Recycling lead from spent lead pastes using oxalate and sodium oxalate and preparation of novel lead oxide for lead-acid batteries

Abstract

A sustainable method, with minimal pollution and low energy cost in comparison with the conventional smelting method, is proposed for treating components of spent lead acid batteries with oxalate and sodium oxalate. The pure lead oxalate precursor of PbC₂O₄ is the only product crystallized in the leaching experiment. Lead oxalate is readily crystallized from the solution due to its low solubility and can be combusted to directly produce lead oxide as a precursor for making new battery pastes. Both lead oxalate and the oxide products have been characterized by means of thermogravimetric analysis (TGA) and X-ray diffraction (XRD). The results show that the lead oxides synthesized at different calcination temperatures are comprise of α-PbO and β-PbO. The batteries assembled using the novel lead oxide powder as the positive active material show good cyclic stability for 50 charge/discharge cycles. The cell using lead oxide powder containing 15 wt% β-PbO exhibits excellent initial capacity, cycling performance and high-rate discharge characteristics and can deliver a discharge capacity of 180 mA h g⁻¹ at 30 mA g⁻¹ and more than 60 mA h g⁻¹ at 240 mA g⁻¹. Within 50 cycles, its capacity loss is low (5%) with excellent cyclic stability.

---

Introduction

A waste lead-acid battery comprises four components: electrolyte (11–30%), lead and lead alloy (20–30%), lead paste (30–40%) and organic matter and plastic material (22–30%). Out of these fractions, the lead paste has a particularly complex composition. It is an oxide-sulphate product with the following approximate composition: 60% lead sulphate, 20–30% lead dioxide, 5–15% lead monoxide and 3% lead alloy. The sulphur content in the paste is about 6%, which causes difficulties when direct pyrometallurgical processing is employed. In the traditional refining process, decomposition of lead sulphate requires a temperature of at least 1180 °C or higher. The smelting process using coal or coke as the fuel is associated with emissions of many gaseous and solid pollutants such as SO₂ and lead fumes. Hence, it poses serious hazards to the environment and human health.

High temperature (>1000 °C) is required to decompose and reduce PbSO₄, which is associated with generation of dilute SO₂ gas streams in addition to lead fumes. New developments in the pyrometallurgical processes have led to recycling in Isasmelt^1,2 or short rotary furnaces,³ which use Fe or soda to remove S in the furnace by forming FeS–PbS matte or Na₂SO₄ containing slag. However, the disposal of hazardous matte or leachable slag is also expensive and harmful to the environment.⁴

Thus, more and more attention has been paid to green recycling processes for spent lead-acid batteries such as those involving hydrometallurgical routes. The most widely used hydrometallurgical method, which consists of pre-treatment, desulphurization, reduction and electro-winning processes, can avoid SO₂ and lead dust emission problems.^4,5 However, the energy efficiency of the electro-winning process is low, with a higher total energy consumption when compared to the traditional pyrometallurgical routes. The suggested H₂SiF₆ or HBF₄ leaching solutions may lead to the emission of fluorine into the environment, which is also unacceptable.

Given the disadvantages of the traditional pyrometallurgical and electro-winning methods, several new hydrometallurgical approaches have been developed.^6–9 The hydrometallurgical recovery process for the treatment of lead paste without electro-winning has not been practically employed so far. Hence, we need to develop an effective, low cost and environment-friendly process for recycling spent lead paste. Attempts have been made to leach the components (PbSO₄, PbO₂ and PbO) of spent lead pastes using citric acid and sodium citrate, as reported in previous studies.^10–12 D. Yang et al.¹³ leached spent lead pastes using sodium citrate and acetic acid as reagents using 30% hydrogen peroxide as the reducing agent for PbO₂. X. Zhu et al.¹⁴ leached spent lead acid battery paste components using sodium citrate and acetic acid. In this leaching process, lead citrate (Pb₃(C₆H₅C₇)₂·3H₂O) was initially synthesized by leaching the spent lead-acid battery paste in a mixed solution containing sodium citrate and other reagents. The as-prepared leady oxide samples simultaneously varied, depending on the calcination temperature used. However, the self-synthesized leady oxide samples comprised mainly β-PbO, which is the major phase contained in the self-synthesized leady oxide instead of α-PbO that is the main component of conventional leady oxide.¹⁵

