Development of a Sustainable Process for Complex Sulfide Ores Containing Anglesite: Effect of Anglesite on Sphalerite Floatability, Enhanced Depression of Sphalerite by Extracting Anglesite, and Recovery of Extracted Pb2+ as Zero-Valent Pb by Cementation Using Zero-Valent Fe

The presence of anglesite (PbSO4) in complex sulfide ores negatively affects the separation of Cu-Pb sulfides and sphalerite (ZnS) due to lead activation, and PbSO4 rejected to tailings dams contaminates the surrounding environment with lead. To address these problems, this study investigated the application of ethylene diamine tetra acetic acid (EDTA) pretreatment extracting PbSO4 to ZnS flotation and the recovery of the extracted Pb2+ as zero-valent Pb by cementation using zero-valent iron (ZVI). The application of EDTA pretreatment could extract ~99.8% of PbSO4, thus depressing ZnS floatability from 82% to 30%. In addition, cementation using ZVI could recover ~99.7% of Pb2+ from the leachate of EDTA pretreatment.


Introduction
In 2015, 193 governments adopted 17 sustainable development goals (SDGs) aiming to eradicate poverty, protect the planet, and ensure peaceful and prosperous lives for everyone by 2030 [1][2][3]. Number 13 of the SDGs-"Climate Action"-envisions drastic carbon dioxide (CO 2 ) emission reductions to combat climate change and achieve a carbonneutral society by 2050 [4]. Copper (Cu), lead (Pb), and zinc (Zn) are classified as critical metals defined by the World Bank as essential in renewable energy and clean storage technologies (e.g., wind turbines, solar photovoltaic (PV) panels, batteries, etc.), and thus their demands are projected to significantly increase in the future to achieve a carbonneutral society [5,6].
Complex sulfide ores, some of the most important sources of critical metals, are typically composed of several metal sulfide minerals such as chalcopyrite (CuFeS 2 ), galena (PbS), and sphalerite (ZnS). In the mineral processing of complex sulfide ores consisting of Cu-Pb-Zn sulfide minerals, flotation has been commonly adopted to produce concentrates of each mineral [7][8][9][10][11][12][13][14][15][16]. Some complex sulfide ores contain not only PbS but also anglesite (PbSO 4 ), the presence of which is problematic for the separation of Cu, Pb, and Zn by flotation due to the unwanted activation of ZnS by Pb 2+ released from PbSO 4 [17][18][19]. In the general flotation circuit of Cu-Pb-Zn sulfide ores, Cu and Pb sulfides are first recovered as froth, and then ZnS is recovered with the assistance of activators (e.g., CuSO 4 ) [20,21]. When PbSO 4 is contained in complex sulfide ores, however, Pb 2+ is readily released from PbSO 4 during conditioning and/or flotation due to its higher solubility compared to PbSthe solubility product (K sp ) of PbSO 4 and PbS is 10 −7.79 and 10 −26.77 , respectively [22]-and activates the surface of ZnS via the formation of PbS-like compounds, as illustrated in Equation (1) [23,24]; ZnS(s) + Pb 2+ = PbS(s) + Zn 2+ (1) The activation of ZnS by Pb 2+ is known to increase the ZnS floatability because of a higher affinity of PbS-like compounds for xanthate compared to ZnS [25][26][27]. As a result, ZnS is recovered as froth together with CuFeS 2 and PbS, making their separation difficult [17]. Therefore, a proper way of depressing ZnS floatability during flotation of complex sulfide ores containing PbSO 4 should be established.
In addition, PbSO 4 is typically distributed into flotation tailings because it has poor affinity for the collector (e.g., xanthate) [28,29]. The disposal of PbSO 4 into tailings storage facilities (TSFs) would cause lead pollution in the surrounding environment [30,31]. That is, the presence of PbSO 4 in complex sulfide ores would cause not only difficulties in the separation of ZnS from Cu and Pb sulfide minerals but also lead pollution in the surrounding environment of TSFs.
To address the above-mentioned problems, this study investigated the improvement of ZnS depression by extracting PbSO 4 and the recovery of extracted Pb 2+ as zero-valent Pb by cementation using zero-valent iron (ZVI). Specifically, this study was devoted not only to optimizing the conditions for the extraction of PbSO 4 by ethylene diamine tetra acetic acid (EDTA) as well as the recovery of extracted Pb 2+ as Pb 0 via cementation using ZVI but also to understanding how the absence/presence of PbSO 4 affected the floatability of ZnS through flotation experiments and surface characterizations of ZnS/PbSO 4 mixtures with and without EDTA pretreatment. Finally, a sustainable flowsheet for the processing of complex sulfide ores containing PbSO 4 with the combination of EDTA pretreatment, flotation, and cementation is proposed.

