Optimizing the Dealkalization Process of Red Mud: Controlling Calcium Compounds to Improve Solid–Liquid Separation Performance

Abstract

The acid neutralization process is widely recognized for its effectiveness in the dealkalization of red mud, and it faces challenges in solid–liquid separation due to the formation of numerous colloidal components. This study investigated the impact of calcium-containing compounds (CaO, CaCl₂, CaCO₃, and CaSO₄) on the solid–liquid separation and the dealkalization efficiency of red mud during the dealkalization process. The sodium leaching efficiency of the red mud reached 95.6% when the red mud was reacted with 8% of sulfuric acid for 10 min with a stirring speed and liquid to solid ratio of 700 r/min and 5:1, respectively. The replacement of sulfuric acid using simulated waste acid reached similar sodium leaching efficiency. However, the filtration rate of red mud becomes exceedingly sluggish using sulfuric acid or simulated waste acid. Adding calcium-containing compounds significantly augments the efficacy of solid–liquid separation in red mud. With a mass content of 2% for CaO or 8% for CaCl₂, the filtration speed experienced a remarkable fivefold and ninefold increase, respectively. Furthermore, a simplification in the composition was observed within the leaching solution derived from red mud, thereby creating favorable conditions for the extraction of sodium. The influence mechanism was investigated with X-ray diffraction, inductively coupled plasma analysis, and scanning electron microscopy. The addition of calcium compounds led to the formation of calcium silicate and iron silicate in the leaching residue, inhibiting the generation of colloidal substances, such as silica gel. Additionally, these compounds increased the size of red mud particles, facilitating the solid–liquid separation process. This study provides valuable technical insights for the dealkalization of red mud.

1. Introduction

Red mud is an alkaline byproduct generated during the alumina production process [1,2,3]. It boasts a pH level ranging from approximately 10 to 12.5 and contains substantial quantities of heavy metals, such as Cd and Cr [4,5], in addition to some radioactive elements, like Th and U [4,6,7,8]. The accumulation of red mud not only engulfs vast swathes of land but also gives rise to leachate that contaminates the surrounding soil and groundwater, thereby posing significant environmental hazards [9,10]. Furthermore, the fine-grained nature of red mud readily gives rise to dust on the surface of landfills, leading to potential air pollution in the vicinity and at lower tuyere levels [11,12]. Owing to its alkalinity, the vegetation struggles to thrive in close proximity to red mud, rendering it susceptible to dam-breaking incidents that pose a grave threat to the surrounding ecological environment [13]. A prime example occurred on 4 October 2010 when a dam ruptured at a red mud tailings pond in Hungary resulting in a severe ecological catastrophe [14]. Approximately 600,000 cubic meters of red mud inundated nearby villages causing extensive ecological and environmental contamination [15,16]. Consequently, it is imperative that effective solutions for managing red mud disposal are identified. The strong alkalinity of red mud plays a pivotal role in its comprehensive utilization. Presently, the dealkalization methods for red mud are beset with various challenges, including exorbitant costs, intricate processes, and arduous filtration and dehydration procedures, all of which impede its industrial applications [17,18]. The development of an efficient and expeditious dealkalization technology is imperative to address the environmental pollution and safety risks stemming from the accumulation of red mud [19].

Acid leaching stands as the most efficient and rapid dealkalization method for red mud [20,21,22,23]. However, several technical challenges hinder the industrial application of this technology in the dealkalization process. One of the major issues lies in the difficulty of separating the solids and liquids after the acid leaching of red mud [24,25,26,27]. This is primarily attributed to the fine particle sizes of the red mud itself. Additionally, during the acid leaching process, silicon and iron are also partially leached alongside sodium. The presence of silicon and iron leads to the formation of colloidal components in acidic solution, resulting in increased stickiness of red mud and significantly impacting the solid–liquid separation performance [28,29]. The composition of the red mud acid leaching solution is complex [30], containing trace amounts of silicon, iron, aluminum, titanium, and other elements besides sodium. Furthermore, there is a severe excess in heavy metal content within the leaching solution which escalates purification costs and complicates reusability [22,31,32]. Despite an abundance of sodium in red muds, the liquid–solid ratio during acid leaching processes tends to be higher to achieve optimal sodium removal effects. Consequently, this results in a relatively low concentration of sodium within the leaching solution. Furthermore, the traditional steaming method for recovery of sodium incurs high costs with no effective alternative currently available.

