Method for manufacturing recycled palladium

Abstract

To provide a method that uses waste liquid containing inorganic sulfur oxide as well as palladium as resource liquid, capable of yielding reduced metal palladium in high yield.SOLUTION: A method for producing recycled palladium includes a reduction step for adding a reductant to a resource liquid that contains inorganic sulfur oxide and a palladium compound and has a pH of 2.5 or more, thereby reducing the palladium compound.SELECTED DRAWING: None

Description

TECHNICAL FIELD

[0001] The present invention relates to a palladium recycling technology. BACKGROUND ART

[0002] Palladium is a platinum group metal used in organic synthesis catalysts for compounds or raw materials thereof used in chemical products (for example, agricultural chemicals, pharmaceuticals, fragrances, dyes, and the like), automotive exhaust catalysts, electronic components, alloys for dental silver fillings, jewelry, and the like. Due to its low output and uneven distribution of production areas, palladium is always in short supply, so the need for its recycling is increasing more and more.

[0003] Several techniques for recovering palladium have been proposed. For example, Patent Document 1 discloses a palladium recovery method characterized by bringing a palladium-containing liquid into contact with a cation exchanger for the purpose of efficiently recovering palladium from waste liquids such as palladium plating washing wastewater. Patent Document 2 discloses a method for recovering palladium from a palladium-containing solution, which is characterized in that, for the purpose of selectively recovering only palladium in high yield from a colloidal mixed solution of stannous chloride and palladium chloride containing ions such as copper, nickel and iron, after oxidizing the palladium-containing solution, the solution is brought into contact with a chelate resin having a thiosemicarbazide residue and / or a chelate resin having a thiouronium residue. PRIOR ART DOCUMENTS PATENT DOCUMENTS

[0004] Patent Document 1 Japanese Unexamined Patent Publication No. 2000-192162 Patent Document 2 Japanese Unexamined Patent Publication No. 2004-83926 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] Palladium wastewater used as an organic synthesis catalyst often contains inorganic sulfur oxides used as reaction stoppers. Furthermore, efficiently reducing palladium wastewater containing inorganic sulfur oxides to produce metallic palladium is particularly difficult, and conventional palladium reduction methods cannot produce metallic palladium in good yield.

[0006] Therefore, the present invention aims to provide a method for producing reduced metallic palladium in high yield using waste liquid containing inorganic sulfur oxides together with palladium compounds as a resource liquid. [Means for solving the problem]

[0007] As a result of diligent research, the inventors of this invention discovered that metallic palladium can be produced in high yield by adjusting the pH of a resource solution containing inorganic sulfur oxides together with a palladium compound to 2.5 or higher, and then reducing the palladium with a reducing agent. This invention was completed by further research based on this finding.

[0008] In other words, the present invention provides inventions in the following embodiments. Item 1. A method for producing recycled palladium, comprising a reduction step of adding a reducing agent to a resource solution containing inorganic sulfur oxides and palladium compounds with a pH of 2.5 or higher, thereby reducing the palladium compounds. Item 2. The manufacturing method according to Item 1, wherein the pH of the resource liquid is 3.5 or less. Item 3. The manufacturing method according to item 1 or 2, wherein the reducing agent is a metal hydride. Item 4. The manufacturing method according to Item 3, wherein the metal hydride is used in an amount equivalent to 30 to 100 times the theoretical amount required to reduce the palladium compound to metallic palladium. Item 5. The manufacturing method according to item 1 or 2, wherein the reducing agent is a metal. Item 6. The manufacturing method according to Item 5, wherein the metal is used in an amount equivalent to 30 to 100 times the theoretical amount required to reduce the palladium compound to metallic palladium. Item 7. A manufacturing method according to item 5 or 6, which does not involve a step of agglomerating the reduced metallic palladium. [Effects of the Invention]

[0009] The present invention provides a method for producing reduced metallic palladium in high yield using waste liquid containing inorganic sulfur oxides together with palladium compounds as a resource liquid. [Modes for carrying out the invention]

[0010] The present invention relates to a method for producing recycled palladium, characterized by including a reduction step in which a reducing agent is added to a resource solution with a pH of 2.5 or higher containing inorganic sulfur oxides and palladium compounds, thereby reducing the palladium compounds. The production method of the present invention will be described in detail below.

