Utilization of AGRO-based extraction liquor in a wood pulping process

Abstract

The present invention relates to a method for utilization of agro-based extraction liquor in a wood pulping process, the method comprises: a) extracting agricultural crops / residues in a sodium hydroxide solution to obtain a slurry of silica-depleted agro-fibers; b) separating the silica-depleted agro-fibers from the slurry to obtain separated silica-depleted agro-fibers and a separated extraction liquor; c) purifying the separated extraction liquor from silica to obtain a purified extraction liquor; and d) adding the purified extraction liquor to a wood pulping process having a recovery cycle for cooking chemicals, thereby using sodium in the purified extraction liquor as sodium make-up in the wood pulping process.

Description

[0001] UTILIZATION OF AGRO-BASED EXTRACTION LIQUOR IN A WOOD PULPING PROCESS

[0002] FIELD OF THE INVENTION

[0003] The present inventive concept relates to a method for utilization of agrobased extraction liquor in a wood pulping process. The present inventive concept also relates to wood pulp and to a pulp mixture manufactured by the method.

[0004] BACKGROUND

[0005] Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibers from wood, fiber crops, wastepaper, or rags. Mixed with water and other chemicals, pulp is the major raw material used in production of different paper products.

[0006] Today, the most common source for pulp for papermaking is wood. Wood pulping processes producing chemical pulp typically have a high production capacity, normally ranging from several hundreds of thousand tons of pulp per year up to over one million tons per year. However, the demand for pulp increases at the same time as many countries have limited accessibility to forest resources. As a result, the interest for non-wood alternatives, such as agricultural materials is increasing.

[0007] Agro-based pulp mills are normally much smaller than wood-based pulp mills, typically producing less than hundred thousand tons of pulp per year. In contrast to wood-based mills, which typically have a recovery cycle for regeneration of cooking chemicals from spent cooking liquor collected in the brownstock washing including washing after oxygen delignification operations, the agro-based mills normally lack a complete recovery cycle due to the high investments needed for such processes. Therefore, the emissions and waste generated, per ton of produced pulp, in agro-based pulp mills are normally substantially higher than that of modern wood pulp mills.

[0008] In a wood kraft pulp process, the spent cooking liquor contains various organic compounds extracted from the wood, mainly originating from lignin and carbohydrates released from the wood material during cooking, and inorganic compounds originating from the cooking chemicals.

[0009] Depending on the type of chemical wood pulping process used, the recovery cycle can contain different subprocesses. As an example, kraft pulp mills have a recovery cycle for regeneration of the so-called white liquor, which contains the active cooking chemicals NaOH and Na2S. The recovery cycle can be divided into four main subprocesses: evaporation, incineration in the recovery boiler, white liquor preparation and lime reburning. The aim of the evaporation is to increase the dry content of the spent cooking liquor by removing water. In the recovery boiler, the concentrated spent cooking liquor is incinerated and heat energy released during the incineration enables generation of steam which subsequently can be converted to electricity and process heat (steam and hot water). A smelt containing the inorganic part of the spent cooking liquor is formed at the bottom of the recovery boiler. In the first step of the white liquor preparation, this smelt is transferred to the smelt dissolving tank where the so-called green liquor is formed. The green liquor mainly contains dissolved Na2S and Na2COs. Na2COs in the green liquor is converted into sodium hydroxide by slaking and causticization reactions using lime. In the slaking reaction, added lime (CaO) forms calcium hydroxide Ca(OH)2 in reaction with water, and in the subsequent causticization reaction, calcium hydroxide reacts with sodium carbonate and forms sodium hydroxide and lime mud (CaCCh). The white liquor formed is taken back to the wood cooking process while the lime mud is washed and reburned into lime in a lime kiln. Without a well working recovery cycle the kraft process would not be an economical or an environmentally sustainable process. Any major disturbances in this process (or any of its subprocesses) may cause production limitations and / or unplanned stops of the whole pulp mill.

[0010] However, agro-based feedstocks usually have high levels of silicon compounds, such as silicon dioxide, and in the commonly used alkaline pulping processes, like the kraft and soda processes, much of the silica ends up in the cooking liquor. The spent cooking liquors from silica containing feedstocks are difficult to handle in the subprocesses of the recovery cycle. Liquors containing high silica content have an increased viscosity and can increase scale formation during evaporation. They can also disturb the causticizing process, decrease lime mud filterability, and negatively affect the operation of the lime kiln. A high intake of silica with the raw material used in the pulping will result in an unwanted accumulation of silica in the lime cycle. To handle these risks, many agro-based pulp plants send some or all the spent cooking liquor to the sewer or an effluent treatment plant.

[0011] Large and more modern agro-based pulp mills may operate a partial or complete recovery cycle, but if using high silica containing feedstocks a very high portion of the lime mud needs to be discharged from the lime cycle and be replaced by new fresh lime as make-up in the process. Often, 100% of the produced lime mud must be discharged / purged out and handled by landfill and only fresh lime is used for slaking and causticization purposes.

[0012] Document W02011072718 describes a process for forming a pulp mixture wherein agricultural crops / residues or pre-processed agricultural crops / residues are introduced to wood material in a wood pulping process. The idea is based on the possibility of taking advantage of sorption of xylan in the agro-based raw material on the wood based pulp fibers and the positive effects xylan provide on the properties of the pulp. However, the above-mentioned challenge with the agro-based materials is not addressed in W02011072718.

[0013] A partial desilication of the feedstock by dirt / soi I removal can be used to lower the intake of silica, but it is not efficient to remove a high proportion of silica present in the feedstock in such pretreatment operations. There are also methods to remove silica from spent cooking liquors by separation of a silica rich phase obtained after, for example aging, pH reduction, reaction of silica with metal ions (e.g. calcium or aluminium). However, these techniques have been developed to enable recovery of cooking chemicals in separate agro-pulping processes having a separate chemical recovery cycle. In practice, small-scale agro-based pulp mills do not have a complete recovery cycle and therefore do not have a use for these desilication techniques. Therefore, agricultural crops / residues are an underutilized resource for pulp production especially in areas / regions which have plentiful forest resouces available.

[0014] There is, therefore, a need for an improved method enabling co-production using both wood and high silica containing agricultural crops / residues in a resource efficient way without needing two separate comprehensive recovery cycles for regeneration of cooking chemicals and with a normal, i.e. not an excessive, use of make-up lime in the lime cycle operation of the recovery cycle.

[0015] Summary of the invention

[0016] It may be desired to incorporate agricultural materials in wood pulp. However, agricultural material typically contains large amounts of silica which is unfavourable in a wood pulping process.

[0017] It is an object of the present inventive concept to overcome this problem, and to provide an improved method for utilization of agro-based material in a wood pulping process. The method is aimed to limit and control silica intake to the recovery cycle and to maximize the utilization of raw materials as well as process chemicals, in particular sodium, from an agro pulping process in a wood pulp process.

