Round 1

Reviewer 1 Report

The manuscript “Recovery, Recycle and Reusability of Critical Raw Materials from Waste Electric and Electronic Equipment: A Review” is a review article that highlights current technological advances in the recovery of rare earth metals from manufactured products such as magnets, solar PV and electronics. Considering the timeliness and rarity of these materials and their need, the paper is relevant, important and interesting. Overall the paper is very well written and organized and should be a valuable addition to the journal Metals. Thus I recommend publishing with some minor suggestions.

 

The article could benefit from a brief section describing what the current state of recycling of rare earth metals is. For example, what percentage of these devices currently gets recycled to recover the material and how much ends up in the landfill? The guess here is that it is probably not high since the article itself is about recycling which will have huge benefits.

 

I feel like the article heavily overuses abbreviations and acronyms which at times detracts from the readability of the paper. For example, there is little benefit by abbreviating “cell phone” as CP, and though it is tedious to type the former so many times, as a reader, it becomes annoying to constantly try to keep these acronyms in order, especially when defined way above in the article. I would recommend keeping only the very frequently used ones. Another example, line 88 PCB in the header… A chemist will read that as polychloro biphenyls even though its previously been defined as printed circuit boards. These are just a couple of examples of this, there are many more.

 

Line 522 has some ambiguous reference to 90 C K (I believe the K is unnecessary)

 

Line 543 the word manifold should probably read many-fold.

 

Section 4 on Additive Manufacturing seems out of place in this article and unnecessary. The article goes from an in-depth survey of recycling processes for these technologies to the concept of 3D printing and using the products of recycling as additives. This article does not benefit from having this discussion at all. The materials produced by these methods have clear applications in the same industries and products from which they came. The authors do not need to “sell” these products in another way. This abrupt change in direction is distracting and not needed. I recommend removing it from this article.

Author Response

We thank the reviewer for the kind comments that helped improve our manuscript.

The manuscript “Recovery, Recycle and Reusability of Critical Raw Materials from Waste Electric and Electronic Equipment: A Review” is a review article that highlights current technological advances in the recovery of rare earth metals from manufactured products such as magnets, solar PV and electronics. Considering the timeliness and rarity of these materials and their need, the paper is relevant, important and interesting. Overall the paper is very well written and organized and should be a valuable addition to the journal Metals. Thus, I recommend publishing with some minor suggestions.

 

The article could benefit from a brief section describing what the current state of recycling of rare earth metals is. For example, what percentage of these devices currently gets recycled to recover the material and how much ends up in the landfill? The guess here is that it is probably not high since the article itself is about recycling which will have huge benefits.

 

The following text has been incorporated in the Introduction:

The current recycling rate of REE is extremely low (only 2% of REEs are recovered by recycling processes against 90% of iron and steel) [1]. Despite their low recycling rates, REE are expected to be used even more in the near future, as their demand grows rapidly. WEEE, batteries and magnets represent a significant opportunity for REE supply chain balance. According to Patil et al [1], at least 10% of REE used in batteries and magnets and 17% in phosphor lighting could be recovered through recycling processes. Finally, around 10 tons of Tb and 230 tons of Nd (both elements are short in supply) are estimated to be recoverable from WEEE streams every year [1].

In the study of Gutiérrez-Gutiérrez et al [2], landfill sites were analyzed and it was observed that REE concentrations did not differ significantly. Ce was the most abundant rare metal with a mean concentration of 17 mg/kg of waste. The concentration of Nd ranged between 8.5 and 12 mg/kg and La between 7 and 10 mg/kg. Significant quantities of Cu and Al (mean concentration of 1500 and 15000 mg/kg respectively) and less quantities of Au and Ag (mean concentration of 0.15 and 3.5 mg/kg respectively) were also observed. The mobility analysis of the critical metals showed that they are not being vertically transported since concentrations remained similar through the whole range of depth in the landfills [2].

 

 

[1]      A. B. Patil, V. Paetzel, R. P. W. J. Struis, and C. Ludwig, “Separation and Recycling Potential of Rare Earth Elements from Energy Systems: Feed and Economic Viability Review,” Separations, vol. 9, no. 3, pp. 1–15, 2022.

[2]      S. C. Gutiérrez-Gutiérrez, F. Coulon, Y. Jiang, and S. Wagland, “Rare earth elements and critical metal content of extracted landfilled material and potential recovery opportunities,” Waste Manag., vol. 42, no. May, pp. 128–136, 2015.

 

I feel like the article heavily overuses abbreviations and acronyms which at times detracts from the readability of the paper. For example, there is little benefit by abbreviating “cell phone” as CP, and though it is tedious to type the former so many times, as a reader, it becomes annoying to constantly try to keep these acronyms in order, especially when defined way above in the article. I would recommend keeping only the very frequently used ones. Another example, line 88 PCB in the header… A chemist will read that as polychloro biphenyls even though its previously been defined as printed circuit boards. These are just a couple of examples of this, there are many more.

Done
A list of symbols has been added after the introduction and the titles (e.g. PCB became Printed Circuit Boards)

 

Line 522 has some ambiguous reference to 90 C K (I believe the K is unnecessary)

Done

Indeed, it was

 

Line 543 the word manifold should probably read many-fold.

Done

 

Section 4 on Additive Manufacturing seems out of place in this article and unnecessary. The article goes from an in-depth survey of recycling processes for these technologies to the concept of 3D printing and using the products of recycling as additives. This article does not benefit from having this discussion at all. The materials produced by these methods have clear applications in the same industries and products from which they came. The authors do not need to “sell” these products in another way. This abrupt change in direction is distracting and not needed. I recommend removing it from this article.

 

The scope of the present review is the reusability of critical raw materials (critical metals and REE) and nonmetal raw materials, such as glass, found in the main components of WEEE, in applications, employed by new, sustainable technologies, such as Additive Manufacturing, that can lead the way to future breakthroughs. The introduction of the aforementioned raw materials, as additives to filaments used for the synthesis of composite materials with the ability to fabricate architectures that are impossible to fabricate through conventional processing techniques, has tremendous potential for the performance and the commercialization of the final products by adding unique characteristics, such as antibacterial properties, enhanced mechanical and magnetic properties, thermal and electrical conductivity.

