Figure 1 Drawing of an ironmaking blast furnace with hot-blast stove. Source: The German Steel Federation (WV Stahl)In order to make steel, iron must be extracted or won from iron ore. Blast furnacing, smelting and direct iron reduction are the current ironmaking processes.

Ironmaking Blast Furnace

Iron is made by reacting iron ore (iron oxide and impurities), coke (a reductant) and limestone (CaCO₃) in a blast furnace.

Iron ores with lower iron content such as taconite are first processed to concentrate the iron level and drive off volatile impurities. The iron ore is dressed or crushed into 0.5-1 in. chunks, which increases surface area for reactions. Magnetic separation is used to remove some of the undesirable minerals or gangue in the crushed ore. Roasting or calcination of the ore oxidizes some of the sulfur, phosphorus and arsenic impurities. Sulfur oxide is volatile and evaporates off or is washed out. Iron is mainly extracted from hematite (Fe₂O₃) and magnetite ores. Natural or direct shipping iron ores contain between 50-70% iron and can be fed directly into the blast furnace. Fe₃O₄ decomposes when heated to ferrous oxide (FeO) and ferric oxide (Fe₂O₃) via Fe₃O₄ → FeO + Fe₂O₃.

A specialized type of coal, called hard coal, is used to make coke, a porous form of carbon. Coke is a reductant or a chemical that can reduce iron oxide to iron metal – especially when combined with heat and oxygen. Hard coal has high-carbon content and low ash, sulfur and phosphorus levels. Coke is made in a coking oven by heating hard coal in the absence of air or oxygen.

The iron ore can be ground further to a fine powder mixed with a fluxing agent (limestone), fine pulverized coke (coke breeze) and a binder, which is formed into granules or pellets and sometimes cooked or sintered to form sinter.

Figure 2 – Schematic of blast furnace with reactions and temperature ranges (Source: Eurotherm).The carbon in the coke reacts with the oxygen to produce a reducing gas, carbon monoxide (CO) according to the following reaction:
2 C(s) + O₂(g) → 2 CO + Heat

In the upper region of the blast furnace where temperature range from 600 to 700 C. the iron ore or iron oxide is reacts with the gaseous CO reductant to produce iron:

Fe₂O₃(s) + 3CO(g) → 2Fe + 3CO₂(g)

Lower in the blast furnace where higher temperatures occur, the iron ore may react directly with the coke or carbon:

2Fe₂O₃(s) + 3C(s) → 4Fe(s) + 3CO₂(g)

The limestone flux decomposes to lime during heating or:

CaCO₃(s) → CaO(s) + CO₂(g)

Lime removes more of the sulfur and silicon, which was not removed during mineral processing. The lime reacts with the excess silicates or iron sulfides, which is then become part of the molten slag:

CaO(S) + SiO₂(S) → Ca₂SiO₄(L)

CaO(S) + FeS(S) → CaS(S) + FeO(S)

CaO(S) + Al₂O₃(S) →→ Ca(AlO₂)₂(L)

Ellington diagrams (Figure 3) can help metallurgical engineers understand whether one metal or element will act as a reducing agent for another. When two lines for oxidation reactions are compared on the Ellingham diagram, the reaction with the lower line has a lower negative free energy and will proceed according to thermodynamic laws. The Ellingham diagram shows that below 710 ° C CO is a better reducing agent for Fe₂O₃, since the free energy of formation of Fe₂O₃ is higher than the formation of CO₂ from CO. Above 710° C, thermodynamics indicates that coke or carbon is a better reducing agent for Fe₂O₃. The Ellingham or Elingham-Richardson diagrams can be used to determine the partial pressures or ratio of carbon monoxide to carbon dioxide required to reduce a metal oxide.

Figure 3 – The Ellingham diagram for various oxides describes the metallurgical thermodynamics behind iron ore reduction. (Source: MIT)The combustion of the carbon or coke produces a great amount of heat, which is thermodynamically needed for the reduction of iron oxide and to melt the iron and slag. The liquid iron and molten slag flows to the bottom of the blast furnace while the slag floats on top of the molten iron and is drawn off separately. The molten iron is tapped off the very bottom of the blast furnace and cast into hot metal or pig iron ingots. The slag byproduct or waste consists of mainly of calcium silicate with varying amounts of aluminum oxide, magnesium oxide, iron oxide and sulfur. The byproduct slag has a variety of end-uses such as slag cement, soil stabilizer, lightweight concrete aggregate, road construction material and abrasive blast media (see Nippon Slag Association, National Slag Association).

Many technologies are leveraged in a blast furnace to enhance iron making yield, throughput and hot metal quality such as modern tuyere and oxygen injection designs, refractories with improved iron and slag resistance, improved burden feeders, gas and energy recovery and advanced process control systems. Emerging technologies are being implemented to reduce or eliminate byproduct slag waste, reduce energy consumption and reduce greenhouse gas release. According to the Iron and Steel Institute, the U.S. steel industry has reduced greenhouse gas (GHG) emissions intensity by 37% and energy intensity (energy/unit of production) by 32% since 1990.

Conclusion

Iron and steel alloys continue to reign as the key engineering materials in automotive, construction, marine and many other industries due to its strength, toughness, versatility and low cost. While the blast furnace remains a tool for ironmaking today, newer smelting and direct iron reduction technologies continues to expand in adoption and emerging reduction processes under development promise to provide additional breakthroughs in productivity, energy efficiency, reduced CO2 and greenhouse gas (GHG) emissions and lower life cycle costs.