Currently, in the lead acid battery industry, leady oxide is required for both the negative and positive electrodes. Therefore, it would be of interest to synthesize PbO, which can yield higher active material utilization and longer cycle-life. For example, J. Wang et al.¹⁶ prepared crystalline α-PbO via the calcination of PbCO₃ and used it as the positive electrode active material. The results showed that the α-PbO powder discharged a capacity of 30% higher than that of the conventional ball-milled leady oxide. M. Cruz et al.¹⁷ prepared a thin α-PbO film using chemical spray pyrolysis from an aqueous solution of Pb(CH₃COO)₂·3H₂O on a lead substrate. When the film deposited on the substrate was used as a positive plate, it maintained a discharge capacity of 100 mA h g⁻¹ upon extended cycling. H. Karami et al.^18,19 synthesized a uniform structured lead oxide via a sonochemical method, which discharged a large capacity of 140 mA h g⁻¹ and even 230 mA h g⁻¹ as the cathode or anode of a lead acid battery. M. Salavati-Niasari et al.²⁰ also reported that lead oxide powder can be prepared by decomposing lead oxalate at 500 °C, but no electrochemical results were reported. In addition, the performance of the lead oxide powder, including β-PbO, has not been reported to date.

In this study, as shown in Fig. 1, spent lead paste was treated with an aqueous oxalate acid and sodium oxalate solution to generate a lead oxalate precursor, which was then separated from the solution. Lead could be recovered as lead oxide powder after calcination of the lead oxalate precursor. Furthermore, the two polymorphs of PbO can be easily prepared. Control over the content of β-PbO in the lead oxide powder was accomplished and new batteries assembled with the novel lead oxide. The results will be used to recover lead in secondary lead industry and produce high-performance lead oxide.

Fig. 1 A flow sheet of the experiments.

Experimental

Materials and methods

Spent lead paste was provided by a secondary lead smelting plant in Hunan Zhuzhou, China. Sodium oxalate (Na₂C₂O₄, AR ≥ 99.5%), oxalic acid (H₂C₂O₄·H₂O, AR ≥ 99.5%) and concentrated sulphuric acid (98%) were obtained from Aladdin of Shanghai, China.

The reduction treatment of the spent lead paste (the quantity of the spent lead paste was 10.00 g) was carried out as follows: 50 mL of 10 wt% H₂C₂O₄·H₂O as the reducing agent was added to spent lead paste in a conical-flask and used to convert lead(IV) to lead(II).²¹ Desulphurization of the samples was carried out as follows: the sample obtained above was added to 150 mL of saturated sodium oxalate solution. The conical-flask was heated with continuous stirring at 200 rpm. In order to transform all the lead compounds in the spent lead paste into lead oxalate, the actual dosage of the leaching agent was twice the stoichiometrically calculated amount. After reduction and desulphurization, the conical-flask containing the lead oxalate slurry was placed in a homoeothermic water bath at 60 °C overnight to allow the crystals to grow. Finally, the lead oxalate was washed with distilled water, vacuum filtered and dried at 110 °C overnight.

The main reactions involved in the process are as follows:

4H₂C₂O₄ + 3PbO₂ = 3PbC₂O₄ + 4H₂O + 2CO₂↑ + O₂↑ (1)

H₂C₂O₄ + PbO = PbC₂O₄ + H₂O (2)

Na₂C₂O₄ + PbSO₄ = PbC₂O₄ + Na₂SO₄ (3)

In eqn (1), oxalic acid, which is known to be a strong reducing agent and acid in previous reports, acts as the reducing agent. Eqn (3) is based on the difference between the ionic products of lead sulphate and lead oxalate: K_sp(PbC₂O₄) < K_sp(PbSO₄).

The recovery efficiency of lead from the spent lead paste was calculated using eqn (4):

In eqn (4), m₁ (g) is the actual mass of PbC₂O₄ obtained from XRD results and m₂ (g) is the theoretical weight of PbC₂O₄. Using the Jade software, the content of PbC₂O₄ was calculated.

