Study on the Influence of Different Particle Sizes of Kaolin Blending with Zhundong Coal Combustion on the Adsorption of Alkali Metal Sodium and Ash Fusion Characteristics

Abstract

Blending kaolin is an effective method to alleviate fouling and slagging during the combustion of Zhundong coal. The influence of blending kaolin with varying particle sizes on the adsorption behavior of alkali metal sodium and the ash fusion characteristics was studied using sodium capture experiments and ash fusion temperature tests. The results indicate that kaolin particle size is a critical factor influencing sodium retention. As the particle size decreases, the sodium retention rate increases accordingly. In the absence of kaolin, the sodium retention rate was only 28.03%. However, when 75–100 µm particles of kaolin were added, the retention rate increased to 43.49%. Further reducing the particle size to 20–63 µm resulted in an additional increase of 10.51%. Additionally, decreasing the kaolin particle size contributed to the noticeable increase in ash fusion temperatures. After 75–100 µm kaolin was blended, the DT and FT of the ash were 1137 °C and 1161 °C, respectively. With 20–63 µm kaolin, the DT increased by 42 °C, and the FT increased by 36 °C. This trend is attributed to the enhanced decomposition and transformation of sulfates in the ash, which promotes the formation of high-melting-point feldspar minerals such as anorthite and gehlenite. These findings provide important data support for understanding the influence of kaolin particle size on the ash fusion behavior during the combustion of Zhundong coal.

1. Introduction

Coal resources are the foundation of global energy supply and remain dominant in the energy structure [1,2]. Despite global decarbonization and the shift toward oil and gas, coal continues to serve as a strategic bridge fuel for energy security, particularly in developing and energy-intensive economies. The Zhundong (ZD) coalfield located in the Junggar Basin of Xinjiang is China’s largest fully explored coalfield, with reserves of 390 billion tons [3]. ZD coal exhibits high volatility, medium-high moisture, and low ash content [4,5] with a notably high alkali metal content that lowers ash fusion temperature and promotes slagging and fouling in boilers [6,7].

Scholars [8,9] have conducted extensive research on coal rich in alkali and alkaline earth metal (AAEM) elements. They found that under the same conditions that ZD coal exhibited a greater tendency for deposition than bituminous coal or even biomass, confirming that the high alkali metal content in the fuel leads to a lower ash fusion temperature. As an efficient adsorbent, kaolin can remove harmful substances such as alkali metals, heavy metals, and fine particulate matter produced during the combustion of ZD coal [10]. Kaolin possesses a typical layered crystal structure, with each layer consisting of an Al-O octahedral layer and a Si-O tetrahedral layer, which are tightly connected by hydrogen bonds [11]. This unique layered crystal structure endows kaolin with exceptional adsorption properties and excellent stability. At low temperatures, kaolinite gradually transforms into amorphous metakaolin through dehydroxylation, which is one of its primary products during high-temperature decomposition [12]. Due to the loss of hydroxyl groups, metakaolin exhibits an unsaturated coordination state. Meanwhile, as the temperature rises, gaseous metals diffuse to the surface of kaolin, and they are adsorbed and simultaneously undergo chemical reactions, generating sodium aluminosilicates (nepheline and albite) [13]. With the temperature rising above 1100 °C, metakaolin converts into crystalline mullite, a mineral with a relatively weak adsorption capacity [14]. The efficiency of kaolin in capturing gaseous alkali metals in boilers is primarily related to the thermal decomposition and transformation behavior of kaolin.

The fusion characteristics of coal ash are generally regarded as the key parameters for evaluating the caking and fouling of ash in boilers. Vassilev et al. [15] demonstrated that an increase in the content of alkaline metals such as Ca, Na, and Fe in coal will lead to a significant decrease in the ash fusion temperature, while an increase in the content of Si and Al has the opposite effect. Zhu et al. [16] found that when the ratio of base to acid was higher than 0.4, this type of additive tended to lower the fusion temperature of the ash. The Si- and Al-based minerals react with gaseous Na to convert the alkali metals in the coal into a solid state, thereby achieving the regulation of the coal ash fusion temperature [17,18]. The research on kaolin additives has shown that their influence on the fusion temperature of ash is closely related to the blending ratio [7]. With a kaolin blending ratio below 3%, the ash fusion temperature decreases significantly; at ratios exceeding 6%, the temperature rises rapidly [19,20]. At low blending ratios of kaolin, its components of Si and Al tend to participate in the formation of low-temperature eutectic mixtures and low-temperature co-crystals, which promotes the release of Ca and Na [21]. At present, most studies have focused on the influence of the proportion of kaolin addition of the transformation of coal ash minerals and the fusion temperature of the ash. However, the grindability of kaolin and coal is inconsistent. After blended and ground, the particle size distribution of kaolin is unclear, and the effects of different particle sizes of kaolin on the transformation of coal ash minerals and the ash fusion temperature are also unknown. Therefore, it is more important to study the influence of kaolin particle size on the adsorption and ash fusion characteristics of alkali during the combustion of ZD coal.

