Platinum from the Asteroids?

In a previous millennium, I wrote a number of articles and several books on the nature of asteroids, the threat they pose to life on Earth, and their economic attractiveness as future resources for exploitation.  I will post a list of those publications separately on this site so that those interested can verify what I write here, both its content and its priority. 

Science fiction author Robert Heinlein, although saying little about mining and resources, wrote of converting asteroids into habitats as early as 1939, in his story “Misfit”.  The idea of mining asteroids appears to have been introduced in 1951 by E. E. “Doc” Smith, writer of the “Lensman” series of science fiction novels, who presents his protagonist, Kimball Kinnison, as the discoverer of a massive platinum deposit in an asteroid.   Frederik Pohl, starting in 1977, set many stories against a background of mining for metals in the asteroid belt.  Former astronaut Brian O’Leary proposed in 1981 that certain near-Earth asteroids might contain economically attractive amounts of platinum-group metals, although he was unable to suggest a workable method of extracting and retrieving this material.   I first suggested actual industrial processes for making asteroidal metals useful, including the platinum-group metals, in 1983.

The Near Earth Asteroids (NEAs) are of the most immediate interest as resources: they are readily accessible from Earth, and their compositional attractions (and diversity) are well documented by chemical analyses of many thousands of meteorites which have fallen to Earth.  The overwhelming majority of these meteorites derive from the NEAs and therefore represent not only samples of the materials that threaten Earth, but also of the resources available to us when we exercise modern technology to exploit them.

The NEA swarm contains thousands of known asteroids, ranging in size from mere rocks (1-10 meters) to bodies over 10 km in diameter.  Their orbits range from inside Mercury’s orbit to a few that roam out to Jupiter and beyond.  Their compositions span all the varieties of meteorites that fall on Earth, including material similar to organic-rich lake bottom sediments, the C-type (carbonaceous) asteroids and the corresponding C chondrite meteorites.  They are rich in organic carbon compounds, magnetite, and water-bearing clay minerals, with elemental sulfur, hydrated mineral sulfates, sulfides, phosphates, and a great variety of minor and trace minerals.  These carbonaceous chondrites are relatively fragile, poor at surviving both passage through the atmosphere and long-term residence on Earth’s surface: a single rain storm will utterly destroy them.

Most of the meteorites in our museum collections are stronger, with very low content of water and other volatiles and far less oxidized minerals, being dominated by silicates of iron and magnesium (olivine and pyroxene), feldspars made of aluminum, sodium, and potassium oxides, a sulfide of iron (FeS) called troilite, and particles of metallic iron-nickel alloys.

Also common in our meteorite collections are “iron” meteorites, composed of natural stainless steel, an alloy of iron, nickel, and cobalt.  Irons contain traces of dozens of rare and strategic elements such as gallium, germanium, indium, and other non-metals which dissolve in iron alloys, and the platinum-group metals (PGMs) such as platinum, osmium, iridium and rhodium, along with a trace of gold.  The NEA swarm contains about 1.8x10¹⁵ grams of PGMs; in more familiar terms, that’s 1.8 billion tonnes of stuff worth about $70,000 trillion dollars at present Earth-surface prices.  Obviously returning even a tiny proportion of that material to Earth would cause the market price to crash.

Once assembled into planets (or even the very largest asteroids), these raw materials melt and “differentiate” by density into layered planets with metal-rich cores, iron-and-magnesium silicate mantles, and volatile-rich crusts. After many generations of melting and recrystallization, rich veins of minerals evolve.  Humans, having grown up on such a differentiated world, expect valuable minerals to be found as rare veins of ore that wind their way tortuously through vastly larger bodies of “dross” (economically uninteresting) materials that must be removed or tunneled through to access the “right stuff” in the veins.  Such expectations may apply to one or a few of the very largest asteroids; they are probably irrelevant to most or all or the entire Near-Earth Asteroid family.  The study of meteorites makes it clear that the large majority of the rocks that fall on Earth have not gone through such a process of differentiation and mineral-vein formation.  But even some “space mining” companies seem to be unaware of the meteorite evidence; they adopt irrelevant models of mining based on terrestrial experience, not meteorite evidence.

Now let us suppose that we wanted to break into that market by extracting PGMs from nearby asteroids and throwing them into the Earth-surface marketplace.  How could we do this?  Since these elements are present in small concentrations in all asteroidal metal alloys, the obvious method is to extract them from the metal and ship them back to Earth in concentrated form (that is, we don’t have to engage in the laborious and complex process of separating them into their individual component elements; we can just ship the mixture back to Earth and let processing plants at home separate them into useful products).  Fortunately, we know how to do all this because it involves technologies we already have on Earth.

