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CHAPTER 7
Basudev Swain
Department of Chemistry, School of Natural Science, Indiana University Southeast, USA
Jinki Jeong, Min-seuk Kim and Jae-chun Lee
Mineral Resources Research Division, Korea Institute of Geoscience and Mineral
Resources (KIGAM), Republic of Korea
Solvent extraction of Pt/Pd from chloride media by
Alamine 300—analysis of equilibrium and mechanism
ABSTRACT
Solvent extraction of Pt and Pd from dilute chloride solutions by Alamine 300 has been
investigated. The distribution of Pt and Pd between aqueous chloride solutions and the organic
solvent containing Alamine 300-TBP diluted in kerosene has been analysed. The separation of Pt
and Pd from chloride solutions was proposed by two different ways: (i) selective extraction of Pt in
the organic phase leaving Pd in the aqueous phase/raffinate, (ii) simultaneous extraction-selective
stripping of Pd and Pt from the loaded organic phase. Different process parameters were
optimised for both studies. From the experimental data, a suitable mechanism involving solvation
of the chloro-complexes of precious metals with the protonated organic species has been proposed
and validated. A very high separation factor (Pt over Pd) of 372 was obtained when the saturated
aqueous solution of NaCl and 5 × 10-4 M each of Pt and Pd was equilibrated with 5 × 10-3 M
Alamine 300 and 5 volume % of TBP in kerosene as an extractant. The separation factor was
observed to be as low of 20 when the aqueous solution containing 0.5 M HCl and 5 × 10-4 M of Pt
and Pd each was used for extraction with 5 × 10-3 M of Alamine 300 and 5 volume % of TBP in
kerosene. The basic problem involved in the separation of Pt and Pd from the aqueous chloride
solutions, like the stripping of the loaded metals from the organic phase, has been overcome
successfully. This involved selective striping of Pt using 0.05 M sodium thiocyanate and Pd using
0.5 M HCl + 0.1 M thiourea solutions from the co-extracted species in the organic phase which led
to the possibility of quantitative separation of the two industrially important metals.
−1−
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INTRODUCTION
Pt group metals (PGMs) and chemical compounds containing them are extremely useful as catalysts in the automobile, chemical and petroleum industries, as conductors in the electrical and electronic industries, in extrusion devices, in dental and medical prostheses and in jewellery. Due to their favourable characteristics such as resistance to corrosion and oxidation, high melting points, electrical conductivity, and catalytic activity PGMs are extremely useful in different industries. For the automotive sector, technology to date incorporates Pt catalysts and there is no substitute so far. Pt demand is projected to a significant increase because of its usage for current automotive technology, fuel cell technology and development of fuel cell vehicles. Consumption of Pt for auto‐catalysts is projected to rise by 5% each year. Hence extraction/separation of PGM either from primary or secondary resources by techno‐economic and eco‐friendly process has always been a matter of great importance. Because of the similar chemistry and formation of highly extractable chlorocomplexes, hydrometallurgical separation of Pt and Pd has been considered a difficult and challenging problem. However, the inertness of the chlorocomplexes of Pt and Pd toward aquation can play an important role in their extraction from acidic solution by an anion‐exchange mechanism with organic base Alamine 300 [1]. In this investigation separation of Pt and Pd from chloride solutions has been proposed in two different ways i.e., (i) selective extraction of Pt to organic phase leaving the Pd in aqueous phase, then stripping Pt and Pd selectively using suitable stripping agent, (ii) selective stripping of either Pd or Pt after extracting both the metals in to organic phase. For both the studies effect of different process parameter was optimised and results discussed. THEORY