Generally speaking, particles with smaller diameters have larger surface areas, more surface-active sites, and stronger adsorption capabilities [22,23]. To investigate the effect of kaolin particle size on the adsorption of alkali metal sodium in ZD coal and its ash fusion characteristics, X-ray diffraction (XRD) was utilized to analyze mineral transformations, scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDS) analysis was conducted on the microscopic morphology and elemental distribution of the ash sample, ash fusion temperatures were measured using an ash fusion point analyzer, and sodium retention rates were determined using inductively coupled plasma optical emission spectrometry (ICP-OES). These findings provide valuable data to mitigate slagging and fouling issues during ZD coal combustion.

2. Materials and Experimental Methods

2.1. Raw Material Preparation

In this study, ZD coal was selected as the experimental raw coal, and the kaolin was used as the blending additive. First, according to the Chinese standards GB/T 212-2008 [24] and GB/T 30733-2014 [25], the proximate analysis and ultimate analysis of raw coal were conducted, and the content of fixed carbon and oxygen were calculated by differences. And the results are shown in

Table 1. Proximate and ultimate analyses of raw coal (wt.%).

Proximate Analysis				Ultimate Analysis
Mar	Ad	Vdaf	FCad *	Cd	Hd	Od *	Nd	St,ad
25.40	9.67	37.05	48.76	47.86	3.16	11.80	1.18	0.93

M: moisture; A: ash; V: volatile matter; FC: fixed carbon; ar: received basis; ad: air-dried basis; daf: dry ash-free basis, d: dry basis, t: total; *: by difference.

. Meanwhile, following the Chinese standard GB/T 219-2008 [26], the ash fusion temperatures of the coal ash were tested. The ash fusion characteristics are evaluated using four characteristic temperatures: deformation temperature (DT), softening temperature (ST), hemisphere temperature (HT), and flow temperature (FT). The data on the ash fusion temperatures of the raw coal are shown in

Table 2. ZD coal ash fusion temperature.

Ash Fusion Temperatures
ZD coal	DT	ST	HT	FT
Temperature/°C	1149	1160	1172	1193

. Furthermore, the ash composition was analyzed under the standard of GB/T 1574-2007 [27]. The analysis results are presented in

Table 3. Chemical composition of ZD coal ash and kaolin (wt.%).

Sample	Oxide Composition
	Na2O	Fe2O3	Al2O3	SiO2	CaO	MgO	SO3	TiO2	K2O
ZD coal	3.20	9.10	18.69	28.44	14.06	5.33	20.16	0.78	0.25
kaolin	1.11	2.83	22.99	68.17	0.30	0.64	0.14	1.20	2.35

, and the proximate and ultimate analysis shows the raw coal has a moisture content of 25.40% and ash content of 9.67%. The deformation temperature of the raw coal is 1149 °C, and the flow temperature is 1193 °C, indicating a relatively low ash fusion temperature. Na₂O accounts for 3.2% of the raw coal ash composition. In contrast, the Na₂O content in the blended kaolin is only 1.11%, while the Al₂O₃ content is 22.99%. The SiO₂ content is as high as 68.17%, so it is classified as a high-silica and -aluminum mineral.

Kaolin contains no calorific value, and adding it to coal may reduce the boiler’s combustion efficiency. In addition, a low blending ratio may lead to an insignificant effect on the ash fusion temperature. Therefore, to increase the ash fusion temperature and minimize the impact of kaolin content on load, kaolin with different particle sizes was blended with ZD coal at a 2% ratio to analyze the influence of particle size on sodium capture and ash fusion characteristics. The particle size of kaolin at 75–100 µm, 63–75 µm, and 20–63 µm were uniformly blended with the raw coal. The mixed samples were labeled as C-K1, C-K2, and C-K3, respectively.

