Search

Cationic reverse flotation of kaolinite from bauxite is an alternative to adequate high silica bauxites to the specifications of the Bayer process. However, the presence of clay minerals influences the slurries rheology decreasing the flotation performance. The objective of this work was to investigate the use of ultrasonic pre-treatment on kaolinite flotation and contrast it with the traditionally used chemical dispersant. Two bauxite samples were characterized in terms of chemical and mineralogical composition, as well as particle size distribution. Flotation was accomplished after conditioning with sodium polyacrylate and/or pre-treatment in ultrasound. Determinations of viscosity, sedimentation yield, and screening followed by the analysis of available alumina and reactive silica were also conducted. By increasing the concentration of solids to more than 20 wt.%, the slurry behavior changed from a Newtonian to a non-Newtonian behavior, the viscosity increased, and the flotation performance decreased. The use of chemical dispersant and ultrasonic pre-treatment increased the flotation performance. The ultrasound had a more marked effect in slurries with higher concentrations of solids. This result is related to the synergic effect of particle dispersion and reduction of the slime coating promoted by kaolinite onto gibbsite particles. [ABSTRACT FROM AUTHOR]

Copyright of Nonferrous Metals (Extractive Metallurgy) is the property of Beijing Research Institute of Mining & Metallurgy Technology Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The diaspore is a typical representative of bauxite resources in China, which is the primary raw material for the Bayer process in alumina production, particularly in regions such as Shanxi, Guangxi, Guizhou, and Henan. Clarifying the phase transformations and reaction mechanisms of the silicon-containing minerals during the Bayer leaching process of diaspore is essential for improving the efficiency of alumina production. This article focuses on montmorillonite, which is one of the silicon-containing minerals of diaspore-type bauxite, investigating the reaction mechanisms and phase changes of montmorillonite under the high-calcium sodium aluminate solution system by using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Magic Angle Spinning Nuclear Magnetic Resonance (MAS–NMR) and Fourier Transform Infrared Spectroscopy (FTIR). The results show that montmorillonite dissolved and transformed into Na6(AlSiO4)6 (hydrated sodium aluminosilicate) under the high-calcium sodium aluminate solution system, and calcium oxide and sodium aluminate in the solution reacted to form (CaO)3Al2O3(H2O)6 (hydrated calcium aluminate). With the increase of reaction temperature, caustic alkali concentration (Nk), and reaction time, hydrated calcium aluminate and hydrated sodium aluminosilicate react and transform into Ca3Al2SiO4(OH)8 (hydrogarnet). Under the optimal reaction conditions of a 120 min reaction time, a temperature of 240 °C, an Nk of 240 g/L, and a CaO–to–SiO2 mass ratio (C/S) of 3.5:1, the montmorillonite reaction degree can reach a maximum of 93.71%. [ABSTRACT FROM AUTHOR]

Research on the digestion kinetics of gibbsite in the Bayer process contributes to understanding the reaction mechanism and promoting the reaction process. In this work, the non-isothermal kinetics of synthesized pure gibbsite digestion in NaOH solution was studied at the heating rates of 2.5 K min−1, 5 K min−1, 7.5 K min−1, and 10 K min−1 by high-pressure differential scanning calorimetry (HP-DSC). The apparent activation energy was calculated using the Ozawa method and the Kissinger method, then the most probable mechanism function of the reaction was determined using the Coats–Redfern method. The apparent activation energies calculated from the Ozawa and Kissinger methods were 62.56 kJ mol−1 and 61.86 kJ mol−1, respectively, indicating that the digestion of gibbsite was controlled by chemical reactions. The integral and differential forms of the most probable mechanism function were G(α) = 1 − (1 − α)1/3 and f(α) = 3(1 − α)2/3, which accorded with systolic spherical models. Furthermore, the kinetics equation of the gibbsite digestion in NaOH solution has been proposed. [ABSTRACT FROM AUTHOR]

The effective recovery of gallium from wastewater discharge in the Bayer process is promising for the long-term development of gallium resources. The adsorption and desorption behavior of gallium (Ga), vanadium (V), and aluminum (Al) ions on a strong acidic styrene cation exchange resin (JK resin) from a simulated Bayer solution was systematically investigated by static experiments. The results showed that the optimum conditions for separating Ga from V and Al were at low temperatures and short contact times, with 78.30%, 15.16%, and 6.63% of the adsorption efficiency at 25 °C and 60 min, respectively, for Ga, V, and Al. The adsorption kinetics of Ga3+ conformed to the pseudo-second order model, and the static saturation adsorption capacity was 18.25 mg/g. The Langmuir model fitted the adsorption isotherm of gallium well, and the maximum adsorption capacity was 1.11 mg/g at 25 °C. FT-IR spectroscopy and XPS showed that the mechanism of the Ga3+ adsorption was only related to the interaction of the oxygen atoms of the amide oxime group (C=NOH). The separation of Ga, V, and Al can be achieved by desorbing 98% of Al with low concentrations of ammonia and 90% of Ga with low concentrations of hydrochloric acid. The results indicate that JK resin is an efficient adsorbent for separating gallium and vanadium in alkaline solutions. [ABSTRACT FROM AUTHOR]

A large batch of cracks appeared in and around the welds of alumina-decomposition tank used in production of alumina by Bayer process. The crack morphology and metallographic structure were analyzed by means of SEM, EDS and optical microscope. As a crack source, pores expanded along grain boundaries and pearlite under the promotion of stresses to form final cracks in alkaline environment, resulting in leakage. The multi-stress cracking mechanism in alkaline environment is strain induced-anodic dissolution. IGPSCC (intergranular pearlitic stress corrosion crack) electrochemical corrosion model for alkali embrittlement of thick wall tank welds is established, which provides a reference for the study of weld cracking in alumina-decomposition tank after long-term operation. [ABSTRACT FROM AUTHOR]

Depending on the ore quality, a washing process can be conducted with the bauxite, which basically consists of scrubbing the ore and screening in order to increase the available alumina grade, i.e., the alumina extractable using the Bayer Process, and reduce the impurity content. Tailings are usually disposed of in a tailings dam in the form of a slurry, which is a mixture of solid particles and liquid, consisting mainly of ultra-fine kaolinite, making the dewatering operation challenging. To reduce the environmental impact, mining companies are studying alternative methods to dewater the tailings, and different dewatering methods are available worldwide. The use of new technologies to dewater the tailings has contributed to facing the challenges of achieving sustainable development with their disposal. The decanter centrifuges are already an option for operations for the Canadian oil sands, gold ore in Peru, and nickel in New Caledonia; they are also being tested for iron ore in Brazil. In the present work, bauxite dewatering using the decanter centrifuge was evaluated to understand more about the behavior of these materials and to investigate the effects of various process parameters on the solid recovery and solid content of the flows, using three different kinds of equipment. The results indicated that decanter centrifuges can be used to achieve a high concentration of solids in the cake, with values ranging from 60% to 80% solids per weight and a great clarification in the liquid phase (centrate) from 0 to 6% solids per weight, values which mean the solid phase is suitable for reutilization in the processing circuit. Additionally, the present work provides a better understanding of how different solid contents feed can affect the behavior of the equipment. [ABSTRACT FROM AUTHOR]

