Method for preparing noble metal-containing solution and quantitative analysis method for noble metal element

Abstract

To provide a method for preparing a noble metal-containing solution that enables a noble metal element in a sample to be dissolved promptly without loss of the noble metal element and without addition of external metal, and further to provide a quantitative analysis method for the noble metal element employing the noble metal-containing solution.SOLUTION: A method for preparing a noble metal-containing solution is provided, the method comprising: a step of melting a solid sample containing a noble metal element by an alkali fusion method to yield a melt; a step of cooling the resulting melt to yield a solidified fused product and leaching the fused product with water to yield a leachate; a first reduction step of mixing and heating the resulting leachate and a weak reducing agent; a second reduction step of mixing and heating a mixed liquid 1 yielded in the first reduction step and a strong reducing agent to yield a mixed liquid 2; a step of performing solid-liquid separation on the mixed liquid 2 to recover a solid; and a step of acid-dissolving the recovered solid to yield the noble metal-containing solution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for preparing a noble metal-containing solution for quantitatively analyzing noble metal elements contained in a solid sample, and a method for quantitatively analyzing noble metal elements. [Background technology]

[0002] In the past, for example, in precious metal recycling businesses, determining (evaluating) the quality (content) of precious metal elements in recycled raw materials, which are samples, has been an important process. High accuracy is required for determining the quality of these precious metal elements. Because precious metal elements have properties that make analysis difficult, such as being sparingly soluble, pretreatment is performed to convert them into a form that allows for highly accurate evaluation of their content. Specifically, as shown in Patent Documents 1-5, for example, the target element in the recycled raw material is converted into a solid that is easily dissolved by chemical techniques, which is then dissolved. The resulting solution is then quantified using a method such as ICP emission spectroscopy, thereby determining the quality of the target element in the sample. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-131128 [Patent Document 2] Japanese Patent Publication No. 2021-135297 [Patent Document 3] Patent No. 2018-169389 [Patent Document 4] Patent No. 2015-232167 [Patent Document 5] Patent No. 2005-308705 Summary of the Invention [Problem to be solved by the invention]

[0004] A typical method for obtaining a form that is easily dissolved is the alkali fusion method. However, this method uses a large amount of flux, which increases the amount of sample liquid, resulting in a worsening of the lower limit of quantitation, and also results in a high-salt sample liquid containing a large amount of flux, which leads to a deterioration in quantitative accuracy. Given this background, a method has been adopted in which precious metal elements are separated from a solid containing a large amount of flux components and precious metal elements after alkali fusion and then dissolved. Patent Documents 1, 2, 4, and 5 employ this method. Separating precious metal elements from the solid involves dissolving the solid and re-solidifying only the precious metal elements from the resulting solution. The solidified precious metal is dissolved, and quantitative analysis is performed using the resulting sample solution.

[0005] Specifically, the method described in Patent Document 1 combines ethanol reduction of precious metal elements with coprecipitation collection to solidify the precious metals. Copper (Cu) is added to the sample, and the precious metal elements are co-precipitated with copper hydroxide, so the prepared solution contains a large amount of Cu. Generally, the more impurities present in the analysis target, the worse the analytical accuracy. The method described in Patent Document 2 involves bromolysis and acid decomposition of a sample, followed by coprecipitation separation using bismuth (Bi) as a coprecipitant. The prepared solution contains a large amount of Bi. The method described in Patent Document 4 involves reducing a sample containing a Cu or iron (Fe) compound with ethanol. The Cu or Fe in the prepared solution is removed by ion exchange. This method is limited to samples containing Cu or Fe. The method described in Patent Document 5 is a coprecipitation separation method using tellurium (Te) and arsenic (As) as coprecipitants. Rhodium (Rh) can be recovered and analyzed, but ruthenium (Ru) and iridium (Ir) cannot be recovered. Furthermore, the prepared solution contains large amounts of Te and As.

[0006] The method described in Patent Document 3 involves alloying the noble metal elements in a sample with tin (Sn) and nickel (Ni) to form compounds that are soluble in acid. The resulting solution contains large amounts of base metal elements Sn and Ni.

[0007] In general, if there is a loss of precious metal elements during the pretreatment process of converting the precious metal elements into a solid form that is easily dissolved and then dissolving this to prepare a quantitative solution, it will be impossible to accurately determine the grade.Furthermore, from the viewpoint of analytical speed, it is desirable that the time required to obtain a quantitative solution be short.