2.2. Sodium Adsorption Characteristic Experiment

A fixed-bed combustion system was employed for the experiment involving kaolin blending to retain sodium, as shown in Figure 1. The system consists of an air supply unit, a heating unit, and a gas-washing unit. The air supply unit is composed of an air compressor and a mass flow meter. The heating unit utilizes a TL1500-1500 dual-zone tubular furnace. The gas-washing unit employs dilute nitric acid and low-concentration sodium hydroxide solutions to absorb the flue gas generated during the reaction.

During the experiment, 4 g of the coal sample was weighed and placed in an alumina crucible. The sample was then heated from room temperature in the tubular furnace, with the airflow rate maintained at 0.2 L/min. The target temperatures were set at 900 °C, 1000 °C, 1100 °C, and 1200 °C, respectively. Below 900 °C, the heating rate was controlled at 10 °C/min, while above 900 °C, it was adjusted to 5 °C/min. After reaching each target temperature, calcination was continued for an additional hour to ensure the completion of the reaction.

2.3. Calculation of Sodium Retention Rate

To investigate the sodium capture performance of kaolin with different particle sizes in raw coal at various temperatures, the chemical extraction method was employed to measure the sodium content in coal ash [28]. The capture performance of sodium in coal ash is represented by the sodium retention rate η, and the calculation formula is shown in Equation (1).

$$\eta ={\displaystyle \frac{M_{Na,H}(d,T)}{M_{Na,D}}}\times 100\%$$

(1)

M_Na,H(d,T) represents the sodium content (wt.%) of the coal ash obtained after adding kaolin with particle size d at temperature T. In contrast, M_Na,D represents the sodium content (wt.%) in the raw ZD coal.

2.4. Experimental Methods

The ash fusion temperatures were tested using an ash fusion point instrument (YHHR-4000, Changzhou Aeolian Technology Co., Ltd., Changzhou, China). Coal ash was shaped into triangular cones (height of 20 mm, bottom edge length of 7 mm) using 10% dextrin as a binder. Ash fusion behavior was examined in an ash fusion temperature analyzer under atmosphere condition. The room temperature was raised to 900 °C, with a heating rate of 15 °C/min. After reaching 900 °C, the heating rate decreased to 5 °C/min. Four characteristic temperatures of ash samples were determined by comparing the real-time shape of each triangular cones with the shape described in Chinese standard (GB/219–2008). The final result AFT values represent the average of three parallel measurements. The coal ash was digested using a microwave digestion system (Multiwire PRO, Anton Paar GmbH, Graz, Austria) with a mixture of nitric acid and hydrofluoric acid. The digestion was followed by multiple acid evaporation treatments, and the sample was diluted to 50 mL with deionized water. The sodium content was tested using ICP-OES (AAA7900, Agilent, Santa Clara, CA, USA) under the following conditions: RF power of 1550 W, plasma gas flow rate of 15.0 L/min, carrier gas flow rate of 1.05 L/min, and sampling depth of 8.0 mm. The mineral composition in coal ash was analyzed by comparing with the standard cards in the Jade 6.5 software package using XRD (SmartLab 9 kW, Rigaku Corporation, Akishima, Japan). During the XRD data collection, Cu Kα radiation with a wavelength of 0.154 nm was employed under operating conditions of 40 kV and 30 mA. The diffraction patterns were recorded with a step size of 0.02°, a scanning speed of 2°/min, and a 2θ range of 3°–30°. SEM-EDS (Phenom Pure, Thermo Fisher Scientific, Bleiswijk, The Netherlands) was used to analyze the microscopic morphology and elemental distribution of coal ash under different reaction conditions, with spectral acquisition conducted using the Phenom ProSuite system. The measurements were performed with an energy resolution of 129 eV (Mn Kα, nominal value) and a spatial resolution of approximately 2 μm at an accelerating voltage of 10 kV.

