Control of Silica Gel Formation in the Acidic Leaching of Calcium Aluminate Slags with Aqueous HCl for Al Extraction

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is devoted to the investigation of critical parameters of HCl leaching of calcium aluminate slag for Al extraction as well as scientific justification of primary reasons for Si gelation to optimize the process.

The topic is relevant and can be of interest for specialists in aluminothermy, hydrometallurgy and leaching technologies.

The paper contains enough details, appropriate citations, coherent conclusions, and I can only mention a small point to be addressed: the designation “4N-Si” in Fig.1 is not described in the text.

On the whole, I recommend accepting the paper.

Author Response

Response to Reviewer 1

We would like to thank the Reviewer for the prompt review of the manuscript. As a reply to the following comment:

1. The paper contains enough details, appropriate citations, coherent conclusions, and I can only mention a small point to be addressed: the designation “4N-Si” in Fig.1 is not described in the text.

Authors want to thank Reviewer for the comment. The designation “4N-Si” in Fig.1 has been described in the text as 99.99 % (high purity silicon) according to the reviewer’s suggestion.

Finally, we would like to point out that we proceeded in altering the title of the manuscript as per the instructions of the other reviewers.

Reviewer 2 Report

Comments and Suggestions for Authors

This study concerns the extraction of alumina from a CaO-Al₂O₃-SiO₂ slag of tailored composition by HCl leaching. The subject is interesting and worthy of being studied. However, some explanations are unclear and the analysis lacks depth. The detailed comments are listed as follows:

The Introduction section lacks the necessary information to understand the necessity and significance of the research. Authors should highlight the novelty of this work compared to the large literature on this topic, in particular in relevance to the different methods for alumina extraction. The authors can follow the references: 'Extraction of alumina from low-grade kaolin in the presence of lime and NaOH via multi-stage hydrothermal process'; 'Extraction and value-added utilization of alumina from coal fly ash via one-step hydrothermal process followed by carbonation'.

Table 1 defines the chemical components of calcium aluminate slag, it is noted that the total components exceed 100%, why? Similar issues also exist in Table 5.

Please confirm the necessity of Table 3, the experimental conditions applied in this study do not seem complicated.

The purpose of the work performed is to optimize the efficiency of the Al leaching process and, at the same time, avoid SiO₂ gelation of the resulting pregnant leaching solution. However, I am missing the necessary information on alumina leaching ratio; and the in-depth analysis should be included to elucidated the mechanism on SiO₂ gelation and avoidance, is it judged only on the particle size of the amorphous SiO₂ precipitates?

Comments on the Quality of English Language

A number of sentences should be rearranged for clarity.

Author Response

Response to Reviewer 2

We would like to thank the Reviewer for the prompt review of the manuscript and for the fruitful discussion. In reply to the raised issues:

1. The Introduction section lacks the necessary information to understand the necessity and significance of the research. Authors should highlight the novelty of this work compared to the large literature on this topic, in particular in relevance to the different methods for alumina extraction. The authors can follow the references: 'Extraction of alumina from low-grade kaolin in the presence of lime and NaOH via multi-stage hydrothermal process'; 'Extraction and value-added utilization of alumina from coal fly ash via one-step hydrothermal process followed by carbonation'.

We have included a new paragraph in the revised manuscript from line 64 to line 74 and also adjusted the paragraph from line 75 to line 83, including the proposed references. We believe that we adequately position our research in the broad field of alternative alumina extraction technologies and highlight the contribution of this research.

 

2. Table 1 defines the chemical components of calcium aluminate slag, it is noted that the total components exceed 100%, why? Similar issues also exist in Table 5.

The answer to this question was already included in the original manuscript and is retained in the revised manuscript in lines 157 – 159. Regarding Table 1: As it is evident from the mineralogical analysis of the slag (Fig 2) Si is not only included in silicate phases but also metallic Si and SiC are included in the slag. These phases will be oxidized to SiO₂ after the fusion of the sample, resulting in gain on ignition effect, as expected. Consequently, a slightly increased value of SiO₂ is expected (due to the 2 moles of O per mole of Si metal and SiC) and the overall mass balance will reflect this increased value. The same explanation applies in Table 5. All residues, as shown in Figure 5 include metallic Si and SiC as impurities.

 

3. Please confirm the necessity of Table 3, the experimental conditions applied in this study do not seem complicated.

We believe that the experimental conditions should be reported clearly inside the text to avoid any confusion or misunderstanding of the linked diagrams.

 

4. The purpose of the work performed is to optimize the efficiency of the Al leaching process and, at the same time, avoid SiO₂ gelation of the resulting pregnant leaching solution. However, I am missing the necessary information on alumina leaching ratio; and the in-depth analysis should be included to elucidated the mechanism on SiO₂ gelation and avoidance, is it judged only on the particle size of the amorphous SiO₂ precipitates?

