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Solvent extraction of cadmium (II) from sulphate solutions using TOPS 99, PC 88A, Cyanex 272 and their mixtures
Hydrometallurgy 74 (2004) 277 – 283 www.elsevier.com/locate/hydromet
Solvent extraction of cadmium (II) from sulphate solutions using TOPS 99, PC 88A, Cyanex 272 and their mixtures B. Ramachandra Reddy *, D. Neela Priya, J. Rajesh Kumar Inorganic Chemistry Division, Indian Institute of Chemical Technology (CSIR), Uppal Road, Hyderabad 500 007, India Received 1 March 2004; received in revised form 2 June 2004; accepted 3 June 2004
Abstract Extraction of cadmium (II) from sulphate solutions using organophosphorous-based extractants—TOPS 99, PC 88A and Cyanex 272 has been studied. Percent extraction of cadmium (II) increased with increasing equilibrium pH of the aqueous phase and extractant concentration. Extraction of Cd (II) by organophosporus extractants involves cation exchange mechanism with the formation of 1:3 metal to reagent complex. Characterization of the solid complex of cadmium with TOPS 99 by FTIR, 31 P NMR supported metal complex formation with the phosphorus-hydroxyl (P-OH) group. Finally, the extraction behavior and possible separation of cadmium and nickel from the mixture of metals was studied. D 2004 Elsevier B.V. All rights reserved. Keywords: Organophosphorus extractants; Solvent extraction; Cadmium (II); Synergism; Separation
1. Introduction Cadmium and its compounds are toxic and poisoning occurs through inhalation and ingestion. In spite of its toxicity, it is used in different industries such as electroplating, pigments, synthetic chemicals, ceramics, metallurgical and photographic products (Cheremisinoff, 1995). Cadmium is primarily produced as a byproduct from mining, smelting and refining of sulphide ore concentrates of Zn. Secondary cadmium is recovered from spent Ni –Cd batteries. Hydrometallurgical methods of treating Cd-containing materials such as Ni – Cd batteries generate leach liquors containing Cd and Ni as major metals. * Corresponding author. Tel.: +91-40-27193510; fax: +91-4027160921. E-mail address: [email protected] (B. Ramachandra Reddy). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2004.06.001
Literature on cadmium solvent extraction, reports the use of solvating and cation exchange type extractants. Wassink et al. (2000) studied the extraction and separation of cadmium and zinc from nickel and cobalt from chloride and thiocyanate solutions using aliquat 336 extractant. Gupta et al. (2001) have reported extraction and recovery of Cd from HCl medium using Cyanex 923. The organophosphorus-based extractants studied for the extraction Cd from other metals such as Zn, Ni were D2EHPA (Casas et al., 1986; Galan et al., 1998), Cyanex 272, Cyanex 301 and Cyanex 302 (Nayak et al., 1995; Avila-Rodriguez et al., 1998; Almela and Elizalde, 1995). In this paper, we report the solvent extraction studies of cadmium from sulphate solutions using the commercial extractants, TOPS 99, PC 88A and Cyanex 272 and their mixtures. Various parameters studied include equilibrium pH, extractant and metal
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concentration, synergism, different salts, loading capacity, stripping of cadmium from loaded organic, extraction from Ni –Cd mixtures and preparation of cadmium –TOPS 99 solid complex. The results of this study are important from the environmental point of view in the treatment and recovery of cadmium from wastes such as spent batteries/industrial wastewaters.
2. Experimental 2.1. Reagents Commercial extractant TOPS 99, (an equivalent of di-(2-ethylhexyl) phosphoric acid, Heavy Water Plant, Talcher, Orissa, India), PC 88A (2-ethylhexyl phosphonic acid mono 2-ethyl hexyl ester, Daihachi, Japan) and Cyanex 272 (bis (2,4,4-trimethyl pentyl) phosphinic acid, Cytec, Canada) were used as received. Distilled kerosene (bp: 165 – 200 jC) was used as the diluent. The sodium salts of the extractants (60% neutralized) were prepared by adding stoichiometric amounts of concentrated NaOH solution to the extractant in kerosene as required. All other chemicals were Analar grade. Stock solution of cadmium (II) sulphate (0.5 M) was prepared in distilled water containing little H2SO4 to prevent hydrolysis of the metal ion and standardised against 0.05 M EDTA solution using Eriochrome Black T as indicator (Vogel, 1962). 2.2. Apparatus A digital pH meter (Digisun DI 707 model) and Atomic Absorption Spectrophotometer (AAS) of Perkin-Elmer Model A 300 were used for the measurement of pH and metal concentration in the aqueous phase. The IR spectrum of the solid complex was recorded using FTIR-Nicolet (USA)-740-spectrophotometer. Phosphorous-31 Nuclear magnetic resonance measurements were carried out with Varian Unity Inova 500 MHz instrument using H3PO4 (85%) as the external standard. 2.3. Solvent extraction procedure Extraction was carried out with 10 mL of the aqueous solution containing 0.01 M metal and 0.1
M Na2SO4 equilibrated with equal volume of sodium salts of the extractants in a separating funnel. The pH of the aqueous phase was adjusted initially to the desired value by adding dilute H2SO4/NaOH solution before equilibration. Initial experiments on contact time showed that 5 min was sufficient to reach equilibrium. In all the extraction experiments after equilibrating for 10 min, the two phases were allowed to separate. The aqueous phase equilibrium pH was measured and the metal concentration was analysed by AAS. The concentration of the metal ion in the organic phase was calculated from the difference between the metal ion concentration in the aqueous phase before and after extraction. The distribution coefficient (D) is taken as the ratio of the concentration of cadmium in the organic phase to that present in the aqueous phase at equilibrium. For determination of the loading capacity of 0.03 M extractants, the loaded organic (LO) phase was filtered through a 1PS separating paper and a suitable aliquot was stripped with 2 M HCl, followed by analysis using AAS. All the experiments were carried out at room temperature (30 F 1 jC). 2.4. Preparation of Cd-TOPS 99 solid complex One hundred milliliters of 0.2 M TOPS-99 in kerosene was taken in a 500-mL beaker and equal volume of 0.1 M cadmium was added. Both phases were stirred for 10 min at an equilibrium pH of f 5.0 and allowed to stand for phase separation. The raffinate was separated and fresh cadmium metal solution was added to the loaded organic phase repeatedly until the organic phase became saturated with cadmium. After each contact, concentration of Cd in the raffinate was analysed. When the raffinate content of Cd is same as the feed, then the LO was considered to be fully loaded with Cd. The loaded organic phase was filtered through phase separator filter paper. Solvent from the LO phase was allowed to evaporate under suction at around 80 jC to yield solid complex. This complex was then washed with 50% ethanol to remove traces of TOPS 99, dried under vacuum and finally in oven maintained at around 90 jC and stored in a desiccator (Biswas and Hayat, 2002).
