Ferrosilicon

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Ferrosilicon alloy

Ferrosilicon is an alloy of iron and silicon with a typical silicon content by weight of 15–90%. It contains a high proportion of iron silicides.[1]

Production and reactions

Ferrosilicon is produced by reduction of

millscale. Ferrosilicons with silicon content up to about 15% are made in blast furnaces lined with acid fire bricks.[2]

Ferrosilicons with higher silicon content are made in

Microsilica
is a useful byproduct.

A mineral perryite is similar to ferrosilicon, with its composition Fe5Si2. In contact with water, ferrosilicon may slowly produce hydrogen. The reaction, which is accelerated in the presence of base, is used for hydrogen production. The melting point and density of ferrosilicon depends on its silicon content, with two nearly-eutectic areas, one near Fe2Si and second spanning FeSi2-FeSi3 composition range.

Physical properties of ferrosilicon[3][4]
Si
mass fraction
(%) 		0 20 35 50 60 80 100
Solidus
point (°C) 		1538 1200 1203 1212 1207 1207 1414
Liquidus
point (°C) 		1538 1212 1410 1220 1230 1360 1414
Density (g/cm3) 7.87 6.76 5.65 5.1 4.27 3.44 2.33

Uses

Ferrosilicon is used as a source of silicon to reduce metals from their oxides and to

electromotors and transformer cores. In the manufacture of cast iron, ferrosilicon is used for inoculation of the iron to accelerate graphitization. In arc welding
, ferrosilicon can be found in some electrode coatings.

Ferrosilicon is a basis for manufacture of

rare-earth metals
. Ferrosilicon is also important as an additive to cast irons for controlling the initial content of silicon.

Magnesium ferrosilicon is instrumental in the formation of nodules, which give ductile iron its flexible property. Unlike gray cast iron, which forms graphite flakes, ductile iron contains graphite nodules, or pores, which make cracking more difficult.

Ferrosilicon is also used in the Pidgeon process to make magnesium from dolomite.

Silanes

Treatment of high-silicon ferrosilicon with hydrogen chloride is the basis of the industrial synthesis of trichlorosilane.

Ferrosilicon is also used in a ratio of 3–3.5% in the manufacture of sheets for the magnetic circuit of

electrical transformers
.

Hydrogen production

See also: Hydrogen production § Ferrosilicon method

The method has been in use since World War I. Prior to this, the process and purity of hydrogen generation relying on steam passing over hot iron was difficult to control.[6] The chemical reaction uses sodium hydroxide (NaOH), ferrosilicon, and water (H2O). While in the "silicol" process, a heavy steel pressure vessel is filled with sodium hydroxide and ferrosilicon, and upon closing, a controlled amount of water is added; the dissolving of the hydroxide heats the mixture to about 200 °F (93 °C) and starts the reaction; sodium silicate, hydrogen and steam are produced.[7] The overall reaction of the process is believed to be:[2][note 1]

2NaOH +
Si
+ H2O → Na2SiO3 + 2H2

Ferrosilicon is used by the military to quickly produce hydrogen for

balloons by the ferrosilicon method. The generator may be small enough to fit in a truck and requires only a small amount of electric power, the materials are stable and not combustible, and they do not generate hydrogen until mixed.[8]

One report notes that this method of hydrogen production wasn't thoroughly investigated for about century despite being reported by the US military in the beginning of 20th century.[2]

Footnotes

  1. ^ The iron is intentionally omitted

References

  1. ISBN 978-3527306732
    .
  2. ^
    S2CID 54929253
    .
  3. ISBN 978-3-540-74193-0
    . Retrieved 25 December 2011.
  4. doi:10.1016/j.matchar.2006.06.003
    .
  5. ISBN 978-0-12-391916-8
    . Retrieved 25 December 2011.
  6. ^ Hydrogen for Airships, A.M. Burgess and Cleveland Industrial Archaeology Society
  7. ^ Candid science: conversations with famous chemists, István Hargittai, Magdolna Hargittai, p. 261, Imperial College Press (2000)
    ISBN 1-86094-228-8
  8. ^ Report No 40: The ferrosilicon process for the generation of hydrogen

Further reading

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