Method for recovering rhodium

Abstract

To improve a recovery rate of rhodium from an acidic aqueous solution containing platinum group metals.SOLUTION: A rhodium recovery method of the present invention comprises: a neutralization step of filtering a mixture of an acidic aqueous solution containing rhodium and platinum group metals other than rhodium and a calcium compound to obtain a first filtrate; a nitrite reaction step of mixing the first filtrate with a nitrite salt to produce a complex ion having nitrite ions coordinated to rhodium in the mixture, followed by filtering the mixture to obtain the second filtrate; and a recovery step of mixing the second filtrate with an ammonium salt or potassium salt to produce a water-insoluble compound containing rhodium, and recovering the water-insoluble compound.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for recovering rhodium from an acidic aqueous solution containing rhodium and platinum group elements other than rhodium.

Background Art

[0002] Platinum group metals used in catalysts, electronic materials, etc. are recycled actively because they are scarce resources and have a large environmental burden during smelting. For example, in Patent Document 1, a method for separating and recovering platinum group metals is proposed in which nitrite is added to an aqueous nitric acid solution containing platinum group metals to form an anionic nitro complex, and the anionic nitro complex is adsorbed on an ion exchange resin.

[0003] In Patent Document 2, a treatment product obtained by bringing a substance containing a platinum group metal into contact with a molten salt containing iron halide is cooled to obtain a solid product, the solid product is treated with water to obtain an aqueous dispersion, and a liquid containing a component containing a platinum group metal is separated from the aqueous dispersion, and a method for recovering a platinum group metal is proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] When recovering rhodium from sludge or scrap containing platinum group metals such as rhodium, these sludge or scrap are dissolved in acid, the filtrate from which the dissolved residue has been separated is reduced to obtain a metal, and after dissolving the metal in acid, impurities such as tin contained in the dissolved solution are neutralized with a basic substance and precipitated and separated. Generally, sodium hydroxide or potassium hydroxide is used as the basic substance. However, as a result of the study by the present inventor, it has been found that when sodium hydroxide or potassium hydroxide is used as the basic substance, there is a limit to increasing the recovery rate of rhodium.

[0006] Therefore, an object of the present invention is to provide a rhodium recovery method with improved recovery rate of rhodium from an acidic aqueous solution containing platinum group metals.

Means for Solving the Problems

[0007] The present invention provides a neutralization step of filtering a mixed solution of an acidic aqueous solution containing rhodium and platinum group metals other than rhodium and a calcium compound to obtain a first filtrate, a nitrous acid reaction step of mixing the first filtrate and a nitrite to generate a complex ion in which nitrite ions are coordinated to rhodium in the mixed solution, and then filtering the mixed solution to obtain a second filtrate, and a recovery step of mixing the second filtrate with an ammonium salt or a potassium salt to generate a water-insoluble compound containing rhodium and recovering the water-insoluble compound.

Effects of the Invention

[0008] According to the method of the present invention, the recovery rate of rhodium from an acidic aqueous solution containing platinum group metals is improved.

Modes for Carrying Out the Invention

[0009] Hereinafter, the present invention will be described based on its preferred embodiments. The present invention relates to a method for recovering rhodium from an aqueous solution containing rhodium and platinum group metals other than rhodium. The method for recovering rhodium of the present invention preferably has the following steps (a) to (c). (a) Neutralization step: A mixed solution of an acidic aqueous solution containing rhodium and platinum group metals other than rhodium and a calcium compound is filtered to obtain a first filtrate. (b) Nitrous acid reaction step: The first filtrate and a nitrite are mixed to generate a complex ion in which nitrite ions are coordinated to rhodium in the mixed solution, and then the mixed solution is filtered to obtain a second filtrate. (c) Recovery step: The second filtrate and an ammonium salt or a potassium salt are mixed to generate a water-insoluble compound containing rhodium, and the water-insoluble compound is recovered. Hereinafter, each step will be described.

