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Article

Selective Recovery of Zinc from Oxide Ores Using Monosodium Glutamate as a Green Lixiviant

by
Yasemin Ozturk
Mining Engineering Department, Hacettepe University, Beytepe, Ankara 06800, Turkey
Minerals 2025, 15(8), 870; https://doi.org/10.3390/min15080870
Submission received: 25 July 2025 / Revised: 9 August 2025 / Accepted: 15 August 2025 / Published: 18 August 2025
(This article belongs to the Special Issue Mineral Processing and Extractive Metallurgy of Sulfide Ores, 2nd Edition)
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Abstract

This study aims to develop an environmentally friendly hydrometallurgical process for the recovery of zinc from zinc oxide ores. The process includes monosodium glutamate (MSG) leaching, followed by zinc recovery from the pregnant leach solution via electrowinning, and the recirculation of the spent solution to the leaching stage. The study investigated the effects of key leaching parameters and identified the optimal conditions as a pH of 9.5, temperature of 70 °C, 5 h leaching time, solid-to-liquid ratio of 50 g/L, particle size of d80 = 115 µm, and initial MSG concentration of 1.0 M. Under these conditions, 82.3 ± 0.05% of the zinc was extracted with minimal co-dissolution of impurities. Subsequent electrowinning at 100 A/m2 for 150 min yielded 74.97 ± 2.43% zinc recovery with 96.4 ± 0.76% purity. The process achieved a current efficiency of 87.08%, while the specific energy consumption was calculated to be 3.98 kWh per kilogram of zinc recovered. The reusability of MSG was examined by recirculating spent electrowinning solution back to the leaching stage. Zinc extraction decreased from 82.2% to 28.5% over three electrowinning–leaching cycles, due to MSG degradation during electrowinning. The results of this study demonstrated that MSG is a selective and effective lixiviant for zinc recovery, while underlining the limitations of its reuse.

Graphical Abstract

1. Introduction

Worldwide, 95% of zinc is extracted from sulfide ores, where sphalerite (ZnS) is typically found alongside copper, lead, and iron sulfides. The zinc oxide ores, consisting of smithsonite (ZnCO3), willemite (Zn2SiO4), and hemimorphite (Zn2SiO3.H2O), are also important sources for zinc production [1].
Zinc oxide ores typically cause less serious metallurgical and environmental problems than zinc sulfide ores. They contain lower levels of arsenic, cadmium, lead, and sulfur. In addition, zinc is recovered from sulfide concentrates through roasting processes. These processes are energy-intensive and environmentally harmful, mainly due to the high temperature roasting of zinc sulfide ores that leads to sulfur dioxide emissions and large energy consumption [2,3]. Various methods, including flotation, gravity separation combined with flotation, hydrometallurgy, and pyrometallurgy, have been utilized for the beneficiation of oxide ores [4]. Froth flotation is widely used in the beneficiation of oxide minerals, due to its operational simplicity and relatively low cost. However, it is often inefficient for lead and zinc oxide minerals [5]. The strong hydration of oxide minerals and the release of metal ions from their surfaces prevent effective separation. In order to achieve efficient flotation, the surface properties of these minerals must be modified [6].
Hydrometallurgical technology utilizing acid and alkaline (such as ammonia and sodium hydroxide) leaching methods for zinc oxide ores has been proposed in a previous paper [7]. Commercial-scale zinc dissolution typically uses sulfuric acid, but the acid leaching of oxidized zinc ores poses some challenges. Zinc carbonates and silicates increase acid consumption and generate silica gel, which complicates the subsequent processes of separation and purification [8]. Ammonia leaching yields various environmental considerations. A significant amount of ammonia is lost during this process, due to the volatility of ammonia, which poses risks to the environment [9]. In contrast, sodium hydroxide leaching causes less environmental damage; however, it is less efficient for extracting zinc from zinc silicate ores [10].
Amino acids have been utilized in extractive metallurgy over the past decade, due to their role as eco-friendly lixiviants for metal recovery. Glycine is the most widely studied amino acid that is used to extract copper [11], gold [12], and zinc [13] from mineral resources. In addition to glycine, monosodium glutamate (MSG) is another amino acid that can act as a lixiviant. Under alkaline conditions, MSG acts as a strong ligand that can chelate various transition metals and improve their extraction efficiency. It is an effective lixiviant due to its wide availability, biodegradability, cost-effectiveness, and reusability [14]. Li et al. showed that alkaline glutamate leaching is highly selective for copper, zinc, and lead. Comparative studies showed that it provides a higher solids-handling capacity than glycine [15]. Perea et al. also demonstrated that using glutamate as a bidentate ligand is more effective than glycine for the recovery of copper from electronic wastes [16]. MSG leaching in alkaline conditions achieved the selective recovery of copper and zinc from electric arc furnace waste [14]. Kostudis et al. recovered 44% of copper from complex polymetallic copper–silver (Cu-Ag) ores through leaching with glutamic acid [17].
The hydrometallurgical process involves the separation and recovery of dissolved metals from the pregnant leach solution (PLS). Zinc can be recovered from PLS using sulfide precipitation [18], solvent extraction [19], ion exchange [20], or electrowinning [21]. Sulfide precipitation is a simple and cost-effective process; however, it has low selectivity, resulting in the co-precipitation of other metals such as iron, copper, and cadmium. In addition, the production of fine colloids complicates the solid–liquid separation process [22]. Solvent extraction offers high selectivity but requires strict pH control and complex phase separation processes. It also poses environmental and economic risks due to solvent degradation and loss [23]. Ion exchange is suitable for dilute solutions and offers good selectivity, but is limited by low resin capacity, mechanical instability, fouling, and high water consumption during regeneration cycles [24]. Electrowinning is an effective method for recovering high-purity zinc, although it is energy-intensive and sensitive to impurities [25]. Selective leaching with MSG generates a pregnant leach solution with low levels of contaminants and, hence, reduces the potential adverse effects on electrowinning efficiency and deposit quality. The compatibility between MSG leaching and the requirements of electrowinning supports the selection of this method as a practical and efficient zinc recovery process.
This study was conducted to develop a novel and environmentally friendly hydrometallurgical process for recovering zinc from zinc oxide ores using MSG as a lixiviant. MSG was selected because of its environmentally friendly characteristics, its high selectivity for zinc over common impurities such as iron and lead, and its potential reusability in a closed-loop process. The proposed process uniquely integrates leaching with MSG, recovering zinc from the PLS through electrowinning, and recirculating the spent electrowinning solution to evaluate the reusability of MSG. This study investigated the effects of various leaching parameters, including temperature, pH, initial MSG concentration, particle size, and the solid-to-liquid ratio, on zinc extraction efficiency. Electrowinning experiments were conducted to evaluate zinc recovery and purity, as well as current efficiency and the specific energy consumption of the process. Finally, the reusability of the MSG was assessed by recirculating the spent electrowinning solution through three leaching–electrowinning cycles to determine its long-term performance in a sustainable metal recovery process.

