WO2004053224A1

WO2004053224A1 - Method for reducing the energy demand during refining of chemical pulp

Info

Publication number: WO2004053224A1
Application number: PCT/SE2003/001924
Authority: WO
Inventors: Lars-Åke HAMMAR; Marie BÄCKSTRÖM; Myat Htun
Original assignee: Stfi, Skogsindustrins Tekniska Forskningsinstitut Ab
Priority date: 2002-12-12
Filing date: 2003-12-10
Publication date: 2004-06-24
Also published as: SE0203673D0; AU2003302773A1

Abstract

The present invention provides a method for treatment of chemical pulpfor the manufacturing of paper or paperboard whereby reducing the energy demand during refining comprising the following steps: a) treatment of the pulp with a complexing agent prior to refining; and b) refining the pulp in at least one step.

Description

Method for treatment of pulp

This invention concerns the technical field of pulp treatment wherein the energy consumption during refining can be decreased. More specific the present invention relates to a method for treatment of pulp with one or more complexing agents before refining/beating wherein said method preferably is used for the manufacture of paper or board with origin in chemical pulp.

Background

Pulp for making paper or related products may be obtained from cellulose and other wood polymers contained in soft wood or hard wood raw material. The raw material, e.g. soft wood, may be chipped into chips and be delignified (if chemical pulp) and subsequently ground or refined into fine particles which in turn gives rise to a pulp (which may be so called chemical pulp if using a chemical process).

A process using mechanical pulp from wood is disclosed in WO 9106700, WO 8906717, US 3023140 and SU 1234483, respectively, where a complexing agent is added before refining. None of the documents discloses essentially chemical pulps and their refining/beating before paper or related products are manufactured. Mechanical pulps all contain wood material, which in turn contain lignin, hemicellulose and cellulose. The purpose of the complexing agents is to soften the stiff lignin and assist in the defibrillation process. Further it is disclosed in US 5,454,907 a method for refining woodchips or beating pulp, wherein energy consumption is reduced by using a refining assisting agent comprising sulfonated chitosan. Nothing is disclosed regarding the use of a complexing agent. The refining assisting agent has a completely different mode of action than e.g. complexing agents. Further, refining is performed at an essentially higher fibre concentration than the beating.

A problem when manufacturing paper and board from chemical pulp is the high energy consumption during the refining (beating) of the pulp. Normally reduction of energy consumption is achieved by raising the pH of the system by addition of alkali such as NaOH. Further a washing step is often needed in connection with the beating of the pulp. This washing step is further time consuming and costly.

Thus there is a need for a process wherein the energy consumption can be reduced without changing too much of the pH, wherein said process also achieves a more efficient processing and a more simple treatment of the pulp (whereby the above mentioned washing step may be omitted), thereby solving one or more of the said problems in the paper manufacturing processes as e.g. set out above. Summary of the invention

The present invention solves the above problems by providing a method for treatment of chemical pulp for the manufacturing of paper or paperboard whereby reducing the energy demand during refining comprising the following steps: a) treatment of the pulp with a complexing agent prior to refining, and b) refining the pulp in at least one step.

The present method relates in particular to unbleached chemical pulps, i.e. they are essentially already delignified and defibrillized. The present method may however also be used in connection with bleached chemical pulps. The treatment is preferably performed before the refining (beating) of the fibres. The fibre material here essentially consists of low concentrations of remaining lignin and hemicellulose, and for the rest cellulose. The complexing agents mode of action is here to make the fibres swell better and make them more sensitive for efficient processing. The refining (beating) is preferably performed in an aqueous environment in which the concentration of the fibres may vary between 2 and 10 %. The refining of wood chips or any fraction of a pulp is normally performed in a steam environment with a chips concentration from 20 to 40%. In the present invention the corresponding effect of complexing agent on the energy consumption is achieved even though the pulp is not washed before the beating. Accordingly it is not necessary to wash the pulp before the beating and thus the whole process can be simplified. Earlier it was thought that the ion-exchanging with a complexing agent required a washing step. Normally an acidic environment is used which calls for a washing step. This is accordingly not necessary in this case as set out above. The complexing agent can follow the fibres through the mill without the need for a specific washing step for removing the complexing agents.

Detailed description of the invention

It is intended throughout the present description that the expression "refiner" embraces any apparatus capable of refining (beating) chemical pulp. Examples of beating apparatuses are beaters and refiners equipped either with refining discs (disc refiners) or a refining plug in a conical housing (conical refiner). A beating apparatus may operate continuously or discontinuously. The beating of pulp is preferably performed using a relatively small amount of energy. The energy consumption may be in the range of approximately 5- 2000 kWh/ADt (air dry tonne).