Minerals and Reagents
Five types of minerals were used in this study: sphalerite (ZnS, Kamioka Mine, Hida, Japan), lead sulfate (PbSO 4, 98% purity), lead carbonate (PbCO 3 , 99% purity), lead chloride (PbCl 2 , 99% purity), and quartz (SiO 2 , 99% purity). The lead compounds and quartz were obtained from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). A mineral specimen of ZnS was characterized using X-ray fluorescence spectroscopy (XRF, EDXL300, Rigaku Corporation, Tokyo, Japan) and X-ray powder diffraction (XRD, MultiFlex, Rigaku Corporation, Tokyo, Japan), and its chemical and mineralogical compositions ware shown in Table 1 and Figure 1, respectively. The XRD pattern of ZnS implied that ZnS was relatively pure due to the absence of the peaks of common minerals such as SiO 2 . In preparation for the flotation experiments, ZnS was ground by a vibratory disc mill (RS 200, Retsch Inc., Haan, Germany) and screened to obtain a size fraction of −75 + 38 µm.  For the flotation experiments, potassium amyl xanthate (KAX, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) and methyl isobutyl carbinol (MIBC, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) were used as collector and frother, respectively. Zinc sulfate (ZnSO4) and sodium sulfite (Na2SO3), purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan), were used as depressants for ZnS. Sulfuric acid (H2SO4, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) and sodium hydroxide (NaOH, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) were used as pH adjusters. For the extraction experiments, ethylene diamine tetra acetic acid (EDTA, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) was used as an extractant. For the cementation experiments, zero-valent Fe (ZVI) powder (Fe 0 , −45 µ m, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) was used as a reductant.

Extraction Experiments of Lead Minerals Using EDTA
The extraction experiments of lead minerals were conducted in 50 mL Erlenmeyer flasks containing 1 g of the lead mineral and an EDTA solution (solid/liquid ratio: 1 g/10 mL). The solutions containing 100, 200, or 500 mM EDTA were used without pH adjustment (natural pH: 11.3). The flasks were shaken in a thermostat water bath shaker at a shaking speed of 120 rpm and an amplitude of 40 mm at 25 °C. After this, the leachates were collected by filtration using 0.2 μm syringe-driven membrane filters and immediately analyzed using an inductively coupled plasma atomic emission spectrometer (ICP-AES, ICPE 9820, Shimadzu Corporation, Kyoto, Japan) (margin of error = ±2%) to measure the concentration of Pb 2+ . The experiments were performed in triplicate to ascertain that the differences observed were statistically significant, and the extraction efficiency of the lead mineral (E ) was calculated using the following equation: where [Pb] and [Pb] are total Pb (mM) and extracted Pb (mM), respectively.

Cementation Experiments of the Extracted Pb 2+
The cementation experiments of Pb 2+ extracted from PbSO4 were conducted in 50 mL Erlenmeyer flasks. A volume of 10 mL of the leachate collected by filtration using 0.2 μm syringe-driven membrane filters after the extraction experiments ([EDTA], 500 mM; S/L ratio, 1 g/10 mL; time, 30 min) was added to the flask, and ultrapure nitrogen gas (N2; 99.99%) was introduced for 15 min to remove the dissolved oxygen (DO) present in the leachate. Then, a known amount of ZVI powder was added to the flask, and N2 was further introduced for 5 min. The flask was tightly capped with a silicon rubber plug and parafilm and shaken in a thermostat water bath shaker at a shaking speed of 120 rpm and

