Chemical leaching − Flexible precipitation for sustainable recovery of fluoride from aluminum electrolysis multisource hazardous waste

Abstract

The spent carbon materials, including anode and cathode, generated from the aluminum electrolysis process contain high fluoride levels and are unavoidable hazardous solid wastes. Chemical leaching, as a promising strategy for large-scale industrial treatment, must address the challenge of secondary recovery of the solution. The co-recovery mechanism preparing various fluorides from acidic and alkaline fluoride-aluminum solutions was investigated. In this process, the F/Al molar ratio and the pH value of the substrate environment are critical factors for achieving synergistic recovery. Different acid and alkali adjustment modalities resulted in a diverse array of products. For instance, adding an alkali to an acid solution generated aluminum hydroxyfluoride hydrate, whereas the reverse sequence yielded cryolite. The precipitation mechanism of acidic solutions is characterized by a complex interplay of multiple substances, whereas the process in alkaline solutions is comparatively straightforward. Specifically, as the pH value increases, the sequence of phase transformations progresses through aluminum hydroxyfluoride hydrate, chiolite, aluminum hydroxide, and cryolite. In addition to pH, localized over-alkalinity was the primary factor contributing to impurity generation. The precipitation phase in an alkaline substrate environment is significantly better controlled. During this process, pure cryolite can be synthesized by adjusting the F/Al molar ratio to 6.0 using the neutralization solution. The synergistic synthesis of fluoride from both acidic and alkaline fluorine-containing solutions is not only feasible but also anticipated to effectively address the significant issues of water swelling and secondary solution recovery that are commonly encountered in industrial applications.