is normally acidified to pH 5-7 with sulphur dioxide water, bisulphite, sulphuric acid or carbon dioxide. Pulps used in acidic papermaking can be acidified to a lower pH than pulps that are used in neutral papermaking.
  • a lower pH gives less brightness loss, however a too low pH can cause problems directly at the point of addition of the acid or later in the process.
  • a low pH may cause dissolution of calcium carbonate and the pH may be too low to give optimum performance for chemicals that are added later or at the same time as the acid, for example dithionite, different retention and sizing systems, etc. Due to these conflicting needs of the process, a non-optimised pH for brightness is typically used.
  • the circulating process water systems of present day pulp and paper mills are closed to a more and more increased level in order to lower the amount of water discharged from the pulping and the papermaking processes.
  • Various impurities enter and accumulate in the water circulations in different phases of the process.
  • Organic impurities dissolve from the fiber raw material and inorganic and organic chemicals are added to the process in different production steps.
  • patent application WO 99/00545 describes a method of reducing the content of calcium in a pulp suspension by alkalizing a suspension of chemical paper pulp to a pH above 8 and adding carbonate ions to the pulp suspension for formation of calcium compounds which are adsorbed on the pulp. Calcium is transferred from the liquid phase to the pulp fibers.
  • the pulp is subjected first to a bleaching sequence at a pH value under 6 and thereafter the pulp is subjected to a bleaching sequence at a pH value above 8, especially a pH of at least 10.
  • carbonate ions are added to the pulp suspension for formation of calcium compounds, which are adsorbed on the pulp.
  • calcium hardness of recycled paper-processing water is controlled by precipitating calcium ion in process water into calcium carbonate through addition of sodium carbonate or sodium hydrogen carbonate to white water short-circulated in a papermaking process and removing the calcium carbonate precipitate from the water.
  • Air or carbon dioxide may be used for removal of the precipitate by flotation.
  • US 3,639,206 describes removal of ligneous colour bodies and suspended fibrous material, whereby waste water effluent is treated first with a calcium salt (CaO, Ca(OH) 2 ). The separated aqueous phase is then contacted with CO 2 gas to precipitate any dissolved calcium salts and a final separation is performed.
  • a calcium salt CaO, Ca(OH) 2
  • US 5,679,220 discloses a method enhancing the deposition and retention of particulate filler on papermaking fibers wherein separate slurries of fibers and Ca(OH) 2 or equivalent thereof are combined with gaseous precipitant (CO 2 ) to form in situ calcium carbonate on the fibers.
  • gaseous precipitant CO 2
  • carbon dioxide may be used for increasing the solubility of calcium carbonate and for the hardening of recycled waters used in the treatment of pulp from waste paper.
  • precipitation of gypsum in an alkaline recycle or broke derived pulp may be prevented by adding carbon dioxide to form bicarbonate ions in the alkaline pulp and to precipitate calcium as carbonate.
  • the pH of a pulp suspension in the stock preparation of a paper machine is stabilized by carbon dioxide.
  • a combination of sodium hydroxide and carbon dioxide provide a significant buffering effect in the pulp.
  • the stabilized pH reduces the dissolution of calcium carbonate at acidic addition points.
  • an apparatus located outside the main process stream is used for combining sodium hydroxide and carbon dioxide at a desired ratio.
  • the resulting non-stochiometric sodium bicarbonate solution has a desired pH value and a desired buffering ability and it can be used for stabilizing the pH of the process and for reducing the calcium hardness of the process waters.
  • An object of the present invention is to reduce the problems of the prior art and to provide a process which does not impair the brightness of a mechanical pulp suspension.
  • Another object of the present invention is to reduce the problems caused by calcium ions in a mechanical pulp suspension.
  • a further object of the present invention is to provide a process for decreasing the dissolution of calcium carbonate in a process for producing paper from mechanical pulp.
  • a further object of the invention is to provide a technically advantageous process for maintaining the brightness of a mechanical pulp suspension after dilution with process water.
  • An object of the present invention is furthermore to provide a process for reducing the decrease of the brightness of a mechanical pulp suspension during storage.
  • An object is also to provide a process for reducing the amount of peroxide bleaching agent needed to reach a target brightness in a peroxide bleached mechanical pulp.
  • the present invention is based on the realization that subjecting a pulp suspension comprising mechanical pulp to an acid-base treatment reduces the risk for brightness loss in the pulp suspension while avoiding excess accumulation of calcium ions in the circulation.
  • an acid-base treatment of a diluted pulp provides both an improvement in the brightness and a reduction in the calcium ion level in the system.
  • the present invention relates to a process for making paper, wherein a pulp suspension, which comprises mechanical pulp, is thickened in a dewatering device and is thereafter diluted with water and pumped to a storage tank, then diluted again and made into paper.
