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, AlFi(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.
In this study, the treated anode cover material was sourced from an aluminum smelter in Liaoning Province, China. The chemical composition of the samples is shown in Table 1. The contents of aluminum, sodium, and fluoride in the cover material are 26.7 wt%, 15.0 wt%, and 35.7 wt%, respectively. Combined with the XRD pattern of the samples shown in Fig. 1(a), the main components of the sample are alumina (Al2O3), cryolite (Na3AlF6), and chiolite (Na5Al3F14). The SEM images and the distribution
The possible chemical reactions of fluoride dissolution during Al3+ solution leaching and AHF crystallization are summarized in Table 3. The reaction Gibbs free energy (?G?) of the above-mentioned reactions at atmospheric pressure in the temperature range of 20–100 °C was calculated, and the relationship between ?G? and temperature in Eqs. (2)–(7) are given in Fig. 3. The results of thermodynamic calculations show that the Gibbs free energy of all reactions at 20–100 °C is negative, and these
In this work, the Al3+ salt solution leaching process was investigated and applied for the resource recovery of waste cover material. The application of this new process is feasible to separate alumina and recycle fluoride from cover material of aluminum smelting. We herein investigated the effects of the Al3+ solution on the leaching of sodium and fluoride. The maximum extraction efficiency of sodium and fluoride of 98.1 % and 95.1 % was achieved when the cover material was leached in an Al3+
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was financially supported by the National Natural Science Foundation of China (Grant numbers 51874086, 51804071, 51904192, 51434005, 51529401, 51804069, 51804070); and the Fundamental Research Funds for the Central Universities (Grant number N2025024).
*Original abstract online at https://www.sciencedirect.com/science/article/abs/pii/S0892687522003508