[0018] According to a first aspect of the present invention, this and other objects are achieved by a method for utilization of agro-based extraction liquor in a wood pulping process. The method comprises: a) extracting agricultural crops / residues in a sodium hydroxide solution to obtain a slurry of silica-depleted agro-fibers; b) separating the silica-depleted agro-fibers from the slurry to obtain separated silica-depleted agro-fibers and a separated extraction liquor; c) purifying the separated extraction liquor from silica to obtain a purified extraction liquor; and d) adding the purified extraction liquor to a wood pulping process having a recovery cycle for cooking chemicals, thereby using sodium in the purified extraction liquor as sodium make-up in the wood pulping process. The method according to the inventive concept is at least partly based on the understanding that by extracting agro-based raw material, which is usually rich in silica, in a sodium-hydroxide solution and subsequently purifying the extraction liquid from silica, a purified extraction liquid is realized which can be reused in a wood pulp process to replace sodium containing make-up chemicals and optionally also at the same time enable sorption of xylan on the wood fibers without causing contamination of cooking chemicals by silica. Hereby, a resource-efficient reuse of the agro-based extraction liquid as well as the wood cooking chemicals is enabled.

[0019] By adjusting the production capacity of the extraction of agro-based crops / residues and the extraction liquor purification, a purified extraction liquor is obtained which may cover up to 100% of the wood pulping process sodium make-up consumption.

[0020] The method comprises extracting agricultural crops / residues in a sodium hydroxide solution to obtain a slurry of silica-depleted agro-fibers. The extraction of agricultural crops / residues may be performed batch wise or continuously.

[0021] The term "extracting" used herein may be referred to as "alkaline extracting" or "alkaline extraction".

[0022] The sodium hydroxide solution may comprise e.g. sodium hydroxide, water, used washing liquor, separated extraction liquor that has been recirculated, leaching / washing filtrate from leaching / washing of silica-depleted agro-fibers and / or washing liquid from washing of the solid phase of the pH-reduced extraction liquor as will be described further down.

[0023] The agricultural crops / residues to be extracted may have a silica content of at least 1000 ppm, preferably at least 2000 ppm, more preferably at least 4000 ppm, most preferably at least 10000 ppm. The silica content may be measured according to method ISO 776:2011.

[0024] White liquor is used as the alkaline solution of cooking chemicals in kraft cooking. However, the inventors of the present invention have realized that it is preferable not to perform the extraction of agricultural crops / residues in white liquor, since the hydrogen sulfide in white liquor can make a pH-reduction of the extraction liquor hazardous (there is a risk of formation of poisonous hydrogen sulfide gas if pH is reduced below 9). To this end, the sodium hydroxide solution used in the extraction of agricultural crops / residues may in the main comprise sodium hydroxide as the alkali source, meaning that the concentration of sodium sulfide in the sodium hydroxide solution should be kept below 0.2 g / kg, preferably below 0.1 g / kg.

[0025] The method may further comprise measuring the alkalinity of the sodium hydroxide solution and / or the separated extraction liquor. This is to control the extraction of the agricultural crops / residues. For the same reason, the method may further comprise measuring the pH value of the sodium hydroxide solution and / or the separated extraction liquor. The pH value of the sodium hydroxide solution may be between pH 10.5 and pH 14, preferably between pH 11.5 and pH 13.5. The pH value of the separated extraction liquor may be between pH 10 and pH 13, preferably between pH 10.5 and pH 13.

[0026] The method may further comprise washing the agricultural crops / residues prior to extraction. Hereby, at least parts of the water-soluble compounds, such as starch and proteins, and ions such as potassium, chloride, nitrate, and phosphate are separated from the raw material prior to extraction in the sodium hydroxide solution. The washing of the agricultural crops / residues may be performed as a water wash at a temperature below 50°C or hot water wash at a temperature between 50 to 90°C.

[0027] The method comprises separating the silica-depleted agro-fibers from the slurry to obtain separated silica-depleted agro-fibers and a separated extraction liquor. The separated extraction liquor may typically comprise sodium ions, silica, xylan, and other organic compounds released from the agricultural crops / residues.

[0028] At least a part of the separated extraction liquor may be recirculated back to the extraction of agricultural crops / residues for reuse of sodium hydroxide and to increase the silica content. In other words, the method may further comprise recirculating at least a part of the separated extraction liquor for reuse in the extraction of agricultural crops / residues. High silica content typically results in high precipitation ability, resulting in improved separation efficiency. To this end , the method may comprise evaporating at least partly the separated extraction liquor prior to the purification to increase the silica content of the separated extraction liquor.

[0029] The method may further comprise repeating, at least one time, steps a) and b) for the separated silica-depleted agro-fibers and recirculating at least a part of the separated extraction liquor(s) for reuse in the extraction of agricultural crops / residues and / or in the extraction of silica-depleted agro-fibers. Hereby, the separated extraction liquor obtained in the extraction of agricultural crops / residues, herein referred to as the first extraction, may be recirculated and reused in the first extraction and / or in the extraction of silica depleted agro-fibers, herein referred to as the second extraction. Similarly, the separated extraction liquor obtained from the second extraction, i.e. the extraction of silica depleted agro-fibers, may be recirculated and reused in the first and / or the second extraction(s). The sodium hydroxide solution used in the first extraction may then be referred to as a first sodium hydroxide solution and the second sodium hydroxide solution used in the second extraction may be referred to as a second hydroxide solution. Stated differently, the method may further comprise: repeating the extraction of the separated silica-depleted agro-fibers in a second sodium hydroxide solution to obtain a second slurry of twice-extracted silica- depleted agro-fibers; separating the twice-extracted silica-depleted agro-fibers from the second extraction liquor to obtain separated twice-extracted silica-depleted agro-fibers and a second separated extraction liquor, and reusing at least a part of the second separated extraction liquor in the extraction of agricultural crops / residues and / or in the extraction of separated silica- depleted agro-fibers.

[0030] If no separated extraction liquor from the second extraction is recirculated back to the first extraction (i.e. the second extraction only having a recirculation of its separated extraction liquor to the second extraction), the second extraction may at least partly use oxidized white liquor instead of NaOH as the alkali source.

[0031] Preferably, the pH of the first sodium hydroxide solution is lower than the pH of the second sodium hydroxide solution. The pH value of the first sodium hydroxide solution may be between pH 10.5 and pH 14, preferably between pH 11.5 and pH 13.5. The consumption of alkali may be higher in the extraction of the agricultural crops / residues than in the extraction of the separated silica-depleted agro-fibers. At least 60%, preferably at least 70% of the alkali may be consumed in the extraction of the agricultural crops / residues. At most 40%, preferably at most 30% of the alkali may be consumed in the extraction of separated silica-depleted agro-fibers. The alkali content may be measured according to a standard method for residual alkali i.e. standard method SCAN-N 33:94

[0032] The method comprises purifying the separated extraction liquor from silica. Purifying the separated extraction liquor from silica may include reducing the silica content by at least 60 %, preferably at least 80 %. The silica content may be measured according to standard method SCAN-N 38:01.

[0033] Purifying the separated extraction liquor from silica may comprise treating the separated extraction liquor with a pH-reducing agent to obtain a pH-reduced extraction liquor comprising a liquid phase and a solid phase; and separating the liquid phase from the solid phase to obtain the purified extraction liquor and a silica- enriched sludge. The reduction of the pH value results in decreased solubility of silica compounds which forces silica to precipitate. The silica-enriched sludge may be washed and the used washing liquid may be recirculated for reuse in the extraction of agricultural crops / residues and / or in the extractions of silica-depleted agro-fibers.