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3. Results and discussion 3.1. Extraction mechanism and species The extraction of Cd (II) (0.01 M) from aqueous sulphate solutions was studied in the equilibrium pH range 3.6 –7.6 using the sodium form of TOPS 99, PC 88A and Cyanex 272 at 0.02, 0.03 and 0.04 M in kerosene. Percentage extraction of Cd (II) increased
Fig. 2. Plot of log D vs. log [extractant], M. Org.: TOPS 99 (5), PC 88A (o), Cyanex 272 (D); Aq.: 0.01 M Cd (II), 0.1 M Na2SO4.
e=
slop
0.6
Fig. 1. Plot of equilibrium pH vs. log D. Org.: TOPS 99 (a); PC 88A (b); Cyanex 272 (c); Aq: 0.01 M Cd (II), 0.1 M Na2SO4.
with increase in the equilibrium pH of the aqueous phase. The plot of equilibrium pH vs. log D (Fig. 1) shows that the plots are linear with a slope of 1.0, 0.9 and 1.3 for TOPS 99; 0.8, 0.6 and 0.7 for PC 88A; 0.6, 0.6 and 0.8 for Cyanex 272 with 0.02, 0.03 and 0.04 M extractants, respectively, indicating the exchange of 1 mol of H+ ion with 1 mol of Cd (II). The influence of extractant concentration on the extraction of Cd (II) (keeping initial pH at 2.85) was studied in the range 0.01 –0.04 M. The corresponding change in equilibrium pH was in the range 3.4 –5.9. Percentage extraction increased steadily with increase in extractant concentration with the three extractants. The increase was 4.3% to 85% in case of TOPS 99, 0.5% to 83.4% in case of PC 88A and 1% to 82.2% in case of Cyanex 272. The plots of log [extractant], M vs. log D (Fig. 2) are linear with a slope value of f 3 indicating the association of three moles of the extractant with the extracted species. Correlation of extraction behavior of Cadmium (II) at 50% extraction using 0.02 M extractant concentration with their pKa values clearly indicate direct relation with pKa values. The corresponding pH1/2 values for TOPS 99, PC 88A and Cyanex 272 are 4.5, 5.3 and 5.8, respectively, indicating that TOPS 99 as an efficient extractant than the other two. The loading capacity of 0.03 M extractants was studied from a feed solution containing 0.1 M Na2SO4 and varying the metal concentrations between 0.005 and 0.04 M. Loading of the extractant with metal increases systematically with increase in metal con-
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centration and reaches maximum at z 0.02 M metal concentration. The corresponding percentage extraction with respect to initial metal concentration decreases from 94% to 37% in case of TOPS 99 and from 98% to 36% in case of PC 88A and Cyanex 272. The loading capacity was calculated to be 1.62 F 0.05 g/L, which is equivalent to 99.1 –93.1% of loading indicating that the extractants are almost pure in form. Further, the plot of log [Cd]Aq, M vs. log [Cd]Org, M for TOPS 99 (Fig. 3) is linear with a slope of f 0.7 indicating that monomeric metal species is extracted into the organic phase. Based on the results, the species extracted into the organic phase appears to be CdA25HA. Extraction of Cd (II) from aqueous sulphate solutions using the acid forms of TOPS 99, PC 88A and Cyanex 272 in kerosene was poor. As a result, the extractants were converted to sodium form (60%) by the addition of sodium hydroxide. Considering the results of Cd (II) extraction as a function of pH, extractant and metal obtained by slope analysis method, the mechanism can be proposed as follows: the neutralization reaction of the extractants written as (Sarangi et al., 1999; Reddy and Bhaskara Sarma, 2001): þ Naþ ðaqÞ þ 1=2ðHAÞ2ðorgÞ ! NaAðorgÞ þ HðaqÞ
ð1Þ
The acidic (Saji John et al., 1999; Ramachandra Reddy et al., 2004) and neutral form of the extractant
Fig. 3. Plot of log metal (Aq) vs. log metal (Org). Org.: 0.03 M TOPS 99; Aq.: 0.1 M Na2SO4.
Table1 Effect of salts on percentage extraction of cadmium (II) S. no Extractant
Salt
Salt concentration (M) 0.1
0.2
0.3
0.5
1.0
Extraction of cadmium (%) 1.
2.
3.
TOPS-99
Na2SO4 NaCl NaNO3 NaSCN PC-88A Na2SO4 NaCl NaNO3 NaSCN Cyanex-272 Na2SO4 NaCl NaNO3 NaSCN
71.0 83.8 75.2 84.1 92.9 93.2 92.5 95.3 99.97 99.98 99.99 99.97
70.7 83.6 75.0 82.9 92.2 93.0 91.7 93.4 99.96 99.96 99.98 99.95
69.6 81.0 74.8 80.7 91.6 92.9 91.0 91.5 99.95 99.95 99.98 99.92
67.2 73.0 73.9 75.6 91.1 92.6 90.1 90.0 99.95 99.94 99.97 99.81
63.0 66.0 73.4 54.3 90.4 92.3 89.3 88.9 99.95 99.92 99.95 99.53
takes part in the extraction process of Cd (II) as given by the equation: þ Cd2þ ðaqÞ þ AðorgÞ þ 3ðHAÞ2ðorgÞ ! CdA2 :5HAðorgÞ þ HðaqÞ
ð2Þ Slope analysis from Figs. 1 and 2 supports the mechanism. 3.2. Effect of salts The effect of salts such as Na2SO4, NaNO3, NaCl and NaSCN (in the concentration range 0.1– 1 M) on the extraction efficiency of 0.035 M TOPS 99, PC 88A and Cyanex 272 was studied for 0.01 M metal (initial pH 2.85) The effect of salts on the percentage extraction of metal by Cyanex 272 and PC 88A is almost negligible. In case of TOPS 99 as extractant and NaCl as salt, the percentage extraction decreases from 83.6% to 66% whereas for NaSCN, the decrease is from 84.1% to 54.3%. The change in equilibrium pH was 5.2 – 5.5 under the present experimental conditions. On the other hand, with NaNO3, the percentage extraction changed marginally from 75% to 73.4%. In case of Na2SO4, the decrease was from 71% to 63%. The general conclusion is that Cd (II) extraction decreases with rise in salt concentration when TOPS 99 is used as the extractant (Table 1).
