Determination of Trace Rare Earth Elements in Tungsten-Molybdenum Ore by Inductively Coupled Plasma-Mass Spectrometry with Microwave Digestion System

Tungsten-molybdenum ore and rare earth are both important strategic resources. It is of great significance to evaluate the contents of rare earth elements in tungsten-molybdenum ore for the development of rare earth resources in minerals. During the pretreatment of tungsten and molybdenum ore samples, the alkali melting flux will introduce the salt matrix, and the tungsten and molybdenum elements in an acid dissolution condition are easy to hydrolyze into tungstic acid and molybdic acid and produce rare earth fluoride.

To establish an analytical method for accurate determination of the 16 rare earth elements including La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, and Sc, in tungsten-molybdenum ore by inductively coupled plasma-mass spectrometry (ICP-MS).

Tungsten-molybdenum ore samples were fast digested by microwave with four acid system (HNO₃-HF-HClO₄-HCl) in a high temperature and high-pressure environment. Then the digestion acid solution was evaporated to the viscous state, and the mixed solution (citric acid-hydrochloric acid) was used to dissolve the complexed tungsten and molybdenum at warm temperature, which effectively avoided the introduction of salt matrix in the pretreatment of tungsten-molybdenum ore samples. The problem of tungsten and molybdenum easily hydrolyzing into tungstic acid and molybdic acid was also solved, and rare earth fluoride in an acid environment was produced. Online internal standard and kinetic energy discrimination (KED) strategy were used in real-time to calibrate the sample analysis by ICP-MS, which solved the interference problems of matrix effect and polyatomic ions in the analysis process.

The 16 rare earth element contents of tungsten molybdenum standard material (GBW07239 and GBW07238) and real samples (1#, 2# and 3#) was efficiently measured. The results showed that the concentrations of 16 rare earth elements were within the scope of the standard value, the contents of real samples were 0.198-41.2µg/g. Relative standard deviation (RSD) of method precision was lower than 2.0%, the method detection limit was 0.0002-0.0087µg/g, and the spiked recovery of real samples was between 80.0% and 114.0%. The absolute value of the logarithmic error between the average value of sample testing and the standard value of reference materials| ΔlgC|≤0, meet the requirements of the geological and mineral industry. The measured contents of rare earth elements in tungsten-molybdenum ore sampled from Henan are 0.198-41.2µg/g, compared with 0.013-5.53µg/g in molybdenite ore from Jilin, 0.68-107.0µg/g in tungsten ore (quartz schist) from Yunnan and 0.071-2.11µg/g in tungsten ore (tourmaline) from Yunnan. The content distribution of rare earth elements has spatial distribution characteristics and research significance for differences in rock types.

The tungsten-molybdenum ore samples are completely dissolved by microwave with the four-acid system. This method satisfies the requirements of the geology and mineral industry and can provide reference for the analysis technology of trace rare earth elements in tungsten-molybdenum ore for high-throughput sample analysis capability.