Comparative Study on the Kinetics of the Isothermal Reduction of Iron Ore Composite Pellets Using Coke, Charcoal, and Biomass as Reducing Agents
Round 1
Reviewer 1 Report
The manuscript is very clear and with high quality, at a high scientific level. I am sending a few comments:
Line 55:
In my opinion, the use of coke in a defined quality (with a high ash content) was not the best solution. For a relevant comparison, it was necessary to use a standard quality coke with a carbon content of about 80 - 83% and an ash content of about 12 - 14%.
Line 73:
Determining of the degree of reduction through weight loss may not be the most accurate, since in addition to weight loss through the reduction of iron oxides, there is also weight loss due to the escape of volatile combustibles from the carbon reducer. More precisely, it is to express the degree of reduction by the ratio of oxygen removal to total oxygen.
Line 79:
Fe-Cr-A1 alloy wire? Shouldn't aluminum be used instead of one in the alloy designation?
Line 96-98:
Only a negligible amount of reduction will occur by solid-solid reactions between carbon particles and iron oxide particles since such reactions are very slow.
This theory is valid at lower temperatures up to 1000 oC, at temperatures of about 1200 oC (this is also the temperature used in the research) the reaction through direct reduction with carbon is much more intense, as part of the mass and some grains are in the liquid state and this mechanism also takes place.
Line 130:
It would be appropriate to supplement the explanation of the differences in structure and also to clarify the differences in kinetics - possibly also on the basis of the reactivity of the individual carbon reducing agents.
Line 211:
It should be added to the conclusion why the reduction with biomass is faster and easier - related to the composition of biomass (low ash content) and its properties - possibly higher reactivity - due to the higher content of volatile combustibles and with higher porosity.
Author Response
Dear Reviewer: 1,
Thank you very much for your comments on our manuscript. I have responded to your comments as follows.
1) Comment: The manuscript is very clear and with high quality, at a high scientific level. I am sending a few comments:
Line 55:
In my opinion, the use of coke in a defined quality (with a high ash content) was not the best solution. For a relevant comparison, it was necessary to use a standard quality coke with a carbon content of about 80 - 83% and an ash content of about 12 - 14%.
Response: Thank you for your suggestion. For a relevant comparison, it is indeed more appropriate to use a standard quality coke with carbon and ash contents as above. In our future works, the characteristics of raw materials will be considered more adequately, and a standard quality coke with a carbon content of about 80 - 83% and an ash content of about 12 - 14% will be adopted for a deeper relevant comparison of the three different iron ore composite pellets. Thank you again.
2) Comment: Line 73:
Determining of the degree of reduction through weight loss may not be the most accurate, since in addition to weight loss through the reduction of iron oxides, there is also weight loss due to the escape of volatile combustibles from the carbon reducer. More precisely, it is to express the degree of reduction by the ratio of oxygen removal to total oxygen.
Response: Thank you for your comment. Determining of the degree of reduction through the degree of reduction by the ratio of oxygen removal to total oxygen is undoubtedly a reliable approach to obtain the precise data of reduction degree. Unfortunately, the precise quantitative characterization of oxygen element is difficult. As is known, the data precision of oxygen content obtained by conventional element analysis methods such as X-ray fluorescence analysis (XRF), atomic emission spectrometry (AES) is not high. Actually, the precise oxygen content data of pellets in our work is hard to obtain due to the limitation of experimental requirements. Therefore, to evaluate the degree of reduction, a pseudo kinetic parameter, the fraction of reaction (m) was introduced, which is defined as the ratio of mass loss at a given time to the maximum mass loss of the pellets. Many previous works (J. Iron Steel Res. Int., 2013, 20: 28–33; J. Iron Steel Res. 2000, 12 (4): 52–58) also evaluated the reduction degree of iron ore through weight loss. Indeed, determining of the degree of reduction through weight loss may not be the most accurate. However, introducing the pseudo kinetic parameter m is still a feasible approach to evaluate the degree of reduction. If the explanation to you is not appropriate and has any question about this, please don't hesitate to let me know, and we will revise it further.
3) Comment: Line 79:
Fe-Cr-A1 alloy wire? Shouldn't aluminum be used instead of one in the alloy designation?
