Drop Dynamics and Speciation in Isolation of Metals from Liquid Wastes by Reactive Scavenging

Abstract

An instrument for emission, fluorescence, and scattering diagnostics of a 22 k W gadair thermal reactor is described. 2-pair parabolic mirrors provide diffraction-limited imaging in field center, with worst-case blur over a 12.8 mm field of view having a radius of 0.044 mm rms. Either full-field filtered or transverse line dispersed viewing can occur simultaneously. Dispersion is achieved using a 2-m spectrographf-matched to the feed optics. Examples of data show non-axisymmetric flow in a nominally laminar flow system, drop shapes in situ, and Mie scattering from soot. Index Headings: Flame diagnostics; Planar laser-induced fluorescence; Spectroscopy in harsh environments; Imaging; Drop vaporization. INTRODUCTIONMetal-bearing solutions and slurries can pose environmental hazards. Among the most acute of these hazards is the accumulation of radioactive wastes, held in underground steel tanks at locations such as Hanford, Washington, and the Savannah River Site at Aiken, South Car01ina.I-~ Vitrification of radioactive wastes, either as glass logs4 or ~e m e n t ,~ is the approach most actively being followed to sequester radionuclides over the millennia required for decay to negligible activity levels. An important step in preparation of vitrified material is removal of water and organic material, resulting in volume reduction. Among the ways to perform solvent removal, that simultaneously destroys organic matter, is -thermal treatment. This process simply consists of spraying the waste into a high temperature (T > 1800 K) reactor and adding powdered sorbent material. Water is vaporized, organics are oxidized to water and carbon dioxide, and metallic species are oxidized. Scavenging of metal ions by kaoliniteG9 prior to cooling and oxide formation is a promising technology for high-efficiency collection of gasphase radionuclides. Under optimized conditions, destructiodrecovery efficiencies ( D E S ) of up to 99.99% of metal ions in an entering waste stream can be scavenged.Considering the hundreds of millions of curies in the millions of gallons of waste,3 a DRE of 99.99% scavenging is entirely unacceptable. Understanding the cause of the 0.01% unscavenged fraction and devising means for its reduction are essential if thermal processing is to be used for nuclear waste treatment. Our hypothesis is that a small fraction of large (>200 km in diameter), that is, "rogue", drops originating from the input liquid stream or formed by coalescence in the reactor are responsible for the 0.01% unscavenged metals. These drops avoid hot zones in the reactor, resulting in either incomplete vaporization or vaporization in a zone with insufficient temperature to vaporize the metal contents. The metals form small particulates that easily circumvent particle traps such as filters and cyclones.I0 Drop dynamics in analytical flames and plasmas has been studied for many years, including work by Hieftje (e& Refs. 11-13), Farnsworth,14 and Olesik (e&,. To test the rogue drop hypothesis requires the ability to generate drops, both as spr...