Phase Transformation and Si/Al Leaching Behavior of High-Silica–Alumina Coal Gangue Activated by Sodium-Based Additives
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsReviewer Comments on Manuscript ID: [minerals-3795804]
Title: Phase Transformation and Si/Al Leaching Behavior of High-Silica-Alumina Coal Gangue Activated by Sodium-Based Additives
General Evaluation
This manuscript presents a comprehensive study on the activation, phase transformation, and selective leaching behavior of high-silica-alumina coal gangue using sodium-based additives. The authors integrate thermochemical treatment, leaching kinetics, and advanced characterization techniques (XRD, SEM, FTIR, ICP-MS) to elucidate the mineralogical evolution and elemental migration mechanisms. The proposed two-step extraction strategy—alkaline activation followed by graded leaching—is scientifically sound and holds great potential for improving the utilization of coal gangue, a major industrial solid waste in China.
The manuscript is generally well-structured, the experimental design is robust, and the conclusions are supported by experimental data. The use of response surface methodology (RSM) for process optimization enhances the technical depth of the study. The insights into the phase transformation pathway—from kaolinite to nepheline and sodium silicate, and eventually to glassy sodium aluminate—are particularly valuable for guiding future work in this area.
Major Comments
1. Scientific Merit and Novelty
The proposed stepwise alkaline activation–selective leaching process is novel and clearly positioned within the context of recent literature. However, the authors are encouraged to further highlight the comparative advantages of their method in terms of energy efficiency and product purity, particularly in relation to traditional techniques such as the Bayer process and acid roasting (e.g., see Ref. [28]).
2. Language and Clarity
While the manuscript is understandable overall, there are a number of grammatical and stylistic issues throughout the text. Examples include:
“the mineral phase are...” → should be “the mineral phase is...”
“it make use of...” → should be “it makes use of...”
A thorough proofreading or professional language editing is strongly recommended to improve fluency and readability.
3. Environmental Considerations
In light of growing attention to green processing, the environmental impacts of the proposed method should be briefly discussed. For example, how are acid waste streams handled? Is the CO₂ emission from Na₂CO₃ decomposition significant? A short paragraph in the conclusion or discussion would help contextualize the sustainability of the approach.
Minor Comments
Abstract
Revise awkward phrasing such as “...goes through the decomposition of silica-aluminate...” → consider “...involves decomposition of the aluminosilicate framework...”.
Table 2.1
Please indicate the units for both elemental composition and proximate analysis (e.g., wt.% for Si, Al, ash, volatile matter, etc.).
Materials and Methods
Section 2.2.1:
Please clarify the following experimental details:
Were the samples heated in open or covered crucibles? This affects Na₂CO₃ volatilization.
Was the activation conducted in ambient air or under an inert atmosphere?
Section 2.3:
Please specify the XRD scan conditions, including:
Radiation source (e.g., Cu Kα),2θ scan range,Step size and scanning rate.
Comments on the Discussion Section
The role of activation temperature is emphasized as dominant, but the discussion could be further enriched by explaining why higher temperatures (>850 °C) result in densification and hinder leaching beyond just phase formation.
Conclusion
The conclusions are well-summarized. Consider adding a brief outlook, such as:
“Future studies may focus on process scale-up and the recovery of additional valuable elements (e.g., Fe, Ti) to realize multi-element resource utilization from coal gangue.”
Recommendation:
Minor Revision
Summary:
This is a solid and meaningful contribution to the field of coal gangue valorization and resource-efficient mineral processing. The study is technically sound, and the findings are novel and practically relevant. With minor revisions related to language, data presentation, and a few clarifications in experimental details, the manuscript will be suitable for publication in Minerals.
Comments for author File: 
Comments.pdf
Author Response
Response to Reviewer 1 Comments
- Summary
We sincerely thank Reviewer 1 for the thorough evaluation of our manuscript entitled “Phase Transformation and Si/Al Leaching Behavior of High-Silica-Alumina Coal Gangue Activated by Sodium-Based Additives”(Manuscript ID: minerals-3795804). We are very grateful for the constructive comments, which have significantly improved the quality of the revised manuscript. Below we provide a detailed point-by-point response. All changes have been highlighted in the revised version.
- 2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: Scientific Merit and Novelty
The proposed stepwise alkaline activation–selective leaching process is novel and clearly positioned within the context of recent literature. However, the authors are encouraged to further highlight the comparative advantages of their method in terms of energy efficiency and product purity, particularly in relation to traditional techniques such as the Bayer process and acid roasting (e.g., see Ref. [28]).
Response 1: Thank you for pointing this out. We agree with the comment. Therefore, we added an explicit, side-by-side comparison with the Bayer process and acid roasting, and we highlighted the advantages of our stepwise route in energy/time input, by-product control, and product purity.
Introduction — explicit comparison with Bayer and acid roasting
Where changed: p. 3, lines 133–142.
What we added (key points): we now state that the Bayer process requires long digestion at high temperature/pressure and generates by-products that can lower alumina quality; acid roasting causes serious equipment corrosion and often induces silica-gel formation that hinders leaching. In contrast, our process requires only 30 min activation at 800 °C under ambient pressure, transforms aluminosilicates into water-soluble Na₂SiO₃ and acid-leachable nepheline, avoids strong-acid treatment, reduces unwanted by-products, and improves both efficiency and product purity (with Ref. [28] cited).
