Improved bauxite residue dealkalization by combination of aerated washing and electrodialysis

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.

Recovery of valuable metals from red mud: A comprehensive review

As a bulk solid waste with high alkalinity, red mud (RM) not only occupies a large amount of land and requires high maintenance costs, but also unavoidably generates serious hazards to the surrounding ecological environment. The comprehensive treatment of RM has become an enormous challenge for the green, low-carbon and high-quality development of the global alumina industry. To minimize the RM destruction to the ecology and the waste of secondary resources, the sustainable utilization of RM was widely investigated in the past decades, especially for the recovery of valuable metals. This paper systematically summarized the research status of recycling valuable metals (Al, Fe, Na, Ti, Sc, Ga, V and RE) from RM in recent years. The recycling technology mainly includes physical beneficiation, hydrometallurgy, pyrometallurgy and electrodialysis. The technical principles and characteristics as well as the current problems of various recovery processes from RM were comprehensively introduced, and the future development directions of sustainable utilization were also prospected. The advantages and disadvantages based on the different aspects of recovery efficiency, energy consumption and environmental impact were also discussed. The proposal of new technologies for the harmless, high-value and full utilization of RM is beneficial to the future research on the comprehensive utilization of bulk industrial solid wastes.

Efficient dealkalization of red mud and recovery of valuable metals by a sulfur-oxidizing bacterium

Red mud (RM) is a highly alkaline polymetallic waste generated via the Bayer process during alumina production. It contains metals that are critical for a sustainable development of modern society. Due to a shortage of global resources of many metals, efficient large-scale processing of RM has been receiving increasing attention from both researchers and industry. This study investigated the solubilization of metals from RM, together with RM dealkalization, via sulfur (S0) oxidation catalyzed by the moderately thermophilic bacterium Sulfobacillus thermosulfidooxidans. Optimization of the bioleaching process was conducted in shake flasks and 5-L bioreactors, with varying S0:RM mass ratios and aeration rates. The ICP analysis was used to monitor the concentrations of dissolved elements from RM, and solid residues were analyzed for surface morphology, phase composition, and Na distribution using the SEM, XRD, and STXM techniques, respectively. The results show that highest metal recoveries (89% of Al, 84% of Ce, and 91% of Y) were achieved with the S0:RM mass ratio of 2:1 and aeration rate of 1 L/min. Additionally, effective dealkalization of RM was achieved under the above conditions, based on the high rates (>95%) of Na, K, and Ca dissolution. This study proves the feasibility of using bacterially catalyzed S0 oxidation to simultaneously dealkalize RM and efficiently extract valuable metals from the amassing industrial waste.

Conversion of alkaline characteristics of bauxite residue by mechanical activated pretreatment: Implications for its dealkalization

Following the strict environmental policies of various countries, the strong alkalinity of bauxite residue (BR) has become a worldwide problem limiting the sustainable development of the global alumina industry. Continuous conversion of solid-phase alkalinity to free alkali is a major challenge for BR dealkalization to reduce its environmental impact. This work aimed to investigate the effect of mechanical grinding pretreatment on the transformation mechanisms of alkaline solids to free alkali at the BR interface under acids leaching, by monitoring the morphology, phase, and speciation transformations of Al and Si using primarily cross-section scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) elemental mapping, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The results indicated that particle grinding wrapped some of the alkaline minerals inside the particles to inhibit its release process. The leaching kinetics revealed the order of the buffering effect of minerals against acids leaching is firstly dissolved by minerals containing Na and Ca via the ion-exchange process, followed by Si and Al through the hydrolysis of the desilicated products. The mineral dissemination characteristics and surface compositions further confirmed the undissolved minerals block the interface reaction between embedded alkaline solids and acids to result in the difficult reaction dissolution of alkaline minerals, which is induced by ball milling. This novel approach provides new insight into the efficient dealkalization of BR on a large scale in the industry.

