Research Article
Received: 3 November 2014
Revised: 14 December 2014
Accepted article published: 22 December 2014
Published online in Wiley Online Library: 21 January 2015
(wileyonlinelibrary.com) DOI 10.1002/jctb.4620
Structural study of a lead (II) organic
complex – a key precursor in a green recovery
route for spent lead-acid battery paste
Wei Zhang,a Jiakuan Yang,a* Xinfeng Zhu,a,b Xiaojuan Sun,a Wenhao Yu,a
Yuchen Hu,a Xiqing Yuan,a Jinxin Dong,a Jingping Hu,a Sha Lianga and
Ramachandran Vasant Kumarc
ABSTRACT
BACKGROUND: Lead citrate is an attractive precursor for the preparation of ultrafine leady oxide from the paste in spent
lead-acid batteries through a novel hydrometallurgical process, since the recovered lead oxide could be recycled for the
production of new lead acid batteries.
RESULTS: Two different metal organic complexes were synthesized from lead sulfate from the paste of spent lead-acid batteries
in a leaching solution at two different initial pH values. Single crystals of the two precursors were obtained by conditioning
and filtering from the leached solutions, and used for single crystal XRD analysis. At an initial pH of 3.5, the chemical
formula of Precursor-I is deduced to be Pb(C6 H6 O7 )·H2 O while at an initial pH of 5.2, the chemical formula of Precursor-II is
Pb3 (C6 H5 O7 )2 ·3H2 O.
CONCLUSIONS: Both Precursor-I and Precursor-II crystallize in the triclinic crystal system, with space group P-1. This structural
study on precursors sheds light on the mechanism of a paste-to-paste recovery route for the recovery of spent lead paste.
© 2014 Society of Chemical Industry
Keywords: lead-acid battery; lead organic complex; hydrometallurgical route; single crystal XRD analysis; recycling
INTRODUCTION
672
The lead-acid battery is currently the most common type of
rechargeable battery owing to its relatively low cost, reliable and
safe performance, and higher recycling ratio compared with other
rechargeable batteries. Lead-acid batteries account for over 80% of
global consumption of lead.1,2 Despite the relative success in lead
recycling, much effort is still needed to solve the massive pollution
issues arising from collection, sorting, treatment and refining of
spent and discarded lead acid batteries, as the improper disposal
of spent batteries is detrimental to the environment because of the
high toxicity of lead.3 – 5
Nearly 95% of all discarded batteries are currently recycled
by pyrometallurgical methods. However, conventional smelting
methods are challenged by the emission of considerable amounts
of sulfur dioxide and lead dust into the environment. In a typical
spent lead paste, lead sulfate (PbSO4 ) is the dominant component
at nearly 60 wt%. Decomposition of lead sulfate requires a carbothermic reduction procedure at high temperature (>1000 ∘ C),
resulting in inevitable and significant emission of hazardous sulfur
dioxide gas and lead particulates.2,4,6 – 9
In order to reduce hazardous emissions during the recycling of
spent lead-acid batteries, several hydrometallurgy methods have
been proposed and studied to recover lead paste from spent
lead acid batteries. The traditional hydrometallurgical methods,
i.e. leaching followed by electro-winning, could partially address
J Chem Technol Biotechnol 2016; 91: 672–679
the inherent pollution issue of the pyrometallurgical method, but
could still be discouraged by the intensive energy demand.10 The
proposed leaching reagents consist of fluorine-containing chemicals, the emission of which would also lead to an additional environmental hazard. A lead hydrometallurgical process was reported
recently, based on a hydrogen–lead oxide fuel cell.11
A novel method of lead recycling by sodium citrate and citric
acid has been developed.12,13 In this process, spent lead paste was
treated with an aqueous citric acid solution to produce insoluble lead citrate precursors, followed by their calcination to recycle PbO/Pb products from the starting material of PbO, PbO2 and
PbSO4 . Since the recovered lead oxide product could be used
as the active material to manufacture new lead-acid batteries,
∗
Correspondence to: Jiakuan Yang, School of Environmental Science and Engineering, Huazhong University of Science and Technology (HUST), 1037 Luoyu
Road, Wuhan, Hubei, 430074, P. R. China. E-mail: [email protected]
a School of Environmental Science and Engineering, Huazhong University of
Science and Technology (HUST), 1037 Luoyu Road, Wuhan, Hubei, 430074,
P. R. China
b Henan University of Urban Construction, Pingdingshan, Henan, 467000, P. R.
