Recycling of polymer membranes comprising metal-containing catalyst material

Abstract

The invention relates to a method (100) for recycling polymer membranes comprising metal-containing catalyst material. The method comprises the following steps: S2: adding water without adding organic solvents to a polymer membrane comprising a metal-containing catalyst material to form a polymer membrane / water mixture, S3: simultaneously increasing the pressure and the temperature of the polymer membrane / water mixture to a pressure between 20 bar and 40 bar and a temperature between 200 °C and 250 °C, a liquid phase and a solid phase being formed, and S5: separating the liquid phase and the solid phase.

Description

[0001] Description

[0002] Reprocessing of polymer membranes containing metal-containing catalyst material

[0003] The invention relates to a process for the recycling of polymer membranes containing metal-containing catalyst material.

[0004] Hydrogen can be obtained from deionized water by electrolysis. The electrolysis can be carried out, for example, as polymer electrolyte membrane electrolysis (PEM electrolysis), in which the reaction spaces for the anode reaction (I) and the cathode reaction (II) are separated by a proton-conducting membrane, the polymer electrolyte membrane, also known as a proton exchange membrane.

[0005] Anode reaction 2 4 H + + 02+ 4 e _ (I) H2(II)

[0006] The polymer electrolyte membrane is typically made of a polymer, particularly an ionomer. Perfluorosulfonic acid-containing polymers, marketed, for example, under the name Nafion™, are widely used.

[0007] Catalyst materials are typically incorporated into or applied to the polymer electrolyte membrane to facilitate water electrolysis more efficiently. A PEM coated with catalyst material is also referred to as a catalyst-coated membrane, abbreviated to CCM. Possible catalyst materials include metals such as iridium, platinum, ruthenium, and palladium, in the form of pure metals or metal compounds.

[0008] Similar polymer electrolyte membranes and catalyst materials are used in fuel cells. Unlike the catalyst materials used in fuel cells, the catalyst materials used in water electrolysis often contain iridium, as iridium is preferentially used to catalyze the oxygen evolution reaction.

[0009] For example, after the end of their service life or in case of faulty manufacturing, reprocessing (recycling) of the polymer electrolyte membranes including catalyst materials to recover the precious metals and membrane polymers is desirable for cost and environmental reasons, among others.

[0010] Previous recycling processes involve a two-stage incineration process at temperatures between 850 and 1000 °C, in which the fluoropolymers are burned, releasing HF and CO2. Due to the aggressive nature of HF, special reactor linings and an extensive downstream gas scrubbing system are required. The resulting ash is usually mixed with other charges or ore concentrates and fed into a multi-stage refining process to separate the precious metals. Such a refining process may include dissolving in aqua regia, i.e., an HCl / HNO3 mixture in a ratio of approximately 3:1.

[0011] However, if metallic iridium catalysts (IrO) are used, the possibility of dissolving in aqua regia is eliminated, as metallic iridium is insoluble. The subsequent separation steps for precious metal separation therefore involve the Na2C>2 oxidation melt process to convert metallic iridium into soluble IrO>2. This process yields only low conversions and must be repeated several times.

[0012] A major disadvantage of the established process chain is the loss of the catalysts used, as these are completely dissolved and must be reprecipitated or pulverized. Due to the high number of process steps, side streams are created, which represent potential loss paths, as they are difficult to integrate into the main process stream.

[0013] Furthermore, the membrane polymers cannot be recycled because they are burned during the initial incineration process. The entire recycling process is also energy- and time-consuming. Furthermore, the use of numerous chemicals produces a number of toxic substances that require separate disposal and pose a danger to the recycling personnel and the environment.

[0014] US Pat. No. 7,255,798 B2 discloses an alternative recycling process in which the CCM to be recycled is treated in a water-solvent mixture at elevated pressure and temperature. Solvents that can be used include methanol, ethanol, n-propanol, isopropanol, n-butanol, 2-butanol, 2-methoxyethanol, 2-ethoxyethanol, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, dioxane, acetonitrile, or mixtures of the aforementioned solvents with a mass fraction between 10% and 80%.

[0015] After this treatment, the catalyst material and the polymer material can be separated and further processed. For example, the polymer material can be used to create a membrane, and the catalyst material can be deposited onto this membrane.

