Enhancing Flotation Performance of Low-Rank Coal Using Environment-Friendly Vegetable Oil

Round 1

Reviewer 1 Report

Review of the article minerals-2388183: Enhancing flotation performance of low-rank coal using environment-friendly vegetable oil 

The manuscript under review seems to be well-written and the described results could be useful for future studies related to the flotation of oxidized coal. However, the authors must be considered some comments before publishing their results. 

-          The Introduction must be rewritten. In my opinion, is not suitable to purify coal by the flotation technique. In such a case, it would be better to float impurities. In addition, is essential to mention in this section that most of the investigation previously published has been directed to treat the presence of oxidized coal as an impurity in the processing of several ores. The presence of oxidized coal, even at very low concentrations, causes high consumption of flotation reagents. It would be worth considering some recent development works in this regard.

- In the same sense that the previous comment, it must be described in detail the effect of the use of diesel as a collector reagent on an oxidized coal surface, as well there is a lot of information in the literature in this regard.

- A proper characterization of the low-rank coal sample must be performed. Which is inorganic carbon content? This must be differentiated from the organic carbon contained in the sample. The use of XRD is recommended for the proper identification of the mineralogical phases present in the sample.

- How the analyses shown in Table 1 were performed? A detailed description should be provided.

- Which was the criteria to classify the 1030# oil as environmentally friendly? What is its origin? An explanation must be provided in this sense.

- Which was the particle size used in the flotation experiments?

- It should be described in more detail how the conditioning of the samples with the different compounds was carried out before measuring the contact angle. This considering that diesel and surely also 1030# oil are insoluble in water. Describe the objective to use boric acid in the preparation of coal samples for contact angle measurements.

- Provide more details of the microelectromechanical balance system (JK99M2). This reviewer searched for information about it without success. At least provide a schematic of the arrangement used for the determination of the particle-bubble interaction.

-    Lines 200-202, are necessary to describe in more detail which are the polar components present in 1530# oil. How is the interaction of these with the hydrophilic sites of the oxidized carbon surface? In this sense, it would be desirable to know the chemical structures of the different compounds that compose the 1530# oil (table 2) for a better explanation of the obtained results.

- Very important, the presence of other impurities in the flotation process, mainly carbonates, is not considered. How affects the presence of carbonates in the flotation process? (in case of existing). To give more validity to the flotation results it is necessary to specifically incorporate the recovery of organic carbon in the concentrate. This is an important fact that must be considered since the main objective of the work is to purify coal.

- The results of the FT-IR analyses do not provide sufficient evidence that diesel or oil-1030# were adsorbed on the oxidized coal surface. Probably related to the use of the potassium bromide pellet technique, which is not the most suitable for a superficial study of coal. This is related to the fact that the coal sample is ground and diluted with KBr, where a new area is formed in the grinding and compression process, therefore, the weak signals from the collectors on the coal surface are overshadowed by the signals that come from of the organic groups present in the low-rank-coal sample. For this reason, no changes are observed in the FT-IR spectra before and after the treatments with the different collectors shown in Figure 4. In this sense, the use of FT-IR in ATR mode is recommended, which is more suitable for surface analysis.

- Lines 243-248. It is not clear how was reached the conclusion that through the XPS results shown in Figure 5, the treatment with 1030# provides more hydrophobicity to the low-rank coal sample. The difference in the oxygen content between the treatments can be related to the change in the atomic proportion of carbon on the surface, where it increased after the treatments with the collectors rich in carbon, therefore the oxygen proportion should decrease since it remains constant during the adsorption process. In this sense, what happens with the oxygen contained in the compounds mentioned in Table 2? Where does the nitrogen reported in Table 4 come from?

Author Response

Dear editor and reviewers:

Thank you very much for your considerations on our manuscript. We would like to re-submit the revised manuscript entitled “Enhancing flotation performance of low-rank coal using environment-friendly vegetable oil” to Minerals.

The manuscript has been carefully rechecked in accordance with the reviewers’ suggestions. The responses to their comments have been prepared and attached herewith. Your criticisms were highly appreciated. Following is our response to the reviewers’ comments. We hope that the revised manuscript is now suitable for publication on the journal.

I look forward to your reply.

Sincerely,

Yaowen Xing

Chinese National Engineering Research Center of Coal Preparation and Purification, Xuzhou 221116, Jiangsu, China

E-mail: [email protected]

Respond to Reviewer #1:

Reviewer #1: Review of the article minerals-2388183: Enhancing flotation performance of low-rank coal using environment-friendly vegetable oil. The manuscript under review seems to be well-written and the described results could be useful for future studies related to the flotation of oxidized coal. However, the authors must be considered some comments before publishing their results.