  • the invention is characterized by subjecting said pulp suspension to an acid-base treatment, wherein the pH of said suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6 and then the pH of said suspension is increased with a weak base by 0.5-3 units to a pH below 8, and wherein said acid is added in connection with said dilution after said dewatering device and said weak base is added before said storage tank or in connection with the first dilution after said storage tank.
  • the pulp suspension which is to be treated according to the invention is a suspension which contains or consists of mechanical pulp in water.
  • the mechanical pulps in question may be bleached or unbleached and they may be selected from stone groundwood, pressurized groundwood, refiner mechanical pulp, thermomechanical pulp, chemithermomechanical pulp, chemimechanical pulp and combinations thereof.
  • the pulp suspension consists of bleached mechanical pulp.
  • the pulp comprising the mechanical pulp used in the present invention has typically been subjected to at least one washing step and one or more subsequent thickening steps in dewatering device(s).
  • the thickened pulp is diluted with water which may be fresh water or process water. Dilution with re-circulated process water increases the amount of organic and inorganic impurities into the pulp suspension, which potentially harms the pulp brightness.
  • the present invention counteracts the harm caused by such impurities.
  • the present invention also relates to the use of an acid-base treatment in a process for making paper form a pulp suspension comprising mechanical pulp.
  • the pulp suspension which has been thickened in a dewatering device, is subjected to an acid-base treatment, wherein the pH of said suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6 and then the pH of said suspension is increased with a weak base by 0.5-3 units to a pH below 8, and wherein said acid is added in connection with a dilution after said dewatering device and said weak base is added before a subsequent storage tank or in connection with the first dilution after said storage tank.
  • the acid-base treatment of the present invention can be utilized in many ways, e.g for adjusting the brightness of the pulp, for reducing the amount of bleaching agent, for reducing the dissolution of calcium carbonate and diminishing the amount of detrimental substances in the process waters.
  • Fig. 1 shows a schematic diagram of an acid/base treatment in a mechanical pulp bleaching process
  • the present invention relates especially to pulps comprising mechanical pulps, and it aims at solving the problem posed by the conflicting needs of providing a low enough pH for obtaining an acceptable brightness, on one hand, and a high enough pH for allowing use of calcium carbonate and other substances, on the other hand.
  • the pH is first lowered with a strong acid and then raised with a weak base, optimized conditions for both brightness and dissolution can be provided.
  • the time during which the pH is at its lowest value is very brief and in any case short enough to avoid excess dissolution of any calcium carbonate contained in the suspension.
  • the weak base is added no more than one minute after the addition of the strong acid.
  • the acid-base treatment of the present invention is located in the process at the stage wherein the mechanical pulp has been produced and optionally bleached and then washed in one or more washing stages.
  • the washed pulp is then thickened in a dewatering device.
  • the thickened pulp is diluted with re-circulated process water in order to facilitate its pumping forward in the process.
  • the process water is typically selected from white water, clear filtrate, bleaching filtrate and mixtures of any of these. Fresh water may also be used as such or added to any of the said process waters.
  • the acid according to the invention is added to the pulp in connection with this dilution.
  • the acid that is used according to the invention is a strong acid. It is typically selected from the group consisting of sulphuric acid, sulphurous acid , hydrochloric acid and bisulphite.
  • the preferred acid is sulphurous acid that is added in the form of SO 2 water. SO 2 in water forms sulphurous acid H 2 SO 3 , which dissociates into H + and HSO 3 - . Thus SO 2 also inherently forms the bisulphite ion. At pH 4, the acid is almost exclusively in the bisulphite form. When bisulphite is added as such, it is typically added as sodium bisulphite. It has been found that adding a weak acid such as carbon dioxide will not provide the desired reduction in brightness loss of the mechanical pulp.
  • Addition of the strong acid in connection with the dilution facilitates uniform mixing of the strong acid throughout the suspension. A uniform distribution is desired in order to avoid local areas of too low or too high pH.
  • the amount of strong acid depends on the desired pH reduction, which in turn depends on the process pH as a whole.
  • the pH of the pulp suspension which is to be subjected to the acid-base treatment, may vary between about pH 10 and pH 4.5, depending on the treatments that the pulp has been subjected to before the acid-base treatment.
  • peroxide bleaching is done at a high pH (pH 10-11) and the pulp may have a pH of about 10 to 9 after the peroxide bleach.
  • the bleached pulp is generally washed and dewatered after the bleach and prior to the acid-base treatment, and the wash water may lower the pH to a level such as 9 to 7 depending on the pH used in the papermaking.
  • a non-bleached mechanical pulp will typically have a pH of 4.5 to 5.5 and the pH may again vary due to washing and dilution with process water having a different pH.
  • the pH of the circulating process water used for dilution will typically have a pH of 4.5 to 5.5, while the waters of a neutral papermaking process typically have pH values between 6.8 and 8.5.
  • the pH of the pulp will initially be low and a lowering of the pH by as little as 0.5 pH units may be sufficient for providing the desired results.