[0008] The present invention was made under the above circumstances, and the problem to be solved by the present invention is to provide a method for preparing a precious metal-containing solution that can be used for quantitative analysis of precious metal elements by ICP atomic emission spectrometry or the like by rapidly dissolving precious metal elements in a sample (with high yield) without the need to add metal from outside, unlike the methods disclosed in patent documents, and that is not limited to samples containing Cu or Fe, and a method for quantitatively analyzing precious metal elements using the precious metal-containing solution. Note that the metals referred to here as "without the need to add metal from outside" do not include metals that cannot be reduced by the strong reducing agent used in the present invention, which will be described later. [Means for solving the problem]

[0009] That is, the first invention to solve the above-mentioned problems is: A step of melting a solid sample containing at least one noble metal element selected from gold, palladium, platinum, rhodium, ruthenium, and iridium by an alkali fusion method to obtain a melt; a step of cooling the obtained melt to obtain a solid melt, and leaching the melt with water to obtain a leachate; a first-stage reduction step in which the obtained leachate and a weak reducing agent having a standard electrode potential ε of −0.5 V or more are mixed and heated; a second-stage reduction step in which the mixed solution 1 obtained in the first-stage reduction step and a strong reducing agent having a standard electrode potential ε of less than −0.5 V are mixed and heated to obtain a mixed solution 2 in which a solid is formed in the liquid; a step of performing a solid-liquid separation treatment on the mixed liquid 2 to recover the solid matter; and a step of dissolving the recovered solid matter in acid to obtain a solution containing a precious metal.

[0010] The second invention is: In the first-stage reduction step, heating is performed within a temperature range from a temperature 10°C lower than the boiling point of the mixed solution 1 to the boiling point, In the second-stage reduction step, the mixture 2 is heated within a temperature range from a temperature 10° C. lower than the boiling point to the boiling point, in the method for preparing a noble metal-containing solution according to the first aspect of the present invention.

[0011] The third invention is In the method for preparing a noble metal-containing solution according to the first or second invention, 1 to 25 parts by mass of the weak reducing agent is mixed with 100 parts by mass of the leachate in the first-stage reduction step.

[0012] The fourth invention is This is the method for preparing a precious metal-containing solution according to any one of the first to third aspects of the present invention, wherein in the second-stage reduction step, 0.05 to 5 parts by mass of the strong reducing agent is mixed with 100 parts by mass of the mixed solution.

[0013] The fifth invention is The method for preparing a precious metal-containing solution according to any one of the first to fourth aspects of the present invention, wherein ethanol is used as the weak reducing agent and sodium borohydride is used as the strong reducing agent.

[0014] The sixth invention is This is a method for preparing a precious metal-containing solution according to any one of the first to fifth inventions, wherein the solid sample is a granular material obtained by subjecting waste containing precious metal elements to a granulation process including a roasting process, a crushing process, and a magnetic separation process.

[0015] The seventh invention is This is the method for preparing a precious metal-containing solution according to any one of the first to sixth aspects of the present invention, wherein in the alkali fusion method, the solid sample is mixed with at least one selected from sodium hydroxide, sodium peroxide, sodium nitrate, and sodium carbonate, and then heated to melt the solid sample.

[0016] The eighth invention is The method for preparing a noble metal-containing solution according to any one of the first to seventh aspects of the present invention does not include a step of adding a metal element that can be reduced by the strong reducing agent.

[0017] The ninth invention is This is an analytical method for quantitatively analyzing the content of one or more precious metal elements in a precious metal-containing solution obtained by the method for preparing a precious metal-containing solution described in any one of the first to eighth aspects of the present invention.

[0018] The tenth invention is This is an analytical method for simultaneously quantitatively analyzing the contents of multiple precious metal elements in a precious metal-containing solution obtained by the method for preparing a precious metal-containing solution described in any one of the first to eighth aspects of the present invention. [Effects of the Invention]

[0019] According to the present invention, it is possible to quickly dissolve the precious metal elements in a sample with high yield, and to obtain a precious metal-containing solution that can be used for quantitative analysis of the precious metal elements by ICP emission spectrometry or the like. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a process flow diagram showing a method for preparing a precious metal-containing solution and a method for quantitatively analyzing precious metal elements according to the present invention. [Figure 2] 1 is an example of a scanning secondary electron microscope photograph (50,000 times magnification) of a solid obtained by performing solid-liquid separation treatment in the method for preparing a noble metal-containing solution according to the present invention. [Figure 3] 1 is a scanning secondary electron microscope photograph (50,000 times) of the residue obtained by "solid-liquid separation" according to Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0021] An embodiment of the invention will be described with reference to FIG. 1, which is a process flow diagram illustrating a method for preparing a noble metal-containing solution and a method for quantitatively analyzing noble metal elements according to the present invention. In the present invention, the noble metal elements include gold (Au), palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), and iridium (Ir).