[0016] A disadvantage of this process is the use of mixtures of water and organic solvents, as this complicates the purification of the recycled polymer and catalyst material. Furthermore, the organic solvents used are hazardous substances, thus posing an increased risk to the personnel involved in recycling and to the environment, particularly given the process conditions of elevated pressure and high temperature. Against this background, the object of the invention is to provide a process for reprocessing polymer membranes provided with a catalyst material, with which the aforementioned problems can be reduced or even completely avoided.

[0017] This problem is solved by the subject matter of the independent claim. The dependent claims relate to preferred embodiments of the inventive solution.

[0018] The invention relates to a process for reprocessing polymer membranes containing metal-containing catalyst material. The process comprises the following steps: adding water, without the addition of organic solvents, to a polymer membrane containing a metal-containing catalyst material to form a polymer membrane-water mixture, simultaneously increasing the pressure and temperature of the polymer membrane-water mixture to a pressure between 20 bar and 40 bar and a temperature between 200°C and 250°C, thereby forming a liquid phase and a solid phase, and separating the liquid phase and the solid phase.

[0019] The polymer membrane comprising the metal-containing catalyst material can be, for example, a polymer electrolyte membrane for use in water electrolysis or in a fuel cell. The polymer membrane can comprise, for example, polymerized perfluorosulfonic acid as the polymer material, or the polymer material can consist of, for example, polymerized perfluorosulfonic acid.

[0020] In addition to the polymer material, the polymer membrane comprises a metal-containing catalyst material, e.g., for catalyzing the anode and / or cathode reaction (I, II) mentioned above in water electrolysis or the anode and / or cathode reaction in a fuel cell that uses hydrogen as fuel. The metal-containing catalyst material can comprise one or more precious metals selected from a group comprising iridium, platinum, ruthenium, rhodium, and palladium, each in the form of metal compounds, e.g., metal oxides, or pure metals.

[0021] The process step of simultaneously increasing the pressure and temperature can be carried out, for example, as a hydrothermal treatment in an autoclave. The pressure in the range between 20 bar and 40 bar and the temperature in the range between 200 °C and 250 °C must be reached simultaneously at least once. The specific treatment duration depends, among other things, on the chemical composition of the polymer material and the size of the polymer membrane pieces to be treated. The holding time can be, for example, 5 to 15 minutes, e.g., 10 minutes. The hydrothermal treatment promotes the separation of the polymer chains, so that the polymer material is dissolved in the water.

[0022] Preferably, the temperature can be increased to a temperature between 220 °C and 230 °C. At temperatures above 250 °C, decomposition of the polymer may occur; at temperatures below 200 °C, the polymer may not sufficiently dissolve.

[0023] The liquid and solid phases can be separated, for example, by centrifugation, preferably at speeds of 10,000 to 15,000 revolutions per minute. Metal-containing catalyst material and / or metal from the metal-containing catalyst material can subsequently be recovered from the separated solid phase.

[0024] The separated liquid phase contains dissolved polymer, which, depending on the concentration, can be reused either directly or after concentration. The concentration is particularly advantageous in the described process because, by using water as a solvent without the addition of organic solvents, there is no need to take different boiling points into account, as is the case with water-alcohol mixtures, for example. The proposed process enables the reprocessing of polymer membranes, in particular polymer electrolyte membranes, and the metal-containing catalyst materials contained therein and / or deposited thereon, by separating the polymer and the metal-containing catalyst material from one another. This enables the recovery and reuse of the polymer and / or the metal-containing catalyst material, if necessary after further processing steps.

[0025] Since both the polymer material and the metal-containing catalyst material are rare and cost-intensive materials, reprocessing is advantageous in terms of resource conservation and cost savings. Compared to the combustion process explained in the introduction, the proposed process has the advantage that not only the metal-containing catalyst material but also the polymer material can be recovered. Both the metal-containing catalyst material and the polymer material can be recovered without any significant losses and in an essentially unchanged state, meaning they can be directly reintegrated into the value chain. This results in further cost advantages and resource savings compared to conventional combustion processes with downstream precious metal refining. In the process described here, the number of process steps is significantly reduced, namely from approx.Reprocessing by incineration reduces the number of 20 process steps to approximately six, thus avoiding waste pathways, especially for iridium. The reprocessing costs for separation (removal process) and the manufacturing costs for precipitation of the new catalyst material are eliminated.

