**1. Introduction**

The increase in the global population, the rapid development of cities, and the current push to decarbonize society are all contributing to the unprecedentedly high demand for metals. For example, the renewable energy and clean energy technologies needed to decarbonize society are more metal and material intensive than conventional fossil-fuel-based technologies [1–3]. In a recent report by the World Bank, zinc (Zn) and lead (Pb) were two

**Citation:** Silwamba, M.; Ito, M.; Hiroyoshi, N.; Tabelin, C.B.; Hashizume, R.; Fukushima, T.; Park, I.; Jeon, S.; Igarashi, T.; Sato, T.; et al. Alkaline Leaching and Concurrent Cementation of Dissolved Pb and Zn from Zinc Plant Leach Residues. *Minerals* **2022**, *12*, 393. https:// doi.org/10.3390/min12040393

Academic Editor: Przemyslaw B. Kowalczuk

Received: 23 February 2022 Accepted: 19 March 2022 Published: 23 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of the 17 materials/metals identified as critical for the clean energy transition to succeed [4]. Unfortunately, high-grade, primary metal resources have become scarce, so alternative sources such as submarine deposits and wastes are currently being explored [5,6].

Zinc plant leach residues (ZPLRs), especially those that were produced using old technologies, are regarded as environmental nuisances and hazardous wastes due to their high amounts of leachable residual hazardous elements such as Pb, Zn, cadmium (Cd), copper (Cu), and arsenic (As) [7–10]. Lead, Cd, and As can cause various illnesses that affect the central nervous system, skin, lungs and kidneys, even in minute amounts, while Cu and Zn are essential micronutrients that are toxic at high concentrations [11–13]. Aside from Zn and Pb, ZPLRs can also contain other critical metals such as cobalt (Co), indium (In), gallium (Ga), and germanium (Ge) [14–16]. The extraction of these metals from ZPLRs serves two purposes: (1) exploitation for economic benefits and (2) the detoxification and clean-up of ZPLRs-impacted sites.

Metal extraction from ZPLRs by hydrometallurgical processes is preferred because they are less energy-intensive (especially for low-grade metallurgical wastes such as ZPLRs) compared to their counterpart, pyrometallurgical processes. Most hydrometallurgical techniques involve the use of strong acids to extract metals of interest. Because these acids are nonselective, they dissolve unwanted elements, the majority of which interferes with succeeding recovery processes, so purification processes (e.g., solvent extraction) are required [17–20].

The leaching of ZPLRs using alkaline lixiviants achieves the selective solubilization of amphoteric elements—Al, Pb, and Zn—leaving iron (Fe), calcium (Ca), and magnesium (Mg) host minerals that constitute a large percentage of ZPLRs undissolved. The dissolution of Pb and Zn under alkaline conditions is due to the formation of complexes with hydroxyl ions (OH−) [21]. In weak to moderately strong alkaline solutions (i.e., pH 6–12), Pb and Zn dissolve as Pb(OH)<sup>3</sup> − with small amounts of Pb(OH)<sup>4</sup> <sup>2</sup><sup>−</sup> and Zn(OH)<sup>3</sup> − with small amounts of Zn(OH)<sup>4</sup> <sup>2</sup>−, respectively. In a strong alkaline solution (i.e., pH > 12), the dominant species are Pb(OH)<sup>4</sup> <sup>2</sup><sup>−</sup> for Pb and Zn(OH)<sup>4</sup> <sup>2</sup><sup>−</sup> for Zn. Many researchers have investigated and successfully extracted Pb and Zn from ZPLRs using alkaline solutions [14,22–24]. The alkaline extractive processes studied are as follows: leaching → solid–liquid separation → metal recovery stages. However, solid–liquid separation by filtration, especially for strong alkaline, is difficult [25]. Thus, some dissolved Pb and Zn from ZPLRs remain in residues if thorough filtration and the washing of leaching residues are not carried out. The residual metals in produced residues are economic losses and at the same time render the produced residues hazardous.

The authors previously developed concurrent-extraction cementation (CEC)—a new metals recovery technique that extracts metals and captures/sequesters them by cementation before solid–liquid separation. Cementation or reductive precipitation is an electrochemical process whereby zero-valent metals or alloys are used to selectively recover redox-sensitive dissolved metals from solution [26,27]. The CEC technique eliminates the need for thorough filtration and extensive washing to remove residual toxic elements in the leaching residues [28,29]. These previous studies, however, were conducted in acidic solutions and Zn could not be cemented by Al metal powder from the leaching pulp or filtered solution because of the competitive effects of proton reduction on cementation [30]. This study, therefore, investigates the CEC of dissolved Pb and Zn from ZPLRs in alkaline (NaOH) leaching pulp using Al metal powder as the cementation agent.

#### **2. Materials and Methods**

#### *2.1. Materials*

Zinc plant leach residues (ZPLRs) from a historic Pb-Zn mine dumpsite in Kabwe, Zambia were used in this study. The total amounts of Pb and Zn in the ZPLRs were around 6.19 wt% and 2.53 wt%, respectively. The major crystalline minerals of Pb were anglesite (PbSO4), cerussite (PbCO3), and esperite (PbCa2Zn3(SiO4)3). Meanwhile, only one crystalline mineral for Zn, zinkosite (ZnSO4), was detected. Ultra-pure Al metal

powder with a median particle size (D50) of 126.8 µm (>99.99%, +50–150 µm, Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used to cement dissolved Pb and Zn from ZPLRs. Detailed characterizations of Al powder and ZPLRs are reported elsewhere [28,31]. Reagent grade NaOH (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used to prepare the alkaline leaching solutions of different concentrations by dissolving and diluting in deionized (DI) water (18 MΩ·cm, Milli-Q® Integral Water Purification System, Merck Millipore, Burlington, VT, USA).

