Study on Reaction Behavior and Phase Transformation Regularity of Montmorillonite in High-Calcium Sodium Aluminate Solution System

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigates the reaction behavior and phase transition laws of montmorillonite in a high calcium sodium-alumina salt solution system were investigated. However, some points need to be further clarified.

1.      The reaction degree of montmorillonite was calculated through the mass of SiO₂, which should be make sense firstly.

2.      The reaction degree and phase change of montmorillonite were calculated throughout the manuscript. How did these changes affect the leaching efficiency of alumina should be further illustrated and discussed, in order to make the whole manuscript meaningful.

3.      In the section of Introduction, the sentence “China is rich in diaspore-type bauxite resources” is not properly linked to the sentence “China imports amount of high-grade bauxite ore every year”, and there is no indication that domestic diaspore-type bauxite is not a high-quality mineral.

4.      In the section of Experimental Material and Method, Equation 2-2 labeling is repeated.

5.      In Figure 3(a) and Figure 4(a), 13 should be changed to 130.

6.      In the section of Results and Discussions, when N_k=240, 260 or 280, the montmorillonite reaction degree is similar, but the mass of SiO2 in the liquid phase increases, and it is not explained here why N_k=260 or 280 was not chosen as the optimal condition for the reaction.

7.      In Figure 6(a), the number 2 above the horizontal coordinate should be removed.

8.      In the section of Conclusion, the optimal reaction time of 120 minutes in a salt bath is not experimentally specified.

9.      Figure 1 is not formatted the same way as the rest of the figures, so adjust them appropriately.

Comments on the Quality of English Language

The English writing is clear and easy-understanding.

Author Response

Summary:                  

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in the re-submitted files.

 

1. The reaction degree of montmorillonite was calculated through the mass of SiO₂, which should be make sense firstly.

Many thanks to the experts for their comments, which will help us in our future work and we will pay more attention to the details in this area in our future study. We agree with the comment. Therefore, we have further elaborated on the relationship between the reaction rate of montmorillonite and its SiO₂ content in the article. In a high-calcium and high-alkali solution system, SiO₂ in montmorillonite dissolves and reacts to form other compounds. By measuring the mass of SiO₂ in montmorillonite before and after the reaction, we can determine the extent of the reaction. Mention exactly where in the revised manuscript this change can be found in line 215.

 

2. The reaction degree and phase change of montmorillonite were calculated throughout the manuscript. How did these changes affect the leaching efficiency of alumina should be further illustrated and discussed, in order to make the whole manuscript meaningful.

Thank you for pointing it out. We agree with the comment. In the high calcium sodium aluminate solution system, the crystal structure of montmorillonite breaks down, causing a large amount of silicon to dissolve and enter the sodium aluminate solution. A portion of the montmorillonite reacts with the sodium aluminate solution to form Na₆(AlSiO₄)₆, resulting in the loss of alkali and aluminum. As the reaction of montmorillonite progresses, Na₆(AlSiO₄)₆ gradually disappears. Meanwhile, the aluminum and silicon entering the liquid phase form Al(OH)₄^⁻ and H₂SiO₄^2⁻. Finally, the Ca(OH)₂ in the solution reacts with Al(OH)₄^⁻ to produce (CaO)₃Al₂O₃·6H₂O, which ultimately reacts with H₂SiO₄²⁻ in the liquid phase to form the insoluble Ca₃Al₂(SiO₄)(OH)₈, leading to further loss of aluminum in the solution.

Overall, during the actual leaching process of bauxite, the montmorillonite in the bauxite reacts with the sodium aluminate solution to produce the insoluble Na₆(AlSiO₄)₆ and (CaO)₃Al₂O₃·6H₂O. The formation of these compounds consumes a large amount of alkali and aluminum, resulting in a decrease in the leaching efficiency of bauxite.

Mention exactly where in the revised manuscript this change can be found in section 3.4.1.

 

 

3. In the section of Introduction, the sentence “China is rich in diaspore-type bauxite resources” is not properly linked to the sentence “China imports amount of high-grade bauxite ore every year”, and there is no indication that domestic diaspore-type bauxite is not a high-quality mineral.

