Recovery of Lead and Zinc from Zinc Plant Leach Residues by Concurrent Dissolution-Cementation Using Zero-Valent Aluminum in Chloride Medium

: Zinc plant leach residues (ZPLRs) contain signiﬁcant amounts of metal compounds of lead (Pb), zinc (Zn), iron (Fe), etc., hence, they are considered as a secondary source of metals. On the other hand, ZPLRs are regarded as hazardous materials because they contain heavy metals that pollute the environment. Resources and environmental concerns of ZPLRs were addressed in this study by removing / recovering Pb and Zn using a concurrent dissolution and cementation technique. To cement the dissolved Pb and Zn in leaching pulp, zero-valent aluminum (ZVAl) was added during ZPLRs leaching in the hydrochloric (HCl)–sodium chloride (NaCl) solution. The resulting cemented metals were agglomerated and separated by sieving. Lead removal increased with increasing both NaCl and HCl concentrations. However, when ZVAl was added, signiﬁcant Pb removal was achieved at a low concentration. Zinc was not cemented out of the pulp using ZVAl and its recovery from ZPLRs was dependent on the HCl concentration only. By applying a concurrent dissolution and cementation technique, both Pb and Zn were removed using a low concentration of NaCl, and most importantly Pb—the most toxic metal in ZPLRs—was captured and separated before the solid-liquid separation, hence, eliminating the need for extensive washing of the generated residues to remove the inherent residual solution. Pb


Introduction
Explosive population growth and its associated economic activities such as massive construction projects to modernize and improve communication, transportation, and agricultural sectors have in recent years led to high demands for metals [1][2][3][4]. To keep up with demands, mining and metals production have also increased at unprecedented levels. Enormous amounts of solid wastes are also mortar and pestle, and then dry-sieved using stainless steel sieves to obtain sample with particles passing 106 µm fraction. Chemical characterization of the ZPLR samples was carried out using both X-ray fluorescence spectroscopy (XRF, EDXL 300, Rigaku Corporation, Tokyo, Japan) and ICP-AES (ICPE-9820, Shimadzu Corporation, Kyoto, Japan) after aqua regia (3 HCl:1 HNO 3 v/v) digestion in a microwave-assisted acid digestion system (Ethos Advanced Microwave Lab station, Milestone Inc., Sorisole, Italy). The amounts of Pb and Zn in ZPLR samples were as high as 6.19% and 2.53%, respectively (Table 1). ZPLR samples also contained significant amounts of other elements such as Si, Fe, Ca, S, Cu, and other elements, as shown in Table 1. The mineralogical composition of ZPLRs was determined by XRD (MultiFlex, Rigaku Corporation, Tokyo, Japan) and crystalline minerals were identified using a full package of the Crystallography Open Database (COD) and MATCH 3.4. The crystalline Pb and Zn minerals in ZPLRs that were detected included anglesite (PbSO 4 ), cerussite (PbCO 3 ), esperite (PbCa 2 Zn 3 (SiO 4 ) 3 ), and zinkosite (ZnSO 4 ), as illustrated in Figure 2. Other minerals detected in the samples are quartz (SiO 2 ), gypsum (CaSO 4 ·2H 2 O), hematite (Fe 2 O 3 ), and goethite (FeOOH). The particle size distributions of lightly pulverized ZPLRs were analyzed using Laser diffraction (Microtrac ® MT3300SX, Nikkiso Co. Ltd., Osaka, Japan) and were found to have a median size (D 50 ) of around 9.6 µm ( Figure 3a).