Calcium salt has been extensively used as a charge neutralizer and has been successfully used for industrial wastewater, metal contamination, and groundwater treatment [33,34,35] due to its good pH adjustment and low cost compared with conventional coagulants [34,36]. In our previous research [37], it was found that the sodium leaching efficiency rate reached 94.31% with 18.4% sulfuric acid and 2% CaO when the filtration speed increased by about 70 times with the addition of CaO up to 4.5%. Furthermore, Zeng Hua et al. [38] found that the synergy of lime and polyacrylamide (PAM) significantly accelerated solid–liquid separation from 0.64 m/h to 2.24 m/h compared with the addition of PAM alone when the turbidity of the supernatant decreased from 61,920 NTU to 232 NTU.

In the current alumina industrial production system, the issue of strong alkalinity in red mud has been a significant concern. In this paper, we first examined the influence of key factors (acid concentration, leaching time, liquid–solid ratio, stirring rate) in the sulfuric acid leaching process on the dealkalization of red mud, successfully achieving effective dealkalization. Furthermore, the low-cost dealkalization of red mud was achieved using simulated waste acid instead of sulfuric acid. Subsequently, we introduced calcium-containing compounds as electrical neutralizers, and this approach significantly improved the filtration performance of red mud without compromising its dealkalization efficiency. Furthermore, microscopic morphological changes in red mud before and after dealkalization were carefully examined. Additionally, the impact of calcium compound additives on the aggregate structure and other properties of red mud were meticulously studied. Ultimately, this study contributes to the development of low-cost, high-efficiency red mud green dealkalization technology and provides theoretical and technical support for maximizing the utilization of red mud and thus, addresses environmental pollution and safety concerns associated with aluminum smelting red mud.

2. Material and Methods

2.1. Materials and Reagents

The red mud sample utilized in the experiment was procured from Shandong Province, China. The red mud sample required pulverizing due to clumping. Literature inquiries and data from an aluminum oxide plant in Shandong indicate that most of the red mud is below −200 mesh. Prior to the commencement of the experiments, the samples underwent crushing, drying, and screening utilizing a 200-mesh sieve. The primary components of the samples are detailed in

Table 1. Main chemical compositions of the red mud.

Compositions	Al2O3	CaO	Fe2O3	TiO2	Na2O	SiO2
Contents/%	20.49	2.46	48.01	5.77	9.14	14.66

. As depicted in Table 1, Na₂O constitutes 9.14%, Al₂O₃ comprises 20.49%, SiO₂ accounts for 14.66%, CaO represents 2.46%, Fe₂O₃ makes up 48.01%, and TiO₂ is present at a level of 5.77% within the red mud, which is categorized as high alkali and high iron red mud. The results of the X-ray diffraction (XRD) analysis of the samples are presented in Figure 1. It is evident that the red mud is primarily composed of hematite, diaspore, anatase, quartz, and sodalite; with sodalite being identified as the main alkali-bearing mineral. Based on the main composition of titanium dioxide waste acid in Shandong Province, a corresponding simulated waste acid was prepared. This simulated waste acid contained a sulfuric acid concentration of 20% and an iron content of 3.6%.

2.2. Experimental Procedure

Figure 2 presents a schematic diagram providing an overview of the red mud treatment process. The leaching experiments consist of the following steps: (1) dealkalization of red mud utilizing sulfuric acid as the leaching reagent; and (2) addition of calcium compounds prior to filtration in order to investigate their impact on the filtration properties of the red mud.

The procedure is outlined as follows: 10 g of red mud was carefully dispersed in a sulfuric acid solution using a thermocapillary magnetic agitator. The leaching experiments were conducted at varying sulfuric acid dosages, leaching temperatures, stirring velocities, and leaching times.

Subsequent to the leaching process, calcium compounds were introduced and allowed to react with the red mud for a duration of 3 min. Upon completion, the resulting slurry underwent filtration by means of a circulating water vacuum pump to facilitate separation into a supernatant liquid and solid residue. The solid residue was subjected to three rounds of washing, followed by collection, drying, and weighing. The leachate was analyzed for concentrations of Na, Si, Fe, and Al.

