How pig iron is made

Charge the stack

Iron ore, coke and limestone are fed in at the top of the furnace, and the stack runs without stopping for long periods. Furnaces are hard to start and stop, so they are kept going. The coke must stay strong, because the column has to stay porous for gas to rise through it.

Blast in hot air

Pipes called tuyeres blow preheated air, sometimes enriched with oxygen, into the lower furnace. The air burns the coke and makes hot carbon monoxide, which is the real worker in the process.

Strip the oxygen off the ore

As ore sinks, the carbon monoxide rising through it takes oxygen away in stages. Near the top, at 200 to 700 °C, the oxide is partly reduced. Around 850 °C it is reduced further, and by the lower furnace, at up to 1,200 °C, iron metal forms. This countercurrent flow is why the furnace is efficient.

up to 1,200 °C

Float off the rock as slag

The heat breaks limestone into lime, and the lime combines with silica from the ore to form silicates. These float on the iron as molten slag, which carries away much of the rock that came in with the ore.

Tap iron and slag

At intervals the furnace is opened at the bottom and a stream of molten iron runs out, with the lighter slag skimmed from its surface. The iron is saturated with carbon, around 4 to 5 percent, which makes it brittle.

Ladle it or cast the pigs

Most iron goes still liquid, in ladle cars, to a steel plant next door. The rest runs onto a pig-casting machine, which makes sticks that snap into pieces of 4 to 10 kilograms. The old way was a sand bed with a main channel feeding many small molds.