Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

- Although the paper deals with recovery of Iodine from steel dust, there si a clear deficiency in information on this dust. Moreover, the raw material underwent processing in this paper was zinc oxide not steel dust; 

The ZnO processed in the paper: It is produced from steel dust by a pyrometallurgical process , but which process ?; 

The authors showed the particle size distribution histogram in Fig 6, but it is important to have an SEM photograph too. 

About the chemical composition of ZnO powder shown in Table 1, How did you get this composition? Is it by analysis of solid powder ? or by digestion in acidic solution followed by ICP? ;

The ZnO contains 0.08 I, Did you analyze the ZnO residue after alkaline treatment, also after acisic treatment?

What is the effect of alkaline treatment on the components of ZnO powder?

What is the effect of adding H2SO4 on the components of ZnO powder?

What about the chemical composition of Iodine produced? the authors presented only the particle size distribution histogram

The authors presented two flowcharts, one for laboratory scale iodine recovery (Fig 2), and the other for the industrial scale (Fig.3).  Fig 2 is not clear. May the authors present a flowchart for the experimental procedure they did. It is important to clear up the experimental procedure. Also more and more clarification of experimental procedures are needed.

Equation 2 is not balanced. It is not clear the importance of reduction of iodine by SO2 after precipitation of I2 by H2O2.

Again a deficiency in information about the steel dust in the Introduction section. The authors told that "production of 1.892 billion tones of steel (annual global production) generates 27 million tones of solid wastes".  This means that every tone of steel produces about 14.3 kg of solid wastes, but these solid wastes include also slag. I think it is better to relate the steel production with dust generated.    

 

Author Response

Response to Reviewer Comments

Dear Reviewer,

First of all, we would like to express our sincere gratitude for reviewing our manuscript entitled "Optimized Iodine Recovery from Steel Dust Using Alkaline Washing and Air Blowing-Out: A Sustainable Industrial Approach" and for providing valuable comments and suggestions. We have carefully considered your feedback and have made corresponding modifications and improvements to the manuscript. Below, we provide a point-by-point response to your comments:

Reviewer Comment 1:

Although the paper deals with recovery of Iodine from steel dust, there is a clear deficiency in information on this dust. Moreover, the raw material underwent processing in this paper was zinc oxide not steel dust.

Response: Thank you for pointing this out. We would like to clarify that the zinc oxide mentioned in the paper is indeed from steel dust, processed through a rotary kiln pyrometallurgical treatment. Essentially, it originates from steel dust. In the revised manuscript, we have made modifications to Fig. 2 to more accurately reflect the source and processing of the raw material.

And we have made the following changes in the original text (Line154-155):

Reviewer Comment 2:

The ZnO processed in the paper: It is produced from steel dust by a pyrometallurgical process, but which process?

Response: We have clarified that the ZnO is produced from steel dust using a Waelz kiln pyrometallurgical process.

And we have made the following changes in the original text (Line 135-137):

“The zinc oxide mentioned in the paper is indeed from steel dust, processed through a rotary Waelz kiln pyrometallurgical treatment.”

 

Reviewer Comment 3:

The authors showed the particle size distribution histogram in Fig 6, but it is important to have an SEM photograph too.

Response: We have added an SEM photograph of the ZnO powder to provide more insight into the microstructure, which can be found in the revised Figure 7.

And we have made the following changes in the original text (Line 211-212):

“Fig. 7. Showing the microscopic structure and morphology of zinc suboxide powder.”

 

Reviewer Comment 4:

About the chemical composition of ZnO powder shown in Table 1, how did you get this composition? Is it by analysis of solid powder or by digestion in acidic solution followed by ICP?

Response: Thank you for your question regarding the chemical composition of the ZnO powder shown in Table 1. The composition was determined using X-ray fluorescence(XRF) analysis, which was conducted directly on the solid ZnO powder without any prior digestion. XRF provided a straightforward and accurate method to analyze the elemental composition of the powder.

Reviewer Comment 5:

The ZnO contains 0.08 I. Did you analyze the ZnO residue after alkaline treatment, also after acidic treatment?

Response: Thank you for your question regarding the iodine content in the ZnO residue after treatment. We did not conduct an analysis of the iodine content in the residue after either the alkaline or acidic treatment. The iodine content in the residue is very low, as almost all the iodine is  transferred to the alkaline washing solution during the treatment process.

Reviewer Comment 6:

What is the effect of alkaline treatment on the components of ZnO powder?

Response: The alkaline treatment primarily affects the iodine content in the ZnO powder by dissolving the iodine into the alkaline solution. The treatment does not affect the zinc oxide itself, and zinc oxide present in the residue remains intact, entering the subsequent zinc recovery process.

