How to actually mine Platinum Group Metals from an asteroid

How to actually mine Platinum Group Metals (PGM) from an asteroid, or my guess at how Planetary Resources might do it:

1) Find a metal-rich rubble pile (if they exist) with a PGM ore concentration of 100ppm (asteroids of that concentration of PGMs supposedly exist).

2) Pick off big metal chunks magnetically or somesuch.

3) After you have a thousand or so tons of metallic ore, put it in the shade to cool off to deep cryogenic temperatures.

4) Then, crush it in a grinder. Cooling the ore to cryogenic temperatures first should significantly reduce the energy needed to grind the ore to a fine powder, but grinding of ore often takes roughly ~10kWh/ton, so it'd take ~10MWh to grind the batch (or about 2 weeks if you have 30kW), although iron-nickel is pretty tough, which is why I included first cryogenically cooling the ore.

5) Grinding may heat up the ore, so it may take several steps where you grind, then cool, then grind.

6) Once the powder is fine enough, you put it in a vibratory centrifuge to clear off the relatively lightweight rocky bits (the metal will be much denser).

7) Then use a magnet to remove the powdered iron and nickle bits.

8) What you have left will be a MUCH more concentrated ore and a lot less material.

9) You can ship this back to Earth (a couple of tons to a few hundred kilograms... small enough to be returned to Earth, perhaps guided by a very small tug and perhaps with a very inexpensive heatshield) for sale or further processing.

10) Or do some wet-processing of the ore to concentrate it to PGMs (like dissolving the ore in acid... PGMs won't dissolve in most acids while everything else will, so you'll be left with fairly pure PGMs) before shipping back to Earth for sale.

However, acid processing may be too chemically intensive, and you might get by with just dry processing of the ore (although it sounds like Planetary Resources intends to do some wet processing, part of their motivation for focusing initially on extracting water).

At the end of this, you'll have about 100kg of PGMs, with a market price of roughly $15-30 million.

Rinse and repeat.

First test firing! No continuous ignition.

This is the first test of a rocket igniter (i.e. small rocket engine) running on compressed air and propane. Using compressed air as the oxidizer is notorious for causing difficulty in keeping a rocket engine lit, as you can see here where the rocket only ignites a few times (by the spark plug) and never stays lit for any length of time. Basically, it appears like the spark just ignites what's in the ignition chamber before it all goes out. I tried some nozzles slightly smaller and slightly larger, and one just very slightly smaller seems to work best. (I also tried increasing the spark frequency from 31Hz to 122Hz, which helped slightly.) All of them seem to work best after the chamber becomes warmer after more testing. I may try running it for longer to warm it up and see if it ever stays lit for an appreciable length of time. I'm just concerned about melting the plastic tubing which brings the propellants into the chamber if I get the chamber any warmer (yeah, I probably should've tried metal plumbing there...). I may try seeing if I can water-cool it a little.

http://youtube.com/7alGMFgz52s

I originally had a lot more trouble with the spark generator producing a whole bunch of EMF which caused the microcontroller to go crazy. The relay which I was using for isolation earlier wasn't helping anymore for some reason, so I switched to an LED/photoreceiver setup which worked pretty well then had its own problems. I separated the power supplies to separate AC circuits and then used a metal mesh to shield the non-sparker side of the test stand, and that seemed to do the trick, 95% of the time, at least (good enough for me, though I can do better). I think I'm slowly wearing out the microcontroller somehow.

If I'm ever able to get it working smoothly, I will make a parts list. I need suggestions about how to keep it lit.
Here's a picture of the chamber with the nozzle unscrewed:

Choked Flow! Mach Diamonds! Schlieren! (but just with compressed air)

Here are a couple "Schlieren" images I made (really just shadow graphs) using a DSLR camera without a lens, the bright video flash (just a bright white LED) on my cell phone, and compressed air going through the oxidizer orifice that will be used for my rocket igniter (I just drilled it totally free hand, no drill press... 1/16" diameter orifice, I think). You can see nice shock diamonds, especially with the last image (which was taken with less pressure drop, actually... I was running my air compressor in "blow down" mode because it's loud). I need to put my pressure gauge closer to the orifice (should be on the orifice side of my valve) to get a better reading of the pressure drop, but I think the first image is at about 100psi, with the next one being significantly less.

I simply projected the shadow of the end of the orifice directly on the DSLR image sensor. I held the orifice pretty close to the camera. To get more contrast, I could have held it further away (allowing the diffracted vs undiffracted light to diverge more, giving more contrast).

And here's what the orifice actually looks like, next to my breadboard that controls the spark generator (indirectly through a relay, ~31Hz PWM) and the valves, with three LEDs hooked up so I can test the code without the valves and stuff connected:

The purple thing is just the FTDI cable used for programming the ATMEGA chip and powering it with my laptop (in the field, it has its own 5V power supply). FTDI is optional, I'll probably remove it once everything is integrated.

I'll post more later.

Calculating vapor pressure of propane during test firing

I will be relying on the vapor pressure of a propane tank to maintain a certain feed pressure for my rocket igniter (which I will measure with a pressure gauge). But will the boiling/evaporating of the propane while it's being used lower the temperature enough that there's a significant reduction in feed pressure?

