Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning

Waste Management xxx (2016) xxx–xxx

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Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning Chuh-Shun Chen a, Yu-Jen Shih b, Yao-Hui Huang a,c,⇑ a

Department of Chemical Engineering, National Cheng-Kung University, Tainan 701, Taiwan Department of Civil and Environmental Engineering, University of Delaware, Newark 19716, United States c Sustainable Environment Research Center, National Cheng Kung University, Tainan 701, Taiwan b

a r t i c l e

i n f o

Article history: Received 28 January 2016 Revised 29 March 2016 Accepted 30 March 2016 Available online xxxx Keywords: Anglesite Lanarkite Hydro-electrometallurgy Waste management Cyclic voltammetry

a b s t r a c t Fly ash that was enriched with lead (Pb), formed as an intermediate in waste lead-acid battery (WLAB) smelting, was recycled by the hydro-electrometallurgy. Characterization of fly ash thereof indicated that the Pb was in the forms of PbSO4 (anglesite) and Pb2OSO4 (lanarkite). Nitric acid and sodium hydroxide were firstly used to study the leaching of the fly ash sample, which was affected by leachant dosage and solid-to-liquid ratio (S/L). At an S/L of 60 g L
1, the leachability of Pb was 43% and 67% in 2 M acidic and basic solutions, respectively, based on an average 70 wt% of Pb in the original fly ash. Anglesite was completely soluble in NaOH and lanarkite was mildly soluble in HNO3. Pb was recovered from the pregnant leach solution within an electrolytic cell constructed with graphite or RuO2/IrO2-coated titanium (Ti-DSA) anodes and a stainless steel cathode. Properties of anodes deposited with lead dioxides were analyzed by cyclic voltammetry. The optimized parameters of electrowinning were 2 M NaOH leachant, a current density of 0.75 A dm
2 and an electrolytic process duration of 120 min, which yielded a Pb removal of higher than 99% and a specific energy consumption of 0.57 W h g
1. This process constitutes an ecofriendly and economic alternative to the presently utilized secondary pyrometallurgy for treating leadcontaining fly ash. Ó 2016 Published by Elsevier Ltd.

1. Introduction Pulverized fly ash from smelting furnaces is a hazardous waste because it normally is composed of toxic heavy metals, such as Pb, Cu, Cd, Hg, and Zn. In Taiwan, the pyrometallurgical recycling of Pb metal from waste lead-acid batteries (WLABs) has created significant amounts of fly ash as a municipal solid waste over many years. The Environmental Protection Administration of Taiwan (EPA) has reported that about 66,000 tons of spent lead-acid batteries were generated in 2014. The WLAB plant in this case study has a capacity of 500 tons of batteries per month. It adopts the pyrometallurgical carbothermic process with carbon to reduce the lead paste from waste batteries at high temperature, producing 12 tons of fly ash per month. Electricity is generated from lead-acid batteries as follows (Blanpain et al., 2014). At the positive electrode:

⇑ Corresponding author at: Department of Chemical Engineering, National Cheng-Kung University, Tainan 701, Taiwan. E-mail address: [email protected] (Y.-H. Huang).

PbO2ðSÞ þ HSO
4 þ 3Hþ þ 2e
$ PbSO4ðSÞ þ 2H2 O E ¼ 1:626
0:089  pH
0:029  pSO4 V

ð1Þ

At the negative electrode:

PbSO4ðSÞ þ Hþ þ 2e
$ Pb þ HSO
4 0

E ¼
0:297
0:029  ðpH
pSO4 Þ V

ð2Þ

The full reaction proceeds from left to right during discharge:

PbO2ðSÞ þ Pb þ 2HSO
4 þ 2Hþ $ 2PbSO4ðSÞ þ 2H2 O 0

ECell ¼ ð1:626 þ 0:297Þ
0:059  ðpH þ pSO4 Þ V

ð3Þ

As a result, the WLAB is composed of mostly electrodes of metallic Pb and PbO2 with H2SO4 as the electrolyte; PbSO4 is deposited on the electrodes during the charge-discharge cycles of working batteries. The chemical issue to be addressed in the recovery of Pb from waste batteries by the pyrometallurgical method is the removal of sulfur (Ma and Qiu, 2015). The following reactions generally proceed during the thermal process in a smelter. Metallic lead is formed by either the reduction of lead dioxide with carbon or the reaction of PbS with PbO (Stevenson, 2004).

PbO2 þ C ! Pb þ CO2

ð4Þ

http://dx.doi.org/10.1016/j.wasman.2016.03.056 0956-053X/Ó 2016 Published by Elsevier Ltd.

Please cite this article in press as: Chen, C.-S., et al. Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.03.056

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C.-S. Chen et al. / Waste Management xxx (2016) xxx–xxx

PbSO4 þ C ! Pb þ CO2 þ SO2

ð5Þ

4Pb þ PbSO4 ! PbS þ 4PbO

ð6Þ

NaOH. The waters for all chemicals were doubly deionized using a laboratory-grade RO-ultrapure water system (resistance >18.3 MX).

