Maleic, glycolic and acetoacetic acids-leaching for recovery of valuable metals from spent lithium-ion batteries: leaching parameters, thermodynamics and kinetics

Environmentally friendly acid-leaching processes with three organic acids (maleic, glycolic and acetoacetic) were developed to recover valuable metals from the cathodic material of spent lithium-ion batteries (LiCoO2). The leaching efficiencies of Li and Co by the maleic acid were 99.58% and 98.77%, respectively. The leaching efficiencies of Li and Co by the glycolic acid were 98.54% and 97.83%, while those by the acetoacetic acid were 98.62% and 97.99%, respectively. The optimal acid concentration for the maleic acid-, glycolic acid- and acetoacetic acid-leaching processes were 1, 2 and 1.5 mol l–1, respectively, while their optimal H2O2 concentrations were 1.5, 2 and 1.5 vol%, respectively. The optimal solid/liquid ratio, temperature and reaction time for the leaching process of the three organic acids was the same (10 g l−1, 70°C, 60 min). The thermodynamic formation energy of the leaching products and the Gibbs free energy of the leaching reactions were calculated, and the kinetic study showed that the leaching processes fit well with the shrinking-core model. Based on the comparison in the leaching parameters, the efficacy and availability of the three acids is as follows: maleic acid > acetoacetic acid > glycolic acid.


Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
The manuscript presents "Leaching of Valuable metals from spent LIBs: Optimization, Thermodynamics and kinetics of leaching". In this study, the authors are using the organic acids which have been not used by other researchers. Although, the work has the scientific novelty in terms of using new leaching agents to leach metals from spent LIBs, the thermodynamical interpretation and kinetic analysis should be more comprehensive to understand the leaching behavior of metals by organic acids such as maleic, glycolic and acetoacetic acid. The same concentration of acids should be employed to compare the leachability to dissolve metals, but the effect of different concentrations (1.0 M for maleic, 2.0 M for glycolic, and 1.5 M for acetoacetic acid) has been compared under the optimized leaching conditions. The comprehensive interpretation of leaching should be conducted with the dissociation constant of acids, activation energy, Gibbs free energy etc.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? Yes

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
In this study, Maleic, glycolic and acetoacetic acids were respectively adopted by the authors during the reductive leaching process for the recycling of valuable metals from spent LIBs batteries. Despite sound results can be obtained, several issues should be taken due consideration before its publication in this journal. 1. The first issue, also the most important one, concerning this manuscript is the logic and arrangement of this text, the introduction part cannot present a clear cue for potential readers and there are also several errors (like line 59, page 1 "i.e. ~500 tons"; line 9, page 2 "hydrometallurgical [12], and biometallurgical methods [13]" I think the bio-leahcing method can be concluded in hydrometallurgical method.). The most recent developments involved with leaching process are also adopted like phosphoric acid as inorganic acid (e.g. Waste Management 85 (2019) Chem. Eng. 2017, 5, 714−721;Waste Manag. Res., 2018, 36(2), 113-120), there are may be also other newly developed acid leaching systems, please update them and give potential readers more precise and clear statement and discussion in second paragraph 2 of page 2. In conclusion, the introduction part should be significant revision. 2. The second issue is the results and discussion section, the 4.1, 4.2 and 4.2.1 sections should be mainly discussed in the experimental section instead of this section. In section 4.3 I do not think it is a thermodynamic analysis, please pay due attention to revise this section; 3. The novelty of this manuscript derives from the adoption of different kinds of organic acid. However, as we all known, organic acids are usually expensive than those of inorganic acids, so is it profitable for the adoption of these expensive acids instead if inorganic ones? What about their environmental impacts regarding TOC and other organic hazardous substances?
Review form: Reviewer 3

