The mainstream process for producing magnesium metal is the Pidgeon Process. Ferro Silicon 75, due to its high silicon content and low impurity content, is the optimal reducing agent choice. The core rationale for its suitability is as follows:
A silicon content of 72%-80% ensures a complete reduction reaction, and its cost is lower than higher silicon grades such as FeSi90, balancing efficiency and economy;
The Pidgeon Process requires harsh conditions of 1150-1250℃ and a vacuum of 1-10 Pa. The chemical stability of FeSi 75% avoids side reactions, ensuring the purity of the magnesium product;
In industrial production, 1.2-1.3 tons of 75FeSi are consumed for every ton of magnesium metal produced, making it the "core of cost and efficiency" in magnesium smelting.
The Role and Reaction Mechanism of FeSi75 in Magnesium Smelting
(1) Core Reduction Reaction
Silicon in FerroSilicon 75 acts as the core reducing element, undergoing a displacement reaction with calcined dolomite (CaO・MgO) to generate gaseous magnesium and calcium silicate. The reaction equation is as follows:
Main Reaction: 2MgO・CaO + Si (from FerroSilicon 75%) → 2Mg↑ + Ca₂SiO₄ (Temperature 1150-1250℃, Vacuum 1-10Pa)
Thermodynamic Basis: The standard Gibbs free energy (ΔG°) of this reaction is -68kJ/mol at 1200℃, satisfying the conditions for a spontaneous reaction; the difference in electrode potential between silicon and magnesium (Si⁴⁺/Si = -0.86V, Mg²⁺/Mg = -2.37V) ensures the reduction priority.
(2) Step-by-Step Analysis of the Reaction Process
Raw Material Pretreatment: Dolomite (CaCO₃・MgCO₃) is calcined at 900-1000℃ to decompose into calcined white (CaO・MgO);
Material Packaging and Pressing: Calcined white and FeSi75 powder (size ≤1mm) are mixed at a mass ratio of 7:1, and a binder is added to press into pellets (diameter 20-30mm);
Vacuum Reduction: The pellets are loaded into a reduction tank and heated to... At 1150-1250℃, under vacuum of 1-10 Pa, silicon reduces MgO to generate Mg vapor.
Condensation and collection: The Mg vapor is cooled by a condenser at the top of the reduction tank, condensing into solid crude magnesium (purity ≥ 99.5%).
Refining and purification: The crude magnesium is refined with molten salt (e.g., a NaCl-KCl-MgCl₂ system) to remove trace impurities, yielding high-purity magnesium (≥ 99.9%).
Quality Requirements of FeSi75
The purity and impurity content of 75# FerroSilicon directly affect the recovery rate and purity of magnesium.
| Indicator Type | Requirement Scope | Impact of Exceeding the Standard |
| Silicon (Si) Content | 72%-80% | 72%: Incomplete reduction reaction, magnesium recovery rate decreases by 5%-8%; >80%: Increased cost, no additional efficiency improvement. |
| Aluminum (Al) content | ≤1.0% (preferably ≤0.5%) | >1.0%: Enhances the reactivity of FeSi75, excessively consumes reducing agent, and increases FeSi75 consumption per ton of magnesium by 5%-10%. |
| Carbon (C) content | ≤0.2% | ＞0.2%: MgC₂ byproduct is formed, causing magnesium purity to drop below 99%, affecting subsequent processing performance. |
| Sulfur (S) content | ≤0.05% | ＞0.05%: Forms MgS inclusions with magnesium, reducing the plasticity and corrosion resistance of magnesium. |
| Phosphorus (P) content | ≤0.04% | ＞0.04%: Induces brittleness in magnesium alloys; unsuitable for high-end magnesium alloys (such as aerospace magnesium alloys) |
| Size | ≤1mm (powder) | ＞1mm: Incomplete mixing with calcined white powder, resulting in incomplete reduction reaction in certain areas and fluctuations in reaction efficiency within the vessel of ±10%. |
Core Impacts of FeSi75 on Magnesium Production
(1) Impact on Magnesium Recovery Rate
When the silicon content of 75% FeSi is 75% and Al ≤ 0.5%, the magnesium recovery rate can reach 85%-90%;
When the silicon content drops to 70%, the recovery rate drops to 78%-82%; when the Al content rises to 1.5%, the recovery rate further drops to 75%-78%.
(2) Impact on Magnesium Purity
High-quality FeSi75# (total impurities ≤ 1.5%) can produce 99.7%-99.9% refined magnesium;
If the carbon content of FeSi75 exceeds the standard to 0.3%, the magnesium purity drops to 99.2%-99.5%, requiring additional refining costs.
(3) Impact on Production Costs
FerroSilicon75 accounts for 60%-70% of the raw material cost in magnesium smelting. A price fluctuation of 1000 RMB/ton corresponds to a change of 1200-1300 RMB in the cost per ton of magnesium.
Although the purchase price of low-impurity 75 FerroSilicon alloy (Al≤0.5%) is 5%-8% higher, it can reduce the consumption of reducing agent and refining costs, resulting in a 3%-5% reduction in the overall cost per ton of magnesium.
Optimization Strategies for the Application of FeSi75 in Magnesium Smelting
(1) Raw Material Selection and Quality Control
FeSi75-Al0.5 grade (Al≤0.5%) should be prioritized, avoiding the use of high-alumina and high-carbon grades.
Incoming Inspection: A spectrometer is used for rapid detection of core indicators such as Si, Al, and C. Small-scale reduction tests are conducted on samples from each batch to verify the reaction activity.
(2) Process Parameter Adaptation
Size: The particle size of FeSi75 powder is controlled at 0.1-1mm, and the mixing uniformity with calcined white powder is ≥95% to ensure sufficient reaction contact;
Reduction Conditions: The temperature is stabilized at 1200±20℃, and the vacuum degree is maintained below 5Pa to avoid incomplete reaction of FeSi75 due to temperature fluctuations.
(3) Impurity Control and Side Reaction Suppression
Carbon Suppression: 0.1%-0.2% limestone (CaCO₃) is added to the pellets to suppress the reaction of carbon with MgO to form MgC₂;
Refining Support: A dual process of "vacuum reduction + molten salt refining" is adopted to remove trace impurities introduced by FeSi75, ensuring that the magnesium purity meets the standards.
Industry Trends: Upgrading Directions of FeSi75 in Magnesium Smelting
Low Hybridization:
High-end magnesium alloys (for aerospace and electronics) have stricter requirements for the impurities in 75FeSi alloy, leading to increased demand for special FeSi alloy 75 with Al≤0.3% and C≤0.1%;
Greening:
Using FeSi75 alloy produced with green electricity can reduce carbon emissions from magnesium smelting by more than 50%, aligning with the global trend towards carbon neutrality;
High Efficiency:
Developing a "FeSi75 + rare earth" composite reducing agent can lower the reduction temperature by 50-80℃ and reduce electricity consumption per ton of magnesium by 800-1000kWh, improving production efficiency.
