JP2856010B2 - Recovery method of rhodium from non-reducible rhodium complex ion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copper refining process or a method for selectively recovering rhodium from a substance containing a platinum group element.

[0002]

2. Description of the Related Art When a rhodium ion does not form a complex salt, or forms a chloro complex salt having a small complex stability constant, it is easily reduced to metal by most reducing agents. As described in JP-A-57-35251 and JP-A-58-45125, reduction can be performed quantitatively by reduction with carbon monoxide or hydrogen gas.

[0003] On the other hand, even if the complex ion is stable, if the core metal ion is easily reduced like gold and the ligand is easily decomposed like cyanide ion, see Japanese Unexamined Patent Publication No.
As described in JP-A-163327, a metal can be deposited by reducing a ligand while decomposing it.

[0004] However, when rhodium ions form a non-reducible complex such as a sulfate complex or a phosphate complex,
The complex salt itself and the ligand are chemically extremely stable, and the redox potential of the rhodium ion itself, which is the core of the complex, is relatively low among the noble metal ions. Recovery of rhodium was extremely difficult.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a method for selectively recovering rhodium as a metal in a high yield from a non-reducible rhodium complex salt, which had been difficult to recover by reduction by conventional methods. It is in.

[0006]

Means for solving the problems according to the present invention is to provide an aqueous solution containing a non-reducible rhodium complex ion in which at least 3 mol / l, preferably at least 6 mol / l, of chloride ion coexists. The standard oxidation-reduction potential is 0.8 V
As described above, nitric acid is added as an oxidizing agent so that the voltage is preferably 1.0 V or more, and the reaction is carried out while maintaining the temperature at 50 ° C. or more and the boiling point or less.
The purpose of the present invention is to maintain rhodium in a range of 5 V and to selectively deposit rhodium mainly by adding a reducing agent.

[0007]

According to the present invention, many stable complexes such as rhodium sulfate and phosphate complexes have a much lower standard oxidation-reduction potential than rhodium ions, so that it is extremely difficult to deposit metal by reduction. However, hexachlororhodate (IV) ion [RhCl ₆ ] ^2- , which is a complex salt formed by oxidation in high concentration chloride ion, and
The property that hexachlororhodate (III) ion generated by its decomposition undergoes a reduction reaction relatively easily,
The standard oxidation-reduction potential of the liquid is adjusted to hexachlororhodium (III)
If the acid ions are reduced but the coexisting base metal is maintained at a value that is not reduced, the reaction utilizes that rhodium alone can be selectively collected as a metal.

[0008] To describe the case of diphosphate _{complexes, [Rh (P 2 O 7} ) 3] 9- + 6Cl - → [RhCl 6] 2- + 3P 2 O 7 4- + e - ·· (1) [RhCl 6] _{^{2- + e - → [RhCl 6}} ] 3- ··· (2) [Rh (P 2 O 7) 3] 9- + 6Cl - → [RhCl 6] 3- + 3P 2 O 7 4- ··· (3) ^{_{[RhCl 6] 3- + 3e -}} → 3Rh + 3Cl - ··· (4) [CuCl 3] - + e - → [CuCl 2] - + Cl - ··· (5)

First, when a rhodium diphosphate complex (tris (diphosphato) rhodium (III) ion is oxidized in the presence of an excess of chloride ion, the reaction of the formula (1) gradually proceeds, and hexachlororhodium ( IV) Generates acid ions.
In this reaction, the equilibrium tilts toward the right side as the chloride ion concentration increases, and the activation energy of the rhodium complex is very high. Therefore, the reaction is quicker as the liquid temperature is higher, and the equilibrium is more complete as the reaction time is longer. Reach In addition, hexachlororhodium
(IV) Since an acid ion cannot be generated unless it is in a strong oxidizing atmosphere, it must be maintained in a strong oxidizing atmosphere during the reaction.

If the chloride ion concentration is not more than 3 mol / l, the hexachlororhodate (III) ion formed by the formula (2) will not be generated in an amount of 50% or more. Since it does not participate in the reaction even if it exists in the inside, the required range of chloride ion concentration is 3 mol / l or more,
It is necessary that the concentration is not more than the saturation concentration. In particular, when the concentration is 6 mol / l or more, the equilibrium of the equation (1) is almost completely inclined to the right side.

Regarding the reaction temperature, even if the chloride ion concentration is sufficiently high, if it is lower than 50 ° C., it takes too much time for the reaction to reach equilibrium, which is not practical. Since evaporation is intense, it is necessary that the temperature range be 50 ° C. or higher and the boiling point or lower, particularly around 90 ° C. where the reaction proceeds completely in 1 hour or less.

