Modeling and Parametric Study of Spent Refractory Material Dissolution in an Aluminum Reduction Cell

Abstract

Utilizing spent refractory material (SRM), generated after the overhaul of aluminum electrolytic cells, as a raw material for producing Al-Si alloys presents an efficient approach towards achieving full resource utilization of SRM. However, a bottleneck restricting this technology has become the dissolution of SRM. Based on the heat and mass transfer mechanism, the shrinkage core model of SRM particle dissolution was established. The effects of alumina concentration, silica concentration, electrolyte superheat, particle temperature, and turbulent kinetic energy dissipation rate on the mass dissolution rate and dissolution time of SRM particles were investigated. Calculation results and experimental data were compared to confirm the accuracy of the established model. The results show that by maintaining low alumina and silica concentrations, increasing the electrolyte superheat and particle preheating temperature, and increasing the electrolyte turbulent kinetic energy dissipation rate, SRM particles can dissolve faster. The dissolution of agglomerated particles is greatly influenced by the turbulent kinetic energy dissipation rate and superheat. The present research provides promising guidance for practical application in predicting particle dissolution time, controlling process parameters, and accelerating the dissolution of SRM particles.

1. Introduction

Spent refractory material (SRM) generated after the overhaul of aluminum electrolytic cells is a hazardous waste, and its main components include alumina, silica, and fluoride salts [1]. On average, every tonne of primary aluminum produced generates 24~30 kg of spent pot lining, of which 50% is SRM [2,3]. From the global primary aluminum production of 69 million tonnes in 2022, approximately one million tonnes of SRM was generated. Large amounts of hazardous waste pose a significant threat to the environment. With the growing global concern for ecological preservation, it becomes imperative to reduce pollutant emissions from the aluminum electrolysis industry. To achieve this, the industry needs to focus on making SRM harmless, reducing it, and utilizing it as resources. This will pave the way for the aluminum industry’s greener and more sustainable development [4,5]. Therefore, the economical and effective recycling of SRM is an urgent issue facing the current aluminum electrolysis industry.

To achieve the harmless reduction in and high-value utilization of SRM, many scholars have conducted relevant research [6,7,8]. Liang et al. [9] used wet acid-free treatment technology to convert fluoride and cyanide in SRM into non-toxic products through leaching and chemical reactions. Renó et al. [10] and Ghenai et al. [11] utilized SRM as an input material for cement production and discovered that fluoride had several benefits in the cement manufacturing process. A lower cement kiln temperature, a lower melting point of raw materials, and a decrease in NO_x and CO₂ emissions were all achieved. However, the Na in SRM restricts its addition amount. Wang et al. [12] used high-temperature vacuum distillation to distil out toxic substances in SRM, and the remaining refractory materials could be recycled. Hu et al. [13] constructed a novel asphalt modifier using SRM as an additive and realized the high-value utilization of SRM.

Although there are various methods to dispose of SRM, achieving high-value recycling of large amounts of SRM is not common. Our previous work [14,15,16] proposed a process for recycling SRM through aluminum electrolytic cells to produce Al-Si alloys. The industrial application has been successfully demonstrated, providing a sustainable solution to SRM disposal and resource utilization in aluminum smelters. However, the current bottleneck of this technology is that the dissolution rate of SRM is relatively slow. As a result, the addition of SRM is not too large, which limits the Si content in the alloys. Additionally, the current efficiency is also slightly reduced. A detailed investigation of the dissolution mechanism and dissolution rate of SRM is therefore necessary.

Currently, the most studied topic is the dissolution of alumina in aluminum electrolytic cells, and there is little research on the dissolution of SRM. Alumina, being a major constituent of SRM, can serve as a reference for studying the dissolution of SRM. Thonstad et al. [17] suggested that the dissolution of alumina particles is governed by heat and mass transfer mechanisms. According to the mass transfer mechanism, Alarie et al. [18] designed a series of experiments to measure the alumina diffusivity in an electrolytic bath and obtained a general Sherwood number equation to predict the dissolution rate of alumina. Kovács et al. [19] investigated the dissolution of one cold alumina particle in a bath based on the heat and mass transfer mechanism. Our previous work [20,21] focused on the behavior of alumina agglomerates in an electrolyte. It was found that the particles took much longer to dissolve than to freeze. Taylor et al. [22] discovered through experiments that the dissolution of agglomerated particles is governed by the heat transfer, while the dissolution of dispersed small particles is governed by the mass transfer. The above-mentioned research on alumina dissolution provides a valuable reference for studying the dissolution process of SRM, but the effect of process conditions and parameters on the dissolution rate is neglected, which is the most concerning part of the production process.

