Apparatus for processing photovoltaic cells, and plant and process for recovering the silver contained therein

Abstract

The invention concerns an apparatus for the treatment in continuous mode of photovoltaic cells. The apparatus comprises at least one basin 12, which contains a process liquid and a pre-defined quantity of photovoltaic cells, and means 14 for moving the photovoltaic cells in the basin. The photovoltaic cells remain resting on the bottom 18 of the basin and remain immersed in the process liquid. The moving means comprise a plurality of push elements 16 which, in use, translate along the basin brushing against its bottom. The invention also concerns a plant and a process for recovering silver present in a mass of photovoltaic cells.

Description

[0001] APPARATUS FOR PROCESSING PHOTOVOLTAIC CELLS, AND PLANT AND PROCESS FOR RECOVERING THE SILVER CONTAINED THEREIN

[0002] TECHNICAL FIELD

[0003] The present invention concerns an apparatus for the treatment of photovoltaic cells, and a plant and a process for recovering silver present in the photovoltaic cells.

[0004] STATE OF THE ART

[0005] Photovoltaic panels represent a strategic resource in the so-called energy transition, i.e. in the transition from the use of non-renewable energy sources, mainly consisting of fossil hydrocarbons, to the exploitation of renewable energy sources.

[0006] The massive installation that has been promoted since the 90s of the twentieth century will soon result in the need to dispose of a large quantity of exhausted photovoltaic panels. The operational lifespan of the photovoltaic panels is in fact estimated at around 25-30 years, after which the panel must be replaced because it is no longer productive. Early replacements may be desirable due to the availability of newly developed photovoltaic panels with higher efficiency, while a proportion of panels must be replaced early due to failures or damages. It is estimated that around 1 billion photovoltaic panels are currently installed in Europe, for a total mass of 25 million tonnes of material to be recycled.

[0007] The correct disposal of the panels is therefore an integral part of the challenge of the energy transition. However, as explained in detail below, such disposal is not easy and presents considerable technical difficulties.

[0008] The most common photovoltaic panels are obtained by hot-rolling a plurality of different layers, including the actual photovoltaic cells.

[0009] Each photovoltaic cell comprises a support substrate of weakly doped silicon. On the front face of the support substrate, an upper layer of silicon is arranged having a doping type opposite to that of the support substrate. For example, if the support substrate has a P-type doping (boron doping), the upper layer has an N-type doping (phosphorus doping). In this way, a P-N junction is formed that separates the charges of reverse polarity (electrons and holes) that are generated when the cell is exposed to light radiation. The upper layer typically has a thickness of less than 1 pm.

[0010] A lower layer of high-doped silicon, of the same doping type as that of the support substrate, is normally placed on the lower face of the support substrate. The lower layer has, for example, a thickness of about 5 pm.

[0011] The photovoltaic cell further comprises a plurality of silver lines, distributed in a relatively even manner on the upper layer, which collect the charges generated in the underlying semiconductor structure. Such silver lines are generally composed of an alloy comprising between 93% and 97% silver. These lines usually have a thickness of about 20 pm and a width of about 100 pm, and can be spaced by a distance of typically 2 mm.

[0012] On the upper layer, around the silver lines, there is usually an antireflection layer, made of silicon nitride (ShN-i) or silicon oxide (SiO?), which allows to limit the losses due to the reflection of the solar radiation.

[0013] The photovoltaic cell also comprises one or more contacts on the side of the lower face of the support substrate, usually a layer of aluminium paste. For example, this lower layer of aluminium may have a thickness comprised between one and a few tens of micrometres, usually 30 micrometres.

[0014] Finally, due to the diffusion of aluminium into silicon that occurs during the heat treatments for the realization of the cell, between the lower layer of silicon and the lower layer of aluminium there may be present an intermediate layer formed by a silicon and aluminium alloy. The thickness of such an intermediate layer may be, for example, comprised between 2 pm and 5 pm.

[0015] In some cases, other silver lines, similar to those described above, may be provided on the lower aluminium layer.

[0016] In light of what has been reported above, it can be well understood how silver is the most precious material among those present in the photovoltaic cells. It can be estimated that, in the mass of the solar panels corresponding to an installed power of 1 MW, there is a quantity of silver worth about 25,000 Euro.

[0017] The processes aimed at recovering silver present both in the exhausted photovoltaic panels and in the production waste of the photovoltaic cells are therefore of extreme interest. Most of these processes are based on hydrometallurgical processes that involve the complete dissolution of silver. In accordance with such processes, the silver passes into solution in the form of an ion, after which it must first be extracted, typically as a salt by addition of further reagents, and subsequently reduced.

[0018] To overcome the disadvantages of such hydrometallurgical processes, FR 3 096833 proposes a process intended to treat scrap of photovoltaic cells each comprising a silicon wafer on the upper surface of which an anti-reflective layer is provided, and a plurality of silver lines.

[0019] The process of FR 3 096 833 comprises, in succession, the two steps of: attacking the anti-reflective layer by immersing the scrap in a solution of hydrofluoric acid (HF) in a concentration comprised between 0.5% and 5%; and attacking the upper layer of the wafer by immersing the scrap, now devoid of the anti-reflective layer, in a solution of sodium hydroxide (NaOH) in a concentration comprised between 1% and 30% .

[0020] This results in the separation of the silver lines. The process then provides for drying the mixture formed by the residues of the wafer and by the separated silver lines; and finally extracting the silver lines in the solid state.

[0021] As can be well understood, this process has the considerable advantage of recovering silver directly in the solid state, so that it can be melted and reused without other intermediate operations.

[0022] However, although it represents a considerable improvement of the prior art, not even the process proposed by FR 3 096833 is devoid of disadvantages.

[0023] In fact, it requires that, in order to make the silver available, the entire mass of the scrap be subjected to two different chemical treatments. In addition, both treatments must be carried out with reagents (hydrofluoric acid and sodium hydroxide) which, although they are used at low concentration, are potentially very dangerous and therefore require precautions and specific plant solutions. Furthermore, the process requires a large quantity of such reagents, as they are very aggressive towards silicon.

