Blast Furnaces and Smelting

Iron does not occur naturally as metal but it occurs plentifully in combination with other elements forming compounds such as iron oxides and carbonates. When these compounds are found en masse they are called iron-ores. Metal is freed from the ore and rock that holds it by using heat and carbon monoxide gas in a furnace.

The chemistry is very simple: the carbon monoxide is generated in the furnace by burning carbon with insufficient oxygen.  The result is carbon monoxide gas, a substance that is hungry for more oxygen. It takes oxygen from the hot oxide ore thus freeing the atoms of iron. In chemical terms the iron oxide is reduced.

How Did a Blast Furnace Work?

Furnaces were strongly built and thick enough to maintain the heat needed for smelting. They were also tall and capacious so that the chemistry could take place continuously over days or weeks.

To begin a smelt, an empty furnace is part filled with coke or charcoal. This is burnt slowly to heat and dry the furnace. When this is completed ore and coke (or charcoal) is tipped into the top alternately to form burning layers within. A temperature of about 1600°C is produced by blasting air into the base.  The blast is provided either by water powered bellows, or a steam powered compression cylinder.

Reduction of the ore takes place at a relatively low temperature but the iron will not melt until 1540°C is reached.  Once this is achieved the iron liquefies and sinks to the base of the furnace where it collects.  It is then tapped at intervals and run out onto a sand floor in the cast house to form cast iron.

The rocky waste also melts in the furnace. It forms a pool of molten slag above the molten iron.  It is tapped periodically for disposal.


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