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The effect agitation on the leaching efficiency of REE from bauxite residue in 1.5 M H2SO4 at 90 °C (with and without microwave pretreatment) within 30 min residence time. The dashed lines represent the maximum leaching efficiency that could be achieved using 3 M HNO3 at 90 °C without microwave pretreatment. Error bars represent the standard error of the mean for four replicates.
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Some rare earth elements (REEs) are classified under critical materials, i.e., essential in use and subject to supply risk, due to their increasing demand, monopolistic supply, and environmentally unsustainable and expensive mining practices. To tackle the REE supply challenge, new initiatives have been started focusing on their extraction from alt...
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Over 100 million tonnes of bauxite residue is produced
annually as a by-product of the Bayer process. Bauxite
residue utilisation represents both a challenge and an
opportunity for the alumina industry. Bauxite residue is a
fine-grained, complex mixture of numerous mineral
phases, which varies based on refinery operating conditions
and bauxite ore....
Citations
... Under atmospheric circumstances, a single acid could not entirely dissolve red dirt, according to this study. Reid et al. (2017) examined the REEs' leaching from red mud from Canada under atmospheric pressure using various acids. For 30 min, HCl, HNO 3 , and H 2 SO 4 were employed at varied leaching settings with concentration of acids ranging from 0.5 to 3 mol/L and temperature ranging from 25°-90°C. ...
... The kinetic modeling of nickel leaching from the spent catalyst was carried out within 6 h, and four models were evaluated based on a literature review for leaching: [30][31][32][33][34] (1) diffusion control through the fluid film (Eq 2); (2) solid product diffusion control (Eq 3); (3) surface chemical reaction control (Eq 4); and (4) porous product layer (Eq 5). In the equations, k 1 , k 2 , k 3 , and k 4 (h À1 ) are the rate constants, t is the time reaction (min), and x is the fractional conversion of nickel. ...
Nickel catalysts are widely applied in the chemical and petrochemical industry due to high catalytic activity and low cost. However, these catalysts are often irreversibly deactivated, leading to residue generation. For this reason, recycling is necessary to reduce the environmental impact of landfills and to obtain valuable metal from the secondary source. Hydrometallurgical recycling routes have been investigated to provide nickel from secondary sources. A catalyst from the petrochemical process contained 44.5% Ni and was calcined at 600°C for 3 h to remove organic compounds. Then, the material was ground, and leaching experiments were performed using H2SO4. The effects of temperature, concentration, solid-liquid ratio, particle size, and time were evaluated. The best leaching conditions were 100°C, 2 mol/L H2SO4, solid-liquid ratio 1/5, d50 8.5 µm in 8 h with 97.4% of nickel leaching (116 g/L). The leach solution was purified using chelating ion exchange resins. Three resins were studied with different functional groups: iminodiacetate, HPPA, and aminophosphonic. The effect of pH in the batch and removing contaminants in a continuous system were evaluated. Iron removal achieved 92.2% efficiency using aminophosphonic resin in a continuous process using four columns. The recovery of nickel in the process reached 94.3%.
... Conversely, (Eq. (5)) is applied to leaching scenarios in which the diffusion process predominates as the rate-controlling step, as illustrated by (Reid et al., 2017). (1 ¡ 3(1 ¡ α) ...
... Since the kinetic behavior presented for almost all REE was very similar ( Fig. 4 and 5), the leaching mechanism was investigated only for the sum of all REE. For the fitting, the experimental data obtained up to 10 min for the US-assisted leaching and up to 20 min for leaching at silent condition, that is, the experimental data obtained up to the time required for each system to approach equilibrium, were considered [46,47]. The results obtained from the shrinking core model fitting are shown in Fig. 6. ...
... Their extractions from bauxite residue could finally be used for making hydrogen-storage alloys (e.g., La), catalysts (e.g., Ce) or magnets (Nd, Pr). Also, the contained Sc (20-135 ppm) could be recovered and used as an alloying agent for highperformance aluminum alloys for future light weight aircraft transportation (Botelho Junior et al., 2020;Botelho Junior et al., 2021;Narayanan et al., 2018;Reid et al., 2017). Efforts to develop a more circular economy progressed by the European Commission has resulted in renewed focus on how the major global challenge of bauxite residue can be used as an alternative resource to ensure a sustainable supply of REEs needed for the energy transition to a low-carbon future (Commission, 2018;Panda et al., 2021). ...