[0024] A solution to the problems left unresolved by FR 3 096 833 is proposed in patent document EP 4400617 Al, on behalf of the same Applicant. EP 4400617 Al describes a process employing a weak acid and ultrasounds. However, this process is rather slow and therefore not advantageous from the point of view of industrial application. Furthermore, EP 4400617 Al merely describes a process without envisaging any apparatus or plant to carry it out.

[0025] In the known solutions, the photovoltaic cells are placed in baskets that are immersed in the basins containing the chemical solutions with the disadvantage that the process is of the batch-type and can be slow. In addition, the loading and unloading of the basket can be complex as the wet cells tend to adhere to the surfaces due to the high surface tension of the water. Finally, the presence of a basket can interfere with the application of the ultrasounds to the cells, reducing their effectiveness in detaching the silver.

[0026] The need is therefore felt for a new type of apparatus, plant and process that improve the known solutions.

[0027] OBJECTS AND SUMMARY OF THE INVENTION

[0028] The object of the present invention is therefore to overcome the drawbacks highlighted above in relation to the prior art.

[0029] In particular, a task of the present invention is to make available an apparatus for the treatment of the photovoltaic cells that facilitates the recovery of the silver contained therein.

[0030] Furthermore, a task of the present invention is to make available an apparatus, a plant and a process for recovering silver from the scrap of photovoltaic cells that do not imply the production of hazardous wastewater.

[0031] Furthermore, a task of the present invention is to make available an apparatus, a plant and a process for recovering silver from the scrap of photovoltaic cells that reduce the times required for the treatment.

[0032] Furthermore, a task of the present invention is to make available an apparatus, a plant and a process for recovering silver from the scrap of photovoltaic cells that allow a treatment in continuous mode of the photovoltaic cells.

[0033] Furthermore, a task of the present invention is to make available an apparatus, a plant and a process for recovering silver from the scrap of photovoltaic cells that allow the photovoltaic cells to be treated without damaging them.

[0034] Furthermore, a task of the present invention is to make available an apparatus, a plant and a process for recovering silver from the scrap of photovoltaic cells that allow ultrasounds to be used to the maximum of their effectiveness.

[0035] Finally, a task of the present invention is to make available an apparatus, a plant and a process for recovering silver from the scrap of photovoltaic cells which, in addition to making possible the advantages described above, allow to maintain the main advantages of the plants and of the processes of the prior art.

[0036] These and other objects and tasks of the present invention are achieved by means of an apparatus in accordance with claim 1, by means of a plant in accordance with claim 4 and by means of a process in accordance with claim 5. Further features are identified in the dependent claims. The features of the dependent claims may be freely combined with each other to meet specific needs. All appended claims form an integral part of the present disclosure.

[0037] In a first aspect thereof, the invention concerns an apparatus for the treatment in continuous mode of photovoltaic cells. The apparatus comprises at least one basin, configured for containing a process liquid and for containing a pre-defined quantity of photovoltaic cells; and means for moving the photovoltaic cells inside the basin. The basin is configured such that the photovoltaic cells remain resting on the bottom of the basin and remain immersed in the process liquid. The moving means comprise a plurality of push elements. The basin and the moving means are configured such that, in use, the push elements translate along the basin brushing against its bottom and pushing the photovoltaic cells.

[0038] The particular structure of the invention allows to effectively perform a treatment in continuous mode on the photovoltaic cells which facilitates the recovery of the silver contained therein.

[0039] Preferably the apparatus further comprises a plurality of ultrasound generators applied at the bottom of the basin.

[0040] The use of the ultrasounds allows to increase the speed and the overall effectiveness of the treatment. Preferably, the apparatus further comprises heating elements configured for maintaining, in use, the process liquid present in the basin at a temperature comprised in a pre-defined range.

[0041] The possibility of controlling the temperature of the process liquid allows to increase the speed and the overall effectiveness of the treatment.

[0042] In accordance with some embodiments, the apparatus comprises two basins arranged in series. The second basin, arranged downstream of the first basin, comprises a plurality of ultrasound generators applied at the bottom.

[0043] In case the apparatus comprises two basins, it is preferable that the ultrasounds are applied in the second basin.

[0044] In a second aspect, the invention concerns a plant for recovering silver present in a mass of photovoltaic cells. The plant of the invention comprises:

[0045] - controlled release means, configured for releasing a pre-defined flowrate of photovoltaic cells; an apparatus for the treatment of the photovoltaic cells, in accordance with what is described above; a rotary trommel screen, downstream of the apparatus, configured for collecting the photovoltaic cells exiting the apparatus; a washing circuit configured for washing with a washing solution the photovoltaic cells in the trommel screen; and

[0046] - means for drying the photovoltaic cells, downstream of the trommel screen.

[0047] The washing circuit is configured for:

[0048] - collecting the dirty washing solution after washing the photovoltaic cells in the trommel screen; and

[0049] - treating the dirty washing solution for separating the solid particles in suspension.

[0050] The overall structure of the system makes it possible to best exploit the characteristics of the apparatus of the invention and to obtain an effective and continuous treatment of the photovoltaic cells.

[0051] In a third aspect, the invention concerns a process for recovering silver present in a mass of photovoltaic cells. The process of the invention comprises the steps of:

[0052] - providing a mass of photovoltaic cells each comprising a silicon wafer upon which silver lines are provided;

[0053] - immersing the photovoltaic cells in a process liquid comprising water, a weak acid and a fluorine salt soluble in water;

[0054] - removing the photovoltaic cells from the process liquid;

[0055] - washing the photovoltaic cells with a washing solution;

[0056] - collecting the dirty washing solution after washing the photovoltaic cells; and

[0057] - treating the dirty washing solution for removing solid particles comprising silver and obtaining a clean washing solution.

