FI122898B - 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

Method for adjusting the pH of cellulosic pulp, making pulp or paper from cellulosic pulp, and using (bi) carbonate removal to reduce acid consumption

FIELD OF THE INVENTION

The present invention relates to a process for adjusting the pH of a cellulosic pulp at a pulp and / or paper mill and to making pulp, paper or board from cellulose pulp so treated. The invention also relates to the use of (bi) carbonate removal to reduce acid consumption in treating cellulosic pulp. The invention uses carbon dioxide to adjust the pH of the pulp.

BACKGROUND OF THE INVENTION

15 Cellulose pulp is produced in a pulp mill by chemical or mechanical treatment of chips. Cellulosic pulp can also be obtained by pulping recycled fibers.

Although several different methods are available, the most commonly used process for the production of chemical pulp is the so-called sulphate process. In this process, the chips are boiled in the presence of sodium hydroxide and sodium sulfide. A very basic mixture of cooked pulp and cooking liquor (black liquor) is obtained. The black liquor is removed and the pulp is typically washed, optionally delignified with oxygen, washed, filtered and re-washed.

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The cooked and possibly oxygen-delignified pulp is still quite dark in color and needs to be bleached for lighter shades. In a bleaching plant, pulp o is typically bleached with acidic or basic bleaching agents and extracted and washed between treatments. Bleaching chemicals include chlorine, hypochlorite, chlorine dioxide, oxygen, hydrogen peroxide, ozone, enzymes, dithionite, etc. Huge amounts of acids and bases are consumed when changing the pH from basic to acidic and again to alkaline or neutral.

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o LO Washing is an important step in removing fibers and chemicals that have been o 35 removed from the fibers during pre-cooking, bleaching and / or leaching operations. Several methods have been developed to improve fiber washing at the pulp mill. One such prior art washing method which is widely used in the art is described in EP-B1-0 296 198 (AGA). The publication relates to an improved 2 washes of a basic mass. The washing of COD-enhancing substances is enhanced by lowering the pH with carbon dioxide added to the pulp and / or the washing water used for pulping. A description of the factory tests using the carbon dioxide washing technology of EP 0 296 198 was given by G. Östberg in his lecture "Use of Carbon Dioxide in 5th Production of Sulphate Pulp", 5th international conf. on new available techniques, Stockholm, June 4-7, 1996.

Carbon dioxide (CO 2) has also been used at various points in the pulping and papermaking process to adjust the pH of the pulp. Carbon dioxide is soluble in water and produces carbonic acid (H2CO3), which in turn decomposes and produces carbonate ions (C032) and bicarbonate ions (HC03). The use of carbon dioxide to adjust the pH of a basic pulp is described, for example, in EP-B1-0 281 273 (BOC).

Enzyme treatment is a relatively new form of pulp treatment, and new types of enzymes 15 are continuously produced for pulp purification and bleaching. The usual enzyme is based on xylanase, which catalyses the hydrolysis of xylan and promotes lignin clearance. Enzyme treatment is usually performed at approximately neutral pH (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 about 4. U.S. Patent 20,5,645,686 to Solvay describes a process for treating a chemical pulp in a xylanase step followed by a bleaching step with peroxyacid. U.S. Patent Application 2005/0150619 (Iogen Bioproducts) describes the use of xylanase after a chlorine dioxide bleaching step.

In pulp processing, chelating agents are sometimes used to remove heavy metals from the pulp. WO 1996/011298 (Sunds Defibrator) describes a process for bleaching a pulp without chlorine and removing o metals from the pulp with a chelating agent. The pulp is treated with a chelating agent at a pH of about 2 to 7 for five minutes to two hours, after which it is washed.

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Adjustment of the pulp pH to various basic, neutral or? Acidic treatments at pulp and paper mills is traditionally performed using strong bases such as sodium hydroxide and strong acids such as sulfuric acid, o ££ Carbon dioxide can be used to produce carbonic acid which is a weak acid and 35 therefore allows for a more precise control of the pH.

As stated by G. Östberg in the above presentation, carbon dioxide is suitable for use in pulp mills for adjusting alkaline solutions to a neutral pH range.

According to Östberg, carbon dioxide can be used to adjust the pH, for example, at positions prior to the chelating step or the enzyme step. When dissolved in water, carbon dioxide produces carbonic acid but also bicarbonate and carbonate ions, which produce a buffering effect on the pulp. By adjusting the pH and washing the 5 masses as described by Östberg, (bi) carbonate ions have a positive effect on the system. However, the present invention has found that when it is necessary to adjust the pH of the buffered pulp after adjusting the pH with carbon dioxide, these same (bi) carbonate ions prevent pH changes and increase the amount of acid or base needed to change the pH.

10 In treatment sequences where the pH of the pulp needs to be changed from basic or neutral to acidic or vice versa, the buffering ions produced by carbon dioxide negatively affect subsequent pH adjustment.

There is a need in the art for methods which allow the use of carbon dioxide as a pH adjusting agent but which do not adversely affect the achievement of the low pH required in the subsequent acidic process. The object of the present invention is to satisfy this need.

