Indium

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Indium is a soft metal with a low melting point. It is used mostly in Indium Tin Oxide (ITO) in flat screens, the oxide mixture is transparent and conducts electricity. The softness and low melting point makes it useful in some special solders for sensitive electronic components, for example semiconductor lasers. In electronic scrap the most common sources are flat panels and soldering of the heat spreader in some CPU:s.

Indium is a quite new metal which have found a lot of technical niches since it's discovery in 1863. In 1924 the total amount of indium recovered was 1 gram. After locating some sources the production increased and in 1930 there had been a pound of metal produced from ores which led to a price only three times as high as platinum. Increased production has lead to lowering of prices and today it is below silver.

Contents

Natural sources

There are no ores where indium is the main metal recovered. Indium is recovered as a byproduct from other processes, mostly zinc, lead and copper mining. Increasing recycling means that more of the indium is also recovered from recycled goods.

Recovery from scrap

One process of recovery of indium from flat panel scrap (ITO) is by a chlorination process with ammonium chloride, NH4 Cl, at 400 °C.

Common scrap objects where indium can be found is in

  • Flat panel displays, in the form of indium tin oxide or ITO.
  • The solder connecting the heat spreader on modern CPU:s.
  • Heated windshields as ITO.
  • Solder for connecting laser diodes.
  • Solder for soldering glass.

Indium can easily be collected from the heat spreader on modern (P4 and later models) CPU:s as shown in this video.

Chemical reactions

Indium is dissolved by nitric acid, hydrochloric acid and sulfuric acid.

Potassium hydroxide doesn't affect the indium metal.

Indium in solution is precipitated by hydroxides ( KOH,NaOH,NH4OH ) as In(OH)3. In the presence of tartaric acid H2C4H4O6 the hydroxide reacts and forms a soluble compound H2C4H2(InOH)O5.

Electrolysis of indium

Indium electrolysis can be done in both hydrochloric acid and sulfuric acid based systems. With proper additions of the chloride salt, electrolysis can be done on molten salt too.

A recipe for a H2SO4 system is given as : 100g/l Indium, pH 2-3, little gelatin as leveller, 3-6 A/dm2 at 60C.

Patents and research referenses

This is the referenses collected from the article "Comparison of indium purification between vacuum refining and electrowinning"

   M. F. MCNamara J. A. Slattery and A. F. Witt, US Patent 4,828,608.
   J. A. Adamski, US Patent 4,559,217.
   R. B. Massy, US Patent 3,180,812.
   S. L. Eriangen, US Patent 3,325,380.
   A. B. I. Bollong and R. P. Bult, US Patent 4,888,051.
   J. G. Harper, US Patent 3,088,853.
   J. R. Mills B. G. Hunt and G. H. Turner J. Electrochem. Soc. 100 (1953) 136.
   B. Heshmatpour and D. A. Stevenson J. Less Common Metals 81 (1981) 329.
   O. Kubaschewski and C. B. Alcock, in “Metallurgical Thermochemistry,” 5th ed. (Pergamon Press, Oxford, 1979) p. 363.
   E. T. Turkdogan, in “Physical Chemistry of High Temperature Technology” (Academic Press Inc., NY, 1980) p. 233.
   J. O'M. Bockris and A. K. N. Reddy, in “Modern Electrochemistry” Vol. 2 (Plenum Press, NY, 1970).
   M. Pourbaix, in “Atlas of Electrochemical Equilibria in Aqueous Solutions” (Pergamom Press, Brussels, 1966) p. 437.

References

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