IRIDIUM

Listing description:

Iridium (pronounced /ɨˈrɪdiəm/, i-RID-ee-əm) is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can even be flammable. The most important iridium compounds in use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in catalysis and in research.

^{Detailed Description}

Iridium was discovered in 1803 by Smithson Tennant in London, England, among insoluble impurities in natural platinum from South America. Although it is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes, it has a number of specialized industrial and scientific applications. Iridium is employed when high corrosion resistance at high temperatures is needed, as in spark plugs, crucibles for recrystallization of semiconductors at high temperatures, electrodes for the production of chlorine in the chloralkali process, and radioisotope thermoelectric generators used in unmanned spacecraft. Iridium compounds also find applications as catalysts for the production of acetic acid. In the automotive industry, iridium is used in high-end after-market sparkplugs as the center electrode, replacing more commonly used metals.

A member of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. Because of its hardness, brittleness, and very high melting point (the ninth highest of all elements), solid iridium is difficult to machine, form, or work, and thus powder metallurgy is commonly employed instead. It is the only metal to maintain good mechanical properties in air at temperatures above 1600 °C. Iridium has a very high boiling point (11th among all elements) and becomes a superconductor at temperatures below 0.14 K.

Chemical properties

Iridium is the most corrosion-resistant metal known: it is not attacked by almost any acid, aqua regia, molten metals or silicates at high temperatures. It can, however, be attacked by some molten salts, such as sodium cyanide and potassium cyanide, as well as oxygen and the halogens (particularly fluorine) at higher temperatures.

While no binary hydrides of iridium, Ir_xH_y are known, complexes are known that contain IrH4−5 and IrH3−6, where iridium has the +1 and +3 oxidation states, respectively.

History

The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. Native platinum used by ancient Ethiopians and by South American cultures always contained a small amount of the other platinum group metals, including iridium. Platinum reached Europe as platina ("small silver"), found in the 17th century by the Spanish conquerors in a region today known as the department of Chocó in Colombia. The discovery that this metal was not an alloy of known elements, but instead a distinct new element, did not occur until 1748.

Occurrence

Iridium is one of the least abundant elements in the Earth's crust, having an average mass fraction of 0.001 ppm in crustal rock; gold is 4 times more abundant, platinum is 10 times more abundant, and silver and mercury are 80 times more abundant. Tellurium is about as abundant as iridium, and only three naturally occurring elements are less abundant: rhenium, ruthenium, and rhodium, iridium being 10 times more abundant than the last two. In contrast to its low abundance in crustal rock, iridium is relatively common in meteorites, with concentrations of 0.5 ppm or more. It is thought that the overall concentration of iridium on Earth is much higher than what is observed in crustal rocks, but because of the density and siderophilic ("iron-loving") character of iridium, it descended below the crust and into the Earth's core when the planet was still molten.

Iridium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich). In the nickel and copper deposits the platinum group metals occur as sulfides (i.e. (Pt,Pd)S)), tellurides (i.e. PtBiTe), antimonides (PdSb), and arsenides (i.e. PtAs₂). In all of these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum group metals, iridium can be found naturally in alloys with raw nickel or raw copper.

Within the Earth's crust, iridium is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa, though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of iridium. Smaller reserves are found in the United States. Iridium is also found in secondary deposits, combined with platinum and other platinum group metals in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department of Colombia are still a source for platinum-group metals. As of 2003 the world reserves had not been estimated.

K–T boundary presence

Main article: Cretaceous–Tertiary extinction event

The K–T boundary of 65 million years ago, marking the temporal border between the Cretaceous and Tertiary periods of geological time, was identified by a thin stratum of iridium-rich clay. A team led by Luis Alvarez proposed in 1980 an extraterrestrial origin for this iridium, attributing it to an asteroid or comet impact. Their theory, known as the Alvarez hypothesis, is now widely accepted to explain the demise of the dinosaurs. A large buried impact crater structure with an estimated age of about 65 million years was later identified under what is now the Yucatán Peninsula (the Chicxulub crater).

Production

Year	Price ($/ozt)[45][46]
2001	415.25
2002	294.62
2003	93.02
2004	185.33
2005	169.51
2006	349.45
2007	440.00

Iridium is obtained commercially as a by-product from nickel and copper mining and processing. During electrorefining of copper and nickel, noble metals such as silver, gold and the platinum group metals as well as selenium and tellurium settle to the bottom of the cell as anode mud, which forms the starting point for their extraction. In order to separate the metals, they must first be brought into solution. Several methods are available depending on the separation process and the composition of the mixture; two representative methods are fusion with sodium peroxide followed by dissolution in aqua regia, and dissolution in a mixture of chlorine with hydrochloric acid.