Addressing the issue that the Bayer process is not suitable for low-grade bauxite, carbochlorination was proposed to recover aluminum and silicon from low-grade bauxite. This study focused on the behavior of aluminum and silicon during the carbochlorination process of low-grade bauxite. The impact of various process parameters on the chlorination efficiency was investigated, and the chlorination mechanism and kinetics of aluminum and silicon chlorination in bauxite were analyzed and discussed. Under optimal experimental conditions, the chlorination efficiency of Al2O3 and SiO2 reached 94.93% and 86.32%, respectively. The carbochlorination of aluminum and silicon in bauxite adhered to a shrinking, unreacted core model governed by gas diffusion within the product layer. This process can be bifurcated into two stages. Additionally, calculations were conducted to determine the apparent activation energy and reaction order of the chlorination processes involving Al2O3 and SiO2. Examining the chlorination mechanism revealed that the bauxite carbochlorination encompasses transformations among various minerals. Notably, the aluminum component prefers to participate in the carbothermal chlorination reaction over silicon. [ABSTRACT FROM AUTHOR]

This paper describes a maintenance scheduling model for digester banks. Digester banks are network-connected assets that lie on the critical path of the Bayer process, a chemical refinement process that converts bauxite ore into alumina. The banks require different maintenance activities at different due times. Furthermore, the maintenance schedule is subject to production-related constraints and resource limitations. Given the complexity of scheduling maintenance for large fleets of digester banks, a continuous-time, mixed-integer linear program is formulated to find the cost-minimising maintenance schedule that satisfies all required constraints. A solution approach that employs lazy constraints and Benders decomposition is proposed to solve the model. Unlike generic implementations of Benders decomposition, we show that the subproblems can be solved explicitly using a specialist algorithm. We solve the scheduling model for realistic scenarios involving two Bayer refineries based in Western Australia. [ABSTRACT FROM AUTHOR]

The SO2 absorption ability of an aqueous suspension of red mud, which is waste from the Bayer process for bauxite, and the possibility of dissolution of metals using sulfurous acid are studied. It is found out that red mud can be used to neutralize sulfur dioxide of any concentration to an environmentally safe level. A statistical mathematical model is developed, and the absorption process is optimized. Under the optimal conditions, the mud is totally neutralized, and the sulfurous acid produced in the presence of an excess amount of SO2 dissolves 35% of all the aluminum in one step. Since iron precipitates as FeSO3, aluminum readily precipitates as hydroxide from solutions of iron-free aluminum salts. [ABSTRACT FROM AUTHOR]

Aluminum is produced from its primary bauxite ore through the Bayer process. Although Al is important nowadays in the development of humanity, its production leads to the generation of a huge amount of waste, called red mud. Globally, the estimation of the stock of red mud is about 4 billion tons, with about 10 million tons located in Turkey. The presence of rare-earth elements (REEs) in crucial materials such as red mud makes it a major source of these elements. A number of methods have been developed for treating red mud, which are employed globally to recover valuable products. The application of a suitable method for REE extraction from red mud is a way to overcome the supply risk, contributing to reducing the environmental issues linked to red mud pollution. The current review summarizes the research on red mud processing and examines the viability of recovering REEs from red mud sustainably, utilizing hydrometallurgy and biohydrometallurgy. [ABSTRACT FROM AUTHOR]

The aluminum hydroxide produced by the Bayer process contains a large amount of water which leads to the consumption of a large amount of heat for moisture removal in the calcination process, resulting in an increased energy consumption. The effects of temperature and microwave power on the dehydration ratio and the dry matter ratio of aluminum hydroxide were investigated. The characteristics of temperature variation during drying were discussed. X-ray diffraction (XRD), scanning electron microscopy (SEM), laser particle size, Fourier transform infrared (FTIR) spectroscopy, and dielectric property analyses were made to characterize the dried materials. The analysis results showed that within the range of bench-scale experimental parameters, the dehydration ratio was higher and the proportion of dry matter was lower at a higher final temperature. Within the range of pilot-scale experimental parameters, the dehydration ratio increased with the increasing microwave power from 500 W to 1500 W. XRD spectra revealed that when the final temperature exceeded 220 °C, a part of the aluminum hydroxide underwent a low-temperature phase transition to boehmite. The SEM images and a particle size analysis showed that there was no significant difference between the morphologies of the powder obtained by microwave drying and conventional drying methods. The powder obtained by both processes had an average particle size of around 80 μm. The dielectric constant and the dielectric loss of the dried material decreased greatly. [ABSTRACT FROM AUTHOR]

Corundum and transition alumina powders are commonly produced through the Bayer process—that is, by digestion of bauxite in a sodium hydroxide solution, followed by gibbsite precipitation, which contains significant amounts of Na+ ions. During the de‐hydroxylation of gibbsite, Na+ is associated with retardation of the structural phase transformations between transition alumina and corundum (α‐Al2O3) phases, although the exact role of Na is unclear. The influence of Na+ ions on the phase transformation of γ‐Al2O3 to α‐alumina was investigated by varying the concentration of NaOH in aqueous γ‐Al2O3 suspensions. The phase transformation behavior of γ‐Al2O3 was monitored through thermal analysis, X‐ray diffraction, scanning electron microscope, X‐ray photoelectron spectrometer (XPS), and transmission electron microscope. As the Na concentration in γ‐alumina suspensions increased, the amount of α‐Al2O3 decreased in samples that were heat treated at 1200°C for 2 h. XPS analysis indicated that washing transition alumina powders with NH4OH solutions had decreased the surface concentration of Na+ and facilitated the phase transformation to α‐Al2O3. [ABSTRACT FROM AUTHOR]

Aluminum is an important metal material in people's modern life, and bauxite is the main raw material for the production of aluminum. With the continuous development of domestic aluminum industry, the demand for bauxite resources is increasing year by year. However, the quality of domestic bauxite resources is poor, the supply is insufficient, and the degree of external dependence of bauxite is high, which brings a serious threat to the supply security of aluminum resources in China and restricts the sustainable development of China's aluminum industry. In order to cope with the scarcity of high-quality bauxite resources in China, the development of complex and difficult-to-process bauxite resources, such as high sulfur and high iron bauxite, is crucial to ensuring the strategic security of China's bauxite resources. China has a large amount of complex bauxite resources, such as high-sulfur and high-iron bauxite, which is more than 1.5 billion tons, accompanied by other strategic metal resources, which is urgently needed to be efficiently developed and advanced technology is used to realize the industrial application of this kind of typical poor and miscellaneous bauxite. Excessive sulfur and iron impurities in bauxite will have non-negligible effects on the process of aluminum extraction by Bayer process, such as corroding equipment and pipes, increasing alkali consumption and red mud, affecting the quality of alumina and so on. Therefore, starting with the present situation and characteristics of high-sulfur and high-iron bauxite resources in China, the current research progress of desulphurization and iron removal process of high-sulfur and high-iron bauxite has been summarized. The main desulfurization technologies such as flotation desulfurization, perorating desulfurization, biological desulfurization and physical iron removal, chemical iron removal and biological iron removal are summarized. The industrial application level of various processes is discussed and several typical examples of industrial application of high-sulfur and high-iron bauxite are given. It will provide theoretical support for the development and utilization of complex bauxite resources in China, make effective use of difficult-to-process aluminum resources, reduce China's dependence on foreign aluminum resources, and improve the supporting force of bauxite resources. [ABSTRACT FROM AUTHOR]

