Review Reports - Interfacial Structure Change and Selective Dissolution of Columbite–(Fe) Mineral during HF Acid Leaching

Round 1

Reviewer 1 Report

The paper of Yang et al studied an interesting aspect related to selective dissolution of columbite during acid leaching using HF. In general, the paper is well written, and the methodology is appropriately explained. In my opinion, the discussions are supported by the result of this study.

The paper can be accepted with minor corrections:

Please add details about the sample used for this study. From where the columbite sample was collected, etc? particle size distribution?
For example: add details about samples filtration for ICP analysis; what diameter (0.45µm or less?)
What type of ICP has been used for the analysis.
The chemical quality of the leachates presented only for Fe and Nb, what about other chemical species?

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors of the article “Interfacial structure change and selective dissolution of columbite mineral during HF acid leaching” experimentally and theoretically analyzed the surface element adsorption, charge distribution, chemical state, and energy changes of the mineral surface during the process of leaching columbite with different concentrations of hydrofluoric acid by X-ray photoelectron spectroscopy (XPS), leaching experiments and density functional theory (DFT) calculation. The results of this article can be rather convincing after significant revision. And one of the main problems, from my point of view, the poor operation by mineralogical terminology and nomenclature. So, the following suggestion should be considered by the authors of this manuscript:

Title: columbite is not a mineral, but a series of minerals called “Columbite-(Fe)-Columbite-(Mn) Series”. If you analyze only Fe-rich columbite, its better to replace “columbite” with “columbite-(Fe)”.
row 29 – columbite is not an “ore”, but “ore mineral”.
row 30 - columbite-(Fe) instead of iron ferrocolumbite, columbite-(Mn) instead of manganese niobium.
row 31 - tapiolite-(Mn) instead of manganese tantalite, and tapiolite-(Fe) instead of iron tantalum.
rows 35-36 – information about the sample used in the manuscript is better to replace to Material and Method section, and the info about calculation of unit cell parameters should be included.
48 – unclear who obtained “the research result”
The introduction section should be more concise. The information about methods (rows 61-93) can be removed.
The novelty of the manuscript is not satisfied with the introduction section.
Materials and Methods - The authors should add a separate section for the characterization of the initial sample, including chemistry and crystallography.
In the Results and discussion section, one can feel a strong lack of its discussion part (ie comparison with the literature overview on this topic or comparison with other minerals / ores at least). At the same time, the results can be expressed in a more concise way.
row 459 – please, check the chemical formulae formatting appropriately.

Author Response

Response to Reviewer 2 Comments

1 Profile of Our Submission

Manuscript ID.	minerals-1077367
Title	Interfacial structure change and selective dissolution of columbite mineral during HF acid leaching
Manuscript Submitted	30/12/2020

We would like to thanks for the reviewer’s endeavor for polish our presentation. As for all the comments, we followed the suggestions and accepted all corrections made by the reviewer.

Answers to the Comments

Point 1: Title: columbite is not a mineral, but a series of minerals called “Columbite-(Fe)-Columbite-(Mn) Series”. If you analyze only Fe-rich columbite, its better to replace “columbite” with “columbite-(Fe)”.

row 29 – columbite is not an “ore”, but “ore mineral”.

row 30 - columbite-(Fe) instead of iron ferrocolumbite, columbite-(Mn) instead of manganese niobium.

row 31 - tapiolite-(Mn) instead of manganese tantalite, and tapiolite-(Fe) instead of iron tantalum.

Response 1: The reviewer is really careful, and the expression of such terms makes the article more rigorous. The expression of mineralogical terms in this paper has been revised.

Point 2: rows 35-36 – information about the sample used in the manuscript is better to replace to Material and Method section, and the info about calculation of unit cell parameters should be included.

Response 2: Material and Method section has been presented as an independent part of the paper, and more details of the calculated parameters have also been added in the manuscript to refine the calculation section.

