Spatiotemporal Evolution of Red Mud Flocculated Structure During Self-Weighted Siltation and Macro–Micro Correlation Modeling

Abstract

In high-concentration tailings backfilling, the stability of the backfill largely depends on the slurry’s self-weighted siltation behavior. Red mud—the strongly alkaline by-product of the Bayer process and a mixture of coarse and fine particles—requires a clear understanding of its self-weight settling–consolidation mechanisms to ensure safe and efficient backfilling. In this study, red mud slurry was selected as the research object, and a multi-scale approach combining sedimentation column tests and microstructural image analysis was employed to reveal the intrinsic relationships among the sediment layer height, sedimentation rate, and pore structure changes during self-weighted siltation. The results show that the flocculated structure of red mud slurry exhibits distinct stratification during self-weighted siltation, including a clarified layer, a structural transition layer, and a dense sediment layer. During the siltation process, the sediment layer height, sedimentation rate, and floc structure of red mud evolve nonlinearly. The addition of flocculants significantly enhances the formation rate of flocculated structures but increases the porosity of the sediment body. At the macroscopic level, this results in a shortened self-weighted siltation time and increased final sediment layer height. To describe the regulatory effect of flocculants on red mud floc structure, a macro–micro predictive model for the sediment layer height was established by introducing a structural enhancement coefficient, considering the effect of flocculants. The model achieved a prediction error within 16%. These findings provide theoretical support for structural control technologies and process optimization of high-concentration fine tailings backfilling, thereby contributing to the sustainable utilization of red mud and the development of environmentally responsible backfilling practices.

1. Introduction

Red mud, a strongly alkaline by-product of alumina production, is a coarse–fine particle mixture whose self-weighted siltation behavior differs markedly from conventional tailings [1,2,3,4]. Its complex mineral composition and wide particle size distribution [5,6,7] lead to poor sedimentation and dewatering, limiting engineering adaptability [8,9,10]. During siltation, red mud often shows blurred interfaces, slow dewatering, and unstable structure formation, reducing filling efficiency and posing environmental risks. These issues make its use in tailings backfilling both technically and environmentally challenging.

Research on the flocculated sedimentation of tailings has mainly focused on the role of polymer flocculants. Laboratory studies [11,12,13] have demonstrated that polymers enhance particle bonding through hydrogen bonding and charge neutralization, and that the sequence of flocculant–coagulant addition influences floc compactness and settling efficiency. In red mud systems, strong alkalinity and salinity further alter inter-particle forces, modifying the pathways of floc formation [14,15]. To understand these mechanisms, advanced techniques such as XRD, CT, and microscopic imaging have been increasingly employed to clarify structure–property relationships [16,17,18,19,20]. Shear tests [21] and molecular simulations [22] have further revealed how particle size ratios and micro-contact forces affect structural evolution. Sedimentation–consolidation models have also been applied to systems such as slag, tailings sand, and tungsten tailings, enabling quantitative assessment of the effects of fine particle content on settling behavior [23,24,25].

The sedimentation column test is a primary method for studying the self-weighted siltation behavior of mud and slurry, and extensive experimental and theoretical research has been conducted by scholars both domestically and internationally. Using such approaches, previous work demonstrated that flocculation accelerates the settling of fine particles [26], which slurries stratify into clarified, sedimentation, and consolidation zones [27,28], and that large-scale monitoring systems can validate consolidation theory under realistic conditions [29]. However, these studies have largely emphasized macroscopic parameters such as the interface position and settlement rate, while paying insufficient attention to the coupling between microstructural evolution and macroscopic behavior. In particular, the spatiotemporal evolution of flocculated structures in coarse–fine mixtures such as red mud, and their influence on large-scale settling responses, remains poorly understood.

To address these issues, this study investigates the evolution of flocculated structures in strongly alkaline, high-concentration red mud slurry and their influence on macroscopic siltation, using macro–micro observations from sedimentation column tests. A flocculation enhancement coefficient (λ) was introduced to establish a macro–micro predictive model for the sediment layer height considering the effect of PAM flocculants. The results deepen academic understanding of structure–behavior coupling in coarse–fine particle systems and provide a theoretical basis for structural regulation and process optimization. The study also enhances the theoretical understanding of red mud behavior in backfilling systems and highlights its significance for sustainability by supporting the sustainable utilization of industrial residues, improving resource efficiency, and reducing environmental risks in mining operations.

2. Materials and Methods

2.1. Test Material

The red mud samples used in this study were collected from the Wujiagou tailing pond in Nanchuan, Chongqing. After collection, the samples were naturally air-dried to a constant moisture content of approximately 40%, and impurities with particle diameters greater than 1 mm were removed prior to testing. The mineralogical composition of the red mud was determined using a Rigaku Ultima IV X-ray diffractometer (XRD). As shown in Figure 1, the main mineral components of the red mud sample are calcite and magnesite (with a total content of 60.4%), accompanied by smaller quantities of quartz, hematite, zircon sand, magnetite, and hydrotalcite.

