Selective Disintegration–Milling to Obtain Metal-Rich Particle Fractions from E-Waste

Round 1

Reviewer 1 Report

The authors investigated the milling rate's effect on the properties of mechanically disintegrated printed circuit boards. Besides, this work demonstrates the content of valuable metals depending on the size of particles obtained. These results are helpful to the recovery of metals from printed circuit boards. The subject is in line with the metals. However, this paper needs revision before publication, and some questions must be answered.

Comment 1: The manuscript is mainly descriptive. This work will be improved if the mechanism of metal distribution in particles with different sizes is analyzed.

Comment 2: It is useful to investigate the metal content in particles of different sizes. However, if the authors could comprehensively consider the distribution proportion of metals in various particles by combining the quality of different particles, it will be more practical.

Comment 3: The particle size in Figure 7 includes parts of “5.6-11.2 mm” and “＞11.2 mm”, but why are there no relevant properties of these two parts in Table 3?

Comment 4: Please indicate the particle size of “Obtained tested particles from PCBs” in the conclusions section. Moreover, the mass ratio of the metals in the obtained particles to the metals in the original PCBs should be given.

Comment 5: The errors in the work need to be modified. For example, “The work demonstrates the dependence of the content of metals and valuable metals depending on the size of obtained particles.” in Line 165.

Author Response

The authors investigated the milling rate's effect on the properties of mechanically disintegrated printed circuit boards. Besides, this work demonstrates the content of valuable metals depending on the size of particles obtained. These results are helpful to the recovery of metals from printed circuit boards. The subject is in line with the metals. However, this paper needs revision before publication, and some questions must be answered.

Answer: Thank you for your evaluation. It's nice to hear such an assessment and we did our best to improve our article according to comments and objective criticism. We have also corrected small style and grammar errors without changing the meaning of the sentences throughout the article regarding the comments and suggestions made by other reviewers.

Comment 1: The manuscript is mainly descriptive. This work will be improved if the mechanism of metal distribution in particles with different sizes is analyzed.

Answer: Thank you for your suggestion. The authors made an introductory study which already leads to large amount of research data. The study is being continued and more detailed descriptions on effects will be represented in future articles. No changes are made in the present article.

Comment 2: It is useful to investigate the metal content in particles of different sizes. However, if the authors could comprehensively consider the distribution proportion of metals in various particles by combining the quality of different particles, it will be more practical.

Answer: Thank you for your comment. The Table 3 demonstrates the metal content measurement results in the various fractions after single and double milling. The additional date would oversaturate the article with results. No changes are made in the present article.

Comment 3: The particle size in Figure 7 includes parts of “5.6-11.2 mm” and “＞11.2 mm”, but why are there no relevant properties of these two parts in Table 3?

Answer: Thank you for your question. The authors focus on possible finest particles production with higher possible metal content to provide useful feedstock for further metals extraction. The authors propose to subject the resulting first-stage milling large sized particles (5.6-11.2 mm and “＞11.2) to the further milling to produce finer particles. In case of many tested metals, it is proposed to perform metal extraction process without mixing produced particles from both stages to save energy. No changes are made in the present article.

Comment 4: Please indicate the particle size of “Obtained tested particles from PCBs” in the conclusions section. Moreover, the mass ratio of the metals in the obtained particles to the metals in the original PCBs should be given.

Answer: Thank you for your valuable demand. The conclusions section is improved by marking results with relevant designations (milling step and fractions). The metal content (MC) is given instead of mass rations of the metals.

Comment 5: The errors in the work need to be modified. For example, “The work demonstrates the dependence of the content of metals and valuable metals depending on the size of obtained particles.” in Line 165.

Answer: Thank you for the indication. The sentence is corrected in the new version.

“The work demonstrates the dependence of common and valuable metal contents on the size of obtained disintegration-milled particles.”

Reviewer 2 Report

The paper deals with the processing of Printed Circuit Boards (PCBs) with the aim of obtaining metals from this type of electronic waste. The article compares two methods of PCB grinding, one-stage and two-stage process. The morphology, particle fraction sizes, bulk density, and metal content in produced particles were measured and compared.