1. The Introduction must be rewritten. In my opinion, is not suitable to purify coal by the flotation technique. In such a case, it would be better to float impurities. In addition, is essential to mention in this section that most of the investigation previously published has been directed to treat the presence of oxidized coal as an impurity in the processing of several ores. The presence of oxidized coal, even at very low concentrations, causes high consumption of flotation reagents. It would be worth considering some recent development works in this regard.

Answer: Thanks for your helpful comments on our paper.

According to reviewer’s comment, we explained the reason why reverse flotation is not adopted for low rank coal flotation, also explained the difference between low rank coal and oxidized coal, and add some new literature on low rank coal flotation in the introduction.

(1) The froth flotation technique, which is based on the differences in surface hydrophobicity between organic matter and mineral matter, is an effective method for such low-rank coal cleaning and upgrading. The target mineral particles can be attached onto the bubble surface and separated with rising bubbles under the premise that the hydrophobicity of the floating particles is higher than that of the sinking particles [1, 2]. In low-rank coal flotation, the impurities are the quartz and a minor proportion of beryl and polylithionite, alongside other silica-rich minerals. If we folate the impurities form low-rank coal, the flotation is considered to be reverse flotation. During the process of reverse flotation, gangue minerals are scrapped out as froth product while coal particles stay in the pulp. Compared with gangue minerals, the coal particles have better hydrophobicity and more likely to adhere to bubbles. Therefore, large amount of depressant should be added in the reverse flotation process to inhibit the floating upward of coal particles. The reverse flotation of low-rank coal has high flotation reagent consumption and poor selectivity. Ding et al. [1] investigated the reverse flotation of subbituminous coal and found that the consumption of reagent was large. Zhang et al. [2] studied the effects of NaCl and grinding fineness on reverse flotation of lignite, and the results showed that the collector consumption was high and the selectivity was poor in the reverse flotation. Therefore, the direct flotation is widely used for upgrading of low-rank coal [5-8].

[1] Xia, Y.; Zhang, R.; Cao, Y.; Xing, Y.; Gui, X. Role of molecular simulation in understanding the mechanism of low-rank coal flotation: A review. Fuel. 2020, 262, 116535

[2] Verrelli, D.; Koh, P.; Nguyen, A. Particle–bubble interaction and attachment in flotation. Chem Eng Sci. 2011, 66(23), 5910-5921.

[3] Ding, K.; Laskowski, J. Coal reverse flotation. Part II: Cleaning of a subbituminous coal. Miner. Eng. 2006, 19(1), 79-86.

[4] Zhang, H.; Liu, J.; Cao, Y.; Wang, Y. Effects of particle size on lignite reverse flotation kinetics in the presence of sodium chloride. Powder Technol. 2013. 246, 658-663.

[5] Niu, C.; Xia, W.; Li, Y.; Bu, X.; Wang, Y.; Xie, G. Insight into the low-rank coal flotation using amino acid surfactant as a promoter. Fuel. 2022, 307, 121810.

[6] Liu, J.; Zhang, R.; Bao, X.; Hao, Y.; Gui, X. Xing, Y. New insight into the role of the emulsified diesel droplet size in low rank coal flotation. Fuel. 2023, 127388.

[7] Liu, J.; Bao, X.; Hao, Y.; Liu, J.; Cheng, Y.; Zhang, R.; Xing, Y.; Gui, X.; Li, J.; Avid, B. Research on mechanisms of improving flotation selectivity of coal slime by adding sodium polyphosphate. Minerals. 2022, 12, 1392.

[8] Huang, G.; Xu, J.; Geng, P.; Li, J. Carrier flotation of low-rank coal with polystyrene[J]. Minerals. 2020, 10(5), 452.

(2) Low rank coal and oxidized coal are two different types of coal. The low-rank coal refers to coal with relatively low coalification degree, which generally has high moisture content and volatile content, and contains a large number of oxygen-containing functional groups on the surface. In contrast, the oxidized coal refers to coal subjected to air oxidation during coal storage or processing, which contains a large number of oxidation products and a low calorific value. Despite their differences, low-rank coal and oxidized coal have certain commonalities, such as more oxygen-containing functional groups and more developed pores on the surface of coal. In order to avoid ambiguity, the research on oxidized coal is deleted in introduction, and the latest research on low rank coal flotation was added. The introduction is modified as follows:

Introduction:

Coal is the predominant fossil fuel in China, and it is expected to maintain its dominant position in the country’s energy structure for the foreseeable future [1-4]. In recent years, rapid industrial development has led to the depletion of high-quality coal resources. On the contrary, although the reserves of low-rank coal are abundant, its effective utilization has not been realized to a great extent [2, 5]. In economically underdeveloped regions, the direct combustion of low-rank coal gives rise to the release of various pollutants, including sulfur compounds, nitrogen oxides, and heavy metals, thereby contributing to environmental pollution. Additionally, this combustion process leads to the generation of a significant volume of ash solid waste [6-9]. Therefore, the rational development and clean utilization of this resource a critical to country’s economic development and environmental protection. Several separation methods, such as magnetic aspiration, gravity separation, bio-beneficiation and froth flotation, have been developed to facilitate clean and efficient utilization of low-rank coal. Compared with other separation methods, froth flotation is an effective separation method for fine coal cleaning, which depends upon the differences in the surface properties of coal and gangue minerals [10-13].

In the process of fine coal flotation, non-polar hydrocarbon oil is commonly utilized as the collector to enhance the surface hydrophobicity of particles, thereby in-creasing the probability of bubble-particle adhesion. Low-rank coal, characterized by well-developed pores and an abundance of oxygen-containing functional groups, including hydroxyl, carbonyl and carboxyl moieties [2, 10, 14], could form stable hydration films on its surface by combining with polar water molecules [15]. This hydration film impedes the spread of the collector on the particle surface, thereby leading to an increased amount of collector dosage and significant rise in economic cost. The low efficiency of flotation, large amount of collector and high cost are the main problems restricting the efficient utilization of fine low-rank coal.

The high cost of low-rank coal flotation using diesel of kerosene as the collector significantly restricts the improvement in the economic efficiency of the coal plant [16, 17]. In recent years, the research on strengthening the flotation process of low-rank coal has attracted extensive attention, especially the development of polar collectors containing oxygen such as alcohols, aldehydes, acids and esters has aroused great interest of researchers [18]. The addition of polar collector can modify the surface of low-rank coal, promote the interaction between particles and bubbles and improve the flotation recovery. Jia and Gui et al [19, 20]. discovered that the polar groups within the collector could interact with the polar hydrophilic sites present on coal surface through the hydrogen bonding, which can enhance the surface hydrophobicity of coal sample, consequently leading to an increase in the flotation recovery. Xia et al. [21] founded that the lubricating oil, including oxygen-containing functional groups, aromatics and long-chain hydrocarbons, can strongly interact with low-rank coal to improve its surface hydrophobicity. Zhu et al. [22] founded that a mixture of fossil oil and oxygen-containing compound (FO) could enhance the low-rank coal flotation com-pared with traditional diesel oil. The FO was more likely adsorbed on coal surface, and improved the hydrophobicity of coal surface, thus promoting the adhesion between bubble and particle. According to previous research, it has been demonstrated that the polar compound collector, a blend of hydrocarbon oil and polar reagent, represents the optimal choice in practical terms. However, it is widely recognized that the diesel, kerosene and other fossil oil, which are commonly used as collectors, are nonrenewable resources that are expected to become depleted in the near future. Furthermore, these petroleum derived compounds are not environment-friendly and may entail numerous environmental hazards [23]. In light of these issues, there is a pressing need to explore alternative, renewable, and environment-friendly collectors. Vegetable oils, such as colza oil, sunflower oil, soybean oil, and olive oil, have been reported as effective collectors for fine coal flotation, owing to their abundant long-chain fatty acids [24-28]. The use of vegetable oils as collectors for low-rank coal flotation presents a significant opportunity for large-scale upgrading of low-rank coal flotation.

In this study, a homemade vegetable oil (1030#) and traditional hydrocarbon oil (diesel) were used as the collectors, and low-rank coal from the coal preparation plant in Zhuanlong Bay was selected as the sample material. The study aims to systematically investigate the underlying mechanisms behind the enhanced flotation of low-rank coal induced by vegetable oil collectors, thereby providing a fundamental basis for the development of new and environment-friendly collectors for low-rank coal flotation.

2. In the same sense that the previous comment, it must be described in detail the effect of the use of diesel as a collector reagent on an oxidized coal surface, as well there is a lot of information in the literature in this regard.

Answer: Thanks for your helpful comments on our paper.

The topic of this paper is flotation of low-rank coal. In order to avoid ambiguity, references about oxidized coal are deleted and the latest references about low-rank coal flotation are added in introduction. Once again, thank you very much for your contributions to our paper.

3. A proper characterization of the low-rank coal sample must be performed. Which is inorganic carbon content? This must be differentiated from the organic carbon contained in the sample. The use of XRD is recommended for the proper identification of the mineralogical phases present in the sample.

Answer: Thanks for your helpful comments on our paper.

