Method development for determination and recovery of rare earth elements from industrial fly ash

Abstract

Rare earth elements (REE) are important in numerous high technology applications and in addition their supply risk is high, which gives rise to studying new sources for rare earth elements. Rare earth element concentrations in industrial fly ash samples collected from Finnish power plants, using a mixture of peat and biomass as a fuel, have been determined. Two sample pre-treatment methods, ultrasound- and microwave-assisted acid digestions, have been applied to fly ash samples. Measurement conditions for inductively coupled plasma-optical emission spectrometry (ICP–OES) have been optimized to reach robust plasma conditions and the reliability of the analysis has been investigated by using a standard reference material, synthetic samples, a method of standard addition, and a comparison analysis with inductively coupled plasma-mass spectrometry (ICP-MS). Most rare earth elements in fly ash samples are enriched compared to concentrations in earth’s crust, and some ashes have total rare earth element concentrations as high as 560 mg kg-1. A leaching procedure using dilute sulfuric acid as a leaching reagent has been developed for the dissolution of rare earth elements from fly ash. Optimization of the sulfuric acid leaching resulted in approximately 70 % of the rare earth elements being dissolved from the fly ash samples. Recovery of rare earth elements from the sulfuric acid leachate has been investigated using oxalate precipitation and liquid-liquid extraction. Conditions for oxalate precipitation were optimized, and it was found to be quantitative but non-selective towards rare earth elements. Liquid-liquid extraction with bis(2-ethylhexyl)phosphoric acid (D2EHPA) was found most suitable to concentrate rare earth elements before precipitation as oxalates. After optimization of process parameters, recovery of light and heavy rare earth elements was achieved. A specific liquid-liquid ex-traction procedure for scandium recovery was also developed.