#### *2.2. Methods*

All batch experiments were done using 200-mL Erlenmeyer flasks, and the volume of leaching solutions of different concentrations (i.e., 0–6 M) was fixed at 50 mL. A leaching solution of known volume was added in a flask before the addition of ZPLRs to obtain a predetermined solid-to-liquid ratio (S/L). In the case of CEC experiments, 0.25 g of Al powder was added together with ZPLRs. The pulp was then shaken in a temperaturecontrolled water bath shaker maintained at 25 ◦C at a shaking speed of 120 strokes/min and a shaking amplitude of 40 mm. After shaking for preplanned durations, the pulp was carefully collected and filtered through 0.20 µm syringe-driven membrane filters. The filtrate was analyzed for Pb and Zn using an inductively-coupled plasma-atomic emission spectrometer (ICP-AES) (ICPE-9820, Shimadzu Corporation, Kyoto, Japan) (margin of error = ±2%). For the CEC experiments, one more step was added to remove cemented and agglomerated Pb and Zn by sieving using a stainless-steel sieve with an aperture size of 150 µm. The cementation products (i.e., +150 µm) were dried in a vacuum oven, digested by aqua regia in a microwave-assisted acid digestion system (Ethos Advanced Microwave Lab station, Milestone Inc., Sorisole, Italy), and the leachates were analyzed for Pb, Zn, and Fe by ICP-AES. Additionally, the cementation products were analyzed by a scanning electron microscope with an energy-dispersive X-ray spectrometer (SEM-EDX) (JSM-IT200, JEOL Ltd., Tokyo, Japan).

To calculate the Pb and Zn removal efficiencies (*ηMe*) from ZPLRs with and without the addition of Al powder, Equations (1) and (2), respectively, were used.

$$
\eta\_{M\varepsilon} = \frac{(V \times \text{C}\_{Me}) + (W\_{cm\varepsilon} \times M\_{cm\varepsilon})}{W\_{\text{S}} \times M\_{\text{s}}} \times 100\tag{1}
$$

$$
\eta\_{M\varepsilon} = \frac{V \times \mathcal{C}\_{M\varepsilon}}{W\_{\text{S}} \times M\_{\text{s}}} \times 100\tag{2}
$$

where *CMe* is the concentration (g/L) of Pb and Zn, *V* is the volume (L) of the leaching solution, *W<sup>S</sup>* is the weight % of either Pb and Zn in ZPLRs, *M<sup>s</sup>* is the mass (g) of the leached ZPLRs, *Mcme* is the mass (g) of cemented and agglomerated particles, and *Wcme* is the weight % of Pb and Zn in cemented and agglomerated particles calculated based on the digested fraction of *Mcme* in aqua regia and analysis of the solution by ICP-AES.

### **3. Results and Discussions**

#### *3.1. Leaching of ZPLRs in NaOH without the Addition of Al Powder*

The effects of leaching time, NaOH concentration, and S/L on Pb and Zn removal efficiencies from ZPLRs were investigated by batch leaching experiments without the addition of Al powder.

The leaching duration effects on Pb and Zn removal was investigated using 3 M NaOH, a 2.5 g/50 mL S/L ratio, and a temperature of 25 ◦C. The results show that the removal efficiencies for Pb and Zn increased with time up to 15 min, beyond which they changed only insignificantly (Figure 1a). At 15 min of leaching time, the Pb removal efficiency was 60.4% and remained the same even when the leaching time was increased to 120 min (i.e., 59.6%). Similarly, the removal efficiency for Zn was around 28% for 15 min of leaching and 25% when the leaching time was prolonged to 120 min. The Pb and Zn removal efficiencies corroborated and correlated with the water-soluble, exchangeable, and carbonate phases

of Pb and Zn approximated by sequential extraction (experimental method and detailed discussion reported by the authors elsewhere [31]) (Figure 2). It is thermodynamically difficult to dissolve Pb and Zn bound to relatively stable phases (e.g., Fe/Mn oxyhydroxide, Fe oxide, and sulfides/organic) in NaOH leaching solution [32,33]. and carbonate phases of Pb and Zn approximated by sequential extraction (experimental method and detailed discussion reported by the authors elsewhere [31]) (Figure 2). It is thermodynamically difficult to dissolve Pb and Zn bound to relatively stable phases (e.g., Fe/Mn oxyhydroxide, Fe oxide, and sulfides/organic) in NaOH leaching solution [32,33]. and carbonate phases of Pb and Zn approximated by sequential extraction (experimental method and detailed discussion reported by the authors elsewhere [31]) (Figure 2). It is thermodynamically difficult to dissolve Pb and Zn bound to relatively stable phases (e.g., Fe/Mn oxyhydroxide, Fe oxide, and sulfides/organic) in NaOH leaching solution [32,33].

