SE1000032A1 - Process for adjusting the pH of a cellulose pulp, preparing cellulose or paper of a cellulose pulp and using (bi) carbonate removal to reduce the supply of acid - Google Patents

Description

2 The document relates to improved washing of alkaline pulp. The leaching of substances that contribute to COD is improved when the pH is lowered by adding gaseous carbon dioxide to the pulp and / or to the water used in the washing of the pulp. A description of factory experiments using the carbon dioxide washing technique in EP 0296 198 was presented by G. Östberg in a lecture entitled "Use of Carbon Dioxide in the Production of Sulphate Pulp" ("Use of carbon dioxide in the production of sulphate pulp") at the 5th International Conference on new available technologies, Stockholm, 4-7 June 1996.

Carbon dioxide (CO2) has also been used at various points in the pulp production and papermaking process to regulate the pH of the pulp. Carbon dioxide is water-soluble and produces carbon dioxide (H2CO3), which in turn dissociates to form carbonate (CO32 ") and bicarbonate ions (HCO3 ') in the pulp. The use of carbon dioxide to adjust the pH of an alkaline pulp has been described in, for example, the publication EP- B1-0 281 273 (Boc).

A fairly new form of pulp treatment is enzyme treatment and new enzymes are produced continuously for purification and bleaching of the pulp. A common enzyme is based on xylanase, which catalyzes xylan hydrolysis and promotes the removal of lignin.

Enzyme treatment is typically performed at a pH of about neutral (pH 6-9) and is typically followed by an acidic bleaching sequence containing one or more chlorine dioxide steps at a pH of about 2 to 4. U.S. Patent 5,645,686 (Solvay) discloses a process for treatment of chemical pulp using a xylanase step followed by a bleaching step with peroxyacid. U.S. Patent Application 2005/0150619 (Iogen Bioproducts) discloses the use of xylanase after a bleaching step with chlorine dioxide.

Chelating agents are sometimes used in pulp treatment to remove heavy metals from the pulp. Publication WO 1996/011298 (Sunds Debrator) describes a process for chlorine-free bleaching of pulp, in which metals in the pulp are removed by means of a chelating agent. The pulp is treated with the chelating agent at a pH of 2-7 for a period of from 5 minutes to 2 hours after which it is washed.

Adjustment of the pH of the pulp to be suitable for various alkaline, neutral and acidic treatments in cellulose and paper mills is traditionally performed by using strong bases, e.g. sodium hydroxide, and strong acids, e.g. sulfuric acid. Carbon dioxide can be used to produce carbon dioxide, which is a weak acid and which therefore enables a more precise regulation of the pH. According to G. Östberg in the above-mentioned lecture, carbon dioxide in pulp mills is suitable for adjusting alkaline liquids within the neutral pH range. Examples of situations where carbon dioxide can be used to adjust the pH are, according to Östberg, before a chelation step or an enzyme step. Dissolved in water, the carbon dioxide forms carbon dioxide, but also bicarbonate and carbonate ions, which have a buffering effect on the pulp. When adjusting the pH and when washing a mass, as described by Östberg, the (bi) carbonate ions have a positive effect on the system. However, the present inventor has observed that when there is a need to adjust the pH of the buffered mass after the carbon dioxide-induced adjustment of the pH, the same (bi) carbonate ions will resist changes in pH and increase the amount of acid or base that required to change the pH.

In treatment sequences where the pH of the pulp needs to be changed from alkaline or neutral to acidic or vice versa, the buffering ions formed by the carbon dioxide will adversely affect the subsequent adjustment of the pH.

There is a need in the art for methods that allow the use of carbon dioxide as a pH regulating agent without adversely affecting the achievement of low pH for a subsequent acidic step. The present invention aims to satisfy this need.

All the documents referred to in this text as well as any document or reference referred to in the documents thus referred to, and all literature, specifications, instructions, product bulletins, material bulletins and the like published by the manufacturer concerning the products and procedures mentioned in this text is hereby expressly incorporated herein by reference.

Summary of the Invention The present invention provides a step for removing (bi) carbonate to reduce the buffering effect of a carbon dioxide-induced pH adjustment. By reducing the buffering effect, a subsequent adjustment of the pH to a low pH value is facilitated. The invention thus has the effect that the amount of strong acid required after the addition of carbon dioxide to achieve a desired low pH value is reduced. Since the strong acid comprises a bleaching chemical, the invention has the effect of reducing the amount of bleaching chemical required. The present invention relates to a process for adjusting the pH of a cellulosic pulp. The process comprises the steps of: producing an alkaline cellulosic pulp, wherein said pulp has been prepared by chemical or mechanical processing and / or storage of recycled fiber and then subjected to washing and optional oxygen delignification; adjusts the pH of the alkaline cellulose mass thus obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acid treatment; and offers a (bi) carbonate removal step between the first and second carbon dioxide induced pH adjustments.

The carbon dioxide-induced adjustments of pH can be made by adding carbon dioxide directly to the pulp. Alternatively, carbon dioxide can be added to water, which is then added to the pulp. Such water is typically water used for diluting and / or washing the pulp.

