Ferrosilicon: properties, grades and applications / Articles

Ferrosilicon (FeSi) is a ferroalloy whose main elements are iron and silicon. It is one of the most massive and strategically important materials in metallurgy, without which it is impossible to imagine the production of high-quality steels and cast irons. Due to the high affinity of silicon for oxygen and its ability to dissolve in iron, ferrosilicon occupies a central place in the processes of deoxidation, alloying and modification of metals.

Chemical Nature and Physicochemical Properties

Ferrosilicon is an alloy of silicon with iron, in which the silicon content can vary from 19% to 92% depending on the grade. The rest is iron and unavoidable impurities: aluminium, carbon, sulfur, phosphorus, manganese, chromium and titanium. The properties of the alloy directly depend on the silicon content. For example, an increase in silicon concentration reduces the density of the alloy: while 45% ferrosilicon has a density of about 5.0 g/cm³, 75% ferrosilicon has a density of 3.5 g/cm³.

The melting point also depends on the composition: for 45% ferrosilicon it is 1330–1220 °C, and for 75% ferrosilicon — 1300–1330 °C. Silicon increases the hardness of steel, tensile strength, elastic and yield limits, as well as increases oxidation resistance and reduces electricity losses.

Classification, Grades and Standards

In Russia and CIS countries, requirements for ferrosilicon are regulated by GOST 1415-93 (previously GOST 1415-78 was in force). According to the standard, ferrosilicon is produced in several grades that differ in the content of the main element and impurities. The most common grades:

FeSi90 — silicon content 87.0–95.0%.
FeSi75 — silicon content 74.0–80.0%.
FeSi70 — silicon content 68.0–74.0%.
FeSi65 — silicon content 63.0–68.0%.
FeSi50 — silicon content 47.0–52.0%.
FeSi45 — silicon content 41.0–47.0%.
FeSi25 — silicon content 23.0–29.0%.

The grade designation also indicates the aluminium content (e.g., FeSi75Al1.5). The lower the impurity content, especially aluminium, the higher the quality and cost of the alloy. Ferrosilicon is supplied in crushed form with lump sizes of 0–10 mm, 10–50 mm, 10–100 mm or 20–200 mm. The mass fraction of the fraction less than 10 mm must not exceed 10%.

Production Technology

Ferrosilicon production is based on the reduction of silica (quartzite) by carbon at high temperatures. Metallurgical coke is used as a reducing agent, and steel chips serve as the source of iron. The process takes place in powerful submerged arc electric furnaces of continuous operation.

The charge, consisting of quartzite, coke and steel chips, is loaded into the furnace. When electric current is passed through the electrodes, a temperature of about 2000 °C develops, at which silicon is reduced from its oxide. Molten ferrosilicon accumulates at the bottom of the bath and is periodically tapped through a taphole. Electricity is one of the key cost items, accounting for about 20–25% of the cost (approximately 8500 kWh per tonne of 75% ferrosilicon). To obtain particularly pure grades with reduced carbon and aluminium content, additional refining of the melt may be used.

Applications

Steel deoxidation. The bulk of ferrosilicon is consumed in ferrous metallurgy for steel deoxidation, i.e., to remove dissolved oxygen from the metal. Silicon, having a high affinity for oxygen, binds it into oxides that pass into the slag, improving the quality and homogeneity of the steel.

Alloying and modification. Ferrosilicon is used for alloying steels, especially electrical, spring, corrosion- and heat-resistant steels, giving them increased strength, elasticity and oxidation resistance. In cast iron production, it is used as a modifier (inoculant) that promotes graphite formation and produces grey cast iron with good machinability.

Production of ferroalloys and master alloys. Ferrosilicon serves as a reducing agent and base for producing other ferroalloys, such as ferrosilicomanganese and ferrosilicochromium.

Welding materials. It is used in the production of electrodes and welding wires for alloying the deposited metal with silicon.

Other applications. Ferrosilicon finds application in the production of thermite mixtures, as well as a source of silicon in the chemical industry and in the production of high-performance concrete (in the form of microsilica captured during smelting).

Safety and Storage

Ferrosilicon belongs to substances of moderate hazard. During crushing and screening, dust is generated that can irritate the respiratory tract. When working with dust, respirators and safety goggles are recommended. The alloy is non-flammable. Ferrosilicon should be stored in dry, closed warehouses or on areas with hard surfacing, protected from moisture and contamination. It is supplied in big bags with polyethylene liners weighing up to 1000 kg.