A Review on Recycling of Waste Lead-Acid Batteries

Lead-acid batteries (LABs) have become an integral part of modern society due to their advantages of low cost, simple production, excellent stability, and high safety performance, which have found widespread application in various fields, including the automotive industry, power storage systems, uninterruptible power supply, electric bicycles, and backup power supplies. Hence, the use of LABs has greatly benefited human society and contributed to advancements in science and technology. However, the extensive use of LABs unavoidably leads to the generation of a significant amount of LABs waste. On one hand, if these waste LABs are not handled properly, any leakage can cause devastating damage to the natural environment and human health. On the other hand, waste LABs represent an important secondary resource for lead, with approximately 64.57% of global lead resources derived from recycled lead, making them a major source of lead worldwide. Moreover, approximately 85% of global lead resources are currently utilized for manufacturing LABs, and the recycling of waste LABs brings favourable prospects for the sustainable development of the energy storage industry. Therefore, the recycling of waste LABs is necessary and inevitable. In this paper, we have comprehensively reviewed the methods of recycling waste LABs. Particularly, we focused on the valuable component of waste lead paste and critically evaluated the pyrometallurgical and hydrometallurgical techniques associated with it. By categorizing and summarizing the characteristics of different methods, we have conducted a detailed comparison of these technologies, aiming to provide a comprehensive assessment of the advantages, disadvantages, status, and trends in LABs recycling technology. Additionally, the paper explores the necessity and impacts of recycling waste LABs from the perspectives of resource, energy, economy, environment, and society. It discusses the challenges faced by waste LABs recycling and presents the development prospects from both technical and non-technical point of views.

1, despite the gradual replacement by lithium batteries, LABs-powered electric bicycles still dominate the market, capturing 77% of China's electric bicycle market as of 2020 [20].Additionally, LABs maintain a significant market share of 20.6% in grid energy storage, second only to lithium-ion batteries (53.8%) in 2021 [21].The grid energy storage sector presents promising prospects for LABs, with a projected market size reaching trillions of dollars in the future [16].
While LABs may have lower energy density, which is 50 watt hours per kilogram, compared to lithiumsulphur batteries (505 watt hours per kilogram), lithium-ion batteries (265 watt hours per kilogram), and sodiumion batteries (150 watt hours per kilogram) [22], their competitive advantage lies in their cost-effectiveness.
Considering the cost factor, LABs exhibit theoretical capacities comparable to many cathode materials used in lithium-ion batteries [16].Therefore, in applications where product volume is not a constraint, LABs still present a rational choice.Moreover, their safety features and low cost make LABs an attractive option.As a result, LABs have garnered significant attention and widespread application, signifying a promising future.
However, the limited lifespan of LABs and their widespread usage result in the generation of substantial waste.Those waste LABs contain hazardous substances, such as sulfuric acid and lead element, which, if not properly handled, can contaminate the environment, including air, soil, and water bodies, posing risks to ecosystems and human health [23][24][25].Consequently, appropriate disposal of waste LABs is paramount.Moreover, waste LABs represent a valuable secondary resource [26].Recycling waste LABs reduces the demand for primary lead ore, mitigates environmental pollution, and conserves energy and water resources consumed during mining and processing.Lead recycling, in particular, can save approximately 55% to 65% of energy comparing with lead mining [25].
In conclusion, the active recycling and utilization of waste LABs serve as effective strategies for resource reutilization, enhancing material using efficiency, and reducing environmental pollution.These measures are crucial for promoting sustainable and eco-friendly development [27].This paper provides an indepth exploration of waste LABs recycling technologies, analyses current challenges, and proposes solutions.
Furthermore, this paper also examines the impact of waste LABs recycling from multiple perspectives, including resources, energy, economy, environment, and society, discussing the present state and future development trends of waste LABs recycling, both technically and non-technically.

Necessity of Recycling
The recycling and utilization of waste LABs undoubtedly contribute to the development of the energy industry, particularly in promoting the use of renewable energy sources.For instance, establishing energy storage systems using recycled LABs allows for the storage of electricity In conclusion, the impacts and challenges of waste LABs recycling need to be comprehensively considered in terms of the resources, energy, environment, economy, and society.These aspects are interconnected and mutually influential.By actively promoting the recycling of waste LABs, we can achieve resource circularity, promote sustainable energy development, protect the environment, create economic benefits, and improve social well-being.Therefore, waste LABs recycling is a task of significant scientific nature, requiring the collective efforts and cooperation of society as a whole.

