Study of Red Mud Desulfurization and External Field Action

To further enhance red mud desulfurization, this paper analyzes red mud desulfurization under the action of ball mills and ultrasonic external elds and studies the red mud desulfurization mechanism. In this study, experiments were conducted using a bottom-blow stirred reactor device. The results show that the suitable red mud slurry concentration is 10 g/L. The raw red mud desulfurization can reach 100% absorption in the rst 25 min, and the absorption capacity decreases gradually with the reaction time. The red mud desulfurization is mainly due to the dissolution of free alkali and metal oxides in the red mud. Under the action of the external eld of the ball mill, the red mud particles can be rened to prolong the desulfurization time. Under the action of the external eld of ultrasonic waves, the red mud slurry is dispersed more uniformly, which can accelerate the reaction rate. Under the effect of an ultrasonic external eld, the red mud slurry is dispersed more uniformly, which can accelerate the reaction rate and dissolve more metal substances in the liquid phase.

SO 2 , as an irritant gas, is harmful to the human body and the environment. Flue gas desulfurization technologies are dry desulfurization, semidry desulfurization, and wet desulfurization . Red mud desulfurization technology can be used for dry adsorption desulfurization by using the characteristics of red mud, such as a large speci c surface area and porous structure. At present, research on the dry desulfurization of red mud is shown as follows. Jin Yan (2021) used red mud instead of CaO for desulfurization in a circulating uidized bed with the highest desulfurization rate of more than 94%.
Hongtao Liu (2011) used red mud-modi ed limestone for desulfurization by adsorption in a tube furnace.
Jian Niu (2021) used red mud as an additive to activated carbon to improve the desulfurization capacity of activated carbon, and the maximum sulfur capacity was increased by 17.9% compared with activated carbon alone. Red mud is highly alkaline, and the slurry made from red mud for desulfurization is a wet desulfurization process. In red mud slurry desulfurization, Xinke Wang (2014) studied the effect of red mud slurry desulfurization and decarbonization on red mud dealkalization. The main components of red mud residue after desulfurization and decarbonization are SiO 2 , Fe 2 O 3, and AlOOH. Wei Peng (2012) obtained the optimal operating conditions for desulfurization of an absorption tower with red mud slurry in an absorption tower. The desulfurization rate was above 95%, and the red mud slurry lost its desulfurization capacity after 10 h. Jinji Yang (2012) performed red mud slurry desulfurization in a desulfurization absorption tower and obtained the optimum operating conditions with a maximum desulfurization rate of 98.8%. Bin Li (2020) used ozone preoxidation of red mud slurry desulfurization and denitri cation in a spray absorption tower, and the desulfurization rate was stabilized at 98% within 1 h. The reaction mechanism of desulfurization and denitri cation was investigated. Lei Tao (2019) used red mud slurry desulfurization in a bubbling reactor and found that the liquid/solid ratio had the most signi cant effect on the desulfurization process. Yu Liu (2022) used a yellow phosphorus emulsion coupled with red mud for desulfurization and denitri cation to optimize the reaction conditions. Under the optimized conditions, the optimal desulfurization and denitri cation rates were as high as 97.9% and 100%, respectively. Yuwei Zhang (2021) established an industrial demonstration of red mud limestone with a desulfurization rate of 98.9%, which can meet ultralow emissions requirements.
In this paper, red mud slurry wet desulfurization is used to study the desulfurization mechanism in a bottom-blow stirred reactor. The existing studies on red mud desulfurization are all about the process of desulfurization, and there is no research on red mud desulfurization under the action of external eld intensi cation. Therefore, to better enhance the promotion of red mud slurry desulfurization, the external eld of mechanical energy of the ball mill and the external eld of the ultrasonic mechanical wave were used in this paper. The effect of the external eld on the desulfurization of red mud slurry was studied.
2 Experimental Section

Experimental procedures
In this study, the red mud slurry desulfurization reaction was carried out in a bottom-blow stirred reactor. The experimental setup is shown in Fig. 1. It mainly includes a gas supply device, absorption device, and analysis device. The speci c experimental process is as follows: SO 2 gas is supplied by a compressed stainless steel bottle, controlled by a gas ow meter, and the SO 2 inlet ow rate is 400 ml/min. SO 2 is re ned by aeration stones, and bubbles escape at the bottom of the reactor to react with the red mud slurry. The stirring paddle in the reactor continuously disperses the red mud and renews the absorption reaction interface. The speed of the stirring paddle was controlled at 350 r/min. The pH change of the slurry was measured by a Thunder Magnetic pH Meter (PHSJ-3F, Shanghai Jingke Company) during the absorption reaction. The SO 2 gas after the reaction was measured by a SO 2 detector (Leibo 3040, Jiangsu Leibo Scienti c Instruments Co., Ltd.) for concentration. The unreacted SO 2 tail gas is passed into the NaOH solution. The desulfurized red mud residue was ltered and dried for characterization and determination.
The absorption reactor was a 1 L four-neck ask with a red mud slurry volume of 700 mL. The red mud is mechanically re ned by a planetary high-energy ball mill (Fritz Instrument Equipment Co., Ltd., Germany) to re ne the particles. The ball milling speed was 300 r/min, and the ball milling time was 1 h. The ultrasonic mechanical wave for the reaction process was provided by a 40 kHz ultrasonic cleaner (CJ-060B, Shenzhen Super Clean Technology Industrial Co., Ltd.).

