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Article

Upcycling Anodic Sludge from Aluminum Anodizing: Leaching Efficiency and Thermal Conversion into Refractory Materials

by
Fausto Acosta
1,
Cristhian Feijoo
2,
Alfredo S. Sangurima-Cedillo
1,
Alicia Guevara
1 and
Carlos F. Aragón-Tobar
1,*
1
Department of Extractive Metallurgy, Escuela Politécnica Nacional, Ladrón de Guevara E11-253, Quito 170525, Ecuador
2
Instituto de Investigación Geológico y Energético (IIGE), Quito 170518, Ecuador
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(18), 8491; https://doi.org/10.3390/su17188491
Submission received: 15 July 2025 / Revised: 19 August 2025 / Accepted: 21 August 2025 / Published: 22 September 2025
(This article belongs to the Special Issue Waste Management for Sustainability: Emerging Issues and Technologies)
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Abstract

Anodic sludges generated in the production of aluminum profiles pose both an environmental and economic problem due to their accumulation in municipal landfills. This study investigates their valorization as a raw material for industry through leaching and calcination processes. The solid residue was characterized both physically and chemically. In the leaching process, concentrations of NaOH (1–2.5 M) and solid percentages (10–30%) were evaluated, achieving a 93.7% recovery of aluminum as sodium aluminate with 2 M NaOH and 10% solids. In the calcination process, the sludges were treated at temperatures ranging from 200 to 1600 °C, and different particle sizes (−3 + 1 mm, −1000 + 400 μm, −400 + 200 μm). The best result from calcination was obtained at 1600 °C, producing a refractory material composed of corundum (α-Al2O3) and diaoyudaoite (NaAl11O17).

1. Introduction

The rapid growth of industrial activities has led to a significant increase in waste generation, posing serious challenges for sustainable development and environmental protection. Among industrial sectors, aluminum processing plays a critical role, yet it generates substantial quantities of anodic sludge (AS) as a byproduct during the anodizing process [1,2,3,4,5,6]. This waste, composed mainly of aluminum hydroxides, metal salts (e.g., Na, K, Mg, Ca), and residual water (80–90%) [7,8,9], is typically disposed of in municipal landfills, leading to high environmental risk and economic costs [10,11,12]. In Europe, disposal can cost as much as USD 2230 per ton, with no clear reuse pathways currently standardized [13]. Moreover, this results in the annual disposal of approximately 100,000 metric tons in the European Union alone [13,14].
The United Nations Sustainable Development Goals (SDGs), particularly those related to responsible consumption and production (SDG 12) and industry, innovation, and infrastructure (SDG 9), call for the transformation of industrial waste into usable materials [15]. In this context, valorizing anodic sludge as a secondary raw material aligns directly with the circular economy principles of reduce, reuse, and recycle (3Rs) and contributes to minimizing landfill dependency and resource extraction [16,17].
Several studies have investigated the reuse of AS by thermal transformation into ceramic or refractory materials, particularly corundum (α-Al2O3) and mullite (3Al2O3·2SiO2) [18,19] through high-temperature sintering or calcination processes [20,21,22,23,24]. These materials are essential in applications such as refractories, ceramics, and catalysts due to their high thermal and chemical resistance [10,13,25,26,27,28,29,30,31,32]. However, most prior work requires the addition of external silica sources or pretreated bauxite, increasing process complexity and cost [13,25,33].
In parallel, hydrometallurgical approaches, especially alkaline leaching, have been proposed to extract aluminum compounds (e.g., sodium aluminate) from AS under ambient or low-temperature conditions. These compounds have further utility in water treatment, paper manufacturing, and zeolite synthesis [4,9,34,35,36,37]. While promising, few studies integrate both pyrometallurgical and hydrometallurgical processes in a unified framework to maximize material recovery and product diversification.
Despite the potential of AS valorization, three main gaps remain in the current literature. First, there is a lack of integrated processing routes that combine leaching and calcination to fully evaluate AS’s potential for generating multiple high-value outputs. Second, there is a limited understanding of how process variables—such as particle size distribution and calcination temperature—affect phase formation and the final properties of refractory materials. Third, there is a shortage of data from Latin American contexts, particularly Ecuador, where industrial waste management infrastructure is still developing and sustainable circular models are urgently needed. Moreover, previous works often focus on lab-scale optimization without considering scale-up feasibility or economic implications in real production settings. This restricts the broader applicability of these technologies in regions with emerging economies.
The objective of this research is to develop a sustainable and circular strategy for managing anodic sludge (AS) waste from an aluminum profile factory in Ecuador. This is pursued through a dual-process approach involving (1) alkaline leaching of AS using varying NaOH concentrations and solid loadings to recover aluminum in the form of sodium aluminate and (2) high-temperature calcination of the residual sludge to synthesize refractory materials, specifically corundum (α-Al2O3) and diaoyudaoite (NaAl11O17). The novelty of this work lies in its integrated processing route that maximizes material recovery from a single waste stream, enhancing sustainability through waste-to-resource conversion. It also provides new data on phase transformations under specific particle size ranges and thermal regimes, which were underexplored in prior studies. Additionally, this is the first comprehensive study of AS valorization from Ecuador, contributing regional insights to the global circular economy discourse.

2. Materials and Methods

2.1. Collection of the AS from the Aluminum Plant

The AS sample (50 Kg) was collected from a 1 ton-heavy bag located in an industrial plant in Ecuador. This residue is stored after the liquid effluent treatment, carried out by the company using flocculation, coagulation, and pressing processes. First, the collected sample was dried at room temperature for 72 h and then at 40 °C for 24 h. Subsequently, it was homogenized and divided into 1 kg fractions.

2.2. Determination of the Physical, Chemical, and Mineralogical Characteristics of the AS

Humidity of the AS was determined by gravimetry. The real density was calculated based on the ASTM D854-23 standard “Test Methods for Specific Gravity of Soil Solids by the Water Displacement Method”. The granulometric analysis by sieving was carried out under ASTM C136/C136M-19, “Standard test method for analysis by sieving of fine and coarse aggregates”. The surface area was estimated by N2 physisorption using the Quantachrome NOVA4200e Surface Area Analyzer (Quantachrome, Boynton Beach, FL, USA). For thermogravimetric analysis (TGA), the Perkin Elmer STA 8000 equipment (PerkinElmer, Marlborough, MA, USA) was used (from 40 °C to 1000 °C at a heating rate of 5 °C/min and atmospheric pressure).
The crystallographic and structural analysis of the AS was carried out by X-ray diffraction (XRD) with a diffractometer (Bruker D8 Advance; Bruker, Bremen, Germany) equipped with a Cu-Kα radiation source (1.5406 Å). The measurements were carried out over a range of 3° to 70°, with a step time of 2 s, and at room temperature (25 °C). Using the TOPAS 5.0 software, the composition of the crystalline phases present was estimated. The chemical characterization was carried out with an X-ray fluorescence (XRF) spectrometer (Bruker S8 Tiger AX15; Bruker).

