A method for separating valuable metals in a retired photovoltaic cell by vacuum joule flash directional oxidation step by step

Abstract

The application relates to the technical field of physical metallurgy and resource recycling, and discloses a method for separating valuable metals in a retired photovoltaic cell through vacuum joule flash evaporation and directional oxidation in stages. First, the broken retired silicon cell is placed in a graphite boat, vacuum joule flash evaporation is carried out, Ag and Al are rapidly vacuumized at high temperature, and the product Al-Ag alloy and residual silicon piece are obtained through condensation; then, the condensation product Al-Ag alloy is heated and melted, directional oxidation is carried out under the condition of air atmosphere, Al in the melt is oxidized to Al2O3, and Ag is not oxidized. 3， The heavy metal Ag and Al2O3 powder are obtained through solid-liquid separation. The application realizes the step-by-step recovery of the valuable metals Ag, Al and Si in the retired photovoltaic cell; the process has the characteristics of simplicity, high efficiency, environmental protection and sustainability.

Description

Technical Field

[0001] This invention relates to the fields of physical metallurgy and resource recycling, specifically to a method for the stepwise separation of valuable metals in decommissioned photovoltaic cells using vacuum Joule flash evaporation-directional oxidation. Background Technology

[0002] Compared to traditional fossil fuels, renewable energy has become a focus of attention due to its green and clean characteristics, and solar energy, as a representative of renewable energy, has been widely used and promoted. Photovoltaic technology, as the main form of solar energy utilization, has experienced rapid industrial development. However, with the exponential growth of global photovoltaic installations, the amount of photovoltaic waste is steadily increasing. Photovoltaic modules generally have a lifespan of about 20-25 years. This also indicates that the first generation of crystalline silicon solar cells, which began to be used in my country in 2000, has reached the end of their service life. Retired photovoltaic modules contain valuable metals such as Ag, Al, and Cu; direct landfilling will cause irreversible environmental damage and resource waste. Therefore, the harmless disposal and resource recycling of retired photovoltaic modules has become an urgent environmental and resource problem to be solved globally.

[0003] Currently, recycling methods primarily rely on chemical methods, which use strong acids and alkalis (such as sodium hydroxide, hydrofluoric acid, and nitric acid) to recover valuable metals from retired photovoltaic cells. While these methods offer high recovery rates, the resulting waste can potentially damage the environment. Furthermore, existing recycling technologies for retired photovoltaic modules often employ high-temperature gasification to remove silver and aluminum from the surface of the cells. Patent (CN116673309A) proposes using gasification to treat waste silicon solar cells. While this method can quickly strip silver and aluminum from the cells, it still has significant technical limitations: slow gasification speed and incomplete separation of aluminum and silver. Existing gasification processes are not fully optimized, resulting in low purity aluminum and silver produced during gasification, which affects subsequent recycling rates and thus limits efficient resource recovery.

[0004] Given the drawbacks of existing chemical methods and traditional gasification recovery processes, such as severe pollution, poor separation effect, low product purity, and high energy consumption, it is necessary to develop a green, environmentally friendly, low-energy-consumption method that can achieve efficient and selective recovery of valuable metals. Summary of the Invention

[0005] To address the challenges of treating waste gas and liquid generated by wet processes and the low efficiency of physical recovery methods, this invention provides a method for the tiered separation of valuable metals from decommissioned photovoltaic cells using vacuum Joule flash evaporation and directional oxidation. The specific steps of the method are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments.

[0006] (2) The retired photovoltaic cell fragments obtained in step (1) are subjected to vacuum Joule flash evaporation. Al and Ag are rapidly vaporized in a vacuum. After cooling, Ag-Al alloy and residual silicon wafers are obtained.

[0007] (3) The Ag-Al alloy obtained in step (2) is heated and melted, and then directionally oxidized at 980~1100℃ for 2~3h under air atmosphere conditions, so that Al in the melt is oxidized to Al2O3, while Ag is not oxidized, and then the heavy metal Ag and Al2O3 powder are obtained by solid-liquid separation.

[0008] Preferably, the size of the decommissioned photovoltaic cell fragments in step (1) is 0.83~0.9mm.

[0009] Preferably, the temperature of vacuum Joule flash evaporation in step (2) is 1750~1900℃ and the time is 20~30s.

