Vitrification of an incinerator blowdown waste containing both chloride salts and carbon

Abstract

A low‐level, mixed‐waste simulant, derived from incinerator blowdown residue, has been vitrified in a series of crucible tests. Major components of this waste simulant included carbon, sodium chloride, silica, and alumina. Hazardous and surrogate radioactive metal complexes were also included. Two different formulations of additives were combined with the waste to facilitate glass formation. These glass formulations included NaNO₃ to serve as a flux and to assist in the oxidation of the carbon. During vitrification, a chloride salt layer was observed on the melt surface which volatilized during the course of heating. Furthermore, significant concentrations of As, Cd, Cs, Ni, Pb, Sb, Se, Tl, and Zn had volatilized during vitrification. It is postulated that this was due, in part, to the formation of volatile metal chlorides. Upon quenching, small metal nuggets, containing Pb, Sb, Ag, and Ni separated from the glasses. This reduction of the metal oxides was caused by the presence of carbon in the simulant. The chemical durability was evaluated by both the Toxicity Characteristic Leaching Procedure (TCLP) and the Product Consistency Test (PCT). The TCLP results were below the limits for the toxicity characteristic for a hazardous waste and also met the Universal Treatment Standards for listed wastes such as F006. The PCT elemental release rates were considerably below the waste acceptance criteria for high level waste glass, which is also indicative of highly leach resistant glasses. Even though the glass products themselves were chemically durable, the formation of secondary waste streams, due to the presence of the chloride and carbon, may pose difficult challenges for the vitrification of incinerator wastes.