EP4291708A1

EP4291708A1 - Method for the chemically assisted mechanical pulping of wood chips or wood shavings

Info

Publication number: EP4291708A1
Application number: EP22702851.1A
Authority: EP
Inventors: Juha FISKARI
Original assignee: Mondi AG
Current assignee: Mondi AG
Priority date: 2021-02-12
Filing date: 2022-01-31
Publication date: 2023-12-20
Also published as: AT524693A1; AT524693B1; WO2022170373A1

Abstract

The invention relates to a method for the chemically assisted mechanical pulping of wood chips, said method comprising: a) producing a feedstock from wood chips; b) bringing the feedstock into contact with steam at a pressure of between 1 and 2 bars and a steam temperature of between 100 and 110°C for 10 to 20 minutes in order to produce a steamed wood chip feedstock; c) bringing the steamed wood chip feedstock into contact with a solution containing a deep eutectic solvent comprising a quaternary ammonium salt and a co-solvent at between 40 and 120°C and for 5 and 15 minutes in order to obtain a pretreated wood chip feedstock, a concentration of the deep eutectic solvent being greater than 92 wt.% and a residual amount of the solution consisting substantially of water, and in which a molar ratio of the quaternary ammonium salt to a co-solvent is selected to be between 1:1 and 1:2; and d) mechanically grinding the pretreated wood chip feedstock at a pressure of between 1 and 3.5 bars and a temperature above 100°C. The invention also relates to a paper or board produced therewith.

Description

PROCESS FOR A CHEMICALLY ASSISTED MECHANICAL DIGESTING PROCESS OF WOOD CHIPS RESPECTIVELY. WOOD CHIPS

The present invention relates to a process for a chemically assisted mechanical pulping process for wood chips or wood chips and to paper or cardboard produced therewith.

In order to obtain a suitable starting material for the production of paper, cardboard or cellulose, two different pulping processes for wood chips or wood chips are usually used in industry. The so-called kraft process is usually used for the chemical digestion of wood chips. Furthermore, wood chips or wood chips can be broken down using what is known as a semi-chemical or semi-chemical digestion process, in which a two-stage process is used. This process is essentially described in the book Bierman's Handbook of Pulp and Paper: raw material and Pulp making, by Pratima Bajpai, 3rd edition, Elsevier Amsterdam, 2018, page 312, in which process a chemical treatment takes place in a first stage (either a sulfite digestion or a kraft digest, which is carried out similarly to the usual chemical digestion process), but which uses reduced temperatures, shortened cooking times and a reduced chemical charge, which has an economic advantage over the kraft process and at in which process mechanical grinding takes place in a second step to make the cooked cellulose more uniform. The aim of this process is, on the one hand, to soften the wood chips and, on the other hand, to partially remove lignin and hemicellulose from the wood pulp. In addition to these two processes for breaking down wood chips or wood chips, a hybrid of the two known processes, the so-called thermo-mechanical digestion, is used, which is characterized on the one hand by a very high energy consumption and on the other hand leads to a and cardboard production leads to particularly well-suited wood pulp.

Finally, there is the mechanical pulping process for wood chips or whole trunks, in which the wood is mechanically ground in a first step. Very little lignin is removed from the wood during this milling step, which is why papers with lower qualities but higher yields can subsequently be produced from the mechanical pulp produced in this way compared to a chemical pulping process. Another disadvantage of mechanical pulp production is that the cellulose fibers contained in the pulp are rather short and stiffer compared to mechanical pulp produced using chemical processes. Also, the fines content and the lignin content are one mechanically produced pulp is higher than in mechanical pulp produced using chemical processes. Of course, there are a wide variety of processes that represent combinations of the mechanical or chemical pulping process and continue to use changes in the pulping temperature, the pulping pressure, the chemicals used and the like to improve the wood pulp quality.

A method has already become known from EP 2 876 202 A1, which replaces at least part of the chemical batch in a conventional semi-chemical wood pulping process or also chemical wood pulping process by using a so-called strong eutectic solvent in order to achieve improved separation of lignin and lignocellulose as a result of the pulping process. The strongly eutectic solvents used in this document are described, for example, in WO 2013/153203 A1 and consist of a salt in its liquid state that consists of a mixture of a proton donor and a proton acceptor, which mixture contains a eutectic with a forms a low melting point, which melting point is significantly lower than that of the individual components. Furthermore, various examples of strongly eutectic solvents can be found in the document and that these can be used in connection with lignocellulosic starting materials, in particular for dissolving the lignin contained in these materials.

