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The influence of nickel ions and triethylbenzylammonium chloride on the electrowinning of zinc from sulphate electrolytes containing manganese ions
Hydrometallurgy 64 (2002) 193 – 203 www.elsevier.com/locate/hydromet
The influence of nickel ions and triethylbenzylammonium chloride on the electrowinning of zinc from sulphate electrolytes containing manganese ions Yavor Stefanov *, Ivan Ivanov Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl.11, 1113 Sofia, Bulgaria Received 8 February 2002; received in revised form 11 April 2002; accepted 12 April 2002
Abstract By means of potentiodynamic dissolution of the deposited zinc, the influence of nickel ions and triethylbenzylammonium chloride (TEBA) on the electrowinning of zinc from sulphate electrolytes is studied. In the presence of a ‘‘Nafion’’ 423 cationic membrane separating the electrode compartments, the influence of nickel ions is weakened. The membrane prevents access to the cathode by the oxidized products from the anode, which intensify the harmful action of nickel ions. The organic additive TEBA inhibits the process of zinc redissolution. The inhibiting action is stronger when the compartments are separated. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Current efficiency; Hydrogen evolution; Nickel redissolution; Zinc deposition
1. Introduction During the electrowinning of zinc from sulphate electrolytes in the presence of nickel ions, a process of redissolution of deposited metal takes place. Nickel codeposits with zinc forming numerous galvanic micropairs. Hydrogen evolves on the nickel zones and the surrounding zinc redissolves. The hydrogen evolution decreases at higher current density and increases at higher acid concentration and temperature. The manganese compounds depolarize the cathodic process and promote the zinc redissolution (Stender and Pecherskaya, 1950; Pecherskaya and
*
Corresponding author. E-mail address: [email protected] (Y. Stefanov).
Stender, 1950; Turomoshina and Stender, 1955; Nikiforov and Stender, 1959; Pomosov et al., 1958). Previous workers (Znamenski and Beziazikov, 1965; Maya and Spinelli, 1971; Maya et al., 1982; Wark, 1979) have shown that an induction period of more than 1 h exists before cobalt and nickel begin to have an effect on zinc deposition current efficiency. After the induction period, the current efficiency decreases rapidly with time. At this point, the corrosion of the deposit begins and continues with an autostimulating mechanism because the impurities incorporated previously into the deposit gradually come to the surface, further increasing the contaminated area on which hydrogen evolves. When the zinc deposit dissolution has been completed, hydrogen evolves on aluminium and a new polarization of the cathode with respect to zinc begins. After many formation and
0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X ( 0 2 ) 0 0 0 3 7 - 3
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dissolution cycles of the zinc deposit, the frequency of the cycles increases, showing that the aluminium cathode is gradually contaminated by the impurities until zinc deposition can no longer occur. If the acidity of the electrolysis bath is increased, the induction time decreases from several hours to a few minutes and the electrochemical system becomes unstable; zinc periodically deposits and dissolves and, under galvanostatic conditions, the cathode potential oscillates. The length of the induction period decreases also with increasing temperature and with decreasing current density (Znamenski and Beziazikov, 1965; Maya and Spinelli, 1971; Wark, 1979; Maya et al., 1982). The behaviour of cobalt and nickel has been interpreted in terms of the formation of local Zn –Co and Zn –Ni galvanic cells by Fosnacht and O’Keefe (1980), Wang et al. (1980) and Fratesi et al. (1980). Both cobalt (Fosnacht and O’Keefe, 1980) and nickel (Wang et al., 1980) were found to produce characteristic changes in zinc deposition polarization curves. A nickel-activated hydrogen evolution peak was found to occur at low current densities during the anodic sweep portion of the cyclic voltammogram (CVAG), the height of which was proportional to the nickel concentration in solution. MacKinnon et al. (1986) found that nickel had little effect on the forward scan but the reverse scan of the voltammograms was characterized by two cathodic peaks, one (first) before the zero current line, while the current was still cathodic, and another (second) after the anodic dissolution of zinc was complete. Both peaks increased with increasing nickel concentration but the second peak was much more sensitive to the nickel concentration than the first. The first peak was associated with vigorous hydrogen evolution. Its presence in the voltammogram was accompanied by a reduction in the corresponding anodic current peak, indicating that some of the deposited zinc had dissolved before the current becoming anodic. The behaviour suggests that a local cell-type impurity interaction occurs in the potential region of the first peak whereby nickel sites act as local cathodes and the adjacent zinc sites as local anodes (MacKinnon et al., 1986). When aluminium contains an alloyed Fe-phase, it has a local catalytic action on the discharge of hydrogen ions. This effect is intensified by the predominant nickel deposition in the Fe-aggregates. In the presence
of Fe-aggregates in the aluminium cathode, the capacity of the double layer is reduced by organic additive triethylbenzylammonium chloride (TEBA), which probably results from its adsorption on these aggregates (Rashkov et al., 1990; Petrova et al., 1991). The nature of the induction period in the course of zinc redissolution in the presence of nickel ions has been explained by the action of the hydrogen bubbles arising on the areas of co-deposited zinc and nickel. Protection ceases at these spots and the action of Zn – Ni galvanic microelements initiates. The organic additive TEBA, added into the electrolyte, reduces the time of attachment of bubbles on the cathodic surface so that their local screening effect is eliminated. Consequently, the cathode protection is maintained during the electrolysis and the local galvanic microelements do not begin their work. As a result, reverse dissolution of the deposited zinc does not start and zinc deposits with high current efficiency and a good quality (Bozhkov et al., 1990a,b; Ivanov, 1993; Wiart et al., 1990). It has been shown that the destabilization of the process of zinc deposition in the presence of nickel in the electrolyte is favoured in the absence of a membrane separating the anodic and the cathodic compartments. It has been concluded that the deleterious influence of nickel ions is enhanced by the presence of anodically formed products—a strong synergetic influence of nickel and oxidized species takes place on the substrate impurities (Cachet et al., 1993). Using cyclic voltammetry and impedance measurements, the influence of an organic additive, triethylbenzylammonium chloride, on the mechanism of zinc deposition and reverse dissolution of zinc deposits in acidic sulphate medium, has been analysed. In the presence of Ni2 + ions, the additive adsorption: (a) inhibits the reaction of zinc deposition, first at the stage of nucleation and then during the growth of zinc deposits; (b) inhibits the hydrogen evolution taking place on a Ni/Zn-containing surface compound; (c) increases the charge transfer resistance and decreases the double layer capacitance over a wide potential range ( 1.38 to 1.58 V); (d) modifies the lowfrequency impedance by magnifying the variation of the number of active sites for zinc deposition with increasing electrode polarization; and (e) stabilizes the galvanostatic deposition of zinc by competing with
Y. Stefanov, I. Ivanov / Hydrometallurgy 64 (2002) 193–203
the formation of the Ni/Zn-containing adsorbate (Cachet et al., 1994a,b). The aim of this paper is to study the influence of nickel ions and triethylbenzylammonium chloride on zinc electrowinning from sulphate electrolytes containing manganese ions.
