A closed loop ammonium salt system for recovery of high-purity lead tetroxide product from spent lead-acid battery paste

A closed-loop ammonium salt system for recovery of high-purity lead
tetroxide product from spent lead-acid battery paste
Mingyang Li a, b
, Jiakuan Yang a, b, c, *
, Sha Liang a, b
, Junxiong Wang a, b
, Peiyuan Zhang a, b
,
Wenhao Yu a, b
, Jingping Hu a, b
, Keke Xiao a, b
, Huijie Hou a, b
, Bingchuan Liu a, b
,
R. Vasant Kumar d
a
School of Environmental Science and Engineering, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan, 430074, China
b
Hubei Provincial Engineering Laboratory of Solid Waste Treatment, Disposal and Recycling, 1037 Luoyu Road, Wuhan, 430074, China
c
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan, 430074, China
d
Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, United Kingdom
a r t i c l e i n f o
Article history:
Received 19 August 2019
Received in revised form
11 November 2019
Accepted 27 November 2019
Available online xxx
Handling editor. Giorgio Besagni
Keywords:
Spent lead-acid battery paste
Ammonium salt system
Filtrate recirculation
Impurity elements
Lead tetroxide
a b s t r a c t
Hydrometallurgical process for recovery of spent lead-acid battery paste shows great advantages in
reducing SO2 and lead particulates emissions than traditional pyrometallurgical process. However, the
hydrometallurgical process usually has drawbacks of high consumption of chemical reagents and diffi-
culty in removing impurities (especially Fe and Ba elements) from the recovered product. In this paper, a
closed-loop ammonium salt system is proposed for spent lead-acid battery paste recovery. Both recir-
culation of leaching reagents and preparation of low-impurity recovered products have been realized.
The spent lead paste is first leached by a mixed solution of ammonium acetate, acetic acid and hydrogen
peroxide. After filtration, the separated lead acetate solution is reacted with ammonium carbonate to
generate lead carbonate via precipitation process. The impurity elements are efficiently removed by pH
control and complexation between acetate ions and impurity elements in the leaching and precipitation
processes. The soluble SO4
2
separated from the precipitation process is removed by adding barium ac-
etate to generate solid BaSO4 by-product. At the same time, the regenerated ammonium acetate filtrate is
separated and re-used in the next-round leaching process in order to realize a closed-loop process. In the
5th round of filtrate recirculation processes, the leaching ratio of lead is maintained at levels higher than
92.7 wt%. Furthermore, high-purity lead tetroxide is prepared by calcination of lead carbonate in air at
450
C. The contents of Fe and Ba in the final recovered lead tetroxide product are as low as 2.7 and
5.2 mg/kg, respectively. The recovered lead tetroxide product meets the specifications for use as an
additive in the positive active materials for making a new lead-acid battery. This study provides a feasible
technology for high-value utilization of spent lead paste.
© 2019 Elsevier Ltd. All rights reserved.
1. Introduction
Lead-acid battery (LAB) has wide applications in the fields of
auxiliary power supply, electric vehicles, starting, lighting and
ignition (SLI) because of its low cost, stable electrochemical per-
formance and high recovery efficiency of spent LABs (Hu et al.,
2018; Zhang et al., 2018). The production of LAB has been
increasing rapidly, and LAB has accounted for a large proportion of
the secondary battery market (Liu et al., 2016; Sun et al., 2017).
With the decreasing of primary lead ore reserves, spent LABs have
become dominant lead resource for new LAB manufacturing
(Zhang et al., 2016). Usually, the spent LABs are divided into four
parts, namely, spent lead paste (30e40 wt%), alloy grid (24e30 wt
%), plastics (22e30 wt%), and spent sulfuric acid electrolyte
(11e30 wt%) (Yu et al., 2019a). Among them, the spent lead paste,
which is the most difficult part to be treated, contains ~60 wt% of
very stable phase Pb(II)SO4, ~25 wt% Pb(IV)O2, ~10 wt% Pb(II)O,
~5 wt% metallic Pb and a small amount of impurity elements such
as Fe, Ba, Sb, etc (Li et al., 2017). Currently, pyrometallurgical
technologies are the most common methods used for recovering
* Corresponding author. School of Environmental Science and Engineering,
Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan,
430074, China.
E-mail address: jkyang@mail.hust.edu.cn (J. Yang).
Contents lists available at ScienceDirect
Journal of Cleaner Production
journal homepage: www.elsevier.com/locate/jclepro
https://doi.org/10.1016/j.jclepro.2019.119488
0959-6526/© 2019 Elsevier Ltd. All rights reserved.
Journal of Cleaner Production xxx (xxxx) xxx
Please cite this article as: Li, M et al., A closed-loop ammonium salt system for recovery of high-purity lead tetroxide product from spent lead-
acid battery paste, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119488

metallic lead from spent LABs (Li et al., 2019a). However, a high
temperature of 1000 C is required for decomposition of the stable
PbSO4 phase, inevitably resulting in the emission of SO2 and lead
particulates (Tian et al., 2017, 2018). Environmentally-friendly
technologies that can replace the commonly used pyrometallurgi-
cal process are in urgent demand (Zhang et al., 2016).