Metals 2020, 10, x FOR PEER REVIEW 3 of 16 ZPLR samples were collected from the historic dumpsite of Pb-Zn mine wastes in Kabwe, Zambia ( Figure 1). The samples were air-dried for 30 days in the laboratory, lightly pulverized with an agate mortar and pestle, and then dry-sieved using stainless steel sieves to obtain sample with particles passing 106 µ m fraction. Chemical characterization of the ZPLR samples was carried out using both X-ray fluorescence spectroscopy (XRF, EDXL 300, Rigaku Corporation, Tokyo, Japan) and ICP-AES (ICPE-9820, Shimadzu Corporation, Kyoto, Japan) after aqua regia (3 HCl:1 HNO3 v/v) digestion in a microwave-assisted acid digestion system (Ethos Advanced Microwave Lab station, Milestone Inc., Sorisole, Italy). The amounts of Pb and Zn in ZPLR samples were as high as 6.19% and 2.53%, respectively (Table 1). ZPLR samples also contained significant amounts of other elements such as Si, Fe, Ca, S, Cu, and other elements, as shown in Table 1. The mineralogical composition of ZPLRs was determined by XRD (MultiFlex, Rigaku Corporation, Tokyo, Japan) and crystalline minerals were identified using a full package of the Crystallography Open Database (COD) and MATCH 3.4. The crystalline Pb and Zn minerals in ZPLRs that were detected included anglesite (PbSO4), cerussite (PbCO3), esperite (PbCa2Zn3(SiO4)3), and zinkosite (ZnSO4), as illustrated in Figure  2. Other minerals detected in the samples are quartz (SiO2), gypsum (CaSO4·2H2O), hematite (Fe2O3), and goethite (FeOOH). The particle size distributions of lightly pulverized ZPLRs were analyzed using Laser diffraction (Microtrac ® MT3300SX, Nikkiso Co. Ltd., Osaka, Japan) and were found to have a median size (D50) of around 9.6 µ m ( Figure 3a).    Reagent grade NaCl and HCl (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used to prepare the leaching solutions of different concentrations by dissolution and dilution using deionized (DI) water (18 MΩ·cm, Milli-Q ® Integral Water Purification System, Merck Millipore, Burlington, Vermont, USA). To simultaneously precipitate reductively (cement) the dissolved Pb 2+ and Zn 2+ in leaching pulp, ultra-pure ZVAl powder (>99.99%, 50-150 µm, Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used (the median particle size (D 50 ) of ZVAl was 126.8 µm). The particle size distribution is shown in Figure 3b. A stainless steel sieve with 150 µm aperture size was used to separate cemented and agglomerated Pb and Zn from the leaching pulp. The sieve size was selected by taking into consideration the particles size ranges of both ZPLRs and ZVAl. In other words, this sieve could only retain cemented and agglomerated particles while passing particles of unreacted ZVAl and particles of undissolved minerals particles of ZPLRs. Reagent grade NaCl and HCl (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used to prepare the leaching solutions of different concentrations by dissolution and dilution using deionized (DI) water (18 MΩ·cm, Milli-Q ® Integral Water Purification System, Merck Millipore, Burlington, Vermont, USA). To simultaneously precipitate reductively (cement) the dissolved Pb 2+ and Zn 2+ in leaching pulp, ultra-pure ZVAl powder (>99.99%, 50-150 µ m, Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used (the median particle size (D50) of ZVAl was 126.8 µ m). The particle size distribution is shown in Figure 3b. A stainless steel sieve with 150 µ m aperture size was used to separate cemented and agglomerated Pb and Zn from the leaching pulp. The sieve size was selected by taking into consideration the particles size ranges of both ZPLRs and ZVAl. In other words, this sieve could only retain cemented and agglomerated particles while passing particles of unreacted ZVAl and particles of undissolved minerals particles of ZPLRs.   Reagent grade NaCl and HCl (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used to prepare the leaching solutions of different concentrations by dissolution and dilution using deionized (DI) water (18 MΩ·cm, Milli-Q ® Integral Water Purification System, Merck Millipore, Burlington, Vermont, USA). To simultaneously precipitate reductively (cement) the dissolved Pb 2+ and Zn 2+ in leaching pulp, ultra-pure ZVAl powder (>99.99%, 50-150 µ m, Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used (the median particle size (D50) of ZVAl was 126.8 µ m). The particle size distribution is shown in Figure 3b. A stainless steel sieve with 150 µ m aperture size was used to separate cemented and agglomerated Pb and Zn from the leaching pulp. The sieve size was selected by taking into consideration the particles size ranges of both ZPLRs and ZVAl. In other words, this sieve could only retain cemented and agglomerated particles while passing particles of unreacted ZVAl and particles of undissolved minerals particles of ZPLRs.

