Pulp production that uses bark residues as raw materials

Abstract

The present invention relates to a method for preparing pulp from one or more wood-based raw materials comprising at least bark-derived residues, comprising performing an alkali treatment step on the bark-based raw material, recovering a solid residue after the alkali treatment step, and cooking the obtained residue alone or in admixture with wood. The present invention also relates to pulp prepared using said treatment.

Description

Technical Field

[0001] The present invention relates to a process for preparing pulp from one or more wood-based raw materials, in particular residues obtained from the extraction of bark. Furthermore, the present invention relates to the pulp thus obtained.

Background Art

[0002] (Description of Related Art) In most industrial processes that utilize wood raw materials, the wood material is debarked before the process begins. The bark of most wood species has a low cellulose content and a relatively high content of components that are harmful to, for example, the pulping process (see, for example, Miranda I, et al. 2012), so discarding the bark is considered advantageous for the overall process. Thus, wood bark is an abundant side stream obtained from the forest industry.

[0003] The bark material from the debarking is typically used as an energy source and has in the past been regarded simply as a low-value solid fuel. However, bark contains beneficial compounds such as tannins in addition to the above-mentioned harmful compounds, and extraction to recover these beneficial components has also been attempted. The extraction is traditionally carried out using hot water, but the addition of alkaline reagents has also been proposed.

[0004] U.S. Patent No. 2,673,798 describes a bark extract obtained by first steaming the bark and then performing extraction using caustic soda. After washing and dilution, the extract is mixed with newsprint and pulped.

[0005] On the other hand, Vazquez et al. 2001 describes the alkaline extraction of softwood bark.

[0006] All of these processes utilize only the bark extract, but the bark residue left after extraction is thought to have limited opportunities for valorization. Therefore, the above description cannot fully utilize the potential of the bark fraction of wood. Furthermore, hot water extraction provides only limited extraction levels.

[0007] International Publication No. 2020 / 084196, which describes the alkaline conditioning of bark, also focuses on the bark extract. This publication mentions the possibility of using the bark residue remaining after extraction, but does not suggest the conditioning of the bark residue. Instead, the bark residue is said to be suitable for use in enzymatic hydrolysis, bleaching, or fibrillation.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0009]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0010] Therefore, there is still a need for a further simpler method that utilizes valuable bark materials, which are generally discarded as waste or used as fuel for heating purposes.

Means for Solving the Problems

[0011] The present invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0012] According to a first aspect of the present invention, there is provided a method for preparing pulp from one or more wood-based raw materials, including raw materials obtained at least from bark.

[0013] According to a second aspect of the present invention, there is provided a method of utilizing the residue remaining after extracting the polyphenol fraction from the bark-based raw material.

[0014] According to a third aspect of the present invention, there is provided a method for preparing pulp that utilizes the residue obtained by subjecting the bark-based raw material to an alkali treatment step.

[0015] The pulp can be prepared using the residue obtained from bark extraction as the sole fiber source, or from a mixture of the bark-based raw material and one or more additional wood-based raw materials.

[0016] The present invention is based on the finding that the residue remaining after extraction of the bark-based raw material has a low content of components harmful to the pulping process and a sufficiently high content of cellulose, which is used as a high-value component in pulping.

[0017] The sufficiently low content of harmful components is achieved by an alkali treatment used to replace the hot water extraction of past processes. This alkali treatment results in a more efficient separation of the extraction compounds and inorganic components together with the polyphenol fraction in which many bark extractions concentrate, leaving a solid residue lacking such harmful components.

[0018] Accordingly, the present invention relates to a process for preparing pulp from one or more wood-based raw materials, one raw material being based on a fiber-rich residue obtained from a bark-based raw material. The fiber-rich residue can be mixed with other wood materials prior to pulping to obtain a mixed product from the pulping process having more general pulp properties. The mixed pulp product can also be obtained by mixing the bark-containing pulp obtained after pulping the fiber-rich residue with conventional wood pulp. Thus, the pulp obtained using the method of the present invention is suitable for processing, for example, in paper and packaging products.

