§ 4.4.3 Solar Oven Distillation
Vacuum distillation is simply boiling off materials using containerless processing in the zero gravity of orbit, or in a refractory container. A batch of material can be suspended in a giant solar oven and heated slowly, eventually reaching extremely high temperatures. Different materials liquify or boil off at different temperatures. After one element finishes boiling off, the temperature can be raised and the next element will be collected. The input material can be either raw minerals or the output from a previous industrial processing step.
The difference between smelting and distillation is that smelting liberates the desired material in liquid or solid form, whereas distillation liberates it in gaseous form.
One research project into processing simulated lunar material on Earth was performed by Rudolf Keller and David B. Stofesky of EMEC Consultants (Export, Pennsylvania) under sponsorship by SSI and NASA JSC, as reported in their paper "Selective Evaporation of Lunar Oxide Components" (with special thanks to Dr. Lee Valentine of SSI, Dr. David S. McKay of NASA JSC, and Prof. Larry Taylor of the Univ. of Tennessee at Knoxville for conducting product analyses by electron microprobe analysis).
Their aim was to gather data on separating the metal oxides as a first step towards separating the metals. From reading the paper, it appears that NASA JSC was funding the study out of interest in getting oxygen and iron, whereas SSI leveraged in some extra money for getting aluminium as a space construction metal.
The experiment was conducted on Earth by putting the material on an electrically heated strip within a high-vacuum jar, and heating the sample to a temperature between 1,200 and 2,000 degrees Celsius (2,200 to 3,600 degrees Fahrenheit), with the volatilized material collected from a target.
In experiments for NASA (seemingly not reported at the time of the above paper), iron oxide completely volatilized out at 1200C. (In turn, iron oxide can be separated into iron and oxygen relatively easily, e.g., electrolysis of the material volatized out from vacuum distillation.)
In the above paper reported at the SSI conference, they experimented with anorthite (CaSi2Al2O8), the most common lunar highlands mineral, with the goal of preparing calcium aluminate as a starting material for the electrolytic extraction of aluminium metal.
First, impurity iron oxide completely volatilized out at 1200C. After that process was completed, the temperature was raised to 1500C and higher, whereby silica (SiO2) also volatilized, as did magnesium oxide which was present in their sample, leaving behind a fairly pure calcium aluminate as the remaining product. The heating time at maximum temperature was typically just 5 minutes. Their experiment indicated that this "approach could be developed into a process to prepare a suitable, reasonably pure raw material for the production of aluminum metal". The resultant calcium aluminate can be readily separated into aluminium, calcium and oxygen by electrolysis after dissolving the calcium aluminate in a CaF2-LiF molten salt electrolyte.
The authors have collected data on volatilization vs. temperature and vacuum pressure for anorthite, anorthosite (with impurities), and MLS-1 (Minnesota Lunar Simulant), as well as silica and calcia. It's interesting to look at the effects of vacuum pressure on the ratios of volatilized products.
It is clear from this work that a containerless processing system in zero gravity with solar heat would be highly desirable over the electrically heated strip in the gravity at Earth's surface.
This process can be made so simple in space by an enterprising sort that surely it will be an integral part of space industry.