Removal of Iron, Zinc, and Copper Impurities from Sodium Aluminate After the Bayer Process

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper systematically investigates the removal mechanisms of iron, zinc, and copper impurities from sodium aluminate solutions after the Bayer process, focusing on the influence of seed crystal specific surface area (SSA) and particle size on purification efficiency. The experimental design is rigorous, and the data are well-supported by XRD, SEM, and EDS analyses, providing practical insights for industrial optimization. However, the study lacks sufficient experimental details, contains inconsistencies in figure annotations, relies on outdated references, and offers limited mechanistic analysis. The conclusions would benefit from quantitative data and economic considerations.

 

1. The "Procedure" section omits key operational details such as mixing ratios of seed crystals to sodium aluminate solution, stirring conditions (e.g., speed, homogenization time), temperature control methods, and measures to mitigate evaporation effects on Na₂O concentration.
2. - Figures 3–6 incorrectly label SSA units as "m³/g" instead of "m²/g."

   - The term "αk" (caustic ratio) is mentioned in Figure 3(b) but not defined in captions or text. 

   - Axis labels lack standardized formatting (e.g., "mg/l" should be "mg·L⁻¹"). 

3. The references include obsolete studies (e.g., 1981) and fail to incorporate recent advancements in impurity removal, such as nanoadsorbents or membrane separation technologies.
4. The discussion inadequately quantifies the contributions of isomorphic replacement, surface adsorption, and co-crystallization mechanisms to impurity removal. No models (e.g., adsorption capacity vs. SSA) or kinetic analyses (e.g., Arrhenius plots) are provided to distinguish dominant mechanisms.
5. Statements like "optimal purification time is 90 minutes" lack quantitative validation. The trade-off between Al₂O₃ loss and impurity removal efficiency at SSA=27.66 m²/g is not explicitly quantified or supported by comparative data tables.
6. Formatting inconsistencies:

   - Units vary (e.g., "wt.%" vs. "wt%"). 

   - Reference entries are incomplete (missing DOIs, page numbers). 

   - Grammatical errors (e.g., "is is detected") and repetitive phrasing reduce clarity.

Author Response

Dear Reviewer,

Thank you for your thorough and insightful comments. We have carefully addressed each of your concerns and made the necessary revisions to improve the manuscript. Please find our responses below:

The paper systematically investigates the removal mechanisms of iron, zinc, and copper impurities from sodium aluminate solutions after the Bayer process, focusing on the influence of seed crystal specific surface area (SSA) and particle size on purification efficiency. The experimental design is rigorous, and the data are well-supported by XRD, SEM, and EDS analyses, providing practical insights for industrial optimization. However, the study lacks sufficient experimental details, contains inconsistencies in figure annotations, relies on outdated references, and offers limited mechanistic analysis. The conclusions would benefit from quantitative data and economic considerations.

 The "Procedure" section omits key operational details such as mixing ratios of seed crystals to sodium aluminate solution, stirring conditions (e.g., speed, homogenization time), temperature control methods, and measures to mitigate evaporation effects on Na₂O concentration.

1. - Figures 3–6 incorrectly label SSA units as "m³/g" instead of "m²/g."

Corrected. Thank you.

   - The term "αk" (caustic ratio) is mentioned in Figure 3(b) but not defined in captions or text. 

We have added definition of caustic ratio.

   - Axis labels lack standardized formatting (e.g., "mg/l" should be "mg·L⁻¹"). 

Corrected.

1. The references include obsolete studies (e.g., 1981) and fail to incorporate recent advancements in impurity removal, such as nanoadsorbents or membrane separation technologies.

A few more recent studies on the purification of aluminate solutions have been added.

1. The discussion inadequately quantifies the contributions of isomorphic replacement, surface adsorption, and co-crystallization mechanisms to impurity removal. No models (e.g., adsorption capacity vs. SSA) or kinetic analyses (e.g., Arrhenius plots) are provided to distinguish dominant mechanisms.

As noted, the current study focuses on preliminary observations and trends. However, we fully acknowledge the importance of developing a more rigorous kinetic and mechanistic understanding. Further kinetic analyses and modeling effortsincluding adsorption capacity correlations with SSA and Arrhenius-type evaluations, are currently underway and will be presented in next paper. This subsequent study will incorporate expanded data and modeling to enable more precise differentiation of dominant removal mechanisms and provide predictive capability.

1. Statements like "optimal purification time is 90 minutes" lack quantitative validation. The trade-off between Al₂O₃ loss and impurity removal efficiency at SSA=27.66 m²/g is not explicitly quantified or supported by comparative data tables.

We acknowledge that the claim regarding 90 minutes as the optimal purification time requires more detailed justification. In our study, aluminium hydroxide was used as an adsorbent and it is also used as seed for crystallization, which introduces a balance between impurity removal and potential Al₂O₃ loss due to prolonged contact time. Based on internal calculations and operational experience from Alumina Ltd, we found that around 90 minutes offers an effective compromise between impurity removal while minimizing Al₂O₃ loss from the aluminate solution.

1. Formatting inconsistencies:

   - Units vary (e.g., "wt.%" vs. "wt%"). 

Corrected

   - Reference entries are incomplete (missing DOIs, page numbers). 

   - Grammatical errors (e.g., "is is detected") and repetitive phrasing reduce clarity.

Corrected.

We appreciate your valuable feedback, which has helped us improve the clarity and structure of the manuscript. We hope that the revised version meets your expectations and can be published as soon as possible.

