Study on Reaction Behavior and Phase Transformation Regularity of Montmorillonite in High-Calcium Sodium Aluminate Solution System

Abstract

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 (N_k), 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 N_k 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%.

1. Introduction

Bauxite is a general term for a series of ores with different compositions, including gibbsite, boehmite, and diaspore [1,2,3]. Over 99% of alumina in the world is extracted and smelted from bauxite [4,5,6]. China’s bauxite mainly consists of diaspore, which is widely distributed in regions such as Shanxi, Henan, Guizhou, Guangxi, and Sichuan. It serves as the primary raw material for the Bayer process in alumina production [7,8]. The average alumina content in China’s diaspore bauxite is 50%, indicating a relatively high-grade [9]. However, this type of bauxite contains approximately 9%–15% SiO₂, resulting in a low alumina–silica ratio (A/S), which is unfavorable for extraction in conventional Bayer solutions. Consequently, the leaching of diaspore bauxite requires higher temperatures and alkali concentrations. During the dissolution of the Bayer process, the silicon-containing mineral in diaspore dissolves in the alkali solution in the form of Na₂SiO₃²⁻ (sodium silicate) and reacts with [Al(OH)₄]⁻ to form Na₆(AlSiO₄)₆ (hydrated sodium aluminosilicate), which enters into the red mud, resulting in an amount of aluminum and caustic alkali loss [10]. Due to the high-silica content and low solubility characteristics of China’s diaspore bauxite, the Bayer process for alumina production requires significantly more energy, making large-scale application challenging. As a result, China needs to import a significant amount of high-grade bauxite each year, which increases costs and restricts the further development of the aluminum industry. China’s diaspore bauxite accounts for 98% of the total reserves in the country [11]. Considering the future development of the alumina industry, there is considerable potential for further research and utilization of diaspore bauxite.

Apart from the aluminum-containing phase, diaspore contains a significant amount of silicon-containing mineral impurities in the form of silicate, including kaolinite, illite, chlorite, pyrophyllite, and montmorillonite [12,13,14,15]. The complex composition of these silicon-containing minerals requires higher pressure and temperature during the Bayer process, which reduces the efficiency of alumina production. When diaspore-type bauxite is used as the raw material for production, the silicon-containing mineral would become the main factor contributing to high-energy consumption. In the Bayer process used for alumina production, the consumption of caustic soda increases with the rising silica content in bauxite ore. This results in the generation of large quantities of red mud, which contains 4%–15% alkali. The accumulation of this red mud poses significant environmental pollution [16]. Therefore, clarifying the phase changes and reaction mechanisms of silica-bearing minerals in monohydrate bauxite within high-calcium sodium aluminate solution is of significant value for the development and utilization of monohydrate bauxite. According to the results of the studies, the silicon-containing minerals of diaspore will react with calcium oxide in the solution and transform into hydrogarnet under the high-calcium sodium aluminate solution [17,18,19]. Geidarov [20] pointed out that the content and morphological evolution of SiO₂ in sodium aluminate solution play a key role in alumina production. However, there has been limited research on the evolution characteristics and mineral phase interaction of silicon-containing mineral phases in bauxite under the high-calcium sodium aluminate solution.

Montmorillonite is one of the silicon-containing mineral impurities found in bauxite [21]. As an associated silicon-containing mineral phase of bauxite, the montmorillonite can reduce the leaching efficiency of the Bayer process, leading to alumina losses. A semi-quantitative XRD analysis of karst bauxite deposits in central Guizhou revealed that montmorillonite constitutes 14%–18% of the total silica minerals, making it the second most abundant silica-containing mineral after kaolinite [22]. In the Yunnan region, XRD and SEM analyses of bauxite claystone identified montmorillonite as the primary mineral [8]. Additionally, significant amounts of kaolinite and montmorillonite were found in Hungarian bauxite [23]. Aldabsheh [24] et al. investigated the dissolution behavior of montmorillonite in alkaline solutions, demonstrating that the leaching degree of aluminum and silicon from montmorillonite exceeds 30% in high-alkali solutions. These studies indicate montmorillonite is a common silicon-containing mineral in bauxite. However, there are few reports on the dissolution mechanism of montmorillonite, especially the structural evolution of montmorillonite under the high-calcium sodium aluminate solution system. Therefore, understanding the reaction behavior and mechanisms of montmorillonite in the high-calcium sodium aluminate solution can help improve the processing of low-grade diaspore-type bauxite in China.

