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Separation of platinum(IV) and palladium(II) from concentrated hydrochloric acid solutions by mixtures of amines with neutral extractants
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JIEC 2627 1–8 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6 7
Separation of platinum(IV) and palladium(II) from concentrated hydrochloric acid solutions by mixtures of amines with neutral extractants Q1 Thi a b
Hong Nguyen a, Chong Ho Sonu b, Man Seung Lee
a,
*
Department of Advanced Materials Science & Engineering, Institute of Rare Metal, Mokpo National University, Chonnam 534-729, Republic of Korea Metallic Resources Technology Laboratory, LS-Nikko Copper Inc., Kyunggi 463-400, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 July 2015 Received in revised form 25 August 2015 Accepted 29 August 2015 Available online xxx
Separation of platinum and palladium from concentrated HCl solutions by extraction with mixture of amines (Aliquat 336/Alamine 336/TOA) and neutral extractants (TBP/TOP/MIBK) have been investigated. The mixture of Aliquat 336 and TBP led to higher Pt(IV) separation factor at 6 M HCl solution. McCabe– Thiele diagram for the extraction of Pt by this mixture was constructed. Low concentration of thiourea in the HCl solution resulted in selective stripping of Pd from the loaded organic. Pt solution with 99.9% purity was obtained by extraction and stripping from the solution containing Pd. A process was proposed to separate the two metals. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Platinum Palladium Hydrochloric acid Separation Solvent extraction
8 9
Introduction
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Platinum group metals (PGMs) are used as metallic catalysts in the automobile for minimizing air pollution. Among PGMs, the demand for Pt and Pd is increasing and thus the extraction and separation of these metals from secondary resources is important [1]. The separation and purification of PGMs is extremely difficult due to their similar structure and chemical behavior [2,3]. Hydrometallurgical method, such as precipitation [4], ion exchange [5,6], and solvent extraction [1,3,7–12] are commonly employed for the separation of these metals. Generally, the solvent extraction is one of the most versatile methods in terms of separation efficiency and environmental impact [7,11]. In treating the secondary resources by hydrometallurgical method, Pt and Pd together with other PGMs are dissolved by using concentrated hydrochloric acid concentration in the presence of an oxidizing agent. Lots of works have been reported on the extraction and separation of Pt and Pd from the leach liquors containing PGMs. Three kinds of extractant including acidic and neutral extractants and amines are employed for the separation of Pt and Pd. The process details, advantages and disadvantages of each
* Corresponding author. Tel.: +82 61 450 2492; fax: +82 61 450 2498. E-mail address: [email protected] (M.S. Lee).
extractant system are listed in Table 1. Literature survey indicates that acidic extractants offer high separation efficiency of Pt and Pd, when the acid concentration in leach liquor is low and concentrated HCl solution can strip the metals from the loaded organic. In the case of neutral extractants, selective extraction of Pd over Pt is possible from weak HCl solution [7] but the co-extraction of Pt is high in the concentrated HCl solution [9]. Use of amines lead to complete extraction both of Pt and Pd and these metals can be separated by selective stripping. The disadvantage of employing amines is the possibility of forming a third phase during the extraction and the complexity of the stripping process owing to the necessity to adjust the composition of the stripping solution. Compared to neutral extractants, amines offer higher extraction efficiency of Pt and Pd [9–21]. However in terms of separation factor, the separation efficiency of Pt and Pd by amines is low and the formation of a third phase should be overcome. Unlike single extractant, mixtures of extractants provide some opportunity to circumvent the above-mentioned disadvantages of amines. However, little information has been reported on the separation of Pt and Pd from HCl solution by using mixtures of amines and other extractants. For this purpose, solvent extraction experiments have been done from a synthetic chloride solution containing only Pt and Pd. The extraction and separation behavior of Pt and Pd was investigated as a function of the nature and concentration of extractant in the mixture, acid concentration, metal ion
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Table 1 A summary on the extraction and separation of Pt and Pd from the chloride leach solution by various extractants. Extractant PC 88A
a
LIX 84Ia
Advantage
Disadvantage
Ref.
0.1 mg/L Pt, Pd
Extraction of Pd over Pt from 1 M HCl + 1 M HClO4 solution.
[13]
1 g/L Pt, Pd
Extraction of Pd over Pt in the pH range of 1–3. Separation factor: 400. Extraction of Pd over other metals at 3 M HCl. Using 0.5 M thiourea + 1 M HCl for stripping of Pd. Extraction of Pd over Pt and Au at pH 2.0. Extraction of Au over Pt at 5 M HCl. Extraction of Pd over Pt at 0.1 M HCl.
Low HCl concentration in leach liquor. Low metal concentration. Using 4 M HCl for stripping. Low HCl concentration in leach liquor. Using 6 M HCl for stripping. Extraction of Pt over Mn, Ni, Fe and Cr using Alamine 336 + TBP. High co-extraction of Fe. Using 6 M HCl for stripping of Pd.
[16]
Synthesized extractant. Using 3% v/v NH4Cl + 5 M ammonia solution for stripping. Low separation efficiency. Synthesized extractant. Poor separation efficiency of Pt and Pd.
[3]
[18]
Poor separation efficiency of Pt and Pd. Losing Pt from precipitation of Fe.
[1] [7]
Separation of Pt and Pd from stripping step. High co-stripping of Pt together with Pd. No information for separation of Pt, Pd from stripping. No information for separation of Pt, Pd and Au from stripping. High co-extraction of Pd. Crucial stripping problems.
[8]
Pd-0.15, Pt-0.55, Mn-0.5, Ni-1, Fe-1.5, Cr-0.1 g/L Pd-5.95, Au-5.42, Pt-10.23 g/L 1000 g/L Pt and Pd
N530a DBSO
Aqueous
b
DMDCHTDGAb 1
Cyphos IL 101/104b Cyanex 923b TBPb
Alamine 336c
0.1 g/L Pt, Pd 2.5 mM of Pt, Pd, Ni, Cu, Pb, Ir, Rh, Ru 50 mM of Pt and Pd Pd-0.15, Pt-0.55, Mn-0.5, Ni-1, Fe-1.5, Cr-0.1 g/L 0.001 M Pt and Pd Pt, Pd
TIOAc
Pt, Pd, Rh, Ir, Au
Alamine 300c
0.01 M Pt and Pd
Co-extraction of Pt and Pd at 8 M HCl. Separation of Pt and Pd using stripping step. Extraction of Pd over other metal at 0.1 M HCl. Using ammonia solution for stripping. Extraction of Pt over Pd at 5 M HCl. Extraction of Pd over other metals at 3 M HCl.
Extractions both of Pt and Pd in the HCl concentration range of 1–5 M. Complete extractions both of Pt and Pd at 15% w/w HCl. Extraction of Pt, Pd, Au over Rh and Ir in the HCl concentration range of 3.5–6 M. Extraction of Pt over Pd using saturated sodium chloride solution at pH 1.5.
[14] [15]
[17]
[19] [20] [11]
a
Acidic extractants. Neutral extractants. c Basic extractants. PC-88A = 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester; LIX 84I = 2-hydroxy-5-nonylacetophenone oxime; N530 = 2-hydroxy-4-sec-octanoyl diphenyl-ketoxime; DBSO = dibutyl sulfoxide; DMDCHTDGA = N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide; Cyphos1IL 101 = trihexyl(tetradecyl)phosphonium chloride; Cyphos1IL 104 = trihexyl(tetradecyl)phosphonium bis-2,4,4-trimethylpentylphosphinate; Cyanex 932 = the phosphine oxides; TBP = tributyl phosphate; Alamine 336 = tri-octyl/ dodecyl amine; TIOA = tri-iso-octylamine; Alamine 300 = tri-n-octyl amine. b
54 55 56 57 58 59
concentration and stripping condition. Extraction isotherm for the extraction of Pt was constructed. The effect of thiourea concentration in the HCl solution on the stripping of Pt and Pd was also investigated. A process flowsheet was developed to obtain pure Pt and Pd from the solution by employing extraction and stripping.
60
Experimental
61 62 63 64 65 66 67 68
A synthetic solution was prepared by dissolving appropriate amounts of PtCl4 (99.9%) and PdCl2 (99.9%) purchased from AlfaAesar. The composition of the synthetic solution was Pt-120 mg/L and Pd-50 mg/L. The acidity of the solution was controlled by HCl (Daejung Co.) solution (1–8 M). TOA (Samchun Pure Chemical Co.), TBP (Yakuri Pure Chemical Co.), TOP (IS Chem Co.), MIBK (Daejung Co.), Decanol (Acros Organics Co.), Aliquat 336 (BASF Co.) and Alamine 336 (BASF Co.) were used without further
69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
Extraction percentage
Table 2 Chemical formula of extractants used in this study.
