Method of recycling spent electrolytes from redox flow batteries

Abstract

In a method for recycling spent electrolytes from redox flow batteries in a cellulose digestion process, an aqueous solution containing between 1 and 20 wt% of at least one spent, optionally modified electrolyte from a redox flow battery is produced, the solution of the optionally modified spent electrolyte from redox flow batteries is adjusted to a pH between 2 and 13 corresponding to a process stream of the cellulose digestion process by adding a mineral acid or base, the solution of the spent electrolyte is introduced into the process stream of the cellulose digestion process, to whose pH the spent electrolyte from redox flow batteries has been adjusted beforehand, and the mixture of the process stream of the cellulose digestion process and the solution of the spent electrolyte from redox flow batteries is heated to temperatures above 90°C, and one or more processes selected from a cellulose digestion such as a kraft process, TMP process, CTMP process or NSSC process, a workup of black liquor from a cooking process, a delignification or a spreading and recovery method of process fluids is conducted after the cellulose digestion, and use.

Description

[0001] METHOD FOR RECYCLING SUBSTANTS FROM

[0002] REDOX-FLO WB ATTER! EN

[0003] The present invention relates to a method for recycling spent electrolytes from redox flow batteries in a cellulose digestion process and to the use of spent electrolytes from redox flow batteries in a cellulose digestion process.

[0004] Plants for the production of paper, cardboard, or pulp, for the delignification of wood and other natural, fibrous materials such as straw, etc., are known, employing a number of known mechanical and chemical pulping processes or combinations thereof. In these processes, the wood or other fibrous raw material is usually first pre-steamed to remove the air contained within it. The pre-steamed material is then impregnated under pressure with a so-called cooking liquor to completely wet the raw material. Such a cooking liquor can be fresh, a mixture of fresh and used cooking liquor, or even used cooking liquor.In the subsequent boiling process, the penetration of the lye into the wood depends on the physicochemical properties of the lye as well as its chemical composition, for example, temperature, pressure, time, surface tension, viscosity, etc. After completion of the boiling process, the lye is drawn off and usually at least partially processed and returned to the boiler for reuse. It is undoubtedly known to those skilled in the art that such drawn-off lyes differ from one another depending on the method used and the raw materials employed, since, for example, organic components of the processed wood or fibrous materials are also introduced into the lye to a certain extent, and the lye can be acidic, basic, or essentially neutral. As soon as this boiling process or...When the delignification of wood or other fibrous natural materials is stopped, a certain residual lignin content from the fibers remains in the liquor. If, as is usually the case, the liquor is subsequently evaporated, its viscosity increases significantly due to the residual components, such as lignin. However, if such used liquor exhibits a steadily increasing viscosity, this negatively impacts the evaporation units, as handling the liquor becomes more difficult, particularly for recycling. To at least partially address this problem, the temperature during evaporation could be increased, which lowers the viscosity and also leads to better wetting of the wood or wood chips being treated.However, increasing the temperature is not only disadvantageous for energy-related reasons, but also due to the limitations imposed by the materials used. Furthermore, it is known that certain viscosity-reducing additives can be used to lower the viscosity of spent cooking liquors. This process is known as salting, and involves the use of substances such as thiocyanates, formamides, acetamides, or sulfoxides to reduce the viscosity of drawn-off liquors, particularly black liquor with varying solids content. The use of such viscosity-influencing substances is also not very economically viable and is therefore rarely, if ever, used in industry for cost reasons.

[0005] In addition to the problem of viscosity changes in cooking liquors described above, numerous other problems, particularly energy-intensive ones, are known to occur during paper and pulp production, such as the reprocessing of the black liquor drawn off after pulping, which is currently mainly burned after the liquid is separated, and the pulping process itself.

