- Email: [email protected]
Electrowinning of cobalt from sulphate solutions contaminated with organic impurities
Hydrometallurgy 65 (2002) 97 – 102 www.elsevier.com/locate/hydromet
Electrowinning of cobalt from sulphate solutions contaminated with organic impurities K.G. Mishra a,*, P. Singh b, D.M. Muir c a
Regional Research Laboratory (C.S.I.R), Bhubaneswar-751 013, India Division of Sciences, Murdoch University, WA 6150, Perth, Australia c C.S. I.R.O. Minerals, PO Box 90, WA 6982, Australia
b
Received 14 August 2001; received in revised form 17 August 2001; accepted 12 April 2002
Abstract The electrowinning (EW) of cobalt from sulphate solution contaminated with organic impurities: tri-n-butyl phosphate (TBP), 2-hexylmethyl decanoic acid (VERSATIC 10), 2-hydroxy-5-nonyl acetophenone oxime (LIX 84), di-2-ethylhexyl phosphoric acid (D2EHPA) and di-2,4,4-trimethylpentyl phosphinic acid (CYANEX 272) has been investigated at 60 jC. Both cyclic voltammetry and flow cell electrolysis have been used. The effect of organic impurities on the cathodic reduction of cobalt is compared with the blank solution. The current efficiency is more than 90% in most cases and is highest in the case of D2EHPA (96%) and lowest with VERSATIC 10 (86%). The cathodic polarization from cyclic voltammetry indicates that the exchange current density (i0) value is higher in the presence of organic impurities than in solutions containing no organic impurities. The organic impurities indicate an inhibition of electrocrystallization process due to suppression of nucleation. Cathodic polarization also indicates that Co2 + reduction occurs slightly at less negative potential in solution containing organic impurities. D 2002 Published by Elsevier Science B.V. Keywords: Electrowinning; Cobalt; Organic impurities; Cyclic voltammetry
1. Introduction Solvent extraction has gained a wide acceptance as one of the new tools of hydrometallurgy. With the growing demand for metals of higher purity and the necessity for treating low-grade ores, which are not amenable to concentration, solvent extraction has become a viable commercial method of recovering valuable metals such as cobalt, nickel, copper, vanadium, titanium and rare earths.
*
Corresponding author. E-mail address: [email protected] (K.G. Mishra).
Although the majority of cobalt is still produced by the conventional route of concentration, smelting and refining, hydrometallurgical routes involving solvent extraction and electrowinning (SX-EW) are rapidly growing in importance for recovering cobalt from oxides and sulphides, waste dumps and many secondary sources. Previous work on this aspect has been carried out by other workers (Prasad et al., 1992; Kuzeci et al., 1994; MacKinnon and Laxman, 1976; Storey and Barens, 1961; Laxman et al., 1997; Brennar et al., 1952). Several plants around the world are successfully using SX-EW process to produce cobalt. As a result of engineering innovations and operational progress, the SX-EW route is a proven technology.
0304-386X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 3 8 6 X ( 0 2 ) 0 0 0 3 6 - 1
98
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
In order to replicate flow conditions in hydrometallurgical plant, the electrolyte is passed through a rectangular cell so that the effect of organic impurities on cobalt morphology and current efficiency could be studied during electrowinning. In this study, an attempt is made to understand the effect of the organic impurities during the electrowinning of cobalt from a sulphate bath since there is little information on the mechanism of adsorption/desorption as well as metal ion reduction in the presence of impurities, such as tri-n-butyl phosphate (TBP), 2-hexylmethyl decanoic acid (VERSATIC 10), 2-hydroxy-5-nonyl acetophenone oxime (LIX 84), di-2-ethylhexyl phosphoric acid (D2EHPA) and di-2,4,4-trimethylpentyl phosphinic acid (CYANEX 272).
jC, at a current density of 200 A/m2 from a solution containing 1.0 M Co2 + , 0.08 M Na2SO4 and 0.50 M MgSO4 at pH 2.5. The cathodic current efficiency was calculated from the weight change of the cathode. The deposit was then carefully removed and analysed by SEM and XRD. Analytical grade CoSO4, Na2SO4, MgSO4 and H2SO4 were used with ultra pure water to prepare the electrolyte. One hundred milliliters of electrolyte was thoroughly shaken for 45 min with 20 mL of LIX 84, VERSATIC 10 (Henkel), D2EHPA (Daihachi), CYANEX 272 (Cytec) or TBP (Merck). The separated organic saturated aqueous phase was used as the catholyte solution for electrowinning and cyclic voltammetry studies. A fresh solution was prepared and sparged with nitrogen for each experiment.