Furthermore, the existing expertise in the composition and characterization of composite materials and 3D printing solutions for the modern industry, as well as the infrastructure with the latest technology equipment, make the Physical Metallurgy Laboratory of the Department of Mechanical Engineering of AUTh an excellent candidate for the implementation of such new technologies. Moreover, as the title of the paper mirrors part of the doctoral dissertation of one of the co-authors (V. Stratiotou-Efstratiadis), which will highlight the optimal protocol for the production of sustainable, innovative and complex 3D printed materials, whose research and commercial value will be evaluated, with more results to be published in scientific journals and conferences, we strongly believe that the content of Chapter 4 needs to remain in the manuscript.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript deals with the very important topic of recycling valuable metals and materials from WEEE, and the in general circularity of WEEE. The review highlights the latest advancements in the field with some chosen components very well. The discussion is based on the latest references and critically addresses the strengths and weaknesses of the suggested processes in the literature. The manuscript is well organized and written with very good English. Therefore I warmly recommend to publish the review in the Metals journal after the minor revisions I have attached.

 

The comments and suggestions:

 

Title: I suggest making it more definitive to describe what is the novelty in this review compared to the other reviews made in CRMs recycling from WEEE. You also have chosen certain cases for discussion and the title could reflect that. Perhaps something related to REE, permanents magnets, PCBs and metals recovered from them, PVs and metals recovered from then, and/or where there recycled materials are used?

 

Introduction:

 

"WEEE contains large quantities of metals and Rare Earth Elements (REE) 34

that " --> metals, for example REE, that…

 

"significant economic benefits" --> economic and societal benefits

 

"Iron and steel constitute about 50% of the waste" --> 50 wt.-%?

 

"the resulting low purity leads to their costly extraction" --> mention also that major share of the REE processing costs come from the separation of the REE from each other because they have similar chemical and physical properties.

 

There are quite many acronyms used, and I understand that it makes sense to make the text easier to read because they are used often. Perhaps the manuscript could benefit from list of symbols?

 

2.1

 

"recyclable metals and REE, such as Cu, Al, Pb, Au, Ag, Pt, Pd, Nd, La, Ce" --> recyclable metals (Cu, Al, Pb, Au, Ag, Pt, Pd) and REE(such as Nd, La, Ce).

 

"a type of secondary mining of precious REE" --> these precious metals and REE

 

Table 5.: Isn't there younger pricing information for the metals?

 

Subheadings in Section 2: I suggest using full names in the headings.

 

2.3: "discovered by General Motors and Sumitomo" --> developed by…?

 

3.1.: "Therefore, in their study [73], five different acids, i.e., HCl, HNO3, H2SO4, acetic acid 255

(C2H4O2) and citric acid (C6H8O7) are used" --> In the study of XXXXX et al. [] (new text paragraph)…were used…

 

Fig.1: Metal removal --> Metal recovery or Product recovery

 

"WPCB content is dissolved" --> metals are leached, but the entire PCB is not probably dissolved?

 

What are the purities of the products coming from the Behnamfard process?

 

"According to the study of Xiao et al [75], the melting of 0.1-8 mm WEEE is performed " --> I would only put this to present tense if the process is actually in use somewhere. This applies to many of the text paragraphs in this section.

 

"Zn, Pb and Ni can be further recovered using selective solvent extraction" --> This is what you mean, right?

 

Fig. 3 caption --> ammonia and thiourea leaching

 

3.2.:

"The profit from the glass recycling is 0.02 $/W" --> What is the profit of the metals? I.e. you should extent the discussion to overall profitability of the suggested processes? Or this the overall value of the recovered materials meaning that the recycling process is not economically feasible? There is the discussion in the First Solar case, but should it be also here?

 

"To further separate REE from the residues of the aforementioned methods" --> you should shortly open up which REEs are present and what are their concentrations.

 

"The flotation tests aim at enriching the residue with Te and In from the CIGS thin film." --> but how about the REEs you first advertised?

 

"The Cd and Te containing solution is directed to the Cd and Te recovery circuit. The same procedure is followed for In. " --> it would be nice to hear about this recovery circuit.

 

"Finally, NaOH is added to the solution to adjust the pH above 12, while a high-performance extractant is used to remove Cu in the form of CuSO4" --> what is the extractant?

 

3.3.:

"The pure REE-loaded leachate " --> How pure and what are the impurities?

 

"Finally, high purity Nd, Pr and Dy oxides from the leach solution can be recovered, using solvent extraction and separation, followed by precipitation [91]. " --> …can be separated with solvent extraction, followed…

 

In section 4 it would be nice to play with some numbers regarding the amount of possibly recycled REEs and other metals and materials (glass) and how much are they needed in these 3D printing processes.

 

5 Conclusions

 

Could you give some generalization which kind of separation technologies and processes are used in state-of-the-art?

Author Response

We thank the reviewer for the kind comments that helped improve our manuscript.

Title: I suggest making it more definitive to describe what is the novelty in this review compared to the other reviews made in CRMs recycling from WEEE. You also have chosen certain cases for discussion   and the title could reflect that. Perhaps something related to REE, permanents magnets, PCBs and metals recovered from them, PVs and metals recovered from then, and/or where there recycled materials are used?