[0006] Paper and board manufacturing processes per se require a variety of additives to obtain a final product with desired physicochemical properties. These additives, while typically used in small quantities, often significantly increase the process costs or necessitate the transport of large quantities of chemical additives to the paper and pulp mill. This is essential to ensure reproducible process control in paper or board production and the maintenance of consistent end-product quality. EP 0 004 928 A1 describes a dispersion for use in pulp production and a process for pulp production using this dispersion.The dispersion contains, in addition to organic cyclic compounds containing keto and / or hydroxy groups, at least one liquid dispersing agent used to disperse the organic cyclic compounds containing keto and / or hydroxy groups, which are usually in powder form, and to use them in the process for cell stem extraction, wherein the dispersion contains a viscosity-increasing substance.

[0007] EP 0 006 201 B1 covers powdered mixtures for use in pulp production, consisting predominantly of one or more organic compounds containing cyclic keto and / or hydroxyl groups, as well as one or more surfactants. These substances are intended to serve as pulping aids. EP 4 180 504 A2 covers lignin-derived compounds and compositions that can be used as electrolytes in redox flow batteries.

[0008] WO 2018 / 146341 A1 discloses a process for treating lignocellulosic material in which at least one lignin-derived process stream is subjected to a chemical decomposition step to obtain modified lignin-derived components that detach from the low-molecular-weight aromatic lignin-derived compounds. The present invention aims to utilize available waste materials in process streams of a cellulose pulping process and thereby not only increase the overall throughput of the pulping process but, in particular, significantly improve its energy balance compared to conventional processes. To achieve this objective, the process according to the invention is essentially characterized in that, in a first step, a preferably aqueous solution containing between 1 and 20 wt.-%, preferably between 2 and 17 wt%, particularly preferably between 3 and 15 wt% of at least one spent electrolyte from a redox flow battery, optionally modified with either a sulfur compound, a phosphorus compound or a nitrogen compound, wherein the electrolyte is selected from the group of compounds with general formula (I) or compounds with general formula (II). where Ri . R sfrom combinations of H, methyl, methoxy, amino, phosphorus, and sulfonate groups, as well as lignin, lignosulfonate, and / or vanillin derivatives are produced; in a second step, the solution of the given modified spent electrolyte from redox flow batteries is adjusted by adding a mineral acid or a base to a pH value between 2 and 13, preferably between 9 and 13, corresponding to a process stream of the cellulose digestion process; in a third step, the solution of the spent electrolyte is introduced into the process stream of the cellulose digestion process, to whose pH value the spent electrolyte from redox flow batteries was adjusted in the second step; and in a fourth step, the mixture of the process stream of the cellulose digestion process and the solution of the spent electrolyte from redox flow batteries is heated to temperatures above 90 °C.preferably heated above 120 °C, most preferably above 150 °C, and one or more processes selected from a cellulose pulping process, such as a Kraft process, TMP process, CTMP process or NSSC process, a processing of black liquor from a boiling process, a delignification or a processing and recovery process of process fluids after the cellulose pulping is carried out. With such a process, it is surprisingly possible not only to lower the viscosity, for example during the evaporation of black liquor and thus to carry out the evaporation completely, but the spent electrolytes from redox flow batteries surprisingly have positive effects in a cellulose pulping process, vrfe one.

[0009] Improvement of wood impregnation, improvement of the sodium and / or sulfur balance in chemical recycling, pH equalization, and the like. In the context of the present invention, the term "electrolyte" means an aqueous solution of at least one electrically active carbon-based component and at least one base or acid. In this case, the acid can be phosphoric, sulfuric, or nitric acid. The base can be sodium or potassium hydroxide. The term "spent electrolyte from redox flow batteries" means an organic electrolyte that was contained in a redox flow battery and consists of at least one organic molecule, regardless of whether the molecule is of fossil or biological origin.In particular, this term also refers to the degradation products that have formed in the redox flow battery during previously completed charge and discharge cycles, or that have arisen within it, for example, through oxidation or addition reactions. The definition of "spent electrolyte from a redox flow battery" used here definitively excludes any spent electrolyte from batteries of non-organic origin, such as vanadium-based batteries. "Spent" in this context means that the electrolyte was previously contained in the redox flow battery and subjected to at least one charge cycle, so that the aforementioned degradation products were at least partially formed.

