Recovery of Titanium and Aluminum from Secondary Waste Solutions via Ultrasonic Spray Pyrolysis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The current work represents the preparation of fine-sized Ti or Al-based products using the Ultrasonic Spray Pyrolysis technique. First, the title of the paper mischaracterizes the work presented in this paper. The work lacks novelty, and the analysis of the results is too descriptive and insufficiently scientific.

The authors write sentences superfluously, making it difficult for the readers to follow. For example, the abstract is unnecessarily too extended, and some paragraphs are too long, such as lines 90-121.

The scale of Figure 3 is unclear. The size of particles appears to be more in the micron range, and stating them as nano-size is too exaggerated. The authors didn’t even show the size distribution of the particles produced. There wasn’t any attempt to explore various conditions determining the size of the product, contrary to their claim of the optimization of the product size. The authors introduced Eq. 1, but it does not affect the results of this study. It is not shown whether this equation is valid, wrong, or indifferent based on this study.

The authors have claimed that this study enhances the scalability and purity of the product material processing without substantive results indicating this is true. In addition, contrary to the author's claim, the work does not promote resource recovery or waste reduction.

Comments on the Quality of English Language

A great deal of editing is required to meet the standards required for publication.

Author Response

Dear Reviewer,

Thank you for your thorough and insightful comments. We have carefully addressed each of your concerns and made the necessary revisions to improve the manuscript. Please find our responses below:

The current work represents the preparation of fine-sized Ti or Al-based products using the Ultrasonic Spray Pyrolysis technique. First, the title of the paper mischaracterizes the work presented in this paper. The work lacks novelty, and the analysis of the results is too descriptive and insufficiently scientific.

We agree with the reviewer that the original title, "Advanced techniques to recover titanium and aluminium from secondary waste solutions", may overstate the scope of the work, particularly with the use of the term “advanced techniques”. To more accurately reflect the content and focus of the study, we propose the following revised title: "Recovery of Titanium and Aluminium from Secondary Waste Solutions via Ultrasonic Spray Pyrolysis”. While we acknowledge that USP is an established method, the novelty of our work lies in applying it specifically for the recovery of Ti and Al from industrial secondary waste streams—an underexplored topic.

The authors write sentences superfluously, making it difficult for the readers to follow. For example, the abstract is unnecessarily too extended, and some paragraphs are too long, such as lines 90-121.

We thank the reviewer for the constructive feedback. We fully agree that concise and focused writing improves readability and overall clarity. Accordingly, we have thoroughly revised the Abstract and Introduction sections to eliminate superfluous language, reduce paragraph length, and improve sentence structure. We have reorganized and shortened overly long paragraphs (including lines 90–121).

The scale of Figure 3 is unclear. The size of particles appears to be more in the micron range, and stating them as nano-size is too exaggerated. The authors didn’t even show the size distribution of the particles produced. There wasn’t any attempt to explore various conditions determining the size of the product, contrary to their claim of the optimization of the product size. The authors introduced Eq. 1, but it does not affect the results of this study. It is not shown whether this equation is valid, wrong, or indifferent based on this study.

We have updated Figure 3 to include a clearly labeled scale bar and improved contrast to better illustrate particle dimensions. Additionally, we have incorporated a new experiment focused on titanium oxide nanopowder, where smaller particles in the nanometer to submicron range are clearly observed and documented. These results are now included in the revised manuscript and discussed accordingly. We also acknowledge the reviewer’s concern about the lack of size distribution data. Unfortunately, these experiments were originally carried out in a separate laboratory as part of the PhD work of Vladimir Damjanović, and the equipment used is no longer accessible to us. As a result, particle size distribution analysis could not be repeated or newly acquired.

The authors have claimed that this study enhances the scalability and purity of the product material processing without substantive results indicating this is true. In addition, contrary to the author's claim, the work does not promote resource recovery or waste reduction.

We appreciate your valuable feedback, which has helped us improve the clarity and structure of the manuscript. We hope that the revised version meets your expectations and can be published as soon as possible.

In the revised manuscript, we have clarified that the implementation of an electrostatic precipitator (ESP)—compared to our previous work using wet collection—has significantly improved powder recovery efficiency and product purity. The ESP enables the dry collection of fine particles, avoiding contamination and loss associated with wet methods. While we acknowledge that quantitative purity analysis was limited, SEM imaging and visual inspection showed a marked improvement in powder homogeneity and cleanliness. Process Our updated experimental setup using the ESP allows for continuous powder collection, which is an important step toward scalable production. We respectfully disagree with the assertion that the work does not contribute to resource recovery or waste reduction. This study utilizes industrial waste-derived precursors, such as titanium oxysulfate and aluminum nitrate from aluminum industry byproducts (e.g., red mud), which are otherwise environmentally hazardous.