China
c Department of Materials Science and Metallurgy, University of Cambridge,
Cambridge, CB2 3QZ, UK
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© 2014 Society of Chemical Industry
Structure of a lead organic complex from spent lead paste
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this novel hydrometallurgical process is termed a ‘paste-to-paste’
recovery route. In our previous study,14 stoichiometric calculations indicated that the chemical formula of lead citrate precursor prepared from the starting material of PbO, and PbO2 is
[Pb(C6 H6 O7 )] · H2 O, and the formula of lead citrate precursor prepared from the starting material of PbSO4 is Pb3 (C6 H5 O7 )2 · 3H2 O.
The synthesis of the lead (II) citrate from individual lead (II) oxide,
lead (IV) dioxide, and lead (II) sulfate is presented as Equations (1),
(2) and (3), respectively. When PbO2 was used as the starting material, hydrogen peroxide was required to reduce PbO2 to PbO before
the conversion to lead citrate.
[ (
)]
(1)
PbO + C6 H8 O7 · H2 O → Pb C6 H6 O7 · H2 O + H2 O
Lead sulfate
Leaching (pH of 3.5 and 5.2)
Conditioning and
Filtration
Direct Filtration
Powder crystal
(1-5 μm)
[ (
)]
PbO2 + C6 H8 O7 · H2 O + H2 O2 → Pb C6 H6 O7 · H2 O
+ O2 ↑ +2H2 O
Single Crystal XRD
(2)
[
]
(
)
3PbSO4 + 2 Na3 C6 H5 O7 · 2H2 O → Pb3 C6 H5 O7 2 · 3H2 O
+ 3Na2 SO4 + H2 O
Crystal data
and Molecular
structure
(3)
In previous reports the formulae of lead citrate precursors were
only deduced using stoichiometric calculations based on the conservation of mass during the reaction. However, the crystal structure was not identified. In this paper, single crystal XRD analysis was explored further to confirm the specific chemical formula
and crystal structure. This fundamental information relating to
lead citrate precursor is critical for understanding the subsequent
combustion-synthesis of lead oxide powder from lead citrate precursors for the preparation of recycled paste in a new battery.
The interaction between lead ions and organic molecules is
commonly studied for its soft–hard character. Kourgiantakis15
synthesized Pb-citrate by the reaction of Pb(NO3 )2 with citric
acid solution at pH of 2 and demonstrated that the novel compound is [Pb(C6 H6 O7 )]n · nH2 O from an X-ray crystallographic
study. Shi16 suggested that the chemical formula of lead citrate is
Pb6 (H2 O)2 (C6 H5 O7 )4 · 3H2 O. Generally, it has been demonstrated
that lead (II) citrate complexes have different formulae and structures depending on the pH of the leaching reagents and the
starting lead compound. However, all of the above reported
lead citrate structure studies are based on a highly soluble lead
salt, Pb(NO3 )2 . In comparison, it would be more enlightening to
investigate the crystal structure of lead citrates produced from
insoluble components which can shed light on the mechanism of
a paste-to-paste recovery route for the recovery of paste in spent
lead-acid batteries, consisting of PbO, PbO2 , and PbSO4 .
Therefore, large crystals of single crystal lead citrate precursor
was synthesized from lead sulfate, the dominant component in a
spent lead-acid battery paste, and chemical formula and crystallographic structure was determined by single crystal XRD. The flow
chart of procedure followed in this study is illustrated schematically in Fig. 1.
EXPERIMENTAL
Stimulated
powder
XRD pattern
Figure 1. Synthesis and analysis of lead (II) citrate precursor prepared from
lead sulfate.
Synthesis of lead citrate precursors
Lead sulfate was leached using a mixed solution of citric acid and
sodium citrate under magnetic stirring at a speed of 650 rpm for
24 h at room temperature (25 ∘ C). In the leaching process, a S/L
mass ratio of 1/10 with respect to the PbSO4 starting material
(solid) and water (liquid) was used at two different initial pH values
of 3.5 and 5.2. The corresponding reactions at the two initial pH
values are designated as Reaction-I and Reaction-II, respectively.