There were typographical errors concerning the numbering of Tables. We hope that having corrected them in the revised manuscript, along with same slight rearrangements, will clarify the information presented. Data on Table 4 present the extraction yields of all major metals from the slag and the chemical analysis of the residue under the optimum conditions is shown in Table 6. According to the data of Table 6 only 12.9% of the alumina remains in the residue, leading to a leaching ratio of 100%-12.9%=87.1% . We have included this number in line 246 of the revised manuscript.

Concerning the mechanism of SiO₂ gelation avoidance, in paragraph 4.2., we refer to the work of E.A. Gorrepati et al. (reference 30) and A. Lazaro et al. (reference 31) who have elucidated the SiO₂ precipitation and agglomeration mechanism of SiO₂ particles in acidic solution of pH<2 (below the isoelectric point of SiO₂). As can be seen from Figure 4 of our manuscript (orange colored line) at the optimum conditions selected, i.e. 15%S/L ratio, the pH of the solution is below 2, and therefore below the isoelectric point. Furthermore, the work of E.A. Gorrepati et al. has shown that accelerating effect of SiO₂ agglomeration below the isoelectric point pH value with the addition of CaCl₂ and AlCl₃ salts. Our system, by definition, contains the same salts as they are the main constituents of the calcium aluminate slag we are leaching. We also went on to further clarify these points by editing and simplifying the text contained in lines 292-308. Moreover we added an explanatory paragraph, linking our findings with the literature on the subject (lines 309-315). By adding this paragraph we hope to have clarified the drive to perform the particle size analysis of the residues presented in figure 6. In this way we provide evidence of the agglomeration phenomenon, which leads to a filterable residue at 120 minutes of leaching. We could provide pictures of the evolution of filtration as testimonial, if the reviewer thinks they will add to the clarity of the text.

 

5. A number of sentences should be rearranged for clarity.

Extensive rearrangements have been made throughout the text. Also, a lot of unnecessary repetitions have been deleted. Overall, we hope you will find the revised manuscript to be more comprehensive and concise.

Finally, we would like to point out that we proceeded in altering the title of the manuscript as per the instructions of the other reviewers.

Reviewer 3 Report

Comments and Suggestions for Authors

     The authors presented a research on Al extraction by HCl solution from slags. The submission is pretty reasonably well prepared. However, I have some comments and suggestions about possible important amendments and improvements.

1.     The title of the submission needs to be revised and rearranged. In the present form the title is quite unintelligible. Please, emphasize a core, an essential research problem, an issue that you have taken.

2.     The language of the submission require improvement, particularly regarding syntax errors, and I suggest in this case an assistant of a native speaker or a professional language service.

3.     The Figure 1 is rather illegible, the Figure 3 as well, so there is need to improve the readability of the Figures.

Comments on the Quality of English Language

The language of the submission require minor improvement, particularly regarding syntax errors.

Author Response

Response to Reviewer 3

We would like to thank the Reviewer for the prompt review of the manuscript and for the fruitful discussion. In reply to the raised issues:

1. The title of the submission needs to be revised and rearranged. In the present form the title is quite unintelligible. Please, emphasize a core, an essential research problem, an issue that you have taken.

Thank you for your comment. We have revised the title of the submission to the following:

“Control of silica gel formation in the acidic leaching of calcium aluminate slags with aqueous HCl for Al extraction”.

We believe the new title is more intelligible and descriptive of the work we performed. Moreover, we also revised the abstract to better fit the revised title.

 

2. The language of the submission require improvement, particularly regarding syntax errors, and I suggest in this case an assistant of a native speaker or a professional language service.

The revised manuscript has been scanned for syntax, typographical and spelling errors. Moreover, repeating sentences have been removed and an overall editing of the text has been performed. Overall, we hope you will find the revised manuscript to be more comprehensive and concise.

 

3. The Figure 1 is rather illegible, the Figure 3 as well, so there is need to improve the readability of the Figures.

Figure 1 has been substituted by a higher resolution image.

Figure 3 has been divided into two diagrams emphasizing in different scale of metals extraction: a) 70 – 90 % of Ca, Al, Fe and Me extraction and b) up to 20 % of Si extraction. In this way the readability of the Figures has been strongly improved.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I am satisfied with all the changes made in the manuscript. I believe that the authors made a great effort to include all the suggestions made and, consequently, the quality of the article has improved considerably.

Author Response

Reviewer's comment: "I am satisfied with all the changes made in the manuscript. I believe that the authors made a great effort to include all the suggestions made and, consequently, the quality of the article has improved considerably".

Author's response: Thank you very much for your precious contribution to the improvement of our work.