Influence of flow rates on the electrogenerative Co2+ recovery at a reticulated vitreous carbon cathode

Chemical Engineering Journal 189–190 (2012) 182–187

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Influence of flow rates on the electrogenerative Co2+ recovery at a reticulated vitreous carbon cathode W.X. Tan a , M.A. Hasnat a,b , Nurul Hanis Mohd Ramalan a , W.M. Soh a , Norita Mohamed a,∗ a b

School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Department of Chemistry, Graduate School of Sciences, Shahjalal University of Science and Technology, Sylhet-3114, Bangladesh

a r t i c l e

i n f o

Article history: Received 10 October 2011 Received in revised form 13 February 2012 Accepted 18 February 2012 Keywords: Electrogenerative Cobalt recovery Reticulated vitreous carbon Flow-by reactor

a b s t r a c t Reticulated vitreous carbon (RVC) was used as the cathode material in order to recover Co(II) ions. When Zn/ZnSO4 was coupled with Co(II)/RVC, a free energy of −274 ± 2 kJ mol−1 was generated, which in turn recovered Co(II) at the RVC surface spontaneously achieving a current efficiency of 82% when the initial Co(II) was 200 mg L−1 . Studies of Co(II) reduction on 100 ppi RVC were accomplished using 500 mg L−1 Co(II) concentration at pH 4.00 and with electrolyte flow rates of 50, 100, 150 and 200 mL min−1 . It was shown that poor removal of <75% cobalt was achieved at all flow rates after 8 h of operation. However, without pH adjustment (initial pH 5.44 ± 0.10) using similar test solutions, a shorter time of 7 h was required to remove more than 99% of cobalt at all flow rates. Therefore, this pH condition was selected for evaluating the performance of the system with different cathode porosities. Mass transport characteristics of the system using cathodes of different porosity were also evaluated. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Cobalt is prevalent and widely used in many industries. Today, most of the world’s cobalt is used predominantly to produce superalloys (Ni/Co/Fe), batteries, hard materials (carbines), magnets, catalysts and so on. From 1995 to 2005, there has been a significant shift in worldwide cobalt demand patterns in terms of end-use products [1]. A high demand of cobalt in rechargeable batteries (lithium-ion batteries) has been observed in recent years mainly due to the extensive use of such batteries as electrochemical power sources in mobile phones, personal computers and other portable electronic devices [2,3]. Cast cobalt-base superalloys are deployed in military and commercial aircraft turbine engines for vanes and other high temperature structural components [4]. Pure cobalt does not exist naturally in a metallic state. Cobalt ores are mainly found as mixed sulfides, either copper or nickel oxide/sulfide mixtures and occasionally cobalt arsenides. Small amounts of it are generally extracted as by-products in the mining and processing of nickel and copper ores. It was reported that considerable amounts of cobalt may also be found in silver, gold, lead and zinc ores; however, their processing did not always lead to cobalt recovery [5,6]. Due to the scarce production of cobalt, it being

∗ Corresponding author. Tel.: +604 6534049/6533576; fax: +604 6574854. E-mail addresses: [email protected], [email protected] (N. Mohamed). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.02.054

least abundant compared to nickel and copper and the high consumption of cobalt in industries, recycling or treatment of cobalt from industrial wastes is deemed important. Recently, several reports have been published regarding hydrometallurgical recovery of cobalt from aqueous solutions [7–11]. With respect to metal recovery, high values of the mass transport coefficient (km ) and large specific electrode area (Ae ) are particularly important to achieve a high removal rate or high conversion of reactant to product. Despite the use of three-dimensional (3D) porous electrodes, convective flow of electrolyte through the electrode surface has been reported as an alternative way to improve mass transport values [12,13]. These features (3D porous electrode and electrolyte flow rate) were incorporated herein to investigate their effects on cobalt removal using an electrogenerative flow-by batch-recycle system. Reticulated vitreous carbon (RVC) has been extensively used over the past decades for studies on metal ion removal [12,14–18]. It is selected as a cathode material in this study due to its properties of exceptional large void volume, large surface area, high porosity, chemical inertness, resistance to very high temperature, and good electrical conductivity [19]. Moreover, its 3D structure makes it a more superior electrode compared to a two-dimensional electrode as the electrode is distributed throughout all three dimensions in contrast to a planar configuration of a two-dimensional electrode. This arrangement can counteract the limitations of low specific surface areas and low space-time yield [20]. Electrolytic methods are mostly employed in industries such as in metal refining and recycling or water treatment. This method