The tittle changed to Sustainable Recovery, Recycle of Critical Metals and Rare Earth Elements from Waste Electric and Electronic Equipment (circuits, solar, wind) and their Reusability in Additive Manufacturing Applications: A Review

Introduction:

"WEEE contains large quantities of metals and Rare Earth Elements (REE) 34 that " --> metals, for example REE, that…

done

"significant economic benefits" --> economic and societal benefits
done

"Iron and steel constitute about 50% of the waste" --> 50 wt.-%?
yes and done

"the resulting low purity leads to their costly extraction" --> mention also that major share of the REE processing costs come from   the separation of the REE from each other because they have similar chemical and physical properties.

done

There are quite many acronyms used, and I understand that it makes sense to make the text easier to read because they are used often.  Perhaps the manuscript could benefit from list of symbols?

done

2.1

"recyclable metals and REE, such as Cu, Al, Pb, Au, Ag, Pt, Pd, Nd, 
La, Ce" --> recyclable metals (Cu, Al, Pb, Au, Ag, Pt, Pd) and 
REE(such as Nd, La, Ce).
done

"a type of secondary mining of precious REE" --> these precious metals and REE
done

Table 5.: Isn't there younger pricing information for the metals?

There is no consistency on the newest pricing (no data for many elements), so it’s the most recent date, remaining consistent for every element

Subheadings in Section 2: I suggest using full names in the headings.
done

2.3: "discovered by General Motors and Sumitomo" --> developed by…?
done…

As part of the Hitachi Corporation, Sumitomo Special Metals developed and currently manufactures and licenses other companies to produce full-density sintered Nd2Fe14B magnets and holds more than 600 patents covering Neodymium magnets [69].

3.1.: "Therefore, in their study [73], five different acids, i.e.,   HCl, HNO3, H2SO4, acetic acid 255 (C2H4O2) and citric acid (C6H8O7) are used" --> In the study of XXXXX   et al. [] (new text paragraph)…were used…

done…

Fig.1: Metal removal --> Metal recovery or Product recovery

done…

"WPCB content is dissolved" --> metals are leached, but the entire PCB 
is not probably dissolved?

Yes. Changed to leached
What are the purities of the products coming from the Behnamfard process?

done….The following text has been added.

Their method presents high efficiency (more than 99 % recovery for Cu, 90 % for Ag and Au and 100 % for Pd), low cost and process time and is environmentally friendly. In their method, the grounded (less than 300 μm) WPCB content was leached by using two consecutive H2SO4 leaching steps in the presence of hydrogen peroxide (H2O2) as oxidizing agents. The solid residue of the first leaching was subjected to a second leaching step and the solid residue of the second leaching was treated by acidic thiou-rea (SC(NH2)2) in the presence of ferric iron (Fe+3) as oxidizing agent. The Cu recovery during the 2nd leaching step was more than 99 %. The precipitation of Au and Ag from acidic thiourea leachate was optimized by using specific amount of sodium borohy-dride (NaBH4) as a reducing agent, reaching 84 and 71 % respectively in the 3rd step. Finally, the leaching of Pd and remaining Au from the solid residue of the third leach-ing step was performed in a (Sodium hypochlorite-) NaClO-HCl-H2O2 leaching system and the precipitation was optimized by using specific amount of sodium borohydride NaBH4 [75].

"According to the study of Xiao et al [75], the melting of 0.1-8 mm 
WEEE is performed " --> I would only put this to present tense if the 
process is actually in use somewhere. This applies to many of the text 
paragraphs in this section.

Corrected throughout the section
"Zn, Pb and Ni can be further recovered using selective solvent 
extraction" --> This is what you mean, right?

Yes and done

Fig. 3 caption --> ammonia and thiourea leaching

done

3.2.:
"The profit from the glass recycling is 0.02 $/W" --> What is the 
profit of the metals?  I.e. you should extent the discussion to overall 
profitability of the suggested processes? Or this the overall value of 
the recovered materials meaning that the recycling process is not 
economically feasible? There is the discussion in the First Solar 
case, but should it be also here?

In the study of Sasala et al, there is no data available for the profit of the recycle of metals.

"The profit from the glass recycling is 0.02 $/W" was deleted

Even at higher cost, they ¹ concluded that the cost was affordable and the recycle process feasible. Replaced it

 

"To further separate REE from the residues of the aforementioned 
methods" --> you should shortly open up which REEs are present and 
what are their concentrations.

it was a typo. There are no REEs and the composition of PV (CdTe and CIGS) is given in Table 4 (176)

"The flotation tests aim at enriching the residue with Te and In from 
the CIGS thin film." --> but how about the REEs you first advertised?

Done…… it was a typo, no REEs but critical metals

To further separate Cd, Te (CdTe PV) Cu, In, Ga, Se (CIGS PV) from the residues

"The Cd and Te containing solution is directed to the Cd and Te 
recovery circuit. The same procedure is followed for In. " --> it 
would be nice to hear about this recovery circuit.

done… changed to:

The Cd and Te containing solution was directed to the Cd and Te recovery circuit, where they could be converted to 99.999% metal with additional purification [84]. The still do not refer to the additional purification

"Finally, NaOH is added to the solution to adjust the pH above 12, 
while a high-performance extractant is used to remove Cu in the form 
of CuSO4" --> what is the extractant?
done…. high-performance Cu extractant (AD-100N). They do not give further information besides this url

https://senowchem.en.made-in-china.com/product/vohEUFuwnHkx/China-High-Performance-Copper-Specific-Extractant-AD-100.html

3.3.:
"The pure REE-loaded leachate " --> How pure and what are the impurities?
done…. The pure REE-loaded (>95%) … (mainly consisted of Dy, Nd, Gd, Pr with minor impurities of B, Nb, Si).

"Finally, high purity Nd, Pr and Dy oxides from the leach solution can 
be recovered, using solvent extraction and separation, followed by 
precipitation [91]. " --> …can be separated with solvent extraction, 
followed…
done…

In section 4 it would be nice to play with some numbers regarding the 
amount of possibly recycled REEs and other metals and materials 
(glass) and how much are they needed in these 3D printing processes.

Done… The following text has been added.