In this work, a mass transfer model and a heat transfer model for the dissolution of SRM particles were established, which describe the diffusion migration control and surface reaction control, respectively. According to the established dissolution model, the effects of electrolyte superheat, alumina concentration, silica concentration, turbulent kinetic energy dissipation rate, and particle temperature on the dissolution rate of SRM particles were analyzed. The dissolution law of SRM obtained through theoretical analysis can provide theoretical guidance for subsequent electrolytic cell process parameter control and process design.

2. Modeling and Methods

There are two processes involved in dissolving SRM particles: the surface reaction and the diffusion of the components, as shown in Figure 1. The slowest step of the surface reaction rate and diffusion rate is the rate-controlling step, and the dissolution rate depends on this rate-controlling step [23,24]. If the chemical reaction rate at a particle surface is greater than the diffusion rate, dissolution is governed by the mass transfer mechanism. Conversely, dissolution is governed by the heat transfer mechanism.

2.1. Mass Transfer Model

The electrolyte flow in an aluminum electrolytic cell is turbulent [25,26]. Assuming that the alumina and silica particles are spherical, the mass transfer process of alumina and silica particles in the electrolyte can be expressed by a dimensionless equation [27]:

$$Sh=2.0+0.52R{e}_{\mathrm{m}}^{0.52}S{c}^{1/3}$$

(1)

where Sh is the Sherwood number, $Sh=k{d}_{\mathrm{s}}/D$; Re_m is the modified Reynolds number, $d_{\mathrm{s}}^{4/3}{\epsilon }_{\mathrm{L}}^{1/3}/v_{\mathrm{L}}$; and Sc is the Schmidt number, $v_{\mathrm{L}}/D$. d_s is the solid particle diameter, k is the mass transfer coefficient, ε_L is the turbulent kinetic energy dissipation rate, D is the diffusion coefficient, and ν_L is the kinematic viscosity of the electrolyte. k is expressed as follows:

$$k=D\left[2.0+0.52{\left(d_{\mathrm{s}}^{4/3}{\epsilon }_{\mathrm{L}}^{1/3}/{\nu }_{\mathrm{L}}\right)}^{0.52}{\left({\nu }_{\mathrm{L}}/D\right)}^{1/3}\right]/d_{\mathrm{s}}$$

(2)

Due to the concentration difference being the driving force for particle dissolution under the mass transfer mechanism, the mass dissolution rate can be expressed as follows [17,28]:

$${\displaystyle \frac{d{m}_{\mathrm{s}}}{d\tau }}=k{\rho }_{\mathrm{L}}(c_{\mathrm{sat}}-c)\pi d_{\mathrm{s}}^2$$

(3)

where m_s is the particle mass, $\pi {\rho }_{\mathrm{p}}/6d_{\mathrm{s}}^3$. c_sat is the saturation concentration and c is the local concentration. τ is time and ρ_p and ρ_L are the particle density and the electrolyte density, respectively.

By combining Equations (2) and (3), a shrinking core model in which particles dissolve under the mass transfer mechanisms can be derived:

$${\displaystyle \frac{d(d_{\mathrm{s}})}{d\tau }}=-{\displaystyle \frac{2k{\rho }_{\mathrm{L}}\left(c_{\mathrm{sat}}-c\right)}{{\rho }_{\mathrm{p}}}}$$

(4)

2.2. Heat Transfer Model

According to the analogy of heat and mass transfer, the heat transfer process of alumina and silica particles can be expressed by a dimensionless equation:

$$Nu=2.0+0.52R{e}_{\mathrm{m}}^{0.52}P{r}^{1/3}$$

(5)

where Nu is the Nusselt number, $h{d}_{\mathrm{s}}/{\lambda }_{\mathrm{L}}$ and λ_L is the thermal conductivity of the electrolyte. Pr is the Prandtl number, $c_{\mathrm{p},\mathrm{L}}{v}_{\mathrm{L}}{\rho }_{\mathrm{L}}/{\lambda }_{\mathrm{L}}$, c_p,L is the specific heat capacity of the electrolyte, and h is the convective heat transfer coefficient between the agglomerate particle and the electrolyte.

