Techniques to separate metal from waste printed circuit boards from discarded personal computers

Waste printed circuit boards contain valuable metals such as Au, Pd, Ag, and Cu that can be reutilized and harmful elements such as Pb, Br, and Cr that must be removed from the viewpoint of environmental conservation. In this research, we examined a method that separates the materials from printed circuit boards contained in discarded personal computers. After cutting the printed circuit boards to a size of 20 × 20 mm, they were heated at 873 K under an Ar atmosphere to remove organic resins containing elements such as C, H, and N. After heat treatment, the printed circuit boards were crushed using a planetary ball mill and the pulverized powders were filtered. The fraction with a granularity of greater than 250 μm was separated into magnetic and nonmagnetic materials by a magnetic field. Because the fraction with a granularity of less than 250 μm contained 39 mass% of C, it was heated at 1273 K in an atmosphere of 95% Ar and 5% O₂ to allow carbon combustion to take place, followed by metal reduction processing at the same temperature in an atmosphere of 97% Ar and 3% H₂. The basicity of the resulting powder was adjusted and the powder was heated at 1773 K under an Ar atmosphere. The proposed method separated the slag and metal, and 80% of the valuable metals contained in printed circuit boards could be collected.     

Pulverization of waste printed circuit boards

A new pulverization method to reduce the volume of waste printed circuit boards is reported. About 50% of printed circuit boards with integrate circuits (ICs) could be pulverized by our method in one 20-min batch, but boards without ICs could not be pulverized. By repeating the process three times, about 95% of printed circuit boards with ICs could be made into a fine powder with particles less than 106µm. A weight-drop test was also performed to examine the strength of the printed circuit boards and clarify the mechanism of pulverization. When a weight was dropped on the solder-welding side of the board ruptures occurred more easily than when the weight was dropped on the IC-mounted side. With a heavy weight, the IC was fractured more easily when the potential energy was low. Where the stress was concentrated, two types of rupture location were found on printed circuit boards with ICs. One was where the IC was connected to the printed circuit board. The other was where the surface had undulations. It also became clear that the fracture of printed circuit boards depends on the impacting weight rather than on the potential energy.