Cornwall Iron Furnace - Blast Furnace

Blast Furnace

In 1742, Grubb replaced his bloomery with a 30-foot (9.1 m) high charcoal-fired cold blast furnace. The blast furnace burned hotter than the bloomery and was able to render molten pig iron ("charcoal iron") from the ore.

A blast furnace relies on the fact that the unwanted silicon and other impurities are lighter than the molten iron that is the main product. Grubb's furnace was built in the form of a tall chimney-like structure lined with refractory brick. Charcoal, limestone and iron ore (iron oxide) were poured in at the top, and air was blown in through tuyeres near the base. The resulting "blast" promotes combustion of the charcoal (more modern furnaces use coke or even anthracite), creating a chemical reaction that reduces the iron oxide to the base metal which sinks to the bottom of the furnace. The exact nature of the reaction is:

Fe₂O₃ + 3 CO → 2Fe + 3CO₂

More precisely, the compressed air blown into the furnace reacts with the carbon in the fuel to produce carbon monoxide, which then mixes with the iron oxide, reacting chemically to produce iron and carbon dioxide, which leaks out of the furnace at the top. In the beginning of the reaction cycle, the hot blast, also called "wind", containing pre-heated gas from Cowper stoves and air, is blasted into the furnace through tuyeres. The wind will ignite the coke and the Boudouard reaction will take place:

C + O → CO
CO + C → 2 CO

The temperature in the furnace typically runs at about 1500 °C, which is enough to also decompose limestone (calcium carbonate) into calcium oxide and additional carbon dioxide:

CaCO → CaO + CO

The calcium oxide reacts with various acidic impurities in the iron (notably silica), forming a slag containing calcium silicate, CaSiO₃ which floats on the iron.

The pig iron produced by the blast furnace is not useful for most purposes due to its high carbon content, around 4-5%, making it very brittle. Some pig iron is used to make cast iron goods, often being remelted in a foundry cupola.

For other purposes further processing is needed to reduce the carbon content to enable iron to be used for tools or as a construction material. There have been various processes for this. The earliest process was conducted in the finery forge. In the late 18th century, this began to be displaced by 'potting and stamping', but the most successful new process of the industrial revolution period was puddling.

This is now done by forcing a jet of high-pressure oxygen into a special rotating container containing the pig iron. Some of the carbon is oxidised into carbon monoxide, CO, and carbon dioxide, CO. This also oxidizes impurities in the pig iron. The container is rotated and the processed pig iron can be separated from the oxidised impurities. Before the mid 19th century, pig iron from the blast furnace was made into wrought iron, which is commercially pure iron. At that period, if steel was needed, particularly pure varieties of iron were heated with charcoal in a cementation furnace to produce blister steel (with about 1-2% carbon). This might be further purified using the crucible technique, but steel was too expensive to use on a large scale. However with the introduction of the Bessemer process in the late 1850s and then other processes, the production of steel was dramatically increased. By the late 19th century most iron was being converted to steel before use.