Recovery of Magnetic Ni Particles from Spent Catalyst Leachate by Direct Cementation

Abstract

An alternative method based on cementation for the recovery of nickel from spent Ni/Al₂O₃ reforming catalyst pregnant leach solution (PLS) was proposed to overcome the limitations of traditional two-step extraction and precipitation processes. Thermodynamic analysis was used to evaluate the potential interference of key reactions, such as nickel and sacrificial metal leaching, with the selective cementation of nickel from the PLS. Key variables in the cementation process were optimized using response surface methodology (RSM) combined with Box–Behnken design (BBD). Under optimal conditions—pH 2.2 ± 0.1, processing time of 15 min, and Al/Ni molar ratio of 2.65—a maximum nickel recovery of 73.2% was achieved. Extensive characterization confirmed the high quality of the cemented nickel product: (i) ICP-OES indicated nickel purity of 99.47%, (ii) XRD patterns verified the presence of pure face-centered cubic nickel, (iii) SEM-EDS and vibrating sample magnetometry confirmed the high purity of the metallic nickel particles.

1. Introduction

In recent decades, there has been a significant increase in the focus on metal recovery from waste and secondary sources due to the rapid depletion of primary metal resources [1]. This is due to the escalating global demand for metals, the difficulty in discovering new ore deposits, and heightened environmental concerns. Recovering metals from these sources is critical not only to conserve resources, but also to reduce the need for new mining operations. Such recovery efforts help mitigate the environmental impacts associated with ore extraction and industrial waste disposal [2,3].

Heterogeneous catalysts play a critical role in a wide variety of industrial processes, including petroleum refining, chemical manufacturing, fertilizer production, and automotive emission control [4]. Their importance in facilitating various conversion processes underscores their significance in the modern industrial landscape. Over time, catalysts degrade due to the accumulation of coke, poisons, or metal impurities such as sulfur, vanadium, iron, zinc, and arsenic [5,6]. This degradation requires cycles of regeneration and reuse to maintain the optimal/commercial viability of the underlying industrial processes. However, when regeneration becomes technically and economically infeasible, significant quantities of spent catalysts accumulate, creating a significant environmental burden [7,8].

In the United States, the European Union, and China, spent catalysts are strictly regulated and classified as solid wastes that require careful disposal due to their hazardous nature [4]. For example, the U.S. Environmental Protection Agency classifies certain catalysts, such as those used in hydroprocessing and hydrocracking, as hazardous waste due to their toxic constituents [4]. Similarly, China’s State Environmental Protection Administration has classified a number of catalysts, including those used in flue gas denitrification and hydrocracking, as hazardous waste. This increased regulatory scrutiny is critical to prevent environmental degradation and protect public health, embodying the principles of sustainable production and green chemistry [4]. The importance of recycling spent catalysts becomes even more critical when one considers that the European Union has identified 30 critical industrial raw materials that are essential to key sectors such as defense, renewable energy, and emerging technologies such as robotics and batteries [9]. These critical raw materials include metals such as nickel, cobalt, vanadium, and platinum, which are essential to the energy industry sector but are in limited global supply, with production controlled by a few countries. The rapid expansion of the new energy sector, particularly electric vehicles, has led to a surge in demand for these critical metals, resulting in a significant increase in their market value [10]. As a result, recycling spent catalysts not only mitigates environmental impacts, but also alleviates resource constraints by recovering these critical metals, which are essential to national security and industrial progress. This sustainable practice supports the global transition to a circular economy, where resource efficiency and environmental stewardship are of paramount significance.

