Molten Salt Electrolysis

This is one possible way to get oxygen for Lunar mare basalt.

(CaCl2/Oxide Solid)

Operation

• Chemical reaction

European Space Research and Technology Centre (ESTEC)

ESTEC has built a prototype oxygen plant in Noordwijk, Netherlands [1]. This process was developed by Metalysis; however, in their process the oxygen is an unwanted by-product [1]. The process is based on the FCC process [3].

Their process:

• Use molten molten calcium chloride salt as an electrolyte [1].
• Crush regolith (or basalt) into powder [1].
• Heat to 950°C [1].
  • The regolith will remain solid at this temperature [1].
• Place regolith, contained in a metal basket, into the molten electrolyte at the cathode (carbon composite catalyst) [1][2].
• Pass a current through the electrolyte [1].
• Oxygen is extracted from the regolith, migrates across the electrolyte, and is collected at the anode (made of graphite) [1][2].
  • 15 hours to extract 75% of the total oxygen [3].
  • 50 hours to extract 96% of the total oxygen [3].
• The oxygen is then vented into the atmosphere in their current setup but eventually they will have it go into a storage system [1].
• A by-product of this process is a “tangle of different metals” [1].

Advantages

• Lower temperature and energy consumption because the regolith remains in a solid state instead of melting [2].
• Much of the oxygen can be extracted [3].
• No toxic by-products [2].
• Minimal processing. The feedstock is a powder and remains a powder at the end of the process [2].
• Silent [1].
• A by-product is a metal sponge that can be crushed, milled, washed, and dried into a variety of metal powders [2].
• Calcium chloride is considered pretty safe.

Disadvantages

• No cast basalt as a by-product (because the regolith isn’t melted).
• Calcium is pretty common but chlorine isn’t. That will have to be imported from Earth.
• I haven’t found data on if the CaCl2 is consumed in the process.

Parameter priorities

1. Able to be constructed/maintained locally
   1. UNKNOWN
2. Minimal reliance on Earth for reagents
   1. UNKNOWN
3. Operable in 1 SLHA
   1. UNKNOWN
4. Ease of maintenance
   1. UNKNOWN
5. Simple
   1. UNKNOWN
6. Durable
   1. UNKNOWN
7. Energy input
   1. Electrolyte heated to 950°C [1].
8. Heat output
   1. UNKNOWN
9. No hazardous materials are produced
   1. UNKNOWN
10. Efficiency
    1. UNKNOWN
11. Processes that extract more than just oxygen are preferred
    1. Process leaves behind a metal sponge from which a variety of metals can be obtained [2].
12. Weight/mass/volume
    1. UNKNOWN
13. Other uses for the reagents
    1. UNKNOWN

Resources

1. European Space Agency (ESA) – ESA opens oxygen plant – making air out of moondust. (www.esa.int/Enabling_Support/Space_Engineering_Technology/ESA_opens_oxygen_plant_making_air_out_of_moondust)
2. Metalysis (www.metalysis.com/technology/)
3. Phys.org – Oxygen and metal from lunar regolith. (phys.org/news/2019-10-oxygen-metal-lunar-regolith.html)

Need to read

• Lomax 2019. Planetary and Space Science Vol 180