2. Materials and Methods

2.1. Materials

Three ore samples with different grades of low, medium, and high were blended to prepare a master composite ore sample (MCS). The blend consisted of 40% of medium-grade samples and 30% each of low- and high-grade samples. The estimated grade values of the resource were taken into consideration during the sample preparation. The chemical composition of the MCS is presented in Table 1. The specific gravity of the ore samples was measured by a pycnometer as 3.36 g/cm3. A detailed mineralogical characterization study was performed on the MCS ore sample for the quantitative evaluation of minerals using the scanning electron microscopy (QEMSCAN) technique.
Monosodium glutamate (MSG, C5H8NO4Na) was utilized as a leaching agent. It was provided by a commercial supplier in a technical grade. The structural formula of the monosodium glutamate is illustrated in Figure 1.
The pH was adjusted using analytical-grade sodium hydroxide (NaOH), as supplied by Merck. Deionized water (Smart Mini, Heal Force, Shanghai, China) was used to prepare the solutions throughout the study.

2.2. Leaching Procedures

This part of the study investigated the optimization of leaching parameters by varying the temperature, pH, initial MSG concentration, particle size, and the solid-to-liquid (S/L) ratio to determine the efficiency of zinc extraction when using MSG as a lixiviant for the master composite ore sample. The experimental parameters and conditions are given in Table 2.
The leaching experiments were performed as kinetic tests to determine the effect of leaching time on extraction efficiency. Aliquots of 1 mL leachate were taken at 30, 60, 120, 180, 240, and 300-min time intervals. Each aliquot was immediately filtered and analyzed with an atomic absorption spectrometer (AAS, 240FS, Varian, Mulgrave, Australia) for the determination of metal content (Zn, Pb, and Fe). At the end of 5 h of leaching, the solid residue was filtered and dried at 65 °C for 2 h, and analyzed with an X-ray fluorescence spectrometer (XRF, Niton XLt, Thermo Scientific, Billerica, MA, USA) to determine the residual metal content (Zn, Fe, and Pb) in the solid residue. X-ray diffractometer analysis (XRD, CuKα radiation, D/MAX-2200 PC, Rigaku, Tokyo, Japan) was conducted on the feed and tailing samples after leaching under optimal conditions to observe the changes in mineralogy, using Jade 7.0 software.

2.3. Zn Recovery by Electrowinning and Recirculation of Spent Electrowinning Solution

This part of the study involves the recovery of the dissolved zinc from the pregnant leach solution by electrowinning and recirculation of the spent electrowinning solution back to the leaching stage. The flowsheet of the closed-circuit operation is illustrated in Figure 2.
The optimum leaching test conditions (pH 9.5, 70 °C, solid-to-liquid ratio = 50 g/L, d80 = 115 µm, leaching time 5 h, and a total volume of 300 mL) were applied to dissolve the zinc. The pH of the solution was adjusted and kept constant throughout the experiments. At the end of leaching, the pregnant leach solution was transferred to an electrolytic cell.
An electrolytic cell containing graphite electrodes (Sigracell PV15 bipolar plates, SGL Carbon) for the anode and cathode was used to deposit zinc metal on the surface of the cathode (Figure 3). Graphite was selected as the electrode material due to its high electrical conductivity and superior chemical resistance. The effective surface area of the cathode was 50 cm2. A constant current of 0.5 A was applied with a power source (LSP-1165, Voltcraft, Hirschau, Germany), and the cell potential was measured during the 150-min period of electrowinning. The electrolyte was continuously stirred at 250 rpm using a magnetic stirrer to distribute the ions uniformly throughout the process.
The cathode was weighed before and after electrolysis to determine the amount of deposited metal. Metal content was determined by XRF analysis. The surface morphology and elemental distribution were further analyzed using scanning electron microscopy (SEM; GAIA3 Oxford XMax 150 EDS, TESCAN, Triglav, Brno, Czech Republic) and energy dispersive x-ray spectroscopy (EDS). Following the process, the spent electrowinning solution was filtered and reused in the subsequent leaching stage.
The current efficiency was calculated using Equation (1) [26]:
C u r r e n t   E f f i c i e n c y   ( % ) =   m f − m i I t n F M × 100  
where mf and mi are the final and initial cathode weight (g), respectively; I is the applied current (A); t is the electrolysis time (s); n is the number of electrons transferred per zinc ion (2 for Zn2+ → Zn0); F is Faraday’s constant (96,485 sA/mol); M is the molar mass of Zn (65.38 g/mol).
Additionally, the specific energy consumption (SEC) was determined using Equation (2) [26]:
S E C kWh kg = I V m t × 3600
where I is the applied current (A); V is the cell potential (V); m is the total zinc mass deposited on the cathode (g); t is the electrolysis time (s).

