Vitrified metal finishing wastes II. Thermal and structural characterisation

Waste filter cakes from two metal finishing operations were heat treated and vitrified. Substantial weight loss during heating was due to emission of water, volatile sulphur-rich and chlorine-rich compounds, and the combustion of carbonaceous components. Estimations of COx, SOx and HCl emissions were based on chemical analyses. Upon cooling from molten, one sample remained amorphous but all others partially crystallised. Crystalline nature was dependent upon waste composition and the level of P2O5 addition. Thermal stabilities of the waste forms were good, but less so than MW, a borosilicate glass developed for its high temperature stability. Mössbauer and FTIR analyses showed that iron environments in the different vitrified waste forms were very similar. Iron was present predominantly as Fe3+, although the exact redox ratio varied slightly between waste forms. Iron in both redox states occupied distorted octahedral coordination polyhedra with similar levels of site distortion. Phosphate networks in the vitreous materials were highly de-polymerised, consisting largely of (PO4)3− monomer and (P2O7)2− dimer units. This explained the high chemical durability of these waste forms and their structural insensitivity to compositional change, underlining their suitability as hosts for the immobilisation of toxic and nuclear wastes.