Utilizing γ- and θ-Alumina from Aluminium Black Dross for Catalytic Applications: Hybrid Sustainable Pyro–Hydrometallurgy and Residue Heat Utilization

This study is aimed at extracting γ- and θ-alumina from recycled black aluminium dross for catalytic applications, employing a novel hybrid pyro–hydrometallurgical approach as a waste management technique. The process begins with the extraction of elements from aluminium black dross, controlled heating at 680 °C leverages residue heat resources, essential in optimizing resource utilization. Further alkali metal halides addition is done to significantly enhance efficiency in melting aluminium–low magnesium (≤ 1%) scraps. Optimization in filtration was conducted at 900 RPM and temperatures near 50 °C to refine the purification process and eliminate impurities. Slow calcination was done at 100 °C for over 40 min, the salt cake allowing dissolution in a 3 M NaOH solution to achieve a critical pH value of 11.72. The precise pH neutralization was done by using a 3:1 H₂SO₄ solution to form aluminium hydroxide through the precipitation method at a pH level of 7, a pivotal precursor in alumina extraction. A detailed analysis of post-low-temperature calcination, ranging from room temperature to 800 °C for 10 min, was done. Furthermore, a thorough examination of residues and extracted γ- and θ-alumina was done using XRD, SEM–EDS, and ICP-OES techniques. About 79% of alumina was recovered from black aluminium dross. The proposed technique extracted alumina with a purity of 94.9% with a particle size of 318 nm.