Treatment of alumina refinery waste (red mud) through neutralization techniques: A review

Electricity-driven dealkalization of bauxite residue based on thermodynamics, kinetics, and mineral transformation

High alkalinity content of bauxite residue is a major factor that hinders resource reutilization and pollutes the environment. Although acid neutralization is a direct and effective method, the amount of acid and secondary waste of sodium salt are still difficult problems to solve. Herein, we innovatively integrated an electric field into the acid neutralization dealkalization of bauxite residue and analyzed the dealkalization behavior by thermodynamics, kinetics, and mineral transformation. The results show that the pH of the anode chamber was maintained at the acidic levels of 3-6 after 30 min of galvanostatic electrolysis, and bauxite residue can realize dealkalization by acid neutralization. In the anode chamber, Na+ was released into the leachate via the reactions of Na3Al3Si3O12 and the removal of encapsulated soluble alkali. The stainless steel wire mesh anode exhibited its superiority and decreased the Na2O content in bauxite residue from 9.48 to 3.13% through convective mass transfer driven by the electric field and steady-state diffusion under stirring. This research provides a promising method for the electricity-driven dealkalization of bauxite residue, thus facilitating the development of multifield coupling theory and the application of electric fields in the alumina industry.

Ecological restoration affects the dynamic response of alkaline minerals dissolution in bauxite residue

Regulating alkalinity is the key process to eliminating environmental risk and implementing sustainable management of bauxite residue. Nevertheless, continuous release of free alkali from the solid phase (mainly sodalite and cancrinite) is a major challenge for long-term stability of alkalinity in amended bauxite residue. In order to understand the dissolution behavior of sodalite and cancrinite, their dissolution kinetics under simulated pH conditions of 8, 9 and 10 were investigated. Additionally, PHREEQC software and shrinking core model (SCM) were employed to analyze the release pattern of saline ions. The results revealed that the ratio of Na/Si and Na/Al values exhibited greater stability in sodalite than in cancrinite. The dissolution of elemental Na, Si, and Al in sodalite and cancrinite was matched with non-chemometric characteristics. The kinetic calculations by the shrinking core model (SCM) suggested that both sodalite and cancrinite exhibited slow dissolution kinetics, and their dissolution processes belong to internal diffusion control and external diffusion control, respectively. pH controlled the dissolution kinetic rates of sodalite and cancrinite mainly by changing their coupled dissolution-precipitation processes. More importantly, these findings can predict the change of alkaline components accurately, thus facilitating the implementation of efficient alkalinity regulation strategies for the ecological restoration of bauxite residue disposal areas.

Synthesis process-based mechanical property optimization of alkali-activated materials from red mud: A review

Repeated red mud (RM) stockpile accidents have sounded an alarm that a healthy alumina industry requires secure RM disposal. Unfortunately, the flawed mechanical properties of RM-based alkali-activated materials (RM-AAM) with bulk RM incorporation have impeded the ideal large-volume, low-risk utilization of RM and the provision of sustainable binders for communities. By reviewing a wide range of studies, this work provides insights into establishing a mature synthesis technique for optimizing the mechanical properties of RM-AAM. Brief evaluations of the nature and the current RM-AAM synthesis systems were conducted. The following emphasis is on addressing the influence characteristics and mechanisms of the known RM-AAM synthesis factors, including RM pre-activation, precursor composition, alkali activator property, preparation process treatment, and curing regime, on the mechanical properties of RM-AAM. Further optimization suggestions on each aspect of the synthesis process and the final complete set of synthesis technology that could best enhance the mechanical properties of RM-AAM were proposed. The general limitations of current research on developing a mature RM-AAM synthesis technique were identified, along with possible solutions.