Leaching-Cementation Experiments in Chloride Solution
Batch leaching experiments for the extraction of Pb and Zn from ZPLRs with and without ZVAl additions were conducted using a 200-mL Erlenmeyer flask. The volume of the leaching solution was at 50 mL for all experiments. Concentrations of NaCl (0-3 M) were varied and acidified with different HCl concentrations (0-0.1 M) to obtain required leaching solutions. Fifty milliliters (50 mL) of leaching solution of a given concentration was initially poured in an Erlenmeyer flask and nitrogen (N 2 ) was purged for 10 min to remove dissolved oxygen (DO). Nitrogen gas (N 2 ) purging was again carried out Metals 2020, 10, 531 5 of 15 for 2 min after the addition of 2.5 g ZPLRs with and without 0.1 g ZVAl that were added before sealing the flask using silicon stoppers and parafilm ® . The flask was then shaken at 4 cm amplitude and 120 min −1 shaking frequency in a water-bath shaker maintained at 25 • C for a predetermined length of time. At the end of the predetermined shaking time, the leaching pulp was carefully collected, and solid-liquid separation was carried out by filtering the collected leaching pulp using a syringe-driven membrane filter-pore size of 0.20 µm-(LMS Co., Ltd. Tokyo, Japan). The filtrate was then analyzed for dissolved Pb and Zn using ICP-AES. In the case where ZVAl was added during ZPLR leaching, additional steps-the separation of cemented and agglomerated product from the leaching pulp by screening using a sieve of aperture size of 150 µm-were carried out. The +150 µm particles (cemented and agglomerated) were thoroughly washed with deionized (DI) water before drying in a vacuum oven at 40 • C for 24 h. Dried +150 µm particles were then digested in aqua regia using a microwave-assisted acid digestion system and the leachate was analyzed for Pb and Zn using ICP-EAS. Furthermore, the +150 µm particles obtained were examined by both XRD and SEM-EDX (JSM-IT200, JEOL Ltd., Tokyo, Japan). All the experimental tests were carried out twice and the average was reported here.
The Pb and Zn removal (R Pb,Zn ) from ZPLRs without and with ZVAl were quantified using Equations (1) and (2), respectively.
where C Pb,Zn is the concentration (g/L) of Pb and Zn, V is the volume (L) of leaching solution, W S is the weight percent (%) of either Pb and Zn, M s is the mass (g) of leached ZPLRs, M cg is the mass (g) of cemented and agglomerated particles, and W cg is the weight percent (%) of cemented and agglomerated particles calculated based on the digested fraction of M cg in aqua regia and analysis of the solution by ICP-AES.

Leachability of Lead and Zinc after Concurrent Dissolution-Cementation
To evaluate the leachability of Pb and Zn from before and after concurrent dissolution-cementation, leachability experiments were conducted according to the toxicity characteristic leaching procedure (TCLP) [26]. For TCLP, 1 g of vacuum-dried treated and untreated residues were equilibrated with 20 mL of acetic acid solution (pH 2.89) in a centrifuge tube shaken at 30 rpm on a rotary tumbler for 18 h. After the predetermined leaching time, the leachate was filtered through 0.20 µm syringe-driven membrane filters and the filtrate was analyzed for dissolved Pb and Zn using ICP-AES.

Concurrent Dissolution-Cementation of Pb and Zn from Zinc Plant Leach Residues
The concentrations of Pb and Zn as a function of time when 2.5 g of ZPLRs were leached in a solution composed of 3 M NaCl and 0.05 M HCl with and without the addition of 0.1 g ZVAl is shown in Figure 4a,b. The concentration of Pb when ZPLRs were leached without ZVAl reached an apparent equilibrium of around 8.5 mM (which represents 59% of total Pb) after just 15 min ( Figure 4a). Pb dissolution from ZPLRs involves the formation of lead-chloride complexes as explained by Equations (3) and (4) [20,[27][28][29]: where PbCl (2−x) x and x are lead-chloride complex(es) and integers from 1 to 4, respectively, all of which depended on the chloride concentration.
in Figure 4a,b. The concentration of Pb when ZPLRs were leached without ZVAl reached an apparent equilibrium of around 8.5 mM (which represents 59% of total Pb) after just 15 min (Figure 4a). Pb dissolution from ZPLRs involves the formation of lead-chloride complexes as explained by Equations (3) and (4) [20,[27][28][29]: where PbCl x (2-x) and x are lead-chloride complex(es) and integers from 1 to 4, respectively, all of which depended on the chloride concentration. The concentration of dissolved Pb when ZVAl was added was 10-fold lower than when only ZPLRs were leached in the same solution. The dissolved concentration of Pb decreased further with increasing the treatment time and reached below 0.048 mM (i.e., 0.1 mg/L) with ZVAl after 4 h. The dramatically lower dissolved concentration of Pb after 15 min and its continued decrease to below 0.1 mg/L with ZVAl could be attributed to its sequestration from the solution via cementation. In other words, an additional chemical reaction-cementation described by the overall reaction (Equation (7)) which is the sum of two half-reactions (i.e., Equations (5) and (6))-occurred concurrently with dissolution reactions, as previously described.