Thank you for pointing it out. We provide a more detailed explanation of the diaspore-type bauxite. The average alumina content in China’s diaspore bauxite is 50%, indicating a relatively high grade. However, this type of bauxite contains approximately 9-15% SiO₂, resulting in a low alumina-silica ratio (A/S), which is unfavorable for extraction in conventional Bayer solutions. Consequently, the leaching of diaspore bauxite requires higher temperatures and alkali concentrations. During the Bayer process, the silica-containing minerals in diaspore-type bauxite dissolve in the alkaline solution as Na₂SiO₃^2- (sodium silicate) and react with Al(OH)₄^- to form Na₆(AlSiO₄)₆ (hydrated sodium aluminosilicate), which enters the red mud, resulting in significant losses of aluminum and alkali. Due to the high silica content and low solubility characteristics of China’s diaspore bauxite, the Bayer process for alumina production requires significantly more energy, making large-scale application challenging. As a result, China needs to import a significant amount of high-grade bauxite each year, which increases costs and restricts the further development of the aluminum industry. China’s diaspore bauxite accounts for 98% of the total reserves in the country. Considering the future development of the alumina industry, there is considerable potential for further research and utilization of diaspore bauxite.

Mention exactly where in the revised manuscript this change can be found in line 35.

 

4. In the section of Experimental Material and Method, Equation 2-2 labeling is repeated.

Thank you for pointing it out. We agree with the comment. Therefore, we have revised the equation 2-2. Mention exactly where in the revised manuscript this change can be found in line 238.

 

5. In Figure 3(a) and Figure 4(a), 13 should be changed to 130.

Thank you for pointing this out, which will help us in our future work and we will pay more attention to the details in this area in our future study. The Figure.3(a) and Figure.4(a) have been modified by changing the horizontal coordinate of 13 to 130. Mention exactly where in the revised manuscript this change can be found in Figure 4 and Figure 5.

 

 

6. In the section of Results and Discussions, when N_k=240, 260 or 280, the montmorillonite reaction degree is similar, but the mass of SiO₂ in the liquid phase increases, and it is not explained here why N_k=260 or 280 was not chosen as the optimal condition for the reaction.

Many thanks to the experts for their comments, which will help us in our future work and we will pay more attention to the details in this area in our future study. We have provided the corresponding explanation regarding this matter. The reaction degree of montmorillonite increases with the rise in alkali concentration (N_k). When N_k reaches 240 g/L and the reaction is allowed to proceed for 120 minutes, the reaction degree of montmorillonite peaks. Beyond this concentration, further increases in N_k do not enhance the reaction degree. Instead, there is an increase in N_k leads to a certain decline in the reaction degree. At the same alkali concentration, the reaction degree of montmorillonite gradually increases with the extension of reaction time. After 120 minutes of reaction, there is no significant upward trend in the reaction degree of montmorillonite.

As the concentration of N_k increases, the quality of SiO₂ in the liquid phase also rises. reaching its peak when N_k is 280 g/L. However, excessively high concentrations of alkali can lead to over-expansion of montmorillonite, compromising the integrity of its crystal structure and diminishing the degree of the montmorillonite reaction. Consequently, the degree of montmorillonite reaction begins to decline when N_k is 280 g/L.

 Mention exactly where in the revised manuscript this change can be found in line 304 and 330.

 

 

7. In Figure 6(a), the number 2 above the horizontal coordinate should be removed.

Thank you for pointing this out, which will help us in our future work and we will pay more attention to the details in this area in our future study. The Figure 6(a) has been modified and the number 2 above the horizontal coordinate has been removed. Mention exactly where in the revised manuscript this change can be found in Figure 7(a).

 

8. In the section of Conclusion, the optimal reaction time of 120 minutes in a salt bath is not experimentally specified.

Thans you for thoughtful suggestion. We will provide the corresponding explanation regarding this matter. In the section 3.1, we provided additional explanation for selecting 120 min as the reaction time for montmorillonite in the salt bath furnace. As the reaction time increases, the reaction degree of montmorillonite rises sharply within the first 30 min, and the increase in the reaction degree begins to level off after 120 min. Meanwhile, the mass of SiO₂ in the liquid phase shows a trend of first increasing and then decreasing over time, with the downward trend stabilizing at 120 min. Overall, the optimal reaction time for montmorillonite in the salt bath furnace is 120 min. Mention exactly where in the revised manuscript this change can be found in line 255.

