As the lightest metal structural material, magnesium demand
continues to rise in aerospace, automotive lightweight, 3C
electronics and other fields. 95% of the world's magnesium
metal is extracted through smelting, and the mainstream
process includes two major technical routes: electrolysis and
thermal reduction (Pijiang method)**. This article will be in-depth
dismantling of magnesium extraction of the underlying logic,
technical difficulties and industry trends, to provide
practitioners with selection reference.
Magnesium resource distribution and
extraction logic
Magnesium is abundant in the earth's crust (about 2.3%), mainly
in dolomite (MgCO₃-CaCO₃), magnesite (MgCO₃), carnallite
(KCl-MgCl₂-6H₂O) and seawater (containing magnesium 0.13%).
The choice of extraction process depends on the characteristics
of the raw material:
Electrolysis: suitable for magnesium chloride (MgCl₂) feedstock,
such as seawater/salt lake magnesium extraction;
Thermal reduction method: applicable to magnesium oxide (MgO)
raw materials, such as dolomite calcination products.
Electrolysis: efficient purification under
chlorine salt system
1. Core process
Raw material pretreatment:
Seawater / salt lake brine by lime milk precipitation, hydrochloric
acid acidification, to produce anhydrous magnesium chloride
(MgCl₂ content > 98%);
Or magnesite chlorination roasting: MgCO₃ + Cl₂ → MgCl₂ +
CO₂↑ (800-1000℃).
Electrolyzer reaction:
The multi-pole tank is energized with direct current (tank voltage
5-7V, current efficiency 85%-90%):
Cathode: Mg²⁺ + 2e- → Mg (liquid)
Anode: 2Cl- → Cl₂↑ + 2e-
Liquid magnesium density is lower than the electrolyte (MgCl₂-KCl-NaCl
molten salt), floating up to the magnesium collection chamber, and
siphoning ingot casting periodically.
2. Technical advantages
Continuous production: daily output of single tank can reach 1-1.5
tons, suitable for large-scale operation;
Controllable purity: the purity of cathode magnesium reaches above
99.95%, which can be directly used in high-end alloy;
Chlorine cycle: anode by-product Cl₂ is reused for raw material
chlorination, forming a closed loop.
3. Pain points and breakthroughs
Energy consumption bottleneck: electricity consumption of about
13,000-15,000 kWh per ton of magnesium, accounting for more
than 50% of the production cost;
Equipment corrosion: high-temperature chlorine gas erodes the
electrode and tank body, and the life of titanium alloy cathode
is only 2-3 years;
Technology upgrade:
Optimization of multi-pole tank structure, current efficiency
increased to 92%;
Inert anode (e.g. SnO₂-based ceramics) reduces chlorine
gas emission by 30%.
Thermal reduction: the classic route for
MgO reduction from ferrosilicon
1. Pidgeon Process (Pidgeon Process) full process disassembly
Calcination of magnesium oxide:
Dolomite (MgCO₃-CaCO₃) is calcined in a vertical kiln (1200℃)
and decomposed into CaO-MgO;
After crushing and sieving, it is mixed with ferrosilicon (75% Si)
and fluorite (CaF₂ flux) in proportion to the compressed dough.
Vacuum reduction furnace reaction:
Load into a heat-resistant steel tank, vacuum to 10-50 Pa,
heat to 1200-1250°C:
2MgO + Si(Fe) → 2Mg↑ + 2CaO-SiO₂ (slag)
Magnesium vapor is crystallized in the condenser (purity
99.8%-99.9%), and the residue can be recycled as
calcium-silicon fertilizer.
2. Core competitiveness
Low investment in equipment: investment in single furnace is
only 1/3 of electrolysis method, suitable for small and
medium capacity;
Strong adaptability of raw materials: low-grade dolomite
(MgO>18%) can be directly processed;
Value-added by-products: each ton of magnesium produces
4-5 tons of calcium and silicon slag, which can be used for
building materials or soil improvement.
3. Development constraints
Intermittent production: single furnace cycle 12-14 hours,
energy consumption 110-130GJ per ton of magnesium;
Ferrosilicon dependence: ton of magnesium consumes 1.1-1.2
tons of ferrosilicon, the cost is affected by the fluctuation
of silicon price;
Environmental protection pressure: calcination link CO₂
emissions up to 5-6 tons / ton of magnesium.
Technical innovation and future trends
1. Low-carbon smelting technology breakthrough
Green power electrolysis: photovoltaic/wind power direct supply
electrolyzer, carbon emission per ton of magnesium can be
reduced to less than 3 tons;
Carbothermal reduction method: coke instead of ferrosilicon,
reaction formula MgO + C → Mg↑ + CO↑ (test stage);
CO₂ mineralization sequestration: Calcination exhaust gas is
injected into calcium-based slag to generate carbonate,
realizing negative emission.
2. Diversification of raw material routes
Waste magnesium alloy regeneration: vacuum distillation
method is adopted to recover waste magnesium, with
energy consumption only 5% of virgin magnesium;
Direct extraction of magnesium from seawater: new ion
sieve adsorbent (e.g. MnO₂-nH₂O) selective extraction of
magnesium ions;
Low-grade ore utilization: microbial leaching technology
to treat tailings with MgO <15%.
3. Intelligent upgrading
Digital twin system real-time optimization of electrolyzer
temperature field and current distribution;
AI predicts the reduction furnace slag phase composition
and dynamically adjusts the ferrosilicon ratio;
Robot automatic slag picking and ingot casting, reducing
manual contact with high temperature environment.
Conclusion
The evolution of magnesium extraction technology is essentially
a game of “efficiency” and “sustainability”. Electrolysis has
dominated the market with its potential for cleanliness, while
thermal reduction has held on to specific markets with its
flexibility. With the maturity of hydrogen metallurgy and carbon
capture technology, magnesium smelting will break through
the shackles of energy consumption and emissions and become
a benchmark for green metal materials. For enterprises, based
on resource endowment, close to the policy-oriented process
selection, will be the key to win the market competition.