Rotary drum recycling of photovoltaic panels

Abstract

The invention relates to methods and apparatus for the recovery of recyclable substance from photovoltaic panels Particularly, the method comprises leaching photovoltaic cell particles with a leachant in a rotating perforated drum rotating inside an external leaching tank.

Description

ROTARY DRUM RECYCLING OF PHOTOVOLTAIC PANELS

[0001] This application claims priority from Australian Provisional Patent Application No. 2024901281 filed 3 May 2024, the content of which is incorporated herein by reference in its entirety.Field of the Invention

[0002] The invention relates to methods and apparatus for the recovery of recyclable substances from photovoltaic panels. Particularly, the method comprises leaching photovoltaic cell particles with a leachant in a rotating perforated drum rotating inside an external leaching tank, followed by recovery of the leachant and separation of the recyclable substances. However, it will be appreciated that the invention is not limited to these particular fields of use.Background of the Invention

[0003] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0004] In the realm of energy production, photovoltaics (PV) stand out as a preeminent, environmentally benign, and enduring method. Presently, these systems contribute to approximately 3% of the worldwide electricity output, wherein crystalline silicon (c-Si) modules (also known as panels) account for more than 90% of the global PV market. Given that it engenders economical electricity generation and facilitates substantial employment opportunities in the non-polluting environment, PV attributes not only fortify ongoing governmental endorsement but also cultivate widespread societal backing for this technology. However, with the swift rise in the manufacture of c-Si PV modules, how to deal with end-of- life (EOL) PV modules has become significant in sustainable development and environmental protection at the end of their estimated lifetime of 25-30 years. The International Renewable Energy Agency (IRENA) has projected that by 2050, the world could see up to 78 million tons of solar panel waste.

[0005] Ensuring the materials embodied in EOL modules are not locked away in landfills has been an early policy priority. Firstly, there is a strong environmental incentive for recycling and recovery of materials from spent Si PV modules due to hazardous substances (such as tin) in them. In addition, valuable elements contained in silicon panels (such as silicon (Si), aluminium (Al), silver (Ag), copper (Cu) and glass) are worth recovering for sustainability and economic benefits. Typically, c-Si PV modules are composed of low iron glass, Al frame, solarcell, encapsulants, back-sheet, and junction box, in order of mass. The recycling regimen for photovoltaic panels principally includes three steps: initial mechanical disassembly to detach the frame and junction box; followed by the excision of the encapsulant, facilitating the segregation of glass and silicon wafer; culminating in the extraction and refinement of the silicon wafer and select metals (such as silver, tin, lead, and copper) via chemical and electrochemical methods. Strategies encompassing this entire suite of processes are designated as “integrated” recycling pathways.

[0006] Normally, pure silicon is the most recyclable material due to its cost and scarcity. However, given the high complexity of c-Si cell recycling, other useful materials such as glass, aluminium and silver are becoming increasingly useful for improving the economic viability of recycling processes. Typically, the front electrode is usually made of silver, and the back electrode is made of silver (N-type) or aluminium (P-type). Although Ag represents only -0.08% of the total weight of the module and is fairly well dispersed throughout the solar cell, it is one of the main cost drivers in industrial production. Besides, Ag content also corresponds to 1 % of the total embedded energy. Thus, EOL PV modules can be viewed as an important source for mining and recovering silver.

[0007] The most used method for silver recovery from solar cells is hydrometallurgy (i.e. , chemical leaching) based on nitric acid. The efficiency of this reaction is basically good with a high purity, even up to 99.999%. Leaching is a key step in silver recovery as it realizes the Ag preliminary separation from the cell in the form of ions and then recycled. Despite its importance, there are no published studies regarding the research on time-evolved internal leaching mechanism analysis and subsequent leaching efficiency tracking.

[0008] Until now, a myriad of studies have been conducted only to explore efficient ways to extract silver for getting an overall leaching recovery efficiency at certain moments. However, these are all realized in the lab scale, such as using beakers, flasks and autoclaves. Beaker systems are commonly employed as the preferred leaching systems and are extensively utilized by myriad researchers. Examples of previous work include conducting the leaching experiments using nitric acid with various concentrations (1-10 mol / L) in the Erlenmeyer flask with a nominal capacity of 250 mL which is placed on a shaker with heating control, or carrying out leaching experiments in a 1 L autoclave equipped with a mechanical stirrer, a reactiontemperature control unit, and a condenser to avoid loss of solution.

[0009] Despite their manoeuvrability, these small-scale system’s limitation in terms of scalability, efficiency, control, and safety make them less suitable for industrial-scale leaching of solar panels.