Inserted text (excerpt): “…the route proposed here shows clear advantages… By contrast, our process only needs 30 minutes of activation at 800 °C under normal pressure, transforming aluminosilicate into soluble sodium silicate and acid-leachable nepheline. This stepwise strategy avoids strong acid treatment, reduces unwanted by-products, and improves both efficiency and product purity.”
Results/Discussion (bridge to mechanism),
Where changed: p. 13, lines 462–471 – added a results-based paragraph that quantifies the advantages (rate, selectivity, Al leaching ratio, purity).
What we added (key points): These revisions directly explicitly contrasting our route with Bayer digestion and acid roasting, and by linking the observed performance metrics (conversion time, >80% Al leaching, fewer secondary phases, higher purity) to the mechanistic basis of the process.These points underscore the resource-efficiency and product-quality advantages of our route over Bayer or acid-roasting schemes.
Inserted text (excerpt): Based on these results, it is evident that the sodium-assisted activation route decisively influences subsequent leaching. Unlike conventional Bayer digestion or acid roasting, where incomplete conversion and amorphous silica residues restrict Al recovery, this method enabled a rapid and selective transformation, achieving complete activation within 30 min at 800 ℃ with direct conversion to nepheline. This unique pathway not only facilitated higher Al leaching ratios (>80%) and minimized secondary phases, but also enhanced energy efficiency and ensured higher product purity compared with traditional methods. To further elucidate the origin of these advantages, a mechanistic study of the activation reaction was subsequently conducted.
Comments 2: Language and Clarity
While the manuscript is understandable overall, there are a number of grammatical and stylistic issues throughout the text. Examples include:
“the mineral phase are...” → should be “the mineral phase is...”
“it make use of...” → should be “it makes use of...”
A thorough proofreading or professional language editing is strongly recommended to improve fluency and readability.
Response 2: Thank you very much for this helpful observation. We carefully re-edited the entire manuscript line-by-line to correct grammar, tense, agreement, word choice, and style, and to improve flow and readability. In brief, we standardized: (i) subject–verb agreement and singular/plural forms, (ii) tense usage (past tense for methods/results; present tense for general statements), (iii) articles and prepositions, (iv) removal of redundancy and run-on sentences, (v) consistent hyphenation, and (vi) uniform chemical notation/abbreviations. Below are representative, line-located examples in the revised manuscript (edits are shown in red in the tracked-changes file):
Updated text in the manuscript (examples):
Abstract (p.1, lines 30–34): “…which gradually encapsulates the sodium silicate. This encapsulation restricts dissolution pathways, thereby leading to system densification.”
Introduction (p.3, lines68–69). We removed repetition and improved flow:
“This entails high energy consumption.” (replacing “And the energy consumption is huge.”). Subject verb agreement / duplication.
Introduction (p.3, lines 90–92). We corrected agreement and tightened the sentence:
“These mineral phases are highly stable and often intergrown, which makes direct utilization difficult.”
Consistency of terminology and hyphenation
Throughout, terms such as “high-temperature,” “water-leaching,” and “acid-leachable” were standardized; chemical symbols and subscripts were unified (e.g., Na₂CO₃, Na₂SiO₃).
In addition to these examples, extensive small edits (singular/plural, tense, articles, punctuation) were made across the manuscript; all such edits appear in red in the tracked-changes file. We believe these revisions significantly enhance fluency and readability.
We appreciate the reviewer’s advice; the manuscript has been thoroughly proofread and linguistically polished in response.
Comments 3: Environmental Considerations
In light of growing attention to green processing, the environmental impacts of the proposed method should be briefly discussed. For example, how are acid waste streams handled? Is the CO₂ emission from Na₂CO₃ decomposition significant? A short paragraph in the conclusion or discussion would help contextualize the sustainability of the approach.
Response 3: Thank you for raising this important point. We agree with the comment. Therefore, we added a concise paragraph that discusses acid-stream handling, CO₂ emissions from Na₂CO₃ decomposition, and the overall sustainability of the proposed route.
Where changed: Discussion/Practical implications, p. 15, paragraph 3, lines 531–545 (added text is marked in red in the revised manuscript).
Updated text:“Besides the phase transformation and product recovery, the environmental impact of this process also needs attention. First, no strong-acid roasting is involved. Acid is only used in the leaching step, and the spent HCl solution can be neutralized with the alkaline liquor generated from dissolved sodium silicate. After neutralization and clarification, the liquid can be reused, while the solid residue is mainly high-purity silica (>95%), which can serve as a by-product. Second, some CO₂ is released when Na₂CO₃ reacts with silicates. Under the chosen conditions (800 °C, 30 min, gangue:Na₂CO₃ = 1:0.8), both the carbonate dosage and holding time are reduced, so the CO₂ emission is lower than that of conventional alkali roasting. The off-gas can be handled with common kiln treatments such as dust removal and alkaline scrubbing, and it can also be connected with CO₂ capture systems at scale. Third, the aluminum-rich filtrate (Al³⁺/Cl⁻) can be further concentrated to produce valuable Al salts such as AlCl₃, which reduces acidic wastewater. Taken together, by avoiding prolonged high-temperature digestion and strong-acid roasting, and by allowing neutralization and reuse of process streams, the proposed stepwise route improves resource efficiency while lessening typical environmental impacts.”