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Fig. 1. Experimental set-up and hydraulic flow circuit of flow-by reactor with a batch-recycle system.

requires the supply of energy to drive the desired chemical processes, but electrolytic processes have limitations when dealing with low metal ion concentrations. In dilute solutions, the current efficiency (CE) of the system is very low due to the side reactions occurring in the system; thus, the power consumption in treating dilute solutions is relatively high and not cost effective [21]. Unfortunately, in wastewater treatment and hydrometallurgical processes, we are typically dealing with dilute metal-bearing solutions ranging from 1 to 1000 mg L−1 [21]. A more attractive alternative electrochemical process would be an electrogenerative process which does not require an external supply of energy. The driving mechanisms of electrogenerative systems are spontaneous electrochemical reactions that occur in galvanic systems. The galvanic system offers a good approach in removal and recovery of metals in wastewater. In electrolytic cells, the selectivity of the system depends on the cathode potential applied to the electrode. Meanwhile, the electrogenerative processes can be made selective for particular reactions by the choice of electrodes used by the system. This paper reports, the recovery of Co2+ from synthetic solutions at a RVC cathode. Here, the RVC cathode has been coupled with a sacrificial Zn anode so that an electroregenerative system is attained. The efficiency of Co2+ recovery has been explained on the basis of thermodynamic and kinetic points of view. 2. Experimental 2.1. Electrogenerative removal of cobalt using a flow-by batch-recycle system Fig. 1 shows the experiment set-up and hydraulic flow circuit of a flow-by batch-recycle system. The system consists of a flowby reactor with two 150 mL reservoirs for the catholyte and anolyte solutions, a Masterflex peristaltic pump (Cole Parmer) and two sets of flowmeters for controlling the catholyte and anolyte flow rates ranging from 50 to 200 mL min−1 . The flow cell has two electrolyte compartments each of dimensions 5 cm × 2 cm × 1 cm separated by an anion exchange membrane Neosepta® AM-01 (Tokuyama Corp.). Reticulated vitreous carbon (RVC) was used as cathode material with dimensions 2 cm × 5 cm × 0.7 cm. The different RVC porosities studied were 60, 80 (Electrolytica Inc.) and 100 (The Electrosynthesis Co.) pores per inch (ppi). The anode was zinc foil (>99% purity, R & M chemicals) with dimensions 2 cm × 5 cm × 0.1 cm. I-C

wafer dicing mounting tape (Pokico Packaging System) was placed between the compartments as a gasket to prevent leakage. Two copper sheets were used as current collectors. The electrodes were attached to the copper current collector sheets with conductive carbon adhesive (Leit-C). The cell components were sandwiched together with six bolts and nuts. The electrolyte solutions were pumped into the cell through the inlet opening at the bottom by a peristaltic pump and flowed out of the cell from the outlet at the top of the cell. In this system, the anolyte and catholyte flowed separately in a closed circuit through the anolyte and catholyte compartments and returned to their respective reservoirs. The circuit was completed when the current collectors were linked up with the Sanwa Digital Multimeter CD800a by external conducting wires. All the electrolytes were prepared in distilled and deionized water using analytical grade reagents (from System® ChemAR® unless otherwise stated). The catholytes of 100 and 500 mg L−1 initial cobalt concentrations were prepared in 0.2 M sodium sulfate and 0.1 M boric acid. The anolyte used was 0.1 M sodium sulfate. The influence of maintaining pH at 4.0 and no pH adjustment on cobalt removal was investigated. Either dilute sulfuric acid or sodium hydroxide solution was used to adjust the catholyte pH to the desired value. Further work was explored to investigate the effect of cathode porosity on cobalt removal. Before each experiment, nitrogen gas was used to purge out dissolved oxygen in both compartments for 15 min and a flux of nitrogen was maintained above the electrolyte surface throughout the experiment to ensure an inert atmosphere. Magnetic bars were used respectively in the catholyte and anolyte compartments (in each experiment) for agitation. All the experiments were carried out at room temperature (∼25 ◦ C). As a test of reproducibility, a particular experimental run was performed three times.

2.2. Analysis At predetermined intervals, the catholyte was analyzed for Co(II) concentration in order to monitor the performance of the system in terms of percentage of cobalt recovered by using PerkinElmer AAnalyst 200 atomic absorption spectrometer at a wavelength of 240.73 nm. The crystallographic patterns of the cobalt deposits were characterized by X-ray diffraction (XRD) using a Siemens D5000 X-ray diffractometer.

