Comprehensive examination of acid leaching behaviour of mineral phases from red mud: Recovery of Fe, Al, Ti, and Si

Red mud represents an environmental and economic liability for the alumina industry in the form of wasted raw material. Although some leaching studies have been performed, there are deficiencies in the current literature regarding the amounts of metals extracted relative to each other, and minimal information regarding silicon contamination of extracts. There is also limited knowledge of extraction efficiencies of different acids (particularly in the case of phosphoric acid) under the same experimental conditions. This study focused on the leaching behaviour of the four most extractable elements present within red mud (iron, titanium, aluminium and silicon). By varying the experimental conditions, acid concentration, and type of acid, a comprehensive dataset of leaching trends was obtained. This allowed for direct comparison of leaching efficiency for the four elements under the same conditions, which was difficult previously due to the variation of experimental conditions and red mud composition between studies. The patterns in recoveries were explained in terms of the reactivities of the mineral phases within red mud and the interaction between the different acids and the reaction surfaces. Out of the four acids studied (nitric, hydrochloric, sulfuric, and phosphoric) phosphoric and hydrochloric acids produced some of the best recoveries for iron (76–78%) and titanium (23–24%), and phosphoric acid also produced the highest recoveries for silicon (49%) and aluminium (50%). The differences observed between the acid types and reaction conditions revealed potential for development of element selective extraction methods. Additionally, explaining leaching behaviour in terms of the mineral phases present allowed easier prediction of expected leaching trends for these four elements, which made this study applicable to red muds with a wide variety of compositions.