Overall, explains that acid is used only in leaching and the spent HCl is neutralized with activation liquor and recycled, yielding high-purity silica as a by-product; (ii) clarifies that CO₂ from Na₂CO₃ decomposition is limited by our short residence time and lower carbonate dosage and can be handled by standard off-gas controls with optional CO₂ capture; and (iii) notes valorization of the Al-rich filtrate to reduce acidic wastewater. These measures ensure that emissions and liquid–solid residues are controllable and recyclable.
Comments 4: Abstract:
Revise awkward phrasing such as “...goes through the decomposition of silica-aluminate...” → consider “...involves decomposition of the aluminosilicate framework...”.
Response 4: Thank you for the suggestion—we agree. We revised the awkward phrasing in the Abstract to use a clearer, field-standard expression.
Where changed: Abstract, p. 1, first paragraph，lines 26–27 (edits shown in red in the tracked file).
Updated text:
“…the activation process involves the decomposition of the aluminosilicate framework, producing soluble sodium silicate and acid-leachable nepheline…”
This replaces the previous wording (“…goes through the decomposition of silica-aluminate…”) and improves precision and readability.
Comments 5: Table 2.1
Please indicate the units for both elemental composition and proximate analysis (e.g., wt.% for Si, Al, ash, volatile matter, etc.).
Response 5: Thank you for this careful suggestion. We agree that the units should be stated explicitly in Table 2.1. We have revised the table caption and column headers to include units and ensured consistent notation across both the elemental composition and the proximate analysis.
What we changed: Table caption now reads “Table 2.1. Elemental composition (wt.%) and proximate analysis (wt.%) of raw coal gangue.”
In the elemental block, the header “Content” is changed to “Content (wt.%)”.
Where to find it in the revised manuscript:
Section 2.1 Materials, Table 2.1 (page with line165–166 for the caption). “Table 2.1. Elemental composition (wt.%) and proximate analysis (wt.%) of raw coal gangue.”
Comments 6: Materials and Methods
Section 2.2.1:
Please clarify the following experimental details:
Were the samples heated in open or covered crucibles? This affects Na₂CO₃ volatilization.
Was the activation conducted in ambient air or under an inert atmosphere?
Response 6: Thank you for this practical suggestion. We agree that the heating configuration and atmosphere affect Na₂CO₃ volatilization and reaction pathways. Therefore, we have added these experimental details to Section 2.2.1.
What we changed & where: Covered vs. open crucibles. We now specify that the mixtures were placed into covered alumina crucibles to minimize volatilization of Na₂CO₃ (Revised MS, p. 4, lines 171–173).
Atmosphere. We also state that activation was conducted in a muffle furnace under ambient air (Revised MS, p. 5, line 175–176).
Updated text in the manuscript:
“The mixtures were placed into covered alumina crucibles to minimize volatilization of Na₂CO₃ and were heated in a muffle furnace under ambient air.” (Section 2.2.1)
These clarifications document the measures taken to suppress alkali loss and reproduce the intended reaction environment, improving the transparency and reproducibility of the procedure.
Comments 7: Section 2.3
Please specify the XRD scan conditions, including:
Radiation source (e.g., Cu Kα),2θ scan range, Step size and scanning rate.
Response 7: Thank you for this helpful suggestion. We agree and have added the full XRD scan conditions to Materials and Methods, §2.3. The revision appears on p. 6, paragraph 1, lines 209-215 where the XRD entry now specifies the radiation source, 2θ range, step size, and scanning rate (see lines corresponding to the XRD method).
Updated text in the manuscript:
“X-ray diffraction (XRD) patterns were collected on a Rigaku SmartLab SE diffractometer using Cu Kα radiation (λ = 1.5406 Å, 40 kV, 40 mA). Scans were recorded over 10–90° (2θ) with a step size of 0.02° and a scan rate of 5° min⁻¹; the diffraction data were used to identify phases and analyze mineral transformation during activation.”
These details now meet the reviewer’s request.
Comments 8: Comments on the Discussion Section
The role of activation temperature is emphasized as dominant, but the discussion could be further enriched by explaining why higher temperatures (>850 °C) result in densification and hinder leaching beyond just phase formation.
Response 8: Thank you for this insightful suggestion. We agree. We expanded the Discussion/Mechanism to explain the microstructural and thermodynamic reasons why temperatures above 850 °C densify the solids and reduce leaching beyond simple phase formation.
Where changed: Discussion / Mechanism study, p. 14, last paragraph, lines 497–507 — wording clarified.
New integrative paragraph added on p. 15, lines 510–520 (immediately after Table 3.3), explicitly linking liquid-phase sintering, glass formation, and leaching kinetics. (All new text is marked in red in the revised manuscript.)