2.3. Evaluation of the Recovery of Sodium Aluminate by Leaching with Sodium Hydroxide and Precipitation with H2O2

To evaluate aluminum recovery as sodium aluminate, different concentrations of sodium hydroxide solutions were tested (1 M, 1.5 M, 2 M, and 2.5 M), and different percentages of solids were used (10%, 20%, and 30%).
For the development of the leaching, samples of 20 g of AS were taken, which were put in contact with the sodium hydroxide solution of different concentrations (1 M, 1.5 M, 2 M, and 2.5 M) until forming a pulp with the required solids content %w/w (10%, 20%, and 30%). The pulps obtained were stirred at 600 rpm for 24 h. At the end of the test, the pulps were filtered with MCE paper (pore size, 45 µm) to obtain the pregnant solutions and the tailings. The wet tailings were washed with 50 mL of distilled water to obtain the washing solutions. The final tailings were dried at 40 °C for 24 h. Then, the amount of aluminum present in the pregnant and washing solutions was quantified by atomic absorption spectrometry with the AAnalyst 300 equipment (Perkin Elmer, Waltham, MA, USA). The remaining aluminum content of the final tailings was analyzed by X-ray fluorescence on the S8 Tiger AX15 equipment. The aluminum recovery in each test was calculated by metallurgical balance.
For the precipitation of aluminum ions, 40 mL of pregnant solution was mixed with 200 mL of H2O2 (30%) and stirred at 800 rpm for 30 min. The pH of the solution was adjusted to 10 to ensure the formation of the Al(OH)3 precipitate according to the Pourbaix diagram for the Al-H2O system at 25 °C, as presented in Miskufova et al. [38]. According to Ahmad et al. [39] and Mahecha-Rivas et al. [40], the formation of Al(OH)3 as bayerite (α-Al(OH)3) and gibbsite (γ-Al(OH)3) phases is expected, as described by Equation (1).
NaAlO2 + 2H2O2 → Al(OH)3 ↓+ NaOH + O2
The precipitate of Al(OH)3 obtained was dried at 110 °C for 3 h and calcined at 1200 °C for 2 h according to the methodology presented in El-Katatny et al. [41] and Tripathy et al. [42]. X-ray diffraction analysis was performed to determine the structure before and after calcination using a D8 Advance diffractometer. The transformation of aluminum hydroxide into alumina is expected, as shown in Equation (2), following calcination at 1200 °C.
2Al(OH)3 → Al2O3 + 3 H2O ↑

2.4. Thermal Transformation of AS at Different Temperatures and Granulometries

With the purpose of investigating the effect of particle size on the recovery efficiency of corundum (α-Al2O3) from AS, three granulometric fractions of different sizes were used: −3 + 1 mm, −1000 + 400 µm, −400 + 200 µm, which were coded as A, B, and C, respectively. The different fractions were calcined at temperatures of 200 °C, 400 °C, 600 °C, 800 °C, 1000 °C, 1200 °C, 1400 °C, and 1600 °C in the Carbolite CTF/17/3000 tubular furnace (Carbolite, Neuhausen, Germany). In all cases, the amount of sample used was 20 g.
The calcined samples were analyzed by X-ray diffraction using a D8 Advance diffractometer to verify the formation of the alumina phase and the other crystalline phases after each process. Additionally, the samples that presented the corundum phase were chosen and analyzed by Scanning Electron Microscopy (SEM-EDX) in the Vegan-Tescan equipment (TESCAN, Brno, Czech Republic) for the determination of the morphology, surface characteristics, and semi-quantification of the elements present in the calcined samples.
Mullite formation was evaluated by calcining a mixture of Al2O3 and SiO2 with a molar ratio of Al2O3/SiO2 = 1.5. The mixture was subjected to calcination at 1200 °C, 1300 °C, and 1500 °C for 2 h using a Carbolite CTF/17/3000 tubular furnace. Phase identification and confirmation of mullite formation were performed by X-ray diffraction.

3. Results

3.1. Physical, Chemical, and Mineralogical Characterization of the AS

The AS sampled was white. The physical characteristics of this material were evaluated based on the properties listed in Table 1:
As presented in Table 2, AS contains approximately 53.7% water, as it originates from a filtration process [43,44,45]. AS is a hygroscopic material capable of retaining water within its structure, often leading to the formation of granules. The sludge exhibits deagglomeration behavior only after undergoing drying and grinding [46].
The apparent density was 0.65 g cm−3, and the real density was 2.2 g cm−3. This difference is due to the tendency of the anode sludge to partially agglomerate. The density obtained is reasonable due to the possible presence of aluminum hydroxides in the form of gibbsite, bayerite, and nostrandite that have this average density value [46].
The particle size determined by d80 was 4.5 mm, which is a high value, since the material partially agglomerates and presents hygroscopic and sticky characteristics. With reference to the surface area of the AS, it was 13.7 m2 g−1, which differs from the research of Mitrakas et al. [2], where a value varying between 60 and 216 m2/g was reported. The difference can be attributed to the fact that this parameter depends directly on the manufacturing stages of the aluminum profiles, such as cleaning, stripping, and anodizing, and these stages vary in each company.
Thermogravimetric analysis of the AS is presented in Figure 1:
Figure 1 shows that between 40 °C and 1000 °C, a weight loss of 35% occurred, and four distinct regions can be identified for analysis. In the first region, which ranges from 40 °C to 140 °C, there is a weight loss of 4% that corresponds mainly to the elimination of moisture absorbed on the surface of the sludge that could not be evaporated. The second region extends from 140 °C to 287 °C, which belongs to the 19.5% loss that, according to Wang et al. [4] and Farinha et al. [25], corresponds to the endothermic transformation of aluminum hydroxide present in AS, which can be in the form of boehmite or gibbsite. The mass loss in the third region, from 287 °C to 582 °C, can be associated with the transition from boehmite to γ-alumina according to Vieira et al. [44], De Moraes et al. [47], and Ghoniem et al. [48]. Finally, the fourth mass loss corresponds to the final transformation into α-alumina.
Figure 2 shows an image of a portion of the aluminum sludge (AS) observed under an electron microscope at 80× magnification. The sludge displays a non-uniform particle size and lacks a well-defined shape.
The chemical composition of AS is shown in Table 2, where it can be observed that the aluminum content is 26.6%, which is in accordance with the concentration determined by other researchers [45]. In addition, there are smaller amounts of other elements, such as sodium (1.5%), calcium (0.5%), and sulfur (2.0%).
Additionally, it is also worth noting that Si, Mg, and P are present in concentrations lower than 1%, and that there is the presence of other metals such as Cr, Ni, and K in trace amounts. These results are like those obtained by Souza et al. [46], Magalhães et al. [49], Mymrin et al. [50], Corker et al. [51], and Kumar et al. [15].
Figure 3 shows the diffractogram of the AS, revealing a predominantly amorphous structure with the presence of peaks in 2θ near 18°, 20°, 37°, and 64°, corresponding to gibbsite and bayerite [2,5,51,52].
The presence of gibbsite is attributed to its preferential precipitation in acidic environments with pH values below 5.8, which correspond to conditions typically found in acid anodizing processes used in aluminum profile manufacturing. Conversely, bayerite is considered to form under neutral to basic conditions (pH > 5.8). As a result, aluminum hydroxide may initially precipitate as gibbsite in anodizing tanks and partially transform into bayerite following neutralization, that is, the mixing of acidic and basic waste streams. It is therefore likely that both polymorphs—gibbsite and bayerite—are present in the aluminum sludge, possibly in an amorphous state.