[0010] In this context, "process stream" refers to any neutral or alkaline liquid or suspension from a cellulose pulping process. The term "process stream" specifically encompasses all liquid fractions drawn from a digester after delignification, regardless of the type of cellulose pulping process. This includes, in particular, liquid fractions from sulfite processes, Kraft processes, or variants of chemo-thermomechanical processes (CTMP) or thermomechanical processes (TMP), as well as neutral semi-chemical sulfite processes (NSSC). The term "process stream" does not imply any reference to the pH value of this fraction but is derived from the technical terminology of papermaking.A process stream includes, for example, both cooking liquor from the power boiling process, which has a pH value greater than 11, and cooking acid from the neutral semi-chemical sulfite boiling process, which can have pH values ​​ranging from acidic to slightly alkaline, with a maximum pH of 11. Furthermore, in this case, the term "process stream" also encompasses process streams from pulp pulping, pulp bleaching, liquor evaporation, chemical and energy conversion, as well as process streams from other processes upstream or downstream of the pulp pulping process.

[0011] In connection with the present invention, it has surprisingly been found that if the spent electrolyte from redox flow batteries, in particular the aqueous solution of a spent electrolyte, is adjusted to a pH value corresponding to the pH value of the respective process stream to which the spent electrolyte is to be added, it is possible to positively influence the process stream using the spent electrolyte from the redox flow battery contained in the aqueous solution in amounts of 1 to 20 wt.%, preferably 2 to 20 wt.%. In the context of the present invention, "positively influence" means a reduction in the electrolyte's viscosity, improved conversion of the ions contained therein, or...Chemicals such as sulfur, nitrogen, phosphorus, CO2 or oxygen, and also a simplified processing of, for example, black liquor, without having to burn excessive amounts of it, are understood.

