JP3724009B2 - Method for back extraction of platinum in tributyl phosphate - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a technique for separating and purifying platinum obtained from a non-ferrous metal smelting step, a platinum-containing catalyst treatment step, and the like by a solvent extraction method.
[0002]
[Prior art]
In a non-ferrous metal smelting step, a platinum-containing catalyst treatment step, and the like, platinum dissolved by an acid is contained in the aqueous solution as platinum ions together with other metals. The separation and recovery of platinum from this aqueous solution is usually carried out by back extraction with water after extraction using tributyl phosphate (hereinafter abbreviated as TBP) as an extractant.
[0003]
However, in this back extraction step with water, the acid extracted into TBP, which is an organic phase, elutes into the aqueous phase at the same time, so that platinum ions are extracted again into TBP. As a result, back extraction is not possible. Become complete. In order to prevent such a phenomenon, a method is known in which platinum ions are back-extracted while the acid liberated in the aqueous phase during back-extraction is neutralized with an alkali.
[0004]
On the other hand, for elements other than platinum, a method of eluting and recovering metal ions, which are difficult to elute in the extractant or ion exchange resin, to low valence ions by reduction is known. For example, Fe ³⁺ ions extracted to di-2-ethylhexyl phosphate are reduced to Fe ²⁺ ions by SO ₂ and eluted, or [IrCl ₆ ] ²⁻ adsorbed on an anion exchange resin is SO _2. Has been carried out by reducing to “IrCl ₆ ] ³⁻ and recovering.
[0005]
[Problems to be solved by the invention]
As described above, a method of back-extracting with water while neutralizing an acid by adding an alkali is known for separation and recovery of platinum extracted in TBP and contained in the form of H ₂ [PtCl ₆ ]. . However, even with this method, it is basically possible to back extract only platinum ions that are easily extracted in the TBP as an adduct, and platinum ions that are difficult to back extract remain in the TBP. There was a problem. In particular, when the proportion of platinum ions that are likely to remain in TBP increases due to an increase in platinum concentration or the like, complete back extraction of platinum is difficult.
[0006]
On the other hand, it is also conceivable to apply a method of recovery by reduction to platinum. However, there is no problem with ions that are not easily converted to metals such as iridium and iron ions, but the platinum (IV) ions that are usually handled are not limited to the valence of the platinum (II) ions due to the reduction. It is easy to be reduced. Therefore, once reduced to a metallic state, platinum powder cannot be completely recovered unless all the organic and aqueous phases are filtered, so this method has not been adopted.
[0007]
In view of such conventional circumstances, the present invention provides a method capable of almost completely back-extracting platinum ions in TBP even if they are in a form that is difficult to back-extract by conventional methods when back-extracting platinum ions in TBP with water. The purpose is to do.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the method for back extraction of platinum in tributyl phosphate provided by the present invention is obtained by mixing tributyl phosphate containing platinum ions and water, and adding a reducing agent and an alkali. It is characterized in that platinum ions are back-extracted into an aqueous phase by adjusting the pH of the mixed liquid in a stirred state to -0.5 to +7 and the redox potential to -300 to +500 mV (vs. Ag / AgCl electrode). To do.
[0009]
[Action]
In general, when metal ions in an aqueous phase are extracted into an organic phase with an extractant, the metal ions are extracted into the organic phase under conditions where they can easily form a compound or adduct with the extractant to form a compound or adduct. In difficult conditions, extraction will be incomplete. Conversely, for metal ions extracted into the organic phase, metal ions are released into the aqueous phase under conditions where it is difficult to form an extractant and a compound or adduct. In many cases, metal ions remain in the water phase, and the target metal ions cannot be completely recovered in the aqueous phase.
[0010]
The most likely cause of this phenomenon is that metal ions in the organic phase react with the extractant to form a polymer. The polymer formation reaction in the organic phase tends to proceed as the metal ion concentration in the organic phase is higher and the basicity of the metal ion is lower. In the case of the same element, the higher the valence of the ion, the more basic Decreases, it becomes easier to form a polymer. Therefore, by reducing the metal ion in the polymer to a low valence ion, the polymer becomes unstable, so that the metal ion is easily back-extracted.
[0011]
Some platinum ions present in the form of H ₂ [PtCl ₆ ] in TBP are also considered to be in a form that is difficult to back-extract by forming a polymer, but according to the above principle, platinum (IV ) By reducing the ions to platinum (II) ions, it is possible that back extraction is possible even if the back extraction is difficult. However, since platinum ions are easily reduced to metals, as described above, conventionally, platinum (II) ions could not be stably formed by reduction.