¼
Extractant
Reagent class
Structure
Aliquat 336
Aliphatic quaternary ammonium salt Tertiary amine Tri-n-octylamine Tri-n-butyl phosphate Tri-n-octyl phosphate Methyl isobutyl Ketone
[(C8H17)3N(CH3)]Cl
Alamine 336 TOA TBP TOP MIBK
purification. Chemical formula of each extractant is presented in Table 2. Kerosene (Samchun Pure Chemical Co.) was used as a diluent. Solvent extraction experiments were performed by mixing equal volumes (10 mL) of aqueous and organic phases for 30 min (extraction and stripping equilibrium were reached within 30 min in initial tests) using a wrist action shaker. After equilibrium, the two phases were separated with a separating funnel. All the experiments were performed at room temperature (25 1 8C). Metal ion concentrations in the aqueous phase before and after extraction were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES; Spectro Arcos). The concentration of metal ions in the loaded organic phase was calculated by mass balance. The distribution ratio (D) was calculated as the concentration of metal present in the organic phase to that present in the aqueous phase at equilibrium. The separation factor was calculated from the ratio of the distribution coefficient. The extraction percentage of a metal is defined as follows
[C8–C10 mixture]3N [CH3(CH2)7]3N [CH3(CH2)3O]3PO [C8H17O]3PO (CH3)2CHCH2C(O)CH3
weight of a metal extracted into organic initial weight of a metal in the aqueous before extraction 100 (1)
The extraction percentage of a metal can be calculated from the distribution coefficients of a metal by using the relation, %E = D 100/(D + (Vaq/Vorg)) where Vaq and Vorg represent volumes of aqueous and organic phases, respectively.
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93
The stripping percentage of a metal is defined as follows
40
3
Aliquat 336 alone Aliquat 336 + Decanol
Stripping percentage (2)
94 95 96
Results
97
Effect of HCl concentration on the extraction with Aliquat 336
98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124
It has been reported that Aliquat 336 can selectively extract Pt from chloride solutions containing Rh, Fe, Ni, Mn, and some minor elements [7,21]. However, little information has been reported on the extraction and separation of Pt and Pd using Aliquat 336. Therefore, three organic systems – (1) Aliquat 336 alone, (2) Aliquat 336 mixed with 0.3 M Decanol and (3) Aliquat 336 mixed with 0.4 M TBP were employed to investigate the extraction and separation behavior of Pt and Pd. The effect of hydrochloric acid concentration on the extraction behavior of Pt and Pd from synthetic solutions containing Pt-120 mg/L and Pd50 mg/L was studied in the HCl concentration range of 1–8 M. In these experiments, the concentration of Aliquat 336 was fixed at 0.01 M and the ratio of organic to aqueous phase was unity. Fig. 1 shows that the extraction of Pt and Pd by Aliquat 336 depended strongly on the concentration of HCl in the solution. Most of Pt and Pd were extracted by Aliquat 336 alone and Aliquat 336 mixed with TBP when the HCl concentration was lower than 3 M. As HCl concentration increased from 3 to 8 M, the extraction percentage of Pd decreased more rapidly than that of Pt. In the case of the system of Aliquat 336 mixed with Decanol, the extraction percentage of Pt and Pd steadily decreased with the increase of HCl concentration from 1 to 8 M. The decrease in extraction of Pt and Pd with the increase of HCl concentration can be explained by the solvent extraction reaction of Pt and Pd by Aliquat 336. Since PtCl62 and PdCl42 are predominant species in the HCl concentration range of 1–8 M, the extraction reaction of Pt and Pd by Aliquat 336 can be represented as Eq. (3) [9,11,22]. nR4 NCl þ MClm n ? ðR4 NÞn ðMClm Þ þ nCl
2
where R4NCl and MClm represent Aliquat 336 and PtCl6 / PdCl42, respectively. The increase in HCl concentration would favor the back reaction of Eq. (3) and thus the extraction percentage of Pt and Pd decreases. 100
Extraction percentage, %
126 125 127 128 129
80
60
40
20 Aliquat 336 alone Aliquat 336+ Decanol
0 0
Pt
Pt
Pd
Pd
2
Aliquat 336+TBP Pt Pd
4
6
30
20
10
(3)
n
Separation factor (DPt/DPd)
Aliquat 336 + TBP
weight of a metal stripped into aqueous ¼ initial weight of a metal in the organic before stripping 100
8
[HCl], M Fig. 1. Effect of HCl concentration on extraction of Pt and Pd by Aliquat 336 alone, Aliquat 336 + 0.3 M Decanol, and Aliquat 336 + 0.4 M TBP. Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 1–8 M; [Aliquat 336] = 0.01 M; diluent = kerosene; O/ A = 1:1.
0 0
2
4
6
8
[HCl], M Fig. 2. Separation factor between Pt and Pd by extraction with Aliquat 336 alone, Aliquat 336 + 0.3 M Decanol, and Aliquat 336 + 0.4 M TBP. Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 1–8 M; [Aliquat 336] = 0.01 M; diluent = kerosene; O/ A = 1:1.
It has been reported that the tendency for the PGM chlorocomplexes to form ion pairs with anion exchangers changes in the following order: [MCl62] > [MCl42] [MCl63] > aqua species [18]. Since most of Pt and Pd exist as PtCl62 and PdCl42 in the HCl concentration employed in this study, Pt was selectively extracted over Pd by Aliquat 336 in this work. The extraction efficiency of Pt and Pd by the three organic systems was in the order Aliquat 336 + Decanol < Aliquat 336 + TBP < Aliquat 336 alone. The variation in the separation factor between Pt and Pd with HCl concentration using the three organic systems is presented in Fig. 2. The high separation factor was obtained with Aliquat 336 alone and Aliquat 336 + TBP systems in the HCl concentration range of 4–8 M, while the separation factor was lower than 5 in the case of Aliquat 336 + Decanol at any HCl concentration range. From the data represented in Figs. 1 and 2, it might be said that Aliquat 336 alone offered the highest separation efficiency among the three organic systems. However, separation of the two phases after extraction with Aliquat 336 alone was poor because of the formation of a third phase or emulsion in the organic phase. The presence of Decanol or TBP in Aliquat 336 suppressed the formation of a third phase and TBP was chosen as an optimum modifier in terms of separation factor (Fig. 2). Highest separation factor (22) between Pt and Pd by the system of Aliquat 336 and TBP was obtained from 6 M HCl where the extraction percentage of Pt and Pd was 80% and 19%, respectively. Therefore, the synthetic solution with 6 M HCl was employed in further experiments.
130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155
Effect of the mixtures of Aliquat 336, Alamine 336 or TOA and TBP
156
Lots of work has been reported on the extraction efficiency of Pt by using single amines [11,23,24]. Especially, Sun and Lee [8] and Mirza [20] have reported that complete extraction of both Pt and Pd is obtained in the HCl concentration range of 1–6 M using single Alamine 336 or TOA. Thus, the extraction behavior and separation efficiency of Pt and Pd by the mixture of Aliquat 336/Alamine 336/ TOA and TBP was studied to find an optimum organic system. The composition of the synthetic solution was Pt-120 mg/L, Pd-50 mg/ L and 6 M HCl. The concentration of amines was kept at 0.01 M in the mixture, while the concentration of TBP was varied from 0 to 1 M. Fig. 3 shows that in the absence of TBP in the mixtures, Alamine 336 and TOA led to high extraction percentage of both Pt and Pd, while Aliquat 336 offered some possibility to separate Pt and Pd. The extraction percentage of Pd decreased more than that of Pt as TBP concentration increased from 0 to 0.4 M and there was
157 158 159 160 161 162 163 164 165 166 167 168 169 170 171
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4 100
100
Pt Pd
Pt Pd 80
Extraction percentage, %
Extraction percentage, %
80
(a) 0.01 M Aliquat 336 +TBP
60
40
20
0
60
(b) 0.01M Alamine 336 + TBP
40
20
0.0
0.2
0.4
0.6
0.8
0
1.0
0.0
[TBP], M
0.2
0.4
0.6
0.8
1.0
[TBP], M 100
Pt Pd
Extraction percentage, %
80
60
(c) 0.01 M TOA + TBP 40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
[TBP], M Fig. 3. Effect of TBP concentration on the extraction Pt and Pd by the mixture of Aliquat 336/Alamine 336/TOA and TBP. Aqueous solution, mg/L: Pt-120, Pd-50, [HCl] = 6 M. [TBP] = 0–1 M; [amine] = 0.01 M; diluent = kerosene; O/A = 1:1.
little change in the extraction percentage of Pt and Pd with the further increase of TBP concentration. These results agreed well with the reported literature which reports antagonism for the extraction of Pt by the mixture of Aliquat 336 and TBP [25]. The decrease in the extraction of Pt and Pd with increasing TBP concentration in the mixture may be due to the interaction between TBP and amines, which reduces the effective concentration of amines. Huang and Bautista [26] have reported that the association degree between TBP and Aliquat 336 is very strong, which is represented as (TBP)2(R4NCl)k, where k is a positive integer. In strong HCl solution, the interaction of TBP and protonated TOA or Alamine 336 is also similar to Aliquat 336. When TBP concentration increases, k value also increases and thus the free concentration of amine which can take part in the solvent extraction reaction is reduced. Hence, the extraction percentage of Pt and Pd by these mixtures decreased with the increase of TBP concentration. The separation factor between Pt and Pd by the three organic systems is presented in Fig. 4. The separation factor between Pt and Pd by Aliquat 336 system almost decreased with the increase of TBP concentration from 0 to 1 M, while the reverse was true in the case of Alamine 336 and TOA. The order of separation efficiency of extractant mixtures followed the Aliquat 336 + TBP > TOA + TBP > Alamine 336 + TBP. Although the presence of TBP in Aliquat 336 results in a decrease in the separation factor of Pt and Pd, the mixture of Aliquat 336 and TBP was chosen as an optimum extractant mixture. The obtained results also indicated that adding 0.4 M TBP to Aliquat 336 was the best condition for the separation of
Pt and Pd, where high separation factor was obtained and the formation of third phase or emulsion was avoided during extraction.
