Sample Treatment Methods for Determination of Rare Earth Elements in Manganese Ore by High-pressure Closed Digestion-Inductively Coupled Plasma-Mass Spectrometry

Abstract

BACKGROUND With advantages of small sample quantity, less acid, and low blank, high-pressure closed digestion-Inductively Coupled Plasma-Mass Spectrometry became the main method in the determination of rare earth elements. However, the composition of manganese ore is complex with varying manganese contents and different valence states of Mn, often with accompanying (co)producing metals and other impurities. The ore is difficult to digest completely using conventional acid-soluble systems, resulting in inaccurate ICP-MS analytical results. OBJECTIVES To optimize the acid-solution system condition of the high-pressure closed digestion method, in order to completely and effectively digest the manganese ore with complex components and improve the accuracy of the results. METHODS Three standards GBW07261, GBW07263, GBW07266 and an unknown sample of manganese ore were digested using three different acid-solution systems of high-pressure closed digestion:Hydrofluoric acid-nitric acid closed digestion, extracted with nitric acid (Method 1); Hydrofluoric acid-nitric acid-hydrogen peroxide closed digestion, redissolved with nitric acid-hydrogen peroxide, extracted with nitric acid (Method 2); Hydrofluoric acid-nitric acid closed digestion, redissolved with hydrochloric acid, extracted with nitric acid (Method 3). The contents of rare-earth elements were then determined by ICP-MS. RESULTS Methods 2 and Methods 3 can completely digest manganese ore samples. The results of rare-earth elements by ICP-MS were similar, and were consistent with the results of conventional sodium peroxide fusion. However, during the experimental process, for samples with high manganese content, Method 3 required repeated additions of hydrochloric acid before the samples were completely digested, whereas Method 2 need only one repeat. The precision of Method 2 was 0.96%-2.68%, and the recovery was 95.0%-107.0%. CONCLUSIONS For different manganese ore samples, Method 2 can completely digest the samples. The determination of rare earth elements was reliable by ICP-MS. Method 2 was the most suitable sample digestion method for the determination of rare earth elements in manganese ores.

Keywords

• manganese ore
• rare earth elements
• high-pressure closed digestion
• hydrogen peroxide
• inductively coupled plasma-mass spectrometry