MATERIAL

Background of the Invention

Field of the Invention

[0001] The present invention concerns a process for preparing pulp from one or more wood-based raw materials, particularly including a residue obtained from an extraction of bark. Further, the invention relates to the pulp thus obtained.

Description of Related Art

[0002] In most industrial processes that utilize wood raw materials the wood material is subjected to debarking before beginning the process. Since the bark of most wood species has a low cellulose content and a relatively high content of components that have a harmful effect on e.g. pulping processes (see e.g. Miranda I, et al. 2012), discarding the bark has been considered advantageous for the overall process. Wood bark is thus an abundant side stream obtainable from forest industries.

[0003] The bark material from said debarking is typically used as an energy source, and has in the past been considered merely as a low- value solid fuel. However, bark contains valuable chemical compounds, such as tannins, in addition to the above mentioned harmful compounds, and an extraction to recover these valuable components has also been attempted. Said extraction is traditionally performed with hot water, although addition of alkaline reagents has also been suggested.

[0004] US 2673798 describes a bark extract obtained by first steaming the bark, and then carrying out an extraction using caustic soda. After washing and dilution, the extract is mixed with newsprint to be pulped.

[0005] Vazquez et al. 2001, in turn, describes the alkaline extraction of softwood bark. [0006] These processes all utilize merely the bark extracts, while the bark residues left after the extraction have been considered to have limited opportunities for valorization. Thus, said publications still fail to fully utilize the potential of the bark fraction of wood. Further, hot water extractions provide only limited extraction levels.

[0007] WO 2020084196, which describes the alkaline cooking of bark, also focuses on said bark extracts. This publication does mention the possibility to use also the bark residue remaining after extraction, but it does not suggest cooking the bark residue.

Instead, the bark residue is said to be suitable for use in enzymatic hydrolysis, bleaching or fibrillation.

[0008] Thus, there is still a need for further and more simple ways of utilizing the valuable bark material that is commonly discarded as waste or used as fuel for heating purposes.

Summary of the Invention

[0009] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0010] According to a first aspect of the present invention, there is provided a process for preparing pulp from one or more wood-based raw materials, including at least a raw material obtained from bark.

[0011] According to a second aspect of the present invention, there is provided a process for utilizing a residue remaining after the extraction of a polyphenol fraction from a bark-based raw material.

[0012] According to a third aspect of the invention, there is provided a process for preparing pulp, utilizing a residue obtained by carrying out an alkaline treatment step on a bark-based raw material. [0013] The pulp can be prepared either using the residue obtained from the bark extraction as the only fibre source, or it can be prepared from a mixture of a bark-based raw material and one or more further wood-based raw materials.

[0014] The present invention is based on the finding that the residue remaining after an extraction of a bark-based raw material has a lowered content of components that are harmful for the pulping process, and a sufficiently high content of cellulose to be used as a high-value component in pulping.

[0015] The sufficiently low content of harmful components is achieved by the alkaline treatment used to replace the hot-water extraction of past processes. This alkaline treatment results in a more efficient separation of extractive compounds and inorganic components together with the polyphenolic fraction that many bark extractions focus on, leaving a solid residue deprived of such harmful components.

[0016] Thus, 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 may be mixed with other wood materials before pulping, to obtain a mixed product from the pulping step, having more common pulp properties. A mixed pulp product may also be obtained by mixing the bark-containing pulp obtained after pulping the fiber-rich residue with conventional wood pulp. The pulps obtained using the process of the present invention are thus suitable for use in e.g. papermaking and packaging products.

[0017] Several advantages are achieved using the present invention. Among others, a viable solution is achieved to valorize industrial fibre-rich bark side-streams. Further, by using bark-based raw materials, pulps can be produced with increased overall yields with respect to the amount of wood used and with specific properties, optimized for certain applications. The bleachability of the pulp is not impaired, even by high bark contents in the pulp. Further, the bark-containing residue gives the resulting paper products a higher bulk, which further enables grammage reduction at constant thickness.

