Processing for the extraction of rare earth elements
Processing schemes for the extraction and/or separation of rare earth elements (REEs) from rare earth containing products such as rare earth mineral ore bodies and intermediate products derived from rare earth mineral ore bodies. The processing schemes may be applied independently or in various combinations to produce end-products that have a very high purity with respect to REEs, including high value critical REEs. The processes may include acid digestion, formation of rare earth oxalate compounds, metathesizing of rare earth oxalate compounds, selective precipitation and/or solvent extraction to form the high purity REE end products.
1. A method for the removal of thorium from an REE-containing acidic solution, the REE-containing acidic solution comprising thorium and at least one rare earth element, the method comprising the steps of:
contacting the REE-containing acidic solution with a phosphinic acid-type extractant to extract at least a portion of the thorium from the REE-containing acidic solution, whereby a thorium-depleted raffinate and a thorium-loaded extractant are formed; and
contacting the thorium-loaded extractant with oxalic acid to strip thorium from the thorium-loaded extractant and form a thorium-depleted extractant.
2. The method recited in claim 1 , wherein the phosphinic acid-type extractant comprises bis(2,4,4-trimethylpentyl) phosphinic acid.
3. The method recited in claim 1 , wherein the phosphinic acid-type extractant is dispersed in an inert hydrocarbon diluent.
4. The method recited in claim 1 , wherein the REE-containing acidic solution comprises nitric acid.
5. The method recited in claim 1 , wherein, during the step of contacting the REE-containing acidic solution with a thorium selective phosphinic acid-type extractant, the REE-containing acidic solution has an acidity of at least about 0.001 M.
6. The method recited in claim 1 , wherein, during the step of contacting the REE-containing acidic solution with a thorium selective phosphinic acid-type extractant, the REE-containing acidic solution has an acidity of at least about 0.5M.
7. The method recited in claim 1 , wherein, during the step of contacting the REE-containing acidic solution with a thorium selective phosphinic acid-type extractant, the REE-containing acidic solution has an acidity of not greater than about 8.0M.
8. The method recited in claim 1 , wherein, during the step of contacting the REE-containing acidic solution with a thorium selective phosphinic acid-type extractant, the REE-containing acidic solution has an acidity of not greater than about 1.0M.
9. The method recited in claim 1 , wherein, during the step of contacting the REE-containing acidic solution with a thorium selective phosphinic acid-type extractant, the REE-containing acidic solution has an oxidation-reduction potential of at least about 1500 mV.
10. The method recited in claim 1 , wherein the acidic solution is formed by digesting a rare earth oxide product in an acid.
11. The method recited in claim 1 , further comprising the step of separating the thorium-depleted extractant from the oxalic acid and recycling the thorium-depleted extractant back to step of contacting the REE-containing acidic solution with the thorium selective phosphinic acid-type extractant.
12. The method recited in claim 1 , wherein substantially all of the thorium in the acidic solution is in the +4 oxidation state.
13. The method recited in claim 12 , further comprising the steps of:
calcining a thorium-containing compound at a temperature of at least about 710° C. in an oxidizing environment to form a rare earth oxide product; and
forming the REE-containing acidic solution by digesting the rare earth oxide product in an acid.