I have the following comments and questions about the article:

1) In my opinion, references 5 and 6 are not relevant.

2) Fig.1 - in what units is the scale - in cm?

3) What is DPS?

4) How did you determine the bulk density? Why was bulk density determined? What is the importance of bulk density when grinding waste?

5) Table 3 shows that the larger fractions = the smaller  bulk density. Do you have any explanation why the bulk density for fraction 0.711-1.4 mm is lower than for fraction 1.4-2.8 mm in single-stage grinding. Similarly, it is with two-stage grinding when the fraction 0.35-0.711 has a smaller bulk density than the fraction 0.711-1.4.

6) I don't really understand Tab.4 - it would need a better explanation.

7)" The morphology of sieved milled particles is represented in figure 3"   Not in picture 3 but in pictures 5 and 6.

8)Fig. 5 and 6 do not have the same magnification and are therefore relatively difficult to compare. The individual components that appear in the images may have been marked.

9)ref[38] GOLJANDIN, D.; SARJAS, H.; KULU, P.; KÄ ERDI, H.; MIKLI, V. Metal-Matrix Hardmetal/Cermet Reinforced Composite 501 Powders for Thermal Spray. Materials Science 2012, 18, 84–89, doi:10.5755/j01.ms.18.1.1348.

  ref [40]GOLJANDIN, D.; SARJAS, H.; KULU, P.; KÄ ERDI, H.; MIKLI, V. Metal-Matrix Hardmetal/Cermet Reinforced Composite 505 Powders for Thermal Spray. Materials Science 2012, 18, doi:10.5755/j01.ms.18.1.1348.

Citation 38 and 40 are the same and correct it

Author Response

This paper reports an interesting study about the mechanical pre-treatment of Printed Circuit Boards (PCBs). This pre-treatment consists in a rough cutting of the PCBs followed by (i) a first operation of shredding/grinding, (ii) the recovery of the fragments/particles < 2.8 mm, (iii) a second operation of shredding/grinding for the fragments/particles > 2.8 mm. The driving idea of such a pre-treatment is to obtain sieved fractions with an increased content of recovered valuable metals (in fact in the finest particles). From this point of view and in the frame of a circular economy, the subject is of high interest. Furthermore, the paper is rather well documented (41 references) and presented. However, it suffers from a series of weaknesses. As a result, the reviewer recommends its acceptance for publication in Metals after moderate revision as stated below:

1）The state-of-the-art is well documented. However, to complete the overview, the method of micronisation (typically grinding in the range 20 – 100 µm), based only on grinding / gravity and magnetic separations and leading to fractions enriched with valuable metals, could be cited. Method for processing and removing electronic waste with a view to recovering the components included in such waste. WO 2016/042469 A1. https://patentimages.storage.googleapis.com/45/af/ff/e70226a3e124b7/WO2016042469 A1.pdf

Answer: Thank you for your valuable evaluation. It's nice to hear such an assessment and we did our best to improve our article according to comments and objective criticism. We have also corrected small style and grammar errors without changing the meaning of the sentences throughout the article regarding the comments and suggestions made by other reviewers.

We have added the additional thesis about the state-of-the-art approaches regarding your suggestion

“A 3.25 mm fraction particle production from PCBs with the help of a rotary cutting shredder and a subsequent three-stage grinding process in the ceramic ball mill allows for manufacturing particles with sizes down to 125 µm [35]. However, the two-stage PCBs crushing into a rotary cutting shredder down to 3.35 mm, then size reduction down to about 1 mm in a four-bladed rotary cutting shredder, and final grounding with the help of an ultra-centrifugal mill (Retsch ZM 200) allowed the production of particles with sizes up to 250 µm prior use for leaching tests [36]. At the same time, the PCBs ham-mer-crushing and grounding with the help of an ultra-centrifugal mill (Retsch ZM 200) provides the production of particles with fraction sizes of 4mm – 212 μm) [37]. Finally, the grounding of PCBs, preliminarily cut into 2 mm pieces, with the help of an LM1-M ring mill (LabTechnics Australia, Victoria, Australia), allowed the production of particles sieved into <365 μm, 365–500 μm, and 500–750 μm with the help of Retch AS200 control sieve shaker [38]. All these methods characterize an inefficient multi-step milling ap-proach with high-energy and human workload consumption.”

