Classifications

• • 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
  • D21H17/20—Macromolecular organic compounds
  • D21H17/33—Synthetic macromolecular compounds
  • D21H17/34—Synthetic macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds
• • 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
• • 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
  • D21H21/00—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties
  • D21H21/14—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties characterised by function or properties in or on the paper
• • 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
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/04—Addition to the pulp; After-treatment of added substances in the pulp
• • 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
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/04—Addition to the pulp; After-treatment of added substances in the pulp
  • D21H23/06—Controlling the addition
• • 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
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/04—Addition to the pulp; After-treatment of added substances in the pulp
  • D21H23/06—Controlling the addition
  • D21H23/08—Controlling the addition by measuring pulp properties, e.g. zeta potential, pH
  • D21H23/10—Controlling the addition by measuring pulp properties, e.g. zeta potential, pH at least two kinds of compounds being added
• • 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
  • D21H23/78—Controlling or regulating not limited to any particular process or apparatus

Definitions

• the invention describes a process for preparing a buffer solution in an industrial papermaking process, wherein the buffer solution is supplied to a main process stream, which is an aqueous pulp suspension or a water stream leading to and/or from said suspension, comprising
• neutral pH corresponds in these processes to a pH in the short circulation of approximately 7-8.5, most preferably 7- 8. This applies to paper produced from chemical, mechanical and recycled pulp, bleached or unbleached. If the papermaking pulp is acidic when entering the stock preparation and the short circulation is run at a neutral or alkaline pH, the traditional way of raising and controlling the pH is to add sodium hydroxide, NaOH. NaOH is a very strong base, which means that only a small amount is needed for pH adjustment.
• NaHCO 3 dissociates in the pulp suspension forming bicarbonate ions, HCO 3 " , which have a buffering effect and therefore counteract any pH decrease.
• NaHCO 3 is a solid powder, which is generally supplied in so called big- bags, and the paper mill needs space for handling, equipment for dissolving and tanks for storage. The NaHCO 3 is messy to work with, when in contact with moisture or water.
• bicarbonate ions were generated by the dissociation of carbon dioxide, CO 2 , in the aqueous pulp.
• Carbon dioxide is a gas, which easily dissolves under alkaline conditions, e.g. in water or a pulp suspension forming carbonic acid and/or bicarbonate ions according to the reaction:
• EP099181 1 describes a process for stabilising the pH of a pulp suspension in the stock preparation of a paper machine by using carbon dioxide.
• the stabilisation is provided by increasing the buffering ability of the paper making pulp suspension by adding a combination of an alkali earth metal hydroxide feed, preferably a NaOH feed, and a carbon dioxide feed which feeds substantially counter each other's pH changing effects.
• the feeds are provided in an amount sufficient to achieve a significant buffering effect of said pulp suspension.
• the process includes adjusting the pH of the pulp suspension by addition of an excess of said hydroxide or said carbon dioxide. Thus the pH is maintained at a desired level throughout the subsequent paper making process.
• the process as disclosed in EP0991811 has some difficulties especially in the addition of the sodium hydroxide and carbon dioxide.
• a local high pH caused by the addition of the sodium hydroxide may cause precipitation of calcium carbonate.
• a high pH may dissolve extractives, which may then precipitate elsewhere in the process.
• a high pH may also cause yellowing of the pulp. It may also be difficult to add sufficiently carbon dioxide to the process.
• the addition of the basic and acidic substances is usually done separately at special dosing points.
• the use of basic and acidic substances often requires preprocessing, e.g. dissolving and diluting, and special equipment for that before the substances are fed into the process. Thereby it may not even be possible to feed the substances in the needed points of the process, or it may require complex equipment.