[0010] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0011] It is an object of at least one preferred form of the present invention to provide a method of recovering a recyclable substance from a photovoltaic cell with improved mixing. It is an object of further preferred forms of the present invention to a method of recovering a recyclable substance from a photovoltaic cell with improved heat and / or mass transfer efficiency. It is an object of yet further preferred forms of the present invention to a method of recovering a recyclable substance from a photovoltaic cell with improved efficiency.Summary of the Invention

[0012] In a first aspect, the invention provides a method of recovering a recyclable substance from a photovoltaic cell, the method comprising the steps of: a. providing crushed photovoltaic cell particles; and b. leaching the photovoltaic cell particles with a leachant in a rotating perforated drum rotating inside an external leaching tank, thereby to provide a solution of the recyclable substance in the leachant.

[0013] The inventors of the present application have surprisingly found a method of recovering a recyclable substance from a photovoltaic cell with improved mixing and more effective heat and / or mass transfer by using a rotating perforated drum. Without wishing to be bound by theory, the present inventors contemplate such a reactor ensures reduced impact on the cells due to the lower degree of collisions between particles. Such configuration offers the possibility of using high solid loadings without negatively influencing the Ag conversion which tends to be the case when impeller-driven reactors are used. In addition, the perforated design in rotary drums enhances leaching efficiency by improving material-solution contact and mass transfer. It facilitates uniform leaching, efficient solid-liquid separation, and controlled processing time. This method is energy-efficient, reduces particle agglomeration, and is adaptable to various scales and materials. The method has the potential to be industrialised. The present inventors have also found surprisingly that using rotary drum reactors interacts with the specific morphology of solar cells in a manner that allows for particularly efficient extraction of valuable materials in the recycling of solar cell panels.

[0014] In some embodiments, the recyclable substance is silicon, aluminium, silver, or copper or any combinations thereof.

[0015] The recyclable substance may be any leachable substance of value found in a photovoltaic cell, such as silicon, aluminium, silver, or copper. Preferably, the recyclable substance is silver.

[0016] Any suitable leachant may be used,. Any suitable concentration may be used having regards to leaching ability, safety and convenience.

[0017] In some embodiments, the leachant is an acid.

[0018] In some embodiments, the leachant is nitric acid. In other embodiments, the leachant is sulfuric acid or methanesulfonic acid.

[0019] In some embodiments, the acid has a concentration of about 0.1M to about 10M in the leachant, for example, about 0.1M to about 0.5M, about 0.5 M to about 1 M, about 1 M to about 1.5 M, about 1.5 M to about 2 M, about 2 M to about 2.5 M, about 2.5 M to about 3 M, about 3 M to about 3.5 M, about 3.5 M to about 4 M, about 4 M to about 4.5 M, about 4.5 M to about 5 M, about 5 M to about 5.5 M, about 5.5 M to about 6 M, about 6 M to about 6.5 M, about 6.5 M to about 7 M, about 7 M to about 7.5 M, about 7.5 M to about 8 M, about 8 M to about 8.5 M, about 8.5 M to about 9 M, about 9 M to about 9.5 M, about 9.5 M to about 10 M, about 0.1 M to about 2 M, about 2 M to about 4 M, about 4 M to about 6 M, about 6 M to about 8 M, about 8 M to about 10 M, about 0.1 M to about 5 M, about 5 M to about 10 M, about 0.1 M, about 1 M, about 2 M, about 3 M, about 4 M, about 5 M, about 6 M, about 7 M, about 8 M, about 9 M, or about 10 M.

[0020] In some embodiments, the leachant is 4M nitric acid

[0021] The leaching process may be carried out at any suitable temperature having regards to reaction kinetics, safety and convenience.

[0022] In some embodiments, the leachant has a temperature of about 40°C to about 100°C, for example, about 40°C to about 50°C, about 50°C to about 60°C, about 60°C to about 70°C, about 70°C to about 80°C, about 80°C to about 90°C, about 90°C to about 100°C, about 40°C to about 60°C, about 60°C to about 80°C, about 80°C to about 100°C, about 40°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C.

[0023] In some embodiments, the leachant has a temperature of about 40°C.

[0024] In some embodiments, the method further comprises a step of crushing the photovoltaic cell to obtain the crushed photovoltaic cell particles.

[0025] The crushed photovoltaic particles may be crushed to any suitable size by any suitable means (crusher or ball mill).

