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So I have this "lost colony" planet where the locals decided they didn't want high-tech.
Intentionally they don't go further than pre-Industrial Age tech.
No internal combustion or steam engines. No electricity. They are not averse to automation as long as it is wind-, water- or muscle-power driven.
They do have retained knowledge of physics, chemistry, biology, etc. but their chosen tech-level limits to some extend what they can do. Planet is Earth-like: Similar gravity and other conditions.

Now for the problem:
There is an abundant, easy to mine and cheap mineral available that can be processed into a much more valuable substance by subjecting it to high (50 bar or more) pressure for several hours. The pressure causes a chemical reaction that converts the mineral.
Additional complication: To prevent the processed stuff from self-igniting in air (it burns like thermite, so this is best avoided...) during pressurization and afterwards for storage and transport it must be kept under water or oil. (Will self-ignite in 2-3 minutes after exposure to air. Short period in air is OK.)
The raw unprocessed mineral is inert and doesn't care whether it is wet or not.

How to do this?

My solution:
Put the mineral in a large bag. Add some weights and sink it in deep water to 500 meters or more. Haul it back up after a few hours and store it in barrels with water or oil.
Water-pressure rises with 1 bar for roughly every 10 meters. And the stuff is kept wet.
That seems to satisfy all the requirements.

Is this feasible? Or did I overlook something?
Can anyone come up with another idea, within the tech-limits of this world?

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  • $\begingroup$ I don't see any reason why your solution won't work. But if you want to go more in depth, here is the full equation for water pressure: physicsforums.com/threads/pressure-under-water.307277 $\endgroup$
    – upfish
    Commented Feb 25, 2015 at 20:46
  • $\begingroup$ If this processed substance is so volatile, why is it valuable? $\endgroup$
    – KSmarts
    Commented Feb 25, 2015 at 21:52
  • $\begingroup$ @KSmarts I suddenly have an image of a magnifying glass dancing before my eyes... $\endgroup$
    – Tonny
    Commented Feb 25, 2015 at 21:53
  • $\begingroup$ @KSmarts Precisely that. Burns at an extreme high temperature which is useful in metallurgy and glass-production. Temperature can be reasonably well controlled by mixing the stuff with non-burning stuff. Nothing else on the planet burns like that. $\endgroup$
    – Tonny
    Commented Feb 25, 2015 at 21:57
  • $\begingroup$ @Tonny Nothing else, except termites. $\endgroup$
    – KSmarts
    Commented Feb 25, 2015 at 22:06

4 Answers 4

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What I don't like of the OP original suggestion is that you would need an elastic vessel, which is definitely not something "low-tech".

You can sink a rigid vessel up to 500 meters, but the structure receiving the pressure is the vessel itself, not the contents. And if the vessel cannot withstand the pressure, it will crack. Think of a submarine going deeper and deeper, the internal pressure in the submarine does not change.

You could leave a few openings in the vessel to let sea water inside and then the internal pressure would be that of the ocean at that depth. But you would contaminate your chemicals and reaction products with sea water; by the time the vessel reaches the 500 mts mark any soluble compound in the reaction would have been dispersed into the ocean.

The only kind of flexible membrane to pass pression from the ocean to the interior of the vessel without contaminating it with sea water would be some kind of animal skin/leather. That said, I don't know how well any leather can resist such pressions without shearing (not to mention the salt water and temperature changes).

Instead, I would go the opposite way. Create a robust, resistent vessel, fill it with gases, and heat it. A lot.

Now, 50 bars are 50 atmospheres. If beginning with a 1 atmosphera interior pressure, and you want to get 50 atmospheres, you would need to multiply the temperature (in kelvin) by 50. In a cold day, it means that you have to get 273*50 = 13.650 K (or about 13.400 C), which is quite a lot, so you will have to cheat.

You will cheat by starting with more than an atmosphera of internal pressure. For example, there are lots of chemical reactions that produce CO2, so, just before sealing your vessel, you pour these chemicals in. If you get 10 atmospheres of initial internal pressure, you only need to head the interior to 273*5 = 1.365 K or 1.100 C, which is not that hot if the system is well designed.

The advantage of the system is that:

a) You don't have to find a 500 meters depth spot in the sea. You can do everywhere you want to, provided you have fuel.

b) Scalability. If a 1 cm layer of your building material (ceramics, metal) cannot stand 50 bar, then you can just make your vessel 10 cm thick. The "sea option" means that the size of the vessel is constrained, and some parts (plastic membrane) cannot be enlarged at all.

Of course, an issue is with such pressures, risks of catastrophic failure are somewhat high. You do not want that thing in the middle of your town. And, wherever it is, you want it to be surrounded by an sturdy wall. Anyway, building a "safety valve" is not that difficult (just make a section of the top thinner enough so it breaks way before the rest of the structure does).