$$h={\lambda }_{\mathrm{L}}\left[2.0+0.52{\left(d_{\mathrm{s}}^{4/3}{\epsilon }_{\mathrm{L}}^{1/3}/{\nu }_{\mathrm{L}}\right)}^{0.52}{\left(c_{\mathrm{p},\mathrm{L}}{\nu }_{\mathrm{L}}{\rho }_{\mathrm{L}}/{\lambda }_{\mathrm{L}}\right)}^{1/3}\right]/d_{\mathrm{s}}$$

(6)

Due to the superheat being the driving force for particle dissolution under the heat transfer mechanism, the mass dissolution rate of particles can be expressed as follows:

$${\displaystyle \frac{d{m}_{\mathrm{s}}}{d\tau }}={\displaystyle \frac{h\pi d_{\mathrm{s}}^2(T_{\mathrm{L}}-T_{\mathrm{liq}})}{{\displaystyle {\int }_{T_{\mathrm{p}}}^{T_{\mathrm{L}}}{c}_{\mathrm{p}}\mathrm{d}T}+\Delta H_{\mathrm{diss}}}}$$

(7)

where c_p is the specific heat capacity of the particle, which is a variable related to temperature. ΔH_diss is the heat absorbed by particle dissolution, which mainly includes particle dissolution enthalpy, water vapor vaporization enthalpy in particles, and component phase change enthalpy [29,30]. T_L, T_p, and T_liq are the electrolyte temperature, agglomerate temperature, and liquidus temperature, respectively.

According to Equations (8) and (9), the shrinkage core model of particles under the heat transfer mechanism can be derived:

$${\displaystyle \frac{d(d_{\mathrm{S}})}{d\tau }}=-{\displaystyle \frac{2h\left(T_{\mathrm{L}}-T_{\mathrm{liq}}\right)}{{\rho }_{\mathrm{a}}\cdot \left({\displaystyle {\int }_{T_{\mathrm{p}}}^{T_{\mathrm{L}}}{c}_{\mathrm{p}}\mathrm{d}T}+\Delta H_{\mathrm{diss}}\right)}}$$

(8)

According to the specific heat capacity data of Al₂O₃ and SiO₂ in NIST-JANAF [31], as shown in

Table 1. Specific heat capacity of Al₂O₃ and SiO₂ at different temperatures.

Temperature (K)	cp,AlO (J·kg−1·K−1)	cp,SiO (J·kg−1·K−1)
298	811.07	743.15
300	815.19	746.15
400	986.29	890.50
500	1089.41	994.05
600	1155.25	1073.62
700	1200.22	1146.13
800	1233.16	1228.35
900	1259.09	1132.47
1000	1280.74	1149.20
1100	1299.59	1165.93
1200	1316.47	1182.68
1300	1331.72	1199.42

, the formulas for the specific heat capacity of Al₂O₃ and SiO₂ are obtained through data fitting.

$$c_{\mathrm{p},\mathrm{AlO}}=1483.79-\mathrm{199,795.92}/T$$

(9)

$$c_{\mathrm{p},\mathrm{SiO}}=1358.46-\mathrm{181,069.5}/T$$

(10)

The heat absorbed by alumina when it heats up can be expressed as follows:

$${\displaystyle {\int }_{T_{\mathrm{p},\mathrm{AlO}}}^{T_{\mathrm{L}}}{c}_{\mathrm{p},\mathrm{AlO}}}\mathrm{d}T=1483.79\cdot \left(T_{\mathrm{L}}-T_{\mathrm{p},\mathrm{AlO}}\right)-\mathrm{199,795.92}\cdot \ln \left(T_{\mathrm{L}}/T_{\mathrm{p},\mathrm{AlO}}\right)$$

(11)

The heat absorbed by silica when it heats up can be expressed as follows:

$${\displaystyle {\int }_{T_{\mathrm{p},\mathrm{SiO}}}^{T_{\mathrm{L}}}{c}_{\mathrm{p},\mathrm{SiO}}}dT=1358.46\cdot \left(T_{\mathrm{L}}-T_{\mathrm{p},\mathrm{SiO}}\right)-\mathrm{181,069}.5\cdot \ln \left(T_{\mathrm{L}}/T_{\mathrm{p},\mathrm{SiO}}\right)$$

(12)

2.3. Calculation Conditions

The particle size of SRM particles is below 200 μm, and the particle size distribution is listed in

Table 2. Particle size distribution of non-agglomerated SRM particles.

