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Electrochemical recovery of antimony and bismuth from spent electrolytes
Separation and Purification Technology 235 (2020) 116169
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Electrochemical recovery of antimony and bismuth from spent electrolytes a
a,b,⁎
V.R. Chithambara Thanu , M. Jayakumar a b
T
Electroplating and Metal Finishing Division, CSIR Central Electrochemical Research Institute, Karaikudi 630 006, Tamil Nadu, India Academy of Scientific and Innovative Research (AcSIR), CSIR Central Electrochemical Research Institute, Karaikudi 630 006, Tamil Nadu, India
ARTICLE INFO
ABSTRACT
Keywords: Bismuth Antimony Electrodeposition Recovery Spent Electrolyte
In the present study, direct electrowinning is adopted for the recovery of antimony and bismuth from a quaternary chloride electrolyte that depicts the spent electrolyte from copper extraction. Reduction behavior of metal ions in electrolytes was studied using linear sweep voltammetry. Electrowinning of antimony and bismuth was carried out under potentiostatic and galvanostatic conditions. Electrodeposition parameters such as current density, potential, and temperature were systematically varied to understand their effect on the recovery of antimony and bismuth. The recovered metals are characterized for their composition and phase purity using XRF, XRD and surface morphology by SEM. By careful choice of conditions, it is possible to recover bismuth and antimony from the chloride bath. At a potential of −0.5 V as well as at a current density of 5 A/dm2, antimony rich deposits can be obtained at ambient bath temperatures.
1. Introduction Reduce, Recycle and Reuse (3Rs) are considered the cornerstones for a sustainable lifestyle and recovery of materials from industrial wastes and end-of-life devices is an important step towards achieving it. Critical elements like antimony and bismuth are widely used in semiconductor, thermoelectric, pharmaceuticals, chemicals, ceramics, pigments and as alloying element. However, their limited natural resources and continuously expanding applications such as thermoelectric demand alternate sources. Extractive metallurgy of non-ferrous metals such as copper, silver, lead from sulfide ores results in solid and liquid wastes, which contain metals with chemical proximity such as Ag, Bi, Sb, Fe and Tl. Such waste can be considered as potential resource and approaches to recover them can have economic and environmental impacts. At present the cost of antimony and bismuth are 8800 US$/t and 2800 US$/t (LMX). Further, these metals and their compounds, particularly, antimony have adverse health effects such as cardiovascular, respiratory issues upon prolonged exposure. Conventional approaches such as disposing underground and burning in air can contaminate the environment and are unsustainable [1]. Therefore there is a pressing necessity to treat wastes and effluents containing them prior to discharge. Further, among all the antimony and bismuth used around the world only 20% is recycled. China contributes to 90% of antimony and 75% of bismuth production and any break-down in economic or political equation between rest of the world with China may hamper global supply of these critical metals.
⁎
There is an increased focus on the recovery of antimony and bismuth from the industrial wastes. Generally, recovery of metals is carried out by various approaches such as solvent extraction, precipitation, ion-exchange, crystallization, electrowinning, etc. Electrodeposition is a low-cost, scalable and simple approach to recover metals that can be electrodeposited from a specific media and does not require sophisticated equipments. The metals most commonly recovered by electrolysis are gold, silver, copper, manganese, iron, nickel, cobalt, zinc, cadmium, tin and lead. The electro-recovery of metals by the electrolysis of solutions of metal compounds, employing an electrolytic cell in which the metal is deposited upon a cathode, has long been known and is widely practiced. Electroplating process can be fine-tuned by suitable choice of parameters, additives, temperature, etc. thereby yielding deposits with desired morphology, composition, functionality and high purity. However, electrowinning has limitations such as slow kinetics, relatively low throughput, etc. Khaliq et al. reviewed the hydrometallurgical and electrometallurgical processing for the recovery of pure base and precious metals [1]. Chen et al. recovered bismuth and arsenic from flue dusts arising from copper extraction via leaching and precipitation and showed a recovery of 90% and 42%, respectively [2]. Several groups have reported recovery of metals from waste generated from metallurgical industries [3,4]. Sumitomo Metal Mining has developed a process to recover antimony and bismuth by ion-exchange and direct electrowinning in metallic form [5]. Hoffmann discussed solvent extraction and ion exchange strategies for the recovery of Sb, Bi, As, Ni
Corresponding author. E-mail addresses: [email protected], [email protected] (M. Jayakumar).
https://doi.org/10.1016/j.seppur.2019.116169 Received 1 July 2019; Received in revised form 16 September 2019; Accepted 4 October 2019 Available online 04 October 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
nitrates were dissolved in hydrochloric acid with heating at 80 °C for 30 min. The clear solution obtained was subsequently diluted with aqueous solution containing known concentration (50–70 g/L) of potassium chloride. The resultant solution had a composition of ~1/10th of actual discharge arising from copper extraction process. Care was taken to avoid formation of any precipitate in the solution. Linear sweep voltammetry studies were performed using 3-electrode configuration using Biologic SP-200 electrochemical workstation. Copper wire was taken as working electrode as it was proposed as cathode for electrolysis. Silver wire acted as quasi-reference electrode and the counter electrode chosen was TSIA (Titanium substrate insoluble anode). Electrodeposition experiments were carried out using two electrode setup with copper cathode (2 × 2 cm) and titanium substrate insoluble anode (TSIA) having inter-electrode distance of 4 cm in 100 ml bath. Choice of electrode was based on their stability in the acidic chloride bath. Electro-recovery was carried out at constant potential and constant current conditions. Electrolysis duration was 1 h and temperature was 303 K except temperature variation studies. All electrolysis experiments were carried out under constant stirring of 300–400 rpm. Post electrolysis the cathode was retrieved, carefully washed with deionized water and dried. Weight of the deposit was calculated from increase in cathode weight. The elemental composition was analyzed by X-ray Fluorescence Spectrometer (Horiba, XGT-5200) and phase purity is analyzed by X-ray Diffractometer-Multipurpose (Rigaku Smart lab guidance). The surface morphology and composition is analyzed with Scanning Electron Microscope (Tescan Vega 3 SBH EDS). The Faradaic efficiency for alloy deposition was calculated according to the following Eq. (1) [28].
