Blast furnace
A blast furnace is a special type of furnace. It is very tall - up to 60 m - and can be up to 15 m by its diameter. Blast furnaces are used on making iron from ore. In many languages the blast furnace is called as "high oven". The blast furnace is the biggest chemical reactor which there is.
It normally is a steel case with bricks made of magnesia lining it. Magnesia is very heat resistant and does not melt. The furnace is cooled with water running inside the channels of the case and lining.
The ironmaking process is simple. Ore is basically iron oxide, so iron is produced by taking the oxygen off the iron, leaving crude iron called pig iron left. This process, where oxygen is stripped off its compounds, is called reduction. The reducing agent is carbon, as it can easily strip the oxygen off the ore in extremely hot temperatures.
1. Hot blast ("wind") from Cowper stoves
2. Melting zone (bosh)
3. Reduction zone of ferrous oxide (barrel)
4. Reduction zone of ferric oxide (stack)
5. Pre-heating zone (throat)
6. Feed of ore, limestone, and coke
7. Exhaust gases
8. Column of ore, coke and limestone
9. Removal of slag
10. Tapping of molten pig iron
11. Collection of waste gases
A blast furnace is fed at the top with ore, carbon in form of coke and limestone, as layers. At the same time hot air called "wind" is blown inside the furnace with special nozzles called "tuyeres" at the bottom of the furnace. This process is called blasting, hence the name "blast furnace". The coke ignites and burns, creating carbon monoxide as there is too little oxygen to make carbon dioxide. The carbon monoxide then reduces the metal oxide to the metal and makes carbon dioxide. This process is used to make iron. The limestone forms a substance called slag with the rock substance of the iron ore.
When the bottonmost part of the furnace called hearth has filled with molten pig iron and molten slag, the iron is drained away. First the slag is removed. This is called skimming. Slag is lighter than iron and floats on it. A hole is drilled on the hearth at the level of the slag and the liquid slag runs into a collecting vessel called slag pot. The iron is then collected. This is called tapping. Similarly a hole is drilled on the bosh and the liquid pig iron is let run away. It is either run directly into a converter for steelmaking, collected into a special railway wagon called torpedo car or cast into molds. When all the pig iron has been drained, the taphole and skimhole are then plugged with fire-resistant clay, which solidifies immediately in the immense heat.
The furnace is again fed with ore, coke and limestone and the reaction cycle begins again. Normally a blast furnace can work continuously for 10 to 20 years. This is called "campaign".
Pig iron contains some carbon and is too hard and brittle to be used right on. It must first be refined into steel for further use. This is made in special furnaces called converters.
