Journal of Hazardous Materials 280 (2014) 191–199

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Recycling of electric arc furnace dust through dissolution in deep eutectic ionic liquids and electrowinning Ashraf Bakkar a,b,∗ a b

Metallurgical & Materials Engineering Department, Faculty of Petroleum & Mining Engineering, Suez University, P.O. Box 43721, Suez, Egypt Department of Environmental Engineering, College of Engineering at Al-Lith, Umm Al-Qura University, Corniche Road, Al-Lith City, Saudi Arabia

h i g h l i g h t s • • • •

Ionic liquid (1 choline chloride:2 urea) dissolved about 60% of Zn and 39% of Pb present in EAF dust. CV studies on the electrolyte formed showed distinct deposition and stripping peaks for Zn and Pb. Zn–Pb alloy was electrowon by application of high static potentials determined from CV diagrams. Lowering Zn and Pb contents allows recycling the dust in iron/steelmaking and in cement synthesis.

a r t i c l e

i n f o

Article history: Received 3 June 2014 Received in revised form 21 July 2014 Accepted 23 July 2014 Available online 10 August 2014 Keywords: Recycling Electrodeposition EAF dust Zinc Deep eutectic solvents

a b s t r a c t The dust waste formed during steelmaking in electric arc furnace (EAF) is rich in ferrous and nonferrous metals. Recycling of this dust as a raw material in iron or steel-making is hazardous and therefore it is mostly dumped. This paper demonstrates recycling of EAF dust through selective dissolution of metal oxides in a deep eutectic ionic liquid. It was found that about 60% of Zn and 39% of Pb could be dissolved from the dust when stirred for 48 h in 1 choline chloride:2 urea ionic liquid at 60 ◦ C. The resultant electrolyte was subsequently fed to a conventional three-electrode cell where cyclic voltammetry (CV) measurements were conducted to describe its electrochemical behavior. Two deposition peaks were determined and ascribed to deposition of zinc and lead. Static potentials were successively applied to electrowin metallic zinc. SEM/EDX investigations showed that the zinc electrowon contained remarkable contents of lead. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Electric arc furnace (EAF) is used for steelmaking from ferrous scraps, as the main charge material, mixed with pig iron and/or direct reduced iron. It produces between 10 and 25 kg of dust per ton of steel [1–5]. This dust consists of metal oxides, lime, and silica. It contains mainly iron, zinc, lead, chromium, and cadmium. Presence of toxic elements such as Pb, Cr, and Cd has led the EAF dust to be categorized as hazardous waste by Environmental Protection Agency (EPA) in United States [1]. The EAF dust is also listed as hazardous waste in the European Waste Catalog [6]. The toxic elements Pb, Cd and Cr leach in water exceeding their maximum limits in groundwater, and this necessitates treating the EAF dust

∗ Correspondence address: Metallurgical & Materials Engineering Department, Faculty of Petroleum & Mining Engineering, Suez University, P.O. Box 43721, Suez, Egypt. Tel.: +20 128 0212000; fax: +20 623 360252. E-mail addresses: [email protected], [email protected] http://dx.doi.org/10.1016/j.jhazmat.2014.07.066 0304-3894/© 2014 Elsevier B.V. All rights reserved.

before landfilling [1,5,7] or storing the dust in appropriate places protected from rain [3]. The type of scrap melted in the EAF predominantly determines chemical composition of the dust generated. When galvanized steel scraps are used in the EAF, most of the zinc ends up in the dust and fume due to its very low solubility in the molten steel and slag [3]. Zinc has higher vapor pressure at steelmaking temperatures, and consequently vapor zinc leaves the furnace along with other gases and fumes of species volatilized at the hot spots in the arc zone and oxygen jet zone, in addition to sucked droplets by bubbles burst at the liquid bath surface [2]. In the meanwhile, simple and complex compounds, like ZnO, PbO and ZnF2 O4 , are formed [1–4]. The gas stream is finally treated and the dust is collected by filters. The main quantitative constituents in the dust are iron and zinc [3]. The latter has low content when the galvanized steel is not intensive in the scrap used in the EAF [5]. The low zinc-bearing iron dust can be recycled in the charge of EAF [8] or blast furnace [9]. However, the use of high zinc-bearing iron dust in the charge of blast furnace leads to such undesirable consequences as