[0022] (Solid sample) The solid sample to be processed by the present method for preparing a precious metal-containing solution and the present method for quantitatively analyzing precious metal elements is not particularly limited as long as it contains precious metal elements. Examples include granular materials (so-called black mass) obtained by recycling waste containing precious metal elements, such as waste electronic circuit boards and catalyst waste. Waste electronic circuit boards are electronic circuit boards discarded for various reasons, such as malfunction, end of life, or manufacturing defects. Electronic circuit boards are used in electronic devices such as mobile phones and personal computers, and in computers in various machines such as vehicles, and are rich in precious metal elements (at least one selected from Au, Pd, Pt, Rh, Ru, and Ir). Examples of catalysts include exhaust gas purification catalysts used primarily in automobiles and catalysts used in the chemical and pharmaceutical fields. These typically contain platinum group metals. The present method for preparing a precious metal-containing solution can solubilize all precious metal elements without any particular limitations. Therefore, by analyzing the solution obtained from the above sample using the present method, the quality of the precious metal elements can be determined in one go.

[0023] Note that precious metal elements and the metal elements described below exist in ionic form in liquids such as water and acids, and these ionic forms of precious metals and metals are also referred to as "precious metal elements" and "metal elements."

[0024] In the recycling process described above, waste containing precious metal elements is heat-treated (roasted) at temperatures of, for example, 500 to 1000°C to convert valuable metals, such as cobalt (Co), contained in the waste into a form that is easily dissolved by acid. Organic matter, such as plastics, is also removed from the waste at this stage. The waste is then granulated by a crushing process using, for example, an impact crusher (e.g., a sample mill, hammer mill, tornado mill, or hammer crusher). The resulting crushed material may be sieved (classified). The crushing and sieving conditions can be appropriately selected from conventionally known conditions. The crushed material and the classified material are subjected to a magnetic separation process, in which magnetic separation is performed to separate a portion of the cobalt, nickel (Ni), and other magnetic materials from the crushed material / classified material. This results in a non-magnetic material, i.e., a granular material (black mass) containing precious metal elements. This is a typical solid sample used in the present invention. The magnetic separation conditions can be appropriately selected from conventionally known conditions.

[0025] The solid sample may also contain metal elements other than the above-mentioned precious metal elements. Since the metal elements can be easily dissolved in acid, by carrying out the present invention, the obtained precious metal-containing solution can be used to determine the grade of the metal elements together with the precious metal elements. The content of the precious metal elements in the solid sample is, for example, 0.001 to 40 mass%. This numerical range applies to the total amount of the precious metal elements when multiple types of precious metal elements are contained. The content of the metal elements other than the precious metals in the sample is, for example, 1 to 80 mass%. This numerical range applies to the total amount of the metal elements when multiple types of metal elements are contained.

[0026] (alkali fusion) Since solid samples contain precious metal elements and are difficult to dissolve, alkali fusion is used in the present invention to turn them into a solution. Known methods can be used for alkali fusion. Convenient alkalis include alkali metal salts such as sodium hydroxide, sodium peroxide, sodium nitrate, and sodium carbonate. The solid sample and alkali are mixed and heated to melting temperatures of approximately 250 to 900°C for approximately 0.5 to 5 hours in a crucible or the like.

[0027] (melt product) Once the alkali fusion of the sample is complete, heating is stopped. The alkali fusion of the sample cools inside the crucible and turns into a solid melt. This melt is soluble in water.

[0028] (Pure water leaching) Although the resulting melt is solid, it is preferable to leach the melt using pure water, since it is easier to handle the reaction in a liquid in the subsequent process. It is also preferable to leach the melt adhering to the wall of the container, such as the crucible, used during alkali fusion using pure water. The resulting leachate contains many substances derived from the flux, such as sodium, in addition to the precious metal elements and other metal elements that constituted the solid sample. Therefore, it is difficult to guarantee sufficient analytical accuracy if the leachate is subjected to quantitative analysis.

[0029] (First stage reduction by mixing a weak reducing agent) In the present invention, the resulting leachate is reduced in two stages, which precipitates precious metal particles. However, if this reduction is carried out in the crucible in which the alkali fusion was carried out, the particles may adhere to the crucible. Therefore, it is preferable to carry out the reduction in a reaction vessel made of a material that does not allow the particles to adhere, such as a beaker.