The alkali content of red mud is mainly sodium, and the potassium content is low. Therefore, sodium leaching efficiency was used to characterize the dealkalization efficiency of the red mud. The sodium leaching rate was calculated using Equation (1).

$${\mathsf{\eta }}_{\mathrm{N}\mathrm{a}}={\displaystyle \frac{m_0\times {\alpha }_{Na}-m_1\times {\theta }_{Na}}{m_0\times {\alpha }_{Na}}}\times 100\%$$

(1)

In this formula, ${\mathsf{\eta }}_{\mathrm{N}\mathrm{a}}$ refers to the sodium leaching efficiency, %; $m_0$ and $m_1$ refers to the weight of the original red mud and picking residue, g; ${\alpha }_{Na}$ and ${\theta }_{Na}$ refers to the Na content of the original red mud and picking residue, %.

2.3. Analytical Methods and Characterization of Samples

In the present investigation, the phase composition of the red mud and picking residue was analyzed utilizing X-ray diffractometry (XRD) with a Bruker-axsD8 Advance instrument from Bruker AXS, Berlin, Germany. The utilization of Cu Kα radiation and a scanning rate set at 6°/min over a range of 2θ from 5° to 70° were employed for this purpose. The XRD instrument operated at 40 kV and 30 mA. XRF analysis of the leaching residue was conducted to analyze the dealkalization effect of the red mud using an X-ray fluorescence analyzer (XRF, PANalytical Axios mAX, PANalytical B.V., Almelo, The Netherlands). The examination of surface micromorphology of the red mud and leaching residue was conducted using scanning electron microscopy (SEM) imaging, which facilitated detailed visualization of the surface features of the samples. Additionally, energy-dispersive spectroscopy (EDS) was employed to identify the spatial distribution of elements. The concentrations of Na, Si, Fe, and Al in the leaching solution were determined using an Emission Spectroscopy of Electrically Coupled Plasmonic Atoms (ICP-MS, PerkinElmer NexION 350D, PerkinElmer, Waltham, MA, USA). Prior to measurement, liquid sample analysis was diluted with 2.5% HNO₃ (TraceMetal™ grade, Fisher Chemical, Pittsburgh, PA, USA). Initially, red mud samples extracted under different conditions were stirred with a magnetic stirrer for 5 min. Subsequently, a volume of 5 mL was transferred to a particle size analyzer for a brief ultrasound treatment lasting approximately thirty seconds.

3. Results and Discussion

3.1. Acid Leaching

3.1.1. Factors Affecting the Leaching Using H₂SO₄

The pivotal factors that may influence the efficacy of desalinization are the concentration of sulfuric acid, the liquid–solid ratio, the stirring time, and the stirring speed. In this investigation, acid leaching experiments were conducted to ascertain the optimal conditions for dealkalization using H₂SO₄ and to analyze the alterations in the red mud during this process. The sodium leaching efficiency of the red mud under the same leaching conditions was used as the evaluation criterion (as shown in Figure 3).

Figure 3a shows the impact of H₂SO₄ concentration on both the sodium leaching efficiency and filtration rate of red mud, which were examined while stirring at 700 r/min for 10 min with a liquid–solid ratio of 2:1. A significant increase in the leaching efficiency of Na as the H₂SO₄ concentration was elevated from 0 to 8%, resulting in a sharp rise from 7.18% to 72.7%. With further increases in H₂SO₄ concentration, there was a slight uptick in the leaching rate of Na to 84.1%. However, this also led to a gradual increase in filtration time from 0.5 min to 34.4 min and 49 min. The reason for this might be that, while sulfuric acid leaches sodium, it also leaches significant quantities of aluminum, iron, silicon, and other elements. This process leads to the formation of a considerable amount of silica gel, which complicates filtration and results in a low sodium leaching efficiency. Consequently, it was determined that the optimal concentration of H₂SO₄ is 8%.

