As part of our work, the study support consists of used batteries. It is a spent battery type PowerSafe SBS-B8 VRLA 12.0 V 31.0 Ah (Figure 1). It is said to be waterproof, since its electrolyte is absorbed and immobilized between each plate by cellulose (a kind of blotting paper). It is generally used for solar-type applications. This battery consists of three compartments connected in series and connected by soldered lead connections. Each compartment is composed of a set of couples of positive lead oxide IV (PbO₂) and negative lead (Pb) electrodes connected in parallel, the electrodes are separated by a paper and bathed in an electrolyte solution of sulphuric acid H₂SO₄.

Figure 1. Used lead acid battery (12 V voltage and 31 Ah capacity).

3.2.1. Glassware

Glassware is the one commonly used in laboratories. These include, among others, 250 mL volumetric flasks, various pipettes and insets, and 100 mL graduated cylinders for measuring solvent volume and preparing solutions, erlenmeyer flasks for leaching, and vacuum flasks for vacuum filtration. Figures 2(a)-(d) illustrate the essential instruments for the preparation and manipulation of solutions in the context of our experiments.

(a) (b)

(c) (d)

Figure 2. Laboratory glassware. (a) fiole jaugee de 250 mL, (b) eprouvette graduee de 100 mL, (c) erlenmeyer de 250 mL, (d) fiole video at 250 mL.

3.2.2. Reagents and Solvents

All the chemicals used are of pure analytical quality and prepared with distilled water, they are solutions of citric acid and sodium hydrated citrate of respective molar concentrations 2.5 mol∙L⁻¹ and 1 mol∙L⁻¹ then a 30% purity hydrogen peroxide solution. All the experiments were carried out at room temperature (1˚C - 25˚C). Figures 3(a)-(c) show the chemicals used for the preparation of solutions needed for experiments.

(a) (b) (c)

Figure 3. Chemical reagents. (a) Bottle containing crystals of hydrated citric acid (C₆H₈O₇H₂O) with a purity of 98%, (b) Vial containing crystals of sodium citrate hydrate (Na₃C₆H₅O₇∙2H₂O) with a purity of 98%, (c) Bottle containing a 50% purity hydrogen peroxide solution.

The leaching and recovery process of lead oxide requires the use of several specialized laboratory equipment to ensure the accuracy and efficiency of operations. The equipment used are as follows:

An OHAUS brand precision balance 0.1 mg used to make high-precision mass measurements (Figure 4(a)).

(a) (b)

(c) (d)

Figure 4. Laboratory equipment. (a) Precision balance (Maximum load 200 g, sensitivity 0.1 mg), (b) magnetic stirrer (speed continuously adjustable from 0 to 2000 rpm., Plate diameter: 120 mm.), (c) Air oven (Voltage: 230 V, 50 - 60 Hz, Power: 600 W, Content: 14 L, Temperature range: 220˚C), (d) Electric oven (Volume: 15 liters, Max temperature: 1200˚C).

A STIRRER brand magnetic agitator used to ensure homogeneous mixing of the reagents, agitation constante of the solution and ensure uniform distribution of the reagents for chemical reaction efficiency by maintaining continuous movement in the solution (Figure 4(b)).A filtering system, used for the separation of solid precipitates from the liquid solution, thus allowing the elimination of unwanted solids while recovering the precipitates formed during the reaction.A Memmert air oven with adjustable temperature, used for sample drying and ensuring controlled evaporation of water without altering the precipitate structure (Figure 4(c)).A Nabertherm brand calcination oven, allowing the thermal decomposition of samples at a precise temperature (Figure 4(d)).

Battery dismantling must be carried out carefully to ensure safety and efficient recovery of components. It is essential to use appropriate tools, such as a saw, knife and hammer, to avoid damaging the internal components. It is advisable to start by carefully removing the plastic case from the battery in order to preserve the electrodes and lead paste. Once the housing is removed, the positive and negative electrodes must be gently separated to avoid any short circuit or accidental discharge. Then it is necessary to use a spatula to recover the lead paste, ensuring that it is placed in a porcelain crucible for the next processing step. It is also important to ensure appropriate protection to avoid contamination or exposure to toxic substances contained in batteries, such as lead and sulfuric acid.

