JP7635966B2 - Palladium extractant and method for separating palladium - Google Patents

Description

The present invention relates to a palladium extractant and a method for separating palladium.

Palladium, platinum, and rhodium are the three platinum group metals that are used in automobile exhaust catalysts. Due to the worldwide spread of exhaust gas regulations and their strengthening, and the increasing demand for these three platinum group metals, they have become metal resources that are difficult to secure in a stable manner. As demand increases, the amount of used automobile exhaust catalysts is also steadily increasing. Since these metals are expensive and valuable as resources, they are recovered and reused after use. In order to supply a constant amount of platinum group metals, it is very important to increase the efficiency of the metal refining process (primary resource) and the separation and purification of platinum group metals from used products (secondary resource).

For the purification of platinum group metals in the separation process, electrolytic deposition, ion exchange, and precipitation methods have been proposed, but solvent extraction is widely used due to its selectivity, economy, and ease of operation. Various extractants for use in solvent extraction have been developed and are in use. Dialkyl sulfide (DAS) is currently used as a known palladium extractant (e.g., Patent Documents 1 and 2), and the former is characterized by its ability to be back-extracted with an aqueous ammonia solution.

JP 10-130744 A Patent Publication 2005-146326

In the actual process of separating and refining platinum group metals using solvent extraction, the components containing platinum group metals present in primary or secondary resources are leached using hydrochloric acid/chlorine gas or aqua regia to produce an acidic solution, which is then subjected to extraction processing. When DAS is used under these conditions, palladium is selectively extracted from the acidic solution containing palladium, platinum, rhodium, and base metals, but a significant amount of undesired metals are also extracted, and there is also the problem of a slow extraction speed.

As described above, the objective of the present invention is to provide a palladium extractant that (1) is capable of selectively extracting palladium from a solution containing a mixture of platinum, rhodium, base metals, etc., and (2) is capable of extracting palladium in a short period of time, and a method for separating palladium using the extractant.

After extensive research, the inventors discovered that the above problems can be solved by using a palladium extractant that contains a specific pincer-type compound as an active ingredient.

The first invention is a palladium extractant characterized by having as its active ingredient thiodibenzene containing two thioether groups represented by the following general formula (1):

（式（１）中、Ｒ^１およびＲ^２は、互いに同一であっても異なっていてもよく、直鎖でも分岐していてもよい炭素数１～１２の鎖式炭化水素基から選ばれる基を表す。）

(In formula (1), R ¹ and R ² may be the same or different and represent a group selected from linear or branched chain hydrocarbon groups having 1 to 12 carbon atoms.)

The second invention is a method for separating palladium, which involves contacting an aqueous phase consisting of an acidic aqueous solution in which palladium is present with an organic phase containing the palladium extractant of the first invention, thereby extracting palladium into the organic phase.

In the second invention, the aqueous phase is preferably an acid leachate obtained by treating waste containing a mixture of various metals, including at least palladium, with an acid to obtain an aqueous solution.

The palladium extractant of the present invention and the method for separating palladium using said extractant (1) make it possible to selectively extract palladium from a solution containing a mixture of platinum, rhodium, base metals, etc., and (2) make it possible to extract palladium in a short period of time.

1 is a graph showing the extraction rate (E%) of Pd(II) versus shaking time in a palladium extraction experiment in the examples. 1 is a graph showing the results of an extraction experiment from a palladium only solution in a hydrochloric acid bath having different hydrochloric acid concentrations in an embodiment. 1 is a graph showing the results of an experiment on selective extraction of palladium from a simulated solution in an embodiment. 2 is a graph showing the results of an experiment on selective extraction of palladium from a solution obtained by acid leaching an automobile exhaust catalyst according to an embodiment of the present invention.

<Palladium Extractant>
The palladium extractant of the present invention is characterized in that it contains, as an active ingredient, thiodibenzene having two thioether groups represented by the following general formula (1):

（式（１）中、Ｒ^１およびＲ^２は、互いに同一であっても異なっていてもよく、直鎖でも分岐していてもよい炭素数１～１２の鎖式炭化水素基から選ばれる基を表す。）

(In formula (1), R ¹ and R ² may be the same or different and represent a group selected from linear or branched chain hydrocarbon groups having 1 to 12 carbon atoms.)