Fig. 1. Linear sweep voltammetry of chloride bath containing antimony, bismuth and iron at a scan rate of 20 mV/s. Inset: Voltammetric curve for the potential range of −0.05 (open circuit potential) to −1.5 V. Working electrode: Cu wire (surface area = 0.35 cm2), Reference electrode: Ag wire, Counter electrode: titanium substrate insoluble anode. Temperature: 303 K. Quaternary electrolyte composition is given in ESI 1. For unitary electrolytes, concentration for bismuth and antimony is as in ESI 1.
wherein significant decrease in Sb and Bi concentrations by ion exchange was reported [6]. Selective adsorption [7] and precipitation using soluble sulfates of barium, strontium and lead [8] are some of the techniques reported. King et al. reported addition of lead monoxide for the removal of bismuth which did not have any significant effect on the levels of Sb, As and Cu [9]. Several research groups have reported recovery of noble and base metals such as Au, Ag, [10–13] and Cu, Zn, Pb [14–18] and critical metals such as cobalt, lithium, rare-earth, [19–23] etc. Leaching followed by solvent extraction, precipitation, electrolysis [13,14], are some of the approaches used to recover these metal values from secondary sources such as industrial waste and end-of-life products. Reports are available on electrodeposition of Bi-Sb based films from chloride based baths [24–26] and non-aqueous electrolytes [27] for thermoelectric applications. In the present work, we carried out electro-recovery of antimony and bismuth from chloride electrolytes that closely depict the reject from copper extraction process. The present study is an academic attempt to understand the recovery of critical metal values utilizing the solution composition shared by industry. Effect of applied current, potential, temperature, electrolysis duration was studied and deposits are characterized by various physicochemical techniques. Element-wise kinetics of electrodeposition was also investigated. For comparison, voltammetry and electrodeposition in unitary electrolytes of bismuth and antimony were also studied.
CEa =
m1 W1
+
m2 W2 It F
+
m3 W3
x100
(1)
wherein CEa is the Current efficiency of alloy deposition, ma is the mass of metal 'a' in the deposit, Wa is the Electrochemical equivalent weight of metal 'a', I is the Current passed (A), t is the Time (sec), F is the Faraday constant (C) 3. Results and discussion 3.1. Linear sweep voltammetry Linear sweep voltammetry (LSV) is a useful technique to have meaningful insights on the redox reaction occurring at the electrode. LSV can offer possible potential to be applied for quantitative recovery of desired metal from a multi-element solution. In the present study, we used a three electrode set-up for LSV to assimilate the potential range for electro recovery of bismuth and antimony. Fig. 1 presents the LSV experiment carried out in the potential range of −0.05 (open circuit potential) to −2.0 V at 303 K at a scan rate of 20 mV/s. In quaternary electrolyte, onset of broad reduction wave was observed at −0.13 V with peak at −0.17 V followed by continuous increase in cathodic current. Voltammetry was also carried out for individual bismuth and antimony electrolytes. LSV of antimony electrolyte shows occurrence of broad reduction wave at −0.064 V and minor peak at −0.16 V. Voltammogram of bismuth electrolyte shows the presence of two reduction peaks; the first sharp one with onset at −0.066 V and peak at −0.12 V, and another wave with onset at −0.16 V and peak at −0.2 V. Minor reduction waves observed at more cathodic potentials could be due to
2. Experimental Simulated solution containing bismuth, antimony and iron was prepared matching the composition of process solution arising from copper extraction from sulfide ores. Typical composition for a process solution from copper extraction is given in ESI 1. Stoichiometric quantities of antimony trioxide, copper chloride, bismuth and iron
Table 1 Electrochemical recovery of antimony and bismuth as a function of applied potential. Electrolysis duration: 60 min. Temperature: 303 K. Voltage (V)
Recovery Obtained (g/m2 h)
Bi (%)
Sb (%)
Fe (%)
Bi wt (g/m2 h)
Sb wt (g/m2 h)
Fe wt (g/m2 h)
Current efficiency (%)
−0.3 −0.5 −1.0
0.727 40.7 152.3
9.1 12.3 48.2
77.8 81.7 50.8
12.9 5.9 0.9
0.067 5 73.5
0.57 33.3 77.5
0.09 2.4 1.3
76.1 49.5 28.5
2
Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
BiCl4 + 3e
(3)
Bi + 4Cl
xSbCl4 + yBiCl4 + 3e
Bi x Sb y + 4Cl
(4)
While the standard redox potentials of bismuth and antimony are separated by 100 mV, the difference is lowered by one order to ~10 mV (0.16 V for BiCl4−/Bi, 0.17 V for SbCl4−/Sb) when complexed in chloride solutions [25]. As a result, electrodeposition of BiSb compounds is more favoured in chloride baths resulting in one reduction peak with higher current. Comparison of voltammograms shows that the current observed in the quaternary electrolyte is nearly equals the combined currents of bismuth and antimony. Further, there is one significant reduction peak in the quaternary electrolyte indicating possible formation of Bi-Sb alloy deposits. The shift of reduction peak to more negative potentials in bismuth-antimony system than observed in unitary electrolyte is consistent with literature [24]. Voltammetric data indicates that a minimum of −0.2 V would be necessary to recover bismuth and antimony from individual and quaternary electrolytes. Further, the continuously increasing cathodic current at potentials > −0.4 V in unitary and quaternary electrolytes clearly indicate that hydrogen evolution reaction competes with electrodeposition of antimony and bismuth. 3.2. Electrolysis experiments 3.2.1. Effect of applied potential Electrolysis experiments were carried out under potentiostatic conditions for the recovery of bismuth and antimony. Potentiostatic method offers a possibility for selective recovery of a particular metal or group of metals when a potential closer to their standard redox potentials is applied. Table 1 and Fig. 2(a) presents the results of electrolysis carried out at three different potentials, −0.3, −0.5 and −1 V. Increase in applied potential results in higher recovery of metals; from < 1 g/m2 h at −0.3 V to ~152.3 g/m2 h at −1.0 V. Antimony recovery increases from 0.57 g/m2 h at −0.3 V to 77.5 g/m2 h at −1 V and bismuth escalates from 0.067 g/m2 h at −0.3 V to 73.5 g/m2 h at −1 V. At optimized potential of –0.5 V, deposits with maximum antimony content can be recovered. Sudden increase in bismuth recovery at higher cathodic potentials can be attributed to its slightly higher reduction potentials. Current efficiency values of 76.1%, 49.5%, 28.5% were observed for the applied potentials of −0.3, −0.5 and −1 V, respectively. This observation clearly indicates that hydrogen evolution strongly concurs with electrowinning of bismuth and antimony thereby lowering current efficiency. At similar potentials, negligible deposition was observed in unitary electrolytes which indicates possibility of other mechanisms in an multi-element electrolyte such as formation of stable Sb-Bi solid solutions, induced codeposition. Although there are reports on electrodeposition of antimony – bismuth alloys, the authors used pure solutions with low metal concentrations (< 0.1 M) to obtain functional thin films [25–27] and the data is not comparable to the present study. 3.2.2. Effect of current density Constant current methodology is widely used in electroplating industry as it ensures constant rate of electrodeposition by maintaining steady mass transport of active material to the substrate. The Table 2 and Fig. 2(b) show the effect of current density on recovery of bismuth and antimony. Recovery data shows that galvanostatic route offers higher recovery than potentiostatic conditions. The amount of material deposited is not precisely proportional to the current applied. The data shows contrasting trends of bismuth and antimony recovery with increase in current density from 2 to 5 A/dm2. Bismuth shows slight increase from 140 g/m2 h to 164.8 g/m2 h, but drops to 106.8 g/m2 h at 5 A/dm2. On the other hand antimony content in the deposit marginally lowers from 55% to 52% at 2–3 A/dm, but at 5 A/dm2 escalates to 72%. Although bismuth enrichment is expected at higher current density, the
Fig. 2. Electrochemical recovery of antimony and bismuth as a function of various parameters (a) applied potential, (b) current density and (c) bath temperature.
