Journal of Radioanalytical and Nuclear Chemistry, Vol. 246, No. 3 (2000) 693696
Palladium(II) extraction by pyridinecarboxylic acid esters M. Wisniewski Institute of Chemical Technology and Engineering, Poznan University of Technology, 60-965Poznan, Poland (Received March 13, 2000)
The commercial extractant Acorga CLX-50 and model individual di-2-ethylhexyl pyridine-3,5-dicarboxylate and 2-ethylhexyl pyridine-3carboxylate in toluene were used for palladium(II) extraction from aqueous HCl solutions. The studies of extraction rate and equilibrium were carried out in systems containing palladium(II) ions in 3.0, 0.1, and 0.1M HCl in the presence of 0.5M sodium chloride and in 0.1M HCl in the presence of 0.16.0M lithium chloride and in 0.1M HCl in the presence 0.13.5M sodium nitrate. The examined extractants can efficiently extract palladium(II) from aqueous hydrochloric acid and nitrate solutions. The extraction is slow and equilibrium is obtained after 2 hours. The best extraction of palladium(II) is observed from 0.1M HCl solution in the presence of 3.5M sodium nitrate. A spontaneous transfer of palladium(II) to the toluene phase without any phase mixing is also observed.
Introduction
Platinum-group metals, which possess specific physical and chemical properties, are applied to many advanced technology e.g., as industrial or automobile catalyst, as electric conductive and corrosion resistant materials. The solvent extraction of palladium(II) inspired a lot of interest in the last ten years and is realised in industry. Therefore, it is very important to develop an effective technique to recover platinum group metals. Various types of extractants are proposed, e.g., derivatives of hydroxyquinoline, hydroxyoximes, dialkyl sulphides, dialkyl dithiophosphinic acids, dialkyl monothiophosphinic acids, aminoacids, pyridine carboxylates and 4-alkylphenylamines. Dialkyl sulphides, hydroxyoximes and aminoacids are well known as extractants of precious metals, which are used in several plants, including the Action Precious Metal Refinery of INCO, Matthey Rustenberg, Lonhro Refiners. The kinetics of palladium(II) extraction with these extractants is slow. Recently ICI proposed the use of Acorga CLX-50 for solvent extraction of palladium(II) from its acidic solutions. It contains hydrophobic pyridine-3-5dicarboxylates of unknown structure and concentration. One may suppose, that also other derivatives of pyridine carboxylic acids can be used for palladium(II) extraction from chloride solutions. The aim of this work was the synthesis of some model individual hydrophobic pyridine carboxylic acid esters and the studies of extraction rate and equilibrium in systems containing palladium(II) ions in 3M HCl, 0.1M HCl, and 0.1M HCl in the presence of 0.5M sodium chloride and various concentrations of lithium chloride and sodium nitrate. For this process the commercial extractant Acorga CLX-50 and the model extractants synthesised by us according to the method described previously, were used. 113
Experimental Reagents
The formulae of the reagents used for palladium(II) extraction are as follows: RO
O
O
C
C
OR
N
1
CH3
(CH2)3 CH CH2 O
O
O
C
C
C2H5
O
CH2 CH (CH2)3 CH3 C 2H 5
N
2 O C
1417
O
CH2 CH
(CH2)3 CH3
C2H5
N
3
where 1, Acorga CLX-50, 2, di-2-ethylhexyl pyridine-3,5dicarboxylate, 3, 2-ethylhexyl pyridine-3-carboxylate. Palladium(II) extraction
Palladium(II) extraction was carried out typically in a disperse system using equal volumes of the aqueous and the organic phases at ambient temperature.
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Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht
M. W
ISNIEWSKI: PALLADIUM(II) EXTRACTION BY PYRIDINECARBOXYLIC ACID ESTERS
The concentration of extractants in toluene was equal to 5.10 M. Aqueous phases contained 5.10 M of palladium(II) in 3.0M HCl, 0.1M HCl, and 0.1M HCl+0.5M NaCl and in 0.1M HCl in the presence of 0.1, 0.5, 1.0, 3.0, 4.0 and 6.0M lithium chloride, and in 0.1M HCl in the presence of 0.1, 0.5, 1.0, and 3.5M sodium nitrate. Organic phases were prepared by dissolution of extractants in toluene. The palladium(II) content was determined in the aqueous phase before and after extraction by spectrophotometry. 3
3
20
Results and discussion
Figures 1 to 3 show the effect of time and the composition of the aqueous phase (hydrochloric acid and sodium chloride content) on percent of palladium(II) extraction. In each case considered the reaction occurs slowly and the equilibrium is achieved after 2 hours. 1 and 3 exhibit similar extraction abilities permitting to transfer about 55% Pd(II) from 0.1M HCl to the organic phase. An increase of the concentration of both HCl and NaCl decreases the extraction. The results are similar for both systems, i.e., containing 3M HCl and 0.1M HCl+0.5M NaCl. The lowest effect is observed for 3.
