Research on Leaching of V and Ni in Spent FCC Catalyst Using Oxalic Acid/H₂O₂ under Microwave-Assisted Conditions

Abstract

In this work, we propose a rapid and facile method (oxalic acid leaching under microwave-assisted conditions) to study the simultaneous recovery of vanadium (V) and nickel (Ni) from spent fluid catalytic cracking (SFCC) catalysts. The central issue in all of these studies is to test the modeling and experimental results of excellent fitting effects of leaching parameters. In order to maximize the recovery of V and Ni, leaching parameters were investigated. Furthermore, response surface methodology (RSM) was applied to optimize the leaching parameters. The optimum conditions obtained were as follows: oxalic acid concentration of 1.8 mol/L; leaching time of 91 min; microwave-assisted power of 500 W; H₂O₂ concentration of 1.1 mol/L. The maximum leaching rates of V and Ni reached the values of 91.36% and 46.35%, respectively. The results showed that microwave energy was very helpful in improving the efficiency of the leaching process and shortening the leaching time by 75%. According to the shrinking core model, test results showed that a surface chemical reaction was the controlling step of the overall reaction kinetics. The activation energy of V and Ni during the leaching reaction was calculated to be 3.28 and 34.41 kJ/mol, respectively.

1. Introduction

Fluid catalytic cracking (FCC) catalysts are used for converting heavy oil-gas into gasoline-blended mixtures [1]. A typical FCC catalyst contains an active matrix (alumina), an inert matrix (kaolin), a rare earth exchanged Y-zeolite, and a binder (silica or silica-alumina) [2]. During the FCC process, contaminant metals (such as, Ni and V) are deposited on the surface of the catalyst, and diffused through the pores of the catalyst [2,3,4]. As a result, poisoning of active sites, pore blocking, or both [2,4] gives rise to a significant activity loss, resulting in the formation of a spent FCC catalyst (SFCC) [5,6]. Ordinarily, these SFCC catalysts have been discarded for landfills. However, the air-, soil-, and marine-related environmental pollution caused by these SFCC catalysts has become a serious global problem [7,8].

SFCC catalysts can be regarded as important secondary resources because they contain a certain amount of vanadium (V) and nickel (Ni) compounds. Our group has studied the occurrence and location of V and Ni in SFCC catalysts; it is believed that vanadium exists in two valence states: V5+ and V4+, V is presented in the form of V₂O₅ and VO₂ in the SFCC catalyst. Because of its mobility, V could be homogenously distributed in the SFCC catalyst. The state of Ni in SFCC catalyst using XPS shows that Ni exists as either NiSiO₃ or NiAl₂O₄, with a small portion as Ni and NiO. In the same study, Ni was found to be concentrated at the SFCC catalyst surface uniformly and hardly migrated into the interior of the particles [9].

Current research is focused on replacing the storage of SFCC catalysts and assessing an environment-friendly and economical method, in which the leaching residue is used as the raw material of geopolymer and metals are recovered [10,11]. Oxalate has been considered as a sustainable reagent that can offer a wide range of selectivity and efficient leaching capabilities for various mixed metals under mild reaction conditions. Although the acidity of oxalic acid is very weak [12], the oxalate has good chelating properties, and therefore, has a strong tendency to form complexes with metal ions [13]. However, these processes have certain disadvantages, such as long reaction times for leaching process and the removal rate of V and Ni is low.

Therefore, it is important to study and develop a method which not only shortens the reaction times but also improve the removal rate of V and Ni. Generally, dipole rotation and migration current loss (existence of free moving electrons) are the two major principles of microwave heating [14,15,16]. Recent experiments on microwave-assisted leaching have indicated that it can be used in metals leaching from industrial residues. This is due to an effective mineral breakage and faster processing. Studies regarding the recovery of metals from industrial residues proved to be significantly faster when microwave energy was applied, and in some cases metal dissolution is greatly enhanced [16,17,18]. However, previous studies on the recovery of valuable metals from SFCC catalysts are limited to the investigation of only a single factor influence on the leaching rate; thus, systematic and comprehensive analysis of various acting factors as well as the impact of their interactions on leaching efficiency is required. At the same time, to further elucidate the role of leaching parameters on the leaching of valuable metals from SFCC catalysts. The kinetics determines the reaction rate and considers the influent parameters on yield and efficiency of reaction directly affecting the engineering of the process [19]. There are a number of models for describing metals leaching from spent catalysts. In these models, the shrinking core model has been widely used in the process of spent catalysts leaching metals [20]. However, few researchers have studied extracted metals by oxalic acid leaching with microwave-assisted methods [21], especially the effect of microwave on leaching kinetic of vanadium and nickel.