The valence electrons selected for the pseudopotential calculation of each atom were O 2s²2p⁴, Fe 3d⁶4s², and Nb 4s²4p⁶4d⁴5s¹. According to the test results of plane wave cut-off energy, the cut-off energy used for FeNb₂O₆ bulk model calculation was 450eV. The integration of the Brillouin zone adopted the Monkhorst-pack scheme, the k points was set to 2×3×1, the Pulay density mixing method was used in the calculation of self-consistent field (SCF), and the convergence accuracy of SCF was 5.0×10^-5eV/atom. The Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm was used in the structural optimization of the FeNb₂O₆ bulk model. The optimization parameters included the convergence criterion of the interaction force between atoms, the internal stress in the crystal and the maximum displacement of the atoms, which were set to 0.1eV/A, 0.2GPa and 5×10^-3A, respectively. When these three parameters were optimized at the same time, the sign of structural optimization was that these parameters all reached the convergence standard.

Point 3: unclear who obtained “the research result”

Response 3: The name of the researcher A. Timofeev has been added to the citation.

The research results of A. Timofeev’s study showed that in the niobium oxide ion aqueous solution, increasing the HF concentration was beneficial to the dissolution of niobium oxide, and the pH value was the main factor controlling the dissolution of niobium oxide.

Point 4: The introduction section should be more concise. The information about methods (rows 61-93) can be removed. The novelty of the manuscript is not satisfied with the introduction section.

Response 4: Based on the research I have done, I decide to keep some references about Mulliken population analysis, core-level energy shift, the combination of DFT calculations and XPS experiments in Introduction section(lines 75-95 originally). And I’m sure that the novelty of the manuscript is satisfied with the introduction section.

Point 5: Materials and Methods - The authors should add a separate section for the characterization of the initial sample, including chemistry and crystallography.

Response 5: The separate section Materials and Methods contains the characterization of the initial sample ( including chemistry and crystallography characterization) has been added to the article.

The FeNb₂O₆ used in this paper was natural columbite-(Fe) from Yichun, Jiangxi province in China, with the space group Pbcn, and the lattice parameters were a=14.266Å, b=5.733Å, and c=5.049Å. Niobium is mainly distributed in ferro-niobium phase FeNb₂O₆. According to the columbite-(Fe) ore by energy dispersive spectrometer (EDS) analysis, the element content distribution of columbite-(Fe) phase is Nb-20.7at.%, Ta 1.1at.% and Fe-8.4at.%.According to the chemical composition, the formula of the ore is Fe_0.84Nb_2.07Ta_0.11O_6.98. The XRD pattern of selected columbite-(Fe) is shown in Figure 1(Figure 4 in attachment).

Figure 1(Figure 4 in attachment). XRD pattern of selected columbite-(Fe)

From the XRD results in Figure 1(Figure 4 in attachment), it could be seen that the selected columbite-(Fe) phase was consistent with the structure of FeNb₂O₆ crystal, so the space group of columbite-(Fe) used in this study was Pbcn, and the lattice parameters were a=14.266Å, b=5.733Å, and c=5.049Å.

Point 6: In the Results and discussion section, one can feel a strong lack of its discussion part (ie comparison with the literature overview on this topic or comparison with other minerals / ores at least). At the same time, the results can be expressed in a more concise way.

Response 6: We have modified the expression of the results and the discussion part to make the discussion part more in-depth.

Point 7: row 459 – please, check the chemical formulae formatting appropriately.

Response 7: We have checked and corrected the chemical formula in this paper.

Author Response File: Author Response.pdf

Reviewer 3 Report

The current investigation concerns the surface behavior of columbite by crystal transformation when the mineral is subjected to the HF leaching using the Mulliken population analysis. Because it deals with columbite mineral, it relates some interest to the readers of the journal, Minerals. Other than that, it does not contribute very much to the recovery of metal, Nb per say. More practical leaching agents for columbite would be sulfuric, hydrochloric, and nitric acids than HF. Some comments could have been beneficial when the effects of sulfate, chloride, or nitrate ions in addition to fluoride are made.

The authors have concluded that the dissolution of Nb is much faster than that of Fe with HF, which is based on the bond strength between Nb-O and Fe-O. However, they failed to recognize the fact that the driving force behind the leaching is more than the bond energy. The complexation of these cations with anions such as F- in the solution phase contributes significantly to the dissolution rate. Also noted is that the Fe ions for complexation with F- is much easier than Nb, and furthermore, the detection of the dissolved Fe-species is not easy with ICP when it is formed complexes.