After separation of the electrodes and recovery of the lead paste, it is ground and then sieved to obtain a fine powder with homogeneous particle size. The recovered and pretreated lead paste is composed of 60% by weight of lead sulfate (PbSO₄), but also lead dioxide (PbO₂), about 26.5% by weight, lead oxide (PbO), 9.5% by weight and finally metallic lead (Pb) about 4% by weight [8]. The synthesis of lead citrate (II) is a key step to convert this paste into a precursor that, by thermal decomposition, will produce lead oxide (PbO). This protocol describes the complete procedure, from dismantling the battery to obtaining lead oxide (PbO).

The 250 mL preparation of 2.5 mol∙L⁻¹ citric acid solution required the withdrawal of 131.25 g of the hydrated citric acid crystal, the 1 mol∙L⁻¹ sodium citrate solution was prepared with a mass of 73.5 g of hydrated sodium citrate. The concentrations of 2.5 mol/L for citric acid and 1 mol/L for sodium citrate were selected based on previous studies that demonstrated their effectiveness in dissolving lead compounds while maintaining a moderate acidity that limits unwanted side reactions. These concentrations ensure good leaching kinetics without excessive consumption of reagents, thus contributing to the environmental and economic viability of the process. Finally, the withdrawal of a volume of 150 mL of 50% hydrogen peroxide solution was necessary for the preparation of 250 mL of 30% hydrogen peroxide.

4.4.1. Reaction of Lead Paste with Leachate Constitutes Solutions of Citric Acid and Sodium Citrate

The recovered and pretreated lead paste was dissolved in an Erlenmeyer flask containing 18 mL of a 2.5 mol citric acid (C₆H₈O₇H₂O) solution∙L⁻¹ and 26 mL of a 1 mol∙L⁻¹ sodium citrate (Na₃C₆H₅O₇∙2H₂O) solution. The chemical reactions resulting from this process are given by the following equations:

4.4.2. Reduction of Lead Oxide IV (PbO₂) with Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) at 30% has been added slowly to the previous slurry to reduce lead oxide IV (PbO₂) to lead oxide II (PbO), facilitating the complete formation of lead citrate II. The addition of hydrogen peroxide must be done in a small amount to avoid a violent reaction. The chemical reaction resulting from this process is given by the following equation:

After the reaction, the solution is filtered to separate solid precipitates from the liquid. Then the precipitate should be placed in an air oven at a controlled temperature to remove moisture. Once this step is completed, the resulting lead citrate is placed in an electric furnace set at 405˚C to proceed with thermal decomposition. After calcination, the lead oxide formed can be recovered.

5.1.1. Lead Paste Recovered from the Dismantled Battery

As shown in Figure 5, the dismantling of the battery has been successfully completed, allowing the recovery of essential components necessary for the synthesis of lead oxide. After carefully removing the plastic housing, the positive and negative electrodes were carefully separated, minimizing the risk of short circuit. The lead paste was efficiently extracted using a spatula and placed in a porcelain crucible (Figure 6). This recovered paste was then ground and sieved to obtain 10 g of fine-grained lead paste (Figure 7). The process took place in compliance with safety standards, ensuring the integrity of materials and safe handling of potentially toxic substances.

Figure 5. Photograph of the dismantled battery.

Figure 6. Photograph of unpretreated lead paste.

Figure 7. Pretreated lead paste on a mass balance 10 g.

5.1.2. Appearance of Lead Citrate Crystals

Leaching is carried out under magnetic agitation at a speed of 650 rpm for five minutes to maintain the total slurry suspension at room temperature (Figure 8).

Figure 8. Photograph of the first stage of leaching of lead paste on a magnetic shaker (black solution).