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Fig. 2. Concentration profiles of Zn(II) and Co(II) in a batch reactor. Anode: Zn plate in 100 ml 200 mg L−1 Na2 SO4 , Cathode: 80 ppi RVC in 100 mL 200 mg L−1 CoSO4 . pH 4.00.

3. Results and discussion 3.1. Spontaneity of the process Fig. 2 shows the decay of Co2+ concentration in the catholyte and the rise of Zn2+ concentration in the anolyte in a batch reactor. As soon as the Zn anode is coupled with the RVC cathode, Zn2+ ions are liberated from the anode and Co2+ is deposited on the RVC cathode surface attaining an electrogenerative process. The state of spontaneity of the redox process may be explained by the free energy change (G◦ ) of the process. It is well known that the oxidation potential of Zn in 1.0 M ZnSO4 is 0.763 V. According to our experiments, the reduction potential of Co2+ on the RVC surface was determined as 0.658 V. Therefore, if the electrode processes are coupled as per the following reactions, Anode : Cathode : Overall :

Zn  Zn2+ + 2e−

E ◦ = 0.763 V

Co2+ /RVC + 2e−  Co/RVC Zn + Co2+  Zn2+ + Co

E ◦ = 0.658 V

E cell ◦ = 1.42 V

(1) (2) (3)

a cell potential (Ecell ◦ ) of 1.421 V was generated at room temperature. This potential supports the free energy change (G◦ = −nEcell ◦ F) of −274 ± 2 kJ mol−1 for this process. Consequently, a large negative free energy suggests that the cobalt ions were recovered at RVC cathode by means of an electrogenerative process. Fig. 2 provides an idea about current efficiency (CE) regarding the electrogenerative cobalt process at the RVC cathode on the assumption that the total zinc dissolution (Zn moles) is proportional to the total charge passed through the circuit, meanwhile, total cobalt deposit (Co moles) is proportional to the charge consumed for Co2+ reduction at the RVC surface. Therefore, in the present research, CE was evaluated using the following Eq. (4) CE =

Total Co moles deposited on RVC cathode × 100 Total Zn moles oxidized in the anode

Fig. 3. Percentage of removal vs. time with 100 ppi RVC from 500 mg L−1 Co(II) at (a) pH 4.00, and (b) without pH adjustment.

(4)

It was observed that after 360 min of the electrogenerative process, 98% of Co(II) was recovered at RVC attaining current efficiency of 82%. The remaining 18% (CE) was used for other side reactions, for example, water reduction (2H2 O + 2e− → H2 + 2OH− ) at the cathode surface as was evidenced from the evolution of gas bubbles on the RVC surface and increase of catholyte pH. 3.2. pH effect on cobalt removal It has been found that the highest percentage removal of cobalt (99.7%) was achieved at pH 4.00 with RVC 80 ppi using a batch reactor. Therefore, on that basis, pH 4.00 is applied herein in conjunction

with varying electrolyte flow rates (50, 100, 150 and 200 mL min−1 ) to study their effects on cobalt removal. The experiments were conducted with an initial cobalt concentration of 500 mg L−1 in the presence of 0.2 M sodium sulfate and 0.1 M boric acid using 100 ppi RVC as cathode. The data obtained are summarized in Fig. 3a. It is surprising to note that the cobalt deposition rate is decreased and that only less than 75% of cobalt is removed after 8 h of operation at all electrolyte flow rates. It is observed that cobalt recovery decreases as the electrolyte flow rate is increased. The poor cobalt removal indicates that the experimental condition at pH 4.00 may not be implementable for the flow-by batch-recycle system. Subsequently, another set of experiments (with no pH control throughout the experiments) was performed to investigate the suitable pH condition for removing cobalt. The composition of electrolyte (500 mg L−1 Co(II) in 0.2 M sodium sulfate and 0.1 M boric acid) gave a resultant pH of 5.44 ± 0.10. At the end of experiments, the pH was <8.40. For work without pH adjustment (Fig. 3b), it is noteworthy that a shorter time of 7 h is required to remove more than 99% of cobalt at all electrolytes flow rates. Cobalt recovery increases as the electrolyte flow rate is increased. However, further increase of electrolyte to 200 mg L−1 resulted in the decrease of cobalt recovery rate. Result in Fig. 3b reveals that a higher pH enhances the cobalt deposition rate. For this reason, this pH condition is selected for further studies. When comparing the overall performance, it can be concluded that poor cobalt recovery at pH 4.00 is due to the reciprocal effects from both the hydrogen evolution reaction (HER) and electrolyte flow rate. It has been reported that HER always influences cobalt