In an advantageous embodiment of the invention, the alkaline mass has been prepared by chemical boiling, typically in a sulphate process.

In one embodiment of the invention, the first carbon dioxide-induced adjustment of the pH in connection with washing of the pulp is performed. The second carbon dioxide-induced pH adjustment is performed before the neutral treatment to adjust the pH of the pulp to suit the neutral treatment in question. The second pH adjustment is typically performed as a step separate from the wash. The purpose of the second pH adjustment is to achieve the desired pH for the subsequent neutral treatment and this is typically not the same as the purpose of a mass wash.

The adjustments of the pH with carbon dioxide form carbonate and bicarbonate ions in the aqueous solution of the pulp. These ions have a buffering effect in the neutral pH range at about pH 6-8, which means that they resist adjustments of pH to a lower pH value. According to the invention, (bi) carbonate ions are removed from the pulp after the first carbon dioxide-induced pH adjustment and before the second carbon dioxide-induced pH adjustment.

In one embodiment of the invention, the (bi) carbonate removal step comprises using water to displace and / or wash away (bi) carbonate ions from the pulp after the first carbon dioxide-induced adjustment of the pH. The (bi) carbonate removal step of the invention may also comprise dewatering and / or compressing the pH-adjusted pulp. Dewatering and compaction are typically followed by subsequent dilution of the pulp with water.

Addition of carbon dioxide after the (bi) carbonate removal step to adjust the pH of the pulp a second time forms new (bi) carbonate ions in the pulp. However, the total number of (bi) carbonate ions in the mass where the pH has been adjusted twice is less than it would have been without the intermediate removal step. As the amount of buffering (bi) carbonate ions in the pulp decreases, the subsequent lowering of the pH by means of a strong acid, in order to be suitable for the acidic treatment of the pulp, requires less acid to overcome the buffering action of the (bi) carbonates.

The present invention also relates to a process for producing cellulose, board or paper from cellulosic pulp which has been treated in a cellulose and / or paper mill in accordance with the inventive pH adjustment plan.

The present invention also relates to the use of (bi) carbonate removal to reduce the consumption of acid in the acidification of an alkaline cellulose pulp.

Brief Description of the Drawings Fig. 1 is a block diagram showing an embodiment of the invention in a sulfate cellulose plant.

Detailed Description of the Invention The present invention relates to a process in which carbon dioxide is used to adjust the pH of a pulp. In one embodiment, the invention relates to treatment sequences in the fiberline in a cellulose plant, where the pH needs to be changed from alkaline to neutral and then to acidic. Acid treatment in the fiber line typically involves bleaching the pulp.

By offering pH adjustment with carbon dioxide divided into two separate carbon dioxide additions and by offering a (bi) carbonate removal step between them, the present invention enables the plant to take advantage of the benefits of adjusting carbon dioxide without adversely affecting subsequent acidification.

Removal of (bi) carbonates after the first carbon dioxide-induced adjustment of the pH reduces the amount of strong acid required in the acidification. This offers clear technical advantages and reduced consumption of chemicals.

The strong acids typically used in a cellulose or paper mill are sulfate-rich acids such as sulfuric acid and residual acid from chlorine dioxide production. The reduction in the amount of acid has a significant effect on the purity of the process. The pulp typically contains small amounts of barium, which forms a harmful coating on the plant in a sulphate-rich environment. In a cellulose factory, the use of sulfuric acid contributes to the factory's total sodium / sulfur balance. Reducing the amount of strong acid can improve the Na / S balance in the factory.

Dividing the carbon dioxide-induced pH adjustment into two separate adjustments can also result in carbon dioxide savings compared to achieving the same pH with an addition of carbon dioxide. The (bi) carbonate removal step also removes organic components from the pulp, which could otherwise consume bleaching chemicals. This improves the process and enables the use of smaller amounts of bleaching chemicals. In addition, the invention enables significant savings in bleaching chemicals, as the strong acid comprises a bleaching chemical, such as chlorine dioxide.

The pulp to be treated according to the present invention is an alkaline pulp in a cellulose and / or paper mill. A chemical pulp is typically produced by chemical boiling of wood ice, such as in a sulphate cooking process. After boiling, the cooking liquid is removed and the mass is washed. The washing is typically followed by delignition with oxygen to release most of the lignin. The oxygen delignification is followed by washing and / or filtration processes after which the pulp is ready for bleaching or other purification processes. Alternatively, the delignification with oxygen is omitted and the pulp may instead undergo one or more further washes. The pulp is dewatered and then dried or transferred to the paper mill to make paper or board from it.

Other pulps, including mechanical and semi-mechanical pulps as well as recycled fibers also undergo alkaline, neutral and acidic treatments and the present invention is suitable for improving the pH adjustment for such pulps as well.

After washing and the optional delignification with oxygen, a sulphate pulp has a pH which varies depending on the previous treatment. After boiling in a sulfate process, the pH is about 12-14. Delignation with oxygen is carried out at a pH of 12 to 14 and the mass coming from delignation typically has a pH of 10 to 7 13. Washing typically lowers the pH slightly even without the addition of carbon dioxide during the washing phase.