3. Results and Discussions

3.1. Ore Mineralogy and Leaching Residue Characterization

Mineralogical analysis of the MCS indicated that both Zn and Pb show complex deportment. The main host mineral for Zn is smithsonite (ZnCO3), some of which is Fe-rich and is likely to be a carbonate species that is intermediate between smithsonite and siderite (FeCO3). The ‘combined smithsonite’ component accounts for 75% of the Zn.
About 10% of the Zn is hosted by a poorly characterized phase that contains Fe, Zn, Mn, and Pb, which is assumed to be an oxide but may also be a carbonate or hydroxide. The remaining 14% of the Zn is hosted by goethite (FeOOH) and/or siderite phases that have been classified into distinct groups based on their BSE (backscattered electron) brightness and their typical Zn and Pb contents. As a combined group, the goethite and siderite phases account for 41% of the sample mass and host 43% of the Pb. Cerussite (PbCO3) accounts for 27% of the Pb, and the poorly characterized Fe, Zn, Mn, and Pb-bearing phase accounts for 26% of the Pb. The MCS sample also contains 10% calcite, 4.5% barite, 1% quartz, and 1% biotite.
The mineralogical changes caused by the leaching process were evaluated using XRD. Figure 4a and Figure 4b illustrate the XRD patterns of the ore, both before and after leaching, respectively.
The XRD pattern presented in Figure 4a confirmed that smithsonite (JCPDS Card No: 8-449), goethite (JCPDS Card No: 17-536), calcite (CaCO3, JCPDS Card No: 5-586), and quartz (SiO2, JCPDS Card No: 5-0590) were present in the MCS ore sample before leaching. However, the presence of siderite (FeCO3) and cerussite (PbCO3) could not be confirmed, likely due to their low concentrations or weak crystallinity.
The XRD pattern of the residue (Figure 4b) revealed the continued presence of goethite, calcite, and quartz, whereas the diffraction peaks corresponding to smithsonite were completely absent after leaching. This observation confirms that smithsonite, the primary zinc-bearing mineral, was completely dissolved under the applied leaching conditions.

3.2. Optimization of Leaching Conditions

MSG is the sodium salt of glutamic acid, a non-essential amino acid containing two carboxyl groups (–COOH) and one amino group (–NH2). In aqueous environments, these functional groups can undergo protonation and deprotonation depending on the pH of the solution [27].
The distribution of species of the glutamate-H2O system as a function of pH is illustrated in Figure 5 [28]. At low pH values (below 2), both carboxyl groups are protonated, and the dominant species is the triprotic cation HOOC–CH(NH3+)–(CH2)2–COOH (H3Glu+). As the pH increases to 2–4, the carboxyl group adjacent to the amino group loses a proton, forming the neutral zwitterion species −OOC–CH(NH3+)–(CH2)2–COOH (H2Glu), which has no net charge. Between pH 5 and pH 9, the side chain carboxyl group also deprotonates, yielding the singly charged anion −OOC–CH(NH3+)–(CH2)2–COO− (HGlu−). Above pH 9, the amino group loses a proton, producing the doubly charged anion −OOC–CH(NH2)–(CH2)2–COO− (Glu2−).
During the leaching processes, glutamate primarily exists as an anionic species and exhibits strong complexation behavior with metal ions. Due to its two carboxyl groups and one amino group, glutamate can act as a tridentate ligand with the potential to coordinate metal ions [29]. Within the studied pH range of approximately 7.1 to 10.5, zinc ions (Zn2+) can form stable complexes with glutamate, such as ZnGlu and Zn(Glu)22−. The complexation process can be described by the following reactions.
Dissociation of zinc carbonate [13]:
ZnCO3(s) + H2O ↔ Zn2+ (aq) + HCO3− (aq) + H+ (aq)
Complex formation of zinc glutamate [14]:
Zn2+ + Glu2− → ZnGlu
Zn2+ + 2Glu2− → Zn(Glu)22−

3.2.1. Effect of Temperature

Temperature is a critical parameter that influences metal extraction efficiency, as it directly impacts the kinetics of leaching reactions [30]. A zinc leaching process was performed at three different temperatures, 30 °C, 50 °C, and 70 °C, over 5 h to investigate the impact of temperature on zinc extraction. All other parameters were kept constant: pH 9.5, MSG concentration 1.0 M, and solid-to-liquid ratio 50 g/L.
Figure 6 illustrates the effect of temperature on zinc recovery over 300 min. At 30 °C, zinc recovery proceeded slowly, reaching only about 35% after 300 min. Lower temperatures reduced the reaction kinetics and inhibited diffusion, which weakened the complexation interaction between MSG and Zn2+. At 50 °C, a significant improvement was observed. Zinc recovery increased steadily over time and reached approximately 63% at 300 min. This indicates that higher temperatures enhanced the reaction rates and improved diffusion at the solid–liquid interface. At 70 °C, the recovery rate was significantly higher throughout the leaching period. Zinc recovery increased rapidly in the initial stages and approached 82.3% by the end of the 300-min period. Most of the leaching occurred within the first 180 min. The experiment performed at 70 °C was repeated to verify the reproducibility of the results. The close agreement between the repeated tests confirms the consistency and reliability of the leaching data at this temperature. The significant increase observed in recovery from 50 °C to 70 °C can be attributed to increased surface complexation and the faster diffusion of ions from the solid–liquid interface [31].
Mineralogical analysis of the ore showed that approximately 14% of the total zinc was associated with refractory phases such as goethite and siderite. These phases were less amenable to MSG leaching under the studied conditions. In contrast, zinc that was present in smithsonite and other carbonate or hydroxide phases was more readily leached, explaining the plateauing recovery values near 83%.