[0019] Several advantages are achieved using the present invention. In particular, a practical solution is achieved for varying the industrial fiber-rich bark side stream. Further, by using a bark-based raw material, the pulp can be produced with an increased overall yield having specific properties optimized for a particular use, with respect to the amount of wood used. The bleachability of the pulp is not impaired even by the high bark content in the pulp. Further, the bark-containing residue gives the resulting paper product a higher bulk, which further enables a reduction in basis weight at a given thickness.

[0020] Using a low content of residue obtained from a bark-based raw material, mixed with conventional wood, cooked, optionally bleached together, or using a low-content bark pulp mixed with conventional wood pulp (<10%, for example, about 5%) has the particular advantage that it increases the pulp yield as a function of the amount of pulp obtained from the selected wood source, while having only a slight, negligible effect on the technical properties of the resulting paper product. Thus, these pulps can be used for any paper or packaging application.

[0021] In such mixtures, a fiber-rich residue obtained from bark-based raw materials, or a higher content (>10%, for example about 20%) of bark-based pulp, makes the pulp particularly suitable for packaging applications. These high-bark pulps have an increased bulk with a given Scott bond that allows for a basis weight reduction at a certain thickness. However, the change in tensile strength during refining induces a loss of drainage and porosity, which makes these pulps most suitable for various applications particularly related to packaging.

Brief Description of the Drawings

[0022]

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Modes for Carrying Out the Invention

[0023] (Definition) In this context, the term "wood-based raw material" is intended to include materials typically used in cooking, i.e., sapwood and heartwood, as well as bark-based raw materials typically separated from wood prior to cooking. Accordingly, the term "bark-based raw material" includes, for example, bark components obtained from debarking of wood, which typically results in a solid mass of bark chips or a mixture of bark chips and other materials, with these other materials present in a content of less than 50 w% of the solid mass. The other materials can be wood materials or non-wood materials. Non-wood materials are typically selected from textiles or cotton, or other fibrous materials suitable as raw materials for the cooking process. Typically this bark-based raw material is untreated and consists of the untreated bark obtained directly from debarking. The term "cooking" is intended to include, inter alia, various forms of chemical pulping aimed at the removal of lignin and other undesirable components. A preferred cooking process is the kraft process. Cooking can utilize various chemicals, with alkaline chemicals being preferred. The cooking process typically begins with an impregnation step using white liquor as a source of cooking chemicals, followed by the actual cooking step, where the impregnated material is heated to cause delignification, and in a further recovery step, additional undesirable components such as soap, volatiles, and fragments are removed. The term "alkali treatment" or "alkali extraction" is then intended to describe a broader process of removing harmful components from the raw material and recovering valuable compounds such as tannins. It can utilize alkaline chemicals similar to kraft digestion such as sodium hydroxide (NaOH) and sodium sulfide (Na 2 2S), with only NaOH being more common and being similar or milder, but not necessarily requiring other sub-steps of the cooking method. The term "bark-containing residue" or "high-fiber residue" is intended to describe the solid fraction obtained from the alkaline treatment of bark-based raw materials, and typically consists of materials derived from bark obtained from debarking of wood raw materials, i.e., containing at least 50 w% of the bark component of said residue. The remaining material can be derived from the same wood material as the bark component or can be wood or non-wood materials that were mixed with the bark-containing raw material or residue prior to cooking.

[0024] Accordingly, the present invention relates to a method for preparing pulp from one or more wood-based raw materials by subjecting a bark-based raw material to an alkaline treatment step, recovering a solid residue after said alkaline treatment step, and cooking and optionally bleaching the resulting residue either alone or as a mixture with one or more additional wood-based raw materials.

[0025] The bark-based raw material is typically selected from the bark of coniferous or broad-leaved tree species, preferably from pine, fir, beech or spruce of coniferous species, or from oak, eucalyptus, acacia, poplar or willow of broad-leaved species, and typically contains less than 5 w% of other materials that generally end with the bark fraction in wood debarking.

