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Ferrosilicon, or ferrosilicium, is a ferroalloy an alloy of iron and silicon with between 15 and 90% silicon. It contains a high proportion of iron silicides. Its melting point is about 1200 °C to 1250 °C with a boiling point of 2355 °C. It also contains about 1 to 2% of calcium and aluminum.

 

Ferrosilicon is used in steelmaking and foundries as a source of silicon in production of carbon steels, stainless steels, and other ferrous alloys for its deoxidizing properties, to prevent loss of carbon from the molten steel (so called blocking the heat); ferromanganese, spiegeleisen, silicides of calcium, 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 cores. In 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 prealloys like magnesium ferrosilicon (FeSiMg), used for modification of melted malleable iron; FeSiMg contains between 3-42% of magnesium and small amounts of rare earth metals. Ferrosilicon is also important as an additive to cast irons for controlling the initial content of silicon.

 

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

 

In contact with water, ferrosilicon may slowly produce hydrogen.

Ferrosilicon is produced by reduction of silica or sand with coke in presence of scrap iron, millscale, or other source of iron. Ferrosilicon with silicon content up to about 15% is made in blast furnaces lined with acid fire bricks. Ferrosilicon with higher silicon content is made in electric arc furnaces. An overabundance of silica is used to prevent formation of silicon carbide. Microsilica is a useful byproduct.

 

The usual formulations on the market are ferrosilicon with 15, 45, 75, and 90% of silicon. The remainder is iron, with about 2% of other elements like aluminum and calcium.

Its CAS number is [8049-17-0].

 

A mineral perryite is similar to ferrosilicon, with its composition Fe5Si2.

Ferrosilicon is used by the military to quickly produce hydrogen for balloons by the ferrosilicon method. The chemical reaction uses sodium hydroxide, ferrosilicon, and water. The generator is small enough to fit 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.

 

The melting point and density of ferrosilicon is dependent on its silicon content.