Response: Thank you for your careful reading of our manuscript. We are very sorry for this mistake in the manuscript. We have corrected to “Fe-Cr-Al alloy wire” as highlighted in the revised manuscript. Thank you again.
4) Comment: Line 96-98:
Only a negligible amount of reduction will occur by solid-solid reactions between carbon particles and iron oxide particles since such reactions are very slow.
This theory is valid at lower temperatures up to 1000 oC, at temperatures of about 1200 oC (this is also the temperature used in the research) the reaction through direct reduction with carbon is much more intense, as part of the mass and some grains are in the liquid state and this mechanism also takes place.
Response: Thank you for your suggestion. At relatively high temperature, reduction occurred by solid-solid reactions between carbon particles and iron oxide particles is indeed should be considered. In our revised manuscript, we have pointed out that solid-solid reaction also plays a certain role in the reduction of pellets when the temperature is relatively high. Thank you again.
5) Comment: Line 130:
It would be appropriate to supplement the explanation of the differences in structure and also to clarify the differences in kinetics - possibly also on the basis of the reactivity of the individual carbon reducing agents.
Response: Thank you for your suggestion. We have supplemented the explanation of the differences in structure and discussed the influence of this structure difference on reduction kinetics of pellets as highlighted in the revised manuscript. Thank you again.
6) Comment: Line 211:
It should be added to the conclusion why the reduction with biomass is faster and easier - related to the composition of biomass (low ash content) and its properties - possibly higher reactivity - due to the higher content of volatile combustibles and with higher porosity.
Response: Thank you for your suggestion. In our revised manuscript, it had been added to the conclusion why the reduction with biomass is faster and easier. Thank you again.
Many thanks for your suggestions.
With best regards,
Sincerely yours，
Shifeng Liu
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Manuscript ID: metals-1103333
Title: Comparative Study on the kinetics of Isothermal Reduction of Iron Ore Composite Pellets Using Coke, Charcoal and Biomass as Reducing Agents
Authors: Xiaoli Yuan et al.
Line 26-45. Authors must add more information about previous researches. Find more articles from Metals or Minerals from 2019-2020 years. It is necessary to describe not only new processes and methods for reduction process, but also other types of resources (waste) - for the production iron products (pig iron, steel, Fe powder, etc.).
For example:
https://www.mdpi.com/2075-4701/10/1/133
https://www.mdpi.com/2075-4701/10/1/32
https://www.mdpi.com/2075-4701/10/11/1501
Line 48. Deleted utilized. Add used. What is bentonite was used in this study? Also from Brazil or from Chine?
Line 49. Where straw was collected?
Table 1. What is the LOI content? Sum of elements are not 100 wt.%
Line 57-65. What is the mass of samples?
Line 66-72. This method does not seem to be ensure. There are chemical methods for determining the degree of the iron metallization. They are more exact. The content of metallic iron in the samples can determined using the potassium dichromate titration method. Why authors used another approach?
Line 94-95. This sentence must add to Introduction section. Add more information from [10-16]. Use one or two references for one sentences.
Line 103. It is necessary to add Boudouard reaction: 2CO ⇌ CO2 + C
Figure 3. Curves must be the same color as points.
Line 121 and Figure 4. The Fe2(SiO4)2 was fayalite phase? It has another chemical formula - Fe2(SiO4).
Figure 5. The article was submitted in 2021. SEM-images from mid-2016. Is it normal to use such old research? The fact of using old SEM-images decrease the credibility of the entire study as a whole.
Figures 6-8. All fitting curves must leave from the zero point. All of these calculations are wrong. All of them need to be redone, although the author's approach is correct.
Figure 9. An equations y=ax+b type and coefficient R2 must be added for each curve.
Section 3. Results. The chemical composition of the pellets after reduction at the optimal parameters for each type of reducing agent must be added.
Conclusion. Chapter (1). Authors must add information about the degree of reduction for Iron pellets for all reductant agent.
References. New sources for 2019-2020 from MDPI journals need to be added. Add more than 10 links.
Author Response
Dear Reviewer: 2,
Thank you very much for your comments on our manuscript. I have responded to your comments as follows.
