Ferrosilicon is a ferroalloy, an alloy of iron and silicon with an average silicon content between 15 and 90 weight percent.[1] It contains a high proportion of iron silicides.
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Ferrosilicon is produced by reduction of silica or sand with coke in presence of scrap iron, millscale, or other source of iron. Ferrosilicons with silicon content up to about 15% are made in blast furnaces lined with acid fire bricks. Ferrosilicons with higher silicon content are made in electric arc furnaces. 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. Microsilica is a useful byproduct.
A mineral perryite is similar to ferrosilicon, with its composition Fe5Si2.
| Si content (wt.%) | 0 | 20 | 35 | 50 | 60 | 80 | 100 |
| Melting point (°C)[2] | 1538 | 1210 | 1410 | 1210 | 1230 | 1360 | 1414 |
| Density (g/cm3 | 7.87 | 6.76 | 5.65 | 5.1 | 4.27 | 3.44 | 2.33 |
The melting point and density of ferrosilicon depends on its silicon content.
In contact with water, ferrosilicon may slowly produce hydrogen.
Ferrosilicon is used as a source of silicon to reduce metals from their oxides and to deoxidize steel and other ferrous alloys.[1] This prevents the 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.[3] 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 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 prealloys like magnesium ferrosilicon (FeSiMg), used for modification of melted malleable iron. FeSiMg contains 3–42% 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.
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.[4] The method has been in use since World War I. A heavy steel pressure vessel is filled with sodium hydroxide and ferrosilicon, closed, and a controlled amount of water is added; the dissolving of the hydroxide heats the mixture to about 200 °F and starts the reaction; sodium silicate, hydrogen and steam are produced. After the cylinder cools and the steam condenses, hydrogen is drained and the glassy sodium silicate is scraped from the cylinder; the cleaning is a laborious operation, taking a soldier an entire day. The silicate scrapings have to be buried, but the tired soldiers often resorted to just dumping them on the ground; cattle then ate them and the army was getting lawsuits because of the damaged cattle. The steel cylinders were also difficult to transport. At the beginning of the World War II the US military sought an alternative; catalyzed decomposition of sodium borohydride was attempted as a replacement instead.[5]