[0026] The recycling process is carried out without the use of organic solvents, such as alcohols, carboxylic acid esters, ethers, ketones, alkanes, aromatic hydrocarbons, glycol ethers, and nitriles. For example, the polymer membrane-water mixture can be formed by adding distilled water without any additives.

[0027] This simplifies the purification of the reprocessed or recycled polymer and catalyst material. Furthermore, the potential hazards for the personnel involved in the recycling process and the environment are reduced. The safety of the process can be increased, especially given the elevated temperature and pressure involved.

[0028] Treatment in water without the addition of organic solvents is particularly advantageous considering the quantities of platinum catalysts involved. Otherwise, the platinum catalysts can react with the aforementioned organic solvents and pose a safety risk. Furthermore, as already mentioned, the subsequent concentration of the polymer solution is significantly simplified.

[0029] Optionally, the polymer membrane containing the metal-containing catalyst material can be crushed before adding the water, for example, into segments with dimensions between 1 mm and 10 mm, preferably between 2 mm and 5 mm. The crushing can be carried out, for example, by cutting or shredding.

[0030] The advantage of comminution is that a more homogeneous polymer membrane-water mixture can be formed, which allows the hydrothermal treatment to be carried out more efficiently, e.g., holding times can be shortened or continuous process control can be simplified.

[0031] According to various embodiments, the polymer membrane can have fiber reinforcement. In this case, the process can comprise separating fibers from the fiber reinforcement before separating the liquid phase and the solid phase. The fibers can be in the form of a supporting fabric and, for example, comprise polyetheretherketone (PEEK) or consist of polyetheretherketone. The fibers can be separated, for example, by sieving, e.g., using sieve plates with a mesh size of 0.1 mm to 0.3 mm.

[0032] By separating the fibers, they can be prevented from remaining in the solid phase, thus obtaining a solid phase with higher purity of the metal-containing catalyst material. This avoids the need for downstream purification of the metal-containing catalyst material to remove the fibers or their components.

[0033] According to further embodiments, the method may comprise processing the separated solid phase.

[0034] Depending on requirements, different metal-containing catalyst materials or the metals they contain, for example metallic iridium and platinum or platinum-containing compounds, can be separated from each other.

[0035] The processing may involve one or more physical and / or chemical process steps, for example treatment with aqua regia, filtration, precipitation by addition of a precipitant, washing, etc.

[0036] The specific selection of processing steps depends on the metal-containing catalyst material present and the desired reprocessing product(s). The processing of the separated solid phase serves to recover the metal-containing catalyst material in metallic and / or chemically bound form, preferably in such a way that direct reuse is possible, for example, again as catalyst material for catalysis in water electrolysis or in a fuel cell. For example, the processing of the separated solid phase can be carried out in such a way that metallic iridium is recovered, e.g., as powdered iridium black. Since iridium-containing raw materials are particularly limited and iridium extraction is complex and loss-making, the recovery of metallic iridium is particularly advantageous.

[0037] According to further embodiments, the method may comprise treating the separated liquid phase with activated carbon.

[0038] The activated carbon treatment enables the purification of the separated liquid phase, for example from very small particles of the metal-containing catalyst material, which could not be separated by centrifugation.

[0039] This allows the purity of the remaining aqueous polymer solution to be increased, so that direct further processing of the polymer solution is possible without further purification, if necessary after concentration.

[0040] According to further embodiments, the method may comprise producing a polymer membrane from the separated liquid phase.

[0041] The separated liquid phase contains an aqueous solution of the polymer material of the polymer membrane. Polymer membranes can be produced from this solution, for example, by forming a layer from the liquid phase, for example, on a support material, followed by drying and, if necessary, annealing. Coating processes for coating from liquid phases, such as spin coating, dip coating, and doctor blade film drawing, can be used for this purpose.

