Dissolution and analytical characterization of Tantalite ore, niobium metal and other niobium compounds

English: Qualitative and quantitative analyses of solid samples using a wet analytical technique, such as ICP-OES, are entirely dependent on the complete dissolution of the samples. Since 1866 niobium and tantalum samples have been successfully dissolved by hydrofluoric acid digestion. However, this method is not preferred in industry due to the dangerous nature of hydrofluoric acid as dissolution reagent. This study was aimed at finding alternative, economically viable and ecologically-friendly conversion methods for the dissolution of Nb metal, Nb2O5, NbF5 and three niobium containing minerals demarcated as Tantalite A, Tantalite B and Sample 1. Acid digestion of Nb and Nb2O5 on a hot plate was investigated with HNO3, HCl, H2SO4, H3PO4 and aqua regia. Both Nb and Nb2O5 dissolved to a greater extent in H2SO4 and H3PO4 (0.94% and 0.36% respectively) than in the other acids but the results were very inaccurate in all the acids. Good recoveries were obtained for NbF5 with recoveries of 95% and 93% from the dissolution of this sample in sulphuric acid and water respectively. Microwave-assisted acid digestion was also investigated with the same acids and high accuracy was obtained with recoveries of 99+% for Nb and Nb2O5 and 100% for NbF5 with H2SO4 digestion. Microwave-assisted digestion of the mineral samples produced 90+% Nb recoveries at digestion periods longer than that for the pure Nb samples. The calibration curves for all the elements showed good linearity as well as high precision (R2 ≥ 0.999) with low y-intercept values which indicated proper background corrections. Flux fusion digestion was investigated with different flux reagents for the dissolution of pure Nb2O5 and the mineral samples. Lithium tetraborate flux produced excellent results for niobium with recoveries of 98.56 to 109.59% Nb2O5 and other major and minor elements. The melt was dissolved in the mixture of H2SO4 and methanol to remove the excess of boron as the B(OMe)3 ester which otherwise resulted in the formation of an insoluble boric acid which coprecipitated with some of the analytes of interest. This dissolution method produced accurate results for pure Nb2O5 (102.76% Nb recovery) and for most of the analytes which were identified through the qualitative analysis in the mineral samples (98.56 to 109.59% Nb2O5, 100.58 to 108.57% Ta2O5, 101.03 to 103.29% TiO2 and 99.23 to 100.55% Fe2O3. Other analytes such as SiO2, ThO2 and WO3 showed lower accuracy with recoveries in the range 61.38 to 85.68%. Good linearity and precision was observed in the standard addition calibration curves for all the elements studied, which illustrates the reliability of the results. Semi-quantitative analyses of the solid mineral samples were done by means of XRF. The XRF results were in good agreement with the results obtained by fusion with Li2B4O7 and ICP-OES analysis. Finally, the mineral samples were qualitatively analysed by X-ray diffraction spectroscopy, Scintillometer, Microscope and a hand magnet to get positive identity of the test samples, mainly the sample demarcated as Sample 1. The XRD patterns for Tantalites A and B were identical and these minerals were radioactive but showed very little magnetic properties. Grains of euxenite, garnet, microlite, tourmaline, quartz, muscovite and manganotantalite were microscopically identified in both Tantalites A and B. From these tests, Tantalites A and B were identified as variations of manganotantalite with small amounts of microlite, euxenite, garnet, tourmaline and accessory of quartz and muscovite impurities. The XRD pattern of Sample 1 showed the presence of manganotantalite, garnet, quartz and muscovite. These minerals were also identified under the microscope. Different from the Tantalite minerals, Sample 1 showed no radioactivity but intense magnetic properties. Sample 1 was identified as a mixture of manganotantalite and garnet with quartz and muscovite impurities.