Recovery of Rare Earth Elements from Mining Tailings: A Case Study for Generating Wealth from Waste

Round 1

Reviewer 1 Report

Dear Authors

Your work is a complete scientific study, the results of which, in my opinion, will be of interest to other scientists. However, the manuscript has a number of minor shortcomings that do not reduce the level and relevance of the study, but the correction of which will improve the quality of the manuscript, enhance the ease of perception of the presented material and increase the reader's interest.

1.) From my point of view, the title of the article does not reflect the fundamental nature of this study and confuses the reader a little.

1.1) The authors use the localization "from the Bagre-Nechi mining district in Colombia" in the title. The accumulation of man-made waste and their involvement in recycling is a global challenge facing all researchers. From my point of view, the localization of the research in the title does not demonstrate the fundamental nature of the research and reduces the degree of global relevance.

1.2.) The same can be said about the specification of the valuable component to be extracted from the waste and the activator (solvent) used. As far as I understand, the technology proposed by the authors and the activators (solvents) used in the study do not limit the possibility of extracting only the valuable components indicated in the manuscript. From my point of view, it is necessary to give a title without much specificity. This will not only demonstrate the fundamental nature and high significance of the study, but will also help improve citation in the future. Too much “focused” articles reduce the possibility of citing them.

1.3.) The title of the article must clearly and concisely reflect the essence of the study and the results obtained by the authors. The authors underestimate the fundamentality, significance and relevance of their research in their title.

Summary of remark (1). From my point of view, the title of the article should sound like a "shot" and attract the attention of the reader. The authors have a very good idea and a great study, but the title understates the importance of the study and confuses the reader a bit. I ask the authors to decide what was paramount in this study. The main merit of the authors must be placed in the title, and everything that was the subject (material) of the study, the way to achieve the goal, should be described in the text.

2.) Despite the fact that the presented abstract reflects the essence of the study, from my point of view, it is a little blurry and framed incorrectly. One gets the feeling that for the annotation, the authors pulled out separate sentences and phrases from the text of the manuscript. The abstract should clearly indicate the purpose of the study, its importance for society (that is, characterize the problem), indicate the methods and materials of the study. Therefore, it is desirable to avoid narrative text in the abstract. Please try to use words and phrases: an analysis has been carried out; carried out; studied; analyzed; developed; proposed; installed and others. It is advisable to start sentences in the abstract with these words and phrases.

3.) In lines 23-33, the narrative is less like a scientific article, but like a symbiosis of sections of economic geography and high school inorganic chemistry. The beginning of the article should catch the reader. And the placement of well-known information makes the beginning boring and does not contribute to attracting attention.

4.) The highly controversial and unproven statements of the authors made in lines 38-41. Links need to be backed up.

5.) I would recommend avoiding group references [1-5], even more so [7-13]. From my point of view, [14,15] or [28-30] can be acceptable, up to three are acceptable, everything more needs to be deciphered. Each work you cite is unique and the studies you cite deserve more proper and careful review to demonstrate (and prove) its importance to current research. It is necessary to demonstrate in detail the essence of each study and their need for your work. From my point of view, citation shows the reader to the depth of the study of the material and each article that the author refers to proves his statement. Just lines 38-41, the authors make a lot of statements, and after each statement it is desirable to provide a proof. Thus, you avoid group links and locally demonstrate to the reader the uniqueness of the link and, in general, the depth of study of the material. Such examples can be continued. Please break group links and demonstrate proof of your claims.

6.) Lines 56-59 are common knowledge. This information slightly overloads the text and distracts the reader's attention. If the authors believe that it is necessary to mention this in the text of the manuscript, then, from my point of view, it is enough to say: The stages of processing REE ores are described [link], and the enrichment stages are specified [26].

7.) In lines 76-78 the authors make two statements. But at the end of the second statement (line 78) a reference is made to the study [33]. From my point of view, this reference proves the first statement of the authors (line 76-77), but the statement that solvent extraction is widely used throughout the world in hydrometallurgical processing is not confirmed by the authors. Such incorrect references or statements without corresponding proof from the manuscript are sufficient. Changes need to be made.