[0012] According to a further development of the invention, the process according to the invention is carried out such that the consumed electrolyte, optionally modified with either a sulfur compound, a phosphorus compound or a nitrogen compound, is selected from a redox flow battery from the group consisting of: anthraquinone (AQ), 9,10-anthraquinone-2,7-disulfonic acid (AQS), 2-methoxyquinone (MQ), 2-methoxyhydroquinone (MHQ), 1,4-benzoquinone, 1,2-benzoquinone-3,5-disulfonic acid, 1,2-hydroquinone-3,5-disulfonic acid, ethoxyhydroquinine (EHQ), (2,5-BiS(N-methylethanolamine>cyclohexa-2,5-dten-1,4-dione (MGQ), (tetrasodium 3,3',3",3"'-((3,6-dihydroxybenzo-1)-1 ,2,4,5-tetrayl)tetrakis(sulfanediyl))tetrakis(propane-1-suifonate)) (MHQS). The specific selection of the consumed, possibly modified, electrolytes ensures that, for example, individual natural substances contained in the process streams of the zeilulose digestion process, such as vanillin, lignin, and the like, are removed.The electrolytes from the redox flow battery are easily accessible, thus contributing to the conversion of the otherwise difficult-to-transform components of the zeil lysis process and completing or supporting it. Furthermore, this process makes it possible to reduce energy consumption, completely recover inorganic fractions of the electrolytes while simultaneously utilizing the organic fractions for energy generation, achieve more efficient delignification, reduce the depolymerization of carbohydrates, and so on.By operating the process in such a way that electrolytes modified with specific chemical groups and used in the process can be selectively improved, for example, by using used electrolytes with a significant sulfur content, or by improving the nitrogen or phosphorus balance by using used electrolytes with a nitrogen or phosphorus content, thus making the overall process more efficient, with higher yields and lower energy consumption. Modification with concentrated aqueous sulfuric acid, among other things, leads to the conversion of components of the black liquor into sodium sulfite and sodium carbonate by the used electrolyte, which contains significant concentrations of sulfur and possibly also sodium, and the resulting products can then be further processed.In the causticization process, the sodium carbonate can ultimately be converted into sodium hydroxide and thus used again as a base. Particularly surprising, however, is that the inorganic fraction of the used spent electrolytes from redox flow batteries can be completely recovered, while the organic fraction can be used, for example, as an energy source in the recovery furnace. The process can also be modified to include the electrolyte contained in the redox flow battery, in which case all substances contained in the redox flow battery housing—namely the electrolyte and the electrically active substance—can be directly used in the recycling process.According to a further development of the invention, the process can be carried out such that the process stream used in the third step of the cellulose pulping process is selected from fresh cooking liquors, such as cooking liquors for sulfite or sulfate cellulose pulping processes, spent liquors from sulfite or sulfate cellulose pulping processes, spent acids from sulfite cellulose pulping processes, process streams containing tall oil lignins, filtrates from alkaline or acidic process steps of cellulose pulping processes, such as filtrates from an ozone or chloride dioxide bath, condensates, or wastewater. The use of spent electrolytes from redox flow batteries in at least one of the process streams of the cellulose pulping process produces a wide variety of effects.For example, the addition of spent electrolyte to spent liquors from sulfite and sulfate cellulose pulping processes can be used as a rheology modifying agent, significantly reducing the viscosity of these liquors. Furthermore, by adding spent electrolytes to process streams using fresh cooking liquors, the sulfur, nitrogen, and / or phosphorus content of the process stream can be optimized beforehand, thus achieving the most complete possible impregnation of the wood chips used in the subsequent steps of the cellulose pulping process. Another advantageous application of spent electrolytes from redox flow batteries is their use as rheology modifying agents in the evaporation of spent liquors.Spent electrolytes are partially evaporated for recovery due to their solids content, and the evaporation residues are subsequently incinerated. If, for example, a spent electrolyte from a redox flow battery is also added to the reprocessing of chemical fluids from the cellulose pulping process, the spent electrolyte can be used as a reprocessing aid for sodium and sulfur, thus significantly reducing the required amount of these substances.

[0013] By adding the spent electrolyte from a redox flow battery to fresh cooking liquor, as is a further development of the invention, it is possible not only to optimally adjust the pH value of the cooking liquor with the spent electrolyte, but also, and more importantly, to create optimal conditions during the cooking process itself and, in particular, to introduce valuable chemicals into the process. This allows for a favorable influence on and optimization of cellulose pulping, especially the cooking of wood chips, both in terms of process duration and wood yield. For example, the wood yield can be increased by 0.25% to 0.75% by using a spent electrolyte. Considering that approximately 20% of the world's total wood harvest is used in paper production, an increase in yield of 0.25% to 0.75% achievable with the process according to the invention is an immensely significant figure.According to a further development of the invention, the process is carried out such that at least one spent electrolyte is added to a process stream from the cellulose pulping process with a kappa value according to ISO 302:2015 between 10 and 100. By adding at least one spent electrolyte to the process stream from the cellulose pulping process with a kappa value according to ISO 302:2015 between 10 and 100, it is ensured that the viscosity of the spent boiling water can be significantly reduced before its processing. Process streams with kappa values ​​between 10 and 100 are process streams that are either obtained directly after the boiling of the wood chips or in the cellulose pulping process, such as black liquor that is added at the beginning of the cellulose pulping process, or that arise in the processing of process streams originating from cellulose pulping processes.

[0014] In order to carry out the process reliably and in particular to prevent an undesirable thickening of the mixture of spent electrolytes from redox flow batteries and a process stream from the cellulose pulping process, the process is carried out in such a way that the mixture of the process stream from the cellulose pulping process and the spent electrolyte is adjusted to a total solids content of 3 g / l to a maximum of 120 g / l, preferably a maximum of 100 g / l, by adding a harmless diluent.