Reductive roasting of arsenic-contaminated red mud for Fe resources recovery driven by johnbaumite-based arsenic thermostabilization strategy

Arsenic-contaminated red mud (As-RM) is a hazardous waste with limited recycling approaches. Generally, through reductive roasting and magnetic separation, RM could be transformed into Fe-rich concentrate for Fe resource recovery. However, due to the poor thermostabilization of As species, reductive roasting of As-RM would cause severe As volatilization pollution together with high As leaching risks from heated residue. Herein, a novel johnbaumite-based As thermostabilization strategy is developed for clean Fe resources recycling from As-RM. We found that in the presence of Ca(OH)₂, the As species in As-RM could be immobilized as thermostable and insoluble johnbaumite (Ca₅(AsO₄)₃OH) at 900 °C, effectively enhancing the As thermostability and insolubility. Introducing 1.5% Ca(OH)₂ into As-RM suppressed the As volatilization ratio from 60.3% to 15.7% during reductive roasting. Meanwhile, the As leaching concentration of the reduced residue was reduced to < 100 µg/L, thus satisfying the Japanese wastewater discharge standard. A concentrate with approximately 67.5% total iron grade was obtained from As-RM through this clean reductive roasting and magnetic separation. Overall, the approach introduced in this work effectively reduces the As diffusion pollution deriving from As-RM thermal reduction, which could contribute to hazardous As-RM reutilization, clean Fe resources recovery, and As pollution mitigation.

Ca-driven stable regulatory of alkalinity within desilication products: Experimental, modeling, transformation mechanism and DFT study

The prevalent pH rebound phenomenon in the bauxite residue alkalinity regulation is primarily caused by the presence of alkaline minerals, including sodalite and cancrinite. Calcium ion is widely used to remove the free alkali for reducing the alkalinity of bauxite residue, but its underlying mechanism on alkaline minerals is still unclear. In this work, we investigated the action mechanism of calcium ion on sodalite and cancrinite by various microspectroscopic methods, and then employed spin-polarized density functional theory (DFT) calculations to reveal the reaction pathways of calcium ion substitution and migration in minerals. The calcium ion can effectively regulate the stability of alkaline minerals by inhibiting alkaline ions release, which respectively enters sodalite and cancrinite by displacing Na adsorbed inside the mineral lattice and on the mineral surface. The entered calcium ion acts as competitive protection against sodium during the neutralization process, thus inhibiting the proton-promoted dissolution of sodalite and cancrinite. Moreover, the amount of entry calcium ion controls their acid neutralization ability. DFT calculations revealed calcium ions readily replaced sodium on the internal channels of minerals rather than on the surface. These new findings contribute to the understanding of potential options to directly stabilize critical alkaline components in bauxite residue.

Alkaliphiles for comprehensive utilization of red mud (bauxite residue)-an alkaline waste from the alumina refinery

The mining industry has powered the human endeavor to make life more innovative, flexible, and comfortable. However, it has also led to concerns due to the increasing amount of mining and associated industrial waste. Special attention is highly desired for its proper management and safe disposal in the environment. The problem has only augmented with the increase in the mining costs because of the investments needed for ecological remediation after the mining operation. It is pertinent that the targeted technologies need to be developed to utilize mining and associated industrial waste as a secondary resource to ensure sustainable mining operations. Every perceived waste is a valuable resource that is needed to be utilized to create additional value. In this review, the case of alkaline bauxite residue (red mud)-alumina refinery waste has been discussed at length. The highlight of the proposed work is to understand the importance of alkaliphile-assisted biomining-a sustainable alternative to conventional metal recovery processes. Along with the recovery of metals, pH reduction of red mud is possible through biomining, which ultimately paves the way for its complete utilization. The unique adaptation strategies of alkaliphiles make them more suitable for biomining of red mud through bioleaching, biosorption, and bioaccumulation, which have been discussed here. Furthermore, we have focused on the potential of the indigenous microflora of red mud for metal recovery in addition to its neutralization. The study of indigenous alkaliphiles from red mud, including its isolation and propagation, is crucial for the industrial-scale application of alkaliphile-based technology and has been emphasized.

Transferring waste red mud into ferric oxide decorated ANA-type zeolite for multiple heavy metals polluted soil remediation