PbS þ 2PbO ! 3Pb þ SO2

ð7Þ

2.2. Fly ash sample and analytical methods

2PbO þ C ! 2Pb þ CO2

ð8Þ

PbO þ C ! Pb þ CO

ð9Þ

Fly ash samples of secondary lead smelting was provided by CUST Ltd. Co. Taiwan, a resource disposal and recycling plant for WLAB. The as-received fly ash was freeze-dried for 72 h and sieved through a 0.5 mm mesh to ensure uniformity and stored at room temperature in amber glass bottles that had been pre-rinsed with n-hexane, A scanning electron microscopy (SEM, JEOL JSM-6700F, Japan), equipped with an energy dispersive spectroscopy (EDS, Oxford INCA 400, UK) was used for the micro-morphology and semi-quantification of the sample. X-ray powder diffractometer (XRD, Rigaku RX III, Japan) was applied to characterize the structural phase of fly ash. Toxicity characteristic leaching procedure (TCLP) method (US EPA SW-846 3rd Ed, Method 1311) was conducted to assess the heavy metal toxicity of fly ash using acetic acid leachant at pH 3.57. Concentrations of Pb and other metals in solutions were measured by an inductively coupled-plasma emission spectrometer (ICP-OES, JY 2000-2, HORIDA) whose acceptable calibrations should have R2 values higher than 0.99.

Therefore, the potential components of fly ash from smelting are Pb, PbS, PbSO4 and oxysulfates, such as PbOPbSO4 (lanarkite) and (PbO)2PbSO4 (Batonneau et al., 2004), which have been characterized as PM10 and PM2.5 (Uzu et al., 2011). A majority of the fly ash is disposed of in landfills, but this method of disposal has recently been prohibited by EPA because heavy metals in the fly ash potentially has been leaching into groundwater and drinking water supplies (Gwenzi and Mupatsi, 2016). The secondary smelter recycling of fly ash with moderate Pb enrichment is conducted, but the volatizing of PbS and the emission of SO2 gas raise healthrelated challenges. Pyrometallurgy, such as smelting systems, still dominate the recovery of both primary and secondary lead globally. Pb that is unintentionally released into aquatic systems accumulates in the food chain with a half-life of more than two decades (Cai et al., 2009; Meadows-Oliver, 2012). Hydrometallurgy is more eco-friendly than the pyrometallurgical disposal of fly ash or its disposal in landfills because it emits nothing, is cost-effective, and saves energy (Kinoshita et al., 2003; Sayilgan et al., 2009; Akcil et al., 2015). Several reagents, such as sulfuric acid, nitric acid, hydrochloric acid, hydrofluoric acid, acetic acid, citric acid, sodium ˇ ová et al., 2013; hydroxide and ammonia (Zhu et al., 2013; Kubon Tang and Steenari, 2016), are utilized to dissolve metals from solid waste. Aqueous Pb leaching can be followed by electrolysis, chemical precipitation, solvent extraction, and ion-exchange to recover the metal (Angelidis et al., 1995; Orhan et al., 2002), depending on the dissolved species and concentration. The electrowinning is a relatively efficient route for separating and purifying Cu, Zn, Ni, and Ag from wastewaters (Antunano et al., 2013; Ma et al., 2014). To the best of the authors’ knowledge, the feasibility of combining leaching and electrochemistry to dispose of fly ash has rarely been discussed (Tao et al., 2014). To solve the problem that is raised by the conventional landfilling of pyrometallurgical byproducts, hydro-electrometallurgy was used to recycle lead from real fly ash from WLABs smelting. The steps were as follows; (1) Pb from waste fly ash was dissolved in acidic and alkaline chemicals, and (2) the pregnant leach solution were electrolytically recovered as metallic Pb on the cathode and lead dioxide on the anode. Experimental parameters included the concentration of leachants (NaOH and HNO3), the material of the electrode and the current density. Graphite or RuO2/IrO2-coated titanium was selected as the anode, because they are highly stable and insoluble. The voltammetric results were presented here to elucidate the variation among the properties of the various used electrodes. 2. Materials and method 2.1. Chemicals All reagents were of analytical grade, and used without further purification. Leachants for dissolution of lead from the fly ash of WLAB were sodium hydroxide (NaOH, Merck KGaA, Germany) and nitric acid (HNO3, 70%, BASF SE, Germany). The voltammetry of lead was conducted in acidic electrolyte using lead nitrate (Pb (NO3)2, Kanto Chemical Co. INC., Japan) and HNO3, and in alkaline electrolyte using lead sulfate (PbSO4, 99%, SHOWA, Japan) and

