Methods for recovering valuable materials

Abstract

To efficiently recover valuable materials. [Solution] The method for recovering valuable materials is a method for recovering valuable materials from an electrochemical device, wherein the electrochemical device has a catalyst made of an oxide of a precious metal, and the method comprises the steps of: reducing the oxide while heating an object having the catalyst; and applying a voltage to the object in contact with a solution containing at least one selected from the group consisting of ionic species and chemical species that can form a complex with the precious metal, and periodically reversing the polarity of the voltage by increasing the absolute value of the voltage over time and then holding it, thereby performing an electrolytic reaction and dissolving the reduced precious metal as an ion in the solution.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a method for recovering valuable materials. [Background technology]

[0002] As the global transition to a carbon-neutral society progresses, hydrogen is attracting attention as an alternative to fossil fuels. One of the currently considered uses of hydrogen is in fuel cells, which generate electrical energy by electrochemically reacting hydrogen and oxygen. Among fuel cells, polymer electrolyte fuel cells (PEFCs), which use a solid polymer membrane as the electrolyte and operate at temperatures below 100°C, were launched in 2009 as a household fuel cell cogeneration system (product name: EneFarm®). Furthermore, PEFCs are beginning to be installed in fuel cell vehicles (FCVs), as well as mobile vehicles such as forklifts, buses, and trucks, and the demand for PEFCs is expected to increase further in the future.

[0003] Simultaneously, development is underway to produce the large quantities of hydrogen used in PEFCs through the electrolysis of water. While there are various methods for this water electrolysis, the main ones being developed are high-temperature steam electrolysis, which uses steam at several hundred degrees Celsius as a raw material, and polymer electrolyte membrane (PEM) type water electrolysis, which operates at around room temperature. PEM type water electrolysis devices have advantages such as not requiring high-temperature operation and being easier to develop cells for because their configuration is similar to that of PEFCs. As a result, megawatt-scale PEM type water electrolysis devices are being introduced in countries around the world.

[0004] PEM-type water electrolyzers are composed of similar components to PEFCs, and the electrolysis reaction of water proceeds in a membrane electrode assembly (MEA) consisting of an electrolyte membrane, an electrode catalyst, and a gas diffusion layer (GDL). To reduce the energy required during the electrolysis reaction, noble metal elements that facilitate hydrogen and oxygen evolution reactions are typically used as electrode catalysts. Currently, the cathode, where the hydrogen evolution reaction occurs, is formed using platinum, and the anode, where the oxygen evolution reaction occurs, is formed using a catalyst containing iridium.

[0005] To achieve a carbon-neutral society in the context of a hydrogen-based society, the introduction of PEFCs and PEM-type water electrolysis devices will inevitably increase, making it essential to secure valuable materials such as precious metal elements used in these devices. Means of securing precious metal elements include the development of new mines, the development of mining technologies, and the development of recycling technologies. However, Japan relies on overseas sources for the majority of its precious metal elements, such as platinum, ruthenium, and iridium, and mining development and mining technology development alone will not be sufficient to address situations such as conflicts and resource competition. Therefore, developing technologies to recover precious metal elements from devices already in circulation is crucial. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 1626036 [Patent Document 2] Patent No. 6652518 [Patent Document 3] Patent No. 6652454 [Patent Document 4] Patent No. 6109769 [Patent Document 5] Japanese Patent Application Publication No. 63-270421 [Non-patent literature]

[0007] [Non-Patent Document 1] Yusuke Koto et al., "Increasing Current Density of CO2 Electrolytic Cells for the Realization of Power to Chemicals," Toshiba Review, Vol. 75, No. 6, pp. 48-51 (November 2020) [Non-Patent Document 2] Mohammad Fathi Tovini et al., “Degradation Mechanism of an IrO2 Anode Co-Catalyst for Cell Voltage Reversal Mitigation under Transient Operation Conditions of a PEM Fuel Cell”, J. Electrochem. Soc., 168, 064521 (2021) [Overview of the project] [Problems that the invention aims to solve]

[0008] The problem that this invention aims to solve is the efficient recovery of valuable materials. [Means for solving the problem]