[0030] The first stage of the two-stage reduction involves mixing and heating the leachate and a weak reducing agent. The weak reducing agent used has a standard electrode potential ε of -0.5 V or higher. The standard electrode potential ε of the weak reducing agent is preferably -0.1 V or higher. The maximum value of the standard electrode potential ε of the weak reducing agent is not particularly limited, but is, for example, +0.9 V. Examples of the weak reducing agent include ethanol (EtOH) with an ε of +0.197 V, methanol with an ε of +0.209 V, ascorbic acid with an ε of +0.34 V, hydrazine with an ε of +0.73 V, and stannous chloride with an ε of +0.15 V. Among these, ethanol is preferred because it has a moderate reducing power and low chemical costs.

[0031] In the heating for the first-stage reduction, the temperature of the mixed solution 1 obtained by mixing the leaching solution and the weak reducing agent is preferably 50°C or higher from the viewpoint of reaction efficiency. From the viewpoints of reaction efficiency and safety, the temperature of the mixed solution 1 is more preferably a temperature at which the mixed solution 1 can be maintained in a gently boiling state so as not to scatter (i.e., a temperature near the boiling point of the mixed solution 1 (for example, a temperature range from a temperature 10°C lower than the boiling point of the mixed solution 1 to the boiling point)). Therefore, it is particularly preferable to heat the mixed solution 1 using a heating means set at a temperature of 100°C or higher and 300°C or lower. From the viewpoints of sufficiently carrying out the first-stage reduction reaction and preparing the precious metal-containing solution described below in a short time, the duration of the first-stage reduction (the time for heating the mixed solution 1) is preferably 10 minutes or higher and 60 minutes or lower. Furthermore, from the viewpoint of reduction reaction efficiency, the pH of the mixed solution 1 at the start of the reduction is preferably 10 to 14.

[0032] Since it is basically unknown how much of the precious metal element is contained in the sample, it is preferable to use a sufficient amount of weak reducing agent. Specifically, for example, in the first reduction stage, it is preferable to mix 1 to 25 parts by mass of weak reducing agent with 100 parts by mass of leachate.

[0033] The inventors believe that in this first-stage reduction, the precious metal elements (present in the form of ions) in mixed solution 1 are reduced, resulting in the formation of fine precious metal particles and numerous nuclei for initiating particle formation. However, even if a large amount of weak reducing agent is added at this stage, the precious metal elements in mixed solution 1 are not completely reduced due to the weak reducing power of the weak reducing agent. Furthermore, substances such as sodium derived from the alkali fusion reagent are not reduced and remain dissolved in mixed solution A. If the solid sample contains metal elements other than precious metal elements, they will be reduced at this stage if their standard electrode potential ε0 is greater than that of the weak reducing agent.

[0034] (Second stage reduction by mixing a strong reducing agent) After the first-stage reduction using a weak reducing agent is completed, mixed solution 1 is mixed with a strong reducing agent, and the resulting mixed solution 2 is heated to perform the second-stage reduction. The strong reducing agent has a standard electrode potential ε of less than −0.5 V (the standard electrode potential ε of a strong reducing agent is typically −2.30 V or higher). Examples of such strong reducing agents include tetrahydroboric acid and its salts (e.g., sodium borohydride (SBH)), which has an ε of −1.79 V, formic acid, which has an ε of −1.99 V, and sulfurous acid, which has an ε of −0.936 V. Among these, sodium borohydride is preferred due to its high reduction rate. Figure 2 shows an example of a scanning secondary electron microscope photograph (50,000x magnification) of the solid obtained after the second-stage reduction and the solid-liquid separation process described below.

[0035] In the heating for the second-stage reduction, the temperature of the mixed solution 2 obtained by mixing the mixed solution 1 and the strong reducing agent is preferably 50°C or higher from the viewpoint of reaction efficiency. From the viewpoints of reaction efficiency and safety, a temperature at which the mixed solution 2 is gently boiled (i.e., a temperature near the boiling point of the mixed solution 2 (for example, a temperature range from 10°C lower than the boiling point of the mixed solution 2 to the boiling point)) is more preferable. Therefore, it is particularly preferable to heat the mixed solution 2 using a heating means set at a temperature of 100°C or higher and 300°C or lower. From the viewpoints of sufficiently carrying out the second-stage reduction reaction and preparing the precious metal-containing solution described below in a short time, the duration of the second-stage reduction (the time for heating the mixed solution 2) is also preferably 10 minutes or higher and 60 minutes or lower. Furthermore, from the viewpoint of reduction reaction efficiency, the pH of the mixed solution 2 at the start of the reduction is preferably 10 to 14. Even in this second-stage reduction, substances such as sodium derived from the alkali fusion reagent are not reduced and remain dissolved in the mixed solution 2.