As mentioned, AM technology is well suited for the fabrication of architectures that are impossible to fabricate through conventional processing techniques [96]. Additives and fillers such as critical metals, REE and glass can give novel properties to composites, whilst using sustainably recovered raw materials. Recycle of critical metals such as Cu, is growing exponentially. More specifically, a total of around 8.7 million tons of Cu per year come from the recycling of “old” scrap (EOL products) and “new” scrap (generated during production and downstream manufacturing processes) [119]. Only 1% of the REE are recycled from end-products, with the rest deporting to waste and being removed from the materials cycle [5]. Only in the Nordic countries, the quantity of critical and rare earth metals, such as Au, Ag, In, Ga, Nd, Pd in WEEE, accounted for a total of 148 tons (in 2015) [120], [121]. As for the nonmetal materials, such as glass, latest value chain data show that the average collection for recycling rate for glass packaging grew to the record rate of 78% in 2019 in the EU [122]. The US Glass Packaging Institute (GPI) currently estimates that the container glass and flat glass industry in the US uses a total of 3.35 million tons of recycled glass [123]. The amount of the aforementioned critical raw materials that can be recovered and recycled from WEEE on a global scale and reused in sustainable new technologies, such as AM, can grow exponentially.

 

5 Conclusions
Could you give some generalization which kind of separation 
technologies and processes are used in state-of-the-art?
done…. The following text has been added.

As the need for sustainability during recycle and reuse of raw materials is more significant than ever before, the state-of-the-art processes need to fulfill some major requirements; the flow-chart of the process needs to be composed of environmentally friendly techniques (such as mechanical removal of parts from WEEE) that are less energy- (and subsequently cost-) intensive, and ensure the use of non-hazardous chemicals during every stage. Furthermore, heat treatment of WEEE must be in compliance with safe-by-design protocols of capturing dangerous and toxic emissions.

Author Response File: Author Response.pdf

Reviewer 3 Report

1 Solar cells are not only CdTe and CIGS, but also monocrystalline silicon, monocrystalline silicon, and amorphous silicon solar cells. It is recommended that the recycling of these cells is also described.

2 The content of "Additive Manufacturing" does not seem to be very closely related to the main content of this article, and it is recommended to delete it.

Author Response

We thank the reviewer for the kind comments.

1 Solar cells are not only CdTe and CIGS, but also monocrystalline  silicon, monocrystalline silicon, and amorphous silicon solar cells.  It is recommended that the recycling of these cells is also described.

 

A thorough review of every type of solar panels, along with their respective recycle paths is given below.

1^st generation PVs

Crystalline silicon Photovoltaic panels

1. a) Monocrystalline silicon (mc-Si) is one of the first photovoltaic cell technologies. The cells are composed entirely of monocrystalline silicon with a crystal structure, allowing better displacement of the photo-stimulated electrons ¹. They present very high yields (present technologies exceed 20%). The manufacture of silicon crystals is complex and requires highly purified silicon, leading to high initial costs ². Monocrystalline silicon cells are very resistant to high temperatures and have a significantly longer life cycle, lasting over 25 years. In addition, they work efficiently, even in low sunlight conditions. However, their efficiency decreases gradually (about 0.5% per year), so the modules may need to be replaced sooner. The main disadvantages of monocrystalline silicon panels are their high capital investment and fragility ³.
2. b) Polycrystalline silicon (pc-Si) cells are made by melting bulk silicon, a faster and cheaper process than that used for monocrystalline cells, resulting in a lower cost. Their lifespan is just as long (more than 25 years) and like monocrystalline cells, they can produce more than 80% of their original power after this period) ⁴. Polycrystalline cells show relatively lower yields, of the order of 15%, compared to monocrystalline. They also have lower resistance to high temperatures and need a larger spatial background to achieve the same power generation ⁵.

2^nd generation PVs

Thin film Photovoltaic panels

Thin-film photovoltaic cells are made by placing one or more films of compounds of photosensitive materials that act as semiconductors on a support substrate such as glass, plastic or metala)               Amorphous silicon (a-Si): In this process, less silicon is used for production compared to mono- and polycrystalline cells, due to the size of the films. The production process is easier, but the overall yield is reduced compared to the aforementioned cells (6-10% vs. 15-20%) ⁶. Due to their ease of construction and low weight, it is often possible to build a multi-level, multi-cell structure to increase efficiency. Amorphous silicon can be deposited on a variety of substrates, which can be flexible and available in different shapes with wider applications ⁴. Amorphous silicon is also less prone to overheating than crystalline and CdTe (see below), which reduces the efficiency of solar cells. In comparison, the temperature constant that correlates the increase in temperature with the decrease in efficiency due to overheating is about 0.5% / K for crystalline silicon cells, while for amorphous silicon cells, it appears less than 0.25% / K ¹. The efficiency of an a-Si cell is significantly reduced by 10 - 30% during the first six months of operation, due to fluctuations in photoconductivity and other losses caused by prolonged exposure to sunlight. As a result, its lifespan is around 15 years.

1. b) CdTe…. (Already exist in the review)
2. c) CIGS…. (Already exist in the review)

