Pretreatment methods for rare earth element extraction in supercritical carbon dioxide

In this study, rare earth elements were recovered from bastnasite concentrate and waste fluorescent lamp phosphors using supercritical carbon dioxide extraction with a nitric acid/tributyl phosphate adduct. Several pretreatment methods were used to alter the crystal structure of the source materials, and thus improve rare earth element recovery. Pretreatment methods included 1) dry roasting of the source material at 730^oC, 2) decomposition with 50% sodium hydroxide solution at 150^oC for 4 hours, and 3) decomposition with 12 M sulfuric acid at 300^oC for 4 hours. Treated and untreated samples underwent supercritical extraction at 24 MPa and 65^oC for 1 hour. Untreated bastnasite and phosphor samples showed <10% rare earth element recovery, but sodium hydroxide digestion resulted in >60% recovery for most rare earth elements. Dry roasting of the bastnasite concentrate was also effective, resulting in >60% recovery for cerium.