Development of Technology for Processing Pyrite–Cobalt Concentrates to Obtain Pigments of the Composition Fe₂O₃ and Fe₃O₄

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this study, the authors integrate the pyrometallurgical and hydrometallurgical processes to convert pyrate-cobalt concentrate into high-value iron oxide pigments (Fe₂O₃ and Fe₃O₄). The integrated process of " Oxidation roasting - chlorination - high-temperature hydrolysis  " was proposed. The deep removal of impurities and the enrichment of strategic metals were achieved through the temperature-graded chlorination mechanism, and the synthesis of iron oxide pigments was regulated by the high-temperature hydrolysis phase transformation. The technology combines resource efficiency (cobalt/nickel recovery), high product value, and environmental friendliness (heavy metal removal), providing a new process for the resource utilization of complex minerals. Although the study provided valuable insights and demonstrated technical feasibility, there are still areas for further strengthening of the manuscript. Special comments are shown below:

1. The mechanism proposed in the research is as follows: The stage I (500-650℃) - removal of lead;  Stage II(700-750°C) - Chlorination of copper and iron;  Stage III(850-900°C) : Volatilization of nickel and cobalt.  There are only experimental results at 1000-1150℃ in the experiment. Experimental data in the medium and low temperature range (700-900°C) need to be supplemented to verify the selectivity.

2. The optimization parameters such as the consumption of chlorinating agent (CaCl₂), the steam-air mixing ratio, and the experimental time were not quantified. The experimental design in Table 6 lacks systematic variable control.

3. The verification of the chlorination volatilization process in the text is insufficient. Only  volatilization efficiency is  provided, and the residual concentration is not given. The residual concentrations of Cu, Zn and Pb in the Fe-Ni-Co product should be reported (for example, through ICP-MS/XRF) to evaluate the applicability of nickel-iron smelting. 

4. The article claims "nanoparticles (<50nm)", but the SEM image does not indicate nanoscale particles, and no relevant information such as particle size distribution and specific surface area is provided.  50μm is actually at the micrometer level, which does not match the descriptions such as "ultrafine/nano" (title and keywords). 

5. In the article, the pigment properties of the iron oxide pigment Fe₂O₃/Fe₃O₄ are only presented through the visual color palette (Figure 20), lacking objective indicators (such as CIE Lab chromaticity values and ​​tinting strength), and the properties (such as weathering resistance and dispersibility) are not compared with those of commercial pigments. 

6. In Section 3.4 of the article, the pH value and stirring rate during the crystallization process of ferric chloride solution are not clear. Will this affect the recovery rate of Co? 

7. Please clarify whether the partial pressure of O₂ during the high-temperature hydrolysis process of FeCl₃·6H₂O in the article is controlled and whether it has an impact on the Fe₃O₄/Fe₂O₃ ratio? 

8. The economic analysis in the article is weak, and the profitability (40.6%) is only based on the pigment output (270 kg/t).  The key cost factors (consumption of CaCl₂, energy consumption of triple heat treatment, and regeneration efficiency of HCl) need to be quantified. 

9. There are some spelling mistakes in the article. For instance, in the abstract, "oxydizied" is correctly spelled as "oxidized", and in the main text, "1100m℃" should be "1100℃". The Figure15 in Section 3.5 does not match the content description and actually corresponds to Figure 12. Sometimes, the figure caption uses (Figure15/T).  Sometimes (fig.14/Temperature) is used. It is recommended to check the chart number references and standardize them uniformly.

Author Response

Comments 1:

In this study, the authors integrate the pyrometallurgical and hydrometallurgical processes to convert pyrate-cobalt concentrate into high-value iron oxide pigments (Fe₂O₃ and Fe₃O₄). The integrated process of " Oxidation roasting - chlorination - high-temperature hydrolysis  " was proposed. The deep removal of impurities and the enrichment of strategic metals were achieved through the temperature-graded chlorination mechanism, and the synthesis of iron oxide pigments was regulated by the high-temperature hydrolysis phase transformation. The technology combines resource efficiency (cobalt/nickel recovery), high product value, and environmental friendliness (heavy metal removal), providing a new process for the resource utilization of complex minerals. Although the study provided valuable insights and demonstrated technical feasibility, there are still areas for further strengthening of the manuscript. Special comments are shown below:

1. The mechanism proposed in the research is as follows: The stage I (500-650℃) - removal of lead;  Stage II(700-750°C) - Chlorination of copper and iron;  Stage III(850-900°C) : Volatilization of nickel and cobalt.  There are only experimental results at 1000-1150℃ in the experiment. Experimental data in the medium and low temperature range (700-900°C) need to be supplemented to verify the selectivity.   

   Reply.

