METHOD FOR A CHEMICALLY ASSISTED MECHANICAL DISCHARGE PROCESS OF WOOD CHIPS OR WOOD SHREDDS

Description

[0001] The present invention relates to a method for a chemically assisted mechanical pulping process of wood chips or wood chips and to a paper or cardboard produced therewith.

[0002] To obtain a suitable raw material for the production of paper, cardboard, or pulp, two different pulping processes are typically used in the industry for wood chips or wood chips. The so-called kraft process is typically used for the chemical pulping of wood shavings. Furthermore, wood chips or wood chips can be pulped using a so-called semi-chemical pulping process, which involves a two-stage process. 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.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 digestion, which is carried out in a similar way to the usual chemical digestion process), in which, however, reduced temperatures, shortened cooking times and a reduced chemical charge are used, which has an economic advantage over the kraft process, and in which process in a second step mechanical grinding takes place to homogenize the cooked cellulose. 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 digesting wood chips and kraft pulp,Wood chips, a mixed form of the two known processes, the so-called thermomechanical pulping, is also used, which on the one hand is characterized by very high energy consumption and on the other hand leads to a wood pulp that is particularly suitable for subsequent paper and cardboard production.

[0003] Finally, there is the mechanical pulping process for wood chips or whole logs, which first involves mechanical grinding of the wood. This grinding step removes very little lignin from the wood, which is why the resulting pulp can subsequently be used to produce paper of lower quality but with higher yields compared to a chemical pulping process. A further disadvantage of mechanical pulping is that the cellulose fibers contained in the pulp tend to be shorter and stiffer than those produced using chemical processes. The fines content and lignin content of a mechanically produced pulp are also higher than those produced using chemical processes.Of course, there are various processes that represent combinations of mechanical or chemical pulping processes and also make use of changes in the pulping temperature, the pulping pressure, the chemicals used, etc. to improve the pulp quality.

[0004] EP 2 876 202 A1 already discloses a process which, in a conventional semi-chemical wood pulping process or chemical wood pulping process, replaces at least part of the chemical charge with the use of a so-called strongly 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, which consists of a mixture of a proton donor and a proton acceptor. This mixture forms a eutectic with a low melting point, which melting point is significantly lower than that of the individual components.The document also provides various examples of strongly eutectic solvents and shows that these can be used in conjunction with lignocellulosic starting materials, in particular for dissolving the lignin contained in these materials.

[0005] 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 unnecessarily included, which may even contribute to a reduction in the subsequent use of chemicals, such as a reduced use of starch.

[0006] However, a further problem, particularly in the production of semi-chemical pulp, is the fact that, compared to kraft pulp, this pulp often still contains residual components of unpulped wood particles. This results in a non-homogeneous starting material, which leads to deteriorated paper quality when subsequently used in paper production. To eliminate this problem, such pulp suspensions are usually subjected to refining to reduce unpulped or only partially pulped wood particles to such an extent that they do not lead to inhomogeneities in the product made from the pulp.A disadvantage of refining, however, is that in addition to the comminution of non-pulped wood particles, a significant portion of the fibers of the feedstock are also ground and thus shortened. This results in the final product, after refining, exhibiting a fiber length distribution corresponding to a Gaussian curve, which is shifted toward shorter fibers relative to the fiber length of the wood material used. However, the presence of longer fibers in the lignocellulosic material, in particular, has the advantage of allowing a better lattice network to be formed in the paper web, resulting in papers with increased stretchability according to ISO 1924-3, for example.

[0007] In the paper and board industry, there is still a need to provide a process that can produce an optimally digested fiber product with the lowest possible use of energy and chemicals, while at the same time ensuring that this fiber product has the most homogeneous fiber length distribution possible, in order to subsequently produce paper or board with precisely defined properties of the end product.

[0008] To solve this problem, a chemically assisted mechanical pulping process for wood chips is provided, which essentially comprises the following steps: a) producing a feedstock from wood chips; b) contacting the feedstock from wood chips with steam at a first pressure between 1 and 2 bar, a steam temperature 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 strongly eutectic solvent comprising a quaternary ammonium salt selected from a choline salt or a proline salt and a co-solvent at a temperature between 40 and 120 °C, preferably 50 to 100 °C and for a period of between 5 and 15 minutes, preferably 5 to 10 minutes, to obtain a pretreated wood chip feedstock, wherein a concentration of the strongly eutectic solvent is greater than 92 mass %.-%, preferably greater than 95 mass%, and a residual amount of the solution consists essentially of water, and wherein a molar ratio of the quaternary ammonium salt to an amide as co-solvent is chosen between 1: 1 and 1: 2; and d) mechanically grinding the pretreated wood chip feed material at a pressure between 1 and 3.5 bar and a temperature above 100 °C.

[0009] With such a process, after a conventional vapor deposition step in which the feed material is heated and air is to be removed from it, the feed material is brought into contact with a strongly eutectic solvent.