The essential step, separating the high-value PGMs from the less-pricey major elements, requires the use of what chemical engineers call the Mond process, named after the German chemist Ludwig Mond.  Large, strong (think “really massive”) pressure vessels are filled with carbon monoxide at moderate temperatures of 100 to 200 ^oC and high pressures, typically about 100 atmospheres.  Under these conditions, iron and nickel (and under somewhat different conditions, also cobalt) react with the carbon monoxide gas to produce gaseous compounds called carbonyls, including iron pentacarbonyl, Fe(CO)₅, and nickel tetracarbonyl, Ni(CO)₄.  These can be condensed together as water-like liquids and then separated by distillation into the two separate carbonyls with extremely high purity, better than 99.9999% for each carbonyl.  Cobalt, under modestly different conditions, can also be extracted and separated as its carbonyl.  The solid residue from this extraction process contains many other elements besides the PGMs (such as gallium, germanium, indium, etc.) and particles of silicates that were once dispersed as impurities in the metal.  Also present are carbides, phosphides, silicides and sulfides that were originally dissolved in or trapped inside the metal grains.  Physical and chemical purification processes can separate these less-valuable materials from the very dense PGM dust.

Fine, now we have several products separated and ready for shipment back to Earth or for in-space use:  1) ultrapure iron, 2) ultrapure nickel, 3) ultrapure cobalt, 4) and fairly pure PGM dust, not separated into elements.  These are supplemented by 5) mixed nonmetals (gallium, germanium, arsenic, indium, etc., and 6) particulate sulfides and phosphides of iron, nickel, and (depending on which type of meteoritic metal alloy was used) perhaps other metals as well.

Having invested in building the processing plant and shipping it to an asteroid, our investors would surely be interested in the value of the products made available.  Here’s a tentative list:

            1) ultrapure iron, 99.9999% pure, which is stronger and more corrosion-resistant than normal steel; Earth-side market value perhaps $2000/tonne, and extremely useful for construction of habitats and pressure vessels in space. The total NEA swarm’s market value is about $70 quadrillion (Earthside market prices).

            2) ultrapure nickel, Earth-side market value would be about $28,000/tonne for normal purity Ni; no premium is assumed for the high purity or for its presence in space.  Total NEA swarm value for Ni would then be another $70 quadrillion.

            3) high-purity cobalt, Earth-side market value about $35,000/tonne for normal-purity cobalt; no premium is assumed for the high purity or presence in space.  Total NEA swarm value for the cobalt content would then be another $70 quadrillion.

            4) Mixed Platinum Group Metals, Earth-side market value about $40/gram; $40 million/tonne.  The total NEA swarm value is about $70 quadrillion.

            5)  Mixed non-metals (semiconductor materials): available for use in space to build solar power electrical generation stations.  The value of this resource category is “just gravy”, and the logical market for them is in space.

            6) Sulfides, carbides, phosphides, silicides, etc. (rare metals and nonmetals) available for use in space.  More “gravy”.

These proportions are valid regardless of what fraction of the total NEA resource base is consumed.

To a good first approximation, the PGM contribution to the value of asteroidal metal resources is a little less than 25% of the total value.  As on Earth, exploitation of ferrous metal resources will be done for the economic benefit of “local” uses of the iron, nickel, and cobalt, not the PGMs.

Now consider what would happen to the revenue stream from importation of PGMs to Earth: even as little as a few thousand tonnes per year would lead to a crash of market prices.  Rather than commanding current prices on the order of $1000 per ounce, PGMs would drop precipitously in price in response to their growing supply.  Importation of larger quantities of PGMs would be self-limiting even if lower market prices should encourage wider use of these metals.  It would then become obvious that PGM mining is a far less compelling reason for mining asteroids than those that are provided by the ferrous metals or volatiles.

Why do I take the time to challenge the assertion that mining platinum group metals is the reason to exploit Near-Earth Asteroids?  Because the idea is attributed to me, reflects no appreciation of how that mining would be done, and is quite incredible as it is generally presented.  Let’s be clear: there is so vast a supply of platinum-group elements in the NEA swarm that exploiting even a tiny fraction of them would cause the market value to crash, bringing to an end the economic incentive to mine and import them.  PGMs are valuable on Earth today because they are rare; in a world in which vastly larger supplies of PGMs are available, their value plummets.  Mining PGMs from asteroids is not a get-rich-quick scheme; it is a way of lowering PGM prices dramatically.

When any self-described asteroid mining company plays the “platinum card” as a reason to begin asteroid resource exploitation, it is misrepresenting the truth and displaying a willful ignorance of the facts laid out above.  Platinum-group metals from asteroids are a real benefit reserved for the distant future, a fringe benefit of mining ferrous metals.  But the initial economic motivation for NEA exploitation is, as I have been saying for many years, the manufacture of chemical propellants to enable future deep-space missions—and to open the asteroids to economic activity.