Accessing Metals from Low-Grade Ores and the Environmental Impact Considerations: A Review of the Perspectives of Conventional versus Bioleaching Strategies

Round 1

Reviewer 1 Report

In their review, the authors describe the potential of metal recovery from low-grade ores and the related environmental impacts of pyrometallurgical, hydrometallurgical and bio-hydrometallurgical processes. In addition, perspectives regarding conventional and bioleaching strategies are discussed.

Overall, the manuscript gives a good overview of challenges and problems arising during the processing of low-grade ores or mine tailings and the comparison of different processes is sufficient. Different review papers are already available, dealing with metal recovery from artificial ores such as industrial ashes, slags and other by-products, with low metal concentrations. An overall review, specific for low-grade ores or mine tailings could add to this.

Nevertheless, in some parts of the manuscript, the authors could be more specific (see points below).

1. Page 2, Line 48-49. The authors mention environmental problems, without closer definition or examples.
2. Page 5, Line 208-220. This paragraph is about phosphate minerals and not about metal mining. I suggest to remove this paragraph
3. Section 3.1. Page 6. This section misses examples for the processing of low-grade ores via pyrometallurgical processes. Within the manuscript, tables for hydrometallurgical and bio-hydrometallurgical processes are given. Could the authors provide also some examples of investigations regarding pyrometallurgical processing of low-grade ores?
4. Page 7, Line 325-333. This paragraph should extend the section 3.1. as the information provided in there, does not fit to hydrometallurgical processes.
5. Page 7, Line 342-343. Please be more specific about negative impacts associated to the use of H₂SO₄.
6. Section 5.1.3. Page 15. The different bioleaching mechanisms and related metabolic activities have already been intensively discussed in different review papers. In the present manuscript, this reflects a repetition of already reported and summarized information. I recommend to skip this paragraph.
7. In addition, the potential recovery of metals from hydrometallurgical and bio-hydrometallurgical lixiviants is completely absent in the present manuscript. This reflects a major difference to pyrometallurgical processes and should be discussed in a separate section. Different methods for metal recovery from leaching solutions are available but differ in efficiency of metal recovery, purity of the recovered metal and applicability leading to additional economic considerations.

Minor comments.

8. Page 6, Line 258. Repetition of “previously”
9. Page 8, Line 383. “Is”
10. Page 10, Line 450. Remove “,”
11. Page 17, Line 774. Figure 3 is not present in the manuscript
12. Page 17, Line 779. Acidithiobacillus ferrooxidans should be written in italic letter
13. Page 19, Line 886. The common bacteria name is Acidithiobacillus ferrooxidans

Overall, I recommend this manuscript for publication after minor revision.

Author Response

Minerals

Accessing metals from low-grade ores and the environmental impact considerations: a review of the perspectives of conventional versus bioleaching strategies

Dear Reviewer

RE: LIST OF CHANGES ON THE POINTS RAISED BY REVIEWER

Thank you for your useful technical comments and suggestions on our manuscript. We have modified the manuscript accordingly.

Detailed corrections are listed below point by point. Points raised by the reviewers are in italics and changes made or my response to those points in normal text.

Reviewer #1

In their review, the authors describe the potential of metal recovery from low-grade ores and the related environmental impacts of pyrometallurgical, hydrometallurgical and bio-hydrometallurgical processes. In addition, perspectives regarding conventional and bioleaching strategies are discussed.

Overall, the manuscript gives a good overview of challenges and problems arising during the processing of low-grade ores or mine tailings and the comparison of different processes is sufficient. Different review papers are already available, dealing with metal recovery from artificial ores such as industrial ashes, slags and other by-products, with low metal concentrations. An overall review, specific for low-grade ores or mine tailings could add to this

• Thank you very much

Nevertheless, in some parts of the manuscript, the authors could be more specific (see points below).

1. Page 2, Line 48-49. The authors mention environmental problems, without closer definition or examples.

• The definition and examples were given as suggested by the reviewer.

2. Page 5, Line 208-220. This paragraph is about phosphate minerals and not about metal mining. I suggest to remove this paragraph

• The paragraph has been removed.

3. Section 3.1. Page 6. This section misses examples for the processing of low-grade ores via pyrometallurgical processes. Within the manuscript, tables for hydrometallurgical and bio-hydrometallurgical processes are given. Could the authors provide also some examples of investigations regarding pyrometallurgical processing of low-grade ores?

• There are not so many examples of metal recovery from low grade using pyrometallurgical processes in literature due to the dependency of pyrometallurgy on smelters. However, the following statement was added. “Because low-grade ores are polymetallic, the different metals in the ore should be re-covered for the process to be profitable. In the case of low copper-bearing minerals with high arsenic and other impurities, this becomes an issue. It results in "dirty" concentrates, which are rejected by smelters if copper concentrates contain more than 0.2 percent arsenic [41,80].”

4. Page 7, Line 325-333. This paragraph should extend the section 3.1. as the information provided in there, does not fit to hydrometallurgical processes.

• The paragraph has been combined with the second paragraph of Section 3.1

5. Page 7, Line 342-343. Please be more specific about negative impacts associated to the use of H₂SO₄.

• An example of negative impact of H₂SO₄ has been described as follows: H₂SO₄ is transported via pipes, and a leak or spill due to a broken pipe can occur as it is piped around the facility, leading to ground water contamination [86]. 