200 201
The effect of TBP/TOP/MIBK concentration in the Aliquat 336 system
202
As mentioned above, the addition of TBP into Aliquat 336 affected the extraction and separation of Pt and Pd from the
203 204
30 Aliquat 336 + TBP
25
Alamine 336 + TBP TOA + TBP
Separation factor (DPt/DPd)
172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199
20
15
10
5
0 0.0
0.2
0.4
0.6
0.8
1.0
[TBP], M Fig. 4. Effect of TBP concentration on separation factor between Pt and Pd by the mixture of Aliquat 336/Alamine 336/TOA and TBP. Aqueous solution, mg/L: Pt-120, Pd-50, [HCl] = 6 M. [TBP] = 0–1 M; [amine] = 0.01 M; diluent = kerosene; O/A = 1:1.
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synthetic solution at 6 M HCl. Therefore, in order to investigate the effect of the nature of neutral extractants, three neutral extractants (TBP, TOP, and MIBK) were added into Aliquat 336 and these mixtures were employed in the extraction experiments. In these experiments, the composition of the aqueous synthetic solution was the same and the concentration of HCl was 6 M. The concentration of Aliquat 336 was fixed at 0.01 M, while that of neutral extractant was varied from 0.1 to 1 M. The organic to aqueous phase ratio was unity. After separation of the phases, a third phase was observed in the system of Aliquat 336 + MIBK at any MIBK concentration, while in the system of Aliquat 336 + TBP/ TOP the third phase was formed at lower 0.2 M TBP/TOP. The difference in the efficiency of a modifier in suppressing the formation of a third phase might be correlated to the structure of TBP, TOP, and MIBK (see Table 2). Phosphoryl group is present in the structure of TBP and TOP [9], while the structure of MIBK consists of carbonyl group. This might be the reason why the mixture of Aliquat 336 + TBP/TOP can prevent the formation of a third phase or emulsion when TBP/TOP concentration was higher than 0.2 M. Fig. 5 shows the effect of TBP/TOP/MIBK concentration in the mixture with Aliquat 336 on the extraction of Pt and Pd. The presence of TOP/MIBK in Aliquat 336 had negligible effect on the extraction of Pt and Pd. On the contrary, the extraction percentage of Pt and Pd decreased steadily with the increase of TBP concentration from 0 to 0.4 M and was constant with the further increase of TBP concentration. Since the extraction percentage of Pd decreased more rapidly than that of Pt with the increase of TBP concentration, high separation factor between Pt and Pd was obtained in the system of Aliquat 336 + TBP (see Table 3). According to the obtained results, 0.4 M TBP was chosen as the optimum condition in terms of separation factor, co-extraction percentage of Pd and phase separation (see Fig. 5 and Table 3). It can be concluded that the presence of 0.4 M TBP in 0.01 M Aliquat 336 can prevent the formation of a third phase and result in high separation efficiency of Pt and Pd from 6 M HCl solution.
240
Effect of Aliquat 336 concentration in the mixture with TBP
241 242 243 244 245 246 247
In order to investigate the effect of Aliquat 336 concentration in the mixture with TBP on the separation of Pt and Pd, Aliquat 336 concentration in the mixture with 0.4 M TBP was varied from 0 to 0.02 M. The composition of the synthetic solution was Pt120 mg/L and Pd-50 mg/L and 6 M HCl. The ratio of organic to aqueous phase was unity. Fig. 6 shows that the extraction percentage of Pt and Pd with 0.4 M TBP alone was 64 and 18%,
Extraction percentage, %
100
80
60
40
20 Aliquat 336+TBP
0 0.0
Aliquat 336+TPO
Aliquat 336+MIBK
Pt
Pt
Pt
Pd
Pd
Pd
0.2
0.4
0.6
0.8
1.0
Table 3 Separation factor between Pt and Pd by the mixture of Aliquat 336 and TBP/TOP/ MIBK. [TOP/TBP/MIBK], M
0.1 0.2 0.4 0.6 0.8 1.0
SF = DPt/DPd TBP + Aliquat 336
TOP + Aliquat 336
MIBK + Aliquat 336
21.4 19.7 22.0 11.9 11.0 11.3
18.7 16.0 13.7 12.6 10.1 7.9
19.2 18.9 20.3 18.8 19.3 17.8
Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 6 M; [Aliquat 336] = 0.01 M; [TBP/ TOPO/MIBK] = 0.1–1 M; diluent = kerosene; O/A = 1:1.
100
80
Extraction percentage, %
205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239
5
Pt Pd
60
40
20
0
0.000
0.005
0.010
0.015
0.020
[Aliquat 336], M Fig. 6. Effect of Aliquat 336 concentration in the mixture with 1 M TBP on the extraction of Pt and Pd. Aqueous solution, mg/L: Pt-120, Pd-50, [HCl] = 6 M. [Amine] = 0–0.02 M; diluent = kerosene; O/A = 1:1.
respectively. When Aliquat 336 concentration increased from 0 to 0.02 M, the extraction percentage of Pt and Pd increased to around 87 and 40%, respectively. The obtained results indicated that the presence of Aliquat 336 in TBP enhanced extraction efficiency of Pt and Pd. The formation of a third phase occurred when the Aliquat 336 concentration was higher than 0.01 M. Therefore 0.01 M Aliquat 336 was selected as an optimum condition in terms of phase separation and extraction efficiency of Pt in further experiments.
248 249 250 251 252 253 254 255 256
Effect of Pt concentration
257
In the separation of Pt and Pd, the concentration ratio of these two metals might affect the separation efficiency. Therefore, in order to investigate the effect of the concentration ratio of Pt to Pd, the concentration of Pt in the mixed solution was varied from 50 to 1000 mg/L. In these experiments, the concentration of HCl was 6 M and that of Pd was kept at 50 mg/L. The experiments were carried using the mixture of 0.01 M Aliquat 336 and 0.4 M TBP. Fig. 7 shows that the extraction percentage of Pt and Pd was constant at 80 and 20%, respectively at any Pt concentration range. It might be concluded that in our experimental range the Pt concentration in the mixed solution did not affect the extraction behavior of Pt and Pd using the mixture of Aliquat 336 and TBP.
258 259 260 261 262 263 264 265 266 267 268 269
Extraction isotherm
270
In order to determine the theoretical number of stages for the complete extraction of Pt from the synthetic solution containing Pt-120 mg/L, Pd-50 mg/L at 6 M HCl, extraction isotherm of Pt was
271 272 273
[TBP/TOP/MIBK], M Fig. 5. Effect of TBP/TOP/MIBK concentration in the mixture with Aliquat 336 on the extraction of Pt and Pd. Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 6 M; [Aliquat 336] = 0.01 M; [TBP/TOP/MIBK] = 0.1–1 M; diluent = kerosene; O/A = 1:1.
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6 100
Table 4 Effect of stripping reagents on stripping of Pt and Pd from loaded Aliquat 336 + TBP Q4 using single thiourea, HCl and thiourea mixed with HCl solution.
80
[Thiourea], M
60
40
20
0
0
200
400
600
800
1000
[Pt]aq, mg/L Fig. 7. Effect of Pt concentration on the extraction of Pt and Pd by the mixture of Aliquat 336 and TBP. Aqueous solution, mg/L: [Pt] = 50–1000, Pd-50, [HCl] = 6 M. [TBP] = 0.4 M; [Aliquat 336] = 0.01 M; diluent = kerosene; O/A = 1:1.
274 275 276 277 278 279 280 281 282 283 284 285 286 287 288
obtained by varying O/A ratios from 1/3 to 3. The aqueous solution was contacted with the mixture of 0.01 M Aliquat 336 and 0.4 M TBP. The obtained results show that the extraction percentage of Pt and Pd increased from 60 and 9% to 94 and 50% with the increase of O/A ratio from 1/3 to 3. Extraction isotherm (Fig. 8) indicates that quantitative extraction of Pt using the mixture of 0.01 M Aliquat 336 and 0.4 M TBP was possible in three stages at an O/A ratio of unity. To verify the estimated number of stages from the McCabe– Thiele diagram, batch simulation experiments of three stages counter-current extraction were carried at an A/O ratio of unity. After three stages of batch simulation experiments, the extraction percentage of Pt and Pd was determined to be 100 and 38%, respectively. The composition of the loaded mixture was 120 mg/L Pt and 19 mg/L Pd.