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Table 1 Typical chemical composition (in wt.%) of EAF dust, as-received and after dissolution in the ionic liquid. Oxide

CaO

SiO2

MgO

Al2 O3

MnO

Fe2 O3

ZnO

PbO

K2 O

TiO2

Cr2 O3

P2 O5

SO3

F

Cl

Br

NiO

CuO

SrO

As-received After dissolution in ionic liquid

30.31 31.18

3.62 11.18

2.81 3.99

0.73 1.98

2.68 4.54

15.58 30.45

25.18 10.05

2.80 1.71

2.10 0.30

0.11 0.44

0.14 0.28

0.27 0.49

4.16 2.13

0.23 0.43

8.91 0.54

0.10 0.08 – 0.09

0.15 0.17

0.04 0.05

an increase in the zinc content of the pig iron and the formation of crusts inside the furnace that interfere the normal operation. Moreover, zinc penetrates the furnace lining through deformation and disintegration and leads to its attack [9]. These aspects, combined with growing concern about environmental issues, led researchers and steelmakers to develop several approaches for recycling EAF dust. These approaches are mainly categorized under hydrometallurgical [10–19] and pyrometallurgical processes [8,9,20–33]. Additionally, various applications for reuse of the EAF dust have been suggested such as incorporation in glass and ceramic products [34–40] and incorporation in synthesis of cement [41–46]. However, among the methods suggested for recycling EAF dust, pyrometallurgical processes are still the ones which have been applied in industry [9,31,32]. Nevertheless, about 60% of the EAF dust generated worldwide is still being dumped [9,31] due to problems associated with pyrometallurgical processes such as high capital and operating costs, loss of iron in slag and higher impurity content of zinc grade produced [9]. As a new approach for using non-aqueous solutions to dissolve selective metals from oxide mixtures, a recent study [47] has investigated the leaching of blast furnace dust using carboxylic acids. The solution dissolved zinc from the iron-rich dust so that the dust can be reused directly in the ironmaking process. However, because the Zn content in the blast furnace dust was too small to be economically processed, the study did not present any trial to recover Zn from the leaching solution. Other approaches have been developed for using air and water-stable ionic liquids along with aqueous solutions in multi-step procedures to extract some ferrous and nonferrous metals [48], palladium [49], uranium oxide [50], and rare earth elements [51]. In a more practical approach, a new class of environmentally friendly ionic liquids has offered not only the possibility of selective dissolution of some metal oxides, but also the subsequent electrodeposition of the metals [52–55]. This new class of ionic liquids, namely deep eutectic ionic liquids or deep eutectic solvents,

is based on combination of choline chloride with hydrogen bond forming compounds such as urea and ethylene glycol [56]. Deep eutectic ionic liquids have been successfully used for electrodeposition of Zn from ZnO [52,53] and from ZnCl2 [57,58]. The aim of the present study was to verify a new approach by which zinc and lead-bearing compounds were dissolved from EAF dust in a deep eutectic solvent, namely 1 choline chloride:2 urea ionic liquid. This was followed by electrowinning of zinc layer containing remarkable contents of lead. The electrodeposition of metallic phase from the ionic liquid was characterized by cyclic voltammetry measurements. In addition, the microstructure and composition of the electrowon metal was described by SEM/EDX investigations. The study included also the assessment of the chemical composition of EAF dust, as-received and after dissolution experiments, by X-Ray fluorescence (XRF).

2. Materials and methods 2.1. EAF dust The EAF dust investigated in this study was obtained from Suez Steel Company, Suez, Egypt. The major oxides composition of asreceived dust was investigated by X-ray fluorescence (XRF), and listed in Table 1. In addition, its mineralogical composition was detected by X-ray diffraction (XRD), see Fig. 1.

2.2. Ingredients of the ionic liquid The deep eutectic ionic liquid consisted of choline chloride and urea. Choline chloride (HOC2 H4 N(CH3 )3 + Cl− , Alfa Aesar, 98+%) and urea (NH2 CONH2 , Alfa Aesar, 98+%) were used as received without further purification or drying. They were handled in the lab atmosphere in open-to-air conditions without using glove box.