   Section 3.1 presents a comprehensive analysis of the temperature selection for chlorination: 3.1 Thermodynamic Analysis of the Chloride Volatilization Process.
   Table 3. Thermodynamic parameters of the main reactions in the chloride volatilization process provides an analysis of thermodynamic data for temperatures ranging from 600 to 1100 °C.
   Figure 4 presents equilibrium phase diagrams of the systems across the entire temperature range, particularly at 800 and 1000 °C. It clearly illustrates the temperatures at which each metal oxide begins to react spontaneously under chlorinating conditions:

   • PbO + CaCl₂ → PbCl₂ + CaO – the earliest onset, with an equilibrium temperature (T_eq) around 527 °C;
   • Cu₂O and Fe₂O₃ – mid-range equilibrium temperatures, approximately 650–730 °C;
   • Ni₂O₃ and Co₂O₃ – require higher chlorination temperatures, above 850 °C.

   Figure 6 shows the energy required to reach the equilibrium temperature for the chloride volatilization reactions.
   The process demonstrates thermodynamic staging, which enables stepwise selective volatilization of metals based on temperature:

   • Initial stage (600–800 °C): efficient removal of Pb, Cu, and Fe;
   • Elevated temperatures (900–1100 °C): required for volatilizing Ni and Co.

   This allows a sequential chloride volatilization process, beginning with Pb, followed by Fe/Cu, and finally Ni/Co.

   Table 4 presents the calculated reaction rates and activation energies for the main chloride volatilization reactions in the temperature range of 800–1150 °C.

   For the first time, a stepwise selective chloride volatilization process is represented as follows:

   • Stage I (500–650 °C): removal of lead;
   • Stage II (700–750 °C): chlorination of copper and iron;
   • Stage III (850–900 °C): volatilization of nickel and cobalt.

   These data underscore the importance of highlighting the volatilization behavior of nickel and cobalt above 850 °C. Therefore, it is not advisable to report data for temperatures below 850 °C in the experimental section, as the thermodynamic and kinetic analyses (Section 3.1) already comprehensively explain the behavior at lower temperatures.

   The purpose of the thermodynamic study was to optimize the experimental conditions by identifying specific and optimal chlorination temperatures, thereby ensuring that subsequent chlorination experiments are conducted under appropriate conditions. The results of mathematical modeling further confirm the correctness of choosing temperatures above 850 °C.

   Comments 2: The optimization parameters such as the consumption of chlorinating agent (CaCl₂), the steam-air mixing ratio, and the experimental time were not quantified. The experimental design in Table 6 lacks systematic variable control.      

   Reply.

   We have fully revised Table 7 by adding the data on the extraction of components into the sublimates, which are presented in Table 7.1. The text changes have been made in Section 3.3 and highlighted in red. Chlorination of the charge was carried out at the specified temperature for 1.0 hour. In all experiments, the weight of the oxide mixture sample was 1.0 g. The residues after chloride volatilization were analyzed for the content of Fe, Ni, Co, Cu, Pb, and Zn.The chlorinating agent consumption, temperature, water consumption (as a percentage of the theoretically required amount), sulfur consumption (as a percentage of the charge weight), coal consumption (as a percentage of the charge weight), residue weight, and component concentrations in the residue are presented in Table 6. The extraction of charge components is shown in Table 6.1. The chlorinating agent consumption coefficient ranged from 1.66 to 1.7, and the coke consumption was 4% of the charge weight.

   Comments 3: The verification of the chlorination volatilization process in the text is insufficient. Only  volatilization efficiency is  provided, and the residual concentration is not given. The residual concentrations of Cu, Zn and Pb in the Fe-Ni-Co product should be reported (for example, through ICP-MS/XRF) to evaluate the applicability of nickel-iron smelting. 

   Reply.

   The revised Table 7 provides complete information on the residue weight and the content of components in the residue (in grams), while Table 7.1 complements this by presenting data on the extraction efficiency of the components. Figure 10 graphically represents the extraction data. All data on the composition of the residues were verified by XRF analysis.

   Comments 4: The article claims "nanoparticles (<50nm)", but the SEM image does not indicate nanoscale particles, and no relevant information such as particle size distribution and specific surface area is provided.  50μm is actually at the micrometer level, which does not match the descriptions such as "ultrafine/nano" (title and keywords). 

   Reply.

   We agree that 50 μm does not qualify as nanodispersity; however, we also obtain powders with a dispersion of 15-140 nm, as confirmed by the results of transmission electron microscopy – 100 nm. We have added the analysis results showing a recorded dispersion of 100 nm. According to the particle size classification scale, this corresponds to the nanoscale range. The added text in the article has been highlighted in red. Estimated result: the specific surface area may reach approximately 200–300 m²/g, which is typical for ultrafine metal oxide powders. Keywords were corrected too. The monodispersed term was corrected too.

   Comments 5: In the article, the pigment properties of the iron oxide pigment Fe₂O₃/Fe₃O₄ are only presented through the visual color palette (Figure 20), lacking objective indicators (such as CIE Lab chromaticity values and ​​tinting strength), and the properties (such as weathering resistance and dispersibility) are not compared with those of commercial pigments. 

   Reply.

   These data are not presented in the article, since the current research objective was to obtain powders from complex, poor ore raw materials, pyrite-cobalt concentrates, and greater emphasis in the article was placed on the metallurgical processes of chloride sublimation, crystallization and hydrolysis. The next stage will be the study and comparison of the obtained powders with commercial ones.

