Preparation of high-purity lead oxide from spent lead paste by low temperature burning and hydrometallurgical processing with ammonium acetate solution

Abstract

Lead sulfate, lead dioxide and lead oxide are the main components of lead paste in a spent lead-acid battery. In addition, there are a few impurities in spent lead paste, which have great influence on the performance of the new battery; therefore, it is necessary to remove them. In this study, a novel approach with low temperature burning and hydrometallurgical processing with NH₄AC is developed to recover lead from spent lead paste. First, some of the impurities are converted to metal oxides by the calcination of spent lead paste at low temperature. Second, the metal oxides are transformed into soluble sulphates by the reaction between the calcination products and dilute H₂SO₄ and H₂O₂ (5.0%). Then, the solids are separated from the solution by filtration; the solids are mainly PbSO₄, BaSO₄ and CaSO₄. NH₄AC is used as the leaching solution for PbSO₄, and CO₂ is introduced to obtain pure PbCO₃. Under the optimized leaching conditions (leaching temperature at 40 °C for 20 min, 10.0 wt% NH₄AC), the lead recovery ratio is about 99.9%. The calcination product of lead carbonate is PbO, and high-purity lead oxide is obtained. The initial discharge capacity of high-purity lead oxide is about 158 mA h g⁻¹, and the capacity loss is less than 2% after 80 cycles.

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1. Introduction

The production of lead is growing fast in China, but the growth rate of secondary lead production is slow. The annual lead production in China increased from 1.2 million tonnes (MT) in 2001 to 4.64 million tonnes (MT) in 2013.¹ In 2013, the lead production was about 3.28 MT and the secondary lead production was only about 1.36 MT in China. In addition, the percentage of secondary lead in total lead production was 29.7%, which is less than that of an industrially developed country. This is because the technology of China's secondary lead was backward and the scales of the plants were generally small. Moreover, the recovery of China's secondary lead is low in efficiency and high in pollution.

As we know, the consumption of lead acid batteries accounts for 84% of lead consumption,² and its lifecycle is generally of two years.³ This results in large amounts of scrap lead-acid batteries being generated, and the number is constantly increasing every year. The scrap amount accounts for more than 90% of scrap lead in the whole society.⁴ Therefore, from the perspective of environmental protection, no matter what level the lead industry developed from, it should pay more attention to recycling of the scrap lead acid battery. It is necessary to design a recovery process with environmentally friendly and low energy consumption.⁵

A typical spent lead acid battery mainly consists of four components: waste electrolyte, polymeric materials, lead alloy grids and lead paste.⁶ Among these, lead paste is the most difficult part to deal with. The main components of spent lead paste are lead sulfate (∼60%), lead dioxide (∼28%), lead oxide (∼9%), metallic lead (∼3%) and a small amount of impurities such as iron, antimony, tin and barium.⁷ Environmentally sensitive recovery of spent lead paste has attracted considerable interest from researchers.^8–10

Recently, increasing attention has been paid to the recovery of spent lead paste on alternative hydrometallurgical processes. In general, spent lead paste is desulfurized using a salt solution^11–14 or organic acid.^8,9 PbO₂ is the second highest constituent in the spent lead paste and can be reduced to Pb(II) to aid subsequent leaching using H₂O₂(aq), FeSO₄ or Na₂S₂O₃. Nedialko K. Lyakov et al.¹⁵ first proposed an investigation of the desulphurization process of spent lead paste by sodium carbonate and sodium hydroxide, the results show that the desulphurization process of spent lead paste proceeds at a higher speed when using NaOH in place of Na₂CO₃. X. Zhu et al.¹⁶ worked out a sustainable method, with minimal pollution and low energy cost in comparison with the conventional melting methods, for treating components of spent lead-acid battery pastes in an aqueous organic acid. In his studies, spent lead paste was treated with an aqueous acetic acid (CH₃COOH) and sodium citrate (Na₃C₆H₅O₇·H₂O) solution to generate a lead citrate precursor, which was then separated from the solution. However, in these previous studies, the impurities in spent lead pastes can easily join and contaminate the precursor, sometimes leading to unacceptably high content impurities in the final leady oxide. The capacity retention ratio of the batteries manufactured with this leady oxide as the active material of the cathode decreased rapidly after 20–30 cycles. The impurities in the leady oxide powders could lead to the deterioration of the battery cycle performance.^17–19 It is urgent to develop a new green lead recovery process with lower energy consumption and to obtain high-purity lead oxide without or with low content impurities. Herein, Junqing Pan et al.¹³ reported a new green hydrometallurgical process for producing high-purity metallic Pb based on a specially designed H₂–PbO fuel cell. The PbO was recovered from the spent lead paste by a desulfurization of lead sulphate and a redox reaction of Pb and PbO₂ with a catalyst. However, processes using H₂–PbO fuel cells have not yet been adopted by industry.

