Pig iron is a foundational material in the metals industry, functioning as an intermediate product created during the primary refinement of iron ore. It represents the crude, initial output from the smelting process, resulting from extracting iron from its mineral forms using high heat and chemical reduction. Pig iron is not typically used for structural applications in its original state. Instead, it serves as a crucial feedstock for nearly all subsequent iron-based metal production, including steel and cast iron.
Chemical Composition and Characteristics
The defining feature of pig iron is its high carbon content, which typically ranges from 3.5% to 4.5% by weight. This percentage of carbon causes the metal to have a significantly lower melting point than pure iron, making it easier to handle in its molten state during smelting. However, the high carbon concentration also renders the solid material hard and brittle. This means pig iron cannot be shaped by hammering or rolling and is unsuitable for most structural purposes.
Beyond iron and carbon, the composition includes various impurities, which are byproducts of the ore and the smelting process. These elements commonly include silicon, manganese, phosphorus, and sulfur, all of which affect the final properties of the metal. Silicon levels can vary between 0.5% to 3%, while manganese is often present between 0.5% and 6%. Sulfur and phosphorus are considered undesirable elements that must be removed or controlled in later refining stages.
Why It Is Called Pig Iron
The name “pig iron” originates from the traditional method of casting the molten metal into transportable ingots. Before modern continuous casting machines were developed, liquid iron was poured into sand molds laid out on the foundry floor. The casting arrangement consisted of one large central channel that fed the molten metal into numerous smaller, perpendicular molds.
Workers noted that this layout visually resembled a sow lying on her side with a litter of piglets feeding from her. The central channel became known as the “sow,” and the smaller ingots branching off were called the “pigs.” Once the metal cooled and solidified, the smaller pieces were broken off the main runner, and the name has persisted in the industry.
The Blast Furnace Production Process
Pig iron production is almost exclusively carried out within a vertical shaft blast furnace designed for continuous operation. The furnace is charged from the top with the three primary raw materials: iron ore, coke, and a flux, typically limestone. Iron ore provides the iron oxides, while coke, a carbon-rich fuel derived from coal, serves both as the heat source and the chemical reducing agent.
A blast of preheated air is injected into the lower section of the furnace through nozzles called tuyeres. The heat generated by the burning coke produces carbon monoxide gas, which rises up through the furnace column. This gas chemically reacts with the descending iron ore, stripping the oxygen atoms away from the iron oxides in a process known as reduction. The limestone flux reacts with impurities like silica and alumina, forming a liquid byproduct called slag.
As the materials move downward, the metallic iron melts and collects in the hearth of the furnace, with the lighter slag floating on top. The iron is periodically tapped from a taphole at the base of the furnace, while the slag is removed through a separate opening. The molten product is the crude pig iron, ready for the next stage of processing.
Conversion and Primary Uses
The vast majority of pig iron is not intended for use as a final product but is immediately converted into steel. This conversion process is necessary to reduce the high carbon content and remove the remaining impurities that make pig iron brittle. Modern steelmaking methods, such as the Basic Oxygen Steelmaking (BOS) process or the Electric Arc Furnace (EAF), involve blowing oxygen into the molten pig iron. The oxygen reacts with and burns off the excess carbon and other elements, refining the metal into steel.
A smaller portion of pig iron is used to manufacture various types of cast iron. This material, often referred to as foundry pig iron, is remelted and mixed with scrap iron and other alloys to achieve the specific composition required for casting finished products. Pig iron is classified based on its intended application: “basic pig iron” is used for steelmaking, and “foundry pig iron” is used for producing cast iron.