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.
Chemical Cleanup & Trace Impurity Removal
After the cracking process has been performed on a mineral concentrate, the next step is to ready the material for individual REE separation and purification. There are a number of steps required here, and the necessary steps taken depend upon the solution chemistry and impurity concentrations.
The steps listed below are for removal of small amounts of impurities:
1. Add sufficient water and acid (usually HCl) to the cracked mineral concentrate to solubilize the REEs
2. Filter off the residue
3. Adjust the pH to precipitate the iron
4. Add sufficient sulfide to precipitate the residual iron, heavy metals, U, Pb, and other insoluble sulfides
5. Add barium chloride and sodium sulfate to co-precipitate any residual radium and other radioactive elements
6. Filter off all precipitated impurities
The solution is now sufficiently clean and ready to be used as solvent extraction (SX) plant feed. The next processing steps depend upon the location of the SX facility. If the next step is located in-house, then the solution is used directly as feed, if not, then the solution or its contents must be made into a transportable form.
For transport as a liquid, the solution REE content must be at an appropriate concentration level, and if not, then heat must be applied to solution evaporating off some of the water.
For transport as a solid:
1. Additional heat is applied to the above solution until the REE chloride solution solidifies. This solid can be easily made back into a solution with water addition. Note, water used for solution remake must be very clean in order to not add any unwanted impurities.
2. The solution can have the REE precipitated as the carbonate with soda ash. Note that this REE carbonate gives off CO2 gas when re-dissolved in acid, the contained sodium is very high, and any impurities in the soda ash will transfer to the REE carbonate.