The recovery of valuable metals is the primary objective in the recycling of spent catalysts. The predominant methods used for this purpose include pyrometallurgy, hydrometallurgy, and hybrid approaches that combine elements of both techniques [11,12,13]. Pyrometallurgy involves the extraction of critical metals by high-temperature smelting, often at temperatures in excess of 1000 °C. While this method eliminates the need for pretreatment processes such as comminution, it is highly energy intensive and produces significant emissions, posing significant economic and environmental challenges [5,14]. Conversely, hydrometallurgy is more commonly used in catalyst recycling. This method involves processes such as leaching, separation, and purification to extract critical metals. Known for its milder reaction conditions and higher metal extraction efficiencies, hydrometallurgy is considered more environmentally friendly. It employs various leaching techniques, including acid, alkaline, and roast water leaching, depending on the specific characteristics of the raw materials. The subsequent purification and separation of metals is achieved by methods such as solvent extraction, ion exchange, cementation, and precipitation. Hydrometallurgy offers several advantages, including lower energy requirements, reduced gas emissions, and minimal waste generation, making it a more sustainable option for catalyst recycling [15,16].

The hybrid approach integrates elements of both pyrometallurgy and hydrometallurgy to leverage the strengths of each. In this process, the material is first subjected to high temperature treatment (pyrometallurgical step) to concentrate valuable metals and break down the complex matrix of spent catalysts or ores. This is followed by hydrometallurgical leaching using selective solvents to recover specific metals from the treated material [14,15].

The advantage of the hybrid approach is its ability to efficiently reduce the volume and complexity of the material during pyrometallurgy, removing volatile components and concentrating the desired metals. Once simplified, hydrometallurgical techniques such as leaching can be more effectively applied. However, the hybrid process has significant drawbacks. The pyrometallurgical step is highly energy intensive, often requiring temperatures in excess of 1000 °C, resulting in high operating costs and significant greenhouse gas emissions. In contrast, hydrometallurgy operates at much lower temperatures, typically below 100 °C, reducing energy consumption and emissions while maintaining high metal selectivity [11,12,13,14].

In the recycling of Ni-Al catalysts, a critical step following the extraction of nickel from spent catalysts is the purification and separation of nickel ions from coexisting impurities in the solution. Solvent extraction is commonly used for purification or metal enrichment in these processes [17,18]. However, this method is associated with high operating costs and significant explosion risks due to the volatile organic solvents involved [19,20,21]. An alternative, more cost-effective, and simpler approach is chemical precipitation. This technique requires precise control of temperature and pH levels and involves the gradual addition of a precipitating agent to the solution, followed by the separation of the resulting salt from the original liquor [22]. Lee et al. successfully extracted nickel and aluminum from spent Raney nickel catalysts using a dilute sulfuric acid solution at ambient temperature. By adjusting the pH of the solution to 5.4, they were able to remove all Al³⁺ ions with a nickel loss of less than 2%. They then achieved a 100% pure recovery of nickel in the form of nickel carbonate from the purified solution [23]. However, research by Moosakazemi et al. showed that chemical precipitation does not always yield high quality nickel hydroxide from spent catalyst leachate [16]. The cementation method, a long-standing technique in the mineral processing industry, has also been used for the purification and recovery of various metals [24]. This process is commonly used in the extraction of primary resources such as gold and copper, where zinc and iron scrap are used in the cementation process for efficient metal recovery [25,26]. The method is based on the potential difference between two metals, where a sacrificial metal dissolves, allowing the target metal ion to be electrodeposited as a solid in the solution [27]. Due to its simplicity, high efficiency, and low operating cost, Moosakazemi et al. were the first to apply this method to recover nickel from spent catalysts in an acidic medium [16]. They used aluminum as a sacrificial metal to recover nickel, with the dissolved aluminum supplementing the aluminum already present from the acidic dissolution of the catalysts. Cementation subsequently became a central technique for metal recovery from spent Ni-Al catalysts. Ebrahimi and colleagues applied this method to the recovery of nickel from deep eutectic solvents generated from the leaching of hydroprocessing catalysts, and successfully obtained high quality nickel powder through the cementation process using aluminum powder [28].