In recent years, many hydrometallurgical processes have been
developed, including reagent leaching followed by electrowinning
(Andrews et al., 2000), alkaline leaching followed by recrystalliza-
tion (Pan et al., 2016), organic acid leaching followed by calcination
process (Zhu et al., 2013a), and other related types of chemical
conversion routes (Ma et al., 2016). Reagent leaching followed by
electrowinning process is used to recovering metallic lead from
spent lead paste. However, leaching reagent of strong acid such as
HCl, H2SiF6, HBF4 and HClO4 causes serious acid corrosion. And the
energy consumption is very high (Li et al., 2019b). Alkaline leaching
followed by recrystallization process can produce high-purity lead
oxide from spent LABs. However, the use of high-concentration
sodium hydroxide solution has safety risks. In addition, the lead
oxide should be mixed with additional metallic lead powder for
using as the leady oxide (mixture of PbO and Pb) for making a new
LAB. Organic acid leaching followed by calcination process is
operated by leaching with citric acid/sodium citrate or acetic acid/
sodium citrate to crystallize lead carboxylate precursor (Zhu et al.,
2013a). Then the crystalline lead citrate precursor is calcined at
300e500 C to produce ultrafine leady oxide powder (Hu et al.,
2016; Li et al., 2012). The obtained leady oxide could be directly
used as the active materials for the LAB production. This new route
can significantly shorten the process from spent LAB to make a new
LAB. However, the impurity elements are not efficiently removed by
this method (Yu et al., 2019b). In addition, the use of organic acid
reagent would increase the cost and potentially limit its applica-
tion. The other related types of chemical conversion routes usually
include desulfurization or acidification, and subsequent leaching
processes to produce a lead-containing solution. The lead-
containing solution is used to produce lead compounds such as
lead carbonate (Zhu et al., 2013b). Finally, the lead carbonate is
calcined to prepare lead oxide.
In summary, there are several common issues, which need to be
considered in current hydrometallurgical processes (Li et al.,
2019b). (1) Strong acid and alkali reagents cause equipment
corrosion and health risks (Tian et al., 2017). (2) The impurities in
the recovered product do not easily meet the specification limits to
meet the standards demanded by the LAB industry. If the impurity
elements such as Fe and Ba are not eliminated, which will have an
adverse effect on the electrochemical property of positive active
materials (PAMs) of a new LAB (Yuan et al., 2016). The limits
required of these impurities are listed in the standard of LAB in-
dustry, and high-purity lead is needed for making new LABs. For
instance, the content of Fe element in the 1# Standard Pb99.994
(GB/T 469e2013, China) is limited to 5 mg/kg. So, the harmful
impurity elements in the spent lead paste should be removed in the
hydrometallurgical process (Liu et al., 2011). (3) Hydrometallurgical
process is usually less cost-effective in operating costs because of
consumption of more expensive leaching reagents than in tradi-
tional pyrometallurgical process. So, reducing the cost of leaching
reagents or recirculating the leaching solution would be essential to
decrease the cost of the hydrometallurgical process (Gao et al.,
2018). Unfortunately, leaching filtrate recirculation in spent LAB
recovery has not been reported in previous studies (Li et al., 2019b).
(4) The recovered products by hydrometallurgical processes are
mainly metallic lead, lead oxide and leady oxide, which are applied
as sources of active materials for LAB. The preparation of higher
value-added lead recovery products will be beneficial to expand the
application of hydrometallurgical process.
Lead tetroxide is usually used as positive material additive for
LAB, which could improve the electrochemical performance of LAB
(McKinley et al., 2002). This paper intends to prepare lead tetroxide
from spent lead paste by hydrometallurgical process, and provide
feasible technology for the high-value utilization of spent lead
paste. Furthermore, by introducing a closed-loop ammonium salt
hydrometallurgical system, the circulation of the filtrate is ach-
ieved, and the dosage of the reagent is reduced. At the same time,
the leaching system is in a neutral environment, reducing the
problem of acid/alkaline corrosion.
2. Experimental
2.1. Raw material
The spent lead paste was provided by Hubei Jinyang Metallur-
gical Co. Ltd., China. After a separation process of crushing and
sieving through an 80-mesh screen, the dried spent lead paste
powder was used as raw material in this study. The main lead-
containing compositions in spent lead paste were measured by
disodium edetate (EDTA-2Na) titration method (Liu et al., 2018).
The spent lead paste is composed of 60.3 wt% Pb(II)SO4, 26.5 wt%
Pb(IV)O2,10.6 wt% Pb(II)O, and 1.6 wt% metallic Pb. It also contains a
small amount of impurities such as Fe, Sb, Ba, Ca, Al, Cu, and Zn,
which come from the LAB manufacturing procedures and the
dismantling of spent LAB. The contents of impurity elements were
analyzed by an inductively coupled plasma optical emission spec-
trometer (ICP-OES 8300, PerkinElmer, America) after digestion of
the spent lead paste specimens by aqua regia, and the contents of
Fe, Sb, Ba, Ca, Al, Cu, and Zn elements in the spent lead paste are
1728.6 ± 113.0, 1143.0 ± 89.0, 1005.0 ± 64.0, 552.1 ± 79.0,
92.0 ± 43.0, 79.0 ± 32.0, 72.0 ± 24.0 mg/kg, respectively.
2.2. The closed-loop hydrometallurgical process
The flowsheet of the closed-loop green process is shown in Fig.1,
including the following steps.
Fig. 1. Flowsheet of the closed-loop hydrometallurgical process.
M. Li et al. / Journal of Cleaner Production xxx (xxxx) xxx
2

2.2.1. Hydrometallurgical leaching
NH4Ac, HAc and H2O2 were used as the leaching reagents for the
main lead-containing components conversion in spent lead paste.