The experimental conditions of leaching and conditioning procedures are summarized in Table 1. As shown in Table 1, the mole
ratio of sodium citrate to citric acid in Reaction-II is higher than in
Reaction-I. Thus the initial pH of Reaction-II (5.2) is higher than that
of Reaction-I (3.5) since sodium citrate solution is a weak alkaline
solution. Moreover, the mixed solution of citric acid and sodium
citrate is a buffer, so the pH after reaction remains almost constant.
After leaching, the reaction vessel with the reactant slurry was
immersed in a water bath at 55 ∘ C for 8 h crystallization. The
lead citrate products after conditioning were rinsed with distilled
water, vacuum filtered and separated, and dried at 65 ∘ C. The two
lead citrate solid products leached at initial pH of 3.5 and 5.2 are
termed Precursor-I and Precursor-II, respectively. For comparison,
the slurry of precursors was also directly filtered and separated
without the conditioning procedure to study morphological differences.
The mass of the lead citrate was determined as follows. The
residual leaching solution was filtered into a volumetric flask of
1000 mL. The analysis of SO4 2− ion in the residual solution was performed by IC (Ion Chromatography, DX-120). The desulphurization
ratio of the starting material, i.e. lead sulfate, was calculated using
Equation (4):
Desulphurization ratio (%)
=
Amount of SO2−
in the residual solution
4
Amount of SO2−
in the starting lead sulfate
4
× 100% (4)
The concentration of lead in the filtrate was determined by
atomic absorption spectroscopy (AAS, novAA400, Analytik
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673
Chemicals and materials
Lead sulfate (PbSO4 , 99.0 wt% purity) was purchased from
Sinopharm Chemical Reagent Co., Ltd. Citric acid monohydrate
(C6 H8 O7 · H2 O, 99.5 wt% purity, Tianjin Kerme Chemical Reagent
Co., Ltd, China) and Na3 C6 H5 O7 · 2H2 O (>99 wt% purity, Sinopharm
Chemical Reagent Co., Ltd, China) were used as received without
further treatment.
J Chem Technol Biotechnol 2016; 91: 672–679
Single crystal
(20-50 μm)
SEM
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W Zhang et al.
Table 1. Design of experimental conditions for leaching and conditioning in a mixed solution of citric acid–sodium citrate at different initial pH
values
Reaction designation
PbSO4 (g)
Reaction-I
Reaction-II
Na3 C6 H5 O7 · 2H2 O(g)
10.0
10.0
C6 H8 O7 · H2 O(g)
12.9
19.4
Jena AG). Since lead citrate product is slightly soluble in the
leaching solution, the amount of lead in the filtrate may lead to a
slight underestimation of the recovery efficiency of the leaching
process. The recovery percentage of lead was calculated using
Equation (5):
Initial pH
25.4
7.4
Crystallization temperature(∘ C)
3.5 ± 0.2
5.2 ± 0.2
55
55
full-matrix least squares using the SHELXTL crystallographic software package.17,18 Anisotropic displacement parameters were
applied to all non-H atoms. The H atoms were included and generated geometrically. Crystallographic images were reproduced
using the software Diamond.
Lead recovery percentage (%)
Mass of lead in the residual solution
× 100%
Total mass of lead in the starting lead sulfate
(5)
Thermogravimetric-differential thermal (TG-DTA) analysis
Thermal analysis of lead citrate precursors was performed in alumina crucibles in a Diamond TG/DTA Instrument (Perkin-Elmer)
under an air flow of 20 cm3 min−1 at a heating rate of 10 ∘ C min−1
starting from room temperature to 600 ∘ C in air.
X-ray powder diffraction (XRD) analysis
The crystal structure of Precursor-I and Precursor-II obtained in
leaching experiments were investigated by powder X-ray diffraction (XRD) technique using an X’Pert PRO X-ray diffractometer
(Philips, PANalytical B.V., Holland) with Cu K𝛼 radiation (𝜆 = 1.54 Å).
Scanning electron microscopy (SEM) analysis
Morphological studies of Precursor-I and Precursor-II were carried
out using SEM (Sirion 200, FEI, Holland) at 10 kV after gold coating.
Single crystal XRD analysis
A suitable single crystal of lead citrate of size 30–50 μm range
was selected and mounted at the end of a thin glass fiber. Diffraction data of Precursor-I and Precursor-II single crystals were collected at 298 K on a Bruker SMART APEX CCD-based diffractometer
with graphite-monochromated Mo-K𝛼 radiation (𝜆 = 0.71073 Å).