8.) Notes on figures 1, 2 and 3

Who made Figures 1, 2 and 3, if by the authors, then it is necessary to indicate (done by the authors), if borrowed, then it is necessary to indicate the source of the borrowing or make an appropriate link to the source. If you have permission to submit a borrowed drawing, please provide this permission to the editor.

9.) From my point of view, the introduction is very voluminous and it is overloaded with information about the method of extraction and about the description of activators (extractors), a lot of unnecessary information that needs to be placed in other sections. From my point of view, the description of a method for extracting a useful component by leaching is more suitable for the methods section. The introduction includes: the relevance of the research topic, a review of the literature on the topic, a statement of the problem, the formulation of the purpose and objectives of the study. Part of the material posted by the authors in the introduction should be placed in the "Methods and Materials" section.

10.) I still did not see that the authors well covered the issue of relevance. The topic the authors touched on is really relevant. As I noted earlier: Involvement in the recycling of technogenic waste from mining and processing is a very topical issue. The development of an innovative technology for extracting a useful component from the waste of processing production is not only a way to reduce the impact of a mining enterprise on the environment, but also an opportunity to increase the life of the enterprise and create additional jobs. After the depletion of reserves and the completion of the development of the deposit, it is possible to begin extracting useful components from waste - this is an increase in the life of the enterprise, and the extraction of a useful component from waste in parallel with the development of the deposit is the creation of additional jobs. The development and implementation of such technology can achieve a multiplier environmental and economic effect.

11.) At the end of the introduction (before the work plan) there are no conclusions on the analysis. This conclusion allows us to characterize the actual question posed, the purpose of the study and the tasks to be solved to achieve this goal. For example: Analyzing the above, it can be noted that ...... is a very topical issue. Therefore, the purpose of this study is ..... and to achieve which it is necessary to solve the following tasks: 1); 2); ..... Such a conclusion allows the reader to understand the vector of the study, and the authors at the end of the study correctly formulate conclusions on it.

12.) Is the "Flowsheet for concentrating monazite from alluvial gold mining residue" proposed in Figures 4 and 5 developed by the authors? If so, then it must be specified. If it is borrowed, then it is necessary to indicate the source of borrowing or make an appropriate reference to the source. If you have permission to submit a borrowed drawing, please provide this permission to the editor.

13.) Section 2.5. What is the mixing speed?

14.) In lines 218-221, the authors state the particle size distribution, then give figure 5 showing the same data. Is this duplication necessary? You have given an excellent description of figure 6, indicating the main mineral and so on. From my point of view, it can also be limited for particle size distribution.

15.) Despite the fact that the manuscript has an impressive list of references, 17 papers are older than 5 years and a lot (41) are older than 10 years out of 54. I observe very weak citation geography. The question posed in the present study is very relevant and universal. Your citation list contains works by scientists from many countries, but as I noted, these studies are quite old. You have undeservedly overlooked recent studies of Balkan, Central European and Russian scientists working in this direction for the last 5 years. I would like to draw your attention to very recent works listed below. Widening of the reference list might raise your citing potential, that is beneficial for both: The Authors and the Journal. Of course, the choice is yours and I encourage you to make your own search in Scopus and/or MDPI search engine.

  -   Zglinicki, K.; Szamałek, K.; Wołkowicz, S. Critical Minerals from Post-Processing Tailing. A Case Study from Bangka Island, Indonesia. Minerals 2021, 11, 352. https://0-doi-org.brum.beds.ac.uk/10.3390/min11040352

   -   Šajn, R.; Ristović, I.; Čeplak, B. Mining and Metallurgical Waste as Potential Secondary Sources of Metals—A Case Study for the West Balkan Region. Minerals 2022, 12, 547. https://0-doi-org.brum.beds.ac.uk/10.3390/min12050547

   -   Rybak, J.; Adigamov, A.; Kongar-Syuryun, C.; Khayrutdinov, M.; Tyulyaeva, Y. Renewable-Resource Technologies in Mining and Metallurgical Enterprises Providing Environmental Safety. Minerals 2021, 11, 1145. https://0-doi-org.brum.beds.ac.uk/10.3390/min11101145