[0018] Using low contents of residue obtained from the bark-based raw-material (<10%, e.g. around 5%) mixed with conventional wood, cooked and optionally bleached together, or using low contents of bark pulp mixed with conventional wood pulp, has the specific advantage of increasing the pulp yield as a function of the amount of pulp obtained from the selected wood source, while having only minor, negligible effects on the technical properties of the resulting paper products. Thus, these pulps can be used for any papermaking or packaging applications.

[0019] Higher contents of said fiber-rich residue, obtained from bark-based raw materials, or of bark-based pulp in such mixtures (>10%, e.g. about 20%) make the pulp particularly suitable for packaging applications. These high-bark pulps have an increased bulk at a given Scott bond, which enables grammage reduction at a constant thickness. However, the change of tensile strength during refining induces loss of freeness and porosity, which makes these pulps most suitable for different applications, particularly related to packaging.

Brief Description of the Drawings

[0020] FIGURE 1 shows the length weighted distributions of unrefined pulps, in Fig. 2A for the reference sample, in Fig. 2B for the 5% residue of bark origin mixed with wood before pulping, and in Fig. 2C for the 20% residue of bark origin mixed with wood before pulping. The residues were obtained by alkaline extraction of bark-based rawmaterials at 160°C.

[0021] FIGURE 2 shows the tensile strength development (Fig. 2 A) during refining for pulps having different bark contents, reaching up to a 100% bark content, as well as the drainage resistance, as the Schopper-Riegler (SR) number (Fig. 2B) at different bark contents in the pulp (Fig. 2B).

[0022] FIGURE 3 shows the tear index (Fig. 3A) and the Scott bond (Fig. 3B) at constant tensile index for pulps with different bark contents.

[0023] FIGURE 4 shows the Scott bond at varying bulk (Fig. 4A) and at varying refining energy (Fig. 4B) for pulps with different bark contents.

[0024] FIGURE 5 shows the light scattering coefficients (Fig. 5A) and the opacity (Fig. 5B) for pulps with different bark contents. Embodiments of the Invention

[0025] Definitions

In the present context, the term “wood-based raw materials” is intended to encompass the materials typically used in cooking, i.e. sapwood and heartwood, as well as the bark-based raw material that typically has been separated from the wood before cooking.

Thus, the term “bark-based raw material” covers the bark component obtained e.g. from the 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. The non-wood materials are typically selected from textiles or cotton, or other fibrous materials that are suitable as raw materials for the cooking process. Typically, this bark-based raw material has not been pre-treated, but consists of the untreated bark that has been obtained directly from the debarking.

The term “cooking” is intended to encompass the various forms of chemical pulping, aiming at, among others, the removal of lignin and other undesirable components. A preferred cooking process is the Kraft process. The cooking can utilize various chemicals, alkaline chemicals being preferred. The cooking process typically begins with an impregnation step, using a white liquor as a source of cooking chemicals, followed by the actual cooking step, where the impregnated material is heated to cause delignification, and further recovery steps, where further undesirable components, such as soap, volatiles and debris, are removed.

The term “alkaline treatment” or “alkaline extraction” is, in turn, intended to describe the broader process of removing harmful components from the raw material, and recover valuable compounds, such as tannins. It can utilize similar alkaline chemicals as kraft cooking, such as sodium hydroxide (NaOH) and sodium sulphide (Na2S), with mere NaOH being more common, and similar or milder temperatures, but does not necessarily require the other sub-steps of the cooking process.

The term “bark-containing residue” or “high- fiber residue” is intended to describe the solid fraction obtained from the alkaline treatment of a barkbased raw material, and typically contains at least 50 w-% of material originating from the bark obtained from the debarking of a wood raw material, i.e. the bark-component of said residue. The remaining materials can be wood or non- wood material either originating from the same wood material as the bark-component, or having been mixed with the barkcontaining raw material or the residue before cooking.