[0026] In one embodiment of the present invention, the pretreatment is carried out on the bark material before cooking and optionally before the alkaline treatment, and the pretreatment includes washing the material with a chelating agent such as strong mineral acid, weak acid, or washing with nitric acid, separating and recovering the solid for the next step. Optionally, the bark can be disintegrated or mechanically opened to improve the access of any acid or chelating agent to the bark material before the pretreatment.

[0027] The purpose of this optional acid washing is to remove or at least reduce the content of inorganic compounds (or ash) from the raw material, although some of these are also removed in the alkaline treatment step.

[0028] The alkaline treatment of the bark-based raw material is carried out with sodium hydroxide (NaOH), sodium sulfide (Na 2S) or sodium carbonate (NaCO 3 ) or more than one alkaline sodium compound, that is, a single such alkaline sodium compound, or a combination of two or all three of these, for example, NaOH, Na 2 S and NaCO 3 using a combination of, to form white liquor.

[0029] The alkali treatment step is essential because the product of such treatment is richer in cellulose than, for example, the product of hot water extraction, while also containing a small amount of extractives and, for example, ash. Thus, since hot water extraction results in the separation of useful compounds from the desired fraction, hot water extraction is excluded from the method of the present invention and replaced by the alkali treatment of the present invention.

[0030] The alkali treatment can be carried out using conditions known from soda cooking or kraft cooking, but milder conditions, for example, lower temperature and lower alkali dosage, are preferred.

[0031] In one embodiment of the present invention, the alkali treatment step is carried out with an effective alkali content (EA) of 10 - 50%, preferably 14 - 30%, most suitably 16 - 25% of the dry mass of the bark. The preferred temperature during the alkali treatment step is 80 - 190 °C, and a more preferred temperature is 90 - 160 °C. Typically, the said conditions are maintained for 15 - 300 minutes, preferably 60 - 180 minutes.

[0032] This alkali treatment results in a polyphenol fraction containing tannins and some lignin, and a fraction of the bark-based material into a fiber-rich residue. These fractions are then separated from each other, that is, after the alkali treatment of the bark-based raw material, the bark residue is recovered and used in the cooking process.

[0033] The fiber-rich residue is typically obtained from said fraction at a yield of 30 - 50 w-% depending on the conditions of the alkali treatment.

[0034] According to a preferred embodiment of the present invention, the residue obtained from the bark-based raw material, optionally pretreated by washing to remove inorganic compounds, is the wood chips obtained from debarked wood, i.e., heartwood or wood chips obtained from trees, or typically a mixture thereof, and is mixed before cooking. The wood chips can be obtained from any coniferous or hardwood species, such as hardwood species like oak, eucalyptus, acacia or willow, preferably from coniferous species, more preferably from pine or spruce.

[0035] When the residue obtained from the alkali treatment is added to the conventionally used wood chips, the yield of the resulting pulp increases by 0.5 - 0.8 w% per 1 w% of the added bark. Preferably, a concentration of 5 - 70 wt% of the residue, calculated from the weight of the entire mixture, more preferably 5 - 50 wt%, most preferably 20 - 40 wt%, is used in the mixture.

[0036] The cooking process results in a pulp having the same bleaching ability and characteristics as the pulp made from only conventional wood chips when using a mixture of wood chips and the fiber-rich residue as the raw material, or when using the fiber-rich residue at a concentration of up to 20 w%.

[0037] In another preferred embodiment, pulping, and optionally bleaching, is carried out using only the residue obtained from the alkali treatment of the bark-based material as the raw material, but the resulting pulp is then mixed with the conventional wood pulp obtained from the pulping of wood chips at a concentration of 5 - 70 w% of the bark-derived pulp, calculated from the weight of the entire mixture, more preferably 5 - 50 w%, most suitably 20 - 40 w%, to obtain a pulp mixture that is very suitable for use in papermaking and packaging applications.

