EP3388556A1 - Method for recovering silver present on a substrate, electrochemically and in aqueous solution - Google Patents

Abstract

A method of recovering silver from an electrically conductive substrate, said method comprising the steps of:
- providing a system comprising:
An electrically conductive substrate on which silver is located, forming a positive electrode,
A negative electrode, made of an electrically conductive material, and
∘ a device electrically connected to the positive electrode and the negative electrode, for controlling the potential or the current of one of the two positive and negative electrodes;

- Immersion of the positive electrode and the negative electrode in an aqueous solution comprising at least one acid and a complexing agent, the aqueous solution having a pH ranging from 2 to 6; and
- Application of a potential or a current to one of the two electrodes, so as simultaneously to dissolve the silver of the positive electrode, pass it in solution and electrodeposit it on the negative electrode.

Description

TECHNICAL FIELD AND STATE OF THE PRIOR ART

The present invention relates to a method for recovering silver present on a substrate.

It relates more particularly to a method for recovering silver, electrochemically, and in aqueous solution.

The present invention finds particular application in the recycling and recovery of photovoltaic panels.

Photovoltaic panels (PV), also called photovoltaic modules, are used to convert solar radiation into thermal or electrical energy. Today, about 90% of photovoltaic panels are made of crystalline silicon. The other photovoltaic panels are thin-layer type (10%).

Typically, a crystalline silicon photovoltaic panel comprises a plurality of photovoltaic cells electrically connected together, encapsulated by transparent polymer layers, and arranged between glass plates and an aluminum frame. Electrodes, for example, made of copper or silver, make it possible to collect the electric current generated by the photovoltaic cells.

Each photovoltaic cell is conventionally formed of a silicon substrate, whose front face is covered with silver metallizations.

A silicon photovoltaic module is therefore mainly composed of glass (74% of the total weight), aluminum (10%), polymer (about 6.5%) and silicon (about 3%), metals (zinc, lead, copper and silver) representing only a negligible part of the mass.

Due to the strong development of photovoltaic panels in recent years, the question of their recycling is fundamental. Since then on August 13, 2012, the Waste Electrical and Electronic Equipment (WEEE) Directive was extended to photovoltaic (PV) panels.

The recovery and recycling objectives of these panels are already easily achieved by the only recovery of the glass and the aluminum frame of the photovoltaic panels.

The recovery of metals (copper, tin, lead, ...), and more particularly silver, which is the metal with the highest added value (nearly 90% of the price of the cell), represents a major challenge for sustaining the recycling sector.

To valorize the money, the current processes involve dismantling the modules chemically and / or thermally and then carrying out a series of treatments to dissolve the various metallic elements in solution, and finally recover the money.

The dissolution of metals is often carried out in very concentrated and sometimes boiling acidic solutions (HF, HNO ₃ , H ₂ SO ₄ ).

For example, in the document TW-A-201328990 the silver recovery process comprises the steps of: grinding the cells, treating the resulting powder in a solution of H ₂ SO ₄ (15 mol / L to 20 mol / L) at 60 to 100 ° C, recovery filtrate containing aluminum and crystallization at 100-150 ° C to obtain metallic aluminum. The powder residues containing silicon and silver are treated with HNO ₃ (4 to 12 mol / L). The filtrate containing the silver is recovered and the silver is reduced in the presence of hypophosphorous acid. Treatment with boric acid at a temperature of 900-1100 ° C makes it possible to form silver ingots. However, this silver recovery process is complex and requires the use of many acids at high concentrations and / or temperatures (above 100 ° C and up to 1100 ° C).

The document TW-A-200836850 proposes to remove silver and aluminum by contactless electrolysis by immersing a photovoltaic cell in a solution containing nitric acid, phosphoric acid, acetic acid and ferric chloride. The anti-reflective layer of the cell is dissolved in the acid phosphoric acid heated to between 100 and 200 ° C. The cell is rinsed with nitric acid to remove the last traces of silver. This process does not evoke the recovery of metals after dissolution by electrolysis. In addition, it uses many acids.