Related references:

35. Arslan, V. Bacterial Leaching of Copper, Zinc, Nickel and Aluminum from Discarded Printed Circuit Boards Using Aci-dophilic Bacteria. Journal of Material Cycles and Waste Management 2021, 23, 2005–2015, doi:10.1007/s10163-021-01274-9.
36. Yazici, E.Y.; Deveci, H. Extraction of Metals from Waste Printed Circuit Boards (WPCBs) in H2SO4–CuSO4–NaCl Solutions. Hydrometallurgy 2013, 139, 30–38, doi:10.1016/j.hydromet.2013.06.018.
37. Sahin, M.; Akcil, A.; Erust, C.; Altynbek, S.; Gahan, C.S.; Tuncuk, A. A Potential Alternative for Precious Metal Recovery from E-Waste: Iodine Leaching. Separation Science and Technology 2015, 150629132750004, doi:10.1080/01496395.2015.1061005.
38. van Yken, J.; Cheng, K.Y.; Boxall, N.J.; Sheedy, C.; Nikoloski, A.N.; Moheimani, N.R.; Kaksonen, A.H. A Comparison of Methods for the Characterisation of Waste-Printed Circuit Boards. Metals (Basel) 2021, 11, 1935, doi:10.3390/met11121935.

2）Reference 12 needs revision. It is not clear if the paper (chapter of book) was published in 2022 or 2021, as conflicting information is available and DOI could be added.

Retreatment of Polymer Wastes by Disintegrator Milling

Priit Kulu and Dmitri Goljandin - September 2021

In: Waste Material Recycling in the Circular Economy - Challenges and Developments DOI: http://dx.doi.org/10.5772/intechopen.99715

but

Dmitri Goljandin & Priit Kulu, 2022. "Retreatment of Polymer Wastes by Disintegrator Milling," Chapters, in: Dimitris S. Achilias (ed.), Waste Material Recycling in the Circular Economy - Challenges and Developments,IntechOpen.

The correct order of authors is: Priit Kulu and Dmitri Goljandin

Answer: Thank you for your indication. Unfortunately, DOI database contain errors. We are using Mendeley Cite in which we corrected and added all required fields. It is chapter of book published in 2021. Mendeley Cite with MDPI citation style doesn’t show DOI number in reference for book chapter. It is possible that the error reappears if the references will be updated on another computer.

The corrected reference:

“12.        Kulu, P.; Goljandin, D. Retreatment of Polymer Wastes by Disintegrator Milling. In Waste Material Recycling in the Circular Economy - Challenges and Developments; Achilias, D.S., Ed.; IntechOpen: Thessaloniki, 2021; pp. 1–23 ISBN 978-1-83969-681-7.”

3）Some notations are not very clear, e.g., X1(>2.8)+X2. In particular, what doesX2 denote? From Figure 3, it seems that an amount X of PCBs fragments is treated in the first disintegration operation, leading to an amount X1 of fraction < 2.8 mm. The remaining part, i.e., X-X1 is treated in the second disintegration operation. If correct, why not use, X, X-X1 (maybe X-X1 = X2?). In all cases, this should be clarified. 2

Answer: Dear reviewer, the “X2” means second milling; “X1” correspondently FIRST milling, but “X1(>2.8)+X2” fraction < 2.8 mm from first milling (X1) which is subjected to the second milling. We will be appreciated if we could keep original designation. No changes re made in the text.

4）In the continuation of the preceding question, could the authors indicate the values of X and X1 in mass (g or kg)? This would help to understand the scale of the study.

Answer: Thank you for your question. The authors used 6 kg of computer motherboards produced by the GIGA-BYTE Technology Co., Ltd. The wastes of PCBs are variable e-wastes in terms of different compositions and metal contents which also changes through the times and can be different for the same PCBs model made by one company in different countries. Therefore, there is not any publicly available database which would exactly define the exact element outcomes from recycling randomly mixed PCBs. Therefore, researchers must find most efficient approaches for recovering precious elements with possible higher probability from randomly collected e-scrap. The present article provides one of the inputs in such data base.

