JP7601129B2 - How to recover valuable materials from solar panels - Google Patents

Description

The present invention relates to a method for recovering valuable materials from solar panels.

In the carbon-neutral (carbon dioxide reduction) era, technology that utilizes solar energy is extremely important, and the role of photovoltaic power generation, which converts solar energy into electrical energy, i.e. solar cells, is expected to become increasingly important in the future.

Demand for solar panels in Japan has been expanding since the introduction of the Feed-in Tariff Scheme in 2012, and demand is expected to continue to grow in the future. The product lifespan of solar panels is generally said to be around 20 to 30 years, and if demand continues at the current pace, the amount of waste is expected to increase dramatically after 2030. In addition, with the recent increase in natural disasters (earthquakes, heavy rains, typhoons, etc.), there is a risk that the amount of waste will increase even before the product lifespan. In this case, the amount of waste is expected to increase even earlier than expected. Therefore, if a large amount of solar panels are discarded in the future, how to dispose of the discarded solar panels is an issue.

Here, the breakdown of the solar cell panel's total weight is as follows: solar cell panel (solar cell module and outer frame (made of aluminum (Al))) 58%, power conditioner and connection box 3%, mounting stand 36%, wiring material 3%. Furthermore, the solar cell module is composed of solar cell cells (solar cell grade silicon (Si)), cover glass, back sheet, sealing material (ethylene vinyl acetate (EVA)) and outer frame (made of Al), with the breakdown being cells 3%, glass 62%, back sheet and sealing material 18%, outer frame 16%, wiring material (copper (Cu), solder) 1%. Furthermore, a small amount of silver (Ag) is used in the electrode material.

The technology to separate the constituent materials of the solar cell panel and make effective use of each of them is important. In particular, because the back sheet and sealing material are firmly bonded to the solar cell, it is particularly important to have technology to recover valuable materials such as Si and Ag after removing the back sheet and sealing material, which are made of organic substances, using means such as heat treatment, and various technologies have been disclosed to achieve this goal.

For example, Patent Documents 1 and 2 disclose a technique in which a solar panel is heated in the presence of oxygen gas in the semiconductor intrinsic conduction region (500°C), then dissolved in an aqueous nitric acid solution, and then hydrochloric acid is dripped onto the panel to simultaneously recover silver chloride (AgCl) and Si from the residue.

Patent Document 3 also discloses a method for recovering constituent materials of solar cell elements contained in a solar cell module, which comprises the steps of transporting solar cell elements including at least a cell portion and a glass substrate, and an EVA sealing material bonded to these, to a continuous heat treatment furnace in which the oxygen concentration in the furnace is maintained at 1.0% by volume or more and 3.0% by volume or less, releasing and removing acetic acid gas, which is a type of EVA decomposition gas, in a preliminary thermal decomposition section set at 300 to 400°C, and then desorbing EVA decomposition gases other than acetic acid in a heat treatment section set at 400 to 500°C to remove the EVA sealing material from the solar cell elements, and separating the cell portion from the glass substrate.

JP 2016-190177 A JP 2016-93804 A JP 2014-108375 A

However, in the techniques disclosed in Patent Documents 1 and 2, heating is performed in the presence of oxygen gas, so that the Si constituting the solar cell and the Ag constituting the electrode material are also oxidized by heating. Silver oxide (Ag ₂ O) and silicon oxide (SiO ₂ ) are relatively stable, so subsequent separation by wet processing such as acid leaching is difficult. In addition, the use of hydrochloric acid to extract Ag poses safety problems in recovery.

Furthermore, while the technology disclosed in Patent Document 3 is effective in removing organic substances such as plastics, it is similar to Patent Documents 1 and 2 in terms of metal recovery, and since heating is performed in an area where oxygen gas is present, albeit at a low concentration of 1.0 to 3.0%, there are concerns about oxidation of the materials that make up the solar cell and electrodes.

The present invention was made in consideration of the above problems, and its purpose is to propose a technology for safely and efficiently recovering valuable materials such as silver and silicon from used solar panels.