[0058] The process of the invention allows to efficiently recover the silver present in the mass of photovoltaic cells.

[0059] The weak acid is selected in the group comprising: citric acid, tartaric acid, lactic acid, malic acid, acetic acid, formic acid, ethylenediaminetetraacetic acid (EDTA), ascorbic acid, butyric acid, maleic acid, malonic acid, and methanesulfonic acid. Advantageously the weak acid is present in the process liquid in a concentration comprised in the range 0.5%-10%, preferably l%-5%.

[0060] The fluorine salt soluble in water is sodium fluoride (NaF) or potassium fluoride (KF). Advantageously the fluorine salt soluble in water is present in the process liquid in a concentration comprised in the range 0.5% -10%, preferably l%-5%.

[0061] The types of reagents and the relative concentrations selected by the Applicant ensure that the process liquid is not hazardous.

[0062] Preferably the process further comprises, after the step of immersing the photovoltaic cells in the process liquid, the step of maintaining the process liquid at a temperature comprised between 25 °C and 85 °C, preferably between 40 °C and 70 °C.

[0063] The possibility of maintaining the temperature of the process liquid at the indicated temperatures allows to increase the speed and the overall effectiveness of the treatment. Preferably the process further comprises the step of applying ultrasounds to the process liquid or to the washing solution.

[0064] The use of the ultrasounds allows to increase the speed and the overall effectiveness of the treatment.

[0065] Preferably the step of treating the dirty washing solution comprises one or more of: decantation, filtration and centrifugation. Advantageously the step of treating the dirty washing solution comprises the addition of calcium chloride (CaCl2).

[0066] The ancillary steps indicated above guarantee optimal treatment of the washing solution.

[0067] Preferably the process further comprises, after the step of treating the dirty washing solution, the step of re-circulating the clean washing solution for washing the photovoltaic cells.

[0068] The step of re-circulating the washing solution allows to strongly limit the consumption of water.

[0069] Further features, purposes and advantages of the present invention will become more evident from the description below.

[0070] BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The invention will be described hereinbelow with reference to some examples, provided by way of non-limiting example, and illustrated in the appended drawings. These drawings illustrate different aspects and embodiments of the present invention and reference numerals illustrating structures, components, materials and / or similar elements in different drawings are indicated by similar reference numerals, where appropriate. Moreover, for clarity of illustration, certain references may not be repeated in all figures.

[0072] Figure 1 shows an axonometric view of an embodiment of the apparatus in accordance with the invention; figure 2 shows an axonometric view of another embodiment of the apparatus in accordance with the invention; figure 3 shows a partially exploded axonometric view of the apparatus of figure 1; figure 4 shows a partially exploded axonometric view of the apparatus of figure 2; figure 5 shows a schematic view of an embodiment of the plant in accordance with the invention; figure 6 shows a schematic view of another embodiment of the plant in accordance with the invention; and figures 7 show enlarged views of two possible embodiments of the detail indicated with VII in figure 4.

[0073] DETAILED DESCRIPTION OF THE INVENTION

[0074] While the invention is susceptible to various modifications and alternative constructions, certain preferred embodiments are shown in the drawings and are described hereinbelow in detail. It must in any case be understood that there is no intention to limit the invention to the specific embodiment illustrated, but, on the contrary, the invention intends covering all the modifications, alternative, same and equivalent constructions that fall within the scope of the invention as defined in the claims.

[0075] The description addresses in detail the peculiar technical aspects and features of the invention, while the aspects and technical features known per se can only be mentioned. In these respects, what is reported above with reference to the prior art remains valid.

[0076] The use of "for example", "etc.", "or" indicates non-exclusive alternatives without limitation, unless otherwise indicated. The use of "comprises" and "includes" means "comprises or includes, but not limited to", unless otherwise indicated.

[0077] The invention defines in a unique manner a processing flow of the photovoltaic cells. In the following discussion, it is understood that, in relation to the direction of this processing flow, the terms "before", "upstream" and the like are defined unambiguously with respect to the terms "after", "downstream" and the like.

[0078] The invention is intended to operate on photovoltaic cells which, in their operational life, are directed towards the prevailing course of the sun. In consideration of this, in the present discussion the term "upper" is used to indicate a part of the photovoltaic cell that, in the operational life, is facing the sun, while the term "lower" indicates a part of the photovoltaic cell, opposite to the upper one, which in the operational life is facing away from the sun.

[0079] In a first aspect thereof, the invention concerns an apparatus 10 for the treatment in continuous mode of photovoltaic cells. The apparatus 10 of the invention comprises at least one basin 12, configured for containing a process liquid and for containing a pre-defined quantity of photovoltaic cells; and means 14 for moving the photovoltaic cells inside the basin 12, wherein:

[0080] - the basin 12 is configured such that the photovoltaic cells remain resting on the bottom of the basin 12 and remain immersed in the process liquid;

[0081] - the moving means 14 comprise a plurality of push elements 16; and

[0082] - the basin 12 and the moving means 14 are configured such that, in use, the push elements 16 translate along the basin 12 brushing against its bottom 18.

[0083] As mentioned above, the invention is intended to treat scrap of photovoltaic cells. In the following, for greater simplicity of exposure, reference will be simply made to "photovoltaic cells", meaning by this indifferently scrap deriving from the recycling of exhausted or end-of-life photovoltaic panels, but also new photovoltaic cells that are defective or in any case deriving from production waste. The photovoltaic cells considered herein each comprise a silicon wafer having an upper surface (intended, in use, to face the sun) and a lower surface (opposite to the upper surface). An anti-reflective layer of silicon nitride (ShN-i) or silicon oxide (SiO?) and silver lines (Ag) are generally provided on the upper surface, while a layer of aluminium paste (Al) and other silver lines are generally provided on the lower surface. The silver lines usually comprise sintered silver powder.

