WO2008132228A1

WO2008132228A1 - A method for reducing the energy consumption at the refining of a pulp suspension in a papermaking process and use of sodium bicarbonate in papermaking

Info

Publication number: WO2008132228A1
Application number: PCT/EP2008/055340
Authority: WO
Inventors: Luminita Claudia Schmid; Eva Ann-Charlotte Jansson; Ralf Jankowiak
Original assignee: Linde Aktiengesellschaft; Papierfabrik August Koehler Ag
Priority date: 2007-04-30
Filing date: 2008-04-30
Publication date: 2008-11-06
Also published as: FI120651B; FI20075301A; FI20075301A0; EP2140064A1

Abstract

The invention relates to a method for reducing the energy consumption at the refining of a pulp suspension in a papermaking process. The reducing of the e nergy consumption is provided by treating said pulp suspension with non-stoichiometric sodium bicarbonate. Due to the treatment with sodium bicarbonate the calcium hardness value of the said pulp suspension is decreased to a level having an advantageous ef fect on the energy consumption and at the same time the pH value is adjusted and maintained at the desired level in the refining and preferably also throughout the subsequent paper making. The invention also relates to use of non-stoichiometric sodium bica rbonate in the papermaking process.

Description

A METHOD FOR REDUCING THE ENERGY CONSUMPTION AT THE REFINING OF A PULP SUSPENSION IN A PAPERMAKING PROCESS AND USE OF SODIUM BICARBONATE IN PAPERMAKING

Field of the invention

The invention relates to a method for reducing the energy consumption at the refining of a pulp suspension in a papermaking process and use of non- stoichiometric sodium bicarbonate in this papermaking process.

Description of related art

Refining is one of the most important unit operations when preparing papermaking fibers for high-quality papers or paperboards. Refining is of importance both to the properties of mechanical pulp and to those of chemical pulp. The chemical environment, like pH and various metal ions etc. during refining and sheet forming, is known to have an influence on the final sheet properties.

A problem when manufacturing paper or board from chemical or mechanical pulp is the high energy consumption during refining of the pulp. Many studies for reducing the energy consumption at the refining have been conducted. US 5,007,985 discloses a process using alkali with the cellulose-containing material for neutralizing acid groups bonded to the fiber wall for reducing the energy consumption during the refining. US 5,454,907 discloses a method for treatment of chemical pulp in the papermaking wherein the energy consumption is reduced by using a refining assisting agent comprising sulfonated chitosan. WO 2004/053224 discloses a method for treatment of chemical pulp wherein a refining assisting agent comprises a complexing agent.

There is still a need to reduce the energy consumption at the refining of a pulp suspension in the papermaking process by more simple and efficient treatment of pulp. The invention

An object of the present invention is to provide the refining of a pulp suspension in a papermaking process wherein the refining is an energy efficient process.

Another object of the invention is to provide for the refining a papermaking pulp suspension having a decreased calcium hardness value.

An object of the invention is also to provide for the refining a pulp suspension having a controlled pH which is maintained at a desired level.

A further object of the invention is to enable refining at optimum conditions regarding pH, ion concentration and related electrical properties of the pulp suspension.

An object of the invention is also to provide a refining which improves the paper quality as regards strength and/or porosity.

The invention according to the present application is defined in the appended claims, the contents of which are included herein by reference.

Consequently, the present invention relates to a method for reducing the energy consumption at the refining of a pulp suspension in a papermaking process. The method is characterized in that a pulp suspension containing solid calcium carbonate is treated with non-stoichiometric sodium bicarbonate for providing in said pulp suspension, as it enters said refining a calcium hardness value below 10 °dH.

The reducing of the energy consumption is provided by treating said pulp suspension with sodium bicarbonate. Due to the treatment with sodium bicarbonate the calcium hardness value of the said pulp suspension is decreased to a level having an advantageous effect on the energy consumption and at the same time the pH value is adjusted and maintained at the desired level in the refining and preferably also throughout the subsequent paper making.

According to the present invention there is provided a method for refining an aqueous pulp suspension wherein the refining is accomplished with less energy. Typically, the hardness value of a pulp suspension is, after a diluting with process water, 10 °dH or higher. This has been found to cause increased energy consumption at refining. The process water from the paper machine used in the diluting contains some solid calcium carbonate used as filler when the paper was produced. For instance, in a non-integrated mill, dry pulp is slushed using process water for dilution. The process water is circulated to slushing. Each time the water comes into contact with the acidic pulp or acidic process chemicals dissolve, which leads to fioormation of free calcium ions. The accumulation of calcium ions due to recirculation of process water causes an even increasing hardness value. This, in turn has been found to increase the energy consumption at refining.