Bauxite is the basic raw material of aluminium used to extract alumina in Bayer's process. With the huge demand and the rapid development of the global alumina industry, bauxite consumption is rising and has reached more than 160 million tonnes per year. With such a high demand for aluminium, it is becoming more difficult to meet the consumption due to the gradual global decline of high-grade bauxite resources. In the current market scenario, treating bauxites containing more than 5% reactive silica (SiO2) is generally considered uneconomic due to excessive soda consumption in Bayer's process. The present study is conducted to examine the influence of the wet beneficiation process in the pre-treatment of low-grade and high-grade bauxite ores and a comparative analysis of the results obtained regarding alumina recovery and silica reduction. For this study, High-Grade (HG) and Low-Grade (LG) bauxite ore samples were collected from different bauxite mines of the East Coast region and subjected to wet beneficiation. Comparative chemical analysis of the before and after beneficiation samples revealed the wet beneficiation process to be significantly effective in the pre-treatment of LG bauxite samples with an average increment of 10.84% in the alumina content and 18.38% average reduction in silica after wet beneficiation. [ABSTRACT FROM AUTHOR]

To develop a better process for synthesizing sodium percarbonate and getting separation of the sodium salt, the equilibrium of the ternary system Na2SO4–H2O2–H2O and Na2CO3–H2O2–H2O at 5 °C was determined using the isothermal method, and the appropriate synthesis conditions for sodium carbonate from a theoretical perspective was discussed. The Na2SO4–H2O2–H2O system phase diagram contains one co-saturated point of Na2SO4·0.5H2O2·H2O and Na2SO4·10H2O. The Na2CO3–H2O2–H2O system phase diagram contains three crystal zones: Na2CO3·10H2O crystallization field, 0Na2CO3·1.5H2O2·H2O crystallization field and Na2CO3·2H2O2·H2O crystallization field. According to the phase diagram, the suitable raw material ratio of sodium percarbonate synthesis was obtained, to get the maximum of product yield, the optimal mass ratio of Na2CO3 to 30% H2O2 is 0.609. [ABSTRACT FROM AUTHOR]

The objective of this work is to develop a vanadium recovery process from a liquor residue of Bayer process. Vanadium finds application in strategic industrial sectors such as steel production and energy storage. The recognized importance of vanadium has pushed academic and industrial research towards the development of technologies for its recovery from different types of secondary sources. The developed process refers to a sodium fluorovanadate sludge from a spent Bayer liquor. The resulting filter cake was characterized to determine its composition and a vanadium recovery process was studied and optimized. This starts with a solubilization of the filter cake by water, followed by a precipitation step of aluminum through pH adjustment with sulfuric acid till 9.2. Vanadium is then recovered as ammonium metavanadate by precipitation with ammonium sulfate using a ratio (NH4)2SO4/salt cake = 2.25 w/w; the precipitate was calcined at T = 500°C to obtain vanadium as V2O5. The results showed an overall vanadium recovery of about 95%, with a purity > 99.6%. The innovative contribution here addressed is represented by the feasibility of producing high-purity V2O5 from the Bayer liquor through a relatively simple precipitation route. [ABSTRACT FROM AUTHOR]

Red mud is an alkaline solid residue produced from the extraction of alumina from bauxite. However, the remaining alumina in red mud will cause secondary loss of aluminum resources. In the Bayer process, high alkalinity leads to severe environmental pollution issues and safety hazards. In this study, alkali and alumina were simultaneously recovered by using NaFeO2 (NF) as an additive in a hydrothermal process. Harmless secondary red mud with low Na2O content was obtained, avoiding the alkalinity pollution compared with conventional alumina extraction process. The influence of hydrothermal conditions on the extraction rate of alkali and alumina in red mud was also systematically studied. The chemical composition and microstructure of red mud before and after the reaction were studied using various characterization techniques such as XRF, XRD, and SEM. The results show that the A/S (molar ratio of Al2O3 to SiO2) in the secondary red mud is reduced to 0.21, the Na2O content is 0.43%, and the recovery rate of alumina is 80% with NF as additive. The product is hydroandradite (Ca3(FexAl1−x)2(SiO4)y(OH)9−y) with less aluminum content and hematite. This work will provide a theoretical basis for achieving secondary resource recovery and handling the environmental pollution issues. [ABSTRACT FROM AUTHOR]

Efficient removal of silica is the key to alumina extraction from coal fly ash (CFA) by the Bayer process, due to the easy formation of sodium aluminosilicate hydrates during alkaline digestion. Considering the main mineral of well-crystallized mullite in CFA, mechanical activation was used in this work to improve the silica removal during reduction roasting–alkaline leaching. The phase transformations occurring in CFA during mechanical activation, reduction roasting, and alkaline leaching were systemically investigated through MAS-NMR, XRD, and SEM-EDS analyses. The results showed that the mean particle size (d(50)) decreased from 17.98 μm in CFA to 2.03 μm in activated CFA for 10 min with prolonging mechanical activation time, with the corresponding Al(IV) content decreasing from 73.82% to 67.85%. After pre-treatment with mechanical activation, the conversion of CFA into hercynite and silica solid solutions (i.e., quartz solid solution and cristobalite solid solution) was improved during reduction roasting with hematite, and the silica leaching ratio increased from ~ 70% to ~ 90% during the alkaline leaching. The obtained leaching residue with an alumina to silica mass ratio of > 10 was a decent raw material for the Bayer process to extract alumina. This study lays the foundation for comprehensive utilization of CFA. [ABSTRACT FROM AUTHOR]

Commercial utilization of high-sulfur diasporic bauxite is challenging because of the hazards of sulfur to the Bayer process. This article discusses a case study of producing alumina from high-sulfur bauxite containing chlorite at an alumina refinery in Zunyi, Guizhou, China. Application of flotation desulfurization and chemical oxidation method can control the iron content of the alumina product and retard equipment corrosion, but there are problems of the sulfur balance and low alumina extraction efficiency. To clarify the reasons for this, the changes of chemical compositions during the Bayer process as well as the mineralogical compositions and occurrences of high-sulfur bauxite, alumina concentrate and red mud were investigated. Pyrite and pyrite symbiotic with diaspore can be easily separated by flotation desulfurization. Most of the diaspore and pyrite in the alumina concentrate can react with the Bayer liquor. The generated sulfur-bearing ions can reprecipitate into the red mud as sulfur-containing desilication products, which partially convert into sulfur-free hydrogarnet because of the substitution of Ca for Na during the digestion process, while the pyrite and diaspore encapsulated by the slowly reacting chlorite layer cannot be digested. [ABSTRACT FROM AUTHOR]