the reduction of other chloro complexes that are more stable. The plausible reaction mechanism in unitary (Equations (2) and (3)) and quaternary electrolytes (Eq. (4)) could be [25];
SbCl4 + 3e
Sb + 4Cl
(2) 3
Separation and Purification Technology 235 (2020) 116169
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Table 2 Electrochemical recovery of antimony and bismuth as a function of current density. Electrolysis duration: 60 min. Temperature: 303 K. Current density (A/dm2) 2 3 5
Recovery obtained (g/m2 h) 312.52 342.57 376.58
Bi (%) 44.8 48.1 28.3
Sb (%) 55.1 51.8 71.4
Fe (%) 0.07 0.08 0.1
Bi wt (g/m2 h) 140.1 164.8 106.8
Sb wt (g/m2 h) 172.2 177.5 269.4
Fe wt (g/m2 h) 0.22 0.27 0.38
Current efficiency (%) 38.3 13.5 13.8
Table 3 Effect of bath temperature on electrochemical recovery of antimony and bismuth with applied potential of −0.5 V and duration of 1 h. Bath temperature (°C)
Recovery obtained (g/m2 h)
Bi (%)
Sb (%)
Fe (%)
Bi wt (g/m2 h)
Sb wt (g/m2 h)
Fe wt (g/m2 h)
Current efficiency (%)
30 50 70
38.67 34.4 23.93
14 35 37.7
78.1 53 55
7 10 7.2
5.47 12.3 9
30.5 18.6 13.2
2.7 3.5 1.73
40.13 68.13 83.18
Table 4 Kinetics of antimony and bismuth recovery from the chloride bath at an applied potential of −0.5 V. Temperature: 303 K. Time (min)
30 60 120 240 360 480
Stepwise Recovery
Cumulative Recovery
Weight of recovered deposits in this step (g/m2 h)
Composition of the deposit Bi%
Sb%
Fe%
140 96.25 215.5 610.25 412.75 196
54 80 75 59 25 16
44 18 24 40 70 83
0.6 0.8 0 0.1 4.2 0.12
Overall weight of the recovered deposits till this step (g/m2 h)
% recovery from the bath Bi
Sb
Fe
140 236.2 451.7 1062 1474 1670
5.0 10.1 20.9 44.9 51.8 48.8
6.1 7.8 13.0 37.4 66.3 82.6
3.4 6.7 6.7 10.1 79.5 80.4
Table 5 Kinetics of antimony and bismuth recovery from the chloride bath at a current density of 5 A/dm2. Temperature: 303 K. Time (min)
30 60 120 240 360 480
Stepwise recovery
Cumulative recovery
Weight of recovered deposits in this step (g/m2 h)
Composition of the deposit Bi%
Sb%
Fe%
219.75 181.25 208.75 289.75 289.25 257.25
34 53 45 56 75 43
63.8 55.8 54 42 24 55
1.5 0.2 < 0.1 0.4 0 1.4
deposits were richer in antimony content. The results show that the enhanced recovery rate of antimony at higher current density favors over the deposition of bismuth. Similar trend is observed in kinetics data presented in Section 3.2.5. Iron shows increasing trend in recovery with current density. Recovery of bismuth and antimony from unitary electrolyte at various current densities were carried out to understand the behavior of individual elements (ESI 2). At a current density of 2 A/ dm2, the average recovery was 40 and 400 g/m2 h for antimony and bismuth, respectively from their unitary solutions (ESI 2). Under identical conditions, the recovery was about 300 g/m2 h from the synthetic quaternary electrolyte with a average deposit composition of 1:1 (Sb: Bi). In other words, the presence of other ions lowers the recovery of bismuth while antimony recovery is enhanced. Recovery in quaternary electrolyte is relatively lower than observed in unitary electrolyte which may be due to interfering cyclic redox reactions such as Fe2+/3+. The calculated Faradaic efficiency values were 38%, 14% and 14% for the current densities of 2, 3 and 5 A/dm2, respectively. The values for constant current experiments are inferior to potentiostatic mode. In case of latter, at applied potentials ≤−0. 5 V, hydrogen evolution reaction (HER) and metal deposition compete equally resulting in %CE ~50%, which degrades at higher potential. Using cyclic voltammetry,
Overall weight of the recovered deposits till this step (g/m2 h)
% recovery from the Bath Bi
Sb
Fe
219.75 401 609.7 899.5 1188 1446
4.9 11.3 17.6 28.4 42.9 45.3
14.0 24.1 35.4 47.5 54.5 68.6
13.8 15.8 16.2 21.7 21.7 36.1
Besse et al. reported the possibility of hydrogen evolution competing with bismuth - antimony electrodeposition at higher overpotentials [25]. In galvanostatic conditions, the potential of electrochemical cell can shift to higher potentials resulting in hydrogen evolution as the primary reaction. While lower current density can suppress HER, experiments at current densities < 2 A/dm2 yielded insignificant deposits from the present bath composition. 3.2.3. Effect of temperature Electrolysis of quaternary electrolyte was carried out at an optimum potential of −0.5 V at various bath temperatures to understand its relation with electro-recovery of antimony and bismuth and the results are presented in Table 3 and Fig. 2(c). The results indicate an inverse proportional relationship between temperature and amount of electrodeposits. Recovery lowered by 50% when temperature was increased from 30 to 70 °C. With increase in temperature, the amount of antimony decreases nearly one-third from 30.5 g/m2 h to 13.2 g/m2 h whereas the recovery of bismuth is the range of 5.4–12.3 g/m2 h. It is well-known that hydrogen evolution becomes more prominent with increase in temperature. In the present case, HER may compete and impede metal deposition on the electrode surface at elevated temperatures which can possibly explain the decrease in bismuth and antimony recovery. 4
Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
However, the authors observed lower current flowing through the electrolytic cell with increase in temperature. Concurrently, current efficiency data shows improvement with temperature from 40% at 30 °C to 68% and 83% at 50 and 70 °C, respectively. Such unusual trend has been reported by some authors; Awee et al. reported increase in Faradaic efficiency with temperature during galvanostatic electrowinning of antimony which was attributed to reduced solution resistance and lowering of viscosity [29]. However, the observed lower recovery and its relation to increase in Faradic efficiency at elevated temperatures remain an open question and necessitate a further detailed study. 3.2.4. Kinetics of bismuth and antimony electrowinning In comparison with other metal recovery methods, electrochemical routes are limited by slower kinetics. It is essential to understand the kinetics of recovery of bismuth and antimony to validate them for possible commercial application. Electrolysis was carried out by potentiostatic mode at −0.5 V and galvanostatic condition at a current density of 5 A/dm2 in which the samples deposited were weighed and analyzed at different durations. Table 4 and 5 and Fig. 3 show the kinetics of deposition at 303 K. During a deposition duration of 30 min at −0.5 V, 140 g/ m2 h of deposit was obtained with a composition of 55% bismuth, 44% antimony and negligible iron. Increase in electrolysis duration results in cumulatively increase in recovery of metals. However, the content of bismuth and antimony in the deposit in each stage was different. As a result, the rate of recovery of antimony was higher than bismuth. At the end of 8 h of electrolysis, close to 83% of antimony and 49% of bismuth originally present in the electrolyte were recovered. Kinetics of iron deposit showed close to 80% recovery. Under galvanostatic mode, close to 220 g/m2 h of metals can be deposited within 30 min. Content of antimony and bismuth in the recovered deposit was 40–65% in most cases. At the end of 8 h, close to 69% of antimony and 45% of bismuth were recovered from the bath. If comparison could be drawn between constant potential and current approaches despite their fundamental differences, antimony recovery is rapid under constant potential conditions whereas bismuth recovery reaches a plateau around 50% in both the cases. Trend in kinetics data concurs the potentiostatic and galvanostatic electrodeposition (Sections 3.2.1 and 3.2.2) wherein bismuth recovery is improved at more cathodic potentials, the rate of antimony deposition is more crucial factor that enhances overall recovery. Based on the kinetics of
Fig. 3. Rate of recovery of antimony and bismuth from the chloride bath at 303 K (a) applied potential of −0.5 V and (b) current density of 5 A/dm2.