It is known that the removal of lignin and lignin cellulose from a wood pulp can offer advantages in that in the subsequent production process of paper and cardboard these substances, which are actually not absolutely necessary for sheet formation, do not have to be carried along unnecessarily, which may even lead to a Reduction of the subsequent use of chemicals, such as a reduced use of starch can contribute.

However, another problem, particularly in the production of semi-chemical pulp, is the fact that, in comparison to kraft pulp, this pulp often still contains residual components of unopened wood particles, resulting in a non-homogeneous starting material that is paper production leads to deteriorated paper qualities. In order to eliminate this problem, pulp suspensions of this type are usually subjected to beating in order to comminute unopened or only partially open wood particles to such an extent that they do not lead to any inhomogeneities in the product made from the wood pulp. However, the disadvantage of grinding is that, in addition to the comminution of unbroken-up wood particles, a not inconsiderable part of the fibers of the input material is also ground up and thus be shortened, so that a final product after grinding has a fiber length distribution corresponding to a Gaussian curve, which is shifted in the direction of shorter fibers in relation to the fiber length of the heating material used. However, in particular the presence of longer fibers in the lignocellulosic material has the advantage that a better lattice network can be formed in the paper web and consequently, for example, papers with increased stretchability according to ISO 1924-3 can be obtained.

In the paper and board industry, there is still a need to provide a process that can provide an optimally broken down fiber product with the lowest possible use of energy and chemicals, and at the same time it can be ensured that this fiber product has a length distribution that is as homogeneous as possible Has fibers in order to be able to produce paper or cardboard with precisely defined properties of the end product as a result.

To solve this problem, a chemically assisted mechanical pulping process for wood chips is provided, which essentially has the following steps: a) producing a feed material from wood chips; b) contacting the wood chip feedstock with steam at a first pressure of between 1 and 2 bar, a steam temperature of between 100 and 110°C for a period of between 10 and 20 minutes to produce a steamed wood chip feedstock; c) contacting the vaporized wood chip feedstock with a solution containing a strong eutectic solvent comprising a quaternary ammonium salt and a co-solvent at a temperature between 40 and 120°C, preferably 50 to 100 ^° C and for a time between 5 and 15 minutes, preferably 5 to 10 minutes to obtain a pretreated wood chip feedstock having a strong eutectic solvent concentration greater than 92 mass. -%, preferably greater than 95% by mass, is selected and a remainder of the solution consists essentially of water, and wherein a molar ratio of the quaternary ammonium salt to a co-solvent is selected between 1:1 and 1:2; and d) mechanically grinding the pretreated wood chip feedstock at a pressure between 1 and 3.5 bar and a temperature above 100°C.

With such processing, after a conventional sputtering step in which the feedstock is heated and air is to be removed therefrom, the feedstock is contacted with a strong eutectic solvent. Such a procedure is known in principle from EP 2 876202 A1, but has proven itself Surprisingly shown that it is essential in such a procedure to choose the reaction time so that an undesired dissolution of the lignin contained in the wood pulp in the strongly eutectic solvent is avoided. If, as is described in the prior art, reaction times of at least 10 minutes are selected, an interaction of the lignin with the strongly eutectic solvent is essentially achieved and the same is broken down or dissolved. Surprisingly, however, it has been shown that when the reaction time is chosen to be as short as possible and the reaction temperature is kept low at the same time, the fact that lignin and lignocellulosic dissolution in the wood chips clearly takes a back seat and primarily a significant softening of the wood chips as such can be achieved without leaching the lignin out of the mechanical pulp, but merely modifying it to the point where it becomes “softer”. If in such a procedure the corresponding chemically softened wood chips! or chemically softened wood chips are subsequently mechanically ground, if necessary after a washing and cleaning step, it is surprisingly possible to obtain a fiber material which, compared to the fiber quality of a reference fiber material or the starting material after the grinding step, has significantly longer fibers and has a lower proportion of fines, which is due to more complete and less damaged fibers. Such a mechanical pulp, which has longer and less damaged fibers and thus a significantly improved fiber quality compared to the prior art, is particularly suitable for use in the paper and cardboard industry due to its homogeneous properties. A further advantage of such a procedure is that the strongly eutectic solvent is kept at the lowest possible temperature and thus on the one hand the energy balance can be optimized and on the other hand any adverse effects on the plant, in particular its tendency to corrode or deterioration, are minimized as far as possible - ten can be, in that the eutectic solvent used is not corrosive, in contrast to the strong bases and acids usually used, corrosion of the Äniagenteie can be prevented, for example, and the use of stainless steel for plant construction can be avoided as a result. This also makes it possible to prevent unwanted discoloration of the paper or cardboard web due to corroded plant parts.