  • the pH will initially be higher and lowering the pH to a desired value between 4 and 6 will require a substantially larger amount of acid.
  • the person skilled in the art is able to adjust the amount of acid according to the desired pH.
  • the strong acid should preferably be diluted before it contacts the pulp suspension to avoid locally very low pH areas.
  • concentration of the solution should preferably be below 2 % and even more preferably below 1 %.
  • SO 2 has been used in the prior art to kill off peroxide after a peroxide bleaching sequence.
  • SO 2 is used in the present invention after a peroxide bleaching stage, it is added in excess of the amount needed only for killing residual peroxide, since it is added also for lowering the pH to provide the acidic shock of the present invention. Lowering the pH improves the brightness of the pulp and the pH is typically lowered to the lowest acceptable level taking into consideration the limits set by a subsequent neutral papermaking process.
  • a weak base is added according to the invention.
  • the base that is used is preferably an alkali metal bicarbonate or carbonate or a mixture thereof.
  • the base is added as a liquid which can be made from NaHCO 3 powder, from Na 2 CO 3 powder or from a combination of CO 2 and NaOH.
  • a buffering effect is provided by addition of sodium bicarbonate which is generated from sodium hydroxide and carbon dioxide.
  • the bicarbonate may also be generated in situ by sequential addition of NaOH and CO 2 to the pulp suspension.
  • the addition points of the two components should be close to each to prevent the pH of either component from significantly affecting the properties of the pulp.
  • the components are typically added before and after a pump.
  • the pH of the pulp suspension may be increased by a non-stoichiometric sodium bicarbonate solution having a pH higher or lower than that of stoichiometric bicarbonate.
  • the bicarbonate solution may be produced on site as described in EP-B1 991 811 and in US-B2 7,175,759 .
  • the concentration of the bicarbonate solution is typically between 2 and 8 % and its pH is typically 8 to 9, preferably 8.3-8.5.
  • the optimum dwell time of the low pH phase of the treatment depends on the process conditions and on the load of impurities in the pulp suspension and in the re-circulated process waters. In case there is a significant amount of solid calcium carbonate circulating with the diluting water, it is important that the dwell time is short, preferably no more than a few minutes, most preferably no more than a minute. A significant amount of solid calcium carbonate is typically present in white water circulated from the paper machine of a process, which utilizes calcium carbonate as a filler. The pH of such process water is typically between 6.8 and 8.3.
  • the weak base should be added before the pulp is sent to storage such as to a storage tank or tower.
  • a short dwell time is typically provided by adding SO 2 water at the suction side of a pump and bicarbonate solution at the pressure side of the same pump.
  • Fresh water, clear filtrate and bleaching filtrate contain less solid calcium carbonate than white water, and a pulp diluted with such process waters can endure longer dwell times to be used. In such cases, the acidic phase may even last several hours. Storage under acidic conditions is good for the mechanical pulp since a low pH causes little loss of brightness.
  • the weak base may be added after the storage tank or tower. In such cases, it is often desirable to add the strong acid to the diluting process water itself before using it for the dilution. This facilitates a uniform distribution of the acid into the pulp suspension. Adding the acid to calcium carbonate rich process waters should be avoided because of the risk of dissolution of the solid calcium carbonate.
  • the pH of the acid treated pulp must be raised.
  • the pH is raised simply as in the prior art by diluting the stored pulp with process waters at a pH close to neutral, solid calcium carbonate in the dilution waters will be dissolved and the accumulation of calcium ions in the re-circulating systems will be detrimental to the process.
  • a weak base such as an alkali metal (bi)carbonate
  • the pH may be raised without detrimental effect on the brightness and without causing unacceptable dissolution of calcium carbonate.
  • the acid-base treatment is performed after the peroxide bleaching.
  • this bleaching typically takes place in the storage tank or tower and in such a case, the weak base is added before the storage tank in order to raise the pH to suit the dithionite bleaching.
  • the pH of the pulp suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6, typically to a pH between 5 and 6 in a neutral process, and then the pH of said suspension is increased with a weak base by 0.5-3 units to a pH which is below 8 and typically is between 6 and 7.5 in a neutral process.
  • acidic paper making the pH is typically reduced with the acid to a value between 4 and 5 and then increased to a value between 5 and 6.
  • the pH reduction with the strong acid precipitates impurities in the pulp suspension and the subsequent pH increase with the weak base allows the precipitated impurities to remain substantially undissolved in the suspension while raising the pH to suit the rest of the process.
  • the acid-base treatment works as a "fixative" by decreasing the amount of organic substances, such as extractives, and metals in the process waters.
  • the impurities precipitate on the pulp fibres with the pH reduction caused by the strong acid and remain substantially precipitated when the pH is raised with the weak base. The precipitations leave the system with the pulp and the circulation waters become cleaner.
  • the strong acid comprises sulphurous acid and the weak base comprises sodium bicarbonate.