[0036] Since the amount of precious metal contained in the sample is generally unknown, it is preferable to use a sufficient amount of strong reducing agent when mixing with a strong reducing agent. Similarly, a sufficient amount of weak reducing agent is also used, which ensures a certain level of reduction reaction, so the amount used can be less than that of a weak reducing agent. Specifically, for example, in the second-stage reduction, it is preferable to mix 0.05 parts by mass or more of strong reducing agent per 100 parts by mass of the mixture. The upper limit of the amount of strong reducing agent to be mixed is approximately 5 parts by mass per 100 parts by mass of the mixture. This is because an excessive amount of strong reducing agent can cause a violent reaction.

[0037] After the second reduction step using "weak reducing agent mixing" and "strong reducing agent mixing," precious metal particles (solid matter) with a primary particle diameter of approximately 40 nm or less are formed. In the present invention, the primary particle diameter is the maximum value of the line segment (limited to the line segment that does not extend outside the particle) connecting two points on the particle outline in a two-dimensional image obtained by observing the particle with a scanning secondary electron microscope at an appropriate observation magnification.

[0038] Regarding the formation of the aforementioned precious metal particles with a primary particle diameter of approximately 40 nm or less, it is believed that in the "first-stage reduction by adding a weak reducing agent," many fine particles of precious metal elements and nuclei for initiating particle formation are generated. Then, in the "second-stage reduction by adding a strong reducing agent," the precious metal elements remaining in the leachate are reduced and precipitate onto the many fine particles and nuclei that have already been generated, and as a result, it is believed that the reduction reaction of the precious metal elements is completed without the generation of precious metal particles with large particle diameters.

[0039] If the solid sample contains metal elements other than precious metal elements, those with a standard electrode potential ε0 greater than that of the strong reducing agent will be reduced by the second reduction stage and precipitate as a solid. Since the final target of quantification is precious metal elements, it does not matter whether the other metal elements precipitate as a solid or remain in the liquid component without being precipitated.

[0040] (solid-liquid separation) After the second reduction step is completed, the mixed solution 2 is subjected to solid-liquid separation to separate the residue (solids) and the filtrate. The specific means for solid-liquid separation are not particularly limited, and known methods can be used. For example, filtration using a filter is preferred. The precious metal particles are recovered as a residue through this solid-liquid separation. The residue may be washed with pure water. If the solid sample contains metal elements other than precious metal elements (those with a standard electrode potential ε0 greater than that of the strong reducing agent), these elements are also included in the residue. These elements may form metal particles alone or alloy particles with the precious metal elements, regardless of their form. Furthermore, substances such as sodium (Na) (ε0 = -2.71 V) derived from the reagent used in the alkali fusion are dissolved and migrate to the filtrate, i.e., are separated from the precious metal elements. Metal elements that cannot be reduced by strong reducing agents (those with a standard electrode potential ε0 lower than that of the strong reducing agent), such as Na, are separated from the precious metal elements in this solid-liquid separation process. Therefore, adding these elements for a specific purpose, such as alkali fusion, is permitted in the present invention.

[0041] (acid dissolution) The precious metal particles, which are the residue recovered in the "solid-liquid separation," are dissolved in an appropriate acid to obtain the precious metal-containing solution of the present invention. The fine precious metal particles produced through the two-stage reduction using the aforementioned "weak reducing agent mixture" and "strong reducing agent mixture" can be completely dissolved in acid within 30 minutes. If the precious metal particles contain other metal elements (those with a standard electrode potential ε greater than that of the strong reducing agent), these metal elements are more easily dissolved in acid (than the precious metal).

[0042] The reason why the noble metal particles dissolve in acid in a short time is not clear, but it is thought that the noble metal particles are in a state where they are easily dissolved, and that the noble metal particles are fine and nano-sized, which increases the surface free energy and makes them unstable, thereby increasing their solubility thermodynamically.

[0043] Examples of acids used in acid dissolution include hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, and mixtures / aqueous solutions of these. When using an acid solution, the acid concentration (the total concentration when multiple types of acids are included) is preferably 30% by mass or more and 90% by mass or less. In addition, during acid dissolution, the mixture of acid and precious metal particles may be stirred or heated.

[0044] According to the present invention, each precious metal element contained in the initial solid sample can be transferred into the precious metal-containing solution of the present invention with a high yield of 95% or more. Furthermore, in the process shown in Fig. 1, the precious metal-containing solution of the present invention can be obtained without performing a process of adding a metal element from outside, such as coprecipitating or alloying the precious metal element with other elements.

[0045] (Quantitative determination of precious metal elements) The obtained precious metal-containing solution can be used as a quantitative solution for quantitative analysis of precious metal elements by ICP-OES, mass spectrometry, atomic absorption spectrometry, etc. These methods allow for simultaneous quantitative analysis of two or more, preferably all, of the precious metal elements in the precious metal-containing solution. The precious metal-containing solution may be appropriately diluted (to a constant volume).