In the current work, the effects of different parameters on the leaching efficiencies of V and Ni from SFCC catalysts using microwave-assisted methods have been studied. Based on the Box-Behnken Design (BBD) experimental design, the response surface method was used to optimize the microwave-assisted leaching conditions for optimal recovery of V and Ni from the SFCC catalyst. In addition, the kinetics of the system was investigated including the presentation of a kinetic model. The activation energy of the optimized kinetic model was determined as well. Furthermore, a comparison is drawn between the microwave-assisted and conventional heating methods. The purpose of this research is to determine an energy-efficient approach for leaching V and Ni from SFCC catalysts using microwave heating.

2. Materials and Methods

2.1. Materials

The SFCC catalyst was obtained from a refinery (Hubei, China). The main chemical composition of the SFCC catalyst was determined using ICP-OES (inductively coupled plasma optical emission spectroscopy, Perkin-Elmer Optima, PerkinElmer Inc., Waltham, MA, USA), and the corresponding results are presented in

Table 1. Elemental analysis of the SFCC catalyst.

Element	Al	Si	Fe	V	Ni	La	Ce
(wt.%)	29.26	17.59	0.48	0.44	0.53	1.67	0.68

. As shown by the results presented in Table 1, the SFCC catalyst was characterized by a small number of recyclable metals, such as vanadium (V; 0.44%) and nickel (Ni; 0.53%); the fresh FCC catalyst base material was composed of aluminosilicates. Therefore, the SFCC catalyst shows a higher content of Al and Si.

Oxalic acid dihydrate (C₂H₂O₄·2H₂O) and hydrogen peroxide (H₂O₂) were purchased from China Xilong Chemical Co., Ltd., Shantou, China. Deionized water was obtained from a purified system (Cascada III·I 20, Pall, New York, NY, USA) and used in the experiments.

2.2. Methods

2.2.1. Preparation of the Catalyst

The contents within the dotted line in Figure 1 are the ones that are discussed in the current work. The samples were crushed, screened, and ground to D50 (median diameter) of less than 17.40 µm [22], which not only avoids any impact on the particle diameter of the leaching, but also facilitates the subsequent use of leaching residue.

2.2.2. Method for Leaching Tests

The conventional leaching experiments were performed in a digital display intelligent temperature-controlled magnetic stirrer (SZCL-2A, Wuhan Keer Instrument Equipment Co., Ltd., Wuhan, China) under normal pressure. The microwave leaching experiments were performed under normal pressure in a microwave furnace (MAS-II plus type; SINEO Microwave Chemistry Technology Co., Ltd., Shanghai, China) that was equipped with a spherical condenser, temperature sensor, and mechanical stirrer. Figure 2 shows the schematic of the MAS-II plus type microwave furnace equipment. In this study, the stirring speed and liquid-to-solid ratio for the conventional and microwave-assisted acid leaching experiments were 500 r/min and 50 g/L, respectively. Acidic leaching of SFCC catalyst is an endothermic process. However, the relationship between the recovery rates of V and Ni and temperature requires more research to further the current understanding about the kinetics of the leaching process. The optimum reaction temperature was considered to be 95 °C for the leaching of V and Ni, which was based upon the results from a previous study [23]. The experimental parameters included the concentration of addition agent H₂O₂ (0–4 mol/L), concentration of oxalic acid (26 mol/L), leaching time (20–240 min), and the response power (150–500 W). After the leaching test, the leaching residue (including the leaching solution and the leaching residue) was separated from the mixture using vacuum filtration, and washed several times using deionized water. The leaching liquid volume was 500 mL. Furthermore, inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the concentration of V and Ni in the leaching solution. The leaching residue was dried at 120 °C for 24 h, and then weighed. The toxin leaching experiment was performed. All the experiments were performed at least thrice, and average values were used for further analysis and reporting.

