Rare earth elements (REEs) are highly important in the modern society as a result of their wide use in various fields of industry. Their high supply risk and increase in demand has led to classification as critical materials, and consequently, new resources for REEs are being prospected widely. Coal fly ash has been suggested as a possible secondary resource for REEs, but very little information is available regarding REE occurrence in biomass or peat derived fly ash.

In this paper, fly ash samples from commercial power plants using peat and biomass as fuel were studied for REEs. The average concentration of REEs was 530 ppm, with up to 920 ppm in one fly ash. Five out of seven fly ashes were identified promising for profitable REE recovery according to outlook coefficients of >0.7 and critical REE content of >30%. Four fly ashes were found to exhibit medium REE enriched distributions, whereas three fly ashes displayed light REE enriched distributions. These enriched distribution patterns, as well as the observed Gd, Ce, Eu, La, and Y anomalies in the patterns, are proposed to originate mainly from the granitic bedrock at the fuel source. Additionally, positive correlation between REEs and iron was found, indicating REE association with iron minerals.