Determination of Rare Earth Elements in Coal-related Samples by Inductively Coupled Plasma-Mass Spectrometry with Acid Dissolution

Coal and coal-bearing rock series can enrich beneficial elements such as rare earth under specific geological conditions, forming coal-related key metal deposits. In recent years, highly enriched rare metal elements such as gallium, germanium, uranium, gold, silver and rare earth elements have been successively discovered in coal. The fly ash produced by the combustion of coal and gangue has a higher enrichment degree of rare earth and other elements. These highly enriched metal elements will become one of the alternative sources of rare earth and other strategic key metals. In order to realize the comprehensive utilization of rare earth elements in coal-related samples, it is necessary to objectively evaluate the content level of rare earth elements in coal-related samples. Therefore, it is of great practical significance to establish a set of multielement quantitative analysis methods suitable for rare earth elements in coal-related samples.

To establish a set of analytical methods for the determination of rare earth elements in coal-related samples, and provide theoretical basis for the formulation and optimization of experimental schemes by analyzing the occurrence state of elements.

The method research of rare earth elements in coal-related samples was carried out by using a high-pressure closed acid dissolution method and semi-closed acid dissolution method respectively. The sample mass, dissolution mode, acid decomposition system and extracting solution were analyzed experimentally. The recovery rates of rare earth elements by different methods were compared and analyzed, and the optimal dissolution method was determined. The interferences and interference elimination methods in the mass spectrometry determination of rare earth elements were discussed in detail. The occurrence state of elements in coal-related samples was analyzed by using XRD and SEM techniques, and the reason why the high-pressure closed acid dissolution method could not completely decompose coal gangue, coal fly ash and other samples was explained.

Through experimental analysis of conditions such as the sampling weight, the sample dissolution method, the acid digestion system, and the redissolving solution composition, an analytical method for the determination of rare earth elements in coal by ICP-MS with nitric acid-hydrofluoric acid high-pressure closed acid solution was established. The detection limits were between 0.01μg/g and 0.03μg/g. This method could be used to achieve accurate determination of 15 rare earth elements in coal, but the rare earth recovery rate for coal fly ash and coal gangue was unstable, with the rare earth recovery rate ranging from 37% to 123%. Adding sulfuric acid and perchloric acid to the original acid dissolution system, and switching to hydrochloric acid solution for redissolution, cannot effectively improve the decomposition efficiency of rare earth elements. In order to solve the problem of low dissolution rate of rare earth elements in such samples, further experimental research on semi-closed acid dissolution digestion method was carried out. The analysis method of rare earth elements in coal-related samples was established by using semi-closed acid dissolution, which determined the accurate resolution of rare earth elements in coal fly ash and coal gangue samples. The detection limits were between 0.02μg/g and 0.05μg/g. The occurrence state of elements in coal-related samples was preliminarily analyzed by X-ray diffraction and scanning electron microscopy. It was revealed that high aluminum minerals were the main reason for the low dissolution rate of rare earth elements in coal-based samples such as coal gangue, which provided a theoretical basis for the formulation and optimization of experimental schemes. Method verification was carried out using standard materials and actual samples. The developed method had good precision (relative standard deviation was 0.05%-9.98%) and accuracy (relative error was from −10.2% to 7.62%).

The two analysis methods investigated in this paper have low detection limit, high precision and accuracy, can realize simultaneous determination of rare earth elements in coal-related samples, and are suitable for large-scale analysis and testing of rare earth elements in samples.