Fig. 1. XRD pattern of the as-received EAF dust

A. Bakkar / Journal of Hazardous Materials 280 (2014) 191–199

2.3. Preparation of ionic liquid Choline chloride and urea with a stoichiometric eutectic molar ratio of 1:2, respectively, were mixed in a glass beaker, and heated gently up to 90 ◦ C using hot plate with magnetic stirrer. Heating associated with magnetic stirring continued until a homogeneous colorless liquid was formed. Then, the liquid was cooled and stored in a stopper-locked conical flask for using in further dissolution of EAF dust. 2.4. Dissolution of EAF dust 10 g of EAF dust was gradually added to 200 ml of the formed 1 choline chloride:2 urea ionic liquid undergone stirring with velocity of 500 rpm at 60 ◦ C in 500 ml conical flask put on a hot plate with magnetic stirrer. Dissolution of EAF dust in the ionic liquid was continued for 48 h, with keeping heating and stirring. Then, the solution was filtered and the solid residue was washed by hot distilled water and alcohol, dried and sampled for analysis. The filtered ionic liquid was stored in a stopper-locked conical flask for using as an electrolyte for cyclic voltammetry and electrowinning experiments. 2.5. CV measurements and electrowinning experiments Cyclic voltammetry (CV) measurements and electrowinning experiments were carried out at 60 ◦ C in a conventional threeelectrode electrochemical cell using the Potentio-galvanostat model “VersaSTAT 3, Princeton Applied Research”, controlled by a PC via the software program “VersaStudio”. The electrochemical cell consisted of ionic liquid-containing beaker with a capacity of 100 ml. This beaker was filled with 60 ml of the electrolyte and covered with a PVC sheet with openings for holding the electrodes. For CV measurements, a platinum sheet with working area of 1 cm2 was connected as a working electrode. A platinized network sheet was connected as a counter electrode and held at 30 mm far from the working electrode. A highly pure silver wire was connected as a reference electrode and suspended at 20 mm far from the other two electrodes. CV runs were carried out in static electrolytes without stirring. For electrowinning experiments, two platinized titanium network sheets (80 mm × 20 mm × 1 mm) were immersed in the electrolyte and connected as a counter electrode. A highly pure copper sheet (80 mm × 10 mm × 0.5 mm) was connected as a working electrode and immersed between the two platinized network sheets with a distance of about 15 mm far from each sheet. The Cu sheet was immersed to allow coating of area of 20 mm × 10 mm from two sides. The silver wire was connected as a reference electrode. Moderate stirring of 50 rpm with magnetic stirrer was applied during electrowinning experiments. Before CV and electrowinning experiments, the electrodes were prepared. Copper strips were pickled in concentrated HCl for 2 min and followed by washing in distilled water, ultrasonic rinsing in acetone and drying. Platinized titanium network sheets, silver wire, and platinum sheet were ultrasonically rinsed in ethanol and acetone and then dried. After electrowinning experiments, electrowon layers on copper substrates were washed by tap water and distilled water, ultrasonically rinsed in ethanol, dried, and stored in a desiccator for next microstructure investigations. For measuring the typical current efficiency, each specimen was weighed before and after electrowinning and the mass (M) of the electrowon alloy was found as: M = weight of the specimen after deposition–weight of the specimen before deposition. Cathodic current efficiency(%)

=

193

Weight of metal deposited × 100 Theoritical weight obtained from Faraday’s law

(1)

The cathodic current efficiency of the alloy was calculated as [59]: Cathodic current efficiency(%) =

M × 100 ealloy Qap

(2)

where M is the mass of the alloy deposit (g), ealloy is the electrochemical equivalent of the alloy, and Qap is the quantity of electrical charge applied (A s). The electrochemical equivalent of the alloy was calculated as: ealloy =

eZn × ePb (eZn × fPb ) + (ePb × fZn )

