Platinum

Platinum is relatively rare in the Earth's crust. Platinum is a siderophile element, attracted to iron, and follows iron to the Earth's core. Only a few regions in South Africa, Russia, and Columbia have ores with platinum concentrations worth mining. 80% of global production is in South Africa.

Some asteroids may contain up to 100 grams of platinum per tonne (100 ppm), 10 to 20 times the concentrations found in South African mines. The November 2019 platinum price is $30,000 per kilogram, so raw asteroidal ore contains up to $3 of platinum per kilogram.

But platinum doesn't just hop out of the ore when you snap your fingers. The process of extracting terrestrial platinum requires vastly more inputs of materials and energy that are plentiful on Earth, but absent on platinum-bearing asteroids.

In South Africa, producing a kilogram of platinum entails:

• one man-year of labor
• 600 tonnes of ore (lots of waste rock and dross with some platinum unextracted)
• 400 tonnes of water
• 200 GJ of energy
• 40 tonnes of C0₂ emissions

Steps in the refining process include :

• Electrorefining to remove copper; the platinum is in the anode mud
• float refining
• electromagnets to remove iron and nickel particles
• Aqua Regia (HCl and H₂SO₄) to separate PGM from Iridium, Ruthenium, and Rhodium
• Ammonium Chloride to remove iron and gold

• Heating to precipitate platinum from the (NH₄)₂(PtCl₆)

• further processing of residues to extract more platinum

Given the enormous inputs of materials and process equipment, the end goal for asteroid extraction probably won't be pure platinum, but the concentrated output of some intermediate step above, mass reduced for more economical shipment to Earth. On Earth, clean water and air and solvents and labor and energy are abundant; they will all need to be manufactured or transported to an asteroid platinum refinery.

So, the question isn't "how much platinum does an asteroid contain?", but

• "how much effort and expense,
  • and kilotons of solvents and other materials
    • are needed to extract one kilogram of platinum from 10 tonnes of asteroid,
      • then deliver that kilogram of platinum to a paying market?"

The minerals buried underneath my back yard (all the way to the core) are priced at 40 trillion dollars, with PNG elements priced at 5 trillion dollars. The "delta V" to the surface is less than the delta V from the asteroid belt. The expense and effort to extract those ranges from absurd to impossible; asteroid extraction may be in that range as well.