This study investigates the recovery of nickel from spent catalysts using the cementation process as a cost-effective alternative to chemical precipitation and solvent extraction. Building on our previous work [16], which focused primarily on the selective leaching and precipitation of nickel and aluminum and presented a preliminary proof of concept for cementation, the current research focuses on optimizing the cementation process for nickel recovery from acidic pregnant leach solutions (PLSs) derived from spent alumina catalysts. Key parameters such as pH, reaction time, and aluminum consumption were optimized using response surface methodology (RSM) in combination with Box–Behnken design (BBD) to improve overall efficiency. In addition, the quality, morphology, crystallinity, and magnetic susceptibility of the cemented nickel powder were thoroughly characterized using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM).

2. Materials and Methods

2.1. Cementation Experiments

Liquid–solid suspensions were prepared by pulping ground aluminum (53 to 150 microns) used as a cementing agent in an acidic (HCl) pregnant leach solution (PLS) containing dissolved nickel (see Ref. [16] for detailed PLS characteristics). The PLS was obtained by leaching a previously ground waste methane reforming catalyst. Both the PLS and the ground aluminum were prepared by the Beneficiation and Hydrometallurgy Research Group at the Academic Center for Education, Culture, and Research (ACECR). The intentional use of Al-based reducing agents aligns with the waste-with-waste methodology [29] and complements the two-stage dissolution process for nickel reduction [16], repurposing waste materials to facilitate metal recovery.

Cementation experiments were conducted in a 200 mL two-neck glass reactor, equipped with a glass reflux condenser, mounted on a hot plate magnetic stirrer. The stirring speed was maintained at 200 rpm, with a slurry volume of 100 mL containing aluminum pulp concentrations ranging from 3 to 15 g/L. After each experiment, solid–liquid separation was carried out using a vacuum filtration device, and the pregnant leach solution (PLS) was analyzed for metal concentration. These experiments aimed to recover micron-sized magnetic Ni particles from the as-received PLS.

To optimize the cementation process, a Box–Behnken design (BBD) combined with response surface methodology (RSM) was employed for statistical modeling and process optimization, using Design-Expert software (DX-7). Based on preliminary experiments, three key variables—pH, time, and Al dosage—were selected to investigate their influence on cementation recovery (

Table 1. The variables and their respective levels for modeling Ni cementation.

Actual Variables	Coded Variables	Levels
		−1	0	+1
pH	A	0	2	3
Reaction time (min)	B	5	10	15
Al/Ni ratio (mol/mol)	C	1.32 (3) *	3.96 (9) *	6.60 (15) *

* denotes the amount of Al (measured in g/L) incorporated to the PLS.

). BBD systematically explores the influence of each process variable at three levels, creating a navigational space between the variables and the corresponding response—here, recovery. RSM, as a key optimization tool, reveals the non-linear and non-monotonic (if any) relationships between output and operating variables, inherent in their interactions and collective impact on the process outcome, helping to determine the optimal values for the variables constrained by the desired outcomes [30]. Experimental reproducibility was ensured by conducting three cementation experiments at the center point of the variables.

2.2. Characterization of Nickel Particles

In each batch, approximately 5 g of Ni powder synthesized by the cementation process was characterized as follows: (1) The phase composition of the resulting Ni powder was determined by X-ray diffraction (XRD) analysis using a Philips PW3710 and X’Pert MPD, operating at 40 kV and 30 mA. Diffraction patterns were recorded over a 2θ range of 10–80° with a step size of 0.02° and a scan rate of 2 °/min. The acquired XRD data were analyzed using HighScore Plus software (v. 3.0.5, PANalytical B.V.). (2) Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) was used to study the morphology and chemical composition of the solid product. SEM-EDS measurements were performed using a VEGA-TESCAN (Czech Republic) at an accelerating voltage of 20 kV. (3) The magnetic properties of the sample were evaluated by vibrating sample magnetometry (VSM) using a VSM-7300 instrument (Meghnatis Daghigh Kavir Co., Kashan, Iran). A magnetic field ranging from −8000 to 8000 Gauss was applied and the corresponding magnetic moment was recorded. (4) Cementation recovery was calculated by determining the initial and final Ni concentrations in the solution using inductively coupled plasma optical emission spectrometry (ICP-OES; Vista-Pro Varian, Australia).