Experimental materials
The concentration of SO 2 gas used in the experiment was 5%, balanced with N 2 (Shenyang Shuntai Special Gas Co., Ltd.). The red mud used in the experiment came from an aluminum factory in Shanxi. The ingredients of the raw materials are listed in Table 1, and the main components are shown in Fig. 7.
To better promote the absorption of SO 2 in red mud, the red mud particles were broken by mechanical ball milling. The absorption reaction rate can be improved by re ning the particles and increasing the speci c surface area of the particles from a macroscopic point of view. The adsorption-desorption isotherms before and after ball milling are shown in Fig. 2. The curves of the red mud samples before and after ball milling showed a typical type IV adsorption-desorption isothermal curve. According to the adsorptiondesorption isotherm, the speci c surface area of the raw red mud is 98.3489 m 2 /g, and the speci c surface area after ball milling is 32.4629 m 2 /g by the Brunauer-Emmett-Teller (BET) gas adsorption method (1990).  Fig. 3, it can be seen that the original red mud is formed by cohesions, agglomerates, and agglomerates to form a loose structure. There are spherical particles of Na 2 O and other substances attached to the surface. In addition, some particles have agglomeration behavior. After ball milling, the red mud agglomerates with sphere-like particles. After ball milling, the particles are tighter, more uniform, and have smaller pores. The collision of steel balls in the ball mill, which transmits more mechanical energy to the particles, can crush and re ne the particles.

The effect of red mud slurry concentration on desulfurization
The in uence of slurry concentration on the desulfurization process was characterized by the change in slurry pH value. As shown in Fig. 4, in the red mud slurry desulfurization process, the pH value changes over time into three stages: a rapid decline stage, a slow decline stage, and a basically unchanged stage.
When the concentration of red mud slurry increases from 7 g/L to 10 g/L, the time of pH drop is prolonged. This means that an increase in concentration can prolong the absorption reaction time and absorb and process more SO 2 . When the slurry concentration of red mud increases from 10 g/L to 15 g/L, the increase in slurry concentration does not prolong the absorption reaction time in the rst 10 min. With the increase in slurry concentration, the free alkali in red mud and other substances that react with SO 2 cannot be dissolved in a short time. With increasing time, 15 g/L red mud can have a longer absorption reaction time. However, as its absorption reaction time is not much different from that of 10 g/L red mud, the red mud slurry concentration is selected as 10 g/L in the following experiments after comprehensive consideration.
3.2 The effect on desulfurization under the strengthening of the external eld 3.2.1 Changes in slurry pH and desulfurization e ciency during red mud desulfurization As shown in Fig. 5, the change trend of the red mud after ball milling is the same as that of the raw red mud. The raw red mud can completely absorb low-concentration SO 2 in the rst 25 min, and the desulfurization rate is maintained at 100%. After 25 min, the absorption of SO 2 in red mud gradually reaches saturation, and the concentration of SO 2 in tail gas increases continuously. After ball milling, red mud desulfurization uses mechanical energy to re ne the red mud particles, promote the decomposition of the red mud particles, and prolong the desulfurization time. It can completely absorb low concentrations of SO 2 in the rst 33 min, and the desulfurization rate is maintained at 100%. After 33 min, the absorption capacity of SO 2 decreases gradually, and the concentration of SO 2 in the tail gas increases continuously.
As a kind of high-frequency mechanical wave, ultrasonication provides a cavitation effect, thermal effect and mechanical effect, which can accelerate desulfurization e ciency. Under the effect of an ultrasonic eld, the desulfurization rate of raw red mud reached 100% in the rst 23 min, and the desulfurization ability gradually decreased after 23 min. Under the action of an ultrasonic eld, the desulfurization rate of ball milled red mud was 100% in the rst 29 min, and the desulfurization capacity decreased gradually after 29 min. The SO 2 in the exhaust gas was detected earlier in the presence of an ultrasonic eld, considering that cavitation of the ultrasonic frequency can accelerate the gas-liquid-solid three-phase reaction rate. At the same reaction time, the pH of the slurry in the presence of ultrasound is signi cantly higher than that in the absence of ultrasound, which indicates that ultrasonic frequency accelerates the reaction rate and the escape of SO 2 gas from the slurry. The existence of ball mills and ultrasonic elds can re ne the red mud particles better and can make full use of the eld to achieve a better desulfurization effect.