• Alkalinity is a measure of the buffering capacity of an aqueous system, i.e. the capacity to prevent pH changes. It can also be seen as a capacity to accept H + or OH " ions.
• the buffering capacity enables the addition of acidic and basic substances to an aqueous system without significant changes in the pH.
• the amounts of bases and acids that can be added without affecting the pH depend on the strength and amount of the base and the acid as well as on the amount of alkalinity. Many industrial processes lack a sufficient alkalinity required for operating properly. Generally alkalinity results from the amounts of hydroxide, carbonate and bicarbonate ions in an aqueous solution according to the equation:
• Alkalinity may also result from ammonia and the conjugate bases of phosphoric, silicic, boric and organic acids. Alkalinity is often expressed in units of mg/1 of CaCO 3 or mmol/1
• the apparatus described in EP 1461499 is located outside the main process stream of the papermaking process.
• the apparatus includes inlet means for a basic substance, an acidic substance and water, individually regulated control means for the inlet means and a reactor for reacting the basic substance, the acidic substance and the water for providing a predefined alkalinity and/or pH in the resulting aqueous buffer solution.
• the apparatus also includes outlet means for supplying the aqueous buffer solution to the main process stream in the papermaking process.
• the basic and acidic substances are capable to form buffering ions.
• a buffer solution is generated in the reactor and fed to the aqueous pulp suspension in the paper making process.
• the alkalinity, pH and/or hardness of the pulp suspension are thereby controlled by the defined alkalinity, pH and/or hardness of the buffer solution.
• the problem is partly solved by the use of a chelating agent which prohibits the reaction of the calcium ion with the carbonate ion and thereby the precipitation.
• the chelating agents are expensive. Additionally the chelating agents are not always degenerated in the following waste water treatment equipment and could end up in the nature. Another possibility to deal with the precipitation is to use softened water from which the calcium ions have been removed. However, this is expensive and not suitable for large industrial processes. Thus, the prior art typically necessitates a periodic cleaning of the equipment with an acid. This is expensive, leads to a shutdown of the process and increases the possibility of accidents.
• the said basic substance comprises an alkali metal hydroxide, an alkali metal carbonate, an alkali metal bicarbonate, an alkali metal phosphate, an alkali metal biphosphate and/or an alkali metal phosphite and/or green liquor or white liquor and the said acidic substance comprises gaseous carbon dioxide or sulphur dioxide or a liquid organic or inorganic acid, preferably selected form sulphurous acid, alum, phosphoric acid, carbonic acid and hydrochloric acid.
• sodium hydroxide as said basic substance and carbon dioxide as said acidic substance are preferred.
• carbon dioxide is dissolved in an aqueous solution, preferably water, with a gaseous pressure which is typically above 10 bar before said basic substance, preferably sodium hydroxide, is introduced in the aqueous solution.
• a gaseous pressure which is typically above 10 bar before said basic substance, preferably sodium hydroxide.
• the dissolution of carbon dioxide in an aqueous solution, preferably water, leads to a lowering of the pH value.
• a controlled addition of sodium hydroxide into the aqueous solution containing dissolved carbon dioxide is a good way for preparing a resulting buffer solution where the pH is below 7.5 all the time.
• the concentration of bicarbonate ions in the resulting buffer solution and hence its alkalinity can be increased by a successive dissolution of carbon dioxide into the buffer solution and subsequent addition of sodium hydroxide.
• the said aqueous solution contains between 10 and 25 grains, preferably between 16 and 20 grams, carbon dioxide per litre before introducing said basic substance.
• the basic sunbstance such as sodium hydroxide is typically fed into the water first to provide an alkaline aqueous solution into which the acidic carbon dixode is introduced. This is because carbon dioxide dissolves much more quickly in an alkaline solution.
• the carbon dioxide is fed into the water before any basic substance has been introduced. This reversed order of addition ensures a low pH at all times and reduces the precipitation problem of the prior art. Since the final buffer is formed in a reactor, the carbon dioxide has sufficient time in which to dissolve even though the pH is low.