Recycling Technologies
The waste LABs mainly consist of the following components: grid plates (pure Pb), organic casing (polypropylene, polyethylene, polyvinyl chloride, etc.), lead paste (PbO, PbO2, and PbSO4), electrolyte (H2SO4), and separators.The specific proportions of these material components are outlined in Overall, a typical process of waste LABs recycling is shown as Fig. 3.  1, lead is primarily distributed in the grid plates and lead paste, making these two components highly valuable for recycling and crucial for the circular utilization of lead resources.Moreover, the separation between lead paste and grid is achieved through the process of air classification [45].
The grid plates mainly consist of lead alloys and can be recycled separately.Typically, the grid plates are produced to pure lead or lead alloys through the smelting process [32,41,46].Due to the relatively low melting point of lead and its alloys, grid plates recycling can be conducted at relatively lower smelting temperatures.In contrast, the lead paste, which contains a complex composition with a significant amount of lead sulphate and various lead oxide species, presents the most challenging part of the waste LABs recycling process and remains a key focus of current research [32,38].
Currently, two methods, namely pyrometallurgical and hydrometallurgical processes, are used for lead paste recovery.Nevertheless, regardless of the method chosen, the purpose of this process is to remove impurities and reduce various lead compounds in the lead paste, ultimately obtaining purified lead or lead oxide In the following sections, this article provides separate discussions on these two methods.

Pyrometallurgical Process
The pyrometallurgical method is widely used for the recycling of waste lead paste, and it is currently a mature technology [47].Generally, this method involves high-temperature treatment of the spent lead paste and the addition of a reducing agent such as coke to promote the reduction reaction of the lead paste in a molten state [37,47].The pyrometallurgical method can be further classified into one-step and two-step processes.
Specifically, the one-step process refers to the simultaneous reduction of lead and desulfurization of lead sulphate during the high-temperature smelting and reduction process (above 1000 °C) [40,42,45].In this process, the organic matter and impurities in the waste lead paste are decomposed, resulting in the formation of liquid metallic lead.The relevant reaction equations are as follows (Eq.( 7)-( 9)): For example, Na2S+ ZnO +CO2 → Na2CO3 +ZnS (15) While the one-step method for recovering lead paste is relatively simple and has a shorter process flow, it operates at very high temperatures (The reduction of PbSO4 requires a temperature of 1499°C or higher.)and generates a significant amount of SO2 [49], resulting in environmental pollution and posing a threat to human health [10].Therefore, considering the aspects of economy, energy consumption, and environment, this method is not competitive and has been gradually phased out.As a result, a two-step process, known as predesulfurization smelting, has been developed.
The two-step process in the recovery of waste lead paste involves a pre-desulfurization step followed by smelting.The purpose of pre-desulfurization is to reduce the sulphur content in the lead paste before the smelting process, thereby minimizing the amount of SO2 generated during subsequent smelting.This is achieved by adding desulfurizing agents such as sodium carbonate, sodium hydroxide, ammonium carbonate, ammonium bicarbonate, and potassium carbonate, which react with the lead paste at room temperature.The reaction equations are as follows (Eq.( 16)-( 20)) [37, 49-51]: PbSO4 (paste) + Na2CO3 (aq) → PbCO3 (paste) + Na2SO4 (aq) (17) PbSO4 (paste) + (NH4)2CO3 (aq) → PbCO3 (paste) + (NH4)2SO4 (aq) (18) PbSO4 (paste) + NH4HCO3 (aq) → PbCO3 (paste) + NH4HSO4 (aq) (19) PbSO4 (paste) + K2CO3 (aq) → PbCO3 (paste) + K2SO4 (aq) (20) Through the pre-desulfurization step, the lead sulphate in the waste lead paste reacts with the desulfurizing agents, resulting in the formation of insoluble lead carbonate or lead hydroxide (referred to as new lead paste) and a byproduct solution of sodium sulphate.The obtained new lead paste is then subjected to reduction smelting, which reduces the sulphur content and the generation of SO2 during the smelting process.
Additionally, the smelting temperature can be lowered, thereby reducing environmental pollution and health Overall, the two-step process is widely adopted in the recovery of waste lead paste as it effectively reduces environmental pollution, improves workplace safety, and decreases energy consumption.It is a more sustainable and environmentally friendly approach.However, it should be noted that despite the use of this method, complete removal of sulphur from the lead paste is not achieved, leading to the production of SO2 [55,

