Tungsten
Last updated: July 21, 2009.
By the light, of the silvery moon... We'll be cuddlin' soon, By
the silvery moon. So run the lyrics to a popular song published in 1909.
Back then, electricity was still relatively new-fangled:
Thomas Edison had opened America's first proper power plant, in
Pearl Street, New York City, only 27 years earlier. These days, thanks largely to Edison's promotion of electricity as a super-convenient
power source, things have moved on. Now we're much more likely to
cuddle by the light of the "silvery" tungsten.
Until low-energy
fluorescent lamps started to become popular, most electric lights were
incandescent: they worked by making a tight coil of wire so hot that
it glowed with white light. The wire had to be super-tough to survive
high temperatures so it was made from tungsten, which has the highest
melting point of all metals. Lighting homes is only one of the things
that tough-old tungsten is good for. You'll also find it
strengthening drills and airplane parts. Let's take a closer look at
this intriguing element and find out some more!
Photo: The glowing white line is the filament of an old-style incandescent lamp.
Really, it's just a very tight coil of thin wire. When electricity passes through the filament, its
high resistance
produces so much heat that it gives off light as well.
Tungsten has proved to be the best metal for surviving such tough treatment over
hundreds of hours of use. It's very hard to take photos like this without
risking damage to your camera—if you're trying it yourself, use a
very short exposure.
What is tungsten?
Tungsten is a silvery-white, hard and strong metal that lives in
group 6 (VIb) of the Periodic Table. Like chromium,
copper, and iron,
it's a transition metal (also called a transition element). In continental Europe,
tungsten is often referred to by the alternative name of wolfram.
Several hundred years ago, German smelters decided this was a good
name because tungsten reacted voraciously with tin-rich oxides
"like a wolf." That's why its chemical symbol is W and not T or Tu.
Where does tungsten come from?
Photo: Mineral rich: A space radar photo of the area around the Chinese city of Zhao Qing (light blue area along the banks of the River Xi Jiang, lower left), one of China's most important mineral-producing regions, rich in tungsten, lead,
zinc and gold. This
photo was taken by the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) onboard the
space shuttle Endeavour on April 17, 1994. Courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).
Tungsten is moderately common in Earth's crust: it's the 57th most
common chemical element out of the 92 that are found in nature—so
roughly in the middle of the list. In practice, you'd need to dig up
up about 1.5 tonnes of rock to get about 1 gram (0.04 ounces) of
tungsten, so you can see mining the stuff is pretty hard work! What
makes it harder is that tungsten doesn't occur naturally in its pure state
but only combined with other metals, mostly in two minerals called
scheelite (a compound of calcium, tungsten, and oxygen with the
chemical formula CaWO4) and wolframite
(an intermediate compound that
includes either iron or manganese plus tungsten and oxygen).
Traditionally, tungsten was extracted from its minerals by a multi-stage process:
- The minerals are reacted with sodium carbonate or sodium
hydroxide to produce sodium tungsate.
- The sodium tungstate is washed with water and reacted with
hydrochloric acid to make tungstic acid (H2WO4).
- The tungstic acid is washed and dried to produce tungstic
trioxide (WO3).
- The tungstic trioxide is heated in a furnace with hydrogen.
The hydrogen reduces the oxide (removes its oxygen atoms) leaving
pure tungsten metal, which can then be worked and shaped (pressed,
hammered, or pulled) into bars, wires, or whatever form is needed.
Much of the world's tungsten is currently bought and sold in the form
of a convenient, white powder called ammonium paratungstate, a complex chemical
salt of ammonium and tungsten with the formula
(NH4)10(H2W12O42)·4H2O.
It's easy to transport and easy to convert into all kinds of
tungsten-based materials. In 2006, a tonne of APT powder cost $230-290 in the United States.
Most of the world's tungsten comes from China. Other important producers of tungsten ore include Austria, Australia, Portugal, and South Korea. Between 2004 and 2007, the United States imported roughly 43 percent of its tungsten from
China, 11 percent from Germany, 10 percent from Canada, 7 percent from Bolivia, and 29 percent from the rest of the world.
What is tungsten like?
Physical properties
Pure tungsten metal is grayish or silvery-white, extremely hard
(roughly similar to hardened steel), extremely strong, and brittle.
It's much more impressive at high temperatures: it becomes ductile
(easy to pull into wires), has the highest melting point of any
metal, expands less than any other metal when heated, and has the
greatest tensile strength of any metal at high temperatures. It has
cubic (isometric) with atoms arranged in the body-centered cubic (BCC) pattern
(one atom in the center of the cube and others positioned at the corners).
Chemical properties
Tungsten is reasonably unreactive (inert). It doesn't dissolve in
water or alcohol, for example, and forms relatively few interesting
compounds. Tungsten carbide (tungsten bonded to carbon, chemical
formula WC) is by far the most important tungsten compound. With a
Mohs hardness value of 9.5, it's almost as hard as diamond (and
significantly harder than pure tungsten).
What is tungsten used for?
Photo: You need incredibly high temperatures to melt metals and weld them together, so the welding tool itself needs
to be made of a metal such as tungsten that can withstand even
higher temperatures. Here, a US Air Force engineer is welding two pieces of
aluminum together using gas tungsten arc welding (GTAW) equipment,
also known as a tungsten inert gas (TIG) welder. Photo by Jonathan Snyder courtesy of US Air Force and
Defense Imagery.
Its physical and chemical properties make tungsten useful in two
particular areas: 1) very hard tools and 2) components and materials that can survive high temperatures.
Tungsten carbide is widely used as a coating for cutting tools such as saws, drill bits,
and jackhammers. Tungsten metal is also used in a similar way to make
strong tool steels and tungsten-steel alloys that are important in
airplane and rocket components.
As we've already discovered, tungsten has long been used to make the glowing filaments in incandescent electric lamps. It's also used in old-style cathode-ray tube
televisions (the hot elements that generate the electron beams are
pumped out by a tungsten element).
There are many other, less well-known uses for tungsten
include the manufacture of paints, dyes, pigments, and
phosphors (chemicals that give off visible light
when they're stimulated with energy of some other kind); as a replacement for
lead in fishing shot and bullets; and as an ingredient in lubricants, rust-resistant
coatings, and fire-resistant compounds. Military uses include making radiation shields and heavy-metal alloys
for various kinds of armaments.
Further reading