(3)

where eZn and ePb are the electrochemical equivalents of the constituent metals, and fZn and fPb are their fractions in the deposit. 2.6. Chemical analyses and microstructure assessments The chemical composition of EAF dust, as-received and after dissolution experiments, was investigated by X-Ray fluorescence (XRF) apparatus with the model “Axios (PANalytical) with SuperQ and Omnian”. X-ray diffraction (XRD) investigations were conducted on EAF dust samples using the diffractometer model “X’Pert PW3020 (PANalytical) with X’Pert Industrie and X’Pert High Score” with Cu target. The dust sample was step-scanned from 5◦ to 80◦ 2 at 0.02◦ 2 steps. Microstructure of electrowon Zn layers was characterized by a “CamScan Series 4 device” scanning electron microscope (SEM), coupled with an energy dispersive X-ray analyzer (EDX). 3. Results and discussion 3.1. Characterization of as-received EAF dust The typical major oxide composition of EAF dust, as-received and after dissolution in the ionic liquid, is presented in Table 1. It is shown that the as-received dust sample tested was rich in oxides of iron and zinc, which are aimed to be recycled. The as-received dust showed also that ZnO content was higher than Fe2 O3 . This is different to most of studies [1,3,4,9] in which Fe is more prevalent than Zn. However, it has been reported that the concentration of Fe varies in between 10 and 45% and the concentration of Zn is in the range 2–46% in EAF dust [1]. The chemical composition of EAF dust depends on the quality of scrap processed, the type of steel produced, and the operating conditions [1]. In addition to Zn and Fe oxides and oxides coming from fluxing additives such as CaO and SiO2 , oxides of heavy metals such as Pb and Cr were detected. These heavy metals are of great interest due to their toxicity and their harmful environmental impact [1,3,5,7], as shown in introduction section above. It is worth mentioning that the oxides provided by XRF analysis (Table 1) are the major forms expressing the total elements included in the oxide forms; for example, total iron (FeO + Fe3 O4 ) is expressed as Fe2 O3 [38]. The mineralogical composition of the dust was revealed by XRD, as shown in Fig. 1. The major phases detected were willmite “Zn2 SiO4 ”, Mn-rich Franklinite or Zn/Mn-spinel “Zn0.75 Mn0.75 Fe1.5 O4 ”, silica “SiO2 ” as quartz and as cristobalite, and iron sulphate “Fe2 (SO4 )3 ”. In addition, there were small peaks for roggianite “Ca16 (Al16 Si32 O88 (OH)16 )(OH) 16 (H2 O)16.48 ”, calcium aluminium oxide hydrate “Ca4 Al2 O7 ·19H2 O”, and hydrotalcite Mg6 Al2 CO3 (OH)16 ·4(H2 O). It is noticed that the XRD analysis can detect crystalline phases present in concentration of about 5 wt.% or more.

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2.0

Current density / mA.cm

-2

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -1500

-1000

-500

0

500

Potential vs. Ag / mV Fig. 2. Cyclic voltammogram of Pt sheet in the electrolyte produced from dissolving the EAF dust in 1 choline chloride:2 urea ionic liquid at 60 ◦ C and scan rate = 10 mV/S. Inset: higher resolution of the forward cathodic polarisation part prior to Zn deposition.