Reviewer Comment 7:

What is the effect of adding H₂SO₄ on the components of ZnO powder?

Response: The addition H₂SO₄ is used to adjust the pH of the iodine-containing waste solution, as the recovery of iodine needs to occur under acidic conditions. At this stage of the process, there is no ZnO powder present, as it has already been separated during the previous treatment steps. Thank you for your interest, and I hope this clarifies your question.

Reviewer Comment 8:

What about the chemical composition of Iodine produced? The authors presented only the particle size distribution histogram.

Response: Thank you for your question regarding the chemical composition of the produced iodine. The obtained crude iodine has a purity of over 99.9%.

And we have made the following changes in the original text (Line 261-262):

“The crude iodine obtained exhibited a high purity of 99.9%.”

 

Reviewer Comment 9:

The authors presented two flowcharts, one for laboratory scale iodine recovery (Fig 2), and the other for the industrial scale (Fig.3). Fig 2 is not clear. May the authors present a flowchart for the experimental procedure they did. It is important to clear up the experimental procedure. Also, more clarification of experimental procedures is needed.

Response: We have revised Fig. 2 to clearly present the experimental procedure.

And we have made the following changes in the original text (Line 154):

 

Reviewer Comment 10:

Equation 2 is not balanced. It is not clear the importance of reduction of iodine by SO₂ after precipitation of I₂ by H₂O₂.

Response: Thank you for pointing out the issue with Equation 2. We have reviewed the equation and ensure it is now balanced in the revised manuscript.

Regarding the reduction of iodine by SO₂ after precipitation with H₂O₂, the reduction step plays a crucial role in recovering iodine from the solution. The initial precipitation with H₂O₂ is used to obtain elemental iodine, while the subsequent reduction by SO₂ helps to further purify and stabilize the iodine in the desired form for easier handing and collection. This step ensures optimal recovery efficiency and quality of the final iodine product.

And we have made the following changes in the original text (Line 151-154):

“Further downstream processing in an absorption tower involves an acidic iodide solution with added SO₂, which reduces iodine (I₂) to iodide (I⁻) after its precipitation by H₂O₂. This reduction step is crucial because it ensures the complete conversion of iodine into a more stable, recoverable form, enhancing the efficiency of iodine recovery. The reaction proceeds according to the following equation:”

Reviewer Comment 11:

There is a deficiency in information about the steel dust in the Introduction section. The authors mentioned "production of 1.892 billion tonnes of steel (annual global production) generates 27 million tonnes of solid wastes." This means that every tonne of steel produces about 14.3 kg of solid wastes, but these solid wastes include also slag. I think it is better to relate the steel production with dust generated.

Response: Thank you for your comment. You are correct that the solid waste generated during steel production includes a wide range of materials, such as slag and dust. To address this, we will revise the Introduction section to provide a clearer distinction between slag and steel dust and specify the amount of dust generated per tonne of steel. This information will strengthen the relevance of our study by focusing specifically on steel dust, the material used in our experiments.

And we have made the following changes in the original text (Line 39):

“The production of 1.892 billion tonnes of steel (annual global production in 2023) generates approximately 27 million tonnes of solid wastes, including slag, scale, and dust [1]. Steel dust, a subset of these solid wastes, typically accounts for approximately 5–10% of total solid waste generated, equating to 1.35–2.7 kg of dust per tonne of steel produced. This dust is particularly significant because it contains various valuable elements, including zinc, lead, cadmium, and iodine, which can be recovered and utilized through advanced processing methods. Efficient recovery from this resource not only addresses environmental concerns but also contributes to resource sustainability and economic benefits.”

With Table 1 added:

Table 1. Distribution of solid wastes in steel production [1]

Type of Solid Waste	Proportion of total solid waste (%)	Waste generated per tonne of steel (kg)
Slag	70–80	10–11
Dust	5–10	1.35–2.7
Scale and Other Residues	10–15	2–3

Once again, we sincerely appreciate your detailed review and valuable comments. We believe that, with your suggestions, the quality of the manuscript has been significantly improved.

Best regards,

Lin Lin

November 26, 2024

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

1. I am not satisfied with the Introduction. I believe the introduction should be more clearly aimed at comparing different methods of the iodine extraction from a steel dust to explain the higher efficiency of the process in question. This idea is directly related to the formulation of the novelty and relevance of the work which are poorly undersood.

2. In my opinion, it is better for the respected authors to consider in the introduction: i) why this element is in steel, ii) what is the iodine chemical state in steel dust, iii) which methods, as close as possible to the method under consideration, are used or have been used to extract this element from dust steel. It seems to me, lines 83-93 do not cover this matter at all.

