Once a 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:
- Add sufficient water and acid (usually HCl) to the cracked mineral concentrate to solubilize the REEs
- Filter off the residue
- Adjust the pH to approximately 3 and most of the iron will precipitate
- Add sufficient sulfide to precipitate the residual iron, heavy metals, U, Pb, and other insoluble sulfides
- Add barium chloride and sodium sulfate to co-precipitate any residual radium and other radioactive elements
- 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. This value is approximately 300 g/l as REO, and if not, then heat must be applied to solution evaporating off some of the water.
For transport as a solid:
- Then 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.
- The solution can have the REE precipitated as the carbonate with soda ash. Note that this REE carbonate gives off CO2 gas when redissolved in acid, the contained sodium is very high, and any impurities in the soda ash will transfer to the REE carbonate.