Tuesday, September 15, 2015

Early asteroid mining

This is a followup to the early lunar mining post.
I assume a suitable asteroid has been delivered to EML1 or lunar orbit for processing. I also assume that a painstakingly detailed dissection with full science yield is not necessary; relevant samples and readings are assumed to have been taken and the rock is available to be destroyed. The mission is in no particular hurry to complete the task, but several groups are to be given a chance at testing process technology.

containment bag, 200kg
grinder arm, 2000kg
solar oven, 600kg
ore sorting, 1000kg
ore processing, 1700kg
cryogenic processing, 1000kg
power, 400kg
radiators, 800kg
storage bags, 800kg
water tanks, 1000kg
LOX tanks, 10,000kg (could be subbed by a visiting ULA ACES-121 tanker)
Total mass: 9.6t with tanker, 19.6t standalone

yield (assuming ideal asteroid composition and 1000 tons material):
540 tons of magnesium silicates (shielding)
224 tons of iron
100 tons of water
100 tons of oxygen
30 tons of carbon
4.5 tons of nickel
600kg of cobalt

 Unlike the surface mission, this mission has effectively no gravity. Material processing methods become radically different thanks to sir Newton and his bothersome but useful laws. We also have a limited amount of material, something in the range of 500-1000t and perhaps 10m diameter. The material is from a C-type carbonaceous chondrite with little to no near-surface water or low-temp volatiles.

 First off, the entire body must be enclosed in a bag to avoid debris or loss of volatiles. Allowing a radius of 8 meters (in case of oblong shapes), 800m² of material is required. Aluminized Spectra seems like a reasonable choice, somewhere around 0.075 kg/m² or around 60kg for the whole bag. Triple that to allow for some strong lines and round off to 200kg for a bag that can distribute tension forces.

 The body has to be crushed, pulverized or otherwise broken apart before heating. An unexpected pocket of frozen nitrogen in a hard crust could cause a bad day when it violently sublimates. However it is done, the tool should avoid any net forces. Single-bit drills, egg-beater grinders and similar designs should be avoided since they will induce a spin or cause the toolhead to walk.
 I think a four-drum grinder would work; this would be an articulated arm with four 'hands' each with a toothed drum at the end. The teeth would be blunt enough to be safe inside the bag (won't cut the bag, should deflect off of it). Crushing force would be rotational inertia in the drum plus compression between each pair of opposing drums. Each pair of drums would be at 90° to each other; the outer pair would chew on the face of the rock and produce a stream of material into the inner pair, which would block any bigger chunks and pass the rest up the tool and into the processing equipment. This tool induces a force that tends to pull the tool into the rock, which is useful and controllable. The articulated arm allows it to reach the full diameter of the body. It would be attached to a ring in the bag, so the reaction force will keep the bag taut and keep an open space around the arm. Something roughly similar to Canadarm (450kg mass, 3300kg payload, 15m reach) seems reasonable, but let's quadruple the mass to account for the grinding heads and an internal auger; call it 2 tons.

 The crushed ore is heated to extract volatiles, using temperature steps to isolate specific fluids. Those are drawn off for cryogenic processing. Water is either frozen for storage or electrolyzed for further use. This needs a fairly simple solar reflector, vacuum pump and a loading mechanism with a decent seal. I prefer a cylinder with doors at either end and an internal auger; a batch can be fed in by the grinder and then fed out by the oven's auger. This section needs to handle temps up to 1000 °C with normal operating temp of perhaps 600 °C; this is within the range of stainless steel and fused silica (quartz glass). I will assume 600kg for this; most of that is the steel and quartz body but a substantial amount is for the vacuum pump and insulation. The solar reflector has trivial mass even accounting for pointing.

 Next, the stabilized ore is separated. Magnetic rakes can pull out anything ferromagnetic; some very high-nickel nodules might be missed. Individual mineral grains could be sorted by electrostatics or fragmentation energy or other means to be developed. This would be an excellent opportunity to test a variety of methods. The goal is to produce at least 90% concentration of each individual mineral so each type can be processed efficiently down the line. I will assume five industry-provided modules with a maximum mass of 200kg each.