   -   Zglinicki, K.; Małek, R.; Szamałek, K.; Wołkowicz, S. Mining Waste as a Potential Additional Source of HREE and U for the European Green Deal: A Case Study of Bangka Island (Indonesia). Minerals 2022, 12, 44. https://0-doi-org.brum.beds.ac.uk/10.3390/min12010044

   -   Rybak, J.; Gorbatyuk, S.M.; Kongar-Syuryun, C.; Khayrutdinov, A.M.; Tyulyaeva, Y.; Makarov, P.S. Utilization of Mineral Waste: A Method for Expanding the Mineral Resource Base of a Mining and Smelting Company. Metallurgist 2021, 64, 851–861. https://0-doi-org.brum.beds.ac.uk/10.1007/s11015-021-01065-5

16.) Conclusions do not match the stated title and abstract. The conclusions are very superficial and do not indicate the final result obtained by the authors. I would like to note that lines 379-385, from my point of view, cannot be conclusions for work. Determining the granulometric or chemical-mineralogical composition of enrichment waste was not the purpose of the study. The authors of this information were an intermediate step to achieve the ultimate goal. And the conclusions should be more specific: developed ...; got.....; achieved .... From my point of view, the conclusion needs to be redone.

Summary.

From my point of view, the authors have carried out a large research work presented in this manuscript. The study touches upon a rather relevant topic: reducing the environmental burden on the environment through the prism of involving man-made waste in the recycling. The authors have developed and proposed a technique and technology for extracting a valuable component from enrichment waste. The introduction of this technology will provide a multiplier effect.

From my point of view, the manuscript is worthy of publication in the open press with the correction of the shortcomings indicated in my remark.

Sincerely

Author Response

Reviewer #1:

From my point of view, the authors have carried out a large research work presented in this manuscript. The study touches upon a rather relevant topic: reducing the environmental burden on the environment through the prism of involving man-made waste in the recycling. The authors have developed and proposed a technique and technology for extracting a valuable component from enrichment waste. The introduction of this technology will provide a multiplier effect.

From my point of view, the manuscript is worthy of publication in the open press with the correction of the shortcomings indicated in my remark.

The authors are grateful for the comments provided by the reviewer. Changes to the manuscript have been highlighted in blue in the revised version of the manuscript.

1. From my point of view, the title of the article does not reflect the fundamental nature of this study and confuses the reader a little.

The title of the article has been changed following the recommendations of the reviewer, in items 1.1, 1.2, and 1.3. The new title is: “Recovery of rare earth elements from mining tailings: a case study for generating wealth from waste”

2. Despite the fact that the presented abstract reflects the essence of the study, from my point of view, it is a little blurry and framed incorrectly. One gets the feeling that for the annotation, the authors pulled out separate sentences and phrases from the text of the manuscript. The abstract should clearly indicate the purpose of the study, its importance for society (that is, characterize the problem), indicate the methods and materials of the study. Therefore, it is desirable to avoid narrative text in the abstract. Please try to use words and phrases: an analysis has been carried out; carried out; studied; analyzed; developed; proposed; installed and others. It is advisable to start sentences in the abstract with these words and phrases.

Abstract has been rewritten. “The growing demand for rare earth elements (REE) driven by their applications in modern technologies has caused the need to search alternative sources of these elements, since their extraction from traditional deposits is limited. A potential source of light rare earth elements (LREE) may be the monazite present in the mining waste generated in the Bagre-Nechí mining district in Colombia as a result of the processing of sands containing alluvial gold. Consequently, in this research, a systematic evaluation has been carried out for the extraction of Ce, La and Nd from a leach liquor obtained from monazite present in alluvial gold mining tailings. The leaching process carried out with HCl indicated the recovery of approximately 90% of La and Nd and ∼ 60% of Ce; the solvent extraction tests of these elements showed that increasing the contact time and pH of the leaching liquor positively affects the extraction of lanthanum, cerium, and neodymium, achieving extractions close to 100% with D2EHPA and to a lesser extent with Cyanex572. McCabe-Thiele diagrams for extraction with D2EHPA indicated the requirement of three stages for the extraction of Ce, La and Nd.”

