Lead Recovery from Flue Dust by Using Ultrasonic-Enhanced Hydrogen Peroxide Water Washing

Abstract

An ultrasonic-enhanced hydrogen peroxide water-washing process was developed to recover lead from raw flue dust (RFD) under neutral conditions. At optimal parameters (40 °C, 30 min, 4 mL H₂O₂, liquid-to-solid ratio 2:1, 240 W ultrasound), the Pb mass fraction in the solid residue increased from 41.68% in the RFD to 68.11%, accompanied by a Pb recovery rate of 97.1%. These values are significantly higher than those obtained under identical conditions without ultrasound (64.07% and 95.93%, respectively). Ultrasound promotes de-agglomeration and generates •OH radicals that accelerate the oxidation of PbSO₃ to insoluble PbSO₄ while concurrently removing impurity cadmium. This research offers a green and efficient alternative to traditional lead recovery methods, fostering sustainable development in the metallurgical industry.

1. Introduction

In the process of lead smelting, a large amount of raw flue dust (RFD) is generated, which contains approximately 50% lead, significantly higher than the grade of typical lead ores (2–5%). As a representative heavy metal pollutant, improper disposal of lead poses serious environmental risks [1,2]. By employing effective processes to recover lead from RFD into the lead smelting system not only reduces the environmental burden but also provides a secondary resource, enhancing resource utilization efficiency and supporting sustainable development [3,4].

Currently, the primary methods for lead recovery from RFD are pyrometallurgical and hydrometallurgical [5,6]. The pyrometallurgical method separates lead through high temperature roasting by exploiting differences in component volatility, yet it suffers from significant limitations including excessive energy consumption and substantial environmental pollution [7]. Recovery of lead through hydrometallurgical technology is the mainstream choice nowadays, the core principle of which is to dissolve impurities such as cadmium, zinc, and iron into the liquid phase while retaining lead in the solid phase, thus realizing the separation of lead from impurities. Among them, cadmium is treated as the main impurity due to its high content (about 10–20%) and toxicity [8,9,10]. In the past studies, acid washing has been the dominant process due to its simplicity of operation and low energy consumption [11,12,13]. Chen et al. used sulfuric acid as an oxidizing agent to separate lead and other metals in the process of soot leaching, which produces acidic wastewater requiring further treatment [14]. Thus, this approach generates a large amount of acidic wastewater that is difficult to treat, imposing economic and environmental burdens [15]. Additionally, partial lead dissolution alongside impurities results in significant lead loss.

To address the drawbacks of acid leaching, a neutral water washing process has been proposed. Although this method can reduce the loss of lead and minimize the generation of acidic wastewater, the impurity removal efficiency was found to be low in the practical operation of the enterprise, and the content of cadmium in the solid phase was still up to more than 4%, which led to unsatisfactory separation of lead impurities. Li et al. [16] achieved good separation of valent metallic elements from flue dust by pressurized oxidative leaching. However, such a method requires the redesign and modification of a closed high-pressure leach vessel and presents certain safety hazards. Guo et al. [17] demonstrated that microwave radiation selectively accelerated the dissolution of arsenic in copper smelting flue dust while reducing acid consumption, but the effect under neutral conditions was not studied. Recently, researchers have introduced oxidants such as hydrogen peroxide (H₂O₂) during water washing [13,18]. This method converts lead sulfite into insoluble lead sulfate while transforming sparingly soluble impurities (e.g., cadmium sulfite) into soluble forms that enter the solution, thereby improving lead recovery and reducing impurity content in the lead residue. However, the limited oxidizing ability of these oxidants results in incomplete phase transformation, preventing lead recovery rates from reaching ideal levels. In addition, due to the phenomenon of encapsulation and agglomeration, impurities are encapsulated inside and are difficult to remove. This observation suggests a potential strategy: transforming lead into an insoluble phase while converting impurities such as cadmium into soluble forms through intensive oxidation. This approach holds promise for achieving efficient lead recovery under neutral conditions.