All these prior art processes require several steps, sometimes with the use of acids at high concentrations and / or at temperatures near the boiling point. This leads to complex processes and / or requiring additional cost for the treatment of waste.

STATEMENT OF THE INVENTION

It is, therefore, an object of the present invention to provide a simple silver recovery process requiring few steps and involving mild conditions in terms of acidity and temperature, while limiting the energy and reprocessing costs, so that they can be transposed to an industrial scale.

• fourniture d'un système comprenant :
  • ∘ un substrat électriquement conducteur sur lequel se trouve de l'argent, formant une électrode positive,
  • ∘ une électrode négative, en un matériau électriquement conducteur, et
  • ∘ un dispositif relié électriquement à l'électrode positive et à l'électrode négative, permettant de contrôler le potentiel ou le courant de l'une des deux électrodes positive et négative ;
• immersion de l'électrode positive et de l'électrode négative dans une solution aqueuse comprenant au moins un acide et un agent complexant de l'argent, la solution aqueuse ayant un pH allant de 2 à 6 ; et
• application d'un potentiel ou d'un courant à l'une des deux électrodes, de manière à simultanément dissoudre l'argent de l'électrode positive, le faire passer en solution et l'électrodéposer sur l'électrode négative.

This object is achieved by a method for recovering silver from an electrically conductive substrate, said method comprising the following steps:

• providing a system comprising:
  • An electrically conductive substrate on which silver is located, forming a positive electrode,
  • A negative electrode, made of an electrically conductive material, and
  • ∘ a device electrically connected to the positive electrode and the negative electrode, for controlling the potential or the current of one of the two positive and negative electrodes;
• immersing the positive electrode and the negative electrode in an aqueous solution comprising at least one acid and a complexing agent for silver, the aqueous solution having a pH ranging from 2 to 6; and
• application of a potential or a current to one of the two electrodes, so as simultaneously to dissolve the silver of the positive electrode, to pass it in solution and electrodeposit it on the negative electrode.

Silver is present on substrate. In other words, the substrate is at least partially covered by silver. Money is available to be dissolved. The silver may be in the form of a continuous film, wires, or a grid.

The substrate does not dissolve in the aqueous solution. It does not participate in electrochemical reactions.

By application of a potential or a current, is preferably meant a constant potential or a constant current.

The method is easy to implement and allows to control, via the application of a potential or a current, the rate of dissolution / recovery of silver. The substrate plays the role of anode, the silver is oxidized and dissolved in solution.

The method makes it possible to reduce the steps compared with the prior art since, in a single unitary step, the silver is removed from the substrate and recovered in metallic form. The dissolution and silver recovery steps are performed in the same tank, in which the electrodes are immersed. The solution does not need to be processed, to be transferred between the electrodissolution step and the electroplating step. This leads to a great simplification of the process, a greater compactness of the installations, a reduction in the number of pipes and other devices for conveying fluids and solids, etc.

The electrodes are connected to a device making it possible to control the potential of one of the two positive and negative electrodes and, more advantageously, to control the potential difference between the two positive and negative electrodes.

In a variant of the invention, the device makes it possible to control the current of one of the two positive and negative electrodes and, more advantageously, to control the difference in current between the two positive and negative electrodes.

Advantageously, the system further comprises a reference electrode, for example Ag / AgCl.

According to one variant, in order to carry out the electrodissolution and the electroplating of silver in a single step, the potential applied to the positive electrode ranges from -0.4 V to +0.2 V vs Ag / AgCl. In this range of potentials, the complexing agent, for example thiourea, will not be degraded.

According to another variant, to achieve in one step the electrodissolution and electrodeposition of silver, the potential applied to the negative electrode ranges from -0.4 V to -1V vs. Ag / AgCl. The choice of the applied potential makes it possible to control the microstructure of the deposit and the rate of dissolution.