[0009] The embodiment of the method for recovering valuable materials is a method for recovering valuable materials from an electrochemical device, wherein the electrochemical device has a catalyst made of an oxide of a precious metal, and the method comprises the steps of: reducing the oxide while heating an object having the catalyst; and applying a voltage to the object in contact with a solution containing at least one selected from the group consisting of ionic species and chemical species that can form a complex with the precious metal, and periodically reversing the polarity of the voltage by increasing the absolute value of the voltage over time and then holding it, thereby performing an electrolytic reaction and dissolving the reduced precious metal as an ion in the solution. [Brief explanation of the drawing]

[0010] [Figure 1] This is a flowchart illustrating an example of a method for recovering valuable materials. [Figure 2] This is a schematic diagram showing examples of items for which valuable materials are recovered. [Figure 3] This graph shows the time variation of the voltage applied to an electrochemical device for the conventional electrolysis-based recovery of valuable materials. [Figure 4] This is a graph showing the time variation of voltage in the embodiment. [Figure 5] It is a schematic diagram showing an electric circuit (equivalent circuit) representing the inside of an electrochemical device. [Figure 6] It is a diagram showing the time change of voltage E. [Figure 7] It is a diagram showing the time change of current i. [Figure 8] It is a diagram showing an example of the time change of voltage E. [Figure 9] It is a diagram showing an example of the time change of current i. [Figure 10] It is a graph representing another example of the time change of a voltage wave in the voltage control method of the embodiment. [Figure 11] It is a schematic diagram showing a configuration example of a PEM type water electrolysis device. [Figure 12] It is a schematic diagram showing a configuration example of a valuable material recovery device. [Figure 13] It is a diagram showing the Ir dissolution rate.

Mode for Carrying Out the Invention

[0011] Hereinafter, embodiments will be described with reference to the drawings. In each of the embodiments shown below, substantially the same constituent parts are denoted by the same reference numerals, and the description thereof may be partially omitted. The drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thicknesses of each part, etc. may be different from the actual ones.

[0012] In this specification, "connect" includes not only directly connecting but also indirectly connecting in some cases, unless otherwise specified.

[0013] One aspect of the embodiment is a method (valuable material recovery method) for recovering a noble metal contained in a catalyst used in an electrochemical device such as a water electrolysis device. The catalyst has a function of promoting an electrolysis reaction such as an oxidation reaction or a reduction reaction by an electrochemical device, an electrolysis reaction of an electrolyte or a solute. Examples of the valuable material recovery method will be described below.

[0014] (Example of Valuable Material Recovery Method) Figure 1 is a flowchart illustrating an example of a valuable material recovery method. The example valuable material recovery method includes an oxide reduction step S1 and an electrolytic dissolution step S2. The oxide reduction step S1 and the electrolytic dissolution step S2 are performed, for example, in sequence.

[0015] Figure 2 is a schematic diagram showing an example of an object containing valuable materials. Examples of objects include MEAs. An MEA has an anode 101, a cathode 102, and an electrolyte membrane 103 between the anode 101 and the cathode 102. The anode 101, cathode 102, and electrolyte membrane 103 are stacked on top of each other. MEAs are used in electrochemical devices such as PEM-type water electrolyzers. The anode 101 and cathode 102 each contain catalysts such as an anode catalyst and a cathode catalyst. The catalyst is made of a precious metal, for example, which is a valuable material. Examples of precious metals include platinum (Pt), iridium (Ir), ruthenium (Ru), rhodium (Rh), palladium (Pd), platinum (Pt), gold (Au), rhenium (Re), etc. The catalyst may also contain an oxide containing at least one of the above precious metals. An example of the electrolyte membrane 103 is a solid polymer electrolyte membrane. The objects are not limited to MEA.

[0016] The oxide reduction step S1 includes reducing a noble metal oxide. Noble metal oxides are used, for example, as catalysts in electrochemical devices, and are used as catalysts for anode 101 (anode catalyst). Examples of noble metal oxides include iridium oxide (IrO2), ruthenium oxide (RuO2), rhodium oxide, palladium oxide, platinum oxide, gold oxide, rhenium oxide, and the like.