2. Experimental details
2.2. Cyclic voltammetry
2.1. Electrowinning
Cyclic voltammetry was carried out in a 150-mL double-walled cell having stationary rotating disc working electrode, an Ag/AgCl reference electrode and a Pt spiral counter electrode. A Luggin capillary having a microtip was placed close to the electrode surface to avoid IR drop. The other end of the Luggin capillary was placed in the saturated KCl solution containing Ag/AgCl reference electrode. The outlet of the double-walled cell was connected to a thermostatic water bath through a peristaltic pump to maintain the electrolyte in the cell at 60 jC. The glassy carbon (GC) (0.0556 cm2), Pt (0.0956 cm2) and SS (0.0707 cm2) working electrodes were polished as already described for the electrowinning studies. A potentiostat (PAR model 362) coupled with a series X – Y recorder (Omnigraphic model RE0092) was employed to generate the cathodic polarization curves.
The electrowinning cell consisted of two rectangular halves made up of perspex having dimensions: 13.0 8.0 3.0 cm with a groove of 1-cm depth. Each groove accommodated an electrode of size 5.0 4.0 cm. A rectangular sheet of microporous ‘Daramic’ separator was placed between the two halves which were made watertight by using Viton O-rings. The electrolyte was circulated through each compartment of the cell from two separate stoppered glass vessels maintained at 60 jC by a thermostatic water bath by using two peristaltic pumps. Unless otherwise mentioned, all experiments were carried out by circulating the electrolyte at a volumetric flow rate of 1.8 L/h. Each cell was also provided with a ‘Teflon’ port through which an Ag/AgCl reference electrode was connected to allow measurement of the electrode potential. During electrowinning, stainless steel (SS) 304 was used as cathode and Pb/Sb (6 – 7%) as the anode material. The cathode prior to its use was polished with successive grades of 50, 120 and 4000 SiC paper to produce a mirror surface finish. The cathode edges were roughened slightly then degreased with acetone and thoroughly washed with water. All the electrowinning experiments were conducted by applying constant current from a regulated power supply. The electrowinning of cobalt was carried out for 2 h at 60
3. Results and discussion 3.1. Cyclic voltammetry The voltametric curves for platinum (Pt), glassy carbon (GC) and stainless steel (SS) in MgSO4 electrolyte background are presented in Fig. 1. Both the Pt and the SS electrodes show clear hydrogen evolution peaks at a scan rate of 10 mV s 1 on the Pt at about
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
99
Fig. 1. Cyclic voltammetric curves for different stationary electrode substrates at a scan rate of 10 mV s 1 in 0.08 M Na2SO4 and 0.50 M MgSO4 at pH 2.5 and 60 jC. (1) GC, (2) SS, and (3) Pt.
