Mineralogical Stabilization of Heavy Metals from Alkali Melting Furnace Fly Ash by Thermal Treatment with Adjustment of Its Chemical Composition

The potential of an inexpensive and simple method to control the solubilities of hazardous heavy metals such as Pb and Cr from CaO rich melting furnace fly ash (MFFA) by addition of silica and alumina with preceding heat-treatment was demonstrated. The addition of SiO2 and Al2O3 to melting furnace fly ash with preceding heat-treatment at 700°C resulted in a decrease in the pHs of the leachates from 12.4 to 11.2 and in a remarkable reduction of leachabilities of heavy metals although the leachability of Cd was below the ICP-AES detection limit (0.04 mg/l) even without the addition. In the CaO-SiO2-Al2O3 system the solubilities of Pb, Cr and Cu were much lower in the area where 3CaO·Al2O3, gehlenite, CaO·Al2O3 and anorthite deposit. The solubility of Zn was also much lower in the area where pseudowollastonite, rankinite and 3CaO·SiO2 as well as the above-mentioned minerals deposit. The leachabilities were fairly well correlated with practical parameters of CaO/(CaO + SiO2 + Al2O3) (or (SiO2 + Al2O3)/CaO) and Al2O3/(CaO + SiO2 + Al2O3) (or Al2O3/CaO) for Pb and CaO/(CaO + SiO2 + Al2O3) (or (SiO2 + Al2O3)/CaO) for Cu in the range of the present study. For Cr and Zn the leachabilities were very low in the range of the present study and no particular correlations were found. The significant reduction in their solubilities was confirmed not being caused by their volatilization during heat-treatment.