2. Experimental details Experiments were carried out in argon atmosphere, in a glass three-electrode cell thermostated at 37 F 1 jC. To separate the anode from the cathode, a ‘‘Nafion’’ 423 membrane (supplied by Dupont de Nemours) of 2 cm diameter was tightly fitted between a Teflon holder and a glass container. The volume of electrolyte contained within the electrodic compartments was approximately 200 mL in the cathodic compartment and 700 mL in the anodic one. When the separator was used, the cathode and the reference electrode were placed in the inner compartment. Two different cathodes of effective surface area 1.0 cm2 each were successively used, whose effective surfaces were vertical and parallel to the symmetry axis of the cell. A pure aluminium cathode was fashioned from a 0.5-cm-diameter cylinder (Johnson Matthey, specpure). The second cathode was prepared from a sheet of aluminium supplied by Riedel de Haen (RdH), of purity about 99.6% and containing several alloying elements, most commonly Fe, Cu, Zn, Si, Pb, Mn, etc., the iron content being approximately 0.2%. Before electrolysis, the cathode surface was polished with emery paper (1200 grade). The reference electrode consisted of a mercury/mercurous sulphate electrode in 0.5 M H2SO4 (SSE), its potential being + 0.655 V (SHE). Both counter anodes, placed in the outer compartment of the cell, were platinium plates of 8.0 cm2 total area. The studies were carried out using a cyclic potentiodynamic technique. Potential scanning at a rate of 180 mV min 1 was performed with an EP 20A ‘‘Elpan’’ potentiostat (supplied with an IR compensator) and an EG 20 ‘‘Elpan’’ scanner. The cyclic voltammograms, CVAGs, were obtained in the range 0.900 to 1.625 V and were recorded by means of an ‘‘Endim 622.01’’ x – y chart recorder. With an increase in number of the scans, the currents for zinc deposition and hydrogen evolution (the height of
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peaks) were increased as a result of the oxide film reduction as well as of the adsorption of nickel ions and organic additive on the aluminium surface. Stabilized voltammograms were obtained in the whole potential range at the seventh scan. The change in the potential was recorded by means of a V 542.1 digital voltammeter. The change in the cathodic current during the deposition was recorded by means of an ‘‘Endim 621.02’’ y – t recorder. The composition of the base electrolyte (BE) was as follows: 130 g L 1 H2SO4, 220 g L 1 ZnSO47H2O and 15.5 g L 1 MnSO4H2O (5 g L 1 Mn2+ ions). Ni2+ ions and triethylbenzylammonium chloride were added to the base electrolyte in the following concentrations:
Ni2+ ions: 5 mg L 1 (added as NiSO46H2O); TEBA: 100 and 600 mg L 1 (added as a standard aqueous solution of triethylbenzylammonium chloride).
All chemicals were Merck products of analytical grade purity, except for TEBA, which was from Fluka (purity 98%).
3. Experimental methods The initial potential for zinc deposition (Ed) was determined by extrapolation to zero current from experimentally obtained cyclic voltammograms (MacKinnon et al., 1990). The electrical charge ( q) required for the dissolution of zinc, deposited during the cathodic process, was determined by graphical integration of the anodic peak on the CVAG. Due to the considerably higher overvoltage of hydrogen evolution on zinc (as compared to that on aluminium), it may be assumed that the anodic dissolution of zinc is not accompanied by a cathodic reaction of hydrogen evolution. The scanning was continued until the anodic current became equal to zero, that is, to the complete dissolution of the deposited zinc. The obtained values of q (in coulombs, C), were plotted as a function of deposition time ( q vs. t). The deposition of zinc was carried out at a potential of 1.600 V vs. SSE on the aluminium cathode, with a duration of 5– 30 min. It was established that
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Table 1 Al—Riedel de Haen Electrolyte
Ed/V
ic/mA cm
Without ‘‘Nafion’’ 423 membrane Base electrolyte (BE) BE + 5 mg L 1 Ni2+ BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA
With ‘‘Nafion’’ 423 membrane
2
CEZ/%
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
1.455 1.475 1.480
1.520 1.500 1.510
112 132 154
98 84 126
72 24 55
78 50 64
1.500
1.520
202
177
55
53
functioning of the numerous galvanic micropairs Zn – Ni (already formed at 1.600 V). The process of redissolution is spontaneous and is not registered on the anodic curve. Because of the redissolution, the amount of the deposited zinc obtained using the anodic peak technique is smaller (Stefanov et al., 1996/1997).
