Red mud is a highly alkaline residue generated during the alumina extraction process, and its strong alkalinity significantly hinders its extensive and comprehensive utilization. The alkali in red mud mainly exists in two forms, namely, soluble alkali and insoluble chemically bound alkali. This study aims to provide a comprehensive overview of the dealkalization process progress for red mud, encompassing acid neutralization technology, salt (ion) precipitation or displacement technology, and metallurgical technology. The mechanisms, advantages, and disadvantages associated with each technology are discussed. To enhance readability for readers, mind maps are included for each dealkalization method of red mud. Based on the above analysis, it is advisable to choose the most suitable dealkalization method for red mud, considering its specific properties and the subsequent application strategy. Recommendations regarding future research on red mud dealkalization are proposed.

Four kinds of Ca-montmorillonite with different layer charge density were used to study the effect of charge density on their hydration properties by molecular dynamics simulation and experiments. The research results of Z-density distribution of water molecules, H_w (hydrogen in water molecules), and Ca in the interlayer of montmorillonite show that the hydration properties of montmorillonite are closely related to its layer charge density. If the charge density is low, the water molecules in the interlayers are mainly concentrated on the sides of the central axis about –1.3 Å and 1.5 Å. As the charge density increases from 0.38_semi-cell to 0.69_semi-cell, the water molecules are distributed −2.5 Å and 2.4 Å away from the siloxane surface (Si-O), the concentration of water molecules near the central axis decreases, and at the same time, Ca²⁺ appears to gradually shift from the vicinity of the central axis to the Si-O surface on both sides in the montmorillonite layer. The simulation results of the radial distribution function (RDF) of the Ca-H_w, Ca-O_w (oxygen in water molecules), and Ca-O_t (the oxygen in the tetrahedron) show that the Ca²⁺ and O_w are more tightly packed together than that of H_w; with the increase of the charge density, due to the fact that the negative charge sites on the Si-O surface increase, under the action of electrostatic attraction, some of the Ca²⁺ are pulled towards the Si-O surface, which is more obvious when the layer charge density of the montmorillonite is higher. The results of the RDF of the O_t-H_w show that with the increase of charge density, the number of hydrogen bonds formed by O_t and H_w in the interlayers increase, and under the action of hydrogen bonding force, the water molecules near the central axis are pulled towards the two sides of Si-O surface. As a result, the arrangement of water molecules is more compact, and the structure is obvious. Correspondingly, the self-diffusion coefficient shows that the higher the layer charge density, the lower the self-diffusion coefficient of water molecules in interlayers is and the worse the hydration performance of montmorillonite. The experimental results of the experiments fit well with the above simulation results.

The correlation between crystal chemistry characteristics and dispersion performance of Ca-montmorillonite with different layer charge density was studied. The four kinds of purified montmorillonite were characterized by XRD, XRF, FTIR, SEM, TEM, BET, TG-DTG, Zeta potential, and Molecular dynamics simulation (MD). The XRD analysis results show that the d₍₀₀₁₎ value of the four kinds of Ca-montmorillonite are 1.51 nm, 1.51 nm, 1.53 nm, and 1.50 nm, respectively, and the calculation results of layer charge density show that the semi-unit cell charge density are 0.38, 0.53, 0.61, and 0.69, respectively. The charge density is positively correlated with the mass content of MgO. The FTIR analysis results show that with the increase of layer charge density, the couple vibration wave number of Si–O–Mg and Mg–O move towards the high frequency region, from 464.70 cm⁻¹ to 466.36 cm⁻¹ and from 517.12 cm⁻¹ to 520.41 cm⁻¹. The SEM analysis results show that the higher layer charge density of montmorillonite, the worse exfoliation between layers and the thicker layer thickness, The images of TEM show that montmorillonites are mainly composed of a clearly layered structure. The results of BET show that with the increase of charge density, the BET surface area, micropore area and total pore volume gradually increase from 32.69 m² g⁻¹ to 64.50 m² g⁻¹, from 4.87 m² g⁻¹ to 18.24 m² g⁻¹, and from 0.06 cm³ g⁻¹ to 0.12 cm³ g⁻¹ , respectively, while PSDs and pore diameter decrease from 7.88 nm to 1.74 nm, and from 7.65 nm to 5.47 nm. The results of TG and DTG show that with the increase of charge density, the removal temperature of free water and inter-layer water increase from 80 °C to 100 °C and 139 °C to 155 °C, respectively. The simulation results of MD show that with the increase of layer charge density, the number of hydrogen bonds increase from 186 to 271 and the adsorption energy increases from −1467.11 to −1874.32 kcal mol⁻¹. The particle size increases from 701 nm(Mt1) to 1576 nm(Mt4) with the layer charge density, and the absolute value of Zeta potential, colloidal value and expansion capacity of montmorillonites suspension decrease with the increase of the layer charge density, which further indicates that the dispersion of montmorillonites decrease with the increase of layer charge density.

In this work, the effect of layer charge density of Na-montmorillonite (Na-MT) and carbon chain length of alkyl ammonium on the structure and gel property of organo-montmorillonite (organo-MT) was studied by using X-ray diffraction (XRD), Fourier transform infrared (FTIR), thermogravimetric (TG) analysis, contact angle test, molecular dynamics (MD) simulation, and gel apparent viscosity determination experiment. The results of XRD show that Na-MT with lower layer charge density is easier to swell after intercalation of alkyl ammonium, and the basal spacing of organo-MT increases with the increase of carbon chain length. The results of FTIR show that the absorption bands at 2924 cm⁻¹ and 2853 cm⁻¹ shift towards low frequency region with the increase of carbon chain length, and the absorption bands at 515 cm⁻¹ and 463 cm⁻¹ move towards high frequency region when the layer charge density increases. The mass loss of organo-MT evidently increases with the increase of layer charge density of Na-MT or carbon chain length of alkyl ammonium. The contact angle test results are well in line with the TG data and reveal that alkyl ammonium with longer carbon chain can significantly improve the hydrophobicity of organo-MT. MD simulation indicates that, when the layer charge density is low, the distribution of alkyl ammonium gradually changes from parallel double layers to partially inclined distribution with the increase of carbon chain length, but when the layer charge density is high, the distribution of alkyl ammonium gradually changes from three layers into four layers. The test results of the apparent viscosity of the gel formed by organo-MT in xylene show that the apparent viscosity of organo-MT gel is negatively correlated with the layer charge density of Na-MT and positively correlated with the carbon chain length of alkyl ammonium.