101

002

185

110

100

Intensity (au)

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40

50

60

70

80

90

2θ Fig. 4. XRD pattern of cobalt deposited on 100 ppi RVC from 500 mg L−1 Co(II) after 7 h of experiment.

deposition in both acidic and basic solutions. According to [22], it is proposed that for pH < 4.0, cobalt deposition goes through an adsorbed intermediate, CoH which will be subsequently reduced to metallic cobalt (Eqs. (5)–(7)). H+ (aq) + Co(s) + e−  CoH(ads)

(5)

H+ (aq) + CoH(ads) + e−  Co(s) + H2(ads)

(6)

Overall :

Co2+ (aq) + 2H+ (aq) + 4e−  Co(s) + H2(ads)

(7)

For pH > 4.0, water reduction (Eq. (8)) leads to an increase of pH in the vicinity of the cathode surface. This can result in the precipitation of cobalt hydroxide, which is then reduced to metallic cobalt (Eqs. (9)–(11)). 2H2 O + 2e−  2OH− (aq) + H2(g)

(8)

Co2+ (aq) + 2OH− (aq)  Co(OH)2(s)

(9)

−

Co(OH)2(s) + 2e  Co(s) + 2OH Overall :

−

(aq)

(10)

Co2+ (aq) + 2H2 O + 4e−  Co(s) + 2OH− (aq) + H2(g) (11)

According to Sasaki and Talbot [23], HER is the parallel reaction during the metallic deposition of the iron-group (Fe, Ni, and Co). It was reported that the rate of hydrogen evolution on rotating disk electrodes was proportional to rotation rate. In other words, the rate of HER is associated with convection. Convection may result from electrode rotation or electrolyte flow. HER (Eqs. (5)–(6)) occurs more readily at lower pH values [24,25]. It is obvious herein that the rate of HER is amplified by these two conditions; acidic medium and convective flow of electrolyte. The low deposition rate of cobalt in acidic solutions is apparently ascribed to an increase in competition for adsorption sites by discharged hydrogen. The hydrogen ions tend to occupy the cathode voids and cling to the surface of RVC which subsequently block these surface sites from cobalt deposition. In the course of cobalt removal, cobalt redissolution is observed taking place. This redissolution may result from the blocking effect produced by the deposits or hydrogen evolution following the reaction below [26]: Co + 2H+  Co2+ + H2

(12)

The XRD pattern in Fig. 4 shows that metallic cobalt is recovered on RVC and no peak for impurities are detected. This is proven by the compatibility between the peaks and the diffraction lines of standard cobalt metal. The main characteristic peaks have been assigned to (1 0 0), (0 0 2), (1 0 1) and (1 1 0) planes respectively. All peaks indicate that cobalt deposits are presented as hexagonal closed packed (hcp) phases.

Fig. 5. (a) Normalized concentration [Ct /C0 ] vs. time from 100 mg L−1 Co(II) with (a) 60 ppi RVC, (b) 80 ppi RVC, and (c) 100 ppi RVC. Inset: plots of ln[Ct /C0 ] vs. time for the data shown.

3.3. Effect of electrode porosity on cobalt removal Fig. 5a–c depicts the plots of normalized cobalt concentration as a function of time for the cobalt removal at an initial Co(II) concentration of 100 mg L−1 Co(II) with the influence of different electrolyte flow rates using 60, 80 and 100 ppi RVC respectively. The insets in Fig. 5 show the plot of ln[Ct /C0 ] vs. time. A model of the concentration–time relationship for three-dimensional electrodes under mass-transport control can be expressed by Eq. (13) [27,28].



ln

C(t) C(0)



=

−Ve km Ae t VR

(13)

where C(t) is the metal concentration at time t, C(0) is the initial metal concentration, km is the mass transport coefficient, Ae is the specific surface area (the active area/unit volume of cathode), Ve is the cathode volume and VR is the volume of electrolyzed solution and t is the experiment time. For 60 ppi RVC, all the curves as illustrated in the inset plots of Fig. 5a show similar profiles in which the Co(II) concentration decay observed involves two kinetic regimes. Before 60 min, the first regime with a slow initial rate is limited by charge transfer. After that, the reaction rate switches to mass transport control until 150 min. The enhanced rate is attributed to the roughness of the deposits as a result of the increase in the surface area or the

