Study on the modes of occurrence of rare earth elements in coal fly ash by statistics and a sequential chemical extraction procedure

Rare earth elements and yttrium (REY) are strategically important materials in the defense, aerospace, automation and electronic industries, and therefore, REY has been referred as the "vitamin of industries." The recovery of REY from coal fly ash has been recognized as a promising way to expand REY sources, and characterization of REY in coal fly ash will be critical for its subsequent exploitation and utilization. The work reported here investigates the modes of occurrences of REY in fly ash. Coal fly ash samples were collected from Guizhou Province. The results of the particle size classification showed that the content of total REY as well as that of individual components increased with a decrease in the particle size, and REY showed a moderate degree of enrichment in the smaller particles. The REY content was found to be a strongly and positively correlated with the content of silicon and aluminum through Pearson correlation and regression correlation analysis. A mathematical model between REY and Si/Al was established and this can be used to predict the content of REY with known Si and Al content. A sequential chemical extraction procedure for REY in coal fly ash was designed, and results from these tests showed that the silicate & aluminosilicate form was the dominant mode of occurrence of REY; this form accounted for 80% of the material. Hence the silicate & aluminosilicate form, which contains the most REY, must be the focus of subsequent recovery processes.