Energy Storage in Earth-Abundant Dolomite Minerals

Round 1

Reviewer 1 Report

1. line 17 : Dolomite is not the resource for phosphoric acid production its aan accompanying mineral
2. High concentration phosphatic dolomite pebbles - is different what was used in the experimenst

line 21 : which dolomite minerals ?

line 24 received contains ?

line 27 de or recarbonATION

LIne 41 calcium magnesium minerals(S)--- there are others as dolomite?

 

line 44 : what is the meaning for carbonates ?

Line 47 clay matrix -- not given in the quantitative determination löater in the papaer

 

Line 65 : how are the carboinates used for sequestartaion ?

Line 71,72 : is dolomite really a mahgnesium source --> seawater desalting a lot of MgCl2 is produced !! a lot of MG

 

Line 78 here is calcite and dolomite mentioned -- no Mg in calcite ?

Line 88 : decarbonisation of dolomite is taking place in 2 reactions!

are the temperatures given as precise and for which graikn size ?

line 108 how can dolomite be used for sequestration ?

line 111 where is the origin of dolomite from Alfa-Aesar ---natural?

line 133 X-ray

line 166 : what is a crbonated dolomite ?

Line 179 LOI is missing , How is the chemical anaysis be performed(ssems to be on the CO2/H2O free - should be added by other techniques !

 

Tabel 2 : the chmeical dolomite contains 8.69 % of Fe-Co-Ni mineral ? 19.74 % of unclassified minerals ?  and more -- doubtful?

tabel 4 in number 4 : Anthorite ?

Sample 4 and 5 in tabel 2 : these are really amphibole samples with quartz and unclassified minerals ? What is the sense of these ?

 

Line 196 : First menationing of a Fe-rich dolomite and even magnesite, Ankerite and dolomite are mentioed : do they occur together or are these fe-containing dolomites ?

Figure 2: First mentioning of pyroxene ? In this tabel Ankerite and Fe-contianing dolomite are mentioned?

line 222 dolomite phase ... were ?

223 : Is there really a amphibole formation during thermo-reaction ?

Line 225 : why is the quartz reduced in its content when it is not consumed by any thermoreaction ?

Figure 3: XRD patterns do only show rather pure dolomite (some quartz) - this is representing which part of the samples formerly described, as there are no other minerals visible in the patterns

Tabel 3 : a Comprison of X-ray peaks is not useful, comparison of lattice volume or lattice parameetrs would be more precise

line 263 : not (to) contribute

Figure 6 and 7 : arrhneius plots probably to small, it could be clearly seen, that the decomposition of dolomite (fig6) makes to different stages!

line 334 and following : It would be intersting to see teh phase formation after recarbonastion in case of dolomite ?

line 353 : please explain if moist or dry materials were carbonated ?

Line 360 : It would be interesting which type of commercial dolomite is used ?

Line 364 : XRD showed onyl calcite and dolomite , in figure 3 no calcite but quartz is shown, where are the other minerals ?

Followinf liones : the more impurities are present, the less is the potential for recarbonation ?

 

Overall intersting paper which needs improvement

 

Author Response

Comments and Suggestions for Authors – REVIEWER 1

 

1. line 17: Dolomite is not the resource for phosphoric acid production it’s an accompanying mineral

Thank you, Reviewer, for the suggestion. It is correct in the manuscript of the revised version.

 

1. High concentration phosphatic dolomite pebbles - is different what was used in the experiments

No, it is same handpicked dolomite labeled as hp-dolo in experimental section 2.

 

1. line 21: which dolomite minerals?

Commercial and handpicked dolomite minerals which is now stated in the abstract.

 

1. line 24 received contains?

Contains dolomite (CaMg(CO₃)₂) phase in high percentage.

1. line 27 de or recarbonATION

re-carbonation

 

1. LIne 41 calcium magnesium minerals(S)--- there are others as dolomite?

No. Only calcium magnesium carbonates or minerals are referred as dolomites.

 

1. line 44: what is the meaning for carbonates ?

Carbonates are the compounds carries CO₃ molecule. For example, CaCO₃ is chemically termed as Calcium Carbonate, where as CaMg(CO₃)₂ is termed as calcium magnesium carbonate.

1. Line 47: clay matrix -- not given in the quantitative determination later in the paper

Clay matrix or referred as unreactive materials like SiO₂ discussed in section 3.1. Thanks to the reviewer for comments and suggestions.

1. Line 65: how are the carbonates used for sequestration?

Carbonates are used for adsorbing CO₂ molecules which are considered to be the greenhouse gases. While adsorbing or absorbing CO₂, a metal becomes, metal carbonate, thus capture the CO₂ and enables the sequestration process.

1. Line 71,72: is dolomite really a magnesium source --> seawater desalting a lot of MgCl2 is produced !! a lot of Mg

Yes, the dolomite has at least 25% magnesium oxide phase could be the source for Mg. Dolomite is used as a source of magnesium metal and magnesia (MgO).

1. Line 78 here is calcite and dolomite mentioned -- no Mg in calcite?

Though calcite has not Mg content, still is usable as heat storage materials. But for the sake of uniformity, we have removed the calcite on line 80 in the revised manuscript.

1. Line 88: decarbonisation of dolomite is taking place in 2 reactions!

The decarbonization of dolomite is given in equation (1) and decarbonization from calcite is now removed from the revised manuscript.

1. are the temperatures given as precise and for which grain size?

Yes, the temperature obtained from the literature reference 15, and the grain sizes of oxides (CaO and MgO) having higher surface areas when compared to the carbonate [CaMg(CO₃)₂] phase. The grain sizes at this precise temperature is correspond to micrometer scales.

1. line 108 how can dolomite be used for sequestration?

The “sequestration” is removed from the revised manuscript (line 110).

1. line 111 where is the origin of dolomite from Alfa-Aesar ---natural?

The dolomites purchased from Alfa Aesar company is chemically processed and purified from the naturally occurring minerals with grain sizes of ~5 mm.

 

1. line 133 X-ray

Thank you. Yes, it now replaced with X-ray in the revised manuscript.

1. line 166: what is a crbonated dolomite?

It is carbonated dolomite and now corrected in revised manuscript. Thank you.

1. Line 179 LOI is missing, how is the chemical analysis be performed (seams to be on the CO2/H2O free - should be added by other techniques!

LOI for one of the commercial sample calcined is calculated with LOI in XRF. Please see the calculation below:

Compound	CaO	MgO	SiO2	P2O5	Fe2O3	Al2O3	CaO + MgO	LOI*
Mass (%)	39.10	9.80	0.22	0.33	5.11		48.90	45.24
*LOI – Loss on Ignition calculation done by considering the CO2 gas release in the calcination stage. This is by weighing the initial sample and then after the complete calcination of sample in STF. The difference of the weight indicates the CO2 release during calcination.

 

1. Table 2: the chemical dolomite contains 8.69 % of Fe-Co-Ni mineral? 19.74 % of unclassified minerals?  and more -- doubtful?

The percentages of Fe-Co-NI and unclassified minerals as reported based on our TESCAN measurement which was performed at Colorado School of Mines. Though the presence of Fe-Co-Ni minerals is due to the trace amounts of transition metals, it is only shown in the commercial dolomite sample where this particular phase may arise from any chemical processing adopted by the Alfa Aesar company in producing pure dolomite samples. On the other hand, the unclassified phase is uniformly seen in both commercial and handpicked dolomites and their calcined counterparts as reported by TESCAN.

1. table 4 in number 4: Anthorite?

It is now corrected as Anorthite in the Table 2, row 4 of the revised manuscript. Thank you for suggestions.

1. Sample 4 and 5 in table 2: these are really amphibole samples with quartz and unclassified minerals? What is the sense of these?

Samples 4 and 5, are the handpicked dolomites subjected calcination via single zone tube furnace under nitrogen and air ambient, respectively. These two samples showed the successful calcination of dolomite phase to amphibole as demonstrated using TESCAN measurements. This make sense to know the phase transformation during calcination process.

1. Line 196: First mentioning of a Fe-rich dolomite and even magnesite, Ankerite and dolomite are mentioned: do they occur together or are these Fe-containing dolomites?

The commercial dolomite in its partial thermal decomposition stage exhibited with number of phases, including Fe-rich minerals. From both XRF and TESCAN results, it is unambiguously understood that the dolomite purchased from Alfa Aesar are Fe-containing minerals and occurred together with other minor phases like magnesite and ankerite.

1. Figure 2: First mentioning of pyroxene? In this table Ankerite and Fe-containing dolomite are mentioned?

Pyroxene is removed from Figure 2 to align with Table 2 of the revised manuscript.

1. line 222 dolomite phase ... were?

The dolomite [CaMg(CO₃)₂] of 49.6% and calcite CaCO₃ of 17.5% as reported in Table 2 and Fig. 2 are considered to be the major contributing phases for the thermal decomposition of CO₂. This statement is now added in lines 226-228 of the revised manuscript.

1. 223: Is there really an amphibole formation during thermo-reaction?

Yes, per the Fig. 2 TESCAN results, it was clearly visible that the amphibole phase formation up to 52% and 63% are seen if the handpicked dolomites calcined (800 ^oC) under nitrogen or air ambient, respectively.

1. Line 225: why is the quartz reduced in its content when it is not consumed by any thermo reaction?

It is reported by Sasse et. al, that the thermal decomposition of SiO₂ occurs between 760 ^oC and 850 ^oC. Since our thermal decomposition process was in this temperature, plausible decomposition of quartz phase was confirmed from TESCAN and XRF results. [SiO diffusion during thermal decomposition of SiO₂, H.-E. Sasse and U. Konig, J. Appl. Phys. 67, 10, 1998; https://doi.org/10.1063/1.345185]. The above reference is now mentioned in the revised manuscript as [33].

1. Figure 3: XRD patterns do only show rather pure dolomite (some quartz) - this is representing which part of the samples formerly described, as there are no other minerals visible in the patterns

The X-ray diffraction profiles of commercial, handpicked, and handpicked-ballmilled samples showed the majority phases of dolomite which matches with the XRF results. In addition, the hanpicked and hanpicked-ballmilled samples exhibited an addition impurity peak of quartz (SiO₂) which is aligned with the TESCAN results. Unlike XRD, the TESCAN capabilities are much advanced and employed widely for the mineralogical studies. So the other mineral such as ankerite and Fe-rich phases as obtained from TESCAN because of its specific capabilities. XRD is a gross analysis and overall depends on the matching phases as reported in the ICDD or ICSD or JCPDS databases. Since the XRD of calcined products are not studied in this paper, the amphibole phase matching as obtained from TESCAN could not be matched with the XRD phase analysis.

1. Table 3: a Comparison of X-ray peaks is not useful, comparison of lattice volume or lattice parameters would be more precise

The lattice parameters and unit cell volume for three samples have been calculated and appended in the Table 3. We have added a text regarding the lattice parameters and unit cell volume information with additional references in the main manuscript. The additional text is also enclosed below with the table.

