MINERALOGY AND GEOCHEMISTRY OF FLY ASH: WHAT'S IN IT?

In general, fly ash consists of refractory (non-combustible) constituents that remain after coal combustion. At boiler temperatures (about 1100 to 1650 ^oC), mineral phases present in feed coal melt, react, or undergo transformations to forms that are stable, with corresponding redistribution of major and trace elements. Most coal fly ash consists of varying proportions of quartz, aluminosilicates such as mullite (Al₆Si₂O₁₃), and iron oxides, within a matrix of aluminosilicate glass resulting from partial melt. Calcium aluminosilicates and/or calcium/magnesium oxides are present in fly ash derived from coal with high calcium contents. Iron present in coal as pyrite is largely retained in coal fly ash as iron oxides, whereas sulfur enters the flue gas, and in many utilities burning pulverized coal, is captured by combining with calcium from limestone feedstock. This process produces calcium sulfate sludge, a flue gas desulfurization (FGD) byproduct. ASTM classification of coal ash for construction purposes is based on the sum of SiO₂ + Al₂O₃, + Fe₂O₃ vs. other major constituents, largely a function of the chemistry, rank, and mineralogy of the feed coal.

In bituminous coals, significant fractions of elements such as arsenic, mercury, and selenium occur in pyrite, and these are partitioned into the gas phase during coal combustion. Element capture, primarily by adhering of gas-phase constituents onto ash particle surfaces, occurs as the flue gas cools. Mercury and selenium are especially volatile, with relatively small fractions of the amount present in feed coal retained in the fly ash. Mercury capture also involves partitioning into unburned carbon and FGD. For less volatile elements such as arsenic, greater proportions of the amount present in feed coal are retained in fly ash. Element concentrations in fly ash are enriched relative to feed coals, as a combined result of mass reduction during coal combustion, and capture and retention of gas phase constituents. Highest trace metal concentrations occur in the finest size-fractions of ash. Despite this enrichment, evidence from standardized leaching protocols and in-situ field tests indicates that trace constituents in fly ash, and coal ash in general, are relatively immobile. This allows for a wide range of beneficial uses discussed in this topical session.