3. Results and Discussion

3.1. Morphology of Coal Ash and Variation in the Ash Formation Rate

Figure 2 illustrates the macroscopic morphology of coal ash under different reaction conditions. At 900 °C, the coal ash appears predominantly grayish white, with the surface exhibiting a distinctly loose texture. When the temperature exceeds 1000 °C, the coal ash without kaolin addition turns reddish brown. Although the ash samples remain relatively loose, noticeable agglomeration is observed. Although the blended samples with added kaolin also showed signs of agglomeration, the extent of molten agglomeration decreased significantly with decreasing kaolin particle size, and the overall ash yield was also noticeably reduced.

Figure 3 shows the microscopic morphology of coal ash after the co-combustion of ZD coal with different kaolin samples at 900 °C and 1100 °C. At 900 °C, a small number of large particles are observed in the coal ash blended with 75–100 µm kaolin, with particle diameters around 30 µm. In contrast, when blended with 20–63 µm kaolin, no large particles are present, and the ash consists of uniformly small particles.

At 1100 °C, the morphology of the coal ash undergoes significant changes. For ash blended with 75–100 µm kaolin, the surface displays large particles exceeding 80 µm in diameter. Additionally, adhesion is observed both among small particles and between small and large particles, which may be attributed to sintering phenomena occurring at elevated temperatures. In contrast, for ash blended with 20–63 µm kaolin, no obvious large particles are observed on the surface; however, the adhesion between particles is more pronounced, suggesting that the smaller particles undergo fusion, resulting in more extensive bonding [29].

Figure 4 presents the ash formation rate for kaolin blending with varying particle sizes. It can be seen that at the same combustion temperature, the ash formation rate is negatively correlated with the particle size of the added kaolin, indicating that a decrease in particle size leads to an increase in the ash formation rate. This may be attributed to the fact that kaolin with smaller particle sizes has a larger specific surface area, which offers more active adsorption sites, thereby enhancing its ability to adsorb gaseous substances and increasing the ash formation rate.

However, as the combustion temperature increases, the ash formation rates of all samples show a significant decreasing trend. This can be explained by the fact that chemical reactions between minerals in the coal ash become more complete at higher temperatures. Additionally, elevated temperatures promote the release of gaseous species, resulting in a reduced ash formation rate as the combustion temperature rises.

3.2. Transformation Process of Coal Ash Minerals

Figure 5 shows the mineral composition of coal ash under different conditions. The chemical reaction equations for the minerals in coal ash are provided in

Table 4. Major mineral-forming reactions in coal ash.

Chemical Reaction Equations	Number
$CaS{O}_4\to CaO+S{O}_2$	(2)
$N{a}_{2}S{O}_4\to N{a}_{2}O+S{O}_2$	(3)
$N{a}_{2}O+A{l}_2{O}_3+2Si{O}_2\to NaAlSi{O}_4$	(4)
$CaO+A{l}_2{O}_3+2Si{O}_2\to C{a}_{2}A{l}_{2}Si{O}_7$	(5)
$N{a}_{2}O+A{l}_2{O}_3+2Si{O}_2\to NaAlS{i}_3{O}_8$	(6)
$MgO+A{l}_2{O}_3\to MgA{l}_2{O}_4$	(7)
$CaO+2Si{O}_2\to C{a}_{2}Si{O}_4$	(8)
$F{e}_2{O}_3+MgO+Si{O}_2\to M{g}_{1.86}F{e}_{0.14}Si{O}_4$	(9)
$CaO+A{l}_2{O}_3+2Si{O}_2\to CaA{l}_{2}Si{O}_8$	(10)

.

Table 5. Main minerals in coal ash.

Number	Molecular Formula	Number	Molecular Formula	Number	Molecular Formula
1	CaSO4	6	Na2Si2O5	11	CaAl2Si2O8 (anorthite)
2	CaO	7	NaAlSiO4 (nepheline)	12	Al2(SiO4)O (kyanite)
3	Na2SO4	8	Ca2SiO4	13	Mg2SiO4
4	Al2O3	9	NaAlSi3O8 (albite)	14	MgAl2O4 (spinel)
5	SiO2	10	Ca2Al2SiO7 gehlenite)	15	Mg1.86Fe0.14SiO4 (Iron magnesium olivine)

lists the main minerals and their chemical formulas.