  • the sulphurous acid is preferably provided by sulphur dioxide gas and the sodium bicarbonate is preferably a non-stoichiometric sodium bicarbonate solution provided by combining sodium hydroxide, carbon dioxide and water in a reactor.
  • sulphur dioxide in water also provides bisulphite HCO 3 - ions.
  • sulphurous acid and sodium bicarbonate provide bisulphite by the reaction: H 2 SO 3 + NaHCO 3 ⁇ NaHSO 3 + H 2 CO 3 .
  • Bisulphite as such is known to have a bleaching effect on pulp suspensions. It is believed that a part of the increased brightness provided by the acid -base treatment of the present invention is due to the formation of bisulphite and similar active ions.
  • Sodium bisulphite may also be formed by a reaction between sulphur dioxide water and sodium hydroxide present in the circulating process waters.
  • dewatering device means a device wherein the consistency of the pulp suspension is increased by removal of water from the suspension.
  • Typical dewatering devices useful in the process are filters such as disc filters, washing presses, Sundblad filters, press filters, etc.
  • the term "storage tank” means a vessel in which the pulp suspension is stored for a short or long period.
  • the storage tank need not be any huge vessel or tower. In some cases a long pipe performs the role of a storage tank and may be equated as one.
  • pulp suspension refers to the aqueous product stream passing though a paper and/or pulp mill.
  • the suspension contains pulp fibres at a consistency which is typically between about 1 and 30 %.
  • the acid-base treatment is typically performed on a pulp suspension having a consistency of 3 to 15 %, preferably 4 to 10%.
  • dilution used in the specification and claims refers to the addition of process waters or fresh water to the pulp suspension to decrease its consistency.
  • the dilution may be performed in one or several steps, for example in a dilution ring.
  • connection with the dilution means addition of the acid or base, respectively, either to the dilution water before the dilution water is added to the pulp, addition of the acid or base simultaneously with the addition of dilution water or addition of the acid or base just before or just after the pump closest after the dilution point.
  • connection with relating to other unit operations means in close relation with, before, after or simultaneously with said unit operation.
  • immediately after the first storage tank means within one minute of the dilution of the pulp with process water after the storage tank.
  • the terms “significant” and “low” used for the content or amount of calcium carbonate in a liquid refers more to the amount present in functional terms more than as an absolute value.
  • the amount of calcium carbonate in a dilution water is significant if it causes variations or fluctuations in the pH of the suspension which it dilutes and/or if it causes a substantial buffering ability in the suspension.
  • the amount of calcium carbonate in a dilution water is considered low, if it does not influence the pH or buffering ability of the resulting suspension.
  • the amount of calcium carbonate in dilution water is typically calculated based on the amount of solids per liter.
  • the solids include, besides filler, also other solids such as fibers. Dilution water having less than 50 mg solids per liter is generally considered as having a low content of calcium carbonate. However, if such water is used in huge amounts, it is evident that the absolute amount of calcium may be significant in the unit operation or suspension in question.
  • the mechanical pulp of the process according to the invention is preferably bleached in the process.
  • the bleaching is typically performed with a bleaching agent selected from the group consisting of hydrogen peroxide, sodium dithionite and combinations thereof.
  • One embodiment of this invention relates to peroxide bleached mechanical pulps that are used in neutral papermaking.
  • the washing steps include dewatering devices such as disc filters or washing presses.
  • the dewatered thickened pulp is diluted with process water to a consistency between 5 and 15% and pumped to a storage tank.
  • the strong acid is added in connection with the dilution after the last washing step.
  • the pH in this embodiment is decreased to at least a pH of 6 and preferably to a pH between 5.3 and 5.8.
  • the weak base is added either before the storage tank or immediately after the tank. When a short acid dwell time is desired, it has been found advantageous to add the acid on the suction side of a pump and the base on the pressure side of the pump. After the addition of base the pH should be at least 5.7 and preferably at a pH between 6 and 7.
  • One further embodiment of the present invention concerns peroxide bleached mechanical pulps that are used for paper production in acidic papermaking. After the peroxide bleaching the pulps are processed as discussed above for peroxide bleached pulps used in neutral papermaking. In acidic papermaking, however, the suitable pH levels are different. After the addition of acid the pH should have decreased to no higher than pH 5 and preferably between 4 and 5. The base is added before the storage tank to increase the pH to a minimum pH of 4.5, preferably between 5.0-5.5.
  • One embodiment of the invention concerns unbleached mechanical pulp, where process waters with a neutral pH, meaning pH 7-8, are used to dilute the unbleached mechanical pulp. Because of the pH of the process waters, the pH of the unbleached mechanical pulp increases and causes brightness loss. By utilizing the acid base treatment according to the invention, the brightness loss can be minimized without other unwanted effects, such as those encountered in the prior art.
