PURPOSE: To decrease the amt. of fuel to be used and to permit the use of raw material ores from coarse to fine grains by combining a preheater for raw ore grains by the waste gases of combustion of a vertical calcining furnace using a vertical calcining furnace and a dust collector and a grain screening device for foamed ore grains using an another dust collector.

CONSTITUTION: Raw ore grains are charged into a gas duct connecting a cyclone 4 and a cyclone 5 through a feed port 6 and disperse in the flow of combustion gases. The ore grains are separated from the combustion gases in the cyclone 5. The separated raw ore grains fall into a gas duct connecting the cyclones 4 and 3, and are dispersed into the flow of the combustion gas from the cyclone 3, and arrive at the cyclone 4. Here, the ores are reseparated and fall into a vertical calcining furnace 1 where the greater part are calcined and expanded by the combustion heat of burners 8. The separated ore grains arrive at a cyclone 2. The foamed ore grains of intermediate grain sizes are separated here and those of fine grains are further introduced into the cyclone 3 and are separated.