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Written by Thomas Vulcan  |

December 21, 2009

Indium: No Screen Test Needed

Details

In our latest edition of the minor metals series, we take a look at indium, a crucial component in LCDs.

• The many electronic faces of indium
• Does smuggling to Japan make a difference?
• Which companies give exposure

 

The Nature Of The Metal

 

Although at number 49 in the periodic table indium lies just one notch up from cadmium, it is not a member of any Toxic Trio.

 

FIG 1

 

As with a number of the strategic metals, though, indium was not only quite a late addition to the table, it was also not used for much until the early part of this last century.

The metal was only "discovered" in 1863 by the duo of German chemists Ferdinand Reich and Hieronymous Richter. It is appropriately named after the indigo blue line that appears when the element is subjected to spectroscopic analysis.

As for its physical characteristics, the metal is a shiny silver color and very soft. Like some of the other strategic metals such as gallium, indium is solid at room temperature but has a relatively low melting point: 313.88 °F (156.6 °C). (Indeed, when indium is alloyed with gallium in the proportions indium 24 percent and gallium 76 percent, the resulting eutectic alloy is liquid at room temperature—quite useful in situations when you don't want to use a toxic metal like mercury.)

Although it had already been used to coat mirrors (equal in quality to those made with a coating of silver), indium really only came into its element during World War II, when it was used to coat bearings in fighter aircraft. (When a bearing is coated with indium, any lubricating oil is consequently distributed evenly over its surface.)

But in the last 20-30 years, commercial use of indium has taken off, particularly with the development of flat-panel displays, semiconductors and photovoltaic cells. Production of the metal, too, has increased over this period: In 1989, the U.S. Geological Survey (USGS) estimated indium refinery production was 115 tonnes; in 2008, that figure had risen to 568 tonnes.

 

Whence The Indium

With an abundance in the Earth's continental crust of 0.05 parts per million (ppm) and its oceanic crust of 0.072 ppm, indium is somewhat more abundant than its lookalike, silver. However, since it does not occur in the same concentrations as silver, indium is never mined in its own right. Instead, like many other strategic and minor metals, indium is mined as a by-product of the production of other ore.

Commercially, "virgin" indium is extracted primarily as a by-product of zinc and tin mining. However, it can be extracted also as a by-product from the production of other metals, including lead, copper and, less extensively, bismuth, cadmium and silver.

Even when it occurs most liberally, indium is an impurity whose concentration can be measured only in ppm. But the technology used to extract the metal has improved so much that it can now be produced from "concentrations as little as 100 ppm of indium per ton material."

However, extraction rates remain small: Historically, less than 20 percent of the indium content in concentrates could be extracted, and even with modern technological improvements, producers still can only extract 30 percent of the "1,500 mt of indium mined every year worldwide," says the Indium Corporation of Utica, N.Y.