The clarification about used PCBs manufacturer and total weight is written in lines 191-195 (2.1. Used materials):

“The article's authors used PCBs waste consisting of disassembled personal computer motherboards (produced by the GIGA-BYTE Technology Co., Ltd., Taiwan, from 2010 to 2015) without central processors. An operator cut a total of 6 kg PCBs into rectangu-lar-shaped pieces with side lengths from about three up to 6 cm and subjected them to disintegration-milling experiments, as shown in Figure 1.”

5）Figures 1 and 3 shows that the PCBs are cut into fragments before disintegration. Could the authors indicate the average size of these fragments?

Answer: Thank you for your question. The PCB cutting scheme for all PCB (it was one type of the mother boards was used) is presented on the figure 1 where is placed measure. Materials and methods part will be modified with new text: “Before the feeding PCB for the disintegration it was manually cut for pieces from ~ 3x3 cm up to 6x6 cm as shown on figure 1.”

The clarification about sizes of PCBs pieces is written in lines 193-195 (2.1. Used materials):

“The article's authors used PCBs waste consisting of disassembled personal computer motherboards (produced by the GIGA-BYTE Technology Co., Ltd., Taiwan, from 2010 to 2015) without central processors. An operator cut a total of 6 kg PCBs into rectangular-shaped pieces with side lengths from about three up to 6 cm and subjected them to disintegration-milling experiments, as shown in Figure 1.”

6）Could the authors confirm that what is plotted on the y-axis of Figure 7 is the relative mass of each considered fraction (e.g., 40% in weight of X1 for >11.2 mm)? If positive, “Concentration of fraction, weight %” could be renamed “Amount of each fraction, weight %”. It would be also useful to indicate in the caption of the figure the absolute values of X1 and X1(>2.8)+X2 (i.e., in g or kg).

Answer: Thank you for your indications. The Y axis labelling is changed to the “Amount of fraction, wt. %”. The total weight of each tested fraction is already mentioned in line 247 “…the powder sample's weight of 500 mg (+/- 0.5 mg).”

7）The experimental results presented clearly show an enrichment in metals in specific fractions. However, from these results, it is difficult to evaluate the potentiality of the proposed approach to recover the metals contained in the PCBs. The information is too scattered in Tables and Figures. To help the reader, could the authors give some concrete examples of separation corresponding to plausible scenarios. Indeed, from Table 3 and Figures 4 and 7, it is difficult to see the efficiency of the recovery of metals. For instance, Figure 3 could be completed with two or three examples illustrating the protocol of separation. This would advantageously complete the last paragraph of the conclusion.

Answer: Dear Reviewer, the aim ff this work is not to investigate the efficiency of the recovery of metals but investigate metal content in certain fractions after PCB pre-treatment by using disintegration system. No changes are made in the text.

8）In further studies, the authors could consider the fate of metals such as tantalum (contained in capacitors) which are lost in the slag when the PCBs are treated by pyrometallurgy in copper metallurgy. It is likely that tantalum accumulates in the finest particles.

Answer: Thank You for your valuable suggestion. In the future studies we will pay additional attention to the tantalum content.

9) ref[38] GOLJANDIN, D.; SARJAS, H.; KULU, P.; KÄ ERDI, H.; MIKLI, V. Metal-Matrix Hardmetal/Cermet Reinforced Composite 501 Powders for Thermal Spray. Materials Science 2012, 18, 84–89, doi:10.5755/j01.ms.18.1.1348.

  ref [40]GOLJANDIN, D.; SARJAS, H.; KULU, P.; KÄ ERDI, H.; MIKLI, V. Metal-Matrix Hardmetal/Cermet Reinforced Composite 505 Powders for Thermal Spray. Materials Science 2012, 18, doi:10.5755/j01.ms.18.1.1348.

Citation 38 and 40 are the same and correct it

Answer: Thank You. Reference is corrected.

