Shaft Furnace

a furnace that has an upright working chamber of circular, elliptical, or rectangular cross section and is used to smelt or roast lumped materials. The heat required for the smelting or roasting process is produced by the combustion of a fuel either directly in the furnace or in an external firebox from which hot combustion products are supplied to the furnace.

Moderate velocities of the gaseous combustion products are characteristic of shaft furnaces. At such velocities, the bulk of the lumped materials (the charge) is not entrained by the ascending gas stream and, in contrast to the case of a fluidized-bed furnace, maintains aerodynamic stability. The countercurrent motion of the charge (from the top to the bottom) and of the gases forced through the charge (from the bottom to the top) and the direct contact between the charge and the hot gases result in good heat exchange and the generation of low-temperature exhaust gases. Consequently, shaft furnaces are characterized by a high thermal efficiency and a relatively high output. Such furnaces are widely used to smelt iron ores (seeBLASTFURNACE PRODUCTION), pig iron (seeFURNACE, CUPOLA), and the raw materials employed in non-ferrous metallurgy, as well as to roast, for example, iron ore (in the direct reduction of iron ore) and limestone.

Figure 1. A shaft furnace: (1) charging device, (2) gas outlet, (3) tuyere, (4) outside crucible, (5) slag notch, (6) matte hole, (7) inside crucible

The shaft furnaces used in nonferrous metallurgy (Figure 1) are designed for continuous operation. They are low (<8 m high) because of the need to carry out smelting without the reduction of substantial amounts of iron oxides and are narrow (<2 m wide in the plane of the tuyeres); the length of the furnaces is 8–15 m. The main components of such a furnace are as follows: a top, through which the charge is loaded and the gaseous combustion products are discharged; a shaft equipped with tuyeres, through which either a blast for fuel combustion or hot gases are supplied; and an inside crucible with a refractory lining, where the molten products collect. The smelts are tapped through an outside crucible or directly from the inside crucible to a forehearth for the stripping of the slags. The forehearth is often equipped for electric heating.

Formerly, shaft furnaces were constructed from metal jackets, through which water for cooling circulated (hence the obsolete name for shaft furnaces used in nonferrous metallurgy: the water-jacket furnace). Later, evaporative cooling came to be used instead of water cooling. The shaft is constructed of thick-walled tubing welded into units. Shaft furnaces are usually not lined since iron slags in nonferrous metallurgy readily dissolve refractory materials.

a firebox for the laminar combustion of moist solid fuel, such as lump peat or firewood. A shaft furnace has a tall loading shaft in which the fuel is dried and heated and volatile substances are partially released owing to the heat of the burning fuel bed below. As combustion progresses, the prepared beds in the upper part of the shaft move down to the combustion zone on the furnace grate. Shaft furnaces are used mainly to fire small boilers, that is, boilers with a steam output of up to 6.5 tons-/hr.