Herein, a novel "waste reclamation for soil remediation" strategy has been developed for both alkaline waste red mud (RM) recycling and heavy metal (HM) polluted soil remediation. Through a direct one-pot hydrothermal reaction process, the Al, Si, alkali, and Fe₂O₃ components in waste RM have been transferred into ferric oxide decorated ANA-type zeolite (Fe₂O₃-ANA). As tested by the HMs polluted soil remediation and oilseed rape planting experiment, when 25 g/kg Fe₂O₃-ANA is added into the Pb²⁺, Cu²⁺, Cr³⁺ and anionic AsO₂^- polluted soil (HM concentration: 100-200 mg/kg), it can effectively suppress the HMs mobility in soil and reduce the bio-accumulation concentrations of HMs in the harvested oilseed rape (reduce ratio: 37.9-69.5%). The detailed mechanism study using energy dispersive X-ray energy spectroscopy, in-depth X-ray photoelectron spectroscopy and density function theory calculation concludes that the Cu²⁺, Pb²⁺ and Cr³⁺ in soil have been adsorbed and trapped in the framework structure of ANA in Fe₂O₃-ANA mainly via the cation exchange process. While the anionic AsO₂^- species are mainly caught by the Fe₂O₃ component in Fe₂O₃-ANA via surface adsorption. Overall, this work firstly transforms waste RM into Fe₂O₃-ANA for soil remediation, which is valuable to waste resource recycling and environmental conservation.

Capture and Reuse of Carbon Dioxide (CO2) for a Plastics Circular Economy: A Review

Plastic production has been increasing at enormous rates. Particularly, the socioenvironmental problems resulting from the linear economy model have been widely discussed, especially regarding plastic pieces intended for single use and disposed improperly in the environment. Nonetheless, greenhouse gas emissions caused by inappropriate disposal or recycling and by the many production stages have not been discussed thoroughly. Regarding the manufacturing processes, carbon dioxide is produced mainly through heating of process streams and intrinsic chemical transformations, explaining why first-generation petrochemical industries are among the top five most greenhouse gas (GHG)-polluting businesses. Consequently, the plastics market must pursue full integration with the circular economy approach, promoting the simultaneous recycling of plastic wastes and sequestration and reuse of CO2 through carbon capture and utilization (CCU) strategies, which can be employed for the manufacture of olefins (among other process streams) and reduction of fossil-fuel demands and environmental impacts. Considering the previous remarks, the present manuscript’s purpose is to provide a review regarding CO2 emissions, capture, and utilization in the plastics industry. A detailed bibliometric review of both the scientific and the patent literature available is presented, including the description of key players and critical discussions and suggestions about the main technologies. As shown throughout the text, the number of documents has grown steadily, illustrating the increasing importance of CCU strategies in the field of plastics manufacture.

Experimental Evaluation of PM Emission from Red Mud Basins Exposed to Wind Erosion

The disposal of industrial and mineral processing residues represents a major concern for human health and the environment as a whole. In order to reduce the impact on soil and groundwater due to the waste leachability, the implementation of environmental regulations worldwide has favored the conversion of the disposal techniques from wet to dry (i.e., dry stacking or dry disposal). Such a change in the storage practice may cause the increase of particulate matter (PM) emission from the dry surfaces of the tailings exposed to wind erosion. Considering the significance of the environmental issue on a global scale and the increasingly stricter orientation of environmental policies, the need for modeling tools capable of estimating the contribution of tailing basins to air pollution becomes apparent. The paper deals with the disposal of red mud resulting from the bauxite processing in the alumina industry. An experimental research was carried with an environmental wind tunnel to estimate the Emission Factor (EF) of the basin surfaces as a function of the main affecting variables (i.e., residue water content and wind velocity). The article reports the results of the experimental test carried out on the red mud from a major basin located in Sardinia (Italy).

Protonation stabilized high As/F mobility red mud for Pb/As polluted soil remediation

The hazardous red mud (RM) with high As/F mobility and heavy metal contaminated soil have constituted severe environmental threats. This work demonstrates a "waste to eco-material" strategy through a reliable and low-cost protonation approach to eliminate the As/F leaching risk of RM, and then recycle it as heavy metal passivators for Pb/As polluted soil remediation. The As/F anions have been immobilized by the protonated Fe/Al (hydr)oxides within RM via the formation of stable As/F compounds during the protonation process, which satisfies the requirement by the World Health Organization (As leaching <0.01 mg/L; F leaching <0.8 mg/L). Moreover, in the oilseed rape pot experiments, by adding 30 g/kg stabilized RM into Pb/As polluted soils (100 ~ 300 ppm), benefited from its large adsorption capacity, approximately 40.9 ~ 49.7% Pb and 40.8 ~ 54.8% As concentrations in the plant are reduced without adverse effects. The whole process for RM treatment and soil remediation is cost-effective, straightforward and eco-friendly without secondary pollution or soil degradation. This research provides a green chemical strategy to address both RM recycling and heavy metal contaminated soil remediation problems, which shows high economic feasibility and ecological benefits.

