Applying Circular Thermoeconomics for Sustainable Metal Recovery in PCB Recycling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors presented an interesting document that, after minor corrections, could become a relevant reference in its field.

 

Fig 1 has some words underlined in red that must be correct. Probably when the flowchart was transformed in figure.

 

In some figures, the small letter are unreadable. Please improve resolution.

 

Citations missing for HSC and Fact Sage.

 

Conclusion is wordy. It could be shortened to summarize the main quantitative findings of the manuscript only.

Author Response

The authors presented an interesting document that, after minor corrections, could become a relevant reference in its field.

Thanks to the reviewer for the comments that have helped to improve the presentation of the paper and its clarity. In addition to adding the changes suggested by the reviewer, as justified below, we have considered reorganizing the sections of the paper for greater clarity for the reader. Thus, the former section 2.2 is now section 2.1, in which the thermodynamic model of recycling is explained. We have changed the order since we think that the complexity of the recycling model is greater than the complexity of the primary extraction, because the data for the primary extraction were obtained from previous papers. Thus, the former section 2.1 is now 2.2. In addition, we have added section 2.3, in which we explain more clearly the different allocations used in the paper. Section 3.1. is now called “Recycling and Primary exergy cost of metals” since part of the explanation of the allocations is now in section 2.3 and part in section 3.1.

Fig 1 has some words underlined in red that must be correct. Probably when the flowchart was transformed in figure.

Thanks for the feedback. Due to the changes explained in the previous paragraph, Figure 1 is now Figure 3. The problem cited by the reviewer has been solved. In addition, we have modified the Figure to better appreciate the analogy between Figure 3 (primary production) and Figure 2 (recycling). Both schemes show metal production, but from different resources. However, Figure 3 has simplified processes because it is not the objective of this paper.

In some figures, the small letter are unreadable. Please improve resolution.

We have modified and increase the resolution of Figure 3 and 5.

Citations missing for HSC and Fact Sage.

We have slightly changed the sentence in the lines 112-113 (“The physical structure was obtained from [34–36], which uses the software HSC chemistry [37] to simulate the recovery of copper, silver, gold, and palladium from PCBs”). We now only cite the HSC, as Figure 1 shows the schematic simulated in this software. However, FactSage was not used directly for the production of this paper.

Conclusion is wordy. It could be shortened to summarize the main quantitative findings of the manuscript only.

We have revised and changed the conclusions to make them clearer.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper is well-written with clear objectives and a well-organized structure. The manuscript paper effectively summarizes the key findings of the research and provides valuable insights into the implications of thermoeconomic analysis for PCB recycling. Here are some suggestions for

the authors can further improve the clarity and comprehensiveness of their paper:

1.      Section 2. Materials and Methods :The authors should to clarify the reason for choosing the four specific metals (Cu, Ag, Au, and Pd) for analysis.

2.      Table 1 and Table 2:, it would be beneficial to place Table 1 and Table 2 before Figures 3 and 4 to enhance understanding and provide a clearer context for interpreting the diagrams.

3.      Section 2.2.4. Environmental Model:, the authors could provide more details about the specific environmental model applied in the methodology to improve clarity of the clcultion of the exergy life cycle cost of resources.

4.      Section 3.1: It might be helpful to double-check the consistency of terminology between "exergy" and "exergy cost" to ensure clarity and avoid any confusion.

5.      Table 4, Table 5, and Table 6: To enhance readability, consider adding endnotes at the bottom of these tables to provide abbreviations used.

6.      Section 3.3. Circular thermoeconomics: Following the discussion on circular thermoeconomics, the authors could provide suggestions and a more comprehensive approach to minimizing non-renewable resource destruction throughout the entire life cycle of products.

Author Response

This paper is well-written with clear objectives and a well-organized structure. The manuscript paper effectively summarizes the key findings of the research and provides valuable insights into the implications of thermoeconomic analysis for PCB recycling.

Thanks to the reviewer for the comments that have helped to improve the quality and clarity of the paper. In addition to adding the changes suggested by the reviewer, as justified below, we have considered reorganizing the sections of the paper for greater clarity for the reader. Thus, the former section 2.2 is now section 2.1, in which the thermodynamic model of recycling is explained. We have changed the order since we think that the complexity of the recycling model is greater than the complexity of the primary extraction, because the data for the primary extraction were obtained from previous papers. Thus, the former section 2.1 is now 2.2. In addition, we have added section 2.3, in which we explain more clearly the different allocations used in the paper. Section 3.1. is now called “Recycling and Primary exergy cost of metals” since part of the explanation of the allocations is now in section 2.3  and part in section 3.1.

