Could the extraction of rare earth elements (REE) be more efficient on asteroids as opposed to on earth?

Question

Could we mine or extract REEs in asteroids more easily than on earth? The following conditions apply:

Ease of extraction does not include getting to the asteroid or transferring materials back to earth. Don't worry about how to get to the asteroid. Don't worry about getting materials back to earth.

My question includes the following stages of mining specifically: Exploration, Discovery, Development, Production, but not Reclamation. Specifically, I don't care about the Reclamation of asteroids once used in mining. The scope of this question includes the extraction of REEs.

Answers should make minimal assumptions about other new technologies. Please do not respond with if correlative technologies. For example answers should not include, "If we could build rockets to get to asteroids than we would clearly be able to build advanced robots too."

Answer 1

Asteroids are unpromising - in fact abyssmal - terrain for mining rare earth elements. They exist in very low concentration - as we know from meteorites - and there are probably no rare earth ores on any of them.

Rare earth elements are strongly lithophile, therefore they accumulate in planetary crusts. Earth may be the best place in the solar system for mining REEs, because we have a high bulk abundance of them and geological processes that led to the formation of ores. And we have a strong gravity which enables placer deposits (where most REEs are mined from monazite sand).

Mars seems to have a somewhat lower bulk abundance of REEs than Earth, except for scandium which is more abundant.

On asteroids bulk abundance of light REEs is typically 100 times lower than in the Earth crust and 20-30 times lower for heavy REEs.

Elevated concentrations of REEs are found in phosphate minerals like whitlockite, but still much lower than in average rocks on Earth.

As far as I know there is only one mineral on asteroids that has a higher concentration of REEs than terrestrial rocks, and that is OLDHAMITE, a calcium sulfide mineral which is extremely rare on Earth but common in a certain class of meteorites: metal-rich ENSTATITE CHONDRITES and rocky ENSTATITE ACHONDRITES.

Oldhamite seems to "suck" all the REEs from the rest of the meteorite and can host 10 to 1000 times more REEs than the matrix. Due to extremely low bulk abundance that is still not enough that it would count as REE ore on Earth, but it already comes close, especially for heavy rare earth elements because we have no good ores for them either. Oldhamite is enriched in heavy REEs compared to light REEs. That is because of the refractory nature of HREEs (Oldhamite forms at high temperature).

It is possible that you find oldhamite on asteroids which is even more enriched in REEs than the ones we know from meteorites. That would count as REE ore, at least for HREEs (which are also the expensive ones anyway).

Unfortunately, the lower the abundance of oldhamite in a meteorite, the more enriched it is in REEs. That is the opposite of what we want. Oldhamite with an enrichment factor of 1000 never constitutes more than fractions of a % in an enstatite meteorite. On the other hand, when oldhamite is abundant it is relatively poor in REEs.

So to find an asteroid that can be mined for REEs it would not only have oldhamite that is unusually rich in REEs, there would also have to be a secondary enrichment of oldhamite. That may happen, because oldhamite has a higher melting point than surrounding rocks, but lower density, so it would float in a melt. I would search for it on a large, massive asteroid where igneous processes took place in the past so that separation of rare earth elements from the bulk and separation of odhamite from the rest of the melt phase could occur. Also, oldhamite needs reducing conditions to form, it does not occur when oxygen abundance is too high. All oldhamite containing meteorites probably formed close to the sun (it is expected to be a common mineral on Mercury).

According to spectral analysis some asteroids may contain large amounts of odhamite on the surface (up to 10%).

But, as I mentioned at the beginning, there is probably nothing in the solar system as good for mining REEs as our placer deposits.Long story short, the answer of your question is "no", but it gave me the rare opportunity to talk about one of my favorite minerals.

Answer 2

Easy Access

Earth is big and the minerals might be very deep in the Earth. That means we need to move a lot of soil to get to the deposit.

Mining asteroids has 2 advantages:

1. Because there are many small asteroids rather than one large planet, the deposits are more likely to be closer to the surface.

2. It is easier to dig a hole in the low gravity environment of the asteroid belt. On Earth, once your hole is half a mile deep, you have to move each shovel of earth half a mile against the Earth's gravity, from the bottom of the hole and over the top. In space you just give that shovelful a little push and it floats all the way up.

