Influence of composition variations on the initial alteration rate of vitrified domestic waste incineration fly-ash

Vitrification as a waste stabilization technology has often been considered applicable only to high-level radioactive waste for which, with the use of suitable additives, it yields a vitreous material with excellent chemical durability. It has become apparent in recent years that some waste forms—notably domestic waste incineration fly-ash purification residues—contain most of the ingredients of a vitrified material, although their composition variations are difficult to control. It is thus important to ensure not only that the materials are suitable for vitrification, but also that the resulting product exhibits acceptable long-term behavior under all circumstances. An initial study showed that, allowing for the compensation changes inherent in the melting process builtby EDF ∗∗, the residue collected by a single fly-ash dust separation defines a composition range within which the suitability of the vitrified material can be verified. “Vitrified material” refers to a melted material that contains no unmelted inclusions after cooling, but that may contain a variable fraction of crystallized phases. Five composition parameters were identified for the long-term behavior assessment: the concentrations of the three major elements (silicon, aluminum and calcium), the total alkali metal (sodium and potassium) concentration, and the sum of the concentrations of two toxic elements (zinc and lead). The other elements were assumed constant at molar ratios representative of industrial wastes. The experimentation plan methodology applied to the composition range identified fourteen materials suilable for developing and validating first-order models of the material components. The fly-ash composition had a very significant effect on the degree and kinetics and crystallization in the vitrified material within the experimental composition range; the cooling rate was the determining factor for some of the fourteen materials studied. Two crystalline phases predominated: spinels rich in chromium, zinc, aluminum, magnesium and iron formed quickly on cooling, and accounted for about 2 vol% of the final material. Gehelenite (Ca 2Al 2SiO 7) crystallized massively in some vitrified materials, accounting for more than half the final product and giving it a rock-like appearance. The effect of composition alone must therefore be distinguished from the effect of crystallization on the leaching behavior. Soxhlet tests were conducted for 14 days according to a protocol based on that o