[0010] Invention principle: Due to the large differences in saturated vapor pressure among Ag, Al, and Si in retired photovoltaic cells (for example, at 1900℃, the saturated vapor pressures of Ag, Al, and Si are 2.30 × 10⁻⁶ respectively), 9 Pa, 3.72 × 10 8 Pa, 2.73 × 10 6 Therefore, it can be placed in a flash furnace for instantaneous vacuum vaporization to separate Ag and Al. Simultaneously, due to the instantaneous heating, the silicon wafer is ensured to remain largely unmelted and unvolatile, thus yielding an Ag-Al alloy and residual silicon wafers. Furthermore, according to the oxygen potential diagram, Al has a much higher affinity for O2 than Ag. Therefore, the Ag-Al alloy can be selectively and directionally oxidized in a low-oxidation atmosphere (air atmosphere) to oxidize metallic Al to Al2O3. Through solid-liquid separation, heavy metal Ag and Al2O3 powders are obtained, ultimately achieving a cascaded and efficient separation of Si, Al, and Ag. The entire process of this invention employs pyrometallurgy, realizing the cascaded separation and recycling of valuable metals from retired photovoltaic cells, characterized by high efficiency and sustainable recycling.

[0011] Compared with the prior art, the present invention has the following beneficial effects: (1) High efficiency: This invention employs a combination of vacuum Joule flash evaporation and directional oxidation to achieve the stepwise separation of Ag, Al, and Si in retired photovoltaic cells. In the vacuum Joule flash evaporation stage, Ag and Al rapidly vaporize and condense from the silicon wafer due to their high vaporization tendency, avoiding the large-scale melting of the silicon substrate. In the directional oxidation stage, the oxidation temperature is controlled at 980~1100℃ and the oxidation time is 2~3h, while an air atmosphere is introduced, allowing Al to selectively oxidize to form the Al2O3 solid phase and float to the top, while Ag remains in the metallic phase and is located at the bottom. Through the control of the above reaction conditions, the oxidation direction of Al and the position of the product are limited, and the solid phase Al2O3 and the liquid phase Ag can be further separated, thereby improving the metal separation efficiency and product purity. (2) Sustainability: This invention does not use strong acid or strong alkali leaching as the main separation method, which reduces the pressure of waste liquid treatment; at the same time, it uses Joule instantaneous heating and selective oxidation to complete the key separation steps, resulting in a shorter process flow and the characteristics of being green, efficient and economically feasible. Attached Figure Description

[0012] Figure 1 This is a flow chart of the vacuum Joule flash evaporation-directional oxidation stepped separation process of the present invention.

[0013] Figure 2 The images show the SEM microstructure and elemental distribution of the upper and lower regions of the Ag-Al alloy obtained after vacuum Joule flash evaporation and condensation. These images are used to characterize the enrichment and distribution of Ag and Al in the condensed alloy.

[0014] Figure 3 The thermodynamic calculation diagrams of the saturated vapor pressures of Ag, Al, and Si as a function of temperature are used to illustrate the preferential vaporization trend of Ag and Al relative to Si during the vacuum Joule flash evaporation stage.

[0015] Figure 4 The graph shows the change in purity of Ag and Al2O3 after directional oxidation for different times at 980℃.

[0016] Figure 5 The graph shows the change in purity of Ag and Al2O3 after directional oxidation for different times at 1060℃. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Example 1 A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, the specific steps of which are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments with a size of 0.83~0.9mm.

[0019] (2) Load 10g of the decommissioned photovoltaic cell fragments from step (1) into a graphite boat and place it on the graphite electrode of a vacuum Joule flash furnace; seal the reaction chamber and start the vacuum pump to evacuate the system pressure to a vacuum state below 1000Pa. Apply a high-power pulsed DC current to the electrodes at both ends of the graphite boat, control the flash evaporation temperature at 1750℃, and the flash evaporation time at 20s. Al and Ag are rapidly vaporized in the vacuum. In this system, the saturated vapor pressure of Si is much lower than that of Al and Ag (see saturated vapor pressure). Figure 3 Therefore, Si is difficult to vaporize, while Al and Ag will vaporize rapidly. In the end, the Si wafer is retained, and the vaporized Al and Ag are condensed in the condenser to obtain Ag-Al alloy and the remaining silicon wafer.

