A Review of Recovery of Palladium from the Spent Automobile Catalysts

Round 1

Reviewer 1 Report

The manuscript under revision address a general review about the state of the art in recovering palladium from spent automobile catalysts. The wide variety of alternatives in both hydrometallurgical and pyrometallurgical routes have been discussed. The topic is certainly interesting and deserves to be considered for publication in the journal Metals. However, prior to be accepted, authors should take into account the following recommendations:

1.  Early introduction section. Please, discuss the interest and framework of your review in the near-medium future under the perspective of moving towards electric vehicles instead of the current ones based on combustion engines.
2.  A general scheme collectig all the alternative routes could be very useful for readers.
3.  As presented, the review is very general and not include enpuogh details for each route. Particular conditions for each alternative treatment should be detailed in analogous terms for all of them. Please, consider to describe temperature, time and palladium recovery, among other parameters, for all the alternatives. It could be added directly within the text or in a complete table. Information about patents, techniques in use or under development could be also very useful.
4. Limitations given by each alternative should be clearly indicated.
5. Conclusions should avoid any image or table, only summarizing the most critical aspect previously described and suggesting future trends or research in order to overcome current limitations.

Author Response

Thanks so much for your review, please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Before publication, these points must be revised:

• In the line 80, in the introduction, please specify the yield of the recovery of palladium in the spent catalysts based on the different methods employed to recover palladium from spent catalysts.
• The equations 1 to 8 are chemical equilibrium equations? Please be clear on that point.
• In the chemical equation (6) please express in the correct subscript way the H2O formula
• In the line the author specifies composite oxides citing CeZr, please, could be important to express the general formula including the oxygen.
• In the line 167 could be interesting to express the general chemical equation of coke formation in the catalyst
• In the line 212 could be important to specify more details about the waste-water treatment system employed in the leaching or extraction to the recovery of palladium
• In the HCl(aq) + oxidant leaching please specify the recovery yield of palladium
• In the line 238, please revise the formula of NaClO3 (sodium chlorate)
• Matte collection was first time mentioned in the Table 7 but was not specified in the text in the points related to the pyrometallurgical methods. Please revise this point.

Author Response

Thanks so much for your time, and please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear authors, there are similar manuscripts about PGMs recovery (Pt or Pd) from spent catalysts. Authors need to highlight differences and novelty with respect other papers.

1) Recovery of platinum group metals from spent catalysts: A review. https://doi.org/10.1016/j.minpro.2015.06.009

2) Recovery of precious metals from electronic waste and spent catalysts: A review. https://doi.org/10.1016/j.resconrec.2018.10.041
3) A review on management of waste three-way catalysts and strategies for recovery of platinum group metals from them. https://doi.org/10.1016/j.jenvman.2021.114383

Table 1. Authors have to indicate the units (tons?, kg? )
Table 2. Units of Figure 1?
Authors need to review all the manuscript. In some parts Palladium is write with capitals and in other parts as palladium

What kind of cars use Pd and what kind of cars use Pt?

Author Response

Thanks so much for your review, and please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

I have reviewed article metals-1617882, “A review of recovery of palladium from the spent automobile catalysts” by Bin Xu, Yufeng Chen, Yujuan Zhou, Bangsheng Zhang, Guiqing Liu, Qian Li, Yongbin Yang, and Tao Jiang. The topic is important from the point of view of the circular economy. Comments and suggestions which authors may find useful in upgrading manuscript are the following:

1. Page 2, Table 1: What is the unit of “Production of palladium”? Information should be added.
2. Page 2, lines 45-50: Not only palladium and platinum but also rhodium is an important component of the active layer in the spent automobile catalyst. What about the price of the rhodium?
3. Page 2, Fig. 1: The unit on Y axis should be $/oz. Line 74: is “golden” but it should be “gold”.
4. Page 5, equations 1-8: Which reactions are carried out with palladium as a catalyst? Information should be added into the manuscript.
5. Page 5, lines 139-154: The ceramic support consists of a cordierite. What is it a cordierite? A short information about a cordierite should be added to the manuscript.
6. Page 10, equation 14: stoichiometric coefficients are incorrect (for hydrogen).
7. References should be corrected: in some references [3,4,12-15,17,19,21,23,40,42,49-51,62,73,81,90,92] pages or the access date are missing.

Author Response

Thanks so much for your review, and please see the attachment.

Author Response File: Author Response.pdf

Reviewer 5 Report

The paper makes a good effort of reviewing conventional methods of recovering Pd. The references are of good quality. Several reviews covering the same topic have been published in the past years. Recent research on the recycling of PGMs is not covered by the manuscript in its present form.