2.3. Characterization Techniques

The ICP-OES was used to determine the contents of V, Ni, Fe, Si, and Al in the leaching residue. Surface morphology was analyzed using scanning electron microscope (SEM; JSM-IT300; JOEL Company, Tokyo, Japan) that was equipped with an INCAx-act energy spectrum analyzer (OXFORD Company, Oxford, UK). Surface areas and pore size distribution were determined using Brunauer–Emmett–Teller method (BET; Nova 3000; Quantachrome Corporation, Boynton Beach, FL, USA).

A pH meter (PHS-3C, LabTech Instrument Co., Ltd., Dongguan, China) was used to measure the pH of the leaching solution.

2.4. Mechanism of Kinetic Model Determination

The process of leaching metals will involve the solid film diffusion and surface chemical reaction. To determine the relationship between operating parameters and the recovery yield of V and Ni, the mechanism of kinetic model is studied based on the experiment data. The dynamic equations for the leaching of V and Ni for solid- liquid reactions include:

(1)

Surface chemical reaction control model:

Kt = 1 − (1 − Xi)^1/3

(1)

(2)

Diffusion controls model:

Kt = 1 + 2 (1 − Xi) − 3 (1 − Xi)^2/3

(2)

where Xi is the leaching rate of V or Ni, t is the reaction time, and k is the rate constant.

2.5. Experimental Design

Response surface method (RSM) is a statistical and mathematical model that is used to optimize a process with the least number of experiments and analyzes the interactions among various parameters [24,25,26].

Three independent variables performed with the Box-Behnken Design (BBD) were selected at three levels, coded as −1 (low), 0 (central point), and 1 (high), as shown by the results presented in

Table 2. Independent factors and levels for design.

Factors	Symbol	Range and Levels
		−1	0	1
H2O2 amount (mol/L)	x1	0	2	4
C2H2O4 concentration (mol/L)	x2	2	4	6
leaching time (min)	x3	20	40	60
microwave power (W)	x4	150	350	550

. The relevance between the independent variables and the dependent response can be fitted using a second-order quadratic polynomial equation (Equation (3)).

$$\mathrm{Y}\left(\text{response}\right)={\alpha }_0+{\sum }_{i-1}^k{\alpha }_i{x}_i+{\sum }_{i-1}^k{\alpha }_{ii}{x}_{ii}^2+{\sum }_{1\le i\le j}^k{\alpha }_{ij}{x}_i{x}_j$$

(3)

where Y stands for the response (% metal recovery), ${\alpha }_0$ represents the content coefficient, ${\alpha }_i$, ${\alpha }_{ii}$, and ${\alpha }_{ij}$ represent the coefficients for linear, quadratic, and interaction effects, respectively, while $x_i$ and $x_j$ stand for the chosen independent factors.

The statistical significance and the capability of the model were analyzed using variance analysis (ANOVA). Design-Expert 8 can actively reflect the relationship between the response and the level of each variable, whereas the three-dimensional (3D) surface graph expresses the fitted polynomial equation. The leaching rates of V and Ni with the microwave-assisted process were represented by the response variables Y₁, and Y₂, respectively.

3. Results

3.1. Statistical Analysis and Model Fitting

According to the Box-Behnken experimental design, all 29 experiments were performed under corresponding conditions, and the experimental results are listed in

Table 3. Experimental design matrix and results.