We appreciate your valuable feedback, which has helped us improve the clarity and structure of the manuscript.

Best regards

Reviewer 2 Report

Comments and Suggestions for Authors

It is not fully clarified in this paper that how ultrasound irradiation is important for preparation of nano powders in this process.

 

Originally, the data and discussion of the influence of parameters (flow rates, temperature, concentration of solutes intensity of ultrasound etc.) on the characteristics of prepared particles are supposed to describe in the paper. Add some results if already obtained. The prepared particles in both cases with and without the ultrasonic irradiation should be shown.

 

Figure 1 showed the nonuniform size distribution of particles and their agglomeration.

How break down the agglomeration of particle?

 

In Fig.1, the drawing is crooked.

The information of the inner diameter of reactor tube and temperature profile in the pyrolysis zone should be added. How treated the exhaust gas?

 

Show the product manufacture, type and number of the electrostatic precipitator.

Author Response

Dear Reviewer,

Thank you for your thorough and insightful comments. We have carefully addressed each of your concerns and made the necessary revisions to improve the manuscript. Please find our responses below:

It is not fully clarified in this paper that how ultrasound irradiation is important for preparation of nano powders in this process.

Ultrasonic irradiation is used to generate fine aerosol droplets from the precursor solution. These droplets are typically in the micrometer range and highly uniform in size. As they pass through the heated reactor (furnace), each droplet undergoes rapid solvent evaporation, precursor decomposition, and particle formation, resulting in the production of oxide powders.

 Originally, the data and discussion of the influence of parameters (flow rates, temperature, concentration of solutes intensity of ultrasound etc.) on the characteristics of prepared particles are supposed to describe in the paper. Add some results if already obtained. The prepared particles in both cases with and without the ultrasonic irradiation should be shown.

We appreciate the reviewer’s suggestion regarding the influence of process parameters. We have included an additional experiment on titanium dioxide synthesis to demonstrate the effect of precursor concentration and temperature on particle characteristics. Regarding ultrasonic irradiation, it is essential for droplet formation; without ultrasound, no aerosol droplets are generated, and thus no particles can be carried into or formed in the furnace. Therefore, a comparison without ultrasound is not feasible. Our current equipment operates at a fixed frequency of 1.7 MHz. We acknowledge this limitation and plan to explore different ultrasound frequencies and intensities in future work to further optimize particle properties.

Figure 1 showed the nonuniform size distribution of particles and their agglomeration. How break down the agglomeration of particle?

We believe that You may be referring to Figure 3, which indeed shows nonuniform particle size due to droplet size variation and agglomeration during collection. Agglomeration is a known issue in spray pyrolysis, especially when particles are collected without dispersion aids. In future work, we plan to investigate ultrasonic dispersion techniques post-synthesis to reduce agglomeration. Additionally, optimization of droplet size through precursor formulation and atomization conditions may improve particle uniformity.

 In Fig.1, the drawing is crooked.

Figure 1 was updated.

The information of the inner diameter of reactor tube and temperature profile in the pyrolysis zone should be added. How treated the exhaust gas?

Diameter of the tubes has been added. Exhaust gas wasn't treated and it was emited to the atmosphere because it doesnt contain any dangerous gas.

Show the product manufacture, type and number of the electrostatic precipitator.

Company Prizma Kragujevas made this equipment using only one electrostatic precipitator.

We appreciate your valuable feedback, which has helped us improve the clarity and structure of the manuscript. We hope that the revised version meets your expectations and can be published as soon as possible.

Best regards

Reviewer 3 Report

Comments and Suggestions for Authors

Authors proposed a manuscript titled "Advanced techniques to recover titanium and aluminium from secondary waste solutions". The manuscript has a good scientific soundness, and it deserves to be published after major revisions.

Issue N. 1

Lines: 130–143, 311–334. The manuscript claims significant novelty in using USP with electrostatic precipitators for TiO₂ and Al₂O₃ synthesis from industrial waste, but this approach has been explored previously. Prior works (e.g., Jokanović et al. [26], Zhao et al. [7]) already explored USP for similar applications. Electrostatic precipitation, while useful, is not a substantial technological leap. Clearly state how this setup is different from existing setups (e.g., energy efficiency, yield, morphology control, impurity tolerance). Quantify improvements where possible.