1) Comment: Line 26-45. Authors must add more information about previous researches. Find more articles from Metals or Minerals from 2019-2020 years. It is necessary to describe not only new processes and methods for reduction process, but also other types of resources (waste) - for the production iron products (pig iron, steel, Fe powder, etc.).
For example:
https://www.mdpi.com/2075-4701/10/1/133
https://www.mdpi.com/2075-4701/10/1/32
https://www.mdpi.com/2075-4701/10/11/1501
Response: Thank you for your suggestion. In our revised manuscript, we have added more information about previous researches (including the information about other types of resources) to the introduction part based on the related references. Thank you again.
2) Comment: Line 48. Deleted utilized. Add used. What is bentonite was used in this study? Also from Brazil or from Chine?
Response: Thank you for your suggestion. In our revised manuscript, we substituted used for utilized. Bentonite used in this study was obtained from China.
3) Comment: Line 49. Where straw was collected?
Response: Straw used in this study was collected from Bishan, Chongqing, China. In our revised manuscript, we added the place of production of straw.
4) Comment: Table 1. What is the LOI content? Sum of elements are not 100 wt.%
Response: LOI content of iron ore was supplemented in our revised manuscript. Due to the fact that Table 1 listed the Fe content instead of Fe2O3 and FeO contents, sum of elements in Table 1are not 100 wt.%
5) Comment: Line 57-65. What is the mass of samples?
Response: The mass of iron ore composite pellets sample was about 200 g in each reduction experiment. We have added this information in our revised manuscript.
6) Comment: Line 66-72. This method does not seem to be ensure. There are chemical methods for determining the degree of the iron metallization. They are more exact. The content of metallic iron in the samples can determined using the potassium dichromate titration method. Why authors used another approach?
Response: Thank you for your comment. Determining the content of metallic iron in the samples using the potassium dichromate titration method is indeed appropriate. Unfortunately, the potassium dichromate titration method is hard to be adopted in our work due to the limitation of experiment requirements and skills at present. Actually, introducing the pseudo kinetic parameter m is also a feasible approach to evaluate the degree of reduction. Many previous works (J. Iron Steel Res. Int., 2013, 20: 28–33; J. Iron Steel Res. 2000, 12 (4): 52–58) evaluated the reduction degree of iron ore through weight loss. In further work, we will adopt the potassium dichromate titration method to determine the content of metallic iron in the samples. Thank you for your suggestion.
7) Comment: Line 94-95. This sentence must add to Introduction section. Add more information from [10-16]. Use one or two references for one sentences.
Response: Thank you for your suggestion. This sentence had been added to Introduction section in our revised manuscript, and more information were expressed in the new manuscript based on these references.
8) Comment: Line 103. It is necessary to add Boudouard reaction: 2CO ⇌ CO2 + C
Response: Thank you for your suggestion. Boudouard reaction was added in our revised manuscript
9) Comment: Figure 3. Curves must be the same color as points.
Response: Thank you for your suggestion. Curves have the same color as points in our revised manuscript.
10) Comment: Line 121 and Figure 4. The Fe2(SiO4)2 was fayalite phase? It has another chemical formula - Fe2(SiO4).
Response: Thank you for your careful reading of our manuscript. It is fayalite phase, and the chemical formula is Fe2(SiO4). We are very sorry for this mistake in the manuscript. We have corrected to “Fe2(SiO4)” in our revised manuscript. Thank you again.
11) Comment: Figure 5. The article was submitted in 2021. SEM-images from mid-2016. Is it normal to use such old research? The fact of using old SEM-images decrease the credibility of the entire study as a whole.
Response: Thank you for your comment. Due to the fact that this work lasts for a long period of time, some images are indeed old. We have improved the quality of the SEM-images in our revised manuscript.
12) Comment: Figures 6-8. All fitting curves must leave from the zero point. All of these calculations are wrong. All of them need to be redone, although the author's approach is correct.
Response: Thank you for your suggestion. All fitting curves have been redone according to the above suggestion in our revised manuscript. The kinetic model parameters were updated correspondingly.
13) Comment: Figure 9. An equations y=ax+b type and coefficient R2 must be added for each curve.