Table 3: Comparison of XRD peaks of dolomite of CO-Dolo, HP-Dolo, and HP-Dolo-BM

Co-Dolo Average Crystallite Size = 552.5 nm			HP-Dolo Average Crystallite Size = 448.3 nm			HPO-Dolo-BM Average Crystallite Size = 354.0 nm			Dolomite ICSD - 202162
2θ	d-value	FWMH	2θ	d-value	FWHM	2θ	d-value	FWHM	(hkl)
30.909	2.89075	0.1344	30.9	2.89125	0.188	30.906	2.89099	0.2253	104
41.078	2.19557	0.1932	41.076	2.19565	0.1856	41.072	2.19588	0.2378	113
44.841	2.01966	0.1696	44.878	2.01805	0.1829	44.868	2.01849	0.2226	202
50.430	1.80814	0.1463	50.450	1.80746	0.2309	50.458	1.80721	0.2937	018
50.982	1.78985	0.1809	51.008	1.7890	0.2195	51.005	1.78911	0.3062	116
Lattice Parameters (Calculated for hkl = (300) and (006)) a = b = 4.8163 Å c = 16.0748 Å Unit Cell Volume = 322.93 (Å)3			Lattice Parameters (Calculated for hkl = (300) and (006)) a = b = 4.8146 Å c = 16.0504 Å Unit Cell Volume = 322.22 (Å)3			Lattice Parameters (Calculated for hkl = (300) and (006)) a = b = 4.8187 Å c = 16.0513 Å Unit Cell Volume = 322.78 (Å)3			a = b = 4.811 Å c = 16.0462 Å Volume = 321.659 (Å)3

The most useful information such as lattice constants and unit cell volume of these dolomite samples have been calculated and compared with PDF Card no. 202162 for CaMg(CO₃)₂. The crystal system with space group R-3:H(148) is adopted [34]. The lattice parameters (a and c) and the unit cell volume of the commercial dolomite samples are slightly larger than the handpicked dolomites and it supports the average crystallites calculations. On the other hand, the ball milling though downsizes the crystallites about 94.3 nm, the lattice constants, and the unit cell volume slightly higher in HP-Dolo-BM when compared to pristine HP-Dolo. This increase in both lattice parameters (∆a/a0 = 0.085%; ∆c/c0 = 0.0056%) and unit cell volume (∆V/V0 = 0.18%) for the HP-Dolo-BM are due to the ball mill induced deformation in the crystal lattice [35].

 

1. line 263: not (to) contribute

Thank you. It is corrected in the main manuscript.

1. Figure 6 and 7: arrhneius plots probably to small, it could be clearly seen, that the decomposition of dolomite (fig6) makes to different stages!

Figures 6 and 7 are now plotted for easy readability and clear visibility. The thermal decomposition of dolomite (Fig. 6) to its constituent oxide and the liberation of CO₂ (mass loss %) is shown only in single stages with temperature.

1. line 334 and following: It would be interesting to see the phase formation after re-carbonation in case of dolomite?

Thank you for the suggestions. Due to the lack of high-pressure and high-temperature re-carbonation facilities, we could not produce large amounts of re-carbonated dolomites samples for XRD. However few milligrams of dolomites after calcination in TGA were subsequently carbonated under low pressure CO₂ flow with flow rate of 100 mL/min. Since the amount of sample re-carbonated are not sufficiently enough for the XRD, we were not successfully completed the phase analysis after re-carbonation treatment. We have also tried with our PARR Instrument’s hydrothermal reactor for this purpose; however, the temperature was the limiting factor, less than 300 ^oC which is well below the complete re-carbonation requirements. We will work this with our collaborators and produce results for future publications.

1. line 353: please explain if moist or dry materials were carbonated?

The dry materials after the calcination run were subsequently carbonated. The appropriate moisture statement is now removed for clarity in lines 377-380 of the revised manuscript.

1. Line 360: It would be interesting which type of commercial dolomite is used?

Handpicked dolomite (labeled as hp-dolo) as it is mentioned in line 386-387 of the revised manuscript.

1. Line 364: XRD showed only calcite and dolomite, in figure 3 no calcite but quartz is shown, where are the other minerals?

The calcite is replaced with quartz in the XRD. Additional statement is now added in lines 392-394.

1. Following lines: the more impurities are present, the less is the potential for recarbonation?

This is true. However, little impurity phases will also avoid the agglomeration process at high temperature and pressure conditions to keep the stability of the dolomites. For the sake of clarity, the impurity statement from lines 402-403 are now removed in the revised manuscript.

 

Overall interesting paper which needs improvement Thank you for constructive suggestions.

Author Response File: Author Response.pdf

Reviewer 2 Report

Major revisions are required.

-The application of the studied materials in Concentrating Solar Power (CSP) plants is mentioned in the Abstract. However, there is no explanation in the manuscript elsewhere. I think a small section on the application of the material in CSP plants must be added to the manuscript.

 

-A preliminary cost analysis or cost comparison among the materials must be carried out.

 

-Error analysis of the measurements must be carried out.

 

-Major conclusion or some numerical results must be added to the Abstract.

 

-What are the specific heat capacities of the materials?

 

-The conclusion of existing literature must be written in the last paragraph of the introduction section in order to show what is already done by others and what is the novelty of the present study.

 

-There are English errors.

 

-The font size of figure 7 is different from other figures. Please use same font size for all figures.

 

-Nomenclature or list of abbreviations can be added.

 

Author Response

 Comments and Suggestions for Authors – REVIEWER 2

Major revisions are required.

 

-The application of the studied materials in Concentrating Solar Power (CSP) plants is mentioned in the Abstract. However, there is no explanation in the manuscript elsewhere. I think a small section on the application of the material in CSP plants must be added to the manuscript.

Thank you for the suggestions. A subsection 2.4 is now added in the revised manuscript, on Applications of the Dolomites in CSPs.

 

 

-A preliminary cost analysis or cost comparison among the materials must be carried out.  

The focus was a laboratory-scale demonstration of the feasibility of using high dolomite phosphatic pebbles in a thermal energy storage application from a thermodynamic point of view.  Economic considerations were not explored in the present study but would be a next step in the industrial utilization of such materials and the design/economic analysis of a new process utilizing these new materials.   

 

-Error analysis of the measurements must be carried out.

Error analysis of the measurements was now carried out and shown in Figures 5, 6, and 7. Thank you.

 

-Major conclusion or some numerical results must be added to the Abstract.

We have added to the statement regarding the major conclusion of the work:

The present results exhibit, for the first time, the potential for using abundant, high phosphatic concentration dolomite possessing long-term cycling behavior for the thermochemical energy storage applications in Concentrated Solar Power (CSP) plants. 

 

-What are the specific heat capacities of the materials?

The specific capacities of the dolomites are 1.1 J/g ^oC.

 

-The conclusion of existing literature must be written in the last paragraph of the introduction section in order to show what is already done by others and what is the novelty of the present study.

Previous studies have shown that natural dolomites have the potential for use in these applications but not the specific high dolomite containing phosphatic pebbles used in the present study.  We have amended Line 106 as:

Based on such high-level recommendations, our project addresses the fundamental and applied aspects of metal/bi-metal carbonates or dolomites which have previously been shown to have potential in thermal energy storage applications [6, 7, 13, 14-17]. The present study is the first examination of the thermochemical energy storage feasibility in high dolomite phosphatic pebbles obtained from the Florida Industrial and Phosphate Research (FIPR) Institute that are quite abundant in Florida.

 

-There are English errors.

The manuscript was reviewed, and the English language corrected by English Professor.

 

-The font size of figure 7 is different from other figures. Please use the same font size for all figures.

Thank you. The font size of Figure 7 and other figures are corrected and kept with the same dimensions.

 

-Nomenclature or a list of abbreviations can be added.

Nomenclature or List of Abbreviations section is added after the Conclusion section.

Author Response File: Author Response.pdf

Reviewer 3 Report

The article “Energy Storage in Earth Abundant Dolomites” is from my point of view, an interesting work analysing and characterizing by different techniques a high phosphatic concentration dolomite, and its application for thermochemical energy storage. The work is well done, although I believe that a major revision should be carried out, especially regarding the use, explanation of the techniques and presentation of results of some of the techniques used.

 

Coments:

Line 14: Calcinatiation must be calcination.

Abstract

Line 23: TESCAN is a producer of scientific equipment, it does not seem appropriate to use this term directly in the introduction where characterization techniques used by their acronyms are being described.

1. Introduction

Line 52: Use the chemical formula of gypsum in the parentheses as in the rest of compounds to homogenize the text.

Line 75: It is a recent publication but not very recent (2017).

Lines 77-79: Cite more references that prove that claim. For example: Applied Energy 138 (2015) 202–215.

Line 119: I think it would be interesting to add a very recent reference similar at [28] but for dolomite: In situ XRD analysis of dolomite calcination under CO2 in a humid environment. August 2020 CrystEngComm. DOI: 10.1039/D0CE00974A.

Line 129: Revise sentences.

2.2 Physicochemical characterizations of dolomites

Line 153: Revise sentences.

Line 159: Kalfa must have been used. Kbeta must have been eliminated with a nickel filter for Cu radiation.

Line 160 and before: All the samples where milled before applied the characterization techniques? Please, explain in the text. Please also describe the size of the particles used for the measurements.

Describe in general in more detail, the techniques used and commented from line 134-163.

Line 176: Cite references for the method used. For example: Automated mineralogy and petrology – applications of TESCAN integrated mineral analyzer (TIMA) Journal of Geosciences, 63 (2018), 47–63, or Minerals2019,9, 333; doi:10.3390/min9060333.

2.4. Section 2.4 should be reorganized and restructured, to be integrated into the introduction, perhaps as a new section at the end of the introduction. Section 2.4 in its current state does not fit well with Section 2. Experimental and Theoretical Methods.

3. Results and Discussion

3.1

Line 212: What conditions? Describe in 2.2 Section.

Were the samples dried before preparing for XRF? In that case, under what conditions. If the samples were not dried, the LOI, in addition to the CO2, would also include the H2O contained in the sample.

Use wt% instead of mass%, seems more appropriate for XRF. Apply to the section and table.

Table 1: Standardize the decimal places, using 2 or 3 in all cases. Don’t use % in results of LOI or CaO+MgO if it already appears in the top row which is wt%.

3.2

Line 229: Same comment on the use of the acronym TESCAN as in the introduction. Maybe use TIMA (TESCAN integrated mineral analyser), like in proposed reference Journal of Geosciences, 63 (2018), or Automated SEM-EDX analysis like in Figure 2. Same comment in Table 2 caption and all the 3.2 section and lines 352, 455 and conclusions.

Line 230 and 231: Use wt% instead of mass% for XRF.

From references proposed previously, I understand that the data shown in Table 2 could be vol%. In 2.2 section was explained the results of table were area%. This creates confusion when comparing with the results previously displayed by XRF. Please explain in the text, the difference between the XRF and SEM-EDX results.

Also explain what the symbol | means in the table. For example in the row numbered as 1, it shows 0.91% | 29.38%, or Magnesite: 23.7% | 2.28%. It should not be shown for that result and for others in the table, first the highest value and then the lowest (highest | lowest)? Please, clarify that nomenclature in the text.