Run	X1	X2	X3	X4	Y1	Y2	Run	X1	X2	X3	X4	Y1	Y2
1	1	1.5	90	450	85.64	30.58	16	1	2	120	550	91.37	46.35
2	0.5	2.5	90	500	78.35	40.36	17	0.5	2	90	450	78.26	42.65
3	1.5	2	90	450	91.39	46.38	18	1	2.5	120	500	91.38	40.38
4	1	2.5	90	550	91.38	40.21	19	1	1.5	120	500	83.59	42.35
5	1	2	90	500	91.35	46.35	20	1	2.5	60	500	76.35	38.66
6	1	1.5	60	500	62.59	30.51	21	1.5	1.5	90	500	86.38	40.28
7	1.5	2	60	500	73.29	40.26	22	1	2	60	450	73.25	37.25
8	0.5	2	90	550	80.39	46.35	23	1	2	90	500	91.37	46.34
9	1	2	90	500	91.36	46.34	24	1	1.5	90	550	81.26	41.28
10	1	2	60	550	81.54	42.38	25	1	2	90	500	91.36	46.36
11	1	2	120	450	91.38	46.37	26	1.5	2.5	90	500	91.38	40.39
12	0.5	2	120	500	80.44	46.33	27	0.5	2	60	500	65.28	38.59
13	1.5	2	120	500	91.39	46.36	28	1.5	2	90	550	91.39	46.35
14	1	2.5	90	450	91.37	40.21	29	0.5	1.5	90	500	58.69	32.84
15	1	2	90	500	91.36	46.35

. According to the data, two quadratic models with interdependent terms were obtained, as given by Equations (4) and (5).

Y (V) = 91.36 + 6.98 × A + 5.71 × B + 8.10 × C + 0.50 × D − 3.67 × AB + 0.74 ×AC − 0.53 × AD − 1.47 × BC + 1.10 × BD − 2.08 × CD − 6.84 × A2 − 5.37 × B2 − 7.44 × C2 + 0.91 × D2

(4)

Y (Ni) = 46.35 + 1.08 × A + 1.86 × B + 3.37 × C + 1.62 × D − 1.85 × AB − 0.41 ×AC − 0.93 × AD − 2.53 × BC − 2.67 × BD − 1.29 × CD − 0.77 × A2 − 6.90 × B2 − 2.19 × C2 − 0.87 × D2

(5)

The statistical results of ANOVA for the response surface quadratic models of V and Ni are listed in Table 1. The results showed that the F-values of the respective model for V and Ni were 11.78 and 8.60, respectively. The data indicated the significance of the model. The prob > F values for the models were <0.0001, which indicate the significance of the model terms. Moreover, the model terms of V, A, B, C, AB, BC, A² and B² and the model terms of Ni, A, B, C, AC, BC, A² and B² were found to be significant. The values of R² were determined to be 0.959 and 0.971, as shown by the results presented in

Table 4. Statistical results of the ANOVA.

Statistical Results	V	Ni
Model F-value	11.19	60.37
Model prob > F	<0.0001	<0.0001
R-squared	0.959	0.971
CV%	3.30	2.74
Adjusted R-squared	0.918	0.942
Adequate precision	15.998	21.700

. In the present study, R² and R² adj. coefficients guaranteed the satisfied adjustment of the quadratic model to the experimental data. The percentage recovery of coefficient of variance (CV) for V and Ni were confirmed to be (in %) 3.30 and 2.74, respectively. The ratio of the standard deviation to the mean value of the observed response, indicated by CV%, represents the degree of reproducibility of the models. The models produced a satisfactory degree of fitting and an adequate precision to measure the ‘signal-to-noise ratio’. They indicated an adequate signal.

3.2. Process Optimization

Comprehensive factor analysis of variance illustrated the main effects and interactions of the evaluated variables. In order to achieve a graphical interpretation of the interactions, it is strongly recommended to use the regression model with 3D surface plots and the 2D contour plots.