[0084] As mentioned above, the apparatus 10 of the invention comprises at least one basin 12, configured for containing the process liquid and the photovoltaic cells, and means 14 for moving the photovoltaic cells inside the basin 12 itself. The basin 12 is configured for subjecting the photovoltaic cells to a chemical treatment or to a chemical-physical treatment, depending on the embodiments. In accordance with some embodiments, such as for example those depicted in the attached figures, the basin 12 assumes an elongated shape. The moving means 14 comprise a conveyor belt 20 that bears the push elements 16 that, in use, translate along the basin 12 brushing against its bottom 18. In other words, the basin 12 and the moving means 14 are configured such that the push elements 16 remain in contact with the bottom 18 of the basin 12 and that, in use, they slide thereon, so as to ensure the movement of the photovoltaic cells along the main direction of the basin 12. In this way the photovoltaic cells are introduced at a first end (inlet) of the basin 12 and the moving means 14 drag them along the main direction of the basin 12.

[0085] Advantageously, as can be appreciated from the attached figures, the basin 12 and the moving means 14 are also configured such that, at the end of the path inside the basin 12, the photovoltaic cells exit from the second end (outlet) of the basin 12, opposite to the first one. In particular, the bottom 18 of the basin 12 has, near the outlet, a ramp-like portion 42, which is inclined by an angle a preferably comprised between 5° and 45°. Advantageously, the moving means 14 are configured such that the push elements 16 brush against the ramp-like portion 42, pushing the photovoltaic cells until they fall outside the basin 12 itself, so feeding another device placed downstream of the basin 12, which will be described further.

[0086] Preferably, near the outlet, the basin 12 comprises a dripper 44, i.e. a small substantially vertical bulkhead that facilitates the correct escape of the process liquid and the fall of the photovoltaic cells themselves, according to the desired path.

[0087] For simplicity of representation, figures 1 to 4 show push elements 16 in the form of vanes having a solid and continuous surface. Preferably the push elements 16 are configured so as to effectively push the photovoltaic cells along the basin 12 and, at the same time, so as to be permeable to the process liquid. In this way, during the operation of the plant, the photovoltaic cells are dragged along the entire basin 12, while the process liquid is simply stirred up, minimizing its leakage downstream of the basin 12. Some examples of embodiments of push elements 16 suitable for the moving means 14 of the invention are shown in figures 7, described in more detail below.

[0088] Figure 7.a shows a push element 16 configured as a comb, i.e. comprising a dense array of rigid or semi-rigid prongs, which are straight and parallel to each other. The shape, the density and the flexibility of the prongs may vary depending on the specific application needs.

[0089] Figure 7.b shows a push element 16 configured as a brush, i.e. comprising a dense array of tufts of flexible or semi-rigid bristles. Preferably the tufts of bristles have a diameter comprised between 3 mm and 5 mm and the distance between them (centre distance) is comprised between 5 mm and 15 mm. However, the shape, the density and the flexibility of the bristles may vary depending on the specific application needs.

[0090] This shape is particularly suitable as the photovoltaic cells resting on the bottom 18 of the basin 12 are arranged essentially along a horizontal direction and therefore orthogonal to the direction of the push elements 16 (comb- or brush-shaped) which can then displace them easily.

[0091] The presence of flexible bristles that slide on the bottom 18 can also conveniently displace the solid silver particles, if present, removing them from the basin 12.

[0092] In accordance with some embodiments (see for example the diagram of figure 5), the apparatus 10 of the invention comprises two basins 12', 12", arranged in series. Advantageously, both basins 12', 12" comprise moving means 14 in accordance with what is described above.

[0093] The first basin 12' is intended to host a chemical treatment of the photovoltaic cells. In particular, the first basin 12' is configured for containing a process liquid comprising water, a weak acid and a fluorine salt soluble in water. In this case, the process liquid is a detaching solution, suitable for promoting the detachment of the silver and of the aluminium from the silicon wafer of the photovoltaic cells.

[0094] The weak acid is selected in the group comprising: citric acid, tartaric acid, lactic acid, malic acid, acetic acid, formic acid, ethylenediaminetetraacetic acid (EDTA), ascorbic acid, butyric acid, maleic acid, malonic acid, and methanesulfonic acid. Preferably the weak acid is present in the process liquid in a concentration comprised in the range 0.5% -10%, even more preferably l%-5%.

[0095] The fluorine salt soluble in water may be sodium fluoride (NaF) or potassium fluoride (KF). Preferably the fluorine salt is present in the process liquid in a concentration comprised in the range 0.5% -10%, even more preferably 17o-5% .

[0096] The combination of these reagents, in relatively low concentrations, allows to have a non-hazardous detaching solution at a weakly acidic or almost neutral pH.

[0097] Preferably the first basin 12' comprises heating elements configured for maintaining, in use, the process liquid of the first basin 12' at a temperature comprised in a pre-defined range. Preferably, in use, the temperature of the process liquid is comprised between 25 °C and 85 °C, more preferably between 40 °C and 70 °C. In a manner known per se, the heating elements are preferably feedback-controlled by temperature sensors. Furthermore, the heating elements, known per se, may comprise one or more of: electrical resistors, induction heaters, microwave generators and the like.

[0098] The first basin 12' can be made, at least partially, of stainless steel or of polypropylene (PP). Polypropylene is preferable in the case where the first basin 12' is intended to operate at temperatures close to room temperature (for example at a temperature comprised between 15 °C and 40 °C). Otherwise, stainless steel is preferable in case the first basin 12' is intended to operate at higher temperatures (e.g. above 50 °C). As the skilled person can well understand, stainless steel is necessary in case the heating elements comprise induction heaters.

[0099] As the skilled person can well understand, the combined chemical action of the weak acid and of the fluorine salt allows to significantly weaken the bond between the silver tracks and the silicon wafer of the photovoltaic cells. In fact, while the weak acid has its own effectiveness, dissolving the elements present in the composition of the silver tracks (but not the silver), the fluoride in solution attacks the surface of the silicon on which the silver tracks are deposited, thus facilitating the detachment of the silver tracks themselves.