3.2. Hydrometallurgical Approaches for Recovering Value from AS

In the leaching experiments, the influence of solids content (10%, 20%, and 30%) and NaOH concentration (1, 1.5, 2, and 2.5 M) on the recovery of aluminum from the AS was studied. Figure 4 summarizes the results of the leaching tests.
As shown in Figure 4, using a 1 M NaOH solution, aluminum recovery was 87.2% at 10% solids, decreased to 85.3% at 20% solids, and dropped sharply to 1.3% at 30% solids. With a 1.5 M NaOH solution, the recoveries were 70.6%, 62.9%, and 0.6% for 10%, 20%, and 30% solids, respectively. At 2 M NaOH, the recoveries were 93.7%, 44.0%, and 5.3% for the same solids contents. A different trend was observed at 2.5 M NaOH. Aluminum recovery decreased from 71.6% to 23.9% as the solids content increased from 10% to 20%, then slightly increased to 34.5% at 30% solids.
In the next step, the recovered NaAlO2 solution was precipitated by adding H2O2, followed by a thermal treatment. The presence of different mineral phases in the precipitate was analyzed before and after calcination at 1200 °C. Figure 5 shows that before calcination, the precipitate exhibited the formation of peaks of the bayerite phase: 20.19° for (1 1 0), 27.81° for (1 1 1), and 40.51° for (2 0 1) and gibbsite phase: 18.29° for (0 0 2), 20.39° for (1 1 0), 37.70° for (3 1 1), and 63.72° for (3 2 4). The diffraction pattern showed an absence of narrowness and sharpness in the peaks expected for a semi-crystalline mixture of bayerite and gibbsite phases [53,54].
After the calcination at 1200 °C for 2 h, the diffractogram of the product showed that all the original bayerite diffraction peaks had disappeared and only the broad diffraction peaks of the gibbsite phase at 37.70° remained. The thermal decomposition of aluminum hydroxide produces metastable oxides whose structure is unsuccessfully crystalline [54].

3.3. Pyrometallurgical Approaches for Value from AS

Calcination tests of AS at 200 °C, 400 °C, 600 °C, and 800 °C mainly revealed the presence of an amorphous phase. These results are consistent with previous studies that identified amorphous aluminum hydroxides using nuclear magnetic resonance (NMR) spectroscopy, in which characteristic peaks corresponding to AlO5 and AlO4 coordination environments were observed [42,55]. Accordingly, higher calcination temperatures are required to promote the formation of crystalline phases [55,56].
Figure 6 shows the X-ray diffractogram of AS samples calcined at four different temperatures (1000, 1200, 1400, and 1600 °C). The diffractogram corresponds to one representative particle size distribution, as all three sizes—A (–3 + 1 mm), B (–1000 + 400 µm), and C (–400 + 200 µm)—exhibited similar diffraction patterns under the same thermal conditions.
As shown in Figure 6, the formation of mineral phases was not influenced by the three particle size fractions tested (A (−3 + 1 mm), B (−1000 + 400 µm), and C (−400 + 200 µm)), which showed the formation of the same mineral phases: corundum and diaoyudaoite. As the calcination temperature increased to 1000 °C, the formation of α-Al2O3 was observed (number 2 on the diffractogram). At temperatures between 1200 °C and 1600 °C, the main crystalline phase formed was corundum (α-Al2O3), followed by the diaoyudaoite phase (NaAl11O17), also known as Na-β-alumina (number 1 on the diffractogram).
Figure 7 shows scanning electron microscopy images and highlights the formation of the corundum phase as a function of calcination temperature.
Figure 7a shows that the untreated original sample lacks a defined structure. In Figure 7b, after calcination at 1200 °C, the initial formation of corundum crystals is observed. As the temperature increased to 1400 °C, the formation of this crystalline phase began, first in the form of flakes and then in a typical hexagonal shape (Figure 7c). In Figure 7d, after calcination at 1600 °C, corundum and diaoyudaoite crystals acquired a regular hexagonal shape.
The mullitization processes were carried out from a precursor mixture with an Al2O3/SiO2 ratio of 1.5, calcined at temperatures of 1200 °C, 1300 °C, and 1500 °C for 2 h. The X-ray diffractograms (Figure 8) show the influence of calcination temperature on the formation of the mullite phase.
It is observed that the samples calcined at 1200 °C and 1300 °C contained approximately 99% α-Al2O3 content; that is, with an Al2O3/SiO2 ratio of 1.5, the mullite phase was not detected in significant amounts. At a temperature of 1500 °C, 50% of the corundum was transformed to sillimanite, while 4% was converted to mullite (3Al2O3 2SiO2).

4. Discussion

4.1. Evaluation of the Hydrometallurgical Approaches for Recovering Value from AS

Regarding the hydrometallurgical approach, the highest aluminum extraction yield was 93.7%, achieved through atmospheric leaching using 2 M NaOH with 10% solids. In contrast, the lowest recovery was 0.62%, observed in the test using 1.5 M NaOH and 30% solids. These results are consistent with those reported by El-Katatny et al. [41], who observed an inverse relation between aluminum recovery and solids concentration.
The trend found related to the percentage of solids is due to the fact that increasing the amount of leaching solution allows the viscosity of the AS to decrease, and as a result, the concentration gradient of the aluminate ions between the surface of the solid and the solution decreases [14,57].
Regarding the influence of the concentration of the NaOH solution, although it is generally considered that increasing the concentration facilitates leaching [14,58], in the present work, there is no direct relation between the alkali concentration at 10% and 20% solids, unlike at 30%, where the relation was direct. This leaching conduct is in accordance with the studies carried out by Cao et al. [59]. Seemingly, the indirect relation between Al recovery and alkali concentration could be attributed to the pH of the system.
According to Hayrapetyan et al. [60], it is a hard task to achieve a homogenous phase when the alkali interacts with the AS; therefore, there could exist various levels of pH at distinct parts of the system. Under certain conditions, it does not allow the alumina to keep dissociating but to start precipitating, which decreases the Al extraction yield.
Regarding the precipitation behavior of the obtained precipitate, it does not align with the transformation sequence of aluminum hydroxide into its various transitional crystalline phases with increasing temperature, as described by El-Katatny et al. [41]. In their study, after precipitation with H2O2, the boehmite crystalline phase (γ-AlO(OH)) was obtained and subsequently calcined to yield the desired α-Al2O3. Apparently, the heating rate and calcination time followed did not allow the formation of the desired alumina, since the type of precursor and the calcination conditions used determine the resulting polymorph characteristics [61].