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation? Reject
Comments to the Author(s) The article entitle "Maleic, glycolic and acetoacetic acids-leaching for recovery of valuable metals from spent LIBs: Leaching parameters, thermodynamics and kinetics" was carefully reviewed. The overall present data is not good and not comparable with literature. In the present form the article is not acceptable for the publication. Title: Maleic, glycolic and acetoacetic acids-leaching for recovery of valuable metals from spent LIBs: Leaching parameters, thermodynamics and kinetics Manuscript ID: RSOS-191061 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The manuscript presents "Leaching of Valuable metals from spent LIBs: Optimization, Thermodynamics and kinetics of leaching". In this study, the authors are using the organic acids which have been not used by other researchers. Although, the work has the scientific novelty in terms of using new leaching agents to leach metals from spent LIBs, the thermodynamical interpretation and kinetic analysis should be more comprehensive to understand the leaching behavior of metals by organic acids such as maleic, glycolic and acetoacetic acid. The same concentration of acids should be employed to compare the leachability to dissolve metals, but the effect of different concentrations (1.0 M for maleic, 2.0 M for glycolic, and 1.5 M for acetoacetic acid) has been compared under the optimized leaching conditions. The comprehensive interpretation of leaching should be conducted with the dissociation constant of acids, activation energy, Gibbs free energy etc.

Reviewer: 2
Comments to the Author(s) In this study, Maleic, glycolic and acetoacetic acids were respectively adopted by the authors during the reductive leaching process for the recycling of valuable metals from spent LIBs batteries. Despite sound results can be obtained, several issues should be taken due consideration before its publication in this journal. 1. The first issue, also the most important one, concerning this manuscript is the logic and arrangement of this text, the introduction part cannot present a clear cue for potential readers and there are also several errors (like line 59, page 1 "i.e. ~500 tons"; line 9, page 2 "hydrometallurgical [12], and biometallurgical methods [13]" I think the bio-leahcing method can be concluded in hydrometallurgical method.). The most recent developments involved with leaching process are also adopted like phosphoric acid as inorganic acid (e.g. Waste Management 85 (2019) (2), 113-120), there are may be also other newly developed acid leaching systems, please update them and give potential readers more precise and clear statement and discussion in second paragraph 2 of page 2. In conclusion, the introduction part should be significant revision. 2. The second issue is the results and discussion section, the 4.1, 4.2 and 4.2.1 sections should be mainly discussed in the experimental section instead of this section. In section 4.3 I do not think it is a thermodynamic analysis, please pay due attention to revise this section; 3. The novelty of this manuscript derives from the adoption of different kinds of organic acid. However, as we all known, organic acids are usually expensive than those of inorganic acids, so is it profitable for the adoption of these expensive acids instead if inorganic ones? What about their environmental impacts regarding TOC and other organic hazardous substances?
Reviewer: 3 Comments to the Author(s) The article entitle "Maleic, glycolic and acetoacetic acids-leaching for recovery of valuable metals from spent LIBs: Leaching parameters, thermodynamics and kinetics" was carefully reviewed. The overall present data is not good and not comparable with literature. In the present form the article is not acceptable for the publication. It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.

Response to the Comments of the Editors and Referees (Manuscript No. RSOS-191061) Dear editor and reviewer,
We would like to thank you for giving us a chance to revise the paper No. RSOS-191061, and also thank the reviewers for giving us constructive suggestions. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made thorough corrections which we hope to meet the requirement. The differences between the revision and the original manuscript are present in the form of 'tracked changes' and uploaded as 'Responds to referees' in the online submission system. The point-by-point answers to the comments and suggestions were listed as below.
To Reviewer #1 Question 1: The manuscript presents "Leaching of Valuable metals from spent LIBs: Optimization, Thermodynamics and kinetics of leaching". In this study, the authors are using the organic acids which have been not used by other researchers. Although, the work has the scientific novelty in terms of using new leaching agents to leach metals from spent LIBs, the thermodynamical interpretation and kinetic analysis should be more comprehensive to understand the leaching behavior of metals by organic acids such as maleic, glycolic and acetoacetic acid. The same concentration of acids should be employed to compare the leachability to dissolve metals, but the effect of different concentrations (1.0 M for maleic, 2.0 M for glycolic, and 1.5 M for acetoacetic acid) has been compared under the optimized leaching conditions. The comprehensive interpretation of leaching should be conducted with the dissociation constant of acids, activation energy, Gibbs free energy etc. Answer: We apologize for our negligence. About the suggestions in this question, we have made some improvement in the revised manuscript.
(1) In order to make the thermodynamic interpretation and kinetic analysis more comprehensive, we have rewritten a lot of these two parts (Section 4.2 and 4.3). In Section 4.2, we have added the purpose of the thermodynamic study, with the meaning of the two thermodynamic parameters (formation energy and Gibbs free energy). We have made the reaction process more specific, as well as rearrange the content of this section to make it more comprehensive. At the end of the section, we have added a summary about the information given by the thermodynamic results. In Section 4.3, we have also added the purpose of the kinetic study, with the meaning of the kinetic parameter (activation energy). We have added more specific information about how to obtain the rate-controlling step according to the equation fitting results. At the end of the section, we have added a summary about the information given by the analysis of thermodynamic results.
(2) We have added the data about Gibbs free energy in the revised manuscript, as can be shown in Section 4.2 as well as Table S7. We have also added the relevant contents in the section of Summary, Introduction and Conclusions (3) In Section 4.3, we have added the interpretations of leaching rate with dissociation constant, Gibbs free energy and activation energy. In Section 4.4, we have compared the leaching efficiencies of metals by the three acids under the same acid concentrations. What's moer, we have compared the dissociation constant of acids, activation energy and Gibbs free energy of the leaching reactions, in order to comprehensively compare the leachability of different organic acids.