The aim of this study is to prepare high purity lead oxide for lead-acid batteries through a new production process. Lead sulfate, lead dioxide and lead oxide are the main components of lead paste in the spent lead-acid battery, and there are small impurities in spent lead paste, which needs to be removed. In this study, a novel approach by low temperature burning and hydrometallurgical processes with ammonium acetate was developed to recover high-purity lead oxide from spent lead paste. Furthermore, the impurities in spent lead paste could be removed by a calcinating–dissolving–separating procedure; the researchers did not consider this issue more in previous studies.

2. Experimental

2.1. Experimental materials

The sample of spent lead pastes were provided by a secondary lead smelting plant in Hunan Zhuzhou, China. Before calcinations, the spent lead paste was washed by distilled water to remove the electrolyte. The chemical composition of the spent lead paste is shown in Table 1. The spent lead pastes in this study are exclusively of lead metal (Pb), because lead metal (Pb) will be oxidized to lead oxide (PbO) after washing, drying and milling.

Table 1 Chemical composition of spent lead paste

Spent lead paste
Element	Pb	Sb	Fe	Ba	Ca
%	70.75	0.037	1.120	2.89	0.656

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Fig. 1 presents the XRD pattern of the spent lead paste before calcinations, which shows that the components of the spent lead paste are PbSO₄, PbO₂, PbO and other compounds (Sb, FeS, CaSO₄ and BaSO₄). Concentrated sulfuric acid (H₂SO₄, 98%) was used to prepare the dilute H₂SO₄ solution with distilled water. Hydrogen peroxide (H₂O₂, 5.0%) was used as the reducing agent. Ammonium acetate (NH₄AC) was used as the desulfating agent during leaching of the precipitate after filtration.

Fig. 1 XRD pattern and image of the spent lead paste before calcination.

2.2. Experiment methods

The objective of this study was to seek a new method to obtain high-purity lead oxide from spent lead paste. The flowsheet for this process is schematically shown in Fig. 2. After calcinations, the solids were reacted with H₂SO₄ and H₂O₂. Then, after filtrating to remove impurities of Fe and Sb compounds, the solid continued to react with NH₄AC solution and almost dissolved. Furthermore, Pb(AC)₂ solution was obtained without the other impurities, the compounds of Ba and Ca. The solution was reacted with CO₂ to get PbCO₃, and finally, it decomposed at low temperature to yield high-purity PbO powder.

Fig. 2 Process flowsheet of preparation of high-purity PbO from spent lead paste.

A schematic of the electric resistance furnace is shown in Fig. 3. The spent lead paste powder was placed in a porcelain crucible, which was placed in the electric resistance furnace. To make the impurities in the spent paste undergo complete oxidation, we introduced oxygen enriched air during the calcination process. The temperature in the furnace was elevated to a given calcination temperature (300–400 °C) by 5 °C min⁻¹, and the spent lead paste was calcined for a given time at the designated calcination temperature in the furnace. Finally, the calcined sample was refrigerated to room temperature.

Fig. 3 Schematic of the electric resistance furnace.

