§ 4.4.1 Electrolysis of Minerals
Electrolysis is a simple process using electric voltage and current to separate minerals into metals, oxygen, and any desired oxides. You melt the minerals, put in electrodes, and apply a voltage. The metals go to the negative electrode (cathode) and the oxygen goes to the positive electrode (anode). Electrolysis can also be used to split up a mineral into metal oxides rather than pure metal, e.g., for making special ceramics. Careful selection of electrode voltages, currents, temperature, and optional additives to the melt can tune the process to extract certain desired substances. Researchers have successfully electrolyzed a variety of synthetic lunar materials with no additives whatsoever, as well as by adding small quanitites of salts (e.g., calcium carbonate, sulfides). There are tradeoffs involved in various electrolysis system designs.
From the General Dynamics mission scenario: "Because of the effects of vacuum on the vapor pressures of metals at various temperatures, metals will be liberated at the cathode in solid, liquid and vapor form. Aluminum, calcium, sodium, potassium and manganese would be in vapor form, iron and silicon may be liquid or solid depending on bath temperature, while titanium will deposit on the cathode in solid form."
"[S]eparation [of the different metals] can also be achieved by vacuum distillation" whereby the resultant metal is heated to the different liquification or boiling temperatures of the different metals to extract each one separately.
Citing a Bureau of Mines research effort, "The electrolysis of lunar soil has been demonstrated using earth derived volcanic rocks to simulate lunar material. This study had the objective of extracting oxygen from lunar soil; and demonstrated that oxygen was evolved at the anode and free metals, including silicon, iron, aluminum and others, accumulated on the cathode."
Aluminum is commonly produced on Earth by electrolysis. About one-third to one-fourth the cost of aluminum is the cost of the electricity for electrolysis. This is why aluminum ore is often shipped by railroad to other states for processing into aluminum, where the electricity price is lower.
NASA/SSI-sponsored "vacuum distillation" experiments on simulated lunar soil is covered elsewhere in the PERMANENT industrial section for producing iron, silicon, and an aluminum-calcium oxide compound. The authors, EMEC Consultants, state that the aluminum and calcium are then readily extractable by electrolysis after dissolving the calcium aluminate in a CaF2-LiF molten salt electrolyte.