In this article, the natural montmorillonite was used as raw materials to analyze in detail the effects of changes in reaction temperature, caustic concentration (N_k), and mass ratio of CaO to SiO₂ (C/S) on the reactivity of montmorillonite. An X-ray diffractometer (XRD) was used to study the physical phase transition characteristics of montmorillonite under the high-calcium sodium aluminate solution system. Scanning electron microscope (SEM), transmission electron microscope (TEM), Magic Angle Spinning Nuclear Magnetic Resonance (MAS–NMR), and Fourier Transform Infrared Spectroscopy (FTIR) were used to study the changes in structural characteristics of montmorillonite.

2. Experimental Material and Method

2.1. Material and Instrument

Montmorillonite [25] is a layered mineral composed of hydrous aluminosilicate. In terms of crystal structure, the unit layer of montmorillonite is a 2:1 crystal water-containing structure, which is composed of two layers of Si-O tetrahedron sheets sandwiching one layer of Al(Mg)-O(OH) octahedron sheets. The montmorillonite used in this article was obtained from the Handan mine in Hebei, China. After crushing, grinding, sorting, and purification, it was dried in an oven at a temperature of 60 °C for 24 h.

The experimental equipment includes the following: the D8 ADVANCE X-ray diffractometer from Bruker company in Germany(Bruker, Karlsruhe, Germany), the SU-8010 scanning electron microscope from HITACHI company in Japan (Hitachi, Tokyo, Japan), the FEI TalosF200x transmission electron microscope from Thermo Fisher Scientific company in the USA (Thermo Fisher Scientific, Waltham, MA, USA), the ZSXPrimus II X-ray fluorescence spectrometer from HITACHI company in Japan (Hitachi, Tokyo, Japan), the Avance III 400 Opizen nuclear magnetic resonance spectrometer from Bruker company in Germany (Bruker, Karlsruhe, Germany), the Nicolet 380e Fourier infrared spectrometer from Thermo Fisher Scientific Company in USA (Thermo Fisher Scientific, Waltham, MA, USA), the ZRY-K01-0.5/10 autoclave from Zhengwei Company in China (Zhengwei, Wenzhou, China), and the ZWYYL12006 salt-bath furnace from Zhengwei Company in China (Zhengwei, Wenzhou, China).

The montmorillonite raw material powder was analyzed by X-ray diffractometer (D8 ADVANCE, Bruker, Germany) (Bruker, Karlsruhe, Germany) as shown in Figure 1. The samples used in the XRD test were ground into powder and fixed in specific sample holder. The testing conditions included a copper target with a tube voltage of 40 kV, a tube current of 30 mA, a scanning angle range of 5° to 90°, and a scanning speed of 2°/min. The main diffraction peaks in the montmorillonite raw material correspond to the phases of Na₀.₃(Al, Mg)₂Si₄O₁₀(OH)₂·nH₂O phase (montmorillonite) and SiO₂ (quartz). It can be seen from Figure 1 that the main component of the montmorillonite raw material is the phase Na₀.₃(Al, Mg)₂Si₄O₁₀(OH)₂·nH₂O (montmorillonite), along with a small amount of SiO₂ (quartz), as indicated by the diffraction peaks. Comparing the theoretical chemical formula of montmorillonite (Al₂O₃·4SiO₂·nH₂O), it can be seen that some cations, such as Na⁺ and Mg²⁺, appear in the interlayers of montmorillonite.