The structure was deduced by direct methods and refined by
RESULTS
Synthesis of lead citrate precursors from lead sulfate
The desulphurization efficiency of leaching PbSO4 in solution of
different pH is show in Fig. 2. It was found that the desulphurization
efficiency of Reaction-II with initial pH = 5.2 was more effective
than that of Reaction-I with initial pH = 3.5. For Reaction-II, desulphurization efficiency of lead sulfate reached 99.6%, compared
with an efficiency of 12.1% in Reaction-I both after 1 h leaching. It is
speculated that: (1) PbSO4 had a higher solubility in the Reaction-II
100
Desulphrization efficiency (%)
= 1-
90
Reaciton-I
Reaciton-II
80
70
60
50
40
30
20
10
0
-10
0
200
400
600
800
1000
Leaching time (min)
1200
1400
Figure 2. Desulphurization efficiency of PbSO4 leaching in solutions of
different initial pH: Reaction-I at pH = 3.5, and Reaction-II at pH = 5.2.
Table 2. Desulphurization ratio, percentage lead recovery, and possible yield of lead citrate precursors in the leaching and conditioning process at
different initial pH values
Reaction
Reaction-I
Reaction-II
Initial
mass of
lead
sulfate
10.00
10.00
Moles of
original
PbSO4
(×10−2 )
3.27
3.27
SO4 2−
concentration
in residual
solution
(mg L−1 )*
3156.7
3159.6
Desulphurization
ratio (%)
100.7
100.8
Lead
concentration
in residual
solution
(mg L−1 ) *
Lead
recovery
(%)
218.9
246.2
96.8
96.4
Mass of
solid lead
citrate
product (g)
13.40
11.30
Equivalent
mass of solid
lead citrate
product (g) **
13.52
11.47
Maximum
possible yield
of solid
lead citrate
product (%) ***
99.1
98.6
674
*Each solution was diluted to 1000 mL.
**Equivalent mass of solid lead citrate product in Reaction-I and Reaction-II calculated according to the chemical formulae Pb(C6 H6 O7 )·H2 O and
Pb3 (C6 H5 O7 )2 ·3H2 O, respectively.
***Maximum possible yield of solid lead citrate product = mass of filtrate cake of lead citrate product/equivalent mass of solid lead citrate
product × 100
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J Chem Technol Biotechnol 2016; 91: 672–679
Structure of a lead organic complex from spent lead paste
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Thermal characteristics analysis
The TG-DTA curves of Precursor-I and Precursor-II are shown in
Fig. 3. According to the TG results, the first stage of weight loss in
ambient atmosphere is observed in the range 90–180 ∘ C, which
is interpreted as dehydration. At this stage, the weight loss of
the Precursor-I is 9.6%, while the weight loss of the Precursor-II
is 4.6%. Between 180 and 250 ∘ C, dehydration continues with the
formation of a C = C double bond, i.e. with transformation of the
citrate into aconitate. In the following stages, weight loss takes
place in the temperature range 270–450 ∘ C, which is ascribed
to the combustion of citrate. This process is accompanied by
four exothermic peaks in the DTA curve for Precursor-I at around
274, 334, 366 and 415 ∘ C, due to the oxidation of both C and
H from the citrate.20 – 22 Exothermic peaks in the DTA curve for
Precursor-II appear at around 295, 332, 359 and 414 ∘ C. Within
the temperature range 150–420 ∘ C, the lead citrate precursor
TG (%)
(a)
100
9.6 %
90
12.2 %
80
45.4 %
11.0 %
70
6.7 %
10
415°C
274°C
334°C
20
366°C
5.9 %
173°C
DTA(uv)
60
0
100
200
300
T (°C)
TG (%)
(b) 100
400
500
600
4.6%
90
12.4%
34.7%
80
12.1%
70
5.6%
10
414°C
332°C
274°C
20
359°C
295°C
60
DTA(uv)
solution at pH 5.2 than in Reaction-I; (2) the reaction mechanism
of lead sulfate with citrate ion is different in the two systems with
different initial pH values.
