Determination of 16 Rare Earth Elements, Niobium, Tantalum, Zirconium, and Hafnium in Paleocontinental Sedimentary Rare Earth Ores in Guizhou by Inductively Coupled Plasma Mass Spectrometry with Alkali Fusion and Secondary Precipitation Separation

Paleocontinental sedimentary rare earth ore samples in Guizhou mainly contains accessory minerals such as anatase, kaolinite, and hematite, which were difficult to completely dissolve using conventional microwave digestion process. By optimizing the sample decomposition and extraction methods, a method mentioned by the title has been proposed. An aliquot (0.1 g) of the sample was placed into a corundum crucible, and about 2.5 g of sodium peroxide was added. After mixing thoroughly, about 0.5 g of sodium peroxide was added on top. The mixture was melted at 750 ℃ for 15 min, and the frit was immersed in 50 mL of the hot solution containing 5 g·L⁻¹ magnesium sulfate and 5 g·L⁻¹ ammonium chloride for 10 min. After boiling for 1-2 min by heating, 5 mL of 50% (volume fraction, the same below) aqueous ammonia solution was added. After stirring thoroughly, the mixture was settled for 60 min, and filtered by the filter paper. The precipitate was dissolved in 40 mL of hot 50% (volume fraction, the same below) nitric acid solution in portions, and the resulting solution was boiled for 1-2 min by heating, whose acidity was adjusted to pH 8-10 with 50% aqueous ammonia solution. After filtering with the filter paper, the precipitate was dissolved in 40 mL of hot 50% nitric acid solution in portions, diluted to 200 mL by 2 g·L⁻¹ tartaric acid solution, and analyzed using inductively coupled plasma mass spectrometer in KED mode. It was shown that heating leaching with the hot solution containing magnesium sulfate and ammonium chloride helped to form magnesium niobate and magnesium tantalate precipitates from niobium and tantalum in alkaline solution. In the aqueous ammonia-ammonium salt system, niobium, tantalum, zirconium, hafnium, and rare earth elements could be quantitatively precipitated. Using ammonia water for secondary precipitation separation and purification could remove most of the matrix elements and salts introduced by reagents. The use of KED mode and online internal standard correction could reduce mass spectrometry interference, especially for scandium element. Under optimized experimental conditions, linear relationships between values of the mass concentration of the 20 analytical elements and mass spectrometry intensity calibrated by internal standard of ¹⁰³Rh were kept in the range of 1.00-1 000 µg·L⁻¹, with detection limits in the range of 0.010-1.66 µg·g⁻¹. Verified test was made on first-class national reference materials, and the relative errors of the determined values of analytical elements were found in the range of 0.030%-16%, giving RSDs (n=7) in the range of 2.1%-14%.