PbSO4 is a major component (60.3 wt%) in the spent lead paste. In
order to know the dissolution ratio of PbSO4 in NH4Ac solution, the
spent lead paste was first leached by sole NH4Ac solution. The ef-
fects of parameters including concentration of NH4Ac and solid/
liquid (S/L) ratio on the dissolution ratio of PbSO4 were studied.
After the optimal parameters of NH4Ac leaching were set, the doses
of HAc and H2O2 were further optimized. 9 g of spent lead paste
(SLP) was used for each batch of leaching. The parameters in the
leaching experiments are shown in Table 1.
The dissolution ratio of lead sulfate was calculated by the
following Equation (1).
Dissolution ratio of lead sulfate
ð%Þ ¼ ½ðM W m wÞ = ðM WÞ 100 % (1)
Where M (kg) and m (kg) represent the mass of the spent lead paste
and leaching residue, respectively; W (wt%) and w (wt%) represent
the mass percentage of PbSO4 in the spent lead paste and leaching
residue, respectively.
The leaching ratio of lead was calculated by the following
Equation (2).
Leaching ratio of lead ð%Þ ¼ ½ðC VÞ = ðM WÞ 100 % (2)
Where C (mol/L) represents the concentration of lead in the
leaching solution, which was measured by EDTA-2Na titration
method; V (L) represents the volume of the leaching solution; M (g)
represents the mass of the spent lead paste; and W (wt%) repre-
sents the content of lead in the spent lead paste.
The leaching ratios of impurity elements were calculated by the
following Equation (3).
Leaching ratios of impurity elements
ð%Þ ¼ ½ðCi VÞ = ðM WiÞ 100 % (3)
Where Ci (mg/L) represents the concentrations of impurity ele-
ments in the leaching solution, which were measured by ICP-OES; V
(L) represents the volume of the leaching solution. The M (g) rep-
resents the mass of the spent lead paste; and Wi (mg/kg) represents
the contents of impurity elements in the spent lead paste.
2.2.2. The preparation of lead carbonate
After the Pb(Ac)2 solution was filtered and separated, the PbCO3
was prepared by adding (NH4)2CO3 into the leaching solution. The
dose of (NH4)2CO3 was set according to the theoretical mole
amount required for the conversion of Pb(Ac)2 solution. The con-
centration of (NH4)2CO3 solution was set as 2 mol/L. This mixture
was stirred for 1 h at room temperature of 25 ± 2 C.
2.2.3. The recirculation of ammonium acetate solution
After the PbCO3 precipitation product was filtered, the filtrate
contained a certain amount of SO4
2
that came from the dissolution
of PbSO4 in the leaching process. The high-concentration of SO4
2
ions would restrain the desulfurization conversion of PbSO4 if the
filtrate was reused directly in the leaching process of next round.
Herein, a method of removing the SO4
2
ions by adding Ba(Ac)2 to
produce BaSO4 by-product was proposed. The concentration of
SO4
2
ions in the filtrate was measured by gravimetric method. The
mole dose of Ba(Ac)2 was according to the theoretical mole amount.
This mixture was stirred for 1 h at room temperature of 25 ± 2 C
and then kept for 24 h before filtration step.
After filtration, the regenerated NH4Ac solution was reused for
the next-round leaching process of spent lead paste. The doses of
HAc and H2O2 were kept identical to the doses that used in the first-
round leaching process. No extra NH4Ac was added into the
leaching process. 9 g of SLP was leached in the leaching process of
each round. Through the desulfurization and leaching, PbCO3 pre-
cipitation and SO4
2
removal processes, a closed-loop hydrometal-
lurgical process could be achieved, as shown in Fig. 1.
2.2.4. The preparation of lead tetroxide product
The calcination process of the synthesized PbCO3 at 400e500 C
for 1e12 h in air was investigated. By optimization of the calcina-
tion conditions, a high-purity product of Pb3O4 was obtained.
2.3. Characterization methods
The crystalline phase of the leaching residues, PbCO3 product,
BaSO4 by-product and Pb3O4 product were studied using a X-ray
diffraction (XRD) technique (Shimadzu, XRD-7000, Japan) with Cu-
Ka radiation of l ¼ 1.54 Å at a scanning rate of 10 per minute in the
2q range of 5e75. The morphology of the Pb3O4 powder was
examined using scanning electron microscopy (SEM) (Sirion200,
Table 1
The parameters in the leaching experiment.
No. Concentration of NH4Ac (mol/L) S/L (g/L) Dose of HAc (mmol/g SLPa
) Dose of H2O2 (mmol/g SLPa
) Time (min)
I-NH4Ac-5.0 5.0 100 0 0 120
I-NH4Ac-6.0 6.0
I-NH4Ac-6.5 6.5
I-NH4Ac-7.0 7.0
I-NH4Ac-7.5 7.5
II-S/L-100 6.5 100 0 0 120
II-S/L-90 90
II-S/L-80 80
II-S/L-70 70
II-S/L-60 60
III-HAc-0 6.5 90 0 5.5 0e360
III-HAc-1.7 1.7
III-HAc-2.7 2.7
III-HAc-3.0 3.0
III-HAc-3.3 3.3
IV-H2O2-1.1 6.5 90 3.0 1.1 0e360
IV-H2O2-2.2 2.2
IV-H2O2-3.3 3.3
IV-H2O2-4.4 4.4
a
SLP: spent lead paste.
M. Li et al. / Journal of Cleaner Production xxx (xxxx) xxx 3

FEI, Netherlands) after coating the samples with gold. The micro-
structure of the Pb3O4 sample was investigated by a high-resolution
transmission electron microscopy (HRTEM) (Tecnai G2 20, FEI,
Netherlands).