After calcination, the samples were reacted with dilute H₂SO₄ (1.08 g mL⁻¹) at 40 °C for 2 h, followed by stirring. Metal oxides reacted with sulfuric acid (H₂SO₄) completely, and produced soluble sulfates. Moreover, 5.0% hydrogen peroxide (H₂O₂) was added as the reductant for PbO₂. The compounds of lead will exist in the form of lead sulfate. After filtration, a solid comprised barium sulfate (BaSO₄), calcium sulfate (CaSO₄) and lead sulfate (PbSO₄) was obtained. An aqueous solution of ammonium acetate (NH₄AC) was used as the desulphurizing agent to react with PbSO₄ in the solid. The reaction (1) is based on the fact that PbSO₄ and CH₃COO⁻ can form the complex ion (CH₃COO)₃ Pb⁻ in NH₄AC solution.

PbSO₄(s) + 2CH₃COONH₄(aq) = (CH₃COO)₂Pb(aq) + (NH₄)₂SO₄(aq) (1)

In the desulphurizing experiment, the effects of the NH₄AC concentration, reacting temperature and reacting time on the recovery rate were investigated. Furthermore, after filtration, the filtrate could be used directly for producing PbCO₃ by introducing CO₂. The precipitate and filtrate through filtration can yield lead carbonate crystals. The recovery rate of lead from spent lead paste was calculated through the eqn (2)–(5) as follows:

(CH₃COO)₂Pb(aq) + CO₂ + H₂O = PbCO₃(s) + 2CH₃COOH(aq) (2)

where m₁ is the mass of original spent lead paste, w₁ is the mass percentage of PbSO₄, w₂ is the mass percentage of PbO₂, w₃ is the mass percentage of PbO, and m₂ is the mass of PbCO₃.

Finally, the high-purity lead oxide was prepared by the calcination of lead carbonate in air at 405 °C for 2 h, based on the thermodynamics analysis.

2.3. 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 Kα 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).

2.4. Battery assembling and testing procedure

The methods of battery assembling and testing are based on our previous study reported in the literature.²⁰ In battery assembling (Fig. 4), the lead oxide powder products act as the positive active material, whereas negative plates were provided by commercial sources. After mixing, pasting, curing and formation processes, each of the dried positive plates was coupled with two commercial negative plates soaked in sulfuric acid solution (1.28 g cm⁻³) electrolyte so that batteries for testing under 2 V/0.2 A h could be made. Both the charging and discharging cycle tests were performed repeatedly at a constant current of 25 mA h with a cut-off terminal voltage of 1.75 V (depth of discharge, DOD = 100%).

Fig. 4 Assumption diagram of the assembly of lead acid battery in laboratory.

3. Result and discussion

Fig. 5 shows the detailed process flow chart for preparing high-purity lead oxide from spent lead paste. The main ideas of this study were to calcine spent lead paste at low temperature, dissolve the calcination products in H₂SO₄ and H₂O₂ and then desulphurize them with NH₄AC to get PbCO₃ crystals, and finally prepare high-purity PbO from PbCO₃ by low temperature burning.

Fig. 5 Detailed process flow chart of prepared high-purity lead oxide from spent lead paste.

3.1. Pretreatment of spent lead paste

When spent lead paste was calcinated in the electric resistance furnace (Fig. 3) with oxygen-enriched air, some of the impurities would react with O₂ to the produce metallic oxide (M + O₂ → M₂O_x). According to the experimental analysis, the components of the calcination products were different using different times and temperatures. For example, the impurities were incompletely oxidized at 300 °C for 2 h, and the impurities were completely oxidized in 400 °C. Taking energy into consideration, high calcination temperature will cause energy waste. Therefore, after carrying multi-group calcination experiments, the optimum burning condition was identified as 405 °C for 2 h. The XRD pattern, image and SEM image of the calcinations product are shown in Fig. 6 and 7(b). Comparing Fig. 6 with the XRD pattern of spent lead paste (Fig. 1), after low temperature burning, Sb and FeS were changed to Sb₂O₃ and Fe₂O₃. The calcination reactions of spent lead paste under oxygen-enriched air are listed as follows:

4Sb + 3O₂ = 2Sb₂O₃ (6)

4FeS + 7O₂ = 2Fe₂O₃ + 4SO₂ (7)

Fig. 6 XRD pattern and image of the calcination product of spent lead paste.