[0010] <(a) Neutralization step> In this step, the acidic aqueous solution to be recovered of rhodium is neutralized with a basic substance to distribute rhodium in the liquid phase. The acidic aqueous solution contains rhodium and platinum group metals other than rhodium (for example, platinum, palladium, ruthenium, iridium, osmium, etc.). The acidic aqueous solution can be, for example, a solution obtained by dissolving a product or sludge containing rhodium and platinum group metals other than rhodium with an acid such as hydrochloric acid, but is not limited thereto. When the acidic aqueous solution is a solution obtained by dissolving rhodium etc. with hydrochloric acid, rhodium etc. generally exist in water in the state of 3- [RhCl6].[[]END]] There is no particular limitation on the concentration of rhodium contained in the acidic aqueous solution, but it can be, for example, 0.05% by mass or more and 0.5% by mass or less. The acidic aqueous solution may contain metals other than platinum group metals in addition to these platinum group metals. Such metals include various transition metals other than platinum group metals. Examples of transition metals include, but are not limited to, iron, tin, zinc, and lead. Before neutralization, the pH of the acidic aqueous solution can be, for example, -1.5 or more and -0.5 or less at 25°C. Unless otherwise specified, when referring to the pH value in this specification, it means the value at 25°C.

[0011] For the purpose of neutralizing the acidic aqueous solution, the acidic aqueous solution is mixed with a calcium compound which is a basic substance. As the calcium compound, a substance that shows basicity in a dissolved state in water is preferably used. Examples of such calcium compounds include, but are not limited to, aqueous calcium compounds such as calcium hydroxide, calcium carbonate, and calcium oxide. These calcium compounds can be used alone or in combination of two or more. Among these calcium compounds, it is preferable to use calcium hydroxide from the viewpoint of being able to successfully distribute rhodium in the liquid phase.

[0012] From the viewpoint of stably existing rhodium in the liquid, the amount of the calcium compound used in this step is preferably such that the pH of the acidic aqueous solution after neutralization becomes 3 or less. From this viewpoint, the amount of the calcium compound used is preferably such that the pH of the acidic aqueous solution after neutralization becomes 2.5 or less, particularly 2.2 or less. Also, the amount of the calcium compound used is preferably such that the pH of the acidic aqueous solution after neutralization becomes 1.0 or more, particularly 1.5 or more, and especially 1.8 or more.

[0013] As the basic substance for neutralizing the acidic aqueous solution, in addition to the above-mentioned calcium compounds, it is also conceivable to use alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. However, when neutralization is carried out using sodium hydroxide, due to the excessive presence of sodium ions in the liquid, in the nitrous acid reaction step described later, the reaction shown by the following formula (I) easily proceeds to the left, rhodium is easily distributed in the solid phase, and the recovery rate of rhodium decreases. Na3[Rh(NO2)6]←→3Na + +[Rh(NO2)6] 3- (I) Rhodium reacts with sodium nitrite to obtain sodium hexanitrorhodium(III). This sodium hexanitrorhodium(III) has a solubility of about 110 g / L in water. However, as the sodium concentration increases, the solubility of sodium hexanitrorhodium(III) decreases due to the common ion effect of cations, so it is considered that the recovery rate of rhodium decreases.

[0014] When neutralization is carried out using potassium hydroxide, due to the presence of potassium ions in the solution, a poorly soluble salt of rhodium, such as K3[RhCl6], which will be described later, is formed, and metals such as tin and lead are incorporated into this poorly soluble salt. In addition, since the rhodium concentration in the solution after neutralization using potassium hydroxide can only be reduced to 1 g / L to 2 g / L, the recovery rate of rhodium is low, and there is a problem that when the tail liquid adhering to the poorly soluble salt is washed with water, rhodium redissolves and the recovery rate of rhodium decreases. On the other hand, when neutralization is carried out using a calcium compound, the nitrous acid reaction step, which will be described later, is not affected, and rhodium can be successfully distributed into the liquid phase.

[0015] When, for example, tin is contained in the acidic aqueous solution, as a result of neutralization by mixing the acidic aqueous solution and the calcium compound, a water-insoluble compound of tin precipitates in the mixed solution. By filtering this mixed solution to remove the water-insoluble compound of tin, a first filtrate containing rhodium and platinum group metals other than rhodium is obtained. There is no particular limitation on the filtration method of the mixed solution, and a filtration method known in the art can be adopted. The distribution ratio of rhodium at this point, that is, the ratio of the total mass of rhodium contained in the first filtrate to the total mass of rhodium contained in the acidic aqueous solution, preferably reaches a high value of 84% or more. For the purpose of increasing the distribution ratio of rhodium at this point, the water-insoluble compound of tin can be washed with water to extract the rhodium incorporated in the water-insoluble compound to the washing liquid side. In this case, the water used for washing is preferably warm water at 60°C or higher and 90°C or lower from the viewpoint of enhancing the extraction of rhodium.