3.2. Dissolution of EAF dust In a comparison observation between the chemical composition of the as-received dust and that of the dust residue after dissolution experiment, Table 1, it is shown that ZnO concentration decreased from 25.18% to 10.05% and PbO concentration decreased from 2.80% to 1.71%. This means that about 60% of Zn and 39% of Pb were dissolved in the ionic liquid. Concentration percentages of some oxides, such as Fe3 O4 , SiO2 and TiO2 , increased in the residual dust at the expense of the dissolved oxides. This could imply that the oxides which remained their concentrations percentage almost unchanged in the residue, such as CaO, were partly dissolved in the ionic liquid but with smaller quantities. The solubility of a range of metal oxides in 1 choline chloride:2 urea ionic liquid has been quantified in a relatively recent study [52]. The solubility of ZnO is 8466 ppm and that of PbO2 is 9157 ppm, whereas Fe2 O3 is very poorly soluble with 49 ppm [52]. Moreover, the solubility of Zn from ZnO is approximately twice that from natural EAF dust [53]. Zn is found in EAF dust not only as simple ZnO, but also as complex oxides with iron and some times with Mn [1,3,4]. Although the solvation of metal oxides in deep eutectic ionic liquid is not so understood as in aqueous solutions [55], it has been reported that the complex anion [ZnClO·(urea)]− is formed when ZnO dissolves in 1 choline chloride:2 urea ionic liquid [52,53,55]. 3.3. Cyclic voltammetery Fig. 2 shows the cyclic voltammogram (CV) of the electrolyte produced by dissolving EAF dust in 1 choline chloride:2 urea ionic liquid on Pt sheet at 60 ◦ C. The potential of Pt electrode was scanned with a rate of 10 mV/s from 0.0 mVvs Ag , as a starting potential, towards the negative potential to −1400 mVvs Ag , and then was reversed and continued to a value of +500 mVvs Ag , at which the potential was reversed again to close a complete cyclic voltammogram at 0.0 mVvs Ag . The cathodic sweep showed two distinct cathodic peaks. The first weak cathodic peak, illustrated in the inset diagram, at −460 mV was ascribed to deposition of Pb. The second cathodic peak, attributed to deposition of Zn, started at −1100 mV. The anodic sweep depicted also two distinct anodic peaks. The first peak, appeared at the onset of anodic polarization and reached its maximum anodic current at −740 mV, was

characteristic of Zn stripping [52,53,57]. The second anodic peak recorded its maximum at −320 mV was attributed to stripping of Pb. In model experiments [52,53] where ZnO and PbO2 or PbCl2 were dissolved individually in 1 choline chloride:2 urea ionic liquids, the CV measurements on Pt electrode have shown deposition and stripping peaks for Zn and Pb similar to those shown in Fig. 2. Although the cathodic sweep showed two distinct cathodic peaks, Pb was possibly co-deposited with Zn during the cathodic sweep within the Zn deposition peak. This was supported by the notice that the charge of Pb anodic peak, expressed by the area enclosed by the Pb anodic wave in Fig. 2, was significantly higher than that of the Pb cathodic peak. However, having two distinct cathodic peaks separated by a wide range of potential difference (∼640 mV) suggests that the electrochemical separation between Pb and Zn is possible by potentiostatic electrodeposition. In addition, stripping of Zn and Pb in two distinct anodic peaks suggests that a two-phase alloy of discrete Zn and Pb phases was formed, although Pb and Zn were co-deposited when the reductive limit was extended to more negative potentials. The binary lead-zinc phase diagram shows that there is no solubility of Pb in solid Zn [60]. In order to demonstrate the effect of magnitude of the reductive potential applied on the composition of the metal or alloy electrowon, a set of CVs were run on the ionic liquid electrolyte and graphed in Fig. 3a and b. Many valuable electrochemical parameters, which were characteristic of the deposition of Zn and Pb, were obtained from these CVs and listed in Table 2. These parameters are: – the cathodic peak potential (Epc ), – the cathodic peak current (Ipc ), – the anodic peak potential (Epa ), – the anodic peak current (Ipa ), – the cathodic charge (Qc ) associated with metal deposition, – the anodic charge (Qa ) associated with metal dissolution, and – the coulombic current efficiency () which equals Qa /Qc . Qc and Qa were determined as the areas enclosed by the cathodic peak and anodic peak, respectively. Fig. 3a illustrates a set of CV diagrams revised at different potentials ranged from −500 mV to −1200 mV. It is shown that, apart from the CV revised at −1200 mV, each CV depicted a single oxidative peak representing stripping of Pb. In addition, as the revising reductive potential increased from −500 mV to −1200 mV, the Qa Pb , as well as the Ipa Pb , increased, see Table 2. This indicates that the deposition of Pb continued after the cathodic peak was reached. However, the current efficiency ( = Qa /Qc ) decreased significantly as the reversing reductive potential increased from −600 mV up to −1000 mV, see Table 2. This indicates the occurrence of side cathodic reactions during polarization to far reductive potential beyond the Pb reduction peak. Further cathodic sweep to −1200 mV, where the Zn reduction peak started clearly, led to a relatively humble appearance of Zn striping peak at −860 mV. Furthermore, the Pb stripping peak monitored for reversal potential of −1200 mV recorded higher values of Qa Pb and Ipa Pb compared to less negative reversal potentials. This evidences the co-deposition of Pb also during the Zn reduction peak. Fig. 3b depicts a set of successive CVs recorded by polarization to more negative reversal potentials. It is shown that the more negative the reversal potential was, i.e. the higher the Zn reductive charge (Qc Zn ) was, the higher was the Zn oxidative charge (Qa Zn ). This implies that the bulk deposition of Zn continued with increasing the negative sweep. Also, co-deposition of Pb continued with sweeping to far negative potential and was evidenced with further increase in Pb oxidative charge (Qa Pb ), see Table 2. Following the variation of the coulombic current efficiency () with the reversal potential, Fig. 4, illustrated that the efficiency varied widely depicting a nearly two-hump profile. This profile can be divided in two successive stages depicting the deposition of monolithic Pb followed by the deposition of Zn and Pb (predominantly Zn). In the first stage, the efficiency reached its maximum when