2.3. Experimental procedure Nitric acid and sodium hydroxide were utilized in examining the leachability of the fly ash samples. (For minimizing the corrosion of electrodes and the production of chlorine gas during the following electrolytic process, nitric acid was selected as the acidic leachant instead of a cheaper hydrochloric acid.) The parameters that most strongly affected the leaching yield of Pb (%) were the ratio of solid to liquid (S/L, 12, 30, 60, 120, and 180 g L
1) and the concentration of the leachants (0.6, 1, and 2 M). The ambient temperature was maintained (27 ± 3 °C) using a water bath. A batch experiment was carried out by adding a known amount of fly ash into 500 mL of leachant; the mixture was mechanically stirred at 450 rpm. At given intervals, 5 mL solution was withdrawn and filtered through a 0.22 lm filter (ADVANTEC Ltd., JAPAN), and the Pb concentration in filtrates was analyzed using ICP-OES. After the fly ash had undergone the leaching test, the supernatants of the leachants were transferred to an electrolytic cell, which was made of polymethylmethacrylate (acrylic) (10  6  12 cm3, in LWH, total reaction solution = 0.3 L) Two electrodes of dimensions 100  2  100 mm3 (LWH), RuO2/IrO2-coated titanium, a dimensionally stable anode (Ti-DSA), or graphite plates as anodes and a plate of stainless steel 316 as the cathode, were connected to a DC power supply. In all cases, the effective surface area was 100 cm2, and the distance between anode and cathode was 56 mm. The temperature of the electrolyte was kept at 27 ± 3 °C using a water bath. Electrolytic variables included the current density (0.75, 1.5 and 3 A dm
2), the species of the leachants and their concentrations. At fixed intervals, the soluble Pb in the samples was analyzed by ICP, and the difference between the Pb concentrations before and after electrolysis was used to obtain the removal efficiency of Pb in the electrolytic (electrowinning) process. Cyclic voltammetry (CV) was conducted within a threeelectrode system using a potentiostat (CHI611C, CH Instrument, Inc., USA) to characterize the anode materials. A silver chloride electrode (Ag/AgCl, BAS Inc., Japan) was used as a reference electrode, which was in contact with the working electrode, graphite or Ti-DSA, in solution. The stainless steel mesh (super-corrosionresistant 316, 0.004500 in wire diameter) served as the counter electrode, which was kept 2 cm from the working electrode. The submerged area of each of the working and the counter electrodes in

Please cite this article in press as: Chen, C.-S., et al. Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.03.056

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C.-S. Chen et al. / Waste Management xxx (2016) xxx–xxx

3.2. Batch experiments on leaching of fly ash Fig. 2 plots the leachability of Pb from fly ash (60 g L
1) vs. time using HNO3 and NaOH. In all leaching experiments, the amount of Pb dissolved increased with time and leachant dose, rapidly reaching equilibrium in 20 min. The efficiency with which 2 M NaOH extracted Pb was 67%, exceeding that of 2 M HNO3, 43%, based on EDS results of revealing that the original fly ash contained

Hg As Cu Cr(III) Cr(VI) Cd Pb Se Ba

ND ND 0.116 <0.02 <0.1 29.6 2960 <0.1 0.016

50

20

40

15

30 10

20

0.1 M 0.3 M 0.6 M 1M 2M

5 0 0

20

40

60

80

10

(a)

100

0

120 70

30 25

60

0.6 M 1M 2M

20

50 40

15

30

10

20

5

10

(b)

0 0

20

40

60

80

100

Leaching efficiency, %

SEM images in Fig. 1a reveal that the particles of fly ash had average diameters of 0.5–1 lm (but a mean diameter d50 based on laser diffraction particle size analysis of 0.25 lm (as shown by Fig. S1 in the supporting materials), and were spherical or flaky. The atomic percentages of Pb, O and S elements, according to the EDS analysis, were 22.2 ± 1.0%, 64.1 ± 2.5% and 13.7 ± 3.1% (molar ratio of Pb/O/S was around 1.6/4.7/1). The Pb content from EDS were rather similar to those by aqua regia digestion. (From the five samplings, both methods indicated an average 70 wt% of Pb in fly ash as listed in Table S1. Meanwhile, the amount of other elements were almost negligible.) Based on the JCPDS standard reference, the fly ash contained two primary crystalline phases of lead sulfate, which were anglesite (PbSO4) and lanarkite (Pb2OSO4), as indicated by the XRD pattern of Fig. 1b. TCLP method was used to verify the mobilities of toxic pollutants in the fly ash, as listed in Table 1. Since the concentrations of lead (2960 mg L
1) and cadmium (29.6 mg L
1) exceeded Taiwan’s regulatory standards (5 and 1 mg L
1 for Pb and Cd, respectively), the fly ash from the secondary lead smelting was identified as a hazardous industrial waste.

Concentration (mg L
1)

0

Leaching efficiency, %

3.1. Characterization of fly ash from WLAB smelting

Element

Pb, 103 ppm

3. Results and discussion

Table 1 Toxicity characteristic leaching procedure test (TCLP) of fly ash.

Pb, 103 ppm

the electrolyte solution (80 mL) was 1.5  3.5 cm2. The voltammetric responses in NaOH/HNO3 solution were obtained using 0.01 M Pb(NO3)2 as an acidic solution or 0.01 M PbSO4 as a basic solution.

