Extractability indices for screening coal combustion byproduct feedstocks for recovery of rare earth elements

A simple, standardized test is needed to screen coal combustion byproducts to determine their suitability for large-scale extraction and recovery of rare earth elements (REE). An ideal commercial process for acid extraction of the lanthanides will employ short residence times with no external heating of the reactor, and minimum quantities of a commodity acid. In this work, a rapid screening test mimicking these conditions was established by using (1) 2 M hydrochloric acid at (2) 20:1 Liquid: Solid ratio and (3) ambient temperature with (4) a reactor residence time of 15 min. The test was applied to seventeen different fly, bottom, landfilled or ponded ashes. REE recovery is directly correlated with Ca and Mg content, and inversely correlated with particle size. Fly ashes derived from sub-bituminous coals are ideal feedstocks due to their relatively high Ca content (> 10%) and small particle sizes, leading to as high as 73% REE recovery after only a 15-min leaching at room temperature with 2 M HCl. After a brief grinding step to break up agglomerates that may be >1 cm in diameter, REE recovery from ponded ash of up to 71% is achieved.