Analytical Determination of REEs in Coal Fly Ash and its Application in Two REE Recovery Processes: Aqueous Leaching and Plasma Arc Gasification

摘要

An Inductively Coupled Plasma Mass Spectrometry (ICP-MS) based analytical method was developed to measure the concentrations of rare earth elements (REEs) in coal fly ash samples. Samples were first digested to release metals, including rare earth elements, from the aluminum-silicate containing glassy phase into aqueous solutions, followed by accurate ICP-MS determination of REEs. A standard reference material for coal fly ash, NIST SRM 1633c, was utilized to assess the method performance. Method recoveries of measured REEs were between 93 – 106%, with an averaged detection limit of 0.010 μg/g among all REEs including scandium and yttrium. Precision of the measured REEs were within 10% in different batches. Eight coal fly ash samples from different power plants with various coal sources were measured for their REEs contents, with a total REE values ranging from 391 to 717 μg/g. This developed method was applied to study the efficiency of two REE recovery processes from the above coal fly ash samples: 1) aqueous leaching: It was observed that, with a mild leaching solution (1% HNO3), a longer leach time (30hr vs. 1hr) lead to higher REE recoveries and, typically the first hour leached majority (>50%) of the total leachable REEs (< 40% of total REEs in ash). For a single REE, the leaching efficiency varied among different coal fly ash samples. 2) Plasma arc gasification: Preliminary data indicated promising REEs separation from the coal fly ash and their enrichment in the metal phase with this process (> 100 μg/g REE).