[0026] In some embodiments, the crushed photovoltaic cell particles have a size of about 1mm to about 10mm, for example, about 1 mm to about 1.5 mm, about 1.5 mm to about 2 mm, about 2 mm to about 2.5 mm, about 2.5 mm to about 3 mm, about 3 mm to about 3.5 mm,about 3.5 mm to about 4 mm, about 4 mm to about 4.5 mm, about 4.5 mm to about 5 mm, about 5 mm to about 5.5 mm, about 5.5 mm to about 6 mm, about 6 mm to about 6.5 mm, about 6.5 mm to about 7 mm, about 7 mm to about 7.5 mm, about 7.5 mm to about 8 mm, about 8 mm to about 8.5 mm, about 8.5 mm to about 9 mm, about 9 mm to about 9.5 mm, about 9.5 mm to about 10 mm, about 1 mm to about 3 mm, about 3 mm to about 5 mm, about 5 mm to about 7 mm, about 7 mm to about 9 mm, about 1 mm, about 2 mm, about 3 mm, about 4 mm, about 5 mm, about 6 mm, about 7 mm, about 8 mm, about 9 mm, or about 10 mm.

[0027] In some embodiments, the external leaching tank is cylindrical and configured to allow the rotating perforated drum to be loaded into the external leaching tank via an upper aperture. The particles may be loaded into the rotating perforated drum prior to its loading into the external leaching tank. Preferably the photovoltaic cell particles are introduced to the perforated drum prior to introduction of heated leachant.

[0028] In some embodiments, the external leaching tank and rotating perforated drum are coaxial.

[0029] In some embodiments, the rotating perforated drum comprises a base and a horizontal wall of mesh, preferably stainless-steel mesh. The rotating perforated drum has perforations of a predetermined size based upon the intended particle size of the crushed PV cells. In certain embodiments, the predetermined size of perforations is not larger than that of the crushed photovoltaic cell particles.

[0030] In some embodiments, the rotating perforated drum is perforated with apertures of about 1mm to about 10mm, for example, about 1 mm to about 1.5 mm, about 1.5 mm to about 2 mm, about 2 mm to about 2.5 mm, about 2.5 mm to about 3 mm, about 3 mm to about 3.5 mm, about 3.5 mm to about 4 mm, about 4 mm to about 4.5 mm, about 4.5 mm to about 5 mm, about 5 mm to about 5.5 mm, about 5.5 mm to about 6 mm, about 6 mm to about 6.5 mm, about 6.5 mm to about 7 mm, about 7 mm to about 7.5 mm, about 7.5 mm to about 8 mm, about 8 mm to about 8.5 mm, about 8.5 mm to about 9 mm, about 9 mm to about 9.5 mm, about 9.5 mm to about 10 mm, about 1 mm to about 3 mm, about 3 mm to about 5 mm, about 5 mm to about 7 mm, about 7 mm to about 9 mm, about 1 mm, about 2 mm, about 3 mm, about 4 mm, about 5 mm, about 6 mm, about 7 mm, about 8 mm, about 9 mm, or about 10 mm.

[0031] In certain embodiments, the crushed photovoltaic cell particles have a size of about 3 about 4 mm. In some embodiments, the rotating perforated drum is perforated with apertures of 2 mm.

[0032] In some embodiments, the rotating perforated drum has a gear which engages with a driving gear at a base of the external leaching tank, thereby to rotate the rotating perforated drum.

[0033] In some embodiments, the rotating perforated drum and external leaching tank are coaxial, with the axis oriented vertically, however, the rotating perforated drum and external leaching tank may also be coaxial with the axis oriented horizontally.

[0034] In some embodiments, the external leaching tank further comprises a temperature control mechanism, which may be for example a water jacket adapted to circulate heated or cooled water around the leachant.

[0035] In some embodiments, the external leachant tank has an inlet port for introducing leachant.

[0036] In some embodiments, the external leachant tank has an outlet port for removing leachant and / or a solution of the recyclable substance in the leachant from the external leaching tank.

[0037] In some embodiments, the external leachant tank has a port located above an upper level of the leachant for removal of gas.

[0038] In some embodiments, the photovoltaic cell particles are introduced to the rotating perforated drum prior to introduction of heated leachant.

[0039] In some embodiments, the volume defined within the rotating perforated drum is about 20% to about 90% of total volume within the external leaching tank, for example, about 20% to about 30%, about 30% to about 40%, about 40% to about 50%, about 50% to about 60%, about 60% to about 70%, about 70% to about 80%, about 80% to about 90%.

[0040] In some embodiments, the volume defined within the rotating perforated drum is about 20% of total volume within the external leaching tank. For example, the volume of the external leaching tank is 10L and the volume of the rotating perforated drum is 2L. A 2L drum for instance would have a capacity of 250g of photovoltaic cell particles crushed to 2-3mm and a permeable area of around 628cm2. The skilled person would be able to calculate the capacity and the permeable area with any defined drum volume.