3. In lines 23-33, the narrative is less like a scientific article, but like a symbiosis of sections of economic geography and high school inorganic chemistry. The beginning of the article should catch the reader. And the placement of well-known information makes the beginning boring and does not contribute to attracting attention.

The authors are grateful for this comment. The following text was included in the introduction, in lines 16-23: “The extraction and processing of mineral resources is a cause of the environmental pollution and deterioration in some regions. After the completion of the geological processes and formation of the Earth’s crust, all elements are in equilibrium. However, the human intervention, even at the stage of geological prospecting, mobilizes the minerals concentrated in the Earth’s crust. A commercial component and associated minerals in the development zone are activated, affecting the human habitat. After the beginning of development, the human habitat transforms into a domain of impact. Accumulated and stored industrial wastes are the most hazardous for the environment”

4. The highly controversial and unproven statements of the authors made in lines 38-41. Links need to be backed up.

Reference for the text between in lines 38-41 “Although the REE content in tailings may be lower than in primary sources, their processing can be justified if environmental benefits are considered, e.g., mine site remediation and land reclamation” is give; now this is on lines 39-42

5. I would recommend avoiding group references [1-5], even more so [7-13]. From my point of view, [14,15] or [28-30] can be acceptable, up to three are acceptable, everything more needs to be deciphered. Each work you cite is unique and the studies you cite deserve more proper and careful review to demonstrate (and prove) its importance to current research. It is necessary to demonstrate in detail the essence of each study and their need for your work. From my point of view, citation shows the reader to the depth of the study of the material and each article that the author refers to proves his statement. Just lines 38-41, the authors make a lot of statements, and after each statement it is desirable to provide a proof. Thus, you avoid group links and locally demonstrate to the reader the uniqueness of the link and, in general, the depth of study of the material. Such examples can be continued. Please break group links and demonstrate proof of your claims.

The authors are grateful for this comment. Group references were improved according to the recommendation throughout the document

6. Lines 56-59 are common knowledge. This information slightly overloads the text and distracts the reader's attention. If the authors believe that it is necessary to mention this in the text of the manuscript, then, from my point of view, it is enough to say: The stages of processing REE ores are described [link], and the enrichment stages are specified [26].

The wording of the text was improved. “REE ores such as monazite are generally processed in several stages (concentration, dephosphorization, leaching and solvent extraction)”. Now this is on lines 62-63

7. In lines 76-78 the authors make two statements. But at the end of the second statement (line 78) a reference is made to the study [33]. From my point of view, this reference proves the first statement of the authors (line 76-77), but the statement that solvent extraction is widely used throughout the world in hydrometallurgical processing is not confirmed by the authors. Such incorrect references or statements without corresponding proof from the manuscript are sufficient. Changes need to be made.

The wording of the text was improved according to the recommendation. “Solvent extraction, or liquid-liquid extraction, is one of the most important separation processes in hydrometallurgy [29]” now this is on lines 72-73.

8. Notes on figures 1, 2 and 3. Who made Figures 1, 2 and 3, if by the authors, then it is necessary to indicate (done by the authors), if borrowed, then it is necessary to indicate the source of the borrowing or make an appropriate link to the source. If you have permission to submit a borrowed drawing, please provide this permission to the editor.

The authors are grateful for this comment. Authorship of figures 1,2 and 3 was added

9. From my point of view, the introduction is very voluminous and it is overloaded with information about the method of extraction and about the description of activators (extractors), a lot of unnecessary information that needs to be placed in other sections. From my point of view, the description of a method for extracting a useful component by leaching is more suitable for the methods section. The introduction includes: the relevance of the research topic, a review of the literature on the topic, a statement of the problem, the formulation of the purpose and objectives of the study. Part of the material posted by the authors in the introduction should be placed in the "Methods and Materials" section.