The overall reaction potential, ∆ 0 , is calculated by subtracting the standard electrode potential of Equation (5) from Equation (6), that is, ∆ 0 − 0.126 − (−1.66) = 1.534 . The standard Gibbs free energy change, ∆ 0 (i. e., ∆ 0 = − ∆ 0 , n number of electrons transferred, F is Faraday's constant, and ∆ 0 is the galvanic cell potential), of Equation (7) is negative (−888.047 kJ/mol) because ∆ 0 is positive indicating that cementation of dissolved Pb 2+ from ZPLRs by ZVAl is thermodynamically spontaneous. In addition, the Al2O3 layer which was inherently present on the surface of ZVAl and passivated the cementation is removed at the acidified chloride solution [30,31]. Hence, simultaneous cementation of dissolved Pb 2+ from ZPLRs occurred, which could explain why Pb 2+ was comparatively lower and was even below 0.1 mg/L with ZVAl during ZPLRs leaching. The concentration of dissolved Pb when ZVAl was added was 10-fold lower than when only ZPLRs were leached in the same solution. The dissolved concentration of Pb decreased further with increasing the treatment time and reached below 0.048 mM (i.e., 0.1 mg/L) with ZVAl after 4 h. The dramatically lower dissolved concentration of Pb after 15 min and its continued decrease to below 0.1 mg/L with ZVAl could be attributed to its sequestration from the solution via cementation. In other words, an additional chemical reaction-cementation described by the overall reaction (Equation (7)) which is the sum of two half-reactions (i.e., Equations (5) and (6))-occurred concurrently with dissolution reactions, as previously described.
The overall reaction potential, ∆E 0 , is calculated by subtracting the standard electrode potential of Equation (5) from Equation (6), that is, ∆E 0 − 0.126 − (−1.66) = 1.534 V. The standard Gibbs free energy change, ∆G 0 (i.e., ∆G 0 = −nF∆E 0 , n number of electrons transferred, F is Faraday's constant, and ∆E 0 is the galvanic cell potential), of Equation (7) is negative (−888.047 kJ/mol) because ∆E 0 is positive indicating that cementation of dissolved Pb 2+ from ZPLRs by ZVAl is thermodynamically spontaneous. In addition, the Al 2 O 3 layer which was inherently present on the surface of ZVAl and passivated the cementation is removed at the acidified chloride solution [30,31]. Hence, simultaneous cementation of dissolved Pb 2+ from ZPLRs occurred, which could explain why Pb 2+ was comparatively lower and was even below 0.1 mg/L with ZVAl during ZPLRs leaching.
Meanwhile, the concentration of dissolved Zn reached an apparent equilibrium after 15 min at around 10.3 mM (i.e., equivalent to around 52% of total Zn) for without and with ZVAl ( Figure 4b). This implied that dissolved Zn from ZPLRs was not cemented on ZVAl as described by Equation (10) (i.e., the summation of two half-cell reactions Equations (8) and (9)) though it is thermodynamically feasible due to negative ∆G 0 (i.e., −519.282 kJ/mol).
The cementation product that was obtained as +150 µm particles were characterized by SEM-EDX and XRD. Figure 5 shows that Pb was cemented on ZVAl and agglomerated. However, Zn was not detected, which confirms that dissolved Zn was not cemented by ZVAl. Further characterization of the cementation product by XRD ( Figure 6) showed that cemented Pb was mainly in a zero-valent Pb (metallic Pb) form and a small amount of oxidized metallic Pb as PbO, which supports the chemical reaction expressed in Equation (7). Meanwhile, the concentration of dissolved Zn reached an apparent equilibrium after 15 min at around 10.3 mM (i.e., equivalent to around 52% of total Zn) for without and with ZVAl (Figure 4b). This implied that dissolved Zn from ZPLRs was not cemented on ZVAl as described by Equation (10) (i.e., the summation of two half-cell reactions Equations (8) and (9)) though it is thermodynamically feasible due to negative ∆ 0 (i.e., −519.282 kJ/mol).