The solid waste, bauxite residue, is the main sustainable development challenge for the alumina industry as large volumes of the material must be maintained indefinitely in long-term storage facilities unless complete site remediation can be achieved. Efforts in bauxite residue reutilization have recently shifted from its use as an additive in cement and ceramics manufacturing to development of processes for the extraction and production of metals and metal compound products. A hydrometallurgical approach is considered here and factors such as reaction temperature, time and acid concentration were examined to systematically characterize the leaching behavior of critical rare earth elements and how they correlate among each other and with major elements in the bauxite residue. The study showed that, within the reaction time, low temperature and low acid concentrations primarily caused release of Al, Si and a minority of La, while a majority of Fe coupled with Ce, Sc or V dissolved when higher temperatures and acid concentrations were applied. The statistical significance of the correlations was confirmed by analysis of correlation coefficients among elemental concentrations in leachates using machine learning mathematical models. Management of silica gel formation is a challenge in the hydrometallurgical treatment of bauxite residue, therefore, the silica gel free formation region was identified. Based on the stability field of silica gel formation, a stepwise acid leaching process for bauxite residue is proposed to facilitate downstream processing of the critical metals.
... The by-product obtained is red mud or bauxite residue. It is then washed to recover caustic soda and then dumped off (Reid et al., 2017;Samal et al., 2013). A schematic representation of the Bayer process has been depicted in Fig. 1. ...
Exhausts or emission from industries/automobiles/indoor appliances is one of the most prominent sources of air pollution. Innumerable noxious gases have been identified and been recurrently treated through various technologies from past many decades. Cumulative studies suggest that air pollutants affect the respiratory and cardiovascular systems along with the central nervous system, may it be directly or indirectly. In particular, acquaintances to such air pollutants in early life can lead to developmental delays and may stunt neurological development. This review presents the recent technologies that have been tested at the laboratory level as well as in situ utilizing one of the abundantly available industrial wastes, i.e. red mud. Unlike the conventional expensive catalysts, red mud provides a cheaper alternative in the treatment of toxic exhaust gases from various sources. Furthermore, the review identifies the gap through which experts from other disciplines can explore the employment of red mud in the comprehensive spectrum of pollution control.
... The exploitation of non-ore-related secondary sources at the end of products' lives, often referred to as 'urban mining', has great potential to safeguard CRM supply [2,[7][8][9][10]. On the other hand, landfilled stocks and freshly produced flows of rare-earth-containing industrial by-products represent additional CRM sources [11]. ...
The lack of high-grade scandium (Sc) ores and recovery strategies has stimulated research on the exploitation of non-ore-related secondary sources that have great potential to safeguard the critical raw materials supply of the EU's economy. Waste materials may satisfy the growing global Sc demand, specifically residues from titanium dioxide (TiO2) production. New technologies are being developed for the recovery of Sc from such residues; however, the possible environmental impacts of intermediary products and residues are usually not considered. In order to provide a comprehensive ecotoxicity characterisation of the wastes and intermediate residues resulting from one promising new technology, acid-resistant nanofiltration (arNF), a waste-specific ecotoxicity toolkit was established. Three ecotoxicity assays were selected with specific test parameters providing the most diverse outcome for toxicity characterisation at different trophic levels: Aliivibrio fischeri (bacteria) bioluminescence inhibition (30 min exposure), Daphnia magna (crustacean) lethality and immobilisation (24 h exposure) and Lemna minor (plant) growth inhibition with determination of the frond number (7 d exposure). According to our results, the environmental impact of the generated intermediate and final residues on the aquatic ecosystem was mitigated by the consecutive steps of the filtration methods applied. High and statistically significant toxicity attenuation was achieved according to each test organism: toxicity was lowered based on EC20 values, according to the A. fischeri bioluminescence inhibition assay (by 97%), D. magna lethality (by 99%) and L. minor frond number (by 100%), respectively, after the final filtration step, nanofiltration, in comparison to the original waste. Our results underline the importance of assessing chemical technologies' ecotoxicological and environmental impacts with easy-to-apply and cost-effective test methods to showcase the best available technologies.
... Table 1 presents the Sc concentration in different Bauxite Residues worldwide as reported in literature. (Reid et al., 2017) It has been estimated that 70% of the world's Sc resources might be found in bauxite minerals and bauxite residue (Petrakova et al., 2016). Scandium is a very rarely concentrated element, making commercially usable deposits of this critical material very rare. ...
... Correspondingly, in Canadian BRs, three different acids were examined. H 2 SO 4 selected as the most appropriate, based on the leaching efficiency, cost and practical part (Reid et al., 2017). In the kinetic model, the process of BR's REEs leaching is described by the expansion of acid through the boundary layer at the boundary of grain. ...
... In the kinetic model, the process of BR's REEs leaching is described by the expansion of acid through the boundary layer at the boundary of grain. Pretreatment of BR in direct microwave exposure before leaching, nano-sized pores were grown on the surface of grains, allowing for an increase in REE leaching efficiency, which means 40,0-64,2% Sc and 54,3-78,7% Nd, no results on recoveries for other main metals are given (Reid et al., 2017). ...