 Here are some suggestions for the authors can further improve the clarity and comprehensiveness of their paper:

 

1. Section 2. Materials and Methods: The authors should to clarify the reason for choosing the four specific metals (Cu, Ag, Au, and Pd) for analysis.

 

Thank you for your feedback, we have added a brief explanation in lines 80-82 (“These metals are the selected products of the recycling process since they concentrate most of the exergy cost of the production of the PCB.”). However, in line 413-418 (“Once the metals and materials were produced, we distinguished between the exergy cost necessary to produce the four recycling metals: Cu, Ag, Au, and Pd, and the rest of the metals and materials. The metals selected for recycling comprise 91\% of the entire non-renewable exergy cost, corresponding to 4.9% for Cu, 7.2% for Ag, 65.8% for Au, and 13.4% for Pd. Therefore, the recycling process is correctly focused since it can recover most of the exergy cost embodied in the PCB”) is explained in more depth the reason for this decision and is also visually appreciated in the Sankey diagrams. (Figures 6 and 7).

2. Table 1 and Table 2:, it would be beneficial to place Table 1 and Table 2 before Figures 3 and 4 to enhance understanding and provide a clearer context for interpreting the diagrams.

We have slightly modified the order of the Figures and Tables. First, we show Figure 1, which shows the Block Flow Diagram of the recycling plant. Secondly, we show Table 1, which defines all the flows shown in Figure 1. Thirdly, we show the Figure 2, i.e., the thermoeconomic diagram, which is derived from Figure 1. And finally, Table 2, which is derived from the thermoeconomic diagram (Figure 2).

 

3. Section 2.2.4. Environmental Model:, the authors could provide more details about the specific environmental model applied in the methodology to improve clarity of the clcultion of the exergy life cycle cost of resources.

 

The former section 2.2.4 is now section 2.1.4 Resorces Model. We have changed the name to be clearer, and we have also tried to improve the clarity of the section by adding. The definition of the unit exergy costs of the resources (lines 161-162: “…unit exergy cost values, i.e., the total exergy required (kWh) to obtain oneckWh exergy entering the limits of the plant.”). On the other hand, some additional explanations are added in the new section 2.3.

 

4. Section 3.1: It might be helpful to double-check the consistency of terminology between "exergy" and "exergy cost" to ensure clarity and avoid any confusion.

 

We have checked when the terminology “Exergy” and “Exergy cost” are used in text. These concepts are defined now several times to enhance the consistency:

 

• “Exergy is the maximum theoretical useful work obtained if a system is brought into thermodynamic equilibrium with the environment” (lines 46-47).
• “...exergy cost reflects the cumulative exergy consumption required to manufacture a product, throughout its life cycle [18]. The exergy cost allows for measuring the relationship between the material and energy dimensions through a single thermodynamic property: exergy” (lines 47-50).
• “The exergy cost represents the cumulative exergy consumption necessary to manufacture a product, considering its life cycle, i.e., from the PCBs until the metals in this case” (lines 89-91).
• “…the unit exergy cost values, i.e., the total exergy required (kWh) to obtain one kWh exergy entering the limits of the plant”. (lines 161-162).

 

5. Table 4, Table 5, and Table 6: To enhance readability, consider adding endnotes at the bottom of these tables to provide abbreviations used.

 

Table 4 has been moved to the appendix (now Table A4) and the legend has been improved by adding the abbreviations used. Table 5 is now Table 4 and abbreviations have been included in the legend. Table 6 has been eliminated because it was used to justify the most appropriate allocation. This discussion has been maintained, but is presented through Table 5, as Table 6 provided very similar information to Table 5. In addition, we have added a caption in Table 7 to enhance the clarity.

6. Section 3.3. Circular thermoeconomics: Following the discussion on circular thermoeconomics, the authors could provide suggestions and a more comprehensive approach to minimizing non-renewable resource destruction throughout the entire life cycle of products.