The present invention which solves the above problems is as follows.
[1] A method for recovering valuable materials from solar panels, comprising:
a heating step of subjecting the solar cell panel from which the outer frame and cover glass have been removed to a heat treatment to volatilize the organic substances constituting the back sheet and the encapsulant and to separate them into visible metal components and residues;
a leaching step of adding an acid to the residue produced in the heating step, heating the mixture, and leaching silver contained in the residue into the acid, followed by washing with water and suction filtration;
an extraction step of adding and mixing a chloride ion-containing substance into the filtrate obtained after the leaching step, and then filtering the resulting mixture to extract metal chlorides;
A method for recovering valuables from a solar panel, comprising:

[2] The method for recovering valuable materials from solar cell panels described in [1] above, in which the heating step is carried out in an inert atmosphere and/or a reducing atmosphere at a temperature of 430°C to 900°C.

[3] The method for recovering valuable materials from a solar panel described in [1] or [2] above, wherein the acid used in the leaching process is nitric acid.

[4] The method for recovering valuable materials from a solar panel described in [3] above, in which the nitric acid used in the leaching process has a mixing ratio of nitric acid to water of 3.0 or more.

[5] The method for recovering valuables from a solar panel described in any one of [1] to [4] above, wherein the chloride ion-containing substance used in the extraction process is an alkali metal chloride.

[6] A method for recovering valuable materials from a solar panel according to any one of [1] to [5] above, in which the extraction process is carried out two or more times.

The present invention makes it possible to safely and efficiently recover valuable materials such as silver and silicon from used solar panels.

FIG. 1 is a diagram illustrating the configuration of a general solar cell panel. FIG. 1 is a diagram showing an example of a process flow of a method for recovering valuable materials from a solar cell panel according to the present invention. FIG. 2 is a diagram showing the percentage breakdown of silver, copper, tin, and lead present in the extract, solution, and residue in Example 1. FIG. 1 is a diagram showing the percentage breakdown of silver, copper, tin, and lead present in the extract, solution, and residue in Examples 2 to 4.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The method for recovering valuable materials from a solar cell panel according to the present invention is characterized by having a heating process in which a solar cell panel from which the outer frame and cover glass have been removed is subjected to a heat treatment to volatilize the organic matter constituting the back sheet and sealing material, and then separate the solar cell panel into a visible metal component and a residue; a leaching process in which an acid is added to the residue produced in the heating process and the residue is heated to leach the silver contained in the residue into the acid, followed by washing with water and suction filtration; and an extraction process in which a chloride ion-containing substance is added to the filtrate obtained after the leaching process, mixed, and the resulting mixture is filtered to extract metal chlorides.

Generally speaking, solar cells are made by connecting together solar cells, the smallest usable units, and protecting them with glass or polymers. This is called a "solar cell module." When solar cell modules are fitted into an outer frame made of aluminum or other material to form a panel, this is called a "solar cell panel."

Figure 1 shows the structure of a typical solar cell panel. Solar cell 3 is made by forming an n-type Si layer on top of a p-type Si wafer made of single crystal or polycrystalline Si, with electrodes etc. arranged on top. The electrodes of solar cell 3 are connected to each other by metal interconnectors 4, and are connected to external electrodes through terminals 6. In addition, a cover glass 2 with a thickness of about 3 mm to 5 mm is arranged on the light-receiving surface side of solar cell 3.

The solar cell 3 interconnected as described above is surrounded by a plastic sealant 5 such as EVA, which seals the cells. A back sheet 7 is attached to the surface of the sealant 5 opposite the cover glass 2. This back sheet 7 is made of organic materials with various properties. An outer frame 1 made of aluminum or the like is fixed to the outside of the solar cell module 8 with an adhesive that is an organic material. In this manner, the solar cell panel 9 is constructed.

In the present invention, the target material (starting material) is a solar cell panel 9 that has reached the end of its useful life, or that has become unusable due to a disaster, breakdown, or defective product during the manufacturing process, and is a mixture containing plate-shaped solar cell 3, back sheet 7, and sealing material 5 after the outer frame 1 has been removed and the upper cover glass 2 has been separated and removed. In other words, the target material is a solar cell module 8 after the cover glass 2 has been removed. This is because if heat treatment is performed while the outer frame 1 or cover glass 2 made of Al or the like is still attached, Si and Ag cannot be sufficiently recovered. Note that the outer frame 1 and cover glass 2 can each be recycled separately, and it is preferable to remove them at this stage.