Line 283-285: The phase diagram of quartz is well known and does not show decompositions at such low temperatures. Reference [37] is a study in very specific environmental and sample deposition conditions, which are not suitable for the comparison made. I think that reference and comment should be removed. The explanation for the 6% difference may come for other reasons, such as inhomogeneities of the sample or the measurement technique itself.

I believe that the technique described in this section provides semi-quantitative results, and should be understood in that sense. Please, make a comment in the section about that.

3.3

In the XRD analysis shown, only the quartz and dolomite phases appear in the samples. However, in the SEM-EDX analysis, other phases appear in high percentage such as Calcite, Amphibole or Ankerite. Make a comment to explain these differences between the results shown and also among the techniques themselves.

Line 308: Please, try to identify the peak at 29.32º 2Theta as calcite or magnesium calcite.

3.6

Line 386: dololmite must be dolomite.

Author Response

Comments and Suggestions for Authors – Reviewer 3 (Round 1)

The article “Energy Storage in Earth Abundant Dolomites” is from my point of view, an interesting work analyzing and characterizing by different techniques high phosphatic concentration dolomite, and its application for thermochemical energy storage. The work is well done, although I believe that a major revision should be carried out, especially regarding the use, explanation of the techniques, and presentation of results of some of the techniques used.

 Comments:

Line 14: Calcinatiation must be calcination.

Round 1 Answer: Thank you for the corrections. It is corrected now in the revised manuscript (line 15).

Abstract

Line 23: TESCAN is a producer of scientific equipment, it does not seem appropriate to use this term directly in the introduction where characterization techniques used by their acronyms are being described.

Round 1 Answer: Thank you for the reviewer’s suggestions. We have now replaced TESCAN with automated mineralogy (automated scanning electron microscopy) with an energy dispersive X-ray spectrometer (SEM-EDX). TESCAN is replaced in all the 16 occurrences in the revised manuscript.

1. Introduction

Line 52: Use the chemical formula of gypsum in the parentheses as in the rest of the compounds to homogenize the text.

Round 1 Answer: Thank you for the reviewer’s constructive suggestions. We have given in parenthesis CaSO₄.2H₂O as the chemical formula for gypsum in the revised manuscript (line number 48).

Line 75: It is a recent publication but not very recent (2017).

Round 1 Answer: Thank you for the reviewer’s suggestions. “very” is now removed from line number 73 of the revised manuscript.

Lines 77-79: Cite more references that prove that claim. For example Applied Energy 138 (2015) 202–215.

Round 1 Answer: Thank you for the reviewer’s suggestions. We have added three more references including the suggested one by the reviewer. Please find our newly added references in line numbers 556-563 of our revised manuscript. For easy readability, we have given those references below.

[7] Valverde, J.M.; Sanchez-Jimenez, P.E.; Perz-Maqueda, L.A. Ca-looping for post-combustion CO₂ capture: A comparative analysis on the performances of dolomite and limestone, Applied Energy, 138 (2015) 202-215.

[8] Perejon, A.; Romeo, L.M.; Lara, Y.; Lisbona, P.; Martinez, A.; Valverde, J.M. The calcium-looping technology for CO₂ capture: On the important roles of energy integration and sorbent behavior. Applied Energy, 162 (2016) 787-807.

[9] Benitez-Guerrero, M.; Valverde, J.M.; Sanchez-Jimenez, P.E.; Perejon, A.; Perez-Maqueda, L.A.; Calcium-looping performance of mechanically modified Al₂O₃-CaO composites for energy storage and CO₂ capture, Chemical Engineering Journal, 334 (2018) 2343-2355.

Line 119: I think it would be interesting to add a very recent reference similar at [28] but for dolomite: In situ XRD analysis of dolomite calcination under CO2 in a humid environment. August 2020 CrystEngComm. DOI: 10.1039/D0CE00974A.

Round 1 Answer: Thank you for the reviewer’s suggestions. We have kept the reference [28], now it is [31], and added the suggested reference with DOI as a reference [32] in our revised manuscript. Please the reference below for easy lookup.

[32] Medina-Carrasco, S.; Valverde, J.M. In situ XRD analysis of dolomite calcination under CO₂ in a humid environment, CrystEngComm. Advance Article (First Published on 31 August 2020), https://doi.org/10.1039/DOCE00974A.

Line 129: Revise sentences.

Round 1 Answer: Thank you for the reviewer’s suggestions. We have revised the statement in line numbers 144-146 in our revised manuscript. For easy reference, the statement is also given below.

A Single-zone Tube Furnace (STF) from Carbolite is used for both the calcination (under N₂ atmosphere) and carbonation (under CO₂ ambient) and operates at a temperature from 25 ^oC up to 800 ^oC with the ramping rate of 6 ^oC/minute. Two additional samples of both commercial and handpicked dolomites after their initial calcination stages were chosen and labeled as CO-Dolo-STF and HP-Dolo-STF.

Round 1 Answer: Thank you for the reviewer’s suggestions. We have rephrased and made two sentences to make more meaningful in line numbers 159-164 of our revised manuscript. The statement is given below for easy reference.

The chemical composition, structural and microstructural characteristics of the dolomite minerals was studied using different techniques such as X-ray Fluorescence (XRF, Rigaku – NEX/QC/Quantez), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM, EDS).

All these metrological techniques confirmed the high percentage of the dolomite mineral phase in the earth-abundant phosphatic pebbles.

2.2 Physicochemical characterizations of dolomites

Line 153: Revise sentences.

Round 1 Answer: Thanks for the reviewer’s suggestions. The sentence is revised now in numbers 165-166 of our revised manuscript. For easy reference, the statement is given below.

The Fourier Transform Infrared Spectra of the dolomite samples have been characterized using Agilent’s CARY 600 FTIR spectrometer.

Line 159: Kalfa must have been used. Kbeta must have been eliminated with a nickel filter for Cu radiation.

Round 1 Answer: Thank you for the reviewer’s suggestions. It is now K(alpha) is used instead of K(beta) in line number 173 of our revised manuscript.

Line 160 and before: All the samples where milled before applied the characterization techniques? Please, explain in the text. Please also describe the size of the particles used for the measurements.

Round 1 Answer: Thank you for the reviewer’s comments and suggestions. We have added the particle sizes obtained from ball milling (about micrometer size) before subjected to characterization measurements. Not all the samples were ball milled. These statements are now added to the line numbers 174-176 of our revised manuscript.

Describe in general in more detail, the techniques used and commented from line 134-163.

Round 1 Answer:  Thanks for the reviewer’s comments. We have added a paragraph of general characterization techniques before elaborating them and can be found in the line numbers 150-156. For easy reference, the text is given below.

In general, multiple metrological and mineralogical techniques have been employed to characterize the different stages (plain commercial and handpicked dolomites and their calcined counterparts) of dolomite samples at room temperatures. Some of these techniques are powder X-ray diffraction, X-ray fluorescence, Fourier Transform Infrared Spectroscopy, automated scanning electron microscopy with energy-dispersive X-ray spectroscopy, thermogravimetric analysis, and BET surface area analysis. The experimental conditions of these techniques and the parameters used to study the dolomites are elaborated in detail in the following paragraphs.

Line 176: Cite references for the method used. For example Automated mineralogy and petrology – applications of TESCAN integrated mineral analyzer (TIMA) Journal of Geosciences, 63 (2018), 47–63, or Minerals2019,9, 333; doi:10.3390/min9060333.

Round 1 Answer: Thanks for the reviewer’s suggestions. The sentence is revised now in line numbers 188-189 of our revised manuscript. For easy reference, the statement is given below.

TIMA software was deployed to study the compositional mapping output results.

Round 1 Answer: Thank you for the reviewer’s suggestion. We have included the reference as suggested in line number 189 of our revised manuscript. Please see this reference below.

[37] Honeyands, T.; Manuel, J.; Matthews, L.; O’Dea, D.; Pinson, D.; Leedham, J.; Zhang, G.; Li, H.; Monaghan, B.; Liu, X.; Donskoi, E.; Webster, N.A.S.; Pownceby, M.I. Comparison of the mineralogy of iron ore sinters using of a range of techniques. Minerals, 9, 333 (2019) 1-17. https://doi:10.3390/min9060333.

2.4. Section 2.4 should be reorganized and restructured, to be integrated into the introduction, perhaps as a new section at the end of the introduction. Section 2.4 in its current state does not fit well with Section 2. Experimental and Theoretical Methods.

Round 1 Answer: Thank you very much for the reviewer’s suggestions. We have moved the Applications of Dolomites in CSPs as 1.1 in the introduction section as suggested and rearranged the references. Please see this is reflected in line numbers 120-132 in our revised manuscript.

1. Results and Discussion

3.1

Line 212: What conditions? Describe in 2.2 Section.

Round 1 Answer: The Line 213-215 is re-written to avoid any confusion regarding the “similar operating conditions”. The statement is given below.

The x-ray fluorescence characterization of CO-Dolo, HP-Dolo (see Figure 1), and their calcined counterparts were carried out to explore the chemical constituents in terms of bulk weight percentages.

Were the samples dried before preparing for XRF? In that case, under what conditions. If the samples were not dried, the LOI, in addition to the CO2, would also include the H2O contained in the sample.

Round 1 Answer: Both the commercial and handpicked dolomites procured are in fully dry conditions and not in a wet or moisturized state. The calcined dolomites are the dried samples further heat treated in a single zone tube furnace. Overall, the completely dried sample was powdered and subjected to XRF measurements. Therefore, the LOI is solely calculated based on the CO₂ gas release at the high-temperature decomposition of the dolomites.

The additional line of the statement is added now in number 146 in our revised manuscript regarding the dry condition of all our samples before characterization measurements.

Use wt% instead of mass%, seems more appropriate for XRF. Apply to the section and table.

Round 1 Answer: Thanks for the reviewer’s suggestions. We have replaced all mass% into wt% both in section and Table 1 (all 17 occurrences in the entire manuscript).

Table 1: Standardize the decimal places, using 2 or 3 in all cases. Don’t use % in results of LOI or CaO+MgO if it already appears in the top row which is wt%.

Round 1 Answer: Thank you for the reviewer’s suggestions. The decimal places now stuck to two and changed in both Table 2 and Table 3.  We have also removed the wt% from the places as suggested by the reviewer.

3.2

Line 229: Same comment on the use of the acronym TESCAN as in the introduction. Maybe use TIMA (TESCAN integrated mineral analyzer), like in the proposed reference Journal of Geosciences, 63 (2018), or Automated SEM-EDX analysis like in Figure 2. Same comment in Table 2 caption and all the 3.2 section and lines 352, 455, and conclusions.

Round 1 Answer: Thank you for the reviewer’s suggestions. We have now replaced TESCAN with automated mineralogy (automated scanning electron microscopy) with an energy dispersive X-ray spectrometer (SEM-EDX). TESCAN is replaced in all the 16 occurrences (including Figures and Tables) in the revised manuscript.

Line 230 and 231: Use wt% instead of mass% for XRF.

Round 1 Answer: Thank you. Yes, we have replaced all mass% to wt% in the entire revised manuscript.

From the references proposed previously, I understand that the data shown in Table 2 could be vol%. In 2.2 section was explained the results of the table were area%. This creates confusion when comparing with the results previously displayed by XRF. Please explain in the text, the difference between the XRF and SEM-EDX results.