Pincer-type metal complexes have been used as catalysts (organic synthesis reactions), fluorescent materials, and electrode materials, but little research has been done on their use as extractants. The inventors have discovered that the palladium extractant of the present invention, which has a specific pincer-type ligand structure, can exhibit excellent selectivity and rapidity as a palladium extractant by utilizing the pincer ligand's characteristic of "the pincer ligand's ability to hold the metal in a crab-like grip."

The palladium extractant of the present invention has a specific thiodibenzene structure represented by formula (1) as an active ingredient. Here, "having as an active ingredient" means that the palladium extractant may contain, in addition to the active ingredient, for example, a solvent or various additives.

The pincer-type extractant of the present invention can separate palladium with a high extraction rate from a solution containing the platinum group elements palladium, platinum, rhodium, and other base metals. The skeleton of the extractant is a compound in which thioether groups have been introduced at two locations on thiodibenzene, which has a sulfur element at the bridging portion (formula (1)), and it is a new compound as a palladium extractant. This extractant is capable of extracting palladium by chelation. It has been demonstrated that the dithioether group-containing dibenzene extractant of the present invention can extract palladium from the leachate of used automobile exhaust catalysts with high selectivity, high efficiency, and rapidity.

In general formula (1), R ¹ and R ² in one molecule may be the same or different, but from the viewpoint of ease of synthesis, they are preferably the same. R ¹ and R ² are hydrocarbon groups having 1 to 12 carbon atoms which may be branched, preferably linear or branched alkyl groups having 1 to 12 carbon atoms, more preferably linear or branched alkyl groups having 6 to 10 carbon atoms, even more preferably linear or branched alkyl groups having 7 to 9 carbon atoms, and even more preferably linear alkyl groups having 7 to 9 carbon atoms.

<Method of separating palladium>
The method for separating palladium of the present invention includes the steps of preparing an organic phase containing the above-mentioned palladium extractant, preparing an aqueous phase consisting of an acidic aqueous solution in which palladium is present, and extracting palladium into the organic phase by contacting the aqueous phase with the organic phase.

(Organic phase containing palladium extractant)
The solvent used in the organic phase containing palladium extractant is a water-insoluble solvent, and two or more solvents may be used in combination.The water-insoluble solvent is not particularly limited as long as it can dissolve the thiodibenzene compound containing two thioether groups represented by general formula (I), and includes mineral oil such as petroleum and kerosene; aliphatic hydrocarbons such as hexane, heptane, octane, and dodecane; aromatic hydrocarbons such as toluene and xylene; halogenated solvents such as carbon tetrachloride, methylene chloride, chloroform, and ethylene chloride.Among them, dodecane, toluene, and kerosene are preferred from the viewpoint of solubility.

The concentration of the thiodibenzene compound containing two thioether groups represented by general formula (I) in the organic phase is not particularly limited except for the upper limit determined by the solubility of the compound, but if the concentration is too low, the palladium extraction effect cannot be obtained, so it is usually used in the range of 0.1 mM to 1 M.

(Aqueous phase consisting of an acidic aqueous solution containing palladium)
In the acidic aqueous solution containing palladium, the acid concentration, for example, the hydrochloric acid concentration, the nitric acid concentration, or the concentration of a hydrochloric acid-nitric acid mixed solution is preferably 2 M or less and 5 M or more, more preferably 1 M or less and 6 M or more, even more preferably 0.5 M or less and 7 M or more, still more preferably 8 M or more, and particularly preferably 0.1 M or less and 9 M or more.

The acid contained in the acidic aqueous solution is not particularly limited as long as it is water-soluble, and inorganic acids can be used. Examples of inorganic acids include hydrochloric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, sulfuric acid, nitric acid, phosphoric acid, and hydrogen peroxide. Two or more types of acids may be contained. From the viewpoint of metal solubility, hydrochloric acid, nitric acid, or a mixture thereof is preferred.

There is no particular limit to the concentration of palladium in the acidic aqueous solution containing palladium, and it is usually about 50 to 1000 ppm.

In the palladium separation method of the present invention, the acidic aqueous solution containing palladium is preferably an acid leachate obtained by treating waste containing a mixture of various metals including at least palladium with an acid to produce an aqueous solution. By using such an acid leachate, it becomes possible to further demonstrate the excellent recognition, selectivity, efficiency, and rapidity of the palladium extraction agent of the present invention. An example of a waste containing a mixture of various metals including at least palladium is an automobile exhaust catalyst.

