Grinding of Australian and Brazilian Iron Ore Fines for Low-Carbon Production of High-Quality Oxidised Pellets

Response to Reviewer X Comments

Manuscript ID: 2879930

1. Summary

We would like to extend our sincere gratitude on behalf of our team for the invaluable feedback and suggestions you provided during the review of our manuscript. Your professional insights have significantly contributed to refining and enhancing the quality of our research. And your peer review comments have been addressed individually in the subsequent sections, and corresponding revisions have been made throughout the manuscript.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Yes

Are the methods adequately described?

Can be improved

Are the results clearly presented?

Yes

Are the conclusions supported by the results?

Yes

3. Point-by-point response to Comments and Suggestions for Authors

We appreciate the opportunity to provide additional information regarding the homemade pelletizing disc apparatus used in our experiments. The machine, designed and constructed by our institution without a specific model number, is characterized by a diameter (Φ) of 1000mm, a rotational speed of 32r/min, a side height (h) of 150mm, and a tilt angle (α) of 47°. To ensure specific dimensions and control variables, the water added to the pelletizing disc machine was calculated based on a predetermined ratio.

Moreover, for the investigation into the optimal dosage of bentonite, water content was meticulously controlled at approximately 8%. This allowed us to explore the influence of bentonite quantity on the pelletization process while maintaining consistency in water content. And during each pelletizing experiment, 3 kg of mixed feed and water were used to form pellets within the specified pelletizing time. The dosage of bentonite was optimized across various ratios.We have made additions in Section 2.2.4. We acknowledge the importance of these details in ensuring the accuracy and reproducibility of our experimental setup. Thanks for pointing out.

Comments 6:

Some errors in the way of writing was detected ：

Lines 43, 47, 67: "And The": no capital letter for "the";  no capital letters for "However, Pellets"; no capital letters for " In addition, Wet"

Lines 45, 76, 100, 127, 175, 424: space missing "benefits.The", "performance.And", "tion.And", "Table 3.From", "ducted.Subsequently", "low.For".

Lines 475, 476 and 478: a double space before loss on ignition. No capital letters for ignition (except if you use the acronym LOI)

Line 122: point is missing between "required" and "So"

Line 414: "con-centrate"? Rather "concentrate"

Lines 121, 176, 177, 180, 181: A space should be found between the number and its measurement unit.

Response to Reviewer X Comments

Manuscript ID: 2879930

1. Summary

We would like to extend our sincere gratitude on behalf of our team for the invaluable feedback and suggestions you provided during the review of our manuscript. Your professional insights have significantly contributed to refining and enhancing the quality of our research. And your peer review comments have been addressed individually in the subsequent sections, and corresponding revisions have been made throughout the manuscript.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Yes

Are the methods adequately described?

Yes

Are the results clearly presented?

Yes

Are the conclusions supported by the results?

Yes

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1:

Abstract

(1) “The higher the hardness, the more grinding energy is required” is not clear. The hardness of which object?

(2) The conclusion “iron ore fines with higher density, lower quantity of soft minerals (clay and goethite) and larger particle size give superior settling and filtering performance” should be based on the real experimental data instead of speculations. However, the evidence is not sufficient in the paper.

(3) “The addition of finely ground Australian ore powder improves the performance of pelletization compared to using a single ore.” It’s difficult to understand “using a single ore”. And the same with “the drop strength of the three blends”?

Therefore, the abstract requires more clarification and re-written. The authors need to ensure the conclusions cover all the key findings in the research.

Response 1:

Thank you for pointing this out. We agree with this comment. Therefore, We have updated the abstract section based on your suggestions.

Comments 2:

In terms of the chemical compositions of iron ores, what’s the criterion to define the iron grade as “high” or the silica content as “low”?

Response 2:

Thanks for pointing it out. In our literature search, we did not find specific standards defining "high iron, low silicon." However, available data suggests that high iron content typically refers to magnetite concentrate with iron content above 65%, hematite concentrate above 60%, and goethite concentrate above 50%. In this study, the iron grade of hematite ore consistently exceeds 60%, categorizing it as high-grade iron ore.

Regarding silicon levels, market standards in China, as observed through research on domestic steel industry platforms, roughly categorize silicon content as follows: silicon below 6% is considered low, 6%-8% is classified as medium, and above 8% falls into the high silicon category. In our research, both Brazilian and Australian ores exhibit silicon levels below 5%, fitting the classification of low silicon resources.

