Separation Process and Microstructure-Chemical Composition Relationship of Cenospheres from Lignite Fly Ash Produced from Coal-Fired Power Plant in Thailand

Round 1

Reviewer 1 Report

The article is proper developed, but it basically doesn't add anything new. Information and conclusions contained in the work are well known and well documented by other researchers. There are difficult to find new data. Some actions are difficult to justify e.g. long soaking of cenospheres in water (several days), the use of the ultrasonic separation method (cenospheres do not tend to agglomerate). The work also does not take into account the differentiation in particles falling down during wet separation (crushed cenospheres, small fly ash particles) and phase composition (results based on the XRF method are only calculated to the oxides from elements – what about amorphous phase?). These shortcomings significant affect the quality of manuscript.

Author Response

Dear Reviewer 1,

Authors are very much thankful to the Reviewers’ valuable comments and suggestions on our work. The manuscript has been revised according to the constructive comments and your kind advices, which are valuable for improving the quality of our manuscript. The changes are highlighted using “Track Changes” and marked in the red color in the revised manuscript. The responses to the reviewer’s comments are included as follows. We sincerely hope that this manuscript can be acceptable to be published on Applied Science.

Comments

The article is proper developed, but it basically doesn't add anything new. Information and conclusions contained in the work are well known and well documented by other researchers. There are difficult to find new data. Some actions are difficult to justify e.g. long soaking of cenospheres in water (several days), the use of the ultrasonic separation method (cenospheres do not tend to agglomerate).

Response: 

The focus of this work is on the separation process and structure-chemical composition relationship of cenospheres separated from high calcium class C fly ash produced from Mae Moh power plant in Thailand. To our knowledge, this is the first report on cenospheres separation from Mae Moh class C fly ash, with high calcium content ~24 wt.%, by adopting the traditional wet separation method (using water as the medium) to separate the lightweight cenospheres. The work was mainly devoted to investigation of process variables affecting the cenosphere yield and their characteristic properties by nature. The attention was on the comparative study on percent cenosphere yield influenced by different preparation methods prior to sedimentation and settling separation. The results have revealed that no significant difference of the percent cenospheres recovery for those adopted methods. The comparison result is obvious and clearly confirmed the limit of physical stirring-settling effect associated with the cenospheres content by nature governing the percent recovery, giving us an understanding to process optimization and development of strategies to recover cenospheres from the high calcium content (level > 20 wt.%) class C fly ash with a controllable quality. Further, the characterization part was focused on the structure-chemical composition relationship, as those characteristic properties of Mae Moh cenospheres were also brought to compare to those reported in the literature; mostly class F fly ash were used to study.

Thus, we have made a slight modification on the Abstract and Conclusions parts to make them more precisely and suit the detail of work in this manuscript.

 

The work also does not take into account the differentiation in particles falling down during wet separation (crushed cenospheres, small fly ash particles)

Response:  Thank you very much for the comment on this point. We do concern this impact parameter that the efficiency of the cenosphere separation process can be further influenced by the broken cenospheres and the small ash particles that become heavier and sink. That is; the crushed cenospheres and fine ash particles could possibly affect the cenosphere recovery yield and the impurity of cenosphere product. This concerning point has been reported in the literature that, using water, it was possible to obtain cenospheres with the fraction composed of intact (undamaged) spheres by 2/3 of volume and a sediment part by 1/3 of volume consisting of small ash particles, cracked or fissured cenosphere particles, semi-coked, coked particles, slag fragments, and magnetic microspheres.

This discussion has been added in more details in the Section 3.1. (Line 243-256, Page 7-8 of the revised manuscript).

 

and phase composition (results based on the XRF method are only calculated to the oxides from elements – what about amorphous phase?). These shortcomings significant affect the quality of manuscript.

Response:  Thank you very much for the comment on this point. About the phase composition, we also do concern in this study. We considered that this point needs to be addressed in the manuscript. We have made more description regarding to this point and added in the Section 3.4. (Line 515-526, Page 16-17 of the revised manuscript).

As the work of this manuscript was more devoted to the developing and optimizing cenospheres separation process and focused on the study of structure-chemical composition relationship, accordingly the title of the manuscript has been modified to suitably express the details of work. The modified title is “Separation Process and Structure-Chemical Composition Relationship of Cenospheres from Lignite Fly Ash Produced from Coal-Fired Power Plant in Thailand”.

Author Response File: Author Response.docx

Reviewer 2 Report

This study considers the recovery of a value-added material from fly ash, such as cenospheres, highlighting the influence of separation method parameters on the yield.   