[0017] The oxide reduction step S1 can also be carried out by chemical methods, such as supplying a solution containing a reducing agent and bringing the reducing agent into contact with the object, supplying a reducing gas and bringing the reducing gas into contact with the object, or bringing an electrolyte into contact with the object, applying a voltage to the object, and carrying out an electrolytic reaction without reversing the polarity of the voltage. The electrolyte may contain at least one ionic species and chemical species that can form complexes with noble metals.

[0018] The electrolytic dissolution step S2 includes supplying a solution containing at least one ionic species and chemical species capable of forming a complex with a noble metal, bringing the solution into contact with an object (e.g., an electrode having an oxide catalyst), applying a voltage to the object in contact with the solution, and periodically reversing the polarity of the voltage to carry out an electrolytic reaction. In other words, the electrolytic dissolution step S2 includes alternating oxidation and reduction reactions. The voltage value can be appropriately set, for example, using an external power source. The electrolytic dissolution step S2 allows the reduced noble metal to be dissolved in the solution as ions. The noble metal dissolved in the solution can then be recovered by separating it from the solution. The solution includes, for example, hydrochloric acid (HCl). The solution is not limited to being supplied after the oxide reduction step S1, but may also be supplied before the oxide reduction step S1.

[0019] In the dissolution of precious metal catalysts such as Pt and Ru reported in Patent Documents 2, 3, and 4, a solution containing components capable of forming complexes with the target precious metal is injected into an electrochemical device. After converting the extremely thin precious metal oxide film present on the surface of the metal catalyst into metal through a reduction treatment, the exposed metal is dissolved through an oxidation treatment. During the oxidation treatment, not only dissolution but also oxide film formation proceeds simultaneously, so the dissolution reaction stops when the metal is covered with an oxide film. The oxide film is removed by performing a reduction treatment again, so by alternating between reduction and oxidation, it becomes possible to dissolve precious metals that would normally not dissolve. In other words, the conventional method is a technique for dissolving precious metals that have a thin oxide on their surface.

[0020] On the other hand, since the catalyst targeted in this embodiment is entirely an oxide, for example, in the case of IrO2, the Ir dissolution reaction will not proceed using the same method as in the conventional technology. Therefore, in this embodiment, in order to dissolve the noble metal, the oxide of the noble metal is reduced to the noble metal in the oxide reduction step S1. In the oxide reduction step S1, for example, IrO2 can be reduced to metallic Ir.

[0021] The oxide reduction step S1 is preferably carried out while heating is performed during the reduction reaction in order to accelerate the reduction reaction, such as the IrO2 reduction reaction. For example, by reducing the oxide of a noble metal using the above method while heating the target material with a heater, the rate of the reduction reaction is increased, and thus the reduction efficiency per unit of time can be improved. The oxide reduction step S1 is preferably carried out at a temperature of 100°C or higher, preferably 150°C or higher. The upper limit of the above temperature is not particularly limited, but for example, if the target material contains a solid polymer electrolyte membrane, it should be 300°C or lower so that the electrolyte membrane does not thermally decompose.

[0022] When a solution containing a reducing agent is brought into contact with the material, for example, if an oxalic acid solution is used, the reduction reaction proceeds according to equation (1). IrO2+2(COOH)=Ir+4CO2+2H2O ΔG=-510.7kJ···(1)

[0023] When a reducing gas is brought into contact with the material, if hydrogen gas is used, the reduction reaction proceeds according to equation (2). IrO2+2H2=Ir+2H2O ΔG=-281.4kJ...(2)

[0024] When electrochemically reducing in an aqueous solution, the reduction reaction shown in equation (3) proceeds by controlling the electrode potential to a potential lower than 0.926 V vs. SHE. IrO2 + 4H + +4e - =Ir+2H2O E O =0.926V vs. SHE···(3)

[0025] In the oxide reduction step S1, the reduction treatment converts IrO2 to metallic Ir, allowing the Ir to be dissolved in the electrolytic dissolution step S2, where electrolysis is performed while reversing the polarity. For electrolytic dissolution to occur, it is necessary to contact the Ir with a solution containing an ionic species or chemical species capable of forming a complex with Ir. Hydrochloric acid containing chloride ions can be used as the solution, but any solution containing chloride ions in an acidic solution, such as sulfuric acid with dissolved sodium chloride, can also be used.

