Metal Production: Mining, Metallurgy, Alloys, and Iron-Carbon Processes
Metal Production Processes
Mining and Metallurgy
Mining involves extracting ore deposits and separating the metal-rich parts from other materials. Metallurgy is the set of processes used to obtain metals from their ores, separating them from other elements. Metallic industries create useful items from metals.
Historical Significance and Recycling
Iron and its derivatives (steel and cast iron) have significant historical importance. Metallurgy also includes obtaining steel from siderite (iron ore). Metals can also be obtained by recycling used products, a cheaper alternative.
Minerals and Metal Extraction
Metals are found in minerals, chemically combined with other elements:
- Oxides: Metal + oxygen (e.g., Hematite)
- Sulfides: Metal + sulfur (e.g., Galena)
- Carbonates: Metal + oxygen + carbon (e.g., Magnesite)
To separate metals, high temperatures are applied in furnaces. Reduction is a chemical reaction where an element combined with oxygen is isolated. Oxidation is a chemical reaction where a mixture generates oxygen. Coal is used in metal scaling due to its high capacity for combining with oxygen and providing heat energy.
A mineral contains both usable metal-rich parts and unusable, poor-quality metal called gangue. The Reimer process (DSPR extraction and enrichment) separates ore from gangue.
Alloys
An alloy is a product obtained from the union of two or more chemical elements, at least one being a metal, which presents metallic characteristics. Alloys have altered crystal structures, reducing plasticity and increasing hardness, mechanical strength, and melting temperature. They also decrease thermal and electrical conductivity. Examples include: Steel (Fe + C), Brass (Cu + Zn), Bronze (Cu + Sn), Cupronickel (Cu + Ni), Alpacas (Cu + Ni + Zn), Invar (Fe + Ni).
Solidification of Metals and Alloys
Pure metals have a fixed melting temperature. During cooling, the temperature remains constant during solidification. Alloys, however, have a melting temperature range depending on the proportions of each element. Eutectic alloys solidify at a constant, lowest temperature, forming a fine mixture of crystals.
Metallurgical Products
- Ferrous: Irons, Steels (alloys and non-alloys)
- Non-ferrous: Pure metals (copper, aluminum, lead, tin, zinc) and alloys (brass, bronze, aluminum alloys, magnesium alloys, titanium alloys, nickel alloys)
Iron Alloys
Pure iron (Fe) has many industrial applications (melting point: 1539°C, density: 7.87 g/cm³, ductile, malleable, good electrical conductor, magnetizable). Industrially, pure iron refers to iron-carbon alloys with less than 0.03% carbon, used in magnetic cores and electrical plates.
Solidification of Iron: Allotropic Varieties
Iron’s atomic arrangement in solid state creates different crystal lattice structures with varying properties. Allotropic varieties are different crystal structures a metal can have:
- Delta variety: Stable above 1539°C.
- Gamma variety (Austenite): Stable between 1390°C and 900°C, face-centered cubic structure.
- Beta variety: Exists between 900°C and 770°C, paramagnetic.
- Alpha variety (Ferrite): Stable below 770°C, body-centered cubic structure, ferromagnetic.
Iron-Carbon Alloys
In iron-carbon alloys, iron can exist in various allotropic forms, and carbon can exist as pure carbon, iron carbide (cementite), or graphite. Different combinations, cooling rates, and carbon proportions create various constituents:
- Ferrite: Alpha iron
- Cementite: Iron carbide (Fe3C)
- Pearlite: Lamellar structure of ferrite and cementite
- Austenite: Gamma iron
- Martensite: A very hard and brittle iron phase formed by rapid cooling of austenite
Siderurgical Products: Steel and Cast Iron
Steel is an iron-carbon alloy with 0.1% to 1.76% carbon. Cast iron has a carbon content between 1.76% and 6.67% and also contains silicon. Cast iron typically has 3% to 4.5% carbon.
Forging: Shaping metal by placing a hot solid mass between two mold halves and applying compressive force.
Molding: Pouring liquid metal into a closed mold and letting it solidify.
Low-carbon steels are weldable and machinable but not suitable for tempering. Medium and high-carbon steels are suitable for heat treatment and applications requiring high mechanical strength. Cast iron contains carbon as iron carbide (white cast iron) or graphite (gray cast iron), which can be laminar, nodular, or spheroidal.
Steel and Cast Iron Production
Steel production from cast iron involves two phases. First, iron ore is smelted in a furnace to produce pig iron (high carbon content and impurities). Second, pig iron is refined to obtain steel or cast iron.
Obtaining Cast Iron: The Blast Furnace
A blast furnace is a tall, refractory-lined structure where iron ore is reduced to pig iron. Hot air is blown in at the bottom, combusting coke and producing high temperatures. Raw materials are iron ore, coke, and limestone.
- Iron ore: Provides oxidized iron.
- Coke: Provides heat and acts as a reducing agent.
- Limestone: Reacts with impurities to form slag.
The blast furnace has four zones:
- Dehydration Zone: Moisture is removed (~400°C).
- Reduction Zone: Iron oxide is reduced to iron (~700°C).
- Carburization Zone: Iron absorbs carbon (~1200°C).
- Melting Zone: Iron and slag melt (~1800°C).
Pig iron is an alloy of iron and carbon (around 4%), with smaller amounts of silicon, manganese, phosphorus, and sulfur.
Obtaining Steel
Two main methods convert cast iron to steel:
Oxygen Converter
Molten cast iron, scrap, and lime are charged into a cylindrical vessel. Pure oxygen is blown into the mixture, oxidizing carbon and impurities. The heat generated keeps the mass molten. Slag is removed and can be used as fertilizer.
Electric Arc Furnace
Cast iron, scrap, and lime are melted using electric arcs between graphite electrodes. This method allows precise temperature and composition control.
Thermal Treatments
Heat treatments modify steel’s properties by controlled temperature changes. Key temperatures are Ac1 (austenite starts forming, 723°C) and Ac3 (complete transformation to austenite, varies with carbon content). Common treatments are:
- Tempering (Quenching): Rapid cooling from austenite to form martensite, increasing hardness and strength but also brittleness.
- Reheating (Tempering): Heating to below Ac1 followed by air cooling, increasing toughness and reducing brittleness.
- Annealing: Heating to a high temperature and slow cooling, reducing hardness and increasing plasticity. Types include regeneration, globular, softening, and stress relief annealing.
- Normalizing: Heating to austenite temperature and air cooling, refining grain structure and improving mechanical properties.