[0011] 1. Reduction process 1-1. Resource liquid containing inorganic sulfur oxides and palladium compounds with a pH of 2.5 or higher In the present invention, the resource liquid containing inorganic sulfur oxides and palladium compounds with a pH of 2.5 or higher is not particularly limited as long as it satisfies these constituent components and pH requirements. A typical example is wastewater from an organic synthesis reaction using a palladium catalyst, which may have its pH adjusted to within the specified range as needed.

[0012] Organic synthesis reactions using palladium catalysts are typified by cross-coupling reactions, specifically including the Murahashi coupling reaction, Negishi coupling reaction, Kosugi-Migita-Stille coupling reaction, Suzuki coupling reaction, and Hiyama coupling reaction.

[0013] The palladium compounds in the resource solution are not particularly limited as long as the palladium element is in an oxidized state. Typical examples of palladium compounds in the resource solution include used palladium catalysts used in the cross-coupling reaction described above. More specific examples of palladium compounds in the resource solution include divalent palladium compounds such as palladium acetate, palladium chloride, palladium iodide, palladium nitrate, palladium oxide, and dinitrodiamminepalladium; palladium complexes with triphenylphosphine, tri-t-butylphosphine, acetonitrile, benzonitrile, etc., coordinated as ligands; and palladium activated carbon. These forms of palladium may be present in the resource solution individually or in combination of multiple types.

[0014] The content of palladium compounds in the resource solution is not particularly limited, but an example of 50 to 5000 mg / L in terms of palladium elemental equivalent is given.

[0015] The inorganic sulfur oxides are not particularly limited. Typical examples of inorganic sulfur oxides include used inorganic sulfur oxides used as reaction termination agents in the cross-coupling reaction described above. More specific examples of inorganic sulfur oxides include sulfates, thiosulfates, dithionites, dithionates, sulfites, and bisulfites. These salts can be salts of any metal other than palladium, specifically alkali metal salts such as potassium salts and sodium salts; and alkaline earth metal salts such as calcium salts. The resource solution may contain one of these inorganic sulfur oxides alone or in combination of multiple types.

[0016] The inorganic sulfur oxide content in the resource liquid is not particularly limited, but an example of the amount of inorganic sulfur oxide in terms of sulfur element is 10 to 25 g / L.

[0017] When adjusting the pH of a waste liquid from an organic synthesis reaction using a palladium catalyst, the pH is not particularly limited as long as it falls within the above range. From the perspective of improving the production rate of metallic palladium, the pH is preferably 3.5 or less, more preferably 3 or less, still more preferably 2.7 or less, and even more preferably 2.6 or less. In the present invention, pH refers to the pH at 25°C.

[0018] Further, regarding the pH of the waste liquid from an organic synthesis reaction using a palladium catalyst that has not been subjected to pH adjustment treatment, for example, the pH is, for example, 4 to 6, preferably about 5 to 6. Therefore, in order to adjust the pH to the above-mentioned preferable pH (specifically, pH 2.5 or more and pH 3.5 or less), an acid may be added to the waste liquid as a pH adjuster. Specific examples of the acid are not particularly limited, and include, for example, inorganic acids such as hydrochloric acid, hydrofluoric acid, sulfuric acid, and carbonic acid; organic acids such as citric acid, acetic acid, propionic acid, oxalic acid, and adipic acid. One of these acids may be used alone, or two or more thereof may be used in combination.