The first crystals of white-colored lead citrate began to appear immediately after the addition of hydrogen peroxide following a violent exothermic reaction (Figure 9). The total transformation of lead oxide into lead citrate was observed after 2 min. This reaction ended as indicated in Figure 10 by the observation of the absence of the appearance of new crystals of lead citrate.

Figure 9. Change in leachate coloration indicating the appearance of the first crystals (white) of lead citrate.

Figure 10. End of the leaching reaction.

5.1.3. Lead Citrate Synthesized and Calcined into Lead Oxide

After filtration, and oven drying of the precipitate, 12.9 g of white lead citrate powder could be collected (Figure 11). The synthesized lead citrate was then placed in an electric furnace at 405˚C or its thermal decomposition allowed to synthesize 5.3 g of yellow-colored lead oxide (Figure 12).

Figure 11. Photographie du citrate de plomb synthétisé sur une balance masse 12.9 g.

Figure 12. Photographie de l’oxyde de plomb synthétisé sur une balance masse 5.3 g.

5.1.4. Efficiency of Transformation of Lead Paste into Lead Oxide

Efficiencyoftheleachingprocess

The leaching process made it possible to synthesize a mass of 12.9 g of lead citrate on a theoretical mass of 14.62 g expected, which corresponds to a leaching yield of 88.23%.

Efficiencyofthermaldecompositionofleadcitrate

From the thermal decomposition of lead citrate, 5.3 g of lead oxide were formed over an expected 6.62 g which corresponds to a yield of 80.02%.

5.2.1. Analysis of the Leaching Process of Used Lead Paste

The leaching process showed notable efficiency with a yield of 88.23%. This obtained result indicates an efficient conversion of lead paste into lead citrate whose thermal decomposition will produce lead oxide.

5.2.2. Analysis of Thermal Decomposition of Lead Citrate into Lead Oxide

The thermal decomposition of lead citrate at 450˚C has also been successful. The return of 80.02% is in line with expectations, although it is slightly below 100%. The losses observed can be attributed to volatilization of some organic by-products and partial carbonization of lead citrate during the calcination process.

Losses are mainly due to volatilization and carbonization of intermediates during calcination. To minimize these losses, it is suggested to optimize the calcination temperature (ideally between 380˚C and 400˚C) and extend the treatment time in order to ensure complete decomposition without overheating. Controlling the heating rate and using a covered crucible can also limit losses by volatilization.

Furthermore comparatively, previous studies have reported yields of up to 97.86% for similar processes [9]. This suggests that, while effective, the method employed could benefit from further optimization, particularly in controlling reaction parameters and experimental conditions to minimize losses.

5.2.3. Desulfurization of Used Lead Paste

Lead sulphate, which makes up about 60% by weight of the used lead paste, is a very stable compound that does not react with common acids such as hydrochloric acid, nitric acid or acetic acid. Therefore, a key pre-treatment is to desulphurize lead sulphate into more reactive compounds such as lead citrate. The pH of the leaching solution plays a dominant role in this process, as too high a pH can lead to the formation of layers of lead citrate that cover the lead sulphate, thus inhibiting its desulphurisation. Conversely, a lower pH promotes the dissolution of these layers, exposing more lead sulfate to the reaction [10]. In the leaching system used, composed of citric acid and sodium citrate, citric acid acts both as a pH adjuster and as an agent involved in the crystallization of lead citrate. During our experiment, these conditions made it possible to efficiently transform the lead sulphate contained in the spent pulp into lead citrate, with a mass obtained of 12.6 g and a yield of 88.23% compared to the theoretical mass of 14.3 g. This performance, close to ideal, reflects a good efficiency of the leaching system, despite minimal losses attributed to the filtration and washing steps, as well as the complexity of the initial lead paste, which contains impurities. These results confirm the effectiveness of the method based on citric acid/sodium citrate solution for lead recovery in the form of lead citrate, thus validating the theoretical principles described.