Experimental work has been performed to investigate the precipitation mechanism of aluminum hydroxide phases from sodium aluminate/sodium carbonate pregnant solutions by carbon dioxide gas purging. Such solutions result from leaching calcium aluminate slags with sodium carbonate solutions, in accordance with the Pedersen process, which is an alternative process for alumina production. The concentration of carbonate ions in the pregnant solution is revealed as a key factor in controlling the nature of the precipitating phase. Synthetic aluminate solutions of varying sodium carbonate concentrations, ranging from 20 to 160 g/L, were carbonated, and the resulting precipitating phases were characterized by X-ray diffraction analysis. Based on the results of the previous carbonation tests, a series of experiments were performed in which the duration of carbonation and the aging period of the precipitates varied. For this work, a synthetic aluminate solution containing 20 g/L free Na₂CO₃ was used. The precipitates were characterized with X-ray diffraction analysis and Fourier-transform infrared spectroscopy.

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.

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.

Bauxite residue (BR), an alkaline industrial waste, is a major byproduct of the alumina production process known as the Bayer process. The Bayer process generates a byproduct known as bauxite residue (red mud). This residue was leached with sulfuric acid in a pilot plant of Mytilineos S.A to recover scandium. Scandium was selectively recovered in pilot-scale experiments using ion exchange. This process generated a raffinate solution containing various dissolved impurities such as aluminum, sodium, calcium, iron, and mainly sulfate ions. The regeneration of the raffinate solution can reduce the cost of the process and minimize the use of H₂SO₄. The potential of raffinate recycling as a technology for reducing the usage of H₂SO₄ in the leaching process was evaluated by neutralizing bauxite residue with a raffinate solution before the leaching step. This study aimed to investigate the feasibility of using a raffinate solution for the neutralization of BR, enabling its reuse and improving the process’s environmental sustainability. The neutralization process decreases the pH value of BR pulp with 50% w/v pulp density from 11 to 6. Experimental investigations were carried out to assess the leaching behavior of bauxite residue compared to neutralized bauxite residue (NBR) using sulfuric acid. The obtained results were compared to evaluate the effectiveness of NBR as a substitute for bauxite residue in the leaching process. The consumption of acid during the leaching of neutralized BR was three times less than the BR leaching. An X-ray diffraction (XRD) analysis of BR and NBR was conducted to determine the mineralogical phases of the materials. The results of the study provide valuable insights into potential ways to reduce the cost of the BR leaching process, while also improving its environmental impact by recycling valuable materials.

The objective of this study is the determination of the various stages of deformation of brecciated marbles, under uniaxial compression, and the application of regression techniques, to investigate the relations of these stages, with physical, dynamical, and mechanical properties. Therefore, a total number of fifteen specimens, prepared from samples obtained from various locations from the southern part of the East Attica Prefecture, Greece. The laboratory program involved the determination of the uniaxial compressive strength (UCS), the crack initiation (C_ci) and crack damage stress (C_cd) levels, the static elastic modulus (E_s), as well as the basic physical and dynamical properties, such as the dry density (γ_d), the effective porosity (neff) and the ultrasonic velocities of both primary (V_P) and secondary (V_S) waves. Results obtained from the mechanical tests reveal that the onset of stable (C_ci) and unstable (C_cd) crack growth varies between 0.19–0.38 and 0.66–0.92 of the UCS values, respectively. Applying simple nonlinear regression models, it was found that crack initiation and crack damage stresses, decrease exponentially with the increase of effective porosity, while increase exponentially with the increase of static elastic modulus, dynamic elastic modulus (E_d), and the primary wave velocity.