[0016] <(b) Nitrous acid reaction step> In this step, the first filtrate and a nitrite are mixed to form a complex ion in which nitrite ions are coordinated to rhodium. Also, nitrite ions are coordinated to platinum group metals other than rhodium to form complex ions. However, since nitrite ions do not coordinate to transition metals other than platinum group metals, such as iron and lead, complex ions of these metals are not formed. Therefore, by forming complex ions, rhodium and platinum group metals other than rhodium can be selectively present in the liquid. At the same time, the transition metals contained in the first filtrate, particularly iron and lead, precipitate as water-insoluble components. Examples of nitrites that can be used in this step include sodium nitrite and calcium nitrite.

[0017] To generate complex ions, it is advantageous to heat the mixed solution of the first filtrate and the nitrite. The heating temperature of the mixed solution can be, for example, 55°C or higher, preferably 60°C or higher, and more preferably 65°C or higher. The heating time can be 30 minutes or longer, preferably 40 minutes or longer, and more preferably 50 minutes or longer.

[0018] When nitrite is added to the first filtrate, the pH of the solution gradually increases. Along with this, transition metals such as iron and lead contained in the first filtrate change into water-insoluble compounds and precipitate. For the purpose of ensuring the precipitation of transition metals, after the mixing of the first filtrate and nitrite is completed, a pH adjuster may be added to the mixed solution to raise the pH of the mixed solution preferably to 8 or higher, more preferably to 8.5 or higher, and even more preferably to 9 or higher. As the pH adjuster, for example, sodium hydroxide and calcium hydroxide can be used. Even when the pH of the mixed solution rises to this extent, the complex ions of rhodium and the complex ions of platinum group metals other than rhodium can stably exist in the solution. In this regard, it is advantageous to use nitrite ions as the ligand of the complex ions.

[0019] In this step, it is preferable to mix the first filtrate and nitrite after adding a water-soluble iron(III) salt to the first filtrate. Conversely, if a water-soluble iron(III) salt is added after adding nitrite, there is a possibility that nitrite ions will be decomposed and nitrous acid gas will be generated. By adding nitrite after adding the water-soluble iron(III) salt and raising the pH, iron precipitates while incorporating other metals. For example, water-insoluble compounds of iron and lead are formed. Thereby, the concentration of transition metals other than platinum group metals present in the solution can be further reduced. From this viewpoint, the addition amount of the water-soluble iron(III) salt is preferably 100% or more, more preferably 300% or more, and even more preferably 600% or more in terms of iron(III) conversion (g) with respect to the amount of lead (g) in the first filtrate. Also, the addition amount of the water-soluble iron(III) salt is preferably 0.15% or less, more preferably 0.10% or less, and even more preferably 0.05% or less in terms of iron(III) conversion (g) with respect to the liquid volume (L) of the first filtrate. As the water-soluble iron(III) salt, for example, iron(III) chloride, iron(III) nitrate, etc. can be used.

[0020] By the above operations, a complex of rhodium and a complex of a platinum group metal other than rhodium exist in the liquid phase, and other transition metals such as iron and lead precipitate in the form of water-insoluble compounds. By filtering the liquid in this state to remove the water-insoluble compounds, a second filtrate containing rhodium and a platinum group metal other than rhodium is obtained. The distribution ratio of rhodium at this point, that is, the ratio of the total mass of rhodium contained in the second filtrate to the total mass of rhodium contained in the first filtrate, preferably reaches a high value of 97% or more.

[0021] <(c) Recovery step> In this step, the second filtrate obtained in the previous nitrous acid reaction step is mixed with an ammonium salt or a potassium salt. By this operation, a water-insoluble compound (NH4)3[Rh(NO2)6] or K3[Rh(NO2)6] is formed from the complex ions of rhodium contained in the second filtrate. However, no water-insoluble compound is formed from the complex ions of platinum group metals other than rhodium. That is, the complexes of platinum group metals other than rhodium are distributed to the liquid phase side. Due to this difference, rhodium and platinum group metals other than rhodium can be separated.