[0046] According to the present invention, it is possible to avoid the situation in which elements coprecipitated with or alloyed with precious metal elements remain in a quantitative analysis sample of precious metal elements and interfere with quantitative analysis measurements such as ICP-OES, mass spectrometry, and atomic absorption spectrometry, as in the prior art. Furthermore, since there is no need to add external metals (except those that cannot be reduced by strong reducing agents), costs for reagents and the like can be reduced. Furthermore, it is possible to rapidly prepare a precious metal-containing solution for analysis in which the precious metal elements in a solid sample have been transferred without loss (high yield). Therefore, the present invention contributes to improving the productivity of precious metal recycling businesses. [Example]

[0047] Hereinafter, examples of the method for preparing and analyzing a noble metal-containing solution according to the present invention will be described in detail, but the present invention is not limited to the contents of these examples.

[0048] [Example 1] (Solid sample preparation) Commercially available standard stock solutions (1000 mg / L) of Rh, Ru, and Ir were diluted 10 times with 0.6 mol / L hydrochloric acid to prepare sample solutions containing 100 mg / L of each precious metal element (excluding Fe and Cu). 10 mL of this solution (1 mg of each precious metal element) was dispensed into an alumina crucible, heated, concentrated, and dried to prepare the sample.

[0049] (alkali fusion) 3 g of sodium hydroxide and 3 g of sodium peroxide were added to the crucible, and the mixture was heated in an electric furnace at 600° C. for 1 hour.

[0050] (Pure water leaching) After the heating, the crucible was allowed to cool to room temperature, and the melt (solid) in the crucible was transferred to a 300 mL beaker along with the crucible body. The melt in the crucible was then dissolved in 150 mL of warm pure water to obtain a leachate, which was then poured into the beaker. The inner and outer surfaces of the crucible body were then washed with a small amount of pure water, and the crucible was removed from the beaker. The washing liquid for the crucible body was added to the leachate.

[0051] (First stage reduction by mixing a weak reducing agent) To the resulting leachate (pH 12.5-13.5), 4 g (5 mL) of ethanol (EtOH) was added as a weak reducing agent (approximately 3500 times the molar ratio of the total amount of precious metal elements). A watch glass was then placed over the beaker with the convex side facing downwards, and the mixture was heated on a hot plate set to 200°C for 30 minutes. The standard electrode potential ε0 of ethanol is +0.197 V.

[0052] (Second stage reduction by mixing a strong reducing agent) 1.5 mL of 15 w / v% sodium borohydride (SBH) aqueous solution (approximately 240 times the molar ratio of the total amount of precious metal elements) was added as a strong reducing agent to the ethanol-reduced mixture 1. The pH of the resulting mixture 2 was 12.5-13.5. A watch glass was then placed over the beaker with the convex side facing downwards, and the mixture was heated on a hot plate set to 200°C for 30 minutes. The standard electrode potential ε0 of sodium borohydride is -1.79 V.

[0053] (solid-liquid separation) After allowing mixed solution 2 to cool to room temperature, solid-liquid separation was performed by suction filtration using a mixed cellulose ester membrane filter with a pore size of 0.45 μm. The residue on the filter was washed three times with 10 mL of pure water. The washings were added to the filtrate. The resulting filtrate was transferred to a 250 mL measuring flask and diluted to a constant volume of 250 mL with pure water. This was designated solution A.

[0054] (acid dissolution) The residue on the filter was transferred to a beaker along with the filter, and 12 mL of hydrochloric acid (HCl 35 w / w%) and 4 mL of nitric acid (HNO3 60 w / w%) were added. Then, a watch glass was placed on the beaker with the convex side facing downward, and the mixture was heated on a hot plate set to 200°C for 15 minutes to dissolve the residue and filter, thereby obtaining a solution. Heating was stopped after it was confirmed that the residue and filter had dissolved.

[0055] (constant volume) The resulting solution was allowed to cool to room temperature, then transferred to a 100 mL measuring flask and diluted with pure water to a constant volume of 100 mL. This was designated solution B.

[0056] (Quantitative determination of precious metal elements) The obtained solutions A and B were measured using an ICP optical emission spectrometer (Agilent, 5800), and the ratio of the amount of precious metal elements in each solution to the amount of precious metal elements in the original sample solution was calculated from the amount of precious metal elements contained in solutions A and B. The calculated ratios of each precious metal element in solution A are shown in Table 1, and the ratios in solution B are shown in Table 2.

[0057] For example, if the measurement result of solution A shows 0.994 mg of Rh, the ratio of Rh in solution A is 99.4%. Note that in calculations, the ratio may slightly exceed 100%, but this is thought to be a measurement error originating from the ICP optical emission spectrometer.