After it is dissolved, iridium is separated from the other platinum group metals by precipitating (NH₄)₂IrCl₆ or by extracting IrCl2−6 with organic amines. The first method is similar to the procedure Tennant and Wollaston used for their separation. The second method can be planned as continuous liquid–liquid extraction and is therefore more suitable for industrial scale production. In either case, the product is reduced using hydrogen, yielding the metal as a powder or sponge that can be treated using powder metallurgy techniques.

Annual production of iridium circa 2000 was around 3 tonnes or about 100,000 troy ounces (ozt).^{[note 3][10]} The price of iridium as of 2007 was $440 USD/ozt, but the price fluctuates considerably, as shown in the table. The high volatility of the prices of the platinum group metals has been attributed to supply, demand, speculation, and hoarding, amplified by the small size of the market and instability in the producing countries.

Applications

The global demand for iridium in 2007 was 119,000 troy ounces (3,700 kg), out of which 25,000 ozt (780 kg) were used for electrical applications such as spark plugs; 34,000 ozt (1,100 kg) for electrochemical applications such as electrodes for the chloralkali process; 24,000 ozt (750 kg) for catalysis; and 36,000 ozt (1,100 kg) for other uses.

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OSMIUM

Listing description

Osmium ( /ˈɒzmiəm/ OZ-mee-əm) is a chemical element that has the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family, and is the densest natural element. Osmium is twice as dense as lead. The density of osmium is 22.59 g/cm³, slightly greater than that of iridium, the second densest element. Osmium is found in nature as an alloy, mostly in platinum ores. Osmium is also used in alloys, with platinum, iridium and other platinum group metals. Those alloys are employed in fountain pen tips, electrical contacts and in other applications where extreme durability and hardness are needed.^[2]

Detailed description

Osmium is an extremely dense, blue-gray, hard but brittle metal that remains lustrous even at high temperatures. Due to its hardness, brittleness, low vapor pressure (the lowest of the platinum group metals), and very high melting point (the fourth highest of all elements), solid osmium is difficult to machine, form, or work. Osmium is generally considered to be the densest known element, slightly denser than iridium.^[3] Calculations of density from the space lattice may produce the most reliable data for these elements, giving a density of 22.562±0.009 g/cm³ for iridium versus 22.587±0.009 g/cm³ for osmium.^[4] The extraordinary density of osmium is a consequence of the lanthanide contraction.^[4]

Osmium possesses quite remarkable chemical and physical properties. It has the highest melting point and the lowest vapor pressure in the platinum family. Osmium has a very low compressibility. Correspondingly, its bulk modulus is extremely high, reported between 395 and 462 GPa, which rivals that of diamond (443 GPa). However, the hardness of osmium is lower than diamond, only 4 GPa.^[5][6][7]

Osmium heptafluoride (OsF₇) and osmium pentafluoride (OsF₅) are known, but osmium trifluoride (OsF₃) has not been synthesized yet. The lower oxidation states are stabilized by the larger halogens. Therefore, the trichloride, tribromide, triiodide and even osmium diiodide are known. The oxidation state +1 is only known for the osmium iodide (OsI), whereas several carbonyl complexes of osmium, such as triosmium dodecacarbonyl (Os₃(CO)₁₂), represent the oxidation state 0.^{[12][13][16][17]}

In general, the lower oxidation states of osmium are stabilized by ligands that are good σ-donors (such as amines) and π-acceptors (heterocycles containing nitrogen). The higher oxidation states are stabilized by strong σ- and π-donors, such as O²⁻ and N³⁻.^[18]

History

Osmium (from Greek osme (ὀσμή) meaning "smell") was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England.^[22] The discovery of osmium is intertwined with that of platinum and the other metals of the platinum group. Platinum reached Europe as platina ("small silver"), first encountered in the late 17th century in silver mines around the Chocó Department, in Colombia.^[23] The discovery that this metal was not an alloy, but a distinct new element, was published in 1748.^[24] Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue.^[25] Joseph Louis Proust thought that the residue was graphite.^[25] Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough material for further experiments.^[25]