56].
The pyrometallurgical smelting method offers several advantages, including its simple operation, fast reaction rate, high adaptability to various raw materials, and suitability for large-scale production.However, it has certain limitations that contradict the principles of sustainability.The pyrometallurgical process is associated with low recovery rates, high energy consumption, and the emission of significant amounts of sulphur dioxide and lead dust.These emissions contribute to environmental pollution and pose risks to the health of workers

Hydrometallurgical Process
Hydrometallurgy has emerged as a viable alternative to pyrometallurgy over the past decades, experiencing rapid development and widespread adoption while gradually replacing pyrometallurgical processes [47].This shift can mitigate the significant drawbacks of pyrometallurgy, including low recovery rates, high energy consumption, and severe environmental pollution.In contrast, hydrometallurgy is regarded as a more environmentally friendly approach, characterized by simpler equipment and processes, as well as high metal recovery rates and wide adaptability.By utilizing aqueous solutions as the medium, hydrometallurgy efficiently and precisely reduces various components of lead paste to pure lead or lead oxide.Furthermore, in order to mitigate environmental pollution, hydrometallurgy incorporates desulfurization pretreatment of lead paste.
Hydrometallurgy can be further classified into two primary methods: conventional hydrometallurgy and electrochemical methods.
Conventional hydrometallurgy typically involves leaching the lead paste, followed by the addition of reducing agents (such as lead powder) to induce the reduction reaction.After undergoing purification processes, the desired lead oxide product is obtained through techniques like crystallization.This method is relatively straightforward and commonly employed in small-scale production environments.As a feature, this method does not require electrochemical assistance [44].

For example, Pan et al.
[50] used a method where waste lead paste and lead powder are dissolved in NaOH solution to form a mixed solution of Na2SO4 and NaHPbO2.Impurities such as iron and barium in the lead paste are removed because they do not dissolve.The solution is then cooled for crystallization to obtain lead oxide.Finally, the lead oxide is dissolved again in a sodium hydroxide solution and subjected to another round of crystallization.The resulting lead oxide has a purity level of over 99.99%, and the lead recovery rate reaches 99.2%.

Wu et al. [57]
initially sulfurized the waste lead paste to PbSO4 (with a conversion rate of 99.99%) using reagents such as H2SO4, H2O2, and NaCl.Subsequently, they carried out a simultaneous process of lead leaching and desulfurization by reacting the sulfurized paste with ammonium acetate, achieving a lead leaching rate of up to 99.94%.In the resulting solution, glycerol was added, and after distillation, the glycerol solution containing lead acetate was obtained.This solution was then reacted with ammonium carbonate as the decarburizer, resulting in the precipitation of over 99.9% of the lead.
Electrochemical methods rely on electrochemical reactions to ultimately reduce the lead paste to pure lead or lead oxide.Depending on the form of the lead paste during electrolysis, electrochemical methods can be further divided into liquid-phase electrodeposition and solid-phase electrolysis.
Liquid-phase electrodeposition begins with dissolving the lead paste, and the resulting solution serves as the electrolyte.By applying an external power source, the lead ions in the solution are reduced at the cathode, depositing as elemental lead on its surface, thereby achieving the recovery and utilization of lead paste.However, it should be noted that prior to leaching, generally, the insoluble components in the lead paste need to be converted into soluble PbO, a process that often involves desulfurization as well [10].
For example, Andrews et al. [58] utilized a mixed solution of sodium chloride and hydrochloric acid to leach lead paste and obtain lead chloride solution.After solid-liquid separation to remove impurities, lead powder was added to further purify the lead chloride solution.Subsequently, the purified solution underwent electrodeposition, resulting in lead with a purity level as high as 99.995% and a recovery rate of 99.5%.