The experimental results of Reaction-I and Reaction-II are shown
in Table 2. This shows that both the desulphurization ratios of
lead sulfate at the two different initial pH values were almost
100%, which indicates that all the insoluble sulfate in the starting material has been completely converted into soluble sodium
sulfate during leaching. Thus, it can be assumed that all the
solid lead citrate is precipitated in the solid product and residual unconverted PbSO4 is negligible. The masses of Precursor-I
and Precursor-II were 13.40 g and 11.30 g, respectively. Since the
initial masses of lead sulfate were identical the significant differences in the weight of the two lead citrate products indicate
that the chemical formulae of the precursors must be different,
and as the mass of the Precursor-I product is greater than that
of the Precursor-II product, this indicates the citrate:Pb mole ratio
of Precursor-I is larger than that of Precursor-II. According to our
previous work,14,19 the formulae of the products from lead sulfate with a solution of citric acid and sodium citrate could be
possibly expressed as Pb(C6 H6 O7 ) · H2 O and Pb3 (C6 H5 O7 )2 · 3H2 O.
Thus, the chemical formula of Precursor-I and Precursor-II can
be assumed to be Pb(C6 H6 O7 )·H2 O (citrate:Pb mole ratio = 1:1)
and Pb3 (C6 H5 O7 )2 ·3H2 O (citrate:Pb mole ratio = 2/3:1), respectively. As shown in Table 2, the calculated equivalent mass of
Precursor-I is 13.52 g, close to the actual mass of 13.40 g, which
indicates that assumption of the chemical formula of Precursor-I
as [Pb(C6 H6 O7 )] · H2 O is reasonable. Similarly, the equivalent mass
of Precursor-II is 11.47 g, close to the actual mass of 11.30 g,
which also demonstrates that the formula of Precursor-II can be
expressed as Pb3 (C6 H5 O7 )2 · 3H2 O. The maximum possible yield of
Precursor-I and Precursor-II is 99.1% and 98.6%, respectively. The
insignificant mass difference of equivalent and actual mass of the
precursors is mainly attributed to the dissolution of lead citrate
precursors in the leaching solution.
0
100
200
300
T (°C)
400
500
600
Figure 3. TG/DTA curves of Precursor-I (a) and Precursor-II (b).
decomposes completely (including the decomposition of any
intermediate lead carbonate),23 and produces PbO and metallic
lead (Pb) after decomposition. For temperatures above 400 ∘ C,
both the weight loss and heat flow values are almost constant even
up to around 600 ∘ C.
As presented in Fig. 3, the total weight loss for Precursor-I is
45.4%, much higher than that for Precursor-II at 34.7%. Assuming the thermal decomposition products of Precursor-I and
Precursor-II are only lead (II) oxide, the calculated mass losses
of Precursor-I and Precursor-II are 46.3% and 36.6%, respectively (Table 3). The results indicate that the values of weight
loss calculated from the chemical formula are consistent with
the results of TG and the calcination experiments conducted
at 370 ∘ C for 1 h. The results of thermal analysis experiments
also show that the formula for Precursor-I and Precursor-II can
be expressed as [Pb(C6 H6 O7 )] · H2 O and Pb3 (C6 H5 O7 )2 · 3H2 O,
respectively.
Table 3. Percentage weight loss of the precursors in the thermal decomposition process
Precursors
Pb(C6 H6 O7 )·H2 O
Pb3 (C6 H5 O7 )2 ·3H2 O
J Chem Technol Biotechnol 2016; 91: 672–679
Calc. weight loss for
complete
decomposition (%)
Expt. weight loss
from TG-DTA (%)
46.3
36.6
© 2014 Society of Chemical Industry
45.4
34.7
Expt. weight loss at
calcination
temperature 370 ∘ C for 1 h (%)
46.5
37.4
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675
Precursor-I
Precursor-II
Chemical
formula
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XRD patterns analysis
The XRD patterns of the Precursor-I and Precursor-II are shown
in Fig. 4. The XRD patterns show that the characteristic peaks of
Precursor-I are quite different from those of Precursor-II, indicating
that the crystal structures are different.
300000
250000
200000
150000
Intensity(Counts)
W Zhang et al.
100000
50000
Precursor-I
0
15000
12000
9000
6000
Precursor-II
3000
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
2θ(°)
Figure 4. XRD patterns of Precursor-I and Precursor-II.
Morphological analysis
The SEM images of Precursor-I and Precursor-II are shown in
Fig. 5. It is seen that the morphology of the citrate precursor
undergoes significant changes after conditioning in a water bath.