The effects of the liquid–solid ratio on sodium leaching efficiency and filtration performance were studied under the optimum sulfuric acid concentration of 8% with a stirring time and stirring rate of 10 min and 700 r/min (as shown in Figure 3b), respectively. As the liquid–solid ratio increases, the dealkalization of red mud initially rises and then falls. When the liquid–solid ratio increases from 2:1 to 5:1, the sodium leaching efficiency of red mud improves from 71.05% to 95.6%. However, when the liquid–solid ratio exceeds 5:1, the sodium leaching efficiency gradually declines to 89.4%. This phenomenon may occur because, at lower liquid–solid ratios, the formation of a large amount of silica gel increases the slurry’s viscosity, making it difficult to filter out some alkalis. When the liquid–solid ratio is excessively high, elements such as iron, aluminum, and silicon can leach along with sodium. This leaching creates adhesive substances that increase the moisture content of the filter cake. As a result, sodium cannot be completely filtered out, leading to a lower sodium leaching efficiency. Additionally, with the gradual increase in the liquid–solid ratio, the filtration speed of red mud also increases, resulting in a reduction of filtration time from 34.4 min to 28 min, and then to 20 min. It was concluded that the optimal liquid–solid ratio is 5:1.

The effects of leaching time on the sodium leaching efficiency and filtration performance were studied under the optimum sulfuric acid concentration and liquid–solid ratio of 8% and 5:1 with a stirring rate of 700 r/min (as shown in Figure 3c). With the extension of leaching time, the dealkalization of red mud first increased rapidly and then decreased rapidly. When the leaching time was less than 10 min, the sodium leaching efficiency of red mud gradually increased from 81.4% to 95.6%. When the leaching time was higher than 10 min, the sodium leaching efficiency of red mud gradually decreased to 75.7%. This may be because the long-term leaching causes the aluminum, iron, and silicon in the red mud to dissolve, and form silica gel in the solution, hindering the filtration of alkali, resulting in a gradual decrease in the sodium leaching efficiency. Meanwhile, the filtration rate of red mud decreased gradually with the extension of leaching time. The filtration time of the red mud was gradually reduced from 22 min to 28 min, and then to 42 min. It was concluded that the optimal leaching time is 10 min.

The effects of leaching time on the sodium leaching efficiency and filtration performance were studied under the optimum sulfuric acid concentration and liquid–solid ratio of 8% and 5:1 with a stirring time of 10 min (as shown in Figure 3d). The sodium leaching efficiency of red mud increased gradually with the acceleration of stirring speed, but it decreased slowly when the stirring speed became too high. When the stirring speed was below 700 r/min, the sodium leaching efficiency improved from 69.1% to 95.6%. However, when the stirring speed exceeded 700 r/min, the sodium leaching efficiency fell to 78.8%. This phenomenon may occur because, at lower mixing speeds, the reaction between red mud and sulfuric acid is insufficient, resulting in a lower sodium leaching efficiency. As the stirring speed increases, the alkali in red mud can react more thoroughly with sulfuric acid, leading to a greater dissolution of alkali. Conversely, if the stirring speed is too high, it can cause excessive dissolution of aluminum, iron, and silicon along with the alkali, resulting in the formation of a significant amount of silica gel. This, in turn, reduces the alkali removal rate from the red mud. The excessive formation of silica gel adversely affects the filtration performance of red mud. As the stirring speed increases, the filtration time for red mud also increases from 10 min to 28 min, and ultimately, to 48 min. It was concluded that the optimal stirring rate is 700 r/min.

In summary, after careful analysis and experimentation under various conditions, it has been established that optimum dealkalization occurs at an H₂SO₄ concentration of 8%, a liquid–solid ratio of 5:1, a stirring time of 10 min, and finally, maintaining a consistent stirrer speed set at exactly 700 r/min. Under these conditions, the maximum sodium leaching efficiency achieved an impressive 95.6%, and the stirring time of the red mud was 28 min.

3.1.2. Factors Affecting Leaching Using Waste Acid

The high cost of the process is primarily due to the significant amount of sulfuric acid used. Based on the main composition of titanium dioxide waste acid in Shandong Province, a corresponding simulated waste acid was prepared. This simulated waste acid contained a sulfuric acid concentration of 20% and an iron content of 3.6%. The study examined how factors such as the amount of simulated waste acid, the liquid–solid ratio, leaching time, and stirring speed affect the sodium leaching efficiency and filtration performance of red mud (as shown in Figure 4).

