Selective silica gel free scandium extraction from Iron-depleted red mud slags by dry digestion

A combination of pyrometallurgical and hydrometallurgical processes showed promising results for the sustainable valorisation of red mud with selective iron recovery. However; high Si content in red mud and its slags produced by pyrometallurgical treatment for the Fe removal makes these secondary resources untreatable with conventional acid leaching routes due to the formation of silica gel. In this study, red mud and slags synthesized by electric arc furnace smelting, which contain both moderate and extensive SiO2, were exposed to dry digestion aiming selective Sc recovery with suppressed Ti and Si dissolution. Various additions of concentrated sulfuric acid were investigated to find out the optimum acid consumption for this process. A promising Sc leaching efficiency was found for acidic slag (~80%), where Ti dissolution was suppressed to <10% and without Si gel formation. The mineralogical content of red mud and the two slags were analysed by QEMSCAN and XRD in order to elucidate the leaching mechanism. Using the findings of this study, an empirical dry digestion leaching model was proposed for each starting material in a comparative manner.