The test specimens belong to a typical coarse–fine particle mixture system. The particle size distribution of the red mud samples was measured using a Mastersizer 2000 laser particle size analyzer, and the results show a wide particle size range and a non-uniform particle distribution. The coefficient of uniformity ($C_u$) is as high as 106.48, indicating a broad particle size range; the coefficient of curvature ($C_C$) is only 0.41, far below the well-graded standard range of 1–3. According to commonly used grading evaluation criteria, red mud is classified as a typical gap-graded soil, mainly due to the insufficient content of medium-sized particles, which leads to poor particle interfill and a loose structure.

According to the Chinese national standard GB/T 50123-2019 Standard for Soil Test Methods [30], the basic physical properties of the milled red mud were measured, mainly using indicators such as the moisture content, dry density, and Atterberg limits to reflect the flow properties of the red mud slurry. The results are shown in

Table 1. Physical properties of red mud.

	Moisture Content (%)	Dry Density (g/cm3)	Liquid Limit (%)	Plastic Limit (%)	Plasticity Index
Red mud	40.19	1.61	44.65	36.70	7.95

.

The polyacrylamide (PAM) flocculants used in the experiments—anionic (APAM), cationic (CPAM), and nonionic (NPAM)—were all obtained from a professional chemical supplier. The molecular weights of APAM and CPAM were both 12 million g/mol, while that of NPAM was 15 million g/mol. To ensure consistent flocculation performance and comparability of experimental results, the optimal flocculant dosage determined by Wang et al. [31] in their study on flocculation and sedimentation was adopted. A quantity of 0.2 g of PAM powder was precisely weighed and added to 1000 mL of deionized water, followed by continuous stirring until complete dissolution was achieved. The resulting solution was prepared as a homogeneous and transparent flocculant with a concentration of 0.02 wt% and stored for subsequent use.

2.2. Test Methods

A settling column is a device used to observe sedimentation patterns [32]. The experimental observation system consisted of a 1 L graduated cylinder, a 300 r/min electric stirrer, and a Leyue Z01-5 high-definition microscopic camera.

Preliminary test results showed that using a settling column with a small diameter caused a pronounced wall-induced retardation effect on the slurry, resulting in faster sedimentation, a thinner sediment layer, and unfavorable conditions for observing the sedimentation process and stratification characteristics. Conversely, an excessively large column size would lead to excessive equipment weight, making sample loading, handling, and observation difficult. After repeated trials, a 1 L graduated cylinder was ultimately selected as the settling column. To facilitate the observation of changes in the sediment layer height, a flexible scale was affixed vertically to the outer surface of the settling column.

Each set of tests was repeated three times, and the average value was taken as the final data. If the difference in settlement height between repeated tests exceeded 5%, an additional test was added to ensure data reliability.

Polyacrylamide-based flocculants (PAM) are the most widely used red mud flocculants in the alumina industry and were also the earliest synthetic organic polymer flocculants applied in the red mud sedimentation–separation process. Four groups of settling column tests were conducted, including a no-flocculant group, an anionic polyacrylamide (APAM) group, a cationic polyacrylamide (CPAM) group, and a nonionic polyacrylamide (NPAM) group, serving as mutual control groups to simulate the self-weighted siltation process of red mud under different operating conditions. The experimental scheme is shown in

Table 2. Experimental design.

Serial Number	Group	Slurry Mass Concentration (%)	Flocculant Dosage	Settling Column Dimensions	Simulated Conditions
1	No-flocculant	20%	0 mL, 0 wt%	Height: 410; Diameter: 70	Backfill drainage
2	APAM	20%	20 mL, 0.02 wt%	Height: 410; Diameter: 70	Deep-cone gravity settling
3	CPAM	20%	20 mL, 0.02 wt%	Height: 410; Diameter: 70	Deep-cone gravity settling
4	NPAM	20%	20 mL, 0.02 wt%	Height: 410; Diameter: 70	Deep-cone gravity settling

.

The specific experimental procedure was as follows:

(1)

Weigh 200 g of dry red mud and mix with 800 g of deionized water, stirring thoroughly to prepare a red mud slurry with a mass fraction of 20%.

(2)

Add the prepared PAM solution (20 mL, 0.02 wt%) into the slurry, stir uniformly with the mixer for 10 min, and then quickly pour the homogeneous mud–water mixture into the standard settling column, filling to a total volume of 1000 mL.

(3)

Continuously record the dynamic changes in the sediment layer height. The position of the water–mud interface was recorded every 1 min for the first 10 min, then every 5 min until 60 min, and subsequently every 10 min until the sediment layer height no longer changed. Meanwhile, microscopic images of the flocculated structures were captured using a high-definition microscopic camera.

Each test was repeated three times, and the average value was taken as the result. If the difference in the sediment layer height among repeated tests exceeded 5%, an additional test was performed to ensure data reliability.