3^rd generation PVs

Thin film Photovoltaic panels using novel materials

The main goal of third generation photovoltaic cells is to produce high efficiency cells that continue to use thin film (2^nd generation) and / or crystalline silicon (1^st generation) technology ⁷. They aim at making photovoltaic technology more efficient, while reducing costs by using new materials such as organic compounds - polymers, mirrors and magnifiers to collect solar radiation in combination with semiconductor alloys, conductive plastics, nanotubes, quantum tubes, printed electronics, etc. ⁸. Most of these technologies are still at the laboratory stage. It is noted that, due to their limited capacity and use, a description of the first two and main categories will be made, with a simple reference to the rest.  a)               Organic photovoltaic cells (OPV), are composed of organic compounds and can be produced on a large scale. By enhancing specific functions and properties of the photovoltaic cell, the organic cells now show efficiencies of 10%. Continuous research and development on these cells have ensured that the yield and lifespan of more than 10 years are tripled in just 10 years ¹. Their construction shows lower costs than the previous categories, due to the abundance of materials, while other advantages include their low weight, large surface area, the ability of the cells to be transparent and to absorb larger amounts of radiation ²,⁵. Like a-Si cells, OPV can be deposited on a variety of flexible substrates. They are also available in different shapes, finding wider application ⁹,¹⁰. They are prone to photochemical degradation and have a relatively short lifespan (approximately 10 years) ¹¹.b)               Concentrator photovoltaics (CPV), focus sunlight on a cell using a parabolic mirror or magnifying glass. By focusing solar radiation on a small area, photovoltaic materials become extremely efficient, while requiring a smaller amount of raw materials with semiconductor properties. Yields can even reach values ​​above 40% ⁷. They are extremely resistant to overheating, as the temperature constant is about 0.01% / K, which allows them to operate at very high temperatures (above 100^oC). They have a long service life (over 20 years) and are easily recyclable. However, expensive materials are required (multi-junction solar cells that use different combinations of semiconductors – including crystalline silicon and rare earths – such as ternary gallium, indium, arsenic alloy GaInAs ¹², ternary gallium, indium, phosphorus alloy GaInP made from solid solutions of indium phosphide and gallium phosphide ¹³, binary gallium, arsenic alloy GaAs ¹⁴), complex construction, and mainly a mechanism for detecting the sun motion and cell movement in a corresponding orbit ¹⁵. Finally, they frequently need maintenance. c)                Other third generation photovoltaic cells include nanotube cells, perovskite cells, quantum dots, printed electronics, Grätzel cells, etc. Table 1 presents the performance, advantages and disadvantages of each technology.

Table 1 Yield, pros and cons of each solar technology

PV cell technology	Yield	Pros / Cons
mc-Si	≥20 %	+ High efficiency+ High availability+ Long lifespan+ Operation in low sunlight conditions- High price / longer payback time
pc-Si	15-17 %	+ High efficiency+ High availability+ Lower price / shorter payback time+ Long lifespan
a-Si	6-10 %	+ Low price / shorter payback time+ Flexibility+ Lower temperature constant (resistant to overheating)+ Construction of multilevel cell structure - potential improvement- Average lifespan- Frequent maintenance- Low efficiency / longer payback time
CdTe	11-14 %	+ Average efficiency+ Potential improvement+ Lower price due to less raw materials / shorter payback time+ Resistant to overheating- Low availability- Cadmium toxicity
CIGS	14-18 %	+ Average efficiency+ Potential improvement+ Lower price due to less raw materials / shorter payback time- Relatively short lifespan- Frequent maintenance- Low availability- Complexity of alloy manufacture upscaling
OPV	8-11 %	+ Low price / short payback time+ Flexibility+ High availability+ Lower temperature constant (resistant to overheating)- Relatively low efficiency- Small production on an industrial scale- Relatively short lifespan
CPV	>40 %	+ Very high efficiency+ Long lifespan+ Very low temperature constant (resistant to overheating)+ Use of multiple cells with known and innovative materials whose properties and / or cost can be improved- High price / long payback time- Mechanism for detecting the sun movement and the movement of cells in a corresponding orbit- Low efficiency in low sunlight conditions- Frequent maintenance

Necessity of photovoltaic systems recycle

Photovoltaic manufacturers use tellurium, gallium, indium, cadmium and selenium to produce thin-film cells. CdTe (cadmium telluride) cells represent 6-7% of photovoltaic systems worldwide, while CIGS cells account for 2% ¹⁶. Their future is controversial, as their ever-increasing demand will result in the future accumulation of hazardous elements in landfills ¹⁷. On the one hand, the isolation, recovery and recycling of rare earths can have environmental benefits, by decongesting landfills of potentially hazardous materials. On the other hand, their reuse in added value materials, which were mentioned above, following the technological requirements, can lead to significant economic benefits. As the market moves towards 3^rd generation photovoltaic systems, there is a decrease in the use of elements for thin film cells, as shown in Table 2 ¹⁸. The dependence of the industry on China, in terms of tellurium is small, but in terms of indium and gallium is much higher, as China holds 20%, 58% and 69% of the mined quantity of the respective aforementioned elements. Other countries for tellurium import are Japan, Belgium and Sweden, for indium, Germany, Italy and the United Kingdom, and for gallium France ^{19  16}, ²⁰.

Most photovoltaic companies based in Europe are owned by American, Chinese and Japanese companies. This is a significant advantage, as it alleviates industry’s dependency of tellurium, indium and gallium on China ¹⁶.

Table 2 Te, In, Ga requirements in the European Union in 2020 and 2030

Element	Annual requirements in Ε.Ε. (tons) in 2020	Annual requirements in Ε.Ε. (tons) in 2030
Tellurium (Te)	150	126
Indium (In)	145	121
Gallium (Ga)	4	3

The short-term strategy that can be followed, is the establishment of subsidiaries of those companies in China, and the exit to other markets besides China, such as the USA. and Japan.

Recycling hard-to-find or expensive materials (mainly rare earths) is the main, long-term strategy. A typical example is indium, which can be recovered from the post-industrial fragments and waste of CIGS cells. It is expected that the amount of indium recycled from the volumes of photovoltaic systems will increase rapidly, due to the systems at the end of their life cycle that begin to enter the waste stream. Similarly, gallium is already being recovered in Belgium from CIGS industrial waste, but recycling, following the introduction of systems into the waste stream, has not been yet developed ¹⁶. The complexity of the applications of rare earths, the difficulty of distinguishing them during mining and the long life cycle of the final product (PVs) have led to less than 1% recycling of the rare earths ²¹. The recycling of large quantities of rare earths has the potential to significantly contribute to their increasing demand, availability and supply ²².

Table 3 presents the requirements of photovoltaic technology in metals in 2020 and the forecast for 2030 according to ¹⁸.

