Method for filtering an effluent in a process for recycling photovoltaic cells

Abstract

The invention relates to a method for filtering an effluent, comprising the following steps: a) supplying the effluent, referred to as first basic effluent, having a pH of greater than 12, comprising water, aluminum in the form of sodium aluminate(s), and silver in the form of solid fragments, b) acidifying the first effluent by adding hydrochloric acid, to form an acidic first solution with a pH of less than or equal to 4, wherein the aluminum is in the form of aluminum chloride, soluble in said first solution, c) filtering the first solution to recover the solid silver fragments on the one hand, and the filtered first solution on the other hand, d) neutralizing the filtered first solution, by adding sodium hydroxide, to form a second solution with a pH of between 6.5 and 8.5, wherein the aluminum is in the form of aluminum hydroxide, e) filtering the second solution to recover aluminum salts in solid form on the one hand, and the filtered second solution on the other hand.

Description

METHOD FOR FILTERING AN EFFLUENT IN A PHOTOVOLTAIC CELL RECYCLING PROCESS FIELD OF THE INVENTION

[0001] The present invention relates to the field of recycling solar modules. It relates in particular to a method of filtering effluents in the context of a process for recycling silicon-based photovoltaic cells. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Silver is the most valuable material in photovoltaic cells. The metal lattice used to collect the electrical charges generated by solar energy is usually formed by thin silver lines a few tens of micrometers thick.

[0003] In document WO2020240126, the applicant proposes a process for recycling photovoltaic cells allowing the recovery of silver in solid form. Even if the vast majority of silver lines can be recovered by this process, a portion of these lines, often in the form of small fragments, may be discharged with the effluents. SUBJECT OF THE INVENTION

[0004] The present invention proposes a solution for highly efficient filtering of effluents and thus, in particular, for recovering the fragmented part of the silver lines. It relates to a simple and economical method for filtering effluents from the recycling of photovoltaic cells, allowing the recovery of materials and the obtaining of purified effluents compatible with discharge into wastewater. BRIEF DESCRIPTION OF THE INVENTION

[0005] The invention relates to a method for filtering an effluent comprising the following steps:

[0006] (a) the supply of the effluent, called the first effluent, basic, with a pH greater than 12, comprising water, aluminium in the form of sodium aluminate(s), and silver in the form of solid fragments,

[0007] b) acidification of the first effluent by the addition of hydrochloric acid, to form a first acid solution with a pH less than or equal to 4, in which the aluminium is in the form of aluminium chloride, soluble in said first solution,

[0008] c) filtration of the first solution to recover the solid fragments of silver on the one hand, and a first filtered solution on the other hand,

[0009] (d) neutralizing the first filtered solution by adding sodium hydroxide to form a second solution with a pH of between 6.5 and 8.5, in which the aluminum is in the form of aluminum hydroxide,

[0010] e) filtration of the second solution to recover aluminum salts in solid form on the one hand, and a second filtered solution on the other hand.

[0011] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: step a) comprises the following sub-steps: a1) providing a photovoltaic cell comprising in particular: - a silicon support substrate having a front face and a rear face, - an upper layer of doped silicon, arranged on the front face, a plurality of silver lines arranged on the upper layer, and at least one anti-reflective layer arranged on the upper layer, adjacent to the silver lines, - a lower layer of heavily doped silicon, arranged on the rear face, and a rear contact layer of aluminum, arranged on the lower layer; a2) immersing the photovoltaic cell in a basic solution, leading to the etching of the contact layer and the detachment of the silver lines by local etching of the underlying upper layer,a3) the separation between:- solid macro-elements comprising the remainder of the photovoltaic cell and a major part of the silver lines, and- the first effluent comprising the basic solution, aluminum in the form of sodium aluminate(s), and a minor part of the silver lines in the form of solid fragments.which step a) comprises the following sub-steps:a1) the provision of a photovoltaic cell comprising in particular:- a silicon support substrate having a front face and a rear face,- an upper layer of doped silicon, arranged on the front face, a plurality of silver lines arranged on the upper layer, and at least one anti-reflective layer arranged on the upper layer, adjacent to the silver lines,- a lower layer of heavily doped silicon, arranged on the rear face, and a rear contact layer of aluminum, arranged on the lower layer;a2) the immersion of the photovoltaic cell,in a basic solution, leading to the etching of the contact layer and the detachment of the silver lines by local etching of the underlying upper layer;a3) the separation between:- solid macro-elements comprising the remainder of the photovoltaic cell and a major part of the silver lines, and- an effluent comprising the basic solution, aluminum in the form of sodium aluminate(s), and a minor part of the silver lines in the form of solid fragments;a4) the immersion of the solid macro-elements in a deionized water solution for their rinsing;a5) the separation between the solid macro-elements, and the first effluent comprising water, aluminum in the form of sodium aluminate(s), and silver line residues in the form of solid fragments;in step b), the first solution has a pH of between 3.5 and 4, preferably equal to 3.8;the method comprises a step d), between step d) and step e),corresponding to the addition of a flocculant in the second solution, to promote agglomeration of the aluminum salts; the method comprises a step f), after step e), corresponding to an additional filtration of the second filtered solution; the second filtered solution corresponds to a second effluent which is discharged into the wastewater., BRIEF DESCRIPTION OF THE FIGURES