Round 1 Answer: Thank you reviewer for the concerns raised. It is assumed that the area% from section 2.2 is removed now in the revised manuscript. Both the XRF (Table 1) and automated SEM-EDX (Figure 2) or Table 2 results show the data in terms of wt%. Since it is not much different from XRD and SEM/EDX results data, no justification is given.

Also, explain what the symbol | means in the table. For example in the row numbered as 1, it shows 0.91% | 29.38%, or Magnesite: 23.7% | 2.28%. It should not be shown for that result and for others in the table, first the highest value and then the lowest (highest | lowest)? Please, clarify that nomenclature in the text.

Round 1 Answer: Thank you for the reviewer’s comments. We have now made it clear and removed “|” (the dividing line) from Table 2. Thank you again for this constructive suggestion.

Line 283-285: The phase diagram of quartz is well known and does not show decompositions at such low temperatures. Reference [37] is a study in very specific environmental and sample deposition conditions, which are not suitable for the comparison made. I think that reference and comment should be removed. The explanation for the 6% difference may come for other reasons, such as inhomogeneities of the sample or the measurement technique itself.

Round 1 Answer: Thank you for the reviewer’s excellent suggestions for the much dilemma on this part whether the reduction may be due to SiO₂ decompositions or something else. We have now removed the ambiguous statements and the associated references from the revised manuscript. Please see the line numbers from 281-286 of the revised manuscript. Thank you again.

I believe that the technique described in this section provides semi-quantitative results and should be understood in that sense. Please, make a comment in the section about that.

Round 1 Answer: Thank you for the reviewer’s comments and suggestions. We have made a statement before beginning sections 3.1 and 3.2. Please see line numbers from 212-215 of our revised manuscript.

3.3

In the XRD analysis shown, only the quartz and dolomite phases appear in the samples. However, in the SEM-EDX analysis, other phases appear in high percentages such as Calcite, Amphibole, or Ankerite. Make a comment to explain these differences between the results shown and also among the techniques themselves.

Round 1 Answer: XRD analysis was done only for the pure commercial, handpicked, and handpicked/ball-milled dolomite samples, whereas SEM-EDX was done for both pure and calcined dolomites. The calcite and Amphibole phases in pure dolomites are very less and below the X-ray detection limit when compared to the predominant dolomite (50-75%) and quartz (30%) phases. XRD and SEM-EDX are two different techniques with some limitations in terms of the detection limit.

Line 308: Please, try to identify the peak at 29.32º 2Theta as calcite or magnesium calcite.

Round 1 Answer: Thank you for the reviewer’s suggestion. We have revisited the XRD analysis and found that the 29.32^o is well matching with the highest intensity peak of calcite and indexed with miller index of (104). Figure 3 is now replaced with an updated figure showing the calcite.

3.6

Line 386: dololmite must be dolomite.

Round 1 Answer: Thank you and the spelling is now corrected in line number 399 of our revised manuscript.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comment ot authors

many answers are not sufficiently given in detail and there are still a lot of uncertanties.

 

there is only on dolomite no other "dolomite minerals?

1. LIne 41 calcium magnesium minerals(S)--- there are others as dolomite?

No. Only calcium magnesium carbonates or minerals are referred as dolomites.

Why are then carbonates!! mentioned

1. line 44: what is the meaning for carbonates ?

Carbonates are the compounds carries CO₃ molecule. For example, CaCO₃ is chemically termed as Calcium Carbonate, where as CaMg(CO₃)₂ is termed as calcium magnesium carbonate.

question was not due to the meaning of carbonates but which others do occur ?

1. Line 47: clay matrix -- not given in the quantitative determination later in the paper

Clay matrix or referred as unreactive materials like SiO₂ discussed in section 3.1. Thanks to the reviewer for comments and suggestions.

Question not answered - quantification of the sample should include that

 

1. Line 65: how are the carbonates used for sequestration?

Carbonates are used for adsorbing CO₂ molecules which are considered to be the greenhouse gases. While adsorbing or absorbing CO₂, a metal becomes, metal carbonate, thus capture the CO₂ and enables the sequestration process.

Where is the metal is the oxide used or is the carboate used ? For sure there is no pure "metal"

 

1. Line 71,72: is dolomite really a magnesium source --> seawater desalting a lot of MgCl2 is produced !! a lot of Mg

Yes, the dolomite has at least 25% magnesium oxide phase could be the source for Mg. Dolomite is used as a source of magnesium metal and magnesia (MgO).

Please specify who is going to use dolomit as source for magnesium metal and pure magnesia

 

1. are the temperatures given as precise and for which grain size?

Yes, the temperature obtained from the literature reference 15, and the grain sizes of oxides (CaO and MgO) having higher surface areas when compared to the carbonate [CaMg(CO₃)₂] phase. The grain sizes at this precise temperature is correspond to micrometer scales.

 

Yes, thats right, but when a decarbonization occurs grain size is smaller, but the question is dealing with the decarbonaization temperature and their relation to the tempüerature.

temperatures cannot be given in that high preciseness!!!

 

The dolomites purchased from Alfa Aesar company is chemically processed and purified from the naturally occurring minerals with grain sizes of ~5 mm.

what is the purity of this dolomite? accrdon table chemical analysis contains >5 % iron oxide etc.

 

It is carbonated dolomite and now corrected in revised manuscript. Thank you.

What is carbonated dolomite ?

 

1. Table 2: the chemical dolomite contains 8.69 % of Fe-Co-Ni mineral? 19.74 % of unclassified minerals? and more -- doubtful?

The percentages of Fe-Co-NI and unclassified minerals as reported based on our TESCAN measurement which was performed at Colorado School of Mines. Though the presence of Fe-Co-Ni minerals is due to the trace amounts of transition metals, it is only shown in the commercial dolomite sample where this particular phase may arise from any chemical processing adopted by the Alfa Aesar company in producing pure dolomite samples. On the other hand, the unclassified phase is uniformly seen in both commercial and handpicked dolomites and their calcined counterparts as reported by TESCAN.

 

 

Please specify the amounts of 8.69 % of Co-Ni-mineral and 19.74 minerals as unclassifies  -  this is more than 25 % of sample - How is this consistent with the chemical analysis table ?

 

1. Sample 4 and 5 in table 2: these are really amphibole samples with quartz and unclassified minerals? What is the sense of these?

Samples 4 and 5, are the handpicked dolomites subjected calcination via single zone tube furnace under nitrogen and air ambient, respectively. These two samples showed the successful calcination of dolomite phase to amphibole as demonstrated using TESCAN measurements. This make sense to know the phase transformation during calcination process.

 

 

Due to 0.22 of SiO2 in the chemical analysis there is no possibility to form amphiboles!!

 

1. Line 196: First mentioning of a Fe-rich dolomite and even magnesite, Ankerite and dolomite are mentioned: do they occur together or are these Fe-containing dolomites?

The commercial dolomite in its partial thermal decomposition stage exhibited with number of phases, including Fe-rich minerals. From both XRF and TESCAN results, it is unambiguously understood that the dolomite purchased from Alfa Aesar are Fe-containing minerals and occurred together with other minor phases like magnesite and ankerite.

question if ankerite or fe-containing dolomite occurs not answered

 

1. Is there really an amphibole formation during thermo-reaction?

Yes, per the Fig. 2 TESCAN results, it was clearly visible that the amphibole phase formation up to 52% and 63% are seen if the handpicked dolomites calcined (800 ^oC) under nitrogen or air ambient, respectively.

 

due to low SiO2 contents it is doubtful!! it also differs with the SiO2 quant. mineral analysis

 

1. Line 225: why is the quartz reduced in its content when it is not consumed by any thermo reaction?

It is reported by Sasse et. al, that the thermal decomposition of SiO₂ occurs between 760 ^oC and 850 ^oC. Since our thermal decomposition process was in this temperature, plausible decomposition of quartz phase was confirmed from TESCAN and XRF results. [SiO diffusion during thermal decomposition of SiO₂, H.-E. Sasse and U. Konig, J. Appl. Phys. 67, 10, 1998; https://doi.org/10.1063/1.345185]. The above reference is now mentioned in the revised manuscript as [33].

 

Please specify the thermal decomposition of SiO2 - must be high-quartz at this temperature ?

 

 

1. Figure 3: XRD patterns do only show rather pure dolomite (some quartz) - this is representing which part of the samples formerly described, as there are no other minerals visible in the patterns

The X-ray diffraction profiles of commercial, handpicked, and handpicked-ballmilled samples showed the majority phases of dolomite which matches with the XRF results. In addition, the hanpicked and hanpicked-ballmilled samples exhibited an addition impurity peak of quartz (SiO₂) which is aligned with the TESCAN results. Unlike XRD, the TESCAN capabilities are much advanced and employed widely for the mineralogical studies. So the other mineral such as ankerite and Fe-rich phases as obtained from TESCAN because of its specific capabilities. XRD is a gross analysis and overall depends on the matching phases as reported in the ICDD or ICSD or JCPDS databases. Since the XRD of calcined products are not studied in this paper, the amphibole phase matching as obtained from TESCAN could not be matched with the XRD phase analysis.

 

 

How can the 25 % of the minerals mentioed above not be seen in the XRD. The explanation due the databases used is not answering the question

 

 

1. Table 3: a Comparison of X-ray peaks is not useful, comparison of lattice volume or lattice parameters would be more precise

Explanation of differences in areas of ......,02 Å without mentioning the preciseness of measurment are useless. Measur,eent and preparation of samples are surely within that limit.

Lattice parameters are similar --> no indication of iron in dolomite what are the iron containing minerals ?

1. line 334 and following: It would be interesting to see the phase formation after re-carbonation in case of dolomite?

Thank you for the suggestions. Due to the lack of high-pressure and high-temperature re-carbonation facilities, we could not produce large amounts of re-carbonated dolomites samples for XRD. However few milligrams of dolomites after calcination in TGA were subsequently carbonated under low pressure CO₂ flow with flow rate of 100 mL/min. Since the amount of sample re-carbonated are not sufficiently enough for the XRD, we were not successfully completed the phase analysis after re-carbonation treatment. We have also tried with our PARR Instrument’s hydrothermal reactor for this purpose; however, the temperature was the limiting factor, less than 300 ^oC which is well below the complete re-carbonation requirements. We will work this with our collaborators and produce results for future publications.

 

Very samll amounts can be measured with a low background holder!"!!

 

The calcite is replaced with quartz in the XRD. Additional statement is now added in lines 392-394.

1. Following lines: the more impurities are present, the less is the potential for recarbonation?

This is true. However, little impurity phases will also avoid the agglomeration process at high temperature and pressure conditions to keep the stability of the dolomites. For the sake of clarity, the impurity statement from lines 402-403 are now removed in the revised

manuscript.

 

Exlanations are done by removing the former explanartions not by improving the quality

 

 

 

 

 

 

 

Author Response

Answers to Comments and Suggestions for Authors – Reviewer 1 (Round 2)

Comment to authors

many answers are not sufficiently given in detail and there are still a lot of uncertainties.

There is only one dolomite no other "dolomite minerals?