[0022] From the viewpoint of successfully forming a water-insoluble compound from the rhodium complex, as the ammonium salt, water-soluble compounds such as ammonium halides such as ammonium chloride and ammonium salts of mineral acids such as ammonium nitrate can be used. From a similar viewpoint, as the potassium salt, water-soluble compounds such as potassium halides such as potassium chloride and potassium nitrate can be used. These compounds may be used alone or in combination of two or more.

[0023] As described above, by mixing the second filtrate with an ammonium salt or a potassium salt, a water-insoluble compound containing rhodium precipitates. By filtering the solution in this state to recover the water-insoluble compound, rhodium can be separated from platinum group metals other than rhodium and recovered. The distribution ratio of rhodium at this point, that is, the ratio of the total mass of the recovered rhodium to the total mass of rhodium contained in the second filtrate, preferably reaches a high value of 99% or more.

[0024] The water-insoluble compound of rhodium recovered in this way can isolate rhodium from the compound. Alternatively, an operation can be performed to remove impurities contained in the compound to improve the purity of rhodium.

[0025] In order to improve the purity of rhodium, it is preferable to perform a post-treatment process described below, for example. (d1) First, a dissolution step is performed in which the water-insoluble compound of rhodium obtained in the above-described recovery step is mixed with hydrochloric acid to obtain a solution in which the compound is dissolved in water. As a result, the rhodium contained in the compound dissolves in water, and the impurities contained in the compound also dissolve in water. (d2) Next, a filtration step is performed in which the solution is mixed with a nitrite to generate a complex ion in which nitrite ions are coordinated to rhodium in the mixed solution, and the mixed solution is filtered to obtain a filtrate. This filtrate contains a complex ion of rhodium. (d3) Next, a water-insoluble compound generation step is performed in which the filtrate containing the complex ion is mixed with an ammonium salt or a potassium salt to generate a water-insoluble compound containing rhodium. This water-insoluble compound is separated and recovered by filtration. By the above operations, the purity of rhodium in the water-insoluble compound containing rhodium can be increased.

[0026] By using hydrochloric acid in the step of (d1), the complex ion [Rh(NO2)6], which is a complex ion containing rhodium, 3- the complex ion can be successfully converted into the complex ion [RhCl6]. 3- The complex ion [RhCl6]3- Generating complex ions is advantageous from the perspective of successfully generating complex ions in the subsequent step (d2) of [Rh(NO2)6]. 3- It is advantageous from the viewpoint of successfully generating complex ions.

[0027] The hydrochloric acid used in the step (d1) preferably has a concentration of 6 mol / L or more, and may be 9 mol / L or more, or may be 12 mol / L or more.

[0028] The amount of hydrochloric acid used only needs to be such that the water-insoluble compound of rhodium is sufficiently dissolved, but preferably the number of moles of hydrochloric acid is 5.8 mol or more, may be 8.7 mol or more, or may be 11.6 mol or more with respect to 100 g of rhodium. Thereby, in the subsequent step (d2), [Rh(NO2)6] 3- Complex ions can be successfully generated.

[0029] In the step (d1), when the water-insoluble compound of rhodium and hydrochloric acid are mixed, NO2, which is a ligand of the complex ion [Rh(NO2)6], is 3- substituted by Cl - and nitrous acid gas is generated. After the generation of nitrous acid gas is no longer observed, aging is performed by raising the liquid temperature to preferably 70°C or more and 100°C or less, and it is preferable to surely perform the substitution of NO2 and Cl. - It is preferable to surely perform the substitution of NO2 and Cl.

[0030] Regarding the details of the nitrite used in the step (d2), the description of the nitrite used in the above-described (b) nitrous acid reaction step applies. Regarding the conditions for generating the complex ion of rhodium using nitrite in the step (d2), the description in the above-described (b) nitrous acid reaction step also applies.

[0031] (d3) The details of the ammonium salt or potassium salt used in the step (d3) are the same as those of the ammonium salt or potassium salt used in the above-described (c) recovery step. The description of the production conditions of the water-insoluble compound in the step (d3) is also applicable to the description in the above-described (c) recovery step.

