The Hall–Heroult process is a vital technology for primary aluminum production and has been used for more than a century (Tarcy et al., 2011). The smelting method uses alumina (Al₂O₃) as raw material and cryolite (Na₃AlF₆) as a solvent to obtain metal aluminum liquid in a smelting cell by electrolysis. With the rapid development of the aluminum electrolysis industry, the amounts of various wastes of aluminum smelting cells are increasing year by year (Liu et al., 2020, Zeng et al., 2021). Anode cover is an important measure to maintain the energy balance of aluminum electrolysis cells (Zhou et al., 2015). Generally, in an aluminum smelting cell, cover material for covering anode carbon block is composed of alumina powder and sintered cryolite particles or powder with a certain granularity and thickness, so as to reduce the heat loss through the anode (Zhang et al., 2010). Therefore, the cover material also acts as a thermal insulation material upon the anode (Ma et al., 2019). Smelter alumina or crushed bath/alumina granular mixtures are used to cover the anode and the central channel. It can protect anodes from air burn, absorb fluoride fumes, control heat loss, and maintain overall heat balance (Li et al., 2017, Tao et al., 2005, Wijayaratne et al., 2011, Zhou et al., 2017).

Although the cover material was originally intended to be recyclable (Taylor et al., 2004, Zhang et al., 2013) in the actual production process, due to the operation of replacing the anode or taking out excess electrolyte, many aluminum electrolytes (e.g., Na₃AlF₆, Na₅Al₃F₁₄, and CaF₂) were brought into the cover material. This leads to the change of composition ratio and affects the crustal strength of cover material. To make the crustal strength, it is necessary to re-add a certain amount of alumina before covering the anode (Groutso et al., 2009, Gudmundsson, 2009). Inevitably, this would result in a continuous accumulation of anode cover materials. However, cover material is classified as a hazardous solid waste because of its high concentrations of fluoride. Currently, there are approximately 400,000 tonnes of waste cover material in China (Zheng, 2019). A large amount of accumulated cover material not only takes up a great amount of space but also wastes resources and threatens the environment. With the development of the world’s aluminum industry, the problem of saving resources and reducing environmental pollution has been widely considered. Life cycle assessment (LCA) has become the standard methodology for systematically assessing the environmental impact with a product (such as cover material), and the treatment and recycling of product is one of the most important aspects. Furthermore, due to increasingly stringent environmental laws, many aluminum smelters are also faced with great pressure, and urgently need a harmless recycling treatment for cover material.

Solid waste in aluminum electrolysis production, most researchers focus on spent pot-lining, carbon dust, and alumina ash, but there is little relevant literature and research on anode cover materials. Most researchers prefer to study how to separate fluoride and graphite carbon from spent pot linings and carbon dust, both of which contain carbonaceous materials, fluoride (e.g., NaF, Na₃AlF₆, and CaF₂), and aluminosilicate. The technologies of treating spent pot linings can be divided into pyrometallurgical and hydrometallurgical methods (Tropenauer et al., 2019). There is high–temperature combustion (Holywell and Breault, 2013, Li and Chen, 2010), acid-base leaching methods (Robshaw et al., 2020, Shi et al., 2012), salt leaching methods (Cao et al., 2014, Lisbona et al., 2012b), flotation (Li et al., 2014) and distillation methods (Wang et al., 2018). They tend to extract graphite carbon and useful fluoride by dissolving them in bases, acids, or solutions of aluminum salts. Shi et al. (2012) used the method of alkali (NaOH) leaching followed by acid (HCl) leaching to separate carbon from cryolite. Li et al. (2019) achieved the separation of carbon and cryolite after a leaching treatment using deionized water and acidic aluminum anodizing wastewater. At present, a suitable flotation agent is rare for the separation of alumina and cryolite. Wang et al. (2020) used sodium oleate as a collector and sodium carboxymethyl cellulose as an inhibitor to separate cryolite and alumina in cover material, but the separation effect was not obvious. For fluorides such as cryolite, Al³⁺ salts, in acidic conditions, have been favored as lixiviants (Besida, 2001, Lisbona and Steel, 2008, Wood et al., 2003; Kaaber and Mollgaard, 1996). Wu et al. (2020) reported that Al³⁺ salts solution can dissolve fluoride from waste aluminum electrolytes and the aluminum hydroxyfluoride hydrate (AHF, AlF_i(OH)_(3-i)·nH₂O) was precipitated from the leachate as a byproduct.

The advantage of this approach is that fluoride can be dissolved and fluoride is recycled as aluminum fluoride (Ntuk and Steel, 2016). Similarly, there is fluoride in the anode cover material. Therefore, it is potentially feasible to use Al³⁺ solution as a lixiviant to separate alumina and recycle aluminum fluoride from the anode cover material. Aluminum fluoride is the most important additive in aluminum electrolysis production. It is mainly used to adjust the cryolite ratio (CR) of aluminum electrolyte and is constantly consumed (Liu, 2006). Thus, it is necessary to continuously supplement aluminum fluoride in electrolytic production to ensure the normal operation of aluminum reduction cell.

In this work, a novel process for effectively treating waste cover materials with an Al³⁺ salt solution was developed. The process is mainly divided into two steps: i) Al³⁺ solution leaching, and ii) fluoride precipitation. The effects of leaching time, leaching temperature, Al³⁺ concentration, liquid to solid ratio (L/S), and stirring speed on the sodium and fluoride extraction were investigated systematically. Then aluminum and fluoride in the leachate were precipitated with alkali solution and aluminum fluoride was prepared further. In addition, XRD and SEM analyses were used to explain the phase transformation and microstructures during the whole process.