3. Results and Analysis

3.1. Macroscopic Evolution of the Red Mud Sediment Layer

The variation in the sediment layer height of red mud exhibits a nonlinear characteristic. Figure 2 illustrates a typical evolution process of the sediment layer height with time. During self-weighted siltation, the sediment layer height of red mud shows a distinct “L”-shaped declining trend, reflecting a pronounced nonlinear feature. Based on the variation in the sediment layer height, the self-weighted siltation process of red mud can be divided into three stages: a rapid settling stage, a slow consolidation stage, and a stable sedimentation stage.

Rapid settling stage (0–35 min): At the initial stage (approximately 2 min), the sediment layer height remains high, the mud–water mixture is uniform, and stratification is indistinct. As siltation proceeds, the water–mud interface gradually becomes clear at around 35 min, accompanied by a marked decrease in the sediment layer height. During this stage, the interface forms rapidly, and the height drops sharply from about 11.5 cm to below 8.0 cm, indicating rapid particle settling. The structure is not yet compacted, and the settling response is dominated by water release and interface descent.

Slow consolidation stage (35–90 min): The settling rate gradually decreases, and changes in the sediment layer height become slower, tending toward stability in the later period, ultimately forming a dense sediment structure. At this stage, the reduced rate of height change indicates increased structural compaction, marking the transition of the settling response from free settling to drainage consolidation dominance.

Stable sedimentation stage (90–210 min and beyond): The sediment layer height remains essentially stable between 7.2 and 8.1 cm, with an extremely low settling rate, cessation of particle movement, and attainment of the final compact state of the flocculated structure, forming a complete and stable sediment body. In some test groups, the stable height was reached as early as 90 min, whereas in others, it required up to 210 min, indicating that different structural development rates exert a significant regulatory effect on the settling response.

Overall, the sediment layer height exhibits an initial rapid decline, followed by gradual convergence, and finally stabilization, reflecting that under self-weight, the red mud undergoes a complete evolution process of settling–consolidation–stable layering. The variation pattern of the sediment layer height can serve as an important indicator for both the progression of settling and the structural formation state.

Figure 3 illustrates the variation trend of the average settling rate during the sedimentation process of red mud, along with the rate characteristics at different stages. Overall, the settling rate decreases markedly with time, showing a clear stage-wise behavior.

In the initial settling stage (0–10 min), particles in the slurry have not yet fully combined to form a stable structure; the flocs are loose, the water content is high, and particles settle rapidly under gravity, leading to a sharp increase in the settling rate, with peak values generally reaching around 20 cm/min. This belongs to the rapid settling phase. At this stage, the structure is not yet compacted, the settling behavior is mainly governed by free settling, the flocculated structure is in the early phase of formation and expansion, particle–particle connections are weak, and the overall settling response is characterized by high mobility and rapid interface descent.

In the intermediate settling stage (10–30 min), adsorption–bridging effects among flocs continue to strengthen, and inter-particle structures gradually become tighter, leading to the onset of structural compaction and stabilization. The settling rate drops significantly in this stage; for example, in some tests, it decreased from 20.4 cm/min in the initial phase to below 2.4 cm/min. The slope of the settling curve decreases, the descent of the water–mud interface slows down, and the stabilization of the flocculated structure begins to constrain particle movement. The rapid attenuation of the settling rate reflects the significant control exerted by structure formation on settling behavior.

In the late settling stage (30–300 min), the flocculated structure becomes further compacted, the degree of sediment layer consolidation increases, the inter-particle porosity decreases markedly, the rate of water expulsion slows, and the settling rate approaches extremely low values before finally stabilizing. In some test groups, the rate dropped below 0.1 cm/min, entering the slow consolidation phase. At this stage, the morphology of the sediment layer is essentially formed, the structural evolution is complete, and the settling response nearly ceases. The rate change remains stable, serving as an important indicator of the stability of the flocculated structure and the final state of sedimentation.

It should be noted that the time divisions of the settling rate stages differ from those of the sediment layer height variation stages. This discrepancy arises because the rate curve reflects the instantaneous movement state of the interface, whereas the height curve represents the cumulative settling effect; the two respond to structural evolution on different time scales.

Therefore, the temporal variation in the settling rate not only reflects the stage-wise characteristics of red mud sedimentation but also captures the complete evolution of the flocculated structure from formation to compaction and finally to stabilization. Changes in the settling rate are a direct response to structural evolution: faster rates indicate that the structure is not yet stable; gradual slowing reflects progressive compaction; and stabilization of the rate signifies the completion of sediment layer formation. Consequently, the settling rate can serve as an important indicator of the development state of the flocculated structure and provides a quantitative basis for elucidating the sedimentation–consolidation mechanism of red mud.