6. Section 5.1.3. Page 15. The different bioleaching mechanisms and related metabolic activities have already been intensively discussed in different review papers. In the present manuscript, this reflects a repetition of already reported and summarized information. I recommend to skip this paragraph.

• Section 5.1.3 has been removed 

7. In addition, the potential recovery of metals from hydrometallurgical and bio-hydrometallurgical lixiviants is completely absent in the present manuscript. This reflects a major difference to pyrometallurgical processes and should be discussed in a separate section. Different methods for metal recovery from leaching solutions are available but differ in efficiency of metal recovery, purity of the recovered metal and applicability leading to additional economic considerations.

• A short section was added as follows:

The petroleum refining processes of hydro-processing and fluid catalytic cracking (FCC) generate massive amounts of spent catalysts containing toxic and valuable metals [199]. These spent catalysts are typically disposed of in approved dumpsites that pose a risk to the environment [200]. These wastes are being investigated as a secondary source of valuable metals such as Ni, V, Mo, Co, W, Al, and others in order to solve environmental issues while also meeting current metal demand. Metal extraction from spent catalyst can be accomplished via a hydrometallurgical or bio-hydrometallurgical process. Mouna and Baral [201] used a bio-hydrometallurgical approach to leach lanthanum from spent fluid catalytic cracking catalyst (SFCCC) using the fungus Aspergillus niger in a study published in 2019. Lanthanum recovery efficiency of 63 % was observed at 1% pulp density but decreased as pulp density increased due to the SFCCC's inhibition effect on A. niger activity. Hydrochloric acid had a high recovery efficiency of 68 % to chemical leaching. In this study, bio-hydrometallurgy (a greener process) was recommended because the leaching efficiencies of the two were comparable. However, because these metals have low recovery rates (less than 50%), efforts must be made to develop technologies that allow for high recovery rates [199,202].

Minor comments.

1. Page 6, Line 258. Repetition of “previously”

• Corrected

2. Page 8, Line 383. “Is”

• Corrected

3. Page 10, Line 450. Remove “,”

• Removed

4. Page 17, Line 774. Figure 3 is not present in the manuscript

• Section 5.1.3 which figure 3 was mentioned has been removed

5. Page 17, Line 779. Acidithiobacillus ferrooxidans should be written in italic letter

• Section has been removed

6. Page 19, Line 886. The common bacteria name is Acidithiobacillus ferrooxidans

• Corrected

 Overall, I recommend this manuscript for publication after minor revision.

• Thank you

The authors would like to thank you once more for the constructive comments that help to improve the quality of our work.

Kind regards

Reviewer 2 Report

The review gives a comprehensive overview on the extraction of metals from low-grade ore with a special focus on tailings. The reviews present recent developments in bioleaching strategies on different levels. The review considers many different aspects such as environmental impact, different strategies for improvements in bioleaching processes, (meta)genomics. However, I am missing one important aspect although not focusing just on metal recovery. What happens to the tailings after extraction? In the article it is presented that the residues are deposited again after extraction. However, also these residues have to be dealt with, current strategies focus on remediation or on the use of the residues e.g. as building materials, aggregates in cement production etc. I think also these aspects should be mentioned in the article to give further perspectives for a holistic approach.

Other comments/typos:

p.13, l.585: I guess sentence should be deleted

l.616 species names in italic letters

table 2: strains instead stains

Author Response

Minerals

Accessing metals from low-grade ores and the environmental impact considerations: a review of the perspectives of conventional versus bioleaching strategies

Dear Reviewer

RE: LIST OF CHANGES ON THE POINTS RAISED BY REVIEWER

Thank you for your useful technical comments and suggestions on our manuscript. We have modified the manuscript accordingly.

Detailed corrections are listed below point by point. Points raised by the reviewers are in italics and changes made or my response to those points in normal text.

Reviewer #2

The review gives a comprehensive overview on the extraction of metals from low-grade ore with a special focus on tailings. The reviews present recent developments in bioleaching strategies on different levels. The review considers many different aspects such as environmental impact, different strategies for improvements in bioleaching processes, (meta)genomics. However, I am missing one important aspect although not focusing just on metal recovery. What happens to the tailings after extraction? In the article it is presented that the residues are deposited again after extraction. However, also these residues have to be dealt with, current strategies focus on remediation or on the use of the residues e.g. as building materials, aggregates in cement production etc. I think also these aspects should be mentioned in the article to give further perspectives for a holistic approach. 

• A paragraph was added as follows: However, reprocessing tailings generates waste, which poses environmental problems. Consequently, not all reprocessed tailings are permanently stored for future reprocessing. Some researchers are investigating its use in the production of bricks as a building material-al in the construction industry [125]. According to Beulah et al. [126], although the bricks are more expensive than traditional bricks, this cost-effective method of producing brick is environmentally friendly. Another example is the use of tailing as a substitute for fine aggregates in the manufacture of cement [127].

 Other comments/typos:

p.13, l.585: I guess sentence should be deleted

• Sentence deleted

l.616 species names in italic letters

• Corrected

table 2: strains instead stains

• Corrected

The authors would like to thank you once more for the constructive comments that help to improve the quality of our work.

Kind regards,