0.25 and 0.50 V and on the SS at 0.63 V. No significant hydrogen evolution appeared at the GC electrode (Fig. 1). These findings are consistent with our earlier study (Mishra et al., 2001) and are also found by others (Gomez et al., 1992; Franaszczuk and Sobkowski, 1989). This study helps to identify the region of hydrogen evolution. Fig. 2 presents the cathodic reduction of cobalt on a stationary SS electrode at a scan rate of 10 mV s 1, as it has both practical and commercial interest, from a solution of 1.0 M Co (II) saturated with various organic impurities at pH 2.5 and 60 jC. The point (A) in Fig. 2 presents the hydrogen evolution peak. The peak is prominent in the case of curves 3, 4, 5 and 6 and the potential is nearly the same as found in curve 2 of Fig. 1. The point (B) in the polarization curves of Fig. 2 corresponds to the nucleation potential (En) of cobalt deposition. The nucleation potential of cobalt is almost the same for all the organic impurities as well as in the organic-free solution and occurs at 0.690 F 0.02 V. This suggests that the organic impurities neither affect significantly the conductivity of the solution nor the double layer at the cathode surface. The negative potential limit was
chosen so that deposition of the metal phase on the stainless steel disc occurred just before the direction of scan was reversed. It can be seen that the cathodic current increases rapidly below 0.690 V. The crossover potential (Ecp) that corresponds to the opencircuit potential indicates that the cathode current returns to zero after the peak. The oxidation current is not shown here. All the features in the cathodic region of the voltammogram are typical for a system involving the cathodic deposition of a metal phase on the stationary, stainless steel substrate. It is readily observed in Fig. 2 that cobalt deposition in the presence of organic impurities on the stainless steel electrode does not occur until the cathode potential reaches about 0.690 F 0.02 V. As there is an appreciable nucleation overpotential (NOP), the difference between nucleation potential (En) and crossover potential (Ecp) indicates the extra energy needed for heterogeneous deposition onto a foreign substrate. After the cathode sweep, cobalt covers the electrode surface, so hydrogen evolution occurs more easily than on the original substrate. When Figs. 1 and 2 are compared, it is clear that cobalt reduction occurs at a more negative overpoten-
100
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
impurities, the exchange current densities (i0) increased by about 0.1 A/m2 with respect to the value obtained for solution without organic impurities (Table 1). The greater change in i0 value is measured with electrolyte saturated with VERSATIC 10 and minimum with TBP that can be considered as a measure of inhibition of nucleation due to absorption onto the electrode surface. The inhibition may be due to the lowering of the catalytic property of the electrode surface in presence of organic impurities. In all cases, the organic impurities appear to change the properties of the double layer on the electrode surface, as a result of which, hydrated metal ions cannot undergo charge transfer as readily as at organic-free surfaces. 3.2. Cobalt electrowinning in presence of organic impurities
Fig. 2. Forward CV scan of cobalt (II) on a stainless steel electrode substrate in solution at 60 jC containing 1.0 M Co2 + , 0.08 M Na2SO4, 0.50 M MgSO4 and saturated with, (1) zero additives (2) CYANEX 272, (3) TBP, (4) LIX 84, (5) D2EHPA, and (6) VERSATIC 10.
tial than the diffusion-controlled hydrogen evolution reaction (peak A). The polarization curves of solutions saturated with organic impurities behave differently. The overvoltage (g) and the log current (I) values from the corresponding forward scan of the CV (between the nucleation potential, En, to the initiation of backward scan) is plotted to get the kinetic parameters using the Tafel equation. Table 1 presents the kinetic parameters obtained from the cathodic polarization curve. As it is not possible to use these data to prepare an effective model on the effect of the organic impurities on the rate of electron transfer, further work is required to understand the relationship quantitatively. As expected, all the tested organic impurities appear to inhibit electrocrystallisation by retardation of nucleation. The polarization curves showed a charge transfer controlled Tafel region at overpotentials higher than 200 mV. In the presence of organic
The current efficiency of cobalt deposition obtained in an electrowinning cell in the presence and absence of organic impurities is also presented in Table 1. SEM photographs of the cathode surfaces are shown in Fig. 3. As can be seen from Fig. 3, each of the organic impurities acts differently on the morphology of cobalt deposits. In general, the current efficiency with organic saturated electrolyte was 94 F 3% at pH 2.5 which was comparable to the organic-free electrolyte. However, VERSATIC 10 gave significantly lower current efficiency of 86% and a different cathode morphology (Fig. 3b) together with a more plate-like texture Table 1 Current efficiency and electrochemical parameters obtained in cobalt electrodeposition with organic saturated electrolytes (Electrolyte = 1.0 M Co2 + , pH 2.5, 60 jC, 0.08 M Na2SO4, 0 M MgSO4) Organic saturated electrolyte
Current efficiency (%)
Kinetic parameters from CV study i0 (A cm 2)
b( F 5) (mV)
a
1.0 M Co2 + reference Organic free electrolyte D2EHPA TBP LIX 84 CYANEX 272 VERSATIC 10
94
1.2 10 7
45
0.65
96
1.2 10 7
45
0.65
96 92 94 92 86
2.5 10 4 3.1 10 5 3.5 10 4 6.9 10 5 7.1 10 4
76 63 74 77 80
0.39 0.47 0.40 0.38 0.37
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
101
Fig. 3. SEM micrographs of cobalt electrodeposition obtained from electrowinning of cobalt (II) from a solution containing 1.0 M Co2 + , 0.08 M Na2SO4, 0.50 M MgSO4, pH 2.5, 60 jC, current density of 200 A/m2 and with, (a) zero additives, (b) VERSATIC 10, (c) D2EHPA, (d) TBP, (e) LIX 84 and (f) CYANEX 272. The insets are lower ( 2.5) magnifications of the main images.