3. Thermodynamics

The cementation process, driven by electrochemical metal/aqueous ion swapping reactions between Ni and Al, facilitates the recovery of metallic Ni from acidic solutions and is widely used for purification and recovery in both mineral processing and e-waste recycling [29,31]. The overall electrochemical reaction between aqueous Ni²⁺ cations and metallic Al in aqueous media is represented in Equation (1). Aluminum was selected for this study due to its cost-effectiveness [32], and the aluminum cations produced from anodic dissolution can be efficiently recovered through conventional processing methods, such as chemical precipitation.

$$3{\mathrm{Ni}}^{2+}+2\mathrm{Al}=2{\mathrm{Al}}^{3+}+3\mathrm{Ni}$$

(1)

In Equation (1), the overall half-reactions involving the reduction of Ni²⁺ to metallic Ni and the oxidation of metallic Al to Al³⁺ are represented by Equations (2) and (3), respectively:

$${\mathrm{Ni}}^{2+}+2{\mathrm{e}}^{-}=\mathrm{Ni};{\mathrm{E}}_{{\mathrm{Ni}}^{2+}/\mathrm{N}\mathrm{i}}^0=-0.257\mathrm{V}$$

(2)

$${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}=\mathrm{Al};{\mathrm{E}}_{{\mathrm{Al}}^{3+}/\mathrm{A}\mathrm{l}}^0=-1.676\mathrm{V}$$

(3)

The lower reduction potential of the Al half-reaction relative to that of Ni suggests that zero-valent Al can be thermodynamically replaced by zero-valent Ni, as a result of the dissolution of Al [29]. Gibbs free energy (Figure 1a) and enthalpy (Figure 1b) calculations for the overall electrochemical reaction between Ni²⁺ and Al were performed over a temperature range of 0–100 °C using HSC 6.0 chemistry software (Outokumpu Research, Finland). Further details are given in

Table A1. Thermodynamic information corresponding to Equation (1) and Figure 1.

Temp. (°C)	3Ni2+ + 2Al = 2Al3+ + 3Ni		Ni + 2HCl = H2(g) + NiCl2		2Al + 6HCl = 2AlCl3 + 3H2(g)
	∆G (kJ)	∆H (kJ)	∆G (kJ)	∆H (kJ)	∆G (kJ)	∆H (kJ)
0	−852.4	−913.6	−0.1	51.4	−995.4	−973.1
5	−851.3	−914.0	−1.1	50.3	−995.7	−981.0
10	−850.1	−914.4	−2.0	49.3	−995.9	−988.5
15	−849.0	−914.7	−2.9	48.3	−996.0	−996.0
20	−847.8	−915.1	−3.8	47.4	−995.9	−1003.3
25	−846.7	−915.4	−4.6	46.6	−995.8	−1010.5
30	−845.5	−915.7	−5.5	45.8	−995.5	−1017.7
35	−844.4	−916.1	−6.3	45.0	−995.0	−1024.9
40	−843.2	−916.4	−7.1	44.2	−994.5	−1032.2
45	−842.0	−916.7	−8.0	43.4	−993.8	−1039.4
50	−840.9	−917.0	−8.8	42.6	−993.1	−1046.8
55	−839.7	−917.3	−9.6	41.9	−992.2	−1054.1
60	−838.5	−917.6	−10.3	41.1	−991.2	−1061.6
65	−837.3	−918.0	−11.1	40.3	−990.1	−1069.1
70	−836.1	−918.3	−11.9	39.6	−988.8	−1076.6
75	−834.9	−918.6	−12.6	38.8	−987.5	−1084.3
80	−833.7	−919.0	−13.3	38.0	−986.0	−1092.0
85	−832.5	−919.3	−14.0	37.3	−984.5	−1099.8
90	−831.3	−919.7	−14.8	36.5	−982.8	−1107.7
95	−830.1	−920.0	−15.5	35.7	−981.1	−1115.7
100	−828.8	−920.4	−16.2	34.9	−979.2	−1123.8

in the Appendix A.