3. Results and discussion
3.1. The leaching of spent lead paste with aqueous ammonium
acetate solution
The spent lead paste is leached by NH4Ac solution firstly. The
chemical reaction in the leaching process is as following Reaction
(4).
PbSO4(s)þ2NH4Ac(aq)/Pb(Ac)2(aq)þ(NH4)2SO4(aq) (4)
The leaching ratio of lead and the dissolution ratio of PbSO4 in
the NH4Ac solution show an increasing trend with increasing
NH4Ac concentration from 5.0 to 6.5 mol/L (Fig. S1(a) in Supporting
Information). When the concentration of NH4Ac solution was
6.5 mol/L, the dissolution ratio of PbSO4 reached the highest value
of 97.4 wt%. When the concentration was higher than 6.5 mol/L, the
dissolution ratio of PbSO4 decreased slightly. The main reason is
that the higher concentration of acetate ion has an adverse effect on
the dissolution of Pb(Ac)2 in ammonium acetate solution. From
Fig. S1(b), it could be observed that the leaching ratio of lead and
dissolution ratio of PbSO4 decreased slightly with the increasing of
S/L ratios when fixing the concentration of NH4Ac as 6.5 mol/L.
However, increasing the S/L ratio is beneficial to reduce the dose of
ammonium acetate in the leaching procedure of spent lead paste.
With the S/L ratio of 100 g/L, the leaching ratio of lead was 63.9 wt%,
which was obviously lower than that of 65.1 wt% at S/L ratio of 90 g/
L. Combined consideration of a high dissolution ratio of lead and a
low dosage of ammonium acetate, the S/L ratio of 90 g/L was pro-
posed in the subsequent leaching process. If the leaching of lead is
only attributed to the dissolution of PbSO4, the leaching ratio of lead
should be 52.3 wt%, which is lower than that of 65.1 wt%. It in-
dicates that the leaching ratio of lead is attributed to both the
dissolution of PbSO4 and the dissolution of other lead compounds
such as PbO (Volpe et al., 2009).
3.2. The leaching of spent lead paste with the mixing solution of
ammonium acetate, acetic acid and hydrogen peroxide
The lead in the spent lead paste was not completely converted
into Pb2þ
by the leaching of NH4Ac solution. HAc and H2O2 could be
added into the NH4Ac solution to promote the conversion of lead
compounds into Pb2þ
. The main reactions are shown in Reactions
(5)e(7).
PbO(s)þ2HAc(aq)/Pb(Ac)2(aq)þH2O(aq) (5)
PbO2(s)þ2HAc(aq)þH2O2(aq)/Pb(Ac)2(aq)þ2H2O(aq)þO2(g) (6)
PbO2(s)þPb(s)þ4HAc(aq)/2 Pb(Ac)2(aq)þ2H2O(aq) (7)
In the mixed leaching solution, the concentration of NH4Ac and
S/L ratio were set to be 6.5 mol/L and 90 g/L, respectively. The ef-
fects of different doses of HAc and H2O2 on the leaching of lead and
impurity elements are shown in Fig. 2. The theoretical dose of HAc
for the reaction with lead oxide, lead dioxide and metallic lead in
the spent lead paste was 3.3 mmol/g SLP. The different doses of HAc
were set as 0, 0.5, 0.8, 0.9, and 1.0 times that of the calculated
stoichiometric dose. Fig. 2(a) shows that the leaching ratio of lead
reached the equilibrium value at about 120 min. After the leaching
process, the final leaching pH were 7.9, 7.8, 7.5, 7.3 and 7.1,
respectively. In general, the leaching ratio of lead increased with
the increasing of HAc dose from 0 to 3.3 mmol/g SLP. The leaching
ratios of impurity elements also increased with the increasing of
HAc dose, shown in Fig. 2(b). This is mainly due to the decrease of
pH in leaching solution, caused by the increase of HAc dose. When
the dose of HAc increased from 3.0 to 3.3 mmol/g SLP, the leaching
ratios of Fe, Sb and Al increased significantly. For instance, the
leaching ratio of Fe element at the HAc dose of 3.3 mmol/g SLP was
twice of that at the HAc dose of 3.0 mmol/g SLP. In order to realize
higher leaching ratio of lead and lower dissolution ratio of impurity
elements simultaneously, a dose of 3.0 mmol HAc/g SLP was used
for the optimized leaching parameters in the subsequent leaching
process.
Fig. 2(c) shows the leaching ratio of lead with different doses of
H2O2. The dose of 1.1 mmol H2O2/g SLP was the stoichiometric dose
for the reaction with lead dioxide in the spent lead paste. When the
dose of H2O2 exceeded 2.2 mmol/g SLP, the leaching ratios of lead
generally maintained at 96.1 wt%. Meanwhile, the leaching ratio of
impurity elements maintained at specific levels at different doses of
H2O2 (Fig. 2(d)). In order to minimize the consumption of H2O2,
2.2 mmol H2O2/g SLP was chosen as the optimal dose of H2O2.
