Recovery of rare earth elements from Alaskan coal and coal combustion products

Abstract

Owing to the monopolistic supply and rapid demand growth of Rare Earth Elements (REEs), cost effective and eco-friendly technologies for extraction of REEs from coal and coal byproducts are being widely explored. Physical separation tests, like magnetic separation, float-sink and froth flotation, were conducted at a laboratory scale, for identification and characterization of REEs in two Alaskan coal samples. The studies revealed that the samples are enriched in critical REEs, and have elevated REE concentrations as compared to average world coal estimates. The selected coal samples from Healy and Wishbone Hill regions were found to possess an overall concentration of 524 ppm and 286 ppm, respectively, of REEs in coal on ash basis and some density fractions have total REE concentrations as high as 857 ppm. Based on the characterization studies, detailed investigations were conducted to enrich the REEs and produce a concentrate for downstream extraction. A three-factor three-level Box-Behnken design for modeling and optimization of froth flotation revealed that the optimum flotation conditions for maximum REE Enrichment in the froth fraction was independent of collector dosage for both coal samples. The response variable was maximized at 4.2% solids and 32.7 ppm of frother dosage for Healy Coal sample and 10% solids and 37.9 ppm of frother dosage for Wishbone Hill Coal sample. A processing flowsheet for REE enrichment in clean coal is proposed, which aims at concentrating REEs in lower density fractions by a combination of dense medium separation and froth flotation processes. The overall REE recovery of the process is calculated to be 76% for Healy and 60% for Wishbone Hill with clean coal fractions enriched in REE concentrations above the cut-off value required for the commercial exploitation. The coals are bound to possess the potential to be used as a REE resource under favorable socio-economic and geo-political scenarios.