J. Cent. South Univ. (2012) 19: 2451-2457 

DOI: 10.1007/s11771-012-1296-7

Stability of [BMIM]HSO₄ for using as additive during zinc electrowinning from acidic sulfate solution

ZHANG Qi-bo(张启波), HUA Yi-xin(华一新)

Key Laboratory of Ionic Liquids Metallurgy (Faculty of Metallurgical and Energy Engineering,

Kunming University of Science and Technology), Kunming 650093, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: The stability of ionic liquid additive 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO₄) during zinc electrowinning from acidic sulfate solution was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and scanning electron microscopy. Compared with the traditional industrial additives, gelatine and gum arabic, [BMIM]HSO₄ has more excellent chemical and thermal stabilities. The inhibition effects of gelatine and gum arabic on the zinc electrocrystallization are observed to markedly weaken due to their part degradation after 12 h longtime successive electrolysis and high temperature (90 ℃) treatments. In contrast, the activity of [BMIM]HSO₄ is practically unaffected after 24 h longtime successive electrolysis and high temperature treatments. These results are corroborated with the corresponding morphological analysis of the cathodic deposits.

Key words: zinc electrowinning; ionic liquid additive; stability; electrochemical measurement; morphology

1 Introduction

Additives are widely used in electrodeposition of zinc and its alloys due to their special functions in the deposition process [1-5]. These additives are found to affect both the deposition and crystal-building processes through their adsorbates at the electrode surface [6]. Appropriate addition is necessary for the formation of fine-grained, smooth, compact deposits [7]. Although traditional colloidal additives such as glue, gelatine and gum arabic [8-10], and some organic [11-16] additives have been widely used in industrial production and achieved good effect, they are readily decomposed and environmental unfriendly for their shortcomings, such as bad chemical and thermal stability and high toxicity. Therefore, the search for better additives with good stability, high efficiency and being environmentally benign is continuing.

Ionic liquids (ILs) are organic salts that are liquids at ambient temperature and comprised entirely of organic cations and organic/inorganic anions. Due to the unique structure characteristics, ILs have many attractive properties and attract a great deal of interest in various fields [17-23]. Some of the most important prosperities of ILs are their thermal stability and avirulence, which make them potential additives for metal electrodeposition. For hydrometallurgical metal electrodeposition, additives will be consumed by incorporation into the deposit, by acid-catalyzed hydrolysis, and by adsorption onto the slimes during the successive electrodeposition process. Therefore, it must be replenished on a continuous basis to assure an optimum instantaneous concentration. Since some important information such as the chemical activity of additives in the electrolyte and the theoretic guide for the replenishment operation can be obtained via their stability studies, it is worthwhile to conduct intensive researches on the stability of additives. However, to our knowledge, scant information is available on the stability of additives during the electrodeposition process. FABRICIUS and SUNDHOLM [24] studied the effects of NaCl and some organic agents on the electrodeposition of copper in 0.71 mol/L CuSO₄ and 1.80 mol/L H₂SO₄ solutions by using the impedance technique. The results showed that the degradation of glue by hydrolysis can be followed using impedance measurements. No significant effects of thiourea and avitone could be detected in the concentration and potential range used in electrorefining.

In our pervious studies, alkylimidazoluim ionic liquids are observed to be excellent leveling agents in zinc electrodeposition [25-26]. The addition of [BMIM]HSO₄ is observed to increase current efficiency and reduce specific electric energy consumption for zinc electrodeposition. It is demonstrated that this additive has a pronounced inhibiting effect on Zn²⁺ electroreduction and leads to more leveled and fine-grained cathodic deposits. However, further study needs to be done under normal industrial conditions such as longtime successive electrolysis to establish its chemical activity. The aim of the present work is to investigate the chemical and thermal stabilities of [BMIM]HSO₄ during zinc electrodeposition from acidic sulfate solution. The results are compared with those exerted by gelatine and gum arabic, which are most commonly used as leveling and brightening agents in industrial zinc electrodeposition process. The investigation is performed using cyclic voltammetry, electrochemical impedance spectroscopy techniques, and scanning electron microscopy (SEM) method.