[0100] The moving means 14 of the first basin 12' are configured such that, at the end of the path inside the first basin 12', the photovoltaic cells fall in the second basin 12". As described above, the push elements 16 are preferably configured for minimizing the movement of the process liquid. However, a minimal downstream leakage of the process liquid is unavoidable. In addition to this, it is also necessary to consider the evaporation of the water present in the process liquid. To counteract the combined effect of these two distinct phenomena, the first basin 12' preferably comprises a circuit 46 for the replenishment of each of the components of the process liquid (typically: water, weak acid and fluorine salt). In this way it is guaranteed that the process liquid with a pre-defined volume and composition is constantly present inside the first basin 12', even after a long period of continuous operation of the apparatus 10.

[0101] In the apparatus 10 of the type depicted in figure 5, a second basin 12", intended to host a physical treatment of the photovoltaic cells, is placed downstream of the first basin 12'. For this purpose, the second basin 12", in addition to the means 14 for moving the photovoltaic cells, also comprises a plurality of ultrasound generators 22 applied at the bottom 18. Preferably the ultrasound generators 22 are configured for generating ultrasounds at a frequency comprised between 20 kHz and 60 kHz, preferably at a frequency of about 27 kHz. Preferably each of the ultrasound generators 22 has a power comprised between 40 W and 120 W, even more preferably between 60 W and 100 W. The number of ultrasound generators 22 provided at the bottom 18 of a single basin 12" depends on the sizes of the latter. Preferably the ultrasonic generators 22 are configured for applying a specific power higher than 50W per each litre of liquid, preferably a specific power of about 400W / litre.

[0102] To this end, the second basin 12" will in this case be sized to have a mainly horizontal development, thus limiting the height of the contained process liquid.

[0103] The second basin 12" is preferably made at least partially of steel, for example stainless steel. In particular, it is preferable that at least the bottom 18 of the second basin 12" is made of steel. This particular embodiment makes it possible to optimize the effect of the ultrasounds. The mechanical characteristics of the steel prevent the vibrations produced by the ultrasonic generators 22 from being damped.

[0104] The second basin 12" is configured such that the introduced photovoltaic cells come to rest directly on the bottom 18, in direct contact with it. The presence of any other intermediate elements, such as containment and / or moving baskets, covers of the bottom 18 or the like, would introduce a significant damping of the vibrations and would therefore imply a drastic decrease in the effectiveness of the ultrasounds. For this reason, the photovoltaic cells introduced in the second basin 12" rest directly on the bottom 18 and the moving means 14 push them along the basin 12 so that they constantly maintain contact with the bottom 18. The process liquid of the second basin 12" is an extremely diluted aqueous solution. At the beginning of the operation of the apparatus 10, this process liquid consists substantially of only water. However, during the operation of the apparatus 10, the supply of the photovoltaic cells from the first basin 12' inevitably entails the passage of a minimum quantity of the relative process liquid, i.e. detaching solution, in the second basin 12".

[0105] As will be better described below with reference to the process of the invention, the mechanical action of the ultrasounds makes it possible to obtain the detachment of the silver tracks and of the aluminium paste from the photovoltaic cells. In this way, the silver and the aluminium detach from the silicon wafer, which remains substantially naked, and form the solid particles that are distributed in the aqueous solution.

[0106] The second basin 12" preferably comprises a circuit 48 for replenishing the aqueous solution. In this way it is guaranteed that even inside the second basin 12" a pre-defined volume of aqueous solution is constantly present, even after a long period of continuous operation of the apparatus 10.

[0107] In accordance with some embodiments, the replenishment circuit 48 is fed directly from the water network in such a way as to introduce water into the second basin 12".

[0108] In other preferred embodiments, the replenishment circuit 48 is instead fed by a treatment system 38 which will be described later. Depending on the desired replacement, the excess process liquid is made to overflow from the second basin 12" to the outlet end together with the photovoltaic cells.

[0109] In the first basin 12' the process liquid (i.e. the detaching solution) must be retained as much as possible and is therefore kept below the level of the outlet of the first basin 12'. Conversely, in the second basin 12" it is not important to minimize the loss of process liquid (i.e. of aqueous solution) that escapes together with the photovoltaic cells, therefore the second basin 12" is filled to the overflow, up to the level of the outlet.

[0110] Preferably the auxiliary circuit 48 and the washing circuit 30 (which will be described later) are integrated and connected to the same treatment system 38.

[0111] Preferably, the replenishment circuit 48 feeds the aqueous solution near the inlet of the second basin 12". Preferably, the replenishment circuit 48 feeds a flowrate of aqueous solution or water sufficient to create a flow inside the second basin 12" suitable for dragging the aluminium particles in suspension downstream. In fact, from the tests carried out by the Applicant it emerged that the action of the ultrasounds is strongly compromised by the presence of the aluminium particles in suspension. For this reason, it is preferable to induce a flow of aqueous solution that constantly removes the aluminium in suspension from the second basin 12". The aluminium is then recovered downstream, through a treatment circuit 38 which will be described below.

[0112] In accordance with other embodiments (see for example the diagram of figure 6), the apparatus 10 comprises a single basin 12. In this case, the single basin 12 present in the apparatus 10 incorporates in itself all the features of the first basin 12' and of the second basin 12", with the sole exception of the replenishment plants 46 and 48, which in the single basin 12 are replaced by an auxiliary circuit 50 which will be described in more detail below. Therefore, for a detailed description of the single basin 12, please refer to the previous description: the skilled person will have no difficulty in combining into a single basin 12 what is reported above separately for the first basin 12' and for the second basin 12", as well as what is reported below in relation to the auxiliary circuit 50.