According to previous literatures, it has been established that the carbon present in coal primarily comprises organic carbon, which is composed of carbonaceous organic materials. Organic carbon represents a predominant constituent of coal, typically accounting for between 50% and 95% of coal's mass. Its high calorific value and chemical reactivity make it a fundamental basis for coal's role as an energy source and chemical feedstock. While coal may also contain trace amounts of inorganic carbon, such as that found in coal ash, these do not constitute its primary component [9, 10]. According to reviewer’s comment, we also added the XRD characterization of the low-rank coal, and identified the minerals composition in the low-rank coal. The X-ray diffraction spectrogram of low-rank coal was showed in Figure 1. The results revealed the quartz constitutes the major mineral component in the low-rank coal sample, followed by a minor proportion of beryl and polylithionite, alongside other silica-rich minerals. This result also indicated that the inorganic carbon content in low-rank coal is small, which is consistent with previous studies.

Figure 1 The X-ray diffraction spectrogram of low-rank coal

[9] Warwick, P. Coal systems analysis: A new approach to the understanding of coal formation, coal quality and environmental considerations, and coal as a source rock for hydrocarbons[M]. New York: Geological Society of America, 2005.

[10] Ward, C. Coal geology and coal technology[M]. London：Black Well Scientific Publications，1984.

4. How the analyses shown in Table 1 were performed? A detailed description should be provided.

Answer: Thanks for your helpful comments on our paper.

According to reviewer’s comment, we added a detailed description about the Table 1. According to the ISO348: 1981(E), the "Methods for Proximate Analysis of Coal," proximate analysis of coal primarily encompasses the assessment of moisture, ash, volatile matter, and fixed carbon content. Through the evaluation of these parameters, preliminary conclusions can be drawn regarding the classification, treatment methods, utilization potential, and industrial applications of coal. The present study employed the 5E-MAG6700 automatic proximate analyzer to perform the analysis. The experimental procedure involved conducting three parallel sample experiments, and the final outcomes were obtained by calculating the average values, as presented in Table 1.

M_ad and A_ad represented the moisture and ash content based on air drying. V_daf and F_daf are the contents of volatile matter and the fixed carbon on the dry ash-free basis. The Mad was 11.47%, which indicated the low-rank coal has high water content. The ash content was 17.68%, which was relatively high and required a de-ashing treatment before combustion. The dry ash-free volatile matter content is 35.02%, which indicated that the coal sample belongs to high-volatile coal with a relatively low degree of metamorphism. By referring to the standard of coal classification, the coal sample used in this study should belong to bituminous or sub-bituminous coal as its dry ash-free volatile matter content from 10% to 37% [11].

Table 1. Proximate analysis of the coal sample.

[11] Chen, P. Coal classification in China: a complete system (part 1). China Coal. 2000, 26 (9), 5–8.

5. Which was the criteria to classify the 1030# oil as environmentally friendly? What is its origin? An explanation must be provided in this sense.

Answer: Thanks for your helpful comments on our paper.

In this study, the 1030# oil utilized was obtained from waste soybean and canola oil sources. The chemical constituents of the 1030# oil was analyzed through Gas Chromatography-Mass Spectrometry (GC-MS), and the results are presented in Table 2. The chemical profile of the 1030# collector demonstrated a highly diverse composition, predominantly comprising esters, with trans-methyl oleate constituting the most abundant component at a relative content of 40.22%. Conventional collectors generally contain alkanes and aromatic hydrocarbons, which can result in significant environmental pollution. In contrast, the utilization of vegetable oils as collectors presents several benefits, such as low sulfur, low nitrogen, and low metal content, renewability, and eco-friendliness. Therefore, vegetable oils offer a promising alternative to conventional hydrocarbon collectors. Vegetable oils are also being marketed as environment-friendly collectors.

Table 2. List of chemical compositions of 1030#.

6. Which was the particle size used in the flotation experiments?

Answer: Thanks for your helpful comments on our paper.

In this paper, the flotation of low-rank coal was carried out with whole particle size range. The particle size and ash content distribution of low-rank coal was shown in Figure 2. The yield of +0.50 mm fraction was 1.07%, and the yield of -0.50 mm fraction was 98.03%. Froth flotation is a high-efficiency physicochemical technique, widely applicable for upgrading of coals for the fractions with sizes of smaller than 0.5 mm based on different surface properties of coal and gangue minerals. Therefore, we can use the flotation for the upgrading of low-rank coal.

Figure 2. Particle size and ash content distribution of low-rank coal.

7. It should be described in more detail how the conditioning of the samples with the different compounds was carried out before measuring the contact angle. This considering that diesel and surely also 1030# oil are insoluble in water. Describe the objective to use boric acid in the preparation of coal samples for contact angle measurements.

Answer: Thanks for your helpful comments on our paper.