Issue N. 2

Line 200-256, 271-296. The comparison of nanopowders from synthetic and waste-derived precursors is mostly qualitative (SEM/EDS) and lacks systematic quantitative metrics (e.g., BET surface area, photocatalytic activity). Without particle size distributions, surface area, porosity, or performance metrics, it’s hard to assess the practical significance of using waste-derived materials. Add particle size statistics, surface area (BET), and application performance comparisons (e.g., in catalysis or ceramics).

Issue N. 3

Hydrogen is used for TiOSO₄ conversion but no mechanistic insight or comparison to oxidizing/inert conditions is presented. Literature suggests both hydrogen and air atmospheres have been used in USP; the benefit here is assumed, not demonstrated. Include a comparative study or at least discussion of reduction vs. oxidation atmospheres on particle properties.

Issue N. 4

Lines 204-215, 239-248. The presence of Fe, Cr, Ni, Mn, etc., is discussed (from corrosion or precursors). However, impurities may dramatically affect the performance in catalysis or electronics. No post-processing or mitigation strategies are discussed. Include potential purification steps (e.g., washing, thermal treatments) or discuss the trade-offs between low-cost waste recovery and material quality.

Issue N. 5

Lines 200-232, 280-284. SEM images (Figs. 3, 6) show particles often in the micrometer range, while the term “nanopowder” is used throughout. Nanopowder should imply primary particles <100 nm. Most are >0.5 µm based on provided figures. Either justify the term with detailed size analysis (e.g., DLS, TEM) or revise the terminology.

Issue N. 6

Lines 138-143, 311-334. Scalability is mentioned, but critical parameters like powder yield, energy consumption, or operational costs are not included. Literature increasingly emphasizes techno-economic evaluation for industrial adoption. Include a brief discussion on process throughput, yield efficiency, and challenges of scale-up (e.g., fouling, reactor design).

Issue N. 7

The potential use in catalysis, ceramics, or coatings is mentioned but no functional testing is performed. Claims about photocatalysis or ceramic applications lack experimental backing. Either include basic performance testing or tone down application claims.

Author Response

Dear Reviewer,

Thank you for your thorough and insightful comments. We have carefully addressed each of your concerns and made the necessary revisions to improve the manuscript. Please find our responses below:

Authors proposed a manuscript titled "Advanced techniques to recover titanium and aluminium from secondary waste solutions". The manuscript has a good scientific soundness, and it deserves to be published after major revisions.

Issue N. 1

Lines: 130–143, 311–334. The manuscript claims significant novelty in using USP with electrostatic precipitators for TiO₂ and Al₂O₃ synthesis from industrial waste, but this approach has been explored previously. Prior works (e.g., Jokanović et al. [26], Zhao et al. [7]) already explored USP for similar applications. Electrostatic precipitation, while useful, is not a substantial technological leap. Clearly state how this setup is different from existing setups (e.g., energy efficiency, yield, morphology control, impurity tolerance). Quantify improvements where possible.

In response, we have revised the abstract and introduction to improve clarity and more accurately position our work in the context of existing literature. While USP and electrostatic precipitation have been previously applied, the novelty of our approach lies in combining these methods for the synthesis of TiO₂ and Al₂O₃ from industrial waste-derived precursors, which has not been thoroughly explored. Compared to our previous work, the use of an electrostatic precipitator significantly improves product quality, enables dry collection, and supports continuous processing. We have also included an additional experiment to further support our findings. Future work will aim to quantify improvements in yield, purity, and process efficiency in greater detail.

Issue N. 2

Line 200-256, 271-296. The comparison of nanopowders from synthetic and waste-derived precursors is mostly qualitative (SEM/EDS) and lacks systematic quantitative metrics (e.g., BET surface area, photocatalytic activity). Without particle size distributions, surface area, porosity, or performance metrics, it’s hard to assess the practical significance of using waste-derived materials. Add particle size statistics, surface area (BET), and application performance comparisons (e.g., in catalysis or ceramics).

We thank the reviewer for this valuable suggestion. We fully agree that quantitative characterization such as BET surface area, particle size distribution, porosity, and application performance testing would strengthen the comparison between synthetic and waste-derived nanopowders. However, due to equipment limitations and the fact that part of the experimental work was conducted in a different laboratory during a PhD project, we were unable to perform these additional measurements at this stage. We have clearly acknowledged this limitation in the revised manuscript and identified it as a priority for future work, where more systematic and quantitative analysis will be conducted to better evaluate the practical performance of these materials.