[0026] The present invention thus relates to a process for preparing pulp from one or more wood-based raw materials, by carrying out an alkaline treatment step on a bark-based raw material, 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 further wood-based raw materials.

[0027] The bark-based raw material is typically selected from the bark of softwood or hardwood species, preferably from the softwood species pine, fir, beech or spruce, or from the hardwood species birch, eucalyptus, acacia, poplar or willow, and typically contains less than 5 w-% of other materials commonly ending up in the bark fraction in the debarking of wood.

[0028] In an embodiment of the invention, a pre-treatment is carried out on the bark material before cooking, optionally already before the alkaline treatment, the pre-treatment including washing the material with strong mineral acids, weak acids, or chelating agents, such as washing with nitric acid, and separating and recovering the solids for the following step. The pre-treatment can optionally be preceded by a disintegration or a mechanical opening of the bark to improve the access of any acids or chelating agents to the bark material. [0029] The purpose of this optional acidic washing is to remove, or at least decrease the content of, inorganic compounds (or ash) from the raw material, although a fraction of these are removed also in the alkaline treatment step.

[0030] 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 (Na2S) or sodium carbonate (NaCCh), i.e. one such alkaline sodium compound alone or a combination of two or all three of these, e.g. as a combination of NaOH, Na2S and NaCO ; forming a white liquor.

[0031] The alkaline treatment step is essential, since the product of such a treatment is more cellulose-rich compared to the products of e.g. hot-water extractions, while containing also low amounts of extractives and e.g. ash. Thus, hot-water extraction is left out of the method of the present invention, and replaced with the alkaline treatment of the invention, since hot-water extraction would result in a separation of also useful compounds from the desired fraction.

[0032] The alkaline treatment can be carried out using conditions known from soda cooking, or kraft cooking, although milder conditions are preferred, such as lower temperatures and lower alkali dosages.

[0033] In an embodiment of the present invention, the alkaline treatment step is carried out in an effective alkali content (EA) of 10-50% of the bark dry mass, preferably 14-30%, most suitably 16-25%. Apreferred temperature during the alkaline treatment step is 80-190°C, a more preferred temperature being 90-160 °C. Typically, said conditions are maintained for 15-300 min, preferably 60-180 min.

[0034] This alkaline treatment results in a fractionation of the bark-based material into a polyphenol fraction, containing tannins and some lignin, and a fiber-rich residue. These fractions are then separated from each other, i.e. after the alkaline treatment of the bark-based raw material, and the bark residue is recovered and used in the cooking step.

[0035] The fiber-rich residue is typically obtained from said fractionation with yields between 30-50 w-% depending on the conditions of the alkaline treatment. [0036] According to a preferred embodiment of the invention, the residue obtained from the bark-based raw-material, optionally pre-treated by washing to remove inorganic compounds, is mixed before cooking with wood chips obtained from debarked wood, i.e. obtained from heartwood or sapwood, or typically a mixture of these. The wood chips may be obtained from any softwood or hardwood species, such as the hardwood species birch, eucalyptus, acacia or willow, preferably from softwood species, more preferably from pine or spruce.

[0037] Adding the residue obtained from the alkaline treatment to the conventionally used wood chips will increase the yield of the resulting pulp by 0.5-0.8 w-% per every 1 w-% of bark added. Preferably, a concentration of 5-70 w-% of residue is used in the mixture, calculated from the weight of the entire mixture, more preferably a concentration of 5-50 w-%, most suitably 20-40 w-%.

[0038] The cooking step, when using a mixture of wood chips and fiber-rich residue as raw material, will result in a pulp having the same bleachability and properties as pulp made from only conventional wood chips, at least when using the fiber-rich residue in a concentration of up to 20 w-%.

[0039] In another preferred embodiment, the pulping, and optionally the bleaching, is carried out using only residue obtained from the alkaline treatment of a bark-based material as raw material, but the obtained pulp is then mixed with conventional wood pulp, obtained from the pulping of wood chips, at a concentration of 5-70 w-% of the pulp of bark origin, calculated from the weight of the entire mixture, more preferably a concentration of 5-50 w-%, most suitably 20-40 w-%, to obtain a pulp mixture that is highly suitable for use in papermaking and packaging applications.

