Evaluation of rare earth elements (REEs) in selected Nigerian coal fly ash: a prelude to extraction and waste management

Abstract

The rising need for rare earth elements (REEs) as critical materials for the development of clean energy technologies, as against the rapid depletion of virgin REE-bearing ores as well as their imbalance in geographical occurrence, calls for thorough search on secondary sources such as coal fly ash, given that the aluminosilicate mineral phase in the waste is enriched in REE particles. To support the geographical diversification of REE sources, there is a need for a comprehensive documentation of REE content and, by extension, the economic potential of fly ash derived from Nigeria's vast coal fields. Eight representative coal fly ash samples generated from coals from Nigeria's major coal belts were collected. Silica and alumina, with respective ranges of 38.1–44.5% and 14–15.98%, accounted for the bulk of the major elements in the samples. Total REE contents in the samples ranged from 874 ppm to 1127 ppm, while the cerium, yttrium, neodymium and lanthanum-dominated rare oxide totals were found to be in the range of 941–2145 ppm across the samples. The outlook coefficients (extractability indices) computed for the samples ranged between 0.8 and 1.3, with 0.7 as the benchmark. The range of percentage of critical REEs in the CFA samples was 28%–36%. This research has successfully explored the relative abundance and distribution of REEs in the studied fly ash samples, providing a theoretical lead for the basis of extraction and waste management.