3.3. Synergistic effect of ammonium acetate and acetic acid in the
leaching process
To explore the synergistic effect of NH4Ac and HAc in the
leaching process, a set of experiments were designed. At the opti-
mized leaching conditions, the dose of NH4Ac and HAc were
72.2 mmol/g and 3.0 mmol/g SLP, respectively. So the total Ac
in
the leaching system was 75.2 mmol [Ac
]/g SLP. To keep the total
amount of Ac
in the leaching system as a constant of 75.2 mmol
[Ac
]/g SLP, the doses of HAc were set as 0, 3.0, 6.0, 15.0, and
30.0 mmol/g SLP, and the corresponding doses of NH4Ac were set as
75.2, 72.2, 69.2, 60.2, and 45.2 mmol/g SLP. The other leaching
parameters were kept the same as the optimized parameters (S/
L ¼ 90 g/L, dose of H2O2 ¼ 2.2 mmol/g SLP, T ¼ 25 ± 2 C,
time ¼ 120 min). As shown in Fig. 3(a), the leaching ratio of lead
first increased slightly from 91.2 to 98.5 wt% with the increasing of
the HAc dose from 0 to 6.0 mmol/g SLP and then decreased sharply
with the further increasing of HAc dose from 6.0 to 30.0 mmol/g
SLP. This result is related with the leaching pH. When the dose of
HAc increased from 0 to 6.0 mmol/g SLP, the pH of the leaching
solution decreased from 7.9 to 6.6, which was beneficial to the
leaching of lead in the system. As the amount of HAc continued to
increase, the amount of NH4Ac decreased accordingly, which had a
significant influence on the dissolution of PbSO4, so the leaching
ratio of lead element dropped significantly. The XRD patterns of the
leaching residues at different amounts of NH4Ac and HAc are
shown in Fig. 4. When the dose of HAc was 0 mmol/g SLP, the
leaching residue was Pb4SO4(CO3)2$(OH)2. It indicated that the
desulfurization of PbSO4 was incomplete. The Pb4SO4(CO3)2$(OH)2
phase was possibly formed by the leaching residues of Pb(OH)2 and
PbSO4 in the drying process in air (Milodowski and Morgan, 1984).
When the dose of HAc was increased to 3.0 mmol/g SLP, the in-
tensity of Pb4SO4(CO3)2$(OH)2 phase decreased. It indicated that
the HAc promoted the leaching efficiency of PbSO4. With a higher
HAc dose of 6.0e30.0 mmol/g SLP, the PbSO4 was the only phase in
the leaching residues, and the amount of PbSO4 was increasing as
the crystalline peak intensity of PbSO4 phase apparently increased,
as shown in Fig. 4(c)e(e). It is consistent with the result of Fig. 3(a).
The different lead-containing phases could be explained by the
fraction diagram of lead element in the NH4Ac-HAc system (Fig. S2
in Supporting Information). When the HAc dose increased from 0 to
30.0 mmol/g SLP, the pH of the leaching system decreased from 7.9
4

to 6.6. The lead-containing phases of Pb(OH)2 and PbO$PbSO4
gradually transformed into PbSO4, which is consistent with XRD
results of the leaching residues. In general, both NH4Ac and HAc
contributed to promote the leaching of lead element. The NH4Ac
played a major role in the dissolution of PbSO4. The HAc solution
was mainly used to adjust the pH for the leaching of other lead-
containing phases, contributing to favorable conditions for the
simultaneous desulfurization and leaching of PbSO4.
Fig. 3(b) shows that the leaching ratios of the impurity elements
at different doses of NH4Ac and HAc. With the increasing of HAc
dose, the leaching ratio of Fe and Al elements increased signiﬁ-
cantly. These results could be explained by the fraction diagrams of
Fe and Al elements in the leaching system by using Medusa Soft-
ware. In the leaching solution of impurity elements-NH4Ac-HAc
system containing 6.53 mol/L Ac
, 6.50 mol/L NH4
þ
and 0.17 mol/L
SO4
2
(Eh ¼ ~0.5 V), the decreasing of pH from 7.9 to 5.2 would
promote the conversion of insoluble Fe2O3 to soluble Fe(Ac)3, and
insoluble Al(OH)3 to soluble Al(OH)(Ac)þ
, respectively (Fig. 5(a) and
(b)). The leaching ratio of Sb element increased ﬁrstly when the
dose of HAc increased from 0 to 15.0 mmol/g SLP (corresponding
pH decreased from 7.9 to 6.0). Similar conclusion of leaching ratio of
Sb element increased with the decreasing of pH could be found in
the literature (Yu et al., 2019b). However, it is inconsistent with the
fraction diagram of Sb element (Fig. 5(c)), where the soluble
Sb(OH)6
-
would be gradually converted to insoluble Sb2O4(s) when
the pH decreased from 7.9 to 6.0. It is mainly because that the
complexation between Sb and Ac
has not been considered in the
database of the Medusa software. Therefore, the simulation result
Fig. 2. The leaching ratios of (a) lead and (b) impurity elements at different doses of HAc (Concentration of NH4Ac ¼ 6.5 mol/L, S/L ¼ 90 g/L, dose of H2O2 ¼ 5.5 mmol/g SLP, and
T ¼ 25 ± 2 C); the leaching ratios of (c) lead and (d) impurity elements at different doses of H2O2 (Concentration of NH4Ac ¼ 6.5 mol/L, S/L ¼ 90 g/L, dose of HAc ¼ 3.0 mmol/g SLP,
and T ¼ 25 ± 2 C).
Fig. 3. Leaching ratios of (a) lead and (b) impurity elements at different doses of ammonium acetate and acetic acid with a total amount of Ac
as 75.2 mmol [Ac
]/g SLP (S/L ¼ 90 g/
L, dose of H2O2 ¼ 2.2 mmol/g SLP, T ¼ 25 ± 2 C, and time ¼ 120 min).