3.2.2. Effect of pH

The pH of the solution significantly influences zinc dissolution, as it substantially affects the speciation of glutamic acid. As shown in Figure 5, glutamate exists in various protonation states—H3Glu+, H2Glu, HGlu−, and Glu2−—depending on pH. These species differ in terms of their ability to form complexes with metal ions. The fully deprotonated Glu2− anion displays the highest affinity for Zn2+ due to its strong electrostatic interaction and ability to form stable complexes [32].
The influence of pH on zinc extraction was investigated within the pH ranges of 7.1 (natural), 8.5, 9.5, and 10.5, with a constant MSG concentration of 1.0 M, 70 °C, and a solid-to-liquid ratio of 50 g/L. Figure 7 illustrates the effects of leaching time and pH on zinc recovery. At pH 7.1 and pH 8.5, zinc recovery remained relatively low throughout the leaching period, especially in the early stages. Lower recoveries were observed at pH 7.1 (67.1%) and pH 8.5 (66.8%), due to the prevalence of protonated glutamate forms (H2Glu and HGlu−), which are less effective in forming complexes with Zn2+ ions.
At pH 9.5, where Glu2− was the main species, zinc extraction significantly improved and reached 82.3% after 5 h, which could be attributable to the formation of stable and soluble Zn–glutamate complexes. These complexes facilitate dissolution and prevent the formation of precipitates. At pH 10.5, the extraction of zinc initially increased but then declined to a lower level compared to pH 9.5, resulting in 63% zinc recovery. This decrease is presumably due to the formation of insoluble zinc hydroxide and/or oxide in the leach solution under alkaline conditions. The precipitation process removes Zn2+ ions from the solution, which limits the formation of soluble complexes and subsequently reduces the overall extraction efficiency.

3.2.3. Effect of Initial MSG Concentration

The amount of lixiviant plays a key role in metal complexation and dissolution behavior. Zinc leaching experiments were performed using MSG concentrations of 0.5 M, 1.0 M, and 1.5 M. The leaching conditions were maintained at a pH of 9.5, a temperature of 70 °C, a solid-to-liquid ratio of 50 g/L, a particle size of 115 µm, and a leaching time of 5 h. The zinc content in the solid material added was kept at 110 mM.
The kinetic profiles presented in Figure 8 demonstrate that zinc extraction improves with increasing MSG concentration. As the MSG concentration increased from 0.5 M to 1.0 M, a significant enhancement in extraction efficiency was observed, and zinc recovery increased from 52.0% to 82.1% within 300 min. This improvement can be attributed to the increased availability of glutamate ligands, which enhanced complex formation with Zn2+ ions and facilitated the dissolution of zinc from the ore matrix.
However, further increasing the MSG concentration to 1.5 M resulted in a slight increase in extraction, reaching 82.7% after 300 min. This limited improvement is likely due to physicochemical constraints within the system, such as the increased solution viscosity at higher MSG concentrations, which can hinder mass transfer and diffusion rates [33]. Considering both the extraction performance and reagent efficiency, an MSG concentration of 1.0 M was determined to be optimal under the experimental conditions.

3.2.4. Effect of Particle Size

Particle size is an important parameter in hydrometallurgical processes because it directly affects the surface area that is available for leaching. To evaluate its effect, zinc extraction was studied using ore particle sizes of 75 µm, 115 µm, and 200 µm. Zinc leaching was conducted at a constant pH of 9.5, an MSG concentration of 1.0 M, a temperature of 70 °C, and a solid-to-liquid ratio of 50 g/L for 5 h.
The data presented in Figure 9 show that there was a significant improvement in the leaching rate when the particle size was reduced from 200 µm to 115 µm. As the particle size decreased, the specific surface area increased, leading to a greater number of reactive sites for the lixiviant to attack [34]. Conversely, reducing the particle size to 75 µm did not lead to a considerable improvement in the extraction rate. This suggests that mass transfer or reaction kinetics may be limiting factors at smaller particle sizes. Despite these differences in leaching kinetics, final zinc recovery values after 300 min were nearly identical for all particle sizes, with recoveries of 82.5%, 82.3%, and 81.8% for 75 µm, 115 µm, and 200 µm, respectively. These results indicate that within the tested range (75–200 µm), particle size did not significantly affect the overall zinc recovery rate under the given leaching conditions.
The flotation process mainly relies on mineral liberation and particle fineness for efficient separation [35]; however, leaching processes can achieve high metal recovery rates, even with larger particles [36]. A particle size of 115 µm represents an effective choice from a practical and economic perspective, as it balances surface area and preparation ease.

3.2.5. Effect of Solid-to-Liquid Ratio

The solid-to-liquid (S/L) ratio is an important parameter that affects both the kinetics of the leaching process and the metal extraction efficiency. Leaching experiments were conducted using S/L ratios of 30 g/L, 50 g/L, and 70 g/L while keeping the conditions constant at 1.0 M MSG concentration, pH of 9.5, temperature of 70 °C, and leaching time of 5 h, to investigate the effect of S/L ratio on zinc extraction efficiency.
As illustrated in Figure 10, zinc recovery increased over time under all conditions, but zinc extraction efficiency decreased as the S/L ratio increased. At an S/L ratio of 30 g/L, zinc extraction was the most efficient. It reached a plateau after approximately 120 min and achieved 82.7% recovery after 300 min. As the solid content increased to 50 g/L, the leaching exhibited favorable kinetics and high efficiency, resulting in 82.2% zinc recovery. In contrast, the S/L ratio of 70 g/L displayed the lowest recovery, with an overall recovery rate of 72.4%. This decline in extraction efficiency was attributed to the reduced availability of MSG per unit mass of solid at higher pulp densities. The resulting lower concentration gradient at the solid–liquid interface reduced the driving force for mass transfer, thereby limiting the diffusion of glutamate ligands into the interior of the ore particles [37]. In addition, an increase in slurry viscosity at higher solids content can hinder mixing and reduce effective contact between reactants.
Considering the higher process water demand and potential operational complexity at lower pulp densities, 50 g/L was selected as the optimal condition. This ratio demonstrated a practical balance between extraction efficiency and process manageability, making it more suitable for potential scale-up and industrial application compared to higher ratios such as 70 g/L, which showed reduced leaching performance.