Ultrasonic waves, which are mechanical waves with frequencies over 20 kHz, have unique cavitation and mechanical effects [19,20]. They can significantly enhance processes such as leaching, removing impurities, and mixing in physical and chemical reactions [21,22]. The cavitation bubbles generated during ultrasonic treatment collapse violently, producing localized regions of high temperature and pressure, which can significantly enhance chemical reaction rates [23]. This makes ultrasonic technology particularly effective in breaking down complex structures and promoting reactions [19,20,21]. When combined with oxidants, ultrasound can promote the generation of free radicals, thereby enhancing oxidation reactions [24,25]. For instance, studies have demonstrated that ultrasonic-enhanced oxidation can effectively improve the leaching efficiency of metals from various ores and industrial wastes by increasing the reaction rate and oxidation efficiency [26]. The synergistic effect of ultrasonic treatment and oxidants can lead to more complete oxidation of metal sulfides and other compounds, facilitating their separation and recovery [27]. Furthermore, ultrasonic technology can help in the dispersion of reactants, preventing the aggregation of particles and improving the contact efficiency between reactants [28,29,30]. In lead recovery from RFD, good particle dispersion facilitates impurity removal and offers more reaction sites for oxidants, promoting oxidation and boosting lead recovery efficiency. However, the effectiveness of ultrasonic H₂O₂ synergistic systems in lead recovery from RFD under neutral conditions, and the intensification mechanism of ultrasonic, have not been reported in the literature.

To address these challenges, this study develops an efficient method for lead recovery from RFD through ultrasonic-enhanced H₂O₂ water washing under neutral conditions. RFD is analyzed to determine its elemental composition and the forms of phases present. The impacts of various factors, including washing time, H₂O₂ dosage, liquid–solid ratio, ultrasonic power, and temperature on lead recovery, are investigated to identify optimal process conditions. The study also employs multiple analytical techniques to demonstrate the enhancement effect of ultrasound on lead recovery and to clarify its mechanism in enhancing the oxidation process. Consequently, an efficient and environmentally friendly lead recovery process is established. This process is anticipated to improve economic benefits for enterprises, promote a green and sustainable transformation in the metallurgical industry, and contribute to eco-friendly societal development.

2. Results and Discussion

2.1. Analysis of Raw Flue Dust

X-ray fluorescence (XRF) analysis of the RFD reveals a complex elemental composition (as shown in

Table 1. Different elements and their contents in raw flue dust by XRF.

Element	Content (wt. %)	Element	Content (wt. %)
Pb	41.68	Si	0.3
Cd	14.33	Ca	0.3
S	7.1	Se	0.3
Cl	1.07	Na	0.2
Zn	1.02	Ag	0.1
Fe	1	Sn	0.1
Bi	0.9	Mn	0.08
Sb	0.8	Al	0.06
Tl	0.7	Cs	0.06
Cu	0.6	Br	0.04
As	0.5	Hg	0.03

). The results show that lead (Pb) is the major component, making up 41.68% of the sample by weight. Alongside lead, significant amounts of cadmium (Cd, 14.33%) and sulfur (S, 7.1%) are also present. Trace elements such as arsenic (As), silicon (Si), calcium (Ca), and others collectively highlight the intricate makeup of RFD. This diversity of elements suggests the presence of multiple lead and cadmium compounds, which can affect lead recovery efficiency during water washing.

The X-ray diffraction (XRD) pattern of RFD confirms the presence of various crystalline phases, as shown in Figure 1. The peaks in the pattern correspond to cadmium sulfite (CdSO₃), cadmium sulfate (CdSO₄), cadmium sulfide (CdS), lead sulfite (PbSO₃), and lead sulfate (PbSO₄) [11]. The coexistence of PbSO₃ and PbSO₄ indicates that lead is present in both slightly soluble and insoluble forms. During water washing, the slight solubility of PbSO₃ may lead to some lead being lost into the solution, thereby reducing the recovery rate. Similarly, the presence of CdSO₃ and CdS implies that these compounds, which are not very soluble, can remain in the solid phase after water washing, acting as impurities that affect the purity of the recovered lead.

X-ray photoelectron spectroscopy (XPS) analysis provides further insight into the chemical states of elements in RFD, as shown in Figure 2. The Pb 4f spectrum (Figure 2b) shows peaks at binding energies of 139.2 ± 0.1 eV and 143.3 ± 0.1 eV, which are characteristic of lead sulfate (PbSO₄). Additionally, peaks at 144.1 ± 0.1 eV and 148.3 ± 0.1 eV indicate the presence of lead sulfite (PbSO₃) [8]. The S 2p spectrum (Figure 2c) reveals multiple peaks, with those at 170.7 ± 0.1 eV and 169.6 ± 0.1 eV corresponding to sulfate (SO₄²⁻), and peaks at 167.2 ± 0.1 eV and 166.0 ± 0.1 eV corresponding to sulfite (SO₃²⁻) [13]. Peaks at 161.8 ± 0.1 eV and 160.7 ± 0.1 eV suggest the presence of sulfur in metallic sulfides. The Cd 3d spectrum (Figure 2d) shows peaks at 405.7 ± 0.1 eV and 412.6 ± 0.1 eV [8,31]. Since the binding energies of cadmium compounds are close, combining S 2p spectral analysis shows cadmium exists as sulfides, sulfites, and sulfates. These results match the XRD findings and highlight the challenges of extracting lead from RFD due to the various compounds that can impact recovery efficiency.