The initial pH of the alkaline cellulose pulp subjected to pH adjustment according to the invention is typically between pH 8 and 13 and most typically between 9.5 and 11.

The neutral treatment, for which the subsequent adjustment of the pH by means of carbon dioxide is carried out, is typically carried out at a pH of between 6 and 9 and most typically at 6.5 to 8. This is the pH to be achieved with the other carbon dioxide induced the pH adjustment while the first carbon dioxide induced pH adjustment adjusts the pH to between pH 7 and 12 and most typically to pH 8.5 to 11.

In order to achieve precise control of the pH of the neutral treatment, the carbon dioxide is advantageously added directly to the pulp during the second adjustment of the pH. However, the carbon dioxide can also be added to water used for diluting the pulp.

The subsequent acidic treatment of the pulp is a treatment that requires a lower pH than the neutral treatment. The acid treatment is typically performed in the range of pH 2 to 7. Sometimes even pH values lower than 2 are desirable. If the neutral treatment is carried out at a pH between about 8 and 9, the acidic treatment can be carried out at a pH of 6 to 7. However, the acidic treatment is typically a treatment carried out at a pH lower than 6 and most typically at a pH of 2-4. The adjustment of the pH to suit the acidic treatment is carried out with a strong acid, ie. an acid that is stronger than carbon dioxide.

Different ions are added to the pulp with the wood or they occur in the chemicals that are added in the process during the various treatments in a cellulose and / or paper mill.

Carbonates and bicarbonates are typically formed during the boiling and oxygenation of the pulp. Some of these (bi) carbonates probably disappear from the pulp with the other impurities removed during the washing operations to which the boiled pulp is subjected. The amount of (bi) carbonate in a sulphate mass can be as high as 10 kg or more calculated as kg CO 2 / t dry mass.

Carbon dioxide used for washing or adjusting the pH of the pulp forms (bi) carbonate ions in the pulp. The present invention is mainly involved in the removal of (bi) carbonates added to the pulp when carbon dioxide is added to the pulp or in its aqueous medium to adjust the pH. It is obvious, however, that the (bi) carbonate removal step of the present invention will also affect (bi) carbonates formed or added in other previous 8 unit actions. In a mass with a high content of (bi) carbonate ions, which originate from positions other than the addition of carbon dioxide, the present invention will have a double beneficial effect.

It is easiest to add carbon dioxide in gaseous form as a gas that contains or consists of carbon dioxide. It should be noted that the carbon dioxide does not necessarily have to be in gaseous form. The invention also works with liquid or even solid carbon dioxide. The carbon dioxide gas may be pure carbon dioxide gas or a gas containing carbon dioxide, e.g. flue gas. Air also contains carbon dioxide, but the amount of carbon dioxide (0.03%) in air is generally too small for industrial purposes.

The amount of carbon dioxide added in a single or combined carbon dioxide-induced adjustment of pH depends on the mass condition, such as the pH of the alkaline mass and the desired pH for the neutral treatment, the presence or absence of interfering chemicals, e.g. other buffering compounds, the amount of carbon dioxide that can be added and dissolved in the pulp, etc. Those skilled in the art will be able to select the right amount of carbon dioxide by monitoring the pH of the pulp. The amount of carbon dioxide added at any point of addition is typically in the range of 0.5 to 15 kg per ton of dry pulp. In general, an amount of 1 - 10 kg of carbon dioxide is sufficient to achieve the desired effect.

When carbon dioxide is introduced into an aqueous medium, the following reactions take place: 1. CO 2 + H 2 O -f H 2 CO 3 2. Hzcos s H * + Hcog; 3. Hco; s H * + C03 * The hydrogen ions that are formed lower the pulp pH. The hydrogen ions will appear in the aqueous medium of the pulp solution and they will also penetrate into the fibers themselves. It is believed that some of the hydrogen ions attach to negatively charged phenolic groups within the fibers and remove cations such as sodium and possibly also calcium from the fibers. It is believed that this mechanism is at least one of those responsible for the improved washing results obtained at lowered pH. The mobile anions, ie. the bicarbonates and carbonates, will mostly be present in the aqueous medium surrounding the fibers. Since the fibers comprise immobile negatively charged groups, these will exert a repulsive force on the negatively charged (bi) carbonate ions. The (bi) carbonate ions thus have a greatly reduced tendency to penetrate the fiber water compared to the tendency of the hydrogen ions. 9 Without wishing to be bound by any theory, it is assumed that the different behavior of hydrogen ions and (bi) carbonate ions relative to fibrers is what makes it possible to selectively remove (bi) carbonate ions from the mass without simultaneously removing the corresponding amount of hydrogen ions with their effect on pH.

In the present invention, the pH of an alkaline mass is lowered by means of carbon dioxide to a value of between pH 6 and 9 for a neutral treatment of the mass.

According to the state of the art, such an adjustment of the pH had been carried out simply by adding carbon dioxide to the pulp until the desired pH was reached. In the present invention, however, carbon dioxide is added in two separate steps with a (bi) carbonate removal step between the steps. (Bi) the carbonate removal reduces the buffering capacity (M-alkalinity) of the pulp.