Briefly, the authors used as raw material the fly ash collected from a station of the lignite coal-fired power plant and after their chemical and physical characterization, it was separated by applying the wet method, obtaining the lightweight cenospheres. In this respect, the method parameters were varied to asses the separation and recovery efficiency. Beside this, the study provides important information related to Al₂O₃ and SiO₂ contents in the collected cenospheres.

The submitted article is interesting, original and within the scope of the journal. In addition, the manuscript is well illustrated and presents in a scientific manner the subject, but some minor changes should be addressed:

1. A general remark is that the reference numbers must be introduced in text before punctuation, not after (please see: https://www.mdpi.com/journal/applsci/instructions).
2. Please specify the number of measurements that you mention at line 141. Did the authors make more measurements in order to check the repeatability of the method?!
3. The title of figure 1 (line 178 to line 188) is too long, it must be presented shortly and the information must be introduced in the text.
4. At table 3 title, the text from line 218 “Five measurements were done and the SD of all data is less than 0.01” can be moved to the bottom of the table as footnote.
5. Table 4 title must be shorter taking into account that the same information is found in text, between lines 253 and 261. I recommend you the table title “Properties of cenospheres with various sizes” Also, the text from table 4 title, “Density of the bulk cenospheres before sieving into these fractions is 1.03 g/cm³.” can be introduce as note or in the text.
6. Table 4 must be introduced after the comments from line 253 to line 254.
7. Figure 5 title is too long. Please describe in the text what each image represents.
8. Figure 6 must be moved after the comment from line 389 to line 391 (“…see Figure 6.”).
9. Figure 7 must be introduced before comments which are starting at line 417.

I consider that this article can be recommended for the publication in Applied Sciences after a minor revision.

Author Response

Dear Reviewer 2,

Authors are very much thankful to the Reviewers’ valuable comments and suggestions on our work. The manuscript has been revised according to the constructive comments and your kind advices, which are valuable for improving the quality of our manuscript. The changes are highlighted using “Track Changes” and marked in the red color in the revised manuscript. The responses to the reviewer’s comments are included as follows. We sincerely hope that this manuscript can be acceptable to be published on Applied Science.

Comments of Reviewer 2

This study considers the recovery of a value-added material from fly ash, such as cenospheres, highlighting the influence of separation method parameters on the yield.   

Briefly, the authors used as raw material the fly ash collected from a station of the lignite coal-fired power plant and after their chemical and physical characterization, it was separated by applying the wet method, obtaining the lightweight cenospheres. In this respect, the method parameters were varied to asses the separation and recovery efficiency. Beside this, the study provides important information related to Al₂O₃ and SiO₂ contents in the collected cenospheres.

The submitted article is interesting, original and within the scope of the journal. In addition, the manuscript is well illustrated and presents in a scientific manner the subject, but some minor changes should be addressed:

1. A general remark is that the reference numbers must be introduced in text before punctuation, not after (please see: https://www.mdpi.com/journal/applsci/instructions).

Response:  Thank you very much for this point. The reference numbers must be put before the punctuations or the full stops. The reference format has been rechecked and revised throughout the entire manuscript.

 

1. Please specify the number of measurements that you mention at line 141. Did the authors make more measurements in order to check the repeatability of the method?!

Response:  There were 3-5 measurements per one cross-sectional image to measure the shell thickness. This sentence has been revised; please see Line 162, Page 4 of the revised manuscript. Also, the mean of the measurement was determined and thus the caption of Figure 2 has been modified; the ‘average’ has been added.

 

1. The title of figure 1 (line 178 to line 188) is too long, it must be presented shortly and the information must be introduced in the text.

Response:   Thank you very much for the suggestion. The length of caption of Figure 1. has been reduced.

 

1. At table 3 title, the text from line 218 “Five measurements were done and the SD of all data is less than 0.01” can be moved to the bottom of the table as footnote.

Response:  Thank you very much for the suggestion. The above sentence has been moved as the Table 3’s footnote.

 

1. Table 4 title must be shorter taking into account that the same information is found in text, between lines 253 and 261. I recommend you the table title “Properties of cenospheres with various sizes” Also, the text from table 4 title, “Density of the bulk cenospheres before sieving into these fractions is 1.03 g/cm³.” can be introduce as note or in the text.

Response:  Thank you very much for the suggestion. The title of Table 4 has been revised as “Properties of cenospheres with various sizes” according to the reviewer’s comment. Whereas the sentence, “Density of the bulk cenospheres before sieving into these fractions is 1.03 g/cm³.”, has been deleted because the same information has been already shown in the text (please see Line 284, Page 9 of the revised version of the manuscript).