The process is carried out in a weakly acidic aqueous solution. By weakly acid means a pH greater than or equal to 2 and less than or equal to 6. Advantageously, the aqueous solution has a pH ranging from 3 to 5. Generally, a pH of less than 2 does not allow the selectivity of the treatment while a pH above 6 decreases the kinetics of silver recovery. Advantageously, the acid is sulfuric acid H ₂ SO ₄ , nitric acid HNO ₃ or a mixture thereof.

The method makes it possible to overcome the constraints related to the use of concentrated acid.

The complexing agent, also called ligand or complexing agent, makes it possible to complex the dissolved silver element in the form of a silver complex. The complexing agent is chosen so as to be sufficiently complexing to promote the electrodissolution of the silver and to facilitate the electroplating of the silver in the electrochemical stability window of the solution. The use of silver complexing agent allows, via silver complex formation, to lower the electrodissolution potential of silver. The complexing agent must be soluble in the electrolytic solution and selective for silver. In addition, it should not degrade in the pH range used.

According to an advantageous embodiment, the complexing agent is thiourea or a derivative thereof. Thiourea as a complexing agent of the metal to be dissolved has many advantages in terms of chemistry, cost or process. It promotes the solubility and stability of the complexing agent, while lowering the electrodeposition potential of silver from 0.799 V to about 0.2 V. In addition, it is regenerated during silver deposition. The complexing agent of the silver is regenerated at the end of the electroplating step. The process can be carried out in a closed cycle, thus limiting the release of chemicals, the cost of production and the environmental impact.

Advantageously, the complexing agent concentration ranges from 0.01 mol / L to 1 mol / L, preferably from 0.05 mol / L to 1 mol / L, and even more preferentially, is of the order of 0.5 mol / L. Such concentrations make it possible to promote the kinetics of dissolution of the metal and the recovery of silver. This complexing agent concentration range is particularly advantageous, since a concentration of less than 0.01 mol / L decreases the complexing nature, whereas a concentration greater than 1 mol / L generally corresponds to the saturation level of the complexing agent. complexing agent solution. Even more advantageously, this concentration is equal to twice the stoichiometry of silver to dissolve.

Advantageously, the solution further comprises an agent promoting the ion transport properties. This is, for example, a sulphate salt. The sulfate salt may be any water-soluble salt of the sulfate ion [SO4 ^2- ], alone or in admixture. In general, the sulfate salt is a water-soluble salt of the sulfate ion [SO4 ^2- ]. It may especially be chosen from the group comprising Na ₂ SO ₄ , K ₂ SO ₄ and CaSO ₄ . For example, the solution may comprise 0.001 to 1 mol / L of sulfate salt, preferably 0.1 to 0.5 mol / L. Salt does not interfere with electrode reactions and does not react with the solvent.

According to one variant, the agent promoting the ionic transport properties may be an additional acid.

Advantageously, the solution further comprises silver chloride or silver nitrate, promoting the initiation of the electrodeposition reaction by the introduction of small amounts of silver dissolved in the electrolytic solution.

Advantageously, the substrate is made of silicon. Silicon is electrochemically inert during the electrodissolution / electroplating step.

Advantageously, the substrate is a plate. The process avoids a preliminary grinding, and energy consuming, of the substrate.

Advantageously, the electrically conductive substrate comes from a photovoltaic cell and the silver forms the metallizations of the photovoltaic cell.

Advantageously, the process is carried out at a temperature ranging from 15 ° C to 60 ° C, and preferably being of the order of 20-25 ° C. These conditions also make it possible to reduce the amount of energy required and to improve safety by compared to the processes of the prior art that can implement acid solutions in boiling. The process can be carried out at room temperature (20-25 ° C) facilitating its use in an industrial environment.

The process can be carried out under air. It is particularly advantageous not to work under a controlled atmosphere and not to use inert gases. The process is industrializable.

Other features and advantages of the invention will appear better on reading the additional description which follows, which relates to a recovery test, by electrochemical dissolution and electrodeposition, of silver present in a photovoltaic cell by means of a electrolytic solution according to the invention.

It is specified that this detailed description, which refers in particular to Figures 1 to 5 as annexed, is given merely by way of illustration of the object of the invention and does not in any way constitute a limitation of this object.