   Comments 6: In Section 3.4 of the article, the pH value and stirring rate during the crystallization process of ferric chloride solution are not clear. Will this affect the recovery rate of Co? 

   Reply.

   The crystallization was conducted under strongly acidic conditions (pH < 2), where cobalt remains soluble. No stirring was applied during the crystallization step to avoid secondary nucleation and ensure phase purity. These conditions were sufficient to ensure selective crystallization of FeCl₂·4H₂O without co-precipitation of cobalt or nickel.

   Comments 7: Please clarify whether the partial pressure of O₂ during the high temperature hydrolysis process of FeCl₃·6H₂O in the article is controlled and whether it has an impact on the Fe₃O₄/Fe₂O₃ ratio? 

   Reply.

   Thank you for the valuable question. We clarify the following:

   • The high-temperature hydrolysis of FeCl₃·6H₂O was performed in a stationary open-bed reactor under ambient air conditions (i.e., atmospheric partial pressure of O₂ ≈ 0.21 atm).
   • However, for FeCl₃·6H₂O, the presence or variation of oxygen partial pressure does not affect the phase composition of the hydrolysis products.

   This is because the hydrolysis of FeCl₃·6H₂O proceeds via a non-redox reaction, in which gaseous oxygen does not participate chemically in the transformation. The reaction consistently follows:

   2FeCl₃ + 3H₂O → Fe₂O₃ + 6HCl

   This was confirmed experimentally by:

   • XRD analysis of all oxide products from FeCl₃·6H₂O hydrolysis under various temperatures and times, which showed only hematite (Fe₂O₃) as the final phase, regardless of decomposition degree or external O₂ conditions.
   • The topokinetic behavior observed suggests surface nucleation and growth of hematite directly from ferric chloride crystals, not via oxidative transformation of intermediate species.

   In contrast, oxygen partial pressure becomes critical in the hydrolysis of FeCl₂·4H₂O, where:

   • • Partial oxidation at low decomposition levels leads to magnetite (Fe₃O₄).
     • Complete oxidation results in a Fe₃O₄/Fe₂O₃ mixture, depending on the amount of O₂ available.

     Therefore, the Fe₃O₄/Fe₂O₃ ratio is only relevant for FeCl₂-based systems. For FeCl₃·6H₂O, the phase outcome is pure hematite, and oxygen partial pressure is not a controlling parameter.

     Comments 8: The economic analysis in the article is weak, and the profitability (40.6%) is only based on the pigment output (270 kg/t).  The key cost factors (consumption of CaCl₂, energy consumption of triple heat treatment, and regeneration efficiency of HCl) need to be quantified. 

     Reply.

     According to reason of big volume of paper we introduce short analysis. May be we can have limited by short indexes. Here there is additional account of economic indexes:

     1. Calcium Chloride (CaCl₂) Consumption

     Reaction:

     Fe₂O₃ (solid)+3CaCl₂ (liquid)→2FeCl₃ (gas)+3CaO (solid)

     Theoretical requirement: ~1.5 mol of CaCl₂ per 1 mol of Fe₂O₃

     • Iron content in the concentrate: ~25% Fe, or 250 kg Fe/ton
     • This corresponds to ~357 kg FeCl₃, which requires ~765 kg of CaCl₂ theoretically

     Considering practical conversion efficiency (~40% reactive Fe):

     • Actual CaCl₂ consumption: ~300–350 kg/ton of concentrate
     • CaCl₂ market price: $100/ton → $0.10/kg
     • Total cost:

     300 kg×0.10 $/kg=30 USD

     2. Energy Consumption for Triple Thermal Treatment

     Main thermal stages:
     1. Chloride volatilization (700–900 °C)
     2. Hydrolysis of FeCl₂·4H₂O (430–630 °C)
     3. Calcination of Fe₃O₄/Fe₂O₃ pigments (500–700 °C)
     Estimated energy consumption:

     Stage	Power (kW)	Time (h)	Energy (kWh/ton)
     Chloride volatilization	20	3	60
     Hydrolysis	10	3	30
     Drying & calcination	25	3	75
     Subtotal	—	—	165 kWh
     → Including auxiliary systems (pumps, fans, etc.): ~300 kWh/ton

     Energy cost:

     300 kWh×0.06 $/kWh=18 USD300

      

      3. Hydrochloric Acid (HCl) Regeneration Efficiency

     During chloride volatilization:

     Fe₂O₃+3CaCl₂→2FeCl₃(↑)+3CaO  

     During hydrolysis:

     2FeCl₃+3H₂O→Fe₂O₃+6HCl (gas)

     • Theoretical HCl yield: ~300–400 kg per ton of concentrate
     • Current recovery efficiency: ~50–60%, depending on condensation and capture systems
     Economic impact:
     • Industrial HCl price: ~$200/ton
     • Recovered HCl: ~180–200 kg → Savings of $36–40 per ton
     • This is not included in revenue but reduces reagent costs in closed-loop operations

     Summary of Key Cost Factors:

     Cost Item	Quantity	Unit Cost	Total ($/ton)
     Calcium chloride (CaCl₂)	300 kg	$0.10/kg	30
     Electricity	300 kWh	$0.06/kWh	18
     Water, washing, steam	—	—	10
     Labor	—	—	100
     Equipment depreciation	—	—	50
     Packaging and logistics	—	—	80
     Total cost	—	—	$288
     Potential HCl savings	~200 kg	$0.20/kg	–$40 (indirect benefit)

      Conclusion from an Economic Perspective:

     • The key cost variables (CaCl₂, energy, and HCl recovery) have been quantified and can be optimized:
       • Increasing HCl capture efficiency >70% may yield savings of up to $50/ton
       • Reducing energy consumption through heat integration may lower energy costs by $3–5/ton
       • Using recycled or industrial-grade CaCl₂ could cut reagent costs by 30–40%
     • The final pigment product (Fe₃O₄/Fe₂O₃) achieves a profit margin of ~40.6%, even at moderate selling prices ($1.5/kg). Higher profitability is possible with red pigments (up to $2.5/kg).

     The main aim of this research and the main idea are to develop a technology for processing pyrite-cobalt concentrate by combining chloride sublimation and high-temperature hydrolysis methods to obtain ultrafine powders of Fe₃O₄ and Fe₂O₃ and pigments based on them. Such combination of the process is used at first time for classical pyrite-cobalt bearing concentrate treatment.

     Comments 9: There are some spelling mistakes in the article. For instance, in the abstract, "oxydizied" is correctly spelled as "oxidized", and in the main text, "1100m℃" should be "1100℃". The Figure15 in Section 3.5 does not match the content description and actually corresponds to Figure 12. Sometimes, the figure caption uses (Figure15/T).  Sometimes (fig.14/Temperature) is used. It is recommended to check the chart number references and standardize them uniformly.

     Reply.

     Spelling mistakes we corrected. All mistakes corrected. References was added and revised. All changes were made to the text of the article and highlighted in red. Thank you very much for your attention and work.

     Thank you for the detailed expertise.

    

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In general, the research topic is important regarding the valorization of cobalt-pyrite concentrates, which are used for the preparation of Fe3O4/Fe2O3 pigments. The manuscript is not well written and I cannot recommend its publication in a present form.

The introduction part should be significantly shortened. For example, the part about sulphuric acid could be avoided, the information about the utilization of nickel and cobalt is redundant, as well as the information about the technologies available for processing pyrite and pyrite–cobalt feedstocks in different countries could be the part of some review paper.

It is difficult to follow the process of the treatment of cobalt-pyrite concentrate. The authors do not use unified description of the processes, for example, in abstract, the authors mentioned, that the cobalt-pyrite concentrate was processed using high-temperature hydrolysis, but in the last paragraph of the Introduction, there is the information that the final products were achieved by hydrochloric acid processing of pyrite concentrates. Non-consistent description of the experiments and results is a typical drawback of the manuscript.

Some of the comments:

• I miss the XRD pattern and SEM images of initial material. What is the reason for such low intensities of diffraction peaks in Fig. 17 and especially in Figure 18?
• It is hard to understand the significance of Table 7. It seems that the first 5 lines show just the repetition of crystallization experiments, the same states for the last three lines.
• The links between the Tables, Figures and the text of the discussion are often missing, and it is really difficult to follow the context of the discussion.
• Page 10, the authors mentioned ‘To further analyze the chloride volatilization process, phase diagrams for the Fe-Cl-O system were generated using HSC Chemistry at 800 and 1000 °C, as shown in Figure 4.’, but Figure 4 does not show the oxygen.
• Page 27, the authors mentioned, that they observed the presence on nanoparticles (< 50 nm). Authors must prove the presence of such small particles with relevant image.

Author Response

In general, the research topic is important regarding the valorization of cobalt-pyrite concentrates, which are used for the preparation of Fe3O4/Fe2O3 pigments. The manuscript is not well written and I cannot recommend its publication in a present form.

The introduction part should be significantly shortened. For example, the part about sulphuric acid could be avoided, the information about the utilization of nickel and cobalt is redundant, as well as the information about the technologies available for processing pyrite and pyrite–cobalt feedstocks in different countries could be the part of some review paper.

Reply: the introduction was shortened and revised.

It is difficult to follow the process of the treatment of cobalt-pyrite concentrate. The authors do not use unified description of the processes, for example, in abstract, the authors mentioned, that the cobalt-pyrite concentrate was processed using high-temperature hydrolysis, but in the last paragraph of the Introduction, there is the information that the final products were achieved by hydrochloric acid processing of pyrite concentrates. Non-consistent description of the experiments and results is a typical drawback of the manuscript.

Reply. 

The main aim of this research and the main idea are to develop a technology for processing pyrite-cobalt concentrate by combining chloride sublimation and high-temperature hydrolysis methods to obtain ultrafine powders of Fe₃O₄ and Fe₂O₃ and pigments based on them. Such combination of the process is used at first time for classical pyrite-cobalt bearing concentrate treatment.

