The blast furnace converts iron ore into liquid iron, known as hot metal or pig iron. This towering structure is continuously fed with iron ore, a carbon-rich fuel (coke), and a mineral additive known as a flux. Limestone (\(\text{CaCO}_3\)) is the most common fluxing material added to the furnace burden. Its function is to act as a purifying agent, separating unwanted substances from the molten metal.
The Necessity of a Fluxing Agent
Iron ore, the raw material for iron production, is never a pure source of iron oxide (\(\text{Fe}_2\text{O}_3\)). It naturally contains non-metallic, rocky compounds, collectively referred to as gangue. The most prevalent impurities are silica (\(\text{SiO}_2\)) and alumina (\(\text{Al}_2\text{O}_3\)).
The coke used as fuel also contributes to the impurity load, leaving behind an ash residue containing similar compounds. If these non-metallic materials were allowed to melt and remain untreated, they would form a highly viscous liquid. This liquid would degrade the quality of the final iron product and interfere with the furnace’s operation by potentially clogging the tap holes and preventing the smooth flow of molten iron.
To prevent this outcome, a chemical agent is intentionally introduced to react with these impurities and chemically bind them into a separate, manageable liquid phase. This is the role of the fluxing agent, which lowers the melting point of the gangue. The addition of limestone ensures that the impurities are systematically removed from the system, yielding a cleaner product.
The High-Temperature Transformation of Limestone
Limestone (\(\text{CaCO}_3\)) is not chemically active enough in its raw state to react effectively with the acidic impurities present in the furnace. Its transformation into a highly reactive compound is triggered by the intense heat as the materials descend. When the temperature reaches approximately 850°C to 1000°C, the limestone undergoes calcination, a thermal decomposition reaction.
During calcination, calcium carbonate breaks down into calcium oxide (\(\text{CaO}\)) and carbon dioxide (\(\text{CO}_2\)): \(\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2\). This reaction is endothermic, meaning it absorbs heat from the furnace environment.
The resulting calcium oxide, commonly known as quicklime, functions as the purifying agent. Calcium oxide is a highly reactive basic oxide, prepared to chemically bond with the acidic impurities. The carbon dioxide released ascends the furnace shaft, where it can participate in other chemical reactions that aid in the reduction of the iron ore.
Slag Formation and Impurity Removal
The newly formed calcium oxide (\(\text{CaO}\)) descends into the lower, hotter zones of the furnace, where it encounters the molten iron and the acidic impurities. The \(\text{CaO}\) acts as a basic flux, readily reacting with the primary acidic impurity, silica (\(\text{SiO}_2\)), to form a new, low-melting-point compound. This compound is calcium silicate (\(\text{CaSiO}_3\)), which is the main component of the molten waste product called slag.
The fundamental reaction is \(\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3\). A similar reaction occurs with alumina (\(\text{Al}_2\text{O}_3\)), where the calcium oxide binds with the impurity to form a calcium aluminosilicate compound, incorporating the gangue into the liquid slag phase. By chemically bonding with the impurities, the flux prevents them from dissolving into the molten iron and forms a fluid mixture that can be easily separated.
A physical property difference allows for effective separation inside the furnace hearth. The molten slag is significantly less dense than the molten iron, causing it to float on top of the heavier iron layer.
The limestone flux also performs a secondary role in removing other detrimental elements, primarily sulfur and phosphorus. Sulfur, which originates mainly from the coke, is absorbed into the slag in the form of calcium sulfide (\(\text{CaS}\)). This desulfurization is a function of the slag’s basicity, which the calcium oxide provides.
The liquid iron and the slag accumulate in the hearth at the bottom of the blast furnace, forming two distinct layers. Periodically, the iron and the slag are tapped through separate holes, with the slag being drawn off from the upper layer. This systematic removal of impurities results in purified pig iron that is ready for subsequent steelmaking processes.