Iron ores exists in multiple forms: iron oxides, iron sulphides, dissolved iron in water and nickel-iron metal.
For a world without oxygen, it will depend on whether there were any water on this world.
For native Nickel-iron which is found on most celestial bodies that does not have any atmosphere nor water (deposits will come from iron-rich meteors, which will not be oxidized, and is available even with total planetary differentiation that sank any primordial iron into the core), all you need is a magnet(you won’t even need magnets if your world is of class M, which means there was a high content of metallic iron, as just sawing off a part of the ground will yield malleable metal) to collect the granules of iron from the regolith.
For iron oxides (which is often found co-located with silicates), you have two choice:
if The world have water in the form of ices or a liquid ocean(as for prebiotic earth), then you could probably dissolve your rocks using an acid(depends on what other elements are found, acids are generally made from the respecting salt/salty materials using diaphragm electrolysis from the anolyte. hydrochloric or sulfuric acids may be used, as these ions are often found in ices, volatiles and prebiotic oceans. The resulting solution is then electrolyzed in a diaphragm cell again, depositing iron on the cathode and regenerating the acids (along with oxygen) in the anolyte. The water and acids in this process is purely catalytic, as it is regenerated in the reaction.
For worlds without any water deposits, if you can’t import any hydrogen/water as a catalyst, the Thermite reaction will be extremely useful when it comes to refining iron. First, most rocks on the surface of many planets will generally be a mixture of alkaline metal oxide, silicon dioxide and aluminum oxide, with the occasional calcium thrown in. This composition generally melts at 1200 degrees C maximum, and depends on the exact rock composition, can either have viscosity as low as cooking oil or as high as taffy candies. Electrolysis of the molten rock using platinum electrodes(as they don’t wear out) yields alkaline metals at the cathode and oxygen gas at the anode, alone with silicate glass. Divalent cations like magnesium, calcium and iron will also be deposited. The high temperatures will force these metals to vaporize, and this mixture of metals will be used to reduce any oxide ores to the metal. 208
Depends on the rock type, the electrolyte will start to thicken as the reaction progresses, and should be replaced when the conductivity of the molten rock have dropped below economical values.
Any iron in this molten rock can be easily harvested, as it will plate out on the cathode first, and the mixture of reducing metals can now be used to reduce the actual iron oxide ore using high temperatures. The trick here, is to use an excess of the reducing metal mixture, so the excess silicates inside the ore is being turned into silicon. The iron then drops out as a molten iron-silicon alloy, which can be purified using oxygen from the rock electrolysis cells, leaving behind silica glass. The slag is primarily sodium and calcium silicate, which can be electrolyzed to recover most of the sodium-calcium metal. Any aluminates would have been converted into aluminum metal before the alkali metal silicates have became irreducible due to being thermodynamically stable, and drops out of the solution as a phase-separated layer from the iron metal.
As This process removes silica from the regolith and iron oxide feedstock, in theory we could separate all the iron, silicon, alkali metals, calcium and oxygen from any given piece of rock. The alkali metal rich slags of the reaction can be electrolyzed, which recovers most of the mobile cations, and the residual silica-alumina reduced to elemental silicon using aluminum metal. The alumina product is then reduced using sodium, regenerating aluminum and oxygen. We have achieved total separation of the rock feedstock, and refined iron from the process.
For the iron sulfides, we can employ thermal separation followed by, depends on whether hydrogen is available, wet chemical or Thermochemical methods to refine the Ferris sulfide product. Pyrite breaks down into ferrous sulfide and elemental sulfur at above 500 degrees C, and the sulfur will start to distill over at 700 degrees C. Vacuum distillation can therefore be employed to reduce all the iron and other metal sulfides to the monosulphide variety.
The wet chemical method of separation uses an acid to dissolve the ferrous sulfide, generating hydrogen sulfide gas and an iron salt. 561