Firstly, a mixture of 4 g of low-rank coal and 50 ml of deionized water was blended and pre-wetted for 5 minutes to prepare the pulp with a concentration of 80 g/L, which corresponds to the concentration used in the flotation kinetic experiment. Secondly, the pulp should be combined with diesel oil and 1030# oil at concentrations of 500, 1000, 2000, and 4000 g/t. The mixture should be vigorously shaken for 10 minutes to ensure thorough mixing of the coal particles and reagents. Subsequently, the mixture is filtered and dried. Afterwards, 1-2 g of air-dried coal sample is taken and positioned at the center of a tablet press. Approximately 5-6g boric acid and place it evenly under and around the coal sample to base and edge the coal sample, as shown in Figure 3. The samples obtained by boric acid substrate edging method are smooth without cracks and can be preserved for a certain time. In this process, boric acid is used for substrate and edging and is not mixed with coal sample, so it has no effect on measurement of the contact angle of coal sample. According to reviewer’s comment, we added a detailed description about the process of contact angle measurements in manuscript.

Figure 3. Boric acid was used to base and edge the coal sample

8. Provide more details of the microelectromechanical balance system (JK99M2). This reviewer searched for information about it without success. At least provide a schematic of the arrangement used for the determination of the particle-bubble interaction.

Answer: Thanks for your helpful comments on our paper.

According to reviewer’s comment, we added the schematic diagram of adhesion force measurement system as shown in Figure 4. In many references, the high-sensitivity microelectromechanical balance system (JK99M2) was used to measure the interaction force between bubbles and coal particles [12, 13]. It was also revised in the manuscript, as shown below:

The high-sensitivity microelectromechanical balance system (JK99M2) in con-junction with a digital camera was utilized to investigate the interaction force between bubble and particle under different types and concentrations, the schematic diagram of adhesion force measurement system was shown in Figure 4. Specifically, low-rank coal flake samples treated with various agents were placed in a transparent tank, and a specific quantity of water was added. The samples were than fixed to the displace-ment platform situated below the microbalance. A 2 ml bubble was generated ate the capillary port of the microbalance, and the force was initialized to 0 at the onset of interaction between bubble and particle. The displacement platform was then gradually moved upwards at a rate of 0.01 mm/s. Upon contact between the coal slice fixed on the displacement platform and bubbles, an instantaneous adhesive force was generated. After the coal samples had moved upwards by 0.3 mm, the sample stage was adjusted to return to its original position at the same speed. To ensure the precision and accuracy of the experimental outcomes, the interaction force measurement was re-peated five times to minimize the experimental error.

Figure 4. Schematic diagram of adhesion force measurement system^]

[12] Zhu, C.; Li, G.; Xing, Y.; Gui, X. Adhesion forces for water/oil droplet and bubble on coking coal surfaces with different roughness. Int J Mining Sci Technol. 2021, 31(4), 681-687.

[13] Li, M.; Xing, Y.; Zhu, C.; Liu, Q.; Yang, Z.; Zhang, R.; Zhang, Y.; Xia, Y.; Gui, X. Effect of roughness on wettability and floatability: Based on wetting film drainage between bubbles and solid surfaces. Int J Mining Sci Technol. 2022, 32(6), 1389-1396.

9. Lines 200-202, are necessary to describe in more detail which are the polar components present in 1530# oil. How is the interaction of these with the hydrophilic sites of the oxidized carbon surface? In this sense, it would be desirable to know the chemical structures of the different compounds that compose the 1530# oil (table 2) for a better explanation of the obtained results.

Answer: Thanks for your helpful comments on our paper.

According to the chemical compositions of 1030#, consisting predominantly of fatty acids and acids, with trans-methyl oleate as the most abundant constituent at 40.22% relative content. Fatty acids contain a lot of oxygen-containing groups, and the oxygen-containing groups can form hydrogen bond with the hydrophilic sites on surface of low rank coal. Effectively promote the dispersion of the reagent and the interaction with the coal surface, so as to improve the hydrophobicity of coal particles and increase the flotation recovery. This is consistent with previous researches [14, 15]. Thanks again for the reviewer's comments, and we will be more rigorous in the future.

[14] Wen, B.; Xia, W.; Sokolovic, J. Recent advances in effective collectors for enhancing the flotation of low rank/oxidized coals. Powder Technol. 2017, 319, 1-11.

[15] Xia, Y.; Rong, G.; Xing, Y.; Gui, X. Synergistic adsorption of polar and nonpolar reagents on oxygen-containing graphite surfaces: Implications for low-rank coal flotation. J. Colloid Interf. Sci. 2019, 557, 276-281.