120

Time, min Fig. 2. Leachability of Pb(II) as a function of time (min) in (a) HNO3 and (b) NaOH. (Fly ash = 60 g L
1, magnetically stirring at 450 rpm, 27 ± 3 °C.)

(a) approximately 70% of Pb (wt%). The leachability was significantly improved by increasing the concentration of NaOH from 0.6 to 2 M, and reached almost its highest value when 0.6 M HNO3 was used. The solubility of lead-rich solid critically affects the extraction of Pb under acidic and basic conditions, and depends on the concentration of protons. The amount of Pb that leached from PbSO4 (anglesite) and Pb2OSO4 (lanarkite), which constitute the fly ash, is predicted from the solubility product constants (Marani et al., 1995).

1 m

(b) Anglesite #72-1389 PbSO4 Lanarkite #72-2069 Pb2OSO 4

10

20

30

40

50

60

70

2þ

PbSO4 $ Pb

þ SO2
log Ksp1 ¼
7:975 4

Pb2 OSO4 þ 2Hþ $ 2Pb

2þ

80

2 ,º Fig. 1. SEM observation and (b) XRD pattern of fly ash sample.

þ SO2
4 þ H2 O log Ksp2 ¼
5:83

ð10Þ ð11Þ

The dissolved lead species (Pb2+, PbOH+, Pb(OH)2(aq), Pb(OH)
3 , Pb (OH)2
4 ) reached equilibrium with protons in water molecules, for which the stability constants, *K1, *b2, *b3 and *b4, were 10
8.2, 10
17.2, 10
28.1, 10
39.7 (Stumm and Morgan, 1996). Sulfuric acid is a diprotic acid whose first proton completely dissociates and second acidic constant (Ka) is 10
1.92. Thus, the molar ratios of Pb2+ and SO2
4 ions to total lead and sulfate can be expressed with 1/aPb and 1/aSO, which are functions of proton concentration. (In the following, the square brackets represent ion activities.)

Please cite this article in press as: Chen, C.-S., et al. Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.03.056

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C.-S. Chen et al. / Waste Management xxx (2016) xxx–xxx þ

PbT ¼ ½Pb  þ ½PbOH  þ ½PbðOHÞ2ðaqÞ  þ ½PbðOHÞ
3  þ ½PbðOHÞ2
4  

   K1 b b b ¼ ½Pb  1 þ þ þ þ2 2 þ þ3 3 þ þ4 4 ½H  ½H  ½H  ½H 

2
SOT ¼ ½HSO
4  þ ½SO2
4  ¼ ½SO4 

! ¼ ½Pb aPb 2þ

 þ  ½H  þ 1 ¼ ½SO2
4 aSO Ka

100

ð12Þ

80 60

ð13Þ

40 20

The effects of Pb hydrolysis and the deprotonation of sulfuric acid on the solubility of solids are related to the corresponding conditional solubility products (Ps). Consequently, the ideal solubility of Pb (S) from PbSO4 and Pb2OSO4 at a particular proton concentration is obtained using the following equations. (The solubility curves of anglesite and lanarkite are depicted in Fig. S2.)

Ps1 ð¼ PbT  SOT Þ

aPb aSO

¼

ð14Þ

a a

8

100

2

¼

ð2SÞ  S

80

½Hþ  a2Pb aSO 2

a a

60

ð15Þ

Under strongly acidic conditions (2 M [H+]), the solubilities of PbSO4 and Pb2OSO4 are 0.27 and 25.4 g L
1, respectively; under strongly basic conditions (2 M [OH
]), the solubilities of PbSO4 and Pb2OSO4 are 1.17  107 and 394 g L
1, respectively (as predicted by the red circles at intersection of 2 M [H+]/[OH
] and solubility curves in Fig. S2). Restated, the dissolution of Pb2OSO4 is responsible for most of the Pb leaching by acidic solutions, and PbSO4 is totally soluble in an alkaline environment. The aforementioned EDS analysis suggested that the elemental ratio of Pb:O:S was 1.6:4.7:1, yielding molar percentages of around 38% for PbSO4 and 62% for Pb2OSO4 in the fly ash. These values are equivalent to 9.8 g of Pb from PbSO4 and 32.2 g of Pb from Pb2OSO4, based on a Pb content of around 70% of Pb in 60 g-fly ash L
1. The huge discrepancy between the theoretical solubility and the solid content in the fly ash suggests that PbSO4 can be completely dissolved in NaOH (Eq. (14)). By contrast, the amount of Pb (32.2 g L
1) exceeds the saturation concentration of Pb2OSO4 (25.4 g L
1) in HNO3, so less Pb than the calculated amount leaches (Eq. (15)). (Notably, [H+] or [OH
] herein was in equilibrium with solids in a pH range that could not be measured using a regular pH-meter, but was lower than 2 M owing to dissolution reactions) Fig. 3 plots the Pb leaching rates at various S/L ratios and the theoretical values that are estimated from equilibrium constants. Increasing S/L considerably impairs the amount of extracted Pb in comparison with those predicted from the solubility product. 2 M NaOH extracts Pb from fly ash to an extent that approximates the ideal dissolvable amount for an S/L of 12 g L
1. However, 2 M HNO3 extracts less than NaOH even at small S/L because of the relatively low solubility of fly ash. Consequently, the strong release of Pb from PbSO4 in bases and the mild release from Pb2OSO4 in acids favors the dissolution of fly ash in alkaline solutions. Mass transport-controlled precipitation and dissolution cause the treatability to decline as S/L increased to 120 and 180 g L
1 in the specified period (Sonmez and Kumar, 2009; Zhu et al., 2013). 3.3. Electrolytic recovery of Pb from acidic and basic leachants An electrochemical system was used to evaluate the electrolytic recyclability of Pb from the pregnant leach solution. As displayed in Fig. 4, the efficiencies of the electrolytic recovery of Pb from NaOH and HNO3 by the two-electrode combination (with Ti-DSA