[0042] Optionally, the separated liquid phase can be concentrated before application of the liquid phase. For this purpose, water can be removed by rotary evaporation, for example. Further optionally, n-propanol can be added to the liquid phase before rotary evaporation, for example in a mass ratio of water:n-propanol = 3:7. The use of n-propanol is preferred due to its boiling point, which is similar to that of water. This can increase the cross-linking of the polymer during the drying of the deposited layer, which facilitates the subsequent formation of membranes and, if necessary, allows the properties of the resulting membrane to be specifically influenced. To avoid cracks during the subsequent membrane drawing, a uniform removal of the solvent is helpful, as this enables uniform cross-linking and uniform adhesion of the polymer chains.Otherwise, the hydrophobic parts of the polymer backbone could undesirably form micelles with n-propanol and the hydrophilic side chains with water.

[0043] Advantageously, polymer membranes can be produced again from the recycled polymer membrane, so that waste can be avoided and resources can be conserved.

[0044] Of course, new polymer material can also be mixed with recycled polymer material to produce polymer membranes.

[0045] According to further embodiments, the polymer membrane-water mixture can be stirred during the step of simultaneously increasing the pressure and temperature.

[0046] The stirring speed can be, for example, 50 to 400 revolutions per minute.

[0047] Stirring has the advantage of creating a more homogeneous polymer membrane-water mixture, which allows the hydrothermal treatment to be carried out more efficiently, e.g., by shortening holding times or simplifying continuous process control. According to further embodiments, at least the step of simultaneously increasing the pressure and temperature can be carried out continuously.

[0048] Preferably, further process steps can be carried out continuously, such as, for example, comminuting the polymer membrane comprising the metal-containing catalyst material, adding water without the addition of organic solvents to the polymer membrane comprising the metal-containing catalyst material, separating fibers of the fiber reinforcement of the polymer membrane, processing the separated solid phase and / or treating the separated liquid phase with activated carbon.

[0049] Continuous process control enables higher throughputs to be achieved, making reprocessing more efficient.

[0050] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of the embodiments, which are explained in more detail in conjunction with the drawings.

[0051] Fig. 1 is a flow diagram of an exemplary process for reprocessing polymer membranes containing metal-containing catalyst material;

[0052] Fig. 2 is a pressure-temperature diagram illustrating the conditions during hydrothermal treatment;

[0053] Fig. 3 is a diagram illustrating the ion exchange capacity of new polymembranes and polymer membranes made from recycled polymer material; and Fig. 4 is a diagram illustrating the equivalent weight of a new polymembrane compared to polymer membranes made from recycled polymer material.

[0054] Figure 1 shows a flow diagram of an exemplary process 100 for reprocessing polymer membranes containing metal-containing catalyst material. This embodiment relates to the reprocessing of a perfluorosulfonic acid polymer membrane with a PEEK support fabric and metal-containing catalyst material. The starting material used was 40 g of a membrane electrode assembly from water electrolysis (Silyzer 300 module) with platinum catalysts on the cathode.

[0055] After starting process 100, in process step S1, the polymer membrane containing the metal-containing catalyst material was shredded into segments with dimensions of approximately 3 mm. A Moco ZG 10 shredder with a throughput of 200 kg / h was used for this purpose.

[0056] Subsequently, in process step S2, 235 g of deionized water (without the addition of organic solvents or other additives) were added to the cut segments to form a polymer membrane-water mixture.

[0057] This polymer membrane-water mixture was subjected to a hydrothermal treatment in process step S3, during which pressure and temperature were increased simultaneously. In the exemplary embodiment, a laboratory autoclave with a volume of 700 ml was used for this purpose, but a continuous process is also possible. In the exemplary embodiment, the hydrothermal treatment was carried out at a temperature of 225 °C and a pressure of 24 bar for 10 minutes. During the hydrothermal treatment, the perfluorosulfonic acid polymer dissolved, which was accompanied by foam formation on the resulting solution and a viscosity of the solution higher than that of water. Figure 2 shows a diagram illustrating possible pressure-temperature conditions for carrying out process step S3. In general, the pressure must be between 20 bar and 40 bar, while the temperature is between 200 °C and 250 °C.In the example, the hydrothermal treatment was carried out at a temperature of 225 °C and a pressure of approximately 23 - 24 bar.

[0058] Referring again to Figure 1, after the hydrothermal treatment, process 100 continued with process step S4, in which the PEEK fibers that did not dissolve during process step S3 were separated. Coarse-mesh sieves with a mesh size between 0.1 mm and 0.3 mm were used for this purpose. If the polymer membrane does not have fiber reinforcement, process step S4 can be omitted.

