Alkali fusion-enhanced metal leaching of blast furnace slag for pretreatment of simultaneous carbon mineralization and rare earth elements recovery

Landfilled waste containing alkaline earth metals is a promising feedstock for CO2 storage and carbon mineralization. It has recently gained attention as a resource for recovering rare earth elements (REEs) at concentrations of several hundred ppm. However, during the leaching of calcium (Ca) and REEs, silicate (Si) contained in landfilled wastes forms an inactive Si-passivation layer on the particle surface, thereby hindering the metal leaching efficiency. In this study, we propose an alkali fusion pretreatment to provide an alternative leaching pathway for blast furnace slag (BFS) and enhance the leaching efficiency of both Ca and REEs. The alkali fusion was conducted by roasting BFS with NaOH at 400, 600, and 800 degrees C, and structural changes, including crystalline and amorphous silicate phase, were investigated. A dramatic leaching improvement (98.5 and 88.2 % for Ca and REEs, respectively) was achieved even at the lowest fusion temperature of 400 degrees C, which was two-fold higher than the leaching of raw BFS. This significant improvement was mainly due to the destruction of silicate phases and the transformation of BFS into 'ready-to-leach' phases. Our study highlights mineralogical engineering as a pretreatment for achieving efficient leaching in the recovery of REEs and the carbon mineralization integrated process.