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  • $\begingroup$ You could use whale skin leather. There must be something magical there that causes it not to "shear" at depths several times deeper than the required 500m. Or perhaps the forces do not work that way ;) $\endgroup$
    – Samuel
    Commented Feb 25, 2015 at 23:17
  • $\begingroup$ Just put the mineral in a bucket full of sea water. Given that the processed stuff can be stored in water, there is little risk that water contaminate the reaction. Otherwise, you can use mercury instead of water. $\endgroup$
    – mouviciel
    Commented Feb 26, 2015 at 9:19
  • $\begingroup$ It doesn't even need to be elastic, merely flexible and watertight. I would think a rubber bag would suffice. $\endgroup$
    – Loren Pechtel
    Commented Feb 27, 2015 at 1:21
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Your idea is good. You might not be considering the decreased temperature underwater, so the reaction may need more time to complete under those conditions. Additionally, as it's likely reacting with the oxygen in the air, it may still oxidize somewhat in water.

If temperature doesn't matter too much you could also place the mineral inside an oil filled bag, suspend the bag in a strong box of water, and freeze the whole thing. You would need very slightly lower than 0­°C to freeze water at that pressure. Ice will push at over 2,000 atmospheres (2120 bar) before it gives up and settles on being ice II or III. Basically, you'll get as much pressure as you can build the container to withstand.

enter image description here

To visualize, it's similar to a pneumatic cylinder. The expanding ice serves as the pressure source, a heavy lid as the regulator, and you'd want an access port in the side. Obviously, this image is not to scale, if the lid were made of iron and had a surface area of 10cm it would need to be 6.5m tall to exert 50bar against the expanding ice. This may seem insane, but it's technically feasible. The seal is not as much of an issue with gas, you're dealing with something more like a highly viscous fluid (the expanding solid ice).

Overall, deep water, or less deep for a denser liquid (like mercury, if abundant on your planet), is a very good solution. Just put it in a flexible container filled with oil to keep all oxygen out.

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  • $\begingroup$ That container would be problematic given the tech-level. You must be able to open it. Any sort of lid or cover would provide an escape point for the pressure. You transfer the pressurization problem to "how to keep the lid in place". And as far as ice is concerned: I need the pressure for hours. Ice would change state quickly if I remember my physics correctly. The oil is a good idea. $\endgroup$
    – Tonny
    Commented Feb 25, 2015 at 22:03
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    $\begingroup$ @Tonny Is the civilization opposed to screw caps? Accessing the container from a smaller screw-capped opening would be easier. While a large heavy 'lid' could be designed to regulate pressure. A sufficient mass could be found such that the ice needed to exert 50 bar to lift it up a channel. I don't see the issue you're talking about with the phase change, pressure is maintained until released. $\endgroup$
    – Samuel
    Commented Feb 25, 2015 at 22:23
  • $\begingroup$ The lid is not as problematic as you perhaps suppose; you just need to make it strong enough not to get crushed when compressed equally from all sides. Then you can take advantage of the external pressure to hold the lid in place, which should make it easier to remove once removed from the pressure tank. $\endgroup$
    – Mike L.
    Commented Feb 25, 2015 at 22:37
  • $\begingroup$ @MikeL. I think you're imagining this incorrectly. A rigid container inside the pressure vessel would not transfer the pressure, that would be pointless. The container needs to be flexible so that the force is transmitted to the minerals inside. $\endgroup$
    – Samuel
    Commented Feb 25, 2015 at 23:19
  • $\begingroup$ @Samuel Sure it would transfer the pressure; you just need to fill it with something incompressible (like a liquid). Then it only needs to flex very little. $\endgroup$
    – Mike L.
    Commented Feb 26, 2015 at 8:45
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Pressure = Force / Surface

Use surface of 0.1 cm2(circle with diameter about 3 mm) and you need only 5 kg weight to create the 50 bar. Take a wooden log, put a nail into it and you have the press.

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The idea with the water seems the best so far as long as your mineral doesn't go through drastic density changes initialized by either the pressure or the polymorphism.

Solids and liquids for usual take a lot of pressure before they noticeably shrink. So what you'd need would merely be an air tight container that's somewhat flexible, in which you fill your mineral, then fill up with oil and seal.

Even simple metal will work as long as it's ductile and the wall is weaker than your lid.
Heck, it doesn't even have to be completely air-tight. Make a small hole in the top/bottom (depending on whether the oil is more or less dense than water) and nature's gonna do what's left to do (with some oil loss of course, but you can optimize the technique further to keep the loss low)

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