Fig. 7 SEM image of spent lead paste (a) and calcination product (b) and precipitate (c), (d) that was the product of calcination reactions with H₂SO₄ and H₂O₂.

In addition, the color of the calcination product changed obviously, compared with that of the original spent lead paste. After calcining, the particle size of the calcination product gave evidence that it had agglomerated.

Fig. 7(c), (d) and 8 present the SEM images and XRD pattern of the solid, which was calcinated and reacted with H₂SO₄ and H₂O₂. The components of the solid were PbSO₄ and a minute quantity of BaSO₄ and CaSO₄. Compared with the calcination product, the impurities in the sample powder were decreased, because Sb₂O₃ and Fe₂O₃ reacted with H₂SO₄ to produce soluble sulfates and lead oxide generated lead sulfate in H₂SO₄ solution. The main reaction principles were as follows:

Sb₂O₃ + 3H₂SO₄ = Sb₂(SO₄)₃ + 3H₂O (8)

Fe₂O₃ + 3H₂SO₄ = Fe₂(SO₄)₃ + 3H₂O (9)

PbO + H₂SO₄ = PbSO₄ + H₂O (10)

PbO₂ + H₂O₂ + H₂SO₄ = PbSO₄ + 2H₂O + O₂ (11)

Fig. 8 XRD pattern of precipitate as the product calcinations with H₂SO₄ and H₂O₂.

After filtration, the soluble sulfates [Sb₂(SO₄)₃, Fe₂(SO₄)₃] were in the filtrate. PbSO₄ was the main component of the precipitate. The SEM images of the precipitate are shown in Fig. 7(c) and (d). It can be observed that the precipitate has an irregular structure with an average size. The particle size was about 100 nm, much smaller than that of the original spent lead paste. Therefore, the precipitate had a higher specific surface area, which was good for the desulphurization reactions in the next experiment.

3.2. Hydrometallurgical desulfurization of PbSO₄

According to the chemical reaction mechanism diagram, lead sulfate was dissolved in ammonium acetate (NH₄AC) solution and generated soluble lead acetate [Pb(AC)₂]. Introducing carbon dioxide (CO₂) into Pb(AC)₂ solution will produce a white precipitate (PbCO₃). As we know, the decomposition temperature of lead carbonate is lower than that of lead sulfate, and the lead carbonate (PbCO₃) can readily convert to lead oxide (PbO) by thermal decomposition at a relatively low temperature of about 400 °C. Moreover, this method can avoid the emission of harmful sulfur oxide (SO₂).

Fig. 9(a) presents the effect of reaction temperature on the variation of the recovery rate of lead from PbSO₄. As shown in Fig. 9, complete precipitation is not achieved, and the recovery rate of lead from PbSO₄ increases from 88.69% at 20 °C to 98.55% at 40 °C. This indicates that the reaction temperature has a significant effect on the recovery efficiency. When the reaction temperature increases (50 °C), the recovery rate of lead does not change. A reaction temperature of 40 °C is found to be optimal for hydrometallurgical desulfurization of PbSO₄.

Fig. 9 The chemical reaction mechanism diagram of this process; effect of temperature (a), time (b) and concentration of ammonium acetate on the lead recovery rate of desulphurization.

The effect of reaction time on the lead recovery rate of PbSO₄ when the reaction temperature was 40 °C is presented in Fig. 9(b). In general, lead recovery rate increased with increase of the reaction time. When the reaction time is long enough, almost all the PbSO₄ can be converted. In addition, the recovery efficiency of lead could reach 98.0% in 20 min. After 20 min, the recovery efficiency of lead was almost invariable. Thus, it is reasonable to set 20 min as the optimal reaction time.