The information provided in the introduction was refined and some information was moved to other sections. As you can see in lines 174-176 “The choice of acid during leaching depends on the subsequent separation process; H₂SO₄ is often chosen to facilitate separation by precipitation, whereas HCl is used for separation by solvent extraction. However, HNO3 is generally avoided due to NOx evolution and production of nitrated wastewater [20]” and 192-197 “Today a large number of extractants are available for use in hydrometallurgy with more than 40 reagents available, of which approximately twelve are in everyday use, a detailed list of these reagents can be found in the work of Flett [ 29 ]. Solvent extraction of individual REE is a complex task due to the fact that these elements have similar physical and chemical properties, many extractable ions are in solutions, so a considerable number of separation steps are usually needed to produce a purified individual lanthanide element”

10. I still did not see that the authors well covered the issue of relevance. The topic the authors touched on is really relevant. As I noted earlier: Involvement in the recycling of technogenic waste from mining and processing is a very topical issue. The development of an innovative technology for extracting a useful component from the waste of processing production is not only a way to reduce the impact of a mining enterprise on the environment, but also an opportunity to increase the life of the enterprise and create additional jobs. After the depletion of reserves and the completion of the development of the deposit, it is possible to begin extracting useful components from waste - this is an increase in the life of the enterprise, and the extraction of a useful component from waste in parallel with the development of the deposit is the creation of additional jobs. The development and implementation of such technology can achieve a multiplier environmental and economic effect.

This topic is covered in the lines 42-45. “In addition, the identification of mining and industrial wastes with potential for utilization is currently a very relevant topic [1 ,4 ,14 ], since the development of innovative processes to extract elements of interest is not only a way to reduce the environmental impact of a company, but also an opportunity to increase the useful life of the company after the depletion of reserves”

11. At the end of the introduction (before the work plan) there are no conclusions on the analysis. This conclusion allows us to characterize the actual question posed, the purpose of the study and the tasks to be solved to achieve this goal. For example: Analyzing the above, it can be noted that ...... is a very topical issue. Therefore, the purpose of this study is ..... and to achieve which it is necessary to solve the following tasks: 1); 2); ..... Such a conclusion allows the reader to understand the vector of the study, and the authors at the end of the study correctly formulate conclusions on it.

The authors are grateful for this comment. The following text was included at the end of the introduction (before the work plan) in lines 90-95: “However, it can be observed that there are no detailed studies to determine the technical feasibility of extracting REE from these monazites, which is a highly relevant issue. Therefore, the purpose of this study was to determine the concentration, dephosphorization and leaching conditions of monazite, in order to subsequently study the solvent extraction conditions of Ce, La and Nd. To achieve this purpose, a monazite concentrate was first obtained”

12. Is the "Flowsheet for concentrating monazite from alluvial gold mining residue" proposed in Figures 4 and 5 developed by the authors? If so, then it must be specified. If it is borrowed, then it is necessary to indicate the source of borrowing or make an appropriate reference to the source. If you have permission to submit a borrowed drawing, please provide this permission to the editor.

Added authorship of the diagrams shown in figures 4 and 5.

13. Section 2.5. What is the mixing speed?

The mixing speed can be seen in line 181. "300 rpm".

14. In lines 218-221, the authors state the particle size distribution, then give figure 5 showing the same data. Is this duplication necessary? You have given an excellent description of figure 6, indicating the main mineral and so on. From my point of view, it can also be limited for particle size distribution.

The wording is improved so as not to generate duplication of information. Now this is on lines 215-216.

15. Despite the fact that the manuscript has an impressive list of references, 17 papers are older than 5 years and a lot (41) are older than 10 years out of 54. I observe very weak citation geography. The question posed in the present study is very relevant and universal. Your citation list contains works by scientists from many countries, but as I noted, these studies are quite old. You have undeservedly overlooked recent studies of Balkan, Central European and Russian scientists working in this direction for the last 5 years. I would like to draw your attention to very recent works listed below. Widening of the reference list might raise your citing potential, that is beneficial for both: The Authors and the Journal. Of course, the choice is yours and I encourage you to make your own search in Scopus and/or MDPI search engine.

The authors are grateful for this comment. The list of references was refined, eliminating some older references, and introducing some more recent ones.

16. Conclusions do not match the stated title and abstract. The conclusions are very superficial and do not indicate the final result obtained by the authors. I would like to note that lines 379-385, from my point of view, cannot be conclusions for work. Determining the granulometric or chemical-mineralogical composition of enrichment waste was not the purpose of the study. The authors of this information were an intermediate step to achieve the ultimate goal. And the conclusions should be more specific: developed ...; got.....; achieved .... From my point of view, the conclusion needs to be redone.

