Extractive Metallurgy for the Heat Treater
Extractive metallurgy is an area of our science that deals with the process and methods of gathering and isolating metals from their ores (i.e., natural mineral deposits) and in some instances refining them. Most heat treaters don’t spend too much time thinking about this subject in their daily lives but nevertheless should know a little about it. Let’s learn more.
Where are the elements found on Earth?[1,2]
For the purposes of mineral extraction, Earth can be thought of as being divided into three parts: its continental crust, oceans and atmosphere. The crust (or outermost shell of the planet) is an extremely thin layer formed when molten rock from below the surface reacts with water and oxygen on the surface, forming minerals (Fig. 1, online). In relative terms, its thickness is akin to that of the skin of an apple (it is less than half of 1% of the Earth’s total mass). In some locations, the crust is around 80 km (50 miles) thick; in others, less than 1 km (0.6 miles).
Oxygen is the most abundant element found on the Earth’s surface, composing almost half of the mass of the crust. Silicon is the second-most-abundant element. Feldspar, an aluminum-rich alloy, is the most abundant mineral present on Earth behind quartz, which composes approximately 60% of Earth’s crust. Other elements are also present in more-or-less-significant amounts (Table 1). There are also small or trace quantities of other elements, such as chromium, nickel, gold and the rare-earth elements.
The world’s oceans are a combination of water, plant and animal life. Water, which makes up about 70% of the Earth’s surface, is composed principally of hydrogen, oxygen, sodium, chlorine and magnesium. Sediment and other materials contribute other elements such as silicon and iron.
By comparison, Earth’s atmosphere (which is made up of five principal layers) consists mainly of oxygen, nitrogen and argon.
Iron ore, principally in the form of hematite (Fe2O3) and magnetite (Fe3O4), is removed from the earth through mining. After extraction, crushing, washing and calcining (which removes carbon dioxide along with other impurities), a blast furnace is used to separate or refine iron from the other elements in the ore.
The blast furnace reduces iron oxide, but the complete reduction reaction requires the addition of coke and limestone to the roasted ore. After the completion of a series of processes in the blast furnace, molten iron is produced. A high-purity iron, such as wrought iron, requires the complete removal of carbon from the melt. Likewise, the steelmaking process requires the removal of certain impurities (e.g., sulfur and/or phosphorous) and the addition of selected elements (e.g., manganese and chromium).
Steelmaking (Fig. 2) is the process of producing steel from raw material, which is typically a combination of iron ore and steel scrap, depending on the primary route chosen. Impurities such as nitrogen, silicon, phosphorous, sulfur and excess carbon are removed, and alloying elements (e.g., carbon, manganese, nickel, chromium, vanadium) are added.
In very general terms, modern steelmaking processes can be divided into two or three major steps, again depending on the processing route chosen. The steelmaking process either employs a blast furnace plus a basic oxygen furnace (BOF) in combination with secondary processing or involves the use of an electric-arc furnace (EAF) plus secondary steelmaking operations. These two methods account for virtually all steel production and are used to lower the carbon content generally to a range of 0-1.5% while adjusting alloy content to a desired composition.
- BOF is a process in which carbon-rich molten (pig) iron is converted into steel by passing oxygen through the bath followed by the addition of alloying elements. The process is called basic due to the chemical nature of the refractories that line the vessel. Slag, the result of additives such as lime and fluxing agents combined with dross (solids) that floats on top, is also controlled to ensure that impurities such as silicon and phosphorus are removed.
- EAF methods feed recycled steel scrap into the furnace, which is then heated through the use of high-power electrodes to melt steel scrap and convert it into liquid steel of a specified chemical composition.
Both EAF and BOF steelmaking processes require the addition of fluxes such as lime and dolomitic lime to control the composition of the slag that forms, which serves as a trap for impurities and provides insulation for the steel bath. Additionally, both processing routes typically pour the liquid steel into a ladle (tapping) for subsequent alloying, refining and reheating prior to casting.