Extraction Experiments of Lead Minerals Using EDTA
The extraction experiments of lead minerals were conducted in 50 mL Erlenmeyer flasks containing 1 g of the lead mineral and an EDTA solution (solid/liquid ratio: 1 g/10 mL). The solutions containing 100, 200, or 500 mM EDTA were used without pH adjustment (natural pH: 11.3). The flasks were shaken in a thermostat water bath shaker at a shaking speed of 120 rpm and an amplitude of 40 mm at 25 • C. After this, the leachates were collected by filtration using 0.2 µm syringe-driven membrane filters and immediately analyzed using an inductively coupled plasma atomic emission spectrometer (ICP-AES, ICPE 9820, Shimadzu Corporation, Kyoto, Japan) (margin of error = ±2%) to measure the concentration of Pb 2+ . The experiments were performed in triplicate to ascertain that the differences observed were statistically significant, and the extraction efficiency of the lead mineral (E Pb ) was calculated using the following equation: where [Pb] tot and [Pb] i are total Pb (mM) and extracted Pb (mM), respectively.

Cementation Experiments of the Extracted Pb 2+
The cementation experiments of Pb 2+ extracted from PbSO 4 were conducted in 50 mL Erlenmeyer flasks. A volume of 10 mL of the leachate collected by filtration using 0.2 µm syringe-driven membrane filters after the extraction experiments ([EDTA], 500 mM; S/L ratio, 1 g/10 mL; time, 30 min) was added to the flask, and ultrapure nitrogen gas (N 2 ; 99.99%) was introduced for 15 min to remove the dissolved oxygen (DO) present in the leachate. Then, a known amount of ZVI powder was added to the flask, and N 2 was further introduced for 5 min. The flask was tightly capped with a silicon rubber plug and parafilm and shaken in a thermostat water bath shaker at a shaking speed of 120 rpm and an amplitude of 40 mm at 25 • C. After this, the leachate was filtered using a 0.2 µm syringe-driven membrane filter, and the filtrate was immediately analyzed using ICP-AES to measure the concentration of Pb 2+ . Meanwhile, the residue was washed thoroughly with distilled water, dried in a vacuum-drying oven at 40 • C, and analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS, JSM-IT200TM, JEOL Co., Ltd., Tokyo, Japan). The experiments were performed in triplicate to ascertain that the differences observed were statistically significant. Pb recovery (R Pb ) was calculated using the following equation:

Flotation Experiments
The flotation experiments were carried out using an agitator-type flotation machine (ASH-F30H, Kankyo-kanri Engineering, Akita, Japan) equipped with a 400 mL flotation cell under the following conditions: pH, 6.5; temperature, 25 • C; pulp density, 5%; impeller speed, 1000 rpm; air flow rate, 1 L/min. In a 500 mL beaker, a model sample containing 15 g of ZnS and 5 g of PbSO 4 or SiO 2 was suspended in 300 mL of distilled water, and the supernatant was decanted to remove fine particles (<38 µm) [17]. After desliming, a model sample was repulped to 400 mL with distilled water in the flotation cell and conditioned for 3 min after adding the following reagents in sequence:~5 kg/t of ZnSO 4 (100 ppm Zn 2+ ), 1 kg/t of Na 2 SO 3 , 20 g/t of KAX, and 20 µL/L of MIBC. Afterwards, air was introduced at a flowrate of 1 L/min, and froth was recovered for 3 min. The recovered froth/tailing products were dried at 105 • C for 24 h and analyzed by XRF to determine the recovery of Zn. The experiments were performed in duplicate to ascertain that the differences observed were statistically significant. An aliquot of the pulp of about 5 mL was collected 3 min after adding the depressant, filtered through a 0.2 µm syringe-driven membrane filter, and immediately analyzed using ICP-AES to check the extent of lead activation of ZnS. Meanwhile, the collected residues were washed thoroughly with distilled water, dried in a vacuum-drying oven at 40 • C, and analyzed by X-ray photoelectron spectroscopy (XPS, JPS-9200, JEOL Co., Ltd., Tokyo, Japan). The XPS analysis was conducted using an Al Kα X-ray source (1486.7 eV) operated at 100 W (voltage = 10 kV; current = 10 mA) under ultrahigh vacuum conditions (~10 −7 Pa). The binding energies of photoelectrons were calibrated using C1s (285 eV) or Zn2p 3/2 (1022.0 eV) for charge correction. The XPS data were analyzed by Casa XPS, and deconvolutions of the spectra were carried out using an 80% Gaussian-20% Lorentzian peak model and a Shirley background [32][33][34][35].
In addition, flotation experiments of ZnS/PbSO 4 mixture after EDTA pretreatment were conducted to check how effective the removal of PbSO 4 was on the depression of ZnS floatability. A mixture of 15 g ZnS and 5 g PbSO 4 was mixed with 200 mL of a 500 mM EDTA solution (i.e., solid/liquid ratio: 20 g/200 mL) and shaken in a thermostat water bath shaker at a shaking speed of 120 rpm and an amplitude of 40 mm at 25 • C for 30 min. After this, the leachate was filtered using a 0.2 µm syringe-driven membrane filter, the filtrate was immediately analyzed using ICP-AES to measure the concentration of Pb 2+ , and the cementation experiments to recover the extracted Pb 2+ in the filtrate were carried out using 1 g/10 mL of ZVI for 24 h under the same conditions and with the same procedures mentioned in Section 2.2.2. Meanwhile, the residue obtained after EDTA pretreatment was washed thoroughly with distilled water and deslimed to remove fine particles [17]. After desliming, the residue was repulped to 400 mL with distilled water, and then flotation was conducted under the same conditions mentioned above. An aliquot of the pulp of about 5 mL was also collected 3 min after adding the depressant (ZnSO 4 ) and filtered through a 0.2 µm syringe-driven membrane filter, and the filtrate and residue were analyzed by ICP-AES and XPS, respectively. Figure 2 shows the flotation results of ZnS mixed with SiO 2 or PbSO 4 . As can be seen, Zn recovery in the presence of SiO 2 was~12% but significantly increased to >80% when PbSO 4 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 4 is higher than that of PbS, a substantial amount of Pb 2+ might be released into the pulp during the conditioning process when the ore contains PbSO 4 [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.

Effects of PbSO 4 on ZnS Floatability
In short, the presence of PbSO 4 in the complex sulfide ores would negatively affect the separation of Cu-Pb from Zn. In addition, some portion of PbSO 4 will remain undissolved and be rejected as tailings due to the poor affinity of PbSO 4 for the collector (e.g., xanthate), which can cause lead contamination of the surrounding environment [28,29,36].
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 Pb 2+ 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 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].

Extraction of PbSO4 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 and ZnS) [37][38][39][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 Pb 2+ extracted was identical to the concentration of EDTA, which indicated that Pb 2+ 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 Pb 2+ but also by other metal ions from soluble minerals such as sulfates, oxides, hydroxides, and carbonates.

Extraction of PbSO 4 Using EDTA
To address the above-mentioned problems caused by the presence of PbSO 4 (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 PbSO 4 , without dissolving metal sulfides (e.g., PbS and ZnS) [37][38][39][40]. Figure 3a shows the extraction efficiency of PbSO 4 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 Pb 2+ extracted was identical to the concentration of EDTA, which indicated that Pb 2+ 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 4 (~98%) was extracted. The rate of PbSO 4 extraction was further studied with 500 mM EDTA, and the result showed that~97% of PbSO 4 was extracted within 30 min (Figure 3b). EDTA could extract not only PbSO 4 but also other Pb minerals like PbCO 3 and PbCl 2 ( 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 Pb 2+ but also by other metal ions from soluble minerals such as sulfates, oxides, hydroxides, and carbonates.

