Effect of Solvent Pre-Treatment on the Leaching of Copper During Printed Circuit Board Recycling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. Reduce background description and increase the description of innovative conclusions obtained in this study.
2. A suggested list of the chemical compositions of raw materials is expressed more clearly.
3. The paper has insufficient discussion on the leaching mechanism. Why can pretreatment strengthen leaching? It is recommended to analyze and discuss from the microstructure.
4. What are the changes in materials before and after pretreatment?
5. Is the raw material used by the author representative? Can three kinds be mixed and processed?
6. The picture is not standard enough and can be improved.
7. Are there similar studies in the literature for comparison and discussion?

Author Response

1. Reduce background description and increase the description of innovative conclusions obtained in this study.

Thank you, we appreciate your comment and have consequently revisited the introduction, eliminating some paragraphs to make it more concise and avoid confusing the reader. Similar enhancements have been applied to the abstract to better represent the work done.

You can find the revisions in Lines: 14-16, 20-24, 57-69, 75-81, 381-383.

2. A suggested list of the chemical compositions of raw materials is expressed more clearly.

Thank you for your suggestion. Regarding the raw materials used for the characterization of the various categories of the PCBs, the chemical compositions are discussed in depth in the supplementary materials.

Regarding the chemical compositions of raw materials, we have we have added a reference to the materials found before the pretreatment phase, lines 287 to 291, as well as Table 2.

3. The paper has insufficient discussion on the leaching mechanism. Why can pretreatment strengthen leaching? It is recommended to analyze and discuss from the microstructure.

Thank you for pointing out this aspect.

The leaching mechanism is not innovative in itself but follows the established results by “D. Dutta, R. Panda, A. Kumari, S. Goel, and M. K. Jha, “Sustainable recycling process for metals recovery from used printed circuit boards (PCBs),” Sustainable Materials and Technologies, vol. 17, p. e00066, Sep. 2018, doi: 10.1016/J.SUSMAT.2018.E00066.” since the study had already investigated the optimized conditions for solvent pretreatment under various conditions and concluded that to best separate the various layers and components of PCBs the operating conditions are 1:2 (w/v) at 90 °C for 60 min under constant N2 flow.  The experimental conditions are detailed in section 2.2, lines 202-204, and an illustrative figure of the experimental setup is shown in Figure 2b.

We appreciate your suggestion regarding the analysis of the microstructure, as it reveals more about the changes undergone by the material during the pretreatment and leaching phase. Since the current paper is already rich in data and complex analysis, we fear that adding more analysis would compromise the understandability of the manuscript and make it more complex for the reader to follow. Additionally, it would not be a perfect fit for the objectives of this research, which is to better understand the material composition of the different PCBs and optimize copper recovery using pretreatment. We will, however, carry out this investigation during our future work and will look into including it in subsequent publications, which are more focused on the microstructure of different PCBs and the impact of leaching and pretreatment.

4. What are the changes in materials before and after pretreatment?

Thank you for your comment. There is no material composition change before and after the pretreatment process. This is mentioned in section 3.2.1 specifically lines 310-320.

An extra paragraph discussing explicitly the lack of change in non-organic material composition after the pretreatment has been added. This can be found in lines 321-323.

5. Is the raw material used by the author representative? Can three kinds be mixed and processed?

We appreciate the issue raised.

The raw materials used simulate the industrial scenario where you have a mixture of different models of motherboard PCBs. The specifics of the motherboards analysed are discussed in the supplementary materials, stating their date of production and the different trends, as well as comparing the composition of different models. Based on that, it is thought that the materials used are representative and similar results would be achieved if other models or even a mixture of different kinds were mixed and pretreated, then leached for material recovery. The organic content represents almost the same percentage of the total wight of the PCB  of  different kinds and since pretreatment works on swelling the organic part to liberate the metals, similar results would be obtained even in case of mixtures. We have also added the ANOVA tests to the results to better support the representativeness of the raw materials used. The ANOVA tests are for the five most dominant materials in lines 258-264. The tests reveal that the results are representative for copper and silicon but not for aluminium, tin and calcium.

6. The picture is not standard enough and can be improved.

Thank you for your remark. We have uploaded all figures separately with higher quality i to ensure they are clear when added to the manuscript during publication.

7. Are there similar studies in the literature for comparison and discussion?

To design the experiment and establish the conditions as well as the solvents to be used, a thorough literature review was carried out. A summarised but representative list of the work done is mentioned in the introduction. Table 1 lists the different solvents used, the operating conditions as well and the type of PCB treated. These studies are then compared, and gaps are identified based on which the working conditions, including the selection of DMSO as a solvent, the particle size and the operating temperature, are decided. The discussion of these studies extends from line 90 to 109. The gaps in these studies are presented in lines 117 to 126. Unfortunately, as identified in the gaps, none of the studies found did cover both the pretreatment of PCBs as well as the leaching process. Thus, a comparison of the copper recovery rates with these studies is not possible.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This study presented an interesting work on printed circuits boards recycling. However, problems also existed.  

