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In the work described in this paper, partially zeolitised coal¯y ash from a Spanish power station has been studied as the immobilising agent of an arc furnace dust waste (s-WA). s-WA is a dust coming from the collecting systems of particulate material in a carbon±steel electric arc furnace (EAF). The hazardous metals normally found in this kind of waste are lead, cadmium, and chromium. In addition, the dust usually has a high zinc content resulting from the use of galvanised scrap. Because of its heavy metal content, s-WA is classi®ed as hazardous waste, according to Spanish regulations. Different zeolitisation procedures, depending on the treatment of the¯y ash with NaOH and KOH alkaline solutions, using re¯ux heating and different times of attack were tested. The zeolitised products were studied using XRD in order to characterise the solid phases produced. The solidi®-cation/stabilisation (S/S) trials of s-WA were carried out using the zeolitised ashes, as well as the original coal¯y ash, in mixtures containing 20% (w/w) of ordinary Portland cement. To evaluate the ef®ciency of the immobilisation process, some speci®c criteria were de®ned, with regard to some physical (compressive strength) and chemical (pH and metal concentrations in TCLP leachates) characterists of the S/S solids. Finally, comparisons among the stabilising mixtures were made and the role of the zeolitisation of¯y ash in the S/S process was evaluated.

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