When the usual mineral processing procedures such as size reduction, gravity, flotation, or magnetics do not allow for efficient separation/recovery of the REE minerals, this is often due to mineralogical issues.
Primary REE minerals can be chemically and physically altered through normal geological processes, with the result being new REE minerals being formed, termed “secondary” or “supergene”. This occurs through recrystallizations, and these non-primary minerals can be, in appearance, very dendritic or having a root-like structure when viewed under the electron microscope. Also, the grain size of these new minerals can often be extremely fine, <10um, and beyond the scope of normal mineral processing procedures. Further, the gangue minerals are altered too, and usually very fine grained. Finally, sometimes the iron content can be very high, causing additional poor efficiencies in most processing techniques.
When this occurs, the REE ore material is often reacted directly with acid, either HCl or sulfuric, in an attempt to completely dissolve the REE minerals. Once dissolved, various chemistry approaches are used to clean up the solution of impurities, and leave only the REEs in solution. The dissolved REEs can be then precipitated with oxalic acid, fired in high temperature furnaces to the oxide, and sold to SX plants as feed. Carbonate precipitation is also utilized.
Although this might appear to be a simple and straightforward process, it has rarely, if ever, proven to be industrially useful. Typical problems are the large amounts of required reagents and their cost, low separation efficiencies between the REE and the impurities, many purification steps required with REE losses between each step, extremely large solution volumes to be processed, radiological contamination from any contained thorium/uranium/daughter products, and finally the neutralization and storage of the tailings.