[0012] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

[0013] The present steps of the effluent filtration method, in accordance with the invention;

[0014]

[0015] Laet lapresent respectively a non-limiting example of a photovoltaic cell intended to be recycled, and the solid elements obtained after a sub-step a2 involved in the supply of the effluent, supply which constitutes step a) of the method according to the invention.

[0016] The figures are schematic representations which, for the sake of readability, are not to scale. In particular, the thicknesses of the layers along the z axis are not to scale with respect to the lateral dimensions along the x and y axes; and the relative thicknesses of the layers to each other are not necessarily respected in the figures. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention relates to a method for filtering an effluent from recycling steps of a photovoltaic cell. The method is illustrated in the.

[0018] The first step a) of the method corresponds to the supply of the effluent, called first effluent, which has a basic character with a pH greater than 12, preferably around 14. Remember that the measurement of the pH of a solution can be carried out using a probe in contact with the solution and connected to a pH meter.

[0019] The first effluent comprises water, aluminum in the form of sodium aluminate(s) (NaAlO2, Na2O·Al2O3, or Na2Al2O4), and silver in the form of solid fragments. The silver fragments typically have dimensions less than or equal to 100 μm, or even 30 μm (with respect to thickness and width), and less than or equal to 2 mm (with respect to length).

[0020] Before describing the following steps of the method according to the invention, two non-limiting examples of implementation of step a), for obtaining the first effluent, are detailed below.

[0021] As stated previously, the filtration method is part of a process for recycling silicon-based photovoltaic cells. Thus, step a) comprises a first sub-step a1) corresponding to the supply of a photovoltaic cell 10, for example at the end of its life, defective or downgraded from the production line.

[0022] Such a cell typically comprises:

[0023] - semiconductor layers required to form a PN junction capable of transforming light energy into electrical charges,

[0024] - metal layers required to collect said charges and form the contacts at the terminals of which a potential difference will be established, and

[0025] - at least one anti-reflective layer placed on the face intended to be illuminated, making it possible to limit losses through reflection of solar radiation.

[0026] In particular, the cell 10 provided in sub-step a1) comprises a support substrate 1 made of silicon, lightly doped, having a front face 1a and a rear face 1b ().

[0027] The doping level (usually P-type, but possibly N-type) is typically around 10 16 cm -3 corresponding to a resistivity of the order of 1 ohm.cm.

[0028] The photovoltaic cell 10 also comprises an upper layer 2 of doped silicon arranged on the front face 1a of the support substrate 1. In particular, the upper layer 2 has a doping type opposite to that of the support substrate 1. The resistivity of the upper layer 2 is typically around 75 ohms / sq. For example, if the support substrate 1 is doped P-type (Boron doping), the upper layer 2 is N-type (Phosphorus doping). It forms, with the support substrate 1, a PN junction, to separate the charges of reverse polarity (electrons and holes) when these are generated in the cell 10 under illumination. The upper layer 2 typically has a thickness less than or equal to 1 μm.

[0029] Advantageously, a lower layer 5 of heavily doped silicon, of the same type of doping as the support substrate 1, is arranged on the rear face 1b of the support substrate 1. The lower layer 5 has for example a thickness around 5 μm with a maximum concentration of dopants of the order of 3x10 18 cm -3 at 4x10 18 cm -3 .