The basic oxygen steelmaking process is based on the CaO-FeO-SiO2 ternary slag system, characterized by a high melting point and low lime dissolution rate, often becoming one of the key factors limiting the efficiency of the converter. The bulk solid waste red mud, produced by the Bayer alumina process and rich in Fe2O3/Al2O3/Na2O, significantly reduces the melting point of the steelmaking slag system and enhances the efficiency of lime dissolution. This study utilized red mud as the main raw material to prepare a fluoride-free flux. An in situ online observation system was used to measure the melting point of the flux and the dissolution rate of lime in the flux. The results indicate that the melting point of the red mud-based flux is below 1200 °C, and under the same conditions, the lime dissolution rate is 10 to 15 times higher than when this flux is not used. Experiments in a 10 kg induction furnace show that using this flux, the dephosphorization rate under conditions without oxygen blowing is close to 40%, far higher than the rate achieved using CaF2. Under oxygen-blowing conditions, the dephosphorization rate using the red mud-based flux is comparable to that of CaF2, and significantly higher than without any flux, especially under high [C] content conditions. The data show that the red mud-based flux has the potential to be widely used as a fluoride-free flux in the steelmaking process. [ABSTRACT FROM AUTHOR]

Purpose: Aluminium industry emits around 1–2% of the world's total greenhouse gas emissions. Up to one-third of those are linked to the thermal energy consumed during its initial process: the alumina refining (Bayer process). Previous studies consider the Bayer process a single stage despite its being made of several reaction stages. This work presents a disaggregated energy analysis of the Bayer process that facilitates to find relationships between the main variables in regular alumina production and the environmental impacts. Methods: Two different thermodynamic simulations of the Bayer process were carried out using Aspen V11 software. The results of these simulations were validated with referenced data, and afterwards, they were used to perform a life cycle assessment. ISO 14040 and 14,044 standards were followed during the analysis. LCA was implemented on SimaPro 9.0, and ReCiPe 2016 Midpoint (H) method was used to calculate environmental impacts. The influence of bauxite mineral form, type of fuel (energy input), and the distance from the mine to the plant was analysed throughout the study. Results and discussion: As expected, the type of fuel was revealed as the most crucial factor in the environmental impact of alumina production, with potential savings of up to 75.5% of CO2-equivalent emissions. Nonetheless, the tendency is diverse for other indicators, such as marine eutrophication or terrestrial acidification. On the other hand, while bauxite transportation always has the same impact on the different environmental indicators, bauxite mineral form affects differently depending on the fuel, causing variations in the CO2-eq emissions from 7.7 to 51.3%. Conclusions: Results indicated that the electrification of heat-demanding processes and the use of renewable power is the most effective approach for reducing environmental impacts. This strategy, however, must be considered in combination with others, as interdependent effects exist on the type of mineral used. These results provide strong evidence of the potential for environmentally friendly strategies in the metal industry, including new processes, alternative fuels, or mineral switching to promote more sustainable aluminium production. [ABSTRACT FROM AUTHOR]

Red mud produced from the Bayer process was used for the high temperature metallurgical processing of ilmenite. The major components of the red mud were Fe2O3, Al2O3, Na2O, and TiO2, which play important roles in the flux material. The process temperature of ilmenite slag was lowered from 1794 K (1521°C) to 1687 K (1414°C), as a preliminary study by a high-temperature microscope. Thermochemical calculations using FactSageTM 7.0 confirmed the effect of red mud on the lowered melting temperature of ilmenite slag. The discrepancy between the observed results of the microscope and the calculated phase diagram was due to the slag viscosity, which was significantly affected by Al2O3 in red mud. The reduction of ilmenite at lower temperatures by the addition of red mud was investigated at 1723 K (1450°C). Ilmenite, pseudobrookite, and associated clay minerals were the major phases in the reduced ilmenites which was confirmed by X-ray diffraction (×RD) analysis. Microscopic images obtained by scanning electron microscopy (SEM) showed variations in the pseudobrookite phases according to the amount of fluxed red mud. The benefits of red mud utilization are discussed with regard to the lower energy consumption and recovery of resources. [ABSTRACT FROM AUTHOR]

Alumina is produced from bauxite, which contains a mixture of various oxides, such as aluminum (Al), iron (Fe), silicon (Si), and titanium (Ti). Bauxite can also be considered a source of several other valuable metals, such as scandium (Sc), vanadium (V), and gallium (Ga). The composition and mineralogy of alumina determine their economic value, but their characteristics vary by locality. The physicochemical characteristics of bauxites can also be influenced largely by weathering processes, even within the same locality. For this reason, the present study was undertaken with the objective of comparing the characterization data of three bauxite samples collected, which will be referred to as D, E, and F, from the Cruz Alta do Pará plateau in northern Brazil. The samples were solubilized by multi-acid digestion and fusion with lithium metaborate to quantify their metal compositions by inductively coupled plasma optical emission spectrometry (ICP-OES). The mineralogical characterization was conducted by X-ray diffraction (XRD), and the phase changes of minerals in bauxite were detected by thermogravimetric analysis (TGA/DTG). The total organic carbon (TOC) technique was used to quantify the C in the samples, and the moisture content was also measured. Alumina was 30 wt.% on average for all samples, good for producing high-purity alumina by hydrometallurgical processes. The results, however, showed high (~20 at.%) silica concentrations in two samples and ~3 wt.% Fe in one sample, which can pose a challenge in the Bayer process. The X-ray diffraction (XRD) analysis showed that gibbsite (Gbs), kaolinite (Kln), anatase (Ant), and hematite (Hem) were the major mineral phases in these samples. The study showed that the samples from the same mine vary in their metal content, especially with regard to Si, and they, thus, need to be processed selectively to maximize their economic value. [ABSTRACT FROM AUTHOR]