40 20

12 30 60 -1 120 g L 180

0 theo

1

4

8

Time, h

Fig. 3. Leachability of Pb(II) in (a) 2 M NaOH and (b) 2 M HNO3 as a function of leaching time and solid to liquid ratio (S/L, g L
1) as compared with the theoretical rate estimated by solubility product constants (based on fly ash containing an average lead of around 70 wt%).

100

30

80

Pb, 103 ppm

2 ½Pb PS2 ð¼ PbT  SOT Þ ¼ Ksp2 ¼ 2 ½H  ½Hþ  2Pb SO  1=3 2 SPb2 OSO4 ¼ Ksp2 ½Hþ  2Pb SO =4

4

(b)

aPb aSO

2þ 2

1

Time, h

S2

SPbSO4 ¼ ðKsp1 aPb aSO Þ1=2  ½SO2
4  þ 2

theo

20

Ti-DSA graphite

60 40

10

(a)

20

0

0 0

20

40

60

80

100

120 100

20

Pb, 103 ppm

2þ

Ksp1 ¼ ½Pb ½SO2
4  ¼

12 30 60 -1 120 g L 180

0

Removal, %

2þ

(a)

80

15

Ti-DSA graphite

10

60 40

(b)

5

Removal, %

2þ

20

0

0 0

20

40

60

80

100

120

Time, min Fig. 4. Effects of anode materials on Pb removal from (a) 2 M NaOH and (b) 2 M HNO3 as a function of electrolytic time. (Fly ash = 60 g L
1, 27 ± 3 °C, cathode = stainless steel; current density = 1.5 A dm
2.)

Please cite this article in press as: Chen, C.-S., et al. Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.03.056

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C.-S. Chen et al. / Waste Management xxx (2016) xxx–xxx

or graphite as the anodes and stainless steel as the cathode) in 2 h are 99%. Nevertheless, as summarized in Table 2, the kobs that is obtained by fitting a first-order rate model to the experimental data indicates that the removal of Pb using a graphite anode is faster than that using Ti-DSA. The formation of lead dioxide on a TiDSA anode in HNO3 (34%) is less than in NaOH (22.4%), and leachant type does not significantly affect the recovery of oxide on the graphite anode (32%). The degree of Pb recovery varied with the combination of electrodes in a manner that was related to the activation-dependent overpotentials of the redox reactions (Nikolic´ et al., 2014). To assess the anodic deposition of lead dioxide in HNO3 and NaOH electrolytes, the cyclic voltammetry (CV) of graphite and Ti-DSA as working electrodes with 0.01 M lead salts was conducted, as presented in Fig. 5. (Pb(NO3)2 was adopted as a lead source in the nitric acid to prevent PbSO4 precipitation.) Based on the equilibrium electrode potentials, the i-E polarization of oxide film formation would probably be overlapped by that of the water oxidation (Trasatti, 2009). In an acidic electrolyte (25 °C, pPb(II) =
log{Pb(II)}) (Bratsch, 1989):

PbO2ðSÞ þ 4Hþ þ 2e
$ Pb þ 2H2 O E ¼ 1:455
0:118  pH þ 0:029  pPbðIIÞ V ðvs: SHEÞ 2þ

ð16Þ

reaction (OER), even though it required a higher overpotential on graphite than on Ti-DSA. Restated, OER dominated the current density in an electrolytic system with the Ti-DSA anode, causing less PbO2 to be deposited on this anode than on graphite (Särkkä et al., 2015). The electrolytic recovery of Pb from the pregnant leach solution was performed using Ti-DSA as the anode and stainless steel as the cathode as shown in Fig. 6. The removal of Pb by surface deposition on electrodes reached >99% in 0.6 M HNO3 (with a final Pb of 0.3 g L
1) but fell to 92% as the leachant concentration increased to 2 M. The final Pb concentrations, which varied insignificantly with NaOH concentration were all about 0.2 g L
1. The rate of Pb removal, however, clearly decreased as the concentration of either HNO3 or NaOH increased. pH (or concentration of [H+]/[OH
]) is known to be an important parameter in electrode reactions (Liu et al., 2013). The specific adsorption of highly concentrated H+/OH
at active sites on the cathode/anode competed with that of lead ions. Increasing proton activity increased the anodic overpotential of Pb2+/PbO2 (Eq. (16)), and reduced the cathodic overpotential of the hydrogen evolution reaction (HER), which dominated the formation of metallic Pb (as predicted with reference to the pe-pH diagram (Fig. S3) for Pb).