[0030] The photovoltaic cell 10 further comprises a plurality of silver lines 3 arranged on the upper layer 2. These silver lines constitute the metal contacts for collecting the charges generated in the underlying semiconductor structure and are distributed relatively uniformly on the upper layer 2. They are generally composed of a silver alloy comprising between 93% and 97% silver. Without this being limiting, these silver lines 3 may have a thickness and a width of between 20 μm and 100 μm, and a length varying from 1 mm to 1 cm. The silver lines 3 are typically spaced apart by a distance of 2 mm.

[0031] Advantageously, the photovoltaic cell 10 also comprises one or more contacts on the side of the rear face 1b of the support substrate 1. In particular, a rear contact layer 7 made of aluminum is usually arranged on the lower layer 5. For example, this rear contact layer 7 may have a thickness of the order of one to a few tens of micrometers. Due to the diffusion of aluminum into the silicon during the heat treatments for producing the cell 10, an interlayer 6, an alloy of silicon and aluminum, may be present between the lower silicon layer 5 and the rear contact layer 7. The thickness of the interlayer 6 may for example vary around 10 μm.

[0032] The photovoltaic cell 10 also comprises at least one anti-reflective layer 4 arranged on the upper layer 2, adjacent to the silver lines 3. Usually, the anti-reflective layer 4 is formed from silicon nitride (SiN) or titanium oxide. In the case of silicon nitride, it has a thickness of the order of 75 nm.

[0033] In a second sub-step a2), the photovoltaic cell 10 is immersed in a basic solution, capable of etching the silicon. For this, the cell 10 is for example placed in a perforated basket and immersed in this solution. Even if we are talking here about a cell 10, it is understood that the steps described can also apply to a plurality of cells 10 (or to a plurality of pieces of cells 10) treated collectively.

[0034] The basic solution used is sodium hydroxide (NaOH) with a concentration between 1 and 30%. This solution is advantageous in that it efficiently etches silicon in particular, and has excellent selectivity towards silver: the silver lines 3 are therefore not degraded during this sub-step a2).

[0035] Sub-step a2) is carried out at a temperature between 20°C and 100°C, preferably at 50°C. The immersion time in the basic solution is typically between 10 min and 1 h. According to an advantageous embodiment, ultrasound is applied during all or part of step a2), at a frequency between 40kHz and 100 kHz, to promote the detachment of the silver lines 3. Preferably, this frequency is defined at 80kHz to limit the breakage of the silver lines 3, intended to be completely detached and free in the solution at the end of this sub-step.

[0036] During sub-step a2), the basic solution etches the aluminum of the rear contact layer 7, according to the following reaction:

[0037] Al(s) + NaOH(aq) + H2O(l) → Na[Al(OH)4](aq) + H2(g)

[0038] Sodium aluminates Na[Al(OH)4] form precipitates in suspension; thus the liquid solution in which the photovoltaic cell 10 was immersed corresponds to a colloidal suspension.

[0039] A second etching phenomenon occurs at the interface between the silicon of the upper layer 2 and the silver lines 3, due to the penetration of the basic solution on the sides of the lines 3 (between the lines 3 and the anti-reflective layer 4) and through said lines 3, which are substantially porous. The application of ultrasound is favorable because it increases the probability of contact between the basic solution and the silicon.

[0040] Sub-step a2) thus leads to the etching of the contact layer 7 and the detachment of the silver lines 3 by local etching of the underlying upper layer 2, without requiring the prior removal of the anti-reflective layer 4, which avoids a prior acid etching step ().

[0041] When the silver lines 3 are all detached, present in solid form in the basic solution, the following sub-step a3) provides for the separation between the solid macro-elements (namely the remainder of the photovoltaic cell and the silver lines) and the first effluent, said first effluent comprising the basic solution, aluminum in the form of sodium aluminates and silver fragments. Indeed, even if a major part of the silver lines (included in the solid macro-elements) is separated from the first effluent to be dried and recovered in solid form, a minor part of these lines, often in the form of small solid fragments, remains in the first effluent. This is explained by the fact that the basket supporting the solid elements is perforated and can possibly allow the smallest silver fragments to pass through.

[0042] According to a first example of implementation of step a), the first effluent therefore comes from sub-step a3).

[0043] In a subsequent sub-step a4), the solid macro-elements are immersed in a deionized water solution, for rinsing.