10. Very important, the presence of other impurities in the flotation process, mainly carbonates, is not considered. How affects the presence of carbonates in the flotation process? (in case of existing). To give more validity to the flotation results it is necessary to specifically incorporate the recovery of organic carbon in the concentrate. This is an important fact that must be considered since the main objective of the work is to purify coal.

Answer: Thanks for your helpful comments on our paper.

According to reviewer’s comment, we added the XRD characterization of the low-rank coal, and identified the minerals composition in the low-rank coal. The findings indicate that quartz constitutes the predominant mineral component in the low-rank coal sample, with a minor proportion of beryl and polylithionite, as well as other silica-rich minerals. Notably, no carbonates were detected in this particular low-rank coal sample.

The carbon present in coal primarily comprises organic carbon, and it typically accounting for between 50% and 95% of coal's mass. Therefore, in this paper, we mainly focus on the recovery of organic carbon in low rank coal. The organic carbon in the low-rank coal has good hydrophobicity, while the impurities such as quartz have hydrophilicity. As a result, flotation can be employed for their separation. Froth flotation is predicated on the contrasting surface hydrophobicity between organic matter and mineral matter. Hydrophilic coal particles adhere to the air bubbles and rise to the froth as clean coal products, whereas hydrophilic impurity mineral particles remain in the tailings, enabling the effective recovery of organic carbon.

Figure 1 The X-ray diffraction spectrogram of low-rank coal

11. The results of the FT-IR analyses do not provide sufficient evidence that diesel or oil-1030# were adsorbed on the oxidized coal surface. Probably related to the use of the potassium bromide pellet technique, which is not the most suitable for a superficial study of coal. This is related to the fact that the coal sample is ground and diluted with KBr, where a new area is formed in the grinding and compression process, therefore, the weak signals from the collectors on the coal surface are overshadowed by the signals that come from of the organic groups present in the low-rank-coal sample. For this reason, no changes are observed in the FT-IR spectra before and after the treatments with the different collectors shown in Figure 4. In this sense, the use of FT-IR in ATR mode is recommended, which is more suitable for surface analysis.

Answer: Thanks for your helpful comments on our paper.

The Fourier Transform Infrared Spectroscopy (FTIR) utilized in analytical chemistry focuses on the mid-infrared region, specifically within the wave number range of 4000~400 cm^-1. When employed for tablet determination, KBr, a commonly used material, exhibits no absorption within the mid-infrared region, thereby avoiding interference with the sample signal. Chen and Xia et al [16, 17]. also employed this methodology to investigate the surface functional groups on coal treated with reagents, resulting in favorable outcomes that demonstrated the method's feasibility. Despite the exposure of the fresh surface during the sample grinding test, the interference caused by grinding can be eliminated by comparing the FTIR results of raw coal. Figure 3 illustrates that after the 1030# treatment, the alkyl group content increased while the oxygen-containing group content decreased, indicating that the reagents were adsorbed onto the coal surface, thereby enhancing the hydrophobicity of coal particles. The reviewer also noted that "no changes are observed in the FTIR spectra before and after the treatments with the different collectors shown in Figure 4." It is important to note that the vertical coordinates of the FTIR spectra represent absorbance, not the content of functional groups. Furthermore, the lack of changes in the FTIR spectra before and after treatments with different collectors suggests the absence of new substance formation and implies that the adsorption of the reagent is of a physical nature. The changes in the content of functional groups are reflected in the percentage changes observed. In addition, the FTIR in ATR mode may be better, but existing experiments are sufficient to show the results of reagent adsorption. Thanks again for the reviewer's comments, and we will be more rigorous in the future.

[16] Chen, S.; Wang, S.; Li, L.; Qu, J.; Tao, X.; He, H. Exploration on the mechanism of enhancing low-rank coal flotation with cationic surfactant in the presence of oily collector. Fuel. 2018, 227, 190-198.

[17] Xia, Y.; Yang, Z.; Zhang, R.; Xing, Y.; Gui, X. Performance of used lubricating oil as flotation collector for the recovery of clean low-rank coal. Fuel. 2018, 239, 717-725.

12. Lines 243-248. It is not clear how was reached the conclusion that through the XPS results shown in Figure 5, the treatment with 1030# provides more hydrophobicity to the low-rank coal sample. The difference in the oxygen content between the treatments can be related to the change in the atomic proportion of carbon on the surface, where it increased after the treatments with the collectors rich in carbon, therefore the oxygen proportion should decrease since it remains constant during the adsorption process. In this sense, what happens with the oxygen contained in the compounds mentioned in Table 2? Where does the nitrogen reported in Table 4 come from?

Answer: Thanks for your helpful comments on our paper.

