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Comment by ginko | Hacker News Reader

>All the surface metal deposits are gone.

Well, they're not so much gone as converted into various structures all over the globe. I see no reason why later civilizations couldn't mine the remains of former cities.

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Pretty much this - mining would be done on old landfill sites and cities, at least for metals. Fossil fuels are a different type of problem - we're not going to get all that carbon back. Conversely maybe a reboot civilization would develop differently - forests to solar thermal to PV and batteries. It would be slower, but certainly possible.

How are you going to do any metalwork with your scavenged metal? Heat it up with a big mirror? Its very very hard without high energy density fuel. The article suggests charcoal (did you read the article?) but thats problematic too.

Just because it would be a slower development path doesn't mean it's impossible. It would be slower, but any given civilization would have only a few viable options for development - so yes, I think you'd find a civilization which values glass and mirrors and polishing technology to get the heat necessary for metal working.

I don't think a civilization built on a shit load of aluminium scrap is going to develop very slowly. Don't forget aluminium is a wonder metal that's only been around for like a hundred years.

All really important metals can be melted at temperatures that can easily be reached with a well built furnace and natural fuel like wood.

Plus, we already know what we did to get to where we are now. We can adapt that knowledge to get back to where we are now. It's going forward we're going to be stumbling in the dark, but we're doing that now anyway.

See my reply to your comment in a sibling thread. A wood fire can hit 1050C. There's plenty of easily-available metal in that range, and building a forced-draft charcoal burner will get you to 1350C. Iron melting points start around 1150C; steel is probably out of the question, but that doesn't mean much.

> How are you going to do any metalwork with your scavenged metal?

With a wood-fired forge. Blacksmithing predates fossil fuels by quite a long time.

We can, make new hydrocarbon fuels. Grow plants that are high in fatty acids that we can run catalytic cracking processes on to make usable fuels. Or, use fermentation and the like to make alcohols from plants with a high sugar content. Again, slower, but at the same time, it would be part of the process.

You can just dry and burn the plants directly. You can replace coal almost directly with dried plants.

Indeed you can. It's definitely the simplest way of doing it. However, by pulling the fatty acids out, etc, you can also pull out a lot of other useful products, like ammonia and hydrogen sulfide. Additionally, a fire will produce a lot of ash and smoke, which both will need to be mitigated in one way or another.

Yeah, low tech wood fires are an air-pollution disaster:

http://www.samharris.org/blog/item/the-fireplace-delusion

It's possible to burn plants cleanly - you need lots of air, and possibly a catalytic converter.

Not something you would do at home, but a power plant can.

We should, today, switch from burning coal to burning switchgrass.

One of the reasons why mining the remains of ancient cities could prove to be impossible, is that a lot of the metal would get oxidized (aka rust)

In the case of rust, there are a lot of chemical similarities between it and hematite (though contamination with iron hydroxides might be a problem), which might (theoretically at least) make it potentially suitable for smelting into pig iron. I don't think any existing industrial processes do this, though.

It's not a problem to remove rust from iron. Just heat it in a low oxygen atmosphere, and add carbon which will react with the oxygen in the iron.

You do need to carefully control the amount of carbon though.

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