Tuesday, February 2, 2016

Interorbital Exchange - part 4, mining metals

Continuing the series, this is a look at what to do next after water mining becomes routine and a network of fuel transfers is available. This post assumes that Lunar water mining is operational, but that material from Mars is not guaranteed.

 While harvesting ice is fairly straightforward and something I expect we can automate, surveying mineral resources for efficient mining is altogether different. It would still be possible to do without humans on-site but I believe sending experts would be more effective. A program of manned exploration alongside current state of the art automation would expand our knowledge of the Moon and our experience with autonomous mining. Still, the general program I will describe could be done with or without people on site.
 Actually making use of these materials will require human hands. We do not have the automation technology available to perform complex assembly, particularly in a challenging environment. Individual steps will be automated as much as feasible, but in the near term most manufacturing processes will require a crew.

Details after the jump.

 Operations would be based at the polar water processing center. An abundant supply of fuel and power is available here along with frequent visits by fuel tugs. Hardware from Earth would be delivered to LEO, boosted to EML1 by a cargo tug and then relayed to the pole by another cargo tug. Some types of processing hardware would be left here, to become part of the resource processing infrastructure. Other types would be used on location to preprocess and reduce the mass that needs to be returned to base. Details depend on what specific hardware and process is being used.
 Travel would be by cargo tug, using suborbital hop flights. Surface transport (rover or moon buggy) might reach 1-2° (30-60km) from the pole, but the terrain is rugged. It would be possible to cut a rudimentary road to allow for safer ground travel if a valuable deposit was found. Until then, exploration would be done by hopping to a target site, mapping / probing the area, taking samples, then hopping back to base. Travel to points within 23° (698km) of base would require only 2km/s round-trip. An 83° (2518km) range would require 3km/s, while access to the opposite pole (5461km) would require 3.4km/s. The reference tug can deliver 29 tons to EML1 or could carry 20 tons to any site on the surface and 60 tons back for 63 tons of fuel. For sites within 700km of the pole that same 20-ton exploration package could bring back 80 tons of ore for 46 tons of fuel.

 The easiest initial harvest would be to run magnetic rakes through the regolith and collect iron nodules. The result would be bags of nearly pure nickel-iron grains. This should be done in the immediate area of the pole first to build up a stock of metals with minimal processing requirements. This can be done fairly easily by automated rovers and should be pursued while other exploration activities occur. Even if no other mining is performed, the material collected with this approach would yield iron and nickel as structural materials suitable for 3d printing (via SLS or thermal deposition of carbonyl), plus platinum group metals to use as electrode plating and catalysts.

 Most Lunar mining schemes are intended to produce oxygen, since most were designed before the widespread existence of water ice on Luna was known. (See for example this NSS article.)With such an abundance of water it makes little sense to go to a lot of effort specifically for O2. Instead, the two main targets will be aerospace metals (aluminum and titanium) and incompatibles (phosphorus, potassium, rare earths, etc.).

 Titanium-rich soils contain the mineral ilmenite, an iron titanium oxide. Much research has been done on this as a resource for producing oxygen and the general distribution and concentration of ilmenite at the surface is known. Concentrations can be as high as 10% titanium by mass, while the mineral itself is 31.6% titanium by mass. Impact sorting can produce an enriched feedstock of 90% ilmenite grains, which is 28.4% titanium by mass. This input would be reduced with hydrogen to form iron metal, titanium dioxide and water. The titanium dioxide is electrochemically extracted in a cell with molten calcium chloride and a carbon anode under the FFC Cambridge process, or is carbothermically reduced and then chloride-processed in the MER process.
 Aluminum-rich soils are formed of anorthosite, a calcium-rich plagioclase composed mostly of CaAl3Si2O8 with a small fraction (less than 5%) of NaAlSi3O8. This material is around 25% aluminum by mass. There are a few options for processing; countercurrent hydrochloric acid with fluoride ion (producing aluminum chloride), calcination (with or without carbon), arc melting and reaction with hydrogen. Partially refined aluminum (reduced alloys of aluminum, iron and silicon) can be further refined by the subhalide method, where aluminum chloride (AlCl3) at 1000-1200 °C reacts with aluminum metal to form aluminum subchloride (3 AlCl); aluminum metal is deposited in a condenser and the AlCl3 is recycled. The traditional ALCOA process can also be used, but this equipment is not easy to scale down and is very energy-intensive.

 Incompatibles are concentrated in KREEP (meaning potassium, rare earth elements and phosphorus). This material is about 0.4% phosphorus and 0.8% potassium by mass (present as oxides, each about 1% by mass). It also contains relatively high concentrations of rare earths, including 15-20 ppm thorium and about the same mass of lithium. This material is mostly located in Oceanus Procellarum and can be seen clearly in maps of the Moon's thorium concentrations. There are four specific craters with very high readings that are worth investigating; all are in the Earth-facing northern hemisphere.
 The main goal is to harvest potassium and phosphorus for hydroponics. Less-useful minerals would be removed and the resulting leftovers would be available for later intensive processing (most likely zone refining). Potassium oxide can react violently with water, so care must be taken; bioavailable forms are as chloride or nitrate. Phosphate can be purified by converting to phosphoric acid with additional processing, resulting in either ammonium phosphate or calcium phosphate.

 With a ready supply of structural metals, parts needed for spares can start to be sourced entirely from lunar materials. New hardware built on the Moon won't be under such extreme pressure to minimize mass, meaning the locally-produced equipment can be built heavy to improve MTBF and reduce spares. Simpler but less efficient PV panels can be constructed using all local materials, expanding the base's power supply. Light metals shipped to EML1 can be used to build extremely light structures that would fall apart under chemical acceleration; this capability would be a significant advance for electric propulsion and possibly for light sails. Oversized propellant tanks and other structures could be built to dimensions that could not be launched from Earth.

 This step represents the first major advance in self-sufficiency. No longer dependent entirely on parts and fertilizer from Earth, the facilities offworld can expand with minimal additional launched mass. There is a strong demand for carbon, nitrogen and chlorine, all of which are available in bulk on Mars. Still, the facilities begin to provide services to customers back on Earth, mostly as satellite maintenance but increasingly for satellite construction.

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