[0032] The post-treatment step including the above steps (d1) to (d3) may be carried out only once, or may be carried out a plurality of times, i.e., two or more times. The higher the number of times the post-treatment step is carried out, the higher the purity of rhodium finally obtained. However, since it takes time and effort accordingly, considering the balance between the purity of rhodium and efficiency, the number of times the post-treatment step is carried out is preferably one or more and five or less times, and more preferably two or more and four or less times.

[0033] After mixing and dissolving (NH4)3[Rh(NO2)6] or K3[Rh(NO2)6], which is a water-insoluble compound containing rhodium obtained in this way, with hydrochloric acid, it is treated with a basic substance to produce Rh(OH)3, which is a hydroxide of rhodium. By reducing the produced Rh(OH)3, metallic rhodium can be obtained. For example, hydrazine and hydrogen gas can be used for the reduction of Rh(OH)3. Only hydrogen gas may be used for the reduction of Rh(OH)3, or hydrazine and hydrogen gas may be used sequentially. In the reduction using hydrogen gas, the atmosphere is preferably 800 °C or higher and 1000 °C or lower, and more preferably 850 °C or higher and 900 °C or lower. Further, the reduction using hydrogen gas may be carried out in an atmosphere of 100% hydrogen gas, or hydrogen gas diluted with an inert gas may be used from the viewpoint of safety.

Example

[0034] Hereinafter, the present invention will be described in more detail by way of examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, “%” means “mass %”.

[0035] 〔Example 1〕 (1) Preparation of an acidic aqueous solution containing rhodium An aqueous hydrochloric acid solution containing the metals shown in Table 1 below in the amounts shown in the same table was prepared. The pH of this acidic aqueous solution was -1.5. In the same table, "-" indicates that the analysis was not performed.

[0036] [Table 1]

[0037] (2) Neutralization step 11.6 kg of calcium hydroxide was added to 58 L of the acidic aqueous solution to neutralize the acidic aqueous solution. The pH of the aqueous solution after neutralization was 2. This aqueous solution was filtered to remove the water-insoluble matter and obtain a first filtrate.

[0038] (3) Nitrous acid reaction step Iron(III) chloride was added to the first filtrate (liquid volume: 66 L) obtained in the neutralization step. The addition amount was adjusted so that the concentration of iron in the solution became 500 mg / L. Next, sodium nitrite was added to the solution. The addition amount was adjusted so that the pH of the solution became 4. After the addition of sodium nitrite, the solution was heated to raise the temperature to 70 °C and this temperature was maintained for 3 hours. As a result, [Rh(NO2)6] 3- complex ions were generated in the solution. Thereafter, the solution was cooled to 25 °C, and then an aqueous sodium hydroxide solution was added to raise the pH of the solution to 9. Next, the solution was filtered to remove the water-insoluble matter and obtain a second filtrate.

[0039] (4) Recovery step Ammonium chloride was added to the second filtrate (liquid volume: 140 L). The addition amount was adjusted so that the concentration of ammonium chloride in the second filtrate became 30 g / L. As a result, (NH4)3[Rh(NO2)6] was precipitated and recovered by filtration. The analytical values of each metal in (NH4)3[Rh(NO2)6], which was the precipitate obtained at this point, were as shown in Table 2 below.

[0040] [Table 2]

[0041] [Example 2] (d1) Dissolution step (NH4)3[Rh(NO2)6] recovered in the (4) recovery step of Example 1 was mixed with a 36% hydrochloric acid aqueous solution to dissolve rhodium in the aqueous solution. The amount of the hydrochloric acid aqueous solution was adjusted so that the amount of the hydrochloric acid aqueous solution was 10 L per 1 kg of metallic rhodium. The thus obtained solution was diluted twice with ion-exchanged water, and then an aqueous sodium hydroxide solution was added to adjust the pH of the solution to 2.

