*3.3. Recovery of Extracted Pb2+ as Zero-Valent Pb by Cementation*

*3.3. Recovery of Extracted Pb*2+ *as Zero-Valent Pb by Cementation*  After EDTA pretreatment, the recovery of Pb2+ from the leachate will not only add economic value but also protect the environment. Cementation is one of the effective methods to recover metal ions as zero-valent metals [41–45]. A general cementation reaction is illustrated in Equation (4): a metal (A0) gives electrons to metal ion (Bb+), driven by After EDTA pretreatment, the recovery of Pb2+ from the leachate will not only add economic value but also protect the environment. Cementation is one of the effective methods to recover metal ions as zero-valent metals [41–45]. A general cementation reaction is illustrated in Equation (4): a metal (A<sup>0</sup> ) gives electrons to metal ion (Bb+), driven by the difference in standard redox potentials of the interacting metals and their ions, and as a result, Bb+ is deposited on the surface of A<sup>0</sup> as B<sup>0</sup> [46–49]:

the difference in standard redox potentials of the interacting metals and their ions, and as

**Figure 3.** (**a**) Extraction efficiency of PbSO4 after 24 h with different concentrations of EDTA (100, 200, or 500 mM) and (**b**) effect of time on PbSO4 extraction with 500 mM EDTA. Note that the dotted

$$n\text{ A}^{0} + m\text{ B}^{\text{b}+} \rightarrow n\text{ A}^{\text{a}+} + m\text{ B}^{0} \text{ (}na = mb\text{)}\tag{4}$$

*n* A0 + *m* Bb+ → *n* Aa+ + m B0 (*na = mb*) (4) To recover the extracted Pb2+ as zero-valent Pb (Pb0) from the EDTA leachate, cementation experiments were carried out using ZVI powder as a reductant after removing DO from the solution by purging with ultrapure nitrogen gas. Figure 5a shows the effects of the ZVI amount on the recovery of the extracted Pb2+ from the leachate after EDTA pretreatment using 500 mM EDTA for 30 min. When the amount of ZVI was 0.5 g/10 mL, ~40% of Pb2+ was recovered, although ~0.18 g/10 mL ZVI was stoichiometrically sufficient to recover all Pb2+ in the solution. As the cementation reaction progressed, the surface of To recover the extracted Pb2+ as zero-valent Pb (Pb<sup>0</sup> ) from the EDTA leachate, cementation experiments were carried out using ZVI powder as a reductant after removing DO from the solution by purging with ultrapure nitrogen gas. Figure 5a shows the effects of the ZVI amount on the recovery of the extracted Pb2+ from the leachate after EDTA pretreatment using 500 mM EDTA for 30 min. When the amount of ZVI was 0.5 g/10 mL, ~40% of Pb2+ was recovered, although ~0.18 g/10 mL ZVI was stoichiometrically sufficient to recover all Pb2+ in the solution. As the cementation reaction progressed, the surface of ZVI was covered with cementation products (i.e., Pb<sup>0</sup> ), which hindered a further cementation reaction. This was probably the reason why 1 g/10 mL of ZVI was required to recover all Pb2+ from the EDTA leachate. *Minerals* **2022**, *12*, x FOR PEER REVIEW 8 of 15

**Figure 5.** (**a**) Effects of the ZVI amount on the recovery of the extracted Pb after 24 h in the leachate after PbSO4 extraction using 500 mM of EDTA for 30 min and (**b**) effects of time on the recovery of the extracted Pb2+ using 1 g/10 mL of ZVI in the leachate after EDTA pretreatment using 500 mM EDTA for 30 min. **Figure 5.** (**a**) Effects of the ZVI amount on the recovery of the extracted Pb after 24 h in the leachate after PbSO<sup>4</sup> extraction using 500 mM of EDTA for 30 min and (**b**) effects of time on the recovery of the extracted Pb2+ using 1 g/10 mL of ZVI in the leachate after EDTA pretreatment using 500 mM EDTA for 30 min.

