Rare Earth Magnet Recycling and Materialization for a Circular Economy—A Korean Perspective

Round 1

Reviewer 1 Report

This is an interesting review article that highlights the work being done in the Republic of Korea on rare-earth recycling, in particular from Nd/Dy magnets using pyrometallurgy. The review is laid out in a logical manner and provides good insight into the ReMAT process. The alloying and distillation techniques have a sound chemical basis and their application to RE recycling is clear. The commentary on the non-technical aspects, for example, International Standards, provides an interesting viewpoint. There are some changes required to make this review more readable and these are highlighted in the attached document. Along with these, I think the authors should make more of their 'circularity' arguments, which are weak - there is no discussion about materials balances (e.g. Graedel's work) or on life-cycle analysis, and there is limited discussion on the energy aspects of pyrometallurgical processes compared with hydrometallurgy.

Comments for author File: Comments.pdf

Author Response

Thanks for all reviewer’s comments on our manuscript named “Rare earth magnet recycling and materialization for a circular economy”. We have tried to compensate our manuscript considering all comments from reviewers. Revised sentences have been revealed with red color.

Reviewer #1

1. There is some change required to make this review more readable and these are highlighted in the attached document.

Response: We agree with the reviewer. It was concisely reflected in our draft, considering the review article.

2. Make more of their circularity arguments, which are weak- there is no discussion about materials balances(e.g. Graedel’s work) or on life-cycle analysis)

Response: The comments are essential to strengthen our motivation. Therefore, each section of “Introduction” and “Materials flow of REEs in Korea” was rearranged and the material flow analysis has been investigated for resource management in Korea. Thus, the weak circular economy can be quantitatively demonstrated with the material flow chart.

3. There is limited discussion on the energy aspects of pyrometallurgical processes compared with hydrometallurgy.

Response:

In Korea, chemical usage strictly restricted rather than electric consumption because electricity production network system has been well established. There are various electricity production routes, such as the renewable energy, atomic generation as well as high efficiency conventional steam power generation. In the aspect of recycling technological competitiveness against China or Australia that develops REEs mineral with chemical process, the hydrometallurgy ought to be developed. Nevertheless, pyrometallurgy is beneficial in the issues of energy consumption and wastes. That is why we are focusing to ReMAT process. Moreover, although comparing between hydrometallurgical and pyrometallurgical processes are quite meaningful works in the viewpoint of energy consumption, there have been only few reports the related background data, since the detailed numerical data is now on evaluated. Thus, it is limited to quantitively compare between them. We demonstrated the difference between hydrometallurgy and pyrometallurgy in “Introduction” section and considering Korea environments, ReMAT based on pyrometallurgy process is the strong candidate.

Author Response File: Author Response.pdf

Reviewer 2 Report

Thank you for the interesting manuscript.

The manuscript is well written, but needs some improvement in some parts.

Some aspects:

Thirst you work out more clearly what were  test you conducted and which results are from other sources. At the moment this quite mixed and needs to be separated more clearly.

Figure 6: the scale could be improved. Is it possible to explain the upper part of the figure. What is it (maybe environment?)?

Figure 8 could be explained better. At the moment it is quite superficial.

In Figure 9 you describe two different phases, but only for the dark one you describe it as alpha MG. What is the other? Are there really only two different phases?

From my point of view it would help the reader, if there is an experimental section and a result part.

Overall the publication has a good potential, but as already stated needs some further polishing.

Than you

Author Response

Response to reviewers

Thanks for all reviewer’s comments on our manuscript named “Rare earth magnet recycling and materialization for a circular economy”. We have tried to compensate our manuscript considering all comments from reviewers. Revised sentences have been revealed with red color.

Reviewer 2

Comment 1:Figure 6 : the scale could be improved. Is it possible to explain the upper part of the figure. What is it (maybe environment?)?

Thanks for comment to make us reconsider about data in figure 6. Actually, each zone has various phases hence we cannot show whole phases in each zone if the scale is improved. Therefore, we will add some comments in detail to distinguish phases in each zone. Upper part of figure 6 is overall microstructure of LME system using Mg. As shown in figure 6, green square is unreacted raw material (Magnet), red square is reaction zone (Magnet/Mg) and blue square is Mg zone where rare earths are diffused from magnet. In order to identify this figure, we will explain about experimental in manuscripts.

(Modification to the manuscripts)

“In order to show diffusion behavior of rare earths to Mg, reaction experiment between Mg and Magnet is observed. Magnet and Mg are put in Fe crucible which is sprayed with BN to remove reactivity with raw material. Then, reaction experiment is observed in high frequency induction furnace during 3h at a temperature of 900^oC where the Magnet to Mg ratio was 1:10. Fig.6 shows the SEM images and corresponding XRD patterns of the specimen. Upper part of Fig.6 is overall microstructure of specimen and it can be divided to three distinct zone. Three distinct zones can be seen in the figure i.e. magnet area (unreacted raw material – only magnet), reaction area (reaction between Magnet/Mg), and Mg area where rare earths are diffused from magnet. NdFeB magnet is composed by RE2Fe14B (gray matrix), RE-oxide (white circular phases in grain boundary) and RE-rich (white rectangular phases in grain boundary) phases as shown in magnet area. Mg penetrated the magnet along the grain bound- aries to decompose the RE-rich phase and RE2Fe14B matrix phase to Fe (gray matrix) and RE2Fe17 (white circular phases on Fe matrix) in the reaction zone. However, RE-oxide in magnet cannot be reduced easily due to affinity with oxygen hence it is remained in grain boundary as shown in reaction zone. Finally, rare earths diffused into the Mg area through the path provided by the Mg (black phases in reaction zone) inside the magnet. It can be seen from the XRD patterns that Mg successfully decomposed the inherent phases of NdFeB magnet (RE2Fe14B, RE-rich, and RE-oxide) which can be seen in the XRD patterns of the magnet area, and rare earths diffused into Mg to form intermetallic com- pounds i.e. Mg12RE (gray phases in Mg area).

Comment 2: Figure 8 could be explained better. At the moment it is quite superficial.

The reason why we put these information is reduction of Mg weight% is quite important in order to reduce the time for evaporation. So we tried to examine the LME process with less quantity of Mg to 1:2, and we will study more in future. Even though we give more information in addition to overall result, it seems to be better to omit Fig.8 in accordance to reviewer’s recommendation.

Comment 3: In Figure 9 you describe two different phases, but only for the dark one you describe it as alpha MG. What is the other? Are there really only two different phases?

Thanks for comment that we didn’t give much information about figure 9. The dominant dark phase of 20mins is Mg and light gray phase is Mg12Nd phase. As time goes by the white particles are increased and it is confirmed as Nd metal. I am sorry to make confuse that we referred only 2phases in system. Actually, there are three phases, Mg, Mg12Nd, Nd.  

(Modification to the manuscripts)

“ At the 20 mins specimen, only dark Mg and light gray Mg12Nd coexist as figure 9. As mentioned above, Mg is easily vaporized by the pressure hence Mg related phases are converted to other phases very fast. In addition Mg12Nd is the most stable phase in this system, Mg including Nd phase exist only as Mg12Nd. Therefore, only after 40 mins of distillation time, a white region, Nd metal, has started to appear in the resultant Mg-Nd alloy which has been highlighted. The proportion of dark region which was previously identified as α-Mg phase and light grey region which was previously identified as Mg12Nd are decreased with increasing time. It is because the phase transformation of Mg, Mg12Nd to Nd are proceeded. Finally, the white region increased with increasing distillation time until it became the dominant phase at distillation times of 120min and above.

Author Response File: Author Response.pdf