Diameter/μm	45	53	75	106	150	200
Percentage/wt%	40.6	11.1	21.0	15.6	6.7	5.0

[15]. When SRM particles are fed into the electrolyte, the electrolyte temperature decreases as the particles absorb heat, which causes some poorly dispersed SRM particles to clump together and form agglomerates. The agglomerates typically form within the first 20 s, with the SRM in the agglomerates hardly dissolving during this period [20,32]. The particle size of these agglomerated particles is generally around 1~1.5 cm, and the particle size distribution is listed in

Table 3. Particle size distribution of agglomerated SRM particles.

Diameter/mm	0.4	1	2	4	6	8	10	12	15
Percentage/wt%	10.8	20.1	19.2	20.6	11.2	7.0	4.7	3.3	3.1

. The mass of agglomerated particles in SRM particles accounts for approximately 84.1% [33,34]. The ratio of silica to alumina in SRM is approximately 9:16.

The physical parameters of the electrolyte, alumina, and silica particles are shown in

Table 4. Initial physical parameters in calculation.

Physical Parameters	Units	Values
Density of electrolyte	kg/m3	2050
Density of alumina particle	kg/m3	4000
Density of alumina agglomerate	kg/m3	2975
Density of silica particle	kg/m3	2600
Density of silica agglomerate	kg/m3	2300
Viscosity of electrolyte	Pa·s	2.51 × 10−3
Alumina concentration	wt%	3
Saturation concentration of alumina	wt%	10
Silica concentration	wt%	0.5
Saturation concentration of silica	wt%	5
Particle temperature	°C	30
Liquidus temperature of electrolyte	°C	950
Superheat	°C	10
Turbulent kinetic energy dissipation rate	m2/s3	0.015
Thermal conductivity of bath	W/(m·K)	1.69
Specific heat capacity of bath	J/(kg·K)	1660
Diffusion coefficient of alumina	m2/s	1.5 × 10−9
Diffusion coefficient of silica	m2/s	1.0 × 10−9

. These values were obtained through experimental measurements, theoretical calculations, and relevant literature [17,34].

Based on the conclusions from Taylor’s experiment [22], the mass transfer model was used to calculate the dissolution of non-agglomerated particles, while the heat transfer model was used to calculate the dissolution of agglomerated particles.

3. Results and Discussion

3.1. Effect of Concentration on SRM Dissolution

To avoid the anode effect and accelerate the dissolution of SRM, the alumina concentration is normally kept at 1.5~3.5 wt%, and the silica concentration is generally controlled at 0.2~1.5 wt%. When SRM is fed into the electrolyte, the SRM particles dissolve into the electrolyte, and the local alumina and silica concentrations in the electrolyte will increase. Changes in alumina and silica concentration affect the dissolution rate of the non-agglomerated SRM particles. The effects of alumina concentrations of 1.5, 2.0, 2.5, 3.0, and 3.5 wt% and silica concentrations of 0.2, 0.5, 1.0, 1.5, and 2.0 wt% on the dissolution process were investigated.

Figure 2 shows the effect of concentration on the mass dissolution rate of non-agglomerated SRM particles. The figure shows that for the same alumina or silica concentration, the mass dissolution rate of alumina or silica particles increases rapidly with increasing particle size. For the same particle size, the dissolution rate of non-agglomerated SRM particles decreases with the increasing concentration. This is because a high concentration leads to a reduction in the concentration difference, which limits the mass diffusion rate of alumina and silica in the electrolyte, resulting in a reduction in the mass dissolution rate of the particles. Therefore, maintaining a low concentration of alumina and silica in the electrolyte contributes to the rapid dissolution of SRM particles. Comparing Figure 2a and Figure 2b shows that the dissolution rate of the alumina particles is faster than that of the silica particles, which is consistent with previous literature [1,15].

Figure 3 illustrates the influence of concentration on the dissolution time of non-agglomerated SRM particles. With consistent particle size, reducing the concentration of alumina or silica surrounding the SRM particles leads to a significant reduction in the dissolution time of the alumina and silica particles. Within the concentration range shown in the figure, non-agglomerated alumina particles can be completely dissolved within 28 s, which is much smaller than 53 s for silica particles.