4.2. Pyrometallurgical Approaches for Recovering Value from AS

The thermal treatment of AS at various temperatures (200 °C to 800 °C) predominantly resulted in the formation of amorphous phases, as indicated by the calcination experiments. This behavior aligns with earlier findings that employed nuclear magnetic resonance (NMR) spectroscopy to detect amorphous aluminum hydroxides, particularly through signals associated with AlO4 and AlO5 coordination environments [46,52]. These observations suggest that significantly higher temperatures are necessary to facilitate the development of crystalline structures [56,57,62].
The samples obtained from calcination at low temperatures did not exhibit metastable crystalline phases such as boehmite at 400 °C, γ-Al2O3 at 600 °C, or δ-Al2O3 at 800 °C. This observation aligns with the findings of Bhattacharya et al. [63], who reported the formation of crystalline structures only at temperatures above 900 °C. However, it contrasts with the results reported by El-Katatny et al. [45] and Saridede et al. [63], who observed evident phase transformations of transitional alumina with increasing calcination temperature. These discrepancies may be attributed to differences in precursor materials, preparation methods, heating environments, and the presence of impurities such as Na2O, all of which are known to influence phase transformations, as discussed in previous studies [63,64,65].
Based on this, the calcination temperature and the amount of sodium present in the AS had an influence on the synthesis of α-Al2O3 and Na-β-alumina (NaAl11O17), which is consistent with previous studies [57,66,67]. The X-ray diffraction patterns (XRD) of the samples calcined at 1600 °C revealed the presence of corundum, as indicated by the characteristic diffraction peaks with Miller indices: (012), (104), (110), (006), (113), (202), (024), (116), (211), (122), (018), (214), (030), and (300), which correspond to the reference pattern JCPDS card No. 46-1212.
The Na-β-alumina (NaAl11O17) phase formed is an isomorph of alumina (Al2O3) [65], and because either a non-stoichiometric amount or an excess of sodium allows the formation of both corundum and diaoyudaoite phases, sodium in the crystalline phases of the final product is of great importance [65,68]. Additionally, the stability of the Na-β-alumina phase is due to the presence of magnesium that functions as a stabilizer [66,69].
Corundum and diaoyudaoite crystals acquire a regular hexagonal shape, which has also been observed in previous studies [70,71]. These results confirm the formation of corundum and diaoyudaoite at 1600 °C after 2 h. Furthermore, the chemical analysis carried out confirms the elemental presence of aluminum and oxygen, originating from α-Al2O3; in addition to other elements such as sodium, which comes from diaoyudaoite, and magnesium, which is part of the initial composition of AS, these results were confirmed by XRD.
It was observed that the samples calcined at 1200 °C and 1300 °C consisted of approximately 99% α-Al2O3, indicating that the mullite phase did not form in significant quantities coincides with what was suggested by Sadik et al. [72], even at an overall Al2O3/SiO2 molar ratio of 1.5 [73]. This behavior can be attributed to the early crystallization of α-Al2O3 during the thermal treatment. Since α-Al2O3 is the most thermodynamically stable and least reactive polymorph of alumina, its formation passivates the alumina phase and inhibits its solid-state reaction with silica. As a result, Al–Si interdiffusion is limited, and the nucleation and growth of mullite are suppressed. Only at a higher temperature of 1500 °C does a partial reaction occur, with approximately 50% of the corundum converting to sillimanite and a minor fraction (~4%) to mullite (3Al2O3·2SiO2) [33,74].

5. Conclusions

The anodic sludge derived from the aluminum anodizing process under study exhibited a high content of the target metal (26.6%). For this reason, two aluminum extraction processes were evaluated in this research: leaching and calcination.
Using the NaOH leaching method on the anodic sludge, aluminum recoveries of up to 94% were achieved. The optimal parameters for the test were a NaOH concentration of 2 M and a pulp solid content of 10%. The leaching product consisted of sodium aluminate. For aluminum precipitation, a 30% H2O2 solution was employed, yielding aluminum hydroxide. Calcination of the precipitate at 1200 °C produced no crystalline phases; therefore, it is recommended to further investigate the pyrometallurgical process conditions to obtain crystalline phases with refractory characteristics.
Calcination of anodic sludge through pyrometallurgical processes promotes the formation of crystalline phases, primarily corundum (α-Al2O3), whose development is strongly influenced by temperature. Initial formation of θ-Al2O3 occurs at temperatures above 1000 °C, transitioning to predominantly α-Al2O3 between 1200 °C and 1600 °C, accompanied by the appearance of Na-β-alumina (NaAl11O17), whose formation primarily depends on the presence of sodium. Optimal crystal formation is achieved at 1600 °C, rendering the material suitable for refractory applications. It is noteworthy that variations in grain size due to calcination temperature do not significantly affect corundum phase formation. Calcination of the precursor mixture for mullite formation, with an Al2O3/SiO2 ratio of 1.5, yields 99% α-Al2O3 at 1200 °C, whereas at 1500 °C, three crystalline–crystalline phases are obtained: sillimanite (50%), corundum (46%), and mullite (4%).
In both hydrometallurgical and pyrometallurgical processes, two products with distinct chemical and mineralogical characteristics are obtained. While a direct comparison of the techniques is not feasible, each product may possess unique properties that warrant evaluation in future studies.

Author Contributions

Conceptualization, A.G.; methodology, A.G., A.S.S.-C. and F.A.; formal analysis, F.A. and A.G.; investigation, F.A.; resources, A.G.; data curation, A.S.S.-C. and F.A.; writing—original draft preparation, F.A. and C.F.A.-T.; writing—review and editing, F.A., C.F. and C.F.A.-T.; visualization, F.A. and A.S.S.-C.; supervision, A.G.; project administration, A.G.; funding acquisition, A.G. All authors have read and agreed to the published version of the manuscript.