1. The novelty of this work must be presented properly, although a long background has been given.

2. For experimental design, the variate should be such as temperature, pressure, or solvent components (which can be controlled and repeated easily) and so on. The differences of experimental results among motherboards, CPU and RAM are very likely depending on their models, production batch, or manufacturer (even their composition could be similar), the representativeness of the results is suspected. 

3. the quality of the Figures should be improved. Such as Figure 6.

Author Response

1. The novelty of this work must be presented properly, although a long background has been given.

Thank you for your comment. Regarding the long background, we have revisited the introduction and eliminated some paragraphs to make it more concise and avoid the reader getting lost. Similar enhancements have been applied to the abstract to better represent the work done.

The gaps identified in the existing research are discussed in detail in the introduction, from line 117 to line 126, as well as in lines 140-142. The research questions are defined in the previous paragraph, lines 143-149. The full picture is obtained when the novelty of this research is then stated clearly in lines 150 to 161.

2. For experimental design, the variate should be such as temperature, pressure, or solvent components (which can be controlled and repeated easily), and so on. The differences of experimental results among motherboards, CPU and RAM are very likely dependent on their models, production batch, or manufacturer (even their composition could be similar), the representativeness of the results is suspected. 

Thank you for your comment. The difference in experimental results between motherboards, CPU, and RAM is due to the different models. The objective of this study is to evaluate how significant these differences are not only among models of the same categories but among the different categories (lines 145-147). During the analysis, we have tried to study different models from different production years to assess how significant the differences are. This is detailed in the supplementary materials, where each category is analysed with its different models. The representativeness of the results is an issue that we reported in the supplementary materials by showing the standard deviation of the average material content for all analysed PCBs (Tables 1,2, and 3 in the supplementary materials).

We have also added the ANOVA tests for the five most dominant materials in lines 258-264. The tests reveal that the results are representative for copper and silicon but not for aluminium, tin, and calcium.

3. The quality of the Figures should be improved. Such as Figure 6.

Thank you for your remark. We have uploaded all figures separately with higher quality to ensure they are clear when added to the manuscript during publication.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In this study, an experimental analysis of the material composition of different categories of PCBs is presented. This research finds that there are significant variances among the different categories of PCBs investigated allowing a more informed handling process of WEEE. The research suggests that solvent pre-treatment using DMSO for PCB particle size between 5.6mm and 2 mm would be a good optimization technique mitigating the drawbacks of treating fine particles while maintaining appealing recovery efficiency. This manuscipt is interesting, but it need modification before further publication.

Detailed comments:

1. The current manuscript presents an investigation of material composition of different categories of PCBs and although the work does use what could be perceived as innovative data analysis techniques, there is a lack of a clear research object of scientific value. Such scientific objective and value need to be more clearly identified and defined in the abstract and the conclusions, and also in the last paragraph of the introduction (where the novel approach or hypothesis can be detailed), and also in the results discussions.
2. The abstract needs to be rewritten. The research status review should be as concise as possible in the abstract and should focus on the research results you have achieved.
3. The introduction is too long and should be summarized as much as possible. In addition, references 1-45 are concentrated in the introduction, while there are no references in the results and discussion section. This is unacceptable in scientific writing. In addition, references 46-49 do not appear in the main text.
4. In Figure 4, the styles of a and b-h are quite different, so they can be divided into different Figures.

Author Response

1. The current manuscript presents an investigation of material composition of different categories of PCBs and although the work does use what could be perceived as innovative data analysis techniques, there is a lack of a clear research object of scientific value. Such scientific objective and value need to be more clearly identified and defined in the abstract and the conclusions, and also in the last paragraph of the introduction (where the novel approach or hypothesis can be detailed), and also in the results discussions.

Thank you for your suggestions, we have added the objectives more explicitly in the abstract lines 19-20 as well as in the conclusion lines 355-357. In addition, connections between the novelty of our research and the objectives were drawn in lines 159-161.

2. The abstract needs to be rewritten. The research status review should be as concise as possible in the abstract and should focus on the research results you have achieved.

Thank you for your suggestion. we have reformulated the abstract as suggested to make it more concise and better highlight the research results.

You can find the revisions in lines 14-16, 20-24

3. The introduction is too long and should be summarized as much as possible. In addition, references 1-45 are concentrated in the introduction, while there are no references in the results and discussion section. This is unacceptable in scientific writing. In addition, references 46-49 do not appear in the main text.

Thank you for your comment, we have revisited the introduction and eliminated some paragraphs to make it more concise and avoid the reader getting lost. The reference list has been updated.

