PURPOSE: To easily purify heavy metal-contng. dust with high efficiency, by mixing, compressing and forming exhaust dust with high-molecular thermoplastic waste matter under the condition that fluid having a temperature higher than the softening point of the high-molecular thermoplastic resin waste matter flows through.

CONSTITUTION: Dust separated from combustion exhaust gas by a dust collector 2 is injected into a mixer 4 by a carrier 3, while thermoplastic high-molecular resin waste matter such as plastics is injected from a quantitative feeder 5 through a crusher 6 into the mixer 4. In the mixer 4, the waste matter and the dust are mixed 30W12sec and concurrently heated at a temperature higher than the softening point of the waste matter. The softened mixture of the waste matter and the dust is blended and compressed under a heated and pressurized condition by a compressor 7, and then pushed into a forming machine 8 where the mixture is formed into a solid pellet having a specified shape. The formed pellet is cooled and caked in a water vessel 9, and then discharged outside the water vessel 9.