High-temperature thermal plasma treatment of monazite followed by aqueous digestion

Abstract

Monazite is a chemically inert, radioactive phosphate mineral that contains the light rare earth metals (Ce, La, Nd, Pr and Y), thorium and uranium. A new process, which improves the extraction efficiency of monazite, is under development. The equilibrium mole composition, in conjunction with the Gibbs free energy, indicates that as monazite is heated in the presence of carbon to a temperature above 1400°C, it decomposes into the rare earth oxides. These temperatures are within the operational limits of a thermal plasma. When monazite is fed to a DC plasma it forms amorphous plasma-treated monazite (PTM), which is more susceptible to chemical attack. This increased chemical reactivity permits dilute mineral acids to digest the PTM, which leads to higher extraction efficiencies of the rare earth elements than when monazite is digested using the conventional processes. When the conventional process for digestion of monazite is adapted to PTM, the extraction of rare earth elements, thorium and uranium is increased significantly.