The Gibbs free energy for the cathodic conversion of Ni²⁺ to metallic Ni and the anodic dissolution of Al (Figure 1a) is markedly negative over the temperature range 0–100 °C, indicating that the cementation reaction with Al, unless kinetically controlled, is spontaneous. As the temperature rises from the normal melting point to the boiling point of the aqueous solvent, the equilibrium constant (or driving force) for the reaction in Equation (1) decreases slightly, indicating a 2.8% reduction in the tendency to form metallic Ni. Nevertheless, within this temperature range, thermodynamics does not prevent the formation of metallic Ni, which remains under an exothermic enthalpic control of the cementation reaction (Figure 1b). Given the acidic nature of the PLS, it is crucial to examine the temperature-dependent behavior of metallic nickel dissolution, as undesirable concurrent leaching during the cathodic reaction could adversely affect cementation efficiency. Figure 1a depicts the variation of the Gibbs free energy for the dissolution of Ni and Al as a function of temperature in an HCl medium. A comparison of the acidic dissolution of Al (Figure 1a) with its anodic dissolution (Equation (1)) is also important in evaluating the potential for excessive sacrificial metal consumption.

The Gibbs free energy for the acid dissolution of Al (Figure 1a, Table A1) suggests that the reaction is spontaneous unless kinetically controlled, with minimal sensitivity to temperature variations. Thermodynamically, neither Equation (1) nor the dissolution of Al is precluded, provided that the kinetics of cementation outpaces that of aluminum leaching and/or there is a sufficient excess of sacrificial Al metal. If these conditions are not met, the metallization of dissolved nickel will be hindered, aborted, or not occur. Therefore, an excess of stoichiometric Al, beyond the requirements of Equation (1), is essential to ensure that the cementation process is not hindered by the depletion of metallic aluminum in the competing acid leaching reaction. In contrast, the Gibbs free energy for Ni dissolution is considerably higher than that for Al dissolution and the redox reaction in Equation (1), approaching the threshold of thermodynamically favorable reactions (ΔG = 0). For example, the Gibbs free energy for Ni dissolution is −0.1 kJ/mol at 0 °C and decreases to −16.2 kJ/mol at 100 °C, indicating that Ni dissolution is less thermodynamically favorable compared to Al dissolution. In addition, the dissolution of Ni in acid is endothermic, which means that the heat generated by the redox reaction and acid dissolution of Al could intrinsically promote the unwanted dissolution of Ni.

4. Results and Discussion

4.1. Characterization of Cemented Ni

The chemical composition of the solid samples after cementation (for test with optimum conditions) was analyzed by ICP-OES. Of the 45 different elements analyzed, Ni was identified as the predominant component, accounting for a substantial 99.47% of the cemented product. Iron, while notable among the trace elements, accounted for only 0.04% of the total. The analytical results confirm that the cemented nickel is of high purity, with only trace amounts of impurities present.

XRD analysis was performed to determine the phases present in the Ni-bearing product sample (Figure 2). The analysis revealed three prominent peaks of high intensity corresponding to the fcc Ni structure according to JCPDS numbers 45–1027 and 04–0850 [33]. These peaks confirm the presence of fcc Ni in the sample. No additional peaks, particularly those indicative of hcp Ni, were detected, confirming that the product contains only fcc Ni. The solid product particles are pure nickel, as confirmed by XRD analysis, which reveals no peaks corresponding to metallic aluminum. This absence can be attributed to the vigorous evolution of hydrogen gas, a consequence of the anodic dissolution of aluminum (Figure 1). The intense gas release disrupts the formation of nucleation sites on the aluminum particles, preventing nickel from being deposited on their surface. As a result, nickel precipitates uniformly in a homogeneous process, yielding isolated pure nickel solids, free of any residual metallic aluminum.

Figure 3 shows the SEM image of the cemented concentrate accompanied by the EDS analysis. The EDS pattern clearly shows that the product is composed entirely of Ni. The image, taken at 10,000× magnification, shows a porous morphology characterized by spherical nuclei perched on irregularly shaped rods. The EDS results are consistent with the elemental and XRD analyses, confirming the exclusive presence of metallic Ni. As expected from above thermodynamic analysis, Al³⁺ with a reduction potential of −1.676 V remained in the solution.