  • the acid is added with the dilution water after the last washing step before the storage tank for the unbleached pulp or in connection to the pump between the washing step and before the storage tank.
  • the pH in the pulp suspension is decreased by a minimum of 0.5 pH units, preferably by 1 to 3 pH units.
  • the weak base is added either before the storage tank or immediately after the storage tank. After the addition of the weak base, the pH of the pulp suspension has been increased by a minimum of 0.5 pH units and preferably by 1 to 3 pH units to a minimum pH of 5.8.
  • the present invention operates in the following manner: the acid-base treatment of the present invention gives an acidic shock to the pulp suspension. That acidic shock precipitates some of the impurities of the process waters, such as colloidal and dissolved organic substances and metals onto the fibres. Due to this precipitation, the process waters have a lower amount of substances, for example metals, that are detrimental for the pulp brightness. The acidic shock also decreases the dissolution of organic substances from the mechanical pulp fibres. This is especially apparent in the case when the base is added after the storage tank.
  • Raising the pH after the acidic shock with a weak base provides mild enough conditions to keep the redissolution of the precipitated substances at a minimum while providing conditions in which soluble substances such as calcium carbonate will remain in solid form.
  • the counter ion effect of the (bi)carbonate base is believed also to play a role in keeping calcium carbonate from dissolving.
  • the use of a weak base which also works as a buffer. This is especially true for the preferred sodium bicarbonate, which increases the alkalinity and , counters unwanted effects that otherwise would be caused by the low pH in the acidic shock.
  • the weak base also raises the pH after the acidic chock to a pH value suitable for the subsequent papermaking.
  • the treatment may be used for adjusting the brightness and/or for reducing the brightness loss of a pulp suspension comprising mechanical pulp.
  • the acid-base treatment may also be used for reducing the amount of peroxide needed to reach a target brightness in a mechanical pulp.
  • the amount of bleaching chemicals could be reduced by as much as 20 to 30 %.
  • the acid-base treatment can also be used for controlling the dissolution of calcium carbonate in a system for making paper from mechanical pulp with calcium carbonate as filler.
  • the treatment according to the invention is also effective in decreasing the amount of detrimental substances in the process waters of a papermaking process.
  • detrimental substances include organic substances measured as COD, lignin compounds, calcium ions, metals and mixtures thereof.
  • a specific embodiment of the invention relates to a process for producing paper from bleached mechanical pulp, which includes the steps of
  • a paper mill was producing high brightness paper made from mechanical pulp with calcium carbonate as filler in a neutral process with pH 7.2 in the head box.
  • the mechanical pulp was peroxide bleached and directly after the peroxide bleaching it was washed to clean the pulp.
  • the cleaned pulp was dewatered in a wash press and then diluted with process water from a tank containing varying mixtures of clear filtrate and white water.
  • the pH was adjusted with SO 2 water to pH 6.4.
  • the diluted pulp was then pumped to a storage tower where the storage time was 2-4 hours. After the storage tower the pulp was further diluted with process water.
  • This dilution water comprised white water having a pH of 7.8 and a fairly high amount of solid calcium carbonate.
  • Calcium carbonate filler was added to the diluted pulp and the pulp was pumped to the mixing chest and then through a series of steps on to the paper machine where paper was made.
  • the problems associated with the dissolution of calcium carbonate could be held at an acceptable level.
  • the paper mill in example 1 changed the treatment of the pulp after the peroxide bleaching to try to decrease the needed amount of peroxide. After the washing step the cleaned pulp was diluted with process water and the pH was adjusted with SO 2 water to 5.4. The pulp brightness increased through these measures, but huge amounts of calcium carbonate dissolved and the calcium levels became intolerably high in the process.
  • the paper mill in example 1 changed the treatment of the pulp after the peroxide bleaching to try to decrease the needed amount of peroxide.
  • the cleaned pulp was diluted with process water and the pH was adjusted with SO 2 to pH 5.4 and then after ca 3 minutes the pH was again increased to pH 6.4 with NaHCO 3 produced on site by combining NaOH and CO 2 .
  • the amount of peroxide used in the peroxide bleaching was decreased with 30%, the same brightness of the pulp as in example 1 was obtained. No higher amounts of calcium carbonate than in example 1 were dissolved and the process operated to satisfaction.
  • the paper mill in example 3 changed the addition point of the base addition to immediately after the storage tower. Also by using this addition point good results were obtained. Compared to example 3 the brightness loss was smaller but the dissolution of calcium carbonate was slightly higher.
  • a paper mill was producing paper from mechanical pulp with kaolin as filler with an acidic process with pH 5 in the head box.
  • the pulp was peroxide bleached and directly after the peroxide bleaching it was washed to clean the pulp.
  • the cleaned pulp was dewatered and then diluted with process water from a tank containing varying mixtures of clear filtrate and white water and the pH was decreased with SO 2 water to pH 5.
  • dithionite was added for further bleaching and the pulp was pumped to a storage tower where the storage time was 2-4 hours.