Updated text (excerpt):
“At ≥ 850 °C, NaAlO₂—with a low melting point—partially melts and forms a viscous liquid that encapsulates Na₂SiO₃, promoting liquid-phase sintering. The resulting glassy sodium-aluminate/silicate film closes surface pores, reduces open porosity and specific surface area, and produces a dense microcrystalline–glassy shell. Consistently, SEM shows smooth, compact particle surfaces with blocked pores, while XRD shows diminished crystallinity and a broad amorphous hump. This densification introduces diffusion-limited leaching: longer diffusion paths and a thicker boundary layer impede the contact between lixiviant and active phases, thereby lowering the dissolution rates above 850 °C.
In addition, the ΔG° data in Table 3.3 indicate that the nepheline-to-Na₂SiO₃/NaAlO₂ conversion becomes only weakly favorable near 800–850 °C, so excessive temperature does not provide a thermodynamic benefit but accelerates sintering and encapsulation, which explains the performance drop at very high temperatures.”
These additions clarify the physical cause (liquid-phase sintering and glassy encapsulation) and its kinetic consequence (diffusion-limited leaching), thereby enriching the discussion as requested.
Comments 9: Conclusion
The conclusions are well-summarized. Consider adding a brief outlook, such as:
“Future studies may focus on process scale-up and the recovery of additional valuable elements (e.g., Fe, Ti) to realize multi-element resource utilization from coal gangue.”
Response 9: Thank you for this constructive suggestion. We agree that a brief outlook will strengthen the paper. Therefore, we have added a forward-looking statement to the Conclusion section.
What we changed & where: Conclusion, p. 16, paragraph 2, lines ~581–586 (marked in red in the revised manuscript).
Updated text in the manuscript: Together with the low-energy and low-emission features of this route, these results also demonstrate its environmental compatibility and suggest a more sustainable pathway for the comprehensive utilization of coal gangue. Future studies may focus on process scale-up and the recovery of additional valuable elements (e.g., Fe, Ti) to realize multi-element resource utilization from coal gangue.”
This addition provides the requested outlook on scale-up and multi-element valorization.
- Additional clarifications
We sincerely thank Reviewer 1 for the encouraging and constructive comments. All points have been addressed with page/line citations, and all changes are highlighted in the tracked‐changes file (red font). We believe the revised manuscript is now clearer and technically stronger, and we remain at the Editor’s and Reviewers’ disposal for any further adjustments.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript proposes the use of sodium carbonate to activate typical high silica aluminum coal gangue. Systematically revealed the mechanism of physical phase transition and ion migration law during the activation process. A graded leaching separation process was proposed for activated products, and effective separation and extraction of silicon and aluminum were achieved using water and hydrochloric acid, respectively. The optimal process parameters were optimized. The research work can provide guidance and assistance for the efficient resource utilization of high silicon aluminum coal gangue. The manuscript has clear ideas, detailed and feasible plans, abundant figures and tables, reasonable analysis, and the obtained data and results can well support the conclusions drawn. However, there are several points in the manuscript that need further improvement:
(1) The response surface should be analyzed in detail in conjunction with experimental results.
(2) The comprehensive experimental results under the optimal leaching conditions should be provided, including the changes in phase and morphology before and after leaching.
(3) The manuscript should double checked to eliminate typos and grammar errors.
Author Response
Response to Reviewer 2 Comments
- Summary
We sincerely thank Reviewer 2 for the thoughtful evaluation of our manuscript entitled “Phase Transformation and Si/Al Leaching Behavior of High-Silica-Alumina Coal Gangue Activated by Sodium-Based Additives” (Manuscript ID: minerals-3795804). We are very grateful for the constructive comments, which have helped us further strengthen the manuscript. Below we provide a detailed, point-by-point response. All corresponding changes have been incorporated in the revised version and highlighted accordingly.
- 2. Point-by-point response to Comments and Suggestions for Authors
Comments 1: The response surface should be analyzed in detail in conjunction with experimental results.
Response 1: Thank you for this constructive suggestion. We agree with the comment. Therefore, we expanded Section 3.1 of the Results and Discussion to provide a more detailed coupling between the response surface methodology (RSM) analysis and the experimental validation. Specifically, we presented the explicit quadratic regression model, residual analysis, and head-to-head comparison between predicted and experimental values, as well as an in-depth interpretation of factor interactions with supporting figures.
Where changed: p. 7–8, lines 254–298 (Section 3.1, Results and Discussion).
What we added (key points):
We introduced the quadratic regression model (Eq. 1) to quantitatively describe the Si extraction response as a function of temperature, time, and Na₂CO₃ ratio.
Added residual probability plot analysis, confirming the high fitting degree (99.997%) and model reliability.
Provided explicit validation of RSM predictions with experimental data (predicted 82.3% vs. experimental 82.1%, relative deviation 0.2%).
Expanded the description of factor interactions (temperature–time, temperature–Na₂CO₃ ratio, and time–Na₂CO₃ ratio) with mechanistic interpretations:
Temperature was identified as the dominant factor controlling phase transformation and Si dissolution.
Excess Na₂CO₃ beyond 1:0.8 reduced efficiency due to product encapsulation and diffusion limitation.
