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?
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?