Technospheric mining of niobium and titanium from electric arc furnace slag

Abstract

Valorization, i.e., converting waste into high-value products, is a key enabler of the circular economy (make, use, recover) as an alternative to the traditional linear economy (make, use, dispose). As countries become more industrialized and developed, their use of natural resources—and hence their industrial waste generation—increases tremendously. Most industrial waste streams contain considerable amounts of strategic materials, such as refractory metals. Such elements are in increasingly high demand but facing supply uncertainty. To tackle the sustainability challenges associated with the supply of such materials and to unveil the hidden value of industrial wastes, technospheric mining is imperative. Here we develop a novel environmentally sustainable pyro-hydrometallurgical process, called “acid-baking water-leaching” to extract niobium (Nb) and titanium (Ti) from an electric arc furnace (EAF) slag. A fractional factorial design methodology and empirical modeling is utilized to design the experimental matrix and to optimize the process. Utilizing the optimum operating conditions, 98.7% Nb and 93.4% Ti extraction efficiency is achieved. With fundamental investigations into the extraction mechanism, it is proposed that Ti and Nb are associated with Ca and exist as CaTiO3. During the extraction process, Ca is transformed into anhydrite (CaSO4) precipitates, forcing the process to proceed to completion.