In particular, the silver recovery process described above and detailed below allows to obtain in a unitary step the electrochemical dissolution of the silver present on a substrate and its recovery in metallic form.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est un cliché obtenu par microscopie électronique à balayage, en électrons rétrodiffusés, d'un substrat en silicium recouvert de connectiques en argent, provenant d'une cellule photovoltaïque,
• la figure 2 est un cliché obtenu par microscopie électronique à balayage, en électrons rétrodiffusés, du substrat en silicium de la figure 1 sur lequel a été réalisé le procédé de l'invention,
• les figures 3 et 4 sont des clichés obtenus par microscopie électronique à balayage, en électrons rétrodiffusés, du dépôt d'argent obtenu avec le procédé de l'invention, sur une électrode en carbone vitreux à différentes échelles, et
• la figure 5 est un spectre d'une microanalyse par Energie Dispersive des rayons X de l'argent électrodéposé sur l'électrode en carbone vitreux.

The present invention will be better understood on the basis of the description which follows and the attached drawings in which:

• the figure 1 is a cliché obtained by electron microscopy with scanning, in backscattered electrons, of a silicon substrate covered with silver connectors, coming from a photovoltaic cell,
• the figure 2 is a cliché obtained by electron microscopy with scanning, in backscattered electrons, of the silicon substrate of the figure 1 on which the method of the invention has been carried out,
• the figures 3 and 4 are clichés obtained by scanning electron microscopy, in backscattered electrons, of the silver deposit obtained with the method of the invention, on a vitreous carbon electrode at different scales, and
• the figure 5 is a spectrum of X-ray Dispersive Energy Microanalysis of silver electrodeposited on the glassy carbon electrode.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Subsequently, the method is described for recovering the silver contained in photovoltaic cells, preferably in crystalline or polycrystalline silicon, and more particularly for recovering the silver present on the front face and sometimes also on the back face of said cells.

The method could be used for any type of electrically conductive substrate not dissolving naturally in the electrolytic solution.

Prior to the implementation of the method, the cells are removed from the photovoltaic panel and separated from the cables, junction boxes and metal frames. The cells are subjected to heat treatment to remove, by calcination, polymer encapsulation materials, such as ethylene vinyl acetate (EVA). The calcination step is, for example, carried out in an oven under air. The heat treatment is, for example, carried out at a temperature between 400 ° C and 700 ° C, more preferably between 450 and 550 ° C. The duration of this treatment may be between 30 and 120 minutes, more preferably between 60 and 90 minutes.

At the end of this pretreatment, the photovoltaic cell consists of an electrically conductive silicon substrate covered by one or more silver electrodes. The silver electrodes are conventionally formed from a silver paste which may comprise a SiO ₂ , B ₂ O ₃ , PbO and ZnO glass powder, silver and a binder.

The cells can also be separated from the electrical connectors. According to a variant, the electrical connectors, composed of a copper core coated with Sn ₆₂ Pb ₃₆ Ag ₂ , for example, and the silver can be recovered using the process of the invention. The different metals (copper, lead, tin) forming very stable complexes with the complexing agent and / or being in a minimal amount and / or not being dissolved in these pH ranges and / or not being electrodeposited with the potential used in the process, the deposit on the negative electrode will be essentially silver. It is therefore particularly advantageous to be selective with respect to money.

The substrate is not ground to implement the process. It can be, for example, cut into a plate of a few cm ² or dm ² .

The substrate has a first face and a second face. The first face, corresponding to the front face of the photovoltaic cell, is covered with silver. The second face corresponds to the rear face of the photovoltaic cell. The second face may include aluminum. The second face will advantageously be covered by a plate to prevent dissolution of the aluminum in solution. For example, aluminum can be protected by a plate. The plate provides physical isolation of the aluminum from the solution. It can be electrically conductive. It will advantageously be chemically inert vis-à-vis the electrolytic solution.