It has now been surprisingly found that refining of the pulp suspension is accomplished with less energy if the calcium hardness value of said pulp suspension is adjusted to below 10 °dH with non-stoichiometric sodium bicarbonate.

According to the preferred embodiment of the invention the calcium hardness value can be decreased below 10 °dH, preferably below 7 °dH, by treating the pulp suspension with non-stoichiometric sodium bicarbonate solution.

According to the preferred embodiment of the invention the treatment of the pulp suspension with non-stoichiometric sodium bicarbonate is performed before the refining. The sodium bicarbonate solution is produced by reacting gaseous carbon dioxide and aqueous sodium hydroxide. The ratio is typically between 0.5 and 2.0, preferably between 1.2 and 1.7, and most preferably 1.4 and 1.6, respectively. The carbon dioxide and sodium hydroxide feeds may be added to the pulp suspension in a pulper or a storage tower, or to the process water used in the dilution. Typically, the sodium bicarbonate comprises non-stoichiometric sodium bicarbonate produced by adding carbon dioxide and sodium hydroxide separately or simultaneously to the pulp suspension. The suspension may, for instance, be treated as it flows in a pipe leading to a pulp dumping chest or the like.

The feeding of sodium bicarbonate obtained by the addition of carbon dioxide and sodium hydroxide in this way corresponds to the one obtainable by an addition of dissolved sodium bicarbonate, but has the advantage that the space consuming and messy handling of solid sodium bicarbonate is avoided. Sodium hydroxide, on the other hand, is a chemical which is abundantly available in the paper mill since it is used for many other purposes. Carbon dioxide gas may be generated on site or may be bought as desired. Feeding of carbon dioxide into the suspension is technically clean and easy.

The carbon dioxide is preferably in gaseous form, although it may be added as an aqueous liquid by dissolving gaseous or solid carbon dioxide in water. Carbon dioxide and sodium hydroxide may be added separately or simultaneously to the pulp suspension or to the process water used in the dilution.

A further advantage resides in the treatment with non-stoichiometric sodium bicarbonate in that the pH value can be adjusted and maintained also by providing a suitable ratio of carbon dioxide to sodium hydroxide. The ratio should be sufficient to provide a significant buffering effect at a pH value between 6.0 and 8.0. Processes for stabilizing the pH of a pulp suspension at a desired pH level are disclosed in US 6,991,705 and US 7,175,759. These processes are suitable for adjusting the pH value also in the present invention. The pulp suspension is preferably buffered by carbon dioxide and sodium hydroxide combination to a pH value below 8.0 and above 6.0, preferably below 7.3 and above 6.8.

As a consequence of the stabilized pH and the increase of bicarbonate ions and decrease of calcium ions in the sodium bicarbonate treated pulp suspension, the electrical properties of the pulp suspension will also change. As is well known to those skilled in the art, the pH and reduced calcium carbonate dissolution affects not only the cation concentration (Ca²⁺) but also influences other electrical properties such as the z-potential of the pulp suspension. As a consequence of the reduced amount of positively charged calcium ions, which compete with cationic chemicals to absorb onto the fibers, the amount of cationic chemicals which can be adsorbed onto the fibers increases and thus, e.g. it will be easier to apply the desired amount of cationic starch to the paper.

According to the present invention the energy consumption at the refining may be reduced by 25 to 70 % compared to the refining of a similar suspension having the same pH but without being treated with said non- stoichiometric sodium bicarbonate.

Refining is used by mills e.g. to improve the strength of the fibers and thus the strength of the paper produced from the refined pulp suspension. Furthermore, the porosity of the paper may be controlled by refining the pulp. Papermakers may set a target porosity to obtain paper having a desired printability and/or coating hold-out.

The pulp suspension may be bleached or unbleached, chemical or mechanical pulp.

Calcium carbonate may advantageously be used as filler for the pulp after the refining, since the stabilized pH will ascertain that the filler remains in solid form in the suspension. Less dissolved calcium carbonate also provides a paper with less problems with pinholes since there is less gas formed in the process. Solid calcium carbonate may also enter the process by being added as filler or enters the process as filler and/or coating pigment of reclaimed fiber. The reclaimed fiber typically comprises also broke.