As shown in Figure 4, flocculants exert a significant influence on the self-weighted siltation process of red mud. The addition of flocculants markedly shortens the siltation time but increases the final porosity of the flocculated structure. Figure 4 shows that, in the initial stage, the sediment layer height is relatively high and then decreases rapidly, with a distinct water–mud interface forming at approximately 35 min. Thereafter, the process enters the slow consolidation stage, during which the sediment layer height gradually approaches stability, ultimately stabilizing between 7.2 and 8.1 cm. In terms of siltation duration, some tests showed consolidation stabilization times of up to 210 min, whereas others reached stability within 90 min, indicating a substantial difference in the settling and consolidation rates. Regarding the settling rate, the highest initial average settling rate reached 0.267 cm/min, while the lowest among the test groups was only 0.118 cm/min, demonstrating that the settling rate has a pronounced effect on the formation of the sediment layer height. Overall, the self-weighted siltation of red mud exhibits a settling behavior characterized by an initial rapid phase followed by a slower phase, leading to a stable final structure. The sediment layer height and settling rate show a strong correspondence, reflecting the dynamic evolution pattern of the settling–consolidation process.

As shown in Figure 5, the sediment layer height in all four test groups exhibited a typical “L”-shaped variation pattern, characterized by a rapid decline followed by gradual stabilization, indicating that the red mud settling process is distinctly stage-wise rather than uniform. Compared with the no-flocculant control group, the polyacrylamide-added groups rapidly formed a distinct water–mud interface during the initial settling phase, whereas the control group showed no obvious stratification. In the final consolidation stage, the flocculant groups reached stability in the sediment layer height within 90 min, with similar final heights across the groups, while the no-flocculant group required 210 min to stabilize and attained a final sediment layer height of only 7.2 cm—significantly lower than that of the flocculant groups. This indicates that flocculants can effectively accelerate the settling rate but have a limited impact on the final sediment layer height. The mechanism lies in the fact that, during the initial settling stage, flocculants promote the rapid aggregation of fine particles into large flocs via adsorption–bridging effects, thereby accelerating settling and producing a clear interface. In the later consolidation stage, the three-dimensional structure of the flocs inhibits excessive compaction, maintaining a higher sediment layer height and significantly improving both the settling efficiency and structural stability.

As shown in Figure 6, the average settling rates of all four test groups exhibited a typical “L”-shaped declining trend, indicating that red mud settling undergoes two distinct processes: a rapid settling stage and a stable consolidation stage. Experimental data show that the addition of flocculants can significantly enhance the settling rate, with the effect being most pronounced for nonionic PAM (NPAM). Within the first 10 min, the settling rate of the NPAM group dropped sharply from 20.4 cm/min to 2.379 cm/min, representing the largest decline among all groups. Between 10 and 30 min, the rate continued to decrease to 0.797 cm/min, again showing the greatest reduction. During 30–300 min, the settling rate of the NPAM group stabilized at 0.0802 cm/min, which remained lower than that of the other groups. Overall, the results indicate that flocculants markedly accelerate the initial settling process of red mud, with NPAM showing the optimal performance in both accelerating settling and stabilizing structure due to its stronger adsorption–bridging capability.

3.2. Spatiotemporal Evolution of Flocculated Structure in Red Mud

The flocculated structure of red mud slurry during self-weighted siltation exhibits a distinct spatially nonlinear stratification pattern. After the red mud slurry is left to stand in the settling column, three layers form from top to bottom: a clarified layer, a structural transition layer, and a dense sediment layer. In the clarified layer, many fine particles remain suspended in the early stage of settling, resulting in turbid water with low transparency; continuous particle settling can be observed near the water–mud interface. The structural transition layer, located at the surface of the sediment layer, contains flocs formed by fine particles under the combined action of gravity and buoyancy. This layer is loosely structured, and drainage channels appear in a crack-like pattern. In the dense sediment layer, red mud particles are highly compacted with significantly reduced porosity, and the particles are in close contact, forming a stable packed structure. The dense structure and restricted drainage channels cause the settling rate to approach extremely low values, and the overall sediment layer height remains essentially stable.

During the self-weighted siltation process, the morphology of the flocculated structure of red mud evolves continuously over time, as shown in Figure 7. The flocculated structure gradually changes from an initially loose and dispersed state to a later dense and stable configuration. In the initial settling stage, inter-particle pores are large, contact points are few, and flocs are loosely arranged, with the overall structure uniformly suspended. As time progresses, the upper suspension layer gradually thins, particle bonding within the lower sediment layer strengthens, pores are progressively filled by fine particles, and the structure becomes increasingly compacted. In the stable stage, the coarse particle skeleton and fine particle infill together form a multi-scale support system. The sediment body is fully densified, particle contacts become tight, and the overall structure tends toward stability. This morphological evolution process reflects the continuity and stage-wise characteristics of red mud transitioning from dispersion to compaction and finally to stabilization under self-weight.

Flocculants have a significant impact on the flocculated structure of red mud. Microscopic observations indicate that the structural characteristics of the structural transition layer determine the self-weighted siltation behavior of red mud. After adding flocculants, the zoned structure of the red mud slurry changes markedly: fine particles, under the adsorption–bridging action of the flocculant, form large and loose flocs that interconnect to create a flocculated structural body. This increases the overall volume and porosity, while the turbidity of the supernatant decreases, though it remains slightly cloudy. As shown in Figure 8, after the addition of PAM, the clarified layer is noticeably clearer than that without flocculant, and particles in the vicinity of the water–mud interface appear as blocky floc networks with uneven surfaces, in contrast to the disordered particle packing of the control group. Within the structural transition layer, distinct pores are formed, providing drainage–consolidation channels for the dense sediment layer. These pores are gradually compressed over the settling time but never completely disappear, further confirming the promoting effect of PAM on the self-weighted siltation of red mud.