Industry represents a fundamental component of modern society, with the generation of massive amounts of industrial waste being the inevitable result of development activities in recent years. Red mud is an industrial waste generated during alumina production using the Bayer process of refining bauxite ore. It is a highly alkaline waste due to the incomplete removal of NaOH. There are several opinions in both the literature and legislation on the hazards of red mud. According to European and national legislation, this mud is not on the list of hazardous wastes; however, if the list of criteria are taken into account, it can be considered as hazardous. The complex processing of red mud is cost-effective because it contains elements such as iron, manganese, sodium, calcium, magnesium, zinc, strontium, lead, copper, cadmium, bismuth, barium and rare earths, especially scandium. Therefore, the selection of an extraction method depends on the form in which the element is present in solution. Extraction is one of the prospective separation and concentration methods. In this study, we evaluated the kinetic modelling of the solid–liquid acid extraction process of predominantly scandium as well as other elements present in red mud. Therefore, three acids (HCl, HNO3 and H2SO4) at different concentrations (10, 20 and 30%) were targeted for the extraction of Sc(III) from solid red mud. Specific parameters of the kinetics of the extraction process were studied, namely the solid:liquid ratio, initial acid concentration, contact time and temperature. The extraction kinetics of Sc(III) with acids was evaluated using first- and second-order kinetic models, involving kinetic parameters, rate constants, saturation concentration and activation energy. The second-order kinetic model was able to describe the mechanism of Sc(III) extraction from red mud. In addition, this study provides an overview on the mechanism of mass transfer involved in the acid extraction process of Sc(III), thereby enabling the design, optimization and control of large-scale processes for red mud recovery. [ABSTRACT FROM AUTHOR]

The influence of process parameters in the three-stage purification of aluminate solution from the Bayer process and aluminum hydroxide was considered in this paper. One of the ways of purification is treating the aluminate solution in order to reduce the concentrations in the starting raw material (solution) and then treating the aluminum hydroxide at a certain temperature and time in order to obtain an alumina precursor of adequate quality. The purification process itself is divided into three phases. The first phase involves the treatment of sodium aluminate with lime in order to primarily remove Ca2+ and (SiO3)2− impurities. Phase II aims to remove impurities of Zn2+, Fe2+, and Cu2+ by treatment with controlled precipitation using specially prepared crystallization centers. In Phase III, Na+ is removed by the process of hydrothermal washing of Al2O3 ∙ 3H2O. In this work, parameters such as temperature (T), reaction time (t), and concentration of lime (c) were studied in order to remove the mentioned impurities and obtain the purest possible product that would be an adequate precursor for special types of alumina. [ABSTRACT FROM AUTHOR]

The mechanism of the condensation reaction of Al(OH)4− with D-gluconate is considered as a starting point in understanding the impact of organic substances on the Bayer process for obtaining alumina from bauxitic ores. Density functional theory (DFT) with a solvation model is used to study a model system of the different complexes formed between D-gluconate molecule and Al(OH)4− with monodentate or bidentate structures. The different mechanisms are studied to determine the most favorable pathway for the formation of the model complex. The DFT results show that the D-gluconate model forms an oxo bridge with aluminum through a first associative condensation step involving a pentacoordinated aluminum bonded to the carbon adjacent to the carbonyl of the ligand, and then a second bond is formed through a similar mechanism to obtain the most favorable bidentate structure. The rate-limiting activation barrier of the most favorable path is found to be 16.9 kcal mol−1 with density functional and 6.5 kcal mol−1 with CCSD(T) on geometries optimized with DFT and dispersion correction. The reaction energy for the preferred mechanism in solution is − 2.3 kcal mol−1. These results are in agreement with experimental observations, proposing the formation of bidentate Al(OH)4− with D-gluconate through condensation reactions in a strongly alkaline medium. [ABSTRACT FROM AUTHOR]

Introduction/purpose: Bauxite residue as a waste product from the aluminium industry produced through the Bayer process is mainly composed of iron oxide, titanium oxide, silicon oxide and undissolved alumina together with a wide range of other oxides and a minor content of rare earth elements, gallium, vanadium and scandium, which vary according to the country of origin of the bauxite. The extraction of valuable elements from bauxite residues and the minimisation of bauxite residues during different treatments are an open research field. Methods: Different hydrometallurgical and pyrometallurgical methods were used for the treatment of bauxite residues. In this study, the results of the hydrometallurgical treatment of bauxite residue from Alumina Zvornik using sulfuric acid and hydrochloric acid will be shown in order to study the change of the mineralogical composition. Leaching efficiency will be calculated using the ICP OES analysis. The XRD-Analysis was used for the characterization of the initial material and solid residues studying the change of the mineralogical phases. Results: Leaching of bauxite residues with sulphuric and hydrochloric acid leads partially to the change of mineralogical structure and the transfer of elements into a liquid phase. Natural precipitation of iron is observed over time. Silica gel formation is confirmed during leaching of bauxite residues with hydrochloric acid. Conclusion: A new research strategy for treating bauxite residue is needed in order to ensure a complete change of the initial minearlogical structure and the most efficient transfer of metals into a liquid phase. [ABSTRACT FROM AUTHOR]

Copyright of Hydrometallurgy of China is the property of Hydrometallurgy of China Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

High-iron and low-silicon red mud is not only an alkaline solid waste from Bayer process alumina production, but it is also a very important secondary iron resource. Magnetization roasting is considered as an effective and typical method for the iron recovery and removal of impurities in red mud. In this work, based on the characteristics of large specific surface area and high porosity of red mud, the kinetics of magnetization roasting and phase transformation of red mud were studied. Thermodynamic analysis results show that the reduction of iron oxide in red mud is more easily promoted by CO as reducing agent at low roasting temperature. The reduction reaction is prone to overreduction, and fayalite and ferrospinel can be formed in the reaction system. The phase transformation and iron reduction mechanism during the roasting process were evaluated. Most of hematite and goethite in the red mud decomposed in the process of magnetization roasting, released CO2, and transformed into strongly magnetic magnetite. The reaction process has some characteristics controlled by homogeneous reaction. The process of magnetization roasting reduction with CO was controlled by the hybrid control dynamics model, and the apparent activation energy was 38.31 kJ·mol−1. [ABSTRACT FROM AUTHOR]

Reductive leaching in the Bayer cycle using iron (2+) allows for Al extraction to be significantly increased through the magnetization of Al-goethite and Al-hematite. However, the use of expensive iron (2+) salts or iron powder as a source of iron (2+) leads to a significant increase in production costs. In this work, the feasibility of a new method, the reductive leaching of bauxite using an electrolysis process, was investigated. The reduction of iron minerals of boehmitic bauxite in both the Bayer solution and purely alkaline solutions was carried out. Experiments were performed using a plate cathode and a bauxite suspension in an alkaline solution, as well as using a bulk cathode with a stainless-steel mesh at the bottom of a cell as the current supply. During the electrolysis process, the potential of the cathode relative to the reference electrode was measured as a function of the current at different concentrations of solid (100–300 g L−1) and suspension temperatures (95–120 °C). It was shown that the current efficiency using the suspension and plate cathode with the predominant deposition of Fe did not exceed 50% even with the addition of magnetite to increase the contact of the solid phase with the current supply. With the use of a bulk cathode, the reduction of iron minerals led predominantly to the formation of magnetite with the efficiency of using the electric current at more than 80%. As a result of the preliminary desilication and electroreduction, it was possible to extract more than 98% of Al from bauxite and to increase the iron content in the bauxite residue to 57–58%. [ABSTRACT FROM AUTHOR]

