Thermodynamic modeling of volatile hazardous metal behavior in the Vortec Vitrification System

The thermochemical equilibrium calculations indicate that at the temperature of a propane--air flame, some volatilization of uranium, plutonium, technetium, and cesium will occur. The expected concentrations of plutonium, technetium, and cesium in the flame will be very low because of the small maximum concentration of these elements in the projected feed materials for the first 30-day test. The quantities volatilized can generally be decreased by operating the flame in a fuel-rich mode, although this will lead to greater carbon monoxide production, which may be more objectionable. The concentrations of chlorine and fluorine, at least at the maximum levels in the projected Vortec feed, are not projected to greatly influence the vaporization rates. Therefore, blending to reduce the concentrations of those elements would most likely not be effective in reducing metal vaporization. Most of the elements vaporized condense by the time the gas cools to 2000 F. These elements would condense either on surfaces near the front of the heat recuperator or on entrained particulates or homogeneously as relatively pure submicron particles. Cesium would be expected to condense at the lower temperatures near the rear of the recuperator, although the expected maximum concentration in the Vortec feed material is extremely low so it should be greatly diluted by other particulates. The elements that condense on other entrained particles will form enriched surface coatings. Particles larger than 10{micro}m or so will be collected in the scrubber. Smaller particles, especially the submicron particles formed from homogeneous nucleation, should be largely collected in the HEPA filter. Deposits formed in the heat recuperator can normally be handled via sootblowing. To reduce handling problems, we suggest that the recuperator be oriented vertically so that the deposits blown off of the heat exchanger fall directly into the molten glass. The large size of the deposits should help to reduce the rate of revaporization, allowing the volatile elements to be removed with the glass. The volatile elements that do not deposit on system surfaces will be concentrated in the smaller particles. Therefore, the HEPA ash will be greatly enriched in these elements. If the HEPA filter is itself sent to a melter, the elements may revaporize and multiply the problems related to metal vaporization significantly. Therefore, the HEPA filters should be disposed of without high-temperature processing. Also, to reduce the formation of these very small particles, it is helpful to include in the feed larger particles to act as condensation nuclei that can then be collected in the scrubber. This can be accomplished by using feed materials with a fraction consisting of particles small enough that they will not be collected in the cyclone in the melter, but large enough that they will easily be collected by the scrubber. This is one advantage that firing bituminous coal has over gas firing; it provides a source of ash particles of the right size range to serve as nucleation sites, but large enough (depending on the coal) so that they can usually be collected efficiently in the scrubber system