102
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
compared to the nodular deposit from organic-free electrolyte (Fig. 3a). The morphology of cathodes in the presence of LIX 84 (Fig. 3e) and D2EHPA (Fig. 3c) show nodules similar to the organic-free cathode but with more dendritic or fibrous structure. With TBP and CYANEX 272, the morphology was again nodular but finer grained. The deposit was less uniform with CYANEX 272 present, suggesting less uniform adsorption of the organic on the surface and the presence of fine oily droplets. In all cases, XRD analysis of the deposits showed the existence of strong h100i, h101i and h110i crystallographic patterns. Hence, the face-centred cubic structure of cobalt predominates, irrespective of the organic impurity.
6. The XRD pattern of the cobalt deposits was not affected by the various organics studied and a strong h100i, h101i and h110i crystallographic pattern predominated.
Acknowledgements The authors extend their thanks to the Australian Government for providing financial support under the TIL program to carry out the research work at Murdoch University, Perth. One of the authors, Dr. Mishra, is grateful to the HRD, CSIR (New Delhi), Dr. R.P. Das, Head, H&EM Division, and Director, RRL (Bhubaneswar) for their support in sanctioning leave to travel to Australia to carry out the research work.
4. Conclusions 1. High background concentration of MgSO4 does not have any effect on the cathodic polarization of cobalt and cobalt reduction occurs at a more negative overpotential than the diffusion-controlled hydrogen evolution reaction. 2. Polarization curves show that organic impurities increase the NOP and inhibit electrocrystallisation of cobalt. 3. Electrokinetic parameters derived from cyclic voltammograms show that the exchange current density is higher in the presence of organic impurities and nucleation is suppressed. 4. The current efficiency is generally unaffected by saturation with trace organic extractants but VERSATIC 10 saturated electrolyte gave significantly lower efficiency for cobalt deposition. 5. The morphology of the cobalt deposit varies with each organic extractant but is generally nodular in appearance.
References Brennar, A., Zenterland, V., Jennings, C.W., 1952. Plating 38, 865. Franaszczuk, K., Sobkowski, J., 1989. J. Electroanal. Chem. 261, 223. Gomez, E., Muller, C., Proud, W.G., Valles, E., 1992. J. Appl. Electrochem. 22, 87. Kuzeci, E., Kammel, R., Gogia, S.K., 1994. J. Appl. Electrochem. 24, 730. Laxman, V.L., MacKinnon, D.J., Brannen, J.M., 1997. J. Appl. Electrochem. 7, 127. MacKinnon, D.J., Laxman, V.L., 1976. CANMET Sctf. Bull. CM76, 10. Mishra, K.G., Singh, P., Hefter, G.T., Muir, D.M., 2001. Metall. Mater. Trans. B 32 B, 2001. Prasad, M.S., Kenyen, V.P., Assar, D.N., 1992. Min. Proc. Extract. Met. Rev. 8, 95. Storey, G.G., Barens, S.C., 1961. J. Inst. Met. 90, 336.