Response: Thank you for your suggestion. equations y=ax+b type and coefficient R2 have been added for each curve in our revised manuscript.
14) Comment: Section 3. Results. The chemical composition of the pellets after reduction at the optimal parameters for each type of reducing agent must be added.
Response: Thank you for your suggestion. In fact, the approximate chemical composition of the pellets after reduction can be inferred from the reduction degree of pellets. In present work, we focus on the comparative study on the reduction kinetics of the three different composite pellets, so the chemical compositions of the pellets after reduction have not been present in this manuscript. In further work, we will systematically study the chemical composition of the pellets after reduction. If the explanation to you is not appropriate and has any question about this, please don't hesitate to let me know, and we will revise it further.
15) Comment: Conclusion. Chapter (1). Authors must add information about the degree of reduction for Iron pellets for all reductant agent.
References. New sources for 2019-2020 from MDPI journals need to be added. Add more than 10 links.
Response: Thank you for your suggestion. Information about the degree of reduction for Iron pellets for all reductant agent were added in our revised manuscript. New sources for 2019-2020 from MDPI journals have been added.
Many thanks for your suggestions.
With best regards,
Sincerely yours，
Shifeng Liu
Author Response File: Author Response.pdf
Reviewer 3 Report
The paper contains valuable contribution for the reduction reaction of composite pellets. It provides a deeper understanding of the reaction kinetics of biomass (carbonized straw) in comparison to coke and char coal.
I recommend the publication of the paper after revision. The following comments and questions should be considered for that:
2.1 Materials, Table 1
- S content of iron ore is untypical high
- Check chemical composition of bentonite. Sum is lower than 100 %. Is this the wet analysis? Is there really no Fe in the bentonite?
2.2 Experimental procedure
- The amount of reducing agent was determined that the molar ratio of reducible oxygen to fixed carbon was equal to 1.2. Add in Table 1 the amount of reducible of the iron ore.
- Equation (1): How was the maximum mass loss of the pellet determined. Explain the procedure and add the values for the three composites.
- The experimental conditions are not fully reproducible. Following information should be added:
- sample mass,
- type and accuracy of the electronic scale,
- conditions for preheating (heating rate, gas atmosphere and flow, pressure),
- conditions during isothermal reaction (gas atmosphere and flow, pressure),
- conditions during cooling (gas flow, pressure and typical cooling time).
- Which analytical method was applied for the determination of the carbon content of the pellets after the reduction?
- Results and discussion
- Figure 3: Does the time include the pre-heating period? How much was the fraction of reaction during pre-heating?
- Table 4: The title of the table might be revised to “Pre-exponential factor and apparent activation energies of different reducing agents”.
- Conclusions
- Is there any conclusion concerning the pre-exponential factor? How do you assess the determined values?
- Was the mechanical stability of the reduced pellets determined? Are there any differences between the three composites observed in the experiments (e.g., decomposition during the experiment)? Such occurrence might also influence the kinetics.
Author Response
Dear Reviewer: 3,
Thank you very much for your comments on our manuscript. I have responded to your comments as follows.
1) Comment: 2.1 Materials, Table 1
S content of iron ore is untypical high
Check chemical composition of bentonite. Sum is lower than 100 %. Is this the wet analysis? Is there really no Fe in the bentonite?
Response: Thank you for your comment. We have checked the characterization result of chemical composition of iron ore, S content of iron ore is indeed 0.200 wt.%. Chemical composition of bentonite was characterized again, Fe content was added to the chemical composition table. Due to the significant loss of ignition, sum is lower than 100 %.
2) Comment: 2.2 Experimental procedure
The amount of reducing agent was determined that the molar ratio of reducible oxygen to fixed carbon was equal to 1.2. Add in Table 1 the amount of reducible of the iron ore.
Equation (1): How was the maximum mass loss of the pellet determined. Explain the procedure and add the values for the three composites.
The experimental conditions are not fully reproducible. Following information should be added:
sample mass, type and accuracy of the electronic scale, conditions for preheating (heating rate, gas atmosphere and flow, pressure), conditions during isothermal reaction (gas atmosphere and flow, pressure), conditions during cooling (gas flow, pressure and typical cooling time).