Al 3+ + 3 -= Al 0 0 = -1.660 (8) Zn 2+ + 2 -= Zn 0 0 = -0.763 (9) 3Zn 2+ + 2Al 0 = 3Zn 0 + 2Al 3+ The cementation product that was obtained as +150 µ m particles were characterized by SEM-EDX and XRD. Figure 5 shows that Pb was cemented on ZVAl and agglomerated. However, Zn was not detected, which confirms that dissolved Zn was not cemented by ZVAl. Further characterization of the cementation product by XRD ( Figure 6) showed that cemented Pb was mainly in a zero-valent Pb (metallic Pb) form and a small amount of oxidized metallic Pb as PbO, which supports the chemical reaction expressed in Equation (7).  The explanation to why Zn could not be cemented by ZVAl in the leaching solution could be (a) the dissolution of cemented Zn by the proton (H + ) (Equation (11)) and (b) the reduction of H + to H2 on ZVAl, which competes with the reduction of Zn 2+ to Zn 0 (Equation (12)).
6 H + + 2Al → 3H 2 + 2Al 3+ In an acidic region, the redox potential of H + /H2 redox pair is higher than that of Zn 2+ /Zn redox pair, indicating that the reaction in Equation (11) (∆ 0 = −6121.203 kJ/mol) occurs, and Zn once cemented on the ZVAl surface would be dissolved [32]. Similarly, since the redox potential of H + /H2 The explanation to why Zn could not be cemented by ZVAl in the leaching solution could be (a) the dissolution of cemented Zn by the proton (H + ) (Equation (11)) and (b) the reduction of H + to H 2 on ZVAl, which competes with the reduction of Zn 2+ to Zn 0 (Equation (12)).
In an acidic region, the redox potential of H + /H 2 redox pair is higher than that of Zn 2+ /Zn redox pair, indicating that the reaction in Equation (11) (∆G 0 = −6121.203 kJ/mol) occurs, and Zn once cemented on the ZVAl surface would be dissolved [32]. Similarly, since the redox potential of H + /H 2 redox pair is higher than that of Al 3+ /Al redox pair, the reaction as shown in Equation (12) (∆G 0 = −8168.614 kJ/mol) also takes place. This reaction consumes the electron supplied from ZVAl and competes with Zn 2+ reduction to Zn (Equation (10)). As a result, these reactions suppress the Zn cementation on ZVAl. The rates and equilibrium of these reactions (Equations (11) and (12)) depend on the H + concentration, hence, suppression of Zn cementation on ZVAl would decrease at higher pH.
To investigate the effects of H + concentration on cementation of Zn 2+ from the solution using ZVAl, simulated (model) acidic and alkaline solutions containing both 8 mM Pb 2+ and 10 mM Zn 2+ , and to mimic the composition similar to what would be obtained by leaching ZPLRs, were prepared by dissolving ZnCl 2 and PbCl 2 (Wako Pure Chemical Industries, Ltd., Japan) in an acidified chloride solution (3 M NaCl and 0.05 M HCl, initial pH = 0.82) and alkaline solution (3 M NaOH, initial pH = 14.5), respectively. To cement both Pb and Zn, 0.15 g of ZVAl was added after N 2 purging. Figure 7a shows the percentage of cemented Pb and Zn from the simulated acidified chloride solution. Only Pb (around 99.7% after 30 min) was cemented out leaving Zn in the solution, which is in line with the results obtained when ZVAl was added during ZPLRs leaching. However, in the alkaline solution around 99.8% of both Pb and Zn were cemented out of the solution (Figure 7b). The SEM-EDX analysis and mapping results showed that both Pb and Zn were deposited on the ZVAl surface ( Figure 8). The results confirm the suppression of Zn cementation, which depends on pH. In the acidic region, Zn cementation is strongly suppressed by the reactions shown in Equations (11) and (12), while in the alkaline region the suppressive effects become negligible because of low H + concentrations.