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3.3. Stripping studies The effect of acid concentration in the range 0.1– 1.0 M on the stripping of Cd (II) from 0.01 M LO (TOPS 99, PC 88A and Cyanex 272 in kerosene) containing 0.56 g/L Cd (II) was studied (Table 2). More than 93% stripping efficiency of metal from loaded organic phase was possible in one stage with 0.1 M HCl and H2SO4 and further increase in acid concentration showed marginal increase in efficiency. On the other hand, in case of oxalic acid, two stages of stripping was necessary to achieve >97% efficiency with acid concentrations z 0.5 M. 3.4. Synergism Blake et al. (1958) used the term ‘synergism’ to describe the discovery of a definite enhancement of extraction of uranium by mixture of an acidic alkyl phosphate and certain neutral organophosporous esters, the resulting mixture giving a better extraction of uranium than either the acid or the neutral phosphate alone. The effect is understood to be the replacement of one extractant in the extracted complex by the other and also due to various other factors. This property of the mixed solvent system was utilised to study the extraction behavior of cadmium using mixture of extractants (TOPS 99, PC 88A and Cyanex 272), keeping the concentration of one of the extractant constant at 0.01 M and varying the other as synergist in the range of 0.005 – 0.02 M (Table 3). In case of TOPS 99 as extractant, the synergistic enhancement factor (SEF) increased and then decreased with increase in concenTable 2 Effect of acid concentration on stripping of cadmium (II) from loaded organic S. no. Loaded organic [Acid] (M) Stripping (%)
1.
TOPS-99
2.
PC-88A
3.
Cyanex-272
() = Two-stage stripping.
0.1 0.5 1.0 0.1 0.5 1.0 0.1 0.5 1.0
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Table 3 Distribution coefficient values of Cd (II) using extractant (0.01M) and synergist mixtures DA, Extractant
Concentration of synergist (M)
DB
Dmix
SEF
0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.011(C) 0.011(C) 0.011(C) 0.011(C) 0.011(C) 0.011(C) 0.011(C) 0.011(C)
0.005(P) 0.010(P) 0.015(P) 0.020(P) 0.005(C) 0.010(C) 0.015(C) 0.020(C) 0.005(T) 0.010(T) 0.015(T) 0.020(T) 0.005(C) 0.010(C) 0.015(C) 0.020(C) 0.005(T) 0.010(T) 0.015(T) 0.020(T) 0.005(P) 0.010(P) 0.015(P) 0.020(P)
Nil 0.01 0.15 0.33 Nil 0.01 0.06 0.31 Nil 0.05 0.17 0.41 Nil 0.01 0.06 0.31 Nil 0.05 0.17 0.41 Nil 0.01 0.06 0.31
0.385 0.722 1.106 1.852 0.388 1.486 1.874 2.119 0.372 0.705 1.058 1.915 0.344 0.558 1.257 1.866 0.338 0.638 1.113 1.948 0.328 0.60 1.039 1.746
8.6 14.4 5.8 5.0 8.6 26.5 17.8 6.0 74.4 14.1 6.1 4.6 68.8 34.9 19.3 5.9 30.7 11.4 6.2 4.6 29.8 27.3 14.6 5.4
T = TOPS 99; P = PC 88A; C = Cyanex 272. DA = Distribution coefficient of cadmium with extractant. DB = Distribution coefficient of cadmium with synergist. Dmix = Distribution coefficient of the mixture. SEF = Dmix/DA + DB
tration of synergist (PC 88A and Cyanex 272). On the other hand, in case of PC 88A and Cyanex 272 as extractants and others as synergist, SEF decreases with increase in synergist concentration. The results indicate TOPS 99 as the best synergist. 3.5. Characterisation studies of Cd-TOPS 99 complex
HCl
H2SO4 H2C2O4
93.9 94.3 95.1 96.0 98.2 98.3 96.2 97.0 99.8
94.6 96.9 99.0 97.7 98.8 99.4 95.5 96.2 97.7
78.3 89.6 98.5 81.4 84.1 89.4 46.0 50.2 88.4
(92.9) (100) (100) (94.2) (97.8) (99.2) (75.9) (98.6) (100)
3.5.1. Infrared studies The Infrared spectra of TOPS-99 and Cd-TOPS 99 solid complex indicate the absence of PU(OH) vibration in the region 2750– 2550 cm 1, the absence of PjO (OH) vibration in the region 2350 – 2080 cm 1, the absence of OUH deformation vibration at 1680 cm 1, the shift of P – O bonded stretching vibration at 1225 to 1189 cm 1, the absence of PUOH bending vibration at 1115 cm 1, the absence of PjO (OH)
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vibration within 2700 –2550 cm 1 and the absence of PUOU(H) antisymmetric stretching vibration at 1040 cm 1 (Peppard and Ferraro, 1959; Bellamy, 1975) suggesting complex formation of Cd (II) by TOPS-99. 3.5.2. 31P NMR spectroscopy Fig. 4 shows the 31P NMR spectra of TOPS 99 and its Cd (II) complex. The 31P NMR of cadmium complex showed a shift from 1.698 ppm (TOPS 99) to 3.016 ppm (Cd-TOPS 99 complex). A broad signal in the 31P NMR spectra of the Cd-TOPS 99 complex was observed although there was a small chemical shift when compared with the extractant (TOPS 99). This
Fig. 5. Plot of equilibrium pH vs. percentage extraction of Cd and Ni from their mixtures. Org.: 0.05 M TOPS 99 (a), PC 88A (b), Cyanex27 (c); Aq.: 0.01 M Cd (5) and Ni (o), 0.1 M Na2SO4.
shift may be attributed to the coordination of cadmium with UPO2 group of TOPS 99 (Lemire et al., 1985). 3.6. Extraction of Cd (II) and Ni (II) from mixture of metals Fig. 4. 31P NMR spectra of TOPS 99 (1) and Cd-TOPS 99 solid complex (2). Org.: 0.2 M TOPS 99; Aq.: 0.1 M Cd (II), 0.1 M Na2SO4.
