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by
  • Biao Shen 1,
  • Xuexin Chen 2,† and
  • Baijun Yan 2,*

Reviewer 1: Anonymous Reviewer 2: Iván A. Reyes Reviewer 3: Anonymous

Round 1

Reviewer 1 Report

see attached

Comments for author File: Comments.pdf

Some (essentially minor) improvement of the English is recommended; see comments in review

Author Response

  • Overall (in particular in the Introduction), I recommend some adaption of the English to its conventional use. Selected examples (line 1 to 60): line 25, oxalic acid; line 32, transition element (rather than transitional element); line 39, two types … deposits … (rather than deposit); line 44/45: dissolved by oxidation (rather than oxidised) and enriched in (instead of enriched into) the formed slag; line 54: To solve (rather than To solving) these problems; line 47: “That stated above” should read “What is stated above”?

Authors’ reply: Sorry for the careless mistake. We have corrected it in the revised manuscript.

  • line 53: CO2 is a toxic gas ??

Authors’ reply: Thanks for the careful comments of reviewer. It was replaced by harmful gas in the revised manuscript.

  • lines 56/57: What is sulfide acid supposed to be?

Authors’ reply: Sorry for our careless mistake. The correct name “sulfuric acid” was used in the revised manuscript.

  • line 62: What kind of “lower organisms” from the marine sediment?

Authors’ reply: Here, the “lower organisms” refer to the microorganism in seawater.

  • What is “the carbonaceous” supposed to be? Carbonate?

Authors’ reply: The reviewer is right. It was replaced by “carbonaceous matter”.

  • Tetravalent vanadium is dissolvable? Which species containing VIV are the authors referring to?

Authors’ reply: Thanks, the original expression is really ambiguous. It was revised as “the carbonaceous matter protects trivalent vanadium (V3+) from being oxidized to tetravalent vanadium (V4+) or pentavalent vanadium (V5+) and forming soluble vanadate.”

  • I do not quite perceive the meaning of the sentence “m0 is the mass of the concentrate …

Authors’ reply: This sentence was revised as “ is the quantity of the concentrate”.

  • 1, ordinate: Leaching (instead of Leacing); line 205: x is (instead of xis)

Authors’ reply: Figure 1 was redrawn, and the mistake in line 205 was corrected.

 

  • lines 222 and 223: It is not quite clear what the authors are referring to with “interfacial chemical reaction step”

Authors’ reply: A explanation was added in the revised manuscript.

  • line 296: It is not quite clear what “k0 is a constant model parameter” stands for.

Authors’ reply: Very agree with the reviewer. The “constant” was deleted in the revised manuscript.

  • legend to Fig. 7: “and” should be inserted between “experimental results” and “model results”?

Authors’ reply: Sorry for our carelessness. It was corrected.

  • lines 309 and 310: urgent (instead of urgently); is promising (instead of “is of promising”)

Authors’ reply: Thanks for the careful review. It was corrected in the revised manuscript.

 

Reviewer 2 Report

Review of the article metals-2569633: Hydrothermal Leaching Kinetics of Vanadium from an Iron Vanadate Mineral by Oxalic Acid

The article deals with the kinetics of the vanadium extraction from Fe3-xVxO4 mineral with oxalic acid as a leaching agent under hydrothermal conditions (90-130 °C). The kinetic analysis is well developed and the information obtained may be useful to potential readers of the journal. However, some comments must be considered before publication, some are minor but others deserve attention.

Comments:

Line 25: change “oxalis” to “oxalic”.

Why was a raw material characterization section not considered? At the least, the elemental analysis and the mineral phase identification by XRD of the Fe3-xVxO4 mineral sample must be provided. In the same sense, the characterization of the residual solids after the hydrothermal leaching process as elemental analysis, XRD, and SEM images would be desirable for a better explanation of the obtained results.

Provide more detail on the autoclave used. Is not clear how was stopped the hydrothermal reaction in every case. The vessel was dismounted for cooling?  