2 Experimental

2.1 Reagents

The zinc electrolyte concentration of 55 g/L zinc and 150 g/L sulfuric acid was prepared by ZnSO₄·7H₂O and H₂SO₄ and the specific experimental procedures were similar as described previously [25, 27]. The ionic liquid additive 1-butyl-3-methylimidazolium hydrogen sulfate, [BMIM]HSO₄, was synthesized in laboratory and the specific synthetic methods were mentioned previously [28]. All the reagents used were of AR grade.

2.2 Electrolysis

Small-scale galvanostatic electrolysis was performed in a rectangular flow cell containing 500 mL electrolyte with dimension of 20 cm×10 cm×8 cm made of plexiglass by chronopotentiometric measurements. The flow rate of the electrolyte with different additives was maintained at 1.2 L/h during the deposition process. A pure vertical planar aluminum (>99.95%) sheet and two parallel lead-silver-calcium-strontium alloy (Ag 0.2%, Ca and Sr 0.1%-0.13%) plates with effective area of 12 cm² were used as the cathode and anode, respectively. The interelectrode distance was 3.0 cm. Zinc was deposited on both sides of the cathode onto a total area of 10 cm². All electrolysis experiments were run in a constant temperature bath at (40±1) ℃. In all cases, the current density was held constant at 400 A/m² during the deposition time. For each experiment, a freshly prepared solution was used and the solution temperature was thermostatically controlled at a desired value.

2.3 Electrochemical measurements

Electrochemical studies were based on the analysis of cyclic voltammetry and electrochemical impedance spectroscopy (EIS). All the electrochemical measurements were conducted by using an electrochemical work station (GAMRY USA, PCl4/300) and carried out at 40 ℃ under atmospheric condition. A conventional three-electrode electrochemical cell was used for these experiments. An aluminum disk electrode (d 4 mm, 99.995%) inserted in a Teflon tube with exposed surface of 0.125 6 cm² was used as working electrode. A platinum  wire (d 1 mm, 99.995%) and a saturated calomel electrode (SCE) mounted inside a Luggin capillary were used as the counter and reference electrodes, respectively. Prior to each experiment, the electrolyte was deoxygenated by bubbling ultrapure argon for at least 10 min. The working electrode was ground with 1 200 grit silicon carbide paper and polished using 0.5 μm high-purity alumina, and then degreased with anhydrous alcohol in an ultrasonic bath for 1 min, washed with double distilled water and finally dried.

Cyclic voltametric experiments were carried out at a constant scan rate of 10 mV/s, from the initial potential of -0.70 V to the final potential of -1.25 V. A typical voltammogram obtained for zinc deposition from an additive-free electrolyte is shown in Fig. 1. The voltammograms are initiated at point A (-0.70 V), scanned in the negative direction and reversed at point C (-1.25 V) in the positive direction. The cathodic current increases sharply once zinc electroreduction begins at point B (about -1.16 V), followed by a current loop as the direction of sweeping is reversed. This reversed scanning results in a decrease in current which subsequently reaches zero at the crossover potential, point D (about -1.05 V) and then the current becomes anodic corresponding to the dissolution of the deposited zinc previously formed. The anodic peak is reached at point E and dissolution is completed on return to point A. The region BCD is called a nucleation hysteresis loop [29]. The current for the initial deposition of zinc (point B) does not become appreciable until well beyond the electroreduction potential of zinc. The potential difference between the electroreduction potential of zinc ion at point B and the crossover potential at point D has been used to define a nucleation overpotential (NOP). The NOP is regarded as an indicator of the extent of polarization of a cathode and has been found to be sensitive to the presence of certain organic additives [11-16].

Fig. 1 Cyclic voltammogram for zinc deposition at aluminum electrode from acidic zinc sulfate solution

EIS technique was used to gain information about the thermal stability of additives. The electrolytes in the absence and presence of 5 mg/L different additives were treated by heating at 90 ℃ for 1 h, and then cooled down to 40 ℃ for tests. In the EIS measurements, a zinc   disk electrode (d 4 mm, 99.995%) inserted in a Teflon tube with exposed surface of 0.125 6 cm² was used as working electrode and the working electrode potential was held at open circuit potential where the natural corrosion of zinc occurred, over a frequency range of 100 kHz-10 mHz with a signal amplitude perturbation of 5 mV.

2.4 Deposit examination

After electrolysis, the deposits were removed from the cell and washed thoroughly with distilled water and dried. A microscope (Tescan Czech, VEGA II XMH) was used to examine the surface morphology.