3.3. Zn Recovery by Electrowinning

3.3.1. Process Efficiency and Product Characterization

Acid and alkaline leaching processes generally dissolve zinc along with other metals present in the ore, such as iron, lead, copper, cadmium, and manganese [10,38]. In order to enhance the efficiency of zinc recovery, these impurities must first be purified through an additional purification step [21]. Therefore, achieving the selective dissolution of zinc is critical for the efficient recovery and purification of this metal.
In this part of the study, the pregnant leach solution, containing 6436 mg/L Zn, 132 mg/L Pb, and 21 mg/L Fe, was utilized for the electrowinning process. The selective dissolution characteristics of MSG resulted in the negligible co-dissolution of lead and iron, enabling direct electrowinning without further purification.
The electrowinning process was carried out at a current density of 100 A/m2 for 150 min, with an average cell voltage of 4.2 V. Following electrolysis, the zinc concentration in the solution decreased to 1670 mg/L, corresponding to a zinc recovery of 74.97 ± 2.43%. The process achieved a current efficiency of 87.08%, while the specific energy consumption was calculated to be 3.98 kWh per kilogram of zinc recovered. Although this value is slightly above the commonly reported average range for industrial zinc electrowinning, which is between 2.8 and 3.4 kWh/kg Zn [39,40], it is still considered relatively favorable.
The metal deposited on the cathode was gray with a metallic luster. It compactly covered the cathode surface. After deposition, it was thoroughly rinsed with deionized water and dried at room temperature. The deposited metal was easily removed from the cathode. XRF analysis revealed that the electrodeposited zinc had a purity of 96.4 ± 0.76%. However, lead was identified as the predominant impurity, with an average content of 3.22 ± 0.75% in the deposit. Figure 11 depicts the SEM and EDS images of the deposited metal. The SEM surface morphology (Figure 11a) demonstrates a dense and uniform structure. EDS analysis (Figure 11b) indicates that the deposited metal is composed of 96.1% zinc and 3.2% lead, which is in agreement with the results obtained from the XRF analysis.
Despite its relatively low concentration in the initial PLS, lead was partially co-deposited on the cathode under the applied electrochemical conditions. Further optimization of operating conditions or the inclusion of minor pretreatment stages may be required to achieve higher-purity zinc that is suitable for high-grade applications [41].

3.3.2. Reusability of MSG in Leaching–Electrowinning Cycles

The potential reusability of MSG was investigated by recirculating the spent electrowinning solution to a new leaching stage. The leaching process was conducted under optimal conditions.
Figure 12 illustrates the impact of recycling on zinc extraction efficiency across three electrowinning–leaching cycles. The first cycle achieved 82.2% of zinc recovery, but the second and third cycles declined sharply to 40.9% and 28.5%, respectively. The decline in efficiency is primarily attributed to the electro-oxidation of MSG under electrochemical conditions.
Previous investigations on amino acids have shown that these compounds can undergo electro-oxidation and produce aldehydes, amines, carboxylic acids, and carbon dioxide. Takayama [42,43] reported that aldehydes are formed in the first stage of the electro-oxidation process and that other products are produced from further oxidation or chemical interactions between one product and another. Studies on the electro-oxidation of α-alanine [44] and glycine [45] suggest a radical mechanism involving the formation of free radicals at the surface of the electrodes, followed by decarboxylation. The resulting intermediates may be hydrolyzed anodically to produce ammonia and an aldehyde, or further oxidized into a carbonium ion, which subsequently hydrolyzes into ammonia and aldehyde in solution. These degradation products are less effective as complexing agents for zinc and may interfere with subsequent leaching reactions.
The results of this study indicate that MSG is an effective leaching agent in zinc leaching in the first cycle. However, its chemical degradation under electrochemical conditions limits its reuse in a closed-loop system. Future research should focus on preventing MSG oxidation or developing regeneration methods to enable its reuse.

4. Conclusions

This research aimed to develop an environmentally friendly hydrometallurgical process to recover zinc from zinc oxide ores. It used MSG as a selective leaching agent. The impact of the leaching parameters, including pH, temperature, MSG concentration, solid-to-liquid ratio, and particle size, was investigated to determine the optimal leaching conditions. The optimal conditions were found to be a pH of 9.5, temperature of 70 °C, S/L ratio of 50 g/L, particle size of 115 µm, and MSG concentration of 1.0 M.
Higher temperatures and increased MSG concentration significantly improved zinc dissolution by enhancing reaction kinetics and increasing ligand availability. However, higher pulp densities and excessive pH levels decreased the extraction efficiency because of mass transfer limitations and the precipitation of zinc compounds. Although finer particle sizes improved leaching kinetics, they had a minor effect on final recovery within the tested range. Under optimal conditions, 82.3 ± 0.05% of the zinc was successfully extracted, while the co-dissolution of impurities such as iron (24.1 mg/L) and lead (23.6 mg/L) remained minimal. X-ray diffraction analysis confirmed the complete dissolution of smithsonite, the main zinc-bearing mineral, under optimized leaching conditions.
Selective zinc leaching enabled the electrowinning process without additional purification steps. The electrowinning process successfully recovered zinc from the pregnant leach solution, with a recovery rate of 74.97 ± 2.43% and a zinc purity percentage of 96.4 ± 0.76%. The process achieved a current efficiency of 87.08% and a specific energy consumption of 3.98 kWh/kg Zn, which is considered favorable compared to conventional electrowinning benchmarks. Lead was identified as the primary impurity in the cathodic deposit (3.22 ± 0.75%). Future work should focus on optimizing operating conditions or including minor pretreatment stages to produce a higher-purity zinc product.
Investigation into the reusability of MSG indicated that, despite its effective performance in the first leaching cycle, its efficiency declined sharply in subsequent uses. Zinc recovery was 82.2% in the first leaching stage and decreased to 40.9% in the second and 28.5% in the third cycle. This decrease in efficiency can be attributed to the electrochemical degradation of MSG during the electrowinning process, which results in the formation of less effective byproducts. Future research should focus on preventing MSG degradation under electrochemical conditions or developing regeneration methods to enhance its reusability.

Funding

This research received no external funding.

