Carbon Mineralization of Steel Slag for Integrated CO2 Storage and Utilization with Recovering Rare Earth Elements

Abstract

As a practical way to reduce anthropogenic CO2 from fossil fuel utilization, carbon capture, utilization, and storage (CCUS) technologies are getting focused these days. Among CCUS technologies, carbon mineralization is considered to have a great impact on CO2 negative emission by producing thermodynamically stable solid carbonates like CaCO3. Furthermore, the increasing demand for rare earth elements (REEs) requires recycling those valuable elements from secondary resources. In this context, Blast furnace slag (BFS) is regarded as a potential resource to give a possible solution for those issues, because of its exploitable amount of Ca and REEs and huge global production. However, the strong bond between amorphous silicates and metals inside of the BFS limits the extraction efficiency of Ca and REEs in the conventional hydrometallurgical way. Additionally, the amorphous silicates could cause the silica–passivation layer and silica–gelation during and after chemical reactions, which hinder mass transfer kinetics. To solve these problems, a sequential process is designed with alkali fusion pre–treatment that could convert insoluble amorphous silicates into soluble phases, and efficiently leach out target metals. In detail, NaOH beads were mixed with particle size controlled BFS with a 1:1 mass ratio and were fused with different temperatures and times (i.e., 400, 600, and 800 °C with 1 and 4 hours, respectively). Next, the water leaching step was applied to the fused materials to remove impurities including Si, Al, and Na which were in the form of soluble phases. Finally, 3 M of HCl leaching step was applied to extract Ca and REEs. As a result, silicon can be eliminated up to 22.26 % through the water–leaching step. The maximum leaching efficiency of Ca and total REEs (i.e., the sum of each amount of Sc, Y, La, Ce, and Nd) were significantly enhanced in the acid leaching step compared to the BFS direct acid leaching with 3 M of HCl. However, there occurred interesting phenomena during acid leaching for raw BFS and alkali fused materials. In detail, two different silica dissolution modes were observed: silica–network breakdown and partial dissolution. In this study, those different leaching mechanisms were reasoned, and the limitation of direct BFS acid leaching was also elucidated, and finally, the optimal alkali fusion condition was proposed.