Over a billion tons of high-sulfur bauxite has not been utilized effectively currently in China, because the pyrite existing in the bauxite poses a range of hazards during the Bayer process. A novel idea was proposed to remove sulfur by the silicon-containing minerals in bauxite reacting with sulfur species in sodium aluminate solution to form sulfur-bearing desilication products (SDSP) for discharge with the red mud in the Bayer process. This study investigated the reaction behavior between kaolinite and different sulfur-containing ions under the simulated Bayer process conditions, elucidating the desulfurization rate variation and formation mechanism of SDSPs. The thermodynamic calculations suggest that the reaction between kaolinite and sulfur-bearing sodium aluminate solution to form SDSPs can occur spontaneously. The experimental results demonstrated that various SDSPs can be produced through the reaction of kaolinite and sulfur-containing ions in sodium aluminate solution during the simulated Bayer process, resulting in various desulfurization efficiencies, while the desulfurization process will not result in additional alkali consumption. Increasing the kaolinite dosage, extending the reaction time, and elevating the reaction temperature all contribute positively to enhancing desulfurization efficiency. Kaolinite reacted with  S 2 O 3 2 −  in sodium aluminate solution to generate Na8Al6Si6O24S2O3·2H2O, achieving a desulfurization rate exceeding 90% under optimized conditions. Under the simulated Bayer digestion process conditions at elevated temperature, the desulfurization rates of kaolinite ranked in ascending order as  S 2 −  <  SO 3 2 −    <  SO 4 2 −  <  S 2 O 3 2 − . Kaolinite reacted with  SO 4 2 −  and  S 2 O 3 2 −  to form cancrinite type SDSPs, and a superior desulfurization rate can be achieved. This work can provide a theoretical foundation and technological support for the efficient utilization of high-sulfur bauxite by the Bayer process. [ABSTRACT FROM AUTHOR]

Bauxite Residue (BR) is an industrial waste generated by the extraction of alumina through the Bayer process. It is usually stored in specially constructed sedimentation ponds. Long-term storage notoriously leads to severe environmental issues due to its high alkalinity which affects neighboring communities. It contaminates phreatic and surface waters through the infiltration of metal laden caustic solution and radionuclides. The air also gets polluted through dispersion of particles which threatens surrounding biodiversity. To manage these environmental concerns, it is necessary to engineer the waste residue into value added products. This paper systematically reviews the mineralogical and chemical characterization of the waste and techniques developed for its recycling. [ABSTRACT FROM AUTHOR]

The Western Indonesia Bauxite Province in Kalimantan is a lateritic bauxite region with a complex history and poorly known sustainable metal contents within the bauxite residue. Bauxite residue produced using the Bayer process contains, notably, scandium. We present new geochemical, mineralogical and geological data from the lateritic bauxite and red mud from the active mine and deposit, which allow us to investigate the behaviour of critical elements during weathering. The geochemical analysis and translated isocon results have shown that the content of scandium in red mud is higher than the average concentration of crustal rocks and is concentrated in the ferrite layer and bauxite residue. A positive correlation between the existence of iron oxyhydroxide mineral in the residual iron-rich layer and red muds with the rare earth elements (REEs) and scandium concentrations may be interpreted as a scavenging effect of mobile REEs. The weathering and leaching processes in bauxite allows the adsorption of the trivalent scandium cation (Sc3+) on goethite and are followed by ionic substitution with other trivalent cations in crystals of hematite. The study illustrates the importance of understanding processes during weathering and laterization for geochemical processes and REE exploration in tropical areas. Supplementary material: Material A, (A) Mineralogical profile of the bauxite, ferrite, and tonalite in Tayan and Mempawah, (B) Ternary diagram of bauxite and their parent rocks, (C) Trace element ratios in bauxite; Material B, Spearman's correlation rank of bauxite and associated rocks from Tayan mine (n = 35); Material C, (A) XRD patterns of representative samples from the Tayan mine and Mempawah deposit, West Kalimantan, (B) Typical normal (left) and continuum (right) removed reflectance spectral curves of samples are available at https://doi.org/10.6084/m9.figshare.c.6689139 Thematic collection: This article is part of the Geochemical processes related to mined, milled, or natural metal deposits collection available at: https://www.lyellcollection.org/topic/collections/geochemical-processes-related-to-mined-milled-or-natural-metal-deposits [ABSTRACT FROM AUTHOR]

Several procedures for extracting content from different waste materials types were investigated, with the aim of evaluating their environmental impact. The waste materials consisted of wastes from bauxite ore processing by means of the Bayer process (red mud, Ajka, Hungary), bauxite ore using the sintering process followed by the Bayer process (brown-red mud, Žiar nad Hronom, Banská Bystrica region, Slovakia) and sulphide ores (metal-rich post-flotation tailing, Lintich, Slovakia). The extraction procedures were carried out with the aim of isolating "mobilizable" fractions using 0.05 M ethylenediaminetetraacetic acid (EDTA) and 0.43 M acetic acid (AA) (representing environmental risk during changes in normal environmental conditions) and "maximum potentially mobilizable" fractions using 2 M HNO3 (representing the total environmental risk). The content of chosen toxic heavy metals (THMs) (Cd, Cr, Cu, Pb, Ni, Zn) and Fe, Mn as metals creating Fe/Mn oxides in the extracts and solutions after microwave digestion was determined using high-resolution continuum source flame atomic absorption spectrometry (HR CS FAAS). On the basis of the results obtained in this study, it is possible to state that different origin of waste materials is reflected in different mobility of toxic heavy metals into the surrounding environment. From the point of view of toxic heavy metals mobility, disposal site of wastes after bauxite processing are much less of a threat to the environment than disposal site of flotation sludge after processing sulphide ores. The single extraction of 0.43 M AA is more effective than the extraction of 0.05 M EDTA for the purposes of determining the content of metals in the mobilizable fraction of tailing waste materials. The mobility of the studied toxic heavy metals in the Lintich tailing decreases in the direction from the lagoon to the dam, which may indicate the fact that the dam serves to a certain extent to inhibit the mobility of metals into the surrounding ecosystem. [ABSTRACT FROM AUTHOR]

Pressure hydrometallurgy has attracted much attention for its characteristics, such as the high adaptability of raw materials and environmental friendliness. Flashing (flash boiling or flash evaporation) refers to the phase change phenomenon from liquid to gas triggered by depressurization, which is an important connection between high-pressure processes and atmospheric ones in pressure hydrometallurgy. This paper takes the flashing process in zinc leaching and alumina Bayer processes as examples, describes the flashing process in pressure hydrometallurgy in detail for the first time, and shows the importance of the flashing process in energy recovery, solution concentration, and liquid balance, as well as increasing equipment life. According to solid holdup (the volume percentage of solid), this paper proposes to divide the flashing process into solution flashing (low solid holdup) and slurry flashing (high solid holdup). A further focus is put on reviewing the state of the art of related studies. The results reveal that the research on the flashing process in pressure hydrometallurgy is scarce and often oversimplified, e.g., ignoring the BPE (boiling point elevation) and NEA (non-equilibrium allowance) in solution flashing and the effect of solid particles in slurry flashing. Computational fluid dynamic (CFD) simulation is a promising tool for investigating the flashing process. Based on the progress made in other fields, e.g., seawater desalination, nuclear safety analysis, and engine fuel atomization, we suggest that solution flashing can be studied using the CFD–PBM (population balance model) coupled two-fluid model, since a wide size range of bubbles will be generated. For slurry flashing, the effect of solid holdup on the bubble nucleation rate and mechanism as well as other bubble dynamics processes should be accounted for additionally, for which a quantitative description is still lacking. Meanwhile, data for validating the numerical method are scarce because of the harsh experimental conditions, and further research is needed. In summary, this work presents an overview of the flashing processes in pressure hydrometallurgy and some guidelines for future numerical studies. [ABSTRACT FROM AUTHOR]

