We continue to review some of the most important materials in heat treatment and metallurgy.
Lanthanum (chemical symbol: La)
Lanthanum (Fig. 1) is a silvery-white, malleable, ductile metal that is so soft it can be sliced with a knife. Exposed surfaces of the metal will quickly oxidize in air and will react with water to form both lanthanum hydroxide (La(OH)3) and hydrogen gas (H2).
Lanthanum is considered moderately toxic and is chemically reactive, making it willing to readily form compounds with carbon, nitrogen, boron, selenium, silicon, phosphorous, sulfur and the halogen elements. Lanthanum is the first element in the lanthanide series of the periodic table – a group of 15 metallic elements – commonly referred to as the rare-earth elements. Lanthanum is the 28th most-abundant element within the Earth’s crust and only about 0.0018%, making it the 3rd most-abundant element of the lanthanide elements.
Lanthanum was first discovered by Swedish chemist Carl G. Mosander when he was able to extract it from cerite, a cerium-rich mineral, in 1839. He added diluted nitric acid to powdered cerium nitrate (Ce(NO3)3) and found a new oxide he called lanthana (La2O3), which is derived from the Greek word “lanthanein” meaning to lie hidden (Fig. 2).
Pure lanthanum wasn’t produced until the early 20th century through the process of electrolysis. As he continued his research into lanthanum, Mosander would later discover another rare-earth metal named didymium, which was later known to be a mixture of two undiscovered elements of neodymium and praseodymium.
Lanthanum sees a wide range of uses in a variety of different industries. For instance, it has prominent use in nickel-metal-hydride rechargeable batteries for hybrid vehicles, in which a small car can require about 10 kg of the element. It can be found within homes in electronics such as televisions, as well as fluorescent and energy-saving lamps.
Lanthanum sees use in the petroleum industry as a cracking catalyst where it splits long-chain hydrocarbons into shorter chains. It is a major component (25%) of mischmetal (Fig. 3), an alloy commonly used for flints in cigarette lighters. When lanthanum is used as an additive to steel, it improves both its malleability and resistance. Lanthanum oxide is a common addition to special optical glasses, such as infrared-absorbing glass, camera and telescope lenses where it gives increased density, refractive index and hardness to the lenses.
Here are a few important facts about lanthanum.
- Atomic number: 57
- Atomic weight: 138.90547
- Melting point: 1191 K (918°C or 1684°F)
- Boiling point: 3737 K (3464°C or 6267°F)
- Density: 6.15 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 6
- Group number: none
- Group name: Lanthanide
- Electron configuration: [Xe] 5d16s2