Yttrium

Yttrium

Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is never found in nature as a free element. Its only stable isotope, 89Y, is also its only naturally occurring isotope.

In 1787, Carl Axel Arrhenius found a new mineral near Ytterby in Sweden and named it ytterbite, after the village. Johan Gadolin discovered yttrium's oxide in Arrhenius' sample in 1789, and Anders Gustaf Ekeberg named the new oxide yttria. Elemental yttrium was first isolated in 1828 by Friedrich Wöhler.

The most important use of yttrium is in making phosphors, such as the red ones used in television set cathode ray tube (CRT) displays and in LEDs. Other uses include the production of electrodes, electrolytes, electronic filters, lasers and superconductors; various medical applications; and as traces in various materials to enhance their properties. Yttrium has no known biological role, and exposure to yttrium compounds can cause lung disease in humans.

Occurrence


Abundance

Yttrium is found in most rare earth minerals, as well as some uranium ores, but is never found in nature as a free element. About 31 ppm of the Earth's crust is yttrium, making it the 28th most abundant element there, and 400 times more common than silver. Yttrium is found in soil in concentrations between 10 and 150 ppm (dry weight average of 23 ppm) and in sea water at 9 ppt. Lunar rock samples collected during the American Apollo Project have a relatively high content of yttrium.

Yttrium has no known biological role, though it is found in most, if not all, organisms and tends to concentrate in the liver, kidney, spleen, lungs, and bones of humans. There is normally as little as 0.5 milligrams found within the entire human body; human breast milk contains 4 ppm. Yttrium can be found in edible plants in concentrations between 20 ppm and 100 ppm (fresh weight), with cabbage having the largest amount.With up to 700 ppm, the seeds of woody plants have the highest known concentrations.

Production

The chemical similarity of yttrium with the lanthanides leads it to being enriched by the same processes and ends up in ores containing lanthanides, forming rare earth minerals. A slight separation is recognized between the light (LREE) and the heavy rare earth elements (HREE) but this separation is never complete. Yttrium is concentrated in the HREE group by virtue of its ionic size even though it has a lower atomic mass.


There are four main sources for REEs:

• Carbonate and fluoride containing ores such as the LREE bastnäsite ([(Ce, La, etc.)(CO3)F]) contain an average of 0.1% of yttrium compared to the 99.9% for the 16 other REEs. The main source for bastnäsite from the 1960s to the 1990s was the Mountain Pass rare earth mine in California, making the United States the largest producer of REEs during that period.
• Monazite ([(Ce, La, etc.)PO4]), which is mostly phosphate, is a placer deposit of sand that is created by the transportation and gravitational separation of eroded granite. Monazite as a LREE ore contains 2% (or 3%) of yttrium. The largest deposits were found in India and Brazil in the early 20th century, making these two countries the largest producers of yttrium in the first half of that century.
• Xenotime, a REE phosphate, is the main HREE ore containing up to 60% of yttrium as yttrium phosphate (YPO4). The largest mine for this mineral is the Bayan Obo deposit in China, making China the largest exporter for HREE since the closure of the Mountain Pass mine in the 1990s.
• Ion absorption clays or Lognan clays are the weathering products of granite and contain only 1% of REEs. The final ore concentrate can contain up to 8% of yttrium. Ion absorption clays are mostly mined in southern China. Yttrium is also found in samarskite and fergusonite.

One method to obtain pure yttrium from the mixed oxide ores is to dissolve the oxide in sulfuric acid and fractionate it by ion exchange chromatography. With the addition of oxalic acid, the yttrium oxalate precipitates. The oxalate is converted into the oxide by heating under oxygen. By reacting the resulting yttrium oxide with hydrogen fluoride, yttrium fluoride is obtained. Using quaternary ammonium salts as extractants, yttrium prefers to remain in the aqueous phase: when the counter-ion is nitrate, the light lanthanides are removed, but when the counter-ion is thiocyanate, the heavy lanthanides are removed. Yttrium salts of 99.999% purity are obtained. In the usual situation, where yttrium is two-thirds of a heavy-lanthanide mixture, there is an advantage to getting it out of the system as quickly as possible, to ease the separation of the remaining elements.

Annual world production of yttrium oxide had reached 600 tonnes by 2001, with reserves estimated at 9 million tonnes. Only a few tonnes of yttrium metal are produced each year by reducing yttrium fluoride to a metal sponge with calcium magnesium alloy. The temperature of an arc furnace of above 1,600 °C is sufficient to melt the yttrium.

Symbol		Y
Atomic Number		39
Atomic Weight		88.90585
Oxidation States		+3
Electronegativity, Pauling		1.22
State at RT		Solid, Metal
Melting Point, K		1795
Boiling Point, K		3611

Appearance and Characteristics

Harmful effects:

Water soluble compounds of yttrium are considered to be slightly toxic, while its insoluble compounds are considered to be non-toxic.

Characteristics:

• Yttrium is a soft, silvery metal. Yttrium usually exists as a trivalent ion, Y3+, in its compounds. Most of its compounds are colorless.
• Yttrium’s properties are very similar to those of the rare earth elements of the lanthanide series. Accordingly, yttrium is classified as one of the rare earth elements.
• It is relatively stable in air as a result of an oxide film which forms on its surface.
• The finely divided metal ignites in air when heated.
• Yttrium reacts with water to form yttrium hydroxide plus hydrogen gas.
• Interestingly, samples of rock and dust brought back from the Apollo moon landings show a high yttrium content. The yttrium content in lunar soil samples ranged from 54 to 213 parts per million. This compares with an average abundance of 33 parts per million in the earth’s crust. 
• Yttrium has an exceptionally high affinity for oxygen, with a free energy of formation for the oxide of 1817 kJ mol-1, probably the greatest of any element. Yttrium also dissolves oxygen gas in relatively high concentrations. 

Uses of Yttrium

• Yttrium is often used in alloys, increasing the strength of aluminum and magnesium alloys.
• It is also used as a deoxidizer for non-ferrous metals such as vanadium.
• Yttrium is used as a catalyst in ethylene polymerization.
• Yttrium-90, a radioactive isotope, is used in treatments for various cancers and is used in precision medical needles to sever pain-transmitting nerves in the spinal cord.
• Yttrium oxide is the most important compound of yttrium. It is used to make the high-temperature superconductor YBCO (yttrium barium copper oxide). This substance becomes superconducting at -178 oC (meaning that it can be kept in a superconducting state using liquid nitrogen, rather than more expensive and more difficult to handle liquid helium).
• Yttrium oxide is also used to make yttrium iron garnets (Y3 Fe5O12) which are very effective microwave filters, blocking some microwave frequencies, while allowing others through in communication devices such as satellites.
• Yttrium doped with europium is used to produce phosphors, which provide the red color in color television tubes.