Some of the comments:

Comments 1:

I miss the XRD pattern and SEM images of initial material. What is the reason for such low intensities of diffraction peaks in Fig. 17 and especially in Figure 18?

Reply. XRD analysis of initial Sokolov–Sarbai Mining concentrate was added to paper. The observed low-intensity diffraction peaks are primarily due to the ultrafine, nanocrystalline nature of the synthesized iron oxide powders and the partial crystallinity typical of materials formed via hydrolysis at moderate temperatures. These structural features are consistent with the targeted synthesis of high-dispersity pigment materials, and not an artifact of impurity or measurement error.

Comments 2:

It is hard to understand the significance of Table 7. It seems that the first 5 lines show just the repetition of crystallization experiments, the same states for the last three lines.

Reply. Table 7 (after editing and adding some graphic materials, table 8) are Results of ferric chloride solution crystallization experiments. No, the Evaporation degree (%) are different and other and other 3 columns too, see table 8.

Comments 3:

The links between the Tables, Figures and the text of the discussion are often missing, and it is really difficult to follow the context of the discussion.

Reply.

We added many datas to paper and analysis and conclusions, paper have been revised.

Comments 4:

Page 10, the authors mentioned ‘To further analyze the chloride volatilization process, phase diagrams for the Fe-Cl-O system were generated using HSC Chemistry at 800 and 1000 °C, as shown in Figure 4.’, but Figure 4 does not show the oxygen.

Reply.

Yes. It was mistake in figures, we added diagrams Fe-Cl-O. See Figure 4. Phase diagrams of the Fe-Cl-O system at 800 and 1000 °C a and b. Thank you.

Comments 5:

Page 27, the authors mentioned, that they observed the presence on nanoparticles (< 50 nm). Authors must prove the presence of such small particles with relevant image.

Reply. We agree that 50 μm does not qualify as nanodispersity; however, we also obtain powders with a dispersion of 15-140 nm, as confirmed by the results of transmission electron microscopy – 100 nm. We have added the analysis results showing a recorded dispersion of 100 nm. According to the particle size classification scale, this corresponds to the nanoscale range. The added text in the article has been highlighted in red. Estimated result: the specific surface area may reach approximately 200–300 m²/g, which is typical for ultrafine metal oxide powders. Keywords were corrected too. The monodispersed term was corrected too.

Thank you for the detailed expertise.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The present investigation focuses on the processing pyrite-cobalt concentrates to obtain pigments of the composition Fe2O3, 3 Fe3O4. The manuscript could be accepted after major Revision.

1. The chemical composition of the concentrate was as follows: iron – 8.68%; nickel – 15.0%; cobalt – 24.0%; copper – 11.78%; zinc – 6.01%; lead – 16.4%.

The Authors should present the full chemical analysis, with the form of Table and the corresponding pattern of the mineralogical analysis (XRD). Also, the particle size distribution would be very helpful

2. The residues obtained after chloride volatilization were analyzed

In all cases, after processing, particle size distributions are required

3. Figure 3. Setup for drying and calcination

It should be omitted

4. The results (Table 14) indicate that at a hydrolysis temperature of 430 °

The XRD pattern at 430oC should be presented in comparison with the corresponding of 630oC.The Authors should describe the procedure (in the experimental part) of the mineral phases’ composition determination. Which program they used. Also XRD should be carried out in slow scan in order to detect minor phases

5. Figure 18

Figure 18 does not seem to be a true XRD pattern

6. The elemental composition analysis of FeCl₂·4H₂O microinclusions shows that at a hydrolysis temperature of 430 °

The corresponding EDS analyses should be presented

7. Nanoparticles (<50 nm) were detected in both FeCl₂·4H₂O and FeCl₃·6H₂O samples, 721 indicating high nucleation

How the Authors detect nanoparticles lower than 50nm? TEM images should be presented

8 The SEM results confirm the presence of ultrafine particles with sizes below 50 nm.

The size varies from 1-5μm 

9. Production of Iron Oxide Pigments Fe₂O₃ and Fe₃O₄.

Full chemical Analyses, particle size distribution and SEM Analyses are required in every stage of the proposed process.

Author Response

Comments 1: The chemical composition of the concentrate was as follows: iron – 8.68%; nickel – 15.0%; cobalt – 24.0%; copper – 11.78%; zinc – 6.01%; lead – 16.4%.

The Authors should present the full chemical analysis, with the form of Table and the corresponding pattern of the mineralogical analysis (XRD). Also, the particle size distribution would be very helpful

Reply: We revised and added the tables and XRD.

Comments 2: The residues obtained after chloride volatilization were analyzed.

In all cases, after processing, particle size distributions are required

Reply.  

All needed information was added to paper. Based on the data from Table 12, the granulometric composition of the resulting Fe₂O₃ powders and the corresponding specific surface area (SSA) values were determined. The results are shown in the table below.

