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Article

The Adjustment for Entrainment Behavior of Kaolinite in Coal Flotation

by
Chengyong Liu
1,
Wenzhe Gu
1,
Haijun Zhang
2,
Quanzhi Tian
3,*,
Hainan Wang
2,
Yuejin Zhou
4 and
Zhicheng Liu
1
1
China National Coal Group Corp., Xi’an 710054, China
2
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China
3
State Key Laboratory of Coking Coal Resources Green Exploitation, China University of Mining and Technology, Xuzhou 221116, China
4
State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221008, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(2), 147; https://doi.org/10.3390/min15020147
Submission received: 29 December 2024 / Revised: 21 January 2025 / Accepted: 26 January 2025 / Published: 31 January 2025
(This article belongs to the Section Mineral Processing and Extractive Metallurgy)
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Versions Notes

Abstract

:
The entrainment of gangue particles always increases the ash content of coal flotation concentrate. In the present work, the adjustment for entrainment of kaolinite in coal flotation was studied focusing on the parameters including solid concentration and frother dosage. The results indicated that the ash contents of concentrates were always higher in the early or late stage (flotation time: 0–30 s or 120–180 s) than that in the middle stage (flotation time: 30–120 s). This is mainly because of the mechanical entrainment in the early stage and the entrainment caused by water transportation in the later stage. It has been confirmed that reducing the feed solid concentration and frother dosage effectively decrease mechanical entrainment in the early stage. Furthermore, better flotation results can be obtained under a high solid concentration and frother dosage in the late stage of the flotation process. Therefore, a novel two-stage flotation process was proposed for the coal flotation. A better flotation performance (combustible recovery: 91.28%; ash content: 4.09%) can be achieved by two-stage flotation, compared to the flotation results (combustible recovery: 93.12%; ash content: 5.16%) of the one-stage flotation process.

1. Introduction

Flotation is a complex separation process which exploits the physicochemical surface properties of mineral particles to separate the valuable minerals from the gangue [1,2,3,4,5]. However, a substantial amount of fine-grained gangue minerals is generated during the crushing and grinding process. These fine particles not only increase the pulp viscosity and reduce flotation efficiency but also become non-selectively entrained into the froth product, thus seriously contaminating the concentrate [6,7,8]. To a large extent, the separation efficiencies between valuable minerals and dispersed gangues are dependent on the degree of entrainment [9]. As for the coal flotation, the entrainment of high-ash slime should be the main reason for the production of the low-grade concentrate [10,11,12]. On the other hand, with the increase in the mechanization of coal mining, the content of fine particles smaller than 0.074 mm increases, which results in a rapid increase in the content of high-ash fine slime in the flotation feed. This would increase the difficulty of fine coal slime separation. Therefore, it has great importance for coal slime beneficiation to reduce the entrainment of high-ash particles and then improve the separation selectivity.
Entrainment in flotation can be considered as a two-step process including the transportation of the suspended solids to the top of the pulp region and the transportation of the entrained particles in the froth to the concentrate [13,14]. Finely sized liberated particles can be carried out in the thin water layer surrounding the air bubble and recovered in the concentrate launder regardless of the hydrophobicities of the particles [15,16,17]. Thus, entrainment of gangue particles can occur when particles are dragged by the interstitial liquid films between air bubbles [18,19,20]. Generally, the entrainment of hydrophilic gangue is mainly controlled by three factors, including the physical properties of the mineral particles, the working conditions of the flotation process, and the type and dosage of flotation reagents [21,22]. For example, Kursun [23] found that the finer the particle size of gangue minerals, the higher the degree of entrainment. Wang et al. [22] studied the effect of gangue particle density on entrainment and found that a decrease in gangue particle density leads to a decrease in particle settling rate, resulting in an increase in the degree of entrainment of gangue in flotation. Yu et al. [11] found that the external energy input would break the “energy barrier”, which resulted in a slime coating and decreasing the coal particle floatability. Akdemir and Sönmez [24] found that increasing the impeller speed raises the energy input in the pulp system and thereby increases the collision frequency between particles and bubbles, ultimately promoting the entrainment of hydrophilic gangue minerals into the concentrate. Wang et al. [25] studied the effect of non-ionic composite collectors on entrainment, and found that span80 enhanced foam stability and increased both water recovery and entrainment, thereby improving the recovery of coal gangue. Wang et al. [26] stated that the frothers changed the bubble size, leading to alterations in the liquid content and liquid velocity at the interface. Subsequently, the altered interfacial liquid velocity modified the drag force, which determined the motion of a particle against the downward gravitational force on the particle due to its weight. As a result, the amount of hydrophilic solid particles remaining in and recovered with the water phase varied, thereby changing the degree of entrainment. Thus, this indicates that reducing the degree of entrainment of gangue particles is an effective measure to improve the purity of the concentrate.
Many scholars have conducted extensive research on how to regulate the entrainment behavior of fine-grained gangue minerals and improve the flotation concentrate indicators [27,28,29,30]. Ni et al. [31] investigated the effect of installing a set of inclined plates in the froth zone on the flotation performance of a flotation column and found that the arrangement of the inclined plates in the froth zone enhanced froth drainage and reduced water entrainment with the fine coal slime particles, both the recovery of fine coal slime and the clean coal ash content decreased. Gong et al. [32] found that polyethylene oxide can significantly reduce the entrainment of fine-grained quartz during chalcopyrite flotation. Polyethylene oxide selectively adsorbs onto the surface of quartz particles, causing flocculation and effectively increasing their apparent particle size, thereby markedly lowering the degree of entrainment of quartz particles. However, few tests have been conducted in the entrainment adjustment during the coal flotation process.
In this investigation, the adjustment of kaolinite entrainment in the coal flotation process was studied, and the effects of feed solid concentration and frother concentration on the entrainment of kaolinite were analyzed. Based on the findings in this study, a novel flotation process was proposed and examined.