[0038] In one embodiment of the present invention, the cooking process is carried out as a conventional kraft pulping using white liquor having an effective alkali content (EA) of 10 - 50%, preferably 14 - 30%, most suitably 16 - 25% of the dry mass of the mixture of wood and fiber-rich residue, and a sulfidity of 30 - 50%, preferably about 40%. The heating step of the cooking process is preferably carried out at 80 - 190 °C, more preferably 150 - 175 °C, and these conditions are maintained for 15 - 300 minutes, preferably 60 - 180 minutes. Since these conditions correspond to conventional kraft cooking conditions, the existing infrastructure can be utilized in this process despite adding residues of bark raw materials to the materials used.

[0039] In addition to the pulp product, the cooking process produces black liquor containing, among other things, many of the chemicals used in the impregnation and cooking processes. Therefore, this black liquor can be recycled by returning it to the cooking process, for example, by recycling the black liquor for the cooking of the present invention.

[0040] In any embodiment of the present invention, one or more bleaching steps follow the cooking step to further prepare the pulp for the desired use. In particular, a combination of oxidation and alkaline bleaching steps is used, and preferred alternatives are oxygen delignification, bleaching with chlorine dioxide, bleaching with hydrogen peroxide, and alkaline extraction enhanced with bleaching chemicals such as hydrogen peroxide. For example, a bleaching sequence including the following steps can preferably be used in the order of the steps: - Oxygen delignification - Bleaching with chlorine dioxide - Alkaline extraction enhanced with hydrogen peroxide, - Further bleaching with chlorine dioxide, and - Bleaching with hydrogen peroxide.

[0041] The present invention also relates to pulp prepared, for example, using the above method, and in particular to pulp prepared by cooking one or more wood-based raw materials containing or consisting of fiber-rich residues obtained from the alkaline treatment of bark-based raw materials optionally mixed with one or more non-wood materials such as straw, reed, bamboo, or bagasse.

[0042] Thus, in one option, the pulp can be prepared by cooking only the above residues.

[0043] However, in one embodiment of the present invention, the pulp is prepared from a mixture containing the residue mixed with wood chips at a concentration of up to 90 w%, preferably 5 - 70 w% (calculated from the weight of the whole mixture) of the residue, more preferably 5 - 50 w%, and most preferably 20 - 40 w%.

[0044] The pulp, or pulp prepared using the above method, can be used, for example, in papermaking, sanitary or textile applications, or in non-woven fabric applications, or in packaging applications. Papermaking includes the manufacture of paperboard, tissue, and specialty papers.

[0045] Paper sheets manufactured from the pulp obtained using the treatment of the present invention were tested. It was found that the technical properties of paper sheets manufactured from pulp not containing the residue of the above bark raw material and the properties of paper sheets manufactured with a low content of about 5 w% of the residue were equivalent. At higher contents of the residue, the high bulk of the paper sheet makes it particularly suitable for packaging applications.

[0046] It should be understood that the disclosed embodiments of the present invention are not limited to the specific structures, processing steps, or materials disclosed herein, but extend to their equivalents that would be recognized by those skilled in the art. It should also be understood that the terms used herein are used only for the purpose of describing specific embodiments and are not intended to be limiting.

[0047] Throughout this specification, reference to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. For example, when a numerical value is referenced using terms such as about or substantially, the exact numerical value is also disclosed.

[0048] As used herein, a plurality of items, structural elements, components, and / or materials may be presented in a common list for convenience. However, these lists should be construed as if each member of the list were individually identified as a separate and unique member. In addition, various embodiments and examples of the invention may be referred to herein along with alternatives for their various components. It is understood that such embodiments, examples, and alternatives should not be construed as virtual equivalents of each other, but rather as separate and distinct expressions of the invention.

[0049] Furthermore, the described functions, structures, or features can be combined in any suitable manner in one or more embodiments. In this description, numerous specific details are provided to provide a thorough understanding of the embodiments of the invention. However, one of ordinary skill in the art will recognize that the invention can be practiced without one or more of these specific details.

[0050] The foregoing examples are illustrative of the principles of the invention in one or more particular applications, but it will be apparent to those skilled in the art that numerous modifications can be made in the embodiments, uses, and details without departing from the principles and concepts of the invention and without losing the ability of the invention to function. Accordingly, the invention is not intended to be limited except as defined by the following claims.