Electrodialysis Applications in Wastewater Treatment for Environmental Protection and Resources Recovery: A Systematic Review on Progress and Perspectives

This paper presents a comprehensive review of studies on electrodialysis (ED) applications in wastewater treatment, outlining the current status and the future prospect. ED is a membrane process of separation under the action of an electric field, where ions are selectively transported across ion-exchange membranes. ED of both conventional or unconventional fashion has been tested to treat several waste or spent aqueous solutions, including effluents from various industrial processes, municipal wastewater or salt water treatment plants, and animal farms. Properties such as selectivity, high separation efficiency, and chemical-free treatment make ED methods adequate for desalination and other treatments with significant environmental benefits. ED technologies can be used in operations of concentration, dilution, desalination, regeneration, and valorisation to reclaim wastewater and recover water and/or other products, e.g., heavy metal ions, salts, acids/bases, nutrients, and organics, or electrical energy. Intense research activity has been directed towards developing enhanced or novel systems, showing that zero or minimal liquid discharge approaches can be techno-economically affordable and competitive. Despite few real plants having been installed, recent developments are opening new routes for the large-scale use of ED techniques in a plethora of treatment processes for wastewater.

Alkalinity neutralization and structure upgrade of bauxite residue waste via synergistic pyrolysis with biomass

Bauxite residues, a large volume solid waste, are in urgent need of effective disposal and management. Especially, strategies to alleviate the high alkalinity of bauxite residue remain a big challenge. Here, we developed a synergistic pyrolysis to neutralize the alkalinity of bauxite residue and upgrade the structure of biomass simultaneously. By cooperating the catalytic feature from bauxite residue, rice straw, a cellulose-enriched biomass, could prefer to produce acidic components under a hypothermal pyrolysis temperature (below 250 °C) and partial oxygen-contained atmosphere as evidenced by the synchronous TGA-FTIR analysis. In return, these in-situ produced acidic components neutralized the bauxite residue profoundly (pH decreased from 11.5 to 7.2) to obtain a neutral product with long-term water leaching stability. Also, a higher pyrolysis temperature led to neutral biochar-based products with well-defined carbonization characteristics. Thus, the biomass-driven pyrolysis strategy provides a potential to dispose the alkalinity issue of bauxite residue and further opportunities for the sustainable reuse and continuing management of bauxite residue.

Variation on leaching behavior of caustic compounds in bauxite residue during dealkalization process

Bauxite residue, a byproduct of alumina manufacture, is a serious environmental pollutant due to its high leaching contents of metals and caustic compounds. Four typical anions of CO₃^2-, HCO₃^-, Al(OH)₄^- and OH^- (represented caustic compounds) and metal ions (As, B, Mo and V) were selected to assess their leaching behavior under dealkalization process with different conditions including liquid/solid ratio (L/S ratio), temperature and leaching time. The results revealed that washing process could remove the soluble composition in bauxite residue effectively. The leaching concentrations of typical anions in bauxite residue decreased as follows: c(CO₃^2-) > c(HCO₃^-) > c[Al(OH)₄^-] > c(OH^-). L/S ratio had a more significant effect on leaching behavior of OH^-, whilst the leaching concentration of Al(OH)₄^- varied larger underleaching temperature and time treatment. Under the optimal leaching, the total alkaline, soluble Na concentrations, exchangeable Ca concentrations were 79.52, 68.93, and 136.0 mmol/L, respectively, whilst the soluble and exchangeable content of As, B, Mo and V in bauxite residue changed slightly. However, it should be noted that water leaching has released metal ions such as As, B, Mo and V in bauxite residue to the surrounding environment. The semiquantitative analysis of XRD revealed that water leaching increased the content of gismondine from 2.4% to 6.4%. The SEM images demonstrated the dissolution of caustic compounds on bauxite residue surface. The correlation analysis indicated that CO₃^2- and HCO₃^- could effectively reflect the alkalinity of bauxite residue, and may be regarded as critical dealkalization indicators to evaluate alkalinity removal in bauxite residue.