Extraction of Rare Earth Elements (REEs) from coal ash of Thar, Pakistan by chemical leaching with Novel Deep Eutectic solvents

Abstract

The supply of rare earth elements is less and its demand is high so we need a secondary source to fulfill this demand. This research adopted coal ash as an alternative resource of rare earth elements. The mineral acids, ionic liquids, and organic acids were used for the extraction of rare earth elements (REEs) from Coal Ash (CA) which were corrosive, expensive, hazardous, and not friendly to the environment. Our approach was to find innovative reagents that were green to the environment and extract REEs from CA as a replacement for mineral acids, ionic liquids, and organic acids. The researchers synthesized a novel Deep Eutectic Solvent (Choline chloride & Humic acid) for the extraction of REEs. The study conducted experiments on the coal of Sindh Engro Thar Coal Mining Company (SECMC), Pakistan. Based on literature and ASTM standard D-2013 coal was crushed and ground to a size of -250 microns. The leaching experiments were designed with two different parameters (1-2 hours) time and temperature (60-90 Celsius) at a constant 500 revolutions per minute (rpm). After leaching, the filtrate was centrifuged at 4000 rpm for 10 minutes. Both the feed and leachate were analyzed using various techniques, X-Ray fluorescence (XRF), X-Ray Diffraction (XRD), Inductively coupled plasma mass spectrometry (ICP-MS), Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electronic Microscope (SEM).  In feed characterization, the XRF results showed that Calcium had the highest percentage which was 35.676%. Calcium sulfate, Diborane, and Magnesium oxide mineral phases were detected by XRD. ICPMS analysis showed the CA sample contained a total of 351.41 PPM of REEs. The concentration of heavy rare earth elements (HREEs) were 119.5 PPM, while the light rare earth elements (LREEs) were 231.91 PPM. Moreover, Lutetium had a PPM value of 96.56 and was the most prevalent REE. In FTIR spectroscopy C, C=O, C-H bond, C-O-C, N-O bond, and SiO functional groups were found. In SEM analysis, luminous and non-luminous features could be seen in the matrix which indicates the presence of minerals by raw CA sample while both luminous and non-luminous features were disseminated in CA dipped with DES sample. The leaching efficiency of the DES was increased by increasing the temperature and time. The highest leaching efficiency we achieved was at 90 degrees Celsius for 2 hours which was 90%, While, at 60 degrees Celsius for 1 hour the DES was at its lower peak and the efficiency was 62%. Our results showed that the DES was effective in extracting REEs from CA.