An upcoming issue of ACerS’ International Journal of Applied Ceramic Technology (available as an “Early View” paper, subscription required) will have an intriguing report from three researchers from the United Kingdom’s Imperial College and a colleague from Tectronics Ltd. regarding a process to render flue glass emissions into an environmentally safe form and, potentially, into useful ceramic applications such as tile and porcelain.
The underlying issue is what to do with the residues from the growing number of dual-purpose operations that extract energy from solid waste, such as municipal trash. These energy-from-waste plants are required to have complex and tightly regulated air pollution control systems to capture and treat the gas emissions they generate. These APC systems typically yield powder residues, that, if put in contact with water, can leach dangerous chemicals. In the U.K. and elsewhere, these dangerous powders currently are stored in special landfills or saltmines.
The researchers wondered if there was a way to make the residues safer. They took an idea already used with medical hazardous waste and nuclear waste material – DC plasma treatment – and tested how it might be applied the APC residues. It turned out that DC plasma, as expected, has the ability vitrify the powder and, in the process, destroy hazards, such as dioxins and furans. The vitrified product is a stable, inert, nonleachable and recyclable glass product. Heavy metals that had been present are volatilized and collected as a very low-volume (5–8 wt%) secondary APC residue in a bag house filter of the DC plasma plant.
The researchers then took the APC residues, mixed in glass-forming additives (SiO2 and alumina) and subjected the material to DC plasma arc treatment. The resultant material was quenched into water to produce a sample with ∼5 mm particles. They then combined the resultant frit with bentonite (a binder) to make various glass-ceramic tiles using a standard powder pressing and sintering route. They found that their test tiles had physical properties comparable to commercially available ceramics such as porcelain and monoporosa, with high bulk density (2.4 g/cm3), low water absorption (<6%) and high flexural strength (∼60 MPa).
The researchers conclude that,
“DC thermal plasma treatment of APC residues is expected to become increasingly economically viable compared with alternative disposal options such as landfill that involves haulage, pretreatment, gate fees and landfill tax . . .
“Glass-ceramic tile production incorporating plasma treated APC residues could provide high quality ceramic tiles. Combined thermal plasma treatment and glass ceramic production from APC residues therefore represents an integrated solution to the management of this problematic waste.”
It should be noted that the commercial-scale plasma facilities are relatively new, and only five or six are in operation in the world. None, to my knowledge, are in the U.S. or Canada, although several are in the planning stages.