The leaching duration effects on Pb and Zn removal was investigated using 3 M NaOH, a 2.5 g/50 mL S/L ratio, and a temperature of 25 °C. The results show that the removal efficiencies for Pb and Zn increased with time up to 15 min, beyond which they changed only insignificantly (Figure 1a). At 15 min of leaching time, the Pb removal efficiency was 60.4% and remained the same even when the leaching time was increased to 120 min (i.e., 59.6%). Similarly, the removal efficiency for Zn was around 28% for 15 min of leaching and 25% when the leaching time was prolonged to 120 min. The Pb and Zn removal efficiencies corroborated and correlated with the water-soluble, exchangeable,

The leaching duration effects on Pb and Zn removal was investigated using 3 M NaOH, a 2.5 g/50 mL S/L ratio, and a temperature of 25 °C. The results show that the removal efficiencies for Pb and Zn increased with time up to 15 min, beyond which they changed only insignificantly (Figure 1a). At 15 min of leaching time, the Pb removal efficiency was 60.4% and remained the same even when the leaching time was increased to 120 min (i.e., 59.6%). Similarly, the removal efficiency for Zn was around 28% for 15 min of leaching and 25% when the leaching time was prolonged to 120 min. The Pb and Zn removal efficiencies corroborated and correlated with the water-soluble, exchangeable,

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*Minerals* **2022**, *12*, x 4 of 13

**Figure 1.** Pb and Zn removal efficiencies from ZPLRs: (**a**) Effects of leaching time when 2.5 g ZPLRs was leached in 50 mL of concentration 3 M NaOH and shaken at 120 strokes per minute in the water bath at 25 °C; (**b**) effects of NaOH concentration when 2.5 g ZPLRs was leached in 50 mL of different NaOH concentration and shaken at 120 strokes per minute in the water bath at 25 °C; and (**c**) effects of S/L ratio when various amounts ZPLRs were leached in 50 mL of 3M NaOH concentration and shaken at 120 strokes per minute in the water bath at 25 °C. **Figure 1.** Pb and Zn removal efficiencies from ZPLRs: (**a**) Effects of leaching time when 2.5 g ZPLRs was leached in 50 mL of concentration 3 M NaOH and shaken at 120 strokes per minute in the water bath at 25 ◦C; (**b**) effects of NaOH concentration when 2.5 g ZPLRs was leached in 50 mL of different NaOH concentration and shaken at 120 strokes per minute in the water bath at 25 ◦C; and (**c**) effects of S/L ratio when various amounts ZPLRs were leached in 50 mL of 3M NaOH concentration and shaken at 120 strokes per minute in the water bath at 25 ◦C. **Figure 1.** Pb and Zn removal efficiencies from ZPLRs: (**a**) Effects of leaching time when 2.5 g ZPLRs was leached in 50 mL of concentration 3 M NaOH and shaken at 120 strokes per minute in the water bath at 25 °C; (**b**) effects of NaOH concentration when 2.5 g ZPLRs was leached in 50 mL of different NaOH concentration and shaken at 120 strokes per minute in the water bath at 25 °C; and (**c**) effects of S/L ratio when various amounts ZPLRs were leached in 50 mL of 3M NaOH concentration and shaken at 120 strokes per minute in the water bath at 25 °C.

**Figure 2.** Phase partitioning by sequential extraction of Pb and Zn for ZPLRs (reprinted with permission from Silwamba et al., [31] copyright (2020) Elsevier) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). **Figure 2.** Phase partitioning by sequential extraction of Pb and Zn for ZPLRs (reprinted with permission from Silwamba et al., [31] copyright (2020) Elsevier) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). **Figure 2.** Phase partitioning by sequential extraction of Pb and Zn for ZPLRs (reprinted with permission from Silwamba et al., [31] copyright (2020) Elsevier) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

To investigate the effects of the NaOH concentration on Pb and Zn removal efficiencies, NaOH in solution was varied from 0 to 6 M, the S/L maintained at 2.5 g/50 mL, and the temperature was maintained at 25 °C for 30 min of leaching time. The removal efficiencies of Pb and Zn increased with higher NaOH concentrations up to 3 M (i.e., Pb and Zn removal of around 60% and 25%), after which, the change in the removal efficiencies To investigate the effects of the NaOH concentration on Pb and Zn removal efficiencies, NaOH in solution was varied from 0 to 6 M, the S/L maintained at 2.5 g/50 mL, and the temperature was maintained at 25 °C for 30 min of leaching time. The removal efficiencies of Pb and Zn increased with higher NaOH concentrations up to 3 M (i.e., Pb and Zn removal of around 60% and 25%), after which, the change in the removal efficiencies To investigate the effects of the NaOH concentration on Pb and Zn removal efficiencies, NaOH in solution was varied from 0 to 6 M, the S/L maintained at 2.5 g/50 mL, and the temperature was maintained at 25 ◦C for 30 min of leaching time. The removal efficiencies of Pb and Zn increased with higher NaOH concentrations up to 3 M (i.e., Pb and Zn removal of around 60% and 25%), after which, the change in the removal efficiencies of Pb and Zn became insignificant (Figure 1b). The reactions between Pb- and Zn-hosting minerals and NaOH in the solution can be described by Equations (3)–(6) [22,32].

$$\text{PbSO}\_4 + 3\text{NaOH} \rightarrow \text{Pb(OH)}\_3^- + \text{SO}\_4^{2-} + 3\text{Na}^+ \tag{3}$$

$$2\text{PbO} + 2\text{NaOH} + \text{H}\_2\text{O} \rightarrow 2\text{Pb(OH)}\_3^- + 2\text{Na}^+ \tag{4}$$

$$\text{ZnSO}\_4 + \text{3NaOH} \rightarrow \text{Zn(OH)}\_3^- + \text{SO}\_4^{2-} + \text{3Na}^+ \tag{5}$$

$$2\text{ZnO} + 2\text{NaOH} + \text{H}\_2\text{O} \rightarrow 2\text{Zn(OH)}\_3^- + 2\text{Na}^+ \tag{6}$$