[0020] (3) The Ag-Al alloy was directionally oxidized and smelted in an air atmosphere at a temperature of 980℃. The smelting time was set to 2h, 2.5h and 3h, so that Al in the Al-Ag melt was oxidized to Al2O3 and floated to the surface of the melt. Solid-liquid separation was performed to obtain solid phase Al2O3 and noble metal Ag. In this embodiment, the purity of metallic Ag at different directional smelting times was 82.71wt.%, 89.45wt.% and 98.74wt.% respectively by mass percentage; the purity of Al2O3 was 98.57wt.%, 99.37wt.% and 99.69wt.% respectively (see Figure 4 ).

[0021] Example 2 A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, the specific steps of which are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments with a size of 0.83~0.9mm.

[0022] (2) 10g of the decommissioned photovoltaic cell fragments from step (1) were loaded into a graphite boat and placed on the graphite electrode of a vacuum Joule flash furnace; the reaction chamber was sealed, and the vacuum pump was started to evacuate the system pressure to a vacuum state below 1000Pa. High-power pulsed DC current was applied to the electrodes at both ends of the graphite boat, and the flash evaporation temperature was controlled at 1800℃ with a flash evaporation time of 25s. Al and Ag were rapidly vaporized in the vacuum and condensed in the condenser to obtain Ag-Al alloy and residual silicon wafers. The obtained Ag-Al alloy was subjected to scanning electron microscopy, and the results are as follows: Figure 2As shown, Ag is enriched by condensation in the lower, higher temperature region, while Al is enriched by condensation in the upper, lower region. This is due to the difference in condensation temperatures between Ag (962℃) and Al (660℃).

[0023] (3) The Ag-Al alloy was directionally oxidized and smelted in an air atmosphere at a temperature of 1020℃. The smelting time was set to 2h, 2.5h and 3h, so that Al in the Al-Ag melt was oxidized to Al2O3 and floated to the surface of the melt. Solid-liquid separation was performed to obtain solid phase Al2O3 and noble metal Ag. In terms of mass percentage, the purity of metal Ag in this embodiment was 86.45wt.%, 96.57wt.% and 99.90wt.%, respectively; the purity of Al2O3 was 99.56wt.%, 99.85wt.% and 99.97wt.%, respectively.

[0024] Example 3 A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, the specific steps of which are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments with a size of 0.83~0.9mm.

[0025] (2) Load 10g of the decommissioned photovoltaic cell fragments from step (1) into a graphite boat and place it on the graphite electrode of a vacuum Joule flash furnace; seal the reaction chamber and start the vacuum pump to pump the system pressure to a vacuum state below 1000Pa, apply high-power pulsed DC current to the electrodes at both ends of the graphite boat, control the flash temperature at 1850℃, and the flash time at 30s, so that Al and Ag are rapidly vaporized in the vacuum and condensed in the condenser to obtain Ag-Al alloy and residual silicon wafers.

[0026] (3) The Ag-Al alloy was directionally oxidized and smelted in an air atmosphere at a temperature of 1060℃. The smelting time was set to 2h, 2.5h and 3h, so that Al in the Al-Ag melt was oxidized to Al2O3 and floated to the surface of the melt. Solid-liquid separation was performed to obtain solid phase Al2O3 and noble metal Ag. In terms of mass percentage, the purity of metallic Ag in this embodiment was 99.85wt.%, 99.94wt.%, and 99.94wt.%, respectively; the purity of Al2O3 was 99.92wt.%, 99.97wt.%, and 99.98wt.%, respectively (see Figure 5 ).

[0027] Example 4 A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, the specific steps of which are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments with a size of 0.83~0.9mm.

[0028] (2) Load 10g of the decommissioned photovoltaic cell fragments from step (1) into a graphite boat and place it on the graphite electrode of a vacuum Joule flash furnace; seal the reaction chamber and start the vacuum pump to pump the system pressure to a vacuum state below 1000Pa, apply high-power pulsed DC current to the electrodes at both ends of the graphite boat, control the flash temperature at 1900℃, and the flash time at 20s, so that Al and Ag are rapidly vaporized in the vacuum and condensed in the condenser to obtain Ag-Al alloy and residual silicon wafers.