At temperature stages of 900 °C and 1000 °C, the mineral composition of coal ash from raw coal combustion primarily consists of anhydrite (CaSO₄), quartz (SiO₂), and sulfates, with only trace amounts of silicate minerals. Upon kaolin addition, the diffraction peaks of certain sulfates in the coal ash transform into those of nepheline (NaAlSiO₄) and gehlenite (Ca₂Al₂SiO₇), as described by the reactions in Equations (2)–(5). At 1000 °C, compared to raw coal without kaolin, the diffraction peak intensity of sodium sulfate (Na₂SO₄) significantly decreases after kaolin addition, while the diffraction peak of albite (NaAlSi₃O₈) intensifies with decreasing kaolin particle size. This is attributed to the high content of silico-aluminous oxides in kaolin, which adsorb sodium in the coal ash to form albite, as shown in Equation (6). As kaolin particle size decreases, the NaAlSi₃O₈ diffraction peak becomes more pronounced, indicating that smaller kaolin particles enhance the reaction between sodium and aluminosilicates. The formation of spinel (MgAl₂O₄) in coal ash after kaolin blending is likely due to the increased Al₂O₃ content from kaolin, which reacts with periclase (MgO) to form MgAl₂O₄, as described in Equation (7). Furthermore, blending with kaolin of 20–63 µm particle size significantly enhances the diffraction peak intensity of calcium silicate (Ca₂SiO₄). Smaller kaolin particle sizes are more conducive to reacting with silicon dioxide to form silicates, as shown in Equation (8) [30].

At 1100 °C, the sulfate content in raw coal decreases significantly, with elements such as Na, Ca, and Mg predominantly present as silicates. The addition of kaolin markedly enhances the intensity of the Ca₂SiO₄ diffraction peak, while the sulfate diffraction peak disappears, attributed to the decomposition of calcium sulfate [31]. Additionally, as kaolin particle size decreases, Fe₂O₃, MgO, and SiO₂ react to form iron magnesium olivine (Mg₁.₈₆Fe₀.₁₄SiO₄), as described by Equation (9). Concurrently, the increased diffraction peak intensity of sodium-containing minerals, such as nepheline, suggests that smaller kaolin particle sizes enhance surface active sites [32], thereby improving the sodium capture efficiency of kaolin.

At the high-temperature condition of 1200 °C, sodium in raw coal predominantly exists as sodium silicate and albite, while calcium primarily occurs as calcium silicate, with a minor fraction as aluminosilicate. Upon kaolin addition, the diffraction peaks of albite and anorthite significantly increase, as described by the reaction in Equation (10). This is likely due to the reaction of Al₂O₃ in kaolin with sodium silicate and calcium silicate, leading to the formation of feldspar-like minerals [33]. Furthermore, as the particle size of blended kaolin decreases, the diffraction peaks of silicate minerals in coal ash largely disappear, while those of high-melting-point minerals, such as anorthite, gehlenite, and albite, are significantly enhanced. This suggests that smaller kaolin particle sizes promote the transformation of low-melting-point silicate minerals into high-melting-point feldspar group minerals.

Figure 6 shows the distribution of elements on the surface of coal ash with different particle sizes of kaolin blended at 1000 °C. In the case of blending 75–100 µm kaolin, there are large particles larger than 80 µm, with loose surfaces and numerous small pores. The surface of the large particles is mainly composed of Si and Al, along with some sodium and a small amount of calcium. It is speculated that the main component is a sodium-calcium-containing aluminosilicate. The particles around the large particles are distributed relatively evenly. The main elements are Ca, S, and O. Based on XRD analysis, the main mineral is calcium sulfate. As the particle size of blended kaolin is reduced to 20–63 µm, the particle sizes of the surface particles of the ash sample become uniform. By comparing C-K1 with C-K3, it was found that after adding large-sized kaolin, the surface Na content of the ash sample was lower than that of the small-sized one. The smaller the particle size is, the higher the content of adsorbed alkali metal Na.

3.3. Analysis of Coal Ash Fusion Characteristics

Table 6. The ash fusion temperature of coal ash and its main minerals.