Chang et al. [59]
proposed a process for the recovery of waste lead paste using an electro-deposition method.Firstly, PbSO4 was converted in the waste lead paste into soluble Pb3(CO3)2(OH)2 or NaPb2(CO3)2OH in Na2CO3 solution.and this process also converted insoluble PbO2 into PbO simultaneously.Then, the lead paste after pre-treated was leached in methane sulfonic acid solution.The leachate was used as the electrolyte, with the addition of 0.5 g/L calcium lignosulfonate and 0.05 g/L bone gum.Finally, lead was deposited on the cathode through electrodeposition, resulting in a purity of 99.996% for the obtained product.Moreover, the lead recovery rate reached 95.28%, and the cathodic current efficiency and specific energy consumption were 99.31% and 598.91 kWh/t Pb, respectively.

Xing et al. [60]
utilized an acidic solution (pH=1) consisting of calcium chloride and ferrous chloride tetrahydrate as the leaching agent to dissolve the lead paste at 90°C, achieving a leaching rate of 99%.Subsequently, an electrodeposition experiment was conducted at a current density of 200 A/m 2 , resulting in lead product with a purity of 99.6%.The cathodic current efficiency was 96.3%, and the specific energy consumption was 85.9 kWh/t Pb.Importantly, this method eliminates the need for a desulfurization step, and the iron ions and waste electrolyte are recyclable.
In contrast, solid-phase electrolysis involves transforming the lead paste into cathode plates or slurry, which are then subjected to reduction through an external circuit, resulting in the production of sponge lead or lead oxide.The choice of electrolyte in this method can include acids, alkalis, or ionic liquids [10, 38, 61].

Fan et al. [62]
developed a method to directly electrochemically reduce waste lead paste in an NH3-NH4Cl solution, resulting in the production of high-purity metallic lead with a purity of 98.3%.The process exhibited an electrical current efficiency of 86.3% and an energy consumption of 689.4 kWh/t Pb, respectively.
Ultimately, the core idea of electrochemical methods is to use the reduction reaction through electrochemistry to convert the lead compounds in the lead paste into pure lead.Regardless of the exist states of the lead paste (solution, slurry, or electrode plate), regardless of variations in the electrolytic cell structure or the use of different additives, the fundamental principles and production objectives remain the same.
With the increasing emphasis on environmental protection and sustainable development,

Challenges and Solutions
The current waste LABs recycling industry still faces significant challenges, including both technological (technology and environment) and non-technological (legislation and society) aspects.Here, we discussed these challenges and their corresponding solutions in both aspects.

• Challenges of technology and environment
The technological challenges primarily revolve around the impact on the environment and human life and health.
Although lead recovery has made significant progress, the environmental issues including the leadcontaining particulates, SO2 gas and lead-containing slag require special precautions in the process of waste Moreover, in low to middle-income countries (LMICs), the hazards associated with waste LABs recycling are amplified substantially.Notably, LABs recycling is the most environmentally damaging industrial process in the world, resulting in an estimated loss of between 2 million and 4. In the face of technological and environmental challenges, the development of more efficient and environmentally friendly new technologies is inevitable.These new technologies should aim for high energy efficiency, high recovery rates, short processing cycles, and environmental friendliness.This would require significant investment and research and development of new production equipment.
In addition, strict adherence to production standards and regulations is necessary during the manufacturing process.Proper disposal of waste LABs should be ensured to prevent any leakage into the environment and avoid serious consequences.Furthermore, it is important to establish long-term production plans and emergency response plans to address various unforeseen circumstances, including ecological restoration and medical support.

