Characterization of Thermochemical Inactivation of Asbestos Containing Wastes and Recycling the Mineral Residues in Cement Products

Abstract

Asbestos Containing Wastes (ACW) were mixed with chemical additives and heated with microwaves. Eternit, Progypsol and Spray coated Amosite were thermo-chemically treated and the mineral residues recycled in mortar. Samples were analysed before and after thermal treatment by Scanning Electron Miscroscopy with Field Emission Gun (SEM FEG) and Energy Dispersive X-ray analysis (EDX) coupled to the SEM FEG. Mineralogy was examined by microprobe. X Ray diffraction (XRD) was used to identify the crystal lattice of products before and after melting. Additives such as borates, phosphates and sodium carbonate were introduced to lower the fusion temperatures and the treatment costs. Thermogravimetric analysis and differential scanning calorimetry were used to estimate melting effects due to the additives. EPR spectroscopy was used to study the environment of iron ions embedded on asbestos surfaces and demonstrate fiber transformation to a more isotropic phase. Magnesium silicates (serpentine group) were found in Eternit and Progypsol samples, and iron silicates (amphibole group) in Spray Coated Amosite samples. Observations by SEM illustrate differences in the structure between raw and treated materials but elemental analyses show the same chemical compositions. XRD patterns confirm the transformation of asbestos fibers into other crystalline materials. Borates are most effective for lowering the temperature of thermal transformation. The ground mixtures begin to melt at 900°C with borates compared to other melting products where the melting temperature is around 1,000–1,100°C. Heating with fusion additives allows irreversible transformation of the asbestos fibrous structure and recycling of the residues as filling materials in mortars. Thermal treatment seems to be the most effective process to transform and inactivate asbestos fibers from starting materials. Recycling treated ACW in mortar shows a decrease of mechanical properties when replacing cement or aggregate, but samples with 10 wt% ACW replacing aggregate show tensile strengths of 10 MPa, close to the value for the reference mortar with no ACW. Reuse of treated ACW would prevent stockpiling in hazardous waste landfills.