Recovery of rare earth elements from coal flyash using deep eutectic solvents as leachants and precipitating as oxalate or fluoride

This paper presents results of the study on the applicability of deep eutectic solvents (DES) for the recovery of rare earth elements (REE) from coal flyash (CFA) assaying REE 0.22%. The combination of DES used was choline chloride (ChCl) with lactic acid (LA) and ChCl with para toluene sulphonic acid monohydrate (pTSA). Besides the synthesis and characterisation of the DES systems, detailed study on their flow behaviour as a function of temperature and viscosity was also made. Screening studies indicated better leaching performance when the molar ratio of ChCl:pTSA and ChCl:LA were 1:1 and 1:2 respectively. Influence of various process parameters like temperature, solid to liquid ratio, reaction time and dilution ratio of DES with water on the leaching efficiency of REE and other impurity ions was investigated using the best molar ratio combination identified for the two systems. Performance of the DES systems were compared with that of individual hydrogen bond donors (HBD) namely, LA and pTSA, hydrogen bond acceptor (HBA) viz. ChCl as well as with H2SO4. The dissolved REE from the DES medium were selectively and quantitatively recovered by chemical precipitation. The DES systems gave REE leachability of about 85–95%. The mixture of ChCl:pTSA(1:1) showed higher leach recovery and faster kinetics compared to ChCl:LA(1:2). Both DES systems gave a decisive 5–8% higher leachability than the individual HBDs (LA and pTSA) and HBA (ChCl) which constitute the respective eutectic. Comparison of leaching performance of REE using DES with that of H2SO4 indicated significant enhancement, about 35%, with the former over the inorganic acid. A probable leaching mechanism of REE with the DES used is also proposed. Precipitation of REE from the DES medium was near complete with oxalic acid and NaF reagents.