Question 2:
Some comments are given to improve the manuscript as follows; -Stoichiometric amount of acids to leach metal components from spent LIBs? Answer: We apologize for our negligence. We have added the chemical equations of the three acidleaching reactions as Eq (7) to (9) in the revised manuscript to illustrate the stoichiometric amount of acids to leach metal components. Question 6: -The formation energy could be expressed as SI unit? Answer: We apologize for our negligence. We have changed the unit of formation energy into SI unit in the Section 4.2 Table S6 as the suggestion.

Question 7:
-Leaching rate (kinetics, not efficiency) should be interpreted with dissociation constant of acids and Gibbs free energy and activation energy to compare the leachability of different organic acids. Answer: We apologize for our negligence. We have added the data about Gibbs free energy in the revised manuscript, as can be shown in Section 4.2 as well as Table S7. We have also added the relevant contents in the section of Summary, Introduction and Conclusions. In Section 4.3, we have added the interpretations of leaching rate with dissociation constant, Gibbs free energy and activation energy. In Section 4.4, we have compared the dissociation constant of acids, activation energy and Gibbs free energy of the leaching reactions, in order to comprehensively compare the leachability of different organic acids.
To Reviewer #2 Question 1: In this study, Maleic, glycolic and acetoacetic acids were respectively adopted by the authors during the reductive leaching process for the recycling of valuable metals from spent LIBs batteries. Despite sound results can be obtained, several issues should be taken due consideration before its publication in this journal. 1. The first issue, also the most important one, concerning this manuscript is the logic and arrangement of this text, the introduction part cannot present a clear cue for potential readers and there are also several errors (like line 59, page 1 "i.e. ~500 tons"; line 9, page 2 "hydrometallurgical [12], and biometallurgical methods [13]" I think the bio-leahcing method can be concluded in hydrometallurgical method.). The most recent developments involved with leaching process are also adopted like phosphoric acid as inorganic acid (e.g. Waste Management 85 (2019) (2), 113-120), there are may be also other newly developed acid leaching systems, please update them and give potential readers more precise and clear statement and discussion in second paragraph 2 of page 2. In conclusion, the introduction part should be significant revision. Answer: We apologize for our negligence. About the suggestions in this question, we have made some improvement in the revised manuscript.
(1) We have rewritten a lot of the section of Introduction to make it more logic and comprehensive, and we have also added a new

Question 2:
The second issue is the results and discussion section, the 4.1, 4.2 and 4.2.1 sections should be mainly discussed in the experimental section instead of this section. In section 4.3 I do not think it is a thermodynamic analysis, please pay due attention to revise this section; Answer: We apologize for our negligence. About the suggestions in this question, we have made some improvement in the revised manuscript.
(1) We have rearranged the contents about the spent material characterization, organic properties, and orthogonal experiment in the section of Experimental Procedure (Section 3.2.1, 3.2.2 and 3.2.3). Moreover, we have rearranged the contents in the Section 3.2 to make it more comprehensive.
(2) In Section 4.3, we have added the calculation of Gibbs free energy of the leaching reactions.
The formation energy of the leaching products in the original manuscript could provide basis for the calculation of Gibbs free energy which is an important thermodynamic parameter of the leaching reactions. We hope that this correction could make the contents appropriate as the thermodynamic analysis.

