The present research article investigates the extraction of aluminum (Al) from an engineered CaO-Al₂O₃-SiO₂ slag by leaching with aqueous HCl under atmospheric pressure conditions. The slag is the by-product of an aluminothermic reduction process to produce metallurgical-grade silicon (Si) in a more sustainable way compared to conventional carbothermic reduction. One major challenge in the acidic leaching of aluminosilicate materials, like the slag treated in this study, is the possibility of SiO₂ gel formation during the leaching process. Extensive SiO₂ gel formation can make the separation of pregnant leach solution (PLS) from the leached residue impossible. Consequently, any acid leaching of aluminosilicate materials must be optimized for avoidance of these gelation phenomena. The present study first explores the leaching behavior of the calcium aluminosilicate slag in relation to the S/L ratio, with all other hydrometallurgical factors remaining stable (20.2% wt. HCl solution, 80 °C, optimized stirring rate), to determine at which value of this parameter SiO₂ gelation occurs. After determining the optimum S/L ratio for this system, an analysis of the behavior of Si in the PLS is presented, and the mechanism of SiO₂ gelation is explained based on critical assessment of these results against data provided from the scientific literature. It was found that the value of the pH of the PLS and the concentration of dissolved CaCl₂ and AlCl₃ are critical factors for the formation of filterable amorphous SiO₂. Under the optimum leaching conditions, PLS containing 37 g/L Al was obtained with concurrent avoidance of gelation phenomena.

Sustainable utilization of Bauxite Residue (BR) is currently one of the greatest challenges being tackled by the alumina industry, due to its high production rates and limited reuse options. The present work is concerned with the use of BR as a candidate metallurgical raw material for iron (Fe) production and aluminum (Al) extraction. In more detail, at first, BR undergoes reductive smelting to extract its Fe content and produce a slag of mainly calcium aluminate composition. In a second step, Al contained in the calcium aluminate phases is extracted hydrometallurgically by leaching with a Na₂CO₃ aqueous solution. The focus of the current study is the optimization of this leaching process, and it was performed in two stages. The first was a laboratory scale investigation on the main parameters affecting the extraction rate of Al. The second stage was performed in pilot scale and incorporated observations and suggestions based on the laboratory scale investigation. Laboratory work showed that more than 50% of aluminum could be easily extracted in less than 1 h, in 5% S/L, at 70 °C and with an 20% excess of Na₂CO₃. Pilot scale work, by successfully applying the suggestions derived from laboratory scale work, achieved an average Al extraction of 68% from a 10% S/L pulp, with a slag of optimized composition in relation to the one used in the laboratory scale.

Primary aluminum production is dependent on the Bayer process for Al₂O₃ refining and the Hall–Héroult process for Al production. Both these processes face serious sustainability challenges, while the overall Al supply chain is inflexible and prone to disruptions. One solution to the aforementioned challenges is the adoption of alternative Al₂O₃ production processes from a variety of primary and secondary raw materials. This research paper explores the potential use of bauxite tailings as an alternative secondary resource for alumina production, with an aqueous HCl process technology. Bauxite tailings are solid waste produced during the mining and beneficiation of certain bauxites before their digestion in the Bayer process. The research was conducted in two stages. Initially, 24 h trials using aqueous 5.9 M HCl solution and a 5:100 g/mL solid-to-liquid (S/L) ratio revealed complete dissolution of Al and near-complete dissolution of Fe. In the second stage, investigations involving higher S/L ratios under the same conditions confirmed that an S/L ratio of 20:100 g/mL represented the optimal parameters for bauxite tailings leaching, producing a 36.8 g/L Al and 25.2 g/L Fe concentration solutions. The PLS produced under the optimum conditions is suitable for downstream processing for the production of AlCl₃·6H₂O.

The Pedersen process is a method to produce alumina from Al-containing sources, and it is a more material-efficient method than the current commercial Bayer process, since the formation of bauxite residue (red mud) is avoided, and the bauxite can be holistically consumed. The smelting reduction (SR) part of the Pedersen process yields pig iron and a calcium aluminate slag, and the latter is a feedstock material for alumina extraction via alkaline leaching. In the present study, three different bauxite ores (Greek, Turkish and Jamaican) were smelted with lime to ease the process and control the slag chemistry and coke for the carbothermic reduction of iron oxides. The slags produced were analyzed with XRD, XRF, and EPMA to identify the phases and chemical compositions. According to the results, the slags composed of Al-containing leachable phases. Moreover, it is shown that the amount and distribution of both the leachable and non-leachable phases in the slags depend on the ore chemical composition. The results are discussed regarding the characteristics and potential leachability of the slags. Standard leaching tests were performed to examine the actual leachability.