Etching after polymer decomposition[edit | edit source]

M. Lippold, S. Patzig-Klein, and E. Kroke, "HF-HNO3-H2SO4/H2O Mixtures for Etching Multicrystalline Silicon Surfaces: Formation of NO2+, Reaction Rates and Surface Morphologies," Zeitschrift für Naturforschung B, vol. 66, no. 2, pp. 155–163, Feb. 2011, doi: 10.1515/znb-2011-0208. Abstract:The reaction behavior of HF-HNO 3 -H 2 O etching mixtures, which are frequently used for texturing silicon surfaces, is significantly influenced by the addition of sulfuric acid. For high concentrations of sulfuric acid, nitronium ions NO 2 + ions have been detected by means of 14 N NMR spectroscopy, and results of Raman spectroscopic investigation have allowed the quantification of the nitronium ions. Maximum etching rates of 4000 - 5000 nm s −1 are reached for HF (40 %)-HNO 3 (65%)-H 2 SO 4 (97%) mixtures with w (40%-HF)/w (65%-HNO 3 ) ratios of 2 to 4 and w (97%-H 2 SO 4 )<0.3. For higher concentrations of sulfuric acid, H 2 SO 4 can be considered as a diluent.

In order to investigate the influence of the sulfuric acid at constant HF and HNO 3 quantities, fuming HNO 3 (100 %) was used and the water in the mixtures successively replaced by H 2 SO 4 . A sudden increase of etching rates was found for sulfuric acid concentrations around 6 mol L −1 correlating with the characteristic color of the etching solutions. Decreased reaction rates at > 7 molL −1 H 2 SO 4 are attributed to high solution viscosities and the formation of fluorosulfuric acid. Generally, in HF-HNO 3 -H 2 SO 4 /H 2 O etching mixtures a reduced dissociation of nitric acid, the formation of nitronium ions, the solubility of neutral nitrogen intermediates (e. g. NO 2 , N 2 O 3 ), as well as other effects influence the attack of silicon surfaces. The structure of etched silicon surfaces was investigated by means of scanning electron (SEM) and laser scanning microscopy (LSM).

The morphologies are influenced most significantly by the relative amounts of sulfuric acid. Unexpectedly, in nitronium ion-containing mixtures rough surfaces with pore-like etching pits are generated. Graphical Abstract HF-HNO 3 -H 2 SO 4 /H 2 O Mixtures for Etching Multicrystalline Silicon Surfaces: Formation of NO 2 + , Reaction Rates and Surface Morphologies

  • When etching Si with HNO3, nitrous oxide, hexaflourocilicic acid, water, H2, NO2, N2O, NH4 form, so we should add something(H2SO4) else to lessen products
  • Si dissolves much more once H2SO4 concentration exceeds 6 mol/L
  • Accommodate this high dissolution rate by increasing solution viscosity and form more flourosulfuric acid
  • With H2SO4 rich solutions, pore-like pits form on surface

Recycling via Crushing/Grinding[edit | edit source]

P. Zhao et al., "A novel and efficient method for resources recycling in waste photovoltaic panels: High voltage pulse crushing," Journal of Cleaner Production, vol. 257, p. 120442, Jun. 2020, doi: 10.1016/j.jclepro.2020.120442.

Abstract: Photovoltaic power generation technology has developed rapidly in the past decade due to its clean and efficient characteristics. However, with the development of photovoltaic power generation technology, a large number of waste photovoltaic panels are generated, but there is no clean and effective method for resources recycling in waste photovoltaic panels. High-voltage pulsing tends to cause fractures at interfaces of materials with different dielectric constants, which has a satisfactory recovery effect on layered materials like photovoltaic panels In this paper, high voltage pulse crushing is used to dissociate and enrich waste photovoltaic panels, the experimental results show that there are differences in the selectivity of different components during high-voltage pulse crushing (selectivity: Ag > Si > glass).

This makes high voltage pulse crushing have good enrichment effect on photovoltaic panels. Most of the high-value elements are enriched to lower grain size, the glass purity of 0.5∼4 mm grain size can be directly recycled while it reaches over 98%. High-voltage pulse crushing has the most obvious enrichment effect on silver, Selectivity increases with the decrease of field strength and pulse number. The enrichment rate of silver in the lower field strength and pulse can reach 3.08, and the recovery rate is 54.07%. As the field strength and pulse number increase, the enrichment rate of silver drops to 1.67 but the recovery rate increases to 89.41%. The silver recovery can be effectively improved by adjusting the electrode gap. High-voltage pulse crushing can effectively enrich and recover the silver in the waste photovoltaic panels, providing convenience for subsequent sorting.

  • Materials of different dieletric constants fracture at interface during high-voltage pulsing, so it is used for PV modules, traditionally used for mineral processing and recycling circuit boards
  • 9.6 million tonnes of waste PV by 2050
  • Against organic solvent, thermochemistry, chemical etching, and mechanical recycling methods: poor dissociation, high cost, reagant harm, pollution, high energy consumption
  • High-voltage pulsing could allow for enrichment
  • Heating in muffle furnace 650 C for 1 hr pyrolyzes EVA and Tedlar backfoil... what about bad byproducts or EVA swelling?
  • High-voltage pulse applied to solid submersed in water, electric field intensity in the solid breaks down but in water does not so a channel of discharge occurs and expands which causes a pressure wave which breaks down the solid
  • Increase voltage, decrease in average particle size that is produced due to more crushing of initially crushed solid, good for amount recovered
  • Electrode gap increase, decrease in electric field intensity so less crushing happens
  • More Ag and Si recovered with higher voltage
  • Most product dissociate at higher number of pulses
  • Ag gets crushed first, then Si, then glass
  • Easier to dissociate between materials with more different dielectric constants
  • Ag gets enriched, no Si recovery

