*3.1. Effects of PbSO<sup>4</sup> on ZnS Floatability*

Figure 2 shows the flotation results of ZnS mixed with SiO<sup>2</sup> or PbSO4. As can be seen, Zn recovery in the presence of SiO<sup>2</sup> was ~12% but significantly increased to >80% when PbSO<sup>4</sup> was present. It was reported that the affinity between ZnS and xanthate is poor, so the surface of ZnS needs to be modified using activators [21]. Since the solubility of PbSO<sup>4</sup> is higher than that of PbS, a substantial amount of Pb2+ might be released into the pulp during the conditioning process when the ore contains PbSO<sup>4</sup> [17]. As a result, lead activation of ZnS could occur (Equation (1)), thus increasing its floatability due to the formation of PbS-like compounds, having a higher affinity for xanthate than ZnS, on the mineral surface.

In short, the presence of PbSO<sup>4</sup> in the complex sulfide ores would negatively affect the separation of Cu-Pb from Zn. In addition, some portion of PbSO<sup>4</sup> will remain undissolved and be rejected as tailings due to the poor affinity of PbSO<sup>4</sup> for the collector (e.g., xanthate), which can cause lead contamination of the surrounding environment [28,29,36]. remain undissolved and be rejected as tailings due to the poor affinity of PbSO4 for the collector (e.g., xanthate), which can cause lead contamination of the surrounding environment [28,29,36].

Figure 2 shows the flotation results of ZnS mixed with SiO2 or PbSO4. As can be seen, Zn recovery in the presence of SiO2 was ~12% but significantly increased to >80% when PbSO4 was present. It was reported that the affinity between ZnS and xanthate is poor, so the surface of ZnS needs to be modified using activators [21]. Since the solubility of PbSO4 is higher than that of PbS, a substantial amount of Pb2+ might be released into the pulp during the conditioning process when the ore contains PbSO4 [17]. As a result, lead activation of ZnS could occur (Equation (1)), thus increasing its floatability due to the formation of PbS-like compounds, having a higher affinity for xanthate than ZnS, on the mineral surface. In short, the presence of PbSO4 in the complex sulfide ores would negatively affect the separation of Cu-Pb from Zn. In addition, some portion of PbSO4 will

*Minerals* **2022**, *12*, x FOR PEER REVIEW 6 of 15

**3. Results and Discussion** 

*3.1. Effects of PbSO4 on ZnS Floatability* 

**Figure 2.** Flotation results of ZnS with SiO2 or PbSO4. **Figure 2.** Flotation results of ZnS with SiO<sup>2</sup> or PbSO<sup>4</sup> .

#### *3.2. Extraction of PbSO4 Using EDTA 3.2. Extraction of PbSO<sup>4</sup> Using EDTA*

To address the above-mentioned problems caused by the presence of PbSO4 (i.e., unwanted activation of ZnS and lead contamination), its extraction prior to flotation was investigated. Among the extractants, EDTA was chosen in this study because of its ability to extract metal ions from sulfates like PbSO4, without dissolving metal sulfides (e.g., PbS To address the above-mentioned problems caused by the presence of PbSO<sup>4</sup> (i.e., unwanted activation of ZnS and lead contamination), its extraction prior to flotation was investigated. Among the extractants, EDTA was chosen in this study because of its ability to extract metal ions from sulfates like PbSO4, without dissolving metal sulfides (e.g., PbS and ZnS) [37–40].

and ZnS) [37–40]. Figure 3a shows the extraction efficiency of PbSO4 after 24 h with different concentrations of EDTA (100, 200, or 500 mM). When the stoichiometric ratio of EDTA/[Pb]*tot* was lower than 1, the amount of Pb2+ extracted was identical to the concentration of EDTA, which indicated that Pb2+ and EDTA formed a 1:1 complex (i.e., [Pb(EDTA)]2–). When the stoichiometric ratio of EDTA/[Pb]*tot* was >1, almost all PbSO4 (~98%) was extracted. The rate of PbSO4 extraction was further studied with 500 mM EDTA, and the result showed that ~97% of PbSO4 was extracted within 30 min (Figure 3b). EDTA could extract not only PbSO4 but also other Pb minerals like PbCO3 and PbCl2 (Figure 4). These minerals are also unfloatable and will be rejected into the TSFs, indicating that their presence in the ores may result in the contamination of the TSFs with Pb. It was confirmed that EDTA pretreatment can extract almost all Pb minerals except for PbS, thus suppressing lead pollution to the surrounding environment of TSFs. It is noted that the concentration of EDTA should be chosen in the consideration of the amount of soluble minerals contained in ores, because EDTA would be consumed not only by Pb2+ but also by other metal ions from Figure 3a shows the extraction efficiency of PbSO<sup>4</sup> after 24 h with different concentrations of EDTA (100, 200, or 500 mM). When the stoichiometric ratio of EDTA/[Pb]*tot* was lower than 1, the amount of Pb2+ extracted was identical to the concentration of EDTA, which indicated that Pb2+ and EDTA formed a 1:1 complex (i.e., [Pb(EDTA)]2–). When the stoichiometric ratio of EDTA/[Pb]*tot* was >1, almost all PbSO<sup>4</sup> (~98%) was extracted. The rate of PbSO<sup>4</sup> extraction was further studied with 500 mM EDTA, and the result showed that ~97% of PbSO<sup>4</sup> was extracted within 30 min (Figure 3b). EDTA could extract not only PbSO<sup>4</sup> but also other Pb minerals like PbCO<sup>3</sup> and PbCl<sup>2</sup> (Figure 4). These minerals are also unfloatable and will be rejected into the TSFs, indicating that their presence in the ores may result in the contamination of the TSFs with Pb. It was confirmed that EDTA pretreatment can extract almost all Pb minerals except for PbS, thus suppressing lead pollution to the surrounding environment of TSFs. It is noted that the concentration of EDTA should be chosen in the consideration of the amount of soluble minerals contained in ores, because EDTA would be consumed not only by Pb2+ but also by other metal ions from soluble minerals such as sulfates, oxides, hydroxides, and carbonates. *Minerals* **2022**, *12*, x FOR PEER REVIEW 7 of 15

**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 line in Figure 3a corresponds to the ratio of EDTA/[Pb]*tot*. cover all Pb2+ from the EDTA leachate. **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 PbSO<sup>4</sup> extraction with 500 mM EDTA. Note that the dotted line in Figure 3a corresponds to the ratio of EDTA/[Pb]*tot*.

**Figure 4.** Extraction efficiency of lead minerals with 500 mM EDTA after 24 h.

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

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 ZVI was covered with cementation products (i.e., Pb0), which hindered a further cementation reaction. This was probably the reason why 1 g/10 mL of ZVI was required to re-

*n* A0 + *m* Bb+ → *n* Aa+ + m B0 (*na = mb*) (4)

*3.3. Recovery of Extracted Pb*2+ *as Zero-Valent Pb by Cementation* 

a result, Bb+ is deposited on the surface of A0 as B0 [46–49]:

line in Figure 3a corresponds to the ratio of EDTA/[Pb]*tot*.

**Figure 4.** Extraction efficiency of lead minerals with 500 mM EDTA after 24 h. **Figure 4.** Extraction efficiency of lead minerals with 500 mM EDTA after 24 h.