In weak to moderately strong alkaline solutions (pH 6–12), the dominant species for Pb and Zn are Pb(OH) − 3 and Zn(OH) − 3 . When the NaOH concentration increases (strong alkaline solution, pH > 12) the equilibrium shifts and the more soluble Pb and Zn hydroxyl complexes Pb(OH) 2− 4 and Zn(OH) 2− 4 , respectively, become more dominant [21]. This explains why Pb and Zn removal efficiencies increased at higher concentrations of NaOH (i.e., 3 M NaOH). Increasing the NaOH concentration beyond 3 M did not improve the Pb and Zn removal efficiencies because almost all the easily extractable Pb and Zn (as determined by sequential extraction) from the ZPLRs were already exhausted. In weak to moderately strong alkaline solutions (pH 6–12), the dominant species for Pb and Zn are Pb(OH)ଷ ି and Zn(OH)ଷ ି. When the NaOH concentration increases (strong alkaline solution, pH >12) the equilibrium shifts and the more soluble Pb and Zn hydroxyl complexes Pb(OH)ସ ଶି and Zn(OH)ସ ଶି, respectively, become more dominant [21]. This explains why Pb and Zn removal efficiencies increased at higher concentrations of NaOH (i.e., 3 M NaOH). Increasing the NaOH concentration beyond 3 M did not improve the Pb and Zn removal efficiencies because almost all the easily extractable Pb and Zn (as determined by sequential extraction) from the ZPLRs were already exhausted. The S/L is another parameter that affects Pb and Zn removal efficiencies from ZPLRs

*Minerals* **2022**, *12*, x 5 of 13

of Pb and Zn became insignificant (Figure 1b). The reactions between Pb- and Zn-hosting

ି + SOସ

ି + SOସ

ଶି + 3Naା (3)

ି + 2Naା (4)

ଶି + 3Naା (5)

ି + 2Naା (6)

minerals and NaOH in the solution can be described by Equations (3)–(6) [22,32].

2PbO + 2NaOH + HଶO → 2Pb(OH)ଷ

2ZnO + 2NaOH + HଶO → 2Zn(OH)ଷ

PbSOସ + 3NaOH → Pb(OH)ଷ

ZnSOସ + 3NaOH → Zn(OH)ଷ

The S/L is another parameter that affects Pb and Zn removal efficiencies from ZPLRs due to changes in the ratio of hydroxyl concertation to Pb and Zn. To investigate the effects of the S/L ratio on Pb and Zn removal efficiencies, leaching experiments were carried out by varying the amounts of ZPLRs (i.e., 1–10 g) added in 50 mL of 3 M NaOH solution and shaking for 30 min in the water bath at 25 ◦C. The results show that Pb and Zn removal efficiencies decrease with increasing amounts of ZPLRs in a 50 mL of 3 M NaOH (Figure 1c). The Pb removal efficiency decreased from 62.5% for 1 g to 22.7% for 10 g of ZPLRs. Similarly, the Zn removal efficiency was negatively affected by the S/L. The Zn removal efficiency decreased from 27.1% for 1 g to 13.3% for 10 g of ZPLRs. This decrease in Pb and Zn removal efficiencies with an increase in the S/L can be attributed to the limited hydroxyl ions available to extract Pb and Zn [34], as highlighted above. due to changes in the ratio of hydroxyl concertation to Pb and Zn. To investigate the effects of the S/L ratio on Pb and Zn removal efficiencies, leaching experiments were carried out by varying the amounts of ZPLRs (i.e., 1–10 g) added in 50 mL of 3 M NaOH solution and shaking for 30 min in the water bath at 25 °C. The results show that Pb and Zn removal efficiencies decrease with increasing amounts of ZPLRs in a 50 mL of 3 M NaOH (Figure 1c). The Pb removal efficiency decreased from 62.5% for 1 g to 22.7% for 10 g of ZPLRs. Similarly, the Zn removal efficiency was negatively affected by the S/L. The Zn removal efficiency decreased from 27.1% for 1 g to 13.3% for 10 g of ZPLRs. This decrease in Pb and Zn removal efficiencies with an increase in the S/L can be attributed to the limited hydroxyl ions available to extract Pb and Zn [34], as highlighted above.

#### *3.2. Concurrent Cementation of Dissolved Pb and Zn in Leaching Pulp of ZPLRs* The concurrent cementation of dissolved Pb and Zn in leaching pulp was conducted

The concurrent cementation of dissolved Pb and Zn in leaching pulp was conducted using Al metal powder. Al is not only environmentally friendly as a cementation agent, but it also has a very low standard electrode potential (i.e., −2.35 V vs. NHE in basic solution), which makes it a thermodynamically good candidate for the cementation of dissolved Pb and Zn [35–37]. The oxide layer (Al2O3) which covers and insulates Al and suppresses the transfer of electrons dissolves at a high pH [30,38]. As previously discussed, Pb and Zn oxides equally dissolve and are complexed with hydroxide, as shown in Figure 3. using Al metal powder. Al is not only environmentally friendly as a cementation agent, but it also has a very low standard electrode potential (i.e., −2.35 V vs NHE in basic solution), which makes it a thermodynamically good candidate for the cementation of dissolved Pb and Zn [35–37]. The oxide layer (Al2O3) which covers and insulates Al and suppresses the transfer of electrons dissolves at a high pH [30,38]. As previously discussed, Pb and Zn oxides equally dissolve and are complexed with hydroxide, as shown in Figure 3.

