Sustainable recycling of spent aluminium electrolyte for AlF ₃ and efficient fluoride recovery via an integrated pyro-hydrometallurgical process

The rapid expansion of the aluminium industry has generated significant quantities of spent aluminium electrolyte (SAE), a hazardous waste rich in aluminium and fluoride, making its eco-friendly recycling a critical priority. To address this, this study introduces a novel integrated pyro-hydrometallurgical process for the sustainable conversion of SAE into commercial purity aluminium fluoride and hydroxyaluminum fluoride. Kinetic analysis confirms that the pyrometallurgical conversion follows an interfacial solid-state double decomposition mechanism governed by the Jander diffusion model. The reaction initiates at the particle interfaces, where the formation of a dense AlF₃/Na₂SO₄ product layer acts as the rate-limiting barrier for ion transport. Guided by thermodynamic equilibrium analysis, a specific temperature range of 600-670 °C was identified to optimize reaction efficiency and minimize material adhesion. Experimental results demonstrated that a baking temperature of 660 °C, a reagent ratio of 0.95:1, and a reaction time of 3 h yielded an optimal aluminium fluoride purity of 98.1%. In the subsequent wet precipitation phase, fluoride recovery reached a remarkable 99.8% under optimized conditions (pH = 7, T = 50 °C). During this stage, pH regulation via NaOH proved essential for facilitating the coordination of F⁻ with Al³⁺ and OH⁻ ions.to form hydroxyaluminum fluoride. Characterization via XRD, XRF, and SEM confirmed the superior phase purity and uniform morphology of the products. This developed process offers a highly efficient, environmentally benign solution for SAE valorization, contributing substantially to resource recovery and the circular economy within the aluminium sector.