Hydrometallurgy 117–118 (2012) 13–17

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The effects of magnafloc and zinc on electrowinning of cadmium in sulfate solutions

A. Biswal, K. Padhy, C.K. Sarangi, B.C. Tripathy ⁎, I.N. Bhattacharya, T. SubbaiahInstitute of Minerals and Materials Technology, Council of Scientific and Industrial Research (CSIR) Bhubaneswar 751 013, India

⁎ Corresponding author. Tel.: +91 674 2582635; fax:E-mail address: bankimtripathy@gmail.com (B.C. Tri

0304-386X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.hydromet.2012.01.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 May 2011Received in revised form 6 January 2012Accepted 13 January 2012Available online 21 January 2012

Keywords:CadmiumElectrowinningCathodic current efficiencyOrganic additivePolarization

The effect of an organic additive magnafloc and/or zinc on the electrowinning of cadmium from acidicsulfate solutions are studied. It is observed that addition of magnafloc increases the current efficiencyand decreases the energy consumption both in the absence and presence of zinc in the solution. Theaddition of magnafloc increases the cathodic current efficiency of cadmium from 84.3% at 5 mg dm−3

to ~97% at 40 mg dm−3 and decreases the energy consumption by 133 kWht−1. Magnafloc when pre-sent in the solution polarizes the cathode causing the electroreduction of cadmium at more negativepotentials. The presence of either magnafloc or zinc or both affects the degree of crystallinity of theelectrodeposits, however, with magnafloc the degree of crystallinity is higher indicating that the de-posits are also more ductile. Scanning electron micrographs of cadmium deposits obtained in the pres-ence of magnafloc show that compact deposits are formed with an instantaneous nucleation andgrowth mechanism. It is evident that the presence of magnafloc decreases the number of grains and in-creases the sizes of the crystallites. However, the presence of zinc in the solution has opposite effect onthe morphology as compared to that of magnafloc.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Cadmium and its compounds are toxic. In spite of its high negativeimpact on the environment its production cannot be avoided due toits use in various industries such as electroplating, pigments, synthet-ic chemicals, metallurgical and electronics industries (Butterman andPlachy, 2002; Cheremisinoff, 1995; Llewellyn, 1989; Plachy, 2003;Safarzadeh et al., 2007). It has various technological applications innickel–cadmium and silver–cadmium storage batteries, functional al-loys and coatings (Lyakishev, 2000), and as control rods in nuclearpower plants (Cook, 1992). Direct production of cadmium metal hasincreased due to the development in its industrial applications(Butterman and Plachy, 2002).

Usually cadmium ores are not available for direct extraction ofcadmium. However most of the zinc ores contain cadmium and inmany zinc production plants cadmium is also produced as a byproduct(Safarzadeh et al., 2007). Due to the increase in the global demand forcadmium, treatment of secondary sources has also taken the lead rolein its extraction (Bartolozzi et al., 1995; Freitas and Rosalem, 2005 andYang, 2003). Cadmium electrowinning is possible from many non cya-nide baths including sulfate baths (Abd El-Halim et al., 1984a; Canaris,1981; Chi-Chung and Young, 1992; Dolati et al., 2005; Safarzadeh andMoradkhani, 2010). Safarzadeh and Moradkhani (2010) have studiedthe effect of zinc on cadmium electrowinning from sulfate solutions.

+91 674 2581637.pathy).

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They found that with increasing concentration of zinc in the solution CEas well as cathode purity decreases. Different organic additives are alsoeffective in forming bright, uniform and fine grained deposit (Abd El-Halim et al., 1984b; Chi-Chung and Young, 1992; Franklin et al., 1998;Narayanan, 1999, 2001). The presence of organic additives also increasethe current efficiency and deposition rate of cadmium and decrease theporosity of the deposits (Abd El-Halim et al., 1984b; Dolati et al., 2005;Franklin et al., 1998). It has also been reported in the literature(Kuznetsov et al., 2006) that the use of surfactants in the electrodeposi-tion of metals controls the quality and structure of the electrodeposits.Although a number research works on cadmium electrowinning havebeen reported in the literature, there are scopes for research to improveupon the existing database available as regards phase analyses, cathodicsurface morphologies and polarization behavior of the cathode.