3. The respected authors is encouraged to formulate zinc suboxide, What is the of this compound?

4. What is the yield of iodine to be obtained according to the reaction (2)?

5. The reaction (3) undergoes only at elevated temperature. At room temperature, iodine is disproportionated differently.

6. If the date in the Tables 2 and 3 are taken from literature, the literature sources should be referred.

7. It is bad when different physical values are denoted by the same letter (e.g., μ is chemical potential and ionic strength).

8. The respected authors use Van’t Hoff reaction isotherm (Equation 9). However, there is no evidence that the system is in chemical equilibrium.

9. The dimension of α is nm. Then why is lgγ, obtained from the second approximation of Debye-Hückel equation, a dimensionless value?

Author Response

Response to Reviewer Comments

Dear Reviewer,

First of all, we would like to express our sincere gratitude for reviewing our manuscript entitled "Optimized Iodine Recovery from Steel Dust Using Alkaline Washing and Air Blowing-Out: A Sustainable Industrial Approach" and for providing valuable comments and suggestions. We have carefully considered your feedback and have made corresponding modifications and improvements to the manuscript. Below, we provide a point-by-point response to your comments:

Reviewer Comment 1·:

I am not satisfied with the Introduction. I believe the introduction should be more clearly aimed at comparing different methods of iodine extraction from steel dust to explain the higher efficiency of the process in question. This idea is directly related to the formulation of the novelty and relevance of the work which are poorly understood.

Response: Thank you for your valuable feedback. We agree that the introduction could benefit from a more focused comparison of different iodine extraction methods from steel dust to better highlight the efficiency and novelty of the process presented in our study. To address this comment, we will revise the introduction to:

Summarize commonly used iodine extraction methods.

Compare these methods with the approach presented in our study, emphasizing the advantages, including efficiency, scalability, and sustainability.

Clarify the relevance of the study in filling existing knowledge gaps.

And we have made the following changes in the original text (Line 69):

“Various methods have been explored for iodine recovery from different sources, including steel dust. For instance, Godo Shigen Co. Ltd. developed a process to recover iodine from organic iodine compounds by converting them into iodide or iodate ions through reduction or oxidation, followed by separation using ion exchange resins. While this method achieves high purity, it is costly and limited in scalability, making it less suitable for complex matrices like steel dust [11,12]. Similarly, Iochem Corporation devised a process to recover iodine vapor from iodine-containing waste via impurity removal and blowing-out techniques, followed by vapor concentration to produce crude iodine with a purity exceeding 99.3%. However, this method requires high iodine concentrations in the initial feedstock, limiting its applicability for low-concentration sources like steel dust [13]. Another approach by Song and colleagues utilized the blowing-out method to recover iodine from phosphorite mines, achieving an annual output of 250 tons of crude iodine. Although effective for phosphorite-based recovery, this method does not adequately address the challenges posed by multi-element, low-iodine-content materials such as steel dust [10,14]. Additionally, Hong et al. employed hydrogen peroxide (H₂O₂) to recover iodine from LCD manufacturing wastewater. This method demonstrated excellent recovery rates of 95% under high iodine (10.2 g/L) and boron (0.82 g/L) concentrations but lacked adaptability for less enriched sources [15].

To further illustrate the advantages and limitations of these methods, Table X summarizes the key characteristics of iodine extraction techniques and compares them to the Greennovo process. The table highlights the unique efficiency and sustainability of the Greennovo method in recovering iodine from steel dust, especially in the context of low-concentration and multi-element materials.

Table 2. Comparison of iodine extraction methods

Method	Process description	Advantages	Limitations	References
Direct Acid Leaching	Iodine is dissolved using strong acids (e.g., H₂SO₄ or HCl).	- Simple setup - Low initial investment	- Low selectivity - High waste generation - Inefficient iodine recovery	[11,12]
Ion Exchange	Iodine is absorbed onto a resin and later eluted.	- High-purity iodine recovery - Efficient for small-scale applications	- Expensive resins - Poor scalability - High operational costs	[11,12]
Solvent Extraction	Iodine is extracted into organic solvents via chemical partitioning.	- Effective in specialized setups - High separation efficiency	- Solvent loss - Environmental concerns - High chemical cost	[13,14]
Blowing-Out Method	Iodine is volatilized and recovered as vapor via air blowing.	- Scalable for certain sources - Moderate recovery rates	- Limited for low-concentration materials - Requires optimized feed composition	[10,14]
Proposed Method	Combines pyrometallurgical zinc enrichment with alkaline washing and oxidative air blowing for iodine recovery.	- High recovery rate (C(I⁻) > 0.6 g/L) - Cost-effective - Utilizes industrial waste heat - Scalable for industrial applications	- Requires initial pyrometallurgical treatment	Current Study