While we acknowledge the absence of universally defined thresholds for "high iron, low silicon," the provided information aligns with industry practices and empirical data. We trust that these clarifications address the reviewer's concerns and contribute to a better understanding of our categorizations.

Comments 3:

Table 1 also includes one type of magnetite concentrate as pellet feed, however, there is no description about it. Also, what about the particle size distribution of the magnetite concentrate?

Response 3:

The description of magnetite concentrate has been incorporated in Section 2.1, and its particle size distribution is provided in Table 2. These additions aim to enhance the comprehensiveness of the manuscript by offering detailed insights into the characteristics of the magnetite concentrate.

Comments 4:

In table 3, it is mentioned that the dominant iron-bearing mineral in B3 is specularite and its amount reaches 79%. How to identify the speculate and hematite, both of which have the same chemical formula Fe₂O₃?

Response 4: Thank you for your valuable feedback. We have conducted a thorough reanalysis of the mineralogy, specifically comparing the distinctions between hematite and specularite in the mineral assemblage(Specularite is a variety of hematite characterized by aggregates of silvery, metallic, specular or mirror-like hematite flakes or tabular, anhedral crystals. While hematite often occurs in hexagonal or elongated shapes. It is opaque, although extremely thin flakes may exhibit translucency and appear red. The mineral shows weak iridescence.). As a result, it has been determined that B2 predominantly comprises 79% hematite, not specularite. This clarification ensures an accurate representation of the mineral composition in our study. We appreciate your guidance and have updated the Table 3 accordingly.

The common hematite is mostly irregular in grain shape. The hematite in the image on the left in Table3 is Martite, a type of hematite formed by low-temperature oxidation of magnetite. It still retains the crystal morphology typical of magnetite.

The hematite in the image on the right in Table3 is irregular hematite formed by the dehydration of goethite.

Comments 5:

Different iron ores have different particle size. Is there any standard to be followed when measuring the Bond work index of iron ore fines?

Response 5:

Thank you for your prompt observation. The added explanations have been incorporated into Section 2.2.1 of the manuscript.

In accordance with the particle size requirements of the Bond ball mill and to ensure a closely matched feed particle size, the iron ore were crushed to -3 mm using a jaw crusher.

The approximate measurement method for the Bond Work Index is outlined as follows：

In this paper, the standard Bond method was used to measure the Bond work index (Wi) by ball milling to estimate the grindability of iron ore fines.

The grindability index, or grindability, of minerals is an indicator of the difficulty of grinding ores and is a fundamental data requirement for industrial grinding machines. It is a characteristic constant of ores that can be determined experimentally. The most widely used method for measuring the grindability index is the ‘Bond Work Index’ proposed by F C Bond. Bond’s ‘crack hypothesis’ suggests that the useful work consumed in grinding is proportional to the geometric mean of the volume and surface area of the product and gives the following famous Bond Work Index practical formula:(1)

Where F₈₀ and P₈₀ are the widths of the square aperture sieve that allows 80 per cent of the feed and product to pass, respectively, in micrometers (μm); W is the energy required to crush a short ton (907.185 kg) of material of size F to size P; W_ib is the Bond Work Index, which represents the energy required to grind a theoretically infinite feed size to a product size of 80 per cent passing 100 μm (or 65 per cent passing 75 μm).

According to the standard Bond test, the Bond Work Index (W_ib) is obtained by dry grinding in a closed circuit ball mill until the circulating load reaches 250 per cent. The Bond ball mill work index (W_ib) (in kW•h/t) is calculated from the following equation:(2)

where P is the particle size required for the test, in micrometers (μm); Gbp is the amount passing the specified test size per revolution, expressed in grams per revolution (g/r).

Specific references can be consulted for further details: Ahmadi and Shahsavari, 2009; Free, McCarter, and King, 2005; Gent et al., 2012; Magdalinovic et al., 2012; Ipek, Ucbas, and Hosten, 2005. These references have been cited in the manuscript.

Comments 6:

What do you mean about “a simulated straight grate process”? How to simulate?

Response 6:

The schematic representation of the simulated system model for the straight grate used in the experiments is illustrated in Figures 1. The specifications of the simulation apparatus are Ø300×500 mm. The straight grate simulation device can replicate the entire process of drying, preheating, roasting, and uniform cooling of pellets on the roasting tray. This includes processes such as air-drying, suction drying, preheating, roasting, and uniform cooling. The temperature and air velocity during these processes are adjustable, providing reliable process parameters for design purposes in engineering firms. Additionally, the bed height can be adjusted within the range of 0 to 450 mm.