The magnetic properties of Ni particles were assessed using vibrating sample magnetometry. Figure 4 shows the hysteresis loop for Ni at room temperature, although the loop is narrow. The sample exhibits ferromagnetic behavior with a saturation magnetization of 44.15 emu/g. The literature suggests that hcp Ni is either nonmagnetic or antiferromagnetic, as evidenced by its lower saturation magnetization compared to fcc Ni [33]. The saturation magnetization value observed in our study is further evidence that the metallic nickel is aggregated according to the fcc crystalline structure, as confirmed by XRD analysis.

Whether some Ni oxidation with dissolved oxygen during cementation or air oxidation may have occurred between the isolation of the solid products from the pulp and their analysis needs to be verified. Magnetometry is able to discriminate the simultaneous presence of NiO and metallic Ni, as reported in the literature, in terms of an exchange bias due to the interfacial interaction between the ferromagnetic properties of Ni and the antiferromagnetic properties of NiO [34]. As shown in Figure 4, the hysteresis loop is symmetric with respect to zero magnetic fields, indicating the absence of an exchange bias effect typically associated with the presence of NiO [35,36]. This bias is usually evidenced by a noticeable shift of the hysteresis loop along the magnetic field axis. The symmetry of the hysteresis loop can be quantified using the following equation:

$${\mathrm{H}}_{\mathrm{C}}=({\mathrm{H}}_{\mathrm{C}}^{+}+{\mathrm{H}}_{\mathrm{C}}^{-})/2$$

(4)

where H_C, $H_C^{+}$, and $H_C^{-}$ are the coercivity and coercive fields at decreasing and increasing field strengths, respectively.

Previous research on Ni/NiO systems has reported H_C values ranging from 80 G in Ni/NiO nanowires to 700 G in partially oxidized Ni nanoparticles [37,38]. The hysteresis loop shown in Figure 4 clearly demonstrates symmetry around the zero field, with a ΔH_C value of approximately 0.65 G. This indicates the absence of partially oxidized NiO-Ni particles. Furthermore, the minimal shift observed in the loop suggests a negligible presence of NiO particles, which typically exhibit significant loop shifts of up to 10 kG [39]. Consequently, our magnetic data strongly suggest that the cementation technique used to recover Ni from PLS primarily yields pure fcc Ni particles.

4.2. Statistical Model and ANOVA Analysis

The experimental factors and response results, with particular emphasis on Ni recoveries, are presented in

Table 2. Experimental design layout and corresponding results for nickel recovery. The symbols A, B, and C represent the parameters for pH, reaction time (in min), and Al/Ni ratio (in mol/mol), respectively.

Run	Coded Factors			Ni Recovery (%)
	A	B	C	Actual	Predicted
1	−1.0	0.0	1.0	74.7	73.4
2	−1.0	1.0	0.0	70.1	71.4
3	0.0	0.0	0.0	75.6	75.2
4	−1.0	0.0	−1.0	60.3	59.4
5	−1.0	−1.0	0.0	63.9	64.3
6	1.0	0.0	1.0	84.6	85.5
7	0.0	0.0	0.0	74.1	75.2
8	0.0	0.0	0.0	75.4	75.2
9	0.0	−1.0	1.0	80.1	80.4
10	1.0	1.0	0.0	80.2	79.8
11	1.0	−1.0	0.0	74.7	74.0
12	0.0	−1.0	−1.0	63.9	64.3
13	0.0	1.0	−1.0	70.2	69.9
14	1.0	0.0	−1.0	64.8	65.5
15	0.0	1.0	1.0	89.1	88.5
16	0.0	0.0	0.0	75.2	75.2
17	0.0	0.0	0.0	75.6	75.2

