In the blast furnace application, the production of iron historically required three important raw materials: iron ore, coal converted to "coke", and chemical grade limestone. In more modern times additional materials have been used such as iron ore pellets and sinter.
Chemical grade limestone is important to the process as it is the primary raw material which helps remove impurities from the iron ore and produces a slag with low melting point and a high fluidity. Consistency of the chemical grade limestone for chemistry and sizing is critical for efficient blast furnace operations and cost control.
Limestone will react with the temperature in the blast furnace as it continues down the furnace to react with sulfur from the iron and produce a slag with the silica formed from the iron ore. The first reaction involved is as follows: CaCO3 = CaO + CO2 The CaO is used to remove the sulfur and react with the silica to produce a fluid slag at the bottom of the furnace.
- High Calcium Chemical Grade Limestone – High quality limestone for the blast furnace process is typically has a 2” x ¾: sizing and consistency in chemistry with CaCO3 analysis typically 97.6%. In some cases 1” x ½” sizing is used.
- Dolomitic Chemical Grade Limestone – Some plants prefer using dolomitic limestone instead of high calcium limestone. The sizing is consistent with the requirements for the high calcium limestone. The MgO content is typically around 21.7% and the CaO is 30.5%.