Figure 5b shows the cementation results of Pb2+ by ZVI in time. As can be seen, almost no cementation occurred for 3 h, and then Pb2+ started being cemented gradually. The

Pb(EDTA)2– + Fe0 = Pb0 +EDTA + Fe2+ (5)

Pb(EDTA)2– +2e– = Pb0 + EDTA4– (6)

Fe0 = Fe2+ + 2e– (7)

of free EDTA that redissolved the cemented Pb0. After 3 h, the concentration of free EDTA ligands was most likely decreased due to its consumption in dissolving Fe oxide films and/or in forming complexes with Fe species released by the cementation reaction. It is important to note that after 24 h, ~97% cementation was achieved. To confirm the recovery of Pb2+ on ZVI, the residue obtained from the cementation experiment using 1 g/10 mL of ZVI for 24 h was analyzed using SEM-EDS. As shown in Figure 6, the deposition of Pb compounds on ZVI powder was observed, suggesting that the extracted Pb2+ was reduc-

**Figure 6.** SEM photomicrograph of the residue of the cementation experiment using 1 g/10 mL of

Equation (5) consists of two half-cell reactions [50]:

ZVI for 24 h with the corresponding elemental maps of Fe, Pb, and O.

tively deposited via the cementation reaction:

Figure 5b shows the cementation results of Pb2+ by ZVI in time. As can be seen, almost no cementation occurred for 3 h, and then Pb2+ started being cemented gradually. The initial slow rate of Pb cementation can have two reasons: (i) the presence of Fe oxide films formed on the ZVI surface that limited the cementation of Pb2+ and (ii) an excess amount of free EDTA that redissolved the cemented Pb<sup>0</sup> . After 3 h, the concentration of free EDTA ligands was most likely decreased due to its consumption in dissolving Fe oxide films and/or in forming complexes with Fe species released by the cementation reaction. It is important to note that after 24 h, ~97% cementation was achieved. To confirm the recovery of Pb2+ on ZVI, the residue obtained from the cementation experiment using 1 g/10 mL of ZVI for 24 h was analyzed using SEM-EDS. As shown in Figure 6, the deposition of Pb compounds on ZVI powder was observed, suggesting that the extracted Pb2+ was reductively deposited via the cementation reaction: ligands was most likely decreased due to its consumption in dissolving Fe oxide films and/or in forming complexes with Fe species released by the cementation reaction. It is important to note that after 24 h, ~97% cementation was achieved. To confirm the recovery of Pb2+ on ZVI, the residue obtained from the cementation experiment using 1 g/10 mL of ZVI for 24 h was analyzed using SEM-EDS. As shown in Figure 6, the deposition of Pb compounds on ZVI powder was observed, suggesting that the extracted Pb2+ was reductively deposited via the cementation reaction: Pb(EDTA)2– + Fe0 = Pb0 +EDTA + Fe2+ (5) Equation (5) consists of two half-cell reactions [50]: Pb(EDTA)2– +2e– = Pb0 + EDTA4– (6)

**Figure 5.** (**a**) Effects of the ZVI amount on the recovery of the extracted Pb after 24 h in the leachate after PbSO4 extraction using 500 mM of EDTA for 30 min and (**b**) effects of time on the recovery of the extracted Pb2+ using 1 g/10 mL of ZVI in the leachate after EDTA pretreatment using 500 mM

Figure 5b shows the cementation results of Pb2+ by ZVI in time. As can be seen, almost no cementation occurred for 3 h, and then Pb2+ started being cemented gradually. The initial slow rate of Pb cementation can have two reasons: (i) the presence of Fe oxide films formed on the ZVI surface that limited the cementation of Pb2+ and (ii) an excess amount of free EDTA that redissolved the cemented Pb0. After 3 h, the concentration of free EDTA

$$\text{Pb(EDTA)}^{2-} + \text{Fe}^{0} = \text{Pb}^{0} + \text{EDTA} + \text{Fe}^{2+} \tag{5}$$

EDTA for 30 min.

Equation (5) consists of two half-cell reactions [50]:

*Minerals* **2022**, *12*, x FOR PEER REVIEW 8 of 15

$$\text{Pb(EDTA)}^{2-} + 2\text{e}^- = \text{Pb}^0 + \text{EDTA}^{4-} \tag{6}$$

$$\mathbf{F}\mathbf{e}^0 = \mathbf{F}\mathbf{e}^{2+} + \mathbf{2}\mathbf{e}^-\tag{7}$$

The above-mentioned residue was also analyzed by XRD to further characterize Pb compounds formed on ZVI (Figure 7). As shown in the XRD pattern, the peaks of Fe<sup>0</sup> , Fe2O3, Pb<sup>0</sup> , and PbO were detected, indicating that Pb2+ was reductively deposited as Pb<sup>0</sup> on the surface of ZVI, as illustrated in Equation (5). The presence of PbO and Fe2O<sup>3</sup> was due most likely to the oxidation of Pb<sup>0</sup> and ZVI surface during drying and storing of the sample [51–53]. The above-mentioned residue was also analyzed by XRD to further characterize Pb compounds formed on ZVI (Figure 7). As shown in the XRD pattern, the peaks of Fe0, Fe2O3, Pb0, and PbO were detected, indicating that Pb2+ was reductively deposited as Pb0 on the surface of ZVI, as illustrated in Equation (5). The presence of PbO and Fe2O3 was due most likely to the oxidation of Pb0 and ZVI surface during drying and storing of the sample [51–53].

pretreatment was applied.

h.