289
Stripping
290 291 292 293 294
The stripping of Pt/Pd from TBP or Aliquat 336 alone has been reported [7,21,25,27]. It has been reported that the mixture of HCl and thiourea solution is better than single HCl or thiourea for the stripping of Pt/Pd from single TBP and Aliquat 336. In order to investigate the separation possibility of Pt and Pd during stripping
Stripping percentage, %
0 0 0 0
0 0 0 0
0.1 0.3 0.5 1.0
0.001 0.001 0.001
0.1 0.3 0.5
Pd
No stripping White precipitates Poor separation phase
No stripping
0 5 6.1
87.1 95.5 100
Loaded organic, mg/L: Pt-120, Pd-19; [HCl] = 0.1–1 M; [thiourea] = 0.001–0.01 M; O/ A = 1:1
from the loaded mixture of Aliquat 336 and TBP, thiourea, HCl, and a mixture of thiourea and HCl solution were tested in the stripping experiments. The loaded mixture containing 120 mg/L Pt and 19 mg/L Pd was employed in these experiments. The volume ratio of the two phases in stripping experiments was fixed at unity. The stripping percentage of Pt and Pd was very low in the case of single thiourea or HCl solution and some precipitates were formed in the organic phases during stripping. Table 4 indicates that the presence of a small amount of thiourea in the HCl solution is indispensable to strip the metals from the mixture of Aliquat 336 and TBP. Pd was selectively stripped from the loaded mixture by employing the mixture of thiourea and HCl solution. As the concentration of HCl in the mixture with thiourea increased, the stripping percentage of the two metals increased. Since the presence of thiourea in the mixture was critical in stripping the metals from the loaded mixture, the effect of thiourea concentration in the mixture with HCl solution was investigated. For this purpose, the concentration of HCl was fixed at 0.5 M and that of thiourea in the mixture was varied from 0.001 to 0.9 M. Fig. 9 shows that the concentration of thiourea in the mixture with HCl had a profound effect on the stripping of Pt from the loaded mixture. Pd was completely stripped at any thiourea concentration range. However, the stripping percentage of Pt increased from 6 to 99.7% with the increase of thiourea concentration from 0.01 to
Pt Pd
80
160
80
Ope
40
0
20
40
PtFeed =120 mg/L
120
0
0.001 0.005 0.01 0.1
100
200
[Ptorg], mg/L
[HCl], M
Pt
A=1 , O/ line g n i rat
60
80
100
120
[Ptaq], mg/L Fig. 8. McCabe–Thiele diagram for the extraction of Pt by the mixture of 0.01 M Aliquat 336 and 0.4 M TBP. Aqueous solution, mg/L: Pt-120, Pd-50, [HCl] = 6 M. [TBP] = 0.4 M; [Aliquat 336] = 0.01 M; diluent = kerosene; O/A =1:3–3.
Stripping percentage, %
Extraction percentage, %
Pt Pd
60
40
20
0 0.0
0.2
0.4
0.6
0.8
1.0
[Thiourea], M Fig. 9. Effect of thiourea concentration in the mixture with 0.5 M HCl on the stripping of Pt and Pd from the loaded mixture of Aliquat 336 and TBP. Loaded organic, mg/L: Pt-120, Pd-19; [HCl] = 0.5 M; [thiourea] = 0.001–0.9 M; O/A = 1:1.
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295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319
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Feed, ppm: Pt-120, Pd-50 [HCl] =6 M 0.001 M Tu + 0.5 M HCl Aliquat 336 + TBP
Loaded
Extraction Pt
1st stripping Pd
organic
0.7 M Tu + 0.5 M HCl
Loaded organic
Stripping solution, ppm:
Pure Pd solution
2nd stripping Pt Pure Pt solution
Pt-8.4; Pd-19.1 Regenerated organic Fig. 10. Process flow sheet for the separation of Pt and Pd from hydrochloric acid solutions by extraction with the mixture of Aliquat 336 and TBP followed by stripping.
320 321 322 323 324 325 326 327 328
330 329 333 332 331 334
0.7 M and was constant with the further increase of thiourea concentration. The stripping reactions with Pt and Pd are represented in Eqs. (4) and (5) [11]. The stripping behavior of Pt and Pd by the mixture of HCl and thiourea solution can be explained by a coordination-substitution reaction between thiourea ((NH2)2CS) and Cl. The Cl in ((R4N)2(PdCl4)) complex is substituted by (NH2)2CS faster than that in (R4N)2(PtCl6) [11]. This is the reason why the stripping efficiency of Pd was higher than that of Pt even at low thiourea concentration. ðR4 NÞ2 ðPtCl6 Þ þ 2ðNH2 Þ2 CS ¼ 2R4 NCl þ PtCl4 ððNH2 Þ2 CSÞ2
(4)
ðR4 NÞ2 ðPdCl4 Þ þ 2ðNH2 Þ2 CS ¼ 2R4 NCl þ PdCl2 ððNH2 Þ2 CSÞ2
(5)
Fig. 9 clearly suggests the possibility of separating Pt and Pd by selective stripping of Pd from the loaded mixture. First, Pd can be Table 5 Regeneration of the mixture of Aliquat 336 and TBP. Extractant
Original organic Stripped organic washed with water
Extraction, %
% Pt stripping
Pt
Pd
Separation factor (DPt/DPd)
80 72
19 2.6
22 93
99.3 100
Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 6 M; Original organic: 0.01 M Aliquat 336 + 0.4 M TBP; stripping reagent: 0.5 M HCl + 0.7 M thiourea; O/A = 1:1.
Table 6 Separation of Pt and Pd from the synthetic chloride solution at 6 M HCl by the extraction with mixture of Aliquat 336 and TBP and stripping with the mixture of thiourea and HCl solution. Parameter
Value
Extraction Extractant HCl concentration O/A ratio Number of stages Extraction percentage
0.01 M Aliquat 336 + 0.4 M TBP 6M 1/1 3 Pt-100%; Pd-38%
1st stripping Stripping reagent O/A ratio Number stage Stripping percentage
0.5 M HCl + 0.001 M thiourea 1/1 1 Pt-7.3%; Pd-99.3%
2nd stripping Stripping reagent O/A ratio Number stage Stripping percentage Purity percentage
0.5 M HCl + 0.7 M thiourea 1/1 1 Pt-100% 99.9%
Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 6 M; [Aliquat 336] = 0.01 M; [TBP] = 0.4 M; diluent = kerosene.
selectively stripped from the loaded mixture by keeping the concentration of thiourea low in the mixture with 0.5 M HCl. Then the pure Pt in the loaded mixture can be stripped by employing a mixture of 0.7 M thiourea and 0.5 M HCl.
335 336 337 338
Regeneration efficiency of organic and a process for the separation of Pt and Pd
339 340
In solvent extraction process, the regeneration of organic phase is important for economic and environment purpose. The effect of washing the stripped organic with water on the regeneration of the organic was investigated. The loaded organic was first stripped using the mixture of 0.5 M HCl and 0.7 M thiourea for complete stripping of Pt and Pd. After washing this stripped organic with water, the washed organic phase was conducted with the fresh aqueous phase containing Pt-120 mg/L and Pd-50 mg/L at 6 M HCl. The obtained results are shown in Table 5. Although the separation factor between Pt and Pd by the stripped organic after washing was enhanced to 4 times, the extraction percentage of Pt and Pd by the stripped organic was lower than that by the original organic. Therefore, some further experiments on the effect of washing time need to be done to restore the extractability of the mixture. A proposed process for the separation of Pt and Pd from hydrochloric acid solutions by the mixture of Aliquat 336 and TBP is presented in Fig. 10. Based on Fig. 10, a series of tests including extraction, selective stripping was carried out and the obtained results are presented in Table 6. The results in Table 6 indicate that the mixture of Aliquat 336 and TBP can be employed for the separation of Pt and Pd from 6 M HCl solution and the purity of Pt in the stripping solution was found to be 99.9%. During the first stripping, some of Pt was stripped together with Pd and these solutions can be treated by acidic extractants [13,15].
341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364
Conclusions
365
The separation of Pt and Pd from synthetic chloride solutions was investigated by solvent extraction. HCl concentration strongly affected the extraction behavior of Pt and Pd by amines or ammonium salt. Both Pt and Pd were extracted by single Aliquat 336 in the HCl concentration range of 1–3 M, while Pt was selectively extracted over Pd in concentrated HCl concentration (4–8 M). Highest separation factor between Pt and Pd was obtained from 6 M HCl solution using the mixture of Aliquat 336 and TBP and the formation of a third phase was prevented. The concentration of Pt had negligible effect on the extraction behavior of the two metals by the mixture of Aliquat 336 and TBP. Complete extraction of Pt was obtained in three stages. The small amount of Pd in the loaded mixture was completely stripped by keeping the concentration of thiourea low in the mixture with HCl. The remaining Pt after the first stripping of Pd was completely stripped by increasing the concentration of thiourea to 0.7 M in the mixture with 0.5 M
366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381
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382 383
HCl. A process flowsheet was proposed to recover pure Pt solution from the chloride solutions containing Pd.