[0026] As described above, the example of the valuable material recovery method, by providing an oxide reduction step S1 different from the electrolytic dissolution step S2, allows for the recovery of valuable materials with high efficiency by reducing the oxide, even when using an oxide of a precious metal as a catalyst. Furthermore, the example of the valuable material recovery method allows for polarity reversal in the electrolytic dissolution step S2 to be performed under simple application conditions, thus improving the operability of polarity reversal.

[0027] In the electrolytic dissolution process S2, a square wave that reverses polarity at a constant period can be used as the applied voltage. However, when electrolysis is performed with a square wave, an extremely large current may flow through the electrochemical device when the polarity is reversed.

[0028] Figure 3 is a graph showing the time variation of the voltage applied to an electrochemical device for the conventional electrolysis-based recovery of valuable materials. The horizontal axis of the graph represents time [seconds]. The vertical axis of the graph represents the voltage [V] applied to the electrochemical device. In the conventional electrolysis-based recovery of valuable materials described in Patent Documents 2, 3, and 4, as shown in Figure 3, voltages X and Y are periodically applied to the electrochemical device in a rectangular wave to dissolve platinum group metals such as platinum and ruthenium. Voltage X is a lower value than voltage Y. Voltage Y is a higher value than voltage X, for example, a positive value. However, with this voltage control method, a spike-like non-Faraday current flows at the moment of switching from voltage X to voltage Y and from voltage Y to voltage X, so there is a problem that electrolysis cannot be performed if the output of the electrochemical device is insufficient. Thus, although precious metal dissolution is possible with the above method, when considering the scaling up of the equipment, a large current flows when the polarity is reversed, which increases the cost of the equipment and necessitates safety mechanisms for large currents.

[0029] Therefore, a voltage control method that enables the melting of precious metals while suppressing the large non-Faraday current that flows when the polarity is reversed will be described below. In this embodiment, instead of a square wave, the polarity of the voltage is periodically reversed by increasing the absolute value of the voltage over time and then holding it, as shown in Figure 4. This suppresses the instantaneous flow of a large non-Faraday current. In other words, in this embodiment, when reversing the polarity of the voltage, the slope of the voltage wave from a reference value such as 0V to the target value of the applied voltage is made larger than that of a square wave. Examples of such voltage waves include trapezoidal waves.

[0030] Figure 4 is a graph showing the time variation of the voltage in the embodiment. The horizontal axis of the graph represents time [seconds]. The vertical axis of the graph represents the voltage [V] applied to the electrochemical device 100. The voltage control method of the embodiment applies a voltage to the electrochemical device 100 and periodically reverses the polarity of the voltage to perform an electrolytic reaction. Therefore, it has a period P1 with a first polarity and a period P2 with a second polarity opposite to the first polarity. When recovering valuable materials, periods P1 and P2 are alternately switched and repeated. The voltage is applied, for example, to an electrode having the catalyst.

[0031] During period P1, the voltage applied to the electrochemical device 100 is gradually decreased from time T1 to time T2 until it is less than or equal to the value of voltage X. Voltage X is lower than voltage Y. Note that voltage X is not limited to a negative value lower than 0V, but may also be a positive value. Figure 4 shows an example where the voltage is gradually decreased from 0V to voltage X, but it may also be gradually decreased from a value different from 0V to voltage X.

[0032] Next, the voltage applied to the electrochemical device 100 is kept below the value of voltage X from time T2 to time T3. Figure 4 shows an example where the voltage is kept at the value of voltage X, but it does not necessarily have to be a constant value. The voltage that is held until time T3 changes to 0V when the application is stopped. At this time, the voltage value may be increased over time from the value of voltage X until it reaches 0V.

[0033] Next, in period P2, the value of the voltage applied to the electrochemical device 100 from time T3 to time T4 is increased over time to a value equal to or higher than the value of voltage Y. Voltage Y is higher than voltage X. Voltage Y is, for example, a positive value higher than 0V. In FIG. 4, an example of increasing the voltage from 0V to voltage Y over time is shown, but it may be increased from a value different from 0V to voltage Y over time.

[0034] Next, the value of the voltage applied to the electrochemical device 100 from time T4 to time T5 is maintained at a value equal to or higher than the value of voltage Y. In FIG. 4, an example of maintaining the value at the value of voltage Y is shown as an example, but it does not necessarily have to be a constant value. The voltage whose value is maintained until time T5 changes to 0V by stopping the application thereafter. At this time, the value of the voltage may be decreased over time from the value of voltage Y to 0V.

