Innovative Application of Microwave Treatment for Recovering of Rare Earth Elements from Phosphogypsum

Some rare earth elements (REEs) are classified as strategic materials because of their increasingly high demand, supply uncertainty, and near zero recycling. For tackling the sustainability challenges associated with REEs, their technospheric mining, i.e., recovery from secondary sources, is imperative. Characterization results indicate that phosphogypsum, a byproduct of the fertilizer industry, contains about 0.03-0.4 wt % REEs. Here, a novel process was developed that utilizes microwave irradiation to enhance the leaching efficiency of REEs from phosphogypsum. Optimal REE leaching was achieved by either microwaving at low power (600 W) and short duration (5 min) or at high power (1200 W) and long duration (15 min). The former creates cracks and pores in the particles, enhancing the infiltration of lixiviant, with minimal conversion of gypsum into less soluble crystals. The latter results in thermal degradation of the PG particles and the release of REEs at the cost of changing the PG crystal structure to less soluble phases. In all cases microwave pretreatment had a positive effect (more than 20% increase) on REE leaching efficiency. At the optimum microwaving conditions [Is min irradiation (2.45 GHz) at 1200 W], 80% Nd, 99% Y, and 99% Dy leaching efficiency was achieved.