In air, the lead oxalate powder samples were calcined for 1 h at 450 °C, 500 °C, 550 °C and 600 °C. The calcination temperature was determined based on the TGA data and the transition point of α-PbO to β-PbO, which is 488.5 °C.

Characterization of materials

The phase composition of the different leady oxides was determined by X-ray diffraction (XRD) analysis using a D/MAX 2550 X-ray diffraction analyzer (Japan) with Cu K_α radiation (λ = 1.54 nm) at 80 mA and 5.0 kV. The crystal morphology of the different leady oxides was examined by scanning electron microscopy (SEM) using an environmental scanning electron microscope ESEM Quanta-200FEG FEI (Holland).

Plate preparation

The self-synthesized lead oxide powders were used as the positive active material in lead acid batteries. In order to measure the performance of the positive active mass, a negative paste from a commercial source was used. The positive pastes were prepared with the components listed in Table 1. We referenced the formula of diachylon of valve regulated lead-acid battery.

Table 1 The composition of the positive paste used for the test batteries

No.	Compound	wt%
1	Lead oxide powder	79.03
2	Aquadag	0.158
3	Fibers	0.158
4	Distilled water	10.54
5	Sulfur acid (d = 1.40 g mL^−1)	10.12

---

The grids used for the test batteries were made of low antimony alloy, Sn–Al–Ca–Pb. The size of the positive plate was 20 × 15 × 1.5 mm and the negative plate was 20 × 30 × 1.5 mm. The pasted plates were immersed in sulphuric acid solution for 5 s (the solution density was 1.15 g mL⁻¹ for the positive plates and 1.06 g mL⁻¹ for the negative plates, respectively). The plates were placed in an oven with a sustained temperature of 100 °C for 5 min, then the temperature of the oven was decreased to 55 °C with a relative humidity of >95%. The curing process was conducted for 24 h. After that, the plates were dried at 60 °C for 5 h.

Battery performance tests

All formation and cycling tests were performed using a NEWARE Cycler (BTS-5V3A Shenzhen NEWARE Electronics Co., Ltd., China). Test batteries were assembled with one experimental positive plate and two negative plates; the cell capacity was restricted by the positive plate, separated by an absorptive glass-mat (AGM) separator. The electrolyte was a H₂SO₄ solution with a relative density of 1.28 g mL⁻¹.

After immersing the plates in the electrolyte for 2 h, formation of the test batteries was carried out in three steps. Firstly, the plate was charged at a current density of 15 mA g⁻¹ to 100 mA h g⁻¹, then it was charged at 25 mA g⁻¹ for another 150 mA h g⁻¹ and finally, charged at 8 mA g⁻¹ for another 100 mA h g⁻¹.

After formation, an initial capacity test was carried out with a constant discharge current of 30 mA g⁻¹ until a terminal voltage of 1.75 V was reached. The charge/discharge cycling tests were performed repeatedly at a constant discharge current of 30 mA g⁻¹ with a cut-off terminal voltage of 1.75 V (depth of discharge DOD = 100%).

Results and discussion

Leaching of spent lead paste using a Na₂C₂O₄–H₂C₂O₄ solution

After a high ball milling process, the dried reddish powder sample of lead paste was separated using a sieving process. Then, the battery plate grid (oversize) was separated from the fine lead paste (undersize). In our laboratory, the powders were washed with distilled water to remove sulphuric acid until the pH of the washing solution was about 6.5. The cleaning process of spent lead paste is shown in Fig. 2. The cleaning process would continue until the number of the first pH-meter was about 6.5 through the wastewater treatment units and the control of the second pH-meter, which can achieve zero-emission of wastewater. This process may also be useful in a factory as it is efficient and environmentally friendly. It also includes wastewater to maximize the conservation of fresh water, which can be recycled.

Fig. 2 A flow sheet of the cleaning process of spent lead paste powder.

In this study, only fine lead paste (less than 150 mesh size) was used in the leaching tests in order to eliminate the negative effect of particle size non-homogeneity. The percentage of PbSO₄, PbO₂, PbO and others was about 67.0%, 27.5%, 5.0% and 0.5%, respectively. The chemical composition of the paste sample was analyzed using a chemical titration method.