Thanks to the Reviewer’s suggestions. We will use “dolomite minerals” since the derivatives or substitutional materials of dolomites are still in the classification of dolomite minerals. Now we have changed the title of the paper with respect to the reviewer’s suggestions as “Energy Storage in Earth Abundant Dolomite Minerals”

1. Line 41 calcium magnesium minerals(S)--- there are others as dolomite?

Round 1 Answer: No. Only calcium magnesium carbonates or minerals are referred to as dolomites.

Why are then carbonates!! Mentioned

Round 2 Answer: Thank you, Reviewer, for the suggestion. We have replaced now “carbonates” with “minerals. Please see in the track changed document which is now in line numbers 17 (in Abstract section) 45 (in the Introduction section).

2. line 44: what is the meaning for carbonates?

Round 1 Answer: Carbonates are the compounds carries CO₃ molecule. For example, CaCO₃ is chemically termed as Calcium Carbonate, whereas CaMg(CO₃)₂ is termed as calcium magnesium carbonate.

the question was not due to the meaning of carbonates, but which others do occur?

Round 2 Answer: Thank you, reviewer, for the clarifications. The carbonates classified into three categories such as calcite minerals, dolomite minerals, and the aragonite minerals. The Encyclopedia Britannica (https://www.britannica.com/science/carbonate-mineral) discusses wide classifications of carbonate minerals and their physical and chemical properties. This reference is now included in the revised manuscript, line numbers 50-52.

3. Line 47: clay matrix -- not given in the quantitative determination later in the paper

Round 1 Answer: Clay matrix or referred to as unreactive materials like SiO₂ discussed in section 3.1. Thanks to the reviewer for comments and suggestions.

Question not answered - quantification of the sample should include that

Round 2 Answer: Thank you, reviewer, for the suggestion and sorry for not answering with full information previously. The quantification of the clay matrix sample contains at least 30% of the quartz (SiO₂) phase in addition to the dolomite mineral phase ~45-50%. These quantification details are now included in the revised manuscript, line numbers 53-55.

4. Line 65: how are the carbonates used for sequestration?

Round 1 Answer: Carbonates are used for adsorbing CO₂ molecules which are considered to be the greenhouse gases. While adsorbing or absorbing CO₂, a metal becomes metal carbonate, thus capture the CO₂ and enables the sequestration process.

Where is the metal is the oxide used or is the carbonate used? For sure there is no pure "metal"

Round 2 Answer: Thanks for the reviewer’s comments. The metal is not available as pure, but as in form of metal oxides. The metal oxide in the first place absorbs carbon dioxide to form metal carbonate. In the reversible process, the metal carbonates release carbon dioxide to become metal oxide. Please see the equation (1) in line number 96 of our manuscript. However, the word “sequestration” is already removed in line number 118 of the revised manuscript based on the previous round review.

5. Line 71,72: is dolomite really a magnesium source --> seawater desalting a lot of MgCl2 is produced !! a lot of Mg

Round 1 Answer: Yes, the dolomite has at least a 25% magnesium oxide phase could be the source for Mg. Dolomite is used as a source of magnesium metal and magnesia (MgO).

Please specify who is going to use dolomite as a source for magnesium metal and pure magnesia

Round 2 Answer: Thanks for the reviewer’s queries regarding the Mg or MgO for industrial use. Rather Mg or MgO directly be used by the Chemical Manufacturing industry, if the Mg or MgO absorbed to form dolomite mineral, CaMg(CO₃)₂ will lead to the removal of Mg from the Phosphatic clays. The removal of Mg or MgO will enable the Phosphate Mine industries to save huge revenue in the production of agricultural products such as diammonium phosphate (DAP). This justification statement is already given in the introduction line numbers 45-47 and 61-63. In addition, dolomite will be deployed to store heat energy in concentrated solar power (CSP) plants which are mentioned in line numbers 75-77 of the revised manuscript.

6. Are the temperatures given as precise and for which grain size?

Round 1 Answer: Yes, the temperature obtained from the literature reference 15, and the grain sizes of oxides (CaO and MgO) having higher surface areas when compared to the carbonate [CaMg(CO₃)₂] phase. The grain sizes at this precise temperature are corresponding to micrometer scales.

 Yes, that's right, but when decarbonization occurs grain size is smaller, but the question is dealing with the decarbonization temperature and their relation to the tempüerature.

temperatures cannot be given in that high preciseness!!!

Round 2 Answer: Thank you for the reviewer’s comments and suggestions. We have now removed the precision in temperature values, and the equation temperature is now specified as ~>600 ^oC (line number 96) in the revised manuscript.

7. line 111 where is the origin of dolomite from Alfa-Aesar ---natural?

Round 1 Answer: The dolomites purchased from Alfa Aesar company is chemically processed and purified from the naturally occurring minerals with grain sizes of ~5 mm.

what is the purity of this dolomite? According to the table chemical analysis contains >5 % iron oxide etc.

Round 2 Answer: Thank you for the reviewer. The purity of commercial dolomite is ~90% with impurities of iron oxide >5% and SiO₂ is of ~2-3%. The commercial dolomite was subjected to XRF analysis (reported as a Table 1 in our revised manuscript) and from our studies, it is reported that the purity of the dolomite phase matches with the industry purity value.

8. line 166: what is carbonated dolomite?

Round 1 Answer: It is carbonated dolomite and now corrected in the revised manuscript. Thank you.

What is carbonated dolomite?

Round 2 Answer: Thank you for the reviewer’s query and sorry for not clarifying the questions asked during the last round of review. In the revised manuscript, “carbonated counterparts” line number 178-179 is rephrased as “carbonated oxide phases”. There are no carbonated dolomites which were the wrong rebuttal statement provided by the authors. Thank you for correcting our mistakes.

9. Table 2: the chemical dolomite contains 8.69 % of Fe-Co-Ni mineral? 19.74 % of unclassified minerals? and more -- doubtful?

Round 1 Answer: The percentages of Fe-Co-NI and unclassified minerals as reported based on our TESCAN measurement which were performed at Colorado School of Mines. Though the presence of Fe-Co-Ni minerals is due to the trace amounts of transition metals, it is only shown in the commercial dolomite sample where this particular phase may arise from any chemical processing adopted by the Alfa Aesar company in producing pure dolomite samples. On the other hand, the unclassified phase is uniformly seen in both commercial and handpicked dolomites and their calcined counterparts as reported by TESCAN.

Please specify the amounts of 8.69 % of Co-Ni-mineral and 19.74 minerals as unclassified  -  this is more than 25 % of the sample - How is this consistent with the chemical analysis table?

Round 2 Answer: Thank you, reviewer, for the critical comments. We carefully analyzed the Table 2. The Fe-Co-Ni phase of 8.69% is only seen in the commercial dolomite sample which is procured with a 90% purity level (with 5% iron oxide) from Alfa Aesar previously mentioned. This commercial dolomite was hand crushed and heated under nitrogen in a single zone tube furnace. On the other hand, our handpicked dolomites from the Phosphate mining site do not contain the Fe- related phases so Table 2 for the HP-Dolo sample or its heat-treated versions, we have not seen any Fe-Co-Ni traces of phases. Regarding the concentration of Amphibole, both the commercial and handpicked dolomite samples show a consistent level of within ~2.0%. Whereas the heat-treated samples, the concentrations of Amphibole increase to 50-60%, which is again maintaining consistency. The increase in Amphibole concentration is merely due to the heating process of dolomite in STF under nitrogen or air ambient for the degradation of the carbonate phase to oxide phases. Overall, the unclassified 25% as shown in commercial dolomite is reduced to less than 15% in handpicked dolomite samples due to the absence of Fe-Co-Ni impurities. We have mentioned this information in our revised manuscript below Table 2.

10. Sample 4 and 5 in table 2: these are really amphibole samples with quartz and unclassified minerals? What is the sense of these?

Round 1 Answer: Samples 4 and 5 are the handpicked dolomites subjected calcination via single zone tube furnace under nitrogen and air ambient, respectively. These two samples showed the successful calcination of the dolomite phase to amphibole as demonstrated using TESCAN measurements. This makes sense to know the phase transformation during the calcination process.

 Due to 0.22 of SiO2 in the chemical analysis, there is no possibility to form amphiboles!!

Round 2 Answer: For the sample 4 [HP-Dolo-STFN2], and sample 5 [HP-Dolo-STFO2], the SiO₂ or quartz phase was identified by the TESCAN are 15.975%, 14.26% and respectively (please see Table 2). These mass percentages in a close match with our XRF results as reported in Table 1, for the sample HP-Dolo-STF, the SiO₂ phase is ~24.1 mass%. With this high concentration of SiO₂ phase and the calcination of dolomites in a tube-furnace end up with Amphibole formation. On the other hand, for the non-calcined [HP-Dolo] sample, even though the SiO₂ concentration of 21.67% (sample 2 in Table) and 32 mass% in Table 1, there was no Amphibole formation. Yet another closer look of calcined commercial dolomite, sample 1 [CO-Dolo-STFN2], the Amphibole formation is less than 2 mass%, due to the low concentration of SiO₂ (0.36%). The same is true for the handpicked and ball-milled sample 3 [HP-Dolo-BM], without further calcination, the Amphibole phase is less than 2% while the SiO₂ is reported in Table 2, with 6.42%. Based on the above analysis, it is very clear that for the formation Amphibole phase, a sample composition parameter with a high concentration of SiO₂ (>15 mass%) and the sample must undergo high-temperature calcination. We have mentioned this analysis part in our Results and Discussion section of our revised manuscript (line number 293-305).

11. Line 196: First mentioning of Fe-rich dolomite and even magnesite, Ankerite, and dolomite are mentioned: do they occur together or are these Fe-containing dolomites?

Round 1 Answer: The commercial dolomite in its partial thermal decomposition stage exhibited with a number of phases, including Fe-rich minerals. From both XRF and TESCAN results, it is unambiguously understood that the dolomite purchased from Alfa Aesar are Fe-containing minerals and occurred together with other minor phases like magnesite and ankerite.

question if ankerite or Fe-containing dolomite occurs not answered

 Round 2 Answer: Thank you for the reviewer’s query. Yes, based on our Table 2 and Figure 2 results, the Fe containing dolomite or Ankerite does occur in our handpicked and commercial samples. These phases are commonly referred to as Ferroan dolomite/ankerite due to the presence of Fe in form of FeCO₃ in dolomite structure [42] An interesting feature is noticed in particular to the Fe containing dolomite samples is now mentioned in our revised manuscript, line numbers 255-259.

[42]: S. St. J. Warne, D. J. Morgan, and A. E. Milodowski, Thermal analysis studies of the dolomite, ferroan dolomite, ankerite series. Part 1. Iron content recognition and determination by variable atmosphere DTA, Thermochimica Acta, 51, 2-3, 1981, 105-111.

12. Is there really an amphibole formation during thermo-reaction?

Round 1 Answer: Yes, per Fig. 2 TESCAN results, it was clearly visible that the amphibole phase formation up to 52% and 63% are seen if the handpicked dolomites calcined (800 ^oC) under nitrogen or air ambient, respectively.

due to low SiO2 contents, it is doubtful!! it also differs from the SiO2 quant. mineral analysis

 Round 2 Answer: Thank you for the reviewer’s query. Please refer to our round 2 answers given for question 10 above.