Hþ þ 2e
$ H2ðgÞ 2þ

In a basic electrolyte:

Pb

PbO2ðSÞ þ 2H2 O þ 2e
$ PbðOHÞ
3 þ OH
E ¼ 0:209 þ 0:029  ðpOH þ pPbðIIÞÞ V ðvs: SHEÞ

ð17Þ

þ 2e
$ Pb

0

E ¼
0:059  pH V ðvs : SHEÞ E ¼
0:126 V ðvs: SHEÞ

ð19Þ ð20Þ

Nevertheless, the activity of the hydroxyl ions affected the efficiency of the reduction of Pb(II) reduction:

PbðOHÞ
3 þ 2e
$ Pb þ 3OH
0

Oxidation of water:

E ¼
0:537 þ 0:089  pOH
0:029  pPbðIIÞ V ðvs: SHEÞ

O2 þ 4Hþ þ 4e
$ 2H2 O E ¼ 1:229
0:059  pH V ðvs: SHEÞ

ð18Þ

After a correction is applied using the reference potential of Ag/AgCl (E0 = 0.197 V) and the corresponding pH, ideal potentials of Pb oxidation in acidic and alkaline solutions are found to be 1.1 and 0.086 V, respectively. In Fig. 5, when the potential was positively scanned, the absence of a complete polarization peak implied that the onset of PbO2 accompanied O2 evolution. When the positive limit of the potential window was increased stepwise, the appearance of cathodic peaks in the backward scan provided evidence that Pb(II) was oxidized at a particular potential during the forward scan. Consequently, in 0.1 M HNO3, the deposition of PbO2 on graphite was initiated at an onset voltage of +1.8 V, which exceeded that on Ti-DSA electrodes (+1.6 V). In 0.1 M NaOH, both electrodes required a potential of approximately +0.7 V to trigger the formation of PbO2. The estimated overpotentials of Pb(II) to PbO2 are 700 mV on graphite and 500 mV on Ti-DSA in acids, and 614 mV on both anodes in bases. Fig. 5e presents scans of the potentials at which Pb redox couple never occurs in acids; the inflection points at +1.44 and +1.28 V correspond to the evolution of O2 at graphite and Ti-DSA electrodes. RuO2/IrO2 are well-known to be efficient in electrolyzing water (Hu et al., 2008). The CV test revealed that the deposition of PbO2 was critically affected by the oxygen evolution Table 2 Effect of anode material on the rate constant of Pb removal and mass ratio of Pb oxides on anode and metallic Pb on cathode (cathode = stainless steel, reaction period = 120 min, fly ash = 60 g L
1, 27 ± 3 °C, cathode = stainless steel; current density = 1.5 A dm
2). Anode

Electrolyte

kobs (h
1)

PbO2 on anode (%)

Pb0 on cathode (%)

Ti-DSA

HNO3 NaOH

0.030 0.042

22.4 34.0

77.6 66.0

Graphite

HNO3 NaOH

0.052 0.055

32.3 32.1

67.7 67.8

ð21Þ

Hence, increasing the concentration of proton/hydroxyl ions favored the dissolution of fly ash in the leaching step, and reduced the rate constant (kobs) of the electrolytic recovery of Pb (Table 3). Different current densities (CDs) were used to examine the efficiency of recovery and the correlated energy consumption (Fig. 7). Increasing CD from 0.75 to 3 A dm
2 did not substantially change the final Pb level (which was less than 0.2 g L
1 after 120 min with both leachants), but improved the removal rate (Table 3). Two key factors were used herein to investigate efficacy of the electrowinning process: these were total current efficiency (g) and the specific energy consumption (SEC) (Dobrev and Rashkov, 1996; Wu et al., 2014).

DM  F  100 I  s  Mw E Is
1 SEC ðW h g Þ ¼  1000 DM  3600

g ð%Þ ¼

ð22Þ ð23Þ

where: I t M

F Mw E

electrical current (A) electrolysis duration (s) difference between quantities of Pb (g) in the electrolyte (obtained as change of the concentration of lead (g L
1), multiplied by the reaction volume) Faraday constant, 96485 (C mol
1) molecular weight of Pb, 207 cell voltage (V)

Although Pb rapidly approaches its minimum concentration as more current is applied, increasing CD considerably limits the g of electrodeposition, as shown in Table 4, indicating that energy is wasted on some undesirable electrochemical reactions, such as the production of gas from the surfaces of electrodes (Zhang et al., 2014). Furthermore, under particular conditions, HNO3 provides a

Please cite this article in press as: Chen, C.-S., et al. Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.03.056

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C.-S. Chen et al. / Waste Management xxx (2016) xxx–xxx

