Determination of Vanadium, Aluminum, Iron, Titanium, Copper, Manganese, Nickel, and Phosphorus in Stone Coal Vanadium Ore by Microwave Digestion-Inductively Coupled Plasma-Optical Emission Spectrometry

Stone coal vanadium ore has become an important source of vanadium products globally. Accurate determination of its composition is a fundamental task for exploration, research, and comprehensive utilization. The contents of V, Al, Fe, Ti, and P in stone coal vanadium ore are on the order of 10⁻² wt%, while those of Cu, Mn, and Ni are on the order of 10⁻⁶ wt%. Inductively coupled plasma-optical emission spectrometry (ICP-OES) performs well for the determination of these elements, but alkali fusion suffers from high background interference and detection limits; acid digestion requires pre-ashing to remove carbon, which is cumbersome; and open acid digestion without ashing tends to leave residual carbonaceous slag. In this work, a ternary acid system of HNO₃-HF-H₂SO₄ (volume ratio 3:2.5:1) and digestion conditions were optimized. Complete wet digestion of the samples was achieved at a maximum digestion temperature of 220℃, with a gradient heating ramp of 30 min and digestion for 1.5 h, followed by stepwise acid evaporation at 180℃ and 260℃. A method for the determination of eight elements in stone coal vanadium ore by microwave digestion coupled with ICP-OES was established. The results indicate that under high-temperature sealed microwave conditions, sulfuric acid provides a sustained high-temperature liquid environment and promotes dehydration-oxidation, hydrofluoric acid breaks the silicate lattice, and nitric acid together with sulfuric acid thoroughly oxidizes the carbonaceous matter. The synergistic action of the three acids enables complete digestion of siliceous carbonaceous stone coal vanadium ore samples. The detection limits of the method range from 0.0005% to 0.003%, with precision (RSD, n=7) of 0.91% to 3.8%. The determined values for certified reference materials agree well with the certified values. The previously employed five-acid open digestion method by our group could not completely oxidise high-carbonaceous matter; however, the residual carbon contained essentially none of the target elements, so the results remained accurate. In contrast, microwave digestion can completely eliminate high-carbonaceous matter, yielding a clear and transparent solution. Both methods satisfy the analytical requirements. For samples with a carbon content exceeding 15%, microwave digestion is recommended as the preferred option; for samples with a carbon content below 15%, particularly when processing large batches with high throughput demands, the five-acid open digestion method is advisable.