The process developed by Greennovo Environmental Technology Co. Ltd. specifically targets the recovery of iodine from low-concentration, multi-element steel dust, overcoming limitations seen in previous methods. The approach integrates pyrometallurgical enrichment to concentrate zinc suboxide powder, followed by an alkaline washing step that extracts iodine into the wastewater stream. By achieving iodide concentrations of C(I⁻) > 0.6 g/L, this process enables efficient iodine recovery through a final oxidation-blowing step with H₂O₂. Scaled for industrial application, the Greennovo method not only demonstrates significant economic benefits but also offers a sustainable pathway for recovering iodine and other valuable resources from steel dust [16]. This process uniquely addresses the challenges of complex matrix separation, low iodine content, and multi-element co-existence, making it a versatile solution for industrial-scale applications.

Despite these advancements, challenges remain. Steel dust contains only trace amounts of iodine, and its complex composition, coupled with variability in the distribution of valuable and hazardous elements, complicates separation and purification. Furthermore, the mechanisms governing iodine extraction in acidic solutions are not yet fully understood, hindering further optimization [17,18]. To address these gaps, this study provides a comprehensive thermodynamic analysis that elucidates the relationship between pH and iodine extraction efficiency. By offering theoretical insights into these interactions, this work not only advances the understanding of iodine recovery mechanisms but also provides practical guidance for improving process efficiency in industrial applications.”

Reviewer Comment 2:

It is better for the respected authors to consider in the introduction: i) why this element is in steel, ii) what is the iodine chemical state in steel dust, iii) which methods, as close as possible to the method under consideration, are used or have been used to extract this element from steel dust.

Response: Thank you for this valuable comment. We agree that addressing why iodine is present in steel dust, its chemical state, and a review of related extraction methods will provide a more comprehensive background and establish the relevance of the proposed method. Below, we outline how these points will be addressed and suggest revisions to the Introduction section.

And we have made the following changes in the original text (Line 53):

“Iodine demand has been increasing steadily due to its broad applications, particularly in X-ray contrast agents and pharmaceutical industries. Additionally, iodine deficiency disorders are rising globally, further driving demand. The global iodine demand is projected to reach 56,400 tons by 2030, yet iodine resources are heavily concentrated in a few regions. For example, the United States consumes 30% of the world’s iodine production but produces only 20% of its consumption [7]. Similarly, China produced only 880.9 tons of iodine in 2022, while its demand reached 7,101.4 tons, leaving a substantial gap that is largely addressed through importation and marine extraction [8,9]. This global scarcity underscores the urgent need to recover iodine from secondary resources [3]. Steel dust, a by-product of the steelmaking process, represents a promising secondary source of iodine. The presence of iodine in steel dust is attributed to the raw materials used in steel production, including ores and chemical additives, as well as environmental deposition during high-temperature processes. Iodine in steel dust predominantly exists as iodide (I⁻) and iodate (IO₃⁻), depending on the oxidation conditions during steel production and subsequent handling. Despite its potential, the concentration of iodine in steel dust is extremely low, approximately 0.0015%, posing significant challenges for separation and purification due to the complex matrix of valuable and hazardous components in the dust. However, recovering iodine from such secondary sources not only supplements the limited global supply but also aligns with sustainable industrial resource utilization practices.”

 

Reviewer Comment 3:

The respected authors are encouraged to formulate zinc suboxide. What is this compound?

Response: Thank you for your comment. The term “zinc suboxide” refers to a low-grade form of zinc oxide mixture, with its composition detailed in Table 1.

Reviewer Comment 4:

What is the yield of iodine to be obtained according to the reaction (2)?

Response: Thank you for your inquiry regarding the yield of iodine in reaction (2). We are pleased to report that the yield of iodine in this reaction exceeds 70%, which is a satisfactory outcome for the process described.

And we have made the following changes in the original text (Line 155-158):

“The yield of iodine in this reaction exceeds 70%, demonstrating the effectiveness of the reduction process. These reactions guide laboratory experiments that simulate the conditions used in larger-scale operations, helping to optimize the parameters for efficient iodine recovery.”

 

Reviewer Comment 5:

The reaction (3) undergoes only at elevated temperature. At room temperature, iodine is disproportionated differently.

Response: Thank you for your insightful comment. We appreciate your observation that the disproportionation reaction of iodine, as presented by the equation , typically occurs under elevated temperature conditions.

However, in the context of our study, we would like to clarify that the reaction we describe, , takes place under alkaline conditions and does not require high temperature. Experimental results indicate that this disproportionation reaction occurs spontaneously even at room temperature in cold alkaline solutions.