 

Issue N. 3

Hydrogen is used for TiOSO₄ conversion but no mechanistic insight or comparison to oxidizing/inert conditions is presented. Literature suggests both hydrogen and air atmospheres have been used in USP; the benefit here is assumed, not demonstrated. Include a comparative study or at least discussion of reduction vs. oxidation atmospheres on particle properties.

We thank the reviewer for this insightful comment. In the revised manuscript, we have added a discussion clarifying the role of hydrogen as a reducing agent that facilitates the transformation of TiOSO₄ into TiO₂ during USP. To address the reviewer's point, we also included results from an additional experiment conducted in an inert (argon) atmosphere, which showed minimal or no TiO₂ formation under the same conditions. This supports our conclusion that hydrogen significantly enhances the decomposition and conversion efficiency of the precursor. Although a full comparative study was beyond the scope of this work, we have acknowledged this as a potential area for further research.

Issue N. 4

Lines 204-215, 239-248. The presence of Fe, Cr, Ni, Mn, etc., is discussed (from corrosion or precursors). However, impurities may dramatically affect the performance in catalysis or electronics. No post-processing or mitigation strategies are discussed. Include potential purification steps (e.g., washing, thermal treatments) or discuss the trade-offs between low-cost waste recovery and material quality.

We thank the reviewer for raising this important point. In the revised manuscript, we have adjusted the discussion of potential applications to reflect that, due to the presence of impurities such as Fe, Cr, Ni, and Mn (from precursors and reactor materials), the synthesized powders are more suited for non-electronic or low-spec catalytic applications.To address contamination from the steel reactor, we integrated ceramic tube, which offers higher thermal stability and reduced risk of introducing metallic impurities. Although purification steps such as washing or post-synthesis thermal treatment were not applied in this work, we have now included a brief discussion of these possible strategies for future material quality improvement.

Issue N. 5

Lines 200-232, 280-284. SEM images (Figs. 3, 6) show particles often in the micrometer range, while the term “nanopowder” is used throughout. Nanopowder should imply primary particles <100 nm. Most are >0.5 µm based on provided figures. Either justify the term with detailed size analysis (e.g., DLS, TEM) or revise the terminology.

We thank the reviewer for pointing out the need for clarification regarding the term “nanopowder.” In response, we have added an additional experiment on TiO₂ synthesis at higher temperature, which demonstrates that particle size can be controlled by adjusting the temperature, resulting in the formation of nanosized primary particles as confirmed by SEM imaging. While some agglomerates appear in the micrometer range, closer observation reveals the presence of nanoparticles within these clusters.

 

Issue N. 6

Lines 138-143, 311-334. Scalability is mentioned, but critical parameters like powder yield, energy consumption, or operational costs are not included. Literature increasingly emphasizes techno-economic evaluation for industrial adoption. Include a brief discussion on process throughput, yield efficiency, and challenges of scale-up (e.g., fouling, reactor design).

We appreciate the reviewer’s comment regarding scalability and techno-economic considerations. While a detailed analysis of powder yield, energy consumption, and operational costs was beyond the scope of the current study, we recognize the importance of these factors for industrial application. These aspects, including throughput and cost analysis, have been clearly identified as important directions for future research and optimization.

Issue N. 7

The potential use in catalysis, ceramics, or coatings is mentioned but no functional testing is performed. Claims about photocatalysis or ceramic applications lack experimental backing. Either include basic performance testing or tone down application claims.

We thank the reviewer for this valid observation. In the revised manuscript, we have toned down the language regarding application claims and now refer to “possible” or “potential applications” in areas such as catalysis, ceramics, or coatings.

We appreciate your valuable feedback, which has helped us improve the clarity and structure of the manuscript.

Best regards

 

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have done a good job in planning and executing experiments in their attempt to recover metal values from spent Li batteries using reduction smelting and chlorination. The paper reads well, and this reviewer recommends the paper be accepted for publication.

 

Fig. 7 is missing.

 

Author Response

Dear Reviewer,

We have addressed your concern regarding the missing Figure 7.

Figures 8 and 9 have been mistakenly named. They are now Figures 7 and 8.

We hope that the revised version meets your expectations and can be published as soon as possible.

Best regards

Reviewer 3 Report

Comments and Suggestions for Authors

Authors repsonded to my issues. However, they should improve the quality of Figure 9.

Author Response

Dear Reviewer,

thank you for your comments. We have improved the quality of Figure 9.

We hope that the revised version meets your expectations and can be published as soon as possible.

Best regards