The substrate, and more particularly the first face of the silver-coated substrate, is electrically connected to a voltage or current source, such as a potentiostat. The potentiostat is preferably used in potentiostatic mode. It could be used in galvanostatic mode. The substrate forms the positive electrode (or anode). Advantageously, all the silver connectors (bus, bar) on the surface of the substrate is electrically connected to the potentiostat.

The negative electrically conductive electrode, itself electrically connected, is also connected to the current or voltage source, and may be a substrate made of graphite, glassy carbon, stainless steel, titanium or a noble metal such as platinum, or indium oxide doped with tin.

The substrate is immersed, at least partially, in the electrolytic solution, and arranged facing the negative electrode (or cathode), itself immersed in the electrolytic solution.

A potential or a current is imposed on the positive or negative electrode, which simultaneously generates the electro-dissolution of the silver at the positive electrode and the electroplating / recovery of the silver in metallic form at the electrode negative.

A reference electrode, for example Ag / AgCl, may also be added to the assembly.

The potential applied to the positive electrode ranges from 0.4 V to +0.2 V vs Ag / AgCl, for example 0 V vs. Ag / AgCl.

The potential applied to the negative electrode ranges from -0.4 V to -1 V vs. Ag / AgCl, for example -0.8 V vs. Ag / AgCl.

Advantageously, the potential or the current is applied for a period of 30 minutes to 5 hours, and preferably for a period of 1 hour to 3 hours. The duration will be, in particular, chosen according to the amount of money to be valued and the potential or current applied.

The electrolytic solution is a weakly acidic aqueous solution (pH of 2 to 6) comprising at least one acid, at least one silver complexing agent and, optionally, at least one sulfate salt for improving the ion transport within the solution. .

According to a particular embodiment, the acid used has a pKa between -7 and 3. It is a Brönsted acid, that is to say an acid capable of releasing at least one proton. For example, sulfuric acid, nitric acid or a mixture of these acids will be used.

When carrying out the process according to the invention, the pH of the acidic solution can be controlled and optionally adjusted to these values by addition of acid.

The complexing agent is preferably thiourea (CAS number 62-56-6).

The solution may further include silver chloride or silver nitrate to facilitate the initiation of the electrodeposition reaction.

The electrolytic solution is preferably devoid of any solvent other than water. The acidic solution is preferably devoid of an oxidizing agent such as, for example, hydrogen peroxide or metal salts (iron or copper sulfate, for example).

The solution may naturally contain dissolved oxygen. We will not add oxygen in addition to that naturally present in solution. The naturally occurring oxygen can also be removed from the solution by bubbling with another gas, such as argon.

The solution has a low viscosity and good ionic conductivity.

The process is advantageously carried out with mechanical stirring, for example between 200 and 1000 rpm.

The various steps of the process are advantageously carried out under air.

After the electroplating, the complexing agent is regenerated. The acid treatment solution can be reused, possibly by adjusting its pH, which reduces the consumption of reagents.

After electro-dissolution / electroplating of the silver metal, the silicon cell is extracted from the bath and it is possible to proceed to a new treatment cycle with a new substrate containing silver to be recovered.

Illustrative and nonlimiting example of an embodiment :

In this example, silicon cells from conventional photovoltaic panels are used.

Firstly, the photovoltaic cells are subjected to heat treatment in order to burn the EVA encapsulation layers. This step takes place in an oven under air at 500 ° C for 1 hour. The cells are also separated from the connectors.

• une cathode de carbone vitreux en tant que contre-électrode (électrode négative),
• une anode formée d'un morceau de cellule photovoltaïque d'une dizaine de cm² environ est portée en électrode de travail (électrode positive), et
• une électrode de référence Ag/AgCl.

The electrolytic system used in the context of this study consists of a cell comprising three electrodes:

• a vitreous carbon cathode as a counter electrode (negative electrode),
• an anode formed of a piece of photovoltaic cell of about 10 cm ² is carried in working electrode (positive electrode), and
• an Ag / AgCl reference electrode.