[0040] In an embodiment of the invention, the cooking step is carried out as a conventional kraft pulping, utilizing a white liquor having an effective alkali content (EA) of 10-50% of the dry mass of the mixture of wood and fiber-rich residue, preferably 14- 30%, most suitably 16-25%, and a sulphidity of 30-50%, preferably about 40%. The heating step of the cooking process is preferably carried out at a temperature of 80-190°C, more preferably 150-175°C, and by maintaining these conditions for 15-300 min, preferably 60-180 min. Since these conditions correspond to conventional kraft cooking conditions, existing infrastructure can be utilized in the present process, despite adding a residue of bark origin to the used materials.

[0041] The cooking step yields, in addition to the pulp product, among others a black liquor, which contains many of the chemicals used in the impregnation and cooking steps. It is therefore a possibility to recycle this black liquor, e.g. back to a cooking step, such as by recirculating the black liquor to the cooking of the present invention.

[0042] In an optional embodiment of the invention, the cooking step is followed by one or more bleaching steps, to further prepare the pulp for the desired applications.

Particularly, a combination of oxidative and alkaline bleaching steps are used, preferred alternatives being oxygen delignification, bleaching with chlorine dioxide, bleaching with hydrogen peroxide and alkaline extraction reinforced by a bleaching chemical, such as hydrogen peroxide. For example, a bleaching sequence including the following steps can be used, preferably in said order of steps:

- oxygen delignification

- bleaching with chlorine dioxide

- alkaline extraction reinforced with hydrogen peroxide,

- a further bleaching with chlorine dioxide, and

- bleaching with hydrogen peroxide.

[0043] The present invention also relates to a pulp prepared e.g. using the above described process, particularly prepared by cooking one or more wood-based raw materials, comprising or consisting of a fiber-rich residue obtained from an alkaline treatment of a bark-based raw material, optionally mixed with one or more non- wood materials, such as straw, reed, bamboo or bagasse.

[0044] Thus, in one option the pulp can be prepared by cooking merely the residue described above.

[0045] In an embodiment of the invention, said pulp has, however, been prepared from a mixture that contains up to 90 w-% of said residue, mixed with wood chips, preferably a concentration of 5-70 w-% of the residue, calculated from the weight of the entire mixture, more preferably a concentration of 5-50 w-%, and most suitably 20-40 w- %. [0046] Said pulp, or a pulp prepared using the above described method can be used e.g. in papermaking, hygienic or textile applications or in non-wovens or in packaging applications, the papermaking including the making of board, tissue and specialty papers.

[0047] Paper sheets made from pulps obtained using the process of the present invention have been tested. The technical properties of paper sheets made from pulps without the above described residue of bark origin and the properties of paper sheets made with low contents of said residue, of about 5 w-%, have been found to be comparable. At higher contents of said residue, the high bulk of the paper sheets will make them particularly suitable for packaging applications.

[0048] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0049] Reference throughout this specification 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 present invention. Thus, 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. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0050] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. In addition, various embodiments and examples of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention. [0051] Furthermore, the described features, structures, or characteristics may 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 embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details.

[0052] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0053] The following non-limiting examples are intended merely to illustrate the advantages obtained with the embodiments of the present invention.

EXAMPLES

Example 1 - Kraft cooking of wood material with varying bark contents

[0054] Spruce bark was subjected to soda cooking in 24% EA, at a temperature of 160°C, for 89min with an H factor of 680, after partial removal of wood particles. The obtained polyphenol fraction was separated from the product mixture. The remaining residue originating from said spruce bark was mixed with spruce chips in the contents shown in Table 1.

[0055] These samples were then subjected to kraft cooking at EA 20%, a sulphidity of 40%, a temperature of 165°C, and H 1200.

