Red mud as secondary source for critical raw materials – Purification of rare earth elements by liquid/liquid extraction
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BACKGROUND Critical raw materials (CRM) are crucial to Europe's economy and essential to maintaining and improving our quality of life due to their usage for production of many devices. Red mud is generated from alumina production where bauxite is digested in hot sodium hydroxide solution during the Bayer process. Red mud can contain considerable amounts of CRM such as rare earth elements (REEs). In the present study, purification of CRM from perturbing, co-extracted elements such as Fe and Al from red mud hydrochloric acid leachates was evaluated. RESULTS A first purification was achieved by removing Fe (>87%) from the acidic leachate using precipitation with NaOH. REEs as well as Al were hardly removed by precipitation (21%, and 33%, resp.). A second purification was achieved using liquid/liquid extraction (LLE) with di-(2-ethylhexyl)phosphoric acid (D2EHPA). Here, four explanatory variables (i.e. LLE organic/aqueous ratio, D2EHPA concentration in kerosene, stripping acid organic/aqueous ratio, HCl concentration) were studied. Finally, the optimal extraction conditions maximizing the economic potential (total metal extracted × economic value of the respective metal) of CRM were determined using a design of experiment approach. CONCLUSION The experimentally determined economic potential extracted corresponded well to the prediction (88%; to the predictions, maximum recovery of 17.18 ± 0.59 US $ t−1). Ultimately, more than 40% of the overall REEs (>62% of the leachable REEs) in red mud were purified using LLE, whereas Al was successfully rejected from the concentrate (∼5% of the overall Al present). © 2017 Society of Chemical Industry