In 1803, Smithson Tennant analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with alkali and acids^[26] and obtained a volatile new oxide, which he believed to be of this new metal—which he named ptene, from the Greek word πτηνος (ptènos) for winged.^[27][28] However, Tennant, who had the advantage of a much larger amount of residue, continued his research and identified two previously undiscovered elements in the black residue, iridium and osmium.^[25][26] He obtained a yellow solution (probably of cis–[Os(OH)₂O₄]²⁻) by reactions with sodium hydroxide at red heat. After acidification he was able to distill the formed OsO₄.^[27] He named osmium after Greek osme meaning "a smell", because of the smell of the volatile osmium tetroxide.^[29] Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.^[25][30]

Occurrence

Osmium is one of the least abundant elements in the Earth's crust with an average mass fraction of 0.05 ppb in the continental crust.^[33]

Osmium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich).^[26] In the nickel and copper deposits, the platinum group metals occur as sulfides (i.e., (Pt,Pd)S)), tellurides (e.g., PtBiTe), antimonides (e.g., PdSb), and arsenides (e.g., PtAs₂); in all these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum group metals, osmium can be found naturally in alloys with nickel or copper.^[34]

Within the Earth's crust, osmium, like iridium, is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa,^[35] though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of osmium. Smaller reserves can be found in the United States.^[35] The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum group metals. The second large alluvial deposit was found in the Ural Mountains, Russia, which is still mined.

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PALLADIUM[Pd]

Listing description:  

Palladium (pronounced /pəˈleɪdiəm/, pə-LAY-dee-əm) is a chemical element with the chemical symbol Pd and an atomic number of 46. Palladium is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it after the asteroid Pallas, which was named after the epithet of the Greek goddess Athena, acquired by her when she slew Pallas.

Detailed description:

Palladium, along with platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as the platinum group metals (PGMs). Platinum group metals share similar chemical properties, but palladium has the lowest melting point and is the least dense of these precious metals.

The unique properties of palladium and other platinum group metals account for their widespread use. One in four goods manufactured today either contain platinum group metals or had platinum group metals play a key role during their manufacturing process.^[2] Over half of the supply of palladium and its congener platinum goes into catalytic converters, which convert up to 90% of harmful gases from auto exhaust (hydrocarbons, carbon monoxide and nitrogen oxide) into less harmful substances (nitrogen, carbon dioxide and water vapor). Palladium is found in many electronics including computers, mobile phones, multi-layer ceramic capacitors, component plating, low voltage electrical contacts, and SED/OLED/LCD televisions. Palladium is also used in dentistry, medicine, hydrogen purification, chemical applications, and groundwater treatment. Palladium plays a key role in the technology used for fuel cells, which combines hydrogen and oxygen to produce electricity, heat and water.

Ore deposits of palladium and other platinum group metals are rare, and the most extensive deposits have been found in the norite belt of the Bushveld Igneous Complex in the Transvaal in South Africa, the Stillwater Complex in Montana, United States, the Sudbury District of Ontario, Canada, and the Norilsk Complex in Russia. In addition to mining, recycling is also a source of palladium, mostly from scrapped catalytic converters. The numerous applications and limited supply sources of palladium result in palladium drawing considerable investment interest.

History

Palladium was discovered by William Hyde Wollaston in 1803. This element was named by Wollaston in 1804 after the asteroid Pallas, which had been discovered two years earlier. Wollaston found palladium in crude platinum ore from South America by dissolving the ore in aqua regia, neutralizing the solution with sodium hydroxide, and precipitating platinum as ammonium chloroplatinate with ammonium chloride. He added mercuric cyanide to form the compound palladium cyanide, which was heated to extract palladium metal.

Palladium chloride was at one time prescribed as a tuberculosis treatment at the rate of 0.065 g per day (approximately one milligram per kilogram of body weight). This treatment did have many negative side-effects, and was later replaced by more effective drugs.^[6]

In the run up to 2000, Russian supply of palladium to the global market was repeatedly delayed and disrupted^[8] because the export quota was not granted on time, for political reasons. The ensuing market panic drove the palladium price to an all-time high of $1100 per troy ounce in January 2001.^[9] Around this time, the Ford Motor Company, fearing auto vehicle production disruption due to a possible palladium shortage, stockpiled large amounts of the metal purchased near the price high. When prices fell in early 2001, Ford lost nearly US$1 billion.^[10] World demand for palladium increased from 100 tons in 1990 to nearly 300 tons in 2000. The global production of palladium from mines was 222 metric tons in 2006 according to USGS data.

Occurrence

Palladium is found in the rare minerals cooperite^[14] and polarite.

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