Time extension beyond 30 min showed diminishing improvement, consistent with SEM evidence of stable particle morphology.
Clarified the optimized conditions (800 °C, 30 min, 1:0.8) and confirmed their reproducibility.
Inserted text (excerpt):“…By coupling the RSM model with experimental verification, the predicted Si extraction efficiency (82.3%) closely matched the measured value (82.1%) under the optimal conditions of 800 °C, 30 min, and 1:0.8 ratio, with a relative deviation of only 0.2%. The residual probability plot further confirmed the high reliability of the model (fit degree 99.997%). Interaction surface analyses (Figs. 4–7) revealed that temperature is the dominant factor, while excessive alkali dosage beyond 1:0.8 caused surface encapsulation and hindered ion diffusion, resulting in decreased efficiency. These additions integrate statistical modeling with experimental evidence, providing a solid basis for optimizing activation parameters.”
Comment 2:The comprehensive experimental results under the optimal leaching conditions should be provided, including the changes in phase and morphology before and after leaching.
Response 2: Thank you for this valuable suggestion. We agree with the comment. In the revised manuscript, we added new results and detailed discussion regarding the phase and morphological changes of the samples before and after acid leaching under the optimized conditions. These additions provide clear experimental evidence for the dissolution behavior of Al-bearing phases and the enrichment of Si in the residue.
Where changed: p. 13, lines 438–452 (Section 3.5, Experimental Study of the Acid Leaching of Aluminum; Figure 12).
What we added (key points):
Incorporated new XRD results showing that nepheline (NaAlSiO₄), the main Al-bearing phase, is nearly completely decomposed after leaching, while quartz remains largely unchanged due to its chemical stability.
Added SEM images before and after leaching, revealing that dense and compact particle surfaces became rough and porous after leaching.
Supplemented with EDS analysis, confirming a significant decrease in Al content in the leached residue and enrichment of Si as the dominant element.
These combined evidences demonstrate the selective dissolution of alumina-bearing phases and the efficient release of Al into solution, while simultaneously enriching the residue with stable silica phases.
Inserted text (excerpt):“…As shown in Figure 12, the XRD patterns indicate that nepheline (NaAlSiO₄), the primary alumina-containing phase, is almost completely decomposed after leaching, whereas quartz remains largely unaffected due to its stability. SEM images further reveal that the particle surfaces, initially dense and compact, became rough and porous after leaching, reflecting the removal of aluminum-bearing phases. Corresponding EDS spectra confirm a marked reduction in Al content with Si becoming dominant in the residue. These results collectively demonstrate that acid leaching effectively dissolves nepheline, enabling efficient aluminum release into solution and producing a silica-rich residue.”
Comment 3:The manuscript should be double checked to eliminate typos and grammar errors.
Response: Thank you for this important comment. We fully agree and have carefully re-edited the entire manuscript line by line to correct grammar, tense, agreement, word choice, and style, and to improve overall readability. Below we summarize the categories of edits, along with representative examples (line-located in the revised file).
All corrections are highlighted in the tracked-changes file.
- Subject–Verb Agreement and Singular/Plural Forms
Original: “these mineral phases has a very stable structure…”
Revised: “these mineral phases have very stable structures…”
Original: “the results shows…”
Revised: “the results show…”
- Tense Usage (Past tense for methods/results; Present tense for general statements)
Original (Methods): “The samples are dried in oven at 105 °C…”
Revised: “The samples were dried in an oven at 105 °C…” (Past tense for experimental description).
Original (Results): “Figure 8 shows that kaolinite is decomposed at 750 °C.”
Revised: “Figure 8 showed that kaolinite was decomposed at 750 °C.” (Past tense for observed results).
Original (General statement): “Coal gangue was a solid waste rich in silica and alumina.”
Revised: “Coal gangue is a solid waste rich in silica and alumina.” (Present tense for general knowledge).
- Articles and Prepositions
Original: “in order to determine dissociation of silica-aluminum elements.”
Revised: “in order to determine the dissociation of silica-aluminum elements.”
Original: “reaction carried out under ambient air atmosphere.”
Revised: “reaction was carried out under ambient air.”
- Removal of Redundancy and Run-on Sentences
Original: “And the energy consumption is huge. Moreover, the added value of products is extremely low.”
Revised: “This entails high energy consumption, and the added value of products is extremely low.”
Original: “These mineral phases are highly stable and often intergrown, which makes direct utilization difficult. They are very stable and often embedded in each other, interwoven with each other, which makes it difficult to be utilized directly.”
Revised: “These mineral phases are highly stable and intergrown, which makes direct utilization difficult.”
- Consistent Hyphenation and Word Choice
Original: “high temperature alkali roasting”
Revised: “high-temperature alkali roasting”
Original: “acid leachable nepheline”
Revised: “acid-leachable nepheline”
Original: “water leaching activation products”
Revised: “water-leaching activation products”
- Uniform Chemical Notation/Abbreviations
Original: “Na2SiO3”
Revised: “Na₂SiO₃”
Original: “Si , 、Al elements”
Revised: “Si and Al elements”
- Word Choice and Style Adjustments
Original: “the samples is were heated…”
Revised: “the samples were heated…”
Original: “the particles appears shows significant collapse.”