Recovery of Extracted Pb 2+ as Zero-Valent Pb by Cementation
After EDTA pretreatment, the recovery of Pb 2+ from the leachate will not only add economic value but also protect the environment. Cementation is one of the effectiv methods to recover metal ions as zero-valent metals [41][42][43][44][45]. A general cementation reac tion is illustrated in Equation (4): a metal (A 0 ) gives electrons to metal ion (B b+ ), driven by the difference in standard redox potentials of the interacting metals and their ions, and a a result, B b+ is deposited on the surface of A 0 as B 0 [46][47][48][49]: To recover the extracted Pb 2+ as zero-valent Pb (Pb 0 ) from the EDTA leachate, cemen tation 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 o the ZVI amount on the recovery of the extracted Pb 2+ from the leachate after EDTA pre treatment using 500 mM EDTA for 30 min. When the amount of ZVI was 0.5 g/10 mL ~40% of Pb 2+ was recovered, although ~0.18 g/10 mL ZVI was stoichiometrically sufficien to recover all Pb 2+ in the solution. As the cementation reaction progressed, the surface o ZVI was covered with cementation products (i.e., Pb 0 ), which hindered a further cemen tation reaction. This was probably the reason why 1 g/10 mL of ZVI was required to re cover all Pb 2+ from the EDTA leachate.

Recovery of Extracted Pb 2+ as Zero-Valent Pb by Cementation
After EDTA pretreatment, the recovery of Pb 2+ 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][42][43][44][45]. A general cementation reaction is illustrated in Equation (4): a metal (A 0 ) gives electrons to metal ion (B b+ ), driven by the difference in standard redox potentials of the interacting metals and their ions, and as a result, B b+ is deposited on the surface of A 0 as B 0 [46][47][48][49]: To recover the extracted Pb 2+ as zero-valent Pb (Pb 0 ) 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 Pb 2+ 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 Pb 2+ was recovered, although~0.18 g/10 mL ZVI was stoichiometrically sufficient to recover all Pb 2+ in the solution. As the cementation reaction progressed, the surface of ZVI was covered with cementation products (i.e., Pb 0 ), which hindered a further cementation reaction. This was probably the reason why 1 g/10 mL of ZVI was required to recover all Pb 2+ from the EDTA leachate.    Figure 5b shows the cementation results of Pb 2+ by ZVI in time. As can be seen, almost no cementation occurred for 3 h, and then Pb 2+ 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 Pb 2+ and (ii) an excess amount of free EDTA that redissolved the cemented Pb 0 . 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 Pb 2+ 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 Pb 2+ was reductively deposited via the cementation reaction: Pb(EDTA) 2− + Fe 0 = Pb 0 + EDTA + Fe 2+ (5) the extracted Pb 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 Pb 2+ by ZVI in time. As can be seen, almost no cementation occurred for 3 h, and then Pb 2+ 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 Pb 2+ and (ii) an excess amount of free EDTA that redissolved the cemented Pb 0 . 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 Pb 2+ 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 Pb 2+ was reductively deposited via the cementation reaction: Pb(EDTA) 2-+ Fe 0 = Pb 0 +EDTA + Fe 2+ (5) Equation (5) consists of two half-cell reactions [50]: Fe 0 = Fe 2+ + 2e -(7) Fe 0 = Fe 2+ + 2e − 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 0 , Fe 2 O 3 , Pb 0 , and PbO were detected, indicating that Pb 2+ was reductively deposited as Pb 0 on the surface of ZVI, as illustrated in Equation (5). The presence of PbO and Fe 2 O 3 was due most likely to the oxidation of Pb 0 and ZVI surface during drying and storing of the sample [51][52][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 Fe 0 , Fe2O3, Pb 0 , and PbO were detected, indicating that Pb 2+ was reductively deposited as Pb 0 on the surface of ZVI, as illustrated in Equation (5). The presence of PbO and Fe2O3 was due most likely to the oxidation of Pb 0 and ZVI surface during drying and storing of the sample [51][52][53].

