Palladium recovery method

Abstract

The present invention addresses the problem of providing a method for efficiently recovering palladium from a palladium-containing material. [Solution] A method for recovering palladium from a palladium-containing material, comprising: a step of dissolving the palladium-containing material in a nitric acid-containing solution to obtain a palladium nitrate solution; a step of passing the palladium nitrate solution through a resin to adsorb palladium onto the resin; and a step of calcining the resin with the adsorbed palladium to recover palladium, wherein acidic gases remaining in the resin layer during the step of passing the solution are vented.

Description

[Technical Field]

[0001] The present disclosure relates to a method for recovering palladium from a palladium-containing material. [Background technology]

[0002] Palladium, a precious metal, is widely used as an essential material in a wide range of industrial fields, including automobile exhaust purification catalysts, electronic components, and gas detection tubes, due to its excellent catalytic properties and stability. Due to its scarcity, market prices have been rising, and stable supply has become an issue. In gas detection tubes, palladium deposits react with specific target gases, enabling highly accurate measurements. However, currently, many used gas detection tubes that have outlived their useful life or are scheduled for disposal at the end of their product life are simply disposed of as waste.

[0003] Regarding the recovery of palladium, for example, Patent Document 1 discloses a method for recovering palladium from a palladium-containing waste liquid containing inorganic sulfur oxides. Specifically, it describes that palladium can be recovered at a high recovery rate by adjusting the pH of the waste liquid and adding a reducing agent and a cationic surfactant to reduce, coagulate, and precipitate the palladium. Furthermore, Patent Document 2 discloses a method in which, since precipitation separation methods from waste liquid containing low concentrations of palladium colloids are not cost-effective, the palladium-containing liquid is oxidized to destroy the colloids, eluting the palladium, and the palladium is selectively adsorbed and recovered on a metal adsorbent. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2015 / 083233 [Patent Document 2] Japanese Patent Application Publication No. 2022-159597 Summary of the Invention [Problem to be solved by the invention]

[0005] At present, it is difficult to say that a technology has been established for efficiently extracting and purifying palladium at high purity from palladium-containing materials contained in gas detection tubes and the like while minimizing the environmental impact, and recovering the palladium as a reusable resource. Therefore, in building a resource circulation system that contributes to a stable supply of palladium, there is a problem that resources are not being used effectively enough. In view of these circumstances, an object of the present disclosure is to provide a method for efficiently recovering palladium from palladium-containing materials that contain palladium.

[0006] The palladium-containing material in the present disclosure is not limited to that derived from gas detection tubes, and the present disclosure can also be applied to recovery from various other forms of palladium-containing material. [Means for solving the problem]

[0007] The gist of the present disclosure is as follows. [1] A method for recovering palladium from a palladium-containing material, comprising: a step of dissolving the palladium-containing material in a nitric acid-containing solution to obtain a palladium nitrate solution; a step of passing the palladium nitrate solution through a resin to adsorb palladium onto the resin; and a step of calcining the resin with the adsorbed palladium to recover palladium, wherein acidic gas remaining in the resin layer is vented during the step of passing the solution. [2] The method for recovering palladium according to [1], wherein in the passing step, the palladium nitrate solution is passed upward through the resin. [3] After passing the palladium nitrate solution through the resin, the palladium is circulated. The method for recovering palladium according to [1] or [2], wherein the palladium nitrate solution is passed through the resin again. [4] The method for recovering palladium according to any one of [1] to [3], wherein the palladium-containing material contains one or more of potassium palladium sulfite, palladium sulfate, and palladium powder. [5] The method for recovering palladium according to any one of [1] to [4], wherein the palladium-containing substance is a palladium deposit contained in a gas detection tube. [6] The method for recovering palladium according to [2], wherein in the passing step, the palladium nitrate solution is passed through the resin at a linear velocity LV of 3.4 to 17.0 m / h. [7] The method for recovering palladium according to any one of [1] to [6], wherein the resin is a chelating resin. [Effects of the Invention]

[0008] According to the present disclosure, a method for efficiently recovering palladium from a palladium-containing material can be provided. [Brief explanation of the drawings]

[0009] [Figure 1] 1 shows a schematic diagram of a palladium recovery apparatus used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] Specific embodiments are shown below, but each configuration and their combinations are merely examples, and modifications such as addition, omission, and substitution are possible as appropriate within the scope of the gist of this disclosure. The present disclosure is not limited to the embodiments. Note that each aspect disclosed in this specification can be arbitrarily combined with other features.