As shown in Figure 9, the microscopic morphology of the structural transition layer varies under different flocculation conditions. Without flocculant, red mud particles exhibit high dispersibility, with pronounced inter-particle gaps and a loose structure. With the addition of nonionic polyacrylamide (NPAM), irregular flocs begin to form between particles, resulting in a relatively compact structure. Under cationic polyacrylamide (CPAM), fine flocs adhere to particle surfaces and undergo pronounced aggregation, forming a porous skeleton-like sediment. In samples treated with anionic polyacrylamide (APAM), particle bonding is denser, pores are fully filled with fine particles, and the structure is markedly compacted. These morphological changes are primarily attributed to the adsorption–bridging action of polyacrylamide molecular chains, which effectively reduce electrostatic repulsion between particles, promote adhesion and aggregation, increase the effective particle size, and thereby significantly accelerate the self-weighted siltation process while enhancing the density of the sediment layer.

As shown in Figure 10, with respect to the water–mud interface, the red mud sediment layer under no-flocculant conditions is mainly formed through the combined effects of particle self-weight and overlying pressure. The particles are relatively dispersed in distribution, and although the particle arrangement in the structural transition layer and the sediment layer is relatively tight, drainage channels are limited, resulting in a slower consolidation process. The addition of PAM markedly improves the structural morphology of the red mud sediment layer, producing a distinct floc network structure. Microscopic observations reveal notable differences in the floc structures formed by different types of PAM. NPAM produces larger and more compact flocs with uniformly distributed pores, which facilitate rapid drainage. In contrast, the flocs formed by anionic (APAM) and cationic (CPAM) polyacrylamides are relatively loose, with irregular pore structures, leading to lower settling and consolidation efficiency.

As shown in Figure 11, different flocculants have a pronounced effect on the microscopic morphology of the flocculated structure in the sediment layer. Without flocculant, particles in the sediment layer are uniformly dispersed, loosely structured, and lack distinct aggregation units. With the addition of nonionic polyacrylamide (NPAM), loose flocs form between particles, and structural porosity is evident. Under cationic polyacrylamide (CPAM), flocs bond more tightly, forming a porous block-like structure. Under anionic polyacrylamide (APAM), particle aggregation is the densest, pores are fully filled with fine particles, and the structural compaction is the highest. These differences primarily arise from variations in the adsorption–bridging action and charge neutralization capability of different PAM types, which lead to differences in particle bonding strength and structural densification, thereby altering the spatial structural characteristics of the sediment layer.

Figure 12 provides an intuitive illustration of the adsorption–bridging mechanism between polyacrylamide (PAM) flocculants and red mud particles. In the initial stage, long-chain PAM molecules adsorb onto the surfaces of red mud particles, forming unstable flocculation monomers. Subsequently, through secondary self-adsorption, these monomers become more stable and further interconnect via bridging, ultimately forming structurally robust flocs. This process not only reduces the distance between particles and increases the effective particle size but also markedly enhances floc stability, thereby effectively promoting the rapid settling and consolidation of red mud particles.

Overall, during the settling process, the flocculated structure of red mud undergoes a temporal evolution from formation to compaction and finally to a dense and stable state. Changes in the sediment layer height provide a direct reflection of this process. The sequential nature of structural development determines the rate and stability of the settling response, and the variation pattern of the sediment layer height can serve as a quantitative indicator of the evolution state of the flocculated structure. This is of great significance for understanding the settling–consolidation mechanism of red mud and for optimizing backfilling processes.

3.3. Macro–Micro Coupling Mechanism of Self-Weighted Siltation in Red Mud

During the self-weighted siltation process of red mud, the evolution of the microscopic structure between particles not only determines the stage-wise characteristics of the settling and consolidation behavior but also profoundly influences the macroscopic response of the final sediment layer morphology. The preceding analyses have revealed the temporal variation patterns of the sediment layer height and settling rate during red mud settling, as well as the evolutionary characteristics of the flocculated structure at different time intervals.

Fundamentally, however, the settling behavior originates from the dynamic reconstruction of the microstructure. The modes of particle aggregation, compaction, and floc connection at the microscopic scale constitute the structural foundation of the macroscopic self-weighted siltation process of red mud. Therefore, in this section, microscopic image observations and zonal analysis of the settling process are employed to examine the spatial distribution characteristics and evolutionary mechanisms of the flocculated structure at different stages, thereby further elucidating the coupling relationship between macroscopic settling behavior and microscopic structural states.

