L5 satellite workshops.

>>>What you could do is melt the metal at the centre of a light
spherical form and after a few months or even years a thick layer of
metal will have deposited itself to form the shell of a spherical
habitat.>>>

This will certainly come in handy, when the time comes to produce larger
workshops. If we wanted to build much larger SPS satellites (or space hotels for
the LEO market), then a much larger workshop with a larger assembly hall, would
be required. The mass of a pressurised vessel is proportional to the cube of its
radius. Where as a 50m diameter pressure sphere might weigh in at 20 tonnes, a
200m diameter sphere, would weigh 1280 tonnes. Interestingly enough, the volume
of the sphere is also proportional to the cube of its radius. So for a Zubrin
sphere, regardless of the size of the sphere, we must ship up from Earth, 0.31
Kg of kevlar or vectran for each cubic meter of workspace space that we create.
Once our operation has been under way for sometime, we might be able to afford
to import 200m spheres from Earth. However, it will always be undesirable to do
so. One option for the production of such a sphere, would be to use the 'waste'
iron, from the oxygen production process. Most (but certainly not all) theorised
oxygen production processes, on the surface of the moon, produce O2 through the
reduction of iron(ii)oxide. This is present within the mineral Illmenite,
FeTiO3. The reduction process is quite simple:

H2(g) + FeTiO3 = H2O(g) + TiO2(s) + Fe(s)

or, CO(g) + FeTiO3 = CO2(g) + TiO2(s) + Fe(s)

or, CH4(g) + FeTiO3 = CO2(g) + 2H2O(g) + 3TiO2(s) + 3Fe(s)

The H2O and/or CO2, is then electrolysed to produce O2 gas and H2 and CO
gas. The O2 is then liquefied and sold to customers in low earth orbit, where it
has high value. The Rutile is either converted into Titanium metal and used as
an SPS construction material, or it is used as a substrate for solar cells. What
about the iron? Far more iron is produced, than is actually likely to be needed.
In fact, for every tonne of LOX produced, 3.5 tonnes of iron are produced. An
ideal use for this iron, would be the vacuum formation of a habitat, as you
suggest. It would have to be purified first though. Even small amounts of
impurities would have the tendency of making the sphere brittle and thus prone
to catastrophic failure. Does anyone know how this could be done? Maybe the
carbonyl process?

When I was about 7 years old, I can remember my grandfather building a vacuum
plating machine. The interior of the machine was simply a hollow, stainless
steel sphere, with some electric heating elements in its centre. Most of the
cost of the machine was related to the cost of the vacuum pumps. My grandfather
would simply wrap Aluminium foil around the elements, place whatever he wanted
plated within the chamber, and switch on the pumps. When the chamber was
evacuated, he would switch the heating elements on. I remember on one occasion,
the vacuum pump failed to completely evacuate the chamber. The work piece was
therefor plated with aluminium oxide.