The residual material contains uranium that the mill can extract to make fuel rods for nuclear power plants. The mill is located in the rural area of Utah where the Bears Ears National Monument protects lands considered sacred to Native Americans, but the facility isn't in the original or revised boundaries of the monument.
President Donald Trump downsized the monument in 2017, triggering backlash from conservation groups.
Information from: Cortez Journal, http://www.cortezjournal.com/