2. Experimental

2.1. Materials

Ultra-low-ash fine coal and kaolinite powder were used in the experiment. The ultra-low-ash fine coal sample was obtained from Inner Mongolia, China, belonging to anthracite. The combustible matter recovery of ultra-low-ash fine coal was 97.12%, which was mainly composed of C (carbon). The kaolinite was obtained from Shanlin Shiyu Mineral Products, and its content of SiO2 and Al2O3 was 83.2%. Firstly, the 1–50 mm size fraction of the received coal sample was separated using sieves. The fraction was subjected to the float-and-sink test to obtain a light fraction (<1.4 kg/L) which was then ground in a rod mill. Thereafter, the coal slimes were screened to obtain a fraction with a particle size lower than 0.125 mm. Then, this fraction of the coal sample was used in this study. In addition, the XRD pattern of the kaolinite product used in this study is shown in Figure 1, indicating that the main phase is kaolinite, and another hydrophilic phase of quartz also exists as an impurity.
The ultra-low-ash fine coal sample and kaolinite show a great difference in ash content, showing 2.88% and 86.63%, respectively. This indicates that the entrainment content of kaolinite can be largely reflected by the ash content of the concentrate. Figure 2a shows the relationship between the cumulative yield and the ash content of the concentrate for the ultra-low-ash fine coal flotation without adding kaolinite (collector dosage: 0; frother dosage: 80 mg/L). The ash content of the concentrate remains almost the same, and combustible recovery can reach 97.18%. In addition, the Microtrac S3500 laser particle size analyzer (Microtrac, Montgomeryville, PA, USA) was used to analyze the size distributions of the ultra-low-ash fine coal and kaolinite (Figure 2b). It can be seen that the <0.045 mm fraction of kaolinite and ultra-low-ash fine coal is 91.60% and 70.01%, respectively.