[0019] As long as the effects of the present invention are achieved, the resource liquid may contain any other components in addition to the above components. Examples of the other components include various products generated in organic synthesis reactions using a palladium catalyst, and specifically include organic substances, metal halides (excluding palladium salts), and the like. Specific examples of the organic substances include aprotic polar solvents used in cross-coupling reactions: N,N-dimethylformamide (DMF), N-methylformamide, N,N-dimethylacetamide (DMA), N-methylacetamide, 1,3-dimethyl-2-imidazolidinone (DMI), N-methyl-2-pyrrolidone (NMP), and hexamethylphosphoric triamide (HMPA). One of these other components may be contained alone, or a combination of two or more thereof may be contained.

[0020] The content of other components is not particularly limited, but the content of organic substances is, for example, 10 to 35% by weight, preferably 15 to 25% by weight.

[0021] 1-2. Reducing agent The reducing agent is not particularly limited as long as it is a substance capable of reducing a palladium compound. Examples include metal hydrides, metals, hydrazine compounds, hydrogen gas, alcohols, and the like. Examples of the metal hydride include sodium borohydride, lithium borohydride, lithium aluminum hydride, sodium aluminum hydride, and the like. Examples of the metal include single metals such as iron, zinc, aluminum, and magnesium, as well as alloys obtained by combining two or more of these metal elements. Examples of the hydrazine compound include hydrazine, hydrazine sulfate, and the like.

[0022] One of these reducing agents may be used alone, or two or more thereof may be used in combination. Among these reducing agents, metal hydrides and metals are preferable. Further, from the viewpoint of significantly improving the aggregability of reduced palladium and increasing the recovery rate of metallic palladium, metals are more preferable, and iron and zinc are even more preferable.

[0023] The particle size of the metal is not particularly limited, but examples include 0.5 to 400 μm. When iron is used as the metal, the particle size can be as follows: For D10, for example, 10 to 200 μm, preferably 20 to 150 μm, more preferably 50 to 120 μm, even more preferably 80 to 110 μm, and even more preferably 90 to 100 μm; for D50, for example, 30 to 350 μm, preferably 50 to 300 μm, more preferably 120 to 250 μm, and even more preferably 150 to 200 μm; and for D90, for example, 50 to 400 μm, preferably 100 to 350 μm, more preferably 200 to 330 μm, and even more preferably 250 to 280 μm. When zinc is used as the metal, the particle sizes include, for example, D10, 0.5 to 8 μm, preferably 1 to 4 μm, and more preferably 23.5 μm; for example, D50, 1 to 10 μm, preferably 2.5 to 7 μm, and more preferably 4 to 6 μm; and for example, D90, 3 to 20 μm, preferably 5 to 15 μm, and more preferably 7 to 10 μm. Note that D10 refers to the particle size at which the cumulative frequency by volume is 10%, obtained by laser diffraction scattering particle size distribution measurement; D50 refers to the particle size at which the cumulative frequency by volume is 50%, obtained by laser diffraction scattering particle size distribution measurement; and D90 refers to the particle size at which the cumulative frequency by volume is 90%, obtained by laser diffraction scattering particle size distribution measurement.

[0024] The particle size of the metal is preferably 50 to 300 μm, and more preferably 75 to 300 μm, in terms of sieve diameter.

[0025] The amount of reducing agent used can be set according to the content of the palladium compound. Specific examples of the amount of reducing agent used include 30 to 100 times the theoretical amount (palladium reduction equivalent) required to reduce the palladium compound to metallic palladium, preferably 60 to 100 times the theoretical amount, more preferably 80 to 100 times, and even more preferably 90 to 100 times.

[0026] 1-3. Redemption Conditions The reduction conditions are not particularly limited as long as the reduction reaction of the palladium compound to metallic palladium proceeds. Examples of temperature conditions include 10-35°C, preferably 20-30°C. Time conditions may vary depending on the reaction scale, but examples include 1-6 hours, preferably 1.5-4 hours.