The assessment of surface hydrophobicity cannot be solely deduced from Table 5. To comprehensively analyze the enhancement of coal surface hydrophobicity, it is imperative to examine the reduction in the proportion of oxygen elements and oxygen-containing groups, as well as the augmentation in the proportion of carbon elements and alkyl groups, by integrating the data presented in both Table 4 and Table 5. This combined analysis allows for a more comprehensive understanding of the improvement in coal surface hydrophobicity.

Table 2 presented a comprehensive characterization of the component content in 1030#. It is important to note that the adsorption of chemical reagents onto the coal surface occurs through physical adsorption. Consequently, the chemical components of both the chemical reagents and coal remain unaltered, thereby ensuring that the proportion of oxygen within the 1030# components remain constant during the adsorption process. The 1030# oil, which contains a higher concentration of hydrophobic groups (specifically, hydrocarbon groups), undergoes adsorption onto the coal surface by interacting with the oxygen-containing groups present on the surface. This interaction leads to an augmentation in the proportion of hydrophobic groups on the coal surface subsequent to adsorption, accompanied by a concurrent reduction in the proportion of oxygen-containing groups. The presence of the N element in Table 4 is likely attributed to experimental error. As a result, the table has been revised accordingly. I appreciate your valuable input and advice, and I will strive for increased rigor in my future academic writing.

Table 4. Surface element composition and relative contents of the coal samples with and without reagents treatment.

We have corrected our manuscript in detail according to the comments. The revisions are marked in red. Once again, thanks very much for your contributions to our paper.

Author Response File: Author Response.docx

Reviewer 2 Report

The work in the manuscript is significant, supporting the conclusions. It is important to expand research on eco-friendly reagents. Flotation kinetics and contact angle measurements are notably interesting. Therefore, I recommend the manuscript for publication after minor revisions:

1.     Page 3, Line 102: What is the size range or P50 of the fine coal?

2.     Section 3.1 and 3.3: The rough surface must be noted. It is expected that a few nanometers of surface roughness can increase attachment and stabilization. However, high height of surface roughness can have the contrary effects. There can be indications of surface roughness effects if the authors provide results of natural floatability (without diesel and 1030#) in Figure 3. Moreover, the authors can conclude if surface roughness effects are negligible or not; this would be very interesting and can add more value to the manuscript. The authors must cite the following work: “Contribution of particle shape and surface roughness on the flotation behavior of low-ash coking coal “, Effects of grinding time on morphology and collectorless flotation of coal particles “, and “Particle–bubble interaction energies for particles with physical and chemical heterogeneities”.

3.     Figure 3: Error bars must be added.

4. Figure 7: Changes in contact angle with diesel concentration are little compared with 1030#. More discussion on this must be provided. The citations provided in Comment 2 can be used to this end.

Author Response

Dear editor and reviewers:

Thank you very much for your considerations on our manuscript. We would like to re-submit the revised manuscript entitled “Enhancing flotation performance of low-rank coal using environment-friendly vegetable oil” to Minerals.

The manuscript has been carefully rechecked in accordance with the reviewers’ suggestions. The responses to their comments have been prepared and attached herewith. Your criticisms were highly appreciated. Following is our response to the reviewers’ comments. We hope that the revised manuscript is now suitable for publication on the journal.

I look forward to your reply.

Sincerely,

Yaowen Xing

Chinese National Engineering Research Center of Coal Preparation and Purification, Xuzhou 221116, Jiangsu, China

E-mail: [email protected]

Respond to Reviewer #2:

The work in the manuscript is significant, supporting the conclusions. It is important to expand research on eco-friendly reagents. Flotation kinetics and contact angle measurements are notably interesting. Therefore, I recommend the manuscript for publication after minor revisions:

1. Page 3, Line 102: What is the size range or P50 of the fine coal?

Answer: Thanks for your helpful comments on our paper.

The “fine coal” mentioned in Page 3, Line 102, refers to the coal particles which size below 0.045mm. The -0.045 mm fractions have the ash content of 32.21%, which is higher than the ash content of raw coal (18.80%). It indicated that the mount of gangue minerals contained in -0.0.45 mm fractions. Gangue minerals possess a propensity to be entrained within the cleaned coal foam through water flow or adhere to the surface of coal particles. Consequently, these occurrences significantly impact the efficiency of flotation processes. To avoid ambiguity, the statement in the paper has been revised as follows: The ash content is correlated with the hydrophilicity of surface, which indicated that flotation recovery of the -0.045 mm fractions is low. The presence of -0.045 mm fractions in the flotation process may not only impede the floatability of coarse coal but also result in clean coal pollution through entrainment and other mechanisms. Thanks again for the reviewer's comments, and we will be more rigorous in the future.