13. Line 225: why is the quartz reduced in its content when it is not consumed by any thermo-reaction?

Round 1 Answer: It is reported by Sasse et. al, that the thermal decomposition of SiO₂ occurs between 760 ^oC and 850 ^oC. Since our thermal decomposition process was in this temperature, plausible decomposition of the quartz phase was confirmed from TESCAN and XRF results. [SiO diffusion during thermal decomposition of SiO₂, H.-E. Sasse and U. Konig, J. Appl. Phys. 67, 10, 1998; https://doi.org/10.1063/1.345185]. The above reference is now mentioned in the revised manuscript as [33].

 Please specify the thermal decomposition of SiO2 - must be high-quartz at this temperature?

Round 2 Answer: Thank you for the reviewer’s query. From our TESCAN and XRF results, it is very consistent that the SiO₂ (quartz) phase reduction was observed, this may be not due to the thermal decomposition of SiO₂ and maybe because of sample inhomogeneity or artifacts in the measurement technique. Since the quartz phase is of no interest for the thermochemical reaction studies, we have not explored further on this SiO₂ phase. Please see the line numbers 287-292 of our revised manuscript.

 Figure 3: XRD patterns do only show rather pure dolomite (some quartz) - this is representing which part of the samples formerly described, as there are no other minerals visible in the patterns

Round 1 Answer: The X-ray diffraction profiles of commercial, handpicked, and handpicked-ball-milled samples showed the majority phases of dolomite which matches the XRF results. In addition, the handpicked and handpicked-ball-milled samples exhibited an additional impurity peak of quartz (SiO₂) which is aligned with the TESCAN results. Unlike XRD, the TESCAN capabilities are much advanced and employed widely for mineralogical studies. So, the other mineral such as ankerite and Fe-rich phases as obtained from TESCAN because of its specific capabilities. XRD is a gross analysis and overall depends on the matching phases as reported in the ICDD or ICSD or JCPDS databases. Since the XRD of calcined products is not studied in this paper, the amphibole phase matching as obtained from TESCAN could not be matched with the XRD phase analysis.

 How can 25 % of the minerals mentioned above not be seen in the XRD? The explanation due the databases used is not answering the question

Round 2 Answer: Thank you for the reviewer’s remarks and further query. The XRD and the SEM/EDX (TESCAN) are two different studies with characterization limitations. XRD has detection limits of minerals that occur with a concentration of less than 1% or in ppm ranges. However, the SEM has greater capabilities of measuring the secondary minerals if they even occur in trace amounts in the dolomite matrix. Our samples, commercial dolomite, handpicked dolomite, and handpicked/ball-milled dolomite, all having high concentrations of dolomite phase of >50%, when compared to the secondary minerals which are less than few percentages as given in Table 1 and Table 2. The XRD of all these samples picked up only the dolomite peak and silica peak and not the trace minerals which are below the detection limit. On the other hand, the TESCAN reported in Table 2 with other secondary minerals even though they are either in the very small concentrations or smaller sizes in ppm-level. Additionally, the XRD peaks sometimes occur in form of doublets, or as shoulder, which is often not indexed by the peak search limitations.

14. Table 3: A comparison of X-ray peaks is not useful, comparison of lattice volume or lattice parameters would be more precise

Explanation of differences in areas of ......,02 Å without mentioning the preciseness of measurement is useless. Measurement and preparation of samples are surely within that limit.

Round 2 Answer: Thank you for the reviewer’s suggestion. We have followed the same XRD sample preparation methods recommended for the powder X-ray diffraction of multiphasic materials. The XRD measurements were carried out for multiple scans and ensured that the precision is well maintained within the limits of instrument capability and human error. The lattice constants calculated based on our XRD dataset are within the precision of 0.0001 Å. This precision data is now given in the parenthesis next to a and c lattice constants in Table 3 of our revised manuscript.

Lattice parameters are similar --> no indication of iron in dolomite what is the iron-containing minerals?

Round 2 Answer: Thanks for the reviewer’s comments. Yes, we do agree that it is no difference between dolomites and Fe-containing dolomites from the XRD patterns since XRD has a detection limit which is discussed as our round 2 answers for the question 14 (please refer to the same).  We have done XRD only for the non-calcined, commercial, and handpicked dolomite minerals. From Table 2, the handpicked dolomite plain and ball-milled samples showed only 0.08% (sample 2) and 0.53% (sample 3) of Fe containing dolomite phase when compared to 49.6% (sample 2) and 74.9% (sample 3) pure dolomite phase. The Fe containing dolomite phase increase to 3.51% for calcined handpicked dolomite (sample 4) and 29.38% (sample 1) for which the XRD was not performed. These details are now added to our revised manuscript, line numbers 327-331.

15. line 334 and following: It would be interesting to see the phase formation after re-carbonation in the case of dolomite?

Round 1 Answer: Thank you for the suggestions. Due to the lack of high-pressure and high-temperature re-carbonation facilities, we could not produce large amounts of re-carbonated dolomites samples for XRD. However few milligrams of dolomites after calcination in TGA were subsequently carbonated under low-pressure CO₂ flow with a flow rate of 100 mL/min. Since the amount of sample re-carbonated is not sufficient enough for the XRD, we were not successfully completed the phase analysis after re-carbonation treatment. We have also tried with our PARR Instrument’s hydrothermal reactor for this purpose; however, the temperature was the limiting factor, less than 300 ^oC which is well below the complete re-carbonation requirements. We will work this with our collaborators and produce results for future publications.

 Very small amounts can be measured with a low background holder!"!!

Round 2 Answer: Thank you reviewer for his/her constructive suggestions. We did procure the zero background Si disc to perform the XRD of small amounts of re-carbonated samples. But in the meantime, our TGA is broken due to the mall functioning of sample beam sensors which needs to have service contract and maintenance which will take a few months due to the Covid-19 pandemic. We assure to the reviewer that we will perform such important re-carbonation and produce results as addendum this manuscript. This very important recommendation is given in our revised manuscript (line numbers 462-466).

16. Line 364: XRD showed only calcite and dolomite, in figure 3 no calcite but quartz is shown, where are the other minerals?

Round 1 Answer: The calcite is replaced with quartz in the XRD. An additional statement is now added in lines 392-394.

Round 2 Answer: The line number in the revised manuscript is now 475-478.

17. Following lines: the more impurities are present, the less is the potential for re-carbonation?

Round 1 Answer: This is true. However, little impurity phases will also avoid the agglomeration process at high temperature and pressure conditions to keep the stability of the dolomites. For the sake of clarity, the impurity statement from lines 402-403 is now removed in the revised manuscript.

 Explanations are done by removing the former explanations not by improving the quality

Round 2 Answer: Two rounds of an answer for the reviewer’s comments and remarks are provided. Thank you to the reviewer for critical comments and constructive suggestions in improving the quality of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript can be accepted

Author Response

Comments and Suggestions for Authors (Reviewer 2 – Round 2)

The manuscript can be accepted

Thank you to the Reviewer for accepting our revised manuscript for publication.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors have considered my comments and made appropriate changes.

Author Response

Reviewer 3 -Comments and Suggestions for Authors

Reviewer 3: The authors have considered my comments and made appropriate changes.

Round 2 Answer: Thank you Reviewer for comments and constructive suggestions that make our manuscript is much better shape now for publication. Thank you once again.

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Comments are given in text.

 

Mineralogical descriptions and measurements are inadequatly described and some conclusions therfore not proven.

 

This part of this interesting manuscript must be improved.

The overall content is acceptable.

Comments for author File: Comments.pdf

Author Response

Answers to Comments and Suggestions for Authors – Reviewer 1 (Round 3)

Comment to authors

The overall content is acceptable

Round 3 Answer: Thank you for the Reviewer’s constructive comments and suggestions throughout the manuscript which made it is now at a publishable standard. Thank you, Sir.

1. Mineralogical description and measurements are inadequately described, and some conclusions therefore not proven. This part of this interesting manuscript must be improved.

Round 3 Answer: Thank you for the reviewer’s suggestions and comments.

2. Line 58: Is this pure dolomite or occurs solid solution – which other dolomite minerals besides CaMg(CO₃)₂ are mentioned?

Round 3 Answer: Thank you for the reviewer’s comments. The dolomite which we received (handpicked) is with 60% purity and the rest of 30% with sand (SiO₂ or quartz). We have discussed only CaMg(CO₃)₂ and not any other minerals. So we have removed the plural form to the singular “mineral” in line number 45 of our revised manuscript. Please see the revised statement in line numbers 44-46. Thank you again.

3. Lines 63-65: Why not use a typical mineral book, which describes all the relevant groups of dolomite type minerals or calcite type minerals? these are minerals and normally the relevant rocks do contain additional minerals.

Round 3 Answer: Thank you for the reviewer’s suggestions. We have used a book reference and also mentioned about this reference in line numbers 51-55. For easy review, we have included both statements of the main text and the reference below.

Late Professor of Mineralogy and Geology from Amherst College, Frederic Brewster Loomis published an e-book, “Field Book of Common Rocks and Minerals” [3] discusses systematic approaches of identifying minerals and rocks, their physical and chemical properties including the composition of minerals, and their classifications based on metals. The phenomenon of metamorphism in rocks and minerals have been widely analyzed and discussed by Kurt Bucher and Martin Frey [4].

[3]   Loomis, F.B.; Field Book of Common Rocks and Minerals, e-book, G.P. Putnam’s Sons, New York, and London, (1948).

[4]   Bucher, K.; Frey, M.; Definition, Conditions, and Types of Metamorphism. In: Petrogenesis of Metamorphic Rocks. Springer, Berlin, Heidelberg (2002).

 

4. Lines 66-69: SiO₂ is not a clay mineral which ones do occur. There is still no quantification of the clay minerals.

Round 3 Answer: Thank you for the reviewer’s comments. We have removed the “clay” from the statement and modified it in line number 57 of the revised manuscript. For ease in review, the statement is reiterated below.

The dolomites are not only difficult to remove from the gangue (phosphatic minerals) but also pose “environmental sustainability”

5. Line 118: The reaction forming two oxides. The reaction consists of two separate dissociations; first Mg-part decarbonates and secondly the Ca- part dissociates. Here CO₂ is set free – this CO₂ should be included in the calculations.

Round 3 Answer: Thank you for the reviewer’s comments and suggestions. In this current manuscript, we have considered the decomposition of metal oxide to metal. Once the MgO and CaO are decomposed from the dolomite phase while releasing the CO₂, the reversible reactions (carbonation) of metal oxides to metal carbonates (dolomites) are considered from our TGA life cycle analysis, Moreover, our XRF studies in Table 1, accountable for the LOI of CO₂ release in calculations of other known phases.

6. Line 124: What type of MgO Layered mineral is meant.

Round 3 Answer: Than you for the reviewer’s query. We have removed the “layered” from the text and replace it with the statement in line numbers 105-107 in our revised manuscript. For easy review, please see below the new statement.

The CO₂ capture behavior on the porous MgO material involves a two-stage adsorption process which is not caused by adsorbed products and the adsorption process is minimized by the inter-particle diffusion resistance [22].