[0113] The single basin 12 is intended to host a chemical-physical treatment of the photovoltaic cells. In use, it contains a process liquid similar to that described above for the first basin 12'. Such a process liquid is a detaching solution comprising water, a weak acid and a fluorine salt soluble in water.

[0114] The auxiliary circuit 50 is configured for:

[0115] - withdrawing the process liquid from the single basin 12;

[0116] - treating the process liquid so as to remove the solid aluminium particles in suspension; and

[0117] - re-introducing the process liquid without aluminium into the single basin 12.

[0118] Preferably, in order to remove the aluminium from the process liquid, the auxiliary circuit 50 comprises one or more of: a hydrocyclone, a centrifuge, a filter, a filterpress. As the skilled person can well understand, the auxiliary circuit 50, after removing the aluminium, constantly re-circulates the process liquid with the weak acid and the fluorine salt. However, even for the single basin 12, a downstream leakage and evaporation are inevitable, which lead to a decrease in the process liquid. Preferably the auxiliary circuit 50 is configured for ensuring the replenishment in the single basin 12 of each of the components of the process liquid (typically: water, weak acid and fluorine salt). In this way, it is guaranteed that the process liquid with a pre-defined volume and composition is constantly present inside the single basin 12.

[0119] In a second aspect thereof, the invention concerns a plant 24 for recovering silver present in a mass of photovoltaic cells, in particular in a mass of scrap of photovoltaic cells. The plant 24 of the invention comprises:

[0120] - controlled release means 26, configured for releasing a pre-defined flowrate of photovoltaic cells; an apparatus 10 for the treatment of the photovoltaic cells, in accordance with what is described above; a rotary trommel screen 28, downstream of the apparatus 10, configured for collecting the photovoltaic cells exiting the apparatus 10; a washing circuit 30 configured for washing with a washing solution the photovoltaic cells in the trommel screen 28; and

[0121] - means 32 for drying the photovoltaic cells, downstream of the trommel screen 28.

[0122] In addition, the washing circuit 30 is configured for:

[0123] - collecting the dirty washing solution after washing the photovoltaic cells in the trommel screen 28; and

[0124] - treating the dirty washing solution for separating the solid particles in suspension.

[0125] The controlled release means 26 may comprise for example a hopper or a different storage container within which a batch of photovoltaic cells to be treated is loaded. As will be clear from the following description, the plant 24 of the invention, as well as the apparatus 10, is configured for operating continuously; therefore, the controlled release means 26 are configured for receiving the photovoltaic cells in input (with loading that can be of the batch type or, in turn, continuous) and for releasing with continuity a pre-defined flowrate of photovoltaic cells in output. This makes it possible to make the operation of the plant 24 independent of any previous treatments carried out upstream. Preferably, the controlled release means 26 allow the released flowrate to be modified as a function of certain specific conditions that affect the treatment speed along the plant 24 itself.

[0126] As mentioned above, downstream of the apparatus 10 there is provided a rotary screen 28 (also known as trommel), in itself widely known to the skilled person, comprising a washing circuit 30. In short, the screen 28 receives in input the photovoltaic cells exiting the apparatus 10 (i.e. the second basin 12" or the single basin 12, as the case may be). The washing circuit 30 is configured so as to spray the photovoltaic cells inside the trommel screen 28 with a washing solution. For example, the washing circuit 30 may comprise one or more showers 34 or other nozzles for dispensing the washing solution.

[0127] In accordance with some embodiments, the washing circuit 30 is fed directly from the water network in such a way as to spray the photovoltaic cells with water.

[0128] In accordance with other preferred embodiments, the washing circuit 30 is also configured for, after treating the dirty washing solution, re-circulating the clean washing solution for washing the photovoltaic cells in the trommel screen 28. This significantly limits the consumption of water.

[0129] The mechanical action of the washing solution allows to remove the solid particles of silver and aluminium that, although detached from the wafer, continue to adhere to the photovoltaic cells due to the veil of liquid that wets them. At the same time, the rotation of the trommel screen 28 allows the photovoltaic cells to be stirred up so as to repeatedly expose all their faces to the washing action.

[0130] As mentioned above, the washing circuit 30 is configured for collecting the dirty washing solution, i.e. comprising the solid particles of silver and of aluminium dragged away from the photovoltaic cells during the washing action carried out in the trommel screen 28. The dirty washing solution is then collected in a special collector 36 and sent to a treatment circuit 38 aimed at removing the solid particles of aluminium and of silver, so as to obtain a clean washing solution again, suitable for being disposed of or re-circulated in the washing circuit 30 itself. The dirty washing solution also comprises the solid particles of aluminium and of silver, as well as the residues of the weak acid and of the fluorine salt that have travelled through the plant 24 together with the photovoltaic cells. In contrast, the clean washing solution is substantially an extremely diluted aqueous solution (e.g. of sodium chloride, i.e. it is salt water), which entrains with itself traces of the other substances with which it has come into contact.

[0131] As the skilled person can well understand, the washing solution has substantially the same composition as the process liquid of the second basin 12". In accordance with some embodiments of the plant (see for example figure 5) the process liquid of the second basin 12" is the washing solution.

[0132] In accordance with some embodiments of the plant 24, the washing circuit 30 also feeds the replenishment circuit 48 of the second basin 12". This significantly limits the consumption of water. In accordance with such embodiments, the same solution serves as a process liquid in the second basin 12" and as a washing solution in the rotary trommel screen 28.

[0133] The treatment circuit 38 of the dirty washing solution preferably comprises one or more of: a decantation basin, a hydrocyclone, a centrifuge, a filter, a filterpress or a knurled sluice box, of the type used by gold prospectors.

[0134] Preferably, the treatment circuit 38 also comprises a line 40 for the addition of calcium chloride (CaCb), suitable for optimizing the treatment of the dirty washing solution, in particular for the removal of the fluorides present.

[0135] The operation of the treatment circuit 38 of the dirty washing solution will be described in more detail below, with reference to the process of the invention.