. The experiments achieved a noteworthy Ni recovery rate from the PLS, bracketed between 59.4% and 89.1%. Response surface methodology (RSM) was used to develop a polynomial (Equation (5)) to relate the process variables to the response of Ni cementation from the PLS. This analysis resulted in a second-order polynomial equation (Equation (5)) based on coded factors including linear effects of the process variables (A, B, and C), two-way interactions (AB, AC, and BC), and quadratic interactions (A², B², and C²), and provides insight into the relative influence of each factor by comparing the coefficients of these variables.

$$R=75.18+4.41A+3.38B+8.66C-0.17AB+1.35AC+0.67BC-3.84A^2+0.89B^2-0.24C^2$$

(5)

where R, A, B, and C are Ni recovery, pH, reaction time (min), and Al/Ni ratio (mol/mol), respectively.

Assessing the contribution of each variable is critical to understanding its impact on the process response, allowing for more precise control and optimization of Ni cementation from PLS. ANOVA (analysis of variance) was used to quantify the influence of each variable and the results are presented in

Table 3. ANOVA results for modeling of the cementation process.

Source	Sum of Squares	df	Mean Squares	F-Value	p-Value	Contribution (%)
Model	921.18	9	102.35	83.60	<0.0001	99.08
A	155.76	1	155.76	127.22	<0.0001	16.75
B	91.13	1	91.13	74.43	<0.0001	9.80
C	600.31	1	600.31	490.31	<0.0001	64.57
AB	0.12	1	0.12	0.10	0.7610	0.01
AC	7.29	1	7.29	5.95	0.0448	0.78
BC	1.82	1	1.82	1.49	0.2619	0.20
A2	62.09	1	62.09	50.71	0.0002	6.68
B2	3.30	1	3.30	2.69	0.1448	0.35
C2	0.24	1	0.24	0.20	0.6697	0.03
Residual	8.57	7	1.22			0.92
Lack of fit	7.00	3	2.33	5.95	0.0588	0.75
Pure error	1.57	4	0.39			0.17
Corr. total	929.75	16				100.00

. The model’s low p-value of <0.0001, combined with a high F-value of 83.60 for Ni recovery, confirms the model’s suitability, with only a 0.01% probability that such a high F-value could result from random variation. Model terms with p-values less than 0.05 are considered significant. While all linear model terms were deemed important, the terms A² and AC showed some degree of significance, in contrast to the AB, BC, B², and C² contributions. Among the variables considered, the Al/Ni ratio has the most significant effect on Ni recovery, followed by pH, and then reaction time within the specified range. Altogether, over 91% of the contribution to the cementation process can be attributed to the significant influence of these three variables. The contribution was determined by calculating the ratio of the adjusted sum of squares to the total sum of squares.

The Ni recovery model showed high accuracy, achieving a confidence level of over 99% as shown in Table 3, indicating a well-chosen set of model terms. The R-squared value for Ni recovery was 0.9908 (see Figure A1), close to unity, indicating a good fit between the model and the observed data. The small discrepancy between the fitted R-squared of 0.9789 and the predicted R-squared of 0.8769 serves as an important indicator of model reliability. In addition, a signal-to-noise ratio greater than 4 [40] indicates reliable data; for the Ni recovery model, this ratio was 34.4.

4.3. Response Surface Analysis

Response surface plots were used to depict the interaction between variables and cementation recovery. In this analysis, one variable was consistently maintained at its midpoint value while the other two variables varied within their experimental ranges. Figure 5a–c shows the effects of these variable interactions on Ni recovery.

Figure 5a illustrates the relationship between time and pH on Ni recovery. Increasing the reaction time from 5 to 15 min across all pH levels results in only a modest improvement in Ni recovery. For instance, at pH levels of 0 and 3, increasing the reaction time from 5 to 15 min increases the recoveries by only 9.7% and 7.4%, respectively. Conversely, the figure highlights that an increase in pH boosts metal recovery at all reaction times. The optimum appears to be around a pH of 3 with a reaction time of 15 min. This notable increase in Ni recovery due to reduced proton concentration limits the unwanted consumption of Al sacrificial acidic dissolution and inhibits the redissolution of cemented Ni [16].

