The Behaviour of Rare Earth Elements from South African Coal Fly Ash during Enrichment Processes: Wet, Magnetic Separation and Zeolitisation

Round 1

Reviewer 1 Report

Manuscript ID: minerals-1310068

Title: The fate of rare earth elements found in South African coal fly ash during processing by wet, magnetic separation and zeolitisation.

Authors: Mero-Lee Ursula Cornelius et al.

Title must be changed. “Extraction/recovery of rare earth elements from South African coal fly ash during processing by wet, magnetic separation and zeolitisation” or “The behavior of rare earth elements from South African coal fly ash during enrichment processes: wet magnetic separation and zeolitisation”

Introduction. If authors write about South Africa CFA, they need to have reference for great research from van der Merwe et al. Ammonium sulphate and/or ammonium bisulphate as extracting agents for the recovery of aluminium from ultrafine coal fly ash [10.1016/j.hydromet.2017.05.015]

Line 124. Section 2. What are the particle size distribution of raw CFA, magnetic and non-magnetic fraction after wet magnetic separation and zeolitisation?

Line 126-134. What equipment used for wet magnetic separation? What the magnetic intensity (T)?

Line 136-140. Why authors used these parameters (8M NaOH, 150 °C for 24 hours)?

Figure 1. Add more peaks of magnetite. Sign all peaks at XRD pattern.

Table 1. What is the carbon content in CFA (wt. %)?

Table 2. Sc and Ga are the main trace elements of aluminum in aluminosilicates. Why didn't the authors measure the gallium content?

Figure 2. Sign more than one peaks of hematite and magnetite minerals. Valeev et al. had find a lot of peaks of hematite in magnetic fraction of CFA [https://doi.org/10.3390/met8070533]

Section 3.2.2.1 This section must be added to materials and methods. Add equation fro yield calculation.

Section 3.2.2. Authors must write about yield of fraction and recovery of magnetite from raw CFA to magnetic fraction.

Equation 1. Where is authors find this equation? Add reference.

Line 323-327. This is the same text from section 2. Remove it.

Authors must write full chemical formulas of minerals in all figures and article text (first time writes name and formula – Quartz (SiO₂), further only formula).

Authors must add chemical reaction of zeolitisation process of non-magnetic fraction (minerals) by NaOH.

Authors must add flowchart to the REE recovery method, write the optimal parameters and yields. What is the yield of CFA after zeolitisation?

Conclusions must have separate paragraph: 1) 2) etc.

Technical errors:

Remove “,” in “wet magnetic separation” in all article text.

The references style in the text are not corresponding to Minerals style https://www.mdpi.com/journal/minerals/instructions.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

1-This manuscript is a little bit lack of novelty. The authors should be explain the specification of this manuscript (like geopolitical importance of REEs, etc.)

2-The introduction section needs to be review and include a paragraph with an emphasis on the need for alternative uses of fly ashes (geopolymer concrete production, alkali activated concrete production, heavy metal removal material, etc.). The authors must spell out the alternative processes available in this front and research gap.  There are several publications in this direction and request to the author to carry out a detailed literature review and include in the manuscript.  Few of these are given below for the authors’ reference.

Mohammad Mastali, Zahra Abdollahnejad, Fernando Pacheco-Torgal, 16 - Carbon dioxide sequestration of fly ash alkaline-based mortars containing recycled aggregates and reinforced by hemp fibers: Properties, freeze-thaw resistance, and carbon footprint, Editor(s): Fernando Pacheco-Torgal, Caijun Shi, Angel Palomo Sanchez, In Woodhead Publishing Series in Civil and Structural Engineering, Carbon Dioxide Sequestration in Cementitious Construction Materials, Woodhead Publishing, 2018, Pages 393-409, ISBN 9780081024447, https://doi.org/10.1016/B978-0-08-102444-7.00016-2.

Soner Top, Hüseyin Vapur, Mahmut Altiner, Dogan Kaya, Ahmet Ekicibil, Properties of fly ash-based lightweight geopolymer concrete prepared using pumice and expanded perlite as aggregates, Journal of Molecular Structure, Volume 1202, 2020, 127236, ISSN 0022-2860, https://doi.org/10.1016/j.molstruc.2019.127236.

Prinya Chindaprasirt, Thammanun Boonbamrung, Apivich Poolsong, Wunchock Kroehong, Effect of elevated temperature on polypropylene fiber reinforced alkali-activated high calcium fly ash paste, Case Studies in Construction Materials, Volume 15, 2021, e00554, ISSN 2214-5095, https://doi.org/10.1016/j.cscm.2021.e00554.

Rishen Roopchund, Jerome Andrew, Bruce Sithole, Using cellulose nanocrystals to improve the mechanical properties of fly ash-based geopolymer construction materials, Engineering Science and Technology, an International Journal, 2021, ISSN 2215-0986, https://doi.org/10.1016/j.jestch.2021.04.008.

Lu Wang, Xunrong Huang, Jianxin Zhang, Feng Wu, Fuhao Liu, Hanghang Zhao, Xiongfei Hu, Xin Zhao, Jingtian Li, Xundiao Ju, Puhui Ji, Stabilization of lead in waste water and farmland soil using modified coal fly ash, Journal of Cleaner Production, Volume 314, 2021, 127957, ISSN 0959-6526, https://doi.org/10.1016/j.jclepro.2021.127957.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This research is of great practical significance and has a guiding role in the extraction and separation of valuable metals from fly ash. However, there are still some problems, which are mainly as follows:  

1. General comment: Author needs to mention the demand for this research in introduction, abstract and conclusion. The novelty and practical applicability of this study should be clearly highlighted in the manuscript.

2. General comment: The experimental section could benefit from a flow diagram of your experimental process to help visualize.If you can supplement the graphic abstract, the article quality will be significantly improved.

3. Experimental: Line 143, there is an ambiguity about "solid products of the magnetic separation and zeolitisation process", and it is recommended to be clear as “zeolitisation process products of non-magnetic fraction”.
4. Line 156- Line 158，This paragraph is recommended to be incorporated into 3.1.1 and 3.1.2. This is the research method and does not need to be listed separately.
5. Line 230- Line 233. The same problem exists here, please incorporate this paragraph in 3.2.1 and 3.2.2.
6. Line 324- Line 327. The same problem exists here, please incorporate this paragraph in 3.3.1 and 3.3.2.
7. Experimental: The content of minor elements in CFA needs to be explained in more detail in the article. The calculation formula for "Relative standard deviation" or the origin of this data should be explained accordingly, and the unit "%" is also.
8. The quality of Figure 1 should be improved, because SEM micrograph is difficult to read.
9. Line 333, “a sodium sulphate crystalline mineral phase called Thernadite.” “Thernadite” should be corrected to “Thenardite”.
10. The author should supplement the content of trace elements in the supernatant liquid collected after the Si extract is precipitated from the filtrate in the article, and explain the phenomenon that there is almost no trace element content in the Si extract precipitate.
11. General comment: Please make sure your conclusions' section underscore the scientific value added of your paper, and/or the applicability of your findings/results, as indicated previously. Please revise your conclusion part into more details. Basically, you should enhance your contributions, limitations, underscore the scientific value added of your paper, and/or the applicability of your findings/results and future study in this session.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have made profound changes to the article. The Supplementary files are made at a very high level. I don't understand why the authors didn't add this infromation to the main text of the article. This is a very interesting data. It would improve the all research.

I would advise the authors to add this part to the article.

Otherwise, the article corresponds to the Minerals level and can be accepted.

Reviewer 3 Report

Many thanks for the author's efforts.  All the questions have been answered, and it can be accepted in the present form.