[0136] Downstream of the rotary trommel screen 28, the plant 24 of the invention finally comprises means 32 for drying the photovoltaic cells. In light of what is reported above, it will be clear to the skilled person that the drying means 32 receive, exiting from the trommel screen 28, photovoltaic cells wetted by a veil of salt water. Incidentally, in this step the photovoltaic cells are essentially reduced to silicon wafers only, since the other components have already been removed upstream. The drying means 32 must simply facilitate the complete evaporation of the water. By way of example the drying means 32 may comprise a conveyor belt 20 and / or a rotary trommel, which are preferably ventilated and / or heated. The photovoltaic cells exiting the drying means 32 are then directed to subsequent processing steps.

[0137] In a third aspect thereof, the invention concerns a process for recovering silver present in a mass of photovoltaic cells, in particular in a mass of scrap of photovoltaic cells. The process of the invention comprises the steps of: - providing a mass of photovoltaic cells each comprising a silicon wafer upon which silver lines are provided;

[0138] - immersing the photovoltaic cells in a process liquid comprising water, a weak acid and a fluorine salt soluble in water;

[0139] - removing the photovoltaic cells from the process liquid;

[0140] - washing the photovoltaic cells with a washing solution;

[0141] - collecting the dirty washing solution after washing the photovoltaic cells; and

[0142] - treating the dirty washing solution for removing solid particles comprising silver and obtaining a clean washing solution.

[0143] Preferably the photovoltaic cells each comprise a silicon wafer having an upper surface (intended, in use, to face the sun) and a lower surface (opposite to the upper surface). An anti-reflective layer of silicon nitride (ShN-i) or silicon oxide (SiCh) and silver lines are provided on the upper surface, while a layer of aluminium paste and other silver lines are generally provided on the lower surface. The silver lines usually comprise sintered silver powder.

[0144] The weak acid is selected in the group comprising: citric acid, tartaric acid, lactic acid, malic acid, acetic acid, formic acid, ethylenediaminetetraacetic acid (EDTA), ascorbic acid, butyric acid, maleic acid, malonic acid, and methanesulfonic acid. Preferably the weak acid is present in the process liquid in a concentration comprised in the range 0.5% -10%, even more preferably l%-5%.

[0145] The fluorine salt soluble in water may be sodium fluoride (NaF) or potassium fluoride (KF). Preferably the fluorine salt is present in the process liquid in a concentration comprised in the range 0.5% -10%, even more preferably l%-5%.

[0146] Preferably the process of the invention, after the step of immersing the photovoltaic cells in the process liquid comprising water, a weak acid and a fluorine salt soluble in water, further comprises the step of maintaining the process liquid at a temperature comprised between 25 °C and 85 °C, preferably between 40 °C and 70 °C.

[0147] In accordance with some embodiments, the process of the invention provides for applying ultrasounds to the process liquid or to the aqueous solution in which the photovoltaic cells are immersed. Preferably, the step of applying ultrasounds provides for one or more of the following characteristics: a frequency comprised between 20 kHz and 60 kHz, preferably a frequency of about 27 kHz; and / or a specific power higher than 50 W / litre, preferably a specific power of about 400 W / litre; and / or a duration comprised between 1 and 20 minutes, preferably a duration of about 5 minutes.

[0148] Preferably the step of treating the dirty washing solution comprises one or more of: decantation, filtration and centrifugation.

[0149] Preferably, the process of the invention provides for re-circulating the clean washing solution for washing the photovoltaic cells.

[0150] Advantageously, the step of treating the dirty washing solution comprises the addition of calcium chloride (CaCl2), an additive useful for the treatment of the washing solution. In particular, the addition of calcium chloride to the washing solution allows to react the traces of (sodium or potassium) fluoride present in the dirty washing solution, obtaining one of the following reactions:

[0151] CaCl2+ 2NaF 2NaCl + CaF2

[0152] CaCl2+ 2KF 2KC1 + CaF2

[0153] As the skilled person can understand, in the first case the addition of calcium chloride allows to obtain a solution of sodium chloride (cooking salt), while in the second case a solution of potassium chloride is obtained. In both cases a calcium fluoride precipitate is then obtained which is an extremely poorly soluble salt and which can therefore be easily removed from the washing solution by centrifugation and / or filtration.

[0154] In accordance with some embodiments of the invention, during the same centrifugation and / or filtration step, (calcium or potassium) fluoride, aluminium and silver are removed from the washing solution, which are then subsequently separated with techniques known per se.

[0155] Differently, in accordance with other embodiments of the invention, prior to the step of adding the calcium chloride, a decantation step is provided. This decantation step allows the silver to be removed, leaving in the solution the aluminium and the fluoride residues to be reacted with the calcium chloride as described above. In this case, the centrifugation and / or filtration step removes the fluoride and aluminium from the washing solution, which are then subsequently separated from each other by techniques known per se.

[0156] In accordance with still other embodiments of the invention, before the step of adding the calcium chloride, and after the decantation step with which the silver is removed, a first centrifugation and / or filtration step is provided. This first centrifugation and / or filtration step allows the aluminium to be removed, leaving in the solution only the fluoride residues to be reacted with the calcium chloride as described above. The subsequent centrifugation and / or filtration step, already described above, removes only fluoride from the washing solution.

[0157] As the skilled person can well understand from the description reported hereinabove, with the sole exclusion of the possible initial loading of the photovoltaic cells into the controlled release means 26, the apparatus 10, the plant 24 and the process of the invention are characterized by a continuous operation. The flowrate of the photovoltaic cells fed by the controlled release means 26 is defined in such a way that each of the components arranged along the path is able to perform its function in the expected cycle times, in such a way that the apparatus 10, the plant 24 and the process overall obtain the desired result. In particular, the optimal flowrate is the maximum flowrate for which each of the components arranged along the path is able to fully perform its function.