• Challenges of legislation and society
The efficient collection of waste LABs is also a crucial challenge.The inadequate collection networks, limited collection infrastructure, lack of recycling awareness and low community participation pose limitations to the recycling of spent LABs.These challenges are not of a technical nature, but they have a significant impact on the overall sustainability of the industry [34, 71, 72].
In many areas, the lack of well-established collection networks and systems is a major barrier.This means that there are insufficient collection points and recycling centres, making it inconvenient for people to deliver their spent LABs to professional recycling facilities [73].Furthermore, due to the lack of a formalized recycling system in some regions, a substantial amount of waste LABs enter the hands of tiny secondary lead enterprises illegally.For example, small illicit secondary lead enterprises are responsible for the disposal of more than forty percent of the nation's scrap lead resources in China, a disposal capacity comparable to that of large-scale enterprises.However, industry concentration is low, and small businesses lack of environmental protection equipment has led to a disregard for zoning and pollution restrictions [70].
Another challenge is the lack of sufficient recycling awareness and participation in recycling.
Community residents may not fully understand the importance and impact of recycling waste LABs.They may lack proper knowledge on how to dispose of waste LABs correctly and may lack the motivation to participate in recycling efforts.The lack of effective publicity and educational activities also hinders public engagement Overall, to establish an effective and sustainable waste LABs collection system, it is crucial to address these non-technical challenges.This requires improving the collection network, strengthening recycling awareness and education, and investing in appropriate collection infrastructure to facilitate the recycling process.
Additionally, enhancing the legal framework and strengthening regulatory measures are of utmost importance.Suitable subsidies and penalties can help curb illegal practices and enhance people's willingness to participate in it.Ultimately, this will lead to the efficient and environmentally friendly recycling and reuse of secondary lead resources.

Prospect
In the context of achieving carbon peak and carbon neutrality, as well as the growing prominence of sustainable development principles, technological advancement has become a critical factor in the waste LABs recycling industry.Within this framework, there is an urgent need to develop cleaner, more efficient, costeffective, streamlined, and user-friendly new technologies.This necessitates increased attention, research interest, and financial investment to drive technological innovation in the field of waste LABs recycling.
Moreover, existing technologies are bound to be replaced by new ones in pursuit of economic benefits.
The establishment of a robust recycling system and network is also of utmost importance.This includes the establishment of dedicated responsible recycling institutions, as well as the development of recycling plans and emergency protocols.Such a recycling system ensures the smooth and uninterrupted lifecycle circulation of LABs, mitigating the risk of fatal issues due to any missing links or failures in the recycling process.Furthermore, it is imperative to enhance relevant policies, regulations, and oversight, which may involve implementing reward and penalty mechanisms.Additionally, strengthening public awareness through educational campaigns is crucial to foster a deeper understanding of the concept of a circular economy.By encouraging proactive public participation in waste LABs recycling and promoting active classification of secondary resources, the difficulty and cost associated with recycling can be significantly reduced.
Enhancing collaboration and cooperation is also of paramount importance.This encompasses not only communication and collaboration among governments, businesses, research institutions, and consumers but also the sharing of technological advancements among different enterprises, regions, and even nations.Particularly, developed countries possessing more advanced recycling technologies should assist developing nations in achieving efficient circularity and recycling within the LABs industry.Moreover, interdisciplinary academic exchanges will further facilitate the development of the waste LABs recycling industry, as interdisciplinary integration is one of the future trends.

Conclusion
The recycling of waste lead-acid batteries (LABs) is crucial for sustainable development.The increased usage of LABs has resulted in significant waste generation, posing risks to the environment and human health.
Recycling waste LABs offers multiple benefits.Firstly, it aids in reducing the release of hazardous substances such as lead and sulfuric acid, thereby minimizing damage to the natural environment and ecosystems.Secondly, waste LABs are a valuable secondary resource, with a substantial portion of global refined lead production derived from the recycling of LABs, highlighting the resource importance of LABs recycling.Thirdly, considering the substantial consumption of lead by LABs globally, recycling the lead from waste LABs holds positive implications for the energy storage sector.Finally, the recycling of LABs not only mitigates environmental risks but also generates significant economic benefits by converting waste into valuable resources.
In this paper, we provided an overview and classified the main technologies currently used in LABs recycling, discussed the challenges faced, and presented relevant solutions.We also analysed the impact of LABs recycling from various perspectives, including resources, energy, economy, environment, and society.
Through a comprehensive examination of both technical and non-technical factors, we have evaluated the status of LABs recycling and identified future trends.
Overall, this study emphasizes the importance of recycling waste LABs and highlights the need for ongoing research and development in this field.By implementing effective recycling practices, we can minimize environmental impacts, preserve precious resources, support the energy storage sector, and contribute to sustainable development.