XRF, XRD, and XPS analyses reveal that RFD has a complex composition. Lead exists as both insoluble PbSO₄ and slightly soluble PbSO₃. PbSO₃ can dissolve somewhat during water washing, risking lead loss. Some cadmium compounds are hard to dissolve and usually end up as impurities in the solid phase. Given the mix of elements and compounds in RFD, advanced and efficient recovery methods are needed to improve lead recovery and ensure the final product’s quality.

2.2. Performance Comparison of Various Leaching Methods

To preliminarily verify if ultrasound and H₂O₂ addition promote lead recovery from RFD, exploratory experiments were conducted under room temperature (20 °C) conditions with a 10 min washing time and a liquid–solid ratio of 2:1, and the results are presented in Figure 3. The experimental setups included conventional water washing (CW), conventional oxidative water washing (CWO), ultrasonic water washing (UW), and ultrasonic oxidative water washing (UWO). During CWO and UWO, 5 mL of H₂O₂ was added. For UW and UWO, ultrasonic power at 120 W was applied.

Statistical significance of lead mass fraction between groups was assessed using a one-sample t-test, and each ultrasound-enhanced group was compared with the conventional group. All experiments were replicated three times and a p-value ≤ 0.05 was considered statistically significant. The differences between CW and CWO (* p ≤ 0.05) and UW and UWO (* p ≤ 0.05) were significant, and it is noteworthy that the differences between CW and UW (** p ≤ 0.01) and CWO and UWO (** p ≤ 0.01) were even more significant.

For CW, the lead mass fraction in the solid phase after washing is 48.36%. The relatively low lead mass fraction in CW may be due to two factors. Firstly, partial lead is lost to the solution, especially in the form of soluble lead compounds such as lead sulfite (PbSO₃). Secondly, certain impurities that are difficult to remove with water washing alone remain in the solid phase, diluting the lead mass fraction. When H₂O₂ is added to conventional water washing (CWO), the lead mass fraction in the solid phase increases to 55.26%. This improvement occurs because the addition of H₂O₂ promotes the oxidation of lead sulfite to lead sulfate (PbSO₄), which has lower solubility and remains in the solid phase, reducing lead loss. Moreover, H₂O₂ may convert some impurities into more soluble forms, enabling their removal from the solid phase, thereby increasing the lead mass fraction. For UW without H₂O₂, the lead mass fraction in the solid phase further increases to 57.75%. Ultrasound effectively disperses particles and prevents aggregation, improving the contact between water and solid particles and promoting the dissolution of soluble impurities. The mechanical effects of ultrasound also help in removing impurities more effectively, resulting in a higher lead mass fraction in the solid phase. The most effective method is ultrasonic water washing with H₂O₂ (UWO), which yields a lead mass fraction of 66.77%. The combination of ultrasound and H₂O₂ creates a synergistic effect [32]. The ultrasonic cavitation enhances the oxidation reaction by generating free radicals and increasing the contact efficiency between reactants [26]. This dual action of ultrasound and H₂O₂ maximizes lead retention in the solid phase and impurity removal, resulting in the highest lead mass fraction.

In summary, both H₂O₂ addition and ultrasonic application enhance lead recovery from RFD. The combination of the two proves to be the most effective approach, highlighting the necessity of studying ultrasonic oxidative water washing for improving lead recovery efficiency.

2.3. Optimization of Ultrasonic-Enhanced H₂O₂ Water Washing Conditions

To optimize the ultrasonic-enhanced H₂O₂ water washing process for lead recovery from flue dust, the effects of temperature, time, H₂O₂ dosage, liquid–solid ratio, and ultrasonic power on lead recovery efficiency were investigated, as shown in Figure 4. The lead mass fraction in the solid phase after washing was analyzed, and the lead recovery rate was calculated based on the difference in lead mass fraction in the solid phase before and after water washing.