The shrinking core kinetic model requires for their correct application a constant size of particle or at the least a narrow interval of particle size. Did the authors consider the variation of the particle size in their leaching experiments and calculations?

Do not confuse recovery (efficiency) with extraction. The first is related to the total vanadium leached compared with the total amount present in the sample. The second is related to the leached vanadium under certain conditions of temperature and concentration of reagents and must be calculated with the total amount of vanadium leached when the reaction reaches the steady state. The data of extraction are the correct for calculating the kinetics of the vanadium leaching.

In the same sense that the previous comment, the reactions at 90 and 100 °C shown in Fig. 1 did not reach the steady state, then, the extraction of vanadium was not the maximum, and the efficiency reported under these conditions is not entirely correct.

Kd and kc in Eqs. 2 and 3 must be described in detail. This would help to understand the origin of Eq. 5.

The statement in Lines 208-210 is not entirely correct. Equations 2 and 3 are only valid in the conversion period (when there is a gradient of concentration between the particle surface and the bulk) and do not imply a change in the leaching mechanism, the reaction only reaches the steady state where the concentration does not change over time (there is no gradient of concentration).

The shrinking core kinetic model considers two cases, when the particle stays constant in size as the reaction progresses, and when the particle decreases in size until the complete dissolution is achieved. In this sense, which is the case that applies to the particles in the leaching of vanadium? According to Eq. 3, only the case when the particle size stayed constant with the formation of ash halo (the diffusion of fluid reagents in the solid of by-products layer is the controlling stage) was tested, however, it would be desirable to consider the equation corresponding to the diffusion of reagents in the fluid film as the controlling stage in the calculations. Although it was proved that the chemical reaction is the controlling stage of the leaching process, considering the equation when the diffusion in the fluid film as the controlling stage would help better explain the results.

Author Response

  • Line 25: change “oxalis” to “oxalic”.

Authors’ reply: Sorry for the careless mistake. We have corrected it in the revised manuscript.

  • Why was a raw material characterization section not considered? At the least, the elemental analysis and the mineral phase identification by XRD of the Fe3-xVxO4 mineral sample must be provided. In the same sense, the characterization of the residual solids after the hydrothermal leaching process as elemental analysis, XRD, and SEM images would be desirable for a better explanation of the obtained results.

Authors’ reply: This is a very constructive suggestion. We have added the XRD pattern of the concentrate. About the characterization of the residual solids, it has been published in our previous paper about the introduction the leaching method, so we just provided the citation of the previous paper in the revised manuscript.

  • Provide more detail on the autoclave used. Is not clear how was stopped the hydrothermal reaction in every case. The vessel was dismounted for cooling?

Authors’ reply: Thanks for the suggestion. The details on the experimental process about the cooling of the autoclave wad added in the revised manuscript.

  • The shrinking core kinetic model requires for their correct application a constant size of particle or at the least a narrow interval of particle size. Did the authors consider the variation of the particle size in their leaching experiments and calculations?

Authors’ reply: This suggestion of reviewer is really correct. In the present study, the concentrate used as the raw material was obtained by magnetic separation. Before the magnetic separation, the roasted pellet was ground and sieved to pass through a standard sieve with pore size of 0.074 mm. So, the particle diameter of the concentrate was less than 0.074 mm, and should distributed in a narrow range.

  • Do not confuse recovery (efficiency) with extraction. The first is related to the total vanadium leached compared with the total amount present in the sample. The second is related to the leached vanadium under certain conditions of temperature and concentration of reagents and must be calculated with the total amount of vanadium leached when the reaction reaches the steady state. The data of extraction are the correct for calculating the kinetics of the vanadium leaching.

In the same sense that the previous comment, the reactions at 90 and 100 °C shown in Fig. 1 did not reach the steady state, then, the extraction of vanadium was not the maximum, and the efficiency reported under these conditions is not entirely correct.

