Iron is found in three key oxides: ferric oxide or wuestite (FeO), ferrous oxide or hematite (Fe2O3), and ferroferric oxide or magnetite (Fe3O4). Magnetite is also called lodestone, which was the first magnetic mineral to be discovered. The two next most important terrestrial ores, goethite (HFeCO3) and siderite (FeCO3), are not found on the Moon (despite certain contaminated lunar samples). As the lunar surface is underoxidised, however, significant quantities of metallic iron exist, which is quite rare on Earth.
More information about hematite, magnetite, and many other lunar minerals is found in the M.5.2. Lunar Minerals: Oxides chapter of the Artemis Data Book.
Firstly, iron is quite magnetic. Ores can be located by magnetic sensing akin to backyard metal detectors, and possibly even harvested, unoxidised iron at least, with simple electromagnets. Iron is often almost linearly linked in concentration to many other lunar elements and minerals, so magnetic detection of iron can be useful in pretty reliably identifying other ores.
Iron compounds also exhibit the Mossbauer effect, where high-energy photons (ie, gamma rays) are absorbed and reradiated by the atomic nucleus without recoil. While this effect has been observed in about one-third of elements, iron-57 can be excited by gamma radiation of a specific frequency which depends on its oxidation state, electron configuration, and chemical environment. It is conceivable that a gamma-ray sensor could be used to examine samples or regions at specific frequencies and measure re-emissions for iron concentrations, although this possibility requires further research.
Iron compounds are used as a colourant in glasses, which look to have a tremendous importance in lunar construction, manufacturing, and export. Peter Kokh has suggested artificial colouring as a way to perk up the dull lunar landscape. Ferrous oxide absorbs infrared radiation, which can be used to block in heat for industrial processes, or to narrow the frequencies of light which can enter into agricultural greenhouses through the very thick panes of glass required.
Ferrosilicon alloys of various compositions are used to make flexible items such as springs, and electrical transformer cores. FeSi, and iron in general, is very useful in reducing lunar minerals to extract many other metals such as cesium, mangesium, and the like.
A favourite method for extraction of most lunar elements of interest is heating in the presence of hydrogen. A process of hydorgen illemite reduction is being considered for a lunar oxygen production pilot plant. Metalic iron is among these elements easily extracted using a mildly endothermic hydrogen reduction process:
Fe2O3 + H2 = 2 Fe3O4 + H2O (1)
Fe3O4 + H2 = 3 FeO + H2O
FeO + H2 = Fe + H2O
The water can then be electrolysised to recover the hydrogen.
2 H20 = 2 H2 + O2
This process is really easy, although the electrolysis of oxygen is relatively energy-intensive. Aerospace electrolysis equipment is very compact and efficient, and is a very well-developed process using layers of electrolyte-impregnated plastic sandwiched between metal meshes.
It is also possible to use a carbon monoxide reduction process, which is the essence of the process in a terrestrial blast furnace, which uses coke to produce the carbon monoxide. On the Moon, on the other hand, many processes such as silicon carbide aluminum reduction will yield large quantities of carbon monoxide. The carbon monoxide is then used to reduce the ore as follows:
3 Fe2O3 + CO = 2 Fe3O4 + CO
2 Fe3O4 + 2 CO = 6 FeO + 2 CO2
FeO + CO = Fe + CO2
Blast furnaces on Earth also include calcium carbonate, which forms calcium oxide to react impurities such as silica and sulphur into slag for easy removal.
The waste CO2 evolved from the reduction of iron now must be recycled.
The carbon dioxide can be partly dissociated into carbon monoxide and oxygen by heating it to 1,100 C with solar concentrators. However, the propensity for these gasses to recombine rules out simply cooling them with radiators and separating them as the liquids form at different temperautes. Instead, it is possible to electrochemically pump the oxygen across a zirconia ceramic membrane by applying a voltage, leaving a mixture of CO2 and CO which can be separated by fractional distillation.
However, this process has its problems. It probably has to be done in batches rather than as a continuous process. The zirconia tubes are brittle and slow, so large numbers are needed. But the big problem is that they consume five times as much power as hydrogen electrolysis does for each kilogram of oxygen liberated from the iron.
Carbon monoxide can also be produced by running the water-gas shift reaction in reverse. Using an iron-chrome catalyst and solar concentrators to heat the apparatus to 400 C, the following reaction takes place:
CO2 + H2 = CO + H2O
It is mildly endothermic, but then we've got unlimited solar energy at our disposal. Continuous removal of the water is necessary to upset the equilibrium and keep the reaction moving along. The water can then be electrolysised.
It is possible to reduce iron oxides to yield oxygen and metallic iron using either hydrogen or carbon monoxide, where the oxygen is then transferred to hydrogen for easy electrolysis. While the hydrogen process removes the intermediary steps involving carbon, both justify consideration if either can be more easily integrated with extraction of other elements, as in the case of production of waste carbon monoxide as in silicon carbide aluminum reduction.
Comprehensive Inorganic Chemistry
McGraw-Hill Encyclopedia of Science and Technology
Lunar Sourcebook: a user's guide to the moon