Table 3 Photovoltaic technology requirements in metals

	Global production in 2010 (thousand tons)	Required quantity (kg) / MW	Global annual requirements in 2020 (thousand tons)	Projection of global annual requirements (thousand tons) in 2030
Tellurium (Te)	0.5	4.7	0.04	0.25
Indium (In)	1.35	4.5	0.04	0.24
Tin (Sn)	261	463.1	4.03	25.01
Silver (Ag)	22	19.2	0.17	1.04
Gallium (Ga)	0.16	0.12	0.001	0.005
Cadmium (Cd)	22	6.1	0.05	0.33
Selenium (Se)	3.25	0.5	0.004	0.026
Copper (Cu)	16200	2194.1	19.09	118.48

Table 4 presents the most recent recorded metal prices and the types of photovoltaic cells to which they apply.

Table 4 Metal prices and their PV cells application

Metals	Price (€/kg) -2021	PV module types that metals find application	Literature
Silver (Ag)	894	c-Si	23
Indium (In)	190-200	a-Si, CIS, CIGS	24,25,26
Gallium (Ga)	200-220	CIGS, CPV and emerging technologies	27
Germanium (Ge)	950-1000	a-Si, CPV and emerging technologies	28
Cadmium (Cd)	2-3	CdTe	29
Tellurium (Te)	90	CdTe	30
Selenium (Se)	20	CIGS	31

Figure 1 presents  the forecast for the annual electrical / electronic waste of photovoltaic systems in Europe until 2050, according to ³.

 

Figure 1 Annual electrical / electronic waste PV in Europe by technology until 2050

Figure 2 presents the cumulative waste volumes of photovoltaic systems at the end of their life cycle, from the five countries that hold the largest share of the solar energy market in 2050, according to the study of ³². There are two scenarios of losses. The regular loss scenario, which assumes a 30-year life cycle for photovoltaic systems without premature wear and attrition, and the early-loss scenario. The latter takes into account the failures in the initial stage of the life cycle, where the probability of structural defects and failures occurrence is high (infantile failures), the failures in the middle of the life cycle, where the probability is relatively small due to the fact that any failure probably has already occurred (mid-life failures) and failures before the end of their life cycle ³², ³³.

 

Figure 2 Cumulative waste volumes of the five main countries for end-of-life PV systems in 2050

pc-Si PV modules Recycle

In Europe, the waste of first-generation photovoltaic systems is expected to reach 33,500 tons by 2040. A silicon photovoltaic unit consists of silicon wafers, metal contacts between each one, supporting layers encapsulating the wafers, front glass and a metal plate or a second glass on the back ³⁴. Regarding the recycling processes of silicon photovoltaic systems, German company Deutche Solar AG, installed a pilot unit in 2004, with the aim of recovering and reusing silicon wafers in new solar cells ³⁵.

In the initial stage of the process, the system is heated in an oven at a suitable temperature for the protection of the semiconductor (600^oC), where the supporting layers are burned and then the wafers, glass and metals are separated manually. Glass and metals, following their respective supply chain, are recycled. The remaining wafers, broken or not, are collected. The broken ones are melted and can be reused to create silicon bars / ingots. The intact ones, after removal of the plating, the anti-reflective coating and the pn junction by chemical etching, are recycled. The refined polycrystalline silicon wafers, which constitute the final product, can be recycled, and re-treated for their use in new a PV system ³⁴.

During the heat treatment, the furnace and the leaching stage consume energy. In addition, quantities of water are consumed during leaching. The outlet streams consist mainly of flue gases of the burner and water waste. During chemical treatment, energy and quantities of chemicals and water are consumed. Outlet streams consist of flue gases and chemical waste, which are discarded. The flow chart, input and output currents are shown in Figure 3 ³⁴, ³⁵. Despite the energy consumption, a system with recycled wafers can reduce EPBT to 1.6 years, from 3.3 years compared to a new system of the same capacity, as shown in Table 5 ³⁴, ³⁵.

 

 

Figure 3 Recycle of pc-Si PV module flow chart

Table 5 Energy requirement & production during production and use of pc-Si PV system (165 Wp power, 72 cells)

	PV w/ new wafers	PV w/ recycled wafers
Si wafer production energy requirement (kWh)	306	-
Recycle energy requirement (kWh)	-	92
Cell process energy requirement (kWh)	49	49
System assembly energy requirement (kWh)	45	45
Total (kWh)	400	186
Annual energy reduction (kWh/year)	120	120
EPBT (year)	3.3	1.6

Granata et al ³⁶ studied the recycling of pc-Si PV by natural methods. After the manual disassembly of the PV system, in order to separate the units from the external frames, approximately 2 kg of the PV module were used as input materials in each test. Specifically, the first physical method involves the crushing of the material by two rotors, followed by heat treatment and the second involves the crushing by two rotors, followed by crushing with a hammer mill. Heating took place at 650^oC for 1 hour. After each method, the materials are sieved and analyzed by XRD and XRF methods ³⁶.According to the 1^st method (crushing, heat treatment - C.HT.), it is calculated by XRD and XRF, that a percentage of 2-3% of the input sample has a diameter of less than 0.08 mm (fine grain), from which zinc, other metals and silicon can be collected, separately recovered and further utilized, as shown in Table 8. The diameter range between 0.08 and 1 mm of the fragments, cannot be considered recoverable glass. For diameters larger than 1 mm, the fragments are considered recoverable glass and thus more than 85% of the mass of the original sample is recovered [56]. According to the 2^nd method (crushing, hammer mill crushing - C.C.), it is calculated through XRD and XRF, that for a diameter range between 1 to 5 mm the fragments are considered recoverable glass and thus more than 70-75% of the mass of the original sample is recovered. The diameter range between 0.08 and 1 mm of the fragments can also be considered recoverable, glass and thus 10% of the mass of the original sample is recovered. Fragments with a diameter smaller than 0.08 mm contain various metals and their oxides, which can be further exploited (Table 6). The 2^nd method allows a higher percentage recovery of the mass of the original sample, as fragments with a diameter between 0.08 and 1 mm are also considered recoverable glass fragments ³⁶.