[0034] The method may further comprise adding a flocculation polymer to the pH- reduced extraction liquor before separating the liquid phase from the solid phase.

[0035] The pH-reduced extraction liquor may have a pH between pH 5 and pH 9.5, preferably between pH 6 and pH 8. Alternatively, the pH-reduced extraction liquor may have a pH above pH 4 and / or a pH below pH 9. The pH may be between pH 4 and pH 9, between pH 5 and pH 8.5, or between pH 6 and pH 8. The pH-reducing agent may be selected from pH-reducing agents that are compatible with the chemical recycling of the wood pulp process. As an example, the pH-reducing agent may comprise sulfuric acid and / or carbon dioxide. Sulfuric acid is beneficial in that it may replace sulfur make-up. Normally, pulp mills have an excess of sulfur, but there are conditions where sulfur also needs to be added as a make-up chemical. Other pH-reducing agents may be organic acids.

[0036] Separating a liquid phase from a solid phase may comprise at least one of filtering, settling, using hydrocyclones and / or centrifugation, preferably decanter centrifugation. Additionally, different separation techniques can be combined to yield a more efficient separation.

[0037] Separating the liquid phase from the solid phase may comprise increasing the dry matter content of the solid phase to at least 10%, preferably at least 20%. A high dry matter content of the solid phase is advantageous in that less silica is transferred to (and contaminating) the wood pulping process at the same time as more sodium can be reused.

[0038] In order to maximize the reuse of sodium, the method may further comprise washing the solid phase of the pH-reduced extraction liquor and adding used washing liquid to the wood pulp process and / or to the extraction of agricultural crops / residues. The used washing liquor may additionally and / or alternatively be added to the extraction of silica-depleted agro-fibers.

[0039] The method comprises adding the purified extraction liquor to a wood pulping process having a recovery cycle for cooking chemicals. The wood pulping process may be a kraft pulping process, such as a softwood kraft pulping process.

[0040] The wood pulping process and the alkaline extraction of agricultural crops / residues may be co-located at the same production site. Alternatively, the wood pulping process and the alkaline extraction of agricultural crops / residues may be carried out at different production sites.

[0041] The sodium in the purified extraction liquor is used to replace at least a part of the sodium containing make-up chemicals, such as sodium hydroxide, in the wood pulping process. To this end, the purified extraction liquor is preferably added in a position of the wood pulping process enabling use of sodium from the purified extraction liquor as sodium make-up in the chemical recovery cycle. The purified extraction liquor may for example be added to a wood cooking process, an evaporation stage in the recovery cycle, a brownstock washing stage, and / or an oxygen delignification stage of the wood pulping process.

[0042] The purified extraction liquor may, in addition to sodium, provide valuable additional xylan to the wood pulp via xylan sorption, since xylan present in the purified extraction liquor will, at least partly, become sorbed on the wood pulp fibers. Sorption of xylan contributes both to increased pulp yield (a higher portion of the agricultural crops / residues is used to produce materials) and enhanced paper properties when the pulp is utilized for paper making purposes. The purified extraction liquor may thus be added in a position of the wood pulping process enabling sorption of xylan, such as to the fiber-line of the wood pulping process, preferably in a position after wood cooking but before oxygen delignification.

[0043] The method may further comprise adding the separated silica-depleted agrofibers to wood pulp fibers of the wood pulp process to obtain a pulp mixture.

[0044] The separated silica-depleted agro-fibers may be added together with the purified extraction liquor. This could be described as a dilution of the silica-depleted agro-fibers with the purified extraction liquor before the addition to the wood pulp fibers. The separated silica-depleted agro-fibers and the purified extraction liquor may be added to the wood pulp fibers at the same time and at the same addition point. Alternatively, the silica-depleted agro-fibers may be diluted with spent liquor from an oxygen bleaching stage, brownstock washing liquid, optionally with addition of alkali in the form of oxidized white liquor and / or sodium hydroxide.

[0045] The method may further comprise leaching and / or washing the silica- depleted agro-fibers before adding to the wood pulp fibers. Hereby, less silica in the silica-depleted agro-fibers is carried over to the wood pulp and more of the residual alkali which still may be present in the silica-depleted agro-fibers can be reused. The used leaching and / or washing liquid / filtrate may be added to the extraction of agricultural crops / residues and / or to the extraction of silica-depleted agrofibers. According to a second aspect of the present invention, there is provided a wood pulp manufactured by the method according to the first aspect.

[0046] According to a third aspect of the present invention, there is provided a pulp mixture manufactured by the method according to the first aspect.

[0047] According to the fourth aspect of the present invention, there is provided a silica-depleted agro-fiber pulp obtained by the method according to the first aspect. The silica-depleted agro-fibers may be used to produce an agro-based pulp which can be used e.g. for production of powdered cellulose. In this case, the silica depleted agro-fibers may be further washed and / or bleached prior to drying.

[0048] According to a fifth aspect of the present invention, use of the silica-depleted agro-fiber pulp of the fourth aspect is provided for production of powdered cellulose.

[0049] According to a sixth aspect of the present invention, there is provided a silica- enriched sludge obtained by the method according to the first aspect.

[0050] According to a seventh aspect of the present invention, there is provided the use of the silica-enriched sludge of the sixth aspect for biogas production and / or fertilization.

[0051] The second to seventh aspects may generally present the same or corresponding advantages as the first aspect. Effects and features of these aspects are largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second to seventh aspects.

[0052] BRIEF DESCRIPTION OF THE DRAWINGS

[0053] These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention.

[0054] Fig. 1 shows a flow chart of a method for utilization of extraction liquor from alkaline extraction of agricultural material according to at least one example embodiment of the inventive concept; Fig. 2a Illustrates a flow chart of a similar method to Fig. 1 with illustration of different options of addition points for the purified extraction liquor in the wood pulp process according to at least one example embodiment of the inventive concept;

[0055] Fig. 2b shows a flowchart of the method in Fig. 2a except that also the separated silica-depleted agro-fibers are added to the wood pulp fibers of the wood pulp process to obtain a pulp mixture according to at least one example embodiment of the inventive concept;

[0056] Fig. 3a shows a flowchart of the alkaline extraction of the agricultural crops / residues and the purification of the separated extraction liquor according to at least one example embodiment of the inventive concept;

[0057] Fig. 3b shows a flowchart of alkaline extraction and separation in two steps according to at least one example embodiment of the inventive concept;

[0058] Fig. 4 shows a flowchart of a method for utilization of separated extraction liquor from alkaline extraction of agricultural material according to at least one example embodiment of the inventive concept.

[0059] DETAILED DESCRIPTION

[0060] Fig. 1 illustrates a flow chart of a method for utilization of extraction liquor from alkaline extraction of agricultural material.

[0061] At SI, the method comprises extracting agricultural crops / residues in a sodium hydroxide solution to obtain a slurry of silica-depleted agro-fibers.

[0062] The agricultural crops / residues to be extracted may have a silica content of at least 1000 ppm, preferably at least 2000 ppm, more preferably at least 4000 ppm, most preferably at least 10000 ppm. The silica content may be measured according to method ISO 776:2011.

[0063] The sodium hydroxide solution may in the main comprise NaOH as alkali source. The sodium hydroxide solution may have a concentration of sodium sulfide below 0.2 g / kg, preferably below 0.1 g / kg.