 

 

9. Figure 1 is not formatted the same way as the rest of the figures, so adjust them appropriately.

Thank you for your valuable suggestions. I will provide an appropriate explanation for this. The XRD pattern in Figure 1 primarily serves to illustrate the main components of the montmorillonite raw material used in this manuscript. In contrast, the other images of XRD are intended to demonstrate the phase and structural changes of montmorillonite under different conditions. Therefore, Figure 1 and the other images are not formatted the same way.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The author studied the phase transition and reaction mechanism of silicon containing minerals during the Bayer leaching process of diaspore. Overall, the paper is well-written, but some content still needs to be slightly modified.

1.    English expression and grammars need to be checked.

2.    In the abstract, what are the optimal reaction conditions?

3.    Please explain the meaning of MAS-NMR.

4.    In line 107, there is a lack of relevant structural diagrams to prove your conclusion.

5.    On line 114, how do you ensure that the molar ratio of NA₂O to AL₂O₃ is 2.

6.    In the Results and Discussion section, there are some repetitions.

7.    When discussing the physical phase transition process of montmorillonite, it is recommended to add a subheading.

8.    In the references, some articles are too outdated.

 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

1.    English expression and grammars need to be checked.

Author Response

Summary:

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in the re-submitted files.

 

1. English expression and grammars need to be checked.

Many thanks to the experts for their comments, which will help us in our future work and we will pay more attention to the details in this area in our future study. We agree with this comment. Therefore, we have checked English grammars and expression. Mention exactly where in the revised manuscript this changed can be found in section of Introduction and Result and Discussions.

 

2. In the abstract, what are the optimal reaction conditions?

Thank you for pointing it out. We agree with this comment. Therefore, we have provided the optimal reaction condition in the abstract. Under the optimal reaction conditions of a reaction time of 120 minutes, a temperature of 240 °C, an alkali concentration (N_k) of 240 g/L, and a calcium-to-silicon ratio (C/S) of 3.5:1, the montmorillonite reaction rate can reach a maximum of 93.71%. Mention exactly where in the revised manuscript this change can be found in line 25.

 

3. Please explain the meaning of MAS-NMR.

Thank you for pointing it out. We agree with this comment. Therefore, we have provided the explain in the section 2.1. Mention exactly where in the revised manuscript this change can be found in section 2.1. MAS-NMR is a nuclear magnetic resonance technique used to analyze the montmorillonite minerals employed in the experiments. The analysis is conducted using a nuclear magnetic resonance spectrometer (Avance III 400 Opizen, Bruker, Germany) to determine the molecular structure and composition of the samples. Mention exactly where in the revised manuscript this change can be found in line 168.

 

4. In line 107, there is a lack of relevant structural diagrams to prove your conclusion.

Thank you for pointing it out. We agree with this comment. It can be seen from Fig.1 that the main component of the montmorillonite raw material is the phase Na_0.3(Al, Mg)₂Si₄O₁₀(OH)₂·nH₂O (montmorillonite). Comparing the theoretical chemical formula of montmorillonite (Al₂O₃·4SiO₂·nH₂O), it can be seen that some cations such as Na⁺ and Mg²⁺ appear in the interlayers of montmorillonite. Meanwhile, we have included image of montmorillonite raw material (Fig.3) as supplementary material, it can be observed that the irregular structure of montmorillonite is primarily enriched with Al, Si, O, Mg, and trace amounts of Na and Ca. The elemental analysis results show a certain consistency with the previous XRD analysis, indicating that montmorillonite contains cations such as Na⁺ and Mg²⁺. Mention exactly where in the revised manuscript this change can be found in line 137 and 156.

 

5. On line 114, how do you ensure that the molar ratio of NA₂O to AL₂O₃ is 2.

Thank you for pointing this out. The molar ratio of the sodium aluminate solution used in this experiment was prepared independently. The sodium aluminate solution used in this study is an industrial-grade solution with a caustic ratio fixed at 2. Subsequently, the corresponding precise amounts of NaOH, Al(OH)₃ and deionized water were weighed according to the required concentration of caustic soda and placed in autoclave (ZRY-K01-0.5/10, Zhengwei, China) to react for 60 min. Mention exactly where in the revised manuscript this change can be found in line 186.

6. In the Results and Discussion section, there are some repetitions.

Thank you for pointing this out, which will help us in our future work and we will pay more attention to the details in this area in our future study. We have made modifications and reductions to some potentially repetitive sentences. Mention exactly where in the revised manuscript this change can be found in section 3.2.

