Quantification of scandium in columbite-tantalite tailings and other scandium compounds

Prof J. H. Potgieter, S. Potgieter-Vermaak, W. Purcell

Columbite-tantalite, a niobium rich mineral found in a number of deposits, is mainly exploited for the large amounts of niobium (31 – 79 % Nb2O5) and to a lesser extend the tantalum present in these ores. Residues or tailings remaining after the processing and the removal of the majority of niobium normally contains large amounts of iron and titanium, smaller amounts of niobium and tantalum, but also small amounts of the two rare earth elements yttrium and scandium. Demand for scandium is steadily increasing due to its unique mechanical and chemical properties and it is currently extensively used in aerospace applications, electronics, optical and the fuel cells industries (scandia stabilized zirconia (ScSZ). Scandium exist in very minuscule quantities in more than 800 types of minerals, which add to the shortage of scandium worldwide. The accurate quantification as well as the development of processes to extract scandium from these scandium-bearing ores are urgently needed. This paper describes the development of analytical methods for the accurate determination of scandium using inductively coupled plasma optical emission spectrometry (ICP-OES) in both natural scandium-containing and synthetic matrices. Scandium was quantified in different organometallic complexes (in a form of Sc(acac)3), inorganic salt ScCl3·H2O, Sc2O3 as well as a columbite-tantalite mineral ore and tailings from a commercial plant. The organometallic compounds were synthesized from ScCl3.H2O salt and different acetylacetone ligands as starting materials. Products were purified and characterized by elemental analysis, melting point and IR. Scandium recoveries ranging from 75.04(3) to 99.96(4) % were obtained for different organometallic complexes while 99.70(5)and 100.1(5) % recoveries were obtained for ScCl3·H2O and Sc2O3 respectively. Low recoveries obtained for some of the synthesized organometallics were possibly due to the contaminated products and/or incomplete synthesis reactions. Optimum experimental conditions were investigated for the digestion of the mineral ore tailings using NH4F•HF flux. Scandium and other elements present in the mineral, namely Nb, Fe, Ti, Al, Mn, Sn, W, Ta, Zr and Y, were quantitatively analyzed using the newly developed optimum conditions (1:10 Sample:flux ratio, 200 °C, 60 min., dissolved in water). Scandium recovery of ~ 94.23 % was obtained.