 Each mineral type benefits from a specific process. Nickel-iron can be extracted with the carbonyl process at reasonable temperatures and high purity. Metal oxides can be reduced with hydrogen (my objections to this method on the lunar surface do not hold weight in this particular application). Oxides in general can be electrolyzed in solid or liquid form. There are other chemical processes that can be used; in particular, any native carbon can be burned off into CO2 as process heat for other steps and then later reduced back to C for compact shipping. As a lower-energy alternative it can be compressed and bottled as CO2, and an intermediate option is to form solid CO2. Since each approach will be different, I will again assume five industry-provided modules of 300kg each. I also assume one water purification module (100kg) and assume that the cryogenic plant can isolate the remaining gases. There are also those volatile compounds at or above water's boiling point (such as sulfur); these can be isolated in a separate module of 100kg.

 Speaking of, one of the main end products of this endeavor is liquid oxygen. In the process of making LOX most other gases can be isolated at fairly high purity and stored as compressed gas or liquids. Mass depends on throughput, but let's assume 1 ton of equipment; that allows for intermediate purification steps and extras like dry ice production.

 Each process requires electrical power. We assign 10kW and assume an alpha of 10kg/kW (Encore 3-junction cells, EOL) or 100kg of panels. Power conversion, emergency battery backup, pointing and distribution eat about three times that, so call it 400kg for the power center.

 Storage requirements depend on the products. That in turn depends strongly on the type. Chondrites are composed of calcium-aluminum inclusions, free metal nodules, chondrules and matrix. CAI's are full of calcium and aluminum oxides with traces of other light metals. Free metal nodules are iron (72-93%) and nickel (5-25%) with about 2% cobalt and trace amounts of platinum group metals and other siderophiles. Matrix is ice, other volatiles, iron sulfides, salts, magnesium oxides and silicates, carbon polymers, organic compounds, presolar grains, glass fragments and other complex bits.

 For a CI that could be about 95% matrix, 5% chondrules and tiny fractions of CAI and free metal. That's 20% water, 25% iron (as oxides), 0.01% free metal, perhaps 2.4% carbon (as complex compounds) and the rest is mostly magnesium silicates.
 Other types might be closer to 5-10% water, 2-3% free metal, 10-20% total iron, 10% non-water oxygen and 1-3% carbon. I will use the high end of these numbers to spec storage requirements.

Using 1000 tons as the upper size of the captured asteroid that's 100 tons of water, 224 tons of iron, 100 tons of oxygen, 30 tons of carbon, 4.5 tons of nickel, 600kg of cobalt and a few kg of platinum-group metals and rare earths. The remaining 540 tons is as magnesium silicates which can be processed further for more oxygen, silicon and to extract some other trace elements. Magnesium by itself may find some use but as stone it is useful for radiation shielding.

I assume the nickel is stored as tetracarbonyl (density 1.32g/cc, 34%Ni by mass, mp -17°C, bp 43°C), requiring 10.03m­³ of low-pressure, controlled temp storage. (Also requires 8.74t CO or 3.75t C + 5t O.) It could instead be stored in bags of fine powder, bulk density about 2.6g/cc, 1.73m³ of bulk storage. Should only be a few kg (4.5) of bags.
The iron can also be stored as pentacarbonyl (density 1.45g/cc, 28.5%Fe by mass, mp -21°C, bp 103°C), requiring 542m³ of low-pressure, controlled temp storage. (Also requires 562t CO or 241t C + 321t O.) It could preferably be stored in bags of fine powder, bulk density about 2.6g/cc, 86.2m³ of bulk storage. Should be a few hundred kg (224) of bags.
The water can be frozen into blocks (density 0.93g/cc) and sealed in aluminized mylar wrap for very little mass. It could also be carefully frozen inside tanks, avoiding overpressure; this would take 108m³. With a tank fraction of 1% that's 1 ton of storage tanks.
The oxygen is stored as a cryogenic liquid (density 1.14g/cc, pressure ~25 bar, temperature 90K), requiring 87.8m³ of high-pressure cryogenic tanks. With a tank fraction of 10% that's 10 tons of storage tanks.
 The carbon and other metals can be bulk-bagged for a few kg (~31) of bags.
 The remaining slag can be bulk-bagged for 540kg.

As a general rule I'm using 10% for high-pressure tanks, 1% for low-pressure tanks and 0.1% for bags.

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