Interactions among the parameters of V leaching rate are shown in Figure 3 and Figure 4. Figure 3A–C and Figure 4A–C show the leaching rate of V for different H₂O₂ solution concentrations. The results demonstrate that the leaching efficiency of V improved significantly from 62.82% to 91.56%, with the increase in the concentrations of H₂O₂ solution from 0 to 1.5 mol/L, which suggests that the addition of H₂O₂ in the reaction system had a significant effect in increasing the extraction of V. This result is mainly attributed to the low valence of V (in the form of VO₂) that was oxidized to high valence (in the form of V₂O₅) in the SFCC catalysts [27]. In this condition, the high valence species could easily react with oxalic acid. As seen in Figure 3A,D,E and Figure 4A,D,E, with different reaction times, the V recovery rate increased almost linearly with the increase in the concentration of oxalic acid from 1.5 mol/L to 2.0 mol/L. However, the slope curve lowered when the concentration of oxalic acid exceeded 2.0 mol/L. This is because a higher acid concentration provides enough H⁺ to accelerate the leaching process, and therefore, more V could be recovered. However, after the reaction stabilized, simply increasing the H⁺ could not further promote the reaction. As seen in Figure 3B,D,F and Figure 4B,D,F, the presented data explain that the factor of time has a more significant influence on leaching rate for vanadium compounds. When the leaching time extended from 0.25 to 1.5 h, a sharp increase in the productivity was observed. Any further extension in the leaching time could not increase the leaching rate significantly and it became practically unchanged. As shown in Figure 3C,E,F and Figure 4C,E,F, the leaching rate of V enhanced with the increasing microwave-assisted power (up to 500 W), and thereafter, the metal leaching stabilized.

Interactions among various parameters of Ni leaching rate are shown in Figure 5 and Figure 6. As seen in Figure 5A–C and Figure 6A–C, the data suggests that the addition of H₂O₂ in the reaction system had an insignificant effect on the extraction of Ni. This is due to the reason that H₂O₂ could not further oxidize Ni compounds in SFCC catalysts. Based on Figure 5A,D,E and Figure 6A,D,E, the recovery of Ni first increased, and then, decreased, which was due to the reason that the value was higher than the stoichiometric value due to the increase in acid concentration. This results in over-saturation due to the existence of large amounts of H⁺ ions, which decreased the nickel recovery [28]. As seen in Figure 5B,D,F and Figure 6B,D,F, extending the leaching time increased the Ni recovery. When the critical time of 1.5 h was extended and the leaching time was continuously extended, the Ni recovery decreased slightly. In this case, the reason could be attributed to the existence of impurities with some increase in the concentration of other ions. It is possible that some of the Ni²⁺ species precipitated out. As shown in Figure 5C,E,F and Figure 6C,E,F, the leaching rate of Ni enhanced with the increasing microwave-assisted power (up to 500 W), and thereafter, the metal leaching stabilized. The phenomenon is similar to the leaching characteristics of V.

Based on the statistical modeling, the optimal conditions for vanadium and nickel recovery were found to be as follows: oxalic acid concentration of 1.8 mol/L; leaching time of 91 min; microwave-assisted power of 500 W; H₂O₂ concentration of 1.1 mol/L. As shown in Figure 7, the actual values were evenly distributed around the predicted values. The desired values of Y₁ and Y₂ were 91.64%, 46.89%, respectively. The experimental results for Y₁ and Y₂ under the same conditions were 91.56% and 46.78%, respectively, which agreed well with the predicted values. In order to test the optimal experimental parameters estimated by the response surface model, three verification experiments were conducted in the optimal process conditions. At the same time, other experimental conditions were confirmed with a liquid-to-solid ratio of 50 g/L and a stirring speed of 500 r/min. The leaching rates of V were fixed as 91.62%, 91.41%, and 91.48% respectively, and the average value was 91.50% with the relative error of 0.07%. In addition, the leaching rates of Ni were 46.46%, 46.49%, and 46.83%, for which the average value of Ni leaching rate was 46.59%, and the relative error is 0.40%. The results above further explain that this model accurately predicts the leaching rates of V and Ni and they also prove the reliable optimal process conditions. The key influencing factors of the leaching rate and their corresponding optimum range can be identified by the single-factor experiment. The response surface method is very effective for studying the importance of each factor and the interaction among different factors and for figuring out the optimal process conditions.

3.3. Leaching Kinetics

The experimental data of leaching processes of V and Ni in oxalic acid are shown in Figures S1 and S2. To determine the optimum kinetic model, the plots of the left-hand sides of Equations (1) and (2) against time for oxalic acid are shown in Figure 8. The apparent ratio constant (k) and multiple regression coefficients (R²) of V and Ni at various leaching temperatures using different kinetic models are listed in

Table 5. Kinetic parameters at different leaching temperatures using different kinetic models.