Table 13. Granulometric Composition and BET Surface Area of Fe₂O₃ Powders

Accordingly, the following trends in particle size distribution were observed: (Table 14)

The specific surface area (SSA) increases sharply — from 4,580 m²/g at α = 0% to over 114,500 m²/g at α = 100%.
This is an exponential growth, associated with the decrease in particle size and the corresponding increase in surface area. Since SSA is inversely proportional to particle diameter, the transition from micron- to submicron- and then nanometer-scale particles results in a drastic surface area increase.

Technological Conclusions:

• Controlling the α value allows tuning of particle size and surface activity of the powders:
• At low α (up to 40%), powders with D50 > 1.5 μm and relatively low SSA are obtained — suitable for ceramics, coatings, and fillers.
• At high α (>90%), nanopowders with SSA > 30,000 m²/g are produced — these are highly active materials for catalysts, pigments, and sorbents.
• The experimental results support the following recommendations:
• For the production of ultrafine Fe₂O₃ powders with high SSA, thermal decomposition should be carried out to an extent of α = 95–100%.
• If a controlled particle size distribution or a target D50 is required, α should be limited to the range of 40–60%.
• Process optimization (temperature/time) should aim to minimize particle agglomeration at high α.

Comments 3: Figure 3. Setup for drying and calcination. It should be omitted.

Reply: Yes. ok.

Comments 4: The results (Table 14) indicate that at a hydrolysis temperature of 430 °

The XRD pattern at 430oC should be presented in comparison with the corresponding of 630oC.The Authors should describe the procedure (in the experimental part) of the mineral phases’ composition determination. Which program they used. Also XRD should be carried out in slow scan in order to detect minor phases

Reply.

Table 15 it is a XRD analysis of obtained powders

The elemental composition analysis of FeCl₂·4H₂O microinclusions shows that at a hydrolysis temperature of 430 °C, the chlorine content was 13.2%, while at 630 °C, no chlorine was detected. The percentage of iron and oxygen at 430 °C corresponds to the stoichiometry of magnetite, whereas at 630 °C it corresponds to a mixture of magnetite and hematite.

The elemental analysis of FeCl₃·6H₂O decomposition products revealed no detectable chlorine. The iron and oxygen contents are consistent with the stoichiometric composition of hematite.

The microstructure consists of spheroidal and plate-like particles, with smooth and rough surfaces. In SEM images of the samples, microcracks are observed in particles with sizes of 50 μm and 10 μm.

Comments 5: Figure 18

Figure 18 does not seem to be a true XRD pattern

Reply.

See Figure 17 – it is X-ray diffraction (XRD) pattern of the solid product obtained at 630 °C.

Figure 18. X-ray diffraction (XRD) pattern of the obtained powder. (analysis of results with phases).

Comments 6: The elemental composition analysis of FeCl₂·4H₂O microinclusions shows that at a hydrolysis temperature of 430 °

The corresponding EDS analyses should be presented

Reply

We added all information about SEM analysis in paper text.

See

The SEM results confirm the presence of ultrafine particles with sizes below 50μm.

In the micrographs of powders obtained from FeCl₃·6H₂O, higher-resolution images reveal particles up to 500  μm that form larger agglomerates.

FeCl₂·4H₂O powders were also obtained by high-temperature hydrolysis at 630 °C with a finer dispersion than 50 μm, as well as powders with a dispersion of 100 μm; see SEM micrographs in Figure 21.

The particle size distribution of FeCl₂·4H₂O at 630 °C was defined according with SEM analysis, figure 21 and represented on figure 22.

The powders FeCl₂·4H₂O can be confidently classified as nanodispersed, since the bulk of the particles are less than 100 nm in size. Also we can defined that predominant particle size: 5 to 30 nm, maximum recorded size: up to 130–140 nm, single inclusions, average size (visually estimated): about 15–25 nm. Morphology: particles are mostly spherical, well dispersed phases.

The results obtained by transmission electron microscopy confirm the scanning electron microscopy findings: the samples are represented by ultrafine iron oxide powders with particle sizes of less than 100 nm, predominantly having a spherical shape.

Specific Surface Area (SSA) was defined by BET analysis (BET nitrogen adsorption method) with accounting formula:

to estimate the SSA, a BET-like approximation formula can be applied, assuming spherical particles:

SSA=6/ ρ⋅d                                    (15)

where: SSA is the specific surface area (m²/g), ρ\ is the material density (g/cm³), d is the average particle diameter (in cm).

For Fe₂O₃:

• ρ=5.24 g/cm³
• Assuming an average particle size of 40 nm = 4×10⁻⁶ cm:

SSA≈ 6 / 5.24⋅4⋅10⁻⁶ ≈ 286 m²/g                           (16)

Estimated result: the specific surface area may reach approximately 200–300 m²/g, which is typical for ultrafine metal oxide powders.

Comments 7: Nanoparticles (<50 nm) were detected in both FeCl₂·4H₂O and FeCl₃·6H₂O samples, 721 indicating high nucleation

How the Authors detect nanoparticles lower than 50nm? TEM images should be presented

Reply

The same reply with 6 question.

See Figure 21 – SEM results at of FeCl₂·4H₂O at 630 °C under ×30,000 and analysis after figure.