The conclusions have been rewritten, considering the suggestions made

Author Response File: Author Response.pdf

Reviewer 2 Report

1. Abstract: “D2EHP” should be rewritten as D2EHPA.

2. Explanation of ICP-OES in line 194 is redundant, because the author has already explained it in line 140.

3. The pulp density in the manuscript is 30 g/L, but the volume of leaching solution used is 0.3 L and the mass of the initial dephosphorized rare earth concentrate is 7.5 g. The two results do not match.

4. Error bars must be added to the figures.

5. The 6 mol/L hydrochloric acid was used for the leaching. However, the pH of the leaching solution during the subsequent solvent extraction was 1.5 to 2.0. Did the pH of leaching solution need to be adjusted before solvent extraction? The authors should have given the H⁺ concentration of leaching solution in manuscript.

6. The unit of legend in Figure 11 is incorrect.

7. Figure 13 duplicates the data in Table 4.

8. The extraction of Ce, La and Nd use D2EHPA and Cyanex 572 both followed a cation exchange mechanism, so why the Cl^- number of D2EHPA and Cyanex 572 involved in the reaction is different? The authors should have done the experiment of Log D vs. Log [Cl^-] to verify it.

9. The recovery and recycling performance of extractants need to be proved by experimental data.

Comments for author File: Comments.pdf

Author Response

Reviewer #2:

The authors are grateful for the comments provided by the reviewer. Changes to the manuscript have been highlighted in magenta in the revised version of the manuscript.

1. Abstract: “D2EHP” should be rewritten as D2EHPA.

The change was made, you can see in lines 11 and 12

2. Explanation of ICP-OES in line 194 is redundant, because the author has already explained it in line 140.

The authors are grateful for this comment. Correction was made, now this is on lines 185

3. The pulp density in the manuscript is 30 g/L, but the volume of leaching solution used is 0.3 L and the mass of the initial dephosphorized rare earth concentrate is 7.5 g. The two results do not match.

The authors are grateful for this comment. Correction was made, the pulp density is 25 g/L, now this is on lines 182

4. Error bars must be added to the figures.

The authors are grateful for this comment. There are no error bars in the figures, because each of the experiments was particular.

5. The 6 mol/L hydrochloric acid was used for the leaching. However, the pH of the leaching solution during the subsequent solvent extraction was 1.5 to 2.0. Did the pH of leaching solution need to be adjusted before solvent extraction? The authors should have given the H+ concentration of leaching solution in manuscript.

The following text was included in the section 3.5.2 in lines 305-306: “the required pH of the aqueous was adjusted using NaOH or HCl solution.”

6. The unit of legend in Figure 11 is incorrect.

The legend of figure 11 was corrected

7. Figure 13 duplicates the data in Table 4.

The authors are grateful for this comment. The idea of showing figure 13 is to have a comparative picture of the leaching efficiency of the elements Ce, La and Nd. While table 4 indicates detailed information of the leaching process, including the leaching efficiency.

8. The extraction of Ce, La and Nd use D2EHPA and Cyanex 572 both followed a cation exchange mechanism, so why the Cl- number of D2EHPA and Cyanex 572 involved in the reaction is different? The authors should have done the experiment of Log D vs. Log [Cl-] to verify it.

The authors are grateful for this comment. The number of Cl- involved in the reactions with D2EHPA and Cyanex 572 is different according to equations 11 and 12, considering the balance and the number of H+ involved according to the slope analysis shown in figures 16 and 17. However, it is necessary to carry out more experiments considering the concentration of Cl- and the concentration of the extracting agents, to be conclusive. which is why it is indicated in line 337 that more research is necessary before drawing conclusions (this research is currently being carried out)

9. The recovery and recycling performance of extractants need to be proved by experimental data.

Table 5 is included to show this information.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The presented version of the corrected article is a complete scientific study and from my point of view will be very interesting to the reader. All remarks are taken into account, which makes the manuscript easier for readers to perceive. The manuscript can be published in the open press in present form.