Influence of the Alkali-promoted phase transformation in monazite for selective recovery of rare-oxides using deep eutectic solvents

Abstract

• High temperature formation of Ce 1 - x Ln x O 2 - x / 2 solid solutions has been demonstrated. • Electrochemical activity of pure rare earth oxides in the DES Ethaline has been proven. • 23.85% Nd 2 O 3 in a 0.1 mol/L glucose solution in Ethaline was dissolved. • No dissolution was reported for pure CeO 2 , La 2 O 3 and PrO 2 /Pr 2 O 3 . • Dissolution of Ce 1 - x Ln x O 2 - x / 2 formed in roasting was not possible because of its inert behaviour. The physico-chemical changes occurring during the high-temperature phase transformation of monazite in the presence of Na 2 CO 3 at 1000 °C for 2 h duration at monazite: Na 2 CO 3 ratios between 1.0 and 5.0, were investigated. The formation of sodium lanthanide phosphates was prevalent above a monazite:alkali ratio of 2, however, below this ratio, the dephosphorization of monazite as Na 3 PO 4 and Ce 1 - x Ln x O 2 - x / 2 solid solutions occur offering unique selectivity for rare-earth oxide separation from the mineral matrix. Cyclic voltammetry of pure CeO 2 , La 2 O 3 , Nd 2 O 3 , and PrO 2 /Pr 2 O 3 was carried out in the deep eutectic solvent Ethaline (1:2 mixture of choline chloride and ethylene glycol) proving the electrochemical activity of these oxides. Electrodissolution of pure oxides and water-leached monazite after high-temperature reaction with a ratio of 1:1 was carried out in a 0.1 mol/L glucose solution in Ethaline showing a preferential solubility of 23.85% for pure Nd 2 O 3 . In contrast, pure oxides of CeO 2 , La 2 O 3 and PrO 2 /Pr 2 O 3 were found to be insoluble. We also observed that electrodissolution of the water leached monazite was not possible because of the inert behaviour of Ce 1 - x Ln x O 2 - x / 2 solid solutions. Avoiding cerium oxidation during the high-temperature process will lead to a method for further selectivity for rare-earth oxide processing using staged electro-chemical winning of oxides.