The reaction between Alamine 300 and HCl can be written as [1]: R3Norg.+HCl aq.→ R3NH+Cl –org. (1a) R3Norg.+HCl aq.(Exces)→ R3NH2Cl 2org. (1b) The amine chloride reaction occurs first as shown in Equation (1a), and in excess acid an additional amount of HCl is extracted according to Equation (1b). The mechanism by which a metal ion is extracted from an aqueous chloride solution using the anion exchange extractant is as follows [2–3]: PtCl62‐aq.+2R3NH+Cl–org.⇔(R3NH)2(PtCl6)org.+2Cl‐aq. (2) PdCl42‐aq.+ 2R3NH+Cl–org.⇔(R3NH)2(PdCl4)org+2Cl‐ aq (3) In presence of saturated NaCl solution can be explained as below. −2−
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PdCl42‐aq. Sat. solution NaCl/HCl PdCl62‐aq. (4) 2PdCl62‐aq.+2R3NH+Cl–org. ⇔ [(R3NH)2(PdCl5)2]org+4Cl‐aq. (5) The equilibrium constant, Kex for the extraction reaction for Equation (2) and (3) can be written as Equation (6), whereas Equation (5) it can be written as Equation (7). Where D = distribution coefficient of each metal in respective equations, can be defined as metal ion concentration in organic phase with respect to metal ion concentration in aqueous phase. Taking logarithms, and re‐arranging Equation (6) Log D = log Kex + 2 log [R3N]org. +2 log [HCl]aq.‐ 2 log [Cl‐]aq. (8) at saturated solution of NaCl, Equation (7) can be re‐arranged as Log D = log Kex + 2 log [R3N]org. +2 log [HCl]aq. (9)
The separation factor for solvent extraction (βsx) is calculated using Equation (11). βsx = DPt / DPd (10)
Where DPt = distribution coefficient of the Pt, and DPd = distribution coefficient of the Pd. METHODOLOGY
Stock solutions of 0.01 M each of Pt and Pd chloride were prepared by dissolving AR H2PtCl6.6H2O and PdCl2 in 0.5 M HCl. Aqueous solutions containing Pt and Pd were used for solvent extraction (SX) by diluting the stock solutions to desired concentration. The commercial extractant, tri‐n‐octyl amine (Alamine 300) was supplied by Cognis USA, Inc., and was used as received. Commercial EP grade kerosene and 5 vol. % tri‐n‐butyl‐
phosphates (TBP) (Junsei Chemicals, Japan, Co.) were used as a diluent and phase modifier, respectively. All other chemicals used were of analytical reagent grade supplied by Junsei Chemicals, Japan, Co. The synthetic aqueous phase (20 ml) containing Pt and Pd of 0.0005 M each was equilibrated with equal volume of Alamine 300 in kerosene by shaking in a separatory funnel for 30 minutes. Unless otherwise stated, these experimental conditions and experimental procedure were kept fixed for all over the investigation. All experiments were carried out at ambient temperature (25 ± 1 °C). The −3−
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concentrations of Pt and Pd in the aqueous phase were measured by ICP‐AES (ICP model; Perkin Elmer, OPTIMA 2100DV) after suitable dilution with 5 vol. % HCl. RESULTS AND DISCUSSION
Effect of HCl concentration on separation
The effect of the HCl concentration on the extraction and separation of Pt and Pd from mixed chloride solution was studied using Alamine 300 as an extractant. The distribution coefficient of Pt decreased from 902.9 to 0.005 while it decreased from 44.36 to 0.31 for Pd as HCl concentration was varied from 0.5 M to 10 M. Because of their similar labile character of chloro complexes, PtCl6 2‐ and PdCl42‐ towards Alamine 300, both the metals show similar kind of quantitative extraction in this range of HCl concentration. Despite of every parameter being constant, both the metals behaved significantly different at 10 M HCl concentration: extraction of both the metals decreased sharply as HCl concentration increased to 10 M. The apparent change in distribution coefficientfor Pd was relatively lower in comparison to Pt. As mentioned in Equation (1b), (2) and (3) at higher concentration of HCl, there