2. Section 3.1 and 3.3: The rough surface must be noted. It is expected that a few nanometers of surface roughness can increase attachment and stabilization. However, high height of surface roughness can have the contrary effects. There can be indications of surface roughness effects if the authors provide results of natural floatability (without diesel and 1030#) in Figure 3. Moreover, the authors can conclude if surface roughness effects are negligible or not; this would be very interesting and can add more value to the manuscript. The authors must cite the following work: “Contribution of particle shape and surface roughness on the flotation behavior of low-ash coking coal “, Effects of grinding time on morphology and collectorless flotation of coal particles “, and “Particle–bubble interaction energies for particles with physical and chemical heterogeneities”.

Answer: Thanks for your helpful comments on our paper.

In fine coal flotation, the floatability of coal is affected by many factors, such as particle surface hydrophilicity, particle size and shape, and surface morphology, which can affect the interaction between bubbles and particles, and thus affect the flotation yield. Although many scientific studies have reported the effects of surface morphology on floatability of different minerals including magnetite, talc, chromite, alumina, quartz, glass beads, sphalerite and chalcopyrite, only a few of them have focused on the flotation of coal samples. Guven and Yin et al [1, 2]. founded that the grinding time has considerable effect on coal particle morphology and the larger the roughness of coal particles, the faster the flotation kinetics. Gomez-Flores et al [3]. investigated the effect of coal particle roughness on bubble−particle interaction energy, and founded that energy barrier between significantly decreased with the increase of surface roughness.

For our paper, on the one hand, the SEM results showed that surface of low-rank coal exhibits a rough and porous surface, owing to the presence of numerous pores and cracks, which indicated that the low-rank coal has a high surface roughness. We also did the natural floatability test of low-rank coal, the low-rank coal particles could not folate without reagent. Therefore, we thought that the surface hydrophilicity of low rank coal is the main reason affecting its flotation. On the other hand, we focused on the mechanism of different collectors on the flotation of low-rank coal. The low-rank coal samples used in the test is the primary coal slime of coal preparation plant, which has not been processed in the grinding process, so the change of roughness is considered to be within the error range. At the same time, according to the reviewer's comments, we also cited the three literatures in this paper to exclude the effect of roughness on the flotation of low-rank coal. Thanks again for the reviewer's comments, which triggered a very interesting thinking for us and made our article more rigorous.

[1] Guven, O.; Kaymakoğlu, B.; Ehsani, A.; Hassanzadeh, A.; Sivrikaya, O. Effects of grinding time on morphology and collectorless flotation of coal particles. Powder Technol. 2021, 339, 117010.

[2] Yin, W.; Zhu, Z.; Yang, B.; Fu, Y.; Yao, J. Contribution of particle shape and surface roughness on the flotation behavior of low-ash coking coal. Energy Sources Part A. 2019, 41, 636-644.

[3] Gomez-Flores, A.; Bradford, S.; Hwang, G.; Heyes, G.; Kim, H. Particle–bubble interaction energies for particles with physical and chemical heterogeneities. Miner. Eng. 2020, 155, 106472.

3. Figure 3: Error bars must be added.

Answer: Thanks for your helpful comments on our paper.

According to reviewer’s comment, we added the corresponding error bars to the curves of cumulated yield and cumulated ash content in Figure 3 to make the data more realistic, which shown as follows. Thanks again for the reviewer’s comments, we will be more rigorous in data processing in the future.

Figure 3. (a) Effect of diesel and 1030# on cumulative yield of low-rank coal flotation. (b) Effect of diesel and 1030# on cumulative ash content of clean coal for low-rank coal flotation.

4. Figure 7: Changes in contact angle with diesel concentration are little compared with 1030#. More discussion on this must be provided. The citations provided in Comment 2 can be used to this end.

Answer: Thanks for your helpful comments on our paper.

The low-rank coal has strong surface hydrophilicity, and the contact angle of raw coal samples without chemical adsorption is only 40.25°. The contact angle increased gradually after diesel was adsorbed on coal surface, which indicated that the diesel oil could improve the surface hydrophobicity of low-rank coal. However, the contact angle on coal surface shows little variation with increasing diesel concentration. This is because diesel consists mostly of non-polar components, while low-rank coal has numerous polar sites on its surface. The non-polar components in diesel cannot interact with the polar sites on the surface of low-rank coal, making it difficult to completely spread on the coal surface. As a result, the hydrophobicity of the low-rank coal surface is only slightly enhanced.

We have corrected our manuscript in detail according to the comments. The revisions are marked in red. Once again, thanks very much for your contributions to our paper.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors addressed in a suitable form all comments made.