 

7. When discussing the physical phase transition process of montmorillonite, it is recommended to add a subheading.

Thank you for providing us with suggestions. We have added additional subheadings in the Results and Discussion section for the analyses of XRD, SEM, TEM, MAS-NMR and FTIR to examine the changes in the mineral phases of montmorillonite. Mention exactly where in the revised manuscript this change can be found in section 3.4.

 

8. In the references, some articles are too outdated.

Thank you for pointing it out. We agree with this comment. We have removed the outdated references, which can be seen in the revised manuscript this change can be found in the reference.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The article is dedicated to the study of the mechanism of montmorillonite transformation in high-calcium aluminate solution, which can occur in the Bayer process during leaching of diaspore bauxites. The work relates to the specific fundamental issue of alumina production.

 

The article has a good structure. The work was carried out using numerous equipment, the experimental data are of a good level. The introduction contains relevant references. It should be noted that the results are scientifically sound, all the results obtained are explained or confirmed by references. The level of the article is sufficient for publication in Minerals. My recommendation is minor revision. The following recommendations could be given:

- It would be better to write the mechanism as reactions in the Conclusion or create good graphical abstract.

- The results of the study can be confirmed by the composition of bauxite residue (red mud). Hydrogarnet and other phases were found in red mud, so you can give corresponding references.

 

The comments according to the article:

l. 17. All abbreviations should be clarified.

l. 49. The sentence is not clear and should be rewritten.

l. 68. reaching = leaching. typo.  

l. 107-108. It is not clear why did you come to this conclusion. It has not been shown; you should prove it.

l. 224, 231, 266. It is better to write the used experimental conditions in the figure captions.

Author Response

Summary:

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in the re-submitted files.

 

17. 17. All abbreviations should be clarified.

Many thanks to the experts for their comments, which will help us in our future work and we will pay more attention to the details in this area in our future study. We have clarified all abbreviations in abstract. Mention exactly where in the revised manuscript this change can be found in line 18.

 

2. 49. The sentence is not clear and should be rewritten.

Thank you for pointing it out. We have rewritten the sentence in line 49. In the Bayer process used for alumina production, the consumption of caustic soda increases with the rising silica content in bauxite ore. This results in the generation of large quantities of red mud, which contains 4-15% alkali. The accumulation of this red mud poses significant environmental pollution concerns. Mention exactly where in the revised manuscript this change can be found in line 69.

 

3. 68. reaching = leaching. typo.  

Thank you for pointing it out, which will help us in our future work and we will pay more attention to the details in this area in our future study. We have changed the reaching to leaching. Mention exactly where in the revised manuscript this change can be found in line 96.

 

4. 107-108. It is not clear why did you come to this conclusion. It has not been shown; you should prove it.

Thank you for pointing it out. We agree with this comment. It can be seen from Fig.1 that the main component of the montmorillonite raw material is the phase Na_0.3(Al, Mg)₂Si₄O₁₀(OH)₂·nH₂O (montmorillonite). Comparing the theoretical chemical formula of montmorillonite (Al₂O₃·4SiO₂·nH₂O), it can be seen that some cations such as Na⁺ and Mg²⁺ appear in the interlayers of montmorillonite. Meanwhile, we have included image of montmorillonite raw material (Fig.3) as supplementary material, it can be observed that the irregular structure of montmorillonite is primarily enriched with Al, Si, O, Mg, and trace amounts of Na and Ca. The elemental analysis results show a certain consistency with the previous XRD analysis, indicating that montmorillonite contains cations such as Na⁺ and Mg²⁺. Mention exactly where in the revised manuscript this change can be found in line 137 and 156.

 

5. 224, 231, 266. It is better to write the used experimental conditions in the figure captions.

Thank you for pointing it out. We agree with the comment. However, since the font in the image listing the experimental conditions is too small to be clearly visible, we have included the experimental conditions in the caption below the figure. Mention exactly where in the revised manuscript this change can be found in Figure 4, Figure 5, Figure 6.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Agree to publish

Comments on the Quality of English Language

 English writing is fine

Reviewer 2 Report

Comments and Suggestions for Authors

The article has been carefully revised and can be accepted directly.

Comments on the Quality of English Language

After careful revision, the English expression of the article has reached a publishable level.