Dephosphorization Behavior of Monazite Concentrates Through Carbothermal Treatment at High Temperatures: Thermodynamic Assessment and Experimental Study

Abstract

The high-temperature, carbothermal treatment of REE-bearing monazite concentrates to remove P in preparation for REE extraction was investigated. The study comprised a series of thermodynamic assessments followed by laboratory-scale experiments at 1400 °C and 1500 °C at 1 atm pressure. Two monazite concentrates with different compositions were used; a RE flotation concentrate from Australia and a high-grade Korean monazite concentrate. The Australian sample contained 38% total RE (49% monazite) as well as goethite (32 wt%) and aluminosilicate gangue constituents. The Korean sample was higher grade, containing 58% RE (91% monazite) with minor gangue components. The P level in the high-grade Korean monazite concentrate was reduced from 27.1% P2O5 to 1.9% P2O5 (93% reduction) following carbothermal reduction at 1400 °C. For the lower grade Australian concentrate and using the same conditions, the P level was lowered from 19.4 to 15.8% P2O5, i.e., only a 19% reduction. Characterization of the calcine for the Australian sample revealed that the remaining P was mainly associated with iron, as Fe2P, whereas the REs were tied up as REE-bearing silicates and aluminosilicates. For the Korean calcine, the REs were mainly present as REE-oxides and REE- silicates. These results indicated that impurities in the monazite concentrate such as Fe, Al, and Si significantly impact P removal during carbothermal treatment via the formation of unwanted metallics, phosphides, and REE-bearing aluminosilicate phases. Based on the results, a generalized monazite carbothermic dephosphorization mechanism, encompassing the effects of various feed impurities, is proposed. Further, the strategic implications for the dephosphorization of monazite using carbothermal treatment at high temperatures are discussed. Graphical Abstract