How the Blast Furnace Works

A blast furnace is built as a tall shaft, wider through the middle and narrowing toward the top and bottom, loaded from above with alternating layers of ore, fuel and flux, and fed air forced in near its base through openings called tuyeres. The forced air is the whole point of the design: an open fire is limited in how hot it can burn by how much oxygen simply drifts to it on its own, but air driven continuously and under pressure into the base of the shaft lets the fuel burn far hotter and far more completely than it otherwise could.

As the charge descends through the shaft it passes through a rising column of intensely hot gas, and by the time it reaches the bottom the ore has been reduced and melted, separating into two layers that can be drawn off separately, molten iron at the bottom and a lighter molten slag, made mostly of the ore's non-metallic impurities and the added flux, floating above it. Because the furnace is loaded continuously from the top as material is drawn off the bottom, it can run for extended periods without being emptied and relit the way a batch furnace must, which is the second sense in which "blast" describes not just the air but the furnace's whole continuous, industrial-scale operation.