Any method, such as cutting with a blade, can be used to obtain the target material from the solar cell panel 9. Furthermore, it is preferable to use the target material cut into flakes after removing the outer frame 1, such as made of aluminum, from the solar cell panel 9 and removing the cover glass 2 on the top surface. In this case, the size of the solar cell panel flakes is not particularly limited.

When recovering valuables such as the constituent materials of the solar cell 3 and expensive Ag from the solar panel 9, the key is to remove the organic substances that make up the back sheet 7 and the sealing material 5, and to extract trace elements efficiently and under simple process conditions. Based on this concept, the inventors have earnestly studied methods for recovering Ag from used solar panel 9. As a result, they have invented a recovery method that includes a heating step at a relatively low temperature of 1000°C or less in an inert atmosphere and/or reducing atmosphere, a leaching step using an aqueous nitric acid solution, and an extraction treatment step by adding a substance containing chloride ions. Each step is explained below.

Figure 2 shows an example of a process flow for the method of recovering valuable materials from a solar panel 9 according to the present invention. The present invention can be divided into a heating process, a leaching process, and an extraction process.

<Heating process>
First, the solar cell panel 9 from which the outer frame 1 and the cover glass 2 have been removed is subjected to a heat treatment to volatilize the organic substances constituting the back sheet 7 and the sealing material 5, and further separate them into visible metal components and residues (heating step). The heating step is preferably performed in an inert atmosphere and/or a reducing atmosphere. Here, the "inert atmosphere" refers to an atmosphere of a gas with low reactivity, specifically a nitrogen atmosphere or an argon atmosphere. Also, the "reducing atmosphere" refers to an atmosphere of a gas having reducing properties, specifically a carbon monoxide (CO)-containing gas atmosphere, a hydrogen-containing gas atmosphere, or a hydrocarbon-containing gas atmosphere.

Furthermore, when the inventors investigated the components of the organic substances that make up the back sheet 7 and the sealing material 5, etc., they found that they were mainly composed of acetic acid and aromatic compounds (benzene, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, benzoic acid, ethyl biphenylacetylbenzoate, octabenzone, icosene, etc.), and when they investigated the boiling points, the highest was 426°C. For these reasons, it is preferable that the temperature (ambient temperature) in the heating process be 430°C or higher, at which point all of these organic substances volatilize.

Metals contained in the solar cell panel 9 include Cu and Ag. The melting points of these metals are 1080°C and 980°C, respectively. Cu is mainly used as a wiring material for the terminals 6, while Ag is used as a wiring material for connecting the solar cell (semiconductor) 3 together. It is preferable that the upper limit of the atmospheric temperature is 900°C, since it is easier to separate and recover Cu as a solid, and it is more efficient and reliable to leave Ag in the residue and recover it in the subsequent leaching and extraction treatment processes. It is more preferable that the atmospheric temperature is between 600°C and 900°C.

The heating time is not particularly limited as long as it is long enough for the back sheet 7 and the sealing material 5 to completely volatilize. The volatilized organic substances can be recovered as tar.

After the above treatment, the metals are separated into visible metals such as Cu, tin (Sn), and lead (Pb) and a residue mainly composed of Si and containing Ag. There are no particular limitations on the method of separation, and it can be carried out by, for example, sieving, gravity separation, or color sorting. The separated metals can be recovered as valuable materials and recycled.

<Leaching process>
Next, an acid is added to the residue produced in the heating step and heated to leach Ag contained in the residue into the acid (leaching step). The acid used in the leaching step is preferably nitric acid. This allows Ag in the residue to be transferred to the solution as silver nitrate (AgNO ₃ ).

When nitric acid is used as the acid, the nitric acid aqueous solution added to the residue preferably has a nitric acid to water ratio of 1.0 or more (i.e., a nitric acid:water ratio of 1:1) and more preferably has a nitric acid to water ratio of 3.0 or more (i.e., a nitric acid:water ratio of 3:1). This allows the Ag contained in the residue to be more effectively leached into the acid.