[0064] The agricultural crops / residues may be washed prior to the extraction (not illustrated in Fig. 1). Hereby, at least parts of the water-soluble compounds, such as starch and proteins, and ions such as potassium, chloride, nitrate, and phosphate are separated from the raw material prior to the extraction in the sodium hydroxide solution. The washing of the agricultural crops / residues may be performed as a water wash at a temperature below 50°C or hot water wash at a temperature between 50 to 90°C.

[0065] At S2, the method comprises separating the silica-depleted agro-fibers from the slurry which now is enriched with silica. In addition to the separated silica- depleted agro-fibers, a separated extraction liquor comprising sodium ions, silica, xylan, and other organic compounds released from the agricultural crops / residues is also obtained.

[0066] At S3, the separated extraction liquid is purified. This step may be performed in two sub-steps: treating the separated extraction liquor with a pH-reducing agent, forming a pH-reduced extraction liquor (S31) and separating a liquid phase from a solid phase of the pH-reduced extraction liquor to obtain a purified extraction liquor and a silica-enriched sludge (S32).

[0067] The reduction of the pH value results in decreased solubility of silica compounds which forces silica to precipitate. The method may for example comprise adding a flocculation agent to the pH-reduced extraction liquor before the separation of the liquid phase from the solid phase of the pH-reduced extraction liquor.

[0068] Purifying the separated extraction liquor from silica may include reducing the silica content by at least 60 %, preferably at least 80 %. Silica content in the extraction liquor may be measured according to method SCAN-N 38:01.

[0069] The pH-reduced extraction liquor may have a pH between pH 5 and pH 9.5, preferably between pH 6 and pH 8. Alternatively, the pH-reduced extraction liquor may have a pH above pH 4 and / or a pH below pH 9.5. The pH may be between pH 4 and pH 9.5, between pH 5 and pH 8.5, or between pH 6 and pH 8.

[0070] The pH-reducing agent may comprise sulfuric acid and / or carbon dioxide. The pH-reducing agent may be selected from pH-reducing agents that are compatible with the chemical recycling in the wood pulp process. The separation of the liquid phase from the solid phase at S3 may be carried out by at least one of filtering, settling, using hydrocyclones and / or centrifugation, preferably decanter centrifugation.

[0071] The separation of the liquid phase from the solid phase of the pH-reduced extraction liquor may be carried out to a dry matter content of at least 10%, preferably at least 20% of the solid phase of the pH-reduced extraction liquor. A high dry matter content of the solid phase of the pH-reduced extraction liquor is advantageous in that less silica is transferred to (and contaminating) the wood pulping process at the same time as more sodium can be reused.

[0072] In order to maximize the reuse of sodium, the method may optionally comprise washing the solid phase of the pH-reduced extraction liquor and adding used washing liquid to the wood pulp process and / or to the extraction of agricultural crops / residues.

[0073] After the purification of the separated extraction liquor, the method comprises adding the purified extraction liquor to a wood pulping process having a recovery cycle for cooking chemicals, thereby using the sodium in the purified extraction liquor as sodium make-up in the wood pulping process (S4). The sodium in the purified extraction liquor is thus used to replace at least a part of the sodium containing make-up chemicals, such as sodium hydroxide, in the wood pulping process.

[0074] In addition to the purified extraction liquor, the separated silica-depleted agro-fibers may be added to the wood pulping process (S4'). The separated silica- depleted agro-fibers may be added together with the purified extraction liquor, i.e. at the same time and at the same addition point. Alternatively, the separated silica depleted agro-fibers may be added separately from the purified extraction liquor, i.e. in a different addition point.

[0075] Fig. 2a. shows flowchart of a similar method to Fig. 1 with illustration of different options of addition points for the purified extraction liquor. As previously described, it is preferable to add the purified extraction liquor in a position of the wood pulping process enabling use of sodium from the purified extraction liquor as sodium make-up in the chemical recovery cycle. To this end, the purified extraction liquor may be added to a wood cooking process, a brownstock washing stage, an oxygen delignification stage and / or an evaporation plant in the recovery cycle of the wood pulping process.

[0076] The purified extraction liquor may, in addition to sodium, provide valuable additional xylan to the wood pulp via xylan sorption, since xylan present in the purified extraction liquor will, at least partly, become sorbed on the wood pulp fibers. Sorption of xylan contributes both to increased pulp yield (a higher portion of the agricultural crops / residues is used to produce materials) and enhanced paper properties when the pulp is utilized for paper making purposes. The purified extraction liquor may thus be added in a position of the wood pulping process enabling sorption of xylan, such as to the fiber-line of the wood pulping process, preferably in a position after wood cooking but before oxygen delignification.

[0077] The silica-depleted agro-fibers may be used to produce an agro-based pulp which can be used e.g. for production of powdered cellulose. In this case, the silica depleted agro-fibers may be further washed and / or bleached prior to pulp drying.

[0078] Fig. 2b illustrates a flowchart of the method in Fig. 2a but in this method also the separated silica-depleted agro-fibers are added to the wood pulp fibers of the wood pulp process, to obtain a pulp mixture.

[0079] The separated silica-depleted agro-fibers may be added together with the purified extraction liquor, for example, at the same time and at the same addition point. Alternatively, the separated silica depleted agro-fibers may be added separately from the purified extraction liquor, in a different addition point.

[0080] Fig. 3a shows a flowchart of alkaline extraction of the agricultural crops / residues and purification of the separated extraction liquor, wherein at least a part of the separated extraction liquor is recirculated to the alkaline extraction to increase the silica content in the extraction liquor. As previously described, high silica content typically results in high precipitation ability, resulting in improved separation efficiency. Fig 3b shows a flowchart of alkaline extraction and separation in two steps. In this embodiment, two separate alkaline extraction steps and separation steps are used. In the first extraction step, a lower pH value may be applied, i.e. to minimize the consumption of the pH-reducing agent but still keep the silica soluble while in the second extraction step, a higher pH value may be applied to dissolve additional silica and dissolve more of the xylan. The method may thus comprise measuring the alkalinity and / or pH of the first and / or second sodium hydroxide solution(s) and / or separated extraction liquor(s) to control the extraction of the agricultural crops / residues and / or extraction of the silica-depleted agro-fibers. The pH value of the first sodium hydroxide solution may be between pH 10.5 and 14, preferably, between pH 11.5 and 13.5.

[0081] The consumption of alkali is typically higher in the first extraction step than in the second extraction step. Typically, at least 60% of the alkali are consumed in the first extraction step and at most 40% of the alkali are consumed in the second extraction step. Preferably, at least 70% of the consumption of alkali occurs in the first extraction step and a maximum of 30% of the alkali consumption occurs in the second extraction step.

[0082] At least a part of the separated extraction liquor obtained in the first separation step is recirculated to the first extraction while at least a part of the separated extraction liquor obtained in the second separation is recirculated to the first and / or the second extraction step(s) to use residual alkali. At least a part of the separated extraction liquor obtained in the second extraction may be added to the wood pulp fibers of the wood pulping process without further purification provided that the separated extraction liquor has a Na:SiO2 ratio higher than 10:1 and a silica content of less than 2 g / kg of liquor.

