Method of recovering rare-earth elements

A method of recovering rare-earth elements from a bauxite residue, which is produced as a by-product in a Bayer process and is used as a raw material,

• the method comprising:
• a leaching step of bringing the bauxite residue into contact with a liquid leaching agent to recover a leachate containing rare-earth elements;
• a pretreatment step of bringing the leachate recovered in the leaching step into contact with a pretreatment liquid containing one of a phosphoric acid ester-based extractant and a phosphonic acid ester-based extractant at a concentration of from 0.01 to 0.1 mol/L, followed by filtration treatment, to thereby recover a pretreated leachate containing the rare-earth elements;
• an extraction step of bringing the pretreated leachate into contact with an extraction treatment liquid containing a phosphoric acid ester-based extractant to recover an extract liquid containing the rare-earth elements;
• and
• a back extraction step of bringing the extract liquid into contact with a back extraction treatment liquid to recover the rare-earth elements,
• the leaching step comprising recovering a leachate having a pH of 0.5 to 1.2 by using an acidic liquid leaching agent,
• the extraction step comprising adjusting the pH of the pretreated leachate to 1.0 to 1.7 and then bringing the pretreated leachate into contact with the extraction treatment liquid under a condition that the pretreated leachate has an oxidation-reduction potential of 300 mV or less adjusted by a method of performing electrolytic reduction treatment,
• wherein the back extraction treatment liquid comprises a 40 to 60 mass % sulfuric acid aqueous solution, and wherein the extract liquid and the sulfuric acid aqueous solution are brought into contact with each other at a volume ratio of extract liquid:sulfuric acid aqueous solution of from 5:1 to 10:1, or
• wherein the back extraction treatment liquid comprises a 4 N to 8 N hydrochloric acid aqueous solution, and wherein the extract liquid and the hydrochloric acid aqueous solution are brought into contact with each other at a volume ratio of extract liquid:hydrochloric acid aqueous solution of from 5:1 to 10:1.