As depicted in Figure 4a, an increase in the dosage of simulated acid from 10 mL to 50 mL resulted in a rise in the sodium leaching efficiency of red mud from 47.91% to 77.94%. However, further escalation of the simulated acid dosage from 50 mL to 60 mL did not yield a significant change in the sodium leaching efficiency of red mud. Subsequent increments in the simulated acid dosage led to a decrease in the sodium leaching efficiency of red mud to 64.44%. As the amount of simulated acid increases, the filtration performance of red mud gradually worsens. The filtration time increased from 3.5 min to 35.5 min, and then to 48 min. The mechanism of action is similar to that of sulfuric acid. It was determined that the optimal simulated acid dosage stands at 50 mL. The optimal amount of waste acid contained approximately 7% sulfuric acid, which is comparable to the optimal amount used in sulfuric acid tests.

In Figure 4b, the impact of the solid–liquid ratio on both the sodium leaching efficiency and filtration time of red mud is depicted. It is evident from the graph that as the liquid–solid ratio increases, there is a continuous enhancement in the sodium leaching efficiency of red mud, accompanied by a reduction in filtration time. Once the liquid–solid ratio reaches 5:1, further increments do not yield significant effects on either the sodium leaching efficiency or filtration time. Transitioning from a 2:1 to a 5:1 liquid–solid ratio resulted in an increase in the sodium leaching efficiency of red mud from 65.44% to 90.67%, while simultaneously reducing the filtration time from 42.4 min to 28.5 min. The mechanism of action is similar to that of sulfuric acid. Consequently, it can be concluded that the optimal liquid–solid ratio stands at 5:1.

Figure 4c illustrates the correlation between the sodium leaching efficiency and filtration time of red mud in relation to stirring time. It is evident that as the duration of stirring increases from 0 to 15 min, the sodium leaching efficiency of red mud escalates from 76.43% to 95.62%. However, once the stirring time surpasses 15 min, there is a noticeable decline in the sodium leaching efficiency of red mud to 62.2%. Furthermore, with an increase in stirring time, there is a corresponding prolongation in the filtration time of red mud from 14.5 min to 31.5 min, and then 48.2 min. Consequently, it can be concluded that the optimal stirring time stands at 15 min.

In Figure 4d, it is evident that the agitation rate exerts great influence on the variation in filtration time of red mud. As the agitation rate escalates from 300 r/min to 800 r/min, the sodium leaching efficiency of red mud surges from 68.38% to 95.62%. However, a further increase in agitation rate precipitates a gradual decline in the sodium leaching efficiency of red mud to 84.44%. Meanwhile, the filtration time of red mud increased slowly from 29.9 min to 31.5 min, then 32.5 min. The mechanism of action is similar to that of sulfuric acid. Consequently, the optimal agitation rate is ascertained to be 800 r/min.

In conclusion, the optimal conditions for dealkalization were determined to be a simulated acid volume of 50 mL, a liquid–solid ratio of 5:1, a stirring time of 15 min, and a stirring rate of 800 revolutions per minute. Under these specified conditions, the maximum achieved sodium leaching efficiency was an impressive 95.62%, and the filtrate time was 31.5 min.

3.2. Acid Leaching with Calcium Compounds

It is evident that the process of acid neutralization proves to be a highly effective technique for the dealkalization of red mud. However, there will be other ion dissolutions during the acid leaching process in the removal of alkali, resulting in complex acid leaching liquid components, especially the dissolution of iron, aluminum, silicon, and other elements, which are easy to form colloids. These will lead to difficulties in both solid–liquid separation and leaching liquid treatment. The effects of different calcium salts on the filtration performance and sodium leaching efficiency of red mud were studied (as shown in Figure 5).