Reviewer 3 Report

See enclosed PDF report

Comments for author File: Comments.pdf

Author Response

This paper reports an interesting study about the mechanical pre-treatment of Printed Circuit Boards (PCBs). This pre-treatment consists in a rough cutting of the PCBs followed by (i) a first operation of shredding/grinding, (ii) the recovery of the fragments/particles < 2.8 mm, (iii) a second operation of shredding/grinding for the fragments/particles > 2.8 mm. The driving idea of such a pre-treatment is to obtain sieved fractions with an increased content of recovered valuable metals (in fact in the finest particles). From this point of view and in the frame of a circular economy, the subject is of high interest. Furthermore, the paper is rather well documented (41 references) and presented. However, it suffers from a series of weaknesses. As a result, the reviewer recommends its acceptance for publication in Metals after moderate revision as stated below:

1）The state-of-the-art is well documented. However, to complete the overview, the method of micronisation (typically grinding in the range 20 – 100 µm), based only on grinding / gravity and magnetic separations and leading to fractions enriched with valuable metals, could be cited. Method for processing and removing electronic waste with a view to recovering the components included in such waste. WO 2016/042469 A1. https://patentimages.storage.googleapis.com/45/af/ff/e70226a3e124b7/WO2016042469 A1.pdf

Answer: Thank you for your valuable evaluation. It's nice to hear such an assessment and we did our best to improve our article according to comments and objective criticism. We have also corrected small style and grammar errors without changing the meaning of the sentences throughout the article regarding the comments and suggestions made by other reviewers.

We have added the additional thesis about the state-of-the-art approaches regarding your suggestion

“A 3.25 mm fraction particle production from PCBs with the help of a rotary cutting shredder and a subsequent three-stage grinding process in the ceramic ball mill allows for manufacturing particles with sizes down to 125 µm [35]. However, the two-stage PCBs crushing into a rotary cutting shredder down to 3.35 mm, then size reduction down to about 1 mm in a four-bladed rotary cutting shredder, and final grounding with the help of an ultra-centrifugal mill (Retsch ZM 200) allowed the production of particles with sizes up to 250 µm prior use for leaching tests [36]. At the same time, the PCBs ham-mer-crushing and grounding with the help of an ultra-centrifugal mill (Retsch ZM 200) provides the production of particles with fraction sizes of 4mm – 212 μm) [37]. Finally, the grounding of PCBs, preliminarily cut into 2 mm pieces, with the help of an LM1-M ring mill (LabTechnics Australia, Victoria, Australia), allowed the production of particles sieved into <365 μm, 365–500 μm, and 500–750 μm with the help of Retch AS200 control sieve shaker [38]. All these methods characterize an inefficient multi-step milling ap-proach with high-energy and human workload consumption.”

Related references:

35. Arslan, V. Bacterial Leaching of Copper, Zinc, Nickel and Aluminum from Discarded Printed Circuit Boards Using Aci-dophilic Bacteria. Journal of Material Cycles and Waste Management 2021, 23, 2005–2015, doi:10.1007/s10163-021-01274-9.
36. Yazici, E.Y.; Deveci, H. Extraction of Metals from Waste Printed Circuit Boards (WPCBs) in H2SO4–CuSO4–NaCl Solutions. Hydrometallurgy 2013, 139, 30–38, doi:10.1016/j.hydromet.2013.06.018.
37. Sahin, M.; Akcil, A.; Erust, C.; Altynbek, S.; Gahan, C.S.; Tuncuk, A. A Potential Alternative for Precious Metal Recovery from E-Waste: Iodine Leaching. Separation Science and Technology 2015, 150629132750004, doi:10.1080/01496395.2015.1061005.
38. van Yken, J.; Cheng, K.Y.; Boxall, N.J.; Sheedy, C.; Nikoloski, A.N.; Moheimani, N.R.; Kaksonen, A.H. A Comparison of Methods for the Characterisation of Waste-Printed Circuit Boards. Metals (Basel) 2021, 11, 1935, doi:10.3390/met11121935.

2）Reference 12 needs revision. It is not clear if the paper (chapter of book) was published in 2022 or 2021, as conflicting information is available and DOI could be added.

Retreatment of Polymer Wastes by Disintegrator Milling

Priit Kulu and Dmitri Goljandin - September 2021

In: Waste Material Recycling in the Circular Economy - Challenges and Developments DOI: http://dx.doi.org/10.5772/intechopen.99715

but

Dmitri Goljandin & Priit Kulu, 2022. "Retreatment of Polymer Wastes by Disintegrator Milling," Chapters, in: Dimitris S. Achilias (ed.), Waste Material Recycling in the Circular Economy - Challenges and Developments,IntechOpen.

