Metal leaching from MSW fly ash before and after chemical and thermal treatments

Municipal Solid Waste (MSW) incineration fly ash was collected in the industrial facilities in the city of Toulouse, equipped with recent gas scrubbing equipment which collects daily 15 t of lime treated fly ash. The fly ash contains a large proportion of water soluble chlorides, besides trace elements such as iron, zinc, titanium and copper. Presently, the fly ash is landfilled after a cement solidification process. Matrix stability and leaching behavior of heavy metals (Cd, Cu) from samples of doped MSW fly ash were examined before and after chemical and thermal treatments. A new phosphate process leading to hydroxylapatite formation is described. To evaluate it's performance as a stabilization process, cupric and cadmium ions were added as typical pollutant tracers and their distribution was studied as a function of different treatment parameters. The hydroxylapatite process used effectively removes chloride ions in the water extract and retains most of the added metal ions in the solid residues. Evaluation of this procedure as a cold process shows 50% reduction in leachable metal ions. Calcination of the solid residues at 900°C results in agglomerated particles with increased resistance to metal ion dissolution. This is attributed to heavy metal incorporation in the hydroxylapatite matrix formed during the thermal treatment. Thus, combined chemical and thermal treatments are efficient for heavy metal stabilization in MSW fly ash.