would be a competition between R3NH2Cl2org. formation and (R3NH)n(MCln)org. formation. In presence of excess HCl the formation of R3NH2Cl 2org. is dominant over the (R3NH)n(MCln)org. formation which decreases the metal ion extraction. Figure 1a shows dependencies of log D vs log[HCl] for Pt and Pd. The plots are linear with a slope of 1.96 for Pt and 0.95 for Pd. It could be presumed from Equation (8) that at excess HCl concentration the Pt gets extracted through the exchange of two moles of chloride with one mole of Pt. Similarly, Pd gets extracted through the exchange of one mole of chloride with one mole of Pd. Figure 1 Effect of (a) log [HCl] (b) log [NaCl] on log D of Pt and Pd. Experimental
condition: 0.0005 M Pt and Pd each, 0.005 M Alamine 300 in kerosene
Effect of NaCl concentration
To understand effect of [Cl‐] and [H+] on the extraction of the two metals separately, experiments were carried out with lower acidity and saturated solution of NaCl in aqueous phase. Separation behaviour of the metals was studied at different pH varying −4−
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in the range 0.3 to 5.5 using saturated solution of NaCl. A remarkable separation was observed at pH 1.5. At this experimental condition 99.27% of Pt was extracted along with 27% of Pd, leading to a separation factor of 365.34. From pH variation study it was observed that lower [H+] and higher [Cl‐] may be suitable for better extraction and better separation. To justify the above argument different experiments were carried out at pH 1.5 and varying NaCl concentration in aqueous phase. As NaCl concentration was varied from 0.05 M to 6.14 M (saturated), the Pt distribution coefficient remained constant at 137.31, whereas distribution coefficient for Pd decreased rapidly from 12.31 to 0.36. From the above studies it could reasonably understood that lower HCl concentration and high NaCl concentrations lead to the higher extraction of Pt and relatively poor extraction of Pd, which is an important innovation in our study added boons to our separation studies. Cotton et al. [4] reported that, Pd(II) can be converted to Pd(IV) in presence of higher chloride ions. As Pd(II) converts to Pd(IV), kinetic inertness increases, hence the complexation reaction with Alamine 300 decreases. This is the probable reason for decreased Pd extraction and remarkable increase in separation factor. Figure 1b shows the dependencies of log D vs log [NaCl] for Pt and Pd. The plots are linear with no slope for Pt but a slope of 1.88 for Pd suggests that Pd(II) requires 2 moles of Cl to convert Pd(IV). This is absolutely in agreement with proposed concept. From above experimental results, two different possibilities for separation of Pt and Pd are expected. (i) Selective extraction of Pt to organic phase leaving Pd in the aqueous phase at pH 1.5 and in presence of saturated solution of NaCl to aqueous phase, then stripping of Pt using suitable stripping agent. (ii) Extraction of both the metals into organic phase, using 0.5 M HCl in aqueous phase without addition of NaCl, and then selective stripping either Pd or Pt using suitable stripping agent Effect of extractant concentration
The competitive extraction behaviour of Pt and Pd was studied with Alamine 300 concentration of 0.0005–0.01 M at the pH of 1.5 and at 6.14 M NaCl concentration (saturated). The distribution co‐efficientof Pt increased steeply from 0.14 to 9999 as against relatively low increase in distribution co‐efficientof Pd (0.03 to 0.709) with respect to Alamine 300 concentration variation. Similarly, the extraction behaviour of Pt and Pd was studied while varying the Alamine 300 concentration in the range 0.001–0.01 M in presence of 0.5 M HCl in aqueous phase, without NaCl addition. The distribution coefficient of Pt increased steeply from 0.88 to 9999, and so was the distribution coefficient of Pd which increased from 0.78 to 6348.2 as