Production of indium also has been reported
in the following countries: West Germany
(Duisburger Kupferhiitte, Duisburg; Dr. W.
Franke, Frankfurt a.M.; and Unterharzer
Berg-und  Hiittenwerke, G.m.b.H., Goslar),
Japan (Mitsui Mining & Smelting Co.), and
the U.S.S.R.
DEFINITION   OF   TERMS, GRADES, AND
SPECIFICATIONS
Indium is a soft, slippery, silvery-white
metal, and can be deposited on harder metals
by rubbing. Some of its other properties are:
Density, 7.31 g./cc.; Brinell hardness, 0.9; melt-
ing point, 156.2 C.; boiling point, 2,000 C.;
tensile strength, 380 p.s.i.; specific heat, 0.057
cal./g./ C.; and crystal structure, face-centered
tetragonal (2, 4).
Indium is usually considered to be nontoxic,
but under certain conditions it is toxic (3, 6).
Laboratory experiments on toxicity using ani-
mals have been conducted. Underskin injec-
tions of indium in quantities from 7 to 30 milli-
grams per kilogram of body weight and venous
injections of indium are toxic. Large quanti-
ties of indium  hydroxide swallowed over a
period of 2 weeks are harmful. Acid foodstuffs
containing indium from indium-coated con-
tainers showed some absorption of the element
by the body. Indium citrate of chloride in-
jected under the skin produced intoxication,
and degenerative changes occurred in the liver,
kidneys, and heart.
TECHNOLOGY
MINERALOGY AND OCCURRENCE
The earth's crust contains about 0.1 gram of
indium per ton, about the same abundance as
silver. It is not found free in nature but has
been detected in many minerals. No deposits
are rich enough to mine for indium alone. In-
dium has been found in about 50 minerals
sporadically, some of which are sphalerite, cas-
siterite, chalcopyrite, coal, galena, hematite,
pyrite, and wolframite. In the United States
indium is found in very small quantities in vari-
ous ores of copper, lead, molybdenum, vanadi-
um, and zinc (5, 6). Sphalerite is the commer-
cial source of indium.
Indium is recovered commercially as a by-
product in processing zinc and zinc-lead ores.
In the metallurgy of zinc, indium, if present,
may appear in variable proportions in any of
the following products: Flue and Cottrell pre-
cipitator dusts, residues from spelter retorts,
rejects from zinc sulfate solution at lithopone
plants, residues from cadmium recovery, lead
blast furnace slags, and crude zinc and lead.

EXTRACTION
In the process developed by The Anaconda
Co. for the recovery of indium, zinc oxide fume
recovered from lead blast furnace slag is
leached with dilute sulfuric acid to remove the
bulk of the zinc, leaving indium in the leach
residue. After settlement in a thickener the
residue is leached in stronger sulfuric acid, to
dissolve 'the indium. After clarification, in-
dium is precipitated from this solution by the
addition of zinc oxide and sodium bisulfite.
The indium precipitate is washed with water
and then leached with strong sodium hydroxide
solution followed by a water wash to remove
excess caustic. The precipitate is then washed
with dilute sulfuric acid to remove residual
zinc. The indium precipitate is then dissolved
in strong sulfuric acid, and heavy metals are
removed with hydrogen sulfide. Aluminum is
added to the clarified pure indium sulfate solu-
tion to precipitate an indium metal sponge that
is recovered, dewatered, and dissolved in 200
Baume hydrochloric acid. The indium chlo-
ride solution is treated with hydrogen sulfide,
filtered, and purified in electrolytic purification
cells. Aluminum is added to the pure indium
chloride solution, and the indium is precipitated
as a pure sponge, which is dewatered and melted
in an aqueous solution of sodium hydroxide, and
cast into marketable shapes.
The American Smelting & Refining Co. re-
covers indium from crude lead-zinc metal (4),
Molten zinc-lead metal containing indium is
treated with lead chloride and sodium chloride.
A slag is formed containing indium chloride,
which is leached with dilute sulfuric acid, and
indium is precipitated as a sponge with zinc
dust. The zinc-indium sponge is melted and
zinc is removed with chlorine. The crude in-
dium is melted, cast into anodes, and refined
electrolytically.
USES
The three largest uses for indium are in semi-
conductor devices, in bearing alloys, and in
special alloys (1, 2, 8). Indium is solderable
and is used in attaching leads to the germanium
body of electronic devices. It also modifies the
electronic properties of germanium. The larg-
est use for indium is in transistors.
A tiny semiconductor device, known as the
Hall Generator, fabricated from indium arse-
nide, has been suggested for use in analog
computers. It also has been reported that in-
dium antimonide may be used in infrared de-
tecting cells for application in missile guidance
systems.
The second largest use for indium is in sleeve-
type bearings where the element gives de-

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INDIUM