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Table 1 Values of km Ae and km calculated from the slopes of inset plots of Fig. 5 and the percentage of cobalt removal after 4 h of experiment. Grade of RVC (ppi)

Linear flow velocity, u × 103 (m s−1 )

Percentage of removal after 4 h (%)

km A × 102 (s−1 )

km × 106 (m s−1 )

R2

60

0.86 1.72 2.58 3.44

99.9 99.9 >99.9 99.9

1.83 2.22 2.13 2.28

4.82 5.84 5.61 6.00

0.992 0.967 0.992 0.986

80

0.86 1.72 2.58 3.44

99.9 99.8 >99.9 99.8

1.30 1.77 2.10 1.93

2.45 3.34 3.96 3.64

0.958 0.994 0.994 0.992

100

0.86 2.58 3.44

99.7 99.6 99.7

1.61 1.71 1.68

2.40 2.55 2.51

0.981 0.992 0.987

3.4. Mass transport correlations For the batch-recycle system, the value of km Ae may be correlated with the linear flow velocity of solution, u, by the empirical expression as shown below [21,32]. km Ae = aub

(14)

0.04 60 ppi RVC 80 ppi RVC 100 ppi RVC

0.03

-1

kmAe (s )

presence of turbulence at the electrode surface [26,29,30]. The results obtained show that for the first 150 min the removal rate fluctuates with the increase in electrolyte flow rates. After 150 min, the deposition rate reaches a plateau in which more than 99% of cobalt is removed until the end of the experiment. At the end of 4 h, the removal percentages at all flow rates are almost similar as presented in Table 1. Similar trends of Co(II) concentration decay are depicted with studies using 80 and 100 ppi RVC in which two kinetic regimes are also observed. For 80 ppi RVC, before 150 min, the removal rate increases as the electrolyte flow rate is increased until an optimum flow rate of 150 ml min−1 as shown by the increase in km Ae values. However, after 150 min, the removal rate becomes constant (at all electrolyte flow rates) as reflected by the plateaus in the curves. This is due to the substantial drop in mass transport rate when nearly 100% of cobalt has been removed. For 100 ppi RVC, before 150 min, a much lower removal rate is observed at 100 mL min−1 . Since the cobalt removal process is not mass transport controlled at 100 mL min−1 , the data is not included in the inset plot of Fig. 5c. The effect of electrode porosity can be better analyzed by calculating the km Ae values. Table 1 shows the km Ae values for various flow rates calculated from the inset plots of Fig. 5a–c from 60 min to 150 min (mass transport region). The values of Ae are taken from the manufacturer’s literature [31]. For 60, 80 and 100 ppi RVC, the Ae values are 3800, 5300 and 6600 m−1 respectively. The data presented shows that 60 ppi RVC exhibits the highest km value among the other grades of RVC. km values were found to decrease with increasing cathode porosity.

0.02

0.01

0.001

0.01

u (m s -1) Fig. 6. Double logarithmic plots of specific mass transport coefficient, km Ae vs. linear flow velocity, u for cobalt removal from 100 mg L−1 Co(II) with different RVC grades.

As shown in Fig. 6, the values of km Ae = aub show marked differences for RVC of different grades. The variation in proportionality constants indicates that mass transport correlation is dependent on the electrode geometry and porosity, fluid flow pattern and the electrochemical reaction [33]. In addition, the values of the velocity exponent, b, point out that RVC appears to be a poor turbulence promoter during cobalt removal under the experimental conditions used. It is hoped that this limitation be overcome by incorporating turbulence promoters in future studies. Since the correlation of km Ae against u deviates far from linearity, further evaluation of mass transport data using dimensionless groups (Sherwood, Reynolds, Schmidt) is therefore inapplicable. The deviation from linearity also indicates that km values are not strongly dependent on electrolyte flow rate especially for studies using 60 and 100 ppi RVC.