Best regards

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents a study on improving the purification efficiency of sodium aluminate solutions by manipulating the surface area and particle size. The removal of Cu, Fe, and Zn increased significantly with increasing surface area. At the same time, the authors conducted various characterization analyses to determine the percentage of different elements. I would recommend a minor revision before it can be published in matels.

 

1. Figure 8c and Figure 10b seem to be the same. Please explain why putting two same images is necessary.
2. Please provide the origin SEM image for Figure 9.
3. From Figure 3 and 6, why does the trend of Na2O caustic content start to increase with time, with an increasing SSA?
4. Figure 4b and 5b miss the Y2 axis title.
5. Line 360-360 and 366 are redundant.
6. Can the author provide a table of details of Figure 11 and 13 as well?

Author Response

Reviewer 2

Dear Reviewer,

Thank you for your thorough and insightful comments. We have carefully addressed each of your concerns and made the necessary revisions to improve the manuscript. Please find our responses below:

This manuscript presents a study on improving the purification efficiency of sodium aluminate solutions by manipulating the surface area and particle size. The removal of Cu, Fe, and Zn increased significantly with increasing surface area. At the same time, the authors conducted various characterization analyses to determine the percentage of different elements. I would recommend a minor revision before it can be published in matels.

1. Figure 8c and Figure 10b seem to be the same. Please explain why putting two same images is necessary.

Corrected.

1. Please provide the origin SEM image for Figure 9.

Provided

1. From Figure 3 and 6, why does the trend of Na2O caustic content start to increase with time, with an increasing SSA?

We believe that the apparent increase in Na₂O caustic content is not due to a process-related phenomenon but is likely the result of analytical variability. Specifically, we suspect that inconsistencies in laboratory handling or measurement technique may have contributed to the observed deviation. Additionally, we can consider the effect of solution evaporation during the synthesis process. Although measures were taken to minimize evaporation, it is likely that a small amount of water was lost, leading to a slight increase in the concentration of caustic soda.

1. Figure 4b and 5b miss the Y2 axis title.

Corrected.

1. Line 360-360 and 366 are redundant.

Corrected.

1. Can the author provide a table of details of Figure 11 and 13 as well?

Unfortunately, we are unable to provide a detailed table corresponding to Figures 11 and 13, as the EDS analyses were conducted several months ago in a different laboratory. At that time, only the graphical outputs were retained, and the raw numerical data necessary to construct a detailed table are no longer accessible.

We appreciate your valuable feedback, which has helped us improve the clarity and structure of the manuscript. We hope that the revised version meets your expectations and can be published as soon as possible.

Best regards

Reviewer 3 Report

Comments and Suggestions for Authors

Reviewer’s comment

 

1. The authors showed the some charts of the results measured using EDS as Appendix B. From the measurements of EDS, the contents of the elements of the substance can be obtained. It is more important the contents of the elements rather than the charts.

   The authors have to summarized the the contents of the elements in Table.

 

2. The authors have to make graphs using the data shown in Appendix A for readers’ understand. Especially, the degree of the decrease in zinc concentration became the specific surface area. The reviewer have attempted fitting of the data about zinc in Appendix A to the following equation.

   ln(C/C_i) = –kt

Where, k and t represent the the first order kinetic constant and time. C and C_i represent the zinc concentration at t and the initial concentration, respectively. k expressed the degree of the decrease in the zinc concentration. The reviewer plots k vs. SSA on zinc. The graph is below.

 

 

The authors can discuss why the decrease rate of Zn concentration increased abruptly in the range from ca. 12 to 20 m2/g on SSA.

 

Comments for author File: Comments.pdf

Author Response

Reviewer 3

Dear Reviewer,

Thank you for your thorough and insightful comments. We have carefully addressed each of your concerns and made the necessary revisions to improve the manuscript. Please find our responses below:

1. The authors showed the some charts of the results measured using EDS as Appendix B. From the measurements of EDS, the contents of the elements of the substance can be obtained. It is more important the contents of the elements rather than the charts.

The charts included in Appendix B are intended to support the consistency and reliability of the EDS measurements, particularly in demonstrating the trends in impurity removal. While we agree that the elemental content values are more critical, the figures were included to visually complement the quantitative data and highlight the spatial distribution and consistency across samples.

1. The authors have to make graphs using the data shown in Appendix A for readers’ understand. Especially, the degree of the decrease in zinc concentration became the specific surface area. The reviewer have attempted fitting of the data about zinc in Appendix A to the following equation.

   ln(C/C_i) = –kt

Where, k and t represent the the first order kinetic constant and time. C and C_i represent the zinc concentration at t and the initial concentration, respectively. k expressed the degree of the decrease in the zinc concentration. The reviewer plots k vs. SSA on zinc. The graph is below.

 The authors can discuss why the decrease rate of Zn concentration increased abruptly in the range from ca. 12 to 20 m2/g on SSA.

We agree that modeling the relationship between zinc concentration and specific surface area (SSA) is important for understanding the underlying mechanisms of impurity removal. While this manuscript focuses on presenting experimental trends and preliminary observations, we would like to note that a comprehensive kinetic analysis, including model fitting and validation, is planned as part of a follow-up article. That work will include expanded datasets and evaluate various kinetic models to better quantify and predict the behavior of zinc and other impurities in relation to SSA and process conditions.

We appreciate your valuable feedback, which has helped us improve the clarity and structure of the manuscript. We hope that the revised version meets your expectations and can be published as soon as possible.

Best regards

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The reviewer agrees to publish this manuscript in Metal.