[0083] Fig 4 shows a flowchart of a method similar to that in Fig. 1. In Fig. 4 it is shown that the method provides the advantages of utilization of sodium, xylan, and TOC from the extraction process of the agricultural crops / residues. The method may be described as a co-production of wood and agro-based feedstocks. The alkaline extraction process of the agro-based feedstocks does not have a separate chemical recovery cycle.

[0084] Examples

[0085] Example 1. Purification of separated extraction liquor with addition of flocculation agent

[0086] A separated extraction liquor (Separated extraction liquor 1A) obtained after alkaline extraction of oat hulls followed by separation of a slurry of silica-depleted agro-fibers was used as starting material for extraction liquor purification trials. The separated extraction liquor was prepared in pilot-scale. The oat hulls had a silica content of 25.5 g / kg. The alkaline extraction was performed in a 5 m3stirred reactor tank using 200 kg of oat hulls (based on dry solids) and 0.1 kg NaOH / kg of oat hulls. The alkaline extraction was performed at 85 °C for 60 minutes. After the extraction, the slurry of silica-depleted oat hulls was separated into separated silica-depleted agro-fibers and separated extraction liquor using a pilot-scale screw press. The separated extraction liquor had composition according to Table 1.1.

[0087] Table 1.1 Composition of the separated extraction liquor including residual alkali, xylan, silica, sodium, and total organic carbon (TOC). Na:SiO2 describes the weight ratio between sodium and silica

[0088] The pH of the separated extraction liquor was reduced by addition of a pH-reducing agent (in this case 1 M sulfuric acid (H2SO4)) to two different pH levels (pH 7 and pH 9). 0.2 kg of separated extraction liquor was stirred in a beaker using a magnetic stirrer during the pH reduction and 10 minutes after the acid addition. Then, a flocculation agent (FennoPol K supplied by Kemira Kemi AB, Helsingborg, Sweden) was added to the pH-reduced extraction liquor under stirring in a beaker using a magnetic stirrer. Finally, the pH-reduced extraction liquor was poured into five 50 mL cylindrical centrifugation tubes (inner diameter 2.8 cm with conical bottom, height of the bottom cone was 1.5 cm) and centrifuged in a laboratory centrifuge (Hettich Rotanta 460R produced by Andreas Hettich GmbH & Co. KG, Tuttlingen, Germany) for 15 minutes at 3000 RCF (Relative Centrifugal Force), followed by decantation of the liquid phase to separate the liquid phase (purified extraction liquor) from the solid phase (silica-enriched sludge). The purified extraction liquors (Purified extraction liquors 1A-1B) had a composition according to Table 1.2.

[0089] Table 1.2 Composition of purified extraction liquor including xylan, silica, and total organic carbon (TOC)

[0090] This example demonstrates that a pH-reduction to pH 7 is most efficient to reduce the silica content in the purified extraction liquor. The silica content was reduced by 85%, while the xylan content only decreased by 27% in the purification process. The dilution of the separated extraction liquor due to the pH reduction was very small, less than 3% in both trials and will, therefore, influence the calculation only marginally. In this example, the separated extraction liquor was obtained after an alkaline extraction without any recirculation of separated extraction liquor. Thereby, silica content in the separated extraction liquor prior to purification (Separated extraction liquor 1A) was quite low (1.0 g SiCh / kg liquor). Example 2. Purification of separated extraction liquor at different residence time and temperature

[0091] Samples of 0.2 kg of Separated extraction liquor 1A were used in extraction liquor purification trials where the pH-level of the separated extraction liquor was reduced to the target pH (pH 6) by addition of a pH-reducing agent (1 M sulfuric acid (H2SO4)) under stirring using a magnetic stirrer. No flocculation agent was added in these trials. The bottles with pH-reduced extraction liquor were then put in a shaking water bath for provide further stirring and temperature control. Two residence time levels (10 minutes and 60 minutes) in the water bath and two temperature levels (20 °C and 85 °C) were evaluated. Finally, the pH-reduced extraction liquors were centrifuged in a laboratory centrifuge (using the same procedure as Example 1) followed by decantation of the liquid phase to separate a liquid phase (purified extraction liquor) from a solid phase (silica-enriched sludge) according to the same procedure as in Example 1. After decantation, final pH was measured at room temperature (20 °C).

[0092] The purified extraction liquors (Purified extraction liquors 2A-2D) had compositions according to Table 2.1.

[0093] Table 2.1 Experimental parameters (Residence time, temperature, and target pH), final pH and composition of purified extraction liquor including silica and total organic carbon (TOC)

[0094] The results in Table 2.1 show that low silica contents in the purified extraction liquor can be obtained using either short (10 minutes) or long residence times (60 minutes) and either low (20 °C) or high (85 °C) temperatures. At a pH slightly above 6, the reduction in silica content was to minor extent favoured by long residence time (60 minutes) and high temperature (85 °C).

[0095] Example 3. Purification of separated extraction liquor from alkaline extraction with recirculation of separated extraction liquor

[0096] In a first trial, 50 g of oat hulls (based on dry solids) was treated in 575 g of a sodium hydroxide solution (sodium hydroxide solutio oat hulls weight-ratio of 11.5:1) containing 6.5 g of added NaOH (130 g NaOH / kg of oat hulls, 100% of the total alkali charge) and deionised water. The alkaline extraction of oat hulls was conducted at 85 °C for 60 minutes in a shaking water bath. After the treatment, silica-depleted agro-fibers were separated from a slurry of silica-depleted agro-fibers by filtration using a Buchner funnel with a filter cloth (Monodur PA 100 N with mesh opening size 100 pm) to obtain a separated extraction liquor (Separated extraction liquor 3A). Part of the separated extraction liquor (375 g) was recirculated to a second trial.

[0097] In the second trial, 50 g of raw material (based on dry solids) was treated in 575 g of a sodium hydroxide solution containing 375 g of the recirculated extraction liquor (15% of the total alkali charge), 5.5 g of added NaOH (85% of the total alkali charge) and deionised water. The residual alkali in the recirculated extraction liquor also contributes to the total alkali charge in the trial, so that the total alkali charge was about the same as in the first trial. The alkaline extraction of oat hulls was conducted at 85 °C for 60 minutes in a shaking water bath. After the treatment, the slurry of silica-depleted agro-fibers was separated by filtration using a Buchner funnel with a filter cloth (Monodur PA 100 N with mesh opening size 100 pm) into silica-depleted agro-fibers and separated extraction liquor (Separated extraction liquor 3B). Part of the separated extraction liquor (375 g) was recirculated to the next trial.

[0098] The procedure in the second trial was repeated three more times (in the third to fifth trial) to obtain separated extraction liquors with higher contents of silica, xylan and TOC than the separated extraction liquor from the first trial. Properties of the obtained separated extraction liquors are presented in Table 3.1.

[0099] Table 3.1 Initial alkali content and composition of extraction liquor including residual alkali, xylan, silica, and total organic carbon (TOC).

[0100] The results in Table 3.1 show that the reduction in alkali content in the sodium hydroxide solution during the alkaline extraction was between 75 and 79% of the initial alkali content all the five trials. Furthermore, recirculation of separated extraction liquor (with residual alkali in the liquor) reduced the NaOH addition from 6.5 g to 5.5 g in this example demonstrating that recirculation of separated extraction liquor gives a more efficient use of alkali in the alkaline extraction. A low residual alkali content in the separated extraction liquor after the alkaline extraction is also beneficial to minimize the consumption of the pH-reducing agent in the following purification of the separated extraction liquor. However, the residual alkali content needs to be high enough to keep silica soluble before purification of the liquor.

