Recovery of Rare Earth Elements from Spent NdFeB-Magnets: Separation of Iron through Reductive Smelting of the Oxidized Material (Second Part)

Round 1

Reviewer 1 Report

1. The background and research progress of this study are lacking, and the innovation of this research method is unclear, and it is suggested to supplement.

2. There are many problems in English expression, and it is suggested to be revised and improved.

3. Line 99: For specific DTA/TG results, please add and analyze by the author.

4. Figure 3 is not clear, please readjust.

5. The thermodynamic analysis of Fig. 4 should consider the change of gas phase composition during the reduction process, that is, the volume fraction of CO and CO2.

6. It can be seen from Figure 11 that the diffraction peaks of iron-containing substances are obvious. What is the content of iron-containing substances in slag phase from experiment V3? Please add analysis results.

7. As a whole, the paper studies the separation of total iron from raw materials. Therefore the title is not suitable and needs to be revised.

8. It is recommended to use SEM-EDS or surface scan to clarify the distribution of elements for the product in Figure 6.

Author Response

Firstly, the authors would like to thank the reviewer for the collaboration in the improvement of the work. The manuscript was revised, and corrections were made based on the reviewer’s suggestions. The responses to the comments of the reviewer are listed next and the corrections made are highlighted in the revised manuscript.Ttached you can find our answers

Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript ID: metals-1879512

Title: Recovery of Rare Earth Elements from spent NdFeB-magnets: Reductive Smelting of the oxidized material (Second Part)

Authors: Hanwen Chung et al.

1.     Authors should use the classic version of the table of contents of the article: 1. Introduction, 2. Materials and methods (you can insert a section about previous research into them or move this part to the Introduction). 3. Results. 4. Conclusions.

2.     Authors should make a Materials and Methods section, including sample characterization, description of the smelting experiments, description of thermodynamic modeling, and description of analytic methods.

3.     Line 102. MDPI prefer if authors use the third-person singular, instead of the first-person singular or plural.

4.     Line 160 Authors write about 100 g of sample for each smelting, however in Table 6, these values for V2 and V3 were 60 g. Why is there a mismatch?

5.     Figure 6. Authors must use a) b) c) in the figures title.

6.     Figures 7-10. Change it to the one table with metals and slags chemical compositions. Why didn’t authors analyze the V3 sample?

7.     Figure 11-12. Why didn’t authors analyze the XRD patterns of V1 and V2 samples?

8.     Authors must add a Discussion section. In this section authors can write about the level of results obtained, the possibility of scaling and implementation. The article lacks a discussion of the obtained experimental data.

9.     Why didn’t authors use the SEM-EDX method to show the slag and metal samples?

10. The references list is very small (only 7 sources, 3 self-citations). It is necessary to make a qualitative literature review and add 15-20 new links from 2019-2022.

Author Response

Dear Reviewer, thank you very much for your invested time and valuable comments. Attached in PDF-file you can find our answers.

Author Response File: Author Response.pdf

Reviewer 3 Report

The research has an appropriate scope to describe the basis of a proposal to treat NdFeB magnets through an oxidation-smelting reduction process. Nevertheless, the analysis provided, from the principles of the process to the presentation of the results, is poor. 

The Ellingham diagram is the perfect tool to make an initial rough analysis. If available, and a tool such as the Factsage equilibrium module is used (calculations shown for the metal phase), it should also be prioritized for the slag phase. At least, the ideal solution model should be considered to describe the slag phase if the data is unavailable in the database used. The thermodynamic analysis can stay at a description level, but estimating the effect of activities, RRE oxides in the slag, and Fe in the metallic solution can be depicted and ideally quantified.

Finally is strongly recommended to present an explicit mass balance of the experiments linked with the thermodynamic analysis to validate, even partially, a preliminary set of chemical reactions that represent the whole process. Such you have done only three experiments and reported only two results, a table that summarizes all the information must be presented, including the recovery of Fe in the metal and RREE in the slag.