The morphology of the montmorillonite raw material was analyzed using scanning electron microscopy (SEM, SU-8010, HITACHI, Japan) (Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM, FEI Talos F200X, Thermo Fisher Scientific, USA) (Thermo Fisher Scientific, Waltham, MA, USA). Prior to testing, the sample powder was uniformly dispersed on conductive adhesive, and any unbound particles were blown away using a bulb syringe to prevent contamination of the electron microscope during vacuum. The conductive adhesive was then fixed onto the sample stage for surface morphology observation. As shown in Figure 2, the microscopic morphology of the montmorillonite particles appears as irregular blocky and earthy aggregates, with particles of varying sizes distributed randomly, resulting in a relatively uniform type of morphology under the microscope. Meanwhile, as shown in Figure 3, it can be observed that the irregular structure of montmorillonite is primarily enriched with Al, Si, O, Mg, and trace amounts of Na and Ca. The elemental analysis results show a certain consistency with the previous XRD analysis, indicating that montmorillonite contains cations such as Na⁺ and Mg²⁺.

The composition of the montmorillonite was analyzed for elemental composition using X-ray fluorescence (XRF, ZSX Primus II, Rigaku, Japan) (Rigaku, Tokyo, Japan) with a 4 kW ultra-thin window (30 µm) rhodium target X-ray tube. The analysis results are shown in

Table 1. Chemical composition of montmorillonite (mass fraction/%).

Element	Al2O3	SiO2	Fe2O3	TiO2	K2O	Na2O	CaO	MgO	SO3	LOI
Content	12.83	62.44	2.2	0.46	0.72	0.8	1.66	2.97	0.09	15.83

, indicating that montmorillonite consists of 62.44% SiO₂ and 12.83% Al₂O₃, along with trace amounts of elements such as Fe, Mg, Ca, Ti, and Na.

MAS–NMR is a nuclear magnetic resonance technique used to analyze the montmorillonite minerals employed in the experiments. The analysis was conducted using a nuclear magnetic resonance spectrometer (Avance III 400 Opizen, Bruker, Germany) to determine the molecular structure and composition of the montmorillonite. The testing conditions for the silicon spectrum were set at 79.5 MHz and 9.4 Tesla, while the aluminum spectrum was tested at 104.3 MHz and 9.4 Tesla.

Fourier transform infrared spectroscopy (FTIR, Nicolet 380, Thermo Fisher Scientific, USA) was used to analyze the chemical composition of the montmorillonite raw material. The sample was prepared using the conventional KBr pellet method, mixing the sample with KBr powder and pressing it into a disk. The testing range was from 4000 to 400 cm⁻¹, with an instrument resolution of 4 cm⁻¹ and a scanning number of 64.

2.2. Configuration of Sodium Aluminate Solution

Preparing sodium aluminate solutions with caustic alkali concentrations (N_k) of 160–280 g/L, respectively. The sodium aluminate solution used in this study is an industrial-grade solution with a caustic ratio fixed at 2. Subsequently, the corresponding precise amounts of NaOH, Al(OH)_3, and deionized water were weighed according to the required concentration of caustic soda and placed in autoclave (ZRY-K01-0.5/10, Zhengwei, China) (Zhengwei, Wenzhou, China) to react. The temperature of autoclave was set at 160 °C, and the reaction time was 60 min.

The caustic (N_k) of sodium aluminate solution was analyzed by chemical titration. The sodium aluminate solution was diluted 5000 times, then 10 mL of 10% BaCl₂, 10 mL of 10% salicylic acid, and phenolphthalein indicator were added before titrating with HCl solution. The reaction Equation (1) is shown below, where V_HCl is the volume of HCl consumed (mL), V₀ is the sample volume (mL), and M_HCl is the molar mass of HCl.

The above steps were repeated three times, and the average of 3 N_k concentrations was multiplied by 5000 to determine the caustic soda concentration of the sodium aluminate solution.

$$N_k(\mathrm{g}/\mathrm{L})={\displaystyle \frac{{31M}_{HCl}{V}_{HCl}}{V_0}}$$

(1)

2.3. Experiment

Montmorillonite (liquid–to–solid ratio L/S = 5 mL/g) and different proportions of calcium oxide (C/S ratio of 2.5, 3, 3.5, 4, 4.5, 5) were added to sodium aluminate solution (N_k of 160 g/L, 180 g/L, 200 g/L, 220 g/L, 240 g/L, 260 g/L, 280 g/L). The montmorillonite raw material contains 1.6% calcium oxide, which is less and negligible. The weighed materials were put into the reaction steel bomb in the salt-bath furnace (ZWYYL12006, Zhengwei, China), along with a specified amount of sodium aluminate solution as the heating medium. The heating and stirring are conducted at designated temperatures of 160 °C, 200 °C, 240 °C, and 280 °C, with the salt-bath furnace set to a speed of 30 r/min for a reaction time of 120 min. After the reaction, the mixture was removed and allowed to cool. Subsequently, solid-liquid separation was performed, and the resulting hydrogarnet solids were washed and dried for future use.