[0041] In some embodiments, the rotating perforated drum is rotated at about 1 rpm to about 50 rpm, for example, about 1 rpm to about 10 rpm, about 10 rpm to about 20 rpm, about 20 rpm to about 30 rpm, about 30 rpm to about 40 rpm, about 40 rpm to about 50 rpm, about 1 rpm to about 5 rpm, about 5 rpm to about 10 rpm, about 10 rpm to about 15 rpm, about 15 rpm to about 20 rpm, about 20 rpm to about 25 rpm, about 25 rpm to about 30 rpm, about 30 rpmto about 35 rpm, about 35 rpm to about 40 rpm, about 40 rpm to about 45 rpm, about 45 rpm to about 50 rpm, about 1 rpm, about 5 rpm, about 10 rpm, about 15 rpm, about 20 rpm, about 25 rpm, about 30 rpm, about 35 rpm, about 40 rpm, about 45 rpm, about 50 rpm.

[0042] In some embodiments, the rotating perforated drum is rotated at 10 rpm.

[0043] In some embodiments, the method has a leaching time of about 5 minutes to about 120 minutes, for example, about 5 minutes to about 15 minutes, about 15 minutes to about 25 minutes, about 25 minutes to about 35 minutes, about 35 minutes to about 45 minutes, about 45 minutes to about 55 minutes, about 55 minutes to about 65 minutes, about 65 minutes to about 75 minutes, about 75 minutes to about 85 minutes, about 85 minutes to about 95 minutes, about 95 minutes to about 105 minutes, about 105 minutes to about 115 minutes, about 5 minutes to about 30 minutes, about 30 minutes to about 55 minutes, about 55 minutes to about 80 minutes, about 80 minutes to about 105 minutes, about 5 minutes, about 15 minutes, about 25 minutes, about 35 minutes, about 45 minutes, about 55 minutes, about 65 minutes, about 75 minutes, about 85 minutes, about 95 minutes, about 105 minutes, about 115 minutes, about 120 minutes.

[0044] In some embodiments, the leaching time is at least 10 minutes.

[0045] In some embodiments, the leaching time is at least about 10 to about 60 minutes

[0046] The extraction time is chosen to maximise recovery of loaded leachant. The leaching takes place at an initially rapid rate and then slows. For example, depending upon the volume of material to be processed, it may be more efficient to extract 10x batches to 90% than to extract x batches to 99%.

[0047] In some embodiments, the method further comprises a step of extracting the recyclable substance from the solution.Definitions

[0048] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains.

[0049] Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusivesense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

[0050] As used herein, the phrase “consisting of’ excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phrase “consisting essentially of” limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter.

[0051] With respect to the terms “comprising”, “consisting of’, and “consisting essentially of”, where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of’ or, alternatively, by “consisting essentially of”.

[0052] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term “about”. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, “%” will mean “weight %”, “ratio” will mean “weight ratio” and “parts” will mean “weight parts”.

[0053] The term ‘substantially’ as used herein shall mean comprising more than 50% by weight, where relevant, unless otherwise indicated.

[0054] The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1 , 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

[0055] Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

[0056] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

[0057] It must also be noted that, as used in the specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

[0058] As used herein, with reference to numbers in a range of numerals, the terms “about,” “approximately” and “substantially” are understood to refer to the range of -10% to +10% of the referenced number, preferably -5% to +5% of the referenced number, more preferably -1 % to +1 % of the referenced number, most preferably -0.1 % to +0.1 % of the referenced number. Moreover, with reference to numerical ranges, these terms should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 1 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, from 8 to 10, and so forth.

[0059] Although exemplary embodiments of the disclosed technology are explained in detail herein, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the disclosed technology be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The disclosed technology is capable of other embodiments and of being practiced or carried out in various ways.Brief Description of the Drawings

[0060] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0061] Fig. 1 shows a roadmap of the leaching process of solar module wastes in the rotary drum reactor.

[0062] Fig. 2 shows a leaching reactor design for experiments of solar panels.

[0063] Fig. 3 shows a bench top scale rotary drum platform for silver leaching.

[0064] Fig. 4 shows SEM analysis of the PV samples: (a), (e) and (i) observed from the front surface for the N-type cell; (b), (f) and (j) observed from the back surface for the N-type cell; (c), (g) and (k) observed from the front surface for the P-type cell; (d), (h) and (I) observed from the front surface for the P-type cell.

[0065] Fig. 5 shows SEM-EDS Mapping analysis of the front and back surface for the N- type cell: (a) and (c) observed from the front surface; (b) and (d) observed from the back surface.

[0066] Fig. 6 shows SEM-EDS Mapping analysis of the front and back surface for the P- type cell: (a) and (c) observed from the front surface; (b) and (d) observed from the back surface.

[0067] Fig. 7 shows the effect of leaching time on Ag leaching efficiency.

[0068] Fig. 8 shows an Ag separation mechanism during the leaching for N-type cell at 1 min, 2 min, 10 min and 30 min.

[0069] Fig. 9 is an EDS analysis of the front surface with time.

