Concentrations and Distribution of Major and Selected Trace Elements in Size-Density Fractionated Fly Ashes

Abstract

The effectiveness of separation methods based on differences ill size and density for isolating and concentrating trace-metal-enriched solid phases was investigated in four fly ashes. Size fractionation of fly ashes showed that about 80-90% by mass of the particles were <45 mu m in size. Also, 50-60% of the mass of fly ash particles occurred in the 2.3-to 2.9-g cm(-3) density range and consisted mainly of mullite, quartz, and glass. About 16-30% of the mass of particles had densities >2.9 g cm(-3). The magnetic particles in this mass consisted mainly of spinels and hematite with the morphology of pyrite framboids. Concentrations of Ca, Ba, Sr, and P were enhanced in the >2.9-g cm(-3) density fractions. Concentrations of trace elements As, Cr, Cu, Mo, Ni, Pb, V, and Zn increased with decreasing particle size. The highest concentrations of Cr, Cu, Ni, and V were found in the >2.9-gcm(-3) portion of the magnetic fractions. The Cr, Ni, and V in these fractions seemed to be associated with spinel. The mass distribution of trace elements observed is apparently due to their volatilization-condensation during coal combustion. Calculations showed that 20-50% of the mass of As, Mo, Pb, Se, V, and Zn and 6-30% of the mass of Cr, Cu, and Ni may be present in surface layers. The trace elemental mass in these surface layers may constitute the readily mobilizable fraction of these trace elements in bulk ash. The nature of the solid phases of the trace elements in these fly ashes (except for Cr, Ni, and V in spinel) could not be determined directly.