[0059] In process step S5, the solid and liquid phases formed after the hydrothermal treatment were separated. The solid phase contained the metal-containing catalyst material, while the perfluorosulfonic acid polymer was dissolved in the liquid phase. Phase separation was achieved by centrifugation at a speed of 11,500 revolutions per minute for a period of 1 h. After phase separation, the solid and liquid phases were further processed separately.

[0060] In process step S6, divided into steps S6a to S6e in the execution step, the solid phase was prepared. For this purpose, aqua regia was added to the solid phase in step S6a and the temperature was raised to approximately 80 °C. This caused the platinum contained to dissolve, allowing it to be separated by filtration in step S6b. Platinum was separated or precipitated from the filtrate in step S6c. The filter residue was washed with deionized water in step S6d until a pH of 7 was reached. After drying in step S6e, powdered metallic iridium was obtained, which can be used again directly as a catalyst without further conversion or purification steps.

[0061] For further processing of the liquid phase obtained in process step S5, an optional treatment with activated carbon was carried out in process step S7. This clarified the polymer solution, i.e., the ionomer phase. After removal of the activated carbon, a purified aqueous solution of the perfluorosulfonic acid polymer was obtained.

[0062] In process step S8, a polymer membrane was again produced from the resulting aqueous perfluorosulfonic acid polymer solution. Process step S8 is divided into steps S8a to S8c.

[0063] The polymer membranes were produced from recycled polymer using the autoclaved polymer solution. In process step S8a, the polymer solution was concentrated in a rotary evaporator at a negative pressure between 60 mbar and 80 mbar and a heating bath temperature of 40 °C. Two different solvent approaches were investigated. Firstly, the polymer solution was concentrated purely in the rotary evaporator by removing the solvent water. This could lead to severe foaming of the polymer solution. Secondly, n-propanol was added to the polymer solution before rotary evaporation in a water to n-propanol ratio of 3:7. Since the perfluorosulfonic acid polymer dissolves well in water, but water is highly polar, another solvent was added to enable crosslinking of the perfluorosulfonic acid during the drying process.

[0064] In the subsequent step S8b, films were drawn from the concentrated solutions onto a polyimide film (Kapton film) using a doctor blade. The doctor blade feed rate was 10 mm / s. In step S8c, the drawn films were dried in a drying oven at 60 °C for 30 min and then annealed at 180 °C for 10 min. Polymer membranes without visible defects were obtained. To characterize the resulting polymer membranes, their ion exchange capacity and equivalent weight were determined.

[0065] Recycling the polymer membranes should not cause any damage to the polymer. The material properties of a polymer membrane made from recycled polymer should be approximately equivalent to those of polymer membranes made from new or juvenile polymer material. To characterize the polymer membranes made from recycled polymer material, the ion exchange capacity IEC and the resulting equivalent weight were determined. The results are summarized below and graphically presented in Figures 3 and 4.

[0066] Figure 3 compares the ion exchange capacity of polymer membranes made from recycled polymer material (designated "IEC rec.") and polymer membranes made from virgin polymer material (designated "IEC new"), each for membrane production from aqueous solution (right side of the bar chart) and from an n-propanol-water mixture (ratio of water to n-propanol = 3 : 7; left side of the bar chart). In the case of polymer membranes made from recycled polymer material, the error bars represent the actual scatter of the measured values ​​for five samples tested, whereas in the case of the polymer membrane made from virgin polymer material they represent the specified range of the manufacturer's specification.

[0067] The comparison according to Figure 3 shows that the ion exchange capacity of the polymer membranes made from recycled polymer material is in the lower range of the specification for a polymer membrane made from virgin polymer material. In other words, the ion exchange capacity is only marginally affected by recycling. Furthermore, it can be deduced from Figure 3 that the ion exchange capacity of the polymer membranes made from recycled polymer material is slightly higher when manufactured from an aqueous solution than when manufactured from an n-propanol-water mixture, whereas when virgin polymer material is used, the ion exchange capacity is independent of whether the membrane is manufactured from an aqueous solution or from an n-propanol-water mixture.