2

1.0

(a)

0.5 +1.4 V

0

+1.6 V +1.7 V +1.8 V

-1

mA cm-2

1

mA cm-2

(b) +1.3 V +1.1 V

0.0

-0.5

+1.9 V

+1.5 V +1.6 V +1.7 V

-2 -1.0 0

400

800

0

1200 1600

400

800

1200

E, mV vs. Ag/AgCl

E, mV vs. Ag/AgCl 10 20

(c)

8

mA cm-2

mA cm-2

10 0 -10 -20

+0.6 V +0.7 V +0.8 V +0.9 V +1.0 V

-800 -400

0

400

(d)

6 4 2 +0.6 V

0

+0.5 V +0.7 V +0.8 V

-2 -4 -800

800

+0.9 V

-400

0

400

800

E, mV vs. Ag/AgCl

E, mV vs. Ag/AgCl 4

(e)

mA cm-2

3 2

Ti-DSA (0 ~ +1.5 V)

1 0 Graphite (0 ~ +1.7 V)

-1 0

400

800

1200

1600

E, mV vs. Ag/AgCl Fig. 5. Cyclic voltammograms of PbO2 formation (with different anodic inversion potentials) in acidic solution (0.01 M Pb(NO3)2, 0.1 M HNO3, pH < 1.5) using (a) graphite and (b) Ti-DSA as working electrodes, and in basic solution (0.01 M PbSO4, 0.1 M NaOH, pH > 12.5) using (c) graphite and (d) Ti-DSA as working electrodes. (e) Comparison of overpotential of oxygen evolution on graphite and Ti-DSA in HNO3. (Sweeping rate = 50 mV s
1; area of working electrode = 1  3.5 cm2.)

g that is only two-thirds of that is yielded by NaOH for Pb removal. Using a CD of 0.75 A dm
2 in NaOH leachant yields a g of around 100% at 60 min. SEC depends on the cell voltage and the current efficiency, which are both functions of time, so the electrolytic operation consumes less energy in NaOH than in HNO3. Based on the above 99% Pb removal, the SEC values for NaOH and HNO3 systems are 0.57 and 0.88 W h g
1, respectively (CD = 0.75 A dm
2, cell voltage = 3.1–3.3 V), which are lower, on average, than those utilized in the electrowinning of heavy metals, such as Co (Kargl-Simard et al., 2003), Ni (Lupi et al., 2006), Zn (Mukongo et al., 2009), Sb (Awe and Sandström, 2013) and Cu (Dutra et al., 2008; Gorgievski et al., 2009).

Fig. 8 displays the morphologies of PbO2 and metallic Pb on the anode and cathode. In HNO3 (Fig. 8a), the deposit is smooth and well-crystallized, whereas in NaOH (Fig. 8b), powdered crystallites (<0.5 lm) are aggregated and adhere to the substrate. PbO2 in acidic and basic electrolysis displays allotropic forms (Carr and Hampson, 1972). In this work, PbO2 from NaOH electrolyte comprised mostly the plattnerite phase (b type), and that from HNO3 electrolyte comprised mostly the scrutinyite phase (a type) (Zhang et al., 2015), as indicated in Fig. 9. On the cathode side, nodular and flaky crystals of metallic Pb, whose morphology was independent of the electrolyte, were generated (Fig. 8c). These observations are consistent with other investigations (Li et al., 2006; Souza et al., 2014; Wu et al., 2014).

Please cite this article in press as: Chen, C.-S., et al. Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.03.056

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C.-S. Chen et al. / Waste Management xxx (2016) xxx–xxx

80

10

40

0.75 A dm -2 1.5 A dm -2 3 A dm -2

10

20

0 25

50

75

100

0

125

25

50

75

100

125 100

20

(b) 2M 1M 0.6 M

10 5 0 0

50

100

150

200

60 40

Pb, 103 ppm

80

15

40

0 0

100

20

60

20

0 0

Pb, 103 ppm

(a)

20

80

15

(b)

10

0.75 A dm -2 1.5 A dm -2 3 A dm -2

20

5

0

0

250

60 40

Removal, %

2M 1M 0.6 M

Pb, 103 ppm

60

20

80

Removal, %

(a)

Removal, %

Pb, 103 ppm

30

100

30

Removal, %

100

20 0 0

50

100

Time, min

150

200

250

Time, min

Fig. 6. Effects of (a) NaOH and (b) HNO3 concentrations on the removal of Pb as a function of electrolytic time. (Current density = 1.5 A dm
2, fly ash = 60 g L
1, 27 ± 3 °C; anode = Ti-DSA, cathode = stainless steel.)

Fig. 7. Effects of current density on the removal of Pb from (a) 2 M NaOH and (b) 0.6 M HNO3 leachants as a function of electrolytic time. (Fly ash = 60 g L
1, 27 ± 3 °C; anode = Ti-DSA, cathode = stainless steel.)

Table 3 Rate constants of Pb recovery from NaOH and HNO3 leachants (fly ash = 60 g L
1, 27 ± 3 °C; anode = Ti-DSA, cathode = stainless steel).

