3.1.3 Slag Distillation

Overview

Parameter Value
Input Al/Mg slag from MRE cell
Input volume ~251 t/day
Metal output ~111 t/day (Al 42 + Mg 48 + Na 18 + K 3)
Final slag ~138 t/day
Energy consumption Induction furnace + condensers

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Slag Processing (Al/Mg/Na/K)

Slag from the MRE cell is a mixture of light metals that floated to the surface of the melt.

Source of Na and K

Feldspars are the main mineral in regolith (40-60%). During electrolysis, they decompose into constituents:

Mineral Formula Yields
Albite NaAlSi3O8 Na + Al + Si + O2
Orthoclase KAlSi3O8 K + Al + Si + O2

Separation occurs by vacuum distillation - each metal boils at its own temperature.

Slag Composition (600 t regolith/day)

Metal Quantity % of regolith Boiling point
Magnesium (Mg) ~48 t/day 8% 1091C
Aluminum (Al) 42 t/day 7% 2519C
Sodium (Na) ~18 t/day 3.3% 883C
Potassium (K) ~3 t/day 0.5% 759C

Separation Principle (Continuous Cascade)

Important: This is a continuous process, not batch. Slag flows continuously, all condensers operate simultaneously.

How It Works

  1. Slag continuously enters the induction furnace (1200C)
  2. At 1200C all volatile metals vaporize simultaneously - K, Na, Mg (with different intensity, proportional to vapor pressure)
  3. Mixed vapors rise into the condenser cascade
  4. Each condenser maintains its own temperature:
    • EQU-031 (700-759C): K condenses, Na+Mg vapors pass through
    • EQU-032 (800-883C): Na condenses, Mg vapors pass through
    • EQU-033 (1000-1091C): Mg condenses
  5. Al remains liquid in the furnace (Tb = 2519C) and is continuously drained

Analogy: This works like a rectification column in oil refining - but for metals. Different fractions condense at different heights in the column (= at different temperatures).

Why Vacuum Helps

In vacuum, boiling points decrease. At 10^-3 atm pressure: - Na boils at ~500C (instead of 883C) - Mg at ~700C (instead of 1091C) - This saves energy and increases the difference between condensation temperatures

Distillation Column Diagram

flowchart TD
    subgraph FURNACE["INDUCTION FURNACE"]
        SLAG[/"Slag Al+Mg+Na+K<br/>from MRE"/] --> IND["EQU-024<br/>Heating to 1200C"]
    end

    IND --> |"Vapors (all except Al)"| K_COND

    subgraph CONDENSERS["CONDENSER CASCADE"]
        K_COND["K Condenser<br/>EQU-031<br/>700-759C"] --> |"Na+Mg vapors"| NA_COND
        NA_COND["Na Condenser<br/>EQU-032<br/>800-883C"] --> |"Mg vapors"| MG_COND
        MG_COND["Mg Condenser<br/>EQU-033<br/>1000-1091C"]
    end

    K_COND --> K_OUT[/"K liquid<br/>to NaK alloy"/]
    NA_COND --> NA_OUT[/"Na liquid<br/>to NaS batteries"/]
    MG_COND --> MG_OUT[/"Mg liquid<br/>to Al-Mg alloys"/]
    IND --> |"Residue"| AL_OUT[/"Al liquid<br/>to mirrors, windings"/]

    style FURNACE fill:#fff3cd
    style CONDENSERS fill:#d4edda
    style K_OUT fill:#cce5ff
    style NA_OUT fill:#cce5ff
    style MG_OUT fill:#cce5ff
    style AL_OUT fill:#cce5ff


DISTILLATION COLUMN (slag processing section)

+-------------+
| Induction   |    Slag Al+Mg+Na+K from MRE
| furnace     |<---------------------------
| EQU-024     |
+------+------+
       | Vapors (all metals except Al)
       v
+-------------+
| Potassium   |    K condenses (Tb=759C)
| condenser   |---> K liquid -> NaK alloy
| EQU-031     |
+------+------+
       | Vapors (Na+Mg)
       v
+-------------+
| Sodium      |    Na condenses (Tb=883C)
| condenser   |---> Na liquid -> NaS batteries
| EQU-032     |
+------+------+
       | Vapors (Mg)
       v
+-------------+
| Magnesium   |    Mg condenses (Tb=1091C)
| condenser   |---> Mg liquid -> Al-Mg alloys
| EQU-033     |
+-------------+

       | Residue in furnace
       v
+-------------+
| Aluminum    |    Al does not evaporate (Tb=2519C)
| liquid      |---> Al -> mirrors, windings, housings
+-------------+

Equipment

Induction Furnace (EQU-024)

Heats slag to 1200°C, MgO ceramic lining.

Potassium Condenser (EQU-031)

Operating temperature 700–759°C. Cooling: radiator (radiation to space).

Sodium Condenser (EQU-032)

Operating temperature 800–883°C. Similar to K, but larger (Na is 7× more than K).

Magnesium Condenser (EQU-033)

Operating temperature 1000–1091°C. Largest unit (Mg — 48 t/day).

Alloys and Flow Table

Alloy Composition Source 1 Source 2 Application
Pure Al 100% Al Ind. furnace (residue) - Mirrors, foil
Al-Mg (housings) 90% Al + 10% Mg Ind. furnace Mg cond. Lightweight structures
NaK (coolant) 78% K + 22% Na K cond. Na cond. Cooling
Fe-Mn (steel) 94% Fe + 6% Mn MRE cell - Frames, rollers
Pure Si 100% Si Zone melting - Electronics

Storage After Condensation

Metal condenses in liquid state and flows into heated accumulator crucibles. There it is maintained liquid until transfer to casting/forming.

Metal Melting point Storage temp Heating type
K 63C >70C Electric (low T)
Na 98C >110C Electric (low T)
Mg 650C >700C Induction
Al 660C >700C Induction

Why induction heating: - Contactless - no heater wear - Uniform heating of entire volume - Easy to regulate - Works with any metals

Accumulator crucibles: - Material: MgO ceramics - local production (withstands all metals) - Capacity: 1-2 t (buffer for several hours) - Drain: bottom stopper or MHD pump

Note: K and Na are stored in sealed containers — they react with oxygen (dome atmosphere is O₂). Double walls + leak detectors.

Product Output

Product Form Application
Aluminum (Al) Liquid -> ingots, wire Mirrors, housings, motor windings
Magnesium (Mg) Liquid -> ingots Lightweight alloys (Al-Mg), MgO ceramics
Sodium (Na) Liquid -> sealed ampules NaS battery anode
Potassium (K) Liquid -> sealed ampules NaK coolant
NaK alloy Liquid metal (stored liquid) Electronics cooling

NaK eutectic: Alloy of 78% K + 22% Na - liquid at -12C. Ideal coolant for space: does not freeze, high thermal conductivity, requires no pressure.

Sulfur Distillation

Obtaining sulfur for NaS batteries.

Parameter Value
Source Sulfides in regolith (troilite FeS)
Method Vacuum sublimation at 450C
Condenser Cold surface (shadow), sulfur deposits as yellow crystals
Remelting Liquid sulfur (>115C) -> to battery workshop

Note: Sulfur is extracted on the titanium line - troilite (FeS) is part of the magnetic fraction.

Sources

  • Vacuum Distillation of Metals - Industrial metal separation methods
  • Alcoa, Rio Tinto - Aluminum producers

See Also