[0070] Fig. 10 shows an Ag separation mechanism during the leaching for P-type cell at 1 min, 2 min, 10 min and 30 min.

[0071] Fig. 11 shows an EDS analysis of the front surface with time.Detailed Description of the Invention

[0072] The skilled addressee will understand that the invention comprises the embodiments and features disclosed herein as well as all combinations and / or permutations of the disclosed embodiments and features.

[0073] A leaching process using the perforated rotary drum design in the solar panel is proposed. As shown Fig. 1 , after the removal of encapsulants (i.e., EVA), the waste photovoltaic modules are crushed into chip-like particles and the rotary drum is applied to perform the leaching process for the selective extraction of Ag. The developed rotary drum reactor is capable of processing 250g of raw materials, a significant scale-up from the traditional capacity.

[0074] The time evolved leaching efficiency and mechanism are obtained by considering the differences of the reactor system and the solar panel type. The leaching results are obtained by ICP (Inductively Coupled Plasma) Spectroscopy analysis and the leaching mechanism is investigated by conducting the Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis on the samples.

[0075] The lab-scale leaching experiments are conducted by using a leaching platform that comprises a 150 ml beaker / flask reactor and a water bath (i.e., a 500 ml beaker on a hot plate magnetic stirrer). For the scale-up consideration, the bench top-scale RPDLR consists of two main parts: a rotary drum and the external leaching tank (10 L), as shown in Fig. 2(b). The drum itself possesses a volume of 2 L and is engineered to be detachable, thereby streamlining the processes of loading and unloading specimens. Encased by a stainless-steel mesh with individual apertures measuring 2 mm, the drum affords a permeable area totalling 628 cm2. Four wheels which are evenly distributed around the base of the drum provide support androtational power for the drum. A variable frequency motor is connected to a gear meshing with one of the supporting wheels of the drum, to provide varying rotational speeds. The perforated drum can be filled with solar panel particles from the end cap and put into the leaching tank. The acid is preheated in a water bath and then introduced into the tank through the pump, partially immersing the drum. Gas is collected in Tedlar bags through a port in the lid of the outer casing. The leachate is collected from the discharge port below.Leaching experiments

[0076] The c-Si PV samples (6 x 6 inches in size) are collected and used. Different layers of solar panels were separated by thermal treatment. After that, the extracted samples are first crushed to the size of 3 ~ 4 mm in a ball mill for 60 s. The leaching solutions are chosen as HNO3 (4 mol / L), which is obtained by diluting 70 % nitric acid with distilled water and heated to a pre-set temperature (40°) in the water bath. In the beaker system, the acid with a volume of 40 mL is preheated to the pre-set temperature (40°C) in the water bath beaker, before loading solar cells in the reactor. In the RPDLR system shown Fig. 3, leaching solutions with a volume of 5 L are introduced into the drum reactor tank after heating to the desired temperature (40°C). All experiments are performed over 1 h using waste solar panel particles of approximately 250 g. Both leaching efficiency measurements are carried out under the rotating speed of 10 rpm. During the experiments, the leachate is taken at certain times to determine the amount of dissolved Ag.Analysis and Characterization

[0077] Samples of cells are examined by SEM and EDS (Bruker Esprit 2.1) analysis to investigate the morphological properties and internal chemical composition. A semiquantitative analysis using inductively coupled plasma mass spectrometry (ICP-MS) is applied to analyse the concentration of Ag in the leachate, which is sampled at each interval two times.Results and discussionsMorphological analysis of the solar panels

[0078] The front and back surfaces of the N-type and P-type samples are observed using SEM-EDS. For N-type cells, as shown in Fig. 4(a) and 4(b), the front exhibits a more lustrous coloration compared to the back. The enhanced brightness of the front side of solar cells can be attributed to the application of a specialized anti-reflective coating. This layer is typically endowed with certain optical properties, including a specific colour or sheen, contributing to the perceived increase in lustre. The primary function of this coating is to minimize light reflection, thereby optimizing the amount of light that penetrates and is subsequently absorbed by the Siwafer, an essential factor in the overall efficiency of solar energy conversion. Each surface is represented by a network that consists of two types of tracks, namely longitudinal tracks (main tracks) with an average width of 718 pm and transversal tracks connecting the main tracks. The shape of longitudinal tracks can be continuous or discontinuous to decrease the usage of Ag in manufacturing as shown in Fig. 4(e) and 4(i). The Ag paste layer consists of bonded spherical microparticles distributed uniformly on both surfaces from Fig. 4(f) and 4(j). Similarly, the downside layer (Si substrate) is also composed of bonded microparticles. Notably, the distribution of silicon particles is much sparser than that of silver particles. Overall, the fact that the back is dimmer than the front is also reflected in the morphological comparison.