[0101] Another advantage of recirculating separated extraction liquor is that the agricultural crop / residue (in this case oat hulls) can be treated in the alkaline extraction at a relatively low consistency (in this case 8%) with a low addition of fresh liquor. This reduces the amount of water that needs to be added in the alkaline extraction, in this case from 11.2 m3 / ton to 3.7 m3 / ton (67 % less water consumption).

[0102] Separated extraction liquor 3E was then used in a purification trial. The same procedure was used as in Example 1, however, in this trial no flocculation polymer was added this trial before centrifugation and decantation. The purified extraction liquor (Purified extraction liquor 3A) had a composition according to Table 3.2.

[0103] Table 3.2 Measured pH level (at 20°C) after decantation and composition of purified extraction liquor including xylan, silica, sodium, and total organic carbon (TOC).

[0104] Na:SiO2 describes the weight ratio between sodium and silica There are several benefits of recirculating the separated extraction liquor. By recirculating part of the separated extraction liquor, less NaOH is needed and the volumetric load on the purification process for purification of separated extraction liquor is also decreased. Finally, an important effect of the recirculation of separated extraction liquor in the alkaline extraction is that the separated extraction liquor entering the purification process is more concentrated, i.e. has a higher content of silica and sodium. A high silica content in the separated extraction liquor enables a higher reduction in silica content in the purification (for Purified extraction liquors 3A and 3B the silica content was reduced by more than 98%).

[0105] The purified extraction liquor 3A was used further in oxygen delignification trials (see Example 5). extraction liquor from a two-step alkaline extraction process with recirculations of extraction li

[0106] Trials exploring a two-step alkaline extraction process of oat hulls raw material were performed.

[0107] In a first step (Step 1) of the first trial (Tl), 50 g of oat hulls raw material (based on dry solids) was treated in 575 g of a first sodium hydroxide solution (sodium hydroxide solutio oat hulls ratio was 11.5:1) containing 6.5 g of added NaOH (29 % of the total alkali charge in Trial 1) and deionised water.

[0108] The extraction of oat hulls was conducted in a closed beaker at 85 °C for 60 minutes in a shaking water bath. After the treatment, a first slurry of silica-depleted agro-fibers was separated by filtration using a Buchner funnel with a filter cloth (Monodur PA 100 N with mesh opening size 100 pm) into silica-depleted agro-fibers and a first separated extraction liquor (LI). Part of the first separated extraction liquor from Trial 1 (375 g) was then recirculated as an alkali source in the first sodium hydroxide solution used in Trial 2.

[0109] In a second step (Step 2) of the first trial (Tl), the silica-depleted agro-fibers from the Step 1 was then treated in 500 g of a second sodium hydroxide solution containing 16 g of added NaOH (71% of the total alkali charge in Trial 1). The silica- depleted agro-fibers were treated at 85 °C for 10 minutes in a closed beaker in a shaking water bath. After the extraction, the second slurry of silica-depleted agrofibers was separated by filtration using a Buchner funnel with a filter cloth (Monodur PA 100 N with mesh opening size 100 pm) into twice-extracted silica-depleted agrofibers and a second separated extraction liquor (L2). Part of the second separated extraction liquor from Trial 1 (100 g) was recirculated to the second sodium hydroxide solution used in Trial 2. Also, part of the second separated extraction liquor from the first trial (153 g) was added to the first sodium hydroxide solution used in Trial 3, thereby using the residual alkali in the second separated extraction liquor as an alkali source in the first alkaline extraction step in Trial 3 to further reduce the total need for new added NaOH in Trial 3.

[0110] In the following trials, this two-step alkaline extraction process for oat hulls raw material was repeated. Instead of only using new added NaOH, parts of the total alkali charge in Trial 2 to Trial 5 were made utilizing recirculated extraction liquors (LI from the first extraction (Step 1) and L2 from the second extraction (Step 2)) according to Table 4.1.

[0111] Table 4.1 Experimental set-up in a two-step alkaline extraction process for oat hulls describing the additions of recirculated extraction liquors and NaOH in Trial 1 to Trial 5.

[0112] It can be seen in Table 4.1 that addition of the second recirculated extraction liquor (L2) to the first extraction (Step 1), has a large effect on amount of new added NaOH in the first extraction. The final extraction liquors from the extraction trials were T5.L1 (Trial 5,

[0113] Liquor from Step 1) and T4.L2 (Trial 4, Liquor from Step 2) which had a composition according to Table 4.2.

[0114] Table 4.2 Initial alkali content and composition of separated extraction liquors including residual alkali, xylan, silica, sodium, and total organic carbon (TOC). Na:SiO2 describes the weight ratio between sodium and silica

[0115] The results in Table 4.2 show that 78% of the alkali added in the first extraction (Step 1) in Trial 5 was consumed during the extraction in Step 1. It is taken into account here that 25% of the weight of oat hulls is dissolved into the sodium hydroxide solution during the first extraction (Step 1), thereby increasing the total weight of the sodium hydroxide solution during the first extraction. The alkali consumption in Trial 5 in the first step was at a similar level as in Trial 5 in Example 3.

[0116] In the second extraction step (Step 2), alkali was consumed to a much lower extent (only 8.4% of the alkali added in Step 2 was consumed during the extraction in Step 2). A high initial alkali content is beneficial to solubilize and more silica and xylan from the agricultural crops / residues. However, a too high addition of alkali in a one-step alkaline extraction would result in a too high residual alkali content in the separated extraction liquor and thus a bad starting point for the purification of the liquor. The need for pH-reducing agent would simply be at a too high level to be reasonable. When instead dividing the alkaline extraction process into more than one extraction with more than one sodium hydroxide solution and using more than one recirculation of extraction liquors it may be possible to a) obtain a low residual alkali content in the first separated extraction liquor i.e. the separated extraction liquor used in the following extraction liquor purification and b) treat the silica- depleted agro-fibers from the first step in a second sodium hydroxide solution with a higher alkali concentration enabling further extraction of silica and xylan.

[0117] The separated extraction liquor from Step 2 in Table 4.2 obtained a high xylan and residual alkali content but a relatively low silica content. In this example, it has been shown that this liquor can be used to replace a high portion of the NaOH added in the first extraction (Step 1), thereby also enabling more silica from the feedstock to reach the extraction liquor purification. On the other hand, the high xylan and residual alkali content makes it interesting to also add this second separated extraction liquor directly to the wood pulping process (without passing it through Step 1 and extraction liquor purification) in a position in the wood pulping process where the residual alkali in the second separated extraction liquor can be used to replace added alkali e.g. in oxygen delignification and / or in alkaline bleaching.

[0118] Separated extraction liquor T5.L1 was used in an extraction liquor purification trial. The purification trial was performed according to the procedure described in Example 1, but without any polymer addition. The liquor was adjusted to pH 7 with IM H2SO4. The purified extraction liquor obtained after centrifugation and decantation (Purified extraction liquor 4A) had a composition according to Table 4.3.