Fig. 3. Palladium(II) extraction from 3M HCl, 0.1M HCl and 0.1M HCl in the presence of 0.5M NaCl by 2-ethylhexyl pyridine-3carboxylate (initial palladium(II) and extractant concentration 3 mol.dm3) . 5 10
Fig. 4. Influence of LiCl concentration on palladium(II) extraction by Acorga CLX-50 (0.1M HCl)
Fig. 1. Palladium(II) extraction from 3M HCl, 0.1M HCl and 0.1M HCl in the presence of 0. 5M NaCl by Acorga CLX 50 (initial 3 mol.dm3) . palladium(II) and extractant Acorga concentration 5 10
Fig. 2. Palladium(II) extraction from 3M HCl, 0.1M HCl and 0.1M HCl in the presence of 0.5M NaCl by di-2-ethylhexyl pyridine-3,5dicarboxylate (initial palladium(II) and extractant concentration 3 mol.dm3) . 5 10
694
Under the best conditions, i.e., HCl concentration equal to 0.1M, similar amounts of palladium(II) (55%) are extracted at equilibrium by 1 and 3. The extraction ability of 2 is twice lower (25%). 3 permits a quicker extraction than 1 and the equilibrium is obtained after 25 and 80 minutes for 1 and 3, respectively. Figures 4 and 5 show the effect of LiCl concentration on palladium(II) extraction from 0.1M HCl with 1 and 3, respectively. A very strong effect of LiCl on extraction with Acorga CLX-50 is observed. An addition of 0.1M LiCl decreases the extraction to about 9%, which then increases to 20% with an increase of LiCl concentration (Fig. 4). The effect of LiCl on Pd(II) extraction with 3 is also negative, but the effect is only significant for LiCl concentration ≥1M (Fig. 5). An increase of LiCl concentration always decreases the extraction. The comparison of the extraction behavior of 1 and 3 at equilibrium extraction is demonstrated in Fig. 6. Atypical and different characters of the experimental curves indicate different extraction behavior of the extractants studied and the complex effect of LiCl concentration. An increase of this concentration varies the content of free chloride ion in the aqueous phase, changing the composition of palladium(II) chlorocomplex. Secondly, an addition of electrolyte increases
M. W
ISNIEWSKI: PALLADIUM(II) EXTRACTION BY PYRIDINECARBOXYLIC ACID ESTERS
the ionic strength and decreases the water activity enhancing the extraction. The latter problem was broadly discussed by B at al. To separate these two different effects the experiments were also carried out in the presence of sodium nitrate (Figs 7 to 9). An increase of the extraction ability both of 1 and 3 was observed. OUVIER
21
Fig. 8. Influence of NaNO3 concentration on palladium(II) extraction by 2-ethylhexyl pyridine-3-carboxylate (0.1M HCl)
Fig. 5. Influence of LiCl concentration on palladium(II) extraction by 2-ethylhexyl pyridine-3-carboxylate (0.1M HCl)
Fig. 9. Influence of nitric ions on extraction of palladium(II) in equilibrium extraction by Acorga (1) and 2-ethylhexyl pyridine-3carboxylate (3)
Fig. 6. Influence of LiCl on extraction of palladium(II) in equilibrium extraction from 0.1M HCl (1, Acorga;
3, 2-ethylhexyl pyridine-3-
carboxylate)
Fig. 10. Spontaneous palladium(II) extraction (without mixing) by Acorga (1) and 2-ethylhexyl pyridine-3-carboxylate (3), 3 . (initial solution 5 10 M Pd(II) in 0.1M HCl)
However, the effect was not very strong because the extraction at equilibrium increased from about 55% to 70% with an increase of NaNO concentration from 0.0 to 3.5M. Similar behavior was observed for 1 and 3 (Fig. 9). All this means that the latter effect is caused by an increase of the ionic strength and an decrease of the water activity. The presence of an ambient electrolyte increases the extraction of palladium(II) with extractants 3
Fig. 7. Influence of NaNO3 concentration on palladium(II) extraction by Acorga CLX 50 (0.1M HCl)
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ISNIEWSKI: PALLADIUM(II) EXTRACTION BY PYRIDINECARBOXYLIC ACID ESTERS
