Phase Transformation of Alumina, Silica and Iron Oxide during Carbothermic Reduction of Fly Ash for Ceramics Production
Round 1
Reviewer 1 Report
Manuscript ID: metals-1282068
Title: Phase transformation of alumina, silica and iron oxide during carbothermic reduction of fly ash
Authors: Qingchun Yu et al.
Title. Authors must add information about product after carbothermic reduction. Phase transformation of alumina, silica and iron oxide during carbothermic reduction of fly ash for ceramics production (for example).
Introduction. Authors must add last articles of these science direction:
- W. Wang et al. Recycling of waste red mud for fabrication of SiC/mullite composite porous ceramics, Ceram. Int. 45 (2019) 9852–9857. doi:https://doi.org/10.1016/j.ceramint.2019.02.024.
- J. Zhu, H. Yan, Microstructure and properties of mullite-based porous ceramics produced from coal fly ash with added Al2O3, Int. J. Miner. Metall. Mater. 24 (2017) 309–315. doi:10.1007/s12613-017-1409-2.
- Y. Yin et al. Synthesis of Al2O3-SiC composite powders from coal ash in NaCl-KCl molten salts medium, Ceram. Int. 42 (2016) 19225–19230. doi:10.1016/j.ceramint.2016.09.087.
- Y. Luo et al. Novel two-step process for synthesising β-SiC whiskers from coal fly ash and water glass, Ceram. Int. (2018) 0–1. doi:10.1016/j.ceramint.2018.03.082.
Line 33-34. Add more reference for this information. Authors must add information about Al extraction for coagulant production [DOI: 10.1016/j.jclepro.2020.123206 10.1080/09593330.2018.1426639]and rare earth elements recovery from CFA [10.1016/j.mineng.2019.01.027 10.3390/min10050451]
Section 2.1 Add full chemical formulas of mullite, silica, iron oxide, calcium lignosulphonate.
Table 1. Why so big LOI content if carbon content only 4,5 wt.%? What is the P and S content?
Figure 1. Did the authors find hematite or is it magnetite peaks?
Line 112-124. What is the mass of pellets?
Line 115. How much was the charcoal powder content in pellet. How authors calculate this amount. Please, add this information.
Section 3. Authors used special software: FactSage v.7.1 or HSC Chemistry 9.9 for thermodynamics analysis?
Figure 4. Why Fe2O3 phase still exists at 1373-1473K?
Figure 8. Bad quality of figure. No definition.
Section 4.2.2. Can authors separate this cotton-shaped condensate from other ceramic materials?
Section 4.4. Add XRD of samples after carbothermic reduction at nitrogen atmosphere.
What the mass of the pellets after reduction at 2273K?
The authors must add a flowsheet of the CFA carbothermic reduction process in the air atmosphere. Write the optimum process parameters (T, duration of reduction) and pellets mass before and after carbothermic reduction. This will make the process clearer for readers.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The article is poorly written and English in multiple places is far from being excellent.
Some sentences are trivial or naive. Examples:
Temperature plays an important role in carbothermic reduction of fly ash.
According to X-ray quantitative interpretation, phase content in mixtures is proportional to its diffraction intensity.
It is found that the alloy profile shows silver, light grey and grey at different sections
Random examples of other rather unfortunate formulations:
line 48: the conversion decreases
line 96: Infrared radiation apparatus
line 144: It is clearly that Fe
line 175: It is clearly that mullite
line 197: Further reduction at 2073 K causes a stronger diffraction peak
line 244: Possible reactions about the condensate
line 247: Alumina and carbon is hard to undergo the reaction
While the final conclusions may be correct many pieces of intermediate information are missing.
A few sentences in the beginning of the abstract discuss the motivation of the work and belong to the introduction. The abstract should start at the sentence beginning with "This work is focused ...".
A word on the environmental and economical aspect of the proposed method of the fly ash utilization would be welcome. We need to use natural wood and a lot of energy. Under what circumstances would it make sense?
The Authors did not hesitate to name the supplier of the charcoal, so may be the could also describe the source of the fly ash - their main object of interest. Did it come from a single power plant or was it a mix of many sources from the named province?
Chemical composition of the raw materials is given but nothing is said about how the figures were obtained.