The effect of the water washing temperature was investigated at a washing time of 10 min, a H₂O₂ dosage of 5 mL, a liquid–solid ratio of 2:1, and an ultrasonic power of 120 W, with the results shown in Figure 4a. A comparison between UWO and CWO reveals that the lead mass fraction in the solid phase initially increases then decreases with temperature, while lead recovery rate declines slowly. These trends are due to the interplay of thermodynamic and kinetic effects [33]. For UWO, at low temperatures, the limited molecular kinetic energy slows chemical reactions. As temperature rises, increased molecular collisions and energy enhance the oxidation of lead sulfite to lead sulfate by H₂O₂ and promote impurity dissolution. Consequently, the lead mass fraction in the solid phase rises with temperature, peaking at 67.53% around 40 °C due to accelerated reaction rates and improved oxidation efficiency. However, when the temperature exceeds approximately 40 °C, the lead mass fraction begins to decline. This is because higher temperatures accelerate the decomposition of H₂O₂ into water and oxygen (2H₂O₂ → 2H₂O + O₂), reducing the H₂O₂ concentration and oxidizing capacity in the solution, which leads to incomplete oxidation of lead sulfite, resulting in more lead being lost to the solution [34]. Additionally, the solubility of certain lead compounds, such as lead sulfate, increases with temperature, further contributing to lead loss in the solution. Under low temperature conditions (<50 °C), the localized high temperatures and pressures generated by ultrasonic cavitation enhance the reaction rates and oxidation efficiency, even at lower bulk temperatures. This means that UWO can achieve higher lead mass fractions at lower temperatures compared to CWO. Furthermore, the mechanical effects of ultrasonic waves help disperse particles and increase the contact efficiency between reactants, ensuring more thorough oxidation and impurity removal. However, at high temperatures (>50 °C), the lead loss is more pronounced in UWO compared to CWO. This is because the local high temperatures and pressures produced by ultrasonic cavitation can synergistically intensify the negative impact of bulk temperature on H₂O₂ decomposition and lead solubility. High temperatures also increase energy consumption. Thus, the optimal temperature is 40 °C. At this temperature, UWO achieves a lead mass fraction of 67.53% and a recovery rate of 98.03%. For CWO, the lead mass fraction is 63.57% and the recovery rate is 96.67%.

Figure 4b demonstrates the effect of water washing time on lead recovery at a washing temperature of 40 °C, a dosage of 5 mL of H₂O₂, a liquid–solid ratio of 2:1, and an ultrasonic power of 120 W. The variations in lead mass fraction between UWO and CWO at different washing times were compared. The results indicate that in the UWO process, the lead mass fraction increases slightly and tends to stabilize around 68.02% after 30 min. The increase is due to the enhanced oxidation of lead sulfite to lead sulfate and more efficient impurity removal as time progresses. The stabilization after 30 min suggests that the reaction approaches equilibrium, with most lead sulfite converted to lead sulfate and impurities dissolved. In contrast, the CWO process shows a more pronounced increase initially but starts to decline after reaching a peak of 66.17% at 30 min. This decline is likely due to the insufficient oxidizing ability of H₂O₂ over time. Without continuous oxidation, unoxidized lead sulfite remains slightly dissolved and is gradually lost to the solution as water washing time extends [35]. Additionally, prolonged water washing may enhance the dissolution of certain lead compounds, contributing to the decrease in lead mass fraction [36]. The consistent cavitation and mechanical effects of ultrasound maintain reaction efficiency, ensuring continuous oxidation and impurity removal, thereby achieving higher and more stable lead mass fraction compared to CWO. The optimized washing time is 30 min. At this point, under UWO, the lead mass fraction is 68.02% and the recovery rate is 97.43%. Under CWO, these values are 66.17% and 95.98%, respectively.

The effects of H₂O₂ dosage were examined under conditions of 40 °C washing temperature, a washing time of 30 min, a liquid–solid ratio of 2:1, and 120 W ultrasonic power. As shown in Figure 4c, the lead mass fraction in the solid phase and the lead recovery rate for both UWO and CWO increase with the dosage of H₂O₂. UWO consistently outperformed CWO, highlighting ultrasonic enhancement. Insufficient dosage leads to incomplete oxidation, causing some of the sparingly soluble lead sulfite to enter the solution, while other sparingly soluble impurities like cadmium sulfite remain in the solid phase. Both factors contribute to a lower lead mass fraction in the solid phase. As the dosage of H₂O₂ increases, its oxidizing ability also enhances, thereby promoting the conversion of lead sulfite into lead sulfate and improving the removal rate of impurities. This explains why, in both methods, the mass fraction of lead increases with the addition of more H₂O₂. The reaction primarily depends on H₂O₂ to oxidize lead sulfite and other sulfite impurities. In UWO, ultrasonic cavitation induces local high temperatures and pressures, accelerating H₂O₂ decomposition and generating more free radicals [37]. These radicals enhance the oxidation reaction, making UWO more efficient than CWO at the same H₂O₂ dosage [38]. The mechanical effects of ultrasound also improve reactant mixing and contact, further boosting oxidation efficiency. Therefore, UWO achieves a higher lead mass fraction than CWO. After a certain H₂O₂ dosage, the lead mass fraction in UWO stabilizes. Excess H₂O₂ decomposes into water and oxygen, reducing its effective concentration. At this point, the reaction rate no longer rises with additional H₂O₂. For UWO, the optimal H₂O₂ dosage is about 4 mL, achieving a lead mass fraction of 68.01% and a recovery rate of 97.11%. For CWO, the corresponding values are 64.19% and 95.89%. Beyond this dosage, increases in lead mass fraction and recovery rate are minimal, indicating diminishing returns.