Funding

The research presented in this study was made possible by the financing of the Department of Extractive Metallurgy (DEMEX) of the Escuela Politécnica Nacional, thanks to the research project PII-DEMEX-02-2020. Tel.: +593-2-297-6300 (ext. 5806).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to thank the Department of Extractive Metallurgy and the Escuela Politécnica Nacional for their valuable support. Their technical assistance and access to laboratory facilities were instrumental in the development of this research.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ASAnodic sludge

References

  1. Ferreira, J.M.F.; Olhero, S.M. Al-rich sludge treatments towards recycling. J. Eur. Ceram. Soc. 2002, 22, 2243–2249. [Google Scholar] [CrossRef]
  2. Mitrakas, M.G.; Bakaloulis, A.K.; Gkinis, K.T. Studies on arsenic adsorption by the use of aluminum anodizing sludge. Desal. Water Treat. 2012, 39, 235–247. [Google Scholar] [CrossRef]
  3. Memon, N.A.; Ahmed, N.; Haq, T.; Ismail, S.; Rashid, U.; Begum, R. An Effective Use of Anodizing Mud for Treatment of Tannery Wastewater as a Coagulation Agent. Int. J. Sci. Res. Publ. 2018, 8, 418–429. [Google Scholar] [CrossRef]
  4. Wang, H.; Zhang, X.; Peng, X.; Luo, L.; Shen, L. Comprehensive Recovery of Aluminum and Calcium from Aluminum-Anodizing Sludge by Physicochemical Process. J. Sustain. Metall. 2021, 7, 1343–1353. [Google Scholar] [CrossRef]
  5. De Magalhães, L.F.; Da Silva, G.R.; Henriques, A.B.; De Freitas, V.A.A.; Peres, A.E.C. Synthesis of Analcime Zeolite from Glass Powder Waste and Aluminium Anodizing Waste. Silicon 2024, 16, 4173–4185. [Google Scholar] [CrossRef]
  6. Abdellatief, M.; Mortagi, M.; Hamouda, H.; Skrzypkowski, K.; Zagórski, K.; Zagórska, A. Influence of silicate modulus and eggshell powder on the expansion, mechanical properties, and thermal conductivity of lightweight geopolymer foam concrete. Materials 2025, 18, 2088. [Google Scholar] [CrossRef] [PubMed]
  7. Shackelford, J.F.; Doremus, R.H. Ceramic and Glass Materials: Structure, Properties and Processing; Springer: New York, NY, USA, 2008. [Google Scholar] [CrossRef]
  8. Busca, G. Structural, Surface, and Catalytic Properties of Aluminas. Adv. Catal. 2014, 57, 319–404. [Google Scholar] [CrossRef]
  9. Ruys, A. Processing, structure, and properties of alumina ceramics. Alum. Ceram. 2019, 4, 71–121. [Google Scholar] [CrossRef]
  10. Novales, A.; Pereira, F.; Hotza, D.; Klegues, O. Aluminum rich sludge as raw material for the ceramic industry. Interceram 2003, 52, 44–46. Available online: https://www.researchgate.net/publication/269931045 (accessed on 15 May 2025).
  11. Álvarez-Ayuso, E. Approaches for the treatment of waste streams of the aluminium anodising industry. J. Hazard. Mater. 2009, 164, 409–414. [Google Scholar] [CrossRef]
  12. Ling, X.; Chen, W.; Schollbach, K.; Brouwers, H.J.H. Valorization of biomass bottom ash in alkali-activated GGBFS-fly ash: Impact of biomass bottom ash characteristic, silicate modulus and aluminum-anodizing waste. Constr. Build. Mater. 2024, 428, 136408. [Google Scholar] [CrossRef]
  13. Marques, D.; Barcelos, R.L.; Parma, G.C.; Girotto, E.; Américo, J.; Pereira, N.C.; Magnago, R.F. Recycled polyethylene terephthalate and aluminum anodizing sludge-based boards with flame resistance. Waste Manag. 2019, 92, 1–14. [Google Scholar] [CrossRef]
  14. Da Silveira Júnior, J.G.; Sakata, R.D.; Onghero, L.; De Matos, P.R.; Rodríguez, E.D.; Simão, L.; De Oliveira, A.P.N.; De Araújo Moura, R.C.; De Campos, C.E.M.; De Castro Pessôa, J.R.; et al. Al-anodizing waste as a supplementary cementitious material for 3D-printed Portland cement. Waste Manag. Bull. 2025, 3, 10–20. [Google Scholar] [CrossRef]
  15. Kumar, K.S.; Srinivas, J.; Ashokkumar, P.; Saravanan, R.; Aravind, V.; Bhat, S.S.; Raj, R.A.; Girimurugan, R. Utilization of Steelmaking By-Products and Aluminum sludges in Alkali-Activated Materials: A Sustainable approach for construction applications. J. Phys. Conf. Ser. 2025, 3045, 012002. [Google Scholar] [CrossRef]
  16. Rajaniemi, K.; Tuomikoski, S.; Lassi, U. Electrocoagulation Sludge Valorization—A Review. Resources 2021, 10, 127. [Google Scholar] [CrossRef]
  17. David, E.; Kopac, J. Aluminum recovery as a product with high added value using aluminum hazardous waste. J. Hazard. Mater. 2013, 261, 316–324. [Google Scholar] [CrossRef] [PubMed]
  18. Lecomte-Nana, G.; Hammas, A. Mullite: Structure and Properties. In Reference Module in Materials Science and Materials Engineering; Elsevier: Amsterdam, The Netherlands, 2021. [Google Scholar] [CrossRef]
  19. Lima, L.K.S.; Silva, K.R.; Menezes, R.R.; Santana, L.N.L.; Lira, H.L. Microstructural characteristics, properties, synthesis and applications of mullite: A review. Cerâmica 2022, 68, 126–142. [Google Scholar] [CrossRef]
  20. Dexpert, H.; Larue, J.F.; Mutin, I.; Moraweck, B.; Bertaud, Y.; Renouprez, A. Thermal Transformation of Transition Aluminas. JOM 1985, 37, 17–21. [Google Scholar] [CrossRef]
  21. Misra, C. Aluminum Oxide (Alumina), Hydrated; Kirk-Othmer Encyclopedia of Chemical Technology: Hoboken, NJ, USA, 2003. [Google Scholar] [CrossRef]
  22. Oliveira, F.A.C.; Livramento, V.; Delmas, F. Novel mullite-based ceramics manufactured from inorganic wastes: I. Densification behaviour. J. Mater. Process. Technol. 2008, 196, 101–108. [Google Scholar] [CrossRef]