Changes in red mud composition during red mud desulfurization
XRF was used to detect the composition changes of red mud in the desulfurization process under various conditions, and the detection results are shown in Table 2. From the XRF results of the red mud in the desulfurization process in Table 1 and Table 2, it is clear that Al and Na are more easily dissolved in the initial stage of the reaction. As the desulfurization reaction proceeds, the slurry becomes more acidic, and the aluminum and silicon in the red mud begin to dissolve in large amounts (Nie et al. 2019). The iron minerals are relatively stable, and the Fe 2 O 3 content is almost unaffected by the desulfurization reaction. With the continuous dissolution of Na, Al and Si, the mass of red mud decreases, which is also the reason for the increase in Fe 2 O 3 content. CaO and TiO 2 were partially dissolved with the reaction. The dissolution rate of Fe, Ti and Ca metal substances increased with the intensi cation of ultrasonication (Agrawal et al. 2021). The Na 2 O content in the red mud of the desulfurization process in Table 2 is represented in Fig. 6. Fig. 6 clearly shows the decreasing content of Na 2 O in the red mud as desulfurization proceeds. For the red mud after 60 min of desulfurization under the above conditions, the content of Na 2 O is <1%, which can meet the composition requirements of cement, brick, geopolymer and other construction materials ). The S content in the desulfurized red mud did not increase signi cantly, indicating that the red mud is mainly dissolved in the liquid phase under the condition of oxidation without the addition of oxygen. Figure 7 shows the SEM image of the red mud with desulfurization time t = 60 min. Compared with the undesulfurized SEM image in Fig. 3, the microscopic morphology of the red mud changed signi cantly after desulfurization. In red mud desulfurization, the solid phase material in the red mud is involved in the reaction, while it is continuously consumed. After desulfurization, the red mud is broken down into many small spherical particles, and the microstructure of the red mud becomes loose as a result. The microstructure of red mud is more uffy after desulfurization by ball milling. The ultrasonically dispersed particles are more uniform, and the desulfurized red mud has more small spherical particles and smaller pores. In short, the ball mill machinery out eld is able to extend the desulfurization time by re ning the red mud particles. The ultrasonic external eld disperses the red mud particles, which can improve the reaction rate and promote the dissolution of metals such as Ti and Fe. However, it also accelerates the escape of SO 2 gas, resulting in no longer absorbing SO 2 at slurry pH = 4. Combining the ball mill and ultrasonic external eld can both accelerate the reaction rate and extend the reaction time compared to generating the external eld alone.

Mechanism of red mud desulfurization
As shown in Fig. 8, the diffraction peaks of sodium aluminosilicate hydrate and calcium aluminosilicate hydrate mainly change during the desulfurization of red mud. In the initial stage of the reaction, hydrated sodium aluminosilicate rst decomposes and dissolves aluminum and sodium. As the desulfurization reaction proceeds, the XRD diffraction peaks of both sodium aluminosilicate hydrate and calcium aluminosilicate hydrate gradually weaken. When desulfurization was carried out for 40 minutes, the pH was approximately 4, and the phases of sodium aluminosilicate and calcium aluminosilicate hydrate were not observed in the desulfurization red mud. The raw red mud will have decomposed Al 3+ combined with OH − in the form of Al(OH) 3 in the desulfurized red mud. Some of the Si is present as a stable structure of amorphous SiO 2 , which is also detected in the desulfurized red mud. After 20 min of reaction, CaCO 3 in the red mud decomposed, and the nal desulfurized red mud did not contain Ca 2+ , which was consistent with the XRF results in Table 2. Part of the hematite (Fe 2 O 3 ) in the red mud is reduced to Fe 2+ by reaction with sulfur dioxide. Fe 2+ is unstable and easily oxidized. Eventually, a part of Fe is present in the desulfurized red mud in the form of titanium iron oxide. Figure 9 shows the infrared spectra of the raw red mud desulfurization process. As shown in Fig. 9, the main mid-infrared band that appears at 900-1000 cm − In summary, the process of red mud slurry desulfurization is divided into three steps: (1)   Adsorption-desorption isotherms before and after ball milling Changes in pH with red mud slurry concentration Slurry pH and SO 2 concentration in tail gas during red mud desulfurization XRD of raw red mud desulfurization process Figure 9 Infrared spectrum of the raw red mud desulfurization process Figure 10 Mechanism of red mud desulfurization in the reactor