[0035] Here, it will be explained that spike-like non-Faradaic current can be suppressed by changing the voltage wave from a rectangular wave to a trapezoidal wave. FIG. 5 is a schematic diagram showing an electric circuit (equivalent circuit) representing the inside of the electrochemical device. When the inside of the electrochemical device is represented by an electric circuit, it becomes an RC circuit in which the resistance R s of the electrolytic solution and the electric double layer (capacitance) C d are connected. FIG. 5 further shows a switch SW that controls the supply of voltage E to the electrochemical device and a current i flowing inside the electrochemical device.

[0036] The voltage E c applied to the electric double layer, the charged electric quantity q, and the capacitance C d satisfy Equation (1).

[0037]

Equation

[0038] When the switch SW is in the on state, if the voltage applied to the electrochemical device is E, the sum of the voltage drop iR s in the electrolytic solution and the voltage E c applied to the electric double layer must be equal to E, so Equation (2) holds.

[0039]

number

[0040] Furthermore, since current i is the amount of electric current flowing per unit time, it can be expressed by equation (3).

[0041]

number

[0042] Substituting equation (3) into equation (2) yields equation (4).

[0043]

number

[0044] Solving the differential equation (4) under the condition that q=0 at t=0 and then differentiating it, we can find the change in current i with respect to time t, as shown in equation (5). This current i is a non-Faraday current.

[0045]

number

[0046] A qualitative illustration of equation (5) is shown in Figures 6 and 7. Figure 6 shows an example of the time variation of voltage E. Figure 7 shows an example of the time variation of current i. From Figures 6 and 7, it can be seen that a large current flows when a voltage is applied.

[0047] Next, we will explain the case where a voltage is applied to an electrochemical device using a trapezoidal wave. In this case, the voltage E gradually increases with respect to time t, so it can be expressed by equation (6).

[0048]

number

[0049] Substituting equation (6) into equation (4) yields equation (7).

[0050]

number

[0051] Solving this under the condition that q=0 when t=0, and then differentiating, we obtain equation (8).

[0052]

number

[0053] Equation (8) can be qualitatively illustrated as shown in Figures 8 and 9. Figure 8 shows an example of the time variation of voltage E. Figure 9 shows an example of the time variation of current i. As is clear from Figures 8 and 9, unlike the case of a square wave, the non-Faraday current gradually increases and eventually becomes a constant value determined by the voltage control conditions and the electrochemical device. Therefore, compared to when the voltage is applied with a square wave, a large instantaneous non-Faraday current can be suppressed by gradually increasing the absolute value of the voltage over time when reversing the polarity. In Figure 4, a trapezoidal wave in which the absolute value of the voltage increases linearly with respect to time was explained, but even when the absolute value of the voltage is increased curvilinearly, the flow of a large instantaneous non-Faraday current can be suppressed. Furthermore, if at least a part of it increases over time, for example, when the absolute value of the voltage is changed in a stepwise manner, the flow of a large instantaneous non-Faraday current can also be suppressed.

[0054] In this embodiment, if the absolute value of the voltage can be increased over time when reversing the polarity of the voltage, a period of no voltage application may be provided, for example, before decreasing or increasing the voltage over time. Figure 10 is a graph showing another example of the time variation of the voltage in the voltage control method of the embodiment. The horizontal axis of the graph represents time [seconds]. The vertical axis of the graph represents the voltage [V] applied to the electrochemical device 100. Figure 10 shows, as an example, an example in which period P3 is provided between period P1 and period P2 in Figure 4, but is not limited to this, and period P3 may be provided before period P1 or before period P2. These periods are repeated in the order of period P1, period P3, period P2 or period P2, period P3, period P1. In period P3, for example, the application of voltage is stopped from time T3 to time TX, and then the value of the voltage is changed over time from time TX to time T4. By providing period P3, it is possible to suppress the backflow of charge charged to the electrodes to the power supply.