*3.2. Concurrent Cementation of Dissolved Pb and Zn in Leaching Pulp of ZPLRs* 

**Figure 3.** Log activity-pH predominant diagram for (**a**) 0.1 mM Al3+ and 0.1 mM SO42<sup>−</sup>, (**b**) 0.1 mM Zn2+ and 0.1 mM SO42<sup>−</sup>, and (**c**) 0.1 mM Pb2+ and 0.1 mM SO42<sup>−</sup> at 25 °C and 1.013 bar created using **Figure 3.** Log activity-pH predominant diagram for (**a**) 0.1 mM Al3+ and 0.1 mM SO<sup>4</sup> <sup>2</sup>−, (**b**) 0.1 mM Zn2+ and 0.1 mM SO<sup>4</sup> <sup>2</sup>−, and (**c**) 0.1 mM Pb2+ and 0.1 mM SO<sup>4</sup> <sup>2</sup><sup>−</sup> at 25 ◦C and 1.013 bar created using the Geochemist's Workbench® with MINTEQ database [39] (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

3.2.1. Effects of Time and NaOH Concentration on Cementation and Distribution of Pb in Leaching Pulp

When 0.25 g of Al powder was added during the leaching of ZPLRs in 3 M NaOH solution, the overall chemical reaction that was thermodynamically expected is expressed by Equation (9), and the net reaction of anodic and cathodic half-reactions are expressed by Equations (7) and (8).

$$\text{Al} + 4\text{OH}^- \rightarrow \text{Al(OH)}\_4^- + 3\text{e}^- \quad -2.35\text{ V} \tag{7}$$

$$\text{Pb} + 4\text{OH}^- \rightarrow \text{Pb(OH)}\_4^{2-} + 2\text{e}^- \quad -0.54\text{ V} \tag{8}$$

$$2\text{Al} + 3\text{Pb(OH)}\_{4}^{2-} \rightarrow 2\text{Al(OH)}\_{4}^{-} + 4\text{OH}^{-} + 3\text{Pb} \tag{9}$$

ି + 3eି – 2.35 V (7)

ଶି + 2eି – 0.54 V (8)

ି + 4OHି + 3Pb (9)

Equation (9) is the cementation reaction of dissolved Pb by added Al powder. The standard Gibbs free energy change, ∆*G* 0 (i.e., ∆*G* <sup>0</sup> <sup>=</sup> <sup>−</sup>*nF*∆*<sup>E</sup>* 0 , *n* is number of electrons transferred, *F* is Faraday's constant, and ∆*E* 0 ( *E* 0 Al/Al(OH) − 4 − *E* 0 Pb/Pb(OH) 2− 4 ) is the galvanic cell potential of Equation (9)), is <sup>−</sup>1047.82 kJ/mol, implying that cementation of dissolved Pb2+ from ZPLRs by Al powder is thermodynamically feasible. The distribution of Pb among the cementation product, solution (i.e., dissolved Pb but uncemented), and undissolved Pb from ZPLRs for different leaching times and NaOH concentrations are shown in Figure 4. Figure 4a shows that the amount of dissolved but uncemented Pb decreased with an increase in leaching time of up to 30 min, where almost 100% of the dissolved Pb was cemented by Al metal powder. This entails that dissolved Pb from ZPLRs was cemented as described in the chemical reaction represented by Equation (9). However, a leaching period longer than 30 min led to an increase in the Pb remaining in the solution. This trend could be attributed to the redissolution of cemented Pb after all the Al has been dissolved and consumed by cementation and other side reactions. standard Gibbs free energy change, ∆ (i. e. , ∆ = െ∆, *n* is number of electrons transferred, *F* is Faraday's constant, and ∆ ( ୪ ୪(ୌ)ర ൗ <sup>ష</sup> െ ୠ ୠ(ୌ)ర <sup>൘</sup> మష ) is the galvanic cell potential of Equation (9)), is −1047.82 kJ/mol, implying that cementation of dissolved Pb2+ from ZPLRs by Al powder is thermodynamically feasible. The distribution of Pb among the cementation product, solution (i.e., dissolved Pb but uncemented), and undissolved Pb from ZPLRs for different leaching times and NaOH concentrations are shown in Figure 4. Figure 4a shows that the amount of dissolved but uncemented Pb decreased with an increase in leaching time of up to 30 min, where almost 100% of the dissolved Pb was cemented by Al metal powder. This entails that dissolved Pb from ZPLRs was cemented as described in the chemical reaction represented by Equation (9). However, a leaching period longer than 30 min led to an increase in the Pb remaining in the solution. This trend could be attributed to the redissolution of cemented Pb after all the Al has been dissolved and consumed by cementation and other side reactions.

ଶି → 2Al(OH)ସ

the Geochemist's Workbench® with MINTEQ database [39] (for interpretation of the references to

3.2.1. Effects of Time and NaOH Concentration on Cementation and Distribution of Pb

When 0.25 g of Al powder was added during the leaching of ZPLRs in 3 M NaOH solution, the overall chemical reaction that was thermodynamically expected is expressed by Equation (9), and the net reaction of anodic and cathodic half-reactions are expressed

color in this figure legend, the reader is referred to the web version of this article).