2.2. Experimental Procedure

The flotation tests were performed using a 1 L XFD laboratory flotation machine (LICHEN, Jinjiang, China), with an impeller speed of 1800 r/min and airflow of 0.15 m3/(m2 min). The collector was absent in the flotation cell because of the natural hydrophobicity of ultra-low-ash fine coal. After adding ultra-low-ash fine coal and kaolinite in the cell, the pulp was conditioned by agitating for 2 min, mixing for another 30 s after adding frother (Sec-octyl alcohol), and then the air was introduced into the cell the products were collected within different intervals. One-stage flotation tests were conducted to study the effects of feed solid concentration and frother concentration on the entrainment of kaolinite. As shown in Figure 3a, froth products (C1, C2, C3, and C4) were collected at 30, 30, 60, and 60 s after conditioning in one-stage flotation. Two-stage flotation tests were performed by feeding twice, as shown in Figure 3b. After floating for 60 s, feed 2 was added into the tailing pulp. Finally, six products (C1, C2, C3, C4, C5, and W) were obtained in the two-stage flotation. Water was added as necessary to maintain a constant pulp level in flotation, and the added amounts in each flotation stage were recorded. The amount of water in the products can be known by the difference between their wet weight and dry weight. The weights of coal and kaolinite in products can be calculated by their quality ratio of ultra-low-ash fine coal and kaolinite in the products based on the ash contents of the products, ultra-low-ash fine coal, and kaolinite.
Entrainment is strongly dependent on water, which is considered the carrier that transfers mineral particles to the concentrate during the flotation process. Furthermore, a linear relationship is commonly observed between the entrainment of fine particles (particularly those smaller than 50 μm) and water recovery [33]. Thus, the degree of entrainment of kaolinite eg and combustible recovery Ec can be calculated by Equations (1)–(4) [34,35].
eg = Rs/Rw
Rs = m(c,i)/m(f,i)
Rw = m(c,w)/m(f,w)
  • eg, the degree of entrainment of kaolinite;
  • Rs, the recovery of kaolinite in a certain period;
  • Rw, the recovery of water in a certain period;
  • m(c,i), the amount of kaolinite in concentrate in a certain period, g;
  • m(f,i), the amount of kaolinite in feed pulp in a certain period, g;
  • m(c,w), the amount of water in concentrate in a certain period, g;
  • m(f,w), the amount of water in feed pulp in a certain period, g.
Ec = γc × (100 − Ac)/(100 − Af) × 100
  • Ec, the combustible recovery of concentrate, %;
  • γc, the yield of concentrate, %;
  • Ac, the ash content of concentrate, %;
  • Af, the ash content of feed, %.

3. Results and Discussion

3.1. Effect of Kaolinite Concentration on Entrainment

The effect of kaolinite concentration on entrainment was explored. The total solid concentration of flotation slurry was fixed at 80 g/L, and the mass fractions of pure coal and kaolinite were designed as 80 + 0, 70 + 10, 60 + 20, and 50 + 30 g, respectively. The flotation results are shown in Figure 4a,b. It can be clearly seen that the ash contents of the concentrate decreased firstly and then increased with the increase in flotation time, for the tests using a high mass ratio of kaolinite in the flotation feed (60 + 20, 50 + 30 g). For the degree of entrainment, it decreased gradually and then achieved equilibrium. These data indicate that the ash content of the concentrate has a close relationship with the ash content of the flotation feed, confirming that the ash content of the concentrate cannot easily meet the requirement if the content of high-ash slime is high in the flotation feed. In addition, it can be also found that the degree of entrainment at the early stage is much higher for the case of 60 + 20 g, compared to other cases. This might indicate that the degree of entrainment of kaolinite could be largely affected by the concentrations of floatable material and gangue mineral.
Figure 4c,d presents the flotation results achieved at different concentrations of flotation feed (40, 60, 80 g/L) under the fixed mass ratio of pure coal to kaolinite (3:1). At the higher solid concentration (60 and 80 g/L), the changes in ash content and the degree of entrainment with the increase in flotation time show a similar trend as evident in the above illustrations. However, the ash content of the concentrate achieved at 40 g/L gradually increased, and the degree of entrainment slowly increased until achieving equilibrium. Therefore, at the initial stage (0–30 s), the degree of entrainment is mainly affected by the solid concentration, and the degree of entrainment was kept as a constant at the late stage (120–180 s). But the ash content of the concentrate became higher at the late stage because of a low solid concentration. Therefore, it would benefit the flotation performances if the low solid concentration was maintained at the initial stage of the flotation process and if a slightly high solid concentration could be maintained at the late stage.