[0055] The time it takes for the voltage to drop from 0V to below the value of voltage X during period P1, and the time it takes for the voltage to rise from 0V to above the value of voltage Y during period P2, are preferably 0.6 seconds or more, and more preferably 1 second or more, respectively. By having a time of 0.6 seconds or more, it is possible to suppress the instantaneous flow of large non-Faraday currents. The upper limit of the above time is not particularly limited, but for example, it is 60 seconds or less. The above time can be controlled, for example, by adjusting the rate of increase of the absolute value of the voltage (slope of the voltage wave) using a voltage control device.

[0056] Next, we will describe below an example of an electrochemical device that can be used to perform a method for recovering valuable materials.

[0057] (First example of an electrochemical device configuration) Figure 11 is a schematic diagram showing an example of the configuration of a PEM-type water electrolysis apparatus. The PEM-type water electrolysis apparatus includes an electrochemical cell unit 1, a power supply unit 2, an anode solution supply line 3, a cathode solution supply line 4, and a temperature controller 9.

[0058] The electrochemical cell unit 1 includes an anode chamber 11, a cathode chamber 12, a diaphragm 13, an electrode terminal 14, and an electrode terminal 15.

[0059] The anode chamber 11 has an anode 101 as shown in Figure 2. The anode 101 can, for example, oxidize water to produce hydrogen ions and oxygen. The anode 101 has an anode catalyst. Examples of anode catalysts include iridium oxide (IrO2) and ruthenium oxide (RuO2). Multiple of these catalyst materials may be included. In the case of a PEM-type water electrolysis apparatus, the anode catalyst promotes, for example, the oxidation reaction of water.

[0060] The cathode chamber 12 has a cathode 102 as shown in Figure 2. The cathode 102 can produce hydrogen, for example, by reducing hydrogen ions. The cathode 102 has a cathode catalyst. An example of a cathode catalyst is platinum (Pt).

[0061] The diaphragm 13 is provided between the anode chamber 11 and the cathode chamber 12. The diaphragm 13 separates the anode chamber 11 and the cathode chamber 12. The diaphragm 13 has an electrolyte membrane 103, as shown in Figure 2, provided between the anode 101 and the cathode 102. An example of the electrolyte membrane 103 is an electrolyte membrane containing a fluorine-based polymer material having sulfonic acid groups.

[0062] The electrode terminal 14 is electrically connected to the anode 101. The electrode terminal 14 is connected to the power supply unit 2, for example, via wiring.

[0063] The electrode terminal 15 is electrically connected to the cathode 102. The electrode terminal 15 is connected to the power supply unit 2, for example, via wiring.

[0064] The power supply unit 2 can apply voltage to the MEA via electrode terminals 14 and 15. The power supply unit 2 can periodically switch the polarity of the voltage applied to the MEA and adjust the value of each voltage. The polarity of the voltage applied to the MEA may be switched by a relay circuit, for example, provided between the power supply unit 2 and electrode terminals 14 and 15. The power supply unit 2 may have a power supply that generates voltage and a voltage control device that adjusts the value of the voltage.

[0065] The anode solution supply line 3 includes an anode channel 30, a tank 31, and a pump 32, and is configured to circulate the anode solution through the anode channel 30. The anode solution supply line 3 connects the inlet and outlet of the anode chamber 11 via the anode channel 30. The tank 31 is located in the middle of the anode channel 30 and can contain the anode solution. The pump 32 is located in the middle of the anode channel 30, for example, downstream of the tank 31, and can control the pressure in the anode channel 30 and the flow rate of the fluid flowing through the anode channel 30. When reducing oxides by supplying a reducing gas, the gas may be supplied to the anode chamber 11 via the anode channel 30 from, for example, a gas supply source (not shown) instead of the anode solution, and discharged via the anode channel 30.

[0066] The electrochemical cell unit 1 is connected to a temperature controller 9. The temperature controller 9 can heat the electrochemical cell unit 1. The temperature controller 9 includes, for example, a heater. For example, by heating the electrochemical cell unit 1 with the temperature controller 9, the oxide reduction process S1 can be performed at a temperature of, for example, 100°C or higher.

