Page 26 - Curriculum Visions Dynamic Book

The chemistry of smelting
The processes that occur inside the blast furnace are quite complex, and different reactions happen at the top and bottom of the furnace.
The objective is to ensure that the right chemical reactions occur in each part of the furnace so that controlled and continuous processing is achieved. The reactions involved in the general principles of iron-making (shown on the previous page) are described here.
After the last reaction, the iron that flows from the bottom of the furnace into the moulds (known as pigs) is about 95% iron with various proportions of carbon and other impurities.
Slag
The waste gases pass out through pipes at the top of the furnace.
Cold iron ore, coke and limestone are added as a charge into the top of the furnace.
The limestone in the charge decomposes to calcium oxide and gives off carbon dioxide gas. The calcium oxide reacts with the non-metal of the ore. For example, it reacts with the silica of the rock to make calcium silicate.
The mixture of rock materials is known as slag. It is a light grey material, less dense than the iron, which is tapped off the furnace from holes above those used to tap the molten iron.
 This picture shows a blast furnace with a pile of slag
in front of it. The slag will be allowed to cool and then carried away to be made into useful road-making materials.
The slag, less dense than the iron, is tapped off.
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