7. Line 136: Which metal carbonates besides Ca and Mg?

Round 3 Answer: Thanks for the reviewer’s query. It is Ca-Mg Carbonates or CaMg(CO₃)₂. Please see the changes in line numbers 112-118 of the revised manuscript. For easy review, please find below the statement.

Based on such high-level recommendations, our project addresses the fundamental and applied aspects of calcium-magnesium carbonates [CaMg(CO₃)₂] or dolomites which have previously been shown to have potential in thermal energy storage applications [6, 10, 16, 17-20]. The present study is the first examination of the thermochemical energy storage feasibility in high dolomite phosphatic pebbles obtained from the Florida Industrial and Phosphate Research (FIPR) Institute that are quite abundant in Central Florida’s phosphate mining sites. Besides the Earth abundancy that makes the system cost-effective, these calcium-magnesium carbonates or high dolomite pebbles possesses high enthalpy of reaction which is suitable for their wide usage in sustainable and green technological applications such as thermochemical energy storage (TCES)) and carbon dioxide capture (Carbon Capture and Storage, CCS) [24-32].

8. Line 181: “in it is”?

Round 3 Answer: Thank you and sorry for the typo. “in it” is now removed from the revised manuscript (line number 139).

9. Lines 249-250: Which standards were used for the quantification otherwise it seems to be semiquantitative?

Round 3 Answer: Thanks for the reviewer’s constructive suggestions. We have added the word “semi-quantitative” in the line numbers 191-193 of our revised manuscript. For easy review, the statement is given below.

TIMA software was deployed to study the semi-quantitative compositional mapping output results of dolomite minerals at different stages of calcination [37].

10. Table 2: Mineralogy still unclear, which minerals are not identified. How can be then quantification minerals which that preciseness – very doubtful!!

Round 3 Answer: Thank you for the reviewer’s comments. Since the automated SEM-EDX results produced with unclassified materials’ phases in percentage, Table 2 is titled as “semi-quantitative results of samples”. The precision of different phase percentages with two decimal places are provided by the expert does perform routinely these analyses at Colorado School of Mines.

11. Section 3.3: There is no crystal structure determinations!

Round 3 Answer: Thank you for the reviewer’s comments and suggestions. We have removed the “Crystal Structure” from the subtitle of Section 3.3. Therefore, section 3.3. is titled “Phase Determination and Crystallite Size Analysis” in the revised manuscript.

12. Figure 3: Calcite?

Round 3 Answer: Yes, the little peak at 29.32^o (commercial dolomite sample) corresponds to the characteristic peak of calcite. Based on the Reviewer 3’s suggestions, we have revisited the XRD analysis and found that the 29.32^o is well matching with the 100% intensity peak of calcite (at 2θ angle of 29.34^o and the d-value of 3.042) and indexed with (hkl) miller index of (104). The handpicked dolomite and its ball-milled counterpart did not show this peak or the peak at this angle is too low to detect by the XRD. This is explained now in line numbers 327-331 in our revised manuscript. For easy review, the statement is given below.

There is one diffraction peak at 29.34^o, d-value 3.026 that is assigned to the maximum intensity XRD peak of calcite (ICSD PDF Card No. 164935) with angle and d-values (29.34^o, d=3.042) matching with miller index correspond to (104).

13. Line 528: Amphibole could not be determined by XRD. How was the OH content amphiboles measured by TESCAN?

Round 3 Answer: XRD and automated SEM-EDX are two different techniques with varied detection limits. XRD could not detect Amphibole because of its phase percentage present in the sample is far less than dolomite or quartz phases. On the other hand, the EDX will measure the samples even in trace amounts, so the Amphibole of less than 2% in the sample is picked by the SEM-EDX. This justification is given in the paragraph, line numbers 296-308. The SEM-EDX study reported by the Colorado School of Mines was the full phase analysis of Amphibole and its OH content. Therefore, Table 2 is titled as “semi-quantitative’ SEM-EDX analysis. We will do full quantitative analysis and the results will be forthcoming.

14. Line 534: Here is pure dolomite with no- Fe content?

Round 3 Answer: Yes, the handpicked samples are primarily shown a pure dolomite phase of 50-75% and the rest contains SiO₂ phase. The Fe contents in terms of FeCO₃ were less than 0.5% especially the handpicked dolomites or its ball-milled counterparts.

15. Line 535: How was the Fe-content do this dolomite determined, what is the Fe- content? again two decimal places.

Round 3 Answer: The Fe-content in dolomite is determined by the four-probe EDX detectors in elemental mapping mode. The EDX detector is based on measuring the X-rays counts while mapping the sample image. The automated SEM-EDX is collecting the phase % or either mass% or volume % and reported for analysis. The elemental composition is the way Fe spectral peak is picked up and reported, however, this capability is not possible in XRD measurements of the same sample.

16. Table 3: Lattice parameters were determined by using only two (hkl) values β. No least-squares refinements β.

Round 3 Answer: Yes, lattice parameter calculations are done by our Rigaku PDXL software which does base on the X-ray diffraction peaks of appreciable intensities of the dolomite phases. However, the structural refinement using the least-square method is currently under progress (slow processing due to COVID-19 and unavailability of resources) and the results will be discussed in our future communications.

17. Table 3: What is the preciseness of the sample preparation and repeats of measurement

Round 3 Answer: The preciseness of the sample preparation and repeating of measurements are within the error limit of less than 1%. Since the commercial and handpicked dolomite samples are as used without further purification the preciseness in preparation is almost 100%. The ball-milled handpicked dolomites, we have followed the same experimental conditions, such as a ball to powder weight ratio, time duration, and medium of milling, the preciseness of preparation is within 5% experimental and human error. The repeating of characterization measurements has been done with the standard operating procedure, so the preciseness was ensured, and it was within the error limit of less than 1%. These statements are discussed in line numbers 366-373 of our revised manuscript.

18. Line 565: Is there incorporation of Fe in the lattice how can this distinguish between deformation due to milling and foreign ion incorporation?

Round 3 Answer: No, there was no incorporation of Fe in our dolomite sample, that milled under a planetary ball mill. Since the ball milling was done in DI water and with only 10 minutes the possibility of Fe incorporation into the sample highly unlikely. This was further confirmed from our in-house SEM-EDX measurement. These statements are now appearing in the line numbers 364-366 of our revised manuscript.

 

 

 

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The paper by Srinivasan et al. focuses on Dolomite obtained from the phosphoric acid production in Florida and how to utilise this ‘waste product’ for thermal energy storage or CO₂ capture. The approach to the subject is interesting, however, poorly conducted. I have listed some of the flaws below, but recommend that the authors improve the manuscript significantly, i.e. language, data treatment and visualisation, and comparison of the data to draw proper conclusions. I can not recommend this article for publication.

The introduction insufficiently highlights why metal carbonates are interesting for the applications such as CSP and CO2 capture. It is mentioned that dolomite is abundant, but that also makes it cheap, which is one of the advantages. Another is the high enthalpy of reaction, which makes them suitable for thermochemical energy storage. There are numerous examples that could be cited; some DOIs are provided:

• 1016/j.energy.2017.11.084
• 1039/c8ta07254j
• 1021/acs.chemrev.8b00315
• 1039/d0ta03080e
• 1016/j.rser.2015.09.026

Line 60: CSP should be defined

Line 61: the CSP plants do not operate at temperatures above 600 °C as they are limited by the molten salt technology used, i.e. 550 °C. However, CSP plants do have the potential to operate at higher temperature if an alternative to molten salt is found. The sentence should be rephrased.

 

Line 81+83: The equilibrium between dolomite and MgCO₃ and CaCO₃ is at a higher temperature than for the equilibrium with MgO, which does not make sense as MgO is formed from decomposition of MgCO_3. It would be good to provide the enthalpy of reaction as well as the CO2 pressure condition for these reactions. Is it 1 bar of CO2?

Line 96-96: the sentence says ‘the we…’, please correct.

 

The materials and methods section need to be detailed enough so that anyone can reproduce them. No details at all are provided on experimental parameters.

Line 145 – 156: This should be included in the experimental section.

Section 3: It would be suitable to provide the Arrhenius equation and explain the terms and where each constant is taken from and thus show how the activation energy is extracted.

Figure 2: needs labelling with a and b. what is going on with the 2 °C curve? It looks like a multiple-step function. Why is 90 mass% highlighted with a dotted line? Use subscript in the ‘table’.

Line 188: How does the ball-milling process enhance the content of dolomite? What could possibly react to form dolomite in the process?

 

Figure 4: It is unclear what sample has actually been investigated by BET.

Figure 6: A difference plot should be provided if quantitative Rietveld refinement has been performed. However, some Bragg reflections are unidentified and quantification is in fact not possible based on the XRD data.

Line 259: The data should be ‘normalised’ or presented in a percentage fraction instead of in a future publication.

Line 270: ‘Further explorations and optimizations are underway.’ These explorations should be included in this research article as the data presented is insufficient.

Line 308: The DOI provided for reference 10 is incorrect. It should be 10.3103/S1067821216030032

Author Response

Reviewer 1 Report:

The paper by Srinivasan et al. focuses on Dolomite obtained from the phosphoric acid production in Florida and how to utilise this ‘waste product’ for thermal energy storage or CO₂ capture. The approach to the subject is interesting, however, poorly conducted. I have listed some of the flaws below, but recommend that the authors improve the manuscript significantly, i.e. language, data treatment and visualisation, and comparison of the data to draw proper conclusions. I can not recommend this article for publication.

The introduction insufficiently highlights why metal carbonates are interesting for the applications such as CSP and CO2 capture. It is mentioned that dolomite is abundant, but that also makes it cheap, which is one of the advantages. Another is the high enthalpy of reaction, which makes them suitable for thermochemical energy storage. There are numerous examples that could be cited; some DOIs are provided:

• 1016/j.energy.2017.11.084
• 1039/c8ta07254j
• 1021/acs.chemrev.8b00315
• 1039/d0ta03080e
• 1016/j.rser.2015.09.026

 

Answers to Reviewer’s comments: Thank you for the Reviewer’s suggestions. We have reworked on the introduction section, language, and more compelling statements of why the dolomites are good candidates for thermochemical energy storage and carbon capture. We have included the suggested statements from the reviewer, and this is reflected in line numbers 104-108 of the revised manuscript and also included the five references given by the reviewer.

Line 60: CSP should be defined

Answers to Reviewer’s comments: CSP is now defined in line 60 and highlighted. Thanks for this suggestion.

Line 61: the CSP plants do not operate at temperatures above 600 °C as they are limited by the molten salt technology used, i.e. 550 °C. However, CSP plants do have the potential to operate at higher temperatures if an alternative to molten salt is found. The sentence should be rephrased.

 Answers to Reviewer’s comments: Thank you for the Reviewer’s suggestion. We have rephrased the statement which now can be found in lines 61-63 (highlighted) per reviewer’s comments and suggestions.

Line 81+83: The equilibrium between dolomite and MgCO₃ and CaCO₃ is at a higher temperature than for the equilibrium with MgO, which does not make sense as MgO is formed from the decomposition of MgCO_3. It would be good to provide the enthalpy of reaction as well as the CO2 pressure condition for these reactions. Is it 1 bar of CO2?