  23. Mymrin, V.; Pedroso, D.E.; Pedroso, C.; Alekseev, K.; Avanci, M.A.; Winter, E.; Cechin, L.; Rolim, P.H.B.; Iarozinski, A.; Catai, R.E. Environmentally clean composites with hazardous aluminum anodizing sludge, concrete waste, and lime production waste. J. Clean. Prod. 2018, 174, 380–388. [Google Scholar] [CrossRef]
  24. Carstens, S.; Meyer, R.; Enke, D. Towards Macroporous α-Al2O3—Routes, Possibilities and Limitations. Materials 2020, 13, 1787. [Google Scholar] [CrossRef]
  25. Farinha, A.; Mendes, R.; Vieira, M. Production of Sintered α-Alumina by Explosive Compaction from Low Temperature Calcinated Aluminum-Rich Sludge. Waste Biomass Valor. 2013, 4, 627–633. [Google Scholar] [CrossRef]
  26. Ribeiro, M.J.; Abrantes, J.C.C.; Ferreira, J.M.; Labrincha, J.A. Predicting processing-sintering-related properties of mullite–alumina ceramic bodies based on Al-rich anodising sludge by impedance spectroscopy. J. Eur. Ceram. Soc. 2004, 24, 3841–3848. [Google Scholar] [CrossRef]
  27. Ribeiro, M.J.; Abrantes, J.C.C.; Labrincha, J.A. Electrical characterization of mullite bodies containing Al-rich anodizing sludge. Mater. Sci. Forum. 2006, 514–516 Pt 2, 1726–1730. [Google Scholar] [CrossRef]
  28. Ribeiro, M.J.; Tulyaganov, D.U.; Ferreira, J.M.F.; Labrincha, J.A. Production of Al-rich sludge-containing ceramic bodies by different shaping techniques. J. Mater. Process. Technol. 2004, 148, 139–146. [Google Scholar] [CrossRef]
  29. Ribeiro, M.J.; Tulyaganov, D.U.; Ferreira, J.M.; Labrincha, J.A. Recycling of Al-rich industrial sludge in refractory ceramic pressed bodies. Ceram. Int. 2002, 28, 319–326. [Google Scholar] [CrossRef]
  30. Labrincha, J.A.; Albuquerque, C.M.; Ferreira, J.M.; Ribeiro, M.J. Electrical characterisation of cordierite bodies containing Al-rich anodising sludge. J. Eur. Ceram. Soc. 2006, 26, 825–830. [Google Scholar] [CrossRef]
  31. Labrincha, J.; Puertas, F.; Schroeyers, W.; Kovler, K.; Pontikes, Y.; Nuccetelli, C.; Krivenko, P.; Kovalchuk, O.; Petropavlovsky, O.; Komljenovic, M.; et al. From NORM by-products to building materials. In Naturally Occurring Radioactive Materials in Construction: Integrating Radiation Protection in Reuse (COST Action Tu1301 NORM4BUILDING); Elsevier: Amsterdam, The Netherlands, 2017; pp. 183–252. [Google Scholar] [CrossRef]
  32. Komarneni, S.; Schneider, H.; Okada, K. Mullite Synthesis and Processing. In Mullite; Wiley-VCH: Weinheim, Germany, 2005; pp. 251–348. [Google Scholar] [CrossRef]
  33. Badanoiu, A.-I.; Stoleriu, S.-P.; Carocea, A.-C.; Eftimie, M.-A.; Trusca, R. Influence of synthesis route on composition and main properties of mullite ceramics based on waste. Materials 2025, 18, 1098. [Google Scholar] [CrossRef]
  34. González, E.; Barbadillo, P.; García, F.; Hernández, A.; Ayala, N. Utilización de los Lodos de Hidróxido de Aluminio Procedente de las Plantas de Anodizado. Patent No. 2130098, 1 March 2000. [Google Scholar]
  35. Sanga, P.; Iradukunda, Y.; Munyemana, J.C. Recycling of alum from water treatment residue and reuse it as a flocculating agent for raw water treatment. J. Geosci. Environ. Prot. 2018, 6, 216–226. [Google Scholar] [CrossRef]
  36. Barakat, M.A.; El-Sheikh, S.M.; Farghly, F.E. Removing Al and regenerating caustic soda from the spent washing liquor of Al etching. JOM 2005, 57, 34–38. [Google Scholar] [CrossRef]
  37. Ye, M.; Li, G.; Liang, J.; Liao, X.; Ren, J.; Fen, X.; Qian, W.; Li, L.; Sun, S. A high-efficiency process for the separation of chromium and aluminum from waste aluminum sludge with a high chromium content using a combined oxidation and dispersion process. Sep. Purif. Technol. 2021, 258, 118083. [Google Scholar] [CrossRef]
  38. Miskufova, A.; Petranikova, M.; Kovacs, M.; Briancin, J.; Havlik, T.; Orac, D. Leaching of Aluminium Dross in Alkaline Solution. In Proceedings of the European Metallurgical Conference, Innsbruck, Austria, 28 June–1 July 2009. [Google Scholar]
  39. Ahmad Zauzi, N.S.; Zakaria, M.; Baini, R.; Rahman, M.; Mohamed Sutan, N.; Hamdan, S. Influence of alkali treatment on the surface area of aluminium dross. Adv. Mater. Sci. Eng. 2016, 2016, 1–4. [Google Scholar] [CrossRef]
  40. Mahecha-Rivas, J.C.; Fuentes-Ordoñez, E.; Epelde, E.; Saldarriaga, J.F. Aluminum extraction from a metallurgical industry sludge and its application as adsorbent. J. Clean. Prod. 2021, 310, 127374. [Google Scholar] [CrossRef]
  41. El-Katatny, E.A.; Halawy, S.A.; Mohamed, M.A.; Zaki, M.I. Surface composition, charge and texture of active alumina powders recovered from aluminum dross tailings chemical waste. Powder Technol. 2003, 132, 137–144. [Google Scholar] [CrossRef]
  42. Tripathy, A.K.; Mahalika, S.; Sarangi, C.K.; Tripathy, B.C.; Sanjay, K.; Bhattacharya, I.N. A pyro-hydrometallurgical process for the recovery of alumina from waste aluminium dross. Minerals Eng. 2019, 137, 181–186. [Google Scholar] [CrossRef]
  43. Correia, A.; Chambino, T.; Gonçalves, L.; Franco, A.; Gonçalves, R.; Gonçalves, A.; Limpo, V.; Delmas, F.; Nogueira, C.; Bartolomeu, F. Municipal wastewater treatment with anodizing solid waste. Desalination 2005, 185, 341–350. [Google Scholar] [CrossRef]
  44. Vieira, S.C.; Ramos, A.S.; Vieira, M.T. Mullitization kinetics from silica- and alumina-rich wastes. Ceram. Int. 2007, 33, 59–66. [Google Scholar] [CrossRef]
  45. Peñafiel, F.; Martínez, R. Síntesis de zeolitas utilizando como materia prima lodos de los procesos de anodizado de aluminio. Tecnol. Marcha 2019, 32, 12–23. [Google Scholar]