Keeping this in view, the present work deals with electrowinning ofcadmium from sulfate solutions in the presence of zinc ion impurityand/or an organic additivemagnafloc. The effect of zinc and/ormagnaflocon the cathodic current efficiency, energy consumption, surface mor-phology, crystal orientations and polarization behavior of the cathodeduring electrodeposition process has been discussed. Polarization studieswere carried out using cyclic voltammetric (CV) techniques to investigatethe polarization behavior of the cathode during electrodeposition in thepresence and absence of the organic additive and zinc. X-ray diffractionstudies were carried out to investigate the changes in the crystal orienta-tions in the cadmium deposits. Scanning electronmicroscope (SEM) andenergy dispersive X-ray (EDX) analyses were carried out to view thesurface morphology and to know the elemental composition of thecadmium deposits.

Fig. 2. Change in cathodic current efficiency showing the effect of magnafloc with andwithout zinc in the solution.

14 A. Biswal et al. / Hydrometallurgy 117–118 (2012) 13–17

2. Experimental

A stock solution of cadmium was made from cadmium sulfate (AR,Merck Chem. Ltd., India) in doubly distilled water. Test solutions wereprepared taking the required amount of cadmium sulfate solutions fromthe above stock solution and sulphuric acid (AnalaR, BDH). All the electro-winning experiments were performed in an electrolytic cell containingaqueous solution of cadmium sulfate (Cd, 100 g dm−3) and sulphuricacid (80 g dm−3) at ambient temperature (30±2 °C) for 3 h at a cur-rent density of 200 A m−2. Calculated amounts of the organic additivemagnafloc-351 and zinc were added to the electrolytic bath in aliquotsfrom freshly prepared stock solutions.

For electrowinning and polarization studies the surface of the cath-odes were polished with 400 and 1200 grade silicon carbide paper andthen rinsed with 1 M HCl and washed with water. A lead sheet wasused as anode and an aluminum sheet was used as cathode during elec-trowinning. The distance between anode and cathode was kept at 3 cm.Constant current was supplied from a regulated power supplier[0–60 V, 2A, DC Power supply, Elnova Ltd., India]. Cell voltage wasrecorded through a multimeter connected to the cell. After electrolysisthe cathode was removed from the cell, washed thoroughly withwater, followed by acetone and then dried in an oven. The cathodic cur-rent efficiency was calculated from the weight gained by the cathode atthe end of the electrolysis.

Polarization studies were carried out in a three electrode cell usingEG&G PAR potentiostat model 273A. Platinum wires were used asworking and auxiliary electrodes and Standard Calomel Electrode(SCE) was used as the reference. All the potentials were measuredagainst SCE. Voltammetric scans were performed in the potentialrange −0.45 to −0.85 V vs SCE at a scan rate of 10 mV s−1. The sur-face morphology of the cadmium electrodeposits were characterizedusing Scanning electron microscope (SEM, Hitachi) and the presence ofthe impurities associated with the cadmium deposits were analyzed byenergy dispersive X-ray analysis (EDX). Changes in the crystal orienta-tions in the cadmium deposits were observed from the X-ray diffracto-grams obtained from X-ray Diffractometer (PAN ANALYTICAL PW 1830).

3. Results and discussion

3.1. Cathodic current efficiency and energy consumption

As shown in Fig. 1 the current efficiency (CE) of the cathode duringelectrowinning of cadmium from pure sulfate solution was 84.3%.

Fig. 1. Effect of zinc on cathodic current efficiency and energy consumption.