Revised: “the particles show significant collapse.”
- Typographical and Formatting Errors
Removed duplicated words (e.g., “warming will would promote…” → “warming would promote…”).
Fixed inconsistent spacing around symbols and units (e.g., “800 ℃” standardized throughout).
Inserted text (excerpt):“…We carefully re-edited the entire manuscript line-by-line to correct grammar, tense, agreement, word choice, and style, and to improve flow and readability. In brief, we standardized: (i) subject–verb agreement and singular/plural forms, (ii) tense usage (past tense for methods/results; present tense for general statements), (iii) articles and prepositions, (iv) removal of redundancy and run-on sentences, (v) consistent hyphenation, and (vi) uniform chemical notation/abbreviations. Representative corrections include: ‘the mineral phase are’ → ‘the mineral phase is’; ‘it make use of’ → ‘it makes use of’; and standardization of chemical formulas (Na₂CO₃, Na₂SiO₃). These revisions collectively enhanced the fluency, clarity, and professional quality of the manuscript.”
- Additional clarifications
We sincerely thank Reviewer 2 for the encouraging and constructive comments. All points have been addressed with page/line citations, and all changes are highlighted in the tracked‐changes file (red font). We believe the revised manuscript is now clearer and technically stronger, and we remain at the Editor’s and Reviewers’ disposal for any further adjustments.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsI reviewed the manuscript; please receive findings regarding the manuscript below.
The manuscript provides a comprehensive overview of China’s coal gangue production, its associated environmental impacts, and the challenges related to its current utilization. The discussion effectively highlights the extensive generation of coal gangue and the resulting environmental hazards, underscoring the urgent need for improved resource utilization. Additionally, the integration of policy context relating to China’s “dual carbon” goals and solid waste management regulations is both timely and relevant.
- The manuscript presents robust statistical data on coal and coal gangue production, emphasizing the scale of the issue.
- The inclusion of environmental risks, such as heavy metal pollution and spontaneous combustion, strengthens the rationale for enhanced utilization strategies.
- The review of traditional coal gangue applications including power generation, construction materials, and backfilling is thorough and well-supported by relevant
- The identification of valuable elemental components within coal gangue and their potential industrial applications offers a forward-looking and innovative perspective.
However, the excerpt would benefit from clearer paragraph segmentation. Currently, the text is presented as a dense block, which hampers readability and obscures the distinction between key topics such as environmental impacts, traditional uses, and policy context. It is recommended that the content be organized into thematic sections for example, Overview of
Coal Gangue Production, Environmental Impact, Traditional Utilization Methods and Limitations, and Policy and Future Directions to improve clarity and coherence.
Moreover, the manuscript contains numerous grammatical errors and typographical mistakes throughout the text. Thorough proofreading and language refinement are necessary to enhance readability and overall presentation.
In conclusion, the manuscript is suitable for publication pending the implementation of the recommended corrections.
I have attached additional comments along with the PDF manuscript containing editorial corrections for reference.
Sincerely,
Comments for author File: Comments.pdf
The manuscript contains numerous grammatical errors and typographical mistakes throughout the text. Thorough proofreading and language refinement are necessary to enhance readability and overall presentation
Author Response
Response to Reviewer 3 Comments
- Summary
We sincerely thank you for your thorough review and constructive comments on our manuscript entitled “Phase Transformation and Si/Al Leaching Behavior of High Silica-Alumina Coal Gangue Activated by Sodium-Based Additives.” We carefully revised the manuscript according to your suggestions. Below we provide a detailed point-by-point response. All corresponding revisions have been incorporated in the revised manuscript, and we believe these changes have significantly improved the quality and clarity of the paper.
For clarity, Reviewer 3’s comments 8–13 (corresponding to the constructive suggestions) are renumbered as Comments 1–6 in our response letter. Reviewer 3’s comments 1–7 were positive evaluations, for which we express our sincere appreciation in the general response above.
- 2. Point-by-point response to Comments and Suggestions for Authors
Comments 1:The manuscript would benefit from thorough proofreading and language refinement, as several grammatical and syntactical errors currently hinder readability. For instance:
“And it optimizes the key parameters by using the response surface method” could be revised to: “Key parameters were optimized using the response surface methodology.”
Additionally, the use of conjunctions such as “And” (line 30) and “But” (line 100) at the beginning of sentences should be avoided in formal academic writing.
Response 1: We sincerely thank the reviewer for pointing out the grammatical and stylistic issues. In the revised manuscript, we have thoroughly proofread the entire text and refined the language to ensure clarity and conciseness. Specifically:
The sentence “And it optimizes the key parameters by using the response surface method” has been revised to “Key parameters were optimized using the response surface methodology” (Lines 21-23 in the revised manuscript).
We have carefully checked and removed conjunctions such as “And” and “But” at the beginning of sentences, including those in line 30 and line 100 of the original manuscript.
A general language polishing has been conducted across the manuscript to improve readability and formal academic tone.
Modification in the manuscript (examples):
Line 32-34: Sentence previously beginning with “And” has been revised.
Original:“… resulting in the densification of the system. And the enhancement of the resistance to acid attack leads to ….”