[0051] The following non-limiting examples are intended merely to illustrate the advantages obtained by embodiments of the invention.

Example

[0052] (Example 1 - Kraft Cooking of Wood Materials with Various Bark Contents) After partially removing the wood particles, the spruce bark was soda - cooked in 24% EA at a temperature of 160 °C for 89 minutes with an H - factor of 680. The resulting polyphenol fraction was separated from the product mixture. The remaining residue derived from the douglas - fir bark was mixed with douglas - fir chips at the contents shown in Table 1.

[0053] Next, these samples were subjected to kraft cooking at EA 20%, sulfidity 40%, temperature 165 °C, and H 1200.

[0054] The screened yield and kappa number were analyzed. These results are also shown in Table 1.

[0055] In another method, after partially removing the wood particles, the douglas - fir bark was subjected to soda - cooking in 15% EA at a temperature of 100 °C for 90 minutes. The resulting polyphenol fraction was separated from the product mixture. The remaining residue was mixed with douglas - fir chips at contents of 5%, 20%, and 100% of the residue obtained from the douglas - fir bark.

[0056] Next, these samples were subjected to kraft cooking at EA 20%, sulfidity 40%, temperature 165 °C, and H 1200.

[0057] The screening yield and kappa number of the 100% bark pulp are also shown in Table 1.

[0058]

Table 1

[0059] As the results show, the use of the same amount of wood chips supplemented with the residue derived from the bark causes an increase in the yield of pulp obtained from kraft cooking.

[0060] (Example 2 - Analysis of Pulp Obtained by Kraft Cooking of Bark - Containing Raw Materials) The above samples were analyzed to obtain results indicating the bleachability of each pulp. Bleaching was carried out using a bleaching sequence including oxygen delignification (O), bleaching with chlorine dioxide (D), alkaline extraction enhanced with hydrogen peroxide (Ep), bleaching with chlorine dioxide (D), and bleaching with hydrogen peroxide (P).

[0061] The results are shown in Table 2 below.

[0062]

Table 2

[0063] From these results, it is clear that a higher bark content did not impair the bleachability of the pulp. Furthermore, the positive increase in yield remained throughout the bleaching sequence.

[0064] The fiber properties of the selected pulp are shown in Table 3, showing the bark-derived residues at 100 °C and 160 °C after alkaline extraction (AE).

[0065]

Table 3

[0066] The length-weighted distribution of the unrefined pulp is shown in Figure 1. Figure 1A represents the reference sample, Figure 1B represents the 5% bark sample, and Figure 1C represents the 20% bark sample.

[0067] Further results of the pulp are shown in Figures 2 to 5. As these results show, the development of tensile strength during refining was relatively poor in 100% bark pulp (Figure 2A), and the drainage resistance increased (SR number; Figure 2B) as the bark content in the pulp increased. At a constant tensile index, the tear index (Figure 3A) was lower, and the Scott bond (z-direction strength; Figure 3B) was higher in the pulp with an increased bark content. A 20% bark content in the pulp increased between 10 and 15% of the bulk with a given Scott bond (Figure 4A), which enables a decrease in basis weight while maintaining a constant thickness. The bark fibers further reduced the refining energy with a given Scott bond (Figure 4B), whereby the fibers remained more bulky. Finally, a 20% bark content in the pulp also resulted in a higher light scattering coefficient (Figure 5A) and a higher opacity (Figure 5B), while the 100% bark pulp had the lowest scattering and opacity.

[0068] The results from Figures 2 to 5 also show that 100% pulp can be useful for improving Scott bond and bulk without sacrificing drainage and porosity. These 100% bark pulps can further be relevant for improving printability by smoothness and formation, as well as improving wet strength and resistance to high filler contents. These 100% bark pulps can also be useful as reinforcing pulp for LWC. Also, the bonding properties can be further improved by metal exchange.

Industrial Applicability

[0069] This method can be used in providing higher versatility in the selection of raw materials for the papermaking process. Further, it can be used in providing further use for the bark side stream that is generally discarded in the pulping process. Further, it can also be used to increase the bulk of the pulp at a given basis weight.

