High pressure caustic leach methods and processes for recovery of scandium and rare-earth oxides

A method of extracting Rare Earth Elements (REE) and/or scandium from ore, REE and/or scandium bearing feedstock, and/or scandium/REE bearing mineral concentrate, the method comprising:

• providing the ore, the REE and/or scandium bearing feedstock, and/or scandium/REE bearing mineral concentrate;
• leaching the ore, the REE and/or scandium bearing feedstock, and/or the scandium/REE bearing mineral concentrate in an alkali solution at a first temperature for a target duration and at a given pressure to produce a leachate slurry, wherein the leaching in the alkali solution is a High Pressure Caustic (HPC) leaching;
• separating a solid residue from the leachate slurry;
• leaching of the solid residue in a mineral acid to form a primary leach solution;
• and
• extracting REE and/or scandium from the primary leach solution,
• wherein the given pressure is between 9.87 ATM and 39.48 ATM, the HPC leaching is performed in a reaction vessel or in an autoclave, and the leaching of the solid residue in a mineral acid comprises stirring for a period of 60 to 1200 minutes at a second temperature between 20° C. and 30° C. and at 1 ATM.