[0119] Table 4.3 Measured pH level (at 20°C) after decantation and composition of purified extraction liquor including xylan, silica, sodium, and total organic carbon (TOC).

[0120] Na:SiO2 describes the weight ratio between sodium and silica

[0121] The purification process removed a high proportion of the silica in the separated extraction liquor. The content of silica in the liquor was reduced by more than 96% providing a purified extraction liquor having a high sodium to silica ratio. As a relatively high proportion of xylan was lost to the silica-enriched sludge it may be advantageous to use at least parts of the second separated extraction liquor from the second extraction (Step 2) directly in the wood pulping process as described above.

[0122] The Purified extraction liquor 4A and Separated extraction liquor T4.L2 were used further in oxygen delignification trials (see Example 5).

[0123] Example 5. Utilization of purified extraction liquor in oxygen delignification of wood pulp trials

[0124] Samples of purified extraction liquor from Example 3 (Purified extraction liquor 3A) and from Example 4 (Purified extraction liquor 4A) and one sample of Separated extraction liquor from the second extraction in Trial 4 in Example 4 (Extraction liquor T4.L2) were used in oxygen delignification of wood pulp trials. Batches of 50 g of never-dried unbleached softwood kraft pulp having a kappa number of 29.8 and an ISO-brightness of 27.0 %-ISO were used in the oxygen delignification trials.

[0125] To each pulp sample, NaOH (addition levels specified in Table 5.1) and 5 g MgSC / kg pulp (g per kg of 100% dry pulp) were added together with deionized water so that a pulp consistency of 10% was obtained. In some trials, part of the water was replaced with liquor samples from Example 3 and 4. In the trial with addition of liquor T4.L2, the residual alkali in the liquor was used as a part of the total alkali charge, thereby replacing some of the NaOH addition in the oxygen delignification step.

[0126] Table 5.1 Wood pulp, NaOH and liquor additions in oxygen delignification trials, liquors prepared in Example 3 and 4.

[0127] * The total alkali charge including the residual alkali in the added separated extraction liquor was 38 g / kg of 100% dry pulp

[0128] The pulp suspensions were charged into steel autoclaves, oxygen gas was added to a pressure of 15 bars and the autoclaves were inserted into a hot-air oven (CRS Reactor Engineering AB, Stenkullen, Sweden). A two-stage oxygen delignification was performed, first stage was performed at 90°C and the second stage at 100°C, in between the two stages the pressure in the autoclaves was reduced to 5 bars. A 40-minute temperature ramp and 30 minutes residence time at 90°C were used during the first stage. For the second stage, a 10-minute temperature ramp and 60 minutes residence time at 100°C were used. Kappa number, pulp viscosity and xylan content were analyzed after oxygen delignification, see results in Table 5.2.

[0129] Table 5.2. Results after oxygen delignification incl. kappa, brightness, viscosity and xylan content

[0130] It is shown in Table 5.2 that addition of purified extraction liquor enhances the xylan content of the oxygen delignified wood pulp. In addition to providing an increase in pulp yield, the increased content of xylan is known to give positive effects on the tensile strength of paper as disclosed in EP2513372. Addition of Separated extraction liquor T4.L2 in oxygen delignification resulted in an even larger increase in xylan content, therefore this liquor can be used to increase pulp yield, increase tensile strength, and to reduce the need for fresh NaOH (or oxidized white liquor) in the oxygen delignification step.

[0131] Example 6. Purification of silica-depleted agro-fibers by leaching and filtration

[0132] In cases where the silica-depleted agro-fibers are not added to the wood pulp but instead used for other applications, a washing of the pulp is needed so that large amounts of sodium and xylan are not wasted.

[0133] A sample of twice-extracted silica-depleted agro-fibers obtained after the second extraction in the fourth trial in Example 4 (Trial 4, Step 2 in Example 4) was used. This sample was a wet cake still containing some sodium hydroxide solution after the filtration step performed in Example 4. The agro-fiber consistency of this wet sample was 15%, remaining parts being carry-over liquor from the second extraction in Trial 4 in Example 4.

[0134] In a first trial, 50 g of this sample was diluted with 50 g of deionised water in a beaker and mixed with a spoon to mix the agro-fibers with the dilution water. The sample was left for 15 minutes at room conditions to allow the alkaline xylan-rich liquid to leach out from the agro-fibers. After leaching the sample was filtered on a Buchner funnel with a filter cloth (Monodur PA 100 N with mesh opening size 100 pm). The filtrate (Filtrate 6A) had a composition according to Table 6.1.

[0135] In a second trial, the leached agro-fiber sample from the first trial was again diluted with 50 g of deionised water and the same leaching and filtration procedure as described above was used. The filtrate from the second leaching trial (Filtrate 6B) had a composition according to Table 6.1.

[0136] Table 6.1 Composition of filtrate including residual alkali, xylan, sodium, and total organic carbon (TOC)

[0137] The results show that with further leaching or washing of the silica-depleted agro-fibers, more residual alkali, xylan, sodium and TOC can be extracted and possibly be returned to the alkaline extraction process if these filtrate / filtrates are used to replace added water in the extractions (Step 1 and / or Step 2). Silica was not measured, but as the starting liquor in this example contains silica it seems likely that also silica to some extent will be returned to the alkaline extraction process. Leaching and / or washing of the silica-depleted agro-fibers can, thus, be used to reduce carry-over of silica in the silica-depleted agro-fibers and re-use more of the residual alkali which is still in the wet silica-depleted agro-fibers after the solid / liquid separation after extraction(s) in alkaline extraction process. Leaching and / or washing can also be performed with a counter-current use of leaching / washing filtrates, thereby reducing the use of added water and increasing the concentration of e.g. xylan and sodium in the filtrate which is returned to the alkaline extraction process.

[0138] Example 7. Purification of silica-enriched sludge

[0139] A solid residue of silica-enriched sludge obtained after centrifugation and decantation of pH-reduced extraction liquor during purification of Separated extraction liquor T5.L1 in Example 4 was used in sludge purification trials. In a first trial, 12.5 mL of the sludge was dispersed in 35 mL of deionised water. The dispersed sludge was transferred into a 50 mL cylindrical centrifugation tube (inner diameter 2.8 cm with conical bottom, height of the bottom cone was 1.5 cm) and centrifuged in a laboratory centrifuge (Hettich Rotanta 460R produced by Andreas Hettich GmbH & Co. KG, Tuttlingen, Germany) for 15 minutes at 3000 RCF. The liquid phase (Liquid 7A) was separated by decantation and had a composition according to Table 7.1.

[0140] Table 7.1 Composition of liquid including xylan, sodium, and total organic carbon

[0141] (TOC)

[0142] As can be seen in Table 7.1, sodium and xylan may be recovered from the silica-enriched sludge. This liquid with sodium and xylan may be returned to the alkaline extraction process.

[0143] Example 8. Theoretical example of co-production combining agro-based and woodbased feedstocks

[0144] A wood pulp mill with a recovery cycle for regeneration of cooking chemicals needs to add makeup chemicals to compensate for losses due to e.g. carry-over from washing of pulp and losses in flue ashes or green liquor dregs. The amount of makeup chemicals per ton of produced pulp varies between different mills. Therefore, this example gives a set of different levels of sodium makeup consumptions.