The effects of the liquid–solid ratio were examined under conditions of 40 °C washing temperature, 30 min washing time, 4 mL H₂O₂ dosage, and 120 W ultrasonic power, with results presented in Figure 4d. As the liquid–solid ratio increased, the lead mass fraction initially rose and then gradually declined in both methods, with UWO consistently outperforming CWO in lead recovery efficiency. This trend can be attributed to the increased liquid–solid ratio providing a more favorable environment for ultrasonic cavitation. The additional water serves as a medium that facilitates the formation and growth of cavitation bubbles, which, when subjected to ultrasonic waves, undergo violent collapse, generating localized high temperatures and pressures [39,40]. These extreme conditions significantly augment the kinetic energy of molecules within the solution, thereby intensifying molecular collisions and interactions. The heightened molecular activity accelerates the oxidation of lead sulfite to lead sulfate and expedites the dissolution of impurities, resulting in a higher lead mass fraction in the solid phase. Furthermore, the intense mixing by ultrasonic waves disrupts particle aggregates, thereby increasing the surface area available for reaction [41]. As a result, UWO achieves superior lead recovery compared to CWO at equivalent liquid–solid ratios. After the liquid–solid ratio reaches around 2, further increases have minimal effect on the lead mass fraction. Excess water dilutes the reactants, lowering their concentration and slowing the reaction. The increased volume also enhances the solubility of unoxidized lead, reducing the lead recovery rate. Thus, the optimal liquid–solid ratio is 2. At this ratio, UWO yields a lead mass fraction of 68.03% and a recovery rate of 97.43%, while CWO gives 64.07% and 95.93%, respectively.

Figure 4e illustrates the lead mass fraction in the solid phase under UWO across varying ultrasonic powers. The lead mass fraction initially increases with ultrasonic power, reaching a maximum of 68.11% at 240 W before decreasing as power continues to rise. However, the lead recovery rate exhibits a gradual decline. This trend can be attributed to the dual effects of ultrasonic cavitation and its impact on the reaction environment. At lower ultrasonic powers, the cavitation effect is relatively mild. As the power increases, the intensity of cavitation increases, leading to more frequent and violent collapse of cavitation bubbles [32]. This generates localized high temperatures and pressures within the solution. These extreme conditions significantly enhance molecular kinetic energy, promoting more frequent and energetic molecular collisions [42,43]. This heightened molecular activity accelerates the oxidation of lead sulfite to lead sulfate and improves the dissolution of impurities, resulting in a higher lead mass fraction in the solid phase. The mechanical effects of ultrasonic waves also play a crucial role. Ultrasound induces intense mixing and breaks up particle aggregates, increasing the surface area available for reaction [44]. This enhanced contact between reactants ensures more thorough oxidation and impurity removal, further contributing to the increase in lead mass fraction. At 240 W, the ultrasonic power is optimal, maximizing the beneficial effects of cavitation and mechanical action. However, when the ultrasonic power exceeds 240 W, the lead mass fraction begins to decline. This can be attributed to several factors. First excessive ultrasonic power can lead to the over-dissolution of lead compounds. Intense cavitation at high power may enhance the dissolution of lead sulfite, enabling it to overcome the solubility barrier and enter the solution. This causes lead loss from the solid phase and explains why the lead recovery rate declines gradually. Second, high ultrasonic power can cause the premature decomposition of hydrogen peroxide. The localized high temperatures and pressures generated by intense cavitation can accelerate the decomposition of H₂O₂ into water and oxygen. This reduces the effective concentration of H₂O₂ available for the oxidation reaction, leading to incomplete oxidation of lead sulfite and a subsequent decrease in lead mass fraction. Thus, the optimal ultrasonic power is 240 W. At this condition, the lead mass fraction in UWO reaches 68.11%, with a lead recovery rate of 97.19%.

2.4. Effects of Ultrasound on Particle Agglomeration and Encapsulation in Lead Recovery

The SEM-EDS and particle size analysis were conducted on the solid phases of RFD, CWO, and UWO under optimal conditions, and the results are shown in Figure 5 and Figure 6, respectively.