The metals other than palladium contained in the acid leaching solution are not particularly limited, and may include alkali metals, alkaline earth metals, transition metals, 3B metals, etc., and in particular, in terms of exhibiting the selectivity of the palladium extractant of the present invention, it is preferable to include at least one selected from platinum, rhodium, lanthanum, rare earths, zirconium, and base metals. This allows rare and useful palladium to be recycled. In addition, since the amount of palladium contained in the acid leaching solution after separation of palladium can be greatly reduced, it can also be used as an operation to remove palladium. Therefore, the present invention can also be used as a pretreatment when isolating useful metals other than palladium from the acid leaching solution obtained by treating waste with acid to turn it into an aqueous solution.

(Extraction conditions)
The extraction temperature is not particularly limited as long as it is equal to or lower than the boiling point of the solvent used, and is usually carried out at around room temperature. The extraction operation is carried out by bringing an organic phase containing a palladium extractant and an aqueous phase consisting of an acidic aqueous solution containing palladium into contact with each other by shaking, stirring, etc. Shaking is usually carried out about 100 to 500 times per minute.
The palladium extractant of the present invention can extract palladium in a short time, and the majority of palladium can be extracted with a shaking time of 30 minutes, and almost the entire amount can be extracted with a shaking time of about 1 hour. The shaking time can be adjusted appropriately depending on the conditions of the extractant and the acidic aqueous solution. The lower limit of the shaking time is preferably 5 minutes or more, more preferably 10 minutes or more, even more preferably 30 minutes or more, and particularly preferably 1 hour or more, and the upper limit is preferably 3 hours or less, more preferably 2 hours or less.

<Synthesis of thiodibenzene extractant having dioctylthioether group>
(1) Method for synthesizing 1,1'-N,N-dimethylcarbamoylthio-2,2'-sulfanediyl-4,4'-tert-butylbenzene

A 100 mL recovery flask was charged with 2,2-thiobis[4-tert-butyl-O-N,N-dimethylthiocarbamoylbenzene] (1.26 g, 2.5 mmol) and diphenyl ether (60 mL) as a hot solvent, and reacted at 235° C. under a nitrogen atmosphere for 15 hours. After completion of the reaction, diphenyl ether was removed by distillation under reduced pressure. The resulting brown solid was purified by flash column chromatography using an eluent of hexane:ethyl acetate=1:2 and a column packed with silica gel to obtain the target solid (yield: 69.5%). The structure of the target product was confirmed by ¹ H NMR measurement.

The analytical results of the obtained target substance were as follows.
¹ H NMR (300 MHz, CDCl ₃ , TMS) δ7.50 (d, 2H, ArH), 7.27 (dd, 2H, ArH), 7.22(d, 2H, ArH), 3.08 (s, 6H, -S-(C=O)-N-(CH ₃ ) ₂ ), 2.99 (s, 6H, -S-(C=O)-N-(CH ₃ ) ₂ ), 1.22(q, 18H, -CH ₃ ))

(2) Synthesis of 2,2'-sulfanediyl-4,4'-tert-butyldibenzenethiol

Lithium aluminum hydride (0.75 g, 19.8 mmol) was weighed and placed in a 200 mL flask, and dehydrated tetrahydrofuran (30 mL) was added. 1,1'-N,N-dimethylcarbamoylthio-2,2'-sulfanediyl-4,4'-tert-butylbenzene (1.0 g, 1.98 mmol) dissolved in dehydrated tetrahydrofuran (20 mL) was slowly added dropwise in a dropping funnel and stirred, and then stirred at room temperature under a nitrogen atmosphere for 1 hour. After the reaction was completed, 2N hydrochloric acid (140 mL) was added little by little in an ice bath to quench the reaction, and the mixture was stirred overnight. The target component was extracted with ethyl acetate (20 mL x 5), washed with water (20 mL x 2) and saturated saline (20 mL), and the organic layer was dehydrated with anhydrous sodium sulfate. The brown oily product obtained by removing ethyl acetate by distillation under reduced pressure was purified by flash column chromatography using an eluent of hexane:ethyl acetate=10:1 and a column packed with silica gel to obtain a colorless transparent oily product (yield: 76.6%). The structure of the target product was confirmed by ¹ H NMR measurement.