The paper reads well, and the presentation is by and large acceptable, but a direct relevance to the improvement of the recovery of Nb is somewhat questionable. Journals concerning the orbital theory could be a better place for the publication of this paper.

Some comments on the editorial improvement are given below:

Lines 37,43, 63,79,88: take out the initials of the author’s names.

Subscripts and superscripts should be handled correctly. For example. Fe3+ and, H2O.

There are two Fig 10s.

Author Response

Response to Reviewer 3 Comments

1 Profile of Our Submission

Manuscript ID.	minerals-1077367
Title	Interfacial structure change and selective dissolution of columbite mineral during HF acid leaching
Manuscript Submitted	30/12/2020

We would like to thanks for the reviewer’s endeavor for polish our presentation. As for all the comments, we followed the suggestions and accepted all corrections made by the reviewer.

Answers to the Comments

Point 1: The current investigation concerns the surface behavior of columbite by crystal transformation when the mineral is subjected to the HF leaching using the Mulliken population analysis. Because it deals with columbite mineral, it relates some interest to the readers of the journal, Minerals. Other than that, it does not contribute very much to the recovery of metal, Nb per say. More practical leaching agents for columbite would be sulfuric, hydrochloric, and nitric acids than HF. Some comments could have been beneficial when the effects of sulfate, chloride, or nitrate ions in addition to fluoride are made.

Response 1: In hydrofluoric acid method, hydrofluoric acid with concentration of 60%~ 70% is used to decompose columbite-(Fe)-columbite-(Mn) series mineral at 90~100℃. The main advantages of hydrofluoric acid method are as follows: the process is simple, the decomposition temperature is low (90~100℃), and the decomposition rate of high grade concentrate is high (98%~99%). Therefore, hydrofluoric acid method is widely used. However, for columbite-(Fe)-columbite-(Mn) series minerals with low grade, the efficiency of Nb and Ta elements recovery using hydrofluoric acid method is not particularly high. Therefore, the purpose of this paper is to indicate the dissolution mechanism of hydrofluoric acid leaching columbite-(Fe) process, and provide reference for the improvement of columbite-(Fe) hydrometallurgy process parameters through the dissolution mechanism of columbite ore mineral.

Point 2: The authors have concluded that the dissolution of Nb is much faster than that of Fe with HF, which is based on the bond strength between Nb-O and Fe-O. However, they failed to recognize the fact that the driving force behind the leaching is more than the bond energy.

Response 2: In addition to the binding energy, the charge transfer and ion coordination in hydrofluoric acid leaching process are also the factors that promote the dissolution of mineral surface. To some extent, the Mulliken population reflects the coordination dissolution tendency of Nb and Fe atoms with F ions on the mineral surface. The different amount of charge transfer between Nb, Fe and F atoms on mineral surface reflects the different influence of F ions on metallic elements on mineral surface. The larger effect of F ion on the charge transfer of Nb atom shows that F ion’s concentration is also the driving force of Nb atom dissolution from mineral surface.

Point 3: The complexation of these cations with anions such as F- in the solution phase contributes significantly to the dissolution rate. Also noted is that the Fe ions for complexation with F- is much easier than Nb, and furthermore, the detection of the dissolved Fe-species is not easy with ICP when it is formed complexes.

Response 3: ICP test was carried out with the model PERKINE 7300DV. The ICP equipment used in this paper can detect ion’s concentration as low as 5ppm in solution, so the ICP is accurate and sensitive enough to detect Fe complexes.

Point 4: Some comments on the editorial improvement are given below:

Lines 37,43, 63,79,88: take out the initials of the author’s names.

Subscripts and superscripts should be handled correctly. For example. Fe3+ and, H2O.

There are two Fig 10s.

Response 4: The comments have been checked and corrected.

Please see my attachment, the highlighted parts in the attached file are the modified parts.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors managed to do a good work during revision, and the current manuscript is suitable for publication in minerals.