[0058] [Comparative Example 1] The "preparation of solid sample," "alkali fusion," and "pure water leaching" were carried out in the same manner as in Example 1, and a leachate according to Comparative Example 1 was obtained.

[0059] 5 mL (4 g) of ethanol was added to the obtained leachate of Comparative Example 1. Then, a watch glass was placed over the beaker with the convex side facing downwards, and the mixture was heated on a hot plate set to 200°C for 30 minutes. Next, 5 mL of ethanol was added again, and the watch glass was placed over the beaker with the convex side facing downwards, and the mixture was heated for another 30 minutes.

[0060] The subsequent steps, "solid-liquid separation," "acid dissolution," "volume determination," and "quantitative determination of precious metal elements," were carried out in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0061] Comparative Example 2 The "preparation of solid sample," "alkali fusion," and "pure water leaching" were carried out in the same manner as in Example 1, and a leachate according to Comparative Example 2 was obtained.

[0062] To the obtained leachate of Comparative Example 2, 1.5 mL of a 15 w / v % aqueous solution of sodium borohydride was added. Then, a watch glass was placed over the beaker with the convex side facing downwards, and the mixture was heated on a hot plate set to 200°C for 30 minutes. Next, 1.5 mL of a 15 w / v % aqueous solution of sodium borohydride was added again. Then, the watch glass was placed over the beaker with the convex side facing downwards, and the mixture was heated for another 30 minutes.

[0063] Subsequent steps included "solid-liquid separation" and "acid dissolution" in the same manner as in Example 1. In Comparative Example 2, the residue did not completely dissolve in the "acid dissolution," but in the "constant volume" step, the undissolved residue was transferred to a 100 mL measuring flask along with the solution, and the volume was adjusted to 100 mL with pure water. Then, as in Example 1, "quantitation of precious metal elements" was performed. The results are shown in Tables 1 and 2. In Comparative Example 2, solution B contained undissolved precipitate, but a test solution for emission spectroscopy analysis was collected from the supernatant of solution B so as not to contain this.

[0064] [Table 1] [Table 2]

[0065] [Consideration] The results shown in Tables 1 and 2 demonstrate that precious metal elements can be reduced and recovered at a high recovery rate by performing a two-stage reduction reaction: a first-stage reduction by mixing a weak reducing agent and a second-stage reduction by mixing a strong reducing agent. Furthermore, it was also demonstrated that the recovered precious metal elements can be easily dissolved (in a short time) in an acid to form a solution, which can be used as a sample solution for ICP atomic emission spectrometry and the like (solution B obtained by adjusting the volume of the solution).

[0066] On the other hand, in Comparative Example 1, the ratio of Ir in Solution B was low, and most of Ir was detected in Solution A, which revealed that the reduction of Ir was incomplete.

[0067] Furthermore, in Comparative Example 2, the amount of precious metal elements in Solution A was below the lower limit of quantitation, indicating that the reduction reaction of the precious metal elements was carried out with high conversion efficiency. However, the ratios of precious metal elements in Solution B all showed low values. This is thought to be due to the residue not being completely dissolved in the solution during the "acid dissolution" step. In ICP atomic emission spectrometry, the solution is aspirated and sprayed into the plasma for measurement, so if the precious metal elements are not completely dissolved in the solution, the measured value will be lower than the true value.

[0068] The residues on the filters after the "solid-liquid separation" in Example 1 and Comparative Example 2 were observed using a scanning secondary electron microscope (JEOL, JSM-6700F, 5 kV, WD (working distance) 7.4 to 7.5 mm, magnification 50,000 times). The former was observed as aggregates of particles with a primary particle diameter of 20 to 30 nm, and the latter was observed as aggregates of particles with a primary particle diameter of about 100 nm.

[0069] A scanning secondary electron microscope photograph (magnification: 50,000 times) of the residue obtained in Comparative Example 2 is shown in FIG. 3. Comparative Example 2 was repeated starting from the "preparation of a solid sample" step. In the "acid dissolution" step, the residue was not completely dissolved after 15 minutes of heating on a hot plate set at 200°C, as described above. Therefore, heating was continued for another 7 hours under the same conditions. However, after 7 hours of further heating, visual observation revealed no substantial change in the amount of undissolved residue from the 15-minute heating step.

[0070] [Example 2] As solid samples, two samples were prepared, which were powdered recycled materials containing precious metal elements and obtained from different waste substrates. 200 mg of each of the two samples (Sample-1 and Sample-2) was weighed into a crucible, and each sample was treated in the same manner as in Example 1 to perform "quantification of precious metal elements." Furthermore, no residue was observed in either sample after "acid dissolution."

