Thermal decomposition mechanism of low-content-fluorite Bayan Obo rare earth concentrate roasted with sodium carbonate and its consequent separation study

Decomposition Process of Nonoxidative Microwave Radiation Roasting of a Mixed Rare Earth Concentrate with Sodium Carbonate

This article introduces an efficient decomposition process that uses sodium carbonate (Na₂CO₃) and activated carbon (C) as additives to decompose Bayan Obo mixed rare earth concentrate (hereinafter to be referred to as RE concentrate) by nonoxidative microwave radiation roasting. The roasting temperature, holding time, and contents of Na₂CO₃ and activated carbon are investigated. The optimum process parameters for decomposition are 800 °C and 30 min. The ratio of m(Na₂CO₃)/m(RE concentrate) is 0.5, and the ratio of m(C)/m (ER concentrate and Na₂CO₃) is 0.2 based on experimental data. Under the above conditions, the decomposition rate (shortened to DR) of RE concentrate is 98.58%, and the removal rates (shortened to CRs) of fluorine (F) and phosphorus (P) reached 80.35 and 46.75%, respectively. These rates are higher than traditional oxidation roasting under the same conditions. The three reasons for the result are the unique microwave heating characteristics, the overall efficient reaction of the mixture (RE concentrate, Na₂CO₃, and activated carbon), and the high nonoxidation rate of cerium. For these reasons and large experimental data, the reaction rate of the mixture is improved, and the efficiency of dilute hydrochloric acid for leaching rare earth elements is enhanced. In this article, the valence of rare earth elements in the roasted ore is all in the form of trivalence. Importantly, this nonoxidative roasted product can avoid the generation of chlorine in hydrochloric acid leaching. Moreover, such a short holding time is scarce in traditional roasting. When the mixture was roasted by utilizing microwave heating, the sinter phenomenon of the roasted product was avoided at high-temperature roasting. Finally, the surface morphology of RE concentrate at different conditions was observed by scanning electron microscopy (SEM) analysis, which can be used to compare the specific differences of roasting methods. According to these results, this process is beneficial for the decomposition of RE concentrate in terms of Na₂CO₃ roasting and is helpful for improving the clean and green technology method of hydrometallurgy.