The reactions are described in eqn (1)–(3). In eqn (1), H₂C₂O₄ acts as a reductant and Pb(IV) turns into Pb(II) in the acidic condition. In eqn (3), Na₂C₂O₄ acts as a desulfating agent and oxalate acid provides acid leaching conditions, which benefits the desulfuration reaction during lead sulfate leaching. The effects of temperature and time on the lead recovery rate of Pb from the spent lead paste are presented in Fig. 3. When the leaching time was under 0.75 h, the reaction was incomplete and the recovery rate was nearly 90% at 90 °C. Therefore, if we want to improve the recovery rate, we must extend the reaction time. The lead recovery rate increased with an increase in leaching temperature, when the leaching time was 2 h, the lead recovery rate can increase up to over 97% at a leaching temperature of 70 °C. When the leaching temperature was over 70 °C, the rate remains almost unchanged. Thus, it is reasonable to settle at 70 °C as the optimal leaching temperature, whereas 2 h was sufficient as an increase in leaching time to 3 h does not significantly increase the leaching efficiency of spent lead paste. The activation time 2 h is, therefore, appropriate for the leaching reaction when economy and practicality are taken into consideration.

Fig. 3 Effects of temperature and time on the lead recovery rate of Pb from spent lead paste.

Crystallization of lead oxalate

In the Na₂C₂O₄–H₂C₂O₄ leaching system, the product of the leaching process was PbC₂O₄ and the lead recovery rate can reach 98.6% under the optimal leaching conditions. Therefore, it can greatly reduce lead pollution to the environment. The XRD pattern of the lead oxalate precursor is shown in Fig. 4(a) and an image and SEM image of the lead oxalate product are shown in Fig. 4(c) and (d), which preliminarily indicate that the leaching product was pure lead oxalate. Therefore, the desulfurization rate in the leaching process was approximately 100%.

Fig. 4 The XRD pattern (a), TGA curve for the decomposition (b), image (c) and SEM image (d) of the lead oxalate precursor.

Characterization of lead oxide

Fig. 4(b) shows the TGA curves in static air of the precursor obtained from the solvothermal reaction and the desulphurization of spent lead paste in a H₂C₂O₄–Na₂C₂O₄ solution. There is only one weight loss step in the temperature range of 341–400 °C. The weight loss at 341–400 °C may be ascribed to the decomposition of oxalate. The weight loss was about 25.00%, which is close to the theoretical value (24.39 wt%) indicated in eqn (5).

2PbC₂O₄ + O₂ = 2PbO + 4CO₂ (5)

According to the reaction equation, the calcination products obtained from the lead oxalate precursor at different temperatures were only lead oxide in air. This is consistent with the change of the TGA curve shown in the paper.

This weight loss was due to the decomposition of lead oxalate to lead oxide because the oxidation number of the lead ions was constant during the thermal decomposition process.

Fig. 5 shows the XRD patterns of the products after calcination for 1 h of the precursor at 450 °C, 500 °C, 550 °C and 600 °C. There is no doubt that high purity lead oxide can be obtained using this method. Moreover, α-PbO was the major phase at low temperatures, which also is the main component of conventional leady oxide.¹⁵ In addition, the amount of β-PbO increases as the temperature increased. The XRD patterns of the calcination–combustion products obtained at 450 °C and 500 °C indicate that they are mostly α-PbO (Fig. 5(a) and (b)) and at 550 °C, the lead oxalate precursors had transformed into both α-PbO and β-PbO phases. At the higher temperature of 600 °C, lead oxalate was completely transformed into β-PbO. As for the lead acid battery industry, α-PbO is the preferred phase. If the proposed process was used in the battery industry, the lower temperature range would be selected for the calcination process. Although the theoretical transition point of α-PbO to β-PbO is 488.5 °C, in the process of our experiments, the calcination products are still α-PbO at 500 °C.

Fig. 5 XRD patterns of the calcination products obtained from the lead oxalate precursor at different temperatures for 1 h: (a) 450 °C, (b) 500 °C, (c) 550 °C and (d) 600 °C.