Studies have been carried out from sulphate solutions containing 0.01 M Cd (II) and Ni (II) using
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0.05 M TOPS 99, PC 88A and Cyanex 272 dissolved in kerosene (Fig. 5). The extraction of Cd (II) starts at lower pH values than that of Ni (II) and increases systematically with rise of pH with all the three extractants. The observed pH0.5 (50% extraction) values for Cd (II) and Ni (II) are 3.9 and 7.4 (DpH (pH0.5 Cd –pH0.5 Ni) = 3.5) with TOPS 99, 5.9 and 7.35 (DpH = 1.45) with PC 88A and 4.2 and 7.5 (DpH = 3.3) with Cyanex 272, respectively, indicating TOPS 99 to be an effective extractant for separation of Cd (II) from Ni (II) considering better separation and cost.
4. Conclusions Solvent extraction studies of Cd (II) from sulphate solutions by organophosphorus-based extractants establish the dependence of cadmium extraction on equilibrium pH of the aqueous phase and extractant concentration. The mechanism of metal transfer involves 1:3 metal to extractant complex formation. Addition of salts showed variable effect on percent extraction of Cd (II). Mixtures of extractants indicated TOPS 99 as the best synergist. The present results demonstrate possible application to the separation of Cd and Ni from the leach liquors of spent Ni –Cd batteries/or any related materials.
Acknowledgements The authors express their sincere thanks to Ministry of Environment & Forests (MOEF), Government of India, New Delhi, India for the financial support. Thanks are also due to Cytec Canada for providing Cyanex-272 free sample. References Almela, A., Elizalde, M.P., 1995. Solvent extraction of cadmium (II) from acidic media by Cyanex 302. Hydrometallurgy 37, 47 – 57. Avila-Rodriguez, M., Cote, G., Mendoza, R.N., Medina, T.I.S., Bauer, D., 1998. Thermodynamic study of the extraction of
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In(III) and Cd(II) by Cyanex 301 from concentrated HCl Media. Solvent Extr. Ion Exch. 16, 471 – 485. Bellamy, L.J., 1975. Infrared Spectra of Complex Molecules, 3rd ed. Chapman & Hall, London. Biswas, R.K., Hayat, M.A., 2002. Solvent extraction of Zirconium (II) from chloride media by D2EHPA in kerosene. Hydrometallurgy 63, 149 – 158. Blake, C.A., Baes, C.F., Brown, K.B., Coleman, C.F., White, J.C., 1958. Proceedings of 2nd International Conference on Peaceful Uses of Atomic Energy, Geneva, vol. 28. 289. Casas, I., Miralles, N., Sastre, A., Aguilar, M., 1986. Extraction of Cadmium (II) by organophosphorous compounds. Polyhedron 5, 2039 – 2045. Cheremisinoff, P.N., 1995. Handbook of Water and Wastewater Treatment Technology. Marcel Dekker, New York (p. 418). Galan, B., Roman, F.S., Irabien, A., Oritz, I., 1998. Viability of the separation of Cd from highly concentrated Ni – Cd mixtures by non-dispersive solvent extraction. Chem. Eng. J. 70, 237 – 243. Gupta, B., Deep, A., Malik, P., 2001. Extraction and recovery of cadmium using Cyanex 923. Hydrometallurgy 61, 65 – 71. Lemire, A.E., Janzen, A.F., Marat, K., 1985. A 31P and 15N NMR study of the extraction of uranyl nitrate by di-2-ethylhexyl phosphoric acid. Inorg. Chim. Acta 110, 237 – 241. Nayak, A.K., Mishra, P.K., Panda, C.R., Chakravorty, V., 1995. Solvent extraction of Zinc (II) and Cadmium (II) by Cyanex 272, 301 and 302 extractants. Ind. J. Chem. Technol. 2, 111 – 112. Peppard, D.F., Ferraro, J.R., 1959. The preparation and infra-red absorption spectra of several complexes of bis (2-ethylhexyl)phosphoric acid. J. Inorg. Nucl. Chem. 10, 275 – 288. Ramachandra Reddy, B., Rajesh Kumar, J., Varada Reddy, A., 2004. Liquid – liquid extraction of tetravalent zirconium from acidic chloride solutions using Cyanex 272. Anal. Sci. 20, 501 – 505. Reddy, B.R., Bhaskara Sarma, P.V.R., 2001. Separation and recovery of cobalt and nickel from sulphate solutions of Indian Ocean nodules using Cyanex 272. Miner. Metall. Process. 18, 172 – 176. Saji John, K., Prasada Rao, T., Ramamohan, T.R., Reddy, M.L.P., 1999. Solvent extraction of tetravalent titanium from acidic chloride solutions by 2-ethylhexyl phosphonicacidmono-2-ethylhexylester. Hydrometallurgy 53, 245 – 253. Sarangi, K., Reddy, B.R., Das, R.P., 1999. Extraction studies of cobalt(II) and nickel(II) from hydrochloric acid solutions using Na-Cyanex 272: separation of Co(II)/Ni(II) by the sodium salts of D2EHPA, PC-88A and Cyanex 272 and their mixtures. Hydrometallurgy 52, 253 – 265. Vogel, A.I., 1962. A Text Book of Quantitative Inorganic Analysis including Elementary Instrumental Analysis, 3rd ed. The ELBS and Longman Green & Co, London. Wassink, B., Dreisinger, D., Howard, J., 2000. Solvent extraction separation of zinc and cadmium from nickel and cobalt using Aliquat 336, a strong base anion exchanger, in the chloride and thiocyanate forms. Hydrometallurgy 57, 235 – 252.