  46. Souza, M.T.; Simão, L.; Montedo, O.R.K.; Raupp Pereira, F.; de Oliveira, A.P.N. Aluminum anodizing waste and its uses: An overview of potential applications and market opportunities. Waste Manag. 2019, 84, 286–301. [Google Scholar] [CrossRef]
  47. De Moraes, G.G.; Oliveira, B.G.; Siligardi, C.; Innocentini, M.D.M.; Oliveira, A.A.M., Jr.; Neto, J.B.R.; Hotza, D.; de Oliveira, A.P.N. Production of foundry filters using Al2O3 from the Al-anodizing process. Int. Ceram. Congr. Part 2010, 62, 119–124. [Google Scholar] [CrossRef]
  48. Ghoniem, M.G.; Sami, T.M.; El-Reefy, S.A.; Mohamed, S.A. The production of high purity alumina from solid wastes obtained from aluminium factories. WIT Trans. Ecol. Environ. 2014, 180, 29–40. [Google Scholar] [CrossRef]
  49. Magalhães, J.M.; Silva, J.E.; Castro, F.P.; Labrincha, J.A. Physical and chemical characterisation of metal finishing industrial wastes. J. Environ. Manag. 2005, 75, 157–166. [Google Scholar] [CrossRef] [PubMed]
  50. Mymrin, V.; Pedroso, D.E.; Pedroso, C.L.; Avanci, M.A.; Rolim, P.H.B.; Carvalho, K.Q.; Catai, R.E. Physical-chemical processes of sustainable construction materials structure formation with iron ore processing tailings and aluminum anodizing sludge. Constr. Build. Mater. 2021, 298, 123698. [Google Scholar] [CrossRef]
  51. Corker, J.; Marques, I.; Resalati, S.; Okoroafor, T.; Maalouf, A.; Fu, Z.; Fan, M. Al-rich industrial waste as new alternative of fumed silica for the manufacture of vacuum insulation panels for building energy conservation. J. Clean. Prod. 2023, 415, 137854. [Google Scholar] [CrossRef]
  52. Yusta, I.; Castellano Calvo, A.; Aranburu, A.; Velasco Roldán, F. Los depósitos de Mn-Al-Fe de la cueva de Lazalday (Zarate, Alava): Composición química y mineralogía. Geogaceta 2009, 47, 31–34. [Google Scholar]
  53. Addai-Mensah, J. Surface and structural characteristics of gibbsite precipitated from pure, synthetic Bayer liquor. Miner. Eng. 1997, 10, 81–96. [Google Scholar] [CrossRef]
  54. Thimmasandra, R. Mechanism of structural transformations during thermal decomposition studies of aluminium hydroxide polymorph. Inernational J. Sci. Res. 2013, 1, 514–518. [Google Scholar]
  55. Isobe, T.; Watanabe, T.; D’Espinose De La Caillerie, J.B.; Legrand, A.P.; Massiot, D. Solid-state 1H and 27Al NMR studies of amorphous aluminum hydroxides. J. Colloid. Interface Sci. 2003, 261, 320–324. [Google Scholar] [CrossRef]
  56. Mercury, J.M.R.; Pena, P.; de Aza, A.H.; Sheptyakov, D.; Turrillas, X. On the decomposition of synthetic gibbsite studied by neutron thermodiffractometry. J. Am. Ceram. Soc. 2006, 89, 3728–3733. [Google Scholar] [CrossRef]
  57. He, L.; Shi, L.; Qizhen, H.; Hayat, W.; Shang, Z.; Ma, T.; Wang, M.; Yao, W.; Huang, H.; Chen, R. Extraction of alumina from aluminum dross by a non-hazardous alkaline sintering process: Dissolution kinetics of alumina and silica from calcined materials. Sci. Total Environ. 2021, 777, 146123. [Google Scholar] [CrossRef]
  58. Xing, W.D.; Ahn, B.D.; Lee, M.S. Treatment of black dross with water and NaOH solution. J. Korean Inst. Resour. Recycl. 2017, 26, 53–60. [Google Scholar] [CrossRef]
  59. Cao, S.; Zhang, Y.; Zhang, Y. Preparation of sodium aluminate from the leach liquor of diasporic bauxite in concentrated NaOH solution. Hydrometallurgy 2009, 98, 298–303. [Google Scholar] [CrossRef]
  60. Hayrapetyan, S.; Mangasaryan, L.; Tovmasyan, M.; Khachatryan, H. Precipitation of aluminum hydroxide from sodium aluminate, by treatment with formalin, and preparation of aluminum oxide. Acta Chromatogr. 2006, 16, 192. [Google Scholar]
  61. Chagas, L.H.; de Carvalho, G.S.G.; San Gil, R.A.S.; Chiaro, S.S.X.; Leitão, A.A.; Diniz, R. Obtaining aluminas from the thermal decomposition of their different precursors: An 27Al MAS NMR and X-ray powder diffraction studies. Mater. Res. Bull. 2014, 49, 216–222. [Google Scholar] [CrossRef]
  62. Rodríguez-Páez, J.E.; Villaquirán, C.; Cobo, J. Estudio de la formación de los complejos intermedios durante la síntesis de alúmina. Mater. Res. 2001, 4, 255–264. [Google Scholar] [CrossRef]
  63. Bhattacharya, I.N.; Gochhayat, P.K.; Mukherjee, P.S.; Paul, S.; Mitra, P.K. Thermal decomposition of precipitated low bulk density basic aluminium sulfate. Mater. Chem. Phys. 2004, 88, 32–40. [Google Scholar] [CrossRef]
  64. Das, B.R.; Dash, B.; Tripathy, B.C.; Bhattacharya, I.N.; Das, S.C. Production of η-alumina from waste aluminium dross. Miner. Eng. 2007, 20, 252–258. [Google Scholar] [CrossRef]
  65. Matori, K.; Loy, C.; Hashim, M.; Ismail, I.; Mohd Zaid, M. Phase transformations of α-alumina made from waste aluminum via a precipitation technique. Int. J. Mol. Sci. 2012, 13, 16812–16821. [Google Scholar] [CrossRef]
  66. Ruiz, V.; Rodriguez, J. The effect of the content of sodium in the obtaining of Na+ β-alúmina using the polymeric precursor. UNAL 2011, 78, 190–195. [Google Scholar]
  67. Kale, M.; Kale, S.; Joshi, C.; Moharil, S. Preparation of sodium beta-alumina using combustion synthesis. I-Manag. J. Mater. Sci. 2020, 7, 37–41. [Google Scholar] [CrossRef]
  68. Liu, Z.; Chen, J.; Wang, X.; Wang, Y.; Wang, D.; Mao, Z. Synthesis and characterization of high ionic-conductive sodium beta-alumina solid electrolyte derived from boehmite. J. Mater. Sci. Mater. Electron. 2020, 31, 17670–17678. [Google Scholar] [CrossRef]
  69. Tan, A.; Kuo, C.K.; Nicholson, P.S. Formation and characterization of Na-β″-alumina single crystal films. Solid State Ion. 1993, 67, 131–136. [Google Scholar] [CrossRef]
