The volatilization behaviour of typical fluorine-containing slag in steelmaking

It was taken as typical steelmaking fluorine-containing slag systems with the remelting electroslag, continuous casting mould flux and refining slag. The volatilization behaviour of each slag system was analysed by thermogravimetric (TG) and mass spectrometry (MS) detection. The results showed that the remelting electroslag volatilized significantly above 1300°C and the volatiles were mainly CaF2, MgF2 with a small amount of SiF4 and AlF3; the continuous casting mould flux volatilization was divided into two stages, in the first stage (500°C∼800°C), CaF2 and Na2O reacted to form NaF, and in the second stage (greater than 1200°C), the CaF2 evaporation was highlighted; for CaF2-CaO-based refining slag, the volatilization was the most significant at the eutectic point 84% CaF2–16% CaO, and the volatility can be reduced by adding 5% SiO2. This research will be guiding significance for the composition and performance control of fluorine-containing slag and metallurgical environmental protection in the steelmaking process.


Introduction
In the introduction section, the research results of Mills, Chen, Zhao and other scholars are listed. But the difference between this work and the previous scholars' work has not been elaborated. Please describe the difference.
2. Page 4, line 45~47 "According to the previous thermodynamic calculation, the first stage was the reaction of CaF2 with Na2O and SiO2 to generate NaF and SiF4 gas, and the second stage was mainly the CaF2 evaporation." The previous research has not been cited, please provide. What is more, please consider whether the experimental conditions of the previous study are the same as this experiment. Otherwise, it should be recalculated and explained in based on this experimental conditions.
3. Page 5, line 7~14 "Different kinds of steel and smelting equipment (VOD and RH) have different requirements for the properties and composition of refining slag." According to the description, authors would like to provide guidance on the VOD and RH slag system though this work. However, during actual steelmaking process, the mass fraction of CaF2 added in RH or VOD slags is usually below 20%, which is different from the experimental conditions in this article. It is recommended to reconsider the background of this study.

Figure 5
The abscissa in the figure is not suitable, it is recommended to revise it to CaO/(CaO+CaF2).

Page 5, line 52~54
The experimental conditions of CaO-CaF2-SiO2 slag system are not provided, please supplement in the Materials and Methods part.
6. Figure 7 The ordinate in the figure is not right, please revise it.

Page6, line 19~22
The author proposed that a small amount of SiO2 could reduce the volatilization of the CaO-CaF2-SiO2 slag system, but the experiment results cannot verify this. The CaF2 contents in R4 and R3 slags are higher than that in R1 slag. The volatilization of R4 and R3 slags are also higher than that of R1 slag. According to these results, it cannot be concluded that the addition of SiO2 in R1 slag reduce the volatilization of the CaO-CaF2-SiO2 slags. It is recommended to use TG-MS experiment to investigate the volatiles and volatilization ratio of the CaO-CaF2-SiO2 slags.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? No Manuscript ID: RSOS-200704 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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RSC Subject Editor:
Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) Dear Authors: Thank you for your consideration to publish your work for Royal Society Open Science journal. The experimental data are very precious. The work investigated the volatilization behavior of fluorine-containing slag systems. However, I recommend major revision because some contradiction can be seen in the descriptions. Please improve the manuscript according to the following comments:

Introduction
In the introduction section, the research results of Mills, Chen, Zhao and other scholars are listed. But the difference between this work and the previous scholars' work has not been elaborated. Please describe the difference.
2. Page 4, line 45~47 "According to the previous thermodynamic calculation, the first stage was the reaction of CaF2 with Na2O and SiO2 to generate NaF and SiF4 gas, and the second stage was mainly the CaF2 evaporation." The previous research has not been cited, please provide. What is more, please consider whether the experimental conditions of the previous study are the same as this experiment. Otherwise, it should be recalculated and explained in based on this experimental conditions.
3. Page 5, line 7~14 "Different kinds of steel and smelting equipment (VOD and RH) have different requirements for the properties and composition of refining slag." According to the description, authors would like to provide guidance on the VOD and RH slag system though this work. However, during actual steelmaking process, the mass fraction of CaF2 added in RH or VOD slags is usually below 20%, which is different from the experimental conditions in this article. It is recommended to reconsider the background of this study.
4. Figure 5 The abscissa in the figure is not suitable, it is recommended to revise it to CaO/(CaO+CaF2).