Fig. 1
Fig. 1 Market share of battery materials for electric two-wheelers in China from 2016 to 2020

Fig. 2
Fig. 2 Five impact aspects of recycling waste lead-acid batteries

Fig. 3
Fig.3The process of waste lead-acid batteries recycling

risks [ 45 ,
52].The smelting temperature of desulfurized lead paste is typically at least 100-150 ℃ lower than that of untreated lead paste [40].Specifically, lead carbonate can decompose at 288 ℃ [49].Other than carbon, certain metals can also act as reducing agents.Liu et al. [53]  treated the waste lead paste with sodium carbonate for desulfurization, and then used lead-antimony alloy as the reducing agent to reduce the waste lead paste under vacuum, achieving both the separation of lead-antimony alloy and the reduction of waste lead paste.Ultimately, the direct recovery rate of Pb from the desulfurized lead paste reached 99.29%.The antimony content in the lead-antimony alloy was reduced from 46% to 0.98%.This method not only enables the efficient and economical recovery of lead paste but also achieves the efficient separation of lead-antimony alloy without generating any waste.Furthermore, a reverse approach is also one of the methods for treating waste lead paste, such as converting all the components of waste lead paste into high-value lead sulphate.Liu et al. [54]  used potassium bisulphate as a roasting agent and achieved the conversion of waste lead paste into lead sulphate under the conditions of 300°C, KHSO4/waste lead paste mass ratio of 1:1, and a holding time of 10 minutes.This method significantly reduces the emissions of carbon dioxide and smoke.The results of the life cycle assessment show that recycling 1.0 ton can reduce carbon emissions by 2.45 tons of carbon dioxide and 0.13 tons of smoke.
hydrometallurgy has gained widespread application as a more environmentally friendly alternative.It not only effectively enhances the recovery rate of lead paste and the utilization of metal resources but also reduces energy consumption and environmental pollution.Therefore, hydrometallurgy plays a crucial role in promoting the sustainable development of secondary lead resources [34, 40, 44, 63].The 10th International Conference on Lead and Zinc Processing (Lead-Zinc 2023) Journal of Physics: Conference Series 2738 (2024) 012019 IOP Publishing doi:10.1088/1742-6596/2738/1/01201912 LABs processing.Nowadays, most of the secondary lead recovery plants are based on pyrometallurgical methods [64].The emission of sulphur oxides, nitrogen oxides and lead-containing particulate matters from the pyrometallurgical route constitute a serious consequence for the local environment and human lives.Despite the fact that the paste smelting process has made important progress in energy efficiency, the issue of dust measured as PM 2.5, containing lead particulates constitute a major lead poisoning threat to humans, animals, birds, soil and drinking water [65].Atmospheric dispersion and deposition are the major transport pathways.An investigation has shown that the lead intakes were 50.1 and 6.2 μg/kg/d for the children and adults which are much higher than the Provisional Tolerable Weekly Intake (PTWI) values in these areas in China [65].

[ 34 ,
41, 71-73].Moreover, the limited collection infrastructure is also a problem.In certain areas, there is a lack of adequate collection points and containers, making it difficult for residents to conveniently dispose of their spent LABs at designated collection points.This inconvenience and lack of proper infrastructure pose challenges to the recycling of spent LABs [71, 72].In addition, Insufficient and inadequate implementation of relevant laws and regulations in certain regions also pose obstacles to the waste LABs recycling industry [74-76].

Table 1 [10, 37, 38].Table 1
Typical phase composition of a waste lead-acid battery Due to the diverse composition and varying properties of different parts of waste LABs, direct processing is difficult to target and often results in low lead recovery rates and severe environmental pollution.Therefore, it is more reasonable to dismantle waste LABs and sequentially separate their different components for targeted recycling processes.The recycling process of waste LABs includes pretreatment, smelting, and electrolytic refining processes.The pretreatment process involves steps such as acid drainage, crushing, and powder separation [

10, 37, 44].
The environmental impact and health hazards associated with the pyrometallurgical method contradict the concept of sustainable development.Therefore, it is crucial to explore alternative approaches that minimize these adverse effects and align with sustainable practices [