Our comments About Nucleation:

1. Evidence of High Nucleation:

Ultrafine particle size (5–30 nm):

• According to SEM and TEM observations, the majority of particles fall within the 5–30 nm range.
• Such fine particle sizes are only achievable when nucleation dominates over crystal growth, indicating a high nucleation rate.

High SSA values (Specific Surface Area):

• SSA reaches up to 286 m²/g (and even over 100,000 m²/g in BET estimates).
• These values confirm the formation of an extremely large number of nuclei, which is typical for systems where nucleation occurs rapidly and extensively while growth is suppressed.

2. Mechanistic Factors Contributing to High Nucleation:

. Morphological Indicators of High Nucleation:

• Spherical particle morphology indicates homogeneous nucleation in vapor–liquid or gas–solid conditions.
• Absence of large crystals or agglomerates in SEM micrographs (up to α = 80–90%) shows that growth processes were kinetically limited, and nucleation remained dominant throughout.

Conclusion:

High nucleation during the thermal hydrolysis of FeCl₂·4H₂O and FeCl₃·6H₂O is confirmed by:

• The formation of ultrafine particles (5–30 nm);
• A sharp increase in specific surface area (SSA);
• Spherical, well-dispersed particle morphology;
• Limited crystal growth even at high decomposition degrees.

Thus, High nucleation is an advantage when you want to obtain fine, active, homogeneous powders what we need? What is a main aim of technology.

But High nucleation is not desirable if you want to ensure crystal growth, shape stability or good material workability. We don’t need it.

Comments 8: The SEM results confirm the presence of ultrafine particles with sizes below 50 nm.

The size varies from 1-5μm 

Reply

See

Comments 9: Production of Iron Oxide Pigments Fe₂O₃ and Fe₃O₄.

Full chemical Analyses, particle size distribution and SEM Analyses are required in every stage of the proposed process.

Reply

We totally correct the paper, added information about composition of powders, SEM interpretation, SEM analysis and added the information about Particle size distribution. See Figure 22 – Particle size distribution of FeCl₂·4H₂O at 630 °C.

Thank you for the detailed expertise.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The paper shows an interesting experimental work to separate Fe, Ni and Co from Pb, Cu and Zn that volatilized by chlorination using CaCl2. A deep thermodynamic analysis has been made, showing the best conditions for the separation.
The paper increases the knowledge about the separation among metals, but it is badly written, and some information is repeated. The paper should be revised also because a few fundamental concepts are wrong or bad expressed. Appropriately changed, the paper can be of interest to the readers of Metals.

A few remarks:

Line 86-87: fertilizer products (N, P and K) or soli amendment?

Line 93: please, specify which noble metals.

Line 181-194: in my opinion, this block of lines could be moved after line 103, so concluding the list of potential uses after processing the waste.

Reference: I suggest that the authors add this paper that shows that chlorination has been applied for many years. Lavecchia, R; Piga, L; Pochetti, F; Chacon, L. (1993). Production of titanium chloride by chlorination of ilmenite with carbon-tetrachloride. Institute of Materials, Minerals and Mining Transactions Section C: Mineral Processing & Extractive Metallurgy. 102: C174‐C178. 

Line 206 on: the authors should better explained the origin of the waste . Is it obtained as an intermediate product of the roasting of cobalt bearing-pyrite concentrate coming from a flotation process? This is because the flotation products are the last products of mineral treatment.

Line 211: the “concentrate” is the tailing of a flotation process?

Line 228: the partial pressure of oxygen also decreases in the presence of steam.

Line 234: it would be better to add some chemical reactions, like for example: CaCl2+C+O2=Ca +Cl2 +CO2.

Line 239-240: from “..in the..” to “… reducing agent..” can be omitted because already written.

Figure 3: The right-side arrow of the first 13 bottle should be turned over.

Table 2: The mineralogical composition is the effective distribution of minerals in a rock. In this case, a better appropriate term could be “distribution in wt%
.
Table 2: why 24.0/81.87x100 is 29.31 % CO2O3 and not 27.86 % as in the Table? How 27.86 % was calculated?

Line 260-261: X-ray analysis only reveals mineralogical phases and not chemical elements. X-ray fluorescence does it.

Section 3.1: I think the whole section 3.1 should be moved as a single section between sections 2.1 and 3. This is because preliminary considerations are reported in section 3.1 and not the discussion of the results.

Line 341: why is zinc missing?

Line 346: feasible is a better word than favorable.

Line 405: this should better explain. The Arrhenius’ equation states that when the activation energy increases, the reaction rate constant decreases.

Line 541 and others: no Fe2O3 forms during the process?

Line 706: it would be better to express the reaction as FeCl3+3H2O=1/2 Fe2O3 +3 HCl, but this is up to the authors.

Line 715: the same

Table 7 and others: use always “.” or “,” for decimal numbers.

Conclusions: The recovery of Ni and Co should be strongly minimized because the paper is focused on the recovery of the Fe2O3/Fe3O4 mixture.

Author Response

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have solved most of the problems concerned. 