... Liu et al. [24] focused on recovering elements (such as aluminum, sodium, iron, etc.) from Bayer red mud and using these residues as raw materials to extract rare elements or produce construction materials. Several researchers investigated the extraction of REEs from red mud, possibly alongside other valuable metals [25][26][27]. Khanna et al. [28] evaluated the suitability of red mud as an iron resource and briefly reviewed its utilization in some commercial and industrial operations. However, these utilization strategies are poorly economics and challenging to implement industrially. ...
In 2021, global alumina production reached over 135 million tons and discharged approximately 200 million tons of red mud, leading to severe environmental safety hazards. More than 70% of raw materials originated from gibbsitic bauxite. This work reviews existing and potential process from open literature to treat gibbsitic bauxite and reduce red mud discharge from the alumina production process, which are divided into two categories. The first strategy involves non-Bayer methods such as smelting reduction, reduction roasting, sub-molten salt, ammonium sulfate roasting, acid leaching, and calcification–carbonation. The current obstacle to implementing these methods is the lack of economics, making industrial application challenging. The second potential industrial application strategy is to modify the current Bayer process, with critical measures involving the enhancement digestion of inert aluminum-bearing minerals, dissociation and recovery of iron minerals, and separation of desilication products. Analyzing the reaction mechanism and process flow clarify that the modified Bayer method is more favorable to the cost-effectively and synergistic extraction of aluminum, iron, and other elements. The red mud obtained by the modified Bayer process can be efficiently utilized. Therefore, a mini-summary of the areas where red mud is expected to be substantially dissipated (e.g., the steel industry, cement building materials, and ecological restoration of red mud stockpiles) is provided. The review can advance the current technology on the comprehensive utilization of gibbsitic bauxite, especially process discharge reduction approaches and large-scale abatement ways for red mud. Our results can contribute to the future development of sustainable green alumina production.
... They found that 80% Nd, 99% Y, and 99% Dy leaching efficiency from PG was achieved at the optimum microwaving conditions of 15 min irradiation (2.45 GHz) at 1200 W. The effects were caused by changes in the microstructure and mineralogy of the PG. Reid et al. [68] demonstrated that during microwave pretreatment, the mineral water was heated and vaporized, which could cause nanosized cracks and pores in the mineral, allowing enhanced infiltration of leaching agent into the particle matrix and facilitating the extraction of elements from PG. The PG morphology had a nonlinear progression with microwave irradiation time, which was due to the factor that the drying rate of microwaved gypsum depended primarily on the particle moisture content (a nonlinear trend) and the microwave power [69]. ...
... The PG morphology had a nonlinear progression with microwave irradiation time, which was due to the factor that the drying rate of microwaved gypsum depended primarily on the particle moisture content (a nonlinear trend) and the microwave power [69]. It was observed that, as the microwave irradiation duration increased, the PG morphology had changed gradually, resulting in increased surface roughness and particle porosity [68]. In contrast, there were no morphological changes when the PG samples were heated in a conventional oven at 100 or 150 • C. The research of Yang et al. [70] had shown that microwave irradiation could induce the formation of CaSO 4 ·0.5H 2 O, which required temperatures exceeding 107 • C, by creating localized hot spots. ...
... In the study of Lindroth et al., when a sample of wet gypsum was exposed to 1400 W for 2 min, a small amount of CaSO 4 ·0.5H 2 O was produced, with no CaSO 4 production [71]. In the study of Adrian Lambert et al. [68], as the microwave power and duration was increased, there was a sharp drop in CaSO 4 ·2H 2 O content, accompanied by a proportional increase in CaSO 4 ·0.5H 2 O and CaSO 4 . Thus, it could be seen that microwave heating and irradiation caused a distinct change in the crystal structure of the PG sample, causing a shift from the doubly hydrated gypsum to hemihydrated gypsum to anhydrous gypsum. ...
As phosphogypsum constitutes a large amount of solid waste material, its purification treatment and comprehensive utilization have close connection with economic development and ecological environmental protection. For the moment, the storage quantity of phosphogypsum is still rising as a result of the increasing phosphate fertilizer production to meet the food demand in China. This paper summarizes the generation process, impurity removal treatment (physical method, chemical method, heat method), high-value utilization (nanometer calcium sulfate whisker, nanometer calcium carbonate) of phosphogypsum material and some existing problems. It puts forward some views on the challenges in this field and the direction of future development. It is hoped that the investigation and summary in this paper could supply some significant information for the impurity removal and high-value utilization of phosphogypsum material as a contribution to sustainability.
