Manufacture of Ferrosilicon
Ferrosilicon is an alloy of iron and silicon containing different proportions of silicon, commonly about 15–90% depending on the grade. It is mainly used as a source of silicon in steelmaking and as a deoxidizing agent.
Manufacturing process of ferrosilicon using a submerged electric arc furnace.
Raw Materials
The principal raw materials required for the manufacture of ferrosilicon are:
- Silica (SiO2): Quartz or quartzite is used as the principal source of silicon.
- Iron source: Iron scrap, steel scrap, iron turnings, or suitable iron-bearing materials may be used.
- Carbonaceous reducing agent: Coke, coal, or charcoal is used as the reducing agent.
Manufacturing Process
Ferrosilicon is manufactured mainly by the carbothermic reduction of silica in a submerged electric arc furnace (SAF). The raw materials are mixed in suitable proportions and charged into the furnace.
1. Charging of Raw Materials
Quartzite or quartz, an iron-bearing material, and a carbonaceous reducing agent such as coke or coal are mixed in appropriate proportions and charged into the electric arc furnace. The composition of the charge is adjusted according to the required grade of ferrosilicon.
2. Reduction of Silica
A high-temperature electric arc is produced by submerged electrodes. The intense heat generated inside the furnace promotes the reduction of silica by carbon. The reduction occurs through intermediate silicon monoxide (SiO) species before silicon is ultimately formed.
SiO2 + C → SiO + CO
SiO + 2C → Si + 2CO
The silicon formed during the reduction process dissolves in molten iron, producing a molten iron–silicon alloy known as ferrosilicon.
3. Formation of Molten Ferrosilicon
The reduced silicon combines with molten iron in the furnace to form the ferrosilicon alloy. The silicon content of the final alloy is controlled mainly by the composition of the furnace charge and operating conditions.
4. Tapping
At suitable intervals, the molten ferrosilicon is tapped from the furnace through the tap hole and collected in suitable refractory-lined containers or casting beds.
5. Cooling, Crushing and Screening
The molten ferrosilicon is allowed to cool and solidify. The solidified alloy is then crushed and screened to obtain the required particle or lump size for industrial applications.
Overall Principle
The manufacture of ferrosilicon can be represented schematically as:
Quartzite (SiO2) + Carbon + Iron-bearing material → Submerged Electric Arc Furnace → Molten Ferrosilicon → Cooling → Crushing & Screening
Important Conditions
- The process is carried out in a submerged electric arc furnace.
- Very high temperatures are required for the reduction of silica.
- Carbonaceous materials act as the reducing agents.
- The furnace charge is carefully controlled to obtain the desired silicon content in the alloy.
- Continuous control of electrical power, charge composition and furnace operation is important for efficient production.
Chemical Composition of Ferrosilicon
The chemical composition of ferrosilicon varies according to its grade, manufacturing process, and intended application. Commercial ferrosilicon is available in several grades, with silicon generally being the major alloying constituent and iron forming the balance. Common grades used in metallurgical applications contain approximately 65–75% silicon (Si).
In addition to silicon and iron, ferrosilicon contains small amounts of aluminum (Al), carbon (C), phosphorus (P), and sulfur (S). The permissible levels of these impurities depend on the grade and the requirements of the steelmaking or foundry process. In particular, phosphorus and sulfur are generally controlled because excessive amounts can adversely affect the quality and properties of steel and cast iron.
| Element | Typical Composition |
|---|---|
| Silicon (Si) | 65–75% |
| Iron (Fe) | Balance |
| Aluminum (Al) | < 2% |
| Carbon (C) | < 0.2% |
| Phosphorus (P) | < 0.04% |
| Sulfur (S) | < 0.02% |
The relatively high silicon content gives ferrosilicon its important deoxidizing and alloying properties. Its effectiveness in metallurgical applications depends not only on the silicon content but also on the grade, impurity levels, particle size, and the specific requirements of the steelmaking or foundry process.
Therefore, the composition of ferrosilicon is selected according to its intended use, with appropriate control of silicon and impurity contents to obtain the desired metallurgical performance.
Uses of Ferrosilicon
- It is widely used as a deoxidizer in steel manufacture.
- It is used as an alloying material in the production of silicon-containing steels.
- It is used in cast-iron production to promote graphitization.
- It is used in the production of other silicon-containing alloys.
- It serves as an important source of silicon in metallurgical processes.
Bangalore University, B.Sc. Fourth Semester