The heating temperature of the residue to which the nitric acid aqueous solution has been added is preferably 200° C. or higher, since Ag can be reliably dissolved in nitric acid as AgNO _3. The heating time is preferably longer than the time required for all of the nitriding gas generated by heating the nitric acid to disappear, and more specifically, is preferably 30 minutes or longer.

It is preferable to wash the residue after the acid leaching treatment with water and then perform suction filtration using filter paper or the like. This allows the solution in which Ag is dissolved to be separated from the residue. The main component of the residue is metallic Si, so it can be recycled as a heating material or metallic Si raw material.

<Extraction process>
Next, the residue after the leaching process is filtered, and a chloride ion-containing substance is added to the obtained filtrate and mixed, and then the resulting mixture is filtered to extract metal chlorides (extraction process). In the extraction process, a chloride ion-containing substance is added to the filtrate and mixed, and then the mixture is filtered to extract metal chlorides. When recovering Ag, Ag ions (AgNO ₃ ) are reacted with chloride ions to be extracted as AgCl. Here, as the chloride ion-containing substance, an acid such as hydrogen chloride water may be used, but it is common to use an alkali metal chloride such as sodium chloride or calcium chloride, or an alkaline earth metal chloride salt, and this is also preferred from the viewpoint of versatility.

After the extraction process, it is preferable to thoroughly wash the solution with water, mix and stir it, and then perform a process of suction filtration. This process may be performed once, but it is more preferable to perform it multiple times to ensure that Ag is extracted. By recovering the AgCl remaining in the filter paper through this filtration, Ag can be separated from the filtrate. In other words, by going through the above processes, Ag can be efficiently recovered from the solar cell panel 9.

Since a large amount of Cu is dissolved in the filtrate after the extraction process, it is preferable to recover Cu by further extraction or electrolysis. Specifically, the above extraction process can be carried out by solvent extraction or electrolysis to precipitate and recover Cu.

The following describes examples of the present invention, but the present invention is not limited to these examples.

Ag was recovered from solar cell panels that had been cut into flakes after the aluminum outer frame had been removed in advance and the cover glass had been removed using a metal blade. In the following Examples 1 to 3, an electric resistance furnace capable of controlling the atmosphere was used in the heating process, a method of adding an aqueous nitric acid solution and heating in the leaching process, and a method of adding an alkali metal or alkaline earth metal chloride salt in the extraction process were used.

Example 1
First, in the heating step, the solar cell panel treated as described above was subjected to a heat treatment in a natural gas (CH ₄ ) atmosphere at a temperature of 1000° C. Then, the solar cell panel was sieved and then manually selected to separate metals including Cu, Sn, and Pb from a residue including Ag. Next, in the leaching step, an aqueous nitric acid solution obtained by mixing nitric acid and water at a ratio of 1:1 was used as an acid, and the aqueous nitric acid solution was added to the residue produced in the heating step, and the residue was heated to 200° C. until no nitriding gas was produced, and Ag contained in the residue was leached into the aqueous nitric acid solution (acid leaching treatment). Then, the residue after the acid leaching treatment was washed with water and suction filtered using filter paper. Next, in the extraction treatment step, sodium chloride was added to the filtrate obtained in the leaching step, and then the filtrate was washed with water and suction filtered using filter paper to extract AgCl. The process from adding sodium chloride to the filtrate to suction filtration was performed once. In this way, Ag was recovered from the solar cell panel. Details of Example 1 are shown in Table 1.

Example 2
Similarly to Example 1, Ag was recovered from the solar cell module. However, the heating step was performed at an atmospheric temperature of 1200°C using an argon atmosphere. In addition, an aqueous nitric acid solution obtained by mixing nitric acid and water at a ratio of 3:1 was used as the acid used in the leaching step. Furthermore, calcium chloride was added in the extraction treatment step, and the process from adding calcium chloride to the filtrate to suction filtration was performed twice. All other conditions were the same as in Example 1. Details of Example 2 are shown in Table 1.

Example 3
Ag was recovered from the solar cell module in the same manner as in Example 1. However, the heating step was performed in a nitrogen atmosphere at an atmospheric temperature of 600°C. In addition, an aqueous nitric acid solution obtained by mixing nitric acid and water in a ratio of 3:1 was used as the acid used in the leaching step. Furthermore, in the extraction treatment step, the process from adding sodium chloride to the filtrate to suction filtration was performed three times. All other conditions were the same as in Example 1. Details of Example 3 are shown in Table 1.