[0015] Surprisingly, it has been found that the use of spent electrolytes from redox flow batteries in the cellulose pulping process can positively influence almost all steps of the process, particularly by introducing ions required for the individual process steps, such as sulfur, nitrogen, etc., through the spent electrolytes. This addition surprisingly makes it possible, for example, to reduce the viscosity of black liquor during its processing, to improve the reprocessing of various alkalis in the cellulose pulping process, such as black liquor, or finally to use the spent electrolytes as an additive for the processing process.

[0016] Due to these surprising effects, the invention further aims at the use of spent electrolytes from redox flow batteries in a cellulose pulping process. The use is essentially characterized in that a preferably aqueous solution containing between 1 and 20 wt.%, preferably between 2 and 17 wt.%, particularly preferably between 3 and 15 wt.% of at least one spent electrolyte from a redox flow battery, optionally modified with either a sulfur compound, a phosphorus compound, or a nitrogen compound, is adjusted by the addition of a mineral acid or base to a pH value between 2 and 13, preferably between 9 and 13, corresponding to a process stream of the cellulose pulping process, wherein the electrolyte is selected from the group of compounds with general formula (I) or compounds with general formula (II). where Ri - Ra The solution of the optionally modified spent electrolyte from redox flow batteries is used in a process stream of a cellulose pulping process at temperatures above 90 °C, preferably above 120 °C, most preferably above 150 °C. The beneficial effects of this use only become apparent when the spent electrolytes are used in a process stream at temperatures above 90 °C.

[0017] A particularly advantageous application is characterized by the modification of the spent electrolytes with either a sulfur compound, phosphorus compound, or nitrogen compound, so that ions required in the cellulose pulping process, such as sulfur, nitrogen, and phosphorus, can be introduced in even larger quantities than they are inherently contained in the spent electrolytes. Solutions of electrolytes from redox flow batteries, containing up to 21 wt% of the at least one spent electrolyte, allow the spent electrolytes to be used in virtually all process streams of the cellulose pulping process, such as in the white liquor before pulping, the drawn-off black liquor, black liquor reprocessing, evaporation, or the like.A preferred use is characterized in that the spent electrolyte from a redox flow battery is used to reduce the viscosity of the process stream in a cellulose pulping process, as an auxiliary substance to accelerate delignification in the cellulose pulping process, and / or as a substance supplementing sodium and sulfur content. Such use makes it possible to utilize waste materials from redox flow batteries effectively and reliably without the formation of undesirable emissions such as N₂O or SO₂, and simultaneously makes the cellulose pulping process more energy-efficient.to achieve lower energy consumption as well as to improve the process conditions in such a way that the viscosity of the materials to be processed is reduced, delignification can be accelerated, the wood yield can be significantly improved5 and / or important components, such as sodium or sulfur content, can be supplemented by the addition of the spent electrolyte.

[0018] The invention is explained in more detail below using exemplary embodiments. These show: 0

[0019] Fig. 1 schematically shows a pulp production facility and markings for a possible addition of spent electrolytes from redox flow batteries.

[0020] Figure 1 schematically shows a pulp processing plant in which markings are placed indicating the points where spent electrolytes from redox flow batteries can be added. A ring marks the points where spent electrolytes from redox flow batteries can be added as pulping aids. Point 1 indicates the point where the spent electrolyte can be added to a pre-treatment. While the pre-treatment primarily removes air from the material to be pulped, ions or materials required for the pulping process can also be added to the material at this point. Another way to add spent electrolytes from redox flow batteries is at the point marked 4, i.e., after the impregnation of the cellulose-containing materials to be pulped.During impregnation, the material is primarily soaked with steam and prepared for digestion. At this stage, ions required for the subsequent digestion can also be advantageously introduced. A third option is to add the necessary lyes directly to the boiler 2 via the black or white lye solutions supplied at the base of the boiler 2, labeled 5 and 3 respectively. When the spent electrolytes are used in white lye, which consists mainly of sodium hydroxide and calcium carbonate, the reaction equilibrium shifts towards sodium ions, as the spent electrolytes contain considerable amounts of sodium. In this way, the pH value in the boiler can be adjusted more precisely, and furthermore, additional ions required for the process, such as sulfur, nitrogen, or phosphorus, are supplied from the spent electrolyte at this stage.Similarly, the spent electrolytes from the redox flow battery can be fed into the reactor via black liquor recycling at the base, particularly with weak black liquor, which is black liquor with no more than 15% solids content. The black liquor, which contains considerable amounts of lignin, is reduced in viscosity by the addition of spent electrolytes from redox flow batteries. Furthermore, in addition to this viscosity reduction, the weak black liquor introduces ions required for digestion into the reactor.