M. F. Azeumo, C. Germana, N. M. Ippolito, M. Franco, P. Luigi, and S. Settimio, "Photovoltaic module recycling, a physical and a chemical recovery process," Solar Energy Materials and Solar Cells, vol. 193, pp. 314–319, May 2019, doi: 10.1016/j.solmat.2019.01.035.Abstract: End-of-life photovoltaic modules can be hazardous wastes if they contain hazardous materials. The main problem arising from this type of waste is the presence of environmentally toxic substances and the poor biodegradability of the waste, which occupies great volumes when landfilled. For these reasons, photovoltaic modules have to be treated before landfilling as required by the legislation. The subject of this paper is the polycrystalline silicon type photovoltaic modules. They were treated with a physical and a chemical process. The physical process was aimed at the recovery of glass, metals, and the polyvinyl fluoride film. The modules were initially shredded with a knife mill and then processed with heavy medium separation, milling, and sieving. The glass (76%) and 100% of the metals were recovered respectively, at a grade of about 100% and 67%. Finally, a flow sheet of the physical process was proposed. The chemical process was aimed at identifying the best conditions which allow the dissolution of the EVA (ethylene vinyl acetate), that is the polymer that attaches the three layers that make up the module, namely the glass, the polycrystalline silicon, and the polyvinyl fluoride support. The experimental factors investigated were: type of solvent, thermal pretreatment, treatment time, temperature, and ultrasound. The best conditions to completely dissolve EVA in less than 60 min were the use of toluene as a solvent at 60 °C combined with the use of ultrasound at 200 W, while the pretreatment at 200 °C appeared to be useless.

F. C. S. M. Padoan, P. Altimari, and F. Pagnanelli, "Recycling of end of life photovoltaic panels: A chemical prospective on process development," Solar Energy, vol. 177, pp. 746–761, Jan. 2019, doi: 10.1016/j.solener.2018.12.003.Abstract: The application of photovoltaics has been rapidly increasing over the past two decades driven by the idea that it could provide a fundamental contribution to the transition from traditional fossil fuels to renewable energy based economies. However, long-term sustainability of photovoltaics will be largely dependent on the effectiveness of the process solutions that will be adopted to recycle the unprecedented volume of end-of-life panels expected to be generated in the near future. Recycling is indispensible to avoid the loss of the valuable materials employed to produce the photovoltaic panels and, at the same time, prevent that harmful elements, including, for example, heavy metals, could be dispersed into the environment through improper disposal practices. In this article, the process solutions proposed over the past two decades to recycle photovoltaic panels are critically reviewed. Main objective is to provide the basis for the identification of the recycling solutions that can effectively sustain the continuous increase of the photovoltaic market. In order to assess the requirements that should be satisfied by the recycling processes, the legislation currently in force to regulate the management of end-of-life photovoltaic panels is reviewed, and the evolution of the PV market over the past two decades is analysed. Based on this analysis, forecasts are derived for the flux of end-of-life panels that will be generated over the coming four decades. A technical survey of the previously proposed recycling processes is successively performed by including, in addition to the analysis of the research studies published in scientific articles, a detailed review of the patented recycling processes. Indications are given to which may be the most promising processes in terms of their economic sustainability and environmental impact.

  • Reduced use of metal in PV modules means recycling them is less valuable
  • Polycrystalline Si PV cells are least expensive to produce, in the market right now
  • Concentrator PV have trackers to move with the sun - added cost
  • Dye-sensitized cells also expand the variety of modules that will eventually become waste
  • Year of manufacture is relevant for recycling and LCA
  • 2036 will see spike in PV waste because 2008-2011 had a spike in PV production
  • Best glass recovery from two blade rotors crushing followed by hammer crushing and thermal treatment
  • Al and Ag recovered by aluminum chloride solution, yielding poly-aluminum-hydroxide-chloride

Recycling Thin Film PV Modules[edit | edit source]

W. Berger, F.-G. Simon, K. Weimann, and E. A. Alsema, "A novel approach for the recycling of thin film photovoltaic modules," Resources, Conservation and Recycling, vol. 54, no. 10, pp. 711–718, Aug. 2010, doi: 10.1016/j.resconrec.2009.12.001.

Abstract: A sustainable recycling of photovoltaic (PV) thin film modules gains in importance due to the considerable growing of the PV market and the increasing scarcity of the resources for semiconductor materials. The paper presents the development of two strategies for thin film PV recycling based on (wet) mechanical processing for broken modules, and combined thermal and mechanical methods for end-of-life modules. The feasibility of the processing steps was demonstrated in laboratory scale as well as in semi-technical scale using the example of CdTe and CIS modules. Pre-concentrated valuables In and Te from wet mechanical processing can be purified to the appropriate grade for the production of new modules. An advantage of the wet mechanical processing in comparison to the conventional procedure might be the usage of no or a small amount of chemicals during the several steps. Some measures are necessary in order to increase the efficiency of the wet mechanical processing regarding the improvement of the valuable yield and the related enrichment of the semiconductor material. The investigation of the environmental impacts of both recycling strategies indicates that the strategy, which includes wet mechanical separation, has clear advantages in comparison to the thermal treatment or disposal on landfills.

  • Estimated life span of thin film modules is 25-35 years
  • At EOL hazardous materials may affect environment, humans
  • The processes explored in this study limit or eliminate chemicals/reagents
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Created May 23, 2022 by Irene Delgado
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