Revised: “… thereby leading to system densification. Moreover, enhanced resistance to acid attack leads to …”
Line 112–113: Sentence previously beginning with “But However” has been revised.
Original: “… use … But However, …”
Revised: “… apply … However, …”
Line 184
Original: “Response surface methodology (RSM) was employed. And the response surface methodology is used to analyze …”
Revised: “Response surface methodology (RSM) was employed to analyze …”
Lines 323–324
Original: “And the characteristic peak of kaolinite can no longer be seen …”
Revised: “The characteristic peak of kaolinite can no longer be seen …”
Line 351
Original: “**And 800 °C these peaks have almost disappeared …”
Revised: “At 800 °C these peaks have almost disappeared …”
Line 548
Original: “And it puts forward a graded leaching and step-by-step extraction process …”
Revised: “It puts forward a graded leaching and step-by-step extraction process …”
Line 627
Original: “The higher the temperature, the faster the diffusion rate will be. And the dissolution rate increases …”
Revised: “The higher the temperature, the faster the diffusion rate will be. The dissolution rate increases …”
Comments 2: The text is dense; methodology, results, and discussion should be more clearly separated.
Response 2: We sincerely thank the reviewer for this valuable suggestion. In the revised manuscript, we have carefully restructured the relevant sections by splitting overly long paragraphs into shorter, focused parts to improve readability and logical flow. The main changes are as follows:
Section 2.1 Materials (Lines 155–168):
The first paragraph now introduces the raw material source and mineralogical composition of the coal gangue.
The second paragraph separately describes the chemical reagents and experimental conditions.
This restructuring avoids overly long paragraphs and makes the section clearer.
Section 2.2.1 Deep activation and silicon extraction experiment (Lines 170–186):
The first paragraph now only covers the pre-treatment steps of coal gangue (crushing, sieving, drying, mixing, and heating).
The following paragraph separately describes the activation conditions and analytical methods (reaction time, temperature, cooling, and characterization techniques such as XRD, FTIR, SEM, and RSM optimization).
This clear separation makes the experimental procedure easier to follow.
Section 3.2 XRD analysis (Lines 315–343, Figure 8):
We now first present the experimental observations of phase transformations under different activation temperatures, followed by a separate paragraph in the Discussion section that interprets the mechanisms (phase transitions and their implications for leaching efficiency).
Section 3.6 Mechanism study (Lines 437–470):
This part has been reorganized into two separate paragraphs. The first focuses on reaction pathways and phase transformations (e.g., sodium carbonate decomposition, nepheline generation, sodium aluminate formation), supported by XRD and SEM evidence. The second emphasizes the practical implications, explaining why the optimal activation temperature should be controlled below 850 °C to ensure efficient silicon and aluminum extraction.
These revisions ensure that methodology, results, and discussion are well connected but clearly separated, significantly improving the logical flow and accessibility of the manuscript.
Comment 3: Scientific Terminology
Certain terms require clarification to improve scientific accuracy. For example:
“Ion migration law” may be more precisely expressed as “ion migration behavior” or “mechanism of ion migration.”
The phrase “gradually encapsulated sodium silicate resulting in the densification of the system” would benefit from further clarification to enhance precision and clarity.
Response: We sincerely appreciate the reviewer’s careful attention to terminology, which helps to improve the scientific accuracy of our manuscript. Following the suggestion, we have revised the expressions as follows:
Lines 17–18 (Abstract):
The term “ion migration law” has been replaced with “ion migration behavior” throughout the manuscript to provide a more precise and widely accepted expression.
Lines 31–33 (Abstract):
The original phrase “gradually encapsulated sodium silicate resulting in the densification of the system” has been rewritten as:
“...., which gradually encapsulates the sodium silicate. which gradually encapsulates the sodium silicate. This encapsulation restricts dissolution pathways, thereby leading to system densification.”
This revised wording is clearer and more precise in describing the physical changes observed.
Comment 4: “The paper addresses the formation of Na₂SiO₃ and NaAlSiO₄; however, it does not provide sufficient thermodynamic or kinetic context for these transformations. The inclusion of pertinent reaction equations and thermodynamic data would improve the scientific rigor and depth of the study.”
Response:
We thank the reviewer for this valuable suggestion. In the revised manuscript we have added the pertinent reaction equations and quantitative thermodynamic analysis to substantiate the proposed mechanism.
Where revised in the manuscript (3.6 Mechanistic Study ofThe Activation Reaction, lines 472-519):
Balanced reactions have been explicitly added to the revised manuscript. The key reactions involved in kaolinite/quartz activation and subsequent transformations with sodium carbonate.In addition, we have included thermodynamic calculations (Table 3.3). Text: A new explanatory paragraph has been added immediately following Table 3.3, linking ΔG°(T) to the mechanistic pathway and microstructural evidence.
Standard Gibbs free energy changes, ΔG°(T), were computed at 700, 750, 800, and 850 °C using HSC Chemistry 9.5 under standard-state conditions (1 bar). The following phases were considered: quartz (SiO₂, s), kaolinite (represented as Al₂O₃·2SiO₂·2H₂O, s), nepheline (NaAlSiO₄, s), sodium carbonate (Na₂CO₃, s, anhydrous), sodium silicate (Na₂SiO₃, s, anhydrous), sodium aluminate (NaAlO₂, s), CO₂(g), and H₂O(g).