  • the optimal pH for dithionite pH in this mill was pH 5.
  • the pulp was further diluted with process water and pumped to the mixing chest and then through a series of steps on to the paper machine where paper was made. To get the final wanted brightness of the pulp large amounts of peroxide had to be added in the bleaching stage.
  • the paper mill in example 5 changed the treatment after the peroxide bleaching to try to decrease the amount of peroxide needed by using an acid-base treatment using an acidic shock with sulphurous acid followed by an increase in pH with NaHCO 3 .
  • the cleaned pulp was dewatered and then diluted with process water and the pH was decreased with SO 2 to pH 4. Shortly thereafter, within about 2 minutes, the pH was increased again with NaHCO 3 to pH 5. After that dithionite was added for further bleaching and the pulp was pumped to the storage tower.
  • the amounts of peroxide needed could be decreased with 20% while keeping the same brightness of the pulp as in example 5.
  • a paper mill was producing paper from mechanical pulp according to the schematic diagram of Fig. 1 .
  • the mill used calcium carbonate as filler in a neutral papermaking process having pH 7 to 8 in the head box.
  • the mechanical pulp 1 was peroxide bleached in bleach tower 2 and directly after the peroxide bleaching it was washed in washer 4 to clean the pulp. Washing water 5 was recirculated up-stream in the process. After the washing step the cleaned pulp was dewatered in a thickener 6, from which the pulp was fed forward in the process and the removed water 7 was recirculated up-stream. The dewatered pulp was diluted with process water 8 comprising a mixture of clear filtrate and white water to a consistency of 7-8.
  • the pH of the diluted pulp was decreased to about 6 by feeding SO 2 water 9 on the suction side of a pump 11.
  • the SO 2 water had a concentration of 0.9 % and the resulting pH was 5.7 to 5.8.
  • Dithionite 10 was added in the pump 11 to provide additional bleaching of the pulp.
  • a non-stoichiometric sodium bicarbonate solution having a pH of 8.5 and a concentration of 4 % was added to the pulp flow to raise the pH to 6.4.
  • the pulp was then fed to storage tower 13 where the storage time was about 2 to.6 hours. After the storage tower the pulp was further diluted with process water and bicarbonate solution to bring the consistency and pH to the desired value for the subsequent neutral papermaking.
  • the resulting pulp had an acceptable brightness and the level of calcium ions in the system was kept at an acceptable level.
  • the process produced paper of good quality with reduced amounts of bleaching agents compared to a similar process without the acid-base treatment.
  • the present invention provides advantageous solutions to the production of paper from mechanical pulp. Based on the above description, the skilled person is able to adjust the conditions and to utilize the invention in manner best adapted to his/her specific needs.

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  • Inorganic Chemistry (AREA)
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  • Wood Science & Technology (AREA)
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Claims (30)

  1. Procédé de fabrication de papier, selon lequel une suspension de pâte qui comprend une pâte mécanique est épaissie dans un dispositif de déshydratation, puis est diluée avec de l'eau et pompée dans un réservoir de stockage, puis de nouveau diluée et transformée en papier, caractérisé par la soumission de ladite suspension de pâte à un traitement acide-base, selon lequel le pH de ladite suspension est tout d'abord réduit avec un acide fort de 0,5 à 4 unités de pH à un pH compris entre 4 et 6, puis le pH de ladite suspension est augmenté avec une base faible de 0,5 à 3 unités à un pH inférieur à 8, et selon lequel ledit acide est ajouté en connexion avec ladite dilution après ledit dispositif de déshydratation et ladite base faible est ajoutée avant ledit réservoir de stockage ou en connexion avec la première dilution après ledit réservoir de stockage.
  2. Procédé selon la revendication 1, dans lequel ledit acide fort est choisi dans le groupe constitué par l'acide sulfurique, l'acide sulfureux, l'acide chlorhydrique et le bisulfite.
  3. Procédé selon la revendication 1, dans lequel ladite base faible est choisie dans le groupe constitué par un bicarbonate de métal alcalin, un carbonate de métal alcalin et leurs mélanges.
  4. Procédé selon la revendication 3, dans lequel ladite base faible comprend du bicarbonate de sodium et/ou du carbonate de sodium produit par combinaison d'hydroxyde de sodium et de dioxyde de carbone.
  5. Procédé selon la revendication 1, dans lequel ledit acide fort comprend de l'acide sulfureux et ladite base faible comprend du bicarbonate de sodium.
  6. Procédé selon la revendication 5, dans lequel ledit acide sulfureux et/ou bisulfite est fourni par du dioxyde de soufre gazeux dans de l'eau et ledit bicarbonate de sodium est un bicarbonate de sodium non stoechiométrique fourni par combinaison d'hydroxyde de sodium et de dioxyde de carbone dans un réacteur.
