Thermal Conversion of Heavy Metal Chlorides (PbCl2, CdCl2) and Alkaline Chlorides (NaCl, KCl) into Phosphate Glasses

Fly ashes resulting from municipal solid waste incineration contain lead and cadmium chlorides (PbCl2, CdCl2) that volatilize during the thermal treatment and vitrification. They are recovered in “secondary ashes”. We report the thermal conversion of lead and cadmium chlorides into phosphate glasses, using ammonium dihydrogen phosphate (NH4H2PO4) as a precursor. Two synthetic batches containing (PbCl2 + CdCl2), and (PbCl2 + CdCl2 + NaCl + KCl) were mixed with NH4H2PO4 in variable amounts. The (NaCl + KCl)/(PbCl2 + CdCl2) ratio in the batches to be converted was chosen to reflect the secondary ash composition. The resulting glasses contain 20−60 mol % P2O5. Elemental analyses of the glasses are compared with the calculated data from two conversion reaction models. The first one involves the reaction of NH4H2PO4 with chlorides only, the second addition involves a high-temperature conversion reaction of chlorides with water. The second mechanism is in good accordance with the experimental results for PbCl2 + CdCl2 mixtures. In the case of PbCl2 + CdCl2 + NaCl + KCl mixtures, the conversion of lead and cadmium chlorides into phosphates is optimum for glasses containing more than 45 mol % P2O5.