In summary, the concentration of alumina and silica around non-agglomerated particles affects their dissolution rate, with higher concentrations leading to slower dissolution rates and longer dissolution times. Maintaining a lower concentration of alumina and silica and accelerating the dispersion and transport of the particles to areas with lower concentrations contributes not only to the rapid dissolution of the particles, but also to the timely replenishment of the alumina and silica consumed by the electrolysis.

3.2. Effect of Electrolyte Superheat on SRM Dissolution

The superheat of the electrolyte is generally controlled at 8~15 °C. The temperature of the electrolyte decreases and superheat reduces when SRM is fed into the electrolyte, attributed to particle dissolution and heat absorption. The dissolution of agglomerated SRM particles is governed by the heat transfer, that is, affected by the superheat. To reveal the influence of different superheats on the dissolution of agglomerated SRM particles, the effects of superheats of 4, 7, 10, 12, and 15 °C on the particle dissolution rate and dissolution time were studied, respectively. The results are illustrated in Figure 4 and Figure 5.

Figure 4 shows that the mass dissolution rate of the agglomerated SRM particles increases exponentially with increasing particle size at the same superheat. The higher the superheat, the larger the gradient of particle dissolution rate with particle size. This is because, with an increase in particle size, the surface area of the particles participating in the reaction also increases, leading to a higher dissolution rate. Therefore, it is important to disperse the SRM particles well in the electrolyte to minimize particle agglomeration, which results in a larger total reaction area that is more conducive to particle dissolution. Additionally, for particles of the same size, the dissolution rate increases significantly with an increase in superheat. The increase in superheat accelerates the dissolution of SRM particles.

Figure 5 shows that for the same particle size, the time needed for particle dissolution increases significantly with decreasing superheat. Similarly, when the superheat is the same, the time needed for particle dissolution increases exponentially with the increase in particle size. This explains why SRM must be broken into fine particles, as it encourages its dissolution in the electrolyte and prevents larger particles from dissolving slowly and forming sediment at the bottom of the cell. Comparing Figure 5a and Figure 5b, it is evident that the time required for the dissolution of silica in SRM is longer than that of alumina when the particle size is the same. Alumina and silica particles with a particle size less than 1 mm can be fully dissolved within a feeding cycle of 144 s. However, in the case of large agglomerated particles, which are around 1cm in size, if the electrolyte has low superheat, then the time taken for these particles to dissolve will be longer, which can be up to 3000 s. This may lead to sediment formation. To prevent this from happening, it is important to ensure the fluidity of the particles, so they can be quickly dispersed and avoid aggregation after being fed. This helps to reduce the formation of large agglomerates and excessive sediment.

3.3. Effect of Particle Temperature on SRM Dissolution

To determine whether preheating of SRM particles is required, the effects of SRM particle temperatures on the particle dissolution process at 30 °C, 100 °C, 200 °C, and 300 °C were studied, respectively. The results are presented in Figure 6 and Figure 7. The figures show that the dissolution rate of the particles increases slightly with increasing temperature of the SRM particles and the dissolution time is shortened. For instance, for a 1 cm SRM particle, when the particle temperature increases from 30 °C to 300 °C, the dissolution time reduces by approximately 300 s. The difference in dissolution time for SRM particles with smaller particle sizes is even smaller. Preheating of the SRM particles has little effect on dissolution time. However, the dissolution time of a 1 cm SRM particle can be shortened by 2300 s when the electrolyte superheat increases from 4 °C to 15 °C. Therefore, increasing the temperature of SRM particles can help speed up the particle dissolution rate and shorten the dissolution time, but the effect is not significant. Compared to preheating SRM particles, increasing the superheat of the electrolyte is more effective in dissolving SRM particles. If there is waste heat that can be used to heat the SRM, the SRM particles can be preheated. There is no need to expend fuel specifically to preheat SRM particles.