Which analytical method was applied for the determination of the carbon content of the pellets after the reduction?
Response: Thank you for your comment. The mass loss in the pellet was originated from the following three processes: the reduction of carbon, the removal of oxygen from iron ore, and the expulsion of volatile matter. In present work, the maximum mass loss of the pellet was determined by adding the maximum mass loss of the three mass loss processes together. The values of the maximum mass loss for the three composites were present in our revised manuscript. The detailed experimental conditions (including type and accuracy of the electronic scale, heating rate, gas atmosphere and flow, et al) of reduction experiments in present work have been supplemented in our revised manuscript. The carbon content of the pellets after the reduction has not been characterized in present work. In further work, we will carry out the carbon analysis to estimate the unused carbon in the carbon iron ore composite pellets after reduction under isothermal condition using infrared carbon and sulfur analyzer. Thank you again.
3) Comment: 3. Results and discussion
Figure 3: Does the time include the pre-heating period? How much was the fraction of reaction during pre-heating?
Table 4: The title of the table might be revised to “Pre-exponential factor and apparent activation energies of different reducing agents”.
Response: Thank you for your comment. In reduction experiment, the pellet samples have not been pre-heated. The pellet sample was dried adequately at 105 °C in a drying oven for 24 h before reduction experiment. The detailed reduction experimental procedure is as following: First, the furnace was heated to given temperatures and held for 10 min to ensure the stabilization of the temperature in furnace. Then, the pellet sample was rapidly put in the basket. After a certain reduction time, the samples were rapidly taken off and cooled down with N2.
The title of table 4 has been revised to “Pre-exponential factors and apparent activation energies of different reducing agents”. Thank you for your suggestion.
4) Comment: 4. Conclusions
Is there any conclusion concerning the pre-exponential factor? How do you assess the determined values?
Was the mechanical stability of the reduced pellets determined? Are there any differences between the three composites observed in the experiments (e.g., decomposition during the experiment)? Such occurrence might also influence the kinetics.
Response: Thank you for your comment. The pre-exponential factor represents the occurrence frequency of the reaction situation. In present work, we mainly focus on the activation energies of reduction in different pellets, so we have not discussed the pre-exponential factors further. The mechanical stability of the reduced pellets have not studied in present work. In fact, the microstructures of the three pellets show obvious difference, the reduced iron ore-biomass and iron ore-charcoal composite pellets exhibit looser structure compared to iron ore-coke composite pellet, which undoubtedly causes the difference of mechanical stability of the three different reduced pellets. Further work should be done to clarify the influence of mechanical stability difference on the reduction kinetics of the pellets. Thank you again.
Many thanks for your suggestions.
With best regards,
Sincerely yours，
Shifeng Liu
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors (Shifeng Liu) answered all the questions of the reviewer in detail. I am grateful to the authors for such complete answers.
The introduction has been greatly improved. Materials and methods have been updated. The kinetic data has been corrected. The conclusions have been finalized. In the references 11 new links were added.
In this form, the article: "Comparative Study on the kinetics of Isothermal Reduction of Iron Ore Composite Pellets Using Coke, Charcoal and Biomass as Reducing Agents" can be accepted into Metals.
I wish the authors to continue their research in this area and use the potassium dichromate titration method to determine the degree of metallization of iron.
Author Response
Dear Reviewer: 2,
Thank you very much for your comments on our manuscript.
Comment:
The authors (Shifeng Liu) answered all the questions of the reviewer in detail. I am grateful to the authors for such complete answers.
The introduction has been greatly improved. Materials and methods have been updated. The kinetic data has been corrected. The conclusions have been finalized. In the references 11 new links were added.
In this form, the article: "Comparative Study on the kinetics of Isothermal Reduction of Iron Ore Composite Pellets Using Coke, Charcoal and Biomass as Reducing Agents" can be accepted into Metals.
I wish the authors to continue their research in this area and use the potassium dichromate titration method to determine the degree of metallization of iron.
Response: Thank you for your comments. We will continue our research in this area and use the potassium dichromate titration method to determine the degree of metallization of iron in the pellet samples. Thank you for your suggestion again.
Many thanks for your suggestions.
With best regards,
Sincerely yours，
Shifeng Liu