Although the reaction time depends on the temperature, even if the temperature is raised to near the boiling point, the reaction is incomplete if the temperature is less than 10 minutes. Conversely, if the temperature and the chloride ion concentration are appropriate, the reaction time is within 3 hours. At the end of the reaction.

As the oxidizing agent, a hexaoxidic rhodate (IV) ion is not generated when the oxidizing agent is too weak, and therefore, a oxidizing agent having a standard oxidation-reduction potential of 0.8 V or more and little self-decomposition is desirable. Of the oxidizing agents satisfying this condition, nitric acid is the most suitable because it is hard to undergo self-decomposition and reductive decomposition.
The standard oxidation-reduction potential during the reaction is 1.0 V or more (25
° C), especially if the voltage is kept at 1.1 V, the reaction proceeds quantitatively.

When nitric acid is used as the oxidizing agent, the amount of self-decomposition is small. If no other reducing substance coexists, an amount of about one redox equivalent or more of rhodium is calculated once. Although the addition is sufficient, the use of an oxidizing agent which easily decomposes or an oxidizing agent having a high standard redox potential requires the continuous addition of a large amount of the oxidizing agent, which complicates the operation.

The hexachlororhodate (IV) ion generated by the formula (1) is gradually decomposed into hexachlororhodium (III) ion as shown in the formula (2) when an excess oxidizing agent coexists. The standard oxidation-reduction potential between the ion and the metal is 0.5 V, which is slightly lower than the standard oxidation-reduction potential between Rh ³⁺ / Rh of 0.758 V, but is an ion which can be reduced quite easily.

What is interesting here is that no matter how high the chloride ion concentration or even if the temperature is maintained at a high temperature for a long time, hexachlororhodate (III) ion does not directly progress from the phosphate complex as shown in the formula (3). In this regard, this is because hexachlororhodate (IV) ion
(III) Since the metal ion having a higher valence than the acid ion is the core, the complex stability constant is high even if it is unstable in redox, and the phosphate complex can be formed as long as it is maintained in a strong oxidizing atmosphere. It is considered that the reaction of generating hexachlororhodate (IV) ions is easier to proceed than the reaction of decomposing to generate hexachlororhodate (III) ions.

Next, in the reduction reaction, after hexachlororhodate (III) ion is generated, a general noble metal reduction method can be applied as it is, and rhodium can be collected as a metal according to the formula (4). . However, as described above, since the standard oxidation-reduction potential is lower than that of general noble metal ions, when rhodium is to be completely reduced, copper ions (Cu ²⁺ / C
u: standard oxidation-reduction potential of 0.337 V) is considerably reduced together.

Therefore, in this reduction step, it is necessary to add a reducing agent so that copper is not reduced, but rhodium is reduced to a standard oxidation-reduction potential of 0.3 to 0.5 V, which is reduced. . As the reducing agent, any substance can be used as long as this potential can be maintained. In particular, when bismuth is used, the standard redox potential of the liquid is automatically reduced to 0 without mechanically controlling the redox potential. Since it is maintained at around 0.4 V and hardly dissolved in hydrochloric acid, there is no wasteful consumption. The higher the chloride ion concentration, the more difficultly reducible dichlorocuprate (I) acid is generated by the reaction of the formula (5), and the more difficult it is to deposit copper metal. Therefore, it is preferable not to perform dilution during reduction.

The rhodium metal obtained by the above reaction can be completely dissolved by heating with aqua regia or sulfuric acid, and can be purified according to a conventional method.

[0020]

【Example】

Example 1 Rh: 0.058, Cu: 1.51, Fe: 4.68, C
r: 0.4, Ni: 1.30 kg of powdery raw material containing phosphoric acid groups, sulfate groups and sodium sulfate bound to each weight%
Was dissolved in 0.6 l of 36% HCl. The chloride ion concentration was 6.3 mol / l, and the standard oxidation-reduction potential of this aqueous solution was 709 mV. Then 61% HNO ₃ : 50 ml
Was added and the temperature was raised to 90 ° C., and the mixture was stirred at that temperature for 1 hour to complete the oxidation. The standard oxidation-reduction potential after oxidation is 110
It was 2 mV.

The liquid temperature is maintained at that temperature, and Fe powder is added so that the standard redox potential of the liquid is maintained at 400 mV.
Further, the mixture was stirred for 30 minutes while adding Fe powder so as to maintain the standard oxidation-reduction potential, and then reduced.