20 All documents referred to herein, and any document or reference cited therein, as well as all literature, specifications, instructions, product information sheets, material information sheets, and the like, as well as all products and methods mentioned herein, are hereby incorporated by reference.

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SUMMARY OF THE INVENTION

The present invention provides a (bi) carbonate removal process cv to reduce the buffer effect e), 30 produced by pH adjustment with carbon dioxide. Reducing the buffer effect facilitates subsequent adjustment of the pH to a low pH. Therefore, the invention has a reducing effect on it

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to the amount of strong acid needed to achieve the desired low pH after the addition of carbon dioxide. When the strong acid comprises a bleaching chemical, the invention has a reducing effect on the amount of bleaching chemical required.

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The present invention relates to a method for adjusting the pH of a cellulosic pulp.

The process comprises the following steps: providing an alkaline cellulose pulp, wherein said pulp is made by chemical, mechanical and / or 4 pulping treatments and subsequently washed and optionally delignified with oxygen; adjusting the pH of the alkaline cellulose pulp thus obtained at least twice with carbon dioxide before neutral treatment and at least once with a strong acid before subsequent acid treatment; and providing a 5 (bi) carbonate removal step between the first and second carbon dioxide pH adjustments.

The carbon dioxide-adjusted pH can be accomplished by directly adding carbon dioxide to the pulp. Alternatively, carbon dioxide can be added to the water, which is then added to the pulp. Such water is typically water used for pulp dilution and / or washing.

In a preferred embodiment of the invention, the basic pulp is prepared by chemical cooking, typically by the sulphate process.

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In one embodiment of the invention, the first carbon dioxide pH adjustment is carried out in connection with pulp washing. Another carbon dioxide pH adjustment is performed prior to the neutral treatment to adjust the pulp pH to that neutral treatment. The second pH adjustment is typically performed as a separate operation from 20 washes. The purpose of the second pH adjustment is to achieve the desired pH value for subsequent neutral treatment and is typically not the same as the target for pulp washing.

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

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In one embodiment of the invention, the (bi) carbonate removal operation comprises the use of water to displace and / or wash away the (bi) carbonate ions in the pulp after the first adjustment of the pH achieved with carbon dioxide.

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The process for removing the (bi) carbonate of the invention may also comprise dewatering and / or compressing the pulp at a pH adjusted to pH. Dewatering and compression are typically followed by subsequent dilution of the pulp with water.

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Addition of carbon dioxide after the (bi) carbonate removal operation to adjust the pH of the pulp a second time produces new (bi) carbonate ions into the pulp. However, in this case, the total amount of (bi) carbonate ions is lower in the pulp at twice the adjusted pH than it would have been without the intermediate 5 removal operations. Since the amount of buffering (bi) carbonate ions in the pulp has decreased, the next lowering of the pH with a strong acid to a level suitable for acid treatment of the pulp requires less acid to overcome the buffering effect of the (bi) carbonates.

The present invention also relates to a process for the production of pulp, cardboard or paper from a cellulosic pulp which has been treated at a pulp and / or paper mill in accordance with the pH adjustment process of the invention.

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

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram illustrating an embodiment of the invention at a sulphate pulp mill.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process using carbon dioxide to adjust the pH of the pulp. In one embodiment, the invention relates to treatment sequences on a pulp mill fiber line where the pH must be changed from basic to neutral and then acidic. The acid treatment on the fiber line typically involves bleaching the pulp.

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Since the pH adjustment with carbon dioxide is effected in two separate increments of carbon dioxide and a (bi) carbonate removal process is performed between them, the present invention enables

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exploits the benefits of pH adjustment with carbon dioxide without adversely affecting subsequent acidification. When (g) of (bi) carbonates is removed after the first adjustment of the pH with carbon dioxide, the amount of strong acid required for acidification 35 is reduced. This produces clear technical advantages as well

The door reduces the consumption of chemicals.

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The strong acids typically used in a pulp or paper mill are high sulfuric acids such as sulfuric acid and waste acid from the production of chlorine dioxide. Reducing the amount of acid plays an important role in the purity of the process. The pulp typically contains small amounts of barium, which causes harmful deposition on the surfaces of the apparatus 5 under high sulfate conditions. At a pulp mill, the use of sulfuric acid affects the mill's overall sodium / sulfur balance. Reducing the amount of strong acid may improve the plant's Na / S balance.

Dividing the carbon dioxide pH adjustment into two separate increments 10 may also save carbon dioxide compared to achieving the same pH with a single carbon dioxide addition. The process of removing (bi) carbonate also removes organic ingredients from the pulp that might otherwise consume bleaching chemicals. This improves the process and allows the use of less bleaching chemicals. Further, when a strong acid comprises a bleaching chemical 15 such as chlorine dioxide, the invention enables significant savings in bleaching chemicals.