considered. The change of the composition of palladium chlorocomplexes is the main factor affecting the extraction. The extractants having a pyridine nitrogen and one or two electron withdrawing substituents can be considered as a solvating or a basic extractant. An increase of the number of substituents decreases the nitrogen basicity retarding in this way the extraction of the ion pairs with palladium anionic chlorocomplexes. Thus, the following equilibrium should be considered: L + H + Cl = L.HCl (1) (2) Pd + Cl − = PdCl − (3) PdCl +2L = PdCl L PdCl − + 2 L ⋅ HCl = PdCl (LH) +2Cl − (4)
A similar phenomenon is also observed for Acorga CLX-50. However, the transfer is significantly slower in respect to that demonstrated for 3. Moreover, a positive effect of the chloride ion concentration (HCl and/or NaCl) is not observed. Conclusions
Acorga CLX-50 and 2-ethylhexyl pyridine-3carboxylate are effective extractants of palladium(II) from acidic chloride solutions. They exhibit, however, different extraction performance caused by a different basicity of the pyridine nitrogen. 2-Ethylhexyl pyridine3-carboxylate is easier protonated than Acorga CLX-50 and can transfer the palladium(II) to the organic phase in the form of solvate PdCl L and chlorocomplex ion pair, e.g., PdCl (LH) and PdCl (LH). As a result, an increase of the extraction rate is observed. Moreover, the of palladium(II) can occur without any mixing PdCl − + L ⋅ HCl = PdCl (LH) +Cl − (5) transfer and the equilibrium is achieved in 72 hours. * PdCl + ( LH ) = PdCl L + 2 H + 2Cl − (6) The negative effects of concentrations of HCl, NaCl, and LiCl show that the palladium(II) is mainly extracted by Reaction (3). The extraction capacity amounts References 0.5 M/M for 1 and 3. An addition of hydrochloric acid and salts containing chloride ions shifts the equilibrium ZYMANOWSKI of Reactions (1) and (2) towards the protonated reagent ñ ZYMANOWSKI and negatively changed chlorocomplexes, respectively. The protonation of the reagent depends, however, on the L AZI REISER basicity of the pyridine nitrogen, and the effect is ISNIEWSKI ZYMANOWSKI ALKOWIAK ICHALEWSKA significantly weaker for 1 in comparison to 3. These ISNIEWSKI ZYMANOWSKI OTE EISSNER reagents contain two and one withdrawing substituents, respectively. This can explain the observed effects of ZYMANOWSKI ROCHASKA LEJSKI LiCl concentration under studied conditions of high Cl ZIWINSKI OTE AUER ZYMANOWSKI concentration; 1 still remains neutral while 3 is already partly protonated. As a result, 1 can not extract anionic OTE AUER AAMACH palladium chlorocomplexes. Such extraction is possible L AZI REISER with the protonated 3. It also explains the overcapacity ZYMANOWSKI AUER OTE ROCHASKA of extraction with 3. ISNIEWSKI AKUBIAK ZYMANOWSKI The conclusion is indirectly supported by the mass transfer of palladium(II) to the organic phase in the ALTON RICE ERMANA OFFMANN system containing 3. A transfer is observed without any AVIES mixing of the phases and obeys the diffusion low. The OHLAND equilibrium is obtained after 72 hours. A spontaneous URGESS ALTON palladium(II) extraction was carried out in a cylinder of ALTON URGESS OWNSON 50 cm capacity (volume of each phase 5 cm; height of organic phase 1.5 cm, aqueous phase 1.4 cm, ALTON URGESS temperature 20 °C). A straight line correlation is ISNIEWSKI ZYMANOWSKI OTE AKUBIAK AUER ZYMANOWSKI OKILI observed between the percent extracted and time (Fig. OITRENAUD 10.). The protonated extractant forms probably the OGACKI AKUBIAK OTE ZYMANOWSKI chlorocomplex ion pair quickly which is transferred to ARCZENKO the organic phase. The solvate PdCl L is then formed in the organic phase. OUVIER OTE IERPISZEWSKI ZYMANOWSKI 0
w
+
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w
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0
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2,0
2,0
4
0
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0
2 i
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2
2,0
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2
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The work was supported by KBN donation for Institute activity
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