Quick guide to Furnace operation.
A 19th century blast-furnace consisted of a tall stack into which the ironstone and coke were charged at the top. In the upper part of the stack any remaining water and carbon dioxide gas were driven off. As the charge descended in the stack the iron ore was reduced to metallic iron. Close to the bottom of the stack, air pipes called tuyeres (pronounced 'tweers') were fitted. Hot air was blasted through these pipes into the stack to promote the necessary chemical reactions. The metallic iron (known as pig iron) was drawn off right at the bottom of the stack. There were two main chemical reactions in the smelting operation.
In the first, the ironstone was reduced by carbon monoxide gas produced by the burning coke.
iron stone + carbon monoxide gas = iron + carbon dioxide gas
Ironstone was never pure and limestone was the third essential raw material in the smelting process, being required to remove the impurities. This was added with the ironstone and coke and its function in the second chemical reaction was to act as a flux to fuse the impurities into a slag. Once inside the stack the limestone calcined into burnt lime which had the ability to combine with the impurities and earthy waste in the ironstone. This formed a fluid slag which floated on top of the molten iron where it was run off separately.
impurities and earthy waste + lime = liquid slag
The slag was not wasted. It was broken up and used as an aggregate in concrete or for road building. It was even used to build road-side walls in some areas.
The molten pig iron drawn off at the bottom of the furnace was either cast into 'pigs' for subsequent use or it was transferred in its molten state to a steel-making plant.