The pulp to be treated in accordance with the present invention is an alkaline pulp at a pulp and / or paper mill. The chemical pulp is typically prepared by chemical cooking of the chips 20, such as the sulphate cooking process. After cooking, the cooking liquor is removed and the mass is washed. The washing is typically followed by delignification with oxygen to release more of the lignin. The oxygen delignification is followed by a washing and / or filtration process, after which the pulp is ready for bleaching or other purification processes. Alternatively, oxygen deletion may be omitted and one or more washes may be performed on the pulp instead. The pulp is then dewatered and dried or sent to a paper mill for the production of paper or board.

Other pulps, including mechanical and semi-mechanical pulps, as well as cg recycled fibers, are also subjected to basic, neutral and acidic treatment, and the present invention is also suitable for adjusting the pH of such pulps.

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After washing and optional oxygen delignification, the sulphate pulp has a pH which varies depending on the preceding treatment. After cooking, the pH in the sulfate process is about 12-14. The oxygen delignification is carried out at a pH of 12 to 35 14, and the pH of the pulp arriving from the delignification is typically 10 to 13. The washing typically lowers slightly the pH even if carbon dioxide is not added to the washing step.

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The pH of the basic cellulosic pulp, when subjected to the pH adjusting measures of the invention, is typically initially between 8 and 13 and most typically between 9.5 and 11. The neutral treatment for which the subsequent pH adjustment with carbon dioxide is carried out is typically carried out at a pH between 6-9 and most typically at a pH of 6.5-8. This pH is that which is to be achieved by the second carbon dioxide pH adjustment, while the first carbon dioxide pH adjustment adjusts the pH to between 7-12 and most typically to pH 8.5-11.

In order to provide accurate pH control for the neutral treatment, carbon dioxide from the second pH adjustment step is preferably introduced directly into the pulp. However, carbon dioxide can also be introduced into the water used to dilute the pulp.

Subsequent acid treatment of the pulp requires a lower pH value than neutral treatment. Acid treatment is typically carried out at a pH in the range of 2 to 15 7. Sometimes pH values of less than 2 are desirable. If the neutral treatment is carried out at a pH of about 8-9, the acidic treatment may be performed at a pH of 6-7. However, the acid treatment is typically one which is carried out at a pH below 6, most typically at a pH of 2-4. When the pH is adjusted to acidic treatment, a strong acid, i.e. an acid stronger than 20 carbonic acid, is used.

Various ions are transferred to the pulp with the wood or are present in the chemicals that are added to the process during the various treatments at the pulp and / or paper mill. Carbonates and bicarbonates are typically formed in pulp cooking and oxygen delignification. Some 25 of these (bi) carbonates are likely to be removed from the pulp together with other impurities in the washing operations performed on the cooked pulp. The amount of (bi) carbonate in the sulphate pulp can be up to 10 kg or more, determined on a scale of kg CO 2 / tonne dry pulp.

Carbon dioxide used for washing or pH adjustment produces (bi) carbonate ions in the pulp. The present invention relates primarily to the removal of (bi) carbonates which

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are introduced into the pulp when carbon dioxide is added to the pulp or to the aqueous medium (¾ of the pulp to adjust the pH. It is understood, however, that the og (bi) carbonate removal process of the present invention also affects the (bi) carbonates generated or added in containing a high concentration of (bi) carbonate ions from steps other than carbon dioxide addition, the present invention has a double advantageous effect.

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Carbon dioxide is most easily added in gaseous form as a gas containing or consisting of carbon dioxide. It should be noted that carbon dioxide does not necessarily have to be gaseous. The invention also works with liquid or even solid carbon dioxide. The carbon dioxide gas may be pure carbon dioxide gas or carbon dioxide containing gas, such as flue gas. Air also contains carbon dioxide, but the amount of carbon dioxide in the air (0.03%) is usually too low for industrial purposes.

The amount of carbon dioxide added in any single or combined pH adjustment with carbon dioxide will depend upon pulp conditions such as the pH of the 10 basic pulp and the desired pH of the neutral treatment, the presence or absence of interfering chemicals such as other buffering compounds, which can be added to and dissolved in the pulp, etc. One skilled in the art will be able to select the correct amount of carbon dioxide by monitoring the pH of the pulp. Typically, the amount of carbon dioxide to be added at any one point is 0.5 to 15 kg per ton of dry pulp. Generally, the amount of 1-10 kg of carbon dioxide is sufficient to produce the desired effect.

When carbon dioxide enters the aqueous medium, the following reactions take place: 1. C02 + H2O -> H2CO3

20 2. H2C03 -> H ++ HCCV

3. HC03-> H ++ C032 "

The resulting hydrogen ions lower the pH of the pulp. Hydrogen ions are present in the aqueous medium of the pulp suspension and also end up in the fibers themselves. It is believed that some of the hydrogen ions attach to the negatively charged phenolic groups in the fibers, displacing the fibers from cations such as sodium and possibly also calcium.