As shown in Figure 13, the no-flocculant group exhibits a loose soil structure and a delayed settling response. The settling stage lasts up to 60 min, during which the sediment layer height slowly decreases from approximately 9.0 cm to 8.2 cm, with a settling rate of only 0.4 cm/min. Microscopic images reveal that the clarified layer remains turbid, particles settle slowly, the structural transition layer has a loose particle arrangement, and the dense layer is not fully compacted, presenting a “porous low-density packing” morphology. Macroscopically, the process is characterized by slow settling and a blurred interface, while microscopically it reflects disordered particle distribution and a lack of effective connections, resulting in poor drainage pathways and hindered settling. The looseness of the structure is the core mechanism behind the delayed settling and reduced height, demonstrating the consistency between macro- and micro-scale responses.

As shown in Figure 14, the APAM group exhibits rapid structural formation, with the rapid settling stage shortened significantly to only 4 min, and the average settling rate increased to 1.98 cm/min. The sediment layer quickly reaches a height of 8.2 cm. Microscopic images reveal a distinct flocculated structure, in which particles are bound together into large block-like aggregates through adsorption–bridging action, and the water–mud interface is clear, indicating a rapid and well-defined settling process. The primary consolidation stage lasts only 20 min, during which pore compression occurs quickly, water is expelled rapidly, and the pores between flocs in the structural transition layer are tightly distributed, forming efficient drainage channels. The acceleration of macroscopic settling is closely associated with the rapid structural formation, indicating that the structural enhancement mechanism effectively drives the transition of settling behavior.

As shown in Figure 15, the CPAM group exhibits moderate structural compactness. The settling stage lasts for 10 min, with a settling rate of 2.37 cm/min. Microscopic images show clearly defined block-like flocs, although some regions display a loose structure with uneven pore distribution. The water–mud interface is distinct but slightly turbid. The primary consolidation stage lasts for 40 min, with a stable compaction process, and the final sediment layer height reaches 8.05 cm. This group demonstrates good coordination between structural compactness and the settling–consolidation response, with the macroscopic process being governed by the quality of floc formation and the strength of particle bonding. The stability of the microstructure directly reflects the “rate–compaction” evolution during settling.

As shown in Figure 16, the NPAM group exhibits the most compact structure and the strongest settling response. The settling stage is the shortest, lasting only 6 min, with the highest settling rate of 3.95 cm/min. Microscopic observations reveal large flocs with a dense structure, uniformly distributed pores, unobstructed drainage channels, the clearest clarified layer, and an extremely distinct water–mud interface. The primary consolidation stage lasts for 44 min, but with a high consolidation rate, water is rapidly expelled, and after secondary consolidation, the sediment layer height stabilizes at 8.1 cm. This group demonstrates the most efficient macroscopic settling behavior and the densest microstructure, highlighting the significant promoting effect of microstructural enhancement on macroscopic settling–consolidation efficiency, forming a typical “strong-structure–fast-response” coupling mechanism.

The four groups of tests demonstrate that the macroscopic response of self-weighted siltation in red mud is highly dependent on the development state of the flocculated structure. The rate of structure formation, degree of compaction, and pore distribution characteristics directly determine the settling rate and consolidation duration, indicating a clear structure–behavior coupling relationship between the macro- and micro-scales. Combined analysis of the microscopic observations and settling curves reveals the synergistic process of particle bonding, pore compaction, and interface migration.

3.4. Macro–Micro Predictive Model for Sediment Layer Height Considering Flocculant Effects

In order to quantitatively characterize the self-sedimentation of red mud slurry under the action of polyacrylamide (PAM) flocculant, and to deeply elucidate the influence mechanism of PAM on the self-sedimentation process, a self-sedimentation prediction model applicable to the situation of a lack of fine parameters such as the pore ratio and floc size is proposed on the basis of the previous theories and combined with the actual data in the present experiments. The flocculation enhancement coefficient ($\lambda$) and the initial sedimentation rate ($v_0$) were introduced during the model derivation to explicitly reflect the microstructural influence, which is difficult to be directly measured, in the prediction of macroscopic sedimentation. The validation analysis of the red mud self-weighted siltation data under the action of different types of PAMs clarifies the effectiveness of the model in quickly evaluating and quantifying the effect of the action of different flocculants. Qin [33] et al., based on the similarity of slurry settlement and self-weighted consolidation as the critical transition point of the settlement and consolidation process, reported the quantitative prediction of the tailing slurry self-weighted siltation process is realized by the analytical solution of the two-phase superposition.

$$S_t={\displaystyle \frac{e_0-e_{AEV}}{1+e_0}}\times h_0+{\displaystyle \sum }_{i=1}^n{\displaystyle \frac{e_{AEV}-e_i}{1+e_{AEV}}}\times h_2$$

(1)

where $S_t$ represents the total settlement, $e_0$ represents the initial pore ratio, $e_i$ represents the pore ratio at the end of the ith layer of consolidation, $h_0$ represents the initial height, $h_2$ represents the thickness of the layered layers, $n$ represents the number of layered layers, and $e_{AEV}$ represents the air entry value (AEV) void ratio, i.e., the void ratio at which tailings particles begin to come into contact with each other and pore water is replaced by air.

