Oak Ridge National Laboratory


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Useful metals could be mined from pond sludge, ORNL finds

OAK RIDGE, Tenn., Aug. 9, 1995 — Studies of pond sludge from the Department of Energy's (DOE's) Oak Ridge K-25 Site suggest there may be wealth in the waste. Tests by researchers at DOE's Oak Ridge National Laboratory (ORNL) show that, if the material is heated and cooled, a variety of minerals in the mud can be mined with a magnet. The remaining material is crushed glass, which could be inexpensively disposed of in a landfill.

Alex Gabbard of ORNL's Metals and Ceramics Division and Charles Malone of the Instrumentation and Controls Division conducted tests on surrogate sludge that contained a dozen nonradioactive metals and nonmetallic elements found in the actual sludge. These elements include aluminum, copper, iron, nickel, silver, and sodium. The surrogate sludge did not contain uranium or other radioactive metals, which are present in the real material.

"Our tests showed that cooking the pond sludge in graphite containers in an electrical resistance furnace has several effects," Gabbard said. "The volume of the material is reduced by more than two-thirds. The material is transformed into glass, or vitrified, in the shape of the container. When the material is a heated liquid, some valuable metals-mainly, iron, nickel, and copper-migrate to outer surfaces as 'gold spots' that can be easily separated magnetically from the rest of the material once it has hardened. Recovering such useful metals is in the spirit of the Resource Conservation and Recovery Act."

Gabbard said the researchers would like to test actual pond sludge samples containing uranium to see if it also precipitates out of the mud as magnetic "gold spots" along with the iron, copper, and nickel. In Gabbard's vision, if this technique actually could mine uranium from the mud, the uranium could be stored for potential use as fuel for nuclear power plants.

The researchers do not yet understand why iron, nickel, copper, and sulfur in the sludge form golden globules that are not strongly attached to the rest of the material. Scanning electron micrographs show that the numerous iron-nickel-copper globules that make the black glass sparkle are surrounded by a sulfur skin. Gabbard thinks this skin may keep the spherical globules from bonding with the molten mud as it solidifies.

The formation and separation of the globules, Gabbard said, are likely due to the oxygen-free conditions created by the treatment method. To dry the sludge, an oxygen scavenger was used to remove oxygen, and the sludge was heated to 2100 degrees F (1150 degrees C) in a nonoxidizing, helium atmosphere in the furnace. By contrast, in situ vitrification processes for turning radioactively contaminated soil into glass in tests at ORNL waste burial grounds add oxygen to the vitrified material.

The researchers also found that the best containers for the sludge were made of graphite, not ceramics. The ceramic containers, called crucibles, became quite brittle and fractured during the heating process, but the graphite crucibles were not visibly affected and were reusable.

Pond sludge at the Oak Ridge K-25 Site is being stored in drums. DOE is evaluating three vendor processes to treat the sludge for disposal at Envirocare in Cleve, Utah. The cost for the treatment, transportation, and disposal is estimated to be approximately $50 million.

While acknowledging the high cost of vitrifying the sludge by electrical resistance heating, Gabbard suggests that vitrification may be an economical approach in the long run.

"The metals in the mud are a resource that could be sold for industrial use," he says. "If uranium can be recovered by this technique, it could be sold for energy production. After the metals are separated out, the remaining crushed glassy material could be reclassified as a waste material that requires low-cost disposal. This approach eliminates the need for long-term monitoring. " .

The research was supported by the Environmental Management and Restoration Program led by J. M. Kennerly at the K-25 Site. The source of its funding is DOE's Office of Environmental Management.

You can learn more about this research and many other exciting projects by visiting ORNL on Oct. 21, 1995, during its Community Day event. Many of our facilities will be open to the public that day. For additional information, call ORNL Public Affairs, 865-574-4160.

ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Martin Marietta Energy Systems, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant. Martin Marietta Energy Systems is a Lockheed Martin company.