The SiO₂ content in montmorillonite solids can be used to assess their reactivity and degree of reaction under specific conditions. Under the high-calcium sodium aluminate solution system, SiO₂ in montmorillonite can be dissolved and react to form other compounds. By measuring the mass of SiO₂ in montmorillonite before and after the reaction, the extent of the reaction of montmorillonite can be determined.

The SiO₂ in the solid phase and the liquid phase were measured by ultraviolet spectrophotometer. The steps were as follows:

Measurement of SiO₂ in the liquid phase: A sample was diluted 100 times, then 2 mL of 5 mol/L HCl, 40 mL of deionized water, 2.5 mL of 5% ammonium molybdate solution, 2 drops of potassium permanganate solution, 5 mL of ethanol, and 20 mL of 5% molybdenum blue reagent were added. The solution was then diluted to the appropriate volume and mixed well. It was finally measured by using an ultraviolet spectrophotometer. The steps were repeated for three consecutive measurements to obtain 3 different sets of data. The average value was calculated and multiplied by 100 to determine the SiO₂ concentration in the liquid phase.

Measurement of acid-soluble SiO₂ mass in the hydrogarnet solids: After the reaction, 0.25 g of hydrogarnet solid was weighed and dissolved in 30 mL of 1 mol/L HCl. The analytical method was the same as that used for the determination of SiO₂ in the liquid phase. The steps were repeated for three consecutive measurements to obtain 3 different sets of data, and the average value was calculated to determine the SiO₂ mass in the hydrogarnet solids.

The reaction extent of montmorillonite (η) was calculated using the following formula Equation (2):

$$\eta ={\displaystyle \frac{C_1+C_2}{C_3}}\times 100\%$$

(2)

where C₁ is the mass of SiO₂ (g) obtained from the acid dissolution of hydrogarnet solid, C₂ is the mass of SiO₂ (g) in the liquid phase, and C₃ is the mass of SiO₂ (g) in the montmorillonite materials.

3. Results and Discussions

This article studied the effects of temperature, the alkali concentration of sodium aluminate solution, and the mass ratio of CaO to SiO₂ (C/S) on the reaction degree of montmorillonite under the high-calcium sodium aluminate solution. The structural changes of the montmorillonite during the reaction process under different experimental conditions are also analyzed.

3.1. Effect of Temperature on the Reaction Degree of Montmorillonite

The reaction degree of montmorillonite was investigated at different temperatures under the conditions of N_k of 240 g/L, C/S ratio of 3.5, L/S ratio of 5 mL/g, reaction time of 120 min, and salt-bath furnace at a speed of 30 r/min.

Figure 3 shows the changes in montmorillonite reaction degree and the mass of SiO₂ in the liquid phase over time at temperatures ranging from 160 to 280 °C. As the reaction time of montmorillonite in the salt-bath furnace is extended, the extent of its reaction increases sharply within the first 0 to 30 min. The increase in the reaction extent begins to level off after 120 min. Concurrently, the mass of SiO₂ in the liquid phase shows a trend of first increasing and then decreasing over time, with the decreasing trend also leveling off at 120 min. In summary, the optimal reaction time for montmorillonite in the salt-bath furnace is 120 min.