3 Results and discussion

3.1 Effect of electrolysis time

The cyclic voltammograms recorded for zinc electrodeposition in the absence and presence of 5 mg/L different additives after longtime successive electrolysis are shown in Fig. 2. Similar to the observations in our previous study [25-26], the addition of additives shifts the cathodic potential corresponding to the Zn²⁺ reduction toward more negative value (increased NOP value), along with the reduction of cathodic process area, denoting an inhibition of the electrocrystallization. This is generally attributed to the surface coverage of the cathode by a strongly adsorbed additive layer, which increases the interfacial viscosity, decreases the mass transfer, and slows down the deposition rate. Therefore, additional energy is required to discharge the metal ions, and consequently, the deposition overpotential increases. Additionally, this inhibition effect of these additives is found to weaken along with an increase in the electrolysis time. As listed in Table 1, in the cases of colloidal additives, their effect on the NOP is observed to weaken markedly after longtime successive electrolysis, indicating that the consumption of these additives during the deposition process takes place. This consumption could be mainly attributed to the degradation by acid-catalyzed hydrolysis due to their poor chemical stability, and also their incorporation into the deposit and adsorption onto the slimes. In contrast, the inhibition action with the addition of [BMIM]HSO₄ after 24 h successive electrolysis slightly decreases compared with the colloidal additives. This could be attributed to its high ionic conductivity and stability, and a wide electrochemical potential window, which ensure it to maintain excellent activity during the deposition process. The pathway for its consumption should be the entrainment into the deposit and trace adsorption on the slimes during the longtime successive deposition process. Therefore, it can be concluded that [BMIM]HSO₄ is of more excellent chemical stability in comparison to those traditional colloidal additives.

Fig. 2 Cyclic voltammograms for zinc deposition from acidic zinc sulfate solutions in absence and presence of different additives after 12 h electrodeposition

Table 1 Effects of different additives on NOP after longtime successive electrolysis

The SEM micrographs of zinc deposits obtained by small-scale electrolysis from sulfate electrolyte in the absence and presence of 5 mg/L different additives after 12 h successive electrolysis are given in Fig. 3. The deposit obtained from the additive-free solution consists of relatively large, coarse grains and grows in the form of pyramidal hexagonal plates with base crystal size of   30 μm (Fig. 3(a)) as typical for zinc electrodeposits obtained for 2 h electrolysis [25-26]. Introduction of   5 mg/L [BMIM]HSO₄ reduces the size of platelets (10 μm) and produces more leveled and fine-grained cathodic deposits (Fig. 3(b)). The decrease in crystallites size is expected to account for the blocking effect of the additive through its cathodic adsorption on the electrode surface that causes a reduction in the growth rate of nuclei. A similar effect can be observed with the addition of gelatine and gum arabic where the deposits obtained are more compact and leveled than the case of additive-free solution (Figs. 3(c) and (d)). However, it is clear that the crystallites are much bigger and their leveling performance is observably weakened in comparison to the case of 2 h electrolysis in our pervious studies [25-26], proving the part consumption of these colloidal additives as discussed above.

Fig. 3 SEM micrographs of zinc deposits after 12 h electrolysis in absence and presence of different additives: (a) Blank; (b) 5 mg/L [BMIM]HSO₄; (c) 5 mg/L gelatine; (d) 5 mg/L gum arabic

3.2 Effect of high temperature treatment

It is generally accepted that the stability of additives is influenced by many factors, such as pH, time of electrolysis and temperature. In order to investigate the thermal stability of these additives in more details, electrochemical impedance spectroscopy technique, as well as morphological investigation, was utilized.

The impedance plots obtained in the absence and presence of 5 mg/L different additives are depicted in  Fig. 4. As can be seen from Fig. 4(a), the impedance spectra consist of a large capacitive loop at high frequencies followed by a small inductive one at medium frequencies and a Warburg impedance at low frequencies. The high frequency capacitive loop is usually attributed to the charge transfer across the double layer        for corrosion reaction, and the inductive loop may be related to stabilization of layer by adsorbed intermediate products of the dissolution reaction on the electrode surface involving additives molecules as well as reactive products [30]. It is worthy noting that the similar profile of the Nyquist plots observed in the absence and presence of the additives indicate that the addition of these additives does not change the mechanism for the dissolution of zinc in the acidic solutions.