Relevant references supporting this observation include:

[1] Palmer D A , Ramette R W , Mesmer R E .Potentiometric studies of the thermodynamics of iodine disproportionation from 4 to 209°C[J].Journal of Solution Chemistry, 1984, 13(10):685-697.DOI:10.1007/BF00649009.

Reviewer Comment 6：

If the data in Tables 2 and 3 are taken from the literature, the literature sources should be referred.

Response: Thank you for your valuable comment. We would like to clarify that the data presented in Tables 2 and 3 were obtained from measurements conducted in our factory, rather than from literature sources. Therefore, there is no need to cite external references for this data.

Reviewer Comment 7:

It is problematic when different physical values are denoted by the same letter (e.g., μ is used for both chemical potential and ionic strength).

Response: Thank you for pointing out the inconsistency regarding the use of the same symbol (μ) for different physical values. We understand the importance of avoiding confusion in notation. To address this issue, we have replaced the symbol for ionic strength with the letter I, ensuring that there is no ambiguity between chemical potential and ionic strength.

Reviewer Comment 8:

The respected authors use Van’t Hoff reaction isotherm (Equation 9). However, there is no evidence that the system is in chemical equilibrium.

Response: Thank you for your valuable comment regarding the use of the Van’t Hoff reaction in Equation 9. In our study, we assumed that the system had reached chemical equilibrium to justify the use of the Van’t Hoff reaction isotherm.

And we have made the following changes in the original text (Line 284):

“Assuming that the solution has reached chemical equilibrium”

 

Reviewer Comment 9:

The dimension of α is nm. Then why is lgγ, obtained from the second approximation of Debye-Hückel equation, a dimensionless value?

Response: Thank you for your insightful question regarding the dimensional analysis in the Debye-Hückel equation. Specifically, you raised the concern about the parameter α, which has the dimension of nm, and how the resulting value of  can be dimensionless.

In the Debye-Hückel equation, although parameters such as the distance of closest approach α have physical units, the constants (A and B) are derived in such a way that they balance out the units in the equation. The parameter combinations are designed to ensure that the argument of the logarithm is dimensionless, which is a fundamental requirement in mathematical operations involving logarithms. The constants A and B depend on temperature, the dielectric constant of the medium, and other physical properties that account for these units appropriately.

The logarithmic function itself must have a dimensionless argument, and therefore, the entire fraction involving ionic strength and parameters such as α\alphaα is arranged to be unit-free. This is why the final value of , which represents the logarithm of the mean activity coefficient, is dimensionless, regardless of the initial dimensions of the input parameters.

Once again, we sincerely appreciate your detailed review and valuable comments. We believe that, with your suggestions, the quality of the manuscript has been significantly improved.

Best regards,

Lin Lin

November 26, 2024

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript "Advanced Process for Iodine Recovery from Steel Dust" provides important scientific results in the field of resource recovery, particularly on the efficient extraction and utilization of iodine from steel dust. It incorporates relevant references and provides detailed reasoning, including the thermodynamic relationship between iodine recovery and pH value. However, upon reviewing the manuscript, I found some areas where greater clarity is required. Please address the comments below for further clarification. Adjusting these elements will improve readability and the overall quality of the manuscript. I accept the manuscript for publication after these minor corrections are made.

Comment 1: In results and discussion section – Section 3.1 (Lines 172–177), the author mentions that the amount of iodine in the solution slowly decreased over time under normal conditions without H2O2, as shown in Figure 4(a). However, from the figure, it seems that the iodine content actually increased slightly with respect to blow time. Also, in Figure 4(b), the author describes that when H2O2 was added, the iodine content decreased quickly at first and then stabilized. Could the author please clarify whether they tried extending the blow time beyond 20 minutes to check if the iodine content remained stable? Or, is the observed variation in iodine content within acceptable limits, and that’s why it is considered stable? Further explanation or additional data on the stabilization point would enhance clarity.

Comment 2: Please proofread the manuscript and double-check the use of subscripts, especially in Section 3.1 (Line 175). For example, “H2O2” should have the numbers written as subscripts, as written in the document.