Example 4
Similarly to Example 1, Ag was recovered from the solar cell module. However, the heating step was performed in an argon atmosphere at an atmospheric temperature of 900°C. In addition, an aqueous nitric acid solution obtained by mixing nitric acid and water in a ratio of 3:1 was used as the acid used in the leaching step. Furthermore, in the extraction treatment step, the process from adding sodium chloride to the filtrate to suction filtration was performed twice. All other conditions were the same as in Example 1. Details of Example 4 are shown in Table 1.

For each of Examples 1 to 4, the amount of extracted Ag recovered (defined as the amount of Ag recovered per unit weight of the solar cell), the percentage of Ag in the residue, and the percentage of Ag in the filtrate were evaluated. The results are shown in Table 2. The percentage distribution of Ag, Cu, Sn, and Pb present in the extract, solution, and residue for each of Examples 1 to 4 is shown in Figure 3A (Example 1) and Figure 3B (Examples 2 to 4). In each of the pie charts in Figures 3A and 3B, the residue (a-1) refers to the material remaining on the filter paper when the residue after the acid leaching treatment in the leaching process was washed with water and filtered under suction, the precipitate (a-2) refers to the material remaining on the filter paper when chloride was added in the extraction treatment process, washed with water, and filtered under suction, and the filtrate (b-3) refers to the liquid that passed through the filter paper when chloride was added in the extraction treatment process, washed with water, and filtered under suction. In addition, in each pie chart in Figures 3A and 3B, the data for the precipitate (a-2) is adjacent to the data for the residue (a-1) in the clockwise direction, and the data for the filtrate (b-3) is adjacent to the data for the precipitate (a-2) in the clockwise direction.

In all of Examples 1 to 4, it was confirmed that Ag was recovered as AgCl in the extraction process. Furthermore, Ag was recovered without difference whether the heating process was performed in an inert atmosphere or a reducing atmosphere. In particular, in Examples 3 and 4, where the heating temperature in the heating process was in the range of 430°C to 900°C, an aqueous nitric acid solution with a nitric acid to water ratio of 3:1 was used as the acid in the leaching process, and further, chloride addition and subsequent suction filtration were performed multiple times in the extraction process, a high Ag recovery amount and an extremely high extraction rate were achieved.

The present invention makes it possible to safely and efficiently recover valuable materials such as silver and silicon from used solar panels.

Reference Signs List 1 Outer frame 2 Cover glass 3 Solar cell 4 Inter-connector 5 Sealing material 6 Terminal 7 Back sheet 8 Solar cell module 9 Solar cell panel

Claims (6)

A method for recovering valuable materials from solar panels, comprising:
a heating step of subjecting the solar cell panel from which the outer frame and cover glass have been removed to a heat treatment to volatilize the organic substances constituting the back sheet and the encapsulant and to separate them into visible metal components and residues;
a leaching step of adding an acid to the residue produced in the heating step, heating the mixture, and leaching silver contained in the residue into the acid, followed by washing with water and suction filtration;
an extraction step of adding and mixing a chloride ion-containing substance into the filtrate obtained after the leaching step, and then filtering the resulting mixture to extract metal chlorides;
A method for recovering valuables from a solar panel, comprising: The method for recovering valuable materials from solar panels according to claim 1, wherein the heating step is carried out in an inert atmosphere and/or a reducing atmosphere at a temperature of 430°C to 900°C. The method for recovering valuable materials from solar panels according to claim 1 or 2, wherein the acid used in the leaching step is nitric acid. The method for recovering valuable materials from a solar panel according to claim 3, wherein the nitric acid used in the leaching process has a mixing ratio of nitric acid to water of 3.0 or more. The method for recovering valuables from a solar panel according to claim 1 or 2, wherein the chloride ion-containing substance used in the extraction process is an alkali metal chloride. The method for recovering valuable materials from a solar cell panel according to claim 1 or 2, wherein the extraction treatment step is carried out two or more times.

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