384
Acknowledgements
385 Q2 The authors would like to thank LS-Nikko, Korea for the financial 386 support. We also express sincere thanks the Korea Basic Science 387 Institute (KBSI), Gwangju branch for providing ICP-OES data. 388 389 390 391 392 Q3 393 394 395 396
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Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6 7
Separation of platinum(IV) and palladium(II) from concentrated hydrochloric acid solutions by mixtures of amines with neutral extractants Q1 Thi a b
Hong Nguyen a, Chong Ho Sonu b, Man Seung Lee
a,
*
Department of Advanced Materials Science & Engineering, Institute of Rare Metal, Mokpo National University, Chonnam 534-729, Republic of Korea Metallic Resources Technology Laboratory, LS-Nikko Copper Inc., Kyunggi 463-400, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 July 2015 Received in revised form 25 August 2015 Accepted 29 August 2015 Available online xxx
Separation of platinum and palladium from concentrated HCl solutions by extraction with mixture of amines (Aliquat 336/Alamine 336/TOA) and neutral extractants (TBP/TOP/MIBK) have been investigated. The mixture of Aliquat 336 and TBP led to higher Pt(IV) separation factor at 6 M HCl solution. McCabe– Thiele diagram for the extraction of Pt by this mixture was constructed. Low concentration of thiourea in the HCl solution resulted in selective stripping of Pd from the loaded organic. Pt solution with 99.9% purity was obtained by extraction and stripping from the solution containing Pd. A process was proposed to separate the two metals. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Platinum Palladium Hydrochloric acid Separation Solvent extraction
8 9
Introduction
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Platinum group metals (PGMs) are used as metallic catalysts in the automobile for minimizing air pollution. Among PGMs, the demand for Pt and Pd is increasing and thus the extraction and separation of these metals from secondary resources is important [1]. The separation and purification of PGMs is extremely difficult due to their similar structure and chemical behavior [2,3]. Hydrometallurgical method, such as precipitation [4], ion exchange [5,6], and solvent extraction [1,3,7–12] are commonly employed for the separation of these metals. Generally, the solvent extraction is one of the most versatile methods in terms of separation efficiency and environmental impact [7,11]. In treating the secondary resources by hydrometallurgical method, Pt and Pd together with other PGMs are dissolved by using concentrated hydrochloric acid concentration in the presence of an oxidizing agent. Lots of works have been reported on the extraction and separation of Pt and Pd from the leach liquors containing PGMs. Three kinds of extractant including acidic and neutral extractants and amines are employed for the separation of Pt and Pd. The process details, advantages and disadvantages of each
* Corresponding author. Tel.: +82 61 450 2492; fax: +82 61 450 2498. E-mail address: [email protected] (M.S. Lee).
extractant system are listed in Table 1. Literature survey indicates that acidic extractants offer high separation efficiency of Pt and Pd, when the acid concentration in leach liquor is low and concentrated HCl solution can strip the metals from the loaded organic. In the case of neutral extractants, selective extraction of Pd over Pt is possible from weak HCl solution [7] but the co-extraction of Pt is high in the concentrated HCl solution [9]. Use of amines lead to complete extraction both of Pt and Pd and these metals can be separated by selective stripping. The disadvantage of employing amines is the possibility of forming a third phase during the extraction and the complexity of the stripping process owing to the necessity to adjust the composition of the stripping solution. Compared to neutral extractants, amines offer higher extraction efficiency of Pt and Pd [9–21]. However in terms of separation factor, the separation efficiency of Pt and Pd by amines is low and the formation of a third phase should be overcome. Unlike single extractant, mixtures of extractants provide some opportunity to circumvent the above-mentioned disadvantages of amines. However, little information has been reported on the separation of Pt and Pd from HCl solution by using mixtures of amines and other extractants. For this purpose, solvent extraction experiments have been done from a synthetic chloride solution containing only Pt and Pd. The extraction and separation behavior of Pt and Pd was investigated as a function of the nature and concentration of extractant in the mixture, acid concentration, metal ion
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Table 1 A summary on the extraction and separation of Pt and Pd from the chloride leach solution by various extractants. Extractant PC 88A
a
LIX 84Ia
Advantage
Disadvantage
Ref.
0.1 mg/L Pt, Pd
Extraction of Pd over Pt from 1 M HCl + 1 M HClO4 solution.
[13]
1 g/L Pt, Pd
Extraction of Pd over Pt in the pH range of 1–3. Separation factor: 400. Extraction of Pd over other metals at 3 M HCl. Using 0.5 M thiourea + 1 M HCl for stripping of Pd. Extraction of Pd over Pt and Au at pH 2.0. Extraction of Au over Pt at 5 M HCl. Extraction of Pd over Pt at 0.1 M HCl.
Low HCl concentration in leach liquor. Low metal concentration. Using 4 M HCl for stripping. Low HCl concentration in leach liquor. Using 6 M HCl for stripping. Extraction of Pt over Mn, Ni, Fe and Cr using Alamine 336 + TBP. High co-extraction of Fe. Using 6 M HCl for stripping of Pd.
[16]
Synthesized extractant. Using 3% v/v NH4Cl + 5 M ammonia solution for stripping. Low separation efficiency. Synthesized extractant. Poor separation efficiency of Pt and Pd.
[3]
[18]
Poor separation efficiency of Pt and Pd. Losing Pt from precipitation of Fe.
[1] [7]
Separation of Pt and Pd from stripping step. High co-stripping of Pt together with Pd. No information for separation of Pt, Pd from stripping. No information for separation of Pt, Pd and Au from stripping. High co-extraction of Pd. Crucial stripping problems.
[8]
Pd-0.15, Pt-0.55, Mn-0.5, Ni-1, Fe-1.5, Cr-0.1 g/L Pd-5.95, Au-5.42, Pt-10.23 g/L 1000 g/L Pt and Pd
N530a DBSO
Aqueous
b
DMDCHTDGAb 1
Cyphos IL 101/104b Cyanex 923b TBPb
Alamine 336c
0.1 g/L Pt, Pd 2.5 mM of Pt, Pd, Ni, Cu, Pb, Ir, Rh, Ru 50 mM of Pt and Pd Pd-0.15, Pt-0.55, Mn-0.5, Ni-1, Fe-1.5, Cr-0.1 g/L 0.001 M Pt and Pd Pt, Pd
TIOAc
Pt, Pd, Rh, Ir, Au
Alamine 300c
0.01 M Pt and Pd
Co-extraction of Pt and Pd at 8 M HCl. Separation of Pt and Pd using stripping step. Extraction of Pd over other metal at 0.1 M HCl. Using ammonia solution for stripping. Extraction of Pt over Pd at 5 M HCl. Extraction of Pd over other metals at 3 M HCl.
Extractions both of Pt and Pd in the HCl concentration range of 1–5 M. Complete extractions both of Pt and Pd at 15% w/w HCl. Extraction of Pt, Pd, Au over Rh and Ir in the HCl concentration range of 3.5–6 M. Extraction of Pt over Pd using saturated sodium chloride solution at pH 1.5.
[14] [15]
[17]
[19] [20] [11]
a
Acidic extractants. Neutral extractants. c Basic extractants. PC-88A = 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester; LIX 84I = 2-hydroxy-5-nonylacetophenone oxime; N530 = 2-hydroxy-4-sec-octanoyl diphenyl-ketoxime; DBSO = dibutyl sulfoxide; DMDCHTDGA = N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide; Cyphos1IL 101 = trihexyl(tetradecyl)phosphonium chloride; Cyphos1IL 104 = trihexyl(tetradecyl)phosphonium bis-2,4,4-trimethylpentylphosphinate; Cyanex 932 = the phosphine oxides; TBP = tributyl phosphate; Alamine 336 = tri-octyl/ dodecyl amine; TIOA = tri-iso-octylamine; Alamine 300 = tri-n-octyl amine. b
54 55 56 57 58 59
concentration and stripping condition. Extraction isotherm for the extraction of Pt was constructed. The effect of thiourea concentration in the HCl solution on the stripping of Pt and Pd was also investigated. A process flowsheet was developed to obtain pure Pt and Pd from the solution by employing extraction and stripping.
60
Experimental
61 62 63 64 65 66 67 68
A synthetic solution was prepared by dissolving appropriate amounts of PtCl4 (99.9%) and PdCl2 (99.9%) purchased from AlfaAesar. The composition of the synthetic solution was Pt-120 mg/L and Pd-50 mg/L. The acidity of the solution was controlled by HCl (Daejung Co.) solution (1–8 M). TOA (Samchun Pure Chemical Co.), TBP (Yakuri Pure Chemical Co.), TOP (IS Chem Co.), MIBK (Daejung Co.), Decanol (Acros Organics Co.), Aliquat 336 (BASF Co.) and Alamine 336 (BASF Co.) were used without further
69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
Extraction percentage
Table 2 Chemical formula of extractants used in this study.
