WO2012136887A2

WO2012136887A2 - A method for manufacturing paper pulp, and paper pulp

Info

Publication number: WO2012136887A2
Application number: PCT/FI2012/050324
Authority: WO
Inventors: Björn KROGERUS; Agneta Fuhrmann; Christiane Laine
Original assignee: UPM Kymmene Oy
Current assignee: UPM Kymmene Oy
Priority date: 2011-04-08
Filing date: 2012-03-30
Publication date: 2012-10-11
Anticipated expiration: 2013-10-08
Also published as: FI123309B; WO2012136887A3; FI20115341A0; FI20115341A7

Abstract

The invention relates to a method for manufacturing paper pulp (10), in which method bleached, chemically pulped cellulose fibres (1) from hardwood are used as raw material. The method comprises extracting (5) xylan from said cellulose fibres (1) by means of sodium hydroxide; adding carbon dioxide into a mixture (2) containing said cellulose fibres (1), dissolved xylan and sodium hydroxide, for precipitating xylan; adding calcium hydroxide into said mixture containing precipitated xylan and cellulose fibres, for forming precipitated calcium carbonate (7); and removing sodium hydroxide from said mixture (4) containing sodium hydroxide, xylan, cellulose fibres (1), and calcium carbonate. Furthermore, the invention relates to paper pulp made by the method.

Description

A METHOD FOR MANUFACTURING PAPER PULP, AND PAPER PULP

Field of the invention The invention relates to a method for manufacturing paper pulp. The invention also relates to paper pulp made by the method, as well as the use of paper pulp made by the method as raw material for paper and paperboard.

Background of the invention

PCC is chemically precipitated calcium carbonate CaCO₃. It can be prepared, for example, from burnt lime, CaO, which is slaked by reacting it with water. When calcium hydroxide Ca(OH)₂ obtained by the slaking reaction is reacted with a precipitating agent, typically carbon dioxide CO_2l precipitated calcium carbonate (PCC) is produced. By varying the process conditions it is possible to alter, for example, the crystalline and granular form of PCC.

The precipitation of calcium carbonate is normally carried out in a batch process by introducing carbon dioxide into a suspension of calcium hydroxide in water, that is, milk of lime, to precipitate the calcium carbonate. Precipitated calcium carbonate (PCC) can also be prepared in a continuous process in a fibre or water flow by introducing milk of lime and carbon dioxide into the flow. A method for precipitating calcium carbonate in fibre raw material for paper is disclosed in European patent EP 1297220.

PCC is commonly used in the manufacture of paper, both as a filler and as a coating pigment, e.g. to improve many properties, such as optical properties and printing properties, of the paper. The addition of the filler also makes it possible to use a smaller quantity of fibre material in the papermaking, which is why the cost savings achieved are typically clearly greater than the costs caused by the addition of the filler. Consequently, it is a common aim to add as much filler as possible into the fibre suspension used for papermaking. However, the increase in the filler content is limited by a deterioration in the strength properties of the paper, among other things. To be able to consider- ably increase the filler contents in the paper, significant quantities of binders should also be added to the fibre suspension. However, the binders are typi- cally expensive, which is why it has turned out to be difficult to increase the filler content in a cost effective way.

Brief summary of the invention

The present invention provides a novel and inventive method for manufacturing paper pulp. Paper pulp made by the method is also presented, as well as the use of paper pulp made by the method as raw material for fine paper and paperboard.

Thanks to the present invention, it is possible to manufacture paper pulp in a cost effective way. The raw materials used in the invention for manufacturing paper pulp are typically inexpensive, because it is possible to use inexpensive additives, that is, sodium hydroxide (NaOH), carbon dioxide (C0₂) and calcium hydroxide (Ca(OH)₂), instead of expensive binders and fillers in the paper pulp made by the method of the invention. The new invention provides for a new method for making paper pulp, the paper pulp made by the method comprising both filler, binder, and fibre material. The method according to the invention comprises one or more of the steps listed hereinbelow:

- extracting xylan by sodium hydroxide from cellulose fibres to an extraction solution,

- precipitating xylan in the extraction solution by means of carbon djox- ide,

- adding calcium hydroxide to the mixture, for precipitating calcium carbonate,

- removing sodium hydroxide from the mixture (washing and precipitating the mixture), and

- circulating sodium hydroxide removed from the solution back to e.g. the xylan extraction step.