Author Response

Response to Reviewer Comments

Dear Reviewer,

First of all, we would like to express our sincere gratitude for reviewing our manuscript entitled "Optimized Iodine Recovery from Steel Dust Using Alkaline Washing and Air Blowing-Out: A Sustainable Industrial Approach" and for providing valuable comments and suggestions. We have carefully considered your feedback and have made corresponding modifications and improvements to the manuscript. Below, we provide a point-by-point response to your comments:

Reviewer Comment 1:

Comment 1: In results and discussion section – Section 3.1 (Lines 172–177), the author mentions that the amount of iodine in the solution slowly decreased over time under normal conditions without H₂O₂, as shown in Figure 4(a). However, from the figure, it seems that the iodine content actually increased slightly with respect to blow time. Also, in Figure 4(b), the author describes that when H₂O₂ was added, the iodine content decreased quickly at first and then stabilized. Could the author please clarify whether they tried extending the blow time beyond 20 minutes to check if the iodine content remained stable? Or, is the observed variation in iodine content within acceptable limits, and that’s why it is considered stable? Further explanation or additional data on the stabilization point would enhance clarity.

Response: Thank you for your insightful comments on the results and discussion in Section 3.1. We appreciate your careful observation regarding the changes in iodine content over time, as depicted in Figure 4(a) and 4(b).

Regarding Figure 4(a), it appears there may have been a miscommunication in our description. You are correct that, from the figure, there seems to be a slight increase in iodine content with respect to blow time. We have revised the manuscript to better describe the observed trend and clarify any potential ambiguity.

In Figure 4(b), concerning the experiment where H₂O₂ was added, the iodine content did indeed decrease quickly at first before reaching a stable point. To address your question about extending the blow time beyond 20 minutes, we conducted preliminary trials, and our observations indicated that the iodine content remained relatively stable after 20 minutes, with only minor fluctuations within acceptable experimental limits. We considered these variations as indicative of a steady state. However, we understand the importance of providing more detailed data, and we have revised the manuscript to include additional discussion on the stabilization point and the criteria used to determine stability.

And we have made the following changes in the original text (Line 193):

 

Reviewer Comment 2:

Comment 2: Please proofread the manuscript and double-check the use of subscripts, especially in Section 3.1 (Line 175). For example, “H2O2” should have the numbers written as subscripts, as written in the document.

Response: Thank you for pointing out the formatting inconsistency regarding the use of subscripts, particularly in Section 3.1 (Line 175). We appreciate your attention to detail.

 

Once again, we sincerely appreciate your detailed review and valuable comments. We believe that, with your suggestions, the quality of the manuscript has been significantly improved.

Best regards,

Lin Lin

November 26, 2024

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors extracted iodine from ZnO produced by Waelz process that used iron/steel dust as a raw material with other additions like coke and maybe lime. The ZnO produced can not be counted as steel dust. It is a product from a new process that used also additions as raw materials. Nevertheless, the authors still insist to title the paper as an extraction from steel dust, and didn't mention anything about the real raw materials used in the experiments in the abstract, neither in the conclusion. Why?

It is a treatment of Waelz ZnO, which is poorly characterized in the paper. The chemical composition in Table 3 has abnormal concentrations such as "24.44 % Cl and 18.25 % K" and the authors didn't discuss why these high concentrations of Cl and K ?.

The authors presented the particle size distribution of the ZnO in the 1st version and after sking about the SEM photograph they presented it in Fig 7. The SEM micrograph shows that the particle size is less than 1 micrometer (Fig 7b) and the agglomerations of ZnO in Fig (7a) is less than 5 micometers. However, the due particle size distribution in Fig 6 shows that the particle size is less than 100 micometers and concentrated at 30-40 micrometers. The SEM micro-graph gives results different than in Fig 6. 

Moreover the SEM investigations should be used to ensure the results of chemical composition in Table 3. EDX analysis had to be done to see if the ZnO has those high concentrations of Cl and K.

What is the effect of alkaline leaching on ZnO?

Zn is not stable in alkaline solution according Pourbaix diagram. Moreover Solid zinc oxide will also dissolve in alkalis to give soluble zincates:

ZnO + 2 NaOH + H₂O → Na₂[Zn(OH)₄] (see Earnshaw A (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 1201–26"

Alkaline leaching also dissolves some elments found in Waelz oxide.

Comments on the Quality of English Language

Quality of English Language is good

Author Response

Response to Reviewer Comments

Dear Reviewer,

Thank you once again for your thoughtful review of our manuscript entitled " Optimized Iodine Recovery from Zinc Suboxide Derived from Steel Dust Using Alkaline Washing and Air Blowing-Out: A Sustainable Industrial Approach ". We appreciate the time and effort you have dedicated to providing constructive comments. In response to your feedback, we have carefully revised the manuscript to address the concerns raised. Below, we provide a point-by-point response to your comments:

Reviewer Comment 1:

The authors extracted iodine from ZnO produced by Waelz process that used iron/steel dust as a raw material with other additions like coke and maybe lime. The ZnO produced can not be counted as steel dust. It is a product from a new process that used also additions as raw materials. Nevertheless, the authors still insist to title the paper as an extraction from steel dust, and didn't mention anything about the real raw materials used in the experiments in the abstract, neither in the conclusion. Why?

