One of the milestones of the mixed waste focus area is to develop at least three technologies that cumulatively can treat at least 90 percent of the mixed waste in DOE's current inventory. The challenge for MWFA in meeting this milestone is that some mixed waste streams contain bad actors--constituents that must be stabilized before disposal but that present technical stabilization difficulties--like mercury-contaminated wastes, salt wastes, incinerator ashes, heavy metals, or pyrophorics.

In seeking an answer to these problems, Argonne National Laboratory, with funding provided by the U.S. Department of Energy's Office of Science and Technology, developed the chemically bonded phosphate ceramic process to create superior waste forms for mixed low-level waste streams that cannot be safely or effectively handled by vitrification.

Delphi Research is contributing to this project by showing how CBPCs can be adapted to stabilize DETOX process residues for disposal. Delphi, which developed and patented the DETOX catalytic chemical oxidation process to treat hazardous and mixed wastes as an alternative to incineration, is gearing up to cold test its DETOX process at DOE's Savannah River Site in South Carolina.

How CBPCs are made

The Argonne technique of making phosphate ceramics is based on a simple acid/base reaction: metal oxide powder, made from magnesium, aluminum, or zirconium, is combined with phosphoric acid, saturated aqueous ammonium, or sodium phosphate. The metal oxide and acid components, to which wastes have been added, react at room temperature and form a hard, dense phosphate ceramic as they are allowed to set for 8 to 21 days, depending on the starting metal oxide. The fully cured phosphate ceramic constitutes the final waste form. Setting and stabilization create a phosphate ceramic in which the mechanisms of waste stabilization are both chemical bonding and physical encapsulation.

Testing various waste components and metal oxides

Surrogates of incinerator ash; chloride, nitrate, sulfate, and carbonate salts; and cement sludge have been studied as the solid waste portion of the phosphate ceramics. These solid wastes have also incorporated chromium, cadmium, nickel, and lead (but not mercury) as heavy-metal contaminants, and naphthalene and 1,2-dichlorobenzene as hazardous organic constituents. The loadings of surrogate incinerator ash and cement sludge ranged from 35 to 70 percent by weight (wt %).

One measure of the hardness of the solid material is its compressive strength. Argonne measured the compressive strengths of magnesium, aluminum, and zirconium phosphates that had ash loadings of 35 wt % each. The aluminum phosphate had the highest compressive strength at 8400 pounds per square inch, followed by magnesium phosphate at 7200 psi, and zirconium phosphate at 4700 psi.

Another property of CBPCs that makes them acceptable hazardous waste forms is their low leachability in aqueous environments. Various magnesium and zirconium phosphate ceramics made with surrogate incinerator ash and cement sludge wastes and containing cadmium, chromium, nickel, and lead were analyzed by the U.S. Environmental Protection Agency's Toxicity Characteristic Leaching Procedure to determine the leachability of each metal from the ceramics. Phosphate ceramic stabilization results in very low amounts of leachable cadmium, chromium, and lead. The reduction in leachability from the unstabilized material for the three metals is 99.5-99.9 percent for cadmium, 99.9-99.97 percent for chromium, and 99.8-99.9 percent for lead. The amounts of these three toxic metals leached from the various ceramics are all well below the regulatory limits set by EPA.

Wastes from the DETOX process

Argonne's CBPC technology is an attractive method of accomplishing stabilization of two secondary wastes produced by Delphi's DETOX process: inert and produced residues, and solid residues from the DETOX solution. Inert residues are materials that cannot be destroyed in the DETOX solution. Produced residues are chemical byproducts of the DETOX process resulting from the destruction of hazardous organics. Because both inert and produced residues are removed together, they are considered a single waste stream. During waste treatment, the DETOX solution itself becomes contaminated with hazardous metals and radionuclide residues, which are solubilized and concentrated in the DETOX solution.

As treatment continues, the metals and radionuclide concentrations increase, as does the total radioactivity in the solution. When the radioactivity of the DETOX solution reaches a certain limit imposed by the waste acceptance criteria of the disposal site, the solid components are precipitated and removed from the unit in a hydrated ferric oxide matrix. The DETOX solution is composed mainly of ferric chloride dissolved in dilute hydrochloric acid. This residue is not compliant with the Resource Conservation and Recovery Act's Land Disposal Restrictions and must be stabilized prior to disposal.

Argonne conducted bench-scale demonstrations to determine what changes needed to be made to the CBPC process to accommodate DETOX solution solid residue as a feed component. Argonne found that its CBPC process successfully stabilized the hazardous metal and uranium/plutonium surrogates that were introduced into the DETOX surrogate residues, resulting in very low leachability, high compressive strengths, and good durability of the final waste form.

The very low leachability of contaminants established Argonne's phosphate ceramic process as being tolerant of a wide range of contaminant concentrations in mixed waste feeds. Coupling Dephi's DETOX process with Argonne's chemically bonded phosphate ceramic technique shows promise of providing a complete, nonthermal capability for high-organic, low-level mixed wastes by destroying the organics and stabilizing the resulting secondary wastes in a very durable, insoluble form.

For more information about Argonne National Laboratory's CBPC process, call Arun S. Wagh, one of the principal investigators, at Argonne's Energy Technologies Division at (708) 252-4295, e-mail arun_wagh@qmgate.anl.gov.

For more information on Delphi Research, contact President Terry Rogers at (505) 243-3188, fax (505) 243-3188. For more information about the DETOX process or the Savannah River demonstration, call Patrick Dhooge, vice president of Delphi Research, also at the above numbers.

Donald Robertson is the environmental compliance officer at Delphi Research, Inc.