Solvent extraction of cadmium (II) from sulphate solutions using TOPS 99, PC 88A, Cyanex 272 and their mixtures B. Ramachandra Reddy *, D. Neela Priya, J. Rajesh Kumar Inorganic Chemistry Division, Indian Institute of Chemical Technology (CSIR), Uppal Road, Hyderabad 500 007, India Received 1 March 2004; received in revised form 2 June 2004; accepted 3 June 2004
Abstract Extraction of cadmium (II) from sulphate solutions using organophosphorous-based extractants—TOPS 99, PC 88A and Cyanex 272 has been studied. Percent extraction of cadmium (II) increased with increasing equilibrium pH of the aqueous phase and extractant concentration. Extraction of Cd (II) by organophosporus extractants involves cation exchange mechanism with the formation of 1:3 metal to reagent complex. Characterization of the solid complex of cadmium with TOPS 99 by FTIR, 31 P NMR supported metal complex formation with the phosphorus-hydroxyl (P-OH) group. Finally, the extraction behavior and possible separation of cadmium and nickel from the mixture of metals was studied. D 2004 Elsevier B.V. All rights reserved. Keywords: Organophosphorus extractants; Solvent extraction; Cadmium (II); Synergism; Separation
1. Introduction Cadmium and its compounds are toxic and poisoning occurs through inhalation and ingestion. In spite of its toxicity, it is used in different industries such as electroplating, pigments, synthetic chemicals, ceramics, metallurgical and photographic products (Cheremisinoff, 1995). Cadmium is primarily produced as a byproduct from mining, smelting and refining of sulphide ore concentrates of Zn. Secondary cadmium is recovered from spent Ni –Cd batteries. Hydrometallurgical methods of treating Cd-containing materials such as Ni – Cd batteries generate leach liquors containing Cd and Ni as major metals. * Corresponding author. Tel.: +91-40-27193510; fax: +91-4027160921. E-mail address: [email protected] (B. Ramachandra Reddy). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2004.06.001
Literature on cadmium solvent extraction, reports the use of solvating and cation exchange type extractants. Wassink et al. (2000) studied the extraction and separation of cadmium and zinc from nickel and cobalt from chloride and thiocyanate solutions using aliquat 336 extractant. Gupta et al. (2001) have reported extraction and recovery of Cd from HCl medium using Cyanex 923. The organophosphorus-based extractants studied for the extraction Cd from other metals such as Zn, Ni were D2EHPA (Casas et al., 1986; Galan et al., 1998), Cyanex 272, Cyanex 301 and Cyanex 302 (Nayak et al., 1995; Avila-Rodriguez et al., 1998; Almela and Elizalde, 1995). In this paper, we report the solvent extraction studies of cadmium from sulphate solutions using the commercial extractants, TOPS 99, PC 88A and Cyanex 272 and their mixtures. Various parameters studied include equilibrium pH, extractant and metal
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concentration, synergism, different salts, loading capacity, stripping of cadmium from loaded organic, extraction from Ni –Cd mixtures and preparation of cadmium –TOPS 99 solid complex. The results of this study are important from the environmental point of view in the treatment and recovery of cadmium from wastes such as spent batteries/industrial wastewaters.
2. Experimental 2.1. Reagents Commercial extractant TOPS 99, (an equivalent of di-(2-ethylhexyl) phosphoric acid, Heavy Water Plant, Talcher, Orissa, India), PC 88A (2-ethylhexyl phosphonic acid mono 2-ethyl hexyl ester, Daihachi, Japan) and Cyanex 272 (bis (2,4,4-trimethyl pentyl) phosphinic acid, Cytec, Canada) were used as received. Distilled kerosene (bp: 165 – 200 jC) was used as the diluent. The sodium salts of the extractants (60% neutralized) were prepared by adding stoichiometric amounts of concentrated NaOH solution to the extractant in kerosene as required. All other chemicals were Analar grade. Stock solution of cadmium (II) sulphate (0.5 M) was prepared in distilled water containing little H2SO4 to prevent hydrolysis of the metal ion and standardised against 0.05 M EDTA solution using Eriochrome Black T as indicator (Vogel, 1962). 2.2. Apparatus A digital pH meter (Digisun DI 707 model) and Atomic Absorption Spectrophotometer (AAS) of Perkin-Elmer Model A 300 were used for the measurement of pH and metal concentration in the aqueous phase. The IR spectrum of the solid complex was recorded using FTIR-Nicolet (USA)-740-spectrophotometer. Phosphorous-31 Nuclear magnetic resonance measurements were carried out with Varian Unity Inova 500 MHz instrument using H3PO4 (85%) as the external standard. 2.3. Solvent extraction procedure Extraction was carried out with 10 mL of the aqueous solution containing 0.01 M metal and 0.1
M Na2SO4 equilibrated with equal volume of sodium salts of the extractants in a separating funnel. The pH of the aqueous phase was adjusted initially to the desired value by adding dilute H2SO4/NaOH solution before equilibration. Initial experiments on contact time showed that 5 min was sufficient to reach equilibrium. In all the extraction experiments after equilibrating for 10 min, the two phases were allowed to separate. The aqueous phase equilibrium pH was measured and the metal concentration was analysed by AAS. The concentration of the metal ion in the organic phase was calculated from the difference between the metal ion concentration in the aqueous phase before and after extraction. The distribution coefficient (D) is taken as the ratio of the concentration of cadmium in the organic phase to that present in the aqueous phase at equilibrium. For determination of the loading capacity of 0.03 M extractants, the loaded organic (LO) phase was filtered through a 1PS separating paper and a suitable aliquot was stripped with 2 M HCl, followed by analysis using AAS. All the experiments were carried out at room temperature (30 F 1 jC). 2.4. Preparation of Cd-TOPS 99 solid complex One hundred milliliters of 0.2 M TOPS-99 in kerosene was taken in a 500-mL beaker and equal volume of 0.1 M cadmium was added. Both phases were stirred for 10 min at an equilibrium pH of f 5.0 and allowed to stand for phase separation. The raffinate was separated and fresh cadmium metal solution was added to the loaded organic phase repeatedly until the organic phase became saturated with cadmium. After each contact, concentration of Cd in the raffinate was analysed. When the raffinate content of Cd is same as the feed, then the LO was considered to be fully loaded with Cd. The loaded organic phase was filtered through phase separator filter paper. Solvent from the LO phase was allowed to evaporate under suction at around 80 jC to yield solid complex. This complex was then washed with 50% ethanol to remove traces of TOPS 99, dried under vacuum and finally in oven maintained at around 90 jC and stored in a desiccator (Biswas and Hayat, 2002).