Fig. 4. X-Ray Diffraction pattern of metals recovered at current density of 2 and 5 A/dm2. Standard antimony (JCPDF file 00-001-0802) and bismuth (JCPDF file 00001-0699) are given for comparison. (0 1 2) plane of samples obtained at 4 A/dm2 is present as inset. 5
Separation and Purification Technology 235 (2020) 116169
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Fig. 5. XRD pattern of samples electrodeposited at applied potential of −1.0 V and −0.3 V. Standard antimony (JCPDF file 00-001-0802) and bismuth (JCPDF file 00-001-0699) are presented for comparison. Inset shows (0 1 2) plane of samples obtained at −1.0 V.
500X
and antimony. The broad peak (0 1 2) at 1 A/dm2 indicates the formation of Bi-Sb alloy [25,30] and the minor shoulder may be due to possible traces of other phases. Fig. 5 shows the X-ray diffraction analysis of recovered deposits at constant potential. Antimony gives high intensity peak with weakly defined shoulder corresponding to (0 1 2) plane. Planes corresponding to (0 1 2), (0 1 5), (1 1 0) and (2 0 2) are strong in electrodeposits obtained from both constant potential and current studies. However, (1 0 4) plane is relatively low in the samples obtained under applied potential. No significant peaks corresponding to oxides of bismuth and antimony were observed in both cases. However low intensity iron oxide peaks observed around 30° are due to Fe2O3. Absence of additional peaks further validates the possible deposition of single phase Bi-Sb alloy.
5kX
(a)
3.2.6. SEM analysis of the electrodeposited metals The scanning electron microscopy were recorded for electrodeposits from current and potential variation studies to understand the underlying relationship between the morphology of the recovered metals and the experimental conditions. Fig. 6 shows the SEM micrographs of samples deposited at current densities of 2 and 5 A/dm2 which shows agglomerated morphology with smaller particles at higher current densities. Fig. 7 shows micrographs of deposits under potentiostatic conditions. At a lower potential of −0.3 V, porous, inter connected structure was observed in the deposits. Increase in potential to −1 V yields less porous deposits which indicates that increase in applied potential results in more compact deposits. For comparison, SEM micrographs of recovered bismuth and antimony from individual baths are also presented in ESI 3 and 4.
(b)
Fig. 6. SEM images of recovered metals at current density of (a) 2 A/dm2 and (b) 5 A/dm2.
deposition at −0.5 V up to 8 h, it is expected that quantitative recovery of antimony may be achieved by the end of 10 h. 3.2.5. XRD analysis of the electrodeposited metals X-ray diffraction is employed to know the crystal phase of elecrorecovered deposits. Fig. 4 presents the X-ray diffraction analysis of recovered deposits under galvanostatic conditions. Standard data of bismuth (JCPDS 00-001-0699) and antimony (JCPDS 00-001-0802) are presented for comparison. The X-ray pattern of the deposits are in between the metallic bismuth and antimony but closer to the latter. The recovered deposits exhibit predominant (2 0 2) plane at 53° which is in constrast to high intensity (0 1 2) line observed for standard bismuth
4. Conclusions In the present work, electrochemical approach to recover antimony and bismuth from chloride based electrolyte is proposed. Linear sweep voltammetry indicates that a minimum of −0.2 V would be necessary to recover bismuth and antimony. Electrodeposition was carried out by varying parameters such as current density, potential, temperature and time. At an optimized potential of −0.5 V and current density of 5 A/ dm2, samples with highest antimony content can be electrodeposited.
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500 X
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Yang, Y.H. Kim, Development of a metal recovery process from Li-ion battery wastes, Hydrometallurgy 79 (2005) 172–181. [24] W.-S. Kang, W.-C. Chou, W.-J. Li, T.-H. Shen, C.-S. Lin, Electrodeposition of Bi2Te3based p and n-type ternary thermoelectric compounds in chloride baths, Thin Solid Films 660 (2018) 108–119. [25] F. Besse, C. Boulanger, J.M. Lecuire, Preparation of Bi1-x Sbx films by electrodeposition, J. Appl. Electrochem. 385–392 (2000). [26] S. Muller, et al., Electrochemical synthesis of Bi1-xSbx nanowires with simultaneous control on size, composition, and surface roughness, Cryst. Growth Des. 12 (2012) 615–621. [27] M. M-Gonzalez,, et al., Electrodeposition of Bi1-xSbx films and 200-nm wire arrays from a non-aqueous solvent, Chem. Mater. 15 (2003) 1676–1681. [28] P. Radhakrishnamurty, Current efficiency of alloy plating and the electrochemical equivalent of an alloy, Bull. Electrochem. 15 (1999) 252–256. [29] S.A. Awe, J.-E. Sundkvist, N.-J. Bolin, Å. 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(a)
(b)
Fig. 7. SEM micrographs of electrodeposited samples at applied potential of (a) −0.3 V and (b) −1 V.