Bauxite residue (BR) is a by-product from the aluminum industry, generated during the Bayer process to obtain alumina. It has valuable iron and alumina contents, with soluble ions that can interact with the cement compounds, which could make BR a new source of supplementary cementitious material. This work aims to determine the influence of different contents of BR in substitution to two types of Portland cement regarding the hydration reactions and chemical shrinkage during the first 48 h of hydration, as well as to monitor the crystalline phases with XRD and the amount of bound water and the hydrates formed with thermogravimetric analysis. The results show the interaction of BR and cement, with the formation of new products, such as N-A-S-H, C-A-S-H, aluminum silicate hydrate (AS), and calcium aluminate hydrate (CAH), that culminate in changes in the shrinkage as BR is added. It was also observed that BR accelerates and enhances the slag reactions due to its high alkalinity and soluble ions content and the fineness of the material added to the cement. [ABSTRACT FROM AUTHOR]

The Bayer process is the main method of alumina production worldwide. The use of low-quality bauxites for alumina production results in the formation of a significant amount of technogenic waste—bauxite residue (BR). The Bayer reductive method is one possible way to eliminate BR stockpiling, but it requires high-pressure leaching at temperatures higher than 220 °C. In this research, the possibility of boehmitic bauxite atmospheric pressure leaching at both the first and second stages or high-pressure leaching at the second stage with the simultaneous reduction of hematite to magnetite was investigated. Bauxite and solid residue after NaOH leaching were characterized using XRD, SEM-EDS, and Mössbauer spectroscopy methods. The first stage of leaching under atmospheric pressure with the addition of Fe(II) species in a strong alkali solution (330–400 g L–1 Na2O) resulted in a partial reduction of the iron minerals and an extraction of more than 60% of Si and 5–25% of Al (depending on caustic modulus of solution) after 1 h. The obtained desilicated bauxite was subjected to atmospheric leaching at 120 °C in a strong alkali solution (350 g L−1) or high-pressure leaching at 160–220 °C using the Bayer process mother liquor in order to obtain a concentrate with a magnetite content higher than 83 wt. %. [ABSTRACT FROM AUTHOR]

The residue from the Bayer process has physicochemical characteristics that allow its use in the production of synthetic aggregate, although some characteristics such as low plasticity and water absorption still need better control. In this work, the influence of clay and silica addition on the sintering of synthetic aggregate produced from bauxite residue was analyzed. Compositions containing bauxite residue-sand-clay were elaborated, whose processing occurred using extrusion and crushing, followed by sintering at 1200 °C for 3 h. The X-ray diffraction analyses indicated the presence of new phases such as nepheline, originating from sodalite, and mullite resulting from the decomposition of kaolinite. Increasing the amount of free silica in the mixture resulted in an aggregate with a higher bulk density and lower water absorption, indicating that the amount of this raw material influenced the aggregate properties. [ABSTRACT FROM AUTHOR]

The Bayer process, which converts bauxite into alumina, results in the primary waste product being a rusty-colored mud called "red mud." In addition to a few other insignificant chemicals, it is made up mostly of oxides of the elements iron, titanium, aluminum, and silica. Concerns about the economy and the environment have motivated enormous work on a worldwide scale to find solutions to problems associated with red mud management. These problems include usage, storage, and disposal. Even though there are a lot of diverse applications for red mud, none of them have been shown to be commercially or economically viable as of now. It is well knowledge that MMCs are strengthened to increase their resistance to wear, and that the wear characteristics may be significantly enhanced by including a tough intermetallic compound into the aluminum matrix. The purpose of the present research project is to investigate the low-cost alternative of making use of red mud as a material for reinforcing structures. This is because red mud is not only readily available but also has all of these different components that are good for strengthening. Experiments have been carried out in a laboratory setting with the purpose of determining the wear characteristics of an aluminum red mud composite when subjected to a variety of various operating situations using a pin-on-disc machine in pure sliding mode. Additionally, the samples were heated in order to improve their wear properties. Under an optical microscope, the worn surfaces of the samples that had already been used were analyzed in order to get a better understanding of how the particle reinforcement influences the wear behavior of the composite. The wear resistance and tensile strength of the composite are both enhanced when red mud particles are dispersed throughout the aluminum matrix. If the cooling medium and heat treatment are chosen appropriately, the composite may have higher wear resistance. In addition, an artificial neural network prediction model (ANN) is used in order to reproduce the correlation that exists between the property parameters. As a consequence of this, the values that were seen experimentally and those that were projected match up incredibly well. These results may serve as a springboard for researchers and industrial designers to make MMC components from industrial waste for use in wearable settings. These components may be made from trash from many industries. [ABSTRACT FROM AUTHOR]

Bauxite residues known as "Red Mud" (RM) are the principal waste of caustic digestion of bauxite from the Bayer Process, whose costs of disposal are expensive and cover 5 % of the total costs of extraction and processing for aluminum production. Nevertheless, this material can be considered an important source of high-value elements, such as rare earths (REEs) and metals, Fe, Ti, Al and others. In this work, the focus has been on the recovery of iron in the form of the compound Fe(II) oxalate. Four types of acids have been used (HCl, H 2 SO 4 , H 3 PO 4 , H 2 C 2 O 4) for iron extraction from RM coming from Montenegro. Hydrochloric acid shows a higher iron extraction capacity, reaching an iron extraction yield in solution of 22.6 %. Sulfuric and phosphoric acid, instead, interacted with RM leading to the formation of sulfonate and phosphate species, inhibiting the leaching ability of individual species. Oxalic acid showed the least amount of iron ions extracted but formed a stable ionic complex in solution, Fe 2 (C 2 O 4) 3 ∙2 H 2 O. Starting from this complex it was possible to recover the corresponding salt by a reduction and precipitation process. Through a pre-treatment with HCl and a subsequent treatment with oxalic acid, it was possible to obtain a better yield of iron oxalate. Starting from the laboratory scale, a CHEMCAD plant was conceptualized with a yield higher than 16 % per pass (repeatable 3 times with a global iron yield>50 %) and obtaining iron(II) oxalate dihydrate with purity up to 96%wt. In a holistic view of the problem, the proposed process can operate in parallel with other procedures proposed in the literature for the recovery of other valuable substances from red mud. Not applicable. • Valorization and reuse of red mud through chemical treatments of acid leaching and reduction. • Selective coordination of Fe3+ ions with oxalic acid and subsequent salt precipitation. • Development of a preliminary procedure for the synthesis of high-purity iron oxalate from RM. • Development of a plant scheme for the production of Fe 2 C 2 O 4 with material and energy balances. [ABSTRACT FROM AUTHOR]