After the precipitated black powder was filtered and washed with water, the precipitate and the mother liquor were analyzed to determine the distribution of the main elements in the precipitate. Rh: 97.0, Cu: 1.0% by weight. Met. Thus, if reduction is performed after nitric acid oxidation under high chloride ion concentration, rhodium ions can be reduced almost quantitatively to metal even if rhodium in the raw material exists as a phosphate complex. Can be. Where F
e strong reducing agent such as powder (standard redox potential-0.44
It can be seen that when V) is used, the copper ions are slightly reduced even when the standard oxidation-reduction potential is maintained at an appropriate value.

Example 2 The procedure of Example 1 was repeated, except that 100 g of bismuth particles were used as the reducing agent, and the stirring time for the reduction reaction was changed to 2 hours. The standard oxidation-reduction potential of the solution after completion of the reduction was 424 mV. After filtering the precipitated black powder and washing with water, the precipitate and the mother liquor were analyzed, and the partition ratio to the precipitate was determined.
Rh: 97.4, Cu: 0.007, Fe: 0.000
6, Ni: <0.002, Cr: <0.004, respectively.

When bismuth (standard redox potential 0.23 V) is used as the reducing agent, the reducing agent itself is higher than the standard redox potential (about -0.1 V) of dichlorocuprate (I) acid.
Rhodium metal can be selectively obtained by suppressing the deposition of copper without performing complicated operations for controlling the oxidation-reduction potential. Of course, iron, nickel and chromium which are harder to reduce than dichlorocuprate (I) acid are not reduced.

Comparative Example 1 The same operation as in Example 2 was performed except that the oxidation treatment with nitric acid was not performed, and no rhodium metal was deposited. Therefore, in order to reduce the rhodium phosphate complex, not only the chloride ion but also the standard oxidation-reduction potential must be 8
It can be seen that it is essential to oxidize while maintaining at or above 00 mV.

Comparative Example 2 The oxidizing agent was changed to nitric acid, NaClO (standard redox potential 1.63 V) was used, and the standard redox potential of the solution was 1200.
The same operation as in Example 2 was carried out except that the addition was continued until the voltage reached mV or more. The rhodium reduction rate was determined by analyzing the precipitate and the liquid, and was found to be 24% by weight. Na as oxidizing agent
The same operation was performed using Ce (NO ₃ ) ₄ (standard oxidation-reduction potential 1.74 V) instead of ClO, but the reduction ratio of rhodium was 25.6% by weight. Thus, it can be seen that when a strong oxidizing agent that is easily reduced and decomposed by hydrochloric acid is used, rhodium cannot be sufficiently decomposed into a form that is easily reduced.

Comparative Example 3 In Comparative Example 2, after adding NaClO, the mixture was left at room temperature for 24 hours without heating to 90 ° C.
0 g was added and the mixture was stirred for 2 hours to perform reduction, but no rhodium was precipitated. Thus, it can be seen that it is necessary to maintain the liquid temperature at 50 ° C. or higher in order to decompose the non-reducible rhodium complex ion.

[0028]

According to the present invention, rhodium can be selectively recovered as a metal in a high yield from a rhodium complex solution in which it has been extremely difficult to recover rhodium by reduction in the conventional method.

Claims (4)

(57) [Claims] A nitric acid is added as an oxidizing agent to an aqueous solution containing a non-reducible rhodium complex ion in which 3 mol / l or more of chloride ions are coexisted so that the standard oxidation-reduction potential is 0.8 V or more. The reaction is carried out while maintaining the temperature below the boiling point, and then the standard oxidation-reduction potential of the aqueous solution is set to 0.3 to 0.5.
A method for recovering rhodium from a non-reducible rhodium complex ion, wherein rhodium is mainly selectively precipitated by adding a reducing agent while maintaining the range of V. 2. An nitric acid as an oxidizing agent is added to an aqueous solution containing a non-reducible rhodium complex ion in which 6 mol / l or more of chloride ions coexist so that a standard oxidation-reduction potential becomes 1.0 V or more. 2. The method for recovering rhodium from a non-reducible rhodium complex ion according to item 1. 3. The method for recovering rhodium from a non-reducible rhodium complex ion according to claim 1, wherein the non-reducible rhodium complex ion is a sulfate complex or a phosphate complex. 4. The method according to claim 1, wherein the reducing agent is bismuth.
3. The method for recovering rhodium from a non-reducible rhodium complex ion according to any one of 3.