3.1.2 MRE Electrolysis

Overview

Parameter Value
Input Silicates (~95% of non-magnetic fraction)
Output O₂, Fe/Si (ferrosilicon), Al/Mg slag
Throughput ~536 t/day
Energy consumption ~10 MW thermal (furnace) + ~108 MW electrical (electrolysis)

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Regolith Feed to Furnace

Between the flow splitter and the solar furnace is a vibrating feeder with dosing system:

  1. Regolith flows into a receiving hopper above the furnace
  2. Vibration drive ensures uniform feed rate
  3. Gate valve regulates feed speed
  4. Regolith drops into the furnace crucible through the loading port

Feed is batch-based: load 500 kg → melt → transfer to MRE cell → load next batch.

Material Flow

flowchart TD
    subgraph FURNACE["SOLAR FURNACE"]
        SUN["Sunlight beam"] --> CRUCIBLE["MgO Crucible"]
        CRUCIBLE --> MELT["Melt 1500°C"]
    end

    subgraph CELL["MRE CELL"]
        MELT --> ELECTROLYZER["Electrolyzer"]
        ELECTROLYZER --> ANODE["Ir Anodes"]
        ELECTROLYZER --> CATHODE["Cathode"]
        ANODE -->|"O₂ ↑"| CAP["Gas hood"]
        CATHODE -->|"Fe/Si ↓"| METAL["Metals ρ=6-8"]
        ELECTROLYZER -->|"floats up"| SLAG["Al/Mg slag ρ=2.5"]
    end

    subgraph OUTPUTS["PRODUCT SEPARATION"]
        CAP -->|"pipe"| COMP["Compressor"] --> TANK[/"O₂ 250t/day"/]
        METAL -->|"MHD valve"| CAST["Foundry"] --> FeSi[/"Fe/Si 35t/day"/]
        SLAG -->|"skimmer"| DIST["Distillation"]
    end

    subgraph DISTILL["SLAG DISTILLATION"]
        DIST --> Al[/"Al 42t"/] & Mg[/"Mg 48t"/] & Na[/"Na 18t"/] & K[/"K 3t"/]
    end

    style FURNACE fill:#fff3cd
    style CELL fill:#d4edda
    style OUTPUTS fill:#cce5ff
    style DISTILL fill:#f0e68c

Solar Furnace

Principle: Concentration of sunlight by mirrors into a single point. On Mercury, solar flux is ~9.3 kW/m² — ~7 times greater than on Earth.

Why not an electric furnace?

Electricity on Mercury is precious — it’s needed for electrolysis, motors, electronics. Sunlight is free and abundant. Mirrors focus 10 MW of thermal energy directly into the crucible, bypassing the light→electricity→heat conversion.

Parameter Value
Type Parabolic concentrator
Mirror area ~1000 m²
Focus temperature 1500–2000°C
Power ~10 MW (thermal)
Efficiency ~90% (no convection in vacuum)

Construction and Materials

An array of flat mirrors directs light to a parabolic reflector. The reflector focuses the beam into the crucible containing regolith.

Safety principle: Mirrors reflect light (operating T <100°C), they don’t absorb it. All thermal energy is concentrated in the MgO crucible¹ (operating T 1500–2000°C, melting point = 2852°C — margin of 852°C). The rest of the structure remains cold (<200°C).

¹ Magnite (MgO) is produced from local magnesium in regolith (slag distillation, 8% content, ~48 t/day).

Advantages of vacuum: - No convection — heat is not carried away by air - No oxidation — crucible doesn’t burn externally - No atmospheric absorption — all light reaches the target

Note: These are ground-based concentrator mirrors on Mercury’s surface, not to be confused with the orbital Dyson Swarm. The first mirror set (~1 t, Kapton) is delivered from Earth, then produced locally from aluminum. See Production.

Furnace role: The solar furnace only melts regolith. Electrolysis and oxygen collection occur in a separate enclosed MRE cell, where melt is transferred via MHD channel.

Molten Regolith Electrolysis (MRE)

Principle: Passing current through molten regolith. Metals are deposited at the cathode, oxygen is released at the anode.

Process Chemistry

Regolith at 1500°C is molten oxides: SiO₂, Al₂O₃, FeO, MgO, CaO. During electrolysis:

  • Cathode (−): Metals are reduced
    • Fe²⁺ + 2e⁻ → Fe (settles to bottom, density 7.8 g/cm³)
    • Si⁴⁺ + 4e⁻ → Si (mixes with Fe → ferrosilicon)
    • Al³⁺ + 3e⁻ → Al (floats, density 2.7 g/cm³)
    • Mg²⁺ + 2e⁻ → Mg (floats together with Al)
  • Anode (+): Oxygen is released
    • O²⁻ → ½O₂ + 2e⁻
Parameter Value
Temperature 1500°C (maintained by solar furnace)
Number of cells 60
Voltage 2–4 V
Current per cell ~600,000 A (600 kA)
Power per cell ~1.8 MW
Power (total) ~108 MW (electrical)
O₂ yield ~40% of regolith mass (~250 t/day)

Power justification (Faraday’s law): 250 t O₂/day = 7.8M mol O₂/day. Each O₂ requires 4 electrons → 31.3M mol e⁻/day. Total current: 31.3M × 96,485 C / 86,400 s ≈ 35 MA. At average voltage 3 V: 35 MA × 3 V = ~105 MW. Actual ~108 MW including electrode thermal losses. For comparison: Hall-Héroult aluminum electrolysis cells operate at 100–600 kA per cell — MRE cells are comparable in scale.