[0029] (3) The Ag-Al alloy was directionally oxidized and smelted in an air atmosphere at a temperature of 1100℃. The smelting time was set to 2h, 2.5h and 3h, so that Al in the Al-Ag melt was oxidized to Al2O3 and floated to the surface of the melt. Solid-liquid separation was performed to obtain solid phase Al2O3 and noble metal Ag. In terms of mass percentage, the purity of metal Ag in this embodiment was 82.71wt.%, 89.45wt.%, and 98.74wt.%, respectively; the purity of Al2O3 was 98.57wt.%, 99.37wt.%, and 99.69wt.%, respectively.

[0030] Example 5 A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, the specific steps of which are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments with a size of 0.83~0.9mm.

[0031] (2) Load 10g of the decommissioned photovoltaic cell fragments from step (1) into a graphite boat and place it on the graphite electrode of a vacuum Joule flash furnace; seal the reaction chamber and start the vacuum pump to pump the system pressure to a vacuum state below 1000Pa. Apply high-power pulsed DC current to the electrodes at both ends of the graphite boat, control the flash temperature at 1750℃, and the flash time at 25s. Al and Ag are rapidly vaporized in the vacuum and condensed in the condenser to obtain Ag-Al alloy and residual silicon wafers.

[0032] (3) The Ag-Al alloy was directionally oxidized and smelted in an air atmosphere at a temperature of 1020℃. The smelting time was set to 2h, 2.5h and 3h, so that Al in the Al-Ag melt was oxidized to Al2O3 and floated to the surface of the melt. Solid-liquid separation was performed to obtain solid phase Al2O3 and noble metal Ag. In terms of mass percentage, the purity of metal Ag in this embodiment was 99.85wt.%, 99.94wt.%, and 99.94wt.%, respectively; the purity of Al2O3 was 99.92wt.%, 99.97wt.%, and 99.98wt.%, respectively.

[0033] Example 6 A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, the specific steps of which are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments with a size of 0.83~0.9mm.

[0034] (2) Load 10g of the decommissioned photovoltaic cell fragments from step (1) into a graphite boat and place it on the graphite electrode of a vacuum Joule flash furnace; seal the reaction chamber and start the vacuum pump to pump the system pressure to a vacuum state below 1000Pa, apply high-power pulsed DC current to the electrodes at both ends of the graphite boat, control the flash temperature at 1800℃, and the flash time at 30s, so that Al and Ag are rapidly vaporized in the vacuum and condensed in the condenser to obtain Ag-Al alloy and residual silicon wafers.

[0035] (3) The Ag-Al alloy was directionally oxidized and smelted in an air atmosphere at a temperature of 980°C. The smelting time was set to 2h, 2.5h and 3h, so that Al in the Al-Ag melt was oxidized to Al2O3 and floated to the surface of the melt. Solid-liquid separation was performed to obtain solid phase Al2O3 and noble metal Ag. In terms of mass percentage, the purity of metal Ag in this embodiment was 86.45wt.%, 96.57wt.% and 99.90wt.% respectively; the purity of Al2O3 was 99.56wt.%, 99.85wt.% and 99.97wt.% respectively.

[0036] Example 7 A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, the specific steps of which are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments with a size of 0.83~0.9mm.

[0037] (2) Load 10g of the decommissioned photovoltaic cell fragments from step (1) into a graphite boat and place it on the graphite electrode of a vacuum Joule flash furnace; seal the reaction chamber and start the vacuum pump to pump the system pressure to a vacuum state below 1000Pa, apply high-power pulsed DC current to the electrodes at both ends of the graphite boat, control the flash temperature at 1850℃, and the flash time at 20s, so that Al and Ag are rapidly vaporized in the vacuum and condensed in the condenser to obtain Ag-Al alloy and residual silicon wafers.

[0038] (3) The Ag-Al alloy was directionally oxidized and smelted in an air atmosphere at a temperature of 1100℃. The smelting time was set to 2h, 2.5h and 3h, so that Al in the Al-Ag melt was oxidized to Al2O3 and floated to the surface of the melt. Solid-liquid separation was performed to obtain solid phase Al2O3 and noble metal Ag. In terms of mass percentage, the purity of metal Ag in this embodiment was 82.71wt.%, 89.45wt.%, and 98.74wt.%, respectively; the purity of Al2O3 was 98.57wt.%, 99.37wt.%, and 99.69wt.%, respectively.

