MP Minerals operates the Mountain Pass, CA rare earths mine. This was the largest rare earths mine in the world at one time. After several shutdowns and two bankruptcies due to low rare earth prices, MP Minerals bought the mine in 2017 and had it producing ore in 2020. The mine already had a "beneficiation" plant, where raw ore goes in and a mix of rare earth elements, plus lots of unwanted tailings, come out. Unlike the plants in China, they have a process that doesn't require huge settlement ponds and vast amounts of water, so it doesn't create too much of a mess. It's mostly a mechanical process, and processes huge volumes of ore, yielding a much smaller volume of concentrate.
What comes out of the benficiation plant needs more separation. That had to be sent to China to be refined, because there were no plants to do the separation in the US. So MP Minerals got the US Department of Defense to fund a separation plant at Mountain Pass. This separates the different rare earths. It's mostly a chemical process. Again, what comes out is a lot less volume than goes in. The Mountain Pass separation plant has been working for a few years now.
What comes out of the separation plant needs more purification and alloying. That's what the new plant in Texas does. That's a modest sized facility in an industrial park. What comes out is metal ready to be formed into magnets. That completes the supply chain.
Owning the whole supply chain insulates MP Minerals from trouble in the middle stages. Such as China's 2024 ban on export of rare earth processing technology.[1] Now that they have the whole supply chain working, they can increase production if desired. It's much easier to duplicate a plant than make a new one for a new process.
[1] https://www.csis.org/analysis/what-chinas-ban-rare-earths-pr...