As shown in Figure 4a, under the solution system of high-alkali and calcium, the dissolution rate of montmorillonite increases with rising temperature, reaching a peak reaction rate at 240 °C. Beyond this temperature, further increases do not significantly enhance the reaction degree of montmorillonite. It can be seen from Figure 4b that the montmorillonite continues to dissolve at the initial stage of the reaction, resulting in a rapid increase in the SiO₂ contained in the liquid phase and reaching a peak. With the increase in reaction temperature, the dissolution rate of montmorillonite increases, resulting in more SiO₂ dissolving into the liquid phase. This Si then reacts with (CaO)₃Al₂O₃(H₂O)₆ (hydrated calcium aluminate) to form Ca₃Al₂(SiO₄)(OH)₈ (hydrogarnet), causing the mass of SiO₂ in the liquid phase gradually to gradually decrease. Lu [10] et al. have found that the hydrogarnet phase in the solution disappears and decomposes into hydrated calcium aluminate when the temperature exceeds 240 °C, indicating that excessively high temperatures are detrimental to the reaction of montmorillonite under alkaline conditions. Under the high-calcium sodium aluminate solution system, elevated temperatures can lead to the decomposition of hydrogarnet, resulting in reverse reactions. The thermodynamics of Lu [16] showed that the increase in reaction temperature was beneficial for raising the silicon saturation coefficient of the increase of hydrogarnet, which can form within a temperature range of 60 to 280 °C, suggesting that temperature does not play a crucial role in the formation of hydrogarnet. The reaction of montmorillonite in sodium aluminate solution under the high-calcium sodium aluminate solution system does not require excessively high temperatures.

In summary, the optimum temperature for the montmorillonite reaction in the high-calcium sodium aluminate solution system is 240 °C.

3.2. Effect of N_k on the Reaction Degree of Montmorillonite

The experiment was carried out in the salt-bath furnace at a reaction temperature of 240 °C, C/S ratio of 3.5, L/S ratio of 5 mL/g, reaction time of 120 min, and the rotational speed of the salt-bath furnace was 30 r/min.

Figure 5a shows the relationship of the montmorillonite reaction degree in sodium aluminate solution as a function of time at different alkali concentrations. The reaction degree of montmorillonite increases with higher N_k concentrations. When N_k reaches 240 g/L, the degree of the reaction peaks after 120 min. Further increases in N_k do not enhance the reaction extent of montmorillonite. Instead, a certain extent of decline is observed in the reaction degree of montmorillonite as N_k continues to rise. At the same alkali concentration, the degree of the reaction gradually increases with extended reaction time. There is no significant upward trend in the reaction degree of montmorillonite after 120 min. Kinsela [26] et al. found that the change of pH under alkaline conditions would not have a significant impact on the structure and hydraulic characteristics of montmorillonite. This suggests that the promoting effect of N_k on the reaction degree of montmorillonite is within a limited pH range. However, this mechanism cannot explain why the increase of N_k promotes the reaction degree of montmorillonite. Kuwahara [27] found that the dissolution rate of montmorillonite under alkaline conditions is correlated with [OH]⁻ ions and that a large amount of protonated Al-OH groups in alkaline solution catalyzed [OH]⁻ ions to destroy the Al-O-Si bonds of montmorillonite, which could explain the increase of caustic alkali concentration is beneficial to the dissolution of montmorillonite in the initial stage of the reaction under the calcium sodium aluminate solution.

It can be seen from Figure 5b that the mass of SiO₂ in the liquid phase increased rapidly and reached a peak within 20 min of the initial reaction of montmorillonite before beginning to decline. Stewart [28] and Xiang [29] found that after prolonged erosion of montmorillonite under highly alkaline conditions, there was a decrease in the thickness of the bilayer of the montmorillonite crystalline layer, causing the colloid produced by montmorillonite hydration to disappear and creating numerous irregular small cracks on the edge surfaces of the matrix. This phenomenon can explain why the dissolution rate of montmorillonite in alkaline conditions increases with the enhancement of N_k. As the concentration of N_k increases, the mass of SiO₂ in the liquid phase also rises, reaching its peak when N_k = 280 g/L. However, excessively high concentrations of alkali can lead to over-expansion of montmorillonite, compromising the integrity of its crystal structure and diminishing the degree of the montmorillonite reaction. Consequently, the degree of montmorillonite reaction begins to decline when N_k is 280 g/L.

Taking the above considerations into account, when the N_k is equal to 240 g/L, it is selected as the optimal condition for the reaction in this article.

3.3. Effect of C/S on the Reaction Degree of Montmorillonite

The experiment was carried out at a temperature of 240 °C, N_k of 240 g/L, L/S ratio of 5 mL/g, and 120 min in the salt-bath furnace with a rotational speed of 30 r/min.