Figure 1: Simulation system setup of the straight grate.

Comments 7:

Why did the preparation of pellets particularly investigate the effect of adding ground Australian iron ores, while the Brazilian iron ores were ignored?

Response 7:

Australia ranks first globally in iron ore reserves and is the largest producer, contributing 35% of the world's iron ore. Choosing Australian ores for study is deemed representative, offering valuable insights and guidance for production. We appreciate your suggestions, and in the future, we plan to extend our research to explore pelletizing performance in other major iron ore-producing nations.

Comments 8:

What are the reasons responsible for the contradiction between the measured Bond work index of six types of iron ores and their microhardness?

Response 8:

The values of the Bond Work Index and microhardness do not correspond on a one-to-one basis; however, there is a positive correlation between them, as evident from Figures 3 and 4. Further validation is provided by Formula 1. We appreciate the reviewer's attention to this correlation.

Comments 9:

It is suggested that the settling curves of A1 and B1 are also demonstrated in Fig. 7.

Response 9:

We think this is an excellent suggestion. We have demonstrated the settling curves of A1 and B1 in Fig.7.

Comments 10:

The abbreviation of the iron ores and blends should be easily distinguished because both Brazilian iron ores and blends were defined as “B1”, “B2” and “B3”.

Response 10:

We appreciate the suggestion. In the manuscript, We have corrected that all blends have been consistently referred to by their full names, such as Base, Blend 1, Blend 2, and Blend 3. To enhance clarity and differentiation, we have specifically designated the Brazilian ores as B1, B2, and B3, where the 'B' stands for Brazil. Figure 7 、8、9、10、11 、12 and Table 6 are corrected.

Comments 11:

From Fig. 8, it is known that different ground Australian ores present different impacts on the quality of green balls. More explanations and discussions need to be provided.

Response 11:

Our primary focus in this study has been to investigate how the three types of Australian ores used can generally contribute to an improved quality of green balls under identical experimental conditions. However, the specific impacts and distinctions between different Australian ores remain unexplored. We appreciate your valuable suggestion, and we are committed to addressing this aspect in our future work. You can anticipate the exploration of these inter-variations among Australian ores in our upcoming research. Once again, thank you for your guidance.

Comments 12:

Also, while talking about the effect of ground Australian ores on the firing performance of pellets, the authors need to pay more attention to the difference in the properties of three types of Australian ores, rather than rough discussion as “Australian ores”.

Response 12:

The reasons for the variations in firing pellet formation and quality among different Australian ores have been elucidated in Section 3.3.2.

Comments 13:

Fig. 10, the mineral phases in the SEM-EDS analysis should be identified.

Response 13:

Thanks for your suggestion. We have incorporated modifications in Section 3.3.4. Specifically, Figures 11 and 12 have been revised to enhance the clarity and comprehensiveness of our findings.

Comments 14:

The paper mentioned that the low-carbon production of high-quality oxidized pellets. How to understand “low-carbon production”?

Response 14:

Because oxidised pellets have become an essential charge material for blast furnace ironmaking due to their uniform particle size, high iron grade, excellent metallurgical properties and high mechanical strength. The oxidised pellet exhibits significant advantages, notably low energy consumption, minimal environmental impact, and high iron grade in the resulting products. Among these advantages, the energy consumption during the oxidation pelletization process is only half that of the sintering process. This highlights the efficiency and environmental benefits of the pelletizing method, contributing to high-grade iron-containing products with reduced energy requirements and environmental pollutants. And The application of oxide pellets in blast furnace smelting can contribute to increased production, coke savings, improved economic indicators of ironmaking technology, reduced pig iron costs, and enhanced economic benefits. The raw iron ore concentrate used for pellet production undergoes mineral processing and grinding. And the production of high-grade, low SiO₂ pellet ore in this study not only directly improves the main operational indicators of the blast furnace but also offers several advantages across various aspects. These include reduced energy consumption (50% lower than sintering ore), lower emissions (dust emissions are one-seventh of sintering production, SiO₂ emissions are one-third, and NO_x emissions are one-fifth), and decreased processing costs (pellet ore processing costs are half of sintering ore).