Precursor-I product shows plate-like morphology with crystals
around 5–10 μm when samples were synthesized at the initial
pH of 3.5 (Fig. 5(a)), and around 10–50 μm with a more uniform
crystal shape and larger crystal size after conditioning (Fig. 5(b)).
The Precursor-II products display irregular flake morphology with
size averaging from 1–5 μm (Fig. 5(c)), and after conditioning they
change to rod-like morphology with the rod diameter ranging
from 20–50 μm with a more uniform crystal shape and larger crystal size (Fig. 5(d)). The morphology of Precursor-I and Precursor-II
differs in shape and size, indicating a different growth mechanism.
(a)
(b)
(c)
(d)
676
Figure 5. SEM images of two precursors: (a) Precursor-I before conditioning;(b) Precursor-I after conditioning; (c) Precursor-II before conditioning;(d) Precursor-II after conditioning.
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J Chem Technol Biotechnol 2016; 91: 672–679
Structure of a lead organic complex from spent lead paste
Table 4. Crystal data and structure refinement for Precursor-I
and Precursor-II
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shows three-dimensional structure with an internal columnar
structure which is consistent with the columnar-like morphology
of Precursor-II observed in the SEM image shown in Fig. 5(d).
Crystal data and structure
refinement
Empirical formula
Precursor-I
Precursor-II
C6 H8 O8 Pb
C12 H16 O17 Pb3
Formula weight
415.31
1053.82
Temperature (K)
298
298
Wavelength (Å)
0.71073
0.71073
Crystal system
Triclinic
Triclinic
Space group
Unit cell dimensions
Volume (Å3 )
Z
P-1
P-1
a = 6.339(4) Å,
𝛼 = 99.121(8)∘
a = 9.7278(11) Å,
𝛼 = 109.038(2)∘
b = 6.460(4) Å,
𝛽 = 102.743(6)∘
b = 9.7620(11) Å,
𝛽 = 98.565(2)∘
c = 12.053(7) Å,
𝛾 = 101.556(8)∘
c = 10.9578(13) Å,
𝛾 = 92.126(2)∘
460.9(5)
968.60(19)
2
2
Density (calculated)
(mg m−3 )
2.992
3.613
Absorption coefficient
(mm−1 )
18.326
26.100
0.08 x 0.05 x 0.03
0.10 x 0.05 x 0.02
1.77 to 26.00
2.00 to 29.50
Absorption correction
None
None
Goodness-of-fit on F2
1.046
1.055
R1 = 0.0539,
wR2 = 0.1282
R1 = 0.0351,
wR2 = 0.0725
Crystal size (mm3 )
Theta range for data
collection (∘ )
R indices (all data)
It is worth noticing that subsequent single crystal XRD analysis
of the precursors requires single crystals 30 to 50 μm in size. In this
study, conditioning in a 55 ∘ C water bath is an effective route for
synthesizing large single crystals. However, in previous literature,
autoclave-based solvothermal synthesis was commonly used to
synthesize single crystals of lead citrate in high pressure saturated
steam.
J Chem Technol Biotechnol 2016; 91: 672–679
CONCLUSIONS
(1) Stoichiometric calculation results indicate that the chemical
formula of Precursor-II leached at pH 5.2 could be expressed
as [Pb3 · (C6 H5 O7 )2 ] · 3H2 O, while Precursor-I at pH 3.5 it is
[Pb(C6 H6 O7 )] · H2 O.
(2) The morphology of the precursors shows a significant
change after conditioning. The morphology of Precursor-I and
Precursor-II differs in shape and dimension due to their different structures. The SEM images of single crystals show uniform
crystal shapes and larger crystal sizes in the range 20–50 μm
after conditioning, which could be used for single crystal
XRD analysis.
(3) The chemical formula of Precursor-I and Precursor-II are
identified by single crystal XRD analysis. Both Precursor-I and
Precursor-II crystallize in the triclinic crystal system, with space
group P-1.
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677
Single crystal XRD analysis
A summary of crystallographic data and refinement parameters
for the precursors is listed in Table 4. X-ray crystallographic studies reveal that there is a crystallographic independent Pb center, a
chelating citrate ligand and a H3 O cation in the Precursor-I asymmetric unit (Fig. 6(a)). Precursor-I crystallizes in the triclinic crystal system, space group P-1 with lattice parameters a = 6.339(4)
Å, b = 6.460(4) Å, c = 12.053(7) Å, 𝛼 = 99.121(8)∘ , 𝛽 = 102.743(6)∘ ,
𝛾 = 101.556(8)∘ , V = 460.9(5) Å3 , Z = 2, R1 = 0.0444, wR2 = 0.1234,
and GOF =1.046. As shown in Fig. 6(b), the packing diagram of this
crystal demonstrates a three-dimensional structure with internal
structure of layers, in agreement with the plate-like morphology
observed in Fig. 5(b).