At higher potential of 1 V and current densities < 5 A/dm2, content of antimony and bismuth were nearly equal in the deposits. Galvanostatic approach yielded higher recovery than potentiostatic conditions in all experiments which was also concurred during kinetic studies. Kinetics of deposition follows the order Sb = Fe > Bi at 5 A/dm2 whereas the order was Sb > Bi > Fe at −0.5 V and data indicates that quantitative recovery of antimony and bismuth can be achieved within 10 h and 15 h of electrolysis, respectively. Increase in bath temperature lowers the electrochemical metal recovery. HER is primary competing reaction to metal deposition even at low potentials and current densities. Surface morphology studies showed that potentiostatic recovery yielded more compact and uniform deposits than galvanostatically recovered samples. X-ray diffraction data concurred the metallic nature of antimony and bismuth in the recovered samples. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgement V.R.Chithambara Thanu thanks financial support through sponsored project SSP-42/2017. The authors thank Central Instrumentation Facility, CSIR Central Electrochemical Research Institute, Karaikudi 630003, India for characterization facilities. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116169.
7
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Electrochemical recovery of antimony and bismuth from spent electrolytes a
a,b,⁎
V.R. Chithambara Thanu , M. Jayakumar a b
T
Electroplating and Metal Finishing Division, CSIR Central Electrochemical Research Institute, Karaikudi 630 006, Tamil Nadu, India Academy of Scientific and Innovative Research (AcSIR), CSIR Central Electrochemical Research Institute, Karaikudi 630 006, Tamil Nadu, India
ARTICLE INFO
ABSTRACT
Keywords: Bismuth Antimony Electrodeposition Recovery Spent Electrolyte
In the present study, direct electrowinning is adopted for the recovery of antimony and bismuth from a quaternary chloride electrolyte that depicts the spent electrolyte from copper extraction. Reduction behavior of metal ions in electrolytes was studied using linear sweep voltammetry. Electrowinning of antimony and bismuth was carried out under potentiostatic and galvanostatic conditions. Electrodeposition parameters such as current density, potential, and temperature were systematically varied to understand their effect on the recovery of antimony and bismuth. The recovered metals are characterized for their composition and phase purity using XRF, XRD and surface morphology by SEM. By careful choice of conditions, it is possible to recover bismuth and antimony from the chloride bath. At a potential of −0.5 V as well as at a current density of 5 A/dm2, antimony rich deposits can be obtained at ambient bath temperatures.
1. Introduction Reduce, Recycle and Reuse (3Rs) are considered the cornerstones for a sustainable lifestyle and recovery of materials from industrial wastes and end-of-life devices is an important step towards achieving it. Critical elements like antimony and bismuth are widely used in semiconductor, thermoelectric, pharmaceuticals, chemicals, ceramics, pigments and as alloying element. However, their limited natural resources and continuously expanding applications such as thermoelectric demand alternate sources. Extractive metallurgy of non-ferrous metals such as copper, silver, lead from sulfide ores results in solid and liquid wastes, which contain metals with chemical proximity such as Ag, Bi, Sb, Fe and Tl. Such waste can be considered as potential resource and approaches to recover them can have economic and environmental impacts. At present the cost of antimony and bismuth are 8800 US$/t and 2800 US$/t (LMX). Further, these metals and their compounds, particularly, antimony have adverse health effects such as cardiovascular, respiratory issues upon prolonged exposure. Conventional approaches such as disposing underground and burning in air can contaminate the environment and are unsustainable [1]. Therefore there is a pressing necessity to treat wastes and effluents containing them prior to discharge. Further, among all the antimony and bismuth used around the world only 20% is recycled. China contributes to 90% of antimony and 75% of bismuth production and any break-down in economic or political equation between rest of the world with China may hamper global supply of these critical metals.
⁎
There is an increased focus on the recovery of antimony and bismuth from the industrial wastes. Generally, recovery of metals is carried out by various approaches such as solvent extraction, precipitation, ion-exchange, crystallization, electrowinning, etc. Electrodeposition is a low-cost, scalable and simple approach to recover metals that can be electrodeposited from a specific media and does not require sophisticated equipments. The metals most commonly recovered by electrolysis are gold, silver, copper, manganese, iron, nickel, cobalt, zinc, cadmium, tin and lead. The electro-recovery of metals by the electrolysis of solutions of metal compounds, employing an electrolytic cell in which the metal is deposited upon a cathode, has long been known and is widely practiced. Electroplating process can be fine-tuned by suitable choice of parameters, additives, temperature, etc. thereby yielding deposits with desired morphology, composition, functionality and high purity. However, electrowinning has limitations such as slow kinetics, relatively low throughput, etc. Khaliq et al. reviewed the hydrometallurgical and electrometallurgical processing for the recovery of pure base and precious metals [1]. Chen et al. recovered bismuth and arsenic from flue dusts arising from copper extraction via leaching and precipitation and showed a recovery of 90% and 42%, respectively [2]. Several groups have reported recovery of metals from waste generated from metallurgical industries [3,4]. Sumitomo Metal Mining has developed a process to recover antimony and bismuth by ion-exchange and direct electrowinning in metallic form [5]. Hoffmann discussed solvent extraction and ion exchange strategies for the recovery of Sb, Bi, As, Ni
Corresponding author. E-mail addresses: [email protected], [email protected] (M. Jayakumar).
https://doi.org/10.1016/j.seppur.2019.116169 Received 1 July 2019; Received in revised form 16 September 2019; Accepted 4 October 2019 Available online 04 October 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
nitrates were dissolved in hydrochloric acid with heating at 80 °C for 30 min. The clear solution obtained was subsequently diluted with aqueous solution containing known concentration (50–70 g/L) of potassium chloride. The resultant solution had a composition of ~1/10th of actual discharge arising from copper extraction process. Care was taken to avoid formation of any precipitate in the solution. Linear sweep voltammetry studies were performed using 3-electrode configuration using Biologic SP-200 electrochemical workstation. Copper wire was taken as working electrode as it was proposed as cathode for electrolysis. Silver wire acted as quasi-reference electrode and the counter electrode chosen was TSIA (Titanium substrate insoluble anode). Electrodeposition experiments were carried out using two electrode setup with copper cathode (2 × 2 cm) and titanium substrate insoluble anode (TSIA) having inter-electrode distance of 4 cm in 100 ml bath. Choice of electrode was based on their stability in the acidic chloride bath. Electro-recovery was carried out at constant potential and constant current conditions. Electrolysis duration was 1 h and temperature was 303 K except temperature variation studies. All electrolysis experiments were carried out under constant stirring of 300–400 rpm. Post electrolysis the cathode was retrieved, carefully washed with deionized water and dried. Weight of the deposit was calculated from increase in cathode weight. The elemental composition was analyzed by X-ray Fluorescence Spectrometer (Horiba, XGT-5200) and phase purity is analyzed by X-ray Diffractometer-Multipurpose (Rigaku Smart lab guidance). The surface morphology and composition is analyzed with Scanning Electron Microscope (Tescan Vega 3 SBH EDS). The Faradaic efficiency for alloy deposition was calculated according to the following Eq. (1) [28].