The first carbon dioxide-induced pH adjustment of the invention can be performed in any desired manner and need not, nor will it typically result in the exact pH desired for the neutral treatment. When performing the second carbon dioxide-induced adjustment of the pH, after the (bi) carbonate removal step according to the invention, this second adjustment of the pH will be monitored to achieve the desired pH for the neutral treatment. The second addition of carbon dioxide will form new (bi) carbonate ions in the pulp. However, the sum of (bi) carbonates in the mass in which carbon dioxide has been added twice will be considerably much less than if the pH adjustment had been carried out as in the prior art by a single pH adjustment using carbon dioxide.

In one embodiment of the invention, the first carbon dioxide-induced adjustment of the pH is performed in connection with a mass washing step. This procedure will have the dual benefit of improving the washing of the alkaline mass and adjusting the pH down to a value closer to the desired pH of the neutral treatment.

The second carbon dioxide-induced pH adjustment is typically performed just prior to the neutral treatment. The carbon dioxide additive is typically monitored to adjust “the pH of the pulp with carbon dioxide to suit the neutral treatment.

The neutral treatment of the pulp can be any treatment carried out on the pulp in the neutral pH range between an alkaline and an acidic treatment.

Such treatments include various treatments at various points in the cellulose and papermaking process, such as an enzyme phase in a cellulose plant, a flotation phase in a decolorization plant and a refining phase in a papermaking plant.

The subsequent adjustment of the pH with a strong acid is performed after said neutral treatment and before said acidic treatment. With the help of the strong acid, the pH of the pulp is adjusted to a pH that is suitable for the acidic treatment. Since the acidic treatment is a bleaching step, e.g. a chlorine dioxide step, the strong acid is typically selected from the group consisting of sulfuric acid, sulfuric acid, hydrogen salt, chlorine dioxide and residual acid from the chlorine dioxide production. As the acid treatment is a resin gluing process, the strong acid comprises alum. Thus, the person skilled in the art chooses strong acid so that it is suitable for the acidic treatment in question.

In many processes, there will be no or only very insignificant unit measures between the neutral treatment and the acid treatment. In a typical pulp bleaching process, no washing step will occur between the neutral and the acidic treatment. However, according to one embodiment of the present invention, a second (bi) carbonate removal step is introduced after the neutral treatment and before the addition of the strong acid. This further removes (bi) carbonate ions and facilitates the achievement of the desired low pH.

It is also possible to offer neutral treatment and carbon dioxide-induced adjustment of pH and (bí) carbonate removal in the middle of a bleaching sequence, e.g. after an initial acid bleaching and alkaline extraction.

As discussed above, carbon dioxide anions formed in the first carbon dioxide-induced adjustment of the pH are selectively removed compared to the hydrogen ions of the acid. It is obvious that the removal step generally does not remove all (bi) carbonate ions, nor will all hydrogen ions remain in the aqueous medium. Due to the different behavior of the ions mentioned, there is a significant difference in speed between their removal.

There are many ways in which (bi) carbonate ions can be selectively removed from the pulp without simultaneously removing corresponding hydrogen ions, i.e. without significantly raising the pH of the pulp. The (bi) carbonate removal of the invention may comprise one, two or more individual steps, which in turn comprise one or more individual ways of removing (bi) carbonate, which in combination achieve the desired reduction of (bi) carbonate. Typical methods of removing (bi) carbonate in accordance with the invention 11 include washing, filtration, constriction, dewatering, compaction and combinations thereof.

(Bi) the carbonate removal may also include a combination of dilution and dewatering. Dilution causes diffusion of any (bi) carbonate ions from the fibrous water to the aqueous medium surrounding the fibers. Dilution without subsequent removal of water from the diluted pulp does not reduce the amount of (bi) carbonates in the pulp.

In some situations, (bi) carbonate removal can be accomplished by precipitating calcium carbonate using means well known to those skilled in the art. However, care should be taken to avoid later dissolution of the precipitated solid.

In one embodiment of the invention, the (bi) carbonate removal step comprises using water for displacement and / or washing of (bi) carbonate ions from the pulp.

An advantageous (bi) carbonate removal step comprises subjecting a pulp of low concentration (eg 1-6%) to an filtration measure and displacing the pulp (bi) carbonate and aqueous medium with rinsing water applied to the filter.

A typical way to remove (bi) carbonate ions is to subject the (bi) carbonate-containing mass to a washing step. During the washing, the (bi) carbonate-containing mass is treated with washing water, after which the water is removed. The removal of water removes a large part of the (bi) carbonate ions. Pulp washing steps are typically performed in washing phases, which include one or more of your washing steps. Thus, a washing apparatus used in a washing phase (Dutch) can have several washing steps in succession with a filtration or dewatering step between the washing steps. The pH of the (bi) carbonate-removing washing water is advantageously chosen so that it does not raise the pH of the pulp.