[0067] The cathode solution supply line 4 includes a cathode channel 40, a tank 41, and a pump 42, and is configured to circulate the cathode solution through the cathode channel 40. The cathode solution supply line 4 connects the inlet and outlet of the cathode chamber 12 via the cathode channel 40. The tank 41 is located in the middle of the cathode channel 40 and can contain the cathode solution. The pump 42 is located in the middle of the cathode channel 40, for example, downstream of the tank 41, and can control the pressure in the cathode channel 40 and the flow rate of the fluid flowing through the cathode channel 40.

[0068] The anode solution and cathode solution each contain at least one ionic species and chemical species capable of forming complexes with noble metals such as iridium and platinum. The anode solution and cathode solution include, for example, hydrochloric acid.

[0069] As described above, the first example of the electrochemical device configuration is a PEM-type water electrolysis apparatus, and the example of a method for recovering valuable materials can be performed without removing the MEA from the PEM-type water electrolysis apparatus. This allows for the recovery of precious metals such as platinum and iridium used in the anode catalyst and cathode catalyst in the same process. This makes it easier to recover valuable materials than, for example, recovering them after destroying and separating the PEM-type water electrolysis apparatus.

[0070] (Second example of an electrochemical device configuration) Figure 12 is a schematic diagram showing an example of the configuration of a valuable material recovery device. The valuable material recovery device 5 includes a liquid tank 50, an MEA 51, a counter electrode 52, and a reference electrode 53.

[0071] The liquid tank 50 can contain an electrolyte solution 6. The electrolyte solution 6 involves supplying a solution containing at least one ionic species and chemical species capable of forming complexes with noble metals, and bringing the solution into contact with the object. The solution includes, for example, hydrochloric acid. The liquid tank 50 has an inlet and an outlet, and the solution may be supplied to the liquid tank 50 through the inlet in the solution supply step S2, and the solution may be discharged from the liquid tank 50 through the outlet. When reducing oxides by supplying a reducing gas, the gas may be supplied through the inlet and discharged through the outlet instead of the solution.

[0072] The MEA51 is removed from other electrochemical devices and mounted and placed inside the liquid tank 50. The MEA51 is immersed in the electrolyte solution 6 contained in the liquid tank 50. Examples of other electrochemical devices include, for example, the PEM-type water electrolysis apparatus shown in Figure 11.

[0073] The counter electrode 52 is placed in the liquid tank 50. The counter electrode 52 is immersable in the electrolyte solution 6 contained in the liquid tank 50. The counter electrode 52 is made of a material that is insoluble in the electrolyte solution 6 and is conductive. Examples of the counter electrode 52 include carbon electrodes. Alternatively, a different MEA may be used as the counter electrode 52, and a voltage may be applied between the two MEAs.

[0074] The reference electrode 53 is placed in the liquid tank 50. The reference electrode 53 is immersable in the electrolyte solution 6 contained in the liquid tank 50. The reference electrode 53 is provided for measuring the voltage applied to the MEA 51. Examples of the reference electrode 53 include a standard hydrogen electrode, etc.

[0075] Each of the MEA 51, counter electrode 52, and reference electrode 53 is electrically connected to the power supply unit 7. The power supply unit 7 can apply voltage to the MEA 51 and counter electrode 52. The power supply unit 7 may be installed inside or outside the valuable material recovery device 5. The power supply unit 7 can periodically switch the polarity of the voltage applied to the anode 101 and cathode 102 of the MEA 51. The polarity of the voltage applied to the anode 101 and cathode 102 may be switched by a relay circuit, for example, provided between the power supply unit 7 and the MEA 51 and counter electrode 52. Further details of the power supply unit 7 can be found by referring to the description of the power supply unit 2 as appropriate.

[0076] MEA51 is connected to a temperature controller 9. The temperature controller 9 can heat MEA51. The temperature controller 9 may be located inside or outside the valuable material recovery device 5. The temperature controller 9 includes, for example, a heater. For example, by heating the electrochemical device with the temperature controller 9, the oxide reduction process S1 can be performed at a temperature of, for example, 100°C or higher.