This mechanism is believed to be at least one of those that produce an improved washing effect of lowered pH. The mobile anions, i.e. bicarbonates cm and carbonates, are mainly present in the aqueous media surrounding the fibers. Because the fibers comprise immobilized negatively charged groups, they exert a repulsive force on the negatively charged cc o- (bi) carbonate ions. Thus, the tendency of (bi) carbonate ions to migrate into the fiber water has decreased significantly compared to that of hydrogen ions.

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05 o 35 Without wishing to be bound by any theory, it is believed that hydrogen and the like

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The difference in behavior of (bi) carbonate ions relative to the fibers is a factor that enables the selective removal of (bi) carbonate ions from the pulp without removing the corresponding amount of hydrogen ions affecting the pH.

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In the present invention, the pH of the alkaline pulp is lowered to pH 6-9 for neutral treatment of the pulp with carbon dioxide. In the prior art, such a pH adjustment would be accomplished simply by adding carbon dioxide to the pulp until the desired pH was reached. However, in the present invention, carbon dioxide is added in two separate steps, between which a (bi) carbonate removal operation is performed. When (bi) carbonate is removed, the buffering capacity (M-alkalinity) of the pulp decreases.

The first carbon dioxide-induced pH adjustment of the invention can be carried out in any desired manner and does not have to, and typically does not, achieve the desired pH for just neutral treatment. When the second pH adjustment is carried out with carbon dioxide following the (bi) carbonate removal process of the invention, this second pH adjustment is monitored to provide the desired pH for neutral treatment. Another addition of carbon dioxide 15 produces new (bi) carbonate ions in the mass. However, the total amount of (bi) carbonates in the pulp that has been adjusted twice is significantly lower than if the pH adjustment had been carried out according to the prior art as a single pH adjustment with carbon dioxide.

In one embodiment of the invention, the first carbon dioxide-induced pH adjustment is performed in connection with a pulp washing operation. This procedure has the dual advantage of not only improving the washing of the basic mass, but also adjusting the pH to a value closer to the desired pH of the neutral treatment.

Another carbon dioxide pH adjustment is typically made just prior to neutral treatment. Addition of carbon dioxide is typically monitored to adjust the pH of the pulp to carbon dioxide neutral treatment.

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cvj The neutral treatment of the pulp can be any treatment that is carried out on the pulp c1, at a neutral pH between alkaline and acidic treatment. Such treatments include a series of treatments that are carried out at different points in the pulp and

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a papermaking process such as an enzyme step at a pulp mill, a flotation step at a de-inking plant and a refining step at a paper mill.

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Subsequent adjustment of the pH with a strong acid is carried out after said neutral treatment and before said acidic treatment. The pH of the pulp is adjusted with a strong acid to a pH suitable for acid treatment. When the acid treatment is a bleaching step, such as a chlorine dioxide step, the strong acid is typically selected from the group consisting of sulfuric acid, sulfuric acid, bisulfite, chlorine dioxide, and waste acid from the production of chlorine dioxide. When acid treatment is a resin sizing process, strong acid is alum. Thus, the strong acid is selected by one skilled in the art to be suitable for the particular acidic treatment in question.

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In many processes, there are no, or very few, separate steps between neutral treatment and acidic treatment. In a typical pulp bleaching process, there is no washing step between neutral and acid treatment. However, according to the present invention, another 10 (bi) carbonate removal step is added after neutral treatment and before addition of the strong acid. This further removes buffering (bi) carbonate ions and facilitates the achievement of the desired low pH.

It is also possible to provide a neutral treatment and a carbon dioxide-lowered pH of 15, as well as (bi) carbonate removal in the middle of the bleaching sequence, such as after initial acid bleaching and basic extraction.

As stated above, the carbonic anions generated in the first carbon dioxide pH adjustment are selectively removed relative to the hydrogen ions of the acid. It is evident that the removal operation generally does not remove all (bi) carbonate ions, nor does all hydrogen ions remain in the aqueous medium. However, due to the different behavior of said ions, there is a significant difference in their removal rates.

There are several ways in which (bi) carbonate ions can be selectively removed from the pulp without simultaneously removing the corresponding hydrogen ions, i.e. without significantly increasing the pH of the pulp. The removal of the (bi) carbonate of the invention may comprise one, two or more individual steps which, in turn, comprise one or more cm ways of removing the (bi) carbonate which, when combined, provide the desired (bi) carbonate reduction. Typical ways of removing (bi) carbonate according to the invention include washing, filtration, displacement, cc removal of water, compression and combinations thereof.

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In addition, (bi) carbonate removal may also comprise a combination of dilution and dewatering. Dilution causes the diffusion of any (bi) carbonate ions from the fiber 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.

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In certain situations, (bi) carbonate removal may be accomplished by precipitation of calcium carbonate by methods well known to those skilled in the art. However, care should be taken to ensure that the precipitated solid does not dissolve at a later stage.

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In one embodiment of the invention, the (bi) carbonate removal operation comprises using water to displace and / or wash (bi) carbonate ions from the pulp. One preferred (bi) carbonate removal operation involves exposing a low consistency (e.g., 1-6%) pulp to a filtration operation and displacing the pulp (bi) carbonate-containing aqueous medium with spray water added to the filter.