Electrowinning of cobalt from sulphate solutions contaminated with organic impurities K.G. Mishra a,*, P. Singh b, D.M. Muir c a
Regional Research Laboratory (C.S.I.R), Bhubaneswar-751 013, India Division of Sciences, Murdoch University, WA 6150, Perth, Australia c C.S. I.R.O. Minerals, PO Box 90, WA 6982, Australia
b
Received 14 August 2001; received in revised form 17 August 2001; accepted 12 April 2002
Abstract The electrowinning (EW) of cobalt from sulphate solution contaminated with organic impurities: tri-n-butyl phosphate (TBP), 2-hexylmethyl decanoic acid (VERSATIC 10), 2-hydroxy-5-nonyl acetophenone oxime (LIX 84), di-2-ethylhexyl phosphoric acid (D2EHPA) and di-2,4,4-trimethylpentyl phosphinic acid (CYANEX 272) has been investigated at 60 jC. Both cyclic voltammetry and flow cell electrolysis have been used. The effect of organic impurities on the cathodic reduction of cobalt is compared with the blank solution. The current efficiency is more than 90% in most cases and is highest in the case of D2EHPA (96%) and lowest with VERSATIC 10 (86%). The cathodic polarization from cyclic voltammetry indicates that the exchange current density (i0) value is higher in the presence of organic impurities than in solutions containing no organic impurities. The organic impurities indicate an inhibition of electrocrystallization process due to suppression of nucleation. Cathodic polarization also indicates that Co2 + reduction occurs slightly at less negative potential in solution containing organic impurities. D 2002 Published by Elsevier Science B.V. Keywords: Electrowinning; Cobalt; Organic impurities; Cyclic voltammetry
1. Introduction Solvent extraction has gained a wide acceptance as one of the new tools of hydrometallurgy. With the growing demand for metals of higher purity and the necessity for treating low-grade ores, which are not amenable to concentration, solvent extraction has become a viable commercial method of recovering valuable metals such as cobalt, nickel, copper, vanadium, titanium and rare earths.
*
Corresponding author. E-mail address: [email protected] (K.G. Mishra).
Although the majority of cobalt is still produced by the conventional route of concentration, smelting and refining, hydrometallurgical routes involving solvent extraction and electrowinning (SX-EW) are rapidly growing in importance for recovering cobalt from oxides and sulphides, waste dumps and many secondary sources. Previous work on this aspect has been carried out by other workers (Prasad et al., 1992; Kuzeci et al., 1994; MacKinnon and Laxman, 1976; Storey and Barens, 1961; Laxman et al., 1997; Brennar et al., 1952). Several plants around the world are successfully using SX-EW process to produce cobalt. As a result of engineering innovations and operational progress, the SX-EW route is a proven technology.
0304-386X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 3 8 6 X ( 0 2 ) 0 0 0 3 6 - 1
98
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
In order to replicate flow conditions in hydrometallurgical plant, the electrolyte is passed through a rectangular cell so that the effect of organic impurities on cobalt morphology and current efficiency could be studied during electrowinning. In this study, an attempt is made to understand the effect of the organic impurities during the electrowinning of cobalt from a sulphate bath since there is little information on the mechanism of adsorption/desorption as well as metal ion reduction in the presence of impurities, such as tri-n-butyl phosphate (TBP), 2-hexylmethyl decanoic acid (VERSATIC 10), 2-hydroxy-5-nonyl acetophenone oxime (LIX 84), di-2-ethylhexyl phosphoric acid (D2EHPA) and di-2,4,4-trimethylpentyl phosphinic acid (CYANEX 272).
jC, at a current density of 200 A/m2 from a solution containing 1.0 M Co2 + , 0.08 M Na2SO4 and 0.50 M MgSO4 at pH 2.5. The cathodic current efficiency was calculated from the weight change of the cathode. The deposit was then carefully removed and analysed by SEM and XRD. Analytical grade CoSO4, Na2SO4, MgSO4 and H2SO4 were used with ultra pure water to prepare the electrolyte. One hundred milliliters of electrolyte was thoroughly shaken for 45 min with 20 mL of LIX 84, VERSATIC 10 (Henkel), D2EHPA (Daihachi), CYANEX 272 (Cytec) or TBP (Merck). The separated organic saturated aqueous phase was used as the catholyte solution for electrowinning and cyclic voltammetry studies. A fresh solution was prepared and sparged with nitrogen for each experiment.