However when zinc was added to the above solution the CE decreasedwith increase in the concentrations of zinc ion resulting in a current ef-ficiency of 79% at a concentration of 10 g dm−3. Similar observationswere made by Safarzadeh and Moradkhani (2010), however the de-crease in CE in theirworkwas reported to be lesser even in the presenceof very high concentrations of zinc in the solution. It is also clear fromthe figure that the increase in zinc ion concentration in the cadmiumsulfate solution increased the energy consumption for cadmium elec-trowinning. It is interesting to note from Fig. 2 that the addition of mag-nafloc increased the CE both in the absence and presence of zinc ion inthe electrolytic solution. The addition of 5 mg dm−3 of magnafloc didnot have any effect on the CE, however further increase in its concentra-tion increased the current efficiency from84.8% at 10 mg dm−3 to ~97%at 40 mg dm−3. Increasing the concentration of magnafloc beyond thisresulted in a decrease in CE (Fig. 2). It can be noted from Fig. 3 that anincrease in the concentration of magnafloc in the electrolytic solutiondecreases the energy consumption during cadmium electrowinning. Itis also clear that increase in the concentration of magnafloc in thepresence of 10 g dm−3 of zinc in the cell solution increased the CEfrom ~82% at 5 mg dm−3 to ~88% at 50 mg dm−3 magnafloc (Fig. 2),which was also reflected in the decreases in energy consumption(Fig. 3). Thus it is pertinent to mention here that magnafloc is

Fig. 3. Change in energy consumption showing the effect of magnafloc with and withoutzinc in the solution.

Fig. 4. Cyclic voltammograms during cadmium electrowinning from sulfate solutions,(1) Blank (–) (2) MF — 40 mg dm−3 (—).

Fig. 6. Linear sweep cathodic polarization during cadmium electrowinning from sulfatesolutions, (1) Blank (2) Zn — 10 g dm−3 (3) [2]+MF−40 mg dm−3.

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responsible for reducing the negative impact of zinc on cadmium elec-trowinning. This was also confirmed by analyzing the morphologicalchanges seen with the cadmium deposits as discussed later.

3.2. Polarization studies

Figs. 4–6 show the current-potential profile of the cathode duringelectroreduction of cadmium from sulfate solutions in the presenceand absence of either magnafloc, zinc or both. The voltammetric curves(Fig. 4) showed typical reduction and oxidation peaks indicating elec-troreduction of cadmium ion and dissolution of the deposited mass.The sharp deposition and dissolution peaks indicated that the wholeprocess was associated with reduction of bivalent cadmium ion tometallic cadmium via a one step two electron transfer process. Similarobservations were also reported in the literature (Abd El-Halim et al.,1984a; Dolati et al., 2005; Narayanan, 1999). It is also clear from the fig-ure that magnafloc polarizes the cathode which is also reflected in thedrop off of both reduction and oxidation currents.

Figs. 5 and 6 show the linear sweep cathodic polarization curveswhich have been derived from their respective cyclic voltammetricscans. It can be clearly seen from Fig. 5 that magnafloc when presentin the solution polarized the cathode causing the electroreduction ofcadmium at more negative potentials which is also reflected in the

Fig. 5. Linear sweep cathodic polarization during cadmium electrowinning from sulfatesolutions, (1) Blank (2) MF — 10 mg dm−3 (3) MF — 40 mg dm−3.

reduction of the cathodic currents. However, the presence of zinc inthe solution had very little impact on the polarization behavior ofthe cathode showing depolarization (Fig. 6). It is also relevant tomention here that when both zinc and magnafloc were present inthe electrolytic solution, it is the magnafloc who is responsible forthe polarization of the cathode towards more negative potentials,which was also reflected in the current–voltage profile with de-creases in cathodic and anodic currents. This indicated that the de-posit pattern and morphology of the cadmium deposits should beaffected more in the presence of magnafloc than zinc. It was alsoclear from all the voltammograms that though the nucleation poten-tial of cadmium electrodeposition was most affected in the presenceof magnafloc, the crossover potential remained constant in all the vol-tammograms and was independent of the concentration of zinc, mag-nafloc or both in the solution.

3.3. Crystal orientations and surface morphology

The presence of impurities and/or organic additives not only af-fected the cathodic current efficiency and energy consumption ofthe electrowinning process, but also affected the deposit morphologywhich contributed to the differences in the current efficiencies andenergy consumption values.