As shown in Figure 6a,b, it can be seen that an excess of calcium oxide is not conducive to the montmorillonite reaction degree under the high-calcium sodium aluminate solution system. The montmorillonite reaction degree and mass of SiO₂ in the liquid phase show an increasing trend when the C/S ratio is less than 3.5, which is attributed to the fact that the swelling and dispersion of montmorillonite into a paste occur during the heating process. It makes the initial montmorillonite surface area increase, resulting in the enhanced dissolution rate of montmorillonite. Xiang [29] et al. found that montmorillonite undergoes swelling in an alkaline solution, although the swelling performance declines rapidly and the dissolution rate is accelerated. However, the addition of excessive calcium oxide in the subsequent solution disrupts the equilibrium system of the solution, making the reaction slurry viscous and reducing the fluidity of the system. This negatively impacts the mass transfer process during the reaction, resulting in a decreased reaction degree of montmorillonite.

In summary, the optimum ratio of calcium oxide added to high-calcium sodium aluminate solution is 3.5.

3.4. The Process of Physical Phase Change of Montmorillonite

3.4.1. XRD Analysis of Montmorillonite

Under the high-calcium sodium aluminate solution system, the structure of montmorillonite will change differently under different experimental conditions. The hydrogarnet solid after 120 min of reaction at different reaction temperatures, N_k and C/S ratios was analyzed by XRD.

It can be seen from Figure 7a that the main components in the hydrogarnet solids after the reaction are hydrogarnet phase and a small amount of hydrated calcium aluminate phase. With the increase of the reaction temperature, the diffraction peak of the hydrated calcium aluminate phase disappears, leaving only the diffraction peaks of the hydrogarnet phase and a small amount of CaCO₃ (calcite) phase in the solid phase. Lu [19] et al. found that the reaction between montmorillonite and calcium oxide was controlled by diffusion and chemical reaction in the temperature range of 220–260 °C, with the hydrogarnet beginning to form after 200 °C. The mechanism of which indicates that high-reaction temperatures are necessary for the formation of the hydrogarnet phase in the montmorillonite. As the reaction temperature increases, the hydrated calcium aluminate phase is transformed into the hydrogarnet phase, while the calcite phase is formed by the reaction of calcium oxide, which is not completely reacted in the system with CO₂ in the air.

It can be seen from Figure 7b that with the increase of calcium oxide content in the solution system, the hydrated sodium aluminosilicate phase gradually disappears, while the diffraction peak of the hydrated calcium aluminosilicate phase appears. It is presumed that excessive calcium oxide content, makes the viscosity of the solution system rise, leading to decreased fluidity and hindered ion exchange. Consequently, the [H₂SiO₄]²⁻ ions in the liquid phase are unable to react with the hydrated calcium aluminate to generate the hydrogarnet phase [10].

It can be seen from Figure 7c that with the increase of N_k, the hydrated calcium aluminate phase in the hydrogarnet solid disappears, while the intensity of the diffraction peaks for the hydrogarnet phase increases. According to past findings [29], the formation of the hydrogarnet phase in the high-alkali system could be regarded as a process of substitution of the tetrahedral [OH]₄⁴⁻ in the hydrated calcium aluminate is substituted by [H₂SiO₄]²⁻, which is independent of the N_k of the alkali solution. It is speculated that the surface edge of the montmorillonite matrix is eroded under high-alkaline conditions, resulting in a large number of small gap cavities [30], thus accelerating the conversion rate of hydrated calcium aluminate into hydrogarnet, which indicates the positive linear relationship between N_k and montmorillonite reaction degree.

3.4.2. Morphological Analysis of Montmorillonite

Figure 8 and Figure 9 show the TEM and SEM morphology analysis of hydrogarnet solid after reaction (temperature of 240 °C, N_k of 240 g/L, C/S ratio of 3.5).

It can be seen that there are large agglomerates in the hydrogarnet solid after 120 min of reaction (Figure 8), which are composed of various types of spherical, lamellar, and other morphology particles interlaced and superimposed. EDS mapping of the spherical particles reveals enrichment of elements such as Ca, Al, Si, and O, suggesting the formation of hydrogarnet. Some areas of the aggregates exhibit an accumulation of Mg, which is presumed to be from the magnesium-containing matrix left over from the montmorillonite reaction.

