Metals and Their Properties

Metals

• Metals are substances characterized by high electrical and thermal conductivity, malleability, ductility, and reflectivity.
• Common metals in Earth's crust include aluminum, iron, calcium, sodium, potassium, and magnesium.
• Most metals are found in ores, but some, like copper, gold, platinum, and silver, are found in their free state because they do not readily react with other elements.

Physical Properties of Metals

• Good conductors of heat and electricity.
• Ductile (can be pulled into wire).
• Malleable (can be pounded into sheets).
• Sonorous (produce a deep sound when struck).
• Solid at room temperature (except mercury).
• Usually shiny with metallic luster.
• High melting points.
• High density (exceptions: lithium, potassium, and sodium).
• Corrode in air or seawater.
• Lose electrons in reactions.

Chemical Properties of Metals

• React with water: Highly reactive metals react vigorously with water, producing hydrogen gas and significant heat.
• React with acids: Metals react with acids, producing hydrogen gas and a salt. Zinc reacting with hydrochloric acid is an example.
• React with bases: Some metals react with bases, producing hydrogen gas and a metal salt. Zinc reacting with sodium hydroxide is an example.
• React with oxygen: Metals react with oxygen when burned, forming metal oxides. Magnesium reacting with oxygen forms magnesium oxide.

Extractive Metallurgy

• Mineral processing: The initial step in separating valuable minerals from the waste rock (gangue) in crude ores.
• Extractive metallurgy: This process involves extracting the metal from the chemical compound form and refining it, removing impurities.
• Smelting: A process in which all constituents are melted and separated into two liquid layers (valuable metals and waste).
• Refining: A process that follows extraction, to further reduce the level of impurities using pyrometallurgical, electrolytic, or chemical methods. Pyrometallurgical refining typically involves oxidizing impurities in a high-temperature liquid bath.

Ferrous Metals

• Ferrous metals include iron and steel, which are vital in construction.
• Chemical composition and structure of ferrous metals are closely controlled during manufacturing.
• Strength and other mechanical properties are highly reliable.
• People involved in construction have limited control over the quality of iron/steel after leaving the fabrication shop.

Historical Background

• Cast iron was the first metal used for structural purposes (e.g., the 30m arch bridge built in England between 1777 and 1779).
• Cast iron bridges were common from 1780 to 1820.
• Wrought iron started replacing cast iron after 1840, with the Brittania Bridge (Wales) being an example.
• Currently (late 1980s), steels are prevalently used in construction, with yield stresses ranging between 165 and 690 MPa.

Manufacture of Pig Iron

• Pig iron is a low-grade iron produced in blast furnaces.
• Blast furnaces are tall and wide, continuously loading iron ore, coke, and limestone at the top.
• Iron ore is an oxide of iron found in nature and associated with rocks.
• Coke is produced by heating coal to remove impurities.
• The heat generated from burning coke melts iron ore and limestone to separate iron from impurities that become slag.
• The molten pig iron is significantly heavier than slag and collects at the bottom of blast furnaces.

Manufacture of Cast Iron

• Cast iron is produced by re-melting pig iron with coke and limestone. A furnace (Cupola furnace) is used for this process. It's similar to a blast furnace, but smaller.
• Scrap iron/steel, silicon, and manganese are added to control the chemical composition.
• The molten metal is poured into molds, forming various shapes (casting).

White Cast Iron

• White cast iron is made by rapidly cooling molten pig iron, combining carbon and iron. The fractured surface is bright white.
• It has high strength and is resistant to abrasion.
• However, it is very brittle and not easy to machine.
• Used in parts like crushers and grinders.

Gray Cast Iron

• Gray cast iron is made by slowly cooling molten pig iron, resulting in graphite flakes.
• It is highly machinable and resistant to abrasion.
• It has a lower ductility and toughness compared to other types.
• Frequently used in intricate castings.

Ductile Cast Iron

• Ductile (also known as nodular or spheroidal graphite) iron is similar to gray iron but has graphite particles that are spherical.
• This significantly improves the ductility and shock resistance compared to gray iron. However, it is more expensive.

Malleable Cast Iron

• Malleable cast iron is annealed white cast iron. Annealing transforms the brittle structure to a malleable form. Its composition is similar to white cast iron, but with slightly more carbon and silicon.
• Malleable iron is ductile, tough, resistant to corrosion, and machinable.
• Used for small castings that need good tensile strength.

Manufacture of Wrought Iron

• Wrought iron is a highly refined iron with slag purposely incorporated but not chemically combined with iron.
• Slag fibers are uniformly distributed throughout the metal, affecting its final properties.
• The puddling process, invented by Henry Cort, was a major development. It involved heating and stirring pig iron with oxidizing substances in a furnace.

Cast Iron vs. Wrought Iron

• Cast iron is made through casting, while wrought iron is made via repeated heating and working.
• Wrought iron is stronger than cast iron, and can resist deformation under pressure and stress.
• Cast iron is easier and cheaper to produce.

Basic Oxygen Process (BOP)

• The Basic Oxygen Process is a steelmaking process that uses an oxygen lance to oxidize impurities in the heated molten iron.
• Impurities are removed by the slag during the process, helping refine the molten steel. Pig iron acts as fuel during this process.