Figure 5a reveals the complex morphology of RFD, where particles aggregate and accumulate with one another to form complex structures. These aggregated structures result in impurities (e.g., cadmium) being encapsulated inside. CWO (Figure 5b) relies on mechanical agitation, which is insufficient to effectively penetrate and disperse these complex structures. A large number of impurities remain entrapped inside, limiting their exposure during oxidation or dissolution, resulting in a lower lead mass fraction in the final lead residue. Ultrasound introduces mechanical and cavitation effects that can significantly enhance the water washing process. The mechanical energy generated by ultrasound disrupts particle agglomeration and breaks them into smaller, more homogeneous particles. This is evident in the SEM-EDS images of UWO-treated samples (Figure 5c), which show a more dispersed and less aggregated particle morphology compared to CWO. The cavitation effect further contributes to this phenomenon by generating localized shock waves and micro-jets that effectively disperse the aggregated particles and expose the impurities encapsulated inside, which makes them more susceptible to chemical reactions and removal during the water washing process.

Laser diffractometer was used to determine the particle size distribution of different samples, each sample was repeated three times and averaged, the results are shown in Figure 6. Particle size distribution analysis further illustrates the intensifying effect of UWO. The D₁₀, D₅₀, and D₉₀ of CWO were 0.877, 4.81, and 106 µm, respectively, which suggests that mechanical agitation not only fails to effectively disperse particles but also increases the particle size and significantly widens the distribution of particle sizes (0.3~110 µm). This means aggravated particle aggregation. Wang et al. [45], in their study on the replacement of high concentration of cadmium by aluminum powder, found that conventional stirring could not effectively disperse the aluminum powder but instead promoted the degree of aggregation. The D₁₀, D₅₀, and D₉₀ of UWO were 0.735, 2.53, and 6.91 µm, respectively. This indicates that UWO not only reduces the average particle size but also narrows the particle size distribution, resulting in a more homogeneous particle population and lower degree of aggregation. The smaller and more homogeneous particles in the UWO-treated samples resulted in a larger surface area available for chemical interaction, which led to increased reactivity and improved lead recovery efficiency and purity.

2.5. Phase Transition Under Ultrasound

To further demonstrate the mechanisms of sonication enhancement, XPS was performed on the raw material and the solids from UWO and CWO, with the results shown in Figure 7.

The Pb 4f spectra (Figure 7b,f) reveal that the UWO exhibits more pronounced and sharper peaks at 139.2 ± 0.1 eV (Pb 4f 7/2) and 143.3 ± 0.1 eV (Pb 4f 5/2), corresponding to the typical signals of PbSO₄. These peaks are significantly purer and more distinct in UWO than in CWO, indicating a more thorough transformation of lead species into PbSO₄. In contrast, CWO shows additional peaks at 144.1 ± 0.1 eV and 148.3 ± 0.1 eV, corresponding to PbSO₃, which are significantly weakened in UWO. This confirms that UWO effectively promotes the oxidation of PbSO₃ to PbSO₄, minimizing lead loss during water washing. The S 2p spectra (Figure 7c,g) further confirm this phase transformation. In UWO, the signals for metal sulfides (S 2p 1/2 at 161.8 ± 0.1 eV and S 2p 3/2 at 160.7 ± 0.1 eV) are substantially weakened compared to CWO. This indicates a significant reduction in sulfide species, consistent with their oxidation to sulfates under UWO conditions. This aligns with the transformation of metal sulfides like CdS to more oxidized forms such as CdSO₄, which are more water-soluble and can be efficiently removed during water washing. The Cd 3d spectra (Figure 7d,h) also reflect the beneficial effects of UWO, with markedly reduced Cd signals compared to CWO. This suggests a decrease in cadmium-containing compounds in the solid phase after UWO treatment, likely due to their oxidation to more soluble forms like CdSO₄, which can be effectively removed during the water washing stage.

EPR analysis (Figure 8) provides further insight into the oxidation mechanism of UWO. EPR, or Electron Paramagnetic Resonance, is a technique used to detect and quantify free radicals. The sample (UWO, CWO) was mixed with DMPO, and analysis was conducted within 2 min of mixing to capture transient •OH radicals, forming a stable DMPO-OH adduct. This adduct exhibits a characteristic 1:2:2:1 quadruplet EPR spectrum. The EPR spectrum of UWO shows distinct signals corresponding to DMPO-OH radicals, indicating the generation of hydroxyl radicals during the process. These highly reactive •OH radicals, stronger oxidants than H₂O₂ alone, are likely formed by the ultrasonic cavitation effect on H₂O₂. They play a crucial role in driving the oxidation of PbSO₃ to PbSO₄ and transforming Cd-containing compounds into more soluble forms, thereby enhancing their removal.