 

1. Table 4 must be introduced after the comments from line 253 to line 254.

Response:  Table 4 has been moved to locate after the text line 279-280, Page 8 of the revised version of the manuscript, which is line 253-254 of the original version.

 

1. Figure 5 title is too long. Please describe in the text what each image represents.

Response:  The caption of Figure 5 has been reduced and slightly modified. Some of the description has been moved to the context part of the revised version of the manuscript.

Line 382, Page 11: The sentence added is “This type of particle was found mixing with the collected cenosphere fraction.”

Line 392-393, Page 11: The sentence added is “Figure 5f is the SEM image of cenosphere obtained from the eight-day soaking condition.”

 

1. Figure 6 must be moved after the comment from line 389 to line 391 (“…see Figure 6.”).

Response:  Figure 6 has been moved up to locate after the text line 456, Page 14 (”…see Figure 6a an 6b”) of the revised version of the manuscript, which is previously in line 389-391 of the original version.

 

1. Figure 7 must be introduced before comments which are starting at line 417.

Response:  Figure 7 has been moved up to locate at the text line 513, Page 16 of the revised version of the manuscript, which is before the text starting at line 417 according to the reviewer’s comment.

 

I consider that this article can be recommended for the publication in Applied Sciences after a minor revision.

Response: We really appreciate your time and valuable comments of pointing out the important issues and revisions needed in the manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

 

Overall assessment

 

1. The manuscript does not present revealing information. Therefore, it was necessary to write what gap in the extensive collection of publications devoted to the cenospheres the authors intended to fill with this manuscript.
2. This manuscript is more devoted to developing an optimal method of separating cenospheres than to characterizing the cenospheres themselves. This should be expressed in the title of the article. The chemical composition of the cenospheres is limited only to determining the content of the oxides of the main elements. The content of trace and radioactive elements was not analyzed. This should also be reflected in the title of Chapters 3.4.
3. If point analyzes of the chemical composition of the cenospheres were performed, it was possible to write in Figs. 4 and 5 the content of elements on the particles of mineral phases grown on the microcospheres.
4. If I understood the content of the manuscript correctly, the authors did not explain how they crossed the wettability threshold of samples subjected to separation. Moreover, the purity of the cenospheres obtained as a result of separation was not checked. Because it is not possible that the cenospheres are not accompanied by inorganic fly ash particles. And then the results of the experiment are completely different.
5. Two samples were tested from one Unit 12 station from one power plant. Is this sufficient research material? In addition, the authors expect that ... " The cenospheres properties reported in this work are expected to be put into directional perspective for cenospheres market positioning and, to some extent, a long- term plan of resource utilization" ...
6. There is no information in the Materials and Methods chapter about the processing of the results. This is what single sentences incorrectly placed in Results are about
7. No discussion of results. If authors choose to compare their results with those of other researchers or cite other authors for various reasons in the Results section, title the section Results and Discussion. And if they decide to use the Results chapter, then there should be the Discussion chapter. I did not notice any results in the discussion.
8. The collection of References and the number of citations are very few and very perfunctory.

 

I omit comments regarding punctuation.                                                            

The manuscript looks more like a commissioning analysis report than a scientific article.

 

I have included detailed comments in the text

 

Comments for author File: Comments.pdf

Author Response

Dear Reviewer 3,

Authors are very much thankful to the Reviewers’ valuable comments and suggestions on our work. The manuscript has been revised according to the constructive comments and your kind advices, which are valuable for improving the quality of our manuscript. The changes are highlighted using “Track Changes” and marked in the red color in the revised manuscript. The responses to the reviewer’s comments are included as follows. We sincerely hope that this manuscript can be acceptable to be published on Applied Science.

 

Comments of Reviewer 3

Overall assessment

1. The manuscript does not present revealing information. Therefore, it was necessary to write what gap in the extensive collection of publications devoted to the cenospheres the authors intended to fill with this manuscript.

Response:  We have added more details about experiment procedures, result processing, and result discussion, regarding the reviewer’s comments. The Abstract, the Result and Discussion and the Conclusion part have been amended throughout the manuscript to describe the work more clearly and tie up the point that we have intended to present. Further, we have worked more on the references to cite them appropriately and increased the number of references.

2. This manuscript is more devoted to developing an optimal method of separating cenospheres than to characterizing the cenospheres themselves. This should be expressed in the title of the article.