2. Experimental details
2.2. Cyclic voltammetry
2.1. Electrowinning
Cyclic voltammetry was carried out in a 150-mL double-walled cell having stationary rotating disc working electrode, an Ag/AgCl reference electrode and a Pt spiral counter electrode. A Luggin capillary having a microtip was placed close to the electrode surface to avoid IR drop. The other end of the Luggin capillary was placed in the saturated KCl solution containing Ag/AgCl reference electrode. The outlet of the double-walled cell was connected to a thermostatic water bath through a peristaltic pump to maintain the electrolyte in the cell at 60 jC. The glassy carbon (GC) (0.0556 cm2), Pt (0.0956 cm2) and SS (0.0707 cm2) working electrodes were polished as already described for the electrowinning studies. A potentiostat (PAR model 362) coupled with a series X – Y recorder (Omnigraphic model RE0092) was employed to generate the cathodic polarization curves.
The electrowinning cell consisted of two rectangular halves made up of perspex having dimensions: 13.0 8.0 3.0 cm with a groove of 1-cm depth. Each groove accommodated an electrode of size 5.0 4.0 cm. A rectangular sheet of microporous ‘Daramic’ separator was placed between the two halves which were made watertight by using Viton O-rings. The electrolyte was circulated through each compartment of the cell from two separate stoppered glass vessels maintained at 60 jC by a thermostatic water bath by using two peristaltic pumps. Unless otherwise mentioned, all experiments were carried out by circulating the electrolyte at a volumetric flow rate of 1.8 L/h. Each cell was also provided with a ‘Teflon’ port through which an Ag/AgCl reference electrode was connected to allow measurement of the electrode potential. During electrowinning, stainless steel (SS) 304 was used as cathode and Pb/Sb (6 – 7%) as the anode material. The cathode prior to its use was polished with successive grades of 50, 120 and 4000 SiC paper to produce a mirror surface finish. The cathode edges were roughened slightly then degreased with acetone and thoroughly washed with water. All the electrowinning experiments were conducted by applying constant current from a regulated power supply. The electrowinning of cobalt was carried out for 2 h at 60
3. Results and discussion 3.1. Cyclic voltammetry The voltametric curves for platinum (Pt), glassy carbon (GC) and stainless steel (SS) in MgSO4 electrolyte background are presented in Fig. 1. Both the Pt and the SS electrodes show clear hydrogen evolution peaks at a scan rate of 10 mV s 1 on the Pt at about
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
99
Fig. 1. Cyclic voltammetric curves for different stationary electrode substrates at a scan rate of 10 mV s 1 in 0.08 M Na2SO4 and 0.50 M MgSO4 at pH 2.5 and 60 jC. (1) GC, (2) SS, and (3) Pt.