The design of the experimental setup is not quite clear. The sentence "The graphite crucible was cut open..." suggests a closed crucible while the drawing in Fig.2 suggests the opposite. Bearing in mind that atmospheric gases and gaseous phases are involved in the discussed reactions this point must be clarified.
Composition of the pellets subjected to the heat-treatment is not given. The text mentions 100g of fly ash, 6.5% (weight or volume) of the binder and stoichiometric amount of charcoal. But stoichiometric with respect to what? How much carbon was actually added?
Where the numbers used for making graphs in Fig.3 come from? They were calculated, OK, but on what basis.
Reaction temperatures deduced from the graphs in Fig.3 are, throughout the article, given with the accuracy of 2 decimal places. This is ridiculous. Owing to the finite accuracy of any thermodynamic data, those temperatures may be safely rounded to full Kelvins or even to tens of Kelvins. This would also bring more clarity to the manuscript.
While Authors declare that "Contents of aluminum, silicon, and iron in the reduced product were determined by chemical analysis ..." no numbers can be found in the manuscript. Why?
Analysis of the processed samples is purely qualitative. The authors notice appearance, disappearance, growth or decrease of the diffraction peaks and draw their conclusions from those observations. Quantitative analysis of the processed samples must be given to support the discussion in the manuscript.
Firstly the Authors must include the chemical composition analysis data, which they declared to have obtained. Secondly, the Authors are in possession of the diffraction data taken with an excellent, state-of-the-art instrument which should allow them to determine the phase composition of their samples quantitatively. The software for the so-called Rietveld analysis is available free of charge on the Internet.
The Authors mention considerable mass loss of their samples during processing but don't give any figures. In my opinion such data must be given and discussed with respect to the reaction pathways proposed in the article.
Attributing the source of Ca in the analysis given in Fig.6 solely to the charcoal and the binder is not correct. The fly ash itself contains non-neglectable amount of Ca.
Fig.8 --Publishing phase diagrams copied from the text-books violates the copyrights.
Sect. 4.4. This is not true that it is difficult to distinguish SiC and SiAlON in the diffraction pattern. SiAlON has several strong diffraction peaks that do not overlap with any of those of SiC.
Twice in the manuscript (lines 70 and 105 ) the Authors mention "previous work" but not point to any of the references.
Author Response
Please see the attacement
Author Response File: Author Response.pdf
Reviewer 3 Report
The presented work has some practical value and reports interesting experimental results. However, I believe that the thermodynamic analysis described in the paper is the weak point of this work. The authors should explain in more detail what is the meaning of this part of the work.
In addition, there are the following comments on the text of the article:
1. In many cases, references to figures in the text are found later than the figures themselves.
2. In my opinion, the authors need to add a link to the source of the diagram in the figure caption 8.
3. Where did the authors get the data in Table 3? This is important - in different sources, such data may differ.
4. Judging by table 3, the authors do not take into account the possibility of the formation of carbon dioxide. Why?
In general, to what extent can the list of reactions in Table 3 be considered exhaustive?
5. Thermodynamic analysis was carried out on the assumption that solid solutions are not formed in the system. Why?
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors answered all questions in detail. Also, there were presented references to studies confirming the answers of the authors. It is very good fact. Overall, the article looks much better.
There is one note on the presentation of the names of Europeans scientists. The authors in the 4th link [4] indicated their full first names, and the surnames were shortened to one letter. Authors need to do the opposite. This is a technical error that does not diminish the scientific value of the article.
Author Response
Thanks for your warning. The names of the authors in the fourth reference was changed. Meanwhile, something about thermodynamics was added in section 3, and six more references were also added. All the revisions were in pink in the revised manuscript.
Reviewer 2 Report
I have no further comments.
Author Response
Thanks for your reviewing.
Reviewer 3 Report
The authors answered my questions very thoroughly. Their answers are based on literary references. Unfortunately, this paper hasn't changed enough as a result. For example, I asked about the data source in Table 3. They answered in their answer, but it was necessary to indicate the data source in the text of the paper. About carbon dioxide - there is no need to explain it to me in a separate letter. Write about it in the text of the paper!
Author Response
As you said, this part of revision should be in the article. Last time authors forgot to do. Thanks for your warning. Now it was added in section 3. Meanwhile, six references were added. All the revision were in pink in the revised manuscript.