[0071] From the measurement results of Solution B, which is the noble metal-containing acid solution according to the present invention, the content (mass %) of each noble metal element in each recycled raw material was calculated using Equation 1. The results are shown in Table 3. Precious metal element content (mass%) = amount of each precious metal element in solution B (mg) / (weight of solid sample (mg)) × 100 (Equation 1)

[0072] Furthermore, the distribution rate (recovery rate by reduction reaction) (%) of the precious metal elements in solution B was calculated using (Equation 2) from the measurement results of solutions A and B. The results are shown in Table 4. Precious metal element distribution rate (%) = amount of precious metal element in solution B (mg) / (amount of precious metal element in solution A (mg) + amount of precious metal element in solution B (mg)) × 100 (Equation 2)

[0073] [Table 3] [Table 4]

[0074] Since no residue was observed in the "acid dissolution" step, it can be seen that all of the precious metal elements in the sample were distributed to either Solution A or Solution B, or both Solutions A and B. The results in Tables 3 and 4 demonstrate that the precious metal elements in the recycled raw materials were recovered with a favorable distribution rate of 95% or more into Solution B, the precious metal-containing acid solution (a fixed volume solution) according to the present invention. In other words, it was confirmed that the present invention can rapidly dissolve the precious metal elements contained in a real sample with a high yield, without the need for a process that involves adding metal from outside, such as coprecipitating or alloying with other elements, to obtain a precious metal-containing acid solution that can be used for quantitative analysis of the precious metal elements. Furthermore, it was found that all of the precious metal elements can be simultaneously quantitatively analyzed by analyzing Solution B, the obtained precious metal-containing acid solution, using an ICP atomic emission spectrometer or the like.

Claims

1. A step of melting a solid sample containing at least one noble metal element selected from gold, palladium, platinum, rhodium, ruthenium, and iridium by an alkali fusion method to obtain a melt; a step of cooling the obtained melt to obtain a solid melt, and leaching the melt with water to obtain a leachate; The obtained leachate and standard electrode potential ε ０ a first-stage reduction step in which a weak reducing agent having a potential of −0.5 V or higher is mixed and heated; The mixed solution 1 obtained in the first reduction step and the standard electrode potential ε ０ a second reduction step in which a strong reducing agent having a potential of less than −0.5 V is mixed and heated to obtain a mixed solution 2 in which a solid is formed in the liquid; a step of performing a solid-liquid separation treatment on the mixed liquid 2 to recover the solid matter; and a step of dissolving the recovered solid in acid to obtain a solution containing a precious metal.

2. In the first-stage reduction step, heating is performed within a temperature range from a temperature 10° C. lower than the boiling point of the mixed solution 1 to the boiling point, 2. The method for preparing a precious metal-containing solution according to claim 1, wherein the second-stage reduction step is performed by heating the mixture 2 within a temperature range from a temperature 10° C. lower than the boiling point of the mixture 2 to the boiling point.

3. 2. The method for preparing a precious metal-containing solution according to claim 1, wherein in the first-stage reduction step, 1 to 25 parts by mass of the weak reducing agent is mixed with 100 parts by mass of the leaching solution.

4. 2. The method for preparing a noble metal-containing solution according to claim 1, wherein in the second-stage reduction step, 0.05 to 5 parts by mass of the strong reducing agent is mixed with 100 parts by mass of the mixed solution 1.

5. 2. The method for preparing a precious metal-containing solution according to claim 1, wherein ethanol is used as the weak reducing agent and sodium borohydride is used as the strong reducing agent.

6. 2. The method for preparing a precious metal-containing solution according to claim 1, wherein the solid sample is a granular material obtained by subjecting a waste material containing precious metal elements to a granulation process including a roasting step, a crushing step, and a magnetic separation step.

7. 2. The method for preparing a precious metal-containing solution according to claim 1, wherein in the alkali fusion method, the solid sample is mixed with at least one selected from sodium hydroxide, sodium peroxide, sodium nitrate, and sodium carbonate, and then heated to melt the solid sample.

8. 2. The method for preparing a precious metal-containing solution according to claim 1, wherein a step of adding a metal element to be reduced by the strong reducing agent is not carried out.

9. An analytical method for quantitatively analyzing the content of one or more precious metal elements in a precious metal-containing solution obtained by the method for preparing a precious metal-containing solution according to any one of claims 1 to 8.

10. An analytical method for simultaneously quantitatively analyzing the contents of a plurality of precious metal elements in a precious metal-containing solution obtained by the method for preparing a precious metal-containing solution according to any one of claims 1 to 8.

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