-2

Current density / mA.cm

-2

(a)

-1200

(b)

-900

-1500

-600

-500

-300 600

500

Reversal potential (mV)

Qc Pb (mAs/cm2 )

Qc Zn (mAs/cm2 )

Qc Interval a (mAs/cm2 )

Qa Pb (mAs/cm2 )

Qa Zn (mAs/cm2 )

 (%)b

Epc Pb (mV)

Ipc Pb (mA/cm2 )

Epa Pb (mV)

Ipa Pb (mA/cm2 )

Epc Zn (mV)

Ipc Zn (mA/cm2 )

Epa Zn (mV)

Ipa Zn (mA/cm2 )

−500 −600 −800 −1000 −1200 −1400 −1600 −1800 −2000

1.10 1.96 1.36 1.38 1.38 1.39 1.41 1.39 1.38

– – – – 25.43 47.17 81.32 131.27 203.83

– – 2.39 16.79 2.19 2.30 2.31 2.32 2.38

0.436 1.28 1.86 3.42 4.05 4.45 4.75 5.88 7.24

– – – – 3.1 29.89 59.22 85.63 115.14

39.64 65.31 49.60 18.82 24.66 67.52 75.22 67.80 58.95

−460 −460 −460 −460 −460 −460 −460 −460 −460

0.201 0.210 0.205 0.217 0.227 0.237 0.249 0.242 0.238

−330 −340 −340 −320 −320 −320 −310 −310 −300

0.083 0.185 0.260 0.477 0.634 0.680 0.7 0.86 1.04

– – – – −1200 −1400 −1580 −1800 −2000

– – – – 0.957 1.866 2.593 3.304 4.293

– – – – −860 −740 −620 −580 −520

– – – – 0.221 1.796 3.263 4.249 5.08

a

A. Bakkar / Journal of Hazardous Materials 280 (2014) 191–199

300

Reversing potential (mV) -1400 -1600 -1800 -2000

0

Reversing potential (mV) -500 -600 -800 -1000 -1200

0

Potential vs. Ag / mV

-1000

Table 2 Electrochemical parameters, characteristic of the deposition of Zn and Pb, obtained from CVs shown in Fig. 3.

b

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

5

4

3

2

1

0

-1

-2

-3

-4

-5 -2000

Potential vs. Ag / mV

Fig. 3. A set of cyclic voltammograms of Pt in the electrolyte produced by dissolving the EAF dust in 1 choline chloride:2 urea ionic liquid with different lower reversal potentials (a) and higher reversal potentials (b).

Fig. 4. Variation of coulombic current efficiency () of deposition with the CV reversal potentials;  is ratio of the stripping charge to the deposition charge obtained from the CV diagrams in Fig. 3.