[0021] After the pulping process, the spent electrolyte from redox flow batteries can be used in particular as a rheology modifying agent, especially at the locations marked with * in Fig. 1.

[0022] The spent electrolyte from redox flow batteries can be added back to the black liquor to reduce its viscosity, particularly during processing. Naturally, it is also possible to add the spent electrolytes from redox flow batteries at any later point during the black liquor evaporation process, since the solids content increases during evaporation. Adding a viscosity-reducing material thus ensures a longer period during which targeted evaporation can occur before the product becomes too solid for effective evaporation and must be incinerated.

[0023] The point after boiling is marked 6 and the one in evaporation 7. Finally, it is also possible to use the spent electrolytes from redox flow batteries as processing chemicals in the cellulose pulping process, as indicated by a > in Fig. 1. For this purpose, the spent electrolytes are added before the recovery boiler at 8. In particular, the sodium- and sulfur-depleted lyes to be reprocessed are brought back to the ion concentrations required for subsequent use, for example, in pre-evaporation or pre-impregnation, by adding a waste product, namely the spent electrolytes. Example 1

[0024] Use and effect of spent electrolytes on the viscosity of spent alkali - comparison of the effect of electrolytes with the effect of water

[0025] A sulfite liquor produced using a sulfite process (LSK 10) was mixed with varying amounts of liquid spent electrolytes from redox flow batteries. The spent electrolytes used were always modified with a sulfur compound, in this case oleum, specifically anthraquinone and methoxyhydroquinone. The spent electrolytes were dosed at 3.5, 7.5, and 15 parts of oven-dried mass per dry solids of the sulfite liquor, with the aqueous solutions of the respective sulfur-modified spent electrolytes exhibiting concentrations of 4.8 g / L, 13.5 g / L, and 100 g / L, respectively. The results were compared with those of the sulfite liquor mixed with 3.5% pure water.Table 1 below clearly shows that, in comparison to sodium sulfite lye (LSK 10), the addition of methoxyhydroquinone (MHQS), modified with oleum, led to a massive reduction in viscosity at all measured temperatures of 20 °C, 50 °C, 70 °C and 90 °C.In contrast, the addition of 3.5% sulfur-modified anthraquinone (AQS) at a concentration of 4.8 g / l aqueous solution showed no effect, especially at high temperatures. However, at higher addition amounts, an effect is clearly recognizable. Compared to pure water, all tested solutions of spent electrolytes had a clearly positive effect on the viscosity of the magnesium sulfite hydroxide solution. The very best results were achieved by adding methoxyhydroquinone (MHQS), which was modified with sulfur and available as a 100 g / l aqueous solution, when a 15% solution of MHQS was added to the magnesium sulfite hydroxide solution.

[0026] Table 1

[0027] Example 2

[0028] Addition of spent electrons from redox flow batteries to spent Kraft sulfate lye as a rheology modifying agent.