The analytical results of the obtained target substance were as follows.
¹ H NMR (300 MHz, CDCl ₃ , TMS) δ 7.33 (d, 2H, ArH), 7.18 (d, 2H, ArH), 7.12 (d, 2H, ArH), 3.99 (s, 2H, -SH), 1.19 (q, 18H, -CH ₃ ))

(3) Synthesis of 1,1'-octylsulfanyl-2,2'-sulfanediyl-4,4'-tert-butylbenzene (compound 1)

2,2'-Sulfanediyl-4,4'-tert-butyldibenzenethiol (0.2 g, 0.55 mmol), potassium hydroxide (0.09 g, 1.65 mmol), and ethanol (30 mL) were added to a 100 mL two-neck flask, 1-bromooctane (0.29 mL, 1.65 mmol) was added dropwise, and the mixture was refluxed under a nitrogen stream for 2 hours. The mixture was cooled in an ice bath and neutralized by adding 3N HCl (1 mL). After distilling off the solvent, the reaction mixture was extracted with dichloroethane (10 mL x 5), and the organic layer was dehydrated with anhydrous sodium sulfate. The target component was then purified by flash column chromatography using an eluent of hexane:acetone (100:1) and a column packed with silica gel to obtain the target product (compound 1) as a yellow transparent oil (yield 64.7%).

The ¹ H NMR measurement results of the obtained target product (compound 1) were as follows.
¹ H NMR (300 MHz, CDCl ₃ , TMS) δ 7.28 (d, 2H, ArH), 7.20(dd, 2H, ArH), 7.05(d, 2H, ArH), 2.90 (t, 4H, -S-CH ₂ ), 1.64 (t, 4H, -S-CH ₂ -CH ₂ ), 1.40 (t, 4H, -S-CH ₂ -CH ₂ -CH ₂ ), 1.26 (m, 16H, -S-CH ₂ -CH ₂ -CH ₂ CH ₂ CH ₂ CH ₂ -CH ₃ ), 1.19 (s, 18H, tert-butyl) ,0.87 (t, 18H, -S-CH ₂ -CH ₂ -CH ₂ -CH ₂ -CH ₂ -CH ₂ -CH ₃ ))

<Palladium extraction experiment>
(1) Study of palladium extraction capacity (evaluation of extraction time)
In this example, an HCl solution (0.1 M) containing Pd(II) (1 mM) was prepared in the aqueous phase, and a solution in which the above-synthesized extractant (compound 1) (1 mM) was diluted with toluene was prepared in the organic phase. 10 mL of each solution was collected in a centrifuge tube and shaken at 300 rpm for an arbitrary time. After shaking, the solution was centrifuged at 3500 rpm for 3 minutes, and the concentration of each metal in the solution was measured by ICP emission spectrometry to determine the extraction rate. The extraction rate (E%) was calculated using the following formulas I and II.

E%=[M] _org / [M] _aq,init ×100 (Formula I)
[M] _org = [M] _aq,init - [M] _aq (Formula II)

The definitions of each element in the above formula I and formula II are as follows:
[M] _aq,init : metal concentration in the aqueous phase before extraction [M] _aq : metal concentration in the aqueous phase after extraction [M] _org : metal concentration in the organic phase after extraction

The change in the extraction rate (E%) of Pd(II) versus shaking time is shown in Figure 1. The vertical axis represents the extraction rate (E%) and the horizontal axis represents the shaking time (hours).
1, it was found that the extraction rate was about 60% at 0.5 hours of shaking, and most of the Pd(II) was extracted after 1 hour. After 1 hour, the extraction rate was maintained at about 100%.

(2) Extraction experiment from a palladium-only solution in a hydrochloric acid bath with different hydrochloric acid concentrations (evaluation of the hydrochloric acid concentration at which a high extraction rate can be maintained)
In this example, an extraction experiment was carried out from an acidic solution containing palladium alone with varying hydrochloric acid concentrations. The extractant (compound 1) was dissolved in a diluent of toluene to make 1 mM, which was used as the organic phase. An equal volume of each palladium solution prepared to a concentration of 1 mM using 0.1M to 10.0M hydrochloric acid as shown in FIG. 2 was added to 10 mL of the organic phase, and palladium was extracted into the organic phase by vigorously shaking (300 rpm) for 3 hours. Thereafter, centrifugation was carried out in the same manner as in (1) above, and the palladium concentration in the aqueous phase was analyzed using an ICP emission spectrometer, and the extraction rate (E%) was calculated based on the obtained results using the above formulas I and II.