deviates from the actual leaching process. The leaching ratio of Sb
element decreased when the dose of HAc further increased from
15.0 to 30.0 mmol/g SLP, which was mainly due to the decrease of
lead leaching ratio. Since the impurities were tightly bound to the
particles of lead compounds in the spent lead paste, the incomplete
leaching of spent lead paste would also affect the leaching of Sb
element. The leaching ratio of Ba element was very low and
remained unchanged, which is consistent with the result of
Fig. 5(d) that the BaSO4 (s) remained in the solid form as the pH
decreased from 7.9 to 5.2. The leaching ratio of Ca element
decreased with the increasing of HAc dose from 0 to 15.0 mmol/g
SLP, and then increased when the HAc dose increased to
30.0 mmol/g SLP. The result could be explained by the fraction di-
agram of Ca element in Fig. 5(e). The fraction of Ca(Ac)þ
decreased
firstly as pH decreased from 7.9 to 5.5 and then increased in the pH
range from 5.5 to 5.2. The leaching ratio of Cu and Zn elements
showed little change when the dose of HAc increased from 0 to
30.0 mmol/g SLP. It was mainly because the Cu and Zn elements
were in the form of soluble substance at the pH range of 7.9e5.2
(Fig. 5(f) and (g)). In summary, the complexation between acetate
ions and metal elements played an important role in the leaching
process of impurity elements, and the pH was the key parameters
influencing the leaching of Fe, Al and Sb elements.
3.4. The precipitation of PbCO3 product from lead acetate leaching
solution and the recirculation of NH4Ac solution
The leaching ratio of lead in the initial leaching process (Con-
centration of NH4Ac ¼ 6.5 mol/L, S/L ¼ 90 g/L, dose of
HAc ¼ 3.0 mmol/g SLP, dose of H2O2 ¼ 2.2 mmol/g SLP,
T ¼ 25 ± 2 C, and time ¼ 120 min) was about 96.2%. The con-
centration of lead in the initial lead acetate filtrate was 0.31 mol/L.
Then PbCO3 was precipitated by adding 16 mL 2 mol/L of (NH4)2CO3
solution into the lead acetate solution (Reaction (8)). The precipi-
tation ratio of the Pb2þ
could reach almost 99.9 wt%. After solid-
liquid separation, the SO4
2
ions in the filtrate (Concentration
of SO4
2
¼ 0.15 mol/L) were precipitated by adding Ba(Ac)2, and the
BaSO4 (Ksp ¼ 1.1 1010
) was easily precipitated (Reaction (9)). The
precipitation ratio of the SO4
2
could also reach almost 99.9 wt%.
After solid-liquid separation, the regenerated filtrate comprising
NH4Ac was reused as the leaching solution for spent lead paste in
the next-round leaching process. Then, the closed-loop experiment
was operated five rounds. The reuse of the regenerated NH4Ac
filtrate could reduce the dosage of chemical reagents in the sub-
sequent lead leaching process, which meets the requirements of
cleaner production (Tian et al., 2017).
Pb(Ac)2(aq)þ(NH4)2CO3(aq) ¼ PbCO3(s)þ2NH4Ac(aq) (8)
(NH4)2SO4(aq)þBa(Ac)2(aq) ¼ BaSO4(s)þ2NH4Ac(aq) (9)
The XRD patterns (Fig. S3(a)) of the prepared products from the
precipitation reactions between lead acetate and (NH4)2CO3 show
that the precipitated product in the five rounds is high-purity
PbCO3. In the five-round filtrate recirculation processes, the solid
by-product was high-purity BaSO4 (Fig. S3(b)), which could be used
as expander material for negative active materials of a new LAB. The
excessive BaSO4 can be also used as other industrial chemicals such
as drilling fluid, radiocontrast agent, pigment, paper brightener,
Fig. 4. XRD patterns of the leaching residues at different doses of ammonium acetate
and acetic acid with a total amount of Ac
as 75.2 mmol [Ac
]/g SLP: (a) 0 mmol HAc
and 75.2 mmol NH4Ac/g SLP; (b) 3.0 mmol HAc and 72.2 mmol NH4Ac/g SLP; (c)
6.0 mmol HAc and 69.2 mmol NH4Ac/g SLP; (d) 15.0 mmol HAc and 60.2 mmol NH4Ac/
g SLP; (e) 30.0 mmol HAc and 45.2 mmol NH4Ac/g SLP (S/L ¼ 90 g/L, dose of
H2O2 ¼ 2.2 mmol/g SLP, T ¼ 25 ± 2 C, and time ¼ 120 min).
Fig. 5. Fraction diagram of impurity elements in NH4Ac-HAc system containing 6.53 M
Ac
, 6.50 M NH4
þ
and 0.17 M SO4
2
(Eh ¼ 0.5 V): (a) 2.7 mM Fe3þ
, (b) 0.3 mM Al3þ
, (c)
0.8 mM Sb3þ
, (d) 0.6 mM Ba2þ
, (e) 1.2 mM Ca2þ
, (f) 0.1 mM Cu2þ
, and (g) 0.1 mM Zn2þ
.
6

and plastics filler (Adityawarman et al., 2005). The impurity ele-
ments in the PbCO3 product and BaSO4 by-product of the five
rounds are shown in Tables S1eS5 of Supporting Information. The
results demonstrate that the contents of impurity elements are
very low.
In general, with the increasing of cycle numbers, the leaching
ratio of lead decreased slightly from 96.2 to 92.7 wt% at the 5th
round filtrate-cycle leaching process (Fig. 6), which was mainly due
to the slight volatilization of NH4Ac and the loss of NH4Ac in solid-
liquid separation. It indicated that a suitable amount of NH4Ac
could be supplemented after the 5th round filtrate-cycle leaching
process. In addition, the total lead recovery ratio in each round
decreased slightly from 96.1 wt% towards 92.7 wt%.