Current density / mA.cm

Qc Interval is cathodic charge in the potential range (from −600 mV to −1100 mV) after end of Pb deposition peak and before the start of Zn deposition peak Q +Q  = Q +Qa Zn +Qa Pb × 100. c Zn

c Pb

c Interval

195

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-4.0

Current density / mA.cm

-2

-3.5 -3.0 -2.5

- 1800 mV

-2.0

- 1600 mV

-1.5 -1.0

- 1400 mV

-0.5 0.0 1

10

100

1000

10000

Time / s Fig. 5. Current-time transients for potentiostatic electrowinning of Zn on copper substrate from the electrolyte produced by dissolving the EAF dust in 1 choline chloride:2 urea ionic liquid at 60 ◦ C; the time step used was 1 s as the fastest measurement.

the reductive potential was revised at −600 mV. Then, the efficiency decayed as the reversal potential increased to be close to the potential of beginning deposition of Zn, namely −1100 mV. In the second stage, where the efficiency was greatly affected by bulk deposition of Zn, the efficiency increased sharply with reversal potential and recorded its highest value at −1600 mV, and then decreased gradually at higher reversal potential values. This latter fall of efficiency with the increase of reversal potential was attributed to possible dissociation of the ionic liquid ingredients [52,57,61]. The current efficiency values estimated from CV diagrams ( = Qa /Qc ) were different from the typical cathodic current efficiency values calculated by equation (2), namely measured current efficiency in Fig. 4. The Zn alloy electrowon for 6 h at −1400 mV and at −1600 mV manifested current efficiency values of 79.8% and 81.5%, respectively. The decrease in current efficiency estimated from CV diagrams can be explained due to a side reaction other than deposition, which is most likely to be the dissociation of water present in the ionic liquid. It is reported that the choline chloride-based ionic liquids contain a few percentages of water [57]. The effect of water dissociation on decreasing the current efficiency appeared significantly in CV measurements. However, this effect diminished in electrowinning experiments that extended for long times within which the water was almost completely dissociated. In addition, the possibility of chemical corrosion reaction of the electrowon metal during the stripping peaks minimizes the estimated efficiency [61]. On the other hand, the decrease in the measured current efficiency for the alloy electrowon at −1800 mV to be 58.5% can be attributed to dissociation of the ionic liquid ingredients, which increased with depletion of metallic species in the electrolyte as a result of metallic deposition for long time. 3.4. Electrowinning Potentiostatic electrodeposition experiments were conducted in order to electrowin the possible metallic species from the deep eutectic ionic liquid used for dissolution of EAF dust. Potentiostatic mode was chosen to apply the valuable results obtained from the CV measurements presented above. Fig. 5 shows the typical current–time transients resulted when potentials of −1400 mV, −1600 mV, and −1800 mV were applied for 6 h on copper sheet substrate. It is observed that, as the reductive deposition potential increased, the resultant current increased. In addition, at all

potentials the current increased to its maxima at the 2nd second due to the sudden deposition of Zn to form nuclei. The following decrease in current can be ascribed to formation of depletion layer of Zn ions whereas the deposition process is mostly controlled by diffusion of Zn ions in the solution to the cathode surface [61]. For higher applied potential (−1800 mV), the current increased markedly at longer time more than 5000 s. This subsequent increase in the current was assumed to be caused by side reactions accompanying the depletion of Zn ions in the electrolyte, where partial dissociation of the ionic liquid is highly possible [61,62]. Fig. 6 exemplifies the microstructure and qualitative composition of the outer surface of Zn layers electrowon at low and high potentials. It is shown that, as the potential increased, the grain size of obtained deposit decreased; Zn layer electrowon at −1800 mV (Fig. 6a) showed microstructure finer than that deposited at −1400 mV (Fig. 6b). Fig. 6c and d shows the typical EDX spectra of the electrowon layers in Fig. 6a and b, respectively. It is clear that the co-deposition of Pb was inherent with Zn electrowon at the potential range applied. This was inferred from the CV results presented above. EDX quantitative elemental analysis showed that the outer planar surface of the electrowon layers at −1400 mV, −1600 mV, and −1800 mV contained Pb contents (in wt.%) of 6.34, 8.71, and 12.88, respectively. A hardly notable small Ca peak in spectrum of layer electrowon at −1400 mV (Fig. 6c) could be attributed to entrapment of some CaO particles, that remained suspended in the electrolyte, in the rough surface deposit. The EDX spectrum of the finer layer electrowon at −1800 mV (Fig. 6d) showed no evidence of Ca or other impurities. However, a noticed short peak of Cu in the latter spectrum resulted from interaction of SEM electron beam with copper substrate. 3.5. Outlook and further recommendations The present study demonstrated an approach showing a successful extraction of Zn and Pb from EAF dust into 1 choline chloride:2 urea ionic liquid followed by direct electrowinning of Zn–Pb alloy from the ionic liquid. Eventually, it can be stated that the approach presented is promising and the following advantages can be claimed: - It allows direct electrowinning of Zn metal which has an economical value albeit containing Pb. Zn–Pb alloys can be used as a master alloy in production of Cu–Zn–Pb alloys; commercially named leaded bronze (89% Cu, 9.25% Zn, 1.75% Pb), leaded brass (Cu, 32–38% Zn, 1.5–3% Pb), and leaded nickel silver (65% Cu, 2%Pb, 25% Zn, 8% Ni) [63]. - The residual dust, after its extraction in the ionic liquid, with lower contents of Zn and Pb oxides and higher contents of insoluble Fe oxides can be recycled in the EAF [8]. This residual dust can also be recycled in a special shaft furnace, namely OxiCup furnace, which has been developed to recycle such dust containing Zn and Pb [34]. - Having lower contents of Zn and Pb, the residual dust will be more preferred than the as-generated EAF dust for incorporation in cement synthesis. Higher ZnO content retards hydration of Portland cement and consequently postpones concrete hardening, as well as lowers the concrete strength [3,41,42]. It is reported that the EAF dust can be used as a component in cement mixtures in instances when Zn and Pb contents are low [43]. - Even if the residual dust will be dumped, decreasing Zn and Pb contents enables stabilization by using Portland cement as a binder for detoxifying the EAF dust prior to its landfill disposal [64,65]. However, about 60% of Zn and 39% of Pb in the dust were extracted, and the electrowon metal was a Pb-contained Zn alloy