Response:  Thank you very much for the suggestion. We agree with this point and the comment was taken. Thus, the title of the manuscript has been modified to suitably express the details of work. The modified title is “Separation Process and Structure-Chemical Composition of Cenospheres from Lignite Fly Ash Produced from Coal-Fired Power Plant in Thailand”.

 

The chemical composition of the cenospheres is limited only to determining the content of the oxides of the main elements. The content of trace and radioactive elements was not analyzed. This should also be reflected in the title of Chapters 3.4.

Response:  The content of trace and radioactive elements of fly ash sample used in this study was not analyzed. However, there are a number of publications reporting on the investigation of trace elements in fly ash from Mae Moh power plant, Thailand. Thus, we have put more description and referred those works in the paragraph mentioning about the metals, leachable trace elements, and radioactive elements in the Section 3.4. Chemical Composition (Line 414-431, Page 11-12 of the revised manuscript).

 

3. If point analyzes of the chemical composition of the cenospheres were performed, it was possible to write in Figs. 4 and 5 the content of elements on the particles of mineral phases grown on the microcospheres.

Response:  Thank you very much for the suggestion on this point. The analyzed spots (by EDX) were marked in the Figures 4a, 4g, 5d, 5e, and 5f. The EDX results of spot analysis have been described in the Section 3.4 (Line 402-413, Page 11 of the revised manuscript).

 

4. If I understood the content of the manuscript correctly, the authors did not explain how they crossed the wettability threshold of samples subjected to separation.

Response: The wettability of material was not mentioned previously in the original version of manuscript. Regarding to this point, we have made more explanation and added to the Section 2.1. (Line 121-128, Page 3 of the revised manuscript). Details described a spontaneous imbibition, which is a measure of particle-level wettability. In this circumstance, water does not experience spontaneous imbibition into dry fly ash; however, wettability of the particles with intra-porous grains and accessible porosity is more pronounced for water to imbibe the ash particles. Thus, in this study mixing procedure was adopted to facilitate such imbibition process for preparing the suspension prior to sedimentation and separation, while the fly ash sample was presumed to have a certain amount of such accessible porosity.

 

-Moreover, the purity of the cenospheres obtained as a result of separation was not checked. Because it is not possible that the cenospheres are not accompanied by inorganic fly ash particles. And then the results of the experiment are completely different.

Response:  Thank you very much for the comment. We do concern on the proportion of cenospheres collected from different conditions. Considering the particle size distribution result, fly ash sample contained the small sized particles (< 50 μm) with the weight fraction of ~74%. Whereas for cenospheres, the particles with size < 45 μm were found in a small fraction of ~7%. This confirmed a possibility that the small particles, if size < 50 μm defined as fly ash, would mix with the collected fraction of cenospheres after separation; this was also observed and reported in the literature. Accordingly, such occurrence could probably affect the recovery yield to some extent. The information about impurities that could have impact on the cenospheres separation efficiency, the cenospheres yield and their quality has been added in Line 243-256, Page 7-8 of the revised manuscript.

In the revised manuscript, we have added an XRF image (Figure 6c) taken for a narrow size fraction (106-250 μm); the image showed the dense, smally fly ash particles that are obviously seen. The discussion upon this point has been added in Line 467-475, Page 15 of the revised manuscript.

 

5. Two samples were tested from one Unit 12 station from one power plant. Is this sufficient research material? In addition, the authors expect that ... " The cenospheres properties reported in this work are expected to be put into directional perspective for cenospheres market positioning and, to some extent, a long- term plan of resource utilization" ...

Response:  The sample from unit 12 used this study was targeted and particularly selected on the basis of material availability and agreement to feasible study on cenosphere separation using fly ash from a single power plant. With a fixed starting material, the experimental variables could be designed systematically to investigate the technical possibilities and study the property comparison in a controllable fashion, regarding to those designed experiments in this manuscript. Upon the scientific research merit, we do agree that the more different sources of sample would definitely give us the more information about cenospheres separation process and their quality in comparative details. Our current focus is also on cenosphere separation by using fly ash from different sources, which are different boilers, different classes, and different coal types.

The sentence that the reviewer mentioned above has been deleted from the Conclusions part.

 

6. There is no information in the Materials and Methods chapter about the processing of the results. This is what single sentences incorrectly placed in Results are about

Response:  Thank you very much for the comment. The sentences about the processing of results periodically found in the Section of Results and Discussion have been moved up to the Section of Materials and Methods, regarding the reviewer’s comments notifying us in the context part of results.

 

7. No discussion of results. If authors choose to compare their results with those of other researchers or cite other authors for various reasons in the Results section, title the section Results and Discussion. And if they decide to use the Results chapter, then there should be the Discussion chapter. I did not notice any results in the discussion.