Washing and filtration replace most of the water surrounding the pulp suspension with fresh water. Thus, a (bi) carbonate removal step of the present invention which comprises washing and filtering, typically removes between 10 and 70% of the (bi) carbonate ions in the pulp.

Dewatering and compaction can remove between 10 and 75% of the (bi) carbonate ions.

The pulp can also be compressed after a filtration or washing step to remove as much of the (bi) carbonate ions as possible. A combination of washing and compression can typically remove between 25 and 90% or more of the 12 (bi) carbonate water. After compaction, the resulting more concentrated pulp is typically diluted with water to allow pumping and further processing of the pulp.

It is advantageous to carry out the (bi) carbonate removal step (s) in such a way as to remove at least 25% of the (bi) carbonate ions formed by the first carbon dioxide-induced adjustment of pH. Most advantageously, the (bi) carbonate removal step (s) removes up to 90% or more of the (bi) carbonates in the pulp. In a typical (bi) carbonate removal process, a combination of several methods of removing (bi) carbonate is used. One can e.g. use a washing press to remove 70 to 85% of the (bi) carbonates formed. Highly efficient washing / filtration / compression steps can remove as much as 95 to 100% of the (bi) carbonates.

The removal percentage of (bi) carbonate in each position depends on the method or methods chosen for the removal, on the operation of the equipment in question, on the amount of water added or removed, on the ionic strength of the pulp, etc. The person skilled in the art realizes that the above percentages are only approximate and that the removal can be improved or impaired by normal optimization of the process.

The percentage of (bi) carbonate removal in a removal step is calculated as a decrease in the (bi) carbonate content (measured as kg CO2 per tonne of dry pulp) in a pulp leaving the removal step compared to the (bi) carbonate content in the pulp when it arrived at said (bi) ) carbonate removal step. In the event that the mass arriving at or leaving the (bi) carbonate removal step is diluted with water, the measurement is counted as if it had been carried out before any dilution had taken place with carbonate-containing water.

The water used for constriction, washing and / or dilution typically has a lower content of (bi) carbonate ions than the mass subjected to the (bi) carbonate removal step.

When the alkaline pulp comprises a sulphate pulp which has been subjected to oxygen degeneration, the (bi) carbonate removal step typically comprises at least one step where the pulp is washed.

The water used for washing, displacing or diluting the pulp in the (bi) carbonate removal step is typically fresh water, condensate, clear filtrate or mixtures of such pure water, or it may comprise washing water / filtrate from pulp washing downstream. In the event that the (bi) carbonate removal step is followed by a washing step, the filtrate from said washing step is advantageously used.

However, water from a washing phase downstream of the neutral treatment can also be used in the (bi) carbonate removal step. Mixtures of pure circulating water can also be used.

In one embodiment of the invention, the (bi) carbonate removal step comprises at least two ways of removing (bi) carbonate. These methods can be selected from constriction, washing, filtration, dewatering and compaction of the pulp.

In one embodiment of the invention, the neutral treatment of the pulp comprises an enzyme treatment or chelating step after the second adjustment of the pH with carbon dioxide. The acidic treatment typically comprises an acidic bleaching treatment with a bleaching chemical selected from chlorine dioxide, ozone, peroxyacid and dithionite.

In an advantageous embodiment of the invention, the neutral treatment comprises an enzyme treatment, wherein the enzyme typically comprises xylanase, and the acidic treatment comprises bleaching with chlorine dioxide. The enzyme treatment is advantageously carried out before a possible bleaching sequence, although it is also possible to carry out the enzyme treatment after an initial chlorine dioxide and extraction sequence.

The acid bleaching treatment typically contains a sequence of phases. An initial chlorine dioxide phase (DO) is typically followed by an alkaline extraction phase (E) and a second chlorine dioxide phase (DI). The sequence may also comprise an ozone phase (O) and / or a peroxyacid phase (Pa). Sometimes alkaline bleaching phases, such as hydrogen peroxide phases (P), are included in the sequences. Mechanical pulps are typically bleached with dithionite and / or hydrogen peroxide and may be subjected to chelation phase (s).

Alkaline, neutral and acidic treatment phases are also found in other situations in the cellulose and papermaking process.

When converting waste paper into pulp for papermaking, the paper is first broken down in a pulp production process which is carried out at a high pH, typically pH 9 to 10, at elevated temperature. Usually the fibers are filtered and dewatered after which they are subjected to flotation in the neutral pH range between 6.5 and 8.5 to remove ink particles. Thereafter, the purified mass can be treated with acidic agents, e.g. bleaching or gluing measures. Utilizing the pH adjustment of the present invention for recycled fibers, the first carbon dioxide-induced pH adjustment is advantageously offered before or within an initial flotation phase. The first adjustment of the pH can be performed in a washing step as a measure for direct adjustment of the pH or in the form of a flotation gas. The second pH adjustment is advantageously performed before a subsequent post-flotation phase. Between the flotation phases, the pulp is washed and dewatered to remove (bi) carbonates and ink particles.

By carrying out in accordance with the present invention the carbon dioxide-induced adjustment of the pH of the iodine in two separate steps, the purified pulp will have a lower buffering capacity than if the pH adjustment had been performed in one step, as in the prior art. Accordingly, in a subsequent acid treatment, the consumption of strong acid according to the present invention will be less.