Dealkalization processes of bauxite residue: A comprehensive review

Bauxite residue is a kind of strong alkaline waste produced in the production of alumina. Its long-term storage poses a potential threat to the environment. With the tightening of environment policies in various countries, the strong alkalinity of bauxite residue has become a bottleneck restricting the sustainable development of aluminum industry all over the world. This review covers the composition characteristics of bauxite residue, and describes the Bayer process in detail, where emphasis is put on the formation of alkaline substances in bauxite residue and its release process in long-term storage. This review focuses on several typical processes for the management of bauxite residue alkalinity in recent decades around the world. The phase transformation mechanisms, merits and limitations, as well as application status are discussed. The potential application values of these typical methods are evaluated based on process characteristics. The large amount and varied characteristics of bauxite residue determine that it is unrealistic to solve the dealkalization problem of all bauxite residue with one method. It is recommended that the appropriate dealkalization process of bauxite residue should be selected according to the characteristics of bauxite residue and regional resources, as well as the planning of subsequent application.

Persulfate activation by modified red mud for the oxidation of antibiotic sulfamethoxazole in water

Different pre-conditioning treatments were evaluated in order to stabilize red mud, a waste product from bauxite processing, for obtaining heterogeneous catalysts (named as B1-B3) that can be employed as suitable activators of sodium persulfate (SPS) for the degradation of sulfamethoxazole (SMX), a model antibiotic, in water. The presence of Fe₃O₄ in the composition of the catalysts was found to be a key factor for a suitable activation of SPS, according to the XPS measurements. The oxidation of SMX was successfully fitted to a pseudo-first-order kinetic model (r² > 0.96), obtaining a 68% removal after 180 min when 0.8 mg/L of SMX was oxidized with 2 g/L of SPS and 2 g/L of catalyst B3. The presence of organic and/or inorganic constituents in the water matrix significantly hindered the degradation rate of SMX, the apparent kinetic constants being from 2 to 3 times lower than that determined in ultrapure water test. The use of ultrasound irradiation coupled to the addition of B3 catalyst improved importantly the SMX oxidation in real aqueous matrices, thus attaining values of removal which almost triplicated the ones obtained in absence of ultrasounds.

Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions

The main purpose of this study was to characterize the mineral and chemical composition of typical red muds in China. Changes in the physicochemical and microstructural properties of red muds collected from the Shanxi and Shandong provinces were investigated after they were immersed in an alkaline NaOH or an acidic HCl solution for 7, 28, and 120 days. The results showed that red mud has a high cation exchange capacity and active physicochemical properties, which can be closely related to its extremely high alkalinity and complex microstructure. The neutralization of red mud with the HCl solution results in the release of Na+ from the red mud particles into the leachate and can effectively decrease the pH value of the filtrate. The neutralization process can result in a significant decrease in the liquid limit, plastic limit and plasticity index, whereas the opposite was observed for the different parameters after the addition of the NaOH solution. In this sense, acid neutralization can significantly improve the cementation property of the red mud. This result will increase the water permeability of the acid-treated soil layer and improve the growth ability of plants. The specific surface area of red mud immersed in the NaOH solution decreased, whereas the specific surface area of red mud immersed in the HCl solution increased. This study contributes to our understanding of red mud properties after the red mud has been subjected to acidic and alkaline treatments, and the results can provide insights into the safe disposal of red mud.

A Review on Comprehensive Utilization of Red Mud and Prospect Analysis

Red mud (RM) is a by-product of extracting of alumina from bauxite. Red mud contains high quantities of alkali-generating minerals and metal ions, which can cause significant environmental damage. Many valuable components such as rare-earth elements, Al, and Fe, in RM are difficult to be utilized owing to their particle size and alkalinity. Thus, developing an economical and efficient technology to consume a large amount of RM can efficiently solve RM disposal issues. This paper systematically reviews the comprehensive utilization methods for reducing RM environmental pollution and divides the comprehensive utilization of RM into three aspects: the effective extraction of valuable components, resource transformation, and environmental application. Based on resource, economic, and environmental benefits, the development of new technologies and new processes with market competitiveness, environmental protection, and ecological balance should be the prerequisite for the low-energy, low-pollution, low-cost, and high-efficiency comprehensive utilization of RM. The direction of future research to solve RM disposal issues is also suggested.

