Ferrosilicon production methods are mainly based on high-temperature reduction reactions. The main process is the reduction of silica (SiO₂) and iron in an electric furnace using a carbon reducing agent to produce a ferrosilicon alloy.
1. Raw material preparation
Silica (SiO₂):
Requirements: Silica content ≥ 97%, low impurity content (e.g., Al₂O₃, CaO) to ensure silicon purity.
Pre-treatment: Crushing to 5-50mm particles to improve reaction efficiency.
Source of iron:
Steel scrap, iron filings, or iron ore (e.g., magnetite) are commonly used.
The role of iron: acts as a carrier for silicon, reduces the reaction temperature, and forms an alloy.
Carbon Reducing Agent:
Coke (preferred): high fixed carbon content (≥80%), low ash content (≤10%).
Others: charcoal, petroleum coke (more expensive, for special requirements).
Auxiliary raw materials:
steel scrap (to regulate the air permeability of the kiln), lime (flux, to reduce the viscosity of slag).
2. Main equipment - submerged arc furnace (electric arc furnace)
Furnace Type:
open or closed arc furnace under flux, with the closed type being the main type (environmentally friendly and with a high level of thermal energy use).
Capacity: usually 10-50 MW, large furnace capacity up to 100,000 tons/year.
Electrodes:
Self-baked electrodes or graphite electrodes up to 1.5 meters in diameter that transmit electrical energy deep into the furnace charge.
Stove Design:
Refractory material lining (e.g. carbon brick, magnesia brick) resistant to high temperatures (1800-2000°C).
3. Production Process
(1) Dosing and loading
Mix silica, iron, coke and auxiliary raw materials in a ratio (e.g. silica:coke≈3:1).
Layer-by-layer loading: coke at the bottom, a mixture of silica and an iron source at the top, to maintain air permeability in the furnace.
(2) High temperature reduction reaction
Reaction temperature: 1600~2000°C, the energy is supplied by electric arc and resistive heating.
The main chemical reactions are:
SiO2+2C→Si+2CO↑ (basic reaction) FeO+C→Fe+CO↑ (reduction of the iron source).
Adverse reactions: Small amounts of intermediates such as SiC and FeSi₂ are formed. It is necessary to control the temperature of the furnace to prevent excessive carbonation.
(3) Melting and delamination
The reduced silicon and iron form an alloy melt (density about 5.2 g/cm³), which sinks to the bottom of the furnace.
The slag (mainly consisting of CaO-SiO₂-Al₂O₃) floats to the top and is discharged regularly.
(4) Pouring and casting
Molten ferrosilicon enters the ladle through the outlet hole.
It is poured into ingots or granulated (water quenching is used to obtain granular ferrosilicon).
(5) Refining (optional)
Oxygen/argon purging: Reduces impurities such as aluminum and calcium, resulting in low-aluminum ferrosilicon (e.g., special grades for the reduction of magnesium metal).
Addition of a slag-forming agent: further separation of impurities.
4. Energy consumption and power requirements
Electricity consumption:
To produce 1 ton of ferrosilicon, 8,000-9,000 kWh of electricity is required, which is 60-70% of the total costs.
Energy sources: Most of them are located in areas with a lot of hydroelectric power (e.g., Yunnan, China, and Norway).
Energy-saving technologies:
Waste heat recovery (use of exhaust gases to preheat raw materials).
Closed-type electric furnaces reduce heat loss.
5. Environmental Protection Measures
Exhaust gas treatment:
Closed electric furnaces collect CO gas (which can be burned to generate electricity or used as a chemical feedstock).
Bag filters capture dust (including SiO₂ particles, which are used in the production of building materials).
Wastewater treatment:
Granular ferrosilicon water wastewater needs to be recycled to prevent silicon powder pollution.
Solid Waste Disposal:
Slag can be used for road construction or as an additive to cement.
6. Special Production Processes
(1) Direct method (one-step method)
Simultaneous reduction of silica and iron, suitable for low- and medium-siliceous varieties (e.g. FeSi45).
Advantages: simple process, low cost; Disadvantages: poor impurity control.
(2) Indirect method (two-step method)
First, industrial silicon (Si≥98%) is produced, then it is melted with iron to produce high-siliceous ferrosilicon (e.g., FeSi90).
Advantages: higher purity; Disadvantages: increased energy consumption.
7. Characteristics of world production
China:
it accounts for more than 60% of the world's production capacity, concentrated in the hydropower regions of the Northwest (Ningxia, Inner Mongolia) and the Southwest (Yunnan).
In recent years, small and obsolete furnaces (<25,000 kVA) have been decommissioned due to the impact of the "dual carbon" policy.
Norway/Russia:
Use of clean energy (hydropower/nuclear) to produce high value-added ferrosilicon (e.g. low-aluminium FeSi75).
8. Technological challenges and innovations
Replacement of raw materials: Attempts to replace coke with biomass coal to reduce carbon emissions.
Intelligent Control:
Optimization of ingredients and oven temperature using artificial intelligence to improve energy efficiency (e.g. 5-10% reduction in energy consumption).
Hydrogen metallurgy tests:
Study of the possibility of using hydrogen for partial replacement of carbon reducing agents in order to achieve environmental friendliness of production (still at the stage of laboratory research).