  70. Huang, J.; Fang, M.; Huang, Z.; Liu, Y.; Yang, J.; Huang, S.; Xu, Y.; Chen, K.; Yi, S.; Zhang, S. Preparation, microstructure, and mechanical properties of spinel-corundum-sialon composite materials from waste fly ash and aluminum dross. Adv. Mater. Sci. Eng. 2014, 789867, 1–10. [Google Scholar] [CrossRef]
  71. Ligon, S.C.; Blugan, G.; Bay, M.C.; Battaglia, C.; Heinz, M.V.F.; Graule, T. Performance analysis of Na-β″-Al2O3/YSZ solid electrolytes produced by conventional sintering and by vapor conversion of α-Al2O3/YSZ. Solid State Ion. 2020, 345, 115169. [Google Scholar] [CrossRef]
  72. Sadik, C.; El Amrani, I.; Albizane, A. Processing and characterization of alumina–mullite ceramics. J. Asian Ceram. Soc. 2014, 2, 310–316. [Google Scholar] [CrossRef]
  73. Foo, C.T.; Salleh, M.A.M.; Ying, K.K.; Matori, K.A. Mineralogy and thermal expansion study of mullite-based ceramics synthesized from coal fly ash and aluminum dross industrial wastes. Ceram. Int. 2019, 45, 7488–7494. [Google Scholar] [CrossRef]
  74. Wang, W.; Shi, Z.; Wang, Z.; Wang, S. Phase transformation and properties of high-quality mullite ceramics synthesized using desert drift sands as raw materials. Mater. Lett. 2018, 221, 271–274. [Google Scholar] [CrossRef]
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Figure 1. TGA-DTA curve of anodic sludge dried at 110 °C for 24 h.
Figure 1. TGA-DTA curve of anodic sludge dried at 110 °C for 24 h.
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Figure 2. Scanning electron microscopy (SEM) of AS at 80×.
Figure 2. Scanning electron microscopy (SEM) of AS at 80×.
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Figure 3. X-ray diffractogram of dried anode sludge (AS) at 110 °C for 24 h.
Figure 3. X-ray diffractogram of dried anode sludge (AS) at 110 °C for 24 h.
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Figure 4. Influence of the solids percentage and the NaOH concentration on the aluminum recovery from AS after 24 h leaching.
Figure 4. Influence of the solids percentage and the NaOH concentration on the aluminum recovery from AS after 24 h leaching.
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Figure 5. Diffractograms of bayerite (α-Al(OH)3) and gibbsite (γ-Al(OH)3) phases before and after calcination at 1200 °C for 2 h.
Figure 5. Diffractograms of bayerite (α-Al(OH)3) and gibbsite (γ-Al(OH)3) phases before and after calcination at 1200 °C for 2 h.
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Figure 6. X-ray diffractogram of AS samples calcined at 1000, 1200, 1400, and 1600 °C. Similar patterns were observed across all particle size fractions: A (−3 + 1 mm), B (−1000 + 400 µm), and C (−400 + 200 µm).
Figure 6. X-ray diffractogram of AS samples calcined at 1000, 1200, 1400, and 1600 °C. Similar patterns were observed across all particle size fractions: A (−3 + 1 mm), B (−1000 + 400 µm), and C (−400 + 200 µm).
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Figure 7. Morphology of (a) AS without treatment, (b) formation of corundum crystals after calcination at 1200 °C, (c) greater presence of corundum crystals at 1400 °C, and (d) hexagonal crystals of corundum and diaoyudaoite at 1600 °C.
Figure 7. Morphology of (a) AS without treatment, (b) formation of corundum crystals after calcination at 1200 °C, (c) greater presence of corundum crystals at 1400 °C, and (d) hexagonal crystals of corundum and diaoyudaoite at 1600 °C.
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Figure 8. X-ray diffractograms of the precursor mixture calcined at 1200 °C, 1300 °C, and 1500 °C for 2 h.
Figure 8. X-ray diffractograms of the precursor mixture calcined at 1200 °C, 1300 °C, and 1500 °C for 2 h.
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Table 1. Physical characterization of the anodic sludge.
Table 1. Physical characterization of the anodic sludge.
PropertyValueUnits
Humidity53.7 ± 0.5%
Bulk Density0.7 ± 0.1g cm−3
True Density2.2 ± 0.1g cm−3
Surface area13.7 ± 0.5m2 g−1
Size particle (d80)4.5 ± 0.2mm
Table 2. Chemical composition of the AS without treatment.
Table 2. Chemical composition of the AS without treatment.
Elementwt (%)
Al26.6 ± 0.2
Na1.5 ± 0.2
Si0.6 ± 0.05
Mg0.6 ± 0.05
S2.0 ± 0.01
Ca0.5 ± 0.05
Fe0.4 ± 0.05
P0.2 ± 0.02
K0.04 ± 0.01
Cr0.02 ± 0.01
Ni0.1 ± 0.01
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Acosta, F.; Feijoo, C.; Sangurima-Cedillo, A.S.; Guevara, A.; Aragón-Tobar, C.F. Upcycling Anodic Sludge from Aluminum Anodizing: Leaching Efficiency and Thermal Conversion into Refractory Materials. Sustainability 2025, 17, 8491. https://doi.org/10.3390/su17188491

AMA Style

Acosta F, Feijoo C, Sangurima-Cedillo AS, Guevara A, Aragón-Tobar CF. Upcycling Anodic Sludge from Aluminum Anodizing: Leaching Efficiency and Thermal Conversion into Refractory Materials. Sustainability. 2025; 17(18):8491. https://doi.org/10.3390/su17188491

Chicago/Turabian Style

Acosta, Fausto, Cristhian Feijoo, Alfredo S. Sangurima-Cedillo, Alicia Guevara, and Carlos F. Aragón-Tobar. 2025. "Upcycling Anodic Sludge from Aluminum Anodizing: Leaching Efficiency and Thermal Conversion into Refractory Materials" Sustainability 17, no. 18: 8491. https://doi.org/10.3390/su17188491

APA Style

Acosta, F., Feijoo, C., Sangurima-Cedillo, A. S., Guevara, A., & Aragón-Tobar, C. F. (2025). Upcycling Anodic Sludge from Aluminum Anodizing: Leaching Efficiency and Thermal Conversion into Refractory Materials. Sustainability, 17(18), 8491. https://doi.org/10.3390/su17188491

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