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Progress in Plasma Processing of Materials, 1997

ISBN Imprimer: 1-56700-093-2



Fly-ashes were processed in a plasma furnace where a DC arc is transferred either between a tungsten or a graphite electrode and the melt inside a graphite crucible acting as a bottom electrode,or between two twin plasma torches.
Fly-ashes from a municipal waste incineration plant were collected from the electrostatic precipitator before acid gas removal so that the chlorine and sulfur contents were around 10 % and 2.5 % respectively.
After preheating of the furnace, fly-ashes are fed through a screw-feeder with a feed-rate around 25 kg/h. Arc intensity was adjusted between 250 and 500 A; power input was from 20 to 70 kW; temperature inside the melt, measured by thermocouple, was between 1350 and 1550 °C. Thermal transfers in the furnace were measured to check the energy balances. Off-gases exit the furnace through a post-combustion chamber to burn possible carbon oxide; then they cross a bag-filter before being washed by sodium hydroxide. More than 70 % of the input material were melted to a glassy product in a compact form and 20 % were collected in the bag filter as condensate. The rest of the input passed into the off-gas as permanent gases.
Analysis of the feed, of the glassy product and of the fume exhaust solids are given with the phase partitioning ratios of chemical elements. For example, in particular conditions, only about 1 % of lead is trapped in the glassy product and even less for cadmium. The influence of reducing potential of the furnace atmosphere on metals contents in the glassy product is approached.
To assess environmental implications, the glassy products, similar to obsidian or basalt, were tested in accordance with French toxicity characteristic leaching procedure (X 31210).
Hydrometallurgical treatments have been applied to the fume collected in the bag-filter so that heavy metals are concentrated, as hydroxides, freed of chlorine, in a filter-cake which is only 3 % of the initial fly-ash mass.This residue could be recycled in the arc furnace while the filtrate, mainly sodium chloride , must be ducted to the waste water treatment station.
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