0.25 and 0.50 V and on the SS at 0.63 V. No significant hydrogen evolution appeared at the GC electrode (Fig. 1). These findings are consistent with our earlier study (Mishra et al., 2001) and are also found by others (Gomez et al., 1992; Franaszczuk and Sobkowski, 1989). This study helps to identify the region of hydrogen evolution. Fig. 2 presents the cathodic reduction of cobalt on a stationary SS electrode at a scan rate of 10 mV s 1, as it has both practical and commercial interest, from a solution of 1.0 M Co (II) saturated with various organic impurities at pH 2.5 and 60 jC. The point (A) in Fig. 2 presents the hydrogen evolution peak. The peak is prominent in the case of curves 3, 4, 5 and 6 and the potential is nearly the same as found in curve 2 of Fig. 1. The point (B) in the polarization curves of Fig. 2 corresponds to the nucleation potential (En) of cobalt deposition. The nucleation potential of cobalt is almost the same for all the organic impurities as well as in the organic-free solution and occurs at 0.690 F 0.02 V. This suggests that the organic impurities neither affect significantly the conductivity of the solution nor the double layer at the cathode surface. The negative potential limit was
chosen so that deposition of the metal phase on the stainless steel disc occurred just before the direction of scan was reversed. It can be seen that the cathodic current increases rapidly below 0.690 V. The crossover potential (Ecp) that corresponds to the opencircuit potential indicates that the cathode current returns to zero after the peak. The oxidation current is not shown here. All the features in the cathodic region of the voltammogram are typical for a system involving the cathodic deposition of a metal phase on the stationary, stainless steel substrate. It is readily observed in Fig. 2 that cobalt deposition in the presence of organic impurities on the stainless steel electrode does not occur until the cathode potential reaches about 0.690 F 0.02 V. As there is an appreciable nucleation overpotential (NOP), the difference between nucleation potential (En) and crossover potential (Ecp) indicates the extra energy needed for heterogeneous deposition onto a foreign substrate. After the cathode sweep, cobalt covers the electrode surface, so hydrogen evolution occurs more easily than on the original substrate. When Figs. 1 and 2 are compared, it is clear that cobalt reduction occurs at a more negative overpoten-
100
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
impurities, the exchange current densities (i0) increased by about 0.1 A/m2 with respect to the value obtained for solution without organic impurities (Table 1). The greater change in i0 value is measured with electrolyte saturated with VERSATIC 10 and minimum with TBP that can be considered as a measure of inhibition of nucleation due to absorption onto the electrode surface. The inhibition may be due to the lowering of the catalytic property of the electrode surface in presence of organic impurities. In all cases, the organic impurities appear to change the properties of the double layer on the electrode surface, as a result of which, hydrated metal ions cannot undergo charge transfer as readily as at organic-free surfaces. 3.2. Cobalt electrowinning in presence of organic impurities
Fig. 2. Forward CV scan of cobalt (II) on a stainless steel electrode substrate in solution at 60 jC containing 1.0 M Co2 + , 0.08 M Na2SO4, 0.50 M MgSO4 and saturated with, (1) zero additives (2) CYANEX 272, (3) TBP, (4) LIX 84, (5) D2EHPA, and (6) VERSATIC 10.
tial than the diffusion-controlled hydrogen evolution reaction (peak A). The polarization curves of solutions saturated with organic impurities behave differently. The overvoltage (g) and the log current (I) values from the corresponding forward scan of the CV (between the nucleation potential, En, to the initiation of backward scan) is plotted to get the kinetic parameters using the Tafel equation. Table 1 presents the kinetic parameters obtained from the cathodic polarization curve. As it is not possible to use these data to prepare an effective model on the effect of the organic impurities on the rate of electron transfer, further work is required to understand the relationship quantitatively. As expected, all the tested organic impurities appear to inhibit electrocrystallisation by retardation of nucleation. The polarization curves showed a charge transfer controlled Tafel region at overpotentials higher than 200 mV. In the presence of organic
The current efficiency of cobalt deposition obtained in an electrowinning cell in the presence and absence of organic impurities is also presented in Table 1. SEM photographs of the cathode surfaces are shown in Fig. 3. As can be seen from Fig. 3, each of the organic impurities acts differently on the morphology of cobalt deposits. In general, the current efficiency with organic saturated electrolyte was 94 F 3% at pH 2.5 which was comparable to the organic-free electrolyte. However, VERSATIC 10 gave significantly lower current efficiency of 86% and a different cathode morphology (Fig. 3b) together with a more plate-like texture Table 1 Current efficiency and electrochemical parameters obtained in cobalt electrodeposition with organic saturated electrolytes (Electrolyte = 1.0 M Co2 + , pH 2.5, 60 jC, 0.08 M Na2SO4, 0 M MgSO4) Organic saturated electrolyte
Current efficiency (%)
Kinetic parameters from CV study i0 (A cm 2)
b( F 5) (mV)
a
1.0 M Co2 + reference Organic free electrolyte D2EHPA TBP LIX 84 CYANEX 272 VERSATIC 10
94
1.2 10 7
45
0.65
96
1.2 10 7
45
0.65
96 92 94 92 86
2.5 10 4 3.1 10 5 3.5 10 4 6.9 10 5 7.1 10 4
76 63 74 77 80
0.39 0.47 0.40 0.38 0.37
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
101
Fig. 3. SEM micrographs of cobalt electrodeposition obtained from electrowinning of cobalt (II) from a solution containing 1.0 M Co2 + , 0.08 M Na2SO4, 0.50 M MgSO4, pH 2.5, 60 jC, current density of 200 A/m2 and with, (a) zero additives, (b) VERSATIC 10, (c) D2EHPA, (d) TBP, (e) LIX 84 and (f) CYANEX 272. The insets are lower ( 2.5) magnifications of the main images.