A typical way to remove (bi) carbonate ions is to perform a washing operation on the (bi) carbonate-containing pulp. In washing (bi), the carbonate-containing pulp is treated with washing water and then the water is removed. Dehydration removes much of the 15 (bi) carbonate ions. Mass washing operations are typically carried out in washing steps comprising one or more washing operations. Thus, in a scrubber used in the scrubbing step, a plurality of scrubbing operations may be performed in sequence, with a filtration or dewatering step between the scrubbing operations. Preferably, the pH of the (bi) carbonate-removing wash water is selected so as not to increase the pH of the pulp.

Washing and filtration replace most of the fibers in the pulp suspension with water with fresh water. Thus, the (bi) carbonate removal process of the present invention, comprising washing and filtration, typically removes from 10 to 70% by weight of (bi) carbonate ions. Dehydration and compression can remove 10-75% of the (bi) carbonate ions.

o The pulp can also be compressed after filtration or washing to remove as much of the (bi) carbonate ions as possible. The combination of washing and 55 30 compression typically removes 25-90% or more of (bi) carbonate-containing water. The thicker mass produced after compression is typically diluted cc with water to allow for pumping and further treatment.

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LO It is preferable to carry out the (bi) carbonate removal operation or operations in such a manner that at least 25% of the pH of the first carbon dioxide is achieved.

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the (bi) carbonate ions generated in the control are removed. Most preferably, the (bi) carbonate removal operation (s) will remove up to 90% or more by weight of (bi) carbonates. In a typical (bi) carbonate removal process, there are 12 combinations of multiple (bi) carbonate removal methods. For example, a washing press can be used to remove 70-85% of the (bi) carbonates formed. Particularly efficient washing / filtration / pressing procedures can remove up to 95-100 (bi) carbonates.

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The percentage of (bi) carbonates that can be removed at any point will depend on the removal method (s) selected, the operation of the equipment, the amount of water added and removed, the ionic strength of the pulp, etc. It will be appreciated by those skilled in the art can be increased or decreased through normal process optimization.

The percentage of (bi) carbonate removed in the stripping operation is calculated as the decrease in the carbonate content of the pulp leaving the stripping step (calculated as 15 values per kg CO 2 per tonne dry mass) compared to the stripping (bi) carbonate content of said stripping (bi) carbonate. If the pulp entering or leaving the (bi) carbonate operation is diluted with water, the measurement shall be carried out on the pulp before any dilution with carbonate-containing water is performed.

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The water used for displacement, washing and / or dilution has a lower concentration of (bi) carbonate ions than the pulp subjected to (bi) carbonate removal.

Where the basic pulp comprises a pulp sulphate pulp which has been delignified with oxygen, the (bi) carbonate removal process typically comprises at least one pulp washing step.

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The water used for washing, displacing or diluting the pulp in the (bi) carbonate removal process is typically fresh water, condensate, clear filtrate, or a mixture of such pure waters, or may comprise washing water / filtrate from subsequent pulping steps. If the delay of the (bi) carbonate removal operation is followed by a washing operation, the filtrate from said o g washing operation is preferably used. However, water from a washing step performed after neutral treatment can also be used

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(bi) in a carbonate removal process. In addition, mixtures of pure circulation water may be used.

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In one embodiment of the invention, the (bi) carbonate removal process comprises at least two ways to remove (bi) carbonate. Such methods may be selected from pulping, washing, filtration, dewatering and compression.

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

In a preferred 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 preferably carried out before any bleaching sequences, although it is also possible to carry out the enzyme treatment after the initial chlorine dioxide and extraction sequence.

The acid bleaching treatment typically comprises a sequence of steps. The initial chlorine dioxide step (D0) is typically followed by a basic extraction step (E) and another chlorine dioxide step (Di). The sequence may also comprise an ozone step (O) and / or a peroxy acid step (Pa). Occasionally, basic bleaching steps such as hydrogen peroxide steps (P) are included in the sequences. The mechanical pulps are typically bleached with dithionite and / or hydrogen peroxide and may be subjected to a chelating step or steps.

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Alkaline, neutral or acidic processing steps are also used at other positions in the pulp and / or paper making process.

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When converting recovered paper to papermaking pulp, the paper is first decomposed in a pulping process which is carried out at a high pH, typically pH 9 to 10, and at an elevated temperature. Typically, the fibers are sorted and dewatered, followed by a flotation at a neutral pH of 6.5 to 8.5 to remove ink particles. The purified pulp can then be treated with g acidic substances, for example in bleaching or bonding operations.

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When the pH adjustment of the present invention is applied to recycled fibers, the first carbon dioxide-induced pH adjustment is typically performed prior to or during the initial flotation step. The first pH adjustment can be carried out in 14 washing operations either by direct pH adjustment or by flotation gas. The second pH adjustment is preferably performed prior to the next post-flotation step. Between the flotation steps, the pulp is washed and dewatered to remove carbonates and ink particles.