[0010] In principle, such a procedure is known from EP 2 876 202 A1. However, it has surprisingly been shown that, in such a process, it is essential to select the reaction time so as to avoid undesired dissolution of the lignin contained in the wood pulp in the strongly eutectic solvent. If, as 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, resulting in its breakdown or dissolution.Surprisingly, however, it has been shown that if the reaction time is kept as short as possible and the reaction temperature is kept low, the dissolution of lignin and lignocellulose in the wood chips becomes significantly less important, and primarily a significant softening of the wood chips as such can be achieved without dissolving the lignin from the wood pulp, but merely modifying it to the extent that it becomes "softer." If, in such a process, the chemically softened wood chips or chemically softened wood chips are subsequently mechanically ground, possibly after a washing and cleaning step, it is surprisingly possible to obtain a fiber material that, compared to the fiber quality of a reference fiber material, is significantly superior to that of a reference fiber material.of the starting material after the refining step has significantly longer fibers and a lower proportion of fines, which is due to more complete and less damaged fibers. Such a pulp, which has longer and less damaged fibers and thus significantly improved fiber quality compared to the state of the art, is particularly suitable for use in the paper and board industry due to its homogeneous properties. A further advantage of this type of process is that the strongly eutectic solvent is kept at the lowest possible temperature, thus optimizing the energy balance and minimizing any adverse effects on the plant, in particular its tendency to corrosion or deterioration.Furthermore, because the eutectic solvent used is non-corrosive, unlike the commonly used strong bases and acids, corrosion of the plant components can be prevented, thus avoiding the use of stainless steel in the plant construction. This also prevents unwanted discoloration of the paper or board web caused by corroded plant components.

[0011] According to a further development of the invention, the process is conducted such that the refining in step d) is carried out at a temperature between 100 and 160 °C, in particular at a temperature that increases from 115 °C to approximately 150 °C during the refining process, by successively increasing the temperature during the refining in step d). Additionally or alternatively, according to a further development, the process is conducted such that the pressure in the refining step is also successively increased from values ​​of 1.0 to 2.5, 3.0, or 3.5 bar. By conducting the process in this way, it is possible to keep the energy consumption in the refining step as low as possible and, furthermore, to precisely match the refining efficiency to the feedstock, whereby a significantly improved quality of the fibers in the pulp compared to the prior art can be achieved with minimal energy consumption.

[0012] According to a further development of the invention, the process is conducted using a choline salt or a proline salt as the quaternary ammonium salt. Using a choline or proline salt, a strongly eutectic solvent can be provided, which exhibits a particularly strong melting point depression and thus allows the process to be conducted at low temperatures and with relatively low energy consumption.

[0013] In particular, if, as is the case in a further 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 with the strongly eutectic solvent can be avoided.

[0014] According to a further development of the invention, an amide is used as co-solvent, in particular an amide selected from the group consisting of urea, formamide, and acetamide. These amides are, on the one hand, strongly basic and thus significantly facilitate the digestion process, and, on the other hand, also contribute to a significant reduction in the melting point of the ammonium salt and thus to a strong or deep eutectic. The use of urea is particularly preferred for ecological reasons, since urea-containing waste products can be disposed of in landfills or even used, for example, as fertilizer for forests or the like.

[0015] By using wood chips or wood chips produced according to the process of the invention, as well as auxiliary materials and additives selected from the group of wet strength agents, flocculants, sizing agents, retention aids, and bleaching agents for the production of paper or cardboard, it is possible to produce a paper or cardboard that has particularly homogeneous fibers and thus consistent properties across the entire length or width of the produced paper. Furthermore, since the fiber length is significantly longer compared to 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.

[0016] The invention is explained in more detail below using exemplary embodiments.

[0017] In the exemplary embodiments, wood chips or wood chips 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. This steaming was carried out 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 wood chips, as well as to raise the temperature of the wood chips and stabilize it at the selected level. Immediately after steaming, the wood chips were soaked in a strongly eutectic solvent. For this purpose, 1) a mixture of choline chloride and urea and 2) of choline chloride and glycerin was used in the exemplary embodiment.The molar ratio of the quaternary ammonium salt to the co-solvent is 1:2 in each case.

[0018] In a first series of tests, the steamed wood chips were soaked at both 50 °C and 70 °C in a strongly eutectic solution consisting of choline chloride and urea, as well as in a solution consisting of choline chloride and glycerin. In a second series of tests, the soaking was carried out in a strongly eutectic solution consisting of choline chloride and glycerin at 70 °C. Soaking in the strongly eutectic solvent was carried out for 10 minutes each time. To remove any excess strongly eutectic solvent from the wood chips after soaking, the material was rinsed with warm water (approximately 40 °C). The resulting wood chips or wood chips were fed to a grinder or refiner and subjected to a grinding process as follows. During a preheating stage, water was injected to adjust the pulp to a mass content of 25%.Grinding was then initiated, with the temperature gradually increasing from an initial temperature of 100 °C to 150 °C. Similarly, the pressure in the grinding chamber was increased from 1 bar to at least 3.0 bar, preferably 3.5 bar. Grinding continued until the specific energy consumption reached 2,000 kWh / t.

[0019] After milling, the samples of the resulting mechanical pulp were removed from the milling machine, rinsed again with warm water, and subjected to thickening. The thickened pulp was analyzed and the fibers characterized.