[0044] A subsequent sub-step a5) consists of the separation between the rinsed solid macro-elements, and the rinsing solution. The latter comprises water, aluminum in the form of sodium aluminate(s), and silver line residues in the form of solid fragments: it can constitute the first effluent, according to a second example of implementation of step a). In this case, the first effluent is expected to be less loaded with aluminum and silver, and potentially of slightly lower basicity than in the first example of implementation. The silver line residues can in particular come from fragments of silver lines broken during rinsing, or from residual fragments which had adhered to solid macro-elements during the solid / liquid separation of sub-step a3).

[0045] Finally, according to a third example of step a), the effluents from sub-step a3) and from sub-step a5) can be mixed and correspond to the first effluent of the invention.

[0046] Returning to the description of the filtration method, a second step b) of acidification is applied to the first effluent, by adding hydrochloric acid in a proportion varying for example between 1 / 14 and 1 / 5 (typically of the order of 1 / 8), to form a first acid solution. The hydrochloric acid can have a concentration of between 20% and 37%, typically around 33%. The pH of this first solution must be less than or equal to 4, preferably between 3 and 4, even more preferably between 3.5 and 4. For example, a pH of 3.8 can be aimed for by taking a continuous reading of the pH via a probe and a pH meter. In such a solution, the aluminum (initially in the form of sodium aluminate(s)) will pass into the form of soluble aluminum chloride (AlCl3), according to the following reaction:

[0047] Al(OH)3(s) + 3 HCl(aq) → AlCl3(aq) + 3 H2O(aq).

[0048] The third step c) corresponds to the filtration of the first solution to recover the solid fragments of silver on the one hand, and the first filtered solution on the other hand. Indeed, since aluminum is in a perfectly soluble form, it is easy and effective to filter the solid fragments remaining in the first solution (which happen to be entirely fragments of silver), for example through a filtration bag with a mesh size of 30 μm. The sodium aluminates of the first effluent would have been at least partially retained by the filter due to their colloidal state, or would even have clogged it, preventing effective filtration.

[0049] The filter bag can be made of a material such as polypropylene (PP). Other mesh sizes or materials for the filter bag can of course be considered, as well as other filtration techniques (e.g., centrifugal separation). Once the silver fragments are extracted, the first filtered solution remains.

[0050] The fourth step d) corresponds to the neutralization of the first filtered solution, by adding sodium hydroxide, the concentration of which is between 10% and 30%. The addition of sodium hydroxide is carried out in a proportion of between 1 / 200 and 1 / 80, preferably 1 / 160, to form a second solution with a pH of between 6.5 and 8.5, preferably a pH of 7.

[0051] In this second solution, the aluminum will pass into the form of aluminum hydroxide according to the following reaction:

[0052] AlCl3(aq) + 3 NaOH(aq) → Al(OH)3(s) + 3 NaCl(aq)

[0053] The aluminium salts are recovered in solid form during the fifth step e) of filtration of the second solution, for example through a filtration bag with a mesh of 250 μm.

[0054] Optionally, the method according to the invention comprises a step d), between step d) and step e), corresponding to the addition of a flocculant to the second solution, to promote agglomeration of the aluminum salts. For example, a commercially available flocculant of the Siebec Flotech L-C03 type can be used. The second solution with added flocculant preferably undergoes stirring, so as to increase the probability of contact between the flocculant and the salts and promote agglomeration of the salts; after which, a settling time is required so that the salts stabilize at the bottom of the tank.

[0055] At the end of step e), we obtain, on the one hand, aluminum salts in solid form, and on the other hand, the second filtered solution.

[0056] Advantageously, an additional filtration step f), after step e), can be applied to the second filtered solution, to further improve the quality of the aqueous solution. The filter used here (e.g. Siebec L50 filter) is much finer than those implemented in the previous steps, with a filtration mesh of less than 10 μm, or even less than 1 μm. The filter material can be chosen from polypropylene (PP) or polyvinylidene fluoride (PVDF). It allows the removal of micro-residues potentially still present at this stage (aluminum, silver, silicon, etc.).

[0057] The second filtered solution from step e) or from step f) is freed from metallic elements and constitutes a second effluent, perfectly compatible with discharge into wastewater. The average concentrations measured by ICP-MS mass spectrometry (Inductively Coupled Plasma Mass Spectrometry) are typically: Al < 1 ppm, Ag < 2 ppm, Si < 50 ppm.