It is interesting to note from the X-ray diffractograms that thepresence of either magnafloc or zinc or both, affected the degree ofcrystallinity of the electrodeposits (Fig. 7). However, with magnaflocthe degree of crystallinity was more when compared with zinc. It is

0 10 20 30 40 50

d

c

b

a

Inte

nsity

, arb

. uni

t

Angle (2θ), degrees

a - no additiveb - only zincc - both zinc and magnaflocd - only magnafloc

Fig. 7. X-ray diffractogram of the cadmium deposit obtained from cadmium sulfatesolutions in the presence of magnafloc and zinc.

16 A. Biswal et al. / Hydrometallurgy 117–118 (2012) 13–17

also interesting to note that in the presence of magnafloc, the interpla-nar spacing (d) decreased due to the increase in theta (Ө) values indi-cating the deposit became more ductile in the presence of magnaflocdue to close packing of the planes. In the absence of magnafloc or zincthe cadmiumdeposits obtained had the order of preferred crystal orien-tation (110) (100) (004) (114) (104). The addition of magnafloc or zincinto the electrolytic solution changed the order of preferred orienta-tions to either (002) (004) (103) (101) (205) or (002) (101) (104)(103) (205). However when both magnafloc and zinc were added tothe solution the cadmium deposit obtained had the order (101) (112)(110) (201) (103). It can be clearly seen that when either magnaflocor zinc were present in the electrolytic solution, higher degree of crys-tallinity was observed and in both the cases (002) plane was the mostpreferred one.

The crystallization process of cadmium electrowinning begins withthe growth of nuclei. The rate and direction of growth of the nuclei af-fects the morphology of the cadmium deposits. In order to confirmthe changes observed in the cadmium electrodeposits in the presence

Fig. 8. Typical SE micrographs of the cadmium electrodeposits. (a) CdSO4, 100 g dm−3+H2SO4, 80 g dm−3. (b) [a]+40mg dm−3 Magnafloc. (c) [a]+40mg dm−3 Magnafloc.

and absence of magnafloc, morphological investigations were madeby SEM (Figs. 8–9). It was observed that the deposit obtained fromaddition-free cadmium solution was not compact and the crystalliteswere of different dimensions showing an acicular growth (Fig. 8a).Whereas in the presence of magnafloc a compact crystalline depositwas obtained (Fig. 8b) indicating an instantaneous nucleation andgrowthmechanism. The hexagonal nature of the crystallites were clear-ly seen when SEM was carried out at lower magnification (Fig. 8c). Itwas also evident from the micrographs (Fig. 8b) that in the presenceof magnafloc, the number of grains decreased and size of the grains in-creased. This is due to the fact that in the presence of magnafloc the nu-cleation rate had decreased and growth had taken place in preferentialdirections resulting in larger nuclei.

It has also been observed that in the presence of zinc the depositobtained was also not compact showing acicular growth (Fig. 9a) andthe size of the crystallites decreased further when compared with thedeposit obtained from addition-free solution indicating an increasednucleation rate with slower growth. However when magnafloc wasadded along with zinc a very compact deposit was obtained (Fig. 9b).EDX analyses indicated that the cadmium deposits mostly contain cad-mium and traces of zinc.

4. Conclusions

• Addition of magnafloc increases the CE and decreases the energyconsumption both in the absence and presence of zinc ion impurity.

• Addition of magnafloc increases the cathodic current efficiency ofcadmium from 84.3% at 5 mg dm−3 to ~97% at 40 mg dm−3 anddecreases the energy consumption by 133 kWht−1.

• Magnafloc when present in the solution polarizes the cathode caus-ing the electroreduction of cadmium at more negative potentials.

Fig. 9. Typical SE micrographs of the cadmium electrodeposits. (a) CdSO4, 100 g dm−3+H2SO4, 80 g dm−3+ZnSO4−10 g dm−3. (b) [a]+40 mg dm−3 Magnafloc.

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• The presence of either magnafloc or zinc or both increases the de-gree of crystallinity and hence ductility of the electrodeposits. How-ever, the degree of crystallinity is more with magnafloc than zinc.

• The cadmium deposits show acicular growth when deposited fromaddition free solution or from solutions containing zinc only. Howeveraddition of magnafloc to these solutions resulted in more compactdeposits.

Acknowledgements

The authors wish to thank Professor B.K. Mishra, Director, IMMTfor his support and encouragement to publish this work.

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