Answers to Reviewer’s comments: Thanks for the reviewer’s comments and suggestions. We have now removed Equations (1) and (2) since they are not relevant to the present study. The relevant two equations are now retained which clearly are the focus our research study of the thermochemical decomposition of dolomite to respective oxide phases and CO₂ release. Please find these equations in line 87 and 89 of our revised manuscript.

Line 96-96: the sentence says ‘the we…’, please correct.

 Answers to Reviewer’s comments: Yes, “the” is removed from the sentence. Thanks to the reviewer for the suggestion. It is not in the line number 102 of the revised manuscript.

The materials and methods section needs to be detailed enough so that anyone can reproduce them. No details at all are provided on experimental parameters.

 Answers to Reviewer’s comments: Thank you for the comments and suggestions. We have improved the Experimental and Theory section by listing all the processing conditions, physicochemical characterizations, etc. in detail. The ASTM method behind the activation energy calculations of dolomite samples has been given with relevant citations.

Line 145 – 156: This should be included in the experimental section.

Answers to Reviewer’s comments: Thank you for the reviewer’s comments. Per the reviewer’s suggestions, we have moved the lines 145-156 to the Methods and Methodology section. The content is now available in line numbers 137-149 which is highlighted in the revised manuscript.

Section 3: It would be suitable to provide the Arrhenius equation and explain the terms and where each constant is taken from and thus show how the activation energy is extracted.

Answers to Reviewer’s comments: The activation energy calculations based the ASTM E1641-16 and the relevant equations are now moved to Section 2. We have clearly given the equations its variables and constants that are used. Please find this item now in line numbers 150-162 (highlighted) and also underneath tables of Figures 6(a), 6(b), and 7 in the revised manuscript.

Figure 2: needs labeling with a and b. what is going on with the 2 °C curve? It looks like a multiple-step function. Why is 90 mass% highlighted with a dotted line? Use subscript in the ‘table’.

Answers to Reviewer’s comments: We have re-worked and plotted all the figures now in using Excel and given as jpeg photos. The dotted lines from Figure 6(a) and 6(b) are now removed. All the Arrhenius plots are now placed in the inset of the TGA profiles of Figures, 6 and 7.

Line 188: How does the ball-milling process enhance the content of dolomite? What could possibly react to form dolomite in the process?

Answers to Reviewer’s comments: We have given the postulates in line numbers 210 to 218. The same text is given below for the reviewer’s perusal.

The handpicked dolomite samples are now exposed to the ball milling in the presence of DI water [HP-Dolo-BM] at room temperature. This is carried out to enhance the dolomite reactive species, and to reduce the inert deadweight SiO₂. To our surprise, these HP-Dolo-BM processed materials showed up to 75% of the pure dolomite phase and reduction of calcite to less than 1%. It is unknown how the ball milling enhances the original sample mass% of HP-Dolo from 50% to HP-Dolo-BM, 75% since it is theoretically not plausible. We hypothesize, that the ball milling is a high energy-intensive mechanochemical process, with which the calcite phase from 17.5% (HP-Dolo) is fully consumed, and that increases the concentration of pure dolomite phase. Another possible reason is the reduction of SiO₂ from 22% to 6% after ball milling which increases the initial concentration of the dolomite phase.

 

Figure 4: It is unclear what sample has actually been investigated by BET.

Answers to Reviewer’s comments: BET is now in Figure 5 of the revised manuscript. We have given two sample’s BET, Commercial dolomite, and handpicked dolomite. Please see line number 289 in the revised manuscript. Thanks to reviewers for comments and queries.

Figure 6: A difference plot should be provided if quantitative Rietveld refinement has been performed. However, some Bragg reflections are unidentified and quantification is in fact not possible based on the XRD data.

Answers to Reviewer’s comments: The XRD phase analysis is now carefully done using the ICSD database. Except for one or two unidentified peaks, all other peaks are properly indexed on the basis of dolomite and quartz phases and is given in Figure 5, line numbers 242 to 253 in the revised manuscript.

Line 259: The data should be ‘normalised’ or presented in a percentage fraction instead of in a future publication.

Answers to Reviewer’s comments: The above statement is removed from the revised manuscript since we have executed the normalizations and other relevant calculations per the suggestions from the reviewer. Thanks to the reviewer for constructive comments and suggestions.

Line 270: ‘Further explorations and optimizations are underway.’ These explorations should be included in this research article as the data presented is insufficient.

Answers to Reviewer’s comments: We have given additional explorations, in terms of FTIR, XRD, XRF, TESCAN, Activation Energy for Carbonation, etc. So, the above statement is now removed from the revised manuscript. Thanks to the reviewers for motivating us to do more explorations needed for the manuscript for successful publication.

Line 308: The DOI provided for reference 10 is incorrect. It should be 10.3103/S1067821216030032

Answers to Reviewer’s comments: The DOI is updated with the reviewer’s suggestions. Now it appears in line number 420 of our revised manuscript. Thanks to the reviewer.

Reviewer 2 Report

The article deals with a very interesting issue and may contribute to the development of energy storage technology or CO2 capture. From a scientific point of view, however, the work has many shortcomings and requires significant improvement.
1. SEM-EDX methods are not the best methods for quantitative analysis (qualitative yes, but not quantitative)
2. Very large scattered results - presented in Table No. 1. Of course, the methods of sample preparation are of key importance, but I am afraid that such large discrepancies result from errors in methodology. I am not convinced that we can actually increase the proportion of pure dolomite from 50% to 75% in the ball milling process.
3. Chart 3 is illegible - similar shades of colors occur. This should be improved.
4. The XRD analysis did not indicate which database was used. Was it e.g. JCPDS? No mention was made of the Rietveld (or other) method that was used to determine the number of phases.
5. The authors wrote that XRD studies are not completed. It may be worth waiting and presenting comprehensive research and reviewing your work again.

Author Response

Reviewer 2 Report:

The article deals with a very interesting issue and may contribute to the development of energy storage technology or CO2 capture. From a scientific point of view, however, the work has many shortcomings and requires significant improvement.

1. SEM-EDX methods are not the best methods for quantitative analysis (qualitative yes, but not quantitative)

Answers to Reviewer’s comments: The results produced are from the TESCAN instrument at Colorado School of Mines which is an advanced and automated mineralogy tool. The salient features of the instrument for quantification analysis are given in lines 137 to 149 of our revised manuscript. We have given below the advantage of such an instrument below for the reviewer’s perusal. The compositional mapping results and analysis are detailed and in section 3.2 of our revised manuscript.

The commercial handpicked, and ball milled dolomite samples and their carbonated counterparts via STF were analyzed using automated scanning electron microscopy (automated mineralogy) at the Colorado School of Mines (CSM). The sample were loaded into the TESCAN-VEGA-3 Model LMU VP-SEM platform and analysis was initiated using the control program TIMA3. Four energy dispersive X-ray (EDX) spectrometers acquire spectra from each point or particle with a user-defined beam stepping interval (i.e., the spacing between acquisition points), an acceleration voltage of 25 keV, and a beam intensity of 14. Interactions between the beam and the sample are modeled through Monte Carlo simulation. The EDX spectra are compared with spectra held in a look-up table allowing a mineral or phase assignment to be made at each acquisition point. The assignment makes no distinction between mineral species and amorphous grains of similar composition. Results are output by the TIMA software as a spreadsheet giving the area percent of each composition in the look-up table. This procedure allows a compositional map to be generated and composition assignments were grouped accordingly.

2. Very large scattered results - presented in Table No. 1. Of course, the methods of sample preparation are of key importance, but I am afraid that such large discrepancies result from errors in methodology. I am not convinced that we can actually increase the proportion of pure dolomite from 50% to 75% in the ball milling process.

 Answers to Reviewer’s comments: Thanks for the reviewer’s comments. We have extensively consolidated the table with more meaningful information in Table 2 and Figure 2 of our revised manuscript. We have given a hypothesis regarding the mass increase for the ball-milled materials and these can be found in line numbers 210 to 218. The same text is given below for the reviewer’s perusal.

The handpicked dolomite samples are now exposed to the ball milling in the presence of DI water [HP-Dolo-BM] at room temperature. This is carried out to enhance the dolomite reactive species, and to reduce the inert deadweight SiO₂. To our surprise, these HP-Dolo-BM processed materials showed up to 75% of the pure dolomite phase and reduction of calcite to less than 1%. It is unknown how the ball milling enhances the original sample mass% of HP-Dolo from 50% to HP-Dolo-BM, 75% since it is theoretically not plausible. We hypothesize, that the ball milling is a high energy-intensive mechanochemical process, with which the calcite phase from 17.5% (HP-Dolo) is fully consumed, and that increases the concentration of pure dolomite phase. Another possible reason is the reduction of SiO₂ from 22% to 6% after ball milling which increases the initial concentration of the dolomite phase.

3. Chart 3 is illegible - similar shades of colors occur. This should be improved.

Answers to Reviewer’s comments: Figure 3 is now reconstructed and now it appears as Figure 2 in our revised manuscript. Thanks for the reviewer’s comments and suggestions.

4. 
   The XRD analysis did not indicate which database was used. Was it e.g. JCPDS? No mention was made of the Rietveld (or other) method that was used to determine the number of phases.

 Answers to Reviewer’s comments: The XRD phase analysis is now carefully done using the ICSD database. Except for one or two unidentified peaks, all other peaks are properly indexed on the basis of dolomite and quartz phases and are given in Figure 5 (line numbers 242 to 253 in the revised manuscript).

The commercial dolomite [CO-Dolo] exhibits a single-phase compound with hkl indices matching CaMg(CO₃)₂, ICSD PDF Card no. 202162. There is one diffraction peak at 29.32^o that has not been assigned to the metal carbonate peak, and this may be an unclassified phase as reported in the TESCAN results of section 3.2. On the other hand, the handpicked dolomite sample and its ball-milled counterpart showed CaMg(CO₃)₂ similar to the pristine dolomite, CO-Dolo sample. Additionally, an impurity phase, quartz (SiO₂) with ICSD PDF Card no. 42498 is matched with three diffraction peaks at 2θ angles of 27^o, 42^o and 69^o, respectively. The relative intensities of the majority peak at ~30.9^o are much smaller for the handpicked dolomites when compared to the commercial samples, thus indicating the handpicked versions are at the smaller average crystallite sizes. Table 3 represents the XRD data (few prominent peak positions) of all three samples mentioned above with respect to the shift in 2θ, d spacing, and full-width at half maximum (FWHM) values corresponding to the assigned dolomite phase with hkl Miller indices.

5. The authors wrote that XRD studies are not completed. It may be worth waiting and presenting comprehensive research and reviewing your work again.

 Answers to Reviewer’s comments: We have done the comprehensive XRD evaluations and come to conclusions regarding various samples and crystallite size determinations. Please find our detailed studies as mentioned for the previous question.

Round 2

Reviewer 2 Report

The article after the corrections is much better. I am thinking about another presentation of Figure 3 so that the counting intensity does not start, e.g. for an HP-Dolo-BM sample from 10,000.
I think the article is suitable for publication in its current form. I see that the authors have done a lot of work to improve.