It is intended throughout the present description that the expression "complexing agent" embraces any compound capable of complexing metal ions in a fibre suspension (said expression also encompasses any chelating agent). The complexing agent may be selected from the group consisting of :DTPA (Diethylene-triamine-pentaacetic acid), EDTA (Ethylene-diamine-tetraacetic acid), HEDTA (Hydroxiethylenethylen-diamine-triacetic acid), NTA (Nitrile- triacetic acid), DHEG (N,N-di(2-hydroxiethyl)glycine), TEA (Triethanolamine), NTP (Nitrile-trimethylenephosphonic acid), MIDA (N-methylimine-diacetate), IDA (Imindiacetate), HEIDA (Disodium-hydroxi-ethylimien-diacetate), DTPMPA (Diethylene- triamine-pentamethylen-phosphonic acid), EACDA (Ethylamine-cyclopentene-1- dithiocarboxylic acid), CDTA (Cyclohexylene-diamine-triacetic acid), Poly-carboxylates (including -phosphonate and -sulphonate) such as POC (Poly-(hydroxycarboylate), multivalent carboxylates such as Na-citrate, Glukonic acidlactone, Na-tartrate, multivalent phosphates such as STPP (Na-tripolyphosphate); MTPP (Bis-phosphonylmethylphosphinic acid) and Poly(sodium-α-hydroxyacrylate. Preferably the complexing agent is DTPA, Diethylene-triamine-pentaacetic acid, or a derivative thereof. The complexing agent is preferably added to the pulp before the beating or direct into the refiner during the refining process. It is also plausible that the complexing agent is added in the mill where the ion exchange may be performed during a relatively short time period. The refining of the pulp may involve more than one refining step, e.g. it may involve two consecutive refining steps after the treatment, preferably through an addition, with a complexing agent such as DTPA. The addition of a complexing agent may also be performed intermediate the refining steps, if two refining steps are used.

The chemical pulps that may be used in the present invention include all types of chemical wood-based pulps, such as bleached, half-bleached and unbleached sulphite, sulphate and soda pulps, kraft pulps together with unbleached, half-bleached and bleached chemical pulps, and mixtures of these. The consistency of the pulp may be any consistency, ranging from low consistency through medium consistency to high consistency. The consistency is preferably from 1 to 10 %. According to a preferred embodiment of the present invention the DTPA is added in a range of from 3 - 30 kg substantially pure DTPA per ADt, preferably 10 to 30 kg of 100% (w/w) DTPA per ADt. The dosage depends upon the amount of multivalent metal ions present in the pulp.

According to a preferred embodiment of the present invention the refining in step b) uses an energy input of from approximately 10 to 2000 kWh/ADt , preferably from approximately 50 to 200 kWh ADt, most preferred 10 to 200 kWh /ADt.

The paper or paper board, obtainable by the method according to the present invention may e.g. be used for the manufacture of liquid board, various communication papers, such as newsprint grades, super-calendered SC-grades and coated communication papers such as light weight coated papers (LWC), MWC (medium weight coated) and HWC (high weight coated) and various packaging papers, sack, kraft- and test liners and various board grades from recycled boards, folding boxboards and solid bleached boards.

The present method according to the present invention does not require any particular washing steps for disposing of the complexing agents. Washing steps are normally very costly when using such in bleaching of pulp. Thus washing steps may be omitted after the addition of complexing agents, thereby solving an additional problem in paper manufacturing processes.

Preferred features of each aspect of the invention are as for each of the other aspects mutatis mutandis. The prior art documents mentioned herein are incorporated to the fullest extent permitted by law. The invention is further described in the following examples in conjunction with the appended figures, which do not limit the scope of the invention in any way. Embodiments of the present invention are described in more detail with the aid of examples of embodiments, the only purpose of which is to illustrate the invention and are in no way intended to limit its extent.

Short description of the figures

Fig. 1 shows the WRV vs. specific refining energy for an unbleached kraft pulp refined with DTPA and without DTPA.

Fig. 2 shows the tensile index vs. specific refining energy input for an unbleached kraft pulp refined with DTPA and without DTPA.

Fig. 3 shows the tensile index vs. apparent density for an unbleached kraft pulp refined with DTPA and without DTPA.

Fig. 4 shows the tensile stiffness index vs. apparent density for an unbleached kraft pulp refined with DTPA and without DTPA. Fig. 5 shows the out-going solids content after the press section vs. specific refining energy input during refining for an unbleached a kraft pulp refined with DTPA and without

DTPA.

Fig. 6 shows the tensile index vs. out-going solids contents after the press section for a unbleached kraft pulp refined with DTPA and without DTPA. The press line load combination was 60 kN/m in the first roll press and 500 kN/m in the first shoe press and 700 kN/m in the second shoe press.

Example

Example 1 : Treatment of chemical pulp with DTPA

In an example, a chelating agent, i.e. a complexing agent, was used to ion exchange the multivalent to monovalent cationic which is associated with the charged groups in the pulp fibers in order to reduce the energy consumption during refining. DTPA, a chelating agent, was added to the pulp (DTPA also alters the ionic form into sodium form.) which was suspended in softened water at 3,5% pulp consistency. The addition of DTPA was 1:1 , i.e. 1 equivalent of DTPA per equivalent of multivalent ions.

In the following table, i.e. Table 1, the result from ion-exchanging a pulp using DTPA is shown. An unbleached kraft pulp with kappa number 31 was used. As can be seen from the table, the amount of Na⁺ is rather low in the untreated pulp, while treatment with DTPA decreases the amount of divalent ions such as Ca²⁺ and increases the amount of Na⁺. DTPA can also remove other divalent ions from the pulp fibers than Ca²⁺.

