Once a REE mineral concentrate has been produced, the contained REEs need to be extracted and made available for separation and purification. This is done using a process termed “cracking” or “mineral decomposition”. There are several ways of doing this, listed below:
1. Sulfuric acid is utilized most often because it is low cost and effective. Here, the mineral concentrate is mixed with sulfuric acid to make a slurry or paste, and passed through a rotary furnace at elevated temperatures. Upon exiting the furnace, the REE minerals are decomposed resulting in a complex mixture of REE sulfates and concentrate impurities. There are also instances of this process being performed in batch. Noteworthy issues in this process are:
- Incomplete REE mineral decomposition due to insufficient furnace residence time
- Some REE mineral grains in the concentrate do not react completely due to size and liberation issues
- Subsequent limited REE solubility due to excess sulfate in the process solutions
- Sulfuric acid decomposition producing SOx and F containing gasses
2. Hydrochloric acid can be used under certain conditions, but it is less effective at mineral decomposition than sulfuric acid. It can be used under high temperatures and pressures in batch mode, but often with limited success. One unique and historic use is for processing roasted bastnäsite, which when roasted transforms into a new fluorite structure mineral containing insoluble cerium. The other contained lanthanides are readily dissolved in HCl, isolated in the usual manner, and further processed. The cerium-containing insoluble residue cannot currently be sold due to the high levels of impurities, but can be further processed with other techniques.
3. Caustic cracking is very efficient at cracking most REE minerals. It is usually performed in batch mode at moderately elevated temperatures. Any contained silica in the mineral concentrate is also solubilized, leading to the usual problems of filtering and phase separations due to silica. The cracked residue contains the REE as hydroxides, which are readily soluble in HCl. Further processing is required to remove the contained impurities.
Note, close chemical analysis during the cracking process is necessary, as some of the REE can be observed to re-precipitate after cracking conditions are relaxed. Precipitates containing REE fluorides and phosphates can be observed as well as a compound called a double sulfate, a mixture of sodium and REE sulfate.