MRE Cell Construction

The MRE cell is an enclosed ceramic vessel, separate from the solar furnace:

Component Description
Shell Steel casing with MgO lining
Lid Ceramic, with ports for anodes
Anodes Iridium rods, immersed in melt from above
Cathode Cell bottom (conductive)
Gas hood Above melt, collects O₂
O₂ outlet Pipe to compressor

Oxygen Collection Principle

  1. Oxygen is released at anodes (O₂ bubbles rise through melt)
  2. Gas collects in hood above melt surface
  3. Pipe channels O₂ to cryogenic compressor
  4. Liquefied O₂ is stored in insulated tanks (−183°C)

Important: The solar furnace is open at the top for the solar beam and only melts regolith. Oxygen is released in the enclosed MRE cell, where melt is transferred via MHD channel.

Product Separation by Density

The melt in the cell spontaneously stratifies into layers of different density:

     ┌─────────────────────────┐
     │       Gas hood          │ ─→ O₂ (pipe to compressor)
     │  ~~~~~~~~~~~~~~~~~~~~   │
     │     Al/Mg (ρ=2.5)       │ ─→ Skimmer (ladle from top)
     │  - - - - - - - - - - -  │
     │    Silicate slag        │    (remains, periodically removed)
     │  - - - - - - - - - - -  │
     │     Fe/Si (ρ=6.0)       │ ─→ MHD valve (magnetic gate)
     └─────────────────────────┘
Output Method Frequency
O₂ Continuous pipe extraction Constant
Al/Mg Skimmer removes crust Every 30 min
Fe/Si MHD valve opens Every 2-4 hours

Reduction order (by Gibbs energy): Cr → Mn → Fe → Si → Ti → Al → Mg → Ca. Metals with low energy (Fe, Mn) are reduced first and sink. Metals on the right (Al, Mg) float into slag.

Note on anodes: Anodes are consumable — oxygen gradually oxidizes the material. Iridium anodes last ~6 months. This is the only component requiring regular import from Earth (mass ~10 kg/year per cell).

Product Separation

Slag Skimmer

Mechanical removal of Al/Mg crust from melt surface.

Component Material Mass Source
Ladle SiC (silicon carbide, melting point 2730°C) 8 kg Local
Lever arm Fe-6%Mn steel 20 kg Local
Actuator NaK hydraulics 12 kg Local
Accumulator Al₂O₃ ceramic 30 kg Local
Total 70 kg 100% local

Localization: Fully local production. SiC is synthesized by the Acheson process from local Si (MRE) and C (LRM zones). Ladle replacement — approximately every 2 years under intensive operation.

MHD Valve (Bottom Fe/Si Drain)

On an automated factory without personnel, an MHD valve is used — the same principle as an MHD pump, but in reverse.

Parameter Description
Principle Magnetic field holds conducting melt
Closed Field creates back-pressure — melt doesn’t flow
Open Field switched off — melt flows under gravity
Regulation Field strength = flow rate
Materials Al coils, MgO ceramic channel — all local

MHD valve advantages:

  • No moving parts in contact with melt
  • No wear — magnetic field doesn’t wear out
  • Precise flow control (from drops to stream)
  • Instant open/close
  • Works at 1500°C (coils are external, conductively cooled)

Iron Refining

Purification of Fe/Si from impurities before delivery to foundry.

Method Description
Vacuum degassing Gas bubbles capture non-metallic inclusions
Vacuum degassing Removal of dissolved gases (vacuum already exists)
Result Clean ferrosilicon for casting

Manganese Extraction

Source: MnO in silicates (0.1% of crust per MESSENGER).

Process: During MRE electrolysis, manganese is reduced at the cathode together with iron:

Parameter Value
Reaction MnO + 2e⁻ → Mn (metallic)
Mn density 7.4 g/cm³ (≈ Fe 7.8 g/cm³)
Behavior Sinks together with Fe/Si
Result Automatic iron alloying

Yield: ~600 kg Mn/day from 600 t regolith (0.1%).

Mn content in iron: ~6% (600 kg Mn in 10,000 kg Fe) — higher than in ordinary steel. This is low-alloy steel with improved mechanical properties.

Silicon Extraction

Source: Ferrosilicon (Fe + Si) — MRE electrolysis product. Silicon is reduced at the cathode together with iron and settles at the cell bottom.

Parameter Value
Si content in regolith ~20% (as SiO₂)
Si yield ~25 t/day (from 600 t regolith)
Product form Ferrosilicon (~15% Si, ~85% Fe)

Silicon Purification for Electronics

Solar panels and microchips require pure silicon. Two methods:

Method Description
Aluminothermy 3SiO₂ + 4Al → 3Si + 2Al₂O₃ (local materials)
Zone refining Purification to 99.9999% (for electronics)

Fiberglass: Produced on a separate line from silicates directly (before MRE electrolysis).

References

  • NASA MRE Research — Molten Regolith Electrolysis studies
  • ESA In-Situ Resource Utilization — Lunar regolith electrolysis research
  • Electrochemistry of Molten Oxides — Academic publications

See Also