[0012]
The present inventors have found that there is a region where platinum (II) ions, which are conventionally considered unstable, depending on pH and redox potential, can exist stably, and based on this finding, pH and redox potential. By controlling and controlling within a certain range, the present invention achieves the method of the present invention in which platinum ions are completely back-extracted with water while preventing precipitation of metallic platinum.
[0013]
In addition, the reaction expected at the time of back extraction is the following [Chemical Formula 1], and the reaction for depositing metal platinum is the following [Chemical Formula 2]:
[Chemical 1]
[PtCl ₆ ] ^2- + 2e ⁻ = [PtCl ₄ ] ^2- + 2Cl ⁻
[Chemical formula 2]
[PtCl ₄ ] ^2- + 2e ⁻ = Pt + 4Cl ⁻
[0014]
The pH at the time of back extraction in the method of the present invention needs to be in the range of −0.5 to +7 . When the pH is less than −0.5, the reduction rate of many reducing agents becomes extremely slow, and the back extraction of platinum ions that are basically easily back-extracted tends to be incomplete due to the high acid concentration. If a reducing agent that works even in such a strong acid is used, the redox potential is locally reduced and the platinum ions are reduced to metal, so total filtration of the organic and aqueous phases is required. It becomes. On the other hand, when the pH exceeds +7 , even if a weak reducing agent is used, the oxidation-reduction potential is lowered and platinum is likely to be precipitated as a metal powder, and the basic salt is likely to be precipitated.
[0015]
It is known that the standard electrode potential of the reaction expected at the time of back extraction shown in the above [Chemical Formula 1] is 480 mV (vs. Ag / AgCl electrode, the same applies hereinafter), and the reaction of [Chemical Formula 2] is 530 mV. . However, in the platinum ion back-extraction reaction in the method of the present invention in which the pH is in the above range, the platinum precipitation reaction does not proceed in the vicinity of −200 mV, and platinum is deposited in the form of powder only when it is lowered to −300 mV. On the other hand, when the equilibrium potential of ## STR1 ## is reached and 500 mV is reached, the reduction reaction becomes extremely slow. Therefore, in the method of the present invention, it is necessary to perform the reduction while controlling the oxidation-reduction potential to be in the range of −300 to +500 mV.
[0016]
However, as a practical problem, depending on the type of reducing agent, it may be difficult to stably maintain the pH and redox potential. For example, when a strong reducing agent is added, even if it falls within the above range at equilibrium, the oxidation-reduction potential tends to decrease temporarily, and platinum tends to precipitate as metal powder. On the other hand, when a weak reducing agent is used, heating or long-term reaction is required even if the addition amount is increased, so that reduction is gradually performed even if a high potential is initially shown. The potential may continue to decrease, and eventually the potential may decrease until platinum is deposited as metal powder.
[0017]
Therefore, in order to stably maintain the target oxidation-reduction potential, it is desirable to use a reducing agent having an appropriate reducing power and low activation energy. Examples of the reducing agent that satisfies such conditions and is easily available industrially include sulfur dioxide, sulfite, and hydrazinium salt. Some organic compound-based reducing agents exhibit a suitable oxidation-reduction potential, but many of them basically have high activation energy, and it is difficult to stably maintain the potential.
[0018]
Among the preferred reducing agents, sulfur dioxide and sulfite have the advantage of promoting the back-extraction reaction and making it difficult to deposit metal platinum by forming platinum ions and sulfite complex ions. Further, the hydrazinium salt has a characteristic that the potential can be stably maintained for any oxidation-reduction potential within a wide range of −1000 to +500 mV depending on pH.
[0019]
The aqueous solution containing platinum (II) ions and excess reducing agent back-extracted with water from TBP by the back-extraction reaction described above is separated from the organic phase, and then the pH is raised as it is to convert platinum into metal powder. It can be deposited and recovered in high yield.
[0020]
【Example】
Example 1
TBP containing 62.7 g / l platinum in the form of H ₂ [PtCl ₆ ] is stirred and mixed with a 5% aqueous solution of hydrazinium dichloride N ₂ H ₆ Cl ₂ at a phase ratio O / A = 1/1 . Sodium bicarbonate NaHCO ₃ was added to increase the pH until the pH of the resulting stirred mixed solution reached 1.05. After confirming that the pH value did not change, the redox potential of the stirred mixed solution was increased. Was measured to be 133 mV.