3.5. Mass balance of impurity elements in leaching and
precipitation processes
The mass balance of lead and impurity elements in the leaching,
precipitation and SO4
2
removal process of five rounds are pre-
sented in Tables S1eS5 of Supporting Information. Fig. 7 shows the
distribution percentages of impurity elements in solid and liquid
phases during the five rounds of regenerated NH4Ac filtrate-cycle
leaching processes. In the leaching process, 99.1e99.3 wt% of Fe,
98.9e99.4 wt% of Ba, 73.6e79.0 wt% of Sb, 32.5e40.5 wt% of Ca,
66.6e73.8 wt% of Cu, 82.0e89.6 wt% of Zn, and 92.9e97.0 wt% of Al
elements in spent lead paste were kept in the solid residues after
leaching process. These results could be explained by fraction di-
agram in Fig. 5. At the pH of 7.3, the Fe, Ba and Al elements are
mainly in the form of insoluble solid substances such as Fe2O3,
BaSO4 and Al(OH)3, while the Cu, Zn and Ca are mainly in the form
of soluble substances. It demonstrates that the percentages of Fe, Ba
and Al elements in the leaching solution is much lower than those
of Cu, Zn and Ca elements. In addition, distribution percentages of
the impurities in the solid and liquid phases fluctuated in a small
range in the five rounds, which also proved that the recirculation of
regenerated NH4Ac filtrate-cycle leaching process was relatively
steady.
Fig. 8 shows the distribution percentages of impurity elements
in solid phase of the PbCO3 product and liquid phases of the filtrate
during the PbCO3 precipitation process. 17.2e21.1 wt% of Fe,
17.5e21.3 wt% of Sb, 35.1e42.4 wt% of Ba, 57.2e61.5 wt% of Ca,
2.9e4.0 wt% of Cu, 6.1e9.5 wt% of Zn, and 17.1e22.4 wt% of Al
element in the lead acetate leaching solution finally entered into
the PbCO3 product. Among these elements, the higher percentages
of Ba and Ca elements in the PbCO3 product are mainly due to the
Fig. 6. Leaching ratios of lead in the leaching processes and total lead recovery ratios in
the five rounds of regenerated NH4Ac filtrate-cycle leaching process.
Fig. 7. Distribution percentages of impurity elements in solid and liquid phases in the
regenerated NH4Ac filtrate-cycle leaching process of the five rounds: (a) Round-1, (b)
Round-2, (c) Round-3, (d) Round-4, and (e) Round-5.
Fig. 8. Distribution percentages of impurity elements in the solid PbCO3 product and
aqueous filtrate solution in the PbCO3 precipitation process of the five rounds: (a)
Round-1, (b) Round-2, (c) Round-3, (d) Round-4, and (e) Round-5.

combinations of Ba2þ
and Ca2þ
with CO3
2
are more stable than the
combination of other elements with CO3
2
.
In order to know the contributions of leaching and precipitation
processes on the impurity removal, the elemental balance in the
first-round leaching and precipitation process is provided in
Table 2. 99.3 wt% of Fe, 98.9 wt% of Ba and 97.0 wt% of Al elements
in the starting raw material of spent lead paste were removed in the
leaching process. For Sb, Cu and Zn elements, the removal ratios of
these impurities in the leaching process also dominated (70.0 wt
%). The removal ratios of Ca element in the leaching and precipi-
tation procedures were 40.5 and 22.9 wt%, respectively. The total
removal ratios of the impurity elements were over 98.0% (except for
Sb of 95.2 wt% and Ca of 63.4 wt%), which were higher than the
reported removal ratios of impurity elements by hydrometallurgi-
cal process in the latest work (Yang et al., 2020). It indicated that the
proposed hydrometallurgical recovery method for spent lead paste
could significantly remove the harmful impurity (especially Fe and
Ba) elements and obtain high-purity products.
3.6. The preparation of lead tetroxide product
The obtained PbCO3 from the precipitation reaction was
calcined to prepare Pb3O4 product. The Gibbs free energy and
chemical reaction rate of the possible Reactions (10)e(14) during
the calcination process in air are shown in Fig. 9. DG value of the
Reaction (10) is negative at temperature of 320 C, indicating that
the decomposition of PbCO3 into PbO begins at the temperature
higher than 320 C. Then the PbO oxidation products (PbO2, Pb2O3,
and Pb3O4) are formed at temperature of from 320 to 400 C. The
rates of these oxidation reactions are in the order of Reaction (13)
Reaction (11) Reaction (12). So it can be inferred that the calci-
nation products should be a mixture of PbO2, Pb3O4, and Pb2O3 at
320e400 C. When the temperature is higher than 400 C, PbO2 is
decomposed to produce Pb3O4 (Reaction (14)). Through the ther-
modynamic analysis, it can be deduced that high-purity of Pb3O4
could be obtained under specifically controlled temperature and
time based on the competition mechanism of oxidation and
decomposition reactions.
