Removal of Magnesium in Zinc Hydrometallurgical System via Freezing Crystallization: From Laboratory Experiments to Industrial Application

: Magnesium (Mg) is not only a typical impurity but also an important valuable metal in the zinc (Zn) hydrometallurgical process. This study proposed the use of freezing crystallization to recover Mg and reduce the Mg 2+ concentration in waste electrolyte solutions, meeting the requirements of Zn hydrometallurgy. The experimental results indicated that the Mg 2+ concentration decreased from 23.60 g/L to 14.28 g/L under optimal conditions at a temperature of 263.15 K, holding time of 90.00–120.00 min, H 2 SO 4 concentration of 143.00 g/L, crystal seed addition of 50.00 g/L, and agitation speed of 300.00 rpm. The crystallization mother liquor was returned to the Zn hydrometal-lurgical process. The crystallization product was a mixture of MgSO 4 · 7H 2 O and ZnSO 4 · 7H 2 O with an aspect of 17. Notably, this method resulted in no discharge of waste gas, waste liquid, or waste residue. Additionally, during the industrial application process, the average removal efﬁciency of Mg 2+ was 40.15%. The concentration of Mg 2+ in waste electrolyte was reduced from 25.00–27.00 g/L to 13.00–15.00 g/L. These results indicated that the method effectively controlled the concentration of Mg 2+ in the waste electrolyte and facilitated the recovery of Mg resources from Zn hydrometallurgy.


Introduction
The enrichment of magnesium (Mg 2+ ) has a significant influence on the zinc (Zn) hydrometallurgical process due to the increased viscosity of the solution.The Zn hydrometallurgical process primarily consists of the leaching-purification-Zn electrolysis stages.Specifically, in the Zn electrolysis process, the presence of Mg 2+ results in "plate-burning" and promotes the hydrogen evolution reaction [1,2].The issue of Mg is a common and unsolved problem in the Zn hydrometallurgical process.Therefore, it is of great significance to study the removal and recovery of Mg [3].
Currently, Zn concentrate pre-treatment involves various methods.Dilute sulfuric acid leaching, Mg fluoride precipitation [4][5][6], the lime neutralization process [2,7], and solvent extraction [8][9][10] are the main approaches used to remove Mg from the Zn hydrometallurgical system.In the Zn concentrate dilute sulfuric acid leaching process, MgO and MgCO 3 are dissolved along with ZnO and ZnCO 3 , resulting in the loss of Zn.Furthermore, during the fluoride salts precipitation process, the fluoride ions act as ligands, which react with Mg to form MgF 2 with low solubility, precipitating the removal of Mg.However, this process affects the Zn electrolysis and poses risks to equipment and operators during industrial operations [11].The lime neutralization method relies on the difference in hydrolysis pH between Zn and Mg, regulating the pH to 10-11 by adding Ca(OH) 2 solution.This process consumes a significant amount of lime, and the by-product, gypsum residue, requires further treatment.Solvent extraction is an effective method for separating special metals from polymetallic solutions.Nevertheless, the organic phase partially remains in the aqueous phase during the extraction process, and reagent costs may limit its application [12][13][14].Presently, considering economic and environmental concerns, the loss of Zn, the introduction of fluoride ions, the enrichment of residual organic phases, the deterioration in the working environment caused by ammonia, and the reagent costs are vital problems during the Mg removal process.The freezing crystallization method, which employs no additional reagents, emerges as a promising trend in the purification processes of Mg solutions for Zn hydrometallurgical plants.Jiang et al. utilized a two-stage heat exchange precooling technology to separate and recover zinc magnesium sulfate complex salt from sulfate solution.The filtered crystallization mother liquor was returned to the precooler as the precooling medium, achieving the crystallization separation of magnesium in sulfate solution with low energy consumption [15].
The key to Mg removal via freezing crystallization lies in the difference in the solubilities of Zn 2+ and Mg 2+ at various temperatures, and this process involves no impurity ions.Additionally, this method offers the following advantages [16][17][18]: (1) it is cost-effective because waste electrolyte serves as the raw material for the preparation of MgSO 4 product, and the crystallized solution is returned to the Zn hydrometallurgical system, significantly reducing the raw material cost and wastewater emissions; (2) it is efficient as it eliminates the need for reagents and reduces operational steps, achieving the removal of Mg 2+ in a single step; and (3) the method features low energy consumption and a rapid process without the addition of ammonium compounds (NH 3 •H 2 O, (NH 4 ) 2 CO 3 , NH 4 •Cl, etc.), thereby preventing harm from ammonium volatilization to both human health and the environment.Ultimately, it leads to a decrease in Mg 2+ concentration in the Zn hydrometallurgical system and the recycling of waste electrolytes.
In the freezing crystallization process, MgSO 4 •12H 2 O forms in the crystallizer, and spontaneous recrystallization into Mg sulfate heptahydrate (MgSO 4 •7H 2 O) occurs during subsequent filtration and drying above 273.55K.The MgSO 4 •7H 2 O crystals exhibit a needlelike morphology [19].In the sulfuric acid leaching of high-magnesium borate minerals, more than 90.00% of Mg 2+ can be removed in the form of the MgSO 4 •H 2 O crystalline phase from sulfuric leachate via the cooling crystallization method [20].Nevertheless, the scale-up test and industrial application of these reported findings have not been conducted.
To determine the optimal experimental conditions for freezing crystallization, investigations were conducted on the effects of temperature, holding time, H 2 SO 4 concentration, and the quantity of added crystal seeds on the Mg removal efficiency.Verification experiments were conducted at different temperatures.Moreover, the industrial application of freezing crystallization was realized.The chemical compositions, mineral morphological characteristics, and phase identification of the crystallization products were thoroughly characterized.This approach ultimately achieved the removal of Mg 2+ from waste electrolytes and the recovery of Mg resources from Zn hydrometallurgy.

Materials
The waste electrolyte was provided by Sichuan Sihuan Zn & Germanium Technology Co., Ltd.(Sichuan, China), and the primary chemical components are listed in Table 1.As shown in Table 1, the waste electrolyte mainly contained Zn, Mg, Mn, Pb, and SiO 2 .The concentration of Mg 2+ was 23.60 g/L, which exceeded the allowable value of 15.00 g/L for Mg 2+ concentration in some Zn hydrometallurgical plants during the Zn electrowinning process.Thus, it is imperative to remove Mg from the waste electrolyte.

Characterization
The concentration of Zn 2+ and Mg 2+ in the crystallization solution system, as well as the Zn and Mg content in the crystallization product, were determined using the ethylenediaminetetraacetic acid (EDTA) titration method.In the industrial application process, %Mg and %Zn contents were measured via ICP (inductively coupled plasma optical emission spectroscopy, ICP-OES, Prodigy, Leeman, Hudson, NH, USA), and the average removal efficiency was assessed.The crystalline structures of the equilibrium solid, obtained from the solid-liquid equilibrium study process, and the crystal products, were characterized via X-ray diffraction (XRD) (Empyrean, PANalytical, Almelo, The Netherlands) in the 2θ range of 20-80 • at a scanning rate of 8 • /min operated at a 40 kV accelerating voltage and a 40 mA current with a Cu Kα radiation source.The morphology and elemental compositions of the crystallization products were characterized via Scanning Electron Microscopy (SEM)/Energy-Disperse X-ray Spectroscopy (EDS) (TESCAN MIRA LMS, Brno, Czech Republic), with an accelerating voltage range of 200 eV-30 keV.The samples were prepared through dispersion in ethanol: a drop of the dispersion was placed on a carbon-coated copper grid and allowed to dry before analysis.

Feasibility Analysis
The principle of freezing crystallization for Mg 2+ removal in waste electrolytes was based on the trend of MgSO 4 equilibrium solubility decreasing with a certain temperature range.This approach achieved the removal of Mg in the Zn hydrometallurgical system.In our previous work, the solubility of MgSO 4 in the quaternary system MgSO 4 -ZnSO 4 -H 2 SO 4 -H 2 O at different temperatures was measured, as shown in Figure 1 during preliminary research [21].The results indicated that the solubility of MgSO 4 decreased with decreasing temperature <343.15K.According to Figure 1, when the temperature was < 323.15

Characterization
The concentration of Zn 2+ and Mg 2+ in the crystallization solution system, as the Zn and Mg content in the crystallization product, were determined using the e diaminetetraacetic acid (EDTA) titration method.In the industrial application %Mg and %Zn contents were measured via ICP (inductively coupled plasma optic sion spectroscopy, ICP-OES, Prodigy, Leeman, Hudson, NH, USA), and the ave moval efficiency was assessed.The crystalline structures of the equilibrium solid, o from the solid-liquid equilibrium study process, and the crystal products, were terized via X-ray diffraction (XRD) (Empyrean, PANalytical, Almelo, The Netherl the 2θ range of 20-80° at a scanning rate of 8°/min operated at a 40 kV accelerating and a 40 mA current with a Cu Kα radiation source.The morphology and elemen positions of the crystallization products were characterized via Scanning Electron copy (SEM)/Energy-Disperse X-ray Spectroscopy (EDS) (TESCAN MIRA LMS Czech Republic), with an accelerating voltage range of 200 eV-30 keV.The samp prepared through dispersion in ethanol: a drop of the dispersion was placed on a coated copper grid and allowed to dry before analysis.

Feasibility Analysis
The principle of freezing crystallization for Mg 2+ removal in waste electroly based on the trend of MgSO4 equilibrium solubility decreasing with a certain temp range.This approach achieved the removal of Mg in the Zn hydrometallurgical In our previous work, the solubility of MgSO4 in the quaternary system MgSO4 H2SO4-H2O at different temperatures was measured, as shown in Figure 1 during inary research [21].The results indicated that the solubility of MgSO4 decreased w creasing temperature <343.15K.According to Figure 1, when the temperature was K, both ZnSO4•7H2O and MgSO4•7H2O appeared in the equilibrium solid phase of ternary system MgSO4-ZnSO4-H2SO4-H2O.Consequently, the crystallization p consisted of Zn and Mg co-crystallization products in the process of freezing cry tion of waste electrolytes, thereby achieving the removal of Mg 2+ .

Experiment Procedures
The removal of Mg from waste electrolytes through the freezing crystallization method includes the following steps: (1) A 200.00 mL waste electrolyte in a beaker was measured and placed in a low-temperature reactor.The magnetic stirring device was activated, and the solution was gradually cooled to the specified experimental temperature.(2) After the reaction was completed, the stirring device was deactivated, allowing for heat preservation and sedimentation.After the crystals were completely settled, the supernatant was extracted using a Millipore filter for the analysis of metal ion concentration.(3) The crystal slurry using a centrifugal filter was separated to obtain a crystallization product and the solution was devoid of Mg 2+ .(4) The solid product was washed with ethanol and subsequently placed in an oven for 24 h at 25 • C for drying.The solid products were analyzed via XRD, SEM/EDS, and chemical element composition.Additionally, the crystallization mother solution was returned to the Zn hydrometallurgical system, and the crystallization products were subjected to dissolution and neutralizing precipitation to separate Zn and Mg.This process was important in the utilization of Mg resources.

Optimization of Freezing Crystallization Experimental Conditions
To determine the optimal experimental parameters for the efficient removal of Mg 2+ from waste electrolytic solution through the freezing crystallization method, the effects of temperature, holding time, H 2 SO 4 concentration, and the amount of crystal seed added on the removal efficiency of Mg 2+ were investigated.The results are shown in Figure 2.Moreover, the crystallization products were characterized via chemical composition, XRD, and SEM-EDS.

Experiment Procedures
The removal of Mg from waste electrolytes through the freezing crystallization method includes the following steps: (1) A 200.00 mL waste electrolyte in a beaker was measured and placed in a low-temperature reactor.The magnetic stirring device was ac tivated, and the solution was gradually cooled to the specified experimental temperature (2) After the reaction was completed, the stirring device was deactivated, allowing for hea preservation and sedimentation.After the crystals were completely settled, the superna tant was extracted using a Millipore filter for the analysis of metal ion concentration.( 3The crystal slurry using a centrifugal filter was separated to obtain a crystallization prod uct and the solution was devoid of Mg 2+ .(4) The solid product was washed with ethano and subsequently placed in an oven for 24 h at 25 °C for drying.The solid products were analyzed via XRD, SEM/EDS, and chemical element composition.Additionally, the crys tallization mother solution was returned to the Zn hydrometallurgical system, and the crystallization products were subjected to dissolution and neutralizing precipitation to separate Zn and Mg.This process was important in the utilization of Mg resources.

Optimization of Freezing Crystallization Experimental Conditions
To determine the optimal experimental parameters for the efficient removal of Mg 2 from waste electrolytic solution through the freezing crystallization method, the effects o temperature, holding time, H2SO4 concentration, and the amount of crystal seed added on the removal efficiency of Mg 2+ were investigated.The results are shown in Figure 2.More over, the crystallization products were characterized via chemical composition, XRD, and SEM-EDS.

Effect of Temperature on the Mg Removal Efficiency
The crystal temperature is a crucial parameter in the freezing crystallization process for waste electrolytes.Different crystallization temperatures lead to variations in the sol ubility of MgSO4 and the equilibrium solid phase.Under specific conditions of initia H2SO4 concentration of 163.70 g/L, holding time of 120.00 min, and agitation speed o

Effect of Temperature on the Mg Removal Efficiency
The crystal temperature is a crucial parameter in the freezing crystallization process for waste electrolytes.Different crystallization temperatures lead to variations in the solubility of MgSO 4 and the equilibrium solid phase.Under specific conditions of initial H 2 SO 4 concentration of 163.70 g/L, holding time of 120.00 min, and agitation speed of 300.00 rpm, the effect of crystal temperature on the removal efficiency of Mg was investigated.Then, the crystal slurry was subsequently subject to solid-liquid separation via the centrifugal filtration method.The concentration of Zn 2+ and Mg 2+ in the crystallized solution was determined through EDTA titration.
As shown in Figure 2A, the concentration of Mg 2+ decreased from 23.60 g/L to 8.50 g/L as the temperature decreased from 298.15 K to 243.15 K.The crystallization of Zn and Mg was influenced by decreasing the crystal temperature.However, the freezing point of the system used in this study was 242.15 K.At a temperature of 243.15 K, the system became cloudy due to the co-crystallization of ice, Mg 2+ , and Zn 2+ .This system was unfavorable for Mg 2+ removal.The concentration of Mg 2+ in the post-crystallization solution at 263.15 K was 13.58 g/L, which was <15.00 g/L threshold required for the Zn electrowinning process for Mg 2+ concentration.
The crystallization products obtained through the freezing crystallization of waste electrolytes at different temperatures were dried at room temperature for 24.00 h.SEM images of the crystal products are shown in Figure 3.These images revealed that the crystallization products were all short rod-shaped crystals, with variations in their length and thickness.Specifically, Figure 3A shows the morphology of the freezing crystallization products obtained at 278.15 K.In detail, the aspect ratio of crystallization product was measured by the ratio scale in SEM images.Results show that the aspect ratios of crystallization obtained from 278.15 K, 273.15 K, and 263.15K were 2.4, 12, and 14, respectively.Thus, the aspect ratio of the crystallization product obtained at 263.15 K was larger than those obtained at 278.15 K and 273.15 K. ustainability 2023, 15, x FOR PEER REVIEW 5 of 12 300.00rpm, the effect of crystal temperature on the removal efficiency of Mg was investigated.Then, the crystal slurry was subsequently subject to solid-liquid separation via the centrifugal filtration method.The concentration of Zn 2+ and Mg 2+ in the crystallized solution was determined through EDTA titration.
As shown in Figure 2A, the concentration of Mg 2+ decreased from 23.60 g/L to 8.50 g/L as the temperature decreased from 298.15 K to 243.15 K.The crystallization of Zn and Mg was influenced by decreasing the crystal temperature.However, the freezing point of the system used in this study was 242.15 K.At a temperature of 243.15 K, the system became cloudy due to the co-crystallization of ice, Mg 2+ , and Zn 2+ .This system was unfavorable for Mg 2+ removal.The concentration of Mg 2+ in the post-crystallization solution at 263.15 K was 13.58 g/L, which was <15.00 g/L threshold required for the Zn electrowinning process for Mg 2+ concentration.
The crystallization products obtained through the freezing crystallization of waste electrolytes at different temperatures were dried at room temperature for 24.00 h.SEM images of the crystal products are shown in Figure 3.These images revealed that the crystallization products were all short rod-shaped crystals, with variations in their length and thickness.Specifically, Figure 3A shows the morphology of the freezing crystallization products obtained at 278.15 K.In detail, the aspect ratio of crystallization product was measured by the ratio scale in SEM images.Results show that the aspect ratios of crystallization obtained from 278.15 K, 273.15 K, and 263.15K were 2.4, 12, and 14, respectively.Thus, the aspect ratio of the crystallization product obtained at 263.15 K was larger than those obtained at 278.15 K and 273.15 K.

Effect of Time on the Mg Removal Efficiency
The holding time of the crystal is another significant factor in the freezing crystal process.Under conditions with an initial H2SO4 concentration of 163.70 g/L, agitation speed of 300.00 rpm, and crystal temperatures of 268.15 K, 263.15 K, and 258.15 K, the effect of holding time on the Mg 2+ remove efficiency was investigated.The concentration of Mg 2+ was measured, and the results are shown in Figure 2B. Figure 2B displays that the residual Mg 2+ concentration was 15.50-16.50g/L at 268.15 K, 14.00-15.00g/L at 263.15 K, and 13.00-14.00g/L at 258.15 K.It was observed that the concentration of Mg 2+ in the crystallization mother liquor fluctuated within a range of 1.00 g/L as the reaction time extended at various temperatures.Thus, considering the comprehensive energy consumption, the optimal holding time for crystallization ranged from 90 min to 120 min.

Effect of Time on the Mg Removal Efficiency
The holding time of the crystal is another significant factor in the freezing crystal process.Under conditions with an initial H 2 SO 4 concentration of 163.70 g/L, agitation speed of 300.00 rpm, and crystal temperatures of 268.15 K, 263.15 K, and 258.15 K, the effect of holding time on the Mg 2+ remove efficiency was investigated.The concentration of Mg 2+ was measured, and the results are shown in Figure 2B. Figure 2B displays that the residual Mg 2+ concentration was 15.50-16.50g/L at 268.15 K, 14.00-15.00g/L at 263.15 K, and 13.00-14.00g/L at 258.15 K.It was observed that the concentration of Mg 2+ in the crystallization mother liquor fluctuated within a range of 1.00 g/L as the reaction time extended at various temperatures.Thus, considering the comprehensive energy consumption, the optimal holding time for crystallization ranged from 90 min to 120 min.

Effect of H 2 SO 4 Concentration on the Mg Removal Efficiency
Under the conditions of a crystal temperature of 263.15K and an agitation speed of 300.00 rpm, the effect of initial H 2 SO 4 concentration on the removal efficiency of Mg 2+ was investigated.The initial H 2 SO 4 concentration of waste electrolyte was regulated by adding CaO, resulting in the initial H 2 SO 4 concentrations of 183.00 g/L, 203.00 g/L, and 223.00 g/L, respectively.The residual Mg ion concentration in the crystallized solution is shown in Figure 2C.The residual Mg 2+ concentration in the crystallized solution gradually decreased with an increased initial H 2 SO 4 concentration of the waste electrolyte (Figure 2C).
For instance, when the initial H 2 SO 4 concentration was 163.70 g/L, the reaction was conducted at 263.15 K for 120.00 min, and the Mg 2+ concentration in the crystallized solution was 13.95 g/L.Thus, increasing the respectively.The residual Mg ion concentration in the crystallized solution is shown in Figure 2C.The residual Mg 2+ concentration in the crystallized solution gradually de creased with an increased initial H2SO4 concentration of the waste electrolyte (Figure 2C) For instance, when the initial H2SO4 concentration was 163.70 g/L, the reaction was conducted at 263.15 K for 120.00 min, and the Mg 2+ concentration in the crystallized solu tion was 13.95 g/L.Thus, increasing the H2SO4 concentration to 223.00 g/L reduced the Mg 2+ concentration in the crystallized solution to 10.21 g/L.The increase in H2SO4 concen tration reduced the solubility of MgSO4 due to the common-ion effect of sulfate anion (SO4 2− ) [22].This effect positively influenced the Mg 2+ removal via crystallization.Waste electrolytes of different Zn hydrometallurgical plants, which typically exhibited H2SO concentrations of 160.00-200.00g/L in the production practice, exhibited excellent adapt ability and higher Mg 2+ removal efficiency.This indicated that the freezing crystallization Mg removal technology has universal applicability and effectiveness for waste electrolytes produced using different Zn hydrometallurgical plants.
The crystalline products obtained through the freezing crystallization of waste elec trolytes at different initial H2SO4 concentrations were characterized via SEM analysis.The results are shown in Figure 4.It was observed that the low-temperature frozen crystalli zation products produced from waste electrolytes under different H2SO4 concentrations all exhibited rod-shaped crystals.At the same magnification, the crystals with an H2SO concentration of 143.00 g/L exhibited a significant difference in diameter compared the crystals with H2SO4 concentrations of 163.70.g/L and 223.00 g/L, and the crystal thickness was uneven.Moreover, the aspect ratios of crystallization product were 5, 14, and 7.14 a initial H2SO4 concentration of 143.00 g/L, 163.70 g/L, and 223.00 g/L, respectively.The re sults indicated that the optimal H2SO4 concentration during the freezing process is 163.7 g/L.

Effect of Adding Amount of Crystal Seed on the Mg Removal Efficiency
Crystal seed preparation: The crystal seed was prepared at a crystal temperature o 263.15 K, agitation speed of 300.00 rpm, and holding time of 120.00 min.The seed con tained 52.78% MgSO4•7H2O and 45.92% ZnSO4•7H2O.
The crystal seed played a crucial role in the industrial crystal process.The effect o the amount of crystal seeds added on the Mg 2+ removal efficiency was investigated (Figure 2D).As shown in Figure 2D, the amount of crystal seeds added exhibited little effect on the residual Mg 2+ concentration in the crystallized solution, and the average concentration of Mg 2+ was 13.00-14.00g/L, regardless of whether the added amount of crystal seed was 10.00 g/L, 25.00 g/L, 50.00 g/L, 75.00 g/L, or 100.00 g/L.Moreover, seed addition induced The crystal seed played a crucial role in the industrial crystal process.The effect of the amount of crystal seeds added on the Mg 2+ removal efficiency was investigated (Figure 2D).As shown in Figure 2D, the amount of crystal seeds added exhibited little effect on the residual Mg 2+ concentration in the crystallized solution, and the average concentration of Mg 2+ was 13.00-14.00g/L, regardless of whether the added amount of crystal seed was 10.00 g/L, 25.00 g/L, 50.00 g/L, 75.00 g/L, or 100.00 g/L.Moreover, seed addition induced the crystallization of MgSO 4 •7H 2 O and ZnSO 4 •7H 2 O, which was beneficial for Mg 2+ removal.
Figure 5 depicts the SEM images of the crystalline products obtained from different experiments conducted with varying amounts of crystal seed.In Figure 5, the morphology of the freezing crystalline product was consistently rod shaped, indicating that the freezing crystalline products of the waste electrolyte exhibited minimal variation under different seed addition conditions.In Figure 5B, the aspect ratio of the crystallization product is about 17.To obtain MgSO 4 with a well-defended crystal form and a higher aspect ratio, an additional amount of 50.00 g/L crystal seeds was selected.
stainability 2023, 15, x FOR PEER REVIEW 7 of 12 of the freezing crystalline product was consistently rod shaped, indicating that the freez ing crystalline products of the waste electrolyte exhibited minimal variation under differ ent seed addition conditions.In Figure 5B, the aspect ratio of the crystallization produc is about 17.To obtain MgSO4 with a well-defended crystal form and a higher aspect ratio an additional amount of 50.00 g/L crystal seeds was selected.

Effect of Agitation Intensity on the Mg Removal Efficiency
Under the conditions of crystalline temperature of 263.15 K, holding time of 90.00-120.00min, H2SO4 concentration of 143.00 g/L, and adding amount of crystal seed of 50.00 g/L, the effects of agitation speed (100.00 rpm, 200.00 rpm, 300.00 rpm, 400.00 rpm, 500.00rpm) on the removal efficiency of Mg and crystalline product morphology were investi gated; the results are shown in Figure 2E.As shown in Figure 2E, the average residua concentration of Mg 2+ in the crystallized solution is 12.00-14.00g/L at different agitation speeds; these results verify that the agitation intensity has little effect on the removal o Mg 2+ .
Figure 6 shows the SEM images of crystal products obtained from different stirring speeds.In Figure 6, the morphology of crystal products is cylindrical.However, there are some differences in the diameter of the crystal at different stirring speeds.The reasons for this could be the following: (1) increasing the stirring speed can promote the migration o solute molecules to the crystal surface, which is beneficial for crystal nucleation and growth; (2) increasing the agitation intensity can increase the fluid shear stress, breaking the crystals, resulting in the crystal diameter becoming shorter at higher agitation inten sity [23].

Effect of Agitation Intensity on the Mg Removal Efficiency
Under the conditions of crystalline temperature of 263.15 K, holding time of 90.00-120.00min, H 2 SO 4 concentration of 143.00 g/L, and adding amount of crystal seed of 50.00 g/L, the effects of agitation speed (100.00 rpm, 200.00 rpm, 300.00 rpm, 400.00 rpm, 500.00rpm) on the removal efficiency of Mg and crystalline product morphology were investigated; the results are shown in Figure 2E.As shown in Figure 2E, the average residual concentration of Mg 2+ in the crystallized solution is 12.00-14.00g/L at different agitation speeds; these results verify that the agitation intensity has little effect on the removal of Mg 2+ .
Figure 6 shows the SEM images of crystal products obtained from different stirring speeds.In Figure 6, the morphology of crystal products is cylindrical.However, there are some differences in the diameter of the crystal at different stirring speeds.The reasons for this could be the following: (1) increasing the stirring speed can promote the migration of solute molecules to the crystal surface, which is beneficial for crystal nucleation and growth; (2) increasing the agitation intensity can increase the fluid shear stress, breaking the crystals, resulting in the crystal diameter becoming shorter at higher agitation intensity [23].

Verification Experiments
To verify the experimental parameters for freezing crystallization of Mg 2+ removal from waste electrolytes and the stability of the Mg 2+ removal effect, repeated validation experiments (parallel experiments) were conducted under the optimized experimental conditions.These conditions included crystal temperatures of 273.15 K, 268.15 K, 263.15 K, and 258.15 K; a holding time of 90.00-120.00min; an initial H 2 SO 4 concentration of 143 g/L; a crystal seed amount added of 50.00 g/L; and an agitation speed of 300.00 rpm.The verification experiments were conducted under these conditions, and the residual Mg 2+ concentration at different crystal temperatures is listed in Table 2 and shown in Figure 7. some differences in the diameter of the crystal at different stirring speeds.The reasons for this could be the following: (1) increasing the stirring speed can promote the migration of solute molecules to the crystal surface, which is beneficial for crystal nucleation and growth; (2) increasing the agitation intensity can increase the fluid shear stress, breaking the crystals, resulting in the crystal diameter becoming shorter at higher agitation intensity [23].

Verification Experiments
To verify the experimental parameters for freezing crystallization of Mg 2+ remova from waste electrolytes and the stability of the Mg 2+ removal effect, repeated validation  experiments (parallel experiments) were conducted under the optimized experimental conditions.These conditions included crystal temperatures of 273.15 K, 268.15 K, 263.15 K, and 258.15 K; a holding time of 90.00-120.00min; an initial H2SO4 concentration of 143 g/L; a crystal seed amount added of 50.00 g/L; and an agitation speed of 300.00 rpm.The verification experiments were conducted under these conditions, and the residual Mg 2+ concentration at different crystal temperatures is listed in Table 2 and shown in Figure 7.As shown in Table 2 and Figure 7, the residual Mg 2+ concentrations were in the ranges of 17.00-18.00g/L, 16.00-17.00g/L, 14.00-15.00g/L, and 13.00-14.00g/L at 273.15 K, 268.15 K, 263.15 K, and 258.15 K, respectively, in the verification experiments.The Zn 2+ concentrations were within the ranges of 39.00-40.00g/L, 38.00-39.00g/L, and 34.00-35.00g/L, respectively.The Zn concentration remained relatively consistent at 263.15 K and 258.15 K.The average content of Mg in the crystallization product ranged from 5.4% to 6.0%, while the average content of Zn ranged from 11.60% to 12.00%.The concentration of Mg 2+ and Zn 2+ in the crystallized solution and the average content of crystallization products remained relatively stable under various temperature conditions.This indicated that lower freezing crystallization temperatures resulted in excellent Mg 2+ removal efficiency.The XRD pattern and SEM/EDS images of the crystalline product obtained from the verification experiment are shown in Figure 8. Figure 8A,B show the XRD pattern and SEM/EDS analysis of the crystalline products, respectively.Figure 8A depicts that the primary phases of the crystalline products are MgSO4•7H2O and ZnSO4•7H2O. Figure 8B shows that the micro-structures of the freezing crystallization products were rod shaped with a dense structure.The EDS pattern confirmed that Mg and Zn elements were evenly distributed within the crystal, indicating that the crystallization product was a double salt composed of MgSO4 and ZnSO4, containing crystal water.As shown in Table 2 and Figure 7, the residual Mg 2+ concentrations were in the ranges of 17.00-18.00g/L, 16.00-17.00g/L, 14.00-15.00g/L, and 13.00-14.00g/L at 273.15 K, 268.15 K, 263.15 K, and 258.15 K, respectively, in the verification experiments.The Zn 2+ concentrations were within the ranges of 39.00-40.00g/L, 38.00-39.00g/L, and 34.00-35.00g/L, respectively.The Zn concentration remained relatively consistent at 263.15 K and 258.15 K.The average content of Mg in the crystallization product ranged from 5.4% to 6.0%, while the average content of Zn ranged from 11.60% to 12.00%.The concentration of Mg 2+ and Zn 2+ in the crystallized solution and the average content of crystallization products remained relatively stable under various temperature conditions.This indicated that lower freezing crystallization temperatures resulted in excellent Mg 2+ removal efficiency.
The XRD pattern and SEM/EDS images of the crystalline product obtained from the verification experiment are shown in Figure 8. Figure 8A,B show the XRD pattern and SEM/EDS analysis of the crystalline products, respectively.Figure 8A depicts that the primary phases of the crystalline products are MgSO 4 •7H 2 O and ZnSO 4 •7H 2 O. Figure 8B shows that the micro-structures of the freezing crystallization products were rod shaped with a dense structure.The EDS pattern confirmed that Mg and Zn elements were evenly distributed within the crystal, indicating that the crystallization product was a double salt composed of MgSO 4 and ZnSO 4 , containing crystal water.The chemical composition of the crystalline product is detailed in Table 3. Combining the data from Figure 8 and Table 3, the analysis indicated that the primary phases of the crystalline products were MgSO4•7H2O and ZnSO4•7H2O, consisting of between 53.37% and 43.79% of the composition, respectively.The results corresponded with the singlefactor experiment.This indicated that the experimental parameters obtained via freezing crystallization of Mg 2+ from waste electrolytes in the Zn hydrometallurgical process, as determined through optimization experiments, exhibited excellent stability.The findings suggest promising potential for the industrial application of Mg 2+ removal through freezing crystallization of waste electrolytes in the Zn hydrometallurgical process.

Industrial Application
The development of the technology for removing Mg via freezing crystallization involved a series of exploratory experiments, verification experiments, and scale-up tests.The industrial application of the freezing crystallization method was conducted at Sichuan Sihuan Zn & Germanium Technology Co., Ltd.(Sichuan, China).The diagram of the freezing crystallization process and the primary device are shown in Figure 9.The chemical composition of the crystalline product is detailed in Table 3. Combining the data from Figure 8 and Table 3, the analysis indicated that the primary phases of the crystalline products were MgSO 4 •7H 2 O and ZnSO 4 •7H 2 O, consisting of between 53.37% and 43.79% of the composition, respectively.The results corresponded with the singlefactor experiment.This indicated that the experimental parameters obtained via freezing crystallization of Mg 2+ from waste electrolytes in the Zn hydrometallurgical process, as determined through optimization experiments, exhibited excellent stability.The findings suggest promising potential for the industrial application of Mg 2+ removal through freezing crystallization of waste electrolytes in the Zn hydrometallurgical process.

Industrial Application
The development of the technology for removing Mg via freezing crystallization involved a series of exploratory experiments, verification experiments, and scale-up tests.The industrial application of the freezing crystallization method was conducted at Sichuan Sihuan Zn & Germanium Technology Co., Ltd.(Sichuan, China).The diagram of the freezing crystallization process and the primary device are shown in Figure 9.
hydrometallurgical process.When the Mg removal device operated for some time, the M concentration in the wet Zn smelting system was reduced to 15.00 g/L.The Mg remov device ceased operation until the Mg concentration in the system was re-enriched to 25.00 g/L.The Mg removal device was reactivated at this point.This cyclic operation en bled the open circuit and recovery of Mg in the Zn hydrometallurgical system.At this time, it was successfully commissioned and operated.In the industrial application process, the waste electrolyte initially passes through a fore cooler to reduce its temperature.The temperature of the waste electrolyte was reduced from 311.15-313.15K to 293.15-298.15K through precooling.The waste electrolyte was transferred to the crystallizer, and the temperature control of the crystallizer was 263.15K ± 0.50 K. Centrifugation was employed to achieve liquid-solid separation of the crystal slurry, and the obtained crystallized liquid was returned to the precooler.
Additionally, it was observed that a highly energy efficient and environmentally friendly treatment method for freezing crystallization products involved a wet method, such as hydrothermal synthesis of magnesium sulfate whiskers [24].However, for enterprises implementing this research project, the fire treatment method was more convenient.The existing rotary kiln device of the enterprise was used to directly recover Mg 2+ , and the emission SO 2 was collected effectively and utilized to prepare H 2 SO 4 , while reducing the investment cost of the device.
Consequently, the solid recovery was accomplished through pyrometallurgical processes using the Waelz method for Zn recycling and Mg recovery.During the industrial application process, the crystallized solution with an Mg 2+ concentration of <15.00 g/L was returned to the Zn hydrometallurgical system.The main indexes during the industrial application process are listed in Table 4.As shown in Table 4, the initial concentrations of Mg 2+ varied, owing to changes in the material content during the industrial application process.For example, the Mg and Zn content changed in the Zn concentrates.Additionally, the Mg removal device operated for a total of 114.00 days, processing 10,990.00m 3 of waste electrolyte and producing 77.77t of Mg metal (measured via factory data).Furthermore, the average concentration of Mg 2+ in the waste electrolyte was reduced from 25.00-27.00g/L to 13.00-15.00g/L, and the average content of Mg in the crystallization product was 8.00%-10.00%.
However, the freezing crystallization device was intermittently operated in the Zn hydrometallurgical process.When the Mg removal device operated for some time, the Mg concentration in the wet Zn smelting system was reduced to 15.00 g/L.The Mg removal device ceased operation until the Mg concentration in the system was re-enriched to ≥ 25.00 g/L.The Mg removal device was reactivated at this point.This cyclic operation enabled the open circuit and recovery of Mg in the Zn hydrometallurgical system.

Conclusions
In summary, the results of this study demonstrated the feasibility of removing Mg 2+ and producing MgSO 4 •7H 2 O and ZnSO 4 •7H 2 O from the Zn hydrometallurgical process via freezing crystallization.First, the solubility of MgSO 4 in the quaternary system MgSO 4 -ZnSO 4 -H 2 SO 4 -H 2 O at different temperatures was measured, and the thermodynamic analysis results indicated that the Mg 2+ in the waste electrolyte was effectively removed via freezing crystallization.Second, the effects of initial H 2 SO 4 concentration, crystal temperature, holding time, the amount of added crystal seed, and agitation speed on the removal efficiency of Mg 2+ were explored.Additionally, the verification experiments under optimized conditions were conducted.The experimental results indicated that the Mg 2+ concentration decreased from 23.60 g/L to 14.28 g/L under the optimal conditions at a crystal temperature of 263.15 K, holding time of 90.00-120.00min, initial H 2 SO 4 concentration of 143.00 g/L, crystal seed addition of 0.05 g/mL, and agitation speed of 300.00 rpm.The residual Mg 2+ concentration in the post-crystallization waste electrolyte met the requirements of Zn hydrometallurgy.XRD and SEM/EDS analysis results indicated that the crystalline product was a co-crystallization of MgSO 4 •7H 2 O and ZnSO 4 •7H 2 O, with a regular columnar microstructure and aspect ratio of 17.Furthermore, the industrial application of the freezing crystallization method was conducted and operated for 114.00 days.In this study, 10,990.00m 3 of waste electrolyte was processed, resulting in the production of 77.77 t of Mg metal.Notably, the average concentration of Mg 2+ in the waste electrolyte decreased from 25.00-27.00g/L to 13.00-15.00g/L, and the average Mg content in the crystallization product ranged from 8.00% to 10.00%.
This study achieved effective control of Mg 2+ concentration and the recovery of Mg resources from Zn hydrometallurgical was realized, with minimal emissions of waste gases and waste residues during the freezing crystallization process.

Figure 1 .
Figure 1.The solubility curve of MgSO 4 in the MgSO 4 -ZnSO 4 -H 2 SO 4 -H 2 O system at different temperatures and H 2 SO 4 concentrations.

Figure 2 .
Figure 2. Effect of different experimental conditions on the Mg 2+ concentration in the crystallized solution: (A) temperature, (B) time, (C) H2SO4 concentration, (D) added amount of crystal seed, and (E) stirring speed.

Figure 2 .
Figure 2. Effect of different experimental conditions on the Mg 2+ concentration in the crystallized solution: (A) temperature, (B) time, (C) H 2 SO 4 concentration, (D) added amount of crystal seed, and (E) stirring speed.
H 2 SO 4 concentration to 223.00 g/L reduced the Mg 2+ concentration in the crystallized solution to 10.21 g/L.The increase in H 2 SO 4 concentration reduced the solubility of MgSO 4 due to the common-ion effect of sulfate anion (SO 4 2− ) [22].This effect positively influenced the Mg 2+ removal via crystallization.Waste electrolytes of different Zn hydrometallurgical plants, which typically exhibited H 2 SO 4 concentrations of 160.00-200.00g/L in the production practice, exhibited excellent adaptability and higher Mg 2+ removal efficiency.This indicated that the freezing crystallization Mg removal technology has universal applicability and effectiveness for waste electrolytes produced using different Zn hydrometallurgical plants.The crystalline products obtained through the freezing crystallization of waste electrolytes at different initial H 2 SO 4 concentrations were characterized via SEM analysis.The results are shown in Figure 4.It was observed that the low-temperature frozen crystallization products produced from waste electrolytes under different H 2 SO 4 concentrations all exhibited rod-shaped crystals.At the same magnification, the crystals with an H 2 SO 4 concentration of 143.00 g/L exhibited a significant difference in diameter compared the crystals with H 2 SO 4 concentrations of 163.70.g/L and 223.00 g/L, and the crystal thickness was uneven.Moreover, the aspect ratios of crystallization product were 5, 14, and 7.14 at initial H 2 SO 4 concentration of 143.00 g/L, 163.70 g/L, and 223.00 g/L, respectively.The results indicated that the optimal H 2 SO 4 concentration during the freezing process is 163.7 g/L.stainability 2023, 15, x FOR PEER REVIEW 6 of 12

Figure 4 .
Figure 4. SEM image of crystal products obtained from different H 2 SO 4 concentrations: (A) 143.00 g/L, (B) 163.70 g/L, and (C) 223.00 g/L.3.1.4.Effect of Adding Amount of Crystal Seed on the Mg Removal Efficiency Crystal seed preparation: The crystal seed was prepared at a crystal temperature of 263.15 K, agitation speed of 300.00 rpm, and holding time of 120.00 min.The seed contained 52.78% MgSO 4 •7H 2 O and 45.92% ZnSO 4 •7H 2 O.The crystal seed played a crucial role in the industrial crystal process.The effect of the amount of crystal seeds added on the Mg 2+ removal efficiency was investigated (Figure2D).As shown in Figure2D, the amount of crystal seeds added exhibited little effect on the residual Mg 2+ concentration in the crystallized solution, and the average concentration

Figure 5 .
Figure 5. SEM images of crystalline products obtained with different amounts of crystal seeds: (A 10.00 g/L, (B) 50.00 g/L, and (C) 100.00 g/L.

Figure 5 .
Figure 5. SEM images of crystalline products obtained with different amounts of crystal seeds: (A) 10.00 g/L, (B) 50.00 g/L, and (C) 100.00 g/L.

Figure 6 .
Figure 6.SEM images of the crystal products were obtained from different stirring speeds.(A) 100.00 rpm, (B) 300.00 rpm, and (C) 500.00rpm.

Figure 6 .
Figure 6.SEM images of the crystal products were obtained from different stirring speeds.(A) 100.00 rpm, (B) 300.00 rpm, and (C) 500.00rpm.

12 Figure 8 .
Figure 8. (A) XRD pattern and (B) SEM/EDS images of crystal products obtained from the verification experiments.

Figure 8 .
Figure 8. (A) XRD pattern and (B) SEM/EDS images of crystal products obtained from the verification experiments.

Figure 9 .
Figure 9. Diagram of the freezing crystallization and primary device used in the industrial application process.Construction of the production workshop began on 18 March 2021.It was completed at the end of July 2021.It commenced commissioning and trial operation on 1 August 2021.At this time, it was successfully commissioned and operated.In the industrial application process, the waste electrolyte initially passes through a fore cooler to reduce its temperature.The temperature of the waste electrolyte was reduced from 311.15-313.15K to 293.15-298.15K through precooling.The waste electrolyte was transferred to the crystallizer, and the temperature control of the crystallizer was 263.15K ± 0.50 K. Centrifugation was employed to achieve liquid-solid separation of the crystal slurry, and the obtained crystallized liquid was returned to the precooler.Additionally, it was observed that a highly energy efficient and environmentally friendly treatment method for freezing crystallization products involved a wet method, such as hydrothermal synthesis of magnesium sulfate whiskers[24].However, for enterprises implementing this research project, the fire treatment method was more convenient.The existing rotary kiln device of the enterprise was used to directly recover Mg 2+ , and the emission SO 2 was collected effectively and utilized to prepare H 2 SO 4 , while reducing the investment cost of the device.Consequently, the solid recovery was accomplished through pyrometallurgical processes using the Waelz method for Zn recycling and Mg recovery.During the industrial application process, the crystallized solution with an Mg 2+ concentration of <15.00 g/L was returned to the Zn hydrometallurgical system.The main indexes during the industrial application process are listed in Table4.

Table 1 .
The contents of the main chemical components in the waste electrolysis/(g/L).
K, both ZnSO 4 •7H 2 O and MgSO 4 •7H 2 O appeared in the equilibrium solid phase of the quaternary system MgSO 4 -ZnSO 4 -H 2 SO 4 -H 2 O. Consequently, the crystallization products consisted of Zn and Mg co-crystallization products in the process of freezing crystallization of waste electrolytes, thereby achieving the removal of Mg 2+ .

Table 1 .
The contents of the main chemical components in the waste electrolysis/(g/L).

Table 2 .
Zn and Mg average concentration and content in the crystallization mother solution and crystal product during the variation experiment.

Table 2 .
Zn and Mg average concentration and content in the crystallization mother solution and crystal product during the variation experiment.

Table 3 .
The primary chemical composition of crystal products was obtained from the verification experiments (%).

Table 3 .
The primary chemical composition of crystal products was obtained from the verification experiments (%).

Table 4 .
The main indexes of freezing crystallization during the industrial application process.