Secondary steelmaking involves refining of the crude steel before casting, and the various operations are normally carried out in the ladle. In secondary metallurgy, alloying agents are added (ladle injection); the metal is stirred (typically with argon bubbling or in some cases electromagnetic stirring); dissolved gases in the steel are lowered (degassing); and inclusions are removed (via such techniques as composition adjustment by sealed argon bubbling) or altered chemically to ensure the quality of the steel produced after casting.[6,7]
Secondary steelmaking is most commonly performed in the ladle and involves deoxidation (or “killing” of the steel) using aluminum or silicon, vacuum degassing, alloy addition, inclusion removal, inclusion chemistry (inclusion control or elimination, modification, desulfurization) and homogenization. It is now common to perform ladle metallurgical operations in gas-stirred ladles with electric-arc heating in the lid of the furnace. Tight control of ladle metallurgy is associated with producing high grades of steel in which the tolerances in chemistry and consistency are extremely tight.[6,7]
A Look at Selected Alloying Additions
Some of the more common additions to basic steel, including the form and purpose, are briefly summarized.
- Carbon: Carbon additions are typically in the form of high-carbon ferroalloys or metallurgical coke. Carbon recarburizers (e.g., anthracite, coke, graphite) are either placed in the ladle before the steel introduction or added very early in the tap if enough turbulence is present. Subsequent trimming additions in the ladle are accomplished using powder-core wire feeders.
- Manganese: Manganese additions (e.g., ferromanganese, silicomanganese, manganese metal) are most often chosen on the basis of the carbon content present. For example, when the steel is well oxidized (i.e., low in carbon), ferromanganese is commonly used. The efficiency of the manganese addition, regardless of how or in what form it is added, is dependent on the steel’s oxygen and sulfur content. Thus, BOF or EAF processes often dictate the type and size.
- Silicon: The strong deoxidizing power of silicon influences the way in which it is used, the condition of the steel, the residual silicon (aim) level, the practice involved and mill practice. Ferrosilicon is the most common addition. It is available in a wide variety of grades (15-95% Si), often with specific aluminum, calcium and boron levels. Steels requiring additions of rare-earth elements (titanium and zirconium, for example) are added for such benefits as inclusion shape control and nitrogen scavenging and benefit from prior ferrosilicon deoxidizing.
- Sulfur: Resulfurization (using iron sulfide, pure sulfur or another sulfide compound) typically occurs in the ladle. If added to the furnace, the suppression of carbon monoxide can be beneficial because steels containing manganese sulfides will require less aluminum for deoxidation.
- Phosphorous: Ferrophosphorus is added to the ladle after the steel has been thoroughly deoxidized to further reduce the concentration of residual silicon-type inclusions present in the final product because silicate inclusions are detrimental to machining.
- Other Elements: Many other elements (e.g., Al, As, Bi, B, Ca, Ce, Cr, Co, Hf, Nb, Cu, H, Pb, Mg, Mo, Ni, N, O, Se, S, Sn, Te, Ti, V, W, Y, Zr) are either present or can be added to steel. Their effects, along with more information on the elements discussed, are well documented in literature (see Reference 8, for example).
Extractive metallurgy and steelmaking practices are fascinating subjects and arguably deserve more of the heat treater’s attention. What is covered here is designed to whet one’s appetite to study these topics in greater depth.
- “All About the Earth’s Crust” (www.thoughtco.com)
- “Elements Found on the Earth’s Surface and How is Iron Extracted from the Earth” (www.reference.com/science)
- Element Abundance (www.slideplayer.com)
- “Where on Earth are the Metals Located” (www.sciencearchive.org.au and www.911metallurgist.com)
- “The Modern Steel Manufacturing Process” (www.thebalance.com)
- Deo Brahma and Rob Boom, Fundamentals of Steelmaking Metallurgy, Prentice-Hall, 1993
- Ghosh, Ahindra, Secondary Steelmaking: Principles and Applications, CRC Press, 2000
- Deeley, P.D., K.J.A. Kundig and H.R. Spendelow, Jr., Ferroalloys & Alloying Additives Handbook, Shieldalloy Corporation, 1981
- The Making, Shaping and Treating of Steel, 9th Edition, Harold E. McGannon (Ed.), United States Steel, 1971
- Mr. Scott Lemoine, Special Process Manager, Rexnord Aerospace, technical and editorial contributions