[0079] The P-type cell shows a similar structure on the front surface Fig. 4(c). The prominent grid patterns are silver paste busbars and fingers, designed for optimal current collection. The P-type semiconductor layer, visible beneath the anti-reflective blue coating, plays a vital role in the photovoltaic process. For the back surface in Fig. 4(d), the grey area is the aluminium (Al) substrate, which plays a dual role: it acts as a back electrical contact and helps in forming a back surface field, enhancing the solar cell’s efficiency by reflecting electrons back into the silicon. The white lines are the soldering pads, made of a silver-containing paste, used to connect the cell to external wires, facilitating the flow of electricity out of the solar cell. Interestingly, Fig. 4(h) shows the finer and denser accumulation of silver particles. In addition, its Si substrate Fig. 4(h) has a more pronounced textured surface as compared to Fig. 4(f), which indicates different levels of surface etching or preparation. The texture affects light trapping and the overall efficiency of the solar cell. Fig. 4(l), at 550* magnification, reveals the granular topology of an Al substrate particle layer, integral to a high-efficiency photovoltaic architecture.

[0080] Fig. 5 shows SEM-EDS elemental mapping of the front and back surfaces coated layer composed of Si and N (SiNx) in the N-type cell, as well as the attached Ag pastes. Also, EDS readings of two faces are shown in the Fig. 5(c)-5(d). These results agree with elemental mapping results, which show that Si and Ag are the main components in it. Similar peak shapes are exhibited in silicon in two faces, indicating that they have similar purity levels. Due to the primary role of the back electrode in solar cells being the collection and transmission of electrical current, there is a propensity for a more concentrated application or higher density of silver, resulting in a pronounced peak for silver in the EDS analysis of the backside compared to the front side. This utilization strategy is indicative of the optimization processes in electrode fabrication, prioritizing conductivity and efficient electron transport pathways.

[0081] Fig. 6 shows the similar composition of the front surface in the P-type cell compared with the N-type cell. The back surface analysis emphasizes the Al existence, agreeing with theelemental mapping results which show that Al is the main component in it. The EDS spectrum also shows prominent peaks for silicon (Si), with a relatively low intensity of the Oxygen (O) peak which indicates the oxide forms of these elements. The presence of Si can be captured because the Al layer cannot be completely uniformly coated on the back of the silicon wafer, which is due to cost considerations and technical limitations of the coating process. What’s more, slight Ag peaks correspond to our abovementioned views that white lines are the soldering pads, made of an Ag-containing paste.Leaching efficiency

[0082] Fig. 7 shows recovery efficiency versus leaching times at 4 mol / L HNO3 concentration, 40°C and 10 rpm of two systems. According to the N-type cell leaching in two distinct systems, two leaching efficiency curves culminate at an equivalent maximum efficiency, indicating a similar capacity for Ag recovery despite the disparity in equipment scale. The identical particle size and solid-to-liquid ratio ensure that the intrinsic leaching characteristics remain constant between the two systems. However, the black line reveals a more pronounced initial slope, suggesting that the smaller volume of the beaker facilitates faster kinetics due to more effective mixing and a higher relative surface area exposed to the leaching agent. Conversely, the red line ascends more gradually, implying that the larger drum, while maintaining the same end-point efficiency, operates with delayed kinetics. This is attributed to the dynamics of scale, where the larger volume necessitates longer times for the leaching agent to permeate through the mass and achieve uniform reaction conditions.

[0083] After that, the feedstock effects on the leaching process in the RPDLR were compared. For the N-type cell, from the first 0.5 min of the reaction, the recovery efficiency begins almost immediately yielding 4.23%. This trend continues for 20 min, yielding 85.19%. This is attributable to the immediate availability of reactive silver sites and the acid’s oxidative strength, ensuring swift silver dissolution. Beyond this phase, the reaction stabilizes as the accessible silver is leached, transitioning the process to a diffusion-controlled regime, where the acid must permeate deeper layers, encountering decreased reactivity. Thus, the leaching rate exhibits a deceleration and eventually stabilizes, reaching a maximum of 100 % after 30 min.

[0084] When investigating the leaching process in P-type cells, the rate manifests a sluggish commencement, followed by an acceleration phase for 20-40 minutes concomitant with the dissolution of the cell surface. After that, it gradually stabilizes and reaches the complete Ag conversion at 50 min. After comparison, the leaching efficiency of the N-type cell increases faster within the first 20 minutes than that of the P-type cell and reaches saturation earlier. These differences are caused by the Al involvement in the P-type cell. Al, being more reactivethan silver, exhibits a competitive advantage in reactions, rendering it more susceptible to oxidation.