[0077] As described above, the second example of the electrochemical device configuration is a dedicated valuable material recovery device for recovering valuable materials. By attaching an MEA removed from another electrochemical device and performing the example of the valuable material recovery method, precious metals such as platinum and iridium used in anode and cathode catalysts can be recovered in the same process. This allows for easy recovery of valuable materials. [Examples]

[0078] (Comparative Example 1, Example 1, Example 2) An IrO2 dissolution test was conducted. An IrO2 sample was used as the sample to be dissolved. In the oxide reduction step S1, hydrogen gas was brought into contact with the sample for 1 hour to reduce the IrO2. In the electrolytic dissolution step S2, the sample that underwent the oxide reduction step S1 was immersed in a hydrochloric acid solution, and electrolysis was performed for 3 hours while alternately reversing the polarity at two potentials: 0.1V (vs. RHE) and 1.5V (vs. RHE). Figure 13 shows the weight of Ir contained in the solution after electrolysis and the Ir dissolution rate calculated from the weight of Ir in the initial sample. For comparison, in a sample in which electrolytic dissolution step S2 was performed without oxide reduction step S1 (Comparative Example 1), the Ir dissolution rate was 0.3 wt%. In a sample in which oxide reduction step S1 was performed at room temperature (Example 1), the Ir dissolution rate increased slightly to 2.0 wt%, and in a sample in which oxide reduction step S1 was performed at 150°C (Example 2), the Ir dissolution rate reached over 99 wt%. It is known that the hydrogen reduction of IrO2 proceeds at a significant rate above approximately 100°C, so the higher IrO2 solubility in the sample subjected to hydrogen reduction at 150°C is a reasonable result.

[0079] Although several embodiments of the present invention have been described above, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. [Explanation of Symbols]

[0080] 1... Electrochemical cell unit, 2... Power supply unit, 3... Anode solution supply line, 4... Cathode solution supply line, 5... Valuable material recovery device, 6... Electrolyte solution, 7... Power supply unit, 9... Temperature controller, 11... Anode chamber, 12... Cathode chamber, 13... Diaphragm, 14... Electrode terminals, 15... Electrode terminals, 30... Anode channel, 31... Tank, 32... Pump, 40... Cathode channel, 41... Tank, 42... Pump, 50... Liquid tank, 52... Counter electrode, 53... Reference electrode, 100... Electrochemical device, 101... Anode, 102... Cathode, 103... Electrolyte membrane.

Claims

1. A method for recovering valuable materials from electrochemical devices, The electrochemical device has a catalyst made of a noble metal oxide, The aforementioned method, A step of reducing the oxide while heating the object having the catalyst, A step of dissolving the reduced noble metal as ions in the solution by applying a voltage to an object in contact with a solution containing at least one selected from the group consisting of ionic species and chemical species that can form complexes with the noble metal, and periodically reversing the polarity of the voltage by increasing the absolute value of the voltage over time and then holding it, thereby performing an electrolytic reaction, Equipped with, Methods for recovering valuable materials.

2. The step of reducing the aforementioned oxide is: A first method of supplying a solution containing a reducing agent and bringing the reducing agent into contact with the object, A second method involves supplying a reducing gas and bringing the object into contact with the reducing gas, or A third method involves bringing an electrolyte solution into contact with the object, applying a voltage to the object, and carrying out an electrolytic reaction without reversing the polarity of the voltage. A method for recovering valuable materials according to claim 1, carried out by the above.

3. The aforementioned object is An electrode having the catalyst, Solid polymer electrolyte membrane, Having, The method for recovering valuable materials according to claim 1.

4. The step of reducing the aforementioned oxide is carried out at a temperature of 100°C or higher. The method for recovering valuable materials according to claim 1.

5. The process of dissolving the aforementioned precious metal is as follows: A first period during which the voltage value is linearly or curvilinearly decreased from a first value to a second value lower than the first value and then held thereafter, A second period during which the voltage value is increased linearly or curvilinearly from a third value that is the same as or different from the first value to a fourth value or higher that is higher than the third value, and then held thereafter. Repeat alternately. The method for recovering valuable materials according to claim 1.

6. During the first period, the time it takes for the voltage value to change from the first value to the second value is 0.6 seconds or more. During the second period, the time it takes for the voltage value to change from the third value to the fourth value is 0.6 seconds or more. The method for recovering valuable materials according to claim 5.

7. The catalyst comprises at least one precious metal selected from the group consisting of platinum, iridium, ruthenium, rhodium, palladium, platinum, gold, and rhenium. The method for recovering valuable materials according to claim 1.

Images