The linear flow velocity for RVC electrode is correlated as: u = Q (Ax ε)−1

(15)

where a and b are empirical constants, Ax is the cross-sectional area, Q is the volumetric flow and ε is the porosity of RVC electrode. Fig. 6 shows the double logarithmic plots of km Ae against u. The best-fit correlation equations for the different RVC grades are: 60 ppi km Ae = 0.052u0.15 with R2 = 0.785 80 ppi km Ae = 0.13u0.32 with R2 = 0.855 100 ppi km Ae = 0.021u0.037 with R2 = 0.768

4. Conclusions The flow-by batch-recycle system had demonstrated the feasibility of cobalt removal from sulfate solutions with convective flow of electrolytes. It was shown that the cobalt removal rate was greatly influenced by pH. Under the experimental conditions, 60 ppi RVC exhibited the highest km value among other grades of RVC. km values were found to decrease with increasing cathode porosity. Further improvements of the system are necessary to overcome limitations especially in the effort of enhancing mass transport rate for a possible scale-up of the system.

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Acknowledgments This study was supported by research grants provided by Universiti Sains Malaysia (No. 1001/PKIMIA/831011 and 1001/PKIMIA/814125) and W.X. Tan would like to thank the Ministry of Science, Technology and Innovation (MOSTI) for a postgraduate research scholarship. M.A. Hasnat who is on study leave from the Department of Chemistry, Shahjalal University of Science and Technology, Bangladesh would like to acknowledge the Third World Academy of Sciences for awarding a USM-TWAS fellowship to conduct research at Universiti Sains Malaysia. References [1] J.P.T. Kapusta, Cobalt production and markets: a brief overview, JOM (2006) 33–36. [2] J. Nan, D. Han, X. Zuo, Recovery of metal values from spent lithium-ion batteries with chemical deposition and solvent extraction, J. Power Sources 152 (2005) 278–284. [3] B. Swain, J. Jeong, J.C. Lee, G.H. Lee, J.S. Sohn, Hydrometallurgical process for recovery of cobalt from waste cathodic active material generated during manufacturing of lithium ion batteries, J. Power Sources 167 (2007) 536–544. [4] W.H. Jiang, X.D. Yao, H.R. Guan, Z.Q. Hu, Carbide behavior during high temperature low cycle fatigue in a cobalt-base superalloy, J. Mater. Sci. 34 (1999) 2859–2864. [5] J. Jandová, H. Vu, P. Dvoˇrák, Treatment of sulfate leach liquors to recover cobalt from waste dusts generated by the glass industry, Hydrometallurgy 77 (2005) 67–73. [6] The Cobalt Development Institute, Cobalt facts-Cobalt in chemicals, 2006, available from http://www.thecdi.com/cdi/images/documents/facts/COBALTFACTS-Chemicals.pdf, accessed 20 Dec 2010. [7] L. Chen, X. Tang, Y. Zhang, L. Li, Z. Zeng, Y. Zhang, Process for the recovery of cobalt oxalate from spent lithium-ion batteries, Hydrometallurgy 108 (2011) 80–86. [8] A. Katsiapi, P. Tsakiridis, P. Oustadakis, S. Agatzini-Leonardou, Cobalt recovery from mixed Co–Mn hydroxide precipitates by ammonia–ammonium carbonate leaching, Miner. Eng. 23 (2010) 643–651. [9] W.X. Tan, N. Mohamed, Electrogenerative removal of cobalt from sulfate solutions using a batch reactor, Clean-Soil Air Water 39 (2011) 460–466. [10] Y. Wang, C. Zhou, Hydrometallurgical process for recovery of cobalt from zinc plant residue, Hydrometallurgy 63 (2002) 225–234. [11] K.I. Dermentzis, A.E. Davidis, A.S. Dermentzi, C.D. Chatzichristou, An electrostatic shielding-based coupled electrodialysis/electrodeionization process for removal of cobalt ions from aqueous solutions, Water Sci. Technol. 62 (8) (2010) 1947–1953. [12] R.C. Widner, M.F.B. Sousa, Electrolytic removal of lead using a flow-through cell with a reticulated vitreous carbon cathode, J. Appl. Electrochem. 28 (1998) 201–207. [13] L.C. Almeida, L.H.S. Gasparotto, N. Bocchi, R.C. Rocha-Filho, S.R. Biaggio, Galvanostatic Pb(II) removal from a simulated wastewater by using a stainlesssteel wool cathode in a flow-through cell: a factorial-design study, J. Appl. Electrochem. 38 (2008) 167–173. [14] C. Ponce de León, D. Pletcher, The removal of Pb(II) from aqueous solutions using a reticulated vitreous carbon cathode cell: the influence of the electrolyte medium, Electrochim. Acta 41 (1996) 533–541.

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