 

Table 6 Metal composition of fragments with a diameter smaller than 0.08 mm according to the 2 methods used in pc-Si PV module

p-Si	Mg (%)	Al (%)	Si (%)	Ca (%)	Mn (%)	Fe (%)	Ti (%)	Zn (%)	Sn (%)
1st method (C.HT)	0.9	4.7	25.3	3.88	2.17	1.67	1.65	2.13	0.48
2nd method (C.C.)	1.64	1.42	31.7	5.59	0.59	1.51	0.06	0.34	0.08

a-Si Thin Film PV modules Recycle

Granata et al ³⁶, use the same 2 methods mentioned above, for the amorphous silicon. During the 1^st method (crushing, heat treatment), the crushing leads to fragments with a diameter larger than 8 mm, from which a significant amount of aluminum is recovered, and then, after heat treatment, due to burning of the polymer layers, there is a rapid reduction of diameter. For diameters larger than 1 mm, the fragments are considered recoverable glass and thus more than 70-75% of the mass of the original sample is recovered. Through XRD and XRF, it is proved that the fragments with a diameter smaller than 1 mm, which constitute 2-3% of the input sample, contain a significant amount of oxides (zinc oxide - ZnO, titanium oxide - TiO2) and metals, due to melting of the anti-reflective surface and the semiconductor layer as shown in Table 9 which can be further utilized. According to the 2^nd method, it is calculated by XRD and XRF, that for a diameter larger than 1 mm, the fragments are considered recoverable glass and thus more than 10% of the mass of the original sample is recovered. For a diameter range between 0.08 to 1 mm the fragments are considered recoverable glass and thus more than 70% of the mass of the original sample is recovered. Fragments smaller than 0.08 mm in diameter contain various metals and their oxides, which can be further exploited (Table 8). The 2^nd method allows a higher percentage recovery of the mass of the original sample, as fragments with a diameter range between 0.08 and 1 mm are also considered recoverable glass fragments ³⁶. 

Table 8 Metal composition of fragments with a diameter smaller than 0.08 mm according to the 2 methods used in a-Si PV module

a-Si	Mg (%)	Al (%)	Si (%)	Ca (%)	Mn (%)	Fe (%)	Ti (%)	Zn (%)	Sn (%)
1st method (C.HT)	1.48	0.5	24.6	4.3	3.06	1.62	0.62	2.07	0.21
2nd method (C.C.)	1.94	<0.01	28.4	5.04	0.37	2.63	0.03	0.24	0.1

 

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Even though a thorough review of every type of solar cell, along with the respective recycle paths was compiled, a mutual decision of the authors of not including the pc- and a-silicon solar cells in the final review was met based on the following rationale. For the purpose of the review submitted, Si is not considered a critical raw material, as there is an abundance of semiconductors and their recycle paths are covered extensively in the literature. More emphasis was given towards 2^nd generation thin film PV, containing critical metals (Cd, Te, Cu, In, Ga, Se)

2 The content of "Additive Manufacturing" does not seem to be very 
closely related to the main content of this article, and it is 
recommended to delete it.

The scope of the present review is the reusability of critical raw materials (critical metals and REE) and nonmetal raw materials, such as glass, found in the main components of WEEE, in applications, employed by new, sustainable technologies, such as Additive Manufacturing, that can lead the way to future breakthroughs. The introduction of the aforementioned raw materials, as additives to filaments used for the synthesis of composite materials with the ability to fabricate architectures that are impossible to fabricate through conventional processing techniques, has tremendous potential for the performance and the commercialization of the final products by adding unique characteristics, such as antibacterial properties, enhanced mechanical and magnetic properties, thermal and electrical conductivity.

Furthermore, the existing expertise in the composition and characterization of composite materials and 3D printing solutions for the modern industry, as well as the infrastructure with the latest technology equipment, make the Physical Metallurgy Laboratory of the Department of Mechanical Engineering of AUTh an excellent candidate for the implementation of such new technologies. Moreover, as the title of the paper mirrors part of the doctoral dissertation of one of the co-authors (V. Stratiotou-Efstratiadis), which will highlight the optimal protocol for the production of sustainable, innovative and complex 3D printed materials, whose research and commercial value will be evaluated, with more results to be published in scientific journals and conferences, we strongly believe that the content of Chapter 4 needs to remain in the manuscript.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

the delivered Review is valuable

Author Response

We thank the reviewer for the kind judgement on our review.

Reviewer 5 Report

The paper executes a literature review on the conditions, parameters, porcesses and properties of the final composite materials with regards to different raw material.

I would like to suggest to the authors to better explain why this study is original, what are the gaps that they try to compensate and how this literature review differenziates from the other ones.

In the introduction, the authors should underline the criticalities regarding the WEEE management and collection. For this please read also the following papers:

• Bruno, G., Diglio, A., Passaro, R., Piccolo, C., & Quinto, I. (2021). Measuring spatial access to the recovery networks for WEEE: An in-depth analysis of the Italian case. International Journal of Production Economics, 240, 108210.
• Isernia, R., Passaro, R., Quinto, I., & Thomas, A. (2019). The reverse supply chain of the e-waste management processes in a circular economy framework: Evidence from Italy. Sustainability, 11(8), 2430.

Another critical aspect regards the selection of the papers analyzed. The authors should explain the "method" adopted to select the papers, to choose them and to analyze them (how researchers read the papers, how these papers have been analyzed and classified...). More in general, the method should be more rigourous.

Implications should be added and authors should define at least some future research directions based on the review realized.

 

 

Author Response

We thank the reviewer for the kind comments that helped improve our manuscript.

 

The paper executes a literature review on the conditions, parameters, processes and properties of the final composite materials with regards to different raw material.