Fig. 4 Impedance plots for zinc in electrolyte containing 5 mg/L different additives: (a) 40 ℃, (b) 40 ℃ (after heating at 90 ℃ for 1 h)

The depressed form of the higher frequency loop reflects the surface inhomogeneity of structural or interfacial origin, such as those found in adsorption processes [31]. The charge transfer resistance (R_ct) is estimated from the diameter of the high-frequency capacitive loop and the corresponding values obtained under different addition conditions are presented in Table 2. As can be observed from Table 2, the resistance value is found to increase in the presence of additives, suggesting that these additive molecules are adsorbed on the cathode/solution interface where the adsorbed molecules partly hinder the active sites of the corrodent. The R_ct obtained with the addition of these additives at a given concentration follows the sequence: [BMIM]HSO₄< gelatine

The complex-plane impedance diagrams obtained from the solution containing 5 mg/L different additives after high temperature treatments are illustrated in    Fig. 4(b). It is observed that the resistance values obtained from colloidal additive-containing solution decrease after high temperature treatment (Table 2). This is based on the fact that the colloidal additives are hydrolysed when electrolyte is heated and thus partly degraded. In contrast, the corrosion inhibition action with the addition of [BMIM]HSO₄ after treatment at 90 ℃ for 1 h is practically unchanged, and the value order of R_ct obtained in the presence of different additives at a given concentration is changed to [BMIM]HSO₄>gum arabic> gelatine. This result reveals that [BMIM]HSO₄ is of more excellent thermal stability in comparison to those traditional colloidal additives.

Table 2 Effects of different additives on charge transfer resistance after high temperature treatment

The typical SEM photomicrographs of zinc deposits after high temperature treatment obtained by 2 h small-scale electrolysis from the electrolyte in the absence and presence of 5 mg/L different additives are shown in Fig. 5. In the case of the additive-free electrolyte, the deposit obtained is relatively rough and in the form of hemispherical hexagonal plates with crystal sizes of 30 μm (Fig. 5(a)) similar to the zinc electrodeposits without high temperature treatment. The activity of [BMIM]HSO₄ is found to be unaffected after the high temperature treatment, as its addition reduces the platelet sizes (6 μm) and gives smooth and compact deposit (Fig. 5(b)). However, with the additions of gelatine and gum arabic, the deposits obtained are quite different from the case of [BMIM]HSO₄. The cathodic deposits mainly consist of two kinds of crystallites with bigger crystallites surrounded by smaller crystallites. These small crystallites are attributed to the adsorption of these additives on the electrode surface, which alters the double-layer structure and decreases the nucleation and growth rate of the nuclei [29, 32], and these big crystallites could be related to the part degradation of these colloidal additives due to hydrolysis. It is also noteworthy that under the same condition, the degradation of gum arabic is found to be more obvious and the relatively larger crystallites obtained indicate that the gum arabic is more readily to hydrolysis. This result is consistent with the results obtained by electrochemical impedance spectroscopy measurements mentioned above.

Fig. 5 SEM micrographs of zinc deposits obtained after high temperature treatment with different additives: (a) Blank; (b) 5 mg/L [BMIM]HSO₄; (c) 5 mg/L gelatine; (d) 5 mg/L gum arabic

4 Conclusions

1) [BMIM]HSO₄ has a more excellent chemical stability in comparison to traditional colloidal additives, such as, gelatine and gum arabic.

2) The decrease in the charge transfer resistance in the presence of gelatine and gum arabic after high temperature treatment is due to their part degradation by hydrolysis. In contrast, [BMIM]HSO₄ shows more excellent thermal stability with the resistance practically unchanged.

3) The morphology of cathodic deposits obtained proves that the activity of [BMIM]HSO₄ is scarcely affected by longtime successive electrolysis and high temperature treatment. However, in both cases, the additions of gelatine and gum arabic are found to be partly degraded and produce deposits with larger grain size and inhomogeneous distribution of crystallites.

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(Edited by YANG Bing)

Foundation item: Project(2011FA009) supported by the Natural Science Foundation of Yunnan Province, China; Project(2011FZ020) supported by the Application Foundation Research of Yunnan Province, China

Received date: 2011-08-22; Accepted date: 2011-11-01

Corresponding author: ZHANG Qi-Bo, PhD; Tel: +86-871-5162008; E-mail: qibozhang@yahoo.com.cn