[0039] Example 8 A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, the specific steps of which are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments with a size of 0.83~0.9mm.

[0040] (2) Load 10g of the decommissioned photovoltaic cell fragments from step (1) into a graphite boat and place it on the graphite electrode of a vacuum Joule flash furnace; seal the reaction chamber and start the vacuum pump to pump the system pressure to a vacuum state below 1000Pa. Apply high-power pulsed DC current to the electrodes at both ends of the graphite boat, control the flash temperature at 1900℃, and the flash time at 25s. Al and Ag are rapidly vaporized in the vacuum and condensed in the condenser to obtain Ag-Al alloy and residual silicon wafers.

[0041] (3) The Ag-Al alloy was directionally oxidized and smelted in an air atmosphere at a temperature of 1060℃. The smelting time was set to 2h, 2.5h and 3h, so that Al in the Al-Ag melt was oxidized to Al2O3 and floated to the surface of the melt. Solid-liquid separation was performed to obtain solid phase Al2O3 and noble metal Ag. In terms of mass percentage, the purity of metal Ag in this embodiment was 99.85wt.%, 99.94wt.%, and 99.94wt.%, respectively; the purity of Al2O3 was 99.92wt.%, 99.97wt.%, and 99.98wt.%, respectively.

[0042] Comparative Example 1 In comparison, the only difference between this comparative example and Example 1 is that the directional oxidation melting temperature in step (3) is modified to 900°C. All other steps are the same as in Example 1. After being processed by the same separation and detection methods, the purity of metal Ag and Al2O3 in this comparative example is lower than that in Example 1.

[0043] The purity of Ag and Al2O3 in this comparative example is lower than that in Example 1. This is because 900°C is below the melting point of Ag, the Ag-Al system melt has insufficient fluidity, the selective oxidation of Al and the flotation separation of Al2O3 are not sufficient, and some unoxidized Al and Ag-Al inclusions remain in the metal phase or solid phase, resulting in obvious entrainment between Ag and Al2O3, which ultimately reduces the purity of the two products.

[0044] Comparative Example 2 In comparison, the only difference between this comparative example and Example 1 is that the directional oxidation melting temperature in step (3) is modified to 1500°C. All other steps are the same as in Example 1. After being processed by the same separation and detection methods, the purity of metal Ag and Al2O3 in this comparative example is lower than that in Example 1.

[0045] The purity of Ag and Al2O3 in this comparative example is lower than that in Example 1. This is because 1500℃ is significantly higher than the directional oxidation temperature range defined in this invention. The Al oxidation reaction is too fast, which easily forms a thick and locally dense Al2O3 layer on the surface of the melt, encapsulating some of the unreacted Ag-Al alloy and causing entrainment. At the same time, the excessively high temperature will increase the risk of melt disturbance and metal volatilization loss, making the solid-liquid interface less stable and resulting in a decrease in the separation effect between solid Al2O3 and Ag.

[0046] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells, characterized in that: The specific steps of the method are as follows: (1) Grind the retired photovoltaic cells to obtain retired photovoltaic cell fragments; (2) The retired photovoltaic cell fragments obtained in step (1) are subjected to vacuum Joule flash evaporation, Al and Ag are vaporized in a vacuum, and after cooling, Ag-Al alloy and residual silicon wafers are obtained. (3) The Ag-Al alloy obtained in step (2) is heated and melted, and then directionally oxidized at 980~1100℃ for 2~3h under air atmosphere conditions, so that Al in the Al-Ag melt is oxidized to Al2O3 and floats to the surface of the melt. Solid-liquid separation is performed to obtain solid phase Al2O3 and noble metal Ag.

2. The method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells according to claim 1, characterized in that: The size of the decommissioned photovoltaic cell fragments in step (1) is 0.83~0.9mm.

3. The method for vacuum Joule flash evaporation-directional oxidation stepwise separation of valuable metals in decommissioned photovoltaic cells according to claim 1, characterized in that: In step (2), the temperature of vacuum Joule flash evaporation is 1750~1900℃ and the time is 20~30s.

Images