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279
3. Results and discussion 3.1. Extraction mechanism and species The extraction of Cd (II) (0.01 M) from aqueous sulphate solutions was studied in the equilibrium pH range 3.6 –7.6 using the sodium form of TOPS 99, PC 88A and Cyanex 272 at 0.02, 0.03 and 0.04 M in kerosene. Percentage extraction of Cd (II) increased
Fig. 2. Plot of log D vs. log [extractant], M. Org.: TOPS 99 (5), PC 88A (o), Cyanex 272 (D); Aq.: 0.01 M Cd (II), 0.1 M Na2SO4.
e=
slop
0.6
Fig. 1. Plot of equilibrium pH vs. log D. Org.: TOPS 99 (a); PC 88A (b); Cyanex 272 (c); Aq: 0.01 M Cd (II), 0.1 M Na2SO4.
with increase in the equilibrium pH of the aqueous phase. The plot of equilibrium pH vs. log D (Fig. 1) shows that the plots are linear with a slope of 1.0, 0.9 and 1.3 for TOPS 99; 0.8, 0.6 and 0.7 for PC 88A; 0.6, 0.6 and 0.8 for Cyanex 272 with 0.02, 0.03 and 0.04 M extractants, respectively, indicating the exchange of 1 mol of H+ ion with 1 mol of Cd (II). The influence of extractant concentration on the extraction of Cd (II) (keeping initial pH at 2.85) was studied in the range 0.01 –0.04 M. The corresponding change in equilibrium pH was in the range 3.4 –5.9. Percentage extraction increased steadily with increase in extractant concentration with the three extractants. The increase was 4.3% to 85% in case of TOPS 99, 0.5% to 83.4% in case of PC 88A and 1% to 82.2% in case of Cyanex 272. The plots of log [extractant], M vs. log D (Fig. 2) are linear with a slope value of f 3 indicating the association of three moles of the extractant with the extracted species. Correlation of extraction behavior of Cadmium (II) at 50% extraction using 0.02 M extractant concentration with their pKa values clearly indicate direct relation with pKa values. The corresponding pH1/2 values for TOPS 99, PC 88A and Cyanex 272 are 4.5, 5.3 and 5.8, respectively, indicating that TOPS 99 as an efficient extractant than the other two. The loading capacity of 0.03 M extractants was studied from a feed solution containing 0.1 M Na2SO4 and varying the metal concentrations between 0.005 and 0.04 M. Loading of the extractant with metal increases systematically with increase in metal con-
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centration and reaches maximum at z 0.02 M metal concentration. The corresponding percentage extraction with respect to initial metal concentration decreases from 94% to 37% in case of TOPS 99 and from 98% to 36% in case of PC 88A and Cyanex 272. The loading capacity was calculated to be 1.62 F 0.05 g/L, which is equivalent to 99.1 –93.1% of loading indicating that the extractants are almost pure in form. Further, the plot of log [Cd]Aq, M vs. log [Cd]Org, M for TOPS 99 (Fig. 3) is linear with a slope of f 0.7 indicating that monomeric metal species is extracted into the organic phase. Based on the results, the species extracted into the organic phase appears to be CdA25HA. Extraction of Cd (II) from aqueous sulphate solutions using the acid forms of TOPS 99, PC 88A and Cyanex 272 in kerosene was poor. As a result, the extractants were converted to sodium form (60%) by the addition of sodium hydroxide. Considering the results of Cd (II) extraction as a function of pH, extractant and metal obtained by slope analysis method, the mechanism can be proposed as follows: the neutralization reaction of the extractants written as (Sarangi et al., 1999; Reddy and Bhaskara Sarma, 2001): þ Naþ ðaqÞ þ 1=2ðHAÞ2ðorgÞ ! NaAðorgÞ þ HðaqÞ
ð1Þ
The acidic (Saji John et al., 1999; Ramachandra Reddy et al., 2004) and neutral form of the extractant
Fig. 3. Plot of log metal (Aq) vs. log metal (Org). Org.: 0.03 M TOPS 99; Aq.: 0.1 M Na2SO4.
Table1 Effect of salts on percentage extraction of cadmium (II) S. no Extractant
Salt
Salt concentration (M) 0.1
0.2
0.3
0.5
1.0
Extraction of cadmium (%) 1.
2.
3.
TOPS-99
Na2SO4 NaCl NaNO3 NaSCN PC-88A Na2SO4 NaCl NaNO3 NaSCN Cyanex-272 Na2SO4 NaCl NaNO3 NaSCN
71.0 83.8 75.2 84.1 92.9 93.2 92.5 95.3 99.97 99.98 99.99 99.97
70.7 83.6 75.0 82.9 92.2 93.0 91.7 93.4 99.96 99.96 99.98 99.95
69.6 81.0 74.8 80.7 91.6 92.9 91.0 91.5 99.95 99.95 99.98 99.92
67.2 73.0 73.9 75.6 91.1 92.6 90.1 90.0 99.95 99.94 99.97 99.81
63.0 66.0 73.4 54.3 90.4 92.3 89.3 88.9 99.95 99.92 99.95 99.53
takes part in the extraction process of Cd (II) as given by the equation: þ Cd2þ ðaqÞ þ AðorgÞ þ 3ðHAÞ2ðorgÞ ! CdA2 :5HAðorgÞ þ HðaqÞ
ð2Þ Slope analysis from Figs. 1 and 2 supports the mechanism. 3.2. Effect of salts The effect of salts such as Na2SO4, NaNO3, NaCl and NaSCN (in the concentration range 0.1– 1 M) on the extraction efficiency of 0.035 M TOPS 99, PC 88A and Cyanex 272 was studied for 0.01 M metal (initial pH 2.85) The effect of salts on the percentage extraction of metal by Cyanex 272 and PC 88A is almost negligible. In case of TOPS 99 as extractant and NaCl as salt, the percentage extraction decreases from 83.6% to 66% whereas for NaSCN, the decrease is from 84.1% to 54.3%. The change in equilibrium pH was 5.2 – 5.5 under the present experimental conditions. On the other hand, with NaNO3, the percentage extraction changed marginally from 75% to 73.4%. In case of Na2SO4, the decrease was from 71% to 63%. The general conclusion is that Cd (II) extraction decreases with rise in salt concentration when TOPS 99 is used as the extractant (Table 1).