**Figure 7.** XRD pattern of the residue of the cementation experiment using 1 g/10 mL of ZVI for 24 **Figure 7.** XRD pattern of the residue of the cementation experiment using 1 g/10 mL of ZVI for 24 h.

of EDTA pretreatment on ZnS depression in flotation were investigated. Flotation experiments using model samples (ZnS/PbSO4 mixture) with and without EDTA pretreatment were conducted (Figure 8). Prior to the flotation experiments, the mixture of 15 g ZnS and 5 g PbSO4 was treated with 500 mM of EDTA for 30 min (solid/liquid ratio: 20 g/200 mL), and ~99.8% of PbSO4 was extracted. The cementation experiments using 1 g/10 mL of ZVI for 24 h were also carried out to recover the extracted Pb2+ in the filtrate obtained after EDTA pretreatment , and ~99.7% of the extracted Pb2+ was recovered. Meanwhile, the residue obtained after EDTA pretreatment was deslimed and repulped to 400 mL with distilled water in the flotation cell, and then the flotation experiments were conducted with ~5 kg/t of ZnSO4 (100 ppm Zn2+) and 1 kg/t of Na2SO3 as depressants at pH 6.5. As shown in Figure 8, the floatability of Zn was clearly depressed from ~82% to ~30% when EDTA

**Figure 8.** Effects of EDTA pretreatment on the floatability of ZnS in the presence of PbSO4.

#### *3.4. Effects of EDTA Pretreatment on ZnS Depression in Flotation 3.4. Effects of EDTA Pretreatment on ZnS Depression in Flotation*

*Minerals* **2022**, *12*, x FOR PEER REVIEW 9 of 15

sample [51–53].

h.

The above-mentioned residue was also analyzed by XRD to further characterize Pb compounds formed on ZVI (Figure 7). As shown in the XRD pattern, the peaks of Fe0, Fe2O3, Pb0, and PbO were detected, indicating that Pb2+ was reductively deposited as Pb0 on the surface of ZVI, as illustrated in Equation (5). The presence of PbO and Fe2O3 was due most likely to the oxidation of Pb0 and ZVI surface during drying and storing of the

As described in Section 3.2, almost all PbSO<sup>4</sup> was extracted with EDTA, so the effects of EDTA pretreatment on ZnS depression in flotation were investigated. Flotation experiments using model samples (ZnS/PbSO<sup>4</sup> mixture) with and without EDTA pretreatment were conducted (Figure 8). Prior to the flotation experiments, the mixture of 15 g ZnS and 5 g PbSO<sup>4</sup> was treated with 500 mM of EDTA for 30 min (solid/liquid ratio: 20 g/200 mL), and ~99.8% of PbSO<sup>4</sup> was extracted. The cementation experiments using 1 g/10 mL of ZVI for 24 h were also carried out to recover the extracted Pb2+ in the filtrate obtained after EDTA pretreatment, and ~99.7% of the extracted Pb2+ was recovered. Meanwhile, the residue obtained after EDTA pretreatment was deslimed and repulped to 400 mL with distilled water in the flotation cell, and then the flotation experiments were conducted with ~5 kg/t of ZnSO<sup>4</sup> (100 ppm Zn2+) and 1 kg/t of Na2SO<sup>3</sup> as depressants at pH 6.5. As shown in Figure 8, the floatability of Zn was clearly depressed from ~82% to ~30% when EDTA pretreatment was applied. As described in Section 3.2, almost all PbSO4 was extracted with EDTA, so the effects of EDTA pretreatment on ZnS depression in flotation were investigated. Flotation experiments using model samples (ZnS/PbSO4 mixture) with and without EDTA pretreatment were conducted (Figure 8). Prior to the flotation experiments, the mixture of 15 g ZnS and 5 g PbSO4 was treated with 500 mM of EDTA for 30 min (solid/liquid ratio: 20 g/200 mL), and ~99.8% of PbSO4 was extracted. The cementation experiments using 1 g/10 mL of ZVI for 24 h were also carried out to recover the extracted Pb2+ in the filtrate obtained after EDTA pretreatment , and ~99.7% of the extracted Pb2+ was recovered. Meanwhile, the residue obtained after EDTA pretreatment was deslimed and repulped to 400 mL with distilled water in the flotation cell, and then the flotation experiments were conducted with ~5 kg/t of ZnSO4 (100 ppm Zn2+) and 1 kg/t of Na2SO3 as depressants at pH 6.5. As shown in Figure 8, the floatability of Zn was clearly depressed from ~82% to ~30% when EDTA pretreatment was applied.

