The Effects of Knife Milling and Ball Milling on Hydrogen Decrepitated Sm2TM17 Sintered Magnet Powder for Short-Loop Recycling
, Oliver Peter Brooks 1, Viktoria Kozak 1, Alexis Lambourne 2, Alexander Campbell 2 and Richard Stuart Sheridan 1Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
The paper entitled “The Effects of Knife Milling and Ball Milling on Hydrogen Decrepitated Sm2TM17 Sintered Magnet Powder for Short-Loop Recycling” presents experimental research in the field of magnetic materials obtained from recycled magnets.
There are some minor recommendations, in order to be published, as following:
- As I understand, you have to types of materials: as received scrap HD and commercial sintered magnets (recycled). At point 2.4. you said only of the milling process of HD powders. At lines 193, 203 and 207 also appear two materials “as-received and recycled sintered magnets”. Please check this, because is unclear!
- Figure 1 A, please be attention to the expression “fully dense material”. Are you sure that you obtained fully dense material after sintering? If yes, please explain!
Best regards!
Author Response
The authors would like to thank the reviewers for their comments, please find responses to each comment below.
Comment 1: As I understand, you have to types of materials: as received scrap HD and commercial sintered magnets (recycled). At point 2.4. you said only of the milling process of HD powders. At lines 193, 203 and 207 also appear two materials “as-received and recycled sintered magnets”. Please check this, because is unclear
Response 1: An additional sentence has been added to differentiate between as-received and recycled material. Lines 193-196.
Comment 2: Figure 1 A, please be attention to the expression “fully dense material”. Are you sure that you obtained fully dense material after sintering? If yes, please explain
Response 2: An additional comment has been added to clarify the density associated with ‘fully dense material’. Line 291. As the density values obtained by the recycled sintered compacts fall in this range, the authors believe the material to be fully dense (See Tables 5 and 8).
Reviewer 2 Report
Comments and Suggestions for Authors
Non-renewable natural resources and other geopolitical factors often limit access to materials used in various technological industries. One potential solution is to recycle materials from end-of-life applications. The topic of this article is current, with significant application potential focused on material recycling, which is important in an economic context. The reviewed article concerns assessment the effect of knife milling, roller ball milling and planetary ball milling on Sm2TM17 sintered magnet HD powder particle size and recycled sinter compact density.
Comments on the article:
- What shape and size were the sinters?
- Have the authors considered using another sintering method, e.g. SPS (for produced magnets), where a very short sintering time is usually used?
- The authors demonstrated that the magnetic properties of the recycled sintered Sm2TM17 603 magnets with 2 and 3 Wt.% Sm addition were far lower than the as-received material. This is the result of the increased carbon content in the recycled material, which resulted in the formation of zirconium carbide in the microstructure. Can additional heat treatment of sintered magnets improve their magnetic properties?
- The darkest phase what was identified as a Co and Fe rich phase (for example Figure 17). It is heterogeneously distributed in the microstructure. Does this affect the magnetic properties?
Minor comments
Page 23 - There are unnecessary rectangles at the bottom of the page.
Line 823; °C - degree Celsius is missing.
Author Response
The authors would like to thank the reviewers for their comments, please find responses to each comment below.
Comment 1: What shape and size were the sinters?
Response 1: An additional comment has been made to clarify the shape and size of the compacts generated. Line 278.
Comment 2: Have the authors considered using another sintering method, e.g. SPS (for produced magnets), where a very short sintering time is usually used?
Response 2: The authors appreciate the suggestion of spark plasma sintering as an alternative technique to form sintered magnets and will take it under consideration as a future research direction. However, as this is not the typical technique applied during primary manufacture of Sm2TM17 sintered magnets that the recycling process discussed in this manuscript is trying to emulate, it is beyond this scope of this particular article.
Comment 3: The authors demonstrated that the magnetic properties of the recycled sintered Sm2TM17 603 magnets with 2 and 3 Wt.% Sm addition were far lower than the as-received material. This is the result of the increased carbon content in the recycled material, which resulted in the formation of zirconium carbide in the microstructure. Can additional heat treatment of sintered magnets improve their magnetic properties?
Response 3: The authors appreciate the reviewers comment on the potential for additional heat treatment to improve the magnetic properties of recycled material. However, as the presence of carbon inhibits the formation of the 1:3R lamellar phase, which is key in distributing copper to the 1:5H cell boundary phase and establishing a domain wall pinning coercivity mechanism, additional heat treatment will not be sufficient to increase magnetic properties without first addressing the carbon impurity issue.
Comment 4: The darkest phase what was identified as a Co and Fe rich phase (for example Figure 17). It is heterogeneously distributed in the microstructure. Does this affect the magnetic properties?
Response 4: An additional passage has been added highlighting the distribution of the Fe/Co-rich phase and its potential effect on the magnetic properties of recycled sintered magnets. Line 801-806.
Minor comments:
Minor comment 1: Page 23 - There are unnecessary rectangles at the bottom of the page
Response to minor comment 1: The presence of these rectangles has been addressed in the editing process.
Minor comment 2: Line 823; °C - degree Celsius is missing.
Response to minor comment 2: Degree Celsius symbol added. Now line 847.
Reviewer 3 Report
Comments and Suggestions for Authors
The topic of the article is extremely timely, the recycling of rare earth metals used in permanent magnets. A process for the short-loop recycling was investigated. Similar technology was already reported (e.g. ref 22), but in the present work the composition of the raw material is different and different milling technique was used. Unfortunately, the magnetic parameters of the recycled magnets are much worse than those of the starting material. The authors explain this by carbon contamination from the wetting agent and Sm loss during processing.The experimental work is careful and correct. Despite of the negative final results the article contains useful information about the HD process, milling of this type of materials as well as the effect of wetting agents.
Remarks/questions
row 16 hydrogen pressure instead of pressure is suggested
row 195 backscattered electron imaging instead of backscattered element imaging
fig.9 Difficult to distinguish the curves. Different type lines and/or line+simbol is recommended.
Metallic contamination (Fe, Cr, Ni, W, etc.) from the ball mill was not reported. It was not investigated, or it was not detected? Please, shortly discuss this problem.
Author Response
Comment 1: row 16 hydrogen pressure instead of pressure is suggested
Response to comment 1: The highlighted section has been amended to 'hydrogen pressure'. Row 16.
Comment 2: row 195 backscattered electron imaging instead of backscattered element imaging
Response to comment 2: The highlighted section has been amended to 'back scattered electron imaging'. Row 199-200.
Comment 3: fig.9 Difficult to distinguish the curves. Different type lines and/or line+simbol is recommended
Response to comment 3: Different line types have been assigned to each surfactant trial to increase visibility with a revised figure.
Comment 4: Metallic contamination (Fe, Cr, Ni, W, etc.) from the ball mill was not reported. It was not investigated, or it was not detected? Please, shortly discuss this problem
Response to comment 4: An additional statement has been added to comment on contamination originating from ball milling equipment. Row 261-263.