Table 4 Current efficiency and specific energy consumption for optimizing electrowinning conditions.

NaOH

HNO3

a b

Conc. (M)

kobs (h
1)a

CD (A dm
2)

kobs (h
1)b

Leachant

2.0 1.2 0.6

0.049 0.083 0.096

0.75 1.50 3.00

0.023 0.043 0.080

Time (min)

CD (A dm
2)

Removal (%)

HNO3

60

2.0 1.2 0.6

0.020 0.030 0.039

0.75 1.50 3.00

0.022 0.042 0.090

0.75 1.50 3.00 0.75 1.50 3.00

72.9 90.8 99.2 99.2 99.6 99.8

65.2 40.5 21.9 44.3 20.2 11.0

0.59 0.96 1.77 0.88 1.92 3.52

0.75 1.50 3.00 0.75 1.50 3.00

74 97.9 99.2 99.1 99.6 99.7

102.2 65.5 33.9 67.7 33.9 17.1

0.38 0.59 1.14 0.57 1.14 2.28

CD = 1.5 A dm
2. Obtained from 2 M NaOH and 0.6 M HNO3 leachants, respectively.

Integrating leaching and electrochemistry for treating the leadcontaining fly ash has several economic and ecological benefits as it can be a viable alternative to the conventional pyrometallurgy, such as: (1) avoiding the gas emission of SOx, CO2 and particulate matter (PM) during the secondary smelting; (2) the recyclable acidic and alkaline electrolytes; (3) clean process with low energy consumption compared to the smelting that requires additional carbon (anthracite coal) as the reductant and high temperature of 1100–1400 °C (Okada et al., 2007; Stevenson, 2004); (4) the absence of air pollution control equipment and the leadcontaining oven slag (Volpe et al., 2009). Accordingly, the primary cost estimate for recovering 1 kg Pb would include 2.5 kg NaOH, the electricity of 0.5 kW h and 33 L water. Moreover, the innovative idea in the present work was to test the optimal conditions of a hydro-electrometallurgy process for effective Pb recovery, based on predictions of the solubility product of lead sulfate compounds and the overpotential of Pb redox on electrodes. 4. Conclusions This work examined the treatability of lead-containing fly ash, which is a pyrometallurgical byproduct of waste lead-acid battery

120

NaOH

60

120

g (%)

SEC (W h g
1)

(WLAB), by acidic/basic leaching and electrochemical processes. XRD and SEM revealed that the target sample with a particular diameter of 0.5–1 lm was composed of around 38% anglesite (PbSO4) and 62% lanarkite (Pb2OSO4) (molar percentages). At an S/L ratio of 60 g L
1, 2 M HNO3 and 2 M NaOH extracted around 43% and 67% of Pb from the fly ash, respectively, based on a lead content of 70%. This discrepancy in leachability was predicted from solubility product constants: PbSO4 completely dissolved in a strong base but was insoluble in strong acid, whereas Pb2OSO4 was moderately soluble only in strong acid. The electrolysis of Pb (II) in the pregnant leach solution was carried out using anode/cathode combinations of graphite/stainless steel and Ti-DSA/ stainless steel. The cyclic voltammetric test revealed that the evolution of oxygen from the anode influenced the oxidation of Pb, causing less PbO2 to be deposited on the Ti-DSA anode (22.4%) than on the graphite anode (34%). The fly ash was eventually recovered as metallic Pb formed on the cathode and PbO2 formed on the

Please cite this article in press as: Chen, C.-S., et al. Recovery of lead from smelting fly ash of waste lead-acid battery by leaching and electrowinning. Waste Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.03.056

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C.-S. Chen et al. / Waste Management xxx (2016) xxx–xxx

anode, and its phase varied with the type of electrolyte (plattnerite in NaOH, scrutinyite in HNO3). The aqueous Pb(II) concentration was lower than 0.2 g L
1 in the end solution.

(a)

Acknowledgements

5 m

The authors would like to thank the Environmental Protection Agency of Taiwan, for financially supporting this research under Contract No. EPA G104-B131. Ted Knoy is appreciated for his editorial assistance. Professor CP Huang, civil and environmental department, University of Delaware, is appreciated for his scientific advice. Appendix A. Supplementary material

(b)

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.wasman.2016.03. 056. References

5 m

(c)

2 m Fig. 8. SEM micromorphology of electrodeposition of PbO2 on Ti-DSA anode in (a) 0.6 M HNO3, and (b) 2 M NaOH leachants (magnification = 5000). (c) Metallic Pb on stainless steel cathode (magnification = 15,000).

α-PbO2 (Scrutinyite, 45-1416) Pb0.9 5O2 (75-2419) β-PbO2 (Plattnerite, 41-1492) Pb0.9 8O2 (75-2414)

Acidic

Basic

10 20 30 40 50 60 70 80 90

2 ,º Fig. 9. XRD characterization of PbO2 deposition on the anode in 0.6 M HNO3 and NaOH leachants.

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