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| Fig. 1: Internal view of a lead-acid battery from an electric-start equipped motorcycle. (Source: Wikimedia Commons) |
Lead-acid batteries (LABs) were the first rechargeable electric battery marketed for commercial use and have remained an industry standard ever since. This is true despite the fact that LABs offer low energy density, typically operating at 30%-40% of the theoretical limit, compared to 90% for lithium-ion batteries. [1] The persistence of LABs in the commercial market stems mainly from the low cost of source materials and ease of production brought about by use of a nonflammable water-based electrolyte. In total, LABs account for 70% of the global energy storage market, with a revenue of 80 billion USD and 600 gigawatt-hours of total production in 2018. [1] It was quickly recognized that the pervasiveness of LAB technology could lead to serious health global implications if left unregulated, with the primary risk being toxic metal pollution of soil and water due to the improper disposal.
Lead is a highly potent neurotoxin known to cause serious and in some cases irreversible neurological damage. Children are particularly susceptible to adverse health effects including damage to the brain and nervous system, slowed growth and development, and hearing and speech problems, all of which can occur at any level of lead exposure. [2] Consequently, the need for effective recycling and management of LABs has been recognized and many countries have taken steps to regulate these processes. In 2014 it was reported that in the United States LABs are recycled at a rate of nearly 99%. [3] However it is important to keep in mind that this 99% recycling rate corresponds to the percentage of LABs that undergo a recycling process, not the efficiency of the process itself. It will be the aim of this report to summarize the most commonly used processes for recycling LABs and highlight the importance of regulated recycling measures. This will be done by discussing each process and the possible modes of pollution and then numerically comparing the amount of Pb pollution (in kg of Pb per kg of processed Pb) that would result from these processes if no waste mitigation regulations were in place. These values will be compared to reported values of pollution for factories in the US which do implement waste mitigation measures.
Generally, LABs consist of a lead dioxide (PbO2) anode, a porous Pb cathode and a dilute H2O + H2SO4 electrolyte solution. Fig. 1 shows an example of the internal view of a typical LAB. The reversible charging/discharging reaction is given by : PbO2+ SO42- + 4H+ + 2e- ↔ PbSO4 + 2H2O. Upon full discharge of the battery both the positive and negative plates are converted into lead sulphate (PbSO4). The waste associated with LABs can be broadly categorized as either liquid waste, solid metallic waste or solid nonmetallic waste. Due to the limited scope of this report only solid metallic waste, i.e. lead-containing waste, will considered. Such waste includes the metallic lead grids and lead paste which form the anode and cathode, as well as lead contaminated battery casings.
The most widely used process for LAB recycling is that of pyrometallurgy which recoverers lead via smelting. [4,5] In this process the spent LABs first undergo battery breaking where the batteries are shredded so that the different forms of waste can be separated based on density and water solubility. The lead extracted after breaking is either in the form of metallic lead grids or lead paste. Depending on the exact form of battery the lead paste will typically consist of some combination of PbO, PbSO4, PbO2, PbO3 and metallic Pb. The aim of the pyrometallurgy process is to remove these oxygen compounds from the lead paste so that pure metallic lead can be extracted and reused. This is done by a smelting operation, typically between working between 1100°C and 1300°C. In the smelt state, pure metallic Pb can be recovered via reduction with carbon powder. Finally, the metallic Pb smelt is combined with the metallic Pb grids and undergoes a refining process to remove any remaining alloys.