The effect of the concentration of the NH₄AC solution on the recovery rate of lead is shown in Fig. 9. As shown in Fig. 9, the recovery rate increased when the concentration of NH₄AC solution increased. The recovery efficiency can reach nearly 99.0% at 10% NH₄AC solution and then efficiency is constant with further increasing of NH₄AC concentration. In summary, the optimal desulfurization conditions of PbSO₄ are as follows: a reaction temperature of 40 °C, reaction time of 20 min and 10 wt% NH₄AC solution. Under the optimal conditions (reaction time of 20 min, reaction temperature of 40 °C and concentration of NH₄AC of 10 wt%) the lead recovery rate can be up to 99.0%. The solid residue in the solution after the desulphurization process consists of a small amount of unconverted PbSO₄ and other insoluble solid impurities (BaSO₄ and CaSO₄).

3.3. Characterization of the precursors

The lead acetate solution was obtained by desulfurization of PbSO₄, as previously discussed. There are many ways to recover lead from the lead salt solution. Karami et al.²¹ discussed the synthesis of nano-structured lead oxide through reaction with lead nitrate solution and sodium carbonate solution by a sonochemical method. In this study, recovering lead from lead acetate solution was carried out by a precipitation reaction, wherein the slurry was produced by introducing carbon dioxide into the solution. The carbonation can be summarized as eqn (12).

Pb(AC)₂ + CO₂ + H₂O = PbCO₃(↓) + 2HAC (12)

After filtering, the precipitate product was dried at 100 °C for 8 h. The XRD pattern of the precipitate product is shown in Fig. 10(a), which is a pure lead carbonate pattern, according to the standard card (JCPD file no. 70-2052).²² The SEM image of the lead carbonate precipitation product is shown in Fig. 11. As shown in Fig. 11, the distribution of particle size is in the range of 0.5–1.5 μm.

Fig. 10 (a) XRD patterns and images of the lead carbonate and lead oxide; (b) TG curve of the lead carbonate decomposition in air.

Fig. 11 SEM images of the lead carbonate product (PbCO₃) and calcined product (PbO).

The TG curve of [Pb(AC)₂] is shown in Fig. 10(b). There is only one weight loss step in the temperature range of 321–400 °C. The weight loss at 321–400 °C can be described as the decomposition of the lead carbonate. The weight loss is about 17.0%, which is close to the theoretical value (16.47%, base on eqn (13)). This weight loss was due to the decomposition of lead carbonate to generate lead oxide, because the oxidation number of the lead ions is constant during the thermal decomposition process.

The XRD patterns of the calcined products from lead carbonate are shown in Fig. 10(a). The results showed that the lead carbonate was decomposed completely at the temperature of 405 °C for 2 h. As shown in Fig. 10(a), the lead carbonate was converted to a crystalline phase of PbO (JCPDS file no. 87-0604).^23,24 No significant impurities were observed, which indicates that pure lead oxide was obtained. The SEM image of the calcined product (PbO) from lead carbonate is shown in Fig. 11. As shown in Fig. 11, the morphology of the calcined products exhibited crystals in the size of 100 nm, much smaller than that of the Barton-pot leady oxide. The smaller particle can provide a larger contact area of active material for the proceeding electrochemical reactions, which is beneficial for battery performance.

The spent lead paste can be used to produce lead oxide powders after simple chemical conversion steps. The purity of the lead oxide product prepared by low temperature calcinations is about 99.5%. This simple recycling method for waste lead acid battery paste has a positive effect on the recovery of lead oxide from the starting materials of spent lead paste. Moreover, compared with other methods, the particle size of high-purity PbO is much smaller. D. Yang et al.¹⁸ have reported that the calcination products from lead citrate precursor particles are 400 nm. Mayer²⁵ has investigated the influence of the particle size on battery performance. Their results indicate that the oversized leady oxides cause great harm to the battery cycle performance. In our previous investigations, pure lead oxides could be directly used as the positive active materials for new lead-acid batteries.^20,26