Figure 5b demonstrates the combined effects of the Al/Ni molar ratio and pH on Ni recovery. Increasing the Al/Ni molar ratio over the entire pH range results in a noticeable, albeit moderate, improvement in Ni recovery. Adjusting the Al/Ni molar ratio from 1.32 to 6.60 results in improvements of 23% at pH 0 and 30.5% at pH 3. It is noteworthy that increasing pH meaningfully improves Ni recovery, especially when compared to the effects of increasing Al consumption. The synergistic effect of these variables is most pronounced at a pH of 3 and an Al/Ni molar ratio of 6.60, where Ni cementation achieves approximately 86%.

Figure 5c illustrates the interaction between the Al/Ni molar ratio and process time on Ni recovery. An increase in both factors results in improved metal recovery; however, the effect of the Al/Ni molar ratio is more significant than that of cementation time. For example, within the first 5 min of the reaction, 63.9% Ni recovery was achieved. Over a 15 min period, an additional 9.9% increase was observed at an Al/Ni molar ratio of 1.32. This data highlights the critical influence of the sacrificial metal dosage on Ni cementation.

A thorough optimization strategy must encompass technical, economic, and environmental considerations. In this study, our goal was to maximize Ni recovery while minimizing operating costs. To achieve this, reaction time and pH were varied within their specified ranges, while the Al/Ni molar ratio was maintained at its lowest practical value to reduce expenses. Optimization was performed using DX-7 software, based on the model outlined in Equation (5) and its constraints. The optimal conditions yielded a Ni recovery of 75.6% at a pH of 2.2 ± 0.1, a process time of 15 min, and an Al/Ni molar ratio of 2.65. To validate these predictions, three additional replicate experiments were carried out under these conditions, resulting in an average Ni recovery of 73.2%, thereby confirming the reliability of Equation (5). It is important to note that the excess Al consumption is primarily attributed to chemical dissolution rather than anodic dissolution.

5. Conclusions

This study explored the recovery of nickel from the pregnant leach solution (PLS) of spent Ni/Al₂O₃ catalysts as an alternative to overcome the limitations of traditional metal recovery techniques such as solvent extraction and precipitation. By successfully implementing the cementation process, we have not only provided a viable alternative, but also achieved remarkable efficiency in recovering high-purity nickel.

Thermodynamic analysis was used to discuss the potential interference of the major interfering reactions (nickel and sacrificial metal leaching) with the selective cementation of nickel from the PLS. The selection of aluminum as the sacrificial metal was driven by its cost effectiveness and the synergistic effects observed in the recovery process. Using response surface methodology (RSM) in conjunction with Box–Behnken design (BBD), we optimized the conditions to enhance nickel recovery. The optimal parameters identified—namely a pH of 2.2 ± 0.1, a processing time of 15 min, and an Al/Ni molar ratio of 2.65—proved to be critical in achieving a nickel recovery rate of 73.2% as powder.

The thorough characterization of the cemented nickel powder in this study highlights its high purity. The ICP-OES analysis revealed a predominant nickel content of 99.47%, with only trace amounts of impurities such as iron. XRD analysis confirmed that the nickel exists in the face-centered cubic (fcc) phase, characterized by distinct high-intensity peaks and the absence of hexagonal close-packed (hcp) structures. SEM-EDS provided further morphological details, showing a porous structure composed entirely of metallic Ni, with spherical nuclei and irregularly shaped rods. The EDS results corroborated the elemental analysis and confirmed the high purity of the nickel powder. In addition, VSM was used to evaluate the magnetic properties of the nickel particles. The sample exhibited ferromagnetic behavior with a saturation magnetization of 44.15 emu/g, consistent with fcc Ni properties. The symmetric hysteresis loop, with a coercive field value of approximately 0.65 G, indicated the absence of NiO-Ni composite structures, suggesting negligible NiO presence and further validating the purity and quality of the fcc Ni produced via the cementation process.