Question 3:
The novelty of this manuscript derives from the adoption of different kinds of organic acid. However, as we all known, organic acids are usually expensive than those of inorganic acids, so is it profitable for the adoption of these expensive acids instead if inorganic ones? What about their environmental impacts regarding TOC and other organic hazardous substances? Answer: We apologize for our negligence. We have noticed that in some literature, the organic acids leachate of spent LIBs can be directly used as the material to resynthesize cathodic material of LIBs through sol-gel method, without any intermediate step or loss of materials. Therefore, a closed-loop process of recovery-resynthesis can be achieved when organic acids are employed as the leaching agents. This process can not only reduce the recovery cost, but also minimize the emission of TOC and other organic hazardous substances. Therefore, we think that the organic acids are profitable in the acid-leaching process. We have added the relevant contents in the section of Introduction, and we plan to conduct relevant study in our subsequent research.
To Reviewer #3 Question: The article entitled "Maleic, glycolic and acetoacetic acids-leaching for recovery of valuable metals from spent LIBs: Leaching parameters, thermodynamics and kinetics" was carefully reviewed. The overall present data is not good and not comparable with literature. In the present form the article is not acceptable for the publication. Answer: We apologize for our negligence and confusion that we have caused. And we would like to thank you for your effort on reviewing our manuscript. In this paper, we report on the availability of maleic, glycolic and acetoacetic acids for recovery of lithium and cobalt from cathode of spent lithium batteries. We report on the effects of leaching parameters (i.e. acid concentration, reductant dosage, solid/liquid ratio, temperature and reaction time) on the leaching efficiencies of these organic acids. We think that there is sufficient novelty in this manuscript, which could also provide valuable information for the recycling practice for the spent LIBs.
The novelty lies in the following aspects: (a) maleic, glycolic and acetoacetic acids are employed for the acid-leaching recovery process for the cathode of spent lithium ion batteries, and there is limited information about the availability of these organic acids; (b) we conducted thermodynamic and kinetic studies to investigate the leaching reaction mechanisms, the thermodynamic and kinetic behavior of the leaching system, as well as the feasibility and extent of the leaching reactions. Especially in the thermodynamic part, we used Materials Studios software to calculate the formation energy of the possible leaching products, in order to recognize the thermodynamic favorable molecular structure of the maleic acid-leaching products. Based on the results, in the revised manuscript, the Gibbs free energy of the leaching reactions was calculated, where there is limited information in the literature. For the kinetic study, we have investigated the shrinking-core kinetic model for the leaching reactions, as well as calculated the activation energy. The thermodynamic and kinetic investigation could help a lot to understand the acid-leaching mechanisms; (c) we compared and ranked the overall efficacy of the three organic acids. In the revised manuscript, the comparison is not only based on the leaching efficiencies, but also based on all the process conditions and thermodynamic/kinetic parameters, including dissociation constants of the acids, reactant concentrations, solid/liquid ratio, Gibbs free energy, activation energy. The rank of the overall availability of the three organic acids could provide important information for the recycling technologies of spent LIBs. Moreover, the comparison method of the availability of organic acids in the acid-leaching process has potential to be further investigated and employed.
We hope that our manuscript could meet the requirement for publication in this journal. Thank you so much for your efforts.
Other revisions (a) Because of the rearrangement of the contents in the manuscript, some numbers of equations, tables and references have been changed in the revised manuscript, as well as in the revised supplementary materials. (b) To avoid language problems, the English text in the manuscript has been polished by a native English-speaking expert (i.e. the seventh author of the paper) again.
That is the list of changes for the revised manuscript. We would like to thank you for giving us a chance to revise the paper, and also thank the reviewers for giving us constructive suggestions. If there is any unaccomplished matter, please contact us.