One of the downsides of high-tech tools, such as computers, is the difficulty and cost of disposing their wastes - used circuit boards, vacuum tubes, transistors, etc. - which contain a combination of organic compounds, potentially leachable contaminants and metals.
Now, thanks to scientists at the U.S. Department of Energy's (DOE) Savannah River Technology Center, Aiken, S.C., a new way has been developed to recover the useful precious and base metals contained in these wastes, allowing the remainder to be immobilized into glass that can be landfilled or used as fillers in concrete, asphalt, brick or tile.
The DOE patented process - a new vitrification technique for stabilizing and reducing the volume of organics-containing waste, such as ion exchange resins, radioactive waste sludges and electronics components - was developed by Dennis Bickford, advisory engineer, and Connie Cicero-Herman, senior engineer. The process adds a catalyst to a waste vitrification process, resulting in useful metals recovery and organic constituents oxidation.
There previously has been no cost-effective process for selectively recovering these metals from organics-containing waste, oxidizing the organics and stabilizing the waste residue for disposal. The new process is suitable for any size waste stream, according to Cicero-Herman, depending only on the melter size used.
Facilities treating waste with large amounts of organic constituents - especially the nuclear service industry, with its need for on-site treatment and volume reduction of ion exchange resins - may find this technique useful.
Because glass is very stable, vitrification has been studied for decades in connection with radioactive wastes and some types of hazardous waste. However, when the waste contains organics or precious metals, it may not form a stable waste. Therefore, vitrification alone is not suitable for treating wastes that contain substantial amounts of organics or precious metals. These constituents must be recovered or destroyed before the waste residue can be converted into stable, environmentally safe glass products.
Bickford's and Cicero-Herman's technique eliminates the need for a separate pre-treatment to remove these constituents from the waste, resulting in significant cost savings:
* it eliminates the need to grind and oxidize waste prior to treatment;
* commercial metals (copper, nickel and tin) and precious metals (gold and silver) are recovered/recycled;
* hazardous materials (asbestos, lead, barium, chromium) are made into harmless products for disposal or construction use; and
* Resource Conservation and Recovery Act (RCRA) disposal costs are eliminated.
To begin this new process, the waste is broken up into small pieces and excess water is removed. Then, the waste is mixed with a transition metal oxide. If the waste composition does not include enough silicates or other glass formers, these also are added.
The transition metal oxide increases the oxidation rate of the organic materials, improving the composition of the glass-forming mixture. As the waste mixture is heated, the catalyst dissolves to oxidize more of the organics. When heated to vitrification temperatures of 1100 degrees Celsius to 1500 degrees Celsius, the metal ions conduct oxygen into the melt to oxidize the remaining organics. In addition, the transition metal oxide reduces metals such as gold, silver, platinum and copper to the metallic state to separate from the melt and allow them to be removed. Ninety-nine percent of gold and platinum group metals and 90 percent of silver are removed.
After the metals are recovered, the remainder of the melt cools, forming a good leaching-resistant glass product that can be disposed of or put to good use. The resulting product is suitable for paving or glazing for tiles or ceramics, which also avoids RCRA disposal costs.
The Savannah River Technology Center is the applied research and development laboratory of the U.S. Department of Energy's Savannah River Site (SRS). SRS, operated by Westinghouse Savannah River Co. (Aiken) team, was built in the 1950s to produce nuclear materials for national defense. While the site continues to fulfill defense responsibilities, today the focus has shifted to waste management, environmental cleanup, the development and application of related technologies, and partnerships with private industry, academia and other government agencies.
The new vitrification process currently is undergoing testing at Clemson University, Clemson, S.C. However, the new method can be put to use by customizing the technique to a user's equipment and to the chemical and physical properties of the user's compositions.
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