How can I get out of the Copper Mill?

I didn't understand David Yell's own solution, but here is another, probably easier way:

Standing all the way at the end at the top, facing the burning debris that's blocking the way, turn right 90 degrees, and, above the stairs you came from, there is a ledge. Climb it, and from the platform you've reached there is a sort of chute down to a secluded area at a lower level.

I knew that as soon as I posted this I would figure it out.

The trick is that, when you are at the top section, near the large oil container with the ladder on the side, if you turn and look over the edge, you can jump off and land on the balance beam.

This will then lead you back down to the camp, from which you can travel to the Sheltered Ridge campsite.