[0029] In this second attempt, since the spent lye from the Kraft process is alkaline and the spent electrolytes are usually acidic, the spent electrolytes from redox flow batteries had to be alkalized to pH values ​​above 11. As spent

[0030] Sulfur-modified anthraquinone (AQS) or methoxyhydroquinone (MHQS) were used as electrolytes, with the concentrations of the spent electrolyte solutions being the same as in Example 1 and the added amounts corresponding to those of Example 1. In the case of spent lye solutions, the addition of sulfur-modified anthraquinone proved particularly advantageous. For example, the addition of 3.5% of the 4.8 g / L sulfur-modified anthraquinone solution reduced the viscosity at 20°C from 731 mPa to 434 mPa and at 90°C from 117 mPa to 71 mPa. It follows that if the spent electrolytes from redox flow batteries are added to a waste black liquor reprocessing solution, the viscosity of this black liquor is significantly reduced by the addition of the spent electrolytes, making the reprocessing both more energy-efficient and simpler.

[0031] Table 2

[0032] Example 3

[0033] Chemical Treatment of Black Liquor: Since the spent electrolytes contain significant concentrations of sulfur and sodium, concentrations which are required as treatment chemicals for black liquor when mixed with black liquor before the recovery boiler, an experiment was conducted using sulfur-modified anthraquinone or sulfur-modified methoxyhydroquinone. The spent electrolytes based on sulfur-modified anthraquinone or sulfur-modified methoxyhydroquinone were subjected to chemical analysis with regard to the ions they contained that may be important in pulp processing, and the following results, as shown in Table 3 below, were obtained. This table clearly shows that the amount of sodium, the amount of sulfur, and so on...the amount of nickel or silicon contained in these spent electrolytes is sufficient to shift the equilibrium towards the salts of sodium, sulfur and silicon-containing materials during alkali preparation.

[0034] Table 3

[0035] In summary, this example, as well as the preceding examples, demonstrates that spent electrolytes from redox flow batteries represent a valuable raw material for use in a cellulose production plant. These spent electrolytes can be used at various points within this plant and can have a variety of positive effects, such as reducing the viscosity of solid-containing alkalis, serving as a reprocessing chemical for alkali recovery, or acting as a pulp digestion additive. The present invention thus makes it possible to utilize a waste material effectively in an industrial process and to process it without leaving any residue.

Claims

Patent claims 1. A process for recycling spent electrolytes from redox flow batteries in a cellulose digestion process, characterized in that in a first step a preferably aqueous solution containing between 1 and 20 wt.%, preferably between 2 and 17 wt.%, particularly preferably between 3 and 15 wt.% of at least one spent electrolyte from a redox flow battery, optionally modified with either a sulfur compound, a phosphorus compound or a nitrogen compound, wherein the electrolyte is selected from the group of compounds with general formula (I) or compounds with general formula (II) wherein Ri .. Rs is selected from combinations of H, methyl, methoxy, amino groups, phosphorus groups, and sulfonate groups, and lignin, lignosulfonate, and / or vanillin derivatives are produced, that in a second step the solution of the optionally modified spent electrolyte from redox flow batteries is adjusted by adding a mineral acid or base to a pH value between 2 and 13, preferably between 9 and 13, corresponding to a process stream of the cellulose digestion process, that in a third step the solution of the spent electrolyte is introduced into the process stream of the cellulose digestion process,the pH value of which the spent electrolyte from redox flow batteries was adjusted to in the second step, and that in a fourth step the mixture of the process stream of the cellulose digestion process and the solution from the spent electrolyte from redox flow batteries is heated to temperatures above 90 °C, preferably above 120 °C, most preferably above 150 °C, and one or, Several processes are selected from a cellulose pulping process, such as a Kraft process, TMP process, CTMP process or NSSC process, a black liquor preparation from a boiling process, a delignification or a processing and recovery process of process fluids after cellulose pulping.