Response: Thank you for your valuable feedback regarding the raw materials used in our experiments and the title of the manuscript. We understand your concern about the terminology used to describe the starting material, which is ZnO produced by the Waelz process.

To clarify, while it is true that the ZnO used in our study is derived from the Waelz process, where steel dust is treated alongside other additives such as coke and lime, we would like to emphasize that the iodine present in the ZnO originates from the steel dust itself. In the Waelz process, the steel dust contains trace amounts of iodine, and through repeated processing and concentration in the rotary kiln, iodine is progressively enriched to a level that becomes extractable.

It is important to note that the additives used in the Waelz process, such as reductants, do not contain iodine. Therefore, the iodine content in the ZnO is solely derived from the steel dust, which, as you pointed out, is not explicitly clarified in the manuscript.

We appreciate your comment, and we will revise the title, abstract, and conclusions to make it clearer that the iodine in the ZnO comes from steel dust treated through the Waelz process. This will help accurately reflect the nature of the material used in our experiments.

And we have made the following changes in the original text (Line1,23-24,335-336):

“Optimized Iodine Recovery from Zinc Suboxide Derived from Steel Dust Using Alkaline Washing and Air Blowing-Out: A Sustainable Industrial Approach”

“The present paper proposes an advanced process to effectively recover and put to full use iodine from steel dust-derived zinc suboxide”

“For that recovery process of valuable metal elements from the steel dust-derived zinc suboxide”

 

Reviewer Comment 2:

It is a treatment of Waelz ZnO, which is poorly characterized in the paper. The chemical composition in Table 3 has abnormal concentrations such as "24.44 % Cl and 18.25 % K" and the authors didn't discuss why these high concentrations of Cl and K ?.

Response: Thank you for your thoughtful review. Regarding your comment about the high concentrations of chlorine (Cl) and potassium (K) in the Waelz ZnO (24.44 % Cl and 18.25 % K), we would like to clarify that these high concentrations are due to the typical practice in steel plants of mixing and treating collected dust from both the sintering head and blast furnace gas. The sintering head dust, in particular, contains elevated levels of chlorine and potassium due to the steelmaking process.

Reviewer Comment 3:

The authors presented the particle size distribution of the ZnO in the 1st version and after sking about the SEM photograph they presented it in Fig 7. The SEM micrograph shows that the particle size is less than 1 micrometer (Fig 7b) and the agglomerations of ZnO in Fig (7a) is less than 5 micometers. However, the due particle size distribution in Fig 6 shows that the particle size is less than 100 micometers and concentrated at 30-40 micrometers. The SEM micro-graph gives results different than in Fig 6.

 

Response: Thank you for your thoughtful review. We would like to clarify the apparent discrepancy between the particle size distribution shown in Figure 6 and the SEM images in Figure 7.

Figure 6 presents the particle size distribution of ZnO, while Figure 7 shows the SEM images of ZnO. Specifically, Figure 7a presents the morphology of ZnO at 2K magnification, and Figure 7b shows the microstructure at 20K magnification. It is important to note that SEM images focus on the microstructure and morphology of the material, whereas the particle size distribution in Figure 6 provides information on the overall size distribution of solid particles.

To obtain images corresponding to the particle size distribution in Figure 6, a lower magnification would suffice, as shown below. However, capturing the morphology and structure of the particles at higher magnifications provides more meaningful insights into their characteristics.

 

Reviewer Comment 4:

Moreover the SEM investigations should be used to ensure the results of chemical composition in Table 3. EDX analysis had to be done to see if the ZnO has those high concentrations of Cl and K.

Response: We appreciate the suggestion to use SEM and EDX analysis to verify the chemical composition results presented in Table 3. However, we opted for XRF analysis due to its higher sensitivity and ability to detect elements in the entire sample, making it more representative of the overall material composition. In contrast, EDX analysis might be affected by particle distribution or localized sampling, which could introduce variability in the results.

To address this, we have added a detailed description of the XRF characterization method in the "Results and Discussion" section, we now include a discussion on the sources of Cl and K concentrations, explaining that these may originate from the high-chlorine and high-potassium components in the steel dust.

And we have made the following changes in the original text (Line 212-222):

“The chemical composition of the zinc suboxide powder is presented in Table 3, where impurities such as F, Cl, and others were recorded. The data in Table 3 were derived from XRF analysis, which provides a comprehensive and representative measurement of the overall sample composition. Fig. 6 shows the particle size distribution, while Fig. 7 displays the microscopic structure and morphology of the zinc suboxide powder.