Thanks to the sodium hydroxide used in the extraction, a sufficient content of xylan can be dissolved into the extraction solution and clearly more than, for example, when xylan is dissolved by means of calcium hydroxide. Thanks to this, the hardwood pulp which is used in the invention and from which xylan is extracted, can consist of unrefined or coarsely refined chemically pulped wood fibre. In an example, the Schopper-Riegler (SR) value of said chemically pulped hardwood fibre is between 10 and 40, for example between 15 and 30.

Bleached birch pulp made by the method of the invention and containing xylan and precipitated calcium carbonate can be used as raw material for e.g. paper and paperboard, advantageously as raw material for fine paper. The method according to the invention for manufacturing paper pulp is presented in claim 1. The paper pulp according to the invention is presented in claim 8. The use of paper pulp according to the invention is presented in claims 9 and 10. With the method according to the invention, it is possible to achieve several advantages. In the following, some advantages are presented, which can be achieved by applying the method of the invention.

- The solution according to the invention makes it possible to increase the bulk potential of the paper pulp, because the reduction in the content of xylan in the inner parts of the fibre, in the method according to the invention, typically increases the bulk of the product made of said paper pulp, compared with methods typically used for making paper pulp.

- The solution according to the invention also makes it possible to increase the strength potential of the paper pulp, because the transfer of xylan from the inner parts of the fibre to the outside of the fibre, preferably to the surface of the fibre, typically improves the strength properties of the product made of said fibres, compared with methods typically used for making paper pulp.

- By the method according to the invention, it is also possible to improve the binding and the retention of precipitated calcium carbonate, thanks to the xylan in the pulp suspension.

- The invention also makes it possible to use an inexpensive natural binder in the fibres to replace expensive synthetic binders. - According to an advantageous embodiment of the invention, the recyclability of sodium hydroxide for re-use brings additional advantages to the method according to the invention.

- The invention makes it possible to prepare so-called combined pulp, that is, paper pulp which already contains all the most important main raw materials for paper, that is, the filler, the fibre material, and the binder.

Description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawing, in which

Fig. 1 shows a system according to an embodiment for producing paper pulp.

Detailed description of the invention

In this application, reference is made to Fig. 1 , in which the following refer- ence numerals are used:

1 chemically pulped bleached cellulose fibres from hardwood,

2 a mixture comprising, among other things, bleached cellulose fibres and dissolved xylan,

3 a mixture comprising, among other things, bleached cellulose fibres and precipitated xylan,

4 a mixture comprising, among other things, bleached cellulose fibres, precipitated xylan, and precipitated calcium carbonate,

5 alkali extraction,

6 precipitation of xylan,

7 precipitation of calcium carbonate,

8 washing and precipitation of pulp, and

10 paper pulp.

In the present invention, the term paper pulp 10 refers to a mixture containing precipitated calcium carbonate (PCC) as a filler, xylan as a binder, and chemically pulped cellulose fibres from hardwood as a fibre raw material. Preferably, the fibre raw material for the paper pulp comprises chemically pulped and bleached cellulose fibres from birch, because birch has a particularly high xylan content. The chemically pulped cellulose fibres 1 used as the raw material may have been made and bleached by a method of prior art. The paper pulp according to the invention is advantageously raw material for paper or paperboard, most advantageously raw material for fine paper.

The term cellulose fibre refers to fibres from hardwood pulp, particularly birch pulp, which cellulose fibres are, in other words, made of hardwood by a chemical pulping method.

Xylan is naturally present, among other things, in the inner parts of cellulose fibres of hardwood. Xylan is an intrinsically good binder, which is why the transfer of xylan from the inner parts of said fibres onto the surfaces of the fibres and/or into the fibre suspension increases the strength properties of the product formed of said fibres.

The cellulose fibres 1 according to the invention may comprise unrefined, slightly refined, and/or wet beaten cellulose fibres. According to a advantageous example, the Schopper-Riegler (SR) value of the cellulose pulp used as the fibre raw material ranges from 10 to 40, for example from 15 to 30. When slightly refined cellulose fibres are used as the raw material, the manufacturing costs of the fibre raw material are typically reduced when compared with more intensively refined cellulose fibres. This is due to the reduced refining costs. According to an advantageous example, the refining degree of cellulose fibres is determined according to the target properties of the paper pulp being manufactured. In an advantageous example, the manufacture of the paper pulp according to the invention is integrated in a pulp mill.