	Kt = 1 − (1 − Xi)1/3			Kt = 1 + 2 (1 − Xi) − 3 (1 − Xi)2/3
	T (K)	k	R2	T (K)	k	R2
V	353.15	6.38 × 10−4	0.952	353.15	1.83 × 10−4	0.887
	358.15	7.00 × 10−4	0.969	358.15	2.26 × 10−4	0.889
	363.15	7.58 × 10−4	0.938	363.15	2.41 × 10−4	0.921
	368.15	7.64 × 10−4	0.979	368.15	3.50 × 10−4	0.986
	373.15	8.00 × 10−4	0.942	373.15	3.59 × 10−4	0.904
Ni	353.15	4.46 × 10−6	0.994	353.15	3.84 × 10−6	0.953
	358.15	2.81 × 10−5	0.993	358.15	2.85 × 10−5	0.858
	363.15	4.04 × 10−5	0.945	363.15	3.18 × 10−5	0.881
	368.15	5.04 × 10−5	0.997	368.15	4.17 × 10−5	0.901
	373.15	7.17 × 10−5	0.983	373.15	6.83 × 10−5	0.886

.

As shown in Figure 8 and Table 5, a better fit is obtained using Equation (1) with a high R² value (>0.93) and there is a good agreement between the proposed kinetic model and the experimental data. This indicates that the control step in the reactions between oxalic acid and metals (V and Ni) under microwave-assisted is the surface chemical reaction.

To calculate the activation energy (Ea), lnk is plotted against 1/T according to the Arrhenius equation [20]. The linear fitting of (lnk) versus (1/T) with a slope of (–Ea/R) is depicted in Figure 9. The activation energies calculated from the Arrhenius plots are 3.28 kJ/mol for V and 34.41 kJ/mol for Ni during the leaching process under microwave-assisted. The apparent activation energy of V in the acid leaching process is lower than that of Ni, this indicates that V tends to be more soluble than Ni, because Ni tends to form both oxalate compounds and oxalate complexes, whereas V forms only oxalate compounds [29].

4. Discussion

According to the experiments on conventional leaching processes, without the addition of H₂O₂, the leaching efficiencies of V and Ni were found to be 73.51% and 41.25%, respectively. The conventional leaching time was found to be 240 min, whereas the C₂H₂O₄ concentration was 2 mol/L. Based on the experiments of conventional leaching process in the presence of H₂O₂, the V and Ni leaching efficiencies were determined to be 91.56% and 46.78%, respectively. Obviously, when the data for conventional and microwave-assisted experiments were compared, it was found that the microwave-assisted approach had a significant positive impact on the extraction of V and Ni from SFCC catalysts.

The results presented in

Table 6. Pore structure parameters of SFCC catalysts before and after acid leaching.

Sample	Specific Surface Area (m2/g)	Average Grain Size (nm)
Fresh FCC catalyst	51.34	20.10
Spent FCC catalyst	34.86	24.23
Conventional leaching residue	58.91	18.48
Conventional leaching with H2O2 leaching residue	72.45	13.56
Microwave-assisted with H2O2 leaching residue	83.33	7.47

show that the microwave-assisted leaching residue had a larger specific surface area and a richer pore structure than the conventional leaching residue. In addition, H₂O₂ had an effect on the specific surface area and pore structure of SFCC catalysts, which is explained by the fact that H₂O₂ will react with V compounds in SFCC catalysts. The oxidation of low-price vanadium resulted in better absorption and conversion of microwave energy than that of the conventional leaching. The reaction mechanism of V in the leaching process is shown in Figure 10a. As a strong oxidant, H₂O₂ can oxidize low-valence metal compounds under acidic conditions [30]. Therefore, the use of H₂O₂ to pretreat SFCC catalysts can improve the efficiency of metal leaching without calcination. When the pH value was ≤1, vanadium pentoxide was dissolved in acid to generate vanadium oxide ions [VO₂]⁺. The [VO₂]⁺ ions of positive pentavalent vanadium can be reduced to [VO]²⁺ ions of positive tetravalent vanadium by C₂H₂O₄. The leaching of Ni depended on the pH and the concentration of oxalate, while it was restricted by the low solubility of nickel (II) oxalate. Moreover, other metals that formed complexes with the oxalate (such as V and Al) would also consume the available oxalate ions, and therefore, affect the solubility of Ni. The related reaction mechanism is shown in Figure 10b, which is consistent with the results reported in Verma [31] and Pathak [32].