Effects of Solution Composition on Pb and Zn Removal from Zinc Plant Leach Residues
Lead and Zn removal from ZPLRs was evaluated for different solution compositions and compared with the removal efficiencies when ZPLRs was leached with and without ZVAl addition. When ZVAl was added during ZPLRs leaching, Pb was extracted into a leaching solution and concurrently cemented and agglomerated. The Pb distribution among the solution (i.e., extracted but uncemented Pb), +150 μm particles (i.e., cementation and agglomerated product), and −150 μm particles (unextracted Pb in residues). Since the amount of Pb that remained in the solution was negligible (in most cases below 0.1 mg/L), Pb removal in a case when ZVAl was added during ZPLRs leaching is referred to as Pb that was extracted, cemented, and separated as +150 μm particles. However, in the case when ZPLRs were leached without the addition of ZVAl, Pb removal is referred to as the Pb that was extracted into a leaching solution. The same definition was also applied to Zn removal with and without ZVAl addition since it was not cemented from the leaching solution, as discussed previously.
Lead removal when ZPLRs were leached without the addition of ZVAl increased with increasing both HCl and NaCl concentrations, as shown in Figure 9. Pb removal steadily increased from around 0% to 28%, 0.5% to 58%, and 0.5% to 72% for 0.01, 0.05, and 0.1 M HCl, respectively, when NaCl increased from 0 to 3 M, respectively. Lead dissolution from anglesite (PbSO4) depends on (1) Cl −

Effects of Solution Composition on Pb and Zn Removal from Zinc Plant Leach Residues
Lead and Zn removal from ZPLRs was evaluated for different solution compositions and compared with the removal efficiencies when ZPLRs was leached with and without ZVAl addition. When ZVAl was added during ZPLRs leaching, Pb was extracted into a leaching solution and concurrently cemented and agglomerated. The Pb distribution among the solution (i.e., extracted but uncemented Pb), +150 µm particles (i.e., cementation and agglomerated product), and −150 µm particles (unextracted Pb in residues). Since the amount of Pb that remained in the solution was negligible (in most cases below 0.1 mg/L), Pb removal in a case when ZVAl was added during ZPLRs leaching is referred to as Pb that was extracted, cemented, and separated as +150 µm particles. However, in the case when ZPLRs were leached without the addition of ZVAl, Pb removal is referred to as the Pb that was extracted into a leaching solution. The same definition was also applied to Zn removal with and without ZVAl addition since it was not cemented from the leaching solution, as discussed previously.
Lead removal when ZPLRs were leached without the addition of ZVAl increased with increasing both HCl and NaCl concentrations, as shown in Figure 9. Pb removal steadily increased from around 0% to 28%, 0.5% to 58%, and 0.5% to 72% for 0.01, 0.05, and 0.1 M HCl, respectively, when NaCl increased from 0 to 3 M, respectively. Lead dissolution from anglesite (PbSO 4 ) depends on (1) Figure S1). This is the possible reason why Pb removal increased when NaCl and HCl concentrations were increased. The semi-quantitative analysis of the residues obtained after treating ZPLRs in a 3 M NaCl and 0.05 M HCl solution with the addition of ZVAl by XRD show the disappearance/decrease of peaks of anglesite, cerussite, gypsum, and other minerals (Supplementary Figure S2).
dissolution is limited at high pH. Meanwhile, the release of Pb from other Pb-minerals such as cerussite (PbCO3) in ZPLRs requires an H + attack in addition to the Cl − concentration, as previously described in Equation (4) ( Supplementary Information, Figure S1). This is the possible reason why Pb removal increased when NaCl and HCl concentrations were increased. The semi-quantitative analysis of the residues obtained after treating ZPLRs in a 3 M NaCl and 0.05 M HCl solution with the addition of ZVAl by XRD show the disappearance/decrease of peaks of anglesite, cerussite, gypsum, and other minerals (Supplementary Figure S2).  The addition of ZVAl during leaching of ZPLRs significantly increased the Pb removal even at low NaCl concentration especially when HCl was increased from 0.01 to 0.05 and 0.1 M (Figure 9). For example, while maintaining HCl at 0.05 M, the addition of ZVAl during ZPLRs leaching increased the Pb removal from 2.5% to 35.5% and 8% to 57% for 0.5 and 1 M NaCl concentration, respectively. Meanwhile, for 0.1 M HCl, the addition of ZVAl during ZPLRs leaching increased the Pb removal from 3% to 69% and 9% to 72% for 0.5 and 1 M NaCl concentration, respectively. The dramatic increase of Pb removal at low NaCl concentration is attributed to the leaching solution not attaining saturated with dissolved Pb 2+ and Pb-Cl complexes. In other words, when ZVAl was added during ZPLRs leaching, dissolved soluble Pb 2+ and Pb-Cl complexes were simultaneously sequestered from the solution by cementation, hence, more Pb could dissolve from the host