Microwave-assisted fluidized sulfate roasting of zinnwaldite for enhanced lithium extraction: Phase transformation and reaction mechanism

To address the limitations of conventional sulfate roasting for lithium extraction from zinnwaldite, including high roasting temperature, long roasting duration, and high energy consumption, a microwave-assisted fluidized roasting-water leaching process was proposed in this study. The effects of roasting temperature, roasting time, Na2SO4 dosage, and CaO dosage on Li leaching efficiency were systematically investigated, and the reaction mechanism was elucidated in detail through thermodynamic calculations combined with XRD, SEM-EDS, BET, and FTIR characterizations. The results showed that a Li leaching efficiency of 92.6 % was achieved at 800°C for 30 min with a mass ratio of ore: Na2SO4: CaO = 1:1.5:0.2. Compared with conventional fluidized-bed roasting under identical operating conditions, microwave-assisted fluidized-bed roasting increased the Li leaching efficiency by 9.5 %, while reducing electricity consumption from 74 to 61 kWh ⋅ kg−1, corresponding to an energy saving of approximately 17.6 %. In addition, sintering of the material was effectively alleviated by microwave heating. SEM and BET analyses demonstrated that a loose and porous structure was developed in the microwave-roasted product, thereby enhancing lithium release and mass transfer during subsequent leaching. These findings are of significance for the green and efficient utilization of zinnwaldite resources.