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3.3. Stripping studies The effect of acid concentration in the range 0.1– 1.0 M on the stripping of Cd (II) from 0.01 M LO (TOPS 99, PC 88A and Cyanex 272 in kerosene) containing 0.56 g/L Cd (II) was studied (Table 2). More than 93% stripping efficiency of metal from loaded organic phase was possible in one stage with 0.1 M HCl and H2SO4 and further increase in acid concentration showed marginal increase in efficiency. On the other hand, in case of oxalic acid, two stages of stripping was necessary to achieve >97% efficiency with acid concentrations z 0.5 M. 3.4. Synergism Blake et al. (1958) used the term ‘synergism’ to describe the discovery of a definite enhancement of extraction of uranium by mixture of an acidic alkyl phosphate and certain neutral organophosporous esters, the resulting mixture giving a better extraction of uranium than either the acid or the neutral phosphate alone. The effect is understood to be the replacement of one extractant in the extracted complex by the other and also due to various other factors. This property of the mixed solvent system was utilised to study the extraction behavior of cadmium using mixture of extractants (TOPS 99, PC 88A and Cyanex 272), keeping the concentration of one of the extractant constant at 0.01 M and varying the other as synergist in the range of 0.005 – 0.02 M (Table 3). In case of TOPS 99 as extractant, the synergistic enhancement factor (SEF) increased and then decreased with increase in concenTable 2 Effect of acid concentration on stripping of cadmium (II) from loaded organic S. no. Loaded organic [Acid] (M) Stripping (%)
1.
TOPS-99
2.
PC-88A
3.
Cyanex-272
() = Two-stage stripping.
0.1 0.5 1.0 0.1 0.5 1.0 0.1 0.5 1.0
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Table 3 Distribution coefficient values of Cd (II) using extractant (0.01M) and synergist mixtures DA, Extractant
Concentration of synergist (M)
DB
Dmix
SEF
0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.045(T) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.005(P) 0.011(C) 0.011(C) 0.011(C) 0.011(C) 0.011(C) 0.011(C) 0.011(C) 0.011(C)
0.005(P) 0.010(P) 0.015(P) 0.020(P) 0.005(C) 0.010(C) 0.015(C) 0.020(C) 0.005(T) 0.010(T) 0.015(T) 0.020(T) 0.005(C) 0.010(C) 0.015(C) 0.020(C) 0.005(T) 0.010(T) 0.015(T) 0.020(T) 0.005(P) 0.010(P) 0.015(P) 0.020(P)
Nil 0.01 0.15 0.33 Nil 0.01 0.06 0.31 Nil 0.05 0.17 0.41 Nil 0.01 0.06 0.31 Nil 0.05 0.17 0.41 Nil 0.01 0.06 0.31
0.385 0.722 1.106 1.852 0.388 1.486 1.874 2.119 0.372 0.705 1.058 1.915 0.344 0.558 1.257 1.866 0.338 0.638 1.113 1.948 0.328 0.60 1.039 1.746
8.6 14.4 5.8 5.0 8.6 26.5 17.8 6.0 74.4 14.1 6.1 4.6 68.8 34.9 19.3 5.9 30.7 11.4 6.2 4.6 29.8 27.3 14.6 5.4
T = TOPS 99; P = PC 88A; C = Cyanex 272. DA = Distribution coefficient of cadmium with extractant. DB = Distribution coefficient of cadmium with synergist. Dmix = Distribution coefficient of the mixture. SEF = Dmix/DA + DB
tration of synergist (PC 88A and Cyanex 272). On the other hand, in case of PC 88A and Cyanex 272 as extractants and others as synergist, SEF decreases with increase in synergist concentration. The results indicate TOPS 99 as the best synergist. 3.5. Characterisation studies of Cd-TOPS 99 complex
HCl
H2SO4 H2C2O4
93.9 94.3 95.1 96.0 98.2 98.3 96.2 97.0 99.8
94.6 96.9 99.0 97.7 98.8 99.4 95.5 96.2 97.7
78.3 89.6 98.5 81.4 84.1 89.4 46.0 50.2 88.4
(92.9) (100) (100) (94.2) (97.8) (99.2) (75.9) (98.6) (100)
3.5.1. Infrared studies The Infrared spectra of TOPS-99 and Cd-TOPS 99 solid complex indicate the absence of PU(OH) vibration in the region 2750– 2550 cm 1, the absence of PjO (OH) vibration in the region 2350 – 2080 cm 1, the absence of OUH deformation vibration at 1680 cm 1, the shift of P – O bonded stretching vibration at 1225 to 1189 cm 1, the absence of PUOH bending vibration at 1115 cm 1, the absence of PjO (OH)
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vibration within 2700 –2550 cm 1 and the absence of PUOU(H) antisymmetric stretching vibration at 1040 cm 1 (Peppard and Ferraro, 1959; Bellamy, 1975) suggesting complex formation of Cd (II) by TOPS-99. 3.5.2. 31P NMR spectroscopy Fig. 4 shows the 31P NMR spectra of TOPS 99 and its Cd (II) complex. The 31P NMR of cadmium complex showed a shift from 1.698 ppm (TOPS 99) to 3.016 ppm (Cd-TOPS 99 complex). A broad signal in the 31P NMR spectra of the Cd-TOPS 99 complex was observed although there was a small chemical shift when compared with the extractant (TOPS 99). This
Fig. 5. Plot of equilibrium pH vs. percentage extraction of Cd and Ni from their mixtures. Org.: 0.05 M TOPS 99 (a), PC 88A (b), Cyanex27 (c); Aq.: 0.01 M Cd (5) and Ni (o), 0.1 M Na2SO4.
shift may be attributed to the coordination of cadmium with UPO2 group of TOPS 99 (Lemire et al., 1985). 3.6. Extraction of Cd (II) and Ni (II) from mixture of metals Fig. 4. 31P NMR spectra of TOPS 99 (1) and Cd-TOPS 99 solid complex (2). Org.: 0.2 M TOPS 99; Aq.: 0.1 M Cd (II), 0.1 M Na2SO4.
