An improved rotor design for hammermills. A sintered body is prepared using a powder for capacitor, having as the main component an earth-acid metal, preferably niobium, containing from 3 mass ppb to 1 mass ppm of phosphorus, and a capacitor is constituted from the sintered body as one part electrode, a dielectric material formed on the surface thereof, and another part electrode. An alternative embodiment contemplates smaller tuning rod subassemblies that include shorter tuning rods and a pair of support plates, whereby several subassemblies are supported along a single support rod. In the dehydrating treatmentretrieving zone, a vortex air current producing mechanism for efficiently drying the object to be treated without rotating a dehydrating treatment chamber is introduced.