Figure 5 depicts the influence of various calcium compounds on the solid–liquid separation rate of red mud after acid leaching. It is apparent that the addition of different calcium compounds does not affect the sodium leaching efficiency of red mud, but consistently reduces the filtration time. In Figure 5a, it can be observed that an increase in the quantity of CaO leads to a decrease in filtration time. Specifically, when the amount of CaO added increases from 0 to 2%, the filtration time decreases from 28 min to 2.5 min. Figure 5b shows that as the CaCl₂ content increases from 0 to 8%, there is a reduction in filtration time from 28 min to 3.87 min. Figure 5c demonstrates that as the amount of CaSO₄ added increases from 0 to 8%, there is a decrease in filtration time from 28 min to 19.5 min. As the percentage of CaCO₃ increases from 0 to 6%, there is a substantial reduction in the filtration time, dropping from 28 min to just 2.2 min. The results indicate that adding calcium salt has a minimal impact on the alkali removal rate of red mud. However, it significantly enhances the filtration performance of red mud. This improvement may be due to the fact that calcium salt can effectively regulate the surface electrical properties of red mud, allowing fine particles to spontaneously agglomerate into larger particles. This process ultimately results in better filtration performance for red mud.

Figure 6 illustrates the impact of the addition of different types of calcium salts on the solid–liquid separation rate of red mud when using simulated acid. It can be observed from Figure 6 that when using simulated acid, the addition of different types of calcium salts has no significant effect on the dealumination rate of red mud, while the filtration time of red mud decreases. Figure 6a shows that as the amount of CaO added increases from 0 to 8%, the filtration time of red mud decreases from 31.5 min to 0.8 min. In Figure 6b, it is shown that as the amount of CaCl₂ added increases from 0 to 8%, the filtration time of red mud decreases from 31.5 min to 2 min. Figure 6c demonstrates that as the amount of CaSO₄ added increases from 0 to 6%, the filtration time of red mud decreases from 31.5 min to 13.7 min. Finally, in Figure 6d, it can be observed that as the amount of CaCO₃ added increases from 0 to 6%, the filtration time of red mud decreases from 31.5 min to 3 min. The mechanism of action is similar to that of sulfuric acid.

3.3. Residue Analysis

XRF analysis was conducted on the original red mud and the leaching residue with the addition of CaO and acid at the optimal leaching conditions. As revealed in

Table 2. XRF analysis of the feed and residue.

	Na	K	Si	Ti	Ni	Fe	Ca
Feed, %	12.63	0.09	6.55	3.56	0.08	26.50	2.98
Residue, %	0.96	0.07	1.41	5.22	0.60	27.90	3.27
	Mg	S	Mn	Cl	Zn	As	Cu
Feed, %	4.47	5.30	0.12	0.13	0.02	/	2.00
Residue, %	4.84	5.86	0.13	0.03	0.02	/	0.16

, The sodium residue of red mud decreased from 12.63% to 0.96%, which proved that most of the alkali in the red mud had been removed. The chemical composition of other elements changed little, indicating that the lime did not affect the leaching of other elements.

In addition, XRD and SEM were used to analyze the phase transformation and surface morphology of the red mud before and after acid leaching (as shown in Figure 7). It is evident that the samples contain gibbsite, hematite, anatase, limonite, and gypsum. As the amount of CaO added increases, there is a gradual decrease in the intensity of diffraction peaks corresponding to gibbsite and hematite. This trend indicates a reduction in the content of gibbsite and hematite within the samples.

The microstructure of the red mud was examined both before and after leaching using SEM. Figure 7A shows the SEM images of the red mud samples with added CaO, CaCl₂, and CaSO₄, respectively. These SEM images illustrate the distinct differences in the red mud’s microstructure before and after acid leaching, respectively. It was observed that the particle size of the red mud significantly increased. The process of acid leaching facilitated agglomeration among red mud particles, resulting in an altered rod-shaped appearance within its microstructure following such treatment. EDS analysis was conducted to investigate the elemental distribution (O, Si, Ca, Na, Al, and Fe) in the red mud both before and after leaching. Upon examination of the images, it is evident that there was a significant increase in the content of Ca following acid leaching. In contrast, the content of Si, Na, Al, and Fe exhibited a decrease to some extent. The SEM-EDS findings are consistent with the results obtained from acid leaching.

3.4. Analysis of the Leachate

Table 3. Concentration of impurity elements (mg/L) in the leachates from acid leaching.

