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464 Noble Metals (Chemistry)

weathering of sedimentary and igneous rocks. It accom- platinum and 60% palladium, whereas those in Canada are
panies silver and the base metals, copper and nickel, as the 43% platinum and 43% palladium, and in South Africa,
sulfides and selenides. Gold is also found in the telluride approximately 64% platinum and 26% palladium.
minerals, calaverite and sylvanite. The largest suppliers of
gold are South Africa, Russia, and Canada, in their order
4. Iridium and Osmium
of importance. In the U.S., about 60% of the gold pro-
duced is from ores and about 40% is a by-product of base Iridium and osmium are both found in the earth’s crust
metal production. Most U.S. production is in the Midwest in concentrations of about 0.001 ppm. These metals are
and West (South Dakota, Nevada, Utah, and Arizona). most often found together in the alloys osmiridium or sy-
serkite (less than or equal to 60% iridium and about 35%
osmium), iridiosmium or neuyanskite (greater than 60%
2. Silver iridium and about 20% osmium), and osmium in lauritite
(Ru, Os)S 2 . Estimates indicate that the Russia is the largest
Silver is estimated at about 0.1 ppm in the earth’s
supplier of iridium and South Africa has the largest supply
crust. Silver minerals are most commonly found with
of osmium.
those of lead, but they can also be found in ores of
copper, zinc, and gold. Ores with silver as the main
component—argentite (Ag 2 S), cerargyrite (AgCl), poly- 5. Rhodium
basite (Ag 16 Sb 2 S 11 ), proustite (Ag 3 AsS 3 ), pyrargyrite
Rhodium is present at about 0.001 ppm in the earth’s crust.
(Ag 3 SbS 3 ), and stephanite (Ag 5 SbS 4 )—are most often as-
Its production is estimated to be as low as 2% and as high
sociated with igneous rocks of intermediate felsic compo-
as 10% of the platinum production. Russia is thought to
sition. Native silver also occurs but most deposits of this
have about half of the world’s supply.
type have been worked out. The majority of the world’s
silver is found in Mexico, U.S., Peru, Canada, Russia,
Australia, and Japan. The major U.S. source is the Coeur 6. Ruthenium
d’Alene in Idaho.
Ruthenium is estimated at 0.001 ppm in the crust and is
isolated with the other noble metals mainly from laurite
(Ru, Os)S 2 .
3. Platinum and Palladium
The platinum group metals (PGM) are usually found to- B. Processing and Production
gether and are thus produced together. Their ratio depends
1. Gold
on the ore location. Platinum is found in about 0.01 ppm in
the earth’s crust. The major forms are sperrylite (PtAs 2 ), Gold is obtained as free metal in placer deposits or by
cooperite (Pt, Pd)S, braggite (Pt, Pd, Ni)S, and as na- mining of vein gold, with a minimum of about 3 ppm gold
tive metals in platinumiridium and palladiplatinum. The being required for its economic recovery. Approximately
ores and metal are found in placer deposits derived from 92–96% of the gold is recovered from the ore and is often
basic/ultrabasic igneous rocks. In sedimentary and ig- isolated with silver to produce dor´e metal. The method
neous rocks the platinum is associated with chromite, of separating gold and silver from each other depends
magnetite, ilmenite, iridium, and osmium. The PGM de- on the relative concentrations. The most popular method
posits in the Merensky Reef in South Africa are found is cyanidation with amalgamation being of secondary
with iron, nickel, and copper ores. The total PGM con- importance.
centration in the ore is in the 4–10 ppm range. In Canada,
the Sudbury Area of Ontario is the major source of PGMs,
2. Silver
where they are a by-product of nickel and copper produc-
tion. The Ural Mountain and East Siberian deposits in the Silver is obtained through open pit and sub-surface mining
Russia are obtained as a by-product of nickel production. chiefly as a low grade ore associated with copper, lead,
Palladium has a concentration of about 0.005 ppm in the and zinc. Silver is produced in the largest volume of all
earth’s crust. Beside cooperite and braggite, palladium is the noble metals, and it is separated by amalgamation and
also found as potarite (PdHg or Pd 3 Hg 2 ) or stibiopalladi- cyanidation.
nite (Pd 3 Sb) or as native palladium. The ratio of platinum
to palladium varies with location but together these two
3. Platinum Group Metals
metals account for about 90% of the PGMs. The Russia
is the largest palladium producer, with Canada and South ThePGMsareisolatedmainlyforplatinumandpalladium.
Africa following. The ores in the Russia are about 30% Iridium, osmium, rhodium, and ruthenium production is