You can find these revisions in lines 57-69, 75-81

Regarding the lack of references in the discussion and results section, the work done is quite new and it addresses the gaps identified in the studies mentioned in the introduction (lines 112 to 126). Unfortunately, none of the studies found covered both the pretreatment of PCBs as well as the leaching process. Thus, a comparison of the copper recovery rates with these studies is not possible, and hence references could not be added.

4. In Figure 4, the styles of a and b-h are quite different, so they can be divided into different Figures.

Thank you for your suggestion, although the styles of figure 4a is different from the rest of them, the reason we have kept all different graphs together in figure 4 is that we believe they are all related and having them all together in the same figure allows the reader to draw comparisons between the different materials discussed in the three categories of PCBs.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This paper investigates the effect of solvent pretreatment (DMSO) on copper leaching efficiency in printed circuit board (PCB) recycling process and explores the optimization effect of different particle sizes. The study experimentally analyses the material composition of different categories of PCBs (motherboard, RAM, CPU), and proposes a pretreatment scheme that balances the recovery efficiency and operational risk. However, I have the following problems and suggestions for this article.

1. The introduction part of this paper points out the current challenges of PCB recycling and the entry point of research, namely the improvement of leaching efficiency by solvent pretreatment. However, the advantages and disadvantages of different leagents of wet metallurgy are not discussed further, and the potential obstacles of solvent pretreatment in practical industrial applications.
2. In this paper, there is no theoretical basis for selecting the specific conditions of solvent pretreatment and leaching, so it is suggested to provide more basis for optimizing these parameters, such as whether the optimal conditions are determined by experiment.
3. The number of repeats of the experiment is not mentioned in the experiment section, and the supplement is recommended to verify the reliability of the results.
4. In the analytical methods section, it is recommended to add more discussion about the limitations and accuracy of these analytical techniques to enhance the credibility of the results.
5. It is recommended to supplement statistical tests (e. g. ANOVA or t-tests) to confirm significant differences in material content for different PCB categories.
6. In this paper, the effect of solvent pretreatment was deeply discussed deeply, but not further exploring the reasons for the unsignificant reduction of organic components after pretreatment, and the specific impact of this on the subsequent leaching process, so a supplementary analysis of influencing factors is suggested.
7. The conclusion section is concise and effectively summarizes the main findings of the study, but suggests to add discussion to the conclusions on the potential impact of the study in practical industrial applications, such as specific recommendations for the PCB recycling industry.

Author Response

1. The introduction part of this paper points out the current challenges of PCB recycling and the entry point of research, namely the improvement of leaching efficiency by solvent pretreatment. However, the advantages and disadvantages of different leagents of wet metallurgy are not discussed further, and the potential obstacles of solvent pretreatment in practical industrial applications.

Thank you for raising the issue. The advantages and disadvantages of the different solvents is discussed briefly in the introduction in lines 94-95, lines 99-101, lines 107-108, and lines 112-116.

As for the leaching agents, the reason not many agents were explored is that we wanted to explore fixed leaching conditions to evaluate the change in the particle size parameter with and without pretreatment. The focus in this study is on the role played by the size effect.

2. In this paper, there is no theoretical basis for selecting the specific conditions of solvent pretreatment and leaching, so it is suggested to provide more basis for optimizing these parameters, such as whether the optimal conditions are determined by experiment.

Thank you for your comment. We have opted to using the optimised conditions that were published by “D. Dutta, R. Panda, A. Kumari, S. Goel, and M. K. Jha, “Sustainable recycling process for metals recovery from used printed circuit boards (PCBs),” Sustainable Materials and Technologies, vol. 17, p. e00066, Sep. 2018, doi: 10.1016/J.SUSMAT.2018.E00066.”  since the study had already investigated the optimized conditions for solvent pretreatment under various conditions and concluded that to best separate the various layers and components of PCBs the operating conditions are 1:2 (w/v) at 90 °C for 60 min under constant N2 flow. The experimental conditions are detailed in section 2.2, lines 202-204, and an illustrative figure of the experimental setup is shown in Figure 2b. Furthermore, the agitation speeds were decided by empirically based on the speeds that allowed a constant and smooth spinning of the magnetic stirring anchor without any disturbances. This is expressed in lines 321-333.

3. The number of repeats of the experiment is not mentioned in the experiment section, and the supplement is recommended to verify the reliability of the results.

Thank you to point out this aspect.

The experiments were carried out once, and the analysis was always made for each sample before and after the pretreatment and the leaching. These numbers of repeats were used to compare the results of the experiments and to support them. The same applies to all the PCBs categories analysed and their data was added to the supplementary material to discuss the trends in the material composition.

4. In the analytical methods section, it is recommended to add more discussion about the limitations and accuracy of these analytical techniques to enhance the credibility of the results.

