Ferrosilicon is an alloy of iron and

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

with a typical silicon content by weight of 15–90%. It contains a high proportion of iron silicides.

Production and reactions

Ferrosilicon is produced by reduction of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...

with coke in the presence of iron. Typical sources of iron are

scrap iron Scrap consists of recyclable materials, usually metals, left over from product manufacturing and consumption, such as parts of vehicles, building supplies, and surplus materials. Unlike waste, scrap has monetary value, especially recovered me ...

or millscale. Ferrosilicons with silicon content up to about 15% are made in

blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheri ...

s lined with acid fire bricks. Ferrosilicons with higher silicon content are made in

electric arc furnace An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc. Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundries for producing cast iron products) up to ...

s. The usual formulations on the market are ferrosilicons with 15%, 45%, 75%, and 90% silicon. The remainder is iron, with about 2% consisting of other elements like aluminium and calcium. An overabundance of silica is used to prevent formation of

silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...

. 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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

. The reaction, which is accelerated in the presence of base, is used for

hydrogen production Hydrogen production is the family of industrial methods for generating hydrogen gas. As of 2020, the majority of hydrogen (∼95%) is produced from fossil fuels by steam reforming of natural gas and other light hydrocarbons, partial oxidation of ...

. The melting point and

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

of ferrosilicon depends on its silicon content, with two nearly-eutectic areas, one near Fe₂Si and second spanning FeSi₂-FeSi₃ composition range. :

Uses

Ferrosilicon is used as a source of silicon to reduce metals from their oxides and to deoxidize steel and other ferrous alloys. This prevents the loss of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

from the molten steel (so called ''blocking the heat''); ferromanganese,

spiegeleisen Spiegeleisen (literally "mirror-iron", — mirror or specular; — iron) is a ferromanganese alloy containing approximately 15% manganese and small quantities of carbon and silicon. Spiegeleisen is sometimes also referred to as ''specular pig ...

, calcium silicides, and many other materials are used for the same purpose. It can be used to make other ferroalloys. Ferrosilicon is also used for manufacture of silicon, corrosion-resistant and high-temperature-resistant ferrous silicon alloys, and silicon steel for electromotors and

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

cores. In the manufacture of

cast iron Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuri ...

, ferrosilicon is used for inoculation of the iron to accelerate graphitization. In

arc welding Arc welding is a welding process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals, when cool, result in a binding of the metals. It is a type of welding that uses a welding powe ...

, ferrosilicon can be found in some electrode coatings. Ferrosilicon is a basis for manufacture of prealloys like magnesium ferrosilicon (MgFeSi), used for production of ductile iron. MgFeSi contains 3–42%

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

and small amounts of

rare-earth metal The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silv ...

s. 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 Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...

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 Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

Silanes

Treatment of high-silicon ferrosilicon with

hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chlorid ...

is the basis of the industrial synthesis of

trichlorosilane Trichlorosilane is an inorganic compound with the formula HCl3Si. It is a colourless, volatile liquid. Purified trichlorosilane is the principal precursor to ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to pr ...

. 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

The method has been in use since

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

. Prior to this, the process and purity of hydrogen generation relying on

steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporizatio ...

passing over hot iron was difficult to control. The chemical reaction uses

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and al ...

(NaOH), ferrosilicon, and water (H₂O). While in the "silicol" process, a heavy steel

pressure vessel A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...

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 and starts the reaction;

sodium silicate Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate , sodium orthosilicate , and sodium pyrosilicate . The anions are often polymeric. These compounds are generally colorless transparent ...

, hydrogen and steam are produced. The overall reaction of the process is believed to be: : 2NaOH + Si + H₂O → 2Na₂SiO₃ + 2H₂ Ferrosilicon is used by the military to quickly produce hydrogen for

balloons A balloon is a flexible bag that can be inflated with a gas, such as helium, hydrogen, nitrous oxide, oxygen, and air. For special tasks, balloons can be filled with smoke, liquid water, granular media (e.g. sand, flour or rice), or light s ...

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.Report No 40: The ferrosilicon process for the generation of hydrogen
/ref> 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.

Production and reactions

Uses

Silanes

Hydrogen production

Footnotes

References

Further reading