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Al + 4OHି → Al(OH)ସ

Pb + 4OHି → Pb(OH)ସ

2Al + 3Pb(OH)ସ

in Leaching Pulp

by Equations (7) and (8).

**Figure 4.** Effects of (**a**) leaching time and (**b**) NaOH concentration on cementation and distribution of Pb in leaching pulp when Al powder was added during ZPLR leaching (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). **Figure 4.** Effects of (**a**) leaching time and (**b**) NaOH concentration on cementation and distribution of Pb in leaching pulp when Al powder was added during ZPLR leaching (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

The effects of the NaOH concentration on the Pb distribution for concurrent cementation of dissolved Pb at 30 min of leaching time is shown in Figure 4b. As expected, the amount of Pb dissolved from ZPLRs and cemented by 0.25 g of Al powder increased with the increase in the NaOH concentration. The cemented Pb increased from 27% for 1 M The effects of the NaOH concentration on the Pb distribution for concurrent cementation of dissolved Pb at 30 min of leaching time is shown in Figure 4b. As expected, the amount of Pb dissolved from ZPLRs and cemented by 0.25 g of Al powder increased with the increase in the NaOH concentration. The cemented Pb increased from 27% for 1 M NaOH to 66.1% for 6 M NaOH, with no dissolved Pb remaining in the solution.

NaOH to 66.1% for 6 M NaOH, with no dissolved Pb remaining in the solution. 3.2.2. Effects of Time and NaOH Concentration on Cementation and Distribution of Zn in Leaching Pulp

The half-reactions represented by Equations (10) and (11) that add up to the overall chemical reaction depicted in Equation (12) were thermodynamically expected when 0.25 g of Al powder was added during the leaching of ZPLRs in 3 M NaOH solution.

$$\text{Al} + 4\text{OH}^- \rightarrow \text{Al(OH)}\_4^- + 3\text{e}^- \quad -2.35\text{ V} \tag{10}$$

$$\text{Pb} + 4\text{OH}^- \rightarrow \text{Pb(OH)}\_4^{2-} + 2\text{e}^- \quad -1.285\text{ V} \tag{11}$$

$$\text{2Al} + \text{3Zn(OH)}\_{4}^{2-} \rightarrow \text{2Al(OH)}\_{4}^{-} + \text{4OH}^{-} + \text{3Zn} \tag{12}$$

Equation (12) is the cementation reaction of dissolved Zn as ZnOH)<sup>4</sup> <sup>2</sup><sup>−</sup> from ZPLRs is cemented by Al whose standard Gibbs free energy change, ∆*G* 0 (i.e., ∆*G* <sup>0</sup> <sup>=</sup> <sup>−</sup>*nF*∆*<sup>E</sup>* 0 , *n* is number of electrons transferred, *F* is Faraday's constant, and ∆*E* 0 (*E* 0 Al/Al(OH) − 4 − *E* 0 Zn/Zn(OH) 2− 4 ) is the galvanic cell potential of Equation (9)), is −616.57 kJ/mol. This means that the cementation of dissolved Zn2+ from ZPLRs by Al powder is thermodynamically favorable. However, the results show that little Zn was cemented by Al metal powder from 7.5 up to 120 min

using various NaOH concentrations because most of the dissolved Zn remained in solution (Figure 5a,b). This could mean that there were some counter-reactions to the cementation reaction. These reactions could arise from co-dissolved elements in the leachate and/or solid residues. from 7.5 up to 120 min using various NaOH concentrations because most of the dissolved Zn remained in solution (Figure 5a,b). This could mean that there were some counterreactions to the cementation reaction. These reactions could arise from co-dissolved elements in the leachate and/or solid residues.

3.2.2. Effects of Time and NaOH Concentration on Cementation and Distribution of Zn

g of Al powder was added during the leaching of ZPLRs in 3 M NaOH solution.

number of electrons transferred, *F* is Faraday's constant, and ∆ ( ୪

The half-reactions represented by Equations (10) and (11) that add up to the overall chemical reaction depicted in Equation (12) were thermodynamically expected when 0.25

ଶି → 2Al(OH)ସ

) is the galvanic cell potential of Equation (9)), is −616.57 kJ/mol*.* This means

that the cementation of dissolved Zn2+ from ZPLRs by Al powder is thermodynamically favorable. However, the results show that little Zn was cemented by Al metal powder

Equation (12) is the cementation reaction of dissolved Zn as ZnOH)42− from ZPLRs is cemented by Al whose standard Gibbs free energy change, ∆(i. e. , ∆ = െ∆, *n* is

ି + 3eି െ 2.35 V (10)

ଶି + 2eି െ 1.285 V (11)

ି + 4OHି + 3Zn (12)

୪(ୌ)ర ൗ <sup>ష</sup> െ

*Minerals* **2022**, *12*, x 7 of 13

Al + 4OHି → Al(OH)ସ

Pb + 4OHି → Pb(OH)ସ

2Al + 3Zn(OH)ସ

in Leaching Pulp

୬

୬(ୌ)ర ൗ మష

**Figure 5.** Effects of (**a**) leaching time and (**b**) NaOH concentration on cementation and distribution of Zn in leaching pulp when Al powder was added during ZPLR leaching (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). **Figure 5.** Effects of (**a**) leaching time and (**b**) NaOH concentration on cementation and distribution of Zn in leaching pulp when Al powder was added during ZPLR leaching (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

When the cementation product was analyzed by SEM-EDX, it was shown that both Pb and Zn were cemented but the intensity for Zn was much lower than Pb (Figure 6). When the cementation product was analyzed by SEM-EDX, it was shown that both Pb and Zn were cemented but the intensity for Zn was much lower than Pb (Figure 6).