¼
Extractant
Reagent class
Structure
Aliquat 336
Aliphatic quaternary ammonium salt Tertiary amine Tri-n-octylamine Tri-n-butyl phosphate Tri-n-octyl phosphate Methyl isobutyl Ketone
[(C8H17)3N(CH3)]Cl
Alamine 336 TOA TBP TOP MIBK
purification. Chemical formula of each extractant is presented in Table 2. Kerosene (Samchun Pure Chemical Co.) was used as a diluent. Solvent extraction experiments were performed by mixing equal volumes (10 mL) of aqueous and organic phases for 30 min (extraction and stripping equilibrium were reached within 30 min in initial tests) using a wrist action shaker. After equilibrium, the two phases were separated with a separating funnel. All the experiments were performed at room temperature (25 1 8C). Metal ion concentrations in the aqueous phase before and after extraction were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES; Spectro Arcos). The concentration of metal ions in the loaded organic phase was calculated by mass balance. The distribution ratio (D) was calculated as the concentration of metal present in the organic phase to that present in the aqueous phase at equilibrium. The separation factor was calculated from the ratio of the distribution coefficient. The extraction percentage of a metal is defined as follows
[C8–C10 mixture]3N [CH3(CH2)7]3N [CH3(CH2)3O]3PO [C8H17O]3PO (CH3)2CHCH2C(O)CH3
weight of a metal extracted into organic initial weight of a metal in the aqueous before extraction 100 (1)
The extraction percentage of a metal can be calculated from the distribution coefficients of a metal by using the relation, %E = D 100/(D + (Vaq/Vorg)) where Vaq and Vorg represent volumes of aqueous and organic phases, respectively.
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The stripping percentage of a metal is defined as follows
40
3
Aliquat 336 alone Aliquat 336 + Decanol
Stripping percentage (2)
94 95 96
Results
97
Effect of HCl concentration on the extraction with Aliquat 336
98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124
It has been reported that Aliquat 336 can selectively extract Pt from chloride solutions containing Rh, Fe, Ni, Mn, and some minor elements [7,21]. However, little information has been reported on the extraction and separation of Pt and Pd using Aliquat 336. Therefore, three organic systems – (1) Aliquat 336 alone, (2) Aliquat 336 mixed with 0.3 M Decanol and (3) Aliquat 336 mixed with 0.4 M TBP were employed to investigate the extraction and separation behavior of Pt and Pd. The effect of hydrochloric acid concentration on the extraction behavior of Pt and Pd from synthetic solutions containing Pt-120 mg/L and Pd50 mg/L was studied in the HCl concentration range of 1–8 M. In these experiments, the concentration of Aliquat 336 was fixed at 0.01 M and the ratio of organic to aqueous phase was unity. Fig. 1 shows that the extraction of Pt and Pd by Aliquat 336 depended strongly on the concentration of HCl in the solution. Most of Pt and Pd were extracted by Aliquat 336 alone and Aliquat 336 mixed with TBP when the HCl concentration was lower than 3 M. As HCl concentration increased from 3 to 8 M, the extraction percentage of Pd decreased more rapidly than that of Pt. In the case of the system of Aliquat 336 mixed with Decanol, the extraction percentage of Pt and Pd steadily decreased with the increase of HCl concentration from 1 to 8 M. The decrease in extraction of Pt and Pd with the increase of HCl concentration can be explained by the solvent extraction reaction of Pt and Pd by Aliquat 336. Since PtCl62 and PdCl42 are predominant species in the HCl concentration range of 1–8 M, the extraction reaction of Pt and Pd by Aliquat 336 can be represented as Eq. (3) [9,11,22]. nR4 NCl þ MClm n ? ðR4 NÞn ðMClm Þ þ nCl
2
where R4NCl and MClm represent Aliquat 336 and PtCl6 / PdCl42, respectively. The increase in HCl concentration would favor the back reaction of Eq. (3) and thus the extraction percentage of Pt and Pd decreases. 100
Extraction percentage, %
126 125 127 128 129
80
60
40
20 Aliquat 336 alone Aliquat 336+ Decanol
0 0
Pt
Pt
Pd
Pd
2
Aliquat 336+TBP Pt Pd
4
6
30
20
10
(3)
n
Separation factor (DPt/DPd)
Aliquat 336 + TBP
weight of a metal stripped into aqueous ¼ initial weight of a metal in the organic before stripping 100
8
[HCl], M Fig. 1. Effect of HCl concentration on extraction of Pt and Pd by Aliquat 336 alone, Aliquat 336 + 0.3 M Decanol, and Aliquat 336 + 0.4 M TBP. Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 1–8 M; [Aliquat 336] = 0.01 M; diluent = kerosene; O/ A = 1:1.
0 0
2
4
6
8
[HCl], M Fig. 2. Separation factor between Pt and Pd by extraction with Aliquat 336 alone, Aliquat 336 + 0.3 M Decanol, and Aliquat 336 + 0.4 M TBP. Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 1–8 M; [Aliquat 336] = 0.01 M; diluent = kerosene; O/ A = 1:1.
It has been reported that the tendency for the PGM chlorocomplexes to form ion pairs with anion exchangers changes in the following order: [MCl62] > [MCl42] [MCl63] > aqua species [18]. Since most of Pt and Pd exist as PtCl62 and PdCl42 in the HCl concentration employed in this study, Pt was selectively extracted over Pd by Aliquat 336 in this work. The extraction efficiency of Pt and Pd by the three organic systems was in the order Aliquat 336 + Decanol < Aliquat 336 + TBP < Aliquat 336 alone. The variation in the separation factor between Pt and Pd with HCl concentration using the three organic systems is presented in Fig. 2. The high separation factor was obtained with Aliquat 336 alone and Aliquat 336 + TBP systems in the HCl concentration range of 4–8 M, while the separation factor was lower than 5 in the case of Aliquat 336 + Decanol at any HCl concentration range. From the data represented in Figs. 1 and 2, it might be said that Aliquat 336 alone offered the highest separation efficiency among the three organic systems. However, separation of the two phases after extraction with Aliquat 336 alone was poor because of the formation of a third phase or emulsion in the organic phase. The presence of Decanol or TBP in Aliquat 336 suppressed the formation of a third phase and TBP was chosen as an optimum modifier in terms of separation factor (Fig. 2). Highest separation factor (22) between Pt and Pd by the system of Aliquat 336 and TBP was obtained from 6 M HCl where the extraction percentage of Pt and Pd was 80% and 19%, respectively. Therefore, the synthetic solution with 6 M HCl was employed in further experiments.
130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155
Effect of the mixtures of Aliquat 336, Alamine 336 or TOA and TBP
156
Lots of work has been reported on the extraction efficiency of Pt by using single amines [11,23,24]. Especially, Sun and Lee [8] and Mirza [20] have reported that complete extraction of both Pt and Pd is obtained in the HCl concentration range of 1–6 M using single Alamine 336 or TOA. Thus, the extraction behavior and separation efficiency of Pt and Pd by the mixture of Aliquat 336/Alamine 336/ TOA and TBP was studied to find an optimum organic system. The composition of the synthetic solution was Pt-120 mg/L, Pd-50 mg/ L and 6 M HCl. The concentration of amines was kept at 0.01 M in the mixture, while the concentration of TBP was varied from 0 to 1 M. Fig. 3 shows that in the absence of TBP in the mixtures, Alamine 336 and TOA led to high extraction percentage of both Pt and Pd, while Aliquat 336 offered some possibility to separate Pt and Pd. The extraction percentage of Pd decreased more than that of Pt as TBP concentration increased from 0 to 0.4 M and there was
157 158 159 160 161 162 163 164 165 166 167 168 169 170 171
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4 100
100
Pt Pd
Pt Pd 80
Extraction percentage, %
Extraction percentage, %
80
(a) 0.01 M Aliquat 336 +TBP
60
40
20
0
60
(b) 0.01M Alamine 336 + TBP
40
20
0.0
0.2
0.4
0.6
0.8
0
1.0
0.0
[TBP], M
0.2
0.4
0.6
0.8
1.0
[TBP], M 100
Pt Pd
Extraction percentage, %
80
60
(c) 0.01 M TOA + TBP 40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
[TBP], M Fig. 3. Effect of TBP concentration on the extraction Pt and Pd by the mixture of Aliquat 336/Alamine 336/TOA and TBP. Aqueous solution, mg/L: Pt-120, Pd-50, [HCl] = 6 M. [TBP] = 0–1 M; [amine] = 0.01 M; diluent = kerosene; O/A = 1:1.
little change in the extraction percentage of Pt and Pd with the further increase of TBP concentration. These results agreed well with the reported literature which reports antagonism for the extraction of Pt by the mixture of Aliquat 336 and TBP [25]. The decrease in the extraction of Pt and Pd with increasing TBP concentration in the mixture may be due to the interaction between TBP and amines, which reduces the effective concentration of amines. Huang and Bautista [26] have reported that the association degree between TBP and Aliquat 336 is very strong, which is represented as (TBP)2(R4NCl)k, where k is a positive integer. In strong HCl solution, the interaction of TBP and protonated TOA or Alamine 336 is also similar to Aliquat 336. When TBP concentration increases, k value also increases and thus the free concentration of amine which can take part in the solvent extraction reaction is reduced. Hence, the extraction percentage of Pt and Pd by these mixtures decreased with the increase of TBP concentration. The separation factor between Pt and Pd by the three organic systems is presented in Fig. 4. The separation factor between Pt and Pd by Aliquat 336 system almost decreased with the increase of TBP concentration from 0 to 1 M, while the reverse was true in the case of Alamine 336 and TOA. The order of separation efficiency of extractant mixtures followed the Aliquat 336 + TBP > TOA + TBP > Alamine 336 + TBP. Although the presence of TBP in Aliquat 336 results in a decrease in the separation factor of Pt and Pd, the mixture of Aliquat 336 and TBP was chosen as an optimum extractant mixture. The obtained results also indicated that adding 0.4 M TBP to Aliquat 336 was the best condition for the separation of
Pt and Pd, where high separation factor was obtained and the formation of third phase or emulsion was avoided during extraction.