According to a further development of the invention, the process is carried out in such a way that the grinding in step d) is carried out at a temperature between 100 and 160° C., in particular at a temperature which increases from 115° to around 150° C. in the course of the grinding, by Temperature is gradually raised during grinding in step d). Additionally or alternatively, according to a further development, the method is conducted in such a way that the pressure in the grinding step is also increased successively from values of 1.0 to 2.5 or 3.0 or 3.5 bar. With such a process control, it is possible to keep the energy consumption in the grinding step as low as possible and continue to match the grinding efficiency exactly to the feedstock, which makes it possible to achieve a significantly improved quality of the fibers in the compared to the prior art with minimal energy consumption to achieve wood pulp.

According to a development of the invention, the method is carried out in such a way that a choline salt or a proline salt is used as the quaternary ammonium salt. If a choline or proline salt is used, a strongly eutectic solvent can be provided which shows a particularly strong reduction in the melting point and thus allows the process to be carried out at low temperatures and relatively low energy consumption.

In particular if, as corresponds to a development of the invention, the choline salt used is a choline chloride or choline bromide or the proline salt is a proline chloride, it can be ensured that no foreign ions are introduced into the digestion system and thus a possibly complicated separation process after the digestion can in any case be held back with the strongly eutectic solvent.

According to a development of the invention, an amide is used as a co-solvent, in particular an amide selected from the group consisting of urea, formamide and acetamide, which amides are strongly basic on the one hand and thus make the digestion process significantly easier and on the other hand also result in a significant reduction in the melting point of the ammonium salt and thus contribute to a strong or deep eutectic. The use of urea is particularly preferred for ecological reasons, since waste products containing urea can be landfilled or even used, for example, as fertilizer for forests or the like.

The invention also relates to the use of wood chips or wood chips produced according to the method of the invention and of auxiliaries and additives selected from the group of wet strength agents, flocculants, sizes, retention aids and bleaches for the production of paper or cardboard. Paper or cardboard produced using wood chips or wood chips produced in this way has particularly homogeneous fibers and thus constant properties over the entire length or width of the paper produced. In addition, since the fiber length is significantly longer than that of conventionally produced papers and the papers also have significantly reduced air permeability, they can be used without special or additional surface treatment, particularly in cases where a particularly dense material is desired. The invention is explained in more detail below using exemplary embodiments.

In the exemplary embodiments, wood chips or wood chips that were produced from Norway spruce (Picea abies) using a reduction device, for example a chipper, were used. In a first process step, the wood chips were subjected to steaming, which steaming for a period of 10 minutes at a pressure of at least 1.0 bar, in particular a pressure between 1.0 and 1.5 bar and a steam temperature of over 100° C, advantageously around 110 °C. The purpose of steaming is to remove air from the wood chips or the wood chips as well as to raise the temperature of the wood chips and to stabilize it at the selected level. Immediately after steaming, the chips were soaked in a strong eutectic solvent. For this purpose, 1) a mixture of choline chloride and urea and 2) of choline chloride and glycerol was used in the exemplary embodiment. The molar ratio of the quaternary Am man! Conversion to the co-solvent is in each case 1:2

In a first series of tests, the steamed wood chips were soaked at 50 °C and at 70 °C in the strongly eutectic solution consisting of choline chloride and urea as well as that consisting of choline chloride and glycerine. In a second series of tests, soaking in a strongly eutectic solution Solution consisting of choline chloride and glycerol at 70 °C. The soak in the strong eutectic solvent was done for 10 minutes each, to remove any excess strong eutectic solvent from the chips after the soak, the material was rinsed with warm water (about 40°C). The wood chips or wood chips obtained in this way were fed to a grinding device or a refiner and subjected to a grinding process there as follows: water was injected in a preheating stage in order to adjust a wood pulp to a mass content of 25%. The grinding was then started and the temperature was gradually increased from an initial temperature of 100° C. to 150° C. and the pressure in the grinding chamber was raised in the same way from 1 bar to at least 3.0 bar, in particular 3.5 bar. The grinding was continued until the specific energy consumption reached 2,000 kWh/t.

After beating, the samples of the resulting mechanical pulp were removed from the beating apparatus, rinsed again with warm water and subjected to thickening. The pulp thickened in this way was analyzed and the fibers characterized. For comparison with the process according to the invention, comparative examples were shown in which only a mechanical pulping process was carried out.