The present invention also relates to a method for reducing the energy consumption at the refining of a pulp suspension in a papermaking process, said method comprising the steps of: - providing a pulp suspension containing solid calcium carbonate at a first pH value;

- combining carbon dioxide and sodium hydroxide in a ratio, which provides sodium bicarbonate at a pH value between 7.0 and 8.5;

- adding said sodium bicarbonate to said pulp suspension for adjusting the pH of said pulp suspension to a second pH value below 8.0 and above 6.0 and adjusting the hardness of said pulp suspension to a value below 10 °dH; and

- refining said pulp suspension at said second pH value and said hardness value.

The present invention relates also to the use of non-stoichiometric sodium bicarbonate for reducing the energy consumption at the refining of a pulp suspension in a papermaking process and furthermore to the use of non-stoichiometric sodium bicarbonate for increasing the strength of paper produced from a refined pulp suspension. The invention also relates to the use of sodium bicarbonate for decreasing the porosity of paper produced from a refined pulp suspension.

The invention will be illustrated with the following examples which should not be considered as limiting the invention in any way.

Example 1

In a paper mill producing paper with calcium carbonate as filler, a kraft pulp suspension is fed to a refining chest. To the pulp is added carbon dioxide and sodium hydroxide in a ratio 1.5-1.6, which provides sodium bicarbonate. The hardness value and the pH of the pulp suspension are measured at refining. The results obtained are shown in the table 1.

Table 1

The hardness value is clearly reduced by using sodium bicarbonate at refining. The results significantly show that the energy consumption at the refining is reduced.

Example 2

A laboratory refining trial was performed to test the influence of the reduced hardness obtainable by sodium bicarbonate. For each refining point, 500 g of hardwood kraft pulp was soaked and disintegrated in water. The hardness of the aqueous pulp was adjusted so that the final hardness was 3 or 22 °dH, respectively. The pH of both pulps was 6.8. Also the conductivity was the same for both hardness levels.

Refining was performed on pulp samples of each hardness level at 0.5 kW during O, 3, 6 and 10 minutes. This corresponds to a specific refining energy of 0, 50, 100 and 166.7 kWh/t, respectively.

After refining, the samples were diluted with soft fresh water for sheet forming. Prior to sheet forming, the hardness of the samples was adjusted so that they all had the same hardness at sheet forming.

The results showed that paper made from the pulp refined at 3°dH was stronger than paper made from the pulp refined at 22°dH when comparing at the same refining time (energy input). This means that it is possible to use less energy to reach a certain strength target when refining pulp at lower hardness. The results indicate that 38% less energy was needed to reach a breaking length of 6000 m when refining the pulp at 3 °dH compared to when refining the pulp at 22 °dH.

Example 3

A paper mill produces high quality paper for printing from kraft pulp. The pulp is refined before web forming in order to increase the surface strength of the paper. The paper strength is monitored and the refining is adjusted to reach a desired surface strength, which will ensure that there will be no problems with picking during printing. Calcium carbonate is used as filler in the paper production.

When no sodium bicarbonate is added to the pulp, the hardness of the pulp during refining varies between 15 and 20 °dH. The energy consumption at refining is considered to be too high.

In order to reduce the energy consumption, a non-stoichiometric NaHCO 3 solution is produced from carbon dioxide and sodium hydroxide in a reactor and the solution is added to the pulp suspension before refining. The addition of sodium bicarbonate reduces the concentration of calcium ions in the suspension due to reduced dissolution of solid filler CaCO₃. Sodium bicarbonate is added in an amount which provides, in the resulting pulp suspension, a hardness value below 10 °dH. When the reduced hardness pulp is refined, the energy consumption at refining to the same paper surface strength as before the sodium bicarbonate addition is reduced.

When the hardness is lowered to a value between 4 and 8 °dH, the desired surface strength is achieved with significantly less refining energy. The savings in refining energy vary between 12 and 30%, depending on paper grade.

Example 4 A mill controls the porosity of its paper by refining the pulp to obtain a target porosity. The pulp is refined in 3 refiners. Calcium carbonate is used as filler and some of it is dissolved as a result of the pulp being slightly acidic and some process chemicals being acidic. The hardness of the pulp varies between 15 and 20 °dH during refining. In the short circulation the hardness is even higher.