[0011] (1. Elution process) In an embodiment of the present disclosure (hereinafter referred to as the present embodiment), a palladium-containing material containing palladium is first dissolved in a nitric acid-containing solution to obtain a palladium nitrate solution. The purpose of this step is to dissolve palladium into a liquid phase and prepare a palladium nitrate solution from a solid or powdery palladium-containing material in order to efficiently separate and recover palladium in a subsequent adsorption step.

[0012] "Palladium-containing materials" is a general term for substances that contain palladium as a constituent, and includes cases in which palladium exists as a specific compound, as a metal (pure palladium), or supported on other substances. Furthermore, palladium-containing materials are not limited to palladium deposits contained in gas detection tubes that have been used or are to be discarded, but may also include other palladium-added catalysts, electronic components, and other substances used in a wide variety of other applications.

[0013] Palladium compounds include inorganic and organic palladium compounds. Inorganic palladium compounds include potassium palladium sulfite, palladium sulfate, palladium chloride, palladium oxide, and palladium nitrate. Organic palladium compounds include palladium acetate and palladium tetrakis. Palladium supports include palladium powder (fine powder of metallic palladium), palladium sponge, palladium foil, wire, and alloys containing palladium as a main component (e.g., palladium-silver alloys). Palladium supports and composite materials include catalysts, electronic components, materials recovered from plating waste solution, palladium-containing components contained in gas detection tubes, and other materials in which palladium is mixed or composited with other substances, in which palladium compounds or metallic palladium are supported on supports such as alumina, carbon, silica, and zeolite. These materials may be subjected to pretreatment such as pulverization, washing, and drying, as necessary.

[0014] The nitric acid-containing solution is used because nitric acid has excellent oxidizing power for dissolving precious metals such as palladium. A nitric acid aqueous solution can be used as the nitric acid-containing solution. When dissolving palladium while suppressing the dissolution of impurities other than palladium, or when slowing the reaction rate, dilute nitric acid can be used. Medium-concentration nitric acid or concentrated nitric acid can be used to promote the dissolution of palladium. A mixed solution of nitric acid and another acid can also be used. For example, aqua regia with a ratio (volume ratio) of concentrated nitric acid to concentrated hydrochloric acid of 1:3, or a mixed solution of nitric acid and sulfuric acid can be used. It is preferable to select these solutions taking into consideration the type of palladium-containing material, the desired dissolution rate, the impact on subsequent processes, cost, safety, etc.

[0015] The concentration of nitric acid is adjusted appropriately depending on the type and amount of palladium-containing material and the dissolution temperature. For example, a wide range of concentrations can be selected, from dilute to concentrated nitric acid. However, excessively high concentrations of nitric acid may increase the load on subsequent processes and the environmental load, so a concentration of approximately 5 to 60 mass % is preferably selected. The dissolution temperature can be set from room temperature to near the boiling point, and the dissolution rate can be increased by heating. The dissolution time depends on the properties of the raw materials and the dissolution conditions, but it usually takes several hours to half a day for sufficient dissolution.

[0016] In the dissolution reaction, palladium reacts with nitric acid and dissolves in the solution as palladium nitrate (Pd(NO3)2). At this time, there is a possibility that other impurity metals may also dissolve at the same time. Therefore, it is preferable to remove insoluble components and impurity residues from the solution by performing a solid-liquid separation operation such as filtration as necessary, and extract a clear palladium nitrate solution.