[0085] Comparative studies of N-type and P-type solar cells within the RPDLR reveal distinct leaching behaviours, analyzed through Scanning Electron Microscopy- Energy Dispersive Spectroscopy (SEM-EDS) to elucidate the degradation patterns. Our analysis reveals that the leaching process is predominantly initiated at the Ag electrode peripheries, with the reactive ingress of nitric acid leading to the efficient detachment of Ag from the Si substrate. Interestingly, N-type cells exhibit a more expedient leaching curve, reaching peak efficiency before P-type cells, a dynamic potentially influenced by the presence of aluminium in the latter.Movement of particles in the rotating drum

[0086] Multiphase flow in a rotary drum can be very complex, typically in several regimes e.g., slipping, slumping, rolling, cascading, cataracting and centrifuging. Among the six motion regimes in the rotary drum, the rolling regime is most commonly applied in industrial applications due to its superior granular mixing and heat transfer and indeed is the more suitable for regime for solar panel leaching, due to the materials' fragile essence, acidic working environment and scale-up considerations. However, its internal hydrodynamics and underlying mechanisms, which play a leading role in the leaching efficiency, are hard to understand due to the solar particle shape and complex solid-liquid interactions.

[0087] The inventors have studied the transportation of chip-like particles in a rotary drum using a computational fluid dynamics-discrete element method (CFD-DEM) approach where particle morphology is described by a super-quadric model. After model validations, the particle-scale information (e.g., active-passive interface, mixing, dispersion, orientation, and contact force) under different rotating speeds has been analysed and the results showed that two anti-directional recirculation liquid vortexes are captured in the rotary drum due to the shear of the bed surface. The active depth, mixing degree, and axial particle dispersion increase with the rotating speed. Chip-like particles tend to orient their long axes parallel to the drum wall and drum axis. This minimizes resistance to leachant flow and without wishing to be bound by theory increases the movement of the leachant over the face of the particles, thereby facilitating dissolution of the silver and other valuable materials.Leaching mechanism

[0088] To understand the internal leaching mechanism, the erosion process of the front surface is analysed by the SEM-EDS Mapping analysis. As demonstrated in Fig. 4, the aggregation of Ag particles within printed electrodes is facilitated through chemical bonding,while the interface between the Si substrate and Ag electrode particles is characterized by a combination of chemical and mechanical adhesions. The separation mechanism of Ag electrodes from specimens via a leaching procedure is visualized through SEM-EDS, as depicted in Fig. 8. Observations reveal that separation initiates through a corrosion at the Ag electrode peripheries, particularly at pre-existing notches or imperfections on the electrode surfaces, which exhibit an increased interfacial area with nitric acid. The size of circular holes set up to optimize light capture and reduce resistance continues to expand outward along its central area due to corrosion. During the leaching process, nitric acid progressively infiltrates the Si substrate via these notches, disrupting the interparticle connections among Ag entities as well as the adhesions between the Si substrate and the Ag electrode. This process continues until complete disengagement of Ag contacts is achieved, resulting in an Ag-devoid substrate, as evidenced in Fig. 8(e).

[0089] EDS analysis of the N-type cell front surface versus time during leaching is present in Fig. 9. As time progresses, there is a noticeable decline in the peak corresponding to silver in the spectra, eventually diminishing to non-detectable levels. This trend is unequivocally attributed to the chemical reaction between silver and nitric acid. Contrary to the trend observed for silver, the peak corresponding to Si demonstrates a gradual increase, ascending from approximately 105 to around 165. As mentioned above, the front side of solar cells is typically coated with a specialized anti-reflective layer, commonly composed of materials such as silicon dioxide (SiC>2), silicon nitride (SislSU), or titanium dioxide (TiC>2). Under conditions of elevated acid concentration, nitric acid interacts with certain components of the anti-reflective coating, notably silicon dioxide and silicon nitride, compromising the integrity of the layer. This degradation results in the exposure of the underlying silicon to the corrosive environment, consequently leading to an observable increase in the silicon peak within EDS spectra.

[0090] The morphological analysis of the P-type cell front surface versus time during leaching is present in Fig. 10. The series of SEM images depicts the gradual leaching of silver from a P-type solar cell substrate over time intervals of 1 , 2, 10, and 30 minutes. Initially, at 1 minute, the surface shows no notable silver separation, with well-defined edges intact. At 2 minutes, slight edge distortions begin to appear, signalling the commencement of the leaching process. By 10 minutes, there are discernible remnants of silver, indicated by small clusters, suggesting that leaching is underway but incomplete. Finally, at 30 minutes, the substrate appears largely silver-free, indicating an effective leaching process. The morphological change corresponds to the time-evolved leaching efficiency variation above.

