Bloomery operation

In a typical bloomery furnace, wood or charcoal is burned, producing a temperature around 1200–1300°C and a furnace atmosphere rich in carbon monoxide (CO). Roasted iron ore, composed largely of iron oxides and silica (SiO₂), is added to this fire. Some of the iron oxide combines with the silica to produce fayalite (2FeO·SiO₂ or Fe₂SiO₄), with a melting point of 1205°C. This is the primary component of the slag, most of which melts and flows to the bottom of the furnace. The rest of the iron oxide is reduced to metallic iron by the carbon monoxide. The resulting small pieces of iron, which are never molten, sinter together to make a sponge-like mass of iron and slag called the bloom. This is removed and hammered on an anvil to force out as much as possible of the slag. The reactions involved are:

The thermodynamic conditions for these reactions are shown in the diagram below. It can be seen that, under equilibrium conditions, there is only a narrow range of CO content in which FeO is reduced to Fe and Fe₂SiO₄ is not reduced (between curves C and B). In this range very little carbon can enter the iron. The bloomery operator takes care to control the CO content of the furnace atmosphere, but a bloomery operates very far from equilibrium conditions, and it is quite possible to make steel or cast iron in a bloomery. Nevertheless the operator generally avoids allowing too much carbon to enter the iron, for while steel is often desirable, there is always the danger that so much carbon might enter the iron that it becomes cast iron, which is useless to a smith.

Much more detailed technical accounts of the bloomery process are given by Gordon & Killick (1992; 1993) and Pleiner (2000: 133–137).

Equilibrium conditions for certain reactions which are important in bloomery smelting. The vertical axis gives the ratio of partial pressures of carbon monoxide (CO) and carbon dioxide (CO2) in the furnace atmosphere. When furnace conditions are above a particular line, the corresponding reaction goes to the right; if below, to the left. Curve A and the curves between A and C (except B) are calculated with the assumption that the atmosphere contains ca. 79% nitrogen. Reproduced from Wagner 2008: 92, Figure 41.
The calculations on which this diagram is based are here.

References