[0017] (2. Liquid passing process) In this embodiment, a palladium nitrate solution is passed through a resin to adsorb palladium onto the resin. The purpose of this process is to selectively separate and concentrate palladium from the palladium nitrate solution obtained in the previous process. This allows for efficient separation of palladium from other metal ions and impurities coexisting in the solution, enabling high-purity recovery in the subsequent process. Ion exchange resins with high selective adsorption capacity for palladium ions, particularly chelating resins and anion exchange resins, are preferably used as the resin. Examples of chelating resins include resins with isothiouronium groups, thiol groups, etc. These resins are typically packed in a column as spherical or granular particles, and the palladium nitrate solution is passed through them.

[0018] The liquid flow rate and flow rate can be optimized depending on the resin performance, column size, palladium concentration, and coexisting impurities. A liquid flow rate that is too slow will reduce the treatment efficiency, while a rate that is too fast may result in a decrease in adsorption efficiency. In addition, since a high nitric acid concentration leads to significant gas generation, as described below, it is preferable to adjust the nitric acid concentration to approximately 10 to 30 mass%. In this concentration range, the pH will be less than 1. Palladium ions are selectively adsorbed by binding to the resin, and palladium is removed from the treated solution (effluent).

[0019] During the liquid-passing step, gases such as nitrogen oxides (NOx) are generated by the decomposition of nitric acid contained in the palladium nitrate solution from the previous step and by the reaction between the resin and components in the solution. These gases cause an increase in pressure within the column, leading to problems such as leakage from the container or rupture of the container. Furthermore, gas bubbles can accumulate in the resin layer, increasing the resistance to liquid passage and hindering uniformity of liquid passage, ultimately reducing the adsorption efficiency of palladium. Furthermore, acidic gases are harmful to the health of workers and can corrode equipment, so appropriate treatment is essential.

[0020] Therefore, in this embodiment, a gas exhaust mechanism is provided to exhaust the generated gas sufficiently. In this embodiment, exhaust means to discharge these acidic gases to the outside of the system. As a specific exhaust method, an adsorption device (such as a vacuum pump) is installed from the top or side of the column. However, natural exhaust may be used without using an adsorption device.

[0021] Furthermore, rather than releasing the exhausted gas directly into the atmosphere, it is preferable to detoxify it using a scrubber (for example, washing with an alkaline aqueous solution) or a catalytic detoxification device, etc., in order to comply with environmental regulations. This makes it possible to maintain stable operation of the adsorption process and ensure a safe working environment.

[0022] In this embodiment, when passing the palladium nitrate aqueous solution through the resin, it is preferable to pass the solution upward (upward flow). Upward flow refers to a method in which the chemical solution is supplied from the bottom of the column and flows toward the top. In upward flow, the liquid pushes up the resin particles, which tends to expand or uniformly pressurize the entire resin bed. The liquid flows uniformly throughout the resin bed without being separated, thereby suppressing short-path (uneven flow path). This maximizes the contact efficiency between the liquid and the resin and improves the adsorption efficiency of palladium. Furthermore, in upward flow, the liquid acts to float the resin, so the pressure loss in the resin bed tends to be smaller than in downward flow. This reduces the pump load and enables processing in a larger column.

[0023] Of particular importance is that this embodiment uses a nitric acid-containing solution, which generates acidic gases such as nitrogen oxides (NOx) due to dissolution reactions and decomposition of components in the solution. In a downward flow system, the generated gas must rise against the liquid flow, which tends to trap it as bubbles within the resin bed, hindering the uniform flow of the liquid and reducing the effective surface area of ​​the resin. By using an upward flow system, the liquid flows from bottom to top, so the generated gas is more likely to be naturally released to the top of the column along with the liquid flow. This prevents gas from impeding liquid flow or reducing adsorption efficiency, enabling stable, continuous operation.

[0024] The liquid passing step using an upward flow can be carried out in the following manner. For example, there is the expanded bed method, in which the entire resin layer is slightly expanded (floated) by adjusting the liquid flow rate, achieving uniform liquid distribution and low pressure loss. With this method, the resin particles move relative to each other to a certain extent, making clogging less likely and allowing the generated gas to be quickly discharged. In this case, a screen or freeboard (space) must be installed at the top of the column to prevent the resin from leaking out. On the other hand, there is the fixed bed method, in which the resin is packed into a column and a fixing device is installed at the top to prevent the resin from floating up, and the liquid can be passed through in an upward direction. In this case, treatment at a relatively high flow rate is possible, but consideration must be given to gas retention and pressure loss.