  7. Procédé selon la revendication 1, dans lequel le pH de ladite suspension de pâte est réduit avec ledit acide fort à un pH compris entre 5 et 6.
  8. Procédé selon la revendication 1, dans lequel le pH de ladite suspension de pâte est augmenté avec ladite base faible à un pH compris entre 6 et 7,5.
  9. Procédé selon la revendication 1, dans lequel ladite réduction de pH avec ledit acide fort précipite des impuretés dans ladite suspension de pâte sur ladite pâte et ladite augmentation de pH avec ladite base faible permet auxdites impuretés précipitées de rester essentiellement non dissoutes.
  10. Procédé selon la revendication 1, dans lequel de l'eau de procédé est recyclée dans ledit procédé et dans lequel au moins une desdites dilutions est réalisée avec de l'eau de procédé recyclée.
  11. Procédé selon la revendication 10, dans lequel ladite eau de procédé comprend de l'eau blanche, un filtrat transparent, un filtrat de blanchiment ou leurs mélanges.
  12. Procédé selon la revendication 11, dans lequel ladite eau de procédé contient du carbonate de calcium solide.
  13. Procédé selon la revendication 12, dans lequel le pH de ladite eau de procédé est compris entre 6,8 et 8,3.
  14. Procédé selon la revendication 13, dans lequel de l'eau de procédé ayant une teneur élevée en carbonate de calcium solide est utilisée pour ladite dilution après ledit dispositif de déshydratation et ladite base faible est ajoutée avant ledit réservoir de stockage.
  15. Procédé selon la revendication 14, dans lequel ladite base faible n'est pas ajoutée plus de 1 minute après ledit acide fort.
  16. Procédé selon la revendication 13, dans lequel de l'eau de procédé ayant une teneur faible en carbonate de calcium solide est utilisée pour ladite dilution après ledit dispositif de déshydratation, ledit acide fort est ajouté en connexion avec ladite dilution et ladite base faible est ajoutée après ledit réservoir de stockage.
  17. Procédé selon la revendication 16, dans lequel ledit acide fort est ajouté à ladite eau de procédé.
  18. Procédé selon la revendication 1, dans lequel ladite pâte mécanique est blanchie ou non blanchie et est choisie parmi la pâte mécanique ordinaire, la pâte mécanique sous pression, la pâte mécanique de raffineur, la pâte thermomécanique, la pâte chimico-thermomécanique, la pâte chimico-mécanique et leurs combinaisons.
  19. Procédé selon la revendication 1, dans lequel ladite pâte mécanique est blanchie avec un agent de blanchiment choisi dans le groupe constitué par le peroxyde d'hydrogène, le dithionite de sodium et leurs combinaisons.
  20. Procédé selon la revendication 19, dans lequel le degré de blancheur cible de ladite pâte blanchie au peroxyde et/ou au dithionite est obtenu avec une plus petite quantité du ou des agents de blanchiment en comparaison d'un procédé semblable sans ledit
    traitement acide-base.
  21. Procédé selon la revendication 1, dans lequel ladite suspension de pâte est blanchie avec du peroxyde d'hydrogène et ledit traitement acide-base est réalisé après ledit blanchiment au peroxyde.
  22. Procédé selon la revendication 1, dans lequel ladite suspension de pâte est blanchie avec du dithionite et ledit traitement acide-base est réalisé en connexion avec ledit blanchiment au dithionite.
  23. Procédé selon la revendication 1, pour la fabrication de papier à partir d'une pâte mécanique blanchie, caractérisé par
    a. la préparation d'une suspension de pâte mécanique,
    b. le blanchiment de ladite suspension de pâte avec du peroxyde d'hydrogène,
    c. l'épaississement de ladite suspension de pâte dans un dispositif de déshydratation,
    d. après ledit dispositif de déshydratation, la dilution de ladite suspension de pâte blanchie avec une eau de procédé comprenant de l'eau blanche à un pH de 6,8 à 8,3,
    e. en connexion avec ladite dilution, la réduction du pH de ladite suspension de pâte à un pH compris entre 5,0 et 6,0 avec du SO2 dans de l'eau,
    f. le stockage de ladite suspension de pâte dans une tour de stockage audit pH réduit,
    g. après le stockage, la dilution de ladite suspension de pâte avec une eau de procédé comprenant de l'eau blanche,
    h. en connexion avec ladite dilution, l'ajout d'une solution de bicarbonate non stoechiométrique à ladite suspension pour obtenir un pH compris entre 5,5 et 6,8,
    i. l'ajout d'une charge de carbonate de calcium à ladite suspension,
    j. le pompage de ladite suspension de pâte par une série d'étapes de fabrication de papier dans une machine à papier, et
    k. la fabrication de papier.