102
K.G. Mishra et al. / Hydrometallurgy 65 (2002) 97–102
compared to the nodular deposit from organic-free electrolyte (Fig. 3a). The morphology of cathodes in the presence of LIX 84 (Fig. 3e) and D2EHPA (Fig. 3c) show nodules similar to the organic-free cathode but with more dendritic or fibrous structure. With TBP and CYANEX 272, the morphology was again nodular but finer grained. The deposit was less uniform with CYANEX 272 present, suggesting less uniform adsorption of the organic on the surface and the presence of fine oily droplets. In all cases, XRD analysis of the deposits showed the existence of strong h100i, h101i and h110i crystallographic patterns. Hence, the face-centred cubic structure of cobalt predominates, irrespective of the organic impurity.
6. The XRD pattern of the cobalt deposits was not affected by the various organics studied and a strong h100i, h101i and h110i crystallographic pattern predominated.
Acknowledgements The authors extend their thanks to the Australian Government for providing financial support under the TIL program to carry out the research work at Murdoch University, Perth. One of the authors, Dr. Mishra, is grateful to the HRD, CSIR (New Delhi), Dr. R.P. Das, Head, H&EM Division, and Director, RRL (Bhubaneswar) for their support in sanctioning leave to travel to Australia to carry out the research work.
4. Conclusions 1. High background concentration of MgSO4 does not have any effect on the cathodic polarization of cobalt and cobalt reduction occurs at a more negative overpotential than the diffusion-controlled hydrogen evolution reaction. 2. Polarization curves show that organic impurities increase the NOP and inhibit electrocrystallisation of cobalt. 3. Electrokinetic parameters derived from cyclic voltammograms show that the exchange current density is higher in the presence of organic impurities and nucleation is suppressed. 4. The current efficiency is generally unaffected by saturation with trace organic extractants but VERSATIC 10 saturated electrolyte gave significantly lower efficiency for cobalt deposition. 5. The morphology of the cobalt deposit varies with each organic extractant but is generally nodular in appearance.
References Brennar, A., Zenterland, V., Jennings, C.W., 1952. Plating 38, 865. Franaszczuk, K., Sobkowski, J., 1989. J. Electroanal. Chem. 261, 223. Gomez, E., Muller, C., Proud, W.G., Valles, E., 1992. J. Appl. Electrochem. 22, 87. Kuzeci, E., Kammel, R., Gogia, S.K., 1994. J. Appl. Electrochem. 24, 730. Laxman, V.L., MacKinnon, D.J., Brannen, J.M., 1997. J. Appl. Electrochem. 7, 127. MacKinnon, D.J., Laxman, V.L., 1976. CANMET Sctf. Bull. CM76, 10. Mishra, K.G., Singh, P., Hefter, G.T., Muir, D.M., 2001. Metall. Mater. Trans. B 32 B, 2001. Prasad, M.S., Kenyen, V.P., Assar, D.N., 1992. Min. Proc. Extract. Met. Rev. 8, 95. Storey, G.G., Barens, S.C., 1961. J. Inst. Met. 90, 336.