Platinum group
From Wikipedia, the free encyclopedia
The
platinum group metals (abbreviated as the
PGMs; alternatively, the
platinoids,
platidises,
platinum group,
platinum metals or
platinum group elements (PGEs)) is a term used sometimes to collectively refer to six
metallic elements clustered together in the
periodic table. These elements are all
transition metals, lying in the
d-block (groups
8,
9, and
10, periods
5 and
6).
The six platinum group metals are
ruthenium,
rhodium,
palladium,
osmium,
iridium, and
platinum.
They have similar physical and chemical properties, and tend to occur together in the same mineral deposits.
[1]
However they can be further subdivided into the iridium-group platinum
group elements (IPGEs: Os, Ir, Ru) and the palladium-group platinum
group elements (PPGEs: Rh, Pt, Pd) based on their behaviour in
geological systems.
History
Naturally occurring platinum and platinum-rich alloys have been known by
pre-Columbian Americans for a long time.
[3]
Though the metal was used by pre-Columbian peoples, the first European
reference to platinum appears in 1557 in the writings of the Italian
humanist
Julius Caesar Scaliger
(1484–1558) as a description of a mysterious metal found in Central
American mines between Darién (Panama) and Mexico ("up until now
impossible to melt by any of the Spanish arts").
[3]
The Spaniards named the metal
platina ("little silver") when they first encountered it in
Colombia. They regarded platinum as an unwanted impurity in the silver they were mining.
[3][4]
Properties
The platinum metals have outstanding
catalytic properties. They are highly resistant to wear and tarnish, making platinum, in particular, well suited for fine
jewelry. Other distinctive properties include resistance to chemical attack, excellent high-temperature characteristics, and stable
electrical properties. All these properties have been exploited for industrial applications.
[5]
Sources
Platinum
Sperrylite (platinum
arsenide, PtAs
2)
ore is a major source of this metal. A naturally occurring platinum-iridium alloy,
platiniridium, is found in the
mineral cooperite (platinum
sulfide, PtS). Platinum in a native state, often accompanied by small amounts of other platinum metals, is found in
alluvial and placer deposits in
Colombia,
Ontario, the
Ural Mountains, and in certain western
American states. Platinum is also produced commercially as a by-product of
nickel
ore processing. The huge quantities of nickel ore processed makes up
for the fact that platinum makes up only two parts per million of the
ore.
South Africa, with vast platinum ore deposits in the
Merensky Reef of the
Bushveld complex, is the world's largest producer of platinum, followed by
Russia.
[6][7] Platinum and palladium are also mined commercially from the
Stillwater igneous complex in Montana, USA.
Osmium
Iridiosmium is a naturally occurring alloy of iridium and osmium found in platinum-bearing river sands in the Ural Mountains and in
North and
South America. Trace amounts of osmium also exist in nickel-bearing ores found in the
Sudbury,
Ontario
region along with other platinum group metals. Even though the quantity
of platinum metals found in these ores is small, the large volume of
nickel ores processed makes commercial recovery possible.
[7][8]
Iridium
Metallic iridium is found with platinum and other platinum group
metals in alluvial deposits. Naturally occurring iridium alloys include
osmiridium and
iridiosmium,
both of which are mixtures of iridium and osmium. It is recovered
commercially as a by-product from nickel mining and processing.
[7]
Ruthenium
Ruthenium is generally found in ores with the other platinum group
metals in the Ural Mountains and in North and South America. Small but
commercially important quantities are also found in
pentlandite extracted from Sudbury, Ontario and in
pyroxenite deposits in South Africa.
[7]
Rhodium
The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium,
silver, platinum, and
gold.
It is found in platinum ores and obtained free as a white inert metal
which is very difficult to fuse. Principal sources of this element are
located in river sands of the Ural Mountains, in North and South America
and also in the copper-nickel sulfide mining area of the
Sudbury Basin
region. Although the quantity at Sudbury is very small, the large
amount of nickel ore processed makes rhodium recovery cost effective.
However, the annual world production in 2003 of this element is only 7
or 8
tons and there are very few rhodium minerals.
[9]
Palladium
Palladium is found as a free metal and alloyed with platinum and gold with platinum group metals in
placer deposits of the
Ural Mountains of
Eurasia,
Australia,
Ethiopia,
South and
North America. However it is commercially produced from nickel-
copper deposits found in
South Africa and
Ontario, Canada.
The huge volume of nickel-copper ore processed makes this extraction
profitable in spite of its low concentration in these ores.
[9][dead link]
Production
The production of pure platinum group metals normally starts from
residues of the production of other metals with a mixture of several of
those metals. One typical starting product is the anode residue of gold
or nickel production. The differences in chemical reactivity and
solubility of several compounds of the metals under extraction are used
to separate them.
[5]
A first step is to dissolve all the metals in
aqua regia forming their respective nitrates. If silver is still present, this is then separated by forming insoluble
silver chloride.
Rhodium sulfate is separated after the salts have been melted together with
sodium bisulfate and leached with water. The residue is then melted together with
sodium peroxide,
which dissolves all the metals and leaves the iridium. The two
remaining metals, ruthenium and osmium, form ruthenium and osmium
tetroxides after
chlorine has been added to solution. The
osmium tetroxide is then dissolved in alcoholic
sodium hydroxide and separated from the
ruthenium tetroxides. All of these metals' final chemical compounds can ultimately be reduced to the elemental metal using
hydrogen.
[5]
