Hafnium
| Topics: |
Geology (main)
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Contents
Background
Hafnium is a bright silver, ductile, lustrous metallic element with a very high melting point. Its atomic number is 72 and its symbol is Hf. Hafnium is the 45th most abundant element in the Earth’s crust with an average crustal abundance of 3 parts per million (ppm). The element was discovered by Dirk Coster and George Charles von Hevesey by separating it from zirconium in 1923.
Hafnium does not react with air, water, acids or bases. It is similar to the element cadmium in that it absorbs neutrons. This feature makes hafnium useful as a control rod material in nuclear reactors.
There is no biological use or benefit for hafnium. It is present in ocean water in very small amounts, specifically 0.008 parts per billion (ppb) by weight. For comparison, hafnium is far more concentrated in the Earth’s crust at 3,300 ppb by weight.
Name
| Previous Element: Lutetium Next Element: Tantalum |
| |
| Physical Properties | ||
|---|---|---|
| Color | silvery | |
| Phase at Room Temp. | solid | |
| Density (g/cm3) | 13.28 | |
| Hardness (Mohs) | --- | |
|
Melting Point (K) |
2495.2 | |
|
Boiling Point (K) |
4723 | |
| Heat of Fusion (kJ/mol) | 25.1 | |
| Heat of Vaporization (kJ/mol) | 571 | |
| Heat of Atomization (kJ/mol) | 619 | |
| Thermal Conductivity (J/m sec K) | 23.2 | |
| Electrical Conductivity (1/mohm cm) | 28.49 | |
| Source | Zircon (silicate) | |
| Atomic Properties | ||
| Electron Configuration | [Xe]6s24f145d2 | |
|
Number of Isotopes |
44 (6 natural) | |
| Electron Affinity (kJ/mol) | 0 | |
| First Ionization Energy (kJ/mol) | 680 | |
| Second Ionization Energy (kJ/mol) | 1440 | |
| Third Ionization Energy (kJ/mol) | 2250 | |
| Electronegativity | 1.3 | |
| Polarizability (Å3) | 16.2 | |
| Atomic Weight | 178.49 | |
| Atomic Volume (cm3/mol) | 13.4 | |
| Ionic Radius2- (pm) | --- | |
| Ionic Radius1- (pm) | --- | |
| Atomic Radius (pm) | 159 | |
| Ionic Radius1+ (pm) | --- | |
| Ionic Radius2+ (pm) | --- | |
| Ionic Radius3+ (pm) | --- | |
| Common Oxidation Numbers | +4 | |
| Other Oxid. Numbers | +2, +3 | |
| Abundance | ||
| In Earth's Crust (mg/kg) | 3.0x100 | |
| In Earth's Ocean (mg/L) | 7.0x10-6 | |
| In Human Body (%) | 0% | |
| Regulatory / Health | ||
| CAS Number | 7440-58-6 | |
| OSHA Permissible Exposure Limit (PEL) | TWA: 0.5 mg/m3 | |
| OSHA PEL Vacated 1989 | TWA: 0.5 mg/m3 | |
|
NIOSH Recommended Exposure Limit (REL) |
TWA: 0.5 mg/m3 IDLH: 50 mg/m3 | |
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Sources: |
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The name hafnium was given by its Danish discoverers, Coster and von Hevesey, and was created from the Latin name Hafnia which means Copenhagen, in honor of the capital city of Denmark.
Sources
Hafnium is retrieved as a by-product from zirconium ore minerals. In a typical zirconium ore, there is a Zr:Hf ratio of about 50:1. The mineral zircon is the primary ore source of hafnium. Most zircon (and, therefore, hafnium) is mined from titanium-rich, heavy-mineral sand deposits. Hafnium and zirconium are both used in nuclear reactors. In this application, each must be pure and free from the other. The manufacture of nuclear-grade zirconium therefore produces hafnium as a by-product and, the manufacture of nuclear-grade hafnium produces zirconium as a by-product. This processing actually produces more hafnium than is consumed. Unused hafnium is stored as hafnium oxide or hafnium metal. Geologists estimate the hafnium resources in the United States total 130,000 tons. (By comparison, zirconium resources are about 14 million tons.) World resources of hafnium are estimated at over 1 million tons. Hafnium is imported to the United States in a variety of forms, including hafnium oxide and scrap metals containing hafnium. The majority of the hafnium imported comes from France. Other world producers of hafnium-bearing minerals include Germany, the United Kingdom, Brazil, China, India, Russia, South Africa, Ukraine, and the United States.
Uses
The most significant use of hafnium is in the production of special alloys known as superalloys. Superalloys are alloys (mixtures) of metals that are designed to withstand high-stress situations, such as very high temperatures and pressures. Such metals can include iron, nickel, chromium, titanium, niobium, hafnium and other metals. Because of its ability to absorb neutrons, it is used to control nuclear reactions in fission reactors, including the nuclear reactors that power nuclear submarines. Hafnium is also used as a “scavenger” metal in the retrieval of oxygen and nitrogen. A scavenger metal is one which aids in the collection of gases without reacting with them to form other compounds.
Substitutes and Alternative Sources
Silver-cadmium-indium alloys can be used in place of hafnium as control rods in nuclear reactors. In the production of superalloys, zirconium can often be used in place of hafnium. In some applications, only hafnium gives the desired qualities and so no substitute is possible. However, the abundance of hafnium in storage (and the fact that its production outpaces its consumption) means there is no immediate danger of running short of this rare element.
Further Reading
- Common Minerals and Their Uses, Mineral Information Institute.
- More than 170 Mineral Photographs, Mineral Information Institute.
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