[0058] The filtration method according to the invention provides a simple solution with two advantages: on the one hand, it is of economic interest, because it allows the recovery very efficiently (without complex equipment) of all the metals from recycled photovoltaic cells; on the other hand, it is of ecological interest because it provides a second filtered effluent, compatible with discharge into wastewater.

[0059] Of course, the invention is not limited to the embodiments and examples described, and variant embodiments can be made without departing from the scope of the invention.

Claims

Method for filtering an effluent from recycling steps of a photovoltaic cell, comprising the following steps:a) providing the effluent, called the first effluent, basic, with a pH greater than 12, comprising water, aluminum in the form of sodium aluminate(s), and silver in the form of solid fragments of silver lines of the photovoltaic cell,b) acidifying the first effluent by adding hydrochloric acid, to form a first acid solution with a pH less than or equal to 4, in which the aluminum is in the form of aluminum chloride, soluble in said first solution,c) filtering the first solution to recover the solid fragments of silver on the one hand, and a first filtered solution on the other hand,d) neutralizing the first filtered solution, by adding sodium hydroxide, to form a second solution with a pH between 6.5 and 8.5,in which the aluminum is in the form of aluminum hydroxide,e) filtration of the second solution to recover aluminum salts in solid form on the one hand, and a second filtered solution on the other hand., Method for filtering an effluent according to claim 1, in which step a) comprises the following sub-steps:a1) providing a photovoltaic cell (10) comprising in particular:- a support substrate (1) made of silicon having a front face (1a) and a rear face (1b),- an upper layer (2) made of doped silicon, arranged on the front face (1a), a plurality of silver lines (3) arranged on the upper layer (2), and at least one anti-reflective layer (4) arranged on the upper layer (2), adjacent to the silver lines (3),- a lower layer (5) made of heavily doped silicon, arranged on the rear face (1b), and a rear contact layer (7) made of aluminum, arranged on the lower layer (5);a2) immersion of the photovoltaic cell (10) in a basic solution, leading to the etching of the contact layer (7) and the detachment of the silver lines (3) by local etching of the underlying upper layer (2),a3) separation between:- solid macro-elements comprising the remainder of the photovoltaic cell and a major part of the silver lines (3), and- the first effluent comprising the basic solution, aluminum in the form of sodium aluminate(s), and a minor part of the silver lines (3) in the form of solid fragments.; Filtration method according to claim 1, wherein step a) comprises the following sub-steps:a1) providing a photovoltaic cell (10) comprising in particular:- a support substrate (1) made of silicon having a front face (1a) and a rear face (1b),- an upper layer (2) made of doped silicon, arranged on the front face (1a), a plurality of silver lines (3) arranged on the upper layer (2), and at least one anti-reflective layer (4) arranged on the upper layer (2), adjacent to the silver lines (3),- a lower layer (5) made of heavily doped silicon, arranged on the rear face (1b), and a rear contact layer (7) made of aluminum, arranged on the lower layer (5);a2) immersing the photovoltaic cell (10) in a basic solution, leading to the etching of the contact layer (7) and the detachment of the silver lines (3) by local etching of the underlying upper layer (2) ;a3) the separation between:- solid macro-elements comprising the remainder of the photovoltaic cell and a major part of the silver lines (3), and- an effluent comprising the basic solution, aluminum in the form of sodium aluminate(s), and a minor part of the silver lines (3) in the form of solid fragments;a4) the immersion of the solid macro-elements in a deionized water solution for their rinsing;a5) the separation between the solid macro-elements, and another effluent comprising water, aluminum in the form of sodium aluminate(s) and residues of silver lines (3) in the form of solid fragments, said other effluent forming the first effluent.; Filtration method according to one of claims 1 to 3, in which, in step b), the first solution has a pH of between 3.5 and 4, preferably equal to 3.

8. Filtration method according to one of claims 1 to 4, comprising a step d), between step d) and step e), corresponding to the addition of a flocculant in the second solution, to promote agglomeration of the aluminum salts. Filtration method according to one of claims 1 to 5, comprising a step f), after step e), corresponding to an additional filtration of the second filtered solution. Filtration method according to one of claims 1 to 6, in which the second filtered solution corresponds to a second effluent which is discharged into the wastewater.

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