Page 5, line 52~54
The experimental conditions of CaO-CaF2-SiO2 slag system are not provided, please supplement in the Materials and Methods part.
6. Figure 7 The ordinate in the figure is not right, please revise it.

Page6, line 19~22
The author proposed that a small amount of SiO2 could reduce the volatilization of the CaO-CaF2-SiO2 slag system, but the experiment results cannot verify this. The CaF2 contents in R4 and R3 slags are higher than that in R1 slag. The volatilization of R4 and R3 slags are also higher than that of R1 slag. According to these results, it cannot be concluded that the addition of SiO2 in R1 slag reduce the volatilization of the CaO-CaF2-SiO2 slags. It is recommended to use TG-MS experiment to investigate the volatiles and volatilization ratio of the CaO-CaF2-SiO2 slags.
Reviewer: 2 Comments to the Author(s) The introduction needs to be improved. The section "3 Materials and Methods" is treated superficially. A short description of the steel plant.
Insert the name of the metallurgical plant from which the slag samples were taken. Insert pictures with the three slag samples. Lines 9-10: Description of the slag samples preparation. Description of the chemical analysis methods. Specify the names of the "chemical pure reagents". Lines 23-26: Present the information in the form of a table. For XRF analysis -specify the type and model of the equipment used and the technical characteristics. Why the XRF analysis method was chosen? Specify the steel types and marks from which the slag samples come. Insert a table with the chemical composition of the steels. Insert the XRF tests for the slag samples before the TG test. Make a comparison of the chemical composition of the slag samples before and after the TG test. Line 41: Insert the decomposition reaction of Na2CO3. Lines 45-47 . Insert the chemical reactions. Lines 52-53. The paper does not refer to the importance of the study for "metallurgical environmental protection", although this is pointed out in the conclusions. Please insert a scientific discussion and explain. Specify the concrete measures resulting from the study, regarding the environmental protection in the steelmaking process.

Recommendation? Major revision is needed (please make suggestions in comments)
Comments to the Author(s) 1.
The introduction needs to be improved. The section "3 Materials and Methods" is treated superficially. A short description of the steel plant.

2.
Insert the name of the metallurgical plant from which the slag samples were taken. Insert pictures with the three slag samples. 3.
Lines 9-10: Description of the slag samples preparation. Description of the chemical analysis methods. Specify the names of the "chemical pure reagents" (in the section "Materials and methods"). Lines 29-31 (page 5): Present the information in the form of a table.

4.
For XRF analysis -specify the type and model of the equipment used and the technical characteristics.

5.
Why the XRF analysis method was chosen? Specify the steel types and marks from which the slag samples come. Insert a table with the chemical composition of the steels. 6.
Insert the XRF tests for the slag samples before the TG test. 7.
Make a comparison of the chemical composition of the slag samples before and after the TG test. 8.
Lines 45-47 . Insert the chemical reactions. Lines 50-51 (page 8): The paper does not refer to the importance of the study for "metallurgical environmental protection", although this is pointed out in the conclusions. Please insert a scientific discussion and explain. 11. Specify the concrete measures resulting from the study, regarding the environmental protection in the steelmaking process. 10. Expand the list of references. E.g: "An Assessment of the Substance Losses from Charge Composition Used to the Steelmaking -Key Factor for Sustainable Steel Manufacturing". Procedia Manufacturing, vol. 32, 2019, p. 15-21. The reference is not found in the bibliography. https://www.sciencedirect.com/science/article/pii/S2351978919302112 In conclusion: The authors treated the observations superficially.
Many observations were ignored. They are marked with red.

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