[0091] In Fig. 11 , the EDS analysis of the front surface with time for P-type cells has further validated our findings. At 1 minute, the Ag peak is prominent, indicating a significant presenceof silver. By 2 minutes, the Ag peak slightly diminishes, reflecting initial silver dissolution. At 10 minutes, the Ag peak reduces further, signifying more advanced leaching of silver. Finally, at 30 minutes, the Ag peak is minimal, suggesting a nearly complete removal of silver. These EDS results, in conjunction with the SEM images, demonstrate a progressive silver leaching mechanism: starting with edge distortion, advancing to the formation of residual silver structures, and ultimately leading to an Ag-free substrate.Conclusion

[0092] It has been shown that:

[0093] 1. The rotary drum reactor used is shown to be useful for the leaching process of solar panels. Such configuration offers the possibility of using high solid loadings without negatively influencing the Ag conversion.

[0094] 2. Each surface of the N-type solar cell is represented by a network similar in all samples that consists of longitudinal tracks (main tracks) and transversal tracks connecting the main tracks. The Ag paste layer and Si substrate consist of bonded spherical microparticles distributed uniformly on both surfaces. The P-type cell shows a similar structure on the front surface but with the aluminium (Al) substrate on the back surface.

[0095] 3. The Ag separation initially took place at the peripheries of Ag electrodes, particularly at the pre-existing notches or damaged areas on the electrode surfaces that are more extensively exposed to nitric acid. As the process evolves, nitric acid advances towards the Si substrate via these notches, culminating in the complete detachment of Ag contacts.

[0096] 4. For the solar panel leaching in different systems, the smaller-scale beaker system facilitates a rapid approach to maximum Ag recovery, while the RPDLR demonstrates slower kinetics, attributed to its larger volume and the resultant mass transfer limitations.

[0097] 5. The N-type cell and P-type cell leaching curves both share the same leaching trends, that is, a steep stage followed by a gently upward stage. The leaching efficiency of the N-type cell increases faster within the first 20 minutes than that of the P-type cell and reaches saturation earlier. These differences are caused by the Al involvement in the P-type cell.

[0098] This mechanistic study not only confirms the effectiveness of the RPDLR in processing large volumes but also highlights its capacity to achieve high Ag recovery rates, underscoring its potential for industrial-scale photovoltaic recycling.

[0099] Although the invention will be described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

CLAIMS:

1. A method of recovering a recyclable substance from a photovoltaic cell, the method comprising the steps of: a. providing crushed photovoltaic cell particles; and b. leaching the photovoltaic cell particles with a leachant in a rotating perforated drum rotating inside an external leaching tank, thereby to provide a solution of the recyclable substance in the leachant.

2. The method according to claim 1, wherein the recyclable substance is silicon, aluminium, silver, copper or any combinations thereof.

3. The method according to claim 1 or claim 2, wherein the recyclable substance is silver.

4. The method according to any one of claims 1 to 3, wherein the leachant is nitric acid.

5. The method according to claim 4, wherein the leachant is 4M nitric acid.

6. The method according to any one of claims 1 to 5, wherein the leachant has a temperature of about 40°C.

7. The method according to any one of claims 1 to 6, wherein the crushed photovoltaic cell particles have a size of about 3mm to about 4mm.

8. The method according to any one of claims 1 to 7, wherein the external leaching tank is cylindrical and configured to allow the rotating perforated drum to be loaded into the external leaching tank via an upper aperture.

9. The method according to any one of claims 1 to 8, wherein the external leaching tank and rotating perforated drum are coaxial.

10. The method according to any one of claims 1 to 9, wherein the rotating perforated drum comprises a base and a horizontal wall of mesh, preferably stainless-steel mesh.

11. The method according to claim 10, wherein the rotating perforated drum is perforated with apertures of about 2 mm.

12. The method according to any one of claims 1 to 11, wherein the rotating perforated drum has a gear which engages with a driving gear at a base of the external leaching tank, thereby to rotate the rotating perforated drum.

13. The method according to any one of claims 1 to 12, wherein the external leachant tank has an inlet port for introducing leachant.

14. The method according to any one of claims 1 to 13, wherein the external leachant tank has an outlet port for removing leachant and / or a solution of the recyclable substance in the leachant from the external leaching tank.

15. The method according to any one of claims 1 to 14, wherein the external leachant tank has a port located above an upper level of the leachant for removal of gas.

16. The method according to any one of claims 1 to 15, wherein the photovoltaic cell particles are introduced to the rotating perforated drum prior to introduction of heated leachant.

17. The method according to any one of claims 1 to 16, wherein a volume defined within the rotating perforated drum is about 20% of total volume within the external leaching tank.

18. The method according to any one of claims 1 to 17, wherein the rotating perforated drum is rotated at about 10rpm.

19. The method according to any one of claims 1 to 18, having a leaching time of at least about 10 minutes.

20. The method according to any one of claims 1 to 19, having a leaching time of at least about 10 to about 60 minutes.

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