Preparation of novel lead oxide powders and their formation into the positive electrode of a lead acid battery

The main component of every battery paste is basic lead sulphate. In the curing process of a lead acid battery, 3PbO·PbSO₄·H₂O and 4PbO·PbSO₄ are produced by mixing lead oxide, sulphuric acid and water. The relative proportions of the two polymorphs of PbO are important in the production of battery plates as we have reported previously.⁶ The results indicated that α-PbO delivers a higher initial capacity and β-PbO yields a relatively improved cycle life. This is due to α-PbO favouring the formation of tribasic lead sulphate (3PbO·PbSO₄·H₂O), while β-PbO promotes the formation of tetrabasic lead sulphate (4PbO·PbSO₄).²² It is well known that the improvement of capacity is related to the 3BS content, which can give the battery a high discharge capacity. In addition, cured plates containing amounts of 4BS exhibit good mechanical strength and a long cycle life.²³ 3BS and 4BS mixed together can form a fibroblastic reticular structure. Therefore, it is very important to determine the optimum ratio of α-PbO and β-PbO in the lead oxide powder for the performance of the lead acid battery. X. Sun²⁴ reported that the oxidizability of leady oxide products prepared from lead acetate (Pb(CH₃COO)₂) was about 99.10%. Therefore, we attempted to use pure lead oxide as the positive materials and have been successful. In our study, the novel lead oxide powder was a mixture of α-PbO and β-PbO (Table 2). The content of α-PbO was higher than that of β-PbO, which was based on the fact that α-PbO is the main component of conventional leady oxide.¹⁴

Table 2 The content of the β-PbO and α-PbO phases in the novel lead oxide powders

Sample	β-PbO (wt%)	α-PbO (wt%)
A	5	95
B	10	90
C	15	85
D	20	80

---

According to the XRD patterns of the samples after formation (Fig. 6(a)), the four samples all have a similar composition, including β-PbO₂ and small amounts of PbSO₄. The content of β-PbO₂ for the four samples was calculated using the Jade software and the amount of β-PbO₂ obtained in sample C was higher than that of the other samples. PbO₂ is the active material found in the positive plates of lead acid batteries.

Fig. 6 XRD patterns of the four samples: (a) after formation and (b) after the charge and discharge test.

The morphology of the positive active material has a vital impact on battery performance. To observe the morphology of the samples after formation, the active materials were stripped from the positive plate after formation. SEM images of the four samples after the formation process are shown in Fig. 7(A–D). The smallest building element of the positive active material structure was the PbO₂ particle. These particles interconnect in different ways. The SEM data show that sample A has a polyhedral structure with a particle size of 100–300 nm. Several small particles feature heterogeneous structures in sample A. The particles in sample B have an irregular spherical morphology with a size of ∼200 nm, i.e. they are much smaller and stacked together. However, the particles in sample C are rod-like and smaller than the other samples, which can provide more contact area for electrochemical reaction, so the positive active material morphology of sample C is beneficial for the initial discharge capacity and cycle life performance of the battery. In sample D, the PbO₂ particles have coalesced into small agglomerates.

Fig. 7 SEM images of the positive active materials formed from the novel lead oxide powders after the formation process ((A) sample A, (B) sample B, (C) sample C and (D) sample D).

Battery performance of the novel lead oxide powders

Fig. 8(a) presents the initial discharge curves (at a current density of 30 mA g⁻¹) for the batteries prepared with the four types of lead oxide samples. As evident from the data shown in the figure, the initial discharge capacity of the battery made with sample C was about 180 mA h g⁻¹. If the utilization of active material is defined as the ratio of the discharged capacity to the corresponding theoretical capacity and the theoretical capacity of pure PbO is 240 mA h g⁻¹, the utilization of sample C was about 75% at a discharge current density of 30 mA g⁻¹. The other samples yield capacities less than 180 mA h g⁻¹. According to Fig. 5(a), the content of β-PbO₂ in sample C was higher than that found in the other samples. Therefore, the initial discharge capacity of sample C was the highest. It has been found that the content of β-PbO₂ determines the performance of the plate.^22,25

Fig. 8 Battery performance of the four samples: (a) the initial discharge capacities, (b) the discharge capacity verses cycle number, (c) discharge capacity verses the current density of the four samples and (d) the curves of potential verses discharge capacity for sample C at different discharge current densities.