the extractant concentration was increased from 0.001–0.01 M. To determine the nature of extracted complex in both the experimental condition a log‐log plot for both the metals were analysed. Figure 2a shows the plot for the dependency of log D on log [Alamine 300] in presence of saturated NaCl in the feed solution with a slope of 1.89 and 0.65 for Pt and Pd, respectively. Figure 3a shows the plot for the dependency of log D on log [Alamine 300] in absence of saturated NaCl in feed solution with a slope of 2.30 and 2.12 for Pt and Pd, respectively. In both the cases the log‐log plot for Pt had a slope of about 2.00, but in presence of saturated solution of NaCl the slope pattern of log‐log plot for Pd was substantially different, hence the extraction reaction for Pt can be represented as in Equation (3) for both the cases, but different mechanism for Pd can be explained with a combination of Equation (4) and (6). −5−
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Eq.=Equilibrium
Pd
Pt
Pd
2c
0
-1
-3.4
2.5
2(c)
3(c)
1.5
0.5
-6
-5
-4
log [Pd]Eq.,M
-3.3
-3.2
log [Pt]Eq.,M
-3
6
logD
4
2
3
2(b)
2
3(b)
1
0
logD
-2
-10
logD
logD
1
logD
3.5
2
0
-8
-6
-4
log [Pt]Eq.,M
4
3
2(a)
2
1
0
-1
-2
-4.0 -3.5 -3.0 -2.5 -2.0
log [Alamine 300]Eq.,M
-5
-4
-3
log [Pt]Eq.,M
3(a)
-2
-1
5
4
3
2
logD
3
Pt
1
0
-3.2 -2.8 -2.4 -2.0
log [Alamine 300]Eq.,M
Figure 2 Effect of (a) log [Alamine 300], (b) log [Pt] and (c) log [Pd] on log D
Figure 3 Effect of (a) log [Alamine 300], (b) log [Pt] and (c) log [Pd] on log D
Effect of metal ion concentration
The extraction of Pt and Pd was carried out using 0.005 M Alamine 300 at initial pH 1.5 and saturated solution of NaCl. As Pt concentration in aqueous phase was increased from 0.0001 to 0.0025 M, the distribution coefficient for Pt decreased from 23398 to 1.60. With further increase in Pt concentration to 0.0025 M in the aqueous phase, the distribution coefficient levelled off and become almost constant due to the saturation of organic phase with, the Pt‐Alamine 300 organometallic complex. The loading capacity of 0.005 M Alamine 300 was found to be 0.0125 g/L for Pt at this condition. The Pd extraction remains constant at 0.017 g/L. As shown in Figure 2b the slope of ‐1.75 for dependence of log D vs log [Pt], indicates existence of Pt as PtCl62‐ species in aqueous phase, which undergo anion exchange reaction and justifies the proposed mechanism above. When only 0.5 M HCl was used in aqueous phase without NaCl, and Pt concentration in the aqueous phase was increased from 0.0001–0.002 M, the distribution coefficient for Pt varied from 65.85 to 1.65. Under this experimental condition the distribution coefficient of Pd decreased from 150.19 to 0.88. When there would be a competition between Pt and Pd for limited Alamine 300 available Pt extraction gets facilitated in comparison to that of Pd. Figure 3b shows dependency of log D vs log[Pt] for both the metals, the slope values of ‐2.45 and ‐2.2 for Pt and Pd, respectively indicate existence of these metals as PtCl62‐ and PdCl42‐ species. −6−
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Figure 4 Verification of proposed model in absence of NaCl for (a) Pt (b) Pd, in presence
of saturated NaCl solution for (c) Pt (d) Pd
As explained above the concentration of Pd in the feed solution was varied for both the studies. When saturated solution of NaCl was used, in the range of 0.0001–0.0015 M Pd variation the Pd concentration in the organic phase increased from 0.002 g/L to 0.085 g/L, while the concentration of Pt in the organic phase was found unaffected. In this range of aqueous feed Pd distribution coefficient was increased from 0.31 to 1.11, whereas Pt distribution coefficient remained constant at 137.31. As shown in Figure 2c the slope of 2.30 for decency of log D vs log [Pd], indicates the existence of Pd as PdCl62‐ species in the aqueous phase which undergoes anion exchange reaction. The distribution behaviours of both the metals were studied using 0.5 M HCl in aqueous phase without using NaCl and varying Pd concentration in aqueous phase from 0.0001 M to 0.001 M. Figure 3c shows dependencies of log D vs log [Pd] for both the metals; the slope of ‐0.66 and ‐0.55 for Pt and Pd, respectively, indicates the existence of Pt as PtCl62‐ species and Pd as PdCl42‐ species. Applicability of proposed model