One can also add carbon dioxide and remove (bi) carbonate according to the present invention after a flotation phase or without totation to adjust the pH of an alkaline pulp to suit a neutral papermaking process. In this way, the consumption of any acidic chemicals, such as bleaching and fixing agents, can be reduced.

It should be noted that recycled fiber, which contains large amounts of calcium carbonate, should not normally be subjected to strongly acidic treatments as calcium carbonate dissolves easily in most acidic treatments. However, the (bi) carbonate ions that are formed and remain in the pulp after the second carbon dioxide-induced adjustment of the pH of the present invention will help maintain the calcium carbonate in solid form even in weakly acidic conditions.

When an alkaline mass is to be refined at neutral pH and glued at acidic pH, as described in the publication EP-B1 0 281 273 (BOC), the pH desired for gluing will be achieved with considerable savings of strong acid (sulfuric acid and alum), if one adjusts the pH with carbon dioxide twice and removes (bi) carbonates between the carbon dioxide-induced adjustments of pH i. in accordance with the present invention.

The pulp treated in accordance with any of the embodiments of the present invention comprises raw materials for the manufacture of paper, board or dry cellulose. The present invention also includes a process for making cellulose, cardboard or paper from the cellulosic pulp so treated. Thus, the invention also relates to a process for the manufacture of cellulose, cardboard or paper from cellulose pulp, which is characterized in that a. An alkaline cellulose pulp is produced in a cellulose or paper mill, where said pulp has been produced by chemical or mechanical treatment and / or storage. of recycled fiber and subsequently subjected to washing and, optionally, delignition with oxygen; b. adjusting the pH of the alkaline cellulose pulp thus obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acid treatment; c. performing a (bi) carbonate removal step between the first and second carbon dioxide induced pH adjustments; and d. treating the pulp after the acidic treatment in a cellulose or papermaking process to form the dried product selected from cellulose, paperboard and paper.

The invention is based on the insight that buffering (bi) carbonate ions can be removed from a mass without simultaneously affecting the pH to any significant extent. Thus, the invention relates to the use of (bi) carbonate removal to reduce the consumption of acid in acidifying an alkaline cellulose pulp. In one embodiment of the invention, the pH of the pulp is lowered twice with carbon dioxide and before the second lowering of the pH, (bi) carbonate ions formed are removed. at the first lowering of the pH.

In an advantageous embodiment of the invention, the pulp comprises an alkaline pulp produced in a cellulose factory by chemical boiling. The lowering of the pH twice with carbon dioxide and the (bi) carbonate removal is followed by an acidification which involves lowering the pH of the (bi) carbonate-containing mass with the aid of a strong acid.

Advantageously, the adjustment of the pH twice with carbon dioxide is followed by an enzyme treatment and the said acidification with said strong acid is typically followed by an acid bleaching phase, such as bleaching with chlorine dioxide.

In the present description and claims, the terms generally have the meaning commonly used in the art, i.e. as set forth in a series of handbooks entitled Papermaking Science and Technology, published by J. Gullichsen and H. Paulapuro (ISBN 952-5216-00-4). Specifically, the following terms have the meanings set forth below. The terms "pulp" and "cellulose pulp" refer to a mixture or suspension of cellulose pulp fibers in an aqueous medium. When referring to the specific aqueous medium present inside the fibers, the term "ervverwater" is used. The pulp in the present description and claims has been prepared by chemical or mechanical treatment of wood chips or by spreading recycled fiber. The chemical treatment is a process for the production of sulphate pulp, a process for the production of sulphite pulp or any other chemical process for the production of cellulose pulp.

The term mechanical mass production includes various forms of mechanical and also semi-mechanical processes. The amount of fiber in the aqueous medium calculated on a weight basis (mass concentration) can vary from less than 1% to over 500/0.

The pH of the cellulose mass and (bi) carbonate content are measured from the aqueous medium. The term "dry pulp" is used to refer to the final product made from the aqueous cellulosic pulp.

The term "alkaline cellulose pulp" refers to a pulp suspension with a pH exceeding 7. The alkaline cellulose pulp has a pH resulting from the boiling, washing and / or delignification treatment with oxygen to which it has been subjected. This pH is typically between pH 8 and 13.

The term "neutral treatment" refers to a treatment in a cellulose and / or paper mill that takes place at a pH close to 7. This includes such treatments as enzyme treatments, chelating phases, flotation phases and refining procedures.In the neutral pH range performed dithionite bleaching can also be counted as neutral treatment.

The treatment is typically a chemical treatment carried out at a pH between 6 and 9.

The term "acid treatment" refers to a treatment in a cellulose and / or paper mill which takes place at a pH lower than 7. The treatment is typically carried out at a pH of between 1 and 6. Such treatments include bleaching treatments such as bleaching with the element chlorine, chlorine dioxide, ozone, dithionite and peroxyacid; gluing with acidic gluing agents such as resin and alum etc.