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When the carbon dioxide pH adjustment for flotation is carried out according to the invention in two separate operations, the buffered capacity of the purified pulp is lower than if the pH adjustment had been performed as a single operation according to the prior art. The subsequent acid treatment according to the invention thus consumes less potent acid.

The carbon dioxide addition and (bi) carbonate removal according to the invention may also be carried out after the flotation step or without flotation to adjust the pH of the basic pulp to a neutral papermaking process. In this way, the consumption of any acidic chemicals such as bleaches and fixatives can be reduced.

It should be noted that recycled fibers containing large amounts of calcium carbonate should not normally undergo highly acidic treatments since calcium carbonate is readily soluble during the most acidic treatments.

However, the (bi) carbonate ions generated and retained in the pulp after the second adjustment of the pH with carbon dioxide help to hold the calcium carbonate in solid form even under mildly acidic conditions.

By grinding the basic mass at neutral pH and gluing it at acidic pH as described in EP-B1 0 281 273 (BOC), the desired pH value for gluing can be achieved while achieving considerable savings in the amount of strong acid (sulfuric acid and alum). , if the pH is adjusted twice with carbon dioxide and (bi) carbonates are removed between the carbon dioxide-adjusted pH-30 adjustments according to the present invention.

c A pulp treated with any embodiment of the present invention (Q comprises a raw material for the production of paper, board or dried pulp.

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The invention also encompasses a process for making pulp, paperboard or paper from cellulose pulp so treated.

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Accordingly, the invention also encompasses a process for making pulp, board or paper from cellulosic pulp which is characterized by providing a basic cellulosic pulp at a pulp and / or paper mill, whereby the basic pulp is obtained by chemical, mechanical and / or pulping treatment and subsequent washing and possibly delignification with oxygen; 5b. The pH of the alkaline cellulose pulp thus obtained is adjusted at least twice with carbon dioxide before neutral treatment and at least once with strong acid before subsequent acid treatment; c. providing a (bi) carbonate removal step between the first and second carbon dioxide-adjusted pH; and 10 d. processing the pulp after an acid treatment in a pulp or paper making process to provide a dried product selected from pulp, board and paper.

The invention is based on the finding that buffering (bi) carbonates can be removed from the pulp without significantly affecting the pH. Accordingly, the invention relates to the use of (bi) carbonate deprotection to reduce acid consumption in acidification of alkaline cellulose pulp.

In one embodiment of the invention, the pulp pH is lowered twice with carbon dioxide and the (bi) carbonate ions generated in the first lowering of the pH are removed before the second pH drop.

In a preferred embodiment of the invention, the pulp comprises an alkaline pulp made by chemical cooking at a pulp mill. After the pH has been lowered twice with carbon dioxide and the (bi) carbonate has been removed, an acidification is carried out comprising lowering the pH of the (bi) carbonate-containing mass with a strong acid. Preferably, the twice lowered pH reduction with carbon dioxide is followed by enzymatic treatment, and said acidification with said strong acid is typically followed by an acid bleaching step such as chlorine dioxide bleaching.

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In the present specification and claims, the terms generally have the meanings they ordinarily have in the art, e.g., those shown in the Handbook of Papermaking Science and Technology, ed. J. Gullichsen and H. Paulapuro o g (ISBN 952-5216-00-4). In particular, the following terms have the meanings given below, p. 35

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The terms "pulp" and "cellulosic pulp" refer to a mixture or suspension of cellulosic pulp fibers in an aqueous medium. When referring to a specific aqueous medium among fibers, the term "fibrous water" is used. In the present specification and claims, the pulp is made by chemical or mechanical treatment of chips or by pulping of recycled fibers. The chemical treatment is a sulphate fiber method, a sulphite fiber method, or any other chemical fiber method. The term mechanical defibration 5 includes many forms of mechanical as well as semi-mechanical processes. The amount of fiber in the aqueous medium, calculated on a weight basis (consistency), can range from less than 1% to more than 50%. The pH and (bi) carbonate content of the cellulosic pulp are determined from the aqueous medium. The term " dried pulp " is used to refer to 10 final products made from aqueous cellulosic pulp.

The term "alkaline cellulosic pulp" refers to a pulp suspension having a pH greater than 7. The alkaline cellulosic pulp has a pH resulting from its cooking, washing and / or oxygen delignification treatments. Typically, this pH is in the range of pH 8-13.

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The term "neutral treatment" refers to a pulp and / or paper mill treatment at a pH close to pH 7. Such treatments include, for example, enzyme treatments, chelating steps, flotation steps and grinding operations. Dithionite bleaching, which is carried out in a neutral pH range, can also be considered as a neutral treatment. Typically, the treatment is a chemical treatment carried out at a pH of 6-9.

The term "acid treatment" refers to a pulp and / or paper mill treatment that takes place at a pH below 7. Typically, the treatment is carried out at a pH of 25 to 1 to 6. Such treatments include acid bleaching treatments such as bleaching with elemental chlorine, chlorine dioxide, ozone, dithionite and peroxy acid, bonding with acidic adhesives such as resin and alum, etc.