3.2. Effect of Frother Concentration on Entrainment

The frother concentration has significant effects on the entrainment of kaolinite because frother is helpful in enhancing the stability of bubbles in flotation and further influences the stability of the froth and the water volume entering into froth products [36]. The changes in flotation indexes with time under different frother concentrations are shown in Figure 5 (feed concentration: 80 g/L). It can be seen that a lower degree of entrainment and ash content of the concentrate can be achieved at the frother concentration of 40 mg/L, showing values of 0.48 and 5.62%, respectively, at the early stage (0–30 s). However, in the middle stage of flotation (30–120 s), the frother concentration of 80 mg/L contributes to a lower degree of entrainment and ash content of the concentrate. In the late stage of flotation (120–180 s), the degree of entrainment became the lowest at 160 g/L. But the ash content of the concentrate was still higher than other cases, reaching 8.23%. In addition, it can also be found that a high frother concentration can help in the obtainment of a high combustible recovery within 180 s [37].
A low frother concentration is helpful in reducing the degree of entrainment and ash content of the concentrate in the early stage of flotation. However, it can also lead to a high degree of entrainment and low combustible recovery in the middle and late stage. The increase in the frother concentration could contribute to the high degree of entrainment in the early stage and low degree of entrainment in the late stage. Therefore, keeping a low concentration of frother in the early stage and a high concentration in the late stage might be more suitable for obtaining a concentrate with low ash content.

3.3. Effects of Aeration Rate and Impeller Speed on the Flotation Performance

Figure 6 presents changes in the ash content of the concentrate and the combustible recovery under different aeration rates and impeller speeds (a). When the weights of pure coal and kaolinite are 60 and 20 g/L in the flotation slurry, the aeration rate and impeller speed have less effects on the flotation performance. This might indicate that these parameters, including aeration rate and impeller speed, cannot improve the froth stability. However, the flotation solid concentration (b) shows a significant effect on flotation performance [38].

3.4. Two-Stage Flotation Process

In Figure 6, it can been seen that these parameters, including aeration rate and impeller speed, have less effects on the flotation entrainment. However, the solid concentration and frother concentration have significant effects on the degree of entrainment during the flotation process. For the conventional one-stage flotation process, the degree of entrainment is higher in the early stage. The ash content of the concentrate becomes higher due to the lower solid concentration and frother concentration in the late stage of flotation. Even though the low flotation solid concentration can reduce the ash content of the concentrate, the flotation capacity becomes extremely low.
In order to solve the problem mentioned in the above analysis, a two-stage flotation process was proposed with an initial feed concentration of 40 g/L and frother dosage of 40 mg/L and a second feed concentration of 40 g/L and frother concentration of 40 mg/L (Figure 3b). The comparison of the one-stage flotation process (feed concentration: 80 g/L; frother dosage: 80 mg/L) and two-stage flotation process are shown in Figure 7. In the early stage (0–30 s), the degree of entrainment was 0.33 which was much lower than that of the one-stage flotation process (0.69). Accordingly, the solid (ultra-low-ash fine coal)–liquid ratio (0.20) was much higher than that of one-stage flotation (0.14). The ash content of the concentrate obtained from the early and late stages in the two-stage flotation process decreased significantly compared to that from the one-stage flotation process. This indicates that the two-stage flotation process can effectively reduce the degrees of mechanical entrainment in the early stage and the entrainment caused by water transportation in the later stage. Therefore, the total ash content of the concentrate decreased from 5.16% (one-stage flotation test) to 4.09% (two-stage flotation process). In addition, the combustible recovery achieved from the two-stage flotation process (91.28%) was slightly lower than that from the one-stage process (93.02%). This might be because the flotation time for the feed added in the second stage became shorter. The combustible recovery can still increase if the total flotation time increases. The two-stage flotation can be a promising process to reduce the entrainments caused by both mechanical action and water transportation for the coal flotation.

4. Conclusions

The factors influencing the degree of entrainment in coal flotation have been explored in the current study. The results indicated that the ash contents of the concentrate were much higher in the early or late stages (flotation time: 0–30 s or 120–180 s) than at the middle stage (flotation time: 30–120). This is mainly because the slow drainage of kaolinite leads to serious mechanical entrainment in the early stage of flotation and the low solid (ultra-low-ash fine coal)–liquid ratio results in the entrainment caused by water transportation in the late stage. The solid concentration and frother concentrations have been confirmed to be the main factors controlling the characteristics of the flotation process. A proper solid concentration is essential to decrease the entrainment of kaolinite. As for the frother, a low concentration can reduce the degree of entrainment in the early stage. However, a high frother concentration contributes to the decrease in the entrainment degree in the late stage of flotation. Thus, based on the above analysis, a novel two-stage flotation process was proposed for coal flotation. The ash content of the concentrate (4.09%) achieved from the two-stage flotation process was much lower than that (5.16%) obtained from the one-stage flotation process, and the combustible recovery (91.28%) obtained by the two-stage flotation process was almost maintained at the value (93.02%) achieved by the one-stage flotation process. Although the two-stage flotation process is promising, more studies should be conducted to optimize the parameters including the solid concentration, frother concentration, etc.