The mill changes its process and starts adding sodium bicarbonate according to the invention into the process in order to reduce the number of refiners and the energy of the refining.

As a result of the change, the dissolution of calcium carbonate is significantly retarded and the hardness is reduced. The sodium bicarbonate is adjusted so as to provide a hardness value between 4 and 8 °dH during refining. The hardness in the short circulation is also lowered to below 10 °dH.

With the sodium bicarbonate addition, the mill only needs to use 2 refiners to obtain the target porosity. The energy required for refining is thus diminished by about 1/3.

Also the paper has significantly less problems with pinholes since there are fewer problems with gas formed in the process.

Claims

1. A method for reducing the energy consumption at the refining of a pulp suspension in a papermaking process, characterized in that a pulp suspension containing solid calcium carbonate is treated with non- stoichiometric sodium bicarbonate for providing in said pulp suspension, as it enters said refining a calcium hardness value below 10 °dH.

2. A method according to claim 1, wherein said pulp suspension comprises chemical and/or mechanical pulp.

3. A method according to claim 2, wherein said pulp suspension has been diluted with process water containing solid calcium carbonate.

4. A method according to claim 2, wherein said pulp suspension contains solid calcium carbonate, which has been added as filler or has entered the process as filler and/or coating pigment of reclaimed fiber.

5. A method according to any one of claims 1 to 4, wherein the calcium hardness of said pulp suspension prior to said treatment is 10 °dH or higher.

6. A method according to claim 1, wherein the pH value of said suspension, as it enters said refining, is below 8.0 and above 6.0, preferably below 7.3 and above 6.8.

7. A method according to claim 1, wherein said sodium bicarbonate comprises a non-stoichiometric sodium bicarbonate solution produced by reacting gaseous carbon dioxide and aqueous sodium hydroxide prior to said treatment.

8. A method according to claims 5, wherein the pH of said sodium bicarbonate solution is adjusted by adjusting the amounts of carbon dioxide and sodium hydroxide, respectively.

9. A method according to claim 7, wherein the ratio of carbon dioxide to sodium hydroxide is varied between 0.5 and 2.0, preferably between 1.2 and 1.7, most preferably between 1.4 and 1.6.

10. A method according to claim 1, wherein sodium bicarbonate comprises non stoichiometric sodium bicarbonate produced by adding carbon dioxide and sodium hydroxide separately or simultaneously to said pulp suspension.

11. A method according to claim lor 3, wherein sodium bicarbonate comprises non stoichiometric sodium bicarbonate produced by adding carbon dioxide and sodium hydroxide separately or simultaneously to said process water, with which said pulp suspension is diluted.

12. A method according to claim 1, wherein said sodium bicarbonate comprises non-stoichiometric sodium bicarbonate produced by adding carbon dioxide and sodium hydroxide separately or simultaneously to said pulp suspension flowing in a pipe.

13. A method according to any one of the preceding claims, wherein the energy consumption at said refining is reduced by 25 to 70 % compared to the refining of a similar pulp suspension having the same pH but without being treated with said non-stoichiometric sodium bicarbonate.

14. A method according to any one of the preceding claims, wherein said pulp suspension is refined and processed into paper.

15. A method according to any one of the preceding claims, wherein said pulp suspension is processed into paper having an increased strength compared to a paper produced from a similar pulp suspension having the same pH but without being treated with said non-stoichiometric sodium bicarbonate.

16. Use of non-stoichiometric sodium bicarbonate for reducing the energy consumption at the refining of a pulp suspension in a papermaking process.

17. Use of non-stoichiometric sodium bicarbonate for increasing the strength of paper produced from a refined pulp suspension.

18. Use of non-stoichiometric sodium bicarbonate for decreasing the porosity of paper produced from a refined pulp suspension.

19. A method for reducing the energy consumption at the refining of a pulp suspension in a papermaking process, characterized in providing a pulp suspension containing solid calcium carbonate at a first pH value combining carbon dioxide and sodium hydroxide in a ratio, which provides sodium bicarbonate at a pH value between 7.0 and 8.5 adding said sodium bicarbonate to said pulp suspension for adjusting the pH of said pulp suspension to a second pH value below 8.0 and above 6.0 and for adjusting the hardness of said pulp suspension to a value below 10 °dH and refining said pulp suspension at said second pH value and said hardness value.

20. A method according to claim 19, wherein said refined pulp suspension is processed into paper.