Using figures mostly from here:
http://en.wikipedia.org/wiki/Propane_(data_page)

propane heat of vaporization:
356kJ/kg (or 356J/gram)

propane liquid molar heat capacity:
98.36J/(mol*K)

propane molar mass:
44.1 gram/mol

propane liquid heat capacity:
98.36J/(mol*K)/(44.1 gram/mol)
=2.23J/(gram*Kelvin)

Thus a 4 second firing consuming .5 grams of propane a second for a total of 2 grams will cool the liquid propane in the tank by:
2grams*356J/gram=712Joules
which will change the temperature of half a liter (roughly 500 grams) of propane by:

712J/(500grams*2.23J/(gram*Kelvin))
=
.639 degree (in K or C)
that's roughly 1 degree F (or Rankine)


It's quite possible that the whole tank won't equalize in temperature, but even so, it does mean there's not a big enough drop in total temperature to lower the overall vapor pressure by a considerable amount over the length of the burn.


Here's a chart of the vapor pressure of propane (and other substances) as it changes with temperature:

Wolframalpha.com says that propane at 77 F has a vapor pressure of about 138.1 psi, and at 76 F, it's 136.1 psi. So, if we're operating the tank at 77 F, we'll get roughly a 2 psi change in feed pressure over the length of the burn, assuming 500 grams of propane in the tank, a feed rate of .5 grams of propane per second and a 4 second burn length. That's low enough that we don't care.

BTW, since we'll be wanting to operate at closer to 80psi for the propane tank, that means we need it to be at 41 Fahrenheit (5 C), which means an ice bath or something like that.
http://www.wolframalpha.com/input/?i=propane+vapor+pressure+at+5+Celsius
Still need to figure this out, but I will be doing some experiments with flow-rates and orifice sizes (I'll be drilling them my own).

Another update, now in 2012

Well, it's been a couple months since the last update. HOWEVER, I now have renewed my subscription to my local hackerspace, acquired the block of brass (for free), have very nearly acquired the coil-on-plug (for free, just have to pick it up), and have some favors I can call on for my local machinist. I also bought a decent CAD program, Alibre Design Personal, which makes certain things a lot easier (like putting threads in my drawing and making a 2D diagram).

And now, I think the valves need 12V and 24V, so that's three power supplies I need (5V for Arduino... could be from laptop, though I may want to be better isolated, 12V for one valve, and 24V for the other, and 12V for the coil-on-plug)

Still need:
*the right taps for my spark plug and my orifices (if I don't just drill them) or my existing adapters
*correct gender for the propane tank adapter
*orifices (again, if I don't drill them)
*propane tank (may have one lying around)

Semi-optional:
*filters
*usb opto-isolator (or, just run it from a cheap PC or semi-broken laptop).
*digital pressure and temperature probes

I've also been thinking about making the throat and nozzle replaceable, perhaps allowing me to 3d-print different nozzles (in steel) for testing different shapes in subscale.

Also been thinking of using two, three, or four servos to do some experiments with vectored thrust, ala V2 and Redstone, etc on a subscale. I've got 2 servos already.

I have to make sure I can safe the system by just turning the power off on everything. I'll have everything hooked up to a power strip I can just switch off or unplug everything if something goes bad. Also, I want a big, red "fire" button. ;)

Fire durations will necessarily be very short, one the order of a few seconds, starting with just two seconds.

I just got a nice digital SLR (not video-capable, though), so better pictures will be up shortly.

EDIT: Most of the stuff I got:

First snow and time for an update...

It has been... almost 7 months since my last update. There's something about summer that makes you want to put projects like this off! I have also been taking a class at the local university about space plasma physics, so that has taken up my time as well (and likely will slow me down until it's finished). Well, I have acquired both the valves (for fuel... propane... and oxidizer... compressed air), and tested them with compressed air. They both work great! They use different voltages, but that's not that bad (12V and 5V, I believe). I also have a manual ball valve, which I plan to use for the propane side for added safety.

I also acquired connectors for connecting a small, camping propane tank to my plumbing, but got the wrong gender attachment. I also have a pressure gauge, which I think works with propane as well.

What I need now is:
*the right gender propane tank attachment piece
*small propane tank
*spark generator (one of those coil-on-plug deals which just needs 12V and a TTL trigger or something like that and outputs a nice spark to my spark plug)
*a block of brass or something like that to machine the igniter out of
*perhaps some orifices to get the right flowrate for each side... possibly could just machine these directly into the brass block, but it'd be more difficult
*filters to put just upstream of the orifices to catch foreign debris and keep it from exploding (which it might anyway)
*5V and 12V power supplies with enough current for everything

Nice-to-have:

*maybe some sort of optoisolators to protect the arduino?
*maybe one or two extra, cheap arduino clones, like Sippino, so I have extras
*digital pressure sensor for the chamber, ideally pressure sensors for the chamber, one for oxidizer, one for fuel
*digital temperature probe for chamber

Eventually, I'd like to get some more cheap activated valves and make a mini cold-gas thruster system, with the little thrusters printed online. That'd be cool.

A few WEEKS of no new progress.

I have made no new progress. But I am committed to following through on this project. I am going to find some valves which can be actuated.