**Figure 7.** XRD pattern of the residue of the cementation experiment using 1 g/10 mL of ZVI for 24

**Figure 8.** Effects of EDTA pretreatment on the floatability of ZnS in the presence of PbSO4. **Figure 8.** Effects of EDTA pretreatment on the floatability of ZnS in the presence of PbSO<sup>4</sup> .

To confirm the depression of ZnS floatability after EDTA pretreatment, a pretreated sample reacted with zinc sulfate for 3 min, a ~5 mL aliquot of the pulp, was collected, and the filtrate and the residue were analyzed by ICP-AES and XPS, respectively. The concentration of Pb2+ in the filtrate without EDTA pretreatment was 5.3 ppm; it decreased to 0.2 ppm with EDTA pretreatment. On the other hand, the concentrations of Zn2+ in the filtrates with and without EDTA pretreatment were 101 and 99.4 ppm, respectively. Based on thermodynamics calculations using Equation (8), the possibility of lead activation of ZnS (Equation (1)) could be evaluated using the measured values of Pb2+ and Zn2+ [17,54,55]. In this study, the changes in free energy were calculated using measured values of Pb2+ and Zn2+ concentrations.

Table 2 shows the calculated results of the changes in free energy based on the equilibrium constants of lead activation of ZnS (Equation (1)). The change in free energy without EDTA pretreatment was negative, while that with EDTA pretreatment was positive. When the change in free energy is positive, the reverse reaction of lead activation (Equation (1)) would occur spontaneously, indicating that lead activation would be limited by EDTA pretreatment. These results support the flotation results that the depression of ZnS floatability was achieved by the pretreatment, which decreased Pb2+ concentration during flotation by extracting PbSO<sup>4</sup> in advance.

$$
\Delta G = -RT\ln K + \ln\left(Z\mathbf{n}^{2+}/\mathbf{Pb}^{2+}\right) \tag{8}
$$

$$K = \frac{K\_{sp}^{ZnS}}{K\_{sp}^{PbS}} \tag{9}$$


**Table 2.** Calculation results of the change in free energy with and without EDTA pretreatment.

<sup>a</sup> The equilibrium constant was obtained from a previous study [54]. b, c, d The equilibrium constants were calculated using Equation (9) by the reported *Ksp* values of *ZnS* and *PbS*, respectively [56–58].

To confirm whether the lead activation of sphalerite was limited by the extraction of PbSO<sup>4</sup> using EDTA, untreated ZnS as well as ZnSO4-pretreated ZnS/PbSO<sup>4</sup> mixtures with and without EDTA pretreatment were analyzed by XPS. The Pb4f7/2 core-level spectra of the samples are shown in Figure 9, and the corresponding curve-fitting parameters are summarized in Table 3. Since the Zn3s peak overlapped with the Pb4f7/2 signals, the spectrum was resolved by curve fitting to subtract the area due to the Zn3s peak [55,59]. As illustrated in Figure 9b and c, the deconvoluted XPS spectra of the residues with and without PbSO<sup>4</sup> extraction showed two types of Pb species: (1) Pb2+—S of PbS (143.6 and 138.6 eV) and (2) Pb2+—SO<sup>4</sup> of PbSO<sup>4</sup> (144.7 and 139.8 eV), while the deconvoluted XPS spectrum of untreated ZnS showed no Pb species (Figure 9a) [26,60]. The decrease in the intensity of Pb2+—SO<sup>4</sup> was in a good agreement with the extraction efficiency of PbSO<sup>4</sup> using EDTA for the ZnS/PbSO<sup>4</sup> mixture (~99.8%). When EDTA pretreatment was applied prior to the flotation experiments, the peak intensity ratio of PbS/ZnS decreased from 2.3 to 0.2, indicating that EDTA pretreatment was effective in limiting lead activation of ZnS. This is in line with the calculated results of the change in free energy based on thermodynamics (Table 2). *Minerals* **2022**, *12*, x FOR PEER REVIEW 11 of 15

**Figure 9.** XPS Pb4f7/2 spectra of (**a**) raw ZnS, (**b**) ZnS/PbSO4 with EDTA pretreatment, and (**c**) ZnS/PbSO4 without EDTA pretreatment. **Figure 9.** XPS Pb4f7/2 spectra of (**a**) raw ZnS, (**b**) ZnS/PbSO<sup>4</sup> with EDTA pretreatment, and (**c**) ZnS/PbSO<sup>4</sup> without EDTA pretreatment.