I would like to suggest to the authors to better explain why this study is original, what the gaps that they try to compensate are and how this literature review differentiates from the others.
The following text has been incorporated in the Conclusions and Future references:

The review presents the main features and characteristics of the critical raw materials (CRM) found in the main components of WEEE. Then, having narrowed them down to those that are widely used in sustainable energy sources (solar, wind) and every-day appliances (pcb) and which can be relatively easily isolated, removed and recycled, this review can serve as a collection of thorough procedures, applied and theoretical, that can lead the way to future breakthroughs. As the need for sustainability during recycle and reuse of CRM is more significant than ever before, the state-of-the-art processes need to fulfill some major requirements; the flow-chart of the process needs to be composed of environmentally friendly techniques (such as mechanical removal of parts from WEEE) that are less energy- (and subsequently cost-) intensive and ensure the use of non-hazardous chemicals during every stage. Furthermore, heat treatment of WEEE must be in compliance with safe-by-design protocols of capturing dangerous and toxic emissions. Most of the widely used REEs’ extraction, recovery and separation from minerals and ores are extremely costly, resulting to the increased price of the final product. The proposed recycle paths and the sustainable added-value, novel composites with aim at tackling this issue, while maintaining the environmental impact at a minimum.

The originality of the review lies on the collection of relatively recent methods and patents on the recycle path of the mentioned CRM and their introduction, as additives to filaments used for the synthesis of composite materials with the ability to fabricate architectures that are impossible to fabricate through conventional processing techniques. By adding unique characteristics, tremendous potential for the performance and the commercialization of the final products can be achieved.

 

In the introduction, the authors should underline the criticalities regarding the WEEE management and collection. For this please read also the following papers [……]:
The following text has been incorporated in the Introduction:

The European Union has increased the priority of regulations on circular economy, WEEE management, sustainability and environmental agenda. As exponentially increased amounts of WEEE are wasted every year, the excessive need for their recovery, reuse, recycling or disposal is crucial. Closed-loop supply chains and infrastructures of recycled critical raw materials need to be supported or financed from each respective country’s initiatives and EU’s as a coordinator, as WEEE European Directives define the requirements for national collection systems but leave to each Country-Member States the responsibility to undertake specific policies and to reach the fixed targets [1]. The analysis of both availability and accessibility of WEEE and the logistics of the closed-loop supply chains, will be proven beneficial for policy authorities, as they will constitute diagnostics data which will help them intervene where needed [1]. As recovery and recycle technologies become more feasible, companies will adopt them and WEEE will be treated on site, with high probability of ensuring availability and accessibility in the aforementioned collection points.

 

Another critical aspect regards the selection of the papers analyzed.   The authors should explain the "method" adopted to select the papers, to choose them and to analyze them (how researchers read the papers, how these papers have been analyzed and classified...). More in general, the method should be more rigorous.

The following text has been incorporated in Chapter 3, before part 3.1:

The main criteria under which the state-of-the-art methods are presented, include the following: the flow-chart of the process needs to be composed of a small number of environmentally friendly techniques (such as mechanical removal of parts from WEEE) that are less energy- (and subsequently cost-) intensive, and ensure the use of non-hazardous chemicals during every stage. Furthermore, heat treatment of WEEE must be in compliance with safe-by-design protocols of capturing dangerous and toxic emissions. Finally, the applicability of each method at a laboratory scale with potential upscaling is a major requirement.

1) Implications should be added and 2) authors should define at least some future research directions based on the review realized.

• The following text has been incorporated in the Conclusion

WEEE is a waste stream of different materials, including multiple hazardous constituents that can be released in the environment if not treated properly. Moreover, as the demand for raw material rises, more recovery and recycle operations, as well as illegal shipment of WEEE, take place in developed and developing countries, which do not always follow the strict regulations and requirements, increasing health and environmental risks [2]. The WEEE Directive has been adopted to reduce these risks by establishing requirements
to ensure the safe collection and environmentally sound treatment of WEEE. The Directive of 2020, dictated that the minimum collection target was 4kg/inhabitant/year of WEEE for countries-members (equal to 33% of WEEE arising per year). Inspection during recovery, recycle paths and shipment of WEEE is mandatory, to ensure safe-by-design procedures. Finally, the WEEE collection points are increased, followed by the collection targets accordingly [2]. However, there are pitfalls, such as extra costs which are not matched by the increased recovery of valuable materials. For some types of WEEE, particularly those containing hazardous substances, these costs are very significant compared to the value of the materials themselves.   

Furthermore, informative and educational campaigns can raise awareness of the advantages of WEEE reusability and lead to improved measures and tailor-made policies, for every country [3]. The logistics and closed-loop supply chains can be analyzed and optimized accordingly, to ensure a balanced, monitored stream of WEEE, based on a circular-economy, zero-waste, sustainable-by-design paradigm.

• The following text has been incorporated in the Conclusion:

Further research will focus on testing some of the most sustainable and environmentally friendly, recent processes, followed by technoeconomic reports on the feasibility of each respective study and its applicability at a larger scale. Moreover, after the appropriate protocol of the critical raw material (CRM) recovery is drafted, the existing expertise in the incorporation of CRM into composite materials and characterization of the final, novel 3D printed composites for the modern industry, as well as the infrastructure with the latest technology equipment, will make the Physical Metallurgy Laboratory of the Department of Mechanical Engineering of AUTh an excellent candidate for the evaluation of the research and commercial exploitation of these new technologies, with further results to be published in scientific journals and conferences.

 

[1]        G. Bruno, A. Diglio, R. Passaro, C. Piccolo, and I. Quinto, “Measuring spatial access to the recovery networks for WEEE: An in-depth analysis of the Italian case,” Int. J. Prod. Econ., vol. 240, no. May, p. 108210, 2021.

[2]        Commission of the European Communities, “Directive of the European Parliament and of the Council on Waste Electrical and Electronic Equipment (WEEE). Impact Assessment,” Brussels, 2008.

[3]        R. Isernia, R. Passaro, I. Quinto, and A. Thomas, “The reverse supply chain of the e-waste management processes in a circular economy framework: Evidence from Italy,” Sustainability, vol. 11, no. 8, 2019.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 5 Report

First of all, I would like to thank the authors for their effort in addressing my comments and suggestions.

The paper is improved a lot and I think that it is ready for the publication

Best regards