The term "(bi) carbonate" refers to one or each of the ions C032 "(carbonate) and HCO3" (bicarbonate). The ions are mostly present in the aqueous medium of the mass. The two ions alternate depending on the pH and precipitation / dissolution of salts of said ions .

The term "(bi) carbonate removal" refers to a treatment of a pulp suspension containing (bi) carbonate ions. During the (bi) carbonate removal, negatively charged (bi) carbonate ions are removed together with countercations other than the hydrogen ion (H *). A selective removal of (bi) carbonate ions is possible, thanks to the affinity of the hydrogen ions for the fibers in the pulp. When removing (bi) carbonate ions without removing the corresponding hydrogen ions, the pH of the pulp is not affected, despite the fact that the buffering capacity of the suspension decreases.

The term "(bi) carbonate removal step" refers to a unit action performed in a device in the cellulose or paper mill. The unit measure may comprise a clean washing step (ie adding water to the pulp and removing water from the pulp) or it may comprise a washing step in combination with a subsequent compression step. Individual reference is made to washing and compaction with "method of removing (bi) carbonate".

The term "pH-induced adjustment of carbon dioxide" refers to the addition of carbon dioxide directly or indirectly to the pulp in such a way that the pH of the pulp changes. When carbon dioxide is added to an alkaline mass, the pH drops because the added carbon dioxide forms carbon dioxide H2CO3 in the aqueous medium. The aqueous medium in which carbon dioxide is added in the present invention may be the aqueous medium of the pulp or it may be an aqueous medium, e.g. water, which is then added to the mass.

The term "buffering capacity" refers to a measure of the resistance of a buffer solution to a change in pH as a result of the addition of hydroxide ions. A buffer solution is an aqueous solution of a mixture of a weak acid and its conjugate base. It has the property that the pH changes very little when a small amount of acid or base is added to it.

The following examples are provided to further illustrate the invention and are not intended to limit its scope. On the basis of the above description, one skilled in the art is able to modify the invention in many ways to achieve the best advantages of the invention for any practical purpose. The chemical amounts have been calculated per ton (metric tons) of dry mass unless otherwise stated.

Example A process according to Fig. 1 is operated in a sulphate cellulose factory which produces sulphate cellulose from softwood. The fiber line comprises a continuous boiler 1, a washing machine (Dutch) 2, a strainer 3, a second washing machine 4, oxygen lignification 5, a first washing press 6, a second washing press 7, a storage tank 8, a tower 189 with enzyme treatment (with xylanase) and a chlorine dioxide bleaching followed by optional extraction and additional bleaching sequences (not shown).

The pulp comes out of the oxygen lignification tower at a pH of 11.5 and a pulp concentration of 10%. Filtrates from the second washing press 7 are used as washing water in the first washing press 6. The washed mass from the washing press 6 is compressed to a concentration of 30%. The pulp is then diluted with water to a concentration of 6% and fed to the second washing press 7, where it is washed with fresh water and then compressed again to a concentration of 30%.

The enzyme treatment 9 requires a pH of 6.5. For the enzyme treatment, the pH is adjusted by adding CO 2 in gaseous form to a tube leading to the tower 9 to achieve an initial pH of 6.5. The enzyme-treated pulp is bleached with chlorine dioxide and its pH is adjusted before the ClO 2 phase with sulfuric acid to reach pH 3 at the outlet of the ClO 2 bleaching tower 10.

Taking the above process as a reference (without admitting that the process as such is part of the state of the art), one changes the procedure for testing the operation of the present invention. For the test runs, CO 2 is added, in the amounts shown in Table 1, to water used to dilute the 30% compressed mass after the first washing press 6. The point for adding CO 2 is marked in Figure 1 with a dashed line between the washing presses 6 and 7. The pH of the pulp at the beginning of the enzyme treatment and the exit from the bleaching tower are maintained at the same level as in the reference (pH 6.5 and 3, respectively) by correspondingly reducing the amount of CO 2 at the beginning of the enzyme treatment and the amount of H 2 SO 4 at the beginning of the CO2 treatment.

Table 1 pH at the exit from the pH at the entrance to the pH at the exit second washing press the enzyme treatment from the CIO2 tower Reference 10.5 6.5 3 Test 1 9.2 6.5 3 Test 2 9.3 6.5 3 Test 3 9 .9 6.5 3 The amount of (bi) carbonate removal in the second washing press 7 in test runs 1 to 3 was between 80 and 87%. The effect of (bi) carbonate removal on the consumption of acid is shown in Table 2. 19 Table 2 CO 2 additive 1 CO 2 additive 2 strong acid before CIOZ kg CO 2 / t mass kg CO 2 / t mass kg H 2 SO 4 / h mass Reference 0 9 6, 1 Test 1 5 3 2.8 Test 2 4 4 3.3 Test 3 2 7 5.0 As is clear from Table 2, the separate pH-induced adjustments of pH decrease with the (bi) carbonate removal between them on amount of strong acid required to adjust the pH of the acidic treatment.

In addition, the division of the CO2-induced pH adjustment into two CO 2 additives can be used to reduce the amount of CO 2 required for the CO2-induced pH adjustment.