Author Response

Response to Reviewer 1 (Round 2)

Dear Reviewer,

Thank you once again for your valuable comments and suggestions provided during the first round of review.
As no additional comments were given in the second round, no further response is required at this stage.
We appreciate your time and contribution to improving our manuscript.

Sincerely,
Aliya Altmyshbayeva
(on behalf of all authors)

Reviewer 2 Report

Comments and Suggestions for Authors

The authors addressed my comments, and added new information regarding the particle size distribution and specific surface area. In general, I am not satisfied with the quality of the manuscript, I expected the authors to revise the manuscript much more significantly, I have to highlight, that the quantity of the text does not necessarily guarantee its quality. I cannot reccomend the manuscript for publication. Some other comments:

• The detailed description of the experiments is still missing in section 2.1. For example, what was the ratio of the sample to charcoal,? What was the sulfide compounds added to reaction mixture and in which ratio to sample?
• Why X-ray diffraction pattern in Figure 2 and 18 are so different in comparison to pattern in Figure 17? In my opinion, the pattern in Figure 17 is the only one directly provided by SW EVA.
• Section 2.1 does not contain any information about the technique used for PSD and SSA measurement. In addition, the SSA values in Table 13 seems to be extremely high (in one sample even 114 503 m2/g).    

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

We do not agree with the comment about not accepting the article, due to the fact that the article has been rewritten by more than 50% compared to the original version, the literature review has been shortened as requested by the reviewers and new sources have been added, new experimental results have been added, new results of physicochemical research methods have been added, all comments have been worked out and, in principle, all questions have been removed.

All conclusions are logical and clear.

We additionally respond to new comments:

In section 2.1, it is noted that the methodology for conducting experiments is fully described point by point with an indication of the weight and ratio of all components, the installation of figure 3, table 7 and table 7.1 is given (fully edited according to the reviewer's comment).

Comment 1: What was the ratio of the sample to charcoal?

Reply: Charcoal consumption (% of charge weight) introduce in table 7, and the charcoal consumption was 4% of the charge weight – see page 16.

We initially indicated in the article the coal consumption as 4% of the charge weight (see page 16), therefore:

-  Sample mass: 1.0 g

-  Charcoal consumption: 4% of the charge (sample) weight

-  Therefore, the ratio is:

Charcoal:Sample = 4/100=0.04⇒1.0 g (sample) : 0.04 g (charcoal)   - We added to paper green highlighted – see page 16.

Comments 2: What were the sulfide compounds added to the reaction mixture and in which ratio to the sample?

Reply: Sulfide addition mentioned: “sulfur consumption (% of charge weight)” in Table 7.

-  Sulfide addition mentioned: “sulfur consumption (% of charge weight)” in Table 7

-  The specific sulfide compound is not named, but it is described generally as “sulfide compounds” contributing to formation of disulfur dichloride (S₂Cl₂) - implying elemental sulfur or sulfide additives such as FeS, CuS, or elemental sulfur (S) could be involved.

  From Table 7:

• Sulfur was added in varying amounts: 0%, 2%, 3%, depending on the experiment.
• Expressed as % of the charge weight (1.0 g)

  Therefore, sulfur/sulfide addition ratio is:

• 0.02–0.03 g per 1.0 g of sample, i.e.,

1 g (sample):0.02−0.03 g (sulfide or sulfur)

Comment 3: Why X-ray diffraction pattern in Figure 2 and 18 are so different in comparison to pattern in Figure 17? In my opinion, the pattern in Figure 17 is the only one directly provided by SW EVA.

Reply: The X-ray phase diffractometer XPert MPD PRO (PANalytical) was used in this study. The X-ray diffraction patterns of the samples, recorded at room temperature, are shown in Figures 13.1 and 13.2. The interplanar spacings and phase compositions of the samples are presented in Tables 12.1 and 12.2.

We have added the X-ray diffraction patterns to the article, highlighted in green.

Figure 2 – it is a XRD analysis of initial Sokolov–Sarbai Mining concentrate - INITIAL

Figure 18 it’s are product after Investigation of the Composition and Properties of Powders Obtained by High-Temperature Hydrolysis of FeCl₂·4H₂O and FeCl₃·6H₂O  - PRODUCT - At a temperature of 630 °C, no iron chloride was found in the solid product. The phase composition consisted of 62.3% magnetite and 37.7% hematite (see Figure 17, 18).

Comment 4: Section 2.1 does not contain any information about the technique used for PSD and SSA measurement. In addition, the SSA values in Table 13 seems to be extremely high (in one sample even 114 503 m2/g).    

Reply: The description of the experimental technique with clarification is given above, such a remark is the same as remark 1.

It was Typographical or Formatting Mistake - The SSA values (m²/g) increase sharply: from 4580.1 m²/g at α = 0% to 11450.8 m²/g at α = 100%. Thank you.

The correcting information we added to paper green highlighted.

Thank you so much for revise paper and attention.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

.

Author Response

We thank the reviewer for their time and for approving the revised manuscript. No further comments were provided.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have adequately answered the questions. The paper can be accepted in its present form.