Figure 2 shows the change in extraction rate when the hydrochloric acid concentration is changed. The vertical axis represents the extraction rate (%) and the horizontal axis represents the hydrochloric acid concentration (M). Although the extraction rate was low in the range of hydrochloric acid concentrations from 1.0 to 7.0 M, the other hydrochloric acid concentrations showed extraction rates of 94% or more. From the above, it can be concluded that palladium can be effectively extracted by applying hydrochloric acid concentrations other than the above 1.0 to 7.0 M.
In addition, the values (M) on the horizontal axis of each plot in FIG. 2 are 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10.

(3) Selective extraction of palladium from a simulated solution (containing 13 types of metals) prepared by selecting the metals contained in automobile exhaust catalysts (evaluation of palladium selectivity)
Thirteen kinds of metals contained in automobile exhaust catalysts were selected, and a 0.1M HCl solution (simulation solution) was prepared so that each metal was 100 ppm. 10 mL of an organic phase in which an extractant (compound 1) was dissolved in toluene to a concentration of 1 mM, and 10 mL of the above-mentioned simulation solution (aqueous phase) were weighed into a 50 mL centrifuge tube and shaken at 300 rpm for 6 hours. Then, centrifugation was performed in the same manner as in (1) above, and the concentration of each metal in the aqueous phase was analyzed with an ICP emission analyzer, and the extraction rate (E%) was calculated based on the obtained results using the above formulas I and II.

The extraction rate (E%) of each metal is shown in Figure 3. The vertical axis shows the extraction rate (E%) and the horizontal axis shows each metal ion.
As shown in Figure 3, palladium was extracted at 99%, and the extraction rates of other metal species were 5.0% or less, indicating high selectivity for palladium. From this result, it can be seen that when a toluene solution containing 1 mM of the extractant (compound 1) is contacted with a simulated solution containing 13 metals, palladium can be extracted at 90% or more, and effectively separated.

(4) Selective extraction of palladium from acid leached solutions of automobile exhaust catalysts (evaluation of palladium selectivity)
The raw solution of the leachate (the leachate obtained by leaching powdered used automobile catalysts with 11.7 M hydrochloric acid and 1 vol% hydrogen peroxide) that contains at least palladium and various other metals discharged from a factory was treated with acid to form an aqueous solution (Al(III): 8720 ppm, Ca(II): 1000 ppm, Cr(III): 106 ppm, Cu(II): 61 ppm, Fe(III): 2147 ppm, Mg(II): 1960 ppm, Ni(II): 170 ppm, Pd(II): 170 ppm, Pb ... ), 472 ppm, Pt(IV): 934 ppm, Rh(III): 71 ppm, La(III): 128 ppm, Ce(III): 7160 ppm) (aqueous phase) and 10 mL of an organic phase in which an extractant (compound 1) was diluted with toluene to a concentration of 4 mM were mixed, and the organic phase and aqueous phase were vigorously shaken (300 rpm) for 6 hours. Thereafter, the concentration of each metal in the aqueous phase was analyzed using an ICP emission analyzer, and the extraction rate (E%) was calculated based on the results obtained using the above formula I and formula II.

The extraction rate (E%) of each metal is shown in Figure 4. The vertical axis shows the extraction rate (E%) and the horizontal axis shows each metal ion.
As shown in Figure 4, palladium was selectively extracted at about 92%, and other metal species were hardly extracted, demonstrating high selectivity for palladium. From these results, it can be seen that when a toluene solution containing 4 mM of the extractant (compound 1) is brought into contact with an acid leaching solution of an automobile exhaust catalyst, palladium can be extracted at a rate of more than 90%, enabling effective separation.

Claims (3)

A palladium extractant comprising, as an active ingredient, thiodibenzene having two thioether groups represented by the following general formula (1):

(In formula (1), R ¹ and R ² may be the same or different and represent a group selected from linear or branched chain hydrocarbon groups having 1 to 12 carbon atoms.) A method for separating palladium, comprising contacting an aqueous phase consisting of an acidic aqueous solution containing palladium with an organic phase containing the palladium extractant according to claim 1, thereby extracting palladium into the organic phase. The method for separating palladium according to claim 2, wherein the aqueous phase is an acid leachate obtained by treating waste containing a mixture of various metals, including at least palladium, with an acid.