The present invention has been described herein with reference to specific embodiments. However, it will be apparent to those skilled in the art that the process (s) may be varied within the scope of the claims.

Claims (9)

A method for adjusting the pH of a cellulosic pulp, characterized in that a. An alkaline cellulosic pulp is produced, said pulp having been prepared by a chemical or mechanical treatment or by storage of recycled fiber and then subjected to washing. and, optionally, oxygen delignification; b. adjusting the pH of the alkaline cellulose pulp thus obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acid treatment; and c. providing a (bi) carbonate removal step between the first and second carbon dioxide-induced pH adjustments. A method according to claim 1, wherein said carbon dioxide-induced adjustment (s) of pH is performed by adding carbon dioxide in gaseous form to the pulp and / or to water used for diluting and / or washing the pulp. A method according to claim 1 or 2, wherein the first carbon dioxide-induced adjustment of the pH is performed in connection with washing the pulp. A method according to claim 1 or 2, wherein the second carbon dioxide induced adjustment of the pH is performed as a step separate from washing the pulp before said neutral treatment to adjust the pH of said pulp to be suitable for said neutral treatment. . A method according to any one of the preceding claims, wherein the adjustment of the pH with a strong acid is performed after said neutral treatment and before said acid treatment and the pH of the pulp is adjusted with said strong acid to be suitable for said acid treatment. . The method of claim 5, wherein there is a second bicarbonate removal step after said neutral treatment and before said adjusting the pH with a strong acid. 10. 11. 12. 13. 14. 15. A method according to any one of the preceding claims, wherein said (bi) carbonate removal step comprises using water to displace and / or wash (bi) carbonate ions from the pulp. A method according to claim 7, wherein said water comprises spray water to which the pulp is exposed in a filter. A method according to any one of the preceding claims 1 to 6, wherein said (bi) carbonate removal step comprises dewatering and / or compression of said pH-adjusted pulp and subsequent dilution with water. A method according to claim 7, 8 or 9, wherein said water used for displacement, washing and / or dilution has a lower content of (bi) carbonate ions than the mass subjected to said (bi) carbonate removal step. A method according to any one of claims 7 to 10, wherein said water is selected from fresh water, condensate, clear filtrate and wash water / filtrate from a downstream wash of pulp. The method of claim 1, wherein said (bi) carbonate removal step comprises at least two (bi) carbonate removal methods selected from washing, filtering, narrowing, dewatering and compressing the pulp. The method of claim 1, wherein said pulp comprises a sulfate pulp subjected to oxygen delignification and said (bi) carbonate removal step comprises at least one pulp washing step. A process according to claim 1, wherein said alkaline cellulose pulp has an initial pH of between pH 8 and 13, the pH is adjusted to between pH 7 and 12 in the first carbon dioxide-induced adjustment of pH, the pH is adjusted to between pH 6 and 9 in the other carbon dioxide induced the pH adjustment and the pH is adjusted to between pH 2 and 7 with the strong acid. Process according to any one of the preceding claims, wherein said strong acid is selected from the group consisting of sulfuric acid, sulfuric acid, bisulate, chlorine dioxide, residual acid from chlorine dioxide production and alum. A method according to any one of the preceding claims, wherein said neutral treatment is selected from an enzyme phase, a chelating phase, a flotation phase, a refining phase and a dithionite bleaching phase. 1 A method according to any one of the preceding claims, wherein said neutral treatment comprises an enzyme treatment after the second adjustment of the pH with carbon dioxide. 1 A method according to any one of the preceding claims, wherein said acidic treatment comprises an acidic bleaching treatment with a bleaching chemical selected from chlorine dioxide, ozone, peroxyacid and dithionite. 1 The method of claim 1, wherein said (bi) carbonate removal step removes at least 25% of the (bi) carbonate ions formed by the first carbon dioxide induced pH adjustment. A process for the manufacture of cellulose, paperboard or paper from cellulosic pulp, characterized by a. Producing an alkaline cellulosic pulp in a cellulose and / or paper mill, said pulp having been produced by a chemical or mechanical treatment and / or a look-up of returns and has subsequently been subjected to washing and, optionally, oxygen lignification; b. adjusting the pH of the alkaline cellulosic mass thus obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acid treatment; c. providing a (bi) carbonate removal step between the first and second carbon dioxide induced pH adjustments; and d. after the acidic treatment, the pulp is treated in a cellulose or papermaking process to form a dry product selected from cellulose, paperboard and paper. 21. Use of (bi) carbonate removal to reduce the consumption of acid in the acidification of an alkaline cellulose pulp. Use according to claim 21, wherein the pH of said pulp is lowered twice with carbon dioxide, and wherein (bi) carbonate ions formed in the first pH lowering are removed before the second pH lowering. Use according to claim 21 or 22, wherein said acidification comprises lowering the pH of a (bi) carbonate-containing mass with a strong acid. Use according to any one of claims 21 to 23, wherein said adjustment of pH twice with carbon dioxide is followed by an enzyme treatment and said acidification with said acid is followed by acid bleaching.