The correct order of authors is: Priit Kulu and Dmitri Goljandin

Answer: Thank you for your indication. Unfortunately, DOI database contain errors. We are using Mendeley Cite in which we corrected and added all required fields. It is chapter of book published in 2021. Mendeley Cite with MDPI citation style doesn’t show DOI number in reference for book chapter. It is possible that the error reappears if the references will be updated on another computer.

The corrected reference:

“12.        Kulu, P.; Goljandin, D. Retreatment of Polymer Wastes by Disintegrator Milling. In Waste Material Recycling in the Circular Economy - Challenges and Developments; Achilias, D.S., Ed.; IntechOpen: Thessaloniki, 2021; pp. 1–23 ISBN 978-1-83969-681-7.”

3）Some notations are not very clear, e.g., X1(>2.8)+X2. In particular, what doesX2 denote? From Figure 3, it seems that an amount X of PCBs fragments is treated in the first disintegration operation, leading to an amount X1 of fraction < 2.8 mm. The remaining part, i.e., X-X1 is treated in the second disintegration operation. If correct, why not use, X, X-X1 (maybe X-X1 = X2?). In all cases, this should be clarified. 2

Answer: Dear reviewer, the “X2” means second milling; “X1” correspondently FIRST milling, but “X1(>2.8)+X2” fraction < 2.8 mm from first milling (X1) which is subjected to the second milling. We will be appreciated if we could keep original designation. No changes re made in the text.

4）In the continuation of the preceding question, could the authors indicate the values of X and X1 in mass (g or kg)? This would help to understand the scale of the study.

Answer:

Thank you for your question. The authors used 6 kg of computer motherboards produced by the GIGA-BYTE Technology Co., Ltd. The wastes of PCBs are variable e-wastes in terms of different compositions and metal contents which also changes through the times and can be different for the same PCBs model made by one company in different countries. Therefore, there is not any publicly available database which would exactly define the exact element outcomes from recycling randomly mixed PCBs. Therefore, researchers must find most efficient approaches for recovering precious elements with possible higher probability from randomly collected e-scrap. The present article provides one of the inputs in such data base.

The clarification about used PCBs manufacturer and total weight is written in lines 191-195 (2.1. Used materials):

“The article's authors used PCBs waste consisting of disassembled personal computer motherboards (produced by the GIGA-BYTE Technology Co., Ltd., Taiwan, from 2010 to 2015) without central processors. An operator cut a total of 6 kg PCBs into rectangu-lar-shaped pieces with side lengths from about three up to 6 cm and subjected them to disintegration-milling experiments, as shown in Figure 1.”

5）Figures 1 and 3 shows that the PCBs are cut into fragments before disintegration. Could the authors indicate the average size of these fragments?

Answer: Thank you for your question. The PCB cutting scheme for all PCB (it was one type of the mother boards was used) is presented on the figure 1 where is placed measure. Materials and methods part will be modified with new text: “Before the feeding PCB for the disintegration it was manually cut for pieces from ~ 3x3 cm up to 6x6 cm as shown on figure 1.”

The clarification about sizes of PCBs pieces is written in lines 193-195 (2.1. Used materials):

“The article's authors used PCBs waste consisting of disassembled personal computer motherboards (produced by the GIGA-BYTE Technology Co., Ltd., Taiwan, from 2010 to 2015) without central processors. An operator cut a total of 6 kg PCBs into rectangular-shaped pieces with side lengths from about three up to 6 cm and subjected them to disintegration-milling experiments, as shown in Figure 1.”

6）Could the authors confirm that what is plotted on the y-axis of Figure 7 is the relative mass of each considered fraction (e.g., 40% in weight of X1 for >11.2 mm)? If positive, “Concentration of fraction, weight %” could be renamed “Amount of each fraction, weight %”. It would be also useful to indicate in the caption of the figure the absolute values of X1 and X1(>2.8)+X2 (i.e., in g or kg).