3.4. Battery performance

In this experiment, batteries were assembled by different leady oxide powder (factory leady oxide and high-purity lead oxide). Fig. 12 obtains the first discharge curves of cell voltage versus capacity at a current density of 30 mA g⁻¹. It is evident that the initial discharge capacity of the high-purity lead oxide is about 158 mA h g⁻¹, whereas factory leady oxide is about 143 mA h g⁻¹. Therefore, the high-purity lead oxide has a higher discharge capacity than factory leady oxide because of its larger specific surface area. As we know, the theoretical capacity of pure PbO is 240 mA h g⁻¹, and the utilization of an active material is defined as the ratio of the discharge capacity and the corresponding theoretical capacity of PbO. The utilization of high-purity PbO is about 65.8%, which is a higher discharge capacity than that in the lead acid battery.²⁷

Fig. 12 Initial discharge capacity of batteries assembled with factory leady oxide and high-purity lead oxide.

The cycle performance of batteries manufactured with factory leady oxide or high-purity lead oxide are shown in Fig. 13. The capacity retention ratios of the assembling batteries made from the two types of lead oxide products show a good cyclic stability in 80 charge/discharge cycles with the depth of discharge of 100%. It can be observed that the discharge capacity of high-purity PbO was quite stable in the 80 cycles. In general, good cycling performance mostly depends on the excellence of the active material particles and their large specific surface area, which will enhance their reactivity with sulphuric acid. It can be observed that the high-purity PbO comprises much smaller particles and has a closely packed structure; this type of scattered granular crystal structure facilitates the expansion of the plate and reduces the stability of the plate structure. The cycle performance of the assembled batteries in this study is significantly better than that previously reported using the novel lead oxide recovered from the spent lead pastes with the acetic and sodium citrate leaching process.¹⁸

Fig. 13 Cycle life of batteries assembled with factory leady oxide and high-purity lead oxide.

Therefore, removing impurities in the recovered lead oxide has more important influences on the performance of the new battery. In summary, high-purity PbO is a superior material for the manufacture of high-quality lead acid batteries with a longer cycle life and a higher discharge capacity. The experimental results are of significance to the development of economic and environmental processes for the recycling of spent lead acid batteries.

4. Conclusion

In this study, a new environmentally friendly route to replace the traditional spent lead paste smelting recycling process is proposed. Furthermore, this method improves the hydrometallurgy technique previously reported. There is an urgent need to develop a new green lead recovery process with lower energy consumption without the use of toxic chemicals and to achieve high-purity lead oxide.

(1) Spent lead paste burning at low temperature can make some impurities (Sb, Fe) convert to their metal oxides and react with H₂SO₄ to generate soluble salts. Moreover, the main components of the spent lead paste all turned to lead sulfate.

(2) Desulfurization of lead sulfate by NH₄AC solution and the thermal decomposition of lead carbonate precursor have been carried out on the laboratory scale. The effects of time, temperature and concentration of NH₄AC on the recovery efficiency were investigated. Under optimal conditions (reaction time of 20 min, reaction temperature of 40 °C and concentration of NH₄AC of 10 wt%), the lead recovery rate can reach 99.0%. Subsequently, other impurities (Ca, Ba) are removed and pure PbCO₃ and high-purity PbO are obtained.

(3) Battery testing results reveal that the initial discharge capacity and cycle life of a lead acid battery manufactured by the high-purity lead oxide are better than that of factory leady oxide. The initial discharge capacity of the high-purity lead oxide is about 158 mA h g⁻¹, and it behaves excellently in terms of cycling stability. The capacity loss is less than 2% in 80 cycles when it is full discharged at 30 mA g⁻¹.

(4) In further study, we will investigate the recovery of impurities from spent lead paste, and improve resource recovery rate and utilization.

Acknowledgements

The authors acknowledge financial support from the Nature Science Foundation of China-Guangdong Joint Fund (NO. U1201234).

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