The SEM images of SFCC catalysts and various leaching residues are shown in Figure 11. It can be found from Figure 11a that metal oxides were deposited on the surface of FCC catalyst, which reduced the surface area of FCC catalyst, and therefore, deactivated it. Comparing the conventional oxalic acid leaching residue in Figure 11b with the SEM-EDS image of the conventional oxalic acid + H₂O₂ leaching residue in Figure 11c, it can be clearly shown that the average grain size of the latter was significantly reduced. Combined with the average grain size analysis presented in Table 6, it can be seen that the conventional stirring of the SFCC catalyst particles would also cause the particles to break, though the reaction of H₂O₂ and vanadium oxide intensified further fragmentation of the particles. The SEM-EDS image of the microwave-assisted leaching residue shown in Figure 11d indicates that when the thermal stress increased to a certain level, the particle size of the leaching residues was further reduced. The EDS images of the SFCC catalyst and various leaching residues are shown in Figure 12. Compared with the content of each substance shown in Figure 12a–c, Figure 12d shows that the microwave-assisted process was beneficial to the leaching of metals. The carbon is from carbon coating; its content changes can be ignored. According to the EDS analysis of leaching residue by C₂H₂O₄, there is still a certain amount of V and Ni in the residue, and the content of Al and Si has little changes. As shown in Figure 12c,d, the contents of V and Ni were below the detection value. At the same time, the contents of Al and Si decreased seriously, indicating that the leaching effect of H₂O₂ and microwave-assisted on Al and Si was obvious. As shown in Figure 13, Figure 13a is a schematic diagram of the SFCC catalyst, Figure 13b under the combined action of oxalic acid and H₂O₂, the SFCC catalyst particles form an unreacted nucleation model, and Figure 13c is fragmentation reaction model of the SFCC catalyst particles under microwave conditions. The possible reason is that a temperature gradient is generated in the SFCC catalyst, which further causes the decomposition of spent catalyst particles, and results in a stronger diffusion of oxalic acid. However, the microwave-assisted process also facilitated the formation of pores on the surface of spent catalyst particles. After the microwave-assisted treatment, significant changes in the behavior of these particles resulted in a faster dissolution of metals from the SFCC catalysts.

5. Conclusions

In this study, the combination of the addition of H₂O₂ and microwaves has been proved to be an effective process to leach metals from SFCC catalysts. The optimized reaction conditions (concentration of C₂H₂O₄, reaction time, microwave power, and concentration of H₂O₂) for the microwave-assisted process have also been reported. The optimum conditions obtained are as follows: oxalic acid concentration of 1.8 mol/L; leaching time of 91 min; microwave-assisted power of 500 W; H₂O₂ concentration of 1.1 mol/L. Under optimized conditions, the leaching rates of V and Ni reach the values of 91.36% and 46.35%, respectively. The apparent activation energies of V and Ni during the leaching process were calculated to be 3.28 and 34.41 kJ/mol, respectively. Moreover, BET and SEM were applied to analyze the mechanism of the microwave-assisted leaching process. It is shown that different activated metals in the SFCC catalysts have diverse characteristics for microwave absorption, which results in local temperature gradient, leading to the separation of V and Ni compounds from the pores of SFCC catalysts and exposing the V and Ni compounds to the surface of SFCC catalysts. Therefore, the leaching agent can easily react with V and Ni compounds. The present investigation explores the advantages of microwave-assisted heating, and the addition of H₂O₂ to the leaching system for the recovery of valuable metals from SFCC catalysts.