The results of the tests and the comparative tests are shown in Table 1 below. In this Table 1, DES 1 denotes the strongly eutectic solution 1 consists of

Choline chloride and urea, DES 2, the strongly eutectic solution consisting of choline chloride and glycerol, Ref. 1 Comparative Example 1 and Ref. 2 Comparative Example! 2. The strongly eutectic solution 1, DES 1, was used at two different temperatures, namely 50 °C and 70 °C in process step c),

Table 1

Fiber quality as results from all mechanical pulp samples after beating

From Table 1 is. clearly seen that the pulps made with the highly eutectic solution DES 1 had a significantly longer fiber length after beating than those made with either the eutectic solution DES 2 or according to the comparative processes. The air permeability of the wood pulp treated with DES 1 is significantly higher than that of all other wood pulps, which can be explained by the reduction in fines that was also observed. Finally, it can be seen from Table 1 that the water retention of the fibers treated with the solutions DES 1 and DES 2 is significantly increased compared to the comparative examples. Furthermore, it can be seen from Table 1 that the effect of treatment with the strong eutectic solvent DES 2 was significantly smaller than that with the eutectic solvent DES 1. here For example, when the pulp treated with DES 2 was compared to Comparative Example 2, the differences in fiber length (length weighted average), fines content, as well as brightness were essentially not significant.

In a further attempt, an attempt was made to clarify the extent to which the lignin was dissolved out of the wood pulp with the procedure according to the present invention, and the wood pulp samples were subjected to a chemical analysis, the results of which are shown in Table 2.

table 2

Lignin and carbon hydrate analysis of all mechanical pulp samples

From Table 2 it can be seen that all of the wood pulp samples treated with the strong eutectic solvents contained essentially equal amounts of lignin and hemicellulose as Comparative Examples 1 and 2. Carbohydrate analysis showed that treatment with DES 1 resulted in low dissolution of the hemicelluloses, with the most important Hemicellulose monomers such as mannose and xylose showed no significant differences from the comparison examples. Only the galacturonic acid content was greatly reduced compared to the comparative examples when treated with the highly eutectic solvent DES 1.

In summary, it can be shown that with the specific procedure according to the present invention, a mechanical pulp can be obtained which, compared to conventionally processed chips, has a significantly greater fiber length and significantly increased water retention, but the lignin content in the mechanical pulp is essentially reduced has remained unchanged. With the present method, German Lich reduced energy use in the grinding step, only about half the energy of a grinding step was consumed in a conventional method, a wood pulp with a significantly improved fiber quality, in particular significantly elongated fibers, obtained.

Claims

patent claims

Claims 1. A method for a chemically assisted mechanical pulping process of wood chips, comprising the steps of: a) preparing a feed material from wood chips; b) contacting the wood chip feedstock with steam at a first pressure of between 1 and 2 bar, a steam temperature of between 100 and 110°C for a period of between 10 and 20 minutes to produce a steamed wood chip feedstock; c) contacting the steamed wood chip feedstock with a solution containing a strong eutectic solvent comprising a quaternary ammonium salt and a co-solvent at a temperature between 40 and 120°C, preferably 50 to 100°C and for a time between 5 and 15 minutes, preferably 5 to 10 minutes, whereby a pre-treated wood chip feed material is obtained, wherein a concentration of the strong eutectic solvent is selected to be greater than 92% by mass, preferably greater than 95% by mass and a residual amount of the solution consists essentially of water and wherein a molar ratio of the quaternary ammonium salt to a co-solvent is chosen between 1:1 and 1:2; and d) mechanically grinding the pretreated wood chip feedstock at a pressure between 1 and 3.5 bar and a temperature above 100°C.

2. Process according to Claim 1, characterized in that the grinding in step d) is carried out at a temperature between 100 and 160°C, in particular at a temperature which increases between 115° and 150°C during the course of the grinding.

3. The method according to claim 1 or 2, characterized in that the grinding in step d) is carried out with a pressure gradient between 1.0 and 3.5 bar, in particular between 2.5 and 3.0 bar.

4. The method according to claim 1, 2 or 3, characterized in that a choline salt or a proline salt is used as the quaternary ammonium salt.

5. The method according to claim 4, characterized in that the quaternary ammonium salt is selected from the group choline chloride, chosine bromide or proline chloride.

6. The method according to any one of claims 1 to 5, characterized in that an amide is used as co-solvent.

7. The method according to claim 6, characterized in that the amide is selected from the group consisting of urea, formamide and acetamide.

8. Use of wood chips or wood chips produced according to one of Claims 1 to 7 and of auxiliaries and additives selected from the group consisting of wet strength agents, flocculants, sizing agents, retention aids and bleaching agents for the production of paper or cardboard.