Quantification and recovery prospects of critical metals and rare earth elements in Victorian brown coal fly ash: A promising secondary source for critical metal extraction

Abstract

Coal fly ash (CFA), an anthropogenic waste generated from the combustion of coal, is in research focus for the potential it holds in extracting critical metals. Within the classification of critical metals, includes a group of 16 commercially significant metals known as rare earth elements (REEs). The supply of REEs is touted as one of the building blocks of the 21st century to realize a sustainable future. High demand projections for the future coupled with market monopolization, provide an impetus to extract REEs from secondary resources such as CFA. In this study, three Victorian brown CFA samples were examined, quantifying 41 metals and 16 REEs. All three CFA samples had an outlook coefficient (i.e., ratio of the relative amount of critical REE to the relative amount of excess REE) > 0.7, with an observed light REE enrichment and Eu anomaly, implicating the samples as a suitable feedstock for REE extraction. Quantification studies rendered Loy Yang CFA, from the largest power station in Australia, to be rich with REEs with a total metal concentration of 418 mg/kg, followed by CFA from other power stations (Yallourn: 73.6 mg/kg and Morwell: 36.1 mg/kg). A comparative study for different methods of calibration during inductively coupled plasma - mass spectrometry analysis (and external vs internal calibration), depicted the reproducible nature of results while using an internal standard. Subsequently an in-house developed multi-element standard addition method is proposed, within an accuracy of 2–50 %. A new sequential extraction method for studying modes of occurrences in CFA, depicting REE association with larger fractionation, is explored, with results rendering REE association with: aluminosilicate bound (∼ 61.2 %) > acid soluble fraction (23.1 %) > crystalline Fe bound (5.9 %) > organic matter bound (5.3 %) > amorphous Fe bound (3.5 % TREE) > Mn-oxide bound (0.9 % TREE) > water soluble mineral bound (0.1 % TREE). This underpins the strong REE association in the residual and acid soluble fraction in brown CFA.