Some amount of rare earth minerals must surely be by-products of base metal mining, no?

I recall reading somewhere that the Dept of Defense contracted some conventional mine in the US to produce something rare that they needed but the mine was unable to profit from, so DoD stepped in and made up the difference. Can't recall the details.

My question is: at what price does it become profitable to extract these minerals from existing stock piles or tailings? Put another way, is it cheaper to develop a primary mine, or does that run head-on into what's already above ground just waiting for the right price?

I think you need to know this before you jump into this sector, no?

ebear