Authors’ reply: This is a very important point in kinetic study, and often ignored by many scholars. Really thanks for the comment. In the present study, the concentrate used as the raw material for leaching vanadium was composed of three minerals, Fe3-xVxO4, pyroxene and small amount of Fe, and only the Fe3-xVxO4 mineral contain vanadium. Therefore, the vanadium leached into the solution just came from the Fe3-xVxO4 mineral. In this case, the measured value for recovery efficiency was considered approximately as the extraction efficiency of vanadium from the Fe3-xVxO4 mineral. In the revised manuscript, this was pointed out.

  • Kd and kc in Eqs. 2 and 3 must be described in detail. This would help to understand the origin of Eq. 5.

Authors’ reply: The meanings of Kd and Kc in Eqs. 2 and 3 was added in the revised manuscript.

  • The statement in Lines 208-210 is not entirely correct. Equations 2 and 3 are only valid in the conversion period (when there is a gradient of concentration between the particle surface and the bulk) and do not imply a change in the leaching mechanism, the reaction only reaches the steady state where the concentration does not change over time (there is no gradient of concentration).

Authors’ reply: As stated in the reply of question 5, the raw material used was composed of three minerals, Fe3-xVxO4, pyroxene and small amount of Fe. So a FeC2O4⋅2H2O would be formed. With the proceeding of the leaching process, a larger amount of FeC2O4⋅2H2O would cover the Fe3-xVxO4 particles, hinder the leaching of vanadium, and decrease the leaching rate significantly.

  • The shrinking core kinetic model considers two cases, when the particle stays constant in size as the reaction progresses, and when the particle decreases in size until the complete dissolution is achieved. In this sense, which is the case that applies to the particles in the leaching of vanadium?According to Eq. 3, only the case when the particle size stayed constant with the formation of ash halo (the diffusion of fluid reagents in the solid of by-products layer is the controlling stage) was tested, however, it would be desirable to consider the equation corresponding to the diffusion of reagents in the fluid film as the controlling stage in the calculations. Although it was proved that the chemical reaction is the controlling stage of the leaching process, considering the equation when the diffusion in the fluid film as the controlling stage would help better explain the results.

Authors’ reply: We really agree the opinion of reviewer about the applicable cases of the shrinking core kinetic model. In our present study, we think that at the initial leaching stage of the leaching process, the particle size decreases with the progress of the reaction, because the quantity of the formed product FeC2O4⋅2H2O is not large, and it can be dissociated from the reactant particles. Whereas, at the later leaching stage of the leaching process, the formed product FeC2O4⋅2H2O may accumulate and adhere to the reactant particles, and the particle size become unchanged or change slightly.

About the diffusion of reactant in the fluid film, we think the resistance to it was extremely small. The first reason is that a relatively stirring speed of 500 rpm was adopted; the second is that a relatively large liquid/solid ratio of 90 was adopted.

 

Reviewer 3 Report

Review. First round

Article: Hydrothermal Leaching Kinetics of Vanadium from an Iron Vanadate Mineral by Oxalic Acid

Authors: B. Shen et al.

Manuscript ID:  metals-2569633

 

Abstract

Line 25, correct the word oxalis for oxalic.

Please define how much this process is better that the traditional. In this case, temperatures used are higher.

Introduction

This part is well written and reflects the overall merit of investigation, with an adequate background to understand the context.

Experimental

How was prepared the samples used in the X-ray fluorescence and by ICP-OES?

Please unify the units. For example authors uses °C and K combined in text.

Discussion

Again, units must to be uniform throughout the text

Why authors chosen the unreacted shrinking core model?

I think that author could pointed why the leaching studies done with different reagents gave a diffusion control instead chemical control, according temperature effect.

For the effect of concentration of leaching agent, author mentioned in text a range from 15 to 30 % with 5 % interval. However, in table 4 they showed a 33% of concentration.

