A shaft furnace (Figure 1) in an old system employs an external combustion chamber to generate the heat required for indurating and introduces the hot gas into the furnace. The green balls, charged from the furnace top, make contact with the hot gas as they descend and exchange heat to increase their temperature. The heated pellets pass a cooling zone before being discharged outside the furnace. The pellets charged from the furnace top come into sufficient contact with the hot gas to ensure high thermal efficiency, which is a feature of shaft furnaces. However, it is difficult to attain a uniform temperature distribution in the furnaces. This results in nonuniform heating of the pellets, causing them to cluster and/or to adhere to the furnace wall, leading to difficulty of operation. In addition, the scale of the plant is limited to about 450 thousand tonnes/ year at maximum, which limits the cost savings. This technology has become obsolete due to the difficulty of increasing the furnace size.
Figure 1. Flow of Shaft Furnace System