As the reaction time progresses, significant changes occur in the morphology of montmorillonite. At the initial stage of the reaction (5 min), the montmorillonite feedstock retains a lamellar structure, and the matrix structure consists of lamellar, acicular, and massive flakes (Figure 9). After 20 min of reaction, the blocky and earthy aggregate structure of montmorillonite begins to disintegrate, with small spherical particles starting to cluster, surrounded by a few irregularly distributed flakes. After 40 min of reaction, the spherical particles in the matrix gradually increase, completely replacing the original lamellar flakes with more pores. With the prolongation of reaction time, the spherical particles form larger agglomerates, enhancing their sphericity while reducing internal voids, thereby increasing the density of the aggregates.

Energy dispersive spectroscopy (EDS) point scanning was conducted in Figure 9, and the relevant results are presented in

Table 2. EDS results of hydrogarnet solid at different times (mass fraction/%).

Location	Ca	Si	Al	Na	O	Mg
1	-	8.13	25.49	–	51.56	14.83
2	23.37	5.16	13.17	–	58.30	–
3	23.75	4.77	12.79	–	58.69	–
4	35.07	2.59	13.53	–	48.81	–
5	25.67	9.63	14.28	–	50.42	–
6	27.14	8.45	13.95	–	50.46	–
7	3.41	11.47	15.66	2.31	59.14	8.02
8	34.86	8.08	15.69	–	41.37	–

. The flaky particles at point 1 are mainly composed of Mg, Al, Si, and O, where there is no Ca element detected, suggesting that montmorillonite has not yet dissolved at the beginning of the reaction. In contrast, the spherical and massive particles at points 2, 3, 4, 5, 6, and 8, where are mainly composed of Ca, Al, Si, and O. According to the XRD analysis results and elemental composition of the hydrogarnet solid, these are identified as determined to be hydrogarnet grains. The difference in element content is speculated to be caused by different silicon saturation coefficients of hydrogarnet at different stages [30]. The irregular lumps at point 7 have a lower concentration of Ca in the main composition, which is judged to be unreacted montmorillonite, suggesting that the montmorillonite is not fully dissolved under the high-calcium sodium aluminate solution system.

In the high-calcium sodium aluminate solution system, the crystal structure of montmorillonite breaks down, causing a large amount of silicon to dissolve and enter the sodium aluminate solution. A portion of the montmorillonite reacts with the sodium aluminate solution to form Na₆(AlSiO₄)₆, resulting in the loss of alkali and aluminum. As the reaction of montmorillonite progresses, Na₆(AlSiO₄)₆ gradually disappears. Meanwhile, the aluminum and silicon entering the liquid phase form Al(OH)₄⁻ and H₂SiO₄²⁻. Finally, the Ca(OH)₂ in the solution reacts with Al(OH)₄⁻ to produce (CaO)₃Al₂O₃·6H₂O, which ultimately reacts with H₂SiO₄²⁻ in the liquid phase to form the insoluble Ca₃Al₂(SiO₄)(OH)₈, leading to further loss of aluminum in the solution.

Overall, during the actual leaching process of bauxite, the montmorillonite in the bauxite reacts with the sodium aluminate solution to produce the insoluble Na₆(AlSiO₄)₆ and (CaO)₃Al₂O₃·6H₂O. The formation of these compounds consumes a large amount of alkali and aluminum, resulting in a decrease in the leaching efficiency of bauxite.

3.4.3. Morphological Analysis of Montmorillonite

Figure 10 shows the ²⁷Al and ²⁹Si MAS–NMR spectra of the montmorillonite feedstock and hydrogarnet solid (temperature of 240 °C, N_k of 240 g/L, C/S ratio of 3.5, L/S ratio of 5 mL/g, and reaction time of 120 min).

