The application of electric disintegration (ED) to the separation of a tantalum (Ta), the rare metal, from the Ta capacitor has been investigated in the study of the high efficiency recycling technology of rare metals from the waste circuit board of electronic scraps, urban mines. In the ED, the underwater explosion is produced by the electric discharge performed directly toward the capacitor in the water. The discharge path induced by ED has been investigated (S. Owada 2014). The passage of discharging current is formed in the water between the electrode of discharge and the surface of capacitor. The discharge occurs also inside the Ta capacitor and the current passes thorough the boundary between a Ta block and the casing of capacitor. However, the mechanism of the separation of materials caused by ED has not been much clarified.
In order to investigate the separation of ED, we have analyzed numerically the pressure inside the Ta capacitor induced by the shock wave generated from the boundary between the Ta and the casing, PMMA. In this our previous study, it was found that pressure fluctuation was generated near the boundary due to the increase and decrease of pressure induced by the shock wave and the expansion wave reflected from the boundary. It is possible consider that this pressure fluctuation causes the separation of a Ta block from the capacitor. In order to clarify the mechanism of ED, it is necessary to study not only the pressure inside of the Ta capacitor but also the motion of separation of the Ta caused by the underwater explosion generated between the electrode and the capacitor. Therefore, the purpose of the this study is to investigate experimentally the motion of a Ta block induced by the underwater explosion.
The model of a Ta block put on a PMMA block located near the explosion point in the water was employed to simulate the Ta capacitor subjected to underwater explosion in this study. The tested Ta block is 2 mm×2 mm×2 mm, and is nearly the same size as the Ta mounted in the capacitor. The underwater explosion was created by the electric discharge. The discharge electrode was installed on the left side of Ta. The discharge voltage is 4.5 kV. The standoff distance L between the electrode of discharge and the Ta block was varied to investigate the effect of L on the motion of Ta block. The tested standoff distance L = 10 mm, 20 mm. The propagation of underwater shock wave and the primary bubble generated by the electric discharge were visualized by the shadowgraph method. A high-speed video camera, Phantom V2010, recorded images with 2.0×105 frames per second at its exposure time of 0.5 μsec.
The underwater shock wave propagated spherically and reflected at the Ta and PMMA blocks. After that, the primary bubble expanded, contracted, resulting in the rebound. The second bubble rebound occurred. The underwater shock wave induced by the bubble rebound also interacted with these blocks. When the primary bubble expanded, the Ta block moved toward the right direction because the water between the bubble and the Ta was pushed by the expanding bubble. The primary bubble contracted and collapsed near these blocks due to the close standoff distance in L = 10 mm. On the other hand, the bubble contracted toward the electrode as the center in L = 20 mm. As the primary bubble contracted, the Ta block was displaced toward the left direction located in the electrode in L = 10 mm and 20 mm. This is caused due to the increasing acceleration of contracting bubble toward the center of it. These displacement of Ta induced by the expansion and contraction of bubble were also observed after the bubble rebound. In L = 10 mm, when the second bubble rebound was generated, the Ta block moved to the left end of PMMA block was accelerated upward due to the bubble expansion, resulting in the separation of Ta from PMMA with the parabolic motion. In the case of L = 20 mm, the Ta located at the left end of PMMA and the parabolic motion of Ta did not appeared due to the longer standoff distance.
Therefore, the separation of a Ta block is induced by not only the underwater shock wave loading but also the primary bubble generated by the underwater explosion in the ED. The motion of Ta block is depended on the standoff distance. More discussion of the time history of the Ta motion measured quantitatively by analyzing sequential images will be reported in the full paper.