the current density reaches its constant value after 2 –3 min. The current efficiency of zinc deposition (CEZ) in the different electrolytes was calculated using the data for the amount of deposited zinc for 30 min ( q) and the current density (ic). Two different series of experiments were carried out. In the first series, the cathodic and the anodic compartments were not separated and in the second series, the compartments were separated by means of a ‘‘Nafion’’ 423 membrane. This membrane is reinforced with Teflon membrane of 1200 equivalent weight copolymer K + ionic form with a thickness of approximately 0.04 cm and with area electric resistance of 4.7 V cm 2 (about 15 V total resistance). We have established (Stefanov et al., 1996/1997) that, in the cases of deposition in a base electrolyte and in the presence of TEBA, the weights of deposits obtained using the anodic peak technique by weight measuring are practically equal. In the case of nickel addition to the electrolyte, the curves obtained by measuring of the deposit weight are close to the curves obtained in the base electrolyte. At a potential of 1.600 vs. SSE, there is no zinc redissolution. This process starts with the decrease in the electrode potential during the reverse scan as a result of the
4. Results and discussion 4.1. Electrodeposition of zinc on aluminium cathode containing iron impurities Figs. 1 and 3 show cyclic voltammograms and Figs. 2 and 4 show the dependence of q vs. t obtained during the zinc deposition on the Al cathode containing Fe impurities (Al—Riedel de Haen). The values of the initial potentials of zinc deposition (Ed), deposition current density (ic) and current efficiency of zinc (CEZ) are presented in Table 1 and the potentials of the cathodic peaks on CVAG in Table 2. Fig. 1 shows the CVAG obtained without separation of the electrode compartments by a membrane. The forward scan in the base electrolyte (BE) reveals
Table 2 Al—Riedel de Haen Electrolyte
Without ‘‘Nafion’’ 423 membrane I peak/V
Base electrolyte (BE) BE + 5 mg L 1 Ni2+ BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA
1.360 1.360 1.390 1.375
With ‘‘Nafion’’ 423 membrane
II peak/V
III peak/V
I peak/V
II peak/V
III peak/V
–
–
–
–
1.415 1.425
1.210 1.250
1.420 1.400
– – –
1.425
1.270
1.400
–
1.230 1.215 1.225
Y. Stefanov, I. Ivanov / Hydrometallurgy 64 (2002) 193–203
197
Fig. 1. Voltammograms, recorded at a sweep rate of 180 mV min 1. Al—Riedel de Haen. No membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
a flattened peak around 1.360 V due to the hydrogen evolution (curve 1). It is stimulated by the iron impurities contained in the aluminium substrate on which the potential of hydrogen evolution is 0.150– 0.200 V lower than the potential on pure Al (Stender and Pecherskaya, 1950). Zinc deposition begins at 1.455 V and the large anodic peak shows that this is the predominant process. The peak at 1.360 V in the forward scan obtained in electrolyte, containing 5 mg L 1 Ni2 + (curve 2) is higher and larger than in BE which is due to the easier hydrogen evolution onto nickel adsorbed preferentially on iron impurities. The potential of hydrogen evolution on nickel is 0.100 V lower than that on iron and 0.300 V—than that on aluminium (Stender and Pecherskaya, 1950). The zinc deposition begins at 1.475 V, but the smaller anodic peak shows that this process is accompanied with hydrogen evolution. A small peak at 1.415 V appears on the reverse scan due to the evolution of hydrogen on nickel deposited with zinc. After the complete zinc dissolution, a high peak appears at 1.210 V, due to the hydrogen evolution on the iron aggregates covered with nickel. With the increase in the scan number, all peaks rise showing that the area of the iron aggregates covered with nickel gradually increases. The nickel coating does not dissolve after potential return to the starting value.
Two large peaks and a plateau appear on CVAG obtained in presence of the organic additive TEBA (100 and 600 mg L 1—curves 3 and 4) added in the electrolyte containing 5 mg L 1 Ni2+ . The hydrogen peaks increase with cycle number because of the deposition of nickel on iron impurities and of the adsorption of TEBA on the cathode surface. It was found that the organic additive TEBA decreases the surface tension and reduces the time of attachment of hydrogen bubbles on the cathode surface thus facilitating their evolution (Bozhkov et al., 1990a,b; Ivanov, 1993; Wiart et al., 1990). With an increase in TEBA concentration, the areas of both peaks become larger. The anodic peaks are smaller because of the inhibition of the zinc deposition. It was found by Stefanov (1999) that an increase of the TEBA concentration resulted in an decreased zinc deposit. Fig. 2 shows the dependence q vs. t obtained during zinc deposition in the electrolyte, in an undivided cell. The amount of deposited zinc decreases two times and the current efficiency decreases from 72% to 24% in the presence of Ni2+ ions (curve 2) compared to the amount obtained in BE (curve 1). This is due to the zinc redissolution, which starts with the decrease in the electrode potential during the reverse scan because of the action of the numerous galvanic micropairs Zn – Ni
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Fig. 2. Dependence of the charge corresponding to the deposited Zn (in coulombs) on time. E = 1.600 V (SSE). Al—Riedel de Haen. No membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
(already formed at 1.600 V). This process is spontaneous and is not seen on the anodic curve. Because of the redissolution, the amount of the deposited zinc obtained using the cyclic voltammograms is smaller. When the organic additive TEBA is added to the electrolyte at a concentration of 100 mg L 1 (curve
3) or 600 mg L 1 (curve 4), the amount of the deposited zinc is larger and the current efficiency increases to 55% because of the inhibition of the zinc redissolution. The mechanism of the action of the TEBA inhibitor is shown in previous studies (Bozhkov et al., 1990a,b; Ivanov, 1993; Wiart et al., 1990). In spite of
Fig. 3. Voltammograms, recorded at a sweep rate of 180 mV min 1. Al—Riedel de Haen. Cell divided by a membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
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199
Fig. 4. Dependence of the charge corresponding to the deposited Zn on time. E = 1.600 V (SSE). Al—Riedel de Haen. Cell divided by a membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
the inhibition of zinc redissolution, a considerable part of the current is wasted in hydrogen evolution. Fig. 3 shows the CVAG obtained in a divided cell. In all cases, the initial potentials of zinc deposition are more negative and the current values are lower (Table 1) than those obtained without separation. This is due to the increased ohmic resistance of the electrolyte in the presence of a membrane. On the CVAG obtained in BE (curve 1), no current peaks are seen. On the CVAG obtained in electrolyte containing 5 mg L 1 Ni2+ ions (curve 2), two peaks are seen: on the direct scan at 1.420 V and on the reverse scan at 1.230 V. Both peaks appear at more negative potentials than the peaks obtained in an undivided cell. The anodic peak is smaller than the peak obtained in the BE because of the hydrogen evolution.
