Paola Pisciella

Chemical durability of glasses obtained by vitrification of industrial wastes

The vitrification of zinc-hydrometallurgy wastes, electric arc furnace dust (EAFD), drainage mud, and granite mud was shown to immobilize the hazardous components in these wastes. Batch compositions were prepared by mixing the wastes with glass-cullet and sand to force the final glass composition into the glass forming region of the SiO2-Fe2O3-(CaO, MgO) system. The vitrification was carried out in the 1400-1450 degrees C temperature range followed by quenching in water or on stainless steel mold. The United States (US) Environmental Protection Agency (EPA) toxic characterization leaching procedure (TCLP) test was used as a standard method for evaluating the leachability of the elements in the glasses and glass-ceramics samples made with different percentages of wastes. The results for EAFD glasses highlighted that the chemical stability is influenced by the glass structure formed, which, in turn, depends on the Si/O ratio in the glass. The chemical durability of jarosite glasses and glass-ceramics was evaluated by 24 h contact in NaOH, HCl and Na2CO3, at 95 degrees C. Jarosite glass-ceramics containing pyroxene (J40) are more durable than the parent glass in HCl. Jarosite glass-ceramics containing magnetite type spinels (J50) have a durability similar to the parent glass and even lower in HCl because the magnetite is soluble in HCl.

Vitrification of electric arc furnace dusts.

Electric arc furnace baghouse dust (EAFD), a waste by-product of the steelmaking process, contains the elements that are volatilized from the charge during the melting (Cr, Pb, Zn, Cu and Cd). The results of leaching tests show that the concentration of these elements exceeds the regulatory limits. Consequently, EAFD cannot be disposed of in ordinary landfill sites without stabilization of the heavy metals. In this work, the vitrification of EAFD, from both carbon and stainless steel productions, were studied. The vitrification process was selected as the inertizing process because it permits the immobilization of the hazardous elements in the glass network and represents an environmentally acceptable method for the stabilization of this waste. Classes of various compositions were obtained by mixing EAFD with glass cullet and sand. The EAFD and the glass products were characterized by DTA, TG, X-ray analysis and by the TCLP test. The results show that the stability of the product is influenced by the glass structure, which mainly depends on the Si/O ratio. Secondary crystallization heat-treatment were carried out on some samples. The results highlighted the formation of spinel phases, which reduced the chemical durability in acid media. The possibility to recover Zn from carbon steel production EAFD was investigated and about 60-70% of metal recovery was obtained. The resulting glass show higher chemical stability than glasses obtained without metal recovery.

The effect of Cr2O3 as a nucleating agent in iron-rich glass-ceramics

In this work, the eAect of Cr2O3 as a nucleating agent, in iron rich glasses has been investigated by means of DTA, XRD and density measurements. By Cr2O3 addition, from 0. 4t o 1.0wt%, a lowering of the crystallisation peak temperature resulted in the DTA trace, the maximum eAect corresponding to 0.7wt%. By evaluating the degree of crystallisation of the glass at 0.7wt% Cr2O3, the highest eAciency in the nucleation process also corresponds. The optimum values for the nucleation and crystallisation time and temperature, determined for 0 . 7wt% Cr2O3 addition, have been 70min at 630C and 30 min at 800C. The crystalline phases formed at different thermal treatment temperatures of the parent glass have been investigated by XRD; the spinel is the only phase after the nucleation; pyroxene is the major phase after the crystallisation. The results of this study have highlighted that a small percentage of Cr2O3 strongly aAects the spinel formation thereby reducing the time and temperature of the thermal treatment and enhancing the degree of crystallisation of high iron content glasses. # 1999 Elsevier Science Limited. All rights reserved

The crystallisation kinetics of iron rich glass in different atmospheres

The crystallisation kinetics of powder and bulk iron-rich glass, based on zinc hydrometallurgy hazardous waste, were investigated in air and nitrogen atmospheres. The activation energy of crystal growth, EC, and the Avrami parameter, n, were estimated by means of diAerential thermal analysis. The kind and amount of crystal phases, formed during the thermal treatment and their relative ratio were evaluated by X-ray diAraction analysis. The kinetics results sustain that the pyroxene phase grows on a fixed number of magnetite nuclei. The activation energy of crystal growth on the surface and in the bulk are similar in air and nitrogen atmospheres. When the powder glass sample is heat-treated in air, surface oxidation of Fe 2+ into Fe 3+ inhibits crystallisation resulting in the decrease of the percentage of crystal phase and the change of the Avrami parameter value. # 2000 Elsevier Science Ltd. All rights reserved.