Leaching of Ca-Rich Slags Produced from Reductive Smelting of Bauxite Residue with Na₂CO₃ Solutions for Alumina Extraction: Lab and Pilot Scale Experiments

Round 1

Reviewer 1 Report

Manuscript ID: minerals-1281377
Title: Alumina recovery from Ca-rich Bauxite Residue slags: Lab- and Pilot scale experiments
Authors: Michail Vafeias et al.

This article needs major revision. It is necessary to substantially supplement the Introduction section by adding new sources. All tables in the article are not in the Minerals style. The article needs to improved the English language. It is necessary to add the conclusions of the article and answer the following specific questions:

Title: Authors must write the methods of bauxite residue treatment: reductive smelting and Na₂CO₃ leaching.

Line 60-65. Don't use so many reference to one sentence. List the references specifically according to the methods and materials for bauxite residue utilization. There is no need to use so much self-citation on Dr. Panias's et al. articles. Use the most recent articles (not conference proceedings) from 2019-2021. Focus on works from Belgium (Riverra R.M. et al.) , Greece (Ochsenkuehn-Petropoulou M. et al.), Russia (Valeev D. and Zinoveev D. et al.), Germany (Friedrich B. et al.).

Table 1. This table does not write in Minerals style (https://www.mdpi.com/journal/minerals/instructions)
The slag composition should be presented in oxides. The amount of oxides should be 100 wt. %. What kind of iron, does it metallic iron or wustite (FeO)? The iron phase must be indicated both in the XRD analysis (Figure 1) and in the table of chemical composition.
Use figure to show "Particle Size Distribution Analysis"

Section 2.1.2. Why authors used Parr reactor system if the leaching were realized in atmospheric pressure?

Line 187. Don't use  "extraction yields"  use "extraction degree". Yield is the another parameter that characterizes the amount of solid residue after leaching.

Section 2.2. Authors must write the exact smelting temperature and duration of smelting. How was the slag cooling? What is the chemical composition of slag?

Table 3. Use the same concentration: g/L or mg/L.

Figure 3. Using a column chart and add the error bars.

Section 3.1.2. Add chemical equations of reaction between slag and Na₂CO₃.

Figure 6. Authors must combine these XRD on one figure, so that readers can clearly see how the intensity of the phases changes depending on the duration of leaching.

Line 437. Why so big difference of Al extraction degree (72% and 64%). This is a very big difference.

Line 460. Don't use this words. This is scientific article, not popular science article.

Section 4. The authors make a description of the process based only on the data of XRD and chemical analysis. Why is the mechanism of the leaching process not confirmed by micrographs using SEM method?

Table 4. Add yield of grey mud after leaching.

Authors should add the section 5: Conclusions

Technical errors:
English level must be improved, change the following words: originating (line 29); repository (line 38); engineered (line 50); adequate ( line 139).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this manuscript, the authors examined the possibility of Bauxite Residue utilization as a precursor for aluminum extraction. The focus of the proposed study is the optimization of the laboratory scale and pilot scale leaching process. Since similar studies have already been done several times, authors should modify their manuscript to make it more suitable for publication. My specific comments are given below:

1. In the introduction segment, in the section from line 57 to line 65, the authors should supplement the text with the most significant results of bauxite leaching from previous studies.
2. In section 2.1.1. please provide information whether a fraction of 0.3 mm was used or a fraction below?
3. Section 2.1.2 should be before 2.1.1 or incorporated with.
4. Can the authors compare results from Table 1 with previous data and compare possible applications?
5. In lines, 500-501 authors concluded that "Al dissolution in alkaline solutions is in general increased with increased temperatures". Can they provide explanations?
6. Since the authors examined the influence of time on aluminum leaching, please process the results with kinetic models.
7. In figure 6, the authors should place all three spectra on the same axes, so that changes in peak intensity can be clearly seen.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper is devoted to the actual topic of processing the Bauxite Residue of the Bayer process. Interesting results are shown regarding the pilot tests, but I cannot recommend the article for publication in the present form due to the serious shortcomings indicated below:

116 EAF occurs for the first time in the text, please specify.

169 The sum of the items listed in the table is 52.22%, what is the rest? Even if you add oxygen to the specified phases, it will still not be enough. For example, oxygen in mayenite is only 38%, and aluminium in it is almost 27%, and you specify it as the main phase, even in tricalcium silicate aluminium is twice as much as according to your chemical analysis. The composition clearly indicates a large amount of calcium excess, although the pure phase of lime or limestone you have not shown.