[0027] 2. Other processes In addition to the reduction step described above, the manufacturing method of the present invention may include any other steps for recovering the reduced metallic palladium.

[0028] Other processes include a coagulation process for the reduced metallic palladium, a solid-liquid separation process, and a washing process.

[0029] The coagulation process can be performed after the reduction process and before the recovery process. In the coagulation process, a coagulant is added to the resource liquid after the reduction process to coagulate the reduced metallic palladium, making it easier to recover. The coagulant is not particularly limited, and examples include cationic surfactants. Cationic surfactants include cationic polyacrylamide, cationic polyacrylic acid ester, cationic polymethacrylate ester, alkyltrimethylammonium salt, dialkyldimethylammonium salt, alkylbenzyldimethylammonium salt, etc. Commercially available wastewater coagulants can also be used as coagulants. These coagulants may be used individually or in combination of multiple types.

[0030] The amount of coagulant added can be such that the final concentration (the amount of coagulant relative to the total amount of the resource liquid and coagulant after the reduction process) is, for example, 0.5 to 2500 mg / L. Alternatively, the amount of coagulant added can be such that, relative to 100 parts by weight of palladium element in the resource liquid, it is, for example, 0.05 to 250 parts by weight.

[0031] In the present invention, when a metal is selected as the reducing agent used in the reduction step, the coagulation of the reduced metallic palladium is significantly improved, making it possible to eliminate the coagulation step or further reduce the amount of coagulant used (specifically, using an amount that results in a concentration of more than 0 mg / L but less than 0.5 mg / L in the resource liquid after the reduction step, or an amount that results in more than 0 parts by weight but less than 0.05 parts by weight per 100 parts by weight of palladium element in the resource liquid).

[0032] In the solid-liquid separation step, the metallic palladium reduced in the reduction step or the metallic palladium agglomerated in the agglomeration step can be separated from the liquid. Any known solid-liquid separation method may be used as appropriate. Specific examples of solid-liquid separation methods include filtration, centrifugation, and decantation, with filtration being preferred. Suction filtration is preferred for filtration. The particle retention capacity of the filter paper or filter used for filtration can be, for example, 0.3 to 2 μm, preferably 0.5 to 1.5 μm, and more preferably 0.8 to 1.2 μm.

[0033] In the washing process, the separated metallic palladium can be washed using a suitable washing solution. [Examples]

[0034] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0035] Aqueous solutions (waste liquid) discharged from a manufacturing process using palladium as a catalyst for organic reactions were used as resource liquids. Palladium compounds were reduced with a reducing agent according to the method described below, and palladium in the resource liquid was recovered. As reducing agents, iron powder or zinc powder as shown in Table 1, or an aqueous solution of sodium borohydride were prepared, and the resource liquids were prepared with the compositions shown in Table 2. The resource liquids contained at least a compound considered to be palladium acetate as a palladium compound, mainly sodium sulfite as an inorganic sulfur oxide, and mainly dimethylformamide as an organic substance. The volume of the resource liquid used in the test was 0.3 L in all cases. Sulfuric acid was added dropwise to the resource liquid to adjust the pH to the level shown in Table 2. The pH-adjusted resource liquid was thoroughly stirred at a liquid temperature of 25°C, and the reducing agent shown in Table 2 was added and reacted for 2 hours to reduce the palladium compounds to metallic palladium (reduction step). In Example 2, a flocculant containing a cationic surfactant was further added as a flocculant after the reduction step.