minerals (e.g., PbSO4), as well as the conversion of intermediate sparingly soluble solid, PbCl2, to more Pb-Cl complexes (Figure 11a,b). The addition of ZVAl during leaching of ZPLRs significantly increased the Pb removal even at low NaCl concentration especially when HCl was increased from 0.01 to 0.05 and 0.1 M (Figure 9). For example, while maintaining HCl at 0.05 M, the addition of ZVAl during ZPLRs leaching increased the Pb removal from 2.5% to 35.5% and 8% to 57% for 0.5 and 1 M NaCl concentration, respectively. Meanwhile, for 0.1 M HCl, the addition of ZVAl during ZPLRs leaching increased the Pb removal from 3% to 69% and 9% to 72% for 0.5 and 1 M NaCl concentration, respectively. The dramatic increase of Pb removal at low NaCl concentration is attributed to the leaching solution not attaining saturated with dissolved Pb 2+ and Pb-Cl complexes. In other words, when ZVAl was added during ZPLRs leaching, dissolved soluble Pb 2+ and Pb-Cl complexes were simultaneously sequestered from the solution by cementation, hence, more Pb could dissolve from the host minerals (e.g., PbSO 4 ), as well as the conversion of intermediate sparingly soluble solid, PbCl 2 , to more Pb-Cl complexes (Figure 11a,b). Zinc removal was, however, independent of the increase of NaCl concentration, as well as the addition of ZVAl but it increased when the HCl concentration increased, as shown in Figure 12. When HCl increased from 0.01 to 0.05 and 0.1 M, Zn removal increased from around 27% to 60% and 70%, respectively. Increasing HCl concentration increased the H + concentration, which in turn increased Zn solubilization from minerals in ZPLRs by an H + attack mechanism (e.g., dissolution of Zn associated with amorphous iron oxyhydroxide phase fraction as determined elsewhere [34]). Zinc removal was not affected by the NaCl concentration. Unlike Pb that forms an intermediate solid (PbCl2) at low chloride concentration and dissolves as the chloride concentration increases, Zn does form solid Zn-Cl species, and it does not complex strongly with chloride. Additionally, Zn removal was not affected by the addition of ZVAl because it was not sequestered (remained in solution) from the solution, as previously discussed. Since Zn was not be recovered by cementation using ZVAl from the leaching pulp, methods such as precipitation as ZnS [35] or electrowinning [36] can be employed to recover Zn from the solution. Unfortunately, these methods are beyond the scope of this study.  Zinc removal was, however, independent of the increase of NaCl concentration, as well as the addition of ZVAl but it increased when the HCl concentration increased, as shown in Figure 12. When HCl increased from 0.01 to 0.05 and 0.1 M, Zn removal increased from around 27% to 60% and 70%, respectively. Increasing HCl concentration increased the H + concentration, which in turn increased Zn solubilization from minerals in ZPLRs by an H + attack mechanism (e.g., dissolution of Zn associated with amorphous iron oxyhydroxide phase fraction as determined elsewhere [34]). Zinc removal was not affected by the NaCl concentration. Unlike Pb that forms an intermediate solid (PbCl 2 ) at low chloride concentration and dissolves as the chloride concentration increases, Zn does form solid Zn-Cl species, and it does not complex strongly with chloride. Additionally, Zn removal was not affected by the addition of ZVAl because it was not sequestered (remained in solution) from the solution, as previously discussed. Since Zn was not be recovered by cementation using ZVAl from the leaching pulp, methods such as precipitation as ZnS [35] or electrowinning [36] can be employed to recover Zn from the solution. Unfortunately, these methods are beyond the scope of this study. Zinc removal was, however, independent of the increase of NaCl concentration, as well as the addition of ZVAl but it increased when the HCl concentration increased, as shown in Figure 12. When HCl increased from 0.01 to 0.05 and 0.1 M, Zn removal increased from around 27% to 60% and 70%, respectively. Increasing HCl concentration increased the H + concentration, which in turn increased Zn solubilization from minerals in ZPLRs by an H + attack mechanism (e.g., dissolution of Zn associated with amorphous iron oxyhydroxide phase fraction as determined elsewhere [34]). Zinc removal was not affected by the NaCl concentration. Unlike Pb that forms an intermediate solid (PbCl2) at low chloride concentration and dissolves as the chloride concentration increases, Zn does form solid Zn-Cl species, and it does not complex strongly with chloride. Additionally, Zn removal was not affected by the addition of ZVAl because it was not sequestered (remained in solution) from the solution, as previously discussed. Since Zn was not be recovered by cementation using ZVAl from the leaching pulp, methods such as precipitation as ZnS [35] or electrowinning [36] can be employed to recover Zn from the solution. Unfortunately, these methods are beyond the scope of this study.