**Table 3.** XPS peak parameters for Pb4f7/2 spectra and relative abundances of Pb species.

139.8 ± 0.05 1.7 PbSO4 0 8.8 48.4 144.7 ± 0.05

139.8 ± 0.05 3.4 ZnS 100 80.5 9.5 a The binding energies of photoelectrons were calibrated using C1s (285 eV) for charge correction. b The binding energies of photoelectrons were calibrated using Zn2p3/2 (1022.0 eV) for charge correc-

The results of this study have significant implications not only for understanding

Based on the findings of this study, a sustainable process flowsheet for complex sulfide ores is proposed (Figure 10). In this proposed flowsheet, PbSO4 is first extracted by EDTA pretreatment before flotation. The residue obtained after EDTA pretreatment is fed to a flotation stage where ZnS floatability is effectively depressed by the conventional depressant for ZnS due to the decrease in the ratio of Pb2+/Zn2+ of the flotation pulp, and then gangue minerals like SiO2 and FeS2 would be disposed of into a tailings dam after the

how the presence of PbSO4 in complex sulfide ores affects the floatability of ZnS but also for proposing a sustainable process flowsheet covering both the improved flotation separation of complex sulfide ores and the detoxification of solid/solution wastes contaminated with Pb species. Firstly, the presence of PbSO4 might have a detrimental impact on the flotation separation of complex sulfide ores due to unwanted lead activation of ZnS that improves its floatability, which could not be depressed with ZnSO4—the conventional depressant for ZnS. Secondly, EDTA washing could extract almost all PbSO4 from the ZnS/PbSO4 mixture, and consequently, the floatability of ZnS decreased due to the limited amount of PbS-like compounds formed on the ZnS surface by lead activation. Finally, cementation using ZVI could recover Pb2+ extracted from PbSO4 during EDTA pretreatment as Pb0, which will not only add economic value but also protect the environ-

**Contents (at.%)** 

**Pretreatment** 

**W/o EDTA <sup>a</sup> Pretreatment** 

**Untreated <sup>a</sup> W/ EDTA <sup>b</sup>**

**(eV) FWHM Assignments** 

**Binding Energy** 

*3.5. Implication of This Study* 

tion.

ment.


**Table 3.** XPS peak parameters for Pb4f7/2 spectra and relative abundances of Pb species.

<sup>a</sup> The binding energies of photoelectrons were calibrated using C1s (285 eV) for charge correction. <sup>b</sup> The binding energies of photoelectrons were calibrated using Zn2p3/2 (1022.0 eV) for charge correction.

#### *3.5. Implication of This Study*

The results of this study have significant implications not only for understanding how the presence of PbSO<sup>4</sup> in complex sulfide ores affects the floatability of ZnS but also for proposing a sustainable process flowsheet covering both the improved flotation separation of complex sulfide ores and the detoxification of solid/solution wastes contaminated with Pb species. Firstly, the presence of PbSO<sup>4</sup> might have a detrimental impact on the flotation separation of complex sulfide ores due to unwanted lead activation of ZnS that improves its floatability, which could not be depressed with ZnSO4—the conventional depressant for ZnS. Secondly, EDTA washing could extract almost all PbSO<sup>4</sup> from the ZnS/PbSO<sup>4</sup> mixture, and consequently, the floatability of ZnS decreased due to the limited amount of PbS-like compounds formed on the ZnS surface by lead activation. Finally, cementation using ZVI could recover Pb2+ extracted from PbSO<sup>4</sup> during EDTA pretreatment as Pb<sup>0</sup> , which will not only add economic value but also protect the environment.