Other considerations and questions are presented in the pdf file with the manuscript.

I encourage you to complete this interesting work with a more profound analysis.

Comments for author File: Comments.pdf

Author Response

Der Reviewer,

thank you very much for valuabled comments and invested time. Attached I am sending my answers in PDF-file.

Best regards

Srecko

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Line 213 ：98.1, 98.9 and 97.5 should be revised to 98.1%, 98.9 % and 97.5%.

Line 215:  96.9, 97.2 and 95.8 should be revised to 96.9%, 97.2% and 95.8 %.

Author Response

Dear Reviewer,

thank you very much for your continuous support and valuble comments.

According to your suggestions we revised our Text. You can find it in yellow and white color in our umproved version in Line 213 and Line 215.

Line 213 ：98.1, 98.9 and 97.5 should be revised to 98.1%, 98.9 % and 97.5%.

Line 215:  96.9, 97.2 and 95.8 should be revised to 96.9%, 97.2% and 95.8 %.

Reviewer 2 Report

The authors did not correct a number of comments:

1) Authors should use the classic version of the table of contents of the article: 1. Introduction, 2. Materials and methods 3. Results 4. Discussion 5. Conclusions

2) Table 1 does not contain chemical compositions on sample V3 (metal and slag). Although, the authors wrote that they did this analysis. "We have analyzed also V3-sample, and gave information later" (c)

3) The authors write that they cannot do XRD analysis because there is very little sample. But it's not true. Figure 6 shows the samples, and it is quite enough for XRD analysis. Moreover, the authors performed SEM-EDS and ICP-OES analyzes. XRD is a non-destructive method. The sample can then be analyzed by other methods. The absence of XRD analysis significantly reduces the value of the article, since the analysis of the reductive smelting occurs due to the analysis of the metal and slag phases.

Author Response

Dear Reviewer,

thank you very much for your valuable comments. According to your Comments we improved our text.

1) Authors should use the classic version of the table of contents of the article: 1. Introduction, 2. Materials and methods 3. Results 4. Discussion 5. Conclusions

We changed it  and now we have classical version of the table of contents of the article:

I added in Discussion:

The final part of this work is metal winning during molten salt electrolysis using the prepared slag concentrate based on mixed oxides of rare earth elements. These results shall be mentioned in the third part of this study.

2.Table 1 does not contain chemical compositions on sample V3 (metal and slag). Although, the authors wrote that they did this analysis. "We have analyzed also V3-sample, and gave information later" (c)

I am sorry because of this misunderstanding. We made SEM and EDS analysis of the sample V3. Unfortunately ICP-OES analysis is not performed. Therefore we can not put some results about Sample V3 in Table 1. 

3. The authors write that they cannot do XRD analysis because there is very little sample. But it's not true. Figure 6 shows the samples, and it is quite enough for XRD analysis. Moreover, the authors performed SEM-EDS and ICP-OES analyzes. XRD is a non-destructive method. The sample can then be analyzed by other methods. The absence of XRD analysis significantly reduces the value of the article, since the analysis of the reductive smelting occurs due to the analysis of the metal and slag phases.

You have right. From Figure 6 is evident that we had enough samples for XRD-analysis. Firstly, we finished ICP-OES analysis and put it in TAble 1. At the same time we begun to study  the hydrometallurgical  treatment of these samples with nitric acid in order to compare two routes. This is part of new paper and doctoral thesis of Ms. Elif Emil-Kaya from the Istanbul Technical University .  These samples were consumed for this work. You have right that the absence of XRD analysis significantly reduces the value of the article, since the analysis of the reductive smelting occurs due to the analysis of the metal and slag phases, but I am sure that we offered many interesting results for our readers. Thank you for your comprehension in this work.

Round 3

Reviewer 2 Report

My opinion remains the same, without XRD data of slag, the article has little valuable information. As well as data on the chemical composition of sample V3, which were not analyzed.

I understand the concerns of the authors, however, the article should be a completed study.

Let the editor of the Metals decide.