[0042] (d2) Filtration step Iron(III) chloride was added to the solution obtained in the dissolution step. The addition amount was adjusted so that the concentration of iron in the solution became 500 mg / L. Next, sodium nitrite was added to the solution. The addition amount was adjusted so that sodium nitrite was 4.8 kg per 1 kg of metallic rhodium. After the addition of sodium nitrite, the solution was heated to raise the temperature to 70°C, and this temperature was maintained for 3 hours. As a result, [Rh(NO2)6] 3- complex ions were generated. Thereafter, the solution was cooled to 25°C, and then an aqueous sodium hydroxide solution was added to raise the pH of the solution to 9. Next, the solution was filtered to remove water-insoluble components to obtain a filtrate.

[0043] (d3) Water-insoluble compound formation step Ammonium chloride was added to the filtrate obtained in the filtration step. The addition amount was adjusted so that ammonium chloride was 1.9 kg per 1 kg of metallic rhodium in the filtrate. As a result, (NH4)3[Rh(NO2)6] was precipitated and recovered by filtration. The analytical values of each metal at this point were as shown in Table 3 below.

[0044] [Examples 3 to 5] The (d2) dissolution step, (d3) filtration step, and (d4) water-insoluble compound formation step in Example 2 were repeated 2 more times (3 times in total, Example 3), 3 more times (4 times in total, Example 4), and 4 more times (5 times in total, Example 5). The analytical values of each metal were as shown in Table 3 below. Regarding Example 4, the steps (5) to (7) described below were also performed.

[0045] (5) Rhodium hydroxide formation step (NH4)3[Rh(NO2)6] obtained in Example 4 was mixed with a 36% hydrochloric acid aqueous solution to dissolve rhodium in the aqueous solution. The amount of the hydrochloric acid aqueous solution was adjusted so that the amount of the hydrochloric acid aqueous solution was 10 L with respect to 1 kg of metallic rhodium. The thus obtained solution was diluted 2-fold with ion-exchanged water, and then an aqueous potassium hydroxide solution was added to adjust the pH of the solution to 12. Then, a 6% nitric acid aqueous solution was added to adjust the pH of the solution to 5 to form rhodium hydroxide. The formed rhodium hydroxide was recovered by filtration.

[0046] (6) Reduction step 1 After adding ion-exchanged water to rhodium hydroxide, an aqueous sodium hydroxide solution was added to adjust the pH of the solution to 12 and the temperature to 70 °C. Hydrazine was added to this aqueous solution to reduce rhodium hydroxide, and black rhodium black was obtained.

[0047] (7) Reduction step 2 The black rhodium black obtained in Reduction step 1 was heated and reduced in hydrogen gas diluted with nitrogen gas (concentration: 3.95 vol%). The heating temperature was 860 °C. Metallic rhodium was thus obtained. The purity of rhodium in the obtained metallic rhodium was 99.98%.

[0048]

Table 3

[0049] 〔Comparative Example 1〕 (1) Neutralization step 2 L of the same acidic aqueous solution as used in Example 1 was prepared, and sodium hydroxide was added thereto to neutralize the acidic aqueous solution. The pH of the aqueous solution after neutralization was 2. This aqueous solution was filtered to remove water-insoluble components, obtaining a first filtrate.

[0050] (2) Nitrous acid reaction step Sodium nitrite was added to the first filtrate (liquid volume: 1.7 L) obtained in the neutralization step. The addition amount was adjusted so that the pH of the solution became 4. After the addition of sodium nitrite, the solution was heated to raise the temperature to 70 °C, and this temperature was maintained for 3 hours. As a result, [Rh(NO2)6] 3- complex ions were generated in the solution. Thereafter, the solution was cooled to 25 °C, and then an aqueous sodium hydroxide solution was added to raise the pH of the solution to 9. Next, the solution was filtered to remove water-insoluble components, obtaining a second filtrate. The distribution ratio of rhodium at this point, that is, the ratio of the total mass of rhodium contained in the second filtrate to the total mass of rhodium contained in the first filtrate, was a low value of 36%. This is presumably because the solubility of sodium hexanitrorhodate(III) decreased due to the high sodium concentration.

[0051] (3) Recovery step Ammonium chloride was added to the second filtrate (liquid volume: 2 L). The addition amount was adjusted so that the concentration of ammonium chloride in the second filtrate became 30 g / L. As a result, (NH4)3[Rh(NO2)6] was precipitated and recovered by filtration. The distribution ratio of rhodium at this point, that is, the ratio of the total mass of rhodium recovered to the total mass of rhodium contained in the second filtrate, was a low value of 48%. The analytical values of each metal at this point were as shown in Table 2 below.