We believe that there are two main ways to minimize the destruction of non-renewable resources:

• Use renewable energies: lines 454-455 (“One option to minimize the destruction of non-renewable exergy is to maximize the use of renewable energies in the primary production and recycling processes.”). This reduces the non-renewable cost by 84%. (line 458-460: Thus, the non-renewable exergy destroyed goes from 1,795,200 kWh in the 2020 case (Figure 6 to 287,211 kWh in the 2050 case (Figure 7), i.e., by using renewable energies, the non-renewable exergy cost is reduced by 84%).
• Delay recycling as long as possible: lines 478-480 (“On the other hand, the high losses (47-53\%) of non-renewable exergy during recycling indicate that these processes should be delayed as much as possible. Many measures can be promoted to achieve this: using fewer materials in production, intensifying product use, extending product lifetime, reuse, remanufacturing or refurbishing, between other.”). We have included the above measures in the new version of the manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

 

The manuscript entitled “Applying Circular Thermoeconomics for Sustainable Metal Recovery in PCB Recycling” provides some good results. Therefore, the current manuscript could be accepted for publication, but after going through a minor revision.      

1.     Keywords should be more eye-catching.

2.     Tabulated comparisons with other analysis methods should be provided.

3.     “Chemicals (F2, K2, L2, M2, N2): mainly sulfuric acid (H2SO4) for electrolysis, nitric acid (HNO3) used in Ag recovery, oxalic acid (C2H2O4) in Au recovery and ammonium chloride (NH4Cl) in PGM recovery” Authors should pay attention to super and subscript.

4.     Conclusions should be more specific.  

5.     English language must be improved by removing grammatical mistakes throughout the whole manuscript.

Comments on the Quality of English Language

 

English language must be improved by removing grammatical mistakes throughout the whole manuscript.

Author Response

The manuscript entitled “Applying Circular Thermoeconomics for Sustainable Metal Recovery in PCB Recycling” provides some good results. Therefore, the current manuscript could be accepted for publication, but after going through a minor revision.

Thanks to the reviewer for the comments that have helped to improve the paper. In addition to adding the changes suggested by the reviewer, as justified below, we have considered reorganizing the sections of the paper for greater clarity for the reader. Thus, the former section 2.2 is now section 2.1, in which the thermodynamic model of recycling is explained. We have changed the order since we think that the complexity of the recycling model is greater than the complexity of the primary extraction, because the data for the primary extraction were obtained from previous papers. Thus, the former section 2.1 is now 2.2. In addition, we have added section 2.3, in which we explain more clearly the different allocations used in the paper. Section 3.1. is now called “Recycling and Primary exergy cost of metals” since part of the explanation of the allocations is now in section 2.3 and part in section 3.1.

1. Keywords should be more eye-catching.

We have modified the order of the keywords according to their importance in the article and we have added others such as “Renewable Energies”, and eliminated others such as “Exergy Cost”, as we believe it was redundant.

2. Tabulated comparisons with other analysis methods should be provided.

Thermoeconomics is based on the use of exergy as the measure of environmental evaluation of resource consumption. As stated in the introduction (lines 57-61: “In this respect, several studies have analyzed recycling recycling from an exergy perspective [15,16,22–25], but they have not applied a thermoeconomic approach to distinguish between renewable and non-renewable exergy resources. Other research has focused on non-renewable exergy during the life cycle of products, but without a circular economy perspective.”), there are few studies that follow the same perspective taken in this paper, and even fewer that take PCBs (used in any electronic product) as an example of recycling (at least to the best of the author's knowledge). As a result, it is difficult to make a comparison. However, we have included a sentence in line 400-403 (“Therefore, the exergy cost of recycling (44,839 or 43,357 kWh) refers to the total exergy cost of recycling the 20 tons of PCBs. These figures correspond to 7.8 and 8.1 MJ/kg of PCB, comparable to the energy consumption of other references which range between 5.6 and 13.7 MJ/kg of PCB [50,51]”) to compare the exergy cost of recycling PCBs in our article with the energy consumption of other articles that also recycle PCBs.

3. “Chemicals (F2, K2, L2, M2, N2): mainly sulfuric acid (H2SO4) for electrolysis, nitric acid (HNO3) used in Ag recovery, oxalic acid (C2H2O4) in Au recovery and ammonium chloride (NH4Cl) in PGM recovery” Authors should pay attention to super and subscript.

Thank you for this suggestion, which helped improve the presentation of the paper. We have modified this sentence, now in the lines 177-180.

4. Conclusions should be more specific.

Thank you for your comment, we have revised and changed the conclusions to make them clearer.

5. English language must be improved by removing grammatical mistakes throughout the whole manuscript.

We have revised the manuscript to correct grammatical mistakes.

 

 

Author Response File: Author Response.pdf