As shown in Figure 10a, the ²⁷Al spectrum of the montmorillonite shows a resonance peak for octahedral aluminum Al(VI) at 4.93 ppm and a resonance peak for tetrahedral aluminum Al(IV) at 57.099 ppm. The Al(IV) peak arises from the substitution of Si⁴⁺ by Al³⁺ in the silicate tetrahedral sheets. After the reaction, significant changes occur in the chemical environment and coordination structure of aluminum in the hydrogarnet solid. The chemical environment and coordination structure of Al in the reacted hydrogarnet solid are changed considerably. New chemical shifts appear at 4.95 ppm and 13.30 ppm for Al(VI) and 61.25 ppm for Al(IV). Additionally, symmetrical spinning sideband peaks are observed at −81.91 ppm and 110.08 ppm [31,32]. Based on the intensity of the NMR peaks, it can be observed that the signal for Al(IV) tends to disappear. This indicates a higher weight ratio of Al₂O₃ to SiO₂ in the hydrogarnet solids after the reaction. With the crystal structure of montmorillonite is destroyed, the SiO₂ component in the montmorillonite is transformed into the hydrogarnet phase in the high-calcium sodium aluminate solution.

As shown in Figure 10b, the ²⁹Si spectrum of montmorillonite features a resonance peak at −93.86 ppm, corresponding to the Q4 environment of silicon. The resonance signal at −109.62 ppm is the impurity component quartz in montmorillonite, which is in agreement with the related report [33]. The disappearance of the signal at −93.86 ppm and the generation of a new resonance peak at −79.76 ppm in the hydrogarnet solid, indicates that the lamellar structure of montmorillonite is destroyed after the reaction. Under the high-calcium sodium aluminate solution system, the SiO₂ within montmorillonite transforms into the hydrogarnet phase, resulting in a shift from the Q4 environment to a Q2 environment for silicon.

3.4.4. FTIR Analysis of Montmorillonite

Figure 11a,b show the results of FTIR analyses of montmorillonite raw materials and hydrogarnet solids (temperature of 240 °C, N_k of 240 g/L, C/S ratio of 3.5, L/S ratio of 5 mL/g, and reaction time of 120 min).

It can be seen from Figure 11 that the Al-OH stretching vibration absorption peak of montmorillonite at 795 cm⁻¹ disappears with the reaction, which indicates a breakdown of the montmorillonite structure. The Si-O-Si stretching vibration absorption peak at 991 cm⁻¹ and the bending vibration absorption peak for H-O-H at 629 cm⁻¹ undergo a blue shift. The peak values shift to higher frequencies, and the intensity of these peaks decreases. This phenomenon is caused by the structural integrity of montmorillonite being compromised, leading to its dissolution in the sodium aluminate solution. As montmorillonite dissolves, silicon is released in the form of [H₂SiO₄]²⁻ ions, which subsequently react with calcium oxide and [Al(OH)₄]⁻ to form hydrogarnet.

4. Conclusions

The article studied the phase and structure changes of montmorillonite in the high-calcium sodium aluminate solution system. By analyzing the mass of SiO₂ in both the solid phase and liquid phase after the reaction, which studied the influence of different experimental factors on the reaction degree of montmorillonite. The experimental results showed that the reaction degree of montmorillonite can reach 93.7% under the optimum conditions of reaction temperature of 240 °C, N_k of 240 g/L, C/S ratio of 3.5, L/S ratio of 5 mL/g, and reaction time of 120 min.

The characterization analysis of the products after the reaction of montmorillonite showed that silicon in montmorillonite existed in the form of hydrogarnet solid. This phenomenon is caused by the solution that montmorillonite dissolved in sodium aluminate solution, with silicon in montmorillonite endowed in the liquid phase as hydrated sodium aluminosilicate. Under the optimal experimental conditions, calcium oxide added in the liquid phase will react with the [Al(OH)₄]⁻ in the solution to form hydrated calcium aluminate. Subsequently, this compound will react with the hydrated sodium aluminosilicate in the liquid phase at elevated temperatures, leading to the formation of hydrogarnet. Finally, both the hydrated sodium aluminosilicate and hydrated calcium aluminate disappear from the solution, and the silicon from montmorillonite is retained in the form of hydrogarnet. The formation of hydrogarnet marks the reaction of montmorillonite under the high-calcium sodium aluminate solution system.