Red mud (RM) is composed of a waste alkaline solution (pH = 13.3) obtained from the production of alumina. It contains high concentrations of hematite (Fe2O3), goethite (FeOOH), gibbsite [Al(OH)3], a boehmite (AlOOH), anatase (Tetragonal–TiO2), rutile (Ditetragonal dipyramidal–TiO2), hydrogarnets [Ca3Al2(SiO4)3−x(OH)4x], quartz (SiO2), and perovskite (CaTiO3). It was shown to be an excellent catalytic mixture for biodiesel production. To demonstrate the value of RM, an environmentally friendly process of transesterification in aqueous medium using waste cooking oil (WCO), MeOH, and waste alkaline solution (WAS) obtained from aluminum production was proposed. Triglycerides of WCO reacted with MeOH at 60 °C to yield mixtures of fatty acid methyl esters (FAMEs) in the presence of 0.019% (w/w) WAS/WCO using the WAS (0.204 mol L−1, predetermined by potentiometric titration) from aluminum production by the Bayer process. The use of the new catalyst (WAS) resulted in a high yield of the products (greater than 99% yield). [ABSTRACT FROM AUTHOR]

Red mud is a highly alkaline solid waste discharged in the alumina production process. Because of its large amount of discharge and high alkalinity, it is mostly stored in dams, occupying a large number of land resources and posing a great safety hazard to the ecological environment. The large-scale consumption of red mud is a global technical problem. Different alumina production processes will produce different types of red mud, mainly Bayer process red mud. In addition to its overall utilization in the field of building materials, agriculture, the environment, and the chemical industry, red mud also contains valuable metal elements, such as titanium, iron, scandium, and aluminum, and is an important secondary mineral resource. This paper focuses on the principle and characteristics of red mud metallurgical treatment for the extraction of valuable components and looks forward to the prospect of large-scale, harmless, and high-value comprehensive utilization technology for red mud in China. [ABSTRACT FROM AUTHOR]

Machine learning approaches were used to predict and optimize the precipitation efficiency in the Bayer process. One thousand five hundred and sixty real operating data points of the precipitation efficiency from Iran Alumina Company were used for the model's development. Radial basis function (RBF) and support vector machine (SVM) networks were applied to develop a black-box model of the process. The input parameters of the models were the concentrations of sodium oxide (Na2Oc) and aluminum oxide (Al2O3), tank temperature, ambient temperature, residence time, and solid content. To create an optimal model, a trial-and-error strategy based on analyzing all potential configurations was used. The network's prediction performance is further demonstrated through model generalization inside the training data domain. The outcomes of both RBF and SVM networks demonstrate a good agreement between the industrial data and the model predicted values when considering statistical measures such as correlation coefficients of more than 0.99999, mean square errors, the absolute average deviation, and the absolute average relative deviation of less than 0.01%. The outcome of the models was used to optimize the operating parameters in such a way as to maximize precipitation efficiency with a minimum concentration of sodium oxide. The results show that the average precipitation efficiency of 42% was increased to 47% at optimized conditions. [ABSTRACT FROM AUTHOR]

The problem that sulfur causes in alumina production using the Bayer process can be eliminated by adding sodium nitrate, and, while the reaction mechanism is not perfect, this paper studies the reaction mechanism of sodium nitrate and different valence sulfurs in a sodium aluminate solution by combining thermodynamic calculations and experiments. Through thermodynamic analysis, it is deduced that sodium nitrate undergoes oxidation reactions with low valence sulfur (S2−, S2O32−, and SO32−) of different valence states in a sodium aluminate solution, while the oxidation effect of S2− is the most obvious. By studying the influence of the sodium nitrate dosage, oxidation time, and oxidation temperature on the different valence sulfurs in a sodium aluminate solution, it is concluded that S2− removal reaches 67.74% under the conditions of 3% sodium nitrate dosage, 260°Coxidation temperature, and 60-min oxidation time. However, the oxidation effect on S2O32− and SO32− is not obvious. The experimental results are consistent with the thermodynamic calculation results. Finally, the reaction mechanism of sodium nitrate with different valence sulfurs in a sodium aluminate solution is described in detail, which provides theoretical support for the desulfurization of high-sulfur bauxite in the production of alumina using the Bayer process. [ABSTRACT FROM AUTHOR]

Bayer red mud (RM) is made from aluminum ore using the Bayer process and is considered an industrial waste. The storage of unused RM can cause environmental and geological problems. Although several studies have focused on RM geopolymers, the amount of RM consumed by these synthetic geopolymers is small (usually <50%). Therefore, in this study, a hybrid geopolymer material with more than 90% RM content was developed and made it possible to apply in engineering construction. The consumption of a considerable amount of RM would reduce the massive accumulation of RM and address the shortage of natural construction materials. Experiments were conducted with ground granulated blast-furnace slag (GGBS) as the activator. Two different active GGBSs were selected to stabilize the RM by geopolymerization. The effects of dosage and type of GGBS on the mechanical properties of RM were investigated by conducting unconfined compression tests and disintegration and compression experiments. The experimental results indicated that the type and dosage of GGBS had a significant effect on the mechanical behavior of RM. Based on the unconfined compressive strength (UCS) values and the results of the disintegration and compression experiments, the optimal RM-GGBS mixed geopolymer was determined by orthogonal analysis. Microscopic analysis showed that under optimal conditions, the amorphous phase participated in the geopolymerization process, forming an amorphous geopolymer that improved the mechanical properties of RM. [ABSTRACT FROM AUTHOR]

Since the quality of bauxite resources has decreased and the organic carbon content has increased, different approaches are explored to remove the organic matter in alumina production. Advanced oxidative processes (AOPs) represent a possibility since they are widely used as an alternative for treating wastewaters to degrade organic pollutant molecules and in hydrometallurgy processes. For this reason, the goal of the project was the ozonation of Bayer liquor for organic matter removal. The ozone concentration was evaluated over time, as well as the H2O2 concentration and temperature. Results showed that the total organic carbon (TOC) removal achieved 19% in the most optimized condition with a kinetic rate of 0.0157 h−1 –21.9 mg/L O3, 0.05 mol/L H2O2 at 80°C. The colour of the liquor changed from dark brown to white‐yellow, indicating that the size of the organic compounds had decreased. Also, 95.4% of degraded TOC formed CO2, and almost 50% of the organic matter was oxalate compounds. The energy required for ozone production versus removed organic matter demonstrated that the technique proposed might be technically and economically feasible to be applied in the Bayer process. The study demonstrates the application of AOP in an extremely alkaline condition. [ABSTRACT FROM AUTHOR]