Sample	Main Mineral Component	Ash Fusion Temperature (/°C)
		DT	ST	FT	HT
C-K1	CaAl2Si2O8, Ca2SiO4, NaAlSi3O8	1137	1142	1156	1161
C-K2	Ca2Al2SiO7 (increase), CaAl2Si2O8 (decrease), Ca2SiO4, NaAlSi3O8	1163	1173	1178	1185
C-K3	Ca2Al2SiO7 (increase), CaAl2Si2O8(decrease), Ca2SiO4, NaAlSi3O8	1179	1186	1194	1197

shows the fusion temperatures of coal ash blended with kaolin at different particle sizes. It can be seen that after blending with 75–100 µm kaolin, the fusion temperature of the coal ash is significantly lower than that of the raw coal, with a DT of 1137 °C and FT of 1161 °C. This is consistent with the XRD results at 1200 °C, where the diffraction peak of Ca₂SiO₄ in the ash blended with 75–100 µm kaolin is significantly weakened, while the diffraction peak of Ca₂Al₂SiO₇ is noticeably enhanced. Since the fusion temperature of Ca₂Al₂SiO₇ is lower than that of Ca₂SiO₄ [34], the addition of 2% 75–100 µm kaolin leads to a decrease in ash fusion temperature.

As the kaolin particle size decreases, the fusion temperature of coal ash gradually increases. Specifically, with the addition of 20–63 µm kaolin, the deformation temperature of the ash increases by 42 °C, and the flow temperature rises by 36 °C. Referring to Table 6, at 1200 °C and with kaolin particle size reduced to 20–63 µm, the diffraction peak of Ca₂Al₂SiO₇ becomes stronger, while that of CaAl₂Si₂O₈ weakens. Since Ca₂Al₂SiO₇ has a higher fusion temperature than CaAl₂Si₂O₈, the formation of more high-fusion-point gehlenite is promoted by smaller kaolin particles. This is considered the main reason for the increase in ash fusion temperature when finer kaolin is blended [35]. Table 6 shows that the DT and HT of the kaolin samples with small particles are significantly higher than those of the samples with large particles. It confirms that an increase in the content of Ca₂Al₂SiO₇ directly leads to a higher fusion temperature of the coal ash.

3.4. Sodium Retention Rate Analysis

Figure 7 shows the influence of blending kaolin with different particle sizes at 1100 °C on the sodium retention rate of ZD coal. The sodium retention rate of coal ash without kaolin addition is only 28.03%. After adding kaolin with a particle size of 75–100 µm, the sodium retention rate increases to 43.49%, which is 15.46% higher than that of the sample without kaolin. When the particle size of kaolin is further reduced to 20–63 µm, the sodium retention rate reaches 54%, an increase of 10.51% compared to the 75–100 µm kaolin-blended sample. As the particle size of kaolin decreases, the rate of increase in sodium retention gradually declines, and the increment in sodium retention rate weakens from 8.47% to 2.04%. This may be attributed to the progressive saturation of surface active sites as the kaolin particle size decreases, which limits the availability of new reactive sites [32].

4. Conclusions

This study investigates the effects of kaolin with various particle sizes on the sodium adsorption behavior and ash fusion characteristics of ZD coal under combustion temperatures ranging from 900 °C to 1200 °C, with a fixed kaolin blending ratio of 2%. A detailed analysis was conducted on the mineral transformation processes in the coal ash and the evolution of ash fusion temperatures. The main conclusions are as follows:

(1)

As the particle size of the blended kaolin decreases, the molten agglomerated state in ZD coal ash transforms into a compact sintered particle structure, and the ash yield gradually increases.

(2)

The addition of kaolin promotes the decomposition of low-melting-point sulfates in coal ash and facilitates their transformation into silicates and aluminosilicates. As the particle size of blended kaolin decreases, the diffraction intensities of sulfates in the ash decrease, while the silicates and aluminosilicates increase accordingly.

(3)

With decreasing kaolin particle size, the ash fusion temperature shows a gradual increase. Blending kaolin with a particle size of 75–100 µm into the ZD coal, its DT and FT were 1137 °C and 1161 °C. However, the particle size was reduced to 20–63 µm, its DT and FT were 42 °C and 36 °C higher, respectively, than those of the larger particle size kaolin. The particle size of kaolin decreases, and the high-melting-point silicates and aluminosilicates increase, thereby raising the ash fusion temperature.

(4)

A reduction in kaolin particle size significantly improves the sodium retention efficiency. Without kaolin addition, the sodium retention rate is only 28.03%, but it increases to 43.49% when 75–100 µm kaolin is blended. However, as the particle size decreases, the increase in sodium retention rate weakens from 8.47% to 2.04%.