Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

In minerals, the most common substitution mechanism involving scandium is the replacement of aluminum and trivalent iron. This is the main reason for the dispersal of scandium in the lithosphere and is explained by the valency of scandium (Sc³⁺) and similarities in ionic radii. Scandium can also substitute for yttrium and the heavy lanthanides (rare-earth elements), despite a larger difference in ionic radii. Incorporation of scandium in minerals containing magnesium, manganese, and divalent iron--or niobium, tantalum, tin, and zirconium--is explained by various coupled (heterovalent) substitutions; for example, Sc³⁺ + (Ti,Sn)⁴⁺ (Fe,Mn)²⁺ + (Nb,Ta)⁵⁺, Sc³⁺ + Na⁺ (Mg,Fe)²⁺ + Ca²⁺, Sc³⁺ + Ca²⁺ Sn⁴⁺ + Na⁺, and Sc³⁺ + P⁵⁺ Zr⁴⁺ + Si⁴⁺. No wonder scandium is a dispersed element! In common rocks, it is mainly present in ferromagnesian minerals like pyroxenes, amphiboles, micas, garnets, and epidote-group minerals.

Owing to its dispersed nature and resulting low concentration, scandium is produced exclusively as a by-product during processing of various ores and has also been recovered from mine tailings. The principal scandium-producing countries are China, Russia, Ukraine, and Kazakhstan. Future resources are known in the U.S., Australia, and Norway. Undiscovered scandium resources are probably very large.

Investigations of the use of scandium in aluminum alloys began in the Soviet Union in the 1970s, although such alloys were patented in the U.S. in 1971. During the 1980s, Russian scientists implemented scandium in several alloy systems. Some of the advantages of using scandium in aluminum alloys are to achieve grain refinement during casting and welding, increased strength from Al₃Sc precipitates, increased resistance to recrystallization, and enhanced superplastic properties. It has been claimed that scandium provides the highest increment of strengthening per atomic percent of any of the other aluminum-alloying elements.

Principal uses for scandium are in high-strength aluminum alloys, high-intensity metal halide lamps, electronics, and laser research. A recently developed application is in welding wire, and future demand is expected to be in fuel cells. Approximately 15 different commercial Al-Sc alloys have been developed in Russia, and some of them are used for aerospace applications. In Europe and the U.S., scandium-containing alloys have been evaluated for use in structural parts in airplanes. The combination of high strength and light weight makes Al-Sc alloys suitable for a number of applications.

But the high price of scandium has so far restricted widespread use of such alloys in the Western world. Uses have been primarily in sports equipment like baseball and softball bats, bicycle frames, lacrosse sticks, and even in handgun frames and cylinders! With the current prices, an addition of typically 0.2 weight % Sc to an aluminum alloy will quadruple the price. However, should the price fall by 90% or more, scandium will be in immediate demand for large-scale production of several alloy types. Of course, such a development is dependent on stable, long-term supplies. I am convinced that scandium's heyday is ahead of us.



Gunnar Raade is a senior curator of minerals at the Geological Museum, University of Oslo, Norway. He has been involved in the description of 13 new mineral species, among them one new scandium mineral (kristiansenite).

SCANDIUM AT A GLANCE

Name: From the Latin Scandia, Scandinavia.

Atomic mass: 44.96.

History: Discovered in 1879 by Lars F. Nilson at Uppsala, Sweden, in the mineral euxenite, which had not yet been found outside Scandinavia.

Occurrence: Not found as a free metal. It occurs in minute quantities in more than 800 mineral species. Today, scandium is usually obtained as a by-product of refining uranium.

Appearance: Silvery white, soft metal.

Behavior: Scandium metal tarnishes in air and burns readily to form scandium oxide. When finely divided or heated, the metal dissolves in water. Scandium is very reactive toward the halogens, forming trihalides. It is mildly toxic by ingestion, and scandium salts are suspected carcinogens.

Uses: Used in mercury vapor lights for high-intensity lighting that approximates sunlight. Its alloys are used in athletic equipment. When added to aluminum alloys, scandium can significantly increase strength and reduce grain size.

YTTRIUM

PAUL C. W. CHU, UNIVERSITY OF HOUSTON AND HONG KONG UNIVERSITY OF SCIENCE & TECHNOLOGY

Life would cease to exist without the existence of some elements in the periodic table. However, life would continue even if yttrium did not exist. While yttrium is not known to exhibit any significant biological function, its compounds have helped provide some modern-day comforts, such as color television and wireless communication. On a personal level, yttrium is the one element that has left a long-lasting imprint on me.

When I was a young boy in Taiwan, I dreamed of building a machine that would move forever once started. I had been fascinated by science fiction writings based on such a machine. I still remember the countless nights in pursuit of such a dream. I connected in parallel the two identical motors that I built and prayed that once one was rotated as the generator, it would produce enough power to keep the other rotating without stopping. Clearly, I failed.

Only later did I learn that one cannot fool Mother Nature. By virtue of the second law of thermodynamics, a perfect machine cannot exist. However, as I grew older, I also learned that perpetual motion is not impossible in the quantum domain, that is, in the microscopic atomic scale. For example, electrons move without friction inside an atom and will never stop, similar to a perpetual machine, although at a microscopic dimension.

LEVITATION A magnet disk rotates in mid-air above a YBCO disc that is cooled by liquid nitrogen.