Alkaline Leaching and Concurrent Cementation of Dissolved Pb and Zn from Zinc Plant Leach Residues

: Zinc plant leach residues (ZPLRs), particularly those produced using old technologies, have both economic importance as secondary raw materials and have environmental impacts because they contain hazardous heavy metals that pose risks to human health and the environment. Therefore, the extraction and recovery of these metals from ZPLRs has both economic and environmental beneﬁts. In this study, we investigated the removal of lead (Pb) and zinc (Zn) from ZPLRs by alkaline (NaOH) leaching and the concurrent cementation of dissolved Pb and Zn using aluminum (Al) metal powder. The effects of the leaching time, NaOH concentration, solid-to-liquid ratio (S/L), and dosage of Al metal powder on the extraction of Pb and Zn were investigated. Pb and Zn removal efﬁciencies increased with increasing NaOH concentrations and decreasing S/Ls. 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. The extraction of Pb and Zn could be attributed to the partitioning of these metals in relatively more mobile phases—water-soluble, exchangeable, and carbonate phases— in ZPLRs. Around 100% of dissolved Pb and less than 2% of dissolved Zn were cemented in leaching pulp when Al metal powder was added. Minerals in the solid residues, particularly iron oxides minerals, were found to suppress the cementation of extracted Zn in leaching pulp, and when they were removed by ﬁltration, Zn was recovered by Al metal powder via cementation.


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][2][3].In a recent report by the World Bank, zinc (Zn) and lead (Pb) were two Minerals 2022, 12, 393 2 of 11 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][8][9][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][12][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][15][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][18][19][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][7][8][9][10][11][12], Pb and Zn dissolve as Pb(OH) 3 − with small amounts of Pb(OH) 4  2− and Zn(OH) 3 − with small amounts of Zn(OH) 4 2− , respectively.In a strong alkaline solution (i.e., pH > 12), the dominant species are Pb(OH) 4 2− for Pb and Zn(OH) 4 2− for Zn.Many researchers have investigated and successfully extracted Pb and Zn from ZPLRs using alkaline solutions [14,[22][23][24].The alkaline extractive processes studied are as follows: leaching → solid-liquid separation → metal recovery stages. Howwever, 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.

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 (η Me ) from ZPLRs with and without the addition of Al powder, Equations (1) and (2), respectively, were used.
where C Me is the concentration (g/L) of Pb and Zn, V is the volume (L) of the leaching solution, W S is the weight % of either Pb and Zn in ZPLRs, M s is the mass (g) of the leached ZPLRs, M cme is the mass (g) of cemented and agglomerated particles, and W cme is the weight % of Pb and Zn in cemented and agglomerated particles calculated based on the digested fraction of M cme in aqua regia and analysis of the solution by ICP-AES.

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].
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].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 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].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].
Minerals 2022, 12, 393 5 of 11 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.
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.

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 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][36][37].The oxide layer (Al 2 O 3 ) 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.In weak to moderately strong alkaline solutions (pH 6-12), the dominant species fo Pb and Zn are Pb(OH) and Zn(OH) .When the NaOH concentration increases (stron alkaline solution, pH >12) the equilibrium shifts and the more soluble Pb and Zn hydrox complexes Pb(OH) and Zn(OH) , respectively, become more dominant [21].This e plains why Pb and Zn removal efficiencies increased at higher concentrations of NaO (i.e., 3 M NaOH).Increasing the NaOH concentration beyond 3 M did not improve the P and Zn removal efficiencies because almost all the easily extractable Pb and Zn (as dete mined by sequential extraction) from the ZPLRs were already exhausted.
The S/L is another parameter that affects Pb and Zn removal efficiencies from ZPLR due to changes in the ratio of hydroxyl concertation to Pb and Zn.To investigate the effec of the S/L ratio on Pb and Zn removal efficiencies, leaching experiments were carried o by varying the amounts of ZPLRs (i.e., 1-10 g) added in 50 mL of 3 M NaOH solution an shaking for 30 min in the water bath at 25 °C.The results show that Pb and Zn remov efficiencies decrease with increasing amounts of ZPLRs in a 50 mL of 3 M NaOH (Figu 1c).The Pb removal efficiency decreased from 62.5% for 1 g to 22.7% for 10 g of ZPLR Similarly, the Zn removal efficiency was negatively affected by the S/L.The Zn remov efficiency decreased from 27.1% for 1 g to 13.3% for 10 g of ZPLRs.This decrease in P and Zn removal efficiencies with an increase in the S/L can be attributed to the limite hydroxyl ions available to extract Pb and Zn [34], as highlighted above.

Concurrent Cementation of Dissolved Pb and Zn in Leaching Pulp of ZPLRs
The concurrent cementation of dissolved Pb and Zn in leaching pulp was conducte using Al metal powder.Al is not only environmentally friendly as a cementation agen but it also has a very low standard electrode potential (i.e., −2.35 V vs NHE in basic sol tion), which makes it a thermodynamically good candidate for the cementation of di solved Pb and Zn [35][36][37].The oxide layer (Al2O3) which covers and insulates Al and su presses the transfer of electrons dissolves at a high pH [30,38].As previously discusse Pb and Zn oxides equally dissolve and are complexed with hydroxide, as shown in Figu 3. 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).