Just one question, why authors did not determine the effect of particle size to complete the kinetic study?  This could help to complete the kinetics equation determined as follows

r0/VM [1 - (1-X)1/3] = A exp(-Ea/RT) [Reac]n t

where; ro is the average radio of particle size, VM is the Molar volume of mineral, [Reac] is the concentration of oxalic acid, n is the order of reaction, A is the frequency factor, Ea is the activation energy, R is the constant and T the temperature in K.

In this late case, particle size could be of great importance, according the mineral dressing to be involved in a large-scale process.

Conclusions

Perhaps this process can reduce harm environmental effects caused in other alternative process. However, readers need to know much more about this. For this case the process works well at higher temperatures, how this can help to reduce environmental impact?

 

Regards

Author Response

  • Line 25, correct the word oxalis for oxalic.

Authors’ reply: Sorry for the careless mistake. We have corrected it in the revised manuscript.

  • Please define how much this process is better than the traditional. In this case, temperatures used are higher.

Authors’ reply: The process of hydrothermal leaching vanadium by oxalic acid is better in two aspects. First, the oxalic acid is environmentally friendly than inorganic acid such as sulfuric acid. Second, this hydrothermal leaching process can extract the trivalent vanadium directly, which can eliminate the oxidation roast step of the traditional process.

  • How was prepared the samples used in the X-ray fluorescence and by ICP-OES?

Authors’ reply: Because the content of vanadium in the concentrate was needed to determine the leaching efficiency of vanadium, the more accurate method ICP-OES was adopted.

  • Please unify the units. For example, authors uses °C and K combined in text.

Authors’ reply: Sorry for our carelessness. In the revised manuscript, the units were unified.

  • Why authors chosen the unreacted shrinking core model?

Authors’ reply: Firstly, the unreacted shrinking core model is a common model to describe the reaction between a fluid phase and a condense phase; Secondly, it is simple and effective; Thirdly, the present study leaching reaction fits well with the assumptions of the unreacted shrinking core model, such as the reactant is dense particle, the particle shrinks gradually with the proceeding of the leaching reaction.

  • I think that author could pointed why the leaching studies done with different reagents gave a diffusion control instead chemical control, according temperature effect.

Authors’ reply: Thanks for this constructive suggestion. We also noticed the difference of the leaching kinetics of vanadium by different authors using different reagents. We think that this difference may be related with the properties of the vanadium-bearing concentrate such as the mineral composition and micro-structure, and with the properties of the product such as the density and their adherence to reactant particles. In summary, this should be a very complex but meaningful topic. In future, we will plan to deal with the present adopted concentrate using different reagents such as sulfuric acid.

  • For the effect of concentration of leaching agent, author mentioned in text a range from 15 to 30% with 5 % interval. However, in table 4 they showed a 33% of concentration.

Authors’ reply: Sorry for the mistake. It was corrected to 30% in the revised manuscript.

  • Just one question, why authors did not determine the effect of particle size to complete the kinetic study? This could help to complete the kinetics equation determined as follows

r0/VM [1 - (1-X)1/3] = A exp(-Ea/RT) [Reac]n t

where; ro is the average radio of particle size, VM is the Molar volume of mineral, [Reac] is the concentration of oxalic acid, n is the order of reaction, A is the frequency factor, Ea is the activation energy, R is the constant and T the temperature in K.

In this late case, particle size could be of great importance, according the mineral dressing to be involved in a large-scale process.

Authors’ reply: This suggestion of reviewer is really constructive. In the present study, the concentrate used as the raw material was obtained by magnetic separation. Before the magnetic separation, the roasted pellet was ground and sieved to pass through a standard sieve with pore size of 0.074 mm. So, the particle diameter of the concentrate was less than 0.074 mm, and should distributed in a narrow range.

In the future, we will plan to change the particle size of the concentrate, and inspect the effect of the average particle size on the behavior of the leaching kinetics.

  • Perhaps this process can reduce harm environmental effects caused in other alternative process. However, readers need to know much more about this. For this case the process works well at higher temperatures, how this can help to reduce environmental impact?

Authors’ reply: Some minor revisions and explanations were made in the whole manuscript to make the features of the present process clear.