Fig. 8(b) shows the cycling performance of the batteries made with the four types of lead oxide samples. Cycling was conducted at a discharge current density of 50 mA g⁻¹. It can be seen that the powder sample C sustains a quite stable capacity within the 50 cycles and the capacity retention of the battery has retained at 95% of the initial discharge capacity. Battery C shows a better cycle performance and the other samples exhibit a gradual decline in capacity on cycling. The discharge capacity of the battery made with powder samples A and B decreases rapidly, which may be related to the lower content of β-PbO in these samples. The data shown in Fig. 6(b) give us grounds to conclude that higher amounts of the β-PbO phase in the lead oxide powder may enhance the cycle life of the battery. However, when the content of β-PbO was about 20 wt%, the battery exhibits poor performance.

To evaluate the high-rate discharge performance of the batteries made with the novel lead oxide powders, their discharge capacities were measured and plotted versus discharge current densities, as shown in Fig. 8(c). As expected, the discharge capacity decreases as the current density increases. Fig. 8(d) shows the battery voltage versus discharge capacity. The tests were conducted at different discharge current densities while at the same charge current density. The charge current density was fixed at 0.5C (1C = 120 mA g⁻¹) and the discharge current density was varied from 0.25C to 3C, with a cut-off potential of 1.75. The discharge performance of the battery with powder sample C was better than others, i.e. its discharge capacity was about 180 mA h g⁻¹ at 30 mA g⁻¹, 130 mA h g⁻¹ at 60 mA g⁻¹, 93 mA h g⁻¹ at 120 mA g⁻¹, 60 mA h g⁻¹ at 240 mA g⁻¹ and 40 mA h g⁻¹ at 360 mA g⁻¹.

After 50 charge/discharge cycles, the batteries prepared with the novel lead oxides (containing 5 wt%, 10 wt%, 15 wt% and 20 wt% β-PbO) were disassembled after a final charge cycle. The positive lead plates were dried at 60 °C for 8 h and samples of the active materials were examined by X-ray diffraction analysis and scanning electron microscopy. The XRD patterns shown in Fig. 6(b) indicate that the active materials scraped from the positive plates mainly comprise PbO₂ and PbSO₄, in which sample C has a lower degree of sulphation.

The SEM images of the four active material samples after the cycling test are shown in Fig. 9(A–D). It can be seen that sample C comprises much smaller particles. In general, good cycling performance mostly depends on the excellent connectivity of the active material particles and their large specific surface area, which will enhance their reactivity with sulphuric acid (electrolyte) more readily. As evident from the SEM images of samples A, B and C shown in Fig. 9, the particle size decreases with an increase in the content of β-PbO in the powder samples. However, in sample D, the particles are much bigger than those observed in the other samples and it can be seen that the crystal particles are less consolidated. This type of scattered granular crystal structure facilitates the expansion of the plate and reduces the stability of the plate structure.

Fig. 9 SEM images of the positive active materials formed from the novel lead oxide powders after the discharge and charge cycle test ((A) sample A, (B) sample B, (C) sample C and (D) sample D).

Conclusions

A hydrometallurgical method is employed in the secondary lead industry. Leaching of spent lead paste using a H₂C₂O₄–Na₂C₂O₄ solution can avoid smelting and electro-winning and can greatly reduces lead pollution to the environment.

Highly purified lead oxalate can be obtained, which can be useful as a precursor where α-PbO and β-PbO can be generated using a process of combustion–calcination at different temperatures and can be directly used in the preparation of lead paste for new batteries. In addition, the different polymorphs of PbO have different properties.

The test results obtained for the batteries made using the lead oxides synthesized at different calcination temperatures indicate that the different polymorphs of PbO have a significant effect on the batteries performance. Thus, control over the α-PbO : β-PbO ratio is of fundamental importance for the production of leady oxide.

Novel lead oxide powders, comprising a mixture of α-PbO and β-PbO in different proportions, were synthesized and test batteries using these oxides were assembled and set to tests. The results obtained show that the lead oxide sample C (with 15 wt% β-PbO content) yields the highest initial discharge capacity, which was 180 mA h g⁻¹ at 30 mA g⁻¹ and 60 mA h g⁻¹ at 240 mA g⁻¹, respectively, and behaves excellently in terms of cyclic stability with the capacity loss being less than 5% for 50 cycles.