[0070] The pulp produced using this method is useful, for example, in papermaking, particularly for packaging applications.

Claims

1. A method for preparing pulp from one or more wood-based raw materials, comprising: performing an alkali treatment step on the bark-based raw material; recovering a solid residue after the alkali treatment step; cooking the bark raw material residue alone or as a mixture with one or more additional wood-based raw materials; A method characterized by the above.

2. The method according to claim 1, wherein the bark-based raw material is selected from the bark of coniferous or broad-leaved tree species, preferably from pine, fir, beech or spruce of coniferous tree species, or from oak, eucalyptus, acacia, poplar or willow of broad-leaved tree species, and typically contains less than 5 w% of other materials.

3. The pretreatment of the bark-based raw material is carried out before the cooking step, optionally before the alkali treatment step, to remove or at least reduce the content of inorganic compounds as ash. The pretreatment includes washing the material with a chelating agent such as strong mineral acid, weak acid, or washing with nitric acid, and separating and collecting the solid for the alkali treatment step. The method according to claim 1 or 2.

4. The alkaline treatment of the bark-based raw material is carried out using one or more alkaline sodium compounds, such as sodium hydroxide (NaOH), sodium sulfide (Na ２ 2S) or sodium carbonate (NaCO ３ 3), or a combination of two or all three of these, preferably using a bark-based raw material having an effective alkali content (EA) of 10 to 50%, more preferably 14 to 30%, most preferably 16 to 25%, according to any of the preceding claims.

5. The method according to any one of the preceding claims, wherein the alkali treatment step is carried out at 80-190 °C, preferably 90-160 °C.

6. The method according to any one of the preceding claims, wherein the alkali treatment is carried out for a treatment time of 15-300 minutes, preferably 60-180 minutes.

7. The method according to any one of the preceding claims, wherein after the alkali treatment of the bark-based raw material, the polyphenol-containing black liquor and the residue rich in bark-derived fibers are separated, the residue is recovered and used in the cooking step.

8. The cooking step is carried out as conventional kraft pulping, preferably by using white liquor having an effective alkali content (EA) of 10-50% and a sulfidity of 30-50%, and typically by carrying out a heating step at a temperature of 80-190 °C, and maintaining under these conditions for 15-300 minutes. The method according to any one of the preceding claims.

9. The method according to any one of the preceding claims, wherein the residue is mixed with wood chips before cooking, and the wood chips are obtained from any coniferous or broad-leaved tree species, such as broad-leaved tree species oak, eucalyptus, acacia or willow, preferably coniferous tree species, more preferably pine or spruce.

10. The residue, optionally pretreated to remove inorganic compounds, is mixed with wood chips before cooking, preferably at a concentration of up to 90 w%, more preferably 5 - 70 w%, even more preferably 5 - 50 w%, most suitably 20 - 40 w% calculated from the weight of the whole mixture, in the method according to any of the preceding claims.

11. The bark pulp obtained by cooking the residue is mixed with a conventional wood pulp obtained by cooking wood chips from any softwood or hardwood species, preferably at a concentration of up to 90 w% of the bark pulp, more preferably 5 - 70 w%, even more preferably 5 - 50 w%, most suitably 20 - 40 w% calculated from the weight of the whole mixture, in the method according to any one of claims 1 - 8.

12. After the cooking step, one or more bleaching steps follow, in the method according to any of the preceding claims.

13. A pulp characterized in that it is formed by cooking one or more wood-based raw materials containing or consisting of residues obtained from the alkaline treatment of bark-based raw materials.

14. The pulp according to claim 13, containing up to 70% by weight of the residue mixed with wood chips.

15. The wood pulp according to claim 13 or 14, prepared using the treatment according to any of claims 1 - 12.

16. Use of the wood pulp according to claim 13 or 14, or the wood pulp prepared using the method according to any of claims 1 - 12, in papermaking, sanitary or textile applications, or in non-woven or packaging applications.

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