3 ms·

Would whatever the rust turns into be any less accessible (ignoring availability of coal) than what we started out with though? I don't pretend to know the enti

by gpm 5y ago

Would whatever the rust turns into be any less accessible (ignoring availability of coal) than what we started out with though? I don't pretend to know the entire "iron cycle", but it seems like it ought to just be turning back into the same sort of minerals that we originally extracted it from?

masklinn 5y ago
Iron rusts over historical timescales, metal deposits form over geological ones.
- gpm 5y ago
  Am I wrong to think of metal deposits as just "rust mixed with rock"? It doesn't seem like the rock part (i.e. the details of what it is mixed with) should be critical for the extraction process?
  - masklinn 5y ago
    > Am I wrong to think of metal deposits as just "rust mixed with rock"? They're concentrated rust mixed with rocks, otherwise it's not economically viable to extract. Like, iron is ridiculously common, relatively speaking: on earth as a whole it's more common than oxygen, for the crust it ranks 4th at 5% by mass, meaning if you went at it randomly you'd need to sift through 20kg of materials to get 1kg of iron. Currently, we exploit formations as low as 15% iron (banded iron formations / taconite), that's the lower limit of the economically feasible, and those results in absolutely enormous amounts of tailings (waste materials). Pre-industrialisation, unless you had no other choice (e.g. only had ironsands to work with) you really wanted to exploit natural (or "direct-shipping") ores, in the 60~70% range, the extraction is way too much work otherwise.
mikewave 5y ago
Sure, if you have a billion years to wait for it to all run through the rock cycle again.
- ncmncm 5y ago
  The process that concentrated the iron ore we rely on does not operate anymore. Fortunately, despite millions of tons of production every year, we are nowhere near using up the ore.
evilduck 5y ago
The problem will be a lack concentrated deposits making post-collapse (and new) industrial efforts much harder. A pile of rust from one tractor in someone's back yard is not going to be worth the effort to mine and refine. Possibly an analogous situation is the history of steel in Japan and their efforts to extract iron from sand, since they don't have significant iron ore to mine on their island.
- gpm 5y ago
  The pile of rust that use to be a tractor, sure, that's not worth much. The pile of rust that use to be a city though... surely that's more concentrated than the rust mixed with rock (or clay) that we originally extracted it from?
  - evilduck 5y ago
    Most likely not: https://en.wikipedia.org/wiki/Iron_ore https://en.wikipedia.org/wiki/Iron_ore, it's almost 50% iron in the worst case and can be up past 70% in the best case. The geological processes that form this ore in the mantle do a pretty good job of concentrating iron by density but it takes geological scales of time to do so and have it lifted back up through the crust. We might have to excavate a lot of earth to get to the concentrated vein of ore but when we find it it's relatively easy to chase and turn into usable iron once you know a little bit about mining and smelting as a society. A city decayed to rust is going to be a thin layer of iron spread out over miles with some hot spots like where a building once stood (but presumably without a map of the city in this distant future scenario), but there won't be a vein of concentrated ore. Distributed rust can definitely be turned back into pure iron but the energy requirements are going to be substantially higher to do so since you're going to have to sift through much more material to collect it, more material to separate and concentrate it, more material to smelt off, and your operations will have to be more mobile to retrieve it over a larger area. That's why I think retrieving iron from our society will be more like extracting iron from ironsands (2-20% iron), and it will have similar effects on that subsequent society that sits between us now and some future point where geology has re-supplied it to the surface millions of years from now.

by gpm 5y ago

masklinn 5y ago

Iron rusts over historical timescales, metal deposits form over geological ones.

gpm 5y ago
Am I wrong to think of metal deposits as just "rust mixed with rock"? It doesn't seem like the rock part (i.e. the details of what it is mixed with) should be critical for the extraction process?
- masklinn 5y ago
  > Am I wrong to think of metal deposits as just "rust mixed with rock"? They're concentrated rust mixed with rocks, otherwise it's not economically viable to extract. Like, iron is ridiculously common, relatively speaking: on earth as a whole it's more common than oxygen, for the crust it ranks 4th at 5% by mass, meaning if you went at it randomly you'd need to sift through 20kg of materials to get 1kg of iron. Currently, we exploit formations as low as 15% iron (banded iron formations / taconite), that's the lower limit of the economically feasible, and those results in absolutely enormous amounts of tailings (waste materials). Pre-industrialisation, unless you had no other choice (e.g. only had ironsands to work with) you really wanted to exploit natural (or "direct-shipping") ores, in the 60~70% range, the extraction is way too much work otherwise.

mikewave 5y ago

Sure, if you have a billion years to wait for it to all run through the rock cycle again.

ncmncm 5y ago
The process that concentrated the iron ore we rely on does not operate anymore. Fortunately, despite millions of tons of production every year, we are nowhere near using up the ore.

evilduck 5y ago

The problem will be a lack concentrated deposits making post-collapse (and new) industrial efforts much harder. A pile of rust from one tractor in someone's back yard is not going to be worth the effort to mine and refine. Possibly an analogous situation is the history of steel in Japan and their efforts to extract iron from sand, since they don't have significant iron ore to mine on their island.

gpm 5y ago
The pile of rust that use to be a tractor, sure, that's not worth much. The pile of rust that use to be a city though... surely that's more concentrated than the rust mixed with rock (or clay) that we originally extracted it from?
- evilduck 5y ago
  Most likely not: https://en.wikipedia.org/wiki/Iron_ore https://en.wikipedia.org/wiki/Iron_ore, it's almost 50% iron in the worst case and can be up past 70% in the best case. The geological processes that form this ore in the mantle do a pretty good job of concentrating iron by density but it takes geological scales of time to do so and have it lifted back up through the crust. We might have to excavate a lot of earth to get to the concentrated vein of ore but when we find it it's relatively easy to chase and turn into usable iron once you know a little bit about mining and smelting as a society. A city decayed to rust is going to be a thin layer of iron spread out over miles with some hot spots like where a building once stood (but presumably without a map of the city in this distant future scenario), but there won't be a vein of concentrated ore. Distributed rust can definitely be turned back into pure iron but the energy requirements are going to be substantially higher to do so since you're going to have to sift through much more material to collect it, more material to separate and concentrate it, more material to smelt off, and your operations will have to be more mobile to retrieve it over a larger area. That's why I think retrieving iron from our society will be more like extracting iron from ironsands (2-20% iron), and it will have similar effects on that subsequent society that sits between us now and some future point where geology has re-supplied it to the surface millions of years from now.