Extraction of Aluminium from Kaolin: a Comparative Study of Hydrometallurgical Processes

Abstract -The increase of aluminium demand globally, raises the interest of developing alternative technologies to produce alumina from non-bauxitic sources, especially clays. This paper studies the extraction of aluminium from kaolin through leaching process, using aqueous solutions of acids and bases, as leaching agents. For this study, crude kaolin excavated in Milos Island, Greece, was used. Before leaching, crude kaolin was calcined; calcination achieves the dehydroxylation of kaolinite, which is the main mineralogical phase of kaolin and its transformation to metakaolin, an amorphous Al-Si phase from which aluminium is easily leached. Calcination of the crude kaolin used in this work was taken place at 850 o C for 2 h, given that these conditions revealed as optimum in the relevant experimental investigation. Leaching of aluminium from the calcined kaolin was performed with aqueous solutions of mineral acids (HCl, HNO 3 and H 2 SO 4 ), organic acids (C 2 H 2 O 4 , C 2 H 4 O 2 and C 6 H 8 O 7 ) and bases (NaOH, KOH and NH 4 OH). All leaching experiments were performed under constant conditions, regardless the leaching agent used. According to the experimental results, acids were proved more effective in aluminium extraction, than bases. However, aluminum extraction was accompanied by simultaneous extraction of Si which comprises an important impurity for the production of alumina from the pregnant liquor. Based on this consideration, oxalic acid was revealed as the most effective leaching agent, yielding more than 75% extraction of Al and less than 5% Si extraction

Distribution of uranium, thorium and potassium in the Bayer process

Uranium, thorium, potassium and their decay product mass flows were analysed in the Bayer process. Gamma-ray spectroscopy was used to measure the radionuclide content in samples provided by Aluminium of Greece and to model their mass flows. We observed that at any analysed stage, the radionuclide content does not exceed the allowed safety limits set in the European Basic Safety Standard. Another important observation is that a minor portion of uranium from bauxites (3%) ends up in alumina, while the rest is accumulated in the bauxite residue (BR). All of the 226Ra (long-lived decay product of uranium), as well as all decay products of thorium accumulated in the BR. We observed accumulation of 40K in the process liquors, while this radionuclide was not found in the alumina

Radiological assessment of the Bayer process

Naturally occurring radionuclides were studied through the Bayer process by calculating their mass flows. Aluminium of Greece (AoG) provided sample materials and plant data from several process stages. Measurements of radionuclide concentrations were carried out by gamma-ray spectroscopy. The performed measurements show that in the specific case of the AoG plant, the majority of the natural radionuclides were introduced with karst bauxites, which showed higher activity concentrations for nuclides compared to lateritic bauxites. Most of these nuclides accumulated in the bauxite residue, while only a minor portion of uranium isotope 238U was found in alumina, corresponding to 3% of its input value. Uranium was observed to partially dissolve in the process liquors similarly to 40K, whereas the latter was not associated with aluminium hydroxide. All the materials studied in the current research work had radionuclide concentrations well below the exemption limits set by EURATOM Basic Safety Standard, indicating that these naturally occurring radionuclides do not pose a radiological hazard for workers of the AoG plant or the public.https://doi.org/10.1016/j.mineng.2019.04.01

Review of Technologies in the Recovery of Iron, Aluminium, Titanium and Rare Earth Elements from Bauxite Residue (Red Mud)

Bauxite residue (BR), also known as red mud (RM), is the main waste product of the Bayer process in producing alumina from bauxite ores and is currently being managed via stockpiling, storage within settling pond and landfilling. However, these approaches are disadvantageous in terms of potential risk for the environment and the processing of RM as raw material brings about its own challenges. In recent years, research has been focused on the reuse of RM and the recovery of metals to transform the waste into a more viable secondary source of raw materials. This paper will further explore and thoroughly review technologies and processing techniques that have been previously available as well as ones that are currently being developed. The scope of this paper will be focused on iron, aluminium, titanium and rare earths

Carbothermic Reduction of Bauxite Residue for Iron Recovery and Subsequent Aluminium Recovery from Slag Leaching.

Industrially and historically, alternative processes were developed to recover alumina from low-grade ores of non-bauxitic origin as well as for economic reasons. The Pedersen process from Norway was adapted towards high ferruginous and high silica containing bauxitic ores. This paper considers the adaptation of the Pedersen pyro-hydrometallurgical route of reductive smelting with lime and coke to recover iron and aluminium from bauxite residue (BR), which is the by-product of the Bayer process where aluminium is typically lost in complex desilication product matrices. Greek BR was firstly smelted in an Electric Arc Furnace (EAF) to recover pig iron (>95 %). The fluxing strategy entailed feeding lime to favour lower operating temperatures (1500–1550 °C) and slag viscosities, and to produce a slag containing calcium aluminate phases (12CaO·7Al2O3, 3CaO·Al2O3 and CaO.Al2O3). The calcium aluminates were identified as thermodynamically favourable in descending order to form NaAlO₂ when leached in Na₂CO₃ solution (ΔGrxn298K= -782, -188 and -68 kJ/mol, respectively). Leaching temperature, Na2CO3 concentrations, residence time and different particle sizes were varied to investigate the leachability of slags for aluminium recovery; up to 43 % aluminium was recovered with 95 % of silicon content remaining in leached residue. SEM studies also aided understanding of interstitial leaching mechanism

EAF carbothermic co-reduction of alumina and silica, for the direct production of Al-Si master alloy

Abstract A key alternative for primary aluminum production could be the carbothermic reduction of alumina. Developed concepts face problems with the high solubility of carbon in liquid aluminum, the presence of the oxycarbide slag and the high volatilization of aluminum at elevated processing temperatures. In order to overcome some of these obstacles, in the framework of the ENEXAL FP7 project, research into the co-reduction of alumina and silica was made, aiming at the direct production of Al-Si master alloys. The latter are key commercial products for all casting applications, accounting for approximately 30% of all aluminum demand. As predicted by the thermodynamic study and verified by lab scale EAF experiments, the presence of silicon in the system suppresses volatilization phenomena, limits the oxycarbide slag formation and reduces carbon solubility in the metal phase

STOCHASTIC MODELING AND SIMULATION OF FIBER EVOLUTION DURING MELT-BLOWING SLAG FIBERIZATION

Abstract Red mud fiberization is a process with remarkable potential, alleviating environmental pressure by transforming an aluminum by-product into mineral wool, thus to various marketable products. A promising mineral wool process is molten slag fiberization via an impinging air jet, which avoids mechanical wear and rotating parts. A high-temperature molten slag stream is subjected to high shear, intensive droplet generation and subsequent fiber elongation: nascent mineral fibers follow independent trajectories towards a collection chamber, often breaking up to form shots (off-spec product) which either fall away from or are embedded into the mineral wool product. Fiber and shot size distributions are pivotal as a means of ensuring mineral wool product quality. This paper focuses on stochastic modeling and simulation of molten slag fiber generation and evolution: the model encompasses published, validated correlations of dimensionless numbers within the well-defined cone of fiber dispersion. Fiber elongation and breakup along trajectories are explicitly studied, considering temperature-dependent slag transport properties and explicit variability of flight trajectory and operating conditions, towards achieving process optimization