The key findings are as follows:
Kaolinite + Na₂CO₃ and quartz + Na₂CO₃ reactions are thermodynamically favorable throughout the roasting window (e.g., at 800 °C, ΔG° = −329.7 and −58.4 kJ mol⁻¹, respectively), confirming the ready formation of NaAlSiO₄ and Na₂SiO₃ as Na₂CO₃ begins to decompose near ~800 °C.
Nepheline + Na₂CO₃ → Na₂SiO₃ + NaAlO₂ + CO₂ is less favorable at 700–800 °C , but becomes slightly exergonic at 850 °C . This explains why the conversion occurs only near the upper temperature limit and, together with the low melting point of NaAlO₂, accounts for the observed encapsulation/densification that decreases leachability above ~850 °C.
These thermodynamic results are consistent with the proposed reaction pathways (Figs. 13) and are corroborated by the XRD and SEM observations.
Comment 5: The final sentence of the abstract would benefit from a more concise and direct summary of the principal technical contribution. For example: “This study elucidates the mineral phase reconstruction and element migration mechanisms involved in sodium-based activation and presents a viable approach for the high-value utilization of coal gangue.” Such a revision would enhance the clarity and overall impact of the conclusion.
Response:
Thank you for the helpful suggestion. We have revised the last sentence of the Abstract to be more concise and impactful. The new closing sentence reads:
“This study elucidates the mineral phase-reconstruction and element-migration mechanisms involved in sodium-based activation and presents a viable approach for the high-value utilization of coal gangue.”
This change highlights the principal technical contribution and practical significance, improving the clarity and overall impact of the abstract.
Comment 6: The study is technically sound and contributes valuable insights; however, revisions are required. Perform thorough proofreading and copyediting to correct language, typographical, and formatting issues. In addition:
Reorganize the text into clear subsections for improved readability.
Clarify ambiguous terms and ensure technical accuracy.
Expand on current extraction techniques’ limitations and potential novel approaches to enrich the manuscript’s scientific contribution.
Response:
We sincerely appreciate the reviewer’s constructive summary and helpful recommendations. In response, we have made the following revisions throughout the manuscript:
Proofreading and Language Editing:
…We carefully re-edited the entire manuscript line-by-line to correct grammar, tense, agreement, word choice, and style, and to improve flow and readability. In brief, we standardized:
- subject–verb agreement and singular/plural forms,
- tense usage (past tense for methods/results; present tense for general statements), (iii) articles and prepositions,
- removal of redundancy and run-on sentences,
- consistent hyphenation, and
- uniform chemical notation/abbreviations. These revisions collectively enhanced the fluency, clarity, and professional quality of the manuscript.”
(For examples: Replaced sentence-initial “And/But” with formal transitions (“Moreover/However”).
Standardized symbols, SI units, subscripts/superscripts, and significant figures; unified chemical notation (e.g., Na₂SiO₃, NaAlO₂).
Harmonized figure/table captions and cross-references; corrected minor typographical and formatting issues.
2.Improved Structure and Readability:
Section 2.1 Materials: split into two paragraphs (raw material & mineralogy; reagents & experimental conditions).
Section 2.2.1 Experiments: split into pre-treatment steps vs activation conditions & analytical methods (XRD/FTIR/SEM, RSM)
Section 3.6 Mechanism study: restructured into two paragraphs: (i) reaction pathways/phase transformations; (ii) practical implications (optimum < 850 °C, effects on leaching).
Long paragraphs were segmented across Results (e.g., XRD discussion) to improve readability (see track changes).
3.Clarification of Ambiguous Terms:
Ambiguous phrases and technical terms have been carefully revised for precision.
For example, “ion migration law” → “ion migration behavior/mechanism”.
Clarified the phrase about densification: “molten NaAlO₂ gradually encapsulates Na₂SiO₃, restricting dissolution pathways and densifying the system.”
4.Context on existing techniques and novelty (added text).
Expanded Discussion on Extraction Techniques:
We added a concise paragraph in the Introduction summarizing limitations of conventional routes (direct acid leaching, Bayer process, alkali fusion–leach, chlorination/volatilization): low selectivity for Si/Al, high reagent/energy consumption (>900 °C), saline effluents, and slow kinetics due to the stable Si–O–Al framework.
We then position the sodium-based activation with a controlled window (< 850 °C) as a mechanistically guided alternative that reconstructs the aluminosilicate network to form reactive Na₂SiO₃/NaAlO₂ intermediates, enabling sequential water/acid leaching under milder conditions. RSM-optimized conditions deliver high dissolution of silica (82.1%) and alumina (92.36%), underscoring the practical value of the approach.
Where revised in the manuscript: Abstract (closing sentence), Sections 2.1, 2.2.1, 3.2, 3.6, Introduction (new paragraph on limitations & novelty), captions, and the overall formatting (see track changes).
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe author revised the manuscript carefully according to all my comments, so it can be accepted as it is.