[0025] In the case of an upward flow, if the flow rate is too fast, the resin may flow out of the column and the bed shape may collapse. It is important to control the flow rate appropriately depending on the density and particle size of the resin and the design of the column. When using an expanded bed, it is preferable to adjust the flow rate so that the optimal expansion rate (e.g., 20 to 50%) is achieved. Specifically, it is preferable to select a range of approximately 3.4 to 17.0 m / h in terms of linear velocity (LV) value.

[0026] Linear velocity (LV) is an index that indicates the speed at which a fluid passes through a flow path, and indicates the volumetric flow rate per unit cross-sectional area, and is calculated using the following formula: Calculation formula: LV(m / h)=Q / A=SV×H Q: Volumetric flow rate (amount of fluid flowing per unit time) A: Cross-sectional area of ​​the flow path SV: Space velocity (ratio of fluid volumetric flow rate (1 / h) to the effective volume of resin) H: Resin layer height

[0027] The linear velocity is preferably about 6.0 to 15.0 m / h, and even more preferably about 8.5 to 11.5 m / h. Because the use of an upflow system tends to cause gas to collect at the top of the column, it is important to appropriately design the exhaust port at the top of the column and provide a line (connected to a scrubber or detoxification device) for efficiently venting the generated gas outside the system. In addition to continuous exhaust, intermittent gas venting is also effective as needed. It is desirable to select these variations by comprehensively considering factors such as the processing volume, palladium concentration, types of coexisting impurities, safety, and facility utilization efficiency.

[0028] After passing the palladium nitrate solution through the resin, it is desirable to pass the effluent, from which the palladium has been removed, through the resin again and recirculate it (a recirculation process). The adsorption reaction is an equilibrium process, and it is difficult to completely eliminate palladium in the solution in a single pass. Trace amounts of palladium may remain in the wastewater, especially when the palladium concentration is low or when the resin's adsorption capacity is approaching saturation. Recirculation substantially extends the contact time between the solution and the resin, further shifting the adsorption equilibrium toward palladium adsorption on the resin. This minimizes the palladium concentration in the effluent and significantly improves the overall palladium recovery rate. Furthermore, the recirculation process significantly reduces the cost and effort required for treating the final effluent. Furthermore, reusing the nitric acid solution reduces emissions from the system and saves on the amount of solution required for new supply.

[0029] On the other hand, while palladium is adsorbed by the resin, other metal ions and nitric acid present in the solution are not. Consequently, these become concentrated in the solution as the recycle process continues. Therefore, depending on the type of impurity, it may inhibit palladium adsorption, significantly change the solution pH, or cause a decrease in the final purity of palladium. Therefore, it is necessary to regularly monitor the type and concentration of impurities and, if necessary, drain a portion of the solution and replace it with fresh solution. In particular, the adsorption efficiency of palladium depends on the concentration of nitric acid, and fluctuations in the nitric acid concentration of the recirculating solution may decrease the adsorption efficiency. Therefore, monitoring to maintain an appropriate nitric acid concentration and adjusting the concentration as necessary are necessary. Selective adsorption and recovery of palladium while managing the concentration and effects of not only palladium but also other coexisting non-adsorbed impurities is a complex aspect not seen in single-component adsorption, and it is preferable to design the process taking into account the type and concentration of impurities.