2. The method according to claim 1, characterized in that the spent electrolyte, optionally modified with either a sulfur compound, a phosphorus compound or a nitrogen compound, is selected from the group consisting of: anthraquinone (AQ), 9,10-anthraquinone-2,7-disulfonic acid (AQS), 2-methoxyquinone (MQ), 2-methoxyhydroquinone (MHQ), 1,4-benzoquinone, 1,2-benzoquinone-3,5-disulfonic acid, 1,2-hydroquinone-3,5-disulfonic acid, ethoxyhydroquinone (EHQ), 2,5-bis(N-methylethanolamine)-cyclohexa-2,5-diene-1,4-dione (MGQ), tetrasodium 3,3',3",3"'-((3,6-dihydroxybenzene-1,2,4,5- tetrayl)tetrakis(sulfandiyi))tetrakis(propane-1-sulfonate)) (MHQS). 3.A method according to claim 1 or 2, characterized in that the process stream used in the third step of the cellulose pulping process is selected from fresh cooking lyes, such as cooking lyes for sulfite or sulfate cellulose pulping processes, spent lyes from sulfite or sulfate cellulose pulping processes, spent acids from sulfite cellulose pulping processes, process streams containing tall oil lignins, filtrates from alkaline or acidic processing steps of cellulose pulping processes, such as filtrates from an ozone or chlorine dioxide bleaching, condensates or wastewater.

4. Method according to claim 3, characterized in that the used electrolyte from a redox flow battery is mixed with fresh cooking lye.

5. A method according to any one of claims 1 to 4, characterized in that at least one spent electrolyte is added to a process stream from the cellulose digestion process with a kappa value according to ISO 302:2015 between 10 and 100.

6. A method according to any one of claims 1 to 5, characterized in that a total The solids content of a mixture of a process stream from a cellulose digestion process and the spent electrolyte from redox flow batteries is adjusted to 3 g / l to a maximum of 120 g / l, preferably 4 to 100 g / l, before heating in the fourth step by adding a harmless diluent, preferably water or an aqueous solution of an acid or base.

7. Use of spent electrolytes from redox flow batteries in a cellulose pulping process, characterized in that a preferably aqueous solution containing between 1 and 20 wt.%, preferably between 2 and 17 wt.%, particularly preferably between 3 and 15 wt.% of at least one spent electrolyte from a redox flow battery, optionally modified with either a sulfur compound, a phosphorus compound or a nitrogen compound, and degradation products formed therefrom, is adjusted by the addition of a mineral acid or base to a pH value between 2 and 13, preferably between 9 and 13, corresponding to a process stream of the cellulose pulping process, wherein the electrolyte is selected from the group of compounds with the general formula (!) or compounds with the general formula (II) where Ri . R sfrom combinations of H, methyl, methoxy, amino groups, phosphorus groups, and sulfonate groups, as well as lignin, lignosulfonate, or vanillin derivatives, that the solution of the optionally modified spent electrolyte from redox flow batteries is prepared in a process stream of a cellulose digestion process at temperatures above 90 °C, preferably above 120 °C, most preferably above 150 °C.

8. Use according to claim 7, characterized in that the spent electrolyte, optionally modified with either a sulfur compound, a phosphorus compound or a nitrogen compound, from a redox flow battery is selected from the group consisting of: anthraquinone (AQ), 9,10-anthraquinone-2,7-disulfonic acid (AQS), 2-methoxyhydroquinone (MHQ), 1,2-benzoquinone-3,5-disulfonic acid, 1,4-benzoquinone, 1,2-hydroquinone-3,5-disulfonic acid, ethoxyhydroquinone (EHQ), methoxyquinone (MQ), (2,5-bis(N-methylethanolamine)cyclohexa~2,5- diene ~1,4-dione (MGQ), (tetrasodium 3 l 3',3'' l 3' , '-((3,6-dihydroxybenzene1,2,4,5tetrayl)tetrakis(sulfanediyl))tetrakis(propane-1-sulfonate)) (MHQS). 9, Use according to claim 7 or 8, characterized in that the spent electrolyte from a redox flow battery is used to reduce the viscosity of the process stream of a cellulose pulping process, as an auxiliary substance to accelerate delignification in the cellulose pulping process and / or as a substance supplementing a sodium and sulfur content.

Images