2.6. Analysis of the Final Solid Phase

To evaluate the quality of solids obtained from different treatments, a series of analyses were conducted on the solids from optimized CWO and UWO processes.

XRF analysis (

Table 2. Comparison of the content of different elements in different samples (wt. %).

Element	Raw Flue Dust	CWO	UWO
Pb	41.68	64.02	67.83
Cd	14.33	2.43	0.23
S	7.1	10.03	9.88
Cl	1.07	0.08	0.08
Zn	1.02	0.05	0.05
Fe	1	0.07	0.05
Bi	0.9	0.75	0.02
Sb	0.8	-	-
Tl	0.7	0.25	0.02
Cu	0.6	0.41	0.03
As	0.5	-	-
Si	0.3	0.16	0.11
Ca	0.3	0.18	0.03
Se	0.3	0.12	0.02
Na	0.2	0.08	0.02
Ag	0.1	0.09	0.01
Sn	0.1	-	-
Mn	0.08	-	-
Al	0.06	0.04	0.01
Cs	0.06	-	-
Br	0.04	-	0.01
Hg	0.03	-	-

) shows that UWO significantly boosts the lead mass fraction in the solid phase to 66.83%, up from 41.68% in RFD. Simultaneously, the cadmium mass fraction drops sharply from 14.33% to 0.23%. Similar reductions are observed for other elements like sulfur, zinc, iron, and various trace elements. This indicates UWO’s high effectiveness in selectively enriching lead and removing multiple impurities, which is critical for enhancing the quality of the lead residue.

XRD analysis (Figure 9) further confirms the phase structure of the UWO-treated solid. Its XRD pattern closely matches the standard PDF card for PbSO₄ (PDF#97-009-2609), with well-defined peaks indicating high-purity crystalline PbSO₄. The absence of significant peaks from other phases, such as PbSO₃ or cadmium compounds, demonstrates that UWO effectively transforms PbSO₃ to PbSO₄ and removes cadmium impurities.

Figure 10 presents the Raman spectral data for three solid phases. In the raw RFD spectrum, a prominent peak is observed in the 980–1090 cm⁻¹ range. This peak corresponds to sulfate ions (SO₄²⁻) in compounds like PbSO₄ and CdSO₄ [40]. It indicates a high content of sulfate compounds in the raw material. Additionally, a broad peak in the 400–600 cm⁻¹ region is attributed to metal–oxygen stretching vibrations in various metal sulfates, reflecting a diverse array of sulfate compounds with different metal–oxygen bonding environments. After CWO, the Raman spectrum shows an increased intensity of the peak in the 980–1090 cm⁻¹ range compared to raw RFD. This suggests a relative increase in the concentration of sulfate compounds such as PbSO₄. The elevated intensity implies that the CWO process aids in converting or retaining more lead in the low-solubility sulfate form, which is beneficial for lead recovery. However, the broad peak in the 400–600 cm⁻¹ region remains largely unchanged, indicating that multiple sulfate compounds are still present, which aligns with the higher impurity content in CWO. The Raman spectrum of the UWO sample shows the most significant changes. The peak intensity in the 980–1090 cm⁻¹ ranges increases substantially compared to both raw RFD and CWO samples. This indicates that UWO effectively promotes the formation of sulfate compounds like PbSO₄. Moreover, the broad peak in the 400–600 cm⁻¹ region becomes narrower and sharper in the UWO spectrum. This narrowing implies a reduction in the variety of metal sulfate compounds and a more uniform sulfate phase, consistent with previous analyses that UWO enhances lead retention in the solid phase as PbSO₄ and improves impurity removal, thereby boosting lead recovery.

3. Experimental

3.1. Materials

The raw flue dust used in this study was bottom-blown furnace flue dust from a lead smelting enterprise in Henan Province, China, and the H₂O₂ used was of analytically pure grade (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). In these experiments, all solutions were prepared using deionized water.

3.2. Experimental Procedure

A total of 50 g of RFD was weighed and placed in a 200 mL beaker with some deionized water, then the beaker was sealed with plastic wrap. An ultrasonic probe (20 kHz, max 1800 W) was positioned vertically in the center of the beaker, 1–1.5 cm above the bottom. The beaker was placed in a water bath and stirred magnetically at 200 rpm. During leaching, H₂O₂ solution was added at a rate of 10 mL/min. Experiments were conducted under different conditions by adjusting parameters like temperature, time, liquid–solid ratio, ultrasonic power (0 W for conventional water washing), and oxidant dosage. After each experiment, the mixture was transferred to a suction flask and vacuum filter to separate the leachate and residue, which were then characterized with various analytical techniques.