200 If covered with a film, then it is not external, but internal diffusion and mixing and concentration will not have a significant role, as well as temperature. As was shown in your work below.

235 "Paste" is better to replace with "residue". 265 Why are different phases selected for X-ray spectrum of pilot samples and laboratory samples?

269 lt?

286 There must be an error here, Figure 3.

293 Why won't they be mixed? In the industry, it is always customary to return the washing water to circulation.

Fig. 6b Here the peak is calcite, and in Fig. 2-this is mayenite at 29 deg.

411 In fact, there are two parallel processes going on here, as indicated in your discussion section, a katoite is beginning to form. And leaching efficiency of aluminium from katoite at this temperature is very low. If the temperature is raised to 120 ^oC, then the efficiency should increase, as shown in the work https://doi.org/10.1016/j.hydromet.2020.105536. And this will already indicate the kinetic mode of the process of extracting aluminium from the katoite.

434 Here, the Al content is not 11%, but 17% in contrast to the laboratory sample, calcium is 26%, silicon is 6.11%, and the total amount is 90.21% (it is also not clear what the remaining 10% is and why the iron content in the solid residues has doubled?). Such a strong difference in composition can lead to such a difference in the results obtained, which is shown in your discussion in Section 4.2. The question arises about the adequacy of the comparison. In my opinion, in order to make comparisons, you should have leached in the laboratory the same sample that was obtained in the pilot conditions.

473-483 These conclusions are far from new and have been repeatedly shown in works on the calcification-carbonation process, references to which are omitted in your work.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have made profound changes. The figures were corrected, a new section was added - conclusions. The authors have added many links to research on red mud recycling. The English language has been greatly improved.
The article can be accepted in Minerals in presented form.

Author Response

Dear Sir/Madam, 

Thank you very much for helping review this manuscript.

According to another reviewer's demands we proceeded in adding:

1. Restructuring the abstract to better describe the manuscript
2. A chemical analysis (Table 7) of a residue from the laboratory scale experiments
3. A new section (Section 4.3) to adress the Si co-dissolution observed in the process.

Moreover we proceeded in minor spelling corrections

Yours sincerely, 

Michail Vafeias (Corrsponding Author)

Reviewer 2 Report

Since the authors have corrected their manuscript, I suggest that it be discussed in its current form in the Minerals Journal.

Author Response

Dear Sir/Madam, 

Thank you very much for helping review this manuscript.

According to another reviewer's demands we proceeded in adding:

1. Restructuring the abstract to better describe the manuscript
2. A chemical analysis (Table 7) of a residue from the laboratory scale experiments
3. A new section (Section 4.3) to adress the Si co-dissolution observed in the process.

Moreover we proceeded in minor spelling corrections as you requested in your last review

Yours sincerely, 

Michail Vafeias (Corrsponding Author)

Reviewer 3 Report

The article has been significantly revised and now the connection between laboratory and pilot tests has become more logical. Although I still have a number of comments:

377 It is also necessary to give the chemical composition of the residue obtained under optimal conditions.

498 In the discussion section, the behaviour of silicon under these conditions is completely omitted, which also goes into solution and just leads to the deposition of katoite (silicon is omitted in the composition of the last on line 431), as well as the desilication product (DSP), which also leads to lower aluminum extraction and increased sodium content in the residue (as can be seen from Table 7). The presence of silicon in the solution also requires additional desilication, which will also lead to additional losses of aluminum.

Author Response

Please see the attachment

Author Response File: Author Response.docx