The method according to the invention comprises one or more of the following steps:

- Alkali extraction 5 of the pulp, in which step xylan is extracted by sodium hydroxide from cellulose fibres 1 into the extraction solution. - Precipitation 6 of xylan, in which step xylan in the extraction solution is precipitated by carbon dioxide.

- Precipitation 7 of calcium carbonate, in which step calcium hydroxide is added to the mixture, to precipitate calcium carbonate.

- The washing and precipitation 8 of the pulp mixture, in which step sodium hydroxide is removed from the mixture. In the precipitation, it is possible to use, for example, a filter. There may be one or more cycles of washing and/or precipitation. Sodium hydroxide is removed from the mixture formed during this step.

- Process of recirculation of the filtrate formed during at least one step of washing and precipitation 8. Thus, sodium hydroxide removed from the solution is recycled back to e.g. the xylan extraction step.

In the alkali extraction 5 of the pulp, bleached cellulose fibres 1 are extracted with sodium hydroxide to dissolve xylan from said fibres to the extraction solution. The extraction can be performed, for example, in connection with or after the last bleaching step of said bleached cellulose fibres 1. According to an advantageous example, the extraction is performed at a fibre consistency of about 10%, for example at a consistency of 3 to 25%. In an example, the extraction is performed at a fibre consistency of 3 to 5%. In an example, the extraction is performed at a fibre consistency of 5 to 15%.

Advantageously, the extraction is performed with 0.25 to 1.25 M sodium hydroxide (NaOH), more advantageously with 0.5 to 1.0 M sodium hydroxide. According to an advantageous example, the extraction time is about one hour at room temperature. The pH of the mixture 2 formed after the extraction may be, for example, between about 12 and 13.

In connection with the extraction 5 according to the invention, xylan is dissolved from hardwood fibres into the extraction solution. Thus, xylan is also dissolved from the inner parts of the fibres, which has an advantageous effect on the bulk of the product to be made of the pulp. According to an advantageous example, the extraction of xylan is continued until 2 to 100%, more advantageously 5 to 50% or 5 to 25% of the xylan in the fibres has been extracted from the fibres into the extraction solution. The precipitation 6 of xylan, in which step xylan is typically precipitated into white polymer, is performed by adding gaseous carbon dioxide to the fibre suspension 2 after the extraction. Thus, precipitated xylan is formed of the xylan in dissolved form. Furthermore, as a result of the reaction, sodium car- bonate is formed as follows:

2 NaOH + C0₂ (g) -> Na₂C0₃ (aq) + H₂0.

The degree of purity of the carbon dioxide used for precipitating xylan is pref- erably between 10 and 100%. A suitable amount of carbon dioxide is added until the pH of the mixture is between 7.5 and 10. In an example, the precipitation of xylan is intensified by adding a component to intensify the precipitation into the mixture. The precipitation 7 of calcium carbonate is performed by calcium hydroxide. The addition of calcium hydroxide into the fibre suspension not only precipitates calcium carbonate but also converts sodium carbonate, formed in connection with the precipitation of xylan, into sodium hydroxide. The reaction, in which calcium hydroxide reacts with formed sodium carbonate in such a way that precipitated calcium carbonate (PCC) and sodium hydroxide are obtained as the final products, is the following:

Na₂C0₃ + Ca(OH)₂ -> 2 NaOH + CaCO₃. The pulp mixture 4 formed by the effect of sodium hydroxide resulting from the reaction between sodium carbonate and calcium hydroxide, and containing precipitated calcium carbonate, precipitated xylan and fibres, is typically very alkaline (pH typically >13). The reaction conditions, by which it is possible to affect the PCC particle size and quality (temperature, pH, time, concentration, among other things) are preferably determined in such a way that the desired, predetermined PCC particle size and shape are obtained. After the precipitation of calcium carbonate, the pulp mixture 4 is thickened and washed 8. In connection with the thickening and washing 8, the aim is typically to retain as high a content as possible of PCC in the pulp to be thickened. According to an advantageous example, the filtering and the consequent thickening of the pulp mixture take place in a filter and/or a funnel. According to an advantageous example, the pulp mixture 4 is washed, in connection with the thickening and washing 8, with a solution, such as water, at least once, more advantageously at least twice or at least three times. Thus, according to an advantageous example, at least part of the formed filtrates are recovered. Preferably, the recovered filtrates or at least part of them are recirculated. Typically, at least the first washing filtrates comprise strong sodium hydroxide, and their pH may be, for example, about 13. According to an advantageous example, the washing of the pulp mixture with the solution is continued until the pH is close to neutral. The sodium hydroxide containing filtrate recovered from the process can be re-used, for example, for the extraction of xylan, or it can be led to another process, or it can be recovered for another further use. Said filtrate recovered from the process can be treated, to increase the degree of purity of the sodium hydroxide.