In Precursor-II, there are three crystallographic Pb atoms,
two citrate ligands, one combined water and two free water
molecules in the asymmetric unit (Fig. 6(c)), which is consistent
with our previously calculated stoichiometric results. Precursor-II
also crystallizes in the triclinic crystal system, space group
P-1 with lattice parameters a = 9.7278(11) Å, b = 9.7620(11) Å,
c = 10.9578(13) Å, 𝛼 =109.038(2)∘ , 𝛽 = 98.565(2)∘ , 𝛾 = 92.126(2)∘ ,
V = 968.60(19) Å3 , Z = 2, R1 = 0.0274, wR2 = 0.0693, and GOF
=1.055. As shown in Fig. 6(d), the packing diagram of the crystal
Implications for the recovery of electronic wastes
Although high purity lead sulfate was utilized in the current investigation the conclusions elucidate a crucial issue in the recycling
of multi-component paste of spent lead-acid batteries as according to our preliminary study it could easily be converted into lead
citrate precursor.24,25 The initial pH value not only has a significant
influence on the crystal structure of lead citrate products, but also
determines the speed of leaching. Moreover, larger single lead citrate crystals are desirable to improve the filtration and separating
efficiency of the slurry, which is crucial for industrial application of
this novel hydrometallurgical process.
The results of stoichiometric calculations indicate that
the chemical formula for Precursor-II may be expressed as
[Pb3 · (C6 H5 O7 )2 ] · 3H2 O with a citrate:Pb mole ratio = 2/3:1,
while the chemical formula for Precursor-I can be deduced as
[Pb(C6 H6 O7 )] · H2 O with a 1:1 mole ratio of citrate : Pb. This indicates that the consumption of citrate ligand for a certain amount
of lead compound is different in a citric acid – sodium citrate
leaching system at different pH values. The crystal structure of
the lead citrate precursors appears to be one of the prominent
factors to manipulate the consumption and operating cost of
citric acid and sodium citrate reagents. In other studies citric acid
was also used to recover lithium from spent batteries,26,27 in which
a metal–citrate complex is also a key precursor. Therefore, this
fundamental research could be of interest in the field of recovery
of electronic wastes, i.e. discarded lithium batteries, by leaching
with an organic acid instead of traditional inorganic acid.26,27
As lead oxides (PbO containing metallic Pb) are formed directly
from the decomposition/combustion of different types of lead
citrate precursors, a deeper understanding of the crystal structure of lead citrate precursor in this study sheds light on the
mechanism of precursor formation and promotes optimization of the paste-to-paste recovery route for the recovery of
spent battery paste in the lead battery industry. This structural study also benefits the lead battery industry and the
environmental control of lead hazard in further understanding the mechanism by which particulates of lead oxides and
sulfates interact with plasma in humans, and the subsequent
absorption or elimination by crystallization.
www.soci.org
W Zhang et al.
(b)
(a)
(c)
(d)
Figure 6. Precursor-I: molecular structure (a) and crystal packing diagram (b); and Precursor-II: molecular structure (c) and crystal packing diagram (d).
ACKNOWLEDGEMENTS
The authors would like to acknowledge funding support from
the international technology cooperation plan of innovation
fund, Huazhong University of Science and Technology (HUST,
2013ZZGH015), the Wuhan Planning Project of Science and
Technology, China (2013060501010168, 2013011801010593, and
2014030709020313). The authors would like to express thanks to
the support from the National Science-technology Support Plan
Projects (2014BAC03B02). The authors would like to acknowledge
the supply of raw materials and the financial support from Hubei
Jinyang Metallurgical Co. Ltd, China. The authors also thank the
Analytical and Testing Center of HUST for various materials analysis facilities employed in this study. The help of undergraduate
students of Yajun Hu, Haifeng Wang, Gaoke Liu and Lu Peng for
the molecular structure analysis of lead citrate precursors is also
gratefully acknowledged.
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