3.4. Effect of Turbulent Kinetic Energy Dissipation Rate on SRM Dissolution

The electrolyte flow in the aluminum electrolytic cell is turbulent. Especially at the feeding hole, the bubbles escape and drive the electrolyte to stir up and down, making the electrolyte flow more turbulent and increasing the turbulent kinetic energy dissipation rate. Turbulence has a significant influence on the dissolution of SRM particles, as it contributes to particle dispersion, heat transfer, and mass transfer. To investigate the influence of the turbulent kinetic energy dissipation rate of the electrolyte on the dissolution of SRM particles, the dissolution was studied at different turbulent kinetic energy dissipation rates, namely 0.001 m²/s³, 0.015 m²/s³, 0.03 m²/s³, and 0.05 m²/s³. The results are presented in Figure 8 and Figure 9.

The figures shows that both types of particles experience an increase in their dissolution rates and a reduction in the dissolution time as the turbulent kinetic energy dissipation rate increases. This demonstrates that an increase in the turbulent kinetic energy dissipation rate contributes to the rapid dissolution of particles. For agglomerated particles, such as 1 cm agglomerated particles, when the turbulent kinetic energy dissipation rate increases from the lowest 0.001 m²/s³ to the largest 0.05 m²/s³, the dissolution time decreases by around 800 s. The turbulent kinetic energy dissipation rate has a major influence on the dissolution of agglomerated particles. During the field tests, it was also found that alumina dissolves faster when the feeding hole is located at the intersection of the anode gap and the center channel. This is due to the fact that the electrolyte in this area is violently stirred up by the bubbles and the turbulent kinetic energy dissipation rate is higher, which is conducive to the dissolution and dispersion of the particles. Therefore, by adjusting the anode structure and the process parameters, appropriately increasing the turbulence intensity and turbulent kinetic energy dissipation rate of the electrolyte will contribute to the rapid dissolution of SRM.

3.5. Model Validation

To check the accuracy of the model, the data from the literature and the experiment are compared with the calculated results of the model established in this work. The dissolution time of a non-agglomerated alumina particle is compared with the results from the literature, as shown in

Table 5. Comparison of calculated alumina dissolution time with the literature data.

Diameter of Particle	50 μm	100 μm	200 μm
Calculated	2.85 s	8.76 s	25.57 s
Lillebuen’s result [35]	/	9.4 s	/
Thonstad’s result [17]	4.10 s	16.64 s	/
Li’s result [21]	3.98 s	16.1 s	27.87 s

. The comparison shows that the calculated dissolution time of alumina is basically consistent with the literature data, indicating that the proposed model is suitable for calculating the dissolution process of non-agglomerated particles.

The comparison of the percentage of cumulative dissolved quantity between the calculated and experimental results is illustrated in Figure 10. It can be seen that the results obtained by calculations and experiments are in good agreement, with an error within 10%, suggesting that the model established is reliable when calculating the dissolution of SRM particles. The calculated results are slightly larger than the experimental data. This is mainly due to the reduction in dissolution rate caused by the decrease in temperature and the increase in concentration during the dissolution process.

4. Conclusions

A mass transfer model and a heat transfer model for the dissolution of SRM particles were established. Based on the established dissolution model, the effects of electrolyte superheat, alumina concentration, silica concentration, turbulent kinetic energy dissipation rate, and particle temperature on the dissolution rate and dissolution time of SRM particles were investigated. The established model was verified and the dissolution law of SRM particles in the electrolyte was theoretically analyzed and explained. The main conclusions are listed below:

(1)

Non-agglomerated SRM particles can dissolve within 50 s, and as the concentration decreases, the mass dissolution rate increases and the time required for complete dissolution is shorter. For example, when the silica concentration in the electrolyte is reduced to 0.2 wt%, the time required for the complete dissolution of non-agglomerated SRM particles is reduced to 33 s. On the premise that no anode effect occurs, the alumina and silica concentrations should be kept as low as possible.

(2)

By increasing the particle temperature and the electrolyte superheat, the mass dissolution rate of the agglomerated SRM particles can be accelerated and the dissolution time shortened. However, increasing the electrolyte superheat has a greater effect on accelerating the dissolution of agglomerated SRM particles than increasing the particle temperature. In practical applications, the electrolyte temperature can be increased accordingly.

(3)

Regardless of whether the SRM particles are non-agglomerated or agglomerated, increasing the turbulent kinetic energy dissipation rate can increase the mass dissolution rate and shorten the dissolution time. The tilted anode can be used to allow more bubbles to escape from the feeding point and increase the turbulent kinetic energy dissipation rate of the electrolyte, thereby accelerating the dispersion and dissolution of the SRM particles.