3.1.7 MgO Ceramics

TL;DR

  • Purpose: Production of refractory ceramics from magnesium
  • Input: ~44 t/day liquid Mg from distillation + ~29 t/day O2 from MRE
  • Output: ~66 t/day MgO ceramics (88% yield)
  • Applications: Crucibles, furnace linings, thermal insulation
  • Note: Remaining ~4 t/day Mg → Al-Mg alloys (see Distillation)

Overview

Magnesium oxide (MgO) ceramics are a critically important refractory material for all high-temperature processes in the factory. With a melting point of 2852C, MgO withstands the extreme conditions of MRE electrolysis, distillation, and metal casting.

Parameter Value
Input Mg ~44 t/day from distillation (remaining ~4 t → Al-Mg alloys)
Input O2 ~29 t/day from MRE electrolysis
Output MgO ceramic blocks/powder (~66 t/day)
Reaction yield 88% (losses from dust, rejects, cracks)
Power consumption ~150 kW (oxidation + sintering)

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Process Diagram

flowchart TD
    subgraph INPUT["INPUT"]
        MG["Liquid Mg<br/>1091C<br/>~44 t/day"]
        O2["O2 from MRE<br/>32 t/day"]
    end

    subgraph OXIDATION["OXIDATION"]
        OX_FURNACE["Oxidation furnace<br/>600-1000C"]
        REACTION["2Mg + O2 -> 2MgO"]
        MGO_POWDER["MgO powder<br/>10-100 um"]
    end

    subgraph FORMING["FORMING"]
        GRANULATE["Granulation"]
        PRESS["Pressing<br/>50-200 MPa"]
        GREEN["Green bodies"]
    end

    subgraph SINTERING["SINTERING"]
        SOLAR["Solar furnace<br/>1500-1800C<br/>2-6 hours"]
        CERAMIC["MgO ceramics<br/>~70 t/day"]
    end

    MG --> OX_FURNACE
    O2 --> OX_FURNACE
    OX_FURNACE --> REACTION
    REACTION --> MGO_POWDER
    MGO_POWDER --> GRANULATE
    GRANULATE --> PRESS
    PRESS --> GREEN
    GREEN --> SOLAR
    SOLAR --> CERAMIC

    style INPUT fill:#fff3cd
    style OXIDATION fill:#ffe4cc
    style FORMING fill:#d4edda
    style SINTERING fill:#cce5ff

Stage 1: Magnesium Oxidation

Reaction: 2Mg + O2 -> 2MgO + heat (deltaH = -1203 kJ/mol)

Liquid magnesium from distillation (1091C) is fed into the oxidation furnace where it contacts oxygen from MRE electrolysis.

Parameter Value
Reaction temperature 600-1000C
Incoming Mg ~44 t/day = 1,811 kmol/day
Required O2 ~29 t/day = 906 kmol/day
Outgoing MgO (theoretical) ~73 t/day
Reaction character Exothermic (releases heat)

Equipment

Component Material Purpose
Furnace shell Fe (steel) Structure, cooling
Lining Al2O3 ceramics Protection from high temperatures
O2 injectors SiC ceramics Oxygen delivery to reaction zone
Heat exchanger Al alloy Heat recovery from reaction

Oxygen source: MRE electrolysis produces ~250 t O2/day when decomposing regolith oxides (SiO2, FeO, Al2O3). For MgO ~29 t/day is needed – the excess is used for dome atmosphere and other purposes.

Stage 2: Granulation and Forming

After oxidation, MgO powder is obtained which must be converted into strong ceramic products.

Granulation

Parameter Value
Method Melt atomization in vacuum
Particle size 10-100 um
Goal Obtain uniform powder for pressing

Forming (Pressing)

Parameter Value
Pressure 50-200 MPa
Method Isostatic pressing (uniform)
Product Green bodies
Equipment Hydraulic press (frame: Fe, dies: SiC)

Stage 3: Sintering in Solar Furnace

Sintering is thermal treatment where a powder compact transforms into dense ceramics without melting.