[0056] The screened yield and kappa numbers were analysed. These results are also shown in Table 1. [0057] In an alternative procedure, spruce bark was subjected to soda cooking in 15% EA, at a temperature of 100°C, for 90 min, after partial removal of wood particles. The obtained polyphenol fraction was separated from the product mixture. The remaining residue was mixed with spruce chips at 5%, 20% and 100% contents of said residue obtained from the spruce bark.

[0058] These samples were then subjected to kraft cooking at EA 20%, a sulphidity of 40%, a temperature of 165°C, and H 1200.

[0059] The screened yield and kappa number for the 100% bark pulp is also shown in Table 1.

Table 1. Pulp samples using residues of bark origin (herein named “bark”) after alkaline extraction (AE) at 100°C and 160°C.

[0060] As the results indicate, the use of the same amount of wood chips, supplemented with residues of bark origin, will cause an increase in the yield of the pulp obtained from the kraft cooking.

Example 2 - Analysis of pulp obtained by kraft cooking bark-containing raw materials

[0061] The above samples were analyzed to give results indicating bleachability of the respective pulps. The bleaching was carried out using a bleaching sequence including oxygen delignification (O), bleaching with chlorine dioxide (D), alkaline extraction reinforced with hydrogen peroxide (Ep), bleaching with chlorine dioxide (D), and bleaching with hydrogen peroxide (P). [0062] The results are shown in the following Table 2.

Table 2. Bleachability with sequence O-D-Ep-D-P, and residues of bark origin after alkaline extraction (AE) at 100 C and at 160 C

* Different starting kappa number for Do-stage causes also variation on ClCE-consumption.

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

0064] The fibre properties of selected pulps are shown in Table 3, and residues of bark origin after alkaline extraction (AE) at 100°C and t 160°C. able 3. Fibre properties

[0065] The length weighted distributions of the unrefined pulps are shown in Figs. 1, with Fig. 1 A representing the reference sample, Fig. IB representing the 5% bark sample, and Fig. 1C representing the 20% bark sample.

[0066] Further results of the pulps are shown in Figs. 2 to 5. As these results indicate, the tensile strength development during refining was relatively poor for the 100% bark pulp (Fig. 2A), and increasing the bark content in the pulps increased the drainage resistance (SR number; Fig. 2B). At constant tensile index, the tear index (Fig. 3A) was lower and the Scott bond (strength in z-direction; Fig. 3B) was higher for pulps with increased bark content. A 20% bark content in the pulps increased between 10-15% the bulk at a given Scott Bond (Fig. 4A), which would allow a grammage reduction while maintaining a constant thickness. Bark fibers further reduced refining energy at a given Scott bond (Fig. 4B), whereby the fibers remained bulkier. Finally, a 20% bark content in the pulps also resulted in higher light scattering coefficients (Fig. 5A) and higher opacity (Fig. 5B), but 100% bark pulps had the lowest scattering and opacity.

[0067] The results from Figs. 2 to 5 also indicate that 100% pulps can be useful in improving Scott bond and bulk, without compromising ffeeness and porosity. These 100% bark pulps can further have a relevance in printability improvement via smoothness and formation, as well as in improvement of wet strength and tolerance for high filler contents. These 100% bark pulps can also be useful as reinforcement pulps for LWC. Also binding properties might be improved further by metal exchange.

Industrial Applicability

[0068] The present process can be used in providing higher versatility in the choice of raw materials for the papermaking process. Further, it can be used in providing further use for the commonly discarded bark side-streams of the pulping process. Further, it can also used to increase pulp bulk at a given grammage.

[0069] The pulps produced using the present process are useful for example in papermaking, and particularly in packaging applications. Citation List

Patent Literature

US 2673798 WO 2020084196

Non-patent Literature

Miranda L, Gominho, J., Pereira, H. (2012) Bioresources 7(3), 4350-4361

Vazquez et al. 2001. Holz als Roh-und Werkstoff 59, 451-456