Recovery of Alkali from Bayer Red Mud Using CaO and/or MgO

Recovering alkali from Bayer red mud is crucial for storage security, resource utilization and environmental protection. In this study, the addition of MgO and/or CaO was conducted to recover alkali from red mud with a hydrothermal method for the first time. A synergistic result with a residual Na2O/SiO2 weight ratio of 0.03 was obtained by adding the blend of CaO and MgO at an appropriate temperature. MgO was found to be more temperature-dependent than CaO when substituting Na2O from red mud due to their different hydration processes. The alkali recovery was controlled by a reaction at a temperature of <200 °C and by internal diffusion at a higher temperature for MgO, but controlled by internal diffusion for CaO in the whole temperature range studied. The formation of hydrotalcite-like compounds with a loose structure was verified with the help of XRD, FTIR, and SEM-EDS. It was proved that both the reaction kinetics and the characteristics of solid products have a significant influence on the recovery of alkali.

Butan-1-ol as an extractant solvent in dispersive liquid-liquid microextraction in the spectrophotometric determination of aluminium

Determining aluminium ions at μg L^-1 scale currently requires either costly analytical techniques such as inductively coupled plasma, and/or graphite furnace atomic absorption spectrometry. Dispersive liquid-liquid microextraction (DLLME) is designed to promote separation and preconcentration, thus making it possible to determine the analyte of interest without significant matrix influence. This study was aimed at the development of a spectrophotometric method to determine Al³⁺ after microextraction of its complex with quercetin. Butan-1-ol was used as a novel extractant solvent in the DLLME process. The parameters influencing complexation and microextraction, such as the amount of quercetin and volume of extractant were evaluated by univariate analysis. In optimised conditions were estimated for the proposed method: linear range from 7.5 to 165.0 μg L^-1, LOD of 2.0 μg L^-1, and LOQ of 7.0 μg L^-1. The accuracy was checked by applying the proposed method to water (NIST SRM-1643e) and rice flour (NIST SRM-1568c) certified reference materials and spike-and-recovery trials with distinct samples (mineral water, green tea, thermal spring water, contact lens disinfecting solution, saline concentrate for hemodialysis and urine).

Analysis of bauxite residue components responsible for copper removal and related reaction products

Bauxite residue is a solid waste produced during alumina production process, and the storage of that in China reached 0.6 billion tons with an increase of more than 70 million annually. Bauxite residue can be used to remove heavy metals from water. This study analyzed components of bauxite residue responsible for copper removal, removal process and accompanying reaction products. Calcite (CaCO₃), hematite (Fe₂O₃) and sulfur-Fe are main components contributing to copper removal. Sulfur in bauxite residue works with iron to remove copper. All these components reacted with copper immediately as bauxite residue was added. Reaction time of sulfur-Fe and carbonate was 5 min and 1 h, respectively. And hematite reacted until complete removal of copper (>2 h). Sulfur quickly reacted with coexisting iron to remove copper, producing chalcopyrite (CuFeS₂), cubanite (CuFe₂S₃) and bornite (Cu₅FeS₄). Carbonate in bauxite residue reacted with copper, producing tenorite (CuO), copper hydroxide (Cu(OH)₂), malachite (Cu₂(OH)₂CO₃), carbonate cyanotrichite (Cu₄Al₂(CO₃,SO₄)(OH)₁₂·2H₂O), chalconatronite (Na₂Cu(CO₃)₂·3H₂O), nakauriite (Cu₈(SO₄)₄(CO₃)(OH)₆·48H₂O) and callaghanite (Cu₂Mg₂(CO₃)(OH)₆·2H₂O). Copper precipitated through reaction with hematite to produce delafossite (CuFeO₂). After removal reaction, the existing forms of copper in bauxite residue comprised carbonate-bound (73.6%-85.7%), iron oxide-bound (5.6%-23.8%), organic matter/sulfide-bound (0.5%-9.0%) and residual forms (0.9%-2.0%). In conclusion, removal of copper using bauxite residue features a more complex reaction than adsorption.