Number	Photos	Na	Fe	Si	Al	Ca
Water leaching		22.347	0.043	34.419	38.241	/
H2SO4 leaching		11,356	14,400	335.4	1532	446
H2SO4-2% CaO		12,085	0.145	0.937	0.329	562

depicts photographs of the leachate obtained from both the conventional acid leaching process and the new leaching process. In order to facilitate comparison, images of the leached solution obtained through direct water leaching were utilized for reference. As shown in Table 3, it is clear that under the same leaching conditions, the leaching solution obtained through the traditional acid leaching process immediately undergoes gelation, almost solidifying to an extent where further processing becomes unattainable. In contrast, the leaching solution derived from the new process remains transparent. Therefore, it can be inferred that the new leaching process presents a more feasible option for red mud dealkalization application.

Table 4. The variation of pH values in red mud leaching residue over time.

The Amounts of CaO/%	Test Days
	0	1	4	6	40
0	4.25	6.73	6.04	5.94	6.40
1	4.54	7.22	7.25	7.42	7.44
2	4.84	7.75	8.15	8.19	8.03
4	8.4	8.48	9.07	8.95	8.83

shows the variation of pH values in red mud leaching residue over time.

The comprehensive elemental analysis results of the red mud leaching solution, obtained through the aforementioned procedure, are presented in Table 3. As shown in Table 3, the Na content in the leached solution by simulated waste acid measures approximately 11 g/L. Additionally, it contains numerous amounts of Ca, Al, Fe, and Si elements. The Fe content is estimated to be around 14 g/L, primarily originating from a titanium dioxide neutralizer with an Fe content of about 36 g/L. The content of Si and Al was 335.4 mg/L and 1532 mg/L, respectively. It is observed that the sodium content in the filtrate decreases with the addition of 2% CaO, resulting in an increase from 11,356 mg/L to 12,085 mg/L. Furthermore, the presence of other elements is either negligible or virtually non-existent. Compared with the simulated waste acid leaching, the chemical composition of the leaching solution with calcium oxide is simple. The potential reactions occurring in the acid-leaching solution can be represented as follows:

Fe₂O₃ + 3H₂SO₄ = Fe₂(SO₄)₃ + 3H₂O

(2)

Fe₃O₄ + 4H₂SO₄ = FeSO₄ + 4H₂O + Fe₂(SO₄)₃

(3)

FeO + H₂SO₄ = FeSO₄ + H₂O

(4)

2NaAISiO₄ + 4H₂SO₄ = Na₂SO₄ + Al₂(SO₄)₃ + 2H₂SiO₃ + 2H₂O

(5)

Ca₂Al₂SiO₇ + 5H₂SO₄ = 2CaSO₄ + Al₂(SO₄)₃ + H₂SiO₃ + 4H₂O

(6)

4. Conclusions

In this paper, we propose a novel approach involving the incorporation of calcium-containing salts as supplementary additives in the acid leaching and dealkalization process of red mud. This innovative method aims to regulate the leaching and migration characteristics of silicates and metal ions. A sulfuric acid leaching test was conducted to investigate the impact of various types and quantities of calcium salt additives on the leaching rate of sodium, filtration performance, and dissolution characteristics of impurities in red mud. The influence mechanism of calcium salt on the acid leaching and dealkalization process was elucidated through XRD and SEM analysis.

The results demonstrated that optimal sulfuric acid leaching conditions (i.e., a liquid–solid ratio of 5:1, a sulfuric acid dosage of 8%, and a stirring speed of 700 r/min) combined with the addition of CaO and CaCl₂ significantly enhanced the filtration rate of the red mud leaching solution. When utilizing a 2% dosage for CaO and 8% dosage for CaCl₂ relative to red mud quantity, filtration speed increased by 5 times and 9 times, respectively, compared to a control group without these additives. Furthermore, compared with using CaO alone, incorporating both additives simplified the composition of the resulting leaching solution from red mud, thereby creating favorable conditions for sodium recovery.

SEM-EDS, XRF, and XRD analysis have revealed that the addition of CaO can effectively inhibit the dissolution of aluminum, iron, and silicon in the leaching solution. This led to the effective inhibition of colloidal substances such as silica gel. Furthermore, the introduction of lime caused a significant increase in the particle size of red mud, which facilitated the filtration process. This study provides valuable technical insights into the dealkalization of red mud.