Response:  For this manuscript, we have intended not to separate the Result part from the Discussion part. We do apologize that we have only put ‘Result’ in the title of Section 3. According to the reviewer’s suggestion, the title of Section 3. has been revised into “3. Results and Discussion”.

 

8. The collection of References and the number of citations are very few and very perfunctory.

Response:  We have worked more on the references to cite appropriately and increased the number of citations for this manuscript. The order of references has been rearranged throughout the entire manuscript.

 

I omit comments regarding punctuation. 

The manuscript looks more like a commissioning analysis report than a scientific article.

I have included detailed comments in the text

Response:  We have also put our reply per the comments of the reviewer providing in the text of the original version of manuscript; please see the file attached. We really appreciate your time and valuable comments of pointing out the important issues and revisions needed in the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The article was significantly improved and the aim of the research was
emphasized. After the corrections, the article is more precise.
The weak point of the article is the lack of results of the phase
composition of the cenospheres, especially the content of the amorphous
phase and the amount of mullite. This is important for the use of
cenospheres in wide range of potential application. At least a comment
should be added to the phase composition. Also, the new title indicate a
link to "structure" but no structural studies are in the work. In fact
the "microstructure" should be pointed.  

 

Author Response

The article was significantly improved and the aim of the research was emphasized. After the corrections, the article is more precise.

The weak point of the article is the lack of results of the phase composition of the cenospheres, especially the content of the amorphous phase and the amount of mullite. This is important for the use of cenospheres in wide range of potential application. At least a comment should be added to the phase composition.

Response:  Thank you very much for the recommendation concerning this particular point. According to the reviewer’s comment, we have added the result of phase composition characterized by XRD, and made a new Section as “3.5. Phase Composition” in the revised manuscript. Thus, several points in the manuscript have been added according to the addition of the phase composition study.

• Abstract: a sentence has been added in Line 26-27, Page 1.
• 2.2 Fly Ash and Cenospheres Identification: a description about XRD method has been added in Line 175-179.
• Section 3.5. Phase Composition; the text in Line 533-576.
• Figure 8 and caption have been added in Line 578-582.
• Conclusions: a sentence has been added in Line 601-603.

 

Also, the new title indicate a link to "structure" but no structural studies are in the work. In fact the "microstructure" should be pointed.  

Response:  The ‘structure’ in the title has been modified into ‘microstructure’.

Author Response File: Author Response.pdf

Reviewer 3 Report

1. In Fig. 1, the authors still use the term "Fly ash content instead of Fly ash yield."
2. I still disagree that the conclusions (line 526-532) present the purpose and scope of the work.
3. I still believe that not checking the mineral composition of the cenospheres after separation (whether they contain significant amounts of inorganic fly ash particles or not) is a drawback of this manuscript. Therefore, I believe that it should be written that the observations of XX are justified under the condition of 100% purity of the cenospheres concentrate.

Author Response

Comments and Suggestions for Authors

1. In Fig. 1, the authors still use the term "Fly ash content instead of Fly ash yield."

Response:  The caption of Figure 1 has been revised. That is; previously there was ‘fly ash-to-water ratio conditions’. The revised one is ‘ash content’, highlighted in green color. We kindly confirm here that the x-axis is for ‘ash content’, whereas the y-axis is for cenosphere recovery (or yield)’.

 

2. I still disagree that the conclusions (line 526-532) present the purpose and scope of the work.

Response:  The comment has been taken and thank you very much for the suggestion. The sentences of conclusion in these Lines have been modified.

 

3. I still believe that not checking the mineral composition of the cenospheres after separation (whether they contain significant amounts of inorganic fly ash particles or not) is a drawback of this manuscript. Therefore, I believe that it should be written that the observations of XX are justified under the condition of 100% purity of the cenospheres concentrate.

Response:  Thank you very much for the recommendation concerning this particular point. Thus, we have added the result of mineral phase composition characterized by XRD, and made a new Section as “3.5. Phase Composition” in the revised manuscript.

Thus, several points in the manuscript have been added according to the addition of the phase composition study.

• Abstract: a sentence has been added in Line 26-27, Page 1.
• 2.2 Fly Ash and Cenospheres Identification: a description about XRD method has been added in Line 175-179.
• Section 3.5. Phase Composition; the text in Line 533-576.
• Figure 8 and caption have been added in Line 578-582.
• Conclusions: a sentence has been added in Line 601-603.

Author Response File: Author Response.pdf