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cm The term "bicarbonate" refers to one or both of the ions C032 '(carbonate) and ^ 5 30 HC03 "(bicarbonate). The ions are predominantly present in the aqueous medium of the pulp.

Said two ions change from one form to another depending on the pH and the precipitation / dissolution of the salts of said ions.

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The term "bicarbonate removal" refers to the treatment of a pulp suspension 8 containing (bi) carbonate ions. In this (bi) carbonate removal, the negatively charged (bi) carbonate ions are removed by counter-cations other than the hydrogen ion (H +). The affinity of hydrogen ions for pulp fibers allows for selective removal of (bi) carbonate ions. When 17 (bi) carbonate ions are removed without removing the corresponding hydrogen ions, the pH of the pulp does not change, although the buffering capacity of the suspension is reduced.

The term "(bi) carbonate removal operation" refers to a discrete operation on a pulp or paper mill device 5. This separate operation may comprise a mere washing operation (i.e., adding water to the pulp and removing water from the pulp), or it may comprise a washing operation combined with a subsequent pressing operation. The individual washing and pressing operations are referred to as "(bi) carbonate removal methods".

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The term "carbon dioxide pH adjustment" refers to the direct or indirect addition of carbon dioxide to the pulp in such a way that the pulp pH changes. When carbon dioxide is added to the basic mass, the pH decreases because the carbon dioxide added produces carbonic acid, H2CO3, in the aqueous medium. The aqueous medium to which carbon dioxide is added in the present invention may be an aqueous medium for the pulp, or it may be an aqueous medium such as water, which is then added to the pulp.

The term "buffering" refers to a unit of measure representing the resistance of a buffer solution to changes in pH when hydrogen ions are added. The buffer solution is an aqueous solution of a mixture of weak acid 20 and its conjugate base. It has the property that the pH changes very little when a small amount of acid or base is added.

The following example is intended to further illustrate the invention and is not intended to limit its scope. From the above description, one skilled in the art will be able to modify the invention in many ways to achieve the best benefits of the invention for any practical purpose. Unless otherwise stated, the quantities of chemicals are calculated per tonne of dry mass.

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EXAMPLE & 30 The process of Figure 1 is used in a pulp pulp mill to make a softwood sulphate pulp. The fiber line comprises a digester 1, a cc scrubber 2, a sort 2, a second scrubber 4, an oxygen delignification 5, a first scrubber 6, a second scrubber 7, a storage tank 8, a tower 9 o £ 5 for enzyme (xylanase) treatment and chlorine dioxide bleaching 35 optionally extraction plus additional bleaching sequences (not shown).

When the pulp leaves the delignification tower, its pH is 11.5 and its consistency is 10%. The filtrate from the second washing press 7 is used as washing water in the first washing press 18 6. The washed mass from the washing press 6 is pressed to 30% consistency. The pulp is then diluted to 6% consistency with water and fed to another washing press where it is washed with fresh water and pressed again to 30% consistency.

5 Enzyme treatment 9 requires a pH of 6.5. For enzyme treatment, the pH is adjusted by adding CO 2 gas to the tower 9 in the conduit to provide an inlet pH of 6.5. The enzyme-treated pulp is bleached with chlorine dioxide and adjusted to pH 3 with sulfuric acid prior to the ClO2 step to reach pH 3 as the pulp leaves the ClO2 bleaching tower 10.

10

Referring to the above process (without implying that the process as such would be included in the prior art), the procedure is modified to test the operation of the present invention. For experimental purposes, CO 2 is added to the water used to dilute the pressed pulp after the first washing press 6 in the amounts indicated in Table 1 to 30% consistency. The insertion point for CO 2 is indicated in Figure 1 by the dashed line between the washing presses 6 and 7. The pH values of the pulp going to the enzyme treatment and exiting the bleaching tower, respectively, are maintained at the same level as in the reference process (pH 6.5 and pH 3, respectively) by similarly reducing the amount of CO 2 and H2SO4 treated with ClO2.

Table 1 pH of the second pH before the pH pulp scrubber at the exit of the ClO2 tower

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Koe 1 9 ^ 2 6/5 3 ^ Koe 2 9/3 6 ^ 5 3 w Koe 3% 9 6 ^ 5 3 05

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In test runs 1-3, the amount of (bi) carbonate removed in the second washing press 7 was g 80-87%. The effect of the removed (bi) carbonate on acid consumption is shown by σ> in Table 2.

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Table 2 19 Addition of CO2 Addition of CO2 Strong acid before 1 kg of CO2 / 2 kg of CO2 / Cl02 mass per tonne mass per kg kg H2SO4 / tonne mass

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Koe 2 4 4 3 ^ 3

Koe 3 2 7 5 ^ 0

As can be clearly seen in Table 2, the separate pH adjustments provided by CO2: between which (bi) carbonate removal was performed reduced the amount of strong acid 5 needed to adjust the pH for acidic treatment. In addition, dividing the pH adjustment provided by CO 2 into two CO 2 increments can also be used to reduce the amount of CO 2 required to adjust the pH caused by CO 2.