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197

Fig. 6. SEM micrograph of Zn layers electrowon at (a) −1400 mV and at (b) −1800 mV; (c) EDX spectrum of area shown in (a); (d) EDX spectrum of area shown in (b).

which has not a specific practical application. In this context, further research is recommended in two directions:

- Firstly, focusing on how to recover Zn and Pb separately. CV measurements shown above illustrated that separate cathodic waves for Zn and Pb were observed and they are separated by ∼640 mV. This implies that the metals can be electrowon separately by application of potentiostatic electrodeposition technique. An alternative technique to recover Pb separately is cementation. A recent study has shown that immersion deposits of Pb on Zn substrate can be obtained from electrolyte produced by dissolving a sample of EAF dust in choline chloride/urea/ethylene glycol ionic liquid. The processed electrolyte was then used for electrowinning of pure Zn [54]. An on going research by the author of this paper showed effective separation of Zn and Pb through electrowinning at two widely-separated static potentials. The results will also involve the cementation of Pb, thereby enabling the electrowinning of Zn with high purity. - Secondly, increasing the solubility of Zn and Pb in the ionic liquid through studying the influence of grain size of the dust. Milling

of the dust might help to increase its solvation. Furthermore, the effect of drying the dust and ionic liquid, prior to the dissolution experiments, on the Zn solubility may be an important point of research.

4. Conclusions A deep eutectic ionic liquid, namely 1 choline chloride:2 urea, was investigated to dissolve metal oxides present in a dust sample generated from an Egyptian electric arc furnace. About 60% of Zn and 39% of Pb found in the dust as complex compounds were selectively dissolved. Cyclic voltammetry measurements showed the possibility to electrowin Zn and Pb and determined the potential range to be applied. Then, Pb-contained Zn layers were electrowon on copper substrates through potentiostatic electrodeposition procedure. The approach presented was found to be promising not only because it enables direct electrowinning of Zn which has a high economical value albeit containing Pb, but also it allows the residual dust – with lower Zn and Pb contents – to be recycled in steelmaking processes, reused in cement synthesis, or stabilized before

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landfilling. Further research was recommended to recover Zn and Pb separately.

Acknowledgements The author would like to thank Prof. Volkmar Neubert for facilitating SEM, XRF and XRD investigations in labs of Clausthal University. Thanks also to World University Service (WUS) program funded by Deutsches Komitee e.V., Germany, for presenting the Potentio-Galvanostat used in this study to the author’s lab in Suez University.

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