The high concentrations of Cl and K observed in the zinc suboxide powder are likely attributed to the mixing of sintering head dust and blast furnace gas dust in steel plants, as these dusts are often collected and treated together. During the steelmaking process, sintering head dust typically contains very high levels of chlorine and potassium, which contribute to the elevated concentrations of these elements in the final product.”

Reviewer Comment 5:

What is the effect of alkaline leaching on ZnO?

Response: Thank you for your insightful comment regarding the effect of alkaline leaching on ZnO. We would like to emphasize that the process described in our manuscript is alkaline washing, not leaching.

The main purpose of alkaline washing is to clean the material surface by removing contaminants like grease, dirt, and oxides. This process prepares the material for subsequent treatments or improves its surface properties. In contrast, leaching aims to extract and recover valuable components, such as metals or minerals, from solid waste or ores.

In our study, the material contains a significant amount of chlorine (Cl). During the alkaline washing process, the hydroxide ions (OH⁻) preferentially react with these halogens (Cl and I) rather than directly interacting with zinc oxide (ZnO). This preferential reaction helps to effectively remove these contaminants from the surface without significantly dissolving ZnO.

 

Reviewer Comment 6:

Zn is not stable in alkaline solution according Pourbaix diagram. Moreover Solid zinc oxide will also dissolve in alkalis to give soluble zincates:

Response: Thank you for your thoughtful comment regarding the stability of zinc in alkaline solutions, as indicated in the Pourbaix diagram, and the potential dissolution of zinc oxide in alkalis.

We acknowledge that, according to the Pourbaix diagram, zinc is not stable in alkaline solutions and can undergo dissolution to form soluble zincates, especially at high pH. Specifically, zinc oxide (ZnO) can react with alkalis to form soluble zincates, such as:

 

However, it is important to emphasize that the process described in our manuscript is alkaline washing, not a high-pH leaching process. The goal of alkaline washing is primarily to clean the surface of the material, removing contaminants like grease, dirt, and halogens. The reaction of ZnO with the alkaline solution in this case is limited to a mild cleaning effect rather than full dissolution, and it does not result in significant amounts of soluble zincates under typical washing conditions.

The high chlorine content in the material causes the hydroxide ions (OH⁻) in the alkaline solution to preferentially react with chloride, rather than promoting the dissolution of zinc oxide. Therefore, the process does not lead to substantial dissolution of ZnO into soluble zincates but rather enhances the cleaning of the material surface.

Reviewer Comment 7:

ZnO + 2NaOH + H₂O → Na₂[Zn(OH)₄] (see Earnshaw A (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 1201–26"

Response: Thank you for your valuable comment regarding the dissolution of zinc oxide (ZnO) in alkaline solutions. As you pointed out, zinc oxide can indeed dissolve in alkalis to form soluble zincates, as described by the following reaction:

ZnO+₂NaOH+H₂O→Na₂[Zn(OH)₄]

This reaction is consistent with the information provided by Earnshaw (1997) in Chemistry of the Elements. We fully acknowledge this dissolution process in alkaline environments.

However, we would like to emphasize that the process described in our manuscript is alkaline washing, not a high-pH leaching process. The main objective of alkaline washing is to remove surface contaminants such as grease, dirt, and halogens, rather than to achieve the dissolution of ZnO. During alkaline washing, the hydroxide ions (OH⁻) primarily interact with surface contaminants, such as chloride from the high chlorine content in the material, rather than significantly reacting with ZnO to form soluble zincates.

Reviewer Comment 8:

Alkaline leaching also dissolves some elments found in Waelz oxide.

Response: Thank you for your comment regarding the effect of alkaline leaching on elements found in Waelz oxide. We would like to clarify that the process described in our manuscript is alkaline washing, not a high-pH alkaline leaching, and we appreciate the opportunity to clarify the distinction between these two processes.

Alkaline washing is a surface cleaning process, primarily designed to remove specific contaminants such as grease, dirt, and halogens from the material. In contrast, alkaline leaching is aimed at extracting and dissolving valuable elements from a material. While alkaline leaching can dissolve some elements found in Waelz oxide, such as zinc, it is not the intended purpose of our process.

In our study, the high chlorine  content in the Waelz oxide material causes hydroxide ions (OH⁻) to preferentially react with these halogens , rather than with the zinc oxide (ZnO) itself. This helps in removing the chloride and iodine contaminants from the material's surface, rather than dissolving the zinc oxide to form soluble zincates, as would occur in a typical alkaline leaching process.

 

Once again, we sincerely thank you for your valuable feedback, which has helped improve the manuscript. We believe the revisions have addressed your concerns and have enhanced the quality of the paper.

Best regards,

Lin Lin

December 2, 2024

Author Response File: Author Response.docx