Parameter Value
Temperature 1500-1800C
Time 2-6 hours
Atmosphere Vacuum (natural Mercury environment)
Heat source Solar furnace (mirror concentration)

Sintering Solar Furnace

Component Material Function
Concentrator mirrors Al Focus sunlight
Crucible MgO (in-house production) Hold workpieces
Thermal insulation MgO felt Reduce heat losses
Vacuum chamber Fe shell + Al2O3 insulation Sintering environment

Sintering results: - Density increases from 50-60% to 90-95% of theoretical - Compressive strength: 100-200 MPa - Thermal shock resistance: excellent (deltaT > 500C)

Material Balance

Reaction: 2Mg + O2 -> 2MgO

Stoichiometric calculation:

Component In/Out Quantity Molar mass kmol/day
Mg (input) -> ~44 t/day 24.3 kg/kmol 1,811
O2 (input) -> ~29 t/day 32 kg/kmol 906
MgO (theoretical) <- ~73 t/day 40.3 kg/kmol 1,811
MgO (actual) <- ~66 t/day 88% yield 1,594

Losses (12%)

Loss source Share Note
Dust carryover during granulation ~5% Collected by filters, returned to process
Forming rejects ~3% Re-formed or re-melted
Cracks during sintering ~4% Thermal shock, non-uniformity

MgO Ceramics Applications

Product Consumption (t/day) Criticality Description
MRE crucibles ~20 Critical Main vessel for melt in MRE furnace (Tmelt 2852C >> Twork 1500-2000C)
Distillation condensers ~10 Critical Ceramic surfaces for metal condensation (K, Na, Mg, Al)
Furnace thermal insulation ~25 High MgO felt/blocks for insulating solar furnaces, MRE, distillation
MHD channels ~5 High Channel lining for transporting molten metals (Al, Fe)
Tundish lining ~5 High Refractory protection of tundish for casting
Reserve/buffer ~5 – Reserve for failures or production increase
TOTAL ~70

Power Consumption

Stage Power Purpose
Oxidation furnace 50 kW Maintain temperature 600-1000C
Granulation 10 kW Atomization and cooling
Forming press 20 kW Hydraulic system
Solar furnace (electrical) 30 kW Mirror pointing, vacuum pumps
Thermal energy (solar) ~40 kW thermal Sintering at 1500-1800C
Transportation 10 kW Conveyors, loaders
TOTAL (electrical) ~120 kW
TOTAL (including solar) ~160 kW equivalent

Note: The solar furnace uses concentrated sunlight for sintering, reducing electrical consumption. Electricity is only needed for mirror pointing and vacuum pumping.

MgO Ceramics Properties

Property Value Comparison with other materials
Melting point 2852C Higher than Al2O3 (2072C), lower than ThO2 (3350C, but toxic)
Density 3.58 g/cm3 Lighter than Al2O3 (3.95 g/cm3)
Thermal conductivity 40-60 W/(m*K) High -> good heat conduction from melt zone; in felt/fiber form -> thermal insulation
Thermal expansion coefficient 13.5 x 10^-6 /K Medium -> thermal shock resistance
Chemical resistance Excellent to basic slags Reacts with acidic slags (SiO2)
Electrical insulation 10^14 Ohm*m Excellent insulator

Quality Control

Parameter Control method Criterion
Powder particle size Laser diffraction 10-100 um (+/-10%)
Compact density Weighing + volume >= 90% of theoretical
Absence of cracks Visual inspection + ultrasound No visible defects
Thermal resistance Thermal shock test (deltaT 500C) No destruction
Chemical composition X-ray diffraction (XRD) > 98% MgO

Automation: The furnace, press, and sintering control system operates automatically. An operator is only required for maintenance and quality control.

Manufacturing Specifics on Mercury

Advantages

  1. Natural vacuum – no oxidation of impurities, no protective atmosphere required
  2. Oxygen surplus – MRE produces ~250 t O2/day, we use ~29 t for MgO
  3. Free heat – solar furnace runs on concentrated sunlight (1 MW/m2 at surface)
  4. Magnesium in regolith – 8% content, distillation yields 48 t/day pure Mg (~44 t → MgO, ~4 t → Al-Mg alloys)

Challenges

  1. Dust in vacuum – during MgO granulation dust does not settle, electrostatic filters required
  2. Thermal shock – rapid temperature swings (day/night: +430C/-180C) require slow ceramic cooling
  3. Fragile green bodies – pressed compacts before sintering are very fragile -> careful transportation required

Sources

  • RHI Magnesita – world’s largest refractory producer, including MgO (blast furnaces, steelmaking converters)
  • Magnesia Mineracao – mining and MgO production from magnesite and seawater
  • NASA Technical Reports – ceramics production in vacuum and microgravity
  • NIST Chemistry WebBook – thermodynamic data for Mg + O2 reaction

See Also