Based on the findings of this study, a sustainable process flowsheet for complex sulfide ores is proposed (Figure 10). In this proposed flowsheet, PbSO<sup>4</sup> is first extracted by EDTA pretreatment before flotation. The residue obtained after EDTA pretreatment is fed to a flotation stage where ZnS floatability is effectively depressed by the conventional depressant for ZnS due to the decrease in the ratio of Pb2+/Zn2+ of the flotation pulp, and then gangue minerals like SiO<sup>2</sup> and FeS<sup>2</sup> would be disposed of into a tailings dam after the recovery of ZnS with the assistance of activators (e.g., CuSO4). Meanwhile, the leachate obtained from EDTA pretreatment is rich in Pb2+ that can be recovered via cementation using ZVI as Pb<sup>0</sup> . To keep up with the high demand for critical metals following the SDGs, mine developments should be in harmony with the environment. This proposed flowsheet can achieve enhanced selective flotation of complex sulfide ores (depression of ZnS floatability) while preventing lead pollution to the surrounding environment of tailings dams (removal of toxic PbSO<sup>4</sup> before flotation) and maximizing the recovery of critical elements by cementation (recovery of Pb<sup>0</sup> using ZVI). *Minerals* **2022**, *12*, x FOR PEER REVIEW 12 of 15 recovery of ZnS with the assistance of activators (e.g., CuSO4). Meanwhile, the leachate obtained from EDTA pretreatment is rich in Pb2+ that can be recovered via cementation using ZVI as Pb0. To keep up with the high demand for critical metals following the SDGs, mine developments should be in harmony with the environment. This proposed flowsheet can achieve enhanced selective flotation of complex sulfide ores (depression of ZnS floatability) while preventing lead pollution to the surrounding environment of tailings dams (removal of toxic PbSO4 before flotation) and maximizing the recovery of critical elements by cementation (recovery of Pb0 using ZVI).

**Figure 10.** Proposed enhanced depression of sphalerite (ZnS) by extracting anglesite (PbSO4) using ethylene diamine tetra acetic acid (EDTA) and recovery of the extracted Pb2+ as zero-valent Pb (Pb0) by cementation using zero-valent Fe (ZVI). **4. Conclusions Figure 10.** Proposed enhanced depression of sphalerite (ZnS) by extracting anglesite (PbSO<sup>4</sup> ) using ethylene diamine tetra acetic acid (EDTA) and recovery of the extracted Pb2+ as zero-valent Pb (Pb<sup>0</sup> ) by cementation using zero-valent Fe (ZVI).

2. The conventional depressants for ZnS, zinc sulfate and sodium sulfite, were not effective in depressing the floatability of ZnS when PbSO4 was present. 3. Almost of all PbSO4 (>97%) was extracted using EDTA, and >97% of the extracted

4. A pretreatment of flotation extracting PbSO4 using EDTA was effective in depressing

5. The proposed method for complex sulfide ores containing PbSO4, a combination of extraction of PbSO4 using EDTA and recovery of extracted Pb2+ as zero-valent Pb by cementation using ZVI, could achieve enhanced selective flotation of complex sulfide ores (depression of ZnS floatability) while preventing lead pollution to the surrounding environment of tailings dams (removal of toxic PbSO4 before flotation) and maximizing the recovery of critical elements by cementation (recovery of Pb0 using ZVI). **Author Contributions:** Conceptualization, K.A., M.I., and N.H.; methodology, K.A.; investigation, A.K.; data curation, A.K. and S.J.; writing—original draft preparation, K.A.; writing—review and editing, M.I., S.J., I.P., and N.H.; visualization, K.A.; supervision, N.H.; project administration, K.A.; funding acquisition, K.A. All authors have read and agreed to the published version of the manu-

1. ZnS floatability increased in the presence of PbSO4.

ZnS floatability.

script.

Pb2+ could be recovered as Pb0 by cementation using ZVI.

In this paper, a pretreatment of flotation to extract PbSO4 for the prevention of lead pollution to the surrounding environment of tailings dams and the depression of ZnS

### **4. Conclusions**

In this paper, a pretreatment of flotation to extract PbSO<sup>4</sup> for the prevention of lead pollution to the surrounding environment of tailings dams and the depression of ZnS floatability combined with the recovery of extracted Pb2+ by cementation was investigated, and the findings of this study are summarized as follows:


**Author Contributions:** Conceptualization, K.A., M.I. and N.H.; methodology, K.A.; investigation, A.K.; data curation, A.K. and S.J.; writing—original draft preparation, K.A.; writing—review and editing, M.I., S.J., I.P. and N.H.; visualization, K.A.; supervision, N.H.; project administration, K.A.; funding acquisition, K.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by JSPS KAKENHI Grant Number JP21J20552.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