Various versions of pyrometallurgy exist for processing spent LABs. The overall efficiency of each process varies but the average efficiency for lead recovery is reported to be 98%. [5] There are three main pollution modes for pyrometallurgy recycling schemes: air emissions, water contamination and soil contamination. Air emissions in the form of lead particulates are released into the air during the smelting phase of processing. Because the lead particles are light enough to be carried through the air they pose a major health risk to people living near LAB recycling facilities. Soil and water contamination mainly occur from the leaching of lead from battery casings that are improperly stored after the breaking phase and from proper disposal of smelting waste which contains trace amounts of lead. [6]
Hydrometallurgy is the second most common form of LAB recycling process. [5] This process is similar to pyrometallurgy in that the spent LABs must first undergo a battery breaking process, resulting in lead paste that must be processed into metallic Pb. This is done by submerging the lead paste into an alkaline reagent such as NaOH and Na2CO3 which is used as an electrolyte. A potential difference is then applied across the electrolyte forcing a chemical reaction to occur which extracts pure lead from the lead paste. Because this process does not require any smelting, the possibility of air pollution is eliminated. However, these recycling schemes are costly and have a lower lead recovery rate, around 96%, when compared to pyrometallurgy schemes. Soil contamination can result from the improper disposal of the electrolyte-sulfur-lead slurry that is a byproduct of the process. As noted by Li et al., the remaining products of this process can contain up to 0.44% lead. [6] Furthermore, the battery breaking phase of this recycling scheme is again associated with water and land contamination if improper storage methods are not employed.
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| Table 1: Calculated Pb emissions (in kg Pb per kg Pb produced) from each recycling scheme assuming unmitigated waste release. Row three shows reported values of Pb emission from US pyrometallurgy factories; adapted from Higgins. et al. [7] |
Using the methods from Li et al. and Higgins et al., the lead emissions for both recycling schemes were calculated assuming that no lead waste mitigation steps are used.[5,7] Note that the water and soil pollution rates reflect worst-case-scenarios very near processing plants and not a general reflection of water and soil Pb pollution.[8] This calculation is meant to represent an upper bound for lead pollutants entering the environment as a byproduct of recycling and highlights the importance of regulated recycling techniques. The results are summarized in Table 1. Rows 1 and 2 show the emissions for each recycling scheme in units of kg of Pb per kg of Pb recycled. For comparison, row 3 was adapted from Higgins et al. and shows the average reported emissions from US pyrometallurgy facilities from 1998 to 2003, again in units of kg of Pb per kg of Pb output. [8]
From Table 1 it is clear that governmental regulations play a major role in reducing lead waste produced by recycling spent LABs. This is even more apparent when one considers the fact that the data presented in row 3 account only for Pb waste that was officially reported by factories. There have been numerous reports indicating that improper disposal and unsanctioned LAB recycling have taken place across the US which further increase the amounts quoted here. [6,8] Furthermore these numbers only account for lead waste generated by recycling and do not include the 1% of LABs which are not recycled.
© Linsey Rodenbach. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. P. Lopes and V. R. Stamenkovic, "Past, Present, and Future of Lead-Acid Batteries," Science 369, 923 (2020).
[2] K.-S. Liu et al., "Neurotoxicity and Biomarkers of Lead Exposure: a Review," Chin. Med. Sci. J. 28, 178 (2013).
[3] "Advanced Sustainable Materials Management: 2014 Fact Sheet," U.S. Environmental Protection Agency, EPA530-R-17-01, November 2016.
[4] A. D. Ballantyne et al., "Lead Acid Battery Recycling for the Twenty-First Century," Roy. Soc. Open Sci. 5, 171368 (2018).
[5] M. Li, J. Liu, and W. Han, "Recycling and Management of Waste Lead-Acid Batteries: A Mini-Review," Waste Manage. Res. 34, 298 (2016).
[6] T. Nedwed and D. A. Clifford, "A Survey of Lead Battery Recycling Sites and Soil Remediation Processes," Waste Manage. 4, 257 (1998).
[7] C. Higgins, H. S. Matthews, and M. Small, "Lead Demand of Future Vehicle Technologies," Trans. Res. D 12, 103 (2007).
[8] W. P. Eckel, M. B. Rabinowitz, and G. D. Foster, "Investigation of Unrecognized Former Secondary Lead Smelting Sites: Confirmation by Historical Sources and Elemental Ratios in Soil," Environ. Pollut. 117, 273 (2002).