The present invention has been described herein with reference to specific embodiments. However, it will be apparent to one skilled in the art that the process or methods may be modified within the scope of the claims.

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Claims (24)

A process for adjusting the pH of a cellulosic pulp, characterized by: a. Providing an alkaline cellulosic pulp, said pulp having been subjected to a chemical, mechanical and / or pulping treatment followed by washing with possibly delignification with oxygen; b. the pH of the alkaline cellulose pulp thus obtained is adjusted at least twice with carbon dioxide prior to neutral treatment and at least once with a strong acid before subsequent acid treatment; and 10c. providing a (bi) carbonate removal step between the first and second carbon dioxide-adjusted pH. A process according to claim 1, wherein said one or more carbon dioxide-adjusted pH adjustments are effected by adding carbon dioxide gas to the pulp and / or water used for pulp dilution and / or washing. The method of claim 1 or 2, wherein the first carbon dioxide-adjusted pH is carried out in connection with pulp washing. The method of claim 1 or 2, wherein the second carbon dioxide pH adjustment is carried out separately from pulp washing prior to said neutral treatment to adjust said pulp pH to a neutral treatment. The process according to any one of the preceding claims, wherein the adjustment of the pH with a strong acid is carried out after said neutral treatment and prior to said acidic treatment and the pH of the pulp is adjusted to be acidic with said strong acid, is another bicarbonate removal procedure after said neutral treatment and prior to said adjustment of the pH of the cc with strong acid. to (Mo 0 LO) The method according to any one of the preceding claims, wherein said (bi) carbonate removal step comprises using water to displace and / or wash CNJ (bi) carbonate ions from the pulp. The method of claim 7, wherein said water comprises spray water added to the pulp in the filter. The process according to any one of claims 1 to 6, wherein said (bi) carbonate removal operation comprises dewatering and / or squeezing said pH adjusted pulp and then diluting with water. The method of 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 pulp subjected to said (bi) carbonate removal operation. The method according to any one of claims 7 to 10, wherein said water is selected from fresh water, condensate water, clear filtrate and wash water / filtrate from subsequent pulp washing. The method of claim 1, wherein said (bi) carbonate 20 removal operation comprises at least two (bi) carbonate removal methods selected from pulp washing, filtration, displacement, dewatering and compression. The method of claim 1, wherein said pulp comprises a pulp of sulfate that has been delignified with oxygen, and said (bi) carbonate removal process comprises at least one pulp washing step. The method of claim 1, wherein said alkaline cm cellulosic pulp is initially pH 8-13, the pH is adjusted to pH 7-12 cl, 30 in the first carbon dioxide pH adjustment, and the pH is adjusted to pH 6-9 in the second carbon dioxide pH: and the pH CC is adjusted to pH 2-7 with a strong acid, co C 10 O [g 15. The process according to any one of the preceding claims, wherein said strong acid is selected from the group consisting of sulfuric acid, sulfuric acid, bisulfite, chlorine dioxide, chlorine dioxide production acid and alum. The process according to any one of the preceding claims, wherein said neutral treatment is selected from an enzyme step, a chelating step, a flotation step, a refining step and a dithionite bleaching step. A process according to any one of the preceding claims, wherein said neutral treatment comprises enzyme treatment after another adjustment of the pH achieved with carbon dioxide. The process 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. The method of claim 1, wherein said (bi) carbonate removal step removes at least 25% of the (bi) carbonate ions generated by the first carbon dioxide pH adjustment. A process for the production of pulp, board or paper from cellulosic pulp, characterized in that a basic cellulosic pulp is obtained at a pulp and / or paper mill, whereby the basic pulp is obtained by chemical, mechanical and / or pulping treatment followed by washing and possibly delignification. oxygen; b. adjusting the pH of the alkaline cellulose pulp thus obtained to at least 25 times with carbon dioxide before neutral treatment and at least once with strong acid before subsequent acid treatment; c. providing a (bi) carbonate removal step between the first and second pH adjustment provided by the carbon dioxide; and c \ j d. processing the pulp after an acid treatment in a pulp or paper manufacturing process to provide a dried product selected from pulp, board and paper. en CL ^ 21. Use of (Bi) carbonate removal to reduce acid consumption in acidification of basic o g cellulose pulp. o 35 (M Use according to claim 21, wherein the pH of said pulp is twice lowered with carbon dioxide and the (bi) carbonate ions generated in the first lowering of pH are removed before the second lowering of pH. The use according to claim 21 or 22, wherein said acidification comprises lowering the pH of the (bi) carbonate-containing pulp with a strong acid. Use according to any one of claims 21 to 23, wherein said pH adjustment twice with carbon dioxide is followed by enzymatic treatment and said acidification with said acid is followed by acid bleaching. δ CM CM O) X X CL CD CM O LT) O) O o CM