Fig. 1. Linear sweep voltammetry of chloride bath containing antimony, bismuth and iron at a scan rate of 20 mV/s. Inset: Voltammetric curve for the potential range of −0.05 (open circuit potential) to −1.5 V. Working electrode: Cu wire (surface area = 0.35 cm2), Reference electrode: Ag wire, Counter electrode: titanium substrate insoluble anode. Temperature: 303 K. Quaternary electrolyte composition is given in ESI 1. For unitary electrolytes, concentration for bismuth and antimony is as in ESI 1.
wherein significant decrease in Sb and Bi concentrations by ion exchange was reported [6]. Selective adsorption [7] and precipitation using soluble sulfates of barium, strontium and lead [8] are some of the techniques reported. King et al. reported addition of lead monoxide for the removal of bismuth which did not have any significant effect on the levels of Sb, As and Cu [9]. Several research groups have reported recovery of noble and base metals such as Au, Ag, [10–13] and Cu, Zn, Pb [14–18] and critical metals such as cobalt, lithium, rare-earth, [19–23] etc. Leaching followed by solvent extraction, precipitation, electrolysis [13,14], are some of the approaches used to recover these metal values from secondary sources such as industrial waste and end-of-life products. Reports are available on electrodeposition of Bi-Sb based films from chloride based baths [24–26] and non-aqueous electrolytes [27] for thermoelectric applications. In the present work, we carried out electro-recovery of antimony and bismuth from chloride electrolytes that closely depict the reject from copper extraction process. The present study is an academic attempt to understand the recovery of critical metal values utilizing the solution composition shared by industry. Effect of applied current, potential, temperature, electrolysis duration was studied and deposits are characterized by various physicochemical techniques. Element-wise kinetics of electrodeposition was also investigated. For comparison, voltammetry and electrodeposition in unitary electrolytes of bismuth and antimony were also studied.
CEa =
m1 W1
+
m2 W2 It F
+
m3 W3
x100
(1)
wherein CEa is the Current efficiency of alloy deposition, ma is the mass of metal 'a' in the deposit, Wa is the Electrochemical equivalent weight of metal 'a', I is the Current passed (A), t is the Time (sec), F is the Faraday constant (C) 3. Results and discussion 3.1. Linear sweep voltammetry Linear sweep voltammetry (LSV) is a useful technique to have meaningful insights on the redox reaction occurring at the electrode. LSV can offer possible potential to be applied for quantitative recovery of desired metal from a multi-element solution. In the present study, we used a three electrode set-up for LSV to assimilate the potential range for electro recovery of bismuth and antimony. Fig. 1 presents the LSV experiment carried out in the potential range of −0.05 (open circuit potential) to −2.0 V at 303 K at a scan rate of 20 mV/s. In quaternary electrolyte, onset of broad reduction wave was observed at −0.13 V with peak at −0.17 V followed by continuous increase in cathodic current. Voltammetry was also carried out for individual bismuth and antimony electrolytes. LSV of antimony electrolyte shows occurrence of broad reduction wave at −0.064 V and minor peak at −0.16 V. Voltammogram of bismuth electrolyte shows the presence of two reduction peaks; the first sharp one with onset at −0.066 V and peak at −0.12 V, and another wave with onset at −0.16 V and peak at −0.2 V. Minor reduction waves observed at more cathodic potentials could be due to
2. Experimental Simulated solution containing bismuth, antimony and iron was prepared matching the composition of process solution arising from copper extraction from sulfide ores. Typical composition for a process solution from copper extraction is given in ESI 1. Stoichiometric quantities of antimony trioxide, copper chloride, bismuth and iron
Table 1 Electrochemical recovery of antimony and bismuth as a function of applied potential. Electrolysis duration: 60 min. Temperature: 303 K. Voltage (V)
Recovery Obtained (g/m2 h)
Bi (%)
Sb (%)
Fe (%)
Bi wt (g/m2 h)
Sb wt (g/m2 h)
Fe wt (g/m2 h)
Current efficiency (%)
−0.3 −0.5 −1.0
0.727 40.7 152.3
9.1 12.3 48.2
77.8 81.7 50.8
12.9 5.9 0.9
0.067 5 73.5
0.57 33.3 77.5
0.09 2.4 1.3
76.1 49.5 28.5
2
Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
BiCl4 + 3e
(3)
Bi + 4Cl
xSbCl4 + yBiCl4 + 3e
Bi x Sb y + 4Cl
(4)
While the standard redox potentials of bismuth and antimony are separated by 100 mV, the difference is lowered by one order to ~10 mV (0.16 V for BiCl4−/Bi, 0.17 V for SbCl4−/Sb) when complexed in chloride solutions [25]. As a result, electrodeposition of BiSb compounds is more favoured in chloride baths resulting in one reduction peak with higher current. Comparison of voltammograms shows that the current observed in the quaternary electrolyte is nearly equals the combined currents of bismuth and antimony. Further, there is one significant reduction peak in the quaternary electrolyte indicating possible formation of Bi-Sb alloy deposits. The shift of reduction peak to more negative potentials in bismuth-antimony system than observed in unitary electrolyte is consistent with literature [24]. Voltammetric data indicates that a minimum of −0.2 V would be necessary to recover bismuth and antimony from individual and quaternary electrolytes. Further, the continuously increasing cathodic current at potentials > −0.4 V in unitary and quaternary electrolytes clearly indicate that hydrogen evolution reaction competes with electrodeposition of antimony and bismuth. 3.2. Electrolysis experiments 3.2.1. Effect of applied potential Electrolysis experiments were carried out under potentiostatic conditions for the recovery of bismuth and antimony. Potentiostatic method offers a possibility for selective recovery of a particular metal or group of metals when a potential closer to their standard redox potentials is applied. Table 1 and Fig. 2(a) presents the results of electrolysis carried out at three different potentials, −0.3, −0.5 and −1 V. Increase in applied potential results in higher recovery of metals; from < 1 g/m2 h at −0.3 V to ~152.3 g/m2 h at −1.0 V. Antimony recovery increases from 0.57 g/m2 h at −0.3 V to 77.5 g/m2 h at −1 V and bismuth escalates from 0.067 g/m2 h at −0.3 V to 73.5 g/m2 h at −1 V. At optimized potential of –0.5 V, deposits with maximum antimony content can be recovered. Sudden increase in bismuth recovery at higher cathodic potentials can be attributed to its slightly higher reduction potentials. Current efficiency values of 76.1%, 49.5%, 28.5% were observed for the applied potentials of −0.3, −0.5 and −1 V, respectively. This observation clearly indicates that hydrogen evolution strongly concurs with electrowinning of bismuth and antimony thereby lowering current efficiency. At similar potentials, negligible deposition was observed in unitary electrolytes which indicates possibility of other mechanisms in an multi-element electrolyte such as formation of stable Sb-Bi solid solutions, induced codeposition. Although there are reports on electrodeposition of antimony – bismuth alloys, the authors used pure solutions with low metal concentrations (< 0.1 M) to obtain functional thin films [25–27] and the data is not comparable to the present study. 3.2.2. Effect of current density Constant current methodology is widely used in electroplating industry as it ensures constant rate of electrodeposition by maintaining steady mass transport of active material to the substrate. The Table 2 and Fig. 2(b) show the effect of current density on recovery of bismuth and antimony. Recovery data shows that galvanostatic route offers higher recovery than potentiostatic conditions. The amount of material deposited is not precisely proportional to the current applied. The data shows contrasting trends of bismuth and antimony recovery with increase in current density from 2 to 5 A/dm2. Bismuth shows slight increase from 140 g/m2 h to 164.8 g/m2 h, but drops to 106.8 g/m2 h at 5 A/dm2. On the other hand antimony content in the deposit marginally lowers from 55% to 52% at 2–3 A/dm, but at 5 A/dm2 escalates to 72%. Although bismuth enrichment is expected at higher current density, the
Fig. 2. Electrochemical recovery of antimony and bismuth as a function of various parameters (a) applied potential, (b) current density and (c) bath temperature.