[0030] (3. Firing process) In this embodiment, palladium is recovered by calcining the resin on which palladium is adsorbed. This process separates and removes resin components from the resin, and allows the adsorbed palladium to be separated and recovered as metallic palladium with high purity. The resin on which palladium is adsorbed is a resin in which palladium is adsorbed after a liquid-passing process. If necessary, the adsorbed resin is washed with water to remove any acidic solution or other impurities adhering to the surface before calcination. Calcination is typically performed using a heating device such as an electric furnace. The calcination temperature is adjusted depending on the type of resin and the form of palladium recovery, but is generally selected to be a temperature at which the resin is completely decomposed and the palladium is reduced to metal, for example, in the range of approximately 400°C to 1000°C. While calcination in air is generally used to promote resin decomposition, subsequent reduction treatment in a reducing atmosphere (e.g., hydrogen, CO) or an inert gas atmosphere (e.g., nitrogen, argon) may be desirable to obtain high-purity metallic palladium. The firing time varies depending on the temperature and size of the furnace, but is usually about several hours.

[0031] By firing, the resin is carbonized, decomposed, and volatilized, and is eventually recovered in the form of metallic palladium or palladium oxide. If the recovered material is of high purity, it can be reused as is. However, if impurities remain or if even higher purity is required, it is possible to combine additional refining steps such as dissolution, electrolytic refining, and hydrometallurgy. [Example]

[0032] Next, examples and comparative examples of the present disclosure will be described. Note that the following examples are merely representative examples, and the present invention is not necessarily limited to these examples, but should be interpreted within the scope of the technical ideas described in the specification.

[0033] Palladium was recovered from a palladium nitrate solution using a palladium recovery apparatus (schematic diagram) shown in Figure 1. Palladium can be finally recovered by calcining the resin that adsorbs palladium, but to avoid complicating the explanation, the calcination step is not described in Figure 1.

[0034] Palladium-containing material was placed in a tank, and 60% by mass nitric acid was added to obtain a palladium nitrate solution. The resulting palladium nitrate solution was diluted with water to 30% by mass and stored in stock tank 1. Next, using circulation pump 2, the solution was supplied from stock tank 1 to transparent resin column 3 in an upward flow from bottom to top. Palladium adsorption resin 4 was used as the filler for resin column 3, and palladium in the palladium nitrate solution flowing upward was captured and adsorbed onto the resin. The solution was circulated within the apparatus at a linear flow rate of 10.2 L / m. Acidic gases generated during handling of the nitric acid solution were discharged outside the system as exhaust gas 5.

[0035] After circulating the solution for about 60 minutes, the color of the solution changed from orange to yellow to colorless. When the solution was analyzed by ICP-OES, the initial palladium concentration was about 147 mg / L, but after about 60 minutes, almost no palladium was detected, suggesting that it was adsorbed onto the resin. Therefore, the treated solution was discharged outside the system as wastewater 6 after palladium recovery. The amount of palladium added in this example was calculated to be 5040 mg from the concentration of the palladium nitrate solution. After that, the resin was calcined to recover palladium, and 4000 mg of palladium was recovered. [Industrial Applicability]

[0036] The present disclosure provides a method for efficiently recovering palladium from palladium-containing materials, and is particularly useful as a technique for recovering palladium from palladium deposits contained in automobile exhaust gas purification catalysts, electronic components, gas detection tubes, and the like.

Claims

1. A method for recovering palladium from a palladium-containing material, comprising the steps of: dissolving the palladium-containing material in a nitric acid-containing solution to obtain a palladium nitrate solution; a step of passing the palladium nitrate solution through a resin to adsorb palladium onto the resin; and a step of calcining the resin on which palladium has been adsorbed to recover the palladium, In the liquid passing step, the palladium nitrate solution is passed upward through the resin at a linear velocity LV of 3.4 to 17.0 m / h, and gas remaining in the resin layer is exhausted.

2. 2. The method for recovering palladium according to claim 1, wherein the palladium nitrate solution after passing through the resin is circulated, and the palladium nitrate solution containing residual palladium is passed through the resin again.

3. 3. The method for recovering palladium according to claim 1, wherein the palladium-containing material comprises one or more of potassium palladium sulfite, palladium sulfate, and palladium powder.

4. 3. The method for recovering palladium according to claim 1, wherein the palladium-containing substance is a palladium deposit contained in a gas detection tube.

5. 3. The method for recovering palladium according to claim 1, wherein the resin is a chelating resin.

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