Answer 3

A lot of the rare earth's are not actually that rare, and there are a variety of them. So knowing what you want to have is an issue. Like all mining, you may being getting some elements as a byproduct while pursuing a different element.

A major problem is that rare earths oxidize easily and that they are hard to purify. On earth a variety of wet methods are used to separate the rare earth bearing minerals from other minerals. Then usually since the rare earth oxides are pretty chemically stable the eventual product is a rare earth oxide rather than a metal. To reduce the rare earth oxides to a metal, clorides in a refining plant are used and that somewhat difficult, corrosive and expensive. Other methods, like molten-salt electrolysis and other electrorefining methods are being explored but are energy intensive and not widely used. The melting point of Rare earth oxides is usually pretty high.

So what are the advantages in space - well you still have to separate out the rare earths which depending on what you want means going through a lot of material and throwing what you don't want away. Wet methods will be difficult, probably somewhat impractical in low or zero gravity. However you do have lots of energy, and also an oxygen free environment. You also have space... and a long mean free path between molecule.

Depending where you are in the solar system energy is pretty cheap, and you could envision large solar smelters that melt and evaporate minerals and aid in the processing and separation of the metals, especially if you can ionize the vapor plume, and impose a magnetic field. The heavy ionized rare earth elements would tend to curve less, and the lighter elements more, so you could spatially separate the elements like a giant mass spectrometer.

Probably there are more realistic approaches, but I think the big advantage for your asteroid mining is the using the vacuum to your advantage and potentially having a lot of energy available.

Answer 4

It will only be more efficient if the REEs stay in space

I know your question says to disregard the issues of getting to and from the asteroid... but with a level of technology anywhere near modern standards, any question about the efficiency of doing something in space will always be dominated by the cost of transportation, especially traversing earth's gravity well. I think it will be quite some time before it will be efficient to gather any resource from space for use on earth. However, it will be incredibly more efficient to mine materials in space for use in space. In the modern context, the sole reason to consider space mining operations is to save on the massive cost of launch systems. It would be a foolish loss of value to bring REEs down to earth! Their value would come from where they are.

Maybe if a wild amazing asteroid composed of a completely pure rare element is discovered... But we would still probably rather use it in spaceborne manufacturing.

Answer 5

Mass spectrometry

On an asteroid, you can use monochromatic light to give rare earth ions a known energy. You can charge up the entire asteroid with an electric charge, pulling them back in. You therefore should be able to conduct mass spec on a factory scale, firing a vast shining arc of ions up through space that neatly bin themselves by atomic mass upon returning to the surface of your asteroid.

Caveat: on Earth we typically think of ground potential as arbitrary, with no "true" voltage. How I reconcile that with the idea of charging up an asteroid, I'm not so sure. Yet dang it - put enough electrons in the thing and positive ions ought to fall back to the surface, no gravity required.

Answer 6

Existing tech and no transportation problem(which is possible), then yes

Lever is - accessibility of energy, different plasma, plasma-chemistry separation methods - those can be exploited. Potencial abundance of energy, and cheap means of its extraction(reflectors in microgravity and non oxidising environment aka vacuum)

You basically convert the whole asteroid in a machine for extraction of materials you are interested in, and SiO2 is one least interesting one and it is the majority of an asteroid mass and it can be used to make energy (solar) collectors and other structural parts/tools (structural strength requirements in microgravity has a different angle, and much reduced for many cases), so as SiO2 can be used as reactive mass as well.

So answer is yes, it possible to make less complex equipment, use less complex (in some sense) processes of extraction than on earth - effectevly less movement for bigger result, energy efficiency will be bad(compared to earth case), but it is wasted anyway and is free and unlimited, so why not.

Answer 7

Rare earths are so rare because the nucleosynthesis bringing to the production of those elements is not the most likely outcome of the processes.

As such those elements in asteroids are not going to be much more concentrated than they are on Earth: at the end asteroids and planets were made out of the same molecular cloud, and REE are also not so light that can undergo a preferential selection based on temperature, like it happened to hydrogen and helium which are more easily captured and kept by cold planets outside the habitable zone.

It an happen that statistical fluctuation concentrate more a certain element somewhere, but finding those fluctuation will nevertheless be a challenge, on top of the already existing ones related to space.