Two peaks (at 1.400 V on the direct scan and at 1.215 to 1.220 Von the reverse scan) are observed on the CVAG obtained in electrolytes containing Ni2+ ions and organic additive TEBA (100 mg L 1—curve 3 and 600 mg L 1—curve 4). Both peaks are smaller than the peaks on the CVAG obtained in the electrolyte containing only Ni2+ ions. The areas of the peaks do not depend on the TEBA concentration. The anodic peaks are smaller than the peak obtained in the BE (curve 1). The tendency to the reduction of the evolved hydrogen is confirmed by the dependence q vs. t (obtained with divided cell) shown in Fig. 4. The deposition current at 1.600 V is significantly lower than the current in the cases without separation. During the deposition in BE (curve 1) and in the
Table 3 Al—Johnson Matthey Electrolyte
Ed/V Without ‘‘Nafion’’ 423 membrane
Base electrolyte (BE) BE + 5 mg L 1 Ni2+ BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA
ic/mA cm With ‘‘Nafion’’ 423 membrane
2
CEZ/%
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
1.545 1.490 1.490
1.525 1.515 1.530
112 102 77
102 84 63
74 39 45
71 55 53
1.510
1.540
66
77
56
43
200
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Table 4 Al—Johnson Matthey Electrolyte
Base electrolyte (BE) BE + 5 mg L 1 Ni2+ BE + 5 mg L 1 Ni2+ + 100 mg L BE + 5 mg L 1 Ni2+ + 600 mg L
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
I peak/V
II peak/V
III peak/V
I peak/V
II peak/V
III peak/V
–
– – – –
–
– – – –
– – – –
–
1.425 1.375
1
TEBA 1 TEBA
–
electrolyte, containing 5 mg L 1 Ni2+ (curve 2), the amount of deposited zinc is practically equal to the amount of zinc deposited in the electrolyte without separation (Fig. 2, curves 1 and 2) but the current efficiencies are higher at 78% and 50%. The addition of TEBA leads to the increase in the amounts of the deposited zinc and of the current efficiencies—to 64% (with 100 mg L 1—curve 3) and 53% (with 600 mg L 1—curve 4). The separation of the electrode compartments leads to an increased zinc current efficiency. The membrane prevents the anode products (H2O2, S2O82 ) passing into the cathode compartment. These oxidized products destabilize the electrodeposition in pure electrolytes (Cachet et al., 1994a) or promote the harmful influence of Ni2+ ions (Stefanov et al., 1996/1997). In the presence of manganese ions in the electrolyte, the process of zinc redissolution is promoted by MnO4 and
1.250 1.240 1.255
1.230 – –
MnO2 formed on the anode, which reach the cathode when the compartments are not separated. The current efficiency of zinc is lower (24%) than in the divided cell (50%). The organic additive TEBA inhibits the action of Ni2+ -ions and of the oxidized products and thereby stabilizes the galvanostatic deposition of zinc, resulting in an increased current efficiency. The inhibition effect is stronger when the electrode compartments are separated because of the absence of the oxidized products in the cathodic compartment. 4.2. Electrodeposition of zinc on pure aluminium cathode Figs. 5 and 7 show the cyclic voltammograms and Figs. 6 and 8 show the dependence q vs. t obtained during the zinc deposition on the cathode of specpure
Fig. 5. Voltammograms, recorded at a sweep rate of 180 mV min 1. Al—Johnson Matthey. No membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
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201
Fig. 6. Dependence of the charge corresponding to the deposited Zn on time. E = 1.600 V (SSE). Al—Johnson Matthey. No membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
Al (Johnson Matthey). The values of the initial potentials of zinc deposition (Ed), deposition current density (ic) and current efficiency of zinc (CEZ) are presented in Table 3 and the potentials of the cathodic peaks on CVAG in Table 4. Fig. 5 shows the CVAG obtained without separation of the electrode compartments with a mem-
brane. In the CVAG obtained in BE (curve 1), no cathodic peaks are seen and the zinc deposition begins at the most negative potential ( 1.545 V). Nickel ions (5 mg L 1) added in the electrolyte depolarize the cathodic reaction to 1.490 V and the anodic peaks are considerably larger (curve 2). Two cathodic peaks are observed on the CVAG, a very small
Fig. 7. Voltammograms, recorded at a sweep rate of 180 mV min 1. Al—Johnson Matthey. Cell divided by a membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
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peak at 1.425 V, on the forward scan, and a larger peak at 1.250 V on the reverse scan, due to the hydrogen evolution. Both peaks increase with cycle number, because of the nickel adsorption on the cathode surface. Due to the absence of iron impurities on the aluminium surface, the amount of nickel is smaller. Both peaks are lower than the peaks obtained with Al (Riedel de Haen). Addition of 100 mg L 1 TEBA in the electrolyte containing 5 mg L 1 Ni2+ (curve 3) leads to the increase of both peaks. At a TEBA concentration of 600 mg L 1 (curve 4), both peaks also increase. The organic inhibitor polarizes the zinc deposition and the anodic peaks are smaller. Fig. 6 shows q vs. t for an undivided cell. The amount of zinc deposited in BE (curve 1) is large and the current efficiency is 74%. The addition of 5 mg L 1 Ni2+ (curve 2) and the TEBA inhibitor (100 mg L 1, curve 3 and 600 mg L 1, curve 4) decreases the zinc amount and the current efficiency to 39%, 45% and 56%, respectively. The current efficiency, in the presence of TEBA, increases due to the inhibition of zinc redissolution. Fig. 7 shows the CVAG obtained in the divided cell, which shows no cathodic peaks. The effect of separation during the deposition on the cathode of specpure aluminium is strongly shown, in comparison
with the deposition on the cathode of aluminium alloyed with iron impurities. Fig. 8 shows the q vs. t plot obtained in a divided cell. All curves are similar to those obtained without separation. The current efficiency in the presence of Ni2 + ions is higher (55%) than the case in the absence of a membrane (39%). The addition of 100 mg L 1 TEBA does not change the current efficiency (53%). With an increase in the TEBA concentration to 600 mg L 1, the current efficiency decreases to 45% as a result of the inhibition of the zinc deposition.
5. Conclusions The zinc electrodeposition in sulphuric acid electrolytes on a cathode of specpure or aluminium containing iron impurities takes place with a high current efficiency. Nickel ions reduce the amount of the deposited zinc and its current efficiency and this effect is stronger during the deposition on the cathode of aluminium containing iron impurities and when the electrode compartments are not separated by an ion exchange membrane. The separation of the cathode
Fig. 8. Dependence of the charge corresponding to the deposited Zn on time. E = 1.600 V (SSE). Al—Johnson Matthey. Cell divided by a membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
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and anode compartments prevents the access of the oxidized anode products reaching the cathode and the current efficiency increases. The addition of triethylbenzylammonium chloride inhibits zinc redissolution in the presence of nickel ions and as a result, the current efficiency increases. This effect is stronger when the deposition takes place on the cathode of aluminium containing iron and when the electrode compartments are separated.
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MacKinnon, D.J., Morrison, R.M., Branen, J.M., 1986. J. Appl. Electrochem. 16, 53. MacKinnon, D.J., Morrison, R.M., Mouland, J.E., Warren, P.E., 1990. J. Appl. Electrochem. 20, 728. Maya, M., Spinelli, P., 1971. J. Electrochem. Soc. 118, 1538. Maya, M., Penazzi, N., Fratesi, R., Roventi, G., 1982. J. Electrochem. Soc. 129, 2695. Nikiforov, A.F., Stender, V.V., 1959. J. Appl. Chem. (Russian) 25 (1), 18. Pecherskaya, A.G., Stender, V.V., 1950. J. Appl. Chem. (Russian) 23 (9), 920. Petrova, M., Bozhkov, Chr., Rashkov, St., 1991. Bulg. Chem. Commun. 24 (3), 317. Pomosov, A.V., Krimakova, E.E., Levin, A.I., 1958. J. Appl. Chem. (Russian) 31 (5), 734. Rashkov, St., Petrova, M., Bozhkov, Chr., 1990. J. Appl. Electrochem. 20, 17. Stefanov, Y., 1999. Thesis, Sofia (in Bulgarian). Stefanov, Y., Ivanov, I., Rashkov, St., 1996/1997. Bulg. Chem. Commun. 29 (2), 302. Stender, V.V., Pecherskaya, A.G., 1950. Non-Ferrous Met. (Russian) 4, 45. Turomoshina, U.F., Stender, V.V., 1955. J. Appl. Chem. (Russian) 28 (4), 372. Wang, Y.-M., O’Keefe, T.J., James, W.J., 1980. J. Electrochem. Soc. 127, 2589. Wark, I.W., 1979. J. Appl. Electrochem. 9, 721. Wiart, R., Cachet, C., Bozhkov, Chr., Rashkov, St., 1990. J. Appl. Electrochem., 381. Znamenski, G.N., Beziazikov, B.N., 1965. J. Appl. Chem. (Russian) 38 (2), 361.