PbCO3¼PbO þ CO2(g) (10)
PbOþ1/6O2(g) ¼ 1/3Pb3O4 (11)
PbOþ1/4O2(g) ¼ 1/2Pb2O3 (12)
PbOþ1/2O2(g) ¼ PbO2 (13)
PbO2 ¼ 1/3Pb3O4þ1/3O2(g) (14)
Fig. 10(a) shows the XRD patterns of the calcination products
under different temperatures of 350e550 C held for 6 h. With
increasing of temperature, the major phases of the calcination
product changed. At 350, 400 and 425 C, the calcination products
were identified to be Pb2O3, PbO, Pb3O4 and PbO2, which is
consistent with the thermodynamic analysis in Fig. 9. Then only
Pb3O4 phase was identified in the calcination product at
450e500 C. With the further increasing of the temperature up to
550 C, Pb3O4 was converted into PbO. Hence, 450 C was applied in
the following calcination experiments to determine the optimal
calcination time. With the extending of calcination time, the phases
of PbO and PbO2 gradually disappeared. When the calcination time
was over 6 h, Pb3O4 was the only identified product. So, the calci-
nation temperature of 450 C and calcination time of 6 h were
optimized for the preparation of high-purity Pb3O4. The contents of
the impurity elements of the synthesized Pb3O4 are shown in
Table 3. The contents of Fe and Ba were 2.7 and 5.2 mg/kg,
respectively, which were lower than the reported data in the pre-
vious literatures (Pan et al., 2016). If assuming the dose of the
prepared Pb3O4 additive in the PAMs of LAB is 20 wt% (Ferg et al.,
2006), the calculated contents of Fe and Ba in PAMs only
increased by 0.5 and 1.0 mg/kg, respectively, which are extremely
Table 2
Removal ratios of impurity elements in the leaching and precipitation processes of
Round-1 (wt%).
Procedure Elements
Fe Sb Ba Ca Cu Zn Al
Leaching process 99.3 77.5 98.9 40.5 71.2 88.6 97.0
Precipitation process 0.6 17.7 0.7 22.9 27.6 10.3 2.4
Total removal ratio 99.9 95.2 99.6 63.4 98.8 98.9 99.4
Fig. 9. (a) Gibbs free energy values (DG) and (b) chemical reaction rates of possible
reactions during the calcination process of PbCO3.
Fig. 10. XRD patterns of the calcination products from the lead carbonate under
different temperatures and time: (a) at different temperature for 6 h, and (b) at 450 C
for different duration time.
8

too low to cause negative effects on the battery performance.
Regarding to the relatively high content of Ca in the Pb3O4 product,
it has no significant negative impact on the battery performance
(Lam et al., 2010; Pavlov, 2011). As for Sb, previous studies had
demonstrated that the Sb2O3 additive could improve the contents
of hydrated PbO2 in PAMs, resulting in a longer cycle life of LAB
(Yang et al., 2017). It indicates that the Sb element in the prepared
Pb3O4 would also not affect the performance of LAB.
Fig. 11 shows the SEM and TEM images of the prepared Pb3O4.
The Pb3O4 product shows agglomerated amorphous particles in
size of about 500 nm. In the high-resolution TEM image (Fig. 11(c)
and (d)), a clear boundary can be observed. The measured inter-
planar distance is 0.291 nm, which is equal to the theoretical
interplanar distance of the (1, 1, 2) plane of Pb3O4 (0.291 nm). The
result is consistent with the identified pure Pb3O4 phase of the XRD
pattern in Fig. 10.
4. Conclusions
In this paper, a closed-loop ammonium salt hydrometallurgical
system was proposed to prepare PbCO3 and high-purity Pb3O4
products from the spent lead paste, which could reduce the dose of
chemical reagents and achieve efficient removal of impurity ele-
ments. In the leaching process, the NH4Ac solution was used for the
dissolution of PbSO4, and the HAc was used for pH adjustment to
promote leaching of lead element. The leaching ratio of lead in the
initial leaching process could reach 96.2 wt% under the optimal
operating conditions of NH4Ac Concentration ¼ 6.5 mol/L, S/
L ¼ 90 g/L, dose of HAc ¼ 3.0 mmol/g SLP, dose of H2O2 ¼ 2.2 mmol/
g SLP, T ¼ 25 ± 2 C, and time ¼ 120 min. 99.3 wt% of Fe and 98.9 wt
% of Ba elements were removed in the leaching process, which was
mainly attributed to the insoluble forms of Fe2O3(s) and BaSO4(s).
The total removal efficiency of the Fe and Ba element in the
leaching and precipitation process was about 99.9 and 99.6 wt%,
respectively. High-purity PbCO3 product and BaSO4 by-product
could be obtained in the five rounds of regenerated NH4Ac filtrate
recirculation processes. High-purity of Pb3O4 product was prepared
by calcination of the PbCO3 at 450 C for 6 h in air atmosphere. This
study provided an effective strategy to recover spent lead paste or
other lead-containing waste.
Author contributions
All authors make contributions to this paper. Mingyang Li car-
ried out the main part of the experiments and wrote the original
draft of this paper. Professor Jiakuan Yang supervised the novelty
and design of experiments. Junxiong Wang, Peiyuan Zhang and
Wenhao Yu provided many helpful suggestions for the experi-
mental investigation and data analysis. Dr. Sha Liang and Professor
R. Vasant Kumar provided instruction on manuscript revisions.
Professor Jingping Hu, Dr. Keke Xiao, Dr. Huijie Hou, and Dr. Bing-
chuan Liu polished the manuscript.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the funding from National Key
Research and Development Program of China (2018YFC1900105).
The authors would like to thank the Analytical and Testing Center of
Huazhong University of Science and Technology (HUST), State Key
Laboratory of Coal Combustion of HUST, and School of Environ-
mental Science and Engineering of HUST, for the supply of in-
struments for materials analysis, and also thank Hubei Jinyang
Metallurgical Co. Ltd., China for providing raw materials of spent
lead paste.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jclepro.2019.119488.
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