Figure 4 shows applicability of proposed model for two different cases studied i.e. in presence of saturated solution of NaCl and in absence of NaCl. Figure 4a and 4b show that the plots of log D vs 2 log [R3N]org.+2 log [HCl]aq.‐ 2 log [Cl‐]aq for Pt and Pd, respectively are linear having a slope about 1. The Kex values were calculated from given figure and were found to be 2.047 (mol/L)‐2 for Pt and 1.357 (mol/L)‐2 for Pd. Figure 4c and Figure 4d show the plots of log D vs 2 log [R3N]org. +2 log [HCl]aq. for Pt and Pd, respectively which also reflect the linear dependence having a slope about 1. The Kex values were calculated from given figure and were found to be 4.856 (mol/L)‐2 for Pt and 0.013 (mol/L)‐2 for Pd. Figure 4 also shows that log‐log model was linear in all cases with 95% confidence level and truly predicted the extraction behaviour. −7−
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Stripping behaviour of metals
To find out suitable stripping reagent different reagents like HCl, HNO3, H2SO4, H2O2, NH4Cl, NaSCN and (NH2)2CS with HCl, at different concentration level were used for stripping studies. Among all extractants NaSCN was found to be the best stripping reagent for Pt whereas for Pd (NH2)2CS with HCl was the best. Selective stripping from the loaded organic generated using 0.005 M Alamine 300 from the aqueous solution containing saturated solution of NaCl at pH 1.5 and 0.0005 M of Pt and Pd each was studied. When 0.1 M NaSCN solution was used for stripping, 99.999% pure Pt was separated as H2Pt(Cl)4(SCN)2 from the mixed chloride solution of Pt and Pd. After Pt stripping, Pd was stripped using 0.5 M HCl and 0.1 M of thiourea and 99.5% pure Pd was separated as Pd(Cl)2((NH2)2CS)2. Stripping from the loaded organic generated using 0.005 M Alamine 300, from the aqueous solution containing 0.5 M HCl and 0.0005 M of Pt and Pd each was studied. A 99.88% of pure Pt can be stripped using 0.05 M NaSCN from loaded organic, selectively. Then using 0.5 M HCl along with 0.01 M thiourea 98.22% pure Pd can be stripped from the Pt stripped loaded organic phase. CONCLUSION
Problem of separating of Pt and Pd from their chloride solutions by SX can be overcome by the proposed process using Alamine 300 in two different ways depending upon the selectivity of metal ions in combination with the suitable extractant and using crucial stripping reagents. Pt can be extracted as [(R3NH)2(PtCl6)]org species using Alamine 300 in acidic chloride media and the loaded organic can stripped by NaSCN as H2Pt(Cl)4(SCN)2. Similarly Pd can extracted as (R3NH)2(PdCl4)org using Alamine 300 in acidic chloride media and the loaded organic was stripped as Pd(Cl)2((NH2)2CS)2. ACKNOWLEDGEMENTS
All the experiment has been carried out in KIGAM. The author Dr. Basudev Swain is thankful to KIGAM for awarding postdoctoral position. REFERENCES
Swain, B., Jeong, J., Kim, S.K. & Lee, J.C. (2010) Separation of Platinum and Palladium from Chloride Solution by Solvent Extraction Using Alamine 300, Hydrometallurgy, 104 (1) pp. 1–7.[1] Ritcy, G.M. & Ashbrook, A.W. (1984) Solvent Extraction Principles and Applications to Process Metallurgy, Part 1, Elsevier, pp. 15–20. [2] Gerhartz, W., Elvers, B., Ravenscroft, M., Rounsaville, J.F. & Schulz, G., Ullmann’s Encyclopedia of Industrial Chemistry, Volume B3, Fifth edition, pp. 6‐44–6‐50. [3] Cotton, F.A. & Willkinson, G. (1996) Advanced Inorganic Chemistry, 2nd edition, pp. 1035–1050. [4] −8−

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