Fly ash from coal combustion: Dependence of the concentration of various elements on the particle size

The dependence of the distribution of various elements on the particle size of coal combustion fly ash is of special interest for use of the fly ash in ultrafine cement, or for recovery of valuable elements from fly ash. Therefore, electrostatic precipitator fly ash from a coal fired power plant was split into particle size classes with mass median diameters of 2.2 µm, 5.4 µm, 9.7 µm, 19 µm and 43 µm by means of an air classifier. The size dependence for the elements Al, As, Ba, Br, Ca, Ce, Cl, Co, Cr, Cu, Fe, Ga, Ge, Hg, K, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Rb, S, Sb, Se, Si, Sn, Sr, Ta, Ti, Th, Tl, U, V, W, Y, Yb, Zn, Zr, and for total carbon, was determined. For several components a considerable dependence of the concentration on the particle size was observed, which can be approximated by power functions. Substantially higher concentration with decreasing particle size was found for total carbon, As, Cu, Ga, Ge, Hg, Sn, Pb, Tl, V and Zn, whereas the concentration of the main mineral components Si, Al, Fe, Ca, K, Na, Mg, Mn and Ti was slightly lower in the fine particles. The increased concentrations have to be considered for critical elements like Pb and Cu when air classified ultrafine fly ash is used in ultrafine cement. Air classification might also be an important pre-processing step in a recovery process for trace elements like Ga, Ge and V.