Decomposition mechanism of a mixed rare earth concentrate with sodium hydroxide in the microwave heating process

摘要

In this paper, a novel process is proposed for extracting rare earth elements through intensifying the decomposition of mixed rare earth concentrates by microwave heating. The objective of the present study was to investigate the phase transition mechanism of a mixed rare earth concentrate with sodium hydroxide in a microwave field. To this end, the decomposition and separation efficiencies of the rare earth elements and fluorine were analyzed. The results showed that the decomposition reaction of the tested mixed rare earth concentrate in the microwave irradiation field occurred in three stages. The rare earth elements initially presented in bastnaesite and monazite were gradually converted to rare earth hydroxides (RE(OH)(3)), rare earth oxides (RE2O3), and composite oxides (Ce(0.5)Hd(0.5)O(1.75)), with fluorine eventually forming sodium fluoride. The decomposition ratio of the mixed concentrate and conversion ratio of the fluoride reached 88.73% and 89.32%, respectively. Furthermore, the decomposition temperature of the mixed concentrate with microwave heating was lower than that required with conventional heating, which indicated that microwave heating could decrease the thermodynamic reaction temperature. The results of the microscopic morphology analysis showed that after microwave heating, the samples exhibited two distinct particle morphologies, which was different from that of a mixed concentrate in any case. The decompositions of bastnaesite, monazite, and fluorite were somewhat non-interfering in nature, and sodium fluoride and calcium phosphate were separated from the rare earth oxides in the decomposition products. These results indicated that the proposed process was conducive to the separation of rare earth elements and fluoride and helpful for improving the efficiency of rare earth leaching.