[0021]
After the aqueous phase and the organic phase were separated, the organic phase was filtered through a filter paper and the filtrate was analyzed. As a result, 0.023 g / l of platinum remained. Therefore, this back extraction rate was 99.96%, and it was found that platinum could be back extracted almost completely. The separated aqueous phase was able to completely recover platinum as metal powder by raising the pH as it was.
[0022]
Example 2
The TBP containing 62.7 g / l of the same platinum as in Example 1, in phase ratio O / A = 1/1 were stirred and mixed with 5% aqueous solution of sulfur dioxide SO _2, the resulting stirred mixture state of liquid NaHCO ₃ was added until the pH reached 0.94, the pH was raised, and after confirming that the pH value did not change, the oxidation-reduction potential of the liquid in the stirred and mixed state was measured and found to be 313 mV.
[0023]
After the aqueous phase and the organic phase were separated, the organic phase was filtered through a filter paper and the filtrate was analyzed. As a result, 0.005 g / l of platinum remained. Therefore, this back extraction rate was 99.99%, and it was found that platinum could be completely back extracted. Although the potential is higher than that in Example 1, it is considered that the back extraction rate was improved by the sulfite ions forming complex ions with platinum ions.
[0024]
Comparative Example 1
When TBP containing 62.7 g / l of the same platinum as in Example 1 was stirred and mixed with water at a phase ratio of O / A = 1/1, the pH of the resulting stirred and mixed liquid was measured to be −0. It was .54. At this point the organic phase was deep yellow and the back extraction was very incomplete.
[0025]
Therefore , NaHCO ₃ was added to increase the pH until the pH of the liquid in the stirring and mixing state reached 1.04. After confirming that the pH value did not change, the oxidation-reduction potential of the liquid in the stirring and mixing state was measured. It was 558 mV.
[0026]
After separating the aqueous phase and the organic phase, the organic phase was filtered through a filter paper and the filtrate was analyzed. As a result, 0.69 g / l of platinum remained. Therefore, this back extraction rate is 98.9%, which indicates that the back extraction is extremely incomplete.
[0027]
Comparative Example 2
TBP containing 62.7 g / l of the same platinum as in Example 1 was stirred and mixed with water at a phase ratio of O / A = 1/1. Sodium hydroxide NaOH and hydrated hydrazine N ₂ H ₄ .H ₂ O Was added to increase the pH of the stirred and mixed solution while lowering the redox potential. At this time , no precipitation of metal powder was observed at a pH of 5.61 and an oxidation-reduction potential of -220 mV of the liquid in the stirred and mixed state .
[0028]
However, when the pH of the liquid in the stirred and mixed state was increased to 7.14 with NaOH, a black precipitate was deposited when the potential dropped to -515 mV. Further, when the pH was raised to 11.05, the oxidation-reduction potential reached -820 mV, and even when NaOH was further added, the potential did not decrease.
[0029]
After the aqueous phase and the organic phase were separated, the organic phase was filtered through a filter paper and the filtrate was analyzed. As a result, 0.001 g / l of platinum remained and the back extraction rate was 99.998%. However, all platinum was distributed as metal powder in the organic phase, the aqueous phase, and the interface thereof, and it was difficult to recover unless all was filtered.
[0030]
【The invention's effect】
According to the present invention, 99.9% or more of the platinum ions, which are contained in TBP and are usually present in a form that is difficult to back-extract, are reversed to the aqueous phase in a single operation without being precipitated as metal powder. Can be extracted. Therefore, the method of the present invention is extremely effective for separation of noble metals, reduction of residence in the platinum group system in the purification field, prevention of reduction in extraction capacity, and the like.

Claims (2)

Tributyl phosphate containing platinum ions and water are mixed, a reducing agent and an alkali are added, the pH of the resulting stirred mixed solution is -0.5 to +7, and the oxidation-reduction potential is -300 to +500 mV. A method for back extraction of platinum in tributyl phosphate, wherein platinum ions are back-extracted into an aqueous phase by adjusting to (to Ag / AgCl electrode). The method for back extraction of platinum in tributyl phosphate according to claim 1, wherein any one of sulfur dioxide, sulfite, and hydrazinium salt is used as the reducing agent.