Table 1.

Metal ion Untreated, mmol/kg Pulp treated with DTPA, mmol/kg Ca²⁺ 37 Ϊ2

Mn²⁺ 1

Na⁺ 7 83

The effect of the counter ion on the energy demand during refining

The above-described developed technique was used to ion-exchange large quantities of unbleached pulp, which was then used in pilot scale trials. The trial was performed at a research paper machine the so called EuroFEX during dynamic conditions. EuroFEX was equipped with commercial-like refiners that make the refining results fully comparable with industrial refiners.

During the last trial four different refining levels were run. The beating was performed with DTPA-treated pulp and untreated. The untreated pulp is the reference case and can be considered as being more or less in calcium form. The reference case was refined in tap water at EuroFEX. The DTPA-treated pulp was refined in softened tap water. The use of softened water was necessary to retain the sodium form of the fibre. After the refining, the fibres were used as raw material for papermaking on EuroFEX using "tap water"(Ca²⁺) according to the STFIs normal procedure with re-circulated white water system. As can be seen by Figure 1 , the level of swelling is improved by changing the counter ions to sodium due to the DTPA-treatment. The measured WRV-value is dependent on the surrounding media, so values for DTPA-treated pulp both in tap water and softened water is presented in the figure. There is a 50 % reduction in energy demand to achieve a certain WRV-value if the fibres are treated with DTPA prior to refining instead of being untreated. WRV corresponds to the swelling level of the fibre and can be considered as a measurement of the beatability of the fibres. Thus, the energy reduction could be registered.

Another way to illustrate the advantages of refining DTPA-treated pulp is to present the relation between tensile index and specific refining energy input, Figure 2. If the goal is to obtain a certain level of tensile index, the results demonstrate the large energy saving potential. If the energy demand to reach a strength index of for example 75 Nm/g the specific energy demand was 125 kWh/t for the reference pulp, while it was 65 kWh/t for the DTPA- treated pulp being in sodium form. The energy reduction in the refining stage is almost 50 %.

Another way is to use this technique to improve the strength properties of the paper that is produced by using the same specific energy input and obtain a stronger product or use this strength improvement to decrease the grammage of the paper, which in turn is also beneficial from an energy point of view. Mechanical properties of paper

Previous results from the laboratory trials showed the potential of energy reduction due to improved swellability of the fibres, no change in the relation between density and paper strength was observed. In a pilot trial using EuroFEX, which includes industrial-like refining, the forming and pressing of the paper web during dynamic conditions showed an improved relation between strength and density, Figure 3. Thus, the potential of energy savings is further improved. The improvement in tensile index is approximately 10 Nm/g. The improvement in strength properties is also reflected in the tensile stiffness index as can be seen in Figure 4. Production capacity

In order to investigate how the production capacity was influenced by the fact that the fibres were refined in the sodium form, but still consolidated (in the forming and press section of the paper machine) in calcium form, different combinations of line loads in the two shoe presses were investigated. Due to the improved swelling when the fibres was ion- exchanged into sodium form, the solid contents after the press section at the pilot paper machine was slightly reduced at a given specific refining energy input. This means that the fibres contain more water after the forming and press section. This is illustrated in Figure 5 where the solids content after the press section for different press line load combinations is shown. As can be seen from the figure, the fibres refined in sodium form have generally a lower solids content than the fibres refined in calcium form at a given specific refining level. However the differences is not larger than there is an overlap between at certain press line load combinations. However, in a commercial paper machine the productivity will most probably not be affected since the need for refining is substantially reduced. The aim is generally to reach a certain strength level on the paper produced. This can be seen in Figure 6, where the tensile index and the out-going solids content for a press line load combination is shown. The figure shows that at a given tensile index, the solids content is approximately the same for the both ionic forms.

This above experiment has thus shown that the ion-exchanging technique prior to refining is a technique which reduces the energy demand. During the above pilot scale trials the ion exchanging was performed in soft water. The soft water was produced with a water softener.

We have also shown that you may use the complexing agent for the treatment of "mill-dirty" pulps as well.

Various embodiments of the present invention have been described above but a person skilled in the art realizes further minor alterations, which would fall into the scope of the present invention. The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. For example, any of the above- noted methods can be combined with other known methods. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

Claims

1. Method for treatment of chemical pulp for the manufacturing of paper or paperboard whereby reducing the energy demand during refining comprising the following steps: a) treatment of the pulp with a complexing agent prior to refining, and b) refining the pulp in at least one step.

2. Method according to claim 1 wherein the complexing agent is DTPA.

3. Method according to claim 2 wherein DTPA is added in a range of from 3 - 30 kg substantially pure DTPA per ADt, preferably 10 to 30 kg of 100% (w/w) DTPA per ADt.

4. Method according to claim 1 wherein the refining in step b) comprises freeing the fibres using en energy input of from approximately 10 to 2000 kWh/ADt, preferably from approximately 50 to 200 kWh ADt, most preferred 10 to 200 kWh /ADt.