the reduction of other chloro complexes that are more stable. The plausible reaction mechanism in unitary (Equations (2) and (3)) and quaternary electrolytes (Eq. (4)) could be [25];
SbCl4 + 3e
Sb + 4Cl
(2) 3
Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
Table 2 Electrochemical recovery of antimony and bismuth as a function of current density. Electrolysis duration: 60 min. Temperature: 303 K. Current density (A/dm2) 2 3 5
Recovery obtained (g/m2 h) 312.52 342.57 376.58
Bi (%) 44.8 48.1 28.3
Sb (%) 55.1 51.8 71.4
Fe (%) 0.07 0.08 0.1
Bi wt (g/m2 h) 140.1 164.8 106.8
Sb wt (g/m2 h) 172.2 177.5 269.4
Fe wt (g/m2 h) 0.22 0.27 0.38
Current efficiency (%) 38.3 13.5 13.8
Table 3 Effect of bath temperature on electrochemical recovery of antimony and bismuth with applied potential of −0.5 V and duration of 1 h. Bath temperature (°C)
Recovery obtained (g/m2 h)
Bi (%)
Sb (%)
Fe (%)
Bi wt (g/m2 h)
Sb wt (g/m2 h)
Fe wt (g/m2 h)
Current efficiency (%)
30 50 70
38.67 34.4 23.93
14 35 37.7
78.1 53 55
7 10 7.2
5.47 12.3 9
30.5 18.6 13.2
2.7 3.5 1.73
40.13 68.13 83.18
Table 4 Kinetics of antimony and bismuth recovery from the chloride bath at an applied potential of −0.5 V. Temperature: 303 K. Time (min)
30 60 120 240 360 480
Stepwise Recovery
Cumulative Recovery
Weight of recovered deposits in this step (g/m2 h)
Composition of the deposit Bi%
Sb%
Fe%
140 96.25 215.5 610.25 412.75 196
54 80 75 59 25 16
44 18 24 40 70 83
0.6 0.8 0 0.1 4.2 0.12
Overall weight of the recovered deposits till this step (g/m2 h)
% recovery from the bath Bi
Sb
Fe
140 236.2 451.7 1062 1474 1670
5.0 10.1 20.9 44.9 51.8 48.8
6.1 7.8 13.0 37.4 66.3 82.6
3.4 6.7 6.7 10.1 79.5 80.4
Table 5 Kinetics of antimony and bismuth recovery from the chloride bath at a current density of 5 A/dm2. Temperature: 303 K. Time (min)
30 60 120 240 360 480
Stepwise recovery
Cumulative recovery
Weight of recovered deposits in this step (g/m2 h)
Composition of the deposit Bi%
Sb%
Fe%
219.75 181.25 208.75 289.75 289.25 257.25
34 53 45 56 75 43
63.8 55.8 54 42 24 55
1.5 0.2 < 0.1 0.4 0 1.4
deposits were richer in antimony content. The results show that the enhanced recovery rate of antimony at higher current density favors over the deposition of bismuth. Similar trend is observed in kinetics data presented in Section 3.2.5. Iron shows increasing trend in recovery with current density. Recovery of bismuth and antimony from unitary electrolyte at various current densities were carried out to understand the behavior of individual elements (ESI 2). At a current density of 2 A/ dm2, the average recovery was 40 and 400 g/m2 h for antimony and bismuth, respectively from their unitary solutions (ESI 2). Under identical conditions, the recovery was about 300 g/m2 h from the synthetic quaternary electrolyte with a average deposit composition of 1:1 (Sb: Bi). In other words, the presence of other ions lowers the recovery of bismuth while antimony recovery is enhanced. Recovery in quaternary electrolyte is relatively lower than observed in unitary electrolyte which may be due to interfering cyclic redox reactions such as Fe2+/3+. The calculated Faradaic efficiency values were 38%, 14% and 14% for the current densities of 2, 3 and 5 A/dm2, respectively. The values for constant current experiments are inferior to potentiostatic mode. In case of latter, at applied potentials ≤−0. 5 V, hydrogen evolution reaction (HER) and metal deposition compete equally resulting in %CE ~50%, which degrades at higher potential. Using cyclic voltammetry,
Overall weight of the recovered deposits till this step (g/m2 h)
% recovery from the Bath Bi
Sb
Fe
219.75 401 609.7 899.5 1188 1446
4.9 11.3 17.6 28.4 42.9 45.3
14.0 24.1 35.4 47.5 54.5 68.6
13.8 15.8 16.2 21.7 21.7 36.1
Besse et al. reported the possibility of hydrogen evolution competing with bismuth - antimony electrodeposition at higher overpotentials [25]. In galvanostatic conditions, the potential of electrochemical cell can shift to higher potentials resulting in hydrogen evolution as the primary reaction. While lower current density can suppress HER, experiments at current densities < 2 A/dm2 yielded insignificant deposits from the present bath composition. 3.2.3. Effect of temperature Electrolysis of quaternary electrolyte was carried out at an optimum potential of −0.5 V at various bath temperatures to understand its relation with electro-recovery of antimony and bismuth and the results are presented in Table 3 and Fig. 2(c). The results indicate an inverse proportional relationship between temperature and amount of electrodeposits. Recovery lowered by 50% when temperature was increased from 30 to 70 °C. With increase in temperature, the amount of antimony decreases nearly one-third from 30.5 g/m2 h to 13.2 g/m2 h whereas the recovery of bismuth is the range of 5.4–12.3 g/m2 h. It is well-known that hydrogen evolution becomes more prominent with increase in temperature. In the present case, HER may compete and impede metal deposition on the electrode surface at elevated temperatures which can possibly explain the decrease in bismuth and antimony recovery. 4
Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
However, the authors observed lower current flowing through the electrolytic cell with increase in temperature. Concurrently, current efficiency data shows improvement with temperature from 40% at 30 °C to 68% and 83% at 50 and 70 °C, respectively. Such unusual trend has been reported by some authors; Awee et al. reported increase in Faradaic efficiency with temperature during galvanostatic electrowinning of antimony which was attributed to reduced solution resistance and lowering of viscosity [29]. However, the observed lower recovery and its relation to increase in Faradic efficiency at elevated temperatures remain an open question and necessitate a further detailed study. 