3.3. Analytical Methods

The element content of the samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5110, Santa Clara, CA, USA). The physical phases of different samples were studied using a powder X-ray diffractometer (XRD, Rigaku dX 2000, Zhaodao, Japan) with an operating voltage of 40 kV and an operating current of 25 mA, using Cu-Kα as the ray source. The scanning angle of the XRD ranged from 10 to 80°, and the scanning speed was 2°/min. The XRD data were also analyzed using MDI Jade v9.0 software. X-ray photoelectron spectroscopy (XPS, PHI-5300, Chanhassen, MN, USA) was used to analyze the binding energy of the different elements in the samples, with an operating voltage of 12 kV and an operating current of 6 mA, using Al-Kα as the ray source. The results detected by the XPS instrument were analyzed using Avantage v5.9931 software and calibrated at the C1s peak position of 284.8 eV. Qualitative elemental analysis was performed using an X-ray fluorescence spectrometer (XRF, Rigaku ZSX Primus, Akishima, Japan). The laser used was a 532, with a laser energy of 6.0 mW, a 600 g/mm grating, an Olympus 50× objective (Olympus 50×/0.5), an integration time of 35 s, and an accumulation number of 6 times. A scanning electron microscope–energy dispersive spectrometer (SEM-EDS, Zeiss Sigma 300, Jena, Germany) was used to study the micromorphology and elemental distribution of different samples. Particle size and distribution of the samples were measured using a laser particle size analyzer (Malvern Mastersizer 3000+ Ultra, Malvern, UK), with ethanol used as a dispersant. A Raman analyzer (WITec alpha 300R, Ulm, Germany) was used to determine the chemical structure of different samples. Electron paramagnetic resonance (EPR, JEOL JES-FA200, Akishima, Japan) analysis was performed using 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) as the spin trapping agent. The instrument parameters wre set as follows: operating frequency: 9182 MHz; central magnetic field: 327.0 mT; power: 0.998 mW; scan width: ±5 mT; modulation amplitude: 1.0 G; scan time: 30 s.

The calculation formula for lead recovery rate is as follows:

$$\mathrm{L}\mathrm{e}\mathrm{a}\mathrm{d} \mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e} (\%)={\displaystyle \raisebox{1ex}{${\omega }_t\times m_t$}\!\left/ \!\raisebox{-1ex}{${\omega }_0\times m_0$}\right.}\times 100\%$$

(1)

where ${\omega }_t$ represents the mass fraction of lead in the solid phase at the end of the reaction; $m_t$ represents the mass of the solid phase at the end of the reaction; ${\omega }_0$ represents the mass fraction of lead in the raw flue dust; and $m_0$ represents the mass of the raw flue dust taken for each experiment.

4. Conclusions

In this study, a method for ultrasonic-enhanced hydrogen peroxide water washing to recover lead from RFD under neutral conditions was developed. The key conclusions are as follows:

(1)

The optimized conditions for ultrasonic-enhanced hydrogen peroxide water washing to recover lead from RFD under neutral conditions were determined as follows: a washing temperature of 40 °C, a washing time of 30 min, an H₂O₂ dosage of 4 mL, a liquid–solid ratio of 2:1, and an ultrasonic power of 240 W. Under these conditions, the lead mass fraction in the UWO-treated lead residue reached 68.11% and a lead recovery rate of 97.19%, significantly higher than that obtained from CWO (lead mass fraction of 64.07%, lead recovery rate of 95.93%).

(2)

Ultrasound enhances lead recovery through two key mechanisms: inhibiting aggregation and unwrapping encapsulated particles, promoting phase transformation via hydroxyl radical generation. Its cavitation and mechanical effects break up particle agglomerates, exposing impurities for removal. Meanwhile, ultrasound action on H₂O₂ generates hydroxyl radicals, boosting oxidation to transform PbSO₃ into PbSO₄ and Cd compounds into more soluble forms like CdSO₄. These synergistic effects improve recovery efficiency and the lead mass fraction in the solid phase after washing.

(3)

The UWO process significantly improves the purity and quality of the final lead residue. The lead mass fraction increased significantly, and major impurities such as cadmium were significantly reduced. The treated solids consisted mainly of highly crystalline PbSO₄ with reduced formation of other sulfates, resulting in a more homogeneous and lead enrichment.

Ultrasonic technology can significantly improve the efficiency of lead recovery from RFD through a combination of physical destruction and chemical oxidation. This highly efficient process offers a more environmentally friendly and efficient alternative to traditional methods, providing insights into moving the metallurgical industry towards greater sustainability.