Turning Limestone into Lime Steel Foundations
Limestone is a sedimentary rock of calcium carbonate (CaCO3) and is composed mainly of the two minerals calcite and aragonite.
It is extracted from the earth’s crust either by underground shaft mining or open-pit mining at ground level. Depending on the limestone’s degree of purity, it may also contain impurities such as silica, clay, silt, and sand. As mined, limestone requires refinement prior to use in the steelmaking and ironmaking processes to react properly. Limestone fines are also used in the production of sinter and direct-reduced iron.
To be useful in the steelmaking and ironmaking processes, limestone is heated in a rotary lime kiln to create what is called “burnt” lime, or calcium oxide (CaO). Limestone is charged at the cold end of the lime kiln and as the cylindrical kiln rotates, it falls toward the burner end. As the limestone travels through the kiln, the burner heats the limestone to remove the carbon and some of the oxygen, transforming it into lime. Upon exiting the kiln, the “burnt” lime is cooled, screened, and sized for use in the next steps of the steelmaking process.
During the blast furnace step of the iron making process, burnt lime is used as the flux material for slag production. Slag and the lime in it are extremely reactive and they collect impurities from the hot metal, such as oxides, inclusions, phosphorus, and sulfur.
Dolomitic lime is a special type of limestone that contains a high amount of magnesium oxide (MgO). Magnesium oxide is a critical component in steelmaking slags because it creates the proper slag chemistry, further increasing the slag’s ability to absorb impurities and enhancing the quality and composition of the steel.