The electrodes are immersed in an aqueous solution of 900 ml comprising 10 ^-3 mol.l ^-1 sulfuric acid (pH = 3), 0.5 mol.l ^-1 thiourea and 0.1 mol.l ^{- 1} of Na ₂ SO ₄ . The solution is stirred under air at 20 ° C. at 200 rpm.

The electrodissolution / electroplating is performed in potentiostatic mode. A constant potential of 0 V is applied to the positive electrode, causing the silver of the positive electrode to dissolve and simultaneously deposit the silver on the negative electrode. The potential is maintained for two hours to extract 16 Coulombs, or 98% of the available silver in the silicon cell.

Scans obtained by scanning electron microscopy (SEM) in backscattered mode highlight the state of the surface of the photovoltaic cell (positive electrode) before ( figure 1 ) and after ( figure 2 ) the implementation of the method of the invention. White corresponds to heavy elements, here silver, and black to light elements, mainly silicon. At the end of the electrochemical treatment, only some residual traces of silver remain.

At the end of the process, a deposit is visible on the glassy carbon electrode. This deposit was analyzed by MEB and Energy Dispersive X-ray (EDX) in order to determine the chemical composition. SEM observations have highlighted the presence of a deposit of money. The presence of silver was confirmed by microanalysis by EDX. The microstructure of the deposit is of the "cauliflower" type. Some sulfur impurities are present in the silver deposit. These impurities are removed with a washing of the silver deposit in water in which the deposit is insoluble.

The electrolysis process worked: about 98% of the initial silver mass of the silicon panel was electrodissolved and electrodeposited.

This example shows that it is possible to recover the silver, by simultaneous electrochemical dissolution and electroplating thereof, under an uncontrolled atmosphere such as air, and in an electrolytic solution according to the invention.

Claims (14)

A method of recovering silver from an electrically conductive substrate, said method comprising the steps of: - providing a system comprising: An electrically conductive substrate on which silver is located, forming a positive electrode, A negative electrode, made of an electrically conductive material, and ∘ a device connected to the positive electrode and the negative electrode, for controlling the potential or the current of one of the two positive and negative electrodes; immersing the positive electrode and the negative electrode in an aqueous solution comprising at least one acid and a complexing agent for silver, the aqueous solution having a pH ranging from 2 to 6; and - Application of a potential or a current to one of the two electrodes, so as simultaneously to dissolve the silver of the positive electrode, pass it in solution and electrodeposit it on the negative electrode. Method according to claim 1, characterized in that the system further comprises a reference electrode, for example Ag / AgCl. Process according to Claim 2, characterized in that the potential applied to the positive electrode ranges from -0.4 V to +0.2 V vs Ag / AgCl. Process according to Claim 2, characterized in that the potential applied to the negative electrode ranges from -0.4 V to -1 V vs. Ag / AgCl. Process according to any one of Claims 1 to 4, characterized in that the aqueous solution has a pH ranging from 3 to 5. Process according to any one of claims 1 to 5, characterized in that the acid is sulfuric acid, nitric acid or a mixture thereof. Process according to any one of Claims 1 to 6, characterized in that the concentration of complexing agent ranges from 0.01 mol / l to 1 mol / l, preferably from 0.05 mol / l to 1 mol / l, and even more preferentially, is of the order of 0.5 mol / L. Process according to any one of Claims 1 to 7, characterized in that the complexing agent is thiourea. Process according to any one of claims 1 to 8, characterized in that the solution further comprises a sulfate salt, preferably selected from Na ₂ SO ₄ , CaSO ₄ and K ₂ SO ₄ . Process according to any one of claims 1 to 9, characterized in that the solution further comprises silver chloride. Process according to any one of claims 1 to 10, characterized in that the process is carried out at a temperature ranging from 15 ° C to 60 ° C, and preferably of the order of 20-25 ° C. Process according to any one of Claims 1 to 11, characterized in that the electrically conductive substrate is made of silicon. Process according to any one of claims 1 to 12, characterized in that the substrate is a plate. Process according to any one of Claims 1 to 13, characterized in that the substrate comes from a photovoltaic cell and in that the silver forms the metallizations of the photovoltaic cell.

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