Since the original model does not account for the influence of flocculants, a revision was made accordingly.

Considering that the settling of red mud undergoes two stages—free settling and self-weight consolidation—and drawing on the concept of the two-stage self-weight settling–consolidation model proposed by Qin et al. [33]., the total settlement ratio $S_t$ can be approximately decomposed into the sum of the contribution from the free settling stage $S_c$ and that from the self-weight consolidation stage $S_s$:

$$S_t=S_s+S_c$$

(2)

In the equation, $S_t$ denotes the total settlement ratio; $S_c$ denotes the percentage of the settlement amount during the consolidation stage (i.e., the drainage consolidation stage) relative to the total settlement amount; and $S_s$ denotes the percentage of the settlement amount during the self-weight settling stage (i.e., the initial particle settling stage) relative to the total settlement amount.

$$S_C=k\cdot S_s$$

(3)

In the equation, $k$ denotes the empirical coefficient. Substituting Equation (3) into Equation (2) yields the following:

$$S_t=S_s+k\cdot S_s=\left(1+k\right)\cdot S_s$$

(4)

Based on the slurry settling test data, the value of $k$ ranges from 0.10 to 0.20, among which $k$ = 0.15 can be recommended as a typical value for slurry systems. This value enables simplification of the structural model and rapid prediction without significantly sacrificing accuracy.

The following equation is defined as follows:

$$S_s={\displaystyle \frac{e_{AEV}-e_i}{1+e_{AEV}}}\times h_0$$

(5)

For ease of calculation, the following approximation is made:

$$e_{AEV}\approx e_C$$

(6)

$$e_i\approx e_0$$

(7)

In the equation, $e_C$ denotes the critical void ratio, referring to the void ratio at which, under the action of the particles’ self-weight, the particles begin to settle and move closer to each other, and the inter-particle spacing is reduced to a critical state.

Substituting Equations (6) and (7) into Equation (5) yields the following:

$$S_s={\displaystyle \frac{e_C-e_0}{1+e_c}}\times h_0$$

(8)

Substituting Equation (8) into Equation (4) yields the following:

$$S_t=\left(1+k\right){\displaystyle \frac{e_c-e_0}{1+e_c}}\times h_0$$

(9)

In the initial settling stage, under the action of flocculants, particles form larger flocs, which accelerate settling and lead to an earlier onset of the particle contact stage. To quantitatively describe this effect, this study assumes that the strength of the flocculant effect can be reflected by the initial settling rate ($v_0$) and an empirical relationship is proposed as follows:

$$e_c={\displaystyle \frac{e_0}{1+\lambda v_0}}$$

(10)

In the original equation, the void ratio $e_0$ is defined directly; in the revised formula, the void ratio $e_0$ is expressed through conversion using the solid mass fraction $s$, and the specific gravity of soil particles $G_s$ is also employed as follows:

$$e_0=G_s{\displaystyle \frac{1-s}{s}}$$

(11)

In the model, the initial void ratio $e_0$ is converted based on the solid mass fraction $s$. The flocculation enhancement coefficient $\lambda$ can also be determined through simple experiments requiring only the initial slurry height $h_0$, the slurry height at 1 min $H_1$, and the final equilibrium height $H_{f.}$ By calculating the initial settling velocity ${\nu }_0$ and the corresponding initial void ratio $e_0$, the critical void ratio at the transition point between the settling and consolidation stages can be estimated, thereby allowing for the back-calculation of the flocculation enhancement coefficient λ:

$$e_c={\displaystyle \frac{H_f}{h_0}}\left(1+e_0\right)-1$$

(12)

$$\lambda ={\displaystyle \frac{\frac{e_0}{e_c}-1}{v_0}}$$

(13)

Substituting Equation (10) into Equation (9) yields the following:

$$S_t=\left(1+k\right){\displaystyle \frac{e_0-\frac{e_0}{1+\lambda v_0}}{1+e_0}}\times h_0$$

(14)

The original data of the NPAM test group were brought into the modified formula to verify the correctness of the formula, taking $h_0$ = 32 cm, $s_0$ = 0.2, $e_0$ = 10.6, 1 min settling rate ${\nu }_0$ = 20.4 cm/min, and taking $\lambda$ = 0.15, $k$ = 0.15, the resulting $H_f$ ≈ 6.7 cm, which is the same as the measured stabilized height of 7.95 cm, with a relative error of about 15.72%.

The original data of the CPAM test group were brought into the modified formula to verify the correctness of the formula, taking $h_0$ = 32 cm, $s_0$ = 0.2, $e_0$ = 10.6, 1 min settling rate ${\nu }_0$ = 20.5 cm/min, and taking $\lambda$ = 0.133, $k$ = 0.15, the resulting $H_f$ ≈ 7.4 cm, and the measured stabilization height of 8.05 cm, with a relative error of about 8.12%. The relative error is about 8.12%.