  24. Utilisation d'un traitement acide-base dans un procédé de fabrication de papier à partir d'une suspension de pâte comprenant une pâte mécanique, dans laquelle une suspension de pâte épaissie dans un dispositif de déshydratation est soumise à un traitement acide base et dans laquelle le pH de ladite suspension est tout d'abord réduit avec un acide fort de 0,5 à 4 unités de pH à un pH compris entre 4 et 6, puis le pH de ladite suspension est augmenté avec une base faible de 0,5 à 3 unités à un pH inférieur à 8, et dans laquelle ledit acide est ajouté en connexion avec une dilution après ledit dispositif de déshydratation et ladite base faible est ajoutée avant un réservoir de stockage ultérieur ou en connexion avec la première dilution après ledit réservoir de stockage.
  25. Utilisation selon la revendication 24 pour l'ajustement du degré de blancheur d'une suspension de pâte comprenant une pâte mécanique.
  26. Utilisation selon la revendication 24 pour la réduction de la perte de degré de blancheur d'une suspension de pâte comprenant une pâte mécanique.
  27. Utilisation selon la revendication 24 pour la réduction de la quantité de peroxyde nécessaire pour atteindre un degré de blancheur cible dans une pâte comprenant une pâte mécanique blanchie.
  28. Utilisation selon la revendication 24 pour le contrôle de la dissolution de carbonate de calcium dans un système de fabrication de papier à partir d'une suspension de pâte comprenant une pâte mécanique.
  29. Utilisation selon la revendication 24 pour la réduction de la quantité de substances nocives dans les eaux de procédé d'un procédé de fabrication de papier.
  30. Utilisation selon la revendication 29, dans laquelle lesdites substances nocives sont choisies parmi les substances organiques et inorganiques colloïdales et dissoutes, y compris les substances organiques mesurées en tant que DCO, les composés de lignine colloïdaux et dissous, les ions calcium, les métaux et leurs mélanges.
EP08760061A 2007-07-03 2008-05-27 Procédé de fabrication de papier et utilisation d'un traitement acide/base dans sa fabrication Active EP2176461B1 (fr)

Applications Claiming Priority (2)

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FI20075503A FI20075503L (fi) 2007-07-03 2007-07-03 Menetelmä paperin valmistamiseksi ja happo/emäskäsittelyn käyttö paperin valmistuksessa
PCT/EP2008/056463 WO2009003770A1 (fr) 2007-07-03 2008-05-27 Procédé de fabrication de papier et utilisation d'un traitement acide/base dans la fabrication de papier

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EP2176461B1 true EP2176461B1 (fr) 2011-07-06

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CN103392039A (zh) 2011-01-25 2013-11-13 巴斯夫欧洲公司 生产用于制造纸制品的半浆的方法
US20120186766A1 (en) * 2011-01-25 2012-07-26 Basf Se Manufacturing a papery article
US20120186762A1 (en) * 2011-01-25 2012-07-26 Basf Se Producing half-stuffs for the manufacture of papery articles
EP2668333B1 (fr) 2011-01-25 2014-10-29 Basf Se Procédé de fabrication d'un produit de type papier

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3639206A (en) * 1969-07-29 1972-02-01 Continental Can Co Treatment of waste water from alkaline pulping processes
DE2752413C2 (de) * 1977-11-24 1979-03-29 J.M. Voith Gmbh, 7920 Heidenheim Verfahren zur Aufhärtung des Wassers bei der Altpapieraufbereitung und Vorrichtung zur Durchführung des Verfahrens
EP0296198B1 (fr) * 1986-12-22 1991-09-18 Aga Aktiebolag Procede de lavage des pates alcalines
SE461860B (sv) * 1989-02-13 1990-04-02 Mo Och Domsjoe Ab Foerfarande foer framstaellning av papper och bestruket papper varvid baspappret innehaaller precipiterat kalciumkarbonat
US5679220A (en) * 1995-01-19 1997-10-21 International Paper Company Process for enhanced deposition and retention of particulate filler on papermaking fibers
AU4271596A (en) * 1995-12-19 1997-07-14 Kvaerner Hymac Inc. Process for treating refiner pulp
FI102911B1 (fi) * 1997-06-13 1999-03-15 Aga Ab Menetelmä massasuspension pH:n stabiloimiseksi ja paperin tuottamiseksi stabiloidusta massasta
SE511794C2 (sv) * 1997-06-30 1999-11-29 Sunds Defibrator Ind Ab Sätt att reducera halten kalcium i vätskefasen i en pappersmassasuspension
FI103520B (fi) * 1998-03-03 1999-07-15 Upm Kymmene Corp Parannetut paperinvalmistusmenetelmät
KR100319491B1 (ko) * 1998-12-08 2002-04-22 김충섭 재활용되는제지공정수의칼슘경도조절방법

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FI20075503A0 (fi) 2007-07-03
ATE515593T1 (de) 2011-07-15
CA2691590A1 (fr) 2009-01-08
FI20075503L (fi) 2008-06-19
EP2176461A1 (fr) 2010-04-21

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