The influence of nickel ions and triethylbenzylammonium chloride on the electrowinning of zinc from sulphate electrolytes containing manganese ions Yavor Stefanov *, Ivan Ivanov Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl.11, 1113 Sofia, Bulgaria Received 8 February 2002; received in revised form 11 April 2002; accepted 12 April 2002
Abstract By means of potentiodynamic dissolution of the deposited zinc, the influence of nickel ions and triethylbenzylammonium chloride (TEBA) on the electrowinning of zinc from sulphate electrolytes is studied. In the presence of a ‘‘Nafion’’ 423 cationic membrane separating the electrode compartments, the influence of nickel ions is weakened. The membrane prevents access to the cathode by the oxidized products from the anode, which intensify the harmful action of nickel ions. The organic additive TEBA inhibits the process of zinc redissolution. The inhibiting action is stronger when the compartments are separated. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Current efficiency; Hydrogen evolution; Nickel redissolution; Zinc deposition
1. Introduction During the electrowinning of zinc from sulphate electrolytes in the presence of nickel ions, a process of redissolution of deposited metal takes place. Nickel codeposits with zinc forming numerous galvanic micropairs. Hydrogen evolves on the nickel zones and the surrounding zinc redissolves. The hydrogen evolution decreases at higher current density and increases at higher acid concentration and temperature. The manganese compounds depolarize the cathodic process and promote the zinc redissolution (Stender and Pecherskaya, 1950; Pecherskaya and
*
Corresponding author. E-mail address: [email protected] (Y. Stefanov).
Stender, 1950; Turomoshina and Stender, 1955; Nikiforov and Stender, 1959; Pomosov et al., 1958). Previous workers (Znamenski and Beziazikov, 1965; Maya and Spinelli, 1971; Maya et al., 1982; Wark, 1979) have shown that an induction period of more than 1 h exists before cobalt and nickel begin to have an effect on zinc deposition current efficiency. After the induction period, the current efficiency decreases rapidly with time. At this point, the corrosion of the deposit begins and continues with an autostimulating mechanism because the impurities incorporated previously into the deposit gradually come to the surface, further increasing the contaminated area on which hydrogen evolves. When the zinc deposit dissolution has been completed, hydrogen evolves on aluminium and a new polarization of the cathode with respect to zinc begins. After many formation and
0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X ( 0 2 ) 0 0 0 3 7 - 3
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dissolution cycles of the zinc deposit, the frequency of the cycles increases, showing that the aluminium cathode is gradually contaminated by the impurities until zinc deposition can no longer occur. If the acidity of the electrolysis bath is increased, the induction time decreases from several hours to a few minutes and the electrochemical system becomes unstable; zinc periodically deposits and dissolves and, under galvanostatic conditions, the cathode potential oscillates. The length of the induction period decreases also with increasing temperature and with decreasing current density (Znamenski and Beziazikov, 1965; Maya and Spinelli, 1971; Wark, 1979; Maya et al., 1982). The behaviour of cobalt and nickel has been interpreted in terms of the formation of local Zn –Co and Zn –Ni galvanic cells by Fosnacht and O’Keefe (1980), Wang et al. (1980) and Fratesi et al. (1980). Both cobalt (Fosnacht and O’Keefe, 1980) and nickel (Wang et al., 1980) were found to produce characteristic changes in zinc deposition polarization curves. A nickel-activated hydrogen evolution peak was found to occur at low current densities during the anodic sweep portion of the cyclic voltammogram (CVAG), the height of which was proportional to the nickel concentration in solution. MacKinnon et al. (1986) found that nickel had little effect on the forward scan but the reverse scan of the voltammograms was characterized by two cathodic peaks, one (first) before the zero current line, while the current was still cathodic, and another (second) after the anodic dissolution of zinc was complete. Both peaks increased with increasing nickel concentration but the second peak was much more sensitive to the nickel concentration than the first. The first peak was associated with vigorous hydrogen evolution. Its presence in the voltammogram was accompanied by a reduction in the corresponding anodic current peak, indicating that some of the deposited zinc had dissolved before the current becoming anodic. The behaviour suggests that a local cell-type impurity interaction occurs in the potential region of the first peak whereby nickel sites act as local cathodes and the adjacent zinc sites as local anodes (MacKinnon et al., 1986). When aluminium contains an alloyed Fe-phase, it has a local catalytic action on the discharge of hydrogen ions. This effect is intensified by the predominant nickel deposition in the Fe-aggregates. In the presence
of Fe-aggregates in the aluminium cathode, the capacity of the double layer is reduced by organic additive triethylbenzylammonium chloride (TEBA), which probably results from its adsorption on these aggregates (Rashkov et al., 1990; Petrova et al., 1991). The nature of the induction period in the course of zinc redissolution in the presence of nickel ions has been explained by the action of the hydrogen bubbles arising on the areas of co-deposited zinc and nickel. Protection ceases at these spots and the action of Zn – Ni galvanic microelements initiates. The organic additive TEBA, added into the electrolyte, reduces the time of attachment of bubbles on the cathodic surface so that their local screening effect is eliminated. Consequently, the cathode protection is maintained during the electrolysis and the local galvanic microelements do not begin their work. As a result, reverse dissolution of the deposited zinc does not start and zinc deposits with high current efficiency and a good quality (Bozhkov et al., 1990a,b; Ivanov, 1993; Wiart et al., 1990). It has been shown that the destabilization of the process of zinc deposition in the presence of nickel in the electrolyte is favoured in the absence of a membrane separating the anodic and the cathodic compartments. It has been concluded that the deleterious influence of nickel ions is enhanced by the presence of anodically formed products—a strong synergetic influence of nickel and oxidized species takes place on the substrate impurities (Cachet et al., 1993). Using cyclic voltammetry and impedance measurements, the influence of an organic additive, triethylbenzylammonium chloride, on the mechanism of zinc deposition and reverse dissolution of zinc deposits in acidic sulphate medium, has been analysed. In the presence of Ni2 + ions, the additive adsorption: (a) inhibits the reaction of zinc deposition, first at the stage of nucleation and then during the growth of zinc deposits; (b) inhibits the hydrogen evolution taking place on a Ni/Zn-containing surface compound; (c) increases the charge transfer resistance and decreases the double layer capacitance over a wide potential range ( 1.38 to 1.58 V); (d) modifies the lowfrequency impedance by magnifying the variation of the number of active sites for zinc deposition with increasing electrode polarization; and (e) stabilizes the galvanostatic deposition of zinc by competing with
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the formation of the Ni/Zn-containing adsorbate (Cachet et al., 1994a,b). The aim of this paper is to study the influence of nickel ions and triethylbenzylammonium chloride on zinc electrowinning from sulphate electrolytes containing manganese ions.
