PURPOSE: To improve the heat exchangeability through a uniform dispersion of granular solid material into heat gas by means of a bent duct extending upward provided on a vertical successively duct connected to a heat gas dust collector to rectify the heat gas with a feed inlet above thereof.

CONSTITUTION: Heat gas generated from a rotary kiln, calcination furnace and the like is inducted to a cyclone 12 from a connection duct 1, turned as shown by the arrow 27, and ascends into a rise duct section from an inner cylinder 16 turning. Then, combustion gas introduced turning into a bent duct section 23 from the rise duct section 22 hits against the vertical side wall of the bent duct section 23 and eliminated in the turning speed component, it is led upward to an extended duct section 24. Accordingly, granular solid material fed to the extended duct section 24 from material feed inlet 18 is mixed evenly with the combustion gas, which enables a better heat exchange before it reaches a cyclone 19 via a horizontal duct section 25.