The original data of the APAM test group were brought into the modified formula to verify the correctness of the formula, taking $h_0$ = 32 cm, $s_0$ = 0.2, $e_0$ = 10.6, 1 min settling rate ${\nu }_0$ = 20.6 cm/min, and taking $\lambda$ = 0.137, $k$ = 0.15 to obtain $H_f$ ≈ 7.17 cm, which was about an 11.69% relative error from the measured stabilized height of 8.12 cm, which was about an 11.12% relative error. The relative error is about 11.69%.

The calculated and measured results of the above three test groups are summarized in

Table 3. Predicted calculated and actual measured settlements for three different PAM test sets.

Group	${\mathit{h}}_0$ (cm)	s0	e0	v0 (cm/min)	λ (cm−1)	$\mathit{k}$	${\mathit{H}}_{\mathit{f}}$ (cm)	(cm)	(%)
CAPM	32	0.2	10.6	20.4	0.15	0.15	6.7	7.95	15.72
NPAM	32	0.2	10.6	20.6	0.137	0.15	7.17	8.12	11.69
APAM	32	0.2	10.6	20.5	0.133	0.15	7.4	8.05	8.12

.

It should be noted that the flocculation enhancement coefficient (λ), while introduced here as a novel parameter to capture the structural effect of flocculants, is essentially an empirical coefficient calibrated from experimental data. At present, λ is not directly correlated with a specific flocculant type or dosage, which limits its capacity for a priori prediction. Instead, it requires preliminary experimental measurements of the settling behavior for calibration before application. This limitation should be explicitly acknowledged, and future work should aim to establish quantitative links between λ and intrinsic flocculant properties (e.g., molecular structure, dosage, and interaction mechanisms) to enhance the predictive applicability of the model.

4. Discussion

This study, by combining settling column tests with microscopic imaging, systematically revealed the spatiotemporal evolution of flocculated structures during the self-weighted siltation of red mud, as well as the macro–micro coupling mechanisms that govern its settling behavior. The experimental results demonstrated that the slurry gradually developed into a three-layer configuration—comprising a clarified layer, a structural transition layer, and a dense sediment layer. Among them, the rapid evolution of the structural transition layer exhibited a strong correlation with the migration rate of the interface, indicating that microstructural formation and reconstruction exert substantial control over the macroscopic settling rate and sediment layer height.

From a dynamic perspective, red mud settling displayed a nonlinear “fast–slow–steady” temporal evolution of the settling rate and sediment layer height, reflecting the progressive densification of the structure and contraction of the drainage channels. The addition of flocculants accelerated structural formation, shortened the settling time, and increased the final sediment layer height; however, it also reduced structural compaction and increased porosity, presenting a dual effect of faster siltation but lower density.

Despite these insights, the environmental sustainability of flocculant use remains a major concern. Residual polyacrylamide (PAM) and its degradation products may persist in water and soil, posing risks of ecotoxicity and long-term accumulation in mining ecosystems. This indicates that synthetic PAM flocculants are not inherently sustainable, and their widespread use raises critical questions for green mining and sustainable development. Future studies should therefore evaluate residual concentrations, clarify the environmental fate of PAM in backfill systems, and explore alternative or biodegradable flocculants with lower ecological impacts.

From a practical standpoint, the present study primarily contributes to academic understanding but provides limited direct guidance for engineering application. Issues such as flocculant selection, dosage optimization, cost–benefit analysis, and trade-offs between settling efficiency and environmental risk remain unresolved. Future research should extend the current framework to integrate structural evolution models with economic and environmental assessments, thereby offering more actionable guidance for the optimization of backfilling technologies in the alumina industry.

5. Conclusions

Through an integrated multi-scale approach combining settling column experiments with microscopic imaging, this study elucidates the macro–micro coupling mechanisms governing the siltation process of red mud. The main conclusions are as follows:

• During self-weighted siltation, red mud slurry develops a distinct flocculated structure stratification, comprising a clarified layer, a structural transition layer, and a dense sediment layer. This stratified structure continuously evolves during the siltation process and forms the fundamental basis for the formation and stabilization of the sediment body.
• The sediment layer height, sedimentation rate, and floc structure exhibit nonlinear temporal evolution, characterized by rapid changes in the initial stage followed by gradual stabilization in the mid-to-late stages. The settling response is strongly coupled with the structural densification process, reflecting typical nonlinear settling–consolidation behavior.
• The addition of flocculants markedly accelerates the formation of flocculated structures, thereby shortening the self-weighted siltation time and increasing the final sediment layer height. However, it also reduces structural compaction and increases sediment body porosity, indicating a dual effect of accelerated siltation and decreased density.
• To quantify the regulatory effect of flocculants on flocculated structures, a macro–micro predictive model for the sediment layer height was developed by introducing a flocculation enhancement coefficient. The model achieved an average prediction error within 16%, indicating good applicability and offering valuable theoretical support for understanding structure-dominated settling processes and guiding the future optimization of high-concentration tailings backfilling.