2. Experimental details Experiments were carried out in argon atmosphere, in a glass three-electrode cell thermostated at 37 F 1 jC. To separate the anode from the cathode, a ‘‘Nafion’’ 423 membrane (supplied by Dupont de Nemours) of 2 cm diameter was tightly fitted between a Teflon holder and a glass container. The volume of electrolyte contained within the electrodic compartments was approximately 200 mL in the cathodic compartment and 700 mL in the anodic one. When the separator was used, the cathode and the reference electrode were placed in the inner compartment. Two different cathodes of effective surface area 1.0 cm2 each were successively used, whose effective surfaces were vertical and parallel to the symmetry axis of the cell. A pure aluminium cathode was fashioned from a 0.5-cm-diameter cylinder (Johnson Matthey, specpure). The second cathode was prepared from a sheet of aluminium supplied by Riedel de Haen (RdH), of purity about 99.6% and containing several alloying elements, most commonly Fe, Cu, Zn, Si, Pb, Mn, etc., the iron content being approximately 0.2%. Before electrolysis, the cathode surface was polished with emery paper (1200 grade). The reference electrode consisted of a mercury/mercurous sulphate electrode in 0.5 M H2SO4 (SSE), its potential being + 0.655 V (SHE). Both counter anodes, placed in the outer compartment of the cell, were platinium plates of 8.0 cm2 total area. The studies were carried out using a cyclic potentiodynamic technique. Potential scanning at a rate of 180 mV min 1 was performed with an EP 20A ‘‘Elpan’’ potentiostat (supplied with an IR compensator) and an EG 20 ‘‘Elpan’’ scanner. The cyclic voltammograms, CVAGs, were obtained in the range 0.900 to 1.625 V and were recorded by means of an ‘‘Endim 622.01’’ x – y chart recorder. With an increase in number of the scans, the currents for zinc deposition and hydrogen evolution (the height of
195
peaks) were increased as a result of the oxide film reduction as well as of the adsorption of nickel ions and organic additive on the aluminium surface. Stabilized voltammograms were obtained in the whole potential range at the seventh scan. The change in the potential was recorded by means of a V 542.1 digital voltammeter. The change in the cathodic current during the deposition was recorded by means of an ‘‘Endim 621.02’’ y – t recorder. The composition of the base electrolyte (BE) was as follows: 130 g L 1 H2SO4, 220 g L 1 ZnSO47H2O and 15.5 g L 1 MnSO4H2O (5 g L 1 Mn2+ ions). Ni2+ ions and triethylbenzylammonium chloride were added to the base electrolyte in the following concentrations:
Ni2+ ions: 5 mg L 1 (added as NiSO46H2O); TEBA: 100 and 600 mg L 1 (added as a standard aqueous solution of triethylbenzylammonium chloride).
All chemicals were Merck products of analytical grade purity, except for TEBA, which was from Fluka (purity 98%).
3. Experimental methods The initial potential for zinc deposition (Ed) was determined by extrapolation to zero current from experimentally obtained cyclic voltammograms (MacKinnon et al., 1990). The electrical charge ( q) required for the dissolution of zinc, deposited during the cathodic process, was determined by graphical integration of the anodic peak on the CVAG. Due to the considerably higher overvoltage of hydrogen evolution on zinc (as compared to that on aluminium), it may be assumed that the anodic dissolution of zinc is not accompanied by a cathodic reaction of hydrogen evolution. The scanning was continued until the anodic current became equal to zero, that is, to the complete dissolution of the deposited zinc. The obtained values of q (in coulombs, C), were plotted as a function of deposition time ( q vs. t). The deposition of zinc was carried out at a potential of 1.600 V vs. SSE on the aluminium cathode, with a duration of 5– 30 min. It was established that
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Table 1 Al—Riedel de Haen Electrolyte
Ed/V
ic/mA cm
Without ‘‘Nafion’’ 423 membrane Base electrolyte (BE) BE + 5 mg L 1 Ni2+ BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA
With ‘‘Nafion’’ 423 membrane
2
CEZ/%
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
1.455 1.475 1.480
1.520 1.500 1.510
112 132 154
98 84 126
72 24 55
78 50 64
1.500
1.520
202
177
55
53
functioning of the numerous galvanic micropairs Zn – Ni (already formed at 1.600 V). The process of redissolution is spontaneous and is not registered on the anodic curve. Because of the redissolution, the amount of the deposited zinc obtained using the anodic peak technique is smaller (Stefanov et al., 1996/1997).