200 201
The effect of TBP/TOP/MIBK concentration in the Aliquat 336 system
202
As mentioned above, the addition of TBP into Aliquat 336 affected the extraction and separation of Pt and Pd from the
203 204
30 Aliquat 336 + TBP
25
Alamine 336 + TBP TOA + TBP
Separation factor (DPt/DPd)
172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199
20
15
10
5
0 0.0
0.2
0.4
0.6
0.8
1.0
[TBP], M Fig. 4. Effect of TBP concentration on separation factor between Pt and Pd by the mixture of Aliquat 336/Alamine 336/TOA and TBP. Aqueous solution, mg/L: Pt-120, Pd-50, [HCl] = 6 M. [TBP] = 0–1 M; [amine] = 0.01 M; diluent = kerosene; O/A = 1:1.
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synthetic solution at 6 M HCl. Therefore, in order to investigate the effect of the nature of neutral extractants, three neutral extractants (TBP, TOP, and MIBK) were added into Aliquat 336 and these mixtures were employed in the extraction experiments. In these experiments, the composition of the aqueous synthetic solution was the same and the concentration of HCl was 6 M. The concentration of Aliquat 336 was fixed at 0.01 M, while that of neutral extractant was varied from 0.1 to 1 M. The organic to aqueous phase ratio was unity. After separation of the phases, a third phase was observed in the system of Aliquat 336 + MIBK at any MIBK concentration, while in the system of Aliquat 336 + TBP/ TOP the third phase was formed at lower 0.2 M TBP/TOP. The difference in the efficiency of a modifier in suppressing the formation of a third phase might be correlated to the structure of TBP, TOP, and MIBK (see Table 2). Phosphoryl group is present in the structure of TBP and TOP [9], while the structure of MIBK consists of carbonyl group. This might be the reason why the mixture of Aliquat 336 + TBP/TOP can prevent the formation of a third phase or emulsion when TBP/TOP concentration was higher than 0.2 M. Fig. 5 shows the effect of TBP/TOP/MIBK concentration in the mixture with Aliquat 336 on the extraction of Pt and Pd. The presence of TOP/MIBK in Aliquat 336 had negligible effect on the extraction of Pt and Pd. On the contrary, the extraction percentage of Pt and Pd decreased steadily with the increase of TBP concentration from 0 to 0.4 M and was constant with the further increase of TBP concentration. Since the extraction percentage of Pd decreased more rapidly than that of Pt with the increase of TBP concentration, high separation factor between Pt and Pd was obtained in the system of Aliquat 336 + TBP (see Table 3). According to the obtained results, 0.4 M TBP was chosen as the optimum condition in terms of separation factor, co-extraction percentage of Pd and phase separation (see Fig. 5 and Table 3). It can be concluded that the presence of 0.4 M TBP in 0.01 M Aliquat 336 can prevent the formation of a third phase and result in high separation efficiency of Pt and Pd from 6 M HCl solution.
240
Effect of Aliquat 336 concentration in the mixture with TBP
241 242 243 244 245 246 247
In order to investigate the effect of Aliquat 336 concentration in the mixture with TBP on the separation of Pt and Pd, Aliquat 336 concentration in the mixture with 0.4 M TBP was varied from 0 to 0.02 M. The composition of the synthetic solution was Pt120 mg/L and Pd-50 mg/L and 6 M HCl. The ratio of organic to aqueous phase was unity. Fig. 6 shows that the extraction percentage of Pt and Pd with 0.4 M TBP alone was 64 and 18%,
Extraction percentage, %
100
80
60
40
20 Aliquat 336+TBP
0 0.0
Aliquat 336+TPO
Aliquat 336+MIBK
Pt
Pt
Pt
Pd
Pd
Pd
0.2
0.4
0.6
0.8
1.0
Table 3 Separation factor between Pt and Pd by the mixture of Aliquat 336 and TBP/TOP/ MIBK. [TOP/TBP/MIBK], M
0.1 0.2 0.4 0.6 0.8 1.0
SF = DPt/DPd TBP + Aliquat 336
TOP + Aliquat 336
MIBK + Aliquat 336
21.4 19.7 22.0 11.9 11.0 11.3
18.7 16.0 13.7 12.6 10.1 7.9
19.2 18.9 20.3 18.8 19.3 17.8
Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 6 M; [Aliquat 336] = 0.01 M; [TBP/ TOPO/MIBK] = 0.1–1 M; diluent = kerosene; O/A = 1:1.
100
80
Extraction percentage, %
205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239
5
Pt Pd
60
40
20
0
0.000
0.005
0.010
0.015
0.020
[Aliquat 336], M Fig. 6. Effect of Aliquat 336 concentration in the mixture with 1 M TBP on the extraction of Pt and Pd. Aqueous solution, mg/L: Pt-120, Pd-50, [HCl] = 6 M. [Amine] = 0–0.02 M; diluent = kerosene; O/A = 1:1.
respectively. When Aliquat 336 concentration increased from 0 to 0.02 M, the extraction percentage of Pt and Pd increased to around 87 and 40%, respectively. The obtained results indicated that the presence of Aliquat 336 in TBP enhanced extraction efficiency of Pt and Pd. The formation of a third phase occurred when the Aliquat 336 concentration was higher than 0.01 M. Therefore 0.01 M Aliquat 336 was selected as an optimum condition in terms of phase separation and extraction efficiency of Pt in further experiments.
248 249 250 251 252 253 254 255 256
Effect of Pt concentration
257
In the separation of Pt and Pd, the concentration ratio of these two metals might affect the separation efficiency. Therefore, in order to investigate the effect of the concentration ratio of Pt to Pd, the concentration of Pt in the mixed solution was varied from 50 to 1000 mg/L. In these experiments, the concentration of HCl was 6 M and that of Pd was kept at 50 mg/L. The experiments were carried using the mixture of 0.01 M Aliquat 336 and 0.4 M TBP. Fig. 7 shows that the extraction percentage of Pt and Pd was constant at 80 and 20%, respectively at any Pt concentration range. It might be concluded that in our experimental range the Pt concentration in the mixed solution did not affect the extraction behavior of Pt and Pd using the mixture of Aliquat 336 and TBP.
258 259 260 261 262 263 264 265 266 267 268 269
Extraction isotherm
270
In order to determine the theoretical number of stages for the complete extraction of Pt from the synthetic solution containing Pt-120 mg/L, Pd-50 mg/L at 6 M HCl, extraction isotherm of Pt was
271 272 273
[TBP/TOP/MIBK], M Fig. 5. Effect of TBP/TOP/MIBK concentration in the mixture with Aliquat 336 on the extraction of Pt and Pd. Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 6 M; [Aliquat 336] = 0.01 M; [TBP/TOP/MIBK] = 0.1–1 M; diluent = kerosene; O/A = 1:1.
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6 100
Table 4 Effect of stripping reagents on stripping of Pt and Pd from loaded Aliquat 336 + TBP Q4 using single thiourea, HCl and thiourea mixed with HCl solution.
80
[Thiourea], M
60
40
20
0
0
200
400
600
800
1000
[Pt]aq, mg/L Fig. 7. Effect of Pt concentration on the extraction of Pt and Pd by the mixture of Aliquat 336 and TBP. Aqueous solution, mg/L: [Pt] = 50–1000, Pd-50, [HCl] = 6 M. [TBP] = 0.4 M; [Aliquat 336] = 0.01 M; diluent = kerosene; O/A = 1:1.
274 275 276 277 278 279 280 281 282 283 284 285 286 287 288
obtained by varying O/A ratios from 1/3 to 3. The aqueous solution was contacted with the mixture of 0.01 M Aliquat 336 and 0.4 M TBP. The obtained results show that the extraction percentage of Pt and Pd increased from 60 and 9% to 94 and 50% with the increase of O/A ratio from 1/3 to 3. Extraction isotherm (Fig. 8) indicates that quantitative extraction of Pt using the mixture of 0.01 M Aliquat 336 and 0.4 M TBP was possible in three stages at an O/A ratio of unity. To verify the estimated number of stages from the McCabe– Thiele diagram, batch simulation experiments of three stages counter-current extraction were carried at an A/O ratio of unity. After three stages of batch simulation experiments, the extraction percentage of Pt and Pd was determined to be 100 and 38%, respectively. The composition of the loaded mixture was 120 mg/L Pt and 19 mg/L Pd.
