Aluminum smelting residue upcycling via targeted fluoride extraction and alkaline-driven cryolite synthesis.

Abstract

Introduction

The electrodes in the aluminum electrolytic cell were composed of carbon-based materials. During the electrolysis process, aluminum oxide functioned as the raw material, while cryolite served as the solvent, leading to the production of liquid aluminum via reduction within the electrolytic cell (Ratvik et al., 2022). The carbon anodes facilitated the conduction of electric current and were consumable materials that required regular replacement (Hou et al., 2024). Simultaneously, the carbon anodes were continuously eroded and washed by the electrolyte and molten aluminum, undergoing uneven combustion and selective oxidation, which caused carbon particles from the anode surface to detach and enter the electrolytic cell, thereby forming spent carbon anode (SCA) (Chen et al., 2022). The continuous accumulation of SCA adversely affected conductivity, increased power consumption, and reduced the lifespan of the electrolytic cell (Mao and Zhang, 2021). Regular dredging operations were implemented to mitigate the adverse effects of SCA on the electrolysis process. The formation of SCA is an inherent and consistently generated byproduct during aluminum electrolysis. Due to its elevated fluoride content, numerous countries classify it as hazardous waste. Unmanaged SCA, characterized by its elevated concentration of fluorides, poses a significant threat to the surrounding ecological environment (Andrade-Vieira et al., 2011). Statistics reveal that approximately 5–15 kg of SCA is generated per ton of primary aluminum output (Yang et al., 2020b). In 2024, global primary aluminum production amounted to 72.86 million tons. Over the past five years, the growth rate of production has slowed and stabilized, with global SCA expected to accumulate at a rate of 70 tons per year (IAI, 2025).

Fluorides represent both a source of hazardous waste and a valuable component with recyclability potential. Various processes for the disposal and recovery of SCA have been developed. High-temperature disposal methods encompass two models: the first involves combusting carbon materials to obtain electrolyte resources, resulting in low product-added value and the emission of CO₂ (g) (Li et al., 2022). The second model exploits the volatility differences between carbon materials and fluorides in an inert gas environment, wherein fluorides are collected through vapor condensation while carbon materials remain at the bottom. This method involves thermal treatment and vacuum distillation processes, which incur significant disposal costs and produce fluorine-containing emissions (Li et al., 2019, Yang et al., 2020a, Zhao et al., 2021a). Alkaline melting converts insoluble fluorides into soluble forms through solid-solid reactions within a specific temperature range, followed by water washing for removal, thereby complicating the process (Yang et al., 2020b, Yao et al., 2020). Combined disposal with other hazardous wastes is commonly employed to recover valuable metals from solidified fluorides, often in conjunction with copper slag (Kuang et al., 2023), electroplating sludge (Yu et al., 2021), red mud (Xie et al., 2020), and aluminum ash (Tang et al., 2024); however, these methods remain at the experimental stage.

Hydrometallurgy encompasses acid leaching, alkali leaching, and combined acid-alkali leaching, all of which effectively dissolve fluorides and facilitate their separation from carbon materials. This versatility in leaching techniques allows for tailored processes that can optimize the recovery of valuable materials from waste carbon sources. However, the acid leaching process inevitably generates HF (g). The addition of Al³⁺ can suppress HF generation but may reduce the reaction rate, typically extending the process to over 12 h (Ma et al., 2023). Alkaline leaching has struggled to remove substances such as CaF₂. The two-step acid and alkaline leaching processes increase the complexity of the procedure and generate significant quantities of waste liquids that necessitate secondary recovery (Ma et al., 2024). Furthermore, hydrometallurgical methods face challenges such as complex process flows, incomplete removal of leaching impurities, and waste liquid generation, all of which hinder large-scale application. Therefore, a straightforward and efficient chemical leaching synthesis process is essential for industrial applications.

This work achieves the harmlessness of SCA and the recovery of fluoride through simple alkaline leaching. The types and forms of fluorides in the SCA were determined via analysis of raw materials and thermodynamic calculations, thereby elucidating the reaction dynamics. The ratio of fluorides involved in the reaction was optimized to reduce reagent usage and enhance efficiency. Key factors affecting the liquid-solid ratio were determined considering the nature of the reaction and the resulting synthetic products. The process for recovering valuable elements was optimized based on the ionic forms present in the alkaline leaching solution. The effects of the pH, aging time, temperature, F/Al ratio, and acid concentration adjustments on the recovered products were systematically investigated. This process successfully established a closed-loop cycle for the safe disposal of SCA, as well as the separation and resynthesis of fluorides.

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