Effects of EDTA Pretreatment on ZnS Depression in Flotation
As described in Section 3.2, almost all PbSO4 was extracted with EDTA, so the effects

Effects of EDTA Pretreatment on ZnS Depression in Flotation
As described in Section 3.2, almost all PbSO 4 was extracted with EDTA, so the effects of EDTA pretreatment on ZnS depression in flotation were investigated. Flotation experiments using model samples (ZnS/PbSO 4 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 4 was treated with 500 mM of EDTA for 30 min (solid/liquid ratio: 20 g/200 mL), and~99.8% of PbSO 4 was extracted. The cementation experiments using 1 g/10 mL of ZVI for 24 h were also carried out to recover the extracted Pb 2+ in the filtrate obtained after EDTA pretreatment, and~99.7% of the extracted Pb 2+ 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 4 (100 ppm Zn 2+ ) and 1 kg/t of Na 2 SO 3 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.

Effects of EDTA Pretreatment on ZnS Depression in Flotation
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 Pb 2+ in the filtrate obtained after EDTA pretreatment , and ~99.7% of the extracted Pb 2+ 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 Zn 2+ ) 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.  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 Pb 2+ 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 Zn 2+ 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 Pb 2+ and Zn 2+ [17,54,55]. In this study, the changes in free energy were calculated using measured values of Pb 2+ and Zn 2+ 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 Pb 2+ concentration during flotation by extracting PbSO 4 in advance. ∆G = −RT ln K + ln Zn 2+ /Pb 2+ (8)  [54]. b, c, d The equilibrium constants were calculated using Equation (9) by the reported K sp values of ZnS and PbS, respectively [56][57][58].
To confirm whether the lead activation of sphalerite was limited by the extraction of PbSO 4 using EDTA, untreated ZnS as well as ZnSO 4 -pretreated ZnS/PbSO 4 mixtures with and without EDTA pretreatment were analyzed by XPS. The Pb4f 7/2 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 Pb4f 7/2 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 4 extraction showed two types of Pb species: (1) Pb 2+ -S of PbS (143.6 and 138.6 eV) and (2) Pb 2+ -SO 4 of PbSO 4 (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 Pb 2+ -SO 4 was in a good agreement with the extraction efficiency of PbSO 4 using EDTA for the ZnS/PbSO 4 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).

Implication of This Study
The results of this study have significant implications not only for understanding how the presence of PbSO 4 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 4 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 ZnSO 4 -the conventional depressant for ZnS. Secondly, EDTA washing could extract almost all PbSO 4 from the ZnS/PbSO 4 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 Pb 2+ extracted from PbSO 4 during EDTA pretreatment as Pb 0 , 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 4 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 Pb 2+ /Zn 2+ of the flotation pulp, and then gangue minerals like SiO 2 and FeS 2 would be disposed of into a tailings dam after the recovery of ZnS with the assistance of activators (e.g., CuSO 4 ). Meanwhile, the leachate obtained from EDTA pretreatment is rich in Pb 2+ that can be recovered via cementation using ZVI as Pb 0 . 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 4 before flotation) and maximizing the recovery of critical elements by cementation (recovery of Pb 0 using ZVI). recovery of ZnS with the assistance of activators (e.g., CuSO4). Meanwhile, the leachate obtained from EDTA pretreatment is rich in Pb 2+ that can be recovered via cementation using ZVI as Pb 0 . 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 Pb 0 using ZVI).

Conclusions
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 floatability combined with the recovery of extracted Pb 2+ by cementation was investigated, and the findings of this study are summarized as follows: 1. ZnS floatability increased in the presence of PbSO4.

Conclusions
In this paper, a pretreatment of flotation to extract PbSO 4 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 Pb 2+ by cementation was investigated, and the findings of this study are summarized as follows: 1.
ZnS floatability increased in the presence of PbSO 4 .

2.
The conventional depressants for ZnS, zinc sulfate and sodium sulfite, were not effective in depressing the floatability of ZnS when PbSO 4 was present.
The proposed method for complex sulfide ores containing PbSO 4 , a combination of extraction of PbSO 4 using EDTA and recovery of extracted Pb 2+ 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 PbSO 4 before flotation) and maximizing the recovery of critical elements by cementation (recovery of Pb 0 using ZVI).