289
Stripping
290 291 292 293 294
The stripping of Pt/Pd from TBP or Aliquat 336 alone has been reported [7,21,25,27]. It has been reported that the mixture of HCl and thiourea solution is better than single HCl or thiourea for the stripping of Pt/Pd from single TBP and Aliquat 336. In order to investigate the separation possibility of Pt and Pd during stripping
Stripping percentage, %
0 0 0 0
0 0 0 0
0.1 0.3 0.5 1.0
0.001 0.001 0.001
0.1 0.3 0.5
Pd
No stripping White precipitates Poor separation phase
No stripping
0 5 6.1
87.1 95.5 100
Loaded organic, mg/L: Pt-120, Pd-19; [HCl] = 0.1–1 M; [thiourea] = 0.001–0.01 M; O/ A = 1:1
from the loaded mixture of Aliquat 336 and TBP, thiourea, HCl, and a mixture of thiourea and HCl solution were tested in the stripping experiments. The loaded mixture containing 120 mg/L Pt and 19 mg/L Pd was employed in these experiments. The volume ratio of the two phases in stripping experiments was fixed at unity. The stripping percentage of Pt and Pd was very low in the case of single thiourea or HCl solution and some precipitates were formed in the organic phases during stripping. Table 4 indicates that the presence of a small amount of thiourea in the HCl solution is indispensable to strip the metals from the mixture of Aliquat 336 and TBP. Pd was selectively stripped from the loaded mixture by employing the mixture of thiourea and HCl solution. As the concentration of HCl in the mixture with thiourea increased, the stripping percentage of the two metals increased. Since the presence of thiourea in the mixture was critical in stripping the metals from the loaded mixture, the effect of thiourea concentration in the mixture with HCl solution was investigated. For this purpose, the concentration of HCl was fixed at 0.5 M and that of thiourea in the mixture was varied from 0.001 to 0.9 M. Fig. 9 shows that the concentration of thiourea in the mixture with HCl had a profound effect on the stripping of Pt from the loaded mixture. Pd was completely stripped at any thiourea concentration range. However, the stripping percentage of Pt increased from 6 to 99.7% with the increase of thiourea concentration from 0.01 to
Pt Pd
80
160
80
Ope
40
0
20
40
PtFeed =120 mg/L
120
0
0.001 0.005 0.01 0.1
100
200
[Ptorg], mg/L
[HCl], M
Pt
A=1 , O/ line g n i rat
60
80
100
120
[Ptaq], mg/L Fig. 8. McCabe–Thiele diagram for the extraction of Pt by the mixture of 0.01 M Aliquat 336 and 0.4 M TBP. Aqueous solution, mg/L: Pt-120, Pd-50, [HCl] = 6 M. [TBP] = 0.4 M; [Aliquat 336] = 0.01 M; diluent = kerosene; O/A =1:3–3.
Stripping percentage, %
Extraction percentage, %
Pt Pd
60
40
20
0 0.0
0.2
0.4
0.6
0.8
1.0
[Thiourea], M Fig. 9. Effect of thiourea concentration in the mixture with 0.5 M HCl on the stripping of Pt and Pd from the loaded mixture of Aliquat 336 and TBP. Loaded organic, mg/L: Pt-120, Pd-19; [HCl] = 0.5 M; [thiourea] = 0.001–0.9 M; O/A = 1:1.
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295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319
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7
Feed, ppm: Pt-120, Pd-50 [HCl] =6 M 0.001 M Tu + 0.5 M HCl Aliquat 336 + TBP
Loaded
Extraction Pt
1st stripping Pd
organic
0.7 M Tu + 0.5 M HCl
Loaded organic
Stripping solution, ppm:
Pure Pd solution
2nd stripping Pt Pure Pt solution
Pt-8.4; Pd-19.1 Regenerated organic Fig. 10. Process flow sheet for the separation of Pt and Pd from hydrochloric acid solutions by extraction with the mixture of Aliquat 336 and TBP followed by stripping.
320 321 322 323 324 325 326 327 328
330 329 333 332 331 334
0.7 M and was constant with the further increase of thiourea concentration. The stripping reactions with Pt and Pd are represented in Eqs. (4) and (5) [11]. The stripping behavior of Pt and Pd by the mixture of HCl and thiourea solution can be explained by a coordination-substitution reaction between thiourea ((NH2)2CS) and Cl. The Cl in ((R4N)2(PdCl4)) complex is substituted by (NH2)2CS faster than that in (R4N)2(PtCl6) [11]. This is the reason why the stripping efficiency of Pd was higher than that of Pt even at low thiourea concentration. ðR4 NÞ2 ðPtCl6 Þ þ 2ðNH2 Þ2 CS ¼ 2R4 NCl þ PtCl4 ððNH2 Þ2 CSÞ2
(4)
ðR4 NÞ2 ðPdCl4 Þ þ 2ðNH2 Þ2 CS ¼ 2R4 NCl þ PdCl2 ððNH2 Þ2 CSÞ2
(5)
Fig. 9 clearly suggests the possibility of separating Pt and Pd by selective stripping of Pd from the loaded mixture. First, Pd can be Table 5 Regeneration of the mixture of Aliquat 336 and TBP. Extractant
Original organic Stripped organic washed with water
Extraction, %
% Pt stripping
Pt
Pd
Separation factor (DPt/DPd)
80 72
19 2.6
22 93
99.3 100
Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 6 M; Original organic: 0.01 M Aliquat 336 + 0.4 M TBP; stripping reagent: 0.5 M HCl + 0.7 M thiourea; O/A = 1:1.
Table 6 Separation of Pt and Pd from the synthetic chloride solution at 6 M HCl by the extraction with mixture of Aliquat 336 and TBP and stripping with the mixture of thiourea and HCl solution. Parameter
Value
Extraction Extractant HCl concentration O/A ratio Number of stages Extraction percentage
0.01 M Aliquat 336 + 0.4 M TBP 6M 1/1 3 Pt-100%; Pd-38%
1st stripping Stripping reagent O/A ratio Number stage Stripping percentage
0.5 M HCl + 0.001 M thiourea 1/1 1 Pt-7.3%; Pd-99.3%
2nd stripping Stripping reagent O/A ratio Number stage Stripping percentage Purity percentage
0.5 M HCl + 0.7 M thiourea 1/1 1 Pt-100% 99.9%
Aqueous solution, mg/L: Pt-120, Pd-50; [HCl] = 6 M; [Aliquat 336] = 0.01 M; [TBP] = 0.4 M; diluent = kerosene.
selectively stripped from the loaded mixture by keeping the concentration of thiourea low in the mixture with 0.5 M HCl. Then the pure Pt in the loaded mixture can be stripped by employing a mixture of 0.7 M thiourea and 0.5 M HCl.
335 336 337 338
Regeneration efficiency of organic and a process for the separation of Pt and Pd
339 340
In solvent extraction process, the regeneration of organic phase is important for economic and environment purpose. The effect of washing the stripped organic with water on the regeneration of the organic was investigated. The loaded organic was first stripped using the mixture of 0.5 M HCl and 0.7 M thiourea for complete stripping of Pt and Pd. After washing this stripped organic with water, the washed organic phase was conducted with the fresh aqueous phase containing Pt-120 mg/L and Pd-50 mg/L at 6 M HCl. The obtained results are shown in Table 5. Although the separation factor between Pt and Pd by the stripped organic after washing was enhanced to 4 times, the extraction percentage of Pt and Pd by the stripped organic was lower than that by the original organic. Therefore, some further experiments on the effect of washing time need to be done to restore the extractability of the mixture. A proposed process for the separation of Pt and Pd from hydrochloric acid solutions by the mixture of Aliquat 336 and TBP is presented in Fig. 10. Based on Fig. 10, a series of tests including extraction, selective stripping was carried out and the obtained results are presented in Table 6. The results in Table 6 indicate that the mixture of Aliquat 336 and TBP can be employed for the separation of Pt and Pd from 6 M HCl solution and the purity of Pt in the stripping solution was found to be 99.9%. During the first stripping, some of Pt was stripped together with Pd and these solutions can be treated by acidic extractants [13,15].
341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364
Conclusions
365
The separation of Pt and Pd from synthetic chloride solutions was investigated by solvent extraction. HCl concentration strongly affected the extraction behavior of Pt and Pd by amines or ammonium salt. Both Pt and Pd were extracted by single Aliquat 336 in the HCl concentration range of 1–3 M, while Pt was selectively extracted over Pd in concentrated HCl concentration (4–8 M). Highest separation factor between Pt and Pd was obtained from 6 M HCl solution using the mixture of Aliquat 336 and TBP and the formation of a third phase was prevented. The concentration of Pt had negligible effect on the extraction behavior of the two metals by the mixture of Aliquat 336 and TBP. Complete extraction of Pt was obtained in three stages. The small amount of Pd in the loaded mixture was completely stripped by keeping the concentration of thiourea low in the mixture with HCl. The remaining Pt after the first stripping of Pd was completely stripped by increasing the concentration of thiourea to 0.7 M in the mixture with 0.5 M
366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381
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382 383
HCl. A process flowsheet was proposed to recover pure Pt solution from the chloride solutions containing Pd.
384
Acknowledgements
385 Q2 The authors would like to thank LS-Nikko, Korea for the financial 386 support. We also express sincere thanks the Korea Basic Science 387 Institute (KBSI), Gwangju branch for providing ICP-OES data. 388 389 390 391 392 Q3 393 394 395 396
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