the current density reaches its constant value after 2 –3 min. The current efficiency of zinc deposition (CEZ) in the different electrolytes was calculated using the data for the amount of deposited zinc for 30 min ( q) and the current density (ic). Two different series of experiments were carried out. In the first series, the cathodic and the anodic compartments were not separated and in the second series, the compartments were separated by means of a ‘‘Nafion’’ 423 membrane. This membrane is reinforced with Teflon membrane of 1200 equivalent weight copolymer K + ionic form with a thickness of approximately 0.04 cm and with area electric resistance of 4.7 V cm 2 (about 15 V total resistance). We have established (Stefanov et al., 1996/1997) that, in the cases of deposition in a base electrolyte and in the presence of TEBA, the weights of deposits obtained using the anodic peak technique by weight measuring are practically equal. In the case of nickel addition to the electrolyte, the curves obtained by measuring of the deposit weight are close to the curves obtained in the base electrolyte. At a potential of 1.600 vs. SSE, there is no zinc redissolution. This process starts with the decrease in the electrode potential during the reverse scan as a result of the
4. Results and discussion 4.1. Electrodeposition of zinc on aluminium cathode containing iron impurities Figs. 1 and 3 show cyclic voltammograms and Figs. 2 and 4 show the dependence of q vs. t obtained during the zinc deposition on the Al cathode containing Fe impurities (Al—Riedel de Haen). The values of the initial potentials of zinc deposition (Ed), deposition current density (ic) and current efficiency of zinc (CEZ) are presented in Table 1 and the potentials of the cathodic peaks on CVAG in Table 2. Fig. 1 shows the CVAG obtained without separation of the electrode compartments by a membrane. The forward scan in the base electrolyte (BE) reveals
Table 2 Al—Riedel de Haen Electrolyte
Without ‘‘Nafion’’ 423 membrane I peak/V
Base electrolyte (BE) BE + 5 mg L 1 Ni2+ BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA
1.360 1.360 1.390 1.375
With ‘‘Nafion’’ 423 membrane
II peak/V
III peak/V
I peak/V
II peak/V
III peak/V
–
–
–
–
1.415 1.425
1.210 1.250
1.420 1.400
– – –
1.425
1.270
1.400
–
1.230 1.215 1.225
Y. Stefanov, I. Ivanov / Hydrometallurgy 64 (2002) 193–203
197
Fig. 1. Voltammograms, recorded at a sweep rate of 180 mV min 1. Al—Riedel de Haen. No membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
a flattened peak around 1.360 V due to the hydrogen evolution (curve 1). It is stimulated by the iron impurities contained in the aluminium substrate on which the potential of hydrogen evolution is 0.150– 0.200 V lower than the potential on pure Al (Stender and Pecherskaya, 1950). Zinc deposition begins at 1.455 V and the large anodic peak shows that this is the predominant process. The peak at 1.360 V in the forward scan obtained in electrolyte, containing 5 mg L 1 Ni2 + (curve 2) is higher and larger than in BE which is due to the easier hydrogen evolution onto nickel adsorbed preferentially on iron impurities. The potential of hydrogen evolution on nickel is 0.100 V lower than that on iron and 0.300 V—than that on aluminium (Stender and Pecherskaya, 1950). The zinc deposition begins at 1.475 V, but the smaller anodic peak shows that this process is accompanied with hydrogen evolution. A small peak at 1.415 V appears on the reverse scan due to the evolution of hydrogen on nickel deposited with zinc. After the complete zinc dissolution, a high peak appears at 1.210 V, due to the hydrogen evolution on the iron aggregates covered with nickel. With the increase in the scan number, all peaks rise showing that the area of the iron aggregates covered with nickel gradually increases. The nickel coating does not dissolve after potential return to the starting value.
Two large peaks and a plateau appear on CVAG obtained in presence of the organic additive TEBA (100 and 600 mg L 1—curves 3 and 4) added in the electrolyte containing 5 mg L 1 Ni2+ . The hydrogen peaks increase with cycle number because of the deposition of nickel on iron impurities and of the adsorption of TEBA on the cathode surface. It was found that the organic additive TEBA decreases the surface tension and reduces the time of attachment of hydrogen bubbles on the cathode surface thus facilitating their evolution (Bozhkov et al., 1990a,b; Ivanov, 1993; Wiart et al., 1990). With an increase in TEBA concentration, the areas of both peaks become larger. The anodic peaks are smaller because of the inhibition of the zinc deposition. It was found by Stefanov (1999) that an increase of the TEBA concentration resulted in an decreased zinc deposit. Fig. 2 shows the dependence q vs. t obtained during zinc deposition in the electrolyte, in an undivided cell. The amount of deposited zinc decreases two times and the current efficiency decreases from 72% to 24% in the presence of Ni2+ ions (curve 2) compared to the amount obtained in BE (curve 1). This is due to the zinc redissolution, which starts with the decrease in the electrode potential during the reverse scan because of the action of the numerous galvanic micropairs Zn – Ni
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Fig. 2. Dependence of the charge corresponding to the deposited Zn (in coulombs) on time. E = 1.600 V (SSE). Al—Riedel de Haen. No membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
(already formed at 1.600 V). This process is spontaneous and is not seen on the anodic curve. Because of the redissolution, the amount of the deposited zinc obtained using the cyclic voltammograms is smaller. When the organic additive TEBA is added to the electrolyte at a concentration of 100 mg L 1 (curve
3) or 600 mg L 1 (curve 4), the amount of the deposited zinc is larger and the current efficiency increases to 55% because of the inhibition of the zinc redissolution. The mechanism of the action of the TEBA inhibitor is shown in previous studies (Bozhkov et al., 1990a,b; Ivanov, 1993; Wiart et al., 1990). In spite of
Fig. 3. Voltammograms, recorded at a sweep rate of 180 mV min 1. Al—Riedel de Haen. Cell divided by a membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
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199
Fig. 4. Dependence of the charge corresponding to the deposited Zn on time. E = 1.600 V (SSE). Al—Riedel de Haen. Cell divided by a membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
the inhibition of zinc redissolution, a considerable part of the current is wasted in hydrogen evolution. Fig. 3 shows the CVAG obtained in a divided cell. In all cases, the initial potentials of zinc deposition are more negative and the current values are lower (Table 1) than those obtained without separation. This is due to the increased ohmic resistance of the electrolyte in the presence of a membrane. On the CVAG obtained in BE (curve 1), no current peaks are seen. On the CVAG obtained in electrolyte containing 5 mg L 1 Ni2+ ions (curve 2), two peaks are seen: on the direct scan at 1.420 V and on the reverse scan at 1.230 V. Both peaks appear at more negative potentials than the peaks obtained in an undivided cell. The anodic peak is smaller than the peak obtained in the BE because of the hydrogen evolution.
