Imagine that you have a big chunk of glass that has blended into it some desirable metal compound. It can be a metal aluminate, silicate, or oxide. Now imagine that there are may be a dozen other metal species in there as well. Some of these metals will have the same oxidation state and similar ionic radii so that rather than having discrete domains of entirely specific composition, you have a dogs lunch of compositions and phases. Now lets say that the metal of interest is measured at the tens to hundreds of ppm.
So, Mr./Ms. chemist, how would you set about retrieving the desired metal from the matrix? I specified glass only to set a reference point with regard to reactivity. Many ores are full of quartz or amphiboles or micaceous silicates or aluminates that are relatively inert and perhaps refractory. So, if you seek metal M which is dispersed in the matrix, dissolution of that matrix to pull out M is going to be challenging.
Ores that are rich in metal sulfides can be roasted in air to liberate the sulfur as volatile SO2, leaving behind a metal oxide. Metal oxides are often rather soluble in strong acid, so recovery of M as a pregnant liquor is messy but straightforward. Recovery of metals that are not found as sulfides or oxides can be less than straightforward.
A natural consequence of having an inert, refractory matrix is that the whole thing can’t digested into an aqueous solution phase, with the possible exception of HF treatment. Only the surface can be attacked and extracted. So, the idea is to increase the surface area. This is commonly done by milling and is called comminution. A rule of thumb is that the milling cost varies linearly with the surface area generated. Milling is commonly a multistage process wherein big chunks are crushed and gradually hammered into small chunks. This is very energy intensive.
There are many ways to recover and transform elemental metals from their ionic form, from electro-winning to chemical reduction. Every element is isolated according to it’s unique chemistry. Rare earth metals are particularly troublesome owing to the fact that they are very often found together in the ore and most of them have a + 3 charge and are of similar ionic radius. A few stand out as exceptions, like cerium which can take a + 4 charge.
Solvent extraction schemes have been developed to take advantage of differential affinity for certain extractants. Solvent extraction technology was advanced in the post WW2 atomic age. Since many rare earths are often associated with uranium and thorium, or vice versa, rare earth extraction technology was developed as a result of U and Th beneficiation.