5. It is recommended to supplement statistical tests (e. g. ANOVA or t-tests) to confirm significant differences in material content for different PCB categories.

Thank you for your both recommendations 4 and 5. We have added the ANOVA tests for the five most dominant materials in lines 258-264. The tests reveal that the results are representative for copper and silicon but not for aluminium, tin and calcium.

6. In this paper, the effect of solvent pretreatment was deeply discussed deeply, but not further exploring the reasons for the unsignificant reduction of organic components after pretreatment, and the specific impact of this on the subsequent leaching process, so a supplementary analysis of influencing factors is suggested.

Thank you for your question. the results obtained from the solvent pretreatment go hand in hand with existing research, which confirms that solvent pretreatment using DMSO is not meant to dissolve the organic components but is meant to swell it, liberating the metals. This has been now been expressed clearly in  lines 171, 310-320. Thus, a change in the composition was not expected.

7. The conclusion section is concise and effectively summarizes the main findings of the study but suggests to add discussion to the conclusions on the potential impact of the study in practical industrial applications, such as specific recommendations for the PCB recycling industry.

Thank you for your suggestion, we discuss how the data obtained can benefit the industry implicitly in the paragraph starting line 372 which states that the data can help optimize models and better target the extraction of specific materials. We did not want to go more in-depth as the conclusion would become longer and less specific to the objectives of the research and its results. In addition, since the study is carried out on lab scale it might be questionable discussing the industrial impact of such an approach. However, we have added a paragraph about the future work (lines 403-408) which discusses the plans to apply this method on a pilot scale to better assess its feasibility for the industry.

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The article addresses an interesting issue related to copper recovery from printed circuit boards (PCB) and presents possibilities for increasing process efficiency, while taking into account environmental impacts and aligning with the goals of green circular economy (CE).
PCBs are key components of electronic devices, containing many valuable metals such as copper, gold, and silver. Effective recycling methods, enabling high recovery rates, play a crucial role in fully utilizing these resources.

The article, although addressing a well-known topic of metal recovery from WEEE, is, in my opinion, original and brings new, valuable information (preliminary solvent treatment of shredded PCB waste), the implementation of which may allow for optimization of the copper recovery process.

The article has a proper structure. The authors demonstrate extensive knowledge in the subject area, which is supported by a significant number of references. The research methodology is described in a comprehensive and clear manner, followed by a presentation of obtained research results. Moreover, the study covers various types of materials (motherboards, RAM, processors), which additionally increases the value of the article.
The conclusions are consistent with the presented research results. Among other things, they confirm the positive effect of solvent application and the influence of particle size on the efficiency of the process.

The Appendix is also noteworthy, as it presents the changing shares of individual elements in the tested materials (PCBs) depending on their origin (personal computer motherboards, RAM, CPU, and chipsets).

Comments:

1. The article mentions a solvent that is not entirely environmentally friendly (DMSO). Could the authors indicate the negative environmental impact of this chemical and specify how advanced the research on alternative solvents is?
2. Please adjust (align) Figure 2 and remove the gap on page 14 between parts a) and b) of Figure 6.

Author Response

1. The article mentions a solvent that is not entirely environmentally friendly (DMSO). Could the authors indicate the negative environmental impact of this chemical and specify how advanced the research on alternative solvents is?

Thank you for your interest in the topic. There is currently a focus on the use of ionic liquids as an alternative to DMSO and other organic solvents since they are more environmentally friendly. In addition, the use of supercritical fluids is an alternative. Both methods are still in their early research phase and face many hurdles, especially because they are a costly alternative. We would suggest two very interesting papers regarding the use of ionic liquids

[1]             Y. Barrueto, P. Hernández, Y. Jiménez, and J. Morales, “Leaching of metals from printed circuit boards using ionic liquids,” J Mater Cycles Waste Manag, vol. 23, no. 5, pp. 2028–2036, Sep. 2021, doi: 10.1007/s10163-021-01275-8.

[2]             C. Vallejos-Michea, Y. Barrueto, and Y. P. Jimenez, “Life cycle analysis of the ionic liquid leaching process of valuable metals from electronic wastes,” J Clean Prod, vol. 348, p. 131357, May 2022, doi: 10.1016/J.JCLEPRO.2022.131357.

 

2. Please adjust (align) Figure 2 and remove the gap on page 14 between parts a) and b) of Figure 6.

Thank you for your kind comment. The figures are uploaded separately to ensure they are of high quality. once the document is finalized, this should be addressed in the editing process.

Reviewer 6 Report

Comments and Suggestions for Authors

A very important study that is very well presented in the paper.

Author Response

Thank you for your kind words

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

This paper has been improved

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been revised properly based on reviewer's comments.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have given good answers to the review comments and have revised the paper well.