• a reciculation of produced buffer solution so that both the basic and acidic additions are made to an aqueous buffer solution having a pH below 7.5.
• the said basic substance preferably sodium hydroxide
• an aqueous solution preferably water
• the resulting basic solution is mixed with a re-circulated buffer solution keeping the pH of the buffer solution below 7.5.
• the said acidic substance preferably carbon dioxide
• the carbon dioxide is added to the recirculating buffer solution before the addition of the basic substance.
• the said re-circulated buffer solution is a recycle stream from a part of the reactor or from downstream of the reactor. In this embodiment the buffering capacity of the buffer solution is used to keep the pH below 7.5 all the time.
• the recirculation flow now buffered by sodium bicarbonate and saturated or over-saturated with carbon dioxide, has a pH below 7.5, preferably between 7.0 and 7.4 depending on the ratio between NaOH and CO 2 , and thus fulfils the invention.
• the possible increase of the pH due to the addition of the basic substance, preferably sodium hydroxide, is compensated by the buffer capacity of the re-circulated buffer solution.
• the pH of the buffer solution is adjusted with an additional portion of the basic substance to a pH above 7.5 after the final addition of the acidic substance.
• a buffer solution with an higher pH could provided to the main process.
• the pH is kept below 7.5 during the buffer production stage and thereby a stable and non-precipitating buffer solution is formed. Afterwards the pH is adjusted to the needs of the main process by the addition of a portion of a basic substance.
• the present invention has one main advantage over the prior art.
• the precipitation of calcium carbonate in the reactor or similar downstream equipment is significantly reduced. Hence there is less need for using expensive chelating agents or periodic cleaning of the equipment.
• the profitability of the papermaking process is increased.
• Figure 1 shows an embodiment of the invention where a re-circulated buffer solution is used.
• the basic substance, sodium hydroxide, 1 and water 2 are fed together in a mixer 3 to get a diluted sodium hydroxide solution.
• the amounts of sodium hydroxide and water are controlled to provide a desired concentration of sodium hydroxide in the solution.
• the acidic substance, carbon dioxide, 4 in the gaseous phase is introduced in the dissolver 5.
• the dissolver 5 the gaseous carbon dioxide is dissolved in a re-circulated buffer solution 6.
• the diluted aqueous solution of sodium hydroxide is fed to the dissolver 5 too.
• the amount of diluted sodium hydroxide is controlled so as to keep the pH below 7.5.
• the carbon dioxide feeding into the buffer solution is adjusted to provide a solution saturated or oversaturated with carbon dioxide.
• the pH of the re- circulated buffer solution is according to the invention below 7.5 at all the time when the basic or acidic substances are mixed together.
• the re-circulated flow of the buffer solution 6 prevents a local increase of the pH above 7.5 due to the introduction of the sodium hydroxide solution. Thereby the precipitation of calcium carbonate is reduced.
• the pH is always below 7.5 when carbon dioxide is introduced.
• the sodium bicarbonate containing buffer solution is created in the reactor 7 where bicarbonate ions are formed according the chemical reaction between sodium hydroxide and carbon dioxide mentioned above.
• the alkalinity and pH of the buffer solution is adjusted to a predetermined value by adjusing the feeds of sodium hydroxide and/ or carbon dioxide. In case a pH above 7.5 is desired in the final buffer solution, the pH may be raised after the final carbon dioxide addition to a pH higher than 7.5 by adding an additional portion of sodium hydroxide.
• the resulting bicarbonate ions containing buffer solution 9 is supplied to the main process stream, the aqueous pulp suspension, for controlling the alkalinity, pH and/or hardness.

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• Chemical & Material Sciences (AREA)
• Inorganic Chemistry (AREA)
• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Paper (AREA)

Priority Applications (1)

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Publications (1)

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Family Applications (1)

Country Status (3)

Citations (5)

• 2008
  • 2008-08-27 FI FI20085797A patent/FI20085797L/fi not_active IP Right Cessation
• 2009
  • 2009-08-26 WO PCT/EP2009/061001 patent/WO2010023224A1/en active Application Filing
  • 2009-08-26 EP EP09782218A patent/EP2329078B1/en not_active Not-in-force

Patent Citations (5)

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