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 an d cadmium chlorides (PbCl2, CdCl2) that volatilize during the thermal treat ment 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 synthe tic batches containing (PbCl2 + CdCl2), and (PbCl2 + CdCl2 + NaCl + KCI) we re mixed with NH4H2PO4 in variable amounts. The (NaCl + KCI)/(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 conv ersion reaction models. The first one involves the reaction of NH4H2PO4 wit h chlorides only, the second addition involves a high-temperature conversio n reaction of chlorides with water. The second mechanism is in good accorda nce with the experimental results for PbCl2 + CdCl2 mixtures. In the case o f PbCl2 + CdCl2 + NaCl + KCL mixtures, the conversion of lead and cadmium c hlorides into phosphates is optimum for glasses containing more than 45 mol % P2O5.