[0068] In addition, the ion exchange capacity was converted into the equivalent weight EW of the polymer membranes (IEC= (V(NaOH) *c (NaOH) ) / mO in mmol / g;

[0069] EW=1000 / IEC in g / mol) and plotted against each other in Figure 4 (the designation “rec. n-Prop / H20” corresponds to the polymer membrane made from recycled polymer material, produced from an n-propanol-water mixture; “rec. H20” corresponds to the polymer membrane made from recycled polymer material, produced from an aqueous solution and “new” corresponds to the polymer membrane made from new polymer material, value independent of production from an aqueous solution or n-propanol-water mixture). The comparison of the equivalent weight of the polymer membranes shows that the values ​​of the recycled membranes are within the manufacturer’s specification and that there is no structural damage to the polymers resulting from recycling.

[0070] It can therefore be concluded that the membranes produced from recycled perfluorosulfonic acid polymer material using the described process do not show any damage to the perfluorosulfonic acid polymer due to the recycling process.

[0071] In summary, the proposed method has the following advantages:

[0072] — higher recovery rates for precious metals of more than 95%,

[0073] — Retention of the iridium catalyst, no new synthesis required,

[0074] — Preservation of the ionomer dispersion, suitable for the production of new membranes, — mild process conditions,

[0075] — no pollutant emissions,

[0076] — resource-saving chemcycling process,

[0077] — simple process chain without special process technology,

[0078] — enables a direct replacement of the previous process step of incineration for the recovery of precious metals and the simultaneous recovery of other valuable materials such as the polymer membrane,

[0079] — Enabling the reduction of emissions and ensuring the availability of rare catalyst metals through highly efficient raw material utilization.

[0080] The proposed process is also suitable for the reprocessing of large quantities of polymer membranes containing metal-containing catalyst material.

[0081] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0082] The term "and / or" as used herein, when used in a series of two or more elements, means that any one of the listed elements may be used alone, or any combination of two or more of the listed elements may be used.

Claims

Patent claims 1. Method (100) for reprocessing polymer membranes containing metal catalyst material, the method comprising the following steps: — S2 : Addition of water without the addition of organic solvents to a polymer membrane containing a metal catalyst material to form a polymer membrane-water mixture, — S3: Simultaneously increasing the pressure and temperature of the polymer membrane-water mixture to a pressure between 20 bar and 40 bar and a temperature between 200 °C and 250 °C, forming a liquid phase and a solid phase, and — S5: Separation of the liquid phase and the solid phase.

2. Method (100) according to claim 1, wherein the metal-containing catalyst material comprises one or more precious metals selected from the group comprising iridium, platinum, ruthenium, rhodium and palladium.

3. Method (100) according to any one of the preceding claims, comprising: — Sl: Comminution of the polymer membrane containing the metal catalyst material before adding the water.

4. Method (100) according to any one of the preceding claims, comprising: — S4: Separation of fibers from a fiber reinforcement of the polymer membrane prior to separating the liquid phase and the solid phase.

5. Method (100) according to any one of the preceding claims, comprising: — S6: Processing of the separated solid phase.

6. Method (100) according to claim 5, wherein metallic iridium is obtained in the step of processing the solid phase.

7. Method (100) according to any one of the preceding claims, comprising: — S7 : Treating the separated liquid phase with activated carbon .

8. Method (100) according to any one of the preceding claims, comprising: — S8: Production of a polymer membrane from the separated liquid phase.

9. Method (100) according to any of the preceding claims, wherein the polymer membrane-water mixture is stirred during the step of simultaneously increasing the pressure and temperature.

10. Method (100) according to one of the preceding claims, wherein at least the step of simultaneously increasing the pressure and temperature is carried out continuously.

11. Method (100) according to any one of the preceding claims, wherein the polymer membrane comprising the metal-containing catalyst material is a polymer electrolyte membrane for use in water electrolysis or in a fuel cell.

12. Method (100) according to any one of the preceding claims, wherein the polymer membrane comprises a perfluorosulfonic acid polymer.

13. Method (100) according to one of the preceding claims, wherein the temperature is increased to a temperature between 220 °C and 230 °C in the step of simultaneously increasing the pressure and temperature of the polymer membrane-water mixture.

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