[0145] In a kraft pulp mill losses of sodium needs to be compensated for, which can be partly of fully fulfilled by a) addition of purified extraction liquors, optionally b) carry-over liquor when adding silica-depleted agro-fibers to the wood pulp and optionally c) separated extraction liquor from the second extraction when a two- step alkaline extraction process is used.

[0146] This example is based on a wood pulp mill producing 750 000 tons of air-dry pulp (ADt) per year receiving purified extraction liquor (composition according to Purified extraction liquor 3A, i.e. 11 kg Na / ton liquor) from an alkaline extraction process. It is assumed that 2 tons of purified extraction liquor is added to the wood pulping process per ton of agricultural crops / residues (in this case oat hulls, based on dry solids). The extraction liquor is added in the oxygen delignification of the wood pulp fibers.

[0147] To enable utilisation of all sodium in the above-mentioned liquor as sodium makeup in the wood pulping process, the capacity of the alkaline extraction of agricultural crops / residues needs to be adapted to the wood pulp production capacity, as shown in Table 8.1.

[0148] Table 8.1 Raw material consumption (bone-dry tons of agricultural crops / residues per year) of an alkaline extraction process providing purified extraction liquor as sodium makeup (between 25-100% of total sodium makeup consumption) to a wood pulping process producing 750 000 tons of air-dry wood pulp (ADt) per year.

[0149] Table 8.1 shows that the sodium makeup consumption in the wood pulping process sets restrictions for the maximum capacity of the alkaline extraction process. If the alkaline extraction process would be larger than the levels displayed in the 100% column, then the wood pulp mill would need to purge sodium which would not be an efficient use of resources. Typically, the wood pulp mill would not use only purified extraction liquor as makeup source so therefore 25-75% share of the total makeup consumption coming from purified extraction liquor is more realistic. However, the wood pulp mill may also use less than 25% of its makeup needs coming from purified extraction liquor.

[0150] It should also be noted that if silica-depleted agro-fibers and / or extraction liquor from a second alkaline extraction step is also added to the wood pulp process the sodium in these materials must also be considered, thereby reducing the maximum capacity / raw material consumption of the alkaline extraction.

[0151] Analytical methods: Sodium content of liquors / filtrates / liquids was analysed according to SCAN-N 37:98 Silica content of liquors / filtrates / liquids was analysed according to SCAN-N 38:01 Silica content of the oat hulls sample was analysed according to ISO 776:2011, the method is developed for pulp samples but can be utilized provided that the residue of the oat hulls after incineration in the muffle oven is complete, i.e. not leaving any non-incinerated organic carbon compounds.

[0152] TOC content of liquors / filtrates / liquids was analysed according to EN 1484:97 Kappa number of pulp samples was analysed according to standard method SS-ISO 302:2016

[0153] Pulp brightness analysed by standard method ISO 3688 and SS ISO 2470-1

[0154] Pulp viscosity (also denoted limiting viscosity) analysed by standard method ISO SS- ISO 5351:2010

[0155] Xylan content (defined and calculated as the sum of the xylan and arabinan contents) in pulp was determined according to standard method ISO 21437:2020. Klason lignin, acid soluble lignin (ISO 21436:2020) and ash content (ISO-1762:2019) was included in the total mass balance to calculate the sugar contents.

[0156] The xylan contents (defined and calculated as the sum of the xylan and arabinan contents) of liq uor- / filtrate- / l iq uid-sam pies where the carbohydrate analysis is based on acid hydrolysis of the sample at the same conditions as in ISO 21437:2021. The preparation of the same is as follows: 5 g of sample is added to a 150 ml beaker, 50 ml of milli-Q water is added followed by neutralisation with 5 M hydrochloric acid (HCI) to pH 7. After neuralisation Milli-Q water is again added to a total sample weight of 84 g (incl. 5 g liquor / filtrate / liquid sample and the added acid). After neuralisation and dilution, the sample is acidified with 3 mL of 72 % (by weight) of H2SO4 and then the procedure is continued according to ISO 21437:2021 after the step of acid addition. Anhydrosugar contents were analyzed using an ICS-6000 instrument calibrated based on solutions with known contents of xylose, arabinose glucose, mannose and galactose. Residual alkali content was analysed according to SCAN-N 33:94. Initial alkali content is based on mass balance considering added NaOH and residual alkali of recirculated extraction liquor / liquors. The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Claims

CLAIMS1. A method for utilization of agro-based extraction liquor in a wood pulping process, the method comprises: a) extracting agricultural crops / residues in a sodium hydroxide solution to obtain a slurry of silica-depleted agro-fibers; b) separating the silica-depleted agro-fibers from the slurry to obtain separated silica-depleted agro-fibers and a separated extraction liquor; c) purifying the separated extraction liquor from silica to obtain a purified extraction liquor; and d) adding the purified extraction liquor to a wood pulping process having a recovery cycle for cooking chemicals, thereby using sodium in the purified extraction liquor as sodium make-up in the wood pulping process.

2. A method according to claim 1, further comprising recirculating at least a part of the separated extraction liquor for reuse in the extraction of agricultural crops / residues.

3. A method according to claim 1 or 2, further comprising repeating, at least one time, steps a) and b) for the separated silica-depleted agro-fibers and recirculating at least a part of the separated extraction liquor(s) for reuse in the extraction of agricultural crops / residues and / or in the extraction of silica-depleted agro-fibers.

4. A method according to any one of the preceding claims, wherein purifying the separated extraction liquor from silica includes reducing the silica content by at least 60 %, preferably at least 80 %.

5. A method according to any one of the preceding claims, wherein purifying the separated extraction liquor from silica comprises treating the separatedextraction liquor with a pH-reducing agent to obtain a pH-reduced extraction liquor comprising a liquid phase and a solid phase; and separating the liquid phase from the solid phase to obtain the purified extraction liquor and a silica-enriched sludge.

6. A method according to claim 5, wherein the pH-reduced extraction liquor has a pH between pH 5 and pH 9.5, preferably between pH 6 and pH 8.

7. A method according to claim 5 or 6, wherein the pH-reducing agent comprises sulfuric acid and / or carbon dioxide.

8. A method according to any one of claims 5 to 7, wherein separating a liquid phase from a solid phase comprises at least one of filtering, settling, using hydrocyclones and / or centrifugation, preferably decanter centrifugation.

9. A method according to any one of the preceding claims, further comprising washing the solid phase of the pH-reduced extraction liquor and adding used washing liquor to the wood pulp process and / or to the extraction of agricultural crops / residues.

10. A method according to any one of the preceding claims, wherein the purified extraction liquor is added to a wood cooking process, an evaporation stage in the recovery cycle, a brownstock washing stage, and / or an oxygen delignification stage of the wood pulping process.

11. A method according to any one of the preceding claims, further comprising: adding the separated silica-depleted agro-fibers to wood pulp fibers of the wood pulp process to obtain a pulp mixture.

12. A wood pulp manufactured by the method according to any one of claims 1-13. A pulp mixture manufactured by the method according to claim 11.

14. Silica-depleted agro-fibers manufactured by the method according to any one of claims 1-10.

15. Use of the silica-depleted agro-fibers of claim 14 for production of powdered cellulose.

16. A silica-enriched sludge obtained by the method according to any one of claims 5-11.

17. Use of the silica-enriched sludge of claim 16 for biogas production and / or fertilization.

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