**Figure 6.** SEM-EDX of cementation product when 2.5 g ZPLRs were leached in 3 M NaOH solution for 30 min with the addition of 0.25 g Al powder: (**a**) SEM microphotography, (**b**) EDX map of Pb, (**c**) EDX map of Zn, and (**d**) EDX map of Al (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

To investigate the effects of co-dissolved elements and solid residues on the cementation of dissolved Zn from leaching solution using Al metal powder, simulated (model) 3 M NaOH solutions containing both 8 mM Pb2+ and 10 mM Zn2+ and filtrate (to eliminate solid residues interference) after the initial addition of Al powder during ZPLR leaching, respectively, were used. The model solution was prepared by dissolving ZnCl<sup>2</sup> and PbCl<sup>2</sup> (Wako Pure Chemical Industries, Ltd., Osaka, Japan) in 3 M NaOH. For the model solution,

1

0.15 g of Al metal powder was added to cement both Pb and Zn. For the filtrate after the concurrent cementation of dissolved Pb and Zn experiments, 0.1 g of Al metal powder was added to the cement residual Zn in the presence of other co-dissolved elements from ZPLRs. Figure 7a shows that 100% of Pb and 100% of Zn in the model solution were cemented out of the solution by Al metal powder. This confirms the thermodynamic feasibility discussed above and that Al metal powder can cement Zn. Likewise, the dissolved Zn in the filtrate after the concurrent cementation experiment was recovered (around 96.9%) by the second portion of Al metal powder added in the filtrate (Figure 7b). This implies that the co-dissolved elements from ZPLRs and from cementation experiments do not affect the cementation of Zn in NaOH solution. The results, however, highlight that solid residue could possibly interfere with and suppress the cementation of Zn during the concurrent cementation of dissolved Pb and Zn experiments. *Minerals* **2022**, *12*, x 9 of 13

**Figure 7.** Amounts of Pb and Zn cemented out from (**a**) 3 M NaOH model solution containing Pb and Zn ions and treated for 15 and 30 min, and (**b**) concurrent cementation of dissolved Pb and Zn ions and cementation of Zn in the filtrate solution after concurrent cementation experiment (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article)*.* **Figure 7.** Amounts of Pb and Zn cemented out from (**a**) 3 M NaOH model solution containing Pb and Zn ions and treated for 15 and 30 min, and (**b**) concurrent cementation of dissolved Pb and Zn ions and cementation of Zn in the filtrate solution after concurrent cementation experiment (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

To investigate the effects of minerals in the solid residues from ZPLRs that could affect the cementation of dissolved Zn from leaching pulp using Al metal powder, 2.5 g (to maintain the same S/L ratio) of the three most abundant minerals in the ZPLRs we used. This included SiO2 (quartz), Fe2O3 (hematite), and Fe3O4 (magnetite), and each mineral was added in a model solution of 3 M NaOH solutions containing both 8 mM Pb2+ and 10 mM Zn2+. To mimic the ZPLR concurrent cementation experiments, 0.25 g of Al metal powder was added to the solution and shaken for 30 min in a temperature-controlled bash shaker. For all of the three solid residues, around 98% of Pb was cemented by Al metal powder (Figure 8). However, Zn cementation was slightly reduced by SiO2 (93%) and significantly suppressed by Fe2O3 (28.5%) and Fe3O4 (27.9%). The slight decrease in cementation in the case of Zn in 2.5 g could be ascribed to dissolved silicate anions that exhibit the properties of nanoparticles suspension (colloid) hence affecting viscosity and the transportation of metal ions on the surface of the Al metal and cementation Al ions away from the surface of Al metals [40]. Meanwhile, the significant suppression by Fe2O3 and Fe3O4 could be attributed to preferential consumption of electrons from Al metal powder to reduce Fe3+ to Fe2+ (i.e., Fe(OH)3 to Fe(OH)2), whose standard electrode potential in basic solution is around −0.54 V. This deduction is also supported by other works highlighting the participation of Fe2O3 and Fe3O4 in electrochemical reactions such as pyrite dissolution, arsenite oxidation to arsenate, and the recovery of gold ions from chloride To investigate the effects of minerals in the solid residues from ZPLRs that could affect the cementation of dissolved Zn from leaching pulp using Al metal powder, 2.5 g (to maintain the same S/L ratio) of the three most abundant minerals in the ZPLRs we used. This included SiO<sup>2</sup> (quartz), Fe2O<sup>3</sup> (hematite), and Fe3O<sup>4</sup> (magnetite), and each mineral was added in a model solution of 3 M NaOH solutions containing both 8 mM Pb2+ and 10 mM Zn2+. To mimic the ZPLR concurrent cementation experiments, 0.25 g of Al metal powder was added to the solution and shaken for 30 min in a temperature-controlled bash shaker. For all of the three solid residues, around 98% of Pb was cemented by Al metal powder (Figure 8). However, Zn cementation was slightly reduced by SiO<sup>2</sup> (93%) and significantly suppressed by Fe2O<sup>3</sup> (28.5%) and Fe3O<sup>4</sup> (27.9%). The slight decrease in cementation in the case of Zn in 2.5 g could be ascribed to dissolved silicate anions that exhibit the properties of nanoparticles suspension (colloid) hence affecting viscosity and the transportation of metal ions on the surface of the Al metal and cementation Al ions away from the surface of Al metals [40]. Meanwhile, the significant suppression by Fe2O<sup>3</sup> and Fe3O<sup>4</sup> could be attributed to preferential consumption of electrons from Al metal powder to reduce Fe3+ to Fe2+ (i.e., Fe(OH)<sup>3</sup> to Fe(OH)2), whose standard electrode potential in basic solution is around −0.54 V. This deduction is also supported by other works highlighting the participation of Fe2O<sup>3</sup> and Fe3O<sup>4</sup> in electrochemical reactions such as pyrite dissolution, arsenite oxidation to arsenate, and the recovery of gold ions from chloride solutions [41,42]