Author Contributions

Conceptualization, H.Z. and Q.T.; methodology, Q.T.; software, H.W.; validation, Q.T.; formal analysis, Q.T.; investigation, C.L., W.G. and Z.L.; resources, C.L.; data curation, Y.Z.; writing—original draft preparation, C.L.; writing—review and editing, C.L.; supervision, C.L.; project administration, C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Authors Chengyong Liu, Wenzhe Gu and Zhicheng Liu were employed by the company China National Coal Group Corp. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Minerals 15 00147 g001
Figure 1. X-ray diffraction pattern of kaolinite used in the present study.
Figure 1. X-ray diffraction pattern of kaolinite used in the present study.
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Figure 2. (a) The relationship between the cumulative yield and ash content for pure coal flotation; (b) particle size distributions of pure coal and kaolinite.
Figure 2. (a) The relationship between the cumulative yield and ash content for pure coal flotation; (b) particle size distributions of pure coal and kaolinite.
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Figure 3. (a) One-stage flotation rate test flowchart and (b) two-stage flotation rate test flowchart.
Figure 3. (a) One-stage flotation rate test flowchart and (b) two-stage flotation rate test flowchart.
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Figure 4. The changes in ash content and degree of entrainment with the increase in flotation time under different conditions: (a,b) different kaolinite concentrations at a fixed solid concentration of 80 g/L; (c,d) different solid concentrations at a fixed mass ratio of pure coal to kaolinite. The legend in the figure represents the weights of pure coal and kaolinite.
Figure 4. The changes in ash content and degree of entrainment with the increase in flotation time under different conditions: (a,b) different kaolinite concentrations at a fixed solid concentration of 80 g/L; (c,d) different solid concentrations at a fixed mass ratio of pure coal to kaolinite. The legend in the figure represents the weights of pure coal and kaolinite.
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Figure 5. The changes in (a) ash content of the concentrate and (b) the degree of entrainment under different frother dosages.
Figure 5. The changes in (a) ash content of the concentrate and (b) the degree of entrainment under different frother dosages.
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Figure 6. The relationship between the combustible recovery and ash content of the concentrate under different conditions (a,b).
Figure 6. The relationship between the combustible recovery and ash content of the concentrate under different conditions (a,b).
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Figure 7. Comparison of one-stage flotation and two-stage flotation: (a) ash content of concentrate, (b) degree of entrainment, (c) solid (ultra-low-ash fine coal)–liquid ratio, and (d) combustible matter recovery.
Figure 7. Comparison of one-stage flotation and two-stage flotation: (a) ash content of concentrate, (b) degree of entrainment, (c) solid (ultra-low-ash fine coal)–liquid ratio, and (d) combustible matter recovery.
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MDPI and ACS Style

Liu, C.; Gu, W.; Zhang, H.; Tian, Q.; Wang, H.; Zhou, Y.; Liu, Z. The Adjustment for Entrainment Behavior of Kaolinite in Coal Flotation. Minerals 2025, 15, 147. https://doi.org/10.3390/min15020147

AMA Style

Liu C, Gu W, Zhang H, Tian Q, Wang H, Zhou Y, Liu Z. The Adjustment for Entrainment Behavior of Kaolinite in Coal Flotation. Minerals. 2025; 15(2):147. https://doi.org/10.3390/min15020147

Chicago/Turabian Style

Liu, Chengyong, Wenzhe Gu, Haijun Zhang, Quanzhi Tian, Hainan Wang, Yuejin Zhou, and Zhicheng Liu. 2025. "The Adjustment for Entrainment Behavior of Kaolinite in Coal Flotation" Minerals 15, no. 2: 147. https://doi.org/10.3390/min15020147

APA Style

Liu, C., Gu, W., Zhang, H., Tian, Q., Wang, H., Zhou, Y., & Liu, Z. (2025). The Adjustment for Entrainment Behavior of Kaolinite in Coal Flotation. Minerals, 15(2), 147. https://doi.org/10.3390/min15020147

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