Two peaks (at 1.400 V on the direct scan and at 1.215 to 1.220 Von the reverse scan) are observed on the CVAG obtained in electrolytes containing Ni2+ ions and organic additive TEBA (100 mg L 1—curve 3 and 600 mg L 1—curve 4). Both peaks are smaller than the peaks on the CVAG obtained in the electrolyte containing only Ni2+ ions. The areas of the peaks do not depend on the TEBA concentration. The anodic peaks are smaller than the peak obtained in the BE (curve 1). The tendency to the reduction of the evolved hydrogen is confirmed by the dependence q vs. t (obtained with divided cell) shown in Fig. 4. The deposition current at 1.600 V is significantly lower than the current in the cases without separation. During the deposition in BE (curve 1) and in the
Table 3 Al—Johnson Matthey Electrolyte
Ed/V Without ‘‘Nafion’’ 423 membrane
Base electrolyte (BE) BE + 5 mg L 1 Ni2+ BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA
ic/mA cm With ‘‘Nafion’’ 423 membrane
2
CEZ/%
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
1.545 1.490 1.490
1.525 1.515 1.530
112 102 77
102 84 63
74 39 45
71 55 53
1.510
1.540
66
77
56
43
200
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Table 4 Al—Johnson Matthey Electrolyte
Base electrolyte (BE) BE + 5 mg L 1 Ni2+ BE + 5 mg L 1 Ni2+ + 100 mg L BE + 5 mg L 1 Ni2+ + 600 mg L
Without ‘‘Nafion’’ 423 membrane
With ‘‘Nafion’’ 423 membrane
I peak/V
II peak/V
III peak/V
I peak/V
II peak/V
III peak/V
–
– – – –
–
– – – –
– – – –
–
1.425 1.375
1
TEBA 1 TEBA
–
electrolyte, containing 5 mg L 1 Ni2+ (curve 2), the amount of deposited zinc is practically equal to the amount of zinc deposited in the electrolyte without separation (Fig. 2, curves 1 and 2) but the current efficiencies are higher at 78% and 50%. The addition of TEBA leads to the increase in the amounts of the deposited zinc and of the current efficiencies—to 64% (with 100 mg L 1—curve 3) and 53% (with 600 mg L 1—curve 4). The separation of the electrode compartments leads to an increased zinc current efficiency. The membrane prevents the anode products (H2O2, S2O82 ) passing into the cathode compartment. These oxidized products destabilize the electrodeposition in pure electrolytes (Cachet et al., 1994a) or promote the harmful influence of Ni2+ ions (Stefanov et al., 1996/1997). In the presence of manganese ions in the electrolyte, the process of zinc redissolution is promoted by MnO4 and
1.250 1.240 1.255
1.230 – –
MnO2 formed on the anode, which reach the cathode when the compartments are not separated. The current efficiency of zinc is lower (24%) than in the divided cell (50%). The organic additive TEBA inhibits the action of Ni2+ -ions and of the oxidized products and thereby stabilizes the galvanostatic deposition of zinc, resulting in an increased current efficiency. The inhibition effect is stronger when the electrode compartments are separated because of the absence of the oxidized products in the cathodic compartment. 4.2. Electrodeposition of zinc on pure aluminium cathode Figs. 5 and 7 show the cyclic voltammograms and Figs. 6 and 8 show the dependence q vs. t obtained during the zinc deposition on the cathode of specpure
Fig. 5. Voltammograms, recorded at a sweep rate of 180 mV min 1. Al—Johnson Matthey. No membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
Y. Stefanov, I. Ivanov / Hydrometallurgy 64 (2002) 193–203
201
Fig. 6. Dependence of the charge corresponding to the deposited Zn on time. E = 1.600 V (SSE). Al—Johnson Matthey. No membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
Al (Johnson Matthey). The values of the initial potentials of zinc deposition (Ed), deposition current density (ic) and current efficiency of zinc (CEZ) are presented in Table 3 and the potentials of the cathodic peaks on CVAG in Table 4. Fig. 5 shows the CVAG obtained without separation of the electrode compartments with a mem-
brane. In the CVAG obtained in BE (curve 1), no cathodic peaks are seen and the zinc deposition begins at the most negative potential ( 1.545 V). Nickel ions (5 mg L 1) added in the electrolyte depolarize the cathodic reaction to 1.490 V and the anodic peaks are considerably larger (curve 2). Two cathodic peaks are observed on the CVAG, a very small
Fig. 7. Voltammograms, recorded at a sweep rate of 180 mV min 1. Al—Johnson Matthey. Cell divided by a membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
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Y. Stefanov, I. Ivanov / Hydrometallurgy 64 (2002) 193–203
peak at 1.425 V, on the forward scan, and a larger peak at 1.250 V on the reverse scan, due to the hydrogen evolution. Both peaks increase with cycle number, because of the nickel adsorption on the cathode surface. Due to the absence of iron impurities on the aluminium surface, the amount of nickel is smaller. Both peaks are lower than the peaks obtained with Al (Riedel de Haen). Addition of 100 mg L 1 TEBA in the electrolyte containing 5 mg L 1 Ni2+ (curve 3) leads to the increase of both peaks. At a TEBA concentration of 600 mg L 1 (curve 4), both peaks also increase. The organic inhibitor polarizes the zinc deposition and the anodic peaks are smaller. Fig. 6 shows q vs. t for an undivided cell. The amount of zinc deposited in BE (curve 1) is large and the current efficiency is 74%. The addition of 5 mg L 1 Ni2+ (curve 2) and the TEBA inhibitor (100 mg L 1, curve 3 and 600 mg L 1, curve 4) decreases the zinc amount and the current efficiency to 39%, 45% and 56%, respectively. The current efficiency, in the presence of TEBA, increases due to the inhibition of zinc redissolution. Fig. 7 shows the CVAG obtained in the divided cell, which shows no cathodic peaks. The effect of separation during the deposition on the cathode of specpure aluminium is strongly shown, in comparison
with the deposition on the cathode of aluminium alloyed with iron impurities. Fig. 8 shows the q vs. t plot obtained in a divided cell. All curves are similar to those obtained without separation. The current efficiency in the presence of Ni2 + ions is higher (55%) than the case in the absence of a membrane (39%). The addition of 100 mg L 1 TEBA does not change the current efficiency (53%). With an increase in the TEBA concentration to 600 mg L 1, the current efficiency decreases to 45% as a result of the inhibition of the zinc deposition.
5. Conclusions The zinc electrodeposition in sulphuric acid electrolytes on a cathode of specpure or aluminium containing iron impurities takes place with a high current efficiency. Nickel ions reduce the amount of the deposited zinc and its current efficiency and this effect is stronger during the deposition on the cathode of aluminium containing iron impurities and when the electrode compartments are not separated by an ion exchange membrane. The separation of the cathode
Fig. 8. Dependence of the charge corresponding to the deposited Zn on time. E = 1.600 V (SSE). Al—Johnson Matthey. Cell divided by a membrane. 1—Base electrolyte (BE); 2—BE + 5 mg L 1 Ni2+ ; 3—BE + 5 mg L 1 Ni2+ + 100 mg L 1 TEBA; 4—BE + 5 mg L 1 Ni2+ + 600 mg L 1 TEBA.
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and anode compartments prevents the access of the oxidized anode products reaching the cathode and the current efficiency increases. The addition of triethylbenzylammonium chloride inhibits zinc redissolution in the presence of nickel ions and as a result, the current efficiency increases. This effect is stronger when the deposition takes place on the cathode of aluminium containing iron and when the electrode compartments are separated.
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