[0158] As the skilled person may well understand, certain factors may speed up or slow down the treatments in certain parts of the plant 24. By way of example, the type of the photovoltaic cells, the concentration of the reagents in the process liquids, the specific power of the ultrasounds, the temperature of the process liquid can have an effect on the speed of the respective treatments.

[0159] Preferably the process of the invention is carried out in a plant 24 and in an apparatus 10 in accordance with the invention. In this regard, the skilled person will have no difficulty in better clarifying some technical features of the process, starting from the technical features described in detail in relation to the apparatus 10 and / or to the plant 24. Similarly, the skilled person will have no difficulty in better clarifying some technical features of the apparatus 10 and / or of the plant 24, starting from the technical features described in detail in relation to the process.

[0160] As the skilled person can well understand, the invention overcomes the drawbacks highlighted above in relation to the prior art.

[0161] In particular, the invention makes available an apparatus 10 for the treatment of the photovoltaic cells that facilitates the recovery of silver contained therein.

[0162] In particular, the invention makes available an apparatus 10, a plant 24 and a process for recovering silver from the scrap of photovoltaic cells that do not imply the production of hazardous wastewater.

[0163] Furthermore, the invention makes available an apparatus 10, a plant 24 and a process for recovering silver from the scrap of photovoltaic cells which reduce the time required for treatment.

[0164] Furthermore, the invention makes available an apparatus 10, a plant 24 and a process for recovering silver from the scrap of photovoltaic cells that allow a treatment in continuous mode of the photovoltaic cells.

[0165] In addition, the invention makes available an apparatus 10, a plant 24 and a process for recovering silver from the scrap of photovoltaic cells that allow the photovoltaic cells to be treated without damaging them.

[0166] In addition, the invention makes available an apparatus 10, a plant 24 and a process for recovering silver from the scrap of photovoltaic cells that allow the ultrasounds to be used to the maximum of their effectiveness.

[0167] Finally, the invention makes available a plant 24 and a process for recovering silver from the scrap of photovoltaic cells which, in addition to making possible the advantages described above, allow to maintain the main advantages of the plants and of the processes of the prior art.

[0168] The description reported above in particular delves into the innovative aspects of the invention and the differences between this and the known solutions. For all other aspects, known by themselves, that the invention shares with the prior art, what is described in the introductory part in relation to the known solutions remains valid.

[0169] In conclusion, all the details can be replaced by other technically equivalent elements; the features described in relation to a specific embodiment can also be used in the other embodiments; the materials used, as well as the contingent shapes and dimensions, can be any according to the specific implementation needs without leaving the scope of protection of the following claims.

Claims

CLAIMS1. Apparatus (10) for the treatment in continuous mode of photovoltaic cells, comprising at least one basin (12), configured for containing a process liquid and for containing a pre-defined quantity of photovoltaic cells; and means (14) for moving the photovoltaic cells inside the basin (12), wherein:- the basin (12) is configured such that the photovoltaic cells remain resting on the bottom (18) of the basin (12) and remain immersed in the process liquid;- the moving means (14) comprise a plurality of push elements (16); and- the basin (12) and the moving means (14) are configured such that, in use, the push elements (16) translate along the basin (12) brushing against its bottom (18) and pushing the photovoltaic cells.

2. Apparatus (10) according to claim 1, further comprising a plurality of ultrasound generators (22) applied at the bottom (18) of the basin (12).

3. Apparatus (10) according to claim 1 or 2, comprising two basins (12', 12") arranged in series and wherein the second basin (12"), arranged downstream of the first basin (12'), comprises a plurality of ultrasound generators (22) applied at the bottom (18).

4. Plant (24) for recovering silver present in a mass of photovoltaic cells, comprising:- controlled release means (26), configured for releasing a pre-defined flowrate of photovoltaic cells;- an apparatus (10) for the treatment of the photovoltaic cells, according to one or more of the preceding claims; a rotary trommel screen (28), downstream of the apparatus (10), configured for collecting the photovoltaic cells exiting the apparatus (10); a washing circuit (30) configured for washing with a washing solution the photovoltaic cells in the trommel screen (28); and- means (32) for drying the photovoltaic cells, downstream of the trommel screen (28); wherein the washing circuit (30) is configured for:- collecting the dirty washing solution after washing the photovoltaic cells inthe trommel screen (28); and- treating the dirty washing solution for separating the solid particles in suspension.

5. Process for recovering silver present in a mass of photovoltaic cells, comprising the steps of:- providing a mass of photovoltaic cells each comprising a silicon wafer upon which silver lines are provided;- immersing the photovoltaic cells in a process liquid comprising water, a weak acid and a fluorine salt soluble in water;- removing the photovoltaic cells from the process liquid;- washing the photovoltaic cells with a washing solution;- collecting the dirty washing solution after washing the photovoltaic cells; and- treating the dirty washing solution for removing solid particles comprising silver and obtaining a clean washing solution; wherein the weak acid is selected in the group comprising: citric acid, tartaric acid, lactic acid, malic acid, acetic acid, formic acid, ethylenediaminetetraacetic acid (EDTA), ascorbic acid, butyric acid, maleic acid, malonic acid, and methanesulfonic acid; and the fluorine salt soluble in water is sodium fluoride (NaF) or potassium fluoride (KF).

6. Process according to claim 5, wherein the weak acid is present in the process liquid in a concentration comprised in the range 0.5% -10%, preferably l%-5%.

7. Process according to claim 5 or 6, wherein the fluorine salt soluble in water is present in the process liquid in a concentration comprised in the range 0.5% -10%, preferably 1 % -5 % .

8. Process according to one or more of claims 5 to 7, further comprising the step of applying ultrasounds to the process liquid or to the washing solution.

9. Process according to one or more of claims 5 to 8, wherein the step of treating the dirty washing solution comprises the addition of calcium chloride (CaCh).

10. Process according to one or more of claims 5 to 9, further comprising, afterthe step of treating the dirty washing solution, the step of re-circulating the clean washing solution for washing the photovoltaic cells.

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