The concurrent cementation of both dissolved Pb and Zn can be applied for the remediation of Pb-Zn mine waste materials or Pb-Zn contaminated soil that do not contain

with less suppression by Fe oxides. Meanwhile, the dissolved Zn can be recovered after

filtration by either cementation using Al or precipitation as ZnS [21].

solutions [41,42]

**Figure 8.** Effects of SiO2, Fe2O3, and Fe3O4 on cementation of Pb and Zn when 2.5 of a given mineral was mixed in 3 M NaOH solution containing 8 mM of Pb2+ and Zn2+ for 30 min (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this **Figure 8.** Effects of SiO<sup>2</sup> , Fe2O<sup>3</sup> , and Fe3O<sup>4</sup> on cementation of Pb and Zn when 2.5 of a given mineral was mixed in 3 M NaOH solution containing 8 mM of Pb2+ and Zn2+ for 30 min (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

article). **4. Conclusions**  This study investigated Pb and Zn removal from ZPLRs in alkaline solution by the concurrent cementation of dissolved Pb and Zn in leaching pulp. The findings are sum-The concurrent cementation of both dissolved Pb and Zn can be applied for the remediation of Pb-Zn mine waste materials or Pb-Zn contaminated soil that do not contain a substantial amount of iron oxide. The most toxic metal, Pb, can be cemented in pulp with less suppression by Fe oxides. Meanwhile, the dissolved Zn can be recovered after filtration by either cementation using Al or precipitation as ZnS [21].

#### marized as below: (1) Pb and Zn removal efficiencies were affected by the leaching time, the NaOH con-**4. Conclusions**

centration, and the S/L ratio. The Pb and Zn removal efficiencies were 62.2% and 27.1%, respectively, when 2.5 g/50 mL (S/L) of ZPLRs were leached in a 3 M NaOH solution for 30 min. This study investigated Pb and Zn removal from ZPLRs in alkaline solution by the concurrent cementation of dissolved Pb and Zn in leaching pulp. The findings are summarized as below:

	- powder. However, around 96.9% was cemented by Al after filtration. The suppression of cementation by Al metal was attributed to solid residues, in particular Fe ox-(3) Around 100% of the dissolved Pb was cemented by Al metal powder for the concurrent cementation of dissolved Pb in the leaching pulp.

M.S., M.I. (Mayumi Ito), N.H., S.J., I.P. and C.B.T.; investigation, M.S.; writing—original draft prep-

aration, M.S.; writing—review and editing, M.S., M.I. (Mayumi Ito), N.H., C.B.T., S.J., I.P., I.N., T.S., and T.I.; supervision, M.I. (Mayumi Ito) and N.H.; project administration, M.I. (Mayumi Ishizuka), S.N. and H.N.; funding acquisition, M.I. (Mayumi Ishizuka)., S.N. and H.N. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was supported partly by the Japan Society for the Promotion of Science (JSPS) CORE to CORE program (M.I. (Mayumi Ishizuka)), Hokkaido University SOUSEI-TOKUTEI Spe-**Author Contributions:** Conceptualization, M.S.; methodology, M.S., T.F. and R.H.; formal analysis, M.S., M.I. (Mayumi Ito), N.H., S.J., I.P. and C.B.T.; investigation, M.S.; writing—original draft preparation, M.S.; writing—review and editing, M.S., M.I. (Mayumi Ito), N.H., C.B.T., S.J., I.P., I.N., T.S. and T.I.; supervision, M.I. (Mayumi Ito) and N.H.; project administration, M.I. (Mayumi Ishizuka), S.N. and H.N.; funding acquisition, M.I. (Mayumi Ishizuka)., S.N. and H.N. All authors have read and agreed to the published version of the manuscript.

cific Research Projects (M.I. (Mayumi Ishizuka)), JSPS Bilateral Open Partnership Joint Research Projects (grant number: JPJSBP120209902) (S.N.), The Japan Prize Foundation (S.N.), and Grants-in-**Funding:** This work was supported partly by the Japan Society for the Promotion of Science (JSPS) CORE to CORE program (M.I. (Mayumi Ishizuka)), Hokkaido University SOUSEI-TOKUTEI Specific

Research Projects (M.I. (Mayumi Ishizuka)), JSPS Bilateral Open Partnership Joint Research Projects (grant number: JPJSBP120209902) (S.N.), The Japan Prize Foundation (S.N.), and Grants-in-Aid for Scientific Research from the Ministry of Education, and Culture, Sports, Science and Technology of Japan (grant number 20K20633) (S.N.).

**Data Availability Statement:** Data is available on request because of the restrictions, as the research is ongoing.

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