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Recovery potential of Scandium and other critical raw materials from European industrial by-products
(2017)
Technological evolution and modern advances in the field of renewable energies are connected to an increasing demand on critical raw materials and especially technological metals.
Recently, interest in the element scandium (Sc) has started to grow due to its positive influence on material properties e.g. in high performance-alloys, solid-oxide-fuel cells and solid-state lasers. Therefore, a rise in value and demand is expected in the near future.
This study focuses on the reliable and sufficient methodology of measuring Sc and other critical raw materials in secondary waste products from the Al2O3- and Ti2O producing industry. It also targets the examination of Sc-bearing phases and their potential influence in the recovery of Sc from above mentioned materials. Essentially, chemical and structural analyses and characterization of European red muds and its Sc-bearing mineral phases are carried out.
Furthermore, analogue investigations are performed for filter cakes and acid slurries that incur during TiO2 production.
Red muds and TiO2 filter cakes from several European countries have been analyzed by X-ray diffraction, X-ray-fluorescence, ICP-MS and -OES. Analytical procedures performed are based on previous studies and different methods are tested and compared to ensure reliable data from ICP-MS and -OES measurements.
Red mud samples were found to contain ~60-100 mg/kg Sc. Main mineral phases are Hematite, Gibbsite, Boehmite, Cancrinite, Diaspore, Perovskite, Rutile, Katoite and Quartz in variable amounts. Samples from TiO2-production contain up to 400 mg/kg of Sc and significant amounts of niobium and cerium. The main mineral phases are Rutile, Calcite, Quartz, Graphite and Iron-Oxide-Chloride. Generally, it is assumed that Sc is associated with iron and titanium-bearing phases such as Ilmenite and Rutile as it was found in REE deposits in China. Hence, These phases are studied in more detail with advanced structural analyses such as X-ray Absorption near edge structure (XANES) and point-based mineral chemistry analyses by electron microprobe. The study is incorporated in the SCALE project (GA No. 730105) funded by European Union's Horizon 2020 research and innovation program and aims to secure a European supply chain for Sc from metallurgical by-products.
The leaching behavior of scandium (Sc) from bauxite residues can differ significantly when residues of different geological backgrounds are compared. The mineralogy of the source rock and the physicochemical environment during bauxitization affect the association of Sc in the bauxite i.e., how Sc is distributed amongst different mineral phases and whether it is incorporated in and/or adsorbed onto those phases. The Sc association in the bauxite is in turn crucial for the resulting Sc association in the bauxite residue. In this study systematic leaching experiments were performed on three different bauxite residues using a statistical design of experiments approach. The three bauxite residues compared originated from processing of lateritic and karstic bauxites from
Germany, Hungary, and Russia. The recovery of Sc and Fe was determined by ICP-OES measurements. Mineralogical changes were analyzed by X-ray-diffraction and subsequent Rietveld refinement. The effects of various parameters including temperature, acid type, acid concentration, liquid-to-solid ratio and residence time were studied. A response surface model was calculated for the selected case of citric acid leaching of Hungarian bauxite residue. The investigations showed that the type of bauxite residue has a strong influence. The easily leachable fraction of Sc can vary considerably between the types, reaching ~20–25% in German Bauxite residue and ~50% in Russian bauxite residue. Mineralogical investigations revealed that a major part of this fraction was released from secondary phases such as cancrinite and katoite formed during Bayer processing of the bauxite.
The effect of temperature on Sc and Fe recovery is strong especially when citric acid is used. Based on the exponential relationship between temperature and Fe-recovery it was found to be particularly important for the selectivity of Sc over Fe. Optimization of the model for a maximum Sc recovery combined with a minimum Fe
recovery yielded results of ~28% Sc recovery at <2% Fe recovery at a temperature of 60 ◦C, a citric acid normality of 1.8, and a liquid-to-solid ratio of 16 ml/g. Our study has shown that detailed knowledge about the Sc association and distribution in bauxite and bauxite residue is key to an efficient and selective leaching of Sc
from bauxite residues.