[0036] For the reducing agent, in Examples 1 and 2, a commercially available VenPure(TM) solution (an aqueous solution prepared by dissolving 12% sodium borohydride in a 40% sodium hydroxide solution) was used, diluted five times with water to form a sodium borohydride aqueous solution. For Examples 3 to 6, commercially available iron powder or zinc powder as shown in Table 1 was used. Specifically, in Example 3, 200 mesh (average 75 μm) iron powder 1 was used; in Example 4, 50 mesh (average 300 μm) iron powder was sieved through a 300 μm sieve, and the sieved iron powder below that was sieved through a 75 μm sieve (i.e., 75-300 μm iron powder 2) was used; and in Example 5, 50 mesh (average 300 μm) iron powder 3 was used. In Example 6, commercially available zinc powder 1 was used. The particle sizes of each iron powder or zinc powder were measured using the laser diffraction scattering particle size distribution method, and the values ​​of D10, D50, and D90, which are indicators of particle size distribution, were recorded. The measurement results are shown in Table 1. As shown in Table 1, in iron powder 2, particles smaller than 75 μm and particles larger than 300 μm from iron powder 3 were removed by sieving, and it can be seen that the particle size distribution range is narrower compared to iron powder 3.

[0037] [Table 1]

[0038] The following measurements were performed on the supernatant and the entire reaction mixture obtained.

[0039] (1) Metallic palladium production rate The amount of palladium dissolved in the supernatant after the reduction (amount of dissolved palladium in the supernatant) was measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES). The metallic palladium generation rate was calculated from this amount and the amount of palladium in the resource solution, based on the following formula. The metallic palladium generation rate represents the proportion of palladium elements present in the resource solution in the form of palladium compounds that were reduced to metallic palladium by the reduction reaction described above. The results are shown in Table 2.

[0040]

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[0041] (2) Palladium leak rate in filtration The entire reaction mixture after reduction was subjected to suction filtration, and the precipitate was filtered off. For suction filtration, quantitative filter paper No. 5C (manufactured by Advantec Toyo Co., Ltd., particle retention capacity 1 μm) was used as the filter paper, and a Buchner funnel and suction bottle were used. The filtered filtrate was mixed with aqua regia and heated to oxidize and dissolve the metallic palladium that had leaked through the filter paper. The amount of palladium element dissolved in the aqua regia solution (amount of dissolved palladium element in the aqua regia solution) was then measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Using the obtained measurement values, the palladium leak rate in palladium filtration was calculated based on the following formula. A lower palladium leak rate indicates better flocculation of the reduced metallic palladium and more efficient recovery. The results are shown in Table 2.

[0042]

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[0043] (3) Filtration recovery rate of metallic palladium The filtration recovery rate of metallic palladium was calculated based on the following formula. The metallic palladium filtration recovery rate represents the proportion of palladium elements present in the resource liquid in the form of palladium compounds that were ultimately recovered as recycled metallic palladium through the reduction process to metallic palladium described above and the recovery process by filtering the reduced Pd metal. The results are shown in Table 2.

[0044]

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[0045] [Table 2]

[0046] As shown in Table 2, by adding a reducing agent to a resource solution containing inorganic sulfur oxides and palladium compounds at a pH of 2.5 or higher (after pH adjustment) and performing a reduction process to reduce the palladium compounds, an extremely high rate of metallic palladium generation was achieved regardless of the type of reducing agent (Examples 1-6). Furthermore, when a metal was used as the reducing agent (Examples 3-6), an extremely high rate of metallic palladium filtration recovery was achieved compared to when a non-metallic reducing agent was used (Example 1), even without the use of a coagulant. This was particularly remarkable, being equivalent to or better than when a coagulant was used (Example 2).

Claims

1. The process includes a reduction step in which a reducing agent is added to a resource solution containing inorganic sulfur oxides and palladium compounds with a pH of 2.5 to 3.5, thereby reducing the palladium compounds. A method for producing recycled palladium, wherein the reducing agent is used in an amount equivalent to 30 to 100 times the theoretical amount required to reduce the palladium compound to metallic palladium.

2. The manufacturing method according to claim 1, wherein the reducing agent is a metal hydride.

3. The manufacturing method according to claim 1, wherein the reducing agent is a metal.

4. The manufacturing method according to claim 3, which does not include a step of agglomerating the reduced metallic palladium.

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