Answer: Thank you for your indications. The Y axis labelling is changed to the “Amount of fraction, wt. %”. The total weight of each tested fraction is already mentioned in line 247 “…the powder sample's weight of 500 mg (+/- 0.5 mg).”

7）The experimental results presented clearly show an enrichment in metals in specific fractions. However, from these results, it is difficult to evaluate the potentiality of the proposed approach to recover the metals contained in the PCBs. The information is too scattered in Tables and Figures. To help the reader, could the authors give some concrete examples of separation corresponding to plausible scenarios. Indeed, from Table 3 and Figures 4 and 7, it is difficult to see the efficiency of the recovery of metals. For instance, Figure 3 could be completed with two or three examples illustrating the protocol of separation. This would advantageously complete the last paragraph of the conclusion.

Answer: Dear Reviewer, the aim ff this work is not to investigate the efficiency of the recovery of metals but investigate metal content in certain fractions after PCB pre-treatment by using disintegration system. No changes are made in the text.

8）In further studies, the authors could consider the fate of metals such as tantalum (contained in capacitors) which are lost in the slag when the PCBs are treated by pyrometallurgy in copper metallurgy. It is likely that tantalum accumulates in the finest particles.

Answer: Thank You for your valuable suggestion. In the future studies we will pay additional attention to the tantalum content.

Reviewer 4 Report

The presented study is devoted to an unconditionally important and relevant topic - improving the processing of electronic waste (first of all, we are talking about the processing of electronic waste using the example of a computer motherboard). The problem of processing such complex composite wastes still does not have a satisfactory solution, and new research in this area can only be welcomed.

The authors in their work obtained new data on how the electronic waste shredding mode affects the efficiency of the separation and recycling process.

In my opinion, the work can be published after correcting some problems with the design (in particular, the list of references should be done more uniformly).

Also, in my opinion, it is necessary to explain in more detail how the authors ensured the possibility of comparing the results of shredding under different modes, given that the motherboards they used were probably made at different times and by different manufacturers, which undoubtedly affected their composition and mechanical characteristics, and in laboratory conditions, it is impossible to take such a quantity of waste so that fluctuations in their composition are sufficiently leveled.

Author Response

The presented study is devoted to an unconditionally important and relevant topic - improving the processing of electronic waste (first of all, we are talking about the processing of electronic waste using the example of a computer motherboard). The problem of processing such complex composite wastes still does not have a satisfactory solution, and new research in this area can only be welcomed.

The authors in their work obtained new data on how the electronic waste shredding mode affects the efficiency of the separation and recycling process.

In my opinion, the work can be published after correcting some problems with the design (in particular, the list of references should be done more uniformly).

Answer: Thank you for your evaluation. It's nice to hear such an assessment and we did our best to improve our article according to comments and objective criticism. We have also corrected small style and grammar errors without changing the meaning of the sentences throughout the article regarding the comments and suggestions made by other reviewers.

The references were made using Mendeley citation system employing the MDPI citation style (recommended by the MDPI). Unfortunately, for some reason the Mendeley software ignores authors corrections in case of references with doi numbers and takes the information with errors in the doi database. Therefore, final version differs on authors and reader computers.

Also, in my opinion, it is necessary to explain in more detail how the authors ensured the possibility of comparing the results of shredding under different modes, given that the motherboards they used were probably made at different times and by different manufacturers, which undoubtedly affected their composition and mechanical characteristics, and in laboratory conditions, it is impossible to take such a quantity of waste so that fluctuations in their composition are sufficiently leveled.

Answer: The authors used 6 kg of computer motherboards produced by the GIGA-BYTE Technology Co., Ltd. The wastes of PCBs are variable e-wastes in terms of different compositions and metal contents which also changes through the times and can be different for the same PCBs model made by one company in different countries. Therefore, there is not any publicly available database which would exactly define the exact element outcomes from recycling randomly mixed PCBs. Therefore, researchers must find most efficient approaches for recovering precious elements with possible higher probability from randomly collected e-scrap. The present article provides one of the inputs in such data base.

Round 2

Reviewer 3 Report

The authors have improved their manuscript which can now be accepted for publication in Metals