Studies have been carried out from sulphate solutions containing 0.01 M Cd (II) and Ni (II) using
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0.05 M TOPS 99, PC 88A and Cyanex 272 dissolved in kerosene (Fig. 5). The extraction of Cd (II) starts at lower pH values than that of Ni (II) and increases systematically with rise of pH with all the three extractants. The observed pH0.5 (50% extraction) values for Cd (II) and Ni (II) are 3.9 and 7.4 (DpH (pH0.5 Cd –pH0.5 Ni) = 3.5) with TOPS 99, 5.9 and 7.35 (DpH = 1.45) with PC 88A and 4.2 and 7.5 (DpH = 3.3) with Cyanex 272, respectively, indicating TOPS 99 to be an effective extractant for separation of Cd (II) from Ni (II) considering better separation and cost.
4. Conclusions Solvent extraction studies of Cd (II) from sulphate solutions by organophosphorus-based extractants establish the dependence of cadmium extraction on equilibrium pH of the aqueous phase and extractant concentration. The mechanism of metal transfer involves 1:3 metal to extractant complex formation. Addition of salts showed variable effect on percent extraction of Cd (II). Mixtures of extractants indicated TOPS 99 as the best synergist. The present results demonstrate possible application to the separation of Cd and Ni from the leach liquors of spent Ni –Cd batteries/or any related materials.
Acknowledgements The authors express their sincere thanks to Ministry of Environment & Forests (MOEF), Government of India, New Delhi, India for the financial support. Thanks are also due to Cytec Canada for providing Cyanex-272 free sample. References Almela, A., Elizalde, M.P., 1995. Solvent extraction of cadmium (II) from acidic media by Cyanex 302. Hydrometallurgy 37, 47 – 57. Avila-Rodriguez, M., Cote, G., Mendoza, R.N., Medina, T.I.S., Bauer, D., 1998. Thermodynamic study of the extraction of
283
In(III) and Cd(II) by Cyanex 301 from concentrated HCl Media. Solvent Extr. Ion Exch. 16, 471 – 485. Bellamy, L.J., 1975. Infrared Spectra of Complex Molecules, 3rd ed. Chapman & Hall, London. Biswas, R.K., Hayat, M.A., 2002. Solvent extraction of Zirconium (II) from chloride media by D2EHPA in kerosene. Hydrometallurgy 63, 149 – 158. Blake, C.A., Baes, C.F., Brown, K.B., Coleman, C.F., White, J.C., 1958. Proceedings of 2nd International Conference on Peaceful Uses of Atomic Energy, Geneva, vol. 28. 289. Casas, I., Miralles, N., Sastre, A., Aguilar, M., 1986. Extraction of Cadmium (II) by organophosphorous compounds. Polyhedron 5, 2039 – 2045. Cheremisinoff, P.N., 1995. Handbook of Water and Wastewater Treatment Technology. Marcel Dekker, New York (p. 418). Galan, B., Roman, F.S., Irabien, A., Oritz, I., 1998. Viability of the separation of Cd from highly concentrated Ni – Cd mixtures by non-dispersive solvent extraction. Chem. Eng. J. 70, 237 – 243. Gupta, B., Deep, A., Malik, P., 2001. Extraction and recovery of cadmium using Cyanex 923. Hydrometallurgy 61, 65 – 71. Lemire, A.E., Janzen, A.F., Marat, K., 1985. A 31P and 15N NMR study of the extraction of uranyl nitrate by di-2-ethylhexyl phosphoric acid. Inorg. Chim. Acta 110, 237 – 241. Nayak, A.K., Mishra, P.K., Panda, C.R., Chakravorty, V., 1995. Solvent extraction of Zinc (II) and Cadmium (II) by Cyanex 272, 301 and 302 extractants. Ind. J. Chem. Technol. 2, 111 – 112. Peppard, D.F., Ferraro, J.R., 1959. The preparation and infra-red absorption spectra of several complexes of bis (2-ethylhexyl)phosphoric acid. J. Inorg. Nucl. Chem. 10, 275 – 288. Ramachandra Reddy, B., Rajesh Kumar, J., Varada Reddy, A., 2004. Liquid – liquid extraction of tetravalent zirconium from acidic chloride solutions using Cyanex 272. Anal. Sci. 20, 501 – 505. Reddy, B.R., Bhaskara Sarma, P.V.R., 2001. Separation and recovery of cobalt and nickel from sulphate solutions of Indian Ocean nodules using Cyanex 272. Miner. Metall. Process. 18, 172 – 176. Saji John, K., Prasada Rao, T., Ramamohan, T.R., Reddy, M.L.P., 1999. Solvent extraction of tetravalent titanium from acidic chloride solutions by 2-ethylhexyl phosphonicacidmono-2-ethylhexylester. Hydrometallurgy 53, 245 – 253. Sarangi, K., Reddy, B.R., Das, R.P., 1999. Extraction studies of cobalt(II) and nickel(II) from hydrochloric acid solutions using Na-Cyanex 272: separation of Co(II)/Ni(II) by the sodium salts of D2EHPA, PC-88A and Cyanex 272 and their mixtures. Hydrometallurgy 52, 253 – 265. Vogel, A.I., 1962. A Text Book of Quantitative Inorganic Analysis including Elementary Instrumental Analysis, 3rd ed. The ELBS and Longman Green & Co, London. Wassink, B., Dreisinger, D., Howard, J., 2000. Solvent extraction separation of zinc and cadmium from nickel and cobalt using Aliquat 336, a strong base anion exchanger, in the chloride and thiocyanate forms. Hydrometallurgy 57, 235 – 252.