The content of impurities (such as, Sb, Fe and BaSO₄) in real lead paste is about 0.5%, these are probably the most difficult to deal with. However, the impurities in the recovered lead oxide greatly influence the performance of the new battery. We will investigate the typical impurities in the spent lead paste in a further study.

Acknowledgements

The authors acknowledge financial support from the Natural Science Foundation of China-Guangdong Joint Fund (No. U1201234) and Development and Reform Commission of Guangdong Province (No. 301-5). We thank the Analytical and Testing Center of South China Normal University for providing the facilities for the experiments and measurements performed within this study.

References

1. T. T. Chen and J. E. Dutrizac, Hydrometallurgy, 1996, 40, 223–245.
2. K. Ramus and P. Hawkins, J. Power Sources, 1993, 42, 299–313.
3. A. Guerrero, A. Romero, R. D. Morales and F. Chavez, Can. Metall. Q., 1997, 36, 121–130.
4. M. Olper, in Eleventh International Lead Conference, 1993, p. 1993.
5. R. David Prengaman, J. Miner. Met. Mater. Soc., 1995, 47, 31–33.
6. Y. Shu, C. Ma, L. Zhu and H. Chen, J. Power Sources, 2015, 281, 219–226.
7. P. Gao, Y. Li, W. Lv, R. Zhang, W. Liu, X. Bu, G. Li and L. Lei, J. Power Sources, 2014, 265, 192–200.
8. P. Gao, Y. Liu, X. Bu, M. Hu, Y. Dai, X. Gao and L. Lei, J. Power Sources, 2013, 242, 299–304.
9. P. Gao, W. Lv, R. Zhang, Y. Liu, G. Li, X. Bu and L. Lei, J. Power Sources, 2014, 248, 363–369.
10. M. S. Sonmez and R. V. Kumar, Hydrometallurgy, 2009, 95, 82–86.
11. M. S. Sonmez and R. V. Kumar, Hydrometallurgy, 2009, 95, 53–60.
12. J. Yang, R. V. Kumar and D. P. Singh, J. Chem. Technol. Biotechnol., 2012, 87, 1480–1488.
13. D. Yang, J. Liu, Q. Wang, X. Yuan, X. Zhu, L. Li, W. Zhang, Y. Hu, X. Sun, R. Vasant Kumar and J. Yang, J. Power Sources, 2014, 25, 27–36.
14. X. Zhu, X. He, J. Yang, L. Gao, J. Liu, D. Yang, X. Sun, W. Zhang, Q. Qang and R. Vasant Kumar, J. Hazard. Mater., 2013, 250–251, 387–396.
15. J. Morales, G. Petkova, M. Cruz and A. Caballero, J. Power Sources, 2006, 158, 831–836.
16. J. Wang, S. Zhong, G. X. Wang, D. H. Bradhurst, M. Ionescu, H. K. Liu and S. X. Dou, J. Alloys Compd., 2001, 327, 141–145.
17. M. Cruz, L. Hernan, J. Morales and L. Sanchez, J. Power Sources, 2002, 108, 35–40.
18. H. Karami, M. A. Karimi, S. Haghdar, A. Sadeghi, R. Mir-Ghaserm and S. Mahdi-Khani, Mater. Chem. Phys., 2008, 108, 337–344.
19. H. Karami, M. A. Karimi and S. Haghdar, Mater. Res. Bull., 2008, 43, 3054–3065.
20. M. Salavati-Niasari, F. Mohandes and F. Davar, Polyhedron, 2009, 28, 2263–2267.
21. L. Li, X. Zhu, D. Yang, L. Gao, J. Liu, R. V. Kumar and J. Yang, J. Hazard. Mater., 2012, 203–204, 274–282.
22. M. G. Mayer and D. A. J. Rand, J. Power Sources, 1996, 59, 17–24.
23. Y. Kamenev, J. Power Sources, 2006, 159, 1440–1449.
24. X. Sun, J. Yang, W. Zhang, X. Zhu, Y. Hu, D. Yang, X. Yuan, W. Yu, J. Dong, H. Wang, L. Li, R. Vasant Kumar and S. Liang, J. Power Sources, 2014, 269, 565–576.
25. H. Bode, Lead-acid Batteries, Wiley, New York, 1997, p. 212.

---