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).
Equation ( 9) is the cementation reaction of dissolved Pb by added Al powder.The standard Gibbs free energy change, ∆G 0 (i.e., ∆G 0 = −nF∆E 0 , n is number of electrons transferred, F is Faraday's constant, and ) is the galvanic cell potential of Equation ( 9)), is −1047.82kJ/mol, implying that cementation of dissolved Pb 2+ 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.
Pb + 4OH → Pb(OH) + 2e -0.54 V (8) Equation ( 9) is the cementation reaction of dissolved Pb by added Al powder.The standard Gibbs free energy change, ∆ (i.e. , ∆ = ∆ , n is number of electrons transferred, F is Faraday's constant, and ∆ ( ) is the galvanic cell potential of Equation ( 9)), is −1047.82kJ/mol, implying that cementation of dissolved Pb 2+ 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.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.

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.
Equation ( 12) is the cementation reaction of dissolved Zn as ZnOH) 4 2− from ZPLRs is cemented by Al whose standard Gibbs free energy change, ∆G 0 (i.e., ∆G 0 = −nF∆E 0 , n is number of electrons transferred, F is Faraday's constant, and

4
) is the galvanic cell potential of Equation ( 9)), is −616.57kJ/mol.This means that the cementation of dissolved Zn 2+ 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.
cemented by Al whose standard Gibbs free energy change, ∆ (i.e. , ∆ = ∆ , n is number of electrons transferred, F is Faraday's constant, and ∆ ( ) is the galvanic cell potential of Equation ( 9)), is −616.57kJ/mol.This means that the cementation of dissolved Zn 2+ 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 counterreactions to the cementation reaction.These reactions could arise from co-dissolved elements in the leachate and/or solid residues.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).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 Pb 2+ and 10 mM Zn 2+ 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 2 and PbCl 2 (Wako Pure Chemical Industries, Ltd., Osaka, Japan) in 3 M NaOH.For the model solution, Minerals 2022, 12, 393 8 of 11 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.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 Pb 2+ and 10 mM Zn 2+ .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 Fe 3+ to Fe 2+ (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 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 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].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 2 (quartz), Fe 2 O 3 (hematite), and Fe 3 O 4 (magnetite), and each mineral was added in a model solution of 3 M NaOH solutions containing both 8 mM Pb 2+ and 10 mM Zn 2+ .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 2 (93%) and significantly suppressed by Fe 2 O 3 (28.5%)and Fe 3 O 4 (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 Fe 2 O 3 and Fe 3 O 4 could be attributed to preferential consumption of electrons from Al metal powder to reduce Fe 3+ to Fe 2+ (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 Fe 2 O 3 and Fe 3 O 4 in electrochemical reactions such as pyrite dissolution, arsenite oxidation to arsenate, and the recovery of gold ions from chloride solutions [41,42] Figure 8. Effects of SiO2, Fe2O3, and Fe3O4 on cementation of Pb and Zn when 2.5 of a given min- eral was mixed in 3 M NaOH solution containing 8 mM of Pb 2+ and Zn 2+ 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).

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 summarized as below: (1) Pb and Zn removal efficiencies were affected by the leaching time, the NaOH concentration, 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.(2) The amounts Pb (62.2%) and Zn (27.1%) that were removed from ZPLRs using NaOH solution correlated and corroborated with the mobile phase fraction (i.e., Pb and Zn bound to water, exchangeable, and carbonates) approximated by sequential extraction.(3) Around 100% of the dissolved Pb was cemented by Al metal powder for the concurrent cementation of dissolved Pb in the leaching pulp.(4) The dissolved Zn was not cemented out in leaching pulp by the addition of Al metal 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 oxides.(5) The concurrent cementation of both dissolved Pb and Zn in alkaline leaching pulp has the potential to be applied for the remediation of Pb-Zn mine wastes or Pb-Zn contaminated soil, provided they contain minimal amounts of iron oxides, which were found to suppress Zn cementation.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].

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 summarized as below: (1) Pb and Zn removal efficiencies were affected by the leaching time, the NaOH concentration, 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.(2) The amounts Pb (62.2%) and Zn (27.1%) that were removed from ZPLRs using NaOH solution correlated and corroborated with the mobile phase fraction (i.e., Pb and Zn bound to water, exchangeable, and carbonates) approximated by sequential extraction.(3) Around 100% of the dissolved Pb was cemented by Al metal powder for the concurrent cementation of dissolved Pb in the leaching pulp.(4) The dissolved Zn was not cemented out in leaching pulp by the addition of Al metal 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 oxides.(5) The concurrent cementation of both dissolved Pb and Zn in alkaline leaching pulp has the potential to be applied for the remediation of Pb-Zn mine wastes or Pb-Zn contaminated soil, provided they contain minimal amounts of iron oxides, which were found to suppress Zn cementation.

Figure 1 .
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 .
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 1 .
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 .
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 .
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 .
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 4 .
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 NaOH to 66.1% for 6 M NaOH, with no dissolved Pb remaining in the solution.

Figure 4 .
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 5 .
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 .
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 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).

Minerals 2022, 12 , x 9 of 13 Figure 7 .
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 .
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 8 .
Figure 8. Effects of SiO 2 , Fe 2 O 3 , and Fe 3 O 4 on cementation of Pb and Zn when 2.5 of a given mineral was mixed in 3 M NaOH solution containing 8 mM of Pb 2+ and Zn 2+ 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).