Acknowledgments

The author would like to thank HUNITEK (Hacettepe University Advanced Technologies Research and Application Center, Ankara, Turkey) for providing the SEM imaging, and the Department of Geological Engineering at Hacettepe University, Ankara, Turkey, for conducting the XRD analysis.

Conflicts of Interest

The author declares no conflicts of interest.

References

  1. Kaya, M.; Hussaini, S.; Kursunoglu, S. Critical review on secondary zinc resources and their recycling technologies. Hydrometallurgy 2020, 195, 105362. [Google Scholar] [CrossRef]
  2. Hitzman, M.W.; Reynolds, N.A.; Sangster, D.F.; Allen, C.R.; Carman, C.E. Classification, genesis, and exploration guides for nonsulfide zinc deposits. Econ. Geol. 2003, 98, 685–714. [Google Scholar] [CrossRef]
  3. Gilg, H.A.; Boni, M.; Cook, N.J. A Special issue devoted to nonsulfide Zn-Pb deposits. Ore Geol. Rev. 2008, 33, 115–116. [Google Scholar] [CrossRef]
  4. Hussaini, S.; Kursunoglu, S.; Top, S.; Ichlas, Z.T.; Kaya, M. Testing of 17-different leaching agents for the recovery of zinc from a carbonate-type Pb-Zn ore flotation tailing. Miner. Eng. 2021, 168, 106935. [Google Scholar] [CrossRef]
  5. Moradi, S.; Monhemius, A.J. Mixed sulphide–oxide lead and zinc ores: Problems and solutions. Miner. Eng. 2011, 24, 1062–1076. [Google Scholar] [CrossRef]
  6. Chen, Y.; Sun, Y.; Han, Y. Efficient flotation separation of lead–zinc oxide ores using mineral sulfidation reconstruction technology: A review. Green Smart Min. Eng. 2024, 1, 175–189. [Google Scholar] [CrossRef]
  7. Frenay, J. Leaching of oxidized zinc ores in various media. Hydrometallurgy 1985, 15, 243–253. [Google Scholar] [CrossRef]
  8. Xu, H.; Wei, C.; Li, C.; Fan, G.; Deng, Z.; Zhou, X.; Qiu, S. Leaching of a complex sulfidic, silicate-containing zinc ore in sulfuric acid solution under oxygen pressure. Sep. Purif. Technol. 2012, 85, 206–212. [Google Scholar] [CrossRef]
  9. Yu, Y.S.; Cui, L.X.; Zhang, L.B.; Wang, Y.F. Efficient mechanochemical leaching of zinc from zinc oxide ores. Trans. Nonferrous Met. Soc. China 2024, 34, 1976–1993. [Google Scholar] [CrossRef]
  10. Chen, A.; Zhao, Z.W.; Jia, X.; Long, S.; Huo, G.; Chen, X. Alkaline leaching Zn and its concomitant metals from refractory hemimorphite zinc oxide ore. Hydrometallurgy 2009, 97, 228–232. [Google Scholar] [CrossRef]
  11. Tanda, B.C.; Eksteen, J.J.; Oraby, E.A. An investigation into the leaching behaviour of copper oxide minerals in aqueous alkaline glycine solutions. Hydrometallurgy 2017, 167, 153–162. [Google Scholar] [CrossRef]
  12. Oraby, E.A.; Eksteen, J.J. The leaching of gold, silver and their alloys in alkaline glycine peroxide solutions and their adsorption on carbon. Hydrometallurgy 2015, 152, 199–203. [Google Scholar] [CrossRef]
  13. Khodaei, H.; Haghshenas, D.F.; Firoozi, S. Selective leaching of zinc from carbonate source using glycine as an ecofriendly lixiviant. Miner. Eng. 2022, 185, 107680. [Google Scholar] [CrossRef]
  14. Prasetyo, E.; Anderson, C.; Nurjaman, F.; Muttaqii, M.A.; Handoko, A.S.; Bahfie, F.; Mufakhir, F.R. Monosodium glutamate as selective lixiviant for alkaline leaching of zinc and copper from electric arc furnace dust. Metals 2020, 10, 644. [Google Scholar] [CrossRef]
  15. Li, H.; Oraby, E.; Jacques Eksteen, J. An alternative amino acid leaching of base metals from waste printed circuit boards using alkaline glutamate solutions: A comparative study with glycine. Sep. Purif. Technol. 2025, 356, 129953. [Google Scholar] [CrossRef]
  16. Perea, C.G.; Restrepo Baena, O.J.; Ihle, C.F.; Estay, H. Copper leaching from wastes electrical and electronic equipment (WEEE) using alkaline monosodium glutamate: Thermodynamics and dissolution tests. Clean. Eng. Technol. 2021, 5, 100312. [Google Scholar] [CrossRef]
  17. Kostudis, S.; Bachmann, K.; Kutschke, S.; Pollmann, K.; Gutzmer, J. Leaching of copper from Kupferschiefer by glutamic acid and heterotrophic bacteria. Miner. Eng. 2015, 75, 38–44. [Google Scholar] [CrossRef]
  18. Liu, Q.; Zhao, Y.; Zhao, G. Production of zinc and lead concentrates from lean oxidized zinc ores by alkaline leaching followed by two-step precipitation using sulfides. Hydrometallurgy 2011, 110, 79–84. [Google Scholar] [CrossRef]
  19. Lupi, C.; Pilone, D. Effectiveness of saponified D2EHPA in Zn(II) selective extraction from concentrated sulphuric solutions. Miner. Eng. 2020, 150, 106278. [Google Scholar] [CrossRef]
  20. Zhou, K.; Wu, Y.; Zhang, X.; Peng, C.; Cheng, Y.; Chen, W. Removal of Zn(II) from manganese-zinc chloride waste liquor using ion exchange with D201 resin. Hydrometallurgy 2019, 190, 105171. [Google Scholar] [CrossRef]
  21. Rudnik, E. Recovery of zinc from zinc ash by leaching in sulphuric acid and electrowinning. Hydrometallurgy 2019, 188, 256–263. [Google Scholar] [CrossRef]
  22. Estay, H.; Barros, L.; Troncoso, E. Metal sulfide precipitation: Recent breakthroughs and future outlooks. Minerals 2021, 11, 1385. [Google Scholar] [CrossRef]
  23. Sole, K. Solvent extraction in the hydrometallurgical processing and purification of metals: Process design and selected applications. In Solvent Extraction and Liquid Membranes: Fundamentals and Applications in New Materials; CRC Press: Boca Raton, FL, USA; Taylor &Francis: Boca Raton, FL, USA, 2008; pp. 141–200. [Google Scholar]
  24. Taute, C.C.; Sole, K.C.; Hardwick, E. Removal of zinc from a base-metal solution using ion exchange at Rustenburg Base Metal Refiners. J. Chem. Technol. Biotechnol. 2014, 89, 919–926. [Google Scholar] [CrossRef]
  25. Xu, X.; Li, D.; Chen, L.; Liu, M.; Liu, C.; Jia, J. Improve the energy efficiency: Effects of additives on longtime zinc electrowinning. Hydrometallurgy 2020, 193, 105326. [Google Scholar] [CrossRef]
  26. Badenhorst, W.D.; Rossouw, C.; Cho, H.; Kerres, J.; Bruinsma, D.; Krieg, H. Electrowinning of iron from spent leaching solutions using novel anion exchange membranes. Membranes 2019, 9, 137. [Google Scholar] [CrossRef] [PubMed]
  27. Neuberger, A. Dissociation constants and structures of glutamic acid and its esters. Biochem. J. 1936, 30, 2085–2094. [Google Scholar] [CrossRef] [PubMed]
  28. Prasetyo, E.; Muryanta, W.A.; Amin, M.; Sudibyo; Al Muttaqii, M.; Bahfie, F.; Handoko, A.S. Glutamate leaching of spent lithium-ion battery cathode in weak acidic-neutral condition: New insight on kinetics and dissolution mechanism. Miner. Process. Extr. Metall. Rev. 2024, 46, 386–399. [Google Scholar] [CrossRef]
  29. Murphy, J.M.; Powell, B.A.; Brumaghim, J.L. Stability constants of bio-relevant, redox-active metals with amino acids: The challenges of weakly binding ligands. Coord. Chem. Rev. 2020, 412, 213–253. [Google Scholar] [CrossRef]
  30. Wang, G.R.; Liu, Y.Y.; Tong, L.L.; Jin, Z.N.; Chen, G.B.; Yang, H.Y. Effect of temperature on leaching behavior of copper minerals with different occurrence states in complex copper oxide ores. Trans. Nonferrous Met. Soc. China 2019, 29, 2192−2201. [Google Scholar] [CrossRef]
  31. Faraji, F.; Amirhossein, A.; Fereshteh, R.; Navid, M. Kinetics of leaching: A review. Rev. Chem. Eng. 2022, 38, 113–148. [Google Scholar] [CrossRef]
  32. Sajadi, A.A. Metal ion-binding properties of L-glutamic acid and L-aspartic acid, a comparative investigation. Nat. Sci. 2010, 2, 85–90. [Google Scholar] [CrossRef]
  33. Zhou, Y.; Zheng, G.; Cen, S.; Liu, R.; Taoac, C. Leaching of phosphate ores with lower dissolution of metallic impurities: The dual role of sodium oleate. RSC Adv. 2023, 13, 10600–10609. [Google Scholar] [CrossRef]
  34. Hursit, M.; Lacin, O.; Sarac, H. Dissolution kinetics of smithsonite ore as an alternative zinc source with an organic leach reagent. J. Taiwan Inst. Chem. Eng. 2009, 40, 6–12. [Google Scholar] [CrossRef]
  35. Markovic, Z.S.; Jankovic, A.; Tomanec, R. Effect of particle size and liberation on flotation of a low grade porphyry copper ore. J. Min. Metall. 2008, 44, 24–30. [Google Scholar]
  36. Souza, A.D.; Pina, P.S.; Santos, F.M.F.; Silva, C.A.; Leão, V.A. Effect of iron in zinc silicate concentrate on leaching with sulphuric acid. Hydrometallurgy 2009, 95, 207–214. [Google Scholar] [CrossRef]
  37. Sadeghi, N.; Moghaddam, J.; Ilkhchi, M.O. Kinetics of zinc sulfide concentrate direct leaching in pilot plant scale and development of semi-empirical model. Trans. Nonferrous Met. Soc. China 2017, 27, 2272−2281. [Google Scholar] [CrossRef]
  38. Jia, L.; Zhong, Y.; Li, K.; Li, B.; Gao, J.; Liu, T.; Wang, F.; Wu, W.; Feng, J. Recovery of zinc resources from secondary zinc oxide via composite ammonia leaching: Analysis of Zn leaching behavior. Chem. Eng. J. 2023, 472, 144930. [Google Scholar] [CrossRef]
  39. Parada, T.F.; Asselin, E. Reducing power consumption in zinc electrowinning. J. Miner. Met. Mater. Soc. 2009, 61, 54–58. [Google Scholar] [CrossRef]
  40. Gomes, R.A.M.; Porto, M.P.; Vieira, K.C.P.; Cardoso Filho, B.J.; Rezende, S.L.S.P.; Ciminelli, V.S.T.; Majuste, D. Use of hybrid renewable energy system with organic photovoltaic cells in zinc electrowinning. J. Clean. Prod. 2021, 293, 125333. [Google Scholar] [CrossRef]
  41. Youcai, Z.; Qiang, L.; Chenglong, Z.; Jiachao, J. Production of ultrafine zinc powder from wastes containing zinc by electrowinning in alkaline solution. Braz. J. Chem. Eng. 2013, 30, 857–864. [Google Scholar] [CrossRef]
  42. Takayama, Y. Studies on amino acids and related compounds Part VII. On the oxidation of various amino acids. Bull. Chem. Soc. Jpn. 1933, 8, 213. [Google Scholar] [CrossRef]
  43. Takayama, Y. Studies on amino acids and related compounds Part XI. The formation of aldehydes by the electrolytic oxidation of α amino acids. Bull. Chern. Soc. Jpn. 1937, 12, 338. [Google Scholar] [CrossRef]
  44. Marangoni, D.G.; Wylie, I.G.N.; Roscoe, S.G. Surface electrochemistry of the oxidation reactions of α- and β-alanine at a platinum electrode. J. Electroanal. Chem. Interf. Electrochem. 1991, 320, 269–284. [Google Scholar] [CrossRef]
  45. Marangoni, D.G.; Smith, R.; Roscoe, S. Surface electrochemistry of the oxidation of glycine at Pt. Can. J. Chem. 1989, 67, 921. [Google Scholar] [CrossRef]
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Figure 1. Structural formula of monosodium glutamate.
Figure 1. Structural formula of monosodium glutamate.
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Figure 2. Flowsheet of closed-circuit operation.
Figure 2. Flowsheet of closed-circuit operation.
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Figure 3. Schematic representation of the electrowinning setup using 300 mL of pregnant leach solution (leached at 1 M MSG, pH 9.5, 70 °C for 5 h, solid/liquid ratio 50 g/L, d80 = 115 µm) containing 6436 mg/L Zn, 132 mg/L Pb, and 21 mg/L Fe, with graphite electrodes at a constant current of 0.5 A for 150 min.
Figure 3. Schematic representation of the electrowinning setup using 300 mL of pregnant leach solution (leached at 1 M MSG, pH 9.5, 70 °C for 5 h, solid/liquid ratio 50 g/L, d80 = 115 µm) containing 6436 mg/L Zn, 132 mg/L Pb, and 21 mg/L Fe, with graphite electrodes at a constant current of 0.5 A for 150 min.
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Figure 4. XRD pattern of the ore sample before (a) and after (b) leaching.
Figure 4. XRD pattern of the ore sample before (a) and after (b) leaching.
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Figure 5. Species distribution of glutamate in a glutamate-H2O system as a function of pH [28].
Figure 5. Species distribution of glutamate in a glutamate-H2O system as a function of pH [28].
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Figure 6. The effect of temperature on the recovery of Zn% as a function of time ([MSG] = 1.0 M, pH 9.5, solid-to-liquid ratio = 50 g/L, d80 = 115 µm).
Figure 6. The effect of temperature on the recovery of Zn% as a function of time ([MSG] = 1.0 M, pH 9.5, solid-to-liquid ratio = 50 g/L, d80 = 115 µm).
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Figure 7. The effect of pH on Zn recovery percentage as a function of leaching time ([MSG] = 1.0 M, 70 °C, solid-to-liquid ratio = 50 g/L, d80 = 115 µm).
Figure 7. The effect of pH on Zn recovery percentage as a function of leaching time ([MSG] = 1.0 M, 70 °C, solid-to-liquid ratio = 50 g/L, d80 = 115 µm).
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Figure 8. The effect of initial MSG concentration on the Zn recovery percentage (pH 9.5, 70 °C, solid-to-liquid ratio = 50 g/L, d80 = 115 µm).
Figure 8. The effect of initial MSG concentration on the Zn recovery percentage (pH 9.5, 70 °C, solid-to-liquid ratio = 50 g/L, d80 = 115 µm).
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Figure 9. The effect of leaching time and particle size on the Zn recovery percentage ([MSG] = 1.0 M, pH 9.5, 70 °C, solid-to-liquid ratio = 50 g/L).
Figure 9. The effect of leaching time and particle size on the Zn recovery percentage ([MSG] = 1.0 M, pH 9.5, 70 °C, solid-to-liquid ratio = 50 g/L).
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Figure 10. The effect of leaching time and solid-to-liquid ratio on Zn recovery percentage ([MSG] = 1.0 M, pH 9.5, 70 °C, 115 µm).
Figure 10. The effect of leaching time and solid-to-liquid ratio on Zn recovery percentage ([MSG] = 1.0 M, pH 9.5, 70 °C, 115 µm).
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Figure 11. SEM (a) and EDS (b) images of the metal deposited on the cathode.
Figure 11. SEM (a) and EDS (b) images of the metal deposited on the cathode.
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Figure 12. Effect of the recirculation of spent electrowinning solution on Zn recovery percentage over three electrowinning–leaching cycles.
Figure 12. Effect of the recirculation of spent electrowinning solution on Zn recovery percentage over three electrowinning–leaching cycles.
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Table 1. Chemical composition of the MCS ore sample.
Table 1. Chemical composition of the MCS ore sample.
Zn%Pb%Fe%Ca%Mg%Si%Mn%
14.774.5327.926.980.292.700.76
Table 2. Leaching parameters and conditions for the extraction of zinc from an MCS ore sample.
Table 2. Leaching parameters and conditions for the extraction of zinc from an MCS ore sample.
ParameterUnitValue
Temperature°C30–50–70
pH-7.1–8.5–9.5–10.5
MSG ConcentrationM0.5–1.0–1.5
Particle Sizeµm75–115–200
S/L Ratiog/L30–50–70
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Ozturk, Y. Selective Recovery of Zinc from Oxide Ores Using Monosodium Glutamate as a Green Lixiviant. Minerals 2025, 15, 870. https://doi.org/10.3390/min15080870

AMA Style

Ozturk Y. Selective Recovery of Zinc from Oxide Ores Using Monosodium Glutamate as a Green Lixiviant. Minerals. 2025; 15(8):870. https://doi.org/10.3390/min15080870

Chicago/Turabian Style

Ozturk, Yasemin. 2025. "Selective Recovery of Zinc from Oxide Ores Using Monosodium Glutamate as a Green Lixiviant" Minerals 15, no. 8: 870. https://doi.org/10.3390/min15080870

APA Style

Ozturk, Y. (2025). Selective Recovery of Zinc from Oxide Ores Using Monosodium Glutamate as a Green Lixiviant. Minerals, 15(8), 870. https://doi.org/10.3390/min15080870

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