[0020] For comparison with the process according to the invention, comparative examples were shown in which only a mechanical digestion process was carried out.

[0021] 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 consisting 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 of all pulp samples after refining Ref. 1 DES 1, 50 °C DES 1, 70 °C Ref. 2 DES 2, 70 °C Water retention of pulp (ml) 340 565 515 260 260 Fiber length (mm) 1.563 1.755 1.756 1.639 1.673 Fiber width (µm) 24,1 23,7 23,6 24,4 24,4 Fiber entanglement (%) 6,4 5,2 5,4 6,0 5,5 Fines (%) 29,60 21,40 21,80 28,10 26,20 Tensile strength index (Nm / g) 27,4 20,8 25,1 28,2 24,3 Tensile strength (mNm 2 < / g) 7,4 7,6 7,6 7,5 6,9 Brightness according to ISO (%) 55,0 45,2 44,7 55,4 53,5 Air resistance Gurley (s / 100 ml) 5,6 1,5 3,1 18 14

[0022] Table 1 clearly shows that the pulps produced with the strongly eutectic DES 1 solution exhibited significantly longer fiber lengths after refining than those produced either with the eutectic DES 2 solution or according to the comparative processes. Thus, the air permeability of the pulps treated with DES 1 is significantly increased compared to all other pulps, which can be explained by the also observed reduction in fines. Finally, Table 1 shows that the water retention of the fibers treated with the DES 1 and DES 2 solutions is significantly increased compared to the comparative examples. Furthermore, Table 1 shows that the effect of treatment with the strongly eutectic DES 2 solvent was significantly lower than that of the eutectic DES 1 solvent.Here, the differences in fiber length (length-weighted average), fines content as well as brightness were essentially not significant when the pulp treated with DES 2 was compared with Comparative Example 2.

[0023] In a further experiment, an attempt was made to clarify to what extent the lignin was dissolved out of the wood pulp using the process 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 carbohydrate analysis of all wood pulp samples Ref. 1 DES 1, 50 °C DES 1, 70 °C Ref. 2 DES 2, 70 °C Lignin (Klason), % 26,4 25,8 26,8 26,4 25,0 Galactose (mg / g) 22,9 19,5 22,9 23,2 19,9 Glucose (mg / g) 44,8 40,0 43,0 44,8 41 Mannose (mg / g) 98,3 93,3 99,2 99,7 92,7 Arabinose (mg / g) 10,9 10,2 10,9 11,3 9,7 Xylose (mg / g) 62,2 57,1 59,9 63,0 56,0 Glucuronic acid (mg / g) 7,2 6,6 5,3 1,7 2,1 Galacturonic acid (mg / g) 15,3 4,5 4,3 15,6 13,3 Carbohydrates (mg / g) 262 231 246 259 235

[0024] Table 2 shows that all pulp samples treated with the strongly eutectic solvents contained essentially the same amounts of lignin and hemicellulose as Comparative Examples 1 and 2. Carbohydrate analysis showed that treatment with DES 1 resulted in a low dissolution of the hemicelluloses, with the main hemicellulose monomers, such as mannose and xylose, showing no significant differences compared to the comparative examples. Only the galacturonic acid content was significantly reduced compared to the comparative examples when treated with the strongly eutectic solvent DES 1.

[0025] In summary, it can be shown that, with the specific process according to the present invention, a wood pulp can be obtained which, compared to conventionally pulped chips, has a significantly longer fiber length and significantly increased water retention, while the lignin content in the wood pulp remains essentially unchanged. Thus, with the present process, a wood pulp with significantly improved fiber quality, in particular significantly elongated fibers, was obtained with significantly reduced energy consumption in the milling step—only approximately half the energy of a milling step in a conventional process was consumed.

Claims

1. A method for a chemically assisted mechanical pulping of wood chips, comprising the steps of: a) producing a feedstock from wood chips; b) contacting the feedstock produced from wood chips with steam at a first pressure between 1 and 2 bar, at a steam temperature between 100 and 110°C for a time period of between 10 and 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 selected from a choline salt or a proline salt, and a co-solvent at a temperature between 40 and 120°C, preferably 50 to 100°C, and for a period of time of between 5 and 15 minutes, preferably 5 to 10 minutes, wherein a pre-treated wood chip feedstock is obtained, wherein a concentration of the deep eutectic solvent is selected so as to be more than 92% by weight, preferably more than 95% by weight and a remaining quantity of the solution substantially consists of water, and wherein a molar ratio of the quaternary ammonium salt to an amide as the co-solvent is selected so as to be between 1:1 and 1:2; and d) mechanically grinding the pre-treated wood chip feedstock at a pressure between 1 and 3.5 bar and a temperature of over 100°C.

2. The method as claimed in 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 rises between 115° and 150°C during the course of the grinding.

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

4. The method as claimed in claim 1, 2 or 3, characterized in that the quaternary ammonium salt is selected from the group: choline chloride, choline bromide or proline chloride.

5. The method as claimed in one of claims 1 to 4, characterized in that the amide is selected from the group consisting of urea, formamide and acetamide.