The absence of a (sub)chapter describing new ways of recovering metals is the only shortcoming of the paper. The authors should look in the work done by:

1. N. Hodnik et al. Nature Communications volume 7, Article number: 13164 (2016) and extended work by K.J.J. Mayrhofer
2. O. Lanaridi et al. Molecules 2021, 26(23), 7204 and extended work by K. Bica-Schroder
3. E. Vasile et al. Ultrasonics sonochemistry 72, 105404 and extended work by S. Stamatin

Author Response

Thanks so much for your review, and please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript under revision includes the second version after modifying accordingly the reviewers comments. All the issues have been reasonably addresses, so now the manuscript can be published without further requirements.  

Author Response

Dear reviewer,

I appreciate those valuable and practical comments you give for this manuscript, and thanks again for your time and kindness.

sincerely

Reviewer 3 Report

Dear authors,

I consider that you have to highlight the novelty. The last and advanced technologies need to be included. For example, the leaching assisted by ultrasound or microwave, the use of ionic liquids, the use of environment sustainable technologies with biodegradable and less aggressive leaching agents or new purification/separation processes after leaching. 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This manuscript is a review on various methods to recover Pd from exhaust catalysts from automobile sector. The manuscript is well written, but it resembles a report from university students, just providing a list of various methods. The manuscript must include a comparative table explaining pros/cons of the various techniques, and especially costs and performance obtained in terms of recovered Pd. This table must commented properly. Otherwise, how can a reader choose the best technique to recover Pd?

Line 139: CeO2-based, please move 2 in the correct position as subscript.

Reviewer 2 Report

The recovery of palladium from spent automobile catalyst has been topic for a long time. Therefore, there have been many review papers about Pt group recovery from spent automobile catalyst. For example, a review paper was published in 2015 as follows;

Dong, H., Zhao, J., Chen, J., Wu, Y., & Li, B. (2015). Recovery of platinum group metals from spent catalysts: a review. International Journal of Mineral Processing, 145, 108-113.

I think the review should give a new perspective, but it is difficult to find it in this manuscript. Furthermore, some references were published before 2010, which are kind of old.

Therefore, unfortunately, I think the present form is not suitable for publication. This manuscript must be revised with recent publications, and re-submitted to Metals.

Good luck!

Reviewer 3 Report

The manuscript under revision addresses the Pd recovery from spent automobile catalysts. The paper reviews the recovering of this precious metal by multiple methods in base of the processing flow, related mechanism and environmental impacts. This issue is certainly interesting and should be considered for publication in the Journal Metals after major revision. Some important faults suggest that authors should modify the following issues:

1. Please revise the given keywords trying to remove all preposition and determinant words: “spent automobile catalyst” instead of “the spent automobile catalyst”; “palladium recovery” instead of “recovery of palladium”; “palladium leaching” instead of “leaching of palladium”. Moreover, remove “PMGs” as keywords because the use of acronyms should be avoided.
2. Novelty of this work should be emphasized. Any other similar work? Previous reviews addressing this topic? Throughout the entire manuscript, no information about patents are mentioned. Not only other previous scientific manuscripts should be revised but also patents should be taken into account and included in the bibliography. A table containing some of the most important patents under exploitation or the most promising techniques could be also very useful for readers.
3. English usage should be deeply improved throughout the entire manuscript. Please, consider to ask for a native English speaker for the revision of the amended manuscript before being send again to the journal.
4. Please, consider to revise the following sentences (lines 29-31) by a native English speaker: “The increasing demand of PGMs causes highly consumption rate of high-grade PGMs ore. The increasingly depletion of high-grade PGMs ore forces people turn to the low-grade ore; while the cost of recovering PGMs from low-grade ore is much expensive and influence environment a lot [3]”. Please, avoid to repeat similar sounds and structures in a few lines.
5. Line 139. Please, modify “2” superscript by subscript in CeO2 formulae.
6. A general scheme about the discussed alternatives for Pd recovery could be very useful for readers, as well as some process flow diagram of principal processes like given in Figure 2. Is it identical for all indicated methods?
7. A general table comparing characteristics of processes (i.e. reactants, operating conditions, efficiency, recovery, cost…) is highly recommended to be included in the review.
8. Paragraph between lines 242 and 248 without any reference.
9. No economic consideration for the above-mentioned techniques are provided, despite authors so indicated in the early manuscript. Please, include this part throughout the entire manuscript. Something similar occurs for environmental aspects, only appearing in some slight comments. Both aspects are critical for selecting the best alternative.
10. Conclusion section should be reformulated as main conclusions and future trends, so providing some information about directions followed in most recent research and for the near future.