3.2.4. Kinetics of bismuth and antimony electrowinning In comparison with other metal recovery methods, electrochemical routes are limited by slower kinetics. It is essential to understand the kinetics of recovery of bismuth and antimony to validate them for possible commercial application. Electrolysis was carried out by potentiostatic mode at −0.5 V and galvanostatic condition at a current density of 5 A/dm2 in which the samples deposited were weighed and analyzed at different durations. Table 4 and 5 and Fig. 3 show the kinetics of deposition at 303 K. During a deposition duration of 30 min at −0.5 V, 140 g/ m2 h of deposit was obtained with a composition of 55% bismuth, 44% antimony and negligible iron. Increase in electrolysis duration results in cumulatively increase in recovery of metals. However, the content of bismuth and antimony in the deposit in each stage was different. As a result, the rate of recovery of antimony was higher than bismuth. At the end of 8 h of electrolysis, close to 83% of antimony and 49% of bismuth originally present in the electrolyte were recovered. Kinetics of iron deposit showed close to 80% recovery. Under galvanostatic mode, close to 220 g/m2 h of metals can be deposited within 30 min. Content of antimony and bismuth in the recovered deposit was 40–65% in most cases. At the end of 8 h, close to 69% of antimony and 45% of bismuth were recovered from the bath. If comparison could be drawn between constant potential and current approaches despite their fundamental differences, antimony recovery is rapid under constant potential conditions whereas bismuth recovery reaches a plateau around 50% in both the cases. Trend in kinetics data concurs the potentiostatic and galvanostatic electrodeposition (Sections 3.2.1 and 3.2.2) wherein bismuth recovery is improved at more cathodic potentials, the rate of antimony deposition is more crucial factor that enhances overall recovery. Based on the kinetics of
Fig. 3. Rate of recovery of antimony and bismuth from the chloride bath at 303 K (a) applied potential of −0.5 V and (b) current density of 5 A/dm2.
Fig. 4. X-Ray Diffraction pattern of metals recovered at current density of 2 and 5 A/dm2. Standard antimony (JCPDF file 00-001-0802) and bismuth (JCPDF file 00001-0699) are given for comparison. (0 1 2) plane of samples obtained at 4 A/dm2 is present as inset. 5
Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
Fig. 5. XRD pattern of samples electrodeposited at applied potential of −1.0 V and −0.3 V. Standard antimony (JCPDF file 00-001-0802) and bismuth (JCPDF file 00-001-0699) are presented for comparison. Inset shows (0 1 2) plane of samples obtained at −1.0 V.
500X
and antimony. The broad peak (0 1 2) at 1 A/dm2 indicates the formation of Bi-Sb alloy [25,30] and the minor shoulder may be due to possible traces of other phases. Fig. 5 shows the X-ray diffraction analysis of recovered deposits at constant potential. Antimony gives high intensity peak with weakly defined shoulder corresponding to (0 1 2) plane. Planes corresponding to (0 1 2), (0 1 5), (1 1 0) and (2 0 2) are strong in electrodeposits obtained from both constant potential and current studies. However, (1 0 4) plane is relatively low in the samples obtained under applied potential. No significant peaks corresponding to oxides of bismuth and antimony were observed in both cases. However low intensity iron oxide peaks observed around 30° are due to Fe2O3. Absence of additional peaks further validates the possible deposition of single phase Bi-Sb alloy.
5kX
(a)
3.2.6. SEM analysis of the electrodeposited metals The scanning electron microscopy were recorded for electrodeposits from current and potential variation studies to understand the underlying relationship between the morphology of the recovered metals and the experimental conditions. Fig. 6 shows the SEM micrographs of samples deposited at current densities of 2 and 5 A/dm2 which shows agglomerated morphology with smaller particles at higher current densities. Fig. 7 shows micrographs of deposits under potentiostatic conditions. At a lower potential of −0.3 V, porous, inter connected structure was observed in the deposits. Increase in potential to −1 V yields less porous deposits which indicates that increase in applied potential results in more compact deposits. For comparison, SEM micrographs of recovered bismuth and antimony from individual baths are also presented in ESI 3 and 4.
(b)
Fig. 6. SEM images of recovered metals at current density of (a) 2 A/dm2 and (b) 5 A/dm2.
deposition at −0.5 V up to 8 h, it is expected that quantitative recovery of antimony may be achieved by the end of 10 h. 3.2.5. XRD analysis of the electrodeposited metals X-ray diffraction is employed to know the crystal phase of elecrorecovered deposits. Fig. 4 presents the X-ray diffraction analysis of recovered deposits under galvanostatic conditions. Standard data of bismuth (JCPDS 00-001-0699) and antimony (JCPDS 00-001-0802) are presented for comparison. The X-ray pattern of the deposits are in between the metallic bismuth and antimony but closer to the latter. The recovered deposits exhibit predominant (2 0 2) plane at 53° which is in constrast to high intensity (0 1 2) line observed for standard bismuth
4. Conclusions In the present work, electrochemical approach to recover antimony and bismuth from chloride based electrolyte is proposed. Linear sweep voltammetry indicates that a minimum of −0.2 V would be necessary to recover bismuth and antimony. Electrodeposition was carried out by varying parameters such as current density, potential, temperature and time. At an optimized potential of −0.5 V and current density of 5 A/ dm2, samples with highest antimony content can be electrodeposited.
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Separation and Purification Technology 235 (2020) 116169
V.R.C. Thanu and M. Jayakumar
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References
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(a)
(b)
Fig. 7. SEM micrographs of electrodeposited samples at applied potential of (a) −0.3 V and (b) −1 V.
At higher potential of 1 V and current densities < 5 A/dm2, content of antimony and bismuth were nearly equal in the deposits. Galvanostatic approach yielded higher recovery than potentiostatic conditions in all experiments which was also concurred during kinetic studies. Kinetics of deposition follows the order Sb = Fe > Bi at 5 A/dm2 whereas the order was Sb > Bi > Fe at −0.5 V and data indicates that quantitative recovery of antimony and bismuth can be achieved within 10 h and 15 h of electrolysis, respectively. Increase in bath temperature lowers the electrochemical metal recovery. HER is primary competing reaction to metal deposition even at low potentials and current densities. Surface morphology studies showed that potentiostatic recovery yielded more compact and uniform deposits than galvanostatically recovered samples. X-ray diffraction data concurred the metallic nature of antimony and bismuth in the recovered samples. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgement V.R.Chithambara Thanu thanks financial support through sponsored project SSP-42/2017. The authors thank Central Instrumentation Facility, CSIR Central Electrochemical Research Institute, Karaikudi 630003, India for characterization facilities. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116169.
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