In the invention, it is essential that there is no attempt in any step to remove xylan from the pulp suspension to be made, but the xylan extracted from the cellulose fibres is allowed to remain in connection with the cellulose fibres in the extraction solution. Thus, when xylan is precipitated in the pulp suspen- sion, the xylan can simultaneously adhere onto the surfaces of the fibres. The precipitation of xylan with carbon dioxide is performed in such a way that the precipitation step 6 is started in a strongly alkaline extraction solution. As the precipitation proceeds, the pH of the extraction solution reduces, thanks to the carbon dioxide addition. Preferably, the pH of the mixture is maintained alkaline (pH >7) during the whole precipitation of xylan.

The xylan precipitation step is followed by a calcium carbonate precipitation step 7, in which step 7 calcium hydroxide is added into the mixture. Also, the calcium hydroxide addition is made to the whole fibre suspension. Thus, cal- cium carbonate is precipitated in the fibre suspension that contains, among other things, cellulose fibres and precipitated xylan. Thanks to the method according to the invention, calcium carbonate is also precipitated onto the surfaces of the fibres. Thus, xylan extracted and precipitated in the pulp suspension in the previous step can act as a binder for the calcium carbonate particles.

According to an advantageous example, paper pulp made by the method of the invention is used as raw material for paper or paperboard, preferably as raw material for fine paper. By the method according to the present invention, it is possible to manufacture paper pulp that is suitable as raw material for fine paper, among other things, and contains at least cellulose fibres, a filler and a binder, in a cost effective way. In the solution according to the invention, the binder used is xylan which is intrinsically involved in the process and which is utilized in an efficient way, thanks to the method of the invention. As a result, it is possible to increase the filler content in a cost effective way so that the costs remain typically lower than when, for example, separate, expensive synthetic adhe- sives are used. The invention is not limited solely to the examples presented in Fig. 1 and in the above description, but the invention is characterized in what will be presented in the following claims.

Claims

Claims:

1. A method for manufacturing paper pulp (10), in which method bleached, chemically pulped cellulose fibres (1 ) from hardwood are used as fibre raw material, characterized in that the method comprises, in the following order: extracting (5) xylan from said cellulose fibres (1 ) by means of sodium hydroxide,

adding carbon dioxide to the mixture (2) containing said cellulose fibres (1 ), dissolved xylan and sodium hydroxide, for precipitating the xylan,

adding calcium hydroxide into said mixture containing precipitated xylan and cellulose fibres, for forming precipitated calcium carbonate (7), and

removing sodium hydroxide from said mixture (4) containing sodium hydroxide, xylan, cellulose fibres (1 ), and calcium carbonate.

2. The method according to claim ^ characterized in that sodium hydroxide removed from said mixture (4) is recirculated for re-use, for example for extracting xylan.

3. The method according to claim 1 or 2, characterized in that the method comprises adding carbon dioxide to precipitate (6) xylan, until the pH of the mixture is between 7.5 and 10.

4. The method according to any of the claims 1 to 3, characterized in that the method is integrated in chemical pulping comprising one or more bleaching steps, wherein said extraction (5) is performed in connection with or after the last bleaching step of said cellulose fibres (1 ).

5. The method according to any of the claims 1 to 4, characterized by extracting (5) 5 to 50% of the xylan contained in said cellulose fibres (1 ).

6. The method according to any of the claims 1 to 5, characterized in that said mixture (4) is washed with a solution, preferably with water, in connec- tion with the removal of sodium hydroxide.

7. The method according to claim 6, characterized in that said cellulose fibres (1 ) have been refined, and that the refining degree of the cellulose fibres (1 ) at the beginning of the extracting is between 10 and 40 in the Schopper Riegler (SR) number scale.

8. Paper pulp (10) manufactured by a method according to any of the claims 1 to 7.

9. Use of paper pulp (10) made by the method according to any of the pre- ceding claims 1 to 7, as raw material for paper or paperboard.

10. The use according to claim 9, characterized in that the paper pulp (10) is used as raw material for fine paper.