[0052] 〔Comparative Example 2〕 (1) Potassium salt formation step When potassium hydroxide or potassium chloride was added to the same acidic aqueous solution used in Example 1, potassium salts of rhodium were formed in both cases. Therefore, inexpensive potassium chloride was used. 130 L of the same acidic aqueous solution as used in Example 1 was prepared, and potassium chloride was added to this acidic aqueous solution to a concentration of 120 g / L to obtain a potassium salt of rhodium (K3[RhCl6]). This aqueous solution was filtered to obtain 47 kg of the potassium salt of rhodium. At this point, the distribution ratio of rhodium, that is, the ratio of the total mass of the recovered rhodium to the total mass of rhodium contained in the filtrate, was a low value of 58%. It is presumed that this is because Rh dissolved during the washing of the potassium salt, resulting in a low recovery rate of rhodium.

[0053] (2) Warm water extraction step 47 kg of the precipitate obtained in the step (1) was mixed with 160 L of warm water at 70 °C to extract rhodium into the aqueous phase. This aqueous solution was filtered to remove water-insoluble components and obtain 170 L of filtrate.

[0054] (3) Nitrous acid reaction step 150 L of the filtrate obtained in the step (2) was adjusted to pH 2.0 with sodium hydroxide, and then sodium nitrite was added. The amount of sodium nitrite added was adjusted so that the pH of the solution became 4. After the addition of sodium nitrite, the solution was heated to raise the temperature to 70 °C, and this temperature was maintained for 3 hours. As a result, K3[Rh(NO2)6] was precipitated. Then, the precipitated K3[Rh(NO2)6] was recovered by filtration. At this point, the distribution ratio of rhodium, that is, the ratio of the total mass of the recovered rhodium to the total mass of rhodium contained in the filtrate, was a high value of 95%. The analytical values of each metal in K3[Rh(NO2)6], which is the precipitate obtained at this point, were as shown in Table 2 below.

[0055] As is clear from the results shown in Table 2 and Table 3, it can be seen that according to the present invention, rhodium can be recovered at a high recovery rate. It can be seen that the purity of rhodium can be further increased by increasing the number of repetitions of the post-treatment process in particular.

Claims

1. A neutralization step of filtering a mixed solution of an acidic aqueous solution containing rhodium and a platinum group metal other than rhodium and a calcium compound to obtain a first filtrate; A nitrous acid reaction step of mixing the first filtrate and a nitrite to generate a complex ion in which nitrite ions are coordinated to rhodium in the mixed solution, and then filtering the mixed solution to obtain a second filtrate; A recovery step of mixing the second filtrate and an ammonium salt or a potassium salt to generate a water-insoluble compound containing rhodium and recovering the water-insoluble compound. A method for recovering rhodium comprising these steps.

2. After the recovery step, a dissolution step of mixing the water-insoluble compound and hydrochloric acid to dissolve the water-insoluble compound and obtain a solution containing rhodium; A filtration step of mixing the solution obtained in the dissolution step and a nitrite to generate a complex ion in which nitrite ions are coordinated to rhodium in the mixed solution, and then filtering the mixed solution to obtain a filtrate; The recovery method according to claim 1, further comprising a post-treatment step having an insoluble compound generation step of mixing the filtrate and an ammonium salt or a potassium salt to generate a water-insoluble compound containing rhodium.

3. The recovery method according to claim 2, wherein the post-treatment step is performed multiple times.

4. The recovery method according to claim 1 or 2, wherein in the neutralization step, the calcium compound is used in an amount such that the pH of the solution becomes 3 or less.

5. The acidic aqueous solution contains tin, The recovery method according to claim 1 or 2, wherein the water-insoluble compound of tin generated in the neutralization step is removed by filtration.

6. The acidic aqueous solution contains iron and lead, The recovery method according to claim 1 or 2, wherein after adding a water-soluble iron (III) salt to the first filtrate obtained in the neutralization step, the nitrous acid reaction step is performed to generate insoluble compounds of iron and lead.

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