Leachability of Lead and Zinc after Concurrent Dissolution-Cementation
To evaluate if the solid residues generated after treatment by concurrent dissolution-cementation meet environmental standards, the leachability of Pb and Zn using TCLP was examined out. The amounts of Pb and Zn leached before (untreated ZPLRs) and after treatment (treated by combined dissolution-cementation technique under the conditions 0.1 M HCl, 2 M NaCl, and 0.1 g ZVAl) were compared with the regulatory thresholds. As illustrated in Table 2, the levels of Pb and Zn that leached from untreated ZPLRs were substantially high: Pb was higher than environmental standards. In contrast, the amounts of Pb and Zn that leached from the residues after treatment by the concurrent dissolution-cementation method were dramatically lower. Leachable Pb (which was about 0.12 mg/L) was lower than the regulatory threshold, which entails the detoxification of ZPLRs.

Conceptual Flowsheet
Based on the results obtained in this study, the conceptual flowsheet for ZPLRs treatment by a concurrent dissolution-cementation technique to remove/recover Pb and Zn by using the HCl-NaCl solution with ZVAl is proposed ( Figure 13). The flowsheet involves the removal of Pb-more toxic heavy metal to human beings than Zn-by cementation using ZVAl before solid-liquid separation. The Zn that remains in a solution can be recovered by precipitation or electrowinning. High removal of Pb and Zn can be achieved using a less concentrated NaCl (even as low as 1 M) solution acidified with 0.1 M HCl by the addition of ZVAl. The generated solid residues may not necessarily need to be washed because the most toxic metal that remains in the solution as a result of the inherent incomplete solid-liquid separation is negligible. In addition, this approach shortens and simplifies the treatment of ZPLRs compared to the conventional approach (i.e., leach, solid-liquid separation, and finally recovery of dissolved metals).
Metals 2020, 10, x FOR PEER REVIEW 13 of 16 To evaluate if the solid residues generated after treatment by concurrent dissolutioncementation meet environmental standards, the leachability of Pb and Zn using TCLP was examined out. The amounts of Pb and Zn leached before (untreated ZPLRs) and after treatment (treated by combined dissolution-cementation technique under the conditions 0.1 M HCl, 2 M NaCl, and 0.1 g ZVAl) were compared with the regulatory thresholds. As illustrated in Table 2, the levels of Pb and Zn that leached from untreated ZPLRs were substantially high: Pb was higher than environmental standards. In contrast, the amounts of Pb and Zn that leached from the residues after treatment by the concurrent dissolution-cementation method were dramatically lower. Leachable Pb (which was about 0.12 mg/L) was lower than the regulatory threshold, which entails the detoxification of ZPLRs. Table 2. Toxicity characteristic leaching procedure (TCLP) leachability tests of untreated ZPLRs and treated residues after concurrent dissolution and cementation treatment.

Conceptual Flowsheet
Based on the results obtained in this study, the conceptual flowsheet for ZPLRs treatment by a concurrent dissolution-cementation technique to remove/recover Pb and Zn by using the HCl-NaCl solution with ZVAl is proposed ( Figure 13). The flowsheet involves the removal of Pb-more toxic heavy metal to human beings than Zn-by cementation using ZVAl before solid-liquid separation. The Zn that remains in a solution can be recovered by precipitation or electrowinning. High removal of Pb and Zn can be achieved using a less concentrated NaCl (even as low as 1 M) solution acidified with 0.1 M HCl by the addition of ZVAl. The generated solid residues may not necessarily need to be washed because the most toxic metal that remains in the solution as a result of the inherent incomplete solid-liquid separation is negligible. In addition, this approach shortens and simplifies the treatment of ZPLRs compared to the conventional approach (i.e., leach, solid-liquid separation, and finally recovery of dissolved metals).

Conclusions
This study investigated Pb and Zn removal from ZPLRs using a concurrent dissolutioncementation technique in acidified chloride solution. The following is a summary of the findings: