Efficient Recovery of Rare Earth Elements and Zinc from Spent Ni–Metal Hydride Batteries: Statistical Studies

Considering how important rare earth elements (REEs) are for many different industries, it is important to separate them from other elements. An extractant that binds to REEs inexpensively and selectively even in the presence of interfering ions can be used to develop a useful separation method. This work was designed to recover REEs from spent nickel–metal hydride batteries using ammonium sulfate. The chemical composition of the Ni–MH batteries was examined. The operating leaching conditions of REE extraction from black powder were experimentally optimized. The optimal conditions for the dissolution of approximately 99.98% of REEs and almost all zinc were attained through use of a 300 g/L (NH4)2SO4 concentration after 180 min of leaching time and a 1:3 solid/liquid phase ratio at 120 °C. The kinetic data fit the chemical control model. The separation of total REEs and zinc was conducted under traditional conditions to produce both metal values in marketable forms. The work then shifted to separate cerium as an individual REE through acid baking with HCl, thus leaving pure cerium behind.


Introduction
Rare earth elements (REEs) refer to the lanthanides (57-71) besides scandium (Sc) and yttrium (Y). REEs are involved in a broad range of products [1][2][3][4]. Electric vehicles and wind energy are becoming more widespread. This means there is a higher demand for REEs. There is a possibility that their supply could be disrupted due to geological paucity, extraction difficulties, and national dependence on their use. Thus, they are considered to be truly essential raw commodities that are riskier to obtain and have a greater economic impact on Europe's monetary union [5]. With regards to the development of sources beyond primary sources, the use of REEs promotes their long-term sustainability. Spent battery consumption is expected to be the fastest-growing category of WEEE. Rechargeable batteries have a life cycle of about 1000 cycles. Every 3-5 years, the cycle turns into On the other hand, ammonium sulfate plays a foremost role in the selective extraction of REEs and Zn from various media. Ammonium compounds have become one of the most popular leachants for extracting rare earth elements in recent years. Moldoveanu and Papangelakis used (NH 4 ) 2 SO 4 to leach and recover REE ions [24,25]. Kim et al. also used a 1.0% concentration of (NH 4 ) 2 SO 4 to leach REEs from Vietnamese ore [26]. The use of ammonium sulfate has the advantages of reduced reagent use and increased final product purity [27].
In this work, suggested procedures for the efficient recovery of REEs from spent nickelmetal hydride batteries were applied to leach all REEs and almost all zinc using ammonium sulfate. The work then shifted to individually separating cerium from the REE cake using HCl. Moreover, statistical studies related to the leaching process were conducted.

Preparation of Spent Nickel-Metal Hydride (Ni-MH) Batteries
The samples recycled in this work were AA-and AAA-type cylindrical spent (Ni-MH) batteries produced in Japan (Figure 1a). These batteries were collected from local scraps. To remove the steel case with its associated battery electrode and the nylon separator from the expended batteries, they were ground and crushed in a scutter cutter using deionized water as the medium. Table 1 shows the battery components of the batteries, their weight, and their proportion. The electrode material spread in the deionized water as the heavier particles, primarily the steel casing, dropped to the bottom. The lighter nylon separator then floated away. After the removal of the float fraction (nylon) from the solution, the suspended fraction (black powder) was wet screened. The electrode powder (approximately 73.17 g) was accumulated and washed repeatedly with water in order to eliminate any possible metals (e.g., Na and K) that could behave as an activator for double sulfate precipitation, which causes REE damage throughout the acid dissolution process, from the working electrolyte [28]. After that, the battery powder was dried at 80 • C for 24 h. The obtained black powder was subjected to calcination at 550 • C to remove organic matter [29]. After milling and calcining, the weight of the battery black powder was detected. The powder was analyzed for its chemical composition, particle size, and morphological characteristics.

Apparatus and Chemicals
Many different analytical techniques were used to characterize the raw or fresh original materials, the leaching residues, and the resulting products. ICP-OES (Ultima Expert, Horiba Scientific, Kyoto, Japan) was used to measure the concentration of REEs and heavy metals in the solution. The morphologies of the black powder and the final products (REEs

Apparatus and Chemicals
Many different analytical techniques were used to characterize the raw or fresh original materials, the leaching residues, and the resulting products. ICP-OES (Ultima Expert, Horiba Scientific, Kyoto, Japan) was used to measure the concentration of REEs and heavy metals in the solution. The morphologies of the black powder and the final products (REEs as double sulfates) were identified using a scanning electron microscope (SEM-EDX) (LEO 1450VP, Carl Zeiss Microscopy GmbH, Jena, Germany). X-ray diffraction (XRD) (D8 Discover Family, Bruker, Billerica, MA, USA) was utilized to investigate the structure of the crystalline materials. Furthermore, the chemicals employed in the leaching and precipitation of the products were analytical-grade compounds. NaOH, (NH 4 ) 2 SO 4 , H 2 SO 4 , HCl, HNO 3 , and H 2 O 2 were supplied by Sigma-Aldrich, St. Louis, MO, USA.

Pre-Treatment and Characterization of the Battery Powder
In the beginning, the black powder of the Ni-MH battery was treated with H 2 O 2 to prevent iron leaching during the leaching process ( Figure 1b); H 2 O 2 acts as an oxidizing agent for the oxidation of Fe(II) to Fe(III) ions. This was performed by treating the battery powder with 30.0% H 2 O 2 using a 4:1 S/L ratio at 100 • C for 2 h. The obtained data showed that no ferrous iron was present in the sample and all iron content was ferric. This may have been due to the fact that ferrous iron reacts easier with ammonium sulfate to form Mohr's salt [30].
REEs, including lanthanum (La), cerium (Ce), neodymium (Nd), and praseodymium (Pr), are essential to the nickel-metal hydride (Ni-MH) battery supply chain since they are found in high concentrations in Ni-MH batteries. REEs may have a secondary resource in the form of recovered metals from work on spent Ni-MH batteries. The REEs were collected from discarded Ni-MH batteries via selective alkaline leaching with ammonium sulfate solution. Almost all the REEs were leached from the electrode active material of the discarded Ni-MH batteries. The chemical composition of the Ni-MH batteries is tabulated in Table 2. The SEM of the obtained black powder is shown in Figure 1c. The latter shows the morphology of the black powder, which clearly demonstrates scattered metal aggregates with a high degree of nickel-based phase intercalation. The leaching procedure was conducted using 1.0 g of Ni-MH black powder, 120 min, and 500 rpm at 90 • C to investigate how much of the spent Ni-MH powder leached REEs at various concentrations of (NH 4 ) 2 SO 4 (50-400 g/L). Using an agitator set to 500 rpm kept the particles in suspension and reduced the lixiviant diffusion layer that formed around them. We found that the dissolution of rare earth ions increased progressively as the concentration of (NH 4 ) 2 SO 4 was raised, up to 300 g/L. The efficiency dissolution process of all REEs remained practically constant even after increasing the concentration of (NH 4 ) 2 SO 4 . As a result, 300 g/L of (NH 4 ) 2 SO 4 was the optimum concentration for leaching in this experiment. With 300 g/L (NH 4 ) 2 SO 4 , 98% Nd, 76.3% Pr, 71.9% Ce, 64.2% La, and 80.7% Zn were leached out in 120 min ( Figure 2).
The leaching procedure was conducted using 1.0 g of Ni-MH black powder, 120 min, and 500 rpm at 90 °C to investigate how much of the spent Ni-MH powder leached REEs at various concentrations of (NH4)2SO4 (50-400 g/L). Using an agitator set to 500 rpm kept the particles in suspension and reduced the lixiviant diffusion layer that formed around them. We found that the dissolution of rare earth ions increased progressively as the concentration of (NH4)2SO4 was raised, up to 300 g/L. The efficiency dissolution process of all REEs remained practically constant even after increasing the concentration of (NH4)2SO4. As a result, 300 g/L of (NH4)2SO4 was the optimum concentration for leaching in this experiment. With 300 g/L (NH4)2SO4, 98% Nd, 76.3% Pr, 71.9% Ce, 64.2% La, and 80.7% Zn were leached out in 120 min ( Figure 2).

Influence of Time
A 300 g/L (NH4)2SO4 and 1:2 (S/L) ratio solution was used to test the time effect on rare earth metal leaching at 90 °C and a pulp density of 100 g/L. The obtained results in Figure 3 demonstrate that the amount of REEs recovered improved as the amount of leaching time increased. A La leaching efficiency of 74.1%was achieved after 180 min; further leaching efficiencies were found as 88.4% for Nd, 89.1% for Pr, and 81.5% for Ce. Any additional increment in the leaching time after 180 min had no influence on the leaching efficiency of the five elements. We found that 180 min of leaching yielded the greatest amount of dissolution of the REEs and also attained about 92% leaching efficiency for zinc.

Influence of Time
A 300 g/L (NH 4 ) 2 SO 4 and 1:2 (S/L) ratio solution was used to test the time effect on rare earth metal leaching at 90 • C and a pulp density of 100 g/L. The obtained results in Figure 3 demonstrate that the amount of REEs recovered improved as the amount of leaching time increased. A La leaching efficiency of 74.1%was achieved after 180 min; further leaching efficiencies were found as 88.4% for Nd, 89.1% for Pr, and 81.5% for Ce. Any additional increment in the leaching time after 180 min had no influence on the leaching efficiency of the five elements. We found that 180 min of leaching yielded the greatest amount of dissolution of the REEs and also attained about 92% leaching efficiency for zinc. REE dissolving efficiencies were studied in the range between 1:1 and 1:6 depending on the solid/liquid ratio. For these trials, the stable dissolution conditions included 300

Influence of The Solid/Liquid (S/L) Ratio
REE dissolving efficiencies were studied in the range between 1:1 and 1:6 depending on the solid/liquid ratio. For these trials, the stable dissolution conditions included 300 g/L (NH 4 ) 2 SO 4 and an incubation time of 180 min with a dissolving temperature of 90 • C. REE leaching efficiency was found to increase with increasing the S/L phase ratio to 1:3, followed by a reduction as the S/L ratio increased from 1:4 to 1:6 ( Figure 4). This may be because REEs precipitate as mono-sulfates. It is clear that all the available REEs reacted with ammonium sulfate in the solution until achieving equilibrium, after which any excess of (NH 4 ) 2 SO 4 had no effect on the REE leaching efficiency. As a result, a S/L phase ratio of 1:3 yielded the highest possible dissolution efficiencies of 91.7, 89.4, 94.7, and 95.9% for Ce, La, Nd, and Pr, respectively, and almost 100% for Zn. 3.2.3. Influence of The Solid/Liquid (S/L) Ratio REE dissolving efficiencies were studied in the range between 1:1 and on the solid/liquid ratio. For these trials, the stable dissolution condition g/L (NH4)2SO4 and an incubation time of 180 min with a dissolving tempe REE leaching efficiency was found to increase with increasing the S/L ph followed by a reduction as the S/L ratio increased from 1:4 to 1:6 ( Figure 4 because REEs precipitate as mono-sulfates. It is clear that all the availabl with ammonium sulfate in the solution until achieving equilibrium, after cess of (NH4)2SO4 had no effect on the REE leaching efficiency. As a resu ratio of 1:3 yielded the highest possible dissolution efficiencies of 91.7, 95.9% for Ce, La, Nd, and Pr, respectively, and almost 100% for Zn.

Influence of Temperature
The effect of temperature on the solubility of various REEs extant in powder was investigated throughout a temperature ranging between 25 a obtained data in Figure 5 show temperature's substantial influence on obta est REE leaching efficiency. At ambient temperature and experimental co

Influence of Temperature
The effect of temperature on the solubility of various REEs extant in spent Ni-MH powder was investigated throughout a temperature ranging between 25 and 150 • C. The obtained data in Figure 5 show temperature's substantial influence on obtaining the highest REE leaching efficiency. At ambient temperature and experimental conditions of 300 g/L of (NH 4 ) 2 SO 4 , 180 min, and a 1:3 S/L phase ratio, 62.81, 55.3, 53.4, and 68.7% of Ce, La, Nd, and Pr were leached, respectively. The dissolution efficiency reached its maximum when raising the leaching temperature to 120 • C and any further raise in temperature did not demonstrate any significant improvement in the leaching of the REEs. Consequently, we found that the satisfactory dissolution temperature of Ce, La, Nd, and Pr was 120 • C under the previously mentioned conditions.
According to a prior examination of the effectiveness of leaching REEs from spent NiMH battery powder using ammonium sulfate, the optimal dissolution conditions for dissolving approximately 99.98% of REEs and almost all zinc are as follows: 300 g/L of (NH 4 ) 2 SO 4 , 180 min, 1/3 for the S:L phase ratio, and 120 • C.
Nd, and Pr were leached, respectively. The dissolution efficiency reached when raising the leaching temperature to 120 °C and any further raise in te not demonstrate any significant improvement in the leaching of the REEs. we found that the satisfactory dissolution temperature of Ce, La, Nd, and under the previously mentioned conditions. According to a prior examination of the effectiveness of leaching RE NiMH battery powder using ammonium sulfate, the optimal dissolution dissolving approximately 99.98% of REEs and almost all zinc are as follo (NH4)2SO4, 180 min, 1/3 for the S:L phase ratio, and 120 °C.

Dissolution Kinetic Analysis of REEs
Three methods can be used to identify the rate of reaction: chemical core surface of the particle, diffusion through the solid product layer, or di fluid. The rate of the procedure is dictated by the progress of the slowest p sion [31]. In the shrinking core model (SCM), the reactions occur on the soli and decline towards the center. Meanwhile, the fluid-solid reaction mod ered the most influential models that have been applied [32]. In the comb the reaction occurs on the outer surface of the solid and declines towards th a solid particle (M) is submerged in a fluid (N) and reacts with the fluid following equation: The required time (t) for solid reaction can be determined using the n the reaction rate is controlled by fluid N diffusion through the ash layer: where x is defined as the fraction of dissolved REEs, D (m 2 /s) represents th the REEs from the ash layer, Co and CB (mol/L) correspond to fluid concen the particle and the concentration of solid reactant, respectively, ro (m) is t radius of the particle, K1 as well as K2 both stand for the rate constant, and t

Dissolution Kinetic Analysis of REEs
Three methods can be used to identify the rate of reaction: chemical reaction at the core surface of the particle, diffusion through the solid product layer, or diffusion via the fluid. The rate of the procedure is dictated by the progress of the slowest phase in succession [31]. In the shrinking core model (SCM), the reactions occur on the solid's outer layer and decline towards the center. Meanwhile, the fluid-solid reaction models are considered the most influential models that have been applied [32]. In the combination model, the reaction occurs on the outer surface of the solid and declines towards the center, when a solid particle (M) is submerged in a fluid (N) and reacts with the fluid based on the following equation: The required time (t) for solid reaction can be determined using the next equation if the reaction rate is controlled by fluid N diffusion through the ash layer: where x is defined as the fraction of dissolved REEs, D (m 2 /s) represents the diffusivity of the REEs from the ash layer, C o and C B (mol/L) correspond to fluid concentration outside the particle and the concentration of solid reactant, respectively, r o (m) is the initial outer radius of the particle, K 1 as well as K 2 both stand for the rate constant, and t (min) denotes the solubilization time. If the chemical reaction controls the reaction rate, the equation of the integrated rate is calculated as follows: Figures 6 and 7 demonstrate diagrams of 1 − 3(1 − x) 2/3 + 2(1 − x) as well as 1 − (1 − x) 1/3 against dissolution time; they indicate how temperature affects REE reaction kinetics. The plots show straight lines for Ce, La, Nd, and Pr at various temperatures. As presented in Figure 6, the kinetic data are not a good fit for the diffusion control model. Nevertheless, the calculated R 2 values in Figure 7 are above those of the diffusion control model, indicating that the kinetic results match well with the chemical control model. The activation energy of the dissolution reaction (E a ) is calculated using the Arrhenius equation and the rate constants of K chem : where E a (kJ/mol) is attributed to the activation energy, A corresponds to the Arrhenius constant, and R is 8.314 J/mol K. Figure 8 illustrates a plot of lnK against 1/T. According to the calculations, the activation energies of cerium, lanthanum, neodymium, and praseodymium in the temperature range of 323-393 K are 38.33, 34.67, 31.43, and 24.44 kJ/mol, respectively, which was determined from the slope. As a result, the obtained results are rather suitable for the model of a shrinking core (SCM) with a chemical reaction as a rate-determining step [33,34].
1 − (1 − ) + 2(1 − ) = = Figures 6 and 7 demonstrate diagrams of 1 − 3(1 − x) 2/3 + 2(1 − x) as well as 1 − (1 − x) 1/3 against dissolution time; they indicate how temperature affects REE reaction kinetics. The plots show straight lines for Ce, La, Nd, and Pr at various temperatures. As presented in Figure 6, the kinetic data are not a good fit for the diffusion control model. Nevertheless, the calculated R 2 values in Figure 7 are above those of the diffusion control model, indicating that the kinetic results match well with the chemical control model. The activation energy of the dissolution reaction (Ea) is calculated using the Arrhenius equation and the rate constants of Kchem: where Ea (kJ/mol) is attributed to the activation energy, A corresponds to the Arrhenius constant, and R is 8.314 J/mol K. Figure 8 illustrates a plot of lnK against 1/T. According to the calculations, the activation energies of cerium, lanthanum, neodymium, and praseodymium in the temperature range of 323-393 K are 38.33, 34.67, 31.43, and 24.44 kJ/mol, respectively, which was determined from the slope. As a result, the obtained results are rather suitable for the model of a shrinking core (SCM) with a chemical reaction as a ratedetermining step [33,34].

Regression and Correlation Results between Leaching Factors and REEs
The Pearson correlation is a multiple linear regression model that measures the strength of the linear relationship between two variables. It has a value between −1 to 1, with a value of −1 meaning a total negative linear correlation between leaching factors and elements. In this study, it was applied in order to investigate the relationship between different leaching factors ((NH 4 ) 2 SO 4 concentration, time, S/L ratio, and temperature) of REEs (Ce, La, Nd, and Pr) and Zn. All factors were considered as explanatory variables along with the leaching efficiencies (%) of REEs as dependent variables. The results of the regression model demonstrate that there was a positive relationship with significant significance between the (NH 4 ) 2 SO 4 conc and all of the explanatory variables (REE leaching efficiency, %). This was inferred from the R 2 value, which indicates the magnitude of the relationship between the set of predictors in the regression and the outcome variable; and the beta coefficient, which represents the degree of variation in the outcome variable for every 1 unit of change in the predictor variable, with its associated p-value and Pearson correlation; alongside covariance, which indicates the value of the reverse relationship. The explanatory variables explain more than 75.0% of the variations in leaching efficiency, signifying that there was a very strong relationship between all REE leaching efficiency variations and the explanatory variables ((NH 4 ) 2 SO 4 concentration, time, and temperature). This was with the exception of Ce and temperature, which had a moderate positive relationship with no significant relationship.
The results also show a strong negative relationship with significant significance between the S/L ratio and La with Nd, but the other REEs had a moderate negative relationship with no statistical significance with the S/L ratio.
Variance inflation factors (VIFs) are used to measure how much the variance of the estimated regression coefficients is inflated as compared to when the predictor variables are not linearly related. VIFs are used to verify how much amount of multicollinearity (correlation between predictors) exists in a regression analysis. The value of the VIF of the model was 1, indicating the non-existence of multicollinearity problems (Tables 3 and 4). Thus, the results indicate the following regression model equations. Figures 9-13 display the scatter plot of the linear regression fit for the leaching efficiency of the five elements against different factors, which confirmed the same results with regards to where the relationship was positive and strong with the (NH 4 ) 2 SO 4 concentration, leaching time, S/L phase ratio, and temperature. It is noteworthy to mention that at the start point there was no leaching and that if any variable was set to equal 0, there was no possible leaching. Additionally, it is of note that the (NH 4 ) 2 SO 4 concentration and time of Ce, Pr, and Zn leaching had a stronger positive effect than temperature, while the impacts of temperature and time on Nd and La were stronger than that of (NH 4 ) 2 SO 4 concentration. On the other hand, it is clear that there was a strong negative relationship between the solid/liquid phase ratio and the leaching efficiency of the REE metals. Table 3. Regression model equations.

Separation of Zinc and REEs Products
For studying the recovery procedures for REEs from the spent Ni-MH battery powder, proper ammonium sulfate leach liquor was prepared using the previously studied optimum leaching conditions. Leached REEs can be recovered through precipitation as their hydroxides or else via selective REE precipitation as sulfates or oxalates. Leached Zn and REEs can be recovered via co-precipitation as their hydroxides at a pH of 8.0. Zn(OH)2

Separation of Zinc and REEs Products
For studying the recovery procedures for REEs from the spent Ni-MH battery powder, proper ammonium sulfate leach liquor was prepared using the previously studied optimum leaching conditions. Leached REEs can be recovered through precipitation as their hydroxides or else via selective REE precipitation as sulfates or oxalates. Leached Zn and REEs can be recovered via co-precipitation as their hydroxides at a pH of 8.0. Zn(OH)2 Figure 13. The scatter plot with linear regression fit for Zn leaching efficiency, %, and factors.

Separation of Zinc and REEs Products
For studying the recovery procedures for REEs from the spent Ni-MH battery powder, proper ammonium sulfate leach liquor was prepared using the previously studied optimum leaching conditions. Leached REEs can be recovered through precipitation as their hydroxides or else via selective REE precipitation as sulfates or oxalates. Leached Zn and REEs can be recovered via co-precipitation as their hydroxides at a pH of 8.0. Zn(OH) 2 can be precipitated from its salt solution by adding a suitable base. An excess of alkali base quickly dissolves Zn(OH) 2 as sodium zincates such as NaZn(OH) 3 and Na 2 Zn(OH) 4 [35]. The Zn was removed from the solution using CO 2 gas to precipitate it as ZnO, as seen in Figure 14a. After removing ZnO, the rare earth elements were precipitated as double insoluble sulfate salts using sulfuric acid, which was validated by XRD, as presented in Figure 14b. There were several phases detected in the solution such as the following salts: Additionally, the EDX spectrum confirmed the structure of REEs, as given in Figure 14c. The concentration of elements in the product was determined to be 45.7% S, 17.5% La, 14.83% Ce, 11.17% Nd, and 10.8% Pr. The SEM image in Figure 14d shows a hexagonal shape of RE sulfate salts, which are fully developed. It shows well-structured and tiny particles of 5-6 µm in length and 1-2 µm in width. can be precipitated from its salt solution by adding a suitable base. An excess of alkali base quickly dissolves Zn(OH)2 as sodium zincates such as NaZn(OH)3 and Na2Zn(OH)4 [35]. The Zn was removed from the solution using CO2 gas to precipitate it as ZnO, as seen in Figure 14a. After removing ZnO, the rare earth elements were precipitated as double insoluble sulfate salts using sulfuric acid, which was validated by XRD, as presented in Figure 14b. There were several phases detected in the solution such as the following salts: NaCe(SO4)2.H2O, NaPr(SO4)2.H2O, NaNd(SO4)2.H2O, NaNd(SO4)2, NaLa(SO4)2H2O, and NaLa(SO4)2. Additionally, the EDX spectrum confirmed the structure of REEs, as given in Figure 14c. The concentration of elements in the product was determined to be 45.7% S, 17.5% La, 14.83% Ce, 11.17% Nd, and 10.8% Pr. The SEM image in Figure 14d shows a hexagonal shape of RE sulfate salts, which are fully developed. It shows well-structured and tiny particles of 5-6 µm in length and 1-2 µm in width.

Separation of Ce(IV) Individually
The dried REE powder was subjected to another leaching process in order to separate cerium individually from the REE mixture (Ce, Ln, Nd, and Pr) using hydrochloric acid [36,37]. For the chemical separation of Ce(IV) from the REEs in the generated mixture (Ce, Ln, Nd, and Pr), the trivalent lanthanides were dissolved in a 10.0% HCl solution, whilst the insoluble Ce(IV) was left behind. The efficiency of Ce(IV) separation was primarily influenced by the pH of the chloride leach liquor and the time required for the REEs combination to dissolve.

Separation of Ce(IV) Individually
The dried REE powder was subjected to another leaching process in order to separate cerium individually from the REE mixture (Ce, Ln, Nd, and Pr) using hydrochloric acid [36,37]. For the chemical separation of Ce(IV) from the REEs in the generated mixture (Ce, Ln, Nd, and Pr), the trivalent lanthanides were dissolved in a 10.0% HCl solution, whilst the insoluble Ce(IV) was left behind. The efficiency of Ce(IV) separation was primarily influenced by the pH of the chloride leach liquor and the time required for the REEs combination to dissolve.

Influence of pH
The solution pH is the most important and effective parameter affecting cerium purity and recovery from a REE mixture. The pH value of chloride solutions was studied in the range from 2.0 to 4.0 that was used for the leaching process with stirring at ambient temperature. The obtained results presented in Figure 15a show that the recovery of Ce(IV) improved gradually as the pH increased from 92.7 to 98.1%; as a result, a chloride leach liquor pH of 3.5 was selected as the sufficient pH value for the recovery of cerium, which was not leached with other associated REEs. leach liquor pH of 3.5 was selected as the sufficient pH value for the recovery of cerium, which was not leached with other associated REEs.

Influence of Time
The impact of the stirring time on the cerium separation efficiency from the REE mixture was studied between 10-60 min (Figure 15b). It was observed that by raising the leaching time from 10 to 40 min, the leaching of the other associated trivalent REEs in the chloride solution at a pH value of 3.5 was increased so that the best dissolution time for cerium recovery from the REEs mixture was 40 min.

Cerium Oxide Separation
The yellowish insoluble Ce(IV) was separated from the chloride leach liquor. Subsequently, several washes with distilled H2O were performed to eliminate the excess of Cl − ions from the yellowish Ce(IV) precipitate. The obtained product was confirmed using SEM-EDX and XRD analysis, as shown in Figure 16.

Influence of Time
The impact of the stirring time on the cerium separation efficiency from the REE mixture was studied between 10-60 min (Figure 15b). It was observed that by raising the leaching time from 10 to 40 min, the leaching of the other associated trivalent REEs in the chloride solution at a pH value of 3.5 was increased so that the best dissolution time for cerium recovery from the REEs mixture was 40 min.

Cerium Oxide Separation
The yellowish insoluble Ce(IV) was separated from the chloride leach liquor. Subsequently, several washes with distilled H 2 O were performed to eliminate the excess of Cl − ions from the yellowish Ce(IV) precipitate. The obtained product was confirmed using SEM-EDX and XRD analysis, as shown in Figure 16.  Figure 17 displays the flowsheet of the recovery process of rare earth elements, zinc, and cerium(IV) from spent Ni-metal hydride batteries.

Conclusions
The recovery of REEs and zinc from spent nickel hydride batteries was performed using ammonium sulfate as a leachant; the optimal conditions of alkali baking were attained using a 300 g/L (NH4)2SO4 concentration, after 180 min of leaching time and a 1:3 solid/liquid phase ratio at 120 °C. The dissolution percentage was 99.98% in both the REEs and zinc; we found the kinetic data to be in good agreement with the chemical control model. After a nearly complete leaching of both the zinc and REEs, the separation between them through conventional techniques was nearly completed. Subsequently, Ce ions were individually separated from the total REE cake using 10.0% of HCl at pH 3.5 for 40 min at ambient temperature. Several washes with distilled H2O were performed to remove the excess of Cl − ions from the yellowish Ce(IV) precipitate. Both XRD and SEM-EDX techniques were applied to detect the pure cerium(IV) product.

Conclusions
The recovery of REEs and zinc from spent nickel hydride batteries was performed using ammonium sulfate as a leachant; the optimal conditions of alkali baking were attained using a 300 g/L (NH4) 2 SO 4 concentration, after 180 min of leaching time and a 1:3 solid/liquid phase ratio at 120 • C. The dissolution percentage was 99.98% in both the REEs and zinc; we found the kinetic data to be in good agreement with the chemical control model. After a nearly complete leaching of both the zinc and REEs, the separation between them through conventional techniques was nearly completed. Subsequently, Ce ions were individually separated from the total REE cake using 10.0% of HCl at pH 3.5 for 40 min at ambient temperature. Several washes with distilled H 2 O were performed to remove the excess of Cl − ions from the yellowish Ce(IV) precipitate. Both XRD and SEM-EDX techniques were applied to detect the pure cerium(IV) product.